On Apr 12, 2019, Phil Hobbs wrote (in article):
I?m 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
Fiber couplers too. (or is that what you mean by a collimator?) When I measure LD's in the wavemeter Jr. I always have to add a 'poor mans' isolator; linear polarizer and 1/4 wave plate. Best is if it's the first element before reflection, (as you know.)
George H.
:
rote:
Hobbs:
l Hobbs
Phil
ater
laser
am-opto-semiconduct
wtf?
=s&f
deeplink=
e d ewant to
.in the
d sucks
mode
then
dium,
ent
um, and
ough it
nome, as
re
o yuite a
upled.
I'd get
modes are
tor of
d be
sonably
m19s
e. It's
one
ns are
quency
enly
des.
How the
ths.
ycles
e at
through
time it
m a comb
aced
n modes
t's
lly
ery well.
ome
bove
t, and if
lf stably.
and
ehaviour.
aser
tage)
rick
ically,
way, and
tion of
sses the
Hi Joe, if I may interrupt. I think you are talking about two different reflections. In photography (which I don't know so well) the light source is off axis from the detector (well, most of the time, with a flash they ar e close.) In the laser diode case the diode is source and detector. People pay ~$1k for Faraday opto-isolators.
George H.
,off
cing
Light propagation is time-reversal symmetric. If you send it back the way it came, it winds up where it began.
Two collimators? Are you talking about anamorphic prisms? A collimator is a lens.
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
The circular polarization trick is always worth a try--it generally gets you about 40 dB (optical) isolation. Problem is, you need >60 dB for real insurance against mode hopping due to back-reflections. That needs a two-stage Faraday isolator.
RF modulation of the diode laser bias current can make that a lot easier.
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 Apr 12, 2019, Phil Hobbs wrote (in article):
[big snip]ly,
tion
diameter free-space light beam connecting them. The free-space beams traverse various interesting optical components, some of which have annoying reflections. The intent is to misdirect the various reflections such that they miss the collimator apertures and so are lost.
Joe Gwinn
Worth a read, I had this open and reading it about four months ago...
They have a blue version in another journal:
High-power broadly tunable grating-coupled external cavity laser in green region
Review of Scientific Instruments 89, 125106 (2018);
Steve
Minor misalignment of the free-space optics is a win, because the available etendue (area-solid angle product) is so large compared with a fibre that you can misalign them without losing signal.
The collimators themselves don't have that luxury.
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
Just returned from two weeks away.
On Apr 14, 2019, Phil Hobbs wrote (in article):
age
ric
ica
way
lic
, 2mmThe lenses typically have (or can be required to have) a larger NA than the fiber, so there should be some wiggle room before light is lost.
Unless some loss of light is acceptable?
With single-mode graded-index fiber, there would be no reason not to point the light cone directly at the lens aperture, as both "object" and "image" are ~ diffraction-limited spots.
I may be missing something critical. Is there an article I should read?
always wondered what physics entity (like energy or momentum) is being conserved by etendue.
Joe Gwinn
Nope. Light propagation is time-reversal symmetric, so in a single mode, if misalignment reduces the reverse coupling by N dB, it reduces the forward coupling by the same factor.
A whole lot of loss.
Conservation of etendue, at bottom, is the same physics as the Heisenberg uncertainty--you can't localize both the real-space and k-space wave function simultaneously to better than a certain bound.
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 Apr 27, 2019, Phil Hobbs wrote (in article):
pac
I aBa
her
ap
axi
ato
u g
tI do see that, now.
Conservation of uncertainty, because both concern waves. I do recall this being mentioned somewhere, but how they got there was not explained. Again, is there an article or book chapter that I should read?
Joe Gwinn
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.