Laser wavelength control under direct modulation

In accordance with the present invention, an in-fiber Bragg grating is coupled to the output of a directly modulated DFB laser. The output of the DFB laser is spectrally broadened, but has peak optical power at the channel wavelength. Typically, the grating is designed to have a substantially vertical "edge", i.e., segment of the transmissivity vs. wavelength characteristic, at a particular wavelength, between a transmission minimum and a transmission maximum. Amounts of light transmitted through and reflected by the grating are compared to adjust the channel wavelength to a desired wavelength at or near the edge of the grating.

FIELD OF THE INVENTION 
The present invention is directed to a system and related method for 
controlling the wavelength of light output from a directly modulated 
laser. 
Optical communication systems are a substantial and fast growing 
constituent of communication networks. The expression "optical 
communication system," as used herein, relates to any system which uses 
optical signals to convey information across an optical waveguiding 
medium, for example, an optical fiber. Such optical systems include but 
are not limited to telecommunication systems, cable television systems, 
and local area networks (LANs). Optical systems are described in Gowar, 
Ed. Optical Communication Systems, (Prentice Hall, New York) c. 1993, the 
disclosure of which is incorporated herein by reference. Currently, the 
majority of optical communication systems are configured to carry an 
optical channel of a single wavelength over one or more optical 
waveguides. To convey information from plural sources, time-division 
multiplexing is frequently employed (TDM). In time-division multiplexing, 
a particular time slot is assigned to each signal source, the complete 
signal being constructed from the portions of the signals collected from 
each time slot. While this is a useful technique for carrying plural 
information sources on a single channel, its capacity is limited by fiber 
dispersion and the need to generate high peak power pulses. 
While the need for communication services increases, the current capacity 
of existing waveguiding media is limited. Although capacity may be 
expanded e.g., by laying more fiber optic cables, the cost of such 
expansion is prohibitive. Consequently, there exists a need for a 
cost-effective way to increase the capacity of existing optical 
waveguides. 
Wavelength division multiplexing (WDM) has been explored as an approach for 
increasing the capacity of existing fiber optic networks. WDM systems 
typically include a plurality of transmitters, each including a 
semiconductor laser diode respectively transmitting signals on a 
designated one of a plurality of channels or wavelengths. The channels are 
combined by a multiplexer at one end terminal and transmitted on a single 
fiber to a demultiplexer at another end terminal where they are separated 
and supplied to respective receivers. 
Generally, a plurality of erbium doped fiber amplifiers are provided at 
nodes spaced along the fiber between the multiplexer and demultiplexer in 
order to regenerate the optical signal transmitted on the fiber. These 
erbium doped fibers optimally amplify in a relatively narrow range of 
wavelengths centered about 1550 nm. Thus, the semiconductor laser 
transmitters preferably transmit at respective wavelengths within this 
range. Since the transmitted wavelengths are relatively close to each 
other, typically less than 1 nm apart, these wavelengths must be precisely 
controlled in order to insure integrity of the transmitted information. 
Frequently, each semiconductor laser transmitter (e.g. a distributed 
feedback, DFB, laser) is operated in a continuous-wave (CW) mode, and an 
external modulator, such as a Mach-Zehnder interferometer, is used to 
generate a series of optical pulses corresponding to the communication 
data. While such schemes allow the wavelength of the emitted light to be 
readily controlled, the external modulator adds considerable expense. 
An alternative solution involves direct modulation of the semiconductor 
laser transmitters, in which the semiconductor laser transmitters are 
turned "on" and "off" in accordance with the communication data. In the 
"on" state, a relatively large bias is applied across the semiconductor 
laser transmitter, while in the "off" a relative low bias is supplied and 
a small amount of light is output. Such changes in applied voltage result 
in corresponding changes in the carrier concentration within the 
semiconductor laser transmitter, which, in turn, alter output wavelength. 
The optical spectrum of a directly modulated semiconductor laser 
transmitter is thus spectrally broadened or "chirped", as shown in FIG. 1. 
Typically, the chirped output has a peak optical power 101 at a single 
wavelength, which can be used as a one of the channel wavelengths in a WDM 
system. Due to the above-described spectral broadening, however, the 
channel wavelength of a directly modulated laser is difficult to stabilize 
and control. 
SUMMARY OF THE INVENTION 
Consistent with the present invention, an optical device is provided 
including a laser having a peak optical power at a first wavelength. A 
filtering element is coupled to the laser and has a transmission 
characteristic as a function of wavelength. The transmission 
characteristic has a transmissivity minimum over a range of transmissivity 
minimum wavelengths, a transmissivity maximum over a range of 
transmissivity maximum wavelengths, and a substantially vertical slope at 
a second wavelength between the ranges of transmissivity minimum and 
maximum wavelengths. Additionally, a control circuit is provided for 
adjusting the first wavelength to be within the range of transmissivity 
minimum and maximum wavelengths.

DETAILED DESCRIPTION 
In accordance with the present invention, an in-fiber Bragg grating is 
coupled to the output of a directly modulated DFB laser. The output of the 
DFB laser is spectrally broadened, as noted above, and has peak optical 
power at the nominal channel wavelength. Typically, the grating is 
designed to have a substantially vertical "edge", i.e., segment of the 
transmissivity vs. wavelength characteristic, at a particular wavelength, 
between a transmission minimum and a transmission maximum. Amounts of 
light transmitted through and reflected by the grating are compared to 
adjust the channel wavelength to a desired wavelength at or near the edge 
of the grating. 
Turning to the drawings in which like reference characters indicate the 
same or similar elements in each of the several views, FIG. 2 illustrates 
a laser wavelength control device 100 in accordance an embodiment of the 
present invention. Laser wavelength control device 100 typically includes 
a semiconductor DFB laser diode 105 generally comprising one or more III-V 
semiconductor materials commercially available from a wide variety of 
suppliers such as Fujitsu, Alcatel, Lucent, and Hewlett-Packard. DFB laser 
105 is directly modulated by laser drive circuit in a conventional manner 
to generate a series of light pulses corresponding to the transmitted 
communication data. 
Light output from DFB laser 105 is supplied via fiber 107 to directional 
coupler 110, which diverts a portion of the transmitted light, e.g. 5%, to 
a filtering element, typically in-fiber Bragg grating 115 commercially 
available from Sumitomo Electric Industries, Ltd. and 3M Specialty Optical 
Fibers, for example. A first portion of the diverted light is transmitted 
through grating 115 to photodetector while a second portion is reflected 
back through coupler 110 to photodetector 122. Photodetectors 120 and 122, 
respectively, generate electrical signals in response to the first and 
second portions of light. These electrical signals are supplied to a 
comparator circuit, including, for example, a differential amplifier. The 
comparator circuit, in turn, outputs a comparison signal to laser control 
processor 130 (typically a general purpose microprocessor such as a 68302 
microprocessor manufactured by Motorola). Based on the comparison signal, 
a program stored in processor 130 supplies an output voltage signal to 
thermo-electric heater 135, which sets the temperature of DFB laser 105 in 
accordance with the received output voltage. 
Generally, the wavelength of light output from DFB laser 105 is inversely 
related to the temperature of DFB laser 105. Accordingly, by changing the 
temperature of thermo-electric cooler 132, processor 130 can alter the 
wavelength of light output from DFB laser 105. 
A detailed description of a method of controlling DFB laser 105 will be 
presented below with reference to FIGS. 2-4. In accordance with one aspect 
of the present invention, the method is carried out in accordance with the 
program stored in processor 130. 
As seen in FIG. 3, grating 115 has a transmission characteristic 200 (and a 
complementary reflectivity characteristic) which is low or at a minimum 
(e.g. 5%) and substantially constant over a relatively wide range of 
wavelengths between a desired wavelength, .lambda..sub.0, and 
.lambda..sub.C. At .lambda..sub.0, the characteristic typically has a 
substantially vertical slope, for example at least 2-5 GHz/dB, and for 
wavelengths greater than .lambda..sub.1, and less than .lambda..sub.0, the 
transmission characteristic is high (e.g., 90%) or at a maximum. DFB laser 
105 emits at peak optical power at channel wavelength .lambda..sub.c, as 
indicated by optical spectrum curve 215 superimposed on the transmission 
characteristic in FIG. 3. The optical spectrum of DFB laser 105 at 10% 
peak power is typically at least equal to half the difference between a 
wavelength associated with 90% peak transmissivity and a wavelength 
associated with 10% peak transmissivity of grating 115 to insure proper 
locking of laser 105 to a particular wavelength, as discussed in greater 
detail below. 
As further shown in FIG. 3, optical power or light intensity at wavelengths 
within range 205, as represented by region 220 beneath curve 215, is 
within the transmission minimum and is reflected by grating 115 to 
photodetector 122, while optical power at wavelengths within range 210, as 
represented by region 230 beneath curve 215, is transmitted through 
grating 115 and sensed by photodetector 120. 
Since .lambda..sub.C is offset from .lambda..sub.0, the electrical signal 
(e.g., current) generated by photodetector 122 exceeds the electrical 
signal generated by photodetector 120, as shown by the larger area of 
region 220 relative to region 230 in FIG. 3. Comparator circuit 125, 
therefore, outputs a comparison signal (e.g., a voltage) to processor 130 
corresponding in magnitude and polarity to the difference in optical power 
received by photodetectors 120 and 122. Processor 130, in turn, retrieves 
a control voltage adjustment in a memory 131, such as a look-up table, 
corresponding to the received comparison signal. Alternatively, the 
control voltage adjustment can be determined based upon a formula, such as 
a proportional integral derivative (PID) formula. Processor 130 then 
outputs a control voltage to thermoelectric cooler 135, which adjusts the 
temperature of DFB laser 105 to thereby shift the wavelength 
.lambda..sub.C in an amount and direction indicated by arrow 225. 
Preferably, the range of wavelengths between .lambda..sub.0 and 
.lambda..sub.1 is greater than the optical spectrum of DFB laser 105 to 
insure that .lambda..sub.C is adjusted to the edge at .lambda..sub.0 
instead of the edge at .lambda..sub.1. 
As a result, the ratio of optical power transmitted through grating 115 
(area of region 230) to optical power reflected by the grating (area of 
region 220) is adjusted to correspond to the condition under which 
.lambda..sub.C equals .lambda..sub.0, as shown in FIG. 4, to thereby 
"lock" laser 105 to .lambda..sub.0. Preferably, processor 130 continuously 
monitors the output of comparator 125, and adjusts the wavelength of DFB 
laser 105 to maintain the desired ratio of output optical power. For 
example, if the desired ratio of output optical power is set to one 
.lambda..sub.C is adjusted to a wavelength .lambda..sub.0 whereby the 
amount of optical power reflected by the grating is the same as that 
transmitted through the grating. Other ratios may also be set as well. 
Thus, .lambda..sub.C can be precisely set to a single desired wavelength 
by appropriately selecting a particular ratio of transmitted to reflected 
optical power. The grating position, i.e., the location of the edge of the 
grating can also be used to set .lambda..sub.C. 
While wavelength control based upon the ratio of transmitted to reflected 
power has been discussed above, it is within the scope of the present 
invention to control laser wavelength based on the reflected to 
transmitted optical power as well. Typically, however, the channel 
wavelength is set to fall outside the range of wavelengths associated with 
the transmission minimum of grating 115, and is at most equal to 
wavelength associated with the lower wavelength edge of the grating, e.g., 
.lambda..sub.0 above. Nevertheless, if necessary, the channel wavelength 
may also be tuned to be at least equal upper wavelength edge of grating 
115 corresponding to .lambda..sub.1 (see FIG. 3). 
In accordance with a second embodiment of the present invention, comparator 
125 in FIG. 1 is omitted, and the outputs of photodetectors 120 and 125 
are supplied directly to processor 130. Accordingly, processor 130 can 
contain a program which carries out the following steps shown in FIG. 5. 
Namely, the program fist compares the outputs from photodetectors 120 and 
122 in step 410. Next, in step 420, the comparison result can be used to 
identify a corresponding control voltage adjustment stored in a look-up 
table, for example. An output control voltage corresponding to the control 
voltage adjustment is then output to thermo-electric cooler 135 to adjust 
the temperature, and thus the wavelength of DFB laser 105 in step 430 so 
that the desired ratio of transmitted to reflected optical power by 
grating 115 is obtained. Optionally, a delay (step 440) can be provided in 
the program to allow the wavelength of DFB laser 105 to stabilize prior to 
comparing the outputs of photodetectors 120 and 122 again (step 410). As 
seen in FIG. 5, the program typically cycles through steps 410, 420, 430 
and 440, thereby insuring that the channel wavelength .lambda..sub.C 
remains set to the desired wavelength .lambda..sub.0. 
A third embodiment of the present invention will next be described in 
conjunction with FIG. 6. In accordance with the third embodiment, 
processor 130 is omitted, and replaced by hardwired circuitry. For 
example, as shown in FIG. 6, the outputs of photodetectors 120 and 122 are 
respectively supplied to current-to-voltage operational amplifiers 520 and 
522, which convert the received current signals to voltages. The 
respective gains of the photodetectors are preferably set so that equal 
currents are respectively output when .lambda..sub.C equals 
.lambda..sub.0. The voltage outputs generated by amplifiers 520 and 522 
are supplied to a differential amplifier 530, which is configured to 
output an appropriate voltage for driving thermo-electric coolerl35. 
In accordance with a fourth embodiment of the present invention, grating 
115 receives substantially the entire optical output from laser 105. For 
example, as seen in FIG. 7, the fourth embodiment is similar to the first 
embodiment shown in FIG. 2, with the exception that the optical output 
from laser 105 is supplied directly to a fiber 710 having grating 115 
embedded therein. In this configuration, grating 115 is said to be "in 
transmission". Couplers 610 and 620 respectively tap off 2% of the optical 
power, for example, to photodetectors 120 and 122. The operation of the 
laser control device in accordance with the fourth embodiment is similar 
to that described above in regard to the first embodiment. 
The fourth embodiment, however, is advantageous in that grating 115 acts to 
eliminate a chirped portion 810 (see FIG. 8) of the output of laser 105. 
The chirped portion of the light constitutes blue-shifted light which 
travels at a different speed down the fiber than the light within the main 
peak 101 (see FIG. 1). Thus, light from one pulse can overlap with another 
pulse at the receiving end of an optical fiber causing increased bit error 
rate probabilities. As shown in FIG. 8, however, the ratio of transmitted 
to reflected optical power by grating 115 can be monitored to adjust the 
channel wavelength to be slightly offset from the substantially vertical 
edge of grating 115. As a result, the chirped portion of the output light 
is reflected back and eliminated from the spectrum of the transmitted 
light, as seen in FIG. 9, and reduced bit error rate probabilities can 
thus be achieved. 
While the foregoing invention has been described in terms of the 
embodiments discussed above, numerous variations are possible. 
Accordingly, modifications and changes such as those suggested above, but 
not limited thereto, are considered to be within the scope of the 
following claims.