Fast switching arbitrary frequency light source for broadband spectroscopic applications
Inventors
Plusquellic, David F. • Douglass, Kevin O. • Maxwell, Stephen E. • Hodges, Joseph T. • Long, David A. • Truong, Gar-Wing
Assignees
National Institute of Standards and Technology NIST • United States Department of Commerce
Publication Number
US-8642982-B2
Publication Date
2014-02-04
Expiration Date
2033-03-14
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Abstract
A fast switching arbitrary frequency light source for broadband spectroscopic applications. The light source may operate near 1.6 um based on sideband tuning using an electro-optic modulator driven by an arbitrary waveform generator. A Fabry-Perot filter cavity selects a single sideband of the light source. The finesse (FSR/ΔνFWHM) of the filter cavity may be chosen to enable rapid frequency switching at rates up to 5 MHz over a frequency range of 40 GHz (1.3 cm−1). The bandwidth, speed and spectral purity are high enough for spectroscopic applications where rapid and discrete frequency scans are needed. Significant signal-to-noise advantages may be realized using the rapid and broadband scanning features of this system in many areas of spectroscopy, e.g., process monitoring and control, reaction dynamics, and remote sensing (e.g., greenhouse gas monitoring, biological/chemical agent screening).
Core Innovation
The invention relates to a fast switching arbitrary frequency light source designed for broadband spectroscopic applications, specifically operating near 1.6 micrometers wavelength. The system uses sideband tuning via a waveguide-based electro-optic modulator driven by a tunable microwave source such as an arbitrary waveform generator, which produces multiple discrete frequencies of light. A selection device, typically a Fabry-Perot filter cavity, selects a single sideband from these frequencies. This configuration enables rapid frequency switching at rates up to 5 MHz over a 40 GHz frequency range, with high bandwidth, speed, and spectral purity suitable for spectroscopy requiring rapid and discrete frequency scans.
The problem addressed is that conventional laser-based methods such as Differential Absorption Light Detection and Ranging (DIAL) suffer from retrieval errors due to measuring only a small number of spectral intervals across an absorption line and from atmospheric changes between on- and off-resonance measurements. Prior art methods for cavity-enhanced or ring-down spectroscopy involve slow frequency stepping and laser stabilization, which are time-consuming and sensitive to low-frequency noise. There is a need for a laser-based method that facilitates maintaining single mode laser operation during tuning, enables fast switching speeds to measure multiple spectral points across broadened lines within minimal time, and improves immunity to noise over scan periods.
The disclosed invention overcomes these drawbacks by providing a laser-based system that enables retrieval of many spectral intervals across a pressure broadened line rapidly enough to complete scans within time intervals shorter than atmospheric changes. It achieves this by combining a waveguide electro-optic modulator driven by a fast tunable microwave source, a Fabry-Perot filter cavity for single sideband selection allowing high-finesse spectral purity with rapid switching, and stabilization of the laser device relative to the selection device. This approach allows for performing cavity-enhanced or ring-down measurements at each cavity mode without requiring laser tuning or lock reacquisition between frequencies, thereby significantly reducing measurement time and enhancing signal-to-noise ratio through rapid broadband scanning.
Claims Coverage
The patent includes multiple independent claims detailing a light source and method for fast switching arbitrary frequency light sources. The claims cover the core components, functional features, and method steps related to rapid frequency tuning for spectroscopy.
Fast switching arbitrary frequency light source with modulation and selection
The light source includes a waveguide-based electro-optic modulator driven by a tunable microwave source having speed substantially commensurate with or exceeding the slower of the modulator's or spectroscopic measurement technique's maximum speed. It further includes a selection device that selects a single frequency component from multiple discrete frequencies generated by the modulator, and a laser device configured to stabilize relative to the selection device.
Splitting device for laser stabilization and frequency sideband generation
A splitting device samples the laser output prior to the selection device splitting it into a lock leg and scan leg. The lock leg has sufficient power to detect reflected power from the selection device and stabilize the laser. The scan leg adds at least one tunable sideband, with only one sideband resonant with the filter cavity mode.
Detection sensitivity improvement by increased optical path length
The system includes a detection sensitivity improvement device that increases total optical path length through a sample volume to improve detection sensitivity, enabling measurement of absorption signals and reference laser power at each microwave tuning step.
Optical interferometer filter cavity with absorbing medium
The selection device can include an optical interferometer such as a Fabry-Perot confocal filter cavity or equivalents (I/Q modulator, Mach-Zehnder interferometer), optionally including an absorbing medium between mirrors, and a cavity enhancement device that selects a single sideband resonant with cavity modes.
Laser servo loop with Pound-Drever-Hall locking
A laser servo loop stabilizes laser frequency relative to the filter cavity, employing Pound-Drever-Hall locking or equivalent frequency modulation methods to maximize throughput and stability.
Method for fast switching arbitrary frequency light source operation
A method comprises providing the waveguide electro-optic modulator, tunable microwave source, laser device, and selection device, driving the modulator, selecting a single frequency component from multiple discrete frequencies, and stabilizing the laser with frequency offset relative to the selection device. It includes splitting the laser output into lock and scan legs, adding tunable sidebands, and using cavity-enhanced spectroscopy measurements such as cavity ring-down or hard target absorption spectrum measurements.
The independent claims collectively cover a fast switching arbitrary frequency light source comprising an electro-optic modulator driven by a fast tunable microwave source, a single sideband selection device (filter cavity or equivalent), laser stabilization means including splitting the output for lock and scan legs, methods for improving detection sensitivity through increased path length, and methods for rapid frequency scanning suitable for broadband spectroscopic applications such as cavity-enhanced measurements.
Stated Advantages
Enables rapid and discrete frequency scans at rates up to 5 MHz over a broad frequency range, reducing measurement time significantly.
Allows retrieval of multiple spectral points across pressure broadened lines in time intervals shorter than changes in atmospheric conditions.
Maintains single mode laser operation during tuning without long lock reacquisition times.
Improves signal-to-noise ratio due to rapid and broadband scanning capability.
Offers high frequency fidelity and spectral purity adequate for sensitive spectroscopic applications.
Demonstrates the best achieved sensitivity and speed for direct absorption and cavity ring-down spectroscopy compared to existing methods.
Documented Applications
Measurement of full direct absorption profiles in sample cells.
Cavity-enhanced or cavity ring-down spectroscopy over path lengths.
Measurement of hard target absorption spectra in atmospheric paths.
Process monitoring and control.
Reaction dynamics studies.
Remote sensing such as greenhouse gas monitoring and biological/chemical agent screening.
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