Balanced optical-radiofrequency phase detector
AssigneeMassachusetts Institute of Technology
InventorsFranz X. Kaertner, Jung-Won Kim
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Abstract
A balanced optical-RF phase detector for the extraction of low-jitter radiofrequency (RF) signals from optical pulse trains is demonstrated. The extraction of the low-jitter radiofrequency signals is based on the precise phase detection by use of a differentially biased Sagnac-loop interferometer and synchronous detection. The phase-error signal from this balanced optical-RF phase detector, which is robust against drifts and photodetector nonlinearities, is used to regenerate low-jitter radiofrequency signals from optical pulse trains. Alternatively, the phase-error signal is used to generate a low-jitter optical pulse train, synchronized with a master radiofrequency signal or to synchronize multiple modelocked lasers with each other.
Claims25
A method for optical-radiofrequency phase detection, the method comprising:
- a) generating an optical pulse train;
- b) generating a radiofrequency electrical signal;
- c) detecting a portion of the optical pulse train and generating a first synchronous electrical detection signal;
- d) directing a second portion of the optical pulse train into a differentially biased Sagnac-loop interferometer;
- e) modulating the phase of the second portion of the optical pulse train in the differentially biased Sagnac-loop interferometer as a function of the frequency of the radiofrequency electrical signal;
- f) detecting an amplitude-modulated output of the Sagnac-loop interferometer and generating a second synchronous electrical detection signal;
- and
- g) synchronously detecting the second electrical detection signal with the help of the first electrical detection signal and generating a phase-error electrical signal having an amplitude proportional to the phase difference between the optical pulse train and zero-amplitude crossings of the radiofrequency electrical signal.
An apparatus for synchronizing an optical pulse train and a radiofrequency electrical signal comprising:
- a) a mode-locked laser for generating the optical pulse train;
- b) an oscillator for generating the radiofrequency electrical signal;
- and
- c) a balanced optical-radiofrequency phase detector comprising:
- i) a first optical coupler positioned and configured to split the optical pulse train from the mode-locked laser into a first and second portion of the optical pulse train;
- ii) a first detector positioned and configured to receive the first portion of the optical pulse train and to generate a first electrical detection signal in response;
- iii) a Sagnac-loop interferometer;
- iv) a second optical coupler positioned and configured to direct the second portion of the optical pulse train into the Sagnac-loop interferometer;
- v) a phase modulator mounted in the Sagnac-loop interferometer for modulating the phase of the second portion of the optical pulse train as a function of the frequency of the radiofrequency electrical signal;
- vi) a second detector positioned and configured to receive the second portion of the optical pulse train emitted from the Sagnac-loop interferometer and to generate a second electrical detection signal in response;
- and
- vii) a double-balanced mixer positioned and configured to receive the first and second electrical detection signal and to generate a phase-error electrical signal that is a function of the comparison.
Dependencies are inferred from the claim text.