What is a Laser Interferometer?

What is a Laser Interferometer? How does it work?

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- GoPhotonics

Feb 4, 2023

A Laser Interferometer is one of the interferometers that employs laser technology and electronic controls to calibrate slip gauges as well as inspect machine parts for straightness, parallelism, and flatness. Laser Interferometry is the technique used in laser interferometers to measure the distance between two objects or the shape of an object with high precision. It is based on the principle of interference, which is the phenomenon that occurs when two waves overlap and combine to create a new wave pattern. 

Construction of Laser Interferometer

The laser interferometer has a fixed unit called a laser head that contains a laser, two photodiodes, and a pair of semi-reflectors. The photodiodes electronically measure the fringe intensity and provide an accurate means for measuring displacement. There is a sliding/ moving unit with a corner cube mounted on it. This corner cube is a glass disk whose back surface has three polished faces which are mutually at right angles to each other. Therefore, this cube will reflect light at an angle of 180o irrespective of the angle at which light is incident on it. The schematic of a laser interferometer is shown in figure 1.

Figure 1: Laser Interferometer

Working of Laser Interferometer

When the laser light falls on the semi-reflector P, it is partially reflected by 90o and then falls on the other reflector S. A part of the light also passes through P and hits the corner cube. The corner cube turns the light by 180o and recombines at the semi-reflector S. If the difference between the two beam paths PQRS and PS is an odd number of half wavelengths, interference will occur at S and a minimum diode output will be obtained. And if the path difference is an even number of half wavelengths, then a maximum output is obtained from the photodiode. That is, each time the moving slide is displaced by a quarter wavelength, the path difference becomes half a wavelength and the output from the photodiode changes from maximum to minimum or vice versa. The sinusoidal output from the photodiode is then amplified and sent to a high-speed counter that is calibrated to give the displacement in terms of millimeters. The second photodiode is used to sense the direction of movement of the slide.

Laser interferometry can be performed with a variety of laser sources, including He-Ne lasers, He-Cd lasers, solid-state lasers, such as Nd:YAG or Nd:YLF lasers, diode lasers, etc. The type of laser used for interferometry depends on the specific requirements of the application, including the level of precision needed, the stability and coherence of the laser light, and the cost and availability of the laser source.

Applications and Advantages

Laser interferometry has a wide range of applications, including testing the shape and surface finish of manufactured parts, measuring the deformation of structures under load, and measuring the distance between objects in space. It is also used in gravitational wave detection, where it is used to measure extremely small changes in the distance caused by the passing of a gravitational wave. They are used for the calibration of machine tables, slides, and axis movements of coordinate measuring machines. 

One of the main advantages of laser interferometry is its high precision and accuracy. It can measure distances with an accuracy of less than a nanometer, making it ideal for many high-precision measurement applications. It is also non-contact, meaning that it can measure the shape of an object without physically touching it, which is useful for measuring fragile or delicate objects.