Andreas Klehr

Novel passivation process for the mirror facets of Al-free active-region high-power semiconductor diode lasers

A novel process for the passivation of mirror facets of Al-free active-region high-power semiconductor diode lasers is presented. Designed for technological simplicity and minimum damage generated within the facet region, it combines laser bar cleaving in air with a two-step process consisting of 1) removal of thermodynamically unstable species and 2) facet sealing with a passivation layer. Impurity removal is achieved by irradiation with beams of atomic hydrogen, while zinc selenide is used as the passivating medium. The effectiveness of the process is demonstrated by operation of 808-nm GaAsP-active ridge-waveguide diode lasers at record optical powers of 500 mW for several thousand hours limited only by bulk degradation.

High-Power Monolithic Two-Mode DFB Laser Diodes for the Generation of THz Radiation

We have devolved 1064 nm high-power monolithic distributed feedback lasers which operate simultaneously on two longitudinal modes. These modes correspond to the fundamental and first-order lateral mode and arise from a 7.5 mum width ridge waveguide supporting both of them. They are further stabilized by a first-order grating built into an InGaP/GaAs/InGaP multilayer structure. The threshold current of the laser is 66 mA, the slope efficiency is 0.5 W/A, and an output power of ~500 mW is reached. Detailed investigations of the intensity distribution of lateral and vertical far fields and the spectral behavior are shown. The longitudinal mode spacing at 260 mW is 0.56 nm corresponding to approximately 150 GHz. THz generation is demonstrated by mixing the two-line laser emission in a LT-GaAsSb photomixer.

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.

New Two-Color Laser Concepts for THz Generation

Two-color semiconductor external cavity laser concepts for terahertz (THz) generation are discussed. By defining three critical characteristics, various two-color laser configurations are experimentally classified with respect to the application of THz generation. According to our experimental results, we suggest a new two-color laser configuration. It is based on an external cavity with an etalon.

Design and realization of high-power DFB lasers

The development of high-power GaAs-based ridge wave guide distributed feedback lasers is described. The lasers emit between 760 nm and 980 nm either in TM or TE polarization. Over a large current range, the lasers exhibit stable operation in a single transversal and longitudinal mode. A maximum continuous-wave output power of about 400 mW, a spectral linewidth below 1 MHz and a side mode suppression ratio greater than 50 dB have been demonstrated at room temperature. The distributed feedback is provided by first or second order gratings, formed in an InGaP/GaAsP/InGaP multilayer structure embedded into the p-AlGaAs cladding layer. Applications of such wavelength stabilized devices in non-linear frequency conversion, spectroscopy and for excitation of atomic transitions are discussed.

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.

High-Power 980-nm Broad-Area Lasers Spectrally Stabilized by Surface Bragg Gratings

We report on broad-area distributed Bragg reflector (DBR) lasers with a stripe width of 90 ¿m providing up to 14-W optical power and 50% maximum conversion efficiency. Ninety-five percent of the power is included within a wavelength range of less than 1 nm. The wavelength shift between threshold and the maximum output power is less than 3.5 nm. The wavelength stabilization is achieved with a 500-¿m -long DBR containing a sixth-order Bragg grating defined by i-line wafer stepper lithography and fabricated by reactive ion etching.

Femtosecond passively modelocked diode laser with intracavity dispersion management

We report on the generation of ultrashort pulses by dispersion management of a passively modelocked external cavity diode laser. Pulse widths down to 200 fs are obtained at 830 nm emission wavelength. We use intracavity dispersion management to increase the spectral bandwidth and compress the strongly chirped pulses externally with a grating compressor.

High peak power optical pulses generated with a monolithic master-oscillator power amplifier

We present results on a monolithic semiconductor-based master-oscillator power amplifier (MOPA) combining a distributed-feedback (DFB) laser and a tapered amplifier on a single chip. The MOPA reaches an output power of almost 12 W at an emission wavelength around 1064 nm in continuous-wave operation. Pulses with a length of around 100 ps can be obtained either by injecting nanosecond current pulses into the tapered amplifier alone or into both the DFB laser and the tapered amplifier. In the latter case, pulses with a width of 84 ps, a peak power of 42 W, and a spectral width of 160 pm are generated.

Ultrashort pulse Yb:LaSc 3 (BO 3 ) 4 mode-locked oscillator

Passive mode-locked laser operation based on an Yb-doped lanthanum scandium borate crystal is demonstrated. Pulse durations as short as 58 fs and 67 fs were achieved applying a Ti:sapphire- and a diode-laser pump source, respectively. The average output powers were 73 mW and 39 mW at a repetition rate of 90 MHz. The laser was broadly tunable from 1028 to 1057 nm in the sub-200 fs pulse regime.