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A Multiple Axis Interferometer is a type of interferometer that uses multiple beams of light to perform measurements. These interferometers measure two and three axes that can increase accuracy, reduce alignment time, and save space in high-performance multi-axis systems. An interferometer works by splitting a single beam of light into two or more beams, which then travel along different paths before being recombined. The recombined beams create an interference pattern that can be analyzed to determine certain properties of the light, such as its wavelength or phase.
A multiple-axis interferometer, as the name suggests, uses multiple beams of light, each traveling along a different axis, to perform measurements. This allows for more accurate and precise measurements than a single-axis interferometer. The construction of a multiple-axis interferometer can be quite complex and requires the use of precision optics, such as mirrors and beam splitters, as well as a sophisticated system for aligning the beams. The instrument also requires a high degree of stability to ensure accurate measurements over time.
The schematic of a three-axis interferometer is shown in figure 1. This interferometer is a tip-tilt interferometer and has a configuration similar to the Michelson interferometer.
Figure 1: Basic layout of a Three Axis Interferometer
A typical three-axis interferometer setup consists of a light source, two target mirrors, a reference mirror, a beam splitter, and a focal plane array (FPA). The source and the target assembly are arranged in a straight line, with the reference mirror located at a perpendicular angle to the line connecting the source and the target mirrors. The focal plane array is placed at the output of the interferometer.
The three-axis interferometer works by using the three mirrors to control the path of the light and create an interference pattern. The light from the source is split by the beam splitter. One of the beams is directed toward the target assembly, and it is then reflected back from the target mirrors. The other beam from the beam splitter reaches the reference mirror and then reflects the light back toward the beam splitter. Both the reflected beams will form an interferogram at the focal plane array. By analyzing the interference pattern, it is possible to obtain information about the properties of the object being tested, such as its surface shape or the presence of stress and strain in a material sample.
The interference occurs between the return signal from the target mirrors and the reference mirror. The reflective surfaces of the target mirror are arranged to allow simultaneous measurement of the return signal from both mirrors. The interference fringes for each of the target mirrors are created by a slight tilt in the mirrors, and the interference patterns are measured using a focal plane array. The change in tip-tilt and piston between the target mirrors and a reference beam splitter can be calculated by measuring the spatial frequency and phase of the interference pattern.
The interference pattern or images obtained is shown in figure 2. The image on the left side is obtained when the two target mirrors are misaligned and there is a relative tip-tilt between the mirrors. As a result, the fringe patterns between the two mirrors are altered, allowing the two mirrors' relative tip-tilt to be adjusted until they are parallel to one another, as seen in the image on the right side.
Figure 2: Images obtained when target mirrors are misaligned (left) and aligned (right)
The relative phase of the two fringe patterns gives a measurement of the distance between the mirrors after the tip-tilt angles of the two mirrors are aligned. When the mirror moves by one-half the wavelength of the laser, the fringe pattern will vary by one cycle. One can accurately determine the distance by precisely measuring the fringe pattern's phase.
There are several types of multiple-axis interferometers, including:
Each type of interferometer is designed to measure specific physical quantities, such as the surface profile, surface roughness, surface curvature, etc. The type of interferometer used is dependent on the specific application and measurement requirements.
Advantages
One of the key advantages of a multiple-axis interferometer is its ability to make measurements in three dimensions. By using three beams of light that travel along mutually orthogonal axes, a multiple-axis interferometer can measure the position, shape, and size of an object in three dimensions.
This interferometer can perform measurements at multiple angles. By using multiple beams of light that travel along different axes, a multiple-axis interferometer can measure the same object from multiple angles, providing a more complete picture of its properties.
Applications
This interferometer is useful in fields such as metrology, where precise measurements of the size and shape of objects are critical. It is also used in astronomy, where understanding the properties of celestial objects requires observations from multiple angles. A multiple-axis interferometer also has applications in quantum optics. For example, a Mach-Zehnder interferometer, which can be seen as a type of 2-axis interferometer, is a fundamental tool for many quantum optical experiments, such as quantum key distribution and quantum computing. They are also used in spectroscopy, microscopy, and telecommunications.
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