Goetz Erbert

Passively mode-locked Yb:KYWlaser pumped by a tapered diode laser.

We demonstrate the operation of a low threshold femtosecond Yb:KYW laser, using a saturable absorber mirror for passive mode-locking and a high brightness laser diode as pumping source. Fourier-limited pulses with a duration of 101 fs are achieved at an output power of 100 mW. The performance of the Yb:KYW laser was also compared to that of Yb:KGW.

1.5 W green light generation by single-pass second harmonic generation of a single-frequency tapered diode laser

More than 1.5 W of green light at 531 nm is generated by single-pass second harmonic generation in periodically poled MgO:LiNbO3. The pump laser is a high power tapered laser with a distributed Bragg reflector etched in the ridge section of the laser to provide wavelength selectivity. The output power of the single-frequency tapered laser is 9.3 W in continuous wave operation. A conversion efficiency of 18.5 % was achieved in the experiments.

Nonlinear properties of tapered laser cavities

The nonlinear phenomena accompanying the process of light generation in high-power tapered semiconductor lasers are studied using a combination of simulation and experiment. Optical pumping, electrical overpumping, filamentation, and spatial hole burning are shown to be the key nonlinear phenomena influencing the operation of tapered lasers at high output powers. In the particular tapered laser studied, the optical pumping effect is found to have the largest impact on the output beam quality. The simulation model used in this study employs the wide-angle finite-difference beam propagation method for the analysis of the optical propagation within the cavity. Quasi-three-dimensional (3-D) thermal and electrical models are used for the calculation of the 3-D distributions of the temperature, electrons, holes, and electrical potential. The simulation results reproduce key features and the experimental trends.

Mounting of high power laser diodes on boron nitride heat sinks using an optimized Au/Sn metallurgy

High power diode lasers have become more and more important to industrial and medical applications. In contrast to low power applications, long cavity lasers or laser bars are used in this field and mounting quality influences considerably laser performance and life time. In this paper we focus on the solder metallurgy and stress-induced laser behavior after mounting. The laser chips have been bonded fluxless epi-side down on translucent cubic boron nitride (T-cBN) using Au/Sn solder. The laser behavior has been tested with different chip metallizations preserving the eutectic solder composition or forming the Au rich /spl zeta/-phase during reflow. The resulting additional stress in the lasing region has been independently indicated by polarization measurements of the emitted light. A metallization scheme has been developed which forms the highly melting /spl zeta/-phase during soldering within a wide process window. This procedure yields better results then using eutectic Au/Sn which has a higher hardness than the /spl zeta/-phase. Laser diodes up to a cavity length of 2000 /spl mu/m and an aperture of 200 /spl mu/m have successfully been mounted on T-cBN. State of the art laser data, excellent thermal stability, high yield and reliability have been obtained.

Optimization of GaAsP-QWs for high-power diode lasers at 800 nm

Tensile-strained GaAsP quantum wells (QWs) embedded in AlGaAs waveguide and cladding layers are an alternative approach for the wavelength range 700 - 800 nm. We will present a detailed experimental and theoretical study of the dependence of the threshold current on the thickness and the strain of the QW for 800 nm. The optimum thickness of the GaAsP QW for a minimum threshold current density is about 14 nm and is thus much larger than for compressively strained QWs. Higher characteristic temperatures T0 can be obtained with even thicker QWs. In order to achieve high optical output powers and good fiber coupling efficiencies, we used broad waveguides with weak optical confinement and small far field divergence. We prepared two structures with 1 micrometers thick Al0.65Ga0.35As (structure A) and 2 micrometers thick Al0.45Ga0.55As (structure B) waveguides, respectively. For structure B, the thickness of the Al0.70Ga0.30As cladding layers must be carefully optimized in order to suppress higher-order transverse modes. Whereas structure B yields a higher maximum cw output power of AR/HR coated broad-area devices, structure A shows a better high-temperature behavior. Aging tests performed at 2 W (100 micrometers stripe width) and 25 degree(s)C suggest a very good reliability of these devices.

3-W high-brightness tapered diode lasers at 735 nm based on tensile-strained GaAsP QWs

Tensile strained GaAsP quantum wells embedded in AlGaAs waveguide structures are used to realize high power, high brightness short wavelength tapered laser diodes. At 735nm these laser diodes show up to 3W nearly diffraction limited output power with a wall plug efficiency of about 40%. Single spectral mode behavior is observed at output power levels up to 1W. From aging test a high realiability with lifetime exceeding 5000 can be derived comparable to results obtained from broad area laser diodes with the same aperture width. There are only small changes of the beam quality during aging. In conclusion it is shown that well designed tapered laser are a step forward to high efficiency, diffraction limited light soruces in the Watt-range which can be easily fabricated in high volumes.

Diode lasers with Al-free quantum wells embedded in LOC AlGaAs waveguides between 715 nm and 840 nm

In this paper, we present results on diode lasers in the wavelength range between 715 nm and 840 nm with Al-free QWs which are embedded in a high-quality AlGaAs LOC broadened waveguide structure with low optical loss and a small vertical far field divergence. The laser structures were grown by LP- MOVPE. We studied tensile-strained GaAsP-QWs as well as compressively strained InGaAsP-QWs with strain compensating barriers. For lasers with GaAsP QWs, the lowest transparency current densities of about 130 A/cm2 were obtained in the wavelength range between 750 nm and 800 nm. Very low transparency current densities were achieved with InGaAsP-QWs at wavelengths above 800 nm. At 810 nm, high output powers (100 micrometer aperture) of about 7 W was achieved with both types of QWs from devices mounted epi up. However, with respect to high temperature operation and reliability tensile- strained GaAsP QWs seem to be the better choice, especially for the wavelength range below 760 nm.

Simulation of single-mode high-power semiconductor lasers

Theoretical studies of aluminum-free RISAS and ARROW type lasers operating at 800 nm and 940 nm, respectively, are presented. At 800 nm, the electron leakage current over the hetero barriers leads to a sub-linear light-current characteristic. In order to obtain a high output power at moderate currents, either the losses must be kept as small as possible, or the barriers for the electrons must be increased, for example by higher p-doping. At 940 nm, the leakage current is not as problematic. In both RISAS and ARROW lasers, excess loss for the higher- order modes is needed to prevent them from lasing. The theoretical maximum single mode power of ARROW lasers obtained with a two-dimensional FEM-solution of the scalar wave equation is lower than found with the effective index method.

Conductively Cooled 1 kW—QCW Diode Laser Stacks Enabling Simple Fiber Coupling

A new mini-bar design densely packed with 13 emitters in a 1.6 mm aperture has been developed. These mini-bars deliver >200 W output. With a very low divergence of 95% power enclosed in 25° they are suitable for fast axis collimation (FAC) lenses with large working distance. The mini-bars are mounted on compact CuW carriers, which are further integrated into a custom stack assembly where they are conductively cooled from both sides. This completely AuSn soldered stack consists of only coefficient of thermal expansion matched materials and was 100 times temperature cycled from -40°C to 85°C, without any change in the PUI characteristic, the spectrum and the far field. The beam propagation factor M 2 of a 12-layer stack is ¿130 in the vertical direction (after FAC) and ¿210 in the lateral direction. Thus, the stack can be coupled to a 1.2 mm multi-mode fiber (M 2 = 260) with a coupling efficiency of 90% using only low-cost lenses.

High-power green light generation by second harmonic generation of single-frequency tapered diode lasers

We demonstrate the generation of high power (>1.5W) and single-frequency green light by single-pass second harmonic generation of a high power tapered diode laser. The tapered diode laser consists of a DBR grating for wavelength selectivity, a ridge section and a tapered section. The DBR tapered laser emits in excess of 9 W single-frequency output power with a good beam quality. The output from the tapered diode laser is frequency doubled using periodically poled MgO:LiNbO3. We investigate the modulation potential of the green light and improve the modulation depth from 1:4 to 1:50.