[time-nuts] Photodiodes for high frequency OPLL

Attila Kinali attila at kinali.ch
Sat Mar 30 14:37:34 EDT 2013

On Sat, 30 Mar 2013 10:00:08 -0800
David McQuate <mcquate at sonic.net> wrote:

> You'll need a photodiode that can detect photons at your lasers' 
> wavelength.  

Yes, of course.

> You may be able to use a photodiode at a shorter than its 
> design wavelength as long as there are not coatings (eg anti-reflection) 
> that block the wavelength of interest.

The 1um photodiodes i had a look at, have all a very steep roll of
above below 900nm, leading to 0 detection at 800nm.

>  You'll need to make sure that 
> both lasers illuminate the same photodiode area, or you won't get a 
> signal at the difference frequency.

Yes. The current plan is to have the beams aligned on the same path.

>  The difference frequency power 
> level is generally pretty low, so operating above the photodiode 
> bandwidth is difficult.  My work was at HP and Agilent, who manufactured 
> the photodiodes we used.

Hmm.. that makes it sound more difficult than i anticipated.
Any ballpark numbers i can expect?

> The photodiode frequency response is primarily limited by the depletion 
> region's capacitance.  The circuit model is simply a current source 
> shunted by a capacitor (perhaps with bond wire inductance to the RF 
> connector) so the RF output current falls with increasing signal 
> frequency. 

So, i can expect it to behave like a first order filter and
assume about 20dB/decade?

				Attila Kinali

The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
		-- unknown

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