2D-LDV Long Range Optics

Some LDV applications, such as measurements in large wind tunnels or large pipes, require a longer focal length than usual to achieve the measuring range. Typically, these applications require focal lengths in the range of 1000 mm and beyond. Using a standard LDV system, even if equipped with a beam expander, results in unacceptably extended measurement volumes of 5-10 mm or more, leading to poor spatial resolution and poor signal quality due to reduced laser intensity. With this Long Range LDV optic, focal lengths of 2 m and more are achievable.

The idea of the optical setup is to maximise the beam distance, in practice up to 400 mm. Of course, such beam distances cannot practically be achieved with a single front lens. Instead, each beam has its own collimating optics to focus the beam within the measurement volume. The receiving optics are positioned in the centre of the beams and operate in backscatter mode. The received light is split into its different wavelengths and coupled into multimode fibres that couple into the photodetectors of an ILA LDV controller or into a BSA.

Both components are shifted with an integrated Bragg cell. The laser sources are two DPSS- Genesis 2W lasers with 514 nm and 488 nm. They are coupled directly into the LDV optics without using single-mode fibres. Thus, almost 1.4 W of laser power per wavelength can be achieved in the measurement volume.