Data Center Four-Channel Multimode Interference Multiplexer Using Silicon Nitride Technology.

The operation of a four-channel multiplexer, utilizing multimode interference (MMI) wavelength division multiplexing (WDM) technology, can be designed through the cascading of MMI couplers or by employing angled MMI couplers. However, conventional designs often occupy a larger footprint, spanning a few millimeters, thereby escalating the energy power requirements for the photonic chip. In response to this challenge, we propose an innovative design for a four-channel silicon nitride (Si3N4) MMI coupler with a compact footprint. This design utilizes only a single MMI coupler unit, operating within the O-band spectrum. The resulting multiplexer device can efficiently transmit four channels with a wavelength spacing of 20 nm, covering the O-band spectrum from 1270 to 1330 nm, after a short light propagation of 22.8 µm. Notably, the multiplexer achieves a power efficiency of 70% from the total input energy derived from the four O-band signals. Power losses range from 1.24 to 1.67 dB, and the MMI coupler length and width exhibit a favorable tolerance range. Leveraging Si3N4 material and waveguide inputs and output tapers minimizes light reflection from the MMI coupler at the input channels. Consequently, this Si3N4-based MMI multiplexer proves suitable for deployment in O-band transceiver data centers employing WDM methodology. Its implementation offers the potential for higher data bitrates while maintaining an exemplary energy consumption profile for the chip footprint.

Photonic Crystal Flip-Flops: Recent Developments in All Optical Memory Components.

This paper reviews recent advancements in all-optical memory components, particularly focusing on various types of all-optical flip-flops (FFs) based on photonic crystal (PC) structures proposed in recent years. PCs, with their unique optical properties and engineered structures, including photonic bandgap control, enhanced light-matter interaction, and compact size, make them especially suitable for optical FFs. The study explores three key materials, silicon, chalcogenide glass, and gallium arsenide, known for their high refractive index contrast, compact size, hybrid integration capability, and easy fabrication processes. Furthermore, these materials exhibit excellent compatibility with different technologies like CMOS and fiber optics, enhancing their versatility in various applications. The structures proposed in the research leverage mechanisms such as waveguides, ring resonators, scattering rods, coupling rods, edge rods, switches, resonant cavities, and multi-mode interference. The paper delves into crucial properties and parameters of all-optical FFs, including response time, contrast ratio, and operating wavelength. Optical FFs possess significant advantages, such as high speed, low power consumption, and potential for integration, making them a promising technology for advancing optical computing and optical memory systems.

Optimizations of Double Titanium Nitride Thermo-Optic Phase-Shifter Heaters Using SOI Technology.

A commercial thermo-optic phase shifter (TOPS) is an efficient solution to the imbalance problem in the fabrication process of Mach-Zehnder modulator (MZM) arms. The TOPS consumes electrical power and transforms it into thermal energy, which changes the real part of the effective refractive index at the waveguide and adjusts the MZM transfer function to work in the linear region. The common model being used today is constructed with only one heater; however, this solution requires more electrical power, which can increase the transmitter system cost. To reduce the system energy cost, we propose a pioneering optimal double titanium nitride heater model under forward biasing at 1550 nm wavelength using the standard silicon-on-insulator technology. Numerical investigations were carried out on the key relative geometrical parameters, heat distribution at the silicon layer, thermal crosstalk, and laser wavelength drift. Results show that the optimal TOPS design can function with a low electrical power of 19.1 mW to achieve a π-phase shift, with a low thermal crosstalk of 0.404 and very low optical losses over 1 mm length. Thus, the proposed device can be used for improving the imbalance problem in MZMs with low electrical power consumption and low losses. This functionality can be utilized to obtain better performances in transmitter systems for data centers and long-range optical communication system applications.

An Optical 1×4 Power Splitter Based on Silicon-Nitride MMI Using Strip Waveguide Structures.

This paper presents a new design for a 1 × 4 optical power splitter using multimode interference (MMI) coupler in silicon nitride (Si3N4) strip waveguide structures. The main functionality of the proposed design is to use Si3N4 for dealing with the back reflection (BR) effect that usually happens in silicon (Si) MMI devices due to the self-imaging effect and the higher index contrast between Si and silicon dioxide (SiO2). The optimal device parameters were determined through numerical optimizations using the beam propagation method (BPM) and finite difference time domain (FDTD). Results demonstrate that the power splitter with a length of 34.6 µm can reach equal distribution power in each output port up to 24.3% of the total power across the O-band spectrum with 0.13 dB insertion loss and good tolerance MMI coupler parameters with a shift of ±250 nm. Additionally, the back reflection range over the O-band was found to be 40.25-42.44 dB. This demonstrates the effectiveness of the incorporation using Si3N4 MMI and adiabatic input and output tapers in mitigating unwanted BR to ensure that a good signal is received from the laser. This design showcases the significant potential for data-center networks, offering a promising solution for efficient signal distribution and facilitating high-performance and reliable optical signal routing within the O-band range. By leveraging the advantages of Si3N4 and the MMI coupler, this design opens possibilities for advanced optical network architectures and enables efficient transmission of optical signals in the O-band range.

A Compact Polarization MMI Combiner Using Silicon Slot-Waveguide Structures.

The study of designing a compact transverse electric (TE)/transverse magnetic (TM) polarization multimode interference (MMI) combiner based on silicon slot-waveguide technology is proposed for solving the high demands for high-speed ability alongside more energy power and minimizing the environmental impact of power consumption, achieving a balance between high-speed performance and energy efficiency has become an important consideration in an optical communication system. The MMI coupler has a significant difference in light coupling (beat-length) for TM and TE at 1550 nm wavelength. By controlling the light propagation mechanism inside the MMI coupler, a lower order of mode can be obtained which can lead to a shorter device. The polarization combiner was solved using the full-vectorial beam propagation method (FV-BPM), and the main geometrical parameters were analyzed using Matlab codes. Results show that after a short light propagation of 16.15 µm, the device can function as TM or TE combiner polarization with an excellent extinction ratio of 10.94 dB for TE mode and 13.08 dB for TM mode with low insertion losses of 0.76 dB (TE) and 0.56 dB (TM) and the combiner function well over the C-band spectrum. The polarization combiner also has a robust MMI coupler length tolerance of 400 nm. These attributes make it a good candidate for using this proposed device in photonic integrated circuits for improving power ability at the transmitter system.

Combining Four Gaussian Lasers Using Silicon Nitride MMI Slot Waveguide Structure.

Transceivers that function under a high-speed rate (over 200 Gb/s) need to have more optical power ability to overcome the power losses which is a reason for using a larger RF line connected to a Mach-Zehnder modulator for obtaining high data bitrate communication. One option to solve this problem is to use a complex laser with a power of over 100 milliwatts. However, this option can be complicated for a photonic chip circuit due to the high cost and nonlinear effects, which can increase the system noise. Therefore, we propose a better solution to increase the power level using a 4 × 1 power combiner which is based on multimode interference (MMI) using a silicon nitride (Si3N4) slot waveguide structure. The combiner was solved using the full-vectorial beam propagation method (FV-BPM), and the key parameters were analyzed using Matlab script codes. Results show that the combiner can function well over the O-band spectrum with high combiner efficiency of at least 98.2% after a short light coupling propagation of 28.78 µm. This new study shows how it is possible to obtain a transverse electric mode solution for four Gaussian coherent sources using Si3N4 slot waveguide technology. Furthermore, the back reflection (BR) was solved using a finite difference time-domain method, and the result shows a low BR of 40.15 dB. This new technology can be utilized for combining multiple coherent sources that work with a photonic chip at the O-band range.

1 × 4 Wavelength Demultiplexer C-Band Using Cascaded Multimode Interference on SiN Buried Waveguide Structure.

Back reflection losses are a key problem that limits the performances of optical communication systems that work on wavelength division multiplexing (WDM) technology based on silicon (Si) Multimode Interference (MMI) waveguides. In order to overcome this problem, we propose a novel design for a 1 × 4 optical demultiplexer based on the MMI in silicon nitride (SiN) buried waveguide structure that operates at the C-band spectrum. The simulation results show that the proposed device can transmit four channels with a 10 nm spacing between them that work in the C-band with a low power loss range of 1.98-2.35 dB, large bandwidth of 7.68-8.08 nm, and good crosstalk of 20.9-23.6 dB. Thanks to the low refractive index of SiN, a very low back reflection of 40.57 dB is obtained without using a special angled MMI design, which is usually required, using Si MMI technology. Thus, this SiN demultiplexer MMI technology can be used in WDM technique for obtaining a high data bitrate alongside a low back reflection in optical communication systems.

A Three Demultiplexer C-Band Using Angled Multimode Interference in GaN-SiO2 Slot Waveguide Structures.

One of the most common techniques for increasing data bitrate using the telecommunication system is to use dense wavelength division multiplexing (DWDM). However, the implementation of DWDM with more channels requires additional waveguide coupler devices and greater energy consumption, which can limit the system performances. To solve these issues, we propose a new approach for designing the demultiplexer using angled multimode interference (AMMI) in gallium nitride (GaN)-silica (SiO2) slot waveguide structures. SiO2 and GaN materials are selected for confining the infrared light inside the GaN areas under the transverse electric (TE) field mode. The results show that, after 3.56 mm light propagation, three infrared wavelengths in the C-band can be demultiplexed using a single AMMI coupler with a power loss of 1.31 to 2.44 dB, large bandwidth of 12 to 13.69 nm, very low power back reflection of 47.64 to 48.76 dB, and crosstalk of -12.67 to -15.62 dB. Thus, the proposed design has the potential for improving performances in the telecommunication system that works with DWDM technology.

A Four Green TM/Red TE Demultiplexer Based on Multi Slot-Waveguide Structures.

A four green transverse magnetic (TM)/red transverse electric (TE) light wavelength demultiplexer device, based on multi slot-waveguide (SW) structures is demonstrated. The device aims to demultiplex wavelengths in the green/red light range with wavelengths of 530, 540, 550, and 560 nm; 630, 640, 650, and 660 nm. This means that the device functions as a 1 × 4 demultiplexer for each polarization mode (TE/TM). The controlling of the light switching between two closer segment SWs under the TM/TE polarization mode was studied by designing a suitable SW structure and setting the right segment length to fit the coupling lengths of the operating wavelengths. The device is composed of six-segment SW units and six S-bends (SB) SW. The key SW and SB parameters were optimized and determined by a full vectorial beam propagation method (FV-BPM). Results show power losses better than 0.138 dB, crosstalk better than -21.14 dB, and an optical spectrum smaller than 9.39 nm, with an overall compact size of 104.5 µm. The device can be integrated in wavelength division multiplexing (WDM) for increasing data bit rate in a visible light communication (VLC) system.

Color image identification and reconstruction using artificial neural networks on multimode fiber images: towards an all-optical design.

The rapid growth of applications that rely on artificial neural network (ANN) concepts gives rise to a staggering increase in the demand for hardware implementations of neural networks. New types of hardware that can support the requirements of high-speed associative computing while maintaining low power consumption are sought, and optical artificial neural networks fit the task well. Inherently, optical artificial neural networks can be faster, support larger bandwidth, and produce less heat than their electronic counterparts. Here we propose the design of an optical ANN-based imaging system that has the ability to self-study image signals from an incoherent light source in different colors. Our design consists of a combination of a multimode fiber and a multi-core optical fiber realizing a neural network. We show that the signals, transmitted through the multimode fiber, can be used for image identification purposes and can also be reconstructed using ANNs with a low number of nodes. An all-optical solution can then be achieved by realizing these networks with the multi-core optical neural network fiber.

An Eight-Channel C-Band Demux Based on Multicore Photonic Crystal Fiber.

A novel eight-channel demux device based on multicore photonic crystal fiber (PCF) structures that operate in the C-band range (1530⁻1565 nm) has been demonstrated. The PCF demux design is based on replacing some air-hole areas with lithium niobate and silicon nitride materials over the PCF axis alongside with the appropriate optimizations of the PCF structure. The beam propagation method (BPM) combined with Matlab codes was used to model the demux device and optimize the geometrical parameters of the PCF structure. The simulation results showed that the eight-channel demux can be demultiplexing after light propagation of 5 cm with a large bandwidth (4.03⁻4.69 nm) and cross-talk (-16.88 to -15.93 dB). Thus, the proposed device has great potential to be integrated into dense wavelength division multiplexing (DWDM) technology for increasing performances in networking systems.

Thermal therapy with magnetic nanoparticles for cell destruction.

In this article we suggest a new concept for cell destruction based upon manipulating magnetic nanoparticles (MNPs) by applying external, low frequency alternating magnetic field (AMF) that oscillates the particles, together with focused laser illumination. Assessment of temperature profiles in a head and neck squamous cell carcinoma sample showed that cells with MNPs, treated with AMF (3 Hz, 300 mW) and laser irradiation (30 mW), reached 42°C after 4.5 min, as opposed to cells treated with laser but without AMF. Moreover, a theoretical model was developed to assess the overall theoretical temperature rise, which was shown to be 50% lower than the experimental temperature. Furthermore, we found that the combination of laser irradiation and AMF decreased the number of live cells by ~50%. Thus, the concentrated assembly of laser heating with AMF-induced MNP oscillations leads to more rapid and efficient cell death. These results suggest that the manipulated MNP technique can serve as a superior agent for PTT, with improved cell death capabilities.

Neural networks within multi-core optic fibers.

Hardware implementation of artificial neural networks facilitates real-time parallel processing of massive data sets. Optical neural networks offer low-volume 3D connectivity together with large bandwidth and minimal heat production in contrast to electronic implementation. Here, we present a conceptual design for in-fiber optical neural networks. Neurons and synapses are realized as individual silica cores in a multi-core fiber. Optical signals are transferred transversely between cores by means of optical coupling. Pump driven amplification in erbium-doped cores mimics synaptic interactions. We simulated three-layered feed-forward neural networks and explored their capabilities. Simulations suggest that networks can differentiate between given inputs depending on specific configurations of amplification; this implies classification and learning capabilities. Finally, we tested experimentally our basic neuronal elements using fibers, couplers, and amplifiers, and demonstrated that this configuration implements a neuron-like function. Therefore, devices similar to our proposed multi-core fiber could potentially serve as building blocks for future large-scale small-volume optical artificial neural networks.

A Photonic 1 × 4 Power Splitter Based on Multimode Interference in Silicon-Gallium-Nitride Slot Waveguide Structures.

In this paper, a design for a 1 × 4 optical power splitter based on the multimode interference (MMI) coupler in a silicon (Si)-gallium nitride (GaN) slot waveguide structure is presented-to our knowledge, for the first time. Si and GaN were found as suitable materials for the slot waveguide structure. Numerical optimizations were carried out on the device parameters using the full vectorial-beam propagation method (FV-BPM). Simulation results show that the proposed device can be useful to divide optical signal energy uniformly in the C-band range (1530-1565 nm) into four output ports with low insertion losses (0.07 dB).

An 8-Channel Wavelength MMI Demultiplexer in Slot Waveguide Structures.

We propose a novel 8-channel wavelength multimode interference (MMI) demultiplexer in slot waveguide structures that operate at 1530 nm, 1535 nm, 1540 nm, 1545 nm, 1550 nm, 1555 nm, 1560 nm, and 1565 nm. Gallium nitride (GaN) surrounded by silicon (Si) was found to be a suitable material for the slot-waveguide structures. The proposed device was designed by seven 1 × 2 MMI couplers, fourteen S-bands, and one input taper. Numerical investigations were carried out on the geometrical parameters using a full vectorial-beam propagation method (FV-BPM). Simulation results show that the proposed device can transmit 8-channel that works in the whole C-band (1530-1565 nm) with low crosstalk (-19.97--13.77 dB) and bandwidth (1.8-3.6 nm). Thus, the device can be very useful in optical networking systems that work on dense wavelength division multiplexing (DWDM) technology.

Targeted Magnetic Nanoparticles for Mechanical Lysis of Tumor Cells by Low-Amplitude Alternating Magnetic Field.

Currently available cancer therapies can cause damage to healthy tissue. We developed a unique method for specific mechanical lysis of cancer cells using superparamagnetic iron oxide nanoparticle rotation under a weak alternating magnetic field. Iron oxide core nanoparticles were coated with cetuximab, an anti-epidermal growth factor receptor antibody, for specific tumor targeting. Nude mice bearing a head and neck tumor were treated with cetuximab-coated magnetic nanoparticles (MNPs) and then received a 30 min treatment with a weak external alternating magnetic field (4 Hz) applied on alternating days (total of seven treatments, over 14 days). This treatment, compared to a pure antibody, exhibited a superior cell death effect over time. Furthermore, necrosis in the tumor site was detected by magnetic resonance (MR) images. Thermal camera images of head and neck squamous cell carcinoma cultures demonstrated that cell death occurred purely by a mechanical mechanism.

Effect of spatial coherence on damage occurrence in multimode optical fibers.

We investigate the influence of spatial coherence on damage occurrence in highly multimode optical fibers using ultraviolet (UV) nanosecond pulses, with the aim of delivering high fluence in the UV. In some cases, the optical damage is initiated below the fiber facet damage threshold and takes place along the propagation path; such damage is believed to be caused by local constructive interference, creating "hot spots." In order to reduce the degree of spatial coherence, we used a large-diameter core (1.5 mm) fiber as a mode scrambler. Different lengths of this large core fiber were used to deliver energy to a fiber core with a smaller diameter (0.6 mm), in which the damage occurrence was observed. The experimental results indicate that there is a correlation between the degree of spatial coherence and the occurrence of optical damages, typically observed a few millimeters from the fiber facet. Numerical simulations, based on the beam-propagation method, support the degradation of spatial coherence, due to the excitation of high-order modes. Finally, by degrading the spatial coherence of the beam, we establish a new record by delivering more than 100 mJ via a 1.5 mm core diameter fiber in the UV, corresponding to ∼26 times the critical power for self-focusing. Our work sheds light on the ability to deliver high energies of nanosecond-pulsed UV laser radiation through multimode optical fibers.

Prospects for diode-pumped alkali-atom-based hollow-core photonic-crystal fiber lasers.

By employing large hollow-core Kagome fiber in a double-clad configuration, the performance of a potentially rubidium vapor-based fiber laser is explored. The absorbed power and laser efficiency versus pump power are calculated utilizing a simple laser model. Our results show that a Kagome-based high-power fiber laser is feasible provided that the value of the collisional fine-structure mixing rate will be elevated by increasing the ambient temperature or by increasing the helium pressure.