# US 10,502,632 B2 · High precision wavelength measurement and control of a tunable laser

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- Number: US 10,502,632 B2
- Document ID: US10502632B2
- Status: Granted
- Kind: B2
- Application number: 15/677,146
- Filed: 2017-08-15
- Published: 2018-01-11
- Granted: 2019-12-10
- Earliest claimed: 2008-02-21
- Assignees: Intuitive Surgical Operations, Inc.
- Applicants: Intuitive Surgical Operations, Inc.
- Inventors: Ryan Seeley; Mark E. Froggatt
- Web page: https://patentgrep.com/patent/US10502632B2

## CPC

- G01J 9/0246: {Measuring optical wavelength}
- H01S 3/1303: {by using a passive reference, e.g. absorption cell (H01S3/139 takes precedence)}
- H01S 3/1305: {Feedback control systems}
- H01S 3/1307: {Stabilisation of the phase}
- H01S 3/1392: {by using a passive reference, e.g. absorption cell (H01S3/1396, H01S3/1398 take precedence)}
- H01S 5/0617: {using memorised or pre-programmed laser characteristics}
- H01S 5/0687: Stabilising the frequency of the laser
- H01S 5/143: {Littman-Metcalf configuration, e.g. laser - grating - mirror}
- H01S 5/0064: {Anti-reflection components, e.g. optical isolators}
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- 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

A tunable laser system includes a tunable laser to be scanned over a range of frequencies and an interferometer having a plurality of interferometer outputs. At least two interferometer outputs of the plurality of interferometer outputs have a phase difference. A wavelength reference has a spectral feature within the range of frequencies, and the spectral feature does not change in an expected operating environment of the tunable laser. Processing circuitry uses the spectral feature and the plurality of interferometer outputs to produce an absolute measurement of a wavelength of the tunable laser and controls the tunable laser based on a comparison of the absolute measurement of the wavelength of the tunable laser with a setpoint wavelength.

## Claims

### Claim 1

A tunable laser control system for controlling a laser output of a tunable laser, the tunable laser arranged to be scanned over a range of frequencies, the tunable laser control system comprising:
an interferometer having a plurality of interferometer outputs, wherein at least two interferometer outputs of the plurality of interferometer outputs have a phase difference not equal to 0° and not equal to 180°;
a wavelength reference having a spectral feature within the range of frequencies, the spectral feature not changing in an expected operating environment of the tunable laser; and
processing circuitry to:
use the spectral feature and the plurality of interferometer outputs to produce an absolute measurement of a wavelength of the tunable laser;
compare the absolute measurement of the wavelength of the tunable laser with a setpoint wavelength to generate an error signal; and
control the tunable laser using a feedback signal based on the error signal,
wherein the processing circuitry comprises an analog feedback loop including first and second analog multipliers coupled to an analog differencer the analog feedback loop arranged to generate the error signal in analog form and control the wavelength of the tunable laser.

### Claim 2 (depends on 1)

The tunable laser control system of claim 1, further comprising:
circuitry arranged to generate set points for interferometer outputs of the plurality of interferometer outputs, wherein the first and second analog multipliers are arranged to multiply the interferometer outputs with a corresponding one of the set points, and wherein the analog differencer is arranged to generate the error signal in analog form based on a difference between outputs from the first and second analog multipliers.

### Claim 3 (depends on 1)

The tunable laser control system of claim 1, wherein the setpoint wavelength is determined based on a set point function that varies periodically with time, and wherein the processing circuitry further comprises:
circuitry arranged to use the error signal to control the set point function so that a periodic setpoint wavelength signal is generated.

### Claim 4 (depends on 1)

The tunable laser control system of claim 1, wherein the tunable laser control system is a non-causal control system.

### Claim 5 (depends on 1)

The tunable laser control system of claim 1, further comprising the tunable laser arranged to be scanned over a range of frequencies.

### Claim 6 (depends on 1)

The tunable laser control system of claim 1, wherein
the absolute measurement of the wavelength of the tunable laser has an accuracy better than an order of picometers, a precision better than an order of femtometers, and a drift better than an order of picometers.

### Claim 7 (depends on 1)

The tunable laser control system of claim 1, wherein the wavelength reference comprises a gas cell, and the spectral feature comprises absorption line data of the gas cell.

### Claim 8 (depends on 7)

The tunable laser control system of claim 7, wherein the processing circuitry is configured to use the spectral feature and the plurality of interferometer outputs to produce an absolute measurement of a wavelength of the tunable laser by:
the processing circuitry being configured to convert the plurality of interferometer outputs into a phase signal representative of a relative measurement of the wavelength of the tunable laser; and
the processing circuitry comprising reference calibration circuitry arranged to correct for accumulated scaling and tracking errors in the phase signal using the absorption line data.

### Claim 9 (depends on 8)

The tunable laser control system of claim 8, wherein the reference calibration circuitry is arranged to identify one or more gas cell lines in the absorption line data.

### Claim 10 (depends on 8)

The tunable laser control system of claim 8, wherein the reference calibration circuitry is arranged to use a difference between neighboring gas cell line values in the absorption line data to identify the absolute measurement of the wavelength of the tunable laser.

### Claim 11

A tunable laser control system for controlling a laser output of a tunable laser, the tunable laser arranged to be scanned over a range of frequencies, the tunable laser control system comprising:
an interferometer having a plurality of interferometer outputs, wherein at least two interferometer outputs of the plurality of interferometer outputs have a phase difference not equal to 0° and not equal to 180°;
a wavelength reference having a spectral feature within the range of frequencies, the spectral feature not changing in an expected operating environment of the tunable laser; and
processing circuitry to:
use the spectral feature and the plurality of interferometer outputs to produce an absolute measurement of a wavelength of the tunable laser;
compare the absolute measurement of the wavelength of the tunable laser with a setpoint wavelength to generate an error signal; and
control the tunable laser using a feedback signal based on the error signal,
wherein the processing circuitry comprises:
a digital feedback loop arranged to digitally determine and generate the feedback signal in digital form, and
an analog feedback loop arranged to generate the feedback signal in analog form and control the wavelength of the tunable laser.

### Claim 12 (depends on 11)

The tunable laser control system of claim 11, wherein the setpoint wavelength is determined based on a set point function that varies periodically with time, and wherein the processing circuitry comprises:
circuitry arranged to use the error signal to control the set point function so that a periodic setpoint wavelength signal is generated.

### Claim 13 (depends on 11)

The tunable laser control system of claim 11, wherein the tunable laser control system is a non-causal control system.

### Claim 14 (depends on 11)

The tunable laser control system of claim 11, wherein the absolute measurement of the wavelength of the tunable laser has an accuracy better than an order of picometers, a precision better than an order of femtometers, and a drift better than an order of picometers.

### Claim 15 (depends on 11)

The tunable laser control system of claim 11, wherein the wavelength reference comprises a gas cell, and the spectral feature comprises absorption line data of the gas cell.

### Claim 16 (depends on 15)

The tunable laser control system of claim 15, wherein the processing circuitry is configured to use the spectral feature and the plurality of interferometer outputs to produce an absolute measurement of a wavelength of the tunable laser by:
the processing circuitry being configured to convert the plurality of interferometer outputs
into a phase signal representative of a relative measurement of the wavelength of the tunable laser; and
the processing circuitry comprising reference calibration circuitry arranged to correct for accumulated scaling and tracking errors in the phase signal using the absorption line data.

### Claim 17

A method for controlling a laser output, comprising:
scanning a tunable laser over a range of frequencies;
detecting a laser output of the tunable laser using an interferometer to generate at least two outputs with relative phases substantially other than 0° and substantially other than 180°;
providing the laser output to a wavelength reference, wherein the wavelength reference generates a spectral feature within the range of frequencies of the tunable laser, and wherein the spectral feature does not change in an expected environment of the tunable laser;
using the spectral feature of the wavelength reference and the at least two outputs to produce an absolute measurement of a wavelength of the tunable laser;
comparing the absolute measurement of the wavelength of the tunable laser with a setpoint wavelength and generating an error signal based on the comparing; and
controlling the wavelength of the tunable laser using a feedback signal based on the error signal,
wherein a digital feedback loop is used to digitally determine and generate the error signal in digital form, and wherein an analog feedback loop is used to generate the error signal in analog form and to control the wavelength of the tunable laser.

### Claim 18 (depends on 17)

The method in claim 17, wherein:
the error signal provides directional information about a direction of the scanning of the tunable laser over the range of frequencies and provides absolute wavelength error information, and
the absolute wavelength error information has an accuracy better than an order of picometers, a precision better than an order of femtometers, and a drift better than an order of picometers.

### Claim 19 (depends on 17)

The method in claim 17, wherein the tunable laser includes one or more laser actuators, wherein the digital feedback loop controls a lower-frequency regime of the one or more laser actuators, and wherein the analog feedback loop controls a higher-frequency regime of the one or more laser actuators.

### Claim 20 (depends on 17)

The method in claim 17, wherein the setpoint wavelength is determined based on a set point function that varies periodically with time, the method further comprising:
using the error signal to control the set point function so that a periodic wavelength control signal is generated for achieving a desired periodic wavelength.