Compact Thermally Tunable Silicon Wavelength Switch

X. Wang, J. A. Martinez, M. S. Nawrocka and R. R. Panepucci Electrical and Computer Engineering,

Florida International University, Miami, FL 33174

Abstract— A wavelength selective photonic switch is developed based on a Si microring resonator using thermo-optic effect. The 10μm-diameter microring resonator uses single-mode strip Si waveguides of dimension 0.25 μm×0.45 μm operating at 1.5 μm. Full-width-half- maximum are in the range 0.1-0.2 nm. Ultra wide tunable range (>6.4 nm) was demonstrated. Average rise delay time of 14 μs with low switching power of 3.15 mW has been achieved with 0.2 nm wavelength tuning and 78 μs, 104 mW for 6.4 nm tuning. Input power was observed to have an effect on the tuning time.

Keywords- microring resonator, thermo-optic effect, silicon-on-insulator, wavelength tuning

I. INTRODUCTION Microring resonators [1] are multifunctional photonic

elements with many device applications such as filters, multiplexers and modulators, making use of their high wavelength selectivity. Because of the large refractive index difference between Si and SiO2, the silicon microring resonators allow ultrahigh light confinement making possible large scale photonic integration [2]. The wide wavelength tunable range taking advantage of silicon’s high thermo-optic coefficient and large free-spectral-range (FSR) of silicon microring resonator has enabled a wavelength reconfigurable multi-channel matrix switch [3].

We designed, fabricated and characterized a wavelength selective photonic switch based on a Si microring resonator using thermo-optic effect. As shown in Fig. 1, the 10μm-diameter microring resonator uses single-mode strip Si waveguides of dimension 0.25 μm×0.45 μm operating at 1.5μm. It has a free-spectral-range of 18 nm and full-width-half- maximum (FWHM) in the range 0.1-0.2 nm. Wavelength tuning is achieved through applying current/voltage on a ~800 Ω integrated micro-heater on top of the microring resonator. By using several input and output rings on a common bus waveguide, with corresponding heaters, multiple signals can be muxed/demuxed.

II. EXPERIMENT AND RESULTS Drop port spectra at different sample temperatures (Fig

2(a)) shows linear temperature dependence of the TE resonance peak wavelengths (Fig 2(b)). Linear relationship of resonance wavelength shift as a function of applied electrical power was

determined (Fig 2(c)) by the spectra taken under constant current injections.

The thermal switching of the microring filter between resonance wavelength (λ0) at room temperature and the predefined target resonance wavelengths (λt) is characterized. A probe signal at the target wavelength was used to measure the tuning and detuning waveforms when a square wave voltage was applied to the micro-heater. Fig 3 shows a typical switch response in (c) and (d) respectively, with heating power shown in (a). The time responses are in good agreement with the simulated response which generated by simulated ring temperature time response. Fig. 4 shows the measured rise/fall delay time as a function of wavelength tuned (Δλ for 3

This work was supported in part by the AFOSR under FA9550-05-1-0232, New Span Opto-Technology Inc. and National Science Foundation Award No.0446571.

Figure 1. (a) Top view microscopic picture of fabricated switch element (b) schematic of the cross section at the ring

Figure 2. (a) Normalized transmitted power spectra at different temperatures (b) resonance peak wavelength vs temperature with linear fit (c) resonance shift vs electrical power applied on the micro-heater

with linear fit.

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resonance peaks along with simulated curves. Average rise delay time of 14 μs with low switching power of 3.15 mW has been achieved with 0.2 nm wavelength tuning and 78μs, 104 mW for 6.4 nm tuning. Fall delay times are usually less than 10 μs.

The switch characteristic was found to be slightly dependant to the input optical power. Figure 5 (a) shows the output power transients at λt = 1575.49 nm for input laser powers from 0.1 to 4 mW. Higher input powers appear to accelerate the peak shift when the resonance peak approaches the target wavelength. We believe this is due to the extra heat generated by increased waveguide absorption inside the ring. This effect was investigated further by measuring the transmission spectra around the resonance peaks at different laser powers. Figure 5(b) shows the peak wavelength red shifts for different resonance wavelengths on the rings. A red shift of

0.013 nm was observed for Peak A when 4 mW of optical power was applied compared to 0.01 mW. This peak shift could induce a reduction in coupling that leads to an output reduction of 10% or greater, consistent with the observations in Fig. 5(a) for times beyond the initial tuning which for higher powers leads to a reduction in transmitted power of approximately 20%.

III. CONCLUSION We demonstrate tunable filter in silicon photonics platform

with tuning times as low as 15 μs and tuning range as wide as 6.2 nm. We report the effect of input powers on the tuning time, believed to be due to optical absorption in the microring resonators. Measured spectral peak shifts versus input power are consistent with the observed effect.

REFERENCES

[1] B. E. Little, J. S. Foresi, G. Steinmeyer, E. R. Thoen, S. T. Chu, H. A. Haus, E. P. Ippen, L. C. Kimerling, and W. Greene, "Ultra-compact Si/SiO2 microring resonator optical channel dropping filters," IEEE Photon. Technol. Lett. 10, 549-551 (1998).

[2] R. A. Soref, “Prospects for novel Si-based optoelectronic devices: unipolar and p-i-p-i lasers”, Thin Solid Films, vol.294, pp. 325-329, 1997

[3] H. Ng, M. R. Wang, D. Li, X. Wang, J. Martinez, R. R. Panepucci, and K. Pathak, “1×4 wavelength reconfigurable photonic switch using thermally tuned microring resonators rabricated on silicon substrate”, IEEE Photon. Technol. Lett., vol. 19, no. 9, pp. 704-706, May 2007

Figure 3. (a) Normalized power applied to the heater (b) simulated peak wavelength of the tuning resonance. Simulated and measured

output power transient at (c) λt = 1574.69 nm and (d) λ0 =1573.89 nm.

Figure 4. Plots of tuning rise delay times of room-temperature

resonance peaks at (a) 1573.89 nm (Peak A), (b) 1592.65 nm (Peak B) and (c) 1611.80 nm (Peak C) as well as plots of fall delay times of the

peaks (d), (e) and (f), respectively. The curves are simulated results and the symbols are measured values. The insets are the corresponding peak spectra. Note the different time scale between rise and fall plots.

Figure 5. (a) Output power transient at λt = 1575.49 nm under the same heating current at different input optical power (b) Peak

wavelength redshifts when scanning laser input wavelength at fix output power. Curves for 3 different peaks (A,B and C) are shown.

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