Symmetric Mach-Zehnder Interferometer Research Articles

Improved error tolerance of programmable photonic integrated circuits for MNIST handwritten digit classification

Programmable photonic integrated circuits (PPICs) can realize linear optical unitary transformation between arbitrary multi-port input and multi-port output for optical computing and signal processing. With the scale increase of PPICs, the accumulation of manufacturing errors will lead to overall performance deviations. A symmetrical Mach-Zehnder interferometer (MZI) circuit with a shorter optical path and lower propagation loss has been proposed recently to replace the traditional asymmetrical MZI circuit. However, no systematic research on the error compensation of symmetrical MZI circuits has been reported yet. In this paper, the error tolerance characteristics of symmetrical MZI circuits and asymmetrical MZI circuits are analyzed and compared in detail. An effective strategy to compensate the 3 dB beam splitter error of a symmetrical MZI circuit is proposed. A deep learning task of the Modified National Institute of Standards and Technology (MNIST) handwritten digit dataset classification was carried out with two hidden layers, which are implemented by rectangular symmetrical MZI circuit structures. The recognition accuracy can be significantly improved with this error compensation method.

Hitless Wavelength-Selective Switch Using a Single Microring Resonator Assisted With a Symmetric MZI

We demonstrate an optical hitless wavelength-selective switch using a single microring resonator assisted with a symmetric Mach-Zehnder interferometer (MZI). A part of the ring waveguide and its symmetrical waveguide are designed to be an equally armed MZI as an on-off switch for the resonant state of the microring. The symmetric MZI is insensitive to the spectrum and it avoids free spectral range (FSR) matching or alignment in use. The FSR of the structure is 2.18 nm and the insertion loss (IL) at the drop-port is 1.8 dB. Our experimental results reveal that the fabricated device permits on-chip hitless rerouting of multi-wavelength signals, which means the intermediate wavelength remains unaffected during wavelength switching.

Thermally-induced optical modulation in a vanadium dioxide-on-silicon waveguide

In this paper, we report phase-pure vanadium dioxide (VO2) deposition on silicon-on-insulator and demonstrate switching/modulation exploiting the phase-change property. We present electrical and optical properties of VO2 during phase transition. Exploiting the phase change property, optical modulation is achieved by thermally tuning the VO2 phase using a lateral micro-heater beside the waveguide. We achieve an optical modulation extinction of 25 dB and a low insertion loss of 1.4 dB using a ring resonator with a VO2 patch. We also demonstrate the switching performance of a symmetric Mach-Zehnder interferometer and present a detailed discussion on the optimal operating point to achieve maximum modulation, higher speed, and lower insertion loss.

High Performance InP-Based Polarization Beam Splitter With Reverse Bias and Injection Current

High performance InP-based polarization beam splitters (PBSs) with a symmetrical Mach–Zehnder interferometer (MZI) are theoretically and experimentally demonstrated. The p-i-n waveguide is exploited to employ the phase shifters in the two arms of the MZI. Injection current and reverse bias inducing electro-optic (EO) effects in each of the MZI arms are analysed in detail when the waveguides are aligned along both the [011] and $[ {01\bar{1}} ]$ directions. Results indicate that the birefringence from reverse bias help to realize the PBS combining with the polarization-independent carrier-induced refractive index change from the injection current. Theoretical prediction shows that the polarization extinction ratio (PER) is over 35 dB at 1550 nm for the PBSs with the waveguides aligned along both directions. Furthermore, the PBS with the waveguides aligned along the [011] direction can be adjusted more easily to work at the best operating point and get the higher birefringence which requires lower reverse bias exhibiting better performance across the C-band compared with the $[ {01\bar{1}} ]$ direction. The fabricated PBSs exhibit a PER over 19 dB (14 dB) in the wavelength range from 1525 to 1570 nm with one arm injected of 2.88 mA (0.2 mA) current and the other arm reversed biased at 5.63 V (7.89 V) when the waveguides aligned along the [011] ( $[ {01\bar{1}} ]$ ) direction. The PER of both PBSs is expected to be 20 dB across the C-band with the help of a birefringence contribution arising from inserting waveguide arms of different widths.

Wavelength-Division Demultiplexing Enhanced by Silicon-Photonic Tunable Filters in Ultra-Wideband Optical-Path Networks

With the recent growth of network traffic demands, a high-capacity optical-path network based on reconfigurable optical add-drop multiplexers (ROADMs) is highly desirable. Among the various technologies available, extending the useable frequency band is one of the most promising solutions since it can enlarge the network capacity with relatively small signal-quality degradation. However, increasing the number of wavelength signals detracts flexible signal-drop capabilities of the ROADM because the effective dynamic range of the digital coherent receiver is restricted by the number of wavelength signals input to the receiver. To overcome this difficulty, we adopt the ROADM structure that sets an optical wavelength-tunable filter in front of each receiver. This scheme retains the effective receiver dynamic range even if a large number of wavelength signals are multiplexed. In this paper, we demonstrate flexible signal drop in a C+L-band optical-path network by using an optical wavelength-tunable filter implemented on a single silicon-photonic chip. The prototype filter is monolithically implemented in conjunction with an input-port selector so as to reduce the total system loss. The filter comprises multiple asymmetric Mach-Zehnder interferometers for wavelength tuning and symmetric Mach-Zehnder interferometers for input-port selection. Its effectiveness is experimentally confirmed by measuring the bit-error ratios of 32-Gbaud/100-Gbps dual-polarization quadrature-phase-shift-keying signals and 32-Gbaud/200-Gbps dual-polarization 16-ary quadrature-amplitude-modulation signals. A single wavelength signal is extracted out of 285 wavelength signals multiplexed over the C+L band and successfully demodulated with negligible penalty in terms of the optical signal-to-noise ratio. The net fiber capacity reaches 57 Tbps.

High-performance InP-based Mach-Zehnder polarization beam splitter with a 19 dB extinction ratio across C-band.

A high-performance InP-based polarization beam splitter (PBS) using a symmetrical Mach-Zehnder interferometer is experimentally demonstrated. The waveguides are aligned along the [011] direction, which results in a small reverse bias required and easy adjustment to realize the PBS. The experimental results indicate that the polarization extinction ratio (PER) is over 19dB in the wavelength range from 1525 to 1570nm with one arm injected with a 4.32mA current and the other arm reversed biased at 5.14V simultaneously. The PER is measured to remain above 18dB in the same wavelength range even when the width varies by ±200 nm.

Application of MZI Symmetrical Structure With Fiber Balls and Seven-Core Fiber in Microdisplacement Measurement

An optical fiber microdisplacement sensor based on symmetric Mach-Zehnder interferometer (MZI) with a seven-core fiber and two single-mode fiber balls is proposed. The rationality and manufacturing process of the MZI sensing structure are analyzed. The fabrication mechanism of the Mach-Zehnder sensor by CO2 laser is described in detail. Experimental results show that temperature sensitivities of the two dips are 98.65 pm/°C and 89.72 pm/°C, respectively. The microdisplacement sensitivities are 2017.71 pm/mm and 2457.92 pm/mm, respectively. The simultaneous measurement of temperature and microdisplacement is demonstrated based on the sensitive matrix. The proposed Mach-Zehnder interference sensor exhibits the advantages of compact structure, simple manufacturing process, and high reliability.

Tolerable performance of silicon photonic optical-serial-to-parallel converter with variable power splitter.

We have proposed and investigated the tolerable performance of an optical serial-to-parallel converter with phase operation (OSPC) with lower power consumption than the conventional scheme for the purpose of achieving reduction of both the processing load and the processing latency of the high-speed packet switching systems in the data center. The target OSPC consists of Mach-Zehnder delay interferometers (MZDIs) and balanced photodetectors with silicon (Si) photonic technology. However, we have confirmed that the performance becomes degraded due to optical propagation losses of the delay lines in MZDIs. To solve this issue, unbalanced splitting ratio of a multi-mode interferometer (MMI)-type couplers in the MZDIs has been investigated, but we have found the necessity of improving the tolerance of the splitting ratio deviation. In this paper, we will analytically and experimentally demonstrate the improvement of the characteristics by using an OSPC with variable splitting ratio optical coupler (VSOC) consisting of a symmetric Mach-Zehnder interferometer (MZI). By observing the transmission spectra of the device with various applied voltage to the heater, controllability of peak-to-valley ratios of transmission spectra was observed. In addition, splitting ratio of the output symmetric MMI almost matches well with that of the optimized peak-to-valley ratios of transmission spectra by using short-optical pulse injection. We have confirmed the tolerable performance of the Si-photonic OSPC by using VSOC with the symmetric MZI.

Silicon PAM-4 optical modulator driven by two binary electrical signals with different peak-to-peak voltages

We demonstrate a silicon PAM-4 optical modulator, which is based on a symmetric Mach-Zehnder interferometer. Two uncorrelated binary electrical signals with different peak-to-peak voltages are applied to the phase shifters of the silicon optical modulator. Accordingly, two different phase shifts are generated in the two arms. After the permutation, there are totally four phase differences between the two arms and the output optical power has four levels. The device can work at 32Gbaud in the wavelength range from 1525 to 1565nm, which is promising for the next-generation high-speed silicon optical link.

Information gain versus interference in Bohr’s principle of complementarity

We study the wave and particle nature in a symmetric Mach-Zehnder interferometer from the viewpoint of quantum information theory. By introducing either the von Neumann or Zurek's model of quantum measurement, we find that the classical correlation can be used to quantify the particle nature since its monotonicity is similar to the path distinguishability. The environment in Zurek's model induces the emergence of the optimal measuring basis, and reduces the classical and quantum correlation comparing to the von Neumann's model. A way is presented analytically to calculate the quantum correlation of a two-qubit separable state other than X-type.

Demonstration of a 4 × 4-port universal linear circuit

We present a silicon implementation of a 4 × 4-port universal linear optical circuit. Instead of predefining the exact functionality of a photonic circuit at design time, we demonstrate a simple generic silicon photonic circuit, combined with electronic control and software feedback, that can perform any linear operation between its four input ports and output ports. The circuit consists of a network of thermally tunable symmetric Mach–Zehnder interferometers with phase and amplitude control, in-circuit optical power monitors, and local software controlled feedback loops. The circuit can be configured using a training mechanism, which makes it self-adapt to implement the desired function. We use the circuit to demonstrate an adaptive, universal beam coupler, as well as a switch matrix.

Improved silica-PLC Mach-Zehnder interferometer type optical switches with error dependence compensation of directional coupler

For the most popular structure of planer lightwave circuit (PLC) 2×2 thermo-optic switches, Mach-Zehnder interferometer (MZI), a full range of splitting ratio errors of directional coupler (DC) are investigated. All the parameters determining the splitting ratio are the dimensions and the refractive indices of the waveguide core and cladding layers. In this work, the coherent relationships between the waveguide size and the refractive indices are analyzed and then the error compensation between the width and the refractive index of waveguide core, and the controllable effect of over clad layer refractive index error upon the MZI-type optical switch are all discovered with numerical calculation and BPM simulations. Then, an MZI-type 2×2 thermo-optic switch having a higher error tolerance is established with the efficient optimizations of all the 3dB-DC parameters. As a result, for the symmetric MZI switch, an insertion loss of 1.5dB and optical extinction ratio of over 20dB are realized for the average tolerance of±5.0%. An asymmetric arm optical phase and unequal arm lengths is also employed to improve the uniformities of insertion loss. The agreements between the designs and the experiments are recognized, leading to a wide adoption of practical silica-PLC optical switch products.

Double Bragg Interferometry.

We employ light-induced double Bragg diffraction of delta-kick collimated Bose-Einstein condensates to create three symmetric Mach-Zehnder interferometers. They rely on (i)first-order, (ii)two successive first-order, and (iii) second-order processes which demonstrate the scalability of the corresponding momentum transfer. With respect to devices based on conventional Bragg scattering, these symmetric interferometers double the scale factor and feature a better suppression of noise and systematic uncertainties intrinsic to the diffraction process. Moreover, we utilize these interferometers as tiltmeters for monitoring their inclination with respect to gravity.

Self-collimation-based photonic crystal Mach–Zehnder add-drop filters

Photonic crystal Mach–Zehnder add-drop filters (PC-MZADFs) based on the self-collimation phenomenon in a two-dimensional (2D) PC are proposed and numerically studied using finite-difference time-domain (FDTD) simulations. Each PC-MZADF is composed of a symmetric Mach–Zehnder interferometer (MZI) with an identical filter in each of its two different optical paths. Zizag-box resonators (ZBRs) and Fano resonators (FRs) are employed as the optical filters in rod-type and hole-type PCs, respectively. It is shown that self-collimated beams with the ZBR and FR resonant frequencies can be dropped or added using multiple-beam interference. We also show that the resonant frequencies of the resonators can be adjusted by varying the radii of their rods or holes. Our results indicate that this device design may constitute an efficient approach to light propagation manipulation and increase the application range of self-collimated beams.

Wavelength-Tunable Filters Utilizing Arrayed Waveguide Gratings for Colorless/Directionless/Contentionless Optical Signal Drop in ROADMs

The wavelength-tunable filter is a key device for creating colorless, directionless, and contentionless reconfigurable optical add-drop multiplexers. Although arrayed-waveguide gratings (AWGs) can realize compact and cost-effective tunable filters, crosstalk in wavelength-division-multiplexing systems increases as the frequency grid spacing decreases. To resolve this problem, we propose a tunable-filter architecture that can eliminate the crosstalk by utilizing two specific (commeasurable) cyclic AWGs in conjunction with multiple switches based on symmetric Mach-Zehnder interferometers and an asymmetric Mach-Zehnder interferometer (AMZI) filter. To verify the technical feasibility of the proposed architecture, we fabricate a monolithic prototype using planar-lightwave-circuit technology. We confirm, for the first time, its excellent performance by measuring insertion loss, polarization-dependent loss, and bit error ratios (BERs). Alternative tunable-filter configurations employing multiple AMZI-based filters are newly investigated for future applications.

First Demonstration of Athermal Silicon Optical Interposers With Quantum Dot Lasers Operating up to 125 °C

We previously proposed a photonics-electronics convergence system to solve bandwidth bottleneck problems among large-scale integrations (LSIs) and demonstrated a high bandwidth density with silicon optical interposers at room temperature. For practical applications, the interposers should be usable under high-temperature conditions or rapid temperature changes so that they can cope with the heat generated by the mounted LSIs. We designed and fabricated athermal silicon optical interposers integrated with temperature-insensitive components on a silicon substrate. An arrayed laser diode (LD) chip was a flip-chip bonded to the substrate. Each LD had multiple quantum dot layers with a 1.3-μm lasing wavelength. The output power was higher than 10 mW per channel up to 100 °C. Silicon optical modulator and germanium photodetector (PD) arrays were monolithically integrated on the substrate. The modulators were structured as symmetric Mach-Zehnder interferometers, which were inherently insensitive to temperature and wavelength. The phase shifters composed of p-i-n diodes were stable against temperature change under a constant bias-current condition. The PD photocurrent was also temperature insensitive, and the photo-to-dark current ratio was higher than 30 dB up to 100 °C. We achieved error-free data links at 20 Gbps and a high bandwidth density of 19 Tbps/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> operating up to 125 °C without adjusting the LDs, modulators, or PDs. The interposers are tolerant of the heat generated by the mounted LSIs and usable over the extended industrial temperature range without complex monitoring or feedback controls. The bandwidth density is sufficient for the needs of the late 2010s.

Independently Tunable Multichannel Fractional-Order Temporal Differentiator Based on a Silicon-Photonic Symmetric Mach–Zehnder Interferometer Incorporating Cascaded Microring Resonators

A multichannel fractional-order temporal differentiator with independently tunable differentiation order based on an integrated silicon-photonic symmetric Mach-Zehnder interferometer consisting of cascaded microring resonators (MRRs) is designed, fabricated, and experimentally demonstrated. By controlling the radii of the MRRs, a multichannel spectral response with uniform channel spacing is obtained, which is used to function as a multichannel temporal differentiator with multiple subdifferentiators. The differentiation order of each subdifferentiator is independently tunable by optically pumping the corresponding MRR, which leads to the phase change in the spectral response due to the two-photon absorption induced nonlinear effect. A five-channel temporal differentiator with a channel spacing of 0.49 nm is fabricated on a silicon-on-insulator chip using a CMOS-compatible process with 193-nm-deep ultraviolet lithography. Independent tuning of the differentiation orders of the subdifferentiators is experimentally demonstrated.

Fast, all-optical logic gates and transistor functionalities using a room-temperature atomic controlled Kerr gate

We demonstrate all-optical multilogic gate operations and transistor functionalities using a Kerr phase gate method in a room-temperature $^{85}\mathrm{Rb}$ vapor. Two symmetric Mach-Zehnder interferometers are constructed in the same vapor cell in which a Raman gain medium is established. We show three basic logic gates (and, or, and not) by controlling the output combinations from the two interferometers. With one weakly driven interferometer acting as the phase control light for a strongly driven interferometer, we further demonstrate optical field-effect transistor functionalities. More complex combinations of this Kerr phase gate method and scheme allow all eight basic logic gate operations including the controlled-not gate to be constructed and implemented.

Temperature-independent broadband silicon modulator

We demonstrate a 20Gb/s temperature-independent silicon modulator based on symmetrical Mach–Zehnder Interferometer. The MMI coupler was used as splitter/combiner in symmetrical MZI for balanced propagation. The ±15°C temperature-independent eye diagrams were measured at 10Gb/s with over 15dB extinction ratio. Based on over 30nm flat optical bandwidth, the broadband modulation was demonstrated from 1530nm to 1560nm. The temperature-independent broadband silicon modulator is adaptable to interconnection and communication systems in practice.

Design of a low-power all-optical NOR gate using photonic crystal quantum-dot semiconductor optical amplifiers

Design and simulation of a novel all-optical logic gate (AOLG) based on photonic crystal quantum-dot semiconductor optical amplifiers (PC-QDSOAs) is reported for the first time. For this purpose, a flat-band slow-light QD-based active photonic crystal waveguide (APCW) is designed and employed for implementation of an all-optical NOR gate in a symmetric Mach-Zehnder interferometer configuration. Then, the performance of the NOR gate is investigated using a comprehensive nonlinear state space model (NSSM). Simulation results demonstrate that PC-QDSOAs present an interesting platform for realizing ultracompact AOLGs with low power consumption.