This invention relates to the area of optical sources which provide output radiation at a multiplicity of wavelengths. This has application in such areas as the optical communications industry where Dense Wavelength Division Multiplexing (DWDM) achieves high data rate transmission by independently modulating data on to a multiplicity of optical beams, each with a different wavelength. These optical beams are then combined and propagated down a single optical fiber. Since the different wavelengths do not significantly interfere with each other the multiple wavelengths are effectively independent communications channels.
Multiple wavelength sources are typically generated by having multiple laser diodes each designed to emit at one of the required wavelengths. Each laser diode may be fabricated so that it emits at a particular wavelength as in the case of Distributed Feed Back (DFB) lasers where the emitting wavelength is determined by the physical spacing of a distributed Bragg grating that is part of the laser diode. Alternately, laser diodes may be fabricated that are capable of emitting over a broad wavelength range and are tuned to a particular wavelength by means of precision temperature control or other means.
An alternative approach to generating multiple wavelengths is to generate a continuum of wavelengths by applying a high power single wavelength source for four wave mixing in a nonlinear medium such as fiber. The non-linear or anhannonic characteristics allow the transformation of the source or pump radiation to other wavelengths.
High power is typically achieved be using a pulsed optical source so that high peak power can be attained with relatively low average power. The spectrum of the input optical pulse will be broadened to provide a continuum of wavelengths. The width of this continuum can be large if long lengths of conventional fiber are used. More recently xe2x80x9cphotonic crystal fiberxe2x80x9d allows an extremely large continuum range to be generated with a relatively short length of fiber. A set of individual wavelengths can be generated from this continuum by routing the optical beam through a set of optical filters, such as distributed fiber gratings. This approach of generating a set of multiple wavelengths by filtering a continuum is inherently inefficient because the wavelengths filtered out essentially are wasted energy.
Another approach described at the SPIE Conference on Optical Fiber Communications, Taipei, Taiwan, July 1998 in a paper titled A Multi-wavelength WDM Source Generated by Four-Wave-Mixing in a Dispersion-Shifted-Fiber by Keang-Po Ho and Shien-Kuei Liaw is to combine the output of two continuous;wave laser diodes that have slightly different wavelengths, amplify the combined signal with a high power Erbium Distributed Fiber Amplifier (EDFA) and apply this to a dispersion shifted fiber for four way mixing to produce a set or comb of wavelengths, whose wave length separation is determined by the difference in wavelength of the two seed laser diodes. Dispersion of a medium refers to the variation of the speed of propagation of radiation with wavelength within the medium. Typically the optical dispersion of a medium exhibits one or more minima at specific wavelengths around which the variation of speed of propagation with wavelength is small. Dispersion shifted media, such as, dispersion shifted fiber is designed to have zero dispersion close to the desired operating wavelength. Here, dispersion shifted medium is also intended to include the situation where a minimum coincides with the desired operating wavelength without specific modification.
This approach, however, still requires a physically long amount of dispersion shifted medium, which requires the system to be physically large which makes it more subject to environmental changes and not compatible with a requirement of being compact. It also requires the use of an expensive EDFA.
Therefore there is an unmet need for an efficient compact method and apparatus for generating a set or comb of wavelengths in manner that is compatible with low cost fabrication and which provides an integrated source of radiation at multiple wavelengths.
This invention provides a means for generating multiple wavelengths in an integrated manner using a resonant cavity containing dispersion shifted non-linear medium and coupled to a pulsed laser source. The dispersion shifted non-linear medium is seeded by at least some of the desired wavelengths. The laser source emits radiation at a particular wavelength and is pulsed in a manner synchronously related to the round trip time of the resonant cavity. By means, such as four wave mixing, the dispersion shifted non-linear medium produces a set of discrete wavelengths. The reflective elements of the resonant cavity are designed to contain the radiation of the laser sources within the resonant cavity and to transmit an equal amount of each of the generated set of wavelengths. This invention provides an apparatus for and method of generating repetitive pulsed radiation with a multiplicity of discrete wavelengths, which includes positioning an optical processing medium in a resonant cavity with reflective elements, generating repetitive pulsed radiation from a pulsed laser source in a pump cavity with reflective elements and coupling the resonant and pump cavities.