High power and high focusing CWCO2 laser using anunstable resonator with a phase-unifying output coupler

A high-power and high-focusing continuous-wave (CW) CO₂ laser with a novel unstable resonator was developed. This resonator contains a step-wise variable reflecting output coupler named the phase-unifying output coupler and a total reflector. The laser was excited by a capacitive-barriered AC discharge called a silent discharge. A linearly polarized laser beam with a diffraction-limited divergence angle of 0.55 mrad was obtained. Very stable CW laser operation at an output power of 5 kW was achieved with a threshold discharge power of 10 kW and a slope efficiency of 18%     

Limiter-discriminator detection of a coherent optical heterodyneM-ary CPFSK receiver

The performance of a coherent optical M-ary continuous-phase frequency-shift-keying (CPFSK) receiver using limiter-discriminator (L-D) detection is investigated. It is shown that L-D detection of CPFSK optical signals offers the best performance for a large normalized IF beat spectral linewidth, ΔνT. When the modulation index is unity, the receiver is immune to laser phase noise and can produce (M/4) exp (-SNR) symbol error probability, which may be considered as the upper bound if the optimal modulation index is used (SNR is the signal-to-noise ratio per symbol). Optimum modulation indexes are 0.8 and 1 at ΔνT=1% and ΔνT=2%, respectively, for M=4, 8, and 16     

Performance parameters of a wavelength-division demultiplexer madewith a single 3×3 coupler optical fiber ring or loopresonator

The performance parameters of a two-channel demultiplexer made with a single 3×3 coupler fiber ring or loop resonator are evaluated theoretically. These parameters include crosstalk, output intensity of the selected channel, and channel separation. Their dependence on the parameters of the coupler and the fiber such as the coupling length of the coupler, kd, the phase change of the fiber delay line, &thetas;, and the round-trip amplitude transmission coefficient of the fiber ring or loop G, which is the product of the amplitude transmission coefficients of the coupler and the fiber delay line and the amplitude gain of the degenerate two-wave mixing, is investigated. It is found that the best performance can be obtained at G=1     

Design of coded CPFSK modulation systems for bandwidth and energyefficiency

Consideration is given to the problems related to the design of M-ary continuous-phase frequency-shift keying (CPFSK) systems with modulation index h=J/M, combined with eternal rate r binary convolution encoders. The following questions are raised and answered: (1) how should different encoder-modulator systems be compared and how can comparable systems be recognized from the system parameters, i.e. M, h, and r?; (2) what are the limits on the information rate per unit bandwidth, versus signal-to-noise ratio, when reliable transmission is required?; (3) how does one choose the system parameters M, h, and r when the overall system has to achieve a specified performance?; and (4) how does one design the external rate r binary convolutional encoder to put in front of the M-ary CPFSK modulation system with h=J/M ? A simple approximation for the bandwidth of a CPFSK signal is given and shown to be sufficiently accurate for system design purposes. The design of the external convolutional encoder is carried out in a novel way that leads to fewer states in the combined encoder-modulator system and thus yields improved performance for a given demodulation-decoding complexity compared to previous approaches for the design of coded CPFSK systems     

An EMQ-switched CO2 laser as a pump source for afar-infrared laser with a high peak power and a high repetitionrate

A modified current pulsed Q (EMQ)-switched CO₂ laser which is Q-switched by a mechanical beam chopper in combination with a pulsed discharge current is discussed. The laser produces a very stable output with a peak power greater than 1 kW at a repetition rate of 1000 p.p.s. for all transitions in the P and R branches of the CO₂ spectrum. A CH₃F laser pumped by the EMQ-switched laser produces 496 μm radiation in a 6.5 W peak, 100 ns pulses at 500 p.p.s. in the lowest loss EH₁₁ mode     

Bounds for abnormal binary codes with covering radius one

The normality of binary codes is studied. The minimum cardinality of a binary code of length n with covering radius R is denoted by K(n,R). It is assumed that C is an (n,M)R code, that is, a binary code of length n with M codewords and covering radius R. It is shown that if C is an (n,M)1 code, then it is easy to find a normal (n ,M)1 code by changing C in a suitable way, and that all the optimal (n,M)1 codes (i.e. those for which M=K(n,1)) are normal and their every coordinate is acceptable. It is shown that if C is an abnormal (n,M) code, then n⩾9, and an abnormal (9118)1 code which is the smallest abnormal code known at present, is constructed. Lower bounds on the minimum cardinality of a binary abnormal code of length n with covering radius 1 are derived, and it is shown that if an (n,M)1 code is abnormal, then M⩾96     

Analysis of N×N passive optical starcoupler based on the normal modes of N input waveguides

An integrated passive N×N optical star coupler on silicon wafer is described. Antiresonant reflecting optical waveguides (ARROWs) are analyzed and utilized as the input and output waveguides of the N×N coupler. Combining the exact solutions of the slab ARROW waveguide with the effective index method, a 5×5 coupler is analyzed. In the slab waveguide analysis, the input waveguides are coupled to their neighbors. The interaction of the waveguides is described in terms of the normal modes of propagation. The resultant field distribution is then diffracted into the free space region which separates the input and output sections. The radiation illuminates the receiving aperture from which the receiving N waveguides branch out, each output element obtaining equal power levels. Different types of loss such as spillover loss and mismatch loss were analyzed and estimated for N=5. A 5×5 star coupler with a transmission efficiency of 56% at a wavelength of 1.3 μm is achievable     

Asymptotic results for partial concentrators

Various switching network construction advantageously use modules known as partial concentrators. A partial concentrator is an n-input, m-output, single-stage switching device in which each input has access to some but not all of the outputs. A partial concentrator is said to have capacity c, if, for any k⩽c inputs, there exist k disjoint paths from the k inputs to some set of k outputs. Here, capacity values achievable for large n when each input has access to exactly M outputs, are considered. For a partial concentrator in which each input has access to exactly M outputs, it is shown that the cost ratio can be made arbitrarily small for any fixed M⩾2. In addition, it is shown that the rate of decrease of the cost ratio with increasing n is logarithmic for M=2, and polynomial for M⩾3     

Velocity filters for multiple interference attenuation ingeophysical array data

The problem of velocity filtering a record of seismic data with the objective of extracting a desired signal by attenuating the coherent interferences traveling at different velocities is considered. A two-dimensional (N-input (N-M+1)-output) processing scheme is used where the (N-M+1) output traces are generated from the N-input traces by multichannel processing of overlapping subsets of M-input races. Each output is generated by using a vector of multichannel arrays filters designed to attenuate multiple coherent interference and random noise. The two-dimensional frequency-wavenumber expression corresponding to the proposed multiple-input-multiple-output processing scheme is derived so that it can be implemented using the two-dimensional fast Fourier transform. Two illustrative examples are included     

Staircase PWM: an uncomplicated and efficient modulation techniquefor AC motor drives

An optimized staircase PWM (pulse-width modulation) technique is presented. The fundamental voltage component (U₍₁₎) is proportional to the staircase amplitude (M). Values of U ₍₁₎ higher than 90% of the corresponding value for the nonmodulated waveform are attained for M=1. The staircase is not a sampled representation of a sine wave and the number of steps and the frequency ratio are selected for a desired output voltage quality. A criterion for evaluating the quality of the output voltage (the weighted relative harmonic content) is presented. The control logic is uncomplicated and implemented by inexpensive complementary metal-oxide semiconductor logic with high noise immunity and reliability. The results have been verified by tests     

Output characteristics of a compact 1 J carbon dioxide laser with aGaussian reflectivity resonator

The authors describe the performance of a compact (41-cm-long) e-beam sustained atmospheric pressure CO₂ laser using a Gaussian reflectivity mirror in an unstable resonator configuration. The device operated on a single longitudinal and transverse mode (W _1L=12.9 mm) at an output energy of around 1 J. The frequency characteristics were studied and acoustic effects were observed as a result of the near-threshold operation     

Continuous wave, Q-switched and mode-locked operation of alaser-pumped Nd:LNA laser

A linear cavity Nd:LNA laser pumped by a Ti:sapphire laser was investigated in a continuous wave (CW), Q-switched and mode-locked regime. A parallel study using a Nd:YAG crystal was also conducted using the same optics and intercavity elements. The slope efficiencies obtained with a 5% output coupler in CW operation are 41 and 55% for Nd:LNA (λ~1.054 nm) and Nd:YAG (λ~1.065 nm), respectively. Mode locking was achieved with a 360-MHz intracavity phase modulator and led to a relative improvement of the Nd:LNA versus the Nd:YAG device. In the Q-switched mode with an intracavity acoustooptic modulator, the performance of the two lasers is almost identical. With 300 mW of absorbed pump power at λ~800 nm, peak powers ~130 W were achieved in a 65-ns pulse with a 5% output coupler     

Optimization of an injection-controlled excimer laser guided byanalytical modeling

The application of an analytical model describing the injection control of pulsed laser systems is successfully demonstrated for the design of a scaled XeF(C→A) excimer laser system. Enhancements to an earlier version o the model which improve the treatment of spatial beam overlap and saturation, unpumped volume, intracavity losses, and a noninteger number of roundtrips in the unstable resonator are described. These result in the accurate simulation of injection-controlled laser performance over a wide range of unstable resonator magnifications, mirror spacings, and intracavity optical losses. Excellent agreement between calculated and experimentally observed energies and temporal profiles of the injection-controlled laser output was obtained     

Markov-modulated flow model for the output queues of a packetswitch

The output queues of an M×N packet switch are studied using a Markov-modulated flow model. The switching element is a central server which sequentially routes packets from the inputs to the outputs. The focus is on systems in which the server speed is such that the bulk of the queuing takes place in the output queues. The conventional point process approach neglects the impact of switching and transmission time. An attempt is made to account for these finite system speeds by using a Markov-modulated continuous flow to approximate the arrival process to an output queue. This model captures the dependency between arrivals at different outputs and reflects the fact that packet arrivals and departures are not instantaneous. The output queue content distribution is obtained, for both infinite and finite buffer systems, from the spectral expansion of the solution of a system of differential equations. Numerical examples and comparisons with the results of an M/M/1 approximation are presented     

On linear structure and phase rotation invariant properties ofblock M-PSK modulation codes

Two important structural properties of block M(=2^' )-ary PSK modulation codes, linear structure and phase symmetry, are investigated. An M-ary modulation code is first represented as a code with symbols from the integer group S_M-PSK=(0,1,2,---,M-1) under modulo-M addition. Then the linear structure of block M-PSK modulation codes over S_M-PSK with respect to modulo- M vector addition is defined, and conditions are derived under which a block M-PSK modulation code is linear. Once the linear structure is developed, the phase symmetry of block M-PSK modulation codes is studied. In particular, a necessary and sufficient condition for a block M-PSK modulation code that is linear as a binary code to be invariant under 2^h/180°M phase rotation, for 1⩽h⩽l is derived. Finally, a list of short 8-PSK and 16-PSK modulation codes is given, together with their linear structure and the smallest phase rotation for which a code is invariant     

Design and analysis of a contention-based lookahead reservationprotocol on a multichannel local area network

The contention-based lookahead reservation (CLAR) protocol can provide fast circuit-switching services that are particularly advantageous for networks supporting integrated services. The delay and throughput performance for message transmission are obtained, and they agree closely with that obtained by simulation. The delay performance of CLAR is similar to that of the M-CSMA protocol for an M-channel network, but only CLAR can give a stable maximum throughput of (M-1)/M independent of the cable length. Moreover, CLAR requires only two sets of transceivers, while M-CSMA requires M. The lookahead reservation technique can provide 9% throughput increase for fixed-size messages and 19% for geometrically distributed messages     

A fast algorithm for computing distance spectrum of convolutionalcodes

A fast algorithm for searching a tree (FAST) is presented for computing the distance spectrum of convolutional codes. The distance profile of a code is used to limit substantially the error patterns that have to be searched. The algorithm can easily be modified to determine the number of nonzero information bits of an incorrect path as well as the length of an error event. For testing systematic codes, a faster version of the algorithm is given. FAST is much faster than the standard bidirectional search. On a microVAX, d_∞=27 was verified for a rate R=1/2, memory M=25 code in 37 s of CPU time. Extensive tables of rate R=1/2 encoders are given. Several of the listed encoders have distance spectra superior to those of any previously known codes of the same rate and memory. A conjecture than an R=1/2 systematic convolutional code of memory 2M will perform as well as a nonsystematic convolutional code of memory M is given strong support     

Asymptotic performance of M-ary orthogonal signals inworst case partial-band interference and Rayleigh fading

It is shown that for worst-case partial-band jamming, the error probability performance (for fixed E_b/N_I) becomes worse with increasing M for (M>16). The asymptotic probability-of-error is not zero for any E_b/N _I(>ln 2), but decreases inverse linearly with respect to it. In the fading case, the error-probability performance (for fixed E_b/N₀) improves with M for noncoherent detection, but worsens with M for coherent detection. For large E_b/N₀ the performance of the Rayleigh fading channel asymptotically approaches the same limit as the worst case partial-band jammed channel. However, for values of M at least up to 4096, the partial-band jammed channel does better. While it is unlikely that an M-ary orthogonal signal set with M>1024 will be used in a practical situation, these results suggest an important theoretical problem; namely, what signal set achieves reliable communication     

Fast cosine transform of Toeplitz matrices, algorithm andapplications

A fast algorithm for the discrete cosine transform (DCT) of a Toeplitz matrix of order N is derived. Only O(N log N)+O(M) time is needed for the computation of M elements. The storage requirement is O(N). The method carries over to other transforms (DFT, DST) and to Hankel or circulant matrices. Some applications of the algorithm are discussed     

High-power CO2 laser using Gauss-core resonator for 6-kWlarge-volume TEM00 mode operation

A novel resonator based on a stable resonator configuration designed to yield a highly focusing beam operating in a large-volume TEM ₀₀ is presented. An output coupler of the new stable resonator has a circular partial-reflection region in the center that is surrounded by an antireflection region. Main-beam mode is determined by the partial-reflection region existing in the “core” of the output coupler. A diffraction beam spread from the center of the optical path in the cavity is amplified by a laser medium. By combining part of the core beam transmitted from the partial-reflection region with the amplified diffraction beam under the phase-unified condition, a diffraction-limited laser beam having a large diameter is obtained. A 6.2-kW linearly polarized output beam with the M² factor of 1.7 is obtained experimentally for a high-power CW CO₂ laser. In laser-materials processing, we can cut 1:mm thin mild steel with a cutting speed as high as 64 m/min, at 5.9 kW