Browse Prior Art Database

Laser Fault Detection

IP.com Disclosure Number: IPCOM000101560D
Original Publication Date: 1990-Aug-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 78K

Publishing Venue

IBM

Related People

Ebler, MB: AUTHOR [+3]

Abstract

A laser drive loop's function is to provide a drive current to the laser for constant optical power. A fault detection scheme is necessary to shut the laser drive down if a component fails in the laser drive loop.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Laser Fault Detection

       A laser drive loop's function is to provide a drive
current to the laser for constant optical power.  A fault detection
scheme is necessary to shut the laser drive down if a component fails
in the laser drive loop.

      The laser bias driver and fault sensor circuits are illustrated
in the figure.  The bias drive for the laser analog power is
performed by an op amp B1.  The negative terminal is connected to a
resistor divider from REF1 of the reference generator A4.  The
positive terminal of the op amp is connected to the laser's back
facet photo detector, and to a resistor (R2) and potentiometer (RT1)
connected to REF1.  The output of the op amp drives the laser through
emitter follower transistor T1 and resistor R1.  This driver loop
will drive the necessary current in the laser to produce a voltage on
RT1 + R2.  Adjusting RT1 thus sets the average output power of the
laser.  Since the back facet photodetector is not calibrated, the
required resistance value can vary over a 5:1 range, thus
necessitating the use of an adjustment pot. This pot is adjusted in
manufacturing using a power meter connected to the laser output.

      The operation of this driver loop is monitored by the fault
detection circuitry.  Comparator B2 is used to detect that the
control loop is open in a way such that the photo diode sense pin is
high (SET AVERAGE POWER).  This fault condition, as with all other
fault senses, start a ramp timer.  If the fault is still present
after the timer expires, the laser is shut off and the '+ fault' line
is latched high.  Comparator C2 senses the fault timer capacitor CC2
to determine that the timer has expired.  I...