Efficient, easy-to-use, planar fiber-to-chip coupling process with angle-polished fibers

Transcription

1 2017 IEEE 67th Electronic Components and Technology Conference Efficient, easy-to-use, planar fiber-to-chip coupling process with angle-polished fibers Djorn Karnick, Nils Bauditsch, Lars Eisenblätter, Thomas Kühner, Marc Schneider, Marc Weber Institute for Data Processing and Electronics (IPE) Karlsruhe Institute of Technology (KIT) Karlsruhe, Germany Abstract We present an efficient and easy-to-use process for a permanent fiber-to-chip coupling arrangement with anglepolished single-mode optical fibers (SMF) to maintain a planar profile while surface-coupling to grating couplers of a silicon photonic integrated circuit (PIC). The SMF are polished with a standard polishing machine to match the appropriate coupling angle. Due to the simplicity of the process, it is suitable for both packaging of photonic devices ready for commercialization and the rapid coupling of components at an early stage of development. The coupling arrangement does not impose additional insertion loss compared to a continuously controlled fiber alignment and remains stable even under strong variation of ambient temperature and humidity. Keywords High-Speed Data Transfer/Communications, Micro-Optical System Integration and Photonic System-in- Package, Optical Waveguide Coupling, Optoelectronic Assembly and Reliability, Silicon and III-V Photonics Packaging, Materials and Manufacturing Technology I. INTRODUCTION To meet the demands on increased data transmission capacity of short-reach networks in future data centers as well as on circuit board chip-to-chip interconnects, optical data transmission systems are receiving continuously growing attention [1]. High data rates can be achieved while keeping the energy consumption and heat dissipation low. The same requirements apply for data acquisition (DAQ) and read-out electronics in detector systems in high-energy physics, photon science or astroparticle physics experiments [2]. Those devices consist of numerous layers of subdetectors to identify various particle properties. Due to the large number of electronic channels, future detector systems are very intense data sources. Therefore, a key challenge in detector instrumentation is to increase the data rate of the front-end electronics and the read-out channels. At the same time, the constraints on energy, volume and mass are stringent, since most of the volume in the detector barrel is occupied by sensor cells and particle traces must not be perturbed by the data processing electronics. Hence, a small foot print and a dense integration of the data read-out system is highly requested. The silicon-on-insulator (SOI) platform allows for a dense integration of components enabled by the high contrast of the refractive indices of silicon and silicon dioxide. Complete circuits can be fabricated in complementary metal-oxidesemiconductor (CMOS) processes [3]. However, the index contrast also yields a severe mismatch of the mode field diameters of standard single-mode fibers (SMF) and the silicon waveguides. To obtain acceptable coupling losses, a mode converter is required. Grating couplers are well-known mode converters. The optical mode is coupled from the ber to the waveguide and vice versa at a small angle with respect to the chip surface normal. In contrast to edge-coupled inverted tapers [4], no dicing or edge polishing is required, which furthermore allows for wafer-scale characterization of components [5]. On the other hand, the overall dimension of a conventional vertical coupling arrangement is considerably larger and thus opposes the goal of dense integration. To maintain a low profile, a horizontal fiber-to-chip coupling arrangement as shown in Figure 1 has been proposed [6, 7]. A SMF is polished at an angle smaller than 45 and the fiber axis is aligned in parallel to the chip surface. By means of total internal re ection at the polished fiber facet, the optical eld couples radially from the ber to the grating coupler. The concept is quite well-known for the coupling of photodetectors and laser diodes [8, 9], but has only recently been introduced for the surface coupling of silicon photonic waveguides. In this paper, we present an efficient and easy-to-use process for the permanent coupling of angle-polished SMF to a photonic integrated circuit (PIC). It is even suitable for laboratory use, where a rapid prototyping or in-situ testing of devices at an early stage of development is required. We show the single steps of the process from the fabrication of anglepolished fibers to the final assembly of a permanent fiber-tochip coupling arrangement. Studies of the coupling stability with respect to ambient temperature and humidity in a climate testing cabinet are presented. Finally, we show the result of the investigation in the long-term stability of the arrangement in a non-stabilized environment. II. FIBER-TO-CHIP COUPLING PROCESS To realize a planar fiber-to-chip coupling assembly, an angle-polished optical fiber is attached to a grating coupler on a PIC. The fiber is mounted on a standard glass v-groove chip and fixed with an appropriate glass lid. Its facet protrudes from the v-groove chip s front so as to reach the grating /17 $ IEEE DOI /ECTC

2 Figure 1. Schematic of the optical path in an angle-polished single-mode fiber. The polishing angle stems from the geometric relation to the desired coupling angle. coupler on the photonic chip. All the components are fixed with a UV-curing adhesive. The assembled v-groove chip arrangement is positioned with a customized vacuum pickup holder mounted on our precision fiber alignment setup. The arrangement is sketched in Figure 2. A. Fabrication of angle-polished fibers The polishing of the fiber facet is achieved by using a rotating grinding and polishing machine (Struers LaboPol-5). To set the polishing angle for the process, we use a specially designed fiber holder produced by a Makerbot 3D-printer as shown in Figure 3. It consists of a cantilever, where a ceramic tile carrying the fibers can be mounted, a handle and an exchangeable socket. The ceramic tile is provided with grooves where fibers can be fixed for mechanical stabilization. This is necessary to avoid bending of the fibers during the polishing process. We have chosen a machinable glass-ceramic to prevent fast deterioration of the abrasive. The socket serves as a gauge to select the targeted polishing angle, which is dependent on the grating coupler s incident angle. It can be calculated from the geometric relations in Figure 1 and Snell s law 90 = + 2 (1) n 1 sin = n 2 sin, (2) where n i (i=1, 2) are the refractive indices of quartz glass at wavelength 1.55 μm (n 1 = ) and air (n 2 =1) while Figure 3. Specially designed holder for polishing up to four fiber specimens. By selecting the appropriate black socket, the polishing angle is set according to the desired coupling angle. and are the angles of refraction in the respective medium. The relation holds as long as the condition for total internal reflection is satisfied. Due to the small difference of the refractive indices between the fiber core and the cladding, a change of the angle of refraction is neglected at the boundary. At first, the fiber end face is coarsely shaped by using abrasive paper. Subsequently, the residual surface roughness is smoothed incrementally by applying diamond suspensions to the polishing wheel with a grain size of 9, 3 and 1 μm. Optimum results are obtained for a duration of 8 minutes per step at a rotation speed of 175 to 200 rounds per minute. Up to four fiber specimens can be mounted on the ceramic tile and processed simultaneously. B. Mounting of polished fibers on v-groove chips Before a polished fiber can be aligned, it is mounted on a standard glass v-groove chip. A schematic of the arrangement is shown in Figure 2. The secondary coating of the fiber is stripped off, so the polished tip may sufficiently protrude from the v-groove chip. Additionally, the primary coating is removed where the fiber is resting in the groove. Mechanical fastening is achieved by a compatible glass lid, which is fixed on the v-groove chip s base with a UV-curing adhesive. The lid is not shown in Figure 2 for clarity. It can be positioned by a vacuum pick-up holder with a customized rubber tip. Figure 2. Schematic of an angle-polished fiber mounted on a v-groove chip. The facet protrudes from the chip s front so it can be aligned to a grating coupler on the photonic chip. Primary and secondary coatings are removed accordingly to fit the fiber in the v-groove. The fiber is mechanically fastened on the chip by a glass lid, which is not shown for clarity. Figure 4. Detail photograph of the fiber alignment setup. The rotation of the fiber is adjusted by a mechanical fiber rotator before it is fixed on the v- groove chip. Subsequently, the glass lid is positioned using a vacuum gripper with an appropriate rubber tip. 1628

3 In the planar arrangement, not only the areal position of the fiber tip is aligned, but also the rotation with respect to the fiber axis. It is adjusted prior to fixing the fiber on the v- groove chip using a customized mechanical fiber rotator. The reference for this alignment is a grating-coupled test waveguide on the photonic chip. Optimization for maximum coupling efficiency is performed at the target wavelength of 1.55 μm. A detail photograph of the setup for aligning the fiber rotation and subsequently attaching the glass lid is shown in Figure 4. The same setup is used for the assembly of the fiber-to-chip coupling arrangement and is described in the following section. C. Assembly of fiber-to-chip coupling arrangement The fiber-to-chip coupling arrangement is assembled with an electro-optic modulator embedded on a multi-component photonic chip. In addition to attaching optical fibers, an electrical contact with bond wires is established. At first, the photonic chip is bonded to a glass substrate. It is elevated by a silicon spacer to minimize the gap between the photonic chip s surface and the fiber cladding. The spacer is a tile of a silicon wafer, which is abraded to the appropriate thickness. Being opaque for UV-radiation, the photonic chip is fixed on top of the spacer with a heat-curing adhesive. The v-groove chips carrying the angle-polished SMF are positioned manually for optimum coupling efficiency using a xyz-positioning stage consisting of stepper motors (Newport MFA Series) and piezo-driven handling stages (PI NanoCube). Handling is facilitated by the vacuum pick-up holders as shown in Figure 4. A small amount of adhesive is applied to the interface between the bottom of the v-groove chip and the substrate. When the alignment is optimized to maximum coupling efficiency, the adhesive is cured by irradiating the interface through the transparent v-groove chip with a UV-source (Dymax BlueWave 50). A penalty on the coupling efficiency due to contraction of the adhesive during the curing process is not observed. Thus, the insertion loss of the planar fiber-to-chip coupling arrangement is not higher than when using translation stages to continuously control the coupling of the bare die. By bonding the fibers also to the chip surface, the mechanical stability of the assembly is improved. To enable applying a bias voltage for characterization purposes, the electrical pads of the chip are connected by wire bonds to a ceramic conductor board. The finalized structure is presented in Figure 5. III. VALIDATION OF COUPLING ARRANGEMENT The fiber-to-chip coupling arrangement described in the previous section is validated upon each stage towards the assembly. The alignment tolerances of the angle-polished fibers are evaluated by both simulations and experiments. After the assembly, the sensitivity towards ambient conditions and long-term stability is investigated. A. Investigation of alignment tolerance To simulate the alignment tolerances, the planar fiber-tochip coupling arrangement is simulated with the software package Zemax OpticStudio. Analysis is made utilizing physical optics propagation method (POP). Since the optical field is leaving or entering the angle-polished SMF radially, it is modelled by a cylindrical lens. The source of the optical field is located at the lens axis. Its shape is given by the Gaussian mode field in the SMF core. The propagation of the field through the lens is calculated and the overlap integral is solved at the location of the grating coupler. The mode accepted by the grating coupler is assumed to have a Gaussian shape. This is a suitable approach, since an optimized component aims for a strong overlap with the mode of an optical fiber [6, 10]. With this model, the coupling efficiency as a function of spatial displacement from the optimum alignment between the fiber and the grating coupler is investigated. That includes the offset in the chip surface plane lateral (x-direction, see Figure 1) and parallel (ydirection) to the fiber axis as well as the gap between the fiber cladding and the grating coupler (z-direction). The impact of a misalignment of the fiber rotation is analyzed, too. Except for the rotation angle, the dependency of the coupling efficiency on the displacement is also determined experimentally. We use a transmission setup consisting of a tunable laser source (Agilent 81689A), a manual polarization controller and an optical power head and interface (Agilent 81623B and 81618A). Moving distances of the xyz-stage are read out of the controller units of the piezo-driven handling stages. The results of both the simulations and the measurement are presented in Figure 6. The solid lines represent the simulation results while the dashed lines are connections of data points obtained by the measurement. Figure 6a) and b) show the coupling efficiency as a function of the areal positioning offset in the plane of the photonic chip surface. An alignment error of 5.0 μm lateral or 4.5 μm parallel to the fiber axis yields a penalty of 3 db. This result shows excellent agreement with the simulation. At an offset in y-direction larger than 6 μm, we note that the measured coupling efficiency decreases less sharply with increasing offset than the simulation predicts. This is due to Figure 5. Close-up photograph of the permanent fiber-to-chip coupling of an electro-optic modulator on a multi-component photonic chip. Two anglepolished fibers are mounted on v-groove chips and attached to grating couplers. The contact pads of the chip are bonded to a ceramic concuctor board. That way, the moduator can be provided with a bias voltage. 1629

4 the real grating coupler not being symmetric in that orientation. That is not taken into account in the simulation. The change of the coupling efficiency dependent on the gap size between the fiber cladding and the grating coupler is shown in Figure 6c). Since the coupling angle with respect to the chip surface normal is non-zero, a variation of the gap size involves also a displacement of the incident field s maximum parallel to the fiber axis. For a proper assessment of the dependency of the coupling efficiency on the gap size, the offset parallel to the fiber axis is compensated. The experimental result shows that the attenuation alternates with the gap size. That is due to the superposition of reflections between the fiber cladding and the grating coupler. However, the experiment and the simulation are in good agreement and show a high tolerance of the coupling efficiency towards the gap size between the grating coupler and the fiber cladding. Even at 30 μm, the penalty is only 1 db. In Figure 6d), the coupling efficiency as a function of the fiber rotation angle is shown. The 3 db-tolerance is determined to be ±4.2. The result cannot be verified experimentally, since we do not have the means for an exact measurement of the rotation angle. B. Experimental verification of the coupling stability After the successful completion of the fiber-to-chip coupling arrangement, we study the coupling stability with changing ambient conditions. A change in the properties of an adhesive due to temperature or humidity might involve a change in volume. This may result in a displacement of the coupling arrangement and thus an increased coupling loss. Measurements of the coupling efficiency with respect to ambient temperature and humidity are carried out in a climate testing cabinet (Weiss Type WK3-340/40) with a gratingcoupled test structure on a photonic chip. The efficiency is obtained by normalizing the optical transmission. Figure 7 shows the coupling efficiency as a function of the ambient temperature at a relative humidity of 30% in the range of 20 C to 80 C. The maximum value is obtained at 20 C, which is also the temperature during the assembly. At each temperature increment, the coupling efficiency is measured only after it has stabilized. Apparently, it is only weakly dependent. The maximum attenuation at 80 C is only 2 db. In Figure 8, the dependency of the coupling efficiency on the relative humidity in the range of 30% to 80% at 20 C is Figure 6. Coupling efficiency as a function of the spatial displacement of the angle-polished optical fiber from the optimum position. Solid lines show simulation results while the data points connected by dashed lines are obtained by an experiment. (a) lateral and (b) parallel offset with respect to the fiber axis. (c) Change of coupling efficiency with increasing gap size w g between the cladding of the optical fiber and the grating coupler. (d) Simulation result of the dependency of the coupling efficiency on the rotation angle of the fiber. 1630

5 shown. The arrangement is given sufficient time to adapt by changing the humidity at a rate of 2% per hour. Upon increasing humidity from 30% towards 50%, the coupling efficiency improves. That indicates that the fiber alignment is slightly displaced at 30% and operates at optimum at 50%. Still, the maximum attenuation due to displacement within the given range of humidity is 0.2 db. We have already shown before, that the attenuation due to both temperature and humidity displacement is fully reversible [11]. In order to assess the long-term stability of the fiber-tochip coupling arrangement, the coupling efficiency in nonstabilized environment is observed. For this study, anglepolished fibers are attached by the aforementioned process to a Mach-Zehnder modulator consisting of depletion-type pnmodulators [11, 12]. Figure 9 shows the coupling e ciency normalized to the highest measured value as a function of the elapsed time since assembly. Each data point represents the measured optical power at the wavelength of the modulator s maximum transmission around 1.54 μm. The re-plugging of ber connectors and polarization adjustment are identified as sources of uncertainty, which explains the scattering of the data points. Apart from this, we observe no significant loss for over 8 months. Consequently, the assembly is considered to be long-term stable. IV. CONCLUSION Packaging and in particular the coupling of single-mode fibers to a photonic chip are an essential step to bringing optoelectronic components to application. We have presented a fiber-to-chip coupling process for grating-coupled silicon photonic integrated circuits using angle-polished fibers which allows for a permanent coupling without the necessity for commercial packaging equipment. The crucial mechanical joints are established by a UV-curing adhesive. Essential steps of the process have been discussed in detail. The alignment tolerances have been studied in simulations and experimentally. The areal 3 db misalignment tolerance is 5.0 μm lateral and 4.5 μm parallel to the fiber axis, respectively. The misalignment of the fiber rotation can be ±4.2 for a 3 db power penalty. The gap size between the fiber cladding and the grating coupler of the photonic chip has only a minor influence on the coupling efficiency. The experimental results are in excellent agreement with the predictions from the simulations. Investigations of the coupling stability yield good reliability of the assembly. A 60 Kelvin temperature change degrades the coupling efficiency by less than 2 db. Sweeping the relative humidity from initially 30% to 80% yields a degradation of only 0.2 db. The observation of the optical coupling of a Mach-Zehnder modulator over more than 8 months shows no significant decrease in coupling efficiency. Therefore, we consider the assembly fabricated with our process long-term stable. ACKNOWLEDGMENTS We would like to thank Tibor Piller for his help on the mechanics for the precision alignment. We also thank Benjamin Leyrer for assisting in the development and improvement of the polishing procedures. Figure 7. Relative coupling efficiency as a function of the ambient temperature at 30% relative humidity in the range of 20 C to 80 C. The values are normalized to the measured value at 20 C. Figure 8. Relative coupling efficiency normalized to the measured value at optimum coupling as a function of the ambient humidity at a temperature of 20 C in the range of 30% to 80%. Figure 9. Evolution of the relative coupling efficiency in non-stabilized environment of the permanent fiber-to-chip coupling arrangement from the time of the assembly. The data points represent the measured coupling efficiency normalized to the highest coupling value. 1631

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