A One‐Bit Optical Numerical Comparator With High Extinction Ratio Based on Graphene–Silicon Microring Resonators

We present a silicon-on-insulator (SOI) based device that exhibits Fano resonance with high extinction slope rate (SR) and extinction ratio (ER). It is constructed by using two cascaded tunable Mach–Zehnder interferometers (MZIs). The first MZI is used to adjust the power splitting ratio of the second MZI. In the second MZI, two add-drop microring resonators (MRRs) are located in each arm of the second MZI, respectively. The MRRs are used to generate a high-Q and low-Q resonance respectively. Due to the interference between these two resonances, a Fano resonance could be implemented. Considering that the optical power splitting ratio and phase difference between the two resonances can be finely adjusted, the ER can be greatly increased. In the experiment, the measured Fano resonance of the fabricated device exhibits simultaneous ER of 41.5 dB and SR of 3388.1 dB/nm. To the best of our knowledge, this is the first time to achieve a Fano resonance with such high ER and SR simultaneously. By adjusting the bias voltage in the fabricated device, a pair of complementary Fano resonance line shapes can be achieved.

We propose and realize a bandwidth and wavelength-tunable all-optical filter with high tuning efficiency based on double opto-mechanical microring resonators (MRRs) assisted Mach-Zehnder interferometer (MZI) structure, which is beneficial to achieve high extinction ratio and shape factor. As the optical field gradient is largely enhanced in the free-hanging MRRs, the opto-mechanical effect could be excited by low power consumption. By injecting the corresponding low resonance powers, the drop transmission of each MRR could be flexibly tuned based on the opto-mechanical effect. Consequently, the synthetic transmission of the microring-assisted MZI could be efficiently manipulated. The results show that by injecting pump powers of 0.40 mW and 1.84 mW, the bandwidth and wavelength of the all-optical filter could be tuned from 0.17 nm to 0.30 nm, and from 1550.09 nm to 1550.60 nm respectively with maintaining a high extinction ratio of 38 dB and a small passband ripple of 1 dB. Especially, the tuning efficiencies of bandwidth and wavelength could realize up to 0.325 nm/mW and 0.277 nm/mW, respectively. The proposed optical filter has dominant advantages of high tuning efficiencies and extinction ratios, small passband ripple and compact footprint, which has significant applications in on-chip all-optical systems.

Efficient optical modulation in ring structure based on Silicon-ITO heterojunction with low voltage and high extinction ratio

Silicon-based optical micro-ring resonators (MRRs) are very popular for many application because of the ultra-compact footprint and easy fabrication. The wavelength-selectivity property of MRRs makes it possible to be used not only as an optical filter with a narrow bandwidth but also as an optical sensor with high sensitivity. This paper gives a review of our recent work on the MRRs on silicon. First, a high-order MRR optical filter with a box-like filtering response is realized by introducing bent directional couplers to have sufficient coupling between the access waveguide and the microrings. A efficient thermally-tunable MRR-based optical filter with graphene transparent nano-heater is realized by introducing transparent graphene nanoheaters, which contact the silicon core directly without any isolator layer. As a result, the heating efficiency, the temporal response and the achievable temperature is improved in comparison with a traditional metal heater. MRRs are also very attractive for the applications of optical sensing and two novel designs of MRRs for optical sensing are presented. One is a digital optical sensor based on two cascaded MRRs with different free spectral ranges (FSRs), which was proposed and realized to achieve extremely high sensitivity. The other is a Mach-Zehnder interferometer (MZI) coupled microring, which has only one resonant wavelength with a high extinction ratio in a very large wavelength span and consequently a very large quasi-FSR (free spectral range) is achieved (FSR > 120 nm). This enables a large measurement range for the change of refractive index. In this paper, we give a review of our recent work on the optical MRRs on silicon and their applications.

Microwave photonic filters are a key fundamental subsystem in microwave photonics. A wideband‐tunable on‐chip microwave photonic filter (MPF), which monolithically integrates a dual‐drive Mach‐Zehnder modulator (DDMZM), is proposed with ultrahigh‐Q Mach–Zehnder‐interferometer (MZI)‐coupled microring resonators (MRRs) in cascade and a high‐speed photodetector (PD). It is the first time an MPF has been developed by introducing U‐bend‐MZI‐coupled MRRs whose resonance wavelength and 3 dB bandwidth are tunable. In particular, the ultrahigh‐Q U‐bend‐MZI‐coupled MRRs are realized with ultralow loss broadened silicon waveguides based on a modified Euler curve. For the fabricated U‐bend‐MZI‐coupled MRRs, which have an effective radius of 29 µm, the Q‐factor is as high as 1.3 × 106, while the free spectral range (FSR) is as large as 113 GHz. Furthermore, the introduction of two cascaded U‐bend‐MZI‐coupled MRRs enables the MPF to have flat‐top bandstop (or bandpass) responses with a high extinction ratio (ER) of 20–35 dB as well as a tunable 3 dB bandwidth of 0.15–3 GHz. Meanwhile, the central frequency can be tuned thermally from 2 to 40 GHz. The work provides a feasible route to realize a compact, high spectral resolution, and wideband‐tunable MPF on silicon, enabling a range of RF applications from wireless communication to flexible high‐frequency microwave photonic signal processing.

Introduction: Today sensor systems based on integrated photonics devices are the most important branch of embedded information and control systems for various functions. The output characteristics of a sensor system are significantly determined by the efficiency of the interrogator. The intensity interrogator based on a microring resonator can provide a high scanning rate and sensitivity that meets the requirements of a wide range of applications. Purpose: To develop an effective sensor system composed of a refractometric sensor and an interrogator located on the same photonic integrated circuit for marker-free determination of the concentration of substances in liquids. Methods: We use the numerical simulation of electromagnetic field propagation in a waveguide system (integrated silicon waveguides on a silicon dioxide substrate) in the research. The simulation has been carried out using the Ansys Lumerical environment, the FDTD (Finite Difference Time Domain) solver. The parameters of the microring resonators were optimized to obtain the coupling coefficients between the waveguides, providing the operation in the critical coupling mode. Results: We propose the concept of a fully integrated photonic sensor system based on micro-ring add-drop resonators. A sensor based on microring resonators has been developed, which consists of two half-rings with a radius of 18 μm, connected by sections of straight waveguides 3 μm long. An interrogator represented by a microring resonator with a radius of 10 µm has been developed. According to simulation results with a broadband source, the achieved sensor sensitivity was 110 nm per refractive index change, or 1350 dB per refractive index change. We propose a technique for choosing the optimal characteristics of the sensor and interrogator targeted to improve the complete system efficiency. Practical relevance: Sensor systems based on photonic integrated circuits can meet the demand for devices characterized by low power consumption, small size, immunity to electromagnetic interference and low cost.

This paper presents a novel micro-ring-resonator-assisted structure as an optical switch. This device couples two outputs of a 1x2 power splitter into a micro-ring resonator (MRR). The double-beam interference will occur in the MRR. If the phase difference between the beams and the coupling coefficients of MRR are set properly, the device can switch the light by applying phase shift on MRR. High extinction ratio or low crosstalk can be achieved, even if the modulation additional loss is large.

Integrated microring resonators are well suited for wavelength-filtering applications in optical signal processing, and cascaded microring resonators allow flexible filter design in coupled-resonator optical waveguide (CROW) configurations. However, the implementation of high-order cascaded microring resonators with high extinction ratios (ERs) remains challenging owing to stringent fabrication requirements and the need for precise resonator tunability. We present a fully integrated on-chip second-order CROW filter using silicon photonic microelectromechanical systems (MEMS) to adjust tunable directional couplers and a phase shifter using nanoscale mechanical out-of-plane waveguide displacement. The filter can be fully reconfigured with regard to both the ER and center wavelength. We experimentally demonstrated an ER exceeding 25 dB and continuous wavelength tuning across the full free spectral range of 0.123 nm for single microring resonator, and showed reconfigurability in second-order CROW by tuning the ER and resonant wavelength. The tuning energy for an individual silicon photonic MEMS phase shifter or tunable coupler is less than 22 pJ with sub-microwatt static power consumption, which is far better than conventional integrated phase shifters based on other physical modulation mechanisms.

We propose and experimentally demonstrate microwave photonic filters (MPFs) with high rejection ratios and large tuning ranges of the central frequency and bandwidth leveraging four cascaded opto-mechanical microring resonators (MRRs). As half waveguides of each MRR are free-hanging in the air, the nonlinear effects in the opto-mechanical MRRs could be efficiently excited. Consequently, the transmission characteristics of the cascaded MRRs could be flexibly manipulated by adjusting the input pump powers. When the resonant wavelengths of every two MRRs are tuned to be aligned, the transmission spectrum of the silicon device is a notch bimodal distribution with high extinction ratios. The optical carrier is fixed at the flat region of the bimodal distribution. Under optical double sideband (ODSB) modulation, MPFs with high rejection ratios could be achieved due to the high extinction ratio of the cascaded rings. Moreover, the central frequency and bandwidth of the MPFs could be tuned by properly adjusting the pump powers. In the experiment, with a low power of 2.56 mW, the MPF central frequency and bandwidth could be tuned from 7.12 GHz to 39.16 GHz and from 11.3 GHz to 17.6 GHz, respectively. More importantly, the MPF rejection ratios are beyond 60 dB. Furthermore, during the bandwidth tuning process, an MPF response with approximately equiripple stopband could be realized. Owing to the dominant advantages of high rejection ratios, large tuning ranges, low power consumption and compact size, the silicon device has many significant applications in on-chip microwave systems.

Realization and optimization of optical logic gates using bias assisted carrier-injected triple parallel microring resonators

In this paper, a resonant system is demonstrated on silicon-on-insulator wafer to achieve tunable Fano resonances. In this system, the Fano resonance originates from the interference of two beams resonant in the microring resonator. The shapes of the Fano resonances are tunable through controlling the phase difference of the two beams. Both large slope and high extinction ratio (ER) are obtained when the phase difference is 0.5π or 1.5π. Experimental results show that Fano resonances with steep slope and ER over 20 dB are achieved in the whole free spectral range by controlling the microheaters to meet the phase condition.

The high extinction ratio mode (de)multiplexer is a pivotal component in high capacity mode-division multiplexing data communication and nascent on-chip intermodal acousto-optic modulators. Up to now, high performance on-chip mode (de)multiplexers are still lacking for integrated AOMs on the lithium niobate-on-insulator platform. In this paper, we propose and demonstrate an innovative scheme to achieve high extinction ratio signal routing for acousto-optic modulation, by leveraging a two-mode (de)multiplexer in conjunction with a high-Q racetrack microring resonator. The integrated devices are fabricated with one-step electron beam lithography and dry etching processes. The demonstrated two-mode (de)multiplexer boasts the excellent intermodal crosstalk below −20 dB and the on-chip insertion loss of less than 1.92 dB within the wavelength range of 1,514–1,580 nm. With the reinforcement of the microring resonator filter, the carrier signal can be suppressed thoroughly and the measured extinction ratio attains over 30 dB. Our proof-of-principle investigations have provided a feasible and compact solution to implement practical intermodal AOMs in LNOI for photonic and quantum information process, microwave photonics, and LiDAR.

All optical tri-state frequency encoded logic gates NOT and NAND are proposed and numerically investigated using TOAD based interferometric switch for the first time to the best of our knowledge. The optical power spectrum, extinction ratio, contrast ration, and amplified spontaneous noise are calculated to analyze and confirm practical feasibility of the gates. The proposed device works for low switching energy and has high contrast and extinction ratio as indicated in this work.

LeTrungThanh Optical Biosensors Based on Multimode Interference and Microring Resonator Structures

In this thesis, we simulate, fabricate and analyze the Si-QD doped Si-rich SiOx/ Si-rich SiCx/ Si-rich SiNx micro-ring waveguide resonator based all-optical modulator using linear free-carrier absorption and nonlinear Kerr effect. In the chapter 2, by integrating with a ring-resonator waveguide, the Si quantum dot doped SiOx strip-loaded waveguide based free-carrier absorption modulator with enhanced FCA loss modulation is demonstrated by different pumping wavelengths. The micro-ring waveguide resonator induced a dark-comb like throughput transfer function in wavelength domain, in which function with a Q-factor of 6×103 can be blue-shifted by varying the photo-excited electron-hole plasma density. When transmitting optical data stream at the central wavelength of any notch in the dark-comb throughput function, the output data-stream can be inverted by pumping the micro-ring waveguide resonator to up-shift the notch away from its original wavelength. As a result, this maximize the extinction ratio of output data stream. The largest FCA loss and highest free-carrier density can be enhanced up to 3.44 cm-1 and 9.29×1016 cm-3, respectively, at excitation intensity as high as 6.7 W/cm2. By adding the micro-ring waveguide resonator, the modulation depth can be further enhanced from 52.5% to 63.5% by up-shifting the transfer function of the micro-ring waveguide resonator with the photo-excited e-h plasma. The maximal wavelength of the transmittance notch shift can be up to 0.033 nm under the same pumping intensity at wavelength of 1563.42 nm. The excited free-carriers inside the micro-ring waveguide resonator is obtained to be ~5.25×1015 cm-3. The pumping wavelength dependent transmission notch linewidth is broadened from 0.26 nm to 0.3 nm when the ring waveguide resonator is pumped by 325-nm HeCd laser. Due to the speed of FCA based modulator is limited by the carrier lifetime which the modulation speed of SiOx:Si-QD is ~ 1 μs. As a result, the ultrafast nonlinear optical Kerr effect is employed in the following work. In the chapter 3, the ultrafast optical Kerr switch with a Si-rich SiC micro-ring resonator is demonstrated. In addition, the nonlinear refractive index of Si-rich SiC at telecommunication wavelengths is firstly estimated by the resonance red-shift, which is still unknown in the literature. With the 12 Gbit/s NRZ-OOK optical pump data-stream, the Si-rich SiC micro-ring resonator shows a great applicability in the real optical communication system. From the maximal inverted probe signal, the red-shift of resonance dip is 0.07 nm corresponding to refractive index change of 1.2×10-4. By the red-shift of resonance dip, the nonlinear refractive index of Si-rich SiC is estimated to be n2=3.14×10-13 cm2/W, which is several orders magnitude larger than that in SiC. From the analyses of XPS and the RSS, the excessive Si concentration of 37.2% which exist in the form of Si-QDs is observed. The existence of Si-QDs buried in the Si-rich SiC matrix can effectively result in a huge enhancement on the nonlinear refractive index, which can be explained by the quantum confinement effect. From the BPM analysis under single-mode condition, the waveguide width and height of 600 nm and 300 nm is determined. The fabricated micro-ring waveguide resonator is obtain with Q=22800 and the transmittance drop of nearly 60% at a wavelength of 1551.08 nm. In the chapter 4, the 12 Gbit/s optical Kerr switch has been demonstrated with a Si-rich SiN micro-ring resonator in the first time. The Si-rich SiN with excessive Si of 23.4% is grown by PECVD process with fluence ratio of [SiH4]/[NH3] equals 0.9. The fabricated mirco-ring resonator of Q=11000 is observed, which provides a field enhancement inside the micro ring resonator. By introducing the 12 Gbit/s NRZ-OOK data stream as the optical pump format, the ultra-fast response up to 83 ps of the Si-rich SiN micro-ring resonator shows a great applicability in the real-world optical communication system. The SNR is degraded from 9.66 dB to 5.32 dB after the wavelength conversion. According to the ultrafast response of nonlinear Kerr effect, the nonlinear refractive index of Si3N4:Si-QD at near-infrared wavelengths for optical telecommunications is instantly modified by the input optical data stream to cause the red-shift on the resonance of the micro-ring, thus providing a high-speed optical switch up to 12 Gbit/s via the cross-wavelength amplitude modulation effect. By analyzing the resonance dip red-shift of 0.13 nm corresponding toδn=2.2×10-4, the nonlinear refractive index of the Si-rich SiN is estimated as n2=δn/Ir=2.17×10-13 cm2/W, which is one order and two orders of magnitude larger than that in Si and SiN, respectively.