Influence of laser coherence on reference-matched laser Doppler velocimetry

Area of Science:

• Optical Metrology
• Laser-based Measurement Techniques

Background:

• Laser Doppler Velocimetry (LDV) is crucial for fluid flow measurements.
• Accurate definition of the measurement volume (probe volume) is essential for LDV precision.
• The coherence length of the laser source impacts probe volume characteristics.

Purpose of the Study:

• To investigate probe length influenced by spectral coherence in a focused LDV setup.
• To propose a novel method for defining LDV probe volumes.
• To evaluate the impact of this method on laser coherence requirements for lidar systems.

Main Methods:

• Analysis of probe length under Gaussian and Lorentzian spectra influence.
• Matching focal point and highest interference point in a strongly focused LDV setup.
• Estimation of -3 dB intensity drop isosurfaces to define probe volumes.

Main Results:

• Isosurfaces of -3 dB intensity drop provide a practical definition for probe volumes.
• The proposed method is applied to a lidar setup.
• Reduced coherence length requirements for the laser source are demonstrated in matched vs. unmatched cases.

Conclusions:

• The -3 dB intensity drop isosurface is a viable alternative for defining LDV probe volumes.
• This approach offers flexibility in laser source selection for lidar applications.
• Optimized probe volume definition can enhance measurement accuracy and reduce system constraints.