Optical Power Distribution Research Articles

Fivefold increased power handling in waveguide UTC-PDs using dual-injection shared CPW PD-arrays

This work presents two circuit-based solutions to enhance the power handling capabilities of waveguide-integrated uni-travelling carrier photodetectors (WG-UTC-PDs). Compared to a baseline WG-UTC-PD, these solutions achieve a fivefold increase in photocurrent before thermal breakdown. First, dual-injection improves the optical power distribution within a baseline WG-UTC-PD, raising the photocurrent threshold before thermal breakdown. Second, an array of four optically parallel WG-UTC-PDs, electrically connected to a single coplanar waveguide (CPW) line, further increases the maximum photocurrent by distributing the input optical power across multiple PDs. The design omits a termination resistor, as the arrays do not rely on a traveling wave configuration, maximizing photocurrent without a 50% reduction of bandwidth. Both 4-PD single-injection and dual-injection arrays were designed, fabricated, and characterized. Compared to a baseline UTC-PD, with a maximum photocurrent of 1.8 mA at a 3 dB bandwidth of 55 GHz, the 4-PD single-injection circuit achieved 5.1 mA at 43 GHz. The dual-injection array further increased the photocurrent to 9.2 mA at a bandwidth of 35 GHz. Electrical reflection measurements confirmed the negative effects of CPW losses on RF performance. These power handling improvements enable compact, high-power integrated solutions for microwave photonics.

The Effect of Lens Shape, Zonular Insertion and Finite Element Model on Simulated Shape Change of the Eye Lens

The process of lens shape change in the eye to alter focussing (accommodation) is still not fully understood. Modelling approaches have been used to complement experimental findings in order to determine how constituents in the accommodative process influence the shape change of the lens. An unexplored factor in modelling is the role of the modelling software on the results of simulated shape change. Finite element models were constructed in both Abaqus and Ansys software using biological parameters from measurements of shape and refractive index of two 35-year-old lenses. The effect of zonular insertion on simulated shape change was tested on both 35-year-old lens models and with both types of software. Comparative analysis of shape change, optical power, and stress distributions showed that lens shape and zonular insertion positions affect the results of simulated shape change and that Abaqus and Ansys show differences in their respective models. The effect of the software package used needs to be taken into account when constructing finite element models and deriving conclusions.

A Systematic Review on Non-Orthogonal Multiple Access (NOMA) Based on Visible Light Communication for Intelligent Transportation Systems

Non-Orthogonal Multiple Access (NOMA) is a cutting-edge technology that permits several users to use the same frequency resources and transfer data at once. Visible light communication also known as VLC, is an innovative technology that offers access to an unoccupied spectrum, enables fast data transmission speeds, thereby making it a complementary addition to current radio frequency technology. Although VLC has its advantages, it also comes with some downsides. One major issue is the modulation bandwidth of LEDs, which can be quite challenging when creating high speed VLC systems. Non-orthogonal multiple access (NOMA), particularly power-domain NOMA (PD-NOMA), has become a viable multiple access approach, ensuring sufficient bandwidth allocation for VLC systems. This study delves into multiple access techniques in VLC systems, in particular NOMA, and discusses its fundamental principles, key ideas and practical implementations like vehicular communication. It also addresses the associated hurdles, such as ensuring equitable distribution of optical power among users, leveraging MIMO technology, and grappling with LED nonlinearity issues. In summary, we underscore the significance of utilizing NOMA-VLC technology to enhance wireless communication systems and optimize spectral efficiency.

A novel design of an all‐optical D flip‐flop based on 2D photonic crystals

AbstractIn this study, we endeavored to create and simulate a D‐type flip‐flop utilizing two‐dimensional photonic crystals featuring a square lattice. This flip‐flop, composed of just two intersecting waveguides, is both compact and straightforward. Constructed from 10 × 10 dielectric rods suspended in air, it boasts low output power in logic mode 0, creating a significant distinction between optical powers in modes 0 and 1, lending it a logical aspect. Optimized for operation at a communication wavelength of 1.55 µm, this flip‐flop proves suitable for optical communication systems. The third window optical fiber communication system is suitable for 1.55 μm wavelength because the fiber has the lowest loss. Our analysis involved calculating the band structure via the plane wave expansion method and determining the optical power distribution using the finite difference time domain method.

Design of the Light Source Layout Optimization Strategy Based on Region Partition and Pre-Bias Compensation for Indoor Visible Light Communication Systems

Optimization of the light source layout is an important issue for indoor visible light communication systems, as it affects the received optical power distribution and user perception. In this paper, we propose a local optimization strategy for the light source layout that balances optimization effectiveness with optimization efficiency. First, we divide the optimization region into multiple sub-regions with different sizes and optimization priorities, where the sizes and optimization priorities of the individual sub-regions are determined based on the effect minimization principle among the sub-regions. We then calculate the pre-bias factor based on the equivalent mapping, which can compensate for the effect of the light sources in the latter optimized sub-region on the source layout optimization in the current sub-region. Finally, we search for the coordinate of a single light source for each sub-region using the variance of the squared distance between the projection of the light source on the receiving plane and the received point as a fitness function. Simulation results show that the proposed optimization strategy performs well when the vertical distance between the ceiling and the receiving plane is not less than 2.85 m.

Raising the stimulated Brillouin scattering threshold power by a longitudinal compression gradient in a fiber amplifier

We propose and demonstrate an analysis of our approach to enhance the threshold power of stimulated Brillouin scattering (SBS) in optical fibers, using a longitudinal compressive strain gradient. We derive analytical expressions for the power spectral density of the backscattered Stokes wave in the general case of passive and amplifying optical fibers, by considering the strain and optical power distributions. Our method provides an accurate prediction of the SBS gain spectrum, which we illustrate with a quantitative comparison between measurements and calculations of the SBS Stokes spectra, before and after applying the compression gradient. Our experimental results demonstrate the successful enhancement of the SBS threshold power by a factor of about three for the passive fiber and two for the amplifying fiber. The enhancement that we manage to calculate in the case of the passive fiber is in perfect agreement with the experimental results.

Multi-point fiber optic laser-ultrasonic transducer based on long-period fiber grating

For ultrasonic non-destructive testing, the development of miniaturized, embeddable, and distributed ultrasonic transducers has always been a hot research topic. In light of this, a novel fiber optic laser-ultrasonic transducer, employing the long-period fiber grating (LPFG), was introduced in this study to facilitate the implementation of embedded and distributed applications. LPFG couples the light transmitted in the optical fiber into the photoacoustic conversion material. After the energy conversion of optic-heat-acoustic, ultrasonic waves can be excited. The finite element models of the mode coupling and photoacoustic conversion of LPFG were established. The transmission spectrum and optical power of LPFG were measured by experiments. The results show that by adjusting the structural parameters of LPFG to make it appear as different transmission depths, the quantitative control of the optical power distribution of the core and cladding can be realized. Two LPFGs, each with distinct coupling coefficients, were fused together and anchored onto aluminum plates to create an ultrasonic transducer with dual micro-ultrasonic sources. An ultrasonic excitation experiment was carried out, and the results revealed that both two LPFGs could excite ultrasonic waves with a peak-to-peak value of about 500 mV and a center frequency of about 3 MHz. The study validates the proposed fiber-optic ultrasonic transducer's steady performance and its potential for multi-point applications.

Cascaded subarray design and control method for power efficient, thermal crosstalk optimized optical phased array.

Thermo-optic phase shifters (TOPSs) are commonly used in large-scale silicon photonic integrated optical phased arrays (OPAs). However, fast-response TOPSs consume relatively high power; the elevated temperature floor in the dense region of the TOPSs introduces thermal crosstalk between optical paths, which undermines the control accuracy. We propose a combined method that involves subarray design in the optical power distribution network and array control method to predict, optimize, and redistribute the phase shifts and mitigates thermal crosstalk. Thermal simulations and an array control method for generic OPA models are discussed. A silicon photonic chip prototype of a 4 × 4 OPA with three-level cascaded subarrays is fabricated to demonstrate the proposed method. The experimental and statistical results show that the method effectively reduces the average total power consumption by 31%, the maximum local temperature by 18.4%, and the thermal crosstalk within the OPA.

SIPAM-4 scheme improves bit error rate performance in MISO X-ray communication system

X-ray communication (XCOM) technology, which uses X-ray as the carrier wave, is expected to solve the problem of reentry blackout region communication with its good penetration characteristics. In order to mitigate the effects of non-linear distortion of the gated X-ray tube (GMXT), the multiple-input single-output (MISO) technique has been introduced in XCOM systems. A pulse amplitude modulation scheme based on superimposed intensity (SIPAM) was proposed and achieved by configuring GMXTs with on/off keying modulation in MISO XCOM system. The feasibility of the proposed SIPAM scheme in XCOM was numerically investigated through Monte Carlo simulations. As the divergence angle of the GMXT increases, the optical power distribution tends to be more uniform in the receiving plane. It can greatly relax the requirements for link alignment. After that, we conduct a proof-of-concept experiment for the SIPAM with four levels (SIPAM-4) scheme by employing two GMXTs and an LYSO-SiPM-based X-ray detector. The experimental results show that when the bit error rate is within the forward error correction limit of 3.8 × 10−3, the SIPAM-4 scheme can reach a maximum communication rate of 1.20 Mbps, which is 20% higher than the conventional PAM-4 scheme. It can be included that the proposed MISO based the SIPAM-4 modulation scheme could improve the performance of the X-ray communication system.

Balance between the diffraction efficiency and process robustness for plasmonic lithographic alignment technology considering the Fabry-Perot resonator effect.

Different from traditional lithography, metal material with high absorptivity and high reflectivity is introduced into plasmonic lithography technology. In particular, a silver/photo resist/silver film stack can form a Fabry-Perot (F-P) resonator structure, which can greatly change the behavior of the light reflection and transmission. Since the silver layer has a strong absorption ability to the alignment probe light with a wavelength of 532 or 633nm, the quality of the alignment signal is seriously affected. In this paper, a thin film Fourier transfer model is established to quantitatively calculate the amplitude and phase information of the diffraction light with different orders. The results show that the diffraction optical power can be enhanced by the thickness optimization of all film stacks, and the maximum wafer quality (normalized diffraction efficiency) can be increased to 25.7%. The mechanism analysis of alignment signal enhancement is based on the F-P resonator phase oscillation amplification effect. However, it can also bring the reverse of both the power and phase for the alignment probe signal when the thickness fluctuation of the F-P resonator exists, which will be a great challenge for through-the-mask moiré fringe alignment technology. To obtain the optical power distribution of the structure surface and image of moiré fringes, a transfer matrix method is given to point-by-point calculate the incidence and reflection of the probe light in the vertical direction. The finite-difference time-domain method is also used to demonstrate alignment performance. It is proved that the subtle fluctuation of the photoresist thickness can make a huge difference to moiré fringes. A balance between the diffraction efficiency and process robustness can be achieved for plasmonic lithographic alignment technology by controlling the thickness range of the F-P resonator structure. In addition, the metal-insulator-metal structure has excellent thickness sensitivity and is applicable to optical signal detection and material property monitoring.

Low-noise φ-OTDR employing nonlinear optical preamplification for distributed acoustic sensing.

A phase sensitive optical time domain reflectometry sensing scheme employing a two-stage nonlinear optical preamplification system is proposed to reduce the blind spot effect in Rayleigh scattering by improving the optical power distribution and to enhance the resolution at locations of low-backscatter intensity measurements, providing a higher signal-to-noise ratio for distributed acoustic measurements; the developed system has been tested in-laboratory and on in-field monitoring of a survey well in Cottessen, The Netherlands. The characterization shows strain noise levels below 1nɛ for a 10kHz sampling rate.

Audio Transmission Using Visible Light Communication Technology with Pre-Equalization Technique

Abstract: The Visible Light Communication (VLC) is the emerging technology that aims to offer high speed internet access in indoor environment. This technology basically uses intensity modulation of current lighting infrastructure, which is provided by Light Emitting Diodes. During the implementation of this technology there may be Inter Symbol Interference (ISI) and power loss in free space channel, in order to overcome this problem and to utilize white light spectrum fully for effective transmission of data several techniques are followed. In this paper, the pre-equalization technique is chosen and implemented for audio transmission. The Pre-equalization of data is performed using MATLAB 2022a Graphical User Interface (GUI) and then feed to designed real-time VLC system. Pre-equalization of audio data improves sound quality and maintained it throughout the transmitting distance which is in Line Of Sight. The optical power distribution of the signal at an indoor environment for chosen white LED for audio transmission is also observed.

High-Performance Electro-Optical Mach–Zehnder Modulators in a Silicon Nitride–Lithium Niobate Thin-Film Hybrid Platform

We analyzed a Mach–Zehnder electro-optical modulator based on a silicon nitride strip–loaded waveguide on 0.5 μm thick x-cut lithium niobate thin film. The optical and radio frequency parameters for two different modulator structures (Type I: packaged with 2 μm thick SiO2 and Type II: unpackaged) were simulated, calculated, and optimized. The Optical parameters included the single-mode conditions, effective indices, the separation distance between the electrode edge and the Si3N4-strip-loaded edge, optical power distribution, bending loss, optical field distribution, and half-wave voltage. The radio frequency parameters included the characteristic impedance, attenuation constant, radio frequency effective index, and −3 dB modulation bandwidth. According to the numerical simulation and theoretical analysis, the half-wave voltage product and the −3 dB modulation bandwidth were, respectively, 2.85 V·cm and 0.4 THz for Type I modulator, and 2.33 V·cm and 1.26 THz for Type II modulator, with a device length of 3 mm.

LED layout optimization in visible light communication system by a hybrid immune clonal bat algorithm

To enhance the flatness of received optical power distribution, a hybrid immune clonal bat algorithm (ICBA) combining the bat algorithm (BA) with an immune clonal algorithm (ICA) has been proposed to optimize the light source layout of indoor visible-light communication (VLC) systems. Taking 16 light-emitting diode (LED) lamps under a rectangular layout as an example, the variance of the received light power on the receiving plane is set as the fitness function. Specifically, the fitness value is optimized by the variables of 18 elements consisting of the LED position parameters and their half-power angles. The results show that compared with the non-optimized case, BA, and ICA, the fitness value of ICBA decrease by 97.70%, 96.77% and 98.81%, respectively. The signal-to-noise ratio (SNR) variance optimized by the ICBA is the lowest, approximately 0.1037 dB, and the variance of illuminance optimized by ICBA is 62.99% lower than that optimized by ICA. Besides, compared with the non-optimized case, the BER performance of this VLC system has also been improved significantly after ICBA optimization. This study will benefit the research and development of indoor VLC systems.

Modal analysis of arbitrary-oriented ridge waveguides in x-cut lithium niobate thin film

We theoretically and numerically investigate the mode properties of ridge waveguides made on lithium niobate (LN) subwavelength thin film by taking into complete account the anisotropic feature of LN crystal. We analyze the effective refractive index of the quasi-transverse-electrical (q-TE) and quasi-transverse-magnetic (q-TM) modes and their difference in arbitrary-oriented waveguides on x-cut lithium niobate on insulator. Waveguide simulations based on full-vectorial finite element method are performed. The geometrical parameters and directions of the optical axis of the LN material for the ridge waveguides are varied to investigate the single-mode condition, optical power distribution and mode hybridization effect in the straight waveguides. The different trends in optical power distribution in LNOI waveguide between q-TE and q-TM at different crystal optical axis angles will be explained in detail.

Comprehensive design method of a soft multifocal contact lens with NURBS.

This study presents a comprehensive method for designing a multifocal contact lens (MCL) with Snell's law and non-uniform rational B-spline (NURBS) curves. Instead of using thin lens approximation, general mathematical formulas have been developed to achieve the accurate coordinates of points on the anterior lens surface profile of the MCL to meet various given optical power distributions. Then the NURBS curve is adjusted to fit these data points to obtain the smooth front lens surface profile. This method not only improves the accuracy of the optical power profiles of MCLs but also reduces the spherical aberration in near/distance optical zones. The experimental results show that the power profiles of soft MCLs agree with those of the simulation results and original design requirements. The proposed method has been proven for the MCL design, and it can be feasibly applied in complex optical lens designs.

Design of a freeform imaging spectrometer based on a solution-diversified automatic design method.

A freeform imaging spectrometer design method is proposed. Only system specifications, including the slit length, numerical aperture, magnification, spectral range, and spectral resolution, are required as design inputs, and imaging spectrometer systems with various optical power distributions and structures are output. Using three-mirror imaging spectrometers as design examples, a series of initial solutions are obtained with a diffraction grating placed at the secondary mirror. In order to understand and explore the utilization of large number of output results, these initial solutions are visualized by mapping from high-dimensional solution space to three-dimensional space using the t-distributed symmetric neighbor embedding (t-SNE) dimensionality reduction algorithm; then a neural network is trained to fit the imaging qualities, showing good generalization performance, and can predict imaging quality for systems with unknown optical power distributions and structures.

Uniformity improvement on received optical power for an indoor visible light communication system with an angle diversity receiver.

In this work, an angle diversity receiver (ADR) structure is proposed to optimize the uniformity of the received optical power distribution in an indoor visible light communication (VLC) system. Taking the rectangular and hybrid layouts with 16 light-emitting diodes as examples, different inclination angles and the number of side detectors are investigated with three diversity combining techniques in a typical room, where the primary reflection of the wall is considered. Simulation results showed that the inclination angles and the number of side detectors would affect the variance and average of the received optical power, and the variance would decrease with the increase of the number of side detectors. In addition, maximal ratio combining is more suitable for the ADR when the variance and average of the received optical power are considered simultaneously. By applying the ADR with five side detectors, the variances of the received optical power will decrease by 81.34% and 86.09% under the rectangle layout and the hybrid layout, respectively. This work will benefit the design and development of the VLC system.

Time-averaged image projection through a multimode fiber.

Many disciplines, ranging from lithography to opto-genetics, require high-fidelity image projection. However, not all optical systems can display all types of images with equal ease. Therefore, the image projection quality is dependent on the type of image. In some circumstances, this can lead to a catastrophic loss of intensity or image quality. For complex optical systems, it may not be known in advance which types of images pose a problem. Here we show a new method called Time-Averaged image Projection (TAP), allowing us to mitigate these limitations by taking the entire image projection system into account despite its complexity and building the desired intensity distribution up from multiple illumination patterns. Using a complex optical setup, consisting of a wavefront shaper and a multimode optical fiber illuminated by coherent light, we succeeded to suppress any speckle-related background. Further, we can display independent images at multiple distances simultaneously, and alter the effective sharpness depth through the algorithm. Our results demonstrate that TAP can significantly enhance the image projection quality in multiple ways. We anticipate that our results will greatly complement any application in which the response to light irradiation is relatively slow (one microsecond with current technology) and where high-fidelity spatial distribution of optical power is required.

Performance-Optimized Components for Quantum Technologies via Additive Manufacturing

Novel quantum technologies and devices place unprecedented demands on the performance of experimental components, while their widespread deployment beyond the laboratory necessitates increased robustness and fast, affordable production. We show how the use of additive manufacturing, together with mathematical optimization techniques and innovative designs, allows the production of compact, lightweight components with greatly enhanced performance. We use such components to produce a magneto-optical trap that captures $\sim 2 \times 10^8$ rubidium atoms, employing for this purpose a compact and highly stable device for spectroscopy and optical power distribution, optimized neodymium magnet arrays for magnetic field generation, and a lightweight, additively manufactured ultra-high vacuum chamber. We show how the use of additive manufacturing enables substantial weight reduction and stability enhancement, while also illustrating the transferability of our approach to experiments and devices across the quantum technology sector and beyond.