Magneto-optical (MO) switches operate with a dynamically applied magnetic field. The MO devices presented in this paper consist of microring resonators (MRRs) fabricated on amorphous silicon-on-garnet platform. Two types of MO switches with MRRs were developed. In the first type, the switching state is controlled by an external magnetic field component included in the device. By combination of MO and thermo-optic effects, wavelength tunable operation is possible without any additional heater, and broadband switching is achievable. The other type of switch is a self-holding optical switch integrated with an FeCoB thin-film magnet. The switching state is driven by the remanence of the integrated thin-film magnet, and the state is maintained without any power supply.

This paper presents a theoretical analysis and experimental confirmation of a tunable ring resonator that can independently change its resonant frequency and bandwidth. The tunable ring resonator comprises a ring resonator, three tunable capacitors, and switches. The resonant frequency changes according to the capacitance of tunable capacitors, and the bandwidth varies by changing the state of the switches. The unique feature of the resonator is that the resonant frequency remains steady when the bandwidth is changed. The fundamental characteristics are shown based on linear circuit simulation and electromagnetic simulation results. The resonator is fabricated using GaAs FET single-pole single-throw switches. The fabricated resonator changes the resonant frequency from 1.5 GHz to 2.0 GHz and the fractional bandwidth from 5% to 30%.

We proposed and theoretically discussed a polarization switch based on a multiple quantum well (MQW) microring resonator (MRR) with an L-shaped waveguide as a dynamic polarization control device. The L-shaped waveguide was useful to obtain the rotation angle of the principal axis of 45° even when the core layer has form birefringence such as an MQW. The MRR functions as both a polarization converter and a wavelength filter. As an MQW core layer, a multiple five-layer asymmetric coupled quantum well (FACQW) was assumed. Polarization switching of an inputted light with a selected wavelength was successfully demonstrated at an operation voltage as low as 0.2 V using the electrorefractive index change in the MRR waveguide. The maximum polarization extinction ratio was more than 35 dB. The high-speed and low-voltage polarization switching can be realized with the proposed MRR polarization switch.

A novel compact multi-input multi-output (MIMO) antenna system with split-ring resonator (SRR), a popular metamaterial structure, is presented. The MIMO antenna system consists of SRRs as radiator elements arranged close to each other on a printed circuit board. We evaluate the antenna characteristics with a single and two SRR elements arranged within various sizes of area. We also analyze MIMO channel capacities of SRR elements by using radiation patterns. The obtained results confirm that the proposed MIMO antenna system can achieve the same channel capacity as a conventional MIMO antenna system but with a 30% smaller footprint area and is very suitable for compact wireless equipment in next-generation wireless systems.

A new design method for a high-order series-coupled microring filter using Chebyshev filter condition was proposed and its application to the design of a wavelength-selective switch (WSS) was discussed. In the proposed method, the propagation loss in a microring resonator, coupling loss at a coupler, and a free spectral ranges (FSR) in a microring resonator are considered for the first time. It was found that for high-order series coupled microring resonators, the WSS designed using Chebyshev condition has more boxlike filter responses and high extinction ratio, compared with that designed using Butterworth condition, in the case where the round-trip loss in a microring is relatively large. In addition, the fourth-order series-coupled microring WSS with boxlike responses was successfully designed, considering Vernier effect for a larger FSR and shift in resonant wavelength.

A novel and compact electro-optic modulator implemented by a combination of a 12 multimode interference (MMI) coupler and an integrated Mach-Zehnder interferometer (MZI) modulator consisting of a microring and a phase modulator (PM) is presented and analyzed theoretically. It is shown that the proposed modulator offers both ultra-linearity and high output RF gain simultaneously, with no requirements for complicated and precise direct current (DC) control.

We demonstrate phase demodulation of 10-Gbps DPSK signals using a silicon micro-ring resonator with a radius of 10 µm and with various coupling gaps for light of ∼1550 nm in wavelength. Influence of the Q factors and transmissions of the resonators on the response speed and power balance of the two output ports is discussed. Furthermore, temperature sensitivity on resonance peak was measured and we discuss its effect on practical demodulation application.

Photonic integrated circuits (PICs) produced by large-scale integration (LSI) on Si platforms have been intensively researched. Since thermal diffusion from the LSI logic layer is a serious obstacle to realizing a Si-based optical integrated circuit, we have proposed and realized athermal wavelength filters using Si slot waveguides embedded with benzocyclobutene (BCB). First, the athermal conditions were theoretically investigated by controlling the waveguide and gap width of the slot waveguides. In order to introduce the calculated waveguide structures to wavelength filters, the propagation losses and bending losses of the Si slot waveguides were evaluated. The propagation losses were measured to be 5.6 and 5.3 dB/cm for slot waveguide widths of 500 and 700 nm, respectively. Finally, athermal wavelength filters, a ring resonator, and a Mach-Zhender interferometer (MZI) with a slot waveguide width of 700 nm were designed and fabricated. Further, a temperature coefficient of -0.9 pm/K for the operating wavelength was achieved with the athermal MZI.

We propose a novel metamaterial antenna that is based on loading a single complementary split ring resonator (CSRR) onto a substrate integrated waveguide (SIW) structure. Negative order and zeroth-order resonance can be observed in the proposed structure. These resonance modes are used to reduce the antenna size. In addition, a high quality (Q) factor of the CSRR-loaded SIW structure can minimize the radiation loss. The -1st, 0th, and 1st resonances are experimentally observed at 6.63, 13.68, and 20.31 GHz with maximum gains of 1.59, 3.97, 6.83 dBi, respectively. The electrical size of the antenna at the -1st resonance is only 42% of the resonance of a square microstrip patch antenna.

In this paper, a rat-race hybrid coupler based on an open complementary split ring resonator (OCSRR) is presented. By embedding the OCSRR in the microstrip transmission line, slow-wave effect is introduced to achieve size reduction. The proposed rat-race coupler size is 37% smaller than the conventional rat-race coupler. Besides, the proposed coupler provides better third harmonic suppression up to 35 dB. The simulated results are compared with the measured data and good agreement is reported.

In this letter, compact loop resonator type circular polarization (CP) antennas with a square ring and an X-shaped meander loop are presented. Both antennas are fed to a microstrip line with electromagnetic coupling. By adjusting the gap and length of a coupled microstrip line, the magnitude and phase conditions of two orthogonal modes for CP can be determined. The proposed antennas show good axial ratios and also good agreements between experimented and simulated results.

We have proposed a tunable laser with silica-waveguide ring resonators. In this tunable laser, a semiconductor optical amplifier was passively aligned and mounted onto a silica-waveguide substrate. The ring resonators can be tuned by controlling their temperatures using the thermo optic heaters formed on them, and there are no mechanically moving parts. Thus, they are sufficiently stable and reliable for practical use. Our tunable laser exhibits a high fiber-output power of more than 15 dBm and a wide tunable range of 60 nm (L-band, 50 GHz spacing, 147 channels). Moreover, a tunable laser with a much wider tunable range of 96 nm using 100-GHz-FSR ring resonators is also reported.

In this review paper we showcase recent activities on silicon photonics science and technology research in Hong Kong regarding two important topical areas--microresonator devices and optical nonlinearities. Our work on silicon microresonator filters, switches and modulators have shown promise for the nascent development of on-chip optoelectronic signal processing systems, while our studies on optical nonlinearities have contributed to basic understanding of silicon-based optically-pumped light sources and helium-implanted detectors. Here, we review our various passive and electro-optic active microresonator devices including (i) cascaded microring resonator cross-connect filters, (ii) NRZ-to-PRZ data format converters using a microring resonator notch filter, (iii) GHz-speed carrier-injection-based microring resonator modulators and 0.5-GHz-speed carrier-injection-based microdisk resonator modulators, and (iv) electrically reconfigurable microring resonator add-drop filters and electro-optic logic switches using interferometric resonance control. On the nonlinear waveguide front, we review the main nonlinear optical effects in silicon, and show that even at fairly modest average powers two-photon absorption and the accompanied free-carrier linear absorption could lead to optical limiting and a dramatic reduction in the effective lengths of nonlinear devices.

We propose and demonstrate a simple one-bus two-ring configuration where the two rings are mutually coupled that has advantages over the one-ring structure. Unlike a one cavity system, it can exhibit near critically-coupled transmission with a broader range of loss. It can also significantly enhance the cavity finesse by simply making the second ring twice the size of the bus-coupled one, with the enhancement proportional to the intensity buildup in the second ring.

A novel low insertion-loss and wideband microstrip bandpass filter has been designed and tested. The basic configuration of this novel dual-mode filter is a square ring resonator with direct-connected orthogonal feed lines, and dual-perturbation elements are introduced within the resonator at symmetrical location. The effects of the size of the perturbation element are studied. A new filter having wider bandwidth and transmission zeros are presented. The proposed filter responses are in good agreement with the simulations and experiments.

We have numerically evaluated the filtering characteristics of two vertically coupled microring resonator filters. In this evaluation we used the finite-difference time-domain method as the numerical analysis method. The structure we designed allows only a specific wavelength to pass. The filtering characteristics of this structure can be altered by changing the layout of the microring resonator. By using this structure the interval between peak wavelengths at a specific wavelength in the output spectrum can be increased. Specifically, the interval between peak wavelengths can be increased from 20 nm to 40 nm at wavelengths near 1.46 µm.

Utilizing the small bending radius of high index contrast optical waveguides, ultra-compact optical devices such as waveguide branch, Mach-Zehnder interferometer, arrayed waveguide grating filter, microring resonator filter, and so on can be realized. We have proposed and demonstrated a vertically coupled microring resonator as an Add/Drop filter, and recently realized a hitless wavelength channel selective switch (hitless tunable Add/Drop filter) using Thermo-Optic (TO) effect of double series coupled dielectric microring resonator. Using a high-index dielectric material as the core, the response time was reduced to 105 µs (rise time) and 15 µs (fall time), which are fifteen-fold and hundred-fold faster than that of polymer material, and the reproducibility by the heat cycle test was also improved to less than 0.01 nm. The tuning range of wavelength selective switch was expanded to 13.3 nm using the Vernier effect, and a large extinction ratio of more than 20 dB was realized. In this review, the principle and recent progress of microring resonator based wavelength selective switch will be introduced and some basic switching circuits required to optical cross connect will be discussed.

A novel wideband bandpass filter with improved stopband characteristics is presented in this paper. Dual-mode square ring resonator is used in the proposed filter. New formulas based on the even- and odd-mode analysis are derived to facilitate the design of transmission zeros of the square ring resonator. A short-circuited stub and a piece of aperture-enhanced parallel-coupled lines are introduced to the input and output of the resonator to lower the passband return loss and widen the stopband of the filter significantly. The filter has a 50% fractional bandwidth, is compact in configuration, and shows remarkably improved performance compared with previously reported filters of the same kind. The measured filtering response shows a good agreement with the simulated result.

This investigation proposes a modified equivalent circuit of single complementary split-ring resonator (CSRR) in planar transmission media and a dual-mode ring bandpass filter (BPF) that uses periodic CSRRs to suppress the spurious response. The proposed modified equivalent circuit consists of lumped elements that can be easily extracted from the measured S parameters. The proposed dual-mode ring BPF has exhibits a wide stopband characteristic owing to the bandgap resonant characteristic of CSRRs in the harmonic frequency of the dual-mode ring BPF. Good agreement with EM simulation and measurement is demonstrated.

A novel millimeter wave quadruple-push oscillator is presented in this paper. The quadruple-push oscillator consists of four identical sub-circuits and a ring resonator that is used as a common resonator. It is well known that there are two orthogonal resonant modes on a one-wavelength ring resonator. According to this resonant characteristic, two orthogonal push-push oscillations can be set up in the quadruple-push oscillator, and there is a phase difference of 90among four sub-circuits due to nonlinear performance. Therefore, the four identical sub-circuits can oscillate at the same fundamental frequency f0, and the fundamental oscillating signal of one sub-circuit has phase differences of 90, 180and 270to that of the others, and the desired fourth harmonic signals can be combined due to their in phase relations, and the undesired fundamental signals, the second harmonic signals, the third harmonic signals and so on can be suppressed when the oscillating signals of the four sub-circuits are added in phase. The principle is firstly explained in this paper, and is proved in the experiment of a Ka-band quadruple-push oscillator. The measured output power of the desired fourth harmonic signal (4f0) was +1.67 dBm at the frequency of 35.8 GHz. The measured suppression of the undesired signals of the fundamental signal (f0), the second harmonic signal (2f0), the third harmonic signal (3f0) and the fifth harmonic signal (5f0) were -18.0 dBc, -17.9 dBc, -17.8 dBc and -35.5 dBc, respectively. The measured phase noise performances at 35.8 GHz were -104.0 dBc/Hz and -82.3 dBc/Hz at the offset frequency of 1 MHz and 100 kHz, respectively.