Laser-activated Optical Bubble Switch Element

In Agilent’s Photonic Switching Platform, resistive heaters control tiny bubbles enabling all-optical switching. This paper reports on the addition of more efficient laser-activated heaters. Measurements confirm improvements in power dissipation and insertion loss stability. Introduction In Agilent’s approach to photonic cross-connect switching [1], a fused silica planar lightwave circuit (PLC) chip is hermetically sealed to a silicon matrix controller chip (MCC), leaving a small gap in between. The former contains intersecting arrays of rectangular waveguides with trenches etched into each crosspoint. The latter consists of a matrix of electrically addressable resistive heating elements, each aligned to a waveguide crossing. A fluid whose refractive index matches the effective index of the waveguides fills the trenches and the gap between MCC and PLC. Thermoelectric coolers control the fluid temperature to ensure accurate index matching. An external temperature-controlled fluid reservoir regulates the fluid pressure inside the sealed area. The structure of a single crosspoint or bubble switch element is shown in Figure 1. When no bubble is present, signals in the form of guided lightwaves can transverse the trench from IN to DROP and ADD to OUT with little loss. Once the electrically controlled resistive center heater generates a vapor bubble, total internal reflection is made possible at the vapor/silica interface of the trench sidewalls. Note that the sidewall metalizations drawn in Figure 1 are not part of the original design; their role in the laser-activated bubble switch element will become clear shortly. Prototypes employing the bubble switch element described have been demonstrated at port counts of 16x32 and 32x32 [2]. Agilent Technologies’ prototypes have already achieved insertion losses below 9 dB, polarization dependent losses between 0.05 dB and 0.23 dB, typical switching times of 7.7 ms, and crosstalk in the –50 dB to –55 dB range. In [3], J. Uebbing et al. investigated the heat and fluid flow in a bubble switch element in more detail. In essence, if the sidewall temperature at the silica-vapor interface is below the vapor temperature of the bubble a thin liquid film, referred to as the dimple in [3, 4], covers this portion of the sidewall. Depending on the implementation and the operating conditions this film may interfere with the total internal reflection occurring at the sidewall. One way to a stable and reliable reflection interface is to provide sufficient heat transfer to the sidewall in the vicinity of the waveguides. In this paper, we propose to heat the sidewall metalization shown in Figure 1 using laser irradiation. This will increase heat transfer to the silica-vapor interface. Once the silica temperature around the waveguide location rises above the vapor temperature, a dry reflecting interface ensures stable reflection perpendicular to the waveguide axis. Experimental Setup For evaluation of the laser-activated bubble switch design, prototype devices with a 4x4 matrix of crosspoints were built. These include crosspoints with sidewall metalizations of 10x100, 30x100, 70x100, and 84x140 μm in size and crosspoints without any metalization to act as conventional design reference. For irradiation, we used a laser diode at 785 nm with 50 mW maximum output power. The laser beam was focused on the metalization of the reflecting sidewall, defined here as the side closer to the IN and OUT paths. At its focus, the laser creates a nearly circular spot of about 38 μm radius. The absorption coefficient of the tantalum metalization is about 0.4. Results We compared bubble switch elements in terms of their insertion loss (IL) characteristic. We obtained this characteristic by measuring the IL in the IN to OUT signal path at 1550 nm while increasing the center heater power at a fixed laser output power. Subsequent plots report IL normalized to its maximum value on a logarithmic scale in decibels. Note that this approach captures only steady-state behavior and excludes any transient switching effects. Fig. 1. Top view of bubble switch element. IN Signal ADD Signal DROP Signal OUT Signal Trench Bubble Optical Waveguide Center Heater Fluid Sidewall Metalization Figure 2 compares the IL characteristic of one conventional and two laser-activated bubble switch elements. For small center heater power, the IL of the conventional design is at the limit of the dynamic measurement range, since no bubble is present. At a heater power of 55 mW, the bubble nucleates and expands into the trench and the IL decreases to a minimum around 72 mW. This type of bubble is called the analog bubble. Because of the dimple, a further increase in heater power causes the IL to increase. At high heater power, the IL decreases again, possibly reaching a second minimum. On the other hand, both laser-activated switch elements show the desirable step-shaped or “digital” IL characteristic—the bubble switch element is either on (small IL) or off (high IL). Once the bubble has nucleated at 55 mW, increasing the center heater power drives the IL monotonically towards its constant optimum, loosely referred to as the digital bubble. This suggests that the adverse effects of thin liquid films on IL have been effectively eliminated with as little as 8 mW laser output power. Moreover, further measurements confirmed the expected superior IL stability of the digital bubble over the analog bubble. Figure 2 also reveals that laser heating with a smaller sidewall metalization is more efficient than with a larger one. Sweeping additionally over the laser output power results in a 2D characteristic. Figure 3 shows such an IL surface for a laser-activated switch element, from which the range of powers for digital bubble operation can easily be extracted. For instance, operation at 60 mW heater power and 10 mW laser power results in a total power consumption of 70 mW; the same as for operation with an analog bubble, but with greatly improved optical loss and stability performance. The surface plot also suggests digital operation points below the bubble nucleation power, which we could verify experimentally. Note that the bubble does not collapse for heater powers below 20 mW under the operating conditions chosen. Fig. 2. Insertion loss characteristic for a conventional (dashed ---), the laser-activated bubble switch with 10x100 μm (solid —) and 30x100 μm (dotted ····) sidewall metalizations at 8 mW laser output power. Fig. 3. IL characteristic for 30x100 μm metalization. Conclusions In this paper, we describe a laser-activated optical bubble switch element. In addition to the center heater used in the conventional bubble switch design, it provides heat to a tantalum metalization next to the sidewall of the switch element by means of laser irradiation. This effectively prevents thin liquid films from forming on the reflecting interface that degrade the reflected signals’ insertion loss. Experimental results showed that with a reduction in total power dissipation over the standard switch element, the proposed design achieves greatly improved insertion loss characteristics. AcknowledgementsThe authors gratefully acknowledge the work of VelodyaAjaev and Professor Bud Homsy (UCSB), and theproject team headed by Julie Fouquet at AgilentLaboratories. We would also like to thank our colleaguesin the Optical Networking Division for their contributionsto the Photonic Switching Platform. References[1] J. E. Fouquet, “Compact optical cross-connect switch based on total internal reflection in a fluid containingplanar lightwave circuit,” Optical Fiber Communications Conference, Technical DigestPostconference Edition, Trends in Optics andPhotonics, vol. 37, part vol. 1, March 2000.[2] S. Venkatesh, J.-W. Son, J. E. Fouquet, R. E. Haven,D. Schroeder, H. Guo, W. Wang, P. Russell, A. Chow, P. F. Hoffmann, “Recent advances in bubble-actuated photonic cross-connect,” Silicon-based andHybrid Optoelectronics IV, January 2002.[3] J. Uebbing, D. Schroeder, S. Hengstler, S.Venkatesh, R. Haven, “Heat and fluid flow in an optical switch bubble,” submitted to the IEEE/ASMEJournal of Microelectromechanical Systems,November 2002.[4] “Simulation Helps Microscopic Bubbles Switch Fiber Optic Circuits,” Fiberoptic Product News, pp. 22-23,January 2003.