Optical Metrology

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 of 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. Free-space reflectometers are used e.g. for characterizing thin-film optical devices.
• 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 directions.
• 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.

Optical metrology uses a wide range of measurement instruments. For calibrating those, special calibration light sources are required, providing light with well-defined properties like optical power, luminance or wavelength, for example. For example, there are certain spectral lamps providing quasi-monochromatic light with a precisely defined wavelength.

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.

In high-level optical metrology, traceability is a fundamental concept. It ensures that a measurement result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. For example, a high-precision wavemeter might be calibrated against a stabilized reference laser (e.g., an iodine-stabilized HeNe laser), which in turn is linked to a primary frequency standard (an atomic clock). This chain connects practical industrial measurements to the definition of SI units, ensuring global consistency and comparability of results.

Obviously, optical metrology becomes particularly challenging when extremely high precision is required. However, the magnitude of that challenge also depends on the circumstances. For example, particularly sophisticated metrology is required for characterizing very large optics. Some traditional techniques can then not be used or need to be specially adapted.