High-speed Electro-optic Modulator Research Articles

A quasi-matching scheme for arbitrary group velocity match in electro-optic modulation

Group velocity and impedance matches are prerequisites for high-speed Mach–Zehnder electro-optic (EO) modulators. However, not all platforms can realize matching conditions, restricting high-speed modulation in many practical conditions. Here, we propose and demonstrate a quasi-matching scheme to satisfy the group velocity and characteristic impedance matches by cascading fast-wave and slow-wave traveling wave electrodes. The effective group velocity can be flexibly adjusted by changing the ratio of fast-wave and slow-wave traveling wave electrodes. Moreover, the quasi-matching scheme is experimentally verified by demonstrating a 6 mm long EO modulator on a thin-film lithium-niobate-on-insulator platform with a silica cladding. The radio frequency signal insertion loss at the boundary of the slow-wave and fast-wave electrodes is less than 0.12 dB. The measured small signal EO response of the quasi-matched EO modulator drops less than 2 dB at 67 GHz, while the measured small-signal EO responses of conventional slow and fast traveling wave EO modulators drop 4 dB at 67 GHz. The measured 100 Gb/s on–off key signal eye-diagrams of the quasi-matched EO modulator also exhibit an overwhelming advantage over conventional schemes. Therefore, our results will open many opportunities for high-speed EO modulators in various platforms.

High-Speed Electro-Optic Plasmonic Modulator for CMOS Non-Contact Wafer-Level Testing

High-Speed Electro-Optic Plasmonic Modulator for CMOS Non-Contact Wafer-Level Testing

Highly efficient lead zirconate titanate ring modulator

Advanced photonic integrated circuits require large-scale integration of high-speed electro-optic (EO) functional components on a chip. Low power consumption and high operation speed are thus key metrics for almost all integrated EO devices. Here, we demonstrated a ring resonator modulator based on lead zirconate titanate (PZT) on a SiO2/Si substrate. The ridge waveguides were employed to keep a large spatial overlap between the optical field and the electric field within the PZT layer. The device exhibits a data rate of 56 Gbit/s and significant tuning efficiency, reaching up to 35.8 pm/V, corresponding to 1.17 V·cm. The demonstration of energy efficient and high-speed EO modulation paves the way for realizing dense PZT photonics integrated circuits.

High-speed mid-infrared silicon-based electro-optic modulator at 2 μm

We demonstrate the Mach-Zehnder interferometer (MZI) silicon-based electro-optic modulators in the short-wave mid-infrared 2 μm band. The waveguide structure and doping position of the devices are optimized by simulation. The devices are manufactured on a 220 nm silicon-on-insulator (SOI) platform, and are measured through self-built short-wave mid-infrared measurement platform. The device with the length of 3.5 mm has a measured modulation efficiency of 1.853 V cm at 3 V reverse bias. The highspeed characterization is also performed, and the device with the length of 3.5 mm has a data rate of over 20 Gb/s at the voltage swing of 2.8 Vpp with on-off keying (OOK) modulation formats. The modulation speed of the 2 mm device can reach over 40 Gb/s with 4-level pulse amplitude modulation (PAM-4) formats at a voltage swing of 2.5 Vpp.

Efficient single-pixel imaging based on a compact fiber laser array and untrained neural network

This paper presents an efficient scheme for single-pixel imaging (SPI) utilizing a phase-controlled fiber laser array and an untrained deep neural network. The fiber lasers are arranged in a compact hexagonal structure and coherently combined to generate illuminating light fields. Through the utilization of high-speed electro-optic modulators in each individual fiber laser module, the randomly modulated fiber laser array enables rapid speckle projection onto the object of interest. Furthermore, the untrained deep neural network is incorporated into the image reconstructing process to enhance the quality of the reconstructed images. Through simulations and experiments, we validate the feasibility of the proposed method and successfully achieve high-quality SPI utilizing the coherent fiber laser array at a sampling ratio of 1.6%. Given its potential for high emitting power and rapid modulation, the SPI scheme based on the fiber laser array holds promise for broad applications in remote sensing and other applicable fields.Graphical

Parasitic conduction loss of lithium niobate on insulator platform

Thin-film lithium niobate on insulator (LNOI) has recently emerged as an exciting platform for nonlinear integrated photonic devices, drawing parallels with the impact of silicon on insulator (SOI) in the electronic device realm. While the influence of interfacial states of SOI on electronic device performance, particularly their contribution to parasitic radio frequency (RF) losses, is well-documented, this aspect remains relatively unexplored in LNOI, despite their shared silicon/oxide interface structure. Here, we investigate the RF loss arising from mobile charges at the silicon/oxide interface of the LNOI platform. We show that this parasitic conduction can be exacerbated, especially during plasma etching, and leads to substantial RF losses. Through strategic incorporation of a trap-rich poly-silicon layer at this interface, we effectively eliminate this parasitic conduction, resulting in the suppression of RF losses. This work establishes crucial guidelines for harnessing LNOI in applications such as high-frequency electro-mechanical devices, high-speed electro-optic modulators, and high-efficiency microwave-to-optics converters.

Integrated lithium niobate microwave photonic processing engine.

Integrated microwave photonics (MWP) is an intriguing technology for the generation, transmission and manipulation of microwave signals in chip-scale optical systems1,2. In particular, ultrafast processing of analogue signals in the optical domain with high fidelity and low latency could enable a variety of applications such as MWP filters3-5, microwave signal processing6-9 and image recognition10,11. An ideal integrated MWP processing platform should have both an efficient and high-speed electro-optic modulation block to faithfully perform microwave-optic conversion at low power and also a low-loss functional photonic network to implement various signal-processing tasks. Moreover, large-scale, low-cost manufacturability is required to monolithically integrate the two building blocks on the same chip. Here we demonstrate such an integrated MWP processing engine based on a 4 inch wafer-scale thin-film lithium niobate platform. It can perform multipurpose tasks with processing bandwidths of up to 67 GHz at complementary metal-oxide-semiconductor (CMOS)-compatible voltages. We achieve ultrafast analogue computation, namely temporal integration and differentiation, at sampling rates of up to 256 giga samples per second, and deploy these functions to showcase three proof-of-concept applications: solving ordinary differential equations, generating ultra-wideband signals and detecting edges in images. We further leverage the image edge detector to realize a photonic-assisted image segmentation model that can effectively outline the boundaries of melanoma lesion in medical diagnostic images. Our ultrafast lithium niobate MWP engine could provide compact, low-latency and cost-effective solutions for future wireless communications, high-resolution radar and photonic artificial intelligence.

2D material platform for overcoming the amplitude–phase tradeoff in ring resonators

Compact and high-speed electro-optic phase modulators play a vital role in various large-scale applications including optical computing, quantum and neural networks, and optical communication links. Conventional electro-refractive phase modulators such as silicon (Si), III-V and graphene on Si suffer from a fundamental tradeoff between device length and optical loss that limits their scaling capabilities. High-finesse ring resonators have been traditionally used as compact intensity modulators, but their use for phase modulation has been limited due to the high insertion loss associated with the phase shift. Here, we show that high-finesse resonators can achieve a strong phase shift with low insertion loss by simultaneous modulation of the real and imaginary parts of the refractive index, to the same extent, i.e., ΔnΔk∼1. To implement this strategy, we demonstrate an active hybrid platform that combines a low-loss SiN ring resonator with 2D materials such as graphene and transition metal dichalcogenide [tungsten disulphide (WSe2)], which induces a strong change in the imaginary and real parts of the index. Our platform consisting of a 25 µm long Gr-Al2O3-WSe2 capacitor embedded on a SiN ring of 50 µm radius (∼8% ring coverage) achieves a continuous phase shift of (0.46±0.05)π radians with an insertion loss (IL) of 3.18±0.20 dB and a transmission modulation (ΔTRing) of 1.72±0.15dB at a probe wavelength (λ p ) of 1646.18 nm. We find that our Gr-Al2O3-WSe2 capacitor exhibits a phase modulation efficiency (Vπ2⋅L) of 0.530±0.016V⋅cm and can support an electro-optic bandwidth of 14.9±0.1GHz. We further show that our platform can achieve a phase shift of π radians with an IL of 5 dB and a minimum ΔT of 0.046 dB. We demonstrate the broadband nature of the binary phase response, by measuring a phase shift of (1.00±0.10)π radians, with an IL of 5.20±0.31dB and a minimal ΔTRing of 0.015±0.006dB for resonances spanning from 1564 to 1650 nm. This SiN–2D hybrid platform provides the design for compact and high-speed reconfigurable circuits with graphene and transition metal dichalcogenide (TMD) monolayers that can enable large-scale photonic systems.

Polymer micro-ring modulator on silicon nitride platform

The development of silicon nitride (SiN) waveguide platforms has enabled the realization of photonic circuits with unprecedented performance. Despite the exciting progress, SiN suffers from intrinsic material properties that block it for high-speed electro-optic (EO) modulators. Here, we present an approach to heterogeneously integrate organic EO polymer on SiN for high-performance modulators. We utilize the high resistivity of SiN to achieving a strong overlap between the optical and electrical field. Furthermore, sol–gel SiO2 side-claddings inserted in waveguides shorten the inter-electrode distance and increase the EO activity to a much higher level. As a result, the fabricated micro-ring modulator exhibits an electrical tunability of 10 pm/V, corresponding to an in-device EO coefficient as 60 pm/V. High frequency tests in the device show a 3-dB bandwidth of 32 GHz and a data rate of 40 Gbps. The proposed approach will enable efficient mass-production of the micro-footprint modulators and promote the development of integrated SiN photonics.

High-speed electro-optic modulation in topological interface states of a one-dimensional lattice

Electro-optic modulators are key components in data communication, microwave photonics, and quantum photonics. Modulation bandwidth, energy efficiency, and device dimension are crucial metrics of modulators. Here, we provide an important direction for the miniaturization of electro-optic modulators by reporting on ultracompact topological modulators. A topological interface state in a one-dimensional lattice is implemented on a thin-film lithium-niobate integrated platform. Due to the strong optical confinement of the interface state and the peaking enhancement of the electro-optic response, a topological cavity with a size of 1.6 × 140 μm2 enables a large modulation bandwidth of 104 GHz. The first topological modulator exhibits the most compact device size compared to reported LN modulators with bandwidths above 28 GHz, to the best of our knowledge. 100 Gb/s non-return-to-zero and 100 Gb/s four-level pulse amplitude modulation signals are generated. The switching energy is 5.4 fJ/bit, owing to the small electro-optic mode volume and low capacitance. The topological modulator accelerates the response time of topological photonic devices from the microsecond order to the picosecond order and provides an essential foundation for the implementation of large-scale lithium-niobate photonic integrated circuits.

High-speed mid-infrared graphene electro-optical modulator based on suspended germanium slot waveguides.

The mid-infrared (MIR) region is attracting increasing interest for on-chip synchronous detection and free-space optical (FSO) communications. For such applications, a high-performance electro-optical modulator is a crucial component. In this regard, we propose and investigate a graphene-based electro-absorption modulator (EAM) and microring modulator (MRM) using the suspended germanium waveguide platform. The modulators are designed for the second atmospheric window (8 to 12 µm). The incorporation of double-layer graphene on the suspended slot waveguide structure allows for the significant enhancement of light-graphene interaction, theoretically achieving a 3-dB bandwidth as high as 78 GHz. The EAM shows a calculated modulation depth of 0.022-0.045 dB/µm for the whole operation wavelength range. The MRM exhibits a calculated extinction ratio as high as 68.9 dB and a modulation efficiency of 0.59 V·cm around 9 µm. These modulators hold promise for constructing high-speed FSO communication and on-chip spectroscopic detection systems in the MIR atmospheric window.

High frequency characterization of PZT thin-films deposited by chemical solution deposition on SOI for integrated high speed electro-optic modulators

The increasing demand for high data rates and low power consumption puts silicon photonics at the edge of its capabilities. The heterogeneous integration of optical ferro-electric materials on silicon enhances the functionality of the silicon on insulator (SOI) platform to meet these demands. Lead zirconate titanate (PZT) thin films with a large Pockels coefficient and good optical quality can be directly integrated on SOI waveguides for fast electro-optic modulators. In this work, the relative permittivity and dielectric loss of PZT thin films deposited by chemical solution deposition on SOI substrates are analyzed at high frequencies. We extract ε r =1650−2129 and tan (δ) = 0.170 − 0.209 for the PZT thin films in the frequency range 1-67GHz. We show the possibility of achieving bandwidths beyond 60GHz via a Mach-Zehnder modulator with V π = 7V, suitable for next generation data communication systems.

Linear Optical Quantum Computation with Frequency-Comb Qubits and Passive Devices.

We propose a linear optical quantum computation scheme using time-frequency degrees of freedom. In this scheme, a qubit is encoded in single-photon frequency combs, and manipulation of the qubits is performed using time-resolving detectors, beam splitters, and optical interleavers. This scheme does not require active devices such as high-speed switches and electro-optic modulators and is robust against temporal and spectral errors, which are mainly caused by the detectors' finite resolution. We show that current technologies almost meet the requirements for fault-tolerant quantum computation.

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.

Domain effects on the electro-optic properties of thin-film barium titanate

On-chip electro-optic modulation is essential to realize complex on-chip optical signal processing. Recent developments in thin-film ferroelectric oxide for high-speed electro-optical modulators have gained considerable interest in understanding and correlating the material property with the electro-optic response. Particularly, the effect of thin film, domain orientation, and polling on the electro-optic response is not well understood. In this article, we present the effect of ferroelectric domains of thin-film Barium Titanate on the electro-optic response in a waveguide configuration. We also show the impact of drive electrode orientation with respect to the in-plane polarization angle in a multi-domain structure. Our theoretical findings corroborate the experimental observations in the literature, which substantiate the theoretical framework.

Characterization of high-speed electro-optic phase modulators based on heterodyne carrier mapping at a fixed low-frequency.

A self-referenced method based on heterodyne carrier mapping is proposed to characterize the modulation efficiency of high-speed electro-optic phase modulators (EOPMs). The heterodyne carrier mapping replicates the optical carrier after phase modulation to an electrical replica, which enables observing the power variation of the optical carrier at a fixed low-frequency in the electrical domain. The modulation depths and half-wave voltages within the frequency range of up to 40 GHz are determined by measuring the amplitude ratio of the mapped low-frequency component at 80 MHz in the cases of on and off single-tone modulation of the EOPM. The measured results are compared to those obtained with the traditional optical spectrum analysis method and the electrical spectrum sweep method to check the consistency and accuracy. Surpassing the heterodyne spectrum mapping (HSM) scheme, our method only requires a single-tone driving of the EOPM under test and completely avoids the roll-off responsivity of the photodetector through the fixed low-frequency detection.

Cost-effective fiber-to-lithium niobate chip coupling using a double-side irradiation self-written waveguide.

In recent years, integrated lithium niobate (LN) chips have been widely used for developing a variety of photonic devices, such as high-speed electro-optical (EO) modulators and frequency comb generators. A major challenge for their practical applications is the high coupling loss between micrometer-scale LN waveguides and optical fibers. Lensed fibers and special taper structures are commonly used to tackle the coupling issue. However, in some situations, these approaches may increase the overall complexity and cost of design, fabrication, and alignment. Here, we propose using the self-written waveguide (SWW), an optical waveguide induced by light irradiation, to cope with this coupling issue. The approach can apply in connecting a single-mode fiber (SMF) to any waveguide surface in principle, even with a large mode-field mismatch, significantly alleviating the tight alignment requirements typically needed for end-fire coupling into integrated waveguides. Our study demonstrates that the coupling loss between a SMF with a mode-field diameter (MFD) of 4.4 µm and a sub-micrometer LN rib waveguide could be dramatically reduced from an initial value of -14.27 dB to -5.61 dB, after double-side irradiated SWW formation. Our proposed approach offers a potential solution for achieving a cost-effective and flexible fiber-to-LN chip optical interconnect.

Integrated lithium niobate polarization beam splitter based on a photonic-crystal-assisted multimode interference coupler.

Lithium niobate on insulator (LNOI) is a promising platform for high-speed photonic integrated circuits (PICs) that are used for communication systems due to the excellent electro-optic properties of lithium niobate (LN). In such circuits, the high-speed electro-optical modulators and switches need to be integrated with passive circuit components that are used for routing the optical signals. Polarization beam splitters (PBSs) are one of the fundamental passive circuit components for high-speed PICs that can be used to (de)multiplex two orthogonal polarization optical modes, enabling on-chip polarization division multiplexing (PDM) systems, which are suitable for enhancing the data capacity of PICs. In this Letter, we design and experimentally demonstrate a high-performance PBS constructed by a photonic crystal (PC)-assisted multimode interference (MMI) coupler. The measured polarization extinction ratio (ER) of the fabricated device is 15 dB in the wavelength range from 1525 to 1565 nm, which makes them suitable for the high-speed and large data capacity PICs required for future communication systems.

Heterogeneous silicon-on-lithium niobate electro-optic modulator for 100-Gbaud modulation

Integrated lithium niobate (LN) electro-optic (EO) modulators are emerging for applications in next-generation optical fiber communication networks. To date, LN crystal waveguides have led the technology for high-speed modulators. On the other hand, on-chip LN modulators are expected to realize scalable signaling devices with mature complementary metal–oxide–semiconductor technology. In this study, a silicon-loaded LN modulator on the insulator substrate featuring a small footprint, a low driving voltage, and high-speed EO modulation is designed and fabricated. No etching or patterning of the LN is required. The measured halfwave-voltage length product is 1.9 V cm with a static modulation extinction ratio of 17.9 dB. The fabricated LN modulator has a modulation bandwidth of 60 GHz and supports high-speed signaling at a data rate up to 200 Gbit/s.

Ultralow voltage, high-speed, and energy-efficient cryogenic electro-optic modulator

Photonic integrated circuits (PICs) at cryogenic temperatures enable a wide range of applications in scalable classical and quantum systems for computing and sensing. A promising application of cryogenic PICs is to provide optical interconnects by upconverting signals from the electrical to the optical domain, allowing a massive data transfer from 4 K superconducting (SC) electronics to the room temperature environment. Such a solution can overcome a major bottleneck in the scalability of cryogenic systems that currently rely on bulky coaxial cables that suffer from limited bandwidth, a large heat load, and poor scalability. A key element to realize a cryogenic-to-room temperature optical interconnect is a high-speed, electro-optic (EO) modulator operating at 4 K with a modulation voltage at the mV scale, compatible with SC electronics. Although several cryogenic EO modulators have been demonstrated, their driving voltages are substantially large (several hundred mV to a few V) compared to the mV scale voltage provided by SC circuits. Here, we demonstrate a cryogenic modulator with ∼ 10 m V peak-to-peak driving voltage and Gb/s data rate, with an ultralow electric energy consumption of ∼ 10.4 a J / b i t and an optical energy consumption of ∼ 213 f J / b i t . We achieve this record performance by designing and fabricating a compact optical ring resonator modulator in a heterogeneous InP-on-Si platform, where we optimize a multi-quantum-well layer of InAIGaAs to achieve a strong EO effect at 4 K. Unlike other semiconductors such as silicon, our platform benefits from the high-carrier mobility and minimal free-carrier freezing of III-V compounds at low temperatures, with a moderate doping level and a correspondingly low loss (intrinsic resonator Q ∼ 272 , 000 ). These modulators can pave the path for complex cryogenic photonic functionalities and massive data transmission between cryogenic and room-temperature electronics.