3.8 Finesse examples

3.8.1 Modulation index

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Figure 20: Finesse example: Modulation index.

This file demonstrates the use of a modulator. Phase modulation (with up to five higher harmonics is applied to a laser beam and amplitude detectors are used to measure the field at the first three harmonics. Compare this to Figure 16View Image as well.

Finesse input file for ‘Modulation index’

laser i1 1 0 n0               % laser P=1W f_offset=0Hz          
mod   eom1 40k .05 5 pm n0 n1 % phase modulator f_mod=40kHz, modulation index=0.05
ad    bessel1 40k n1          % amplitude detector f=40kHz          
ad    bessel2 80k n1          % amplitude detector f=80kHz          
ad    bessel3 120k n1         % amplitude detector f=120kHz          
xaxis eom1 midx lin 0 10 1000 % x-axis: modulation index of eom1 

yaxis abs                     % y-axis: plot ‘absolute’ amplitude  

3.8.2 Mirror modulation

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Figure 21: Finesse example: Mirror modulation.

Finesse offers two different types of modulators: the ‘modulator’ component shown in the example above, and the ‘fsig’ command, which can be used to apply a signal modulation to existing optical components. The main difference is that ‘fsig’ is meant to be used for transfer function computations. Consequently Finesse discards all nonlinear terms, which means that the sideband amplitude is proportional to the signal amplitude and harmonics are not created.

Finesse input file for ‘Mirror modulation’

laser i1 1 0 n1               % laser P=1W fxoffset=0Hz   
space  s1 1 1 n1 n2           % space of 1m length
bs     b1 1 0 0 0 n2 n3 dump dump % beam splitter as ‘turning mirror’,  normal incidence
space  s2 1 1 n3 n4           % another space of 1m length
fsig sig1 b1 40k 1 0          % signal modulation applied to beam splitter b1
ad    upper    40k n4         % amplitude detector f=40kHz        
ad    lower   -40k n4         % amplitude detector f=-40kHz        
ad    harmonic 80k n4         % amplitude detector f=80kHz        
xaxis sig1 amp lin 1 10 100   % x-axis: amplitude of signal modulation 
yaxis abs                     % y-axis: plot ‘absolute’ amplitude  

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