Ultrawideband coherent noise lidar range-Doppler imaging and signal processing by use of spatial-spectral holography in inhomogeneously broadened absorbers

Abstract

We introduce a new approach to coherent lidar range-Doppler sensing by utilizing random-noise illuminating waveforms and a quantum-optical, parallel sensor based on spatial-spectral holography (SSH) in a cryogenically cooled inhomogeneously broadened absorber (IBA) crystal. Interference between a reference signal and the lidar return in the spectrally selective absorption band of the IBA is used to sense the lidar returns and perform the front-end range-correlation signal processing. Modulating the reference by an array of Doppler compensating frequency shifts enables multichannel Doppler filtering. This SSH sensor performs much of the postdetection signal processing, increases the lidar system sensitivity through range-correlation gain before detection, and is capable of not only Doppler processing but also parallel multibeam reception using the high-spatial resolution of the IBA crystals. This approach permits the use of ultrawideband, high-power, random-noise, cw lasers as ranging waveforms in lidar systems instead of highly stabilized, injection-seeded, and amplified pulsed or modulated laser sources as required by most conventional coherent lidar systems. The capabilities of the IBA media for many tens of gigahertz bandwidth and resolution in the 30-300 kHz regime, while using either a pseudo-noise-coded waveform or just a high-power, noisy laser with a broad linewidth (e.g., a truly random noise lidar) may enable a new generation of improved lidar sensors and processors. Preliminary experimental demonstrations of lidar ranging and simulation on range-Doppler processing are presented.

Unambiguous range-Doppler LADAR processing using 2 giga-sample-per-second noise waveforms

“…The full array of Doppler bins is produced sequentially by stepping the reference beam frequency shift, which was 10 Hz, with AOM1 and repeating the programming/readout sequence at a 1 Hz experimental update rate. This process was repeated for 1000 Doppler bins giving a total experimental duration of $1000 s. However, given the massive spatial parallelism afforded by S2 materials, it is conceivable that this process could be performed in parallel as described by [Harris et al [5] and Li et al [13]], thus taking only a single integration period of 100 ms to perform the processing. Allowing dwell time between sequences allows the material memory to be restored, based on its population decay-after 100 ms of dwell time the material memory is restored by $86 dB.…”

“…For discussions on how such a LADAR system would perform on real targets, such as distant or low-crosssection targets, we refer the reader to Ref. [13]. The S2-based LADAR system can enable the use of a high-power, random-noise, broadband cw laser transmitters, defining a dramatically different paradigm for long range coherent LADAR systems.…”

“…The Doppler resolution is equal to the 3 dB full width of this profile. Acousto-optic techniques can be employed to create an array of frequency shifted references for multi-channel Doppler filtering and the reader is referred to the following references for these approaches as well as more in-depth discussions and treatises on Doppler processing [5,13].…”

“…This approach can actively process arbitrary analog wideband signals to determine delays between waveforms (range processing), as well as coherently integrate continuous or segmented waveforms to increase the SNR and to determine target velocities (Doppler processing). A detailed theoretical description of this approach is given by Li [13].…”

Broadband analog to digital conversion with spatial-spectral holography

“…Because this modified absorption spectrum persists for several milliseconds in typical SSH materials, readout can occur on a timescale much slower than the original highbandwidth recording event. Several microwave processing operations have been demonstrated utilizing slow readout of SSH materials, including radar signal processing, noise lidar, and spectral analysis [10][11][12][13][14][15]. One example material for SSH-ADC is Er 3+ :LiNbO 3 , which has a 200 GHz inhomogeneous linewidth and a homogeneous linewidth of a few kHz at 1.5 K in a 5 kG magnetic field [8], enabling direct capture of broadband signals with time-bandwidth products up to 10 8 .…”