Optimization of Q-switched lasers

The authors extend the standard rate equation analysis to obtain expressions for the maximum peak power, maximum pulse energy, and minimum pulsewidth of a single Q-switched output pulse; the maximum power efficiency of a repetitively Q-switched laser; and the corresponding cavity output couplings. Results are obtained analytically and numerically, and a comparison of the two sets of results is made. As a first step in this process the authors derive general expressions for the peak power, pulsewidth, pulse energy, and power efficiency. The authors next differentiate these expressions in order to find the maxima or minima that optimize the parameter of interest. Differentiation is done with respect to the cavity output coupling     

Data reduction method for Q measurements of strip-lineresonators

A technique that enables the attenuation constant α of a microstrip line to be found from the measured return loss curve as a function of frequency for a half-wavelength resonator is described. Data averaging is incorporated and the effects of dispersion are included. This method does not require the use of an equivalent resonator model for the stripline, nor does it rely on graphical techniques     

Integrated Q-switched multiple-cavity glass waveguidelaser

A novel Q-switching scheme, using rapid variation of the path difference between the cavities of a multiple-cavity resonator, is described. A thermooptic phase modulator was used to switch the cavity loss of a Y-junction glass waveguide laser between high and low states. Q-switched pulses with durations of 5 μs and peak powers of 70 mW were obtained     

Acoustooptic Q-switching of erbium lasers

Experiments were performed on Q switching of erbium lasers at 3 μm with an acoustooptical modulator. Different laser crystals of YAG:Er and YSGG:Cr:Er have been investigated. The highest fraction of single pulse Q-switch energy to free-running mode energy was 1%. Pulse durations of 50 ns were obtained. Multiple switching with up to 14 Q-switch peaks per flashlamp pulse could be generated, containing up to 19% of the energy of the free-running mode     

Robustness of mean E{X} and R charts

The effect of nonnormality on E{X} and R charts is reported. The effect of departure from normality can be examined by comparing the probabilities that E{X} and R lie outside their three-standard-deviation and two-standard-deviation control limits. Tukey's λ-family of symmetric distributions is used because it contains a wide spectrum of distributions with a variety of tail areas. The constants required to construct E{X} and R charts for the λ-family are computed. Control charts based on the assumption of normality give inaccurate results when the tails of the underlying distribution are thin or thick. The validity of the normality assumption is examined by using a numerical example     

I and Q decomposition of self-noise in square-lawclock regenerators

A method for obtaining power spectra for the self-noise components in phase and in quadrature to the desired generated clock signal for PAM systems is described. A previously recognized cross spectrum is also discussed. Results can be expressed either in a closed form or as the sum of rapidly convergent series, depending on the signalling waveform used. A computation of the in-phase and quadrature spectra for Nyquist pulses for excess bandwidth factors ranging from 0.1-0.9 is included, along with demonstration showing that for signalling waveforms with even or odd symmetry, cross power spectrum is zero at all frequencies. It is also shown that if the cross power spectrum is not zero due to asymmetry in the signalling waveform, sampling keyed to other than zero-crossing of the timing wave can give a lower timing jitter     

Short-pulse, high-power Q-switched fiber laser

A high-power, laser-diode-pumped, Q-switched fiber laser operating at 1.053 μm which is suitable for use in time-multiplexed fiber sensor applications is described. The laser emits >1-kW pulses at 1.053 μm with 2-ns duration at up to 1-kHz repetition rates for an adsorbed pump power of only 22 mW at 810 nm. Tunable Q-switched operation over a 40-nm wavelength range has also been demonstrated     

Analysis of E-H plane tee junction using avariational formulation

A three-port equivalent network for an E-H plane tee junction is determined taking into account the effect of waveguide wall thickness and considering the contribution of the dominant mode to the imaginary part of the self-reaction. The parameters of the three-port equivalent network are determined. From a knowledge of the equivalent network parameters, the net impedance loading, reflection coefficient, and coupling are evaluated for an E-H plane tee junction. A comparison between theoretical and experimental results is also presented     

Theory of the optimally coupled Q-switched laser

The general equations describing Q-switched laser operation are transcendental in nature and require numerical solutions, which greatly complicates the optimization of real devices. Here, it is shown that, using the mathematical technique of Lagrange multipliers, one can derive simple analytic expressions for all of the key parameters of the optimally coupled laser, i.e. one which uses an optimum reflector to obtain maximum laser efficiency for a given pump level. These parameters can all be expressed as functions of a single dimensionless variable z, defined as the ratio of the unsaturated small-signal gain to the dissipative (nonuseful) optical loss, multiplied by a few simple constants. Laser design tradeoff studies and performance projections can be accomplished quickly with the help of several graphs and a simple hand calculator. Sample calculations for a high-grain Nd:YAG and a low-gain alexandrite laser are presented as illustrations of the technique     

Light-to-input nonlinearities in an R-LED series network

The light-to-current (L-I) and light-to-voltage (L-V) differential nonlinearities in the simple network of a customary LED and an external resistor R in series are analyzed and calculated theoretically and compared with experimental data. Particular emphasis is placed on the influence of the log-arithmetic slope ν of the L-I characteristic and the bias current I upon the ratio of the corresponding nonlinearity parameters. It is thus deduced that, for a given optical power P, over superlinear portions of the L-I curve (ν>1) the L-I linearity is typically better than its corresponding L-V linearity. On the contrary, when the L-I dependence is sublinear (ν<1) the voltage driving scheme may ensure for the R-LED network, or the LED alone, a local L-V response much more linear than the L-I response, provided that appropriate (optimum) I and/or R values are chosen     

Electrochemical C-V profiling of heterojunctiondevice structures

Electrochemical capacitance-voltage profiles of multiple (In,Al) GaAs heterostructures have been measured and compared with the results of a numerical calculation of the apparent charge density based on a one-dimensional Poisson solver. The calculation, using layer thicknesses, dopings, and heterojunction band discontinuities obtained from MBE growth calibrations, is in overall agreement with the measured data. The largest discrepancy occurs between the expected and measured heterojunction band discontinuity. This difference is consistent with an electrolyte/semiconductor interface which is not planar on a scale comparable to the layer thickness     

Effects of axial nonuniformity in modeling Q-switchedlasers

Generic Q-switched laser pulses are calculated using a point model and a traveling wave model. Results indicate that the point model approach commonly used in rate equation modeling is inadequate for large initial inversions, large internal losses, or large fractional outcoupling. The point model typically overestimates peak power and energy and distorts the pulse shape. A simple traveling wave model is developed which easily describes these cases. The optimum outcoupling to maximize peak power varies significantly between the two models     

Transient M/M/1 queue variance computation using generalized Q functions

A generalized Q function representation for the transient M/M/1 queue variance is developed. The expression is highly accurate and computationally efficient, and an upper bound on the error is easily calculated. For a Q function relative error of 2×10-¹² using Parl's method, the relative error of the variance is generally less than 10^-9. Average execution time is of the order of 70 ms per point on a VAX 11/750, but faster execution times can be obtained by using larger Q function relative errors     

A Q-switched diode-pumped neodymium yttrium aluminumborate laser

A report is presented on the first Q-switched laser operation of a room temperature CW diode-pumped Nd:YAB laser. The intense peak fundamental power obtained by Q-switching significantly enhances the green light output from the Nd:YAB laser through the second harmonic conversion process. A peak green power exceeding 3 W was obtained with a CW diode laser input power of 180 mW, as compared to a green output power of only about 1 mW under pure CW operation. Nd:YAB shows considerable promise as a room temperature self-doubling material. Diode-laser-pumped Nd:YAB lasers show promise as compact, and reliable green sources for practical applications     

Parameter extraction from I-V characteristics ofsingle MOSFETs

A method is presented to extract the bias-dependent series resistances and intrinsic conductance factor of individual MOS transistors from measured I-V characteristics. If applied to groups of scaled channel length devices, it also allows determination of the effective channel length together with the transversal field dependence of the carrier mobility. The method is exactly derived from conventional MOS theory based on the gradual channel approximation, and the deviations from such an ideal case are studied by means of two-dimensional device simulations. Experimental results obtained with n- and p-channel transistors of conventional as well as LDD type are presented to show the correctness of the proposed extraction procedure     

Analysis of coplanar E-H planeT-junction using dissimilar rectangular waveguides

An analysis of a T-junction that differs from conventional H -plane T-junctions in that the T arm is rotated by 90° and coupling takes place through an inclined slot is presented. Since use of standard X-band waveguides results in such a T-junction operating above 11.7 GHz, nonstandard waveguide dimensions have been considered to bring down the operating frequency to 9.375 GHz. The effect of a change of the broad dimension of the primary feed waveguide on the resonant conductance is evaluated. The variations of resonant length with the angle of inclination of the slot, and coupling with frequency, are presented     

DC I-V characteristics of field emitter triodes

A simple model that is applicable to Spindt-type emitter triodes is presented. Experimentally, it has been observed that the gate current at zero collector voltage follows the same Fowler-Nordheim law as the collector current at high collector voltage, and that for low emission current densities, the sum of gate and collector currents is constant for any collector voltage and is given by the Fowler-Nordheim current I_FN. Based on these observations, a simple model has been developed to calculate the I-V characteristics of a triode. By measuring the Fowler-Nordheim emission, emission area and field enhancement can be obtained assuming a value for the barrier height. Incorporating the gate current, the collector current can be calculated from I_c=I_FN-I_g as a function of collector voltage. The model's accuracy is best at low current density. At higher emission currents, deviations occur at low collector voltages because the constancy of gate and collector currents is violated     

Nonlinear parasitics in MODFETs and MODFET I-Vcharacteristics

A large-signal analysis of the source and drain resistance of MODFETs is reported. Velocity saturation in the two-dimensional electron gas (2DEG) and hypothetical rectifying effects in the n⁺-AlGaAs-i-GaAs interface are accounted for. Rectifying effects are found to be either absent or negligible. Current limitations in the 2DEG lead to the observed compression of the transconductance at large gate voltages, and an improved fit of the MODFET I-V characteristics is demonstrated using an approximate analytic formulation of the current-limited parasitic resistance. The high-frequency dependence of the source and drain resistance is also reported. A decrease of the source impedance for frequencies increasing from 1-30 GHz is predicted and can reach 30%, depending on the device structure. Such a frequency decrease of the parasitics is consistent with the reported increase of the effective transconductance of MODFETs at microwave frequencies. The reported frequency and current-limited parasitic models rely on parameters that can either be measured or calculated and are therefore appropriate for CAD applications     

Effect of finite lower level lifetime on Q-switched lasers

Modeling of Q-switched laser operation for four-level lasers with finite lower-laser-level lifetime is presented. Extraction efficiency is reduced and pulse shape altered in comparison to the case of infinitely fast lower-level relaxation. This reduction in efficiency is dependent on the ratio of the lower state lifetime to the cavity lifetime, the number of times above threshold, and occupancy factors of the upper and lower laser manifolds. The model is applied to the case of a Nd:YAG laser operating at 1.064 μm. At worst, the extraction efficiency is 68% of that expected for infinitely fast relaxation     

A new method of determination of the I-Vcharacteristics of negative differential conductance devices

A method for mapping the complete I-V characteristic of a negative differential conductance (NDC) device has been investigated. This method employs the measurable positive differential conductance (PDC) portions of the DC I-V curve together with the measured conductances at a fixed DC bias voltage in the PDC region with different RF signal levels using a standard semiconductor analyzer. The NDC regime of the I-V curve is numerically constructed from the measured conductances at a fixed DC bias voltage in the PDC region with different signal levels using a large-signal nonlinear-circuit analysis