In order to detect directly faint planets around other stars it is important to develop techniques to cancel the central starlight. We report progress in terms of null depth on the visible nulling experiments at JPL, essential for NASAs future space missions as well as for an upcoming sounding rocket based experiment. A level of 1 Million to 1 nulling ratio (1e-6 rejection ratio) has been obtained for the first time for visible laser light, and a level of 100000 to 1 nulling ratio has been obtained for a relatively large (5%) bandpass at 650nm. Using a combination of a fiber array and deformable mirror a null of 1e-6 would correspond to a contrast of 1e-9 at a distance of 2-3$\lambda$ in the airy disk plane, only 10x away from TPFs goal. The configuration used was a fiber fed Mach Zender type interferometer, using 2 or 3 mirrors in each two arm, placed in an enclosed air container. For broadband light we used tilted dispersion glass plates in the nuller arms and an avalanche photon diode module to cover the large dynamic range. We describe a variety of conditions that have to be met and optimized to reach very deep nulls and sub-nanometer optical path difference stability, such as optical alignment, symmetry in the two arms, mechanical stability and vibration isolation.

The Planet Detection Testbed is designed to simulate the architecture and operation of a space-borne four-telescope nulling interferometer. Constructed in the form of a dual chopped Bracewell interferometer together with star and planet sources, it reproduces the principal features of the flight beam combiner designed at JPL for the proposed TPF-I Formation Flying Interferometer. The aims of the testbed are to demonstrate stable four-beam nulling and planet detection at representative star-planet contrast ratios. In the flight design, starlight between 7 and 17 micron wavelength is to be nulled, and 2 to 3 micron starlight is used for fringe tracking and phasing the interferometer. There are also metrology systems and alignment systems which are required for deep and stable nulling. The testbed reproduces these features; 2 to 3 micron light from a thermal source is used for fringe tracking, and nulling and planet detection is performed at 10 microns. The testbed also incorporates laser metrology and other systems enabling continuous control of beam alignment. The ultimate goal is to simulate planet detection at star-planet contrast ratios of order $10^{-7}$ during full rotations of the telescope array using the phase chopping method. The latest results from the testbed are presented including four-beam nulling experiments at null depths of $10^{-5}$ and planet signal detections at similar contrast ratios.

The Darwin and TPF-I missions are Infrared free flying interferometer missions based on nulling interferometry. Their main objective is to detect and characterize other Earth-like planets, analyze the composition of their atmospheres and their capability to sustain life, as we know it. Darwin and TPF-I are currently in study phase. A number of mission architectures of 3 and 4 free flying telescopes are evaluated on the basis of the interferometer's response, ability to distinguish multiple planet signatures and starlight rejection capabilities. The characteristics of the new configurations are compared also to the former, more complex Bowtie baseline architectures as well as evaluated on base of their science capability.

Nulling interferometry is one of the promising techniques for the study of extra terrestrial planets. This technique will be applied in the future space missions Darwin and TPF-I, and from the ground with GENIE. The nulling interferometry techniques require high symmetry of the interfering beams, to obtain the required contrast (typically $10^6$ to detect terrestrial exo-planets in the thermal infrared). In this paper we consider the polarization symmetry issue, such as polarization rotation and polarization phase shifts occurring on slightly misaligned optics. We study the consequences of these symmetry requirements on a nulling interferometer design. We find the relation between the misalignment tolerances and the achievable nulling, and we show that this tolerance is highly dependent on the interferometer configuration (the way beams turn right, left, up or down in the interferometer arms). It is typically of the order of the arcminute (not the arcsecond) for a $10^6$ contrast. We present a analytical and numerical analyses.