Zachary Cole

Coherent integration of 0.5 GHz spectral holograms at 1536 nm using dynamic biphase codes

Spectral hole-burning-based optical processing devices are proposed for coherent integration of multiple high-bandwidth interference patterns in a spectral hole-burning medium. In this implementation, 0.5 GHz spectral holographic gratings are dynamically accumulated in Er3+:Y2SiO5 at 4.2 K using a 1536 nm laser frequency stabilized to a spectral hole, along with commercial off-the-shelf components. The processed data, representing time delays over 0.5–2.0 μs, were optically read out using a frequency-swept probe; this approach makes possible the use of low-bandwidth, large-dynamic-range detectors and digitizers and enables competitive processing for applications such as radar, lidar, and radio astronomy. Coherent integration dynamics and material advances are reported.

Multigigahertz range-Doppler correlative signal processing in optical memory crystals

Analog optical signal processing of complex radio-frequency signals for range-Doppler radar information is theoretically described and experimentally demonstrated using crystalline optical memory materials and off-the-shelf photonic components. A model of the range-Doppler processing capability of the memory material for the case of single-target detection is presented. Radarlike signals were emulated and processed by the memory material; they consisted of broadband (> 1 GHz), spread-spectrum, pseudorandom noise sequences of 512 bits in length, which were binary phase-shift keyed on a 1.9 GHz carrier and repeated at 100 kHz over 7.5 ms. Delay (range) resolution of 8 ns and Doppler resolution of 130 Hz over 100 kHz were demonstrated.

Dynamics of broadband accumulated spectral gratings in Tm 3+ :YAG

High-bandwidth accumulated spectral gratings are experimentally studied in Tm3+:YAG by the stimulated-photon-echo technique with a mode-locked picosecond Ti:sapphire laser system. The experimental results show that the spectral grating builds up and decays on the time scale of the metastable-state lifetime (∼10 ms), provided that the time interval of accumulating shots is of the order of the excited-state lifetime (800 µs). An echo efficiency of the order of 0.1% was achieved with pulse intensities 2 orders of magnitude less than those needed for a single-shot process. These results fit well an analytic solution of the Bloch equations and a three-level system relaxation model.

Amplification of high-bandwidth phase-modulated signals at 793 nm

Amplification of high-bandwidth phase-modulated optical signals from integrated-optics phase modulators at 793 nm is experimentally demonstrated using an injection-locking technique. Off-the-shelf wide-bandwidth integrated-optics modulators are power limited at 793 nm owing to photorefractive damage of the LiNbO3 waveguides. Typical optical input powers for these devices at this wavelength are less than 10 mW with optical output powers typically less than 1 mW. To amplify the outputs of these modulators, we injected the phase-modulated light into an antireflection-coated 100-mW single-mode diode laser. With the injection-locking technique, small-signal gains of 23 dB are demonstrated with good signal fidelity up to bandwidths of 3 GHz. A bandwidth limitation is found at approximately 3 GHz for sinusoidal phase-modulated signals, above which signal fidelity is seriously degraded. This limitation is significantly less than the measured relaxation oscillations of ∼5.6 GHz, suggesting a new limitation to injection locking of phase-modulated signals. Amplification of binary-phase-shift-keyed-modulated signals to 6 Gbit/s is also demonstrated with no bit errors over the 256-bit test sequences.

Microwave spectral analysis using optical spectral hole burning

A microwave spectrum analyzer capable of capturing multi-GHz spectra with sub-MHz resolution and unity probability of intercept based on optical spectral hole burning materials is proposed and initial demonstrations presented.

Range-Doppler Imaging Using an Analog Optical Signal Processor with Agile Waveform Sets

A range-Doppler radar signal processor based upon a spatial-spectral holographic analog optical signal processor is discussed. Such a system has a variety of advantages over conventional range-Doppler signal processors including increased instantaneous bandwidths (> 20 GHz), enhanced dynamic range over those bandwidths, and the ability to process a variety of wideband radar waveforms directly at the radar carrier frequency without down conversion. Range-Doppler ambiguity functions are measured showing range resolution < 15 cm, Doppler resolution < 130 Hz, and > 15 dB enhanced imaging due to agile waveform sets.

Analog optical signal processing of baseband codes in Tm:YAG up to 10 Gb/s

Aspects of analog signal processing are explored using baseband codes from 1 to 10 Gb/s modulated onto a 378 THz optical carrier and processed by spectral holographic techniques in Tm:YAG. Results include processing of signals buried in additive noise, variation of time delays over 5 /spl mu/s, and material signal losses as low as /spl sim/1 dB//spl mu/s.

Multigigahertz range-Doppler correlative processing in crystals

Spectral-spatial holographic crystals have the unique ability to resolve fine spectral features (down to kilohertz) in an optical waveform over a broad bandwidth (over 10 gigahertz). This ability allows these crystals to record the spectral interference between spread spectrum waveforms that are temporally separated by up to several microseconds. Such crystals can be used for performing radar range-Doppler processing with fine temporal resolution. An added feature of these crystals is the long upper state lifetime of the absorbing rare earth ions, which allows the coherent integration of multiple recorded spectra, yielding integration gain and significant processing gain enhancement for selected code sets, as well as high resolution Doppler processing. Parallel processing of over 10,000 beams could be achieved with a crystal the size of a sugar cube. Spectral-spatial holographic processing and coherent integration of up to 2.5 Gigabit per second coded waveforms and of lengths up to 2047 bits has previously been reported. In this paper, we present the first demonstration of Doppler processing with these crystals. Doppler resolution down to a few hundred Hz for broadband radar signals can be achieved. The processing can be performed directly on signals modulated onto IF carriers (up to several gigahertz) without having to mix the signals down to baseband and without having to employ broadband analog to digital conversion.

Broadband Photonic Signal Processing of LIDAR Noise Waveforms

We present a novel broadband photonic signal processor, capable of sensing and processing a wide variety of waveforms including analog optical noise, for coherent LIDAR range processing. The device relies on the spectral and spatial sensing capabilities of rare-earth ion doped crystals to perform correlative signal processing. The processed results are extracted using a highly-coherent, actively-stabilized low - power frequency-swept laser, allowing broadband information to be detected and digitized at low bandwidths with high dynamic range. Time-of-flight resolution of ∼450 ps, signal-to-noise ratios of > 40 dB and up to 1us delays are demonstrated using 8 ms long waveforms. The optical noise waveforms are created by modulating RF noise onto the carrier, using up to 6 Gbps pseudo noise coding and 2 GHz analog noise waveforms.

Real-time wideband optical processing of S-and X-band signals for advanced radar systems

Real-time, wideband (>1 GHz) analog signal processing is experimentally demonstrated utilizing electro-optical devices and spectrally selective optical materials. Broadband signals are processed directly at S-band and X-band carrier frequencies without any down conversion.