✔ Continuous and exact variation of focal length within a wide focal length range
✔ Aberration-free for monochromatic light
✔ Suitable for high power applications
✔ Scalable in aperture and spectrum (from UV to IR)
✔ Thin and lightweight
✔ Temperature-independent (between -20°C and 70°C)
Variable Focus Moiré Lenses consist of specially structured cascaded Diffractive Optical Elements (DOEs). These lenses are continuously adjustable over a wide focal length range by simply rotating the elements. Variable Focus Moiré Lenses exhibit aberration-free performance with monochromatic illumination and can be adapted for wavelengths from the UV to the IR. Typical applications include laser optical assemblies such as variable beam expanders, or as elements within a zoom lens, eliminating the need for complicated translating optomechanics.
Variable Focus Moiré Lenses feature a pair of specially structured diffractive optical elements, fabricated via standard photolithographic techniques. These lenses also consist of a simpler design and construction than alternative liquid, photo-acoustic, or nanomaterial techniques. The simple construction also makes the Moiré lenses insensitive to changes in temperature and vibration resistant.
The plates are also thin, lightweight, and can be fabricated from a variety of materials including those which transmit in the UV or IR spectral regions. Focusing over a wide focal length range is achieved by a simple rotation of the plates, yielding aberration free spots when used with monochromatic illumination.
The first order diffraction efficiency n1 of a Moiré lens depends on the chosen twisting angle θ, resulting in the corresponding optical power:
The total efficiency is a product of the diffraction efficiency and the transmission efficiency nt, which is approximately 96%. For a twisting angle of 90º, the diffraction efficiency is 81%, therefore the total efficiency is around 78%.
To create a custom Variable Focus Moiré Lens, the following is required:
Provided that the photolithographic processing unit of the DOE surface is in the wavelength range (which is true outside of the UV range), the optical power (D) of a Variable Focus Moiré lens can be calculated as follows:
Here θ, stands for the current twist angle of the DOEs and A denotes the clear aperture of the lens.
Equation 1 shows that the diffraction efficiency of the Variable Focus Moiré lens for a twist angle of ±90º (θ = ±π⁄2) is higher than 80%. For this lens with an aperture of the optical power range is as follows:
This means that the optical power of the Variable Focus Moiré lens is inversely proportional to the aperture. The NA has a constant value throughout the modulation of its optical power (NA = 0.24 in example described above).
Variable Focus Moiré Lenses can be manufactured for wavelengths from the UV through the IR. The Moiré pattern of the DOEs is required to be designed for a specific wavelength; maximum efficiency is achieved when the structure heights are equal to an integer multiple of the design wavelength (2π phase shift). The limiting factor in the DOE design is the transmission properties of the substrate. Fused Silica is commonly used for UV or visible applications, whereas Germanium is a common choice for IR applications.