# US 10,371,499 B2 · Laser swept source with controlled mode locking for OCT medical imaging

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- Number: US 10,371,499 B2
- Document ID: US10371499B2
- Status: Granted
- Kind: B2
- Application number: 13/976,229
- Filed: 2011-12-27
- Published: 2014-03-27
- Granted: 2019-08-06
- Earliest claimed: 2010-12-27
- Assignees: Axsun Technologies, Inc.
- Applicants: Bartley C. Johnson; Dale C. Flanders
- Inventors: Bartley C. Johnson; Dale C. Flanders
- Web page: https://patentgrep.com/patent/US10371499B2

## CPC

- G01B 9/02091: Tomographic interferometers, e.g. based on optical coherence
- A61B 5/0066: {Optical coherence imaging}
- A61B 5/6852: {Catheters}
- G01B 9/02004: using frequency scans
- G01N 21/4795: {spatially resolved investigating of object in scattering medium (in vivo A61B)}
- H01S 3/1112: Passive mode locking
- H01S 5/06216: {Pulse modulation or generation}
- H01S 5/0657: {Mode locking, i.e. generation of pulses at a frequency corresponding to a roundtrip in the cavity}
- H01S 5/141: {using a wavelength selective device, e.g. a grating or etalon (H01S5/146 takes precedence)}
- H01S 3/06791: {Fibre ring lasers (fibre laser gyrometers G01C19/72)}
- H01S 3/083: Ring lasers {(fibre ring lasers H01S3/06791)}
- H01S 3/094026: {for synchronously pumping, e.g. for mode locking}
- H01S 3/1062: {using a controlled passive interferometer, e.g. a Fabry-Perot etalon}
- H01S 3/1109: Active mode locking
- H01S 3/1118: Semiconductor saturable absorbers, e.g. semiconductor saturable absorber mirrors [SESAMs]; Solid-state saturable absorbers, e.g. carbon nanotube [CNT] based
- H01S 5/02251: using optical fibres
- H01S 5/0265: {Intensity modulators (intra-cavity modulators H01S5/0625)}
- H01S 5/041: {Optical pumping}
- H01S 5/0622: {Controlling the frequency of the radiation}
- H01S 5/146: {using a fiber as external cavity}
- G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D)
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

## Abstract

An optical coherence analysis system uses a laser swept source that is constrained to operate in a mode locked condition. This is accomplished by synchronously changing the laser cavity's gain and/or phase based on the round trip travel time of light in the cavity. Many high-speed wavelength swept laser sources emit pulses synchronized with the round trip time of the cavity as part of a nonlinear optical frequency red shifting process. Stable pulsation is associated with smooth tuning and low relative intensity noise. Addition of mode-locking methods to this class of lasers can control and stabilize these lasers to a low clock jitter and RIN state, and in specific cases allow long-to-short wavelength tuning in addition to the usual short-to-long (red shifting). The laser may comprise a SOA (410), a tunable Fabry-Perot-Filter (412) as one reflector and an Output coupler (405) in an optical fiber (406) to adjust the cavity length.

## Claims

### Claim 1

An optical coherence imaging method, comprising:
providing a laser swept source having a laser cavity that is less than a meter long and defined between a Fabry-Perot tunable filter and an output coupler at one end of a fiber;
controlling a swept mode-locked operation of the laser swept source by actively modulating cavity gain by applying a bias current to a gain element in the laser cavity, synchronized with a round trip time of the laser cavity to produce one laser pulse per round trip, and generating a swept optical signal by tuning the Fabry-Perot tunable filter with a tunable filter sweep rate that is several orders of magnitude smaller than a laser cavity round trip rate and the tunable filter sweep rate is greater than 20 kHz, and the laser cavity round trip rate is greater than 1 GHz;
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.

### Claim 2 (depends on 1)

A method as claimed in claim 1, wherein the laser swept source further comprises an intracavity phase modulator configured to be driven at a rate related to the round trip time of the laser cavity.

### Claim 3 (depends on 1)

The method as claimed in claim 1, wherein the bias current has one of: a square or a sinusoidal waveform.

### Claim 4

An optical coherence imaging method, comprising:
providing a laser swept source having a laser cavity that is less than a meter long and defined between a Fabry-Perot tunable filter and an output coupler at one end of a fiber, the laser swept source further comprising a semiconductor optical amplifier (SOA) chip having a gain section, and an intracavity phase modulator configured to be driven at a rate related to the round trip time of the laser cavity, wherein the phase modulator is integral with the SOA chip and configured to be driven at a harmonic of the round trip frequency of the laser cavity by a bias source different from a bias source for the gain section;
controlling a swept mode-locked operation of the laser swept source by actively modulating cavity gain by applying a bias current to the gain section synchronized with a round trip time of the laser cavity to produce one laser pulse per round trip, and generating a swept optical signal;
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.