Alan Mar

All optical millimeter-wave electrical signal generation using an integrated mode-locked semiconductor ring laser and photodiode

The first monolithic photonic integrated circuit for all-optical generation of millimeter (mm)-wave electrical signals is reported. The design integrates a mode-locked semiconductor ring diode laser, an optical amplifier, and a high-speed photodetector into a single optical integrated circuit. Signal generation is demonstrated at frequencies of 30, 60, and 90 GHz.

Single transverse mode operation of electrically pumped vertical-external-cavity surface-emitting lasers with micromirrors

We report an electrically pumped vertical-external-cavity surface-emitting laser (VECSEL) that is designed for wafer-scale fabrication. Single-mode continuous-wave operation is demonstrated at a wavelength of 970 nm. The device structure incorporates a curved micromirror output coupler that is produced using a micromolding process. In addition to outlining the VECSEL fabrication process, we quantify its spatial and spectral modal characteristics.

Fiber-Optically Controlled Pulsed Power Switches

The development and testing of fiber-optically controlled trigger generators (TGs) based on high gain photoconductive semiconductor switches (PCSSs), constructed from high resistivity GaAs, are described in this paper. The TGs are optimized to trigger the high voltage switches (HVSs) in pulsed power systems, where they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future pulsed power systems are even more dependent on triggering, as they consist of many more HVS and, in some cases, produce shaped pulses by independent timing of the HVS. The goal of the PCSS TG is to improve timing precision and replace high voltage trigger cables or line-of-sight optics with fiber-optic trigger control. The PCSS trigger has independent EMP-free timing control via 200-mum-diameter optical fibers. This design is simpler than other TG because optical isolation allows PCSS triggers to be remotely located near the HVS at any voltage. PCSS can improve the performance of prime power HVS, diverters, and diagnostics by supplying trigger pulses with subnanosecond jitter and rise time that are more precise and easily adjusted than the conventional TG. For pulse-charged HVS, the PCSS TG can generally derive their trigger energy from the stray fields of the HVS. High gain PCSS capabilities for producing pulsed power TG have been demonstrated previously (not all simultaneously): 220 kV, 8 kA, 350-ps rise time, 100-ns pulsewidth, 50-ps rms jitter, and 10-kHz repetition rate. Furthermore, PCSS has previously triggered a 300-kV trigatron with 100-ps rms jitter.

HIGH-POWER MODE-LOCKED SEMICONDUCTOR LASERS USING FLARED WAVEGUIDES

We describe the use of flared waveguide diode lasers for obtaining increased output power under mode‐locked operation. The flared waveguide expands the optical mode from a narrow region which gives a single lateral optical mode, to a wider multimode region for higher pulse saturation energy. Flared gain and flared absorber section geometry devices are compared to devices with conventional uniform waveguides. Using flared gain section devices, improvements in both pulse energy (6.8 pJ) and pulsewidth (3.3 ps) were measured compared to uniform waveguide devices. Peak powers of over 2 W are obtained, which, to our knowledge, is the highest peak power obtained directly from mode‐locked single stripe diode lasers.

High-speed, low-threshold InGaAsP semi-insulating buried crescent lasers with 22 GHz bandwidth

The authors have fabricated high-speed, low-threshold 1.3 mu m InGaAsP semi-insulating buried crescent lasers with a CW 3 dB modulation bandwidth of 22 GHz and a threshold current as low as 6.5 mA at room temperature. This is the highest 3 dB modulation bandwidth ever reported for the planar-type semiconductor laser. These results were achieved by implementing a submicron photolithographic process in the channel etching to reduce the cavity width and a polyimide dielectric layer under the bonding pad area to minimize the electrical parasitics.<<ETX>>

Properties of high gain GaAs switches for pulsed power applications

High gain GaAs photoconductive semiconductor switches (PCSS) are being used in a variety of electrical and optical short pulse applications. The highest power application, which we are developing, is a compact, repetitive, short pulse linear induction accelerator. The array of PCSS, which drive the accelerator, will switch 75 kA and 250 kV in 30 ns long pulses at 50 Hz. The accelerator will produce a 700 kV, 7kA electron beam for industrial and military applications. In the low power regime, these switches are being used to switch 400 A and 5 kV to drive laser diode arrays which produce 100 ps optical pulses. These short optical pulses are for military and commercial applications in optical and electrical range sensing, 3D laser radar, and high speed imaging. Both types of these applications demand a better understanding of the switch properties to increase switch lifetime, reduce jitter, optimize optical triggering, and improve overall switch performance. These applications and experiments on the fundamental behavior of high gain GaAs switches is discussed. Open shutter, infra-red images and time-resolved Schlieren images of the current filaments, which form during high gain switching, are presented. Results from optical triggering experiments to produce multiple, diffuse filaments for high current repetitive switching are described.

Optically activated switches for low jitter pulsed power applications

Optically activated high voltage switches are commonly used in pulsed power systems for reliable low jitter, multichannel and multiswitch (low inductance) applications. In addition to low jitter switching, optical activation provides a high degree of electrical isolation between the triggering and switching power systems simplifying pulsed power design. The disadvantages of optical triggering for large-gap gas switches are the optical energy, line-of-sight optics, and system maintenance required to obtain reliable operation. This paper describes two technologies which can reduce or eliminate these disadvantages and provide more flexible optically activated switches for pulsed power systems. One approach is to reduce the optical trigger energy requirement for gas gap switches. Shorter optical pulses require less energy to initiate a plasma discharge. An experiment is being assembled to trigger a 50-100 kV gas gap switch with 120 fs wide optical pulses. Lower trigger energy has also been demonstrated by the introduction of metallic aerosols into a gas gap W. Frey (1997). The apparatus will be added to this experiment to reproduce these results and determine the optical energy and power density requirements over a range of wavelengths and pulse widths. The status of this experiment will be discussed. A second approach uses solid state switching in two configurations: (1) as the main switch and (2) as a trigger. High-gain photoconductive semiconductor switches (PCSS) are practical for some direct pulsed power switching applications. We have demonstrated switching up to 220 kV and 8 kA. Higher power optically activated switching can be obtained by combining solid state and gas gap switching technologies. Multimegavolt (MMV) switches can be triggered with fiber-optically triggered, remotely located PCSS. By placing the compact PCSS trigger extremely close to the trigger point, reliable, low-jitter, high power switching is achieved with low energy fiber-optic trigger systems that can easily be controlled and adjusted from a remote control center. Power for the trigger system can be derived from the electrical fields near the trigger, so all electrical cables to the trigger system are eliminated and replaced with 100 micron diameter fibers that trigger and monitor the operation of the system. Results from experiments with PCSS triggered gas gap switches and the design for a PCSS triggered multimegavolt switch will be reported. PCSS switching properties including new picosecond pulse results and fabrication procedures for improved longevity will also be described. A 120 fs wide 780 nm laser pulse was used to radiate THz bandwidth pulses with a GaAs PCSS operating in the linear mode. New approaches for PCSS contact fabrication are being developed and tested to simplify the growth procedure, increase the current per filament capability, and improve device longevity. Progress continues to make PCSS a more useful component for pulsed power applications.

Sub-nanosecond avalanche transistor drivers for low impedance pulsed power applications

Ultra compact, short pulse, high voltage, high current pulsers are needed for a variety of non-linear electrical and optical applications. With a fast risetime and short pulse width, these drivers are capable of producing sub-nanosecond electrical and thus optical pulses by gain switching semiconductor laser diodes. Gain-switching of laser diodes requires a sub-nanosecond pulser capable of driving a low output impedance (5 /spl Omega/ or less). Optical pulses obtained had risetimes as fast as 20 ps. The designed pulsers also could be used for triggering photo-conductive semiconductor switches (PCSS), gating high speed optical imaging systems, and providing electrical and optical sources for fast transient sensor applications. Building on concepts from Lawrence Livermore National Laboratory, the development of pulsers based on solid state avalanche transistors was adapted to drive low impedances. As each successive stage is avalanched in the circuit, the amount of overvoltage increases, increasing the switching speed and improving the turn on time of the output pulse at the final stage. The output of the pulser is coupled into the load using a Blumlein configuration.

Pulsed- and DC-Charged PCSS-Based Trigger Generators

Prior to this research, we have developed high-gain, GaAs, photoconductive semiconductor switches (PCSSs) to trigger 50–300 kV high voltage switches (HVSs). We have demonstrated that PCSSs can trigger a variety of pulsed power switches operating at 50–300kV by locating the trigger generator directly at the HVS. This was demonstrated for two types of DC-charged trigatrons and two types of field distortion mid-plane switches, including a ±100 kVDC switch produced by the High Current Electronics Institute (HCEI) used in the linear transformer driver. The lowest rms jitter obtained from triggering a HVS with a PCSS was 100 ps from a 300 kV pulse-charged trigatron. PCSSs are the key component in these independently timed, fiber-optically controlled, low jitter trigger generators (TGs) for HVSs. TGs are critical sub-systems for reliable, efficient pulsed power facilities because they control the timing synchronization and amplitude variation of multiple pulse forming lines that combine to produce the total system output. Future facility scale pulsed power systems are even more dependent on triggering, as they consist of many more triggered HVSs and produce shaped-pulses by independent timing of the HVSs. As pulsed power systems become more complex, the complexity of the associated trigger systems also increases. One means to reduce this complexity is to allow the trigger system to be charged directly from the voltage appearing across the HVS. However, for slow or DC charged pulsed power systems this can be particularly challenging as the DC hold off of the PCSS dramatically declines. This paper presents results seeking to address HVS performance requirements over large operating ranges by triggering using a pulsed charged PCSS based TG. Switch operating conditions as low as 45% of self break were achieved. A DC charged PCSS based TG is also introduced and demonstrated over a 39 kV – 61 kV operating range. DC charged PCSS allow the TG to be directly charged from slow or DC charged pulsed power systems. GaAs PCSSs and neutron irradiated GaAs (n-GaAs) PCSSs were used to investigate the DC charged operation.

High-speed and low-relative-intensity noise 1.3 mu m InGaAsP semi-insulating buried crescent lasers

The dependence of static and dynamic performance on active layer doping concentration in 1.3- mu m InGaAsP semiinsulating buried crescent (SIBC) Fabry-Perot lasers were investigated experimentally. The optical loss in the active region is one of the dominant mechanisms in determining the threshold current for doped active layer lasers. These SIBC lasers have a 3 dB modulation bandwidth of 19 GHz for pulsed operation and 16 GHz for continuous-wave (CW) operation, and a relative intensity noise below -150 dB/Hz for biased current at 120 mA. The doped active lasers show an initial small degradation rate at 65 degrees C operation, which gives an acceptably long operation lifetime. >