Precision measurement of the refractive index of carbon dioxide with a frequency comb

Direct high-precision measurement of the effective optical path length of multi-pass cell with optical frequency domain reflectometer

Multi-pass cells (MPCs) are commonly used in trace-gas detection and weak spectrum measurement. It is essential to accomplish a high-precision measurement of MPCs' effective optical path length (EOPL). A direct high-precision measuring method of MPCs' EOPL with optical frequency domain reflectometer (OFDR) was reported and demonstrated in this paper. Several important parameters of a MPC, such as EOPL and base length, were derived with high-precision by identifying the complicated signal of OFDR. The MPC's EOPL was also verified with the prevailing absorbance method. The results showed that the MPC's EOPL measured by each of these two methods is highly consistent. However, the relative uncertainty with the OFDR dramatically decreased 2 orders of magnitude (about 0.0085%) than that with the absorbance method. It demonstrated that the OFDR method with fewer measurement links is more conducive to a direct measurement. The performances of beam spread and stray light in the White-cell were also evaluated with the method.

Multipass cell based on confocal mirrors for sensitive broadband laser spectroscopy in the near infrared

We report on broadband absorption spectroscopy in the near IR using a multipass cell design based on highly reflecting mirrors in a confocal arrangement having the particular aim of achieving long optical paths. We demonstrate a path length of 314 m in a cell consisting of two sets of highly reflecting mirrors with identical focal length, spaced 0.5 m apart. The multipass cell covers this path length in a relatively small volume of 1.25 l with the light beam sampling the whole volume. In a first application, the absorption spectra of the greenhouse gases CO(2), CH(4), and CO were measured. In these measurements we used a femtosecond fiber laser with a broadband spectral range spanning the near IR from 1.5 to 1.7 μm. The absorption spectra show a high signal-to-noise ratio, from which we derive a sensitivity limit of 6 ppmv for methane observed in a mixture with air.

Multi-wavelength generation by self-seeded four-wave mixing

A cost effective method of generating multi-wavelength based on the cascaded four wave mixing effect is experimentally demonstrated. The proposed scheme is free from external tunable laser sources and pump modulators, resulting from the use of a broadened linewidth tunable dual wavelength erbium-doped fiber laser as intracavity pump. In this configuration, the number of four wave mixing cascades becomes larger in tandem with the increment of erbium-doped fiber amplifier output power. When its output power is set at 20.57 dBm, six waves having optical signal to noise ratio larger than 10 dB are generated. The six waves are stable with peak power fluctuations less than 1 dB within 30 minutes period and tunable with wavelength spacing ranging from 1.03 nm to 11.31 nm.

Measurement of the dispersion of air and of refractive index anomalies by wavelength-dependent nonlinear interferometry

We carry out wavelength-dependent second harmonic interference experiments using thin films of an organic dye as nonlinear optical sources. While the measured difference of refractive index between the fundamental and second harmonic wavelengths follows the theoretical expectation for air in a wide spectral region, anomalous dispersion is observed when the second harmonic light lies in the absorption band of the dye. The sensitivity of the technique to small refractive index variations may prove useful for sensing applications as well as for testing models of light dispersion in weakly dispersing and absorbing media.

Femtosecond frequency comb based distance measurement in air

Interferometric measurement of distance using a femtosecond frequency comb is demonstrated and compared with a counting interferometer displacement measurement. A numerical model of pulse propagation in air is developed and the results are compared with experimental data for short distances. The relative agreement for distance measurement in known laboratory conditions is better than 10(-7). According to the model, similar precision seems feasible even for long-distance measurement in air if conditions are sufficiently known. It is demonstrated that the relative width of the interferogram envelope even decreases with the measured length, and a fringe contrast higher than 90% could be obtained for kilometer distances in air, if optimal spectral width for that length and wavelength is used. The possibility of comb radiation delivery to the interferometer by an optical fiber is shown by model and experiment, which is important from a practical point of view.

Precision refractive index measurements of air, N2, O2, Ar, and CO2 with a frequency comb

We report precision measurements of the refractive indices of dry air, N(2), O(2), Ar, and CO(2), performed by using a frequency comb as the light source in a Mach-Zehnder interferometer setup. Improved dispersion formulas for all gases are derived with a sensitivity level of 10(-9). These results are valid for a wavelength range from 740 to 860 nm and are in good agreement with measurements from other groups.