Systems and methods for low probability of detection radars using incoherent light sources

Abstract

A system and method are provided for low probability of detection (LPD) covert radar / lidar detection. A transmitter transmits a broadband light that is in a solar-radiation spectrum, and the broadband light is modulated using a constant amplitude modulation scheme such as M-ary phase shift keying (PSK), which makes the transmitted signal difficult to distinguish the signal from the background radiation. The radar return light from a target is collected and the target is detected by a cross-correlation calculation between the received light and the modulation pattern used to encode the transmitted signal. Further, in a turbulent environment, adaptive optics can be used to improve the detection probability.

Description

SYSTEMS AND METHODS FOR LOW PROBABILITY OF DETECTION RADARSUSING INCOHERENT LIGHT SOURCESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a PCT application that claims priority to U.S. Provisional Application No. 63 / 588,634 filed on October 6, 2023, which is expressly incorporated by reference herein in its entirety.GOVERNMENT SUPPORT

[0002] This invention was made with government support under 2244365 awarded by the National Science Foundation. The government has certain rights in the invention.FIELD

[0003] The present disclosure generally relates to covert radars, and in particular, relates to an inventive concept for low probability of detection (LPD) and covert radars employing optical incoherent sources.BACKGROUND

[0004] A covert radar is a system for remote sensing that detects a target without revealing the presence of the radar to potential adversaries. In this scenario, the primary parts (or parties) are: Alice, the transmitter; Bob, the receiver; and Willie, the interceptor.

[0005] The transmitter, Alice, emits electromagnetic signals (e.g., radar signals) in a covert manner (e.g., using low power or frequency hopping to minimize detection). The goal is to gather information about a target without revealing Alice's location or intentions.

[0006] The receiver, Bob, receives signals resulting from transmissions by Alice. Bob processes these signals to extract information including the presence of the target. Additional information that can be extracted includes the target's location, speed, and other relevant characteristics.

[0007] The interceptor, Willie, attempts to detect Alice's electromagnetic emissions. For example, Willie might try to intercept the signals to gather intelligence about Alice’s activities or to determine the position of her radar system. Willie’s success depends on how well Alice employs covert techniques and how sophisticated are Willie's interception methods.

[0008] The low probability of detection (LPD) radars are of relevance for both commercial and defense applications. The LPD radar can have an emission waveform that is difficult to detect and intercept by a non-cooperative intercept receiver (e.g., Willie). Examples of intercept strategies that are active areas of research include, e.g., various machine-learning approaches.

[0009] Additionally, quantum radars can be used as an alternative to the LPD radars to improve the target detection probability in a low signal-to-noise ratio regime and beat the quantum limit. However, the complexity of implementation for quantum radars can be high.

[0010] It is with these observations in mind, among others, that various aspects of the present disclosure were conceived and developed.SUMMARY

[0011] The challenges presented by making a radar both covert and practical are addressed by the proposed systems and methods.

[0012] In some aspects, the techniques described herein relate to a system for low probability of detection (LPD) covert radar / lidar detection, the system including: a transmitter that includes: a broadband incoherent optical source that generates light that is incoherent and has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, a modulator that modulates the light according to a predefined modulation scheme to generate a modulated optical signal, and output optics that transmit the modulated optical signal from the transmitter through free space towards a target.

[0013] In some aspects, the techniques described herein relate to a receiver, including: input optics that guide reflected / scattered light from a target into the receiver to provide a received optical signal; a balanced detector that mixes the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal; an analog-to-digital converter that converts the electrical signal to a digital signal; and one or more processors that process the digital signal to detect the target, wherein the reflected / scattered light is incoherent light that has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, and the reflected / scattered light has been modulated according to a predefined modulation scheme.

[0014] In some aspects, the techniques described herein relate to a method for low probability of detection (LPD) covert radar / lidar detections, the method including: receiving, at inputoptics of a receiver, reflected / scattered light from a target to provide a received optical signal; mixing, at a balanced detector, the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal; converting, by an analog-to-digital converter, electrical signal to a digital signal; and processing, at one or more processors, the digital signal to detect the target, wherein the reflected / scattered light is incoherent light that has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, and the reflected / scattered light has been modulated according to a predefined modulation scheme.

[0015] In some aspects, the techniques described herein relate to a radar system including: an incoherent source-based transmitter; an expanding telescope; a radar return compressing telescope; a balanced heterodyne detector including a local oscillator laser-based balanced detector; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations including: generating, by the incoherent source-based transmitter, a phase- modulated signal; transmitting, via the expanding telescope, the phase-modulated signal to yield a transmitted signal, wherein the phase-modulated signal is hidden in solar radiation; receiving, via the radar return compressing telescope, a broadband signal including the transmitted signal reflected off a target; and detecting, via the balanced heterodyne detector, a reflected beam by interacting a local oscillator laser with the broadband signal.

[0016] In some aspects, the techniques described herein relate to a method of operating a radar system, the method including: generating, via an incoherent source-based transmitter, a phase-modulated signal; transmitting, via an expanding telescope, the phase-modulated signal to yield a transmitted signal, wherein the phase-modulated signal is hidden in solar radiation; receiving, via a radar return compressing telescope, a broadband signal including the transmitted signal reflected off a target; and detecting, via a balanced heterodyne detector, a reflected beam by interacting a local oscillator laser with the broadband signal.

[0017] In some aspects, the techniques described herein relate to a radar system including: an optical transmitter including an incoherent source-based transmitter and an expanding telescope; an optical receiver including a radar return compressing telescope a balanced heterodyne detector including a local oscillator laser-based balanced detector; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operationsincluding: generating, by the optical transmitter, a phase-modulated signal; transmitting, via the optical transmitter, the phase-modulated signal to yield a transmitted signal, wherein the phase- modulated signal is hidden in solar radiation; receiving, via the optical receiver, a broadband signal including the transmitted signal reflected off a target; and detecting, via the optical receiver, a reflected beam by interacting a local oscillator laser with the broadband signalBRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0018] In order to describe the manner in which the above-recited and other advantages and features of the disclosure can be obtained, a more particular description of the principles briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only exemplary embodiments of the disclosure and are not therefore to be considered to be limiting of its scope, the principles herein are described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0019] FIG. 1 illustrates a monostatic broadband incoherent light source based radar, in accordance with some embodiments.

[0020] FIG. 2 illustrates aa block diagram for a first implementation of a monostatic / bistatic broadband incoherent light radar, in accordance with some embodiments.

[0021] FIG. 3 illustrates a flow diagram of a method for implementing a monostatic / bistatic broadband incoherent light radar, in accordance with some embodiments.

[0022] FIG. 4 illustrates a block diagram for a second implementation of a monostatic broadband incoherent light radar, in accordance with some embodiments.

[0023] FIG. 5A illustrates a plot of detection probabilities from a first day of a case study of a monostatic broadband incoherent light radar, in accordance with some embodiments.

[0024] FIG. 5B illustrates a histogram plot of received power of a case study of a monostatic broadband incoherent light radar, in accordance with some embodiments.

[0025] FIG. 5C illustrates a plot of detection probabilities from a second day of a case study of a monostatic broadband incoherent light radar, in accordance with some embodiments.

[0026] FIG. 6 illustrates a block diagram of a computing device, in accordance with some embodiments.DESCRIPTION OF EXAMPLE EMBODIMENTS

[0027] Various embodiments of the disclosure are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the disclosure.

[0028] Additional features and advantages of the disclosure will be set forth in the description which follows, and in part will be obvious from the description, or can be learned by practice of the herein disclosed principles. The features and advantages of the disclosure can be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the disclosure will become more fully apparent from the following description and appended claims, or can be learned by the practice of the principles set forth herein.

[0029] The disclosed technology addresses the need in the art for low probability of detection (LPD) covert radar / lidar detection.

[0030] For example, the systems and methods disclosed herein can use an LPD radar that uses an incoherent source. Further the systems and methods disclosed herein can use a monostatic configuration (i.e., both transmitter and receiver are in the same location). The systems and methods disclosed herein can use an LPD radar that hides the radar signal in solar radiation and use constant amplitude modulation, such as M-ary PSK, to make it extremely difficult to distinguish the signal from background radiation. The incoherent source beam can be phase modulated, amplified, and transmitted over a noisy, lossy, and atmospheric turbulent channel towards the target. The reflected beam (radar return beam) can be collected by the compressing telescope and detected by the radar’s receiver, which uses a balanced heterodyne detector. In the balanced heterodyne detector, the received broadband light interacts with a local oscillator (LO) laser that has a center wavelength (e.g., a wavelength of 1550 nm). In view of the transmitted signal being hidden in the solar background radiation (e.g., having a similar amplitude, spectrum, and power distribution) the transmitted signal will be very difficult for Willie’s detector (i.e., the hypothesized interceptor) to either detect or intercept the radar signal.

[0031] The disclosed technology addresses the need in the art for an LPD and covert radar by using a broadband, incoherent light source. This light source enables the radar signal to hide in solar radiation. Further, detection by an interceptor can be avoided by using a constant amplitude modulation scheme, such as BPSK, to modulate the broadband source according to amodulation pattern that is shared with the receiver. At the radar receiver, the presence of the target is detected by calculating a cross-correlation between the received signal and the modulation pattern.

[0032] To demonstrate the proposed concept, a case study was performed using a free-space optical (FSO) testbed. The case study experimentally demonstrated that the broadband, incoherent light sources can be indeed used to operate the LDP and covert radars in a strong turbulence regime, in a desert environment. To improve the detection probabilities in the presence of atmospheric turbulence adaptive optics (AO) have been used. The case study shows that the use of AO can improve the tolerance to turbulence effects, even in strong turbulence.

[0033] The proposed LPD radar technique has been evaluated in a telecommunication window centered at 1550 nm, also known the C-band. The systems and methods disclosed herein are not limited to the C-band, and other FSO-compatible transmission wavelength windows can be used for the LDP radar, including, e.g., L-band, 2.2 pm, 3.5-4.1 pm, and 10 pm.

[0034] According to certain non-limiting examples, the proposed incoherent source-based LPD radar or covert radars concept is illustrated in FIG. 1, which corresponds to the monostatic scenario (when both transmitter and receiver are placed on the same location). However, this concept is also applicable on the bistatic scenario. According to certain non-limiting examples, the LPD radar uses a broadband incoherent light source and modulation scheme that allows the radar signal to hide in solar radiation. For example, the light source can be an incoherent light source with a Gaussian power distribution, and the modulation scheme can us constant amplitude modulation, such as M-ary phase shift keying (PSK) modulation, to make it extremely difficult to distinguish the signal from background radiation. In the transmitter of the radar, the incoherent source beam can be phase modulated, amplified, and then propagated through an expanding telescope. The radar signal is launched from the transmitter into a noisy, lossy, and atmospheric turbulent channel towards the target.

[0035] At the receiver of the radar, the reflected beam from the target (i.e., the radar return beam) is collected by a compressing telescope and detected using a balanced heterodyne detector, which mixes coherent light from a local oscillator (LO) laser with the received broadband signal that is reflected / scattered from the target. Given that the transmitted signal is hidden in the solar background radiation and given the distribution of the transmitted signal is the same as the solar radiation, the radar signal is difficult for an interceptor to detect.

[0036] FIG. 1 illustrates a non-limiting example of system 100 in which target 112 is detected using low probability of detection (LPD) and covert radar techniques that use an incoherent, broadband light source. LPD radar / lidar system 102 includes modulated incoherent source 104, out-coupling optics 106, in-coupling optics 116, LO laser based balanced detector 118, and detection processor(s) 124.

[0037] Modulated incoherent source 104 generates a modulated optical signal by modulating light from an incoherent broadband source to encode on the modulated optical signal modulation pattern 108. Out-coupling optics 106 transmit the modulated optical signal as a free-space optical (FSO) beam (e.g., FSO beam 110) towards target 112.

[0038] Reflected / forward scattered beam 114 from target 112 is collected the LPD radar / lidar system 102 and guided to in-coupling optics 116. For example, in-coupling optics 116 can include a free-space-to-optical-fiber in-coupler. The received light can include both ambient light and the reflected / scattered light. The received light can be detected using a balanced detector (e.g., LO laser based balanced detector 118) in which the received light is mixed with coherent light from a local oscillator (LO) laser, and the mixed light is detected using balanced photoreceivers. The resulting electrical signal (e.g., detected signal 120) from LO laser based balanced detector 1 18 will include noise from the ambient light and a time-delayed modulation pattern 108.

[0039] Using modulation pattern 108 and detected signal 120, detection processor(s) 124 can use a cross-correlation method to detect target 112. As discussed above, detected signal 120 can include a radar return signal, which is a time-delayed modulation pattern 108, with a random signal due to ambient light, which can be modeled as additive noise. The crosscorrelation between the radar return signal and the modulation pattern 108 peaks when the time offset for the modulation pattern 108 matches the time delay in the reflected / scattered light for the modulation pattern 108, which corresponds to the range of target 112 (e.g., At —where At is the time delay, R is the range to target 112, i.e., the distance between target 112 and LPD radar / lidar system 102, and c is the speed of light).

[0040] Willie's detector 122 is a detection system that is trying to intercept the signals from the LPD radar / lidar system 102, and LPD radar / lidar system 102 is a covert radar / lidar system that seeks to avoid detection by Willie's detector 122. For example, LPD radar / lidar system 102 can use broadband light that is within the solar radiation spectrum. Further, as discussed belowwith reference to FIG. 5B, the power spectral density of FSO beam 110 can be selected to blend in with ambient solar radiation, making it difficult for Willie's detector 122 to detect FSO beam 110. Further, the modulation scheme that is selected to modulate FSO beam 110 can use constant amplitude modulation (e.g., binary phase shift keying (BPSK) modulation), which further increases the difficulty for Willie's detector 122 to detect FSO beam 110.

[0041] The systems and methods disclosed herein provide a low probability of detection (LPD) and covert radars / lidars that use optical incoherent sources. Herein, the terms “lidar” and “radar” are used interchangeably to refer to systems that perform detection and / or ranging using electromagnetic radiation that spans a part of the electromagnetic spectrum (e.g., light or radio waves), and the term “radar signal” refers to the electromagnetic radiation that spans the part of the electromagnetic spectrum (e.g., light or radio wave).

[0042] According to certain non-limiting examples, the LPD / covert radar hides the radar signal in solar radiation by employing the broadband optical source. For example, the broadband optical source can have a spectrum ranging more than 30 nm. For example, the broadband optical source can be an Erbium-doped fiber amplifier source. Alternatively, the light emitting diode (LED) or the superluminescent diode (SLD) can also be used as an incoherent light source. Further, the electromagnetic radiation (radar signal) can be modulated using a constant amplitude modulation format at high speed. A receiver can then detect the presence of the target using a cross-correlation method.

[0043] As discussed below with reference to FIG. 5A, FIG. 5B, and FIG. 5C, a case study / experiment was performed demonstrating a non-limiting example of the proposed concept in an outdoor free-space optical (FSO) environment. This case study also illustrates the improved tolerance to atmospheric turbulence effects resulting from the use of adaptive optics (AO). This case study also illustrates that the LPD / covert radar concept applied in the systems and methods disclosed herein can be used even in harsh environments such as for a strong turbulent FSO channel in a desert environment.

[0044] FIG. 2 illustrates a non-limiting example of a system for low probability of detection (LPD) covert radar / lidar detection applicable to both monostatic and bistatic radar concepts. Covert radar system 200 includes a transmitter (i.e., TX 202), an FSO beam 204, an FSO channel 206, a receiver (i.e., RX 208), and an adaptive optics (AO) subsystem (i.e., adaptive optics system 228). TX 202 includes broadband source 210, modulator 214, and expanding telescope 216. RX 208 includes beam splitter 226, coupling optics 218, detector 220, and targetdetection processor 222 that generates detection results 224 based on the cross-correlation of the signal from detector 220 and modulation pattern 238. Adaptive optics system 228 includes AO processor 212, deformable mirror 230, and wavefront sensor 232.

[0045] According to certain non-limiting examples, TX 202 includes a broadband optical source (e.g., broadband source 210) that generates light that is incoherent and has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm. Further, TX 202 includes a modulator (e.g., modulator 214) that modulates the light according to a predefined modulation scheme to generate a modulated optical signal. Additionally, TX 202 includes output optics (e.g. coupling optics, such as a collimator, and expanding telescope 216) that transmit the modulated optical signal from the transmitter through free space towards a target. For example, the predefined modulation scheme can use constant amplitude modulation to encode a pattern on the light from the broadband optical source. Further, the constant amplitude modulation can be phase shift keying (PSK) modulation and the pattern can a digital code that is encoded on the light is encoded a bit rate that is in a range of 2-50 Gbps, for example. The light that is incoherent light generated by broadband source 210 can have a spectrum between a wavelength of 1500 nm and 2200 nm and a bandwidth between 5 nm and 50 nm, for example.

[0046] According to certain non-limiting examples, RX 208 includes input optics (e.g., compressing telescope 240 and coupling optics 218 ) that compresses (reduces the beam size) reflected light from the target into the receiver to provide a received optical signal. Further, RX 208 can include a balanced detector (e.g., detector 220). A purpose of the coupling optics 218 is to couple the compressed optical beam into the fiber, which is then used as the input of detector 220. Detector 220 can be a balanced detector that mixes the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal. Further, detector 220 can include an analog-to-digital converter that converts the electrical signal to a digital signal.Additionally, RX 208 can include one or more processors (e.g., target detection processor 222) that process the digital signal to detect the target.

[0047] According to certain non-limiting examples, the modulation scheme used to modulate the light uses constant amplitude modulation to encode a pattern (e.g., modulation pattern 238) on the light from the broadband optical source, and the target detection processor 222 uses a cross-correlation between the pattern and the digital signal to detect the target.

[0048] According to certain non-limiting examples, adaptive optics system 228 includes a deformable mirror (e.g., deformable mirror 230)arranged in an optical path of the reflected / scattered light before the balanced detector (e.g., detector 220).

[0049] Further, adaptive optics system 228 can include a the beam splitter 226, which can be implemented as a partially reflected mirror or dielectric mirror, arranged in the optical path between compressing telescope 240 and detector 220, wherein the smaller portion of the compressed reflected light beam is passed to deformable mirror 230, while the rest to detector 220. Additionally, adaptive optics system 228 can include a wavefront sensor (e.g., wavefront sensor 232) that measures the wavefront of the reflected / scattered light using either the part of the reflected / scattered light or the remainder of the reflected / scattered light. Adaptive optics system 228 can also include a processor (e.g., target detection processor 222) that controls a deformation of the deformable mirror to realize a desired wavefront shape for the wavefront of the reflected / scattered light.

[0050] According to certain non-limiting examples, adaptive optics system 228 increases the amount of the reflected / scattered light that is coupled into the receiver by changing a shape of a wavefront of the reflected / scattered light.

[0051] According to certain non-limiting examples, adaptive optics system 228 reduces the wavefront distortion of the received beam and improves the coupling into the fiber.

[0052] According to certain non-limiting examples, adaptive optics system 228 dynamically adapts to wavefront distortions of the reflected / scattered light, the wavefront distortions being caused by atmospheric turbulence between the receiver and the target.

[0053] According to certain non-limiting examples, adaptive optics system 228 dynamically adapts to wavefront distortions of the reflected / forward scattered light, the wavefront distortions being caused by atmospheric turbulence between during the propagation of the optical beam through free space.

[0054] According to certain non-limiting examples, expanding telescope 216 can launch FSO beam 204 from TX 202 through free-space FSO channel 206 towards target 234. Upon reaching target 234, a portion of FSO beam 204 (e.g., reflected / forward scattered beam 236) is scattered back towards RX 208. For example, the optical signal from TX 202 can transition to free-space propagation from an optical fiber, a photonic waveguide, free-space optics, or other optical format that is used to generate, condition, modulate, and otherwise prepare the optical signal.

[0055] According to certain non-limiting examples, a high-power EDFA can amplify the output from modulator 214 to a signal level of 100 mW (or less depending on the solar radiation level) before launching FSO beam 204 into free space via coupling optics and expanding telescope 216 (and a periscope when needed). The high-power EDFA can compensate for the insertion loss of the modulator. This amplification is optional and can be omitted (e.g., when the optical signal has a large enough amplitude without additional amplification).

[0056] According to certain non-limiting examples, broadband source 210 can be an amplified spontaneous emission (ASE) noise source that generates power spectral density having a Gaussian distribution (e.g., the same as for solar radiation). Alternatively, the light emitting diode (LED) or superluminescent diode (SLD) can be used instead. Consequently, the optical signal can be extremely difficult to intercept, when the broadband optical signal is a low-power phase modulated signal over a large bandwidth (e.g., 39.2 nm).

[0057] According to certain non-limiting examples, the optical signal can be modulated using a modulator to generate a binary phase shift keying (BPSK) signal that encodes a transmitted pattern / sequence (e.g., modulation pattern 238), which is known to both the receiver and the transmitter. The bit rate for the modulation can be, e.g., 10 Gbps or 20 Gbps.

[0058] The modulation scheme used by modulator 214 can be a modulation scheme described in I. B. Djordjevic, Advanced Optical and Wireless Communications Systems, 2ndEdi. (Springer Nature Switzerland AG, Switzerland, 2022) (hereinafter "Advanced Optical and Wireless Communications System”), which is incorporated herein by reference in its entirety, and more particularly, the modulation scheme used by modulator 214 can be a modulation scheme described in chapter 5 of Advanced Optical and Wireless Communications Systems. Detector 220 can be a coherent detector as described in sections 3.1.9 and 6.8 of Advanced Optical and Wireless Communications Systems.

[0059] FIG. 3 illustrates an example method 300 for low probability of detection (LPD) covert radar / lidar operation. Although the example method 300 depicts a particular sequence of operations, the sequence may be altered without departing from the scope of the present disclosure. For example, some of the operations depicted may be performed in parallel or in a different sequence that does not materially affect the function of the method 300. In other examples, different components of an example device or system that implements the method 300 may perform functions at substantially the same time or in a specific sequence.

[0060] According to some examples, the method includes generating an optical signal at process 302. For example, TX 202 illustrated in FIG. 2 may generate an optical signal. Covert radar system 200 includes a transmitter (e.g., TX 202) that generates a broadband optical signal and modulates the optical signal according to a specified modulation pattern 238 and modulation scheme (e.g., binary phase shift keying (BPSK) or some other constant amplitude modulation scheme). This can all be done using fiber-optic components, photonic waveguides (e.g., a photonic system on a chip) or free-space optics.

[0061] According to certain non-limiting examples, .TX 202 hides the radar signal in solar radiation and use constant amplitude modulation, such as M-ary PSK, to make it extremely difficult to distinguish the signal from background radiation. The incoherent source beam is phase modulated, amplified, and with the help of expanding telescope transmitted over noisy, lossy, and atmospheric turbulent channel towards the target.

[0062] According to some examples, the method includes generating a broadband optical signal at block 304. For example, broadband source 210 illustrated in FIG. 2 may generate a broadband optical signal. To provide low probability of detection (LPD) radar signal, a broadband source is used to generate the optical signal. The broadband source generates light in a bandwidth range within the solar spectrum, such that the signal is difficult to distinguish from solar radiation, making interception of the signal challenging. The combination of the broadband signal in the solar spectrum together with the modulation and signal processing techniques described below can make it difficult to detect the radar signal.

[0063] According to certain non-limiting examples, a broadband signal is generated using an ASE noise source (EDFA). The EDFA source output follows a Gaussian distribution, which is the same as for the solar radiation. Consequently, the optical signal can be extremely difficult to intercept, when the broadband optical signal is a low-power phase modulated signal over a large bandwidth (e.g., 39.2 nm).

[0064] According to some examples, the method includes modulating the optical signal at block 306. For example, modulator 214 illustrated in FIG. 2 may modulate the optical signal. The radar signal can be modulated using constant amplitude modulation, such as M-ary PSK, to make it extremely difficult to distinguish the signal from background radiation.

[0065] According to certain non-limiting examples, the optical signal can be modulated using a modulator to generate a binary phase shift keying (BPSK) signal that encodes a transmittedpattern / sequence (e.g., modulation pattern 238), which is known to both the receiver and the transmitter. The bit rate for the modulation can be, e.g., 10 Gbps, 20 Gbps, or 50 Gbps, or lower.

[0066] According to some examples, the method includes amplifying the modulated optical signal at block 308. For example, EDFA 404a illustrated in FIG. 4 may amplify the modulated optical signal. According to certain non-limiting examples, an EDFA is placed after the modulator. For example, this amplifier can compensate for the insertion loss of the modulator. As indicated by the dashed line this amplification step is optional, and this step can be omitted, e.g., when the optical signal has a large enough amplitude without additional amplification.

[0067] According to certain non-limiting examples, A high-power EDFA amplifies the output beam from the phase modulator to a signal level of 100 mW (or less depending on the solar radiation level) before launching into free space by a beam expander (and a periscope when needed).

[0068] According to some examples, the method includes launching the modulated optical signal to a free-space channel at block 310. For example, expanding telescope 216 illustrated in FIG. 2 may launch the modulated optical signal to a free-space channel. The optical signal transitions to free-space propagation from an optical fiber, a photonic waveguide, free-space optics, or other optical format that is used to generate, condition, modulate, and otherwise prepare the optical signal.

[0069] According to certain non-limiting examples, the optical signal is launched to the free space using a beam expander and a periscope.

[0070] According to some examples, the method includes propagating the signal through the channel at block 312. For example, FSO channel 206 illustrated in FIG. 2 may propagate the signal through the channel.

[0071] According to some examples, the method includes applying adaptive optics to correct wavefront distortions due to atmospheric turbulence at block 314. For example, adaptive optics system 228 illustrated in FIG. 2 may apply adaptive optics to correct wavefront distortions due to atmospheric turbulence. According to certain non-limiting examples, adaptive optics system 228 improves performance in the presence of atmospheric turbulence effects. Adaptive optics system 228 can include wavefront sensor 232 and deformable mirror 230 operating in a servo loop. Wavefront sensor 232 detects wavefront distortions. Adaptive optics (AO) processor 212calculates the corresponding correction signals for deformable mirror 230, and these correction signals are communicated to deformable mirror 230 to control the deformation of deformable mirror 230 to compensate for the wavefront distortions due to atmospheric turbulence. For example, beam splitter 226 can divert 8% of the reflected / scattered light as a beacon signal to operate the adaptive optics system 228.

[0072] According to some examples, the method includes detecting the radar return light (e.g., reflected light in monostatic radar or forward scattered light in bistatic radar) at process 316. Herein, the term "reflected / scattered light" refers to light that is reflected from the target and / or light that is scattered by the target in a forward direction, which can also be used interchangeably with “radar return signal” and can be further shortened to "returned light" or “radar return.” For example, RX 208 illustrated in FIG. 2 may detect the reflected / scattered light. After propagating through the FSO channel (e.g., FSO channel 206), the light can be collected by a condensing telescope, such as a reflective telescope or a refractive telescope. The collected light can then be coupled into an optical fiber, a photonic waveguide, or another optical system for detecting the received light and converting it into a digitized electrical signal that can be processed by a computer.

[0073] According to certain non-limiting examples, reflected / forward scattered beam 236 is captured using a compression telescope and passed to an optical bench where it is coupled into an optical fiber. The received signal can be amplified by pre-amp EDFA and detected using a balanced detector to generate an RF signal. The resulting RF signals are then converted from analog to digital signals using an analog-to-digital converter (ADC).

[0074] According to some examples, the method includes coupling the reflected / scattered light into the receiver at block 318. For example, coupling optics 218 illustrated in FIG. 2 may couple the reflected / scattered light into the receiver. According to certain non-limiting examples, reflected / forward scattered beam 236 is captured using a compression telescope and passed to an optical bench where it is coupled into an optical fiber.

[0075] According to some examples, the method includes amplifying the optical signal at block 320. For example, EDFA 404b illustrated in FIG. 4 may amplify the optical signal. According to certain non-limiting examples, the received signals are amplified by pre-amplifier EDFA.

[0076] According to some examples, the method includes detecting the optical signal to generate an electrical signal at block 322. For example, detector 220 illustrated in FIG. 2 may detect the optical signal to generate an electrical signal. According to certain non-limiting examples, the received signal feeds a balanced detector where it is mixed with coherent light from 1550 nm laser and detected using the balanced heterodyne detector. The RF signal from the balanced detector is then converted from the analog domain to a digital signal using an ADC.

[0077] According to some examples, the method includes processing the electrical signal to detect a target at process 324. For example, target detection processor 222 illustrated in FIG. 2 may process the electrical signal to detect a target. According to certain non-limiting examples, a cross-correlation method is used to detect the target. The detected signal can include both an ambient light component and a reflected / forward scattered component, which is a time-delayed copy of the modulation pattern 238. The ambient light component is uncorrelated with the modulation pattern 238 and can be modeled as additive noise. The cross-correlation between the returned signal (e.g., reflected beam in monostatic radar or forward scattered beam in bistatic radar) and the modulation pattern 108 peaks when the time offset for the correlation calculation matches the time delay of the modulation pattern for the reflected / scattered light. This peak in the cross-correlation value indicates the presence of the target, and the time of peak indicates the range of the target.

[0078] FIG. 4 illustrates an example implementation of the systems and methods disclosed herein. Further, this example implementation was used in a case study demonstrating the efficacy of the systems and methods disclosed herein.

[0079] The case study was performed using FSO transmissions from a room in a building to a second building rooftop, which are separated by distance “d,” which is approximately 750 m. Corner cube 412 is a retroreflector that is used a target for proof of concept. The radar beam is pointed from the ECE Dept, building room 549 towards the second (Meinel) building where a retroreflector (corner cube 412), on the rooftop is placed, which reflects beam back to the window of the ECE room 549.

[0080] The proposed incoherent source-based LPD radar or covert radars concept is illustrated in FIG. 4, which corresponds to the monostatic scenario (when both transmitter and receiver are placed on the same location). The systems and methods disclosed herein can use an LPD radar that hides the radar signal in solar radiation and use constant amplitude modulation, such as M-ary PSK, to make it extremely difficult to distinguish the signal from background radiation. The incoherent source beam is phase modulated, amplified, and with the help of expanding telescope transmitted over noisy, lossy, and atmospheric turbulent channel towards the target. These components can be the optical transmitter. The reflected beam (radar return beam) is collected by the compressing telescope and detected by the radar’s receiver based on balanced heterodyne detector, which interacts the LO laser at 1550 nm with the received broadband signal. The LO laser can be tuned to any other wavelength component in the broadband signal. These components can be the optical receiver. Given that the transmitted signal is hidden in the solar background radiation and given the distribution of the transmitted signal is the same as the solar radiation, it will be challenging for Willie’s detector to either detect or intercept the radar signal.

[0081] A low-power phase modulated signal is generated on a light beam with a large bandwidth (i.e., about 39.2 nm). The wideband light is generated using an amplified spontaneous emission noise source (e.g., incoherent source 402). Because the light output from incoherent source 402 follows Gaussian distribution, which is the same as for the solar radiation it can be difficult to detect and intercept. Alternatively, in place of incoherent source 402, any other thermal light source can be used instead, such as a light emitting diode (LED) or superluminescent diode (SLD).

[0082] In the case study, a 39-nm bandwidth Erbium-Doped Fiber Amplifier (EDFA) was used as incoherent source 402, and modulator 406 generated a binary phase shift keying (BPSK) signal to encode a transmitted pattern / sequence, which is known to the receiver. The data is organized in packets and the receiver employs the cross-correlation method to determine the presence of radar return signal.

[0083] Next, the light is modulated using a constant amplitude modulation scheme. Here, modulator 406 modulates the light from incoherent source 402 using binary phase shift keying (BPSK) modulation, followed by EDFA 404a to compensate for the insertion loss of the Mach- Zehnder (MZ) modulator. Here, a high-power EDFA (i.e., EDFA 404a) amplifies the output beam from phase modulator 406 to provide a signal level of 100 mW (or less depending on the solar radiation level) before launching the light into free space by beam expander 408 and periscope 410.

[0084] The beam from periscope 410 propagates toward the other building and is received back by periscope 410 after reflection from the target (i.e., corner cube 412) on the rooftop ofthe other building. When there is a line of sight the periscope is not needed. In a room in the first building, the returning beam first propagates through the AO subsystem (e.g., the AO subsystem includes deformable mirror 420, wavefront sensor 424, and adaptive optics processor 434).

[0085] After reflecting from corner cube 412 and returning to periscope 410, the beam is captured using a compression telescope (i.e. beam compressor 414) and passed through various mirrors 416 and other optics before being coupled into the fiber via fiber coupler 418. The received signals are then amplified by EDFA 404b which feeds into balanced detector 426. Balanced detector 426 mixes the received incoherent signal with coherent light from a local oscillator (LO) laser (i.e., laser 422), which is used as a reference signal for balanced detector 426.

[0086] The RF output of balanced detector 426 is then screen captured by real-time scope 430, which functions as an analog-to-digital converter (ADC). Offline digital signal processing (DSP) was performed by computer 432 to apply the cross-correlation method to detect the target.

[0087] In the case study, after fiber coupler 418 couples the received light into an optical fiber, he received light is amplified by a low-noise, high-gain amplifier (i.e., EDFA 404b). Finally, the received optical signal is converted into the electrical domain with the help of balanced detector 426 by beating it against a 1550 nm local oscillator (LO) laser signal from laser 422. The radio frequency (RF) output from balanced detector 426 then feeds a real-time oscilloscope (i.e., real-time scope 430) where a screen-capture from real-time scope 430 is sent to computer 432 which analyzes the waveform for the presence of the target.

[0088] The AO subsystem improves performance in the presence of atmospheric turbulence effects. The AO subsystem includes wavefront sensor 424 and deformable mirror 420 operating in a servo loop. Wavefront sensor 424 detects wavefront distortions. Adaptive optics processor 434 calculates the corresponding correction signals for deformable mirror 420, and these correction signals are communicated to deformable mirror 420 to control the deformation of deformable mirror 420 to compensate for the wavefront distortions, which are due to atmospheric turbulence. In the case study, system cost was reduced by using an 8% pickoff from the radar return signal as a beacon signal to operate the AO subsystem, which is enabled by using a constant amplitude modulation format.

[0089] Various results from the case study are summarized in FIG. 5 A, FIG. 5B, and FIG. 5C.

[0090] FIG. 5A shows target detection probability for different atmospheric turbulence realizations on a first day. The horizontal axis represents the index of the different atmospheric turbulence realizations. The vertical axis shows the target detection probabilities when the bit rate R was 10 Gbps (20 Gbps) for cases with and without using adaptive optics and the false alarm probability is set to 106.

[0091] FIG. 5A summarizes the target detection probabilities corresponding to the LPD radar in a strong turbulence regime for different realizations of turbulence. In this case study, incoherent source-based detection probability results were compared for cases using the BPSK at lOGb / s and 20Gb / s, with and without AO. For the output beam (transmit signal), the bandwidth exceeded 30nm and the power was 100 mW. The background solar radiation measured in the 76-mm diameter aperture of the compressing telescope was 41.36 mW. The AO is designed to operate in a weak turbulence regime for astronomic applications.Nevertheless, it provided relevant improvements in target detection probabilities in strong turbulence for the proposed incoherent source-based radar technique. As can be seen in FIG. 5 A, the 10 Gb / s case provided better detection probabilities compared to the 20 Gb / s case.

[0092] FIG. 5B shows a histogram of received power, that is collected in front of the compressing telescope, which is similar to Rayleigh distribution, indicating that in this case the turbulence of the FSO channel falls within a strong atmospheric turbulence regime.

[0093] FIG. 5C shows the target detection probability for different atmospheric turbulence realizations on a second day— not the first day corresponding to FIG. 5 A. The results illustrated in FIG. 5C are for experiments performed in the strong turbulence regime for different channel realizations. FIG. 5C shows a comparison of the target detection probability results for 10 Gb / s and 5 Gb / s signaling rates. In this case, the transmit power was 100 mW, while solar radiation measured in the 76-mm diameter aperture of the compressing telescope was 37.08 mW. The adaptive optics feature was turned off. As can be seen in FIG. 5C, the 5 Gb / s case exhibits better tolerance to the turbulence effects compared to the 10 Gb / s case.

[0094] FIG. 6 shows an example of computing system 600. The computing system 600 can be AO processor 212, target detection processor 222, adaptive optics processor 434, and / or computer 432. The computing system 600 can perform the functions of method 300. The computing system 600 can be part of a distributed computing network in which severalcomputers perform respective steps in method 300 and / or the functions of covert radar system 200. The computing system 600 can be connected to the other parts of the distributed computing network via connection 602 or communication interface 628. Connection 602 can be a physical connection via a bus, or a direct connection into processor 604, such as in a chipset architecture. Connection 602 can also be a virtual connection, networked connection, or logical connection.

[0095] In some embodiments, computing system 600 is a distributed system in which the functions described in this disclosure can be distributed within a datacenter, multiple data centers, a peer network, etc. In some embodiments, one or more of the described system components represents many such components each performing some or all of the function for which the component is described. In some embodiments, the components can be physical or virtual devices.

[0096] Example computing system 600 includes at least one processing unit (CPU or processor) 604 and connection 602 that couples various system components including system memory 608, such as read-only memory (ROM) 610 and random access memory (RAM) 612 to processor 604. Computing system 600 can include a cache of high-speed memory 606 connected directly with, in close proximity to, or integrated as part of processor 604. Processor 604 may essentially be a completely self-contained computing system, containing multiple cores or processors, a bus, memory controller, cache, etc. A multi-core processor may be symmetric or asymmetric.

[0097] Processor 604 can include any general -purpose processor and a hardware service or software service, such as services 620, 622, and 624 stored in storage device 614, configured to control processor 604 as well as a special-purpose processor where software instructions are incorporated into the actual processor design.

[0098] To enable user interaction, computing system 600 includes an input device 630, which can represent any number of input mechanisms, such as a microphone for speech, a touch- sensitive screen for gesture or graphical input, keyboard, mouse, motion input, speech, etc. Computing system 600 can also include output device 626, which can be one or more of a number of output mechanisms known to those of skill in the art. In some instances, multimodal systems can enable a user to provide multiple types of input / output to communicate with computing system 600. Computing system 600 can include a communication interface 628, which can generally govern and manage the user input and system output. There is norestriction on operating on any particular hardware arrangement, and therefore the basic features here may easily be substituted for improved hardware or firmware arrangements as they are developed.

[0099] Storage device 614 can be a non-volatile memory device and can be a hard disk or other types of computer-readable media that can store data that are accessible by a computer, such as magnetic cassettes, flash memory cards, solid state memory devices, digital versatile disks, cartridges, random access memories (RAMs), read-only memory (ROM), and / or some combination of these devices.

[0100] The storage device 614 can include software services, servers, services, etc., that when the code that defines such software is executed by processor 604, it causes the system to perform a function. In some embodiments, a hardware service that performs a particular function can include the software component stored in a computer-readable medium in connection with the necessary hardware components, such as processor 604, connection 602, output device 626, etc., to carry out the function.

[0101] For clarity of explanation, in some instances, the present technology may be presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software.

[0102] Any of the steps, operations, functions, or processes described herein may be performed or implemented by a combination of hardware and software services or services, alone or in combination with other devices. In some embodiments, a service can be software that resides in memory of an AO processor 212, target detection processor 222, adaptive optics processor 434 and performs one or more functions of method 300 when a processor executes the software associated with the service. In some embodiments, a service is a program or a collection of programs that carry out a specific function. In some embodiments, a service can be considered a server. The memory can be a non-transitory computer-readable medium.

[0103] In some embodiments, the computer-readable storage devices, mediums, and memories can include a cable or wireless signal containing a bit stream and the like. However, when mentioned, non-transitory computer-readable storage media expressly exclude media such as energy, carrier signals, electromagnetic waves, and signals per se.

[0104] Methods according to the above-described examples can be implemented using computer-executable instructions that are stored or otherwise available from computer-readable media. Such instructions can comprise, e.g., instructions and data that cause or otherwise configure a general-purpose computer, special-purpose computer, or special-purpose processing device to perform a certain function or group of functions. Portions of computer resources used can be accessible over a network. The executable computer instructions may be, e.g., binaries, intermediate format instructions such as assembly language, firmware, or source code. Examples of computer-readable media that may be used to store instructions, information used, and / or information created during methods according to described examples include magnetic or optical disks, solid-state memory devices, flash memory, USB devices provided with non-volatile memory, networked storage devices, and so on.

[0105] Devices implementing methods according to these disclosures can comprise hardware, firmware and / or software, and can take any of a variety of form factors. Typical examples of such form factors include servers, laptops, smartphones, small form factor personal computers, personal digital assistants, and so on. The functionality described herein also can be embodied in peripherals or add-in cards. Such functionality can also be implemented on a circuit board among different chips or different processes executing in a single device, by way of further example.

[0106] The instructions, media for conveying such instructions, computing resources for executing them, and other structures for supporting such computing resources are means for providing the functions described in these disclosures.

[0107] For clarity of explanation, in some instances the present technology may be presented as including individual functional blocks including functional blocks comprising devices, device components, steps or routines in a method embodied in software, or combinations of hardware and software.

[0108] Any of the steps, operations, functions, or processes described herein may be performed or implemented by a combination of hardware and software services or services, alone or in combination with other devices. In some embodiments, a service can be software that resides in memory of a client device and / or one or more servers of a content management system and perform one or more functions when a processor executes the software associated with the service. In some embodiments, a service is a program, or a collection of programs thatcarry out a specific function. In some embodiments, a service can be considered a server. The memory can be a non-transitory computer-readable medium.

[0109] In some embodiments the computer-readable storage devices, mediums, and memories can include a cable or wireless signal containing a bit stream and the like. However, when mentioned, non-transitory computer-readable storage media expressly exclude media such as energy, carrier signals, electromagnetic waves, and signals per se.

[0110] Methods according to the above-described examples can be implemented using computer-executable instructions that are stored or otherwise available from computer readable media. Such instructions can comprise, e g., instructions and data which cause or otherwise configure a general-purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Portions of computer resources used can be accessible over a network. The computer executable instructions may be, e.g. binaries, intermediate format instructions such as assembly language, firmware, or source code.Examples of computer-readable media that may be used to store instructions, information used, and / or information created during methods according to described examples include magnetic or optical disks, solid state memory devices, flash memory, USB devices provided with nonvolatile memory, networked storage devices, and so on.

[0111] Devices implementing methods according to these disclosures can comprise hardware, firmware and / or software, and can take any of a variety of form factors. Typical examples of such form factors include servers, laptops, smart phones, small form factor personal computers, personal digital assistants, and so on. Functionality described herein also can be embodied in peripherals or add-in cards. Such functionality can also be implemented on a circuit board among different chips or different processes executing in a single device, by way of further example.

[0112] The instructions, media for conveying such instructions, computing resources for executing them, and other structures for supporting such computing resources are means for providing the functions described in these disclosures.

[0113] Although a variety of examples and other information was used to explain aspects within the scope of the appended claims, no limitation of the claims should be implied based on particular features or arrangements in such examples, as one of ordinary skill would be able to use these examples to derive a wide variety of implementations. Further and although somesubject matter may have been described in language specific to examples of structural features and / or method steps, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to these described features or acts. For example, such functionality can be distributed differently or performed in components other than those identified herein. Rather, the described features and steps are disclosed as examples of components of systems and methods within the scope of the appended claims.

[0114] The operations of the systems disclosed herein can be performed via a computing device having a processor, a bus, a computer-readable memory and so forth. The term one or more processors or processor can encompass one or more of an application-specific integrated circuit (ASIC), a field programmable gate array (FPGA), or a more traditional computer processor. In many cases, radar systems as disclosed do not use standard computer processors but use ASICs or FPGAs to perform their operations.

[0115] While the present disclosure has been described with reference to various embodiments, it should be understood that these embodiments are illustrative and that the scope of the disclosure is not limited to them. Many variations, modifications, additions, and improvements are possible. More generally, embodiments in accordance with the present disclosure have been described in the context of particular implementations. Functionality may be separated or combined in blocks differently in various embodiments of the disclosure or described with different terminology. These and other variations, modifications, additions, and improvements may fall within the scope of the disclosure as defined in the claims that follow. To illustrate, the proposed concept is directly applicable to the bistatic radars, in which transmitters and receivers are separated. In this scenario, the forward scattered beam from the target is used instead of the reflected beam in the monostatic scenario.

[0116] Claim clauses include:

[0117] Clause 1. A system for low probability of detection (LPD) covert radar / lidar detection, the system comprising: a transmitter that includes: a broadband optical source that generates light that is incoherent and has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, a modulator that modulates the light according to a predefined modulation scheme to generate a modulated optical signal, and output optics that transmit the modulated optical signal from the transmitter through free space towards a target.

[0118] Clause 2. The system of clause 1, wherein the predefined modulation scheme uses constant amplitude modulation to encode a pattern on the light from the broadband optical source.

[0119] Clause 3. The system of clause 2, wherein the constant amplitude modulation is a phase shift keying modulation and the pattern is a digital code that is encoded on the light is encoded a bit rate.

[0120] Clause 4. The system of clause 3, wherein the spectrum of the light is between a wavelength of 1500 nm and 2200 nm and the bit rate in a range from 2 Gbps to 50 Gbps, or lower.

[0121] Clause 5. The system of any of clause 1 through clause 4, further comprising: a receiver that includes: input optics that couple returned light (reflected beam from the target in the monostatic radar or the forward scattered beam in the bistatic radar) from the target into the receiver to provide a received optical signal, a balanced detector that mixes the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal, and an analog-to-digital converter that converts the electrical signal to a digital signal; and one or more processors that process the digital signal to detect the target.

[0122] Clause 6. The system of clause 5, wherein the predefined modulation scheme uses constant amplitude modulation to encode a pattern on the light from the broadband optical source, and the one or more processors use a cross-correlation between the pattern and the digital signal to detect the target.

[0123] Clause 7. The system of clause 5, further comprising: an adaptive optics system that increases an amount of the returned light that is coupled into the receiver by changing a shape of a wavefront of the radar return beam.

[0124] Clause 8. The system of any of clause 5 through clause 7, further comprising an adaptive optics system that comprises: a deformable mirror arranged in an optical path of the return light before the balanced detector, a beam splitter 226, which can be implemented as a partially reflected mirror or a dielectric mirror, arranged in the in optical path between the deformable mirror and the balanced detector, wherein the small portion of the beam is used to operate the adaptive optics and the rest to detect the target, a wavefront sensor that measures the wavefront of the returned light, and a processor that controls a deformation of thedeformable mirror to realize a desired wavefront shape for the wavefront of the radar return beam.

[0125] Clause 9. The system of clause 7, wherein the adaptive optics system dynamically adapts to wavefront distortions of the returned light beam, the wavefront distortions being caused by atmospheric turbulence between the receiver and the target.

[0126] Clause 10. A receiver, comprising: input optics that compress and guide the returned light beam from a target into the receiver to provide a received optical signal; a balanced detector that mixes the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal; an analog-to-digital converter that converts the electrical signal to a digital signal; and one or more processors that process the digital signal to detect the target, wherein the returned light beam is incoherent light that has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, and the return light has been modulated according to a predefined modulation scheme.

[0127] Clause 11. The receiver of clause 10, wherein the return light has been modulated using constant amplitude modulation to encode a pattern on the incoherent light, and the one or more processors use a cross-correlation between the pattern and the digital signal to detect the target.

[0128] Clause 12. The receiver of clause 11, wherein the constant amplitude modulation is a phase shift keying modulation and the pattern is a digital code that is encoded on the light is encoded a bit rate.

[0129] Clause 13. The receiver of clause 12, wherein the spectrum of the backscatter light is between a wavelength of 1500 nm and 2200 nm and the bit rate in a range from 2 Gbps to 50 Gbps, or lower.

[0130] Clause 14. The receiver of any of clause 10 through clause 13, further comprising an adaptive optics system that comprises: a deformable mirror arranged in an optical path of the return light before the balanced detector, a beam splitter arranged in the in optical path between the deformable mirror and the balanced detector, wherein the small portion of the beam is used to operate the adaptive optics and the rest to detect the target, and a processor that controls a deformation of the deformable mirror to realize a desired wavefront shape for the wavefront of the radar return beam.

[0131] Clause 15. The receiver of clause 14, wherein the adaptive optics system dynamically adapts to wavefront distortions of the backscatter light, the wavefront distortions being caused by atmospheric turbulence between the receiver and the target.

[0132] Clause 16. A method for low probability of detection (LPD) covert radar / lidar detection, the method comprising: receiving, at input optics of a receiver, the return light beam from a target to provide a received optical signal; mixing, at a balanced detector, the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal; converting, by an analog-to-digital converter, electrical signal to a digital signal; and processing, at one or more processors, the digital signal to detect the target, wherein the return light is incoherent light that has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, and the return light has been modulated according to a predefined modulation scheme.

[0133] Clause 17. The method of clause 16, further comprising: generating, at a broadband optical source of a transmitter, light that is incoherent and has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, modulating, by a modulator, the light according to a predefined modulation scheme to generate a modulated optical signal, and transmitting the modulated optical signal from the transmitter through free space towards the target.

[0134] Clause 18. The method of clause 17, wherein the predefined modulation scheme uses constant amplitude modulation to encode a pattern on the optical signal, which is known to the receiver.

[0135] Clause 19. The method of clause 18, wherein the constant amplitude modulation is a phase shift keying modulation format and the pattern is a digital code that is encoded on the light is encoded a bit rate.

[0136] Clause 20. The method of clause 19, wherein the spectrum of the light is between a wavelength of 1500 nm and 2200 nm and the bit rate in a range from 2 Gbps to 50 Gbps, or lower.

[0137] Clause 21. The method of clause 16, wherein the predefined modulation scheme uses constant amplitude modulation to encode a pattern on the optical signal, and processing, the digital signal to detect the target includes using a cross-correlation between the pattern and the digital signal to detect the target.

[0138] Clause 22. The method of clause 16, further comprising: modifying a wavefront of the radar return beam using an adaptive optics system that comprises: a deformable mirror arranged in an optical path of the return light beam before the balanced detector, a beam splitter arranged in the in optical path between the deformable mirror and the balanced detector, wherein the small portion of the beam is used to operate the adaptive optics and the rest to detect the target, a wavefront sensor that measures the wavefront of the radar return beam using either the part of the return light beam or the remainder of the radar return beam, and a processor that controls a deformation of the deformable mirror to realize a desired wavefront shape for the wavefront of the radar return beam.

[0139] Clause 23. A radar system comprising: an incoherent source-based transmitter; an expanding telescope; a radar return compressing telescope; a balanced heterodyne detector comprising a local oscillator laser-based balanced detector; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: generating, by the incoherent source-based transmitter, a phase-modulated signal; transmitting, via the expanding telescope, the phase-modulated signal to yield a transmitted signal, wherein the phase-modulated signal is hidden in solar radiation; receiving, via the radar return compressing telescope, a broadband signal comprising the transmitted signal reflected off a target; and detecting, via the balanced heterodyne detector, a reflected beam by interacting a local oscillator laser with the broadband signal.

[0140] Clause 24. The radar system of clause 23, wherein the one or more processors comprises one of an application-specific integrated circuit, a field programmable gate array, or a computer processor.

[0141] Clause 25. The radar system of clause 24 or clause 23, wherein the radar system comprises a low probability of detection radar.

[0142] Clause 26. The radar system of any of clause 23 through clause 25, wherein the local oscillator laser has an operating wavelength that is in a C-band from 1535 nm to 1565 nm, in an L-band from 1565 nm to 1625 nm, 2.2 pm, 3.5-4.1 pm, 10 pm, 1 pm, 850 nm, or in a visible wavelength region.

[0143] Clause 27. The radar system of any of clause 23 through clause 26, wherein the phase- modulated signal is generated using a constant amplitude modulation.

[0144] Clause 28. The radar system of any of clause 23 through clause 25, wherein the detecting, via the balanced heterodyne detector, of the reflected beam based on the broadband signal occurs by using cross-correlation.

[0145] Clause 29. The radar system of any of clause 23 through clause 27, further comprising: an adaptive optics subsystem comprising a deformable mirror and a wavefront sensor operated in a servo loop, wherein the computer-readable storage medium stores additional instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: estimating the azimuthal phase distortions of the wavefront caused by the atmospheric turbulence using the wavefront sensor, executing the program routine to determine the forces needed for the actuators to deform the mirror, deforming, based on wavefront distortions determined in the broadband signal, the deformable mirror to compensate for the wavefront distortions.

[0146] Clause 30. The radar system of any of clause 23 through clause 29, wherein the incoherent source-based transmitter comprises an Erbium-Doped Fiber Amplifier (EDFA).

[0147] Clause 31. The radar system of clause 30, wherein the EDFA produces signals having a bandwidth larger than 30 nm.

[0148] Clause 32. The radar system of any of clause 23 through clause 31, wherein the expanding telescope comprises a beam expander and a periscope. When there exists the line-of- sight between the transmitter and target the pericope is not needed.

[0149] Clause 33. The radar system of any of clause 23 through clause 32, wherein the incoherent source-based transmitter comprises an Erbium-Doped Fiber Amplifier (EDFA) that generates signals having a bandwidth greater than 30 nm and wherein the phase-modulated signal is generated using a constant amplitude modulation to yield a broadband phase- modulated signal that is hidden in the solar radiation.

[0150] Clause 34. The radar system of clause 33, wherein the incoherent source-based transmitter comprises a light-emitting diode or a superluminescent diode.

[0151] Clause 35. The radar system of any of clause 23 through clause 34, wherein the radar system comprises a bistatic radar in which transmission components and receiver components are separated and wherein the broadband signal comprises a forward scattered beam from the target.

[0152] Clause 36. A method of operating a radar system, the method comprising: generating, via an incoherent source-based transmitter, a phase-modulated signal; transmitting, via an expanding telescope, the phase-modulated signal to yield a transmitted signal, wherein the phase-modulated signal is hidden in solar radiation; receiving, via a radar return compressing telescope, a broadband signal comprising the transmitted signal reflected off a target; and detecting, via a balanced heterodyne detector, a reflected beam by interacting a local oscillator laser with the broadband signal.

[0153] Clause 37. The method of clause 36, wherein the one or more processors comprises one of an application-specific integrated circuit, a field programmable gate array, or a computer processor.

[0154] Clause 38. The method of clause 36 or clause 37, wherein the radar system comprises a low probability of detection radar.

[0155] Clause 39. The method of any of clause 36 through clause 38, wherein the local oscillator laser has an operating wavelength in one or more of a C-band from 1535 nm to 1565 nm, an L-band from 1565 nm to 1625 nm, 2.2 pm, 3.5-4. 1 pm, 10 pm, 1 pm, 850 nm and a visible wavelength region.

[0156] Clause 40. The method of clause 36 through clause 39, wherein the phase-modulated signal is generated using a constant amplitude modulation.

[0157] Clause 41. The method of clause 36 through clause 40, wherein the detecting, via the balanced heterodyne detector, of the reflected beam based on the broadband signal occurs by using cross-correlation.

[0158] Clause 42. The method of clause 36 through clause 41, further comprising: an adaptive optics subsystem comprising a deformable mirror and a wavefront sensor operated in a servo loop, wherein the computer-readable storage medium stores additional instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: estimating the azimuthal phase distortions of the wavefront caused by the atmospheric turbulence using the wavefront sensor, executing the program routine to determine the force needed for the actuators to deform the mirror, deforming, based on wavefront distortions determined in the broadband signal, the deformable mirror to compensate for the wavefront distortions.

[0159] Clause 43. The method of clause 36 through clause 42, wherein the incoherent sourcebased transmitter comprises one of an Erbium-Doped Fiber Amplifier (EDFA).

[0160] Clause 44. The method of clause 36 through clause 43, wherein the incoherent sourcebased transmitter comprises a light-emitting diode or a superluminescent diode.

[0161] Clause 45. The method of clause 43, wherein the EDFA produces signals having a bandwidth larger than 30 nm.

[0162] Clause 46. The method of clause 36 through clause 45, wherein the expanding telescope comprises a beam expander and a periscope.

[0163] Clause 47. The method of clause 37 through clause 46, wherein the incoherent sourcebased transmitter comprises an Erbium-Doped Fiber Amplifier (EDFA) that generates signals having a bandwidth greater than 30 nm and wherein the phase-modulated signal is generated using a constant amplitude modulation to yield a broadband phase-modulated signal that is hidden in the solar radiation.

[0164] Clause 48. The method of clause 36 through clause 47, wherein the radar system comprises a bistatic radar in which transmission components and receiver components are separated and wherein the broadband signal comprises a forward scattered beam from the target.

[0165] Clause 49. A radar system comprising: an optical transmitter comprising an incoherent source-based transmitter and an expanding telescope; an optical receiver comprising a radar return compressing telescope a balanced heterodyne detector comprising a local oscillator laser-based balanced detector; one or more processors; and a computer-readable storage medium storing instructions which, when executed by the one or more processors, cause the one or more processors to perform operations comprising: generating, by the optical transmitter, a phase-modulated signal; transmitting, via the optical transmitter, the phase- modulated signal to yield a transmitted signal, wherein the phase-modulated signal is hidden in solar radiation; receiving, via the optical receiver, a broadband signal comprising the transmitted signal reflected off a target; and detecting, via the optical receiver, a reflected beam by interacting a local oscillator laser with the broadband signal.

[0166] Clause 50. The radar system of clause 49, wherein the radar system comprises a bistatic radar in which transmission components and receiver components are separated and wherein the broadband signal comprises a forward scattered beam from the target.

[0167] Clause 51. The radar system of clause 49 or clause 50, wherein the incoherent sourcebased transmitter comprises one of an Erbium-Doped Fiber Amplifier (EDFA) that generates signals having a bandwidth greater than 30 nm or a light-emitting diode or a superluminescent diode.

Claims

CLAIMSWhat is claimed is:

1. A system for low probability of detection (LPD) covert radar / lidar detection, the system comprising: a transmitter that includes: a broadband optical source that generates light that is incoherent and has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, a modulator that modulates the light according to a predefined modulation scheme to generate a modulated optical signal, and output optics that transmit the modulated optical signal from the transmitter through free space towards a target.

2. The system of claim 1, wherein the predefined modulation scheme uses constant amplitude modulation to encode a pattern on the light from the broadband optical source.

3. The system of claim 2, wherein the constant amplitude modulation is a phase shift keying modulation and the pattern is a digital code that is encoded on the light is encoded a bit rate.

4. The system of claim 3, wherein the spectrum of the light is between a wavelength of 1500 nm and 2200 nm and the bit rate in a range from 1 Gbps to 50 Gbps.

5. The system of claim 1, further comprising: a receiver that includes: input optics that couple a radar return beam from the target into the receiver to provide a received optical signal, a balanced detector that mixes the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal, and an analog-to-digital converter that converts the electrical signal to a digital signal; and one or more processors that process the digital signal to detect the target.

6. The system of claim 5, wherein the predefined modulation scheme uses constant amplitude modulation to encode a pattern on the light from the broadband optical source, and the one or more processors use a cross-correlation between the pattern and the digital signal to detect the target.

7. The system of claim 5, further comprising: an adaptive optics system that increases an amount of the radar return beam that is coupled into the receiver and reduces the wavefront distortions by changing a shape of a wavefront of the radar return beam.

8. The system of claim 5, further comprising an adaptive optics system that comprises: a deformable mirror arranged in an optical path of the radar return beam before the balanced detector, a wavefront sensor that measures the wavefront of incoming light, a beam splitter arranged in the in optical path between the deformable mirror and the balanced detector, the beam splitter including a partially reflecting mirror or dielectric mirror that reflects less than 20% of the radar return beam as the incoming light to the wavefront sensor and passes a remainder of the radar return beam to the input optics, and a processor that controls a deformation of the deformable mirror to realize a desired wavefront shape for the wavefront of the radar return beam.

9. The system of claim 7, wherein the adaptive optics system dynamically adapts to wavefront distortions of the radar return beam, the wavefront distortions being caused by atmospheric turbulence between the transmitter and the target and between the target and the receiver.

10. A receiver, comprising: input optics that compresses and guide a radar return beam from a target into the receiver to provide a received optical signal; a balanced detector that mixes the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal; an analog-to-digital converter that converts the electrical signal to a digital signal; and one or more processors that process the digital signal to detect the target, whereinthe radar return beam is incoherent light that has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, and the radar return beam has been modulated according to a predefined modulation scheme.

11. The receiver of claim 10, wherein the radar return beam has been modulated using constant amplitude modulation to encode a pattern on the incoherent light, and the one or more processors use a cross-correlation between the pattern and the digital signal to detect the target.

12. The receiver of claim 11, wherein the constant amplitude modulation is a phase shift keying modulation and the pattern is a digital code that is encoded on the light is encoded a bit rate.

13. The receiver of claim 12, wherein the spectrum of the radar return beam is between a wavelength of 1500 nm and 2200 nm and the bit rate in a range from 1 Gbps to 50 Gbps.

14. The receiver of claim 10, further comprising an adaptive optics system that comprises: a deformable mirror arranged in an optical path of the radar return beam before the balanced detector, a wavefront sensor that measures the wavefront of incoming light, a beam splitter arranged in the in optical path between the deformable mirror and the balanced detector, the beam splitter including a partially reflecting mirror or dielectric mirror that reflects less than 20% of the radar return beam as the incoming light to the wavefront sensor and passes a remainder of the radar return beam to the input optics, and a processor that controls a deformation of the deformable mirror to realize a desired wavefront shape for the wavefront of the radar return beam.

15. The receiver of claim 14, wherein the adaptive optics system dynamically adapts to wavefront distortions of the radar return beam, the wavefront distortions being caused by atmospheric turbulence between the transmitter and the target and between the target and the receiver.

16. A method for low probability of detection (LPD) covert radar / lidar detection, the method comprising: receiving, at input optics of a receiver, a radar return beam from a target to provide a received optical signal; mixing, at a balanced detector, the received optical signal with a coherent optical signal from a local oscillator to provide a mixed signal that is detected by a balanced photoreceiver to generate an electrical signal; converting, by an analog-to-digital converter, electrical signal to a digital signal; and processing, at one or more processors, the digital signal to detect the target, wherein the radar return beam is incoherent light that has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, and the radar return beam has been modulated according to a predefined modulation scheme.

17. The method of claim 16, further comprising: generating, at a broadband optical source of a transmitter, light that is incoherent and has a spectrum in a solar-radiation spectrum with a full width at half maximum of at least 10 nm, modulating, by a modulator, the light according to a predefined modulation scheme to generate a modulated optical signal, and transmitting the modulated optical signal from the transmitter through free space towards the target.

18. The method of claim 17, wherein the predefined modulation scheme uses phase shift keying modulation to encode a digital code on the light from the broadband optical source.

19. The method of claim 16, wherein the predefined modulation scheme uses constant amplitude modulation to encode a pattern on the optical signal, and processing, the digital signal to detect the target includes using a cross-correlation between the pattern and the digital signal to detect the target.

20. The method of claim 16, further comprising: modifying a wavefront of the radar return beam using an adaptive optics system that comprises: a deformable mirror arranged in an optical path of the radar return beam before the balanced detector, a wavefront sensor that measures the wavefront of incoming light, a beam splitter arranged in the in optical path between the deformable mirror and the balanced detector, the beam splitter including a partially reflecting mirror or dielectric mirror that reflects less than 20% of the radar return beam as the incoming light to the wavefront sensor and passes a remainder of the radar return beam to the input optics, and a processor that controls a deformation of the deformable mirror to realize a desired wavefront shape for the wavefront of the radar return beam.

Images