J
Joseph Gwinn
Guest
On Apr 12, 2019, Phil Hobbs wrote
(in article<a8KdnfNtO80RSi3BnZ2dnUU7-fXNnZ2d@supernews.com>
:
Iâm not visualizing this. In my mental picture, the reflection misses the
lens, and so cannot get to the hapless laser diode.
..
I think we have different optical systems in mind. .
What if one has two collimators, one expanding the beam, the other
compressing the beam, squinting at one another from the same side, but facing
one another. Would not the coma cancel out?
Joe Gwinn
(in article<a8KdnfNtO80RSi3BnZ2dnUU7-fXNnZ2d@supernews.com>
:On 4/12/19 12:25 PM, Joseph Gwinn wrote:
On Apr 12, 2019, Phil Hobbs wrote
(in article<J_-dndTKptWxHi3BnZ2dnUU7-b2dnZ2d@supernews.com>
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-semiconduct
or
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.
Iâm not visualizing this. In my mental picture, the reflection misses the
lens, and so cannot get to the hapless laser diode.
..
I think we have different optical systems in mind. .
What if one has two collimators, one expanding the beam, the other
compressing the beam, squinting at one another from the same side, but facing
one another. Would not the coma cancel out?
Joe Gwinn