AlGaAs vertical-cavity surface-emitting laser responses to 4.5-MeV proton irradiation

We have irradiated single- and multimode AlGaAs vertical-cavity surface-emitting laser (VCSEL) arrays operating at a nominal wavelength of 780 nm with 4.5-MeV protons and doses ranging from 10 to 30 Mrad in the active region. We observed a peak power reduction of about 2% per Mrad in the 14-/spl mu/m aperture, multimode VCSELs. Single-mode VCSELs having an aperture of 6 /spl mu/m exhibited a smaller peak power reduction of 0.4%-1% per Mrad. A slight shift in the current threshold was observed only for the multimode VCSELs at dose levels above 10 Mrad. First results indicate a reduced VCSEL peak laser power output that is dominated by a temperature shift caused by the radiation induced increase in resistive heating. In contrast, the power reduction in edge-emitting lasers is dominated by the enhanced radiation induced nonradiative recombination rate. The VCSEL irradiation was performed with a focused ion micro beam that was rastered over the device surface, ensuring a very uniform exposure of a single device in the array.     

The use of atmospheric pressure MOVPE for the growth of high performance uncooled 1300 nm DFB lasers

Manufacture of high performance uncooled 1300 nm distributed feed-back (DFB) lasers operating single mode over the −40 to +85°C range requires control of the wavelength variation across a 2″ wafer to less than 10nm and preservation of grating definition during processing and regrowth. We have used atmospheric pressure metalorganic vapor phase epitaxy, without substrate rotation to achieve the necessary uniformity. Material was assessed using photoluminescence, x-ray diffraction, transmission electron microscopy, electrochemical current/voltage profiling, and secondary ion mass spectroscopy. The devices are based on a strained quantum well structure with an n-type grating layer to provide gain coupling. The best result gave a wavelength spread across 32×32 mm center square of a 2″ InP wafer of 3 nm. Buried heterostructure DFBs manufactured with high yield in this way operate from −40 to +85°C, with thresholds at 85°C as low as 18 mA.     

Multilevel differential polarization shift keying

We analyze the performance of coherent optical communication systems which employ polarization shift keying (PolSK) modulation, introducing for the first time the requirements for multilevel, differential PolSK. For M-ary signal constellations with M>2, the three-dimensional (3-D) nature of the signal constellations within Stokes space requires the use of double-differential modulation and detection (M-DDPolSK). Coding constraints on the signals place an additional restriction on such systems. We present detailed analysis and simulations for a 6-DDPolSK modulation scheme in which the signal points are located at the vertices of an octahedron in Stokes space     

Generating, capturing and processing supply current signatures from analogue macros in mixed-signal ASICs

Transient Response Testing is a powerful test technique for analogue macros in mixed-signal electronic systems which with some enhancement can be particularly useful for testing deeply buried circuit structures. Supply current testing is finding widespread application in the digital domain and its use in the analogue domain may lead to integrated test methodologies for mixed-signal systems. This paper shows that by utilizing both these techniques, and a low-cost test shell, deeply buried analogue macros can be partitioned, tested using Transient Response Testing and the resulting response accurately captured from the total device supply current. It also contains an analysis of the noise on the supply current, due to digital circuit activity during testing, and demonstrates a test response analysis technique which is insensitive to it.     

Finite field inversion over the dual basis

In this transaction brief we consider the design of dual basis inversion circuits for GF(2^m). Two architectures are presented-one bit-serial and one bit-parallel-both of which are based on Fermat's theorem. Finite field inverters based on Fermat's theorem have previously been presented which operate over the normal basis and the polynomial basis. However there are two advantages to be gained by forcing inversion circuits to operate over the dual basis. First, these inversion circuits can be utilized in circuits using hardware efficient dual basis multipliers without any extra basis converters. And second, the inversion circuits themselves can take advantage of dual basis multipliers, thus reducing their own hardware levels. As both these approaches require squaring in a finite field to take place, a theorem is presented which allows circuits to be easily designed to carry out squaring over the dual basis     

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.<>     

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     

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