Background removal from Raman spectra by an intracavity active-tuning element for a laser
Inventors
Pohl, Ken • King, Matthew • Hopkins, Adam J. • Ford, Alan
Assignees
Interested in licensing this patent?
MTEC can help explore whether this patent might be available for licensing for your application.
Publication Number
US-9905990-B1
Publication Date
2018-02-27
Expiration Date
Abstract
A system, apparatus, and method for multiple wavelength Raman interrogation laser generation and Raman spectra acquisition. An intracavity laser tuning subsystem is integrated into the laser cavity. The tuning subsystem allows switching between at least two laser output frequencies in a manner effective for good identification and separation of Raman spectra from non-Raman spectra, including auto-fluorescence from the sample and background. The tuning subsystem can be implemented in different ways in the cavity. It does not require material alteration of the line-narrowing components. Also, processing of acquired raw signal from the multiple wavelength interrogation can further assist effective Raman spectra identification and separation.
Core Innovation
The problem addressed in Raman chemical detection is that non-Raman signals can dominate, particularly fluorescence and other non-Raman spectra such as Rayleigh scattering. This reduces the ability to extract Raman spectra from collected spectra when interrogating solids, liquids, solutions, or mixtures at stand-off distances. The disclosed approach aims to separate Raman spectra from non-Raman spectra by using multiple laser wavelengths and wavelength-dependent spectral processing.
The invention operates a single non-diode, pulsed laser instrument and generates at least a first and a second wavelength for Raman interrogation. The second wavelength is offset from the first wavelength using a tuning element inside the laser cavity, where the tuning element comprises an intracavity etalon having an adjustably controllable pathlength and an actuator that changes the angle of the etalon relative to the optical path. The wavelength switching is tuned within the laser gain bandwidth, and spectral acquisition is synchronized with the intracavity tuning state switching.
Collected scattered or excited light from the target sample is measured across a spectral range at each wavelength state. The invention detects differences between the collected spectra at the first and second wavelengths to separate Raman spectra from non-Raman spectra, and uses the separated Raman spectra for Raman spectroscopy purposes. The document also describes processing for filtering and gating of raw spectral information and comparison of Raman spectra at multiple laser wavelengths to isolate non-Raman background, including processing based on peak changes between measurements.
Claims Coverage
The document includes one independent method claim. The claim set centers on intracavity etalon-based wavelength switching inside a single non-diode pulsed laser instrument and difference-based spectral processing to separate Raman from non-Raman spectra, with additional optional constraints and processing features in dependent claims.
Intracavity etalon wavelength switching for two-state Raman interrogation
Operating a single non-diode, pulsed laser at a first wavelength and directing it onto a target sample, collecting scattered or excited light across a spectral range, and operating the same laser at a second wavelength offset from the first wavelength using a tuning element inside a laser cavity comprising an intracavity etalon with an adjustably controllable pathlength and an actuator coupled to change the etalon angle relative to the optical path or the distance between etalon parallel plates.
Difference detection to separate Raman from non-Raman spectra
Collecting scattered or excited light across the spectral range during operation at the first and second wavelengths and detecting differences between the collected spectra at the first and second wavelengths to separate Raman spectra from non-Raman spectra, and using the separated Raman spectra for Raman spectroscopy purposes.
Synchronized pulsed laser generation and spectral acquisition in a fixed or portable instrument
Synchronizing pulsed laser generation with spectral acquisition to promote Raman spectral extraction and analysis, and combining the laser generation and spectral acquisition in a fixed or portable instrument, with controller software programmable for switching between first and second states, switching speed, length of time in either state, correlation between type of laser and acquisition, and synchronization with laser generation and spectral signal acquisition.
Multiwavelength gain-bandwidth constrained spacing within Raman extraction
Operating such that each of the first and second wavelength sets includes multiple wavelengths, including at least two laser wavelengths spaced by a specified fraction or multiple of full-width at half maximum (FWHM), with more than two wavelengths and spacing that may be equal or unequal.
Peak-change processing for non-Raman background isolation
Further processing collected data to isolate a non-Raman background by using peak changes between measurements at multiple wavelengths.
Switching with motion-synchronized pulse count differences across wavelength positions
Tuning by switching between two or more wavelengths with motion synchronized such that the number of laser pulses differs between a first and a second wavelength position.
Across the independent claim and its dependents, the central inventive elements are intracavity etalon actuator-based switching of a single non-diode pulsed laser between first and second wavelength states, synchronized pulsed laser generation with spectral acquisition, and difference detection of spectra to separate Raman from non-Raman spectra, with optional multiwavelength spacing constraints, peak-change background isolation, and pulse-count synchronization with wavelength-position switching.
Stated Advantages
Improved gain bandwidth at stand-off distances.
Separates Raman spectra from non-Raman spectra, including fluorescence/background, using differences between spectra at different wavelengths.
Promotes Raman spectral extraction and analysis by synchronizing pulsed laser generation with spectral acquisition.
Enables Raman spectroscopy purposes using separated Raman spectra obtained from wavelength-dependent spectral differences.
Documented Applications
Non-destructive interrogation of solids, liquids, solutions, or mixtures for chemical detection at stand-off distances.
Comparison of known materials and identification of unknown materials in Raman spectra.
Interested in licensing this patent?