Optical Interconnect Applications Research Articles

Circular-core single-mode polymer waveguide for high-density and high-speed optical interconnects application at 1550 nm.

We demonstrate circular-core 1550 nm single-mode polymer waveguides with a graded-index profile fabricated by commercially available UV-curable epoxies using so-called mosquito method. The relative index difference ∆n of the waveguides was designed to be 0.46% in order to guarantee both single-mode operation and good compatibility with standard single-mode fiber. Accurate refractive index tuning of monomer for core construction was realized by mixing the core and cladding epoxies properly. The core pitch of the waveguides was chosen to be 50 μm to satisfy the requirements for high-density on-board optical interconnects. Both the optical characteristics and high-speed performances of the waveguides were comprehensively studied at 1550 nm. The measured transmission and coupling loss are 0.79 dB/cm and 0.78 dB, respectively. The waveguides exhibit an inter-channel crosstalk as low as -45 dB, and a 3 dB misalignment tolerance larger than ± 4 μm on the input and output facet in both horizontal and vertical directions. NRZ signal at a data rate of 25 Gb/s was transmitted over a 10 cm-long waveguide. There is no obvious degradation on the eye diagram due to the insertion of the waveguide and error free transmission was successfully obtained. Our results imply that the fabricated single-mode polymer waveguides have good potential in high-density and high-speed optical interconnects application.

Design of full-duplex and multifunction bidirectional CMOS transceiver for optical interconnect applications

This work presents a full-duplex and multifunction bidirectional transceiver for optical interconnect application. The transceiver utilizes a common limiting amplifier/gain stage, thus reducing total chip area and total power consumption. While providing a full-duplex bidirectional transmission with the aid of a hybrid circuit between the electrical input/output (I/O) and the optoelectronic signals from the transmitter and receiver circuits, it also allows for a half-duplex operation with the aid of a switch between the transimpedance amplifier signals and the transmitter electrical input from the I/O port. The multifunction bidirectional CMOS transceiver is designed in a 0.13 µm Si-CMOS technology, with power dissipation of 79 and 54.4 mW for the transmitter and receiver, respectively. It shows a 3-dB bandwidth of 5.58 and 5.69 GHz for the transmitter and the receiver respectively and with a 3-dB gain of 66.14 and 69.6 dB, in full-duplex mode. The transceiver operates up to 7 Gb/s in full-duplex mode.

Micron-scale Raman amplifier: another progressive step towards the maturity of silicon photonics

Notice of redundant publication "Micron-scale Raman amplifier: another progressive step towards the maturity of silicon photonics" It has been brought to the attention of the editors that this article contains redundancies when compared with an earlier article published by the authors in Datta T and Sen M 2017 Superlattices Microstruct. 109 107–16. The original article was not referenced. These concerns were identified after the article appeared as accepted but before publication of the final version. As a member of COPE this was been investigated in accordance with the COPE guidelines and it was agreed a notice of redundant publication should be applied. A new model for micron scale all-silicon amplifier has been proposed based on stimulated Raman scattering (SRS) in a slotted photonic crystal waveguide (SPCW). Silicon nanocrystal material is considered to be embedded in the slot of the SPCW to exploit its giant Raman gain coefficient. Moreover, the gain has further been enhanced by using the ultra-high optical confinement of the SPCW. Results of simulations show that the threshold power of a 3 μm long SPCW based Raman amplifier is in the order of merely 500 μW, while the CW Stokes is launched with a power of 50 μW. The simulations also show that a merely 1.5 μm long SPCW can offer ≈3 dB gain even at the 400 Gbps pulse repetition rate. However, pulse shape becomes disrupted by the SRS induced temporal broadening which has further been reduced considerably by operating the Stokes at a different resonance peak in the SRS gain spectrum. With these tremendous performances, the SPCW based Raman amplifier proves its potentiality for application in on-chip optical interconnects and also opens up a pavement for realizing the long desired micron-scale all-silicon Raman laser.

A vertical coupler for InP active components

A discussion about a coupler of a InP active component on a silicon-on-insulator (SOI) wafer is presented for applications in optical interconnects. A model for the design and evaluation of the coupler is developed using a coupling length based a model solver, the model solver is specifically suited for high index contrast waveguides with small area, which show more accurate for compact integrated optical devices. The photonic coupler is fabricated using microelectronics equipment for compatibility towards future generation electronic integrated circuit processing. Measured coupler efficiency is 65% to valid our model. This will be helpful for the design of InP active components.

Sn-based waveguide p-i-n photodetector with strained GeSn/Ge multiple-quantum-well active layer.

We report on Sn-based p-i-n waveguide photodetectors (WGPD) with a pseudomorphic GeSn/Ge multiple-quantum-well (MQW) active layer on a Ge-buffered Si substrate. A reduced dark-current density of 59 mA/cm2 was obtained at a reverse bias of 1V due to the suppressed strain relaxation in the GeSn/Ge active layer. Responsivity experiments revealed an extended photodetection range covering the O, E, S, C, and L telecommunication bands completely due to the bandgap reduction resulting from Sn-alloying. Band structure analysis of the pseudomorphic GeSn/Ge quantum well structures indicated that, despite the stronger quantum confinement, the absorption edge can be shifted to longer wavelengths by increasing the Sn content, thereby enabling efficient photodetection in the infrared region. These results demonstrate the feasibility of using GeSn/Ge MQW planar photodetectors as building blocks of electronic-photonic integrated circuits for telecommunication and optical interconnection applications.

Experimental Investigation of Stub Resonators Built in Plasmonic Slot Waveguides

In this work, we focus on stub resonators embedded in plasmonic slot waveguides. The resonators have potential applications in optical interconnects and sensors. We fabricate the samples by electron beam lithography and lift-off. We use a scattering matrix based model to quantify the optical power output from the samples. We measure the properties of the resonators by coupling light in and out of the slot waveguides by optical antennas, making use of a cross-polarization based setup utilizing a supercontinuum source and a high numerical aperture objective lens operating in the telecom-wavelength range. Our model agrees well with the measured data. Further development on the stub resonators can be made by using the methods in this paper.

Chip-integrated plasmonic Schottky photodetection based on hybrid silicon waveguides

We numerically and theoretically investigate the plasmonic Schottky photodetection in a novel hybrid silicon-on-insulator waveguide system, which consists of the silicon waveguides and detection area with the metal stripes and doped silicon film on the silicon dioxide substrate. The results illustrate that the fundamental TE mode in the silicon waveguide can be effectively coupled into the metal/silicon waveguide with the excitation of surface plasmon polaritons (SPPs). The coupling is suppressed for the TM mode due to the mismatch between the electric field distributions of the TM and SPP modes. It is found that the coupling efficiency from the TE to SPP mode is dependent on the width and height of the silicon waveguide and can significantly approach ~36.1%. The ultracompact configuration yields a high responsivity of ~21.7 mA/W and low dark current of ~0.45 μA for the photodetection at the communication wavelength. The plasmonic Schottky photodetector could find favorable applications in the chip-integrated optical interconnects and signal processing.

Demonstration of km-scale orbital angular momentum multiplexing transmission using 4-level pulse-amplitude modulation signals.

By designing and fabricating two kinds of orbital angular momentum (OAM) fibers, we demonstrate two OAM modes (OAM+1 and OAM-1) multiplexing transmission and demultiplexing in OAM fiber links. Moreover, we also experimentally demonstrate 4-level pulse-amplitude modulation (PAM-4) signal transmission using two OAM modes multiplexing in a km-scale OAM fiber and achieve a bit-error rate (BER) below 2×10-3 without multiple-input-multiple-output (MIMO) digital signal processing (DSP). The obtained results with favorable data-carrying OAM multiplexing transmission performance show potential application in km-scale short-reach optical interconnects.

Comprehensive vertical-cavity surface-emitting laser model for optical interconnect transceiver circuit design

Directly modulated vertical-cavity surface-emitting lasers (VCSELs) are commonly used in short-reach optical interconnect applications. To enable efficient optical interconnect transceiver systems operating at data rates up to 25 Gb/s and beyond, cosimulation environments, which allow for the optimization of driver circuitry with accurate compact VCSEL models, are necessary. A comprehensive VCSEL model, which captures thermally dependent electrical and optical dynamics and provides direct current, small-, and large-signal simulation capabilities with self-consistency, is presented. The device’s electrical behavior is described with an equivalent circuit, which captures both large-signal operation and electrical parasitics, while the optical response is captured with a rate-equation-based model. Bias and temperature dependencies are incorporated into both key electrical and optical model parameters. Experimental verification of the model is performed at 25 Gb/s with a 990-nm VCSEL to study the impact of bias current level and substrate temperature.

Deep proton writing of high aspect ratio SU-8 micro-pillars on glass

Deep proton writing (DPW) is a fabrication technology developed for the rapid prototyping of polymer micro-structures. We use SU-8, a negative resist, spincoated in a layer up to 720μm-thick in a single step on borosilicate glass, for irradiation with a collimated 12MeV energy proton beam. Micro-pillars with a slightly conical profile are irradiated in the SU-8 layer. We determine the optimal proton fluence to be 1.02×104μm−2, with which we are able to repeatably achieve micro-pillars with a top-diameter of 138±1μm and a bottom-diameter of 151±3μm. The smallest fabricated pillars have a top-diameter of 57±5μm. We achieved a root-mean-square sidewall surface roughness between 19nm and 35nm for the fabricated micro-pillars, measured over an area of 5×63.7μm. We briefly discuss initial testing of two potential applications of the fabricated micro-pillars. Using ∼100μm-diameter pillars as waveguides for gigascale integration optical interconnect applications, has shown a 4.7dB improvement in optical multimode fiber-to-fiber coupling as compared to the case where an air–gap is present between the fibers at the telecom wavelength of 1550nm. The ∼140μm-diameter pillars were used for mold fabrication with silicone casting. The resulting mold can be used for hydrogel casting, to obtain hydrogel replicas mimicking human tissue for in vitro bio-chemical applications.

Design of oxide-confined and temperature stable long wavelength Vertical Cavity Surface Emitting Laser for optical interconnects

One of the important attributes of Vertical Cavity Surface Emitting Laser for applications in optical interconnects is stable high temperature performance. In addition to temperature insensitivity, power dissipation is other major concern in the field of on-chip optical interconnects. With ever increasing chip size and functionalities, researchers are aggressively looking for temperature stability and power reduction in each department. Also the future emerging system of on-chip devices require long wavelength laser for high capacity applications. This paper deals with the designing of oxide-confined VCSEL structure operating at long wavelength i.e. 1310nm. Atlas Silvaco frame work is used to design VCSEL and to analyze its electrical and thermal behavior with state of the art modules. The static properties of the proposed VCSEL are analyzed at different oxide aperture diameter and temperature. It is observed that the future obligatory performance can be attained with this proposed design of VCSEL structure model even operating at low current densities and reduced oxide-aperture diameter equal to 3μm. The average error between the simulated and analytical power conversion efficiency is less than 5%.

Silicon Photonic Integrated Circuits for Wavelength-Division Multiplexing Applications

Silicon photonics will provide low-cost, high-bandwidth and compact optical components for a wide range of applications in optical communications and interconnects. One of the cited key advantages is the capability of wavelength-division multiplexing (WDM). However, the nature of high-index contrast of silicon photonic devices leads to significant challenges when implementing on-chip WDM filters, which is one of the key components in WDM circuits. In this paper, we review several demonstrated silicon photonic WDM circuits based on monolithically integrated silicon nitride (SiN) arrayed-waveguide gratings (AWGs) and thermally tunable silicon microring filters. The use of SiN waveguides with lower index contrast than silicon waveguides enables the realization of high-performance AWGs. Meanwhile, they can evanescently couple to silicon waveguides with high efficiency. The thermally tunable silicon microrings can be used as modulators and wavelength (de)multiplexing filters to implement versatile WDM circuits. Reconfigurability of channel spacing and central wavelengths is achieved by individual tuning of the rings. In this paper, we review silicon photonic circuits for multiple-channel modulators, polarization-insensitive WDM receiver, and variable optical attenuators with multiplexer.

Experimental Demonstration of Directive Si3N4 Optical Leaky Wave Antennas With Semiconductor Perturbations

Directive optical leaky wave antennas (OLWAs) fabricated in CMOS-compatible semiconductor planar waveguide technology have the potential to provide high directivity with electrical tunability promising for modulation and switching capabilities. We experimentally demonstrate directive radiation from a silicon nitride (Si3N 4) waveguide-based OLWA. The OLWA design comprises a crystalline silicon (Si) nanowire array buried inside a Si 3N4 waveguide. Each Si nanowire has a width of 260 nm and a height of 150 nm. The OLWA is designed to exhibit a directive radiation pattern at telecom wavelengths. The measured radiation pattern at the wavelength 1550 nm has its maximum emission intensity at the angle of 85.1° relative to the waveguide axis and a half-power beam width of approximately 5.0°, which are consistent with our theoretical predictions. The results indicate that the TM mode is more radiative than the TE mode in our fabricated devices. Also, the radiation pattern of the measured OLWA shows dependence on wavelength, which is typical of leaky wave antennas. The device is promising in chip-to-space optical interconnect applications.

Advances in communications using optical vortices

An optical vortex having an isolated point singularity is associated with the spatial structure of light waves. A polarization vortex (vector beam) with a polarization singularity has spatially variant polarizations. A phase vortex with phase singularity or screw dislocation has a spiral phase front. The optical vortex has recently gained increasing interest in optical trapping, optical tweezers, laser machining, microscopy, quantum information processing, and optical communications. In this paper, we review recent advances in optical communications using optical vortices. First, basic concepts of polarization/phase vortex modulation and multiplexing in communications and key techniques of polarization/phase vortex generation and (de)multiplexing are introduced. Second, free-space and fiber optical communications using optical vortex modulation and optical vortex multiplexing are presented. Finally, key challenges and perspectives of optical communications using optical vortices are discussed. It is expected that optical vortices exploiting the space physical dimension of light waves might find more interesting applications in optical communications and interconnects.

Silicon-plasmonic internal-photoemission detector for 40 Gbit/s data reception

Silicon-plasmonics enables the fabrication of active photonic circuits in CMOS technology with unprecedented operation speed and integration density. Regarding applications in chip-level optical interconnects, fast and efficient plasmonic photodetectors with ultrasmall footprints are of special interest. A particularly promising approach to silicon-plasmonic photodetection is based on internal photoemission (IPE), which exploits intrinsic absorption in plasmonic waveguides at the metal–dielectric interface. However, while IPE plasmonic photodetectors have already been demonstrated, their performance is still far below that of conventional high-speed photodiodes. In this paper, we demonstrate a novel class of IPE devices with performance parameters comparable to those of state-of-the-art photodiodes while maintaining footprints below 1 μm2. The structures are based on asymmetric metal–semiconductor–metal waveguides with a width of less than 75 nm. We measure record-high sensitivities of up to 0.12 A/W at a wavelength of 1550 nm. The detectors exhibit opto-electronic bandwidths of at least 40 GHz. We demonstrate reception of on–off keying data at rates of 40 Gbit/s.

Simultaneous Wavelength- and Mode-Division (De)multiplexing for High-Capacity On-Chip Data Transmission Link

We present designs of wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) optical links using mode de/multiplexers (DE/MUXs) based on multimode interference (MMI) couplers with a tilt joint as a phase shifter. The properties of WDM-MDM links with three wavelengths and two optical modes are numerically studied by using 3-D beam propagation method. In the first design, the wavelength combiner is also an MMI coupler type. The insertion loss of the design is around 6 dB because of the inherent loss of the MMI combiner. The size of the main passive optical parts (mode DE/MUXs and wavelength combiner) of the design is only 10.4 × 114.2 μm 2 , making the design promising for future compact and high-capacity optical interconnection applications. In another design, arrayed waveguide gratings are used for wavelength multiplexing, leading to a lower insertion loss of the optical connections. For both designs, the mode crosstalk between the two different modes for the same wavelength are below -22 dB.

Design and fabrication of 1.55 μm broad area slotted single-mode Fabry–Perot lasers

We present a single-mode laser on a p-InP substrate suitable for bonding on silicon-on-insulator (SOI) wafer. The laser can realize single mode lasing with etching perturbing slots by standard photolithography and an inductively coupled-plasma (ICP) etching technique without any regrowth steps. The parameters were designed using the simulation tool “cavity modeling framework” (CAMFR). The single mode of 1539 nm wavelength at the threshold current of 130 mA with the maximum output power of 3.9 mW was obtained at 10 °C in continuous-wave operation. The simple technology, low cost and the single-mode characteristics make the broad area slotted single-mode FP laser a promising light source on the silicon-based optical interconnection applications.

Integrated photonics with programmable non-volatile memory.

Silicon photonics integrated circuits (Si-PIC) with well-established active and passive building elements are progressing towards large-scale commercialization in optical communications and high speed optical interconnects applications. However, current Si-PICs do not have memory capabilities, in particular, the non-volatile memory functionality for energy efficient data storage. Here, we propose an electrically programmable, multi-level non-volatile photonics memory cell (PMC) fabricated by standard complementary-metal-oxide-semiconductor (CMOS) compatible processes. A micro-ring resonator (MRR) was built using the PMC to optically read the memory states. Switching energy smaller than 20 pJ was achieved. Additionally, a MRR memory array was employed to demonstrate a four-bit memory read capacity. Theoretically, this can be increased up to ~400 times using a 100 nm free spectral range broadband light source. The fundamental concept of this design provides a route to eliminate the von Neumann bottleneck. The energy-efficient optical storage can complement on-chip optical interconnects for neutral networking, memory input/output interfaces and other computational intensive applications.

Precise identification of directions on Si{0 0 1} wafer using a novel self-aligning pre-etched technique

Micromirrors with a tilt angle of 45° are widely used in optical switching and interconnect applications which require 90° out of plane reflection. Silicon wet bulk micromachining based on surfactant added TMAH is usually employed to fabricate 45° slanted walls at the direction on Si wafers. These slanted walls are used as 45° micromirrors. However, the appearance of a precise 45° wall is subject to the accurate identification of the direction. In this paper, we present a simple technique based on pre-etched patterns for the identification of directions on the Si surface. The proposed pre-etched pattern self-aligns itself at the direction while becoming misaligned at other directions. The direction is determined by a simple visual inspection of pre-etched patterns and does not need any kind of measurement. To test the accuracy of the proposed method, we fabricated a 32 mm long rectangular opening with its sides aligned along the direction, which is determined using the proposed technique. Due to the finite etch rate of the plane, undercutting occurred, which was measured at 12 different locations along the longer edge of the rectangular strip. The mean of these undercutting lengths, measured perpendicular to the mask edge, is found to be 13.41 μm with a sub-micron standard deviation of 0.38 μm. This level of uniform undercutting indicates that our method of identifying the direction is precise and accurate. The developed method will be extremely useful in fabricating arrays of 45° micromirrors.

Design and analysis of a multichannel transceiver for high-speed optical interconnects

In this paper, we designed a multichannel transceiver for high-speed optical interconnect applications. The proposed multichannel transceiver is designed in 0.13 µm CMOS technology. The multichannel transceiver consists of 4-channel transmitter (Tx) and 4-channel receiver (Rx). The simulated Tx has a 3-dB BW of 5.85 GHz with a voltage gain of 35 dB and the measured 3-dB BW is 5.20 GHz. The simulated Rx has the 3-dB BW of 5.95 GHz with a transimpedance gain of 80 dBΩ and the measured 3-dB BW of Rx is 5.25 GHz. To improve frequency operation of Rx, transimpedance amplifier (TIA) active feedback has been applied. Utilization of an active feedback in TIA significantly improves the frequency operation, where 3-dB BW has been improved from 3.27 to 11 GHz with transimpedance gain of 65 dBΩ. Furthermore, to improve the output light of performance temperature sensing circuit has also been integrated in the same silicon substrate with transceiver chip to provide stable output light power temperature range of −30 ~ 100 °C.