Designing a APD based receiver for use with a TOF laser range finder

You've missed the point that avalanche photo diodes amplify the current generated by a single photon - initially a single hole/ electron pair - by a process of avalanche multiplication, in which one of the charge carriers moving through the lattice picks up enough energy to create more hole/elecron pairs.

Any "dark current" is multiplied in the same way. The diodes break down when the reverse voltage across the diode is high enough that even the dark current is multiplied up to a current which can creat run-away warming in the lattice.

There are specialised parts for a small market, so they end up expensive and not widely available.

Stabilising the temperature of the APD also stabilises the avanalnche gain at a given voltage, which can be helpful.

No. If your incoming light is well collimated, it is much better to use a lens to focus it onto a small area photodiode, with a low dark current and a low capacitance.

Sergio Cova at the Milan Polytechnic has published a number of good papers on avalanche photodiodes and single photon avalanche diodes over the years - check out Applied Optic and the Review of Scientific Instruments.

A search on his name on Google Scholar

-- Bill Sloman, Nijmegen

You should be able to find some cheaper apd's, and probably get one as a sample if you write a convincing email. Take a look at a buyer's guide, Photonics Spectra or Laser Focus World, and try some emails. These things are going for a few dollars in medium quantities, so samples shouldn't be a big deal.

If you're going to be working in background brighter than moonlight, an APD may not be worth the trouble; just a fast PIN diode would work as well and not need the high voltage.

If you do use an apd, current limit the supply!

A large area diode will have a lot of capacitance and be slow.

What's your laser like? Range? Optics? Resolution?

John

Hi Bill -

So dark current is the only current flowing through the diode when reverse biased and not exposed to any light, correct?

But I think there's more to it than that. One particular project I looked at cooled the APD down to, if I'm remembering right, 77K. That's cold!!

I will check him out. However, I should be clear that I'm not looking to count single photons, and I think the techniques used for counting single photons differs a bit from what I'm attempting to do.

Thanks,

-Michael

Hi John - I was hoping you would weigh in. I spent a good deal of time reading through the google groups archive of posts related to APDs, and your name popped up a good number of times. I'll check those resources out.

My understanding of APDs is that they vastly increase your capabilities. Specifically, they'll allow you to do things like decrease your transmitted power while maintaining the same sensing range as was available with a PIN diode. So I figure I might as well start with the more powerful solution, and if that ends up being overkill, I can step it down a bit.

I hope for this device to work in all lighting conditions. My plan is to use a diode of a very specific frequency and find a filter that'll knock everything else out. To be honest, though, I haven't spent much time working on the optics side of things, as I'm an EE, so I'm not as familiar with that stuff.

Which brings up a question: what does a typical power supply for these things look like? My understanding is that it doesn't need to be able to source much current at all (1ma maybe?) but that it needs to have a large voltage - 100-200V typically. The eventual goal for this device is for it to be battery powered, probably operated off of a single 5V supply.

Well - that wouldn't be any good! What use are the really large APDs then? Digi-Key lists them as $1500 or so - so surely somebody must have a good reason for using them!

I haven't chosen a laser yet. I thought it'd be best to choose a laser to match whatever APD I end up with. My hope is to stay with a visible wavelength though, and keep it very low power (5mw or less). I want this thing to be very safe. I've been thinking I'd use some laser driver chips designed for optical communication to drive the laser.

My desired range is 0-5 meters. More would be awesome, but I'll survive with that. Really, I would be happy with just a couple meters of range, but I think this device will be alot more useful if I can get a higher range.

As for optics - I haven't put much thought into it just yet. I thought I'd use a collimating lens for the laser's output, and then some filter as I mentioned earlier in front of a large lens in front of the APD.

Regarding resolution - I hope to get a centimeter resolution or better. My plan is to use some of the Time to Digital Converter (TDC) chips made by Acam.

Recommended reference: Building ElectroOptical Systems, Phil Hobbs High resolution range finding at relatively short ranges is most often done with phase measurement rather than pulse echo time measurement. You will soon discover why when you attempt to generate and observe ultrafast pulses. Echo time for 1 cm in free space is 67 picoseconds. Measuring 1 degree of phase at 41 MHz is generally easier than measuring 67 ps. Paul Mathews

The main usefulness of analogue-mode APDs is in the range from the practical upper limit of photon counting (say 10-100 MHz average count rate, or about 1-10 pW in the visible) to the lowest photocurrent where shot-noise limited SNRs are possible with reasonable bandwidths, say about 5 uW. Within that range, by adjusting the APD bias, you can get a big SNR improvement with an APD, though you may not get to the shot noise.

APDs slow down at high gains, because it takes awhile for the avalanche to build up. If you can get afford to get two matched APDs, and run one in the dark, you can temperature-compensate the gain by servoing on the amplified dark current.

Cheers,

Phil Hobbs

phase difference range finding. I remember talking with a guy from MIT's Lincoln Labs about it - and he said that they use TOF range finding, and that they'll even watch for things like 2 different ranges present at the same position, which often indicates something like a car parked beneath a tree. As I recall, their receiver counts single photons returned.

The TDC chips I plan on using should take care of the timing part. Generating really crisp pulses for the laser is one of my bigger worries, though I think some of the commercially available laser driver chips should be able to help me out there.

-Michael

A dinky 850 nm vcsel driven by an eclips lite gate will give you a few milliwatts with an optical risetime of 100 ps or less. The receiver and the timing will be a bigger problem.

John

Hi Phil - first off, just to be sure: by analogue mode - you are referring to analogue mode as opposed to Geiger mode, correct?

Also - what do you mean by servoing the amplified dark current?

Thanks,

-Michael

Hi John - pardon my ignorance - but what is an eclips lite gate? Googling for it turns up nothing. Googling for light gate turns up some mechanical devices that might be able to cut off an optical signal, but I'm not finding any solid sources of information.

Thanks,

-Michael

As John mentioned in another post,

"A dinky 850 nm vcsel driven by an eclips lite gate will give you a few milliwatts with an optical risetime of 100 ps or less. The receiver and the timing will be a bigger problem."

Converting the pulse timing to range is a non-trivial problem. TOF converters suffer from jitter, so averaging will be needed. This will take time, and eventually you hit a barrier where further improvement in SNR will simply take too long.

If you are interested in looking at a newer approach, the Binary Sampler allows you to overcome the averaging limit, and it gives more accurate results much faster. For example, with very simple circuitry, you can obtain greater than 1 ps rms resolution in 1 second at 1 MHz. Running at 41MHz should give a corresponding improvement.

An ideal timebase method is the heterodyne technique. This has been difficult to achieve due to the need for low timing jitter in the offset frequency. A regular DDS may give a jitter of 300ps rms or more, which is unusable.

ADI now has the AD9540, which is a very low jitter clock synthesizer. I have not had time to try it, but it looks very impressive. It offers femtosecond level timing jitter and 48-bit frequency tuning word resolution for under $10.00. If these specs are true, it would solve the frequency offset problem and make the Binary Sampler extremely useful in applications where accurate measurements are needed in signals with large timing jitter.

You can see an early version of the Binary Sampler at

The concept has been considerably improved since this was posted. If you are interested in more information, you can contact me at the address shown on the contact page.

Regards,

Mike Monett

ECLinPS - ON-Semiconductor's recent version of emitter-coupled logic

You can buffer the emitter-follower outputs with wideband discrete transistors if you want a bit more current - I used the 5GHz BFR92 (NPN) and BFT92 (PNP) some twenty years ago. Farnell still stock them, but nowadays they have 10GHz parts and some items in a list that is supposed to go up to 45GHz.

-- Bill Sloman, Nijmegen

Hi,

That binary sampler looked interesting, im trying to average a time interval signal down to sub picosecond resolution, but my data's standard deviation is nearly 10 nanoseconds. I do however have ~10 million points per day to play with.

It looks to me like it effectivly does the same thing as creating a line of best fit with equal number of points above and below, as opposed to line with minimum absoulute error or square error. have I got this right ? however im not familiar with mathmatical techniques to find such a line of best fit, is there any code around for doing such techniques ?

its just a one off physics measurement experiment im doing. At the moment im just doing a fft to find the signal im looking for wich shows up as modulation of time interval of

For certain values of "newer", as 45 years maybe.

Yes. I argued with MM over this for some number of years, and gave up. You saw it right away.

A good laser interferometer could do what you want. But if you want to use tof, be aware that it takes extreme measures to get a signal chain like yours down to 1 ps per degree C drift. The tof chip you suggest using is going to be far, far worse, as will the laser+driver, the pin/apd amplifier, and whatever comparator you use.

What's the physics here? Could you use an incremental, as opposed to absolute, position measurement system? Could you use some other distance measuring scheme, capacitance maybe?

I just don't think you can hold < 1 ps for any usable amount of time using tof as described.

John

Yep.

You run both diodes from the same bias supply, and servo the voltage to keep the amplified dark current of the dark diode constant. That keeps the multiplication gain pretty well constant too. Protection circuits are required to avoid blowing up expensive APDs.

Cheers,

Phil Hobbs

hmm i see.

for me its 2 seperate projects, the

ive had a play about with this binary sampling but I cant get it to do any better than normal averaging, ive introduced a filter in software wich mimics the comparator, although it reduces the noise the signal almost disapears too, unless I increase the slew rate to many times what is needed and then the noise just gets through again. ive also added slew rate limit detection etc.

maybe it is dependant on the signal and noise ? my test noise is just from a weighted random number generator. my test signal is just a sinewave many times lower than the noise.

there might be other ways I can play around with it but the only effect its had so far is to reduce the recovered SNR.

Colin =^.^=

"Binary sampling" [1] has bad statistical properties. It servoes on the median of the signal, not on the average; so it really screws up if the noise is not perfectly symmetric. It looks sort of like delta-sigma, but d-s sums the signal and the feedback into the integrator, and the integrator feeds the comparator, so d-s does indeed servo on the average. BS sums the integrator output plus the signal into the comparator, which changes things.

John