Armin Liero

Mode-locked InGaAs-AlGaAs disk laser generating sub-200-fs pulses, pulse picking and amplification by a tapered diode amplifier.

Almost chirp-free pulses with a duration of 190 fs were achieved from a mode-locked semiconductor disk laser (SDL) emitting at approximately 1045 nm. Pulse shaping was different from the soliton-like mode-locking process known from lasers using dielectric gain media; passive amplitude modulation provided by a fast saturable absorber was essential. The spectrum of the absorber had to be matched to the gain spectrum within a few nm. A tapered diode amplifier was demonstrated to be a device for both picking and amplifying SDL pulses. The pulse repetition rate of the SDL output was reduced from 3 GHz to 47 MHz.

Root-Cause Analysis of Peak Power Saturation in Pulse-Pumped 1100 nm Broad Area Single Emitter Diode Lasers

Many physical effects can potentially limit the peak achievable output power of single emitter broad area diode lasers under high current, pulse-pumped operation conditions. Although previous studies have shown reliable operation to high pump levels (240 A, 300 ns, and 1 kHz), power was found to saturate. We present here results of a systematic study to unambiguously determine the sources of this power saturation. A combination of detailed measurements and finite element device simulation were used for the diagnosis. We find that the measured power saturation is dominated by electron leakage caused by band bending at high bias due to the low mobility of the p-type waveguide. However, the power saturation is only fully reproduced when longitudinal spatial hole-burning is included. Higher powers are expected to be achieved if higher energy barriers and lower confinement factors are used to mitigate leakage and longitudinal hole-burning, respectively.

GaN HEMT Potential for Low-Noise Highly Linear RF Applications

This paper presents a study of the capability of gallium-nitride (GaN) high-electron mobility transistors (HEMTs) to achieve low noise and high linearity performance. A packaged GaN HEMT was measured in a 50 - Omega system at 2 GHz. Noise figures slightly above 1.8 dB were achieved together with a record third-order intercept point of 54 dBm. The same configuration yields a maximum output power of 30 W, with 50% power-added efficiency. This combination of high power and low-noise performance allows the realization of highly linear low-noise amplifiers, which could significantly reduce protection and filter efforts at receiver inputs.

Laser-assisted processing of VIAs for AlGaN/GaN HEMTs on SiC substrates

Vertical interconnect accesses (VIAs) were fabricated between the source electrode on the front and the ground on the backside of high-power microwave AlGaN/GaN high-electron mobility transistors (HEMTs) on /spl sim/400-/spl mu/m-thick silicon carbide substrates. Through-wafer microholes with an aspect ratio of up to /spl sim/ 8 were drilled using pulsed UV-laser machining and subsequently metallized using electroplating. The successful implementation of the laser-assisted VIA technology into device processing was proven by dc and RF characterization. When biased at 26 V, a saturated output power of 41.6 W with an associated power-added efficiency of 55% at 2 GHz was achieved for a 20-mm AlGaN/GaN HEMT with through-wafer VIAs.

Highly robust X-band LNA with extremely short recovery time

GaN-based low-noise amplifiers (LNAs) recently were shown to provide high ruggedness together with low noise figure. These LNAs allow for simplified receiver architectures, e.g., since no limiter is required to protect the input. This paper for the first time presents an investigation of the recovery time of a highly rugged GaN LNA. The X-band LNA is shown to survive input overdrive powers up to 46 dBm under pulsed and 40 dB under cw conditions, with a noise figure of 2.8 dB. Extremely short recovery times below 10 ns were simulated and proved to be below the measurement resolution.

Laser driver switching 20 A with 2 ns pulse width using GaN

A GaN-HEMT-based circuit is presented capable of switching 20 A of current with less than about 0.5 ns rise and fall time. This demonstrates the potential of GaN transistors for high-current switching applications, even if breakdown voltage requirements are low. The current driver is used to realize an optical pulse picker generating 10 ps optical pulses of more than 30 W with a variable repetition rate between 1 kHz and 100 MHz.

On the Recovery Time of Highly Robust Low-Noise Amplifiers

Recently, GaN-based low-noise amplifiers (LNAs) were shown to provide high ruggedness together with low noise figure. Since no limiter is required to protect the input, these LNAs allow for simplified receiver architectures. This paper presents an in-depth analysis of the recovery time of a highly rugged LNA. Recovery time is measured in the time domain, and an analytical approximation is developed that allows to estimate and optimize recovery. A new measurement setup is established in order to determine the impact of the overdrive pulse on LNA gain. An X-band LNA is shown as an example. It survives input overdrive powers of up to 46 dBm under pulsed and 40 dBm under continuous wave conditions, with a noise figure of 2.8 dB. Extremely short recovery times below were simulated and measured.

Compact sources for the generation of high-peak power wavelength-stabilized laser pulses in the picoseconds and nanoseconds ranges

Diode lasers are ideally suited for the generation of optical pulses in the nanoseconds and picoseconds ranges by gainswitching, Q-switching or mode-locking. We have developed diode-laser based light sources where the pulses are spectrally stabilized and nearly-diffraction limited as required by many applications. Diffraction limited emission is achieved by a several microns wide ridge waveguide (RW), so that only the fundamental lateral mode should lase. Spectral stabilization is realized with a Bragg grating integrated into the semiconductor chip, resulting in distributed feedback (DFB) or distributed Bragg reflector (DBR) lasers. We obtained a peak power of 3.8W for 4ns long pulses using a gain-switched DFB laser and a peak power of more than 4W for 65ps long pulses using a three-section DBR laser. Higher peak powers of several tens of Watts can be reached by an amplification of the pulses with semiconductor optical amplifiers, which can be either monolithically or hybrid integrated with the master oscillators. We developed compact modules with a footprint of 4×5cm2 combining master oscillator, tapered power amplifier, beam-shaping optical elements and high-frequency electronics. In order to diminish the generation of amplified spontaneous emission between the pulses, the amplifier is modulated with short-pulses of high amplitude, too. Beyond the amplifier, we obtained a peak power of more than 10W for 4ns long pulses, a peak power of about 35W for 80ps long pulses and a peak power of 70W for 10ps long pulses at emission wavelengths around 1064nm.

Compact ps-pulse laser source with free adjustable repetition rate and nJ pulse energy on microbench

A new compact picosecond light source is presented. It consists of a master oscillator, an ultra fast pulse picker element and integrated high-frequency electronics arranged on a micro bench with a footprint of 5 cm x 4 cm. The master oscillator is a 10 mm long laser consisting of 200 μm long saturable absorber, 1500 μm long gain, 8000 μm long cavity, 200 μm long DBR and 100 μm long monitor sections. Pulses with a width of 7 - 10 ps (depending on output power) and a peak power of 1 W are generated by hybrid mode locking. Despite the 10 mm long cavity, the repetition rate is still 4.3 GHz, which is too high for many applications. In order to reduce the repetition rate to values between 1 kHz and 100 MHz, one has to pick single pulses out of the continuous pulse sequence generated by the mode-locked laser. For this purpose a semiconductor pulse picking element based on a ridge waveguide, which acts as an optical gate, and a tapered section for pulse amplification is developed. Selective pulse picking with a free choice of the repetition rate is achieved by driving the RW section by a high frequency GaN high-electron mobility transistor with low capacitances and high current density. If a current pulse with a width of about 200 ps (smaller than the spacing of the pulses) is injected, the RW section becomes transparent and an injected optical pulse can pass the RW section. This pulse is subsequently amplified by injecting short current pulses with a width of 2 ns and a peak current up to 20 A into the tapered section. Pulse energies in the nJ range at a frequency of 16 MHz are obtained.

Assessment of the limits to peak power of 1100nm broad area single emitter diode lasers under short pulse conditions

High power diode lasers are the root source of optical energy in all high performance laser systems. As their performance advances, diode lasers are increasingly taking the place of other sources. Short pulse, sub-microsecond-class, high power lasers are important for many applications but historically, diode lasers have not been able to reach high enough peak pulse powers with adequate reliability, limited by physical effects such as facet failure. By combining robust facet passivation with thick super large optical cavity waveguides, greatly increased optical output power can be achieved. We present here the results of a study using commercial high current short pulse sources (>200A, <500ns) to assess the performance and endurance limits of high power broad area devices. We find that our lasers can be driven with a peak power density of over 110MWcm-2 without failure for more than 3×107 pulses. For example, on testing to 240A, single emitter 200μm stripe 1100nm broad area devices reach 124W (46μJ) without failure, and 60μm stripes reach 88W. In practice, high injection effects such as carrier accumulation in waveguide typically limit peak power. We review these remaining limitations, and discuss how they can be overcome.