[time-nuts] Photodiodes for high frequency OPLL

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Mar 30 19:28:59 EDT 2013


Yes, the beat (difference) frequency of the 2 lasers has to lie within 
the photodiode electrical passband.

With a suitable low noise tunable LO (laser) the frequency spectrum 
beyond the photodiode electrical bandwidth can be explored.

Bruce

lists at lazygranch.com wrote:
> If I can rephrase that it it, the diode needs to see the difference signal of the mixer?
> -----Original Message-----
> From: Bruce Griffiths<bruce.griffiths at xtra.co.nz>
> Sender: time-nuts-bounces at febo.com
> Date: Sun, 31 Mar 2013 11:43:48
> To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
> Reply-To: Discussion of precise time and frequency measurement
> 	<time-nuts at febo.com>
> Subject: Re: [time-nuts] Photodiodes for high frequency OPLL
>
> The detectors don't have to be fast enough to keep up with optical
> carrier frequency as long as the incident optical power has a component
> at a much lower frequency.
>
> Bruce
>
> ed breya wrote:
>    
>> Ooops - never mind. I wrote before my memory was updated. My
>> experience in E-O stuff was years ago using AM at relatively low
>> frequency, and nowhere near the lasers and microwave/gigabit/sec stuff
>> - I didn't think the detectors were fast enough to actually keep up
>> with the optical carrier frequency. I was also picturing
>> wavelength/spatial separation with interference in order to allow
>> relatively slow detectors to see it, or mixing in nonlinear optical
>> materials.
>>
>> Ed
>>
>> Bruce Griffiths wrote:
>>
>> A photodiode is in fact a nonlinear device for optical fields as it is
>> essentially a linear optical power detector.
>> The output is proportional to the incident optical power not the field
>> amplitude.
>> Photomixers are routinely used in wide range of diverse application such
>> as translating the frequency fluctuations of the (Mie) scattered light
>> due to Brownian motion of the colloidal particle sizes to baseband. The
>> size of the scattering particles can be inferred from the shape of the
>> resultant frequency spectrum.
>>
>> An interferometer of itself (without a detector) is a linear device that
>> merely superimposes optical fields and will of itself produce no
>> difference frequency output.
>>
>> Bruce
>>
>> ed breya wrote:
>>      
>>> I don't think that you can effectively directly mix two laser
>>> wavelengths in a semiconductor light detector and get a useable IF -
>>> it's hard enough just to get the tens of GHz modulation signals out
>>> above the noise floor, let alone a tiny difference signal between
>>> hundreds of THz. You need an optical interference or nonlinear device
>>> up front to do the "mixing" and get the wavelength discrimination,
>>> while the optical detector(s) serve as the first IF O-E transducer.
>>>
>>> My knowledge of this stuff isn't up to date - maybe nowadays there are
>>> detector devices and methods that take care of this directly, but I
>>> don't think so.
>>>
>>>        
>>
>>
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