By exploiting the small bend radius achievable using high-index-contrast silicon photonic wire waveguides, we demonstrate a new low power thermo-optic switch arranged in a dense, double spiral geometry. Such a design permits the waveguide length to be extended for increased phase shift, without the need for increased heated volume. This provides an effective means to reduce the power consumption of thermo-optic switches, as well as a compact geometry desirable for the development of switch arrays. A low switching power of 6.5 mW was obtained for a spiral-path Mach-Zehnder interferometer device having a 10% - 90% rise time of 14 μs. The switching power is shown to be reduced by more than 5 times compared to a Mach-Zehnder interferometer employing a conventional straight waveguide geometry.
© 2009 OSA
There is considerable interest in new planar waveguide optical switching technologies for applications such as optical cross-connect (OXC) and reconfigurable optical add-drop multiplexer (ROADM) development. Since a large number of 1 × 2 or 2 × 2 thermo-optic switches may be cascaded in these components, it is critical that they have compact footprint, low power consumption and can be produced at low cost. Thermo-optic switches have traditionally been fabricated using silica–based waveguides due to their low insertion loss, low birefringence and inexpensive fabrication. However, these devices require relatively large switching powers (typically several hundred mW per switch) due to the low thermo-optic coefficient of SiO2 (∂n/∂T ~10−5 K−1). Furthermore, the low-index-contrast between the waveguide core and the surrounding cladding material (typically ~1%) limits the bend radii and minimum size of these waveguides, leading to rather large switch arrays exceeding several square centimeters in size .
To overcome these limitations, polymer and silicon-based switches have been reported with greatly reduced switching powers, made possible by their large thermo-optic coefficients of ~-1 × 10−4 K−1 for polymer  and ~1.86 × 10−4 K−1 for silicon . Silicon-on-insulator (SOI) based devices have particularly attracted attention due to their compatibility with complementary-metal-oxide-semiconductor (CMOS) fabrication processes permitting monolithic integration of control electronics on-chip, and the high refractive index contrast between the silicon waveguide core and the lower SiO2 cladding (Δn ~2). The high index contrast permits submicrometer size waveguides to be formed, referred to as silicon photonic wires, which support small bend radii down to a few micrometers. This has led to the miniaturization of planar waveguide circuits with dimensions up to two orders of magnitude smaller than those using traditional low-index-contrast waveguide systems [4,5]. Switch geometries based on ring resonators [6,7] and Fabry Pérot cavities  have been reported in SOI and offer ultra-compact size. However, most work to date has been focused on Mach-Zehnder interferometer (MZI) switches due to their insensitivity to wavelength for broadband operation. MZI switches have been reported using SOI waveguides with cross-sections varying from several micrometers [3,9–11] to those employing sub-micrometer photonic wires [12,13]. Compared to silica-based switches these devices have yielded comparably low switching powers (~30-200 mW) and have demonstrated reduced switching times (~5-100 μs).
In this paper, we present a new means of further reducing the power consumption of thermo-optic switches by exploiting the small bend radii achievable with silicon photonic wire waveguides. We demonstrate a new MZI structure that contains densely folded waveguides for increased overlap of the waveguide volume with the induced temperature distribution. The high waveguide density allows increased waveguide path length to be contained in a small volume, thus allowing increased phase shift to be induced for a given applied thermal power. Furthermore, the device does not require minimized heater width to obtain low switching power as in a conventional design. This reduces the heater’s susceptibility to defects, alleviates limitations with power rating and increases the heater fabrication tolerance.
2. Theory and design
A common MZI-based thermo-optic switch configuration consists of a thin film heater deposited on a protective cladding material covering one of the interferometer arms, as illustrated in Fig. 1 . The switching power is defined as the electrical power delivered to the heater electrode that induces a phase shift Δφ = π at the MZI output. The thermally induced phase shift of the light traveling through the heated waveguide of length LH, which has undergone a temperature change ΔT is given by:Fig. 1), the heated volume also scales with LH, resulting in the requirement of increased power to the heater to maintain a given ΔT. These effects cancel one another and a length independent switching power results. To circumvent this limitation, we have developed MZI switches employing densely coiled photonic wire waveguides, as shown in Fig. 2 . In such a geometry, the active waveguide length can be increased independently of the heated volume through an increase in the waveguide packing density. This increases the overlap of the waveguide volume with the induced temperature distribution for increased heating efficiency. Furthermore, the laterally diffused heat from the electrodes, which is lost in a conventional straight waveguide design, can contribute to the temperature rise in the adjacent waveguides, thereby improving the switching power further.
To examine these concepts, both straight and spiral waveguide 1 × 1 MZI switches were designed and fabricated on a common test die using 0.26 × 0.45 μm2 photonic wire waveguides of varying lengths from 2 mm to 6.3 mm. The photonic wires were surrounded with a 1.5 μm and 2 μm thick upper and lower SiO2 cladding, respectively. For the straight waveguide devices the heater electrode had a width of 10 μm and varying length. For the spiral waveguide switches the heater electrode was identical for all devices, having a 10 μm stripe width, a 12 μm electrode pitch, and a total length of 1.355 mm (see Fig. 2). With an active waveguide length LH = 6.3 mm for both the straight and spiral waveguide designs, Eq. (1) yields a temperature change of ΔT = 0.67 K, required to obtain a phase shift of Δφ = π. Here, a wavelength of 1.55 μm was assumed and ∂Neff / ∂T ~1.82 × 10−4 was obtained from mode expansion-based optical simulations for the quasi transverse magnetic (TM)polarized waveguide mode used in this work. To examine the behavior of the two configurations, thermal simulations were performed using a two-dimensional, finite difference thermal solver . The double spiral waveguide device is first considered. The device cross-section of Fig. 2 was simplified to the model geometry shown in Fig. 3(a) to approximate the average overlap of the temperature distribution with the waveguide volume. This model includes fifty-nine 0.26 × 0.45 μm2 silicon photonic wire waveguides of 2 μm pitch, positioned on a 2 μm thick buried SiO2 lower cladding. The underlying silicon substrate was taken to be 20 μm thick for the simulation and assumed to be a perfect heat sink with zero temperature at its lower boundary. The photonic wires were covered with a 1.5 μm thick SiO2 upper cladding. The heater element of Fig. 2 was broken into 12 separate straight segments of width 10 μm. A total thermal power PT, was applied to the heaters, where each element received 1/12 of this value. This configuration replaces the spiral waveguide path of Fig. 2(b) with a parallel array of straight waveguides, thereby allowing a two-dimensional thermal simulation to be used. Since the waveguide and electrode spacing and widths are the same as for the spiral layout, the results should be quantitatively accurate.
The simulated steady-state temperature distribution was calculated for various values of PT. Figure 3(a) shows the results for the case of PT = 6.1 mW, while Fig. 3(b) shows the cross-sectional temperature distribution in the x-direction along the plane of the photonic wire waveguides (at y = 0). It is observed that the waveguides positioned directly beneath the electrodes are heated to the highest temperatures, while the waveguides located between them are heated to above 90% of this temperature, benefiting from the lateral heat diffusion. The average temperature induced in the plane of the photonic wires equals the required π–phase shift temperature change of 0.67 K calculated above, at a power of 6.1 mW.
For the straight waveguide design, a similar calculation was performed where a single 10 μm wide heater element of length 6.3 mm was centered over a single photonic wire waveguide of the same length. It was found that an applied power of 31 mW was required to induce a 0.67 K temperature change in the photonic wire plane (y = 0). This yields a switching power of more than 5 times that of the spiral waveguide device.
3. Experiment and discussion
To experimentally demonstrate the proposed concept, devices were fabricated on unibond SOI wafers obtained from SOITEC with a 0.26 μm thick silicon core layer and a 2 μm thick buried oxide (BOX). Using electron beam lithography, the silicon layer was patterned and then etched to the underlying BOX layer using an inductively coupled plasma etching system with a SF6/C4F8 chemistry. After the etching process was complete, a 1.5 μm thick SiO2 cladding was deposited over the device surface. Finally, Cr/Au heaters of thickness 20/60 nm were deposited using an electron beam deposition system and defined by a lift-off process. For effective fiber-to-waveguide coupling the silicon input waveguide was tapered from the 0.45 μm wide device waveguide to 0.15 μm at the waveguide facet. The small width of the tapered section causes the waveguide mode to adiabatically expand in both the vertical and horizontal directions, thereby improving the mode overlap with the input fiber.
A lensed optical fiber was used to deliver light with a wavelength of 1550 nm from a tunable laser source to the inverse taper mode transformer of the device. The light transmitted through the MZI was collected and focused onto a high speed InGaAs photodetector using a 10 × objective lens. The passive properties of the device were first measured. The propagation loss was evaluated using a straight waveguide fabricated on the same chip as the device and the Fabry-Pérot resonance technique. It was found to be 0.5 ± 0.1 dB/mm, where a three dimensional finite difference time domain simulation was used to estimate the facet reflectivity to be ~20%. We believe that with optimization of the waveguide patterning process and incorporation of sidewall smoothing techniques [15,16] the propagation loss can be significantly improved. The loss penalty of the double spiral waveguide originating from bend loss (including the increased mode-sidewall interaction in the curved sections) was found to be less than 0.5 dB for devices with a minimum bend radius of 4 μm. The insertion loss of the longest 6.3 mm MZI device, defined as the sum of the splitting/combining loss and the propagation loss through the MZI arms, was found to be ~6 dB. The optical coupling loss from the tapered fiber to the waveguide was ~3 dB, indicating effective mode expansion near the device facet.
To evaluate the optical switching power requirements of the devices a signal generator was used to supply a triangular wave voltage signal to the heater electrode, while the transmitted optical signal was monitored by displaying the high speed photodetector output on a digital oscilloscope. This process was performed on both straight and spiral waveguide MZIs of various path lengths. The spiral devices all had the same circular area for the active waveguide arm and identical heater patterns, while the straight waveguide designs had the same electrode width and varying length. The experimental results in Fig. 4 show that the switching power is independent of active waveguide length in the straight waveguide case and is a decreasing function of waveguide length in the spiral geometry. The dashed lines illustrate the fit of the data using a constant value for the straight waveguide configuration and a 1/LT dependence for the spiral design. The measured switching powers are in good agreement with the calculations above, lying within 10% of the predicted values. The discrepancy is mainly attributed to contact resistance between the electrical probes and the heater pads causing power to be lost at the probing points.
The switching curve obtained for the most densely spiraled MZI device, with waveguides spaced at 2 μm, an active waveguide length of 6.3 mm and a heater resistance of ~80 Ohms, is shown in Fig. 5 . Low switching power of ~6.5 mW and an on/off extinction ratio of ~22 dB was obtained. The switching power is less than 5 times that obtained for our straight waveguide devices with an average value of 36 mW. It is also significantly less than that reported for other silicon photonic wire MZI switches employing conventional geometries, which demonstrate values varying between 45 and 90 mW [12,13].
The frequency response of the devices was then examined by applying a square wave voltage signal to the heaters of magnitude sufficient to induce a π-phase change in the MZI output. The device response is shown in Fig. 6 for the 6.3 mm long spiral device. A 10% to 90% rise time of ~14 μs is obtained, which corresponds to a −3 dB frequency cutoff of ~25 kHz. A similar measurement was performed on a straight waveguide device of 6.6 mm length, which demonstrated a rise time of ~12 μs, indicating that the lateral heat diffusion in the spiral design has little impact on the switching speed. The frequency response of the device is fast compared to silica and polymer-based switches with typical rise times on the ms scale and lies within the lower range reported for silicon-based switches (5 - 100 μs). We believe that the switching speed of the device may be further improved by decreasing the lower SiO2 cladding thickness. However, a minimum value should be maintained for low mode leakage loss into the substrate (above ~0.7 μm for our design).
The observed performance of the devices is encouraging and may be improved by further optimization of the heater electrode and waveguide cladding design. The reduced power consumption of the spiral design originates from its ability to use the laterally diffused heat from the electrodes and the increased proportion of waveguide length per heated volume. For a conventional straight waveguide MZI device the former parameter can be increased by reducing the heater width. However, the reduction of switching power obtained in this way will ultimately be limited by lateral heat diffusion from the electrode. For example, the calculated switching power for our straight waveguide MZI switch with an infinitely narrow heater is 14.1 mW, being reduced by only a factor of ~2 compared to that having a 10 μm heater width. This value is still ~2.3 times greater compared to that predicted for the spiral design. Furthermore, in a practical thermo-optic switch the heater width will typically be confined to values greater than a few micrometers to prevent high susceptibility to defects and high sensitivity of resistance to width variations.
We have demonstrated a new means of reducing the power consumption of thermo-optic switches by using folded waveguide geometries. The reduced power consumption of the double spiral design originates from the high thermo-optic coefficient of silicon, the ability to use the laterally diffused heat from the electrode and the increased waveguide density in the heated volume. The heater design does not rely on minimized width as in a conventional design, thereby improving its susceptibility to defects and fabrication variance. We have shown that waveguides of several millimeters can be fabricated in a small circular area of tens of micrometers in radius, which is beneficial for compact switch layout geometries where a large number of Mach-Zehnder interferometer devices may be cascaded. For our most densely coiled waveguide structure, having 6.3 mm of active waveguide length contained in a 65 μm circular radius, a low switching power of 6.5 mW was obtained with a 10–90% rise time of 14 μs. The switching power was reduced by over 5 times compared to a conventional straight waveguide switch with similar design parameters.
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