Scanning Laser Sensors

Introduction

A scanning laser sensor measures range from the phase shift between two laser beams. It is a type of range sensor.

How it works

• a beam splitter divides a laser beam of wavelength $\lambda$ into two beams
• the reference beam travels a distance $L$ to a phase measuring device
• the other beam travels a distance $D$ to a reflecting surface and back, so its total path is $D^{\prime }=L+2D$
• the extra distance $2D$ causes a phase shift $\theta$ between the two beams

Key equations

$$\frac{2D}{\lambda }=\frac{\theta }{360^{\circ }}$$ $$\boxed{D=\frac{\theta }{360^{\circ }}\cdot \frac{\lambda }{2}}$$

The solution is unique only when $\theta <360^{\circ }$, which means $2D<\lambda$. If the phase shift is more than one full wave, the device cannot tell how many full waves there are.

Beaming laser sensors

The wavelength of laser light is very small, for example 632.8 nm for a helium neon laser. So the range without ambiguity is very small, and this is not useful for robots.

The solution is to modulate the amplitude of the laser light with a waveform that has a much longer wavelength. Then you measure the phase shift of this longer wave, and $\lambda$ in the equation becomes the modulation wavelength.