SPI Laser

Impact of Laser Diode Reliability

Most of the laser diode technology has become offered to cost-sensitive industrial applications. Sophisticated industrial systems not able to afford significant downtime need diode reliability at a satisfactory price point. The strict quality control and enormous production capacity of telecom laser-diode manufacturers promise to satisfy these needs for wavelengths which range from 800 to 1000 nm and high output powers, particularly in aluminum gallium indium arsenide (AlGaInAs) diodes.

Into 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 general failure rate follows Equation 1. Generally, AlGaInAs laser-diode technologies are apt 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 according to manufacturing processes, emission wavelength, and device geometry.

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 for this early screening process, however, requires robust laser technology so as not to ever introduce new failure mechanisms.

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 the 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 information 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) together with FIT rates for assorted 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. Even though the 5-W failure rate is more than the 500-FIT telecom standard, the corresponding 375,000-h MTBF exceeds typical industrial diode specifications. To the knowledge, this is basically the only broad-area laser-diode technology to possess undergone multicell testing and demonstrated telecom reliability at any output power.

Single-mode 810- to lasers that are 850-nm

Step-stress testing is a very common way 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 whilst the diode heatsink temperatures are stepped up by 10°C at 200-h intervals. The conclusion for 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 start of rapid degradation by 90°C suggests the devices have entered an innovative new operating regime, and would no further follow a model according to Equation 1. Their robustness permits an aggressive but economical burn-in screening condition to ensure all infant failures are extinguished before shipment.

Single-mode laser diodes operating at center wavelength bands of 810, 830, and 850 nm are widely useful for printing, metrology, inspection, and beam transmission (such as for instance range-finding, illumination, targeting, and free-space communications) applications. High-performance AlGaAs or GaAs active-emitting-region diodes are somewhat more difficult to fabricate than InGaAs QW lasers. Considering that the active region is unstrained, threshold-current density in AlGaAs or GaAs emitters is higher. The bigger photon energy is almost certainly going to spontaneously create semiconductor lattice defects, although the larger bandgap of AlGaAs raises device series resistance. Despite these handicaps, reliable single-mode AlGaAs emitter laser diodes operating at significantly more than 200 mW are feasible.

A comprehensive 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 with 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 those data yields predicts reliability at 150 mW and 25°C of 810 FIT (60% confidence) or, equivalently, an MTBF of at the least 1.2 × 106 h. If a credit card applicatoin were to require higher reliability or different operating conditions, an even more extensive multicell test could improve these statistically limited values.

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