Laser Interferometry

Summary

Laser interferometry includes many different measurement methods that are all based on the unique interference properties of laser lights. These techniques are used to measure distance, velocity, vibration, and surface roughness in industry, military, and scientific research.

Definition and Basic Principles

Laser interferometry is a technique that is used to make extremely precise difference measurements between two beams of light by measuring their interference pattern. One beam is reflected off a reference surface, and the other either reflects from or passes through a surface to be measured. When the beams are recombined, they either add to (constructive interference) or subtract from (destructive interference) each other to yield dark and light patterns that can be read by a photosensitive detector. This interference pattern changes as either the relative path length or the relative wavelength or frequency of the two beams changes. For instance, the path lengths might vary because one object is moving, yielding a measurement of vibration or velocity. If the path lengths vary because of the roughness of one surface, a “map” of surface smoothness can be recorded. If the two beams travel through different media, then the resulting phase shift of the beams can be used to characterize the media.

Lasers are not required for interferometric measurements, but they are often used because laser light is monochromatic and coherent. It is principally these characteristics that make lasers ideal for interferometric measurements. The resulting interference pattern is stable over time and can be easily measured, and the precision is on the order of the wavelength of the laser light.

Background and History

The interference of light was first demonstrated in the early 1800s by English physicist Thomas Young in his double-slit experiment, in which he showed that two beams of light can interact like waves to produce alternating dark and light bands. Many scientists believed that if light were composed of waves, it would require a medium to travel through, and this medium (termed “ether”) had never been detected. In the late 1800s, German-born American physicist Albert Michelson designed an interferometer to measure the effect of the ether on the speed of light. His experiment was considered a failure in that he was not able to provide proof of the existence of the ether. However, the utility of the interferometer for measuring a precise distance was soon exploited. One of Michelson's first uses of his interferometer was to measure the international unit of a meter using a platinum-iridium metal bar, paving the way for modern interferometric methods of measurement.

Until the mid-twentieth century, atomic sources were used in interferometers, but their use for measurement was limited to their coherence length, which was less than a meter. When lasers were first developed in the 1960s, they quickly replaced the spectral line sources used for interferometric measurements because of their long coherence length, and the modern field of laser interferometry was born.

How It Works

The most common interferometer is the Michelson interferometer, in which a laser beam is divided using a beam splitter. The split beams travel at right angles from each other to different surfaces, where they are reflected to the beam splitter and redirected into a common path. The interference between the recombined beams is recorded on a photosensitive detector and directly correlates with the differences in the two paths the light traveled.

In the visible region, one of the most commonly available lasers is the helium-neon laser, which produces interference patterns that can be visually observed, but it is also possible to use invisible light lasers, such as those in the X-ray, infrared, or ultraviolet regions. Digital cameras and photodiodes are routinely used to capture interference patterns, and these can be recorded as a function of time to create a movie of an interference pattern that changes with time. Mathematical methods, such as Fourier analysis, are often used to help resolve the wavelength composition of the interference patterns. In heterodyne detection, one of the beams is purposefully phase-shifted a small amount relative to the other, and this gives rise to a beat frequency, which can be measured to even higher precision than in standard homodyne detection. Fiber optics can be used to direct the light beams, and these are especially useful to control the environment through which the light travels. In this case, the reflections from the ends of the fiber optics have to be taken into account or used in place of reflecting mirrors. Polarizers and wave-retarding lenses can be inserted in the beam path to control the polarization or the phase of one beam relative to the other.

While Michelson interferometers are typically used to measure distance differences between the two reflecting surfaces, there are many other configurations. Some examples are the Mach-Zehnder and Jamin interferometers, in which two beams are reflected off of identical mirrors but travel through different media. For instance, if one beam travels through a gas and the other beam travels through a vacuum, the beams will be phase-shifted relative to each other, causing an interference pattern that can be interpreted to give the index of refraction of the gas. In a Fabry-Perot interferometer, light is directed into a cavity consisting of two highly reflecting surfaces. The light bounces between the surfaces multiple times before exiting to a detector, creating an interference pattern that is much more highly resolved than in a standard Michelson interferometer. Several other types of interferometers are described below in relation to specific applications.

Applications and Products

Measures of Standards and Calibration. Because of the accuracy possible with laser interferometry, it is widely used to calibrate length measurements. For example, the National Institute of Standards and Technology (NIST) offers measurement of gauge blocks and line scales for a fee using a modified Michelson-type interferometer. Many commercial companies also offer measurement services based on laser-interferometer technology to measure vacuum wavelength, relative deviation from wavelength, the difference between longest and shortest wavelength, and a visual diagram displaying these criteria. Typical services are for precise measurement of mechanical devices, such as bearings, and linear, angular, and flatness calibration of other tools, such as calipers, micrometers, and machine tools. Interferometers are also used to measure other laser systems' wavelength, coherence, and spectral purity. Some organizations offer calibration certifications that fulfill traceability and accreditation requirements for official research.

Dimensional Measurements. Many commercial laser interferometers are available for purchase and can be used for measurements of length, distance, and angle. Industries that require noncontact measurements of complex parts use laser interferometers to test whether a part is good or to maintain precise positioning of parts during fabrication. Laser interferometers are widely used for these purposes in the automotive, semiconductor, machine tool, and medical and scientific parts industries.

Vibrational Measurements. Laser vibrometers make use of the Doppler shift, which occurs when one laser beam experiences a frequency shift relative to the other because of the motion of the sample. These interferometers are used in many industries to measure vibration of moving parts, such as in airline or automotive parts, or parts under stress, such as those in bridges.

Optical Metrology.Mirrors and lenses used in astronomy require high-quality surfaces. The Twyman-Green and Fizeau interferometers are variations on the Michelson interferometer, in which an optical lens or mirror to be tested is inserted into the path of one of the beams, and the measured interference pattern is a result of the optical deviations between the two surfaces. Other industrial optical testing applications include the quality control of lenses in glasses or microscopes, the testing of DVD reader optical components, and the testing of masks used in lithography in the semiconductor industry.

Ring Lasers and Gyroscopes. In recent decades, laser interferometers have started to replace mechanical gyroscopes in many aircraft navigation systems. In these interferometers, the laser light is reflected off of mirrors such that the two beams travel in opposite directions to each other in a ring, recombining to produce an interference pattern at the starting point. If the entire interferometer is rotated, the path that the light travels in one direction is longer than the path length in the other direction, and this results in the Sagnac effectan interference pattern that changes with the angular velocity of the apparatus. These ring interferometers are widely available from both civilian and military suppliers.

Ophthalmology. A laser interferometry technique using infrared light for measuring intraocular distances (eye length) is widely used in ophthalmology. This technique is also referred to as partial coherence interferometry (PCI) and laser Doppler interferometry (LDI) and is an area of active research for other biological applications.

Sensors. Technology based on fiber-optic acoustic sensors to detect sound waves in water has been developed by the US Navy and is commercially available from manufacturers. The Hubble Space Telescope uses fine guidance sensors (FGSs) in its pointing control system. FGSs are white-light shearing interferometers that help direct and point the teliscope in space.

Gravitational Wave Detection.General relativity predicts that large astronomical events, such as black hole formation or a supernova, will cause “ripples” of gravitational waves that spread out from their source. Several interferometers have been built to measure these tiny disturbances in the local gravitational fields around the Earth. These interferometers typically have arm lengths on the order of miles and require a huge engineering effort to achieve the necessary mechanical and vibrational stability of the lasers and the mirrors. Building space-based gravity-wave-detecting interferometers became important in space exploration in the early 2010s. These instruments are not subject to the same seismic instability as Earth-based systems and have higher precision than similar microwave-based technology. The US-Germany collaborative mission Gravity Recovery and Climate Experiment (GRACE) and its follow-on mission (GRACE-FO) featured a laser ranging interferometer (LRI). This marked the first successful use of LIR in space.

Research Applications. Laser interferometers are used in diverse forms in many scientific experiments. In many optical physics applications, laser interferometers are used to align mirrors and other experimental components precisely. Interferometers are also used for materials characterization in many basic research applications, while ultrasonic laser interferometers characterize velocity distributions and structures in solids and liquids. A more recent technology development is interferometric sensors, which monitor chemical reactions in real-time by comparing laser light directed through waveguides. The interference pattern from the two beams changes as the chemical reaction progresses. This technique is often referred to as dual-polarization interferometry.

Careers and Course Work

Basic research on laser interferometry and its applications is conducted in academia, many metrology industries, and government laboratories and agencies. A doctorate is generally required for research careers in new and emerging interferometry methods in academia or as a primary investigator in industry. Graduate work should be in physics or engineering. The undergraduate program leading into the graduate program should include mathematics, engineering, computer, and materials science classes.

For careers in industries that provide or use commercial laser interferometers but do not conduct basic research, a master's or bachelor's degree is sufficient, depending on the career path. Senior careers in these industries involve leading a team of engineers in new designs and applications or guiding a new application into the manufacturing field. In this case, the focus of coursework should be on engineering. Mechanical, electrical, optical, or laser engineering will provide a solid background and an understanding of the basic theory of interferometer science. Additional courses should include physics, mathematics, and materials science. A marketing position in laser interferometry requires a bachelor's degree focused on business. However, a strong background in engineering or physics will set applicants apart. Technical jobs in maintenance, servicing, or calibration of laser interferometers do not require a bachelor's degree. They may involve the assembly of precision optomechanical systems or machining precision parts.

Social Context and Future Prospects

The development of increasingly precise interferometers in the first several decades of the twenty-first century, such as for gravitational-wave measurement, spurred corresponding leaps in mechanical and materials engineering since these systems require unprecedented mechanical and vibrational stability. Laser interferometers are increasingly used to characterize nanomaterials, pushing the limits of resolution of laser interferometers. As the cost of lasers and optical components continues to decrease, the use of laser interferometers in many industrial manufacturing applications will likely increase. They are an ideal measurement system in that they do not contain moving parts, so there is no wear on parts, and they do not mechanically contact the sample being measured.

Active research is conducted in the field of laser interferometric sensors, which have potential applications in the military and manufacturing industries. Commercial applications for laser sensors will open up in surveillance areas as acoustic laser interferometry technology is developed.

Interferometry has many applications in environmental science and the fight against climate change. Oil and gas companies can use sensors for leak and gas detection during drilling. Accurate measurement of the changes on Earth's surface allows scientists to monitor the impact of change efforts. Their ability to capture nanometer-scale measurements allows scientists detailed and early understanding of changes in geological topography.

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