Objectives

Figure 1. The Achromatic Nulling Testbed (ANT) includes three testbeds to explore technology for broadband deep nulling.

The Achromatic Nulling Testbed (ANT) was developed to address the technology related to achieving deep, broadband, dual-polarization, two-beam mid-infrared nulls. The two-beam nuller is the basic building block of all flight architectures that have been considered so far. Three approaches to achromatic phase shifting were investigated, with the aim of demonstrating, through one of the approaches, two-beam nulling to a level of 1 part in 100,000 with a 25% bandwidth in the 7-11 µm range. A longer-term objective was the development of a cryogenic nulling interferometer that would meet the above requirements while operating at 40 K.

Testbed Description

Three different methods of implementing achromatic phase delays were investigated: (1) pairs of dispersive glass plates to introduce a wavelength-dependent delay; (2) A through-focus field-flip of the light in one arm of the interferometer; and (3) successive and opposing field-reversals on reflection off flat mirrors in a periscope arrangement. These methods were tested in the same lab, on adjacent optical benches, using a common mid-infrared laser and white-light source. The overall strategy was to develop an error budget for each approach, model the optics, isolate and measure the contributing noise sources, and compare the achievable null depths with the model predictions. A view of the testbed is shown in Figure 1, and results are shown in Figure 2.

The optical breadboards were vibrationally isolated from the optical table that they were mounted on, and the table itself was further isolated from the floor. Pathlength fluctuations were monitored and maintained at a level of less than a few nanometers. The Achromatic Nulling Testbed used laser metrology and automated alignment algorithms to scan across the zero path-difference position and locate the interference null. Pathlength dither algorithms were used to locate the null and maintain the stability of the fringes. The algorithms that were used improved the stability of the testbed and reduced the overall time required for each measurement.

In each of the approaches that were considered, an achromatic phase shifts of 180 degrees should be straightforward to accomplish, however in the field-reversal approaches, (2) and (3) above, a pupil-dependent polarization and/or amplitude mismatch can occur, which must then be corrected using spatial filters. Mid-infrared single-mode fibers were therefore used with these approaches.

Figure 2. Experimental data.

Ancillary optical components and detectors were developed for the Achromatic Nulling Testbed. A new high-flux continuum argon arc lamp was built to increase the dynamic range of the measurements. Work was also undertaken to improve the low-light level limit of measurements through the development of a 10-µm camera with high dynamic range. Components for the balancing of intensities and phases were also tested. Moreover, as noted previously, new single-mode fibers made from chalcogenide glass were used in several of the testbeds.

The principal investigator of the Achromatic Nulling Testbed was Dr. Robert Gappinger at the Jet Propulsion Laboratory.

Results

Deep broadband mid-infrared nulling was demonstrated with the ANT using three approaches to implement achromatic phase shifts. The most successful was the use of periscope mirrors, yielding an average rejection ratio of 51,000:1 at 20% bandwidth and 27,000:1 using a 25% bandwidth. Results using a 20% bandwidth are shown in Figure 2. The through-focus approach yielded a rejection ratio of 2000:1 with a 17% bandwidth. Pairs of glass plates provided a rejection ratio of 10,000:1 with a 25% bandwidth.

Tests using the same optics, but with narrow-band 10-µm laser light and mid-infrared polarizers routinely yielded null depths of 200,000:1. The laser nulling results suggested that a factor of 4 improvement in broadband nulling was possible. There were undoubtedly subtle wavelength-dependent effects that were different in one arm of the interferometer than in the other that limited the overall performance.

Timeline

The Achromatic Nulling Testbed began operations in 2003. By 2005 it had explored both the through-focus approach and the use of pairs of dispersive glass plates. From 2005 through 2007 the effort focused on developing and improving the results from the periscope mirror design. However, in May 2007 the Adaptive Nuller demonstrated deeper and more broadband nulls than could be achieved with the ANT (yielding rejection ratios of 82,000:1 over a 32% bandwidth). It seemed clear that the best approach to achromatic phase shifting was through adaptive nulling, and so the operation of the ANT was brought to a close in December 2007. The testbed was dismantled in January 2008.

Future Technology Development

Of the methods of achromatic phase shifting that were tested by the ANT, the approach using the periscope mirrors produced the best results. Although these results fell short of the goal of 100,000:1 at 25% bandwidth, this goal appears to be well within reach of the Adaptive Nuller. (An adaptive nuller used in conjunction with a periscope phase shifter would be a viable approach, although it is entirely possible that an adaptive nuller alone may be sufficient.) Future developments in broadband mid-infrared nulling will be devoted to improving the performance of the Adaptive Nuller.