Speaking* of green laser diodes.

[Phil Hobbs]

On 4/11/19 1:26 PM, George Herold wrote:

On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater system.
These seem to be the only good green laser diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=

There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple longitudinal modes.
That's not necessarily a bad thing--it makes them much less sensitive to
mode hopping due to backreflections. You sure don't want to use them
for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the above?
It's just because I'm having a hard time understanding how one can
get lasing in different longitudinal modes.
(My simple model is that the longitudinal mode with the highest gain
'wins' and sucks all the gain out of the other modes... The single
mode DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then those would
occupy different areas in the gain medium, and I can then picture
the thing lasing in different modes, (using different portions of the
gain medium, and not 'sucking' the gain away from each other... Though
it does get a little complicated looking... and I can imagine that the
modes interact with each other some, as you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are nodes and
antinodes of the standing wave, leading to spatial hole burning, where
the carriers are locally depleted near the antinodes. Different modes
have different antinode positions, but they do overlap quite a lot,
which makes the longitudinal modes strongly coupled.

OK, Thanks.
Lets see if the cavity was ~1mm (1000um) I'd get ~2000 wavelengths
in the thing and the longitudinal modes are separated by 500nm/2000
~0.25 nm (plus or minus a factor of 2) Can I see the modes with a
spectrometer? That would be fun.

You can see them by using a DVD as a grating. Use a reasonably broad
beam and come out of the DVD near grazing.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Lasse Langwadt Christense]

torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:

On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater system.
These seem to be the only good green laser diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=

There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple longitudinal modes.
That's not necessarily a bad thing--it makes them much less sensitive to
mode hopping due to backreflections. You sure don't want to use them
for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the above?
It's just because I'm having a hard time understanding how one can
get lasing in different longitudinal modes.
(My simple model is that the longitudinal mode with the highest gain
'wins' and sucks all the gain out of the other modes... The single
mode DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then those would
occupy different areas in the gain medium, and I can then picture
the thing lasing in different modes, (using different portions of the
gain medium, and not 'sucking' the gain away from each other... Though
it does get a little complicated looking... and I can imagine that the
modes interact with each other some, as you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are nodes and
antinodes of the standing wave, leading to spatial hole burning, where
the carriers are locally depleted near the antinodes. Different modes
have different antinode positions, but they do overlap quite a lot,
which makes the longitudinal modes strongly coupled.

OK, Thanks.
Lets see if the cavity was ~1mm (1000um) I'd get ~2000 wavelengths
in the thing and the longitudinal modes are separated by 500nm/2000
~0.25 nm (plus or minus a factor of 2) Can I see the modes with a
spectrometer? That would be fun.


You can see them by using a DVD as a grating. Use a reasonably broad
beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s

 

[Phil Hobbs]

On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:

torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater system.
These seem to be the only good green laser diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=

There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple longitudinal modes.
That's not necessarily a bad thing--it makes them much less sensitive to
mode hopping due to backreflections. You sure don't want to use them
for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the above?
It's just because I'm having a hard time understanding how one can
get lasing in different longitudinal modes.
(My simple model is that the longitudinal mode with the highest gain
'wins' and sucks all the gain out of the other modes... The single
mode DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then those would
occupy different areas in the gain medium, and I can then picture
the thing lasing in different modes, (using different portions of the
gain medium, and not 'sucking' the gain away from each other... Though
it does get a little complicated looking... and I can imagine that the
modes interact with each other some, as you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are nodes and
antinodes of the standing wave, leading to spatial hole burning, where
the carriers are locally depleted near the antinodes. Different modes
have different antinode positions, but they do overlap quite a lot,
which makes the longitudinal modes strongly coupled.

OK, Thanks.
Lets see if the cavity was ~1mm (1000um) I'd get ~2000 wavelengths
in the thing and the longitudinal modes are separated by 500nm/2000
~0.25 nm (plus or minus a factor of 2) Can I see the modes with a
spectrometer? That would be fun.


You can see them by using a DVD as a grating. Use a reasonably broad
beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s

It's a little tougher with laser modes, but perfectly doable. It's very
instructive to put a photodiode where it measures just one mode and see
just how noisy it is. Typically the fluctuations are of order unity.

I've also used it to measure the tuning range of single-frequency diodes
between mode jumps. You just tune till the spot suddenly changes
position, and voila.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Jeroen Belleman]

On 2019-04-11 23:08, George Herold wrote:

On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater system.
These seem to be the only good green laser diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=

There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple longitudinal modes.
That's not necessarily a bad thing--it makes them much less sensitive to
mode hopping due to backreflections. You sure don't want to use them
for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the above?
It's just because I'm having a hard time understanding how one can
get lasing in different longitudinal modes.
(My simple model is that the longitudinal mode with the highest gain
'wins' and sucks all the gain out of the other modes... The single
mode DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then those would
occupy different areas in the gain medium, and I can then picture
the thing lasing in different modes, (using different portions of the
gain medium, and not 'sucking' the gain away from each other... Though
it does get a little complicated looking... and I can imagine that the
modes interact with each other some, as you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are nodes and
antinodes of the standing wave, leading to spatial hole burning, where
the carriers are locally depleted near the antinodes. Different modes
have different antinode positions, but they do overlap quite a lot,
which makes the longitudinal modes strongly coupled.

OK, Thanks.
Lets see if the cavity was ~1mm (1000um) I'd get ~2000 wavelengths
in the thing and the longitudinal modes are separated by 500nm/2000
~0.25 nm (plus or minus a factor of 2) Can I see the modes with a
spectrometer? That would be fun.


You can see them by using a DVD as a grating. Use a reasonably broad
beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's very
instructive to put a photodiode where it measures just one mode and see
just how noisy it is. Typically the fluctuations are of order unity.

I've also used it to measure the tuning range of single-frequency diodes
between mode jumps. You just tune till the spot suddenly changes
position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes?
How the Heck does That happen!? It implies multiple optical
path lengths. I know the carrier density (how much current you are
pushing through the diode) can change the optical length*... but
still at one time it seems like there should be one optical length.

Is it lasing at many modes at once.. or jumping around between
modes all the time?

I think if it's the later, there's some chance you could do
some external feedback and make it single mode at low
currents, (above threshold)

George H.

That question seems to make sense only if you think of lasers
along the lines of stimulated emission of identical photons.

I tend to think of it as oscillations in a medium that has gain
over a certain bandwidth that covers many modes at once. Each
mode has a slightly different *frequency*. The path lengths are
the same, but the wavelengths are not.

Jeroen Belleman

 

[George Herold]

On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:

On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater system.
These seem to be the only good green laser diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=

There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple longitudinal modes.
That's not necessarily a bad thing--it makes them much less sensitive to
mode hopping due to backreflections. You sure don't want to use them
for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the above?
It's just because I'm having a hard time understanding how one can
get lasing in different longitudinal modes.
(My simple model is that the longitudinal mode with the highest gain
'wins' and sucks all the gain out of the other modes... The single
mode DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then those would
occupy different areas in the gain medium, and I can then picture
the thing lasing in different modes, (using different portions of the
gain medium, and not 'sucking' the gain away from each other... Though
it does get a little complicated looking... and I can imagine that the
modes interact with each other some, as you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are nodes and
antinodes of the standing wave, leading to spatial hole burning, where
the carriers are locally depleted near the antinodes. Different modes
have different antinode positions, but they do overlap quite a lot,
which makes the longitudinal modes strongly coupled.

OK, Thanks.
Lets see if the cavity was ~1mm (1000um) I'd get ~2000 wavelengths
in the thing and the longitudinal modes are separated by 500nm/2000
~0.25 nm (plus or minus a factor of 2) Can I see the modes with a
spectrometer? That would be fun.


You can see them by using a DVD as a grating. Use a reasonably broad
beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's very
instructive to put a photodiode where it measures just one mode and see
just how noisy it is. Typically the fluctuations are of order unity.

I've also used it to measure the tuning range of single-frequency diodes
between mode jumps. You just tune till the spot suddenly changes
position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes?
How the Heck does That happen!? It implies multiple optical
path lengths. I know the carrier density (how much current you are
pushing through the diode) can change the optical length*... but
still at one time it seems like there should be one optical length.

Is it lasing at many modes at once.. or jumping around between
modes all the time?

I think if it's the later, there's some chance you could do
some external feedback and make it single mode at low
currents, (above threshold)

George H.
*Besides the heating effects of current.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:

On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
~2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.

OK, I think it's a mistake in my model.
Should I think of it as more gain than one mode can handle?
On a microscopic level, (as you said above) there are nodes
in any single mode. An electron hanging around at a node,
looking to make a transition to the valence band, may
find a different mode to dance with.

Thanks for the links. I'll check 'em out.

George H.

I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages, <https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Cheers

Phil Hobbs

George H. *Besides the heating effects of current.

Cheers

Phil Hobbs

-- Dr Philip C D Hobbs Principal Consultant ElectroOptical
Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics,
Photonics, Analog Electronics Briarcliff Manor NY 10510

http://electrooptical.net http://hobbs-eo.com



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Thursday, April 11, 2019 at 5:23:13 PM UTC-4, Jeroen Belleman wrote:

On 2019-04-11 23:08, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater system.
These seem to be the only good green laser diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=

There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple longitudinal modes.
That's not necessarily a bad thing--it makes them much less sensitive to
mode hopping due to backreflections. You sure don't want to use them
for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the above?
It's just because I'm having a hard time understanding how one can
get lasing in different longitudinal modes.
(My simple model is that the longitudinal mode with the highest gain
'wins' and sucks all the gain out of the other modes... The single
mode DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then those would
occupy different areas in the gain medium, and I can then picture
the thing lasing in different modes, (using different portions of the
gain medium, and not 'sucking' the gain away from each other... Though
it does get a little complicated looking... and I can imagine that the
modes interact with each other some, as you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are nodes and
antinodes of the standing wave, leading to spatial hole burning, where
the carriers are locally depleted near the antinodes. Different modes
have different antinode positions, but they do overlap quite a lot,
which makes the longitudinal modes strongly coupled.

OK, Thanks.
Lets see if the cavity was ~1mm (1000um) I'd get ~2000 wavelengths
in the thing and the longitudinal modes are separated by 500nm/2000
~0.25 nm (plus or minus a factor of 2) Can I see the modes with a
spectrometer? That would be fun.


You can see them by using a DVD as a grating. Use a reasonably broad
beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's very
instructive to put a photodiode where it measures just one mode and see
just how noisy it is. Typically the fluctuations are of order unity.

I've also used it to measure the tuning range of single-frequency diodes
between mode jumps. You just tune till the spot suddenly changes
position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes?
How the Heck does That happen!? It implies multiple optical
path lengths. I know the carrier density (how much current you are
pushing through the diode) can change the optical length*... but
still at one time it seems like there should be one optical length.

Is it lasing at many modes at once.. or jumping around between
modes all the time?

I think if it's the later, there's some chance you could do
some external feedback and make it single mode at low
currents, (above threshold)

George H.

That question seems to make sense only if you think of lasers
along the lines of stimulated emission of identical photons.

Well, that is how I think about it. There's nothing 'wrong' with
the photon picture of nature. And sometimes it's a much easier
picture.

I tend to think of it as oscillations in a medium that has gain
over a certain bandwidth that covers many modes at once. Each
mode has a slightly different *frequency*. The path lengths are
the same, but the wavelengths are not.

Hmm, there's some Fabry-Perot (FP) cavity that defines the modes.
Feedback is needed, it's not a single pass gain medium.
(I think... It was years ago, I tried to measure the
output facet reflection coef. of a diode laser.. 90% ?)

90% reflection works out to some 'specificity' in the
FP cavity...
It's been too long, so from here,
http://web.mit.edu/2.710/Fall06/2.710-wk8-a-ho.pdf

Page 6 does the finesse, and I guesstimated the FSR above..
(Well I didn't put in the index of refraction (or the factor of two))

So once one of these modes gets above threshold.. (which one that is
may be a little random.) It starts to lase... and stimulated emission
as you say, takes over. Maybe it's a failure of my model (which is fine
by me) but I can't see how a laser does multiple longitudinal modes
at once. Some oscillation, progression between modes.. that I could
understand. (It could even be chaotic mode hopping.)

George H.







> Jeroen Belleman

 

[Phil Hobbs]

On 4/11/19 5:08 PM, George Herold wrote:

On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
~2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.

I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.

Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.

I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages, <https://hololaser.wordpress.com> and
<http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
<http://hololaser.kwaoo.me/laser/ECDL-test.html>

Cheers

Phil Hobbs

George H. *Besides the heating effects of current.

Cheers

Phil Hobbs

-- Dr Philip C D Hobbs Principal Consultant ElectroOptical
Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics,
Photonics, Analog Electronics Briarcliff Manor NY 10510

http://electrooptical.net http://hobbs-eo.com

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:

On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
~2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.


I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages, <https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with one
diode laser is that the position of the coupling lens is the most
tweaky adjustment. And putting a layer of teflon tape on the
threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

Cheers

Phil Hobbs

George H. *Besides the heating effects of current.

Cheers

Phil Hobbs

-- Dr Philip C D Hobbs Principal Consultant ElectroOptical
Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics,
Photonics, Analog Electronics Briarcliff Manor NY 10510

http://electrooptical.net http://hobbs-eo.com



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 4/11/19 9:43 PM, George Herold wrote:

On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
~2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.


I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages, <https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with one
diode laser is that the position of the coupling lens is the most
tweaky adjustment. And putting a layer of teflon tape on the
threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

George H. *Besides the heating effects of current.

Yup, backreflections from the lens (and even the window in the package)
are a problem. Stick-slip in the threads is a nuisance, though I
haven't tried your teflon tape trick.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

To make the high power blues and greens usable we often need cylindrical lens pairs or anamorphic prisms. The diode die is quite wide in the big ones, and a fast axis divergence of 35 degrees or more from the uncollimated diode is not unusable. To get the traditional "tight" beam from these we use a 3-4 mm focal length lens.

Steve

 

[George Herold]

On Friday, April 12, 2019 at 9:36:19 AM UTC-4, srober...@gmail.com wrote:

To make the high power blues and greens usable we often need cylindrical lens pairs or anamorphic prisms. The diode die is quite wide in the big ones, and a fast axis divergence of 35 degrees or more from the uncollimated diode is not unusable. To get the traditional "tight" beam from these we use a 3-4 mm focal length lens.

Steve

Hi Steve, if that was in response to my lens comment, I should clarify
and say I was talking about the lens in a ECDL. It controls the
coupling between the diode and the grating.

From Phil's link. The osram PL520 works in a ECDL.

http://hololaser.kwaoo.me/laser/ECDL-test.html#PL520

Fun stuff.

George h.

 

[George Herold]

On Friday, April 12, 2019 at 10:45:22 AM UTC-4, bill....@ieee.org wrote:

On Friday, April 12, 2019 at 10:23:18 PM UTC+10, Phil Hobbs wrote:
On 4/11/19 9:43 PM, George Herold wrote:
On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
~2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.


I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages, <https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with one
diode laser is that the position of the coupling lens is the most
tweaky adjustment. And putting a layer of teflon tape on the
threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

George H. *Besides the heating effects of current.

Yup, backreflections from the lens (and even the window in the package)
are a problem. Stick-slip in the threads is a nuisance, though I
haven't tried your teflon tape trick.

Teflon (PTFE) tape is available from plumbing suppliers.

It lubricates the thread, so you can do the joints up more tightly - to the point of bursting the joint if you overdo it (or so I was warned - I've never done it or seen it done).

My experience was that it took six layers of tape to make a water-tight seal (which might have been cheap screw threads that had big gaps to fill - optical instruments may get better quality taps and dies than plumbing supplies).

--
Bill Sloman, Sydney

In this case a single layer of teflon tape is used on the threads of a
Thor labs lens tube. Setting the lens position 'just right' is
critical. The tape mitigated the backlash. Which makes finding and
setting the correct position much easier.

George H.

 

On Friday, April 12, 2019 at 10:23:18 PM UTC+10, Phil Hobbs wrote:

On 4/11/19 9:43 PM, George Herold wrote:
On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
~2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.


I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages, <https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with one
diode laser is that the position of the coupling lens is the most
tweaky adjustment. And putting a layer of teflon tape on the
threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

George H. *Besides the heating effects of current.

Yup, backreflections from the lens (and even the window in the package)
are a problem. Stick-slip in the threads is a nuisance, though I
haven't tried your teflon tape trick.

Teflon (PTFE) tape is available from plumbing suppliers.

It lubricates the thread, so you can do the joints up more tightly - to the point of bursting the joint if you overdo it (or so I was warned - I've never done it or seen it done).

My experience was that it took six layers of tape to make a water-tight seal (which might have been cheap screw threads that had big gaps to fill - optical instruments may get better quality taps and dies than plumbing supplies).

--
Bill Sloman, Sydney

 

[Joseph Gwinn]

On Apr 12, 2019, Phil Hobbs wrote
(in article<J_-dndTKptWxHi3BnZ2dnUU7-b2dnZ2d@supernews.com&gt:

On 4/11/19 9:43 PM, George Herold wrote:

On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductor
s-inc/PL-520_B1/PL520_B1-ND/5719264
DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?

https://www.osram.com/apps/product_selector/#!/?query=*&sortField=

&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&f
ilters=color,True%20Green%20(513-545%20nm)&deeplink=
There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.
OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s

It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.


I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages,<https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with one
diode laser is that the position of the coupling lens is the most
tweaky adjustment. And putting a layer of teflon tape on the
threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

George H. *Besides the heating effects of current.

Yup, backreflections from the lens (and even the window in the package)
are a problem. Stick-slip in the threads is a nuisance, though I
haven't tried your teflon tape trick.

As for back reflections from windows (and maybe lenses), there is a trick
from photography to cause reflections to fall in harmless places. Basically,
if one displaces object to one side the image will migrate the other way, and
back reflections will no longer re-enter the lens. This is an application of
the Scheimpflug
principle:<https://en.wikipedia.org/wiki/Scheimpflug_principle>

Joe Gwinn

 

On Saturday, April 13, 2019 at 1:29:07 AM UTC+10, George Herold wrote:

On Friday, April 12, 2019 at 10:45:22 AM UTC-4, bill....@ieee.org wrote:
On Friday, April 12, 2019 at 10:23:18 PM UTC+10, Phil Hobbs wrote:
On 4/11/19 9:43 PM, George Herold wrote:
On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?query=*&sortField=&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&filters=color,True%20Green%20(513-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
~2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.


I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages, <https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with one
diode laser is that the position of the coupling lens is the most
tweaky adjustment. And putting a layer of teflon tape on the
threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

George H. *Besides the heating effects of current.

Yup, backreflections from the lens (and even the window in the package)
are a problem. Stick-slip in the threads is a nuisance, though I
haven't tried your teflon tape trick.

Teflon (PTFE) tape is available from plumbing suppliers.

It lubricates the thread, so you can do the joints up more tightly - to the point of bursting the joint if you overdo it (or so I was warned - I've never done it or seen it done).

My experience was that it took six layers of tape to make a water-tight seal (which might have been cheap screw threads that had big gaps to fill - optical instruments may get better quality taps and dies than plumbing supplies).

In this case a single layer of teflon tape is used on the threads of a
Thor labs lens tube. Setting the lens position 'just right' is
critical. The tape mitigated the backlash. Which makes finding and
setting the correct position much easier.

Presumably the backlash represents situations where the metal-to-metal contact had welded or dug in, and you've had to apply a lot of torque to break the weld, which has then moved the two threads along for an appreciable distance before they dig in again.

Putting in a lubricating layer - so you don't get metal-to-metal contact or a risk of local welding - could be expected to minimise this.

A couple of layers of Teflon tape might work better than just one - obviously, if you try too put in too much you won't be able to get the threads to engage, but anything short of that should serve the purpose.

--
Bill Sloman, Sydney

 

[Phil Hobbs]

On 4/12/19 9:36 AM, sroberts6328@gmail.com wrote:

To make the high power blues and greens usable we often need cylindrical lens pairs or anamorphic prisms. The diode die is quite wide in the big ones, and a fast axis divergence of 35 degrees or more from the uncollimated diode is not unusable. To get the traditional "tight" beam from these we use a 3-4 mm focal length lens.

Steve

I often just aperture them to get rid of the astigmatism. In the lab I
usually use a Mitutoyo microscope lens as a DL collimator. You can get
fast-axis collimating lenses, but they aren't easy to use.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 4/12/19 12:25 PM, Joseph Gwinn wrote:

On Apr 12, 2019, Phil Hobbs wrote
(in article<J_-dndTKptWxHi3BnZ2dnUU7-b2dnZ2d@supernews.com&gt:

On 4/11/19 9:43 PM, George Herold wrote:

On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil Hobbs
wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-semiconductor
s-inc/PL-520_B1/PL520_B1-ND/5719264
DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?

https://www.osram.com/apps/product_selector/#!/?query=*&sortField=

&sortOrder=&start=0&filters=producttype,Visible%20Laser%20Diodes&f
ilters=color,True%20Green%20(513-545%20nm)&deeplink=
There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want to
use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and sucks
all the gain out of the other modes... The single mode
DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium, and
not 'sucking' the gain away from each other... Though it
does get a little complicated looking... and I can
imagine that the modes interact with each other some, as
you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite a
lot, which makes the longitudinal modes strongly coupled.
OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd get
2000 wavelengths in the thing and the longitudinal modes are
separated by 500nm/2000 ~0.25 nm (plus or minus a factor of
2) Can I see the modes with a spectrometer? That would be
fun.


You can see them by using a DVD as a grating. Use a reasonably
broad beam and come out of the DVD near grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s

It's a little tougher with laser modes, but perfectly doable. It's
very instructive to put a photodiode where it measures just one
mode and see just how noisy it is. Typically the fluctuations are
of order unity.

I've also used it to measure the tuning range of single-frequency
diodes between mode jumps. You just tune till the spot suddenly
changes position, and voila.
Right, I've done that with a spectrometer and single mode diodes.

Can I ask a question about these multiple longitudinal modes? How the
Heck does That happen!? It implies multiple optical path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc cycles
per round trip. OPL is very nearly the same for all, but they're at
different frequencies.


I know the carrier density (how much current you are pushing through
the diode) can change the optical length*... but still at one time it
seems like there should be one optical length.

OPL is measured in metres. The eigenmodes of a given cavity form a comb
in 1/lambda. In the absence of dispersion, that's an equally spaced
frequency comb.


Is it lasing at many modes at once.. or jumping around between modes
all the time?

Generally it's lasing in many modes at once, though I suppose it's
possible to make a system that has one mode at a time. A nominally
single-frequency laser can hop between modes, as we both know very well.


I think if it's the later, there's some chance you could do some
external feedback and make it single mode at low currents, (above
threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold first, and if
the gain delta between modes is big enough, it'll maintain itself stably.

I recommend Wolfgang's pages,<https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser behaviour.

It's somewhat rambling and poorly organized, but his plots of laser
noise vs. temperature and frequency are super illuminating.
Specifically, he evaluates a lot of diodes for ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with one
diode laser is that the position of the coupling lens is the most
tweaky adjustment. And putting a layer of teflon tape on the
threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

George H. *Besides the heating effects of current.

Yup, backreflections from the lens (and even the window in the package)
are a problem. Stick-slip in the threads is a nuisance, though I
haven't tried your teflon tape trick.
As for back reflections from windows (and maybe lenses), there is a trick
from photography to cause reflections to fall in harmless places. Basically,
if one displaces object to one side the image will migrate the other way, and
back reflections will no longer re-enter the lens. This is an application of
the Scheimpflug
principle:<https://en.wikipedia.org/wiki/Scheimpflug_principle

Joe Gwinn

Sort of. In a laser collimator, light reflected back down the beam axis
hits the laser exactly, regardless of minor lens misalignment.

It can help a bit with the first-surface reflection from the collimator,
especially if the first surface is concave.

Problem is, in a LD collimator the coma builds up pretty fast as you go
off axis.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

bill.sloman@ieee.org wrote in
news:12e4709e-5bd9-4199-98be-8132be15ca28@googlegroups.com:

On Friday, April 12, 2019 at 10:23:18 PM UTC+10, Phil Hobbs wrote:
On 4/11/19 9:43 PM, George Herold wrote:
On Thursday, April 11, 2019 at 9:04:55 PM UTC-4, Phil Hobbs
wrote:
On 4/11/19 5:08 PM, George Herold wrote:
On Thursday, April 11, 2019 at 4:40:35 PM UTC-4, Phil Hobbs
wrote:
On 4/11/19 3:51 PM, Lasse Langwadt Christensen wrote:
torsdag den 11. april 2019 kl. 20.24.56 UTC+2 skrev Phil
Hobbs:
On 4/11/19 1:26 PM, George Herold wrote:
On Thursday, April 11, 2019 at 11:39:57 AM UTC-4, Phil
Hobbs wrote:
On 4/10/19 12:15 PM, George Herold wrote:
On Wednesday, April 10, 2019 at 7:21:53 AM UTC-4, Phil
Hobbs wrote:
On 4/9/19 8:05 PM, George Herold wrote:
*I think it was speff and a new 3-D laser theater
system. These seem to be the only good green laser
diodes
https://www.digikey.com/product-detail/en/osram-opto-
s
emiconductors-inc/PL-520_B1/PL520_B1-ND/5719264



DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
data sheet here?
https://www.osram.com/apps/product_selector/#!/?
query=
*&sortField=&sortOrder=&start=0
&filters=producttype,Vi
sible%20Laser%20Diodes&filters=color,True%20Green%20
(5
13-545%20nm)&deeplink=



There's a deeplink at the end so be careful.

George H.

Most short-wavelength diode lasers have multiple
longitudinal modes. That's not necessarily a bad
thing--it makes them much less sensitive to mode
hopping due to backreflections. You sure don't want
to use them for tunable-diode spectroscopy, though.

Cheers

Phil Hobbs
Say Phil, did you mean multiple transverse modes in the
above? It's just because I'm having a hard time
understanding how one can get lasing in different
longitudinal modes. (My simple model is that the
longitudinal mode with the highest gain 'wins' and
sucks all the gain out of the other modes... The single
mode DL's I use have many longitudinal modes.)

Whereas if there were different transverse modes, then
those would occupy different areas in the gain medium,
and I can then picture the thing lasing in different
modes, (using different portions of the gain medium,
and not 'sucking' the gain away from each other...
Though it does get a little complicated looking... and
I can imagine that the modes interact with each other
some, as you said in your later post.

George H.

Nope, longitudinal. In a Fabry-Perot laser, there are
nodes and antinodes of the standing wave, leading to
spatial hole burning, where the carriers are locally
depleted near the antinodes. Different modes have
different antinode positions, but they do overlap quite
a lot, which makes the longitudinal modes strongly
coupled.

OK, Thanks. Lets see if the cavity was ~1mm (1000um) I'd
get ~2000 wavelengths in the thing and the longitudinal
modes are separated by 500nm/2000 ~0.25 nm (plus or minus
a factor of 2) Can I see the modes with a spectrometer?
That would be fun.


You can see them by using a DVD as a grating. Use a
reasonably broad beam and come out of the DVD near
grazing.

the DVD trick is neat, https://youtu.be/EoAZ-u6hn6g?t=1m19s



It's a little tougher with laser modes, but perfectly
doable. It's very instructive to put a photodiode where it
measures just one mode and see just how noisy it is.
Typically the fluctuations are of order unity.

I've also used it to measure the tuning range of
single-frequency diodes between mode jumps. You just tune
till the spot suddenly changes position, and voila.
Right, I've done that with a spectrometer and single mode
diodes.

Can I ask a question about these multiple longitudinal modes?
How the Heck does That happen!? It implies multiple optical
path lengths.

No. As you noted upthread, you can have 2000, 2001, 2002, etc
cycles per round trip. OPL is very nearly the same for all,
but they're at different frequencies.


I know the carrier density (how much current you are pushing
through the diode) can change the optical length*... but
still at one time it seems like there should be one optical
length.

OPL is measured in metres. The eigenmodes of a given cavity
form a comb in 1/lambda. In the absence of dispersion, that's
an equally spaced frequency comb.


Is it lasing at many modes at once.. or jumping around
between modes all the time?

Generally it's lasing in many modes at once, though I suppose
it's possible to make a system that has one mode at a time. A
nominally single-frequency laser can hop between modes, as we
both know very well.


I think if it's the later, there's some chance you could do
some external feedback and make it single mode at low
currents, (above threshold)

A lot of lasers are single-mode near threshold. At any given
temperature, there will be one mode that crosses threshold
first, and if the gain delta between modes is big enough,
it'll maintain itself stably.

I recommend Wolfgang's pages,
https://hololaser.wordpress.com> and
http://hololaser.kwaoo.me>, for a lot more about diode laser
behaviour.

It's somewhat rambling and poorly organized, but his plots of
laser noise vs. temperature and frequency are super
illuminating. Specifically, he evaluates a lot of diodes for
ECDL operation at
http://hololaser.kwaoo.me/laser/ECDL-test.html

Huh.. I'll have to read a bunch of that. My experience with
one diode laser is that the position of the coupling lens is
the most tweaky adjustment. And putting a layer of teflon tape
on the threads of the Thor labs lens helps a bunch in removing
hysteresis in the threads. (and other wobbles)

George H.

George H. *Besides the heating effects of current.

Yup, backreflections from the lens (and even the window in the
package) are a problem. Stick-slip in the threads is a nuisance,
though I haven't tried your teflon tape trick.

Teflon (PTFE) tape is available from plumbing suppliers.

It lubricates the thread, so you can do the joints up more tightly
- to the point of bursting the joint if you overdo it (or so I was
warned - I've never done it or seen it done).

My experience was that it took six layers of tape to make a
water-tight seal (which might have been cheap screw threads that
had big gaps to fill - optical instruments may get better quality
taps and dies than plumbing supplies).

The pipe ends teflon tape is typically used on have a thread
placed on them which is slightly tapered on the first few threads.

Also with internal threaded pipes in the family. So the match up
starts out very open and loose and works toward a full on
interference fit.

Six layers is fine, because teflon has a 'cold flow' attribute, so
the seal eventually would compress out literally hydraulically
because of the closure of the tapers.

 

George Herold <gherold@teachspin.com> wrote in news:ecb652d8-2646-4c1a-
9a08-d812c8e24746@googlegroups.com:

> The tape mitigated the backlash.

Much better term to use in this instance than was "hysteresis".