Optical Coherent Transient True-Time-Delay and Processing
An optical coherent transient material acts as a spectral filter with high spectral resolution and broad processing bandwidth. Over the full bandwidth, the amplitude and phase of the filters frequency channels are fully adjustable with a resolution given by the homogeneous linewidth. The material can be programmed by individually addressing each channel with a narrow band laser beams or simultaneously with a temporally structured laser pulse sequence. A programmed material will process incoming broadband optical beams by taking their Fourier decomposition, spectrally multiplying it by the materials programmed frequency response, and then emitting a processed temporal waveform. Optical coherent transient materials thus offer an unmatched ability to control the phase relations of the frequency components of complex light pulses. A novel application that exploits this phase control capability of coherent transient is broadband true-time-delay regeneration for phase-array antennas.[1]
Coherent transient true-time-delay regeneration (CTDR) enables the storage and processing of thousands of broadband delay elements in a single compact crystal. Each delay element can be programmed with a unique delay that is a true time delay over the full bandwidth of the coherent transient material. Depending on the material and how it is employed, the processing bandwidth can easily exceed 10 GHz. The maximum allowable delay is roughly the inverse of the material's homogeneous bandwidth, which is essentially the spectral resolution of the coherent transient spectral filter. Delays as large as tens of microseconds can be obtained in certain materials. The temporal resolution of CTDRs can be sub-picoseconds. These attributes of a coherent transient true-time delay allow each element of a large phased-array-radar to delay arbitrary broadband waveforms with high precision. This translates to enabling phase array radar systems that can transmit broadband signals to a target location with high accuracy and high resolution with ultra-low average sidelobe levels or, conversely receive such signals from a particular source.
To program a CTDR, each delay element is illuminated by two temporally separated optical reference pulses, whose relative delay correspond to the desired delay for that element. See Figure 1. These reference pulses can be brief pulses whose duration is less than the inverse of the bandwidth over which the true-time delay is to operate. Alternatively, the reference pulses can be made up of chirped pulses. The use of low intensity chirped reference pulses eliminates the somewhat impractical requirement to produce intense brief reference pulses to obtain efficient storage and processing. It also allows fine temporal delays to be generated by means of frequency shifting between the programming chirped pulses, eliminating the need for fine temporal control of the delay between the programming pulses.
The full temporal processing capabilities of each CTDR device could be utilized by introducing reference pulses with complex encoding/decoding that make the array antenna radar sensitive to the temporal structure of the received or transmitted wave. CTDRs can simultaneously perform true-time delay and temporal and spatial signal processing of the radar signals. Multiple patterns of time delays that effectively steer the array towards different directions can be programmed and stored in the coherent material using a variety of techniques. Angular multiplexing can also be used to fan out the delay associated with different antenna elements into different directions, allowing a single input signal to be routed to the appropriate antenna element with the appropriate delay.
Proof of concept demonstrations of angular and frequency multiplexing, chirped pulse programming, delay control by frequency offset chirped pulses, and simultaneous delay and processing have been performed. Using coherent transients to control true-time delay antennas offers optical control of the microwave signal, by a fiber-compatible, compact, and programmable delay unit, with the ability to regenerate a continuously variable high resolution time delay for high bandwidth, arbitrary, optical amplitude and/or phase encoded waveforms with rapid programming and low latency times, high density storage capacity, and the option to store multiple delays per spot or frequency channel, enabling single access, multi-directional beam-steering capacity. Furthermore, advanced applications of CTDR technology would enable an array antenna capable of high time-bandwidth product signal cross-correlation, and/or spatial-signal processing. CTDR dispersion and path-length compensation capabilities could greatly reduce antenna design and manufacturing tolerances, and greatly reduce system costs.
[1] K. D. Merkel and W. R. Babbitt, Optics
Letters 21, 1102-1104 (1996).
