Frequency locking device

A frequency locking method comprising:

• supplying a beam of continuous wave, coherent radiation of frequency f.sub.0 ;
• phase modulating the beam, in response to a compound signal which is the sum of a signal at frequency f.sub.2 and a FM subcarrier at frequency f.sub.1 that is phase modulated at frequency f.sub.3, to produce a phase modulated beam;
• filtering the phase modulated beam by means of a filter whose transfer function has a characteristic frequency f.sub.f ;
• detecting the filtered beam to produce a detector signal;
• adjusting the ratio of f.sub.0 /f.sub.f to a fixed value N in response to the component of the detector signal substantially at frequency (Jf.sub.1 +Kf.sub.2 +Lf.sub.3), where (J+K) is an odd integer and L an even integer;
• and
• adjusting the ratio of f.sub.1 /f.sub.f to a fixed value M in response to the component of the detector signal substantially at frequency (Qf.sub.1 +Rf.sub.2 +Sf.sub.3), where (Q+R) is an even integer and S an odd integer.

A method for calibrating a multimode filter whose transfer function has characteristic frequencies Nf.sub.f and (N.+-.M)f.sub.f, said method comprising:

• supplying a beam of continuous wave, coherent radiation of frequency f.sub.0 ;
• phase modulating the beam, in response to a compound signal which is the sum of a signal at frequency f.sub.2 and a FM subcarrier at frequency f.sub.1 that is phase modulated at frequency f.sub.3, to produce a phase modulated beam;
• supplying the phase modulated beam to the filter to filter the phase modulated beam according to the transfer function;
• detecting the filtered beam to produce a detector signal;
• adjusting the radiation frequency so as to set the ratio of f.sub.0 /f.sub.f to the fixed value N in response to the component of the detector signal substantially at frequency (Jf.sub.1 +Kf.sub.2 +Lf.sub.3), where (J+K) is an odd integer and L an even integer;
• and
• adjusting the subcarrier frequency to lock it to the mode spacing of the filter so as to set the ratio of f.sub.1 /f.sub.f to a fixed value M in response to the component of the detector signal substantially at frequency (Qf.sub.1 +Rf.sub.2 +Sf.sub.3), where (Q+R) is an even integer and S an odd integer, thereby determining the mode spacing of the filter.

A frequency locking device for measuring the refractive index of a gas, said device comprising:

• a source of a beam of continuous wave, coherent radiation of frequency f.sub.0 ;
• means for phase modulating the beam, in response to a compound signal which is the sum of a signal at frequency f.sub.2 and a FM subcarrier at frequency f.sub.1 that is phase modulated at frequency f.sub.3, to produce a phase modulated beam;
• an optical cavity through which the phase modulated beam passes, said cavity having a transfer function that has a characteristic frequency f.sub.f ;
• means for letting the gas into the optical cavity to change the transfer function as a function of the refractive index of the gas;
• a detector responsive to the filtered beam to produce a detector signal;
• first means responsive to the component of the detector signal substantially at frequency (Jf.sub.1 +Kf.sub.2 +Lf.sub.3), where (J+K) is an odd integer and L an even integer, for adjusting the ratio of f.sub.0 /f.sub.f to a fixed value N;
• second means responsive to the component of the detector signal substantially at frequency (Qf.sub.1 +Rf.sub.2 +Sf.sub.3), where (Q+R) is an even integer and S an odd integer, for adjusting the ratio of f.sub.1 /f.sub.f to a fixed value M;
• and
• means for measuring the subcarrier frequency before and after gas is let into the optical cavity to determine the refractive index of the gas.

A frequency locking method for measuring the refractive index of a gas, said method comprising:

• supplying a beam of continuous wave, coherent radiation of frequency f.sub.0 ;
• phase modulating the beam, in response to a compound signal which is the sum of a signal at frequency f.sub.2 and a FM subcarrier at frequency f.sub.1 that is phase modulated at frequency f.sub.3, to produce a phase modulated beam;
• passing the phase modulated beam through an optical cavity, said cavity having a transfer function that has a characteristic frequency f.sub.f ;
• detecting the filtered beam to produce a detector signal;
• adjusting the ratio of f.sub.0 /f.sub.f to a fixed value N in response to the component of the detector signal substantially at frequency (Jf.sub.1 +Kf.sub.2 +Lf.sub.3), where (J+K) is an odd integer and L an even integer;
• and
• adjusting the ratio of f.sub.1 /f.sub.f to a fixed value M in response to the component of the detector signal substantially at frequency (Qf.sub.1 +Rf.sub.2 +Sf.sub.3), where (Q+R) is an even integer and S and odd integer;
• measuring the subcarrier frequency;
• letting the gas into the optical cavity to change the transfer function as a function of the refractive index of the gas;
• adjusting the ratio of f.sub.0 /f.sub.f to a fixed value N in response to the component of the detector signal substantially at frequency (Jf.sub.1 +Kf.sub.2 +Lf.sub.3), where (J+K) is an odd integer and L an even integer;
• and
• adjusting the ratio of f.sub.1 /f.sub.f to a fixed value M in response to the component of the detector signal substantially at frequency (Qf.sub.1 +Rf.sub.2 +Sf.sub.3), where (Q+R) is an even integer and S an odd integer;
• measuring the subcarrier frequency to determine the refractive index of the gas.