1886 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 30, NO. 21, NOVEMBER 1, 2018 Vertical Optical Coupling Structure Based on Polymer for Small Size and High-Speed Photodetectors Carlos Viana , Zerihun Gedeb Tegegne , Jacopo Nanni , Giovanni Tartarini, Catherine Algani, Frédéric Marty, Nicolas Pavy, and Jean-Luc Polleux Abstract—This letter presents a new collective and passive thin-film 3D vertical optical coupling structure, which is fab- ricated based on SU-8 polymer. This technology compensates for the alignment errors of the present passive optical coupling solutions. The structure is collectively manufactured on a matrix of small-sized photodiodes on wafer level to vertically couple with a single-mode optical fiber. The proposed structure shows a coupling efficiency between the 10-μm active area diameter pho- todetector and single-mode fiber of 94% and a 1-dB alignment tolerance of ±9 μm at 1550 nm. Its wafer-level mass-production capability and manufacturing process simplicity make this novel coupling approach a potential solution for short- and long-range optical communication applications. Index Terms— Optical couplers, optical interconnections, passive alignment, polymer, vertical optical couplers. I. I NTRODUCTION S HORT and long range optical communications are playing a great role in our day to day life. As a result, high speed optoelectronic components have a large growing market to be implemented in such systems. Hence, there is a continuous effort to increase the performance (data rate) and reduce the costs of such optoelectronics devices simultaneously. The dimensions of these components regularly define their speed (the smaller the device size means higher the speed). When the dimensions of those active devices are shrunk down and speed is increased, the optical coupling and packaging is then a key challenge. Manuscript received July 16, 2018; revised August 20, 2018; accepted September 6, 2018. Date of publication September 19, 2018; date of current version November 6, 2018. This work was supported in part by SATT IdF- Innov as a maturation project and in part by FUI-ORIGIN and FUI-MORF projects. (Corresponding author: Zerihun Gedeb Tegegne.) C. Viana is with ICON Photonics, 77420 Champs-sur-Marne, France (e-mail: carlos.viana@icon-photonics.com). Z. G. Tegegne, C. Algani, F. Marty, and N. Pavy are with Université Paris-Est, ESYCOM, ESIEE Paris, 93160 Noisy-le-Grand, France (e-mail: zerihungedeb.tegegne@esiee.fr; catherine.algani@cnam.fr; frederic.marty@esiee.fr; nicolas.pavy@esiee.fr). J. Nanni is with Université Paris-Est, ESYCOM, ESIEE Paris, 93160 Noisy-le-Grand, France, and also with the Department of Electrical, Electronic and Information Engineering, University of Bologna, 40136 Bologna, Italy (e-mail: jacopo.nanni3@unibo.it). G. Tartarini is with the Department of Electrical, Electronic and Information Engineering, University of Bologna, 40136 Bologna, Italy (e-mail: giovanni.tartarini@unibo.it). J.-L. Polleux is with ICON Photonics, 77420 Champs-sur-Marne, France, and also with Université Paris-Est, ESYCOM, ESIEE Paris, 93160 Noisy-le-Grand, France (e-mail: jl.polleux@esiee.fr). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2018.2871268 The conventional vertical optical coupling techniques such as butt-coupling and microlens (based on active alignment) are performance-dependent on the device aperture diameter which will have an important impact on the total cost of the module [1]–[5]. When coupling 10 × 10 μm 2 active area photodetector with multimode fibre (MMF) at 850 nm, a coupling efficiency as low as to 35 % is expected as in [6]. VCSEL butt-coupling to a MMF and SMF can be found up to 80 % and 47 % coupling efficiency respectively; with its lateral tolerances for 1 dB alignment tolerance of ±5 μm and 2 μm resp. [6]. The use of microlenses or other complex light guiding or confinement system is also considered to improve the coupling efficiency [1], [2], which adds however extra cost and complexity since one more elements must be placed and aligned between the device and the fiber. Direct integration techniques have been also developed in order to explore the collective fabrication and integration at the chip/wafer level. An example for such approach is a vertical optical coupler to planar photodiode using an amorphous Si waveguide grating coupler with SMF of 850 nm, which is presented in [8] with the coupling efficiency of 80 %. Another example is the monolithic lenses fabricated in the back of the VCSEL/PD substrate [3]. The main drawback of the latter solution is that it is restricted to transparency of substrate and also puts constraints on the substrate thickness which limits the active area and the device bandwidth. Recently, direct integration of polymer materials fabricated on the top of devices got a great interest regarding potential on the cost-performance trade-off point of view [5], [9]–[11]. Those techniques are based on UV light with a photomask transfer method (at a wafer level scale process) to create polymer pillars on the top of the active devices with precision less than 1μm. Although these are promising techniques, the dimensions are still high compared to active areas of the high speed photodetectors when the side is shrinking down to less than 10 μm. This letter presents a novel vertical optical fiber coupling technology for small size active area and high-speed photodetectors. It proves a passive optical alignment method in a collective wafer-level manufacturing (mass-production) process. The proposed structure is a vertical conical-shape taper for top side illuminated photodetectors. The first part of this letter describes the design and simulation results of the polymer coupling structure. The second part describes the technology and manufacturing process. Finally, 1041-1135 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.