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Principle of the ML-CEAS technique for coupling a femtosecond laser and a high finesse cavity

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Guillaume MÉJEAN


Emission spectrum of a femtosecond laser oscillator

A mode-locked laser emits short pulses, for example 100 femtoseconds (1 fs = 10-15s) in our case, with a very stable repetition rate of about 80 MHz. The short pulse duration corresponds, if one considers the Fourier transform principles, to a broad laser emission spectrum, about 10 nm in our case. Again due to the Fourier transform properties, on should easily understand that the periodicity in time of laser pulses must also give a periodic signature in the frequency domain. Indeed the broad laser spectrum consists of a comb of frequencies, a periodic structure composed by some 100 000 modes (again in our case), separated by 1/Δt where Δt is the time interval between two pulses.


Principle of coupling of a femtosecond laser emission into a high finesse optical cavity


The spectrum transmitted by the cavity depends of the good match between the modelocked laser comb modes and the comb of the transmission resonances (modes, too) of the high finesse cavity.



The principle of «ML-CEAS» rests on the correspondence between these two combs of modes. In fact, when one the length of the high finesse cavity is modified, one may observe beating patterns of varying period. The match of the two combs occurs then only for separated groups of modes more or less far apart in frequency.


When the cavity length is exactly equal to that of the laser, the mode combs are tuned together, and the laser spectrum is then transmitted by the cavity with no beating or distorsion! We are then at the "magic point" configuration. In order to remain at this magic point, different locking strategies may be applied...


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