Optical Interconnect Applications Research Articles

Low-loss c-axis oriented Zn0.72Mg0.28O nonlinear planar optical waveguides on silicon.

A material platform of highly c-axis oriented Zn1-xMgxO thin films is developed for nonlinear planar waveguides and electro-optic modulators on Si. Mg content in the film greatly influences the quality of film growth. The second harmonic generation measurement and Maker-fringe analysis reveal that the second-order nonlinear susceptibility tensor element χ33 of the annealed Zn0.72Mg0.28O is approximately 4.2 times larger than that of ZnO. The propagation loss of 633 nm wavelength light in the annealed air/Zn0.72Mg0.28O/SiO2 slab waveguide is 0.68 ± 0.09 dB/cm and 0.48 ± 0.03 dB/cm for the TE0 and TM0 modes, respectively. These results suggest the great potential of the c-axis oriented Zn0.72Mg0.28O nonlinear planar waveguides for applications in on-chip optical interconnects.

Optical Polymer Waveguides Fabricated by Roll-to-Plate Nanoimprinting Technique

The paper reports on the properties of UV-curable inorganic-organic hybrid polymer multimode optical channel waveguides fabricated by roll-to-plate (R2P) nanoimprinting. We measured transmission spectra, refractive indices of the applied polymer materials, and optimized the R2P fabrication process. Optical losses of the waveguides were measured by the cut-back method at wavelengths of 532, 650, 850, 1310, and 1550 nm. The lowest optical losses were measured at 850 nm and the lowest average value was 0.19 dB/cm, and optical losses at 1310 nm were 0.42 dB/cm and 0.25 dB/cm at 650 nm respectively. The study has demonstrated that nanoimprinting has great potential for the implementation of optical polymer waveguides not only for optical interconnection applications.

High-resolution beam scanning technique with microlens array and adaptive fiber-optics collimator.

Conventional beam scanning systems employing a microlens array (MLA) suffer from the problem that only discrete diffraction angles can be addressed because of the periodic structure of the MLA. In this paper, an adaptive fiber-optics collimator (AFOC) that continuously adjusts the position of light source (optic fiber output) is used in front of the periodic structure as a moving linear phase shifter to overcome this discrete scanning angle problem. By introducing the AFOC into the beam scanning system employing MLA, a beam scanning system with continuous scanning capability and high resolution is fulfilled. Theoretical simulations and experimental results both demonstrate the continuous high-resolution scanning capacity of the beam scanning system employing both MLA and AFOC. The proposed beam scanning system is expected to find wide applications in space optical communication, optical interconnection, power projection, and coherent beam combining.

Dynamical tuning for single mode whispering gallery mode microcavity lasing

The dynamically modulated lasing mode is extremely significant for laser applications in optical interconnections, on-chip communications and trace detection. However, tunable micro/nano laser (especially for single mode laser) is still a challenge although enormous efforts have been devoted. The coexisting modes could be selected out through light field coupling between two adjacent cavities based on Vernier effect, which could provide a compact and suitable approach for single mode lasing operation in various kinds of compound microcavity. Herein, a spherical whispering gallery cavity with small size and low threshold was rolled along a hexagonal tapered microrod to adjust the lasing mode. The coupling mechanism was explored based on static and time-resolution photoluminescence measurements in situ. The tunable lasing mode mainly derives from resonant mode of microsphere according to carrier recombination analysis. Such a spectral control establishes to realize a lasing mode selection flexibly in real-time micromanipulation.

Investigation on roughness-induced scattering loss of small-core polymer waveguides for single-mode optical interconnect applications.

We investigated the roughness-induced scattering loss (LossR) of small-core polymer waveguides fabricated using the photolithography method, both theoretically and experimentally. The dependence of LossR on the roughness parameter, waveguide dimension, operation wavelength, refractive index difference and distribution, polarization sensitivity, sidewall angle, and bending radius were studied. The surface roughness of both the sidewall and the top/bottom of the fabricated waveguides were measured using laser confocal microscope, and the results showed that the averaged sidewall roughness was approximately 60 nm, which is 3 times that of the top/bottom surface. As a result, the sidewall roughness-induced LossR is 9 times that induced by the top/bottom roughness. The calculated value of LossR agrees well with the measured value. LossR increases rapidly with the decrease in the waveguide width, especially when the waveguide width is reduced below 10 µm, at which the LossR is approximately 0.3 dB/cm. On the other hand, the dependence of LossR on the waveguide height is negligible. Our results provide guidance for developing single-mode polymer waveguides with low loss for optical interconnect applications.

Research of Optical Interconnection Technology in Datacenter Networks

In recent years, with rapid development of cloud computing, the business volume of datacenter has increased unprecedentedly and the development of datacenter is facing great challenges. At this point, more efficient interconnection schemes are needed to maintain the increasing demand for communication bandwidth. With its great bandwidth advantage, optical fiber provides low-loss and low-cost solutions for telecom networks. At the same time, compared with the electric interconnection based on copper wire, optical switching technology greatly improves the transmission rate of the communication network. In this paper, the interconnection structure of the datacenter network is studied. Aiming at the interconnection for different top-of-racks (ToR), the c-Through and Helios structures based on photoelectric hybrid technology are reported. Moreover, some structures based on all-optical interconnection such as Proteus, OSMOSIS and IRIS are mentioned. For the interconnection in ToR, arrayed waveguide grating (AWG) based passive optical interconnection and the passive optical couplers-based interconnection schemes are described. Meantime, the main principles, advantages and disadvantages of various structures are analyzed. Finally, the future development of optical interconnection applications is stated.

Liquid crystal bifocal lens with adjustable intensities through polarization controls.

In this Letter, transverse and longitudinal liquid crystal bifocal lenses (LCBLs) are proposed to continuously control the relative intensity of two foci through a simple polarization control. The modulation of a LCBL comes from the geometric phase control and is designed through the principle of holography, where the object wave is a light field from two foci respectively formed by the left-circular polarized (LCP) and right-circular polarized (RCP) light, and the reference wave is the incident plane wave. Constructed millimeter-scale LCBLs are verified experimentally, and the foci are precisely formed at the preset plane. Besides, the relative intensity can be easily controlled with different weights of LCP and RCP light. The proposed strategy overcomes the shortcomings of previous bifocal lenses, such as a complex design method, a long optimization time, and an unchangeable relative intensity, and it is expected to find potential applications in parallel optical processing and optical interconnections.

Near-zero-index media as electromagnetic ideal fluids

Near-zero-index (NZI) supercoupling, the transmission of electromagnetic waves inside a waveguide irrespective of its shape, is a counterintuitive wave effect that finds applications in optical interconnects and engineering light-matter interactions. However, there is a limited knowledge on the local properties of the electromagnetic power flow associated with supercoupling phenomena. Here, we theoretically demonstrate that the power flow in two-dimensional (2D) NZI media is fully analogous to that of an ideal fluid. This result opens an interesting connection between NZI electrodynamics and fluid dynamics. This connection is used to explain the robustness of supercoupling against any geometrical deformation, to enable the analysis of the electromagnetic power flow around complex geometries, and to examine the power flow when the medium is doped with dielectric particles. Finally, electromagnetic ideal fluids where the turbulence is intrinsically inhibited might offer interesting technological possibilities, e.g., in the design of optical forces and for optical systems operating under extreme mechanical conditions.

Design and analysis of polarization independent MMI based power splitter for PICs

An ultracompact and low loss polarization-independent 1 ​× ​2 power splitter based on multimode interference (MMI) is designed and analyzed using Eigenmode expansion (EME) and variational FDTD method on silicon on insulator. The numerical tool shows that the excess loss of the power splitter is 0.04 ​dB for TE and 0.06 ​dB for TM polarization at wavelength 1550 ​nm while the polarization differential loss (PDL) is ​< ​0.07 ​dB over wavelength 1500 ​nm–1600 ​nm. Moreover, the optimized MMI structure has a fabrication tolerance of width ±150 ​nm and a length of ±800 ​nm. In addition, polarization-independent arbitrary power splitter (APS) is designed using cascaded MMI and tapered waveguide as a phase shifter having excess loss ​< ​0.16 ​dB for TE and <0.21 ​dB for TM mode at wavelength 1550 ​nm. The proposed devices have broad applications in optical interconnect and switches for PICs.

Athermal Polysilicon Mach-Zehnder Interferometer in Bulk CMOS

This study demonstrates the first athermal polysilicon Mach-Zehnder interferometer (MZI) based on strip and subwavelength grating waveguides in commercialized bulk CMOS without any process modification. As-fabricated MZI exhibits a temperature sensitivity of ±4 pm/°C over 1500~1600 nm wavelength range with a slope of -0.055 pm/°C/nm. This wavelength filtering device is therefore immune from the temperature gradient issue arising from the nearby electronic circuits, thus is promising for practical on-chip optical interconnect and sensor applications in bulk CMOS.

Thermally-Reconfigurable Silicon Photonic Devices and Circuits

Reconfigurable photonic integrated devices and circuits are very important for many applications. Among various mechanisms for realizing reconfigurability, thermo-optic effect is popular because of the availability for many materials, the design simplicity and the fabrication ease. In particular, silicon has a large thermo-optic coefficient as well as a high heat conductivity, and thus it is promising to realize efficient thermally-reconfigurable silicon photonic integrated devices and circuits. Recent progress is reviewed in this paper. First, thermally-tunable silicon photonic filters based on different structures are summarized, including microring resonators (MRRs) and MRRs-assisted Mach-Zehnder interferometers (MZIs). Second, silicon photonic switches based on MZIs and MRRs are reviewed. Finally, a review is given for thermally-reconfigurable silicon photonic integrated circuits developed for the applications in quantum photonics, microwave photonics, and optical interconnects.

Quarter-Millimeter Propagating Plasmons in Thin-Gold-Film-Based Waveguides for Visible Spectral Range

A series of long-range plasmonic waveguides based on a thin three-layer Al2O3/gold/Al2O3 film operating in the visible spectral range have been studied experimentally and theoretically. For long-range plasmonic waveguides having identical structure but different lengths, the propagation length of plasmonic waves was determined to be $\text{250} \pm \text{15} \mu {\text{m}}$ at $\lambda = \text{765} {\text{nm}}$ and was in a very good agreement with the theoretical value obtained through numerical modeling. The demonstrated propagation length is the highest one for the gold film-based plasmonic waveguides described in the literature for a uniform (non-photonic-crystal) substrate in the UV, visible, and near IR (up to $\text{1} \mu {\text{m}}$ ) spectral ranges, which is due to high technological quality of the fabricated waveguides and optimized three-layer structure. The proposed waveguides could find applications in optical interconnects and guided-wave optical sensors.

2D materials integrated with metallic nanostructures: fundamentals and optoelectronic applications

Abstract Due to their novel electronic and optical properties, atomically thin layered two-dimensional (2D) materials are becoming promising to realize novel functional optoelectronic devices including photodetectors, modulators, and lasers. However, light–matter interactions in 2D materials are often weak because of the atomic-scale thickness, thus limiting the performances of these devices. Metallic nanostructures supporting surface plasmon polaritons show strong ability to concentrate light within subwavelength region, opening thereby new avenues for strengthening the light–matter interactions and miniaturizing the devices. This review starts to present how to use metallic nanostructures to enhance light–matter interactions in 2D materials, mainly focusing on photoluminescence, Raman scattering, and nonlinearities of 2D materials. In addition, an overview of ultraconfined acoustic-like plasmons in hybrid graphene–metal structures is given, discussing the nonlocal response and quantum mechanical features of the graphene plasmons and metals. Then, the review summarizes the latest development of 2D material–based optoelectronic devices integrated with plasmonic nanostructures. Both off-chip and on-chip devices including modulators and photodetectors are discussed. The potentials of hybrid 2D materials plasmonic optoelectronic devices are finally summarized, giving the future research directions for applications in optical interconnects and optical communications.

Laser Characteristics for VCSELs for 77 K and 4 K Optical Data Applications

Small-sized vertical-cavity surface-emitting laser (VCSEL) offer the possibility of very low power consumption along with high reliability for cryogenic data transfer. Cryogenic data transfer has applications in focal plane array cameras operating at 77 K, and at the lower temperature of 4 K for data extraction from superconducting circuits. A theoretical analysis is presented for 77 K and 4 K operation based on small cavity, oxide-free VCSEL sizes of 2 to $6~\mu \text{m}$ , that have been shown to operate efficiently at room temperature. Temperature dependent operation for optimally-designed VCSELs are studied by calculating the response of the laser at 77 K and 4 K to estimate their bias conditions needed to reach modulation speed for cryogenic optical links. The temperature influence is to decrease threshold for reducing temperature, and to increase differential gain for reducing temperature. The two effects predict very low bias currents for small cavity VCSELs to reach needed data speed for cryogenic optical data links. Changing the number of top-mirror pairs has also been studied to determine how cavity design impacts speed and bit energy. Our design and performance predictions paves the way for realizing highly efficient, ultra-small VCSEL arrays with applications in optical interconnects.

Split-enabled 350–630 Gb/s optical interconnect with direct detection NOMA-CAP and 7-core multi-core fiber

The ever-growing data traffic volume inside data centers caused by the popularization of cloud services and edge computing demands scalable and cost-efficient network infrastructures. With this premise, optical interconnects have recently gained more and more research attention as a key building block to ensure end-to-end energy efficient solutions, offering high throughput, low latency and reduced energy consumption compared to current networks based on active optical cables. An efficient way for performing such optical interconnects is to make use of multi-core fibers (MCFs), which enables the multiplexing of several spatial channels, each using a different core inside the same fiber cladding. Moreover, non-orthogonal multiple access combined with multi-band carrierless amplitude and phase modulation (NOMA-CAP) has been recently proposed as a potential candidate to increase the network capacity and an efficiency/flexibility resource management. In this paper, using direct detection we experimentally demonstrate the transmission of NOMA-CAP signals through a 2 km MCF with 7 spatial channels for high capacity optical interconnect applications. The results show negligible transmission penalty for different total aggregated traffics ranging from 350 Gb/s to 630 Gb/s.

Broadband optical modulation in a zinc-oxide-based heterojunction via optical lifting

Broadband electro-absorptive optical modulation in a zinc-oxide (ZnO)-based heterojunction is demonstrated. The presence of high-density two-dimensional electron gas (2DEG) at the bottom of conduction band offset in a MgZnO-CdZnO heterojunction is shown to exhibit electrically tunable optical absorption in the regions well below the material bandgap. Electrons confined near the heterojunction are lifted across the potential barrier on optical excitation. Optical modulation with an extinction ratio of 8 dB is reported at an operating wavelength of 527 nm. The extinction ratio remains around 7–8 dB over a wide bandwidth of 115 nm, providing an excellent opportunity to explore ZnO-based heterojunctions to realize a broadband optical modulator for applications in optical communication and interconnects.

Fluorescent incandescent light sources from individual quadrilateral ZnO microwire via Ga-incorporation.

By means of nanophotonics principle, the thermal radiation can be tailored, thus, traditional tungsten lamp light source can glow the vitality and the vigor due to the low-efficiency approaching to commercial fluorescent or light-emitting diode bulbs. However, too far by demanding exacting terms, such as high-temperature thermal radiation (∼ 3000 K), high-vacuum encapsulation technology, restricted spectrally controllable source and so on, tungsten-based incandescent lamp filament has greatly limited the application in lighting, diagnosis and treatment, communication, imaging, etc. Herein, individual Ga-doped ZnO microwires (ZnO:Ga MWs) were successfully synthesized, which can be utilized to construct typical incandescent sources. By adjusting the Ga-incorporation, lighting colors are tuned in the visible spectral band. Especially, by incorporating Au quasiparticle nanofilms, the incandescent lighting features can further be modulated, such as the emission peaks, the modulation of lighting regions. Therefore, individual ZnO:Ga MWs based incandescent emitters can undertake a new function of the oldest, affordable and easily prepared light sources. While preliminary, individual ZnO:Ga MWs being treated as efficient incandescent light sources, can also open up intriguing scientific questions, and possible applications of linear, transparent, flexible displays and optical interconnects with electronic circuits.

Circular core single-mode 3-dimensional crossover polymer waveguides fabricated with the Mosquito method.

We experimentally fabricate circular core 3-dimensional (3D) crossover single-mode polymer optical waveguides using a photomask free unique fabrication technique named the Mosquito method for realizing channel shuffling. The 3D crossover structure is accomplished by forming four cores (2 ch. × 2 ch.) with different heights: the last two channels cross over the first two channels with horizontal and vertical core bending. We compare the insertion losses between the fabricated 3D single-mode crossover waveguide and 3D S-bend core waveguides fabricated separately, which correspond to the lower and upper channels in the crossover waveguide. Then, we investigate the effect of the core crossover on the loss, and find that almost negligible additional loss is observed. The average insertion losses of this 6-cm long 3D crossover single-mode waveguide are 3.95 and 3.81 dB at 1310 nm, and 5.74 and 4.80 dB at 1550-nm wavelength, for the lower and upper channels, respectively. The interchannel crosstalk in this crossover waveguide is observed to be lower than -40 dB, while the 1 dB radial alignment tolerance is ± 1.7 and ± 2.1 µm at 1310 and 1550 nm, respectively. These results suggest that the fabricated circular core single-mode 3D crossover polymer waveguides could have a great impact for high-bandwidth-density on-board and inter-chip optical interconnect applications.

Perspective: optically-pumped III–V quantum dot microcavity lasers via CMOS compatible patterned Si (001) substrates

Direct epitaxial growth III–V quantum dot (QD) structures on CMOS-compatible silicon substrates is considered as one of the most promising approaches to achieve low-cost and high-yield Si-based lasers for silicon photonic integration. However, epitaxial growth of III–V materials on Si encounters the following three major challenges: high density of threading dislocations, antiphase boundaries and thermal cracks, which significantly degrade the crystal quality and potential device performance. In this review, we will focus on some recent results related to InAs/GaAs quantum dot lasers on Si (001) substrates by III–V/IV hybrid epitaxial growth via (111)-faceted Si hollow structures. Moreover, by using the step-graded epitaxial growth process the emission wavelength of InAs QDs can be extended from O-band to C/L-band. High-performance InAs/GaAs QD micro-disk lasers with sub-milliwatts threshold on Si (001) substrates are fabricated and characterized. The above results pave a promising path towards the on-chip lasers for optical interconnect applications.

Analysis of Optical Integration between Si3N4 Waveguide and a Ge-Based Optical Modulator Using a Lateral Amorphous GeSi Taper at the Telecommunication Wavelength of 1.55 µm

We report on the theoretical investigation of using an amorphous Ge0.83Si0.17 lateral taper to enable a low-loss small-footprint optical coupling between a Si3N4 waveguide and a low-voltage Ge-based Franz–Keldysh optical modulator on a bulk Si substrate using 3D Finite-Difference Time-Domain (3D-FDTD) simulation at the optical wavelength of 1550 nm. Despite a large refractive index and optical mode size mismatch between Si3N4 and the Ge-based modulator, the coupling structure rendered a good coupling performance within fabrication tolerance of advanced complementary metal-oxide semiconductor (CMOS) processes. For integrated optical modulator performance, the Si3N4-waveguide-integrated Ge-based on Si optical modulators could simultaneously provide workable values of extinction ratio (ER) and insertion loss (IL) for optical interconnect applications with a compact footprint.