We present designs of a diffractive polarizer having low zeroth-order reflectivity that is compact, potentially mass-producible and cost-effective, and compatible for high-power applications. It consists of subwavelength grating structures superimposed over a diffraction grating. Using rigorous coupled wave analysis, we optimized the parameters of multilevel grating structures to achieve antireflection for both incident TE and TM polarization states with one polarization passing in the zeroth order and the other into the first and higher orders. We focused on polarizer designs for the 1.31-µm wavelength range, and the theoretical values for zeroth-order reflection were calculated to be 0.01% for TE and 0.29% for TM modes. The zeroth-order transmission efficiencies were 97.2% for TE and 0.01% for TM modes. A prototype of one design was fabricated and tested to verify the functionality of the device, and the zeroth-order reflection was determined to be 1.2% and 3.5% for the two modes.
This paper describes a compressive sensing strategy developed under the Compressive Optical MONTAGE Photography Initiative. Multiplex and multi-channel measurements are generally necessary for compressive sensing. In a compressive imaging system described here, static focal plane coding is used with multiple image apertures for non-degenerate multiplexing and multiple channel sampling. According to classical analysis, one might expect the number of pixels in a reconstructed image to equal the total number of pixels across the sampling channels, but we demonstrate that the system can achieve up to 50% compression with conventional benchmarking images. In general, the compression rate depends on the compression potential of an image with respect to the coding and decoding schemes employed in the system.
With this work we show the use of focal plane coding to produce nondegenerate data between subapertures of an imaging system. Subaperture data is integrated to form a single high resolution image. Multiple apertures generate multiple copies of a scene on the detector plane. Placed in the image plane, the focal plane mask applies a unique code to each of these sub-images. Within each sub-image, each pixel is masked so that light from only certain optical pixels reaches the detector. Thus, each sub-image measures a different linear combination of optical pixels. Image reconstruction is achieved by inversion of the transformation performed by the imaging system. Registered detector pixels in each sub-image represent the magnitude of the projection of the same optical information onto different sampling vectors. Without a coding element, the imaging system would be limited by the spatial frequency response of the electronic detector pixel. The small mask features allow the imager to broaden this response and reconstruct higher spatial frequencies than a conventional coarsely sampling focal plane.
In the low-to-mid IR wavelength range there is a need for high performance, cost effective aspheric optics. Silicon has many advantages including high transmission and a high refractive index, but it can be very difficult to diamond turn. The resulting fabrication errors reduce efficiency and increase scattering and stray light. Wafer-based lithographic techniques can be used to make diffractive and refractive elements in both silicon and germanium. Advantages of diffractive structures such as: thinner elements, highly aspheric and even non-rotationally symmetric phase functions and chromatic compensation make this an attractive technology compared to diamond turning. In addition, wafer based fabrication makes these elements cost-effective in many applications. At Digital Optics Corporation, we have designed and fabricated wafer-based optics for use in the 1.3-14 micron range. In this paper, we will discuss the design, fabrication and evaluation of several product categories including a diffractive germanium beamshaper, a diffractive silicon aspheric lens, and a diffractive silicon spiral lens.
This paper presents the passively aligned Wavesetter (PAWS) locker: a micro-optic subassembly for use as an internal wavelength locker. As the wavelength spacing in dense wavelength division multiplexing (WDM) decreases, the performance demands placed upon source lasers increase. The required wavelength stability has led to the use of external wavelength lockers utilizing air-spaced, thermally stabilized etalons. However, package constraints are forcing the integration of the wavelength locker directly into the laser module. These etalons require active tuning be done during installation of the wavelength locker as well as active temperature control (air-spaced etalons are typically too large for laser packages). A unique locking technique will be introduced that does not require an active alignment or active temperature compensation. Using the principles of phase shifting interferometry, a locking signal is derived without the inherent inflection points present in the signal of an etalon. The theoretical background of PAWS locker will be discussed as well as practical considerations for its implementation. Empirical results will be presented including wavelength accuracy, alignment sensitivity and thermal performance.
Etalons having one surface which is highly reflective have been used for a variety of applications. By varying the coating type and carefully controlling the thicknesses of the coatings on the lower reflectance side, one can obtain interesting and useful properties. One example is a low finesse but highly efficient element having a reflectance which is very sinusoidal with respect to wavelength. By adding additional layers, functions which are asymmetric about the reflectance peak with respect to wavelength can be obtained, including behavior which approximates a sawtooth reflectance as a function of wavelength. Such devices are easily fabricated at the wafer scale, and can be used in wavelength monitoring and control applications such as wavelength lockers for tunable lasers.
When considering a roadmap for technology development, the essence of the problem can be framed by two simple questions: 1) Where have we been? and 2) Where do we need to go? The first question is relatively easy to answer with some research, but answering the second requires significantly more effort. In this paper, we address these questions as they relate to the fabrication of diffractive and refractive micro-optics. A brief historical overview of micro-optics fabrication is presented, followed by our predictions on the future of the field. Examples of future applications, technical challenges, and supporting technologies required for manufacturing of different types of micro-optics are discussed.
In order to successfully transition optical processor prototypes from research laboratories to commercial markets, new packaging and manufacturing technologies will be needed. One approach which has been discussed is the use of reflective, off-axis diffractive optical elements (DOEs) in place of refractive optics. In this type of architecture the reflective DOEs are placed predominantly on a single planar surface which faces a second surface on which active devices such as laser diodes, spatial light modulators, and detector arrays are located. Light is deflected away form the surface normal of the planes so that it can propagate from one device to the next within the processor. This offers potential benefits of compact size, low cost mass production, and generic system designs. We are investigating the design, manufacture, and performance of optical processors which combine DOEs and FLC-VLSI spatial light modulators in this type of architecture.
Diffractive optical elements (DOEs) have many advantages over refractive optical elements including the ability to implement exotic function (such as flat-tops, line generators and splitting and combining functions), the ability to easily incorporate a variety of functions in to one element, lower volume and less weight. In addition to these advantages, diffractives offer 3 potential positive characteristics which are sometimes cited as drawbacks. These are: diffraction efficiency, dispersion and cost. In some applications, such as wavelength division multiplexing (WDM) or other applications in which it is desirable for different wavelengths of light to be affected in different manners, the highly dispersive nature of diffractives is an advantage. In other applications when the spectral width of the illumination is large (e.g. laser diodes when the case temperature varies over a wide range), the dispersion of DOEs can be a disadvantage. Diffraction efficiency, defined as the power diffracted into the desired diffraction order divided by the power incident on the DOE, can be very high or low depending on the application and design procedure. This paper focuses on these 3 potential advantages of diffractives. In the remainder of this paper each characteristic is discussed individually in order to show how the negative effects of each can be minimized and the positive effects enhanced.
Binary optical elements are finding increased use in a wide range of applications. Fabrication of binary optical elements generally remains a time consuming and expensive process. Even in high volume, costs can be prohibitive, especially for elements larger than a few millimeters in diameter. Replications techniques, such as injection molding, have recently begun to show promise as a means of manufacturing binary optical elements, and doing so at a significant cost reduction over conventional photolithographic means. We report on recent improvements in the application of injection molding techniques to the replication of binary optical elements. Several examples of binary elements fabricated by these techniques, as well as present process capability, will be discussed.
Diffractive optics is becoming a standard part of the optical designer's toolkit. The transition from design to manufacturing, especially for elements larger than a few millimeters in diameter, has been impeded by the relatively high cost of producing diffractive elements by standard photolithographic means. Replications techniques, such as injection molding, have the potential to significantly lower the cost for such elements. We report on results of the application of injection molding techniques to the replication of diffractive elements. Several examples of diffractives fabricated by these techniques, as well as present process capability, are discussed.
Diffractive optics have the potential to play a key role in several areas of head mounted displays. They can reduce size and weight while providing some unique optical functions that would be difficult to implement with conventional refractives. There are four areas in which diffractive optics may contribute: Magnifier optics, combiner optics, head and hand tracking, and optical data interface. This paper is primarily concerned with the introduction of a new image combiner element based on Babinet's principle.
The design and fabrication of a low cost laser diode to fiber optic coupler is discussed. A single diffractive optical element was used to provide uniform coupling efficiency over a 40 nm bandwidth. The element was optimized to maintain constant coupling efficiency with small tilts and decenters. An iterative method referred to as radially symmetric iterative discrete on-axis (RSIDO) encoding was used to determine optimum fringe placement and profile.