Zhi Zou

Tunable two-stage self-coupled optical waveguide resonators.

We report tunable two-stage self-coupled optical waveguide (SCOW) resonators composed of a pair of mirror-imaged single SCOW resonators connected by a phase shifter in between. Experimental results show that the coupled-resonator-induced-transparency and high-order bandstop filtering characteristics can be obtained in the transmission spectra of the devices with two different configurations. The resonance spectrum can be tuned by using either a p-i-p microheater or a p-i-n diode in the phase shifter. Our theoretical modeling based on the transfer matrix method has a good agreement with the experimental results.

Continuously tunable reflective-type optical delay lines using microring resonators.

We present a reflective-type optical delay line using waveguide side-coupled 13 microring resonators terminated with a sagnac loop reflector. Light passes through the microring resonator sequence twice, doubling the delay-bandwidth product. Group delay is tuned by p-i-p type microheaters integrated directly in the microring waveguides. Experiment demonstrates that the delay line can potentially buffer 18 bits and the delay can be continuously tuned for 100 ps with a power tuning efficiency of 0.34 ps/mW. Eye diagrams of a 20-Gbps PRBS signal after 10 and 110 ps delays are also examined.

Tunable silicon Fabry-Perot comb filters formed by Sagnac loop mirrors.

We experimentally demonstrate tunable silicon comb filters based on Fabry-Perot (FP) resonators composed of two Sagnac loop mirrors. The comb filter resolves up to 54 comb lines with a 115 GHz channel spacing over a spectral range from 1510 to 1560 nm. The comb line extinction ratio is ~26.3 dB and the quality factor is ~57,000 around 1550 nm wavelength. Electrical tuning is enabled via periodically interleaved PN junctions embedded inside the FP resonator. The comb lines are blue shifted by ~0.92 nm (one channel spacing) with a 5 mA forward-bias current and red-shifted by ~0.05 nm with a -10 V reverse-bias voltage.

CMOS-compatible temperature-independent tunable silicon optical lattice filters.

We present a CMOS-compatible athermal tunable silicon optical lattice filter composed of 10 cascaded 2 × 2 asymmetric Mach-Zehnder interferometers. Active tuning experiments show that the filter central wavelength can be red-/blue-shifted by 13.1/21.3 nm with power consumption of 77/96 mW on top/bottom arms. Temperature shift measurements show that the thermal-sensitivity of the filter central wavelength before active tuning is as low as -1.465 pm/°C. The thermal-sensitivity is varied within 26.5 pm/°C to -27.1 pm/°C when the filter central wavelength is tuned in the wavelength range of 1534 nm to 1551 nm. We use the transfer matrix method to theoretically model the lattice filter and its thermal-sensitivity before and after tuning is analyzed and discussed.

Investigation of Coupling Tuning in Self-Coupled Optical Waveguide Resonators

We investigate the influence of coupling strength on self-coupled optical waveguide (SCOW) resonators. Experimental results reveal that the SCOW resonator transmission spectrum can exhibit single-channel or dual-channel stopbands with the band splitting level determined by the two coupling coefficients. Electrically tunable SCOW resonators comprising 2×2 Mach-Zehnder interferometer couplers are also demonstrated, which shows a similar change trend upon coupling tuning.

60-nm-thick basic photonic components and Bragg gratings on the silicon-on-insulator platform.

We demonstrate integrated basic photonic components and Bragg gratings using 60-nm-thick silicon-on-insulator strip waveguides. The ultra-thin waveguides exhibit a propagation loss of 0.61 dB/cm and a bending loss of approximately 0.015 dB/180° with a 30 μm bending radius (including two straight-bend waveguide junctions). Basic structures based on the ultra-thin waveguides, including micro-ring resonators, 1 × 2 MMI couplers, and Mach-Zehnder interferometers are realized. Upon thinning-down, the waveguide effective refractive index is reduced, making the fabrication of Bragg gratings possible using the standard 248-nm deep ultra-violet (DUV) photolithography process. The Bragg grating exhibits a stopband width of 1 nm and an extinction ratio of 35 dB, which is practically applicable as an optical filter or a delay line. The transmission spectrum can be thermally tuned via an integrated resistive micro-heater formed by a heavily doped silicon slab beside the waveguide.

Tunable spiral Bragg gratings in 60-nm-thick silicon-on-insulator strip waveguides.

We demonstrate spiral integrated Bragg gratings (IBGs) in 60-nm-thick strip waveguides on the silicon-on-insulator (SOI) platform. The length of the spiral IBG is 2 mm, occupying an area of 147 × 141 μm2 with a minimum bending radius of 20 μm. Experiments show that the spiral IBGs exhibit a single narrow transparent peak with a Q-factor of 1 × 105 in a broad stopband, induced by the phase shift of the S-junction at the spiral center. This phenomenon is analogous to the electromagnetically induced transparency (EIT) effect. The transparent peak can periodically shift in the stopband upon heating of the S-junction using a TiN-based heater on top. The peak transmittance and Q-factor are dependent on the reflectivity of the spiral IBG. The transparent peak can be completely eliminated under a certain tuning power, and the spiral IBG hence behaves as a bandstop optical filter. The bandwidth is 0.94 nm and the extinction ratio is as high as 43 dB. The stopband can also be shifted by heating the Bragg gratings using a separate TiN heater. The experimental results agree well with the modeling results based on the transfer matrix method.

Channel-spacing tunable silicon comb filter using two linearly chirped Bragg gratings

We propose a novel silicon-based comb filter with tunable channel spacing using a Michelson interferometer consisting of a pair of apodized linearly chirped Bragg gratings (ALC-BGs). The channel spacing of the proposed comb filter can be continuously tuned with a large tuning range by changing the effective refractive index of one of the ALC-BGs through the thermo-optic effect. Our numerical calculation shows that the channel spacing can be continuously tuned form 8.21 nm to 0.19 nm with an out-of-band rejection ratio of greater than 30 dB.

Experimental demonstration of self-coupled optical waveguide (SCOW)-based resonators

We experimentally demonstrate a self-coupled optical waveguide (SCOW)-based optical resonator. Transmission spectra reveal the resonators can exhibit split, broadened, or enhanced resonance dips. The SCOW resonators can be used for second-order optical filters.

Selective excitation of microring resonances using a pulley-coupling structure

We present a method to selectively excite resonances in microring resonators by using a pulley-coupling structure. Experimental results reveal only certain resonances are excited, due to the dispersions of coupling-coefficient and microring internal-loss.