Breaking the Traditional Rules of Lens Design

Achromatic lenses are some of the most commonly used optical components. Historically, they have consisted of two lens elements cemented together to correct for chromatic aberration and bring at least two wavelengths of light to a common point of focus. Single-material aspheric achromats are a novel type of lens that break that assumed rule by using a unique geometry, rather than multiple materials, for achromatic performance. They feature a broader bandwidth than traditional achromats, no coefficient of thermal expansion (CTE) difference between multiple materials, and compatibility with high-power lasers because no cement is required to bond multiple materials.

Rules are Meant to be Broken

Achromatic lenses are typically composed of two lens elements cemented together: a positive crown glass element with a low refractive index and a negative flint glass element with a high refractive index. Single-material aspheric achromats circumvent the need to use multiple materials through an innovative and complex aspheric geometry (Figure 1).

Figure 1: Layout of a single-material aspheric achromat from MATLAB. Only half of the lens aperture is illustrated.

Designing and optimizing these lenses required custom optical design code to be developed because available optical design programs, such as Zemax OpticStudio^® and Synopsis’ Code V, lacked the ability to define these complex aspheric surfaces. The custom MATLAB code features a higher degree of design freedom by avoiding constraints of the commonly-accepted mathematical representations of aspheric surfaces. More details on custom code development can be found in our published conference proceedings from SPIE LASE 2020.¹

Benefits of Single-Material Aspheric Achromats

Single-material aspheric achromats offer a number of benefits over conventional, multiple-material achromats. Minimizing chromatic focal shift is of particular concern for achromatic lenses, as it is typically desired to have multiple wavelengths focus at a single spot on the optical axis with minimal chromatic spread of other intra-band wavelengths. Figure 2 compares the nominal chromatic focal shift of a single material achromat and an equivalent traditional doublet, revealing that the single material achromat significantly outperforms the conventional doublet on axis.

Figure 2: The 10 µm nominal chromatic focal shift of a 100mm focal length, 0.097 NA single material achromat is a 20X improvement over the 200µm nominal chromatic focal shift of #32-327, a 100mm focal length, 0.013 NA achromatic doublet.

Single material achromats are compatible with higher-power lasers over conventional multiple material achromats, primarily because no optical cement is required to bond different materials. The laser damage threshold of the cement is typically the limiting factor for the laser damage threshold of conventional achromatic doublets.²

The lack of cement also makes single material achromats more thermally stable, as there are no CTE differences between multiple materials. CTE differences between the crown and flint glasses in a conventional achromatic doublet may lead to thermally-induced stress and defocus.

Manufacturing Challenges

The unique geometry of single-material achromats makes them more difficult to manufacture than many other optical components. Edmund Optics^® has been diamond turning single-material achromats out of Zeonex E48R, a commonly used optical plastic; they can also be diamond-turned from other materials, including germanium and silicon.

These lenses must be manufactured through diamond turning, rather than conventional grinding and polishing, to form the required geometry. Some iteration is required to fine-tune the alignment between the front and back surfaces. This is accomplished through a feedback loop of profilometry data and kinematic location of diamond-turned datum including flat annular zones and the outer diameter.¹

Testing Real-World Performance

To evaluate how closely the performance of the manufactured single-material achromats matched their nominal performance, the chromatic focal shift of 18 sample lenses was determined by measuring the position of best focus at different wavelengths from 450-650nm using an Optikos MTF test bench and a series of 10nm bandwidth optical filters, as shown in Figure 2. Full details of the testing can be found in the SPIE LASE 2020 conference proceedings.¹

The chromatic focal shift values of the 18 single-material aspheric achromat samples ranged from 49 to 75µm. For reference, the equivalent achromatic doublet from Figure 2 was measured using an identical method and had a chromatic focal shift of around 210µm. While the single-material achromats had some performance degradation due to manufacturing tolerances when compared to their nominal performance, they significantly outperformed the conventional achromatic doublet by a factor of three (Figure 3).

Figure 3: Comparison of the measured chromatic focal shift of 18 different single-material achromatic lenses with that of a traditional achromatic doublet shows that the single-material achromats have significantly less chromatic focal shift