Canon PV-01 Brochure

Canon laser Doppler velocimeters, equipped with diffraction gratings, take advantage of an optical system
(diffraction laser light Doppler method) that will not dependent on the laser wavelength. Therefore, the
measurement precision is unaffected by fluctuations in the semiconductor laser wavelength due to changes in
temperature. Not only are these sensors environmental stability they can be used for a wide variety of applications;
since we use an E/O frequency shifter, they are capable of taking measurement from a still as well as a running state.

The laser light emitted by a semiconductor laser is positioned
so that it is a linearly polarized beam with respect to the zaxis.
Then, a collimator lens makes it into a parallel light
beam. The grating array direction of this parallel light beam is
the direction of the y-axis, and the diffraction grating with a
grating pitch d divides the beam into two light beams by a
diffraction angle of q. Here, the equation d sin θ = λ holds.
These two light beams pass through a first lens and are then
irradiated into an E/O frequency shifter consisting of 
electrooptical
crystals. After that, they are diffracted by a second lens
before irradiated to the object moving at a velocity of V,
which is being measured. The incidence angle of the two
irradiated light beams is θ'.
The first and second lenses form an afocal optical system with
a magnification of m, and the aberration is corrected so that
sin θ / sin θ' = m.
The two light beams that are now modified to have a
frequency difference fR by the E/O frequency shifter are then
irradiated to the object to be measured, and the diffused
beams coming back from the object proceed through the
second lens and a condensing lens, eventually reaching the
photo diode. The beat signal (i.e., the Doppler frequency) of
these light beams containing the speed information obtained

here is F=2V/md+fR, independent of the laser frequency; in
addition, it is possible to measure the velocity from an object
that is originally in a still state.
By signal-processing this Doppler frequency, the unit displays
the velocity and sends the F/V output and pulse output.

Canon