Coherent Raman Spectroscopy Of Gases In Negative Curvature Hollow-Core Fibers

As an important tool for material analysis,Raman scattering is a process of inelastic scattering of photons which carries the structure information of scatterers.However,the cross sections of Raman scattering are usually small.The signals,especially those from spontaneous Raman scattering of gases,are weak and susceptible to interference from fluorescence or Raman background of optical components,sample cells and other components.In order to avoid the influence of the background signals and improve the sensitivity and resolution of gas Raman spectroscopy,the stimulated/coherent Raman spectroscopy of gases in negative curvature hollow-core fibers（NC-HCFs）was carried out.Tunable single-frequency external cavity diode lasers（ECDL）of a variety of center wavelengths were built.Together with tunable dye laser,lock-in amplifier,cryogenic CCD detector and other experimental instruments,an experimental setup of NC-HCF enhanced gas stimulated/coherent Raman spectroscopy was implemented,by which Raman gain,Raman loss,stimulated Raman scattering,CARS,etc.of gases can be studied.Continuous-wave（CW）SRGS and stimulated Raman loss spectroscopy（SRLS）of various gases such as H₂,CO₂ and CH₄ were demonstrated.In SRGS,the Raman transition was pumped by a dye laser,the Stokes light was produced by an ECDL,which is also used as a probe laser.In SRLS,a diode-pumped solid-state laser（DPSSL）acted as the probe laser.The application of single-longitudinal-mode lasers enabled a spectral resolution of 20 MHz.Compared with free-space SRGS,the usage of a 2 m-long gas-filled NC-HCF increased the sensitivity by a factor of nearly a thousand.The spectrum of 0.6 MPa pure CO₂ was obtained with 186 m W transmitted pump power and 5 ms lock-in time constant,the signal to noise ratio was 567,and the sensitivity was about 20 times the shot noise limit.The stimulated Raman gain profiles of CO₂₁ band under pressures between 0.2 MPa to 1.0 MPa were Lorentzian.The stimulated Raman gain profiles of CO₂ 2₂ band exhibited asymmetry,which progressively disappeared as the pressure increased.The stimulated Raman loss profiles of CH₄₁ band appeared to merge towards one effective line above 0.2 MPa due to line mixing.The Raman gain of CO₂ and CH₄ in the hollow fiber pumped by 100 m W CW lasers reached the order of 10^-3,when the pumping laser power was further increased,cascade stimultated scattering was observed in the fiber with a single pulsed pump laser with peak power of several kilowatts.We showed for the first time the application of NC-HCF to CW scanning CARS with a signal boost of more than a million times.Scanning CARS has the same spectral resolution of 20 MHz as SRGS.Phase matching of fundamental fiber modes was achieved by adjusting the gas pressure,for which the optimized pressure for pure CO₂ was about 0.6 MPa.The anti-Stokes photon generation rate using a 152 m W pump laser and a 1.4 m W Stokes laser was～3×10⁹ photons/s.According to high intensity of the anti-Stokes beam,a lock-in amplifier instead of a photon counter was used to detect the signals from photomultiplier tube.The signal to noise ratio was as high as 25000 with 5 ms lock-in time constant,and shot noise limited sensitivity was achieved.SRGS and CARS of H₂ mixture were simultaneously measured along the Stokes and anti-Stokes beams respectively,the Raman profiles measured by both these techniques almost overlapped.Meanwhile,the experients verified the advantage of using the deep-cooled CCD in detecting the CARS signal,which further reduced the pumping power by an order of magnitude.The above results confirm the enhancement effect of NC-HCF on coherent Raman spectroscopy of gases:high-resolution coherent Raman signals can be obtained using 100-m W-class CW lasers;the threshold power of stimulated Raman is reduced to several kilowatts;and the signal-to-noise ratio of CARS spectra is significantly improved because of the strong signal level and the weak influences of the common Raman and fluorescence background.These may provide new approaches for high-resolution molecular spectroscopy and high-sensitivity Raman gas analysis.