Ultra-broadband Silicon Photonic Multimode Interference Coupler

Ultra-broadband Silicon Photonic Multimode Interference Coupler Robert HALIR1*, Pavel CHEBEN2, Jose M. LUQUE-GONZÁLEZ2, Jose D. SARMIENTOMERENGUEL1, Jens H. SCHMID2, Gonzalo Wangüemert-Pérez1, Dan-Xia XU2, Shurui WANG2, Alejandro ORTEGA-MOÑUX1, and Íñigo MOLINA-FERNÁNDEZ1 1Universidad de Málaga, ETSI Telecomunicación, Campus de Teatinos s/n, 29071 Málaga, Spain 2National Research Council of Canada, Ottawa, Ontario K1A0R6, Canada * [email protected] In integrated optics, multimode interference couplers (MMIs) are used as light-wave splitters and combiners in a wide variety of devices ranging from spectrometers to sensors and coherent optical receivers. While their design and operation is generally well understood [1], the operational bandwidth remains limited. Here we present a sub-wavelength structured MMI, shown in Fig. 1(a), that overcomes this limitation and experimentally demonstrate a bandwidth exceeding 300nm at telecom wavelength, with more than 500nm bandwidth potentially attainable (as per 3D-FDTD simulations). Referring to Fig. 1(b), in a MMI a light-wave with mode profile 𝑓𝑖𝑛 is launched into the multimode region, exciting several higher order modes 𝜑𝑖. These modes propagate with different phase constants 𝛽𝑖 forming images of the input field 𝑓𝑖𝑛. A two-fold image is formed at the position 𝑧 = (3/2) 𝐿𝜋, with 𝐿𝜋= 𝜋/(𝛽1− 𝛽2) the beat-length of the two lowest order modes. By placing the output waveguides at this position a 3 dB coupler is obtained. However, the beat-length 𝐿𝜋 is wavelength dependent, so the position of the images moves as wavelength is tuned, limiting the device bandwidth [1]. We use a sub-wavelength grating (SWG) metamaterial to overcome this limitation. SWGs behave as equivalent homogenous media with controllable refractive index, and have found widespread application in silicon photonics [2]-[5]. Broadband 3 dB couplers based on SWG directional couplers [6]-[7] and SWG adiabatic couplers [8] have been recently demonstrated, with bandwidths between 100 nm and 130 nm. In our SWG MMI [Fig. 1(a)], we exploit the anisotropy of the SWG to experimentally demonstrate a 3 dB coupler that covers 300nm of bandwidth around a wavelength of 1.55 μm. 3D FDTD simulations show that more than 500nm can be achieved. The anisotropy of the SWG medium is analogous to that of a uniaxial crystal: two waves polarized along the 𝑥 axis (electric field parallel to the silicon segments) and 𝑧 axis (electric field perpendicular to the silicon segments) experience different equivalent indexes, 𝑛𝑥𝑥 and 𝑛𝑧𝑧 [Fig. 1(a)]. The device can thus be modelled as a conventional MMI composed of a homogenous anisotropic material [Fig. 1(b)]. The beat length of such an anisotropic MMI is given by 𝐿𝜋≈ (4W2)/3𝜆 · 𝑛𝑧𝑧 2/𝑛𝑥𝑥, and is significantly shorter and less wavelength dependent compared to a conventional MMI [9]. Fig. 1. (a) Scanning electron microscope image of a sub-wavelength patterned multimode interference coupler engineered for ultra-broadband operation. (b) Schematic top view of a 22 multimode interference coupler. Fig. 2. Measured performance of the device shown in Fig. 1(a), revealing high-performance operation over a bandwidth in excess of 300nm, actually limited by the setup wavelength scanning range. Figure 2 shows the performance of the sub-wavelength patterned MMI, measured using a broadband source and an optical spectrum analyser. Both imbalance (power difference between the two outputs) and excess loss (power lost in the device) are below 1 dB over the full measurement range (1380nm – 1700nm). The phase error (deviation from the ideal 90 phase shift between the outputs) is smaller than 5. Even an ideal (simulated) conventional MMI device only exhibits a comparable performance over a limited bandwidth of 200nm. We believe that such ultra-broadband MMI devices will pave the way towards broadband on-chip systems for diverse applications ranging from optical sensing to data communications. We acknowledge funding from the Ministerio de Economía y Competitividad, Programa Estatal de Investigación, Desarrollo e Innovación Orientada a los Retos de la Sociedad (cofinanciado FEDER), Proyecto TEC2013-46917-C2-1-R, Proyecto TEC2016-80718-R, and the Universidad de Málaga. References [1] L. B. Soldano, and E. C. Pennings, Optical multi-mode interference devices based on selfimaging: principles and applications, J. Lightw. Technol., vol. 13, pp. 615-627, 1995. [2] P. Cheben et al., Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers, Opt. Lett., vol. 35, pp. 2526-2528, 2010. [3] R. 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