Lidar Phase Modulation for Interference Suppression
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Solution Overview
Problem
Existing lidar systems face challenges in achieving low-noise reception of useful signals due to strong interference reflections and scattering, particularly from internal sources and their vicinity, which overwhelm the desired signals and limit spatial resolution and wind speed measurement accuracy.
Innovation Solution
A lidar measurement system employing a continuous-wave laser source with an interferometer arrangement and stepped phase modulation, where the optical path lengths in the measurement and reference branches are matched to suppress interference signals, allowing for almost quantum-noise-limited detection of signals from distant spatial areas.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If internal interference reflections and scattering are suppressed using conventional methods, then signal-to-noise ratio improves, but measurement precision deteriorates due to loss of useful signals
Solution Approach 1:
The patent applies dynamic phase modulation to the reference beam, where the phase of the reference light is continuously varied according to a predetermined phase function. This dynamic modulation allows the system to distinguish between interference signals (which remain static) and useful scattered light signals (which acquire time-varying phase characteristics), thereby suppressing interference while preserving measurement precision.
Solution Approach 2:
The patent changes the phase parameter of the reference beam dynamically over time according to a predetermined phase function. By modulating the phase parameter and subsequently demodulating it during signal processing, the system can selectively enhance useful signals while suppressing static interference reflections and scattering, resolving the contradiction between noise reduction and measurement accuracy.
2Reliability
If optical path length matching is used to suppress interference, then interference signal suppression improves, but device complexity increases due to additional optical components
Solution Approach 1:
Instead of physically adjusting optical path lengths with additional components, the patent changes the phase parameter of the reference beam dynamically. This parameter-based approach achieves interference suppression through phase modulation and demodulation in the signal processing domain, avoiding the need for complex optical path adjustment mechanisms while maintaining effective interference rejection.
Solution Approach 2:
The patent replaces mechanical or physical optical path adjustment methods with electronic phase modulation and signal processing. By using phase modulation in the optical domain and corresponding demodulation in the electrical domain, the system achieves interference suppression without requiring additional mechanical adjustment components or complex optical path matching hardware.
3Length of stationary object
If continuous-wave laser sources are used with long coherence lengths, then measurement range increases, but spatial resolution deteriorates due to inability to distinguish scattered light from different distances
Solution Approach 1:
The patent applies dynamic phase modulation to the continuous-wave laser reference beam, creating time-varying phase characteristics that encode distance information. By modulating the phase according to a predetermined function and subsequently demodulating the scattered light signals, the system can distinguish signals from different spatial locations along the measurement path, achieving both long measurement range and high spatial resolution with a continuous-wave source.
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 flexible and high-resolution detection of particle and object movement with selectable spatial resolution, effectively suppressing interference and enhancing signal quality by converting interference signals into easily manageable electrical DC or narrow-band signals.
Implementation Method 1
Known lidar systems for determining wind speeds are based on the detection of the Doppler frequency shift of the scattered light caused by the scattering of laser light on particles moving with the wind
Implementation Method 2
the Doppler frequency shift is determined interferometrically by superimposing the scattered light received at a measuring device with direct light from the laser source used
Implementation Method 3
continuous-wave laser sources are used, the beam of which is focused on the desired target distance with the aid of a suitable optical system, such as a lens or a telescope
Implementation Method 4
The photoreceiver assembly is adapted to output detector signals representative of light incident thereon emanating from the CW laser source
Data Source
Figure 1
AI summary
The invention relates to a stray-light tolerant lidar measurement system and a method for detecting the presence/movement of particles using a stray-light tolerant lidar measurement system. The measurement system comprises an interferometer assembly having a continuous-wave laser source (2), a photodetection arrangement (7) and optical components The continuous-wave laser source (2) further comprises a laser light generating component (20), a downstream optical phase modulator (21) and a control unit (22) which is connected and adjusted to the optical phase modulator (21), to deliver a control signal, which corresponds to a pseudo-noise signal defined by a predetermined phase function (Θ(t)). In addition, the interferometer assembly comprises an evaluation unit (9) which is coupled and adjusted to the interferometric photodetector assembly (7), to detect the detector signals of same and to determine the presence and/or movement of particles and/or objects from the detector signals wherein, in the case of two wavelengths emitted from the laser light generating components (20), the detector signals are detected separately in two wavelength regions, and wherein the phase function (Θ(t)) has a predetermined almost step-like time course and the evaluation unit (9) scans the detector signals in synchrony with the timing of the steps of (Θ(t)).