Intra-cavity optical four-wave mixer and optical communications system using the same

In accordance with the invention, an optical four-wave mixer for producing a phase-conjugated signal comprises a source of optical input signals, a fiber laser for receiving the signals, and a detector for selectively detecting the frequency-shifted signals produced by four-wave mixing. The laser can be a rare-earth doped fiber laser with a fiber cavity phase matched to the input signals. The frequency-shifted output signals have an inverted spectral waveform as compared with the input signals. The mixer can be made in compact form with a cavity length as small as 100 m and can provide inverted signals at the same intensity as the input signals, making the mixer particularly useful for providing spectral inversion in an optical communications system.

FIELD OF THE INVENTION 
This invention relates to a device for efficient optical four-wave mixing. 
It is particularly useful for reversing the effect of dispersion in an 
optical communications systems. 
BACKGROUND OF THE INVENTION 
Optical communications systems are becoming increasingly important in the 
high speed transmission of large amounts of information. A typical optical 
communications system comprises a source of modulated optical input 
signals, a length of optical fiber coupled to the source, and a receiver 
for optical signals coupled to the fiber. The input signals are typically 
in the form of digital pulses which are transmitted with minimum 
attenuation in guided modes along the axis of the fiber. 
One difficulty with optical communications systems is dispersion. Different 
wavelength components of a pulse are transmitted with slightly different 
facility with the consequence that a sharp, symmetrical pulse at the 
input, after traveling many kilometers, becomes deformed and 
unsymmetrical. In the absence of preventative measures, a pulse will 
eventually degrade to a point where its initial location in a binary 
sequence is indeterminate. 
It has been proposed that dispersion can be reduced by midspan spectral 
inversion of propagating pulses, i.e. at the midpoint of the fiber path 
inverting the pulse waveform so that the higher frequency portion has the 
shape of the lower frequency portion and vice versa (effectively a 
180.degree. rotation of the pulse waveform about its center wavelength). 
As a result, after the inverted pulse travels over the second half of the 
communications path, the additional dispersion will reverse much of the 
distorting effect of the dispersion that occurred during the first half. 
One approach to spectral inversion is through the use of a phenomenon known 
as four-wave mixing. When the pulse is co-propagated along a fiber with 
high power (5-50 mW) narrow band light near the pulse wavelength, a second 
pulse is produced at a wavelength slightly different from the original 
pulse. The frequency-shifted second pulse has an inverted waveform as 
compared to the initial pulse. Unfortunately, the four-wave mixing 
arrangements heretofore known require tens of kilometers of co-propagation 
and produce inverted pulses 10-25 dB down from the input pulse. 
Accordingly, there is a need for an improved four-wave mixer providing a 
stronger inverted pulse in a more compact arrangement. 
SUMMARY OF THE INVENTION 
In accordance with the invention, an optical four-wave mixer for producing 
a phase-conjugated signal comprises a source of optical input signals, a 
fiber laser for receiving the signals, and a detector for selectively 
detecting the frequency-shifted signals produced by four-wave mixing. The 
laser can be a rare-earth doped fiber laser with a fiber cavity phase 
matched to the input signals. The frequency-shifted output signals have an 
inverted spectral waveform as compared with the input signals. The mixer 
can be made in compact form with a cavity length as small as 100 m and can 
provide inverted signals at the same intensity as the input signals, 
making the mixer particularly useful for providing spectral inversion in 
an optical communications system. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The advantages, nature and various features of the invention will appear 
more fully upon consideration of the illustrative embodiments now to be 
described in detail in connection with the accompanying drawings. In the 
drawings: 
FIG. 1 is a schematic diagram of an optical four-wave mixer in accordance 
with one embodiment of the invention; 
FIG. 2 is a spectral diagram showing the various optical signals associated 
with the operation of the device of FIG. 1; and 
FIG. 3 is a schematic diagram of an optical communications system employing 
the device of FIG. 1 for spectral inversion. 
It is to be understood that these drawings are for purposes of illustrating 
the concepts of the invention and, except for graphs, are not to scale.

DETAILED DESCRIPTION 
Referring to the drawings, FIG. 1 is a schematic diagram of an optical 
four-wave mixer 8 comprising a source 9 of optical input signals of center 
wavelength .lambda., a fiber laser for receiving the input signals, and, a 
detector 17 provided downstream of the laser for selectively responding to 
the frequency-shifted four-wave mixing signals produced in the laser 
cavity. In typical applications the source will provide a digitally 
modulated sequence of input pulses at a constant repetition rate. 
The laser can be composed of a rare-earth doped fiber 11, a laser cavity 12 
defined by a pair of fiber Bragg gratings 13 and 14 and a pumping source 
15. In this particular embodiment, a coupler 16 is provided for supplying 
input optical signals to the laser, and a coupler-reflector arrangement 18 
is provided for reflecting pump radiation back through the rare-earth 
doped fiber. The center wavelength of the laser should be different from 
the signal wavelength .lambda. so that the input signal, the laser light, 
and the mixing signal can all be separated, but the laser wavelength 
should also be within .+-.10% of .lambda.. Isolators 19 and 20 are 
advantageously provided to prevent reflection back into the input source 
and the laser cavity. 
Preferably, the rare-earth doped fiber is Er/Yb fiber, the pumping source 
is a 1060 nm Nd cladding laser, and the laser cavity comprises 100 m to 5 
km of dispersion shifted fiber. The Bragg gratings can be two 0.5 nm wide 
fiber grating reflectors tuned to resonate at the minimum dispersion 
wavelength (1535 nm) of a 1 km length of dispersion shifted fiber. The 
detector can utilize a Fabry-Perot filter to selectively transmit the 
mixing pulses. For maximum conversion efficiency, the cavity is phase 
matched with the input signals as by choosing the minimum dispersion 
wavelength of the dispersion shifted fiber equal to the input wavelength 
.lambda.. 
In typical operation, a sequence of input pulses at a constant repetition 
rate are fed into the laser cavity. The laser output, prior to filtration, 
includes the input pulses, laser light, and four-wave mixing pulses which 
are inverted (conjugated) as compared with the input pulses and 
frequency-shifted to the other side of the laser light in a spectral 
diagram. 
The device can be operated with the laser in either continuous wave 
operation or with the cavity adapted for mode-locked operation. For an 
input pulse source, the laser is preferably mode-locked at a repetition 
rate equal to the input pulse repetition rate, some integral multiple n of 
the repetition rate, or some integral fraction 1/2 of the repetition rate. 
In the continuous wave case, each input signal will generate a four-wave 
mixed output signal. In the mode-locked case, them can be a mixed pulse 
for each input pulse or for every nth pulse. 
FIG. 2 is a spectral diagram of unfiltered output showing an input signal 
A, the laser light B and the conjugated output signal C. While the input 
signal here is at a longer wavelength than the laser, it can also be at a 
shorter wavelength. In general, when propagating through the erbium fiber 
prior to the dispersion shifted fiber, higher conversion efficiency is 
gained when the input signal is on the long wavelength side of the laser. 
When the input signal propagates through the dispersion shifted fiber 
first, the conversion efficiency is greater for an input signal on the 
short wavelength side of the laser. In general, the greater the frequency 
shift, the lower the conversion efficiency. The highest conversion 
efficiency was observed for an input signal traveling through the Yb/Er 
first, with 1060 nm pump power of 2.4 watts. Conjugate conversion 
efficiency as high as OdB was observed for a shift of 9.8 nm. 
FIG. 3 is a schematic diagram of the preferred use of the FIG. 1 device for 
the spectral inversion of propagating signals in an optical communications 
system. Specifically, FIG. 3 illustrates an optical communication's system 
comprising a source 30 of modulated optical input signals, a first optical 
path 31, such as a length of optical fiber and a second optical path 32 
through a similar optical medium to receiver 33 of optical signals. 
Disposed between similar, approximately equal optical paths is a four-wave 
mixing device 34 of the type shown in FIG. 1 for conjugating the spectral 
form of propagating signals. The effects of dispersion in the path from 31 
are thus inverted, and these effects are essentially reversed as the 
conjugated pulses travel over a similar path 32 to the receiver. The 
receiver is adapted for selectively detecting the frequency-shifted, 
conjugated signals produced by four-wave mixing. 
The subject four-wave mixing device can also be modified for parametric 
amplification of the input signals. In this instance, the detectors, or 
receivers of the system are adapted to selectively utilize the amplified 
signal of wavelength .lambda. (peak A of FIG. 2) rather than the conjugate 
signal (peak C). 
It is to be understood that the above-described embodiments are 
illustrative of only a few of the many possible specific embodiments which 
can represent applications of the principles of the invention. Numerous 
and varied other arrangements can be made by those skilled in the art 
without departing from the spirit and scope of the invention.