Wavelength tunable CW Raman fibre ring laser operating at 1486-1551nm

A continuous-wave, wavelength tunable Raman fibre ring laser has been demonstrated operating at 1485-1551 nm with peak output power of over 400 mW, pumped at 1420 nm by a 1.58 W fibre laser     

Estimation of co-channel signals with linear complexity

To increase wireless system capacity using co-channel signals and multiple receiver antennas, we develop the partitioned Viterbi algorithm (PVA). The PVA estimation complexity increases linearly with each additional co-channel signal rather than exponentially as it does with joint maximum-likelihood sequence estimation (MLSE). The estimation problem involves multiple signals simultaneously transmitted and observed through slow-fading, frequency-selective channels. Although transmission is assumed to be in bursts according to a time-division multiple-access scheme, more than one signal can occupy the same time and frequency slot (these signals are referred to as “co-channel” signals). Separation and estimation of the symbol bursts is accomplished by exploiting channel differences, PVA estimation consists of a set of Viterbi detectors, one per signal, that operate in parallel with cross-coupling to allow approximate interference cancellation by means of tentative decisions. The forward filter of a decision feedback equalizer (DFE) is used to “prefilter” received signals prior to PVA estimation. Prefiltering delays the energy of interfering signals so that tentative decisions become reliable enough to use. Simulation results show PVA performance remains near-optimal with respect to the performance of joint MLSE     

On-Line Error Detection for Bit-Serial Multipliers in GF(2m)

In this paper error detection is applied to four finite field bit-serial multipliers. It is shown that by using parity prediction, on-line error detection can be incorporated into these multipliers with very low hardware overheads. These hardware overheads are generally independent of m and comprise only a handful of gates, so for large values of m these overheads are particularly low. The fault coverage of the presented structures has been investigated by simulation experiment and shown to range between 90% and 94.3%.     

Large modulation depth, single-moded quantum well waveguide modulator operating around 1.57 mu m

The performance of long wavelength single-mode waveguide modulators suitable for monolithic integration with a quantum well laser is reported. The device operated between 1.560 mu m and 1.570 mu m. The guiding layer was formed by a 0.27 mu m quaternary layer at the centre of which were four 50 AA In/sub 0.53/Ga/sub 0.47/As quantum wells separated by 100 AA InP barrier layers. Lateral confinement was obtained by etching ridges in the top InP contact layer. For devices of 500 mu m length, a modulation depth of 19 dB was obtained at a wavelength of 1.568 mu m with a reverse bias voltage of only 3 V and an internal loss of 2.5 dB.<>     

High frequency GaAs switched capacitor filter implemented with GaAs insulated gate FET switches

A switched capacitor filter implemented with 5 mu m GaAs IGFET switches and GaAs MESFET operational amplifiers is presented. The circuit is clocked at 25 MHz. By scaling to 1 mu m IGFETs, a switching speed of about 625 MHz should be attainable. Use of GaAs IGFET switches is shown to greatly reduce power consumption and complexity of the circuit. The low frequency switching instability of the GaAs IGFET is shown to be of no consequence in this application.<>     

On the polynomial residue number system [digital signal processing]

The theory of the polynomial residue number system (PRNS), a system in which totally parallel polynomial multiplication can be achieved provided that the arithmetic takes place in some carefully chosen ring, is examined. Such a system is defined by a mapping which maps the problem of multiplication of two polynomials onto a completely parallel scheme where the mapped polynomial coefficients are multiplied pairwise. The properties of the mapping and the rules of operations in the PRNS are proven. Choices of the rings for which the PRNS is defined are also studied. It is concluded that the PRNS can offer significant advantages in those digital signal processing (DSP) applications that involve multiplication-intensive algorithms like convolutions and one-dimensional or multidimensional correlation     

Millisecond pulsars: nature's most stable clocks

The author describes the role pulsars might play in time and frequency technology. Millisecond pulsars are rapidly rotating neutron stars: some 20 km in diameter, 1.4 times as massive as the Sun, and spinning as fast as several thousand radians per second. Radio noise generated in a pulsar's magnetosphere by a highly beamed process is detectable over interstellar distances, as a periodic sequence of pulses. High-precision comparisons between pulsar time and terrestrial atomic time show that over intervals of several years, some millisecond pulsars have fractional stabilities comparable to those of the best atomic clocks. The author briefly reviews the physics of pulsars, discusses the techniques of pulsar timing measurements, and summarizes the results of careful studies of pulsar stabilities     

Intensity noise in long-wavelength superluminescent sources

Noise in broadband 1.3-μm superluminescent diodes (SLDs) is investigated experimentally, using a balanced detector arrangement to determine the excess noise factor as a function of photodetector current. Measurements were made in both the low-frequency 1/f, regime (<500 kHz) and the high-frequency quantum noise spectral region. The data at higher frequencies are in agreement with predictions of the quantum amplifier model, with values of the spontaneous emission coupling factors ranging from 1.2 to 1.9. It is also found that noise for one polarization of the light is uncorrelated with the noise for the orthogonal polarization over the 0-1 MHz frequency range. This implies that the 1/f, noise is not related to carrier density (gain) fluctuations in the active region of the device. An integrated optic chip design to compensate for the excess intensity noise in fiber gyroscopes is proposed     

Microwave signal generation using an optical self-heterodyne technique

A new technique for mixing optical waves to generate microwave-frequency signals using an injection laser diode and a fibre-optic interferometer is reported. A beat signal is produced by interfering light from the laser with light from the same laser emitted earlier at a different frequency. The interferometer consists of an evanescent-field fibre coupler joined to a length of single-mode optical fibre. The laser is tuned by pulsed or bipolar current waveforms superimposed on a DC bias current. Feedback from an external cavity stabilises the laser frequency and reduces its linewidth. Efficient mixing at difference frequencies from 250 MHz to 2 GHz is obtained, with spectral widths of less than 1% of the centre frequency.     

𝒬-switched neodymium-doped phosphate glass fiber lasers

The operation of a short-pulse, 𝒬-switched, neodymium-doped fiber laser operating at 1.054 μm is described experimentally and theoretically. The laser is efficiently pumped with a single-stripe AlGaAs laser diode and emits >1 kW pulses. It is seen that due to high gain short pulses with high energy extraction efficiency can be obtained. The feature of broad emission lines associated with rare-earth-doped glasses is exploited to demonstrate tunable, 𝒬-switched operation over a 40-nm tuning range