P. Ressel

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.

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.

20W continuous wave reliable operation of 980nm broad-area single emitter diode lasers with an aperture of 96μm

High power broad area diode lasers provide the optical energy for all high performance solid state and fiber laser systems. The maximum achievable power density from such systems is limited at source by the performance of the diode lasers. A crucial metric is the reliable continuous wave optical output power from a single broad area laser diode, typically for stripe widths in the 90-100 μm range, which is especially important for users relying on fibered multi-mode pumps. We present the results of a study investigating the reliable power limits of such 980nm sources. We find that 96μm stripe single emitters lasers at 20°C operate under continuous wave power of 20W per emitter for over 4000 hours (to date) without failure, with 60μm stripe devices operating reliably at 10W per stripe. Maximum power testing under 10Hz, 200μs QCW drive conditions shows that 96μm stripes reach 30W and 60μm stripes 21W per emitter, significantly above the reliable operation point. Results are also presented on step-stress-studies, where the current is step-wise increased until failure is observed, in order to clarify the remaining reliability limits. Finally, we detail the barriers to increased peak power and discuss how these can be overcome.

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.

1060 nm DBR tapered lasers with 12 W output power and a nearly diffraction limited beam quality

High-brightness narrow line-width 1060 nm tapered lasers with an internal distributed Bragg reflector were realized. The devices reach a maximal output power of 12 W with a narrow spectral line-width below 40 pm (95% power). A nearly diffraction limited beam quality was measured up to a power of 10 W. The vertical structure is based on an InGaAs triple quantum well (TQW) active region embedded in a 4.8 μm broad AlGaAs super large optical cavity. This leads to a narrow vertical divergence of 15° (FWHM). Tapered devices were processed a total length of 6 mm consisting of 2 mm long ridge waveguide (including 1 mm DBR mirror) and 4 mm tapered sections. A full taper angle of 6° was manufactured. The input currents to both sections can be independently controlled. The devices had a conversion efficiency of about 50%. A first reliability test showed failure-free operation at 5 W without a deterioration of the beam quality and the spectral properties.

Surface recombination and facet heating in high-power diode lasers

Surface recombination velocities and surface temperatures at front facets of standard broad-area lasers emitting at 808nm were investigated by time-resolved two-color photoluminescence and micro-Raman spectroscopy. Surface recombination velocities in the range between <105 and 106cm∕s are determined for devices with tailored surface properties. The results clearly show that increased surface recombination velocities are accompanied by increased facet temperatures. Reabsorption of light generated in the diode lasers leads to an additional enhancement of facet heating for surfaces of minor structural quality. The methodological approach presented here paves the way for improved analytical access to diode laser facet properties.

High-power, high-efficiency 1150 nm quantum well laser

Edge emitting diode lasers with highly strained InGaAs quantum wells and GaAs waveguide layers emitting at 1150 nm were investigated focusing on the impact of the waveguide design on the laser performance. Using a thick GaAs waveguide layer broad area devices with low vertical divergence of 20/spl deg/ FWHM and reliable operation at a power level of 80-mW//spl mu/m stripe width were demonstrated.

Tapered lasers emitting at 650 nm with 1 W output power with nearly diffraction-limited beam quality

High-brightness tapered lasers emitting around 650 nm were developed. Devices 2 mm long with a200-microm-long straight section, 1800-microm-long tapered section, and 4 degrees taper angle reached 1 W output power in CW operation with a nearly diffraction-limited beam quality.

5.6-W Broad-Area Lasers With a Vertical Far-Field Angle of 31

Highly efficient 670-nm high-power broad-area laser diodes with a single InGaP quantum-well embedded in AlGaInP waveguide layers and n-AlInP and p-AlGaAs cladding layers are presented. The developed vertical layer structure leads to a vertical far-field angle of 31deg. At 15degC, 100 mu-m-wide broad-area lasers reach an output power of 5.6 W limited by thermal rollover. The conversion efficiency was 41% at 1.5 W. A 7600-h reliable operation at 1.5 W and a mean time to failure of about 37550 h will be reported.

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High-brightness tapered lasers and amplifiers at 670 nm with output powers up to 1 W and nearly diffraction limited beam quality were realised. The devices consist of a 750 &mgr;m long straight section and a 1250 &mgr;m long tapered section. Devices with a taper angle of 2°, 3° and 4° were manufactured. The material quality was studied in a long-term test for ridge-waveguide lasers. Devices with 7.5 &mgr;m ridge width show reliable operation at 100 mW output power over more than 10000 h. At a temperature of 15°C a tapered lasers with an angle of 4° reached an output power of 1 W at a current of 2.1 A. The highest conversion efficiency for this device was 24%, the peak wavelength of the emission was 668 nm and the spectral width was smaller than 0.2 nm. The beam propagation factor was M2 = 1.7 (1/e2) and M2 = 3.0 (second moments). At 500 mW output power, master-oscillator power-amplifier (MOPA) devices showed also a nearly diffraction limited beam quality with M2 < 1.5 and reliable operation with degradation rates as low as 7x10-6 h-1 over 1200 h. The spectral line-width in this arrangement is determined by the master oscillator and is suitable for high-resolution spectroscopy.