Birefringent interferometer

Details Birefringent interferometer Birefringent interferometer

2000-06-30

2002-07-16

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

Spectroscopy

Reexamination Certificate

active

06421131

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is spectral imaging, specifically spectral measurement which utilizes optical interference to determine spectral content, and which is capable of determining the spectral content at every point in a one- or two-dimensional image or scene.
2. Description of the Related Art
Optical interference is widely used in instruments such as the Michelson interferometer, the Mach-Zender interferometer, the Twyman-Greene interferometer, the Sagnac interferometer, and others. These divide incident light into two or more beams traveling along different paths, which are then recombined. An optical path difference is developed between the paths, which results in constructive or destructive interference, depending on the wavelength of light and on the optical path difference (OPD). The intensity pattern resulting from this interference is termed an interferogram. By observing the interferogram while varying the path difference, one can deduce the wavelength of light present in a monochromatic incident beam, or the amount of each wavelength component in a polychromatic beam. This spectrum is obtained by means of a Fourier transform of the intensity signal, for which reason these instruments are often termed Fourier transform spectrometers, or FTS instruments.
The spectral resolving power of an FTS instrument depends directly on the range of OPD that can be produced. Usually, the OPD is varied by mechanical means, such as actuators or piezeo-electric crystals. This involves the motion or rotation of one or more optical elements, such as mirrors or windows, which leads to high cost and complexity given the need to control the OPD variation to a small fraction of a wavelength of light. Other means are also used for varying path length. In some cases an electro-optic material is present in one or more of the optical paths, and the OPD is varied electro-optically. However, this normally results in a limited range of adjustment, as the modulation range of most electro-optic modulators is limited to approximately one wavelength of light. In other cases, one path contains a cell or vessel which may be evacuated or pressurized with gases, to produce an OPD change via the change in refractive index.
It is common practice to incorporate some means for measuring the change in OPD, often via a laser reference beam. In these arrangements, the laser travels along the same paths as the light being analyzed, or travels an equivalent path located adjacent the aperture region, and its output is observed as the OPD is varied. This is especially common in mechanically-varied systems, or in systems which provide a large change in OPD, for which it is otherwise difficult to determine the exact OPD achieved by the system. These systems are generally complex, mechanically delicate, and thermally sensitive.
Most FTS systems provide a spectrum for the incident beam as a whole, and cannot provide spectra for each point in a two-dimensional input scene; thus they are termed non-imaging spectrometers. However, a few imaging FTS systems have been built. Such systems typically sense the interferogram using a pixelated detector such as a CCD or CID sensor. Cabib teaches in U.S. Pat. No. 5,835,214 the use of low-finesse Fabry-Perot interferometers and interferometers which split the incident beam into a finite number of beams, for the purpose of imaging an entire scene at once and obtaining spectral information about each pixel. Cabib describes several interferometers, in U.S. Pat. No. 5,835,214, in U.S. Pat. No. 5,784,162, and in U.S. Pat. No. 5,539,517. Depending on the optical design of the instrument, a given pixel on the sensor may correspond with a given point in the scene being imaged for all OPD settings, or it may not. For example, an instrument is commercially available from Applied Spectral Imaging, in Migdal Haemek, Israel. In this instrument, the relationship between pixel location and scene location varies as the OPD is changed. A careful accounting must be made to determine which pixel corresponds to which image point, for each different OPD, before the spectra can be calculated. A related problem arises because the sensor has discrete pixels: as the OPD is varied, the sensor location corresponding to a given scene location will move from being within a given pixel, to the adjacent region which lies between pixels. This causes image smear, and mixing of the spectral content of adjacent pixels. Further, it is not easy to acquire images of objects that move between successive exposures, since registration error cannot be corrected by a simple Cartesian shift.
Another type of interferometer described by Buican in U.S. Pat. No. 4,905,169 avoids many of the problems of classical interferometers. It uses a photo-elastic modulator (PEM) or equivalent device to imprint a time-varying retardance on a beam of polarized incident light, which then passes through a linear polarizer after which its intensity is measured at a photodetector. By Fourier analysis of the intensity signal, the spectral content of the incident light is determined. This instrument acts as an interferometer, based on polarization. The time-varying retardance is equivalent to an OPD between the components polarized along the ordinary and extraordinary axes of the PEM, and the analysis at the polarizer generates the equivalent of an interferogram.
Buican's instrument offers the benefits of simplicity, absence of moving parts, and ruggedness. Consequently, it can be built more economically than present-day alternatives. However, there are a few severe limitations. PEM devices provide an adequate range of OPD only when operated in resonant mode. This means that the glass or crystal element involved is excited with transducers at or near the frequency of mechanical resonance, which is typically in excess of 10 kHz, and more commonly in the range 50-80 kHz. Since the PEM undergoes an OPD excursion of up to 16 wavelengths, one would need to measure the interferogram intensity a minimum of 32 times per OPD sweep to achieve the Nyquist sampling criterion. Because the OPD is swept twice during each PEM oscillation, a total of 64 readings must be taken in 100 microseconds or less, or 1.6 microseconds per reading. This is possible with high-speed non-imaging detectors such as photodiodes or photomultiplier tubes (PMT), but not with imaging detectors like CCD or CID sensors. These sensors have much slower readout rates, since each readout involves digitizing many pixels. While very short exposure times are possible, the overall time per frame is normally 1000 microseconds or more when acquiring a continuous stream of images. This is 600× too slow to use in Buican's instrument.
Accordingly, Buican teaches that this instrument may be used in flow cytometry experiments as a non-imaging spectrometer. He further teaches in U.S. Pat. No. 5,117,466 that it is possible to combine this non-imaging instrument with optical scanning means, to obtain a two-dimensional image of a scene with spectra at each point. However, the need for scanning means subvert the inherent simplicity and ruggedness that this system offers.
Other spectral imaging systems have utilized band-sequential approaches, where an imaging detector is coupled with a spectral filter. Examples based on acousto-optic tunable filter (AOTF) elements include Lewis et. al., in U.S. Pat. No. 5,377,003, and Chao et. al. in U.S. Pat. No. 5,216,484.
Kaye teaches in U.S. Pat. No. 4,272,195 a wavelength measuring system comprising a single liquid crystal cell which is driven with a varying voltage while the incident light passes through polarizers on either side of the cell, through the cell, and onto a detector. By adjusting the drive voltage, the retardance of the cell is altered, producing a series of maxima and minima at the detector. The maxima and minima are counted to determine the wavelength of quasi-monochromatic incident light. For light with a finite bandwidth, an estimate of the bandwidth is obtained by observing th

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