Optical Metrology
Definition: the science and technology of performing measurements with light
More specific term: frequency metrology
German: optische Metrologie
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Author: Dr. Rüdiger Paschotta
Optical metrology is the science and technology concerning measurements with light. Such measurements can either target properties of light and light sources or properties of objects such as dimensions, distances and temperatures. There is no strict boundary between those fields, because often one uses measured properties of light not just to characterize a light source, but for other purposes – for example, optical frequency metrology is used for ultraprecise optical clocks.
Some examples of optical metrology are:
- Optical distance measurements with lasers can be based on, e.g., interferometers or measurements of the time-of-flight of light pulses. This is an example for dimensional metrology.
- Highly precise angular measurements are possible with autocollimators, particularly with electronic autocollimators based on lasers.
- Optical profilometers are widely used for measuring surface topographies, e.g. in semiconductor chip production and for the quality control in optical fabrication. Form metrology also uses various other kinds of instruments for measuring surface shape (contour) and surface roughness.
- Optical time-domain reflectometers are used for inspecting fiber-optic links – for example, finding faulty fiber splices or fiber connectors.
- Optical powers can be measured with photodiodes, thermal power meters, or other equipment. Optical irradiance and other illumination measurements can address either some pure physical quantity such as an optical intensity (power per unit area) (radiometry) or something like a perceived brightness (photometry). Integrating spheres are utilized for radiation emitted in a wide range of direction.
- Spectral optical properties are measured with devices like spectrographs or other spectrometers, wavemeters and self-heterodyne setups.
- Optical frequency metrology deals with high-precision measurements of optical frequencies. One can produce ultraprecise optical clocks, surpassing the performance of cesium atomic clocks.
- Optical temperature sensors may be based on the analysis of the thermal emission of hot bodies, or rely on the measurement of occupation probabilities for energy levels of atoms or molecules.
- Fiber-optic temperature and strain sensors allow for distributed sensing, often of temperature and strain combined. They can be used, for example, for measurements in industrial processing plants, bridges and tunnels, buildings, oil and gas pipelines and power transmission lines.
Typical Qualities of Optical Metrology
In many cases, optical metrology can be extremely precise and is ultimately limited by laser noise or quantum noise in detection.
Optical measurements are usually quite fast and suitable e.g. for in-process metrology, i.e., for monitoring industrial production processes.
Generally, optical measurements are non-destructive. Even very sensitive parts can be checked without touching them (non-contact methods), i.e., without a risk of damage.
Suppliers
The RP Photonics Buyer's Guide contains 45 suppliers for optical metrology equipment. Among them:
Laser Quantum
Laser Quantum’s femtosecond lasers for metrology and frequency combs metrology and frequency combs have enabled optical frequency measurements with 20 significant figures, the highest accuracy demonstrated to date. Ultrafast laser offerings include the taccor, a turnkey femtosecond laser with 1 GHz repetition rates and 15 fs pulses, and the gecco, a low maintenance 84 MHz laser.
TOPTICA Photonics
TOPTICA offers laser sources for various applications and optical metrology, including time and frequency measurements, tests of fundamental constants and tests of fundamental theories, a term interferometry, laser-based trace gas analysis, and the measurement of smallest magnetic fields.
The high precision measurements that are mentioned here require tunable lasers, most of the time with very narrow linewidth and long term stability. In addition, special electronics modules are needed to perform reliable and most advanced laser stabilization. Photonicals – additional laser related accessories – help to characterize or to manipulate the laser light. Many, if not most experiments mentioned here already successfully use our products.
Menlo Systems
As the pioneer in the optical frequency comb technology, Menlo Systems offers a full product line from the compact and fully automated SmartComb to the ultra-low noise optical frequency comb FC1500-ULNplus. Our patented figure 9® mode locking technology ensures lowest phase noise and long-term reliable operation.
HBM FiberSensing
Optical interrogators for accurate and precise measurements of Fiber Bragg Grating signals.
Suitable for large scale sensing networks, HBM FiberSensing interrogators provide precise and high resolution static and dynamic measurements 24/7 through reliable software interfaces.
Compatibility with catman software enables hybrid optical/electrical sensing to be easily managed.
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See also: light, frequency metrology, optical clocks, interferometers, distance measurements with lasers, optical profilometers, laser noise, spectrometers, wavemeters, radiometry, photometry
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