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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Multigigahertz range-Doppler correlative processing in crystals

Todd L. Harris, Wm. Randall Babbitt, K.D. Merkel, R. Mohan, Zachary Cole, Andy V. Olson

Published Jun 17, 2004• 1 citation
Publisher

Abstract

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.

DOI
10.1117/12.542431

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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.
I experimentally demonstrated a high bandwidth spatial-spectral holographic (SSH) scanner. Scanners or true time delay lines find their applications in phased-array antennas, radar range- Doppler processing and time-frequency ambiguity function analysis. A typical example of such a device is an acousto-optic deflector (AOD), which has limited bandwidth due to Bragg match conditions, frequency dependent acoustic attenuation of available materials and limitations of piezoelectric transducer technologies.\nThe system proposed in this thesis breaks through the bandwidth limitation of acousto-optic technology, yet resembles the function of an AOD since both operates as a scrolling scanner. It uses a material with large inhomogeneous bandwidth to record space-dependent time-delays as spatial spectral holograms. The recording of the spatial-spectral holograms utilizes a Galvo scanning (GS) mirror and a chirped laser.\nIn Chapter 2, I experimentally show that a GS mirror can be sufficiently stable for the holographic recording process. After reviewing the relevant physics of the spatial-temporal holographic recording medium, the cryogenically-cooled rare earth doped crystals, in Chapter 3, I give further derivations that are useful in explaining the subsequent experimental results. Chapter 4 describes an efficient and stable numerical scheme for simulating the coherent light-atom interaction in a two dimensional inhomogeneously-broadened crystal, allowing a search for the optimum experimental geometry for the recording experiment. Chapter 5 integrates the Galvo scanning mirror with the Tm3+:YAG crystal, and gives the experimental demonstration of the ¯first high bandwidth (1:5GHz bandwidth with 20 resolvable spots) spatial-spectral holographic scanner. This system uses one laser for the proof-of concept experiment. Finally, in Chapter 6, I explore the prospect for the future development of the high bandwidth SSH scanner. This chapter also gives the design and demonstration of a two-laser stabilization circuit, with which we can extend our ability to realize the full version of the high bandwidth SSH scanner system.
Summary form only given. When phased array antenna systems operate with broadband (multiple gigahertz) signals, true time-delay devices are needed to solve the problem of beam squint. Optical coherent transient technology, also known as spatial-spectral holography, offers a unique way of using the space and frequency dimensions of inhomogeneously broadened absorbers to handle thousands of high-resolution delays in a compact volume. We demonstrated broadband true-time delay in a Tm/sup 3+/:YAG crystal, which offers delays up to a microsecond on signals having several tens of GHz bandwidth with sub-picosecond delay resolution.
Unique spectroscopic properties of isolated rare earth ions in solids offer optical linewidths rivaling those of trapped single atoms and enable a variety of recent applications. We design rare-earth-doped crystals, ceramics, and fibers with persistent or transient "spectral hole" recording properties for applications including high-bandwidth optical signal processing where light and our solids replace the high-bandwidth portion of the electronics; quantum cryptography and information science including the goal of storage and recall of single photons; and medical imaging technology for the 700-900 nm therapeutic window. Ease of optically manipulating rare-earth ions in solids enables capturing complex spectral information in 105 to 108 frequency bins. Combining spatial holography and spectral hole burning provides a capability for processing high-bandwidth RF and optical signals with sub-MHz spectral resolution and bandwidths of tens to hundreds of GHz for applications including range-Doppler radar and high bandwidth RF spectral analysis. Simply stated, one can think of these crystals as holographic recording media capable of distinguishing up to 108 different colors. Ultra-narrow spectral holes also serve as a vibration-insensitive sub-kHz frequency reference for laser frequency stabilization to a part in 1013 over tens of milliseconds. The unusual properties and applications of spectral hole burning of rare earth ions in optical materials are reviewed. Experimental results on the promising Tm3+:LiNbO3 material system are presented and discussed for medical imaging applications. Finally, a new application of these materials as dynamic optical filters for laser noise suppression is discussed along with experimental demonstrations and theoretical modeling of the process.
We report on the experimental demonstration of a multi-gigahertz bandwidth RF spectrum analyzer exhibiting a resolution below 20 MHz, based on spectral hole burning in a rare-earth ion-doped crystal. To be compatible with demanding real-time spectrum monitoring applications, our demonstrator is designed to reach a high time resolution. For this purpose, we implemented the so-called “rainbow” architecture in which the spectral components of the incoming signal are angularly separated by the crystal, and are then acquired with a pixelated photodetector. The Tm3+:YAG crystal is programmed with a semiconductor DFB laser which frequency scan is servo-controlled and synchronized with the angular scan of a resonant galvanometric mirror, while a high-speed camera is used to acquire the spectra. In the perspective of future implementation within a system, the crystal is cooled below 4 K with a closed-cycle cryostat. With this setup, we have been able to monitor and record the spectrum of complex microwave signals over an instantaneous bandwidth above 20 GHz, with a time resolution below 100 μs, 400 resolvable frequency components and a probability of intercept of 100 %.
We report on the experimental demonstration of a multi-gigahertz bandwidth RF spectrum analyzer based on spectral hole burning in a 3 K-cooled rare-earth ion-doped crystal. We implemented the so-called “rainbow” architecture in which the optically carried spectral components of the incoming signal are angularly separated by the crystal, and are then acquired with a pixelated photo-detector. With this setup, we have been able to monitor and record the spectrum of complex microwave signals over an instantaneous bandwidth above 20 GHz, with a time resolution below 100 μs, 400 resolvable frequency components and a 100% probability of intercept. RF pulsed signals in the μs range are perfectly analyzed with this high time-resolved set-up. The best achievable sensitivity for pulsed signals is computed and compared with another spectral hole burning technique.
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.