Novel approach for manufacturing of continuously shaped diffractive optical elements

Diffractive optical elements (DOE) play an important role for laser beam shaping in industry, for example in lithography or parallel laser material processing. Typically such applications require high damage threshold and low background illumination (high contrast and efficiency). Usual DOE with binary phase (step-like) profiles are made microlithographically and suffer from substantial scattering on profile derivative discontinuities. That gives also tendency to lower damage threshold as compared to intrinsic material values. The LIMO approach is based contrarily on a proprietary, non etching material processing and is suitable for manufacturing of high-precision free programmable continuous surface profiles in optical glasses and crystals. We report on linear symmetric diffractive beam splitter 1:11 with high homogeneity and efficiency > 95% and discuss also other DOE designs. The design data, simulations with measured surface profiles and experimental intensity distributions are in very good agreement. Furthermore we report on a new type of optical attenuator composed from two DOE gratings. Its dynamic transmission range is 0.3% to 98%. The required lateral DOE shift is only 5 - 10 μm in the present design, so that the device can be very fast and applicable for dynamic intensity stabilization.

Optical microsensors for fluid flow diagnostics

Recent advances in compression molding of glass optical elements for mass production offer the potential of extending this technology to elements with micro and nano scale features. In this research, glass diffractive optical elements (DOEs) with lateral features in the order of 10 µm and vertical height of 330 nm were fabricated using a fused silica glass mold and a special low Tg (glass transition temperature) glass material K-PG325. Molded DOEs were studied using an atomic force microscope (AFM) and scanning electron microscope (SEM) to evaluate the glass molding process capability. Optical testing of the molded DOEs was a further demonstration of the effectiveness of the molding process for high volume micro and diffractive optical component fabrication. The combination of two high-precision, high-volume processes, i.e., semiconductor batch process for optical mold making and glass molding for DOE replication, is an effective alternative manufacturing method for high-quality, low-cost optical components. The reported experiment is a detailed illustration of the glass molding process capability. With further process optimization a robust manufacturing process can be developed for mass production of diffractive and micro glass optical elements.

Researchers from China have proposed a hollow top-hat beam (HTB) diffractive optical element (DOE) shaping method, for use in space uplink chaotic communications systems (SUCCS). Simulation results show that by shaping the beam into a HTB, the bit error-rate (BER) improvement is substantial when compared to the traditional hollow Gaussian beam (HGB). The results could be significant for the optimisation of designs relating to SUCCS. Laser communication is a type of communication technique that, unsurprisingly, uses laser beams as a signal carrier, allowing the transmission of images, sound and other data in free space. It has been conceptualised since the development of the first lasers in the 1960s. When compared to more traditional microwave communication, laser communication has the advantages of allowing faster transmission due to higher data-rate (in principle anywhere from 1,000 to 100,000 times faster), a larger communication capacity, smaller terminal size and lower power consumption. It is anticipated that, due to its higher data-rate, laser communication will be the preferred candidate for the problem of bottlenecking when it comes to space uplink communications. Prof Mi Li (left of centre) and his research group. (a) original Gaussian distribution of laser beam (b) hollow Gaussian beam (HGB) shaped by DOE (c) hollow top-hat beam (HTB) shaped by DOE. Space chaotic laser communication system. Reducing the complexity of designing laser communications systems is a hot topic in the research field of free-space communication. Optical antennas play an important role, where they function as beam collators, expanders and convergers – all of which can improve the performance of free-space communication. One of the most commonly used antennas is the Cassegrain reflective optical antenna, featuring a short mirror body, high magnification and elimination of spherical aberrations (which result in blurry images due to numerous undesired focal points). Cassegrain antennas do come with a drawback, however, in that the secondary mirror can block part of the transmission beam, affecting the transmission efficiency of the optical system. The work presented in this issue of Electronics Letters is applicable to space chaotic laser communications systems, which are more complex than ground optical fiber chaotic communication systems. The authors use a DOE to shape a carrier beam into a hollow, top-hat shape (see inset images). DOEs have been widely used in the field of beam-shaping due to the advantages of high diffraction efficiency, flexible design and simple structure. Intelligent shaping of the beam can eliminate the problem of secondary mirrors in a Cassegrain antenna blocking part of the transmission beam. It can also be used to improve the transmission efficiency of the optical sub-system, meaning that the design pressure on other sub-system parameters is eased. DOEs can be used to shape and split laser beams in an energy-efficient manner and are often used in production facilities for laser material processing, or in medical laser treatments and diagnostics. They also see use in lighting, printing and lithography. In short, DOEs are frequently used in areas where shaping and splitting with minimal light loss is necessary. In order to effectively demonstrate how their suggested HTB improves the quality of the communication system, the authors use BER to evaluate their work. They also compare the HTB to a HGB, which they used in previous research works, to see if there is any difference in quality when considering the two shapes for SUCCS. The research considers strong and weak atmospheric conditions, as atmospheric turbulence also plays a role in the BER of signals moving between Earth and orbit. The work shows that, at higher beam power, BER is reduced by around 10−0.9 when using the HTB shaping method at a low chaotic mismatch and 10−1.5 at a high chaotic mismatch. In both scenarios the HTB shaping outperforms the HGB shaping, showing that the HTB is more suitable for SUCCS. In recent years there have been a number of exciting achievements in the free-space laser communication field. Laser communication has moved from concept to practice, Mbps rates to Gbps rates and we are now seeing the next iteration of communication technology. Research scientists are carrying out research work on small-size, high-speed laser communication stations. The work presented here not only improves the efficiency of the beam, but also makes steps towards reducing the design difficulty of other sub-systems within the communication terminal. The authors hope that this work will also allow a meshing of 5G ground wireless communications and space laser communications networks, and even pave the way for easier realisation of 6G in the future.

In recent years surface-relief Diffractive Optical Elements (DOEs), a specialized class of diffractive optical elements, have revolutionized design of optical systems and instruments. Surface-relief diffractive optical components improve performance of the existing optical systems and increase systems compactness and reliability. Surface-relief DOEs can be fabricated as multilevel binary optical elements, or as continuous surface-relief optical elements. A surface-relief DOE is being designed such that incident light interacts with the topography of the surface-relief microstructure including the size and shape characteristics of surface profile features such as indentations and projections as well as their dimensional relationship, to perform a desired optical function. Binary optical elements are being made by the same microlithographic and ion etching techniques used to produce VLSI circuits. Computer-generated continuous surface-relief DOE is recorded by direct laser-beam or e-beam writing in photoresist. Continuous surface-relief DOE can be also recorded in photoresist by interferometric (holographic) technique. After development recorded in photoresist surface-relief microstructures are being ion-beam etched into quartz or other inorganic optical substrates.

Diffractive optical array element for realizing uniform spot on the plane non-perpendicular to the optical axis with geometrical shape transform

This paper is dedicated to the development of synthesis methods for diffractive optical elements (DOEs) on non-planar surfaces. To form 3D images, a set of color 2D frames is used, visible to an observer from different observation points. The developed DOEs can be called a stereogram with a 360-deg view when the DOE is illuminated with white light. Methods are developed for computing the phase function of the DOE and are applicable to a wide range of convex surfaces. However, the manufacturing technology used limits this range to conical and cylindrical surfaces. The microrelief of the flat DOE is produced using electron-beam lithography. A video recorded using optical elements produced demonstrates the capabilities of the technologies developed. These DOEs can be used to protect brands against counterfeit.

Novel optical materials and manufacturing methods are especially important in the fast growing terahertz (THz) market, where new solutions for shaping and redirecting of radiation are widely investigated. An innovative method of manufacturing refractive and diffractive optical elements (DOEs) for THz spectral range using commercially available and cost-effective devices and materials is presented. The 3D-printing method is used to produce molds, which are then filled with paraffin. The optical elements obtained with this method outperform many commercial ones, especially in higher frequencies. Moreover, a different diffractive element design (as kinoforms of higher order) is proposed to match the resolution limitations of 3-D printing.

The uniform illumination of holographic screens during their recording process is commonly realized by using only the quasi-constant, inner part of the gaussian intensity profile of a very strongly expanded laser beam. This technique is characterized by a very low efficiency (about 5%, depending on the required uniformity). We present a method, which uses refractive, micro optical beamshaping elements in order to create a rectangular, extraordinarily uniform, flat-top intensity profile with minimal phase aberrations. This allows the use of about 80%-90% of the provided optical power for the illumination of the holographic screen. To ensure the required quality of the illumination wave, a spatial frequency filtering has to be applied. For certain holographic applications, requiring a combination of high beam-divergence and high optical power, conventional pinholes are either too thin or too sensitive. To solve this problem, we present a new concept of dielectric pinholes, based on optical microstructures. The combination of beamshaping elements and dielectric pinholes allowed us to extend the available parameter range during the recording of holograms.

Cylinder symmetry of test objects allows grazing incidence tests. Here we propose the use of two diffractive optical elements (DOEs) instead of a single DOE in perpendicular incidence. The splitting of the optical function of the DOEs allows low spatial frequencies in the diffractive elements. In fact, the diffractive structures are curves parallel to the meridian of the cylinder lens. Here an interferometric test is described where identical DOEs serve as beamsplitter and combiner. The undiffracted wavefront serves as reference. Phase shifting evaluations are possible by axial translations of one of the DOEs. The effective wavelength can be chosen within wide limits. Actually, we used a pitch of 5µm, which is identical to the effective wavelength of the test. A surface deviation of 2.5µm corresponds to a fringe distance. The interferometric test enables rms values of better than 20 nm for any aspheric geometry of the meridian curve, even with lenses having numerical apertures up to 0.9.

Pattern transfer for diffractive and refractive microoptics

A method for computing and synthesizing a diffractive optical element (DOE) on a cylindrical surface is proposed. The computer-generated hologram is a phase reflecting DOE, which, when illuminated with white light, displays a 3D image with a 360° viewing angle. The optical element consists of hogels with the size of about 50 microns, which are partly filled with diffraction gratings of various periods and orientations. The DOE is synthesized using electron-beam technology with a resolution of 0.1 micron. Flat optical elements can be replicated using standard equipment employed to produce rainbow relief holograms. When placed on a cylindrical surface, optical elements made on a flexible base display a 3D image observed at 360° viewing angles. The DOE developed can be used to protect objects against counterfeit.

Laser beam shaping refers to the process of altering the spatial distribution of a laser beam's intensity to achieve a desired irradiance profile at a target plane. This manipulation is an essential step in ensuring the beam's characteristics align with the specific requirements of the application. Various approaches, such as the use of refractive and diffractive optical elements, are used to reshape the beam while maintaining efficiency.

Free-space optical interconnects for microelectronics and multiprocessor parallel computing systems offer better scalability because less silicon area will be occupied by interconnections and they have a faster clock speed and higher bandwidth at power levels than do purely electronic systems. For stability and ruggedness optoelectronic systems that combine optics with electronics based on free space optical interconnection need to be packaged. In packaging these types of systems, diffractive optical elements (DOEs) are often used because their fabrication is compatible with VLSI fabrication and they can be flexibly designed by computer. The volume of the packaged system depends on the f-number of the DOE utilized in the system: the smaller the f-number, the more compact the packaged system. Lower f- number DOEs can be obtained by stacking more than one DOE together. Stacking also helps to achieve larger diffraction angles to facilitate longer distance interconnects. The problems to be solved are (a) accurate alignment between DOEs on the opposite sides of one substrate or between DOEs on separate substrates and (b) bonding or assembly of separate substrates. In this presentation, the alignment and assembly procedures for stacking more than one DOE together will be discussed, and the experimental results shown.

Snapshot Compressive Spectral Imaging Systems (SCSI) compress the scenes by capturing 2D projections of the encoded underlying signals. A decoder, trained with pre-acquired datasets, reconstructs the spectral images. SCSI systems based on diffractive optical elements (DOE) provide a small form factor and the single DOE can be optimized in an end-to-end manner. Since the spectral image is highly compressed in a SCSI system based on a single DOE, the quality of image reconstruction can be insufficient for diverse spectral imaging applications. This work proposes a multishot spectral imaging system employing a double-phase encoding with a double DOE architecture (DoDo), to improve the spectral reconstruction performance. The first DOE is fixed and provides the benefits of the diffractive optical systems. The second DOE provides the variable encoding of the multishot architectures. The work presents a differentiable mathematical model for the multishot DoDo system and optimizes the parameters of the DoDo architecture in an end-to-end manner. The proposed system was tested using simulations and a hardware prototype. To obtain a low-cost system, the implementation uses a deformable mirror for the variable DOE. The proposed DoDo system shows an improvement of up to 4dB in PSNR in the reconstructed spectral images compared with the single DOE system.