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Inexpensive Laser Diode and Regulator Diagnostic Circuit

IP.com Disclosure Number: IPCOM000121790D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 3 page(s) / 87K

Publishing Venue

IBM

Related People

Cato, RT: AUTHOR

Abstract

Whenever a solid-state laser diode is used in a product, it is desirable for the system's processor to be able to run a diagnostic test to determine if the laser diode is functioning correctly. This insures that the system can check itself out and report to the user if anything is wrong. The circuit below is an inexpensive way to provide the function.

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Inexpensive Laser Diode and Regulator Diagnostic Circuit

      Whenever a solid-state laser diode is used in a product,
it is desirable for the system's processor to be able to run a
diagnostic test to determine if the laser diode is functioning
correctly.  This insures that the system can check itself out and
report to the user if anything is wrong.  The circuit below is an
inexpensive way to provide the function.

      Solid-state laser diodes require a regulator circuit to insure
that the correct amount of light power is emitted. Without
regulation, a laser diode's output power will vary too much.  The
regulator circuit involves using a photodiode (that is usually built
into the same package as the laser diode) to sense the laser diode's
optical power output.  The photodiode invariably provides input to an
amplifier of some kind.  The photodiode amplifier's output is fed to
some kind of error amplifier that compares a signal representing a
laser diode's current to the photodiode amplifier's output. This
error signal is used to control the current through the laser diode.

      The subject diagnostic circuit attaches to the output of the
photodiode amplifier (PDA).  The design of the remainder of the laser
diode regulator circuit is not of consequence.  The subject
diagnostic circuit will not significantly load a low output impedance
amplifier (see the figure).

      The circuit assumes that the output of the PDA goes positive
when light strikes the photodiode as the result of the laser diode
turning on.  The same concept will work with a negatively-going
turn-on signal from the PDA.  The diodes would be reversed, and the
polarity of the reference voltages would be reversed.

      The output of the PDA drives resistor network node "A" through
diode D1.  Node A is the top of resistor divider R1 and R2.  The node
between R1 and R2 is "B".  The voltage waveform at B is the same as
a...