Method for servocontrolling an optical device comprising a laser and a cavity, making it possible to compensate for an amplitude modulation introduced by a phase modulator

A method for the servo control of an optical device comprising a cavity (C) exhibiting resonance around a center frequency ƒc, a laser (L) and a phase modulator (PM), said method being designed to servo-control said cavity (C) to said laser (L) or vice versa and to compensate for an amplitude modulation introduced by said phase modulator (PM), said method comprising the following steps:

• A. using said laser (L) to generate laser radiation (LL) at an optical frequency ƒ within said resonance,
• B. varying a difference δν between said optical frequency of said laser radiation and said center frequency, such that said optical frequency scans said resonance, said difference being controlled by a parameter of an element of said device, and for each difference δνi
• i. modulating, at a modulation frequency ƒmod, a phase of said laser radiation, through a modulation phase φmod, with the phase modulator (PM),
• ii. injecting the phase-modulated radiation into said cavity (C),
• iii. using a photodiode (PDt, PDr) to detect radiation reflected or transmitted by said cavity and generating an electrical signal (St, Sr) representative of the intensity of said detected radiation,
• iv. demodulating said electrical signal at said modulation frequency ƒmod by synchronously generating a first demodulated signal and a second demodulated signal representative of the demodulated electrical signal, respectively at a first demodulation phase φdem,1 and at a second modulation phase
• ϕ
• dem
• ,
• 2
• ⁢
• ϕ
• d
• ⁢
• e
• ⁢
• m
• ,
• 2
• ≈
• ϕ
• d
• ⁢
• e
• ⁢
• m
• ,
• 1
• ±
• k
• ,
• where
• ⁢
• k
• ∈
• [
• 0
• ;
• 2
• ⁢
• π
• ]
• is different from said first phase, and by filtering the first and the second signal so as to retain only a DC component of the first demodulated signal Vϵ1, called error signal 1, and of the second demodulated signal Vϵ2, called error signal 2,
• C. computing a function 1 ƒ2 and a function 2 ƒ2 respectively representing a change in the error signal 1 and a change in the error signal 2 as a function of said difference for a given value of said first demodulation phase,
• D. repeating steps B) and C) by varying said first demodulation phase between each repetition until, for a value of the first demodulation phase, called decoupling phase
• ϕ
• dem
• ,
• 1
• =
• ϕ
• dec
• ,
• said function 1 or said function 2 has a plateau on a portion of values of said difference comprising 0, said function out of 1 or 2 having the plateau being called PL function, the function not having the plateau being called SP function;
• E. at said decoupling phase, varying said difference so as to observe an extremum, for what is called a zero difference, said intensity of the light radiation detected by said photodiode, and servo-controlling said element to a value of said parameter that makes it possible to maintain this intensity extremum,
• F. at said decoupling phase, modulating the phase of said laser radiation, with the phase modulator (PM), using what is called an additional periodic signal at what is called an additional modulation frequency ƒadd in addition to that at said modulation frequency fmod;
• G. minimizing, for the SP function, an amplitude of a first harmonic of the additional signal, by varying said first demodulation phase, this minimum being reached for what is called a first additional demolition phrase
• ϕ
• dem
• ,
• 1
• =
• ϕ
• dec
• ⁢
• 2
• ,
• and, at said additional phase, servo-controlling said element to a value of said parameter that makes it possible to maintain this intensity extremum,
• H. at said decoupling phase
• ϕ
• dem
• ,
• 1
• =
• ϕ
• dec
• and at said zero difference, varying said first modulation phase until the PL function is canceled out, and maintaining the canceling out of said PL function by servo-controlling said phase modulator.