______________________________________________________________
3
The need for a laser filter
The TCF is a temperature stabilised etalon device, designed to reduce the
intensity of spurious secondary laser modes found on many modern commercial
solid state laser light sources. These secondary modes are very weak, with an
intensity typically 10
-8
to 10
-11
of the laser intensity, but unfortunately they lie in
the region of interest for Brillouin spectroscopy measurements.
Fig. 1. Shows the spectrum of a 532 nm solid state laser source measured using
the JRS high contrast tandem interferometer TFP-1. The main laser peak has
been strongly attenuated relative to the secondary peaks. A Brillouin spectrum
measured in the presence of a strong elastic component will inevitably show
these secondary peaks in the background, making interpretation of the
spectrum difficult.
The TCF strongly attenuates these modes, in many cases reducing them below
the detection threshold.
The largest attenuation provided by this filter for the unwanted modes is close
to 600. As shown also in Fig. 1, the device is expected to provide an extinction
factor of 2 for components as close as 1.8 GHz from the primary radiation and
-40
-30
-20
-10
0
10
20
30
40
10
1
10
2
10
3
10
4
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
int
ens
it
y (
PM
T
c
ount
s)
frequency shift (GHz)
T
CF
-1
e
ta
lo
n t
ra
ns
mit
ta
nc
e
Fig. 1 : A dramatic need for a spectral cleaning. Violet curve shows the
spectrum of a commercial laser source, showing several additional modes
extending out as far as 100 GHz, as seen by means of a high contrast Fabry-
Pérot interferometer. The red curve shows the theoretical transmission
curve of a TCF-1 unit.
