Laser Supported Fibre Array Alignment with Individual Fibre Fine Positioning

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1 Laser Supported Fibre Array Alignment with Individual Fibre Fine Positioning J.H.C. van Zantvoort, S.G.L. Plukker, E.C.A. Dekkers, G.D. Khoe, A.M.J. Koonen and H. de Waardt COBRA Research Institute, Eindhoven University of Technology 6 MB Eindhoven, The Netherlands j.h.c.v.zantvoort@tue.nl Abstract An innovative design is presented enabling fine positioning of each individual fibre in a fibre array used in multi in- and output ports photonic integrated circuits. With this technology, the performance of fibre arrays can improve by eliminating the eccentricities of the lenses deposited on the individual fibres and the inaccuracies of the supporting V- groove substrates. The alignment accuracy is in the sub-micron range and this concept is based on metal deformation by laser-welding induced local heat. It enables us to precisely introduce stepby-step deformations in the constructions (despite the already secured position of the fibre tips and the adjustment construction). The smallest fine-tuning step is. µm and the fibres are aligned over a range of 1 µm µm. The performance of different types of regular commercially fabricated fibre arrays, with fibres mounted in silicon V-groove substrates has been determined in order to compare these with the performance of the proposed fibre array design. Introduction Efficient fibre-to-chip coupling is an indispensable requirement for high-performance opto-electronic devices. For the third generation monolithically integrated photonic components, indium phosphide is the ideal substrate material [1]. Due to the high refractive index, more functions per unit area can be implemented in comparison with glass- or polymer-based devices. Active and passive components can be integrated on one single indium phosphide based chip; moreover, indium phosphide is suitable for integration of electrical circuit functions. However, due to the high index contrast between the waveguides and mono-mode fibres, mode matching is necessary to reduce losses in mode mismatch between the small elliptical widely divergent field of the waveguide (typical near field waveguide dimensions are µm x.6 µm) compared to the 1 times larger circular field of a single mode fibre with a narrow angle of acceptance. As a result of advanced integration technologies, multiple optical components such as arrayed waveguide gratings for wavelength (de)multiplexing, modulators, switches, couplers (passive components), and laser diodes, photo detectors and semiconductor optical amplifiers (active components) can be integrated into multi-function devices, which require multiple fibre connections []. The bottleneck in this multiple fibre connection is the inaccuracy of fibre arrays, due to the core eccentricity of the individual fibres. In [] fibres are partially metal coated. The core position can be aligned by rotating the fibre. In previous work we characterised the eccentricities of individual fibres. The measured eccentricities vary from. µm till µm. Fibres with similar core eccentricity can be selected and mounted in a V-groove by rotating the fibres according to the direction of the eccentricity [4]. To overcome the problem of core eccentricity, a design is developed to assemble fibre arrays whereby each individual fibre can be aligned independently. This paper is organised as follows. First, the laser adjustment mechanism is explained. After this the design of the array assembly is sketched. In the next paragraph the assembly process of a prototype of the fibre array is described. Subsequently, this prototype is characterised in the next paragraph. Commercial types of fibre arrays are characterised in the following paragraph. Conclusions and recommendations are given in the last chapter. Laser adjust technology Laser welding is acknowledged at present as the most stable technology for fixation of fibres and lenses in optoelectronic devices [], [6]. During the welding process, the solidification shrinkage of the welded parts causes misalignment of the components [7], [8]. Laser beam balance and symmetry are used to minimise these post-weld-shifts [9]. The laser hammering process that compensates for the postweld-shifts is proven to be an effective application for single fibre components. For multi-channel devices, laser hammering is effectively applied in [1]. In the design presented here, laser contraction technology is used. With this technique, laser-supported adjustment with the possibility of fine-tuning the position of already secured parts due to precisely chosen step-by-step deformations in the construction is possible and demonstrated for coupling fibre arrays to photonic chips [11]. Laser contraction is based on elastic and plastic material deformation. When a piece of material (see figure 1, picture 1 schematically presented by part (a)) is clamped mechanically to a solid surrounding (represented by both black bars (b)) and it is locally heated by a laser beam over the whole cross-section of the material (picture ), the heated zone expands elastically ((c), picture ). When the temperature of the material is further increased, the material deforms plastically if the expansion of the material is obstructed (picture ). After the heated part of the material has cooled down to room temperature, the shrinkage arises from the plastic deformation of the heated part (picture 4). In addition when this shrinkage is obstructed, tensile stress ((σ) schematically presented in picture ) will be generated in the opposite direction to the applied force due to the plastic deformation of the material piece. In this condition expansion or shrinkage of the piece can occur, depending on the tensile //$. IEEE 66 Electronic Components and Technology Conference

2 1 (b) (a) piece of material (b) 1 Laser beam (c) Laser beam mm 4 mm mm (σ) Fig. 1 Schematic presentations of laser support adjust mechanism. This mechanism is based on elastic and plastic material deformation. If a section of a piece of material is heated up, the material will expand or shrink, depending on the tensile stress in the material and the forces generated by the plastic deformations. stress in the material and the forces generated by the plastic deformations. This is experimentally verified, by measuring the displacement of a mechanical construction as a function of welded position, laser pulse duration and laser beam diameter of an Nd:YAG laser source. The smallest displacement step is. µm. These parameters are sufficient for the proposed application. Fibre array assembly design In figure, an artist's impression is given of the design. The fibres (1) are guided through holes in the adjustment frames () and clamped temporary in a silicon V-groove substrate (). After alignment of the fibres in the longitudinal z direction (for the definition of the x, y and z directions see figure 4), the fibres are fixed to the adjustment frames. The physical dimensions are mm x mm x mm (h x w x l). The dimensions cross sections of the tuning frames are 1. mm x. mm. The accuracy of the fibre array is determined by the method of measuring the core position of the individual fibres. The core positions of the fibres are measured actively by launching laser light into the fibres. A reference fibre, opposite to the launched fibre, connected to an x-y-z translation stage, receives the emitted light and the maximum value of optical received power is a direct measure of the core positions in the directions. The translation stages in lateral x and longitudinal z directions are temperature-stabilised high precision stages. The absolute position is measured by optical linear scales made of zerodure. The transversal y direction is measured with a piezo-electric translation stage with active feedback. (σ) 67 Fig. Design to align the fibres individually in a fibre array. The four fibres (1) are mounted in a V-groove () and the four fibre ends are mounted in four separated Y-junction shaped align frames (). By locally heating the branches of the frames, the fibre tips can be aligned in the desired direction. This results in core position determination with a precision of.1 µm. Further methods for measuring the x-y core centres of fibre arrays by using direct comparison of the fibre core centre position to a lithographically patterned template is published in [1]. Alternatively, fibre arrays can be compared with a standard reference array to reduce the measuring time [1]. Depending on the measured actual IR (Infra red) spot position of the individual fibres of the pre-assembled fibre array, the fibre tips can be aligned based on the previously described concept of laser welding the desired position by shrinkage of material of the tuning frames. A photograph of the set-up is given in figure. The fibre array is located at position (1) (see figure ). The reference fibre for measuring the core position of the individual fibres of the fibre array is located at position (). The x and z positions are measured by device (). The y position is measured with the piezo-electric translation stage (4). The energy of a 1 W Nd:YAG laser is guided to a focusing head () and the position of the weld spot to the tuning frames is adjusted with a mirror (6). Device (7) is a microscope to view the fibre tips. Assembly of device Different types of lensed fibres have been characterised in order to select the most suited fibres. The best coupling efficiency between single mode fibres and InP-based waveguides is -.8 db for fibres with grinded and polished tapered wedge-shaped top angles of 9º and hemi-spherical lens radii of µm. However the selected fibres have a Ni/Au metallisation over a length of mm. Due to this metallisation the fibres are bent from the mechanical axis. The deviation is approximately µm 6 µm and therefore the Electronic Components and Technology Conference

3 1 4 7 Y Fig. Overview of the laser adjust set-up. (1) position of the fibre array. () is the reference measurement fibre. () are the translation stages for the x and z direction. (4) is the piezo-electric translation stage in the y direction. () focusing head of the Nd:YAG laser energy. (6) adjust mirror for the weld spot position. Device (7) is a microscope to view the fibre tips. first prototype is assembled with cleaved SMA8 fibres to test the proof of principle. In this prototype the fibre pitch is µm and the fibres are guided through precisely drilled holes in the adjustment frames (with a diameter of µm), which are precisely aligned to the silicon V-groove. The fibres are temporarily clamped in the already mounted silicon V-groove. An accuracy of alignment in the longitudinal z-direction can be reached within 1 µm, by using a high magnification zoom microscope. The fibres are aligned using a fibre gripper mounted on a differential micrometer. After aligning the fibres in the longitudinal z direction, the fibres are fixed to the adjustment frame by using a two component, room temperature curing, and low viscosity adhesive, specifically designed for glass bonding and suitable for bonding of metals. With this procedure it is also possible to assemble tilted fibre arrays to achieve low reflections in the optical devices [14]. The layer of adhesive between the fibres (diameter of 1 µm) and the holes (diameter of µm) is symmetrically round and approximately - 6 µm thick. This layer thickness will impart the greatest lap shear strength to a joint [1]. In figure 4 is given the fibre numbering and coordinate system used in this paper. The spot positions have been measured of the pre-assembled device. Two fibres have to be adjusted over a relatively large range. In table 1 the measured results are shown. The maximal deviation in the positive lateral x direction is 1.6 µm for the second fibre and the maximal deviation in the transversal y direction is more than µm for Pre-assembled array Final assembled array Fibre X [µm] Y [µm] X [µm] Y [µm] > Fig. 4 Fibre numbering and definitions of the x and y axis used in this paper. Table 1 Measured core positions of the four fibres of the pre-assembled fibre array and the final core positions of the laser adjusted fibre array fibre 1 (a) fibre (c) X (b) (d) Fig. Relative displacements of the four fibres after the laser adjust fine positioning process. - fibre fibre 4 68 Electronic Components and Technology Conference

4 the fourth fibre. (the measurement range in the y direction is limited to µm, therefore an exact measurement is not possible). In the fine adjustment process, first fibre 4 is laser adjusted in the negative y direction until the fibre reaches the measurement range in the y direction. During the adjustment process, the four fibre pitch positions are measured. As a result of these measurements, we adjusted the fibres in the following order:,, 4, 1,,, 4,, 1,, 4, and finally fibre 1. The fibre core pitches of µm in the x direction and the minimum deviation in the y direction is obtained with the minimum absolute movements of all the four fibres (see figure ). Fibre 1 is adjusted 6. µm in the positive x direction and 7.6 µm in the negative y direction, fibre is adjusted 9. µm in the negative x direction, and 4.6 µm in the positive y direction, fibre is.4 µm adjusted in the negative x direction and 1.8 µm in the positive y direction and fibre 4 is adjusted over a range of 6 µm in the negative y direction and 7.9 µm in the positive x direction. In comparison with the initial core positions of the pre-assembled fibre array, the final fibre core positions are tilted. After deciding which fibre has to be aligned in which direction, the alignment procedure of the fibres is as follows; the laser beam spot is focused on the concerned position of the tuning frame and the first heating at energy of.7 J is given. This procedure is repeated by increasing the laser energy. We observed very small steps by energy in the range of.8 J till 1. J. Displacement in the order of a few micrometers is measured by laser energy levels of 1.7 J J. The heating process can introduce shrinkage or expansion on the branches of the Y-junction (see figure 6). Therefore, the fibre position can be moved back and forward, depending on the location on the branch. During this experiment we observed that if the laser spot is near the centre of the tuning frame, shrinkage (-) occurs and if the laser spot is directed at the outer part of the Y-junction shaped tuning frames, expansion (+) occurs; see figure 6. In previous work [11] we used laser adjusting to align fibre arrays to optical chips. The material of the alignment construction was invar due to the low coefficient of thermal expansion and therefore suitable for optoelectronic packaging [16]. The adjustment frames however, were made of steel, because of the resilient properties. We only observed shrinkage of this material. In the fibre array design, all components, including the adjustment frames are made of invar for mechanical stability of the fibre core positions as a function of ambient room temperature fluctuations. The minimum thermal expansivity of invar occurs near room temperature [17]. The expansivity increases with increasing temperature and its magnitude is almost equal to ordinary metals and alloys [18] and therefore this material can be used for the laser adjustment mechanism. The mechanism that caused shrinkage of the invar tuning frame at the centre of the tuning frame can be explained by the fact that in the centre of the tuning frames the stiffness may be less compared with the positions outside the Yjunction shaped frame, and thus closer to the quadrangular frame. If the stiffness is higher and the stress is high at this position, only expansion of the material is possible by heating up the material. This topic is subject to further research. Fibre position Fig. 6 Shrinkage (-) or expansion (+) of the fibre core direction in the centre of the adjust frame as a function of the weld position of the laser spot in the Yjunction shaped adjust frame. Fig. 7 Photograph of the realised fibre array. Fibre core positions are laser adjusted within an accuracy of ±. µm. Characterisation of assembled fibre array The fibre array has been mounted on a 6-axis piezoelectric controlled manipulator and positioned in front of an InP-based optical chip with looped waveguides. (see figure 8). Fibre 1 and fibre are connected to a laser source and fibre and fibre 4 are connected to optical power meters. The laser light emitted by fibre 1 is launched in the waveguide and guided through the chip to fibre 4; the laser light transmitted by fibre is also guided by another looped waveguide to fibre. Fibre combination 1-4 is optimally aligned in all 6 degrees and the fibre array is shifted over a range of 4 µm in the lateral x and transversal y direction. In figure 9 and figure 1 the normalised transmission curves are given for displacement in the x and y direction, respectively. In the lateral x displacement, the difference in both peak levels are. µm and the difference in both peak levels in the y displacement shift has been measured to be. µm. Note that the physical dimensions of the InP-based waveguides are µm in the lateral x direction and only.6 µm in transversal y direction. Transition loss from a cleaved fibre to such a waveguide is calculated to be 1 db by applying the Gaussian beam approximation [19]. The measured transmission loss of 4 db for both fibre pair combinations is composed as follows. Twice the transition loss of fibre to waveguide ( x 1 db) makes db. The waveguide losses of the looped waveguides are estimated to be 1. db. The distance between 69 Electronic Components and Technology Conference

5 1 4 Fig. 8 Enlarged photograph of prototype assembled fibre array aligned to an InP-based optical chip (photograph left). Fibres 1 and are connected to a laser source fibres and 4 are connected to optical power meters. The light is transmitted from fibres 1 and through the chip with optical looped waveguides (photograph right) and the light is received by fibres and 4. Normalized transmission [db] -, -1-1, , -1 -,, 1 1, X displacement fibre array [µm] Fig. 9 Normalised transmission curve of both fibre pairs combination 1-4 and - as a function of lateral displacement shift. Normalized transmission [db] -, -1-1, , -1 -,, 1 1, Y displacement fibre array [µm] Fig. 1 Normalised transmission curve of both fibre pairs combination 1-4 and - as a function of transversal displacement shift. the aligned fibre array and chip facet is 1 µm. All four fibres are mounted within an accuracy of 1 µm in the longitudinal z direction. Due to this separation of 1 µm, additional losses of x 1. db appear approximately. Therefore transformation of the mode field of the fibre is necessary. To compare the proposed fibre design with regular fabricated fibre arrays, the Fig. 11 Fibre array type A assembled with drawn fibre tips. Fig. 1 Fibre array type B assembled with mechanical polished fibre tips. array type N [-] Dx [µm] Dy [µm] Dz [µm] η [db] A A B B Table Measured results of two different fibre tips assembled 4- and 8- fibre arrays. performance of commercial lensed fibre arrays has been measured in the next paragraph. Characterisation of commercially available fibre arrays Two different types of commercial lensed fibre arrays have been characterised. The first type A is assembled with drawn tapered fibres (see figure 11), the other type B is assembled with grinded and polished hemi-spherical fibres (see figure 1). From both arrays A and B, arrays with 4 and 8 fibres are characterised. First the IR spot distribution is measured. Next the coupling efficiency is determined. The fibre arrays are connected to an InP-based optical chip with waveguide loops (same chip used with the characterisation of the assembled fibre array in the previous chapter). The input fibres concerned are connected to a laser source, the light is transmitted through the waveguide loops and the light is coupled into the output fibres concerned. The general conclusion is that the maximum IR spot distribution of array type B is smaller than type A. In table are given the type of array, number of fibres N, the maximal IR distribution between the fibres in lateral x direction Dx, transversal y direction Dy, longitudinal z direction Dz and the minimum coupling loss between the fibres and an InP-based waveguide η. The coupling losses for arrays, assembled with four fibres are 6 db and. db for all fibres simultaneously for fibre array types A and B, respectively. For arrays with eight fibres, the measured losses are 7.8 db and 6. db for all fibres simultaneously for array types A and B. These losses are composed of 4. db due to the relatively large lens radii of the used fibres, which are 14 µm 16 µm and the other remaining losses introduced by the inaccuracy of the focused IR spot. By using an innovative V-groove design and epoxy 7 Electronic Components and Technology Conference

6 dispensing method these inaccuracies can also be improved []. Conclusions Laser-assisted adjustment is successfully demonstrated for aligning fibres in an array. The fibres can be aligned over a range of 1 µm µm. The smallest fine-tuning step is. µm, and with this technology four fibres have been aligned with a deviation of ±. µm in the transversal y direction and ±. µm in the lateral x direction. In the longitudinal z direction, the fibres have been aligned within a deviation of less than ±. µm. Commercially lensed fibres with radii of µm and tapered wedge-shaped angles of 9º have the best coupling efficiency to InP-based optical waveguides of.6 µm x µm. The coupling loss is.8 db. By using these fibres and the proposed laser adjustment technology, a fibre array can be assembled with a coupling efficiency to an InP waveguide of - db. The coupling loss of commercial fibre arrays are in the order of. db 7.8 db. These losses are due to the relatively large lens radii of 14 µm 16 µm and the inaccuracy of the focused IR spot. Recommendations When the ordered fibres without Ni/Au metallization have arrived, a new design will be tested in which the individual Y-junction shaped alignment frames will be modified in a cross shaped frame with the tuning beams perpendicular to each other. Thus the movement of the fibres in the x-and y- direction is better defined, and as a result of this a higher accuracy of the fibre pitch is expected. Also decreasing of the fibre pitch of. mm to. mm will be investigated. Another issue is to investigate the shrink and expand behaviour of the tuning frames after all internal stresses have been reduced by annealing the pre-assembled design. Acknowledgment This work has been supported by the Netherlands Organization for Scientific Research (NWO) through the NRC photonics grant. References 1. Yoshimura, H., Future photonic networks and the role of InP-based devices, in Proc. IEEE Int. Conf. Indium Phophide and related materials,, pp Smit, M.K., InP Photonic integrated circuits, in Proc. 1 th Ann. Meeting IEEE Lasers Electro-Optics Soc., vol., Nov. 1-14,, pp Kaneko, S. et al, Novel fiber alignment method using a partially metal-coated fiber in a silicon V-groove, IEEE Photon. Technol. Lett., vol. 1, no 6, pp , Jun.. 4. Zantvoort, J.H.C. van et al, Fiber array pigtailing and packaging of an InP-based optical cross connect chip, IEEE J. Select. Topics Quantum Electron., vol., no., pp. 1-19, Sep/Okt Song, M.K. et al, Laser weldability analysis of highspeed optical transmission device packaging, IEEE Trans. Comp. Packag., Manufact. Technol. B, vol. 19, pp , Nov Ghaoui, G.M. et al, A method of achieving improved radial alignment in an optical package utilizing laser welding techniques, in IEEE Electron. Comp. Conf., 1989, Proceedings,9th, 1989, pp Wang, S.C. et al, Post-weld-shift in semiconductor laser packaging, in IEEE Proc. 49 th Electron. Comp. and Technol. Conf., 1999, pp Cheng, W.-H, et al, Defect formation mechanisms in laser welding techniques for semiconductor laser packaging, IEEE Trans. Comp. Packag., Manufact. Technol. B, vol. 19, no. 4 pp , Nov Hsu, Y-C, et al, A novel fiber alignment shift measurement technique employing an ultra high precision laser displacement meter in laser-welded laser module packaging, in IEEE Proc. Electr. Comp. and Technol. Conf., 4, pp Kang, S.-G et al, Fabrication of semiconductor optical switch module using laser welding technique, IEEE Trans. Adv. Packag., Vol., pp 67-68, Nov Zantvoort, J.H.C. van. et al, Fiber array-to-photonic-chip fixation and fine tuning using laser support adjustment, IEEE J. Select. Topics Quantum Electron., vol. 8, no. 6, pp , Nov/Dec. 1. Bernussi, A.A. et al, High-precision characterization of single-mode optical fiber arrays, IEEE J. Lightwave Technol., vol 1, no 6, pp , Jun.. 1. Ozawa, K. et al, High-speed measuring equipment of fiber core position of optical fiber array using piezo actuator, in IEEE Int. Conf. on robotics and automation., vol 1, pp , May Grard, E. et al, High performance packaging technique used for clamped gain semiconductor optical amplifier array modules fabrication, in IEEE Proc. Electr. Comp. and Technol. Conf. 1998, pp Jeong, B. et al, Analysis on stress in the fiber-array with FEM, in Proc. OFC, vol. 1, pp Wang, S. C. et al, Effect of Au coating on joint strength in laser welding for invar-invar packages, in IEEE Proc. Electr. Comp. and Technol. Conf. 1996, pp Chanchani, R. et al, Temperature dependence of thermal expansion of ceramics and metals for electronic packages, IEEE Trans. on Comp. Hybrids, and Manufact. Technol. vol. 1, no. 4 pp dec Nakamura. Y, The invar problem, IEEE Trans. On Magnetics., vol 1, no 4, pp Juli Kawano, K. et al, A new confocal combination lens method for a laser-diode module using a single-mode fiber, IEEE J. Lightwave Technol., vol. LT-, no 4, pp , Aug Jeffery, C.C. et al, Modified passive alignment of optical fibers with low viscosity epoxy flow running in V- grooves, in IEEE Proc. Electr. Comp. and Technol. Conf. 4, pp Electronic Components and Technology Conference