Optical Lenses Introduction

Spherical single lenses

Spherical single lenses are a good choice for many applications where aberrations are not very important because they are simple and inexpensive lens types. For simple applications, standard plano-convex lenses, plano-concave lenses, double-convex lenses, and double-concave lenses are sufficient.
For better performance, the contour of lenses are optimized, which can still maintaining a spherical surface when reduce aberrations.
The use of multiple lens components in a composite optical system can achieve additional performance improvements. These multicomponent optical systems often utilize meniscus lenses within them, although they are rarely used alone.
For very demanding applications, spherical single lenses will not perform as well as achromatic lenses (for both broadband and monochromatic sources) or aspherical lenses (for monochromatic sources).

Standard Single Lenses

A variety of basic single-lens designs: Plano-convex, Double-Convex Lenses, Plano-concave, and Double-concave Lenses. Each of these lenses is suitable for a different application.

Plano-convex and Double-Convex Lenses are positive lenses (i.e., they have a positive focal length) that focus collimated light to a point, while Plano-concave and Double-concave Lenses are negative lenses that diverge collimated light.

The shape of each single lens makes the aberrations smaller for a specific conjugation ratio, which defined as the ratio of object distance to image distance (they are called conjugation distance).

Plano-convex Lenses

Plano-convex lenses are suitable for situations where one conjugate distance is more than five times that of the other. The properties of this lens shape are suitable for situations with infinite conjugate ratios (collimation of focused or point sources).

Plano-convex Lenses

Plano-convex Lenses are suitable for situations where one conjugate distance is 0.2 to 5 times that of the other. The performance of this lens shape is suitable for the same distance between objects and images.

Plano-concave Lenses

Plano-concave lenses are suitable for situations where one conjugate distance is more than five times that of the other. They introduce negative spherical aberrations and can be used to balance the spherical aberrations introduced by a single lens with a positive focal length.

Double-Concave Lenses

Double-Concave Lenses have a negative focal length and are often used to increase the divergence of converged light.

How to minimize the spherical aberrations?

To minimize spherical aberrations, the lens should be placed making the greater curvature surface to orient the further conjugate points.

For Plano-convex and Plano-concave lenses used with an infinite conjugate ratio, this means that the surface should be oriented towards the collimated beam (as shown in the figure above).

f of a lens is defined as the focal length divided by the aperture diameter, which has a significant effect on the degree of aberration.

A lens with a smaller f (a “fast” lens) introduces significantly more aberrations than a lens with a larger f (a “slow” lens).

Lens shape becomes important when f is below about f/10, and other lenses (such as achromatic and aspherical lenses) that can replace spherical single lenses with f below about f/2 should be considered.

Best Form Lenses

Best Form Lenses are designed to minimize spherical aberrations and comas (aberrations introduced by light not on the optical axis) while still using the sphere surface to form the lens.
The spherical design makes these best form lenses easier to manufacture than aspherical lenses, reducing costs.
Each surface of the best form lens is polished to give it a different radius of curvature, providing better performance for a spherical single lens.
For small-diameter input beams, contour lenses even have diffraction properties.
These lenses are often used in high-power applications where achromatic bonded lenses cannot be used.

Shaped-lenses

Shaped lenses are designed to minimize spherical aberrations and comas, while still using the sphere surface to form the lens. These lenses are optimized for infinitesimally large conjugate ratios and are well suited for focusing collimating beams or collimating point sources.

Spherical Aberrations and Comas VS. Front surface curvature

The diagram is a plot of the coma and spherical aberrations with the curvature of the front of the lens (curvature is the reciprocal of the radius of curvature). The minimum spherical aberration almost coincides with the zero coma handicap; The curvature at which this minimum occurs is key to the “shape” design.

Meniscus and multicomponent lens systems

Meniscus lenses are commonly used in multicomponent optical systems to modify the focal length without introducing significant spherical aberrations. The optical performance of a multicomponent lens system is usually significantly better than a single lens. In these systems, the aberrations introduced by one component can be corrected by subsequent optical components. These lenses have a convex surface and a concave surface, and they can be positive or negative lenses.

Positive-meniscus Lense

Positive meniscus lenses are usually used with another lens in composite optical assemblies. When used in this structure, positive meniscus lenses shorten the focal length and increase the numerical aperture (NA) of the system without introducing significant spherical aberrations.

Negative-meniscus Lense

Positive meniscus lenses are usually used with another lens in composite optical assemblies. When used in this structure, a positive meniscus lens increases the focal length and reduces the numerical aperture (NA) of the system without introducing significant spherical aberrations.

Figure 1 shows the performance improvements that can be achieved with a multicomponent lens system.

A single Plano-convex lens with a focal length of 100mm produces a spot size of 240μm (Figure 1a). In addition, a single lens produces a spherical aberration of 2.2mm, defined as the distance between the focus margin (the beam is at the edge of the lens focus) and the paraxial focus (the light is in the middle of the lens focus).

By combining two Plano-convex lenses with a focal length of 100mm, the effective focal length is 50mm, the focused spot size is reduced to 81μm, and the spherical aberration is reduced to 0.8mm (Figure 1b).

However, a better way is to combine a convex lens of f = 100mm with a positive meniscus lens of f = 100mm. Figure 1c shows the results: the focused spot size is reduced to 21μm, and the spherical aberration is reduced to 0.3mm.

Note that the convex surfaces of the two lenses should face away from the imaging point.

Poor Performance: 240um Focus

Good Performance: 81um Focus

Excellent Performance: 21um Focus

Figure 1 the performance improvements

When to choose an achromatic lens?

Achromatic lenses are a good choice for any demanding optical application because they offer substantially better performance than spherical single lenses.

Achromatic double-bonded lenses are sufficient for most infinite conjugation applications, and double-bonded lenses are an ideal choice for finite conjugation.

However, the adhesives used in these optical components reduce their damage thresholds and limit their availability in high-power systems.

Air-spaced double lenses are ideal for high-power applications because they have a larger damage threshold than achromatic glued lenses.

In addition, an air-spaced double lens has two more design variables than a double-glued lens because the inner surface of the lens does not need to have the same curvature. These additional variables make the air-spaced double lens far better than the double-bonded lens in terms of transmission wavefront error, spot size, and aberration. However, air-spaced double lenses are also more expensive than double-glued lenses.

Achromatic triplet lenses can be designed for both finite and infinite conjugate ratios. In the middle of these triplet lenses is a low-index optical element that is glued between two identical high-index external optical elements. They are capable of correcting both axial and transverse chromatic aberrations, and their symmetrical design provides better performance than glued double lenses.

Double-glued Lens

Achromatic double-bonded lenses have much more advantages over simple single lenses. They include reduced chromatic aberration, improved off-axis performance, and smaller focal spots. These double lenses have a positive focal length and are optimized for infinite conjugate ratio.

Air-spaced Double Lens

Air-spaced double lenses perform better than double-bonded lenses, because their lenses are separated. These optics components are ideal choice for high-power applications because they have a larger damage threshold than double-bonded lenses. These lenses have a positive focal length and are optimized for infinite conjugate ratios.

Double-glued Lens Pairs

Achromatic doublet pairs have the advantages of achromatic lenses while being optimized for finite conjugation. These lens pairs are ideal choice for image relay and magnification systems.

Achromatic-triplet Lenses

Achromatic triplet lenses perform better than achromatic doublets. An achromatic triplet lens is a simple lens that can corrects for all major chromatic aberrations. The Steinheil triplet lens is optimized for finite conjugate ratio, while the Hastings triplet lens is optimized for infinite conjugate ratio.

Lens material

Our broad optical component manufacturing capabilities allow us to manufacture lenses in a wide range of optical materials. The following table can help you choose the right lens for your specific wavelength.

Material	Transmission range	Description
N-BK7	350nm-2.0um	N-BK7 is a RoHS compliant borosilicate crown glass. It may be an optical glass commonly used for high-quality optical components.
UV Fused Silica	185nm-2.1um	Ultraviolet fused quartz provides high transmittance in the deep ultraviolet region and has very low fluorescence levels compared to natural quartz, making it is an ideal choice for applications in the ultraviolet to near infrared bands. In addition, UV fused quartz has better homogeneity and lower coefficient of thermal expansion than N-BK7 material.
N-SF11	420nm-2.3um	N-SF11 is a RoHS compliant heavy flint glass with a high refractive index and a low Abbe number. This glass exhibits higher dispersion than N-BK7, but it has many other properties comparable to N-BK7
CaF₂	180nm-8.0um	Calcium fluoride has a low refractive index and is mechanically stable and environmentally stable. With its high damage threshold, low fluorescence and high uniformity, it is an ideal choice for any demanding application that requires these properties.
BaF₂	200nm-11.0um	Barium fluoride is similar in nature to calcium fluoride, but it is more resistant to high-energy radiation. But it has poor resistance to water quality damage.
Si	1.2-8um	Silicon has high thermal conductivity and low density. However, because it has a strong absorption band at 9um, it is not suitable for CO₂ laser transmission applications
ZnSe	600nm-16.0um	Because zinc selenide has a wide transmission band and low absorption in the red part of the visible spectrum, it is often used in optical systems that combine CO₂ lasers (operating at 10.6um) with inexpensive helium-neon lasers.
Ge	2.0-16um	Germanium is an ideal choice for infrared laser applications. The element is inert to air, water, bases and acids (except nitric acid), but its transmission properties are very sensitive to temperature.
MgF₂	200nm-6.0um	Magnesium fluoride is a very strong and durable material, which is very useful in high pressure environments. It is commonly used in machine vision, microscopy and industrial applications.
PTFE	30um-1.0mm	PTFE has a low dielectric constant of about 1.96 at 520GHz, and a refractive index of 1.4, making the material particularly useful in THz range applications. The THz range is defined as the frequency range from 300 GHZ to 10 THZ, or the wavelength range from 30um to 1mm.