Regenerative mode locked laser swept source for OCT medical imaging
View Patent ↗An optical coherence analysis system uses a laser swept source that is constrained to operate in a mode locked condition using regenerative mode-locking. This is accomplished by synchronously changing the laser cavity's net gain and/or phase based on time varying intensity of the swept optical signal generated by the laser. This produces a stable pulsation behavior, which is associated with smooth tuning (low optical frequency reference clock jitter) and low relative intensity noise (RIN).
1. An optical coherence imaging method, comprising:
providing a laser swept source to generate a swept optical signal in a laser cavity that is less than a meter long;
detecting time-varying intensity of the swept optical signal;
using the detected time-varying intensity of the swept optical signal as feedback to control a mode-locked operation of the laser swept source by modulating an intracavity element in the laser cavity based on the roundtrip travel time of light in the laser cavity;
transmitting the swept optical signal to an interferometer having a reference arm and a sample arm, in which a sample is located;
combining the swept optical signal returning from the sample arm and the reference arm to generate an interference signal;
detecting the interference signal; and
generating image information of the sample from the detected interference signal.
2. A method as claimed in claim 1 , wherein controlling the mode-locked operation of the laser swept source comprises controlling a bias current to an optical gain element that amplifies light in the laser cavity of the laser swept source in response to the time-varying intensity of the swept optical signal.
3. A method as claimed in claim 1 , wherein controlling the mode-locked operation of the laser swept source comprises modulating a phase of optical signals in the laser cavity of the laser swept source in response to the time-varying intensity of the swept optical signal.
4. A method as claimed in claim 1 , wherein controlling the mode-locked operation of the laser swept source comprises controlling the laser cavity of the laser swept source to reduce a number of pulses circulating in the laser cavity.
5. An optical coherence imaging method, comprising:
providing a laser swept source to generate a swept optical signal in a laser cavity that is less than a meter long;
detecting time-varying intensity of the swept optical signal;
using the time-varying intensity of the swept optical signal as feedback to control a mode-locked operation of the laser swept source;
transmitting the swept optical signal to an interferometer having a reference arm and a sample arm, in which a sample is located;
combining the swept optical signal returning from the sample arm and the reference arm to generate an interference signal;
detecting the interference signal; and
generating image information of the sample from the detected interference signal; and
wherein controlling the mode-locked operation of the laser swept source comprises modulating net gain of the laser cavity of the laser swept source in response to the time-varying intensity of the swept optical signal.
6. An optical coherence analysis system comprising:
a swept laser source for generating a swept optical signal that is frequency tuned over a tuning band in a laser cavity that is less than a meter long, the laser comprising an intracavity element in the laser cavity;
a detector for detecting time-varying intensity of the swept optical signal, in which a mode-locked operation of the swept laser source is controlled by modulating the intracavity element in response to the detected time-varying intensity of the swept optical signal which is based on a roundtrip travel time of light in the cavity;
an interferometer for dividing the swept optical signal between a reference arm and a sample arm leading to a sample; and
a detector system for detecting an interference signal generated from the swept optical signal from the reference arm and from the sample arm.
7. A system as claimed in claim 6 , wherein the swept laser source comprises:
a gain medium for amplifying light within the laser cavity of the swept laser source to generate the swept optical signal; and
a tuning element for controlling a frequency of the swept optical signal to sweep across a scanband, and
wherein a gain of the gain medium is modulated in response to the detected time-varying intensity of the swept optical signal.
8. A system as claimed in claim 7 , wherein the mode-locked operation of the swept laser source is controlled by modulating bias current to the gain medium in response to the detected time-varying intensity of the swept optical signal.
9. A system as claimed in claim 6 , wherein the swept laser source comprises:
a gain medium for amplifying light within the laser cavity of the swept laser source to generate the swept optical signal;
a tuning element for controlling a frequency of the swept optical signal to sweep across a scanband, and
a phase modulator, which is the intracavity element, for controlling a phase of the light within the laser cavity that is modulated in response to the detected time-varying intensity of the swept optical signal.
10. A system as claimed in claim 6 , wherein the swept laser source comprises:
a gain medium for amplifying light within the laser cavity of the swept laser source to generate the swept optical signal;
a tuning element for controlling a frequency of the swept optical signal to sweep across a scanband, and
wherein the intracavity element is a cavity gain control element that modulates a net gain of the laser cavity and that is modulated in response to the detected time-varying intensity of the swept optical signal.
11. A system as claimed in claim 6 , wherein the intracavity element of the swept laser source is controlled to reduce a number of pulses circulating in the laser cavity.
12. A system as claimed in claim 6 , further comprising an amplifier for amplifying the detected intensity of the swept optical signal, and a filter for bandpass filtering the detected intensity of the swept optical signal.
13. A system as claimed in claim 12 , wherein a passband center frequency of the filter is inverse of a roundtrip travel time of light in the laser cavity or a multiple thereof.
14. A system as claimed in claim 6 , further comprising a delay for delaying the detected intensity of the swept optical signal that is used to control the intracavity element of the swept laser source to delay match the detected intensity of the swept optical signal with pulses circulating within the laser cavity of the swept laser source.
15. A system as claimed in claim 6 , further comprising a phase locked loop for controlling the intracavity element and the mode-locked operation of the swept laser source in response to the detected time-varying intensity of the swept optical signal.