Apparatus and method for laser bias and modulation control

The apparatus (10) includes a laser driver (16) that receives low speed and high speed digital and analog input signals (12, 14), and biases and modulates the laser diode (22) accordingly. The laser driver (16) receives and responds to input from a feedback circuit (36, 38), which includes an average power control portion (36), and a modulation power control portion (38). The average power control portion (36) of the feedback circuit generally determines the DC component of the laser diode output, and compares it to a reference average. The modulation power control portion (38) of the feedback circuit generally determines the maximum and minimum peaks in the laser diode output, takes their average and compares the average to the DC component of the laser output from the average power control portion (36). The laser diode (22) is then driven by the laser driver (16) in response to the comparisons.

TECHNICAL FIELD OF THE INVENTION 
This invention relates in general to the field of transmission of 
information on optical fibers. More particularly, the present invention 
relates to apparatus and a method for laser bias and modulation control 
for an hybrid telephony and video telecommunications system. 
BACKGROUND OF THE INVENTION 
In the field of telecommunications, laser diodes have been used to generate 
intensity-modulated light pulses for transmission on optical fibers. Such 
laser diodes are operated and controlled by a DC bias current and a 
modulation current to intensity-modulate the laser diode about a DC bias 
point. However, laser characteristics may change in a number of ways. The 
operation and optical output power of the laser diodes tend to be 
sensitive to variations in ambient temperature and age of the component. 
For example, the laser threshold of the laser diode typically increases 
with increasing temperature and age. Therefore, to maintain a constant 
average optical power output, the DC bias current must be increased. In 
addition, the efficiency of the conversion of signal modulating electrical 
current to optical power, also known as slope efficiency, tends to 
decrease with increasing temperature. As a result, to maintain the same 
slope efficiency or constant peak-to-peak optical signal power, the 
modulation current must also be increased. FIG. 1 illustrates these known 
relationships between the laser diode optical power output and 
temperature. 
In most fiber optics applications, both the average optical power output 
and peak-to-peak optical signal power output the laser diode must be 
relatively constant despite the ambient temperature reading or the age of 
the diode. One solution to the temperature variation problem is to attempt 
to maintain the temperature of the laser diode by insulation and 
thermo-electric cooling. However, the added cost, power consumption and 
bulk are unfeasible for high-volume, low-cost applications. Another 
solution provides expected performance curves based on temperature 
readings and/or age. Due to variations inherent in laser diodes, the 
assumption that all laser diodes operate similarly under the same 
conditions is unrealistic and produces inaccurate performance predictions. 
The constant optical average and peak power requirements are even more 
important in systems where both lower speed digital signals and high speed 
frequency division multiplexed digitally encoded signals are transmitted 
via optical fibers. An example of such a system is a fiber-to-the-curb 
system where both telephony channels and switched video channels are 
transported. Optical transmission systems carrying switched video signals 
require a number of parameters to be held constant for satisfactory 
performance. These parameters include optical average and peak power as 
well as recovered signal linearity. 
When the modulation current approaches or goes below the laser threshold, a 
phenomenon known as clipping arises. FIG. 2 illustrates clipping of the 
composite signals with frequency ranging from zero to approximately 600 
MHz, for example. It is apparent from FIG. 2 that when clipping occurs, an 
asymmetric output waveform with severe distortions is produced. When 
clipping is experienced in a system transporting high frequency signals, 
such as switched video signals, the carrier-to-distortion ratio of the 
recovered signal decreases. The result is degraded optical link 
performance or complete signal loss. The degradation and signal loss are 
especially traumatic in systems with high efficiency modulation schemes. 
Therefore, a perfect video image at one instant may deteriorate very 
quickly the next. 
In U.S. Pat. No. 5,268,916 to Slawson et al ., a control system is provided 
to control the laser output over a wide temperature range by monitoring 
the output of the laser diode. However, in this circuit, the clipping 
phenomenon cannot be detected and avoided. Therefore, should the laser 
high speed source amplitude increase excessively, the modulation current 
would drive the laser diode below the threshold where it operates as a 
light emitting diode or LED. This results in severe distortion of both the 
high speed and low speed signal output signals from the laser diode and 
the unacceptable degradation or loss of the recovered signal. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, apparatus and a method for laser 
bias and modulation control are provided which substantially eliminate or 
reduce disadvantages and problems associated with prior systems. 
In one aspect of the present invention, the apparatus includes a laser 
driver that receives the low speed and high speed digital and analog input 
signals, and biases and modulates the laser diode accordingly. In 
addition, the laser driver receives and responds to input from a feedback 
circuit, which includes an average power control portion, and a modulation 
power control portion. 
The average power control portion of the feedback circuit generally 
determines the DC component of the laser diode output, and compares it to 
a reference average. The modulation power control portion of the feedback 
circuit generally determines the maximum and minimum peaks in the laser 
diode output, takes their average and compares the average to the DC 
component of the laser output from the average power control portion. The 
laser diode is then driven by the laser driver in response to the 
comparisons. 
In another aspect of the present invention, a back facet monitor diode is 
used to monitor the laser diode output. A current-to-voltage converter 
then takes the output from the back facet diode and converts it to a 
voltage level. This voltage level is provided to the average power control 
portion and the modulation power control portion of the feedback circuit. 
In yet another aspect of the present invention, a method to bias and 
control the laser diode includes the steps of receiving low and high 
speed, digital and analog input signals. These input signals include a 
hybrid digital and analog signals in the form of frequency division 
multiplexed signals. The laser diode is driven by a laser driver according 
to the input signals, and the output of the laser diode is monitored. The 
laser diode output is provided to a feedback control circuit which 
determines the average, and the maximum and minimum peaks of the laser 
diode output. These parameters are then compared with predetermined 
reference values. The laser driver thus biases and modulates the laser 
diode in accordance with the comparisons. 
An important technical advantage of the present invention provides 
apparatus and method for monitoring and preventing clipping and kinking. 
The laser is thus capable of providing a constant output regardless of 
temperature variations and aging components. As a result, high frequency 
signals including switched video signals may be satisfactorily transmitted 
via optical fibers.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawings, FIG. 3 illustrates a top level block 
diagram of apparatus for laser bias and modulation control, indicated 
generally at 10 and constructed according to the teaching of the present 
invention. In general, apparatus for laser bias and modulation control 10 
monitors the output of the laser diode 22, provides it to a feedback 
circuit, compares the outputs of the feedback circuit to references 
voltages or signals, and forces the output of the feedback circuit to 
equal the reference voltages in response to the comparison to avoid 
clipping and kinking in the operation of the laser diode. In particular, 
the average bias signals and the modulation signals are monitored and 
controlled in this manner. 
Referring to FIG. 3, apparatus for laser bias and modulation control 10 
receives hybrid digital and analog signals in the form of a frequency 
division multiplexed signal. The hybrid signals are shown as low speed 
digital signals 12 and high speed signals 14 at the input of a laser 
driver 16. In particular, the input signal components include low speed 
baseband digital sequences at approximately 20 megabaud, for example, and 
high frequency RF signals, ranging approximately 50 to 1000 MHz, for 
example. The high frequency RF signals may be in either analog or 
digitally modulated carrier formats. 
Laser driver circuit 16 is responsive to the frequency division multiplexed 
signal and provides a laser drive current 18 for a laser module 20. Laser 
module 20 includes a laser diode 22 which provides a light output to an 
optical fiber (not shown) for transmission through a communication network 
from its front facet and also transmits light into a monitor photodiode 24 
through its back facet. Laser driver 16 intensity-modulates laser diode 22 
about a bias current level in such a manner as to not drive the laser 
below the lasing threshold, a phenomenon commonly called clipping. Laser 
diode 22 is also controlled in such a manner as to not operate laser diode 
22 in a region of operation where distinct changes in slope efficiency 
occur, a phenomenon commonly called kinking. 
A low input impedance current-to-voltage converter shown as a 
transimpedance amplifier (Tz amp) 26 is coupled to monitor photodiode 24 
and converts a sample current therefrom into a voltage proportional to the 
laser light produced by laser diode 22. The output voltage contains the 
sum of all Fourier components including low and high frequency signals. A 
three-way splitter 28 coupled to transimpedance amplifier 26 divides the 
transimpedance amplifier output signal into three signals each having 
one-third power of the transimpedance amplifier output. In particular, 
three-way splitter provides an output port contains a composite 
DC/digital/analog signal 30 and two AC coupled ports contains 
digital/analog signals 32 and 34 for further signal analysis and 
processing. Composite DC/digital/analog signal 30 is provided to an 
average power control path or circuit 36 and digital/analog signals 32 and 
34 are provided to a modulation power control path or circuit 38. 
Average power control path 36 includes a lowpass filter coupled to an 
averager circuit 42, which also receives input from a reference adjust 
circuit 44. Lowpass filter 40 primarily filters out the majority of the 
modulating signals from digital and analog signals 30. In addition, 
lowpass filter 40 provides an impedance match between the low 
characteristic impedance of three-way splitter 28 and the high impedance 
of averager circuitry 42. 
Averager 42 is an integrator/comparator which compares the filtered 
transimpedance amplifier output voltage with a reference voltage provided 
by reference adjust circuit 44. If the reference voltage is greater than 
the filtered transimpedance amplifier output voltage, the average laser 
drive power is increased. If, on the other hand, the reference voltage is 
less than the filtered transimpedance amplifier output voltage, the 
average laser drive power is decreased. The reference voltage required is 
defined as a function of the monitor photodiode current, the 
transimpedance amplifier gain, and the desired average laser optical 
output power. In general, the monitor photodiode current output for a 
given laser output power is defined by the manufacturer of laser module 
20, and the other two parameters are then set accordingly. 
An Average laser bias control circuit 46 is coupled to averager 42 and 
receives an input therefrom indicative of the desired average laser drive 
power. Laser diode 22 in laser module 20 is controlled by the output of 
average laser bias control 46 accordingly. 
Also coupled to three-way splitter 28 as a part of modulation power control 
path 38 are two pre-emphasis circuits 50 and 52. Pre-emphasis circuits 50 
and 52 in effect pre-distorts the signals to ensure that the frequency 
response of positive and negative peak detection circuits 54 and 56 is the 
same for all frequencies. Pre-emphasis circuit 50 receives digital/analog 
signals 32 from three-way splitter 28 and generally provides compensation 
for the loss the high frequency analog RF signals experience in the 
feedback loop. The loss is implementation specific and may be the result 
of the use loss dielectric printed circuit board material and low cost 
detection diodes in a positive peak detector 54 and a negative peak 
detector 56 coupled to pre-emphasis circuits 50 and 52. The analog RF and 
digital input signals are also amplified in the process of applying the 
gain slope. 
Positive and negative peak detection circuits 54 and 56 measure the maximum 
and minimum absolute voltage levels of the composite digital/analog 
converted signals 32 and 34, respectively. Positive and negative peak 
detectors 54 and 56 generates output voltages which are provided to a peak 
detection averager circuit 58. Peak detection averager circuit 58 averages 
the output voltages to find a midpoint voltage. The resultant signal may 
be summarized by the equations: 
##EQU1## 
where P.sub.H and P.sub.L are positive (maximum) and negative (minimum) 
peaks from positive and negative peak detectors 54 and 56, the relation 
(P.sub.H -P.sub.L)/2 finds the midpoint of the peak detectors, mPAV.sub.G 
is the value the optical feedback circuit adjust the peak detection output 
(P.sub.H -P.sub.L)/2 to be equal to. 
The midpoint voltage from peak detection averager circuit 58 is provided to 
a difference detector circuit 60. Difference detector circuit 60 also 
receives the average power voltage from averager 42 of average power 
control path 36. Difference detector 60 finds the difference between the 
average power and the averaged modulation power and provides it to a 
reference adjust circuit 62. In effect, the output from averager 42 of 
average power control path 36 is used as the reference voltage for 
difference detector 60 in this determination. 
The output from difference detector 60 is conditioned by reference adjust 
circuit 62, and analog and digital laser modulation circuits 64 and 66 so 
that the signal is suitable for use by laser drivers 16. The conditioned 
analog and digital modulation control output signals 68 and 70 from analog 
and digital laser modulation circuits 64 and 66, respectively, are then 
provided to laser driver 16 along with average power control signal 48 
from average laser bias control circuit 46. 
Constructed in this manner, apparatus for laser bias and modulation control 
10 is capable of maintaining a constant and linear output power from a low 
cost laser despite varying ambient temperature and component aging. In 
addition, clipping and kinking phenomena in laser operation may be 
monitored and corrected. 
FIGS. 4-7 are more detailed schematic diagrams of an embodiment of 
apparatus for laser bias and modulation control 10. Referring first to 
FIG. 4, laser module 20 includes laser diode 22 which generates and 
transmits light to an optical fiber (not shown) and back facet monitor 
photodiode 24. Monitor photodiode 24 receives the light and generates a 
proportional sample photo current. Transimpedance amplifier 26 is coupled 
to monitor photodiode 24 to receive the photo current. 
Low input impedance transimpedance amplifier 26 includes transistors 100 
and 102, resistors 104-112, and capacitors 114 and 116, which form a 
cascade amplifier. The cascade amplifier configuration is particularly 
useful for high-frequency applications for its good gain, high bandwidth, 
and input/output isolation. A transistor 120 and resistor 122 form a 
buffer amplifier for the cascade stage. A resistor 124 and an inductor 126 
apply feedback which sets the gain and frequency response of 
transimpedance amplifier 26. Transimpedance amplifier 26 further includes 
a capacitor 128 coupled in parallel with resistor 110 and a resistor 130 
coupled in series with resistor 108. 
Coupled to transimpedance amplifier 26 is a transformer-based three-way 
splitter 28. Three-way splitter 28 includes transformers 140-146, 
resistors 148-152,and capacitors 154-160. It is known that such a splitter 
may also be resistor-based, the implementation details thereof is not 
described herein. 
Average power control path 36 receiving a first output of three-way 
splitter 28 and provides it to lowpass filter 40. Lowpass filter 40 
includes a capacitor 166 and an inductor 168. 
Coupled to lowpass filter 40 is averager circuit 42 and reference adjust 
circuit 44. Averager 42 and reference adjust 44 include an operational 
amplifier 170 coupled to resistors 172-176 at its noninverting input 
terminal, and a resistor 178 and a capacitor 180 at its inverting 
terminal. Averager 42 and reference adjust 44 are coupled to average laser 
bias control circuit 46, which are formed by a transistor 184 and 
resistors 186-190. 
Averager 42 in effect compares the filtered transimpedance amplifier output 
voltage with a reference voltage determined at the junction of resistors 
172 and 174. If the reference voltage is greater than the filtered 
transimpedance amplifier output voltage, the average laser drive power is 
increased by forcing more current through resistor 184 into the base 
terminal of transistor 186, and vice versa. The resultant average bias 
control current is applied to laser module 20 through an inductor 190. 
Recall that the reference voltage required is a function of the monitor 
photodiode current, the transimpedance amplifier gain, and the desired 
average laser optical output power, where the monitor photodiode current 
is a known value obtained from the laser module manufacturer. 
Referring also to FIG. 5, an embodiment of modulation power control path 38 
is shown. Pre-emphasis circuit 50, coupled to a second output of three-way 
splitter 28, includes a transistor 200, resistors 202-210, capacitors 
214-220, and an inductor 224. Pre-emphasis circuit 50 in effect forms a 
gain-sloped negative feedback amplifier for the positive peak detection 
signal path. The gain is set by resistors 208 and 210, while the gain 
slope, where increasing gain as the signal frequency increases, is set by 
inductor 224 and capacitor 216. The negative peak detection path similarly 
operates with pre-emphasis circuit 52 that includes a transistor 230, 
resistors 232-240, capacitors 244-250, and an inductor 254. 
The circuit implementation of positive peak detector 54 may be similar to 
that of negative peak detector 56. Positive peak detector 54 includes an 
operational amplifier 260, diodes 262 and 264, resistors 266-272 coupled 
to the noninverting input terminal of operational amplifier 260, resistors 
276-282 and capacitors 286 and 288 coupled to the inverting input terminal 
of operational amplifier 260, and a resistor 290 and a capacitor 292 
forming a feedback loop from the operational amplifier's 260 output to the 
inverting input terminal. Negative peak detector 56 includes an 
operational amplifier 300, diodes 302 and 304, resistors 306-324, and 
capacitors 328-332. 
The response times of both detectors 54 and 56 are set according to the 
signal component of the frequency division multiplex that has the longest 
duration between transitions. Accordingly, the lower frequency digital 
signal determines the longest time constant required of peak detectors 54 
and 56. For example, where the longest time between transitions is on the 
order of 244 nanoseconds, the resistive value for resistor 280 may be 
162K.OMEGA., for both resistors 282 and 278 may be 75.OMEGA., and the 
capacitive values for capacitors 288 and 286 may be 0.47.mu.f and 82 pf, 
respectively. 
In operation, operational amplifier 260 amplifies the detected signals and 
provides temperature and power supply variation compensation. Resistors 
276 and 290, and capacitor 292 with operational amplifier 260 integrate 
and amplify the detected peak signal level. It should be noted that the 
time constant of integration is much less than the above-described longest 
time between transitions in the frequency multiplex signal to prevent 
inadvertent interaction therebetween. The operational amplifier reference 
voltage is determined by the values of diode 264 and resistors 266, 268, 
270, and 272. The operational amplifier reference voltage is responsive to 
and compensates for temperature variations and power supply variations. 
Negative peak detector 56 operates in a similar manner. 
The output of both peak detector circuits 54 and 56 are provided to peak 
detection averager circuit 58 which determines a midpoint voltage from the 
maximum and minimum detected values. Peak detection averager 58 includes 
an operational amplifier 340 and resistors 342-352, which serve to provide 
amplification and DC level shifting of the positive and negative peak 
detector output voltages. Operational amplifier 360, resistors 362-366, 
and a resistor 368 provide further amplification, DC level shifting, and 
inversion of the peak detector output voltages. 
The output from peak detector averager 58 is provided to difference 
detector 60 along with the output from averager 42 of the average power 
control path 36. The difference between the average power and the averaged 
modulation power is determined by difference detector 60. Referring to 
FIG. 6, difference detector 60 may include an operational amplifier 380, 
resistors 382-384, and a capacitor 386 coupled in a noninverting 
configuration. 
Constructed in this manner, the output from averager 42 is used as the 
reference voltage for difference detector 60 and applied through resistor 
382 to the noninverting input of operational amplifier 380. The reference 
voltage is compared to the peak detection averager output voltage of the 
modulation power control path. The modulation power control path peak 
detection average voltage is applied to resistor 384. Operational 
amplifier 380, resistors 384 and 382, and capacitor 386 of peak detection 
averager 58 act to force the modulation control peak detector averaged 
voltage to equal the average power control path average voltage. In this 
manner, linear laser operation is achieved. 
The difference detector output is first conditioned by analog and digital 
reference adjust and modulation control circuits 62-66 before providing it 
to laser driver 16. Analog reference adjust and laser modulation control 
circuit 62 and 64 include an operational amplifier 400, and resistors 
404-418. Similarly, an operational amplifier 420 and resistors 422-440 
form the digital counterpart circuit 62 and 66. 
An embodiment of laser driver 16, divided into an analog laser driver 450 
and a digital laser driver 452, is shown in FIG. 7. Laser driver 16 is 
best described by specifying the control currents generated by laser 
driver 16. The laser driver circuitry consists of two main subsections. 
Recall that average laser bias control 46, shown in FIG. 4, provides an 
average bias control current through inductor 190 to laser module 20. 
Inductor 190 acts as an RF choke to force RF modulating currents through 
laser diode 22. Along with the average bias control current, laser driver 
16 generates a digital laser driver modulation current and an analog laser 
driver modulation current. 
The analog laser driver modulation current is generated by analog laser 
driver 450. The high speed analog input signal 14, assumed to have been 
highpass filtered, is first adjusted by an impedance matched pin diode 
attenuator circuit 456 of analog laser driver 450. Diodes 458-464, 
resistors 466-472, capacitors 476-484, and an inductor 490 form current 
controlled attenuator 456. After being adjusted by attenuator 456, the 
analog input signal is amplified by a broadband negative feedback 
amplifier 500, with a bandwidth of 50 to 1000 MHz, for example. As shown, 
amplifier circuit 500 includes transistor 504, resistors 506-514, 
capacitors 520.524, and an inductor 530. It should be noted that the 
amplifier gain response may be made to yield increased gain at higher 
frequencies and thus be used to correct for low cost laser relative 
intensity noise increase with increasing modulation frequency. 
The output from amplifier 500 may then be impedance matched through a 
resistive attenuator 532 comprising 534-538. The resultant voltage is then 
converted to current by resistor 540 and capacitor 542 before application 
to laser diode 22. 
The digital laser driver modulation current is applied through digital 
laser driver 452. The low speed input digital signal 12 is "squared up" 
and DC level shifted through by a circuit configuration including 
transistors 550-554, resistors 558-566, a capacitor 570, and a diode 572. 
The signal is then filtered by a lowpass filter formed by inductors 576 
and 578 and a capacitor 580 to remove high frequency signal components 
from the RF spectrum. In addition, resistors 582 and 584, and capacitors 
588 and 588 and 590 aid in providing AC terminations for the lowpass 
filter. 
Transistor 594 is used to control the digital modulating current level. The 
modulating current is sensed across a resistor 596 and a portion of the 
resultant voltage is fed to an operational amplifier 598 through resistors 
600-606. This voltage level is compared to the digital modulation control 
signal, which is provided to the noninverting input of operational 
amplifier 598, and transistor 594 is forced to adjust the modulating 
current as required by drive application through resistors 610 and 612, 
and capacitors 614 and 616. 
Constructed in this manner, the optical transport system may operate at the 
optimal modulation index. Therefore, the carrier to noise ratio is 
maximized, and the laser transmitter distortion interference is minimized. 
In addition, full laser transmitter performance may be directly monitored 
and measured by examining the levels of feedback in the apparatus. The 
instant apparatus may be implemented in low power and low cost systems 
without cooling requirements. Wide temperature ranges and operation 
variability due to aging components are also tolerated and compensated. 
Furthermore, alarming and remote control features may be easily 
incorporated in the instant apparatus. Alarming including an 
analog/digital modulation depth alarm and a laser aging alarm may be 
implemented. For example, an analog modulation depth alarm may be 
constructed by monitoring the voltage drop across resistor 468 in FIG. 7. 
Too small a voltage drop indicates either the high speed signal level into 
laser driver 16 is excessive or the average laser bias current is too low. 
Too large a voltage drop indicates either the laser driver high speed 
input level is too low or that the average laser bias current is too high. 
The monitored voltage drop would then be compared, by operational 
amplifier circuits implementing a difference detector or comparator, to a 
reference voltage. When the sensed level is too high or too low with 
respect to the reference voltage, an alarm is sent to the system. 
Similarly, a digital modulation depth alarm may be constructed by 
monitoring the voltage drop across resistor 596 of FIG. 7. A laser aging 
alarm may be constructed by monitoring the voltage drop across resistor 
190 of FIG. 4. An alternative to the operational amplifier difference 
detector or comparator circuit is to permit a microprocessor to monitor 
the voltage levels through an analog-to-digital converter and determine 
the alarm status. The microprocessor may then communicate the alarm status 
to the rest of the network. 
Remote control or status monitoring capability implementation may require 
microprocessor control. Therefore, analog-to-digital and digital-to-analog 
converters may be used to monitor and provision the reference voltages 
used in the feedback paths. For example, the voltages at the junction of 
resistors 172-176 (FIG. 4) for average power reference voltage, at the 
junction of resistors 422-426 (FIG. 6) for digital modulation power 
reference voltage, and at the junction of resistors 402-406 (FIG. 6) for 
analog modulation power reference voltage, require provisionability. A 
calibration of the respective sensed feedback values with laser output 
parameters, such as optical output power, and optical modulation depth 
(ratio of peak-to-average optical power), is required. This allows the 
communication network to optimize the performance based on requirements 
of, for example, reducing DC power consumption or optimizing communication 
system signal-to-noise ratios under changing traffic loading conditions. 
Although the present invention has been described in detail, it should be 
understood that various changes, substitutions and alterations can be made 
thereto without departing from the spirit and scope of the present 
invention as defined by the appended claims.