Differential Absorption Lidar Multiplexed Wavelengths
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional differential absorption lidar apparatuses face challenges in measuring gas concentrations with high precision and high reception S/N ratio, especially when mounted on artificial satellites, due to difficulties in maintaining laser light coherence and stability, and the Doppler frequency shift caused by satellite movement, which affects signal detection and measurement accuracy.
Innovation Solution
A differential absorption lidar apparatus using an incoherent lidar with a continuous wave (CW) system, where two wavelengths are multiplexed and intensity-modulated with distinct baseband frequencies, allowing for concurrent transmission and reception, direct detection of scattered light, and extraction of signal components to determine gas concentration from amplitude or power differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coherent lidar with heterodyne detection is used to achieve high reception S/N ratio, then measurement precision is improved, but device complexity and difficulty of maintaining laser coherence increase
Solution Approach 1:
The patent segments the measurement process by separating the two wavelengths into distinct time slots (time-division multiplexing). The first wavelength is transmitted during a first time slot and the second wavelength during a second time slot, allowing independent optimization of each wavelength's transmission parameters without requiring complex coherence maintenance between them.
Solution Approach 2:
The patent changes the operational parameters by using intensity modulation instead of coherent detection. The modulation frequencies are set to distinct baseband frequencies, allowing direct detection without heterodyne mixing. This parameter change eliminates the need for maintaining optical coherence while preserving measurement precision through differential absorption at the two wavelengths.
2Length of stationary object
If high transmission power laser light with small line width is used for long-distance measurement, then measurement range is extended, but reliability decreases under strict operational constraints
Solution Approach 1:
The patent employs dynamic time-division multiplexing where the transmission parameters (wavelength, power, modulation frequency) can be independently adjusted for each time slot. This allows optimization of transmission power and line width requirements for each wavelength separately, improving reliability under varying operational constraints while maintaining long-distance measurement capability.
3Measurement precision
If Doppler frequency shift compensation is implemented for moving platforms, then measurement accuracy is maintained, but device complexity increases
Solution Approach 1:
The patent extracts and eliminates the Doppler frequency shift problem by using direct detection with baseband modulation frequencies instead of optical carrier frequencies. Since the modulation frequencies are in the baseband (much lower than optical frequencies), the Doppler shift becomes negligible and does not require active compensation, simplifying the system while maintaining measurement accuracy.
4Adaptability or versatility
If incoherent lidar with pulse system is used to reduce laser constraints, then operational flexibility is improved, but reception bandwidth increases requiring large bandwidth receivers
Solution Approach 1:
The patent uses periodic continuous wave transmission with time-division multiplexing instead of pulse transmission. Each wavelength is transmitted continuously during its allocated time slot with intensity modulation, eliminating the need for large reception bandwidth while maintaining operational flexibility. The periodic nature of the modulation allows narrowband filtering and direct detection.
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 enables high-precision gas concentration measurements with improved S/N ratio and reduced impact from Doppler frequency shifts, even when the apparatus is moving, by maintaining consistent irradiation positions and enhancing transimpedance gain in the light receiver.
Implementation Method 1
a first light source (1a) for generating a first CW light signal with a first wavelength (λON) having a large absorption coefficient with respect to a target, and a second light source (1b) for generating a second CW light signal with a second wavelength (λOFF) having a small absorption coefficient
Implementation Method 2
a first light intensity modulator (4a) for subjecting the first CW light signal to intensity modulation with a first modulation signal having a first frequency (fm1) in a baseband, and a second light intensity modulator (4b) for subjecting the second CW light signal to intensity modulation with a second modulation signal having a second frequency (fm2) in a baseband
Implementation Method 3
one wavelength is set to be the one having a large absorption coefficient regarding gas to be measured, and the other wavelength is set to be the one having a small absorption coefficient
Implementation Method 4
differential absorption lidar apparatus for transmitting laser light with at least two wavelengths in the air and receiving scattered light from a target (gas to be measured, such as carbon dioxide, ozone, or water vapor), and measuring the concentration of the target in the air from a difference in intensity of received light regarding each wavelength
Implementation Method 5
a light multiplexer (6) for multiplexing the first and second CW light signals output and subjected to intensity modulation by the first and second light intensity modulators (4a and 4b)
Implementation Method 6
a transmission optical system (8) for forming light signals with two wavelengths output and multiplexed by the light multiplexer (6) in a predetermined beam size and beam shape, and radiating the formed light signals in an air
Implementation Method 7
a reception optical system (9) for receiving scattered light from a target
Implementation Method 8
a light receiver (10) for directly detecting the scattered light output from the reception optical system (9) and converting the scattered light into an electric signal
Implementation Method 9
a signal processor (13) for detecting a concentration of the target from a difference in one of an amplitude and an electric power between time wavelengths of two digital signals, after the two digital signals are extracted by the filters (12a and 12b) respectively
Data Source
AI summary
A differential absorption lidar includes: a light signal generation unit for generating first, second CW light signals with first, second wavelengths having different absorption coefficients with respect to a target, a light intensity modulation unit for subjecting the first, second CW light signal to intensity modulation with a first, second CW modulation signal having a first, second frequency in a baseband, a radiation unit for multiplexing the first, second CW light signals with the intensity modulated, forming the multiplexed light signals with two wavelengths in a predetermined beam size, and radiating the light signals, a reception unit for directly detecting scattered light from the target and converting the scattered light into an electrical signal, and a signal processing unit for extracting only the first, second frequency components from the electric signal, and detecting the concentration of the target from a difference in an amplitude of time wavelengths between two signals.