FIELD OF THE INVENTION

This invention relates to the field of spectroscopy and more particularly to a compact Raman spectroscopy system and method for providing enhanced quantitative analysis for process control

RELATED PATENTS

This invention disclosed, herein, is related to that subject matter recited and taught in U.S. Pat. No. 10,359,313, the contents of which are incorporated by reference, herein.

BACKGROUND OF THE INVENTION

Raman spectroscopy is a well-known technique that can be used to observe vibrational, rotational and other low-frequency modes in molecules. Raman scattering is an inelastic process whereby monochromatic light typically provided by a laser interacts with molecular vibrations phonons or other excitations resulting in the energy of the laser photons being shifted up or down. Due to conservation of energy the emitted photon gains or loses energy equal to energy of the vibrational state.

Many Raman measurements suffer from fluorescence, which forces usage of longer wavelength (lower energy) excitation lasers to mitigate against the fluorescent signal overwhelming the Raman signal, thereby making the latter impossible to extract. Usage of longer excitation wavelengths facilitates extraction of Raman signals from fluorescent samples but at a cost of reduced sensitivity in the silicon CCD detectors that capture spectrometer signals.

Known Raman probes that capture a Raman spectra covering the entire range of wavenumbers from 0 cm−1to 4000 cm−1can be accomplished by use of:

(1) a single laser source with a large spectrometer incorporating long sensors to capture photons at all relevant wavenumbers with sufficient resolution;

(2) a single laser source with multiple spectrometers/detectors that each cover a different wavelength range (e.g. silicon and InGaAs);

(3) multiple laser sources with a single spectrometer, or

(4) multiple laser sources with separate spectrometers configured to capture multiple Raman spectra each covering a smaller range of wavenumbers.

Examples of the use of multiple laser techniques are disclosed by:“Novel Pressure-Induced Molecular Transformations Probed by In Situ Vibrational Spectroscopy”, Yang Song,“Applications of Molecular Spectroscopy to Current Research in the Chemical and Biological Sciences”, Mark T. Stauffer (ed.), Oct. 5, 2016. Chapter 8,“Spatially Compressed Dual-Wavelength Excitation Raman Spectrometer,” J. B. Cooper, S. Marshall, R. Jones, M. Abdelkader, and K. L. Wise, Applied Optics, 53, 3333 (2014);“Dual Wavelength Raman Spectroscopy: Improved Compactness and Spectral Resolution,” J. Kiefer, https://www.americanpharmaceuticalreview.com/Featured-Articles/354604-Dual-Wavelength-Raman-Spectroscopy-Improved-Compactness-and-Spectral-Resolution/ posted 16 Oct. 2018.“Raman Fusion Spectroscopy: Multiwavelength Excitation for Compact Devices, J. Kiefer, SciX 2019. (13-18 Oct. 2019).“Apparatus and Method for Composite Raman Multispectral Spectrometry”, BRUNEEL, Jean-Luc, BUFFETEAU, Thierry, DAUGEY, Nicolas, RODRIGUEZ, Vincent, WO2019220047 (2019) andU.S. Pat. No. 10,359,313, which is assigned to the Assignee of the Assignee of the instant application and whose contents are incorporated by reference, herein.

Each of the referred-to references focuses on capture of multiple Raman spectra using a dual laser configuration. However, the references fail to disclose the means for selecting wavelengths used in the spectral analysis based on the material selected and the characteristics of the spectrometer utilized.

Hence, there is a need in the industry for a compact Raman probe and spectrometer system that provides for improved quantitative analysis using two or more probe laser wavelengths and a method for selecting the laser probe wavelengths to enhance a quantitative analysis of a target material under investigation for different applications.

SUMMARY OF THE INVENTION

The Raman spectral concatenation concept described herein allows use of a single, relatively compact spectrometer to collect both Fingerprint and Stretch Raman spectra and a method for selecting the laser source wavelengths to provide for enhanced quantitative analysis of target material under investigation. In accordance with the principles of the invention, the Fingerprint spectrum is captured using one excitation wavelength, whereas the Stretch spectrum is captured using a second wavelength selected based on a quantum efficiency of the spectrometer and the first wavelength. The selection of one or more of the laser source wavelengths in the manner disclosed, herein, provides for enhanced signal to noise ratio so as to enhance the performance of a quantitative analysis of a target material under investigation.

A compact dual-wavelength Raman probe configured to provide two separate laser wavelengths selected in a manner to provide enhanced quantitative analysis of material under investigation is disclosed

Described herein are embodiments in which two laser sources may be integrated within the housing of a Raman probe and embodiments in which two laser sources are utilized, wherein the wavelengths of the laser sources or probes are selected in a manner based, in part, on the quantum efficiency of the spectrometer used for the analysis of the light reflected by the target material.

In accordance with the principles of the invention, the wavelengths of the probe lasers used in a dual-wavelength Raman probe are selected based on the quantum efficiency of a spectrometer comprising a single detector array (silicon, InGaAs, or any other detector array) within the spectrometer.

In accordance with the principles of the invention, the probe laser wavelengths may be determined for different applications based on the quantum efficiency of the spectrometer and the material in a target object such that a desired Raman spectra is substantially coincident to a peak of the detector quantum efficiency and, hence, achieving higher signal to noise ratio.

In accordance with the principles of the invention, the selection of Raman excitation wavelengths based on the quantum efficiency of the spectrometer allows shifting of both the Fingerprint and Stretch regions of the Raman spectrum to wavelengths at which silicon detectors (or similar detectors) have relatively high quantum efficiency.

In one aspect of the invention, Raman spectra of each of the two laser sources may be captured separately and subsequently concatenated, or stitched together, to provide a single spectral scan encompassing the entire range of data, including the Fingerprint and Stretch regions, wherein the signal-to-noise ratio of the Raman signal in the Stretch region is enhanced.

In accordance with one aspect of the invention analyzing each data set independently is also possible, while collecting the spectra from both excitation wavelengths simultaneously may also be possible.

The compact dual-wavelength Raman probe, disclosed herein, may include optics to configure the output beam of each laser source to have an elliptical cross-section, approximating a shape of the elongated emission region of the laser near-field rather than a circular cross-section. The laser beams are transmitted to a target object or material under investigation and the resultant scattered signal light is transmitted by the compact Raman probe via an optical beam that may also have a corresponding elliptical cross-section. An optical fiber incorporating a core having dimensions approximating those of the returned scattered light beam, transmits the returned scattered light to the entrance aperture of a spectrometer.

In accordance with the principles of the invention of the dual-wavelength Raman probe disclosed herein, the dual-wavelength Raman probe may include external cavity lasers (ECLs) that may be integrated into the probe as wavelength-stabilized laser sources. See, for example, U.S. Pat. No. 9,059,555, “Wavelength-Stabilized Diode Laser,” the contents of which are incorporated by reference herein. Or may retain the ECLs external to the Raman probe.

In accordance with the principles of the compact Raman probe disclosed herein, a distributed Bragg reflector (DBR) or distributed feedback (DFB) lasers may comprise a wavelength-stabilized laser source that may be integrally incorporated into the Raman probe or may be retained external to the Raman probe.

In accordance with the principles of the compact dual-wavelength Raman probe disclosed herein, the light emitted by a laser may be used as the pump source for a non-linear optical (NLO) conversion to produce a different wavelength, e.g., by second-harmonic generation (SHG), third-harmonic generation (THG), or any other non-linear optical process.

In accordance with the principles of the invention of a compact dual wavelength Raman probe, the selection of a second wavelength is based, in part, on a first wavelength and a spectral efficiency of the spectrometer used to collect the Raman signal.

