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All Optical Tutorials on Thorlabs.comLens TutorialSelecting the Proper LensLenses are subject to numerous types of aberrations; chromatic aberration and spherical aberration are two common examples. By properly choosing the lens design or using multielement systems, the effects of aberrations can be decreased and the performance of the system can be improved. Many of Thorlabs lens styles are available in multiple highquality optical materials; once a lens type is chosen, the substrate and/or antireflection coating can be chosen for a particular wavelength region. Spherical SingletsSpherical singlets are a good option for many applications where aberrations are not a great concern. Thorlabs offers several singlet designs: PlanoConvex, BiConvex, PlanoConcave, and BiConcave. Each of these lenses is suited for different applications.
Figure 1: Aberrations may be reduced by using multielement systems MultiElement Lens SystemsFigure 1 shows the performance gains that can be achieved by using multielement lens systems. For example, a single element planoconvex lens with a focal length of 50 mm produces a spot size of 240 µm (Figure 1a). By combining two planoconvex lenses, each with a focal length of 100 mm, for an effective focal length of 50 mm, the focused spot size is decreased to 81 µm (Figure 1b). Meniscus Lenses An even better option, however, is to combine the f = 100 mm planoconvex lens with a positive meniscus lens with a focal length of 100 mm. Positive meniscus lenses are designed to minimize spherical aberration. Meniscus lenses have one convex and one concave surface. When used in combination with another lens, a positive meniscus lens will shorten the focal length and increase the numerical aperture (NA) of the system. Figure 1c shows the results: the focused spot size is reduced to 21 µm, and the transverse and lateral aberrations are also greatly reduced. Note that the convex surfaces of both lenses should be facing away from the image. Positive Meniscus Lens
Negative meniscus lenses are commonly used in beam expanding applications since they increase the divergence of the beam without introducing significant spherical aberration. Combining a negative meniscus lens with another lens increases the focal length and decreases the NA of the system. Negative Meniscus Lens Achromatic Doublet Lens Achromatic Doublet LensesAchromatic doublets offer several advantages over simple singlet lenses. These include a minimization of chromatic aberration, improved offaxis performance, and smaller focal spots. For any application with demanding imaging applications or laser beam manipulation needs, these doublets should be considered. Figure 2: Focusing white light with a planoconvex and an achromatic doublet lens Nearly Constant Focal Length Across a Wide Range of Wavelengths Since the index of refraction of a material depends upon the incident wavelength, a single lens will have a blurred focal length and circle of least confusion (spot size) when using a white light source. This phenomenon is known as chromatic aberration. An achromatic doublet can partially compensate for chromatic aberration since it is a combination of two lenses, each with a different index of refraction, thus leading to a partial cancellation of this aberration. Figure 2 shows the effect on focal length for a number of different wavelengths incident on both a planoconvex singlet and achromatic doublet. The circle of least confusion is reduced from 147 µm to 17 µm by replacing the singlet with the achromatic doublet. Achieve a Tighter Focus for Monochromatic Light Achromatic doublets also have other advantages over single lens designs for applications with monochromatic light. For example, Figure 3 compares the focusing of a monochromatic beam by a planoconvex lens and an achromatic doublet. As can be seen, the circle of least confusion for the doublet is 4.2 times smaller than that from the singlet. Figure 3: Focusing a monochromatic beam with both a planoconvex and achromatic doublet lens Figure 4: Offaxis performance for a planoconvex and an achromatic doublet lens Superior OffAxis Performance The performance of achromatic doublets is also not as compromised as it will be for a singlet lens if the beam is not propagating through the exact center of the lens. Figure 4 shows two 25 mm diameter, 50.0 mm focal length lenses, one of which is a planoconvex 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. Both lateral and transverse aberrations are reduced for the achromatic doublet; the lateral displacement of the focal points is over six times smaller while the circle of least confusion is also significantly smaller. Figure 5: Spherical aberration and coma vs. front surface curvature. Best Form LensesBest Form lenses are ideal for use at infinite conjugates in highpower applications where achromatic doublets are unsuitable due to the cement between elements. These 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. Each side of the lens is polished so that it has a different radius of curvature. Figure 5 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 minimal spherical aberration nearly coincides with the zero coma point; the curvature where this minimum occurs is the basis for a “best form” design. BestForm Lens Aspheric LensesAspheric lenses offer diffractionlimited spot sizes, an advantage over 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 which deviate from a sphere. This deviation is designed to eliminate spherical aberrations when light is refracted at large angles. Aspheric lenses have two important applications: laser diode collimation and fiber coupling. Figure 6: 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, when used as shown in Figure 6. Fiber Coupling When coupling light into a fiber, it is often necessary to focus a collimated beam of light to a diffractionlimited 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. Lens Material SelectionThorlabs' lenses are manufactured using a variety of optical materials. The table below should aid with the selection of a lens best suited for use at a particular wavelength. To view transmission vs. wavelength plots for either the uncoated material or the material with an appropriate coating, please click on the appropriate icon below.  
