Study of the Mechanism of "Smile" in High Power Diode Laser Arrays and Strategies in Improving NearField Linearity

High power diode lasers have found increased applications in pumping of solid state or fiber laser systems for industrial, military and medical applications as well as direct material processing applications. The non-linearity of the near-field of emitters (or the so called “smile” effect) in a laser diode array poses significant challenges in optical coupling and beam shaping and has become one of the major roadblocks in broader applications of laser arrays. Increasing the near-field linearity of a pumping laser diode array enables the laser system manufacturer to improve the laser system compactness, optical coupling efficiency, power, and beam quality while at the same time reducing manufacture cost in the laser system. Therefore, the near-field linearity of a laser bar is one of the key specifications of laser array products and improving the near field performance is especially important in order to increase production yield, reduce cost and gain competitiveness. In this paper, we will study the mechanism of “smile” in high power diode laser arrays and discuss the strategies and ways to achieve low “smile”. Introduction High power diode lasers (HPDLs) offer a variety of applications due to their higher electrical-optical conversion efficiencies, compact sizes and long life-times than the most prominent types of lasers by nearly an order of magnitude. High power semiconductor lasers, including single emitters, arrays, stacks, and two dimension area array stack have found increased applications in pumping of solid state laser systems for industrial, science and technology research, military, antiterrorism, entertainment display and medical applications as well as direct material processing applications such as welding, cutting, and surface treatment [ 1 , 2 , 3 , 4 ]. With continuing improvement of the power, electrical-optical conversion efficiency, reliability, and manufacturability of high power semiconductor lasers, and decreasing manufacturing cost, many new applications of high power semiconductor lasers are being enabled [5]. The three key performance measures of high power semiconductor lasers are power, efficiency, and reliability. For diode laser arrays, the near field linearity is as important as the above three parameters as the near field linearity of a laser array as a pumping source significantly affects power, efficiency, compactness, beam quality and thermal management of a laser system, such as diode pumped solid state lasers and fiber lasers. If near field linearity of a laser array is poor, in other words, the smile is very large, the coupling efficiency of the laser array to a fiber array or micro-optics such as a fast axis collimation lens is very low. A typical 808 nm high power semiconductor laser array in a conduction cooled package has 19 to 49 individual emitters and the typical CW output power ranges from 30W to 80W. Figure 1 is a schematic diagram of a high power semiconductor laser array. To achieve high efficiency and high CW power, Cu is commonly used as the heatsink (anode block) for laser arrays. As it is known, copper has higher thermal conductivity in a variety of heatsink materials, such as Copper-Tungsten alloy, aluminum, etc. Effects of heatsink materials and packaging process on and the “smile” performance of laser diode array are discussed in more details later in this paper. Figure 1 A schematic diagram of a high power semiconductor laser array (laser bar) Near field non-Linearity of laser diode array The individual emitters of high power diode lasers arrays (HPDLAs) are not perfectly aligned linearly because of the coefficient of thermal expansion (CTE) mismatch among the different layers of a bare bar, the packaging process and CTE mismatch between the laser bar and the bonding heat sink. Figure 2 shows a magnified “smile” image of a typical good diode-laser array. As can be seen from Figure 2, a typical good laser diode array is nearly linear. This is a better emitting light source for beam coupling. Figure 2 Enlarged “smile” image of a typical good laser diode array The individual emitters form a curvature, which can be concave or convex, corresponding to “smile” (Figure 3 (a)), 978-1-4244-4476-2/09/$25.00 ©2009 IEEE 837 2009 Electronic Components and Technology Conference “cry” (Figure 3 (b)), or other types (Figure 3(c), (d)). Both “smile”, “cry” or others are commonly called “smile” due to historical reason. Due to the significant impact of “smile” on HPDLAs performance, reliability and cost, it is very important to reduce the smile [6].