laser diodes
Posted by
Elliot Burke
on 2000-03-17 09:10:34 UTC
http://www.repairfaq.org/sam/laserlia.htm#liaint4
The above link contains a great many errors about lasers as light sources
for interferometry.
The HeNe laser makes a fine light source for interferometry, stabilization
is only needed for certain applications if the proper sort of HeNe is chosen
(non polarized, about 6" long).
The trick is that a short unpolarized laser will support exactly two modes
of oscillation. These will be on opposite sides of the peak of the gain
curve, and orthogonally polarized. They are separated with a polarizer, and
are pretty stable in wavelength. If the cavity temperature changes, the
modes will hop, but given enough time the laser will stabilize in one mode
for quite a long time (in a lab environment).
Some people just have to have things that stay stable over a wide range of
temperatures, vibration, and so on, they will need the stabilized laser.
The line width of the HeNe laser is about 0.5 ppm, so if air pressure,
temperture, and humidity are known, the distance measurement can be of ppm
accuracy.
The HP laser interferometer didn't use stabilized lasers in its original
form, but were made stable by cavities made of zero expansion glass.
There is extensive literature on this subject, mostly in Applied Optics
dating from the 1970's. It is not on the web as far as I know. If you have
access to a research library it wouldn't take long to go through the year's
end indexes of AO.
Laser diodes of the 650 nm wavelength class can have quite long coherence
lengths, their problem is that wavelength varies with temperatures, by 0.25
nm /degree, thus about 0.04%/degree. This doesn't matter much for
wavefront measuring interferometers, but is death for distance measurements.
For precision applications they are temperature controlled with
thermoelectric coolers or with external cavity techniques.
What the world needs is a good quadrature readout head for diode lasers.
Does anyone know of one? Does anyone want one but me?
Elliot Burke
The above link contains a great many errors about lasers as light sources
for interferometry.
The HeNe laser makes a fine light source for interferometry, stabilization
is only needed for certain applications if the proper sort of HeNe is chosen
(non polarized, about 6" long).
The trick is that a short unpolarized laser will support exactly two modes
of oscillation. These will be on opposite sides of the peak of the gain
curve, and orthogonally polarized. They are separated with a polarizer, and
are pretty stable in wavelength. If the cavity temperature changes, the
modes will hop, but given enough time the laser will stabilize in one mode
for quite a long time (in a lab environment).
Some people just have to have things that stay stable over a wide range of
temperatures, vibration, and so on, they will need the stabilized laser.
The line width of the HeNe laser is about 0.5 ppm, so if air pressure,
temperture, and humidity are known, the distance measurement can be of ppm
accuracy.
The HP laser interferometer didn't use stabilized lasers in its original
form, but were made stable by cavities made of zero expansion glass.
There is extensive literature on this subject, mostly in Applied Optics
dating from the 1970's. It is not on the web as far as I know. If you have
access to a research library it wouldn't take long to go through the year's
end indexes of AO.
Laser diodes of the 650 nm wavelength class can have quite long coherence
lengths, their problem is that wavelength varies with temperatures, by 0.25
nm /degree, thus about 0.04%/degree. This doesn't matter much for
wavefront measuring interferometers, but is death for distance measurements.
For precision applications they are temperature controlled with
thermoelectric coolers or with external cavity techniques.
What the world needs is a good quadrature readout head for diode lasers.
Does anyone know of one? Does anyone want one but me?
Elliot Burke