4.3 Finesse examples

4.3.1 Optical beat

View Image

Figure 23: Finesse example: Optical beat.

In this example two laser beams are superimposed at a 50:50 beam splitter. The beams have a slightly different frequency: the second beam has a 10 kHz offset with respect to the first (and to the default laser frequency). The plot illustrates the output of four different detectors in one of the beam splitter output ports, while the phase of the second beam is tuned from 0° to 180°. The photodiode ‘pd1’ shows the total power remaining constant at 1. The amplitude detectors ‘ad1’ and ‘ad10k’ detect the laser light at 0 Hz (default frequency) and 10 kHz respectively. Both show a constant absolute of ∘ ---- 1∕2 and the detector ‘ad10k’ tracks the tuning of the phase of the second laser beam. Finally, the detector ‘pd10k’ resembles a photodiode with demodulation at 10 kHz. In fact, this represents a photodiode and two mixers used to reconstruct a complex number as shown in Equation (82View Equation). One can see that the phase of the resulting electronic signal also directly follows the phase difference between the two laser beams.

Finesse input file for ‘Optical beat’

const  freq 10k       % creating a constant for the frequency offset
laser  l1 1 0  n1     % laser with P=1W at the default frequency
space  s1 1n 1 n1 n2  % space of 1nm length
laser  l2 1 $freq  n3 % a second laser with f=10kHz frequency offset
space  s2 1n 1 n3 n4  % another space of 1nm length
bs     b1 0.5 0.5 0 0 n2 n5 dump n4 % 50:50 beam splitter 
space  s3 1n 1 n5 n6  % another space of 1nm length
ad     ad0 0 n6       % amplitude detector at f=0Hz
ad     ad10k $freq n6 % amplitude detector at f=10kHz
pd     pd1 n6         % simple photo detector
pd1    pd10k $freq n6 % photo detector with demodulation at 10kHz

xaxis l2 phi lin 0 180 100 % changing the phase of the l2-beam

yaxis abs:deg         % plotting amplitude and phase 

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