Traveling-wave Electrooptic Modulator Research Articles

Integrated thin-silicon passive components for hybrid silicon-lithium niobate photonics

A silicon photonics platform with a reduced silicon layer thickness, which is suitable for hybrid thin-film lithium niobate traveling-wave electro-optic modulators, is used to design low loss waveguides, precise directional couplers, high-quality-factor silicon microring resonators and broad-top coupled microring filters. These designs are verified with experimental measurements and show a way to include such components without requiring a second layer of crystalline silicon of different thickness for this purpose.

Capacitively-Loaded Thin-Film Lithium Niobate Modulator With Ultra-Flat Frequency Response

Traveling-wave electro-optic (E-O) modulators based on thin-film lithium niobate (TFLN) have attracted much attention recently, as low half-wave voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\pi }$ </tex-math></inline-formula> ) and wide modulation bandwidth can be realized with a small footprint. A flat E-O response of the modulator relies on low microwave loss, perfect velocity match and suitable terminal resistance. In this Letter, we present velocity-matched TFLN modulator based on low-loss capacitively-loaded traveling-wave electrodes (CL-TWEs) with an on-chip terminal resistor. The effect of termination resistance on the E-O frequency response is theoretically analyzed and experimentally confirmed. The TFLN modulator with 6-mm modulation length exhibit a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{\pi }$ </tex-math></inline-formula> of 2.7 V and a 6.4-dB electrical bandwidth over 160 GHz. By adopting a termination resistance slightly lower than the characteristic impedance of the CL-TWEs, an ultra-flat E-O response has been demonstrated with fluctuation less than 1-dB up to 50 GHz, consistent with the theoretical prediction.

Design and Manufacture of Traveling-wave Electro-optic Modulator for X-band LFM Signal Generation

In this paper, a photonic-based microwave system technology is described, and a traveling-wave electro-optic modulator is designed and manufactured as a key component. The fabricated modulator is composed of a metal diffusion waveguide for optical transmission and a planar waveguide electrode on lithium niobate substrate for microwave transmission. The electro-optic response bandwidth of I and Q channels in a fabricated dual parallel Mach-Zehnder modulator were measured for 27.67 and 28.11 GHz, respectively. Photonic four times up-converted X-band frequency and linear frequency modulated signal were confirmed using the fabricated electro-optic modulator by S-band input signal. The confirmed broadband signal can be applied to a microwave system for surveillance and high-resolution ISAR imaging.

The Design of 50 GHz Gallium Arsenide Electro-Optic Modulator Arrays for Satellite Communications Systems

Considerations are presented for the design of GaAs traveling-wave electro-optic modulator arrays for space data-link applications. Central to the modulator design is a low loss folded optical configuration giving direct, straight-line radio frequency (RF) access at one end of the device, with all fiber-optical ports at the opposite end. This configuration is a critical enabler for the close-packed monolithic modulator arrays needed for multi-channel applications. It also leads to much more compact packaging, improved fiber handling and contributes to high modulation bandwidths with low ripple by eliminating directional change in the RF feed arrangements. Both single Mach-Zehnder (MZ) and monolithic dual-parallel (IQ) modulators have been assessed up to 70 GHz, with bandwidths around 50 GHz achieved with a low-frequency ON/OFF voltage swing (Vπ) of 4.6 V (a voltage. length product of 8.3 Vcm). The folded devices can be significantly more compact than conventional ‘straight in-line’ modulators, while a modest array of devices (e.g., ×4) can be accommodated in a package of similar dimensions to a single modulator. Design considerations for monolithic arrays of independently addressed MZ modulators (each with its own input fiber) are discussed and practical configurations proposed.

A Parity-Time-Symmetric Optoelectronic Oscillator Based on Non-Reciprocal Electro-Optic Modulation

In this paper, we propose and experimentally demonstrate a parity-time- (PT-) symmetric optoelectronic oscillator (OEO) based on non-reciprocal electro-optic modulation for the generation of a microwave signal with a low phase noise. In the proposed OEO, two mutually coupled optoelectronic loops with one having a gain and the other a loss are implemented based on the copropagating and counterpropagating modulation in a traveling-wave electro-optic Mach-Zehnder modulator (MZM), to achieve PT symmetry. Due to the non-reciprocity of the travelling-wave MZM, the modulation indices are different for the copropagating and counterpropagating light waves in relative to the applied microwave signal, which is then interpreted as different microwave round-trip gains for the mutually coupled optoelectronic loops. Once the gain/loss coefficient is greater than the coupling coefficient, the PT symmetry is broken, and single-mode oscillation is achieved. The proposed OEO is experimentally demonstrated. A 10-GHz microwave signal with a phase noise of −110 dBc/Hz at an offset frequency of 10 kHz and a sidemode suppression ratio of 42 dB is generated. The work provides a new solution to overcome the mode selection problem in a long-cavity OEO. Since a single physical loop is employed, the proposed PT-symmetric OEO has the simplest architecture as compared to any existing PT-symmetric OEOs.

Quantum analysis of a traveling-wave electro-optic phase modulator in the presence of the phase noise from a radio frequency oscillator and a laser

Quantum analysis has been done for the traveling-wave (TW) electro-optic phase modulator in the presence of the phase noise of a radio frequency (RF) oscillator and a laser source, in which the Hamiltonian for an active section of the phase modulator system is derived. The effect of RF phase noise from the oscillator and a phase velocity mismatch (PVM) between optical photons and the RF field are included in the Hamiltonian. The time evolution of the field operator of the laser in the TW electro-optic phase modulator is derived by the Heisenberg equation of motion. The action of a TW electro-optic phase modulator in the presence of the two distinct weak noises—classical phase noise because of RF and quantum phase noise caused by a laser—has been evaluated and explained in terms of the second-order temporal correlation function and power spectrum. The power spectrum has been written as a function of the semiconductor laser linewidth, an enhancement factor in the laser linewidth, and the RF oscillator linewidth. The results are applied to analyze the basic model of a frequency-coded quantum key distribution (FC-QKD) system. The quantum bit error rate (QBER), the first-order optical intensity correlation function, and the spectrum of intensity fluctuation have been calculated for the partially coherent regime to evaluate the performance of the FC-QKD system.

A heterogeneously integrated silicon photonic/lithium niobate travelling wave electro-optic modulator.

Silicon photonics is a platform that enables densely integrated photonic components and systems and integration with electronic circuits. Depletion mode modulators designed on this platform suffer from a fundamental frequency response limit due to the mobility of carriers in silicon. Lithium niobate-based modulators have demonstrated high performance, but the material is difficult to process and cannot be easily integrated with other photonic components and electronics. In this manuscript, we simultaneously take advantage of the benefits of silicon photonics and the Pockels effect in lithium niobate by heterogeneously integrating silicon photonic-integrated circuits with thin-film lithium niobate samples. We demonstrate the most CMOS-compatible thin-film lithium niobate modulator to date, which has electro-optic 3 dB bandwidths of 30.6 GHz and half-wave voltages of 6.7 V×cm. These modulators are fabricated entirely in CMOS facilities, with the exception of the bonding of a thin-film lithium niobate sample post fabrication, and require no etching of lithium niobate.

Quantum model for traveling-wave electro-optical phase modulator

We develop a quantum model of a traveling-wave electro-optical phase modulator by considering a parametric interaction of the optical and radiofrequency waves in the modulator medium by means of the electro-optic effect. The model includes a possible dispersion of the modulator medium in the optical domain. In the case of negligible dispersion, we derive a simple analytic expression for the evolution operator of the optical field, which has been proposed previously in the literature on the basis of an analogy to classical field transformation. We study by means of a perturbative approach the influence of small dispersion on the field evolution operator and formulate conditions of negligibility of modulator dispersion. We demonstrate the signature of dispersion in phase modulation: breaking the symmetry of sidebands with respect to the mean photon number. We obtain the law of transformation of a multimode coherent state by a phase modulator, which is important for understanding the structure of signal states in quantum cryptographic schemes utilizing phase modulation of coherent optical beams.

High-Speed Traveling-Wave Modulator Based on Graphene and Microfiber

The two-dimensional graphene has an extremely high carrier mobility that can potentially offer an electrical bandwidth of over 500 GHz. However, the bandwidth of the reported graphene-based modulators is only a few tens of gigahertz due to the limitation of the RC constant rather than the carrier transit time. In this paper, a high-speed traveling-wave electro-optic modulator based on a graphene/microfiber structure is proposed and investigated for the first time to the best of our knowledge. The all-in-fiber electro-optic modulation in the graphene/microfiber-modulator is achieved by changing the Fermi level of graphene during its interaction with the evanescent wave of the microfiber. The active length of the modulator is 1.372 mm with a V π of 4.9 V. The coplanar strip electrodes have a characteristic impedance of 54 Ω and a microwave attenuation of 0.967 dB/mm. The mismatch between the microwave and the optical group velocity is 7.38%. A very high speed of 82 GHz 3-dB bandwidth is achieved.

Ring resonator-based traveling-wave electro-optic modulator integrated with asymmetric Mach–Zehnder interferometer

We proposed and theoretically investigated a ring resonator-based traveling-wave electro-optic modulator integrated with asymmetric Mach–Zehnder interferometer (AMZI). The AMZI improved the modulation sensitivity and response of the modulator. A 2.93-fold increase in modulation sensitivity was achieved when compared with conventional Mach–Zehnder (MZ) modulators. A traveling-wave analysis of this modulator was presented for the first time. The simulation results showed that the modulator had superior performance compared to conventional ring modulators and MZ modulators. A modulation up to 275 GHz was achieved in the presence of both microwave loss and velocity mismatch.

Dynamically reconfigurable nanoscale modulators utilizing coupled hybrid plasmonics.

The balance between extinction ratio (ER) and insertion loss (IL) dictates strict trade-off when designing travelling-wave electro-optic modulators. This in turn entails significant compromise in device footprint (L3dB) or energy consumption (E). In this work, we report a nanoscale modulator architecture that alleviates this trade-off while providing dynamic reconfigurability that was previously unattainable. This is achieved with the aide of three mechanisms: (1) Utilization of epsilon-near-zero (ENZ) effect, which maximizes the attainable attenuation that an ultra-thin active material can inflict on an optical mode. (2) Non-resonant coupled-plasmonic structure which supports modes with athermal long-range propagation. (3) Triode-like biasing scheme for flexible manipulation of field symmetry and subsequently waveguide attributes. By electrically inducing indium tin oxide (ITO) to be in a local ENZ state, we show that a Si/ITO/HfO2/Al/HfO2/ITO/Si coupled-plasmonic waveguide can provide amplitude modulation with ER = 4.83 dB/μm, IL = 0.03 dB/μm, L3dB = 622 nm, and E = 14.8 fJ, showing at least an order of magnitude improvement in modulator figure-of-merit and power efficiency compared to other waveguide platforms. Employing different biasing permutations, the same waveguide can then be reconfigured for phase and 4-quadrature-amplitude modulation, with actively device length of only 5.53 μm and 17.78 μm respectively.

High-Power Broadly Tunable Electrooptic Frequency Comb Generator

Broadband traveling-wave electrooptic modulators made of lithium niobate have reached a high level of technological maturity. They can provide simultaneously low Vπ, sustain high power (both optical and RF) and yet provide low propagation loss. By combining together these features, we present a high-power handling, broadly tunable, electrooptic frequency comb generator. The device produces between 60 and 75 lines within -10 dB bandwidth over its full tuning range-from 6 to 18 GHz-and can handle up to 1 W of optical input power. This optical frequency comb platform is very well suited for applications in RF photonics and optical communications that require independent RF and optical tuning as well as high-repetition rates but moderate bandwidth.

Structure and microwave properties analysis of substrate removed GaAs/AlGaAs electro-optic modulator structure by finite element method

In this paper, structure and microwave properties of a substrate removed GaAs/AlGaAs traveling wave electro-optic modulator structure were analyzed and simulated by using the finite element numerical technique for lower loss, simultaneous matching of optical and microwave velocities and impedance matching with 50 Ω. The effects of core layer thickness, claddings thicknesses, and width of the modulator on the microwave effective index n m were investigated, the characteristic impedance Z C, the microwave losses α, and the half-wave voltage-length product V π L were calculated. The results of the simulation suggest that the electrical bandwidth of 22 GHz and the optical bandwidth of 48 GHz can be obtained for fully matched, lower loss structure, which correspond to a 13 V·cm drive voltage.

Band-Pass Non-TEM Mode Traveling-Wave Electro-Optical Polymer Modulator for Millimeter-Wave and Terahertz Application

High-frequency electro-optical modulator is critical for enabling signal processing and distribution in the next generation cloud-computing, tele-medicine, and telecommunications. In this paper, substrate integrated waveguide (SIW) is exploited as an alternative fundamental transmission line structure in support of electrical signal for the design and development of millimeter-wave and terahertz (THz) traveling-wave polymeric electro-optic (EO) modulator. Optical and full-wave electromagnetic analyses are carried out and structure optimization is made on the basis of such analyses in order to obtain millimeter-wave transmission characteristics and optical response. Compared to its conventional TEM-mode transmission lines, this bandpass non-TEM mode SIW-based EO modulator presents numerous advantages, namely compact structure, low transmission loss, low driving power, simple packaging and flat optical response over a wide frequency range.

Mode Coupling Between Substrate Integrated Waveguide and Coplanar Waveguide for Traveling-Wave Electrooptical Modulator

A traveling-wave electrooptical modulator that makes use of a mode coupling mechanism between a substrate integrated waveguide (SIW) and coplanar waveguide (CPW) is proposed for millimeter-wave (mmW) radio-over-fiber applications. In this structure, SIWs are added at the input/output ports of conventional modulators from which mode conversion and signal transfer take place between lossy CPW traveling-wave electrodes and low-loss mmW SIW terminals. Such a gradual coupling from SIW to CPW in the modulation path creates a nonuniform field overlap integral and gives the ability of using the modulator at higher frequency. A laser micromachining technique for a transparent LiNbO3 substrate and a titanium in-diffusion technique for achieving a good optical field confinement are used to fabricate the SIW and optical waveguide, respectively. The bending structure of the SIW, fabrication of pad resistors, and broadband transitions are considered in the proposed modulator. 7-GHz bandwidth was measured for the proposed modulator at 60 GHz.

Picosecond Optical Switching Using RF Nonlinear Transmission Lines

We propose a novel sub-ps optical switch composed of an interferometer in which the phase difference between the arms is controlled by a pair of electrooptic modulators (EOMs) driven by integrated nonlinear transmission lines (NLTLs). As a proof-of-concept we demonstrate 70 ps switching using discrete LiNbO3 traveling wave EOMs and commercially available NLTLs capable of delivering a 35 ps falling edge. This technique is simple and robust, and can potentially be extended to sub-ps switching by integrating the NLTL into the EOM waveguide. This fast switch can be used to implement an Optical Time Division Multiplexing (OTDM) network architecture.

Wideband, efficient optical serrodyne frequency shifting with a phase modulator and a nonlinear transmission Line

We report shifting of the frequency of an 850 nm laser with an instantaneous bandwidth of (350-1650) MHz and an efficiency between 35% (minimum) to 80% (best at frequencies around 600 and 1500 MHz) by phase modulation with a sawtooth waveform ("serrodyne frequency shifting"). We use a fiber-coupled traveling wave electro-optical modulator driven by a nonlinear transmission line.

Design and Modeling of Traveling-Wave Electro-Optic Polymer Modulator for Ultrahigh Speed Applications

We present a complete model and design approach for high speed electro-optic (EO) polymer modulators. A general numerical model for arbitrary travelling-wave EO modulators is built, and the dispersion characteristic of each polymer layer in the device stack is carefully considered and integrated into the model. A velocity compensation layer is added to allow more room for increasing the waveguide core index to optimize half-wave voltage of the device. Microstrip mode analysis is also included to estimate the influence of different polymer stack to the E filed distribution inside the waveguide core. The device has been successfully designed and achieves more than 45 GHz EO bandwidth with a half-wave voltage of 3.2 V. A 100 Gbps EO modulator is under development.

Microwave Photonic Signal Detection Using Phase-Matched Optical Rectification in an AlGaAs Waveguide

We demonstrate a novel approach for detecting microwave-modulated optical signals through velocity-matched optical rectification in second-order optical nonlinear materials. Optical intensity modulation is down-converted to microwave frequency (5-20 GHz) using a traveling-wave GaAs electrooptic modulator as an optical square-law detector. Phase matching between microwave and optical waves is achieved through a velocity-matched slow-wave asymmetric coplanar-strip electrode structure.

Travelling-wave electro-optic modulators with arbitrary frequency response utilising non-periodic polarisation reversal

A new design to realise travelling-wave electro-optic modulators, which utilise a non-periodically polarisation-reversed structure to produce an arbitrary frequency response, is proposed. A flat and broadband electro-optic modulator compensated for both velocity mismatch and degradation by microwave loss is presented.