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High-power Laser Diode Modules Pump Disk Lasers

Date: 2016-11-10
Click: 71

Laser diode systems with expansion-matched heatsinks provide reliability and a price-to-performance ratio that makes them suitable for pumping disk lasers and other pumping applications.

ROBERT ROFF, VIOREL NEGOITA, HAIYAN AN, THILO VETHAKE, STEFAN HEINEMANN, GEORG TREUSCH, and TINA GOTTWALD

Diode-pumped solid-state lasers (DPSSLs) are established tools in many industries based on their low cost of ownership, high efficiency, and high reliability. As beam quality and power improved, DPSSLs also made inroads into the laser cutting market, replacing CO2 lasers. Recent advances enable power scaling to several tens of kilowatts continuous-wave (CW), which make a wide range of applications possible, from material processing to airborne defense systems. Cold, high-precision processing with ultrashort pulsed lasers further accelerates the market penetration of DPSSLs.

High-power Laser Diode Modules Pump Disk Lasers
FRONTIS. To build a high-power laser diode pumping module, TRUMPF first creates a chip-on-submount (CoS) assembly with the laser diode soldered to the copper-tungsten (CuW) submount and wire-bonded to an electrical n-contact (a); 12 CoS assemblies are then mounted on a common cooler (b).

Disk lasers andfiber lasers are the two predominant DPSSL designs. A fiber laser is designed with a fiber-optic cable, typically with a 0.3 mm diameter and 30 m in length, serving as the laser resonator, while disk lasers use a thin disk with a typical diameter of 10 mm and a thickness of less than 1 mm as the laser resonator. Both designs mitigate thermal lensing through efficient heat removal, which is essential for high beam quality and high power.

The requirements for pump diodes are quite different for each design based on the specific dimensions of the laser resonator. Fiber lasers require high-brightness diodes with a beam quality of less than 90 mm*mrad to couple into the small diameter fiber. The pump radiation is absorbed over the length of the fiber. Disk lasers are pumped with a typical beam quality of 450 mm*mrad. Multiple paths through the thin disk are required for efficient absorption. Achieving brightness and power scaling of pump diodes that balance performance with cost has been a challenging engineering task for decades.

Both laser systems use a ytterbium (Yb)-doped active laser medium that demonstrates a broad absorption peak around 940 nm and a narrow absorption peak around 970 nm. The exact wavelengths depend on the specific laser design—a typical absorption spectrum of a ytterbium: yttrium aluminum garnet (Yb:YAG) disk laser is shown in Fig. 1. The broad absorption spectrum is centered at approximately 937 nm and, based on a 90% width of ~8 nm, the diode wavelength is easily matched. For high beam quality at high power and short pulses, a low quantum defect and short resonator are essential. This is achieved by pumping at 969 nm; however, the narrow pump band with a 90% width of approximately 1 nm requires wavelength-stabilized pump modules.

High-power Laser Diode Modules Pump Disk Lasers
FIGURE 1. Across the absorption spectrum of the Yb:YAG disk crystal, pumping is possible in the 940 nm absorption band by using a non-wavelength-stabilized lease system or at 969 nm with a wavelength-stabilized system.

Modules for pumping

High-power laser diode modules with power up to 12 kW CW are now available for optical pumping of thin disk lasers.1-3This family of pump modules is based on a novel platform that solves most, if not all, of the problems associated with traditional microchannel cooler stacks. Traditional microchannel coolers are designed to have both electric current and cooling water flow through and around the copper cooling channels with no isolation between them. This design sets up an electro-corrosion effect that occurs near the inlet and outlet O-ring seals and can lead to early failure (water leaks).

For power scaling, individual coolers are stacked with O-rings in between, leading to very compact and high-power pump modules. However, the issues of electro-corrosion, leaking O-rings, and limited beam quality remain. The platform design employs an aluminum nitride (AlN) layer with structured metallization in the cooler that serves as the common mounting base for multiple diode lasers. It also effectively isolates the electric current from the water and enables hermetic sealing such that the water is contained in one monolithic cooler structure.

The laser bar incorporates single-quantum-well indium gallium arsenide (InGaAs)/aluminum gallium arsenide (AlGaAs) grown by MOCVD and is optimized for low internal loss, high power conversion efficiency (PCE), and low divergence in both axes. The laser bar with dimensions of 10 × 4 mm has 55 emitters and a fill factor of 55%. The packaging technology is optimized for high reliability at high output power, and low smile for good beam quality.

The bar is mounted to an expansion-matched CuW submount with hard-solder gold tin (AuSn) to eliminate solder creep, which appears when soft, solder-like indium (In) is used. Alignment of the laser front facet to the submount is critical for good heat-sinking and to avoid disturbing the emission of the laser. Five-axis alignment is used to position the laser to the submount and to level the laser to the solder interface for a void-free solder joint.

Bonding parameters were developed to induce solder to wet up to the front facet, providing for good thermal contact. Careful control of bond tool surface specification and cleanliness yield a typical smile for this assembly of less than 1.5 μm. Visual inspection, interference surface measurements, and scanning acoustic microscopy (SAM) imaging are used to verify the bond quality.

This element, called chip-on-submount (CoS), is completed with an n-contact by wire bonds. Wirebonding the n-side of the diode laser greatly reduces the mechanical stresses that can be caused by other methods used to make the n-contact. The CoS is then tested with conditions similar to those found later in the pump module, screening out the small fraction of devices with minor degradation. Twelve CoS devices are mounted on the common cooler. The flat surface of the cooler enables an automated assembly in one reflow process. The cooler consists of symmetric pairs of AlN and copper (Cu) layers, which provide excellent mechanical stability as well as rigidity against environmental influences like temperature cycles (see frontis).

The inner structure uses diamond-shaped copper islands with altering cooling channels that reconnect, unlike the known microchannels. This design minimizes issues caused by potential blockages and reduces the flow speed inside the channels compared to microchannels. The cooling water flows through the inner copper structure of the cooler and is completely isolated from the outside AlN layers. Therefore, the electro-corrosion effect found in non-insulated microchannel coolers is eliminated. In addition, the cooler supplies water to all the laser diodes and works as a manifold, eliminating the need for O-rings.

The assembly is expansion matched with the GaAs laser bar, mitigating any stress issues found in other systems with non-expansion-matched materials. The AlN is coated with a thick copper layer for mounting the CoS and to provide low electrical impedance at a current of 220 A.

Output power and scaling

The emission of the individual bars has a fast-axis divergence of approximately 40° (95% power content), a slow-axis divergence of 8° (95% power content), and is oriented in a plane parallel to the cooler surface. A custom optical element called PriFAC (Prism FAC) collimates the fast axis and deflects the beam by 90° perpendicular to the mounting plane (see Fig. 2). After collimation, the divergence is less than 2 mrad (90% power content) including a pointing accuracy of 0.5 mrad. The individual beams are 0.14 × 1 cm and are displaced in one axis by 1 cm.

High-power Laser Diode Modules Pump Disk Lasers
FIGURE 2. Custom optics known as PriFACs collimate and redirect the beams of the individual laser diode in fast axis.

 

 

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