Raman-shifted wavelength-selectable pulsed fiber laser with high repetition rate and high pulse energy in the visible.

L Xu,D P Shepherd,D J Richardson,Q Kang,S Alam

doi:10.1364/oe.25.000351

Abstract

A high-pulse-energy, diffraction-limited, wavelength-selectable, visible source, based on Raman frequency shifting of a frequency-doubled Yb-doped fiber laser, has been studied. The relative length-scaling laws of Raman gain and self-phase modulation push the design towards short fiber lengths with large core size. It is experimentally demonstrated that the Raman clean-up effect in a graded-index multi-mode fiber is not sufficient to obtain diffraction-limited beam quality in the short fiber length. Thus, a large-core photonic crystal fiber is used to maintain diffraction-limited performance and output pulse energies of ~1 μJ, at a 1-MHz repetition rate and 1.3-ns pulse-width are successfully achieved. This step-tunable visible source should find applications in photoacoustic microscopy.

Highlights

Photoacoustic microscopy (PAM) is an emerging technology that offers unprecedented highcontrast imaging based on physiological differences in tissue optical absorption [1,2,3]
The relative length-scaling laws of Raman gain and self-phase modulation push the design towards short fiber lengths with large core size
It is experimentally demonstrated that the Raman clean-up effect in a graded-index multi-mode fiber is not sufficient to obtain diffraction-limited beam quality in the short fiber length

Summary

Introduction

Photoacoustic microscopy (PAM) is an emerging technology that offers unprecedented highcontrast imaging based on physiological differences in tissue optical absorption [1,2,3]. By choosing largemode-area (LMA) photonic crystal fiber (PCF), ~1-μJ energy, 1-MHz repetition rate, 1.3-ns pulses with single-mode beam quality and relatively narrow spectral bandwidth have been achieved at the Raman Stokes wavelengths of 560 nm (2nd Stokes) and 575 nm (3rd Stokes), when pumped by a frequency-doubled Yb-doped-fiber master oscillator power amplifier (MOPA) system. These wavelengths are targeted as they are located in the absorption spectra of oxy and deoxy haemoglobin

Experiment

Results and discussion

Conclusion