Abstract
We demonstrate a high-energy pulsed Raman fiber laser (RFL) with an emission wavelength of 1.44 μm, corresponding to an absorption peak of water. Microsecond pulses with >20 mJ/pulse and >40 W peak power were generated, well above the threshold for tissue coagulation and ablation. Here, we focus on the optical characterization and optimization of high-energy and high-power RFLs excited by an ytterbium fiber laser, comparing three configurations that use different Raman gain fibers, but all of which were prepared with a one-side opened, free-run mode without output mirrors. We show that the free-run configuration can generate sufficiently high energy without requiring a closed cavity design. Experimental RFL characteristics corresponded well with numerical simulations. We discuss the Stokes beam generation process in our system and loss mechanisms mainly associated with fiber Bragg gratings.
Highlights
High-power fiber lasers have gained growing attention for medical therapeutic applications [1,2,3]
We demonstrated high-energy pulsed Raman fiber laser (RFL) operating at 1436-nm wavelength using a pump wavelength of 1090 nm and designing 5-step Stokes transition systems
The use of the Raman filter fiber significantly enhanced the conversion process to the 5th-order Stokes beam (1436 nm), resulting in P1436/Pout = 43.9% when used alone and 90.1% when preceded by a segment of HI1060
Summary
High-power fiber lasers have gained growing attention for medical therapeutic applications [1,2,3]. A continuous-wave (CW) fiber laser with an output power of ~380 mW at 1480-nm wavelength was demonstrated for tissue marking and biopsy guidance [7] Performance of this system was inhibited by the low laser energy available at the tissue surface, inefficient heat deposition due to thermal diffusion under CW optical irradiation, and spectral mismatch between the laser wavelength and the absorption peak of water. In the SRS process, the amount of the Stokes frequency shift is intrinsically determined by the irradiated medium in which a quantum conversion increases proportionally with the complex part of the third-order nonlinear permittivity [10,11] As this allows a design flexibility for laser output spectra, a great deal of effort has focused on developing RFLs with a spectral range of 1400~1500 nm, mostly as pump lasers for optical communications. Experimental results were found to be in agreement with numerical simulations that were obtained by solving coupled ordinary differential equations with a discrete step evolution along the longitudinal axis of the Raman gain fiber
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