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AlGaInAs Broad-area Lasers Exhibit Outstanding Reliability

Low failure rates at high output power make broad-area AlGaInAs lasers suitable for industrial applications, write Victor Rossin, Toby Strite, Erik Zucker and Teh-Hua Ju of JDS Uniphase.
Massive investment driven by the 1990s telecom boom stimulated great progress in the available output power of single-mode AlGaInAs pump lasers. From 1997 to 2002, the peak operating power of 980 nm chips increased five-fold, while reliability was maintained at or below 200 failures in time (FIT; ~0.2% annual failure rate). Less attention was directed towards applying these advances to the AlGaInAs broad-area lasers used widely in fiber lasers and other industrial applications. The current telecom crash has led to this situation being redressed. This article presents the current state of the art for broad-area AlGaInAs diode laser reliability and output power.
Diode structure and performance
The AlGaInAs lasers described below feature an InGaAs strained-layer quantum well (QW) active region and a separate confinement heterostructure waveguide region. Through fine epitaxial adjustments, the basic structure can emit throughout the 900-1000 nm wavelength range at broadly similar efficiency and reliability. The laser diode chips are of a planar design, simplifying both the laser fabrication and the junction-side-down mounting to a heat sink. Straightforward processing forms the 100 µm emitting aperture, and a simple and robust coating technology greatly enhances facet durability.

Figure 1 plots the key L-I, V-I and power conversion efficiency characteristics of 100 µm aperture lasers at 25 ºC. Threshold current values around 300 mA with a slope efficiency just above 1 W/A are typical. Power conversion efficiency peaks around 60% just above 2.5 W optical output power. An output power of 8 W can be sustained at 9 A drive current. The maximum CW output power levels for individual devices are in the 8-12 W range, limited by catastrophic optical damage, heat-sink thermal resistance and heat-sink temperature. The inset shows the typical multimode output spectrum featuring a smooth power distribution within a FWHM band of several nanometers.

The 100 µm AlGaInAs lasers feature a Gaussian-shaped spatial light output. Typical far-field values are 26.5º FWHM in the plane perpendicular to the QW, and ≤12º FWHM in the plane parallel to the QW, allowing efficient fiber-coupling in the production environment of around 80%. Figure 2 illustrates a typical far-field power distribution for a laser diode operating at 4 W.
Overview of diode laser reliability
Over the past decade, telecom vendors learned to manufacture laser diodes whose reliability is well understood. The low failure rates of advanced diode technologies make it uneconomical to perform reliability testing under normal operating conditions. Instead, multicell testing under highly accelerated conditions is used to develop a statistically relevant device reliability model. Multicell tests generally probe the upper temperature and drive current ranges to deduce the sensitivity of diode reliability to each parameter. Analysis of the observed failure rates under high stress produces a model from which failure rates at normal operating conditions are extrapolated.

Diodes fail either suddenly (infant or random failure regimes) or through gradual degradation over an extended period of time (wear-out regime). Several root causes may contribute to failures within any regime.

Infant failure regime Infant failures occur early in a laser s working life. Such failures usually arise from damage or imperfections introduced during device fabrication or intrinsic semiconductor defects. A rigorous burn-in at high drive current and ambient temperature conditions is the accepted technique to screen out infant failures. The overstress must be severe enough to eliminate diodes subject to infant mortality over an economically viable burn-in period, but must not introduce any new failure modes. Therefore, only a robust diode laser technology can be effectively screened in production for infant mortality. For the lasers discussed below, routine screening, including a short burn-in, was performed prior to multicell testing to remove infant failures from the studied population.

Constant random failure rate regime Sudden failures are the predominant failure mode in this regime. The root cause is randomly dispersed epitaxial defects. Therefore the failures are expected to be statistically random in nature. The random failure rate is accelerated by temperature and drive current, and can generally be fitted to equation 1, where EA, m and the proportionality constant are the independent fitting parameters investigated by multicell testing. Within this regime, failure rates are most commonly quoted in FIT, where one FIT corresponds to a single device failure per billion hours of deployment. A useful yardstick is 1000 FIT, corresponding to ~1% of the device population failing per one year deployment period. Telecom laser diodes are typically required to have 500-1000 FIT reliability, but
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