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Segmented, or slow-wave electrodes have emerged as an index-matching solution to improve bandwidth of traveling-wave Mach Zehnder (MZM) and phase modulators on the thin-film lithium niobate on insulator platform. However, these devices require the use of a quartz handle or substrate removal, adding cost and additional processing. In this work, a high-speed dual-output electro-optic intensity modulator in the thin-film silicon nitride and lithium niobate material system that uses segmented electrodes for RF and optical index matching is presented. The device uses a silicon handle and does not require substrate removal. A silicon handle allows the use of larger wafer sizes to increase yield, and lends itself to processing in established silicon foundries that may not have the capability to process a quartz or fused silica wafer. The balanced MZM has an interaction region of 10 mm, shows a DC half wave voltage of 3.75 V with a 92 mW $\text{P}\pi $ , an ultra-high extinction ratio of roughly 45 dB consistent with previous work, and a fiber-to-fiber insertion loss of 7.47 dB with a 95 GHz 3 dB bandwidth. [ABSTRACT FROM AUTHOR]

Traveling-wave electro-optic (E-O) modulators based on thin-film lithium niobate (TFLN) have attracted much attention recently, as low half-wave voltage ($\text{V}_{\pi }$) 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 $\text{V}_{\pi }$ 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. [ABSTRACT FROM AUTHOR]

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 $\mu \mathrm{m}$ 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. [ABSTRACT FROM AUTHOR]

A frequency-region selective light absorber is theoretically investigated by an electro-optical waveguide cavity coupled plasmonic micro-structure. A spectrally separated broadband near-unity absorption in the visible and the narrow band absorption peak in the near-infrared regime are simultaneously achieved due to the different resonant behaviors located in the different positions of the stereostructure. This spatial and frequency selective light absorption shows impressive distinct behaviors during the electro-optical (EO) operation. The absorption band in the visible maintains well while the absorption peak in the longer wavelength range presents a sensitive response to the external voltage. Based on the numerical simulation, a spectral shift related sensitivity reaches 7.1 nm/V for the infrared absorption peak when the applied voltage is launched. Meanwhile, the absorption efficiency is still keeping in the high level with the intensity exceeding 97.5%. These findings open a facile way for differential light absorption and spectral manipulation via the well-design of spatial geometry-related resonant metamaterial. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a novel lateral-zigzag PN junction for silicon modulator working in C and O bands. The designed PN junction exhibits characteristics of large bandwidth, high modulation efficiency, and easy fabrication. For demonstration, it is embedded in a 220 nm silicon rib waveguide on a 2 $\mu \text{m}$ thick buried oxide layer to form a micro-ring modulator. The measured 3 dB electro-optic bandwidth of the fabricated modulator is 42 GHz while the resistor-capacitor (RC) bandwidth is measured and calculated to be 174 GHz. Clear and open eye diagrams are obtained for 50 Gb/s non-return zero (NRZ) and 100 Gb/s four-level pulse amplitude modulation (PAM4) signals. The modulation power consumption is only 14 fj/bit. [ABSTRACT FROM AUTHOR]

Heterogeneous integration of electro-optic (EO) crystal thin film on insulator has become an attractive platform to achieve high-performance and compact modulators. While there has been some impressive research on the development of such modulators, a critical consideration is the relatively complex heterogeneous-integrating process. In this work, we demonstrate EO modulators based on lead zirconate titanate (PZT) crystal thin film on SiO2/Si substrate, where the PZT is epitaxially-grown via the straightforward chemical solution deposition method. The presented device exhibits a voltage–length product VπL of 1.4 V·cm and a propagation loss of 1.8 dB/cm at the wavelength of 1550 nm. The measured 3-dB bandwidth of a 5 mm-long device is 12 GHz, which can be improved by further optimizing the travelling-wave electrodes. The proposed technique should enable low-cost and mass-production of power-efficient modulators and to promote the development of photonic integration. [ABSTRACT FROM AUTHOR]

Compact thin-film lithium niobate (TFLN) modulators with low half-wave voltage $\text{V}_{\pi }$ are highly desirable for many applications. In particular, modulators with $\text{V}_{\pi }~\sim ~1$ V would allow direct driving by CMOS circuits, but they normally require a long modulation length of several centimeters. Folded TFLN modulators exhibit reduced $\text{V}_{\pi }$ together with a small device footprint. However, previously reported schemes suffer from complicated device fabrication procedure to maintain the relative direction of electric field with respect to the TFLN crystal polarization after electrode folding. In this work, a meandered X-cut TFLN Mach–Zehnder (MZ) modulator with specially designed capacitively-loaded traveling-wave electrodes (CL-TWEs) is demonstrated. Interdigitated T-rails are employed to reverse the electric field in the folded region for continuous accumulation of optical phase shift. Ultra-low $\text{V}_{\pi }$ of 1.08 V is achieved on an 8-mm-long modulator containing three-segments of 7-mm-long modulation sections, together with a 3-dB electro-optic bandwidth around 43 GHz. The broadband meandered modulator with ultra-low half-wave voltage as well as a compact footprint has the potential to be directly driven by CMOS electronics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Thin film lithium niobate on insulator (LNOI), as an emerging photonic material, provides a promising platform to realize compact, high-speed, and high-efficient electro-optic (EO) modulators. However, due to the large coupling loss caused by the large mode field mismatch between standard single mode fibers and LNOI single mode waveguides, edge or grating couplers are indispensable to LNOI modulators. In this paper, we propose a novel edge coupler employing staircase structure. The proposed coupler is feasible with i-line lithography and avoids high precise alignment. Moreover, such coupler can be applied to wafer-scale manufacture with low cost. The simulation results show a low coupling loss less than 1.60 dB can be achieved at 1.55 μm wavelength for transverse electric (TE) polarized light signals. Specifically, benefited from the novel staircase design, the proposed edge coupler shows a unique property of polarization extinction ratio larger than 10 dB, which will further improve the modulation efficiency and extinction ratio of the EO modulators based on X-cut LNOI. [ABSTRACT FROM AUTHOR]

We propose and numerically demonstrate an ultrafast secure key distribution based-on random DNA coding and the chaos synchronization in two identical electro-optic oscillators (EO) with common injection. High-quality synchronization between the responses of EOs with extremely sensitivity to the mismatching of time delay and phase bias is observed. It is noted that the random DNA coding and the delay DNA-XOR in the post-processing can greatly amplify the randomness of the synchronized sequences acquired by symmetrically dual-threshold sampling the correlated physical entropies, and enhance the security of key distribution by providing additional encryption. According to our study, high rate key distribution of over 10 Gb/s can be fulfilled with a good consistency, which can be used for the commercial communication networks. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a polarization-insensitive single sideband modulator based on silicon microring modulators (MRM). The proposed modulator splits and modulates the two orthogonal polarization states of an input laser in a loopback structure, with an on-chip silicon polarization splitter rotator (PSR), overcoming the polarization dependence of the silicon photonic modulator. The IQ configuration of the modulator enables single sideband modulation, thus improving the resistance of the modulated signal to chromatic dispersion and extending the transmission reach. The adoption of an MRM relieves the bandwidth limitation in polarization-diverse versions of SiP Mach-Zehnder modulators (MZM). Our experiments validate the proposed modulator polarization insensitivity and transmission performance. [ABSTRACT FROM AUTHOR]

Seamless integration of terahertz wireless communications with existing optical access networks can potentially offer tens of gigahertz of channel bandwidth. However, the timing-jitter exhibited by the electronics-based transceivers is a key transmission impairments that tends to ceil the performance of these hybrid access networks, especially at high transmission baud rates. In this work, we propose a sinc-Lorentzian Nyquist pulse shape that outperforms the conventional raised-cosine and the better-than raised-cosine pulse shapes under the influence of timing-jitter up to 35% of the symbol duration. Experimental demonstrations are carried out using a 311 GHz photonic-terahertz system operating at a standard bit rate of 1.44 Gbit/s to investigate the robustness of the three pulse shapes against timing-jitter. It is confirmed that the proposed pulse shape is highly-tolerant to timing-jitter as well as the nonlinearity exhibited by terahertz systems, and hence, can improve the bit error rate performance by an order of magnitude when the average timing-jitter is as large as 0.24 ns. The proposed pulse shape is a critical step for designing ultra-high-speed terahertz communications links with improved robustness to timing-jitter. [ABSTRACT FROM AUTHOR]

Chip-scale electro-optic (EO) frequency combs are expected to play an essential role in future high-capacity optical communications systems and next-generation mobile communications. The application requires integrated EO frequency comb generators featuring good spectral flatness, high modulation bandwidth, low driving voltage and low insertion loss simultaneously. Here, we demonstrate a flat-top optical flat comb (OFC) generator based on high-performance lithium niobate on insulator modulators. The OFC generator shows a low on-chip loss (2.1 dB), a low driving voltage over a broad frequency range, and a large 3-dB EO bandwidth. Moreover, with consuming power of less than 2W, the presented device produces 13 lines with a power variation of less than 1.2 dB and a line spacing of 31 GHz which can further be extended to 67 GHz. [ABSTRACT FROM AUTHOR]

In this work, we design, model, and characterize the Bragg grating modulator (BGM) on silicon-rich nitride and thin-film lithium niobate (SRN-TFLN) platform. In particular, the slow-light effect on the electro-optical modulation response is modeled and experimentally validated. The presented modulator shows a 3-dB electro-optical (E-O) bandwidth beyond 40 GHz, which agrees well with the simulation results. High-speed modulation up to 60 Gbps and 70 Gbps are experimentally obtained with extinction ratio (ER) of 4.7 dB and 2.7 dB, respectively. A measured modulation efficiency of 2.47 pm/V is also achieved. With optimized slow-light effect and proper design, the proposed BGM has great potential in high-speed data transmission. [ABSTRACT FROM AUTHOR]

We fabricate a membrane InP-based electro-absorption modulator (EAM), in which an InGaAsP-based multiple-quantum-well (MQW) absorption region is buried with an InP layer, on Si-waveguide circuits. By optical coupling between the MQW absorption region and Si core, a low-loss and large-absorption-length (300-μm-long) supermode waveguide is designed to suppress electric-field screening at high optical input power. The EAM is fabricated by combining direct bonding of the MQW layer and regrowth of the InP layer on a thin InP template bonded on a silicon-on-insulator wafer. The fabricated membrane EAM shows an on-chip loss of less than 4 dB at wavelengths over 1590 nm and temperatures from 25 to 50 °C. Since the membrane lateral p-i-n diode structure is beneficial for reducing the RC time constant of a lumped-electrode InP-based EAM, the EO bandwidth of the EAM is around 50 GHz without a 50-ohm termination up to fiber-input power of 10 dBm. Using the device, we demonstrate clear eye openings for 56-Gbit/s NRZ and 112-Gbit/s PAM4 signals at temperatures from 25 to 50 °C. [ABSTRACT FROM AUTHOR]

We report a slow-light silicon modulator that enables high-speed PAM operation without using an electrical digital-to-analogue converter (DAC). Bragg grating resonators, integrated into each arm of a Mach-Zehnder modulator, enhance the phase modulation through the slow light effect. The optical 4-level PAM signal is generated by driving directly the segmented phase shifter design with two binary signals. This modulator presents an ultra-compact footprint (LSL-MZM = 570 μm), low energy consumption (73 fJ/bit), large electro-optic bandwidth (>40 GHz). Up to 90 Gb/s is achieved over an nm-range spectral operation bandwidth (Δλ = 2 nm). Compared to other low-energy resonator-based modulators, such as micro-rings, this operating bandwidth confers higher stability with a potential operating temperature range of ΔT = 50 °C. We further examine the robustness of the proposed design to fabrication variations by measurements of spectral properties across the wafer. This modulator is of particular interest for applications, such as short-range data communications that require multiple compact and energy-efficient modulators on a single chip. [ABSTRACT FROM AUTHOR]

We present the design and characterization of O-band and C-band silicon photonic (SiP) traveling wave Mach-Zehnder modulators (TW-MZM) allowing 220 Gbps/λ net rate operation. The designed modulators show over 45 GHz 3-dB E-O bandwidth with a single-segment design. In the O-band, with simple linear feed forward equalization, we transmit net 203 (200) Gbps signal over 2 km (10 km) of single-mode fiber (SMF) below the hard-decision forward error correction (HD-FEC) BER threshold of 3.8 × 10−3. With the aid of nonlinear Volterra equalizer and one 2.3Vpp driving signal, we transmit net 225 (216) Gbps PAM8 signals assuming 20% overhead soft-decision FEC with a normalized general mutual information (NGMI) threshold of 0.8798 over 2 km (10 km) of SMF. The C-band design enables net 220 Gbps in B2B and net 215 Gbps over 500 m of SMF above the specified NGMI threshold. These results are the highest reported net rate for SiP MZM in an intensity modulation direct-detection (IM/DD) system, fabricated entirely in a commercial foundry. [ABSTRACT FROM AUTHOR]

Silicon photonics can be used to increase the versatility of wavelength division multiplexing (WDM). Ultra-dense wavelength division multiplexing (uDWDM) shrinks channel spacing between WDM channels to decrease guard bands and increase spectral efficiency. Microring modulators (MRMs) provide a simple solution for uDWDM transmitters - they require no multiplexer/demultiplexer. When used with comb lasers, MRMs provide extremely compact and low power solutions. The spectral efficiency is limited by interchannel interference (ICI) from incomplete isolation of channels. We report an MRM-uDWDM transmitter achieving channel spacing comparable to the symbol rate in each WDM channel, reaching a spectrum occupation of 80%. By choosing the correct operating point, we demonstrate experimentally our fabricated MRM chip design causes no discernable ICI power penalty. To the best of our knowledge, this is the first demonstration of MRM-uDWDM to support such dense channel spacing. [ABSTRACT FROM AUTHOR]

Wideband and polarization-insensitive operation are key issues for electro-optic modulators. An electro-absorption modulator based on graphene-silicon nitride hybrid waveguide has been proposed. Geometric parameters are investigated through finite element method to enhance the interaction between the optical mode and the graphene. For a 200-µm-long hybrid waveguide, a modulation depth (MD) of 20 dB can be obtained when Femi level varies from 0.2 to 0.6 eV. The MD difference between the transversal electric polarization and transversal magnetic polarization (ΔMD) can be restrained to 4 × 10−3 dB at 1550 nm. A low average ΔMD of 0.2 dB within the wavelength range from 1100 to 1645 nm can be obtained. The 3 dB-bandwidth of 53 GHz and a power consumption is 0.687 pJ/bit can be obtained. The proposed modulator has potentials for on-chip signal processing. [ABSTRACT FROM AUTHOR]

Phase noise can be modulated onto the output of a laser with an electro-optic phase modulator (EOM) to create a highly incoherent broadened source with low intensity noise. This technique leaves a small but finite fraction of the coherent carrier power that can be highly detrimental in applications requiring incoherent light. This article shows that the carrier suppression in a laser broadened by this technique can be calculated for an arbitrary noise probability density function. The carrier suppression can be varied experimentally by adjusting the noise voltage standard deviation Gvσ and saturation voltage Vsat of the amplifier that amplifies the noise source that drives the EOM. Simulations show that suppressions better than −30 dB are attainable for reasonable tolerances in Gvσ, for example an EOM with a Vπ of 4.7 V, a Vsat of 6.3 V, and Gvσ/Vπ  =  0.73 ± 0.03. Even greater tolerances can be achieved with a higher Vsat (21 V), lower Vπ (3.5 V), and Gvσ/Vπ ≈ 3.2 ± 2.3. This model aids in selecting these parameters such that the carrier suppression is resilient to fluctuations in these voltages due to temperature variations and aging of the components. The model predictions are validated by testing a fiber optic gyroscope interrogated with a broadened laser for various Gvσ and Vsatand inferring the carrier suppression from its measured noise. A −44-dB carrier suppression was observed for a nonlinear amplifier with Gvσ  =  3.43 V, Vsat  =  6.3 V, and an EOM with Vπ  =  4.7 V, in agreement with predictions. [ABSTRACT FROM AUTHOR]

A Serrodyne microwave frequency translator implemented using a microwave photonic technique is presented. It is based on two dual-drive Mach–Zehnder modulators connected in parallel where one of the modulators is operated as an optical phase modulator driven by a sawtooth wave. It is capable of simultaneously shifting the frequency and phase of an input microwave signal. The frequency translator is designed to largely suppress the undesired carrier and sidebands and to minimise the frequency translation loss. Results on the frequency translator demonstrate −1  to 1 MHz frequency translation of multiple microwave signals with a large spurious suppression ratio of 34 to 41 dB and a wide bandwidth of 2 to 18 GHz. Simultaneous frequency and phase shifting operation is also demonstrated. [ABSTRACT FROM AUTHOR]

Nanophotonics, driven by low processing power and high density integration, is appearing as the next logical step for microwave photonics. Optical frequency combs (OFCs), which play a vital role in integrated microwave photonics, have not yet generated using nano-structured devices. In this paper, we propose OFC generation based on all-optical modulation of nanophotonic structures. A theoretical model based on temporal coupled mode theory is developed to describe cascaded all-optical photonic crystal intensity and phase modulators. Using a modulation light with a sinusoidal waveform, a separate carrier light is modulated to generate an OFC. By manipulating the modulation power and device dimensions, a flat OFC that spans over a 600 GHz is delivered. The proposed system offers OFC generation with tunable comb line spacing using devices with high density integration capabilities. [ABSTRACT FROM AUTHOR]

Hybrid Silicon and Lithium Niobate photonic integration platform has emerged as a promising candidate to combine the scalability of silicon photonic with the high modulation performance of Lithium Niobate. Here, we report a hybrid Silicon and Lithium Niobate Mach–Zehnder modulator integrated with a thermal-optical bias controller and an on-chip RF terminator. The device demonstrates high electro-optical bandwidth of up to 60 GHz, low half-wave voltage of 2.25 V, and low optical on-chip loss of 2 dB, DC biasing half-wave voltage of 1.93 V (or biasing power of 23.77 mW), with reliable and stable biasing characteristics. On-off keying modulation up to 100 Gbit/s and four-level pulse amplitude modulation up to 120 Gbit/s has been demonstrated with excellent performance. The scheme, with its low modulation voltage, low biasing power consumption, low optical insertion loss, large bandwidth, and its flexibility and simplicity of designing, packaging, and testing, can provide an excellent platform on which future high performance complex optical modulators can be developed. [ABSTRACT FROM AUTHOR]

We reviewed recent breakthroughs on silicon photonic for 100Gbaud operation. Recent progress on high-speed Ge photodetector and carrier depletion modulator promises 100Gbaud optical transceivers with all-silicon material platform for commercial applications. We achieved high performance high-speed all-silicon photonics carrier-depletion Mach–Zehnder modulation by co-optimization of doping and device design assisted with an accurate electro-optical (EO) model. We reported all-silicon Mach–Zehnder modulator with a measured 6 dB EO bandwidth of >60 GHz by using a medium doping and 2 mm long phase shifter. We experimentally demonstrated 120Gbaud QPSK and 100Gbaud 32QAM operations using a high performance all-silicon in-phase/quadrature (IQ) modulator with extinction ratio of >25 dB, moderate ${{\boldsymbol{V}}_{\boldsymbol{\pi }}}$ of 6.3 V, and 6 dB electro-optic bandwidth of 50 GHz employing practical Nyquist filter and linear compensation in commercial arbitrary wave generator (AWG) and optical modulation analyzer (OMA). We studied both performance optimization and limitation. Our results show that BER performance can be optimized by pre-equalization (Pre-EQ) method for bandwidth-limited silicon photonic modulators. However, the performance on BER and modulation loss is strongly affected by the equalization bandwidth due to peak-to-average-power-ratio (PAPR). The frequency at fast roll-off of transmitter response is more critical than 3 dB electro-optic bandwidth when Pre-EQ is used for bandwidth compensation. [ABSTRACT FROM AUTHOR]

A PC based cost effective electro-optic type flow measurement technique is designed and developed. The anemometric principle i.e. the effect of heat loss on a heating element due to flowing fluid is utilized for the measurement of flow rate of liquid in the process industry. A constant voltage p-n junction diode is used as a basic sensing element and the output of the sensing element is amplified and conditioned with the help of the instrumentation amplifier and signal conditioning circuit. Transmission of electrical signals in remote locations suffers from measurement errors due to electromagnetic induction effect, stray capacitance effect etc. The transmission of electrical signal is also not safe for the industries where an inflammable area is present between fields to remote locations and optical transmission is preferred in such areas. To overcome these, an electro-optical system has been developed. The process variable is first sensed electrically. This electrical signal of the transducer is converted into an optical signal using an electro-optic amplitude modulator and then it is transmitted in the form of an optical signal from field to control room through inflammable areas. The modulated signals are demodulated in the control room or remote location and then send to the PC through an optoisolator circuit and DAS card. The experiment has been performed and the theory as well as the mathematical equation has been derived to explain the operation of the proposed system. The overall PC based flow indicator system is also found to have linear characteristics with very good repeatability within tolerable percentage error.The experimental result is reported in the paper. [ABSTRACT FROM AUTHOR]

Electrical interconnects are becoming a bottleneck in the way towards meeting future performance requirements of integrated circuits. Moore’s law, which observes the doubling of the number of transistors in integrated circuits every couple of years, can no longer be maintained due to reaching a physical barrier for scaling down the transistor’s size lower than 5 nm. Heading towards multi-core and many-core chips, to mitigate such a barrier and maintain Moore’s law in the future, is the solution being pursued today. However, such distributed nature requires a large interconnect network that is found to consume more than 80% of the microprocessor power. Optical interconnects represent one of the viable future alternatives that can resolve many of the challenges faced by electrical interconnects. However, reaching a maturity level in optical interconnects that would allow for the transition from electrical to optical interconnects for intra-chip and inter-chip communication is still facing several challenges. A review study is required to compare the recent developments in the optical interconnects with the performance requirements needed to reach the required maturity level for the transition to happen. This review paper dissects the optical interconnect system into its components and explains the foundational concepts behind the various passive and active components along with the performance metrics. The performance of different types of on-chip lasers, grating and edge couplers, modulators, and photodetectors are compared. The potential of a slot waveguide is investigated as a new foundation since it allows for guiding and confining light into low index regions of a few tens of nanometers in cross-section. Additionally, it can be tuned to optimize transmissions over 90° bends. Hence, high-density opto-electronic integrated circuits with optical interconnects reaching the dimensions of their electrical counterparts are becoming a possibility. The latest complete optical interconnect systems realized so far are reviewed as well.

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. [ABSTRACT FROM AUTHOR]

Dual frequency comb (DFC) spectroscopy using electro-optic comb generators stands out for its flexibility, easy implementation, and low cost. Typically, two combs with different line spacing are generated from a common laser using independent electro-optic comb generators. This approach minimizes the impact of laser phase noise; however, the distinct paths followed by the two combs ultimately limit the attainable signal-to-noise ratio and long-term stability of the system. In this work, a common-path DFC is generated using a single modulator driven by an arbitrary waveform generator, thus enabling a remarkable increase of the system stability (up to 0.8 s of integration time) while maintaining high flexibility. The proposed technique is experimentally validated by implementing a dual frequency comb with 3000 lines, covering an optical bandwidth of 4.5 GHz, and demonstrating an optical-to-radiofrequency compression factor of 7500. Our system is able to measure extremely narrowband optical features (in the MHz range) with an accuracy only limited by the master laser stability. [ABSTRACT FROM AUTHOR]

For 64-Gbaud operation and beyond, we developed a power efficient high-bandwidth coherent driver modulator composed of a linear four-channel ultra-low power CMOS driver IC and an InP-based dual-polarization IQ modulator. The CMOS driver was fabricated in 65-nm CMOS technology and showed power dissipation of <1 W owing to the use of an open-drain configuration and a stacked current-mode architecture. Moreover, by optimizing the temperature of the thermoelectric cooler that controls the modulator operating temperature, the coherent driver modulator achieved consumed power of <2.5 W at case temperatures of −5 to 75 °C and <2.8 W under −5 to 85 °C. As far as we know, this is the lowest power dissipation for a coherent driver modulator with an InP modulator chip. In addition, by employing a circuit that suppresses temperature dependence of RF characteristics in the CMOS driver, we achieved the 3-dB electro-optical bandwidth of >44 GHz and the variation of 2.0 GHz under the 25° to 85 °C condition. Furthermore, we obtained good back-to-back bit-error-rate performance in up to dual polarization 80-Gbaud 16-QAM modulation. [ABSTRACT FROM AUTHOR]

The excellent optical and unique material properties of lithium niobate have long established it as a prevailing photonic material, especially for the long‐haul telecom modulator and wavelength‐converter applications. However, conventional lithium niobate optical waveguides are bulky, hence large‐scale photonic circuit implementations are impeded and high power requirements are imposed. To address these shortcomings, thin‐film lithium niobate technology has been a topic of intense research in the last few years and a plethora of ultracompact devices with significantly superior performances than the conventional counterparts have been demonstrated. These efforts have rejuvenated lithium niobate for novel electro‐, nonlinear‐, and quantum‐optic applications. Herein, the most recent advancements of this booming field are summarized and a perspective for future directions is given. [ABSTRACT FROM AUTHOR]

The ability of an electrooptic (EO) sensor to perform near-field mapping of a 20 GHz pyramidal horn antenna without disturbance on the radiation behavior is demonstrated. The electric field is scanned on a plane close to the horn aperture with spatial resolution less than $\boldsymbol {0.1\lambda _0}$. The dynamic range reaches more than 40 dB. The measured magnitudes and phases of near-field pixels enable the computation of the far-field radiation pattern. The results are in excellent agreement with theory, numerical simulations and actual far-field measurement. The different approaches lead to the same antenna gain with a confidence interval lower than 1 dB. Furthermore, the behavior of the electromagnetic wave from reactive to radiative region is investigated. The weak invasiveness of the probe is also analyzed. [ABSTRACT FROM AUTHOR]

A novel nanocavity-based polymer-on-insulator (POI) electro-optic modulator (EOM) is proposed. It consists of a polymeric photonic-crystal nanobeam cavity (PCNC) with ultra-low index-contrast (n $_{\text{cavity}}$ /n $_{bg}$ = 1.17). Based on three-dimensional (3D) finite-difference time-domain (FDTD) method, the PCNC design and optimization are investigated theoretically. A high quality-factor (Q) of 3.4 $\,\times\, 10^4$ and small mode-volume of 22.8 ${({\lambda }/n_{SEO}})^{3}$ can be achieved. In order to judge the efficiency of the cavity design for electro-optic (EO) modulation, the optical modes and the electric field distribution are computed using an electromagnetic finite-element solver. Benefiting from the fast and strong EO effect in polymers, the modulator shows an EO modulation efficiency of up to 16 pm/V, which is over an order of magnitude higher than that in lithium niobate (LN) PCNCs. Moreover, the device is only 80  $\mu$ m in length, leading to a voltage-length product $V_{\pi }$ × L = 0.05 V $\cdot$ cm, which is much smaller than those of Mach-Zehnder modulators. To the best of our knowledge, this is the first on-chip POI-based EOM that features both ultra-compact size and high modulation efficiency. Hence, it is potentially an ideal platform for applications in optical communications, electric-field sensing, and tunable photonic circuits. [ABSTRACT FROM AUTHOR]

Electro-absorption and electro-optic characteristics of InAs/InP quantum-dash active region-based waveguide, emitting at ∼1600 nm is investigated. Two major peaks were observed in the change of absorption spectrum with a maximum value of 7070 cm−1 at a bias voltage of -8V with an excellent uniform extinction ratio of ∼15 dB across the wavelength range of operation (1460–1620 nm). The effect of temperature on electro-absorption (EA) measurement suggests a strong influence resulting in merging of two major change of absorption spectrum peaks with higher temperature. Furthermore, electro-optic measurements indicate a change in refractive index and its efficiency values of ∼2.9 × 10−4 and ∼0.5 × 10−4 V−1, respectively, hence exhibiting a low chirping factor of 0.9 and 1.5 at bias voltages of −2 V and −4 V, respectively. As a quasi-three-dimensionally confined structure possessing both quantum well- and quantum dot-like features, the quantum dash waveguide showed superior electro-absorption and electro-optic properties compared to quantum dots and close to that of quantum wells, while attaining low chirp and broad wavelength range of operation. This paves a way for potential realization of quantum dash-based EA and electro-optic modulator for future optical access networks, particularly operating in wide C- to L-band region. [ABSTRACT FROM AUTHOR]

A high-efficiency Mach-Zehnder modulator (MZM) using InGaAsP phase shifters and a semiconductor optical amplifier (SOA) using multiple-quantum-well (MQW) are integrated on Si-waveguide circuits. Membrane structure is easy to optically couple to the widely-used 220-nm-thick Si waveguides, because both layers have comparable effective refractive indices. To integrate different bandgap III-V compound semiconductors; InGaAsP-based MQW for SOA and InGaAsP bulk for MZM, we employ epitaxial regrowth on thin-InP layer that is directly bonded on a silicon-on-insulator (SOI) wafer. Their thicknesses are smaller than the critical thickness (∼430 nm) for epitaxial growth of the InP-based layers bonded on the SOI wafer. This fabrication procedure is beneficial for the wafer-level integration of InP-based devices with different bandgaps on Si-photonic circuits. Furthermore, membrane lateral p-n and p-i-n diode structures improve the modulation efficiency of the MZM and reduce the power consumption of the SOA due to the high optical confinement factor and small active area. The integrated 300-μm-long SOA and MZM with 500-μm-long phase shifters show a fiber-to-fiber lossless operation with the SOA current of only 24 mA at room temperature. The modulation efficiency of the compact MZM is high enough for eye opening with non-return-to-zero (NRZ) signals from 28 to 40 Gbit/s. By using the SOA to amplify the signals modulated by the MZM, the fiber output power of −3.4 dBm was obtained without any significant pattern effect. [ABSTRACT FROM AUTHOR]

We demonstrate a 222 GBd on-off-keying transmitter in a short-reach intra-datacenter scenario with direct detection after 120 m of standard single mode fiber. The system operates at net-data rates of >200 Gb/s OOK for transmission distances of a few meters, and >177 Gb/s over 120 m, limited by chromatic dispersion in the standard single mode fiber. The high symbol rate transmitter is enabled by a high-bandwidth plasmonic-organic hybrid Mach–Zehnder modulator on the silicon photonic platform that is ribbon-bonded to an InP DHBT 2:1 digital multiplexing selector. Requiring no driving RF amplifiers, the selector directly drives the modulator with a differential output voltage of 622 mVpp measured across a 50 Ω resistor. The transmitter assembly occupies a footprint of less than 1.5 mm × 2.1 mm. [ABSTRACT FROM AUTHOR]

We propose an electro-optic approach for transparent optical networking, in which frequency channels are actively transformed into any desired mapping in a wavelength-multiplexed environment. Based on electro-optic phase modulators and Fourier-transform pulse shapers, our all-optical frequency processor (AFP) is examined numerically for the specific operations of frequency channel hopping and broadcasting, and found capable of implementing these transformations with favorable component requirements. Extending our analysis via a mutual-information–based metric for system optimization, we show how to optimize transformation performance under limited resources in a classical context, contrasting the results with those found using metrics motivated by quantum information, such as fidelity and success probability. Given its compatibility with on-chip implementation, as well as elimination of optical-to-electrical conversion in frequency channel switching, the AFP looks to offer valuable potential in silicon photonic network design, as well as the realization of high-dimensional frequency-bin gates. [ABSTRACT FROM AUTHOR]

We present an efficient optical modulator with reverse-biased III-V/Si hybrid metal-oxide-semiconductor (MOS) optical phase shifters. By applying a reverse bias on a III-V/Si hybrid MOS capacitor, we found that the Franz–Keldysh effect and carrier depletion contribute to an efficient optical phase modulation. Compared with forward-biased MOS-type optical modulators based on carrier accumulation, the reverse-biased III-V/Si hybrid MOS capacitor shows a small depletion capacitance, which improves the trade-off relationship between modulation bandwidth and energy consumption. We have numerically analyzed the proposed optical modulator in terms of wavelength detuning as well as doping concentrations in the Si and III-V layers. After carefully tuning the doping concentration in Si and III-V layers, a small $V_{\pi }L$ of 0.17 Vcm with a modulation bandwidth of above 200 GHz was predicted. We also experimentally demonstrated the proposed optical modulator that exhibited a $V_{\pi }L$ of 0.12 Vcm. The reverse-biased III-V/Si hybrid MOS optical modulator showed a capacitance half that of the forward-biased III-V/Si hybrid MOS optical modulator with a comparable modulation efficiency, resulting in the enhancement of the modulation bandwidth by a factor of two with the energy per bit reduced by half. [ABSTRACT FROM AUTHOR]

Short reach communication systems, such as datacenters and access networks, exhibit steep capacity growth and require opto-electronic technologies with small size, low complexity, and low power consumption. Here, we demonstrate large-scale arrays of reflective surface normal electroabsorption modulators (SNEAMs). With very small active volumes, SNEAMs enable ultra-wide electro-optic bandwidth (>>65 GHz). We show modulation at 25 Gbit/s with 1 Vpp drive voltage on packaged SNEAMs and ultra-high bit-rate modulation at 107 Gbit/s with bare chips. These modulators are polarization-independent and have very low total input/output coupling loss of 0.7 dB to single-mode-fibers. We package SNEAM arrays with arrayed waveguide gratings into wavelength division multiplexing transmitters. Due to their broad wavelength range of modulation, SNEAMs do not need power hungry wavelength tuning or locking systems. Future co-integration of SNEAM arrays with low-power electronic driver arrays will enable high-capacity, low-power electro-optic engines. [ABSTRACT FROM AUTHOR]

We achieved high performance high speed silicon photonics carrier-depletion Mach-Zehnder modulation with commercial foundry by co-optimization of doping and device design assisted with an accurate electro-optical (EO) model. We demonstrated high performance IQ modulators operating at 85 Gbaud 16 QAM and 64 Gbaud 64 QAM with extinction ratio of over 25 dB. For the design of the high performance all-silicon carrier depletion modulator, we developed modeling and design tools to provide not only accuracy, but also efficiency in the simulation of distributed optical and electronic characteristics of travelling waveguides with different designs of optical and microwave waveguides under various doping conditions, which allow the co-design of velocity phase match between optical and microwave waveguides and the impedance match between microwave travelling waveguide and terminal impedance. Our experimental characterization test data agreed well with the model simulation data. More recently, with practical Nyquist filter and linear compensation in commercial arbitrary wave generator (AWG) and optical modulation analyzer (OMA), we demonstrated 100 Gbaud 32 QAM with an all-silicon IQ modulator, which has 6 dB electro-optical bandwidth of 50 GHz and BER achieving FEC threshold with a modern FEC, showing the potential for Tb/s applications. [ABSTRACT FROM AUTHOR]

Lithium niobate is an excellent photonic material with large $\chi ^{\!(2)}$ and $\chi ^{\!(3)}$ nonlinearities. Recent breakthroughs in nanofabrication technology have enabled ultralow-loss nanophotonic devices based on a lithium-niobate-on-insulator (LNOI) platform. Here we present an overview of recent developments in the LNOI platform for on-chip optical frequency comb generation. These devices could lead to new opportunities for integrated nonlinear photonic devices for classical and quantum photonic applications. [ABSTRACT FROM AUTHOR]

A recirculating loop that includes an acousto-optic frequency shifter (AOFS) constitutes a versatile procedure to generate an optical frequency comb (OFC) from a continuous wave (cw) laser. This scheme, implementable with conventional fiber equipment, is capable of producing spectra composed of at least hundreds of coherent lines using a single-stage system. In this letter, we summarize the method to generate these acousto-optic frequency combs and briefly describe a simple model to optimize their performance. We also overview their main reported applications, paying special attention to those based on the peculiarities exhibited by the optical field at the output of the fiber loop. [ABSTRACT FROM AUTHOR]

Classical optical frequency combs have revolutionized a myriad of fields, from optical spectroscopy and optical clocks to arbitrary microwave synthesis and lightwave communication. Capitalizing on the inherent robustness and high dimensionality of this mature optical platform, their nonclassical counterparts, so-called “quantum frequency combs,” have recently begun to display significant promise for fiber-compatible quantum information processing (QIP) and quantum networks. In this review, the basic theory and experiments of frequency-bin QIP, as well as perspectives on opportunities for continued advances, will be covered. Particular emphasis is placed on the recent demonstration of the quantum frequency processor (QFP), a photonic device based on electro-optic modulation and Fourier-transform pulse shaping that is capable of realizing high-fidelity quantum frequency gates in a parallel, low-noise fashion. [ABSTRACT FROM AUTHOR]

We report on the analysis and experimental study of a silicon photonic single microring modulator (MRM) assisted Mach-Zehnder interferometer for high-speed four-level pulse amplitude modulation (PAM-4). The static extinction ratio (ER) of the proposed device is 18 dB larger than that of a single MRM with the same design parameters. Under 0 V reverse bias voltage, the measured 3-dB electro-optic bandwidth of the device is 20 GHz. Based on the enhancement of the ER, the measured 40 Gb/s on-off keying eye diagram of the device shows larger modulation amplitude than that of the single MRM. We further present 76 Gb/s PAM-4 using the proposed device in a back-to-back configuration, with the measured bit error rate below the hard-decision forward error correction threshold of 3.8 × 10-3.

We report a silicon photonic modulator based on a Mach-Zehnder interferometer (MZI) loaded with a microring modulator (MRM) on one arm and a microring resonator (MRR) on the other arm. The device is operated with the resonances of the MRM and the MRR overlapped to improve the extinction ratios and the optical modulation amplitudes. The operating principle is studied in detail using an analytical model. The measured 3-dB electro-optic bandwidth of the device is 27.4 GHz under 0-V reverse bias voltage. In the back-to-back configuration, the modulator has error-free performance up to 38 Gb/s using on-off keying (OOK) modulation without digital signal processing (DSP). At 56 Gb/s, using four-level pulse amplitude modulation (PAM-4) with DSP, the measured bit error rate (BER) is below 1 × 10-6. We further present successful 56 Gb/s OOK transmission and 80 Gb/s PAM-4 transmission over 2 km of standard single mode fiber, with measured BERs below the hard-decision forward error correction threshold of 3.8 × 10-3.

A new technique for tripling the frequency of a microwave signal is presented. It is based on an integrated dual-parallel Mach–Zehnder modulator, which generates two orthogonally polarised RF modulated optical signals with different modulation indexes, and a polarisation-dependent optical attenuator. The photodetector output fundamental RF components generated by the two orthogonally polarised RF modulated optical signals have the same amplitude but an opposite phase, which are cancelled, leaving the frequency tripled microwave signal and the unwanted fifth order harmonic. The new frequency tripler has a simple structure, a wide bandwidth, and a high electrical spurious suppression ratio. Experimental results are presented demonstrating three times frequency multiplication of different-frequency microwave signals with electrical spurious suppression ratio of over 20 dB. The phase noise and the stability performance of the frequency tripler are also investigated experimentally.

We propose and experimentally demonstrate a monolithically integrated $1\times 4$ reconfigurable electro-optic tunable interleaver in X-cut lithium niobate thin film by cascading three asymmetrical Mach-Zehnder interferometers into two stages. Over a wide wavelength range from 1530 nm to 1605 nm, our fabricated device exhibits polarization-insensitive center wavelengths and channel spacing of ~50 GHz, slightly different switching voltages of 4.5 V and 6.7 V, and extinction ratios of 8–15 dB and 7–13 dB for the transverse electric (TE) and the transverse magnetic (TM) polarized input light, respectively. [ABSTRACT FROM AUTHOR]

A dual-slot silicon electro-optic modulator based on indium tin oxide is proposed with high modulation efficiency, low loss, and broadband operation leveraging the epsilon-near-zero (ENZ) effect. The design is verified by numerical simulation with Lumerical Solutions. The active modulation region consists of two slots for enhancing the overlap between the optical mode and the ENZ region to improve the performance of the electro-absorption modulator. By combining the confinement of the dual-slot waveguide and the ENZ effect, we can obtain a high modulation efficiency of 1.44 dB/ $\mu$ m at the wavelength of 1.55  $\mu$ m. The modulator with a modulation region length of only 10  $\mu$ m demonstrates a low insertion loss and a high extinction ratio, with the corresponding values of 0.37 and 14.4 dB, respectively. Through the optimized design, the efficient coupling can be achieved between the silicon waveguide and the dual-slot waveguide with the coupling loss of only 0.12 dB. Moreover, the modulator can operate in a wide spectrum, covering the S-, C-, and L-band. The dual-slot electro-optic modulator thus provides various merits including high modulation efficiency, ultracompact footprint, broad optical bandwidth, and complementary metal–oxide–semiconductor compatibility. [ABSTRACT FROM AUTHOR]