An active retrodirective antenna array wherein a reference array element is used to generate a phase reference which is replicated at succeeding elements of the array. Each element of the array is associated with a phase regeneration circuit and the phase conjugation circuitry of an adjacent element. In one implementation, the phase reference circuit operates on the input signal at the reference element, a voltage controlled oscillator (VCO) output signal and the input pilot signal at the next array element received from a transmission line. By proper filtering and mixing, a phase component may be produced to which the VCO may be locked to produce the phase conjugate of the pilot signal at the next array element plus a transmission line delay. The same phase conjugation process occurs at the next element where the proper phase reference is regenerated by mixing samples of the input pilot and transmitted signal. In another implementation, particularly suited for large arrays in space, two different input pilot frequencies are employed. Their difference is the phase reference of the system, and a local oscillator is used in obtaining this difference, which is in the IF range. The two pilot frequencies are selected in accordance with particular criteria to insure proper phase addition and elimination of local oscillator components. Appropriate mixing and filtering is performed to achieve phase conjugation and phase reference replication.
An active retrodirective antenna array having central phasing from a reference antenna element through a "tree" structured network of transmission lines utilizes a plurality of phase conjugate circuits (PCCs) at each node and a phase reference regeneration circuit (PRR) at each node except the initial node. Each node virtually coincides with an element of the array. A PCC generates the exact conjugate phase .phi.*.sub.1 of an incident signal .phi..sub.1 in accordance with the relation R (2.phi..sub.0 -.phi..sub.1) where R is equal to the reciprocal of 1-2/n, and n.gtoreq.4, using a phase locked loop which combines the phases .phi..sub.1 and .phi.*.sub.1 in an up-converter, divides the sum by 2 and mixes the result with the phase .phi..sub.0 in a down-converter for phase detection by the phase .phi.*.sub.1 from the loop oscillator divided by n. The PRR extracts the phase .phi..sub.0 from the conjugate phase .phi.*.sub.1 by mixing .phi.*.sub.1 divided by 2 and divided by n in a down-converter and then mixing the phase .phi..sub.1 divided by 4 with the result of the down-converter in two cascaded up-converters. Both the PCC and the PRR are not only exact but also free from mixer degeneracy.
A system and method for automatically generating a return beam in the direction of a received beam. The inventive system (10) includes a phased array antenna (12) for receiving a radio frequency signal having a first wavefront from a first direction. In response to this signal, the invention (10) provides a second signal having a second wavefront. The second signal is a phase conjugate of the first signal and is transmitted in a reverse direction relative to the direction of the first signal. In the illustrative embodiment, the invention includes a plurality of phase conjugators each of which are disposed in a transmit/receive module and coupled to an associated radiating element. Each of the phase conjugators includes a mixer (60) having the input signal as a first input thereto. The input signal has a first frequency and a first phase. A second signal having a frequency equal to twice the first frequency is input to each mixer (60) from a reference frequency module (30) such that the output of the mixer includes a component representative of the negative of the first phase. The output of each mixer (60) is filtered to extract a signal component having a negative phase relative to the input signal. These signal components are then transmitted as the phase conjugated wavefront. This process will occur for each signal when multiple signals are received within the field of regard of the phased array antenna (12).
An energy transmission arrangement emits a microwave energy signal to a target location on the basis of a pilot signal received from the target location. The arrangement includes a plurality of pilot antennas for receiving the pilot signal and a plurality of antenna elements for transmission of the energy signal. The pilot signal as received at a first pilot antenna is multiplied at a phase conjugation circuit and divided into a plurality of transmission signals corresponding to the plurality of antenna elements. A target direction is calculated based on a phase difference of the pilot signal as received at each of the other pilot antennas and a phase adjustment circuit is provided for adjusting the phases of each of the transmission signals on the basis of the calculated target direction. Electrical energy is then supplied from energy collecting means to a plurality of amplifiers and amplified respectively, on the basis of a corresponding one of the transmission signals, to be supplied to respective antenna elements to be emitted as the microwave energy signal. According to this arrangment, the phase conjugation circuit is active to receive the pilot signal as received at the first pilot antenna as well as the pilot signal as received at a predetermined reference point in the path of the pilot signal for calculating a difference signal for effecting phase correction of the microwave energy signal.
A laser provides light of a first frequency .omega.+.DELTA. along a first optical path, the light being focused by optics onto a target disposed at the primary focal plane of the optics. A laser amplifier is disposed along a second optical path which receives light reflected from the target and processed through a portion of the optics. This amplifier transmits, substantially unamplified, light at the first frequency .omega.+.DELTA. and amplifies light substantially at a second frequency .cndot.. Phase conjugation apparatus is disposed along the second optical path and provides light which is the phase conjugate of light incident thereupon. The phase conjugated light is provided at the second frequency .omega. and directed back through the laser amplifier for amplification thereof and through the optics to the target. The phase conjugated light self-corrects for optical distortions introduced into the light caused by the optics and laser amplifier, resulting in a nearly diffraction-limited high-power light beam impinging upon the target. Four specific embodiments are provided which employ stimulated Brillouin scattering, three-wave mixing, four-wave mixing and photon echo processes, respectively, in the phase conjugation apparatus.
Apparatus and method for determining velocity and range of a target within a field of view. A velocimeter and tracker (10, 100) includes a laser (12, 12') that produces modulated coherent light, a portion of which illuminates targets (22, 22') coming into a field of view (24, 24') of the velocimeter and tracker. Coherent light reflected from the target travels back along a detection path toward a phase conjugate mirror (40, 40') as a probe wave (26, 26'). A higher intensity portion of the coherent light produced by the laser is split into equal parts that are directed toward the phase conjugate mirror from opposite directions and interact with the probe wave to produce a phase conjugate light signal that travels back along the detection path and is also reflected from the target. Light reflected by the target experiences a Doppler phase shift as a function of target velocity. First, second, and higher order phase shifted signals reflected from the target are imaged on photo diodes (68, 68'), producing an electrical signal having components corresponding to the frequency differences of the various order light signals. As a function of the sums and differences of the frequency components of the electrical signal, the range and velocity of the target are determined.
