System and method of phase-locked fiber interferometry
View Patent ↗A system and method are provided for optical homodyne detection in an optical fiber interferometer. A detection signal is obtained by interfering an optical data signal with a phase-modulated optical reference signal. The modulator for the optical reference signal is phase-locked to an oscillatory modulation waveform. In embodiments, the modulator includes a piezoelectric element. In more specific embodiments, the modulator is a piezoelectric optical fiber-stretcher.
1. A method for measuring an optical data signal collected from a sample in the signal arm of an optical fiber interferometer having a signal arm and a reference arm that are optically combined onto a signal photodetector, comprising:
injecting a source beam of light from a light source into the signal arm;
generating an oscillatory modulation waveform having a frequency ƒ SG ;
driving a modulator in the reference arm with the oscillatory modulation waveform;
phase-locking the modulator to the oscillatory modulation waveform;
in the modulator, phase-modulating a reference beam of light from the light source so as to produce oscillatory phase modulations with a frequency ƒ PM that are phase-locked to the oscillatory modulation waveform; and
obtaining from the signal photodetector an oscillatory detection signal with frequency ƒ PM that is responsive to a combined optical signal comprising the optical data signal and the modulated reference beam.
2. The method of claim 1 , further comprising obtaining at least a phase measurement of the optical data signal by demodulating the oscillatory detection signal.
3. The method of claim 1 , wherein the modulator is phase-locked by adding an error signal to the oscillatory modulation waveform that drives the modulator.
4. The method of claim 1 , wherein the modulator comprises a fiber-stretcher in which a length of optical fiber is wrapped around a piezoelectric cylinder.
5. The method of claim 1 , wherein the oscillatory modulation waveform drives the modulator to produce a multiplicity of oscillations in the phase modulation for each oscillation in the oscillatory modulation waveform, whereby the frequency ƒ PM is more than twice the frequency ƒ SG .
6. The method of claim 1 , further comprising collecting the optical data signal by scanning an optical field in the signal arm of the interferometer.
7. The method of claim 6 , wherein the scanning is conducted so as to collect two-dimensional or three-dimensional image data.
8. The method of claim 1 , further comprising interferometrically generating the error signal.
9. The method of claim 8 , wherein the interferometric generating of the error signal comprises:
on a reference photodetector, interfering unmodulated light from the light source with a portion of the phase-modulated reference beam;
obtaining from the reference photodetector an oscillatory error-detection signal; and
low-pass filtering the oscillatory error-detection signal.
10. The method of claim 1 , wherein the optical data signal is collected in a multiplicity of sequential time intervals, and method further comprises determining changes in the optical data signal over time.
11. An optical homodyne detection system, comprising:
an optical fiber interferometer having a signal arm and a reference arm that are optically combined onto a signal photodetector, wherein the signal arm includes a sample region, and wherein the signal and reference arms are conformed to receive light from a light source;
an optical phase-modulator in the reference arm for producing phase-modulated light;
a signal generator configurable to generate an oscillatory modulation waveform;
a source of an error signal; and
a controller configurable to add the error signal to the oscillatory modulation waveform to produce a driving signal and to drive the optical phase-modulator with the driving signal, wherein:
the source of the error signal comprises a reference photodetector configured to produce a reference interference signal in response to an optical input in which a portion of the light from the light source is combined with a portion of the phase-modulated light from the phase-modulator;
the system further comprises a low-pass filter configured to filter the reference interference signal, thereby to produce the error signal; and
the error signal is fed back to the controller in a feedback loop that phase-locks the phase-modulator to the oscillatory modulation waveform.
12. The optical homodyne detection system of claim 11 , wherein the phase-modulator comprises a fiber-stretcher in which a length of optical fiber is wrapped around a piezoelectric cylinder.
13. The optical homodyne detection system of claim 11 , wherein the phase-modulator is conformed to respond to the driving signal by producing a multiplicity of oscillations in the phase modulation for each oscillation in the oscillatory modulation waveform, whereby a frequency ƒ PM of the phase modulation is more than twice a frequency ƒ SG of the oscillatory modulation waveform.