Laser locking (control loops with two feedback paths.)

[George Herold]

Laser locking (control loops with two feedback paths.)

So I finally had a user ask about side locking our diode laser.
(That’s where you lock the frequency to the side of an absorption
feature.)
Now, you can change the laser frequency in two ways. There’s a piezo
stack that changes the angle of a diffraction grating. And you can
change the laser current.
The electronics is all set up to lock the laser with the piezo.
Signal chain looks like,

Photodiode->low pass (1 pole, tc = 100ms)->
DC offset->gain->modulation input of piezo control.

With some other bits of gain adjustment sprinkled in there. (The low
pass is working as both integrator and gain (PI), you crank up the
overall loop gain till it oscillates and then back off a bit.)
This works fine, up to ~3kHz the oscillation frequency.

Now I’ve heard tell of a trick where I break the error signal into a
low frequency and high frequency part. And then send the high
frequency part into the laser current modulation input.
It seems I should pick off the error signal before the lowpass (P/I
part of signal chain).(?)
But I'm wondering how to deal with the 'break frequency'
What frequency for the HP?
And do I roll off the 'DC' part at the break frequency too?
(1 pole each)
Or can I leave the rest of the 'DC' signal chain the same if I pick
the right frequency?

Thanks,
George H.

 

[George Herold]

Too many variables.

What are the characteristics of the modulation you get from the piezo vs.
modulating the laser current?

As long as the change is small they are both approximately linear.
Modulating the current also changes the amplitude... but I actually
take the difference of two photodiode signals to get the error
signal... so to first order the amplitude change caused by current
modulation shouldn't be that much of an issue.

(Hmm maybe I can generate freq vs 'voltage' scans for both the piezo
and the current.)

Why does your piezo loop tend to oscillate at around 3kHz?

Well back in the dim past I did a back of the envelope calculation and
figured this was the self resonant frequency of the piezo stack and
the piece of Aluminum that it is pushing around. (Ratio of mass of
aluminum vs mass of piezo to the one half power times the unloaded SRF
of the piezo.)
The Piezo is part number AE0203D04F made by Tokin and a rather long
link to a data sheet,

http://store.bravoelectro.com/redir...s.pdf&osCsid=cgek9fio38jfi297es1j5g8b0rq258qm

SRF ~ 261 kHz. I have no idea if the simple mass scaling is correct..
but about the right number came out the far side of the calculation.
The aluminum and grating are part of a flexure... I sorta wondered if
the spring constant is different too.... But I'm not sure how I get
the spring constant for either the piezo or the flexure, and the mass
was easy to measure. (I did try and do some measuments of the flexure
spring constant using the piezo as the sensor, very 'squishy'
measuments IIRC)

Why can't you just control the laser current?

Hmm... OK that's a good question. I'll have to try it!
But for long term DC drifts it's better to change the piezo (grating
angle.)

Do you want to have closed-loop control using the laser current, with
increased loop bandwidth, or do you just want to push the laser around
open loop at those high frequencies?

Oh for sure closed loop control with higher bandwidth. It'd be cool to
be able to really bang on the table and have the thing stay locked!

I think I've got a paper describing how someone else did this...(Carl
Weiman and Leo Hollberg?) it might be in here, (another long link...
to a RSI paper)
http://www.google.com/url?sa=t&rct=...cil_Sq6Fm7JQ4ypXg&sig2=5lnQLOJC42fKCF_VCTMF8Q

But sometimes it's more fun to 'invent' your own method and then see
what someone else did.

George H.

 

[George Herold]

Here's one leading candidate in the list of things that I'd try, then:

Make a block, call it "laser", with a frequency-steering signal in, and a
frequency out.

Inside of that block, take the frequency-steering signal and run it
through matched low- and high-pass filters.  Make the cutoff frequency
lower than the piezo resonance.  Take the low-pass filter, run it through
a notch at the piezo resonance frequency, and feed it to the piezo.  Take
the high-pass filter, and feed it to the diode current.  Jigger gains
around so that your GHz/whatever (I'm not assuming digital or analog at
this point -- GHz/volt, GHz/ADC count, whatever).

If your laser current response stays flat up to a much higher frequency
than the piezo does, then the overall response of your "laser" block
should also be flat out that high, probably with a hiccup around the
frequency where you transition from piezo to laser current, and possibly
around the piezo resonance, too (although you can damp that one out by
playing with your notch and the cutoff frequency).

Now wrap a loop around that.  Figure that when you knock on the table
you'll see it in amplitude -- your cavity will be changing, and you'll be
fixing it by changing the laser current, so you'll see it in amplitude.

Have fun.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

Hi Tim, Thanks for that! I logged in to report that I tried locking
with just current modulation... one peice at a time so to speak. And
that worked fine, I could bang a bit more on the table. But the
current loop oscillates at ~20kHz when I crank up the gain. I don't
understand that at all! The current modulation electronics has a
bandwdith that's near 1 MHz, so the 20kHz might be for some 'real'
physics reason. Modulating the current changes the wavlength through
thermal effects. I have no idea what the thermal time of the laser
diode is. Would 50us be a reasonable time? (retorical question no
answer expected.)
I'm going to try measuring the current to frequency modulation
parameter as a function of frequency. Hey I might learn someting
today!

If I get around to closing the 'double loop', I may have more
questions....
It's not clear to me where I should put the integrator.

Having friday fun,

George H.

 

[George Herold]

Hi Tim,  Thanks for that!  I logged in to report that I tried locking
with just current modulation... one peice at a time so to speak.  And
that worked fine, I could bang a bit more on the table.  But the
current loop oscillates at ~20kHz when I crank up the gain.  I don't
understand that at all!  The current modulation electronics has a
bandwdith that's near 1 MHz, so the 20kHz might be for some 'real'
physics reason.  Modulating the current changes the wavlength through
thermal effects.  I have no idea what the thermal time of the laser
diode is.  Would 50us be a reasonable time? (retorical question no
answer expected.)
I'm going to try measuring the current to frequency modulation
parameter as a function of frequency.  Hey I might learn someting
today!

If I get around to closing the 'double loop', I may have more
questions....
It's not clear to me where I should put the integrator.

Having friday fun,

George H.- Hide quoted text -

- Show quoted text -

Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode!

George H.

 

[Joerg]

Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode!

20kHz? That's like molasses. Why so low? And it should not cause it to
oscillate.

 

[Joerg]

If he's already got substantial phase shift elsewhere, then the photodiode
rolling off would cause oscillation somewhere around 20kHz.

The fact that it happens at _exactly_ 20kHz just means that, sans
photodiode, he's got about 45 degrees of margin at 20kHz.

But where does all that phase margin fall through the cracks? Unless
everything rolls off fast, of course. 20kHz BW for the photodiode sounds
really low, unless it is one the size of a dinner plate.

 

[George Herold]

20kHz? That's like molasses. Why so low? And it should not cause it to
oscillate.

--
Regards, Joerg

http://www.analogconsultants.com/- Hide quoted text -

- Show quoted text -

Ja Ja, The photodiode design is from 10+ years ago. I hadn't heard of
Phil H. then, let alone read his book.

I've got at least 3 projects now that can use a faster photodiode.

Oh for the above you have to keep the intensity low in order to not
saturate the atomic transistion. So a fairly large PD (0.25" diam),
at zero bias, and 1 M Ohm of gain. (for a 3-5 volt level signal) And
only a 1 MHz opamp (opa124... it has a bad noise gain peak.)

George H.

 

[George Herold]

But where does all that phase margin fall through the cracks? Unless
everything rolls off fast, of course. 20kHz BW for the photodiode sounds
really low, unless it is one the size of a dinner plate.

--
Regards, Joerg

http://www.analogconsultants.com/- Hide quoted text -

- Show quoted text -

Grin, well not quite dinner plate size. ~6-7mm diam.

George H.

 

[George Herold]

freq error -o-> prop. gain - + --> laser
            |                A
            '-> integrator --'

But with two feed back paths should there be an integrator in each
loop?

Not to worry first I need a faster PD.

If your loop is crapping out at 20kHz with your 20kHz photodiode, chances
are that even with a better photodiode in there you'll need some
derivative action to push much above 20-ish kHz:

freq error -o-> prop. gain - + --> laser
            |                A
            o-> integrator --+
            |                A
            '-> derivative --'

If you're doing this in analog, or if you're sampling good and fast in
digital-land, you'll almost certainly want to band-limit the derivative.

Oh all analog.

 

[George Herold]

This uses an optical-feedback Phil circuit that was discussed some here. It has
orders of magnitide more bw than comparable low-noise TIAs.

http://www.highlandtechnology.com/DSS/PH200DS.shtml

I learned a lot working on this. Like, jfets have lots of gate leakage ifthe
drain voltage is high. That gotcha is in AoE but I missed it.

--

John Larkin                  Highland Technology Incwww..highlandtechnology.com  jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators- Hide quoted text -

- Show quoted text -

Yeah, can you release the price for a PH200?
(When I tried, your marketing people wanted my mothers maiden name
and
part of my SS# :^)

1 MHz at 1uA is that 1Meg Ohm gain?

For one project (Rb magnetometer) I'd like ~1MHz at 100kohm gain.

Something the about the same would work for this laser locking.

Going from 10kHz to 1MHz is only a factor of 10^4 in Cap*GBW... :^)

George H.

 

[George Herold]

Interestingly, the analog controls guys tend to do


                            |            |
                            |            |
                            +----int----->|sum--------
                            |            |
                            |            |
                            +----der----->|

because it's easier for people to tune.

That's how I've always done it.

But now (I think, according to Phelan)

error -o->-+- prop. gain---+----gain?--->|

| | |
| | |
| +----int----->|sum--------
| |
| |
+-----------------Neg FB----->|

Hmm Phelan's at work... I might have screwed that up.
(Is there gain in JL's gain of one path?)

George H.

 

[Phil Hobbs]

That's how I've always done it.

But now (I think, according to Phelan)


| | |
| | |
| +----int----->|sum--------
| |
| |
+-----------------Neg FB----->|

Hmm Phelan's at work... I might have screwed that up.
(Is there gain in JL's gain of one path?)

What I usually do is to make both I and T loops more or less
integrating, with the T loop dominating at

very low freq, and make sure the I loop rails safely.
Then just run them together. The T loop will keep the I loop
centred, and nobody has any excess phase down at 0.1 Hz
or wherever the low frequency cross is.

Cheers

Phil Hobbs

(Via Google Groups, from the Carnival Miracle at Port Canaveral--
heading
for the beach bar. Having a daughter in the travel industry means we
can't
afford not to go. )

 

[Joerg]

Grin, well not quite dinner plate size. ~6-7mm diam.

That should be a lot more zippy than 20kHz if connected to a somewhat
reasonable TIA. Or did you give it a hefty dose of Ambien?

 

[Joerg]

Really? We're not supposed to keep pricing a secret; people will find out
anyhow. We do like to keep a record of who downloads manuals or gets pricing,
but the motives are pretty benign. Our registration form is minimal, and we
never spam.

$1986, qty 1. I think The Brat priced it at her birth year.

Now you'll have lots of folks banging on your door at Otis Street,
wanting to see her

The transresistances are 10M and 100K on the two ranges.


That's the low gain range. It's good there, but the real performance is on the
high gain range, where it gets over 1 MHz bandwidth at 10M equivalent and very
low noise, numbers like 100x better than most of the stuff out there.

Sometimes it's best not to have all the gain in the first (TIA) stage.
Opamps are fairly cheap these days and the following ones don't have to
be very fancy. Maybe something George could look at.

 

[Joerg]

Tim Wescott wrote:

[...]

From "On Governors", James Clerk Maxwell, 1868:

"But if the part acted on by centrifugal force, instead of acting
directly on the machine, sets in motion a contrivance which continually
increases the resistance as long as the velocity is above its normal
value, and reverses its action when the velocity is below that value, the
governor will bring the velocity to the same normal value whatever
variation (within the working limits of the machine) be made in the
driving-power or the resistance."

Or, translated into modern English, integrators.

He follows that a bit later with:

"The first and third cases are evidently inconsistent with the stability
of the motion; and the second and fourth alone are admissible in a good
governor. ...

Unfortunately not with many governors voted into office these days.

 

[Phil Hobbs]

Now you'll have lots of folks banging on your door at Otis Street,
wanting to see her


Sometimes it's best not to have all the gain in the first (TIA) stage.
Opamps are fairly cheap these days and the following ones don't have to
be very fancy. Maybe something George could look at.

Yup. You don't want the first stage output
to be more than about half a volt at lowest
nominal photocurrent, because you stop gaining
SNR and can start getting squirrelly behaviour.

The one John's talking about uses fancy homemade
optocouplers, with their photodiodes wired in series.
The parlour trick is to avoid the low f_T of transistors
running at nanoamp I_C levels. Even the late lamented
BFG25A has an f_T of a few megahertz down there, and
that's not even counting C-B cutoff.

Cheers

Phil Hobbs
(Back on the ship)

 

[Joerg]

Yup. You don't want the first stage output
to be more than about half a volt at lowest
nominal photocurrent, because you stop gaining
SNR and can start getting squirrelly behaviour.

500mV for the lowest signals? I usually try to keep it under 1Vpp for
the highest.

The one John's talking about uses fancy homemade
optocouplers, with their photodiodes wired in series.
The parlour trick is to avoid the low f_T of transistors
running at nanoamp I_C levels. Even the late lamented
BFG25A has an f_T of a few megahertz down there, and
that's not even counting C-B cutoff.

"Late" as in clanging of the last order bell? Please say it ain't so,

Cheers

Phil Hobbs
(Back on the ship)

Who let you back on and what's your excuse for being AWOL?

 

[Phil Hobbs]

500mV for the lowest signals? I usually try to keep it under 1Vpp for
the highest.

Not lowest-lowest, but lowest nominal. In a normal resistive-FB TIA,
shot noise equals Johnson noise at 50 mV, so you lose 1dB if the
output is 200 mV, 0.25 dB at 400 mV. Normally the other parts
of the system are way more expensive than the TIA, so from a
cost-benefit POV you want the TIA to be unobtrusive.

"Late" as in clanging of the last order bell? Please say it ain't so,
that this one won't bite the dust ... <sob, sniffle>

Alas, 'tis true. You can still get BFT25As, but they're not as
good.

Cheers

Phil Hobbs

Who let you back on and what's your excuse for being AWOL?

I had a 6-hour pass, sir. Honest.

 

[George Herold]

Really?

OK not that bad, I can't recall the details.

We're not supposed to keep pricing a secret; people will find out

anyhow. We do like to keep a record of who downloads manuals or gets pricing,
but the motives are pretty benign. Our registration form is minimal, and we
never spam.

$1986, qty 1. I think The Brat priced it at her birth year.




The transresistances are 10M and 100K on the two ranges.




That's the low gain range. It's good there, but the real performance is on the
high gain range, where it gets over 1 MHz bandwidth at 10M equivalent andvery
low noise, numbers like 100x better than most of the stuff out there.

1Mhz at 10M Ohm that is impressive! I only need something 100 times
worse, so there's some chance I can make it happen.

George H.

 

[George Herold]

Only if you want things screwed up!  With an integrator in each loop
you'll have an uncontrollable, metastable mode -- said mode being the
difference between the two integrator states, and metastable because
it'll be integrating.  So what will happen is that your loop will hold
just fine until either the piezo or the current goes to the positive
rail, and the other one goes to the opposite rail.

Besides, if you take my suggestion on this you'll have a DC-blocking
filter between the one "laser" input and the current -- only the piezo
will respond to the integrator at any rate.

Hi Tim, thanks again for the sage advice.

So at the moment the gain vs freq looks like,

~1Hz
L ^ |
O |--
G | \
| \
g | \
a | \
i | \ ~1kHz
n | \|
+--------\---------> gain=1
LOG frequency

So If I make a break point (LF to piezo and HF to current)
at say 100 Hz do I still want the high frequency part of the gain to
have a one pole roll-off... or maybe the gain can be flat (for a
while.... I gotta roll off the HF eventually.)

Not to worry, I’ll play around and maybe learn something.

I gathered that, it being the size of a dinner plate (well, for
cockroaches, perhaps).  I'm not on top of the physics of photodiodes, but
I thought that -- given low enough impedances -- your bandwidth is more
limited by your amplifier than by your photodiode itself.  Am I all wet
here?

As far as the photodiode design goes, I don’t know of any ‘new’
tricks*. But the old rule of thumb (for TIA’s) is that the max.
frequency is the geometric mean of the RC ‘frequency’ and the opamp
GBW. For my pedestrian circuit, with unbiased photodiode C=~700pF,
R=1Meg, GBW =1.5MHz,(opa124) for which I get an f(rc)~240Hz and
sqrt(f(rc)*GBW)~19kHz.. (I thought the opa124 had a 1MHz GBW, but the
number was then a bit off.)

George H.

*well there are ways to reduce the capacitance.
reverse bias the PD, bootstrapping, and then Phil H.'s cascode between
the PD and TIA. I've never tried the cascode.