Optical phased array system with closed-loop compensation of atmospheric turbulence and noise effects

Abstract

A system for illuminating a target in the presence of atmospheric turbulence includes a coherent beam combining (CBC) subsystem, an atmospheric wavefront sensor (AWS) subsystem, a multichannel photodetector (MCPD) subsystem, an auxiliary light source, and a receiving optical aperture. The CBC subsystem forms multiple CBC subbeams and a reference beam, all of which are in a wavelength range different from the wavelength range of the auxiliary light source. The auxiliary light source can be a laser or a portion of the solar spectrum. The AWS controller provides closed-loop compensation of atmospheric turbulence for the low-power auxiliary reflected and reference beams. The MCPD controller provides closed-loop compensation of high-frequency laser noise and transfers wavefront corrections from the reference beam to the high-power CBC laser. The frequency of the MCPD controller is at least one order of magnitude greater than the bandwidth of the AWS controller.

Description

[Technical Field]

[0001] The present invention relates to optical phased array (OPA) systems, and in particular to closed-loop compensation of the effects of atmospheric turbulence and noise in such systems. [Background technology]

[0002] Optical phased array (OPA) systems designed to illuminate targets with high-power free-space lasers must overcome limitations posed by wavefront distortions caused by propagation through a turbulent atmosphere.

[0003] Atmospheric turbulence mitigation has traditionally followed one of two approaches. The first involves using a laser, a deformable mirror (DM), and a wavefront sensor (WFS). The WFS measures the residual aberrations in the light reflected from the target and sends a closed-loop control signal to the DM. This approach is difficult to implement in practice due to nonlinear effects that limit the output power of solid-state lasers and the difficulty of fabricating a DM that can sustain the thermal load of a high-power laser while operating at a high mechanical bandwidth.

[0004] A second approach to overcoming the limitations of laser power scalability is to illuminate the target with multiple partially coherent laser subbeams formed by a coherent beam combining (CBC) subsystem. The optical phases of the subbeams are adjusted to add coherently at the target's surface. However, the effects of laser noise and atmospheric turbulence perturb the relative phases between the subbeams and must be constantly compensated for to maintain phase coherence on the target.

[0005] U.S. Patent No. 7,343,098 to D.R. Gerwe et al., entitled "Fiber Optic Phased Array and Associated Method for Accommodating Atmospheric Perturbations with Phase and Amplitude Control," issued March 11, 2008, provides an optical fiber phased array and control method for controllably adjusting the phase and amplitude of optical signals emitted by multiple optical fiber amplifiers to compensate for atmospheric perturbations. The disclosed configuration uses open-loop control that is adversely affected by errors in calibration and operation.

[0006] U.S. Patent Application Publication No. US2021 / 0294109 to D. Golubchik et al., entitled "Coherent Beam Combination (CBC) Systems and Methods," published September 23, 2021, discloses a CBC system that includes an array of beam sources that generate coherent beams directed toward a target and an interferometry technique called Target-in-the-Loop Interferometry (TILI). However, TILI approaches are typically limited in achievable target range and susceptible to variations in target reflectivity. Summary of the Invention

[0007] The present invention provides closed-loop compensation of both atmospheric turbulence and high-frequency laser noise, enabling a high-power OPA system to illuminate long-range targets with multiple CBC sub-beams. The OPA system includes an electro-optic modulator for wavefront compensation and a practical deformable mirror that does not need to be configured to be driven by a high-power laser.

[0008] According to one aspect of the presently disclosed subject matter, an optical phased array (OPA) system for illuminating a target in the presence of atmospheric turbulence is provided, the OPA system including a coherent beam combining (CBC) subsystem configured to form multiple at least partially coherent CBC sub-beams and a reference beam. The CBC sub-beams and the reference beam have optical wavelengths in a first band. The OPA system also includes a beam director for directing the CBC sub-beams to illuminate the target, an auxiliary light source for illuminating the target with light having optical wavelengths in a second band, a receive optical aperture configured to receive a portion of light reflected from the target at the first and second bands of optical wavelengths, an atmospheric wave-front sensor (AWS) subsystem optically coupled to the receive aperture and the reference beam, and a multi-channel photodetector (MCPD) subsystem optically coupled to the CBC sub-beams and the reference beam. The AWS subsystem includes a closed-loop AWS controller for providing atmospheric turbulence compensation, and the MCPD subsystem includes a closed-loop MCPD controller for compensating for the effects of phase noise in the CBC sub-beams and for transferring atmospheric turbulence compensation from the reference beam to the CBC beam.

[0009] According to some embodiments, the frequency of the MCPD controller is at least an order of magnitude greater than the bandwidth of the AWS controller.

[0010] According to some aspects, the OPA system further comprises a CBC sub-beam phase compensator in communication with the MCPD controller.

[0011] According to some embodiments, the MCPD subsystem comprises an array of photodetectors having at least one pixel for each of the CBC sub-beams.

[0012] According to some embodiments, the bandwidth of the AWS controller is 10 kilohertz or less.

[0013] According to some aspects, the reference beam is collimated.

[0014] According to some embodiments, the reference beam overlaps with all of the CBC sub-beams at the entrance plane of the MCPD subsystem.

[0015] According to some aspects, the reference beam undergoes a phase modulation that is different from the phase modulation of the CBC sub-beams.

[0016] According to some embodiments, the AWS subsystem comprises a deformable mirror (DM) and a DM actuator.

[0017] According to some embodiments, the surface of the DM is divided or continuous.

[0018] According to some aspects, the AWS subsystem comprises a Shack-Hartmann sensor, a pyramidal detector, or a phase diversity sensor.

[0019] According to some aspects, the AWS subsystem comprises an optical filter that attenuates light passing through the receive optical aperture whose wavelengths are outside the second band of optical wavelengths.

[0020] According to some embodiments, the OPA system comprises a dichroic beam splitter.

[0021] According to some aspects, the auxiliary light source is a laser.

[0022] According to some embodiments, the supplemental light source is the solar spectrum of the sun. [Brief explanation of the drawings]

[0023] The present invention is herein described, by way of example only, with reference to the accompanying drawings.

[0024] [Figure 1] FIG. 1 is a schematic diagram of an exemplary OPA system according to the present invention. [Figure 2]FIG. 12 is a cross-sectional view of the overlap between the OPA reference beam and each of the CBS sub-beams at the entrance plane of the multi-channel photodetector (MCPD). DETAILED DESCRIPTION OF THE INVENTION

[0025] FIG. 1 shows a schematic diagram of an exemplary OPA system (100) according to the principles of the present invention. The CBC subsystem (110) generates multiple CBC sub-beams (115), shown as dashed lines, and an OPA reference beam (125), shown as a dashed-dotted line. These beams are typically generated by a fiber seed laser and master oscillator power amplifier (MOPA) configuration, which are at least partially coherent and split into multiple CBC sub-beams. The sub-beams and the OPA reference beam are generated by the same seed laser and therefore have a common center wavelength, denoted λ1, although some relative phase noise is generally contributed by thermal fluctuations and acoustic vibrations. By way of example, λ1 may be equal to 1070 nanometers (nm), and the relative phase noise may have a bandwidth of up to 10 kilohertz (kHz).

[0026] The auxiliary beam (185) is generated by the illumination source (180), which may be a narrowband laser source having a central wavelength denoted by λ2. In an alternative embodiment, the illumination source (180) may be a broadband source, such as the solar spectrum of the sun. In the latter case, the system (100) generally incorporates a bandpass filter that selects a portion of the solar spectrum whose central wavelength is again denoted by λ2.

[0027] Table 1 below describes the line symbols used in FIG. 1 for each of the three beam types: CBC sub-beams, OPA reference beams, and auxiliary beams.

[0028] [Table 1]

[0029] The CBC sub-beam phase compensator (130) adjusts the relative optical phases of the CBC sub-beams (115) using a feedback signal provided by a multi-channel photodetector (MCPD) subsystem (170). In some embodiments, the optical phase modulator (120) applies phase modulation to the reference beam (125), simplifying subsequent extraction of relative phase data in the MCPD subsystem (170) by avoiding the need to compensate for amplitude variations.

[0030] An adaptive optics (AO) head (140) converts the CBC sub-beams (115) and the OPA reference beam (125) into free-space optical beams, each of which is collimated by a collimator optic (142), and the reference beam (125) is collimated by a large aperture collimator optic (145).

[0031] The auxiliary beam (185) illuminates the target (210), and a portion of the beam (185) is reflected back toward the aperture (165). Light from the receiver optical aperture (165) passes through a dichroic beam splitter (190) and enters the Turbulence Correction System (TCS) subsystem (150). As described below, the latter processes the light to determine and correct phase distortions caused by atmospheric turbulence.

[0032] Light received from the aperture (165) is reflected by a deformable mirror (DM) (156) whose surface is controlled by a DM actuator (158). The DM surface may be continuous or have discrete mirror segments. The DM-reflected light at wavelength λ2 passes through a beam splitter (152) to a wavefront subsystem (WFS) (154). The WFS (154) may be implemented, for example, by a Shack-Hartmann sensor, a pyramidal detector, or a phase diversity sensor. In this exemplary embodiment, the beam splitter (152) is a dichroic beam splitter that transmits wavelength λ2 and reflects wavelength λ1. Typically, an optical filter (not shown) is placed along the optical path to the WFS (154) to attenuate light whose wavelengths are outside a band centered around wavelength λ2.

[0033] The WFS determines optical distortion due to atmospheric effects and sends a correction signal (155a) to the AWS controller (155), which controls the DM actuator (158). The AWS controller (155) has a closed-loop bandwidth, for example, of 10 kilohertz (kHz) or less, which is low enough to be within the frequency response of the DM actuator (158), but high enough to follow phase variations produced by atmospheric turbulence, which have characteristic time constants on the order of milliseconds.

[0034] The large aperture collimator optics (145) directs the reference beam (125) toward the beam splitter (152) and reflects a portion of the reference beam toward the DM (156). The DM impresses the phase compensation necessary to correct for optical fluctuations on the reference beam and then reflects the light toward the dichroic beam splitter (190). The latter reflects the light toward the MCPD subsystem (170). Note that the DM-reflected reference beam entering the MCPD subsystem includes a phase shift induced by the DM's surface, including the atmospheric fluctuation correction measured by the WFS (154). The optical power of the reference beam is relatively low, on the order of a few watts or less, so the DM does not need to handle high laser intensities. The DM may be implemented by a microelectromechanical systems (MEMS) device, such as the model "Multi-3.5-DM" available from Boston Micromachines Corp.

[0035] The collimated sub-beams (115) are partially reflected and partially transmitted by the dichroic beam splitter (190). The surface (190a) of the dichroic beam splitter (190) typically has a reflectivity of at least 99.9% for a band of optical wavelengths centered at wavelength λ1. Most of the optical energy in the CBC sub-beams (115) is reflected by the surface (190a) of the dichroic beam splitter (190). This energy is directed by the beam director (160) to illuminate the target (210). A small portion of the optical energy in the CBC sub-beams, typically less than 0.1%, is transmitted directly through the surface (190a) to the MCPD subsystem (170). This portion of the CBC sub-beam that does not reach the target (210) carries information about the relative phase shift between the sub-beams, which may be caused, for example, by high-frequency laser noise fluctuations in the CBC subsystem (110). The magnitude of the wavelength difference, |λ2-λ1|, is designed to be large enough to allow efficient splitting by the dichroic surface (190a).

[0036] 2 shows a cross-sectional view of the overlap between the OPA reference beam and each of the CBS sub-beams as they enter the MCPD subsystem (170). The boundary (125c) of the reference beam cross section is large enough to encompass all of the sub-beam boundaries (115c). In this way, the electromagnetic field of each of the CBS sub-beams (115) interferes with the electromagnetic field of the OPA reference beam (125) at the entrance face of the MCPD subsystem.

[0037] All beams in Figure 2 have wavelengths in a band centered at λ1. In one exemplary embodiment, the dichroic beam splitter (190) is configured to have a reflection-to-transmission ratio similar in value to the ratio of intensities between the main beam and the reference beam. For example, the main beam has 1000 times more power than the reference beam, and the dichroic beam splitter (190) has a transmission of 0.1%. In this way, the transmitted portion of the reference beam is similar in intensity to the reflected portion of the reference beam.

[0038] Returning to Figure 1, the light entering the MCPD subsystem (170) includes the DM reflected reference beam (125) and a portion of the CBC sub-beam (115). Optical interference between the beams allows the high-speed MCPD (174) to determine the optical phase of the CBC sub-beam relative to the reference beam. Phase fluctuations caused by laser noise have frequencies in the range of, for example, 100 Hz to 10,000 Hz. The high-speed MCPD (174) can be implemented using a high-speed CMOS camera or photodiode array with at least one pixel for each of the CBC sub-beams.

[0039] The MCPD pixel signals are used to calculate the phase difference between the individual sub-beams and the reference beam. To remove ambiguity in the intensity-to-phase conversion, the phase of the reference beam can be modulated, resulting in a modulation of the light intensity at each pixel of the high-speed MCPD (174). With a sufficiently high dynamic range detector, it is possible to measure the interference signal even when the intensity ratio between the main beam and the reference beam is as large as 10,000.

[0040] The MCPD output signal (175a) is sent to the MCPD controller (175), which operates in a closed loop with the CBC sub-beam phase compensator (130). Note that the output from the MCPD controller (175) is simultaneously used to compensate for high frequency laser noise and to copy atmospheric wavefront corrections from the reference beam to the CBC beam.

[0041] The frequency of the MCPD controller (175) is typically at least an order of magnitude greater than the bandwidth of the AWS controller (155) and at least an order of magnitude greater than the laser phase noise, thereby avoiding undesirable resonance or control instability between the two controllers.

[0042] The description of various embodiments of the present disclosure is presented for illustrative purposes, but is not intended to be exhaustive or limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terms used herein are selected to best explain the principles of the embodiments, practical applications, or technical improvements to technology found in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. 1. An optical phased array (OPA) system for illuminating a target in the presence of atmospheric turbulence, comprising: a coherent beam combining (CBC) subsystem that forms a number of at least partially coherent CBC sub-beams and a reference beam, the CBC sub-beams and the reference beam having optical wavelengths in a first band; a beam director for directing the CBC sub-beams to illuminate the target; an auxiliary light source for illuminating the target with light having a second band of optical wavelengths; a receiving optical aperture configured to receive a portion of light in the first band and the second band of optical wavelengths reflected from the target; an atmospheric wavefront sensor (AWS) subsystem optically coupled to the receive aperture and the reference beam; a multi-channel photodetector (MCPD) subsystem optically coupled to the CBC sub-beam and the reference beam; Equipped with the AWS subsystem comprises a closed-loop AWS controller for providing atmospheric turbulence compensation, and the MCPD subsystem comprises a closed-loop MCPD controller for compensating for effects of phase noise in the CBC sub-beams and for transferring the atmospheric turbulence compensation from the reference beam to the CBC beam. Optical phased array system.

2. 2. The optical phased array system of claim 1, wherein the frequency of the MCPD controller is at least an order of magnitude greater than the bandwidth of the AWS controller.

3. 10. The optical phased array system of claim 1, further comprising a CBC sub-beam phase compensator in communication with the MCPD controller.

4. 2. The optical phased array system of claim 1, wherein the MCPD subsystem comprises an array of photodetectors having at least one pixel for each of the CBC sub-beams.

5. 2. The optical phased array system of claim 1, wherein the AWS controller has a bandwidth of 10 kilohertz or less.

6. The optical phased array system of claim 1 , wherein the reference beam is collimated.

7. 2. The optical phased array system of claim 1, wherein the reference beam overlaps with all of the CBC sub-beams at an input plane of the MCPD subsystem.

8. 10. The optical phased array system of claim 1, wherein the reference beam undergoes a phase modulation that is different from the phase modulation of the CBC sub-beams.

9. 10. The optical phased array system of claim 1, wherein the AWS subsystem comprises a deformable mirror (DM) and a DM actuator.

10. 10. The optical phased array system of claim 9, wherein a surface of the DM is divided or continuous.

11. 10. The optical phased array system of claim 1, wherein the AWS subsystem comprises a Shack-Hartmann sensor.

12. 10. The optical phased array system of claim 1, wherein the AWS subsystem comprises a pyramid detector or a phase diversity sensor.

13. 2. The optical phased array system of claim 1, wherein the AWS subsystem comprises an optical filter that attenuates light that passes through the receive optical aperture, the light having a wavelength outside the second band of optical wavelengths.

14. 10. The optical phased array system of claim 1, comprising a dichroic beam splitter.

15. The optical phased array system of claim 1 , wherein the auxiliary light source is a laser.

16. 10. The optical phased array system of claim 1, wherein the supplemental light source is a solar spectrum of the sun.

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