Registers – Coded record sensors – Particular sensor structure
Reexamination Certificate
2000-11-28
2003-10-07
Frech, Karl D. (Department: 2876)
Registers
Coded record sensors
Particular sensor structure
C235S455000
Reexamination Certificate
active
06629638
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a logic system based controller and more specifically a controller that utilizes state machines, logic and/or a microprocessor for an electro-optic system.
Electro-optic systems are known in the art for providing an interface between electronic and optically-based systems. Such electro-optic systems are used in a variety of applications including telecommunications, remote sensing, medical devices, and in other fields as well.
FIG. 1
a
illustrates a conventional electro-optic system
100
for use with one of the aforementioned systems. The electro-optic system
100
includes a laser
110
and an analog bias controller
120
. Typically, an electronic system produces a modulating signal which is combined
112
with a laser bias current I
DC
. The combined signal is injected into the laser
110
which produces a modulating optical output signal, P
O
, that typically is an intensity modulated signal in which the light intensity varies as a function of the amplitude of the modulating signal. In analog systems, the bias current I
DC
is selected such that it biases the laser
110
at an operating point where the laser output P
O
exhibits a linear relationship to the input bias I
DC
(herein referred to as the laser's transfer function). The modulating signal varies the bias point above and below this operating point thereby producing a corresponding change in the intensity of the output power, P
O
, of the laser
110
.
Continuing with
FIG. 1
, the laser
110
may include a monitoring photodiode (not shown) either separate or integrated into the same package with the laser
110
. The monitoring photodiode produces a photodiode current I
PD
indicative of the laser output power P
o
. The photodiode current I
PD
is supplied to an analog bias controller
120
which includes circuitry designed to measure the photodiode current I
PD
and to return the laser bias I
DC
to a predefined current level.
FIG. 1
b
illustrates a graph showing three transfer functions of a laser at three operating temperatures T
1
, T
2
, T
3
. The graph illustrates laser output power P
O
along the y-axis, and laser bias current I
DC
along the x-axis. The first trace
152
illustrates the laser's transfer function at a first operating temperature T
1
. The second trace
154
and the third trace
156
similarly illustrate the laser's transfer function at a second and a third operating temperature, T
2
and T
3
, respectively. Each of the transfer functions
152
,
154
, and
156
also include a corresponding threshold operating point I
th1
, I
th2
, and I
th3
, respectively. The threshold operating points I
th1
, I
th2
, and I
th3
, indicate the laser bias current level I
DC
at which the laser produces appreciable output power. As can be seen, a laser's threshold operating point varies greatly with operating temperature. A change in the operating temperature of the laser shifts the transfer function laterally along the x-axis.
A shift in the operating temperature can produce a significant change in the laser output power. For example, a laser bias current I
DC
selected to bias the laser at operating point I
Q1
produces a laser output power P
T1
in the center
162
of the linear region of the transfer function
152
. At operating point I
Q1
, The laser will operate as intended to produce a substantially linearly varying output power P
O
when excited by a modulating signal. If the operating temperature of the laser operating temperature changes to T
2
, the laser output power P
T2
for the same bias current I
Q1
drops significantly as shown by operating point
163
. In this case, the linear operating region of the laser and electro-optic system
100
is limited to the linear region below operating point
163
, in the case of negative current swings below I
Q1
. The analog bias controller
120
contains circuits to compensate for change in temperature, but with a limited degree of accuracy. The analog bias controller
120
also requires manual adjustments, has limited control capabilities, and is subject to significant temperature drift.
In view of the aforementioned inadequacies of the prior art, the need exists for a new system and method for controlling an electro-optic system over varying temperatures of operation.
SUMMARY OF THE INVENTION
It is an advantage of the present invention to provide a system and method for controlling an electro-optic system by which the threshold operating point of the system gains and other parameters can be accurately established and the laser transfer function controlled over a variety of different operating temperatures of a laser module.
It is another advantage to provide a system for subtracting errors due to dark current and aging of a laser-monitoring photodiode.
It is yet another advantage to provide a system for driving a laser utilizing both a low frequency circuit to drive DC current in addition to a high frequency circuit for driving a high frequency AC/DC signal to set the operating point of a laser.
A further advantage is to provide a system and method for calibrating and initializing the electro-optic system upon each power up of the system.
It is another advantage to provide a system for continuously monitoring and adjusting parameters of an electro-optic system while the system is fully operational.
Still another advantage of the present invention further is to provide a system and method for enabling a link characterization process to be performed between two electro-optic systems.
Electro-Optic System Controller
In an exemplary embodiment, the present invention is an electro-optic system for driving a laser, monitoring the laser operation, and maintaining the laser parameters within acceptable limits over temperature and device variations. The electro-optic system of an exemplary embodiment includes a signal input coupled to a pre-amplifier via a test system switch (TSS). The output of the preamplifier is fed into a combiner and coupled to a laser module input. A laser module of an exemplary embodiment includes a laser bias input, a laser output, a temperature sensor output, and a monitoring photodiode sensor output. In other embodiments, the laser module may further include a power sensor and a cooler/heater component. The test system switch, the preamplifier and the laser module are controlled by a controller circuit.
The controller circuit of an exemplary embodiment may be a digital processor, a microprocessor, or an ASIC device that is capable of generating control data. The processor of the exemplary embodiment includes a communication port that may be connected to an external host computer. The processor of the exemplary embodiment further includes memory or an input for connection to a memory component. In other embodiments of the present invention the processor may include a port for storing/receiving digital data to/from an external source.
The processor controls the test system switch utilizing switch control lines. In an exemplary embodiment, the test system switch may connect one of a data signal, a signal generated from a signal generator, or a reference ground to the input of the preamplifier. The signal generator is controlled by the processor. The test system switch of an exemplary embodiment also includes a switched or dedicated line for connecting a coded input signal to a code detector of the controller circuit. The code detector identifies the reception of an encoded signal received by the electro-optic system and outputs coded data to the processor.
The preamplifier of an exemplary embodiment includes an amplifier connected to the switched input from the test system switch, a high frequency voltage variable resistor, and a high frequency voltage controlled current source (HF-VCCS) that is connected to the signal input of the laser. The preamplifier is coupled to the processor and may have its gain and offset characteristics changed by means of a dual digital to analog converter (DAC) that accepts digital control signals from the processor. The dual DAC
Brown Martin Haller & McClain LLP
CEYX Technologies
Frech Karl D.
Nowlin April A.
LandOfFree
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