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Browse Prior Art Database

Fibre Optical Laser Impulse Driver

IP.com Disclosure Number: IPCOM000114238D
Original Publication Date: 1994-Nov-01
Included in the Prior Art Database: 2005-Mar-28
Document File: 4 page(s) / 112K

Publishing Venue

IBM

Related People

Newman, EL: AUTHOR

Abstract

A typical fibre optic data link consists of a transmitter, a length of fibre optical cable and a receiver. The transmitter is often a semiconductor diode - for instance, a laser diode or a Light Emitting Diode (LED). The receiver could consist of a photo diode (e.g., PIN diode or avalanche diode). Electronic circuitry conditions the signals at both ends so that the system can transfer data. The light path will suffer from attenuation due to the optical connections from the diodes to the fibre, connections between lengths of fibre in the path, and attenuation inherent in the fibre itself. There are also a number of other nonlinear distorting effects. There is also a delay in the system due to the propagation of light in the optical fibre medium.

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This is the abbreviated version, containing approximately 52% of the total text.

Fibre Optical Laser Impulse Driver

      A typical fibre optic data link consists of a transmitter, a
length of fibre optical cable and a receiver.  The transmitter is
often a semiconductor diode - for instance, a laser diode or a Light
Emitting Diode (LED).  The receiver could consist of a photo diode
(e.g., PIN diode or avalanche diode).  Electronic circuitry
conditions the signals at both ends so that the system can transfer
data.  The light path will suffer from attenuation due to the optical
connections from the diodes to the fibre, connections between lengths
of fibre in the path, and attenuation inherent in the fibre itself.
There are also a number of other nonlinear distorting effects.  There
is also a delay in the system due to the propagation of light in the
optical fibre medium.

      By using a laser/PIN diode combination with the electronic
circuitry described below considerable improvement can be achieved.
(Modern laser diodes nearly always come as a laser/PIN diode
combination because of the laser sensitivity to temperature, etc.)
The average optical power of the laser diode is maintained by
monitoring the laser output through the PIN diode.  A pulse driver
circuit (Fig. 2) superimposes the pulse drive into the laser.

      The laser diode optical output power is kept constant by the
servo loop with the PNP and NPN transistors P1 and N1 (Fig. 1).  The
resistor R1 turns on N1 which turns on P1.  This turns the laser on
and the laser PIN diode starts to conduct.  (The PIN diode current is
proportional to the laser light output when the diode is reverse
biased.)  The PIN diode current tends to generate a voltage across R1
which tends to turn N1 off.  The capacitor C1 acts as a dominant pole
and the loop stabilises with the laser giving a constant light
output.  The time constant of the servo loop is much lower than the
lowest frequency of data to be injected.  In this way the data acts
on essentially constant characteristics maintained by the servo loop.

      The pulse driver circuit works by having two time constants.
One time constant is when the base of transistor N2 is switched high
and the other when N2 is switched low.  The positive going time
constant is short, and the negative going time constant is long.  The
positive going time constant is influenced by the resistor R3 and the
capacitor C2.  The negative going time constant is mainly determined
by R2 + R3 and C2.  The positive going time constant is also
significantly influenced by the output impedance of the emitter of N2
and the forward biased slope resistance of the laser diode.  It is
also significantly influenced by the sum of the series lead
inductances and the capacitance of the laser diode.  The laser drive
impulse is determined by the positive goi...