Silicon Electro-optic Modulator Research Articles

On-chip silicon electro-optical modulator with ultra-high extinction ratio for fiber-optic distributed acoustic sensing

Ultra-high extinction ratio (ER) optical modulation is crucial for achieving high-performance fiber-optic distributed acoustic sensing (DAS) for various applications. Bulky acousto-optical modulators (AOM) as one of the key devices in DAS have been used for many years, but their relatively large volume and high power consumption are becoming the bottlenecks to hinder the development of ultra-compact and energy-efficient DAS systems that are highly demanded in practice. Here, an on-chip silicon electro-optical modulator (EOM) based on multiple coupled microrings is demonstrated with ultra-high ER of up to 68 dB while the device size and power consumption are only 260 × 185 μm2 and 3.6 mW, respectively, which are at least two orders of magnitude lower than those of a typical AOM. Such an on-chip EOM is successfully applied to DAS with an ultra-high sensitivity of −71.2 dB rad2/Hz (4 pε/√Hz) and a low spatial crosstalk noise of −68.1 dB rad2/Hz, which are very similar to those using an AOM. This work may pave the way for realization of next-generation ultra-compact DAS systems by integration of on-chip opto-electronic devices and modules with the capability of mass-production.

Plasmon-Assisted Si-ITO Integrated Electro-Optical Rib-Shape Modulator

An integrated plasmonic indium-tin-oxide (ITO)-based silicon electro-optical modulator has been designed, fabricated, and characterized. A sandwich structure shows a 40 GHz bandwidth at the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-2$</tex-math></inline-formula> dB level and an extinction ratio of about 1.8 dB. The device is fabricated on a silicon-on-insulator wafer. A silicon waveguide is excluded from the electric circuit. Instead, an ITO layer is used both as a bottom electrical contact and as an electric field-sensitive medium. As a gate oxide, we used a silicon dioxide layer capped with a gold top electrode to form a capacitor. The dielectric rib inside the ITO-Au capacitor forms a coupling interface between the TE photonic and plasmonic modes.

Compact Slow-Light Integrated Silicon Electro-Optic Modulators With Low Driving Voltage

In this work, we report two slow-light enhanced silicon optical modulators with electro-optic (EO) phase shifter lengths of 500 μm and 150 μm that were fabricated using the AIM Photonics foundry. Tunable power splitters give a threefold increase in the extinction ratio (ER). An eye diagram operating at 10 Gbps was achieved using a very low signal voltage of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> = 500mV. The energy per bit is 57.5 fJ/bit and 45 fJ/bit for the 500 μm and 150 μm modulators, respectively. The nonlinearity of the devices were measured yielding spur-free dynamic range of 113.2 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> . The 150 μm length MZM is among the smallest optical modulators using Bragg gratings for slow light ever reported and results in an estimated modulation efficiency of 0.33 V·cm.

Integrated Slow-Light Enhanced Silicon Photonic Modulators for RF Photonic Links

An integrated RF-photonic link that consists of a slow-light enhanced silicon (Si) electro-optic modulator, a single mode Si optical waveguide and a silicon-germanium (SiGe) photodetector was fabricated using a Si photonic standard foundry service. The third-order intermodulation distortion (IMD3) of the RF signals and the link spur-free dynamic range (SFDR) were characterized through a two-tone test method. The Si Mach-Zehnder modulator architecture is designed to have unequal arms to operate on a simplified driving scheme. As a result, on-chip thermal heaters can be omitted, and the modulator can configure near its quadrature point by adjusting the wavelength to the modulator. Owning to enhanced light-matter interaction in the slow-light region, an increased slope efficiency is observed in the modulator. By selecting the optimal DC bias to the grating and the reference arms of the modulator, a SFDR of 96 dB/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> can be obtained in the RF link at the slow light region. The slow-light modulator also displays increased modulation efficiency, allowing for a reduced footprint for large array integration.

Recent Progress in Silicon-Based Slow-Light Electro-Optic Modulators.

As an important optoelectronic integration platform, silicon photonics has achieved significant progress in recent years, demonstrating the advantages on low power consumption, low cost, and complementary metal–oxide–semiconductor (CMOS) compatibility. Among the different silicon photonics devices, the silicon electro-optic modulator is a key active component to implement the conversion of electric signal to optical signal. However, conventional silicon Mach–Zehnder modulators and silicon micro-ring modulators both have their own limitations, which will limit their use in future systems. For example, the conventional silicon Mach–Zehnder modulators are hindered by large footprint, while the silicon micro-ring modulators have narrow optical bandwidth and high temperature sensitivity. Therefore, developing a new structure for silicon modulators to improve the performance is a crucial research direction in silicon photonics. Meanwhile, slow-light effect is an important physical phenomenon that can reduce the group velocity of light. Applying slow-light effect on silicon modulators through photonics crystal and waveguide grating structures is an attractive research point, especially in the aspect of reducing the device footprint. In this paper, we review the recent progress of silicon-based slow-light electro-optic modulators towards future communication requirements. Beginning from the principle of slow-light effect, we summarize the research of silicon photonic crystal modulators and silicon waveguide grating modulators in detail. Simultaneously, the experimental results of representative silicon slow-light modulators are compared and analyzed. Finally, we discuss the existing challenges and development directions of silicon-based slow-light electro-optic modulators for the practical applications.

Hybrid WDM-MDM transmitter with an integrated Si modulator array and a micro-resonator comb source.

We demonstrate a multi-channel silicon photonic transmitter based on wavelength division multiplexing (WDM) and mode division multiplexing (MDM). The light source is realized by a silicon nitride (Si3N4) Kerr frequency comb and optical modulation is realized by silicon electro-optic modulators. Three wavelengths and two modes are employed to increase the optical transmission capacity. The accumulated data rate reaches 150 Gb/s. The dense integration of WDM and MDM components with a compact optical comb source opens new avenues for the future high-capacity multi-dimensional optical transmission.

Micro-Nano Electro-Optic Modulator Structure Based on the Si/SiGe/Si Material

The modulation power consumption and the modulation efficiency are the key parameters of the electro-optic modulator, which directly affect the electro-optic modulator's photoelectric properties. Improving the performance of the electro-optic modulator, a micro-nano electro-optic modulator structure based on the Si/SiGe/Si material is proposed in this paper, which has low power consumption and high efficiency. After the plasma dispersion effects and the thermo-optic effects are analyzed, we can know that the performance of the electro-optic modulator could be affected by the carrier concentration and the temperature of modulator. Silicon Germanium (SiGe) material is attached to the common Silicon (Si) electro-optic modulator, and a large injection ratio is obtained from the Si/SiGe/Si double hetero-junction. With the modulation region's carrier concentration rise, and the working voltage and the power consumption of modulator all are reduced. The jugged active region structure is attached to the common Si electro-optic modulator, and the probability of inelastic collision among carriers is decreased, so the temperature rise of modulator can be reduced. The thermal-optic effects are weakened, and the modulation efficiency is increased. The simulation results show that the working voltage of the jugged SiGe modulator is less than that of the Silicon modulator at the same refractive index differences, and the jugged SiGe modulator has lower modulation power consumption; the jugged SiGe modulator's effective refractive index differences are more than the Silicon modulator's effective refractive index differences at the same working voltage, and the jugged SiGe modulator has higher modulation efficiency. Therefore, this jugged SiGe modulator is a micro-nano electro-optic modulator with lower power consumption and higher efficiency.

Silicon ring resonator electro-optical modulator utilizing epsilon-near-zero characteristics of indium tin oxide

One crucial component in optical communication systems is the optical modulator. It links between the electric and optical domains as it transforms the electric signal into an optical stream. Electro-optical modulation is a very popular scheme. Recently, indium tin oxide (ITO) has been intensively used in optical modulators due to its epsilon-near-zero characteristics. A silicon electro-optic ring resonator modulator is proposed in terms of the outspread application of ITO. An extinction ratio of about 14 dB as well as an insertion loss of 0.075 dB are achieved at a standard telecommunication wavelength of 1.55 microns. The design has low losses, high efficiency, and compact size.

Silicon p-n-Diode Based Electro-Optic Modulators

A method for forming p-n-diode based silicon electro-optic modulators using local oxidation is tested. It is shown that the local oxidation of silicon allows forming a rib waveguide as a smoothed trapezoid, in contrast to the classical technique of creating a rib waveguide by plasma-chemical etching. The main advantages of the approach used are described: controllability and reproducibility of critical design parameters of the modulators (width and height of the waveguide rib), low surface roughness, and the possibility of using approaches to forming a modulating p-n-diode of combined type in a rib waveguide, which are standard for planar technologies.

A Dual-Slot Electro-Optic Modulator Based on an Epsilon-Near-Zero Oxide

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.

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

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

Progress of radiation effects of silicon photonics devices

Silicon photonics is a fundamental technology, which has great potential applications in optical interconnection for telecom, datacom, and high performance computers, as well as in bio-photonics. Currently considered are the photonics integrated circuits that are able to work in harsh environments such as high energy equipment and future space systems including satellites, space stations and spacecraft. The understanding of the radiation effects of the photonics devices is critical for fabricating radiation hardened photonic integrate chips and maintaining the performance of the devices and the systems. In this paper, the recent progress of the radiation effects of silicon photonic components is summarized. The effects of the high energy particles that possibly degrade the performance of the device are explained, and the response of the passive and active device under radiation are reviewed comprehensively, including waveguides, ring resonators, modulators, detectors, lasers and optical fibers and so on. For passive devices, radiation-induced effects include accelerated-oxidation of the structures, radiation-generated lattice defects, and amorphous densification or compaction in the optical materials. The effective refractive index of the passive device may change consequently, leading the working frequency to shift, the optical confinement to decrease, and the optical power to leak, which accounts for the extra loss or other performance degradation behaviors. For photodetectors and lasers, radiation-induced displacement damage will be dominant. The induced point defects localized in the silicon layer bring about deep level in the forbidden band, acting as generation-recombination centers or trap centers of tunneling effect, which will compensate for either donor or acceptor levels, degrading the response of these optoelectronic device significantly. The plasma dispersion effect is the mainstream approach to building the silicon electro-optic modulators, which will suffer ionization damage in the high energy particle environment, because the interface-trapped hole caused by ionizing radiation reduces the carrier concentration in the depletion region and even induces the pinch-off of the p-doped side of the modulator, which may result in device failure. To improve the radiation hardness of the silicon photonic device, the passivation of the surface, optimization of the waveguide shape, and the choice of appropriate thickness of the buried oxide layer are possible solutions, and more effective approaches are still to be developed.

High-speed hybrid plasmonic electro-optical absorption modulator exploiting epsilon-near-zero effect in indium-tin-oxide

Using transparent conducting oxides, such as indium-tin-oxide (ITO), for optical modulation has attracted research interest because of their epsilon-near-zero (ENZ) characteristics at the telecom wavelengths. Utilizing ITO in multilayer structure modulators, optical absorption of the active ITO layer can be electrically modulated over a large spectrum range. Although they show advances over common silicon electro-optical modulators (EOMs), they suffer from high insertion losses (ILs). To reduce ILs and device footprints without sacrificing bandwidth and modulation strength, slot waveguides are promising options because of their high optical confinement. We present the study and design of an electro-optical absorption modulator based on an electrically tuning ITO carrier density inside an MOS structure. The device structure is based on the dielectric slot waveguide with an ITO plasmonic waveguide modulation section. By changing the dimensions, the effective refractive indices for the slot mode and the off-state mode of the plasmonic section can be matched. When applying electric field to the plasmonic section (on-state), carriers are generated at the ITO-dielectric interface that results in changing the layer where the electric field is confined from a transparent layer into a lossy layer. A finite difference time-domain method with perfect matching layer absorbing boundary conditions is taken up to simulate and analyze this design. An extinction ratio of 15.5 dB is achieved for a 10-μm-long modulation section, at the telecommunications wavelength (1.55 μm). This EOM has advantages of simple design, easy fabrication, compact size, compatibility with existing silicon photonics platforms, as well as broadband performance.

Compact Silicon Electro-Optical Modulator Using Hybrid ITO Tri-Coupled Waveguides

Silicon based electro-optical modulators are essential for optical communication systems. In this paper, we present a silicon electro-optical modulator, which is based on tri-coupled waveguides. Two of these waveguides are silicon-on-insulator slot waveguides separated by a hybrid Indium Tin Oxide intermediate waveguide. The silicon-on-insulator slot waveguides reaps the advantages of the high mode confinement. The power-splitting mechanism can be electrically tuned through applying external electric field to the intermediate plasmonic waveguide. The tuning mechanism is designed such that it will both change the coupling conditions and introduces additional intrinsic losses at the telecommunication wavelength (1550 nm). The modulator was optimized by three-dimensional full finite difference time domain electromagnetic simulations. Extinction ratio of 6.14 dB and insertion losses of 0.06 dB are realized at 21 μm modulator length; as well as, extinction ratio of 11.43 dB and insertion losses of 1.65 dB are realized at 34 μm modulator length. The proposed silicon electro-optical modulator can potentially play a key role in the next generation of the onchip electronic-photonic integrated circuits.

LeTrungThanh Optical Biosensors Based on Multimode Interference and Microring Resonator Structures

LeTrungThanh Optical Biosensors Based on Multimode Interference and Microring Resonator Structures

(Invited) Future Applications of SiGeSn and GeSn

Silicon photonics is considered to be an attractive pathway to drastically reduce power consumption in information technology based on classical Si microelectronics. Group IV alloys, namely Ge and SiGe on Si, are heavily investigated for the integration of modulators and detectors on Si [1,2]. However, higher integration levels of optical data transfer on the chip will require the integration of low power laser. The latter is approached by a wide range of technologies to deposit III/V structures on Si as well as by bonding III/V lasers Si (see ref. 3-7 in ref. [3]). Recently, a direct bandgap as well as optically pumped lasing was demonstrated for GeSn alloys deposited on Ge virtual substrates on Si (100) 8” wafers [3]. Here, efforts are reported of this ongoing endeavor towards an electrically pumped group IV laser. Moreover due to the predicted low effective mass for electrons in the Г-valley, resulting in high electron mobility as well as high carrier injection velocity, the material bears notable potential for high speed, low power circuitry. A novel epitaxial growth technology, namely, reactive gas source epitaxy has been applied to grow GeSn and SiGeSn film and heterostructures in a commercial 8” LPCVD tool, using Ge2H6, Si2H6 and SnCl4 has source gases in an N2 ambient. PH3 and B2H6 are used as sources for n- and p-type doping. Hetero- and quantum well structures containing SiGeSn/GeSn and strained Ge / GeSn layer, respectively, have been deposited at low growth temperatures (down to 325°C) and Sn concentrations up to 14.5%. Careful structural investigation were performed using X-ray diffractometry and transmission electron microscopy. Thick GeSn films grown on Ge will release about 80% of the strain. Interestingly, misfit dislocations are found typically only at the lowest interface towards the virtual Ge substrate, the interface structure within Hetero- and quantum wells structures appears to be free from extended defects. Optical pumping was performed using a ND-YAG laser with pulse duration of 5 ns. PL spectra are taken using a FTIR spectrometer equipped with a InSb detector. To improve the performance of optically pumped laser obtained from GeSn Fabry-Perot devices fabricated using simple mesa stripes [3], the GeSn mesa lines were undercut by RIE processing. This yields efficient strain relaxation in the undercut area. We have then an increase in the energy splitting between Г and L valley for electrons and a mode overlap of ~94% leading to an improved performance of these laser in comparison to the simple Fabry Perot devices previously reported [3]. These laser operate up to 135K, additionally the lasing efficiency is doubled and threshold pump power is cut by half. Similarly, micro-disc laser have been fabricated. Multi-mode lasing involving several TE and TM modes is observed up to 140K. The threshold is found at a pump power of ~225 kW/cm2. A minimum Sn concentration of ~ 8.5% is found to be required to obtain lasing in these relaxed GeSn structures. Light emitting diodes have been processed from p-i-n GeSn homojunctions. Light emission was observed up to 300K. First SiGeSn layers exhibiting a direct band gap, verified by comparing PL data and a joint density of state model are discussed. SiGeSn/GeSn p-i-n double heterostructures were grown, labelling the next milestone on the path towards an electrically pumped group IV laser. Moreover, the potential of GeSn alloys for the fabrication of low power, high frequency electronics is discussed. In particular, Bandstructure calculation reveal low effective masses, high mobilities and a high carrier injection velocity for electrons in the Gamma valley of the conduction band, making this material most interesting for the monolithic integration of optical and electronic devices. Acknowledgement The staff at PGI-9 and the clean room team at FZ-Jülich is acknowledged for their support. Special thanks to continuous input of Jeremy Witzens (RWTH) and Zoran Ikonic (Univ. Leeds) in analysing the data and modelling GeSn devices. References S. Assefa, F. Xia, F. and Y.A. Vlasov, “Reinventing germanium avalanche photodetector for nano-photonic on-chip optical interconnects. Nature 464, 80–84 (2010). Xu, Q., Schmidt, B., Pradhan, S. & Lipson, M. Micrometre-scale silicon electro-optic modulator. Nature 435, 325–327 (2005). S. Wirths, R. Geiger, N. von den Driesch, G. Mussler, T. Stoica, S. Mantl, Z. Ikonic, M. Luysberg, S. Chiussi, J. M. Hartmann, H. Sigg, J. Faist, D. Buca and D. Grützmacher, Nature Photonics 9, 88–92 (2015)

Silicon electro-optic modulator based on an ITO-integrated tunable directional coupler

Indium-tin-oxide (ITO) has attracted great attention because of its electrically-induced epsilon-near-zero (ENZ) characteristics, which has allowed us to develop electro-absorption optical modulators. As an extended application of ITO in a silicon optical modulator, we propose a silicon electro-optic modulator based on an ITO-integrated tunable directional coupler. An ITO block placed at the center of the two-core silicon directional coupler plays a key role in attenuating the optical power of the guided modes. Strong confinement of the optical field at the ENZ layer of ITO with the nonzero imaginary part of ITO’s permittivity let the guiding light experience high absorption as it propagates along the directional coupler. Numerical simulations reveal that the highest modulation depth of 5 dB is achievable at the ENZ region of ITO at a 1.55 μm wavelength. We can modulate the optical signals on an entire C-band ranging from a 1.530 to 1.565 μm wavelength with an on/off extinction ratio of larger than 4.6 dB.

Design investigation to improve voltage swing and bandwidth of the SiGe driver circuit for a silicon electro-optic ring modulator

Abstract. This paper reports on a new SiGe driver IC to address the low breakdown voltage level of modern BiCMOS transistors. An optical modulator driver IC in SiGe 250 nm technology with a supply voltage of 4.5 V is presented. This driver IC consists of pre- and main driver stages where a newly modified cascode topology and capacitance degeneration technique is employed to meet current application requirements; high voltage swing at high datarate. The simulation results show a differential output voltage swing of 3.9 Vp-p at 14 Gbps data rate, according to the FDR Infiniband standard.

Analysis and simulation of ring resonator silicon electro-optic modulators based on PN junction in reverse bias

The theory of silicon optical modulators of ring resonators based on PN diode in reverse bias is primarily discussed. It secondarily provides a full-featured simulator to investigate the behavior of such modulators. Wave equation for ring structure will be solved by using the conformal transformation method and the matrix method as it was used to analyze bent planar optical waveguides. Power coupling between ring and straight waveguides will be calculated by coupled theory of nonparallel waveguides based on experimental results. The time response demonstrates the capability of this device to operate correctly at up to 10 Gbs−1 bitrate, and the frequency spectrum analysis of device shows a >17 dB drop in transmission at the resonant wavelength of ∼1574.9 nm.

Silicon Electro-Optic Modulators Using Quantum Tunneling Structures

Injection-type silicon electro-optic modulators are widely used in onchip optical interconnections due to its high modulation efficiency. However, as the modulator dimension approaches the wavelength limit, it becomes hard to improve the modulator speed by shrinking the device. A new approach to further scale down the dimension of an intrinsic device in injection-type electro-optic modulators without suffering from subwavelength confinement problem is proposed. The device consists of stacked PN or PiN cells connected by tunneling structures to enable current conduction. The diffusion capacitance is reduced due to the reduction of quasi-neutral region width in each device. In this paper, a detail study is given to the case of a two-cell modulator. A twofold of capacitance reduction and speed improvement are observed. Further speed improvement is expected by splitting the modulator into more cells, and the simulation approach for the tunneling structures in such an optical device is also discussed in details.