Nested microring resonators (NMRRs) have long been explored for their ability to enhance spectral shaping in single-ring designs, enabling phenomena such as Fano resonances and EIT-like responses through variations in coupling schemes. While these advances have expanded studies over intensity and phase, group delay (GD) behavior, particularly the realization and tunability of negative GD, remains underexplored. This work investigates a nested cross-coupled microring resonator (NeXMRR), which incorporates a directional input coupler and a cross-coupled output coupler within a nested topology. Analytical modeling and schematic-level simulations, benchmarked against conventional NMRRs, reveal that the NeXMRR achieves deeper resonance dips, distinct phase shifts, and sustained negative GD and dispersion across a wide range of coupling and feedback conditions, with GD values exceeding 500 ps and dispersion spanning moderate to extreme regimes. In addition, the asymmetric cross-coupled layout provides broader NGD operating windows and improved tolerance to coupling and dimensional variations, yielding higher delay-per-ring efficiency than prior nested configurations. These findings demonstrate how cross-coupling reshapes the temporal response of nested resonators, expanding the design space for photonic integrated circuits. The NeXMRR’s ability to deliver tunable delay lines, dispersion control, and slow/fast-light effects positions it as a promising architecture for next-generation optical signal processing.

This paper presents a numerical characterization of a Cross-Coupled Add-Drop Ring Resonator (XMRR) derived from our previously reported cross-coupled microring architecture. The mathematical formulation was established in earlier work; here, we focus on standalone simulation-based validation. Intensity, phase response, group delay, and chromatic dispersion were analyzed under asymmetric coupling conditions. Results indicate that the XMRR preserves the fundamental spectral and temporal characteristics of a conventional add-drop micro-ring resonator, including identical resonance envelopes and delay magnitudes. The primary distinction is a deterministic resonance displacement of approximately 1 nm induced by the engineered cross-coupling pathway. Although this offset does not intrinsically enhance standalone filtering metrics, it introduces a controllable phase shift that has been shown to enable richer spectral responses when implemented in nested configurations, as demonstrated in our recent work. The XMRR therefore serves as a phase-engineered building block for advanced photonic interconnect architectures.

Coupled ring resonator architectures are widely used in photonic integrated circuits for spectral filtering, sensing, and microwave photonics. Double-ring resonators enable interference-induced resonance splitting, allowing multi-dip spectral responses beyond single-ring designs. This work presents a systematic numerical comparison of recently proposed cross-coupled (X-CRR) and conventional coupled ring resonator (CRR) architectures based on their all-arbitrary coupling-parameter spaces and resulting spectral characteristics. Analytical coupling conditions are used to predict one-to four-dip response regions, which are validated through full-wave varFDTD simulations. The results show that X-CRR facilitates higher-order multi-dip responses under lower coupling strength requirements, while the CRR offers greater flexibility for lower-order responses. These findings reveal a fundamental trade-off between implementation convenience and coupling flexibility, providing design guidelines for interference-based spectral engineering, including emerging non-Hermitian photonic systems.

We report a numerical approach to optimize the Non-cascaded Multifunctional Optical Filter (Cross and Direct- coupled All-Pass Filters in MZI arms) with tunable bandwidth (0 to pi radians), constant Free Spectral Range (FSR) and <= 1% ripple.

We introduce and analyze a novel optical resonator architecture designed for optical sensors and networks. Our theoretical model, supported by our derived equations, aligns closely with our numerical simulations, confirming the expected resonance conditions. Through circuit simulations, we successfully replicated eight distinct intensity and phase response profiles, further validating the design. Our preliminary results demonstrate the architecture’s strong potential for advanced optical sensing and network applications, with key features such as resonance dips, phase variations, and peak splitting enhancing its versatility. Additionally, our observations of "slow light" effects in the group delay indicate promising benefits. Future works will focus on experimental validation and optimization for seamless integration into advanced optical sensor and network technologies

We report the fabrication, testing, performance, and engineering device analysis of a Photonic Integrated Circuit (PIC)-based Linearized Optical Frequency Discriminator filter (LOFD) intended for 5G application using Scanning Electron Microscope (SEM) images. The device was fabricated using electron beam lithography. Besides the positive results, we also discuss the engineering analysis of the structural errors in the chip due to (i) waveguide gap and (ii) waveguide discontinuity in some areas of the Photonic chip.

Electromagnetically induced transparency (EIT) is a phenomenon where a narrow transparency window appears within an absorption band of a material due to quantum interference. This effect has significant applications in various fields, including slow light devices, optical filters, and sensors. This paper compares three prominent EIT implementation resonator structures: single direct-coupled, double coupler add-drop, standard double resonator (SDR), coupled ring reflector (CRR) and self-coupled optical waveguide (SCOW). We analyze their underlying mechanisms, performance characteristics, and suitability for different applications, supported by recent research from 2020 onwards. We also present simulated transmission spectra, using Lumerical Interconnect, for each configuration to visually illustrate their distinct features.

Almost all current microring resonator (MRR) based sensors rely on a wavelength shift detection scheme. This involves (i) expensive tunable lasers as an input source and (ii) optical spectrum analyzers as a detector, which leads to overall bulky configuration, high cost, and complex arrangement. Here, we propose a low-cost, tapered waveguide MRR-based gas sensor that uses a simple intensity-level detection method. It employs (1) a low-cost laser source and (2) a photodetector. The MRR is coated with graphene to detect the specific target element. This design can significantly reduce the overall cost and complexity of the sensing system.

We investigate the general phasor characteristics of the recently reported crisscrossed coupled-ring reflector (X-CRR) and compare it with the typical coupled-ring reflector (CRR) configuration. We observe that they distinctly have different phasor features under arbitrary parameter conditions. We also (i) discuss the importance of normalized frequency range in generating the complete phasor picture of these configurations and (iii) show that in most cases the X-CRR appears to be a spread-out, rotated version of the phasor diagram of the CRR. These phasor features imply that X-CRR possesses more interesting functionalities for various potential telecom applications.

Previously, we demonstrated an electronic circuit analogue of one of Special Relativity's (SR) phenomena called the Relativistic Aberration of Light (RAL) (European Journal of Physics, 42, 015605, 2021), which describes the change in the angle an observer sees a light source relative to their direction of motion at relativistic speeds. It used typical bulky laboratory equipment such as (i) function generators, (ii) oscilloscopes, and (iii) power supplies together with our all-pass filter (APF)-based electronic circuit analogue to perform experiments. In this paper, we present a novel smartphone-based experimental set-up performing the same experiment, but we replace the bulky and expensive laboratory equipment with a low-cost and compact smartphone system that can function as both function generator and oscilloscope. Our smartphone system consists of (i) an Android 8.0 (Oreo) application and (ii) an ESP32-based external module that may be wired or wirelessly interfaced for oscilloscope and signal generation functions. The setup was able to carry out the experiment, however the sampling rate was only limited to 8.5kHz, but with the added input channel, phase shift calculation was much more consistent, albeit with a slight offset of -15 degrees due to the added buffer circuit between the ESP32 and APF circuit. We hope that through our work, we expand the toolset of physics educators and researchers, particularly those in developing countries, especially with our system's considerations of equipment accessibility, affordability, and simplicity.

Recently, we reported a general technique that uses a “cross-coupling structure” as a new vehicle to manipulate the resonance features of the coupled resonators. The technique is important for the (i) search for new optical analogues of Quantum Coherence Effects (QCEs) and (ii) generation of fresh and richer optical behaviors of the coupled resonators that could open future applications in optical fiber communication. Here, we review two new configurations based on this technique. The first configuration uses a modified standard double resonator (SDR) which we refer to as Cross-coupled SDR (CC-SDR). It is a new analogue circuit of QCE that generates a new QC phenomenon we call Cross-coupled Resonator Induced Shifted Absorption (CRISA). It is the first-time a cross-coupled structure has been suggested in studying QCE. Its core mechanism is due to its “cross-coupling structure”. Second, we present a modified coupled ring reflector (CRR) configuration where its original directional couplers (DCs) are replaced with cross-coupler(s) to connect the two ring resonators. We refer to it as crisscrossed-assisted CRR (or X-CCR). We review the unique characteristics of these two configurations and discuss their applications as optical devices.

We present a novel framework referred to as Concept Connectivity that aids in educating and engaging students by presenting the topic of the Special Theory of Relativity (STR) in a coherent and unified manner. It uses different analogue implementations of the STR coming from seemingly distinct fields of study such as (i) Optics, (ii) Photonics, and (iii) Electronics to connect not only to the concepts of the STR but to the various concepts from these different fields. In these analogue implementations, the fundamental characteristics of the different STR phenomena can be mimicked in many different ways. Concept Connectivity has two major benefits. First, from an educational perspective, undergraduate students can (i) understand advanced physical phenomena (like STR) from different points of view, (ii) bridge together different learnings or concepts from Physics, Optics, Photonics, and Electronics, and (iii) learn hands-on knowledge and engineering skills from Optics and Electronic experimentations when these analogues are incorporated in undergraduate physics lectures and laboratory courses. In this way, Concept Connectivity contributes to the growing pedagogical approaches used in science education with an emphasis on Photonics, Optics and Electronics. Second, from a research perspective, Concept Connectivity provides undergraduate students with a rare “taste of research experience” related to the challenge of merging different concepts in STR using principles in Optics, Photonics, and Electronics.

We derive, for the first time to the best of our knowledge, more general and analytical expressions for the reflected complex electric field, intensity, and phase response of a recently reported multifunctional, tunable cross%coupled microring resonators. This allows us to investigate the effects of the parameter asymmetry between the two ring-to-bus directional couplers on the overall behavior of the device. We focus on its effects on the group delay and dispersion properties of the reflected light. Specifically, we explore numerically their profiles when the values of the two directional couplers are varied. Here, we show that it has interesting characteristics such as (i) group delay reveals the existence and the associated parameter conditions for the “slow light” and “fast light”, and (ii) dispersion becomes flat across a wide band when the couplers' asymmetry is increased. These properties are advantageous in optical fiber-based communication applications.

We propose a simple and yet effective compensation technique to address the potential back-reflected signal coming from the output port of the minimalist-designed, linear optical field frequency discriminator (FD) having a unique folded-back arrangement. Specifically, we consider the engineering problems associated with the combined effects of (i) back reflection signal coming from the output port of the Y-junction divider, and (ii) non-optimized coupling condition of the Y-junction power divider. Our proposed compensation technique solves these problems by using a reflector (or mirror) with variable reflectance positioned at one of the outputs of the single Mach-Zehnder interferometer (MZI) device. By tuning the value of the mirror reflectance to an optimal value, we can compensate for the effects of these above-mentioned problems.

We summarize a unique publication-driven Research Experience for Undergraduates program in optics/photonics that uses electronic circuit analogue-based research to strengthen students’ engagement in research. At present, we have published 4 journal and conference papers.

We report, for the first time to the best of our knowledge, a technique that employs ′cross-coupling structure' in the cascaded microring resonators (MRRs) to bring new optical analogues of quantum coherence phenomena. The new configuration resembles a standard double resonator (SDR) but uses cross-coupled–based directional couplers (DCs) instead of the typical direct-coupled–based DCs. One of its unique features is that it does not exhibit the typical SDR's signature quantum coherence analogue effects like coupled-resonator induced transparency (CRIT) or coupled-resonator induced absorption (CRIA). On the contrary, it produces a new very narrow reflection dip positioned at single-pass phase shift θ = − π/2 which we refer to as cross-coupled resonator induced shifted absorption (CRISA). We compare extensively CRISA's characteristics with the CRIT, CRIA, and Autler-Townes splitting (ATS) found in SDR. The technique opens up new configurations with richer optical behaviours that could find potentially still unexplored applications.

Previously, we reported a simple, low-cost, thin–film–based optical analogue of the Thomas Effect in Special Relativity (SR) using an ideal Michelson-Gires-Tournois interferometer (MGTI). This opens the door for other SR related phenomena to be studied analogously. Toward this goal, there are two important tasks. First, it is imperative to conduct a comprehensive evaluation of the technical limitations of this platform under two non-ideal (oftentimes unavoidable) operating conditions, namely: (i) an imperfect Gires-Tournois resonator (GTR), and (ii) the presence of interferometric error due to mismatch in the path-length arm difference of the MGTI. Second, it is also important to develop a technique to recover the direction information of the Thomas angle which is lost during the intensity measurement. Here, we report that (i) the GTR's imperfect back mirror must be fabricated with a minimum reflectance r0 value of greater than 0.9534 to limit the maximum phase deviation error by only less than 1%, (ii) the extracted Thomas angle is more sensitive to changes in interferometric error δl than the GTR imperfection. However, perfect extraction of the Thomas angle can be achieved for special range of the interferometric error δl, and (iii) the mimicked Thomas angle is distorted when the front reflectance coefficient r1 is greater than 0.7. Lastly, we introduce a method to recover the directional information of the Thomas angle by using a counter-intuitive additional positive/negative interferometric error. This method introduces no new optical components, and has high tolerance to fluctuation of the interferometric error.

We present a comprehensive parameters sensitivity analysis of the minimalist-design frequency discriminator, FD filter involving three parameters namely, (i) the back reflection coefficient “ $b$ ”, (ii) mirror reflectance “ $a$ ”, and (iii) the split-power ratio of the Y-combiner “ $r$ ”. We demonstrated that the measured linearity (bandwidth utilization, BWU and Normalized Deviation, ND) are negatively impacted by the increase in parameters b and r, but can be compensated by decreasing the value of the mirror reflectivity “ $a$ ”.

We present a new, multifunctional, tunable coupled-ring reflector (CRR) device where the two cascaded microring resonator-based All-Pass Filters (APFs) are now interconnected using a "crisscrossed port connection arrangement and then directly coupled" by a third directional coupler. We derive analytically and investigate numerically its intensity response. We show that it has many unique features such as: (i) asymmetric stopband/passband property, (ii) tunable dual-narrowband passband filter, and (iii) a unique quasi-interleaver characterized by dual-narrowband passband filter interleaved with broadband stopband filter. We also establish the critical parameter relationships that describe these rich responses and compare them to the typical CRR.