A collimated beam is light with weak divergence, meaning it’s a flow of photonics that move in parallel to one another, without dispersing. The beam remains concentrated in a specific direction, and its energy is evenly distributed along its path. This distinct characteristic of collimated beams makes them valuable in different industries, such as scientific research, engineering, and medical applications.
In this article we will discuss how collimated light beams are created, lenses and coatings that can be applied to manipulate a broad spectrum of wavelengths, and practical applications.
How Collimated Light is Created
To achieve collimated light, there are two theoretical methods: a) positioning an extremely tiny source precisely at a distance equal to the focal length of an optical system with a positive focal length or b) observing the point source from an infinitely distant location. In reality, neither of these situations is achievable. As well, according to diffraction theory, even if one of these conditions were met, there would still be a certain degree of spreading or divergence.
To minimize divergence of a collimated beam, two factors must be balanced: focal length of the collimating system and size of the light source. The diagram below demonstrates the approximate divergence of a collimated beam:
In the real world, light is collimated with a collimator device, which essentially is a lens or curved mirror where the focal length or curvature radius is chosen such that the originally curved wavefronts become flat. Of course, the beam radius at the position of the lens or mirror should be large enough to obtain a low divergence. Any residual divergence can be fine adjusted via the position of the lens or mirror along the beam direction. The collimation can be checked, for example, by measuring the evolution of beam radius over some distance in free space with certain kinds of interferometers.
Lenses used in Broad Spectrum Light Collimation
There are different types of lenses used to generate a collimated beam, each with their own disadvantages and advantages. We will discuss a few options below.
Several types of plano lenses exist
- Plano-convex lenses are the most basic. They have positive focal lengths and are close to the optimum shape for use as collimating condensers and as focusing lenses for collimated beams.
- Plano-concave lenses also have one flat surface (plano) and one concave surface. They are often used to diverge or spread-out light beams.
- Plano-plano lenses are lenses where both surfaces are flat (plano). They are used for non-focusing applications, such as beam splitting or beam shaping.
Each type of plano lens has its specific properties and applications, making them suitable for different optical needs. Plano lenses are typically a cost-effective option when your minimum divergence specification can be relaxed, such as systems that do not require the beam to stay collimated for long distances.
Aspheric lenses have a varying curve across the lens, whereas traditional lenses have a circular shape and could be part of a larger circle or sphere. Aspheric lenses tend to be thinner and flatter compared to their traditional lens counterparts.
Aspheres are often used to collimate light that is leaving a fiber or laser diode. The surface of an asphere is designed to eliminate spherical aberration, as spherical aberration is often what prevents a single spherical lens from achieving diffraction limited performance when focusing or collimating light for monochromatic sources.
Achromatic lenses are particularly good for collimating when a broad spectrum of wavelengths is present. They typically consist of two optical components cemented together, usually a positive low-index (crown) element and a negative high-index (flint) element.
Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths, whereas an achromatic doublet brings red and blue light to the same focal point.
An achromatic lens comes in a variety of configurations, most notably, positive, negative, triplet, and aspherized. It is important to note that it can be a doublet (two elements) or triplet (three elements); the number of elements is not related to the number of rays it can correct. In other words, an achromatic lens designed for visible wavelengths corrects for red and blue, independent of it being a doublet or triplet configuration. Below are diagrams outlining the four varieties of achromatic lenses.
Achromatic lenses provide users with the ability to regulate the field of view, collection efficiency, and spatial resolution of their setup. They also enable the configuration of illumination and collection angles, which is beneficial for sampling purposes.
Parabolic and Elliptical Reflectors
When light rays with a specific orientation hit the surfaces of either parabolic or elliptical mirrors, they create a bundle of reflected rays. This bundle converges at a single point known as the focus.
- Parabolas: Light that originates from a point source at the focus is reflected into a parallel ("collimated") beam, leaving the parabola parallel to the axis of symmetry. The same effects occur with sound and other waves. This reflective property is the basis of many practical uses of parabolas.
- The parabola has many important applications, from a parabolic antenna and car headlights, to the design of ballistic missiles and reflector telescopes. It is frequently used in physics, engineering, and many other areas.
- Ellipses: While parabolic reflectors concentrate parallel light rays to a single focal point, elliptic mirrors have two focal points. They collect light rays generated from a primary focal point and direct it to a secondary focal point.
- When a light source is placed at F1 and reflected, it always reaches F2. Additionally, the time it takes for the light to travel from F1 to F2 is the same and can be calculated by dividing 2a by the speed of propagation. This interesting property has various implications. For example, if an explosion occurs at F1, its energy will concentrate at the second focus, F2. This principle is utilized in triggering a hydrogen bomb by igniting an atomic bomb at F1. It is also the principle behind the effective and non-invasive technique for crushing kidney stones.
How Different Lenses Work Together
A collimator lens system, composed of a set of lenses, can be employed to create a collimated beam. In this method, the light source is situated at one end of the system, and the lenses are arranged in a way that they refract and concentrate the light, resulting in a parallel beam. This technique is commonly utilized in laser diodes, telescopes, and other optical devices that necessitate a collimated beam.
To collimate a diverging beam, we can use lenses with different focal lengths. The resulting diameter of the collimated beam increases as the focal length becomes longer. Assuming an initial tight focus and the subsequent expansion of the beam over a long distance, the distance between the focus and the collimation lens should be equal to the focal length. Using this information, the radius of the collimated beam can be determined by multiplying the half-angle of the beam divergence (or more precisely, its tangent) by the distance.
The diagram below shows a beam of initial diameter x1 being shrunk to a final diameter of x2 and the distance between the two lenses is d. If we wanted to expand the beam, the plano-concave (negative focal length) lens would be placed first and have focal length f1, and the plano-convex lens would be placed second and have focal length f2.
A variety of anti-reflective (AR) coatings can be applied to a lens depending on the desired wavelength.
An AR coating reduces the reflection of the light from the surface. AR coatings are used to reduce reflection loss and hence improve transmission efficiency, while at the same time reducing stray light and ghost images.
AR coatings are applied via a series of layers adhered to the front and back of the lenses. These layers block certain wavelengths of light, helping to reduce reflection.
Applications for Collimated Beams
Collimated beams find practical use in various fields such as scientific investigations, laser advancements, sensors, medical imaging, and industrial processes like laser cutting and welding.
Typical applications that require collimated light include:
Analyze light-matter interactions and study substance properties. They are employed in sample illumination, beam alignment, measuring absorption and transmission properties, and studying emission and fluorescence properties.
- Laser Technology
Used for tasks like laser cutting, welding, drilling, and marking. Collimated beams maintain a consistent beam diameter over long distances, enabling precise and accurate laser processing. Collimated beams have diverse applications in laser technology, including beam generation, propagation, manipulation, and beam delivery.
- Optical Communication
Used to transmit data through optical fibers. They ensure efficient data transmission over long distances without significant signal loss. Collimated beams play a crucial role in fiber optic transmission, amplification, free-space optical communication, and optical alignment.
- Medical Imaging (CT and PET Scans)
Used specifically in CT and PET scans. In CT scans, collimated X-ray beams produce accurate and detailed images by acquiring multiple cross-sectional slices of the body. Collimated beams contribute to consistent radiation, improved spatial resolution, artifact reduction, and dose optimization. In PET scans, collimators enhance spatial resolution, sensitivity, and reduce background noise.
- Astronomy (Telescopes)
Vital for proper functioning of telescopes, ensuring focused and sharp images. They are utilized in reflective and refractive telescopes, astronomical imaging, and calibration and alignment of telescopes.
- Industrial Inspection
Aid in industrial inspection by providing precise measurements of product dimensions, surface quality, and defects. They are also used for accurate imaging, analysis, dimensional measurements, non-destructive testing, and laser profiling.
Provide a stable light source for high-resolution imaging with minimal distortion or aberration. They are used for illumination, laser scanning microscopy, and optical tweezers.
Learn More About Collimated Light
Dayy Photonics offers fiber coupled light sources but also free space products with a collimated beam. We can customize our light sources to provide the desired beam diameters and minimum divergence angle.
To learn more about our light source options for collimated output, contact DAYY Photonics to talk about the specialized needs in your application.