Interferometric Coupling of the Keck Telescopes with Single-Mode Fibers

In situ measurement of chromatic dispersion in an optical hypertelescope: a laboratory demonstration

Optical interferometry is a technique capable of achieving better spatial resolution than the world's largest monolithic telescopes at a fraction of the cost. Most interferometer architectures split the imaging bandwidth into a number of channels in order to prevent image degradation due to a large spectral bandwidth. An optical hypertelescope permits a much broader spectral bandwidth on a single channel than a conventional interferometer. However, a broader spectral bandwidth becomes more sensitive to differential chromatic dispersion, and this dispersion must be measured and corrected in order to maintain a high signal-to-noise ratio. A prototype dispersion measurement system is presented that is capable of measuring chromatic dispersion in an eight aperture hypertelescope. The optical design, calibration, data acquisition, and dispersion measurement process are described in detail. This system is capable of measuring differential dispersion to better than λ/100R M S and is scalable to a system with an order of magnitude more apertures.

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.

Astrophotonics: a new era for astronomical instruments

Astrophotonics lies at the interface of astronomy and photonics. This burgeoning field has emerged over the past decade in response to the increasing demands of astronomical instrumentation. Early successes include: (i) planar waveguides to combine signals from widely spaced telescopes in stellar interferometry; (ii) frequency combs for ultra-high precision spectroscopy to detect planets around nearby stars; (iii) ultra-broadband fibre Bragg gratings to suppress unwanted background; (iv) photonic lanterns that allow single-mode behaviour within a multimode fibre; (v) planar waveguides to miniaturize astronomical spectrographs; (vi) large mode area fibres to generate artificial stars in the upper atmosphere for adaptive optics correction; (vii) liquid crystal polymers in optical vortex coronographs and adaptive optics systems. Astrophotonics, a field that has already created new photonic capabilities, is now extending its reach down to the Rayleigh scattering limit at ultraviolet wavelengths, and out to mid infrared wavelengths beyond 2500 nm.

Mid-infrared guided optics: a perspective for astronomical instruments

Research activities during the last decade have shown the strong potential of photonic devices to greatly simplify ground based and space borne astronomical instruments and to improve their performance. We focus specifically on the mid-infrared wavelength regime (about 5-20 microm), a spectral range offering access to warm objects (about 300 K) and to spectral features that can be interpreted as signatures for biological activity (e.g. water, ozone, carbon dioxide). We review the relevant research activities aiming at the development of single-mode guided optics and the corresponding manufacturing technologies. We evaluate the experimentally achieved performance and compare it with the performance requirements for applications in various fields of astronomy. Our goal is to show a perspective for future astronomical instruments based on mid-infrared photonic devices.

Laboratory demonstration of an infrared-to-visible up-conversion interferometer for spatial coherence analysis

We experimentally demonstrate the possibility of retrieving the spatial coherence of an infrared source by using an up-conversion interferometer. Sum-frequency generation in Ti-diffused periodically poled lithium-niobate waveguides in both arms of the interferometer is used to convert the infrared into the visible domain. The fringe contrast of the interference pattern in the visible domain permits us to resolve the spatial separation of two uncorrelated pointlike infrared sources, which simulate a binary star. The validity of these measurements is confirmed through a simultaneous comparison with a reference interferometer working in the infrared domain.