# US 12,181,589 B2 · Manipulating the optical phase of a laser beam

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- Number: US 12,181,589 B2
- Document ID: US12181589B2
- Status: Granted
- Kind: B2
- Application number: 17/817,859
- Filed: 2022-08-05
- Published: 2023-04-06
- Granted: 2024-12-31
- Earliest claimed: 2021-10-05
- Assignees: Massachusetts Institute of Technology
- Applicants: Massachusetts Institute of Technology
- Inventors: Andrew Benedick
- Web page: https://patentgrep.com/patent/US12181589B2

## CPC

- G01S 19/44: Carrier phase ambiguity resolution; Floating ambiguity; LAMBDA [Least-squares AMBiguity Decorrelation Adjustment] method
- H01S 3/0092: {Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity (nonlinear frequency conversion per se G02F1/35)}
- G01S 19/04: providing carrier phase data
- G01S 19/46: the supplementary measurement being of a radio-wave signal type
- G02F 1/353: {Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams}
- G02F 1/3532: {Arrangements of plural nonlinear devices for generating multi-colour light beams, e.g. arrangements of SHG, SFG, OPO devices for generating RGB light beams}
- G02F 1/395: {in optical waveguides}
- H01S 3/10053: {Phase control}
- H01S 3/2391: {emitting at different wavelengths}
- G02F 2203/50: Phase-only modulation
- H01S 3/06754: {Fibre amplifiers (H01S3/06708 takes precedence)}
- H01S 2301/03: Suppression of nonlinear conversion, e.g. specific design to suppress for example stimulated brillouin scattering [SBS], mainly in optical fibres in combination with multimode pumping
- G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- 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
- G02F: OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS

## Abstract

Binary-phase-shift-key, phase-modulated waveforms with gigahertz bandwidths, suitable for kilowatt-class fiber amplifiers, can be narrowed back to the source laser's linewidth via second-harmonic, sum-frequency, or difference-frequency generation in a second-order nonlinear crystal. The spectrum of an optical signal phase-modulated with a pseudo-random bit sequence (PRBS) waveform recovers its original optical spectrum when frequency-doubled using second-harmonic generation (SHG). Conceptually, the PRBS waveform is cancelled by the SHG process, and the underlying laser spectrum is converted to the second-harmonic wavelength as though the PRBS modulation were not present. The same cancellation is possible with sum-frequency generation (SFG) and difference frequency generation (DFG), making it possible to construct high-power, narrow-linewidth lasers at wavelengths from the visible to the long-wave infrared. Using ytterbium-, erbium-, thulium-, and neodymium-doped fibers with SHG, SFG and DFG processes allows generation of high-power beams with very narrowband optical spectra and wavelengths from below 400 nm to beyond 5 μm.

## Claims

### Claim 1

A method comprising:
modulating a phase of a seed beam at an input wavelength with a phase modulation alternating between phase shifts of 0 and π;
amplifying the seed beam with a fiber amplifier; and
interacting the seed beam with a second-order nonlinear medium to generate an output beam at an output wavelength different than the input wavelength, the second-order nonlinear medium canceling the phase modulation from the output beam.

### Claim 2 (depends on 1)

The method of claim 1, wherein the phase modulation is a pseudo-random binary sequence.

### Claim 3 (depends on 1)

The method of claim 1, wherein the input wavelength is from 970 nm to 2100 nm and the output wavelength is from 485 nm to 1050 nm.

### Claim 4 (depends on 1)

The method of claim 1, wherein amplifying the seed beam with the fiber amplifier comprises amplifying the seed beam to a power level of at least 300 mW in a single-mode fiber.

### Claim 5 (depends on 1)

The method of claim 1, wherein amplifying the seed beam with the fiber amplifier comprises amplifying the seed beam to a power level of at least 30 W in a few-moded optical fiber.

### Claim 6 (depends on 5)

The method of claim 5, further comprising:
transmitting the seed beam from the fiber amplifier to the second-order nonlinear medium via an optical fiber longer than 1 meter.

### Claim 7 (depends on 1)

The method of claim 1, wherein interacting the seed beam with the second-order nonlinear medium comprises generating the output beam as a second harmonic of the seed beam.

### Claim 8 (depends on 1)

The method of claim 1, wherein the seed beam is a first seed beam, and further comprising:
modulating a phase of a second seed beam with the phase modulation; and
amplifying the second seed beam,
wherein interacting the first seed beam with the second-order nonlinear medium comprises interacting the first seed beam and the second seed beam in the second-order nonlinear medium.

### Claim 9 (depends on 8)

The method of claim 8, wherein interacting the first seed beam and the second seed beam in the second-order nonlinear medium comprises generating the output beam at a carrier frequency equal to a sum of a carrier frequency of the first seed beam and a carrier frequency of the second seed beam.

### Claim 10 (depends on 8)

The method of claim 8, wherein interacting the first seed beam and the second seed beam in the second-order nonlinear medium comprises generating the output beam at a carrier frequency equal to a difference of a carrier frequency of the first seed beam and a carrier frequency of the second seed beam.

### Claim 11 (depends on 8)

The method of claim 8, further comprising:
aligning the phase modulation applied to the first seed beam in time with the phase modulation applied to the second seed beam.

### Claim 12 (depends on 1)

The method of claim 1, further comprising:
modulating the seed beam with a communications signal in addition to the phase modulation.

### Claim 13

A system comprising:
a seed laser to generate a seed beam at an input wavelength;
a phase modulator, in optical communication with the seed laser, to modulate a phase of the seed beam with a phase modulation alternating between phase shifts of 0 and π;
an optical amplifier, in optical communication with the phase modulator, to amplify the seed beam; and
a second-order nonlinear medium, in optical communication with the optical amplifier, to generate an output beam at an output wavelength different than the input wavelength via a second-order nonlinear interaction, the second-order nonlinear interaction canceling the phase modulation.

### Claim 14 (depends on 13)

The system of claim 13, wherein the phase modulation is a pseudo-random binary sequence.

### Claim 15 (depends on 13)

The system of claim 13, wherein the input wavelength is from 970 nm to 2100 nm and the output wavelength is from 485 nm to 1050 nm.

### Claim 16 (depends on 13)

The system of claim 13, wherein the optical amplifier is configured to amplify the seed beam to a power level of at least 1 kW.

### Claim 17 (depends on 13)

The system of claim 13, wherein the second-order nonlinear medium is configured to generate the output beam as a second harmonic of the seed beam.

### Claim 18 (depends on 13)

The system of claim 13, wherein the seed laser is a first seed laser, the seed beam is a first seed beam, the optical amplifier is a first optical amplifier, the phase modulator is a first phase modulator, and the optical amplifier is a first optical amplifier, and further comprising:
a second seed laser to generate a second seed beam;
a second phase modulator, in optical communication with the second seed laser, to modulate a phase of the second seed beam with the phase modulation; and
a second optical amplifier, in optical communication with the second phase modulator and the second-order nonlinear medium, to amplify the second seed beam,
wherein the second-order nonlinear interaction is between the first seed beam and the second seed beam.

### Claim 19 (depends on 13)

The system of claim 13, wherein the second-order nonlinear medium is disposed in an enhancement cavity having a cavity length based on a repeat time of the phase modulation.

### Claim 20 (depends on 13)

The system of claim 13, further comprising:
an amplitude modulator, in optical communication with the seed laser and/or the phase modulator, to modulate the seed beam with a communications signal.