Optics: measuring and testing – By dispersed light spectroscopy – With aperture mask
Reexamination Certificate
2002-02-01
2004-07-13
Lester, Evelyn (Department: 2873)
Optics: measuring and testing
By dispersed light spectroscopy
With aperture mask
C356S323000, C356S330000, C359S236000, C250S339070
Reexamination Certificate
active
06762833
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates in general to spectrum analyzers and in particular, to a spectrum analyzer and encoder employing spatial modulation of radiation dispersed by wavelength.
Radiation spectral analysis is presently carried out in a number of ways. Dispersive and Fourier transform based analyzers are for high resolution and can be used for many different applications so that they are more versatile than existing application-specific instruments and procedures. While these analyzers offer superior spectral performance, they tend to be expensive, slow and are not portable. For most applications, these instruments offer a resolution which is largely unnecessary. Many applications require measurements only at several wavelengths so that most of the data taken over the entire complete spectrum using these instruments is discarded and not used at all for such applications. Such analyzers may also be too large and heavy for many practical applications.
In contrast, a non-dispersive approach to spectral analysis employs interference filters of fixed frequency passbands to perform given analytical functions. To perform the measurement, the light signal containing a number of wavelength components is propagated through one or more interference filters which are characterized by a center wavelength and bandwidth. The non-dispersive approach is advantageous over the Fourier transform and dispersive spectrum analyzers in that the non-dispersive approach is cheaper and measures the minimum amount of spectral data required to perform a given analytical function. However, if the analytical function requires a significant number of filters, the system's signal-to-noise ratio is reduced as the total energy measured in a given filter over time is inversely related to the number of filters. Furthermore, if a spectrum analyzer using this approach is configured for a first application, the filters used in the device may have to be replaced, or the number of filters changed, in order to adapt the analyzer to a second application. Therefore, even though the non-dispersive approach may be cheaper and does not measure unnecessary data as compared to the dispersive and Fourier transform approaches, the present non-dispersive approach has its limitations.
Another type of optical spectrum analyzer, which is best described as a hybrid between dispersive and non-dispersive instruments, is the Hadamard spectrometer. The Hadamard spectrometer includes a spatial light modulator, comprised of a disc made of an opaque material with slots therein that reflect or transmit light, where the slots have uniform transmittance or reflectance, a technique which is generally referred to as a binary amplitude modulation or chopping. A light beam is dispersed according to wavelength onto the disc so that different wavelength components may be reflected by or transmitted through different slots. The disc is rotated about a rotation axis and the slots are selectively spaced at different radii from the axis to form a number of different channels for detecting corresponding wavelength components of the beam. The light reflected or transmitted by all of the channels are directed to the same detector. In order to differentiate the intensity of the wavelength component transmitted or reflected by a slot for one channel from that of another wavelength component for another channel, the disc is sequentially stepped through an application specific number of steps, each step defining a binary pattern, which defines one equation in a system of simultaneous equations for the amplitudes of the wavelength components. This set of simultaneous equations is then solved to yield the intensity for each channel prior to any specific analytical function, an approach which is cumbersome and time consuming.
None of the above approaches is entirely satisfactory. It is, therefore, desirable to provide an improved spectrum analyzer where the above-noted disadvantages are avoided or significantly diminished, in particular, where the encoding and demodulation are both generalized and significantly simplified such that the details of the spectrum analyzer can be rendered to a single application specific hardware component.
SUMMARY OF THE INVENTION
This invention is based on the recognition that system performance can be greatly improved by replacing the slots in the Hadamard disc by dispersed radiation filters that modulate the amplitudes of the wavelength components as digitized replicas of smooth functions which render the encoded wavelength components substantially orthogonal to one another. In this manner, the time consuming practice of solving a system of simultaneous equations is unnecessary to deconvolve the encoded signal. For this purpose, the modulation functions of the dispersed radiation filters are such that they cause the amplitudes of given modulated wavelength components to change between three or more distinct levels of contrast as the modulator is rotated about an axis, the preferred configuration having the largest number of distinct levels of contrast that is practical.
In many practical embodiments, digitized versions or replicas of the smooth functions may be used instead of analog ones while retaining most of the advantages of the invention. The small interference effects caused by digitization can either be ignored, or accounted for and corrected as required by the application. The modulation functions of two dispersed radiation filters for two different channels are considered to be substantially orthogonal to each other when it is possible to differentiate the contribution to the detector signal caused by the wavelength component in one of the two channels from that caused by the wavelength component in the other channel without having to solve a simultaneous system of equations. The definition also allows for a finite number of corrections to compensate for the effects of digitization. As explained below, for different applications, this definition may be applied differently. In this manner, the time required to decode the amplitude coefficients is greatly reduced. Furthermore, the modulators can be made with filters specifically adapted for certain analytical functions. If modulators with different characteristics are called for to perform a different analytical function, it is a simple matter to replace the modulator by one with the desired properties without altering the system apparatus. Another advantage of the invention lies in that the dispersed radiation filters of a modulator are at fixed spatial relationship to each other, so that wavelength calibration is reduced in determining the alignment of the dispersed image onto the radius of the disc.
One embodiment of the spectrum analyzer of this invention according to one aspect of the invention comprises a source providing radiation, where the radiation includes a plurality of selected wavelength components. Radiation from the source is collected and focused to form an image dispersed by wavelength onto a plane. A two-dimensional spatial modulator positioned in the plane modulates amplitudes of the wavelength components to encode each component, so that the amplitude of each encoded component is a function of time. Each of the functions has three or more distinct levels of contrast. The encoded beam is collected and directed towards a detector and signals generated by the detector are analyzed. Preferably, at least two of the modulation functions for encoding two corresponding wavelength components are substantially orthogonal, so that the amplitudes of the two encoded components present in the total detector signal may be distinguished from each other without solving a system of simultaneous equations.
Another aspect of the invention and useful for the above-described spectrum analyzer is a two-dimensional spatial radiation modulator adapted to be rotated about an axis. The modulator has a plurality of dispersed radiation filters at different radii from the axis for modulating amplitudes of different wavelength components of an incide
Advanced Photometrics, Inc.
Lester Evelyn
Parsons Hsue & de Runtz LLP
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