W.R. Babbitt

CHIRPED-PULSE PROGRAMMING OF OPTICAL COHERENT TRANSIENT TRUE-TIME DELAYS

Programming an optical coherent transient true-time delay device with two frequency-chirped pulses provides a novel means of performing broadband (> >GHz) true-time delay with a wide dynamic range of delays with fine temporal resolution. We have demonstrated true-time delays exceeding 2micros with sub-100-ps resolution. Chirped-pulse programming has the advantages over the previously proposed brief pulse programming [Opt. Lett. 21 1102 (1996)] of reduced instantaneous power requirements and the ability to control the true-time delay by frequency shifting the programming pulses.

Optical pulse shaping using optical coherent transients

Using multiple temporally-overlapped, frequency offset and phase-tuned, linear frequency chirps, a new method of multi-GHz optical coherent transient optical pulse shaping and processing in inhomogeneously broadened rare-earth doped crystals is proposed. Using this technique with properly chirped laser sources, multi-GHz processing can be controlled with conventional low-bandwidth electronics and optical modulators. Specifically, this technique enables pulse shaping in the MHz to THz frequency regime with time-bandwidth-products exceeding 100,000, filling the gap between the operating regimes of femtosecond pulse shaping and analog electronics. The low bandwidth (~20 MHz) proof-of-concept demonstrations presented in this paper include pulse train creation, self-convolution, auto-correlation, and chirped pulse compression.

Optical coherent transient continuously programmed continuous processor

A novel technique for continuously programming an optical coherent transient spatial-spectral signal processor is proposed. The repeated application of two spatially distinct optical programming pulses to a nonpersistent hole-burning material writes an accumulated spatial-spectral population grating. An optical data stream is introduced on a third beam, resulting in a processor output signal that is spatially distinct from all the input pulses. Programming and processing take place simultaneously, asynchronously, and continuously. In the case of true-time delays, the efficiency that is achievable with currently available materials is of the order of that predicted for a perfect photon-gated device.

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

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.

RF spectrum analysis in spectral hole burning media

We demonstrate an RF spectrum analyzer based on spectral-hole burning (SHB) that operates with unity probability of intercept and resolution under 100 kHz. An SHB crystal, which consists of rare-earth ions doped into a crystal host, records the power spectrum of an RF signal modulated onto an optical carrier as a series of spectral holes that persist for about 10 ms. While the crystals homogeneous and inhomogeneous linewidths place the fundamental limits on resolution and bandwidth, respectively, the practical limits depend on the lasers used to interrogate the record stored in the crystals absorption profile. Up to now, SHB spectrum analyzers have used chirped beams from externally modulated, stabilized lasers, which have linewidths of under 10 kHz but cannot chirp over much more than octave bandwidths, or directly modulated diode lasers, which can chirp over more than 20GHz but have linewidths of about 1 MHz. Switching to chirped fiber lasers, which have natural linewidths of under 2 kHz and chirping linewidths on the order of 10 kHz, produces a measurement with fine resolution without any laser stabilization. In addition, by chirping the fiber laser with a sufficiently fast piezo, the resulting chirp could extend over tens of gigahertz in under 10 ms, yielding both fine resolution and broad bandwidth without extraordinary stabilization schemes.

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.

Demonstration of highly efficient photon echoes.

Highly efficient two-pulse and stimulated photon echoes are experimentally demonstrated in an absorbing medium. Recall efficiencies of 235% and 150% with excellent signal contrast are measured for two-pulse and stimulated photon echoes, respectively, in a Tm:YAG crystal with an absorption length of 3.8. The reported efficiencies do not include any correction for decay or beam profile. We believe that this is the first demonstration of an uncorrected echo efficiency greater than 100% with good signal fidelity in an optically thick medium.

Smart pixels with smart illumination

Smart pixels with smart illumination is a new concept in sensor array technology based on structured built-in illumination and optoelectronic feedback. It offers many new possibilities and potential advantages over more-traditional sensor arrays. We discuss an edge-detection system as an example of how smart illumination can advantageously be used to achieve a variety of functions. We also present initial experimental results from a fabricated chip based on this concept. The chip includes an integrated array of photodetectors and LEDs. The output of each pixel can be controlled based on the feedback received by its dual detectors.

Analytic solutions of the Maxwell–Bloch equations for high photon-echo efficiency of multiple pulse sequences

We derive a method for predicting the efficiency of stimulated photon echoes in optically thick media. Analytic solutions to the Maxwell–Bloch equations are derived for multiple pulse sequences in which the pulses are either temporally brief or have a low pulse area. This method retains the temporal and spatial information of the recalled waveform, and it provides a nonintensive computational method for determining the recall efficiency of stimulated photon echoes. The case considered is when the medium is optically thick and the recall efficiency is high, but with minimal distortion of the data. The results are in good agreement with a full space, time, and frequency numerical integration of the Maxwell–Bloch equations.

A model for optoelectronically interconnected smart pixel arrays

Theoretical results for modeling source-based smart pixel arrays are presented. Attention is focused on modeling the nonlinearities in sources, as well as electrical and optical couplings and interactions within a single pixel. A matrix formalism is used to generalize the theory to include the behavior of an entire smart pixel array consisting of interacting pixels with arbitrary optical and electrical interconnections. A vertical cavity surface emitting laser (VCSEL)-based smart pixel array is used for experimental demonstration of the application of the theory in the design and implementation of an optoelectronic flip-flop, where the nonlinear characteristics of the VCSELs are utilized in a positive feedback scheme to achieve bistability.