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Objective: In this paper, the performance of pulse position modulation (PPM), nonreturn zero modulation (NRZ), and return zero modulation (RZ) at signal-to-noise ratio and bit error rate in free space optical communication is compared. This comparison is performed to obtain the most effective modulation in atmospheric attenuations. Also, the effect of increasing transmitter power on the bit error rate and signal-to-noise ratio is investigated. Methods: Utilizing a light source with a wavelength of 1550 nm, the system is simulated in MATLAB by choosing the Kim data transmission link model with different visibility values of 0- 10 km. The analytical equations of free space optical communications are implemented by selecting appropriate parameters. The effects of weak, moderate, and strong atmospheric attenuation, geometric loss, and different transmitter powers (1-5 mW) on bit error rate and signal-to-noise ratio are investigated for all three modulations. Results: The results show that PPM is the most effective modulation compared to other modulations used in this study and shows a better performance in the mentioned atmospheric attenuation conditions within the given values of bit error rate and signal-to-noise ratio. Conclusion: Modulations with lower average power will perform better than other modulations in bit error rate and signal-to-noise ratio under the same atmospheric attenuation conditions. In PPM modulation, increasing the transmitter power causes more reduction in the bit error rate than other modulations used in this paper. Therefore, using this modulation to optimize the power budget in free space optical communications will be appropriate.

In this paper, a compact all-optical five-channel demultiplexer (DEMUX) using 2D photonic crystal hexagonal cavities is proposed. The proposed DEMUX comprises bus/drop waveguides, photonic crystal waveguides containing four consecutive 120° bends, two power splitters, and hexagonal resonant cavities. By adjusting the radius and refractive index of the inner/coupling rods, position, and radius of the scattering rods, the operating wavelength and the output power of each port are managed. Normalized transmission spectra and calculations of band structure are calculated by the finite-difference time-domain (FDTD) and plane-wave expansion (PWE) methods, respectively. The resonant wavelengths of the proposed structure are 1328.7 nm, 1331.9 nm, 1335.4 nm, 1340.5 nm, and 1344.4 nm, respectively. Calculation results show that the average quality factor, optical transmission efficiency, passbands of channels, minimum and maximum crosstalk of nearly 6236, 95.98%, 0.22 nm, − 18.04 dB, and − 50.2 dB, respectively. The operating wavelength range of DEMUX is 1328 nm to 1345 nm and the average channel spacing between the channels nearly about 3.93 nm. The footprint of the device is 240 μm2. The advantageous features of the proposed structure are valuable for integration based on popular planar technology and optical communications systems.

In this paper, the response time of all-optical AND logic gate using the triangular photonic crystal lattice is investigated. The proposed logic gate consists of a photonic crystal nano-resonator formed by changing the size of the dielectric rods. The structure benefits the interference effect mechanism. The contrast ratio of the photonic crystal AND logic gate is obtained as 6 dB. In addition to simplicity, the designed nano-resonator increases the bit rate of logic gate. The delay time and footprint of logic gate are respectively 0.32 ps and 146 µm2. The proposed photonic crystal AND logic gate can operate at a bit rate of 3.12 Tbit/s

In the present study, the propagation of electromagnetic waves in a square-lattice photonic crystal waveguide (PCW) is investigated using the finite-difference time-domain (FDTD) method. Then, the plane wave expansion (PWE) method is utilized to calculate the 2D photonic crystal band structure. To realize the desired waveguide, nano-line defects are introduced. The results of the numerical simulations and optimization scanning indicate that for the proposed photonic crystal structure consisting of silicon circular dielectric rods with a radius of 84 nm, a band gap can be achieved in the wavelength range of 1.34 μm

In this paper, with dispersion engineering, two waveguides with silicon core and SiO2 cladding are proposed to generation the supercontinuum spectrum and optical frequency combs. By injecting a pulse with a peak power of 800 (W) and a pulse duration of 50 (fs), the output supercontinuum spectrum is obtained from a wavelength of 1100 (nm) to 4000 (nm). Also, by applying a maximum power of 1 KW and a pulse width of 100 (fs), optical frequency combs based on a supercontinuum with a broadband frequency combs have been obtained. Our proposed structure has good potential for engineering and achieving flatness dispersion which there are two zero dispersion wavelengths at 1890 (nm) and 2850 (nm). This flat dispersion can be very useful to achieve the desired output spectrum. Due to the materials used and the flat structure of the proposed waveguides, these waveguides can be used for integrated optical circuits as well as applications in optical communications, spectroscopy, and sensors.

Due to the importance of all-optical demultiplexers (DEMUXs) in optical communication networks, this study aims to design and simulate a novel type of DEMUX using 2D photonic crystals (2D-PhCs). The proposed structure consists of one input waveguide and eight output channels considering linear and point defects. To design this DEMUX, silicon dielectric rods with the refractive index of 3.45 are employed in the air background, the radius of which is equal to R = 0.21Ʌ (Ʌ = 0.522µm) where Ʌ is the lattice constant. The photonic band gap (PBG) and output spectrum of the structure are analyzed by the plane wave expansion (PWE) and finite-difference time-domain (FDTD) methods, respectively. The simulation results indicate the average quality factor and transmission efficiency are 3,602 and 98%, respectively, and inter-channel crosstalk ranges from − 60.5 to -25.5 dB.

In this paper, the design and simulation of an ultra-fast 4 × 2 encoder have been optimized in a new and unique way using the wave interference technique based on two-dimensional photonic crystal structures. To create the given structure, nano-resonators (NR) were used in combination with curved 2-branch waveguides to take advantage of wave interference as well as reduce losses and light scattering in the structure to achieve the desired results. Due to its special design, the proposed structure has been symmetrical with a structural size of about 149 µm2. This symmetry caused the same results to be obtained in different states of the used encoder. The results from the structure simulation indicated a contrast ratio of 7.88 dB, a delay time of 0.21 ps, and a bit rate of 4.761 Tbit/s. All simulations were performed with a central wavelength of 1550 nm and an input power intensity of about 1 mW/µm2. To optimize the structure and evaluate the results obtained from it, the changes were made in the structural parameters such as the size of the radius of dielectric rods, the size of NRs, and the value of lattice constant, and the effects of wavelength, operating frequency, and input power intensity on the simulation results were investigated. The analysis of the results of simulations and optimizations indicated the design of a completely principled and logical structure with very suitable results in comparison with other structures proposed in this field so that it can be used appropriately in integrated optical circuits and can be considered as the basic structure for other designs. In the present research, the plane-wave expansion method was used to extract and analyze the photonic bandgap and the finite-difference time-domain method to obtain the results of the output spectrum of the proposed structure.

The split of the sunlight spectrum by the bandgap energy of multi-junction solar cells is a highly effective way to increase solar cell efficiency. The reason is that the energy of photons is effectively absorbed, and there is a reduction in solar cell loss. In this contribution, we report on the performance of a double-junction copper gallium diselenide/copper indium gallium diselenide (CGS/CIGS) solar cell with a cadmium sulfide (CdS) buffer layer simulator. The J–V characteristics and the external quantum efficiency were simulated under AM1.5 illumination. Increased efficiency was seen as a result of the change in the thickness of layers and different molar ratio amounts of gallium, and the optimal amount of each factor was obtained. In this study, a single CGS solar cell was used as the top cell and a single CIGS solar cell as the bottom cell in the tandem configuration, which showed conversion efficiencies of 16.175% and 15.696%, respectively. Finally, solar cell efficiency of 32.3% was obtained in the double-junction state, an increase of 6% compared with the reference cell.

In this paper, we first present the design of a nano-ring resonator-based filter, in which scattering rods are used at the corners of the structure. By choosing a suitable radius for the dielectric rods in the nano-ring resonator, low channel spacing and high-quality factor parameters have been achieved at 1586.8 nm wavelength. Then, using this filter, a 2-channel demultiplexer is developed. In the proposed demultiplexer, two lattice constants are used: a1 for the main structure and a2 for the structure in the nano-ring resonator. The difference in the lattice constants results in an increase in the quality factor. Some of the advantages of this 2-channel demultiplexer include an average quality factor of 5443, average channel spacing of 0.35 nm, and central wavelengths of 1554.5 nm and 1557.1 nm, respectively, for the first and second channels. Moreover, the minimum and maximum inter-channel cross talks are − 17.63 dB and − 12.1 dB, respectively. Due to the 2.6 nm inter-channels spacing, this structure can be exploited in optical integrated circuits, WDM and DWDM systems.

Using a two-dimensional photonic crystal lattice, a compact and simple structure has been introduced in the present study to construct an all-optical half adder. The designed structure operates under the interference effect. The phase difference has been created in different states by putting the point defects and the difference in the length of the input waveguides. To do analyses, the finite-difference time-domain method and the plane wave expansion have been used. The advantages of this design include the small dimensions, high possibility of construction, compatibility with the silicon-based technology, and equality of the output power in the logic states 0 and 1. The contrast ratio calculated for the port Sum and the port Carry has been equal to 9.3 and 8.22 dB. The response time has been equal to 0.22 ps, generating the data transfer rate of 4.55 Tbps.

In this paper, 2-channel and 4-channel optical demultiplexers have been designed based on photonic crystal nano-ring resonator. The structure of both demultiplexers is based on the use of a photonic crystal filter. The mean quality factor of 2828, mean transfer factor of 95%, mean cross-talk of − 25.07 dB, and an inter-channel distance of 7.4 nm are among the structural advantages of the 2-channel demultiplexer. The advantages of the 4-channel demultiplexer include the mean quality factor of 4525, mean channel power transfer factor of 95%, and maximum and minimum channel cross-talk of − 19.6 and − 40.4, respectively. Also, the inter-channel difference is 7.8 nm and mean channel width is 0.375 nm. Moreover, the simplicity of the structure and the small size are among the characteristics of these designs. The proposed 2-channel and 4-channel demultiplexers can be used in WDM systems, especially DWDM systems.

In the present paper, a four-channel optical demultiplexer (DMUX) based on two-dimensional photonic crystal has been presented. In this optical demultiplexer, filtering and wavelength separation were performed using point defects between the output and input waveguides. To design the optical demultiplexer, a 31 × 21 resonance filter with a lattice constant (Λ) of 0.54 μm and the radius of the dielectric rods of 0.2Λ was first designed, and then it was expanded to obtain a four-channel 31 × 41 demultiplexer. The output wave spectrum was measured for four channels in the range of 1541.5 nm to 1557.6 nm. The average quality factor of 2567, the average crosstalk of − 25 dB, and the transmission coefficient of higher than 97% were obtained.

The present paper examines a photonic crystal-based ring resonator created using a triangular lattice to design a biosensor with a sensing cavity based on an optical filter. Using this structure, the central wavelength of 1547 nm can be filtered with a transmission coefficient of 95%. Additionally, the maximum value of ultra-high-quality factor (Q) in the proposed structure is about 3740 at 1496 nm wavelengths. Moreover, the minimum detection limit and maximum sensitivity for the proposed structure are calculated to be 7.15 × 10–8 RIU (refractive index unit) and 561 nm/RIU, respectively. In this paper, the effects of structural parameters, including refractive index, lattice constant, and the radius of the rods in the resonator core on the central wavelength of the filter, transmission coefficient, and quality factor have been investigated. The plane wave expansion (PWE) method has been used to extract the photonic bandgap and the finite-difference time-domain (FDTD) method has been used to study the optical behavior of the photonic crystal structure.

Supercontinuum spectrum generation is a process in which laser beam in femtoseconds and high power (kilowatts) is converted into a broad-spectrum beam of light after passing through a specific environment. Of course, achieving this range comes with many limitations. In this paper, photonic crystal fibers are used as a substrate for input pulse due to the ability to control dispersion and loss, and creating single-mode operating conditions. One of the main factors for the formation of supercontinuum spectra of injection pulses is maintaining the nonlinear performance of this type of fiber by controlling the effective mode area and also using chalcogenides (nonlinear coefficients about 100 times higher than silica) in their structure. In the proposed structure, a photonic crystal fiber with silica base element and air cavities with hexagonal structure with the center of Ge11.5As24Se64.5 chalcogenide element have been used to provide the nonlinear property of the structure. Also, in this structure, a ring of Ge20Sb15Se65 chalcogenide elements has been used to reduce the effective mode region and create a flat dispersion curve at a wavelength of 1300 nm (second telecommunication window). The input pulse power is 10 kW and its width is 50 femtoseconds, which has caused the range of the supercontinuum from 800 nm to 1900 nm. This structure can be used to provide the required wavelengths as a carrier in a wavelength division multiplexing (WDM).

Power efficiency and ability to high-speed modulation enables LEDs to transmit data in optical wireless communications. Optical camera communication (OCC) is a special type of VLC in which imaging sensor is using as a receiver instead of photodetector. Some challenges of such systems are the low data transmission rate and the light flickering. Recently under-sampled modulations are developed. In this paper to overcome flickering issue, we used one of undersampled modulation named UPSOOK which enable us to modulate data with a frequency many times higher than the frame rate. Due to the presence of color filters in these sensors, the data transmission rate in OCC systems can be increased by using RGB LED and WDM. On the other hand, using windowing method to find optimum region for data detection, bit error rate (BER) decreased.

In this paper, we investigate the effect of the number of quantum-dot (QD) layers on the performance of the 1.3um QD-vertical-cavity surface-emitting lasers (QD- VeSELs). QD rate equations and thermal conduction equation is solved self-consistently to consider the effect of self-heating on the characteristics of the QD- VeSEL. Results demonstrate that a further increase in the number of QD layers leads to a decrease in self-heating of the laser which enhances the maximum achievable output power and 3-dB modulation bandwidth of the laser. It is also shown that with an increase in the number of QD layers, the roll-over of the optical power occurs at higher injection currents which leads to improve in the dynamic and static behavior of the laser.

In this study, we have designed and simulated a photonic crystal (PhC) nonlinear AND logic gate. First, a suitable photonic crystal ring resonator (PhCRR) structure is designed and simulated that can be used for obtaining the switching power intensity after applying the nonlinear Kerr effect. In this structure, GaAs dielectric rods have been used in the air. The plane wave expansion (PWE) method is used to obtain the guided modes in the proposed structure and the two-dimensional finite-difference time-domain (2D-FDTD) method is also employed to simulate the structure. According to the results obtained from the numerical calculations, the minimum switching power intensity for the structure is equal to 800 W/μm2. Using the designed PhCRR and the obtained switching power intensity, an optimal compact photonic crystal AND logic gate based on the Kerr effect is designed and simulated which exhibits a good performance. The minimum contrast ratio, the response time, and the total area of the structure are 11.04 dB, 1.8 ps, and 194.56 μm2, respectively.

An optical comparator is an important logic circuit used in digital designs. Photonic crystals are among the platforms for implementing different kinds of gates and logic circuits, and they are structures with alternating refractive indices. In this paper, an optical comparator is designed and simulated based on a square lattice photonic crystal. In the design of this comparator, a small-sized structure is used. The simulation results show that in the proposed comparator, there is a high difference between logical values “0” and “1”, which are defined based on the optical power level. Due to the small size of this comparator and the adequate difference between logical values “0” and “1”, this structure suits photonic integrated circuits with high accuracy. The proposed structure footprint is 149.04 µm2, and the calculated rise time for this circuit is less than 0.4 ps.

A hollow-core photonic bandgap fiber (HC-PBF) with high relative sensitivity and low confinement loss was designed. Some destructive circumstance such as propagation losses and mode interference can disrupt performance of the PBF. By considering optimum size of the hollow-core radius, we were able to improve confinement loss and the relative sensitivity. By optimization of the shape and size of the closest row of air holes to the hollow core, the quality of the mode distribution in the hollow-core was well improved. Simulation results confirm that, at an optimal and reasonable core radius, the relative sensitivity and confinement loss of the proposed gas sensor were improved to 96.5% and 0.11 dB/m, respectively. In addition, in order to better matching of optical power between single mode fiber (SMF) and HC-PBF, we could reduce the destructive effects of optical mode mismatch, by mode interference suppression. Furthermore, by optimization of fiber structural specifications such as air filling fraction and lattice constant, the PBF was changed to a single-mode waveguide. Considering the operation wavelength 1530 nm which is very close to the acetylene gas absorption wavelength, this fiber is appropriate to be a high sensitivity gas sensor to detect absorbing gases in the middle infrared range.

Despite a significant growth in their efficiency in the last decade, perovskite solar cells suffer from low absorption power in the infrared range which forms a wide range of the solar spectrum. To find solutions to improve the performance of solar panels and the efficiency of their absorption, this research examined the effects of using nanostructures and plasmonic nanoparticles for modification of perovskite solar cells' active layer to create a wider absorption spectrum. For this purpose, the effects of different factors, such as the composition, radius and location of plasmonic nanoparticles on light absorption, open-circuit voltage, short-circuit current density and power conversion efficiency (PCE), were considered. Silver spherical nanoparticles were selected to be placed in the absorbent layer because of their low loss and high absorption power. By selection of optimum size, radius and position of nanoparticles and the location of metallic nanoparticles inside the transporting layers of the carrier, an improvement in the absorption and a 43% increase in the PCE of the perovskite solar cell were achieved.

Chalcogenide-based photonic crystal fibers (PCFs), due to their nonlinear properties, are capable of producing supercontinuum spectra, which can be used in the wavelength division multiplexing (WDM). These types of fibers highly absorb telecommunication wavelengths and hence cannot be used to transmit information over the corresponding wavelengths. We have studied the dispersion engineered chalcogenide PCF numerically, which is important to produce ultra-flat broadband supercontinuum (SC) spectra in all-normal dispersion areas. A 1 mm long, hexagonal chalcogenide PCF made from As2S3/Ge20Sb15Se65 and silica glass pumped at 1550 nm provided a SC bandwidth of 5000 nm with pump power of 1 kW. Here, we have reached zero-dispersion wavelength (ZDW) at third telecommunication window, which in turn has given us a broad spectrum at this window with reasonable efficiency.

In this report, four different metallic grid connectivity models, namely graphene, aluminum, gold, and silver for the copper gallium diselenide/copper indium gallium diselenide (CGS/CIGS) dual-junction solar cell, are discussed. In this structure, it is assumed that the metal contact with the aluminum-doped zinc oxide (ZnO:Al) layer forms an ohmic contact. This assumption is true since the metal grids are sandwiched. The sandwich metal grid is such that at first a metal layer having an ohmic contact with ZnO:Al is deposited, then the desired metal layer is placed on the metal layer as contact. In fact, this metal grid replaces the transparent conductor oxide (TCO) in the structure. Using a metal grid instead of a TCO will increase the transparency and thus be more absorbent, because the TCO also has some absorption and does not allow much light to enter the absorber layer. Based on the results of the simulation and comparison of the performance of the metal patterns, we conclude that by choosing the graphene ohmic contact, we will achieve higher efficiency and absorption. The CIGS dual-junction solar cell features were obtained by graphene metal grid, short circuit current of 14.7403 mA/cm2, open circuit voltage of 1.99054 V, fill factor of 38.78%, and efficiency of 11.38%.

In this paper, an ultracompact all-optical encoder based on a photonic crystal nanoresonator was designed. The proposed structure consists of several waveguides and two nanoresonators. The nanoresonators were designed by reducing the radius of the dielectric rods. To analyze the all-optical encoder, plane-wave expansion and finite-difference time-domain methods were, respectively, applied to calculate the bandgap diagram and to obtain the transmission and propagation of optical field. The contrast ratio, delay time, data transfer speed, and total footprint of the logic gate equaled 9.51 dB, 0.24 ps, 4.16 Tb/s, and 148 µ m 2 , respectively. In addition to these parameters, two new parameters were investigated: the range of optical power required, and the frequency range for better logic gate efficiency. Due to the ultracompacted size, low power consumption, low delay time, and simplicity of structure, this all-optical encoder is suitable for use in low-power optical integrated circuits.

Efficiency has been known to be one of the most important factors in a solar cell. This article presents the results of a simulation performed on a perovskite/CIGS dual-junction solar cell. In this report, first, a top solar cell consisting of a perovskite absorber layer is simulated using the pn-junction; the separation and transfer of carriers in this structure are done by the internal electric field. The pn-junction has a discharge area smaller than the pin-junction, which increases carrier recombination and reduces optical losses. The perovskite band gap of 1.9 eV is considered, and the efficiency is 21.65% using the Au electrode. Then, the bottom solar cell is fabricated with a CIGS absorbent layer with a 1.4 eV band gap and an efficiency of 11.46%. After simulating and evaluating the performance of the top and bottom solar cells independently, both cells were simulated and examined for the dual-junction state. Since the perovskite and CIGS band gaps are both adjustable, these two materials can act as a proper partner for an absorbent layer in a dual-junction solar cell. In this structure, instead of the usual connection of p-i-n and n-i-p perovskite, n-type and p-type homojunction perovskite connection is used, in which the transfer and separation of carriers are done by an internal electric field. Due to the fact that in this structure, the discharge area is smaller, the recombination of carriers is increased, and the light losses are reduced, which will increase the absorption and efficiency of the cell. The results show that in the tandem design, we encounter an increase in Voc (2.25 V), thus increasing the efficiency of the solar cell (30.71%).

In this paper, we investigated the concept of the Sagnac effect in passive integrated optical gyroscope base photonic crystal ring resonator for angular velocity sensing. This configuration utilizes one 3 dB coupler, two Bus waveguides, and five ring resonators for rotation sensing. The structure has been designed using dielectric silicon rods which are embedded in air. The transmission efficiency of the photonic crystal ring resonator at 1551 nm is about 96% with quality factor and bandwidth values equal to 4326 and 0.35 nm, respectively. By measuring the power of the output port, it is possible to estimate the phase shift and then to measure the rotation rate. The central wavelength of structure is assumed to be equal to 1569.9 nm. The structure characteristics have been investigated by using the two-dimensional finite-difference time-domain method. The footprint of the structure is approximately 279 μm2.

In visible light communications with the optical orthogonal-frequency-division multiplexing (O-OFDM) technique, the high peak-to-average-power-ratio (PAPR) is an important issue due to the limited dynamic range of light emitting diode (LED)-based light sources. To address this problem, we propose a hybrid OFDM-pulse time modulation (PTM) scheme where bipolar O-OFDM samples are converted into the digital PTM formats of pulse width modulation (PWM) and pulse position modulation (PPM) for the intensity modulation of LEDs. We convert the DC biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) samples to PTM and show that DCO-OFDM-PTM offers improved bit error rate (BER) performance. For example, for 16-quadrature amplitude modulation (QAM) at the BER of 10−3, the signal-to-noise ratio (SNR) gains are ∼ 1.35 dB and ∼ 1 dB for DCO-OFDM-PPM and DCO-OFDM-PWM compared to ACO-OFDM-PPM and ACO-OFDM-PWM, respectively. Simulation results show that DCO-OFDM-PPM displays an improved BER performance compared with DCO-OFDM-PWM for both lines of sight (LOS) and diffuse configurations. For instance, for an ideal LOS channel and for 16-QAM at the BER of 10−3, the required SNR values are ∼ 4.5 and ∼ 10.2 dB for DCO-OFDM-PPM and DCO-OFDM-PWM, respectively.