Multimode Waveguide Research Articles

Wavefront shaping for reconfigurable beam steering in lithium niobate multimode waveguide.

Reconfigurable photonic devices are important constituents for future optical integrated circuits, where electro-optic manipulation of the light field in a lithium niobate (LN) waveguide is one of the promising solutions. Herein, we demonstrate a paradigm shift of the beam steering mechanism where reconfigurable beam steering is enabled by the wavefront shaping technology. Furthermore, this strategy is fully compatible with the electro-optic tuning mechanism of the LN multimode waveguide, where microstructured serrated array electrodes are employed to fine tune the output beam upon its reconfigurable output position. Our results provide new, to the best of our knowledge, insight for molding the flow of light in multimode waveguides and shed new light on beam steering photonic devices.

3 × 40 Gbit/s All-Optical Logic Operation Based on Low-Loss Triple-Mode Silicon Waveguide.

Information capacity of single-mode fiber communication systems face fundamental limitations imposed by optical nonlinearities. Spatial division multiplexing (SDM) offers a new dimension for upgrading fiber communication systems. Many enabling integrated devices, such as mode multiplexers and multimode bending with low crosstalk, have been developed. On the other hand, all-optical signal processing (AOSP) can avoid optical to electrical to optical (O–E–O) conversion, which may potentially allow for a low cost and green operation for large-scale signal processing applications. In this paper, we show that the system performance of AOSP can be pushed further by benefiting from the existing technologies developed in spatial mode multiplexing (SDM). By identifying key technologies to balance the impacts from mode-dependent loss, crosstalk and nonlinearities, three-channel 40 Gbit/s optical logic operations are demonstrated using the first three spatial modes in a single multimode waveguide. The fabricated device has a broadband four-wave mixing operation bandwidth (>20 nm) as well as high conversion efficiency (>−20 dB) for all spatial modes, showing the potential for a large-scale signal processing capacity with the combination of wavelength division multiplexing (WDM) and SDM in the future.

Ultrasonic Bubble Imaging in Molten Salt Using a Multi-Mode Waveguide and Time Reversal

Direct ultrasound imaging of technical processes in harsh environments, such as high-temperature melts, is not possible beyond the destruction limits of ultrasound transducers. A waveguide between the measured medium and the sensitive ultrasound transducer allows acoustic access to the former while protecting the latter. State-of-the-art single-mode waveguides allow for acoustical coupling between a transducer and the measured medium but are unsuited for real-time imaging since mechanical scanning is necessary. In contrast, multimode waveguides can transmit complex 2-D signals to an imaged region. While multimode waveguides require adaptive wavefront correction to compensate for the complex propagation inside, they allow the reduction of mechanical complexity with the application of the time-reversal virtual array (TRVA) method. This calibration method takes advantage of the time invariance of sound propagation in linear media to identify and compensate for the propagation inside the waveguide. This work proposes a novel <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> calibration procedure for system identification based on a traversing passive point scatterer. We characterize the imaging capacity of such a TRVA inside a molten nitrate salt at about 300 °C, exceeding the array’s operating temperature by 250 °C. By real-time imaging rising gas bubbles in the melt for the first time, we demonstrate the method’s potential for monitoring dynamic processes, such as metallurgical bubble column reactors with hot and opaque melts.

Efficient silicon integrated four-mode edge coupler for few-mode fiber coupling

Here, we designed a broadband, low loss, compact, and fabrication-tolerant silicon-based four-mode edge coupler, composed of a 1×3 adiabatic mode-evolution counter-taper splitter and a triple-tip inverse taper. Based on mode conversion and power splitting, the proposed structure can simultaneously realize efficient mode coupling from TE0, TM0, TE1, and TM1 modes of multimode silicon waveguides to linearly polarized (LP), LP01,x , LP01,y , LP11a,x , and LP11a,y , modes in the few-mode fiber. To the best of our knowledge, we proposed the first scheme of four LP modes coupling, which is fully compatible with standard fabrication process. The 3D finite-difference time-domain simulation results show that the on-chip conversion losses of the four modes remain lower than 0.62 dB over the 200 nm wavelength range, and total coupling losses are 4.1 dB, 5.1 dB, 2.1 dB, and 2.9 dB for TE0-to-LP01,x , TM0-to-LP01,y , TE1-to-LP11a,x , and TM1-to-LP11a,y , respectively. Good fabrication tolerance and relaxed critical dimensions make the four-mode edge coupler compatible with standard fabrication process of commercial silicon photonic foundries.

Four wave mixing in multimode hollow core waveguides with a two-color pump for the thorium nuclear clock

Four wave mixing in multimode hollow core waveguides with a two-color pump for the thorium nuclear clock

Properties of liquid-crystal wave-guiding structures.

The paper presents the results of an experimental and numerical study of some properties of multimode liquid crystal waveguide structures. The nematic 4-cyano-4'-pentylbiphenyl (5CB) was used as a liquid crystal. A description of the experiments performed and some of the techniques used are given. Scattering diagrams are presented that characterize the features of propagation in a multimode liquid-crystal waveguide of one and many modes. It is shown that when an external repetitively pulsed electric field is switched on, the attenuation and size of inhomogeneities decrease. To explain this effect, the classical theory of liquid crystal director fluctuations is used. For the first time some properties of a liquid-crystal waveguide are described with explicit allowance for the two-dimensional Frederiks model. The two-dimensional nature of the liquid crystal director reorientation effects in our case (including under the action of an electric impulse-periodic field) required the involvement of the three-dimensional theory of light scattering in an LC waveguide and, as a consequence, the study of two-dimensional scattering diagrams in experiments, which make it possible to consider the two-dimensional nature of the behavior of the LC director. The relevance and importance of such studies are related both to the practical use and prospects of using liquid crystal materials in various high-speed and low-energy integrated-optical devices, for example, in communication elements, modulators, and sensors.

Silicon Nitride External Cavity Laser With Alignment Tolerant Multi-Mode RSOA-to-PIC Interface

We demonstrate an external cavity laser formed by combining a silicon nitride photonic integrated circuit with a reflective semiconductor optical amplifier. The laser uses an alignment tolerant edge coupler formed by a multi-mode waveguide splitter right at the edge of the silicon nitride chip that relaxes the required alignment to the III-V gain chip and equally splits the power among its two output waveguides. Both the ground and first order mode are excited in the coupler and reach the quadrature condition at the waveguide junction, ensuring equal power to be coupled to both. Two high-quality-factor ring resonators arranged in Vernier configuration close a Sagnac loop between the two waveguides. In addition to wideband frequency tuning, they result in a longer effective cavity length. The alignment tolerant coupler increases the alignment tolerance in the two directions parallel to the chip surface by a factor 3 relative to conventional edge couplers, making it ideal for gain chip integration via pick-and-place technology. Lasing is maintained in a misalignment range of &#x00B1;6 &#x03BC;m in the direction along the edge of the chip. A Lorentzian laser linewidth of 39 kHz is achieved.

Phased array antennas of coupled multimode waveguides of hexagonal cross-section with flat-topped radiation patterns

Phased array antennas of coupled multimode waveguides of hexagonal cross-section with flat-topped radiation patterns

Design of the Ultra-Compact Silicon Multimode Waveguide Bends with Arbitrary Width and Radius Based on the Back Propagation Neural Network

Design of the Ultra-Compact Silicon Multimode Waveguide Bends with Arbitrary Width and Radius Based on the Back Propagation Neural Network

Thin-Film Lithium Niobate Based Acousto-Optic Modulation Working at Higher-Order TE1 Mode

Acousto-optic modulation (AOM) is regarded as an effective way to link multi-physical fields on-chip. We propose an on-chip AOM scheme based on the thin-film lithium niobate (TFLN) platform working at the higher-order TE1 mode, rather than the commonly used fundamental TE0 mode. Multi-physical field coupling analyses were carried out to obtain the refractive index change of the optical waveguide (&gt;6.5×10−10 for a single phonon) induced by the enhanced acousto-optic interaction between the acoustic resonator mode and the multimode optical waveguide. By using a Mach-Zehnder interferometer (MZI) structure, the refractive index change is utilized to modulate the output spectrum of the MZI, thus achieving the AOM function. In the proposed AOM scheme, efficient mode conversion between the TE0 and TE1 mode is required in order to ensure that the AOM works at the higher-order TE1 mode in the MZI structure. Our results show that the half-wave-voltage-length product (VπL) is &lt;0.01 V·cm, which is lower than that in some previous reports on AOM and electro-optic modulation (EOM) working at the fundamental TE0 mode (e.g., VπL &gt; 0.04 V·cm for AOM, VπL &gt; 1 V·cm for EOM). Finally, the proposed AOM has lower loss when compared with EOM because the electrode of the AOM can be placed far from the optical waveguide.

Ultrafast heterodyne mode imaging and refractive index mapping of a femtosecond laser written multimode waveguide.

We demonstrate imaging of individual modes in a femtosecond laser written multimode waveguide by spatial-heterodyne interferometry and decomposition in data post-processing. Despite the spatial and temporal overlap between multiple waveguide modes, we show the extraction of amplitude for each individual mode and their corresponding temporal dynamics. The mode imaging scheme is effective with the presence of intermodal interference and can be prospective for sensing of ultrafast phase and refractive index fluctuations. We also reconstruct the two-dimensional transverse refractive index map of the multimode waveguide leveraging all the imaged modes and substantiate the reconstructed index map by simulation.

Design of an ultrasharp silicon multimode waveguide bend assisted by multiregional subwavelength grating structures

Sharp multimode waveguide bends are beneficial to improve the integration density of the on-chip mode-division multiplexing (MDM) systems, which can be used to increase the optical information transmission capacity. A method of designing a multimode bending structure based on subwavelength gratings is proposed in this paper, through the mode conversion and matching between the straight and bending waveguide. The proposed bending structure is composed of a 90-deg arc-bend with a radius of 5 μm combined with the shallow-etched grating structures on the top, and two-mode converters on the input and output straight waveguide realized also with the help of nonuniform grating-like structures, achieving a total effective bending radius of 8.25 μm and high transmission efficiencies of the first three TE modes (TE0 − TE2) in a wide wavelength range of 1500 to 1600 nm. At the center wavelength of 1550 nm, the calculated excess losses of the three modes TE0, TE1, and TE2 are 0.21, 0.19, and 0.59 dB, respectively, and the crosstalks for all modes are less than −26.58 dB. More importantly, we add a non-uniform grating converter in the straight waveguide region which pre-deviates the mode fields inside the straight waveguide to help further reduce the size of the bending part to only 5 μm, which will be helpful for large scale integration when a great number of bends are cascaded.

Mid-Infrared Frequency Generation via Intermodal Difference Frequency Generation in AlGaAs-On-Insulator Waveguides

We investigate theoretically mid-infrared (MIR) generation via difference frequency generation in multimode AlGaAs-on insulator (AlGaAs-OI) waveguides. The large refractive index difference between the AlGaAs core and the silica cladding shrinks the modes size down to the sub-μm2 scale, and, together with AlGaAs strong second-order nonlinear polarization, empowers strong nonlinear effects. As a result, efficient MIR generation is obtained in few-cm long waveguides with sub-μm2 transverse section, where higher order modes are exploited to achieve the phase-matching condition. These observations suggest that multimode AlGaAs-OI waveguides could represent a novel promising platform for on-chip, compact MIR sources.

Optical Waveguide Sensors for Measuring Human Temperature and Humidity with Gel Polymer Electrolytes.

In this work, multimodal responsive optical waveguide sensors using a stable cross-linking gel polymer electrolyte are successfully designed and fabricated by bottom metal-printing technology. Temperature and humidity sensing characterization based on the polymer electrolyte is simulated and analyzed. The multimodal responsive properties of the photonic chip are defined based on the analysis of ion relaxation dynamics: optical phase variable to monitor temperature and optical attenuation variable to detect humidity. In the supervising temperature (36.0-38.0 °C) and relative humidity (45-65%) range, the temperature and humidity sensitivities of the device are measured as 0.5π rad/°C and 1.14 dB/% RH, respectively. The fast-response time for both temperature and humidity of the multifunctional sensor can be obtained as 4.21 ms and 1.32 s, respectively. These findings provide a feasible scheme for the design and application of temperature and humidity sensors in potential medical treatment. From gel polymer electrolytes to multimode monitoring applications, the application exploration of high stability and ultrafast response multimode waveguide sensors is gradually being carried out. This study has great significance for the comprehensive monitoring of sophisticated human physical signs by multimodal responsive waveguide sensors.

Direct-access mode-division multiplexing switch for scalable on-chip multi-mode networks

Abstract By leveraging mode-division multiplexing (MDM), capacity of on-chip photonic interconnects can be scaled up to an unprecedented level. The demand for dynamic control of mode carriers has led to the development of mode-division multiplexing switches (MDMS), yet the conventional MDMS is incapable of directly accessing an individual lower-order mode that propagates in a multi-mode bus waveguide, which hinders its scalability and flexibility. In this paper, we propose and demonstrate the first direct-access MDMS as a novel platform for scalable on-chip multi-mode networks. At first, the highly efficient mode exchangers are developed for TE0–TE2 and TE1–TE2 mode swap, which are then employed to realize the direct-access mode add-drop multiplexers with high performances. The direct-access MDMS is then achieved based on the proposed mode add-drop multiplexers, which can be used for dynamically adding and dropping any selected mode carrier in a three-channel MDM. Moreover, the novel direct-access scheme is also adopted to simultaneously harness wavelength and mode carriers, leading to a wavelength/mode-hybrid multiplexing system with an enhanced link capacity of twelve channels. To further verify the utility of the MDMS, a multi-mode hubbed-ring network is constructed, where one hub and three nodes are organized within a ring-like multi-mode bus waveguide. The reconfigurable network traffic of 6 × 10 Gbps data streams are obtained by using three eigen modes as signal carriers. The measurement results show low bit-error rates (&lt;10−9) with low power penalties (&lt;3.1 dB).

Characterization of intra-mode forward Brillouin scattering induced by vector high-order optical mode in nanofibers

We present the characteristics of intra-mode forward stimulated Brillouin scattering (FSBS) induced by HE11 and high-order optical mode (HOM) in nanofibers, respectively. Multiple transverse acoustic modes (TAMs) would be excited in the FSBS process. However, only a few specific low-order TAMs participate in the interaction of different optical mode pairs and then contribute to Brillouin gain. Meanwhile, the changes of dominating Brillouin peaks with the nanofiber radius are shown in detail, which further clearly confirms the difference in the gain characteristics of different intra-mode Brillouin gain spectrums. Especially, several obvious maximum and minimum gain values exist at the nanofiber radius of around 0.8 μm. The HOM induced intra-mode FSBS is of great importance for understanding of the phonon–photon interaction in multi-mode waveguides.

Supersymmetry-enhanced stark-chirped rapid-adiabatic-passage in multimode optical waveguides.

We propose a method to efficiently pump an excited mode of a multimode optical waveguide starting from a fundamental-mode input by combining Stark-Chirped Rapid Adiabatic Passage (SCRAP) and Supersymmetry (SUSY) transformations. In a two-waveguide set, we implement SCRAP by modulating the core refractive index of one waveguide, which is evanescently coupled to its SUSY partner. SCRAP provides an efficient transfer of light intensity between the modes of different waveguides, while SUSY allows to control which modes are supported. Using both techniques allows to achieve fidelities above 99% for the pumping of the excited mode of a two-mode waveguide. Additionally, we show that SCRAP can be exploited to spatially separate superpositions of fundamental and excited modes, and how SUSY can also improve the results for this application.

Ultra-narrow passband-tunable filter based on a high-Q silicon racetrack resonator

An ultra-narrow narrow passband-tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on a SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multimode waveguide racetrack, and the Q factor is measured as high as 8.1×105. The structure of the device is based on a thermally tunable Mach-Zehnder interferometer coupled racetrack. The tunability of the bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range is about 0.28 nm (35 GHz), with the footprint of 0.21mm2.

Ultra-compact multimode waveguide bend with shallowly etched grooves.

In this work, an ultra-sharp multimode waveguide bend (MWB) based on gradient shallowly etched grooves is proposed and demonstrated. With a bending radius of only 5.6 μm, our shallowly-etched-groove multimode waveguide bend (SMWB) can enable low excess loss and low-crosstalk propagation with the four lowest-order TE mode-channels, simultaneously. In the simulation, the excess losses of the proposed 90°- SMWB for TE0-TE3 are all below 0.46 dB and the inter-mode crosstalks are lower than -18 dB in 1500 nm-1600 nm. Furthermore, the measured results of the fabricated 90°- SMWB show that the excess losses for TE0-TE3 are less than 1 dB and the inter-mode crosstalks are all below -14 dB in 1510 nm-1580 nm. Such a proposed device thus provides a promising solution for ultra-compact MWBs in multimode silicon photonics.

Broadband Terahertz Photonic Integrated Circuit with Integrated Active Photonic Devices

Present-day photonic terahertz (100 GHz–10 THz) systems offer dynamic ranges beyond 100 dB and frequency coverage beyond 4 THz. They yet predominantly employ free-space Terahertz propagation, lacking integration depth and miniaturisation capabilities without sacrificing their extreme frequency coverage. In this work, we present a high resistivity silicon-on-insulator-based multimodal waveguide topology including active components (e.g., THz receivers) as well as passive components (couplers/splitters, bends, resonators) investigated over a frequency range of 0.5–1.6 THz. The waveguides have a single mode bandwidth between 0.5–0.75 THz; however, above 1 THz, these waveguides can be operated in the overmoded regime offering lower loss than commonly implemented hollow metal waveguides, operated in the fundamental mode. Supported by quartz and polyethylene substrates, the platform for Terahertz photonic integrated circuits (Tera-PICs) is mechanically stable and easily integrable. Additionally, we demonstrate several key components for Tera-PICs: low loss bends with radii ∼2 mm, a Vivaldi antenna-based efficient near-field coupling to active devices, a 3-dB splitter and a filter based on a whispering gallery mode resonator.