Introduction
Since the 13th century, lenses have typically been spherical, as they were easy to manufacture by grinding a glass body. Aspheric lenses are superior in many cases because they reduce spherical aberrations and improve focus. However, their higher complexity makes them harder to produce consistently and at high quality. In the following, we will discuss some common challenges with aspheric lenses, give advice on finding the right aspheric lens for key applications, and introduce our manufacturing partner, GH Optics.
Manufacturing Aspheric Lenses
Traditionally, aspheric lenses were individually ground and polished from glass preforms in complex processes, such as CNC polishing, magneto-rheological finishing, or diamond turning technology. Precision glass molding, however, is a less complex manufacturing process. In this method, optical glass blanks are heated until the surface is soft enough to be pressed into an aspheric mold.
To ensure that the lenses produced meet the required detailed dimensions exactly and reproducibly, the mold must meet high requirements. A lot of effort and high initial costs are necessary to create a mold with an extremely smooth surface without indentation or bumps and enough durability for many pressing processes in consistent quality. Additionally, manufacturers must consider material shrinkage when the finished pressed lens cools down and design the mold accordingly.
Once these challenges are overcome and the mold is finished, suppliers can produce aspheric lenses at a much lower cost than those of conventional manufacturing techniques. Aspheric lenses offer further savings in applications where a single aspheric lens can replace an entire set of multiple spherical lenses and thus make the overall optical system less complex, more compact, and lighter – often even with increased optical performance.
Read more about the basics on Aspheric Lenses
Use Case 1: The Right Aspheric Lens for Collimation
Collimation is the process of aligning light rays to be parallel, which is crucial for applications like laser diodes. Aspheric lenses are ideal for this due to their ability to handle the elliptical and diverging geometry of laser diode light.
Numerical Aperture and FWHM Value of the Aspheric Lens
The numerical aperture (NA) is a key metric for selecting an aspheric lens because it directly impacts the lens’s performance in terms of light-gathering ability, resolution, and its suitability for specific applications requiring precise control over light. The lens must have an NA larger than the aperture of the laser beam’s fast axis to avoid clipping and loss of light. If the numerical aperture of the laser diode is not indicated in the data sheet, it can be approximated by halving the larger of the two aperture angles of the beam ellipse and calculating the sine of the resulting angle.
Matching the aspheric lens to the laser diode’s beam width is essential for achieving the desired outcomes. It is important to know how the divergence or beam width of the laser diode is defined in the data sheet. For some applications, the part of the laser beam covered by the 50% or FWHM value for the divergence is not sufficient; in this case, you must refer to the 87% or 1/e² value. If this value cannot be found in the data sheet, you can approximate it for a Gaussian beam profile with 1/e² ≈ FWHM x 1.7. If you want to capture the entire beam diameter (i.e., 99% of the contained power), you must multiply the FWHM value by a factor of 2.576.
Focal Length of the Aspheric Lens
The effective focal length (EFL) reflects the unique ability of the lens to correct aberrations and control the behavior of light, leading to improved optical performance with aspheric lenses. The beam diameter Ø can be approximated with the formula Ø ≈ 2 x NA x EFL. Use this formula to determine the required focal length of the lens from the desired diameter of the collimated beam. Remember that the resulting beam is elliptical – the beam diameter is larger in the “fast axis”!
Matching the Wavelength of the Laser
In the data sheet of a molded aspheric lens, you will usually find a “design wavelength” – with the optical performance of the lens optimized for this wavelength. If the lens is used for other wavelengths, its values for the effective focal length (EFL) will change, as well as for other parameters such as the back focal length (BFL) or wavefront error (WFE). Here you can roughly assume that with a smaller wavelength compared to the design wavelength value, the focal lengths become shorter, while the focal length values increase when the actual wavelength is above the value of the design wavelength. How a different wavelength affects the wavefront error of the aspheric lens is best simulated with one of the common optical simulation programs such as Zemax or Oslo.
Read more about finding the right aspheric lens for Collimation
Use Case 2: The Right Aspheric Lens For Fiber Coupling
For directing collimated laser diode light into an optical fiber, the aspheric lens must have an appropriate focal length to ensure efficient coupling. We assume here that your laser beam is already collimated to a certain beam diameter, i.e., it no longer diverges. In order to couple this collimated beam into your fiber core, its effective focal length (EFL) must be sufficient. Otherwise, too much optical energy ends up in your fiber’s cladding or is even lost completely. The appropriate focal length is calculated with sufficient accuracy by dividing your beam diameter by twice your fiber’s numerical aperture: EFLlens = Øbeam / 2 x NAfiber.
Matching Definition Criteria Between Components
For good coupling, the definition criteria of the fiber’s numerical aperture and the beam diameter should match. For example, your fiber’s numerical aperture is usually defined for 99% of optical energy; so in this case, your beam diameter should also be defined for 99%. If you only have a 50% or FWHM value for your beam diameter, you can approximate its 99% value for a Gaussian beam profile by multiplying it by a factor of 2.576. From its 1/e² value you get its 99% value by multiplying by a factor of 1.515.
Finding the Minimal Focal Length
The formula “EFLlens = Øbeam / 2 x NAfiber” provides your minimum focal length for your aspheric lens. Falling below this threshold means that not all your power will be effectively coupled into your fiber core; so it’s essential not to exceed this minimum value significantly because an increased focal length leads to a larger diameter of the laser spot exiting the far end of your fiber.
Read more about finding the right asperic lens for fiber coupling
Sourcing Aspheric Lenses
As molding aspheric lenses requires a lot of knowledge, experience, and upfront investment, manufacturers that can deliver them in larger quantities, affordably and in consistent high quality is very limited. To serve the needs of our customers, AMS Technologies cooperates with GH Optics. Founded in 2021, GH Optics is a leading manufacturer of aspheric lenses and optical solutions. They specialize in the design, development, and series production of precision optics, including aspheric lenses for visible and infrared light. With precision molding, advanced coating technologies as well as proprietary processes and tools, GH Optics delivers world-class quality at affordable prices. The product spectrum consists of aspheric lenses for visible light and infrared with various glass types and coatings. Besides optical assemblies, such as mounted lenses or thermal imaging modules, GH Optics also offers custom lenses, coatings, and assemblies.
Read more about our partner GH Optics
Conclusion
Aspheric lenses offer significant advantages in reducing aberrations and improving optical performance with aspheric lenses. Whether for collimation or fiber coupling, selecting the right lens is crucial for achieving the desired results. Together with GH Optics, we can provide high-quality aspheric lenses and optical solutions to meet various application needs. If we can’t find the right lens for your project even in our wide portfolio of standard lenses, we can have a custom solution designed and built for you.
What are your requirements? Let’s discuss and find the right lenses for your project.
