Balancing optical path lengths in broadband fiber interferometers

Broadband chromatic dispersion in fiber-coupled optical interferometry

Chromatic dispersion is a well-known technical challenge in optical interferometry, and the issue is exacerbated when using optical fibers for beam transport. The important sources of chromatic dispersion in a fiber-coupled optical interferometer are investigated using a Mach-Zehnder interferometer operating between 975-1650 nm, with particular attention paid to various dispersive effects in fibers. The compensation of chromatic dispersion is also investigated, and a compensation strategy using bulk glass and fiber stretching is described. A notional dispersion budget is presented for a fiber-coupled interferometer operating in the near infrared, showing that dispersion can be compensated to the λ/20 RMS level over a nearly 700 nm wide bandpass.

Forward fiber Fourier transform spectrometer modeling and design with PZT phase modulation real-time compensation

Piezoelectric ceramic transducers (PZTs) are often applied in all-fiber Fourier transform spectrometers (FFTSs) to realize phase modulation. Combining a PZT and optical fiber features, a theoretical FFTS modeling is established. We systematically deduced the main causes of spectral errors and the factors of the instrumental resolution, then designed a new FFTS system and provided real-time compensation methods for spectral errors. We creatively employed two Mach-Zehnder interferometers by winding the sensing arms of the two on the same PZT cylinder to realize the dual optical path experiencing the same and simultaneous phase modulation. A processing circuit is designed to achieve selective sampling of the test interferogram, avoiding the spectral artifacts generated by the PZT phase discontinuities. The narrow line width of the reference spectrum is obtained at the same time for real-time calibration of spectral shift resulting from dispersion and other phase errors. By the comparison of the original interferogram and spectrum with those after compensation, the experiments validated that the system design could effectively compensate for the spectral errors caused by PZT and optical fiber physical limitations. Finally, the improved ideas are discussed if the FFTS prototype is to be applied to test a real-time gas spectrum.

System for precise balancing and controlled unbalancing of fiber-optic interferometers

A system for accurate balancing and controlled unbalancing of the optical path difference in all-fiber optical interferometers is described. Interferometers with various arm lengths (1-30 m) and with initial optical path differences of as much as 1 cm have been successfully balanced within a 5-microm range. In addition, the proposed system allows for controlled unbalancing of arbitrary all-fiber optical interferometers with a precision better than 5 microm.

Laboratory imaging stellar interferometer with fiber links

We report on the implementation of an end-to-end imaging demonstrator to validate the concept of optical fiber use in high-resolution aperture synthesis in astronomy. As the coherent transport of light has been conclusively investigated, this study is focused primarily on the accuracy and reliability of the experimental data. In the framework of the European Space Agency project Optical Aperture Synthesis Technologies (OAST), the telescope array's complexity is minimized such that a realistic concept for a future space mission can be proposed. The reconstructed images are obtained by use of a particular self-calibration procedure that was designed to process the visibility contrast and the related weak phase information (one phase closure for each visibility triplet).