# Digital Optical Frequency Modulation

Discussion in 'Electronic Basics' started by Radium, Sep 16, 2006.

2. ### John O'FlahertyGuest

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.

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.

4. ### John O'FlahertyGuest

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.

5. ### GrandpaGuest

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

6. ### Guest

Well, technically there is a change in the frequency of the light, but
it's very small compared to the optical frequency (probably about 200
THz). Any RF modulation will give you sidebands on the 200THZ at the
modulation frequencies, though these modulation frequencies (100MHz)
are probably well within the linewidth of the laser. If you had a 5MHz
bandwidth to your 70MHz modulation, you'd see +/-70MHz sidebands, 5MHz
Wolfgang

7. ### Sam GoldwasserGuest

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

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8. ### John O'FlahertyGuest

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.

9. ### Bob MayGuest

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

10. ### Don BoweyGuest

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

Learn Bobby, learn.

Don