Design Of Electro-optic Modulators Research Articles

Design of BTO-Based Compact Electro-Optic Modulator

We present a compact electro-optic (EO) modulator design exploiting the strong Pockels effect of Barium Titanate (BaTiO3 or BTO). This proposed structure, using parallel-plate electrodes tightly sandwiching the EO media, could achieve V[Formula: see text]L value as low as 35[Formula: see text]V[Formula: see text]m, a significant reduction from current photonic-integrated Pockels EO modulators with V[Formula: see text]L value around 1[Formula: see text]V⋅cm. Compared to plasmonic EO modulators, this proposed structure offers much lower optical loss. The small footprint and low-loss properties of this modulator design allow for its future embodiment in photonic-integrated CMOS circuits.

Analysis and Design of Plasmonic-Organic Hybrid Electro-Optic Modulators Based on Directional Couplers

We present a detailed simulation study on plasmonic-organic hybrid electro-optic modulators based on coupled symmetric or asymmetric plasmonic slots. An electro-optic polymer is exploited as an active material, and the device is compatible with a silicon photonics platform. The proposed device operates at 1550 nm wavelength, typical of data center or long-haul telecommunication systems. The device performance in terms of area, plasmonic losses, optical bandwidth, intrinsic modulation bandwidth and energy dissipation are comparable to already proposed Mach-Zehnder solutions, but with potentially better extinction ratio, coupling losses due to photonic-plasmonic transitions, and flexibility in exploiting, without any performance penalty, asymmetric slots to shift the ON and OFF states bias. Finally, the bias dependence of the modulation chirp is investigated, comparing through and cross-coupling configurations.

Design and optimization of high-speed silicon-based electro-optical modulator in mid-infrared band (Invited)

Design and optimization of high-speed silicon-based electro-optical modulator in mid-infrared band (Invited)

Design and analysis of electro-optic modulators based on high contrast gratings in AlGaN/GaN heterostructures

Recently high electron mobility transistor (HEMT) inspired III-V electro-optic modulator topologies were proposed for realizing high speed electro-optic modulators leveraging plasma dispersion effect due to the 2D electron gas (2DEG) present at the III–V heterostructure interface. The 2DEG is highly confined at the interface, extending to very low depths in the bulk (≈10 nm) and therefore has limited spatial overlap with the optical mode. In this paper, we propose a novel modulator design to boost the 2DEG-light interaction, wherein the HEMT is embedded within a high contrast grating (HCG) mirror. We present an analytical model extending the conventional HCG model to multi-layer structures and observe good agreement with rigorous coupled-wave analysis. We explore the design space for identifying optimal device topology and present geometries that produce a change in reflectivity as large as 70% for C- and L-band wavelengths. We also present results of sensitivity analysis and observe low variation in device performance due to geometry variation arising from device fabrication imperfections. The device platforms presented here are suitable for designing high efficiency electro-optic modulators by incorporating the HEMT HCG into a Fabry–Perot cavity.

Theory of Coupled Harmonics and Its Application to Resonant and Non-Resonant Electro-Optic Modulators

We present a unified theory for analyzing the frequency conversion between an RF voltage and an optical carrier in an electro-optic modulator. Our new approach results in the electro-optic coupled harmonic equation (EOCHE) that encompasses the electro-optic behavior of common types of modulators (lumped, traveling wave, and resonant) and effortlessly provides closed-form solutions for the small-signal frequency response and the large-signal intermodulation effects. In particular, the small-signal electro-optic frequency responses of lumped and traveling-wave Mach--Zehnder modulators, and resonant microring modulators are derived. We show that the concept of velocity-mismatch only applies to special cases when designing traveling-wave modulators. Therefore, to generalize the design of electro-optic modulators we propose a universal concept called the “effective RF phase” that governs how the optical power is exchanged between different RF harmonics of light. Using the EOCHE, we also propose a generalization of the widely used transfer matrix method for photonic circuits to fully capture the RF mixing effects in integrated photonic circuits, which is analogous to the steady-state AC analysis of electrical circuits. The simplicity and versatility of our approach make it a holistic tool for designing high-speed electro-optic modulators.

High-performance racetrack resonator in silicon nitride - thin film lithium niobate hybrid platform.

In this paper, we propose an electro-optic modulator design in a hybrid Si3N4-X-cut LiNbO3. The modulator is based on a modified racetrack resonator and performs at both DC and heightened frequencies. Here the driving electrodes are defined along the straight section of the racetrack. This is done to maximize modulation and minimize modulation-cancelation that occurs in a conventional X-cut LiNbO3-based resonator due to the directional change of the electric field in the micro-ring. The single bus racetrack resonator is formed in a hybrid Si3N4-LiNbO3 platform, to guide the optical mode. The fabricated device is characterized and has a measured tunability and intrinsic quality factor (Q) of 2.9 pm/V and 1.3 × 105, respectively. In addition, the proposed racetrack device exhibits enhanced electro-optic conversion efficiency at modulation frequencies that match with the racetrack's optical free spectral range (FSR). For example, at the modulation frequency of 25 GHz, which corresponds to the fabricated device's optical FSR frequency, a ∼10 dB increase in electro-optic conversion efficiency is demonstrated. With the enhancement, our measured device demonstrates a conversion efficiency comparable to non-resonant thin-film LiNbO3 devices that possess RF electrodes that are 10 times longer in length.

Ultra-fast silicon electro-optic modulator based on ITO-integrated directional coupler

An optical modulator is a very essential component in optoelectronic communication system as it generates an optical data stream out of an applied electric signal. Designing devices with subwavelength dimensions turned out to be a challenge constrained by the ‘diffraction limit’. Plasmonic-based devices had overcome this issue, reaching miniature footprint but introducing high propagation losses. Alternative materials such as conducting transparent oxides (TCOs) act as an optical dielectric material with an excess of free carriers. By adjusting the gating potential on such materials, an epsilon-near-zero state can be reached and the material exhibits a metal-like behavior, in terms of the optical losses. A very popular TCO material is indium tin oxide (ITO). In this work, ITO is utilized in electro-optic modulator design based on a directional coupler device with a slot waveguide platform. The design introduces an ITO/HfO2 combination in the coupler section waveguide. An extinction ratio more than 9 dB and an insertion loss less than 0.9 dB are achieved at a telecommunication standard wavelength of 1.55 μm. Additionally, an average energy per bit (E) of 4.355 fJ/bit as well as an operating bandwidth up to 1 THz, under optimal conditions, are attained.

低驱动、高调制X-Cut铌酸锂电光调制器的优化设计

低驱动、高调制X-Cut铌酸锂电光调制器的优化设计

UV-laser induced birefringence in olygoetheracrylate photopolymers

Laser induced birefringence was studied during coherent treatment by two coherent UV laser beams at wavelength 355nm. Principal role of the border separating photo-solidified and the surrounding non-solidified liquid-like fragments is demonstrated. The magnitudes of laser induced birefringence for probing 1150nm wavelength were varied within 10−4…10−2. It is demonstrated that one can operate by values of the birefringence versus polymer solidification time. The UV-induced birefringence achieves a saturation after about 45days. The reversible birefringence after switching off of the UV-solidification beams was equal to about 18% with respect to initially photo-stimulated. The discovered effect may be used for design and fabrication of electro-optical modulators operated by UV laser light. A principal physical mechanism is determined by an interaction between two coherent UV- nanosecond laser pulses at wavelengths of 355nm (third harmonic of Nd:YAG laser). The discovered effects allow to propose a new type of UV-operated photopolymer materials for optical recoding of information.

Highly efficient graphene-on-gap modulator by employing the hybrid plasmonic effect.

We propose a highly efficient graphene-on-gap modulator (GOGM) by employing the hybrid plasmonic effect, whose modulation efficiency (up to 1.23dB/μm after optimization) is ∼12-fold larger than that of the present graphene-on-silicon modulator (∼0.1 dB/μm). The proposed modulator has the advantage of a short modulation length of ∼3.6 μm, a relatively low insertion loss of ∼0.32 dB, and a larger modulation bandwidth of ∼0.48 THz. The physical insight is investigated, showing that both the slow light effect and the overlap between graphene and the mode field contribute. Moreover, an efficient taper coupler has been designed to convert the quasi-transverse electric mode of conventional silicon waveguide to the hybrid plasmonic mode of GOGM, with a high coupling efficiency of 91%. This Letter may promote the design of high-performance on-chip electro-optical modulators.

Techniques of surface optical breakdown prevention for low-depths femtosecond waveguides writing

We demonstrated technique of direct femtosecond waveguide writing at record low depth (2-15 μm) under surface of lithium niobate, that play a key role in design of electrooptical modulators with low operating voltage. To prevent optical breakdown of crystal surface we used high numerical aperture objectives for focusing of light and non-thermal regime of inscription in contrast to widespread femtosecond writing technique at depths of tens micrometers or higher. Surface optical breakdown threshold was measured for both x- and z- cut crystals. Inscribed waveguides were examined for intrinsic microstructure. It also reported sharp narrowing of operating pulses energy range with writing depth under the surface of crystal, that should be taken in account when near-surface waveguides design. Novelty of the results consists in reduction of inscription depth under the surface of crystals that broadens applications of direct femtosecond writing technique to full formation of near-surface waveguides and postproduction precise geometry correction of near-surfaces optical integrated circuits produced with proton-exchanged technique.

Design of electro-optic modulators based on graphene-on-silicon slot waveguides.

We present a graphene-on-silicon (GoS) suspended vertical slot waveguide. By changing the Fermi level of graphene, the variation in the effective refractive index (RI) of the waveguide is a factor of two larger than that in the traditional GoS rib waveguide. The improvement is due to the light-intensity enhancement and the poor confinement of the optical mode in the slot nanostructure. We design Mach-Zehnder interferometer (MZI) and microring modulators based on the GoS suspended vertical slot waveguide. Our calculations show that the modulators can be energy-efficient and footprint-compact due to the large phase shift of the propagating mode in the waveguide after applying a gate voltage on the graphene. Fabrication of our design is easy and CMOS-compatible. It paves the way for chip-integrated electronic-RI modulators.

Design of Mid-infrared electro-optic modulators based on aluminum nitride waveguides

We propose and theoretically evaluate the performance of aluminum nitride (AlN) waveguides as mid-infrared (mid-IR) electrooptic (EO) modulators. The device configurations considered include a strip waveguide modulator and a horizontal slot waveguide modulator. The device dimensions were optimized by finite element analysis of the mode field and electric fields in order to calculate the modulation efficiency and the loss. By applying 15 V on the modulator, a maximum effective waveguide index change of over 2 × 10–5 is obtained using the slot waveguide structure. The modulator has a reasonable loss of ∼2 dB/cm at 2.5 μm wavelength with oxide cladding absorption taken into account. The modulation efficiency of the slot waveguide is found to be twice of the normal strip waveguide. The results also indicate that EO effect in AlN works well over a wide range of wavelengths in mid-IR.

Closed-form expressions for the optimization of high speed electro-optical modulators

Compact closed-form mathematical expressions for the propagation parameters (characteristic impedance, effective permittivity and attenuation) of coplanar waveguides (CPW) are presented. The formulas take into account asymmetric structures, thick electrodes and two-layered substrates, and thus are perfectly tailored for the optimization of electro-optical modulators. Excellent agreements, within a range of frequencies up to 100GHz, are found with full-wave finite elements results from commercial software (HFSS) and with measures reported in the literature, for material and geometrical CPW parameters of practical relevance for the design of electro-optical modulators.

Modeling and Design of High-Speed Ultralow Voltage GaAs Electro-optic Modulators Enabled by Transparent Conducting Materials

We present a comprehensive modeling study of a high-speed gallium arsenide electro-optic modulator with ultralow switching voltages and large modulation bandwidths enabled by transparent conducting (TC) electrodes. The driving voltage, optical insertion loss, and modulation bandwidth of the TC-enabled modulator are systematically analyzed. Optimized designs for both a top-down and a side conduction geometry using Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> as both buffer and side cladding layers are presented. The results predict half-wave voltages from 0.5 down to 0.2 V, optical insertion losses of 6-10 dB, and optical 3 dB modulation bandwidths from 25-50 GHz for a top-down conduction geometry and 15-30 GHz for a side conduction geometry, assuming that proper impedance transforming parts and terminations are used. The use of benzocyclobutane as side cladding layers in the top-down conduction geometry to realize direct impedance matching was also explored. The corresponding modulation bandwidths are 13 GHz for 0.5 V case and 6 GHz for 0.2 V case, mainly limited by RF-optical wave velocity mismatch.

Compact closed-form expressions for the propagation parameters of a finite ground width coplanar transmission waveguide with thick electrodes

Compact closed-form mathematical expressions are presented for the effective permittivity, characteristic impedance and attenuation constant of coplanar waveguides with thick electrodes and high-frequency dispersion. Excellent agreements, for a wide range of frequencies, up to 100 GHz and for conductor thicknesses up to 40 µm are found with full-wave finite-element results from commercial software (HFSS) in a margin of geometric parameter values and frequencies of interest for the design and optimisation of high-speed electro-optical modulators. The formulae are simple and specially tailored to the narrow gap widths and thick electrodes typical of electro-optical modulator applications. Other approaches to closed-form formulae for CPW parameters are also compared and analysed.

Achieving Higher Modulation Efficiency in Electrooptic Polymer Modulator With Slotted Silicon Waveguide

Silicon slot waveguide based Mach-Zehnder interferometric modulators with electrooptic polymers in the slot have the advantage of low half-wave voltage-length product (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> *L). Several key aspects of this unconventional electrooptic polymer modulator design to optimize the modulator performance are studied in this work. Both computer simulation and experiments have been conducted to understand the relationship between modulator performance such as modulation efficiency, optical loss and the waveguide design parameters. Techniques to achieve efficient poling of electrooptic polymers in the silicon slot waveguide have been developed. The doping of the silicon to enhance conductivity for efficient poling and the trade-off between conductivity and optical loss are experimentally investigated. Surface passivation of silicon nanophotonic structures has been found to be effective in improving poling efficiency. By applying these techniques to a silicon slot waveguide Mach-Zehnder modulator, a low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> *L of 0.52 V ·cm has been achieved. Finally travelling wave electrode designs have been evaluated and the results show that the bandwidth is mainly limited by the attenuation of the radio frequency signal in the electrode and a standard coplanar waveguide electrode design is able to reach 20 GHz in modulators of silicon slot waveguide embedded in electrooptic polymer.

Simulation study of surface-plasmon-resonance electro-optic light modulator based on a polymer grating coupler

An electro-optic light modulator design based on a grating-coupled surface-plasmon-resonance structure is numerically investigated using finite-difference time-domain simulation. The thickness effect of a dielectric layer acting as a waveguide on metal structures is investigated. The results show that the new structure has much higher modulation index than the conventional one and can implement an electro-optic modulator with low operating voltage.

High Δn strip-loaded electro-optic polymer waveguide modulator with low insertion loss

We demonstrate a novel electro-optic polymer modulator design which shows a record low optical insertion loss of 5.7 dB at 1550 nm. The modulator consists of a high numerical aperture passive waveguide which is converted to a strip-loaded electro-optic polymer waveguide through refractive index tapers. The device is fabricated using all wet-etch techniques which results in low excess loss from roughness created during fabrication and, employs new low loss passive sol-gel materials. The fabricated device also shows a low half-wave voltage of 2.8 V.

LONG RANGE SURFACE PLASMON DEVICES DESIGN USING SUBWAVELENGTH METAL GRATING

In this paper we review a new electro-optic devices design strategy using long range surface plasmon (LRSP) excitation along subwavelength metal grating. It is shown that LRSP can be excited on extremely thin subwavelength metal grating embedded in symmetric dielectric ambient. Due to coupling and propagation of LRSP between the metal grating nanowires, a super-narrow reflection dip can be obtained. Compared with conventional LRSP along metal thin film, much narrower resonance is achieved through decreased damping from the existence of large dielectric gaps between the grating nanowires. This interesting phenomenon can be used to design electro-optics devices with improved performance. Examples of electro-optic modulator design with lower insertion loss and low operating voltage and spectral notch filter design with very narrow spectral width will be shown. Its application in refractive index sensing is also discussed.