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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.
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