Waveguide Core Research Articles

Millimeter-long metamaterial surface-emitting antenna in the silicon photonics platform.

Integrated optical antennas are key components for on-chip light detection and ranging technology (LIDAR). In order to achieve a highly collimated far field with reduced beam divergence, antenna lengths on the order of several millimeters are required. In the high-index contrast silicon photonics platform, achieving such long antennas typically demands weakly modulated gratings with lithographic minimum feature sizes below 10 nm. Here, we experimentally demonstrate a new, to the best of our knowledge, strategy to make long antennas in silicon waveguides using a metamaterial subwavelength grating (SWG) waveguide core loaded with a lateral periodic array of radiative elements. The mode field confinement is controlled by the SWG duty cycle, and the delocalized propagating mode overlaps with the periodic perturbations. With this arrangement, weak antenna radiation strength can be achieved while maintaining a minimum feature size as large as 80 nm. Using this strategy, we experimentally demonstrate a 2-millimeter-long, single-etched subwavelength-engineered optical antenna on a conventional 220 nm SOI platform, presenting a measured far-field beam divergence of 0.1° and a wavelength scanning sensitivity of 0.13°/nm.

Monolithic integrated waveguide device with dual functions of electro-optic modulation and optical amplification.

An organic polymer-based monolithic integrated waveguide device with dual functions of electro-optic (EO) modulation and optical amplification is demonstrated. In this Letter, the dual functions are achieved by employing EO polymer as the waveguide upper cladding and organic optical amplified material as the waveguide core layer. Based on this dual-functional waveguide structure, the waveguide amplifier and the EO switch are successfully designed and fabricated on a SiO2-Si substrate. An optical gain of 5.8 dB at the wavelength of 1535 nm is obtained in the dual-functional polymer waveguide, and the EO switch based on the dual-functional polymer waveguide also presents a high-speed response time of ∼10ns.

Terahertz mode selector based on multimode resonances in corrugated waveguides

We propose a magnetically switchable terahertz (THz) mode selector based on four transverse mode resonances in corrugated waveguides. The theoretical and numerical results show that two passbands can be obtained in the transmission spectra around the resonance points. The passband of 0.9611–1.0006 THz outputs a single second-order transverse mode with the highest ratio of 99.03% while the passband of 0.8712–0.9111 THz contains the multiple transverse mode output with the first two modes. A tunable THz mode selector with frequency band of 0.5572–0.5926 THz is realized by filling the waveguide core with a liquid crystal (E7). The widest working frequency range of the mode selector reaches 28.4 GHz when the molecular steering angle θ lies between 71.22° and 90°.

A Cost-effective Liquid Core Waveguide Based on a Concentrated Acid Medium for Colorimetric Determination of Sulfide.

A cost-effective long-path absorption liquid core waveguide (LCW) device was fabricated from a typical Teflon tube, and using a high refractive index (RI) medium of acid. It was incorporated in a flow injection-gas diffusion system for the sensitive determination of sulfide via methylene blue (MB) chemistry. The gas diffusion unit could prevent some interferences in samples. The limit of detection of 0.13 μmol L-1 was achieved without using a preconcentration procedure. The proposed device was successfully applied for sulfide determination in canal water samples with 88.7 - 103% recovery.

Waveguide Mach-Zehnder Interferometer for Temperature and Concentration Sensing

Mach-Zehnder interferometer (MZI) based on optical waveguide is proposed and demonstrated, in which a waveguide core with a core expander and a collector is inscribed in a photoresist SU-8 film by ultrafast laser-induced two-photon polymerization (TPP) technique, whereas the cladding of the waveguide is fabricated by the standard photolithographic technique. Interference occurs in this MZI when the light is scattered into the cladding by the core expander and then collected by the core collector, in which the roles of the core expander and the collector are investigated in details. By integrating with a microchannel, a waveguide-based optofluidic MZI sensor is realized, which exhibits a redshift at a sensitivity of 0.618 nm/°C with the increase of temperature, and blueshifts at sensitivities of -0.102 nm/wt% and -0.092 nm/°Bx for concentration measurement of sodium chloride and sucrose solutions, respectively. Experimental results demonstrate the effectiveness of this sensor for temperature and concentration measurement, manifesting its promising potential for biomedical, chemical, and environmental applications.

Writing 3D Waveguides With Femtosecond Pulses in Polymers

We present novel waveguide writing concepts in bulk PMMA. The writing relies on laser induced modification tracks that are completely surrounding a waveguide core. We found the optimal parameters to construct highly reproducible, single-mode waveguides with minimal propagation losses down to 0.6 dB cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Employing the best geometry, we demonstrate 2D and 3D Y-splitters that are the building blocks for creating complex optical networks such as sensors or lab-on-chip devices in polymer materials.

Analysis of Kerr comb generation in silicon microresonators under the influence of two-photon absorption and fast free-carrier dynamics

Kerr frequency comb generation relies on dedicated waveguide platforms that are optimized towards ultralow loss while offering comparatively limited functionality restricted to passive building blocks. In contrast to that, the silicon-photonic platform offers a highly developed portfolio of high-performance devices, but is deemed to be inherently unsuited for Kerr comb generation at near-infrared (NIR) telecommunication wavelengths due to strong two-photon absorption (TPA) and subsequent free-carrier absorption (FCA). Here we present a theoretical investigation that quantifies the impact of TPA and FCA on Kerr comb formation and that is based on a modified version of the Lugiato-Lefever equation (LLE). We find that silicon microresonators may be used for Kerr comb generation in the NIR, provided that the dwell time of the TPA-generated free-carriers in the waveguide core is reduced by a reverse-biased p-i-njunction and that the pump parameters are chosen appropriately. We validate our analytical predictions with time integrations of the LLE, and we present a specific design of a silicon microresonator that may even support formation of dissipative Kerr soliton combs.

Performances characterization of unsaturated polyester resin/polymethylmethacrylate waveguide for refractive index measurement

Investigation of refractive index (RI) and temperature response of unsaturated polyester resin (UPR)/polymethylmethacrylate (PMMA) based waveguides is presented. Three type waveguide structures were fabricated which are straight waveguide (sensor A), curved waveguide (sensor B) and half-filled core waveguide (sensor C). Simple fabrications were done by engraving PMMA sheets using CNC engraving machine to form grooves with cross section of 1×1 mm2. The grooves were then filled with UPR as core material. To investigate the RI response, the waveguides were immersed in glucose solution with various concentration values having RI of 1.33 to 1.3547. Meanwhile, for temperature response characterization, the sensors were placed in distilled water while heated using magnetic stirrer with temperature variation of 35°C-85°C with increment of 5°C. The results showed that the sensors showed good RI sensitivity and linearity with the highest sensitivity of -48.476 dB/RIU. It was also observed that the sensors were temperature dependent. The highest error due to temperature effect is -1.8×10−4 RIU/°C.

Determining thermo-mechanical stress sources of an integrated optical device

In this study, we analyze the thermo-mechanical behavior of a multi-functional integrated optical chip (MIOC) via the finite element method (FEM). MIOC is a kind of fiber optic component which has a high thermal sensitivity. So; during temperature changes its mechanical resistance has a considerable critical significance. To investigate the thermo-mechanical effects, a three-dimensional device structure is modeled and time-dependent Von-Mises stress analyses are carried out via the multiphysics approach in the simulation environment. The simulation results are evaluated regarding the change of the crystal structure, epoxy layers, and waveguide core region. It is demonstrated that structural configuration has a crucial impact on the mechanical stability of the integrated optical device. Because of this reason, case D is accepted as a preferable geometrical structure for mass production. Also, the most dominant stress effect arises from the epoxy layer interface between the fiber pigtail carrier and lithium niobate (LiNbO3) chip called region 1. In this context, we investigate the influence of extreme thermal conditions on the induced stress on region 1. Apart from the mechanical strength of the devices, optical transmission is another subject to consider. Two claims arise on this point: power flow through the waveguide core is temperature-dependent and it is also related to the stress-optical effect. Finally, we compare the simulation results with the experimental results and there is certainly a consistency between the two separate analyses. Furthermore; it can be said that manufacturing more than 500 devices facilitates the quantitative analysis of device performance.

Characterization of Multicore Integrated Active Waveguides Written in an Er3+/Yb3+ Codoped Phosphate Glass

A new methodology to completely characterize active multicore waveguides is proposed. The method is applied to two-core waveguides written in an Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> co-doped phosphate glass by femtosecond laser inscription. Determination of the core diameter and refractive index variation is based on asymmetric excitation of the dual core waveguide and measurements of the intensity output distribution. Moreover, a single core waveguide has also been written for characterization purposes and signal enhancement measurements carried out on it have allowed the determination of the glass active properties. Finally, enhancement measurements on the two-core waveguides for different pump powers and core-to-core separations have been performed, together with simulations of the experiments using the parameters determined in the characterization procedure. The good agreement between measurements and simulations supports the validity of the characterization method presented. The proposed characterization technique is suitable for any multicore symmetric structure regardless the fabrication method and the number of cores, allowing the benefits of multi-core fibre designs to be transferred to integrated active structures.

Optimization of Fabrication Process for SiON/SiOx Films Applicable as Optical Waveguides

In this paper, the analysis of silicon oxynitride (SiON) films, deposited utilizing the plasma enhanced chemical vapor deposition (PECVD) process, for optical waveguides on silicon wafers is presented. The impact of N2O flow rate on various SiON film properties was investigated. The thickness and refractive index were measured by micro-spot spectroscopic reflectometry and confirmed by spectroscopic ellipsometry. The chemical composition of SiON films was analyzed using Secondary Ion Mass Spectrometry (SIMS). The surface roughness was analyzed using Atomic Force Microscopy (AFM). Increasing the N2O flow rate during deposition caused the deposition rate to increase and the refractive index to decrease. By changing the flow rate of gases into the chamber during the PECVD process, it is possible to precisely adjust the oxygen (O2) ratio and nitrogen (N2) ratio in the SiON film and thus control its optical properties. This was possibility utilized to fabricate SiON films suitable to serve as a waveguide core for optical waveguides with a low refractive index contrast.

Complex spectral filters in silicon waveguides based on cladding-modulated Bragg gratings.

Spectral filters are important building blocks for many applications in integrated photonics, including datacom and telecom, optical signal processing and astrophotonics. Sidewall-corrugated waveguide grating is typically the preferred option to implement spectral filters in integrated photonic devices. However, in the high-index contrast silicon-on-insulator (SOI) platform, designs with corrugation sizes of only a few tens of nanometers are often required, which hinders their fabrication. In this work, we propose a novel geometry to design complex Bragg filters with an arbitrary spectral response in silicon waveguides with laterally coupled Bragg loading segments. The waveguide core is designed to operate with a delocalized mode field, which helps reduce sensitivity to fabrication errors and increase accuracy on synthesized coupling coefficients and the corresponding spectral shape control. We present an efficient design strategy, based on the layer-peeling and layer-adding algorithms, that allows to readily synthesize an arbitrary target spectrum for our cladding-modulated Bragg gratings. The proposed filter concept and design methodology are validated by designing and experimentally demonstrating a complex spectral filter in an SOI platform, with 20 non-uniformly spaced spectral notches with a 3-dB linewidth as small as 210 pm.

High-sensitivity microvolume UV absorption spectrometry for routine analysis of small-volume biological samples.

Substantial improvement of microvolume UV absorption spectrometry in sensitivity, robustness and ease of operation was achieved for routine biological applications. A unique microtubing-based absorption cell (208μm internal diameter) featuring enhanced light transmission with a liquid core waveguide technique provided dramatically enhanced absorption sensitivity, proportional to the extended path length (50mm, from the typical 1mm), whilerobust measurement performance was attained by implementation of preventive measures against bubble trapping along the light path. For pBR322 plasmid DNA, absorbance at 260nm wasreliably measurable down to 0.1ng/μl with repeatability typically 2-3% RSD. The detection limit was 0.03ng/μldsDNA, far lower than the current state-of-the-art∼1ng/μl. Sample consumption for each measurement was 2.4μl. Automated operation implemented for the first time in microvolume spectrophotometry facilitated the ease in handling with small-volume biological samples.

Novel 900 nm diode lasers with epitaxially stacked multiple active regions and tunnel junctions

A multi-active-region bipolar-cascade edge-emitting laser emitting at nearly 900 nm is presented. The three active regions and two tunnel junctions located in a single waveguide core share the same third-order vertical mode. A slope efficiency of 3.6 W/A was measured with a threshold current density of 230 A/cm2. The epitaxial layer stack developed features with very low internal optical losses of 0.7 cm−1. The voltage extrapolated to vanishing current is only 0.3 V larger than 3 times the voltage of 1.4 V originating from the photon energy.

Sol–Gel Waveguide-Based Sensor for Structural Health Monitoring on Large Surfaces in Aerospace Domain

The potential of sol–gel-based optical sensors is investigated for applications in the aerospace domain. To this aim, a low-cost and non-intrusive sol–gel sensor based on waveguides, arranged as a 2D matrix structure, is fabricated by UV photolithography for delamination and damage detection. Two different organic–inorganic sol–gels were selected to fabricate the photonic device: TiO2–SiO2 and ZrO2–SiO2, acting as the waveguide core and the cladding, respectively. A systematic study was performed to determine the manufacturing parameters controlling their properties. The results show that large surfaces can be functionalized via sol–gel methods using the direct laser-writing approach. The structures are characterized in terms of refractive index, and the guiding properties were investigated through simulations and experiments, indicating an excellent behavior regarding the light guidance in a straight waveguide or in the 2D matrix structure grid. Additionally, preliminary tests show that the presence of impact can be easily detected after damage through the induced optical losses on large surfaces. This proof of concept sensor is a promising tool for structural health monitoring. To achieve the ultimate goal, the integration of this photonic sensor will be later performed on aircraft wings.

Hybrid electro-optic modulator combining silicon photonic slot waveguides with high-k radio-frequency slotlines

Electro-optic (EO) modulators rely on the interaction of optical and electrical signals with second-order nonlinear media. For the optical signal, this interaction can be strongly enhanced using dielectric slot–waveguide structures that exploit a field discontinuity at the interface between a high-index waveguide core and the low-index EO cladding. In contrast to this, the electrical signal is usually applied through conductive regions in the direct vicinity of the optical waveguide. To avoid excessive optical loss, the conductivity of these regions is maintained at a moderate level, thus leading to inherent RC limitations of the modulation bandwidth. In this paper, we show that these limitations can be overcome by extending the slot–waveguide concept to the modulating radio-frequency (RF) signal. Our device combines an RF slotline that relies on B a T i O 3 as a high-k dielectric material with a conventional silicon photonic slot waveguide and a highly efficient organic EO cladding material. In a proof-of-concept experiment, we demonstrate a 1 mm long Mach–Zehnder modulator that offers a 3 dB bandwidth of 76 GHz and a 6 dB bandwidth of 110 GHz along with a small π voltage of 1.3 V ( U π L = 1.3 V m m ). We further demonstrate the viability of the device in a data-transmission experiment using four-state pulse-amplitude modulation (PAM4) at line rates up to 200 Gbit/s. Our first-generation devices leave vast room for further improvement and may open an attractive route towards highly efficient silicon photonic modulators that combine sub-1 mm device lengths with sub-1 V drive voltages and modulation bandwidths of more than 100 GHz.

Topological inverse design of nanophotonic devices with energy constraint.

In this paper, we introduce an energy constraint to improve topology-based inverse design. Current methods typically place the constraints solely on the device geometry and require many optimization iterations to converge to a manufacturable solution. In our approach the energy constraint directs the optimization process to solutions that best contain the optical field inside the waveguide core medium, leading to more robust designs with relatively larger minimum feature size. To validate our method, we optimize two components: a mode converter (MC) and a wavelength demultiplexer. In the MC, the energy constraint leads to nearly binarized structures without applying independent binarization stage. In the demultiplexer, it also reduces the appearance of small features. Furthermore, the proposed constraint improves the robustness to fabrication imperfections as shown in demultiplexer design. With energy constraint optimization, the corresponding spectrum shifts under ±10 nm dimensional variations are reduced by 17% to 30%. The proposed constraint is unique in simultaneously taking both geometry and electric field into account, opening the door to new ideas and insights to further improve the computationally intensive topology-based optimization process of nanophotonic devices.

Tunable delay line based on high-performance three-dimensional switches using graphene heaters

We propose a novel, to the best of our knowledge, tunable delay line based on high-performance three-dimensional (3-D) switches driven by graphene heaters. The 3-D switches, which are constructed with a vertical directional coupler (VDC), provide continuous tuning functionality of the proposed delay line by tuning the coupling ratio of the VDC. The use of the graphene electrode by placing it in direct contact with the upper waveguide core, combined with the air slots formed on both sides of the waveguides, causes the switch to have a quite low power consumption of 3.6 mW. Our designed device, which has a footprint of 20 m m × 6.8 m m , is capable of providing a continuous tunable delay from 0 to 150 ps with minimum operating bandwidth of &gt; --> 3.34 G H z , from 0 to 100 ps with minimum operating bandwidth of &gt; --> 5 G H z , and from 0 to 50 ps with minimum operating bandwidth of &gt; --> 10 G H z . The cross talk for different delay cases is less than − 21.6 d B over the whole C-band and less than − 45.2 d B at 1550 nm.

Behavior of the Fused Quartz Suspended Core Acoustic Waveguide Sensor in Gamma Radiation

Previously, a fused-quartz acoustic waveguide named the suspended core waveguide with tight-field confinement in a small diameter was developed and its sensing potential in harsh environment was demonstrated with the fabricated periodic acoustic fiber Bragg grating. In this paper, the acoustic sensor was exposed to around 70 Gy/h gamma radiation at room temperature for 3550 h at Oak Ridge National Laboratory. For comparisons, a thermocouple and two optical fiber Bragg gratings were tested under the same conditions. The central frequency reading of the acoustic sensor was found to encounter a relatively fast decrease of 0.5 kHz in the first 800 hours. The reading then became stable, centering at 478.5 kHz with a fluctuation of ±0.2 kHz, and responded to small environmental temperature variations less than 1.2 °C. The major effect to the fluctuation was concluded to be radiation-induced material compaction and expansion. For optical fiber Bragg gratings, the same one-directional fast change of readings in the first 500 hours were observed as well. Although the optical gratings were able to track temperature changes, they also showed continuous drifts. The survivability and consistency of the acoustic sensor under long-term gamma radiation could lead to new sensing methods in nuclear applications.

Selectively excited bimodal interferometer for highly sensitive refractive index sensing

A selectively excited bimodal interferometer was developed for highly sensitive detection of refractive index changes. Numerical simulation revealed that multimode waveguides with tall, narrow cross-sections are suitable for realizing a highly sensitive bimodal (fundamental and lateral first-order modes) interferometer, owing to deeper penetration of the lateral first-order mode through its side surfaces into the cladding medium. A device with an optimum design was determined by simulation and fabricated by electron-beam lithography using a negative photoresist as the material for the waveguide core. The detection of glucose was demonstrated, and the sensitivity and detection limit was found to be 2547 rad / RIU (RIU—refractive index unit) and 9.8 × 10 − 6 RIU, respectively.