Chirped pulse amplification (CPA) is a technique allowing amplification of ultra-short light pulses to a very high energy/power level in a solid state amplifier. To avoid pulse distortion or break up due to the optical nonlinearity of the amplification medium, the pulse is stretched (typically by at least several hundred times) by passing it through a strongly dispersive element and thereby “chirping” it, or delaying its spectral components in respect to each other. Stretching greatly reduces peak optical power for the same pulse energy. After amplification, the pulse is compressed back to its original duration. Such pulse compression is typically accomplished using bulk optics (prisms and/or gratings) for the same reason of avoiding nonlinear distortion. Unfortunately, existing bulk optics compressors are typically very bulky, with a physical length of 1 meter or more.
Recent development of a new type of optical fiber, called photonic bandgap fiber (PBGF) opened up the possibility of an entirely new type of pulse compressor. Unlike conventional optical fiber having a glass core, PBGF has a hollow core for guiding light (which may contain air or a vacuum if the fiber core is evacuated) and consequently has an optical nonlinearity roughly 1000 times less than conventional fiber. At the same time, PBGF can have a very large chromatic dispersion per length. These optical characteristics permit a segment of PBGF to be substituted for the bulk optics pulse compressor used in conventional CPA systems, keeping the advantage of low pulse distortion while allowing a much more compact and potentially all-fiber design where all parts of the CPA system (stretcher, amplifier and compressor) are segments of different types of fiber spliced together.
The substitution of PBGF fiber for the bulk optics compressors used in conventional CPA systems has been accomplished experimentally. However, the potential advantages have been only partially recognized by such prior art designs as either the compressed pulse width could not be reduced below 1 picosecond, or the stretching ratio of the pulses entering the amplifier did not exceed 40 times (which would limit ultimately achievable amplified power/energy for the same amplifier design). Further compression of the pulse width by the PBGF or stretching by the dispersion compensating fiber used in this device results in unacceptably high degrees of pulse distortion due to the difficulty of matching the relatively high dispersion slope of the PBGF with the opposite dispersion slope of the dispersion compensating fiber. A CPA system design using a PBGF fiber compressor is also known that uses a stretcher consisting of one or multiple nonlinearly chirped fiber Bragg gratings (CFBG). This system design has the advantage of being able to match the dispersion slope of the stretcher with the dispersion slope of the PBGF. However, this system is not free of disadvantages. First, group delay ripple of the CFBGs can easily exceed several picoseconds, introducing distortion that can not be compensated. Second, exact compensation might require multiple CFBGs with multiple independently tunable segments each, greatly increasing complexity and cost of the overall system.
Clearly, there is a need for an all-fiber CPA system capable of amplifying high energy pulses to the same amplitude as conventional CPA systems using bulk optic compressors without excessive distortion. Ideally, such a CPA system would use only dispersion compensating fiber to stretch the pulses prior to amplification, and not rely upon nonlinearly chirped fiber Bragg gratings in the pulse stretcher which require multiple tuning steps and can introduce unwanted distortions in the amplified pulses. Finally, such a CPA system should be simple and compact in structure, robust, and easy to install and use.
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