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Browse Prior Art Database

Laser Driver

IP.com Disclosure Number: IPCOM000101289D
Original Publication Date: 1990-Jul-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 87K

Publishing Venue

IBM

Related People

Block, TR: AUTHOR [+4]

Abstract

The laser used in this application emits a beam from both of its mirror surfaces. The front beam is directed into the fiber, but the rear beam, which is proportional to the front beam, is used to monitor the optical power of the front beam. A photodiode is mounted behind the back facet of the laser. This photodiode changes the rear beam into photocurrents. This current is the monitor diode current Im used in the laser driving scheme. Once the laser is above its threshold, the current, Im, is linearly proportional to the laser's optical power.

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Laser Driver

       The laser used in this application emits a beam from both
of its mirror surfaces.  The front beam is directed into the fiber,
but the rear beam, which is proportional to the front beam, is used
to monitor the optical power of the front beam.  A photodiode is
mounted behind the back facet of the laser.  This photodiode changes
the rear beam into photocurrents.  This current is the monitor diode
current Im used in the laser driving scheme.  Once the laser is above
its threshold, the current, Im, is linearly proportional to the
laser's optical power.

      Every laser diode has a unique threshold current. Above this
current the optical power increases linearly with input current.
However, the value of this threshold current changes with
temperature.  So in order to maintain a constant optical power, a
control loop using the monitor diode current becomes necessary.

      Once the laser is supplied with the threshold current, it
begins lasing; then a small transient current can be switched through
the laser.  This switching transient current provides data that is
then transmitted through the laser.

      The figure illustrates a schematic of the LASER drive scheme.
Operational Amplifier 1 controls the drive to the base of Q1.  As
this voltage goes up, the NPN provides more current into the LASER,
increasing the optical power, and as the base voltage of the NPN goes
down, the laser current decreases, thereby decreasing the optical
power.

      The feedback to the operational amplifier is a current
proportional to the optical power.  This current, Im, provides a
voltage drop across R2.  This voltage is referred to the voltage
generated across R1.  As the environment changes, the laser optical
power changes for a given amount of current; when this occurs, Im
changes.  As Im changes, the voltage across R2 changes and the drive
to the laser is adjusted to provide the original optical power.  R2
is an adjustable resistor which allows for a large tolerance in the
laser monitor diode efficiency.

      Opamp 2 keeps the voltage across R5 cons...