PI：Yikai Su，Shanghai Jiao Tong University

NSFC Grant No.: 60407008、60777040

The control of the propagation speed of light may show a significant impact in the photonics field, particularly on all-optical delay line, large capacity optical storage, optical packet switching, etc. With the rapid development of fiber communications and photonic interconnections, optical control of light speed is becoming important. The current optical storage technologies are typically based on a complex and inefficient conversion procedure as follows: optical signal is firstly converted to electrical signal, and this electrical signal is used for storage, the electrical signal is converted back to optical signal when read out. If one can control the speed of light by removing the complex O-E-O conversion, this major bottleneck in optical packet switched networks is eliminated, thus power efficient large-capacity optical storage could be realized. Today’s common technique is to use optical fiber delay lines. However, a fiber can only realize fixed delay of optical signals, showing disadvantages in many applications where tunable delay is required. Therefore the research on controllable and tunable optical delay line becomes important.

The photonics technology is developing towards large-scale integration. Silicon, a matured and predominate material for electronic integration, has also been considered as a candidate for integrated photonics. Silicon photonics not only take the advantage of CMOS fabrication technology, but also enable the compatibility of photonics and electronics, thus paving the way for large-scale integrated opto-electronic systems, optical interconnections and optical computing. In silicon-based integrated optoelectronic systems, optical buffering may play important roles in routing, storage, and logic processing in all-optical communications and processing systems. If all the components such as laser, electro-optical modulator and optical buffer could be realized in one chip, large-scale opto-electronic integration would enter into mass production stage.

On-chip optical buffers based on nano-waveguide might show significant impact on future integrated photonic systems. The recently developed silicon-on-insulator (SOI) structure has been proved to be an excellent platform for monolithic integration of photonic devices due to its high index contrast between the silicon core and the silica cladding, which allows strong confinement of light and enables ultra-compact devices. Some on-chip delay lines based on silicon waveguide have been demonstrated including cascaded microring resonator based all pass filters (APF) structure, coupled resonator optical waveguides structure, stimulated Raman scattering (SRS) controlled slow-light delay line in silicon waveguide, as well as slow-light in silicon resonator enhanced by SRS.

A research group at Shanghai Jiao Tong University (Qiang Li, Fangfei Liu and Yikai Su), has experimentally investigated the delay performances of three widely used modulation formats in optical communications--non-return-to-zero (NRZ), return-to-zero (RZ) and differential phase shift keying (DPSK) with pseudo random bit sequence (PRBS) pattern at different data rates. These data pass through a 20-μm-radius single side coupled silicon microring resonator, which is controlled by a CW pump signal to induce thermal nonlinear effect.

The 20-μm-radius silicon microring resonator in the experiment is fabricated on an SOI wafer with a 250-nm-thick silicon slab on top of a 3-μm silica buffer layer to prevent the optical mode from leaking to the substrate. The cross section of the silicon waveguide is 450×250 nm with a mode area of about 0.1μm2 for transverse-electric (TE) optical mode in such a high-index-contrast structure. The microring is side coupled to the straight waveguide with an air gap of 120 nm between the straight waveguide and the microring. The device is fabricated by E-beam lithography followed by reactive ion etching. The surface roughness is reduced by oxidizing 20Å of silicon surfaces using wet chemistry. The scanning electron microscope (SEM) photo of the silicon microring resonator is shown in Fig. 1.

The experiment setup is depicted in Fig. 2. The pump and the probe signal sit at two adjacent resonances in the vicinity of 1550 nm. A Mach-Zehnder modulator (MZM) is driven by an electrical PRBS signal of 27-1 pattern length. We used the 27-1 PRBS to facilitate the measurements of the delay, and experimental result shows that there is little difference if a longer pattern length is used. This MZM is biased at quadrature and null points of the transmission curve for generating NRZ and DPSK signals, respectively. The RZ signal is obtained by cascading a pulse carver through driving a second MZM using a sinusoidal signal with a same frequency as the data rate; hence the duty cycle of the RZ signal is 50%. The pump light is amplified by a high power erbium doped fiber amplifier (EDFA) followed by an attenuator to adjust the pump power. Both the pump light and the probe signals are coupled through a 3-dB coupler to the microring resonator by the vertical coupling system. The output signal of the microring resonator is amplified using two cascaded EDFAs and the probe signal is separated from the pump wave using a bandpass filter and fed to an oscilloscope to record the waveforms. When the probe signal is DPSK format, we use a Mach-Zehnder delay interferometer (MZDI) to demodulate the DPSK signal. As the gold grating coupler is polarization dependence, two polarization controllers are inserted before the coupler to make sure the input pump and probe lights are in TE mode.

The delay performances of the three formats were measured. In particular, the characterization of RZ Gaussian-like pulse at data rates of 1 Gb/s and 5 Gb/s is provided here as RZ shows the best performance in maintaining the pulse shape. We obtained the delays by comparing the peak position of the recorded pulse after normalizations. The distortion for the 10-Gb/s RZ signal is too severe to identify the delay. Fig. 3(a) shows the delay of the 1-Gb/s and the 5-Gb/s RZ signals as a function of the pump power when the RZ signals are initially at the center of the resonance. Fig. 3(b) (c) are the corresponding waveforms at typical pump powers. The maximum delay is about 110 ps for the 1-Gb/s signal and 70 ps for the 5-Gb/s signal; the threshold of the pump power is about 0 dBm (about -10 dBm into the microring resonator).

A single microring resonator is the building block for more complicated nano-waveguide based structures, which can support slow light operations. These structures consist of periodic spaced resonators with or without intercoupling, such as single-channel side-coupled integrated spaced sequence of resonators (SCISSOR), double-channel SCISSOR and inter-coupled resonators. Our proposed optically tuning method might be used in these slow-light structures to tune the resonances of the microring resonators over a wide range.

Publications:

Yikai Su, Fangfei Liu, Qiang Li, Ziyang Zhang, Min Qiu, “System Performance of Slow-light Buffering and Storage in Silicon Nano-waveguide,” Invited Paper, in Proc. APOC 2007, 6783-111.

Fangfei Liu, Chun Jiang, Min Qiu, Yikai Su, “Widely Tunable Slow-light Delay Line Using Parametric Assisted Silicon Microring Resonator,” in Proc. AOE 2007, paper SC2.S7. 3.

Fangfei Liu, Qiang Li, Ziyang Zhang, Min Qiu, Yikai Su, “Optically Tunable Delay Line in Silicon Microring Resonator Based on Thermal Nonlinear Effect,” submitted to IEEE JSTQE

Lilin Yi, Yves Jaouen, Weisheng Hu, Yikai Su, Sebastien Bigo, “Improved Slow-Light Performance of 10 Gb/s NRZ, PSBT and DPSK Signals in Fiber Broadband SBS,” Optics Express, Vol. 15, No. 25, pp. 16972-16979

Lilin Yi, Yves Jaouën, Weisheng Hu, Junhe Zhou, Yikai Su and Erwan Pincemin, "Simultaneous Demodulation and Tunable-Delay of DPSK Signals using SBS-based Optical Filtering in Fiber," Optics Letters, Vol. 32, No. 21, Nov. 1, 2007, pp. 3182-3184

Lilin Yi, Weisheng Hu, Yikai Su, Mingyi Gao, Lufeng Leng, “Design and System Demonstration of a Tunable Slow-light Delay Line Based on Fiber Parametric Process," IEEE Photon. Technol. Lett., vol.18, no.24, Dec 15, 2006, pp. 2575-2577.

Fangfei Liu, Yikai Su and Paul L. Voss, “Optimal Operating Conditions and Modulation Formats for 160Gb/s Signals in a Fiber Parametric Amplifier Used as a Slow-light Delay Line Element," in Proc. OFC 2007, paper OWB5.

Lilin Yi, Yves Jaouen, Weisheng Hu, Yikai Su and Sebastien Bigo, “10-Gb/s Slow-light Performance Based on SBS Effect in Optical Fiber Using NRZ and PSBT modulation formats," in Proc. ECOC 2007, paper 6.6.2.

Lilin Yi, Weisheng Hu, Junhe Zhou, Yikai Su and Erwan Pincemin, “Simultaneous Demodulation and Slow-light Delay of DPSK signals at Flexible Bit-Rates Using Bandwidth-Tunable SBS in Optical Fibre," in Proc. ECOC 2007, paper 6.6.1.

Lilin Yi, Li Zhan, Yikai Su, Weisheng Hu, Lufeng Leng, Yuejiang Song, Haigen Shen and Yuxing Xia, “Delay of RZ PRBS data based on wide-band SBS by phase-modulating the Brillouin pump,” in Proc. ECOC 2006, paper We3.P.30.

Lilin Yi, Weisheng Hu, Yikai Su, Lufeng Leng, Jian Wu, Xiangqin Tian, Guangtao Zhou, Li Zhan, “Propagation of 10-Gb/s RZ Data through a Slow-Light Fiber Delay-Line Based on Parametric Process,” in Proc. OFC 2006, paper OFH3.