This PDF file contains the front matter associated with SPIE Proceedings Volume 12889, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Inverse design has proved to be a powerful tool in photonics for compact, high-performance devices. To date, applications have been limited to linear systems but have rarely been investigated or demonstrated in nonlinear regimes. In addition, the "black box” nature of inverse design techniques hinders the understanding of the optimized structure. Here, we propose an inverse design approach to amplify the efficiency of on-chip photon pair generation. We implement this strategy based on the open-source package EMopt. Our method employs a multi-frequency co-optimization strategy and calculates gradients with respect to the design parameters via the adjoint method. The resulting efficiency enhancement stems not only from the field intensification due to the confinement of light from high-quality factor cavity resonances but also from the improvement of phase-matching conditions, along with coupling between the cavity and waveguide mode considered in the design. We demonstrate the capability of the proposed method by fabricating and characterizing an optimized device that enables the efficient generation of photon pairs. Our design follows the fabrication constraints and can be used for scalable quantum light sources in large-scale computing and communication applications. Interestingly, the shape of the proposed design can also be explained by the effective potential method. The proposed optimization technique can be generalized to other nonlinear processes for compact frequency-mixing devices on-chip.

We show the first demonstration of a hybrid external cavity diode laser (ECDL) using aluminum nitride (AlN) as the wave-guiding material. Two devices are presented, a near-infrared (NIR) laser using a 850 nm diode and a red laser using a 650 nm diode. The NIR laser has ≈1 mW on chip power, 6 nm of spectral coverage, instantaneous linewidth of 720±80 kHz, and 12 dB side mode suppression ratio (SMSR). The red laser has 15 dB SMSR.

We report the broadband luminescence enhancement of erbium ions embedded in a non-resonant hybrid waveguide photon-emitter interface on silicon. The measured emission rate enhancement of > 300 is enabled by a large radiative Purcell factor across the spectrum in the telecommunications C-band. This enables the shortening of the lifetime of erbium from 3 ms to 8 μs, effectively providing brighter on-chip photon sources. The platform efficiently collects the photon emission of erbium in a single-mode waveguide and out-couples it to a low-loss photonic waveguide using a reverse nano-focusing method. This hybrid approach opens a large range of possible emitter-waveguide platform combinations for heterogeneous and hybrid quantum emitter integration, towards scalable on-chip quantum networks.

Integrated solid-state lasers have the advantage of offering narrow linewidth, well below the MHz range, making them ideal for various applications, from sensing to telecommunication. High coherence free-running lasers are difficult to characterize, due to the combination of a reduced linewidth with potential frequency drift. Conventional techniques such as optical self-heterodyning solutions require decorrelation lengths of several hundreds of km, and standard RF phase noise measurements are not compatible with slow varying effects due to the presence of frequency drift. In order to analyze the coherence of ion-exchanged co-doped Er-Yb integrated glass lasers during free-running operations, we developed a direct measurement of the optical phase by analyzing heterodyne signals in the time domain. From the analysis of the phase evolution in time, we estimate the linewidth of ion exchange glass lasers, estimated to be 1 kHz, while being able to discriminate optical contributions from slow varying processes generating frequency drift.

Integrated photonic modulators have recently attracted interest for optical neural network (ONN) applications. Networks of this kind offer an interesting approach to overcoming the expected digital computational efficiency saturation of the classical von Neumann architecture. Most ONN geometries only employ a 1D waveguide approach, which suffers from a limited number of I/O channels. Using a 2D architecture operating at near- IR wavelengths offers the ability to process a much larger data volume. Here, we present an out-of-plane reconfigurable plasmonic modulator, capable of tuning the phase of incident light using a simple architecture with no moving parts. The device consists of a compact layered structure, using an indium tin oxide (ITO) active material to electronically control the permittivity, generating signal manipulation as a function of voltage. Through numerical simulations, we achieve a π phase shift with an input voltage of 14 V at an operating wavelength of 780 nm. This work provides a robust solution to electro-optic phase modulation at visible and near-IR wavelengths, enabling applications in several important areas such as optical computing and virtual reality.

The centrosymmetric structure of stoichiometric silicon nitride inhibits the realization of second-order nonlinear processes in this low-loss, complementary-metal-oxide-semiconductor fabrication-compatible platform. Nevertheless, linear electro-optic modulation is an essential functionality desired for implementation in photonic integrated circuits. This study presents the successful achievement of electro-optical modulation in a silicon nitride microring resonator, employing thermally assisted electric-field poling. With an inscribed electric field of 100 V/μm within the silicon nitride waveguide, an effective second-order susceptibility of 0.45 pm/V is induced. Leveraging silicon nitride as the active material for electro-optic modulation, we determined the operational bandwidth of the device, constrained by the electrode design, to be 78 MHz. Furthermore, we demonstrate the capability of the device to modulate data at bitrates of up to 75 Mb/s. Our findings highlight the potential of linear electro-optical modulation in the silicon nitride integrated platform.

We introduce cutting-edge monolithically integrated photonic designs realized in a high-performance silica-on-silicon planar lightwave circuit (PLC) platform. Systems-on-chip require integration from a dozen to a few hundred optical functions, necessitating component and wafer level optimizations. Our closed-loop feedback framework enables us to achieve low propagation losses (<0.009 dB/cm), efficient fiber coupling (0.5 dB/facet), temperature stability (< 10 pm/°C), wavelength-independent operation, as well as tight polarization and phase control. Due to the lack of two-photon absorption, low scattering, and negligible absorption, our silica-on-silicon platform is well-suited for high-pump power applications in LiDAR and accelerated computing. We discuss how these characteristics allow us to monolithically integrate high-performance optical building blocks such as K-clocks, cascaded lattice filters, polarization-beam splitters, and optical hybrid components into systems-on-chip for advanced photonics applications. We demonstrate the versatility and robustness of the platform by discussing examples of monolithically integrated chips used in AI/computing accelerators, and advanced vision applications based on LiDAR. Based on exceptional optical characteristics achieved through our platform, our systems-on-chip have emerged as high-performance and scalable solutions, capable of meeting the rigorous demands imposed by a wide range of applications.

Silicon nitride (S_i3N₄) is a leading platform in integrated photonics, providing unique passive functionalities, while maintaining compatibility with complementary metal-oxide-semiconductor processes and scalable and low-cost high-volume production. However, the moderate index contrast of the S_i3N₄ platform makes it difficult to implement efficient vertical surface grating couplers. In this work, we present efficient and robust fiber-chip grating couplers on native and hybrid S_i3N₄ platforms. Minimum coupling losses between -5 dB and -3 dB were measured for single-etch fabricated devices near 1.55 μm wavelength. Moreover, by leveraging the amorphous-silicon overlay on top of the S_i3N₄ platform, we develop hybrid single-etch grating couplers, with a coupling loss approaching 1 dB. The demonstrated grating couplers are promising for SiN integrated photonics, enabling a rapid and cost-effective chip interfacing with standard optical fibers. This opens up novel opportunities for rising applications, including optical communications, nonlinear optics, or quantum information sciences.

The interest in the development of silicon photonic integrated devices is rapidly expanding from the telecom/datacom sector to new emerging application domains such as artificial intelligence or quantum photonics. Silicon benefits from CMOS-compatible fabrication processes and a high index contrast. However, the implementation of new functionalities or the achievement of a superior performance necessarily requires the integration of new materials in current silicon photonics platforms. In this context, phase change materials have been established as promising material technologies for optical switching. In this work, the benefits and challenges for enabling optical switching with VO2/Si and GST/Si devices will be analyzed and discussed.

Photonic Integrated Circuits (PICs) play a crucial role in shaping the future of quantum technology, communications, and sensor applications. With a transparency window ranging from ultraviolet to visible to mid-infrared light and a high bandgap of 6.2 eV, aluminium nitride (AlN) is ideal for a wide range of optical applications.

Within the upcoming PIC platform, we have designed, fabricated, and examined various ring resonators, comprised of coupling structures, waveguides and ring resonators, tailored for the optical L-band (1565 nm – 1625 nm). The arrangement of the coupling structures for incoupling the light from a laser source and outcoupling of the light to a detector allows for automatic probing and mapping of various structure modifications. With grating couplers integrated on the chip, these optical structures can be linked to a tunable laser source and a detector via optical fibers.

We compare the fabrication results of the optical nanostructures for the AlN-based devices with previous results from Si₃N₄-based structures. To ensure the ideal structure dimensions and to minimize deviations from the simulated design values, the AlN dry etch process has been investigated and improved.

Silicon-rich nitride (SRN) devices provide higher optical nonlinearity than stoichiometric silicon nitride. Their growth using CMOS-compatible chemical vapor deposition allows their composition to be tunable. Conventional SRN typically utilizes silane gas which introduces absorption overtones at the 1.55μm wavelength region. As is also the case with stoichiometric silicon nitride, high temperature annealing can be used to reduce Si-H based absorption. An alternate approach towards eliminating Si-H absorption is by replacing silane gas with deuterated silane. The substitution of Si-H with Si-D induces a blue shift in the wavenumber of the bond absorption, thus removing the absorption overtone at the telecommunications region. Consequently, deuterated SRN provides lower material losses compared to non-deuterated SRN, while providing a design degree of freedom for tailoring its linear and nonlinear refractive indices. We present the material properties for deuterated SRN and its application towards linear and nonlinear photonic devices. We demonstrate improved device losses when deuterated SRN is used compared to non-deuterated SRN. We further quantify the optical properties and nonlinearity of grown films and demonstrate low power parametric wavelength conversion in deuterated SRN ring resonators.

Aluminum nitride (AlN) is a promising photonics material contributed by its wide transparency window and remarkable nonlinear optical property. Moreover, its nonlinear effect can be further enhanced by doping Scandium (Sc). Such nonlinear optical property brings potential for high efficiency in nonlinear optical generation processes, such as 2nd harmonic generation and frequency comb generation. Although the nonlinear optical property of Sc-doped AlN looks promising, its waveguide is facing challenge on loss reduction. In this work, we report Sc-doped AlN photonic integrated circuit with reduced waveguide loss of 6 dB/cm around 1550 nm. The waveguide has Sc doping concentration of 10%. Its etching process is tailored through a design of experiment (DoE) approach to achieve smooth surface. An annealing process is also applied to patterned waveguide for optical loss reduction. A loaded Q of 1.41×10⁴ has also been reported from microring resonator on the same wafer. The reported result paves the way towards low-loss Sc-doped AlN for photonic integrated circuits.

Future mobile and terrestrial communication systems B5G/6G are strongly expected to heterogeneously realize typically diversified performances, i.e. high-data-rate, high-mobility, low-latency, high-capacity, massive-connectivity and low-energy in order to satisfy the highly diversified application requirements. To achieve those goals the operation band of B5G/6G should be primarily in the millimeter-wave (mmW) range. Generation and distribution of mmW with traditional methods is limited by electronic bottleneck and associated complexity. Consequently broad bandwidth, simple, efficient, and cost-effective photonic mmW-over-fiber (mmWoF) transmission systems are solutions for B5G/6G. The spectral purity of mmW carriers is necessary. Numerous approaches have been proposed to generate pure mmW signals. Compared with other technologies, quantum dash or dot (QD) coherent comb lasers (QD CCLs) have great advantages for mmW generation because QD-CCLs with low power consumption and chip-scale integration capacity with silicon can provide multiple highly correlated and low noise optical channels. In this paper we will present our developed InAs/InP QD-CCLs around 1550 nm with the channel spacing from 10 GHz to 1000 GHz and the output power up to 50 mW. By using a C-band QD CCL and based on the single- and dual-optical carrier modulation schemes, an up to 16-Gb/s mmWoF optical heterodyne wireless signal at 28 GHz through a 25-km single mode fiber before the mmW carrier is optically synthesized remotely for detection over a 2-m free space. The data capacity and performance of the proposed mmWoF link can be significantly increased by utilizing a duplex mmWoF link with MIMO and WDM technique, which provides a cost-efficient and promising solution for Terabit/s capacity mmWoF fronthaul systems of B5G/6G networks.

The design of a high-efficiency grating coupler for silicon nitride (SiN) photonic integrated circuits is presented. Our devices use a high-index amorphous silicon (α-Si) overlay and subwavelength metamaterial apodization to achieve superior coupling efficiency compared to SiN-only gratings. The high index contrast of the α-Si layer results in increased grating strength, making it possible to radiate more power off-chip within the mode field diameter (MFD) of a standard single mode optical fiber. Simultaneously, through the use of subwavelength grating (SWG) apodization, we shape the radiated field to optimize the overlap with the Gaussian-like profile of the fiber mode at an operating wavelength of 1.31 μm. The overlap is further improved by considering a focalizing scheme, wherein the grating is designed to couple to a fiber placed a certain distance away from the chip. In this configuration, the constraint imposed on the length of the grating by the MFD of the fiber mode is relaxed, allowing for a longer structure that will diffract more power off-chip. By combining the use of an α-Si overlay with SWG apodization and beam focalization, we achieve a coupling efficiency of - 1.3 dB.

Coherent detection plays a major role in optical communications for data transfer. The data extraction is performed by acquiring phase, amplitude, and frequency information from an intermediate frequency signal generated by combining a low-power incoming optical signal with a high-power optical signal from a local oscillator laser with known phase and frequency. Heterodyne or balanced detection is the most popular method to convert the received optical signal into an electrical form due to several advantages over the homodyne technique including high receiver sensitivity, frequency selectivity, spectral efficiency, ease of data extraction, and good tolerance to fiber impairments. One of the key components in a heterodyne detection system is a 90- degree optical hybrid, which separates incoming signals into an in-phase and a quadrature component for coherent demodulation. Multimode Interferometers (MMIs), especially 2 x 4 MMIs, are usually employed to realize an optical hybrid due to their fabrication tolerance and passive nature. MMIs with subwavelength gratings (SWGs) can offer low loss and broad bandwidth in ultracompact size. In this study, we propose a 2 x 4 tapered MMI that operates at a center wavelength of 1550nm with SWGs on a silicon-on-insulator platform. The period of SWG is chosen as 215 nm and has a duty cycle of 0.55. We demonstrate an MMI with a total length of 57.91 μm, i.e multimode section of 34.62 μm long and width of 11.79 μm gradually increasing to 12.813 μm, while offering a broadband performance from 1450 nm to 1650nm with transmission power variations within ± 1 dB.

A chalcogenide waveguide platform with the combination of GeAsSeTe (IG3)/GeAsSe (IG2) on wet-/dry-etched silicon pedestals is reported. Chalcogenide glasses offer high index contrast and compact footprints, while the post-processing challenges are addressed using Si pedestals. The integration of IG2/IG3 chalcogenides on Si demonstrates ultralow loss of 0.08 ± 0.02 dB/cm at a wavelength of 10 μm. We have measured a thermo-optic coefficient of 1.1x10^-4 K^-1, which is comparable to Si and GaAs. The combination of simplified fabrication, minimal propagation losses and a strong thermooptic coefficient, positions this waveguide platform as a promising candidate for on-chip tunable long-wave IR spectrometers for practical applications in biomedical diagnostics and environmental sensing.

On-chip spectrometer operating in the mid-infrared (MIR) regime (λ = 2 – 14 μm) enables the miniaturization of a chemical sensing platform that identifies compounds based on their unique molecular fingerprints. Germanium-on-Silicon (Ge-on- Si) material system is a suitable candidate for its transparency in the MIR spectrum and compatibility with silicon processing. As chemical sensing is conducted by having the mode evanescent field interacting with the analyte, the design of Ge-on-Si waveguide for a compact footprint (small bending radius) and large evanescent field coverage is necessary. However, the bending radius of the Ge-on-Si waveguide is limited to hundreds of micrometers due to the low refractive index contrast between germanium and silicon. In this work, we demonstrate a 3 μm thick Ge-on-Si waveguide, with ~89° sidewall angles and a high gap aspect ratio of 10 (resolvable gaps of 300 nm). Different types of Ge-on-Si devices are fabricated including in-plane distributed Bragg grating (DBR) structures, cascaded Fabry-Perot resonators, and polarization splitters. We show that over-etching the Si lower cladding is able to reduce bending loss by ~10x, allowing us to decrease the bending radius to ~50 μm. Designs of 32 waveguide geometries for single mode propagation from 5.5 μm to 11 μm are presented, each of which is integrated with grating couplers operating at specific peak wavelengths. Our measurements show high consistency between the simulated and measured peak wavelengths of the grating couplers, with an inter-chip standard deviation of σ_λ ⁄ λ_peak <1%

Optical isolators play a pivotal role in photonic integrated circuits (PICs) by safeguarding lasers against reflections, ensuring their stability. Over time, diverse architectures rooted in various phenomena have been explored, yet many faced limitations like bulkiness, integration challenges, or restricted bandwidth. The emergence of magnetoplasmonics presents a promising tool for the next generation of optical isolators, offering efficiency and adaptability. In this study, a novel optical isolator design capitalizes on Metal-Dielectric-Metal (MDM) waveguides, employing a configuration where a Magneto-optic (MO) layer is sandwiched between two metal layers. Under external magnetization in a Transverse Magneto-Optic Kerr Effect (TMOKE) setup, the Long-Range Surface Plasmon Polariton (LRSPP) plasmonic mode becomes asymmetric in its intensity profile distribution, thus following different paths in the opposite senses of propagation. This innovative approach, inspired by magnetoplasmonics, showcases potential for enhanced optical isolation in future photonic systems.

This work describes the development of a fast, sensitive and reliable device for the detection of outdoor and indoor Volatile Organic Compounds (VOCs) based on the Magneto-Optical Surface Plasmon Resonance (MO-SPR). The novelty of the proposed approach is based on the enhancement of the surface plasmon resonance by applying a magnetic field to modulate the optical signal using the transverse magneto-optical Kerr effect leading to improved sensitivity. The device is based on a modular benchtop MOSPR equipment integrated into an SPR platform with imaging capabilities. The detection area consists of a magneto-optical material based on a Co-Au alloy with increased stability and sensitivity previously developed by our group. The sensitivity of the detection area is further augmented by adding a sensing matrix that includes functional nanoparticles with magneto-plasmonic properties. An array of spots of gas-sensitive polymers and metallic oxides embedded in a sensing matrix provides a specific “fingerprint” after interacting with individual VOCs. The measured response is analyzed using a multivariate data analysis approach to extract relevant information regarding the type and quantity of the compound. The device is tested on ethanol, toluene and xylene as model VOCs.

With extreme field confinement in an ultrathin conducting layer, plasmons in 2D materials exhibit strong light-matter interactions, enabling the exploration of molecular vibration modes, specifically in infrared (IR) absorption spectroscopy. However, since high sensitivity in plasmons often results in a shorter resonant bandwidth, detecting molecular fingerprints across a broadband spectrum becomes fundamentally limited. Here, we demonstrate broadband surface-enhanced IR absorption (SEIRA) spectroscopy using a 2D materials-based gap plasmon resonator, specifically adopting Ti₃C₂T_x MXene. Within the MXene(Ti₃C₂T_x)-insulator(SiO₂)-metal(Au) nanostructures, the MXene-based gap plasmon (MGP) modes are produced across the entire mid-IR spectrum. These modes exhibit a wavelength reduced by more than ten-fold compared to the wavelength in a vacuum, amplifying the light-matter interactions. Furthermore, using the MGP resonator, we demonstrate sensing of vibrational mode in polymethylmethacrylate (PMMA). This finding reveals that the MGP resonator can sensitively detect molecular fingerprints of the PMMA across the entire MIR range. Notably, the MGP resonator can also identify C-H bonding in the SWIR range, a detection that is challenging for other 2D material plasmonic devices. Our findings pave the way for developing sensitive sensors for the broadband spectrum and other optical applications in the SWIR range.

As guided-wave circuits continue to increase in complexity, designing efficient and compact on-chip building blocks for these circuits continues to be a crucial research and development objective for many photonic platforms. Despite this critical requirement, the best-performing devices still require computationally intensive simulations that can take up to days, with no guaranteed results. To address this challenge, we introduce a novel, data-driven, and extremely rapid eigenmode expansion (EME) method for designing compact and efficient integrated photonic devices. In contrast to typical EME, our method models a given waveguide geometry using a pre-calculated dataset of optical scattering matrices and effective indices, therefore easily parallelized to computational accelerators like GPUs. This results in individual device simulation times of 10s of milliseconds, representing a speedup of more than 1000x over traditional methods. We then couple this approach with nonlinear iterative optimization methods and demonstrate the design and optimization of highly efficient nanophotonic devices, including tapers, 3dB splitters, and waveguide crossings within ultra-compact footprints. For all three categories of devices, we verify the response of the final geometry using 3DFDTD simulations and demonstrate state-of-the-art metrics, including below 0.05dB of insertion loss, near-perfect mode matching to the desired output, and broadband operation capabilities of over 100nm. Our unique combination of efficient and physically accurate device simulation methods, together with nonlinear optimization, enables the design of high-performance and ultra-compact photonic building blocks. These capabilities present avenues for developing more complex and previously elusive optical functionalities with unprecedented computational efficiency.

We propose a novel methodology employing deep photonic networks comprising cascaded Mach-Zehnder Interferometers (MZIs) to illustrate the proficiency of on-chip polarization handling. By applying gradient-based optimization techniques to tailor specific phase profiles within successive layers of MZIs, we demonstrate the functionality of devices adept at power division in both polarization-dependent and polarization-independent modalities. In silico simulations underscore the cutting-edge performance metrics achieved, encompassing a bandwidth exceeding 120 nm centered at 1550 nm, an extinction ratio surpassing 15 dB, and transmission bands characterized by flat-top profiles. These results prove the comprehensive capabilities of our deep photonic network ecosystem in polarization management, thereby unveiling promising prospects for advanced optical applications necessitating versatile polarization handling capabilities.

The rapid development of photonic applications calls for scalable, miniaturized power efficient integrated circuits. Thin film lithium niobate (TFLN) now emerges as a major photonic platform for integration of advanced functionalities such as based on nonlinear optics. We report experimentally efficient second harmonic generation in periodically poled lithium niobate waveguides and design rules for nonlinear frequency conversion, including a study on tolerances on the dimensions and poling parameters. Our work aims at establishing reliable and versatile nonlinear building blocks for scalable TFLN photonic integrated circuits.

This work demonstrates the implementation of a microwave photonic technique for generating a frequency-quadrupled microwave signal. The approach involves utilizing a parallel Dual-Drive Mach-Zehnder Modulator (DDMZM) driven by an electrical signal, while applying appropriate dc biasing to the DDMZM electrodes to suppress or eliminate the odd harmonic components of the sidebands in the optical domain. The DDMZM is operated at its maximum transmission point (MATP). To eliminate the optical carrier, an optical notch filter with the same central wavelength as the carrier is employed at the output of the DDMZM. This setup yields two optical sidebands. By detecting the beat signal at a photodetector (PD), a fourfold increase in frequency relative to the input RF signal is achieved. The input microwave signal frequencies of 15 GHz are utilized, resulting in output microwave signal frequencies of 60 GHz, 120 GHz, respectively. The generated frequency-quadrupled microwave signal can find applications in areas such as high-speed communication systems, radar systems, wireless networks, and satellite communications, where higher frequency signals are desired.

In this paper, we demonstrate near-C-band semiconductor optical amplifiers (SOAs) integrated on silicon photonic chips using photonic wire bonds (PWBs). PWBs are three-dimensional, nano-printed, freeform, polymer waveguides which provide efficient coupling between optical components. The SOAs used in this work were 975μm long and 400μm wide, with a 1.54μm wide, 1.9μm thick active region. Measurements on a connectorized SOA are presented, showing a peak on-chip gain of 10.6dB at 1510nm when applying a 150mA bias current to it (here we have not calibrated out the coupling losses at the two SOI-waveguide/PWB interfaces nor have we calibrated out the losses at the two PWB/SOA interfaces, indicating that the gain of the SOA is significantly higher than the measured 10.6dB). The PWB connectorized SOA has a wavelength-dependent gain which was measured from 1480nm to 1555nm, the peak gain being obtained at 1510nm. In addition, the gain depends on the bias current applied, increasing with higher bias currents but saturating when the bias current exceeds 150mA. The PWB-connectorized SOA is also sensitive to the power of the input signal, the gain was larger for lower input powers (i.e., for powers below about -4.9dBm). Varying the polarization state of the input to our PWB-connectorized SOA changed the measured gain by 5.85dB.

We present a high-tolerance multi-step-long-period diffraction grating (MSLP-DG) for the integration of III-V and silicon photonics (SiPh). The MSLP-DG can be easily fabricated on III-V chips and enables efficient emission of diffraction light from the facet of III-V chips through its multi-step and long-period structure. An optical coupling is achieved by combining III-V chips with MSLP-DG and SiPh chips with conventional diffraction gratings. Compared with the direct butt-coupling scheme between waveguides, the grating coupler offers a larger beam size, allowing improved misalignment tolerance. The fabricated grating coupler demonstrates tolerances of over ±3.5 μm in both vertical and horizontal directions, enabling cost-effective passive assembly. As a proof-of-concept experiment, we fabricated and operated an InP-SiPh integrated external cavity wavelength-tunable laser (ECTL) using an MSLP-DG. This work demonstrates the potential application of the proposed MSLP-DG for the cost-effective integration of III-V and SiPh in various integrated devices.

We report on an integrated-optic 10 x 10 filter available for an add/drop multiplexer of a variable-capacity optical orthogonal frequency division multiplexing (OFDM) signal. The filter was composed of ten switches and ten delay lines sandwiched by two slab star couplers and was fabricated by use of a silica waveguide. The filter functions as both optical discrete Fourier transform and inverse discrete Fourier transform circuits for demultiplexing and multiplexing the optical OFDM signal, respectively. We show measured one input by multi-output and multi-input by one output transmittance regarding the filter. We achieved the filter that could process the optical OFDM signal ranging from 10 to 50 Gbaud per channel. The filter is helpful to the prospective adaptive optical network where the guard bands are decreased, and the channel number, symbol rate per channel and modulation formats are flexibly changed corresponding to the traffic and transmission distance.

The use of composite materials has seen widespread adoption in modern aerospace industry. This has been facilitated due to their combined favorable mechanical characteristics, namely leveraging their low weight, high stiffness and increased strength. Wide adoption of composites requires an effort to avoid costly and cumbersome autoclave-based manufacturing processes. The up and coming “out-of-autoclave” composite manufacture processes also have to be optimized, to allow for consistent high quality of the parts produced as well as keeping the cost and production speed as low as possible. This optimisation can be achieved offline as well as by trying to have constant monitoring and controlling the resin injection and curing cycles.

Capitalizing on the benefits of Silicon Photonic Integrated Circuits (PICs), namely the fast response, miniature size, ability to operate at high temperatures, immunity to electromagnetic interference (allowing carbon fibers in composites), and their compatibility with CMOS fabrication techniques, a passive PIC based temperature sensor embedded in a composite tool is demonstrated, used to produce RTM-6 composite parts.

The design and development methodology of the PIC based sensor (fabricated in an Multi Project Wafer run of 220 nm Silicon-on-Insulator (SOI) platform and based on periodic Bragg grating elements) as well as the experimental results and comparison with the industry standard thermocouples, during a thermal cycling of the tool are presented. We measured the embedded PIC temperature sensor to have sensitivity of around ~85 pm/°C, while the RTM-6 fabrication cycle requires the tool to operate up to 185°C.

Currently, grating couplers are widely used for the coupling from Single Mode Fiber SMF to Silicon Photonic waveguides. They have relatively good efficiency and they also allow surface testing of the components which is very important for the rapid testing and yield estimation of Si-Photonics circuits and their industrial wide spread. Recently, there has been an interest in Silicon waveguides with large cross section to reduce the maximum intensity in the guide and hence reduce the non-linear effects especially the two-photon absorption in Silicon ring resonators. Increasing the waveguide depth allows also lower diffraction for the output optical beam in chip-to-chip interconnection when using external mirror for beam routing and in optical sensing applications. When using the grating coupler with such deeply etched waveguides, higher order modes are usually generated in the guide and these modes are sometimes not desirable in the optical circuit to guarantee a specific optical performance. In this work, we present a design of an optical circuit for coupling power from Single Mode Fiber to the fundamental mode of a deeply etched Si waveguide with a depth of 500 nm at 1310 nm wavelength. The silicon guide is covered by silicon dioxide layer. The excitation is achieved through a grating structure designed for this purpose. The waveguide is multimode in the vertical direction and the higher order modes in this direction are filtered using a mode filter. The mode filter is based on the structure of a symmetric 3-waveguides directional coupler in which the 2 outer guides are designed to match the propagation constant of the higher order mode to be filtered. The proposed structure (grating and mode filter) achieves a coupling efficiency of about 37% for the fundamental mode of the deeply etched guide and a higher order mode rejection ratio greater than 28 dB. The structure performance is analyzed using the Finite Difference Time Domain FDTD technique.

External cavity semiconductor laser was extensively studied in academia and industry due to their ability to tune the laser output characteristics which is suitable for different applications. Previously reported models used effective reflectivity term to incorporate the external cavity interactions with the output beam profile as a plane wave. Despite the practical observations detecting the multi-beam spatial nature of the output profile for numerous laser structures. In this work, the effective reflectivity equation is investigated while considering the multi spatial beam profile. The emitted beam profile follows a flat top Gaussian distribution. The flat top Gaussian is segmented as the weighted sum of higher order Hermite Gaussian modes. The total coupling efficiency is investigated upon reflection from a flat and (50, 100, 200 μm) spherical mirror configurations with the aid of Huygens Fresnel integration and ABCD matrix. The simulation results indicate that the spherical mirror configurations achieve maximum coupling when placed at near and far positions from the emitted beam, while the flat mirror requires a near position to accomplish maximum coupling. For the flat mirror configuration, the effective coupling efficiency 3 dB coupling decreases by 76% when comparing a beam profile consisting of one beam to eight beam profile. A 100 μm spherical mirror results in similar behavior which is attributed to the diffraction phenomena. The effective reflectivity is calculated for both plane wave and multi spatial beams beam profiles where the effective reflectivity envelope was modulated by the coupling efficiency of multi spatial beams beam profiles.

We present a simple optical fiber sensor with a MOF-on-MOF (Co-ZIF-90-on-ZIF-8 in this work) coating for water vapor sensing and detection, with a sensing mechanism based on the refractive index variation as a function of analyte adsorption within the MOF layer. A seeding layer approach was employed to grow the MOF-on-MOF layers with high quality for optical sensing application. The resulting sensor exhibits stable sensitivity to water vapors with excellent reversibility of vapor adsorption and desorption. The evaluated adsorption and desorption times of water vapor are approximately 48 seconds and 29 seconds, respectively. The demonstrated bi-layer MOF-based optical sensor provides the potential to further develop easy-to-fabricate sensors as well as multiplexed and distributed sensors in an inexpensive and reproducible way.

The construction of optical receivers using heterodyne detection techniques is a significant challenge due to the need for complex and power-intensive DSP approaches. Additionally, it is practically difficult to construct a local oscillator laser at the receiver that has the same frequency as the carrier laser for homodyne detection. Therefore, we propose a polarization multiplexed self-coherent detection method that sends both the message signal and the carrier through the same fiber while utilizing polarization diversity. Quadrature Amplitude Modulation is used for modulating the message signal. The system is capable of transmitting a data rate of 150 Gbps. We demonstrate an integrated polarization stabilizer that consists of cascaded Mach-Zehnder interferometers with a fast-tracking algorithm that can track the polarization of an incoming signal at the receiver and an integrated polarization converter based on Stokes vector that minimizes the effect of mixing of signals of orthogonal polarizations in the optical fiber. The algorithm used to feed the polarization controller can change the polarization of an incoming signal at the receiver to the polarization it had when it left the transmitter. Consequently, reducing the impact of PMD (Polarization mode dispersion) on transmission in fiber.

The MASER, analogous to a laser but operating in the microwave range, amplifies electromagnetic waves through stimulated emission. Recent advancements in optically pumping Masers have reinvigorated the field, overcoming limitations such as high vacuum, strong magnetic fields, and extreme temperatures. This work presents a miniaturised hairpin cavity for microwave antenna design utilising optical Maser sources. The miniaturised cavity demonstrates its performance through simulations and experimentally by combining optical pumping with an optimised setup. The analysis reveals successful resonance at 1.45GHz, with high Q-factors and smaller volumes, facilitating the miniaturisation of Maser cavities. Furthermore, a novel toroidal resonator is investigated via simulation.

Single photon counting detectors are extremely important in the evolution of quantum technologies. The existing devices for the low-flux measurements are bulky and their implementation cannot be made with small footprints. Integrated photonics aims to allow the miniaturization of these setups. We present simulation results for the design of a single 1x10 multimode interference coupler (MMI) in terms of the power imbalance between the output waveguides, optical losses, and tolerance on the operating wavelength. This component acts as the fundamental building block of a photonic integrated circuit (PIC) in the TriPleX platform, acting as an optical divider that is able to bring down the power to ratio levels of 1:10^-5. The central operating wavelength is 850 nm. This PIC is based on five cascaded 1x10 multimode interference couples (MMIs) in a novel function for bringing the power to an exceptionally low, and consistent level with repeatable and reproducible results. The fabricated photonic chips have been characterized in lab settings. The two best-performed PICs have been packaged and incorporated in a laboratory setup with embedded reference standards for optical power measurement in a technique referred to as "self-calibration". They were tested in system settings, where they successfully demonstrated that we have achieved a linear splitting ratio of 1:10^-9 by cascading nine splitters.

The strong electro-optic properties of lithium niobate have long established optical modulators based on the material as the technological backbone for long-haul telecommunication networks. The recent advancements in thin-film lithium niobate (TFLN) permit the fabrication of compact waveguide structures, to produce low-voltage and high-bandwidth modulators, as well as provide potential integration with silicon photonics. Here we propose the use of cascaded apodized gratings to enhance the optical group index of the modulator. By cascading inward, and outward Bragg gratings etched directly into the TFLN waveguide, an optical passband is established between the two Bragg frequencies of the cascaded grating sections. The group index of the passband can be engineered to provide an enhancement to the modulation efficiency by effectively increasing the device interaction length between the optical field, and the RF field. We demonstrate the potential for a 5x reduction in drive voltage while maintaining greater than a 5 nm optical bandwidth.

Recent advancements in active reconfigurable photonic devices have spurred interest in quantum information applications, ranging from computation to communications and sensing. Universal photonic processors (UPPs) play a crucial role in this domain, enabling the implementation of arbitrary unitary transformations on input photonic states. Common architectures for UPPs involve intricate interferometric meshes, with the reconfigurable Mach-Zehnder interferometer (MZI) as the fundamental building block.
In this work, we present the realization of an 8-mode UPP using direct femtosecond laser writing (FLW) as the fabrication platform. FLW allows rapid and cost-effective prototyping of waveguides in glass-based substrates, achieving low insertion losses (down to 0.13 dB cm⁻¹ for propagation and 0.2 dB per facet for coupling), a critical requirement for quantum applications.
By incorporating compact curved deep isolation trenches and stable, efficient thermal phase shifters, we have reduced the size of the MZI unit cell compared to the current state-of-the-art in FLW fabrication. This reduction improves integration density and circuit complexity with respect to the current state-of-the-art devices for this fabrication platform. The phase shifters exhibit minimal power dissipation (∼ 38mW) and thermal crosstalk (∼ 20 %). The device operates at a wavelength of 925 nm, making it compatible with state-of-the-art quantum dot single-photon sources. It features 28 MZIs and 56 thermal phase shifters, with total insertion losses below 3 dB. Additionally, we describe a calibration process combining conventional methods with a machine learning optimization procedure, enabling the realization of unitary transformations with an average amplitude fidelity surpassing 0.99, showcasing the high precision required for quantum photonic applications.

Optical metasurfaces are engineered 2D electromagnetic structures enabling flat optical elements with properties not readily found in nature. Their unit cells, meta-atoms, usually are represented by a set of electric and magnetic multipoles. All-dielectric-based metasurfaces have recently attracted significant attention owing to their virtually lossless transmission properties at optical frequencies. A majority of reported dielectric metamaterials are composed of relatively simple meta-atoms such as spheres, cubes, and cylinders, whose electromagnetic response is dominated by the electric dipole. However, magnetic dipoles and higher-order multipoles may enable new optical properties and functionalities, including directional scattering, beam steering, and new frequency generation. Despite impressive progress in the field of optical metamaterials and nanofabrication technologies, engineering meta-atoms that support such higher-order resonances is still challenging. Here, we demonstrate that designed titanium dioxide meta-atoms can enable dominant magnetic dipole response. We apply a machine-learning model to predict a meta-atom shape with a strong magnetic dipole resonant mode at the operating wavelength of 750 nm. Using finite-element-based numerical simulations implemented in COMSOL Multiphysics, we found that the optimized meta-atom is robust against experimental variations and conditions such as a non-perfectly collimated incident beam, nanofabrication inaccuracies, and an added substrate. The meta-atoms have been fabricated using two approaches, focused ion beam lithography and an electron beam lithography followed by reactive ion etching, and characterized using white light spectroscopy. To the best of our knowledge, this is the first experimental realization of a machine-learning-based optimization of a magnetic dipole mode at optical frequencies.

A primary constraint in the major photonic integration platform of Silica-on-Silicon, especially when combined with fabrication approaches like Direct Laser Writing is the optical waveguides' low refractive index contrast, leading thus to limitations for efficient coupling with currently available state-of-the-art single photon emitters such as semiconductor nanowires with quantum dots (NWQD). We propose and demonstrate a novel approach to drastically enhance the light coupling between silica based Laser-written channel waveguides and NWQDs, by incorporating an optical microsphere in their intermediate space. It is demonstrated that the induced photonic nanojet action of a suitably designed microsphere illuminated by the NWQD, excites efficiently the channel waveguide's modes and can enable light coupling to a degree even above 50%. The proposed method is reasonably robust to imperfections and misalignments and could be implemented by current state-of-the-art micro/nano patterning techniques. It is anticipated that the practical implementation of the method will allow the integration of multiple quantum emitters in silica based hybrid integrated circuits thus enabling their scalability towards for quantum computing and sensing applications.

Mid-infrared (MIR) is a promising spectral regime for gas and biochemical sensing since each molecule exhibits a unique vibrational absorption spectrum at this band. While integrated photonics on silicon-on-insulator (SOI) has become a successful platform, it cannot cover a broad MIR range, mainly limited by oxide absorption. Recently, a germanium-on-insulator (GOI) has emerged as a promising integrated photonics platform with broadband transparency covering 2-14 μm wavelengths. Germanium (Ge) and yttrium oxide (Y₂O₃), which exhibit low loss at the MIR regime, are used as a core and box insulator, respectively. However, the prevailing crosstalk issue of integrated photonics becomes more problematic at MIR due to extended evanescent fields with a longer wavelength. To address this issue, we propose using subwavelength gratings (SWGs), which can be effectively represented by homogenized anisotropic metamaterials. We arranged the SWGs in the cladding and formed an extreme skin-depth (eskid) waveguide, whose skin-depth is suppressed for transverse-electric (TE) mode. We then optimized SWG parameters to achieve an exceptional coupling that can completely suppress the crosstalk, i.e., zero crosstalk. Anisotropic dielectric perturbation via SWG metamaterials allowed different field components to compensate for each other, making the overall coupling coefficient zero. We optimized our SWG-based eskid waveguide scheme near 4.2 μm wavelength, where we can directly apply it for CO₂ sensing with its strong absorption. We expect our eskid scheme on the GOI platform to improve the overall performances of MIR photonic devices, especially for MIR molecular sensing applications.

Refractive index (RI) sensors based on silicon nitride on insulator (SiNOI) waveguide platform are designed and fabricated. SiNOI offers many advantages among which CMOS compatibility, low propagation losses, tolerance to temperature and fabrication variations as well as wide transparency range. The designed RI sensors include micro-ring resonators (MRRs), Mach-Zehnder Interferometers (MZIs) and loop-terminated MZIs (LT-MZIs) operating at both visible and near-infrared wavelengths. The sensors include strip and slot based sensing arms for chemical and biological sensing. These different components and the whole spectrum were designed and optimized using finite difference eigenmode (FDE) and finite difference time domain (FDTD) solvers. The sensors were fabricated using electron beam lithography in a SiN multi-project wafer (MPW).

High-resolution astronomical spectroscopy carried out with a photonic Fourier transform spectrograph (FTS) requires long asymmetrical optical delay lines that can be dynamically tuned. For example, to achieve a spectral resolution of R = 30,000, a delay line as long as 1.5 cm would be required. Such delays are inherently prone to phase errors caused by temperature fluctuations. This is due to the relatively large thermo-optic coefficient and long lengths of the waveguides, in this case composed of SiN, resulting in thermally dependent changes to the optical path length. To minimize phase error to the order of 0.05 radians, thermal stability of the order of 0.05° C is necessary. A thermal control system capable of stability such as this would require a fast thermal response and minimal overshoot/undershoot. With a PID temperature control loop driven by a Peltier cooler and thermistor, we minimized interference fringe phase error to +/- 0.025 radians and achieved temperature stability on the order of 0.05° C. We present a practical system for precision temperature control of a foundry-fabricated and packaged FTS device on a SiN platform with delay lines ranging from 0.5 to 1.5 cm in length using inexpensive off-the-shelf components, including design details, control loop optimization, and considerations for thermal control of integrated photonics.

The combination of photonic integrated circuits with MEMS allows to change the effective refractive index with unique ultra-low power dissipation characteristics. This enables high integration density applications like photonic quantum computing under cryogenic conditions. The here introduced fabrication technology can universally be applied to all common material platforms, such as silicon, silicon nitride or lithium niobate. The technological flexibility allows to use the IPMS PIC-technology (silicon nitride based) or cooperate very well cross-platforms with partners. The fabrication concept is successfully demonstrated and enables low-voltage devices with very low power consumption and modulation speeds up to megahertz.

Polarization handling devices are essential elements in photonic integrated circuits. In this paper, a phase shifter for applying different phase shift on transverse electric (TE) and transverse magnetic (TM) modes is presented. By optimizing a strain-enhanced InGaAsP multiquantum well structure the birefringence is increased and the phase shift between TE and TM modes is enlarged. The simulation results indicate that with a low driving voltage of 2 V and a small length of 2 mm, a 2π phase shift difference between TE and TM modes over 100 nm wavelength around the c-band is achieved.