What Is a Laser Interferometer?
A laser interferometer is a device that uses light interference to measure distances and shapes with extremely high accuracy.
The laser light output from the built-in light source of the device is divided into two parts by a special mirror, known as a beamsplitter. One beam is directed onto the surface of the sample, while the other beam is reflected by the beamsplitter.
At this stage, there is a difference in the optical path length between the two laser beams. When these beams overlap, the interference pattern changes according to the phase of the waves. This pattern, being sensitive to the optical path length, is also affected by the surface roughness of the sample.
Therefore, by analyzing the interference pattern, the shape of the sample’s surface can be determined. Owing to its high measurement accuracy and non-contact nature, laser interferometers are used in various fields, including industry and scientific research.
Applications of a Laser Interferometer
Laser interferometers are used to analyze the surface topography of solid samples. They are employed in precision-required contexts, such as analyzing the surfaces of camera lenses and contact lenses, and measuring the topography of DVDs and glass.
These devices can measure various surface types, including flat, spherical, and hemispherical, making them versatile in many fields. However, they are less effective for measuring liquid samples or unpolished samples due to susceptibility to external vibrations, fluctuations in surface, and roughness.
Principle of a Laser Interferometer
The operation of a laser interferometer is based on the principle of light interference. In these devices, light from a single laser source is split into two optical paths: the “reference optical path” and the “measurement optical path.”
When the split light recombines, light interference occurs. The detection of changes in brightness due to this interference allows for the measurement of minute variations in the length and shape of the optical path.
Specifically, the brightness alteration corresponds to the phase difference of the light waves. Perfect phase alignment (zero phase difference) results in maximum brightness, while a phase difference of half a wavelength (180 degrees) results in interference cancellation and minimal brightness.
Changes in the length of the measurement optical path create a phase difference with the reference path. Detecting this difference enables the accurate measurement of the shape and movement of the object being examined.
Features of a Laser Interferometer
1. Analyzing Phase Differences Between Two Separate Light Paths
A laser interferometer analyzes a sample’s surface by examining the interference pattern created by the overlapping of two divided light beams within the device. The phase difference between these waves, which occurs when peaks and valleys of the waves align or misalign, significantly influences the resultant wave’s strength.
When the phase difference is an exact multiple of the light’s wavelength, the overlapping waves amplify. Conversely, if the phase difference equals a multiple of half the light’s wavelength, the waves cancel each other out.
2. Capability to Detect Minute Thickness Changes of Less Than 1 µM
Given that the wavelength of light in a laser interferometer is about 630 nanometers, changes in the optical path of just a few hundred nanometers can alter the interference pattern. This sensitivity allows for the detection of surface thickness variations smaller than one micrometer.
3. Essential Vibration Countermeasures
One of the key characteristics of a laser interferometer is its ability to measure surfaces non-destructively, as the sample is unlikely to be affected by the laser beam. However, these devices are sensitive to minor vibrations, necessitating the use of an anti-vibration table to shield them from vibrations and shocks.