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
wide, on your 200THz.
Wolfgang
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.
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