Mode-locked multi-mode fiber laser pulse source

Abstract

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.

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       59 . A multi-mode fiber amplifier system providing discrimination between a fundamental mode and undesired higher-order modes, said amplifier system comprising: 
 a light source for producing a light beam, and    a linear multi-mode fiber amplifier for receiving said light beam and comprising a multi-mode doped optical fiber having a V-number equal to or higher than 2.41 with respect to an input signal, capable of supporting a fundamental mode and higher-order modes, having a radius of curvature such that the higher-order modes of said input signal experience substantially increased bend losses as compared with the fundamental mode of said input signal.    
   
   
       60 . An amplifier system according to  claim 59  wherein said fiber comprises a coiled fiber.  
   
   
       61 . An amplifier system according to  claim 60  wherein said coiled fiber has a constant radius of curvature.  
   
   
       62 . An amplifier system according to  claim 59  wherein said light source comprises a continuous wave light source.  
   
   
       63 . An amplifier system according to  claim 59  wherein said light source comprises a pulsed light source.  
   
   
       64 . An amplifier system according to  claim 59  wherein said multi-mode optical fiber comprises a double-cladding structure.  
   
   
       65 . An amplifier system according to  claim 59  wherein said multi-mode optical fiber comprises a core having a diameter of between 28 microns and 50 microns.  
   
   
       66 . An amplifier system according to  claim 59  wherein said multi-mode fiber amplifier has an M 2  value less than 2.0, where M 2 =1 denotes diffraction-limited beam quality.  
   
   
       67 . An amplifier system according to  claim 59  wherein the fiber amplifier is side pumped.  
   
   
       68 . A linear multi-mode fiber amplifier comprising: 
 a cylindrical support member, and    a doped multi-mode optical fiber having a V-number equal to or higher than 2.4 with respect to an input signal, capable of supporting a fundamental mode and a plurality of higher-order modes, said fiber being wound onto said support with a radius such that said higher-order modes of said input signal experience substantially increased bend loss as compared with the fundamental mode of said input signal.    
   
   
       69 . An amplifier system according to  claim 68  wherein said wound fiber has a constant radius of curvature.  
   
   
       70 . A multi-mode fiber amplifier according to  claim 68  wherein said multi-mode optical fiber comprises a double-cladding structure.  
   
   
       71 . A multi-mode fiber amplifier according to  claim 68  wherein said multi-mode optical fiber comprises a core having a diameter of between 28 μm and 100 μm.  
   
   
       72 . A multi-mode fiber amplifier according to  claim 68  wherein said multi-mode fiber amplifier has an M 2  value less than 2.0, where M 2 =1 denotes diffraction-limited beam quality.  
   
   
       73 . An amplifier system as in  claim 68 , further comprising a seed beam having a controlled launch condition for preferential excitation of the fundamental mode.