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Sony has increased the resolution with a
unique optical system that increases the number
of diffractions and has acquired a 138 nm
signal wavelength in their highest resolution
product. In this optical system, the primary
signal wavelength is 1/4 of the scale lattice
wavelength and, by increasing the diffraction
efficiency by using a volumetric phase
hologram for the diffraction grating used
as the scale, Sony is able to achieve a high
signal quality. Furthermore, this optical detection
system is designed so that error does not
occur even if there are variations in semiconductor
laser wavelength due to temperature
changes or changes in pressure and humidity,
which change the index of refraction of air.
The scale is made from a Neo ceram that has
a low coefficient of thermal expansion that is
about 1/10 that of ordinary glass.
Figure 2 shows the detection principles used
by the Laserscale products. The laser beam
emitted from the semiconductor laser active
area passes through a polarized beam splitter
(PBS) and is divided into two beams, the
P wave and the S wave. The split laser beam
impinges on a hologram lattice in which a 550
nm period pattern has been engraved on the
glass surface. The laser beam is diffracted and
then impinges on a mirror with a quarter wave
plate. When the beam is reflected from the
mirror, the S wave is converted to a P wave,
and the P wave is converted to an S wave.
These beams return to the hologram lattice
and are diffracted a second time. Since the
P and S waves have been switched, the two
beams, which return to the polarized beam
splitter, do not return to the active (emission)
area but rather proceed to the detector and
are combined. Since the two optical paths
are symmetrical in the left/right direction,
the influences of atmospheric fluctuation and
pressure changes are cancelled.
The double diffraction means that the synthesized
wave switches between dark and bright
four times each time the hologram lattice
moves 550 nm, which is one period of the lattice.
As a result, a precise primary signal with a
period of 138 nm is detected by the photodetector.
That signal has a sinusoidal form, and a
2-phase primary signal can be extracted optically.
This primary signal is divided electronically
by the interpolator's signal-processing
LSI. A resolution of 34 pm is achieved by
dividing by 4000 and a resolution of 17 pm is
achieved by dividing by 8000.
A highly reliable 790 nm wavelength Sony
laser diode is used in the optical block.
Furthermore, a special-purpose IC that integrates
the light source, polarized beam splitter,
and photodetectors is used to create a
compact detector unit.
Figure 2 Laserscale Detection Principles
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Laserscale Achieves
a Resolution of 17 Picometers
Laserscale Stability
Vol.54