Multimode Waveguide Research Articles

Multi-mode Interference Waveguide Spectrometer with Optimized Performance

Multi-mode Interference Waveguide Spectrometer with Optimized Performance

Integrated subwavelength bimodal interferometer using a multilayer hyperbolic metamaterial.

Bimodal interferometry can be implemented in a photonic integrated waveguide by inserting structures supporting-at least-two modes to connect an input and an output single-mode waveguide. The length of the bimodal section is inversely proportional to the index difference between the involved modes, which can be quite small in multimode dielectric waveguides. We propose and numerically demonstrate an ultrashort bimodal interferometer by embedding a multilayer hyperbolic metamaterial in a subwavelength gap separating two dielectric waveguides. We use the large index difference (>1.5) between the bulk and the plasmonic-guided modes of the metamaterial to reduce the total length of the interferometer to less than 1 µm. Our system, which is potentially fabricable with standard nanofabrication tools, could be used to build ultra-compact integrated bimodal interferometers for signal processing and biosensing.

Monolithic dispersion engineered mid-infrared quantum cascade laser frequency comb

The high-power quantum cascade laser (QCL) frequency comb capable of room temperature operation is of great interest to high-precision measurement and low-noise molecular spectroscopy. While a significant amount of research is devoted to the longwave spectral range, shortwave 3–5 μm QCL combs are still relatively underdeveloped due to the excessive material dispersion. In this work, we propose a monolithic integrated multimode waveguide scheme for effective dispersion engineering and high-power-efficiency operation. Over watt-level output power at room temperature with a wall plug efficiency of 7% and robust dispersion reduction is achieved from a quantum cascade laser frequency comb at a wavelength approximately 4.6 μm. Narrow beatnote linewidth less than 1 kHz and clear dual-comb multiheterodyne comb lines manifest the coherent phase relation among the comb modes which is crucial to fast molecular spectroscopy. This monolithic dispersion engineered waveguide design is also compatible to an efficient active–passive optical coupling scheme and would open up a new research playground for ring comb and on-chip dual-comb spectroscopy.

The Simulation of Mode Control for a Photonic Lantern Adaptive Amplifier

A photonic lantern is a low-loss device that connects a single multimode waveguide to multiple single-mode waveguides and can enhance the beam quality of a fiber laser by adaptively controlling the optical parameters (amplitude, phase, polarization) at the input. In this work, we combined the gains and losses of individual modes within the fiber amplifier and introduced a mode content parameter at the amplifier’s output as an evaluation function to simulate mode control effects. Mode competition within the gain fiber can degrade the control effect of the fundamental mode and lead to it taking a longer time for the control to converge. Optimal parameters, such as the gain fiber length and pumping method, were identified to improve control effectiveness. Specifically, an optimal gain fiber length of 8 m was determined, and backward pumping was found to achieve higher pumping efficiency and better control results. The system demonstrated significant power amplification potential and could stabilize mode control under different pumping powers ranging from 50 W to 5 kW. In conclusion, our research demonstrates that an adaptive fiber amplifier based on a photonic lantern can achieve a stable, high-power, large-mode-field, near-fundamental-mode output from the gain fiber. Although mode competition within the gain fiber can degrade the control effect of the fundamental mode and cause the control to take a longer time to converge, these aspects should be further studied to improve the control’s effectiveness. These findings contribute to the development of advanced simulation models that guide high-power mode control experiments and deepen our understanding of physical processes in science and technology.

Shape sensing endoscope fiber

Minimally invasive and robotic surgeries are growing areas that benefit patients through reduced recovery time. Medical fiber optics play an important role in these procedures by enabling instrument navigation, imaging, sensing, power delivery, and diagnostics in a small form factor. One route to further miniaturization is to combine these functions, or a subset of these functions, into a single strand of optical fiber. In this work, we present a fiber and fan-in device that enables shape sensing, imaging, power delivery, and potentially additional sensing capabilities, such as temperature and/or pressure, in the same waveguide. The refractive index profile of the multimode waveguide in our fiber is similar to step index fibers used in laser delivery and is suitable for imaging applications; however, it also contains seven single mode cores twisted in a helix and with quasi-continuous Bragg gratings along their entire length, such as are used in fiber shape sensing. We first calibrate the transmission matrix of the multimode waveguide to enable the formation of a focused spot at the distal end of the fiber with a spatial light modulator. A second calibration allows us to reconstruct the shape of the fiber using optical frequency domain reflectometry in the twisted shape sensing cores. We show that these multiple functions can be performed simultaneously with our device and that changes in the curvature of the fiber correlate with the quality of the distal spot produced through the fiber, which is an important step towards maintaining the imaging calibration as the fiber is manipulated.

Silicon Loop-Type Multimode Waveguide Structure With Fan-Out Output for Photonic Reservoir Computing

Silicon Loop-Type Multimode Waveguide Structure With Fan-Out Output for Photonic Reservoir Computing

Dissipative Kerr soliton formation in dual-mode interaction Si3N4 microresonators

Dissipative Kerr soliton (DKS) microcombs based on multi-mode Si3N4 waveguides turn into an ideal tool that is compact and has precision for optical communication, precision spectroscopy, and frequency metrology. However, spatial waveguide mode interaction leads to local disturbances of dispersion, which may hinder DKS microcombs formation. In this letter, we generate the DKS microcomb in a dual-mode interaction Si3N4 microresonator without suppressing spatial waveguide mode interaction. The spatial waveguide mode interaction is investigated in the dual-mode interaction Si3N4 microresonator with a cross-sectional area of 800 × 1700 nm2. DKS microcomb is deterministically generated in the microresonator using an auxiliary light heating method. Furthermore, an integrated microcomb frequency measurement system is designed based on the DKS microcomb for frequency metrology.

Achromatic Silicon Photonic Waveguide Lenses for Ultra‐Broadband Multimode Spot‐Size Conversion

AbstractThe rapid development of silicon photonics inspired the emergence of on‐chip geometric optics. As one of the most crucial elements, waveguide‐lenses have attracted much attention. Nevertheless, waveguide‐lenses often suffer from large chromatism due to the strong waveguide dispersion, especially for the cases with multiple guided‐modes. In this paper, achromatic waveguide‐lenses available for multimode photonics are proposed and are further utilized for realizing multimode spot size converters (MSSCs) as an example. The present MSSC shows low excess losses < 0.4 dB and low inter‐mode crosstalks <−14 dB within an ultra‐broad bandwidth of 1450–1650 nm, which is realized for the first time by comprising a positive waveguide‐lens and a negative waveguide‐lens. In addition, the present MSSC is applied successfully for realizing high‐performance multimode waveguide corner‐bends. To the best of the knowledge, the proposed MSSC is the unique one with an ultra‐compact footprint and the largest bandwidth for low crosstalks as well as low losses, indicating that on‐chip geometric optics potentially provides a new perspective for on‐chip light manipulation desired in various applications.

Compact and low-crosstalk multimode waveguide crossing utilizing subwavelength holey metamaterial waveguides

To further increase the transmission capacity of on-chip optical communication systems, hybrid division multiplexing technology has emerged as a crucial alternative solution, in which multimode waveguide crossings are highly desired. In this paper, we propose and experimentally demonstrate a compact multimode (i.e., three different modes) waveguide crossing that employs subwavelength holey metamaterial waveguides (SHMWs). The used SHMW, formed by inserting subwavelength periodic holes into a multimode interference (MMI) coupler, deservedly exhibits synergetic advantages of the two kinds of structures, enabling an attractive three-mode (e.g., TE0, TM0, and TM1) waveguide crossing with flexible design, small size, and good performance. Simulation results show that the realized device has a low insertion loss (< 0.74 dB), low reflection loss (<−13.1 dB), and low crosstalk (<−31.6 dB) at a central wavelength of 1550 nm for all the modes with a compact footprint of 27.4 µm × 27.4 µm. The experimental results prove that insertion losses are as low as 0.72 dB, 0.27 dB, and 0.90 dB for TE0, TM0, and TM1 mode, respectively, with the corresponding crosstalk below −38 dB at 1550 nm. The proposed device can be widely applied in photonic integrated circuits to construct photonic systems with the abilities of mode control and multiplexing.

Long-distance strong coupling of magnon and photon: Effect of multi-mode waveguide

Coupled mode theory predicts that the long-distance coupling between two distant harmonic oscillators is in the weak coupling regime. However, a recent experimental measurement observed strong coupling of magnon and critically-driven photon with a distance of over two meters. To explain the discrepancy between theory and experiment, we study long-distance coupling of magnon and photon mediated by a multi-mode waveguide. Our results show that strong coupling is achieved only when both critical coupling and multi-mode waveguide are involved. The former reduces the damping while the latter enhances the coupling strength by increasing the pathways of coupling magnon and photon. Our theory and results pave the way for understanding the long-distance coherence and designing the magnon-based distributed quantum networks.

Low-loss and low-cross talk polarization-insensitive multimode silicon waveguide crossing.

A polarization-insensitive multimode silicon waveguide crossing is investigated and experimentally characterized in this Letter. By employing the particle swarm optimization (PSO) algorithm and finite difference time domain (FDTD) method, the lengths and widths of the waveguides in the proposed device are optimized for attaining wide bandwidth, small insertion loss (IL), low cross talk (CT), and compact size. Measurement results reveal that the footprint of the presented device is 11.92 μm × 11.92 μm. From 1520 to 1600 nm, the measured insertion loss and cross talk are smaller than 0.67 dB and -28.6 dB in the case of the TE0 mode, lower than 0.65 dB and -28.7 dB in the case of the TE1 mode, less than 0.48 dB and -36.3 dB in the case of the TM0 mode, and lower than 0.62 dB and -28 dB in the case of the TM1 mode.

Ultra-broadband and high extinction ratio silicon TM-pass polarizer based on grating-assisted antisymmetric multimode waveguide

We propose an all-silicon TM-pass polarizer using a grating-assisted antisymmetric multimode waveguide (GAMW). The GAMW design facilitates the conversion of the undesired TE0 mode into the back-reflected TE1 mode, enabling efficient transmission of the desired TM0 mode with low insertion loss (IL) and high extinction ratio (ER). Furthermore, by cascading multiple GAMW structures, the device can filter TE0 mode over a broader spectral range effectively. The simulation results indicate that the single-GAMW-based polarizer achieves IL < 0.17 dB and ER > 40 dB in the wavelength range from 1480 nm to 1620 nm. When cascaded, the device expands the bandwidth to 275 nm (1435 nm–1710 nm), maintaining IL < 1 dB and ER > 40 dB simultaneously. The bandwidth can be further improved by cascading more GAMW structures working at different central Bragg wavelength. Limited by the measuring equipment, the measured wavelength range is between [1470 nm, 1630 nm]. The results of experiment demonstrate that the fabricated single-GAMW-based polarizer achieves IL < 0.65 dB and ER > 28.4 dB in 144 nm bandwidth ranging from 1470 nm to 1614 nm. For the cascaded-GAMW-based polarizer, IL < 0.83 dB and ER > 28.8 dB is achieved throughout the entire measuring wavelength range.

Polarized and Evanescent Guided Wave Surface-Enhanced Raman Spectroscopy of Ligand Interactions on a Plasmonic Nanoparticle Optical Chemical Bench.

This study examined applications of polarized evanescent guided wave surface-enhanced Raman spectroscopy to determine the binding and orientation of small molecules and ligand-modified nanoparticles, and the relevance of this technique to lab-on-a-chip, surface plasmon polariton and other types of field enhancement techniques relevant to Raman biosensing. A simplified tutorial on guided-wave Raman spectroscopy is provided that introduces the notion of plasmonic nanoparticle field enhancements to magnify the otherwise weak TE- and TM-polarized evanescent fields for Raman scattering on a simple plasmonic nanoparticle slab waveguide substrate. The waveguide construct is called an optical chemical bench (OCB) to emphasize its adaptability to different kinds of surface chemistries that can be envisaged to prepare optical biosensors. The OCB forms a complete spectroscopy platform when integrated into a custom-built Raman spectrograph. Plasmonic enhancement of the evanescent field is achieved by attaching porous carpets of Au@Ag core shell nanoparticles to the surface of a multi-mode glass waveguide substrate. We calibrated the OCB by establishing the dependence of SER spectra of adsorbed 4-mercaptopyridine and 4-aminobenzoic acid on the TE/TM polarization state of the evanescent field. We contrasted the OCB construct with more elaborate photonic chip devices that also benefit from enhanced evanescent fields, but without the use of plasmonics. We assemble hierarchies of matter to show that the OCB can resolve the binding of Fe2+ ions from water at the nanoscale interface of the OCB by following the changes in the SER spectra of 4MPy as it coordinates the cation. A brief introduction to magnetoplasmonics sets the stage for a study that resolves the 4ABA ligand interface between guest magnetite nanoparticles adsorbed onto host plasmonic Au@Ag nanoparticles bound to the OCB. In some cases, the evanescent wave TM polarization was strongly attenuated, most likely due to damping by inertial charge carriers that favor optical loss for this polarization state in the presence of dense assemblies of plasmonic nanoparticles. The OCB offers an approach that provides vibrational and orientational information for (bio)sensing at interfaces that may supplement the information content of evanescent wave methods that rely on perturbations in the refractive index in the region of the evanescent wave.

High-performance and ultra-wide bandwidth silicon TM polarizer with suppressed reflection utilizing a tailored contra-mode conversion Bragg grating.

We present a compact TM polarizer with high polarization extinction ratio (PER), low loss, and suppressed reflection over an ultra-wide bandwidth on a 220 nm silicon-on-insulator (SOI) platform. The device utilizes a contra-mode conversion Bragg grating (CMC-BG) with strong polarization dependence embedded in a multimode waveguide. Through a sophisticated grating design incorporating tailored chirping and apodization profiles to match modal properties, we have achieved, by simulation, a compact device footprint of 34.72 × 1.22 µm2 and an ultra-wide bandwidth of 346 nm with PER > 40 dB and an insertion loss (IL) < 1 dB, a 5-fold increase over our previous design. Particularly notable is the polarizer's ability to suppress reflection to <-15 dB across an extended bandwidth exceeding 450 nm. Experimental measurements confirm the excellent performance of the fabricated TM polarizer, with IL < 1.2 dB (0.5 dB) and PER > 30 dB over a bandwidth of 336 nm (268 nm).

Fully tunable microwave photonic narrow bandpass filter using an on-chip dual-drive microring resonator

We propose and experimentally demonstrate a fully tunable microwave photonic narrow bandpass filter based on phase modulation to intensity modulation (PM-IM) conversion. In the filter implementation, an on-chip dual-drive microring resonator (MRR) is a key component. This resonator leverages a multimode waveguide to enable a high Q-factor. A metallic micro-heater and a lateral PN junction are simultaneously created for resonance wavelength tuning. When one driving signal is applied to the micro-heater, a large tuning range of the resonance wavelength is resulted; when another driving signal is applied to the PN junction, a fast tuning speed of the resonance wavelength is caused. By jointly using two different tuning mechanisms, the realized microwave photonic filter features a large frequency tuning range as well as a fast tuning speed. In addition, the filter bandwidth can also be tuned. A silicon-based dual-drive high-Q racetrack MRR chip is designed, fabricated, and evaluated. By incorporating the chip in a microwave photonic filter system, a bandpass filter with a narrow bandwidth of 1.27 GHz is achieved. An ultra-wide frequency tuning range from 3 to 51 GHz, an ultra-fast tuning speed less than 0.54 ns, and a tunable bandwidth from 1.27 to 4.47 GHz is experimentally demonstrated. This fully tunable filter offers significant potential in future radar and next-generation wireless communication applications.

Clearing a path for light through non-Hermitian media

Abstract The performance of all active photonic devices today is greatly limited by loss. Here, we show that one can engineer a low loss path in a metal-clad lossy multi-mode waveguide while simultaneously achieving high-performance active photonic devices. We leverage non-Hermitian systems operating beyond the exceptional point to enable the redistribution of losses in a multi-mode photonic waveguide. Consequently, our multi-mode waveguide offers low propagation losses for fundamental mode while other higher order modes experience prohibitively high losses. Furthermore, we show an application of this non-Hermitian waveguide platform in designing power-efficient thermo-optic phase shifters with significantly faster response times than conventional silicon-based thermo-optic phase shifters. Our device achieves a propagation loss of less than 0.02 dB μm−1 for our non-Hermitian waveguide-based phase shifters with high performance efficiency of P π ⋅ τ = 19.1 mW μs. In addition, our phase shifters have significantly faster response time (rise/fall time), τ ≈ 1.4 μs, compared to traditional silicon based thermo-optic phase shifters.

Ultrahigh Q SOI ring resonator with a Hermite or B-spline curved waveguide

Two new, to the best of our knowledge, designs for a huge, single-mode operation, ultrahigh quality factor SOI ring resonators based on the 90° bend of the B-spline curve and the Hermite curve were demonstrated. The ring resonators comprise multimode straight strip waveguides (MMWG), single-mode strip waveguides (SMWG), 90° bends, and tapers. The TE0-TE0 transmission loss of the bends can reach 0.000013 dB and 0.000091 dB, and the TE0-TE1 excitation ratios reach −48dB and −50dB, respectively, when the bends were designed using the Hermite curve or B-spline curve. Two SOI ring resonators of about 3.00 cm length consisting of the structures mentioned above are manufactured. The first was made up of Hermite curves with 90° bends. It could achieve a Q factor of 4.65×106 and a free spectrum range (FSR) of 0.025 nm. The second one employed B-spline curves with 90° bends. It could reach a Q factor of 4.37×106 and FSR of 0.022 nm.

Six-wave interaction in multimode waveguides with Kerr nonlinearity with allowance for the Gaussian structure of pump waves

The quality of wavefront conjugation in the case of six-wave interaction in two-dimensional multimode waveguides with Kerr nonlinearity is analyzed under the condition that one of the pump waves excites a zero waveguide mode and the amplitude distribution of the other pump wave at the waveguide end facet varies according to the Gauss law. It is shown that in a waveguide with infinitely conducting walls, the half-width of the modulus of the point spread function of a six-wave radiation converter is completely determined by the transverse size of the waveguide and weakly depends on the width of the Gaussian pump wave. In a parabolic-index profile waveguide, a decrease in the width of the Gaussian pump wave at the waveguide ends commonly leads to a monotonic decrease in the half-width of the point spread function modulus.

Ultra-Sharp Multimode Waveguide Bends Based on Superellipse Curves and Shallowly Etched Nanoslots Designed by an Intelligent Algorithm

Ultra-Sharp Multimode Waveguide Bends Based on Superellipse Curves and Shallowly Etched Nanoslots Designed by an Intelligent Algorithm

Ultra-high extinction ratio polarization beam splitter using an antisymmetric grating-assisted multimode waveguide

We have proposed and demonstrated a polarization beam splitter (PBS) with an ultra-high extinction ratio (ER) utilizing an antisymmetric grating-assisted multimode waveguide (AGMW) and an asymmetric directional coupler (ADC) on a silicon-on-insulator (SOI) platform. The AGMW structure is designed to facilitate the conversion of the forward TE0 mode into the backward TE1 mode. Upon injection of the TE0 mode, it transforms into the backward TE1 mode, subsequently undergoing conversion back to a TE0 mode through the ADC and ultimately dropping out from the designated port. In contrast, the injected TM0 mode traverses the device with minimal impact. The simulation results indicate that, for the TE0 mode, a bandwidth of 70 nm with an insertion loss (IL) below 0.65 dB and an ER over 40 dB is achieved. Similarly, for the TM0 mode, the bandwidth with the IL below 0.13 dB and an ER over 40 dB is 90 nm. Experimental validation confirms that, within the measured wavelength range from 1500 to 1580 nm, the bandwidth with an ER exceeding 30 dB is 61 nm for the TE0 mode and 78 nm for the TM0 mode.