Directly modulated semiconductor laser having reduced chirp

In accordance with the present invention, an in-line fiber Bragg grating is coupled to the output of a directly modulated DFB laser. The grating preferably rejects chirp induced frequencies of light emitted by the DFB laser. Accordingly, light transmitted through the grating is spectrally narrowed and has a higher extinction ratio, thereby decreasing bit error rate probabilities.

FIELD OF THE INVENTION 
This application is related to copending application entitled "Laser 
Wavelength Control Under Direct Modulation", incorporated herein by 
reference. 
The present invention is directed to a system and related method for 
narrowing a spectrally broadened output of a directly modulated laser. 
Optical communication systems are a substantial and fast growing 
constituent of communication networks. In a typical optical communication 
system, information bearing optical signals are transmitted along an 
optical fiber. The optical signals are frequently generated by operating a 
laser in a continuous-wave (CW) mode, and modulating the emitted light 
with an external modulator, such as a Mach-Zehnder interferometer. 
Although such external modulation schemes effectively encode the optical 
signals with communication data, the external modulator is expensive and 
inserts additional loss into the system. Such loss, however, can be 
compensated in long haul networks with optical amplifiers, which further 
add to the cost of the system. 
Shorter haul networks, however, are more cost sensitive than long haul 
networks. Accordingly, in order to reduce the cost of these networks, 
semiconductor distributed feedback (DFB) directly modulated lasers have 
been proposed. These lasers are turned on and off directly in accordance 
with the communication data, thereby eliminating the need for an external 
modulator. Further, since DFB lasers can generate a high power optical 
output, few, if any, optical amplifiers are required. 
When the DFB laser is in the "on" state, however, a relatively large 
current is injected into the semiconductor laser, while in the "off" state 
a relative low current is injected and a small amount of light is output. 
Such changes in current result in corresponding changes in the carrier 
density within the laser, which, in turn, alter output frequency and 
spectrally broaden or "chirp" the emitted light. 
As shown in FIG. 1, the optical spectrum of a directly modulated 
semiconductor laser has a main intensity peak 101 at the intended channel 
frequency. The optical spectrum, however, is spectrally broadened to 
include a subsidiary peak 102 at chirp-induced frequencies lower than the 
channel frequency. 
The chirped signal includes relatively significant "blue" (higher frequency 
at peak 101) and "red" (lower frequency at peak 102) components, which can 
propagate through an optical fiber at different speeds due to chromatic 
dispersion. Accordingly, light from one pulse can overlap with a 
successive pulse at the receiving end of an optical fiber causing 
increased bit error rate probabilities. Directly modulated lasers 
therefore limit the distance of short haul optical communication systems. 
SUMMARY OF THE INVENTION 
Consistent with the present invention, an optical device is provided which 
comprises a semiconductor laser configured to be coupled to a first end 
portion of an optical communication path, and a drive circuit coupled to 
the semiconductor laser. The drive circuit supplies an electrical signal 
to directly modulate the semiconductor laser. 
The optical device further includes an optical receiver configured to be 
coupled to a second end portion of the optical communication path to 
thereby receive the light emitted by said semiconductor laser. In 
addition, an in-line fiber Bragg grating is provided in the optical 
communication path. The in-line fiber Bragg grating is spaced a first 
distance from the first end portion of the optical communication path and 
a second distance from the second end portion of the optical communication 
path, such that the first distance is less than said second distance. 
Moreover, the in-line fiber Bragg grating is provided in an 
in-transmission configuration to substantially reject chirp-induced 
frequencies.

DETAILED DESCRIPTION 
In accordance with the present invention, an in-line fiber Bragg grating is 
coupled to the output of a directly modulated DFB laser. The grating 
preferably rejects chirp induced frequencies of light emitted by the DFB 
laser. Accordingly, light transmitted through the grating is spectrally 
narrowed has a higher extinction ratio and narrower spectrum. Accordingly, 
lower bit error rate probabilities can be achieved. 
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 simplified schematic diagram of an optical communication system in 
accordance with the present invention. The optical communication system 
includes transmitter 210 coupled to an optical communication path, such as 
an optical fiber 220, which supplies optical communication signals to 
receiver 230. As generally understood, receiver 230 can include 
photodiodes (not shown) for sensing the optical signals and other 
appropriate circuitry. As further shown in FIG. 2, optical fiber 220 
includes in-line fiber Bragg grating 240 provided in an "in transmission" 
configuration. That is, light transmitted through the grating is passed to 
receiver 230, while reflected light is rejected. In-line fiber Bragg 
grating 240 is commercially available from Sumitomo Electric Industries, 
Ltd. and 3M Specialty Optical Fibers, for example. 
It is understood that combiners, couplers, star distribution networks, 
switching elements, optical amplifiers, signal regenerators, 
reconditioners, add/drop multiplexers, and repeaters and the like may be 
present in the optical communication system and coupled to the optical 
communication path without any loss of generality of applicability for the 
principles of the present invention. 
Transmitter 210 is shown in greater detail in FIG. 3. Transmitter 210 
typically includes a drive circuit 315 that turns laser 325 "on" and "off" 
to transmit communication data as a series of optical pulses on optical 
communication path 220. Laser 325 is typically a DFB laser generally 
comprising III-V semiconductor materials and commercially available from a 
wide variety of suppliers such as Fujitsu, Alcatel, Lucent and 
Hewlett-Packard. 
As seen in FIG. 4, output spectrum 410 of directly modulated laser 325 is 
superimposed on transmissivity characteristic 415 of in-line fiber Bragg 
grating 240. Typically, transmissivity characteristic 415 has a low 
transmissivity over a first range of frequencies substantially between 
.nu..sub.1 and .nu..sub.2, a high transmissivity segment 419 over a second 
range of frequencies greater than or equal to .nu..sub.3, and an 
intermediate portion 421 having a relatively steep slope typically at 
least 1 dB/GHz and preferably 2-5 dB/GHz or more over a relatively narrow 
range of frequencies between .nu..sub.2 and .nu..sub.3. The transmissivity 
characteristic has an additional transmission maximum over frequencies 
less than .nu..sub.4. 
As further shown in FIG. 4, in-fiber Bragg grating 240, in the 
in-transmission configuration, is preferably designed so that 
chirp-induced frequencies, represented by the cross-hatched area 425 
beneath optical spectrum curve 410, typically fall within 
reduced-transmission portions of the transmissivity characteristic, for 
example, between .nu..sub.1 and .nu..sub.3. Typically, the difference in 
frequency between .nu..sub.1 and .nu..sub.3 is about 40 GHz. Moreover, 
this portion of the transmissivity characteristic preferably has a loss 
greater than 3 dB, and is typically greater than 10 dB. 
The grating is further designed so that the peak optical power frequency 
.nu..sub.P is greater than .nu..sub.3. As a result, the chirp-induced 
frequencies of light emitted by laser 325 are substantially rejected by 
grating 240, while the channel frequency, i.e., .nu..sub.P, is 
transmitted. As shown in FIG. 5, the resulting spectrum of light 
transmitted to receiver 230 has a single intensity peak 510 at channel 
frequency .nu..sub.P, is narrowed significantly, and is substantially free 
of the chirp-induced frequencies. Typically, in order to insure that 
modulation induced sidebands of the transmitted signal , not shown in FIG. 
5, are not rejected by grating 240, the grating is typically designed to 
have a low loss (i.e., high transmissivity) over a range of frequencies 
.nu..sub.p .+-..nu..sub.mod, where .nu..sub.mod is the modulation 
frequency of laser 325. The loss is preferably less than 1 dB and is 
typically less than 0.1 dB. 
Moreover, the ratio of transmitted light in the on-state to transmitted 
light in the off-state (the "extinction ratio") can be achieved with the 
present invention with a smaller current swing than that required by a 
conventional directly modulated DFB laser. As noted above, current is 
continuously supplied to the laser in both the on and off states, although 
the on-current is necessarily significantly more than the off-current. In 
the conventional directly modulated laser, the off-current must be 
diminished substantially to a current slightly above the threshold current 
of the laser in order to insure that the combined amounts of intended 
channel light and the chirp light is sufficiently low. Consistent with the 
present invention, however, the chirped light is rejected by in-line fiber 
Bragg grating 240, and, therefore, a sufficiently low light intensity can 
be achieved even at higher off-currents. As a result, the off-current need 
not be diminished to the same extent as in the conventional directly 
modulated scheme, and smaller on-to-off current swings can be tolerated, 
thereby requiring a simpler laser driver circuit. Conversely, the same 
swing of the laser could yield a higher extinction ratio. 
Improved bit error rates (BER) associated with the present invention will 
now be discussed with reference to FIG. 6, which illustrates log(BER) as a 
function of received optical power from a directly modulated DFB laser. 
Curves 810 and 820 represent BER characteristics of conventional directly 
modulated DFB lasers. In particular, curve 810 corresponds to the BER of 
optical signals received over 107 km of fiber having a dispersion of 17 
ps/nm/km, and curve 820 represents the BER of optical signals detected 
adjacent the laser output (i.e., in, a "back-to-back" configuration). 
Curves 830 and 840, however, correspond to BER characteristics of optical 
signals transmitted in accordance with the present invention: over 107 km 
of the fiber (curve 830) and detected directly at the output of the 
grating in the back-to-back configuration (curve 840) due to the higher 
extinction ratio. 
As further seen in FIG. 6, for any given level of received optical power, 
the BER associated with the present invention is significantly lower than 
that of the conventional directly modulated laser. Moreover, the BER of 
optical signals transmitted in accordance with the present invention over 
107 km of fiber, which may have been expected to be relatively high, is 
actually reduced considerably, and is even lower than the BER of a 
conventional directly modulated laser measured in the back-to-back 
configuration. 
Returning to FIG. 3, by providing in-line fiber Bragg grating 240 closer to 
transmitter 210 than receiver 230, for example within 3.0 meters from the 
output of laser 325, a feedback control circuit 260 can be provided in 
transmitter 210 for regulating the frequency of light output from laser 
325 in accordance with the intermediate portion 421 or "edge" of the 
transmissivity characteristic of in-line fiber Bragg grating 240. 
Preferably, the difference between .nu..sub.1 and .nu..sub.2 should be 
relatively large, typically at least 40 GHz, in order to insure that the 
frequency is adjusted to the appropriate edge of in-line fiber Bragg 
grating 240 
Feedback control circuit 260 can include photodetectors (not shown) for 
sensing light reflected by grating 240 via coupler 262 and light 
transmitted through grating 240 via coupler 264. Electrical signals 
generated by these photodetectors can then be used to adjust the frequency 
of laser 325, for example, in a manner similar to that described in the 
above cited patent application. In addition, a back facet monitoring 
photodiode (not shown) may be provided adjacent a read facet of laser 325 
to monitor optical power output. 
It is further noted that light can be reflected by grating 240 back to 
laser 325, thereby degrading performance of the laser. Accordingly, as 
further shown in FIG. 3, an optical isolator 335 is typically provided 
between the output of laser 325 and in-line fiber Bragg grating 240 in 
order to prevent any reflected light from reaching laser 325. 
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.