A low power Mach-Zehnder interferometer thermo-optic switch using free-standing silicon-on-insulator strip waveguides is demonstrated. The air gap provides thermal isolation between the waveguide interferometer arms and the underlying silicon substrate. The highly confined optical modes of the strip waveguides enable miniature heated cross-sections. The heating efficiency from on-chip resistive heaters is enhanced. Measurements of fabricated devices using 100 μm arm lengths at 1550 nm wavelength result in a switching power of 540 μW, a 10% - 90% switching rise time of 141 μs, and an extinction ratio of 25 dB.
©2010 Optical Society of America
Advances in optical switching technologies are essential for future high performance optical networks. Optical cross connect (OXC) and optical add-drop multiplexing (OADM) nodes enable routing, restoration, and reconfiguration in wavelength division multiplexing (WDM) systems. A large number of 1 × 2 and 2 × 2 switches may be used in the implementation of these components. Therefore, it is desirable to develop optical switches with small footprint, low power consumption, low insertion loss, and low cost, with switching times on the order of milliseconds or less [1–3].
Optical switches based on silicon are particularly attractive. Highly confined optical modes allow for high density integration and waveguide bends with micrometer scale radii of curvature . The relatively large thermo-optic coefficient of silicon, ∂n/∂T = 1.86 × 10−4 /K at 300 K (Ref. 5), and high thermal conductivity, kth = 148 W/(m∙K) (Ref. 6), enable Mach-Zehnder interferometer (MZI) thermo-optic switches with arm lengths as short as tens of micrometers, switching powers as low as tens of milliwatts, and switching times as fast as microseconds [7–10]. Folded silicon waveguides have been used to demonstrate 6.5 mW switching power and 14 μs switching time using interferometer arm lengths of 6.3 mm compacted into 130 μm diameter spirals . The folded waveguides circumvent length independent switching power. Direct heating of the silicon waveguide through the use of doped silicon has also been used to demonstrate 6 mW switching power and 600 ns switching time .
For applications where millisecond scale switching times are acceptable, it is desirable to investigate methods to trade off switching time for a reduction in switching power. Efforts in this direction include the etching of deep trenches to prevent heat diffusion  and the etching of v-grooves that yield suspended waveguides to provide thermal isolation between the waveguides and the silicon substrate . A greater temperature increase occurs for a given electrical heating power resulting in approximately an order of magnitude reduction in switching power, at the expense of switching speed, for the v-groove case. In reference , 10 mW switching power and 1.1 kHz bandwidth are achieved using suspended silicon-on-insulator (SOI) rib waveguides with relatively large mode cross-section. In this paper, we present MZI thermo-optic switches using free-standing SOI strip waveguides for the interferometer arms. The cross-sectional dimensions of the silicon waveguide core are 450 nm × 250 nm. The submicrometer squared cross-section and thermal isolation enhance the temperature increase in the waveguide core. Consequently, the heating power from on-chip resistive heaters contributes to the thermo-optic effect more efficiently. We demonstrate submilliwatt switching power and submillisecond switching speed using an arm length of 100 μm.
This paper is organized as follows. The design of the MZI thermo-optic switches is discussed in section two. The fabrication process for the test devices is described in section three. The experimental results are discussed in section four. Finally, concluding remarks are given in section five.
A schematic of the MZI thermo-optic switch with free-standing SOI strip waveguides is shown in Fig. 1 . It consists of a symmetric Y-junction on the input, two free-standing SOI strip waveguides for the interferometer arms, and a 3-dB directional coupler on the output. The bottom cladding for the silicon waveguide core is buried oxide (BOX). The top and side cladding is PECVD silicon dioxide. The SOI strip waveguide is released from the silicon substrate and surrounding silicon dioxide cladding leaving an air gap. The air gap reduces heat conduction to the silicon substrate since the thermal conductivity of air, equal to 0.026 W/(m∙K) (Ref. 5), is much smaller than the thermal conductivity of silicon.
Several factors are considered for the design of the width and height of the free-standing SOI strip waveguide. In Fig. 1, the lateral cross-sectional dimension, labeled w, and vertical cross-sectional dimension, labeled h, are designed to be much larger than the guided wave optical mode so that the effect of the truncation of the silicon dioxide cladding is minimized. At the same time, it is desirable to keep the cross-sectional dimensions small in order to keep the heated cross-section small so that the temperature rise from the resistive heaters is maximized. A design that ensures high yield during fabrication is also considered. Mode analysis using beam propagation method (BPM) shows that the lateral dimension of the quasi-TE mode for a silicon strip waveguide, of width equal to 450 nm and height equal to 250 nm, embedded in silicon dioxide is approximately 600 nm. In this work, w is equal to 2.9 μm and h is equal to 2.1 μm.
Balanced platinum resistive heaters which have the same width w as that of the released arms are deposited on top of the released SOI waveguides. Application of electrical power to the resistive heaters results in a change in effective index of the free-standing SOI waveguide mode due to the thermo-optic effect. The change in the effective index of the guided mode, Δne, is equal to the product of the change in the effective index with temperature, ∂ne/∂T, and the temperature change, ΔT. The induced phase shift in the heated arm affects the phase interference in the output 3-dB directional coupler. Consequently, optical power is redistributed between the two output ports.
Test circuits for optical characterization are fabricated on an SOI wafer with top silicon thickness equal to 250 nm and buried oxide thickness equal to 1 μm. The fabrication process is shown schematically in Fig. 2 . The layout for the silicon waveguide core is defined in hydrogen silsesquioxane (HSQ) resist using electron beam lithography. The resist patterns are transferred to the silicon device layer using inductively-coupled plasma etching with HBr chemistry. A 1.1-µm-thick SiO2 top cladding is then deposited by plasma enhanced chemical vapor deposition (PECVD). Next, a 150 nm thick nickel mask is evaporated on top of the PECVD SiO2. The patterns for the free-standing SOI waveguide interferometer arms are written on the Ni mask using Ga+ focused ion beam (FIB) milling. A subsequent SF6 reactive ion etch releases the interferometer arms by etching the SiO2 anisotropically and the silicon substrate isotropically. The Ni mask is removed with HNO3 solution. The platinum heaters and contact pads are deposited using ion-assisted chemical vapor deposition (IACVD) with the FIB. The IACVD deposition of platinum is based on the decomposition of Pt containing metal-organic vapor (methylcyclopenta-dienyl [trimethyl] platinum) by focused Ga+ ion bombardment. The resistivity of IACVD platinum is 1-2 × 10−5 Ω∙m. Platinum heaters with a nominal thickness equal to 200 nm, width equal to 2.9 μm, and length equal to 100 μm are deposited on the released interferometer arms. The total contact pad-to-pad resistance is 1.6 kΩ.
Figure 3 is an optical micrograph of the fabricated switch. At the input and output ports of the switch, cantilever couplers are used to couple light between tapered optical fibers and the photonic circuit . The total footprint of the thermo-optic switch, excluding contact pads, cantilever couplers, and waveguide feed-lengths, is approximately 320 µm by 28 µm. The inset of Fig. 3 shows a scanning electron micrograph of the free-standing SOI waveguide interferometer arm. Also shown are two silicon dioxide struts which provide mechanical stability. The struts prevent deformation of the released interferometer arms, due to residual stress, which can lead to non-uniform and discontinuous resistive heaters.
For comparison purposes, a similar unreleased thermo-optic switch is fabricated. The unreleased switch has the same dimensions and resistive heaters as the released switch. However, the two interferometer arms are not released from the substrate.
The fabricated thermo-optic switch is characterized by optical transmission measurements. Infrared light from a tunable continuous wave (CW) laser is fiber connected to a polarization controller which outputs linearly polarized transverse electric (TE) light at 1550 nm with cross-polarization rejection ratio greater than 17 dB. Tapered optical fibers with tip diameter approximately equal to 2 µm are butt-coupled to the input and output cantilever couplers. The output light is detected using a photodetector and measured by a power meter. Electrical power is delivered to one arm of the interferometer via the on-chip Pt resistive heater contact pads. The heated arm is on the same side of the switch as the output port labeled port 1.
Figure 4(a) is a plot of the optical transmission versus electrical heating power. The measured power to switch from maximum to minimum optical transmission on port 2 is 540 µW. The corresponding changes in the index of refraction and temperature of the silicon core are approximately 7.7 × 10−3 and 41.7 K, respectively. The change in the index of refraction of the SiO2 cladding is neglected because the thermal refractive coefficient of SiO2 is an order of magnitude less than that of silicon . The measured extinction ratio is 25 dB. The total insertion loss of the switch, relative to the transmission of back-to-back tapered fibers, is 6 dB. An estimated fiber-to-chip coupling loss of 1.6 dB per connection, obtained from linear regression of insertion loss versus length , yields a device insertion loss approximately equal to 2.8 dB. Figure 4(b) is a plot of the measured 10% - 90% switching rise time for port 2 of the released device as the heating power is switched to produce a change in the optical intensity from minimum to maximum. The optical output intensity is recorded using an oscilloscope. The measured rise time is 141 μs, corresponding to a 3-dB bandwidth of 2.48 kHz.
As a comparison, the similar thermo-optic switch, with interferometer arms not released from the substrate, is measured. The unreleased switch has similar insertion loss and extinction ratio as the released switch. The measured switching power, however, is 31.2 mW, which is more than one order of magnitude higher than the released version. The measured rise time of the unreleased version of the switch is 39 μs.
We present submilliwatt switching power and submillisecond switching time in Mach-Zehnder interferometer thermo-optic switches by employing free-standing SOI waveguides in 100 μm long interferometer arms. The air gap provides thermal isolation between the waveguide interferometer arms and the silicon substrate. The strip waveguides enable miniature heated cross-sections. This approach effectively trades off switching speed for reduced switching power. At 1550 nm wavelength, measurements of fabricated devices demonstrate a switching power of 540 μW, a 10% - 90% rise time of 141 μs, an extinction ratio of 25 dB, and a device insertion loss approximately equal to 2.8 dB. Applications include optical cross connect (OXC) and optical add-drop multiplexing (OADM) nodes in future high performance optical networks.
This work was supported by National Science Foundation Grant ECCS-0725657.
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