[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.
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