The output of a laser frequency comb is composed of 100,000+ perfectly spaced, discrete wavelength elements or comb teeth, that act as a massively parallel set of single frequency (CW) lasers with highly stable, well-known frequencies. In dual-comb spectroscopy, two such frequency combs are interfered on a single detector yielding absorption information for each individual comb tooth. This approach combines the strengths of both cw laser spectroscopy and broadband spectroscopy providing high spectral resolution and broad optical bandwidths, all with a single-mode, high-brightness laser beam and a simple, single photodetector, detection scheme. Here I will touch on the application of this system for open-path measurements of atmospheric trace gases (CH4, CO2, CO, NH3, water, ethane, and N2O) and volatile organic compounds (acetone, isopropanol, propane) with field applications targeting industrial oil and gas monitoring and agriculture.
The effect of gas pressure, fiber length, and optical pump power on an acetylene mid-infrared hollow-core optical fiber gas laser (HOFGLAS) is experimentally determined in order to scale the laser to higher powers. The absorbed optical power and threshold power are measured for different pressures providing an optimum pressure for a given fiber length. We observe a linear dependence of both absorbed pump energy and lasing threshold for the acetylene HOFGLAS, while maintaining a good mode quality with an M-squared of 1.15. The threshold and mode behavior are encouraging for scaling to higher pressures and pump powers.
We have now demonstrated and characterized gas-filled hollow-core fiber lasers based on population inversion from
acetylene (12C2H2) and HCN gas contained within the core of a kagome-structured hollow-core photonic crystal fiber.
The gases are optically pumped via first order rotational-vibrational overtones near 1.5 μm using 1-ns pulses from an
optical parametric amplifier. Transitions from the pumped overtone modes to fundamental C-H stretching modes in both
molecules create narrow-band laser emissions near 3 μm. High gain resulting from tight confinement of the pump and
laser light together with the active gas permits us to operate these lasers in a single pass configuration, without the use of
any external resonator structure. A delay between the emitted laser pulse and the incident pump pulse has been observed
and is shown to vary with pump pulse energy and gas pressure. Furthermore, we have demonstrated lasing beyond 4 μm
from CO and CO2 using silver-coated glass capillaries, since fused silica based fibers do not transmit in this spectral
region and chalcogenide fibers are not yet readily available. Studies of the laser pulse energy as functions of the pump
pulse energy and gas pressure were performed. Efficiencies reaching ~ 20% are observed for both acetylene and CO2.
We investigate femtosecond pulse propagation in photonic crystal fiber, reporting the generation of tunable femtosecond soliton pulses. For sufficiently broad spectral content, stimulated Raman scattering transfers energy from the higher frequency spectral components to lower frequencies, resulting in a continuous self-frequency shift to longer wavelengths. Power dependent spectral analysis reveals a well-formed soliton at peak powers exceeding 100 W. Background-free intensity autocorrelation measurements confirm soliton formation with a duration of < 90 fs and with an energy conversion efficiency of 60%. Numerical solutions were performed based on a generalized nonlinear Schrodinger equation that included the effects of dispersion, self-steepening, optical shock formation, self-phase modulation and stimulated Raman scattering. The resulting spectra from the simulations are in excellent agreement with the measured spectra, and are consistent with the intensity autocorrelation measurements.
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