Telecom Band Research Articles

Quantum frequency conversion and single-photon detection with lithium niobate nanophotonic chips

The lithium niobate on insulator (LNOI) platform has revolutionized lithium niobate materials, and a series of quantum photonic chips have exhibited unprecedented performances. Quantum frequency conversion (QFC) that enables quantum state preservation during frequency conversion is crucial in quantum technology. This work demonstrates a low-noise QFC process on the LNOI nanophotonic platform, connecting telecom and near-visible bands. An internal conversion efficiency of 73% and an on-chip noise count of 900 counts per second (cps) are achieved. Furthermore, the preservation of quantum statistical properties is verified, indicating the QFC chip’s promise for extensive applications of LNOI integrated circuits in quantum information. Based on this chip, we constructed an upconversion single-photon detector with a detection efficiency of 8.7% and a noise of 300 cps, paving the way to integrated on-chip single-photon detection. The realization of a low-noise QFC device also provide a pathway for practical chip-scale QFC-based quantum systems in heterogeneous configurations.

III–V selective regrowth on SOI for telecom lasers in silicon photonics

To realize fully integrated silicon photonics (Si photonics), reliable III–V light sources that can be efficiently coupled with Si/SiN waveguides are essential. Here, based on a monolithic InP/silicon-on-insulator (SOI) platform, we developed a selective regrowth scheme and constructed a regrowth platform for on-chip lasers that can be efficiently coupled with Si/SiN waveguides. InP and InGaAs/InP multi-quantum wells (MQWs) were regrown on the regrowth template on SOI as well as patterned commercial InP wafers in the same growth run for comparison. A flat (001) top surface after regrowth with a low roughness of 0.38 nm was obtained on SOI. Benefitting from the high quality of MQWs regrowth, strong photoluminescence emission at telecom band can be obtained on both growth templates. Also, multi-wavelength emission on the same chip can be potentially achieved by designing various regrowth openings. Furthermore, the large material volume with vertical stacking structure and intimate placement of MQWs and the Si layer of SOI allow for the potential demonstration of electrically pumped lasers and efficient light coupling between them and Si/SiN waveguides. Therefore, the demonstrated regrowth method provides a promising solution for the monolithic integration of III–V on-chip lasers on Si.

Measurement of the Kerr Nonlinear Refractive Index and its Variation Among 4H - SiC Wafers

The unique material property of silicon carbide ($\mathrm{Si}\mathrm{C}$) and the recent demonstration of low-loss $\mathrm{Si}\mathrm{C}$-on-insulator integrated photonics platform have attracted considerable research interests for chip-scale photonic and quantum applications. Despite the impressive progresses made in $\mathrm{Si}\mathrm{C}$ photonics, some of its beneficial photonic properties are yet to be fully explored. Here, we carry out a thorough investigation of the Kerr nonlinearity among $4H$-$\mathrm{Si}\mathrm{C}$ wafers from several major wafer manufacturers, and reveal that their Kerr nonlinear refractive index can be significantly different. By eliminating various measurement uncertainties in the four-wave mixing experiment, the best Kerr nonlinear refractive index of $4H$-$\mathrm{Si}\mathrm{C}$ wafers is estimated to be approximately 4 times, instead of the prior estimate of 2 to 3 times, of that of stoichiometric silicon nitride in the telecommunication band. In addition, experimental evidence is developed that the Kerr nonlinearity in $4H$-$\mathrm{Si}\mathrm{C}$ wafers can be stronger along the $c$ axis than that in the orthogonal direction. Our examination of the Kerr nonlinear refractive index also compels a useful correction to the existing model in high-index-contrast waveguides; otherwise, considerable errors can be introduced.

Degenerate optical parametric amplification in CMOS silicon

Silicon is a common material for photonics due to its favorable optical properties in the telecom and mid-wave IR bands, as well as compatibility with a wide range of complementary metal–oxide semiconductor (CMOS) foundry processes. Crystalline inversion symmetry precludes silicon from natively exhibiting second-order nonlinear optical processes. In this work, we build on recent works in silicon photonics that break this material symmetry using large bias fields, thereby enabling χ(2) interactions. Using this approach, we demonstrate both second-harmonic generation (with a normalized efficiency of 0.20%W−1cm−2) and, to our knowledge, the first degenerate χ(2) optical parametric amplifier (with an estimated normalized gain of 0.6dBW−1/2cm−1) using silicon-on-insulator waveguides fabricated in a CMOS-compatible commercial foundry. We expect this technology to enable the integration of novel nonlinear optical devices such as optical parametric amplifiers, oscillators, and frequency converters into large-scale, hybrid photonic–electronic systems by leveraging the extensive ecosystem of CMOS fabrication.

Design and Theoretical Analysis of Highly Negative Dispersion-Compensating Photonic Crystal Fibers with Multiple Zero-Dispersion Wavelengths

This paper presents a highly negative dispersion-compensating photonic crystal fiber (DC-PCF) with multiple zero dispersion wavelengths (ZDWs) within the telecommunication bands. The multiple ZDWs of the PCF may lead to high spectral densities than those of other PCFs with few ZDWs. The full-vectorial finite element method with a perfectly matched layer (PML) is used to investigate the optical properties of the PCFs. The numerical analysis shows that the proposed PCF, i.e., PCF (b), exhibits multiple ZDWS and also achieves a high negative chromatic dispersion of −15089.0 ps/nm·km at 1.55 μ m wavelength, with the multiple ZDWs occurring within the range from 0.8 to 2.0 μ m range. Other optical properties such as the confinement loss of 0.059 dB/km, the birefringence of 4.11 × 10 − 1 , the nonlinearity of 18.92 W − 1 k m − 1 , and a normalized frequency of 2.633 was also achieved at 1.55 μ m wavelength. These characteristics make the PCF suitable for high-speed, long-distance optical communication systems, optical sensing, soliton pulse transmission, and polarization-maintaining applications.

Inverse design of slow light devices at telecommunication band based on metamaterials using a deep learning attempt

Slow light devices have important applications in many fields such as optical communication, optical storage, optical signal shaping and synchronization. Although a variety of high-performance slow light devices can be purchased in the market, it is still a worthwhile research topic to design slow light devices according to specific wavelengths and requirements. Here we employ machine learning method to inverse design electromagnetically induced transparency (EIT) based slow light devices in communication band using metal–dielectric hybrid metamaterials. Three characteristic points of transmittance as well as the group refractive index and bandwidth are chosen as input parameters. By replacing the complex fully connected layer with a one-dimensional convolutional neural network (1DCNN) layer to optimize the fully connected network, the proposed model can break the limitation of passive modulation and find the best slow light structure parameters. The slow light parameters could reach a bandwidth of 38.71 nm and average group refractive index of 10.51. In addition, the model can predict hybrid metamaterial structure parameters of slow light devices in the communication band from 1400 nm to 1600 nm. By combining active and passive modulation technologies, our proposed method improves the adjustment range of design parameters of slow light devices. The procedure can be potentially applied in the design of other nano optical devices.

Ultrafast Carbon Nanotube Photodetectors with Bandwidth over 60 GHz

The future interconnect links in intra- and inter-chip require the photodetector with high bandwidth, ultra-wide waveband, compact footprint, low-cost, and compatible integration process with silicon complementary metal-oxide-semiconductor (CMOS) technology. Here, we demonstrate a CMOS-compatible carbon nanotube (CNT) photodetector that exhibits high responsivity, high bandwidth and broad spectral operation over all optical telecommunication band based on high-purity CNT arrays. The ultrafast CNT photodetector demonstrates the 100 Gbit/s Nyquist-shaped on-off-keying (OOK) signal transmission, which can address the demand for high-speed optical interconnects in and between data centers. Furthermore, the photodetector exhibits a bandwidth over 60 GHz by scaling down the active area to 20 {\mu}m2. As the CNT photodetectors are fabricated by doping-free process, it also provides a cost-effective solution to integrate CNT photonic devices with CNT-based CMOS integrated circuits. Our work paves a way for future CNT-based high-speed optical interconnects and optoelectronic integrated circuits (OEICs).

Gallium-doped zinc oxide: nonlinear reflection and transmission measurements and modeling in the ENZ region

Strong nonlinear materials have been sought after for decades for applications in telecommunications, sensing, and quantum optics. Gallium-doped zinc oxide is a II–VI transparent conducting oxide that shows promising nonlinearities similar to indium tin oxide and aluminum-doped zinc oxide for the telecommunications band. Here we explore its nonlinearities in the epsilon near zero (ENZ) region and show n 2,eff values on the order of 4.5 × 10−3 cm2GW−1 for IR pumping on 200–300 nm thin films. Measuring nonlinear changes in transmission and reflection with a white light source probe in the near-IR while exciting in the near-IR provides data in both time and wavelength. Three films varying in thickness, optical loss, and ENZ crossover wavelength are numerically modeled and compared to experimental data showing agreement for both dispersion and temporal relaxation. In addition, we discuss optimal excitation and probing wavelengths occur around ENZ for thick films but are red-shifted for thin films where our model provides an additional degree of freedom to explore. Obtaining accurate nonlinear measurements is a difficult and time-consuming task where our method in this paper provides experimental and modeled data to the community for an ENZ material of interest.

Cladding pumped bismuth-doped fiber amplifiers operating in O-, E-, and S-telecom bands.

Bismuth-doped fibers (BDFs) are considered nowadays as an essential part of the development of novel optical amplifiers, which can provide a significant upgrade to existing fiber optic telecommunication systems, securing multiband data transmission. In this paper, a series of BDF amplifiers (BDFAs) for O-, E-, and S-telecom bands based on a cladding pumping scheme using low-cost multimode semiconductor laser diodes at a wavelength of 0.7-0.8 µm were demonstrated for, it is understood, the first time. The developed BDFAs are characterized by a high peak gain of >25-30 dB in the corresponding telecom bands and a relatively low noise figure of 5-6 dB. Comparative analysis shows that most of the parameters of cladding pumped BDFAs are close to those of the best core pumped ones. This research opens up new opportunities for utilizing Bi-doped fibers as a key element of cost-effective and ready-to-work BDFAs for various practical applications.

Coherent Control of a Nuclear Spin via Interactions with a Rare-Earth Ion in the Solid State

Individually addressed Er$^{3+}$ ions in solid-state hosts are promising resources for quantum repeaters, because of their direct emission in the telecom band and compatibility with silicon photonic devices. While the Er$^{3+}$ electron spin provides a spin-photon interface, ancilla nuclear spins could enable multi-qubit registers with longer storage times. In this work, we demonstrate coherent coupling between the electron spin of a single Er$^{3+}$ ion and a single $I=1/2$ nuclear spin in the solid-state host crystal, which is a fortuitously located proton ($^1$H). We control the nuclear spin using dynamical decoupling sequences applied to the electron spin, implementing one- and two-qubit gate operations. Crucially, the nuclear spin coherence time exceeds the electron coherence time by several orders of magnitude, because of its smaller magnetic moment. These results provide a path towards combining long-lived nuclear spin quantum registers with telecom-wavelength emitters for long-distance quantum repeaters.

GaN as a Material Platform for Single‐Photon Emitters: Insights from Ab Initio Study

AbstractGaN with atom defects is a rising material platform for single‐photon emitter (SPE) recently due to their room‐temperature working conditions, high emission rate, narrow emission line‐width and mature processing technology. However, the SPE mechanism still remains unclear to date, which greatly hinders the progress of GaN based SPE. Herein, systematic ab initio calculations predict and identify two kinds of intrinsic point defects NGa and NGaVN in GaN, which can be response for the widely observed SPEs. The formation energy, band structure, transition mechanism, and orbital composition are systematically investigated. The results indicate that NGaVN° possesses a large zero‐phonon line (ZPL) of 1.98 eV and a short lifetime of 3.56 ns, which may be one of the most possible origins of SPEs in visible wavelengths. The calculation results coincide well with our measured ZPL (1.92 eV) and lifetime (1.14 ns), as well as the experimental results reported previously. This work also predicts that NGa0 and NGaVN+1 can produce SPEs at fiber telecommunications band (0.84 and 0.94 eV, respectively). These results give deep insights into the SPE emission mechanism in GaN and bridge the gap between the realized SPEs and the underlying physical mechanism.

Multilayer all-polymer metasurface stacked on optical fiber via sequential micro-punching process

Abstract Metasurface technology is revolutionizing the field of optics and pursuing expanded functions via technical developments, such as the integration of multiple metasurfaces with optical fibers. Despite several attempts to realize metasurface-on-fiber platforms, negligible fiber-facet areas pose a serious obstacle to efficient and precise fabrication. Herein, we demonstrate a novel sequential micro-punching process that enables rapid and precise stacking of multiple polymer metasurfaces on the end face of a single-mode optical fiber. Mesh-type nanohole metasurfaces are fabricated on a 1.8-μm-thick polymethyl methacrylate (PMMA) layer via e-beam lithography, and the PMMA layer is separated from the substrate and prepared in the form of a membrane using the external frame. Furthermore, the PMMA metasurfaces are sequentially punched through the fiber and stacked on top. Employing a micro-punching process, we demonstrate highly efficient all-polymer metalenses and orbital angular momentum (OAM) metasurfaces coupled with single-mode fibers operating in the telecommunication band. A 1550 nm laser beam passing through three metalens layers stacked on the fiber is focused at a distance of 135 μm with 83% efficiency. In addition, the 1550 nm beam passing through three OAM metasurfaces on the fiber is converted into a perfect vortex beam with a topological charge of 3. We believe that our proposed micro-punching process will cause a breakthrough in the fabrication of metasurface-integrated optical fibers that will be utilized in a wide range of applications.

Optomechanical Simulation of a Parametric Oscillator

We study an optomechanical device supporting at least three optical modes in the infrared telecommunication band and three mechanical vibration modes. We model the coherent driving of each optical mode, independently of each other, to obtain an effective Hamiltonian showing the different types of parametric processes allowed in the device. We propose a bichromatic driving scheme, in the lossy optical cavity regime, under a mean field approximation, that provides the quantum simulation of a parametric oscillator with optical control of its parameters.

Passive Amplification and Noise Mitigation of Optical Signals Through Talbot Processing

Noise is one of the rare aspects of experimental work that crosses all boundaries. It is present from scientific fields like ultrafast optical signal detection to applied fields such as image processing, or even in our day-to-day lives when we are simply trying to have a conversation in a loud room. In all these cases, incoherent, stochastic noise tends to drown a signal we aim to detect, and various techniques may need to be employed to improve the clarity of the waveform, which is characterized by the signal-to-noise ratio (SNR). Yet, considering the ubiquity of noise in scientific and technology fields, it may be surprising how few methods there exists for denoising a signal. Active amplification techniques alone cannot be employed for weak, noisy signals, since the SNR is inevitably degraded due to fundamental laws of physics, while bandpass filtering schemes necessarily lead to an attenuation of the signal. In this article, we review recent advances on the concept of passive amplification techniques based on the Talbot effect to enhance the noise properties of signals through coherent energy redistribution. We demonstrate the basic framework starting from pulse repetition rate multiplication with the Talbot effect. We then extend this theory to show the principle behind passive amplification of periodic waveforms, and then how this idea can be extended to arbitrary (generally, aperiodic) signals. Methods for passive amplification of both the time-domain and the frequency-domain representations of the signal of interest are reviewed. While here we focus on the application of the technique for optical signals in the standard telecommunication band (near wavelengths of 1550 nm), the proposed denoising scheme relies on widely available wave manipulations, such that it may offer exciting opportunities for any kind of physical wave support, such as acoustics, plasmonics and other regimes of the electromagnetic spectrum, like microwaves or X-rays.

Engineered octave frequency comb in integrated chalcogenide dual-ring microresonators

Octave-spanning Kerr combs bridging the spectral windows of the near-infrared region (NIR) and the mid-infrared (MIR) region are expected in a number of applications, including high-capacity coherent optical communications, and gas molecular absorption footprints. Here, we propose novel concentric dual-ring microresonators (DRMs) for advanced dispersion engineering to tailor the comb spectral profile. The dispersion can be flexibly engineered not only by the cross-section of the DRMs, but also by the gap between concentric dual-ring microresonators, which provides a new path to geometrically control the spectral profile of the soliton Kerr combs. An octave-spanning Kerr soliton microcomb with multi-dispersive waves has been achieved numerically covering from the telecommunication band (1224 nm) to the mid-infrared band region (2913 nm) with a −40 dB bandwidth of 1265 nm. Our results are promising to fully understand the nonlinear dynamics in hybrid modes in DRMs, which helps control broadband comb formation.

Relaxed-tolerance subwavelength grating coupler

Short-wavelength mid-infrared (MIR) silicon photonics at 2–2.5 μm wavelengths has a wide range of applications in optical communications, chemical analysis, and environmental monitoring. In this spectral region, subwavelength grating (SWG) couplers can be fabricated by using commercial multi-project wafer (MPW) services, due to their larger device feature sizes compared with those in the telecommunication band. However, conventional SWG couplers are still sensitive to variations in dimensions produced by errors in fabrication processes, restricting the fabrication reproducibility of the devices. Here, we demonstrated a relaxed-tolerance SWG coupler that has relaxed fabrication tolerance to overcome this limitation. The approach is based on the design of the SWG coupler with dual-hole structures to reduce the influence of random fabrication variations on the grating effective refractive index. Theoretically, the slope of the center wavelength shift of the relaxed-tolerance SWG coupler to the variation of the subwavelength hole size is 0.58, while that is 0.63 for the conventional SWG coupler. We demonstrated the relaxed-tolerance SWG coupler with a peak coupling efficiency of −6.2 dB at a wavelength of 2.038 μm with a 1-dB bandwidth of about 30 nm. Additionally, fabrication reproducibility and optical fiber coupling tolerance are also presented. The study paves a promising way toward developing short-wavelength MIR grating couplers with high performance, excellent repeatability, and cost-efficiency.

Third-Order Optical Nonlinearities of 2D Materials at Telecommunications Wavelengths

All-optical signal processing based on nonlinear optical devices is promising for ultrafast information processing in optical communication systems. Recent advances in two-dimensional (2D) layered materials with unique structures and distinctive properties have opened up new avenues for nonlinear optics and the fabrication of related devices with high performance. This paper reviews the recent advances in research on third-order optical nonlinearities of 2D materials, focusing on all-optical processing applications in the optical telecommunications band near 1550 nm. First, we provide an overview of the material properties of different 2D materials. Next, we review different methods for characterizing the third-order optical nonlinearities of 2D materials, including the Z-scan technique, third-harmonic generation (THG) measurement, and hybrid device characterization, together with a summary of the measured n2 values in the telecommunications band. Finally, the current challenges and future perspectives are discussed.

Blazed subwavelength grating coupler

Short-wavelength mid-infrared (2–2.5 μm wave band) silicon photonics has been a growing area to boost the applications of integrated optoelectronics in free-space optical communications, laser ranging, and biochemical sensing. In this spectral region, multi-project wafer foundry services developed for the telecommunication band are easily adaptable with the low intrinsic optical absorption from silicon and silicon dioxide materials. However, light coupling techniques at 2–2.5 μm wavelengths, namely, grating couplers, still suffer from low efficiencies, mainly due to the moderated directionality and poor diffraction-field tailoring capability. Here, we demonstrate a foundry-processed blazed subwavelength coupler for high-efficiency, wide-bandwidth, and large-tolerance light coupling. We subtly design multi-step-etched hybrid subwavelength grating structures to significantly improve directionality, as well as an apodized structure to tailor the coupling strength for improving the optical mode overlap and backreflection. Experimental results show that the grating coupler has a recorded coupling efficiency of − 4.53 dB at a wavelength of 2336 nm with a 3-dB bandwidth of ∼ 107 nm . The study opens an avenue to developing state-of-the-art light coupling techniques for short-wavelength mid-infrared silicon photonics.

Broadband Second Harmonic Generation in a z-Cut Lithium Niobate on Insulator Waveguide Based on Type-I Modal Phase Matching

We numerically investigate a second harmonic generation (SHG) in a z-cut lithium niobate on insulator (LNOI) waveguide based on type-I mode phase matching (MPM) between two fundamental modes. A mode overlap factor that is close to unity is achieved and the normalized SHG efficiency reaches up to 72.1% W−1cm−2 at the telecommunication band, together with a large spectral tunability of 2.5 nm/K. Moreover, a bandwidth of about 100 nm for the broad SHG in a 5 mm-long LNOI ridge waveguide is demonstrated with this scheme. This stratagem will inspire new integrated nonlinear frequency conversion methods for versatile applications.

Plasmonic Resonance Enabling 2D Perovskite Single Crystal to Detect Telecommunication Light

AbstractHigh performance photodetectors (PDs) based on two‐dimensional (2D) perovskite single crystals (SCs) have been demonstrated with improved stability with respect to those based on their three‐dimensional counterparts. However, due to their large energy band gaps, response of 2D perovskite PDs is limited in the ultraviolet and visible range. In this study, a broadband PD based on (PEA)2PbI4 (PEA = C6H5(CH2)2NH3+) 2D perovskite SC is demonstrated by incorporating thermally annealed Au nanoparticles (NPs). Introduction of Au NPs brings forward enhancement in photocurrent over a broadband wavelength range with the dark current being maintained. In the light absorption range of (PEA)2PbI4, photocurrent enhancement is owing to the enhanced localized electric field induced by Au NPs. Beyond the (PEA)2PbI4 absorption range of 650–900 nm, the photocurrent enhancement factors reach 1000% in average, which arises from the hot holes produced by Au NPs. It is also emphasized that the response wavelength of the PD has been extended to the telecommunication band; specifically, the light signals at 1310 nm can be converted to stable on‐off electric signals by the plasmonic PD. This work promotes the development of broadband, sensitive solution‐processed perovskite PDs, providing a promising strategy for realizing photodetection in the telecommunication range.