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Inductance Tolerant Laser Drive and Modulation Circuit

IP.com Disclosure Number: IPCOM000106438D
Original Publication Date: 1993-Nov-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Radcliffe, JK: AUTHOR

Abstract

Disclosed is a method of biasing and modulating a laser used in optical communications system that is tolerant of parasitic lead inductance in the packaging. The laser modulation current is caused to arrive at the laser in the form of a traveling wave on a transmission line as shown in Fig. 1. At the load end the transmission line is terminated with a resistor whose value is chosen to match the characteristic impedance of the transmission line when combined with the forward dynamic impedance of the laser. A typical laser impedance is in the 3 to 5 ohm range.

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Inductance Tolerant Laser Drive and Modulation Circuit

      Disclosed is a method of biasing and modulating a laser used in
optical communications system that is tolerant of parasitic lead
inductance in the packaging.  The laser modulation current is caused
to arrive at the laser in the form of a traveling wave on a
transmission line as shown in Fig. 1.  At the load end the
transmission line is terminated with a resistor whose value is chosen
to match the characteristic impedance of the transmission line when
combined with the forward dynamic impedance of the laser.  A typical
laser impedance is in the 3 to 5 ohm range.

      The transmission line has a Thevenin equivalent circuit of its
characteristic impedance and twice the traveling wave voltage.  When
the line impedance is combined with the termination impedance the
effective source impedance is twice the transmission line impedance.
It is the high source impedance which renders this circuit tolerant
of inductance in the laser leads.

      An example of the invention is shown in Fig. 2.  The laser is
biased by a conventional current source isolated by a small inductor.
The source of the modulation current is a conventional current switch
with the unused collector tied to the positive power supply.  The
transmission line, 50 ohms in this example, is connected between the
current switch and the laser and is terminate at the source end to
absorb reflections from the circuit driving the laser.  In this...