Reliable laser-diode technology impacts the industrial-laser marketplace
Advances have allowed 980-nm AlGaInAs laser diodes to move from telecom to nontelecom applications across a gamut of wavelengths and output powers. A survey of AlGaInAs industrial-laser applications focuses on reliable high-power diode technology.
In the 1990s, laser-diode development mainly targeted high-dollar-volume telecom applications. Much of this technology is now available to cost-sensitive industrial applications. Sophisticated industrial systems unable to afford significant downtime need diode reliability at an acceptable price point. The strict quality control and large production capacity of telecom laser-diode manufacturers promise to fulfill these needs for wavelengths ranging from 800 to 1000 nm and high output powers, particularly in aluminum gallium indium arsenide (AlGaInAs) diodes.
Diode lasers can exhibit both sudden (infant or random failure regimes) and gradual degradation (wearout regime). Infant failures arise from an intrinsic semiconductor defect or damage/imperfections introduced during device fabrication. A rigorous burn-in (high drive current, high temperature) screens out infant failures. The high stress of this early screening process, however, requires robust laser technology so as not to introduce new failure mechanisms.
In the random failure regime, epitaxial defects are the predominant cause of sudden failure. The AlGaInAs random failure rate is proportional to temperature and drive current, as:
where Tj is the diode junction temperature, I is the diode drive current, and kB is the Boltzmann constant. The Arrhenius factor (EA), exponential current acceleration (m), and proportionality constant are fitted by measuring device failure rates under various temperature and current accelerations. Random failure rates are commonly quoted as failures in time (FIT). One FIT corresponds to a single device failure per 109 hours of deployment. One thousand FIT is a useful yardstick, equating to approximately 1% of the device population failing annually. The more-intuitive mean time before failure (MTBF) can be easily calculated from FIT rates.
In the wear-out regime, AlGaInAs-based lasers show negligible degradation except during extremely accelerated high-current and/or high-temperature operation. Since infant failure and wear-out do not occur in AlGaInAs diodes, the overall failure rate follows Equation 1. Generally, AlGaInAs laser-diode technologies tend to have EA = 0.45 eV, or in other words, a failure rate roughly tripling each 20°C. On the other hand, the current acceleration (m) and proportionality factor vary widely depending on manufacturing processes, emission wavelength, and device geometry.
Multimode QW lasers
Multimode InGaAs quantum-well (QW) laser diodes are mainly used to pump ytterbium (Yb)-based fiber lasers for telecom and industrial applications. Medical and material processing are growing niches for direct and fiber-coupled InGaAs diodes because of their high output power and brightness. Because they were developed for telecom, multimode InGaAs laser diodes are backed by extensive reliability testing across a wide range of operating temperatures and drive currents (see Table 1).
The data from multimode 100-µm-wide stripe InGaAs QW diodes yield a best fit to Equation 1 for EA = 0.43 eV and m = 5.5, which permits the calculation of total accelerated device hours (far right column of Table 1) and the FIT rates for various operating conditions (see Table 2). The model yields 377 FIT (60% confidence) at typical operating conditions of 4 W and 25°C heatsink (Tj = 48°C). Overall, multicell lifetest results prove telecom reliability to 4 W. While the 5-W failure rate is greater than the 500-FIT telecom standard, the corresponding 375,000-h MTBF exceeds typical industrial diode specifications. To our knowledge, this is the only broad-area laser-diode technology to have undergone multicell testing and demonstrated telecom reliability at any output power.
Single-mode 810- to 850-nm lasers
Single-mode laser diodes operating at center wavelength bands of 810, 830, and 850 nm are widely used for printing, metrology, inspection, and beam transmission (such as range-finding, illumination, targeting, and free-space communications) applications. High-performance AlGaAs or GaAs active-emitting-region diodes are considerably more challenging to fabricate than InGaAs QW lasers. Because the active region is unstrained, threshold-current density in AlGaAs or GaAs emitters is higher. The higher photon energy is more likely to spontaneously create semiconductor lattice defects, while the larger bandgap of AlGaAs raises device series resistance. Despite these handicaps, reliable single-mode AlGaAs emitter laser diodes operating at more than 200 mW are feasible.
Step-stress testing is a common technique to rapidly assess both device reliability and robustness of 810- to 850-nm laser diodes (see Fig. 1). In this test, the laser output is maintained at constant 150-mW power while the diode heatsink temperatures are stepped up by 10°C at 200-h intervals. The conclusion of the step-stress test is that 150-mW single-mode AlGaAs technology is extremely robust, showing no ill effects even when operated to a heatsink temperature of 70°C. The onset of rapid degradation by 90°C suggests the devices have entered a new operating regime, and would no longer follow a model based on Equation 1. Their robustness permits an aggressive but economical burn-in screening condition to ensure all infant failures are extinguished before shipment.
An extensive reliability test involved a total of 120 lasers run at constant current (averaging 150 mW output power) and 60°C heatsink temperature for 1500 h. Zero failures and no noticeable wearout were observed. Applying the EA = 0.45-eV rule of thumb, each device hour at 60°C equates to approximately 6 h at 25°C. Analysis of these data yields predicts reliability at 150 mW and 25°C of 810 FIT (60% confidence) or, equivalently, an MTBF of at least 1.2 × 106 h. If an application were to require higher reliability or different operating conditions, a more extensive multicell test could improve these statistically limited values.
Multimode 808-nm pumps
The 808-nm pump wavelength has long been used for Nd:YAG diode-pumped solid-state (DPSS) lasers. Like their InGaAs cousins, broad-area AlGaAs-based 808-nm pumps are also increasingly used for medical and materials processing applications. An MTBF of more than 10,000 h is preferred for DPSS lasers; more when several diodes are used. Because of the requirements of short wavelength, high power, and high brightness, many 808-nm vendors fall short.
In accelerated lifetests for 20 100-µm-stripe 808-nm lasers, zero failures and negligible degradation are observed after nearly a year of continuous operation, generating a lower bound of at least 175,000-h MTBF at 1.2 W and 25°C operation (see Fig. 2). Similar, but less extensive 808-nm lifetest data exist for 100-µm-stripe lasers operated at 2.5 W, 70°C, and 200-µm 808-nm stripe lasers at 3.5 W, 60°C. In each case, a conservative lower bound of MTBF equal to or greater than 100,000 h is established at rated operating power and 25°C. Because DPSS and other 808-nm applications do not require the reliability of multimode InGaAs, or even single-mode AlGaAs diodes, 808-nm pump reliability has not been comprehensively studied. As with single-mode AlGaAs emitters, if better reliability were required, as for satellite-based DPSS lasers, more extensive life testing of multimode 808-nm pumps would easily improve the demonstrated MTBFs.
Fiber-coupled multimode diodes
Many industrial customers prefer "plug- and-play" fiber-coupled laser-diode solutions. For years, the printing industry has consumed high-brightness 830-nm fiber-coupled multimode diodes for high-resolution "computer-to-plate" printing and other applications. High-power fiber-laser manufacturers require reliable, low-cost 920- or 970-nm pigtailed pumps, especially when output powers from 102 to 103 W are desired. Because expensive industrial systems can go down if only a couple of diodes fail, excellent reliability at low cost is demanded. Stacked semiconductor laser arrays (bars) have failed to meet the more stringent industrial reliability requirements of high-power industrial laser systems, and cannot approach the brightness of individual diode emitters. A new generation of super high-power fiber-laser and DPSS solutions will rely on pigtailed single emitters to reach new performance levels at acceptable reliability.
Reliability testing of one laser-diode variety used in low-cost, high-brightness industrial pigtailed products demonstrated zero failures and negligible degradation for standard 1.4 W, 25°C diode operation (see Fig. 3). A version of the standard package passed numerous telecom-inspired robustness tests including vibration, 100 × -40°C to 70°C temperature cycling, 80°C/110-day high-temperature storage, damp heat 40°C/95% relative humidity/56-day storage, and 106-times on/off power-cycling testing. Tens of thousands of devices already deployed for multiple years strongly corroborate the individual results of diode reliability and package robustness established by the in-house testing. A similar 920-nm 100-µm fiber-coupled product rated for 2.5-W operation is scheduled for upgrade to at least 5-W 920-nm fiber-coupled output power in 2003.
The authors thank their colleagues Jim Darchuk and Alex Schoenfelder for their contributions to this article.
TOBY STRITE is European business development manager, and VICTOR ROSSIN, ERIK ZUCKER, and MATTHEW PETERS are chip development managers at JDS Uniphase, 90 Rose Orchard Way, San Jose, CA 95134 Toby Strite can be reached at [email protected]