Optical Metrology

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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. 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 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.

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.

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.

Special Challenges

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 they not be used or need to be specially adapted.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 54 suppliers for optical metrology equipment. Among them:

SmarAct

METIRIO is a high-precision optical encoder designed and manufactured for closed-loop nanopositioning to achieve maximum miniaturisation, high resolution and speed. With a readhead volume of just 6.6 × 5.1 × 1.7 mm, it is one of the smallest encoders available, and can achieve resolution of less than one nanometer. The readhead is available unmounted, or on a standard or flexible PCB. Fully packaged and connectorized designs are available, and can be made UHV compatible or fully custom.

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.

HBK FiberSensing

Optical interrogators for accurate and precise measurements of Fiber Bragg Grating signals.

Suitable for large scale sensing networks, HBK 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.

Silentsys

The optical frequency discriminator (OFD) system of SILENTSYS smartly delivers a voltage signal that is proportional to the excursions of optical frequency of the input laser beam. This turn-key module is suitable for laser frequency noise characterization and/or for laser frequency stabilization to drastically reduced its optical full width at half maximum linewidth. The OFD features ultralow noise performances being successful in achieving frequency noise level as low as 0.1 Hz²/Hz with >60 dB noise reduction; and that is achieved in a compact and user-friendly package.

This product is available in a huge wavelength range from UV, VIS to NIR, with one or two optical modules inside to be a very versatile tool.

Enhance your laser with simplicity!

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-ULN^plus. Our patented figure 9^® mode locking technology ensures lowest phase noise and long-term reliable operation.

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