Digital Optical Frequency Modulation

Check out the first diagram on the page. It show an FM RF carrier being generated, which then goes to a comparator, which turns the light source on and off at the variable RF carrier frequency. There's no change in the light frequency.

-- John

So the "frequency" of the optical signal is the rate at which the light turns on and off?

I thought the "frequency" of the optical signal is determined by the wavelength of the light.

Light of a shorter wavelength has a higher frequency than light of a longer wavelength.

That depends what you mean by "the optical signal". If you mean the signal carried by the optical beam, then yes.

The frequency of the light is another thing. What you have is a double modulation scheme- the 70 MHz carrier is frequency modulated by the signal, and then the light source is pulse modulated by the FM carrier.

True, but that's not the frequency they're talking about. Look at the diagram. Doesn't it indicate what I said? The only change of frequency of the light itself would be the (relatively) tiny spectral spreading implied by turning the light on and off at 70 MHz.

-- John

You may read Jesse Zheng=B4s "Optical Frequency Modulated Continuous Wave (FMCW) Interferometry" Springer 2004 , page 233. Grandpa

Of course, pure optical frequency modulation is possible and used.

Semiconductor (diode) lasers change their freuqency as a function of drive current. So, they are vert easy to modulate.

It's also very easy to implement (at least in principle) for other types of lasers. One technique is to put a phase mouldator inside the cavity of the laser. Applying a signal to the phase modulator will result in frequency modulation of the optical output over up to the free spectral range (c/2L) of the laser.

We have built microchip solid state lasers that can be frequency modulated at 10s of GHz. One implementation of these lasers is about the same size and looks similar to a composite crystal green laser pointer laser with LiNbO3 instead of KTP.

--- sam | Sci.Electronics.Repair FAQ:

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snipped-for-privacy@gmail.com wrote:

That would be true for simple amplitude modulation in a steady state, but they are doing pulse modulation using a frequency modulated FM, so the picture would be a little more complicated, involving continuous spectra from the pulses. In any case, as I mentioned, it would amount to a teensy bit of vibrato on the laser's voice.

-- John

So you've assumed that there is only one frequency with this transmission system? How niave you are!

-- Yeppie, Bush is such an idiot that He usually outwits everybody else. How dumb!

Notice how I quoted your post so mine would not be irrelevant and useless?

Learn Bobby, learn.

Don