Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
2002-03-12
2004-07-13
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C359S326000, C385S017000
Reexamination Certificate
active
06762877
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical switching and, more particularly, to a technique for selectively frequency translating optical channels in an optical network.
BACKGROUND OF THE INVENTION
All-optical wavelength conversion plays an important role in transparent wavelength division multiplexing (WDM) networks. For example, it enables better network utilization, network transparency to bit-rates and to packet formats, as well as simpler network management. Until recently, two wavelength conversion techniques have been popular involving either cross-gain or cross-phase modulation. However, these solutions are limited by the single input or output capability of cross-gain or cross-phase converters.
The first proposed wavelength-interchanging cross-connects are based on cross-gain or cross-phase modulation. These designs comprised a central optical space switch and dedicated or shared tunable wavelength converters based on cross-gain or cross-phase modulation. The wavelength converters were placed at inputs, outputs, or shared between inputs and outputs, of the central optical space switch. In this configuration, individual wavelength channels were usually switched in two steps. In one step, the space switch directed individual wavelength channels to the appropriate fibers. In another step, wavelength channels were converted to appropriate wavelengths by a single wavelength conversion operation. For this reason, and for future reference, these previous designs are called single-stage wavelength conversion architectures.
Single-stage designs optimally exploit cross-gain and cross-phase wavelength conversion, while minimizing non-negligible conversion impairments. Yet these single-stage architectures often lead to large photonic wavelength converter requirements. In fact, a non-blocking wavelength-interchanging cross-connect with F fibers and W wavelengths per fiber would require F.W converters based on cross-gain or cross-phase modulation.
Wave-mixing can also provide photonic frequency conversion. Two forms of wave-mixing frequency converters have been demonstrated, which are either based on four-wave mixing or on difference-frequency generation. Four-wave mixing is a nonlinear optical process based on third order nonlinear susceptibilities. It can be described as the interactions of any subset of three waves from a larger set of input waves in a nonlinear medium. For each subset of three interacting waves, a resulting wave is produced with an amplitude proportional to the product of the amplitudes of the interacting waves, and a phase and frequency linearly dependent on the phases and frequencies of the interacting waves.
Four-wave mixing frequency converters have been demonstrated in passive media such as glass fibers and in active media such as semiconductor optical amplifiers. They provide bulk frequency-mirroring and phase conjugation, by mapping each input optical frequency, f
in
, to an output frequency, f
out
=2f
p
−f
in
, where f
p
is the frequency of a pump wave. However, the generation of in-band cross-talk severely limits the use of four-wave mixing for bulk frequency-conversion.
Difference-frequency generation is another form of wave-mixing. Unlike four-wave mixing, it is based on second order nonlinear susceptibilities. It is explained by the interaction of each wave from a set of input waves at different frequencies with a high power pump wave in a nonlinear medium. Like four-wave mixing, the interaction of each input wave with the pump wave yields a resulting wave with an amplitude proportional to the product of the amplitudes of the pump and input waves, and a phase and frequency linearly dependent on the phases and frequencies of the input and pump waves. Specifically, difference-frequency generation enables bulk frequency conversion and maps each input wave at some input frequency, f
in
, to another wave at output frequency, f
out
=f
p
−f
in
, where f
p
is the frequency of the pump wave. However, unlike four-wave mixing, difference-frequency generation does not generate in-band cross-talk terms. For this reason, difference-frequency generation is a practical alternative to providing wave-mixing bulk frequency conversion, along with other optical signal processing functions.
Bulk frequency conversion, or the ability to simultaneously change the frequencies of several input, waves, is a major advantage of wave-mixing frequency conversion. Unfortunately, this important feature of wave-mixing is unused in single-stage wavelength conversion architectures. Therefore, there is a need for other architectures capable of leveraging the potential of wave-mixing, and of reducing the costs of wavelength-interchanging cross-connects.
One proposed solution involves a family of switches optimized for wave-mixing. These cross-connects provide wavelength conversion in a multi-stage manner, and convert the wavelengths of channels through cascades of elementary frequency conversions. In this proposed solution, 2×2 elements are space switches, while inter-stage connections involve fixed wavelength conversions. This proposed solution is rearrangeably non-blocking, and its converter requirements are half those of dedicated converter architectures (i.e., the all-optical converter requirements are F.W/2). In spite of this improvement, converter requirements remain 0(F.W). This is still too large for practical cross-connects.
In view of the foregoing, it would be desirable to provide a technique for implementing wavelength-interchanging cross-connects which overcomes the above-described inadequacies and shortcomings in an efficient and cost effective manner.
SUMMARY OF INTENTION
According to the present invention, a technique for selectively frequency translating optical channels in an optical network is provided. In one exemplary embodiment, the technique is realized as a method for selectively frequency translating optical channels in a system having W optical frequencies. The method comprises selectively directing an optical channel operating at a respective one of the W optical frequencies based at least in part upon the respective optical frequency of the optical channel. The method also comprises shifting the respective optical frequency of the selectively directed optical channel by an amount defined by ±2
i
&Dgr;f, wherein &Dgr;f is an optical frequency spacing between adjacent optical channels, and i=0, 1, . . . log
2
W−1.
In accordance with other aspects of this exemplary embodiment of the present invention, wherein the optical channel is a first optical channel and the selectively directed optical channel is a first selectively directed optical channel, the method may further beneficially comprise selectively directing a second optical channel operating at another respective one of the W optical frequencies based at least in part upon the respective optical frequency of the second optical channel, wherein the respective optical frequency of the second selectively directed optical channel is the same as the respective optical frequency of the first selectively directed optical channel after it has been shifted.
In accordance with further aspects of this exemplary embodiment of the present invention, wherein the system comprises a plurality of optical waveguides for communicating the W optical frequencies, and wherein a first respective one of the plurality of optical waveguides communicates the first optical channel prior to being selectively directed, the method may further beneficially comprise selectively directing the first optical channel based at least in part upon the first respective one of the plurality of optical waveguides. Analogously, wherein a second respective one of the plurality of optical waveguides communicates the second optical channel prior to being selectively directed, the method may further beneficially comprise selectively directing the second optical channel based at least in part upon the second respective one of the plurality of optical waveguides.
In accor
Dasylva Abel C.
Kodaypak Prasad
Montuno Delfin Y.
Wang Guoqiang G. Q.
Lee John D.
Nortel Networks Limited
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