Often referred to as Laser Doppler Anemometry or LDA, Laser Doppler Velocimetry (LDV) is a technique to measure flow velocity of a fluid. A non-destructive real-time method related to Particle Image Velocimetry (PIV), a single CW laser beam with high coherence is first split in two and then refocused. The focus region or waist is positioned within the fluid flow which contains natural or seeded reflective particles. The natural coherence of the laser beam generates a series of linear interference fringes in the area it is recombined. Particles passing through the beam waist reflect the light which is refocused into a photodetector and measured. The intensity of the reflected light varies due to the Doppler shift caused by the difference between incident and reflected light, and is therefore proportional to the velocity of the fluid passing through the beam.
The laser beam therefore needs to have a long enough coherence length and a high level of pointing stability to ensure a static interference pattern in the region of the flow. It should also exhibit stable output as the system is measuring small fluctuations in the reflected light. A single longitudinal mode (SLM) laser is ideal provided it is locked to that single mode and does not suffer mode hop which causes sharp output changes that would invalidate results.
The opus and gem lasers are ideal for this technique as the coherence length is around 0.7 cm, covering small scale flow samples, while the torus SLM laser with a coherence length of >100 m is perfect for larger tests such as those in wind tunnels.