Methods and apparatus for swept-source optical coherence tomography
In one embodiment of the invention, a semiconductor optical amplifier (SOA) in a laser ring is chosen to provide low polarization-dependent gain (PDG) and a booster semiconductor optical amplifier, outside of the ring, is chosen to provide high polarization-dependent gain. The use of a semiconductor optical amplifier with low polarization-dependent gain nearly eliminates variations in the polarization state of the light at the output of the laser, but does not eliminate the intra-sweep variations in the polarization state at the output of the laser, which can degrade the performance of the SS-OCT system.
1. A clock generator for generating a clock signal from laser light, the clock generator comprising:
an interferometer having a dual optical output and an input, the interferometer receiving laser light at the input and the interferometer having an optical imbalance;
a balanced photoreceiver comprising a pair of photodetectors, each photodetector in optical communication with a respective one of the dual optical outputs of the interferometer, the balanced photoreceiver having an output;
a zero-crossing detector having an electrical input in electrical communication with the output of the balanced photoreceiver, the output of the zero-crossing detector being the generated clock signal;
an analog-to-digital converter in electrical communication with the zero-crossing detector, the analog-to-digital converter clocked by the generated clock signal; and
a digital control system configured to stabilize a drive frequency of a frequency tuning element of the laser using a control signal derived from the generated clock signal.
2. The clock generator of claim 1 further comprising a bandpass filter in electrical communication between the output of the balanced photoreceiver and the input of the zero-crossing detector.
3. The clock generator of claim 2 further comprising a frequency multiplier having an input in electrical communication with the output of the bandpass filter and having an output in electrical communication with the input of the zero-crossing detector.
4. The clock generator of claim 3 further comprising a second bandpass filter having an input in electrical communication with the output of the frequency multiplier and an output in electrical communication with the input of the zero-crossing detector.
5. The clock generator of claim 1 wherein the interferometer is a Mach Zender interferometer.
6. The clock generator of claim 5 wherein the interferometer has two arms and wherein one arm is ΔL larger than the other so as to generate a phase difference.
7. A clock generator for generating a clock signal from laser light, the clock generator comprising:
an interferometer having a dual optical output and an input, the interferometer receiving laser light at the input and the interferometer having an optical imbalance;
a pair of photodetectors, each photodetector in optical communication with a respective one of the dual optical outputs of the interferometer, and each photodetector having an output;
a first zero-crossing detector having an output and having an electrical input in electrical communication with the output of the first photodetector;
a second zero-crossing detector having an output and having an electrical input in electrical communication with the output of the second photodetector;
an exclusive OR gate having a first input terminal in communication with the output of the first zero crossing detector, and having a second input terminal in communication with the output of the second zero crossing detector, and having an output being the generated clock signal.
8. The clock generator of claim 7 further comprising a first bandpass filter in electrical communication between the output one photodetector and the input of the first zero-crossing detector and a second bandpass filter in electrical communication between the output of the other photodetector and the input of the second zero-crossing detector.
9. The clock generator of claim 7 wherein the interferometer is a phase splitting interferometer and the optical outputs of the interferometer have equal intensity but are shifted by 90 degrees.
10. A method of tunable filter linearization comprising:
selecting a filter frequency such that one or more mechanical resonance frequencies of a tunable filter are avoided, wherein the filter frequency is a fundamental frequency;
combining a plurality of harmonically related sinusoids of the filter frequency;
tuning an amplitude and a phase of the combined sinusoids to generate a drive waveform, wherein the drive waveform compensates for non-uniformities in amplitude and phase responses of the tunable filter;
setting a laser repetition rate for a tunable laser using the drive waveform, the tunable laser in optical communication with the tunable filter; and
generating optical coherence tomography data using light emitted from the tunable laser.