Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2000-02-04
2004-03-23
Font, Frank G. (Department: 2877)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S029021, C372S038070, C359S199200, C323S304000
Reexamination Certificate
active
06711189
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optoelectronic transmitters, and more particularly to optoelectronic transmitters having automatic power control and biasing circuitry for driving semiconductor lasers.
BACKGROUND OF THE INVENTION
Optoelectronic transceiver modules provide an interface between an electrical system and an optical transfer medium such as an optical fiber. Correspondingly, most optoelectronic transceiver modules contain electrical and optical conversion circuitry for transferring data to and from the electrical system and the optical transfer medium.
Normally, transceiver modules use laser diodes which produce coherent light for performing high speed data transfers between the electrical system and the optical transfer medium. Typically, each laser diode is packaged with optical power-monitoring circuitry. For example, the HFE4081-321 diode package by Honeywell, Incorporated, contains both a laser diode for transmitting data and a photodiode for performing power-monitoring.
The power-monitoring photodiode within the diode packaging provides a monitor current I
m
which varies as the optical power being generated by the laser diode changes. Normally, the changes in the monitor current I
m
are directly proportional to the changes in the optical power generated by the laser diode. However, the ratio of monitor current I
m
with regard to the laser diode's optical power can vary widely from one diode package to the next. Therefore, each diode package must be tested individually in order to determine its specific ratio of monitor current I
m
to laser diode optical power.
The primary purpose of providing a monitor current I
m
is for ensuring that, during operation, the laser diode is lasing. The minimum current which must be supplied to the laser diode to cause lasing is the threshold current I
th
.
When the current being supplied to the laser diode is less than the threshold current I
th
, the laser diode is operating in the LED mode. In the LED mode, the current supplied to the laser diode is only sufficient to excite atoms in the laser diode's cavity which cause light to be emitted similar to that produced by light emitting diodes (LEDs).
When the current supplied to the laser diode reaches a level greater than or equal to the threshold current I
th
, the laser diode's efficiency of converting electrical current into light will increase dramatically and thus the laser diode changes from the LED mode of operation to the lasing mode of operation.
Referring to the drawings,
FIG. 1
illustrates typical output power versus input current curves, or P-I curves, for three individual semiconductor lasers A, B and C. One of the primary difficulties with semiconductor lasers is that each individual laser has its own unique set of output characteristics. In
FIG. 1
, the horizontal axis (I) represents the drive current input to the semiconductor laser, and the vertical axis (P) represents the corresponding optical output power delivered by the laser. As can be seen, a uniform DC input current I
Q
supplied to each of the individual semiconductor lasers A, B and C results in a different amount of optical output power, P
QA
, P
QB
, and P
QC
, being delivered by each of the lasers. Furthermore, since the linear operating range for each semiconductor laser has a different slope, a given change in the input current ±&Dgr;I will cause a different change in the output power ±&Dgr;P for different semiconductor lasers.
These variations in the slope efficiency of the semiconductor lasers can be seen in FIG.
1
. The uniform DC operating current, or quiescent current, I
Q
is applied to each of the three lasers A, B, and C, and an identical alternating current signal I
SIG
is superimposed thereon wherein I
sig
=I
Q
±&Dgr;I. I
SIG
causes a periodic change in the input current±&Dgr;I above and below the quiescent current I
Q
. The magnitude of the &Dgr;I applied to each semiconductor laser in
FIG. 1
is identical between the three semiconductor lasers A, B, and C. On the output side, however, the resultant changes in the output power ±&Dgr;P
A
, ±&Dgr;P
B
and ±&Dgr;P
C
generated due to the changes in the input current vary from one laser to the other. As is clear in
FIG. 1
, &Dgr;P
A
is greater than &Dgr;P
B
, and &Dgr;P
B
is greater than &Dgr;P
C
. These variations in the output characteristics of individual lasers raise a significant barrier to designing a standard, reliable optoelectronic transmitter for mass production.
Ideally, each optoelectronic transmitter of a particular design will have similar output characteristics. The optical output of the transmitter is to represent a binary data signal comprising a serial string of 1's and 0's. A binary 1 is transmitted when the optical output of the transmitter exceeds a certain power threshold, and a binary zero is transmitted when the optical output power of the transmitter falls below a certain power threshold. Maximizing the difference in transmitted power levels between 1's and 0's improves the reliability of the transceiver and improves the signal-to-noise ratio at the receiver input at an opposite end of a data link. Thus, in a transceiver design incorporating a semiconductor laser as the active optical element, the transmitter should include provisions for optimizing the output characteristics of the semiconductor laser. Furthermore, these output characteristics should be the same from one transceiver to another. The IEEE standard for Gigabit Ethernet is an example of a standard for data communications over an optical fiber which requires such uniform transmitter characteristics. Therefore, the optimizing circuitry should also normalize the output characteristics of the transmitter to a well-defined standard.
In generating the optical output signal, the transmitter driver circuit receives a binary voltage signal from the host device. The driver circuit converts the input voltage signal to a current signal that drives the semiconductor laser. A signal voltage corresponding to a binary 1 must be converted to a current supplied to the semiconductor laser sufficient to cause the semiconductor laser to radiate an optical output signal having an output power level above the power threshold corresponding to the transmission of a binary 1. Similarly, a signal voltage corresponding to a binary 0 must be converted to a current level supplied to the semiconductor laser which will cause the semiconductor laser to radiate an optical output signal having an output power level below the power threshold corresponding to the transmission of a binary 0. However, due to variations in the P-I characteristics from one semiconductor laser to another, the current levels necessary to produce the desired output power levels will vary depending on the individual characteristics of each individual semiconductor laser. U.S. Pat. No. 5,638,390, issued to Gilliland et al., discloses a design and method for stabilizing an optoelectronic transceiver having a laser diode. U.S. Pat. No. 5,638,390 is hereby incorporated by reference.
In general, variations in the P-I characteristics of individual semiconductor lasers can be compensated for by employing bias and AC drive circuits which adjust the input current driving the semiconductor laser. There are two components to the laser control circuits. The first component, automatic power control (APC), involves establishing the average DC input current, or quiescent operating current I
Q
. I
Q
establishes the average output power, or quiescent operating power P
Q
that will be radiated by the semiconductor laser. The second component, laser slope compensation, involves determining the change in the input current, +&Dgr;I, necessary to cause a desired change in the output power ±&Dgr;P to establish the optical power levels corresponding to digital 1's and 0's respectively.
FIG. 2
shows the identical P-I curves for semiconductor lasers A, B, and C as shown as in FIG.
1
. How
Anguelov Evgueniy
Gilliland Patrick B.
Torres Luis
Ward Mike A.
Evans Steven M.
Font Frank G.
Rodriguez Armando
Stratos Lightwave, Inc.
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