Shenghui Lei

A Vision for Thermally Integrated Photonics Systems

Thermal management has traditionally been relegated to the last step in the design process. However, with the exponential growth in data traffic leading to ever-greater levels of component integration and ever-higher levels of energy consumption, thermal management is rapidly becoming one of the most critical areas of research within the ICT industry. Given the vast use of optics for efficient transmission of high-speed data, this paper focuses on a new thermal solution for cooling the components within pluggable optical modules. Thermally Integrated Photonics Systems (TIPS) represents a new vision for the thermal building blocks required to enable exponential traffic growth in the global telecommunications network. In the TIPS program, existing thermal solutions cannot scale to meet the needs of exponential growth in data traffic. The main barriers to enabling further growth were identified and a research roadmap was developed around a scalable and efficient integrated thermal solution. In particular, the effects of replacing inefficient materials and large macroTECs with better thermal spreaders and μTECs are investidated. In addition, new forms of μChannel cooling into the package to more efficiently remove the heat generated by the lasers and the TECs are being studied which can lead to future photonic devices that can be deployed in a vastly more dense and integrated manner to address the requirements of future telecommunication networks.

Reducing thermal crosstalk in ten-channel tunable slotted-laser arrays

Given the tight constraints on the wavelength stability of sources in optical networks, the thermal crosstalk between operating devices in a ten-channel thermally-tunable slotted laser array for DWDM applications has been investigated. It was found experimentally the current standard thermal solution with the laser array chip mounted on an AlN carrier does not allow for wavelength stability of ± 25 GHz ( ± 2 K) with a temperature rise of 5 K measured in a device with 100 mA (CW) applied to a neighbouring laser (device spacing = 360 µm). A combined experimental/numerical approach revealed solid state submounts comprising diamond or highly ordered pyrolytic graphite are inadequate to reduce crosstalk below an allowable level. Numerical simulations of advanced cooling technologies reveal a microfluidic enabled substrate would reduce thermal crosstalk between operational devices on the chip to acceptable levels. Critically our simulations show this reduced crosstalk is not at the expense of device tunability as the thermal resistance of individual lasers remains similar for the base and microfluidic cases.

Towards AlN optical cladding layers for thermal management in hybrid lasers

Aluminium Nitride (AlN) is proposed as a dual function optical cladding and thermal spreading layer for hybrid ridge lasers, replacing current benzocyclobutene (BCB) encapsulation. A high thermal conductivity material placed in intimate contact with the Multi-Quantum Well active region of the laser allows rapid heat removal at source but places a number of constraints on material selection. AlN is considered the most suitable due to its high thermal conductivity when deposited at low deposition temperatures, similar co-efficient of thermal expansion to InP, its suitable refractive index and its dielectric nature. We have previously simulated the possible reduction in the thermal resistance of a hybrid ridge laser by replacing the BCB cladding material with a material of higher thermal conductivity of up to 319 W/mK. Towards this goal, we demonstrate AlN thin-films deposited by reactive DC magnetron sputtering on InP.

Apparent slip enhanced magnetohydrodynamic pump

Reliable thermal management is a key aspect in the operation of telecommunications equipment, both photonic and electronic. In liquid-cooled thermal management, conventional pump designs incorporate moving parts, introducing reliability concerns. The magnetohydrodynamic (MHD) phenomenon can be used to fabricate highly reliable, compact, solid-state pumps that avoid moving parts and make it suitable for rack level integration in telecommunications equipment. However, pumping efficiency and performance is limited by pressure losses in the pump. Here we explore the performance of an MHD pump demonstrating apparent hydrodynamic and thermal slip on its major walls (perpendicular to the magnetic field) that can be achieved using micro-nano structured surfaces to reduce pressure losses due to friction. Our initial analysis suggests that, while the pump section performance can be significantly enhanced by apparent slip, the overall pump performance is strongly dictated by the nature of pressure losses associated with the fringing magnetic field at the pump inlet and outlet such that enhancement due to apparent slip is only manifested for relatively long pump sections.

2D Lattice Boltzmann Simulation of Droplet Jumping in a Viscous Fluid

In order to investigate the coalescence and out-of-plane jumping of two incompressible droplets on a non-wetting surface surrounded by an incompressible fluid with matched viscosity in the low Ohnesorge number regime, a two-dimensional lattice Boltzmann phase-field model is implemented. An interfacial force of potential form is used to model the internal surface tension force and capture the fluid-surface interaction, viz. the contact-line dynamics. We evaluate the simulated velocity fields and interface shape evolution during coalescence and the subsequent jumping event. We confirm that the coalescence dynamics of the binary droplet system is similar to the case where the outer fluid viscosity is small compared to that of the droplet fluid, as is the case of condensed water droplet jumping on superhydrophobic surfaces in a gaseous ambient. An argument is also developed to demonstrate that the dynamics in 2D, when appropriately scaled, should be approximately equivalent to the corresponding 3D case. A simple drag model is used to capture the rapid velocity decay of the jumping droplet as it moves away from the surface into the viscous fluid. The results suggest the possibility of experimentally observing coalescence-induced droplet jumping in liquid-liquid systems that may be potentially exploited for microfluidic applications.Copyright

Mixing enhancement due to viscoelastic instability in serpentine microchannels at very large Weissenberg numbers

The flow of shear-thinning viscoelastic fluids is investigated experimentally in a serpentine microchannel at very large Weissenberg numbers (Wi > 104) undergoing elastic instability. The effects of geometric curvature on local flow instability and the consequent heat transfer enhancement are reported. Unlike previous studies where fluids with large zero-shear viscosities (up to 300 mPa.s) were used, we employ a working fluid with a lower viscosity (η0 = 9 mPa.s) more suited to microfluidic heat transfer applications while exhibiting viscoelastic characteristics. This results in Elasticity number (EI = Wi/Re) flows an order of magnitude larger than previously reported in the literature with apparent viscosities close to the solvent viscosity under flow conditions. Detailed Micro Particle Image Velocimetry (μPIV) measurements reveal the local enhancements due to instantaneous flow structures which result in vigorous local mixing at sub-critical Reynolds numbers. In addition the pressure drop increase is moderate as mixing occurs locally and the flow is maintained undisturbed elsewhere throughout the flow path.

ALN thin-films as heat spreaders in III–V photonics devices Part 2: Simulations

In the paper, we aim to solve the thermal problems appearing in integrated silicon photonics by using high thermal conductivity Aluminium Nitride (ALN) as a thermal spreading layer located around the ridge of a hybrid III-V laser on silicon in comparison to the existing encapsulation material benzocyclobutene (BCB). Here, to facilitate the design of reliable hybrid semiconductor lasers, we first develop and implement a multiphysics electro-thermo-mechanical model within a finite element environment COMSOL. A phenomenological model of laser operation is used to numerically capture all the thermal and electrical characteristics of the lasers. In terms of the hybrid devices, the simulated thermal resistance agrees well with our device measurements presented in Part 1 of this work. We also demonstrate that the use of the ALN heat spreader can significantly reduce the thermal resistance. Moreover, a linear elastic model is employed for a mechanical analysis of the entire laser structure. The maximum allowable stress is estimated using the Christensen criterion. We find that the process-dependent residual stress dictates the device stress field. In the current design, the BCB encapsulation layer is at risk of failure around the InP waveguide. For AlN spreaders, lower film processing temperatures are key to reduce the stress in the deposited film. We further perform a parametric study on Tref to determine the maximum allowable deposition temperature of AlN/BCB. The simulations suggest that Tref should not exceed 59 °C and 69 °C for ALN and BCB respectively to avoid mechanical failure in the devices.

Design of heated-micro-resonator rings

To facilitate the design the heated-micro-resonator rings, we develop a numerical model. Particularly, an etching model based on a diffusion equation is implemented to distinguish the etched and non-etched regions. The simulations reveal that air trenches can significantly increase the thermal resistance and, as a result, reduce the power consumed in the heater to get the same tuning performance. It is found that the tunability of MRR is exponentially enhanced with the underetch level as RL > 0.825, whereas the tuning efficiency is almost the same, as RL <; 0.825. The tuning efficiency is ~0.1 nm/mW without air trenches, but it becomes 20× larger at the etched-through extreme.

Numerical simulations of viscoelastic flow over a confined cylinder in microchannels

In this study, several 2D numerical simulations on a non-Newtonian flow over a confined cylinder placed in a rectangular microchannel are carried out at different Weissenberg (Wi) numbers. In particular, the Oldroyd-B model implemented in open source code OpenFOAM is employed to capture the three basic ingredients of polymer rheology, viz., anisotropy, elasticity and relaxation. Numerical calculations indicate that the flow structure particularly in the downstream is influenced by the presence of the cylinder. As Wi or the channel height increases, the velocity-recovery length required increases. It is observed that both the pressure drop across the channel and the elastic stress magnitude in the downstream grow exponentially with Wi. However it is interesting to observe that recirculation zones appear at Wi = 1.2 with a modest increase in pressure drop compared to Newtonian flow.