Symmetry Free Design (SFD)
Conventional optical design is based on combinations of optics with planar surfaces, such as prisms and flat mirrors, and cylindrical and spherical surfaces, such as lenses and curved mirrors. Optical designers push performance far beyond that available by single plano and spherical elements in a number of ways:
- Spherical aberrations in high-NA systems are minimised by ingenious multiplet lens designs
- Crude beam shaping can be achieved by combining lenses in ways that exaggerate the effects of spherical aberration
- Multi-beam optical systems are fabricated by mechanically assembling arrays of large numbers of prisms and lenses
These approaches are taken to overcome the limitations imposed by conventional optical fabrication techniques (i.e. grinding and lapping flats, grinding and polishing spheres) but each incurs a significant cost, in terms of additional elements, mechanical assembly and the impact on system performance of an increased number of surfaces.
The availability of a process to generate entirely freeform optical surfaces would allow many serial (e.g. multiplet) and parallel (e.g. discrete lens array) optical systems to be reduced to a single, robust, monolithic, mechanical element. One of the best examples of this is the lens in a CD player read head: originally a heavy and expensive glass multiplet costing several hundred dollars, these are now small, light and fabricated in million-off quantities as plastic molded aspheres with unit price well below one dollar.
Truly freeform fabrication offer a transformational change in the capability of refractive optics, overcoming the restrictions of conventional optics in three key ways:
- Removal of shape restrictions
- Removal of symmetry restrictions
- Monolithic parallel integration of optical elements
Removal of shape restrictions, by moving from spheres to aspheres, enables improved system performance, along with reduced system cost and complexity. It also provides a new design flexibility that enables the creation of optical functions, such as Gaussian to flat-top beam transformers, that cannot effectively be fabricated using spherical surfaces. Removal of symmetry restrictions enables fabrication of astigmatic and anamorphic optics that are particularly important for asymmetric light sources such as diode lasers.
Monolithic parallel integration of elements into prism and lens arrays avoids mechanical assembly costs, reduces adhesive-related issues such as cure time and outgassing and improves lens pitch tolerance, routinely achieving pitch error of well below 1µm. The gains available by using freeform optics extend well beyond these three key advantages, however. Using a single surface, or a pair of surfaces in a single element, to combine multiple optical functions enables a reduction in system cost, weight, complexity, assembly time, whilst maximising optical performance and efficiency. A simple example of this is the PowerPhotonic SmileSAC, which is mechanically almost identical to a standard diode laser bar slow-axis collimator array, but incorporates the additional function of diode bar smile correction, increasing system performance with no increase in mechanical complexity or assembly time.
Freeform application: Diode laser bar slow-axis collimator (SAC), with integrated smile correction
Freedom from the restrictions of standard fabrication techniques also enables greater freedom in the design tools that can be used – there is no need to adhere to standard asphere and acylinder descriptions: optical surfaces can be defined as generally as a point cloud or a nurb surface, providing the designer with unlimited scope for the realisation of new functions.
The simplest benefit of freeform optics is often the most powerful: freedom from the restrictions and compromises inherent in using the closest match of available catalogue components. When choice of lens radius of curvature is combined with conic constant and lens array pitch, stock lens arrays can only address a tiny fraction of the design space. Freeform provides the freedom to specify exactly what your design needs.