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

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.

Ultrafast laser-scanning optical resolution photoacoustic microscopy at up to 2 million A-lines per second

“…For example, arbitrary sequences of pulses could be used to investigate Grüneisen mediated effects 23 or measure blood oxygen saturation at a single wavelength by exploiting the different saturation intensities of oxy-and deoxyhemoglobin. 11 A tunable output can also be achieved with fiber lasers by exploiting the nonlinear effects exhibited by optical fibers such as stimulated Raman scattering 24,25 or supercontinuum generation in photonic crystal fibers. 6 Being able to generate a wide range of wavelengths would allow the spectroscopic identification and quantification of specific chromophores, e.g., to enable the measurement of blood oxygenation in vivo.…”

Visible Raman-Shifted Fiber Lasers for Biophotonic Applications

“…Pumping shorter fiber lengths with higher peak-powers increases the relative impact of SPM, XPM and FWM. This effect was demonstrated in the results presented by Xu et al [27], where scaling the pulse energy of the 575 nm Stokes shift to the microjoule-level resulted in a 3 dB spectral bandwidth of 12.3 nm, when using standard single-mode fiber. This broad spectral bandwidth leads to poor spectral extinction between the individual Stokes shifts and results in power loss in applications where the output is spectrally filtered.…”

Wave of single-impulse-stimulated fast initial dip in single vessels of mouse brains imaged by high-speed functional photoacoustic microscopy

“…We employed a picosecond laser here because it provided greater instantaneous power compared with a nanosecond laser of the same pulse energy, making it more easily cause the SRS effect. 26 Since the Raman gain coefficient of KGW is dependent on the polarization of the pump laser, a zero-order half-wave plate (WPH05M-532, Thorlabs) was used to align the pump laser's polarization with the a axis of the KGW crystal. The output beam was collimated by another lens and filtered by a band-pass filter (575/25 nm BrightLine, Semrock) to selectively pass the first Stokes line.…”

“…35 Our motivation for developing a Raman laser was the limited choices of wavelengths for high-repetition-rate lasers. 26,36 SRS effectively shifts the laser wavelength with a conversion efficiency usually one order of magnitude higher than commercial optical parametric oscillator systems. 37 Compared with other popular Raman materials, KGW possesses many desirable features, 38 such as convenient operation, a high Raman gain coefficient, a high thermal damage threshold, a high thermal conductivity, and low thermal lensing, of which the last two factors are most critical for developing stable high-repetition-rate Raman lasers.…”