Abstract:
An optical communications system comprises a first node comprising a phased array transmitter for generating an optical beam and a receiver, and a second node comprising a phase conjugate mirror for returning the optical beam to be detected by the receiver of the first node. The phased array transmitters allow for electronic steering of the beams in a way that is much faster and with a potentially smaller physical footprint than the mechanical systems. The phase conjugate mirrors return the received beams of photons back over the exact path they were sent from the phased array transmitters, ensuring continuity of communication even in the presence of atmospheric turbulence.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 62/340,582, filed on May 24, 2016, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    As a general rule, free-space optical communication systems have a number of advantages over other communication systems. They offer a level of security that is not present in a radiofrequency system; the optical communication signal beam can be highly collimated and directed at the intended optical receiver. In addition, optical communication links can transmit at very high data rates. 
         [0003]    Former methods to produce such free-space optical communications systems involved the use of multiple optical receivers and high powered, mechanically steered laser optical transmitters. Since the laser beam could not be electronically steered, its direction was controlled by a mechanical gimbal system, for example. The receivers of the optical communication links in former approaches were typically based on photodiodes with optics to gather the light from the optical transmitter. 
         [0004]    This approach is fine when the channel is stable and the platforms are not moving quickly. However, in the presence of disturbances such as atmospheric disturbance, or in the presence of motion such as encountered in mobile applications, the optical receiver must also be mechanically steered to maintain alignment to the transmitter. Further, due to the atmosphere, light from the laser is refracted in directions that do not fall on the optical receiver. Thus, mechanical steering is required at both ends of the channel. Further, multiple receivers are often required for redundancy to ensure that the optical communications link is robust against movement and disturbances. 
       SUMMARY OF THE INVENTION 
       [0005]    The previous approach has drawbacks. Mechanical steeling of the optical transmitter and optical receiver is required to maintain alignment between the transmitter and receiver, and multiple receivers are needed to account for channel disturbances such as atmospheric refraction. Moreover, mechanically steered systems are large, making it difficult to have a laser optical transmitter and receiver at each end if handshaking is desired to enable error handling and correction; and without this feedback mechanism, connectivity cannot be confirmed and information can be lost if the optical communication link drops out. For example, when communicating from an unmanned aerial vehicle (UAV) to the communications node on the ground, the link can drop when a cloud goes between the UAV and the ground node, yet the UAV will have no knowledge that the channel was lost. 
         [0006]    The present invention can be used to create a highly reliable, high speed communication links between two or more nodes. These nodes can be on many different platforms including, but not limited to, terrestrially based platforms (between mobile computing devices (e.g., cell phones), between the mobile computing devices and the cell site/tower (network), other moving platforms (e.g., automobiles, aircrafts, UAVs) and in space for communications between or to satellites. The channel is optical although various wavelengths of electromagnetic radiation could be used assuming a phased array emitter and a phase conjugate mirror are available at the desired wavelength of operation. 
         [0007]    The present invention uses phased array transmitters and phase conjugate mirrors to complete the optical communications links. The phased array transmitters allow for electronic steering of the beams in a way that is much faster and with a potentially smaller physical footprint than mechanical systems. The phase conjugate mirrors return the received beams of photons back over the exact path they were sent from the phased array transmitters, ensuring continuity of communication even in the presence of atmospheric turbulence. 
         [0008]    This solution can provide robust handshaking between the transmitter and the receiver. It can also account for link disturbances due to such effects as atmospheric disturbance and eliminates the need for mechanically steering either the transmitter or receiver. 
         [0009]    The phase controlled light source and the phase conjugate mirror can be small and use very little power. These characteristics are critical for mobile applications. This small size and low power requirement make it possible to use this architecture in numerous applications. 
         [0010]    A number of advantages can be obtained when employing the invention. For example, the implemented system can have a small form factor: The solid state nature of the phase controlled transmitter and phase conjugate minors enable extremely small transmitters and receivers. This enables free-space communications between mobile nodes including cell phones and other small devices. Further, power consumption can be lower. The beam is steered electronically and the phase controlled transmitter directs the light directly at the mirror. The phase conjugate mirror reflects the light straight back down the transmit channel, so there is little or no loss on the return trip. Thus, only a small amount of light is required to maintain handshaking between the transmitter and receiver. Finally, solid state steering of the transmit beam and phase conjugate reflection of the mirror enable high speed steering of the communications channel without the need for mechanical steering systems that are extremely slow in comparison. This will be a completely solid state communications and position tracking system. This allows two platforms to move relative to each other at a rather rapid rate while still maintaining communications. For example, this would enable a cell communications architecture in a light based communication medium. In addition, relative position (range, elevation, azimuth) and velocity can be determined, enabling a host of applications not possible with a straight forward communications channel implementation. 
         [0011]    In general, according to one aspect, the invention features an optical communications system. The system comprises a first node comprising a phased array transmitter for generating an optical beam and a receiver, and a second node comprising a phase conjugate mirror for returning the optical beam to be detected by the receiver of the first node. 
         [0012]    In embodiments, the first node further comprises a phase conjugate mirror for returning the beam from the second node back to the first node, and the second node comprises a receiver for detecting the beam from the phase conjugate mirror of the first node. 
         [0013]    Other nodes can be provided, each having a phase conjugate mirror, wherein the first node communicates steering the optical beam between the other nodes in a time division multiplexed fashion. 
         [0014]    The first node can transmit information to the second node by encoding the information into the optical beam. And, the second node can transmit information by encoding the information into the optical beam returned by its phase conjugate mirror back to the first node. 
         [0015]    Further, the first node can determine a position of the second node by determining a time of flight of the optical beam to the second node and detected by the receiver of the first node. 
         [0016]    In some examples, the receiver of the first node is a phased array receiver. 
         [0017]    In general, according to another aspect, the invention features an optical communications system. The system comprises a first node comprising a first phased array transmitter for generating a first optical beam, a first phase conjugate mirror, and a first receiver. A second node then comprises a second phased array transmitter for generating a second optical beam to be returned by the first phase conjugate mirror, a second phase conjugate mirror for returning the first optical beam to be detected by the first receiver, and a second receiver for detecting the second optical beam returned by the first phase conjugate mirror. 
         [0018]    In general, according to still another aspect, the invention features an optical communications method. The method comprises transmitting a first optical beam from a first node with a phased array transmitter, returning the first optical beam with a phase conjugate mirror at a second node, and detecting the optical beam from the phase conjugate mirror at the first node. 
         [0019]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
           [0021]      FIG. 1  is a block diagram showing a master node with a transceiver including a phased array transmitter and receiver and a target node having a receiver with a phase conjugate mirror; 
           [0022]      FIG. 2  is a block diagram showing a phased array transmitter of a master node scanning between multiple network target nodes and communicating with the target nodes in a time division multiplexed fashion; 
           [0023]      FIG. 3  is a block diagram showing a master node with a transceiver including a phased array transmitter and phase conjugate mirror, and two target nodes having a receiver with a phase conjugate mirror; 
           [0024]      FIG. 4  is a block diagram showing two master nodes operating as peers, each with a transceiver including a phased array transmitter and phase conjugate mirror, supporting two links operating at different wavelengths to avoid interference; 
           [0025]      FIG. 5  is a block diagram showing time of flight tracking of the node and providing position and time information in a GPS denied environment; 
           [0026]      FIGS. 6A and 6B  show an example of a nonlinear metasurface structure that enables a phase conjugate mirror through the properties of the material; and 
           [0027]      FIGS. 7A, 7B, 7C, 7D and 7E  show a second embodiment of the phase conjugate mirror that combines a photonic antenna structure with engineered non-linear elements at the nodes of the antennae. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0029]    As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Further, the singular forms and the articles “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms: includes, comprises, including and/or comprising, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Further, it will be understood that when an element, including component or subsystem, is referred to and/or shown as being connected or coupled to another element, it can be directly connected or coupled to the other element or intervening elements may be present. 
         [0030]      FIG. 1  shows a master node  50  with a transceiver including a phased array transmitter  102  and receiver  104 , and a target node  60  having a receiver with a phase conjugate mirror  180 . 
         [0031]    In more detail, a transmitter  102  includes an optical signal source  120  that generates an optical signal in the infrared, visible, or ultraviolet wavelengths. Often, the optical signal source  120  is a laser with a generally long coherence length. In the case of a communications system, the optical signal source  120  may further include an optical signal modulator  121  for modulating the optical signal from the source  120  to encode information for transmission. 
         [0032]    The light generated by the optical signal source  120  is divided into separate optical signals and provided to a beam steering transmitter phase shifting array  122 . In general, the beam steering phase shifting array  122  shifts the phase of the optical signals to steer the beam over a range of angles. 
         [0033]    The optical signals from the phase shifting array  122  are then provided to the transmitter phased array  150 . It includes a two dimensional array of optical transmission elements  158 . In the illustrated example, the transmitter phased array  150  of optical transmission elements  158  comprises a two dimensional, n by m regular array of optical transmission elements  158 . In some examples, n and m are each greater than a 100 or even greater than a 1000. 
         [0034]    The transmission elements  158  emit optical signals, each with a defined phase shift relative to the others provided by the beam steering transmitter phase shifting array  122 . This allows the beam to be steered, under the control of the transmitter steering controller  170 - 1 . 
         [0035]    Phased array transmitters operate such that the phase of the signal at each element  158  is separately controlled. Signals radiated by the various elements constructively and destructively interfere with each other in the space in front of the array  150 . In directions where the signals constructively interfere, the signals are reinforced, whereas in directions where the signals destructively interfere, the signals are suppressed, thereby creating an effective radiation pattern of the entire array that favors a desired direction. The phases at the various elements  158 , and therefore the direction in which the signal propagates, can be changed very quickly, thereby enabling such a system to be electronically steered, for example to sweep over a range of directions. 
         [0036]    According to the reciprocity theorem, a phased array of elements can be used to receive signals preferentially from a desired direction. By electronically changing the phasing, a system can sweep the receive pattern over a range of directions to ascertain a direction from which a signal originates, i.e., a direction from which the signal&#39;s strength is maximum. 
         [0037]    Sun, Watts, et al., (hereinafter “Watts”) describe a phased array of optical elements. (See U.S. Pat. No. 8,988,754 and Sun, Watts, et al., “Large-scale nanophotonic phased array,” Nature, Vol. 493, pp. 195-199, Jan. 10, 2013, the entire contents of each of which are hereby incorporated by reference herein for all that it discloses.) Each optical antenna emits light of a specific amplitude and phase to form a desired far-field radiation pattern through interference of these emissions. 
         [0038]    The generated uplink beam  160  can then be transmitted to a target node  60 . In the illustrated example, the target node  60  might be in an unmanned aerial vehicle (UAV) or a satellite, for example. In one example, the information transmitted by the phased array transmitter  182  on the uplink beam  160  might be flight control information in the example of a UAV target node  60 . In more detail, the uplink beam  160  received by the phase conjugate mirror  180  is also decoded by a decoder  186 . This information is provided to a flight control computer  190  in order to control the operation of the target node  60 . 
         [0039]    Importantly, the target node  60  includes a phase conjugate mirror (PCM)  180 . 
         [0040]    A phase conjugate mirror  180  works generically as follows. Unlike the usual Fresnel reflection, PCM reflects the incident light exactly where it came from, i.e., the wave vector directions of the incident and reflected light are negatives of each other. In this way, it functions like an electronic retroreflector. The device is extremely useful because it not only returns incident light to its source but it also refocuses light despite turbulence, optical distortion and scattering from surfaces. PCM relies on the nonlinearity nature (x) of high intensity laser beams interacting with pure or doped crystals. Typically, two anti-parallel laser beams nonlinearly interfere to create a high energy standing wave in the region of overlap. According to the Bragg condition an impinging beam of light (incident beam  160 ) in the overlapping region must create a reflected beam  162  such that the wave vectors of the incident beam  160  and the reflected beam  162  equal the sum of the wave vectors of the two laser beams which is zero. Thus the reflected beam has the exact opposite direction of the incident beam. Additionally, the reflected beam can have higher energy than the incident beam where the additional energy is due to energy transfer from the two laser beams. 
         [0041]    In one example, the target node  60  modulates the phase conjugate mirror  180  in order to transmit information back to the master node  50  in the downlink beam  162 . For example in one specific example, the target node  60  includes a sensor such as possibly a video camera  182 . The video data generated by the camera  182  is provided to an encoder/modulator  184  that encodes the video data for transmission. Then, the encoder modulates the phase conjugate mirror  180  in order to transmit that information back down to the master node. For example in one embodiment, the encoder encodes the information by modulating the on/off state, the wavelength of the return beam  162 , the phase of the return beam  162 , and/or amplitude of the return beam  162  from the phase conjugate mirror  180 . 
         [0042]    In the illustrated example, the light of the downlink beam  162  is then received by a receiver  104  of the master node  50 . In the illustrated example, the receiver  104  is a phased array receiver. This allows the receiver to steer its receive pattern to track the target node  60 . 
         [0043]    In more detail, detection elements  148  of a receiver phased array  140  receive the returning optical signal  162  from the phase conjugate mirror  180 . The detection elements  148  of a receiver phased array  140  collect the light and provide the light to a receiver phase shifter  124 . In the illustrated example, the transmitter phased array  140  of optical detection elements  148  comprises a two dimensional, n by in regular array of optical detection elements  148 . In some examples, n and m are each greater than a 100 or even greater than a 1000. 
         [0044]    The receiver phase shifter  124  applies a phase shift to the feed from each one of the detection elements  148  and then combines the optical signals for detection by an optical signal detector  126 . 
         [0045]    Detection of the returning optical signal  162  ensures the integrity of the optical communications link between the master node  50  and the target node  60 . The link integrity is validated since the photons transmitted in the uplink  160  from the phased array transmitter  102  are returned in the downlink  162  by the phase conjugate minor and then detected by the phased array receiver  140 . 
         [0046]    Moreover, in the illustrated example, the signal from the optical signal detector  126  is further demodulated by the demodulator  127 . In the illustrated example, the demodulator extracts the video data that was encoded in the returning optical signal  162  by the modulation of the phase conjugate mirror  180  in response to the video data from the camera  182 . 
         [0047]    According to the invention, steering controllers  170 - 1 ,  170 - 2  are provided for each of the transmitter  102  and the receiver  104 . These steering controllers  170 - 1 ,  170 - 2  each generate respective steering signals  172 - 1 ,  172 - 2  to guide the uplink beam  160  to the target node  60  and receive the downlink beam  162  from the target node  60 . 
         [0048]    The present system can also be used to limit power dissipation, especially when battery power must be used, such as in the example of a UAV. One of the key advantages of the phase array transmitter  102  and phase conjugate mirror  180  architecture is that photons that make it to the mirror  180  can be exactly returned down the link. This means that a low power transmitter will be able to find and focus on the mirror  180  without the need for a supplemental high power laser to perform the search function. The phase array transmitter  102  can sweep across the field of view and when the phase conjugate mirror  180  returns photons, the transmitter  102  will know where the mirror  180  is because it knows where it scanned the beam. The transmitter  102  can immediately lock onto the mirror  180  with very little power dissipation due to the efficiency of the reflected signal. Once the connection between the transmitter  102  and mirror  180  is made, the transmitter  102  can continue to scan over small angles from the confirmed location, measuring the reflected signal returned from the mirror  180  to maintain lock. The return signal will confirm that the link is intact and when there is interference, the transmitter  102  will know immediately due to the handshaking architecture. The transmitter  102  can reseed information as necessary. 
         [0049]    For systems in benign environments where the transmitter  102  and minor  180  are not moving, the data rate can be extremely high. For more turbulent environments where the transmitter  102  and mirror  180  (or transceivers) are moving relative to each other, the data rate will be reduced by the amount of time necessary to maintain lock. However, since the steering is fast and solid state, and the reflection is completely directed back to the transmitter, the loss of channel lock will be short and lock can be rapidly reestablished. 
         [0050]      FIG. 2  shows a phased array transmitter of a master node  50  scanning between multiple network target nodes  60 - 1 ,  60 - 2 ,  60 - 3  and tracking the target nodes in a time division multiplex fashion. 
         [0051]    In the illustrated example, a single master node  50  establishes separate communication links with three separate target nodes  60 - 1 ,  60 - 2 ,  60 - 3 . Each of the separate target nodes includes a respective phase conjugate mirror  180 , a phase conjugate mirror demodulator  186  and a phase conjugate mirror modulator  184 . This allows the target nodes to decode information transmitted on the respective uplinks  160 - 1 ,  160 - 2 ,  160 - 3  from the master node  60  and also transmit information on the corresponding downlinks  162 - 1 ,  162 - 2 ,  162 - 3  to the master node  50 . 
         [0052]    One specific example, each of the separate communications links  160 - 1 / 162 - 1 ,  160 - 2 / 162 - 1  and  160 - 3 / 162 - 3  are maintained by the master node  50  in a time division multiplexing scheme. Specifically, the master node  50  successively establishes links with each of the separate nodes  60 - 1 ,  60 - 2 ,  60 - 3 , in a round robin fashion. 
         [0053]    For context, in the previous embodiment of  FIG. 1 , the validation of the communications link  160 - 162  could be performed by the master node  50 , when it received the downlink beam  162  returned by the phase conjugate mirror  180  at its phased array receiver  104 . The target node  60 , however, could not validate the link except for the uplink portion  160 . 
         [0054]      FIG. 3  is a block diagram showing a master node with a transmitter  102  and phase conjugate mirror  180 -M and target nodes, each having a receiver  102  with a phase conjugate mirror  180 . 
         [0055]    In the embodiment, both the master node  50  and the target nodes  60  each validate their respective communications links  160 / 162 . In more detail, the master node  50  includes a phased array transmitter  102  as described previously; it transmits the uplink beams  160 - 1 ,  160 - 2  to the target nodes  60 - 1 ,  60 - 2 , respectively. In the illustrated example, this can occur in a time multiplex fashion. Moreover, as described previously, the uplink beams  160 - 1  and  160 - 2  can contain information such as UAV control signals from the master node  50  to each of the target nodes  60 - 1 ,  60 - 2 . 
         [0056]    The uplink beams  160 - 1  and  160 - 2  are received by the respective phase conjugate mirrors  180 - 1 ,  180 - 2  of each of the target nodes  60 - 1 ,  60 - 2 . As before, a phase conjugate mirror demodulator  186  can decode the incoming beam and extract any commands or other information that is contained in the uplink beams  60 - 1 ,  60 - 2 . 
         [0057]    Through the operation of the phase conjugate mirrors  180 - 1 ,  180 - 2 , for each of the target nodes  60 - 1 ,  60 - 2 , the downlink beams  162 - 1 ,  162 - 2  are returned to the master node  50 , and possibly encoded with information from the modulators  184 - 1 ,  184 - 2 . 
         [0058]    In the illustrated example, the master node  50  also includes a phase conjugate mirror  180 -M. This phase conjugate mirror returns the downlink beams  162 - 1  as secondary uplink beams  63 - 1 ,  163 - 2 . 
         [0059]    As before, the downlink beams  162 - 1 ,  162 - 2  from each of the target nodes  60 - 1 ,  60 - 2  also transmit information from the respective target nodes. This functionality is provided by the phase conjugate mirror modulators  184 - 1 , and  184 - 2  in each of the target nodes. The information contained in the downlink beams  162 - 1 ,  162 - 2  can be extracted by the master node using its phase conjugate mirror demodulator  186 -M. 
         [0060]    For each of the target nodes  60 - 1 ,  60 - 2 , an additional phased array receiver is provided. Specifically target node  60 - 1  includes a phased array receiver  102 -i, whereas target node  60 - 2  includes a phased array receiver  102 - 2 . These phased array receivers can have the same construction as the phased array receiver  102  of the master node  50  as described previously. 
         [0061]    These secondary uplink beams  163 - 1 ,  163 - 2  as received by the target nodes by the respective phased array receivers  102 - 1 ,  102 - 2  allow the target nodes  60  to validate the respective optical links. Specifically, in this example, a photon will be transmitted from the phased array transmitter  102  of the master  50  then returned by one of the respective phase conjugate mirrors  180 - 1 ,  180 - 2  of the target nodes  60 - 1 ,  60 - 2  and then be received by the phase conjugate mirror  180 -M of the master. This will provide the master node with the validation of the corresponding link to that node. Further, when the phase conjugate mirror  180 -M of the master node  50  returns that beam as the secondary uplink beam  163  to the respective phased array receiver  102 - 1 ,  102 - 2  of the respective target node  60 - 1 ,  60 - 2 , then the target nodes will receive confirmation that the link has been validated in both the uplink and downlink directions. 
         [0062]      FIG. 4  shows two nodes operating as peers, each with a transceiver including a phased array transmitter  102  and phase conjugate mirror  180 , supporting two links possibly, operating at different wavelengths to avoid interference. 
         [0063]    In more detail, each of the nodes  50 - 1 ,  50 - 2  includes a respective phased array transmitter  102 - 1 ,  102 - 2  and a phased array receiver  104 - 1 ,  104 - 2 . These phased array transmitters  102  and receivers  104  are similar to those as described previously. Each of these nodes, however, further includes a respective phase conjugate mirror  180 - 1 ,  180 - 2 . Specifically, node  50 - 1  includes a phase conjugate mirror  180 - 1 ; node  50 - 2  includes phase conjugate mirror  180 - 2 . 
         [0064]    The provision of phase conjugate mirrors  180  in each of the nodes  50  allows each node to independently validate the optical communication links between the nodes. For example, master node  50 - 1  transmits the uplink beam  160 - 1  using its phased array transmitter  102 - 1 . The phase conjugate mirror  180 - 2  of the second node  50 - 2  returns the downlink beam  162 - 2 . This downlink beam is then received by the phased array receiver  104 - 1  of the first node  50 - 1 . Any information transmitted in the uplink beam  160 - 1  can be decoded by the phase conjugate mirror demodulator  186 - 2  of the node  50 - 2 . In this way, information can be transmitted from the first node  50 - 1  to the second node  50 - 2  and the first node can validate that that information was received by the second node  50 - 2 . 
         [0065]    At the same time, the second node  50 - 2  can validate the communications link with the first node  50 - 1 . Specifically, the downlink beam  160 - 2  generated by the phased array transmitter  102 - 2  of the second node  50 - 2  is transmitted to the phase conjugate mirror  180 - 1  of the first node  50 - 1 . This phase conjugate mirror  180 - 1  returns the beam as an uplink beam  160 - 2  that is received by the phased array receiver  104 - 2  of the second node  50 - 2 . Any information contained in the downlink beam  160 - 2  from the second node  50 - 2  can be extracted by the phase conjugate mirror demodulator  186 - 1  at the first node  50 - 1 . As a result, the second node  50 - 2  also has confirmation that the communications link to the first node  50 - 1  is also operational. 
         [0066]      FIG. 5  shows time of flight tracking of the target node  60  that can be used to provide position and time information, such as in a Global Navigation Satellite System, e.g., Global Positioning System (GPS), denied environment. 
         [0067]    The use of the phase conjugate mirror  180  on the target node  60  allows the position of the target node  60  to be resolved by the master node  50 . Specifically, the transmitter steering controller  170 - 1  controls the phased array transmitter  102  to track the target node  60 . This provides both azimuth and elevation information given the two-dimensional tracking of the two-dimensional phase array  150 . Similar information is also provided by the receiver steering controller  170 - 2 . 
         [0068]    On the other hand, the range to the target  60  is resolved by calculating the time-of-flight of the uplink beam  160  and the downlink beam  162  to and from the target node  60 . In the illustrated example, a synchronous detector  190  determines the delay between the uplink beam  160  and the downlink beam  162  to extract the range to the target  60 . A positioning system  192  then resolves the position of the target such as in a GPS denied environment, by combining the information from the steering controllers  170 - 1 ,  170 - 2  and the range information provided by the synchronous detector  190 . 
         [0069]    Since the phase conjugate mirror  180  at the target node  60  reflects all the photons directly down the communications link, this ranging can be done without the need for a high power laser such as that necessary in a LiDAR ranging system where most of the light is not reflected and the reflective surfaces are of unknown reflectance and angle. This greatly reduces the power necessary to determine the distance between the master node  50  and the target node  60 . This could be valuable in applications where both distance and angle between nodes and communications between nodes are desired. For example, self driving automobiles could benefit from this feature where the automobiles would need to know their position relative to each other as well as be able to communicate information about their status to ensure that they do not collide. 
         [0070]    PCM  180   
         [0071]      FIGS. 6A and 6B  show nano structures with high nonlinearly that could be used to make a phase conjugate mirror on a chip. The high nonlinearity (χ) of laser interaction with materials is essential for high performance PCMs. In the 700-900 nm (red visible to IR) range, a directed fluidic assembly (DFA) technology using Engineered Optical Materials (EnMats) is used in one embodiment. The schematics of potential EnMat structure for use in a phase conjugate mirror are shown. In  FIG. 6A , the nanorod array serves as a nanophotonic structure optimized to maximize the intensity of the incoming light in the nanospheres, which are core-shell structures containing nonlinear material with intrinsically high χ. In  FIG. 6B , also shown is a more complex EnMat that relies on an array of non-metallic Fano resonators made from a low loss Si core and nonlinear transparent CdS shell. 
         [0072]    Other PCM materials include plasmonic metasurfaces (thin conductor-dielectric composite materials) used to realize high value of nonlinearity (Lee et al., “Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions,”, Nature, vol. 511, no. 7507, pp. 65-69, 2014). They report creating a 28 layer multi quantum well (MQM) structure, which, when pumped with a light of frequency ω, emits at frequency 2ω. The resulting high nonlinear response makes it suitable for use in manufacturing phase conjugate minors. The highest nonlinearity occurs in the infrared. 
         [0073]      FIGS. 7A-7E  show one layer of PCM assembly using EnMat structures shown in  FIGS. 6A and 6B , and scanning electron micrographs of the surface.  FIG. 7A ,  FIG. 7B  and  FIG. 7C  show the layout of nano structures of  FIG. 6A .  FIG. 7D  identifies the caps and pillars for the layouts of  FIG. 7A ,  FIG. 7B  and  FIG. 7C .  FIG. 7E  gives a rough idea of the nano surface, which is about 1 micrometer in length and width. These architectures offer significant reduction in SWaP (Size, Weight and Power) in PCM manufacturing. 
         [0074]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.