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Optical position sensing, minimizing jitter

Hi:

I have to design an improved version of a rotating wheel optical position sensor. There is a slit of about 1mm width at 44mm radius on the wheel with tangential velocity of about 110m/s rotating at 400Hz.

The past version used a IRLED shining through a similar slit onto a PIN3CD photodiode through a 350Hz wheel. The rest of the circuitry is shown here:

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Is hasn't much bandwidth, but has a lot of gain (4Meg effective transimpedance gain) which was needed to deal with the relatively low light reaching the detector from the LED.

In the new detector, I plan to use a 665nm VCSEL to create a well focussed beam. The main goal is to get a much faster optical edge off the slit. So the optical risetime will be roughly equal to the beam diameter / the velocity. For instance, with a 100um beam:

(100*10^-6 m) / (110 m/s) = 910ns

Question: If you wanted some 't' precision of position sensing accuracy (jitter/noise less than 't' seconds) what would the risetime of the optical edge have to be? Equal? Smaller? Larger Ok? Let's say we want to shoot for 85ns sensing jitter. What risetime of the optical signal is needed?

The exact requirement is presently still poorly specified in my application with something like 85ns being a possible future extreme. So for academic purposes let's shoot for now at a detector bandwidth of at least 3x what is implied by the 910ns risetime, ie. about 1MHz bandwidth.

First of all, I plan to use only a transimpedance stage and no second gain stage since I should have a reasonable fraction of 1.5mW of light hitting the photodiode (say, 0.5mW). I will use a UDT BPW-34B 12pF (at

10V bias, sensitivity 0.45A/W @ 650nm) photodiode. Thus, I will get a photocurrent of 225uA. A 22k transimpedance will give a 5V pulse. To get a BW of 1MHz this is easily done with something like an OPA356.

The next question is, how best to deal with this photodiode signal to get the minimum jitter from the comparator output?

There would appear to be two possible strategies:

  1. Since the beam power and alignment will change very little once it's all set up, tune the gain of the transimpedance stage or the optics to give a pulse of peak amplitude 'A' and set a fixed comparator threshold of A/2. Then the threshold should be roughly where the maximum rate of change is occuring in the optical signal, leading to minimum jitter.

Is it detrimental to use too much gain and saturate the opamp, for the purpose of getting a faster slew rate just before the opamp saturates, then putting the comparator threshold nearer the saturation voltage?

  1. Use some sort of adaptive trigger threshold circuit.

I'd like to avoid the complexity of the adaptive trigger. Since there is little fluctuation in optical signal strength, option 1 seems acceptable.

  1. Other?

Comments appreciated.

Thanks for input.

Good day!

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_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
Reply to
CC
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(snip)

Since you do not need anything like a linear measure of the light level, but only a time that it passes through some fixed amplitude, you might think about how you can use a gracefully saturating amplifier in place of the opamp to magnify that part of the optical signal. Opamps are inherently slow, to allow closed loop operation, and you have no need of that. Think about the choices based on common base amplifiers or emitter coupled logic or limiter amplifiers. These all run open loop, are very fast, and handle overload without slowing down, much.

Reply to
John Popelish

To estimate your position jitter, you really have to take account of laser speckle and mode partition noise. VCSELs are typically highly multimode devices, with as many as 12 transverse modes running simultaneously. (They're usually single longitudinal mode due to their very short cavities.) Multimode devices produce focused spots that dance around at megahertz rates, which will be a significant source of jitter and 1/f noise.

To do this really right, you need a single transverse mode device such as an index-guided, cleaved-cavity laser diode. Single longitudinal mode will help too, but is less important than a single spatial mode, since the total intensity noise is typically *dramatically smaller* than the noise of any individual mode. This is because the bias current sets the total pumping rate, so the sum of the mode intensities tends to stay pretty constant--the power just wanders around from mode to mode.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Right. But if the beam is spatially averaged over the detector, then what matters is the total of power distributed throughout the modes, which will be constant to within the noise specification of the emitter.

Where the noise would begin to appear in a signal is if you sample a spatial fraction of the beam approaching the spatial resolution of the transverse mode interference pattern. Then you can have meaningful portions of the total beam power sometimes present in your sampling area, and sometimes not present, leading to substantial noise.

Now the question is whether the slit interrupting the beam qualifies as a sampling of a spatial fraction that will lead to such observed noise. It might, when only a small fraction of the beam is passing. In fact then, we might surmise that the noise due to sampling a fraction of the total beam area will be in inverse proportion to the area sampled.

But for the amplitude midpoint and above regions of the signal pulse, I expect that since this is sampling => 50% of the beam area, that the noise should be very minor.

It would be interesting to take a look for this noise though. The actual VCSELs I am interested in are Firecomms 665nm visible devices. They will be quoting me for some samples (should have gotten the quote yesterday...) And they seemed to suggest that single TEM mode devices are available. We will see. I will likely obtain some OPTEK 850nm devices as well, but don't hope to deploy them since aligning an invisible beam is obviously more difficult.

Thanks for input.

Good day!

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
Reply to
CC

Hello John,

Or use a regular common source architecture and employ a few of those hot TV tuner transistors. Those can do edges in the sub-nsec range.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

It isn't important only for large obscurations, believe me.

As a data point, I once had a *single longitudinal mode* argon laser (also single transverse mode because of the much larger frequency spacing) whose noise went up by *20 dB* when half the beam was obscured by a knife edge. The side modes were still noisy even though they weren't quite oscillating.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

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