Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
2002-04-04
2003-11-25
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Spectroscopy
Reexamination Certificate
active
06654125
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to the field of diagnostic spectroscopy, and more specifically, to a method and apparatus for providing a laser reference for Fourier transform spectrometers. More specifically, the present invention is a subsystem including a vertical cavity surface emitting laser (VCSEL) and selected components making the subsystem useable as a reference laser.
BACKGROUND OF THE INVENTION
In interferometry such as that used for optical spectroscopy, reference lasers are used to provide the ability to obtain digitized interferogram points that are equivalently spaced in position, which is a requirement of Fourier transform algorithms. The industry standard reference is the helium neon laser because of its inherent lasing wavenumber stability and its relatively small size and low cost when compared to other gas lasers.
International Publication WO 00/49690 to Singh et al. and entitled “Compact Wavelength-Independent Wavelength-Locker for Absolute Wavelength Stability of a Laser Diode” discusses the wavelength stabilization of a laser diode by tapping a fraction of the laser diode's output and passing it through a narrow band power splitter to two detectors. The signal from the two detectors is compared, and predetermined control signals are used to maintain a constant lasing wavelength. The need for wavelength stability for wavelength division multiplexed (WDM) transmission systems is disclosed. Application of laser diodes for spectroscopic purposes is not disclosed. In addition, the patent application disclosed wavelength control through the utilization of optical feedback that is obtained from a dedicated diode laser control system. The control of wavelength stability requires significant additional electronics.
International Publication WO 01/20371 to Watterson et al. and entitled “Wavelength Reference Device” describes an apparatus for use in calibrating a tunable Fabry-Perot filter or tunable VCSEL to a precise absolute frequency and maintenance of that frequency using optical feedback derived from a Michelson interferometer. The drawbacks of this method are that the optical feedback system must be included in any application of a VCSEL, and a method of absolute lasing wavelength determination must be available. The desirable low cost feature of VCSELs relative to existing laser reference technology is negated by the described optical feedback system. The VCSEL lasing wavenumber control is provided by an additional optical feedback system. Further, the apparatus of Watterson et al. relies on absolute frequency to maintain a lasing wavenumber.
U.S. Pat. No. 6,069,905 to Davis et al. and entitled “Vertical Cavity Surface Emitting Laser Having Intensity Control” describes the incorporation of a photo detector into a VCSEL package for the purposes of intensity control. This method focuses solely on optical power and intensity regulation and control, which is not critical for the purposes of the application of VCSELs as references for interferometric spectrometers. Davis et al. do not disclose the control and correction of VCSEL lasing wavenumber shifts.
U.S. Pat. No. 5,267,152 to Yang et al. and entitled “Non-invasive Method and Apparatus For Measuring Blood Glucose Concentration” describes the use of solid state lasers as sources of electromagnetic radiation for the non-invasive measurement of blood glucose concentration. Yang et al. do not describe the use of a solid state laser as a wavenumber reference for interferometry. The control of the solid state laser current, voltage and temperature are discussed because the measurement of blood glucose concentration, as described in this patent, is dependent on these parameters.
U.S. Pat. No. 5,933,792 to Andersen et al. and entitled “Method of Standardizing a Spectrometer” describes the use of a standardization sample to determine a characteristic shape, which embodies the difference in response of an instrument over time or between instruments, for absorbance and wavenumber calibration. The limitation of this method is that the characteristic shape is used to correct spectra obtained at later times or on different instruments. The spectra themselves are not inherently correct. The disclosed apparatus does not deal with wavenumber calibration through control and correction of the optical component that determines the spectral wavenumber axis. Instead, it requires a characteristic shape that embodies the spectral differences to correct the spectral wavenumber axis.
In addition, U.S. Pat. No. 5,933,792 discloses a method of standardizing a Fourier transform infrared (FTIR) spectrometer that uses a HeNe laser as its reference. It does not discuss the use of a VCSEL as the reference for the FTIR spectrometer. The HeNe laser has relatively high cost, high power, generates more heat and occupies a large volume relative to a VCSEL. The present invention discloses the method and apparatus of a subsystem or subassembly necessary for successful use of a VCSEL as a reference for an interferometer in optical spectroscopy.
SUMMARY OF THE INVENTION
The present invention is directed to a subsystem for use in interferometry for optical spectroscopy applications which makes possible the use of a vertical cavity surface emitting laser (VCSEL) to serve as an accurate and precise reference laser as an alternative to the industry standard HeNe laser. The present invention offers substantial cost, size, heat and power consumption reductions compared to the HeNe laser. In preferred embodiments, the present invention makes feasible the use of the VCSEL as a reference for an interferometer by incorporating electronics to drive the VCSEL, a photodetector sensitive to the VCSEL output, and an algorithmic wavenumber shift estimation and correction algorithm or method which utilizes a known sample.
A preferred embodiment of the present invention is a subassembly for use in an optical spectroscopy system. The subassembly preferably includes an interferometer having optical components for receiving light and passing light along a defined light path. The optical components preferably include a beamsplitter that separates the light from a source into two portions and means for introducing a pathlength difference between the two portions. A vertical cavity surface emitting laser, including electronics to drive the vertical cavity surface emitting laser and project a beam therefrom is preferably operatively mounted on the interferometer with the beam following the defined light path to act as a reference laser for the interferometer. The interference pattern of the laser is received by a photodetector so that pathlength differences and an accurate digitized interferogram may be constructed for a sample under analysis. The vertical cavity surface emitting laser preferably includes means for temperature control and means for current control connected thereto along with computing means having therein an algorithm for correcting wavenumber drift by the vertical cavity surface emitting laser.
In preferred embodiments, the algorithm for correcting wavenumber drift by the vertical cavity surface emitting laser includes factors derived from spectroscopic analysis of a reference sample utilizing the interferometer and vertical cavity surface emitting laser of the subassembly. At least a portion of the generated spectrum is then compared to a known spectrum for the reference sample. The comparison can include analysis of the relative difference between the generated spectrum and the known spectrum of the reference at selected wavenumbers. Other types of algorithms can be utilized which rely on such methods as employing a derivative-based determination of wavenumber location of spectral features, a center of gravity based determination of wavenumber location of spectral features, an interpolation-based determination of wavenumber of location of spectral features or a wavenumber shift versus wavenumber regression to determine shift correction.
In an alternative embodiment, the algorithm for correcting wavenumber drift by a vertica
Maynard John D.
Ridder Trent
Connolly Patrick
Crompton David
Crompton Seager & Tufte LLC
Inlight Solutions, Inc
Turner Samuel A.
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