All-optical compact silicon comb switch

We demonstrate a 1x2 all-optical comb switch using a 200 μm diameter silicon ring resonator with a switching time of less than 1 ns. The switch overcomes the small bandwidth of the traditional ring resonator, and works for wavelength division multiplexing applications. The device has a footprint of ~0.04 mm and enables switching of a large number (~40) of wavelength channels spaced by ~0.85 nm. ©2007 Optical Society of America OCIS codes: (130.3120) Integrated optical devices; (230.2090) Electro-optical devices; (230.5750) Resonators References and Links 1. K. K. Lee, D. R. Lim, and L. C. Kimerling, “Fabrication of ultralow-loss Si/SiO2 waveguides by roughness reduction,” Opt. Lett. 26, 1888-1890 (2001). 2. Y. A. Vlasov, and S. J. McNab, “Losses in single-mode silicon-on-insulator strip waveguides and bends,” Opt. Express 12, 1622–1631 (2004). 3. V. R. Almeida, C. A. Barrios, R. R. Panepucci, and M. 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Lett. 28, 1302-1304 (2003). #83391 $15.00 USD Received 24 May 2007; revised 12 Jul 2007; accepted 13 Jul 2007; published 18 Jul 2007 (C) 2007 OSA 23 July 2007 / Vol. 15, No. 15 / OPTICS EXPRESS 9600 22. B. G. Lee, B. A. Small, K. Bergman, Q. Xu, and M. Lipson, “Transmission of high-data-rate optical signals through a micrometer-scale silicon ring resonator,” Opt. Lett. 31, 2701-2703 (2006). 23. B. A. Small, B. G. Lee, K. Bergman, Q. Xu, J. Shakya, and M. Lipson, "High data rate signal integrity in micron-scale silicon ring resonators," CLEO 2006, May 2006, CTuCC4. 24. D. Xu, A. Densmore, P. Waldron, J. Lapointe, E. Post, A. Delâge, S. Janz, P. Cheben, J. H. Schmid, and B. Lamontagne, “High bandwidth SOI photonic wire ring resonators using MMI couplers,” Opt. Express 15, 3149-3155 (2007). 25. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23, 123-129 (1987). 26. F. Y. Gardes, G. T. Reed, N. G. Emerson, and C. E. 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Recent breakthroughs in the field of silicon photonics include low-loss and compact siliconon-insulator (SOI) passive waveguide devices [1-2], high speed silicon modulators [3-7], Ge on SOI detectors [8], silicon Raman lasers [9, 10], silicon broadband amplifiers based on fourwave mixing [11], and hybrid III-V silicon lasers [12]. These devices have provided the possibility to construct CMOS compatible optical interconnection systems with low power consumption, high bandwidth and low latencies [13-15]. A critical device for the implementation of such systems, that remains to be demonstrated, is a broadband and compact silicon switch operating in the nanosecond regime [16]. Fast modulation and switching in silicon has been achieved using the plasma dispersion effect [4-7, 17], where the index of refraction of silicon is changed by generating free carriers. However, the demonstration of low powered and compact active silicon devices has remained a challenge due to the small index change that can be attained by this carrier dispersion effect. We have recently demonstrated the use of highly confined resonant structures for overcoming this limitation by confining both optical field and electrical carriers in a small region [18]. Using 10 μm diameter ring resonators, we have demonstrated 12.5 Gbps electro-optical modulators [6]. However in such high quality factor devices, the optical bandwidth is sacrificed, and therefore these devices are not suitable for Wavelength Division Multiplexing (WDM) and other broadband signal applications. Although cascaded ring resonators can be used for multiple-wavelength operation, the resonance of each resonator is difficult to control due to fabrication and temperature variations [19]. Therefore, an efficient and compact silicon switch suitable for WDM is needed at present. In this paper, we propose and demonstrate a technique of comb switching by using a single ring resonator for WDM applications. The ring resonator has a relatively small free spectral range (FSR) corresponding approximately to the wavelength spacing used in dense WDM (0.8 nm), and can simultaneously switch on and off a large number of wavelength channels. Therefore, the advantage of the high quality factor is maintained while allowing the traditional small bandwidth limitation to be overcome. The demonstrated switch has a switching time of less than 1 ns and a footprint of only 200 μm x 200 μm. We fabricated a 250 nm thick silicon-on-insulator ring resonator with a 200 μm diameter and a 450 nm width using electron beam lithography and plasma reactive ion etching. Subsequent deposition of oxide serves as the top cladding. The ring is coupled to two straight waveguides with the same cross section, one acting as an input port and through port, and the other acting as a drop port, as shown in Fig. 1. The gaps between the waveguides and the ring’s outer edges have a distance 400 nm and 430 nm for the through port and drop port, respectively. The gap difference is set to satisfy the critical coupling condition, which requires that the coupling to the ring from the input waveguide is equal to the sum of waveguide loss per round in the ring and the coupling to the drop waveguide from the ring #83391 $15.00 USD Received 24 May 2007; revised 12 Jul 2007; accepted 13 Jul 2007; published 18 Jul 2007 (C) 2007 OSA 23 July 2007 / Vol. 15, No. 15 / OPTICS EXPRESS 9601 [20]. Off resonance, the input light is directly transmitted to the through port. At resonance, i.e. when the circumference of the ring corresponds to an integer multiple of guided wavelengths, the input light will be coupled into the ring and collected by the drop waveguide. The resonant wavelengths of the ring resonator can be changed by altering the index of refraction, resulting in light switching between the drop port and the through port. Note that all of the resonance wavelengths are shifted simultaneously, enabling the switching of multiple wavelengths simultaneously using a single ring resonator. Fig. 1, Top-view microscopic picture of the fabricated device. We measured the quasi-TM transmission spectrum of this device using a tunable continuous wave laser operating at the telecommunication wavelengths. The power from the laser is fixed at 0 dBm. In order to increase the coupling efficiency between the optical fiber 

Optical switches for dense wavelength division multiplexing (DWDM) are key components for all optical networks and optical interconnects on chip [1].Silicon photonics has attained much attention in recent years owing to the maturity of silicon in the electronic industry and its possibility to combine both photonic and electronic devices all on one chip [2].However due to the small index change of the silicon that can be attained by carrier dispersion effect, silicon as active devices remains a challenge.High Q devices can alleviate this limitation [3].For example, our group has demonstrated 12.5 GHz electro-optical modulators by using 10 µm diameter ring resonators [4].High Q devices, on the other hand, operate by sacrificing the optical bandwidth, and therefore are usually not suitable for DWDM and broadband applications.Although cascaded ring resonators can be used for multiple wavelengths, the resonance of each resonator is difficult to control due to fabrication and temperature variations [5].A silicon optical switch for DWDM applications is still missing at present.
In this project, we propose and demonstrate a technique of comb switching by using a single ring resonator for DWDM applications.The ring resonator's free spectral range (FSR) corresponds to the wavelength spacing in DWDM, and therefore can simultaneously switch all of the channels in DWDM applications.Therefore, the high Q advantage of the ring resonator is kept without suffering from the traditional small bandwidth limitation.We demonstrate a device with a working area of 200 µm × 200 µm and a switching time of less than 1 ns.
We fabricated a silicon-on-insulator ring resonator with a 200 µm diameter and a 450-nm-wide by 250-nm-high rectangular cross section by electron beam lithography and subsequent plasma reactive ion etching (see Figure 1).The ring is coupled to two straight waveguides with the same cross section, one acting as an input port and through port, and the other acting as a drop port.The minimum distance between waveguides and ring outer edges is 400 nm.

Optics & Opto-Electronics
The quasi-TM transmission spectrum shows that the FSR of this ring is 0.83 nm at a wavelength around 1530 nm, and a quality factor Q = 18,500.The transmission to the through and drop ports is highly wavelength sensitive.On resonance, the light is coupled into the ring and collected by the drop waveguide.If the refractive index of the ring waveguide is changed, the light, initially on resonance, is redirected to the through port.Note that all of the resonance wavelengths will be shifted simultaneously, which enables switching of multiple wavelengths using a single ring resonator.In Figure 2, we show 40 channels, with the transmission drop of the through port about 15 dB and the transmission rise of the drop port about 15 dB on resonance.
To demonstrate the concept of comb switching, we use two continuous-wave tunable lasers, both on resonance with the ring resonator.We use a ƒs pulse laser centered at a wavelength 400 nm to generate free carriers by shining it on the top of the ring.At this wavelength the laser is strongly absorbed by the silicon layer and free electron-hole pairs are excited in less than 1 ps.The generated electronhole pairs modify the refractive index and the absorption of silicon, and therefore tune the cavity resonance.Figure 3 shows the time dependence of the probe signal transmission.The amount of modulation depth is ~ 90% for both the transmission and drop ports.The switching time is measured to be 100 ps, limited by our detector.The switch-off time is determined by free-carrier lifetime of the photo-excited carriers and is measured to be ~ 500 ps [3].
The probe resonance is shifted by ∆λ = -0.5 nm, which corresponds to an effective index change of -6.5 × 10 -4 induced by a carrier concentration of ∆N = ∆P = 2.2 × 10 17 cm -3 [6].We estimate that the actual power absorbed by the ring resonator to excite this carrier concentration is only 8 pJ [3].
In conclusion, we propose and demonstrate an all optical silicon switch by using a ring resonator on a silicon chip.
We have demonstrated a 1 × 2 all optical switch with a wavelength spacing of ~ 0.83 nm, a switch time of ~1 ns and a small area of 200 µm × 200 µm.

Figure 1 :
Figure 1: Top-view microscopic picture of the fabricated device.

Figure 2 :
Figure 2: Transmission spectra of the ring resonator.Figure3: Experimental temporal response of the probe signal for wavelength 1541.48 nm (grey curves) and 1542.30nm (black curves).

Figure 3 :
Figure 2: Transmission spectra of the ring resonator.Figure3: Experimental temporal response of the probe signal for wavelength 1541.48 nm (grey curves) and 1542.30nm (black curves).