FTu1D.2.pdf Frontiers in Optics/Laser Science 2016 © OSA 2016

A compliant polymer interface with 1.4dB loss between standard fibers and nanophotonic waveguides

Tymon Barwicz1,*, Alexander Janta-Polczynski2, Shotaro Takenobu3, Jean-François Morissette2,4,

Bo Peng1, Yoichi Taira1, Hidetoshi Numata5, Swetha Kamlapurkar1, Sebastian Engelmann1, Paul Fortier2, and Nicolas Boyer2

1IBM T.J. Watson Research Center, 1101 Kitchawan Rd., Yorktown Heights, NY 10598 USA 2IBM Bromont, 23 Boul. de l’Aeroport, Bromont, QC J2L 1A3 Canada

3Asahi Glass Co., AGC Electronics, Technol. Gen. Div. 1150 Hazawa-cho, Kanagawa-ku, Yokohama, 221-8755 Japan 4Université de Sherbrooke, Sherbrooke, Qc Canada

5IBM Research – Tokyo, 7-7 Shin-Kawasaki, Saiwai-ku, Kawasaki, Kanagawa, 212-0032 Japan *tymon@us.ibm.com

Abstract: We demonstrate improved performance in interfacing standard fibers with nanophotonic waveguides through a mechanically compliant polymer interface. We show -1.4dB peak transmission with 0.8dB penalty over the ~100nm bandwidth measured and all polarizations. OCIS codes: (130.3120) Integrated optics devices; (250.5300) Photonic integrated circuits

Challenges in cost and scalability of photonic packaging hinder the deployment of photonic circuits. To reach the potential of silicon photonics, disruptive improvements are required in cost-efficiency and in scalability of both optical port count and manufacturing volume. We have recently proposed a novel approach to interfacing standard fibers with nanophotonic waveguides [1]. As shown in Fig. 1, it involves polymer waveguides lithographically defined on a flexible ribbon. This ribbon is assembled to a custom ferrule, which defines a standard fiber connector interface. This sub-assembly is picked and placed on a nanophotonic die with standard microelectronics tools. Self-alignment structures are defined on the precision-molded ferrule as well as on the lithographically defined ribbon and chip to provide the optically required ±1-2 μm final alignment despite the initial ±10 μm placement uncertainty of high-throughput tools [2]. Our first optical demonstration provided a peak transmission from standard fiber to Si waveguide of -2.4 dB with 1.5 dB penalty over a 100 nm bandwidth and all polarizations [3]. Here, we report on notably improved performance. As shown in Fig. 2(d), the peak transmission is now -1.4 dB with 0.8 dB penalty over the ~100 nm bandwidth measured and all polarizations. The key enhancements over [3] were in ribbon-to-chip assembly, as described in [4], and in polymer waveguide design and fabrication. The wafer interface remained unchanged.

The optical path through the compliant polymer interface is shown in Fig. 1 (c)-(f). Optical fibers are butt-coupled to polymer waveguides in a standard fiber interface. The optical confinement is then increased adiabatically to a

Fig.1. Schematic of the compliant polymer interface in (a) exploded view and (b) after assembly to the photonic chip. Lithographically defined waveguides on a flexible ribbon interface standard fibers to nanophotonic waveguides. Self-alignment

structures enable assembly with standard high-throughput tools. The optical path is illustrated through the waveguide cross-sections of (c) to (f), whose locations are shown in (b). The current implementation has 12 parallel optical waveguides.

FTu1D.2.pdf Frontiers in Optics/Laser Science 2016 © OSA 2016

routing cross-section with wider width. Finally, the polymer cladding is terminated lithographically to expose the waveguide core for efficient adiabatic coupling to the chip. There, a non-linear change in the width of a Si nanotaper controls the mode evolution. The main issue found in [3] was larger than expected scattering loss from the abrupt changes in the effective optical cladding experienced near the chip edge and the start of the Si nanotaper. This was addressed here by increasing the confinement in the polymer waveguide and increasing the optical adhesive thickness between the polymer waveguide and the chip at assembly. The exposed polymer waveguide is now 7.5 μm wide with a short adiabatic transition added from the previous 6 μm wide cross-section, which was kept for routing to ensure single-mode operation in the region where the cladding is symmetric. The results are shown in Fig. 2. As in [3], automated assembly was used between ribbon and chip but active alignment was used between ribbon and a standard fiber connector. Relying on standard connector pin alignment between fiber and polymer is expected to add up to 0.5 dB loss. Performance through all loopbacks is shown before and after preliminary stressing through 25 cycles of -40 to 85ºC. The change in measured loss with stressing is not significant in view of the measurement repeatability.

We have demonstrated improved optical performance with a peak fiber to Si transmission of -1.4 dB, wide spectral bandwidth, and low <0.5 dB polarization dependence. The adiabatic aspect of the compliant polymer interface provides much wider spectral bandwidth than diffractive vertical couplers and its mechanical compliance points towards improved package reliability when compared to rigid direct fiber-to-chip connections.

Acknowledgments The photonic chips were fabricated in the Microelectronics Research Laboratory at the IBM T.J Watson Research Center. We thank Yan Thibodeau and René Maheu for help with characterization, Francis Gagné for help with assembly, and Marie-Claude Paquet for help with adhesive selection.

References [1] T. Barwicz and Y. Taira, "Low-cost interfacing of fibers to nanophotonic waveguides: design for fabrication and assembly tolerances," IEEE

Photonics Journal 6, 6600818, (2014). [2] T. Barwicz et al, “Assembly of mechanically compliant interfaces between optical fibers and nanophotonic chips,” in proc. of 2014 ECTC

(IEEE, New York, 2014), pp. 179-185. [3] T. Barwicz et al, "Optical Demonstration of a Compliant Polymer Interface between Standard Fibers and Nanophotonic Waveguides," in

Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2015), paper Th3F.5. [4] N.Boyer et al, “Sub-Micron bondline-shape control in automated assembly of photonic devices,” accepted for presentation at the IEEE

Electronic Components and Technology Conference (ECTC), Las Vegas, NE, May 31 - June 3, 2016.

Fig. 2 (a) Experimental results were obtained with automated assembly of ribbon to chip but active alignment of ribbon to fiber connector as a ferrule was not attached to the ribbon. (b) Optical micrograph of the loopbacks on chip arranging the 12 ports into 6 roundtrip pairs. The roundtrip between port 5 and 6 includes a ring resonator identifying the polarization

on chip. Its spectral response is shown in (c). The transmission loss in all other ports is shown in (d) before and after preliminary environmental stressing for the principle states of polarization (PSP). Fiber to Si loss is defined as half the

roundtrip loss from and to the fiber connector. The measurement repeatability is estimated at ±0.2 dB.