A newly developed interferometer for characterization of aspheric surfaces is presented. The interferometer is based on
the Fizeau configuration and uses a sub-Nyquist CCD camera as a detector. Due to the camera design, the instrument is
capable of recording very dense fringe patterns of up to 4 fringes/pixel. This enables processing of interferograms with
hundreds of fringes in the field of view. Thus, the interferometer can be used to measure many types of aspheric surfaces
using a standard transmission sphere as a reference. However, the main obstacle associated with this kind of
interferometer is caused by the presence of so called "re-trace" errors, which can be significant. Such errors occur from
un-equal optical paths the reference and test beams travel through in the optical system of the interferometer. A ray
tracing procedure has been developed to subtract the influence of the optical system of the interferometer on the
measurement. This method of error compensation results in reducing measurement errors to λ/5 PV (peak to valley) for
the full range of fringe densities with the low order aberrations not exceeding λ/10. We present measurements of test
surfaces illustrating the effectiveness of the error compensation procedures as well as preliminary measurements of
multiple aspheric surfaces.
The paper deals with the calibration error of unequal phase changes across the interferogram in phase shifting interferometry, i.e., tilt-shift error. For its detection the lattice-site representation of phase shift angles is used. The error can be readily discerned using (N+1) algorithms. Four and five frame algorithms are considered. The influence of experimental parameters on the error detection sensitivity is discussed. Numerical studies are complemented by experimental results.
The Terrestrial Planet Finder Interferometer (TPF-I) concept is being studied at the Jet Propulsion Laboratory and the TPF-I Planet Detection Testbed has been developed to simulate the detection process for an earthlike planet orbiting a star within about 15 pc. The testbed combines four beams of infrared light simulating the operation of a dual chopped Bracewell interferometer observing a star and a faint planet. This paper describes the results obtained this year including nulling of the starlight on four input beams at contrast ratios up to 250,000 to 1, and detection of faint planet signals at contrast ratios with the star of 2 million to 1.
The Planet Detection Testbed developed at the Jet Propulsion Laboratory is being used to test direct optical detection of an Earth-like planet using nulling interferometry. Operating at infrared wavelengths, the testbed produces four near-identical beams simulating a distant star and planet. The testbed is reconfigurable to simulate different telescope array designs that are being studied. Many of the systems which will be needed for the space application of nulling stellar interferometry are incorporated. The goal of the testbed is to simulate the planet detection process which requires both a long detection period of many hours to overcome the thermal background noise and also high instrument stability to avoid introducing noise signals that could be mistaken for a planet. Numerous control systems are needed to maintain the optical path differences to about 2 nm and maintain beam alignments in shear and tilt. The testbed emulates functions of the fringe-tracking and metrology systems envisioned for the flight system including finding and tracking the fringe, controlling vibration and allowing for changing conditions. The relationship of the testbed to flight conditions is discussed and the latest results are presented showing planet detection in the presence of bright starlight.
We present newly developed, compact, self-contained fringe projection interferometer for use with phase step algorithms. The interferometer uses a polarization beam splitter and a computer controlled variable retarder to manage the phase shift. The well compensated path difference permits use of relatively low-coherence sources. The in-line setup contains no mechanical parts and provides a compact, rigid design and nearly perfect common path geometry. The phase shift is realized in a few milliseconds, which enables fast data collection. The interferometer can be used as a sinusoidal fringe projector for topography measurements, or as a shearing interferometer for testing optical elements. Other applications that use phase step technique and require similarly short data acquisition times, versatility and stability are also possible. Examples of surface topography measurements are presented.
A mathematical procedure is described for identifying the modes, or nondiffracting beams, of (alpha) -power GRIN. The general procedure is used to find two novel analytical solutions, which contain as particular cases previously known modes.