There are applications in the fiber optics field in which a high power, low noise, broadband light source (continuum) is of particular interest. For example, efforts are now being made toward “spectral slicing”; that is, using a single optical source to generate a plurality of signals of different wavelengths (i.e., wavelength division multiplexed (WDM) signals). Such an application thus has the potential for replacing many lasers with a single light source. Other applications include, but are not limited to, frequency metrology, device characterization, dispersion measurements made on specialty fibers, and the determination of transmission characteristics of gratings. All of these various diagnostic tools may be greatly enhanced by the availability of a broadband source with the ability to create a plurality of different signal wavelengths.
In general, continuum generation involves the launching of relatively high power laser pulses into an optical fiber, waveguide or other microstructure, wherein the laser pulse train undergoes significant spectral broadening due to nonlinear interactions in the fiber. Current efforts at continuum generation, typically performed using light pulses having durations on the order of picoseconds (10−12 sec) in kilometer lengths of fiber, have unfortunately shown degradation of coherence in the generating process.
A relatively new type of germanium-doped silica fiber with low dispersion slope and a small effective area, referred to hereinafter as “highly nonlinear fiber”, or HNLF, has recently been developed. Although the nonlinear coefficients of HNLF are still smaller than those obtained with small core microstructured fibers, the coefficients are several times greater than those of standard transmission fibers, due to the small effective area of HNLF. Continuum generation using an HNLF and a femtosecond fiber laser has been reported from various sources. One prior art arrangement utilizes an HNLF-based continuum source formed from a number of separate sections of HNLF fiber that have been fused together, each having a different dispersion value at the light source wavelength and an effective area between five and fifteen square microns. Another type of HNLF-based continuum source uses a post-fabrication process to modify the dispersion values of the HNLF and further extend the spectral boundaries of the generated continuum.
For the particular application of spectral slicing, the spectral phase accumulated during the continuum generation process must be removed to achieve the desired “ultrashort” (e.g., fs or sub-picosecond) output pulse width. More particularly, dispersion compensation is required to nullify the negative dispersion created during signal propagation through HNLF. In the past, this dispersion compensation has been accomplished by using bulk optic components, such as prism pairs. It is preferred to provide an “all fiber” solution, eliminating the need for bulk optics. However, at short wavelengths (i.e., wavelengths shorter than the zero dispersion wavelength of HNLF at approximately 1300 nm-1400 nm), the continuum exiting the HNLF is negatively chirped and requires a positive dispersion fiber for recompression. Achieving positive dispersion in fibers at short wavelengths is difficult, and usually requires a microstructured fiber or photonic bandgap fiber. Both of these fibers, however, have a relatively small effective area Aeff, on the order of 5-10 μm2, and as a result suffer from nonlinearities.
Thus, a need remains in the art for an all-fiber arrangement for compressing spectrally sliced components from a continuum source down to fs levels.