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
li...@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.griffi...@xtra.co.nz>
Sender: time-nuts-boun...@febo.com
Date: Sun, 31 Mar 2013 11:43:48
To: Discussion of precise time and frequency measurement<time-nuts@febo.com>
Reply-To: Discussion of precise time and frequency measurement
<time-nuts@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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