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 Laser diodes are an ever-increasing part of
everyday life. Indeed, when people go through the course of a single day
of their lives, they will encounter perhaps hundreds or even thousands of
laser diodes. Commonplace examples of the use of laser diodes include
CD and DVD drives, barcode scanners, laser pointers, construction alignment
devices, and police traffic radar.
Most laser diodes can be easily damaged if
their nominal voltage or current parameters are exceeded. In fact,
products that contain laser diodes often seem to mysteriously fail, with
no apparent provocation. A close examination into the failure modes of
these devices has revealed power surges during power-up/power-down
sequences and electrostatic discharge (ESD) events as two major causes of
laser diode failure.
Power-up/power-down failures
When power is being turned on or off on a
product, internal circuits can be operating outside their intended
internal power supply range for a brief period of time. As one example,
rail-to-rail operational amplifiers used in laser diode drivers may be
specified to operate with power supplies ranging from 2.7 to 5.5 volts.
But the manufacturer makes no guarantee or representation regarding what
the operational amplifier will do between a supply voltage of 0 and 2.7
volts. Because of this, current or voltage regulation circuits may go out
of balance during power-up and power-down, and often apply an over-current
or over-voltage condition to the laser diode that is integrated within the
product. These over-current or over-voltage conditions may stress the
laser diode, such that each power-up or power-down cycle accumulates in
the form of device fatigue. Eventually, the laser diode may fail from the
fatigue, leading to what the user experiences as a mysterious failure.
Electrostatic discharge failures
Electrostatic discharge (ESD) may also
cause mysterious device failures, and there are many ways in which ESD
might come in contact with a product. One of the most common ways that ESD
is generated occurs when a person walks across a carpeted floor, and then
touches something. A discharge can occur to anything being touched,
ranging from things that are not sensitive to ESD, such as doorknobs, to
things that are very sensitive to ESD, such as electronic products.
Studies have shown that, when an ESD discharge occurs, the discharge
voltage can range anywhere between 4,000 and up to as much as 32,000 volts
DC, depending on environmental conditions, clothing being worn, type of
flooring surface, and other factors. When an laser diode that is designed
to operate with a terminal voltage of 2.2 volts experiences an ESD
discharge of thousands of volts, the result can be immediately destructive.
Alternatively (and what happens more commonly), the laser diode will have
some latent damage, with dramatically reduced lifetime. Later, when it
fails in the field, the user will often blame it on infant mortality or
some other cause, not realizing that the actual damage began earlier in
the product's lifetime.
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