Active cooling solutions for high power laser diodes stacks

Yoram Karni*, Genady Klumel, Moshe Levy, Yuri Berk, Yaki Openhaim, Yaakov Gridish, Asher Elgali, Meir Avisar, Moshe Blonder, Hila Sagy, Alex Gertsenshtein

Semiconductor Devices; P.O.Box 2250, Haifa, 3102, Israel; *

ABSTRACT

High power water cooled diode lasers find increasing demand in biomedical, cosmetic and industrial applications, where very high brightness and power are required. The high brightness is achieved either by increasing the power of each bar or by reducing the emitting area of the stacks. Two new products will be presented: Horizontal CW stacks with output power as high as 1kW using 80 W bars with emitting area width as low as 50 µm; Vertical QCW stacks with output power as high as 1.2kW using 120 W bars. Heat removal from high power laser stacks often requires microchannel coolers operated with finely filtered deionized (DI) water. However, for certain industrial applications the reliability of this cooling method is widely considered insufficient due to leakage failures caused the highly corrosive DI water.

Two solutions to the above problem will be discussed. A microchannel cooler-based package, which vastly reduces the corrosion problem, and a novel high-power laser diode stack that completely eliminates it. The latter solution is especially effective for pulsed applications in high duty cycle range.

Keywords: diode laser, repetitive cw operation, qcw operation, laser diode reliability High power diode laser,

microchannel cooling

1. INTRODUCTION

High power laser diodes have become viable component for a variety of laser applications in industry, medical and cosmetic applications and solid state laser pumping. These applications require high average optical powerTypical output power per bar is 50 – 100 W in CW mode and 100 – 150 W ( peak) in QCW mode. These devices operate with electrical-to-optical efficiencies in the range of 50% in most of the available components and up to 75% in the lab. As a result, a substantial amount of waste heat is generated and heat fluxes of the order of 1kW/cm2 are common in the industry today. As device technology continues to improve and optical output powers continue to increase, heat removal by laser diode packages becomes one of the main limitations.

The methods of removing large amounts of waste heat in a laser diode package are normally based on copper heat exchangers. These are micro channel cooler (MCC) and jet cooler packaging technologies, the former is made from multiple layers of copper, which are bonded to give the delicate micro structured heat exchangers . The laser diode bar is often soldered directly to the cooler or to a subassembly that consists of a heat spreader , CTE matched to the laser diode bar. The latter configuration allows hard solder to be more easily used and improves the reliability of the laser diode with minimal reduction of the thermal conductivity of the device.

Several laser manufacturer offer laser diode arrays that are based on laser diode bars mounted on MCC’s. These slim subassemblies are normally stacked so as to obtain high output power and high brightness arrays. Since the subassemblies are electrically connected in a serial fashion, a non conducting cooling fluid is mandatory for preventing current leakage and for inhibiting the galvanic degradation of the coolers. Nevertheless, it has been often observed that this type of packages suffers from two characteristic degradation modes. On one hand, due to he high efficiency of the MCC for heat removal, it has been found that the package solders experience severe stresses when the laser is operated in a relative slow repetitive mode. This leads to thermal mechanical fatigue of the solder. A novel approach, that uses hard solder technology was shown to solve this problem [1]. An additional degradation mode is electrochemical oxidation of the MCC, which occurs on the water inlet and outlet holes and which leads to water leakage. This degradation mode is the main failure mechanism for these MCC based laser diode arrays.

* karni@scd.co.il; Phone:(972) 4-990-2731; Fax: (972) 4-990-2686;

High-Power Diode Laser Technology and Applications VI, edited by Mark S. Zediker,Proc. of SPIE Vol. 6876, 687604, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.765156

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Efforts have been made to optimize the coolant conditioning in order to extend the life time of the heat exchangers[2,3]. The copper exchangers which are in direct contact with the water may go through the following electrochemical reaction:

2 Cu +O2 +4 H+ 2 Cu+2 +2 H2O EMF = 0.55 V.

And

4 Cu +O2 +4 H+ 4 Cu+1 +2 H2O EMF = 0.51 V

The resulting cupric and cuprous ions react with hydroxide ions to give the copper oxides CuO and Cu2O, respectively.

When an electrical potential is applied between the copper heat exchangers a very rapid reaction may occur. However, if the conductivity of the coolant is kept low, the electrochemical reaction is limited to the leakage current. It is normally recommended to use DI water with 0.1-0.5 MOhm·cm resistivity, which typically results in a vertical stack resistance of ~100-200 kOhms between adjacent MCC’s. Further increase of the water resistively will result in lower PH and will generate oxidation by the H+ ions [2]. When about 2V are applied between the MCCs a typical electrochemical reaction current is of the order of 10-5 Amp. This is equivalent to dissolution of 0.2 grams of copper per year. When the MCC is gold plated or if other material is deposited on the MCC this reaction will be partly inhibited. Nevertheless, due to erosion of these layers the electrochemical reaction is inevitable and will eventually lead to failure of the device.

In this paper, two solutions to the above problem will be discussed: A MCC-based package, based on SCD's standard MCC in which the electrical circuit is isolated from the cooling circuit, thus vastly reducing the corrosion problem, and a novel high-power high duty cycle laser diode stack that completely eliminates it.

2. MICROCHANNEL COOLER-BASED LASER DIODE WITH NON ISOLATING COOLANT

A Laser Diode Stack (LDS) using non isolating coolant was fabricated. The LDS is designed to replace existing MCC based LDS without affecting the overall performances and in particular the thermal resistance, the stack dimension and output power. Two configurations are presented: a vertical and a horizontal LDS. The vertical LDS was designed to maintain the narrow pitch which characterizes MCCs.

The ZOFAR subassembly, which is a package of a MCC mounted laser diode bar with floating electrical contacts is illustrated in Figure 1. A laser bar is soldered on one side to a heatspreder which is mounted on a MCC. The heatspreader is made of insulating material and is coated on the top side by a thick layer of metal. Electrical leads are drawn from the upper side of the metalized heat spreader and from the upper side of the laser bar exceeding behind the MCC. Insulating sheets are positioned between and below the leads thus providing insulation among themselves and from the MCC. If the ZOFAR is used in a vertical configuration an additional insulating layer is positioned on the upper lead. The subassembly of a laser diode bar mounted on MCC with its electrical contacts is so called barcooler.

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Figure 1 Illustration of ZAFOR MCC

The additional heatspreader between the laser bar and the microchannel cooler serves two purposes, electrical insulation of the bar from the MCC on one hand and improvement of the package reliability, as it has been described before, on the other [1]. However, the additional width results in thicker subunits and reduces the overall brightness of the vertical stack. In order to reduce the MCC pitch to the original dimension, a special MCC was designed at SCD. In the new design ,shown on Fig 3, a step- like niche was made on the front side of the MCC. This step reduces the overall width of the subassembly and thus, it maintains the relatively small pitch which characterizes the MCC packages.

Figure 2 Illustration of ZAFOR MCC with a step on the front side

Figure 4 illustrates the stacking of the vertical stack with the contacting washers on the back. Figure 5 shows a 12 bar horizontal stack with crossing contacts on the back.

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Figure 3 Vertical stack assembly

Figure 4 12-bar Horizontal stack ( 80 W bars)

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3. RESULTS

ZAFOR units were assembled into vertical and horizontal stacks and single bar test modules. The lasers were characterized and tested for lifetime The results of their power and efficiency performance, thermal resistance and overall durability are described in this section. A visual comparison is made between ZOFAR and standard barcoolers that have gone through similar tests.

Throughout all tests, the ZAFOR laser diodes were operated using untreated tap water that was only circulated through a 10- micron particle filter. Standard barcoolers, which employ the same MCCs and are cooled with the standard recommended coolant: 8-8.5 PH, 0.1-0.5 Mohm resistance deionized water, were tested and served as a reference.

ZAFOR subunits were operated during 1000 hours on a lifetime test system. After 1000 hours with no power degradation the MCC were dismounted and examined. Figure 5 shows the water inlet holes of a ZAFOR barcooler after 1000 of operation with untreated tap water and a standard barcooler after similar operation time. One can easily observed that while the ZAFOR MCC is completely intact, the MCC of the standard package already shows damage which indicate the beginning of corrosion.

Figure 5 Water inlet holes of a ZAFOR MCC (right) and in a standard MCC (left) after 1000 and 500 operation

hours respectively The performance of a 1 mm 808 nm bar on a ZAFOR barcooler is depicted in Figure 6. These results as well as the thermal resistivity of 0.35 K/W are very similar to those obtained on a standard barcooler [1]. The similar performances were foreseeable as there is no change in the thermal interface of the two designs. It should be emphasized that while the standard barcoolers were operated with the optimized coolant conditions, the coolant of the ZOFAR barcoolers was standard tap water. By appropriate choice of water additives the durability of the ZOFAR cooler can be further improved.

A horizontal stack was constructed from 12×80 watt 808 nm laser bars mounted in ZOFAR sub units. The stack is shown in Figure 4. The power and efficiency of the stack are shown in Figure 7. The spectrum of the horizontal stack was measured at full power operation and is shown inFigure 8 The high efficiency and the narrow spectrum obtained at this high power are good indication of the quality and homogeneity of the mounting technology.

The stack was tested at an operational current of 85 A for more than 700 hours (

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Figure 9). During the first operational stage a minor degradation of ~ 4% was observed. However, after the first degradation the power reaches stabilization. This degradation is explained as the result of to laser diode bars as an extended bar burn- in process.(Yoram please rewrite the previous sentence)

010203040506070

10 15 20 25 30 35 40 45 50 55 60

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Figure 6 power and efficiency vs current plots of a 1 mm bar on ZAFOR MCC

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Figure 7 Power and efficiency of a 12 bars horizontal stack.

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Spectrum ( 25 C, 85A)

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Figure 8 Spectrum of a 12 bar horizontal stack at optical output power of 1 kW

12 bars linear laser diode array lifetime testing at 85 A

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Figure 9 Life time test of a 12 bar STACK

4. LASER DIODE STACK FOR HIGH POWER, HIGH DUTY CYCLE OPERATION

Passively cooled packages are known for their superior reliability over MCC packages. Passively cooled laser diodes on CS mounts have been proven to operate for many thousands of hours. However, these packages are limited to relatively low output power and they cannot be densely stacked.

On the other hand QCW stacks can produce very high peak power with excellent brightness. It was also demonstrated in the past that this stack can be very reliable and demonstrates excellent endurance under lifetime and environmental stresses [4]. These packages are limited, however, to relatively low average power and duty cycle.

A new laser diode stack was designed to enable operation of high power QCW stacks in high duty cycle conditions. The new stack, shown in Figure 10 is a robust pack ,based on SCD gold tin technology [4] and is fabricated with bars

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positioned with 1.7 mm pitch. The stack head is convectively liquid cooled through an array of fins protruding from its rear side. Since the laser diode head is completely isolated from the coolant and since the fins spacing is of the order of tenths of mm, only a coarse particle filter is required for the coolant. Moreover, the robust design of the laser diode head should make it more stable towards environmental stresses and water leakage and it should serve as a more suitable platform for mounting of optical elements.

The thermal resistance of a 10 bar stack is 0.2 K/W, equivalent to a 20 K increase in junction temperature when emitting 1 kW peak power at 10% duty cycle. A typical power vs. current curve is shown in Figure 11.

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Figure 11 Power vs. Current plot of a 10 bars stack operated at 10% duty cycle.

5. SUMMARY

Novel designs of laser diode packages were demonstrated. One design, which is based on a microchannel cooler completely eliminates the leak electrochemical current, while preserving the excellent heat removal performances of the MCC. The other solution, which is mainly designed for high duty cycle QCW operation mode, is a robust water cooled package. It is expected that the new designs will contribute to solving the most common failure mode of water cooled laser diode stacks.

Figure 10 Ten bar water cooled stack assembly

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REFERENCES

1 Genady Klumel, et al., “Reliable high power diode lasers - thermomechanical fatigue aspects”, proc. of SPIE 6104-2 (2006). 2 Georg Truesch et. al., “Reliability of Water Cooled High Power Diode Laser Modules,” proc. of SPIE 5711, 132-141 (2005). 3 John Haake and Brian Faircloth, “Requirements for Long Life Micro-Channel Coolers for Direct Diode Laser Systems,” proc. of SPIE 5711, 121-131 (2005). 4 Nir Feldman, et al., Highly efficient and reliable 1 kW QCW laser stacks with diffraction limited fast axis beam collimation”, proc. of SPIE 6456-42 (2007).

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