1. Field of the Invention
The present invention relates generally to the field of optical communications, and more specifically, to modulation of optical signals.
2. Discussion of Related Art
Communication networks increasingly rely upon optical fiber for high-speed, low-cost transmission. Optical fibers were originally envisioned as an optical replacement for electronic transmission media, such as high-speed coaxial cable and lower-speed twisted-pair cable. However, even high-speed optical fibers are limited by the electronics at the transmitting and receiving ends. For switching purposes, operating speeds are generally rated at a few gigabits per second, although 40 Gb/s systems have been prototyped. Such high-speed electronic systems are expensive and still do not fully exploit the inherent bandwidth of fiber-optic systems, which can be measured in many terabits per second.
All-optical transmission systems offer many intrinsic advantages over systems that use electronics within any part of the principal transmission path. Wavelength division multiplexing is a commonly used technique that allows the transport of multiple optical signals, each at a slightly different wavelength, on an optical fiber. The ability to carry multiple signals on a single fiber allows that fiber to carry a tremendous amount of traffic, including data, voice, and even digital video signals. For example, the use of wavelength division multiplexing, in combination with time division multiplexing, permits a long distance telephone company to carry thousands or even millions of phone conversations on a single fiber. Wavelength division multiplexing makes it possible to effectively use the fiber at multiple wavelengths, as opposed to the costlier option of installing additional fibers. Using wavelength division multiplexing, optical signals can be carried on separate optical channels with each channel having a wavelength within a specified bandwidth. It is advantageous to carry as many channels as possible within the bandwidth where each channel corresponds to an optical signal transmitted at a predefined wavelength.
U.S. Pat. No. 4,655,547 to Heritage, et. al., entitled xe2x80x9cShaping Optical Pulses by Amplitude and Phase Masking,xe2x80x9d which is herein incorporated by reference, discloses how an input optical signal can be spatially divided into frequency channels, for example with a diffraction grating. Then, the separated channels are independently operated upon by a segmented modulator. U.S. Pat. No. 5,132,824 to Patel et al., entitled xe2x80x9cLiquid Crystal Modulator Array,xe2x80x9d which is also herein incorporated by reference, discloses using liquid-crystal modulators to manipulate optical pulses. After separating the input optical signal into channels, each channel is separately phase-modulated or amplitude-modulated. The performance of wavelength division multiplexing systems is optimal when signal strength or intensity of each channel is adjusted dynamically. A system and a method for dynamically adjusting the intensity of each channel is needed.
In accordance with the present invention, a modulation system is presented that can, in some embodiments, achieve dynamic intensity modulation of each channel in a wavelength-division multiplexed optical communication system. The wavelengths may be spatially separated into channels and individually modulated by changing the polarization state of each channel and using the polarization states to selectively combine or filter channels and achieve the desired intensity modulation.
An exemplary embodiment of the present invention includes two birefringent wedges, two lenses, and two dispersive elements (e.g., diffraction gratings) arranged symmetrically at two opposing sides of a segmented polarization modulator. Each segment of the polarization modulator can be made to alter the polarization direction of an incident beam of light by a specified angle. A half-wave plate may be inserted between the second dispersive element and the second birefringent wedge to eliminate polarization-dependent loss. Additionally, a parallel birefringent plate may be inserted after the second birefringent wedge to compensate for any polarization mode dispersion.
A more compact embodiment of the invention can use a reflective surface on the polarization modulator to redirect the beams of light through a dispersive element, a lens, and a birefringent wedge which the beams passed through to reach the reflective surface. In some embodiments, two prisms may be placed around the polarization modulator, instead of a reflective surface, to direct the channels back in the direction from which they came. Alternatively, an aperture may be used to prevent all of the output signals from entering the signal transfer medium, thereby achieving the desired attenuation.