Reflectometers

Author: the photonics expert Dr. Rüdiger Paschotta

Reflectometers are instruments for measuring the reflectance (or reflectivity) of objects, i.e., the fraction of the incident optical power which is reflected. Here, we consider only optical reflectometers, probing the reflection of light (possibly in the infrared or ultraviolet spectral region), and not other kinds of reflectometers e.g. for X-rays, for particles like neutrons or for electronic signals.

Free-space Reflectometers

A free-space reflectometer allows one to measure the reflectance of samples with a free-space light beam. Generally, the reflectance can substantially depend on the angle of incidence on the sample. Therefore, many optical reflectometers allow one to measure the reflectance for a wide range of incidence angles, but some devices work only with a fixed angle, which of course allows the use of a much simpler optical setup without moving parts.

Spectroscopic Reflectometers

Generally, the reflectance also depends on the optical wavelength. Therefore, a reflectometer ideally allows one to measure the reflectance also as a function of wavelength in a wide spectral range. Such a device may be called a spectroscopic reflectometer, which can be considered as a type of spectrophotometer. However, for some purposes it is sufficient to work with a fixed wavelength or with a fixed wavelength interval.

Spectroscopic reflectometers are needed, for example, for characterizing dielectric mirrors and other kinds of dielectric coatings. Here, spectral resolution is important, since e.g. errors in thin-film growth can spectrally shift or distort reflection bands. Similar applications involve thin-film semiconductor devices such as VECSEL gain structures or SESAMs.

Calibration

The calibration of a reflectometer becomes critical when reflectance values very close to unity (perfect reflectance) are to be measured. Refined methods can be used to optimize such calibration procedures; for example, one may use an optical setup where one can alternately measure the optical power of light reflected at the sample and the power of light from the same source getting to the same photodetector through some reference path. For very high reflectance values, however, – for example, in the context of supermirrors – one has to resort to other methods, e.g. the cavity ring-down technique.

Specular and Diffuse Reflections

Reflectometers are often used for specular reflections, but some (called optical backscatter reflectometers) also work with diffusely scattered light. There are other devices for optical scattering measurements, which can distinguish specular reflections from scattered light and provide detailed information also on the latter.

Polarization

For oblique (non-perpendicular) incidence on the sample, the polarization of the test light can also be relevant. An instrument may work either with linearly polarized or with unpolarized light, or possibly with other defined polarization states.

A related technique is ellipsometry, where however one measures not only the reflections, but also the polarization state of the reflected light. Therefore, ellipsometers must contain additional optical hardware for polarization analysis.

Spatially Resolved Measurements

The reflectance of a sample can also depend on the position on the sample. Some reflectometers work with tightly focused light and can measure the reflectance as a function of the position with relatively high resolution.

Another potentially useful feature is the ability to work with samples having curved surfaces.

Fiber-optic Reflectometers

Another kind of optical reflectometers is often used with fiber optics, and particularly in the context of optical fiber communications. These are usually optical time-domain reflectometers, which in addition to the reflectance also provide temporal resolution: one can measure at what time a certain reflection intensity is reached, and that reveals at what location the corresponding reflection occurred. Such fiber-optic devices are very helpful for checking fiber-optic links, particularly for locating faults. See the article on optical time-domain reflectometers for more details.

More to Learn

Encyclopedia articles:

• reflectance
• reflectivity
• optical time-domain reflectometers
• spectrophotometers