Binary Optical Elements Research Articles

Axial Multifocus Beams Formed by Binary Optical Elements

Axial multifocus beams (AMBs) potentially offer convenience in scientific and industrial applications. However, the general approach used for forming AMBs based on the Fresnel zone plate limits the conceivable applications. In this paper, we introduce a custom-made method to generate AMBs by the point source method to obtain initial phase distribution and a modified Gerchberg-Saxton algorithm to iteratively optimize the phase distribution. Meanwhile, the sidelobe is also constrained in the design simultaneously. Here, foci numbers, positions, and intensity distributions of the AMBs have been designed by binary optical elements, which can be controlled readily and effectively. Both numerical simulations and experiment results show the validity of the proposed method.

Detour phase Talbot array illuminator

In this Letter, we propose a simple and effective approach for transforming a conventional Talbot array illuminator (TAI) with multilevel phase steps into a binary-phase TAI (BP-TAI) through detour phase encoding. The BP-TAI is a binary (0 π) phase-only diffractive optical element, which can be utilized to generate a large-scale focal spots array with a high compression ratio. As an example, we design a square BP-TAI with the fraction parameter β = 15 for achieving a square multifocal lattice with a high compression ratio β2. Theoretical analysis and experimental results demonstrate that the detour phase encoding is efficient for designing the BP-TAI, especially with the high compression ratio. Such results may be exploited in practical large-scale optical trapping and X-ray imaging.

Multiring pure-phase binary optical elements to extend depth of focus.

Multiring pure-phase binary optical elements (BOEs) are widely used to extend the depth of focus (DOF) in many optical applications. Although researchers have designed various BOEs to extend the DOF, few theories and experiments have been reported to validate the performances of different N-ring pure-phase BOEs to realize the DOF as long as possible. In this paper, aberration theory is used to obtain the simple and straightforward initial phase, and a novel modified Gerchberg-Saxton algorithm is presented for generating N-ring 0-π-phase BOEs to optimally extend the DOF. Theoretical, numerical, and experimental results demonstrate that the DOF can be extended with increased N in the same NA.

Binary code DOE optimization for speckle suppression in a laser display.

Using a laser light source in imaging devices provides a wide color gamut, high brightness, resolution, and efficiency. At the same time, it creates a speckle pattern that deteriorates the image quality. This paper is related to the application of the moving binary code diffractive optical element (DOE) for speckle suppression in a laser display. Analytical optimization of DOE parameters is made in this paper. The optimal DOE pitch is found analytically. Limitation of the DOE code length is shown, and the highest possible code length is estimated. Application of the compound Barker code for the DOE design is suggested. As an alternative, the M-sequence and minimum peak side-lobe codes are considered. The expected residual speckle contrast ratio after application of those codes is estimated for the number of codes. A comparison of the different codes is done, and recommendations for speckle suppression DOE design are provided.

Using the spatial light modulator as a binary optical element: application to spatial beam shaping for high-power lasers.

The liquid crystal spatial light modulator (SLM) is an effective active beam-shaping device through adjusting each pixel transmittance to improve the spatial beam quality of the output laser, which can also be used as a binary optical element (BOE) with each pixel transmittance 0 or 1 to realize spatial beam shaping for high-power lasers. We present and demonstrate an efficient shaping method of the SLM used as BOE based on diffraction principle. The method can be used to control the output nearfield actively by compensating the spatial nonuniformity of transmission and amplification in the high-power laser system. Results show the output nearfield beam quality improves significantly after shaping by using this method with the fluence contrast changing from 22% to 11.3% within only two shots in the single-shot operation laser.

Multiple-plane image formation by Walsh zone plates.

A radial Walsh filter is a phase binary diffractive optical element characterized by a set of concentric rings that take the phase values 0 or π, corresponding to the values + 1 or -1 of a given radial Walsh function. Therefore, a Walsh filter can be re-interpreted as an aperiodic multifocal zone plate, capable to produce images of multiple planes simultaneously in a single output plane of an image forming system. In this paper, we experimentally demonstrate for the first time the focusing capabilities of these structures. Additionally, we report the first achievement of images of multiple-plane objects in a single image plane with these aperiodic diffractive lenses.

Multifunctional binary diffractive optical elements for structured light projectors.

A set of diffractive optical elements for multiple-stripe structured illumination was designed, fabricated and characterized. Each of these elements with a single layer of binary surface relief combines functions of a diffractive lens, Gaussian-to-tophat beam shaper, and Dammann beam splitter. The optical investigations of laser light patterns at 20° fanout angle reveal up to 88% diffraction efficiency, high contrast, and nearly diffraction limited resolution. The developed technology has the potential for reducing complexity, number of optical components, power consumption and costs of structured light projectors in mobile and stationary 3D sensors.

Flexible binary phase photon sieves on polyimide substrates by laser ablation.

A binary phase diffractive optical element photon sieve is fabricated by direct laser ablation of a thin, flexible polyimide substrate with a nanosecond-pulsed ultraviolet laser. The binary phase photon sieve operates at 633nm and was designed with 19 rings and a focal length of 400mm. The total time to fabricate the photon sieves was tens of seconds. The surface properties of the laser-processed areas are examined, and the optical performance of the photon sieve is characterized and compared to FDTD simulations. By optimizing the laser fluence and travel distance between laser pulses, features with sub-wavelength surface roughness were achieved. The photon sieve showed good focusing ability with suppressed side-lobes. When the fractional area of photon sieve pinholes was made to approach 50%, the binary sieve diffraction efficiency approached 11%, matching the highest value reported in the literature for a photon sieve. Thus, this Letter demonstrates both high efficiency and lightweight diffractive optics suitable for space satellite and other applications, with capabilities for low cost and high throughput fabrication.

Extended focus depth for Gaussian beam using binary phase diffractive optical elements.

A novel technique to improve the focus depth of a Gaussian beam is presented in this paper. The improvement is based on two-step beam shaping using a cascade of binary phase diffractive optical elements (BPDOEs). The first BPDOE transforms the incident Gaussian beam into a high-order radial Laguerre-Gaussian beam (LGp0). Then the second BPDOE rectifies the obtained LGp0 beam and gives rise to a quasi-Gaussian one in the focal plane of a converging lens. This resulting quasi-Gaussian beam exhibits a lower divergence and larger focus depth compared to the pure Gaussian beam having the same beam waist. These results open new possibilities in laser beam manufacturing and micromachining, and in applications that need an extended focus depth.

Spark-generated microbubble cell sorter for microfluidic flow cytometry.

High-speed and accurate cell sorting is of great significance for cell analysis regarding both bioresearch and clinical application. Different from the jet-in-air sorting of commercial flow cytometers, sorting in fully enclosed and disposal microfluidic chips can avoid aerosols and crosscontamination, thus contributing to the improvement of biosafety and test accuracy. However, current microfluidic sorters usually require complicated structures, or otherwise cannot attain high throughput. In this article, a sorting mechanism for microfluidics is proposed for the first time based on the jet flow induced by the spark-generated cavitation microbubble that can be easily realized by a pair of electrodes. The sorter was integrated into a microfluidic chip based on three-dimensional (3D) hydrodynamic focusing and a binary optical element (BOE) for laser illumination. Besides, several aspects of the sorting mechanism were studied to optimize the device. It achieved a switching time of 250 μs at the sample flow velocity of 5 m/s and performed the continuous operation at 200 Hz. Both the stability of fluorescence signals and the viability of cells were basically maintained. To conclude, this work explores a new on-chip sorting mechanism which possesses the merits of simple structure, easy control, and fast switching. © 2018 International Society for Advancement of Cytometry.

Diffraction symmetry of binary Fourier elements with feature sizes on the order of the illumination wavelength and effects of fabrication errors

When building spot array binary Fourier diffractive optical elements (DOEs) having feature sizes on the order of the wavelength, we noticed remarkable variations in the experimental diffraction efficiency compared to the simulation results. Even with the use of high-cost electron beam lithography and rigorous Fourier modal method simulations, there appear to be no publications, to the best of our knowledge, showing close agreement in diffraction efficiency between the simulation and experimental results. In this Letter, we show that the diffraction symmetry of binary Fourier DOEs can be an efficient and consistent metric for evaluating the limit of the thin-element approximation and the effects of fabrication errors.

Focusing anomalies with binary diffractive optical elements.

Binary diffractive optics have been extensively studied to date as tools for arbitrary laser beam shaping and experimentally implemented with etched transparent optics and spatial light modulators. Here we demonstrate that a simple one-step binary optic is able to enhance the intensity of a focused beam, displaying some counterintuitive focusing anomalies. We explain these effects by considering the optical aberrations in binary diffractive optics and outline how this may be exploited for further improvements in refractive/diffractive combinations for super-resolution microscopy.

The minimum number belts of DOE for Bessel–Gauss beams to generate a strong longitudinally polarized beam

In this paper, we minimize the number of belts on the diffractive binary optical element (DOE) to generate a strong longitudinally polarized beam by focusing a radially polarized Bessel–Gaussian beam. Based on a combination of the three-belt DOE and a high-numerical aperture parabolic mirror focusing system, a needle of strong longitudinally polarized beam can be achieved. The transverse full width at half maximum of this longitudinally polarized beam is 0.40 λ and the longitudinal field power fraction reaches 85%. In alternative focusing elements, three belts are the minimum number of the DOE for Bessel–Gauss beams.

Binary zero-order diffractive and anti-reflective optical elements in silicon for the mid-infrared.

We propose a novel method for the design of binary two-level diffractive optical elements that provide an efficient diffractive function while reducing the reflectivity of a high-index interface. The structure can be fabricated with a single patterning step and is particularly useful for the mid-infrared spectral range. The concept is based on zero-order transmission gratings using subwavelength microstructures. To demonstrate the concept a Dammann grating has been realized in silicon and characterized in the mid-infrared by scanning the far-field intensity distribution.

Transversely polarized sub-diffraction optical needle with ultra-long depth of focus

We generated purely transversely polarized sub-diffraction optical needles with ultra-long depth of focus (DOF) by focusing azimuthally polarized (AP) beams that were modulated by a vortex 0–2π phase plate and binary phase diffraction optical elements (DOEs). The concentric belts’ radii of the DOEs were optimized by a hybrid genetic particle swarm optimization (HGPSO) algorithm. For the focusing system with the numerical aperture (NA) of 0.95, an optical needle with the full width at half maximum (FWHM) of 0.40λ and the DOF of 6.23λ was generated. Similar optical needles were also generated by binary phase DOEs with different belts. The results demonstrated that the binary phase DOEs could achieve smaller FWHMs and longer DOFs simultaneously. The generated needles were circularly polarized on the z-axis and there were no longitudinally polarized components in the focal fields. The radius fabrication errors of a DOE have little effect on the optical needle produced by itself. The generated optical needles can be applied to the fields of photolithography, high-density optical data storage, microscope imaging and particle trapping.

Detection of the binary optical element based on ptychography

Due to the extremely high diffractive efficiency and flexible design freedom, binary optical element can realize specific function in the optical system in comparison with the traditional refractive optical element. Ptychography, which is a typical lensless optical imaging technology with simple structure, has the advantages of the extensible imaging range and high resolution. The topography of binary optical element can produce the phase difference between the illumination and transmission fields. The features of binary optical element are based on the complex amplitude modulation. So we can obtain the complex transmission function by using ptychography to realize the phase retrieval. In this paper, we propose a detection method for binary diffractive optical element based on ptychography. An improved ptychography optical system is designed by using the combination of variable aperture and lens to control the illumination field. Because the illumination field is a diverging spherical wave, the diffractive patterns can avoid the high contrast and the reconstruction result will contain more details of the sample. The proposed method can not only inspect a large region of the binary optical element, but also calibrate its feature size, such as step height. Compared with the traditional binary optical element detection methods, the proposed method can simplify the system structure, and it can be applied to special environment by using lensless imaging technology. The increasing of the diffraction pattern numbers can acquire the topography of the large size sample and improve the detection efficiency. Taking a phase step plate for sample, the simulations are conducted to analyze the influences of step height and noise on the recovery result. The results show that the detection range of step height is less than 1.5. We can realize a preferable sample reconstruction when the noise of diffraction pattern is less than 5%. A computer-generated holography (CGH) is reconstructed by using the extended ptychographic iterative engine. The diameter of illumination filed is selected to be about 2 mm in order to obtain a large detection region of the sample. The surface micro topography of CGH can be shown through the m 1.98 mm1.98 mm recovery result. More details can be obtained by changing the diameter of illumination filed about 1.6 mm. The recovery result is quite accurate and the error of step height is less than 30 nm compared with the result of white light interference detection. The simulation and experimental results verify the feasibility of this method. When the requirement for accuracy is not extremely high, the proposed method can obtain a satisfactory image quality. In addition, we hope to improve the proposed method, which can be more accurate to detect different types of optical elements in the future research.

Generation of closely located light spots using specular Airy laser beams

We conduct a comparative numerical study of the formation of closely located light spots in the focal plane of diffraction gratings and binary optical elements matched with Hermite-Gaussian modes and specular Airy beams. It is shown that while gratings allow a set of uniform focal spots to be generated with high accuracy, the resulting pattern quickly deteriorates when displaced from the focal plane. Due to their modal properties, Hermite-Gaussian beams are low-sensitive to defocusing, but produce focal spots different in size and intensity. Specular Airy beams offer a trade-off solution - they produce a more uniform intensity pattern of light spots than the Hermite-Gaussian modes, at the same time showing a lower sensitivity to defocusing. Experiments with a tunable laser have confirmed the above-mentioned advantages of the specular Airy beams in comparison with the Hermite-Gaussian modes, also showing good spectral stability of the manufactured diffraction optics.

Micro-structuring of glassy carbon for precision glass molding of binary diffractive optical elements

Precision glass molding is a more cost efficient process for the large volume manufacturing of highly complex optical surfaces than direct manufacturing. Glassy carbon (GC) molds are used for precision glass molding, because they can be operated at temperatures up to 2000°C. Used today mainly for manufacturing aspheric lenses, we consider here material technology for diffractive optical element (DOE). For diffractive optics the surface structuring is in the micrometer range and a surface roughness Ra lower than 20 nm is required. We introduce a reactive ion etching process with a titanium hard mask. Fused silica (FS) molds with identical optical functionality were fabricated for comparison. All molds were used for precision glass molding of a low Tg glass L-BAL42. We will compare GC and FS as mold materials in terms of quality and robustness. Optical performance measurements of the molded glass DOEs are shown and are in good agreement with the theoretical predictions. The results confirm that precision glass molding based on GC molds is a very promising technology to economically fabricate small structures in glass for DOEs.

Creation of an anti-imaging system using binary optics.

We present a concealing method in which an anti-point spread function (APSF) is generated using binary optics, which produces a large-scale dark area in the focal region that can hide any object located within it. This result is achieved by generating two identical PSFs of opposite signs, one consisting of positive electromagnetic waves from the zero-phase region of the binary optical element and the other consisting of negative electromagnetic waves from the pi-phase region of the binary optical element.

A Microflow Cytometer with a Rectangular Quasi-Flat-Top Laser Spot.

This work develops a microflow cytometer, based on a microfluidic chip for three-dimensional (3D) hydrodynamic focusing and a binary optical element (BOE) for shaping and homogenizing a laser beam. The microfluidic chip utilizes sheath flows to confine the sample flow along the channel centerline with a narrow cross section. In addition to hydrodynamic focusing, secondary flows are generated to strengthen the focusing in the vertical direction. In experiments, the chip was able to focus the sample flow with cross sections of 15 μm high and 8–30 μm wide at 5 m/s, under the condition of the sample flow rates between 10 and 120 μL/min. Instead of using the conventional elliptical Gaussian spot for optical detection, we used a specially designed BOE and obtained a 50 μm × 10 μm rectangular quasi-flat-top spot. The microflow cytometer combining the chip and the BOE was tested to count 3, 5, and 7 μm fluorescence microbeads, and the experimental results were comparable to or better than those derived from two commercial instruments.