Diode Lasers for Pumping Solid State Lasers – State-of-the-Art and New Developments Stefan Heinemann Fraunhofer USA, Center for Laser Technology ph: 734-738-0500, fax: 734-354-3335, email: [email protected] Abstract: The requirements for diode pumping of disk, slab and fiber lasers is discussed. The state-of-the art and means for scaling of power and brightness to fit the specific laser designs will be reviewed. ©2008 Optical Society of America OCIS codes: 140.2020 Diode lasers, 140.5560 Pumping Introduction Development of new solid-state lasers is guided mainly by optimizing the beam quality and the optical efficiency for increased average output powers and/or shorter pulses. The volume and propagation of the required laser mode and the pump mode must therefore be designed to closely match each other in the laser active medium. Each of today’s major resonator designs - thin disk, fiber, slab and rod - therefore require a design of the pump diodes, which is tailored for maximum mode overlap. Diode lasers are characterized by output power, brightness, cooling and package design and their impact on optimized mode overlap and integration into the laser system is discussed. Slab lasers require low brightness and high power pump diodes, whereas fiber lasers require high brightness and lower power diode lasers. Disk and end-pumped rods require medium brightness and high power diodes. Overview Broad area emitters are the major diode design for efficient high power pumping of solid-state lasers. A diffraction limited beam is emitted parallel to the pn junction (fast axis) from an aperture of about 1.2 m with a divergence angle of less than 600 (1/e2). Larger apertures and smaller divergences were realized over the last several years making lower NA fast axis collimation more efficient and improving the reliability due to decreased power densities at the facet. Perpendicular to the pn junction (slow axis) multiple transverse modes are emitted with a typical divergence of 100 (1/e2). The emitter width is defined by the photolithographic processing and is typically in the range of 90 m to 200 m. The maximum output power per emitter is determined by the damage threshold of the facet coating, catastrophic optical damage (COD) occurs for excessive power densities (>100mW/ m), and the temperature of the pn-junction, typically not to exceed 1130F (450C) for long reliability. Sophisticated coatings, i.e. NAM (non absorbing mirrors)  or E2  significantly increase the COD level and >220mW/ m are reported for commercial devices . Elevated junction temperatures reduce the electro-optical efficiency of the diode thus resulting in an even higher temperature and enhance the growth of lattice dislocations, likely to cross the laser active area. Electro-optical efficiencies used to be around 50% but recently 71% are reported  and around 60% are commercially available for 9xxnm devices. Diodes emitting at 808nm are less efficient (around 55%) than diodes emitting at 9xxnm due to the materials used in the epitaxy, GaAlAs/GaAs for 808 versus InGaAlAs/GaAs for 9xxnm, and thus emit about 1/3 less power than 9xxnm diodes. Diodes emitting in the range from 1,500nm to 1,900nm (InGaAs/InP) show about 1/3 of the output power of 9xxnm devices. A high electro-optical efficiency, an excellent mirror coating and a diode package with low thermal impedance are the main drivers for the improved output power and reliability of today’s commercial diode lasers. Packaging Conduction cooled packages with a single bar reach about 80W cw, while actively cooled heatsinks, typically micro-channel cooling plates made from copper, are commercially rated for about 120W (24mW/ m) cw per bar at 9xxnm, but >500W are achieved. SE with a 100 m emitter are rated for about 12W (240mW/ m), but >20W are reported . The advanced coatings of SE, developed for the telecom, no thermal crosstalk of individual emitters and a package with low thermal impedance enable the superior output power per unit length of emitting area of the SE compared to the bar. Mini-bars are again in between bars and SEs. Thermo-mechanical design of the diode packaged on the heatsink is essential for high output power and high reliability. Based on 120W from a single bar with a contact area of 20mm2 mounted p-side down, a heat density of 400W/cm2 must be handled. The coefficient of thermal expansion of the diode (about 6.8*10-6K-1) must be closely matched to the one of the heatsink to minimize thermally induced stress. Standard diode packages deploy copper as a heatsink for high heat conduction and soft solders, preferably Indium, to mitigate the stress resulting in a typical thermal impedance of 0.6K/W. Copper microchannel coolers show a thermal impedance of about 0.4K/W leading to a temperature gradient between the pn-junction and the coolant of about 32K for a 120W device. Cavitation, relatively high pressure drops and electro-corrosion need to be addressed for microchannel coolers through proper water management, typically Di water with a controlled ph value is used. The soft Indium solder allows a reliability of 10,000+ hours under continuous operation, but shows electromigration under high stress, induced either by high current densities or by hard pulsing of the diode. Hard solders, such as eutectic AuSn, in combination with expansion matched heatsinks are becoming increasingly popular achieving 50,000+ hours lifetime at slightly higher thermal impedance. The diode is soldered onto a submount, typically ALN or alike, which is than bonded with a soft solder on the main heatsink. Beam Quality Broad area diodes are divided in 3 groups – single emitters (SE), mini-bars, and bars. Bars are typically 10mm wide and 1.5 to 2 mm long with a varying number and size of emitters determining its fill factor, output power and beam quality. The beam quality is highly asymmetric with about 440mm*mrad (M2=1,457) in slow axis, and M2=1 in fast axis. Micro-optical lens arrays are applied for bars with a fill factor of less than 50% improving the beam quality to 175mm*mrad (M2=583) but reducing the output power and increasing complexity and costs respectively. For a symmetric beam quality, micro-optical components are used to cut the beam several times in slow axis and to rearrange the sections on top of each other in fast axis direction. SEs are typically 0.4mm wide and 4-6mm long with a 100 m broad emitter yielding a beam quality of 4.4mm*mrad (M2=15) in slow axis and thus represent the diode with the best beam quality. Mini-bars comprise short bars or bars with low fill factor and are typically configured with slow axis collimation lenses for a medium to high beam quality. Stacking in fast axis is done to first scale the output power and secondly, symmetrizing the beam quality. Brightness is defined as power density at the facet divided by the square of the numerical aperture. It is highest for SE (200 kW/(mm*NA)2), lowest for a bar (0.2 kW/(mm*NA)2) and in between for a mini-bar. However, the absolute power is much higher for bars than it is for SE. Bars are commercial available as stacks with typically up to 12 bars stacked on top of each other resulting in 1,400W from an emitting area of 10mm by 20mm. The dark zones, caused by the heatsinks, are optically filled by symmertrization of beam quality or by optical interleaving of multiple stacks. High output powers with SEs require handling of many diodes and lenses. Many laser designs deploy fiber coupled pump diodes for homogenous pumping with a rotational symmetry. Optical symmetrization and a high optical fill factor in near and far field are required. Bars and stacks typically deploy slow axis microlens arrays and symmetrization techniques but the brightness is limited as described above. SEs offer the highest brightness and eliminate the need for any optical symmetrization, which is replaced by a tailored mechanical arrangement of the SEs, thus maximizing optical efficiency and minimizing complexity. More than 100 SEs can be arranged for coupling into a 200 m fiber, NA=0.2 and >350W are demonstrated from the fiber . Further refinement of components and assembly techniques as well as wavelength multiplexing are under development and will soon enable kilowatt systems making the direct diode a viable alternative to many lamppumped solid-state lasers. Classification of Pump Diodes Low brightness and high power are based on stacks delivering up to 1kW from a 400 m fiber, high brightness diodes are based on mini-bars or SEs delivering typically 30W (200W) from a 100 m (200 m) fiber, both at a single wavelength and fiber NA=0.22. Dense wavelength multiplexing based on Volume Bragg Grating is deployed for scaling the output power in the pump band at identical beam quality. 600W are reported from a 200 m fiber, NA=0.2 . Laser designs requiring a high brightness pump source, such as fiber lasers, are pumped with fiber coupled single emitters, whereas slab lasers are pumped with stacks. Disk lasers require a medium brightness and are pumped with (beam shaped) stacks. End-pumped rods require a higher brightness and are pumped with beam shaped bars, stacks or often fiber coupled devices. Side pumped rods require only a low brightness and are pumped with bars. Summary Output power, beam quality and reliability of diode lasers were greatly improved over the last several years. Driven by efficiency improvements and hard solder, output powers now range from 10W from a single emitter to >1kW from stacks. Package designs are becoming more standardized with the bar and the single emitter being the dominant configuration. Reliabilities from 20,000 to 200,000 hours are commercially available. Fiber coupled devices now reach the beam quality and output power of lamp pumped solid-state lasers. 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