text: The present invention relates in general to optical amplifiers for amplifying ultrashort pulses of optical radiation. It relates in particular to an apparatus for generating a chirped seed-pulse having a selectively variable wavelength for chirped-pulse amplification by a regenerative amplifier.
  Chirped pulse amplification technology has enabled generation of highly energetic ultra-short laser pulses. Ultrashort pulses are generally considered to be pulses having a duration of about 10 picoseconds (ps) or less. In chirped pulse amplification, an ultrashort xe2x80x9cseedxe2x80x9d pulse is positively xe2x80x9cchirpedxe2x80x9d (frequency modulated). The positive chirp stretches the duration of the pulse thereby reducing the peak power in the pulse. The pulse is then amplified in an optical amplifier such as a regenerative amplifier. Reducing the peak power in the seed-pulse enables the seed pulse to be amplified by several orders of magnitude while keeping the peak power in the amplified pulse below levels that can cause damage or undesirable nonlinear optical effects in optical components of the amplifier. Following amplification, the amplified pulse can be negatively chirped to reduce the duration and increase the peak power of the pulse.
  Ultrashort seed pulses are generated by a class of lasers generally termed ultrafast lasers. These are lasers that utilize gain media having a relatively broad gain-bandwidth, for example about ten percent (10%) or greater of a nominal gain wavelength. One generally preferred ultrafast laser is a laser employing titanium-doped sapphire (Ti:sapphire) as a gain-medium. Gain-media used in the regenerative amplifiers include Ti:sapphire, neodymium-doped YAG (Nd:YAG), and ytterbium-doped YAG (Yb:YAG) in bulk (crystal) form, and erbium (Er) and Yb-doped glasses in optical-fiber form.
  The high-energy ultra-short pulses generated by chirped pulse amplification have made possible a variety of fruitful researches and applications in several branches of science including materials processing, spectroscopy, medicine and biology. The variety of applications has created a need for ultrashort pulses at a variety of wavelengths. Indeed, a variety of wavelengths may be required in a single application. If the wavelengths are sufficiently different, this can require different amplifiers having different gain-media, with each gain-medium requiring a specific laser to provide an appropriate seed-pulse.
  The quality (proximity to the transform limit) of an amplified pulse can usually be no better than the quality of the seed-pulse that is amplified. The cost of a seed-pulse laser, can be between about 25% and 35% of the cost of a complete chirped-pulse amplification system. Accordingly, there is a need for an ultrafast laser that is capable of delivering seed pulses at wavelengths that match the gain-bandwidth of a variety of commonly used gain media. This could significantly reduce the cost of operating chirped-pulse amplifiers at different wavelengths, and could extend the range of applications for ultrafast laser pulses.
  The present invention is directed to a method of delivering a seed-pulse to an optical amplifier. In one aspect, the present invention comprises generating an ultrashort optical pulse having a relatively narrow spectral bandwidth. The ultrashort optical pulse is delivered to an optical fiber configured to convert the narrow-bandwidth, ultrashort pulse into a second optical pulse having a continuous wavelength-spectrum (continuum) extending over a bandwidth at least about 20 times greater than the bandwidth of the ultrashort narrow-bandwidth pulse. A portion of the continuous wavelength-spectrum of the second pulse is converted into to a third optical pulse having a center wavelength within the selected portion of the continuous spectrum and having a duration longer than the duration of the second pulse. The third pulse is delivered as the seed-pulse to the optical amplifier.
  In apparatus for carrying out the method of the present invention, the continuous-spectrum-generating optical fiber delivers the second optical pulse to a pulse stretcher. The pulse stretcher is arranged to carry out both the selection of the spectrum portion and the conversion of the selected-spectrum portion of the second pulse into the third optical pulse.
  In another aspect of the present invention, the pulse stretcher includes a diffraction grating and is configured such that the second pulse is incident at an angle on the diffraction grating. The second pulse is diffracted from the diffraction grating, with different wavelengths of the continuous spectrum being diffracted at different angles. The optical system is further configured such that those diffracted wavelengths within the selected spectral portion are recombined on a common path by the diffraction grating after following paths of different lengths through the optical system. The path-length of the different paths is inversely related to the wavelength. The recombined wavelengths form the third pulse.
  In a preferred embodiment of the apparatus, the diffraction grating is rotatable for selectively varying the incidence angle of the second pulse thereon. Selectively varying the incidence angle selectively varies the spectral portion selected by the optical system, thereby selectively varying the center wavelength of said third pulse.