Speaking* of green laser diodes.

[George Herold]

*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.

 

[Phil Hobbs]

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

--
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

 

On Wednesday, April 10, 2019 at 9:21:53 PM UTC+10, 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.

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.

If you do want a single mode output you do have to stabilise the diode temperature - the temperature inside the diode where the gain medium is, though since you can't get at it, it in practice you do tend to stabilise the external temperature around the diode as well as the power dissipation in the diode junction.

--
Bill Sloman, Sydney

 

[George Herold]

On Wednesday, April 10, 2019 at 8:42:38 AM UTC-4, bill....@ieee.org wrote:

On Wednesday, April 10, 2019 at 9:21:53 PM UTC+10, 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.

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.

If you do want a single mode output you do have to stabilise the diode temperature - the temperature inside the diode where the gain medium is, though since you can't get at it, it in practice you do tend to stabilise the external temperature around the diode as well as the power dissipation in the diode junction.

--
Bill Sloman, Sydney

The forward voltage of the diode is 6-7 volts! At currents of
~>100 mA... there's going to be a bit of heat to get rid of.
I'm thinking a TEC will be required.

George H.

 

[George Herold]

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.

Hmm OK, I don't have any experience with multi mode lasers.
(well some HeNe's)
So the coherence length is going to stink.
A tunable diode laser thing won't work because there are too many nearby
modes? I need a good diode laser book, or review article.

George H.

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

 

On Wednesday, April 10, 2019 at 11:06:53 PM UTC+10, George Herold wrote:

On Wednesday, April 10, 2019 at 8:42:38 AM UTC-4, bill....@ieee.org wrote:
On Wednesday, April 10, 2019 at 9:21:53 PM UTC+10, 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.

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.

If you do want a single mode output you do have to stabilise the diode temperature - the temperature inside the diode where the gain medium is, though since you can't get at it, it in practice you do tend to stabilise the external temperature around the diode as well as the power dissipation in the diode junction.

The forward voltage of the diode is 6-7 volts! At currents of
~>100 mA... there's going to be a bit of heat to get rid of.
I'm thinking a TEC will be required.

My 1996 millidegree controller - with TEC - got adapted to keep a laser diode at constant temperature to allow it to stay in a single mode, pointing in the same direction.

The application didn't need a stable wavelength, but it did need the laser beam to point in exactly the same direction, and mode-hopping shifted the beam direction around. Not all that much, but enough to matter.

Even back then, the literature was full of TEC-based laser-diode temperature stabilising circuits. Since then there have been a couple of integrated circuits that handle most of the job, not all that well, but well enough for a lot of applications.

--
Bill Sloman, Sydney

 

[Phil Hobbs]

On 4/10/19 9:03 AM, 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.
Hmm OK, I don't have any experience with multi mode lasers.
(well some HeNe's)
So the coherence length is going to stink.
A tunable diode laser thing won't work because there are too many nearby
modes? I need a good diode laser book, or review article.

Yup, there are probably a lot of modes. What's more, there will be a
lot of mode-partition noise--generally much more than the total
intensity noise of the laser. The reason is interesting: the pumping
rate constrains the total output power, but not the way it's partitioned
between the modes (hence the name).

The modes are strongly coupled, and power moves back and forth between
them. Thus etalon fringes and that sort of thing will demodulate the
mode partition noise into huge amounts of RIN, even though the
undisturbed beam is actually pretty quiet-looking.

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

 

[Winfield Hill]

Phil Hobbs wrote...

Yup, there are probably a lot of modes. What's more, there will be a
lot of mode-partition noise--generally much more than the total
intensity noise of the laser. The reason is interesting: the pumping
rate constrains the total output power, but not the way it's partitioned
between the modes (hence the name).

The modes are strongly coupled, and power moves back and forth between
them. Thus etalon fringes and that sort of thing will demodulate the
mode partition noise into huge amounts of RIN, even though the
undisturbed beam is actually pretty quiet-looking.

Are there simple optical accessories that can quiet the situation?


--
Thanks,
- Win

 

[George Herold]

On Wednesday, April 10, 2019 at 11:42:09 AM UTC-4, Winfield Hill wrote:

Phil Hobbs wrote...

Yup, there are probably a lot of modes. What's more, there will be a
lot of mode-partition noise--generally much more than the total
intensity noise of the laser. The reason is interesting: the pumping
rate constrains the total output power, but not the way it's partitioned
between the modes (hence the name).

The modes are strongly coupled, and power moves back and forth between
them. Thus etalon fringes and that sort of thing will demodulate the
mode partition noise into huge amounts of RIN, even though the
undisturbed beam is actually pretty quiet-looking.

Are there simple optical accessories that can quiet the situation?


--
Thanks,
- Win

An attenuator will give less noise... and less signal :^)
Otherwise I think you are stuck. I think they put apertures
inside the laser cavity to suppress some of the transverse
modes. From here,
https://www.researchgate.net/post/What_is_the_difference_between_single_mode_laser_and_multimode_laser

George H.

 

[George Herold]

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.

--
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

 

[Jon Elson]

On Tue, 09 Apr 2019 17:05:35 -0700, 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

Great! You have to buy 200 pieces, minimum!

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
It is a crystal doubler (or maybe tripler) on an IR diode.

Jon

 

[speff]

On Wednesday, 10 April 2019 14:33:03 UTC-4, Jon Elson wrote:

On Tue, 09 Apr 2019 17:05:35 -0700, 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

Great! You have to buy 200 pieces, minimum!

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
It is a crystal doubler (or maybe tripler) on an IR diode.

Jon

Avnet Europe has 70 of the 30mW ones in stock for 24 EUR each, but they
want 60 EUR for shipping.

--Spehro Pefhany

 

First of all, if you want some to play with, Google "DTR's Laser Shop" and don't tell me how your stunned to see 1 Watt Green diodes..

Next up, Littman Metcalf cavities using a grazing incidence grating configuration for feedback works fine up to modest currents.

In the newer multimode, high power blues and greens, it is not unusual to see 3.5 nm FHWM spectrums unless there is feedback.

Even the little Osram single modes tune a few nanometers. One of my friends likes dunking the modern single mode diodes in LN2 to get drastic wavelength shifts while running at low currents.

Steve

 

[George Herold]

On Wednesday, April 10, 2019 at 2:33:03 PM UTC-4, Jon Elson wrote:

On Tue, 09 Apr 2019 17:05:35 -0700, 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

Great! You have to buy 200 pieces, minimum!

Well they don't have stock. You can probably buy
direct from Osram.. (200 min. :^)

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
It is a crystal doubler (or maybe tripler) on an IR diode.

No, I think this is a 'direct' 'bandgap', maybe a riff on blue
laser diodes? (With 7 V forward there is more going on than just
a band gap.)

George H.

Jon

 

[Phil Hobbs]

On 4/10/19 2:32 PM, Jon Elson wrote:

On Tue, 09 Apr 2019 17:05:35 -0700, 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

Great! You have to buy 200 pieces, minimum!

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
It is a crystal doubler (or maybe tripler) on an IR diode.

Jon

Nope. I'm using their 488 nm version, and it's just a wide bandgap
diode. Infrared diodes don't drop 6-9V.

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 4/10/19 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.

The rate equations are second order--the mode amplitude goes like the
time integral of the population inversion, which goes like the time
integral of the pumping rate minus the total amplitude. So depending on
the coefficients, it can oscillate. Some lasers, such as N2, have no
stable operating points--they always produce nanosecond pulses.

Getting rid of this problem is one reason for liking VCSELs. (Fast
modulation is the other reason.)

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 4/10/19 12:22 PM, George Herold wrote:

On Wednesday, April 10, 2019 at 11:42:09 AM UTC-4, Winfield Hill wrote:
Phil Hobbs wrote...

Yup, there are probably a lot of modes. What's more, there will be a
lot of mode-partition noise--generally much more than the total
intensity noise of the laser. The reason is interesting: the pumping
rate constrains the total output power, but not the way it's partitioned
between the modes (hence the name).

The modes are strongly coupled, and power moves back and forth between
them. Thus etalon fringes and that sort of thing will demodulate the
mode partition noise into huge amounts of RIN, even though the
undisturbed beam is actually pretty quiet-looking.

Are there simple optical accessories that can quiet the situation?


--
Thanks,
- Win

An attenuator will give less noise... and less signal :^)
Otherwise I think you are stuck. I think they put apertures
inside the laser cavity to suppress some of the transverse
modes. From here,
https://www.researchgate.net/post/What_is_the_difference_between_single_mode_laser_and_multimode_laser

George H.

It's multiple longitudinal modes, so apertures don't help. Modern
diodes are all index-guided, so transverse modes are pretty well
controlled. You do get spontaneous emission in higher transverse modes,
which interfere with the main beam and causes "wiggle noise".

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 4/10/19 11:41 AM, Winfield Hill wrote:

Phil Hobbs wrote...

Yup, there are probably a lot of modes. What's more, there will be a
lot of mode-partition noise--generally much more than the total
intensity noise of the laser. The reason is interesting: the pumping
rate constrains the total output power, but not the way it's partitioned
between the modes (hence the name).

The modes are strongly coupled, and power moves back and forth between
them. Thus etalon fringes and that sort of thing will demodulate the
mode partition noise into huge amounts of RIN, even though the
undisturbed beam is actually pretty quiet-looking.

Are there simple optical accessories that can quiet the situation?

Not really. You could select one mode with an etalon and apply
amplitude feedback, but the fading can be pretty deep so I wouldn't
expect that to work well. The best method to get a quiet green laser is
to use a doubled diode-pumped YAG.

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

 

[Jon Elson]

Phil Hobbs wrote:

On 4/10/19 2:32 PM, Jon Elson wrote:
On Tue, 09 Apr 2019 17:05:35 -0700, 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

Great! You have to buy 200 pieces, minimum!

DK link provided.
The output spectrum is terrible! 1-2 nm wide! wtf?
It is a crystal doubler (or maybe tripler) on an IR diode.

Jon


Nope. I'm using their 488 nm version, and it's just a wide bandgap
diode. Infrared diodes don't drop 6-9V.

OK, well, this is new(er) technology. Always something new being created in
opto-electronics!

Jon

 

[George Herold]

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.

George H.

The rate equations are second order--the mode amplitude goes like the
time integral of the population inversion, which goes like the time
integral of the pumping rate minus the total amplitude. So depending on
the coefficients, it can oscillate. Some lasers, such as N2, have no
stable operating points--they always produce nanosecond pulses.

Getting rid of this problem is one reason for liking VCSELs. (Fast
modulation is the other reason.)

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