On Wednesday, October 23, 2002 at 3:38:22 AM UTC+2, Phil Hobbs wrote:
>The best method for fringe surfing is to
> arrange two photodiodes so that they see opposite phases of the fringe
> pattern--e.g. when PD1 is on a bright fringe, PD2 is on a dark fringe.
> Wire the photodiodes in back-to-back parallel (anode to cathode), and
> connect this combination between the inverting input of an op amp and
> ground.

Phil, that simple setup sounds very appealing but I have been struggling to understand how that would work.
Could you detail how the "Wire the photodiodes in back-to-back parallel (anode to cathode)" works because as I understand it, that would add the charges measured by the two photodiodes while I would normally want to subtract one from the other, no?
Many thanks
Loic

On 2023-02-08 10:51, HoloLab wrote:
> On Wednesday, October 23, 2002 at 3:38:22 AM UTC+2, Phil Hobbs
> wrote:
>> The best method for fringe surfing is to arrange two photodiodes so
>> that they see opposite phases of the fringe pattern--e.g. when PD1
>> is on a bright fringe, PD2 is on a dark fringe. Wire the
>> photodiodes in back-to-back parallel (anode to cathode), and
>> connect this combination between the inverting input of an op amp
>> and ground.
>
> Phil, that simple setup sounds very appealing but I have been
> struggling to understand how that would work. Could you detail how
> the "Wire the photodiodes in back-to-back parallel (anode to
> cathode)" works because as I understand it, that would add the
> charges measured by the two photodiodes while I would normally want
> to subtract one from the other, no? Many thanks Loic
>

Well, it turns out that I'm still stalking these silent corridors
occasionally, and glad to see visitors with actual optics to discuss. ;)

Fringe surfing is usually done naïvely by using one photodiode,
dithering around a dark fringe, detecting the fundamental component of
the resulting photocurrent with a lock-in, and feeding that back to some
actuator to zero it out. (The actuator can be lots of things, e.g. a
piezo mirror in an interferometer or a current-tuned diode laser.)

The problem with that approach is that you're servoing around a point
where your SNR is zero. (Not 0 dB, _zero_, i.e. all noise and no
signal.) The error signal builds up only quadratically with mistuning,
so the null is very poorly determined, making the lock very noisy. You
can use some huge dither to get round this, but that usually screws up
the measurement in other ways.

The two photodiode approach requires a spatial fringe pattern, e.g. a
slightly misaligned interferometer. If the two PDs straddle a bright
fringe, subtracting the photocurrents (by wiring the PDs in parallel,
usually) [*] gives a signal that goes linearly through zero when the
fringe is exactly centered. If you pick the separation right (roughly
the 70% points), you get a nice sharp null signal with lots of SNR.

Feeding back on that gives you a good locking signal without needing dither.

AC approaches are possible but much more complicated--making a moving
fringe pattern needs an acousto-optic modulator or the equivalent, and
you need a fair bit of RF signal processing to get that right.

(It's far from impossible--I did something vaguely like that for my
thesis long ago, but it wasn't a quick or easy job.)

Cheers

Phil Hobbs

[*] The best way of wiring PDs in parallel is to wire them in series
between opposite-polarity bias supplies. ;) Either way, the currents
subtract, which is the key point.

--
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 2023-02-08 20:28, Phil Hobbs wrote:
> On 2023-02-08 10:51, HoloLab wrote:
>> On Wednesday, October 23, 2002 at 3:38:22 AM UTC+2, Phil Hobbs
>> wrote:
>>> The best method for fringe surfing is to arrange two photodiodes so
>>> that they see opposite phases of the fringe pattern--e.g. when PD1
>>> is on a bright fringe, PD2 is on a dark fringe. Wire the
>>> photodiodes in back-to-back parallel (anode to cathode), and connect
>>> this combination between the inverting input of an op amp
>>> and ground.
>>
>> Phil, that simple setup sounds very appealing but I have been
>> struggling to understand how that would work. Could you detail how
>> the "Wire the photodiodes in back-to-back parallel (anode to
>> cathode)" works because as I understand it, that would add the
>> charges measured by the two photodiodes while I would normally want
>> to subtract one from the other, no? Many thanks Loic
>>
>
> Well, it turns out that I'm still stalking these silent corridors
> occasionally, and glad to see visitors with actual optics to discuss. ;)
>
> Fringe surfing is usually done naïvely by using one photodiode,
> dithering around a dark fringe, detecting the fundamental component of
> the resulting photocurrent with a lock-in, and feeding that back to some
> actuator to zero it out.  (The actuator can be lots of things, e.g. a
> piezo mirror in an interferometer or a current-tuned diode laser.)
>
> The problem with that approach is that you're servoing around a point
> where your SNR is zero.  (Not 0 dB, _zero_, i.e. all noise and no
> signal.)  The error signal builds up only quadratically with mistuning,
> so the null is very poorly determined, making the lock very noisy.  You
> can use some huge dither to get round this, but that usually screws up
> the measurement in other ways.
>
> The two photodiode approach requires a spatial fringe pattern, e.g. a
> slightly misaligned interferometer.  If the two PDs straddle a bright
> fringe, subtracting the photocurrents (by wiring the PDs in parallel,
> usually) [*] gives a signal that goes linearly through zero when the
> fringe is exactly centered.  If you pick the separation right (roughly
> the 70% points), you get a nice sharp null signal with lots of SNR.
>
> Feeding back on that gives you a good locking signal without needing
> dither.
>
> AC approaches are possible but much more complicated--making a moving
> fringe pattern needs an acousto-optic modulator or the equivalent, and
> you need a fair bit of RF signal processing to get that right.
>
> (It's far from impossible--I did something vaguely like that for my
> thesis long ago, but it wasn't a quick or easy job.)
>
> Cheers
>
> Phil Hobbs
>
> [*] The best way of wiring PDs in parallel is to wire them in series
> between opposite-polarity bias supplies. ;)  Either way, the currents
> subtract, which is the key point.
>

I should add that if you have a Mach-Zehnder interferometer, where the
two output beams are conveniently separate, you don't need a spatial
fringe--you can just detect the two beams separately and subtract as
above.

That's the basis for a very good but little-known laser locking technique.

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

Greetings Phil and many thanks for your reply.

I will experiment with wiring the detectors in series between opposite-polarity bias supplies, seems like a simple and sound setup, just what I needed :)
My application is about stabilizing long exposures of holography setups; nothing new but I like to revisit things by myself.

From what I understand now:

- the single detector approach has the advantages of not requiring a specific fringe spacing and enabling simple phase stepping the interferometer (by inserting a bias voltage in the loop) but has the disadvantage of requiring fringe contrast calibration and is prone to error if the laser output power varies.
Here is a nice article on that setup: https://www.researchgate.net/publication/263582518_A_Laser_Interferometer_for_the_Undergraduate_Teaching_Laboratory_Demonstrating_Picometer_Sesitivity

- the two detector approach is immune to laser output power fluctuations but requires proper fringe spacing

In both cases I don't quite see why it is much more complicated to introduce a (low frequency) carrier frequency in the optical setup by means of an additional mirror on a transducer in order to improve the "reactivity" of the loop to some external vibration as suggested above but I might be missing something.

Best regards
Loic

PS: if anyone is interested in holography here is a link to my channel on what I do in holography (purely amateur, nothing to sell):
https://www.youtube.com/@hololab7368/videos

On 2023-02-09 02:48, HoloLab wrote:
> Greetings Phil and many thanks for your reply.
>
> I will experiment with wiring the detectors in series between
> opposite-polarity bias supplies, seems like a simple and sound setup,
> just what I needed :)
>
> My application is about stabilizing long exposures of holography
> setups; nothing new but I like to revisit things by myself.
>
> From what I understand now:
>
> - the single detector approach has the advantages of not requiring a
> specific fringe spacing and enabling simple phase stepping the
> interferometer (by inserting a bias voltage in the loop) but has the
> disadvantage of requiring fringe contrast calibration and is prone to
> error if the laser output power varies. Here is a nice article on
> that setup:
> https://www.researchgate.net/publication/263582518_A_Laser_Interferometer_for_the_Undergraduate_Teaching_Laboratory_Demonstrating_Picometer_Sesitivity
>
> - the two detector approach is immune to laser output power
> fluctuations but requires proper fringe spacing

Or else a rotation mount, so that you can match a fixed photodiode
spacing to your actual fringes--if the diodes are dx apart, twisting the
mount effectively gives you dx cos theta, which can be significantly
smaller.

There's also nothing that says the diodes have to be looking at the same
fringe, just that they be on opposite slopes, and that you not have a
full fringe across the diameter of the diode.

>
> In both cases I don't quite see why it is much more complicated to
> introduce a (low frequency) carrier frequency in the optical setup by
> means of an additional mirror on a transducer in order to improve the
> "reactivity" of the loop to some external vibration as suggested
> above but I might be missing something.

Jiggling stuff is usually frowned upon in holography setups, I believe. ;)

You could do it with a photoelastic modulator and a lock-in, and look
for the second harmonic to go to zero. That puts you halfway up a
bright fringe, so you don't have the SNR problem of surfing dark fringes.

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 two detector approach is immune to laser output power
> > fluctuations but requires proper fringe spacing
> Or else a rotation mount, so that you can match a fixed photodiode
> spacing to your actual fringes--if the diodes are dx apart, twisting the
> mount effectively gives you dx cos theta, which can be significantly
> smaller.

Well I was thinking of using slit-style apertures in front of the (BPW43) Photodiodes in order to optimize its efficiency, but that would prevent rotating them. Anyhow, this is not much of an issue since adjusting the distance between the photodiodes and a diverging lens does the trick as well

> There's also nothing that says the diodes have to be looking at the same
> fringe, just that they be on opposite slopes, and that you not have a
> full fringe across the diameter of the diode.

Indeed

I have started to design a circuit, partly based on the article previously mentioned and your parallel wiring proposal of the diodes, if you may confirm this is what you meant:

https://flic.kr/p/2og5WDR

Thank you
Loic

On 2023-02-09 13:59, HoloLab wrote:
>>> - the two detector approach is immune to laser output power
>>> fluctuations but requires proper fringe spacing
>> Or else a rotation mount, so that you can match a fixed photodiode
>> spacing to your actual fringes--if the diodes are dx apart,
>> twisting the mount effectively gives you dx cos theta, which can be
>> significantly smaller.
>
> Well I was thinking of using slit-style apertures in front of the
> (BPW43) Photodiodes in order to optimize its efficiency, but that
> would prevent rotating them. Anyhow, this is not much of an issue
> since adjusting the distance between the photodiodes and a diverging
> lens does the trick as well
>
>> There's also nothing that says the diodes have to be looking at the
>> same fringe, just that they be on opposite slopes, and that you not
>> have a full fringe across the diameter of the diode.
>
> Indeed
>
> I have started to design a circuit, partly based on the article
> previously mentioned and your parallel wiring proposal of the diodes,
> if you may confirm this is what you meant:
>
> https://flic.kr/p/2og5WDR

Pretty much. You probably don't need the resistor between the PDs and
the summing junction, and the offset pot doesn't add anything useful, I
don't think.

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

> Pretty much. You probably don't need the resistor between the PDs and
> the summing junction, and the offset pot doesn't add anything useful, I
> don't think.
> Cheers

Many thanks.
Here were my reasonings (which are likely flawed!):

- Resistor R5 between the PDs and the summing junction is there to yield a gain equal to R4/R5
- The offset (Bias) pot is necessary because when the PDs are at equilibrium the output of the loop is 0volts, which is not the best working position for a piezo, since negative voltages may occur

Again, thank you for your help. Once I get a working setup I will share it (schematics, PCB, etc)

On 2023-02-10 16:37, HoloLab wrote:
>
>> Pretty much. You probably don't need the resistor between the PDs
>> and the summing junction, and the offset pot doesn't add anything
>> useful, I don't think. Cheers
>
> Many thanks. Here were my reasonings (which are likely flawed!):
>
> - Resistor R5 between the PDs and the summing junction is there to
> yield a gain equal to R4/R5

Photodiodes are basically current sources, so R5 just adds noise.

> - The offset (Bias) pot is necessary because when the PDs are at
> equilibrium the output of the loop is 0volts, which is not the best
> working position for a piezo, since negative voltages may occur

Bias the other end of the piezo negative.

>
> Again, thank you for your help. Once I get a working setup I will
> share it (schematics, PCB, etc)
>

Looking forward to seeing it!

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

Many thanks Phil for your good pointers.
So I dug
Here is a new revision of the schematics

Many thanks Phil for your good pointers.

So I dug a bit on the topic of transimpedance amplifiers and found this good article: https://www.analog.com/en/technical-articles/optimizing-precision-photodiode-sensor-circuit-design.html

So I revised my circuit (new version heer https://flic.kr/p/2oguPQL) with the following changes:

1/ removed the useless resistor after the PDs and added selectable pre-scaling on the first amplifier stage for best noise performance
2/ added a second op amp to act as an adder in order to add the piezo bias AFTER the various gain stages. This has the advantage of not amplifying the bias as gains are adjusted

Thank you for any feedback!
Loic

On Thursday, 9 February 2023 at 07:48:59 UTC, HoloLab wrote:
> In both cases I don't quite see why it is much more complicated to introduce
> a (low frequency) carrier frequency in the optical setup by means of an additional
> mirror on a transducer in order to improve the "reactivity" of the loop to some
> external vibration as suggested above but I might be missing something.

IME "introducing" something where it wouldn't have been otherwise will always be "more
complicated" than not doing so; the issue is whether the extra complication is worth it.

Are you actually limited by vibration problems?

Thanks

Henry

On Sunday, February 12, 2023 at 11:53:55 PM UTC+1, Henry Nebrensky wrote:
> On Thursday, 9 February 2023 at 07:48:59 UTC, HoloLab wrote:
> > In both cases I don't quite see why it is much more complicated to introduce
> > a (low frequency) carrier frequency in the optical setup by means of an additional
> > mirror on a transducer in order to improve the "reactivity" of the loop to some
> > external vibration as suggested above but I might be missing something.
> IME "introducing" something where it wouldn't have been otherwise will always be "more
> complicated" than not doing so; the issue is whether the extra complication is worth it.
>
> Are you actually limited by vibration problems?

Not really but I am currently working with photopolymer films which are very slow, potentially yielding long exposure times, which increase the chance of a random phase shift and slow thermal drifts. So I am putting a bit of energy on the fringe-locking topic :)

Third iteration of the circuit. I moved the bias to the negative end of the piezo, as suggested by Phil, in order not to clip the piezo driving voltage in case bias voltage is large:
https://flic.kr/p/2ogEQci

On Sunday, 12 February 2023 at 23:31:15 UTC, HoloLab wrote:
> Not really but I am currently working with photopolymer films which are very slow, potentially yielding long exposure times, which increase the chance of a random phase shift and slow thermal drifts. So I am putting a bit of energy on the fringe-locking topic :)

I've never worked with photopolymer... can it work in the Denisyuk geometry? That can be more tolerant of noise and air movement around the laser.

Thanks

Henry

On Monday, February 13, 2023 at 11:27:13 PM UTC+1, Henry Nebrensky wrote:
> I've never worked with photopolymer... can it work in the Denisyuk geometry? That can be more tolerant of noise and air movement around the laser.

It sure can, see an example here (https://youtu.be/QGvfRizX10E) of a full colour Denisyuk I made on PP film. The hologram is at the top of the video, the real object at the bottom, for comparison purposes.

Although the Denisyuk setup is simple, robust and yields realistic holograms, it is not suitable for all applications. See an example here (https://youtu.be/qrpcj7Es1_c) of a transmission interferogram of an entire room that I also recorded on PP. If you look at the way the interference fringes move, you can guess that the main source of vibrations is a rigid body movement of the optical table with respect to the room, which severely degrades the hologram quality at recording time, and which should be compensable by a fringe-locker :)

Hi,

Unfortunately this old thing struggles with youtube - I 'll try to have a
look at those soon.

On Tuesday, 14 February 2023 at 09:39:52 UTC, HoloLab wrote:

> ... If you look at the way the interference fringes move, you can guess
> that the main source of vibrations is a rigid body movement of the
> optical table with respect to the room, which severely degrades the
> hologram quality at recording time, and which should be compensable by a
> fringe-locker :)

I'm probably missing something obvious, but what then are you locking to?
Taking some off the ref. beam and bouncing off the wall?

Thanks

Henry

Le lundi 20 février 2023 à 23:17:22 UTC+1, Henry Nebrensky a écrit :
> I'm probably missing something obvious, but what then are you locking to?
> Taking some off the ref. beam and bouncing off the wall?

I'll sample some of the object beam with a beam-splitter mounted on a post which is sitting on the ground of the lab, then mix it with the ref beam which is on the optical breadboard.
I have actually just completed the prototype fringe-locker circuit and transducer this evening. As youo can see the PCB is mounted on a (3D printed) holder which can be tilted to best align with the fringes.
Stay tuned for some real fringe locking tests!

https://flic.kr/p/2ohYP1Y
https://flic.kr/p/2oi2uvp
https://flic.kr/p/2oi4Cqx

> https://flic.kr/p/2ohYP1Y
> https://flic.kr/p/2oi2uvp
> https://flic.kr/p/2oi4Cqx

One more pic of the mirror on the piezo transducer:
https://flic.kr/p/2ohYP1x

Greetings, here is a video of the fringe locker in action: https://youtu.be/0IrlpE1sPkQ
And the project page: https://github.com/Loic74650/FringeLocker

This first version works ok but could certainly work better I think. In particular a more powerful output stage with a stiffer piezo would likely improve the frequency response of the control loop.
BOM total is just under USD26, so I called it the "Poor Man's Fringe Locker" :))

Many thanks for the inputs, in particular @Phil Hobbs

On 2023-03-19 16:24, HoloLab wrote:
> Greetings, here is a video of the fringe locker in action: https://youtu.be/0IrlpE1sPkQ
> And the project page: https://github.com/Loic74650/FringeLocker
>
> This first version works ok but could certainly work better I think. In particular a more powerful output stage with a stiffer piezo would likely improve the frequency response of the control loop.
> BOM total is just under USD26, so I called it the "Poor Man's Fringe Locker" :))
>
> Many thanks for the inputs, in particular @Phil Hobbs
>
Cool, well done.

You need more loop gain, though--probably a good 20 dB more. It should
be able to really lock them suckas. If the loop wants to oscillate at
higher gain, you can use more feedback capacitance.

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

> Cool, well done.
>
> You need more loop gain, though--probably a good 20 dB more. It should
> be able to really lock them suckas. If the loop wants to oscillate at
> higher gain, you can use more feedback capacitance.
>
> Cheers
>
> Phil Hobbs

OK many thanks, I will try asap and report back!

On 2023-03-20 15:33, HoloLab wrote:
>> Cool, well done.
>>
>> You need more loop gain, though--probably a good 20 dB more. It should
>> be able to really lock them suckas. If the loop wants to oscillate at
>> higher gain, you can use more feedback capacitance.
>>
>> Cheers
>>
>> Phil Hobbs
>
> OK many thanks, I will try asap and report back!
>

The capacitance of your piezo stack is probably in the 10-nf range--it's
worth measuring that, because R_out * C_piezo is probably the dominant
pole in the loop right now.

If you need any help with frequency-compensating the loop, ask.

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 capacitance of your piezo stack is probably in the 10-nf range--it's
> worth measuring that, because R_out * C_piezo is probably the dominant
> pole in the loop right now.
>
> If you need any help with frequency-compensating the loop, ask.
> Cheers

Hmm, you've lost me there (that's how little I know about electronics!).
What do you mean by "probably the dominant pole in the loop right now"?
I have increased the capacitance on the output low pass and I can now increase the gain without oscillating.
Now I have increased the output gain by "a lot" but somehow it does not seem to improve much; I might be missing something

On 2023-03-20 17:03, HoloLab wrote:
>> The capacitance of your piezo stack is probably in the 10-nf
>> range--it's worth measuring that, because R_out * C_piezo is
>> probably the dominant pole in the loop right now.
>>
>> If you need any help with frequency-compensating the loop, ask.
>> Cheers
>
> Hmm, you've lost me there (that's how little I know about
> electronics!). What do you mean by "probably the dominant pole in the
> loop right now"? I have increased the capacitance on the output low
> pass and I can now increase the gain without oscillating. Now I have
> increased the output gain by "a lot" but somehow it does not seem to
> improve much; I might be missing something

Frequency compensation is the gentle art of tuning a feedback loop so
that it responds gracefully but quickly to small changes in both control
inputs and external forcing.

That is, if you make a small but abrupt change in either the setpoint or
the output loading, you want the controlled variable (fringe position
here) to recover quickly and smoothly, without much overshoot and
(ideally) with zero DC error.(*)

The loop properties depend almost entirely on the *loop gain*. You
notionally break the feedback loop someplace convenient, while keeping
all the DC levels the same, introduce a perturbation (e.g. a small step
from an LTspice voltage source), and calculate the response of the rest
of the loop up to the other side of the break. (It's the same anywhere
in the circuit.)

If you post the details of the forcing function (that 0.6 Hz sine wave)
and how it's connected, the capacitance of the piezo, and the
peak-to-peak output from the photoreceiver, we can work out a sensible
frequency compensation scheme that'll give you better fringe stability.

Cheers

Phil Hobbs

(*) The restriction to small perturbations avoids getting mixed up with
the other main problem in control systems, namely *windup*.

--
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