Embodiments of the invention relate generally to optical communications and, more particularly, to a dispersion compensator that may be used in an optical communications network.
Fiber-optic networks are increasingly being used in many industries, most notably telecommunications and computer networks. Transmission speeds and distances can at times, however, be limited based on various factors. One of these factors is chromatic dispersion, which occurs when a pulse of light traveling down an optical fiber broadens.
Such pulse broadening typically occurs as different wavelength components or colors within the pulse move at different speeds along the fiber, with the longer wavelength components traveling faster than the shorter wavelength components. Thus, a pulse may broaden and ultimately may overlap with another pulse, thereby distorting the data in a signal. This effect may become increasingly pronounced at high bit rates, as additional factors may contribute to chromatic dispersion (e.g., temperature, humidity, aging, and stress of the fiber).
In an effort to reduce chromatic dispersion and allow for longer transmission distances and greater throughput of data, several techniques are used. One technique is to use a dispersion compensating fiber (DCF) that can introduce sufficient negative dispersion into the transmission link thereby offsetting the positive dispersion accumulated by the pulse traveling through the fiber. However, a given portion of fiber generally requires a unique length of DCF in order to provide the correct amount of compensation. As such, DCFs are not readily tunable as changing properties of a DCF often requires changing the DCF length itself, which is a process that can be time-consuming and inefficient.
Another technique that is often used includes the use of dispersion compensation gratings. One type of grating is a chirped in-fiber Bragg grating, which reflects each wavelength component at different points to compensate a dispersed pulse. Like DCFs, however, the amount of dispersion compensation provided cannot be adjusted easily. Moreover, the gratings may sometimes over-compensate or under-compensate at certain frequencies.
Accordingly, chromatic dispersion reduces the efficiency of fiber optic networks by limiting transmission distances and throughput of data. Known methods to solve this problem such as use of DCF and dispersion gratings, may have drawbacks, such as not being easily adjusted and/or not providing a suitable amount of compensation.
In the absence of using any chromatic dispersion compensation technique, it may be difficult to detect the transmitted data over long distances at very high data rates (i.e. >>10 Gb/s) at the receiving end. Embodiments utilize the optical phase conjugation (OPC) property in silicon waveguides to compensate chromatic dispersion effect in optical fibers. This enables high-speed optical data to propagate over long distance as for example in metro and long haul communication networks. In one embodiment the silicon based OPC may be placed near the middle of an optical link (or mid-span) to realize chromatic dispersion compensation.
The OPC function may be achieved through four-wave mixing (FWM), a nonlinear optical effect in silicon. Referring now to
According to embodiments, a silicon waveguide device 24 may be placed, for example, at mid-span of the fiber 22. A laser 23 may provide a continuous wave laser beam as a pump signal at wavelength λ1 to produce the FWM effect. That laser may be fabricated on the same substrate as the waveguide device or provided separately. The pump signal Al and the lower power input signal centered at λ2 carrying the data 20 are co-linearly coupled into the silicon waveguide device 24. Due to the nonlinear interaction between these beams (degenerated four wave mixing), a new signal, which is the optical phase conjugate of the input signal, centered at wavelength λ3 is produced and exits the waveguide together with the pump and signal beams.
The wavelengths of the pump 23, the input signal.λ2 and the converted conjugate signal λ3 satisfy the following relation: 1/λ3=2/λ1−1/λ2. An optical filter 26 may be used to separate the converted signal from the pump and the input signal. The newly generated signal at λ3 contains the optical phase conjugate of the original input signal λ2. That is, the higher frequency components in the original signal λ2 become lower frequency components in the newly generated signal λ3 and vice-versa. Therefore, the frequency components that were traveling slowly in the first half-span are now the ones traveling faster in the second half-span, thus compensating for accumulated chromatic dispersion.
The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.