# US 12,015,238 B2 · Injection-locked laser system

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- Number: US 12,015,238 B2
- Document ID: US12015238B2
- Status: Granted
- Kind: B2
- Application number: 16/965,854
- Filed: 2019-01-30
- Published: 2021-02-04
- Granted: 2024-06-18
- Earliest claimed: 2018-01-30
- Assignees: M Squared Lasers Limited
- Applicants: M Squared Lasers Limited
- Inventors: Gareth Thomas Maker; Graeme Peter Alexander Malcolm; Stephen Webster
- Web page: https://patentgrep.com/patent/US12015238B2

## CPC

- H01S 3/2308: {Amplifier arrangements, e.g. MOPA}
- H01S 3/2383: {Parallel arrangements}
- H01S 3/0816: {having 4 reflectors, e.g. Z-shaped resonators}
- H01S 3/10053: {Phase control}
- H01S 3/10092: {Coherent seed, e.g. injection locking}
- H01S 3/1305: {Feedback control systems}
- H01S 3/1307: {Stabilisation of the phase}
- H01S 3/137: for stabilising of frequency
- H01S 3/139: by controlling the mutual position or the reflecting properties of the reflectors of the cavity {, e.g. by controlling the cavity length}
- H01S 3/1625: {titanium}
- H01S 3/1636: {Al2O3 (Sapphire)}
- H01S 3/2391: {emitting at different wavelengths}
- 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)}
- H01S 3/1003: {tunable optical elements, e.g. acousto-optic filters, tunable gratings}
- 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 method and system for injection-locking multiple optical amplifiers is disclosed. A master laser is employed to generate a continuous-wave output field. Optical modulators then produce first and second seed optical fields from the continuous-wave output field. The first and second seed optical fields provide an input to injection lock one or more optical amplifiers, optionally at different operating frequencies. Since the first and second seed optical fields are generated from the continuous-wave output field then the output fields of the optical amplifiers exhibit a high phase-coherence with each other and with the continuous-wave output field. Employing the first and second optical fields reduces the requirement to induce large frequency shifts on a single optical field. Techniques for phase-locking the output of the injection-locked laser systems are also provided to further reduce phase noise within the systems.

## Claims

### Claim 1

An injection-locked laser system (1, 25) comprising:
a master laser (2) that generates a continuous-wave output field (4) having a frequency (f0);
one or more optical modulators (5, 5a, 5d) employed to produce from the continuous-wave output field (4) a first seed optical field (6) having a first frequency (f1) and a second seed optical field (7) having a second frequency (f2);
wherein the first seed optical field (6) provides an input to injection lock one or more optical amplifiers (8a-c) at the first frequency (f1), the one or more optical amplifiers (8a-c) producing one or more continuous-wave output fields (12a-c) for the injection-locked laser system (1, 25) at the first frequency (f1)
and the second seed optical field (7) provides an input to injection lock one or more optical amplifiers (8d-f) at the second frequency (f2), the one or more optical amplifiers (8d-f) producing one or more continuous-wave output fields (12d-f) for the injection-locked laser system (1, 25) at the second frequency (f2);
the injection-locked laser system (1, 25) further comprising a first phase lock control loop (27a) that provides a first feedback signal (29a) to one of the one or more optical modulators (5, 5a) to phase lock the continuous-wave output field (12a), produced at the first frequency (f1), of a first optical amplifier (8a) to the continuous-wave output field (4) and;
the injection-locked laser system (1, 25) further comprising a second phase lock control loop (27b) that provides a second feedback signal (29b) to one of the one or more optical modulators (5, 5d) to phase lock the continuous-wave output field (12d), produced at the second frequency (f2), of a second optical amplifier (8d) to the continuous-wave output field (4).

### Claim 2 (depends on 1)

An injection-locked laser system (25) as claimed in claim 1 wherein the injection-locked laser system (25) comprises N optical modulators employed to produce from the continuous-wave output field (4) N seed optical fields having a frequency (fN).

### Claim 3 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the first frequency (f1) equals the frequency (f0) of the continuous-wave output field (4) plus a modulation frequency (Δf1) introduced by the one or more optical modulators (5, 5a).

### Claim 4 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the second frequency (f2) equals the frequency (f0) of the continuous-wave output field (4) minus the modulation frequency (Δf2) introduced by the one or more optical modulators (5, 5d).

### Claim 5 (depends on 3)

An injection-locked laser system (1, 25) as claimed in claim 3 wherein the modulation frequency (Δf) introduced by the one or more optical modulators (5, 5a, 5d) is between 0.1 and 40 GHz.

### Claim 6 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the one or more optical modulators (5, 5a, 5d) comprises an electro-optic modulator (EOM) or an acoustic-optic modulator (AOM).

### Claim 7 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the first seed optical field (6) is mode-matched with the one or more optical amplifiers (8a, 8b, 8c) injection locked by the first seed optical field (6).

### Claim 8 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the second seed optical field (7) is mode-matched with the one or more optical amplifiers (8d, 8e,8f), injection locked by the second seed optical field (7).

### Claim 9 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the injection-locked laser system (1, 25) further comprises one or more frequency lock control loops (13, 19) to frequency lock one or more of the optical amplifiers (8a-c) to the first seed field (6).

### Claim 10 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the injection-locked laser system (1, 25) further comprises one or more frequency lock control loops (13, 19) to frequency lock one or more of the optical amplifiers (8d-f) to the second seed field (7).

### Claim 11 (depends on 1)

An injection-locked laser (1, 25) system as claimed in claim 1 wherein the first phase lock control loop (27a) comprises a first detector (34a) arranged to generate a first electrical beat signal (33a) from a first optical beat signal (32a) generated from the combined output field (12a) of the first optical amplifier (8a) and the continuous-wave output field (4).

### Claim 12 (depends on 11)

An injection-locked laser (1, 25) system as claimed in claim 11 wherein the first phase lock control loop (27a) further comprises a reference oscillator (35) employed to generate a first reference signal (36a) having a frequency equal to a frequency of the first electrical beat signal (33a).

### Claim 13 (depends on 12)

An injection-locked laser (1, 25) system as claimed in claim 12 wherein the first phase lock control loop (27a) further comprises a first frequency mixer (37a) employed to generate a first error signal (38a) from the first electrical beat signal (33a) and the first reference signal (36a).

### Claim 14 (depends on 13)

An injection-locked laser (1, 25) system as claimed in claim 13 wherein the first phase lock control loop (27a) further comprises a first voltage-controlled oscillator (41a) a control signal (40a) for which is generated from the first error signal (38a).

### Claim 15 (depends on 14)

An injection-locked laser system (1, 25) as claimed in claim 14 wherein an output signal (42a) from the first voltage-controlled oscillator (41a) provides the feedback signal (29a) for the one of the one or more optical modulators (55a).

### Claim 16 (depends on 14)

An injection-locked laser system (1, 25) as claimed in claim 14 wherein the first phase lock control loop (27a) further comprises a first frequency synthesiser (43a) located between the first voltage-controlled oscillator (41a) and the one of the one or more optical modulators (5, 5a).

### Claim 17 (depends on 14)

An injection-locked laser system (1, 25) as claimed in claim 14 wherein the first phase lock control loop (27a) further comprises a first voltage-controlled phase shifter (46a) a control signal (40a) for which is generated from the first error signal (38a).

### Claim 18 (depends on 17)

An injection-locked laser system (1, 25) as claimed in claim 17 wherein an input signal (47a) for the first voltage-controlled phase shifter (46a) is generated by a first fixed frequency oscillator (48a).

### Claim 19 (depends on 17)

An injection-locked laser system (1, 25) as claimed in claim 17 wherein the first phase lock control loop (27a) further comprises a first frequency synthesiser (43a) located between the first voltage-controlled phase shifter (46a) and the one of the one or more optical modulators (5, 5a).

### Claim 20 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the first phase lock control loop (27a) further comprises a first reference oscillator (35) employed to generate a first reference signal (36a) a having a first frequency fA.

### Claim 21 (depends on 20)

An injection-locked laser system (1, 25) as claimed in claim 20 wherein the first phase lock control loop (27a) further comprises a first Direct Digital Synthesiser (DDS) (52) employed to generate a second reference signal (55) a having a second frequency fB.

### Claim 22 (depends on 21)

An injection-locked laser system (1, 25) as claimed in claim 21 wherein the first frequency fA equals the frequency of the first electrical beat signal (33a) minus the second frequency fB.

### Claim 23 (depends on 20)

An injection-locked laser system (1, 25) as claimed in claim 20 wherein the first phase lock control loop (27a) further comprises a first frequency mixer (37a) employed to generate a first intermediate frequency signal (53) from the first electrical beat signal (33a) and the first reference signal fA.

### Claim 24 (depends on 23)

An injection-locked laser system (1, 25) as claimed in claim 23 wherein the first phase lock control loop (27a) further comprises a second frequency mixer (37b) employed to generate an error signal (38) from the first intermediate frequency signal (53) and the second reference signal fB.

### Claim 25 (depends on 24)

An injection-locked laser system (1, 25) as claimed in claim 24 wherein the first phase lock control loop (27a) further comprises a first voltage-controlled oscillator (41) a control signal (40) for which is generated from the first error signal (38).

### Claim 26 (depends on 21)

An injection-locked laser system (1, 25) as claimed in claim 21 wherein the first Direct Digital Synthesiser (DDS) (52) is further employed to generate an output signal (55).

### Claim 27 (depends on 26)

An injection-locked laser system (1, 25) as claimed in claim 26 wherein the first phase lock control loop (27a) further comprises a third frequency mixer (37c) employed to generate the feedback signal (29) to one of the one or more optical modulators (5, 5a) from an output signal (42) from the first voltage-controlled oscillator (41) and the output signal (55) of the first DDS (52).

### Claim 28 (depends on 1)

An injection-locked laser system (1, 25) as claimed in claim 1 wherein the injection-locked laser system (1, 25) further comprise one or more frequency doubling stages arranged to frequency double one or more of the output fields the one or more optical amplifiers (8) and or a component of the continuous-wave output field (4).

### Claim 29 (depends on 28)

An injection-locked laser system (1, 25) as claimed in claim 28 wherein the first optical beat signals (33a) is generated after the one or more frequency doubling stages.

### Claim 30

A method for injection-locking two or more optical amplifiers (8a-f) the method comprising:
generating a continuous-wave output field (4) having a frequency (f0);
employing one or more optical modulators (5, 5a, 5d) to optically modulating the continuous-wave output field (4) to produce a first seed optical field (6) having a first frequency (f1) and a second seed optical field (7) having a second frequency (f2);
employing the first seed optical field (6) to injection lock one or more optical amplifiers (8a-c) at the first frequency (f1)
employing the one or more optical amplifiers (8a-c) to produce one or more continuous-wave output fields (12a-c) at the first frequency (f1);
employing the second seed optical field (7) to injection lock one or more optical amplifiers (8d-f) at the second frequency (f2);
employing the one or more optical amplifiers (8d-f) to produce one or more continuous-wave output fields (12d-f) at the second frequency (f2);
employing a first phase lock control loop (27a) to provide a first feedback signal (29a) to one of the one or more optical modulators (5, 5a) to phase lock the continuous-wave output field (12a), produced at the first frequency (f1), of a first optical amplifier (8a) to the continuous-wave output field (4); and
employing a second phase lock control loop (27b) to provide a second feedback signal (29b) to one of the one or more optical modulators (5, 5d) to phase lock the continuous-wave output field (12d), produced at the second frequency (f2), of a second optical amplifier (8d) to the continuous-wave output field (4).

### Claim 31 (depends on 30)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 30 wherein the method further comprises mode-matching the first seed optical field (6) with the one or more optical amplifiers (8a-c) injection locked by the first seed field (6).

### Claim 32 (depends on 30)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 30 wherein the method further comprises mode-matching the second seed optical field (7) with the one or more optical amplifiers (8d-e) injection locked by the second seed field (7).

### Claim 33 (depends on 30)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 30 wherein the method further comprises frequency locking one or more of the optical amplifiers (8-c) to the first seed field (6).

### Claim 34 (depends on 30)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 30 wherein the method further comprises frequency locking one or more of the optical amplifiers (8d-f) to the second seed field (7).

### Claim 35 (depends on 30)

A method for injection-locking two or more optical amplifiers as claimed in claim 30 wherein generating the first feedback signal (29a) comprises generating a first optical beat signal (32a) from the combined output field (12a) of one of the optical amplifiers (8a) and the continuous-wave output field (4).

### Claim 36 (depends on 35)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 35 wherein generating the first feedback signal (29a) further comprises generating a first electrical beat signal (33a) from the first optical beat signal (32a).

### Claim 37 (depends on 36)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 36 wherein generating the first feedback signal (29a) further comprises generating a first reference signal (36a) having a frequency equal to a frequency of the first electrical beat signal (33a).

### Claim 38 (depends on 37)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 37 wherein generation of the first feedback signal (29a) further comprises generating a first error signal (38a) from the first electrical beat signal (33a) and the first reference signal (36a).

### Claim 39 (depends on 38)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 38 wherein generation of the first feedback signal (29a) further comprises employing the first error signal (38a) to generate a control signal (40a) for a first voltage-controlled oscillator (41a).

### Claim 40 (depends on 39)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 39 wherein an output signal (42a) from the first voltage-controlled oscillator (41a) provides the first feedback signal (29a) for one of the one or more optical modulators (5, 5a).

### Claim 41 (depends on 40)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 40 wherein the frequency of the first feedback signal (29a) is multiplied before arriving at the one of the one or more optical modulators (5, 5a).

### Claim 42 (depends on 38)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 38 wherein generation of the first feedback signal (29a) further comprises employing the first error signal (38a) to generate a control signal (40a) for a first voltage-controlled phase shifter (46a).

### Claim 43 (depends on 42)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 42 wherein a first fixed oscillator (48a) provides an input signal (47a) for the first voltage-controlled phase shifter (46a).

### Claim 44 (depends on 43)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 43 wherein an output signal (49a) from the first voltage-controlled phase shifter (46a) provides the feedback signal (29a) for the one of the one or more optical modulators (5, 5a).

### Claim 45 (depends on 44)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 44 wherein the frequency of the first feedback signal (29a) is multiplied before arriving at the one of the one or more optical modulator (5, 5a).

### Claim 46 (depends on 43)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 43 wherein the frequency of the input signal (47a) is multiplied before arriving at the first voltage-controlled phase shifter (46a).

### Claim 47 (depends on 38)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 38 wherein generating the first feedback signal (29a) further comprises generating a first reference signal (36a) having a frequency fA.

### Claim 48 (depends on 47)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 47 wherein generating the first feedback signal (29a) further comprises generating a second reference signal (55) having a frequency fB.

### Claim 49 (depends on 48)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 48 wherein the first frequency fA equals the frequency of the first electrical beat (33a) signal minus the second frequency fB.

### Claim 50 (depends on 48)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 48 wherein generating the first feedback signal (29a) further comprises generating a first intermediate frequency signal (53) from the first electrical beat signal (33a) and the first reference signal fA.

### Claim 51 (depends on 50)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 50 wherein generating the first feedback signal (29a) further comprises generating a first error signal (38) from the first intermediate frequency signal (53) and the second reference signal fB.

### Claim 52 (depends on 51)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 51 wherein generating the first feedback signal (29a) further comprises generating a control signal (40) for a first voltage-controlled oscillator (41) from the first error signal (38).

### Claim 53 (depends on 52)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 52 wherein generating the first feedback signal (29a) further comprises frequency mixing an output signal (42) from the first voltage-controlled oscillator (41) and an output signal (55) from a first Direct Digital Synthesiser (DDS) (52).

### Claim 54 (depends on 30)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 30 wherein the method further comprises frequency doubling one or more of the output fields (12a-f) the one or more optical amplifiers (8a-f) and or a component of the continuous-wave output field (4).

### Claim 55 (depends on 54)

A method for injection-locking two or more optical amplifiers (8a-f) as claimed in claim 54 wherein the method further comprises generating the first (32a) optical beat signals after frequency doubling the one or more of the output fields (12a-f) of the one or more optical amplifiers (8a-f) and or a component of the continuous-wave output field (4).