Chirped-pulse terahertz spectroscopy
Inventors
Gerecht, Eyal • Plusquellic, David F. • Douglass, Kevin O.
Assignees
National Institute of Standards and Technology NIST • University of Massachusetts Amherst
Publication Number
US-8748822-B1
Publication Date
2014-06-10
Expiration Date
2032-01-10
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Abstract
Terahertz spectroscopy methods that are fast and have excellent spectral resolution and that do not require background correction of the instrument response without sample are disclosed. In one instance, the methods include phase coherent chirp pulse generation and phase coherent detection.
Core Innovation
The invention provides terahertz spectroscopy methods and systems that enable fast measurements with excellent spectral resolution and without requiring background correction of the instrument response without sample. The methods include generating phase coherent terahertz chirp pulses, coupling them to a gas in a gas cell, and detecting the induced Free Induction Decay (FID) emission from the gas in a phase coherent manner using phase coherent detection techniques.
The problem being solved is the limitation of existing terahertz spectroscopy techniques which either have excellent spectral resolution but slow response time, or are fast but lack sufficient spectral resolution, and additionally require background correction of instrument response without sample. There is a need for terahertz spectroscopy methods and systems that combine speed, resolution, and do not require background correction.
Claims Coverage
The patent includes multiple independent claims covering methods and systems for terahertz gas sensing and signal processing. The main inventive features span phase coherent chirp pulse generation and detection, phase coherence maintenance, and system components for effective heterodyne detection.
Phase coherent terahertz chirp pulse generation and detection
The method includes generating phase coherent terahertz chirp pulses, coupling them to gas in a gas cell, and detecting Free Induction Decay (FID) induced in the gas using phase coherent receivers phase locked with a frequency standard reference.
Phase coherent heterodyne detection with frequency switching
The method further comprises selecting terahertz chirp pulse and local oscillator frequencies to detect FID over frequency ranges, switching these frequencies substantially together while maintaining phase coherence to cover extended frequency ranges, and repeating until coverage is complete.
System for chirped pulse terahertz spectroscopy with phase coherent components
The system comprises a terahertz chirped pulse source, gas cell, heterodyne terahertz receiver that is phase coherent with the chirped pulse source, an output component, and a frequency standard reference used for generating the pulses and phase locking the local oscillator in the receiver.
Analysis and control components with phase coherence
The system includes an analysis component that compares detected signals to predetermined spectra for gas component detection and performs phase correction for line shapes. A control component manages pulse and local oscillator frequency selection and switches frequencies maintaining phase coherence during detection.
Quasi-optical coupling and detector types
The system features quasi-optical coupling of FID emission and local oscillator outputs to detectors, heterodyne receivers that can be sub-harmonic with Schottky barrier diode detectors, and hot electron bolometer (HEB) detectors to achieve low noise and efficient detection.
Maintaining phase coherence for averaging and subtraction
The method includes maintaining phase coherence between repeated terahertz chirp pulses and the phase coherent detection signals using frequency standard references, enabling coherent averaging and subtraction of generated terahertz signals for improved sensitivity and signal processing.
The claims collectively cover inventive methods and systems for fast, high-resolution terahertz gas sensing employing phase coherent chirped pulses with heterodyne detection, system components ensuring phase locking and frequency control, and signal processing techniques that improve sensitivity and accuracy without requiring background subtraction.
Stated Advantages
Fast terahertz spectroscopy measurements with excellent spectral resolution.
Elimination of the need for background correction of the instrument response without sample.
Absolute specificity and sensitivity through phase coherent detection and frequency standard referencing.
Capability to maintain phase coherence for long measurement periods, enabling coherent averaging and improved signal-to-noise ratios.
Use of low local oscillator power hot electron bolometer detectors with high sensitivity.
Documented Applications
Detection of toxic industrial chemicals and materials with low false alarm rates for homeland defense.
Human breath analysis for medical diagnosis in clinical and emergency settings.
Greenhouse gas emission monitoring for accurate carbon emission assessment and mitigation.
Ion and radical concentration profiling in chemical vapor deposition and etching for materials engineering and nanofabrication.
Research on biomolecules to improve predictive models for drug efficacy and discovery.
Measurement of solid samples such as nickel oxide to observe anti-ferromagnetic resonance at terahertz frequencies.
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