figure to the right shows another ESD protection means commonly employed to protect laser
diodes. In this scheme, a Zener diode is connected anti-parallel with the
laser diode. There are several problems with this technique.
One problem is that Zener diodes are notoriously
slow devices, and are not able to react to nanosecond-level pulses that
can be seen during ESD events.
The second, and more serious, problem is that it is
believed to be impossible to choose a Zener voltage that is close enough
to the forward-bias lasing threshold of the laser diode, and whose Zener
voltage would track the laser diode’s lasing threshold voltage throughout
temperature ranges likely to be experienced by a laser diode.
In our testing, we could not find a Zener diode that
would protect a laser diode. We even tried Transient Voltage Suppressor
(TVS) devices whose function is similar to a Zener diode, but are made
specially to protect electronic devices from power surges. However, we did
not find any TVS devices that were effective at protecting laser diodes
In fact, in our research we learned something very
interesting about TVS devices. Most TVS devices were not designed to
protect against ESD. They were designed to protect against
lightning-induced power surges. Lightning itself can produce surges with
very fast rise-times. But by the time lightning comes in contact with an
electrical product, it is after having traveled through power distribution
systems whose inductance, capacitance and resistance tends to spread out
the event over tens of microseconds.
If you look at device datasheets you can find a clue
as to which devices were designed to work with ESD, and which ones were
designed to work with lightning and other surges. If the datasheet has a
current specification with time specified as “8/20 microseconds”, chances
are that the device was not designed to protect against ESD.
ESD polarity terminology used on this
The term “positive-ESD” is used to
mean electrostatic discharge (ESD) whose voltage polarity would tend
to forward-bias a laser diode. “Negative-ESD,” means ESD whose
voltage polarity would tend to reverse-bias a laser diode.