Laser Diode Life Testing

Life test studies are used to collect laser diode lifetime data under carefully controlled operating conditions. These data are then used to develop statistical models that can predict the laser lifetime under the intended operating conditions. Within the telecommunications industry, standards for life test studies are developed and promulgated by Telcordia Technologies.

Laser diode life test studies are type- and application-specific. They require the periodic measurement of a variety of device parameters including operating current, optical output power, threshold current and forward voltage under accelerated aging conditions. Accelerated lifetime testing can be accomplished by using higher settings for temperature, injection current or optical power. However, temperature acceleration is most common.

Life Test Modes

The various modes for life test studies are listed below:

  • Constant current aging or ACC mode (automatic current control) . The laser current is held constant for the duration of the test.
  • Constant power aging or APC mode (automatic power control) . Laser output power is held constant by continuously adjusting the current to maintain constant output power. Output power is measured either with an external photodetector or using an internal monitor photodiode if one is available within the laser package. Constant power aging is the life test mode most often used since it closely resembles the typical operational mode of a laser diode.
  • Periodic sample testing - When lifetime testing laser diodes at a higher temperature, typical procedures burn the diode in at constant current and high temperature, then reduce the temperature of the laser to evaluate relative changes in output (generally, laser diodes do not lase at temperatures greater than 100°C). In this type of test, lasers are operated in constant current mode during the high temperature aging. The sampling interval may be varied over the duration of the very long-term test to reduce the amount of data collected. In these cases, sample measurements may be obtained every hour at the beginning of the test period and every few days after the test has been running for months.

Life Test Systems

Many modern reliability and life test systems are designed to house different devices and package types within a single system, thus facilitating faster testing, increased throughput, and captive control. Important considerations for choosing a laser diode life test system include:

  • Device protection - Reliability test systems are designed to ensure that the DUTs are protected from damaging surges/ transients, overcurrents, reverse currents, etc.
  • Current control - Perhaps the most important characteristic of laser diodes is their efficiency - the degree to which they emit light when current is injected into the device. Therefore, a device under test must be subjected to a precisely known and repeatable current. This is most easily guaranteed when each device is driven by an independent current source.
  • Temperature control - Laser diode properties such as threshold current are very sensitive to changes in temperature. For example, a GaAlAs laser diode operating at wavelengths around 850 nm experiences threshold increases of ~1% for every 1°C rise in temperature, while an InGaAsP laser operating at wavelengths around 1300 nm sees threshold increases of ~2% for each 1°C rise in temperature. For this reason, device reliability tests require precise and repeatable control over temperature as well as good temperature uniformity. TECs, water cooling, air cooling and embedded temperature sensor arrays are some of the methods used to control temperature in laser diode test systems.
  • Output power - A device's reliability can be evaluated by monitoring its output power along with changes in the output power over time and under different conditions. Reliability and test systems estimate reliability by monitoring device output power with either external detectors or the device's internal monitor photodiode.
The threshold current is the point along the LI curve where the device starts to emit light or begins to lase
Figure 1. The threshold current is the point along the LI curve where the device starts to emit light or begins to lase. According to the Telcordia standards, it can be calculated by three different methods: second derivative, first derivative, and two-segment fit algorithms.
  • LI curves - The LI curve (Figure 1) is one of the most useful tools for determining a device's performance and performance changes that occur over time or in relation to other devices, i.e., comparative LI curves. As such, this is one of the key graphs produced by test systems.
  • Repeatability - Repeatable performance is an absolute requirement for laser diodes. Thus, life test systems must exhibit minimal variations in input currents, output power measurements, and temperature control to ensure that device changes or failures are attributable to the device rather than to inconsistencies in the test and analysis equipment.
  • Modularity - Test systems that are modular in design are more adaptable to the many variables and modifications associated with laser diode reliability and life testing - such as differences in package type, pinout, number and type of electrical signals required, higher or lower temperature requirements, and differing output powers associated with different devices. Separating modules for different aspects of the test enables flexibility. Modular test systems facilitate troubleshooting, streamline repairs, and updating the functionality of the system. The separate modules in a modular life test system include: device carriers to house different devices, electronics to provide current and temperature controls, and software to monitor performance and modify test parameters.
  • Scalability - It is advantageous to select a life test system that scales with increased production and customer growth. It is not uncommon for production and test requirements to double or even triple; the ability to expand an existing life test system to meet increased demand can save significant time and expense.

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