会社のニュース Optical Lenses: An In-Depth Look at Types and Functions

Optical Lenses: An In-Depth Look at Types and Functions

The very name "lens" reveals its light-transmitting nature. Lenses are mostly made of transparent materials. While these materials may be opaque to visible light, they allow light of specific wavelengths to pass through. Therefore, a lens can be considered a light-transmitting device for specific wavelengths. For example, the common CO2 field lens is made of Gallium Arsenide (GaAs), which is opaque to the human eye but acts as a lens for CO2 laser light.

Next, we will delve into the diverse types and functions of lenses. The core function of a lens lies in its refraction of light, enabling the focusing of parallel light and the collimation of point light sources. Lenses come in various shapes, commonly convex and concave lenses. Convex lenses are characterized by being thicker in the center and thinner at the edges, and are divided into concave-convex, plano-convex, and double-convex types. Concave lenses are the opposite, being thinner in the center and thicker at the edges, including double-concave, plano-concave, and convexo-concave types. It is important to note that the classification of convexo-concave lenses can change depending on the degree of their curvature.

In the laser industry, we frequently encounter various types of lenses, such as focusing lenses, collimating lenses, and beam expanders.

Focusing Lenses

A focusing lens is one that focuses a parallel beam into a point light source and is widely used. Additionally, there are special focusing lenses, such as aspheric or achromatic lenses, to meet specific application needs.

• 1.Aspheric Focusing Lenses: These are lenses used to eliminate spherical aberration, including combined aspheric focusing lenses and single aspheric lenses. Spherical aberration refers to the uneven focusing of light due to the spherical shape of a lens; that is, light rays near the center of the lens focus at a different point than rays at the edges. This prevents the entire beam from concentrating at a single point, instead spreading it over a longer distance, which affects cutting quality. To solve this problem, one can use focusing lenses composed of two or three lens elements combined to correct spherical aberration, or use single aspheric lenses. Among these, the single aspheric lens is the best choice but is more expensive. Such combined aspheric lenses and single aspheric lenses were common during the era of YAG cutters, but their use has gradually declined with the popularity of fiber lasers.

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• 2.Single-Element Focusing Lenses: This term is used relative to combined aspheric focusing lenses. A single-element lens consists of one lens piece and has a simple structure. However, as it can only correct part of the spherical aberration, its effectiveness may not match that of multi-element combined focusing lenses. Against the backdrop of increasing adoption of fiber lasers, the use of single-element focusing lenses has gradually decreased, but they still maintain a certain market demand.

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• 3.Spherical Aberration Characteristics of Single-Element Lenses & Comparison with Combined Aspheric Lenses: Single-element focusing lenses, primarily composed of one lens piece, have a simple structure but can only correct part of the spherical aberration, potentially making their performance slightly inferior to multi-element combined focusing lenses. However, they still maintain a certain market demand amidst the trend of growing fiber laser adoption. On the other hand, combined aspheric lenses achieve spherical aberration correction by cleverly combining positive and negative lens elements. Specifically, when a positive lens is combined with a negative lens, and the positive spherical aberration value of the positive lens exactly offsets the negative spherical aberration value of the negative lens, this lens combination can effectively eliminate spherical aberration. This is the unique working principle of combined aspheric lenses.

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• 4.Achromatic Lenses: Due to the differing refractive effects of lens materials on light of different wavelengths, chromatic aberration can be encountered in practical applications. For example, during the coaxial vision alignment in fiber laser marking or welding machines, if a standard lens is used, you might find that when the CCD field of view is clear, the cutting or welding result is not ideal, requiring fine-tuning of the focal position to achieve satisfactory processing. However, while adjusting the focus for the best processing result, the field of view becomes blurry again. This is primarily caused by chromatic aberration. Specifically, convex lenses have a stronger refractive ability for shorter wavelengths and a weaker one for longer wavelengths; concave (negative) lenses are the opposite, having a stronger diverging ability for shorter wavelengths and a weaker one for longer wavelengths. Based on this understanding, lens systems composed of convex and concave lenses can be designed to eliminate the effects of chromatic aberration. It should be noted, however, that due to the relatively smaller demand for achromatic lenses, their price is usually higher.

Collimating Lenses: Principle and Function

A collimating lens, as the name suggests, is a lens that transforms a point light source into a parallel beam. Its working principle is exactly the opposite of a focusing lens. When a point light source is placed at one focal length of a focusing lens, a parallel beam is formed on the other side of the lens. This conversion process is the fundamental function of the collimating lens.

Fiber Collimating Lenses: Application and Adjustment

Fiber collimating lenses play a key role in applications such as fiber cutting heads and fiber welding heads. If certain applications require the elimination of spherical or chromatic aberration, then combined beam collimating lenses can be used to meet this need.

Beam Expanders

Furthermore, beam expanders are common optical components whose function is to magnify the beam. Although both collimating lenses and beam expanders output parallel beams, their working principles and structures differ. A collimating lens takes a point source as input and outputs a parallel beam, and the point source must be placed at one focal length of the lens. A beam expander, however, takes a parallel beam in and outputs a parallel beam out, merely magnifying the parallel beam, and the source position has little influence on it. For specific designs and applications of beam expanders, you can refer to my other articles for a deeper understanding.

Line Generator Lenses: Application

The function of a line generator lens is to transform a parallel beam into a longer line of light, causing it to spread out in a fan-shaped manner. This type of lens has potential application value in detecting product flatness. By turning on the light and scanning the product, any raised or recessed parts will block the light, thereby revealing the product's flatness.

Line Light Collimating Lenses: Application

The line light collimating lens is designed to accurately collimate a parallel beam into a linear parallel light. This process involves using a cylindrical concave lens to diverge the parallel light, and then a cylindrical convex lens whose focal point coincides with the virtual focal point of the concave lens, thereby collimating the beam. Additionally, this type of line light collimating lens can also be used to detect surface flatness, although its specific application might vary depending on the situation.

Wedge Plates: Application

A wedge plate is a lens with an angle between its front and back surfaces, meaning they are not parallel. When a laser passes through such a lens, the beam is deflected at a certain angle. This characteristic is utilized in oscillating welding heads. When the wedge plate rotates, the deflected laser beam also rotates, tracing out a circular pattern, thus forming a ring-shaped spot. By combining two wedge plates, the diameter of this ring can be adjusted. The diameter size depends on the relative deflection angles of the two plates.