In this paper, we propose a two-dimensional metal-dielectric grating with dielectric nanodisks on a thin gold film structure for refractive index sensing due to its near unity absorption at 1050 nm wavelength. The perfect absorption mainly originates from excitation of the horizontal magnetic dipole mode in the metal-dielectric structure. The results show that the sensitivity and full width half maximum are 560 nm/RIU and 11.13 nm over the sensing range of 1.33 to 1.38, respectively. Obviously, the corresponding figure of merit is calculated to be 50.3 RIU<sup>-1</sup>, which shows a high sensing performance. Moreover, it also shows excellent performance by measuring the light intensity change in the reflected light at a certain wavelength. The proposed structure has great potential application in biological sensing, integrated photodetectors, chemical applications and so on.
Grating is a very useful diffraction optical element. Highly efficient and broadband gratings are required in current ultrafast femtosecond pulsed lasers. This paper mainly focused on gold-plated reflective gratings. We proposed a rectangular grating with high efficiency and broadband under Littrow incidence at the center wavelength of 800nm for TM polarization. Under Littrow incidence, the duty cycle and the groove depth of the grating determine the diffraction efficiency and bandwidth of the gratings. By the Rigorous Coupled Wave Analysis(RCWA) and simulated annealing algorithm, we analyzed the effects of different grating parameters on grating diffraction efficiency and bandwidth. In this paper, we designed a rectangular grating with duty cycle 0.324, groove depth 1466nm and thickness of connection layer 2146nm, respectively. Its diffraction efficiency of the -1 order approached 98% in broadband. And we analyzed the tolerance of our grating to improve the efficiency of the grating. This kind of grating has very important value in the future application of high power laser.
With the rapid development of precision manufacturing, the optical non-contact three-dimensional measurement method for detecting the morphology of tiny objects has gradually become a hot topic with the advantages of high speed, high precision, large measuring range and high repeatability. When the depth of the microgroove reaches a certain range, the general three-dimensional measurement method cannot be used since the depth of focus is usually not deep enough. In this paper, we proposed a new method of detecting based on the grating projection for detecting phase, which introduced a novel diffractive optical device called Dammann zone plate for measuring three-dimensional shape of tiny objects with an extend of focal length. Dammann grating can produce a finite array of uniform intensity spots in the Fourier transforming plane by modulating the transverse position of the transition points of the binary optical phase. Using this feature, the Dammann zone plate takes advantage of the periodic coding details of the Dammann grating for producing an axial multi-focus system with equal intensity in the focusing system when combined with a focusing lens. This experiment, the depth of focus is greatly extended due to the introduction of Dammann zone plate, which can measure deeper grooves. Therefore, we can collect more three-dimensional information of the tiny object with a CCD camera, and then we can obtain more accurate three-dimensional morphologic profile. This method is simple, accurate and robust for practical applications, so it is highly interesting for detecting deeper grooves of tiny objects.
As an important means to obtain three-dimensional depth information of target, optical measurement has been widely used in face recognition, machine manufacturing, aerospace and other related fields in the past decades. Optical three-dimensional imaging and depth measurement is a fast and non-contact method for reconstructing three-dimensional imaging and depth measurement of objects based on optical means and digital image processing analysis. In this paper, a three-dimensional measurement module of transversely rotating combined Dammann grating is proposed, which generates interleaved high-density dot-matrix structured light for three-dimensional imaging and measurement. The measurement module consists of integrated components of laser and beam expander, collimating lens, four transversely rotating combined Dammann gratings with different beam splitting ratios, and objective lens. The laser emits a laser beam which is collimated by a collimating lens. Four Dammann gratings are used to generate four non-staggered dot-matrix by splitting them, and then the high-density staggered projection dot-matrix for three-dimensional measurement and imaging are projected by the objective lens. The measurement module has the advantages of simple structure, high output dot-matrix density, staggered projection dot-matrix edges, and easy integration into mobile devices. This technology may reduce the complexity, number of optical elements, power consumption and cost of structured light projectors in mobile and fixed 3D sensors.
Nowadays, more and more people worldwide suffer from cataracts, an eye disease. The intraocular lens is a precise optical component, which can be implanted into the eyes through the surgery to replace the turbid lens of eyes with the target of providing good vision correction for many cataract patients. Scientists have developed many types of intraocular lenses for correcting vision of cataract patients, but many of them have slightly poor imaging effect because of fewer axial focal points. Dammann zone plate is a very useful optical element, which is formed by introducing the phase modulation of Dammann grating into Fresnel zone plate. The characters of Dammann zone plate enable it to produce a series of axial focal spots distribution with equal intensity in a certain range. In this paper, we introduced Dammann zone plate into the traditional intraocular lens and designed a kind of intraocular lens. And we mainly researched the focusing depth and imaging effect. We find that the intraocular lens with deeper focal depth and better imaging effect can be obtained by the optimal combination of the two parts. This multifocal intraocular lenses enable both the near and far objects to focus on the retina. With the Dammann zone plate, the intraocular lens has deeper focal depth, which allows those cataract patients to get a better visual clarity and can provides better visual quality for them. Therefore, this design has a wide prospect for the treatment of cataract in the future.
A 1×5 transmission grating splitter with triangular structure under normal incidence at the wavelength of 1550 nm is presented in this paper. In order to further increase the efficiency, the material of the designed grating is MgF<sub>2</sub>. The whole transmitted diffraction efficiency of the gratings is over 99% with uniformity better than 0.3%. The designed parameters of this triangular grating are employed by the rigorous coupled-wave analysis and the simulated annealing algorithm. This grating has a large tolerance for fabrication with better performance, which should be highly interesting for practical applications.
Spectrum plays an essential role in spectral imaging technology. To obtain the spectral information of image, two high - density diffraction gratings which substitute the prism are introduced in the Sagnac loop to form the polarization Sagnac interferometer (PSI). Usually, it’s difficult for prism to achieve wider angle of spectral line and higher resolution, the presented Sagnac loop with high-density gratings has advantages of wide spectral and high resolution. Meanwhile, the dispersion generated by grating is more uniformed than the prism. The two parallel beams exit from the Sagnac loop and the pass through the linear polarizer and finally polymerized on the focal plane array (FPA) by an imaging lens. This compact Sagnac loop with two high-density diffraction gratings is a new way to obtain the spectral-resolved image, which should be interesting for practical applications.
With the release of IphoneX, compact 3D optical measurement has become a popular technology. An important application of compact 3D optical measurement is to realize 3D imaging of targets. A compact three-dimensional optical measuring instrument can project a coded or structured light pattern onto an object to achieve the purpose of three-dimensional imaging. Apple's solution is to design diffractive optical elements using algorithms (such as the Gerchberg-Saxton algorithm) to produce a fan-out staggered dot matrix projection pattern in which some dot matrix produce a lateral offset from adjacent dot matrix. In this paper, a new method is proposed to generate the interlaced lattice projection pattern, i. e. the transverse odd-even combinational Dammann grating method. This method produces the above pattern by two Dammann gratings placed perpendicular to the optical axis, which is different from the scheme proposed by Apple. The advantage of this scheme is that the overal structure is simple and the design cost is reduced, so it is easy to mass-produce, and its lateral combinational structure is conducive to the miniaturization and integration of devices, so it is convenient to integrate in various interconnected devices. For example, mobile phones with 3D face recognition using this technology are not only easy to use, but also highly secure.
The direct laser writing lithography technology is an efficient way to make the large-sized diffraction gratings. It has the advantages of high efficiency, low cost and high flexibility. For further improvement the performance of the direct laser writing technology, we introduced the two-dimensional Dammann grating into the direct laser writing system. The Dammann grating can create a finite array of uniform intensity spots so that the efficiency of the writing can be increased. In addition, we also proposed a way of rotating the two-dimensional Dammann grating. By the geometric relationship, the expressions of the rotation angle can be derived. Considering the efficiency, the uniformity and the price of the 1D Dammann, we proposed the rotating 2D Dammann grating technology based on the 1D Dammann grating. While the rotation angles of 1D Dammann grating and the 2D Dammann grating are different. The efficiency of laser writing based on 2D Dammann is quite higher than the 1D-Dammann laser writing. We can use this method to fabricate the large-sized diffraction gratings efficiently.
Edge extraction has found applications in various image processing fields, such as in pattern recognition. In this paper, a new method is proposed for edge extraction of three-dimensional objects in optical scanning holography (OSH). Isotropic and anisotropic edge extraction of 3D objects is simulated using spiral phase plates in OSH operating in an incoherent mode. We propose to use a delta function and a spiral phase plate as the pupil functions to realize isotropic and anisotropic edge extraction. Our computer simulations show the capability of extracting the edges of a given 3D object by spiral phase filtering in OSH.
A highly efficient reflective 1×3 splitting grating with triangular structure operating in 1.064μm wavelength under normal incidence for TE polarization is designed. The schematic of the grating has four layers. The first layer with SiO2 is triangular structure. Rigorous coupled wave analysis (RCWA) and Simulated Annealing (SA) algorithm are adopted to design and analyze the properties. The theoretical efficiency is nearly about 99%. The bigger error tolerance is also analyzed by rigorous coupled wave analysis. These reflective gratings as splitters should be useful optical elements in the field of high-power laser as well as other reflective applications.
Displacement laser interferometers and grating interferometers are two main apparatus for the micron-nanometer displacement measurement over a long range. However, the laser interferometers, whose measuring scale is based on the wavelength, are very sensitive to the environment. On the contrast, the grating interferometers change the measuring scale from wavelength to grating period, which is much stable for the measurement results. But the resolution of grating interferometer is usually lower than that of laser interferometer. Therefore, further investigation is needed to improve the performance of grating interferometer. As we known, the optical subdivision is a main factor that affects the measurement resolution. In this paper, a grating interferometer with high optical subdivision is presented based on the Littrow configuration. We mainly use right angle prisms accompanied with plane mirrors to make the measuring lights diffracted by the grating scale for many times. An optical subdivision factor of 1/24 can be obtained by this technique. A main difficulty of this technique is that the grating scale should be with high diffraction efficiency. Fortunately, the measuring light is incident on the grating scale at the Littrow angle, the grating scale can be designed with very high efficiency easily in this condition. Compared with traditional grating interferometers, this kind of grating interferometer can greatly increase the measuring resolution and accuracy, which could be widely used in nanometer-scale fabrications and measurements.
Beam splitters are widely used in various optical modern systems for separating optical wave into different directions. We have proposed a novel slanted grating for beam splitter at the central wavelength of 1550nm, which can be used in the optical communication. With the simulated annealing algorithm, beam splitter slanted grating can be optimized by using the rigorous coupled wave analysis (RCWA). The diffraction process can be analyzed by the simplified modal method. The simplified modal method, without complicated calculation, reduces the difficult diffraction process into a vividly and physical modal. We have derived an analytical expression which can provide an insightful physical description of the simplified modal method for the slanted grating. Compared with the rectangular grating, the slanted grating has the asymmetric physical structure. Therefore, the odd grating mode can also be excited in the slanted grating under normal incidence. The odd grating mode, which only exists in the asymmetric structure, plays the role of breaking the symmetric field distribution in the output plane. The physical analytical expression of mode conversion and coupling for the slanted grating can be obtained to interpretation the asymmetric field distribution. Numerical results obtained by the rigorous coupled wave analysis verified the validity of the simplified modal method. We expect the modal method for the slanted grating set forth in this work should be helpful for the tremendous potential application of the slanted grating.
In this paper, we develop a binocular three-dimensional measurement system using a Dammann grating. A laser diode and a Dammann grating are employed to generate a regular and square laser spot array. Dammann array illuminator is placed between two cameras and narrowband-pass filters are embedded in the project lens to eliminate the interference of background light. During the measurement, a series of laser spot arrays are projected toward the target object and captured by two cameras simultaneously. Similar to stereo vision of human eyes, stereo matching will be performed to search the homologous spot which is a pair of image points resulting from the same object point. At first, the sub-pixel coordinates of the laser spots are extracted from the stereo images. Then stereo matching is easily performed based on a fact that laser spots with the same diffraction order are homologous ones. Because the system has been calibrated before measurement, single frame three-dimensional point cloud can be obtained using the disparity of homologous points by triangulation methods. Finally, three-dimensional point clouds belong to different frame which represent different view of the object will be registered to build up an integral three-dimensional object using ICP algorithm. On one hand, this setup is small enough to meet the portable outdoor applications. On the other hand, measurement accuracy of this system is better than 0.3 mm which can meet the measurement accuracy requirements in most situations.
Optical encoders and laser interferometers are two primary solutions in nanometer metrology. As the precision of encoders depends on the uniformity of grating pitches, it is essential to evaluate pitches accurately. We use a CCD image sensor to acquire grating image for evaluating the pitches with high precision. Digital image correlation technique is applied to filter out the noises. We propose three methods for determining the pitches of grating with peak positions of correlation coefficients. Numerical simulation indicated the average of pitch deviations from the true pitch and the pitch variations are less than 0.02 pixel and 0.1 pixel for these three methods when the ideal grating image is added with salt and pepper noise, speckle noise, and Gaussian noise. Experimental results demonstrated that our method can measure the pitch of the grating accurately, for example, our home-made grating with 20μm period has 475nm peak-to-valley uniformity with 40nm standard deviation during 35mm range. Another measurement illustrated that our home-made grating has 40nm peak-to-valley uniformity with 10nm standard deviation. This work verified that our lab can fabricate high-accuracy gratings which should be interesting for practical application in optical encoders.
Femtosecond laser technology is one of the frontiers in the fields of nonlinear optics, advanced manufacturing etc. The method of femtosecond pulse compression is an important research content. To compress the femtosecond pulse, we need to use negative dispersive elements to compensate the positive dispersion of Ti: sapphire crystal and other optical elements. For this purpose, we propose a miniature double-line-density grating pair in which the line density of the second grating is twice of the first one, the output pulse propagates along the way back. The density of the gratings is high, which will have a high diffraction efficiency and can compensate a high GVD ( group velocity dispersion ) in a small distance. The first grating is transmitted and the second one is reflective, the device will not occlude the beam propagation. With the pair of the gratings, the input positive chirped 89fs pulse is neatly compressed into the Fourier transform limited 44fs pulse with no spectral spatial walkoff and dispersion. It can be used for compression in laser cavity or out of the cavity. The gratings are easy to adjust and the structure is simple and compact, which has widespread interest in practical applications.
This paper will report our recent works on fabrication, evaluation, and applications of gratings. We are using the Dammann parallel laser writing facility for fabrication of gratings. High-efficiency reflective gratings and large-sized grating are fabricated. We have fabricated high-power reflective laser vortex grating with expectation of a new laser drilling effect for laser fusion in the future, which is just evaluated by our developed method. These gratings are essential elements for high-power laser systems and other high-demanding metrology applications.
A 3×3 high divergent 2D-grating with period of 3.842μm at wavelength of 850nm under normal incidence is designed and fabricated in this paper. This high divergent 2D-grating is designed by the vector theory. The Rigorous Coupled Wave Analysis (RCWA) in association with the simulated annealing (SA) is adopted to calculate and optimize this 2D-grating.The properties of this grating are also investigated by the RCWA. The diffraction angles are more than 10 degrees in the whole wavelength band, which are bigger than the traditional 2D-grating. In addition, the small period of grating increases the difficulties of fabrication. So we fabricate the 2D-gratings by direct laser writing (DLW) instead of traditional manufacturing method. Then the method of ICP etching is used to obtain the high divergent 2D-grating.