High-reliability high-efficiency 976-nm diode laser pump sources

High-reliability high-efficiency 976-nm diode laser pump sources E. Stiers, M. Kanskar Alfalight, Inc, 1832 Wright St., Madison, WI 53704 Email: [email protected] ABSTRACT Long term lifetest data is presented for Al-free active region 980 nm multimode laser diodes configured as chip-onsubmount devices, as packaged fiber-coupled devices, and as multi-emitter laser bars on a microchannel cooled heatsink. Single emitter devices have been tested in chip-on-submount form. A first set of 120 of these devices were tested in a five-cell matrix at varying junction temperatures and optical output levels to obtain measured values for both random and wear-out failure model parameters. A second set of 187 packaged lasers were placed on accelerated lifetest to measure FIT data. In both cases, the devices were operated for up to 9,000. Another set of chips were packaged and tested inside a fiber-coupled, TEC cooled, 14 pin butterfly case as part of a Telcordia qualification process. These devices were operated for up to 5,000 hours with no failures and no degradation of either the chip or the package. Bar devices with a 20% fill factor were mounted on microchannel heatsinks and tested for one second on, one second off quasi CW operation for 4,000 hours. This test condition places a thermal expansion cycle stress on the devices, however once past the initial burn-in period very little degradation is seen in the output characteristics of the device. Key Words: Al-free, semiconductor laser diode, reliability, lifetest, pump source, multimode laser

1. INTRODUCTION Reliable diode lasers emitting optical power at approximately 980 nm are required for applications such as pumping erbium doped fiber amplifiers, dual-clad fiber lasers, or solid-state lasers for military, medical, printing, and industrial use. Reliable diode lasers may be constructed using Al-free materials and a large optical waveguide design, and this reliability can be confirmed by rigorous testing. Al-free active devices, which use the InGaAsP/InGaP/GaAs material system may have superior power conversion efficiency1 when compared to traditional Al-containing structures due to factors such as lower series resistance2 and higher thermal conductivity. The use of the Al-free material system also allows the inclusion of strain-compensated quantum wells which provide high gain while maintaining high reliability3. The absence of Al leads to a lowered surface-recombination velocity at the device facets which lowers facet temperatures. When these materials are used in conjunction with a broadened-waveguide design record high optical power densities of 18.5 MW/cm2 have been achieved4.

2. RELIABILITY TESTING FOR CHIP-ON-SUBMOUNT DEVICES In order to determine the reliability characteristics for Alfalight’s Al-free active single-emitter laser diodes two tests were performed: (1) A five-cell matrix for determining wear-out and random failure rates, and (2) An accelerated lifetest for determining lifetime and FIT (failure in time, the number of expected device failures per 109 operation hours) reliability

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High-Power Diode Laser Technology and Applications III, edited by Mark S. Zediker, Proc. of SPIE Vol. 5711 (SPIE, Bellingham, WA, 2005) · 0277-786X/05/$15 · doi: 10.1117/12.591030

data. Early results from these tests were discussed earlier5, we now present results from a significantly longer period of testing. A five-cell matrix test was performed using 120 chip-on-submount devices from three different epitaxial growth runs and three different device fabrication runs in order to determine both the wear-out failure rate and the random failure rates. This test was conducted in compliance with the Bellcore GR-468 standard. These diodes had a 2mm long cavity with 100 µm wide emission aperture and were bonded p-side down with AuSn solder onto CuW submounts. Each diode was burned in before lifetest and screened by wafer qualification. Devices in each matrix cell were operated in automatic power control (APC) mode at different powers and junction temperatures as follows: Cell 1: 1.5 W and 62°C, Cell 2: 1.5 W and 90°C, Cell 3: 1.5W and 118°C, Cell 4: 1W and 90°C, Cell 5: 2W and 90°C. These conditions stress the diodes well beyond their designed operating point and are chosen to induce device failures within the timeframe of the test. Data collected from this test is shown in Figure 1 below. 2W 90C APC Lifetest

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Figure 1: Data collected from five-cell lifetest matrix to 9000 hours. Temperature is varied from cell to cell in the horizontal direction, optical power output is varied in the vertical direction.

Assuming that the failure mode for these devices is dominated by bulk defect propagation, the expected failure rate in time is assumed to be controlled by the factors shown in Equation 1 below, where kb is the Boltzmann constant, Tj is the laser diode junction temperature, P is the optical power output of the diode, and n and EA are fitting parameters.

FailureRate(T j , P) ∝ e

− E A k bT j

Pn

(1)

Given the data from the five-cell test it is possible to fit the unknown parameters Ea and n from Equation 1. The results of this parameter fitting are outlined in Table 1 below. Wear-out failure is characterized by a slow change in diode properties over time, while random failures are characterized by sudden device failures.

Proc. of SPIE Vol. 5711

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Activation Energy, Ea Power Exponent, n

Wear-out Failure 0.64 eV 2.2

Random Failure 0.53 eV 2.3

Table 1: Measured wear-out and random failure parameters extracted from the multi-cell matrix shown in Figure 1.

In addition to the five-cell matrix test, a second set of chip-on-submount devices were placed on lifetest to determine FIT values due to random failures. These devices are from five different epitaxial runs and three different processing runs. Accelerated testing conditions were used to obtain reliability results in a reasonable amount of time. At the time of this writing, over 1,868,617 device hours from a total of 187 devices operating at 3A and heatsink temperature of 70°C have been collected, and only one sudden failure has been observed. Data collected from the longest running cell is shown in Figure 2 below.

Figure 2: Accelerated lifetest data for one test cell from an accelerated life test sample. These devices are operated at a heatsink temperature of 70 °C.

Given the experimentally determined degradation rates of the activation energy, Ea , and the power exponent, n, from the five-cell test described above, the long-term reliability parameters for these devices can be calculated from the following equations:

FIT =

Nγ ⋅ 10 9 TotalDeviceHours

E ML(T2 ) = ML(T1 ) ⋅ Exp  A  kB

 1 1   −    T2 T1  

(3)

(4)

n

 E  1 1  P FIT (T , P) = FIT (T0 , P0 )  Exp  A  −   P0   k B  T0 T 

(5)

Where ML is the median life, kB is the Boltzmann constant, N is the total number of suddenly failed devices, T is the junction temperature, P is the output power and γ is a statistical weighing parameter that determines the confidence

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level in the FIT value for a given number of realized failures and the values are outlined in Bellcore GR-468. The median life and FIT values are first determined for devices operating at the accelerated lifetime testing temperature, and these results are then translated to the expected lifetime for a device operated at room temperature via Equations 3-4. With the end of life defined as 20% increase in operating current, linear extrapolations from the past few thousand hours of operation were made to project time to failure. Lognormal plots were generated to determine median life. Assuming values of Ea=0.64 eV and n=2.2 which were determined from the five-cell matrix test described earlier, the projected median life is calculated to be over 75.5 years (39 FIT at 60% confidence level) for operation at 2W and at 25°C heatsink temperature.

3. PACKAGED 14-PIN, TEC COOLED, FIBER-COUPLED DEVICES A set of 27 laser diode chips of the kind described in the chip-on-submount tests were packaged in fiber-coupled, thermo-electric cooled (TEC), 14-pin packages in order to confirm the reliability of the chips, the package, and the assembly process. These devices were then tested in an accelerated lifetest system at a heatsink temperature of 70°C (25°C chip temperature due to the TEC cooling) at an APC power of approximately 2 Watts. These tests were performed as part of a Telcordia qualification of the 14-pin multimode package. The Telecordia specification requires 2,000 hours of operation with less than a 10% increase in current required to maintain constant power. Data from one test cell of 11 devices is shown in Figure 3 below, demonstrating zero failures to 5,000 hours. The current required to maintain a constant power level actually decreased during this time period, preventing the calculation of FIT data for these devices without collecting many more hours of data. 3.0 2.5

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4. QUASI-CW PULSED LIFETEST FOR BAR DEVICES


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Mounted bar data has been collected for 18 emitter, 20% fill-factor devices, emitting at 980nm, and tested under quasiCW conditions at 40W optical power output. These bars had the same transverse structure as the single emitter devices described earlier. A total of twelve bars were used in these tests. These devices have a 2 mm cavity length and had eighteen 100 micron emitters arrayed on 500 micron centers across a 1 cm wide bar. The front facet was coated with a 3% reflectivity coating and the back facet was coated with a 97% reflectivity coating. The bars were fully metalized and individually mounted pside down on a microchannel cooled copper heatsink. Each mounted diode bar was tested for over 4,000 hours at a controlled current of approximately 43A, resulting in approximately 40W of optical power. The heatsink temperature throughout the test was 25C. The current was pulsed on for one second, then turned off for one second. This slow pulsing allows the bar to fully thermally equilibrate at both the on and off temperatures and therefore maximizes the amount of thermal expansion stress placed on the device. Over the course of the test, each bar underwent approximately 7.2 million such cycles. The bars were tested under automatically controlled current conditions (ACC) throughout the test. These devices were not screened before being placed on the lifetest system, and three devices failed early in the test. These early failures are most commonly due to either defects present on the facets of the laser diode, damage during handling, or failures of the bonding process. Such early failures are screened at the factory and removed from the population in a commercial situation. After a burn-in period of approximately 120 hours, the devices reach a steady state of operation for the remainder of the 4,000 hours of operation. The results of this test are shown in Figure 4 below.

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Time (hours) Figure 4: Power under ACC conditions for Al-free active, 20% FF bars operated at approximately 40W optical power output. After a burn-in period of approximately 120 hours the devices reach a stable output power.

Periodically throughout the test the optical power of each device was brought to 40W in an automatic power control (APC) condition and the resulting drive current was recorded. As with the ACC results described above a burn-in period during the first 120 hours was observed followed by a long period of stable operation. This data is shown in Figure 5 below. The failures observed during this burn-in period are the same devices which exhibited early failure in the ACC data.

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5. CONCLUSIONS Long-term lifetest data has been collected for Al-free active region laser diodes configured as chip-on-submount, 14-pin packaged single-emitter devices, and 20% fill-factor bars mounted on a microchannel cooler. Stable operation of the chip-on-submount devices has been demonstrated. A five-cell test has been run to 9,000 hours, and degradation parameter values of Ea=0.64 eV and n=2.2 have been extracted. An accelerated lifetime test has been run for up to 9,000 hours and a lifetime of over 75.5 years for operation at 2W and at 25°C heatsink temperature has been estimated. Packaged 14-pin devices have been tested as part of a Telcordia qualification process, and have been operated to 5,000 hours with no degradation. The current required to maintain a constant power level actually dropped over this time period, making the estimation of FIT data impossible within the timescale of the test. Operation of the bar devices has been demonstrated to 4,000 hours under quasi-CW conditions, with stable current and optical power values after an initial burn-in period of approximately 120 hours.


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6. ACKNOWLEDGEMENTS The authors would like to acknowledge valuable technical assistance from E. Boyer, Z. Dai, D. Forbes, C. Galstad, T. Goodnough, M. Lardinois, D. L. Lindberg III, M. Klaus, M. Martin, R. Mitchell, M. Nesnidal, T. Peppich, and A. Quandt of Alfalight. We would also like to thank Professors D. Botez and L. Mawst from the University of Wisconsin – Madison for their valuable discussions. We would also like to thank P. Henning, Dr. I. Michler, and Dr. D. Wolff of Jenoptik for their expertise in packaging and testing the laser diode bars discussed in this paper.

REFERENCES 1. D. Botez, L.J. Mawst, A. Bhattarcharya, J. Lopez, J. Li, T.F. Kuech, V.P. Iakovlev, G. I. Suruceanu, A. Caliman, and A. V. Syrbu, Electronics Letters, 32, 2012 (1996). 2. A. Al-Muhanna, L.J. Mawst, D. Botez, D.Z. Garbuzov, R.U. Martinelli, and J. Connolly, Appl. Phys Lett., 71, 1142 (1997). 3. T. Fukunaga, et al, Appl. Phys. Lett. 69, 248 (1996). 4. J.K. Wade et al, Appl. Phys. Lett. 72, 4 (1998). 5. M. Kanskar, et al, Proceedings of SPIE, 4995, 196 (2003).

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