Path Modulation for Terahertz Spectrometer Signal Stability
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Solution Overview
Problem
Spurious thermal and mechanical disturbances cause variations in path length, leading to instability in demodulation signal-to-noise ratio in scanning CW terahertz spectrometers, which affects the accuracy of electromagnetic signal measurement and absorption loss characterization.
Innovation Solution
A system and method that modulates the path length of electromagnetic signals by altering the length of propagation paths using a spool and actuator, allowing for controlled path length differences during signal transmission and reception, and utilizing a processor to calculate the complex index of refraction through Discrete Fourier Transformation of received signal samples.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If path length is kept fixed for stable measurement, then measurement stability is improved, but sensitivity to thermal and mechanical disturbances worsens
Solution Approach 1:
The patent applies dynamics by transforming the static path length configuration into a dynamic, time-varying path length that is modulated at a specific frequency. The optical path length is continuously changed using a spool and actuator mechanism, allowing the system to actively counteract thermal and mechanical disturbances through frequency-domain separation of the modulation signal from low-frequency noise
Solution Approach 2:
The patent implements periodic action by modulating the optical path length at a specific frequency (e.g., 100 Hz) using a spool and actuator. This periodic modulation creates a time-varying path length difference between reference and measurement arms, allowing the system to encode the measurement signal at a frequency that can be distinguished from low-frequency thermal and mechanical disturbances through frequency-domain filtering
2Measurement precision
If path length is modulated to improve signal-to-noise ratio, then demodulation signal-to-noise ratio is improved, but path length stability worsens
Solution Approach 1:
The patent implements periodic action by modulating the optical path length at a specific frequency (e.g., 100 Hz) using a spool and actuator. This periodic modulation creates a time-varying path length difference between reference and measurement arms, allowing the system to encode the measurement signal at a frequency that can be distinguished from low-frequency thermal and mechanical disturbances through frequency-domain filtering
Solution Approach 2:
The patent applies feedback by using the known modulation waveform to demodulate the received signal. The system correlates the received signal with the reference modulation waveform to extract the measurement information, effectively using the modulation itself as a reference signal for coherent detection and noise rejection
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the stability and accuracy of magnitude and phase retrieval in spectrometers by mitigating path length instabilities and suppressing signal amplitude errors, allowing for precise measurement of absorption signatures.
Implementation Method 1
an optical fiber propagation path may be wound about the spool, and the actuator may be configured to alter the diameter of the spool, and thereby alter the length of the respective propagation path
Implementation Method 2
the receiver is configured to receive the electromagnetic signal and another electromagnetic signal for mixing therewith
Implementation Method 3
The processor may be configured to receive a sequence of samples of the received electromagnetic signal, and Discrete Fourier Transformation process the sequence of samples
Implementation Method 4
a modulator configured to modulate the electromagnetic signal transmitted by the transmitter, where the modulator may be configured to modulate the electromagnetic signal at a frequency (e.g., ωm), which may be above the 1/f noise region of the receiver
Data Source
AI summary
A system includes a transmitter is configured to transmit an electromagnetic signal through a sample cell (including a sample medium) to a receiver, which is configured to receive the electromagnetic signal and another electromagnetic signal for mixing therewith. Propagation paths of the signals to the transmitter and receiver include a first propagation path of the electromagnetic signal to the transmitter, and a second propagation path of the other electromagnetic signal to the receiver. The arrangement, which is located along either or each of the propagation paths of signals to the transmitter and receiver, is configured to alter the length of a respective propagation path. And the processor configured to recover an amplitude and phase of the transmitted electromagnetic signal, and calculate a complex index of refraction of the sample medium as a function of the amplitude and phase of the transmitted electromagnetic signal.