Tunable Mach-Zehnder Interferometer Research Articles

Self-adaptive integrated photonic receiver for turbulence compensation in free space optical links

In Free Space Optical (FSO) communication systems, atmospheric turbulence distorts the propagating beams, causing a random fading in the received power. This perturbation can be compensated using a multi-aperture receiver that samples the distorted wavefront on different points and adds the various signals coherently. In this work, we report on an adaptive optical receiver that compensates in real time for scintillation in FSO links. The optical front-end of the receiver is entirely integrated in a silicon photonic chip hosting a 2D Optical Antenna Array and a self-adaptive analog Programmable Optical Processor made of a mesh of tunable Mach–Zehnder interferometers. The photonic chip acts as an adaptive interface to couple turbulent FSO beams to single-mode guided optics, enabling energy and cost-effective operation, scalability to systems with a larger number of apertures, modulation-format and data-protocol transparency, and pluggability with commercial fiber optics transceivers. Experimental results demonstrate the effectiveness of the proposed receiver with optical signals at a data rate of 10 Gbit/s transmitted in indoor FSO links where different turbulent conditions, even stronger than those expected in outdoor links of hundreds of meters, are reproduced.

Towards interpretable quantum machine learning via single-photon quantum walks

Variational quantum algorithms represent a promising approach to quantum machine learning where classical neural networks are replaced by parametrized quantum circuits. However, both approaches suffer from a clear limitation, that is a lack of interpretability. Here, we present a variational method to quantize projective simulation (PS), a reinforcement learning model aimed at interpretable artificial intelligence. Decision making in PS is modeled as a random walk on a graph describing the agent’s memory. To implement the quantized model, we consider quantum walks of single photons in a lattice of tunable Mach–Zehnder interferometers trained via variational algorithms. Using an example from transfer learning, we show that the quantized PS model can exploit quantum interference to acquire capabilities beyond those of its classical counterpart. Finally, we discuss the role of quantum interference for training and tracing the decision making process, paving the way for realizations of interpretable quantum learning agents.

Discretely Tunable (2594, 2629, 2670 nm) GaSb/Si3N4 Hybrid Laser for Multiwavelength Spectroscopy

AbstractA discrete, tunable photonic integrated laser is showcased for multiwavelength spectroscopy of CO2, H2S, and H2O. The laser combines an AlGaInAsSb/GaSb type‐I quantum well‐reflective semiconductor optical amplifier with a Si3N4 photonic integrated circuit (PIC). Operating at room temperature, the laser emits at 2670.42, 2629.12, and 2594.27 nm, with mW‐level average powers. The PIC employs a novel approach for achieving switching between three distinct emission wavelengths by using two cascaded tunable Mach–Zehnder interferometers (MZIs) that are connected to three spiral‐shaped narrow‐band distributed Bragg reflectors (DBRs). Phase tuning the arms of the MZIs allows the laser emission wavelength to be switched between the DBRs, while the DBRs themselves offer fine‐tuning of the emission wavelength. This design enables a simpler tuning mechanism with fewer control variables compared to hybrid PIC‐based lasers using Vernier architectures or a combination of multiple semiconductor amplifiers. Low‐loss and broadband MZIs are achieved by employing two interconnected directional couplers, which utilize asymmetric waveguides in the coupling region to effectively control the phase. Besides achieving state‐of‐the‐art performance for Si3N4‐based integrated lasers at 2.6 µm wavelength region, the demonstrated performance of passive components, including MZIs and spiral DBRs, opens new possibilities for mid‐infrared PIC technology.

Modeling and Analysis of Device Orientation, Analog and Digital Performance of Electrode Design for High Speed Electro-Optic Modulator

Electro-optic modulators (EOMs) are crucial devices for modern communication enabling high bandwidth optical communication links. Traveling wave electrodes are used to obtain high-speed modulation in these EOMs. We present the electrode design and analysis along with the study of effects of changing orientation on device performance for a thin-film lithium niobate tunable Mach–Zehnder interferometer (MZI) that offers sub-THz bandwidth operations. High velocity and impedance matching with low RF attenuation, high third-order SFDR (∼121 dB/Hz2/3) and a low half-wave voltage length product (1.74 V.cm) have been achieved for a bandwidth of 136 GHz. High-speed digital modulation using multi-level signal formats (PAM-2, QAM-4 and QAM-16) with low BER for 400 Gbps data has been demonstrated to assess the digital performance of the device.

Spectra reconfiguration of a silicon microring resonator via double injection with different angles.

A reconfigurable silicon microring filter, which is constructed by cascading a tunable Mach-Zehnder interferometer and a double injected silicon microring resonator with a variable input angle, is proposed and investigated. The spectra reconfigurations of the optical filter were simulated and analyzed using the transmission matrix method. The results show that when keeping the perimeter of the microring constant, the free spectral range (FSR) of the filter can be multiplied by adjusting the angle between the two injections. Moreover, by changing the coupling coefficients of the microring and the optical power ratio between the two injections, different types of spectral responses such as square, sinusoidal, and flat-top interleaver can be obtained. The device is simple and easy to integrate, and its capabilities to expand the FSR and spectral reconfiguration may have great potential in reconfigurable integrated optic chips.

Optical Spectral Shaping Based on Reconfigurable Integrated Microring Resonator-Coupled Fabry–Perot Cavity

We demonstrate a multifunctional optical spectral shaper on a silicon photonic chip. The proposed structure is composed of a tunable microring resonator (MRR) and three tunable Sagnac loop mirrors (SLMs), which can be regarded as a large SLM incorporating an MRR-coupled Fabry–Perot (FP) cavity. To realize flexible tuning, thermo-optically tunable Mach-Zehnder interferometer (MZI) couplers are used to replace conventional directional couplers with fixed coupling coefficients. By controlling the tunable MZI couplers, the proposed structure can be reconfigured as various functional structures including an all-pass MRR with tunable coupling regime, a tunable FP cavity, an SLM incorporating FP cavity, and an MRR-coupled FP cavity. This flexible reconfiguration allows for multifunctional optical spectral shaping including tunable comb filtering, Fano and EIT-like resonances. In particular, the complementary flat-top comb filtering responses can be theoretically obtained in the two ports of this device with same free spectral range (FSR) value, which enable the (de)interleaver operation. The experimental results show that the implemented all-pass MRR has a maximum extinction ratio (ER) of 38.9 dB and the corresponding quality factor is 6.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> . The comb filter based on configured FP cavity can be switched from narrowband response (0.06 nm) to wideband flat-top response (0.23 nm), with ERs up to 10.6 dB and 22 dB, respectively. Moreover, a dynamic conversion between Lorentz, EIT, and Fano resonance line shapes based on the coupled resonant system can be realized. The demonstrated integrated device holds great potential for various applications such as filtering and switching in optical wavelength-division multiplexing (WDM) communication networks.

Generation of dissipative solitons and self-similar pulses by a mode-locked fiber laser using a bandwidth tunable Mach-Zehnder interferometer filter

We suggest and demonstrate a scheme to generate dissipative solitons and self-similar pulses in a fiber laser mode-locked by nonlinear polarization rotation (NPR). The scheme is based on inserting a bandwidth tunable Mach-Zehnder interferometer (MZI) filter in the cavity. The MZI filter is fabricated by cascading two 3 dB optical couplers (OCs). The propagating light is split into two arms in the first OC and is combined again in the second OC. The free spectral range of the MZI, and the corresponding bandwidth of the MZI filter, depend on the optical path difference between these two arms. By applying stretching on one of the arms, both dissipative solitons and self-similar pulses have been obtained both numerically and experimentally. Furthermore, upon exploiting the dependence of the peak wavelength of the MZI filter on the amount of stretching, dissipative solitons with tunable wavelength have also been obtained.

Electronic Mach-Zehnder interference in a bipolar hybrid monolayer-bilayer graphene junction

Graphene matter in a strong magnetic field, realizing one-dimensional quantum Hall channels, provides a unique platform for studying electron interference. Here, using the Landauer-Büttiker formalism along with the tight-binding model, we investigate the quantum Hall (QH) effects in unipolar and bipolar monolayer-bilayer graphene (MLG-BLG) junctions. We find that a Hall bar made of an armchair MLG-BLG junction in the bipolar regime results in valley-polarized edgechannel interferences and can operate a fully tunable Mach-Zehnder (MZ) interferometer device. Investigation of the bar-width and magnetic-field dependence of the conductance oscillations shows that the MZ interference in such structures can be drastically affected by the type of (zigzag) edge termination of the second layer in the BLG region [composed of vertical dimer or non-dimer atoms]. Our findings reveal that both interfaces exhibit a double set of Aharonov-Bohm interferences, with the one between two oppositely valley-polarized edge channels dominating and causing a large amplitude conductance oscillation ranging from 0 to 2e2/h. We explain and analyze our findings by analytically solving the Dirac-Weyl equation for a gated semi-infinite MLG-BLG junction.

Electronic Mach-Zehnder Interference in a Bipolar Hybrid Monolayer-Bilayer Graphene Junction

Graphene matter in a strong magnetic field, realizing one-dimensional quantum Hall channels, provides a unique platform for studying electron interference. Here, using the Landauer-B\"uttiker formalism along with the tight-binding model, we investigate the quantum Hall (QH) effects in unipolar and bipolar monolayer-bilayer graphene (MLG-BLG) junctions. We find that a Hall bar made of an armchair MLG-BLG junction in the bipolar regime results in valley-polarized edge-channel interferences and can operate a fully tunable Mach-Zehnder (MZ) interferometer device. Investigation of the bar-width and magnetic-field dependence of the conductance oscillations shows that the MZ interference in such structures can be drastically affected by the type of (zigzag) edge termination of the second layer in the BLG region [composed of vertical dimer or non-dimer atoms]. Our findings reveal that both interfaces exhibit a double set of Aharonov-Bohm interferences, with the one between two oppositely valley-polarized edge channels dominating and causing a large-amplitude conductance oscillation ranging from 0 to $ 2 e^2 / h$. We explain and analyze our findings by analytically solving the Dirac-Weyl equation for a gated semi-infinite MLG-BLG junction.

Single-photon detection and cryogenic reconfigurability in lithium niobate nanophotonic circuits

Lithium-Niobate-On-Insulator (LNOI) is emerging as a promising platform for integrated quantum photonic technologies because of its high second-order nonlinearity and compact waveguide footprint. Importantly, LNOI allows for creating electro-optically reconfigurable circuits, which can be efficiently operated at cryogenic temperature. Their integration with superconducting nanowire single-photon detectors (SNSPDs) paves the way for realizing scalable photonic devices for active manipulation and detection of quantum states of light. Here we demonstrate integration of these two key components in a low loss (0.2 dB/cm) LNOI waveguide network. As an experimental showcase of our technology, we demonstrate the combined operation of an electrically tunable Mach-Zehnder interferometer and two waveguide-integrated SNSPDs at its outputs. We show static reconfigurability of our system with a bias-drift-free operation over a time of 12 hours, as well as high-speed modulation at a frequency up to 1 GHz. Our results provide blueprints for implementing complex quantum photonic devices on the LNOI platform.

Ultra-narrow passband-tunable filter based on a high-Q silicon racetrack resonator

An ultra-narrow narrow passband-tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on a SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multimode waveguide racetrack, and the Q factor is measured as high as 8.1×105. The structure of the device is based on a thermally tunable Mach-Zehnder interferometer coupled racetrack. The tunability of the bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range is about 0.28 nm (35 GHz), with the footprint of 0.21mm2.

Widely stretchable soliton crystals in a passively mode-locked fiber laser.

We present the first direct demonstration of a new type of stable and extremely elastic soliton crystals, the bond length and bond strength of which can be individually controlled in a wide range. The stretching and compressing can be repeated many times, conserving the overall structure by incorporating a highly asymmetric tunable Mach-Zehnder interferometer into a specially designed passively mode-locked fiber laser. The temporal structure and dynamics of the generated soliton crystals were measured using an asynchronous optical sampling system with picosecond resolution. We demonstrated that a stable and robust soliton crystal can be formed by two types of primitive structures: single dissipative solitons and (or) pairs each consisting of a dissipative soliton and a pulse with a lower amplitude. Continuous stretching and compression of the soliton crystal by an extraordinarily high factor of more than 30 has been demonstrated, the smallest recorded separation between the pulses being as low as 5 ps, corresponding to an effective repetition frequency of 200 GHz. Collective pulse dynamics, including soliton crystal cracking and transformation of crystals comprising high/low-amplitude pulse pairs to the crystals of similar pulses, has been observed experimentally.

Integrated Thermally Tuned Mach-Zehnder Interferometer in Z-Cut Lithium Niobate Thin Film

We propose and demonstrate a type of highly efficient tunable Mach-Zehnder interferometer (MZI) in Z-cut lithium niobate thin film (TFLN) based on the thermo-optical effect. By placing the electrode directly above the TFLN waveguide, a more uniform and higher heat distribution was obtained, compared with the conventional horizontal structure. Both the principle and design strategies of the proposed scheme are described. The process flow of the device fabrication is demonstrated in Z-cut TFLN. The measured static results accord well with the simulations. About 0.121 nm/V tuning efficiency is observed, which is nearly double the tuning efficiency based on the electro-optic effect, and may prove useful when high switching speed is not the primary consideration. The presented thermo-optic tuning scheme can be extended to other scenarios easily and may satisfy many different application requirements in integrated lithium niobate photonics (LNPs).

Quantum Hall Valley Splitters and a Tunable Mach-Zehnder Interferometer in Graphene.

Graphene is a very promising test bed for the field of electron quantum optics. However, a fully tunable and coherent electronic beam splitter is still missing. We report the demonstration of electronic beam splitters in graphene that couple quantum Hall edge channels having opposite valley polarizations. Theelectronic transmission of our beam splitters can be tuned from zero to near unity. By independently setting the beam splitters at the two corners of a graphene p-n junction to intermediate transmissions, we realize a fully tunable electronic Mach-Zehnder interferometer. This tunability allows us to unambiguously identify the quantum interferences due to the Mach-Zehnder interferometer, and to study their dependence with the beam-splitter transmission and the interferometer bias voltage. The comparison with conventional semiconductor interferometers points toward universal processes driving the quantum decoherence in those two different 2D systems, with graphene being much more robust to their effect.

Dual Tunable MZIs Stationary-Wave Integrated Fourier Transform Spectrum Detection.

In order to resolve spectral alias due to under sampling in traditional stationary-wave integrated Fourier transform (SWIFT) spectrometers, an all-on-chip waveguide based on dual tunable Mach-Zehnder interferometer (MZI) stationary-wave integrated Fourier transform technology (DTM-SWIFT) is proposed. Several gold nanowires are asymmetrically positioned at two sides of zero optical path difference and scatter the interference fringes information, which can avoid aliasing of spectral signals and help to gain high spectral resolution. A systematic theoretical analysis is carried on in detail, including the optical distribution characteristics based on multi-beam interference, stationary-wave theorem and signal reconstruction method based on the FT technology. The results show that the method can complete a resolution of 6 nm for Gauss spectrum reconstruction using only 6 gold nanowires, and a resolution of 5 cm−1 for Raman spectrum reconstruction using 25 gold nanowires.

Integrated silicon reconfigurable optical transmitter.

We demonstrate an integrated silicon reconfigurable optical transmitter based on the reconfigurability of the Mach-Zehnder interferometer (MZI). By incorporating modulators into the tunable MZI structure and manipulating the operation states, different modulation formats, including amplitude/phase modulated binary/quaternary signals, as well as polarization multiplexed signals, can be generated as required, to accommodate different transmission links. For a proof-of-concept demonstration, the microring modulators are adopted, and we experimentally generate a 10 GBaud on-off keying (OOK) signal, four-level pulse amplitude signal, and polarization division multiplexing OOK signal using the same transmitter. The device is promising for a next-generation intelligent optical link.

Graphene Quantum Dots Assisted Optically Tunable In-Core Mach-Zehnder Interferometer

An optically tunable in-core Mach-Zehnder interferometer (MZI) based on the photoexcitation dynamics of nitrogen-doped graphene quantum dots (NGQDs) is demonstrated. The tunable MZI is fabricated by depositing NGQDs into a femtosecond laser ablated microgroove in the core. Combining both the high sensitivity to the external refractive index change of the MZI itself and the quantum confinement effect in quantum dots, a tuning range of ~90 nm is achieved as the pump power density of an ultraviolet (UV) LED varies from 0 to 41.1 mW/cm2, which accounts for 85% of the free spectrum range (FSR) of the MZI with a length of $205~\mu \text{m}$ . Such an optically tunable MZI with high integration, tuning efficiency and wide FSR can be suitably used in edge filter demodulation and tunable laser.

A tunable Mach–Zehnder interferometer based on dual micro-cavity photonic crystal fiber for load measurement

Optical fiber technologies are widely engaged in variety of industries, machines and production lines due to their precise results and low cost. Optical sensors are fabricated using different types of optical fibers. In this paper, a new fabrication approach of, in-fiber, tunable Mach–Zehnder interferometer with dual micro-cavities using a photonic crystal fiber (PCF) has been proposed for load measurement application. A large mode area (LMA-10) PCF is used to splice between two equal lengths of single-mode fibers using fusion splicing technique. Different parameters such as arc power, length of the PCF and the overlap gap between samples have been considered to control the fabrication process. Ellipsoidal shape micro-cavities were experimentally achieved parallel to the propagation axis having dimensions of (24.92–62.32) µm of width and (3.82–18.2) µm of length. Results showed that higher sensitivity values of 0.15 nm/N and 0.32 nm/N were achieved with elliptical width of 18.2 µm. The simplicity of sensor fabrication process, controlled parameters of cavity creation, small and compact size and high sensitivity of large mode area that the PCFs exhibit, add more advantage for load measurement applications.

Microring resonator-assisted Fourier transform spectrometer with enhanced resolution and large bandwidth in single chip solution

Single chip integrated spectrometers are critical to bring chemical and biological sensing, spectroscopy, and spectral imaging into robust, compact and cost-effective devices. Existing on-chip spectrometer approaches fail to realize both high resolution and broad band. Here we demonstrate a microring resonator-assisted Fourier-transform (RAFT) spectrometer, which is realized using a tunable Mach-Zehnder interferometer (MZI) cascaded with a tunable microring resonator (MRR) to enhance the resolution, integrated with a photodetector onto a single chip. The MRR boosts the resolution to 0.47 nm, far beyond the Rayleigh criterion of the tunable MZI-based Fourier-transform spectrometer. A single channel achieves large bandwidth of ~ 90 nm with low power consumption (35 mW for MRR and 1.8 W for MZI) at the expense of degraded signal-to-noise ratio due to time-multiplexing. Integrating a RAFT element array is envisaged to dramatically extend the bandwidth for spectral analytical applications such as chemical and biological sensing, spectroscopy, image spectrometry, etc.

Optimization of thermo-optic phase-shifter design and mitigation of thermal crosstalk on the SOI platform.

We first optimize the design and compare the performance of thermo-optic phase-shifters based on TiN metal and N++ doped silicon, in the same SOI process. The designs don't require special material processing, show negligible loss, and have very stable power consumption. The optimum TiN design has a switching powerPπ=21.4 mW and a time constantτ=5.6 µs, whereasPπ=22.8 mW andτ=2.2 µs for the best N++ Si design, enabling 2.5x faster switching compared to the metal heater. Doped-Si-based heaters are therefore the most practical and efficient on standard SOI. In addition, to optimize the layout density of highly integrated dies, we experimentally characterize internal and external thermal crosstalk for tunable Mach-Zehnder interferometers (MZIs) based on both heater designs for various power, distances, and etching patterns. Deep trenches are the best structures not involving special fabrication techniques to mitigate heat leakage affecting phase-sensitive devices close to heaters. Given the numerous applications of thermal tuners, this work is relevant to almost all silicon photonics designers.