Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
2001-05-04
2003-10-21
Bruce, David V. (Department: 2882)
Optics: measuring and testing
By light interference
Spectroscopy
C356S450000, C250S339070, C250S339080
Reexamination Certificate
active
06636319
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to spectrometers and, more particularly, to a spectroscopic apparatus that digitizes an analog signal of widely varying range by combining the outputs of two or more ADCs that simultaneously digitize differently scaled versions of the same analog signal to provide a combined set of digitizations that cover the range with enhanced sensitivity and accuracy.
2. Discussion of Related Art
Spectroscopy is the science of identifying a sample's relative degree of transmission, absorption, or reflection over a range of radiation frequencies. Spectroscopy can involve various ranges of radiation, i.e. visible radiation, mid-infrared radiation, ultraviolet radiation, and so on. Everybody is familiar with visible radiation, or visible “light”. Spectroscopy, however, often involves other ranges of radiation. Chemists, for example, often use mid-infrared radiation to determine the molecular content of a “sample” because different molecules absorb different amounts of the frequencies contained in such radiation. Each molecular species has a spectral “fingerprint” in the mid-infrared.
An instrument known as a spectrometer (or spectrophotometer) enables spectroscopic analysis. Earlier “dispersive” spectrometers rotated a dispersing element (grating or prism) through an arc so that all wavelengths within a desired range are presented to a detector. The industry subsequently developed spectrometers that use an interferometer to create a composite signal called an interferogram—a signal containing all frequencies in the entire spectrum—and then analyze the magnitude of each particular frequencies in that composite signal using the relatively complicated but well known mathematics of the Fourier Transform. Such interferometer-based spectrometers are often called Fourier Transform Infrared spectrometers, or simply FTIR spectrometers.
FIG. 1
shows a typical FTIR spectrometer
100
. The device shown includes an interferometer module
110
, a detector
120
, and suitable sampling electronics
130
. The FTIR spectrometers made by the assignee of this patent application use Michelson interferometers, as do others.
FIG. 2
shows a typical Michelson interferometer
110
in more detail. The basic principles and general operation of Michelson interferometers are well known. Nonetheless, the detailed construction of an exemplary Michelson interferometer is set forth in U.S. Pat. No. 3,936,193, the entire disclosure of which is hereby incorporated by reference in its entirety.
As shown in
FIG. 2
, a Michelson interferometer generally comprises a beamsplitter BS (e.g. a half silvered mirror), a fixed mirror M
1
, and a movable mirror M
2
. Light from a source enters the interferometer at point E. The beamsplitter BS is placed at an angle of 45° with respect to the incoming light E such that it splits the light into two components: An “A” component travels through the beamsplitter to the fixed mirror M
1
. A “B” component is refelected by the beamsplitter toward the movable mirror M
2
. Both beams ultimately recombine (A+B) on the other side of the beam splitter, where they travel through a sample, and ultimately are focused onto a detector.
If mirrors M
1
and M
2
where alway at the same distance from the beamsplitter, the relative intensities of the wavelengths in the resultant beam (A+B) would be equal to their relative intensities in the initial beam E. At other unequal distances, the difference in the length of paths A and B will cause phase displacements of the various wavelengths such that, at the varying positions of movable mirror M
2
, each wavelength will optically interfere with itself to a degree proportional to the phase displacement resulting from such movement.
The movable mirror may be positioned at a point where the two mirrors are at an equal distance from the beam splitter, a position known as the Zero Path Difference of ZPD point. At this point, none of the incoming radiation is attenuated by phase displacement and the outgoing beam is at maximum intensity and equal to the incoming beam. During operation, the movable mirror M
2
is scanned back and forth across the ZPD point position, as shown, thereby creating the interferogram by variably attenuating the plurality of wavelengths within the incoming beam E throughout its travel. Alternatively, in a different type of Michelson interferometer, a refractive wedge is interposed between the mirror M
2
and the beamsplitter and moved to accomplish an interferometric scan without actually moving mirror M
2
.
The interferometer-modified radiation is passed through a sample or reflected from a sample, where it is further modified, and finally focused on a light sensitive detector that generates a fluctuating electrical signal, or detector signal, that is proportional to the intensity of the incident radiation.
Analog-to-digital conversion is a vital part of signal treatment because the detector signal has a very large dynamic range (i.e. is comprised of both low-level signals and high-level signals) and because accurate digitization of the low-level signals is necessary for good sensitivity and accuracy. Traditionally, this has resulted in the use of high-dynamic-range digitizers, sometimes combined with gain-switching. However, these prior approaches result in certain other problems.
High-dynamic-range digitizers are expensive, and tend to go out of adjustment over time. In particular, the monotonicity of digitization often degrades with time, especially around the zero voltage point of bipolar digitizers. Unfortunately, the low-level signal portion of instrument signals is usually near this zero voltage point, so this degradation seriously affects instrument performance. In addition, practical circuits generally have some semi-coherent noise resulting from the digital circuitry portion of the digitizer. In traditional digitizers, this noise is especially harmful because it is neither perfectly coherent (in some instruments such as the Fourier-transform infrared spectrometer, velocity variations cause the digitization rate to jitter), nor is it completely random.
Gain-switching is a method for utilizing the full range of a single digitizer during the low-level signal portion of the instrument's measurement, while preventing digitizer overflow during the high-level portion of the instrument's measurement. Gain-switching depends on a switchable-gain amplifier which must switch gain and stabilize during the short interval between digitizations (typically 10 to 30 microseconds in the case of a spectrometer). The design and commercial production of such a switch-gain amplifier is very difficult, and actual commercial realizations exist but have limitations. Maintaining exact gain, phase coherence, low noise, and zero stability over time while supplying an adequately short settling time has proven to be a formidable task.
Accordingly, there remains a need for a spectroscopic method and apparatus of using inexpensive digitizers to digitize an analog detector signal of very large dynamic range, with increased sensitivity and accuracy over that range, without requiring expensive, high-dynamic-range digitizers and without requiring gain-switching.
SUMMARY OF THE INVENTION
In a first aspect, the invention may be regarded as a method of operating an FTIR spectrometer to digitize an analog detector signal comprising the steps of passing an IR source signal through an interferometer; receiving an analog detector signal corresponding to the IR source signal; processing the analog detector signal with a low gain circuit to produce a low gain analog signal; processing the analog detector signal with a high gain circuit to produce a high gain analog signal; digitizing the low gain analog signal with a first ADC converter to produce a set of low gain samples; digitizing the high gain analog signal with a second ADC converter to produce a set of high gain samples, some of the high gain samples being below a predetermined threshold and some of the high gai
Auth Gerald
Haddad Roger
Vidrine Warren
Andras Joseph C.
Artman Thomas R
Bruce David V.
Myers Dawes Andras & Sherman LLP
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