Electrostatically actuated micro-fluidic optical cross-connect switch

An electrostatically actuated micro-fluidic OXC switch based on PLC and MEMS technologies has been developed. The motion of the fluid is controlled by the displacement of a mechanical diaphragm driven by an electrostatic actuator in order to ensure low power consumption and enhanced reliability. Specific technological developments have been made for the fabrication of 2 /spl times/ 2 and 8 /spl times/ 8 switches, in particular in the assembly of the two wafers and for the release process. The approach has bee validated by the good optical results obtained on test devices and the good mechanical behaviour of the membranes. Further characterisations will be performed on 8 /spl times/ 8 and 32 /spl times/ 32 switches and will benefit on complementary developments that are still on progress in particular fluid filling.


Introduction
Optical cross-connect (OXC) switches are considered key components for future all-optical fiber networks because they avoid electrical conversion of the optical signals and allow network flexibility by providing provisioning and protection functions.Several solutions have already been proposed based on various technologies such as integrated optics, moveable optical fi bers or waveguides, moveable micro-mirrors, microfl uidic switches.Devices using planar lightwave circuits (PLC) are very promising for moderate size OXC because they avoid light divergence in free-space and accurate control of mirror defl ection.The existing solutions rely on inkjet technology [1] or thermocapillarity effect [2].This paper proposes an alternative that exhibits all the advantages of previous PLC based configurations but avoid thermal action on the index matching fluid in order to erihance reliability.

Device description
The device is based upon the combination of Planar Lightwave Circuit (PLC) technology, MEMS technology and Microfluidic technology.The principle of the elementary 2x2 switch relies on Total Internal Reflection from the sidewalls of a trench etched at the crosspoints of two waveguides (Fig. I).Depending on the position of an index matching fl uid in the trench, the propagation of light is modifi ed and a switching function can be provided.The transmitting and reflective states are respectively obtained when the trench is filled or empty.In this device, the motion of the fluid is controlled by the displacement of a mechanical diaphragm driven by an electrostatic actuator.As shown on Fig. 2, this elementary 2x2 switch is used to realize NxN optical cross-connect switches.This approach offers key attributes.This MEMS-type solution exhibits wavelength and polarisation insensitivity and very low crosstalk.The PLC configuration eliminates optical losses due to beam divergence and allows the use of a low cost fiber-chip connection.The electro-mechanical actuation ensures low power consumption and enables to avoid thermal action on the index matching fluid.As a consequence, erihanced reliability can be expected. .

Characterisation of 2x2 and 8x8 switches
With an appropriate index matching fluid, the measured optical losses are between 0.09 and 0.41dB/gap (index liquid, waveguide/gap angle gap width) for the transmitting state and between 1.9 and 3.8 dB (waveguide/gap angle) for the reflective state.Assuming propagation losses <0.05 dB/em and coupling loss < O.SdB, we can expect insertion losses respectively < 4.3 dB for 8x8.In addition, it must be highlighted that we have theoretically demonstrated that these results can be easily improved by changing the waveguide structure, leading respectively to 2.5dB and 4dB for 8x8 and 32x32.The optical crosstalk was measured to be <-50dB for 2x2 and about -30/-40dB for 32x32.The transmitting state exhibits a PDL (Polarisation Dependent Loss) of between 0.04 and 0.06dB for 2x2 (waveguide/gap angle).As a result, 0.45 to 0.6dB can be expected for 32x32.For the reflective state, the PDL can be higher: 0.06 to 1.27dB (waveguide/gap angle), but this figure can be optimised with a good design.
Fig. 6 shows the behaviour of the membrane when it is actuated to provide the switching function.actuator in order to ensure low power consumption and enhanced reliability.Specific technological developments have been made for the fabrication of 2x2 and 8x8 switches, in particular in the assembly of the two wafers and for the release process.The approach has been validated by the good optical results obtained on test devices and the good mechanical behaviour of the membranes.Further characterisations will be performed on 8x8 and 32x32 switches and will benefit of complementary developments that are still on progress in particular on fluid filling.

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Transmlttlna State" (gap filled with Index matching fluid) Electrostatic gap Gap In transmitti ng state Optical �uides I Gap in reflective state j v L lementary"' 2x2e located at the cro ..__,

Figure 1 :Figure 3 :
Figure 1 : Principle of the 2x2 elementary switch Figure 2 : NxN Optical Cross-Connect switch Technological developments and fabrication of 2x2 and 8x8 switchesThe fabrication of the NxN switches requires the fabrication and assembling of two wafers (Fig.3).The planar lightwave circuit, including the optical waveguides and the trenches, is fabricated on a first wafer.The electromechanical structure, including the moving membrane and the electrostatic gap, is fabricated on a second wafer.Then the two wafers are assembled using a silicon direct bonding (SDB) technique.Elementary 2x2 switches and 8x8 switches were fabricated (Fig.4,5).The optical trench is 5-20J.1mlarge, the diameter and thickness of membrane are respectively I 00-SOOJ.l.m and 1-3 JliD, the electrostatic gap is 1-5 Jlm.The fabrication benefits from the know-how in Si02/Si PLC technology, SOl MEMS technology and SDB technology.However, specific technological developments have been necessary.A specific planarisation and bonding technique has been developed for the assembling.The technique is now capable of satisfying the severe specifications : high surface quality resulting from planarisation of the silica layers, high bonding energy (500-1000mJ/m2) to ensure good bonding despite the high stresses existing in the silica layers and protection against membranes breaking during assembling steps.In addition, good results have been achieved in other key steps.In particular, membranes with

Figure 5 :Figure 6 :
Figure 5 : Photography of a 2x2 elementary switch Conclusion