Optical filter
Inventors
Spuler, Scott M. • Repasky, Kevin S. • Nehrir, Amin R.
Assignees
NASA Langley Research Center • Montana State University Bozeman • University Corp for Atmospheric Research UCAR • National Aeronautics and Space Administration NASA
Publication Number
US-11243295-B2
Publication Date
2022-02-08
Expiration Date
2036-04-27
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Abstract
A filter for a micropulse differential absorption LIDAR is provided. The filter comprises an etalon including a free spectral range substantially the same as a difference between a first laser wavelength and a second laser wavelength, the etalon further including a finesse providing substantial background noise suppression and substantially constant transmission of the first laser wavelength and the second laser wavelength over a predetermined range of wavelengths, and a first filter having a first filter bandpass selected to include the first laser wavelength and the second laser wavelength.
Core Innovation
The invention provides a filter for a micropulse differential absorption LIDAR comprising an etalon and a first filter. The etalon is designed with a free spectral range that is substantially the same as the difference between a first laser wavelength and a second laser wavelength, and it possesses a finesse that provides substantial background noise suppression while delivering substantially constant transmission of both wavelengths within a predetermined range. The first filter has a bandpass selected to include both the first and second laser wavelengths, thereby allowing effective isolation and measurement of return signals corresponding to those wavelengths.
The problem addressed by the invention relates to limitations in prior LIDAR technologies, particularly the inability to continuously and accurately measure atmospheric water vapor at high vertical and temporal resolutions, especially under challenging conditions such as daytime, cloud cover, and rapidly changing atmospheric conditions. Existing instrumentation suffered from problems like inadequate background light suppression, low efficiency, limited duty cycle due to slow wavelength switching, poor signal isolation, and poor filter performance at higher altitudes.
The core innovation enables a micropulse differential absorption LIDAR to have improved background noise suppression, robust operation across a wider range of atmospheric conditions, and simultaneous or rapid sequential measurement at two laser wavelengths. This is critical for enhancing the accuracy and temporal resolution of molecular species measurements, such as water vapor profiling, without the trade-offs of previous designs. The architecture of the filter, especially the use of an etalon with matched free spectral range and appropriate finesse in combination with a bandpass filter, is pivotal to achieving these performance improvements.
Claims Coverage
The patent contains two independent claims that define distinct inventive features relating to the optical filter and its method of use in micropulse differential absorption LIDAR systems.
Filter comprising an etalon and a bandpass filter for two laser wavelengths
The invention features a filter for micropulse differential absorption LIDAR that includes: - An etalon with a free spectral range substantially matching the difference between a first and a second laser wavelength. - The etalon possesses a finesse that provides substantial background noise suppression and substantially constant transmission at both the first and second laser wavelengths over a predetermined range. - A first filter with a bandpass selected to include both the first and second laser wavelengths. This combination allows selective filtering of return signals corresponding to the required laser wavelengths for accurate measurements.
Method for filtering a LIDAR return signal using the etalon and bandpass filter
The main inventive feature is a method for filtering a return signal from a micropulse differential absorption LIDAR that comprises the following steps: 1. Filtering the return signal using an etalon, where the etalon has a free spectral range substantially equal to the difference between a first and second laser wavelength and a finesse for noise suppression and consistent transmission of both wavelengths. 2. Filtering the return signal received from the etalon using a first filter with a bandpass selected to include both laser wavelengths. This method ensures only the target wavelengths are transmitted for measurement, reducing background noise.
In summary, the independent claims cover a filter architecture and a corresponding filtering method that together enable micropulse differential absorption LIDAR systems to efficiently and accurately isolate dual wavelengths of interest, while suppressing background noise and maintaining measurement fidelity.
Stated Advantages
Substantial background noise suppression is achieved, allowing effective discrimination of return signal photons against a bright background during daytime and cloudy conditions.
The filter provides substantially constant transmission of the first and second laser wavelengths over a predetermined range, supporting robust operation even with laser or etalon fluctuations.
The system allows rapid switching between online and offline wavelengths without the need for tuning, improving duty cycle and enabling accurate measurement during rapidly changing atmospheric conditions.
Provides for improved eye safety, mechanical and thermal stability, and increased operational efficiency for the LIDAR device.
Documented Applications
Micropulse differential absorption LIDAR used for detecting water vapor or other molecular species in the atmosphere.
High-resolution and continuous measurement of water vapor vertical profiles in the lower troposphere and planetary boundary layer for weather forecasting and climate science.
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