3
PICO
SCALE
FUNDAMENTALS
3.2 Measurement principle
3.2.1 Michelson interferometer
The
PICO
SCALE
displacement sensor is based on a Michelson interferometer with an extremely
compact and stable reference arm.
.
λ/4
λ/2
Interferometer
signal
Displacement
0
vanishing
sensitivity
high
sensitivity
Sensor head
Target
mirror
Figure 3.2: Measurement principle of a Michelson interferometer. Constructive and destructive
interference occur due to a displacement of the target mirror. To circumvent these
blind spots
the
PICO
SCALE
utilizes sinusoidal phase
modulation, see section 3.2.2.
The interferometer is driven by a laser, which is a coherent light source, that is required if a
Michelson interferometer is operated with (very) unequal arm lengths as it is the case for the
PICO
SCALE
. At a beam splitter, the light is divided into two parts. One part is re
fl
ected at a
fi
xed
reference mirror and guided back to the beam splitter. The other part of the light hits the target
mirror. This light is also re
fl
ected while picking up a relative phase with respect to the reference
beam.
I
nside the beam splitter, both re
fl
ected beams interfere and the coherent superposition is
recorded by a photo detector.
I
n the
PICO
SCALE
system, the beam splitter is located in the sensor head, whereas the light
source, a stabilized, distributed-feedback (DFB) laser diode and the photo detector are located in
the controller housing. The wavelength of the laser is around 1550 nm.
I
nside the controller, a
fi
ber network distributes the laser light to three independent measurement channels. The output
power of each channel is around 150 µW.
17
PicoScale User Manual
