摘要： Selective detection and chemical analysis of trace gases at concentration levels of parts per million (ppm) and below is of critical importance in environmental monitoring and medicine. Spectroscopic techniques offer high gas-type selectivity and are widely used for measuring the concentration of specific molecular species. Raman spectroscopy provides two key advantages. First, the pump wavelength can be freely chosen, independently of the absorption lines of the gas. Second, the highest Raman frequency shift of any gas is the vibrational transition of H2 at 125 THz, which means that all known Raman active molecules can be detected with one spectrometer. We report coherent anti-Stokes Raman spectroscopy (CARS) in single-ring hollow-core photonic crystal fibre (SR-PCF) using a dual-colour pump. The long collinear path-length offered by SR-PCF strongly enhances the efficiency of the Raman interactions, and furthermore the gas-filled SR-PCF has a zero dispersion wavelength which is pressure tuneable. By selecting an appropriate pressure, the dispersion can be arranged to be anomalous in the visible and at the same time normal in the ultraviolet. Under these circumstances, by fine-tuning the pressure to minimize dephasing, the Raman coherence prepared by seeded pumping in the visible can be used for phase-matched generation of an anti-Stokes signal from a second pump in the ultraviolet. This spectrally separates the 'preparation' pump from the 'read-out' pump. The low dephasing rate in the ultraviolet permits efficient generation, along the whole fibre length, of anti-Stokes signals from all known Raman-active gas molecules, which is a significant advantage compared to other spectroscopic techniques. The dual-colour pump scheme allows the main detection limit of CARS imposed by the non-resonant background (NRB) to be overcome. It exploits the fact that the NRB is a quasi-instantaneous effect, present only when pump and seed pulses overlap. Although this is also a requirement of Raman excitation, the resulting coherence wave has a lifetime of nanoseconds before it relaxes through intermolecular collisions. Therefore, by appropriately delaying the read-out pump pulse, it is possible to suppress the NRB. Figure 1(a) shows the CARS spectrum for two trace gases, H2 (500 ppm) and CH4 (600 ppm), mixed with the buffer gas N2, measured when the read-out pump overlaps with the Stokes seed. Figure 1 (b) shows the CARS spectrum for 20 ppm of H2 and 350 ppm of CH4, measured when the pump was delayed by 2 ns with respect to the Stokes seed. As can be seen from Fig.1, the dual-colour-pump CARS scheme in SR-PCF reduces the NRB by 120 dB, improving the detection limit by 15 dB and allowing detection of trace gas concentrations as low as 20 ppm with only 20 kW of peak pump power. This novel dual-colour-pump CARS approach has great potential in environmental monitoring and medical applications, offering high sensitivity, short acquisition times (several seconds) and simultaneous multi-species sensing.