Abstract:
Fiber delivery systems are desirable to provide convenient delivery of an output beam from a laser system to a target distanced from the source. For ultra-short pulse lasers, a limiting factor in fiber delivery is the dispersion of the optical fiber. A fiber delivery system for ultra-short pulses that uses a photonic crystal fiber to Fprovide the appropriate dispersion is described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Ser. No. 09/317,652, filed Mar. 24, 1999 now U.S. Pat. No. 6,236,779, which application is fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to fiber delivery of ultra-short pulses, and more particularly to the use of photonic crystal fibers in an ultra-short pulse delivery system. 
     2. Description of Related Art 
     Fiber delivery systems are desirable for laser systems to provide convenient delivery of an output beam to a target distanced from the source. In particular, for ultra-short pulse lasers, a limiting factor in fiber delivery is the dispersion of the optical fiber. 
     At wavelengths of less than 1.27 microns, all step-index fibers have normal dispersion. In this regime, the ultra-short pulses broaden substantially while propagating in a fiber of lengths as short as a few meters. Prism or grating pairs, which provide anomalous dispersion, have been used to compensate the dispersion of the fiber. However, this increases complexity and cost and in the case of grating pairs, is inefficient. Additionally, with a tunable laser, the prism or grating pair requires adjustment as the wavelength is tuned. 
     There have been suggestions to use photonic crystal fibers to shift the zero dispersion wavelength to shorter values. In “Group-velocity dispersion in photonic crystal fibers”, by D. Mogilevtsev, T. A. Birks and P. St. J. Russell, in Optics Letters 23, 1662 (1998) it is suggested that this may be useful in telecommunication systems. In “Efficient visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm”, by J. K. Ranka, R. S. Windeler and A. J. Stentz, Postdeadline paper at CLEO 1999 (Optical Society of America), it is shown that in combination with a Ti:sapphire laser, novel non-linear effects are possible. 
     There is a need for a fiber delivery system for delivering ultra-short laser pulses. As a result, there is a need for a fiber that has an appropriate value of dispersion at wavelengths where common ultra-short pulse lasers operate. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a fiber delivery system for delivering ultra-short pulses. 
     Another object of the present invention is to provide a fiber delivery system with a fiber that has an appropriate value of dispersion at wavelengths where common ultra-short lasers operate. 
     These and other objects of the invention are achieved in a system that delivers sub-picosecond pulses. Included is a source that produces an output beam of sub-picosecond pulses at a wavelength no greater than 1.27 microns. A photonic crystal fiber is coupled to the source to receive the output beam. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic diagram of one embodiment of the present invention illustrating a laser and a photonic crystal fiber. 
    
    
     DETAILED DESCRIPTION 
     The present invention can utilize several sources of ultra-short pulses in the wavelength range between 700 and 1270 nm. The most popular is the Ti:sapphire laser, however other sources include optical parametric oscillators and the Cr doped colquiriites such as LiSAF, LiCAF, LiSCAF and LiSGAF. Also included are longer wavelength ultra-short pulse sources, which are then frequency doubled. Examples include frequency doubled Erbium doped fiber lasers, frequency doubled optical parametric oscillators and frequency doubled Forsterite lasers. Finally, there are sources of sub-picosecond pulses at wavelengths between 1000 and 1100 nm such as Nd or Yb doped glass. 
     Additionally, to deliver low power sub-picosecond pulses without substantial pulse broadening, the present invention utilizes a photonic crystal fiber designed to have nearly zero dispersion at the wavelength that the laser operates. For example, a Ti:sapphire laser operating at a wavelength of 800 nm with transform limited pulses with duration of 100 fs, will have a bandwidth of 7 nm. Typical step-index fibers have a normal dispersion D of −120 ps/nm-km at 800 nm. The pulse will broaden by an amount D times the bandwidth, or 840 fs, for each meter of fiber it passes through. To prevent significant broadening the dispersion of the fiber should be kept between −20 and +20 ps/nm-km. Clearly the lower the absolute value of the dispersion, the longer the fiber that can be used without broadening the pulse. 
     As the power of the pulse is increased, nonlinear effects will become important. Nonlinear effects, such as self phase modulation (SPM), Raman generation or continuum generation will broaden the bandwidth of the pulse. This distortion of the pulse is clearly undesirable for a sub-picosecond pulse delivery system. A small amount of SPM can be compensated, however, by choosing a fiber with a small amount of anomalous dispersion. When the correct balance is chosen, the pulse becomes a soliton and can propagate long distances in the fiber without changing pulse duration. This is clearly a desirable situation for a sub-picosecond pulse delivery system. To obtain a soliton with a given pulse duration and energy, the dispersion and the core size of the fiber must be chosen appropriately. Consider the Ti:sapphire laser operating at a wavelength of 800 nm with transform limited pulses with duration of 100 fs. For a photonic crystal fiber with a dispersion D of +100 ps/nm-km and a core size of 10 microns, the N=1 soliton will have a peak power of 13 kW. At a repetition rate of 80 MHz this corresponds to 100 mW of average power. 
     Referring now to FIG. 1, one embodiment of the present invention is a system  10  that delivers sub-picosecond pulses. System  10  includes a source  12  of sub-picosecond pulses as described above and a photonic crystal fiber  14  coupled to source  12 . Suitable sources  12  include but are not limited to a mode-locked Ti:sapphire laser, a synchronously pumped OPO, a mode-locked Cr-doped colquiriite laser, a mode-locked fiber laser, a mode-locked Forsterite laser, a mode-locked Nd-doped glass laser, a mode-locked Yb-doped glass laser and the like. A first optical device  16  is positioned between source  12  and fiber  14 . First optical element  16  couples an output beam from source  12  into an input end of fiber  14 . Suitable first optical elements  16  include but are not limited to a lens, a waveplate, an attenuator, a filter, a polarizer and combinations thereof. 
     A second optical device  18  is positioned at an output end of fiber  14  to reduce the divergence of the output beam from  14  fiber and deliver the output beam to a selected target  20 . Suitable second optical elements  18  include but are not limited to a lens, a waveplate, an attenuator, a filter, a polarizer, an acousto-optic modulator, an electro-optic modulator, a scanner, a microscope and combinations thereof. 
     Photonic crystal fibers typically preserve the polarization of a linearly polarized input beam that is oriented correctly with respect to fiber  14 . To orient the polarization, a half waveplate can be used. Further, an attenuator, which may consist of a polarizer and waveplate, can be used to adjust the power delivered to fiber  14 . The output of photonic crystal fiber  14  may be directed to an attenuator, or an acousto-optic or electro-optic modulator to modulate the intensity of the output beam. A scanning system may be used to deflect the direction of the beam. Further, the output of fiber  14  may be directed to an optical instrument including a microscope. 
     In a second embodiment, fiber  14  is a large core photonic crystal fiber. Typical fibers have core sizes of 1-2 microns in radius. As the power of the ultra-short pulses in fiber  14  is increased, nonlinear effects begin to broaden the bandwidth and distort the pulse. A fiber  14  with a larger core size can deliver higher peak power pulses without pulse distortion. For a given length of fiber  14  with twice the core size, four times the power can be delivered with a comparable amount of pulse distortion. A practical upper limit is placed on the core size, since the bending losses also increase with larger core size. 
     In a third embodiment, fiber  14  consists of a dispersion flattened photonic crystal fiber. In a typical fiber, the dispersion will remain between −20 and +20 ps/nm-km for less than 100 nm. In dispersion flattened fiber  14 , the dispersion remains small over a larger range of wavelengths. When used in conjunction with a tunable source of sub-picosecond pulses, such as a Ti:sapphire laser, fiber  14  allows the delivery of sub-picosecond pulses over a large range of wavelengths. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.