Laser locking

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Phil Hobbs

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Mar 18, 2011, 10:24:12 AM3/18/11
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So, I have this one-week-per-month consulting gig in New Mexico, working
with a geophysical instruments startup. (It's a great outfit, one that
you'll be hearing more about.)

One of the things I'm doing for them is developing highly stable lasers
for, *ahem*, hostile and size-constrained environments. I have a few
books on laser locking, e.g. Ohtsu, but I really need a more recent
summary of the field so I don't miss anything important.

Any suggestions for reviews or monographs on laser locking?

Thanks

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Salmon Egg

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Mar 18, 2011, 11:59:32 AM3/18/11
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In article <4D836B0C...@electrooptical.net>,
Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:

> So, I have this one-week-per-month consulting gig in New Mexico, working
> with a geophysical instruments startup. (It's a great outfit, one that
> you'll be hearing more about.)
>
> One of the things I'm doing for them is developing highly stable lasers
> for, *ahem*, hostile and size-constrained environments. I have a few
> books on laser locking, e.g. Ohtsu, but I really need a more recent
> summary of the field so I don't miss anything important.
>
> Any suggestions for reviews or monographs on laser locking?
>
> Thanks
>
> Phil Hobbs

That is what libraries and laser conferences are for. But you know that.
Of course you can hire a consultant. Tony Siegman who is on this group
from time to time is emeritus. Intota offers some consulting. I am not
up to snuff on the subject.

Bill

--
An old man would be better off never having been born.

Phil Hobbs

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Mar 18, 2011, 12:08:39 PM3/18/11
to

Thanks.

If I had a library or laser conference right handy, that would be great.
;)

I don't need someone to do it for me. I just need to make sure I know
what relevant things other folks have done, to double-check that my
approach is the right one for the job. For instance, although I'm using
R-T locking with an optically-contacted ULE glass etalon, it might be
that there's an approach using molecular absorption lines that would
work better or be cheaper overall. A bunch of things like that have
been tried, and my most recent reference is over a decade old.

Besides, it's been too quiet round here, so I thought I'd ask.

Cheers

Lostgallifreyan

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Mar 18, 2011, 12:55:29 PM3/18/11
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Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote in
news:4D838387...@electrooptical.net:

> Besides, it's been too quiet round here

Until today, perhaps. :) My only excuse is that I haven't been here for ages.

Skywise

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Mar 18, 2011, 11:39:43 PM3/18/11
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Lostgallifreyan <no-...@nowhere.net> wrote in
news:Xns9EACAC29E74...@216.196.109.145:

> Until today, perhaps. :) My only excuse is that I haven't been here for
> ages.

What do you mean ages? Just hop in that TARDIS of yours and pop
around the space-time continuum.

And don't allow any bloody Daleks follow you!!!

:)

Brian
--
http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism
Seismic FAQ: http://www.skywise711.com/SeismicFAQ/SeismicFAQ.html
Quake "predictions": http://www.skywise711.com/quakes/EQDB/index.html
Sed quis custodiet ipsos Custodes?

AES

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Mar 19, 2011, 12:36:13 AM3/19/11
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In article <4D838387...@electrooptical.net>,
Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:

>
> I don't need someone to do it for me. I just need to make sure I know
> what relevant things other folks have done, to double-check that my
> approach is the right one for the job. For instance, although I'm using
> R-T locking with an optically-contacted ULE glass etalon, it might be
> that there's an approach using molecular absorption lines that would
> work better or be cheaper overall. A bunch of things like that have
> been tried, and my most recent reference is over a decade old.
>

For those kinds of questions, Jan Hall would be much more the guy to
chat with -- and he's approachable, and a serious need might also be a
motivator for him.

--AES

Phil Hobbs

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Mar 19, 2011, 6:21:53 AM3/19/11
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I was visiting JILA earlier this week, coincidentally, and discussed the
problem with Jan, Jun Ye, and Dana Anderson. Great bunch of folks who
do amazing things. They're working in kind of a different regime,
though....locking to doubly-forbidden absorption lines in lattices of
trapped strontium atoms, for instance. Bose-Einstein condensations too.
Magic.

I got to visit the clock guys at NIST as well, which was neat.

What I was looking for here was a book or review paper published in the
last 10 years or so.

Lostgallifreyan

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Mar 19, 2011, 12:28:28 PM3/19/11
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Skywise <in...@oblivion.nothing.com> wrote in
news:3AVgp.77869$BQ7....@newsfe22.ams2:

> Lostgallifreyan <no-...@nowhere.net> wrote in
> news:Xns9EACAC29E74...@216.196.109.145:
>
>> Until today, perhaps. :) My only excuse is that I haven't been here for
>> ages.
>
> What do you mean ages? Just hop in that TARDIS of yours and pop
> around the space-time continuum.
>
> And don't allow any bloody Daleks follow you!!!
>
>:)
>
> Brian

That TARDIS is SO nonfunctional, sadly...
In other news, apparently I am no longer the only Lostgallifreyan on the net.
襘O I had to get my old domain back before it got nabbed. This was easy
because not even the domain name squatters were prepared to hang on to it,
but suddenly it seems that at least two people were about to use it. But this
isn't about lasers either...

Lostgallifreyan

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Mar 19, 2011, 12:32:43 PM3/19/11
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Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote in
news:4D8483C1...@electrooptical.net:

> Bose-Einstein condensations too.
> Magic.
>

Awesome. It seems like magic to me too, all I know is that they slow light to
a crawl, and that alone is weird enough.

Actually, these new discoveries have two aspects that seem at odds, one is
that they sometimes confirm things I've suspected for decades, like
macroscopic quantum solids existing on scales people can look at unaided, but
other times the findings are totally strange. It's hard to understand because
I don't get much info that I can grasp. Sometimes I suspect the only way to
really grasp it is to be there, to see how it connects to the world as
experienced. Quantum or no quantum, I doubt that's changed much.

Helpful person

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Mar 19, 2011, 2:51:16 PM3/19/11
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On Mar 19, 9:32 am, Lostgallifreyan <no-...@nowhere.net> wrote:
> Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote innews:4D8483C1...@electrooptical.net:

As one with no knowledge of the conditions to slow light down I have a
question. Presumably the materials have a very high refractive
index. Can conditions exist to create a lens?

www.richardfisher.com

Phil Hobbs

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Mar 19, 2011, 3:18:22 PM3/19/11
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Slow light is usually made using a very narrow resonance in e.g.
rubidium vapour. The Kramers-Kronig relations (known as the causality
condition in electronics) give the relationship of phase to amplitude
variation. Group velocity is d_omega/d_k, which is very small in the
interior of a very sharp resonance.

There are other methods too, e.g. people have used spectral hole burning
to record and play back an optical pulse, so if you're willing to use a
Hollywood definition of propagation velocity, it can be as small as you
like.

Phil Hobbs

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Mar 19, 2011, 3:18:54 PM3/19/11
to

I'm not an expert, but AFAIK 'slow light' is usually made using a very

narrow resonance in e.g. rubidium vapour. The Kramers-Kronig relations
(known as the causality condition in electronics) give the relationship
of phase to amplitude variation. Group velocity is d_omega/d_k, which
is very small in the interior of a very sharp resonance.

There are other methods too, e.g. people have used spectral hole burning
to record and play back an optical pulse, so if you're willing to use a
Hollywood definition of propagation velocity, it can be as small as you
like.

Cheers

Salmon Egg

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Mar 19, 2011, 4:16:02 PM3/19/11
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In article
<652f443e-2cf3-4617...@17g2000prr.googlegroups.com>,
Helpful person <rrl...@yahoo.com> wrote:

Having worked on microwaves long before optics, the use of slow wave
structures such as helices and coupled resonator wave guides were not
strange. The basic idea was that the radial part of the wavefunction had
a modified bessel function rather than a bessel function dependence. Not
only that, but it was possible to have interactions when phase and group
velocities were in opposite directions. That is how backward wave
oscillators operated.

Not having studied modern slow light, is there a quantum version of the
regular vs modified bessel function approach to describe slow light?

Salmon Egg

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Mar 19, 2011, 4:22:32 PM3/19/11
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In article <4D85017...@electrooptical.net>,
Phil Hobbs <pcdhSpamM...@electrooptical.net> wrote:

> Slow light is usually made using a very narrow resonance in e.g.
> rubidium vapour. The Kramers-Kronig relations (known as the causality
> condition in electronics) give the relationship of phase to amplitude
> variation. Group velocity is d_omega/d_k, which is very small in the
> interior of a very sharp resonance.

In Bode's book on feedback amplifiers, he discussed realizability of
network functions. I believe those are equivalent to the Kramers-Kronig
relations. I do not know which version was discovered first.

Jürgen Appel

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Mar 20, 2011, 6:31:13 AM3/20/11
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Hello Phil,

>>> One of the things I'm doing for them is developing highly stable lasers
>>> for, *ahem*, hostile and size-constrained environments. I have a few
>>> books on laser locking, e.g. Ohtsu, but I really need a more recent
>>> summary of the field so I don't miss anything important.

> If I had a library or laser conference right handy, that would be great.
> ;)
>
> I don't need someone to do it for me. I just need to make sure I know
> what relevant things other folks have done, to double-check that my
> approach is the right one for the job. For instance, although I'm using
> R-T locking with an optically-contacted ULE glass etalon, it might be
> that there's an approach using molecular absorption lines that would
> work better or be cheaper overall. A bunch of things like that have
> been tried, and my most recent reference is over a decade old.

As far as I know Pound-Drever-Hall-locking to an ULE cavity in vacuum is
still the state-of-the art technique to lock away frequency noise at high
sideband frequencies down to the few Hz-range. It's main advantage is that
it gives a great signal-to noise ratio and therefore needs little averaging
time. That makes it appropriate for fast feedback loops. For higher sideband
frequencies than your feedback can achieve (for example there is a nasty
180° phase shift in the transfer function for frequency modulation via diode
injection current somewhere between 1-10 MHz for most diodes) it might make
sense to protocol your error signal and correct your measurement instead, if
that is possible.

In optical atomic clock applications very low sideband frequency phase noise
(<-> frequency drifts) matters too, and thus the next step then is to lock
your stable laser to a frequency comb and from there to a frequency chain of
oscillators with lower and lower sideband frequency noise - the last and
slowest step is the lock to an atomic transition.

If the requirements are not so high, or if the power consumption or the
budget don't allow this, finding some kind of an atomic transition line
close to your laser frequency indeed gives an alternative way to avoid long
term drifts of your resonator.

Among the newer approaches that I have heard about to replace the medium- to
high phase-noise sideband frequency locks by
* Ultra-High-Q glass resonators (such as bottle-resonators or disk-
resonators made out of single crystals or glas).
* Temperature-stabilized fiber interferometers.

Unfortunately I don't have references at hand right now, but maybe I can
find some during the week.

It really depends what kind of measurements you are going to do, to what
kind of phase/frequency noise you are sensitive to, but of course you also
know that.

All the best,
Jürgen

Jürgen Appel

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Mar 20, 2011, 6:49:18 AM3/20/11
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Salmon Egg wrote:


> Not having studied modern slow light, is there a quantum version of the
> regular vs modified bessel function approach to describe slow light?

As long as it's linear optics, the quantum mechanical equations are
identical to the classical ones and therefore their solutions are as well.

To see any quantum optical effects that are more interesting than what you
could get with a set of beam splitters you need nonlinearities.

Cheers,
Jürgen

Jürgen Appel

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Mar 20, 2011, 7:00:58 AM3/20/11
to
Helpful person wrote:

> As one with no knowledge of the conditions to slow light down I have a
> question.

As already mentioned for slow light no Bose-Einstein-Condensate is needed.
All you need is a spectrally narrow transparency window (or gain) in your
medium, then the Kramers-Kronig relations which relate Dispersion and
absorpton to each other lead to a group velocity < c.

The effect is first described already in
Sommerfeld, A. Über die Fortpflanzung des Lichtes in dispergierenden Medien
Annalen Der Physik, 1914, 349, 177-202

and more specifically in

Garrett, C. G. B. & McCumber, D. E. Propagation of a Gaussian Light Pulse
through an Anomalous Dispersion Medium Physical Review A, 1970, 1, 305-313


[Bose-Einstein-Condensates]


> Presumably the materials have a very high refractive
> index. Can conditions exist to create a lens?

Yes. That's actually one of the main problems when one wants to couple them
to optics: Huge optical densities in the center and strong density gradients
together with a size that is not much bigger than an optical wavelength make
BECs more often than not behave more than a scatterer than a nice optical
element.

Cheers
Jürgen

Lineshape

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Mar 20, 2011, 8:49:04 PM3/20/11
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On Mar 19, 3:18 pm, Phil Hobbs

One of the other methods to slow light that's received some attention
recently through stimulated Brillouin scattering in optical fibers,
which provides an amplifying resonance. Long fibers and interaction
lengths mean low control powers, and you can get pretty large
bandwidths as well (albeit with reduced slowing, but the fractional
delays are similar). Any telecom fiber and control wavelength will
do, so SBS is a practical and relatively easy way to observe the
effect.

Frank.

p.ki...@ic.ac.uk

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Mar 23, 2011, 5:02:41 PM3/23/11
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Salmon Egg <Salm...@sbcglobal.net> wrote:
> but it was possible to have interactions when phase and group
> velocities were in opposite directions.

And quite a popular subject these days it is too :-)

--
---------------------------------+---------------------------------
Dr. Paul Kinsler
Blackett Laboratory (Photonics) (ph) +44-20-759-47734 (fax) 47714
Imperial College London, Dr.Paul...@physics.org
SW7 2AZ, United Kingdom. http://www.qols.ph.ic.ac.uk/~kinsle/

George Herold

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Mar 28, 2011, 1:46:59 PM3/28/11
to
On Mar 18, 10:24 am, Phil Hobbs

I don't have any nice references to suggest. But do you know about
(DAVLL laser locking). (Some atrocious acronym.) This again uses an
atomic line, but you use a B-field and polarized beam splitter to
separate two Faraday rotation signals. It’s a about the same
sensitivity as side locking to some Doppler free hyperfine
transition, But It’s a lot more stable. Pound on the table stable!
You can also tune it over the whole doppler broadened absorption
feature.

George H.

Phil Hobbs

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Mar 29, 2011, 7:54:17 PM3/29/11
to

Thanks, George.
I don't recognize the acronym, but Zeeman splitting an atomic line is
mentioned in the references I already have. I bought a few books on
diode lasers in the mean time, so I have a fair amount of reading to do.

I think I've managed to convince the client to do a bit of work on
thermoacoustic fridges, which will be very interesting. I haven't done
much thermodynamics since grad school, and that was all physicsy stuff
like Mayer cluster expansions, not red-blooded stuff like heat engines.
Besides, I've forgotten most of it. ;)

Cheers

George Herold

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Mar 30, 2011, 11:31:52 AM3/30/11
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On Mar 29, 7:54 pm, Phil Hobbs

Hmm, You might also call it resonant Faraday rotation. I guess it's
related to the Zeeman splitting. (My atomic physics is not very
good.)

Say anyone read the Atomic Physics text by Budker etal?

"ATOMIC PHYSICS
An Exploration Through Problems and Solutions"

It sounds like the perfect book for an atomic physics lab.

>
> I think I've managed to convince the client to do a bit of work on
> thermoacoustic fridges, which will be very interesting.  

Do they use those to liquify natural gas? Or do you have some other
use in mind?

George H.

I haven't done
> much thermodynamics since grad school, and that was all physicsy stuff
> like Mayer cluster expansions, not red-blooded stuff like heat engines.
>   Besides, I've forgotten most of it. ;)
>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
>

> email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net- Hide quoted text -
>
> - Show quoted text -

Phil Hobbs

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Mar 30, 2011, 12:05:28 PM3/30/11
to

Some interesting work has been done on using thermally-pumped
thermoacoustic fridges to liquefy stranded gas, i.e. gas fields out in
the middle of nowhere that are too small to justify building a pipeline.
Projections based on data from a pilot plant suggest that a full scale
setup could liquefy over 80% of the gas by burning the other 15-20%.

Sure beats flaring it off.

Acoustic fridges driven by voice coils or linear motors are a lot more
flexible, but far bulkier and not as tough. The parlour trick will be
to make the fridge work over the entire range of ambient conditions,
because really all you can adjust is the heater power, and possibly the
gas pressure. On the other hand, it's OK if the efficiency is less at
smaller delta-T values, so I think it's probably possible.

Back in the day, thermoacoustic fridges were all standing-wave devices,
but (iirc) Greg Swift of Los Alamos figured out how to build
travelling-wave devices by putting a Helmholtz resonator on the cold
end. (A Helmholtz resonance is of the mass-spring type, like blowing
across the mouth of a beer bottle--the note is far lower in frequency
than the organ pipe resonance. The mass is the air in the neck, and the
spring is the air in the wide part.) By tuning the resonance, you can
get the phase shift you need to approximate a travelling wave. They
even make true travelling wave devices using loops.

Fun stuff.

Phil Hobbs

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Mar 30, 2011, 12:15:54 PM3/30/11
to
George Herold wrote:
> On Mar 29, 7:54 pm, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>> George Herold wrote:
>>> On Mar 18, 10:24 am, Phil Hobbs
>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>>> So, I have this one-week-per-month consulting gig in New Mexico, working
>>>> with a geophysical instruments startup. (It's a great outfit, one that
>>>> you'll be hearing more about.)
>>
>>>> One of the things I'm doing for them is developing highly stable lasers
>>>> for, *ahem*, hostile and size-constrained environments. I have a few
>>>> books on laser locking, e.g. Ohtsu, but I really need a more recent
>>>> summary of the field so I don't miss anything important.
>>
>>>> Any suggestions for reviews or monographs on laser locking?
>>
>>>> Thanks
>>
>>>> Phil Hobbs
>>

>>


>>> I don't have any nice references to suggest. But do you know about
>>> (DAVLL laser locking). (Some atrocious acronym.) This again uses an
>>> atomic line, but you use a B-field and polarized beam splitter to
>>> separate two Faraday rotation signals. It’s a about the same
>>> sensitivity as side locking to some Doppler free hyperfine
>>> transition, But It’s a lot more stable. Pound on the table stable!
>>> You can also tune it over the whole doppler broadened absorption
>>> feature.
>>
>>> George H.
>>
>> Thanks, George.
>> I don't recognize the acronym, but Zeeman splitting an atomic line is
>> mentioned in the references I already have. I bought a few books on
>> diode lasers in the mean time, so I have a fair amount of reading to do.
>
> Hmm, You might also call it resonant Faraday rotation. I guess it's
> related to the Zeeman splitting. (My atomic physics is not very
> good.)
>

Oh, okay, so it's a phase sensitive method rather than just +- slope
detection. Nice. I'll go see if I can find something on it.

George Herold

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Mar 30, 2011, 5:41:38 PM3/30/11
to
On Mar 30, 12:15 pm, Phil Hobbs

No, it's not phase sensitive. When I type DAVLL into google the
first hit (from Davidson college) pretty well sums up my
understanding.

George H.

>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
>

George Herold

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Mar 30, 2011, 6:15:02 PM3/30/11
to
On Mar 30, 12:05 pm, Phil Hobbs

Yeah, a few years ago I was job searching and ran across an add by a
gas company looking for someone to help make thermo-acoustic
resonators. I spent a weekend reading about 'em. But didn't apply for
the job. They wanted someone with ten years of experience after
all.

George H.


>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
>

George Herold

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Mar 30, 2011, 6:38:30 PM3/30/11
to
On Mar 30, 12:05 pm, Phil Hobbs

Can you change the gas mix? (I'm not sure if that 'buys' you
anything.. It's mostly just the ideal gas law?)

George H.

>
> Back in the day, thermoacoustic fridges were all standing-wave devices,
> but (iirc) Greg Swift of Los Alamos figured out how to build
> travelling-wave devices by putting a Helmholtz resonator on the cold
> end.  (A Helmholtz resonance is of the mass-spring type, like blowing
> across the mouth of a beer bottle--the note is far lower in frequency
> than the organ pipe resonance.  The mass is the air in the neck, and the
> spring is the air in the wide part.)  By tuning the resonance, you can
> get the phase shift you need to approximate a travelling wave.  They
> even make true travelling wave devices using loops.
>
> Fun stuff.
>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
>

Phil Hobbs

unread,
Mar 30, 2011, 9:07:41 PM3/30/11
to
George Herold wrote:
> On Mar 30, 12:05 pm, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>> George Herold wrote:
>>> On Mar 29, 7:54 pm, Phil Hobbs
>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>>> George Herold wrote:
>>>>> On Mar 18, 10:24 am, Phil Hobbs
>>>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>>>>> So, I have this one-week-per-month consulting gig in New Mexico, working
>>>>>> with a geophysical instruments startup. (It's a great outfit, one that
>>>>>> you'll be hearing more about.)
>>
>>>>>> One of the things I'm doing for them is developing highly stable lasers
>>>>>> for, *ahem*, hostile and size-constrained environments. I have a few
>>>>>> books on laser locking, e.g. Ohtsu, but I really need a more recent
>>>>>> summary of the field so I don't miss anything important.
>>
>>>>>> Any suggestions for reviews or monographs on laser locking?
>>
>>>>>> Thanks
>>
>>>>>> Phil Hobbs

Yes, and that can help a fair bit by changing the ratio of the thermal
and viscous diffusion lengths. It turns out that you win by putting
some heavy gas like xenon in with your helium--you get better thermal
diffusion than with the heavy gas and lower viscosity than with just helium.

I'm budgeting a week to come up to speed, a couple of days to find
parts, and a week and a half to simulate a few designs and see what
works best. Then we'll try building something and see.

Cheers

Phil Hobbs

George Herold

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Mar 31, 2011, 10:01:28 AM3/31/11
to
On Mar 30, 9:07 pm, Phil Hobbs

Sounds like an exotic gas mix. Is SF6 as good as Xenon?

It's amazing how slow diffusion is. As a post doc we used a CO2 laser
to pump a FIR laser. (You'd put different organic gases into the FIR
tube..) The gas for the CO2 laser was expensive. And I figured we'd
save money by mixing it ourselves. I put in the standard mixture of
He, N2 and CO2. Didn't work. I tried again... utter failure again.
The tank of mixed gas was set asside. Perhaps a month later a grad
student was taking data deep into the night and ran out of CO2 gas.
With nothing to loose he took the tank I'd mixed off the wall and
hooked it up. Worked perfect!

Turns out it takes weeks for the gases to diffuse within the tank.

George H.

>
> I'm budgeting a week to come up to speed, a couple of days to find
> parts, and a week and a half to simulate a few designs and see what
> works best.  Then we'll try building something and see.
>
> Cheers
>

> Phil Hobbs- Hide quoted text -

Phil Hobbs

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Mar 31, 2011, 10:11:22 AM3/31/11
to

I doubt it. Stuff with internal degrees of freedom is never as good for
acoustics, on account of the inelastic collisions. Also the hot HX is
around 1100 C, so I'd worry about galloping corrosion. Krypton is
cheaper, however. Also these fridges are going to be small, so even at
30 bar, it'll only need a litre or so of xenon (at STP).

>
> It's amazing how slow diffusion is. As a post doc we used a CO2 laser
> to pump a FIR laser. (You'd put different organic gases into the FIR
> tube..) The gas for the CO2 laser was expensive. And I figured we'd
> save money by mixing it ourselves. I put in the standard mixture of
> He, N2 and CO2. Didn't work. I tried again... utter failure again.
> The tank of mixed gas was set asside. Perhaps a month later a grad
> student was taking data deep into the night and ran out of CO2 gas.
> With nothing to loose he took the tank I'd mixed off the wall and
> hooked it up. Worked perfect!
>
> Turns out it takes weeks for the gases to diffuse within the tank.

The Royal Society has a large glass tube with two colours of water in
it, that has been diffusing since about 1850, iirc. It's nowhere near
completely mixed.

Lineshape

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Mar 31, 2011, 11:07:40 AM3/31/11
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> > email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net-Hide quoted text -

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> > - Show quoted text -


As you've probably found, DAVLL is dichroic atomic vapor laser lock.
It operates by essentially subtracting the Doppler broadened profiles
obtained for different polarizations under magnetic field (~100 Gauss
or so). It does have a fairly large capture range (Dopper-ish), but
not large tunability. Because of the large capture range, it is
stable against mechanical disturbances, but some reports have claimed
it is not so stable under temperature fluctuations (see Reeves et al,
Appl. Opt. 45, p372, 2006, accessible here) -

http://galileo.phys.virginia.edu/research/groups/sackett/publications/Reeves_06.pdf

Despite the temp fluctuations they observed, I know many groups do use
it successfully.

For large capture range and tunability in our cold atom work, we use a
beat frequency method that gives a few GHz tunability with ~200MHz
capture range without modulating the laser (the zero-crossing/
derivative signal to lock to is generated without modulation). As it
is beat-frequency-based, it requires one laser locked to a reference
for absolute stability; in most atomic work that is readily
available. I've never published that method (partly because I'm
guessing someone else has already done it - for all I know it's well-
known outside my field and by everyone on this list), but the cold
atom groups I've told about it have adopted it.

Essentially the beat signal from a photodetector is split and
recombined in a mixer; one cable in this RF interferometer arm is
longer, so that the DC output of the mixer is sinusoidal as the
frequency is changed. Since the interferometer is in rigid components
(e.g. SMA cable), it's very stable and we easily lock to sub MHz.

Frank.

Phil Hobbs

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Mar 31, 2011, 11:24:45 AM3/31/11
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Thanks, it's an interesting method. I'll read up on it some more.

Jürgen Appel

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Apr 2, 2011, 11:59:58 AM4/2/11
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Phil Hobbs wrote:

>> For large capture range and tunability in our cold atom work, we use a
>> beat frequency method that gives a few GHz tunability with ~200MHz
>> capture range without modulating the laser (the zero-crossing/
>> derivative signal to lock to is generated without modulation). As it
>> is beat-frequency-based, it requires one laser locked to a reference
>> for absolute stability; in most atomic work that is readily
>> available. I've never published that method (partly because I'm
>> guessing someone else has already done it - for all I know it's well-
>> known outside my field and by everyone on this list), but the cold
>> atom groups I've told about it have adopted it.
>>
>> Essentially the beat signal from a photodetector is split and
>> recombined in a mixer; one cable in this RF interferometer arm is
>> longer, so that the DC output of the mixer is sinusoidal as the
>> frequency is changed.

Nowadays there are digital Phase-Frequency-Discriminator chips around that
can divide down your beat signal from the GHz range. Using such a chip has
the advantage that the lock offset can be changed easily - you don't have to
change the cable in your RF-interferometer and don't need many expensive RF-
components like splitters and phase shifters (->
http://arxiv.org/abs/0809.3607).

But of course you only get frequency stability if you have a stable
reference laser within the bandwidth of your detector.

>> Since the interferometer is in rigid components
>> (e.g. SMA cable), it's very stable and we easily lock to sub MHz.

Using a phase-locked beat signal, it's easy to lock the _relative_ frequency
to a Hertz (given enough averaging time). The catch is to get the reference
laser stable and the limit are the optical path-length fluctuations (air
movement etc) up to the beat-signal detector.

DAVLL can be a method to lock the reference. Its advantage is more its large
capture range than its absolute precision. I have no experience how such a
system would work in a mobile application with changing magnetic fields and
temperatures. As far as I know the method that is state-of-the-art for
locking to atomic lines nowadays is modulation transfer spectroscopy.

Cheers,
Jürgen


Lineshape

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Apr 3, 2011, 8:57:10 PM4/3/11
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The RF interferometer doesn't need a cable change if a VCO is placed
at the front (tunable over ~capture range), but we don't end up using
one because the main places we lock (various atomic hyperfine
frequencies) all are fortuitously captured with the same length
cable. It is a little expensive, but for commercial parts the price
is offset since no lock-in or modulation is required. I'm sort of
fortunate that where I work, many of the people are RF photonics guys
who tend to surplus "old GHz-level" microwave parts, which they
consider DC....

My old group used a counter/divide technique for beat locking, though
I never really knew the details of it. I'll take a look at your
paper, seems nice.

Of course, it would be best to have an inexpensive portable fs
frequency comb to lock wherever one desired.

Frank

George Herold

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Apr 26, 2011, 12:23:07 PM4/26/11
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> > > email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net-Hidequoted text -

>
> > > - Show quoted text -
>
> As you've probably found, DAVLL is dichroic atomic vapor laser lock.
> It operates by essentially subtracting the Doppler broadened profiles
> obtained for different polarizations under magnetic field (~100 Gauss
> or so).  It does have a fairly large capture range (Dopper-ish), but
> not large tunability.  Because of the large capture range, it is
> stable against mechanical disturbances, but some reports have claimed
> it is not so stable under temperature fluctuations (see Reeves et al,
> Appl. Opt. 45, p372, 2006, accessible here) -
>
> http://galileo.phys.virginia.edu/research/groups/sackett/publications...

>
> Despite the temp fluctuations they observed, I know many groups do use
> it successfully.
>

FWIW, I was browsing the latest RSI.(March, 2011). There's an article
from (I assume) Will Happers group at Princeton. By artful choice of
the linear polarization and 1/4 waveplate angles, they are able to
remove the first order temperature dependence from the DAVLL
technique.

"Temperature-insensitive laser frequency locking near absorption
lines" N. Kostinski etal, RSI, 82, 033114

George H.
> Frank.- Hide quoted text -

Phil Hobbs

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Mar 26, 2022, 7:54:02 PMMar 26
to
Phil Hobbs wrote:
> AES wrote:
>> In article<4D838387...@electrooptical.net>,
>>   Phil Hobbs<pcdhSpamM...@electrooptical.net>  wrote:
>>
>>>
>>> I don't need someone to do it for me. I just need to make sure I know
>>> what relevant things other folks have done, to double-check that my
>>> approach is the right one for the job.  For instance, although I'm using
>>> R-T locking with an optically-contacted ULE glass etalon, it might be
>>> that there's an approach using molecular absorption lines that would
>>> work better or be cheaper overall.  A bunch of things like that have
>>> been tried, and my most recent reference is over a decade old.
>>>
>>
>> For those kinds of questions, Jan Hall would be much more the guy to
>> chat with -- and he's approachable, and a serious need might also be a
>> motivator for him.
>>
>>     --AES
>
> I was visiting JILA earlier this week, coincidentally, and discussed the
> problem with Jan, Jun Ye, and Dana Anderson.  Great bunch of folks who
> do amazing things.  They're working in kind of a different regime,
> though....locking to doubly-forbidden absorption lines in lattices of
> trapped strontium atoms, for instance.  Bose-Einstein condensations too.
> Magic.
>
> I got to visit the clock guys at NIST as well, which was neat.
>
> What I was looking for here was a book or review paper published in the
> last 10 years or so.
>
> 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

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