A fast-adaptive Huffman coding algorithm

The Huffman code in practice suffers from two problems: the prior knowledge of the probability distribution of the data source to be encoded is necessary, and the encoded data propagate errors. The first problem can be solved by adaptive coding, while the second problem can be partly solved by segmenting data into segments. However, the adaptive Huffman code performs badly when segmenting data into relatively small segments because of its relatively slow adaptability. A fast-adaptive coding algorithm which tracks the local data statistics more quickly, thus yielding better compression efficiency, is given     

PET: a proton/electron telescope for studies of magnetospheric,solar, and galactic particles

The proton/electron telescope (PET) on SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer) is designed to provide measurements of energetic electrons and light nuclei from solar, Galactic, and magnetospheric sources. PET is an all solid-state system that will measure the differential energy spectra of electrons from ~1 to ~30 MeV and H and He nuclei from ~20 to ~300 MeV/nucleon, with isotope resolution of H and He extending from ~20 to ~80 MeV/nucleon. As SAMPEX scans all local times and geomagnetic cutoffs over the course of its near-polar orbit, PET will characterize precipitating relativistic electron events during periods of declining solar activity, and it will examine whether the production rate of odd nitrogen and hydrogen molecules in the middle atmosphere by precipitating electrons is sufficient to affect O₃ depletion. In addition, PET will complement studies of the elemental and isotopic composition of energetic heavy (Z>2) nuclei on SAMPEX by providing measurements of H, He, and electrons. Finally, PET has limited capability to identify energetic positrons from potential natural and man-made sources     

High-power single-mode 2.0 μm laser diodes

Data are presented on high-power single-mode index-guided laser diodes fabricated from a strained-layer InGaAs-InGaAsP double quantum well heterostructure epitaxial design. The total maximum power and external efficiency achieved are 50 mW and 43%, respectively. The far-field is measured to be 31° by 46° in the parallel and perpendicular directions, yielding an aspect ratio of 1.5 for the single-mode laser diode. The optical output of the laser diode is a multi-longitudinal mode spectrum spanning 1.98-2.00 μm at an output power of 50 mW CW. The characteristic temperature of the laser diode is 48 K     

Increased linear dynamic range by low biasing the Mach-Zehndermodulator

Operating a Mach-Zehnder modulator at an optical bias below the conventional 50% (quadrature) bias is investigated. Theoretical distortion curves as a function of bias have been calculated and experimentally verified. These curves show that (single octave) linear dynamic range increases as the bias is lowered, so long as optical power at the detector can be maintained just below the saturation level or laser noise limit. This method thus provides a way for very large optical power to be used to benefit linear dynamic range. Theoretically, 20 dB of excess optical power, attenuated through low biasing, can result in an increase in linear dynamic range of 15 dB     

Analysis of laser pulse chirping in mode-locked vertical cavitysurface-emitting lasers

Mode-locked vertical cavity lasers have a large cross-sectional area and consequently a large saturation energy and large peak powers. The authors analyze excess optical bandwidth generation in these lasers and find that self-phase modulation due to optical pumping and gain saturation is the dominant factor in inducing laser pulse chirping. The large magnitude of the chirp makes intracavity prism-pair compensation difficult. Adjustment of the cavity length has a major impact on the pulse chirping, as observed experimentally. Proper adjustment can result in a large linear frequency chirp which can be compensated using external pulse compression techniques     

Competing effects of well-barrier hole burning and nonlinear gainon the resonance characteristics of quantum-well lasers

The effects of the quantum capture and release of carriers from quantum wells (QWs) on the resonance response of QW lasers are investigated from a model of well-barrier hole burning with built-in nonlinear gain. Significant similarities and contrasts with the conventional single-mode model are noted in both the large-signal transient behavior and in the small-signal resonance characteristics. The competition between well-barrier hole burning and nonlinear gain is explored by studying of time responses, phase portraits, frequency transfer functions; and contour maps of constant resonance frequency, damping rate, and 3-dB bandwidth in the parameter spaces defined by the nonlinear gain coefficient versus the ratio of relaxation times for capture and release of carriers by the wells. A systematic treatment of the well-barrier model is presented along with these predicted dynamical trends     

Simulated and observed backscatter at P-, L-, and C-bands fromponderosa pine stands

The authors compared the output of the Santa Barbara microwave canopy backscatter model to polarimetric synthetic aperture radar (SAR) data for three ponderosa pine stands (ST-2, ST-11, and SP-2) with discontinuous tree canopies near Mt. Shasta, California, at P-band (0.68-m wavelength), L-band (0.235-m wavelength), and C-band (0.056-m wavelength). Given the SAR data calibration uncertainty, the model made good predictions of the P-HH, P-VV, L-HH, C-HH, and C-HV backscatter for the three stands, and the P-HV and L-HV backscatter for ST-2 and SP-2. The model underestimated C-VV for the three stands, and P-HV, L-HV, and L-VV backscatter for ST-11. The observed and modeled VV-HH phase differences were ≃0° for the three stands at C-band and L-band, and for SP-2 at P-band. At P-band, the observed and modeled VV-HH phase differences were at least -80° for ST-2 and ST-11, which indicates that double-bounce scattering contributes to the total backscatter for the two stands     

A note on the PQ theorem and the extrapolation of signals

The problem of determining a bandlimited function from its values on a finite interval is ill-conditioned in the sense that although the pertinent inverse map exists, it is discontinuous at every point. Whenever certain closely related general problems are well conditioned in the sense that the inverse operator is continuous, they can be solved using a special case of a known algorithm. In particular, attention is directed to the relation between the PQ theorem, its Hilbert space projection-operator setting, and later work     

Assessment of coupled-mode theory for tapered optical coupler

The accuracy and the scope of validity of different coupled-mode formulations for a tapered optical waveguide coupler are assessed by comparison with the beam propagation method (BPM). It is demonstrated that the coupled-mode theory (CMT) based on local modes is valid only for small tilt angles, whereas an improved formulation that takes into account the wavefront-tilt effect yields highly accurate results for relatively large tilt angles. The radiation loss at the input and the output junctions can also be estimated by a simple mode-matching method combined with the improved CMT. The coherent coupling between two junctions,which is neglected in the coupled-mode formulations, is observed in the BPM simulations.<>