Dean Faklis

Spectral properties of multiorder diffractive lenses

Diffractive lenses have been traditionally designed with the first diffracted order. The spectral characteristics of diffractive lenses operating in higher diffracted orders differ significantly from the first-order case. Multiorder diffractive lenses offer a new degree of freedom in the design of broadband and multispectral optical systems that include diffractive optical elements. It is shown that blazing the surface-relief diffractive lens for higher diffraction orders enables the design of achromatic and apochromatic singlets. The wavelength-dependent optical transfer function and the associated Strehl ratio are derived for multiorder diffractive lenses. Experiments that illustrate lens performance in two spectral bands are described, and the results show excellent agreement with the theoretical predictions.

Broadband imaging with holographic lenses

Broadband imaging systems that contain holographic lenses can be lightweight and have large apertures. We report a Fresnel diffraction analysis of an imaging system that consists of three lenses of arbitrary dispersion. A general solution is found for the wavelength dependence of the lenses to simultaneously correct the imaging system for both longitudinal and lateral paraxial chromatic aberration. As a special case, we describe an optical system that uses holographic lenses to produce a well-corrected image in broadband light. Experimental results that demonstrate the system performance in both laser and broadband illumination are reported.

Diffractive optics technology for display applications

Diffractive optics technology offers optical system designers new degrees of freedom that can be used to optimize the performance of optical systems. For example, the zone spacing of a diffractive lens can be chosen to impart focusing power as well as aspheric correction to the emerging wavefront. The surface (or blaze) profile within a given zone determines the diffraction efficiency of the element, or in other words, determines how the incident energy is distributed among the various diffraction orders. In this review, we present the fundamental properties of diffractive lens systems that can be useful for display applications. We also review briefly the application of multi-order diffractive lenses and subwavelength structured surfaces.

High-efficiency replicated diffractive optics

Diffractive optics technology offers optical system designers new degrees of freedom that can be used to optimize the performance of optical systems. The zone spacing of a diffractive lens can be chosen to impart focusing power as well as aspheric correction to the emerging wavefront. The surface (or blaze) profile within a given zone determines the diffraction efficiency of the element, or in other words, determines how the incident energy is distributed among the various diffraction orders. Unless the zone profile is generated with high fidelity, incident energy will be distributed into extraneous diffraction orders, which generally reduces the optical system performance. Several diffractive optical components have been fabricated using replication techniques that provide high-efficiency and accurate wavefront generation. Typical minimum efficiency measurements at the design wavelength for diffractive zone spacings greater than 10 micrometers are 95% or above. For minimum zone features as small as 5 micrometers , the measured efficiency is greater than 85% at the design wavelength. The integrated diffraction efficiency, which is a weighted measure of the efficiency across the clear aperture, is typically 2 - 3% more than the efficiency measurement at the minimum feature.

DIFFRACTIVE LENSES CREATE NEW OPPORTUNITIES

FAKLIS PROVIDES AN OVERVIEW OF DIFFRACTIVE OPTICS INCLUDING CHARACTERISTIC PROPERTIES AND FABRICATION METHODS. HE ALSO CITES KEY APPLICATION AREAS AND DESCRIBES SEVERAL CHALLENGES CURRENTLY FACING DIFFRACTIVE LENS MANUFACTURERS.

Application of diffractive optics to laser scan lenses

The control of optical distortion is useful for the design of a variety of optical systems including those used for laser scanning. A lens used for focusing a scanned laser beam onto a flat image field with constant intensity profile must also satisfy the f-(theta) condition, i.e., the image height is proportional to the input field angle itself, so that the scan velocity across the image plane remains constant. The lens needs to be free from coma, astigmatism, and field curvature and must have a prescribed amount of distortion. We describe the design and development of a diffractive f-(theta) lens and present experimental verification of the theoretical predictions.

Broadband Imaging With Combinations Of Holographic And Conventional Lenses

Broadband imaging systems that contain holographic lenses can be lightweight and have large apertures. We report a Fresnel diffraction analysis of an imaging system that consists of three lenses of arbitrary dispersion. A general solution is obtained for the wavelength dependence of the lenses to simultaneously correct the imaging system for both longitudinal and lateral paraxial chromatic aberration. As a special case, we describe an optical system that uses holographic lenses to produce a well-corrected image in broadband light. Experiments are reported that demonstrate system performance in both laser and broadband illumination.

Effects of diffraction efficiency on the performance of diffractive relay optics

The requirements for head-mounted displays (HMDs) continue to become more demanding. Increased light throughput, reduced weight and higher optical performance mandates the use of new optical technologies. Diffractive optical elements can provide the optical designer additional degrees of freedom to develop more sophisticated lightweight color HMD systems. A method of design is discussed along with several manufacturing methods for producing diffractive elements including single-point diamond turning and laser pattern generation. Due to the unique characteristics of diffractive elements regarding efficiency and diffraction orders, it is vitally important to properly characterize and test these systems. The efficiency of a diffractive element can have a profound impact on the performance of the optical system; therefore, it is necessary to accurately measure the diffraction efficiency and correctly interpret the impact on the MTF. An example of a hybrid diffractive/refractive relay lens is presented to demonstrate the relationship between the MTF and diffraction efficiency.

Diffractive optics technology for space-based sensors

Diffractive (or binary) optics offers unique capabilities for the development of high- performance, low-weight optical systems for space-based sensors. The basic operating principles of diffractive optical elements along with fabrication methods suitable for production of diffractive elements for space-based applications are described. Several potential applications where diffractive optics may serve as a key technology for improving the performance and reducing the weight and cost of sensors for the Geostationary Earth Observatory will be discussed. These applications include the use of diffractive/refractive hybrid lenses for the Lightning Mapper Sensor, diffractive telescopes for narrowband imaging and subwavelength structured surfaces for antireflection and polarization control.

Multichannel imaging using diffractive optics

A multiple imaging system can produce a two-dimensional array of images from a single input object. The basic components of our implementation are a high-performance Fourier processor and a suitably designed diffraction grating that is placed in the frequency phase. We have made use of a single diffractive element Fourier transform lens, corrected for coma, astigmatism, and field curvature. The lens is designed to have the proper amount of distortion to produce an optical transform. The optical processing system consists of two of these lenses spaced a distance equal to the sum of their focal lengths. We employ an iterative algorithm to produce the necessary phase diffraction grating that generates a multiplicity of equal energy plane waves from a single incident plane wave. The method is based on an iterative Fourier- transform scheme that makes use of known constraints to synthesize the phase structure. The constraints include the known Fourier modulus (array of point sources), the phase-only requirement in the grating plane, and fabrication requirements such as discrete phase levels. The iterative algorithm provides an efficient means to calculate grating profiles that give rise to large arrays of images. System performance over a relatively wide field-of-view is characterized by calculating the modulation transfer function at various points in the image plane.