Objectives

Figure 1. Three Single-Mode Chalcogenide Fibers Developed for TPF-I by the Naval Research Laboratory.

Spatial filters are an essential technology for nulling interferometry, significantly reducing the optical aberrations in wavefronts, making extremely deep nulls possible. The most basic form of a spatial filter is a simple pinhole, and pinholes have indeed been used to achieve the deepest laser nulls so far at mid-infrared wavelengths. However, pinholes only operate well over a narrow bandwidth and so are ill-suited for broadband spatial filtering for science instruments. In addition, they do not reject very low spatial-frequency wavefront aberrations. The development of improved broadband techniques for spatial filtering at mid-infrared wavelengths may be crucial to the success of TPF-I.

The developed spatial filters should have a single-mode throughput of near 50% and a model suppression of 25 dB for non-fundamental modes. The wavelength range of 7 to 17 microns should be accommodated using no more than two spatial filters, each with their own separate wavelength coverage within that range.

Description of Research

Spatial filters may be implemented in a variety of ways, including single-mode fiber-optics made from chalcogenide glasses, metallized waveguide structures micro-machined in silicon, or through the use of photonic crystal fibers. By promoting the parallel development of various spatial filter technologies, it is hoped that TPF-I will demonstrate the necessary spatial filter performance in the 7 to 17 µm spectrum using no more than two technology types. The development of mid-infrared spatial filters was funded by TPF-I from 2003 through 2008. The performance of the single-mode filters that were developed under contract with TPF-I were tested and characterized in-house at JPL. The scope of the work included prototypes of hollow waveguide filters designed by Christopher Walker at the University of Arizona; completed chalcogenide fibers designed by Dr. Jas Sanghera at the Naval Research Laboratory (NRL), shown in Fig. 1; and completed silver halide fibers designed by Prof. Abraham Katzir at Tel Aviv University (TAU) in Israel.

Results

Although fiber optics at near-infrared wavelengths are extensively used by the telecommunications industry, low-loss, mid-IR, single-mode spatial filters are not yet commercially available. The fibers that have been developed for TPF-I represent the state of the art. About 16 mid-infrared single-mode fibers were delivered and tested at JPL, showing excellent single-mode behavior at 10 µm.

The 20-cm long chalcogenide fibers developed by the Naval Research Laboratory were shown to demonstrate 30 dB rejection (a factor of 1000) of higher order modes and have an efficiency of 40%, accounting for both throughput and Fresnel losses. The transmission losses were measured at 8 dB/m, and the fibers are usable up to a wavelength of about 11 µm. The chalcogenide fibers developed at the Naval Research Laboratory were used successfully in the Achromatic Nulling Testbed and are currently in use in the Adaptive Nuller testbed.

The 10-20 cm long silver halide fibers that were developed by Tel Aviv University were shown to demonstrate 42 dB rejection (a factor of 16,000) of higher order modes with transmission losses of 12 dB/m. This high rejection of higher-order modes was accomplished with the addition of aperturing of the output of the fibers, made possible by the physically large diameter of the fibers. Silver halide fibers should in principle be usable up to a wavelength of about 18 microns, although the lab tests at JPL were conducted only at 10 microns. This is the first time silver halide fibers were demonstrated to have single-mode behavior.

Timeline

Work on hollow waveguides, begun in 2003, ceased in 2005, when it became apparent that single-mode fibers would be the most viable technology to support. The contract work with the Naval Research Laboratory was successfully completed in 2005. Work up until 2008 then focused on maturing the technology for Silver Halide fibers.

Future Technology Development

An important step in future work will be to demonstrate single-mode performance over the entire wavelength band that TPF-I will use (6-20 microns). To date, experiments that test single-mode behavior have been limited to a narrow wavelength range near 10 microns. It would be extremely useful to verify and validate the short-wavelength performance of the chalcogenide fibers and the long-wavelength performance of silver-halide fibers. The spatial filtering capabilities of photonic crystal fibers should also be investigated for use at mid-infrared wavelengths, because of the improved throughput that they may provide.