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Laser encoders can be used in robotic, control, and automation systems. In this paper, periodic nonlinear error of laser encoder resulting from imperfect polarized laser beam is mathematically analyzed and modeled by using the Jones matrix analysis for a robotic system. The photocurrents of each photodetector are mathematically obtained and simulated and the nano-displacement of scale grating is calculated by arctangent method. In addition, with the aim of compensating the periodic nonlinear error, the effect of reducing grating period of scale grating is examined. Simulation results show that there is a direct relationship between the grating period and periodic nonlinear error. Results reveal that by changing the grating period from 1 pm to 0.4 pm, the periodic nonlinear error reduces from 14.8 nm to 5.8 nm.

In visible light communications (VLC) system, the linearity of the light emitting diode is an issue when being intensity modulated with an analogue signal. In optical orthogonal frequency division multiplexing (O-OFDM) based VLC systems, the source nonlinearity imposes a tight restriction on the achievable peak-to-average-power-ratio. To address this issue, we propose a hybrid OFDM-PTM (pulse time modulation) scheme, where a bipolar O-OFDM signal is converted into digital PTM formats for switching ‘on’ and ‘off’ the light source. The simulation results demonstrate that the proposed scheme offers an improved bit error rate performance compared to the traditional asymmetrically clipped O-OFDM (ACO-OFDM).

A two dimensional photonic crystal biosensor implemented by waveguides and microcavity is theoretically investigated. The designed structure has high quality factor about 15,000 and sensitivity approximately 141.67 nm/RIU, which are important parameters in biosensing applications. Also there is a linear dependency between resonant wavelength shift and refractive index changes. Since water is the main component of human organism, the temperature and wavelength dependence of proposed microcavity is investigated. The results show that the structure has good temperature stability. The temperature sensitivity is about −0.0142 nm/°C.

In this paper, a biosensor is designed and optimized based on photonic crystal nano-ring resonator. The ring resonator is formed by two consecutive curves and is sandwiched by two waveguides. For analyzing this biosensor, two-dimensional finite-difference time-domain method and plane-wave expansion approach are applied. Sensing mechanism of the biosensor is based on the intensity variation scheme. When the sensing hole has different refractive indices, output transmission spectrum is shifted. Intensity variations occurred at the wavelength of 1,482.7 nm. The results show that in this biosensor for a unit change in the refractive index, intensity of the transmission spectrum is reduced by 14.26 units.

In this paper an ultra-compact all-optical encoder is presented by using a two-dimensional photonic crystal. The designed logic gate is based on the interference effect. The proposed structure consists of several photonic crystal waveguides connected by 2 nano-resonators. The nano-resonators are designed to reduce the size of the radius of the dielectric rods. The contrast ratios and delay time for the proposed all-optical encoder are respectively 6 dB and 125 fs. The size of the structure is equal to 132 µm2. Equality of the output power in the logic states “one”, the small dimensions, the low delay time, compact and simple structure have shown that the logic gate is suitable for the using in optical integrated circuits. Full Text: PDF References A. Salmanpour, Sh. Mohammadnejad, A. Bahrami, "Photonic crystal logic gates: an overview", Optical and Quantum Electronics. 47, 2249 (2015). CrossRef S. C. Xavier, B. E. Carolin, A. p. Kabilan, W. Johnson, "Compact photonic crystal integrated circuit for all-optical logic operation", IET Optoelectronics. 10, 142 (2016). CrossRef Y. Miyoshi, K. Ikeda, H. Tobioka, T. Inoue, S. Namiki, K. Kitayama, "Ultrafast all-optical logic gate using a nonlinear optical loop mirror based multi-periodic transfer function", Optics Express. 16, 2570 (2008). CrossRef D. K. Gayen, A. Bhattachryya, T. Chattopadhyay, J. N. Roy, "Ultrafast All-Optical Half Adder Using Quantum-Dot Semiconductor Optical Amplifier-Based Mach-Zehnder Interferometer", Journal of Lightwave Technology. 30, 3387 (2012). CrossRef A. Mohebzadeh-Bahabady, S. Olyaee, "All-optical NOT and XOR logic gates using photonic crystal nano-resonator and based on an interference effect", IET Optoelectronics. 12, 191 (2018). CrossRef Z. Mohebbi, N. Nozhat, F. Emami, "High contrast all-optical logic gates based on 2D nonlinear photonic crystal", Optics Communications. 355, 130 (2015). CrossRef M. Mansouri-Birjandi, M. Ghadrdan, "Full-optical tunable add/drop filter based on nonlinear photonic crystal ring resonators", Photonics and Nanostructures-Fundamentals and Applications. 21, 44 (2016). CrossRef H. Alipour-Banaei, S. Serajmohammadi, F. Mehdizadeh, "Effect of scattering rods in the frequency response of photonic crystal demultiplexers", Journal of Optoelectronics and Advanced Materials. 17, 259 (2015). DirectLink A. Mohebzadeh-Bahabady, S. Olyaee, H. Arman, "Optical Biochemical Sensor Using Photonic Crystal Nano-ring Resonators for the Detection of Protein Concentration", Current Nanoscience. 13, 421 (2017). CrossRef S. Olyaee, A. Mohebzadeh-Bahabady, "Designing a novel photonic crystal nano-ring resonator for biosensor application", Optical and Quantum Electronics. 47, 1881 (2015). CrossRef F. Parandin, R. Malmir, M. Naseri, A. Zahedi, "Reconfigurable all-optical NOT, XOR, and NOR logic gates based on two dimensional photonic crystals", Superlattices and Microstructures. 113, 737 (2018). CrossRef F. Mehdizadeh, M. Soroosh, H. Alipour-Banaei, "Proposal for 4-to-2 optical encoder based on photonic crystals", IET Optoelectronics. 11, 29 (2017). CrossRef M. Hassangholizadeh-Kashtiban, R. Sabbaghi-Nadooshan, H. Alipour-Banaei, "A novel all optical reversible 4 × 2 encoder based on photonic crystals", Optik. 126, 2368 (2015). CrossRef T. A. Moniem, "All-optical digital 4 × 2 encoder based on 2D photonic crystal ring resonators", Journal of Modern Optics. 63, 735 (2016). CrossRef S. Gholamnejad, M. Zavvari, "Design and analysis of all-optical 4–2 binary encoder based on photonic crystal", Optical and Quantum Electronics. 49, 302 (2017). CrossRef H. Seif-Dargahi, "Ultra-fast all-optical encoder using photonic crystal-based ring resonators", Photonic Network Communications. 36, 272 (2018). CrossRef S. Olyaee, M. Seifouri, A. Mohebzadeh-Bahabady, and M. Sardari, "Realization of all-optical NOT and XOR logic gates based on interference effect with high contrast ratio and ultra-compacted size", Optical and Quantum Electronics. 50, 12 (2018). CrossRef C. J. Wu, C. P. Liu, Z. Ouyang, "Compact and low-power optical logic NOT gate based on photonic crystal waveguides without optical amplifiers and nonlinear materials", Applied Optics.51, 680 (2012). CrossRef Y. C. Jiang, S. B. Liu, H. F. Zhang, X. K. Kong. "Realization of all optical half-adder based on self-collimated beams by two-dimensional photonic crystals", Optics Communications. 348, 90 (2015). CrossRef A. Salmanpour, S. Mohammadnejad, P. T. Omran, "All-optical photonic crystal NOT and OR logic gates using nonlinear Kerr effect and ring resonators", Optical and Quantum Electronics. 47, 3689 (2015). CrossRef E. H. Shaik, N. Rangaswamy, "Single photonic crystal structure for realization of NAND and NOR logic functions by cascading basic gates", Journal of Computational Electronics. 17, 337 (2018). CrossRef

Flammable and/or toxic gas sensors can be used as safety measuring in gas production facilities, especially in oil rigs. The gas sensors detect gas leaks capable of causing fire, explosion, and toxic exposure. Here we proposed an index-guiding photonic crystal fiber for gas sensing that have a broad spectral transmission band and so is capable to detect more gas condensate components. The dependence of relative sensitivity and confinement loss on the fiber parameters is numerically investigated by finite element method (FEM). Introducing a hollow high index ring with an air hole in the center of fiber simultaneously enhances the relative sensitivity and achieves low confinement loss. In addition, we prove that increasing the diameter of holes located in the inner rings, improve the relative sensitivity and increasing the ring diameter holes located in the outer rings, greatly reduces the confinement loss. Placing hexagonal holes instead of circular holes in the innermost ring, the relative sensitivity is effectively enhanced. The relative sensitivity at wavelength of λ = 1.33 μm that is in the Methane absorption line is enhanced to value of 13.23%. The confinement loss is also improved to 3.77 × 10−6.

The most important aim of nanotechnology development is to construct atomic-scale devices, and those atomic-scale devices are required to use some measurements that have ability to control and build in the range of these dimensions. A method based on super-heterodyne interferometers can be used to access the measurements in nano-scale. One of the most important limitations to increase the resolution of the displacement measurement is nonlinearity error. According to the base and measurement signals received by optical section of super-heterodyne interferometer, it is necessary for circuits to reconstruct and detect corresponding phase with target displacement. In this paper, we designed, simulated, and implemented the circuits required for electronic part of interferometer by complementary metal-oxide-semiconductor (CMOS) 0.5 μm technology. These circuits included cascade low-noise amplifiers (LNA) with 19.1 dB gain and 2.5 dB noise figure (NF) at 500 MHz frequency, band-pass filters with 500 MHz central frequency and 400 kHz bandwidth, double-balanced mixers with 233/0.6 μm ratio for metal-oxide-semiconductor field-effect transistors (MOSFETs), and low-pass filters with 300 kHz cutoff frequency. The experimental results show that the amplifiers have 19.41 dB gain and 2.7 dB noise factor, mixers have the ratio of radio frequency to local oscillator (RF/LO) range between 80 and 2500 MHz with intermediate frequency (IF) range between DC to 1000 MHz, and the digital phase measurement circuit based on the time-to-digital converter (TDC) has a nanosecond resolution.

In this paper, we design and characterize a novel small size four-channel biosensor based on the two-dimensional photonic crystal with introducing waveguides and nano-cavities in the hexagonal lattice of air pores in the silicon slab. By removing a group of air pores, waveguides are achieved, and nano-cavities are shaped by modifying the radius of air pores. Highly parallel operation of this biosensor due to the special architecture is the capability of the designed structure. The biomaterials which are suspended in a liquid medium inside nano-cavities cause effective refractive index changes which lead to the resonant wavelength shift in the output terminal. According to results, with increasing the refractive index of nano-cavities, resonant wavelengths shifts to longer values. For biochemical sensing like DNA molecule and protein and for the refractive index detection, this novel designed biosensor can be utilized.

In the present study, we investigate the structure of Developed Three-Longitudinal Mode Heterodyne Interferometer (DTLMI). Then, the output relations of each part are obtained considering Polarizing Beam Splitter (PBS) leakage. According to this computational study, the identity of error in phase measurement is examined through simulations. According to these investigations, the error is as frequent changes around the real value and its amplitude is proportional to the leakage of Polarizing Beam Splitter (BPS). The results reveal that 2% leakage causes 3.18 nm and 2.05 nm errors at high and low target speeds, respectively. Target speed is also a determinant factor in the generated error type, so that in the speeds higher than a particular limit, 45 degree shift is seen in the periodic error and the amount of error will be larger.

Photonic crystal fibers (PCFs) are widely used in optical communications systems, nonlinear devices, gas sensors, high power transmission, and so on. Photonic crystal fiber with minimum values of confinement loss, nonlinear effects, and chromatic dispersion is used in optical communication. But in some applications, high nonlinear coefficient is required. This paper presents new design of index-guiding photonic crystal fiber (IGPCF) with characteristics appropriate for nonlinear applications. In the present design with a square-hexagonal nano-structure having air holes with unequal diameters, nearly zero dispersion at the wavelength range of 1540 to 1550 nm is achieved. The simulation results reveal that the slope of chromatic dispersion is obtained as −0.05 ps/(km.nm). The confinement loss is less than 10−12 dB/cm and the nonlinear coefficient reaches 8.380 W−1.km−1.

We propose a nano-plasmonic thin-film solar cell (TFSC) as a receiver for the visible light communication (VLC). The presented design is a novel and optimized structure that can be used instead of photodetectors and power supplies in the VLC. The plasmonic TFSC that described in this work is capable of increasing the light absorption in comparison with the conventional thick Si solar cell without plasmonic nanostructures. However, plasmonic nanoparticles can improve the absorption of Si solar cell in a visible range of solar spectra. In this paper, different situations for inserting nanoparticles in an active layer are considered and the absorption is calculated as a function of particle size, particle location, and particle number, in order to account for all possible cases within the active layer.

In this article, we calculated and modeled the gain of In0.53Ga0.47As/InP avalanche photodiode (APD) based on a device mechanism and carrier rate equations using transfer matrix method (TMM). In fact, a distributed model was presented for calculating impact ionization (I2) and relating different sections of the multiplication region. In this proposed model, recessive equations were used, and device gain is considered proportional to the number of output photo-electrons and photo-holes. By comparison of simulated results with experimental data available in literature, it has been demonstrated the capability of the developed model as a powerful tool for simulating APDs’ behavior and interpreting their experimentally measured characteristics.

Nano-metrology has a crucial role in order to produce nano-materials and devices with a high degree of accuracy and reliability. Laser heterodyne interferometers are non-contact, high-resolution measurement systems which are commonly used in the displacement measurement systems. In this paper, an approach based on neural networks (NNs) for nonlinearity modeling in a three-longitudinal-mode laser heterodyne interferometer (TLMI) for length and displacement measurements is presented considering the experimental deviation parameters. We model nonlinearity errors that arise from elliptically and non-orthogonally polarized laser beams, rotational error in the alignment of the laser head with respect to the polarizing beam splitter axis, rotational error in the alignment of the mixing polarizer, and unequal transmission coefficients of the polarizing beam splitter. Here, we use the neural network algorithms including radial basis function (RBF) and multi-layer perceptron (MLP) networks and stacked generalization method. The simulation results show that multi-layer feed forward perceptron network and stacked generalization method is successfully applicable to real noisy interferometer signals. The one-hidden layer network with 5 neurons gives a good quality of fit for the training and test sets for the measurement system with RBF and MLP networks and three MLP networks with one-hidden layer for stacked generalization method. The numbers of neurons and hidden layers are selected for the best mean square error (MSE) and minimum time consuming.

In this article, an octagonal photonic crystal fiber is proposed, in which the core region and four air holes in the inner ring have a refractive index of 1.43. Compared with the hexagonal photonic crystal fiber, the octagonal photonic crystal fiber has less confinement loss. To study the propagation characteristics of the proposed photonic crystal fiber, the finite-difference time-domain method with perfectly matched layer boundary conditions has been used. By properly optimizing the design parameters of the proposed octagonal photonic crystal fiber, an ultra-flattened nearly zero dispersion of 0 ± 0.1 ps/nm.km in the wavelength range of 1.25 μm to 1.65 μm is made feasible, and for wavelengths shorter than 1.65 μm, confinement losses of less than 7 × 10−7 dB/km have been achieved.

In this paper, a new simple structure of index-guiding photonic crystal fiber (PCF) is designed and presented. In this PCF, dispersion, confinement loss, and effective mode area characteristics are investigated in the second communication window (1.55 μm). Since 1.55 μm wavelength is widely used in optical communication systems, we try to optimize the PCF characteristics in this wavelength by designing an index-guiding PCF and three versions of optimized PCF. The results show that the dispersion is obtained very close to zero around 4.6×10−4 ps/(nm·km). Also, the confinement loss is 2.303×10−6 dB/km and effective mode area is as small as 2.6 μm2.

In this paper, we present an interferometry method for refractive index determination in membranes of fuel cells. This technique is based on the use of an improved laser heterodyne interferometer. The photocurrents of the avalanche photodiodes, resulting from reflected beams of the optical head, are led to the signal conditioner and digital signal processing sections. The optical path difference between the target and reference paths is fixed, and as a result, the phase shift is calculated in terms of the refractive index shift. In addition, nonlinearity of this system is analyzed and modeled with different neural networks and adaptive filter algorithms. For neural networks, the radial basis function (RBF), the multi-layer perceptron (MLP), and the stacked generalization method are simulated. In adaptive filter algorithms, the least mean square (LMS), the normalized least mean square (NLMS), the recursive least squares (RLS), and the affine projection algorithm (APA) are applied. The simulation results indicate that the RLS method is faster and contains minimum mean square error (MSE) compared to the other approaches. Also, comparison between two main approaches shows that the nonlinearity of refractive index determination can be effectively modeled with adaptive filter algorithms.

Quantitative analysis has been applied extensively for modeling and compensation of periodic nonlinearities since the invention of heterodyne laser interferometers. In this article, we mathematically analyze and modele the nonlinearity resulting from non-orthogonality and ellipticity of the polarized laser beams for a modified high-resolution nano-displacement measuring system based on a three-mode laser interferometer. Also, we present a simple method to compensate for the periodic nonlinearity in the modified system based on a two-mode type nonlinearity reduction method. The results reveal that the factor of nonlinearity reduction can reach a value of 99.9% for some special cases of deviations resulting from non-ideal polarized light. In addition to the nonlinearity compensation, we show that the resolution of the displacement measurement in the modified system is doubled and quadrupled with respect to the conventional three- and two-mode laser heterodyne interferometers, respectively.

Noncontact displacement measurement is generally based on the interferometry method. In the semiconductor industry, a technique for measuring small features is required as circuit integration becomes denser and the wafer size becomes larger. An interferometric system known as a three-longitudinal-mode heterodyne interferometer (TLMI) is made of two main parts: optical setup and electronic sections. In the optical part, the base and measurement signals having 500-MHz frequency are produced, resulting from interfering three longitudinal modes. The secondary beat frequency to measure the displacement in the TLMI is about 300 kHz. To extract the secondary beat frequency, wide-band amplifiers, doublebalanced mixers (DBMs), band-pass filters (BPFs), and low-pass filters (LPFs) are used. In this paper, we design the integrated circuit of a super-heterodyne interferometer with total gain of 56.9 dB in size of 1030 μm×1030 μm.

Laser heterodyne interferometer is one kind of nano-metrology systems which has been widely used in industry for high-accuracy displacement measurements. The accuracy of the nano-metrology systems based on the laser heterodyne interferometers can be effectively limited by the periodic nonlinearity. In this paper, we present the nonlinearity modeling of the nano-metrology interferometric system using some adaptive filters. The adaptive algorithms consist of the least mean squares (LMS), normalized least mean squares (NLMS), and recursive least squares (RLS). It is shown that the RLS algorithm can obtain optimal modeling parameters of nonlinearity.

In this study, the authors investigate theoretically a periodic non-linearity effect and its compensation in a three-longitudinal-mode heterodyne interferometer (TLMI). An analytical formula of the frequency mixing error in the TLMI is obtained based on the Jones calculus taking into account experimental parameters. We find that the non-ideal polarisation states and imperfect alignment of the optical setup play the major role to produce four- and eight-cycle non-linearities. The operation of non-linearity compensation system on the four- and eight-cycle non-linear errors is also presented.

In this research, an optimization approach is presented to decrease the dark current in GaAs/AlGaAs QWIPs. The dark current noise is reduced by increasing Al density in barriers, decreasing detector dimensions and increasing the periodic length of the structure. In addition, increasing the number of periods can reduce both the dark current and responsivity. Therefore, devices can be optimally designed through judicious choice of these parameters. An optimal photodetector structure is designed and simulated to achieve low dark current (11nA) and detectivity of 1.4A—1012cm(Hz)1/2/W which is an order of magnitude greater than the present values.

This paper presents a new low-noise high-accuracy laser range finder based on the Time-Of- Flight (TOF) method. A Q-switched Nd:YAG laser at 1064 nm as a source and a SLIK avalanche photodiode as a detector are used. The optical section of the system including ZnS/MgF2 optical filter, designed by C ++ and Zemax software, has been successfully implemented and tested. The quality factor of the optical filter is obtained about 2.53. Using the 1.5 MW Nd:YAG laser, the lowest detectable optical power is limited to about 8.14 nW. The absolute range finding error in the range of 0.3-20 km is also measured asr= ± 50 cm. Furthermore, the sampling rate of the distance measurement can be adjusted between 1 and 20 samples per second.

The effects of periodic nonlinearity, intermodulation distortion, phase detection uncertainty and electrical cross-talk in a three-longitudinal-mode laser heterodyne Doppler interferometer (TLMI) displacement measurement system are presented. A frequency-path model consisting of three frequency components and two paths is also designed. As a result, there will be six frequency-path elements and 21 distinct interference terms. The periodic nonlinearity in the TLMI is mathematically modelled and simulated. Simulation results are compared with the typical two-mode heterodyne interferometer to confirm the designed model performance. The authors demonstrate that the nonlinearity of TLMI can be greatly reduced using the same compensation method as used in a two-frequency interferometer proposed by Hou et al.

In this paper, nonlinearity error of a laser encoder is modeled by using the Jones matrix analysis. In the nano-metrology system we suppose that polarizing beam splitters are imperfect. The intensity of interference signals on photodetectors are obtained and simulated by MATLAB software. The nano-displacement of scale grating is simulated by Arctangent method. We show that maximum nonlinearity error phase is equal to 0.07166 rad when the pitch of scale grating and wavelength of the laser are 1 μm and 685 nm, respectively.

Inorganic hole-transporting materials (HTMs) profoundly improve the stability and robustness of the solar cell compared to that expected from organic HTMs. In this work, the efficiency of a perovskite solar cell is greatly enhanced using a hexagonal nanoprism array of inorganic metal oxide HTM. We found that the introduced structure increases the efficiency of the mesoscopic configuration by 42% higher than that of the planar perovskite solar cell with the same thickness of the active layers. The reason of this enhancement in power conversion efficiency is the better carrier transportation in the mesoscopic configuration. The periodic array of mesoscopic hole-transporting layer is vastly tunable and reveals unique capabilities of designing a particular array, which can enhance the efficiency of the perovskite solar cell. We also incorporated plasmonic nanoparticles into the absorber layer and found that they significantly could improve the performance of the solar cell. The inclusion of spherical nanoparticles improves the light absorption of the bare mesoscopic solar cell. An increasing number of nanoparticles enhances the light trapping inside the active layer and as a result improves the probability of light absorption.

An air-core photonic bandgap fiber (PBGF) for gas sensing with high sensitivity is designed. The coupling core modes and surface modes in air-core photonic bandgap fiber cause large propagation losses and will deteriorate the performance of PBGF. There is a relationship between the air-core radius and the surface modes. According to the simulation results, at a special core radius, no surface modes supported by fiber and there is only core modes. By designing Λ=2.63 μm, d=0.94Λ, and R CORE =1.132Λ, where Λ is the distance between the adjacent air holes, the fiber is single-mode, no surface modes supported with fiber, and more than 90% of the optical power is confined in the core. We consider the operation wavelength λ=1.55 μm that close to the absorption wavelength of acetylene gas. This fiber is suitable to be used to detect acetylene gas and as a sensor node in the optical network protection system of pipelines carrying oil, gas, and other important resources.

A new design of photonic crystal fiber based on octagonal array of air holes in the silica background is proposed for using as a sensor node in the optical network protection system of gas pipelines. The aim of our design is achieving more sensitivity and lowering the confinement loss. In addition, introducing a hollow high index ring with an air hole in the center of fiber simultaneously enhances the relative sensitivity and achieves low confinement loss. The dependence of sensing properties on the fiber parameters is numerically investigated by finite element method (FEM). We achieve the optimum design that has the relative sensitivity of 9.33% and the confinement loss of 6.8×10-4 dB/m, at wavelength of λ=1.5μm. The results prove the ability of the proposed fiber in the optical wireless sensor networks as a gas or liquid sensor nod.

Laser heterodyne interferometer is a common nano-metrology system which is used for high-accuracy displacement measurements in industry. Measurement accuracy in this system is limited by the periodic nonlinearity error. In this paper, the nonlinearity error of the nano-metrology interferometric system based on three-longitudinal-mode laser heterodyne interferometer has been modeled by artificial neural network (ANN) and hybrid genetic algorithm-neural network (hybrid GA-ANN). The real code version of genetic algorithm (GA) is used. Genetic operators and parameters are set and designed accurately for optimizing the neural network. The results indicate that nonlinearity error can be effectively modeled by hybrid GA-ANN method and contains minimum mean square error (MSE) compared to the neural network.

In this paper, we propose a two-dimensional photonic crystal multi-channel biosensor. This biosensor is implemented by a waveguide and some cavities which has the capability of highly parallel operation due to special architecture. The architecture is obtained by lattice shift of single hole around the cavities. Each channel has different resonant cavity and filling analyte to selected hole causes independently shift in resonant wavelength. Also, this biosensor has high sensitivity and quality factor. The waveguide and cavities are respectively created by omitting a raw of water holes and one hole, respectively. This biosensor is consisted from hexagonal lattice of water holes in silicon on insulator slab. Trapping DNA molecules and protein within the selected hole, causes spectral shift in the output terminal. The plane wave expansion (PWE) and finite difference time domain (FDTD) method are chosen to perform simulation.

In this paper, we report the numerical analysis of a nanostructured photonic crystal fiber for making an efficient dispersion-compensating medium. For our computational studies, the core of the proposed structure is made up of As2Se3, and the cladding structure is a mixture of air holes and holes filled with silica. Our simulations indicate that the chromatic dispersion and the confinement loss at the wavelength of 1.55 µm is −3280 ps/nm/km and 1.71 × 10−6 dB/m respectively. Moreover, the relative dispersion slope at the wavelength of 1.55 µm is computed to be 0.00357 nm−1 that closely matches that of the conventional fibers at the third window. The effect of changing the dimensions of the holes in the first ring and also the distance between the adjacent holes (Λ) in the same ring on chromatic dispersion are also studied. The presented structure is an efficient dispersion-compensating medium. Copyright © 2016 John Wiley & Sons, Ltd.

In this paper, we illustrate the design and simulation of photonic crystal (PhC) nano-pressure sensor with high sensitivity and resolution. The sensor is based on two-dimensional photonic crystal (2D-PhC) and has hexagonal lattice of air holes in Si. A waveguide is directly coupled to nanocavity and is configured by eliminating one line of air holes. The nanocavity is formed by modifying the radius of one air hole in the center of lattice. This sensor is designed to use in 1450–1550nm wavelength. Simulation results show that resonant wavelength of nanocavity shifts to longer wavelength with increasing the pressure. The sensitivity of sensor and minimum detectable pressure are respectively obtained as 11.7nm/GPa and 13nN which is improved compared to previous researches.

In this paper, In 0.53 Ga 0.47 As/InP avalanche photodiode (APD) gain is calculated based on the device mechanism and carrier rate equations, using transfer matrix method (TMM). In fact, a distributed model is presented for calculating impact ionization and relates different sections of the multiplication region. In proposed model, recessive equations are used and device gain is considered proportional to the number of output photo-electrons and photo-holes. Comparison of simulation results with experimental data available in literature has demonstrated the capability of the new developed model as a powerful tool to simulate the APDs' behavior and to interpret their experimentally measured characteristics.

The optical interferometry is the most frequent technique which has been widely used in metrology systems with subnanometer resolution. In this chapter, first we have discussed an overview of principals of interferometry, interfere phenomenon in the laser interferometers and their applications. A comparison of homodyne interferometers, advantages and disadvantages with heterodyne interferometers was also investigated. The optical setup and electronic sections of two- and three-longitudinal-mode heterodyne interferometers was then described. A frequency-path model of the nano-displacement measurement system based on the laser heterodyne interferometers was presented. This model is described by the AC reference, AC interference, DC interference and optical power components of the multi-mode, multi-path interferometer. The main parameters affecting the displacement measurement accuracy were discussed in the fourth section. The most important errors are related to imperfect alignment of the optical setup and nonideal polarized modes. The periodic nonlinearity is the principal limitation of the laser interferometers involving polarized light. Therefore, we have analyzed and modelled the periodic nonlinearity resulting from ellipticity and nonorthogonality of the polarised modes, deviation angle and unequal transmission­reflection coefficients of the PBS and a rotation angle of polarizer with two main approaches; plane wave and matrix method based on the Jones calculus. Then by using two polarizers oriented under +45º and -45º, a half-wave plate, and another avalanche photodiode, the periodic nonlinearity has been considerably decreased. In addition it was shown that the first-order nonlinearity can be effectively reduced compared to the second-order nonlinearity.

Periodic nonlinearity is the main limitation on the accuracy of the nano-displacement measurements in the heterodyne interferometers. It is mainly produced by non-ideal polarized beams of the leaser and imperfect alignment of the optical components. In this paper, we model the periodic nonlinearity resulting from non-orthogonality and ellipticity of the laser beam by using combination of neural networks such as stacked generalization method and mixture of experts. The ensemble neural networks used for nonlinearity modeling are compared with single neural networks such as multi layer percepterons and radial basis function. Keywords : interferometer, nonlinearity modeling, mixtur e of expert, nano-displacement, neural network. 1. Introduction Polarization-encoded laser heterodyne interferometers are used in the basis of metrology and control in numerous high precision displacement measurement applications such as semiconductor fabrication, calibration, geodesy, precision cutting, lattice consta nt measuring and robotic systems [1-3]. The stability of the laser source, vibration, temperature variation and air turbulence are the main environmental limitation affecting the accuracy of the nano-scale optical interferometers. If all of the above conditions can be kept good enough, then the practical limitations will be given by the noise and the nonlinearity [4-6]. Non-orthogonality of the polarizing radiations, elliptical polarization of the laser head, imperfect polarizing beam-splitter (PBS) and other components produce periodic nonlinearity error [7-11]. Many efforts have been done to model and compensation of the nonlinearity error by inserting more complex optical setup and complex computational analysis [12-18]. Neural networks as artificial intelligent approach can be also used for this reason. Comparing wi th the conventional methods, this approach is a simple technique which adds less no ise and error to the system. Li et al . have first modeled and corrected nonlinearity in the homodyne interferometers based on the single neu ral networks in 2003 [19]. Then, Heo et al. employed a recursive least square (RLS) method combined with a neural network back-propagation algorithm to compensate the nonlinearity in the heterodyne interferometer in 2007 [20]. Neural networks can be generally classified into two categories: single and combining neural networks. Combining classifiers are proposed to achieve higher accuracy and to improve the classification performance of a single classifier. In classification combinations, it is expected that each classifier has different error with the other experts. Consequently, combining results of these experts could cover all of problem space, correctly and the overall performance is improved by combining the outputs in some ways [21].

In this paper, a simple method to reduce the nonlinearity in the high-resolution nano-displacement measuring system using a modified laser interferometer is presented. By using a retarder plate and improved optical head, in additional to the nonlinearity compensation, the resolution of the displacement measurement is doubled and quadrupled compared to the conventional three- and two-mode laser interferometers, respectively. In the particular case, the nonlinearity of 18.4 nm is reduced to a value of 140 pm.

Misalignment and deviation angle in the optical interferometer directly decrease the accuracy of nanometrology system by producing a periodic nonlinearity error. In this paper, the effect of non-orthogonally polarized beams on the displacement measurement error in the advanced nanometrology system is presented. A system for reduction of non-orthogonality effect including linear polarizer, half-wave plate and modified signal conditioner, based on the nonlinearity compensation in a two-mode laser interferometer, is also proposed. The simulations results confirm that the peak error in the nano-displacement measurement is considerably decreased from 0.89 nm to 15 pm by using the modified optics and signal conditioner.

In this paper, a frequency-path model for three-longitudinal-mode interferometer (TLMI) is presented. The model describes two main error sources that affect the accuracy of displacement in heterodyne interferometers. Unwanted leakage between the reference and the target paths and inter-modulation error due to the nonlinearity of current to voltage converter, double balanced mixer, and pre-amplifier are two main error sources in the TLMI. Using a three-longitudinal-mode stabilized He-Ne laser, six frequency-path components and 21 distinct interference terms is produced.

In this paper, we demonstrate and theoretically investigate a compact two-dimensional (2D) photonic crystal biosensor implemented by a waveguide and cavity. Biomaterials such as DNA molecules and proteins trapped inside a hole cause resonant wavelength shifting at the output terminal. The quality factor and sensitivity were obtained at about 4000 and 1.63 nm/fg, respectively. Also, we investigated this structure as a bulk refractive index sensor with a sensitivity of about 165.45 nm/RIU (refractive index units). Then, we modified the structure as a multichannel biosensor. This biosensor has the capability of highly parallel operation because of special architecture that was obtained by lattice shifting of a single hole around the cavity. Each channel had a different resonant cavity wavelength and the filling of analyte in selected holes caused resonant wavelength shifting, independently. Plane wave expansion (PWE) and finite difference time domain (FDTD) methods were used to analyze and compute the sensor characteristics.