2. Handling Precautions and Reliability
2-2-2. Reliability Theory
The well known bathtub curve describes the failure rate for most electrical devices (Fig. 2). In the initial stage, the failure ratio starts off high and decreases over time. After this in the random failure stage, the failure rate is constant. The last stage is the wearout period in which the failure rate steadily increases. The lifetime of equipment using semiconductors is much shorter than the devices themselves which usually do not reach the wearout stage. Through the years improvements in laser diode technology have extended average lifetime to a level of typical semiconductor devices. More importantly, the failure rate in the initial stage has been reduced by upgrading materials, improvements in processing, and the use of screening technology.
When a photodetector such as a laser diode operates in the forward direction, the increase in current which does not contribute to light emission causes the light emission characteristics to change over time. Among these characteristics, the device lifetime is generally defined by the time-dependent change in the optical power output vs. operating current characteristics, which have a large effect on the drive circuit. This is shown in Fig. 3(a). When an automatic power control (APC) drive which maintains a constant optical power output is performed, light emission at the constant optical power output PO is no longer possible at time t5.
In consideration of the effect on systems using laser diodes, Sony usually defines the laser diode life as the time when the operating current becomes 1.2 times the initial value (Fig. 3(b)). However, as shown in Fig. 3, this does not mean the laser diode is no longer able to emit light.
The laser diode reliability is closely related to the operating temperature and the degradation speed (rise in drive current per unit time: ΔIOP/Δt) rises exponentially with the operating temperature. This relationship can be expressed as follows.
Based on the relationship between the temperature and drive current rise ratio, the semiconductor laser in general is given as;
Therefore, near room temperature, the lifetime drops to approximately 1/2 for a temperature rise of 10ºC. Product sizes are becoming more compact, so sufficient care should be taken to thermal design to suppress laser diode temperature rises.
The average lifetime of laser diodes is generally expressed by the mean time to failure (MTTF) obtained from high temperature accelerated life test data and the Weibull chart. An explanation of the Weibull chart and the mean time to failure are given below. The bathtub curve like Fig. 2 can be expressed by the Weibull distribution function resulting in the following probability density function: ƒ (t).
From this, the cumulative failure rate is given by;
The failure rate is as follows.
This t0 corresponds to MTTF. The failure rate is given for m < 1, m = 1, or m > 1, which corresponds to a decreasing, constant, or increasing over time. The failure rate for the initial stage decreases with time, for the random stage is constant, and for the wearout stage increases with time. The distribution depends on the size of m (shape parameter).
Life tests are conducted on element lots screened using fixed methods to confirm that elements satisfy the demanded average lifetime and have no initial failures, and the relationship between the cumulative failure rate and the drive time is investigated.
This relationship is called the Weibull chart. The m value (shape parameter) is given by the inclination of the line and the intersection of the line indicates the average lifetime. In the case of lots which entered the random failure stage (m = 1), the mean time to failure (MTTF) is the time when 63.2% of the entire samples reach to failure. Fig. 4 shows the Weibull plot in each shape parameter.