The Ultimate Guide to Choosing achromatic cemented lens

An achromatic lens is a type of optical lens designed to limit the effects of chromatic and spherical aberration. Chromatic aberration occurs when different wavelengths of light are refracted by different amounts, causing a failure to focus all colors to the same convergence point. This results in a blurred image with color fringes around the edges. Achromatic lenses are engineered to bring two wavelengths, typically red and blue, into focus in the same plane, thereby significantly reducing chromatic aberration.

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Achromatic lenses are usually made by combining two types of glass with different dispersion properties:

1. Crown Glass: A type of glass with low dispersion.
2. Flint Glass: A type of glass with high dispersion.

These two or more elements are cemented together to form a doublet lens. The combination of these materials helps to counteract the dispersion of light, effectively minimizing chromatic aberration.

Structure and Principle

A Positive Achromatic Lens is usually a doublet, made up of a positive low-refractive index element (such as crown glass) and a negative high-refractive index element (such as flint glass). This combination allows the chromatic aberration of one lens to be neutralized by the other, achieving the correction of chromatic aberration.

Applications

These lenses are widely used in fluorescence microscopy, image relaying, detection, and spectroscopy, among others. They provide almost constant focal lengths across a broad wavelength range, and compared to single lenses, they produce smaller light spots and clearer imaging.

Advantages

• Chromatic Aberration Correction: Effectively focuses two principal wavelengths of light, significantly reducing chromatic aberration.
• Improved Image Quality: Delivers clearer imaging and finer light spots compared to single lenses.
• Diverse Coating Options: Offers a selection of coatings such as VIS, NIR, SWIR to suit various application needs.

Manufacturing and Materials

Creation of Positive Achromatic Lenses involves the precise bonding of two selected materials, commonly N-BK7 and SF5 glass. The lens design parameters including radius of curvature, center thickness, and others are meticulously calculated to ensure optimal optical performance.

Typical Specifications (Example)

• Diameter: 50.80mm
• Effective Focal Length (EFL): 150.00mm
• Coating: Anti-Reflective Coating AR@400-700nm
• Materials: N-BK7/SF5
• Back Focal Length (BFL): 140.40mm
  Radius of Curvature (R1/R2/R3): 83.20mm, -72.10mm, -247.70mm respectively
• Center Thickness (CT): 15.00mm
• Surface Quality: Ranges from 40-20 to 60-40 depending on specifications

With precision imaging capabilities and chromatic aberration correction, Positive Achromatic Lenses are indispensable components in advanced optical systems, particularly in applications where image quality is of paramount importance.

Negative Achromatic Lenses are specially designed optical lenses for correcting chromatic aberrations, typically made by bonding two different types of glass materials'a low refractive index crown glass and a high refractive index flint glass. Unlike their counterpart, the Positive Achromatic Lenses, negative achromatic lenses primarily function to disperse, not focus, light rays.

Structure and Working Principle

The negative achromatic lens consists of a positive-dispersion crown glass lens paired with a negative-dispersion flint glass lens. The design aims to counteract the chromatic aberration produced by one lens with that produced by another, thus effectively correcting chromatic aberration. These lenses play a crucial role in various optical systems requiring light to diverge.

Application Fields

Negative achromatic lenses have a wide range of applications in optics, such as laser beam expanders, optical relay systems, and more. They offer a stable diverging angle across a wide wavelength and can produce a smaller and clearer spot and image compared to single lenses.

Advantages

1. Effective Chromatic Aberration Correction: The lens can disperse light rays of different wavelengths onto the same plane, significantly reducing chromatic aberration issues.
2. Superior Imaging Quality: Compared to single lenses, negative achromatic lenses provide clearer imaging quality and produce smaller light spots.
3. Diverse Configurations: Depending on different usage requirements, lenses can be configured with various coating options suitable for visible light, near-infrared (NIR), short-wave infrared (SWIR), and other wavelengths.

Manufacturing Materials

In production, negative achromatic lenses usually employ materials like N-BK7 and SF5. Lens manufacturing involves meticulous design of many parameters, such as the radius of curvature, center thickness, and edge thickness, to ensure optimal optical performance.

Typical Specifications

• Diameter: 50.80 mm
• Effective Focal Length: -150.00 mm
• Coating: Enhanced reflectivity coating for the 400-700 nm band
• Materials: Typically N-BK7 and SF5 glass
• Back Focal Length: -140.40 mm
• Radius of Curvature: R1 -83.20 mm, R2 72.10 mm, R3 247.70 mm
• Center Thickness: 15.00 mm
• Surface Quality: Varies from 40-20 to 60-40

Overall, negative achromatic lenses play a vital role in optical systems that require high precision diversion of light and correction of chromatic aberrations.

Achromatic Triplet Lenses represent an advanced optical technology specifically designed for the effective correction of chromatic aberrations and other types of optical anomalies. These lenses are composed of three distinct lens elements, typically two elements made of high refractive index materials encasing one made of a lower refractive index material. This arrangement not only significantly reduces aberrations, including distortion and spherical aberrations, but also provides clear, high-quality imaging results.

Structure and Working Principle

Achromatic Triplet Lenses usually feature a symmetrical three-element design, consisting of two high refractive index glasses (such as crown glass) and one low refractive index glass (like flint glass) bonded together through a precise adhesion process. This structural layout enables the lens to efficiently correct chromatic aberration and further reduce aberrations, such as pincushion distortion and spherical aberration, through its symmetry.

Application Areas

With their excellent imaging properties, Achromatic Triplet Lenses are extensively used in fields that demand high-quality imaging. These include fluorescence microscopy, spectroscopy, surface inspection, and life sciences imaging, among others. The lenses are capable of providing excellent color correction and high-resolution image quality across a wide wavelength range.

Advantages

1. Chromatic Aberration Correction: The Achromatic Triplet Lenses can precisely adjust light of different wavelengths to the same focal plane, significantly reducing the occurrence of chromatic aberrations.
2. Reduced Aberrations: Thanks to the ingenious symmetrical design and precise manufacturing processes, distortions such as pincushion distortion and spherical aberration are effectively controlled and minimized.
3. High-Resolution Imaging: These lenses offer high-definition and high-quality imaging solutions for a variety of precision optical applications.

Manufacturing Materials and Processes

The production of Achromatic Triplet Lenses involves the precise bonding of lenses made from different types of materials. Typical lens materials include traditional optical glass, ultraviolet-grade fused silica (JGS1), infrared-grade fused silica (JGS3), and calcium fluoride (CaF2), among others. Key lens parameters, such as the radius of curvature, central and edge thickness, are meticulously designed to ensure optimal optical performance.

Typical Specifications

• Manufacturing Materials: Various, including optical glass, ultraviolet-grade fused silica, infrared-grade fused silica, and calcium fluoride.
• Dimensional Tolerances: Typically, ±0.03mm for standard factory specifications, with precision manufacturing achieving up to ±0.01mm.
• Center Thickness Tolerance: ±0.03mm as the standard factory specification, with manufacturing limits reaching ±0.02mm.
• Radius of Curvature Tolerance: ±0.3% as the standard factory specification, with manufacturing limits reaching ±0.2%.
• Surface Quality: Achieving a 20-10 level under factory standards, improving to a 10-5 level for higher demands.
• Irregularity: The common standard is 1/5 Lambda, with the limit for higher demands being less than 1/10 Lambda.
• Centration Deviation: Under normal factory conditions, centration can be controlled within 3 arcminutes (Arcmin), with manufacturing limits tightening to 1 Arcmin.

Achromatic Triplet Lenses play a crucial role in modern optical systems, especially in applications requiring high-precision imaging and chromatic aberration correction. Their high-quality design and manufacturing make them the preferred choice for many advanced optical applications.

Aspheric Achromatic Lenses merge the advantages of both aspheric and achromatic lenses, creating a sophisticated optical component. This unique combination allows them to deliver exceptional image quality and precise chromatic aberration correction.

Structure and Working Principle

These lenses are typically composed by bonding together two lenses: one achromatic lens and one aspheric lens. The design of the aspheric lens is aimed at mitigating the wavefront errors produced by traditional spherical lenses, thereby achieving more accurate image quality, reducing the RMS spot size, and approaching the diffraction limit.

Manufacturing and Material Selection

Commonly, these lenses are made from photosensitive polymers and glass optical components, with the polymer applied to one surface of the bonded lens pair. This method not only enables the lenses to be manufactured quickly within a short timeframe but also offers flexibility similar to traditional multi-element assemblies. However, the working temperature range of Aspheric Achromatic Lenses is quite narrow, restricted from -20°C to +80°C, and they are not suitable for Deep Ultraviolet (DUV) spectral transmission.

Key Advantages

1. Chromatic Aberration Correction: They effectively correct chromatic aberration, precisely focusing light of different wavelengths onto the same plane.
2. Reduction of Aberrations: Their aspheric design significantly reduces spherical aberration and wavefront errors, enhancing image quality.
3. Cost-Effectiveness: Compared to conventional multi-element optical systems, these lenses provide greater value for money.

Application Areas

Aspheric Achromatic Lenses are widely used in various high-precision optical systems, such as:

• Fiber focusing or collimation
• Imaging relay systems
• Detection and scanning systems
• High numerical aperture imaging systems
• Laser beam expanders

Technical Specifications

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• Materials: Photosensitive polymers and glass optical lenses
• Operating Temperature Range: From -20°C to +80°C
• Main Applications: Including fiber focusing, imaging relays, detection scanning, and high numerical aperture imaging, among others

With their ingenious design and efficient manufacturing process, Aspheric Achromatic Lenses demonstrate outstanding optical performance and a broad spectrum of applications, making them an indispensable key component in modern precision optics and vision systems.

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Lens Comparison

Thorlabs offers a wide variety of lenses with very different properties to match the needs of almost any application. However, it is important to choose lenses that are appropriate for a given system. As a general rule, spherical singlets are the most inexpensive, but they suffer from spherical aberration and other monochromatic aberrations. In addition, their single-element design means that they exhibit chromatic aberration that prevents optimum performance with broadband light. For correction of chromatic aberration, achromatic lenses are ideal. These multi-element optics also offer improved aberration correction for monochromatic light. For the best performance with monochromatic laser sources, aspheric optics are recommended. The surfaces of these lenses deviate from spherical sections, allowing for optimal aberration correction. 

Table 1.1 gives an overview of the lenses offered by Thorlabs. More details can be found in the Spherical Singlets, Achromatic Lenses, Aspheric Lenses, and Lens Materials tabs on this page.

Table 1.1  Lens Overview Lens Focal Length Conjugate Ratio Chromatic Aberration Correction Applications Spherical Singlet Lenses Plano-Convex Positive 5X - Infinite - Focusing a Collimated Beam;
Collimating a Point Source Bi-Convex Positive 0.2X - 5X - Relay Imaging (Real Object and Image);
Focusing a Divergent Beam Plano-Concave Negative 5X - Infinite - Diverging a Collimated Beam;
Collimating a Convergent Beam Bi-Concave Negative 0.2X - 5X - Relay Imaging (Virtual Object and Image);
Diverging a Convergent Beam Best Form Positive Infinite - Focusing a Collimated Beam;
Collimating a Point Source Achromatic Lenses Cemented Achromatic Doublet Positive Infinite Good Broadband Focusing and Collimation;
Improved Monochromatic Performance Air-Spaced Doublet Positive Infinite Better Broadband Focusing and Collimation;
Optimized On-Axis Performance;
High-Power Applications Cemented Doublet Pair Positive 1X - 3.33X Good Broadband Relay Imaging (Real Object and Image);
Improved Monochromatic Performance Cemented Achromatic Triplet Positive 1X - Infinite Best Broadband Focusing, Collimation, and Relay Imaging;
Correction of All Primary Chromatic Aberrations Aspheric Lenses Aspheric Lenses Positive Infinite - Optimized On-Axis Performance;
Laser Diode Collimation;
Fiber Coupling Aspheric Lens Pairs Positive 1X - 3.66X - Optimized On-Axis Performance;
Relay Imaging (Real Object and Image) Aspheric Condensers Positive Infinite - Light Collection;
Collimation of Incoherent Light

Spherical Singlets

Spherical singlets are a good option for many situations where aberrations are not a great concern, as they are the simplest and most inexpensive type of lens to produce. For simple applications, standard plano-convex, plano-concave, bi-convex, and bi-concave lenses are sufficient. For better performance, best form lenses are optimized to reduce aberrations while still retaining spherical surfaces. The use of multiple lens elements within a compound optical system can lead to further performance improvements. Meniscus lenses are often employed in these multi-element optical systems, although they are rarely used alone. For the most demanding applications, spherical singlets will not perform as well as achromatic lenses (for both broadband and monochromatic sources) or aspheric lenses (for monochromatic sources). More details about these other types of lenses can be found on the Achromatic Lenses and Aspheric Lenses tabs.

Standard Singlets

Thorlabs offers several basic singlet designs: Plano-Convex, Bi-Convex, Plano-Concave, and Bi-Concave. Each of these lenses is suited for different applications. Plano-convex and bi-convex lenses are positive (i.e., they have a positive focal length) and will bring collimated light to a focus, while plano-concave and bi-concave lenses are negative and will cause collimated light to diverge. Each singlet lens shape minimizes aberrations for a certain conjugate ratio, defined as the ratio of the object distance to the image distance (these are called conjugate distances).

Table 2.1  Positive Lenses Plano-Convex Lenses Bi-Convex Lenses Plano-convex lenses are best used where one conjugate distance is more than five times the other conjugate distance. The performance of this lens shape is best for an infinite conjugate ratio (focusing collimated light or collimating a point source). Bi-convex lenses perform best when one conjugate distance is between 0.2 and 5 times the other conjugate distance. The performance of this lens shape is best when the object and image distances are the same. Table 2.2  Negative Lenses Plano-Concave Lenses Bi-Concave Lenses Plano-concave lenses are best used when one conjugate distance is more than five times the other conjugate distance. They introduce negative spherical aberration and can be used to balance the positive spherical aberration introduced by positive focal length singlets. Bi-concave lenses have a negative focal lengths and are commonly used to increase the divergence of converging light.

Minimizing Aberrations

To minimize spherical aberration, a lens should be oriented so that the surface with the greatest curvature is facing the furthest conjugate point. For plano-convex and plano-concave lenses used at infinite conjugate ratios, this means that the curved surface should face the collimated beam (as shown in the drawings in Tables 2.1 and 2.2). The f-number of a lens, defined as the focal length divided by the aperture diameter, has a significant impact on the magnitude of image aberrations. Lenses with a small f-number ("fast" lenses) introduce significantly more aberrations than lenses with a large f-number ("slow" lenses). Lens shape becomes important for f-numbers below about f/10, and alternatives to spherical singlets (such as achromatic lenses and aspheric lenses) should be considered for f-numbers below about f/2.

Best Form Lenses

Figure 2.3  Spherical Aberration and Coma vs. Front Surface Curvature

Best form lenses are designed to minimize spherical aberration and coma (an aberration introduced for light not on the optical axis) while still using spherical surfaces to form the lens. The use of a spherical design makes best form lenses easier to manufacture than aspheric lenses (described on the Aspheric Lenses tab), reducing costs. Each side of a best form lens is polished so that it has a different radius of curvature, providing the best possible performance for a spherical singlet. For small input beam diameters, best form lenses are even capable of diffraction limited performance. These lenses are commonly used in high-power applications where cemented achromatic lenses are not an option (see the Achromatic Lenses tab for more information).

Table 2.4  Best Form Lenses Best form lenses are designed to minimize aberrations while still using spherical surfaces to form the lens. These lenses are optimized for an infinite conjugate ratio and are ideal for focusing collimated light or collimating a point source.

Figure 2.3 shows a plot of coma and spherical aberration as a function of the curvature of the front face of a lens (the curvature is the inverse of the radius of curvature). The minimum spherical aberration nearly coincides with the zero coma point; the curvature where this minimum occurs is the basis for a 'best form' design.

Meniscus Lenses and Multi-Element Lens Systems

Meniscus lenses are commonly used in multi-element optical systems to modify the focal length without introducing significant spherical aberration. The optical performance of multi-element lens systems is often significantly better than the performance of single lenses. In these systems, aberrations introduced by one element can be corrected by subsequent optics. These lenses have one convex and one concave surface, and they can be either positive or negative.

Table 2.5  Meniscus Lenses Positive Meniscus Lenses Negative Meniscus Lenses Positive meniscus lenses are typically used in cominbation with another lens in a compound optical assembly. When used in this configuration, a positive meniscus lens will shorten the focal length and increase the numerical aperture (NA) of the system without introducing significant spherical aberration. Negative meniscus lenses are typically used in combination with another lens in a compound optical assembly. When used in this configuration, a negative meniscus lens will increase the focal length and decrease the numerical aperture (NA) of the system.

Figure 2.6 shows the performance gains that can be achieved by using multi-element lens systems. A single element plano-convex lens with a focal length of 100 mm produces a spot size of 240 µm [Figure 2.6 (a)]. In addition, the single lens introduces 2.2 mm of spherical aberration, defined as the distance betwen the marginal focus (where rays on the very edge of the lens focus) and the paraxial focus (where rays in the center of the lens focus). By combining two plano-convex lenses with focal lengths of 100 mm, for an effective focal length of 50 mm, the focused spot size is decreased to 81 µm and the spherical aberration is reduced to 0.8 mm [Figure 2.6 (b)]. An even better option, however, is to combine the f=100 mm plano-convex lens with a positive f=100 mm meniscus lens. Figure 2.6 (c) shows the results: the focused spot size is reduced to 21 µm and the spherical aberration is reduced to 0.3 mm. Note that the convex surfaces of both lenses should be facing away from the image point.

Figure 2.6  Improved Performance of Multi-Element Systems

Achromatic Lenses

Figure 3.1  Focusing White Light with a Plano-Convex and an Achromatic Doublet Lens

Achromatic lenses, or achromats, consist of two or three lens elements and offer significantly better performance than simple singlet lenses. The lenses in an achromatic doublet or triplet are either cemented together or have an air gap between them and typically include both positive and negative elements with different indices of refraction. This multi-element design offers a number of advantages, including reduced chromatic aberration, improved imaging of monochromatic light, and improved off-axis performance. The different kinds of achromatic lenses and their properties, such as conjugate ratio and damage threshold, are described at the bottom of this page. For any application with demanding imaging or laser beam manipulation needs, these achromats should be considered.

Reduced Chromatic Aberration

Since the index of refraction of a material depends upon the incident wavelength, the focal length of a single lens depends on the incident wavelength. This leads to a blurred focal spot when singlet lenses are used with a white light source. This phenomenon is known as chromatic aberration. An achromatic lens can partially compensate for chromatic aberration by virtue of its multi-element design.

The constituent optical elements of an achromatic lens generally include both positive and negative lenses with different amounts of dispersion. If the material dispersion values and focal lengths of these constituent lenses are chosen carefully, a partial cancellation of the chromatic aberration can be achieved. Typically, achromatic lenses are designed to have the same focal length for two wavelengths at opposite ends of the visible spectrum. This results in a nearly constant focal length across a wide range of wavelengths.

The use of achromats is beneficial for any broadband imaging application that utilizes a large wavelength range. Figure 3.1 shows the effect on focal length for a number of different wavelengths incident on both a plano-convex singlet and achromatic doublet. The diameter of the focal spot is reduced from 147 µm to 17 µm by replacing the singlet with the achromatic doublet.

Improved Imaging for Monochromatic Light

When an optical system is used with monochromatic light, the chromatic aberration discussed above is inconsequential. However, spherical singlets can still introduce significant monochromatic aberrations, such as spherical aberration and coma. The multi-element design of achromatic lenses reduces these aberrations and leads to significantly increased image quality and tighter focusing of monochromatic light. For example, Figure 3.2 compares the performance of a plano-convex lens and an achromatic doublet in focusing a monochromatic beam. As can be seen, the diameter of the focal spot produced by the doublet is 4.2 times smaller than that produced by the singlet.

Figure 3.2  Focusing a Monochromatic Beam with Both a Plano-Convex and Achromatic Doublet Lens
Figure 3.3  Off-Axis Performance for a Plano-Convex and an Achromatic Doublet Lens

Superior Off-Axis Performance

For spherical singlets, the effect of off-axis aberrations can significantly compromise the performance of the lens if the beam is not propagating through the exact center of the lens. Achromatic lenses are less sensitive to centration, meaning that off-lens-axis beams are focused to almost the same spot as on-axis beams. Generally, achromatic triplets are even better than doublets at correcting for these off-axis effects.

Figure 3.3 shows two Ø25 mm, f=50.0 mm lenses, one of which is a plano-convex spherical singlet and the other is an achromatic doublet. Each lens has one beam propagating along the optical axis and another propagating parallel to the axis but offset by 8 mm. The achromatic doublet reduces both lateral and transverse aberrations; the lateral displacement of the focal points (circled in the diagram) is reduced by a factor of six and the focal spot diameter is also significantly smaller.

Selecting an Achromatic Lens

Achromatic lenses are a good choice for any demanding optical application, as they offer substantially better performance than spherical singlets. Cemented achromatic doublets are sufficient for most applications at infinite conjugates, and cemented doublet pairs are ideal for finite conjugates. However, the cement used in these optics reduces their damage threshold and limits their usability in high-power systems. Air-spaced doublets are ideal for high-power applications, as they have a greater damage threshold than cemented achromats. In addition, air-spaced doublets have two more design variables than cemented doublets because the interior lens surfaces do not need to have the same curvature. These extra variables allow the performance of air-spaced doublets to far exceed the performance of cemented doublets in terms of transmitted wavefront error, spot size, and aberrations. However, air-spaced doublets are also more expensive than cemented doublets.

Achromatic triplets can be designed for both finite (Steinheil Triplet) and infinite (Hastings Triplet) conjugate ratios. These triplets consist of a low-index center element cemented between two identical high-index outer elements. They are capable of correcting both axial and laterial chromatic aberration, and their symmetric design provides enhanced performance relative to cemented doublets.

Table 3.4  Achromatic Lenses Cemented Doublets Air-Spaced Doublets Achromatic doublets offer several advantages over simple singlet lenses. These include a minimization of chromatic aberration, improved off-axis performance, and smaller focal spots. These doublets have positive focal lengths and are optimized for an infinite conjugate ratio. Air-spaced doublets offer even better performance than cemented doublets, as they are optimized with respect to the lens separation. These optics are ideal for high power applications, as they offer a greater damage threshold than cemented doublets. These doublets have positive focal lengths and are optimized for an infinite conjugate ratio. Doublet Pairs Achromatic Triplets Achromatic doublet pairs offer the advantages of achromatic lenses, while being optimized for finite conjugates. These pairs are ideal for image relay and magnification systems. Achromatic triplets offer even better performance than achromatic doublets. An achromatic triplet is the simplest lens that corrects all primary chromatic aberrations. Steinheil Triplets are optimized for finite conjugate ratios, while Hastings Triplets are optimized for infinite conjugate ratios.

Aspheric Lenses

Aspheric lenses offer optimized on-axis performance at an infinite conjugate ratio, an advantage over spherical singlets and achromatic doublets. While individual spherical lenses can refract light at only small angles before spherical aberration is introduced, aspheric lenses are designed with curved surfaces that deviate from a sphere. This deviation is designed to eliminate spherical aberrations when light is refracted at large angles. As such, aspheric lenses are ideal for applications like laser diode collimation and fiber coupling that require a small f-number and large numerical aperture (NA). However, aspheric lenses are made from a single material and suffer from chromatic aberration. As such, they are typically used for monochromatic applications.

Figure 4.1  Theoretical Diffraction-Limited Spot Size

Theoretical Diffraction-Limited Performance

Figure 4.1 shows ray tracing results for a 780 nm beam at the image plane of an ASL lens (f = 79.0 mm at 780 nm). The Airy disk has a diameter of 6.538 µm, and is depicted by a black circle. Since all the rays (in blue) are well within the diameter, the theoretical spot size is diffraction limited.

Aspheric lenses have several particularly important applications, including laser diode collimation, fiber coupling, and light collection.

Figure 4.2  Collimating a Laser Diode Output with an Aspheric Lens

Collimating Laser Diodes

In laser diode systems, difficulties with aberration correction are compounded by the beam's high divergence angle. Because of spherical aberration, three or four spherical singlet elements are often required to collimate the light from a laser diode. A single aspheric lens can collimate the highly divergent emission of a laser diode without introducing spherical aberration, as shown in Figure 4.2. Again, the flatter side of the optic should face the source for optimum performance.

When choosing an aspheric lens for collimation of a laser diode, the first step is to determine the numerical aperture of the diode. This value is given by the sine of the largest FWHM divergence angle of the laser light. Then, an aspheric lens should be chosen that has roughly twice the numerical aperture of the laser. This will ensure that the aspheric lens collects as much light as possible (much of which is outside the FWHM divergence angle).

Fiber Coupling

When coupling light into a fiber, it is often necessary to focus a collimated beam of light to a diffraction-limited spot. Typically, single spherical elements and achromatic doublets are not capable of achieving such a small spot size; spherical aberration is the limiting factor rather than diffraction. Since aspheric lenses are designed to eliminate spherical aberration, only diffraction limits the size of the focal spot.

When choosing an aspheric lens for coupling light into a single mode fiber, the diffraction-limited spot size should be matched to the mode field diameter (MFD) of the fiber. The required focal length for the lens can easily be calculated from the MFD and the beam diameter. If an aspheric lens is not available that provides an exact match, then choose the aspheric lens with a focal length that is shorter than the calculation yields. Alternatively, if the clear aperture of the aspheric lens is large enough, the beam can be expanded before the aspheric lens, which has the result of reducing the spot size of the focused beam.

Light Collection

Many applications, such as microscopy, make use incoherent lamps and high-power LEDs as illumination sources. These applications benefit from the efficient collection of as much light as possible, suggesting the use of a large aperture lens to collimate the output of the source. Unfortunately, large aperture lenses tend to introduce more aberration than smaller lenses, reducing the quality of the resulting collimated light. Aspheric condenser lenses are ideal for efficient light collection, as they offer large diameters and numerical apertures as well as the reduced spherical aberration of an aspheric design.

Table 4.3  Aspheric Lenses Aspheric Lenses Aspheric Collimators Aspheric lenses focus or collimate light without introducing spherical aberration into the transmitted wavefront. Molded aspheric lenses are economical and available in both glass and plastic. For better performance, precision polished aspheric lenses introduce substantially less wavefront error and are offered with larger diameters. Aspheric collimators are designed to collimate divergent light with diffraction-limited performance. We offer fixed focus and adjustable focus fiber collimators as well as laser diode collimation tubes. Aspheric Lens Pairs Aspheric Condensers Aspheric lens pairs are designed for near aberration-free finite conjugate imaging. These pairs are ideal for image relay and magnification systems. Aspheric condensers are designed for high-efficiency illumination applications. They offer reduced spherical aberration with large apertures and low f-numbers. They are ideal for collimating light from a lamp or LED.

Lens Materials

Thorlabs' wide breadth of optics manufacturing capabilities allows us to offer lenses made from a variety of optical materials. Table 5.1 should aid with the selection of a lens best suited for use at a particular wavelength. To view transmission plots for the uncoated materials, please click on the appropriate icon below. For more details on optical substrates, please see our Optical Substrates tutorial.

For more cylinder lensinformation, please contact us. We will provide professional answers.

Table 5.1  Lens Materials Material Transmission Description Transmission Plots N-BK7 350 nm - 2.0 µm N-BK7 is a RoHS-compliant borosilicate crown glass. It is probably the most common optical glass used for high quality optical components. UV Fused Silica (UVFS) 185 nm - 2.1 µm UV-grade fused silica offers high transmission in the deep UV and extremely low fluorescence levels compared to natural quartz, making it an ideal choice for applications from the UV to the near IR. In addition, UV fused silica has better homogeneity and a lower coefficient of thermal expansion than N-BK7. N-SF11 420 nm - 2.3 µm N-SF11 is a RoHS-compliant dense-flint glass with a high index of refraction and a low Abbe number. This glass exhibits higher dispersion than N-BK7 but many of its other properties are comparable. Calcium Fluoride (CaF2) 180 nm - 8.0 µm Calcium fluoride has a low refractive index and is mechanically and environmentally stable. It is ideal for any demanding applications where its high damage threshold, low fluorescence, and high homogeneity are beneficial. Barium Fluoride (BaF2) 200 nm - 11.0 µm Barium fluoride's properties are similar to those of calcium fluoride, but it is more resistant to high-energy radiation. It is, however, less resistant to water damage. Silicon
(Si) 1.2 - 8.0 µm Silicon offers high thermal conductivity and low density. However, since it has a strong absorption band at 9 microns, it is not suitable for use in CO2 laser transmission applications. Zinc Selenide (ZnSe) 600 nm - 16.0 µm Due to its wide transmission band and low absorption in the red portion of the visible spectrum, Zinc Selenide is commonly used in optical systems that combine CO2 lasers, operating at 10.6 µm, with inexpensive HeNe alignment lasers. Germanium (Ge) 2.0 - 16 µm Germanium is well suited for IR laser applications. The element is inert to air, water, alkalis, and acids (except nitric acid), but its transmission properties are highly temperature sensitive. Magnesium Fluoride (MgF2) 200 nm - 6.0 µm Magnesium Fluoride is an extremely rugged and durable material, making it useful in high-stress environments. It is commonly used in machine vision, microscopy, and industrial applications. PTFE 30 µm - 1.0 mm PTFE has a low dielectric constant of approximately 1.96 @ 520 GHz and an index of refraction of 1.4. The material is especially useful for application in the terahertz range, which is defined as the frequency range from 300 GHz to 10 THz, or the wavelength range of 30 μm to 1 mm.