Re: Single mode laser from a multi mode diode laser, how?

Jürgen Appel <jappel@xxxxxxxxxxxxxxx> writes:

alex schrieb:


On a different note I thought the only important criteria for
holography was for the coherence lenght to be reasonably long for good
interference fringes. Why must the laser be single mode?...

It does not have to be, as long as the multiple modes are similar enough in
frequency. But these requirements can be rather strict:

If you have a dual-mode laser with wavelength lambda1, lambda2 and both
waves are in phase at one point, their relative phase a distance l away is
l*2*pi(1/lambda1 + 1/lambda2). So if the modes are different only by 1 nm
at a wavelength of lets say 600 nm (1 part in 600), the coherence length (l
for 180° phase shift) is reduced to 180 µm (600*2 wavelengths).

On the other hand: If you have a single-mode external cavity diode laser,
the frequency usually is stable to less then 10 MHz over the exposure time
of an hologram, giving a coherence length of 30 m.

But having built ECDL: I would not even bother starting to build one out of
a laser diode that is not a single-longitudinal-mode diode. And since a
standard laser diode without external feedback operated on a well
stabilized current supply and temperature usually has a linewidth of <100
MHz for doing holography an ECDL is overkill. You only mast muke sure (or
hope for it ;-) that no mode jumps occur during the exposure time.

Keep in mind that for the case of a single-spatial-mode laser
(which is what we are dealing with), the longidtudinal mode spacing
or frequency between adjacent modes is c/2l where l is the length of
the cavity. This also means the (useful) coherence length is less than
the cavity length - a mm or less for a typical diode laser; a 10s of
cm for a HeNe laser. But that same laser operating in a single
longitudinal mode can have a coherence length of 100s of meters.

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