We all know that laser diodes are widely used in daily life, so do you know the working principle of common laser interferometers on the market? What are the main functions of the laser diode in it? Let's take a look at it.

Principle of laser interferometer

In fact, interferometer uses the "coherence" of light to measure tiny displacement, angle or deformation.

Light comes from one source and is divided into two paths through the beam splitter, one is reference light and the other is measurement light. After the two beams of light are superimposed again at the detection end, light and dark interference fringes will be formed due to different path differences.

The change of fringe reflects the path difference, that is, the displacement change.

What's the laser diode doing here?

It is the light source of interferometer, which is responsible for emitting coherent light.

Compared with the old He-Ne laser, the laser diode has smaller volume, lower power consumption, faster start-up and more optional wavelengths. At present, many small or portable interferometers use it as light source, such as common semiconductor laser diodes in 650nm, 780nm, 1310nm and 1550nm.

The most important thing of interferometer is the coherence length of light. Generally, DFB or ECL laser diode can provide coherence length of several meters or even tens of meters, which is enough for most ranging and displacement detection. Ordinary multimode LD may not be stable enough, which will lead to blurred stripes, so choose single mode or narrow line width model.

The power stability is high. As long as the driving constant current and temperature control are well done, the output power of the laser diode can be stabilized within 1%. The intensity of interference fringes is naturally stable.

The wavelength is controllable and adjustable, and some systems require the interference fringes to change with the wavelength to realize multipoint measurement or phase demodulation. At this time, the DFB laser diode can adjust the temperature and current to fine-tune the wavelength, which is very convenient.

In the interferometer, the laser diode is like the optical heart of the system, which is responsible for emitting coherent and stable light with controllable wavelength. It is small in size, low in power consumption and convenient in modulation, which enables the original complex laboratory-level interference system to enter the industrial field.