Finite Difference Beam Propagation Method Research Articles

Hourglass-Shaped Fiber-Optic Mach-Zehnder interferometer for pressure sensing

A fiber-optic pressure sensor based on an hourglass-shaped structure (HSS) is proposed and demonstrated. A Mach-Zehnder interferometer (MZI) is constructed in the inside cavity of the HSS. The HSS consists of a silica frame and two photoresist diaphragms covered on side holes of the frame. Pressure response of the MZI is analyzed by theoretical calculation, the mechanical deformation of the HSS is simulated by using the finite element method, and the spectral characteristics of the sensor are simulated by using the finite-difference beam propagation method. Experimental results indicate that the pressure sensitivity of the sensor with a size of 0.2 × 0.2 × 0.5 mm3 reaches –1.27 nm/kPa with a linear response range of 0–35 kPa. In addition, repeatability, time stability, the cross-responses to ambient refractive index (RI) and temperature, and response time of the sensor are evaluated experimentally. With advantages of high sensitivity, compact size, and insensitivity to ambient RI, the proposed sensor shows promising potential in optofluidic and biomedical sensing fields.

Optimization Method for Center Frequency Accuracy of High-Channel-Count Arrayed Waveguide Grating in Dense Wavelength Division Multiplexing Systems

The arrayed waveguide grating (AWG) is an essential component in dense wavelength division multiplexing (DWDM) systems. With advancements in optical communication technology, the number of AWG output channels has rapidly increased. However, achieving high center frequency accuracy (CFA) for these channels has become a significant challenge. This paper presents a design and optimization approach for a high-channel-count AWG based on the silica platform and the finite difference beam propagation method (FD-BPM). The causes of center frequency deviation are analyzed, and an optimization method is proposed to adjust the constraint relationship and geometric position of the output waveguides in order to improve CFA. Simulation results demonstrate the excellent optical performance of the designed AWG, achieving a CFA of up to 0.04 times the output channel frequency spacing. The insertion loss, insertion loss uniformity, and adjacent output channel crosstalk are simulated as 1.51 dB, 0.38 dB, and 23.81 dB, respectively. The optimized CFA could compensate for center frequency deviations caused by manufacturing errors and temperature fluctuations. The findings of this research hold potential for extending the optimization to other material platforms and contributing to the development of DWDM systems in the field of optical communication.

Experimental and Modeled Output Characteristics of a Compact, Passively Q-Switched Tm:YLF Laser

We measure the output characteristics of a compact, laser diode end-pumped Tm:YLF laser, operated in continuous wave (CW) mode, or passively Q-switched (PQS) using various Cr2+:ZnS saturable absorbers. A numerical Finite-Difference Beam Propagation Method (FD-BPM) is developed and used to model the laser CW and pulsed performance. Good agreement between the experiment and simulated output is demonstrated. The 39 mm-long laser resonator utilizes a 20.2 mm-long Tm:YLF crystal, a 1908 nm volume Bragg grating (VBG) reflector to restrict the emission wavelength, and an intra-cavity negative lens to reduce the energy fluence in the saturable absorber. A laser utilizing a Cr2+:ZnS saturable absorber with 91.9% unsaturated transmission generates 9.8 mJ PQS pulses with 8.7 ns full-width-half-maximum (FWHM) pulse duration. Maximum average output powers of 6.1 W and 6.5 W are generated in PQS and CW operation, respectively.

Design of low-loss Nb2O5-based grating coupler in scanning differential laser-Doppler integrated probe for cross-sectional velocity distribution measurement

We present an Nb2O5-based grating coupler that employs multiple Nb2O5 / SiO2 layers to improve the optical input/output power efficiency of a scanning differential laser-Doppler integrated probe for two-dimensional cross-sectional velocity distribution measurements. The multiple Nb2O5 / SiO2 layers are designed for both the transmitting grating coupler and the receiving grating coupler. To demonstrate the effect of the designed multiple layers, the field distribution around the grating coupler is simulated as a feasibility study of the concept. The results indicated that the input/output power at the grating coupler can be improved using multiple Nb2O5 / SiO2 layers over the input/output angle of 0 to 20 deg and the wavelength of 1530 to 1570 nm. The increase of the input/output power and the focusing from the transmitting grating coupler were confirmed by the simulation using the finite-difference time-domain method and beam propagation method.

Optical Field Transmission Analysis of an All-Fiber Signal Combiner With a Dumbbell Shape

In this paper, the optical field transmission characteristics of an all-fiber signal combiner with a dumbbell shape and its application in laser beam combining are analyzed theoretically. Based on the three-layer waveguide model, the mode characteristics of a tapered multi-core fiber and a tapered single core fiber are analyzed respectively. On this basis, the working principle of an all-fiber signal combiner with a dumbbell shape is introduced from the perspective of mode evolution. The finite-difference beam propagation method is used to numerically analyze optical field envolution process in the combiner. Influences of the waist diameter, taper length and geometric arrangement are analyzed in detail. The calculation results show that an all-fiber signal combiner with a dumbbell shape can realize high brightness laser beam combining and the beam quality can reach the theoretical limit.

Optical properties and thermal stability of the H+-implanted Dy3+/Tm3+-codoped GeS2–Ga2S3–PbI2 chalcohalide glass waveguide

Abstract Optical waveguides play a vital role in the manufacture of various optical devices due to their unique performances and high-degree integration. We report on the preparation and characterization of the planar waveguides in the Dy3+/Tm3+-codoped GeS2–Ga2S3–PbI2 chalcohalide glass. The waveguide was formed by the 400 keV H+-ion implantation with a dose of 8 × 1016 ions/cm2. Its thermal stability was studied by annealing at 260°C for 1 h. The changes in the nuclear energy loss with the implantation depth were simulated by the stopping and range of ions in matter (SRIM 2013). The dark-mode characteristics of the waveguide were measured by the prism coupling method. The refractive index distribution of the optical waveguide was reconstructed by the reflectivity calculation method. The modal profile of the waveguide structure was calculated by the finite-difference beam propagation method (FD-BPM). The thermally stable proton-implanted Dy3+/Tm3+-codoped GeS2–Ga2S3–PbI2 chalcohalide glass waveguide is expected to be applied in mid-infrared integrated optical devices.

Temperature and Pressure Sensor Based on Polished Fiber-Optic Microcavity

A single-optic-fiber temperature and pressure sensor based on a side-polished optical fiber microcavity coated with a polydimethylsiloxane (PDMS) film has been proposed and fabricated. This sensor integrates two Mach-Zehnder interferometers (MZIs), where the first MZI is formed by path 1 in the inside coating, and the second MZI is constructed by path 1 and path 2 in the waist region. The formation mechanism of the dual MZIs is simulated by using the finite-difference beam propagation method, and the sensitivities of the dual MZIs are predicted by simulations. Experimental results indicate the temperature sensitivity of –1.19 nm/℃ for the first MZI and pressure sensitivity of –3.96 nm/MPa for the second MZI, respectively. By monitoring the dual wavelength shifts corresponding to the dual MZIs, temperature and pressure can be simultaneously measured. Due to the good sensitivities, simple fabrication and robust structure, the proposed sensor shows promising potential for optofluidic sensing and ocean detection.

Terahertz beam splitter based on mode coupling of subwavelength waveguides

A broadband terahertz waveguide type beam splitting scheme for multi-port outputs is proposed, which is realized by uniform mode coupling of the multiple subwavelength waveguides. The full-vectorial finite-difference beam propagation method (BPM) is adopted to numerically analyze the mechanism and splitting performance of the beam splitter, and the influence of the supporting structure between the waveguides and the outer dielectric ring on its transmission performance is studied. In particular, a 1 × 8 beam splitter is designed with a coupling zone length of 1.02 mm, which can couple the input THz wave into eight output ports uniformly. The insertion loss of the device is 9.3 dB at the center frequency of 0.54 THz and the bandwidth can reach 0.21 THz with insertion loss lower than 10 dB.

Fabrication of 2 × 1 signal combiner for high power lasers based on D-shaped fiber pair

The all-fiber signal combiner based on incoherent beam combination is the key component to realize the power amplification of fiber laser. At present, the all-fiber combiner is generally fabricated by splicing a tapered fused fiber bundle with an output fiber. During the tapering process, the size of the cladding and core of the input fiber will become smaller at the same time, which will inevitably cause the core mode to change to the cladding mode. To reduce the degree of taper, the method of hydrofluoric acid corrosion is generally used to remove part of the cladding of the input fiber to make the diameter of the optical fiber smaller. Even so, the tapering process is still unavoidable. Therefore, we have proposed a new manufacturing method for the all-fiber signal combiner. This method uses a CO2 laser to ablate part of the cladding of the input optical fiber to produce a D-shaped fiber. Then, the planes of the two D-shaped optical fibers are pressed tightly and then heated to get fused. Finally, the D-shaped fiber pair is cut and spliced with an output fiber to form an all-fiber combiner. This novel method avoids the tapering process and hardly changes the core size of the input fiber. The relationship between the core distance of the input fiber and the beam quality of the signal combiner manufacturing by the new method is studied by the finite-difference beam propagation method. We also compared the combining ability and beam quality of the combiner made based on the D-shaped fiber and that made by the tapered fiber bundle. Both the simulation and experimental results showed that the beam quality of the combiner made based on the D-shaped fiber was better than that made by the traditional method.

Optical Power Splitter Based on Single Mode Directional Coupler Waveguide Using SnO2 Nanomaterial

Optical power splitter based on waveguide had been simulated numerically using Finite Difference Beam Propagation Method (FDBPM). Proposed waveguide was designed in the form of simple directional coupler waveguide. The waveguide was contained SnO2 nanomaterial as film or the guide part and the other supporting material as cladding with lower refractive index such as flint glasses. The waveguide used 2 μm of width to establish single-mode waveguide. The structure of waveguide is divided into three parts such as input, coupling and output part. While the waveguide was modified with angle in input and output parts to avoid coupling between waveguides. Furthermore, the proposed waveguide was analysed by varying the angle and coupling length. The analysed result shows that the waveguide has best performance in angle of 0.5 degrees and coupling length of 300 μm when the propagation loss was around 0.53%. Using the parameter, the output distribution percentage of waveguide approached 55%:44.5%. This performance indicated that the proposed waveguide can be used as optical power splitter. The application is very useful for optical telecommunication networking development.

Underwater fiber-optic salinity and pressure sensor based on surface plasmon resonance and multimode interference.

In this paper, an underwater fiber-optic sensor based on surface plasmon resonance (SPR) and multimode interference (MMI) is presented for simultaneous measurement of salinity and pressure. This sensor is based on a single-mode-multimode-single-mode-multimode-single-mode structure with a gold film deposited on the middle single-mode fiber and the fiber structure is wrapped around an elastic cylinder to constitute a sensing head. In the fiber structure, the SPR region produces a resonance dip to measure salinity, and the independent MMI region achieves narrow and salinity-insensitive interference dips to measure pressure. Performance of the sensor is predicted by calculation, and the MMI spectrum is simulated by using the finite-difference beam propagation method. By experimental tests for salinity and pressure, the sensitivities of 0.36 nm/‰ and -1.42nm/MPa are achieved, respectively, and the cross talk is also proved to be insignificant. This study provides an important application direction for SPR-MMI sensors and a prospective method for ocean detection.

Design and analysis of 2 × 1 photonic temporal multiplexer using SOI ring resonator for switching and logical applications

In this work, we designed a reconfigurable 2 × 1 temporal multiplexer (MUX) using a 1 × 1 microring modulator (MRM) configured in silicon on insulator (SOI) platform. To evaluate the performance of the MUX, logical operations such as NOT, OR, AND, NOR, and NAND are demonstrated. The numerical functionality of the device is assessed using the finite-difference beam propagation method (FD-BPM). The 1 × 1 MRM has a ring radius of 6 μm having a free spectral range (FSR) of 18 to 19 nm. The transmission characteristics and coupling factors of the all-pass and add-drop configuration are analyzed, especially for switching and logical circuit applications. The voltage tuning across the P-i-N junction is achieved by varying the carrier concentration across the intrinsic region, which corresponds to the change in refractive index. Various design parameters are tuned for this configuration, which offers a switching voltage of 0.79 V with modulation speed of 139 GHz. Further, the logical operations of OR, AND, NOR, and NAND with an extinction ratio of 10.25, 11.40, 18.83, and 16.06 dB is reported.

Macroscopic wave-optical simulation of dielectric metasurfaces.

We propose a novel method for the wave-optical simulation of diffractive optical elements (DOEs) like metasurfaces or computer-generated holograms (CGHs). Existing techniques mostly rely on the assumption of local periodicity to predict the performance of elements. The utilization of a specially adapted finite-difference beam propagation method (BPM) allows the semi-rigorous simulation of entire DOEs within a reasonable runtime due to linear scaling with the number of grid points. Its applicability is demonstrated by the simulation of a metalens and a polarization-dependent beamsplitter, both based on effective-medium metasurfaces. A comparison shows high conformity to rigorous simulations.

Enhanced Raman intensity in ZnS planar and channel waveguide structures via carbon ion implantation

Planar and channel waveguides in ZnS single crystal are fabricated by 6.0 MeV C ion implantation with fluences of 5 × 1014 and 1 × 1015 ion/cm2 at room temperature. The optical modes for the planar waveguides are measured by using a prism coupler by the well-known m-line method. The near-field light intensity profiles for the planar and channel waveguides are measured by the end-face coupling setup at 633 nm and 1539 nm. The reflectivity calculation method and the finite difference beam propagation method are used to reconstruct the refractive index profile of the planar waveguides and near-field intensity distribution of planar and channel waveguides. Raman images obtained with a 532-nm laser are collected to evaluate the damage to the ZnS crystal and waveguide layers.

Combined thermomechanical and optical simulations of planar-optical polymer waveguides

In this work, we describe a theoretical approach for combined thermal, mechanical and optical simulation and analysis of planar polymer waveguides. We consider a finite element approach for thermal and stress/deformation simulation. Also, a Crank-Nicholson finite difference beam propagation method (CN-BPM) is implemented to perform the optical simulation. The results of the finite element (thermo-mechanical) analysis are coupled with the CN-BPM results to carry out the optical simulation of poly(methyl methacrylate) (PMMA) waveguides as function of temperature. For thermal simulation, a model was designed where a polysilicon microheater was added to the upper cladding of the PMMA waveguides to vary the temperature between 20 °C and 200 °C. Thus, the impact of the induced temperature gradients on the refractive index modulation of the PMMA waveguides and the corresponding change in numerical aperture are obtained. In addition, the temperature gradients influence the beam intensity profiles and the movement of the primary eyes within the optical waveguides, thus, impacting the optical properties. Furthermore, the thermally induced mechanical stress and deformation were calculated for transverse and axial directions. In the next step, validation of the model by systematic experimental studies will be performed. In general, our approach provides a toolbox for more comprehensive multi-physics theoretical analysis of polymer-optical waveguides which, in future, can be extended to more complex and functional structures as required for flexible sensor networks, as example.

Selective excitation in multimode fiber using elliptical few mode fiber

The endless growth of the Internet and its applications compels us to introduce new technologies and solutions for increasing data bandwidth demands. In this paper, we propose a technique for selective excitation of fifteen mode groups in a multimode fiber (MMF) using an elliptical few mode fiber (FMF). The elliptical FMF was designed using finite difference beam propagation method and inserted as a pre-compensator before a MMF channel in a numerical optical system to evaluate its modal properties and transmission quality. For performance comparison, the elliptical FMF-pre-compensated MMF optical fiber system is compared with a non-pre-compensated optical consisting of only a MMF. The obtained results show that the elliptical FMF improves the modal purity of the channels and the bit error rate (BER).

The design and analysis of a CMOS-compatible silicon photonic ON–OFF switch based on a mode-coupling mechanism

A directional-coupler-based 1 $$\times$$ 1 silicon-on-insulator photonic ON–OFF switch with a complementary metal–oxide–semiconductor (CMOS)-compatible driving voltage is proposed in this paper. The directional coupling of the switch is accomplished by a carrier injection method with the help of a P-i-N diode phase shifter. The advantage of using the directional coupler as a switch is its smaller layout requirement, which makes it more suitable for use in integrated photonic applications. The ON–OFF switch has potential applications in programmable photonic switch fabrics, where the OFF-state behavior is used to prevent the input from being propagated to the output. The proposed switch offers a wide range of cross power-coupling coefficient $$(\kappa ^2)$$ values, ranging from 0 to 0.3 for the ON-state and from 0.79 to 1 for the OFF-state. The finite-difference beam propagation and two-dimensional (2D) finite-difference time-domain methods are used for photonic simulations. The results are compared analytically using the coupled mode theory with the help of the MATLAB software package.

Gerbang logika X-OR pada pandu gelombang optik single mode y-branching dengan menaruh sebagian bahan tak-linier di dalam cladding

An Optical Logic X-OR gate in Y-Branching single mode waveguide which nonlinear material superimposed partially in the cladding region was analyzed by means of FD-BPM (Finite Difference Beam Propagation Method). The whole proposed waveguide dimension is 50 x 3900 µm and for each fixed parameters such as film width w = 2.0 µm, angle θ = 0.2290, film refractive index nf =1.552, cladding refractive index nc=1.550, λ=0.5145 µm, nonlinear coefficient (partially superimposed in the cladding region) a = 6.377x10-12 m2/V2 and propagation constant β/k0 =1.55087373. Scheme of laser intensities propagation from input channel to the output channel showed that the yield of the optical logic X-OR gate exhibited the following output powers. Â

Broadband second-harmonic generation in a tapered PPLN waveguide

We demonstrate a period poled tapered lithium niobate waveguide and study second harmonic generation (SHG) in this device for the purpose of broadening the quasi-phase matching (QPM) acceptance bandwidth. The finite-difference beam-propagation method is used to simulate the guided modes and calculate the effective indices. The simulation results show that by tapering the width of the cross section linearly, the phase mismatch between a specific input wavelength and its SHG signal can be varied along the propagation length. Ideal SHG phase-matching conditions for a wide range of input wavelengths in communication band from 1 542.5 nm to 1 553.5 nm can be satisfied in different positions of the waveguide.

Characterization of optical waveguide in chalcogenide glass formed by helium ion implantation

In this work, an optical waveguide in the chalcogenide glass (80GeS2–15Ga2S3–5Sb2S3) is fabricated for the first time and its optical properties are investigated. A one-dimensional chalcogenide waveguide structure has been formed by 550 keV He+ ion implantation with a dose of 4 × 1016 ions/cm2 at room temperature. The effective refractive indices of the guided modes at the wavelength of 632.8 nm were measured with the aid of the Model 2010 Prism Coupler system. The refractive index profile, a critical factor in the analysis of waveguide structure, was reconstructed by the reflectivity calculation method. It was found that a positive change of refractive index occurred in the waveguide region, and an optical barrier with decreased refractive index was built at the end of the ion trajectory, i.e., producing a type of “well + barrier” structure. The SRIM 2013 program was utilized to display the profile of the energy loss versus the penetrated depth when the incident ion beam implanted. The near-field intensity distribution for the fundamental mode along the TE polarization was simulated through the finite-difference beam propagation method. From our experiments, these data provide a great deal of useful information about the manufacture of the chalcogenide waveguide structure.