[time-nuts] Photodiodes for high frequency OPLL

J. Forster jfor at quikus.com
Sat Mar 30 13:52:54 EDT 2013


My understanding is that you want to operate photodiodes with high reverse
bias for the best frequency response. The bias widens the space charge
layer, thereby reducing the capacitance of the device. The high electric
fields in the SCL region also sweeps the hole-electron pairs, produced by
photon injection, out faster, hence improving the response as well. Such
devices are best operated with very low capacity wiring into a virtual
ground.

Of course, there are limits as, at some point, the device will avalanche.
When that ocurrs, the device will have gain akin to a photomultiplier.

Some devices are actually designed to operate in this mode, but, since the
thing is so near to unstable requires careful control of temperature and
oher parameters.

-John

================


> You lose me at damping per decade? Is damping the right word? Do you mean
> high frequency rolloff?
>
> Most texts on photodiodes go into bootstrapping them to reduce the effect
> of capacitance. But if you design fully differential amplifier circuits,
> they have the same effect as bootstrapping. Jerald Graeme's "Photodiode
> Amplifiers" goes into this. I'm not a fan of Gain technology as a company,
> but Graeme's book are good texts.
>
> The bootstrapping circuits use simpler buffers to keep the voltage
> potential across the diode low, while the fully differential circuits
> depend on gain bandwidth product to do the same.
>
> -----Original Message-----
> From: Attila Kinali <attila at kinali.ch>
> Sender: time-nuts-bounces at febo.com
> Date: Sat, 30 Mar 2013 12:48:01
> To: <time-nuts at febo.com>
> Reply-To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: [time-nuts] Photodiodes for high frequency OPLL
>
> Moin,
>
> I'm currently reading up some stuff on optical PLLs and am stuck with some
> details i cannot find any data on.
>
> The goal is to make two lasers locked with about 7GHz of offset to
> eachother.
>
> So far, i figured out that PIN photodiodes can go up to several 100MHz
> transition frequency and avalanche photodiodes are available up to 2GHz.
> The only photodiodes that go higher are those for the telecom range
> at 1-1.5um, which is a bit low for my needs.
>
> But, i have seen descriptions of such OPLL that work with multi GHz
> offsets in the 700-800nm range.
>
> Now the question is: Do these use special made photodiodes with a very
> high transition frequency or do they just treat the transition frequency
> as a 3dB point and amplify the signal after detection?
> (They do not describe the detection circuit at all, so i guess it must
> be something readily available)
>
> If it's the former, any idea where i could get such photodiodes?
>
> If it is the later, how much damping can i assume with PIN or avalanche
> photodiodes per decade?
>
>
> Thanks in advance!
>
> 			Attila Kinali
>
>
> --
> The people on 4chan are like brilliant psychologists
> who also happen to be insane and gross.
> 		-- unknown
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