Bragg cell laser intensity modulation: effect on laser Doppler velocimetry measurements

Area of Science:

• Physics
• Optical Engineering
• Fluid Dynamics

Background:

• Laser Doppler velocimetry (LDV) is a common technique for non-intrusive velocity measurement.
• Acousto-optic Bragg cells are typically used to frequency-shift one beam in LDV systems.
• Intensity modulations from Bragg shifting can introduce errors in velocity readings.

Purpose of the Study:

• To identify and characterize intensity fluctuations in Bragg-shifted laser beams used in LDV.
• To explain the physical mechanism behind these intensity modulations.
• To propose a method for mitigating these spurious effects on velocity measurements.

Main Methods:

• Development of a physical model to describe intensity modulations.
• Analysis of the relationship between laser beam diameter and acoustic wavelength in the Bragg cell.
• Experimental or simulation-based validation of the model and proposed mitigation strategy.

Main Results:

• Bragg shifting can introduce intensity fluctuations at twice the Bragg cell frequency.
• The amplitude of these fluctuations depends on the ratio of laser beam diameter to acoustic wavelength.
• A configuration procedure was developed to minimize these adverse effects.

Conclusions:

• Intensity modulations from Bragg shifting are a significant source of error in LDV.
• Understanding the physical model allows for the minimization of these errors.
• The presented configuration procedure enhances the accuracy of LDV velocity measurements.