optical heterodyne detection for spectrometer

[Jamie M]

Hi,

I came across optical heterodyne detection on wikipedia:

http://en.wikipedia.org/wiki/Optical_heterodyne_detection

Basically hitting a photodiode with two closely matched
light sources, and then the electrical signal from the
photodiode will output the frequency difference between
the two light sources. Could something like this be
rigged up to function as a spectrometer? If one signal
was a reference light, a light from a supercontinuum
laser that is swept over the desired light frequency range,
then it might be similar to a spectrometer with a rotating
grating and a photodiode. Would there be a way to operate
in continuous mode so no frequency sweep is required?

cheers,
Jamie

 

[George Herold]

Hi,

I came across optical heterodyne detection on wikipedia:

http://en.wikipedia.org/wiki/Optical_heterodyne_detection

Basically hitting a photodiode with two closely matched
light sources, and then the electrical signal from the
photodiode will output the frequency difference between
the two light sources.  Could something like this be
rigged up to function as a spectrometer?  If one signal
was a reference light, a light from a supercontinuum
laser that is swept over the desired light frequency range,
then it might be similar to a spectrometer with a rotating
grating and a photodiode.  Would there be a way to operate
in continuous mode so no frequency sweep is required?

cheers,
Jamie

Sure, that's exactly how I measure the linewidth of our diode lasers.

(LW about 1 MHz on the second time scale.... Mostly jitter, I think of
the optics in the signal path, but it could be something else.
On shorter time scales the LW looks to be
~1-200kHz, but that's close to the 'scope resolution.)
And then of course I'm measureing the LW of two lasers, and assuming
it's shared equally.

George H.

 

[Bret Cannon]

"Jamie M" wrote in message
Hi,

I came across optical heterodyne detection on wikipedia:

http://en.wikipedia.org/wiki/Optical_heterodyne_detection

Basically hitting a photodiode with two closely matched
light sources, and then the electrical signal from the
photodiode will output the frequency difference between
the two light sources. Could something like this be
rigged up to function as a spectrometer? If one signal
was a reference light, a light from a supercontinuum
laser that is swept over the desired light frequency range,
then it might be similar to a spectrometer with a rotating
grating and a photodiode. Would there be a way to operate
in continuous mode so no frequency sweep is required?

cheers,
Jamie

Yes, it has been done by Jun Ye at JILA in Boulder, CO. He and his
colleagues combined a pair of femtosecond frequency combs with slightly
different spacings on a photodiode and used an rf frequency analyzer to
measure the optical spectrum change due to molecular absorption of one of
the beams. A femtosecond comb is a phase locked set of optical waves where
each "tooth" in the comb has a width below 1 Hz, the teeth are separated by
say 100 MHz, and the frequencies range from say 3E14 Hz to 6E14 Hz (1000 nm
to 500 nm in wavelength). The second femtosecond comb has the "teeth"
separated by 100 MHz + 50 Hz. A pair of "teeth", one from each comb, are
phase locked to be 50 MHz apart, then the beat from the next pair of teeth
is 50 MHz + 10 Hz, etc. So the beats from the (6E14-3E14)Hz/100 MHz = 3E6
pairs of teeth in the two frequency combs are mapped into the range between
50 MHz and 80 MHz.

Bret Cannon