Laser micro welding for fiber-chip-coupling modules with lensed fiber ends for photonic communication systems

Transcription

1 Laser micro welding for fiber-chip-coupling modules with lensed fiber ends for photonic communication systems O. Krips, U. H. P. Fischer (member IEEE) Heinrich-Hertz-Institut für Nachrichtentechik Berlin GmbH Einsteinufer 37, Berlin, Germany, 1. ABSTRACT. We designed and fabricated butterfly (BFY) modules with an optical coupling set-up, which is well suited for small-scale production and rapid prototyping of waveguide-fed InP and GaAs OEICs like diode lasers, semiconductor amplifiers and optical modulators. The light output of these OEICs is collected with high efficiency into tapered single mode fibers with lensed end. 2. INTRODUCTION Future optical communication systems will use the high bandwidth of optical fiber in the optical frequency domain. Fast transmitter and receiver modules are basic elements of these systems, which should be able to transmit terabits/s of information via the fiber. Experiments with optoelectronic integrated circuits (OEICs) in laboratory test beds and field tests require a special packaging that respects system requirements such as high RF data rate and low insertion loss. Several concepts for fiber-chip-coupling with tapered fibers had been proposed [1, 2] in the in the past, like laser micro welding [3, 4] which is now becoming standard for industrial high volume manufacture [5]. The investment costs for this laser welding equipment are considerably high. For laboratory use and rapid prototyping a flexible design is needed which is able to adapt different OEICs with changing dimensions to an existing module type and a reconfigurable mechanical set-up for the fiber-chip-coupling. Additionally an RF-connection to the OEIC with very high modulation bandwidth should be implemented. 3. COUPLING CONCEPT A comparison of the optical fields of a butt ended standard monomode fiber (SMF) and of edge emitting laser diodes shows a great mismatch (see fig. 1). This mismatch is the reason for the very low coupling efficiency of ca. 10%. This low efficiency can be overcome by a better adoption of the two optical mode fields with lenses. In our case we used lenses made at the end of the fiber by melting the glass fiber and pulling it. This kind of fiber end is called fiber taper and works like a lens with diameters from 20 µm to 50 µm (fig. 2). With this tapered fibers a coupling efficiency of more than 50% can be reached. For the adjusting of the tapered fiber a Fig. 1: Optical far field of SMF and laser diodes Fig. 2 :Tapered SMF fiber :37Page 1/5

2 patented new low cost fiber-chip-coupling concept has been developed by Fischer, Peters et. al. [6,7]. It includes a reusable multipurpose packaging for rapid prototyping and low volume production in laboratory environment. In this article, we describe the further development of this module into a miniaturized package (DIP24M6,10x12x30 mm) which is an off-spring of the industrial DIL-housing family. The module includes one optical input and one RF-input (Wiltron V-connector), several DC-inputs and is temperature controllable. The OEIC is placed at one end of the miniature optical coupling bench on a subcarrier, which is fixed by screw (fig. 3). The optical fiber is fed into a metal cannula (ø 350 µm) and fixed by glue or solder. One end of the ferule is locked at the end of the optical bench in a clamp. To induce a parallel adjustment of the fiber tip to the waveguide of the OEIC the cannula is fixed with an angle of nearly 15 to the x and y direction (fig. 4). The cannula acts as a spring in x and y direction with a minimal force of 3 N due to an x and y offset of 300 µm. With the help of two adjusting needles the tapered fiber end can be moved precisely in x and y direction by piezo translators or differential micrometer screws. The z direction is adjusted by longitudinal shifting of the fiber in the metal cannula. After optimal adjustment of the tapered fiber in front of the OEIC waveguide a screw or laser Fig. 1: Coupling machine for laser welding Fig. 3: lateral translation mechanism for the welding flange welding fixes the adjusting needles. To correct the polarization direction of polarization maintaining fibers the tapered fiber is rotated around the z-axis. All adjustments are made in a specially designed patented manipulator tool for fixing in an easy way the x, y and z as well as the rotational positions. A patented new low cost fiber-chip-coupling concept has been developed by Fischer, Peters et. al. [6,7].It includes a reusable multipurpose packaging for rapid prototyping and low volume production in laboratory environment. In this article, we describe the further development of this module into a miniaturized package (DIP24M6,10x12x30 mm) which is an off-spring of the industrial DIL-housing family. The module includes one optical input and one RF-input (Wiltron V-connector), several DC-inputs Fig. 2: Laser focussing heads and is temperature controllable. The OEIC is placed at one end of in position to the miniature optical coupling bench on a subcarrier, which is BFY module fixed by screw (fig. 1). The optical fiber is fed into a metal cannula (ø 350 µm) and fixed by glue or solder. One end of the ferule is locked at the end of the optical bench in a clamp. To induce a parallel adjustment of the :37Page 2/5

3 fiber tip to the wav egui Flanschvariante 2 mit en de of Fig. 5: BFY-Module configuration the OEI C the cannula is fixed with an angle of Fig. 5: used flange types: a) lap-weld, b) fillet-weld nearly 15 to the x and y direction (fig. 2). The cannula acts as a spring in x and y direction with a minimal force of 3 N due to an x and y offset of 300 µm. With the help of two adjusting needles the tapered fiber end can be moved precisely in x and y direction by piezo translators or differential micrometer screws. The z direction is adjusted by longitudinal shifting of the fiber in the metal cannula. After optimal adjustment of the tapered fiber in front of the OEIC waveguide a screw or laser welding fixes the adjusting needles. To correct the polarization direction of polarization maintaining fibers the tapered fiber is rotated around the z-axis. All adjustments are made in a specially designed patented manipulator tool for fixing in an easy way the x, y and z as well as the rotational positions. 4. MODULE SET-UP The module consists of the miniature optical bench, which contains the OEIC, the tapered fiber, and the adjusting needles. The bench is located on top of a peltier cooler for the thermal stabilization of the OEIC. The laser bias, the peltier currents, and the temperature sensor are fed via the DIL-pins and a thick film circuit. The RF is fed via a semi-rigid cable from the Wiltron V-type connectors directly to the OEIC without a glass bead. This guaranties a good RFresponse up to 50 GHz as already shown [8]. We implemented in one module type a kind of coupling bench where the fixing of the adjusting needles is performed by laser welding, while in a second variant fixing by screws (fig. 3) is applied for reusable rapid prototyping. For the operation of waveguide integrated photodiodes (WGPD) with RF response of up to 50 GHz [9] there is no need for temperature control and the peltier cooler is removed. The OEIC carrier is fixed also by a screw so that it can be removed from the module quickly. 5. TEMPERATURE MANAGEMENT To avoid the mechanically twist of the optical bench, all used materials are made of stainless steel. To compensate the different extension coefficients of the OEIC (InP), the fiber, and the cannula, the carrier heat sink is made of copper. The used peltier cooler has a heat pump performance of 2.2 W at 20 C. The peltier cooler is fixed by glue at the bottom side of the lid of Steuergerät MM45 Druckstück Druckwippe the housing to prevent heat circulation inside the module. Additionally a DIL-cooler segment with up to 10 C/W heat spreading capacity can be fixed on the top of the module to be used in high temperature environment (70 C) :37Page 3/5 Kraftsensor ELF Stift des Flansches

4 lfd. Nr. Festigkeit [N] PK-Nr. Parameter 1 >100 2 >100 3 > ENVIRONMENTAL TESTS We performed long-term temperature tests with several modules between +15 C and +40 C and between 20 C and +70 C. To monitor the coupling stability of the modules we used in all tests WGPD chips and a stabilized laser source (HP 8153A). The output signal of the WGPD is not temperature dependant like that of laser diodes and exhibits only minor polarization sensitivity (±1 db). The temperature behavior of the module is shown in fig. 4 where a maximum output variation of ±0.1 db with temperature is to be noticed. After several temperature test runs no significant degradation (<0.2 db) of the coupling efficiency was detected. 7. SUMMARY We designed and fabricated a series of DILsized modules for one-sided fiber-chip coupling by a patented set-up. Electrical Tab. 1 : Festigkeiten der Flanschverbindungen modulation signals up to 50 GHz can be fed via RF-connectors to the OEIC. The modules have been tested in a reliability stress program. The package shows good temperature stability and is well suited for rapid prototyping and low volume production for use in laboratory environment 7. REFERENCES 1 F An = 10N; P Laser = 1,2kW; t Laser = 18ms; Überlapp- Schweißpunkt (PK 1) 2 F An = 10N; P Laser = 0,9kW; t Laser = 12ms; Kehl- (PK 2) 3 F An = 10N; P Laser = 2,4kW; t Laser = 2ms; Kehl- (PK 3) 4 F An = 5N; P Laser = 2,4kW; t Laser = 2ms; Kehl- (PK 4) Betrag von Sges [µm] 1 J.P. Hall, M.Q. Kearly, A.J. Moseley and M.J. Goodwin: Micropackaging Technologies for optoelectronic components, ECOC 1997, Edinburgh, UK, Conf.3 Publ. No. 6,0 448 IEE, vol. 4, pp (1997) 5,0 4,0 3,0 2,0 1,0 2 A.R. Mickelson, N. Basavanhally, Y-C.Lee: Optoelectronic Packaging, Wiley &Sons, New York, pp45-58, M. Becker, R. Günther, R-. Staske, R. Olschewsky, H. Gruhl and H. Richter: Laser micro welding and micro melting for connection of optoelectronic micro-components in Laser in Engineering, Laser 93, W. Waidelich, Ed., Springer-Verlag, Berlin, 1994, pp ,0 4 D.S. Alles: Trends in Laserpackaging, Proceedings of 40 th PK1 PK2 PK3 PK4 Electronic Components and Technology Conference, Las Parameterkombination (PK) Vegas, pp , 1990 Fig. 4: Dislocations measured for several pressure forces 5 D.S. Bargar: An Automated Fiber Alignment, Fixing and Hermetic Sealing System, SPIE Vol 994 Optoelectronic Materials, Devices, Packaging and Interconnects II, pp12-17, U. H. P. Fischer (member IEEE), K. Peters, R. Ziegler, D. Pech, A. Kilk,G. G. Mekonnen, G. Jacumeit.: Packaging of OEICs with Tapered Fibers for Optical Communications Systems with up to 45 GHz Modulation Bandwidth, NOC 99 Delft, Netherlands, Proceedings of the European Conference on Networks and Optical Communications 1999: Broadband Access and Technology, pp , :37Page 4/5

5 7 K. Peters, HHI, Patent DE , Patent DE D. Rohde, H. Ehlers, Th. Rosin, R. Ziegler, R.-P. Braun, G. Großkopf, J. Braunstein, W. Bronner: Optic/Millimeter-Wave Converter For 60 GHz Radio-Over-Fiber Systems, MIOP 97, Conf. Proc., p (1997) 9 A. Umbach, D. Trommer, A. Siefke, and G. Unterbörsch, 50GHz Operation of Waveguide Integrated Photodiode at 1.55µm, Proc. of the 21st ECOC, Brussels, Belgien, Sept. 1995, p ff., :37Page 5/5