Novel resonantly-enhanced nonlinear frequency conversion scheme for cladding-pumped fiber lasers

Abstract

High power continuous-wave (CW) laser sources emitting in the visible spectral region have a diverse range of applications in areas such as laser processing of materials, projection displays, medicine and sensing. The standard method of accessing the visible wavelength region is via nonlinear frequency conversion of nearinfrared solid-state and fiber lasers. In the CW regime, the most popular method for generating visible output is via intracavity second harmonic generation in a diode-pumped 'bulk' solid-state laser. This approach exploits the relatively low resonator losses and hence high intracavity powers that can be achieved in these lasers to achieve high second harmonic conversion efficiency and output powers in multi ten-watt regime. Further power scaling of such sources, however, is rather challenging due to the effects of heat generation in the laser medium, which lead to beam distortion and increased resonator loss. Cladding-pumped fiber lasers benefit from a geometry that is relatively immune to the effects of thermal loading and hence offer the prospect of much higher power levels in the visible regime via nonlinear frequency conversion. Unfortunately, the technique of intracavity second harmonic generation is not well-suited to cladding-pumped fiber lasers, since they have rather high resonator losses. One solution to this problem is to employ the technique of external resonant cavity second harmonic generation. This approach has been successfully applied to CW fiber sources, but suffers from the drawback of added complexity since a single-frequency fiber master-oscillator power-amplifier is required and the master-oscillator and resonant cavity lengths must be actively stabilized to ensure that the resonance condition is maintained at all times.

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