Diffractive optics technologies further advance photonic systems

The proliferation of diffractive optics technologies into military and consumer markets has been driven by recent advances in the modeling, fabrication, and performance characterization of diffractive components. Diffractive optical elements (DOEs) offer additional degrees of freedom by enabling control over the propagation, dispersion, and polarization of light in photonic instruments.¹ As such, they are an integral part of several novel photonic products and systems, including gaming devices, heads-up displays, high-power lasers, and instrumentation for exoplanet research.²

Diffractive optics provide the developers of photonic instrumentation with additional flexibility in systems design, leading to hardware solutions with reduced size and weight, as well as enhanced performance. They are playing an increasingly important role in the spectral regions where optical-quality vitreous materials are sparse or not readily attainable. This especially applies to long-range wavelengths, from mid-wave IR (MWIR) to terahertz (THz), a range in which DOEs perform exceptionally well.

The incorporation of DOEs allows for significant reductions to the size and weight of broadband and multi-band imaging optical systems.³ Figure 1(a) presents an example of a dual-band IR objective lens.⁴ This design uses a six-element Petzval objective lens containing two lens groups. Each lens group is based on three lens components, made of three different materials: zinc sulfide, zinc selenide (ZnSe), and gallium arsenide (GaAs), respectively. These lenses are designed to operate within the spectral ranges of 3–5 and 8–12μm. The weight of the six optical components comprising the lens is estimated to be 2.82kg.

Figure 1. (a) Example of a dual-band IR design using a six-element Petzval objective lens containing two lens groups,⁴and (b) a similar setup incorporating a diffractive optical element (DOE), a diffractive doublet. GaAs: Gallium arsenide.

Figure 1(b) presents our alternative design for a dual-band IR objective lens, with optical specifications that match the original lens-based design.⁴ By using DOEs, we have been able to reduce the number of lens components from six to three; the weight of the optical components by three times (to 0.99kg); and the axial length by 10%. The design also eliminates highly toxic materials (such as GaAs) and employs only two commonly used IR materials (germanium and ZnSe). These significant improvements would not have been possible without the use of diffractive doublet technology.¹ A diffractive doublet, comprising two DOEs made of dissimilar materials, allows diffraction efficiency to be extended over a broad spectral range. Similar to a refractive doublet, the optical power of a diffractive doublet is the difference between the optical powers of the two components comprising the doublet. Figure 2 compares the broadband diffraction efficiency of a diffractive doublet designed for operation over the mid- and long-wave IR spectral regions with the diffraction efficiency of a diffractive singlet. At 80% efficiency, the diffractive doublet has a spectral bandwidth of 8.8μm, compared to only 2.5μm for the diffractive singlet.

Figure 2. Diffraction efficiencies of diffractive structures.

Significant breakthroughs have been made in the fields of structured laser illumination and beam control using DOEs.⁵ Recent developments in electronically controlled spatial phase modulators (SPMs) provide the foundation for fundamentally new ways of controlling spatial light distributions. For example, intracavity placement of an SPM allows for on-demand control of the laser beam modal structure.⁶

By developing and employing a coherent laser field enhancement technique,⁷ we have been able to achieve unprecedented flexibility in electronically controlled structured light distributions. Figure 3 shows the schematic layout of our optical system, which produces coherent modifications of laser radiation based on the precise control of diffractive structures. This optical system contains two diffractive optical components as well as two relay lenses for modifying the input coherent laser beam. By controlling the phase and amplitude transmission characteristics of the two DOEs, a variety of output field distributions can be produced. Figure 4 shows three different irradiance patterns of structured light produced in the output plane of the optical system, based on coherent laser field enhancements. These structured illumination patterns, which can be dynamically adjusted using SPMs, could be employed in a variety of interactive gaming applications, in LIDAR systems, and in sensors for autonomous robotic navigation.

Figure 3. Schematic layout of an optical system for producing electronically controlled structured light distributions. DOE 1 diffracts the laser beam. Lens 1 modifies the phase factor of the light. DOE 2 introduces an optical phase delay to the beam. Lens 2 introduces a phase transformation, allowing the beam to be refocused onto the output plane with a structured light distribution.

Figure 4. Irradiance patterns produced by coherent enhancements of laser radiation using DOEs.

In summary, due to the many advancements and growing sophistication in the field of photonic instrumentation, we expect diffractive optics to evolve as an increasingly important technology. DOEs, capable of performing over a broad range of wavelengths, provide optical designers and instrumentation developers with additional choices in system architectures. To incorporate these innovations into the next generation of volume-produced photonic instruments and products, we intend to tailor their designs to specific applications, including manufacturing and assembly.