optical elements (original) (raw)

Summary: This comprehensive overview explains what optical elements are, categorizing them as the fundamental building blocks of optical systems. The article presents two main ways of categorization: by the physical principle of operation (such as refraction, reflection, diffraction, and interference) and by their purpose in a system (such as focusing, imaging, beam steering, spectral filtering, and polarization control).

Key types of optical elements are discussed, including lenses, prisms, mirrors, diffraction gratings, optical filters, and optical fibers, with links to more detailed articles.

What are Optical Elements?

Optical elements are the fundamental building blocks of optical systems. They influence light (often in the form of light beams) in controlled ways to manipulate its propagation, direction, optical intensity, or spectral composition. While specific types of optical elements are explained in depth in separate Encyclopedia articles, here we give an overview.

See also the article on optical fabrication, i.e., the fabrication of optical elements.

Optical elements are mostly passive; active elements are modulators, for example, where an electrical control signal is applied.

Sometimes, devices are assembled from multiple optical elements, and then called optical elements themselves. For example, Lyot filters are made from multiple birefringent plates.

The article focuses on bulk optical elements, i.e., not e.g. on optical elements realized in photonic integrated circuits.

There is not a single appropriate way to categorize all optical elements. In the following, we categorize them (a) by the used operation principles (such as refraction or diffraction), and (b) by purposes.

Categorization by Operation Principles and Technologies

Refractive Optical Elements

Many optical elements utilize refraction of light at surfaces, often between air and some optical material, or sometimes at an interface between materials — in any case, an interface where the refractive index changes. The most important types are lenses and prisms.

See the article on refractive optical elements for details.

Reflective Optical Elements

Mirrors of various types reflect light. Some have curved surfaces, which allows focusing or defocusing. Reflection may be obtained with dielectric coatings (where interference effects are also important), metal coatings or other means. Mirror sizes range from microns to many meters in large telescopes. Reflection can be nearly complete or partial, e.g. for beam splitters and for output coupler of lasers.
Prisms are also widely used as reflectors, e.g. corner cube prisms for precise retroreflection.

See the article on reflective optical elements for details.

Diffractive Optical Elements

Diffraction gratings utilize diffraction of light at periodic structures. Their angular dispersion is often utilized, e.g. in spectrometers.
Fresnel zone plates act like lenses, but using diffraction rather than refraction.
Photonic metamaterials and metasurfaces often utilize diffraction.
Holographic elements use diffraction inside an optical medium.
Other types of diffractive optical elements are used in micro-optics.

See the article on diffractive optics for details.

Elements Using Interference

Some optical elements exploit interference effects — for example, dielectric mirrors and interference filters.

Elements Using Scattering and Diffusion

Various types of diffusers distribute light based on scattering. They are used for illumination purposes, for example.

Photonic Crystals and Metastructures

Although photonic crystals can be based purely on refraction, they deserve an extra category due to utilized special higher-level physical phenomena. The same applies to photonic metastructures such as photonic metamaterials and photonic metasurfaces.

Nonlinear Optics

Optical nonlinearities can be utilized for nonlinear frequency conversion, for example. One often uses nonlinear crystal materials or optical fibers.

See the articles on nonlinear optics and nonlinear frequency conversion for details.

Fiber Optics

Besides free-space propagation between optical elements, light can also be guided using optical fibers. See the articles on fiber optics and fibers for details.

Categorization by Purpose

Focusing and Imaging

Many optical elements serve focusing and imaging purposes — for example, lenses, objectives, and axicons (for elongated focuses in the form of Bessel–Gauss beams).

Energy Collection and Concentration

In some cases, light needs to be collected and concentrated not for imaging purposes, but e.g. in the context of utilizing solar energy or directing generated light. Examples are Fresnel lenses and parabolic mirrors.

Illumination

Illumination means the controlled distribution of light, e.g. in rooms or in devices like displays. Besides light sources, this requires a wide range of optical elements, such as reflectors, condensers,beam collimators, light guides and diffusers.

Beam Steering and Deflection

Mirrors are the classical beam steering elements, but there are also galvanometer mirrors, MEMS devices with mirrors and various others.

Beam Splitting and Combining

In many cases, beam splitters, e.g. in the form of partially transmissive mirrors or polarizers, are used to split light beams. The same elements can often also be used for beam combining.

Beam Shaping and Homogenization

The optical intensity profiles of light beams often need to be reshaped with various kinds of beam shapers. Profiles can be smoothed and homogenized with beam homogenizers, diffusers and engineered diffractive optics. In some cases, mode control is performed, e.g. using optical resonators.

Wavefront Shaping

Phase corrector plates can apply a fixed but spatially varying correction of the optical phase, e.g. to compensate certain optical aberrations.

Spatial light modulators are active elements, allowing active control of phase changes. Adaptive optics uses controllable deformations, for example in imaging and beam directing applications.

Attenuation and Power Control

Optical attenuators are used for reducing the optical power, often of light beams.

Isolation and Nonreciprocal Control

Optical isolators allow light to pass in one direction but not the opposite direction. Non-reciprocal elements like Faraday rotators are used for such purposes, although reciprocal polarization optics are sufficient in some cases.

Spectral Control

There are optical filters with wavelength-dependent absorption and interference filters, dispersive elements like diffraction gratings and many others, used for controlling the optical spectrum of light.

Polarization Control

Polarization optics includes optical elements used primarily for manipulating the polarization of light. Often, this is done for other final purposes — for example, for realizing optical isolators or for modulating other properties of light such as optical power or optical phase.

See the article on polarization optics for details.

Coupling and Interfacing

One often needs to couple light between systems, such as free-space light beams and fibers or other kinds of waveguides. Examples are fiber collimators, gradient-index lenses and coupling prisms.

Modulation

Various types of optical modulators are used for controlling specific properties of light, such as optical power, optical phase, beam direction, or the polarization. Control is usually done via some input electrical signal.

See the article on optical modulators for details.

seealso: refractive optical elements, reflective optical elements, diffractive optics, polarization optics, nonlinear optics, fiber optics, custom optics, optics fabrication

Frequently Asked Questions

What is an optical element?

An optical element is a fundamental building block of an optical system. It manipulates light in a controlled way, for example by influencing its propagation direction, optical intensity, or spectral composition.

How can optical elements be categorized?

Optical elements can be categorized in different ways, most commonly by their physical operating principle (e.g., refraction, reflection, or diffraction) or by their main function in an optical system (e.g., focusing, beam splitting, or spectral filtering).

What is the difference between active and passive optical elements?

Most optical elements are passive, meaning their properties are fixed. Active elements, such as optical modulators, can have their properties altered by an external control signal, for example an electrical voltage, to dynamically control the light.

What are refractive optical elements?

Refractive optical elements use the principle of refraction, which is the bending of light at an interface between materials with different refractive indices. Common examples are lenses for focusing light and prisms for dispersing or deflecting it.

What are some typical functions of optical elements in a system?

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