Infrared block upconversion spectrometer

On a sunny day (Mon, 09 Sep 2013 11:12:03 -0400) it happened Phil Hobbs wrote in :

How do you 'chirp' (that is frequency modulate no?) an optical pulse?

Back in Silicon Valley, a friend of one of my colleagues invented/created an 'optical' amplifier. When described to me, I actually envisioned the same scenario - the formative days of radio. What happened to that amplifier?

Send it through something dispersive (i.e., different index of refraction vs. frequency). Some examples include: non-ideal (physical) transmission lines, waveguides, and gravity waves (ripples on a pond).

What they usually do is point the beam at a diffraction grating, which reflects different frequencies at different angles (within an octave; of course, it wraps around after that, because the grating is periodic, which wouldn't be normal for dispersion), then collect the different angles with a mirror so they become a beam again, but displaced in position and therefore time. This beam can be amplified (preserving the frequency and phase) and sent back through a diffraction grating and focus mirror to restore the original pulse. This is how world record power levels are achieved, because the single pulse cannot be amplified as-is (it would burn a hole through the amplifying medium).

Tim

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OK, space has L and C so is a transmission line. We are used to transmitting CW and pulsed RF, but apparently we can send true rectangular pulses too.

As if the unequal-arm beam-splitter interferometer weren't mind-boggling enough already, now think about running unidirectional pulses through it.

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The usual way is to use two diffraction gratings facing each other, so that the longer wavelengths (which get diffracted through a bigger angle) have to travel further, i.e. their path is a sharper Z. You can also just run it through a fibre, but the dispersion then is the other way round. In fact the grating trick was invented to complement the self-phase modulation and dispersion of a fibre, in order to make compressed pulses. Works great.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
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hobbs at electrooptical dot net 
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Both semiconductor and rare-earth doped fibre amplifiers are in wide use today. The main differences between them and RF amps is that they're much noisier and have no front/back isolation.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Perhaps Phil can provide some analysis showing that "unidirectional" pulses are approachable as we approach light frequencies.

At "lower" frequencies we do this...

...Jim Thompson

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As I understand it, there is no necessity to chirp. If you wanted to have a chirped pulse, you would send the original "30 fs pulse in the

800 nm region" through a dispersive medium. You will, however, end up with a longer chirped pulse. That chirped pulse could then be fed ubti a de-chirper to regain the compressed pulse.

The point Phil is trying to make is that the narrow base band pulse already contains a broad band of infrared frequencies that could be used to check IR frequency transmission with the single pulse.

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Sam 

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Any crystal that has a quadratic nonlinearity has to generate 'DC' as well as second harmonic. The parlour trick will have been to make it all go in the same direction, which it normally wouldn't do.

Interfering the first and second harmonic in a SHG medium, with a bit of a chirp, would impose phase correlations that might make that work--you'd have a linear phase delay with distance at each frequency, i.e. each wavelength would have a transverse k vector corresponding to the same angle of incidence. That would make them all want to go in the corresponding direction, but it wouldn't happen by accident.

I'll have to get the actual paper and find out.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

On a sunny day (Mon, 09 Sep 2013 13:25:15 -0400) it happened Phil Hobbs wrote in :

OK I see.

Thanks, I appreciate that, just learned something,

Sure -- for your domain, look up ridged horn antennas. Dunno, you might've used them before.

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com