On-chip Waveguide Research Articles

High-responsivity on-chip waveguide coupled germanium photodetector for 2 μm waveband

Recently, the emerging 2 μm waveband has gained increasing interest due to its great potential for a wide scope of applications. Compared with the existing optical communication windows at shorter wavelengths, it also offers distinct advantages of lower nonlinear absorption, better fabrication tolerance, and larger free carrier plasma effects for silicon photonics, which has been a proven device technology. While much progress has been witnessed for silicon photonics at the 2 μm waveband, the primary challenge still exists for on-chip detectors. Despite the maturity and compatibility of the waveguide coupled photodetectors made of germanium, the 2 μm regime is far beyond its cutoff wavelength. In this work, we demonstrate an efficient and high-speed on-chip waveguide-coupled germanium photodetector operating at the 2 μm waveband. The weak sub-bandgap absorption of epitaxial germanium is greatly enhanced by a lateral separation absorption charge multiplication structure. The detector is fabricated by the standard process offered by a commercial foundry. The device has a benchmark performance with responsivity of 1.05 A/W and 3 dB bandwidth of 7.12 GHz, which is able to receive high-speed signals with up to 20 Gbit/s data rate. The availability of such an efficient and fast on-chip detector circumvents the barriers between silicon photonic integrated circuits and the potential applications at the 2 μm waveband.

Rayleigh backscattering-based simultaneous linewidth narrowing of a multi-wavelength DFB laser array with an arbitrary wavelength spacing.

Simultaneous linewidth narrowing of a multi-wavelength laser array with an arbitrary wavelength spacing based on Rayleigh backscattering is experimentally demonstrated. Rayleigh backscattering from a single 30 m high numerical aperture fiber (HNAF) is employed to simultaneously narrow the linewidths of a DFB laser array consisting of four distributed feedback (DFB) semiconductor lasers with different wavelengths. Experimental results show that the instantaneous linewidths of the four DFB lasers can be simultaneously narrowed from megahertz to kilohertz no matter whether the wavelength spacing between the lasers is equally spaced or not, verifying the self-adaptivity of Rayleigh backscattering on laser linewidth narrowing. The method demonstrated here is also applicable for on-chip waveguides without wavelength dependence, providing a more compact narrow linewidth laser array for the wavelength-multiplexing division system and other promising applications.

Dielectric optical waveguide fabricated on a transparent substrate

Transparent glass substrates are routinely used in the fabrication of metasurfaces, augmented reality (AR), virtual reality (VR), and holographic devices. While readily compatible with photolithographic patterning methods, when electron beam (E-Beam) techniques are used, field distortion and stitching errors can result due to the buildup of charge. A common approach to overcome this issue is to deposit a thin conductive polymer layer (E-Spacer). However, if high-voltage E-Beam is used to achieve nano-features, the polymer conductivity is not sufficient. We have shown that by using chromium (Cr) as an overcoating conductive layer on the resist, we can achieve accurate and seamless patterning in multiple writing fields and used the method to fabricate on-chip Si3N4 waveguides on SiO2. This technique has the potential to enable the fabrication of large-scale integrated photonic systems on transparent or dielectric substrates.

On-chip multifunctional self-configurable quadrilateral MZI network

Photonic integrated circuits have garnered significant attention in recent years. To enhance the functional versatility of these devices, researchers have introduced the concept of reconfiguration into photonic integrated circuits. Inspired by field programmable gate arrays in the electrical domain, programmable photonic chips employing various topologies have been developed. However, users still encounter challenges when utilizing these devices, as they need to understand the internal structure and principles of the chip and individually adjust the tunable basic units within the topology network. In this paper, we employ the quadrilateral topological network based on the on-chip Mach–Zehnder interferometer as a black box to realize a highly self-reconfigurable optical signal processor. By leveraging this approach, we achieve positive real-valued matrix computation, optical routing, and low-loss optical energy splitting. Our demonstration effectively showcases the immense potential of on-chip programmable photonic waveguide meshes.

Quantitative gas pressure measurement by molecular spectroscopy using chip-based supercontinuum in the mid-infrared.

We demonstrate the quantitative pressure measurement of gas molecules in the mid-infrared using chip-based supercontinuum and cepstrum analysis without additional measurements for baseline normalization. A supercontinuum generated in an on-chip waveguide made of chalcogenide glass having high nonlinearity passes through CO gas and provides a transmission spectrum. The gas absorption information is deconvoluted from the original supercontinuum spectral information containing temporal fluctuation by cepstrum analysis and extracted simply by applying a bandpass filter in the temporal domain. The gas pressure estimated from the extracted absorption information is consistent with the value measured by a pressure gauge within a difference of 1.25%, despite spectral fluctuations in the supercontinuum baseline comparable to the spectral depth of the gas absorption lines.

Slow-light-enhanced on-chip 1D and 2D photonic crystal waveguide gas sensing in near-IR with an ultrahigh interaction factor

Nanophotonic waveguides hold great promise to achieve chip-scale gas sensors. However, their performance is limited by a short light path and small light–analyte overlap. To address this challenge, silicon-based, slow-light-enhanced gas-sensing techniques offer a promising approach. In this study, we experimentally investigated the slow light characteristics and gas-sensing performance of 1D and 2D photonic crystal waveguides (PCWs) in the near-IR (NIR) region. The proposed 2D PCW exhibited a high group index of up to 114, albeit with a high propagation loss. The limit of detection (LoD) for acetylene (C2H2) was 277 parts per million (ppm) for a 1 mm waveguide length and an averaging time of 0.4 s. The 1D PCW shows greater application potential compared to the 2D PCW waveguide, with an interaction factor reaching up to 288%, a comparably low propagation loss of 10 dB/cm, and an LoD of 706 ppm at 0.4 s. The measured group indices of the 2D and 1D waveguides are 104 and 16, respectively, which agree well with the simulation results.

Multiplexing of bias-controlled modulation modes on a monolithic III-nitride optoelectronic chip.

III-nitride optoelectronic chips have tremendous potential for developing integrated computing and communication systems with low power consumption. The monolithic, top-down approaches are advantageous for simplifying the fabrication process and reducing the corresponding manufacturing cost. Herein, an ultraviolet optical interconnection system is investigated to discover the way of multiplexing between emission and absorption modulations on a monolithic optoelectronic chip. All on-chip components, the transmitter, monitor, waveguide, modulator, and receiver, share the same quantum well structure. As an example, two bias-controlled modulation modes are used to modulate video and audio signals in the experiment presented in this Letter. The results show that our on-chip optoelectronic system works efficiently in the near ultraviolet band, revealing the potential breadth of GaN optoelectronic integration.

All-optical self-manipulation of light flow in on-chip topological waveguides based on synthetic dimension.

Topological photonic crystals provide a new platform for designing nanophotonic devices with robustness. Especially, all-optical devices, which use the light controlling light, based on nonlinear topological photonic crystals, have not been reported yet. In this article, we numerically investigate the robust self-manipulation of light flow in silicon topological photonic crystal waveguides based on the Kerr nonlinearity of silicon and topological edge states of photonic crystal waveguides. By adjusting the intensity of incident light at a communication wavelength of 1550 nm, the transmission path of the light flow in waveguides can be effectively controlled, and such manipulation is immune to some disturbances of nanostructures and thus shows the robustness. The results indicate that nonlinear topological photonic crystals have potential applications in on-chip integrated all-optical photonic devices.

On-chip single-photon chirality encircling exceptional points

Exceptional points (EPs), typically defined as the degeneracy points of a non-Hermitian Hamiltonian, have been investigated in various physical systems such as photonic systems. In particular, the intriguing topological structures around EPs have given rise to novel strategies for manipulating photons and the underlying mechanism is especially useful for on-chip photonic applications. Although some on-chip experiments using lasers have been reported, EP-based photonic chips working in the quantum regime have been largely elusive. Here, we propose a single-photon experiment to dynamically encircle an EP in on-chip photonic waveguides possessing passive anti-parity-time symmetry. Photon coincidences measurement reveals a chiral feature of transporting single photons, which can act as a building block for on-chip quantum devices that require asymmetric transmissions. Our findings pave the way for on-chip experimental study on the physics of EPs as well as inspiring applications for on-chip non-Hermitian quantum devices.

3D trapezoidal edge coupler with high efficiency and tolerance based on Silicon-on-Insulator

Edge couplers connecting the optical fiber and the on-chip waveguide are indispensable devices in integrated photonics. In this study, a 3D edge coupler with trapezoidal cross-section is proposed and designed. High coupling efficiencies of the edge couplers are verified by theoretical calculations. The 3D trapezoidal edge couplers are fabricated on the thick-silicon layer of a Silicon-on-Insulator (SOI) substrate by using 3D grayscale lithography. Over the communication band (C-band), the fiber-to-chip measured results indicate that the maximum coupling efficiency of the 3D edge coupler is 0.68/0.99 dB for the TE/TM mode. The misalignment tolerances are less than 0.65 dB and 0.90 dB for the TE and TM modes within ±5 μm in the horizontal direction. The same measured values are less than 0.60 dB for both the TE and TM modes within 4 μm in the vertical direction. The excellent coupling characteristics of the 3D edge coupler are attractive for the large-scale photonic integration.

Design of high sensitivity on-chip temperature waveguide sensor based on sensitive cladding

Design of high sensitivity on-chip temperature waveguide sensor based on sensitive cladding

Fiber-to-Chip Three-Dimensional Silicon-on-Insulator Edge Couplers with High Efficiency and Tolerance.

The edge coupler is an indispensable optical device for connecting an external fiber and on-chip waveguide. The coupling efficiency of the edge coupler affects the effective integration of optical circuits. In this study, three-dimensional (3D) edge couplers with high efficiency and tolerance are proposed. The high coupling efficiency of the 3D edge couplers is verified by theoretical calculations. Three couplers are fabricated on a thick-silicon platform via 3D grayscale lithography. At the 1550 nm band, the fiber-to-chip experimental data show that the maximum coupling efficiencies of the three edge couplers are 0.70 dB and 1.34 dB, 0.80 dB and 1.60 dB, and 1.00 dB and 1.14 dB for the TE and TM modes, respectively. At the 1550 nm band, misalignment tolerances measurement data reveal 0.8 dB/0.9 dB tolerance of ±5 μm in the horizontal direction, and 1.7 dB/1.0 dB tolerance of ±2 μm in the vertical direction for TE/TM mode. This study provides a new idea for the design of 3D edge couplers and demonstrates significant superiority in research and industrial applications.

Towards a photonic integrated all-optical phase regenerator.

All-optical phase regeneration aims at restoring the phase information of coherently encoded data signals directly in the optical domain so as to compensate for phase distortions caused by transceiver imperfections and nonlinear impairments along the transmission link. Although it was proposed two decades ago, all-optical phase regeneration has not been seen in realistic networks to date, mainly because this technique entails complex bulk modules and relies on high-precision phase sensitive nonlinear dynamics, both of which are adverse to field deployment. Here, we demonstrate a new, to the best of our knowledge, architecture to implement all-optical phase regeneration using integrated photonic devices. In particular, we realize quadrature phase quantization by exploring the phase-sensitive parametric wave mixing within on-chip silicon waveguides, while multiple coherent pump laser tones are provided by a chip-scale micro-cavity Kerr frequency comb. Multi-channel all-optical phase regeneration is experimentally demonstrated for 40 Gbps QPSK data, achieving the best SNR improvement of more than 6 dB. Our study showcases a promising avenue to enable the practical implementation of all-optical phase regeneration in realistic long-distance fiber transmission networks.

Numerical Investigation and Design of Optical On-Chip Waveguide with Engineered Dispersion for Generation of Supercontinuum-Based Frequency Combs

Numerical Investigation and Design of Optical On-Chip Waveguide with Engineered Dispersion for Generation of Supercontinuum-Based Frequency Combs

Recent Progress in Infrared Absorption Spectroscopy for Gas Sensing With Discrete Optics, Hollow-Core Fibers and On-Chip Waveguides

Infrared absorption spectroscopy (IAS) is currently a powerful tool for gas sensing. Gas sensors based on IAS show advantages of high sensitivity, good selectivity and in-situ measurement. Typical infrared gas sensors include sensors based on discrete optics, hollow-core fiber sensors and on-chip sensors, suitable for different application environments. Discrete sensors and hollow-core fiber sensors have a relatively large volume and a heavy weight, but due to large light-gas interaction length (tens of meters or even kilometers), their sensitivity is obviously higher than on-chip gas sensors. Mature complementary metal oxide semiconductor (CMOS) fabrication process promotes the development of miniaturized on-chip sensors, which is portable in size, low in power consumption and light in weight. But the small on-chip sensor volume limits the interaction effect between light and gas analyte. Sensing techniques such as slow-light enhanced absorption spectroscopy and surface-enhanced infrared absorption spectroscopy are proposed to improve the on-chip sensing performance. In this paper, we review the gas sensing principle based on IAS briefly and the recent progress of the three kinds of sensors using different spectroscopy techniques. Finally, some conclusions and an outlook of IAS gas sensors are reached.

Observation of Topological Transmission in Terahertz Domino Waveguide Array

In this paper, we propose on-chip terahertz domino waveguide array structures which can enable the topological effect and enhance the robustness of terahertz transmission. Terahertz topological effect (include topologically-protected edge states and binding states) is excited by constructing kink and antikink structures in designed domino waveguide arrays. Furthermore, the robustness of topological waveguide arrays is successfully demonstrated. The topological transmission effects have been experimentally demonstrated using scanning near-field terahertz microscopy. Our study provides a new approach to study the topological photonic transport of waveguide arrays in the terahertz regime and has potential application in terahertz integrated circuit.

Ultra-Wideband Mid-Infrared Chalcogenide Suspended Nanorib Waveguide Gas Sensors with Exceptionally High External Confinement Factor beyond Free-Space.

On-chip waveguide sensors are potential candidates for deep-space exploration because of their high integration and low power consumption. Since the fundamental absorption of most gas molecules exists in the mid-infrared (e.g., 3-12 μm), it is of great significance to fabricate wideband mid-infrared sensors with high external confinement factor (ECF). To overcome the limited transparency window and strong waveguide dispersion, a chalcogenide suspended nanorib waveguide sensor was proposed for ultra-wideband mid-infrared gas sensing, and three waveguide sensors (WG1-WG3) with optimized dimensions exhibit a wide waveband of 3.2-5.6 μm, 5.4-8.2 μm, and 8.1-11.5 μm with exceptionally high ECFs of 107-116%, 107-116%, and 116-128%, respectively. The waveguide sensors were fabricated by a two-step lift-off method without dry etching to reduce the process complexity. Experimental ECFs of 112%, 110%, and 110% were obtained at 3.291 μm, 4.319 μm, and 7.625 μm, respectively, through methane (CH4) and carbon dioxide (CO2) measurements. A limit of detection of 5.9 ppm was achieved for an averaging time of 64.2 s through the Allan deviation analysis of CH4 at 3.291 μm, leading to a comparable noise equivalent absorption sensitivity of 2.3 × 10-5 cm-1 Hz-1/2 as compared to the hollow-core fiber and on-chip gas sensors.

Aluminum nitride waveguide beam splitters for integrated quantum photonic circuits

We demonstrate integrated photonic circuits for quantum devices using sputtered polycrystalline aluminum nitride (AlN) on insulator. On-chip AlN waveguide directional couplers, which are one of the most important components in quantum photonics, are fabricated and show the output power splitting ratios from 50:50 to 99:1. Polarization beam splitters with an extinction ratio of more than 10 dB are also realized from the AlN directional couplers. Using the fabricated AlN waveguide beam splitters, we observe Hong–Ou–Mandel interference with a visibility of 91.7%±5.66%.

On-Chip Optical Waveguide Visible-Light Power Detector Based on Photothermal Effect of PPy/SU-8 Photopolymer

In this work, an all-optical on-chip polymer waveguide visible-light power detector is proposed based on photothermal effect of polypyrrole (PPy) nanoparticles. PPy/SU-8 waveguide material is prepared and the strong optical absorption of the photopolymer in the visible-light region is beneficial for efficient photothermal energy conversion. The photothermal characteristics of PPy/SU-8 are analyzed, and through selective absorption specificity of the PPy/SU-8 photopolymer in Vis-NIR region, the performances of the multimode interference (MMI) waveguide device by thermo-optic (TO) tuning are simulated. The photothermal energy conversion coefficient of the PPy/SU-8 waveguide is obtained as 2.5 K/mW. The actual sensitivity of the on-chip light power detector is measured as 2.3 nm/mW. This technique could be used to achieve ultra-compact on-chip all-optical detectors in future.

Ultralow-noise Terahertz Detection by p–n Junctions in Gapped Bilayer Graphene

Graphene shows strong promise for the detection of terahertz (THz) radiation due to its high carrier mobility, compatibility with on-chip waveguides and transistors, and small heat capacitance. At the same time, weak reaction of graphene's physical properties on the detected radiation can be traced down to the absence of a band gap. Here, we study the effect of electrically induced band gap on THz detection in graphene bilayer with split-gate p-n junction. We show that gap induction leads to a simultaneous increase in current and voltage responsivities. At operating temperatures of ∼25 K, the responsivity at a 20 meV band gap is from 3 to 20 times larger than that in the gapless state. The maximum voltage responsivity of our devices at 0.13 THz illumination exceeds 50 kV/W, while the noise equivalent power falls down to 36 fW/Hz1/2.