Investigating a Hypothetical Semiconductor Laser Bar with a Damaged Single Emitter Using a Laser Diode Simulation/Emulation Tool

This study demonstrates the use of Barlase, a semiconductor laser diode emulation tool, to emulate the by-emitter degradation of high power semiconductor laser diodes. Barlase is software that uses a LabView control interface. In this study, a hypothetical laser diode bar (multiple emitters) was used to investigate a damaged single emitter randomly located in the bar and its behavior analyzed within the bar. It should however, be noted that, this scenario is valid for devices at the start of the aging process only. When all other relevant effects that affect the performance of laser diodes bars are allowed to interact over time, high levels of defects can also play important role in the degradation process. The results of this simulation scenario show the successful implementation of Barlase in the by-emitter analysis of laser diodes.


INTRODUCTION
Due to several applications emerging in the use of high-power laser bars (Steel, 2008), there is a greater demand on their improved reliability and durability.Apart from their traditional applications such as pumping solid-state lasers, material processing (Schulz and Poprawe, 2000), printing, medicine and entertainment; they have found other uses in the light detection and ranging and free space optical communications (Chazan et al., 1998).
Barlase presents an attempt to understand further, the by-emitter degradation analysis technique developed over recent years (Xia et al., 2002;Tomm et al., 2002;Bull et al., 2005;Lim et al., 2007Lim et al., , 2009Lim et al., , 2005;;Bream et al., 2006).The tool is an addition to the by-emitter analysis technique where the effects of certain factors that affect the degradation of laser emitters/bars can be investigated.
In this study, Barlase (Amuzuvi and Attachie, 2013) is used to perform a by-emitter analysis of a laser bar when an emitter within the bar is damaged.The objective of this study is to investigate and analyze various scenarios of defects in the operation of a laser bar, especially if there is a localized defect like a defective emitter within the bar.

MATERIALS AND METHODS
Bars are made up of multiple emitters and therefore there was a need to find an innovative way to include the interactions between individual emitters within the bar.This gave rise to the Barlase concept (Fig. 1), where a bar is considered as a monolithic block of multiple emitters connected in parallel with each other with a common voltage connected across them, Fig. 2 (Amuzuvi, 2013).
Each emitter is biased with a common voltage, but the emitter currents and powers change depending on the details of the individual emitters and their environment.

RESULTS AND DISCUSSION
In Amuzuvi and Attachie (2013), Barlase was used to investigate a practical scenario, where a laser bar has a random low-level of defects distributed across the bar.In this study, another practical scenario is being investigated where one random emitter has a high level of defects.The bar being considered is the same as in Amuzuvi and Attachie (2013), an (8) emitter bar (Fig. 2).Emitter #3 was randomly selected to represent a "damaged" emitter.The level of defects introduced in  emitter #3 was chosen to be 2×10 16 cm -3 -an order of magnitude higher than that found in the other 7 emitters.Again, multi-emitter simulations were carried out in constant current mode for bar currents of 2, 4, 6, 8 and 10 A. Table 1 indicates the levels of QW trap densities assigned to each emitter in the bar.
Figure 3 shows the P-I characteristic of the bar together with the P-I and P-V characteristics of each of the individual emitters.The threshold current and slope efficiency for the bar are also shown as legend in Fig. 3a.The threshold current and slope efficiency have been calculated for each individual emitter from the emitter P-I curves in Fig. 3b. Figure 3c also shows the power versus individual emitter voltages.The variations of apparent threshold/threshold current and apparent slope/slope efficiency of individual emitters are plotted as a function of emitter number in Fig. 4. The P-I curve of the emitter with the high level of defects is lower than that of the other emitters, as expected.The threshold current in emitter number 3 is 16% higher, whilst the slope efficiency is only 0.25% lower compared to the other emitters.
Figure 5 shows the distribution of current, power and maximum QW temperature for each emitter across the bar for a total bar current of 2 A. Figure 6 shows the same quantities for a total bar current of 10 A. From these graphs, the relationship between the current, power and maximum QW temperature of the emitter with the high level of defects can be seen to be different in comparison to the standard emitters.The higher level of defects in emitter number 3 causes its current to be 0.35% higher, its output power to be 1.0% lower and its maximum QW temperature to be just over 0.5 K greater than the other emitters.
Barlase has therefore been used in this scenario to gain more knowledge and insight into the interaction between emitters in a laser bar when defective emitters are present in the bar.

CONCLUSION
The variations in the operating condition and environments of the individual emitters were seen to affect the performance of the other emitters and of the bar as a whole.This scenario investigated using Barlase demonstrates the effect that a damaged emitter will have on a laser bar via the distribution of power and current competition within the bar.From this scenario, the capabilities of Barlase has clearly been investigated and demonstrated, indicating the increase in temperature and the corresponding decrease in power when a damaged emitter is present in a bar.

Fig. 1 :
Fig. 1: Flow chart showing the communication between emitters in Barlase

Table 1 :
Values of QW trap densities assigned to each emitter in the