Traveling-wave Electrode 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 modulation efficiency sinusoidal vertical PN junction phase shifter in silicon-on-insulator

In this paper, a sinusoidal vertical PN junction phase shifter on a silicon waveguide is designed, and the results demonstrate that modifying the shape of the PN junction significantly increases the area of the depletion region within the standard waveguide width of 500 nm, thereby enhancing the overlap between the depletion region and optical waveguide modes under reverse bias conditions. Furthermore, by adjusting the sinusoidal amplitude (A) of the doping contact interface, it is observed that when A=0.065µm, the resulting sinusoidal PN junction most effectively enhances the interaction between carriers and photons, leading to the highest modulation efficiency and the lowest loss. Based on this, further adjustment of the doping concentration distribution in the waveguide was conducted using a doping compensation method. It is observed that setting the doping concentration at 3×1018cm−3 in the heavily doped region and at 1×1018cm−3 in the lightly doped region enables the phase shifter to achieve high modulation efficiency while maintaining low loss. This is attributed to the highest optical intensity being concentrated in the central region of the waveguide, as well as to the positive correlation between doping concentration and modulation efficiency. The final designed device with a length of 1.5 mm successfully attained a low V π L of 0.58V⋅cm, resulting in high modulation efficiency. By employing traveling wave electrodes and ensuring that the effective refractive index of the radio frequency (RF) matches the optical group index (OGI), circuit-level simulations were conducted. The device exhibited a 3 dB bandwidth of 8.85 GHz and eye diagrams of up to 40 Gbit/s, with a maximum extinction ratio (ER) of 8.27 dB and a bit error rate (BER) of 8.83×10−6, which can be widely used in the field of high-speed silicon optical modules.

Mid-infrared barium titanate electro-optic modulators based on a germanium-on-silicon platform

Mid-infrared electro-optic (EO) modulation efficiency and high-frequency performance of barium titanate (BTO) modulators on a germanium-on-silicon platform are investigated. Leveraging its exceptional Pockels coefficients, BTO exhibits remarkable EO modulation capabilities in both transverse-electric (TE) mode for a-axis growth and transverse-magnetic (TM) mode for c-axis growth. At the wavelength of 3.8 µm, the V π ⋅L for a-axis oriented BTO (TE polarization) is 1.90 V·cm, and for c-axis oriented BTO (TM polarization), it is 2.32 V·cm, which have better performance than those of the Pockels effect based EO modulators from literature. In addition, the high-frequency EO response of the modulator is simulated, and a 3-dB EO bandwidth of 62.82 GHz with the optimized traveling wave electrodes is achieved, showing promise in the high-speed MIR applications such as free-space optical communications.

Highly efficient silicon modulator via a slow-wave Michelson structure.

The weak free carrier dispersion effect significantly hinders the adoption of silicon modulators in low-power applications. While various structures have been demonstrated to reduce the half-wave voltage, it is always challenging to balance the trade-off between modulation efficiency and the bandwidth. Here, we demonstrated a slow-wave Michelson structure with 1-mm-long active length. The modulator was designed at the emerging 2-μm wave band which has a stronger free carrier effect. A record high modulation efficiency of 0.29 V·cm was achieved under a carrier depletion mode. The T-rail traveling wave electrodes were designed to improve the modulation bandwidth to 13.3 GHz. Up to 20 Gb/s intensity modulation was achieved at a wavelength of 1976 nm.

Lithium Niobate Electro-Optic Modulation Device without an Overlay Layer Based on Bound States in the Continuum.

Electro-optic modulation devices are essential components in the field of integrated optical chips. High-speed, low-loss electro-optic modulation devices represent a key focus for future developments in integrated optical chip technology, and they have seen significant advancements in both commercial and laboratory settings in recent years. Current electro-optic modulation devices typically employ architectures based on thin-film lithium niobate (TFLN), traveling-wave electrodes, and impedance-matching layers, which still suffer from transmission losses and overall design limitations. In this paper, we demonstrate a lithium niobate electro-optic modulation device based on bound states in the continuum, featuring a non-overlay structure. This device exhibits a transmission loss of approximately 1.3 dB/cm, a modulation bandwidth of up to 9.2 GHz, and a minimum half-wave voltage of only 3.3 V.

Broadband Thin-Film Lithium Niobate Electro-Optic Modulator

Recently, thin-film lithium niobate electro-optical modulators have developed rapidly and have become the core solution for the next generation of electro-optical problems. Compared with bulk lithium niobate modulators, these modulators not only retain the advantages of lithium niobate materials, such as low loss, high extinction ratio, high linear response and high optical power handling capabilities, but can also effectively improve some performance parameters, such as the voltage bandwidth performance of the modulator. Unfortunately, the extremely small electrode gap of thin-film lithium niobate EO (electro-optic) modulators causes metal absorption, resulting in higher microwave losses. The electro-optical performance of the modulator, thus, deteriorates at high frequencies. We designed traveling-wave electrodes with microstructures to overcome this limitation and achieve a 3 dB electro-optical bandwidth of 51.2 GHz. At the same time, we maintain low on-chip losses of <2 dB and a high extinction ratio of 15 dB. It is important to note that the devices we manufactured were metal-encapsulated and passed a series of reliability tests. The success of this modulator module marks a key step in the commercialization and application of thin-film lithium niobate modulation devices.

High-speed mid-infrared Mach–Zehnder electro-optical modulators in lithium niobate thin film on sapphire

Abstract In this study, high-speed mid-infrared Mach–Zehnder electro-optical modulators in x-cut lithium niobate (LN) thin film on sapphire were designed, simulated, and analyzed. The main optical parameters of three types of Mach–Zehnder modulators (MZMs) (residual LN with thickness of 0, 0.5, and 1 μm) were simulated and calculated, namely, the single-mode conditions, bending loss, separation distance between electrode edge and lithium niobate waveguide edge, optical field distribution, and half-wave voltage–length product. The main radio frequency (RF) parameters of these three types of MZMs, such as characteristic impedance, attenuation constant, RF effective index, and the –3 dB modulation bandwidth were calculated depending on the dimensions of the coplanar waveguide traveling-wave electrodes. The modulations with residual LN thickness of 0, 0.5, and 1 μm were calculated with bandwidths exceeding 140, 150, and 240 GHz, respectively, and the half-wave voltage–length product achieved was 22.4, 21.6, and 15.1 V cm, respectively. By optimizing RF and optical parameters, guidelines for device design are presented, and the achievable modulation bandwidth is significantly increased.

Equivalent circuit model of the traveling wave electrode for lithium niobate thin film Mach-Zehnder modulators.

We propose an equivalent circuit model of the traveling wave electrode for lithium niobate thin film (TFLN) Mach-Zehnder modulators, in which the distributed capacitance and conformal mapping techniques are applied to calculate the microwave refractive index, microwave loss, and characteristic impedance. Their accuracies are verified by comparing with the results of the finite element method, and the relative errors are less than 3.282%, 1.776%, and 5.334%, respectively. The influence of the electrode's structural parameters on the modulation performances is analyzed, and a 3dB modulation bandwidth around 84GHz with an 8-mm-long traveling wave electrode is obtained.

High-speed 2D beam steering based on a thin-film lithium niobate optical phased array with a large field of view

An on-chip optical phased array (OPA) is considered as a promising solution for next generation solid-state beam steering. However, most of the reported OPAs suffer from low operating bandwidths, making them limited in many applications. We propose and demonstrate a high-speed 2D scanning OPA based on thin-film lithium niobate phase modulators with traveling-wave electrodes. The measured modulation bandwidth is up to 2.5 GHz. Moreover, an aperiodic array combined with a slab grating antenna is also used to suppress the grating lobes of far-field beams, which enables a large field of view (FOV) as well as small beam width. A 16-channel OPA demonstrates an FOV of 50°×8.6° and a beam width of 0.73°×2.8° in the phase tuning direction and the wavelength scanning direction, respectively.

Segmented Silicon Modulator With a Bandwidth Beyond 67 GHz for High-Speed Signaling

We demonstrate an all-silicon segmented modulator with traveling-wave electrodes, achieving an electro-optic bandwidth beyond 67 GHz and a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_\pi$</tex-math></inline-formula> of 5V. We show that dividing a long optical modulator into shorter segments can help achieve a higher bandwidth without compromising modulation efficiency or optical loss. We analyze the bandwidth limitation due to the delay mismatch between the driving and optical signals. We show that the impact of misalignment of driving delays between segments on the overall bandwidth of the modulator can be described by a finite impulse response filter. Using this modulator, we achieve optical transmission of 120 Gbaud 8-level amplitude shift keying with coherent detection. Taking into account the forward error correction overhead, this yields 336.4 Gb/s net on a single polarization. Our design has the potential for achieving 1 Tb/s using dual-polarization IQ modulation.

Implementation, Modelling and Verification of High-Speed Mach-Zehnder Phase Modulators in an Open Access InP Foundry Platform

We present the introduction of high-speed phase modulators based on the quantum-confined Stark effect to the generic InP foundry platform at Fraunhofer HHI. An overview of the technological integration of the high-speed phase Mach-Zehnder modulators (MZM) to the existing generic foundry process is described. In addition, an electro-optical behavioral model for the high speed MZM, which gives insight into the influence of design parameters on performance, is discussed. The presented MZM structure with a traveling wave electrode length of 5 mm has a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_\pi$</tex-math></inline-formula> of 1.4 V and a small signal electro-optic 3 dB bandwidth of 32 GHz. Large signal RF operation up to 80 Gbps is demonstrated. To the best of our knowledge, these are the highest-performance MZMs in an InP open access integrated photonics foundry platform.

Over-85-GHz-Bandwidth InP-Based Coherent Driver Modulator Capable of 1-Tb/s/λ-Class Operation

We developed an InP-based coherent driver modulator (CDM) with a flexible printed circuit (FPC) RF interface. The CDM has a 3-dB electro-optic (EO) bandwidth of over 85 GHz, including the evaluation board loss, which is sufficient for 1-Tb/s/λ-class operation. Furthermore, we obtained good back-to-back bit-error-rate (BER) performance in modulations up to 144-Gbaud dual-polarization 16-QAM, and we confirmed the CDM's capability for operation over the 150-Gbaud class. With the FPC RF interface, a package's roll-off frequency above 100 GHz was demonstrated in both measured and simulated results. The CDM includes an InP-based n-i-p-n heterostructure modulator chip with a differential capacitively loaded traveling-wave electrode (CL-TWE) and a 4-channel linear SiGe BiCMOS driver IC with an open-collector configuration and low wire inductance. The modulator chip has an EO 3-dB bandwidth of over 70 GHz, which is an improvement of about 30 GHz over that of a conventional p-i-n structure. In addition, to facilitate future 200-Gbaud-class operation, a simulation with a reduced GND via diameter confirmed that the package's roll-off frequency can be improved to more than 120 GHz. Moreover, by reducing the CL-TWE period and the metal's DC resistance, the n-i-p-n modulator chip achieve an EO 3-dB bandwidth of 90 GHz or more.

Attojoule/bit folded thin film lithium niobate coherent modulators using air-bridge structures

Coherent technology has been employed in long-haul transmission systems in the past decade, with growing demand for capacity at ever-lower costs per bit. High-performance coherent modulators with high data rates, wide bandwidth, small footprint, and low power operation are highly desired. Toward this end, we propose a folded thin-film lithium niobate (TFLN) dual-polarization in-phase quadrature modulator featuring a low half-wave voltage of 1 V and a compact footprint of 4 × 8 mm2. To suppress RF wavefront distortion and optimize high-frequency electro-optic performance, we utilize air-bridge structures in the U-turns of the traveling-wave electrodes. As a demonstration of the long-haul transmission capacities with our device, we present driverless 703 Gb/s/λ line-rate transmissions, with a subcarrier modulation scheme, over a 1120 km single-mode fiber link. Here, for the first time, to our knowledge, our device allows for attojoule-per-bit level electrical energy consumption over transmission distances above 1000 km. The device opens opportunities for much lower-cost and capacity-intensive coherent systems that consume ultra-low power, support high data rate, and work in small spaces.

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.

Compact thin-film lithium niobate modulators using slotted coplanar waveguide electrode suitable for high-volume fabrication

Lithium niobate (LN) is a good candidate for fabricating modulators due to its superior material characteristics. However, the application of traditional LN modulators is limited due to their large footprint and low modulation efficiency resulting from weak optical confinement. In recent years, with the development of the thin-film lithium niobate (TFLN) platform and LN etching technology, the size of the optical mode of the TFLN modulator is 20 times smaller than that of the traditional LN modulator. Furthermore, TFLN modulators have demonstrated a wide bandwidth, low half-wave voltage and small footprint in recent reports. The length of the TFLN modulators can be further reduced by employing a folded design and therefore applicable to compact transceiver package, such as being packaged in the quad small form factor pluggable double density transceiver. In this paper, we report on a folded TFLN modulator fabricated from a 4 inch LN wafer, which is suitable for large-volume fabrication. A fiber-to-fiber insertion loss of 2.5 dB and a voltage–length product of 1.85 V cm have been achieved. The measured electro-optic response curve has a 2.3 dB roll-off at 40 GHz, and the simulated 3 dB bandwidth reaches 65 GHz. Compared to traditional coplanar waveguide traveling wave electrodes, the slotted coplanar waveguide traveling wave electrode (slotted CPW-TWE) design adopted in this work allows adjusting the high-speed characteristics and modulation efficiency with more flexibility. This is the first time a slotted CPW-TWE design has been applied in a folded TFLN modulator.

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.

Compact 100GBaud driverless thin-film lithium niobate modulator on a silicon substrate.

Electro-optic (EO) modulators with a high modulation bandwidth are indispensable parts of an optical interconnect system. A key requirement for an energy-efficient EO modulator is the low drive voltage, which can be provided using a standard complementary metal oxide semiconductor circuity without an amplifying driver. Thin-film lithium niobate has emerged as a new promising platform, and shown its capable of achieving driverless and high-speed EO modulators. In this paper, we report a compact high-performance modulator based on the thin-film lithium niobate platform on a silicon substrate. The periodic capacitively loaded travelling-wave electrode is employed to achieve a large modulation bandwidth and a low drive voltage, which can support a driverless single-lane 100Gbaud operation. The folded modulation section design also helps to reduce the device length by almost two thirds. The fabricated device represents a large EO bandwidth of 45GHz with a half-wave voltage of 0.7V. The driverless transmission of a 100Gbaud 4-level pulse amplitude modulation signal is demonstrated with a power consumption of 4.49fj/bit and a bit-error rate below the KP4 forward-error correction threshold of 2.4×10-4.

A 50-GHz Bandwidth Traveling-Wave Mach-Zehnder Modulator With Built-In Feedback Equalization

A 50–GHz bandwidth traveling-wave Mach-Zehnder modulator (TWMZM) was designed with a new equalization technique, which takes advantage of the intrinsic delay in a traveling-wave electrode (TWE) and a local negative-feedback circuit. With this idea, given the availability of flip-chip bonding, a long TWE can be equalized as a single stage or be segmented into a few stages, each being equalized separately to optimize the performance of the entire device. The detailed analysis of this equalization technique is presented. As a demonstration, a 1.6–mm long equalized TWMZM was designed using a commercial silicon photonic process and a 0.13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu \mathrm{m}$</tex-math></inline-formula> SiGe process. A 2–stage cascaded design with equal equalization was adopted and optimized for high bandwidth and high extinction ratio. The circuit was designed using the multi-physics electronic-photonic integrated circuit (EPIC) design flow we developed recently, wherein both electronic and photonic parts were modeled and the EPIC circuit was simulated for system level performance evaluation. Simulation results showed that the equalization technique increased the 3-dB EO bandwidth of the TWMZM from 28 to 50.4 GHz, an 80% increase. Large signal performance with non-return to zero signals upto 100 Gb/s was evaluated and the results showed significant improvements in the eye quality with equalization. The performance evaluation with pulse amplitude modulation (PAM)-4 signal was also carried out and the results showed successful operation at 106 Gbaud PAM-4 for the equalized TWMZM, makes it a promising candidate for future 200 Gbps-per-lambda optical transceivers.

High-speed compact folded Michelson interferometer modulator.

We propose and experimentally demonstrate a novel compact folded Michelson interferometer (FMI) modulator with high modulation efficiency. By folding the 0.5 mm-long phase shift arms, the length of the modulation area of the FMI modulator is only 0.25 mm. Meanwhile, the traveling wave electrode (TWE) is also shorter, which decreases the propagation loss of the RF signal and contributes to a small footprint. The Vπ-L of the present device is as low as 0.87 V·cm at -8 V bias voltage. The minimum optical insertion loss is 3.7 dB, and the static extinction ratio (ER) is over 25 dB. The measured 3-dB electro-optical (EO) bandwidth is 17.3 GHz at a -6 V bias. The OOK eye diagram up to 40 Gb/s is demonstrated under 2 V driver voltage.

Design of a High-Bandwidth Traveling-Wave III–V/Si Hybrid MOS Optical Modulator

The III–V/Si hybrid Metal-Oxide-Semiconductor (MOS) optical modulator is promising for high-efficiency, low-energy and high-speed optical modulation. As of now, all the III–V/Si hybrid MOS optical modulators demonstrated were driven by the RC-limited lumped electrodes. In order to break the RC limitation posed by the driving scheme, we designed a traveling-wave III–V/Si hybrid MOS optical modulator. Based on the equivalent circuit model developed, the traveling-wave electrode geometry, doping concentration and equivalent oxide thickness were optimized numerically to achieve the velocity and impedance matching for high modulation bandwidth with a low <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_{pp}$</tex-math></inline-formula> of 2 V. High modulation bandwidths of 75 GHz and 102 GHz were predicted for carrier-accumulation mode and carrier depletion mode, respectively. The expected high modulation bandwidth makes the III–V/Si hybrid MOS optical modulator more attractive in the high-data-rate optical transmission.