Constant Optical Path Difference Research Articles

Design and characteristics of a cavity-enhanced Fourier-transform spectrometer based on a supercontinuum source.

We report the in-house fabrication of a high-resolution Fourier-transform spectrometer (FTS) for the spectroscopy of molecules in the gas phase at resolutions down to 0.002 cm-1 working in the spectral range from 5880 cm-1 (1.7 μm) to 15 380 cm-1 (650 nm). The FTS employs a supercontinuum as a broadband light source and a He:Ne laser with a homemade frequency-stabilization scheme as the spatial reference for the sampling of the interferogram on a constant optical path difference (OPD) grid. The sampling of the two lasers is performed at constant time intervals, and the resampling process is performed at the software level. The resampling of the interferogram on a constant OPD grid relies on cubic approximations of the He:Ne interference pattern to determine its zero-crossings. The use of an invariant in the sampling process allows us to perform on-the-fly data treatment. Both the hardware aspect and the data processing are described with, in each case, an original approach. We also report the successful coupling of the FTS with a high finesse optical cavity with effective mirror reflectivities of 99.76%, allowing us to reach sensitivities down to 6.5 × 10-8 cm-1 with a root-mean-square accuracy of 0.0017 cm-1 on the position of the Doppler-broadened transitions with a mean transition width of 0.046 cm-1 for spectra recorded at a spectral resolution of 0.015 cm-1. The sensitivity of the instrument per spectral element, once normalized, represents the best sensitivity reported in the literature for Fourier-transform incoherent broadband cavity-enhanced absorption spectroscopy with a supercontinuum light source.

Birefringent prism based Fourier transform spectrometer

This paper presents the design of a rugged and compact Fourier transform spectrometer (FTS) utilizing a birefringent prism, a pair of polarizers and a linear CCD array. This design improves on existing FTS by eliminating moving parts and spreading the optical path difference (OPD) spatially (rather than temporal scanning), making the system smaller, more reliable, and dramatically reducing measurement times. Both the theoretical models for the design and experimental results of the prototype are presented. The optical performance is tested using LEDs of known wavelengths, with the fringe counting technique employed during interferogram acquisition to ensure accurate sampling of the interferogram at constant OPD intervals. Reconstructing the spectra showed that the detected wavelengths deviated from the actual wavelengths by less than 1 nm.

A Miniaturized Lamellar Grating Based Fourier Transform Spectrometer With Electrostatic Actuation

We report a miniaturized oscillating Fourier transform spectrometer (FTS), of which the novel features are specially designed suspension springs with reduced thickness and a square honey-combed platform which supports the movable grating facets. The proposed design is electrostatically driven by parallel plate actuators at its resonant frequency of 230 Hz. It uses lamellar grating to modulate the incident radiation and the movable part of the grating is capable of bidirectional out-of-plane motion. A maximum displacement of about 100 is achieved with an 80 V peak-to-peak driving voltage superimposed on a DC bias of 110 V in ambient pressure. The optical performance of the model is tested using a tunable infrared (IR) laser source. Fringe counting technique is employed in the IR interferogram acquisition process to ensure accurate sampling of the interferogram at constant optical path difference (OPD) intervals. The reconstructed spectra yielded detected wavelengths which deviated by less than 1 nm from the actual wavelengths, indicating excellent wavelength accuracy and an impressive resolving power.

Resolving fringe ambiguities of a wide-field Michelson interferometer using visibility measurements of a noncollimated laser beam

An actively stabilized interferometer with a constant optical path difference is a key element in long-term astronomical observation, and resolving interference fringe ambiguities is important to produce high-precision results for the long term. We report a simple and reliable method of resolving fringe ambiguities of a wide-field Michelson interferometer by measuring the interference visibility of a noncollimated single-frequency laser beam. Theoretical analysis shows that the interference visibility is sensitive to a subfringe phase shift, and a wide range of beam arrangements is suitable for real implementation. In an experimental demonstration, a Michelson interferometer has an optical path difference of 7 mm and a converging monitoring beam has a numerical aperture of 0.045 with an incidental angle of 17 degrees. The resolution of visibility measurements corresponds to approximately 1/16 fringe in the interferometer phase shift. The fringe ambiguity-free region is extended over a range of approximately 100 fringes.

Dispersion in stellar interferometry: simultaneous optimization for delay tracking and visibility measurements

In long-baseline optical stellar interferometry, it is necessary to maintain optical path equality between the two arms of an interferometer in order to measure the fringe visibility. There will be errors in matching the optical paths because of a number of factors, and it is desirable to use an automatic system to monitor and correct such path errors. One type of system is a delay tracker, based on imaging of the channeled spectrum. The tracking algorithm is designed to maintain a fixed number of fringes, ideally linearly spaced, across the observed spectral band. This results in a constant optical path difference, which may be incompatible with the requirement of path equality for the measurement of fringe visibility. In a practical interferometer that uses an optical path-length compensator operating in air, there is a complication since air paths introduce differential dispersion. This dispersion can be compensated for by including dispersion correction. By modifying the operation of an appropriately designed dispersion corrector, we show that it is possible to make the optical path difference zero at the measurement wavelength and, at the same time, to produce linearly spaced channel fringes across the tracking band.

Infrared point-diffraction interferometer

A point-diffraction interferometer (PDI) for use in the infrared is discussed. It is shown that the PDI is simple and easy to use and also yields fringes of constant optical path difference similar to those obtained with a Twyman–Green interferometer. The fabrication of the PDI is described, and typical results obtained using the interferometer at a wavelength of 10.6 μm are shown.