Video of a flashing cavity

(How to make videos with Finesse)

Andreas Freise
2013

The following example uses the Virgo arm cavity to demonstrate how to make a video from Finesse outputs. The optical setup consists of a two-mirror Fabry-Perot cavity and a laser beam which is injected into the cavity. The laser beam is intentionally set up to be not in the right shape nor alignment for a perfect injection into the cavity.

The input file

#-----------------------------------------------------------------------
# Finesse input file to create a video output.
# The optics is based on an old file of the Virgo interferometer.
# The cavity below is based on the (old) Virgo north arm cavity.
# The input beam is misaligned and mismatched.
# 31.01.2013, Andreas Freise 
#-----------------------------------------------------------------------

l laser 0.5 0 0 nin
tem laser 1 0 .1 40
gauss g1 laser nin 18m 0
s s1 1m nin nEO1
# Modulator for RF modulation frequency
mod eom1 $fMI .4 1 pm 0 nEO1 nEO2
const fMI 6.3  
s s2 1 nEO2 nMNI1

% the following optics were taken from an old model
% of the Virgo north arm cavity
m MNI 0.8819765 0.118 0 nMNI1 nMNI2   
s sN1 2999.5 nMNI2 nMNE1
m MNE 0.99995 50u 0 nMNE1 nMNE2
attr MNE Rc 3602.50
s sN2 1.77 nMNE2 nout

# use Hermite-Gauss modes
maxtem 7
# compute cavity eigenmodes
cav cN MNI nMNI2 MNE nMNE1
# computing the beam shape in transmission
beam b1 nout
xaxis b1 x lin -8 8 100
x2axis b1 y lin -8 8 100

% to create a video, we can do an image sequency with Finesse
% the follwoing two commands do that for you:
x3axis MNE phi lin -90 90 100
video

% use Gnuplot to create png files
gnuterm png
% some Gnuplot settings to format the images
GNUPLOT
set cbrange[0:2e-3]
set nosurface
#set colorbox vertical
#set colorbox user origin .85,.1 size .04,.8
set pm3d
set view map
set palette gray
set format z '%.1g'
set format cb '%.1g'
set size ratio 1
unset grid
# the following commands strip all axes and tic labels 
# to produce a small video without ornaments.
# Remove these if you like a normal apperance with
# labels, a colorbar, etc.
set format ""
unset colorbox
unset label
unset title
unset xlabel
unset ylabel
unset key
unset tics
unset border
set term pngcairo size 100,100 monochrome enhanced
set lmargin 0
set bmargin 0
set rmargin at screen 0.99999
set tmargin at screen 0.99999
END
 

The cavity acts as a spatial filter that decomposes the mismatched and misaligned beam into cavity eigenmodes. Each eigenmode can be resonantly enhanced in the cavity for a specific cavity lengths. When the cavity length is tuned by moving one of the mirrors along the optical axis (just by a half of a wave-length, in this case about 500nm) we can see flashes of the different mode shapes in transmission of the cavity. Many experimentalists enjoy seeing this spectacle one their monitors in the control rooms. With Finesse we can create a similar looking video.

Finesse simulation: A laser beam is injected into a two mirror cavity. The beam is not matched perfectly to the cavity, so that when the cavity tuning is changed, different beam mode shapes are detected in transmission.

The above input file used the `xaxis' and `x2axis' commands to create an image of the transmitted laser beam. The `x3axis' and `video' commands then create a series of such images for different detunings. We use Gnuplot to create and save 'png' files of each image, creating an image sequence which you can turn into a video using an external tool. Shown above is an `animated gif' which has been created using Photoshop.