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. An apparatus for tunable filter linearization comprising:
a mechanically actuated tunable filter; and
a pair of phase-locked sine-wave generators having an output in electrical communication with the mechanically actuated tunable filter, wherein one of the pair of phase-locked sine wave generators operates at twice the frequency of the other of the pair of phase-locked sine wave generators.
2. The apparatus of claim 1 further comprising a power amplifier electrically connected between the mechanically actuated tunable filter; and the pair of phase-locked sine-wave generators.
3. The apparatus of claim 1 wherein the mechanically actuated tunable filter is one of a piezo and a micro-electromechanically actuated Fabry-Perot filter.
4. The apparatus of claim 1 wherein each of the pair of phase-locked sine wave generators is a digital direct synthesis sine wave generator.
5. A method for tunable filter linearization comprising the steps of:
providing a mechanically actuated tunable filter; and
generating an output signal from a pair of phase-locked sine-wave generators; and
applying the generated output signal to the mechanically actuated tunable filter,
wherein one of the pair of phase-locked sine-wave generators operates at twice the frequency of the other.
6. The method of claim 5 wherein the mechanically actuated tunable filter is a Fabry-Perot filter.
7. The method of claim 5 further comprising the step of amplifying the generated output signal prior to applying it to the mechanically actuated tunable filter.
8. A method for tunable filter linearization comprising the steps of:
choosing a fundamental frequency that avoids the mechanical resonance frequencies of a tunable filter;
summing two or three harmonically related sinusoids of the fundamental frequency; and
tuning the amplitudes and phases of the summed sinusoids to compensate for non-uniformities in amplitude and phase responses of the tunable filter.
9. The method of claim 8 wherein the summing is performed by Fourier synthesis.
10. The method of claim 8 further comprising collecting optical coherence data using an electromagnetic radiation source within which the tunable filter is a component thereof.
11. The method of claim 8 further comprising generating light from a laser having a cavity, the tunable filter disposed within the cavity.
12. The method of claim 10 further comprising generating an image using the collected optical coherence tomography data.
13. A linearized tunable filter comprising:
a mechanically actuated tunable filter having a resonance frequency; and
a linearization module in electrical communication with the mechanically actuated tunable filter, the linearization module:
generates a fundamental frequency that avoids the mechanical resonance frequencies of the tunable filter;
sums two or three harmonically related sinusoids of the fundamental frequency; and
generates amplitudes and phases of the summed sine waves to compensate for non-uniformities in the amplitude and phase responses of tunable filter.
14. The linearized tunable filter of claim 13 wherein the step of summing uses Fourier synthesis.
15. The linearized tunable filter of claim 13 further comprising a gain element in optical communication with the mechanically actuated tunable filter.
16. The linearized tunable filter of claim 15 further comprising an optical delay element in optical communication with the gain element.