Method and apparatus for reducing adverse effects of optical beat interference in optical communication systems

A method and apparatus for reducing noise in optical communication systems, wherein the frequency of light output from a signal laser is modulated or dithered to broaden the optical spectrum of the signal laser. The broadening allows error-free data transmission within signal bands, despite the presence of optical beat interference. Different dithering tones are emitted by each laser in the network. The dithering tone for each signal laser in an optical network is chosen so that distortion resulting from the dithering tone falls outside the signal bands.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the field of optical 
communications systems and more particularly to a method and apparatus for 
reducing adverse effects of optical beat interference in lightwave 
transmission systems. 
2. Background of the Invention 
Over the past decade, the popularity of passive optical networks (PONs) has 
grown. By sharing the cost of a "feeder" among many users, PONs promise to 
be more economical than other fiber-to-the-curb (FTTC) architectures. 
Subcarrier-multiplexed (SCM) PONs, where each channel is allocated a 
different RF subcarrier, have been proposed due to the ease with which 
user channels can be independently accessed. Unfortunately, the 
performance of a SCM-PON can be seriously degraded by optical-beat 
interference (OBI). 
OBI occurs when light from two sources is detected in a photodetector, 
creating beat signals at a "difference" frequency. The result is that 
phase noise is converted into intensity noise. Unlike time-division 
multiple access (TDMA) systems, where careful timing prevents light from 
different lasers from arriving at a receiver at the same time, in a 
SCM-PON, light from multiple lasers will arrive at the receiver 
simultaneously. If two of these lasers have optical frequencies differing 
by a subcarrier frequency of one of the channels, then OBI can severely 
degrade the signal-to-noise ratio (SNR) of that channel. See, for example, 
Wood and Shankaranarayanan, "Operation of a Passive Optical Network with 
Subcarrier Multiplexing in the Presence of Optical Beat Interference," J. 
Lightwave Technology, vol. LT-11, no. 10, pp. 1623-1640, October 1993. 
Choosing lasers so that their frequencies do not coincide is difficult 
when uncooled lasers are to be used, as temperature variations may cause 
the wavelength of each laser to change by well over 20 nanometers. 
To reduce the effects of OBI, it is advantageous to broaden the optical 
spectrum of the transmitter's light source because noise in an RF channel 
will be inversely proportional to the optical bandwidth (assuming that the 
bandwidth of the RF channel is much less than the optical bandwidth). When 
each laser carries only a single channel, overmodulating the channel 
(optical modulation depth, m&gt;1) can sufficiently broaden the optical 
spectrum of a distributed feedback (DFB) laser so that error-free 
transmission can be achieved even in the presence of OBI. (Wood and 
Shankaranarayanan, "Operation of a Passive Optical Network with Subcarrier 
Multiplexing in the Presence of Optical Beat Interference"). Error-free 
transmission of a frequency shift keyed (FSK) signal has been demonstrated 
using Fabry-Perot lasers, where a signal laser was 13 dB weaker than the 
combined power of interfering lasers. See R. D. Feldman, T. H. Wood and R. 
F. Austin, "Operation of a Subcarrier Multiple Access Passive Optical 
Network with Multimode Lasers in the Presence of Optical Beat 
Interference," paper TuQ5, Optical Fiber Communication Conference, San 
Diego, Calif., 1995. When an amplified, light emitting diode (LED) is 
used, there is less than a 1 dB penalty from OBI because of the LED's 
broad optical spectrum. Unfortunately, LEDs have a limited modulation 
bandwidth, which limits the subcarrier frequencies that can be used. 
In a system where each remote optical network unit (ONU) serves more than 
one home, each laser may carry multiple RF channels, with each channel 
serving a different user. In this scenario, over-modulating the data 
channels may not be an acceptable option because distortion or clipping 
induced noise will fall within the signal band, and may cause errors. 
The present invention expands upon the method and apparatus for reduction 
of optical interference set forth in U.S. Pat. No. 5,373,385 to Darcie, et 
al. Accordingly, U.S. Pat. No. 5,373,385 is incorporated into this 
document by reference. 
SUMMARY OF THE INVENTION 
In a PON system wherein each laser carries multiple RF channels, the 
present invention uses out-of-band dithering tones to broaden the laser's 
optical spectra sufficiently so that transmission is error-free despite 
the presence of OBI. The optical modulation depth (OMD) of each dithering 
tone is greater than 1. In addition, the frequency of each dithering tone 
is chosen so that distortion from each dithering tone does not fall within 
the signal band, and therefore does not cause errors within the signal 
band. Different dithering tones are used by each laser in the PON 
transmitting information upstream.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows an optical transmission system according to the present 
invention, wherein data signals and dithering signals are input to 
transmitters T. The outputs of the transmitters T are connected to a 
passive optical splitter/combiner 2 via optical fibers F. The passive 
optical splitter/combiner 2 is connected to a receiver 4 via an optical 
fiber 3. 
Dithering tones are generated by modulating the wavelength or optical 
frequency of light emitted from lasers used to transmit signals in a PON. 
Optical frequency modulation may be accomplished by appropriate use of 
techniques well known in the art. As described in U.S. Pat. No. 5,373,385, 
there are various ways to modulate the optical frequency of a laser. For 
example, where the laser is a semiconductor laser, a phenomenon known 
within the art as "chirp" may be used to systematically modulate the 
optical frequency of a laser. Chirp is the incidental modulation of the 
light's wavelength or frequency that occurs during direct intensity 
modulation of a light source. In other words, the wavelength of light 
output from a laser varies with the amount of electrical current provided 
to the laser. The optical frequency can also be modulated by 
systematically varying the operating temperature of the laser, or by using 
an external modulator. 
In an illustrative embodiment of the invention using the system shown in 
FIG. 1, each transmitter T includes a semiconductor Fabry-Perot laser, as 
shown in FIG. 2a. FIG. 2a shows a semiconductor Fabry-Perot laser 
transmitter having an electrical combiner 7. The electrical combiner 7 
combines the DC bias, RF signals, and dithering signal for input to the 
laser L.sub.1. The output of the laser L.sub.1 is emitted into the optical 
fiber F. Dithering signals input to the lasers cause the lasers' optical 
frequency to vary. In this way the lasers emit dithering tones. The 
optical modulation depth (OMD) of each dithering tone on each of the 
signal lasers is 1.4. Note that because the modulation depth is greater 
than 1.0, the laser will be driven below threshold, i.e. will be turned 
completely off so that no light is emitted. This is known as clipping. 
Note that the dithering tones are not required to be sine waves. The 
dithering tones can vary in frequency, or can cover a range of 
frequencies. Dithering tones can also carry information. In other words, a 
signal emitted by one of the signal lasers can function as both a 
dithering tone and an information carrier. 
Dithering tones and signal bands are chosen such that, for each laser, the 
cross products of the dithering tone and the payload signal or signal band 
emitted by that laser do not fall within the signal bands of any of the 
lasers in the PON. Otherwise, dithering induced distortion, including 
clipping-induced impulse noise, will cause errors to occur within data 
transmitted across the PON. Mathematically, this means that for each 
laser, the frequency of the dithering tone of that laser, plus or minus 
the frequency of the signal band for that laser, is not within any of the 
signal bands of any of the lasers in the PON. The frequency and/or 
intensity of the dithering tone may either remain constant or vary. 
In addition, dithering tones for each laser are chosen such that the 
dithering tone for each laser within the PON is unique with respect to the 
dithering tones of the other lasers within the PON. This is necessary 
because, where two or more lasers within a PON emit identical dithering 
tones, optical frequency fluctuations induced by the identical dithering 
tones will be correlated. For example, where the dithering tones are sine 
waves, and the same frequency is used for each dithering tone, then an 
amount of noise reduction will depend on the frequency of the sine wave 
and the relative delay between the beams' paths. If the difference in path 
length from the lasers to the receiver corresponds to an integral number 
of cycles of an identical dithering tone, then a particular tone might not 
give the desired performance improvement. 
In another embodiment of the invention using the system shown in FIG. 1, 
externally modulated signal lasers are used instead of Fabry-Perot 
semiconductor lasers. In this embodiment, each transmitter includes an 
externally modulated signal laser, as shown in FIG. 2b. In FIG. 2b, the 
laser L.sub.2 is connected to an intensity modulator 5 and an optical 
frequency modulator 6. The intensity modulator 5 modulates the intensity 
of light output from laser L.sub.2 according to RF signals, and the 
optical frequency modulator modulates the optical frequency of the light 
output from laser L.sub.2 according to a dithering signal. The resulting 
light is then emitted into optical fiber F. 
FIG. 3 shows radio frequency spectra of an optical network according to an 
embodiment of the invention both with and without dithering tones. As 
shown in FIG. 3, signal channels span from 600 MHz to about 780 MHz, and 
the dithering tones fall between 990 MHz and 1010 MHz. Different dithering 
tones are applied to each laser. A first laser has a sine wave at 1000 
MHz. The dithering tone of a second laser is white noise filtered to fall 
between 990 and 1010 MHz. As can be seen from FIG. 3, the dithering tones' 
OMD is much greater than that of the signal channels. The OMD of each 
dithering tone is 1.4, and the OMD of the signal channels is less than 1. 
Cross products of the dithering tones and the signal channels lie outside 
the signal channels between approximately 210 MHz and 410 MHz. 
While this invention has been described in conjunction with the above 
outlined specific embodiments thereof, it is evident that many 
alternatives, modifications and variations will be apparent to those 
skilled in the art. Accordingly, the preferred embodiments of the 
inventions as set forth above are intended to be illustrative, not 
limiting. Various changes may be made without departing from the scope and 
spirit of the invention.