This application relates to compensation for optical dispersion, and more specifically, to techniques and systems for reducing polarization-mode dispersion in optical media such as optic fiber links.
Dispersion in optical transmission media such as optic fibers can cause optical waves of different characteristics to travel at different speeds. An optical pulse with optical components of different characteristics, therefore, can be broadened after propagation through a distance through a dispersive optical medium. The dispersion is undesirable for applications where information is encoded, processed, and transmitted through optical pulses because the pulse broadening caused by the dispersion can limit the transmission bit rate, the transmission bandwidth, and other performance factors of an optical communication system.
Different dispersive effects can occur in an optical waveguide. For example, the material dispersion in a waveguide can cause different spectral components within an optical pulse to travel at different group velocities. Different waveguide modes in a waveguide can also experience a waveguide dispersion to travel at different group velocities. In addition, some optical materials used for transporting optical pulses may be birefringent so that light with different polarizations can experience different indices of refraction. This can cause a polarization-mode dispersion (xe2x80x9cPMDxe2x80x9d) in optical waveguides independent of other dispersive effects. Typical causes for PMD in some fibers include, among others, imperfect circular core and unbalanced stress in a fiber along different transverse directions perpendicular to the fiber core.
PMD is one of key limitations to the performance of high-speed optical fiber communication systems at or above 10 Gbits/s due to the fiber birefringence. Fibers with significant PMD (e.g., about 1 to 10 ps/kmxc2xd) are used in various fiber networks, particularly in those that were deployed in 1980""s and early 1990""s. Hence, the compensation of PMD is desirable for high-speed transmission that uses such PMD fiber systems.
The techniques and devices of this application include optical devices that use a nonlinearly-chirped Bragg grating formed in a birefringent material in a dual-pass configuration. One embodiment of the devices includes a grating coupled to an optical wave-guiding path and a polarization rotator in the optical wave-guiding path to rotate a light polarization by about 90 degrees. The grating is adapted to have a periodic spatial pattern that changes nonlinearly with a position along an optical path from a first grating distal end to a second grating distal end in the grating. The grating also provides different refractive indices for light polarizations along first and second polarization axes that are substantially perpendicular to the optical path. The optical wave-guiding path has a first distal end coupled to receive light from the first grating distal end and a second distal end coupled to direct that light to the second grating distal end.
The grating or the optical wave-guiding path may be implemented by using optical fibers or waveguides formed on substrates. In a fiber implementation, for example, such a device may include a first optic fiber to transport optical energy and to exhibit optical birefringence for light polarizations along first and second polarization axes that are substantially perpendicular to the fiber.
A fiber grating is formed between a first location and a second location in the first fiber to have a periodic spatial pattern that changes nonlinearly with a position along the first fiber. This fiber grating is operable to effectuate different relative delays in reflected optical spectral components of a common polarization of different wavelengths at different positions along the fiber grating that meet Bragg conditions. It is also operable to effectuate different relative delays between two reflected optical spectral components of different light polarizations respectively along the first and second polarization axes at a common wavelength. Thus, the fiber grating can interact with an input optical signal traveling from the first location towards the second location in the first fiber to produce a first optical signal by reflection in a direction from the second location towards the first location.
This fiber implementation may also include a second fiber having a first distal end and a second, opposite distal end. In addition, a polarization rotator is coupled between the first and second distal ends in the second fiber to rotate a light polarization by about 90 degrees. The first distal end is coupled to the first fiber at a position to receive the first optical signal through the first location. The second distal end is coupled to the first fiber at a different position to direct the first optical signal, after passing through the polarization rotator, back into the fiber grating at the second location. The fiber grating then operates to reflect the first optical signal for the second time to produce an output optical signal directing from the first location towards the second location.
These and other embodiments and associated features are set forth in the accompanying drawings, the description, and the claims.