Apparatus and method for producing rapid and accurate laser phase shifts

• US 10,642,075 B1
• Filed: 09/27/2017
• Issued: 05/05/2020
• Est. Priority Date: 09/27/2017
• Status: Active Grant

• Current Assignee: BAE Systems Information and Electronic Systems Integration Incorporated (BAE Systems Plc)
• Original Assignee: BAE Systems Information and Electronic Systems Integration Incorporated (BAE Systems Plc)
• Inventors: Bennett, Aaron W, Creeden, Daniel J, Kaylor, Brant M, Setzler, Scott D
• Primary Examiner(s): Bolda, Eric L
• Application Number:
  US15/717,328
• Time in Patent Office: 951 Days
• CPC Class Codes: G01S 17/10 using transmission of interrupted, pulse-modulated waves determination of distance by phase measurements G01S17/32
• CPC Class Codes: G01S 17/26 wherein the transmitted pulses use a frequency-modulated or phase-modulated carrier wave, e.g. for pulse compression of received signals
• CPC Class Codes: G01S 17/36 with phase comparison between the received signal and the contemporaneously transmitted signal
• CPC Class Codes: G01S 7/484 Transmitters
• CPC Class Codes: G01S 7/4911 Transmitters
• CPC Class Codes: G02F 1/0121 Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
• CPC Class Codes: G02F 2201/58 Arrangements comprising a monitoring photodetector
• CPC Class Codes: G02F 2203/11 involving infrared radiation
• CPC Class Codes: H01S 3/0085 Modulating the output, i.e. the laser beam is modulated outside the laser cavity
• CPC Class Codes: H01S 3/10038 Amplitude control
• CPC Class Codes: H01S 3/10053 Phase control
• CPC Class Codes: H01S 3/10092 Coherent seed, e.g. injection locking
• CPC Class Codes: H01S 3/1304 by using an active reference, e.g. second laser, klystron or other standard frequency source H01S3/139 takes precedence; automatic control of electronic generators H03L7/00
• CPC Class Codes: H01S 3/1305 Feedback control systems
• CPC Class Codes: H01S 3/1307 Stabilisation of the phase

Abstract

A phase controller for rapid, accurate, stable phase shifting of a continuous wave (cw) laser output combines and adjusts reference paths from before and after an EOPM to obtain maximum constructive interference when the EOPM control voltage is zero. A control voltage V for maximum destructive interference is then determined and regulated to produce and maintain a 180 degree phase shift. The output phase can then be shifted by switching the control voltage to the output of a voltage shifter that shifts V by a specified percentage. The phase shifter can divide the control voltage in half to provide a 90 degree phase shift. The cw laser can function as a seed to a pulsed laser, thereby controlling the pulse phases. Quadrature phase laser pulse pairs can be used for quadrature LiDAR detection. Embodiments include a plurality of voltage shifters for 4-phase quadrature shifting and/or shifting between arbitrary phase values.

Claims

1. A phase control apparatus for accurately shifting the output of a continuous wave (cw) laser between two phases, the apparatus comprising:

a first voltage-controlled phase shifter configured to receive an input laser signal from the cw laser and provide a laser seed beam and a reference signal;

a first phase shifter control system, comprising;

an adjustable first control voltage source, configured to generate a first control voltage;

a voltage shifter, configured to shift the first control voltage by a specified percentage; and

a voltage selector configured to select between an output voltage from the first control voltage source and an output voltage of the voltage shifter, and to apply the selected output voltage to control the first phase shifter;

a second voltage-controlled phase shifter, configured to adjust a phase of the reference signal to produce a phase adjusted reference signal;

an adjustable second control voltage source, configured to apply a second control voltage to the second voltage-controlled phase shifter;

a detector, configured to provide a detector output that is responsive to the input laser signal and the phase adjusted reference signal, such that the detector output varies according to constructive and destructive interference between the input laser signal and the phase adjusted reference signal; and

a feedback system configured to adjust the first control voltage source so as to minimize the detector output.

2. The apparatus of claim 1, wherein the first and second phase shifters are electro-optic phase modulators.

3. The apparatus of claim 1, wherein the cw laser is an infra-red laser.

4. The apparatus of claim 1, wherein the detector is a photodiode detector.

5. The apparatus of claim 1, wherein the feedback system includes a servo that adjusts the first control voltage source.

6. The apparatus of claim 1, further comprising a switch configured to enable and disable control of the first control voltage source by the feedback system.

7. The apparatus of claim 1, wherein the feedback system comprises:

a signal generator that imposes a time-variation onto the second control voltage, thereby imposing the time-variation onto the detector output; and

a frequency discriminator configured to discriminate and isolate a component of the detector output upon which the time-variation has been imposed, said frequency discriminator being configured to receive a reference signal from the signal generator.

8. The apparatus of claim 7, wherein the frequency discriminator is a lock in amplifier.

9. The apparatus of claim 1, wherein the voltage shifter is configured to reduce the first control voltage by fifty percent.

10. The apparatus of claim 1, wherein the apparatus comprises a plurality of voltage shifters, and the voltage selector is configured to select between any of the output voltage from the first control voltage source and the output voltages of the voltage shifters.

11. The apparatus of claim 10, wherein the apparatus comprises three voltage shifters and is able to shift the output beam between four quadrature phases.

12. A method of shifting a phase of a continuous wave (cw) laser output, the method comprising:

directing the cw laser output through a phase control apparatus to produce a laser seed beam having a laser seed phase, the phase control apparatus being controlled by a first control voltage;

detecting an interference between the cw laser output and the laser seed beam, and determining therefrom a value V of the first control voltage at which a seed beam phase shift of the laser seed beam is 180 degrees as compared to a phase of the laser seed beam when the first control voltage is zero;

configuring a voltage shifter to provide a shifted output voltage that is a specified fraction of V, said fraction being designated as F;

regulating V to maintain the seed beam phase shift at 180 degrees; and

when a change of the output phase is desired, disabling the regulation and causing a voltage selector to direct the shifted output voltage to the phase control apparatus in place of the first control voltage, thereby changing the seed beam phase shift to a value equal to 180 degrees times F.

13. The method of claim 12, further comprising causing the voltage selector to select the first control voltage and switching the feedback system on, thereby returning the seed beam phase shift to 180 degrees.

14. The method of claim 12, further comprising periodically repeating the step of detecting the interference between the cw laser output and the laser seed beam, and determining therefrom the value V of the first control voltage at which the seed beam phase shift is 180 degrees.

15. A method of obtaining quadrature LiDAR detection, the method comprising:

providing a pulsed laser, a continuous wave (cw) laser, and a phase control apparatus configured to direct an output of the cw laser through a voltage-controlled phase shifter to provide a seed beam to the pulsed laser;

detecting an interference between the output of the cw laser and the seed beam, and determining therefrom a value V of a first control voltage at which a seed beam phase shift of the seed beam is 180 degrees as compared to a phase of the seed beam when the first control voltage is zero;

configuring a voltage shifter to provide a shifted output voltage that is one half of V;

regulating V to maintain the seed beam phase shift at 180 degrees;

causing the pulsed laser to emit a first laser pulse;

detecting a first LiDAR response as a first component of the quadrature LiDAR detection;

causing the voltage selector to direct the shifted output voltage to the phase shifter in place of V, thereby changing the seed beam phase shift to approximately 90 degrees;

causing the pulsed laser to emit a second laser pulse; and

detecting a second LiDAR response as a second component of the quadrature LiDAR detection.

16. The method of claim 15, wherein the pulsed laser is an infrared laser.