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Temperature Effect
Reversible and Irreversible Temperature Effects
The higher temperature leads to an increase of temporary thermal recombination centers within the semiconductor but there are also non-reversible transformations caused by high temperature.
In a short-time scale, an increased temperature generally reduces the gain, increases the wavelength and the threshold current. It can also increase the propensity for mode-hopping, that can be seen as kinks in the electrical and optical curves and sudden changes in the predominant wavelength in the spectrometer.
In a longer time scale the high temperature kicks loose dislocation migration and propagation processes within the semiconductor crystal that irreversibly detriment the properties of the laser. This process is self-invigorating since more defects lead to lower efficacy and hence more heat. In most cases the propagation of pre-existing lattice defects and impurities does not help to improve the laser efficiency but makes it worse.
While the microscopic degradation mechanisms are complex and depend of the specific laser diode design, there are however some common patterns that we can look for.
The most obvious behavior is that a laser diode, for a given forward current and ambient temperature, will decreases its laser power output a certain rate with time.
Burn-In
In many cases the first couple of hours there is a relative strong shift in parameters that tend to saturate after a while. This can be a problem, since although some applications are more tolerant for shifts in parameters such as wavelength, ouptut power or even far field beam pattern, for most applications there is no much wiggle room for the laser device to shift its parameters after integration in the optical set-up.
This kind of strong initial aging behavior can be minimized by annealing steps during the device fabrication but sometimes there is no way around performing a burn-in of the device that can take few hours of continuous operation at certain forward current and ambient temperature settings. The exact burn-in conditions depend on the laser and they always aim at saturating those initial aging processes so that no major changes are visible during the operation on customer’s site. Since burn-in time is expensive, there is an economical interest to find the optimum parameters to maximize initial aging saturation at the shortest possible time.
Laser diode burn-in is usually done after die soldering to its heat sink and within the package that holds the contact bonds. One of the reasons is the added bonding mechanical stress to the laser diode, other reason is the effect of package materials on an initial strong aging as well. For laser diodes with shorter wavelengths (green, blue and UV laser diodes) the reason is that they need to be operated in a hermetically sealed package to prevent facet degradation.
Another reason for an initial burn-in is the observation that laser diodes with underlying serious problems that have not been detected at the optical inspection nor at the laser characterization do show up at the burn-in. Their behavior deviates from an initial aging that saturates during the burn-in. The light output decreases at a faster speed than the rest of the production population and the saturation does not set in.
The behavior that random microscopic contaminations, dislocations or other degradation mechanisms often follow a similar pattern. The reasons for this behavior are multiple and vary from one diode design to another and it is mandatory to know the detailed construction to understand the degradation mechanisms properly.
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