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Conference on Lasers and Electro-Optics

Broadband true time-delay with optical coherent transients

Ming Tian, Randy R. Reibel, William Randall Babbitt

Published Jan 1, 2001• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cleo.2001.947934

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High bandwidth scanner based on spatial-spectral holograms

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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.

•Jan 1, 2010•0 citations
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.
Summary form only given. Accurate true-time delays are essential to correctly steer modern phased-array antennas. Optical coherent transient technology has the ability to store a variable true-time delay as a spatial-spectral holographic population grating in an inhomogeneously broadened absorber (IBA). Typically, two optical pulses program the delay. A third optical pulse or waveform applied to the IBA generates a delayed replica of itself. The delay is typically defined by /spl tau//sub 21/=t/sub 2/-t/sub 1/, the arrival time difference of the first two pulses. However, we find that this prediction for the delay of the emitted signal only holds for input optical pulse areas much less than pi. Experimental investigation and simulations were performed to observe the variation in arrival time of the output signal.
Summary form only given. A practical device creating a variable RF true-time delay (TTD) for signals on an optical carrier would be useful in array antennas. An optical coherent transient (OCT)-TTD device could provide high-resolution delays (/spl sim/1 ps) over a wide range (/spl sim/1 /spl mu/s), for high bandwidth signals (/spl sim/10 GHz) with currently available materials. In previously demonstrated OCT-TTD devices, two optical pulses separated in time and space program a delay in an absorbing medium as a spatial-spectral grating on an inhomogeneously broadened transition (IBT). In the paper, we experimentally demonstrate widely tunable OCT-TTD of a continuous optical data stream without changing the timing between the programming pulses.
A novel technique for programming broadband true-time delays that uses two frequency-offset temporally overlapped linear frequency-chirped pulses to produce periodic spectral gratings in an inhomogeneously broadened absorber is presented. Advantages of this technique include its ability to use chirped pulses that are longer than the coherence time of the crystal, less stringent laser frequency-stability requirements for grating accumulation, lower power requirements, a simplified system design, and the ability to tune broadband (multigigahertz) delays over a wide dynamic range (picoseconds to microseconds).
A technique is proposed and experimentally demonstrated that achieves simultaneous optical pattern waveform storage and programmable time delay for continuous real-time signal processing by use of optical coherent transient technology. We achieve variable-time-delay and broadband signal processing by frequency shifting of two chirped programming pulses, the chirp rate of one being twice that of the other, without using brief reference pulses and without changing the timing of the programming sequence. We demonstrate the technique experimentally in Tm(3+): YAG at 5 K for 40-MHz chirps by performing temporal signal convolution with true-time delays that vary over a 250-ns range.
We have demonstrated a technique for programming variable time delays of a pattern waveform in a spectral grating. By frequency shifting the chirped programming pulses, correlation and delays over 450 ns were achieved without changing the programming sequence timing. The demonstrated bandwidth of 40 MHz only utilized /spl sim/0.2 % of the inhomogeneously broadened absorber's bandwidth. Thus, this technique has the potential to allow versatile, all-optical reception and processing of > 10 GHz signals in broadband array antenna beamforming.
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.