Laser diodes typically fail as the result of two distinct damage mechanisms:
Optical overstress
One of the damage mechanisms is optically related, and occurs when the laser diode is producing light (referred to as “lasing”), and the optical energy density exceeds the laser diode’s integral mirrors’ reflective capacity. When this occurs, the mirrored surface permanently loses its reflectivity, and the laser diode no longer functions properly. In layman’s terms, this could be thought of as the laser light becoming so intense that it “vaporizes” the mirror surface.
Electrical overstress
The second damage mechanism is related to failure of a laser diode’s P-N junction itself. A severe over-current or over-voltage power surge can cause localized heating and other harmful phenomena, which, under extreme conditions, can actually fracture the laser diode die (we have seen this under a microscope, brought on by high levels of ESD).
Both of these damage mechanisms can be provoked by an over-voltage or over-current condition. Low-power laser diodes, that is, laser diodes whose optical output power is below around 200 mW, are particularly sensitive to ESD. This is because they are designed to be inherently fast devices. Indeed, low-power laser diodes are often directly modulated and used for fiber-optic communication with data rates in the gigahertz range. Thus the P-N junction and optical elements of a laser diode can react very quickly to changes in voltage or current.
Therefore, in order to be effective, an ESD protection device and method should preferably be implemented as a proactive measure, by preventing the over-voltage or over-current condition from happening in the first place, not by reacting to it once it has occurred.
Current vs. voltage profile
The figure to the right shows the current vs. voltage profile of a typical low-power laser diode. It can be seen that the profile is similar to other types of diodes and semiconductor devices.
Starting from zero volts, applying incremental positive increases in voltage (i.e., those voltages that would tend to forward bias the laser diode), very little current flows until around 1.8 volts is reached.
Further incremental positive increases from around 1.8 volts causes current flow to increase at a roughly exponential rate. However, the laser diode does not emit laser light until the current exceeds a “lasing threshold,” which, for the laser diode referred to here, occurs at around 30 milliamps and at around 2.2 volts. With further incremental positive increases in voltage, current flow continues to increase, while the optical power emitted by the laser diode increases at a rate that is roughly proportional to current.
Once the maximum design current for a particular laser diode is reached (which is around 35 milliamps and 2.4 volts for this laser diode), further increases in current will likely result in failure, caused by one or both of the damage mechanisms described above. Thus it is important to completely prevent voltage, and thus current, from increasing beyond the absolute maximum rating for a particular diode. In most cases, a low-power laser diode will be destroyed if the absolute maximum ratings are exceeded, even for a brief period of time.
Note that the figure shows only the current vs. voltage profile for positive voltages, that is, voltages that would forward-bias the laser diode. Laser diode manufacturers recommend that negative voltages, that is, voltages that would tend to reverse-bias the laser diode, be avoided.
The data sheet for an exemplary laser diode lists an absolute maximum reverse voltage of 2 volts. In order to protect this laser diode from being damaged by ESD, the protection means should limit positive voltages to around 2.4 volts and negative voltages to around 2.0 volts or less. These voltages are used as a reference throughout the rest of this discussion.
