Ring-assisted Mach-Zehnder Interferometer Research Articles

High-performance silicon nitride (de)multiplexer based on ring-assisted MZIs.

We present the design and experimental characterization of the first multistage ring-assisted Mach-Zehnder interferometer (RAMZI) lattice (de)multiplexer implemented with silicon nitride optimized for four channels with a spacing of 100 GHz in the L-band. The device comprises two RAMZI stages to provide a sharp box-like response characterized by a shape factor of 0.9, a flat passband over the entire channel, and a crosstalk level better than -14 dB. The maximally flat passband of the demultiplexer enables a passband width twice that of the maximum spectral excursion defined in the NG-PON2 standard.

Self-configuring programmable silicon photonic filter for integrated microwave photonic processors

Reconfigurable photonic filters show great promise as a potential solution to meet the evolving needs of future microwave communication systems. By integrating high-performance filters into programmable microwave photonic processors, they can provide significant benefits for signal processing applications. The development of an algorithm that can automatically characterize and reconfigure the filter using a single optical input and output port is essential for this purpose. This paper presents an optimization technique for a fully tunable ring-assisted Mach–Zehnder interferometer filter. The proposed filter design eliminates the need for monitoring components and employs a novel algorithm that operates independently in each ring by switching between the two arms of the filter. In addition, the filter can be configured to implement different filter architectures, allowing for flexible filtering requirements. Measurements were performed using the device as an interleaver, implementing different types of infinite impulse response filters in the optical and radio frequency domains. Side-coupled integrated spaced sequence of resonator filters were also implemented by reconfiguring the same device. These results demonstrate the exceptional reconfigurability of the filter design proposed herein in terms of bandwidth and central frequency.

Microwave Sensing Aqueous Solutions Based on Ring-Assisted Mach–Zehnder Interferometer

In this work, we present a novel tunable and ring-assisted Mach–Zehnder interferometer (RAMZI) to realize the highly sensitive and contactless microwave measurement of liquid samples. The designed RAMZI consists of a split ring (SR) and a feedback line for the destructive interference at the resonant frequency of odd or even mode of the SR after appropriately tuning the signal balance. An equivalent circuit model is built to study and explain its working principle. The achieved quality factor is up to 3.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times \,\,10^{5}$ </tex-math></inline-formula> with water loaded. The minimum ethanol concentration level is measured to be 0.005 mole fraction. With the empirical relation between the sensor resonance and the sample permittivity, the complex permittivity of a liquid can be extracted.

Multiband Signal Receiver by Using an Optical Bandpass Filter Integrated with a Photodetector on a Chip

Photonic integration brings the promise of significant cost, power and space savings and propels the real applications of microwave photonic technology. In this paper, a multiband radio frequency (RF) signal simultaneous receiver using an optical bandpass filter (OBPF) integrated with a photodetector (PD) on a chip is proposed, which was experimentally demonstrated. The OBPF was composed of ring-assisted Mach-Zehnder interferometer with a periodical bandpass response featuring a box-like spectral shape. The OBPF was connected to a PD and then integrated onto a single silicon photonic chip. Phase-modulated multiband RF signals transmitted from different locations were inputted into the OBPF, by which one RF sideband was filtered out and the phase modulation to intensity modulation conversion was realized. The single sideband with carrier signals were then simultaneously detected by the PD. A proof-of-concept experiment with the silicon photonic integrated chip was implemented to simultaneously receive four channels of 8 GHz, 12 GHz, 14 GHz and 18 GHz in the X- and Ku-bands. The performance of the integrated microwave photonic multiband receiver-including the receiving sensitivity, the spurious free dynamic range, the gain and the noise figure across the whole operation frequency band-was characterized in detail.

Fabrication-robust silicon photonic devices in standard sub-micron silicon-on-insulator processes.

Perturbations to the effective refractive index from nanometer-scale fabrication variations in waveguide geometry plague high index-contrast photonic platforms; this includes the ubiquitous sub-micron silicon-on-insulator (SOI) process. Such variations are particularly troublesome for phase-sensitive devices, such as interferometers and resonators, which exhibit drastic changes in performance as a result of these fabrication-induced phase errors. In this Letter, we propose and experimentally demonstrate a design methodology for dramatically reducing device sensitivity to silicon width variations. We apply this methodology to a highly phase-sensitive device, the ring-assisted Mach-Zehnder interferometer (RAMZI), and show comparable performance and footprint to state-of-the-art devices, while substantially reducing stochastic phase errors from etch variations. This decrease in sensitivity is directly realized as energy savings by significantly reducing the required corrective thermal tuning power, providing a promising path toward ultra-energy-efficient large-scale silicon photonic circuits.

Ultraflat bandpass, high extinction, and tunable silicon photonic filters.

We report on the design and the demonstration of silicon photonic ultraflat bandpass filters with low insertion loss and high out-of-band rejection for an operation near the 1550 nm wavelength band. These filters are based on cascading low (2nd) order Ring-Assisted Mach-Zehnder Interferometer (RAMZI) filter stages. The cascade design enables high out-of-band rejection while keeping the unit cells of each stage low order to be more tolerant to fabrication imperfections. The characterization of filters shows an insertion loss of ∼1 dB, an in-band ripple of <0.1 dB, an out-of-band rejection of >50 dB for a filter 3-dB bandwidth of ∼1.1 nm, and tunable up to ∼6 nm. We also investigate the filter's spur-free dynamic range at high input optical powers, which is important for RF photonics applications, and quantify a dynamic range of >60 dB for a laser power as high as ∼11.6 mW sent to the filter. Such integrated filters are promising for applications in pump wavelength rejection in four-wave mixing photon pair generation, and in RF antenna remoting where multiple RF signals are carried on different coarse wavelengths to be separated.

Ultra-high-linearity integrated lithium niobate electro-optic modulators

Integrated lithium niobate (LN) photonics is a promising platform for future chip-scale microwave photonics systems owing to its unique electro-optic properties, low optical loss, and excellent scalability. A key enabler for such systems is a highly linear electro-optic modulator that could faithfully convert analog electrical signals into optical signals. In this work, we demonstrate a monolithic integrated LN modulator with an ultra-high spurious-free dynamic range (SFDR) of 120.04 dB · Hz 4 / 5 at 1 GHz, using a ring-assisted Mach–Zehnder interferometer configuration. The excellent synergy between the intrinsically linear electro-optic response of LN and an optimized linearization strategy allows us to fully suppress the cubic terms of third-order intermodulation distortions (IMD3) without active feedback controls, leading to ∼ 20 dB improvement over previous results in the thin-film LN platform. Our ultra-high-linearity LN modulators could become a core building block for future large-scale functional microwave photonic integrated circuits by further integration with other high-performance components like low-loss delay lines, tunable filters, and phase shifters available on the LN platform.

A Microwave Frequency Measurement System Based on Si3N4 Ring-Assisted Mach-Zehnder Interferometer

A microwave frequency measurement system based on a ring assisted Mach-Zehnder interferometer (RAMZI) with a large measurement range and high accuracy is proposed and experimentally demonstrated. By using single-sideband modulation, the microwave signal with unknown frequency is converted to an optical sideband signal, which is used to probe the two complementary output optical powers of the ring-assisted Mach-Zehnder interferometer, then a fixed frequency-to-power mapping is established by obtaining a highly linear amplitude comparison function (ACF). High-precision frequency measurement with a root mean square error (RMSE) of less than 30 MHz was achieved in a frequency range of 5.2 GHz, and relative low-precision frequency measurement with a RMSE of less than 102 MHz was achieved in a frequency range of 11 GHz. Besides, in order to break the trade-off between measurement range and measurement accuracy, a multi-band measurement method was implemented and frequency measurement range of 5∼39 GHz with RMSE lower than 37 MHz was achieved.

A Wavelength-Selective Multiwavelength Ring-Assisted Mach–Zehnder Interferometer Switch

Wavelength-selective switches have been proposed for datacenter use to enhance the scalability of their networks to help in meeting ever-increasing traffic demands. We have previously demonstrated a 4 × 4 ring-based crossbar silicon photonic switch in which each cross-point contained three ring pairs to partition the free spectral range (FSR) into three equal regions to reduce wavelength tuning range per ring pair—thereby reducing both the tuning power consumption and stress on the rings—while maintaining full routing flexibility. However, the question of scalability remains for such a crossbar switch in which 96 signal pads—one routed to each ring—are required to fully control it. In this article we present a 4-port silicon photonic ring-assisted Mach–Zehnder interferometer (RAMZI) switch with multiple-sized rings per switching elements in a Benes network configuration to reduce the number of electrical pads required to 36 signal pads for the equivalent number of wavelength-selective switch elements per switch cell. The switch is 500 μm × 3 mm in size and is packaged on a custom PCB. Another advantage the RAMZI switch has over the crossbar switch is that the loss through the switch is not path-dependent due to its balanced path configuration. In the crossbar switch, the difference between the shortest and longest paths is 2(N − 1) switching elements. Finally, we present results from the fabricated switch and the outlook for further scaling of the switch architecture.

Flexible ultra-wide frequency microwave down-conversion based on re-circulating four-wave mixing in a semiconductor optical amplifier.

A flexible ultra-wide frequency photonic-assisted method is proposed for microwave harmonic down-conversion based on re-circulating four-wave mixing (RFWM) in a semiconductor optical amplifier (SOA). The proposed down-converter consists of a RF-driven electro-optic modulator (EOM) and a RFWM-intensified optical local oscillator (LO) located in a ring-assisted Mach-Zehnder interferometer (R-MZI). In the optical LO, the optical carrier is first modulated by a low-frequency electrical LO through an EOM for triggering high-order harmonics sideband generation in the optical domain through the FWM effect in the SOA, and the generated harmonics sidebands are further intensified by re-circulating the FWM products back to the EOM and the SOA successively with an amplified ring loop. The RFWM-intensified optical LO enables accurate tunable and ultra-wide frequency operation of down-conversion by simply adjusting the low-frequency electrical LO. In the experiment, the RFWM-based optical LO is operated with wide spectrum of more than 0.8 nm (15-dB bandwidth) and 1.2 nm (20-dB bandwidth) and tunable frequency spacing from 4 GHz to 12 GHz. The microwave frequency conversion is successfully demonstrated in the RF range of 5-40 GHz down-converted to IF band below 2 GHz with a low-frequency electrical LO at about 4.8 GHz.

Controllable trapping and releasing of nanoparticles by a standing wave on optical waveguides.

Based on the balance between the scattering force and the trapping force of an evanescent field of a standing wave on silicon waveguides, we propose a structure for controllable trapping and releasing of nanoparticles, which can act as pause operation for nanoparticle flow control. The design is realized by the cascade of an optical switch with a structure of a ring-assisted Mach-Zehnder interferometer (RAMZI) and a Sagnac loop reflector which connects to one output of the switch. Through thermal tuning, with a tiny refractive index change of 4.3×10-4 on a ring resonator, the output of a RAMZI can be switched between two ports. As for the release state of the nanoparticle flow, the light is guided to the port without a reflector. There is no standing wave or traps formed on a waveguide. Therefore, the scattering force dominates, which drives particles moving forward to output ports. Otherwise, for trapping a state, the light will be reflected by the Sagnac loop and form a stationary standing wave which provides an array of traps for nanoparticles. Most importantly, the structure can switch its state to trap or sequentially release particles without losing the control of samples which, to the best of our knowledge, has not been realized before. With the statistical description of particle motion, the balance between trapping and releasing is distinguished by the trapping time and tuned by reflectance. The feasibility of our design is verified using the three-dimensional finite-difference time domain and Maxwell stress tensor methods. Our structure possesses the merits of high compactness and time effectiveness and, thereby, it is highly suitable for on-chip optical manipulation of nanoparticle flow control, which brings great potential in integrated on-chip optofluidics.

Volterra Series Based Linearity Analysis of a Phase-Modulated Microwave Photonic Link

An analytical technique based on Volterra series for systematically computing distortion in analog microwave photonic (MWP) links is presented in this paper. In the analysis, we express both modulator and photodetector responses in the MWP link as Taylor series expansions, which are then nested to give rise to Volterra series for systems with memory. Closed-form expressions of the link photocurrent and optical power based on the nth-order Volterra kernels are derived from the functional Taylor series of the MWP link. These expressions are applicable in both direct detection and phase-modulated conditions hitherto referred to as memoryless case and with memory, respectively. The synthesized Volterra kernels are primarily used to determine key figures of merit such as nth-order harmonic distortion, third-order intermodulation distortion (IMDn), and third-order intercept point (OIPn) for an arbitrary MWP link. We apply the technique to a photonic link having a ring-assisted Mach-Zehnder interferometer (RAMZI) filter and compare its OIP3 performance with that of an MZI discriminator. Within a bandwidth of 20 GHz, it is shown that RAMZI has 10-dB better OIP3 performance than MZI. We also illustrate that OIP3 and IMD measurements done using three-tone tests would result in more accurate predictions of MWP link parameters than standard two-tone tests.

LeTrungThanh Optical Biosensors Based on Multimode Interference and Microring Resonator Structures

LeTrungThanh Optical Biosensors Based on Multimode Interference and Microring Resonator Structures

Reconfigurable Sorting of Nanoparticles on a Thermal Tuning Silicon Based Optofluidic Chip

We numerically demonstrate a scheme of optofluidic sorting of nanoparticles on a silicon-based lab-on-a-chip system, with reconfigurable and multilevel sorting size thresholds. Size-dependent sorting of nanoparticles originates from the size-dependent distribution of trapping potential wells, which determines whether a nanoparticle can jump back and forth between parallel waveguides in weakly coupling condition. In numerical modeling, we find that it is easier for larger ones to transfer and the size threshold is power ratio dependent. By setting a thermal tuning ring-assisted Mach-Zehnder interferometer ahead, we can adjust effectively the input power ratio of the parallel waveguides as well as the potential well distribution across them, working as a thermal tuning reconfigurable sorting unit. Consequently, a tree cascaded unit with different thresholds is also presented to be a reconfigurable multilevel sorting unit. The proposed design offers a simple and ultracompact scheme for the reconfigurable and multilevel optofluidic sorting of nanoparticles on thermo-optofluidics chip for the first time.

Silicon Photonic Ring-Assisted MZI for 50 Gb/s DAC-Less and DSP-Free PAM-4 Transmission

We present the design and characterization of a silicon photonic ring-assisted Mach-Zehnder interferometer for four-level pulse amplitude modulation (PAM-4) short-reach transmission without the use of digital-to-analog converter (DAC) or digital signal processing (DSP). The PAM-4 optical signals are generated by driving two microring modulators with independent 2-level radio frequency signals. The device is operated at 25 Gbaud and the PAM-4 optical signals are transmitted over 2 km of standard single mode fiber. The bit error rate is estimated to be lower than the hard-decision forward error correction threshold of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , showing a successful DAC-less and DSP-free PAM-4 transmission at 50 Gb/s.

Differential Microring Modulators for Intensity and Phase Modulation: Theory and Experiments

We theoretically and experimentally demonstrate a ring-assisted Mach–Zehnder interferometer modulator for intensity and phase modulation, leveraging both Mach–Zehnder modulator and single microring modulator. Differential drive of two microring modulators nested in a Mach–Zehnder interferometer offers several advantages for both intensity modulation and phase modulation. We first present a complete theoretical study of this differential ring modulator, including the operation principles, high-speed characteristics, system performance, and energy consumption. We then experimentally verify that this differential ring modulator provides low-chirp, low drive voltage, and large extinction ratio for intensity modulation, and low-chirp, low drive voltage, and drive-voltage insensitivity for phase modulation.

Slot Waveguide Ring-Assisted Mach–Zehnder Interferometer for Sensing Applications

We experimentally demonstrate a ring-assisted Mach–Zehnder interferometer (RAMZI) design with slot waveguides for bulk index sensing using sucrose solution. Devices were implemented with both directional couplers (DCs) and multimode interferometer couplers (MMIs). The RAMZI designs with DCs were found to have a sensitivity of 203.3 ± 15.3 nm/RIU, while 177.2 p 12.0 nm/ RIU was demonstrated with MMIs-coupled RAMZI. The free spectral ranges (FSRs) of DCs and MMIs-coupled RAMZI were measured to be 24.3 and 34.3 nm, respectively. The FSRs demonstrated can be as wide as 7.9 times larger than the equivalent microring resonator.

Ultralinear heterogeneously integrated ring-assisted Mach–Zehnder interferometer modulator on silicon

A linear modulator is indispensable for radio frequency photonics or analog photonic link applications where high dynamic range is required. There is also great interest to integrate the modulator with other photonic components, to create a photonic integrated circuit for these applications, with particular focus on silicon photonics integration in order to take advantage of complementary metal–oxide–semiconductor compatible foundries for high-volume, low-cost devices. However, all silicon modulators, including the highest performing Mach–Zehnder interferometer (MZI) type, have poor linearity, partially due to the inherent nonlinearity of the MZI transfer characteristic, but mostly due to the nonlinearity of silicon’s electro-optic phase shift response. In this work, we demonstrate ultralinear ring-assisted MZI (RAMZI) modulators, incorporating heterogeneously integrated III–V multiple quantum wells on silicon phase modulation sections to eliminate the nonlinear silicon phase modulation response. The heterogeneously integrated III–V/Si RAMZI modulators achieve record-high spurious free dynamic range (SFDR) for silicon-based modulators, as high as 117.5 dB·Hz2/3 at 10 GHz for a weakly coupled ring design, and 117 dB·Hz2/3 for a strongly coupled ring design with higher output power. This is a higher SFDR than typically obtained with commercial lithium niobate modulators. This approach advances integrated modulator designs on silicon for applications in compact and high-performance analog optical systems.

Compact silicon photonic interleaver based on a self-coupled optical waveguide.

We propose and experimentally demonstrate a new scheme to realize an on-chip silicon photonic interleaver by using a self-coupled optical waveguide (SCOW). Benefiting from the high-order filtering property of a multistage SCOW resonator, the device has a smaller footprint and higher extinction ratio compared to conventional ring-assisted Mach-Zehnder interferometer interleavers. Its high fabrication tolerance is also demonstrated in this paper. The operation principle of the proposed interleaver is theoretically analyzed. The designed device is fabricated on a silicon-on-insulator wafer under standard complementary metal oxide semiconductor compatible fabrication processes. Experimental results show that 20 dB extinction ratio and about 8 dB insertion loss can be achieved in the entire C-band without any thermo-optic tuning, verifying the effectiveness of the proposed device as an on-chip interleaver with a compact footprint and high extinction ratio.

Switching of nanoparticles in large-scale hybrid electro-optofluidics integration.

We numerically demonstrate the scheme of independent optofluidic switching of nanoparticles on a silicon-based lab-on-a-chip system, using an electronic logic activated ring-assisted Mach-Zehnder interferometer (RAMZI). By using the carrier injection method with a tiny refractive index change of 8.00×10^-4 to adjust the phase delay of a ring resonator sitting on one arm of the MZI, the light passing through could be switched to any output port of MZI followed by a directional coupler (DC). Meanwhile, the trapping force and scattering force of the guided lightwave could provide the actuation for sample delivery. Therefore, the switching logic of the guided mode is mapping to its loaded sample of nanomaterials. Our structure possesses high compactness, scalability, and time-effectiveness and, thereby, it is very appropriate for on-chip optical manipulation. The introduction of the RAMZI and cascaded RAMZIs in an optofluidic chip can form a scalable switching module with an independent electronic logic trigger signal, and make the chip dynamically configurable and scalable, which is very critical and opens a new horizon for the large-scale hybrid electro-optofluidics integration of a lab-on-a-chip system.