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High Power Laser Diode Module for Fiber Laser Pumping Source High Power Laser Diode Module for Fiber Laser Pumping Source Akira Sakamoto,1 Hirokuni Ogawa,2 Shinichi Sakamoto,3Yuji Yamagata,4 and Yumi Yamada4 High power laser diode modules are essential devices for fiber laser systems because properties of the modules directly affect the performance of the fiber laser systems. We have realized high power and high reliable laser diode modules by combining assemble technology developed for optical communication devices and high power laser diode chips developed by OPTOENERGY Inc. 1. Introduction Therefore, it is essential to manage the COD power Fiber laser systems of various power ranges from level in order to ensure high reliability and high output several watts to over kilo-watt are widely used all over power operation. The process chain of COD mecha- the world as light sources for marking or metal pro- nism is schematically shown in Fig. 2. Dielectric film cessing. High power laser diode modules are essential coating on laser facets are applied for the purpose of devices for these fiber laser systems because proper- not only for facet reflectivity control but also for pro- ties of the modules directly affect the performance of tecting the semiconductor surface from oxidation. If the fiber laser systems. We have realized high power and highly-reliable laser diode modules by combining Current assemble technology developed for optical communi- 100 µm cation devices and high power laser diode chips devel- Dielectric oped by OPTOENERGY Inc. p-Clad 2. High-power laser diode p-Waveguide Active (QW) n-Waveguide 2.1 Device structure n-clad Figure 1 shows the schematic structure of 900 nm range high power multi mode laser diode (LD) based on InGaAs/AlGaAs materials. Laser structure consist- ing of an active layer, a waveguide layer and a cladding Beam layer is formed on GaAs single crystal substrate by epitaxial growth technique. Light emitting width of LD defined by lateral current confinement structure is Fig. 1. Schematic structure of multimode semiconductor laser set to be 100 μm in order to match the core diameter diode chip. of an optical fiber for efficient optical coupling. Laser mirror facets with flatness of atomic layer level are Facet oxidation formed by using cleavage technique. Laser facets are anti-reflection (AR) coated on front side and high-re- flection (HR) coated on rear side. The laser light is Light absorption mainly emitted from the front AR-coated facet. The maximum operable power of these LDs are, in gener- Non-radiative recombination al, limited by the catastrophic optical damage (COD) on laser mirror generated when the optical density ex- Facet heating ceeds the material limit 1) 2) 3) 4) 5) 6). Band gap energy redution 1 Applied Electronics Technology Department of Optics and Electronics Laboratory Catastrophic Optical Damage 2 Production Department, Fiber Laser Business Development Division 3 Silicon Technology Department of Optics and Electronics Laboratory 4 Optoenergy Inc. Fig. 2. Degradation process of LD facet mirror. Fujikura Technical Review, 2015 11 Panel 1. Abbreviations, Acronyms, and Terms. GaAs–Gallium Arsenide CW–Continuous Wave InGaAs–Indium Gallium Arsenide WPE–Wall Plug Efficiency AlGaAs–Aluminum Gallium Arsenide LD–Laser Diode CoS–Chip on Submount COD–Catastrophic Optical Damage DCH–Decoupled Confinement Heterostructure MTTF–Mean Time To Failure special consideration is not paid for the laser facet for- promising structure for high power and high reliabili- mation process, high-density of surface states are gen- ty LDs. erated in the laser facet due to the oxidation of the semiconductor surface 7) 8). Such surface states cause LDs are bonded on submounts using AuSn eutectic non-radiative carrier recombination generated by light solder with epi-side down configuration (called Chip- absorption in laser facet region and that induced local on-Submount: CoS) as shown in Fig.4. The submount heating on a laser output facet. The local heating on is high thermal conductivity type with thermal expan- the laser facet further causes band gap shrinkage fol- sion coefficient matched to the GaAs substrate. Figure lowed by optical absorption increase. Then such a 5 shows light output versus current (I-L) characteris- positive feedback loop of facet heating mechanism tics measured for CoS LD under CW and pulsed driv- eventually result in catastrophic damage of laser fac- ing conditions. In the case of CW driving condition, ets. This process chain is so called COD. Therefore, COD is not observed because the light output is limit- key point to prevent the COD and to improve the reli- ed by power saturation due to heating of the LD chip. ability at high output power operation is suppressing For accurate evaluation of COD level, therefore, the local heating on laser facets in every conceivable way, such as laser facet passivation to prevent surface state generation 9), minimization of light absorption 10) 11) 12), and light density reduction at laser facet 1). OPTOENERGY realized high reliability of these la- sers with long-term stable operation at high output power by applying unique LD design called decoupled confinement heterostructure (DCH) 13). Figure 3 shows band-gap diagram, and corresponding carrier distribution and optical mode profile for DCH and con- ventional separate confinement heterostructure (SCH). The advantage of DCH is that it can reduce the optical density at the active layer without any pen- Fig. 4. LD chip on submount (CoS). alty in carrier confinement. This is an essential condi- tion to achieve high COD level, as well as high power 40 Pulse and high temperature operation. Therefore, DCH is a 35 CW 30 Mode Mode 25 Carrier Carrier Peak intensity 20 Band-gap Band- Optical Power (W) 15 gap 10 915 nm-6 mm Energy 5 Light Intensity Band-gap Tpkg =15deg.C 0 0 51015202530354045 50 index Separated Decoupled Current (A) Confinement Confinement Heterostructure Heterostructure Fig. 5. Comparison of I-L characteristics between CW and (Conventional) (Patented) pulse operation for 6 mm-CoS under package temperature Fig. 3. Comparison of conventional structure and DCH concept. Tpkg of 15deg.C. 12 pulsed operation of LD is necessary in order to avoid measured at room temperature are as high as 60 % at the thermal influence. The light output power under 13 W for 4 mm-LDs and 55 % at 15 W for 6 mm-LDs, pulsed operation reached 38 W at 50 A driving current respectively. without COD. This result proved the advantage of DCH structure for highly reliable COD free operation. Light beam divergence properties of LDs is also an important characteristics similar to the light output 2.2 Electro optical properties power because it directly influences fiber coupling ef- Long cavity design is widely used as a simple solu- ficiency. The beam divergence properties measured tion to increase LD practical output. However, there is for vertical and horizontal directions are shown in a trade-off relationship between practical output power Fig.8. For vertical direction, single transversal mode and wall plug efficiency (WPE) in terms of cavity with Gaussian profile and constant divergence angle length. Therefore, there is an optimal laser cavity de- 27° (FWHM) are obtained without any driving current sign dependent on laser applications. For this reason, dependency. On the other hand, horizontal transver- we chose two different cavity length of 4 mm and 6 sal mode shows multi mode oscillation and have the mm. 4mm cavity LDs are suitable for WPE preferred injection current dependency. This is due to so-called application because they have a feature of high WPE thermal lens effect caused by the refractive index in- with low operation current. On the other hand, 6 mm crease at the center area of waveguide through partial cavity LD is suitable for power preferred application temperature increase. because of superiority in high power operation due to efficient heat dissipation. Light output power and WPE Table 1. Properties of 4 mm-CoS and 6 mm-CoS. characteristics measured for 4 mm and 6 mm-cavity Parameter Unit Laser characteristics LDs are shown in Fig. 6 and Fig. 7, respectively. Typi- Cavity length mm 4 6 cal value of laser characteristics are listed in Table 1. CW output power W 13 15 Practical output power in 4 mm-LDs is 13 W, while that Power conversion efficiency % 60 55 Horizontal beam divergence in 6mm-LDs reaches to 15 W. Practical output powers ° < 11 < 11 of these LDs are at the level of world top-class among @95% power single emitter lasers with 100 μm wide stripe. WPEs 16 80 14 70 13 W 12 60 10 50 8 40 WPE (%) Optical Power (W) 6 30 4 20 Intensity (arb. umit) 2 Optical Power 10 WPE 0 0 -60 -40 -20 0 20 40 60 02486 10 12 14 16 Angle(degrees) Current (A) Fig. 6. Characteristics of 4 mm-CoS. 18 90 16 15 W 80 14 70 12 60 10 50 8 40 WPE (%) 6 30 Intensity (arb. umit) Optical Power (W) 4 20 Optical Power 2 WPE 10 0 0 -30 -20 -10 0 10 20 30 0246 8101214116 8 Angle(degrees) Current (A) Fig.8. Typical beam divergence pattern of 4mm-CoS measured Fig. 7.Characteristics of 6 mm-CoS. in perpendicular ( ) and horizontal (//) directions. Fujikura Technical Review, 2015 13 This thermal lens effect tends to be suppressed in these multi-cell accelerated aging test results, we esti- long cavity length LDs such as 6 mm than 4 mm under mated mean time to failure (MTTF) for the condition the same driving current condition, because of re- of rated optical power and submount temperature of duced junction temperature. 25 deg.C. Estimated MTTF values are summarized in Table 2. Therefore, horizontal beam divergence angle at practical output power of 15 W in 6 mm-LD is achieved MTTF of random failure mode for 4mm-LDs is esti- to be 11°, that is almost the same divergence angle of mated to be 560,000 hours for 13 W operation condi- the 4 mm-LD at 13 W output condition.
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