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.

Description

FIELD OF THE INVENTION The present invention relates to the amplification of single mode light pulses in multi-mode fiber amplifiers, and more particularly to the use of multi-mode amplifying fibers to increase peak pulse power in a mode-locked laser pulse source used for generating ultra-short optical pulses. The present invention relates to the use of multi-mode fibers for amplification of laser light in a single-mode amplifier system. BACKGROUND OF THE INVENTION 1. Background Relating to Optical Amplifiers Single-mode rare-earth-doped optical fiber amplifiers have been widely used for over a decade to provide diffraction-limited optical amplification of optical pulses. Because single mode fiber amplifiers generate very low noise levels, do not induce modal dispersion, and are compatible with single mode fiber optic transmission lines, they have been used almost exclusively in telecommunication applications. The amplification of high peak-power pulses in a diffraction-limited...

AI Technical Summary

Benefits of technology

This laser utilizes cavity designs which allow 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. The shortest pulses may also be generated by taking advantage of nonlinear polarization evolution inside the fiber. The long-term stability of the cavity configuration is ensured by employing an environmentally stable cavity. Pump light from a broad-area diode laser may be delivered into the multi-mode fiber by employing a cladding-pumping technique.

According to yet another embodiment of the present invention, MM optical fibers allow the construction of fiber optic regenerative amplifiers and high-power Q-switched lasers. Further, MM optical fibers allow the design of cladding-pumped fiber lasers using dopants with relatively weak absorption cross sections.

Problems solved by technology

However, early work on fiber lasers did not attract considerable attention, because no methods of generating diffraction-limited coherent light were known.

In general, modal dispersion is the most detrimental effect limiting the transmission bandwidth of multi-mode (MM) optical fibers, since the higher-order modes, in general, have different propagation constants.

However, in the amplification of short-optical pulses, the use of SM optical fibers is disadvantageous, cause the limited core area limits the saturation energy of the optical fiber and thus the obtainable pulse energy.

Any further increase in core diameter requires a further lowering of the NA of the fiber and results in an unacceptably high sensitivity to bend-losses.

However, in general, amplification experiments in MM optical fibers have led to non-diffraction-limited outputs as well as unacceptable pulse broadening due to modal dispersion, since the launch conditions into the MM optical fiber and mode-coupling in the MM fiber were not controlled.

In this technique, however, severe ode-coupling was a problem, as the employed MM fibers supported some 10,000 modes.

Hence, only very poor modal discrimination was obtained, resulting in poor beam quality.

Also, the system of DiGiovanni did not take into account the fact that gain-guiding induced by dopant confinement can in fact effectively guide a fundamental mode in a MM fiber.

In fact, the system of DiGiovanni et al. is not very practical, since it considers a MM signal source, which leads to a non-diffraction-limited output beam.

Further, only a single cladding was considered for the doped fiber, which is disadvantageous when trying to couple high-power semi-conductor lasers into the optical fibers.

Indeed, in SM fibers, gain-guiding is irrelevant due to the strong confinement of the fundamental mode by the wave-guide structure.

However, in MM optical fibers., the confinement of the fundamental mode by the waveguide structure becomes comparatively weaker, allowing for gain-guiding to set in.

However, this work was limited to the use of MM fibers as soliton Raman compressors in conjunction with a nonlinear spectral filtering action to clean-up the spectral profile, which may limit the overall efficiency of the system.

However, hollow-core fibers have an intrinsic transmission loss, they need to be filled with gas, and they need to be kept straight in order to minimize the transmission losses, which makes them highly impractical.

One of the limitations of this technique is that, in the compression grating, a SM fiber with a limited core area is employed.

However, Gambling et al. found low levels of mode-coupling only in liquid-core fibers.

On the other hand, mode-coupling in MM solid-core fibers was found to be severe, allowing for the propagation of a fundamental mode only in mm lengths of fiber.

The inventors are not aware of any prior art using MM fibers to amplify SM signals where the output remains primarily in the fundamental mode, the primary reason being that amplification in MM fibers is typically not suitable for long-distance signal propagation as employed in the optical telecommunication area.

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