Wavelength Reconfigurable Photonic Switching ... - SPIE Digital Library

Wavelength Reconfigurable Photonic Switching Using Thermally Tuned Micro-Ring Resonators Fabricated on Silicon Substrate Michael R. Wang*a, Han-Yong Ng a, Daqun Li b, Xuan Wang c, Jose Martinez c, Roberto R. Panepucci c, Kachesh Pathakd a Dept. of Electrical and Computer Eng., Univ. of Miami, Coral Gables, Florida 33124; b New Span Opto-Technology, Inc., 16115 SW 117 Ave., A-15, Miami, Florida 33177; c Dept. of Electrical and Computer Eng., Florida International Univ., Miami, Florida 33174 d U.S. Army Space & Missile Defense Command, Huntsville, Alabama 35807 ABSTRACT Micro-ring resonators have been traditionally fabricated using expensive III-V materials such as InP or GaAs. Device tuning is typically to utilize the electro-optic effect of the III-V materials that usually leads to complex device layer structures. As another tuning approach, thermo-optic tuning of micro-ring resonators is commonly achieved by heating up the whole chip. In general, it is more challenging to achieve highly localized heating on a common chip for independent tuning of multiple micro-ring resonators residing on the same substrate. To address these issues, we describe the development of wavelength reconfigurable photonic switching using thermally tuned micro-ring resonators fabricated on a low-cost silicon-on-insulator substrate. Independent tuning of multiple micro-ring resonators, spaced at 250 µm, is realized with highly localized micro heaters (50×50 µm2 per heater area) fabricated on the same silicon substrate. Owing to the large thermo-optic effect of silicon (∆n/∆T=1.8×10-4 K-1), 8 mA heating current is sufficient to tune a micro-ring resonator with a 3-dB spectral line width of 0.1 nm by 2.5 nm while creating a minor peak shift of less than 0.04 nm for an adjacent resonator. The switching response time is about 1 ms. A 1×4 wavelength reconfigurable photonic switch device has been demonstrated. With a resonator diameter of approximately 10 µm (greater than 18 nm in free spectral range of each micro-ring resonator), larger port-count switch matrix with wavelength reconfiguration on a small device foot print is feasible for the development of large-scale integrated photonics. Keywords: Photonic Switching; Micro-Ring Resonators; Thermo-Optic Tuning; Optical Communications; Silicon

1. INTRODUCTION Micro-ring resonators (MR) are commonly used as optical filters in optical communication applications 1-2. In this application, four MRs with a diameter of 10 µm were used to implement a wavelength multiplexing/demultiplexing photonic switch on a silicon-on-insulator (SOI) platform. Compared to III-IV type material compounds, SOI is a low cost material and its compatibility with existing microelectronic fabrication processes makes it the attractive material for MR-based device fabrications 3. Furthermore, the high refractive index offered by SOI allows for the realization of micron-sized MRs with well separated resonant peaks to achieve the desired narrow-linewidth wavelength filtering function. MRs can be tuned electro-optically or thermo-optically with the latter commonly found for tuning of MRs fabricated on silicon substrates 4. In comparison to whole substrate heating for MR tuning, it is more challenging to achieve highly localized heating on a common chip for independent tuning of multiple MRs that reside on the same substrate. In this paper, we report our continuing development of a 1×4 photonic switch implemented on a low-cost SOI substrate 5 that is capable of independent MR wavelength tuning using localized micro-heaters fabricated on the same substrate.

2. DESIGN AND FABRICATION The 1×4 photonic switch was fabricated on a SOI platform with a 3 µm buried SiO2 layer using a laterally-coupled configuration. In contrast to vertical coupling, lateral coupling allows all waveguide structures to be fabricated on the same silicon guiding layer, leading to a reduction in fabrication complexity. Having all waveguides on the same layer Nanoengineering: Fabrication, Properties, Optics, and Devices IV, edited by Elizabeth A. Dobisz, Louay A. Eldada, Proc. of SPIE Vol. 6645, 66450I, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.734799

Proc. of SPIE Vol. 6645 66450I-1

also simplifies micro-heater fabrication. As a result, only a single layer deposition is required to fabricate the microheaters. As illustrated in Fig. 1, the 1×4 photonic switch device consists of a main throughput waveguide, four output straight waveguides, and four MRs placed at each of the intersections between the output waveguides and the main waveguide. All the waveguide structures were designed to have a rectangular cross section with width and height of 450 nm and 250 nm, respectively. The MR-to-waveguide lateral gaps in the coupling regions were designed to be 250 nm. The output waveguides as well as their respective MRs have an adjacent separation of 250 µm to accommodate fiber array interconnection. The MRs were designed with a diameter of 10 µm, giving rise to a free spectral range (FSR) as determined by Eq. (1).

λ2 FSR = 2π rneff

(1)

where λ is the MR resonant wavelength, neff is the MR effective index of refraction, and r is the MR radius. As seen in Fig. 2, a transverse-electric (TE) polarization mode simulation shows a FSR of close to 18 nm at 1550 nm wavelength with our design parameters in three-dimensional finite-difference time domain.

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Fig. 1 Schematic diagram of the 1×4 tunable photonic switch showing the coupling waveguides, MRs, and micro-heaters with electric pads

Fig.2 Simulation result of a MR with a diameter of 10 µm using TE polarization light at 1550 nm

To achieve local thermal tuning, Nichrome (resistivity = 1.50×10-6 Ω·m at room temperature) was used to fabricate the micro-heaters. The micro-heaters were designed to have a meandering shape as shown in Fig. 1 (inset) to cover the local area (50 µm × 50 µm) on top of each MR. The linewidth and thickness of the heater wires were designed to be 5 µm and 0.1 µm. To minimize power dissipation outside the heating area, a low resistance metal (Ti-Au) was chosen to pattern the conductive wires and the electric pads. With a resistivity of about 2.44×10-8 Ω·m, it is estimated that each Ti-Au wire pair loop will create a resistance of about 13.5 Ω. By comparison, the resistance of each micro-heater is about 900 Ω. This indicates that about 98.5% of the electric heating power supplied is dissipated in the heating elements. To further evaluate the performance of the micro-heaters, we used FEMLAB simulation software to model the heat dissipation and distribution of a three-dimensional device structure with a micro-heater in cylindrical symmetry, as seen in Fig. 3. Steady state analysis was used to analyze heat transfer and dissipation to the MRs and to the neighboring areas of the micro-heaters. It was concluded through the simulation that an intensity level of 26 mW/µm2 was needed to increase the local temperature by 60 oC. This value translates to a driving current of about 9 mA. As indicated in Fig. 4, the heating was found to remain locally with negligible thermal effect on adjacent MRs. Through transient state analysis, it was found that the time required for the temperature increase to reach to a steady state is about 1 ms.


360

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Lateral Dimension (m)

Fig. 3 Simulation of temperature distribution around a micro heater on SiO2 layer

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Fig. 4 Horizontal temperature variation outwards from the center of a MR

3. DEVICE FABRICATION The fabrication of our device employs standard silicon micro-electronic fabrication procedures. SOITEC SOI wafers with 200 nm thick silicon layer over 3 µm buried oxide layer were used. A spin-on-glass resist from Dow Corning, FOX 12, was spun on the wafer and baked for 2 min at 200 oC. Pattern exposure was carried out in a Leica VB6 e-beam system at 100 kV and 1 nA of exposure current with a beam step size of 5 nm. Development time was 2 min with MIF300 from Aldrich. Reactive ion etching was done in a PlasmaTherm 770 ICP. We used a Chlorine and Argon plasma mixture and etched the FOX resist for 90 seconds with 20% over-etch. To protect the etched structure, a 1 µm thick SiO2 cladding layer was coated over the entire patterned surface. After MR fabrication, the chip was diced and then the facets were polished to optical finish. Next, Nichrome micro-heaters were layered on top of each MR through evaporation. Finally, the Ti-Au conductor patterns were evaporated on the top to provide electrical connections to the micro-heaters. Fig. 5 shows the scanning electron microscopic (SEM) image of a fabricated MR at the intersection of its two coupling waveguides, while an enlarged SEM picture of the coupling gap between MR and waveguide is given in Fig. 6.

Fig. 5 SEM image of a fabricated MR with input and output coupling waveguides

Fig. 6 SEM picture of the coupling gap between a MR and its coupling waveguide


4. DEVICE CHARACTERIZATION The fabricated MR device was characterized using a tunable laser (Photonetics, Tunics-BT) with a scanning step size of 0.01 nm over the wavelength range of 1520 nm – 1580 nm. Due to the polarization sensitivity nature of MRs, the device was designed to work for TE polarization mode only. The input light polarization was maintained through a polarization controller and a polarization maintaining fiber butt-coupled to the input port of the device. The temperature of the chip was stabilized at 22 ± 0.01 °C through a Peltier thermoelectric cooler (TEC) mounted on the back side of the device to eliminate any thermal effects caused by ambient temperature fluctuations. An objective lens was used on the output side to couple the output signal to a photodetector (Agilent, 81626B) for testing. An adjustable iris diaphragm was placed between the objective lens and the detector to serve as a spatial light filter to eliminate unwanted optical noises. A LabVIEW program was written to tune the laser source and to record the output signals through a computer data acquisition card. The FSR of the four MRs were measured by recording each output individually while tuning the wavelength of the laser source. The laser source was tuned from 1520 nm to 1580 nm with a step size of 0.05 nm while keeping the chip at a constant temperature to prevent output peak drifts due to temperature change. Fig. 7 shows the output spectrum of a MR recorded from a single output port. A FSR of about 18 nm was observed around 1550 nm for all four MRs, which agrees well with our simulation result. The full linewidth at half maximum (FWHM) of the resonant peaks was about 0.1 nm as seen in Fig. 8.

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1883.1 1883.2 1883.3 1883.4 1883.8 1883.8 1883.7 1883.8 1883.0 1884 1884.1 Wavelength (11111)

Wavelength (11111)

Fig. 7 Transmission spectrum of a MR showing a FSR of approximately 18 nm at 1550 nm

Fig. 8 Spectral profile of a resonant MR revealing a FWHM of roughly 0.1 nm

The MRs were first tuned by global heating with the TEC mounted on the back side of the chip. This provides an accurate assessment of resonant wavelength peaks versus temperature of the MRs. Fig. 9 shows the output spectrum of a MR at different temperatures. A total wavelength shift of about 1.38 nm was observed for a temperature change of 16 degrees. As expected, a linear relationship between resonant wavelength peak and heating temperature was obtained as seen in Fig. 10, yielding a thermal tuning coefficient of approximately 0.08 nm/°C for the MRs. To implement 1×4 photonic switching, local heating with micro-heaters rather than global heating with TEM was employed. This was done by sending heating currents to the corresponding micro-heaters while maintaining a constant temperature at the back side of the chip with the TEC. To characterize the 1×4 photonic switch, the following ITU grids were chosen as the output wavelength channels: 1562.43 nm, 1562.83 nm, 1563.23 nm, and 1563.63 nm (λ1, λ2, λ3, and λ4, respectively). There are a total of 4! = 24 switching states for a 1 × 4 switch. Fig. 11 shows one switching state with output ports 1, 2, 3, and 4, tuned to wavelengths λ3, λ2, λ1, and λ4, respectively. Although the MRs were designed to have identical dimensions, it was found that the resonant peaks of the four MRs vary by 1.38 nm due to ring size differences caused by fabrication tolerance.


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Fig. 9 Resonant spectrum of a MR at various temperature levels

Fig. 10 Relationship between MR resonant peak and MR temperature

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Fig. 11 One switching state of the 1×4 photonic switch showing input wavelengths of λ1, λ2, λ3, and λ4 routed to output ports 3, 2, 1, and 4, respectively

As seen in Fig. 11, fabrication tolerance also causes the MRs to have different resonant amplitudes. The original resonant peaks of the four MRs, when stabilized globally at 22 °C with the TEC, are 1562.24 nm, 1561.22 nm, 1560.86 nm, and 1561.44 nm, respectively. In addition, the resistances of the four corresponding micro-heaters are not identical either (929 Ω, 959 Ω, 958 Ω, and 909 Ω measured for micro-heaters 1, 2, 3, and 4, respectively). Due to these variations, the heating currents applied to the micro-heaters need to be determined individually in order to tune the four MRs to the four chosen channel wavelengths, respectively. Table 1 summarizes the driving currents required to tune the MRs of each output port to the four predefined channel wavelengths. As expected, the wavelength tuning of a MR is a quadratic function of its heating current since the electric power dissipated is a quadratic function of the current provided. The total power consumption of the device was about 135 mW.


Table 1 Heating currents required to tune the four MRs to the four output wavelengths λ1 (1562.43 nm)

λ2 (1562.83 nm)

λ3 (1563.23 nm)

λ4 (1563.63 nm)

Port 1 (MR 1)

3.19 mA

4.29 mA

5.21 mA

5.96 mA

Port 2 (MR 2)

4.77 mA

5.73 mA

6.44 mA

6.86 mA

Port 3 (MR 3)

6.00 mA

6.60 mA

7.14 mA

7.79 mA

Port 4 (MR 4)

5.05 mA

5.78 mA

6.54 mA

6.80 mA

To confirm independent MR tuning, various heating currents were provided to a MR while monitoring the resonant peak shift of an adjacent MR. No resonant peak shift of a MR was found if heating currents of less than 4 mA were provided to an adjacent MR, as indicated in Fig. 12. It was found in Fig. 12 that heating an MR with 8 mA current causes the resonant peak of an adjacent MR to shift by less than 0.04 nm. This peak shift corresponds to a peak power drop of about 30%. As indicated in Table 1, 8 mA is higher than the maximum current required to tune any of the four output ports to the farthest wavelength channel, λ4. As a result, the crosstalk of the thermally tuned 1×4 photonic switch is less than 0.04 nm.

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Fig. 12 Resonant peak shift of a MR with various heating currents provided to an adjacent MR

Finally, the switching speed of the device was evaluated using a high-speed photodetector (Thorlabs, DET 410) and an oscilloscope. A square-wave voltage signal was used to modulate the heating current (turning it ON and OFF) so that a MR was either pulled off resonance or pushed on resonance as indicated in Figures 13 (a) and (b), respectively. In either case, a similar switching speed of approximately 1 ms was found for the four MRs. It is anticipated that an increase in adjacent micro-heater gap and/or a decrease in micro-heater area will further reduce the switching speed of the device.


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Time (Ills)

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Fig. 13 Off-resonance tuning speed (a) and on-resonance tuning speed (b) of the thermally tuned photonic switch

5. CONCLUSION A 1×4 micro-ring resonator based photonic switch has been developed using low-cost silicon platform. Independent wavelength tuning of multiple resonators was achieved through micro-heaters fabricated on the same device substrate with a minimum crosstalk. The device has a FSR of 18 nm and a FWHM of about 0.1 nm. The switching speed of the device is on the order of 1 ms. The high integration potential and no-moving-part switching mechanism of the photonic switch device makes it highly attractive to a wide variety of applications in optical communication and sensor networks.

ACKNOWLEDGEMENT This research was sponsored by the Missile Defense Agency and managed by the U.S. Army Space & Missile Defense Command.

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