3
PICO
SCALE
FUNDAMENTALS
becomes possible to extract the quadrature
signal, which is a two dimensional signal, dependent
on the target mirror position, depicted in
fi
gure 3.3. The two extracted sinusoidal signals are
phase-shifted by 90 degrees and thus are in quadrature. The X and Y components are shown in
fi
gure 3.3, left. Constant movement of the target mirror produces a circle in a two dimensional
representation. The right part of the
fi
gure shows both signal components simultaneously and
it can be seen that any blind spot in one signal is compensated by a high sensitivity point in the
other. Thus, the sensitivity is maintained high during movement.
Furthermore, the phase relationship between the two components allows to unambiguously in-
fer the movement direction. While an increasing relative position of the target mirror leads to
a counter-clockwise rotation of the X-Y vector, a decreasing position leads to a clockwise rota-
tion. Thus, sinusoidal phase modulation interferometry is perfectly suited for high-resolution,
long-range position measurements.
3.2.3 Working range
During the initial adjustment phase it is required to de
fi
ne a so-called working range. This is the
region where the target mirror is expected to be during the measurement. The setting of the
working range is mapped to internally stored con
fi
guration values and an appropriate setting sub-
sequently leads to highest accuracy and performance of the
PICO
SCALE
position measurements.
3.3 Traceability
Although the DFB laser has a very high spectral purity, spectral width below 10 pm, the emitted
wavelength is not constant due to unavoidable thermal drifts. Since the measured target position
is directly dependent on the emitted wavelength, a feedback system stabilizing the wavelength of
the DFB laser is mandatory.
I
n order to stabilize the temperature of the laser diode, it is equipped
with a thermo-electric cooler (TEC). Besides stabilizing the temperature, the TEC element can be
used to actively control and tune the temperature and thus the emitted wavelength. The wave-
length is locked to an absorption line of a gas absorption cell, according to the standards N
I
ST
SRM 2517a or SRM 2519a. This reference cell is
fi
lled either with acetylene or hydrogen cyanide
and allows to determine the wavelength of the laser within a range of lower than
±
1 pm. The gas
absorption cells are extremely insensitive to environmental parameters like temperature, humid-
ity, pressure and also on electro-magnetic
fi
elds. Thus, these cells o
ff
er a persistent and reliable
wavelength standard to stabilize the wavelength of the DFB laser diode.
The gas absorption cell is integrated in the
PICO
SCALE
controller housing. One part of the mod-
ulated laser light is guided through the reference cell and the transmitted signal is recorded and
evaluated. A feedback loop controls the laser chip temperature such that the wavelength is con-
tinuously kept at an absorption line.
3.4 Environmental compensation
When the interferometer is used in a vacuum environment, no environmental compensation is
necessary. When the interferometer system is operated in ambient conditions, variations in the
19
PicoScale User Manual
