For a long time, aspherical lenses have been considered optimal for refracting light but at the same time too expensive to manufacture – inhibiting their broad use. Today our manufacturing partner’s glass molding technology enables the production of aspheric optics in high volume while maintaining the highest quality at an affordable price. AMS Technologies offers a broad selection of molded-glass aspheric optics in sizes ranging from 0.25 mm to 23 mm and covering the spectral range from 400 nm right up to 14 µm.

Standard design assemblies are available for our most popular lens types, but any asphere in our portfolio can be mounted into a custom assembly of your choice. For large volume applications we recommend to call your regional AMS Technologies office, so that we can tailor a solution based on your technical needs. From concept through prototyping, volume production, and global distribution, AMS Technologies has the optical knowledge and manufacturing expertise to be your optics partner every step of the way.

Molded lenses are used in a variety of photonics products: barcode scanners, laser diode to fiber couplings, optical data storage and medical lasers, to name a few. In many of these applications, the material of choice is optical glass because of its durability and performance stability over a wide environmental range. High power transmittance is also an added advantage. Our aspheric lenses are inspected and optically tested to ensure complete customer satisfaction. Visual cosmetic inspection for scratch/dig is performed on 100% of all lenses per MIL-PRF-13830B. Most lenses are guaranteed to pass 40/20 scratch/dig, but other inspection criteria, such as 60/40 or 20/10, can be provided upon request.

**Improve performance and lower overall cost**

Traditional spherical lenses have a simple shape that can be described as an arc of a circle and can be specified using only a radius of curvature. Although these lenses are simple to manufacture and inexpensive to use, they suffer in performance due to a phenomenon called “spherical aberration”. This inherent defect is due to the fact that a spherical shape is not the ideal shape for a focusing or collimating lens. The ideal case is a more complex shape that is typically defined using a radius of curvature, a parabolic term (conic), and several high order coefficients.

The complex shape of aspheric lenses allows for correction of spherical aberration. This provides better quality collimated beams for collimating applications, a smaller spot size for focusing applications, and better image quality for imaging applications. In fact, in many cases just a single aspheric lens can take the place of several conventional spherical lenses, leading to a lighter, more compact, less expensive, and better performing optical system. Aspheres are now a viable design option for many applications.

Based on our manufacturer partner’s long experience in molding high precision glass aspheric lenses for the industrial, scientific, communications, medical and defense markets, our portfolio contains over 35 standard molded aspheric lens types. Customization and design of new lenses is also available if our off-the-shelf lenses should not fit your requirements.

**Choosing the right aspheric lens for diode collimation**

Due to the way that the laser cavity is constructed in edge emitting diode lasers, light is emitted in a diverging, elliptical geometry – so the divergence is typically specified in both the x- and y-axes separately. The axis with the larger divergence is called the “fast axis” and the axis with the smaller divergence is called the “slow axis”.

When selecting a lens to collimate the laser. ﬁrst consider the numerical aperture (NA) of the lens. If the application requires a high amount of the laser light to be coupled through the system, a lens with a high enough NA must be chosen. The NA of a lens is a measure of the maximum amount of divergence that the lens can capture from the laser. Ideally, a lens should be used that has an NA higher than the NA of the laser's fast axis. if not, the laser will "clip" the lens causing some of the light to be wasted. To convert the NA to the divergence angle (and vice-versa), use this formula:

NA = n • sin(φ)

In most cases, n= 1 since the NA of the laser is defined in air. Therefore, solving for φ the equation is simplified to:

φ = sin-1 (NA)

It is important to note that φ is the half angle of the divergence cone and is given at the marginal ray (not 1/e2 or half angle half max). After the minimum NA is determined, next consider what beam diameter is preferred. Although ray-tracing is necessary to precisely determine the beam diameter for a given NA source with a particular lens, it can be approximated with the following formula:

BeamDiameter ≅ 2 • EFL • NA

where EFL is the effective focal length of the lens and NA is the numerical aperture of the source (not the NA of the lens).

Remember that most edge emitting diodes are elliptical, so the beam diameter will be different in the x-axis versus the y-axis. Use the formula above to calculate the beam diameter in both axes to determine the shape of the collimated, elliptical beam.

**Important Note:**

Some laser manufacturers give the NA of the source in different terms, such as half max (50% point) or 1/e2 (87% point). Whatever type of number is entered into the formula for the NA of the source will be the same type of number given for the beam diameter. For example, if the half width half max NA for a laser is used with the above formula, you will get the full width half max beam diameter. There is no simple way to convert from a half max number or a 1/e2 beam diameter to a full beam diameter for a specific source because it depends on the intensity profile of the source itself. A reasonable approximation, though, for most edge emitting diode lasers is to assume a Gaussian beam profile. Using this beam profile, you can convert the beam diameters as follows:

1. To convert a half max beam diameter to a full beam diameter (i.e. 99% power contained), multiply the diameter by 2.576.

2. To convert a 1/e2 beam diameter to a full beam diameter (i.e. 99% power contained), multiply the diameter by 1.517.

**Choosing the right aspheric lens for fiber coupling**

Another common use for aspheric lenses is to couple laser light into optical fibers. Choosing the right lens or lenses to do the coupling is important to maintain high efficiency in the optical system. The guide below is intended to show how best to do this while using off-the-shelf components. This guide assumes that the input laser light has already been collimated (not diverging) and the fiber is multimode (single fiber requires more extensive modeling for optimum coupling efficiency).

When selecting a lens to focus light into a fiber, first consider what focal length lens is needed. Let’s revisit the formula given previously:

Beam Diameter ≅ 2 EFL • NA

Solving for EFL it becomes:

EFL ≅ Beam Diameter / 2 • NA

where NA is the numerical aperture of the fiber that is used for the coupling. It is important to note that the EFL value that is calculated above is the minimum EFL needed to couple the light completely into the fiber. Longer EFL lenses can be used, but the spot size on the fiber tip will become larger when longer EFL lenses are employed. Therefore, it is best practice to use the shortest EFL lens possible that is larger than the minimum value. It is important to remember that the equations and calculations used above are just approximations. In order to determine the exact optical properties from coupling a collimated beam into an optical fiber, a ray-traced optical design is necessary.