A
laser utilizes a cavity design which allows the stable generation of
high peak power pulses from mode-locked multi-mode
fiber lasers, greatly extending the peak
power limits of conventional mode-locked single-mode
fiber lasers. Mode-locking may be induced by
insertion of a saturable absorber into the cavity and by inserting one or more mode-filters to ensure the oscillation of the fundamental mode in the multi-mode
fiber. The probability of damage of the absorber may be minimized by the
insertion of an additional
semiconductor optical power limiter into the cavity. To amplify and compress optical pulses in a multi-mode (MM)
optical fiber, a single-mode is launched into the MM fiber by matching the
modal profile of the fundamental mode of the MM fiber with a
diffraction-limited optical mode at the launch end, The fundamental mode is preserved in the MM fiber by minimizing mode-
coupling by using relatively short lengths of step-index MM fibers with a few hundred
modes and by minimizing fiber perturbations.
Doping is confined to the center of the fiber core to preferentially amplify the fundamental mode, to reduce
amplified spontaneous emission and to allow
gain-guiding of the fundamental mode.
Gain-guiding allows for the design of systems with length-dependent and power-dependent diameters of the fundamental mode. To allow pumping with high-power
laser diodes, a double-clad
amplifier structure is employed. For applications in nonlinear pulse-compression,
self phase modulation and dispersion in the optical fibers can be exploited. High-power optical pulses may be linearly compressed using bulk
optics dispersive
delay lines or by chirped fiber Bragg gratings written directly into the SM or MM
optical fiber. High-power cw lasers operating in a single near-
diffraction-limited mode may be constructed from MM fibers by incorporating effective mode-filters into the
laser cavity. Regenerative fiber amplifiers may be constructed from MM fibers by careful control of the recirculating mode. Higher-power Q-switched fiber lasers may be constructed by exploiting the large energy stored in MM fiber amplifiers.