Objectives

Figure 1. Schematic of the adaptive nuller.

The variations in amplitude and phase that may be present across a broad wavelength band make nulling extremely challenging. The Adaptive Nuller is designed to correct these variations, matching the intensity and phase between the two arms of the interferometer, as a function of wavelength, for each linear polarization. This will allow high performance nulling interferometry, while at the same time substantially relaxing the requirements on the nulling interferometer's optical components. The goal is to correct the intensity difference across the band of 8-12 microns to less than 0.2% rms (1 σ) between the interferometer's arms. The phase difference across the same band is to be corrected to < 5nm rms (1 σ). This overall correction is consistent with a null depth of 1 part in 100,000.

Testbed Description

The adaptive nuller uses a deformable mirror to adjust amplitude and phase independently in each of about 12 spectral channels. A schematic of the adaptive nuller is shown in Fig. 1, as it would be used to adjust the intensity and phase of one beam in a two-beam nuller. The incident beam is first split into its two linear polarization components, and is also divided into roughly a dozen spectral channels. These beams are then directed onto a deformable mirror, where the piston of each pixel independently adjusts the phase of each channel. Tilt in the orthogonal direction may also be independently adjusted, and, by means of controlled vignetting at a subsequent aperture, provides an independent adjustment of the intensity in each channel. The various component beams are recombined to yield an output beam that has been carefully tuned for intensity and phase in each polarization as a function of wavelength. If the adaptive nuller is used to balance beams entering a nulling interferometer, matching tolerances on optical components in that interferometer are substantially relaxed. The ultimate null depth and stability are now determined by the performance of the adaptive nuller, under active control, that can be monitored and readily characterized, and optical components need only be of sufficient quality that the two arms of the interferometer are matched in intensity and phase to within the capture range of the Adaptive Nuller.

It is important to note that all designs under consideration for TFP-I include a single-mode spatial filter through which the combined light is passed before being detected. The wavefront from the star is incident on the collecting apertures of the instrument and delivered by the respective beam trains to a central beam combiner that couples the combined light into a single-mode filter. With just a single mode for each polarization state, the problem of nulling the on-axis light is simplified. Higher order wavefront aberrations that would reduce the visibility of the fringes (depth of the null) are rejected by the spatial filter. Small errors in tilt in each arm of the interferometer thus translate into small errors in received intensity. The adaptive nuller is not designed to adjust wavefront errors across each pupil, as these are rejected independently by the spatial filter. The adaptive nuller technology demonstration only addresses wavelength and polarization dependent amplitude and phase errors.

The adaptive nuller will use a broadband thermal source to generate light with a spectral width > 3 �m in the 7-12 µm wavelength band. This light will be put through a simple interferometer with one arm holding the adaptive nuller components, and the other serving as a reference arm. There will be intensity and phase dispersion in this interferometer due to normal manufacturing tolerances as well as intentionally added optical material in one arm.

Figure 2. Experimental Layout for a Mid-Infrared Adaptive Nuller.

The tests are performed in the mid-infrared, but without independent control of each polarization. Although Wollaston prisms are included in the design, there are no Wollaston prisms in the mid-infrared testbed. The polarization is selected at the source and only one polarization is treated to compensate for intensity and phase. The testbed results are for one polarization only, because if one polarization can be compensated, then it would be straightforward to compensate both using the same overall approach. It is simply a matter of cost. At mid-infrared wavelengths, the only material that can be used to make a Wollaston prism is cadmium selenide (CdSe). As this is a very expensive material to manufacture for optical components, the Adaptive Nuller was first built to control the two polarizations independently at near-IR wavelengths using quartz Wollaston prisms that are relatively inexpensive (Peters et al. 2005). This test showed that the concept would also work at mid-IR wavelengths. A Zemax model was nonetheless developed and tested to predict the performance of a mid-infrared CdSe Wollaston prism. Thus for the mid-infrared demonstration, the polarization is selected at the source, and there are no Wollaston prisms in the experiment. This simplification in the layout does not detract from the importance of the results.

The principal investigator of the Adaptive Nuller is Dr. Robert Peters at the Jet Propulsion Laboratory.

State of the Art

As noted below, the Adaptive Nuller has now completed its primary goal and represents the state of the art in this field. It is also worth noting that a visible/near-infrared proof-of-concept experiment was completed as the first stage of this development effort. The proof-of-concept was done with null depth requirements that were relaxed due to the lab conditions, and the amplitude control was scaled to the wavelength and beam size used. This visible/near-infrared test exceeded its performance target of 5% and 15 nm, achieving intensity control to 2% and phase control to 2 nm. There had been no previous capability demonstrated in this area.

Progress to Date

The mid-infrared Adaptive Nuller has demonstrated a null depth of 1.2 x 10^-5, over a bandwidth of 32%. This is the deepest broadband null ever achieved by a mid-infrared nulling interferometer, and almost attains the TPF-I flight requirement. In March/April 2007, the testbed achieved its primary goal of demonstrating phase compensation to better than 5 nm rms across the 8-12 micron band and intensity compensation to better than 0.2%. Long term plans now include improving the attainable null depth and the possible design of a cryogenic version of this testbed.