Referencing of the beating spectra of frequency combs
There is provided a method for referencing and correcting the beating spectrum generated by the interference of the components of a frequency comb source. The proposed method allows monitoring of variations of a mapping between the source and the beating replica. This can then be used to compensate small variations of the source in Fourier transform spectroscopy or in any other interferometry application in order to overcome the accuracy and measurement time limitations of the prior art. Constraints on source stability are consequently reduced.
1. A system for correcting a beating interference signal, the system comprising:
a source system for producing a frequency comb having frequency components non-uniformly distributed in a source spectrum, the frequency components to be interfered to produce the beating interference signal having a beating spectrum with beating components, a portion of the beating spectrum having a relation with a portion of the source spectrum, the frequency comb having at least one variable frequency comb parameter;
at least one narrowband filter receiving the frequency comb and having a known frequency passband in the source spectrum for isolating a subset of frequency components among the frequency components, the subset of frequency components interfering to provide a reference beating component subset;
a phase/frequency analyzer for monitoring at least one of a phase and a frequency of the reference beating component subset;
a model calculator receiving the at least one of the phase and the frequency for determining a correction function to be applied to the beating interference signal, the correction function to correct for the at least one variable frequency comb parameter;
an acquisition unit for recording the beating interference signal; and
a correction unit receiving the recorded beating interference signal and the correction function for correcting the recorded beating interference signal using the correction function.
2. The system as claimed in claim 1 , wherein the source system comprises a first frequency comb source and a second frequency comb source each having uniformly distributed frequency components and combined to provide the frequency comb.
3. The system as claimed in claim 2 , wherein the first frequency comb source and the second frequency comb source each comprises a mode-locked laser.
4. The system as claimed in claim 1 , wherein the narrowband filter comprises a passive band-pass filter receiving and filtering the frequency comb to provide the reference beating component subset.
5. The system as claimed in claim 4 , wherein said passive band-pass filter comprises a fiber Bragg grating filter used in reflection.
6. The system as claimed in claim 1 , wherein the narrowband filter comprises an active filter.
7. The system as claimed in claim 6 ,
wherein the source system comprises a first frequency comb source and a second frequency comb source each having uniformly distributed frequency components and combined to provide the frequency comb; and
wherein the active filter comprises:
a reference continuous wave source;
a combiner for combining and interfering the reference continuous wave source and the first frequency comb to provide a first intermediate interference signal;
a first detector for detecting the first intermediate interference signal to provide a first intermediate reference signal with beating components;
a combiner for combining and interfering the reference continuous wave source and the second frequency comb to provide a second intermediate interference signal;
a second detector for detecting the second intermediate interference signal to provide a second intermediate reference signal with beating components;
a filter for filtering, in the beating spectrum, the beating components of the first intermediate reference signal to retain a first reference beating component subset and the beating components of the second intermediate reference signal to retain a second reference beating component subset, the first reference beating component subset and the second reference beating component subset interfering to provide the reference beating component subset.