40 Gbit/s low-loss silicon optical modulator based on a pipin diode

Silicon Photonic Phase Shifters and Their Applications: A Review

With the development of silicon photonics, dense photonic integrated circuits play a significant role in applications such as light detection and ranging systems, photonic computing accelerators, miniaturized spectrometers, and so on. Recently, extensive research work has been carried out on the phase shifter, which acts as the fundamental building block in the photonic integrated circuit. In this review, we overview different types of silicon photonic phase shifters, including micro-electro-mechanical systems (MEMS), thermo-optics, and free-carrier depletion types, highlighting the MEMS-based ones. The major working principles of these phase shifters are introduced and analyzed. Additionally, the related works are summarized and compared. Moreover, some emerging applications utilizing phase shifters are introduced, such as neuromorphic computing systems, photonic accelerators, multi-purpose processing cores, etc. Finally, a discussion on each kind of phase shifter is given based on the figures of merit.

Plasmon-assisted polarization-sensitive photodetection with tunable polarity for integrated silicon photonic communication systems

To establish high-bandwidth chip-to-chip interconnects in optoelectronic integrated circuits, requires high-performance photon emitters and signal receiving components. Regarding the photodetector, fast device concepts like Schottky junction devices, large carrier mobility materials and shrinking the channel length will enable higher operation speed. However, integrating photodetectors in highly scaled ICs technologies is challenging due to the efficiency-speed trade-off. Here, we report a scalable and CMOS-compatible approach for an ultra-scaled germanium (Ge) based photodetector with tunable polarity. The photodetector is composed of a Ge Schottky barrier field effect transistor with monolithic aluminum (Al) source/drain contacts, offering plasmon assisted and polarization-resolved photodetection. The ultra-scaled Ge photodetector with a channel length of only 200 nm shows high responsivity of aboutR = 424 A W^-1and a maximum polarization sensitivity ratio of TM/TE = 11.

Si racetrack optical modulator based on the III-V/Si hybrid MOS capacitor

We have fabricated a Si racetrack optical modulator based on a III-V/Si hybrid metal-oxide-semiconductor (MOS) capacitor. The III-V/Si hybrid MOS optical phase shifter was integrated to a Si racetrack resonator with a coupling length of 200 µm and a coupling gap of 700 nm. The fabricated Si racetrack resonator demonstrated a small VπL of 0.059 Vcm. For 10-dB optical intensity modulation, the Si racetrack resonator showed a 60% smaller driving voltage than a Mach-Zehnder interferometer modulator with the same phase shifter, leading to a better balance between high energy efficiency and large modulation bandwidth.

Heterogeneously integrated ITO plasmonic Mach-Zehnder interferometric modulator on SOI

Densely integrated active photonics is key for next generation on-chip networks for addressing both footprint and energy budget concerns. However, the weak light-matter interaction in traditional active Silicon optoelectronics mandates rather sizable device lengths. The ideal active material choice should avail high index modulation while being easily integrated into Silicon photonics platforms. Indium tin oxide (ITO) offers such functionalities and has shown promising modulation capacity recently. Interestingly, the nanometer-thin unity-strong index modulation of ITO synergistically combines the high group-index in hybrid plasmonic with nanoscale optical modes. Following this design paradigm, here, we demonstrate a spectrally broadband, GHz-fast Mach–Zehnder interferometric modulator, exhibiting a high efficiency signified by a miniscule V_πL of 95 V μm, deploying a one-micrometer compact electrostatically tunable plasmonic phase-shifter, based on heterogeneously integrated ITO thin films into silicon photonics. Furthermore we show, that this device paradigm enables spectrally broadband operation across the entire telecommunication near infrared C-band. Such sub-wavelength short efficient and fast modulators monolithically integrated into Silicon platform open up new possibilities for high-density photonic circuitry, which is critical for high interconnect density of photonic neural networks or applications in GHz-fast optical phased-arrays, for example.

Low-loss optical waveguides made with a high-loss material

For guiding light on a chip, it has been pivotal to use materials and process flows that allow low absorption and scattering. Based on subwavelength gratings, here, we show that it is possible to create broadband, multimode waveguides with very low propagation losses despite using a strongly absorbing material. We perform rigorous coupled-wave analysis and finite-difference time-domain simulations of integrated waveguides that consist of pairs of integrated high-index-contrast gratings. To showcase this concept, we demonstrate guiding of visible light in the wavelength range of 550-650 nm with losses down to 6 dB/cm using silicon gratings that have a material absorption of 13,000 dB/cm at this wavelength and are fabricated with standard silicon photonics technology. This approach allows us to overcome traditional limits of the various established photonics technology platforms with respect to their suitable spectral range and, furthermore, to mitigate situations where absorbing materials, such as highly doped semiconductors, cannot be avoided because of the need for electrical driving, for example, for amplifiers, lasers and modulators.

2D modeling of silicon optical PN phase shifter

In this paper, a 2D model of a silicon lateral PN optical phase shifter is presented, which can be used for multiple-parameter study and optimization of device performance without the need for any commercial numerical tools. The model shows good agreement with technology computer-aided design (TCAD) simulation and can be used to calculate the phase shift, absorption loss, modulation efficiency, and insertion loss of the phase shifter. Multiple-parameter study includes the waveguide dimensions, operating wavelength, cladding material, doping concentrations, junction offset, and applied voltage. The model employs the effective index method to determine the mode properties and construct the 2D mode field. The PN diode is modeled by taking into account the fringing electric field at the core-cladding interface, which results in a wider depletion region near the interface. Multiple-loss components are discussed, and the scattering loss and free-carrier absorption are modeled using Payne-Lacey and Soref models, respectively. The model uses 2D modal overlap with 2D carrier distribution across the waveguide to calculate the phase shifter performance metrics. The algorithm used to model the 2D nature of the PN diode depletion region is presented in detail and uses mathematical and analytical formulas instead of numerical methods, making the model faster and easy to implement, with accuracy on par with commercial tools.

Design of a 90 GHz SOI Fin Electro-Optic Modulator for High-Speed Applications

Introducing high speed networks, such as the fifth generation of mobile technology and related applications including the internet of things, creates a pressing demand for hardware infrastructure that provides sufficient bandwidth. Here, silicon-based microwave-photonics presents a solution that features easy and inexpensive fabrication through a mature platform that has long served the electronics industry. In this work, the design of an electro-optic modulator is proposed where the ‘fin’ structure is adopted from the domain of electronics devices, with emphasis on the high speed of operation. The proposed modulator is customized to provide a bandwidth of 90 GHz with a small phase shifter length of 800 μm and an optical insertion loss of 4 dB. With such a speed, this proposed modulator fits high-speed applications such as modern tele-communications systems.

7 nm/V DC tunability and millivolt scale switching in silicon carrier injection degenerate band edge resonators

We demonstrate electro-optical tuning of degenerate band edge resonances in Si photonic waveguides for applications including tunable filters, low voltage switches, and modulators. Carrier injection modulation is enabled by introducing periodic Si slabs to electrically connect the resonator to P and N dopants. Measured devices yield a large DC tunability of 7.1 nm/V and a peak switching slope of 206 dB/V. Digital data transmission measurements at 100 Mb/s show 3 dB of switching with a swing voltage of 6.8 mV, 91.4 aJ/bit switching energy, and 1.08 pJ/bit holding energy.

High-frequency electro-optic measurement of strained silicon racetrack resonators

The observation of the electro-optic effect in strained silicon waveguides has been considered a direct manifestation of an induced χ(2) nonlinearity in the material. In this work, we perform high-frequency measurements on strained silicon racetrack resonators. Strain is controlled by a mechanical deformation of the waveguide. It is shown that any optical modulation vanishes, independent of the applied strain, when the applied voltage varies much faster than the carrier effective lifetime and that the DC modulation is also largely independent of the applied strain. This demonstrates that plasma carrier dispersion is responsible for the observed electro-optic effect. After normalizing out free-carrier effects, our results set an upper limit of (8±3) pm/V to the induced high-speed effective χeff,zzz(2) tensor element at an applied stress of -0.5 GPa. This upper limit is about 1 order of magnitude lower than previously reported values for static electro-optic measurements.

High-efficiency Si optical modulator using Cu travelling-wave electrode

We demonstrate a high-efficiency and CMOS-compatible silicon Mach-Zehnder Interferometer (MZI) optical modulator with Cu traveling-wave electrode and doping compensation. The measured electro-optic bandwidth at Vbias = -5 V is above 30 GHz when it is operated at 1550 nm. At a data rate of 50 Gbps, the dynamic extinction ratio is more than 7 dB. The phase shifter is composed of a 3 mm-long reverse-biased PN junction with modulation efficiency (Vπ·Lπ) of ~18.5 V·mm. Such a Cu-photonics technology provides an attractive potentiality for integration development of silicon photonics and CMOS circuits on SOI wafer in the future.

Enhanced conductivity of sol-gel silica cladding for efficient poling in electro-optic polymer/TiO2 vertical slot waveguide modulators

We report the enhanced conductivity of sol-gel silica under-cladding for efficient poling of electro-optic (EO) polymer in a hybrid EO polymer/TiO2 vertical slot waveguide modulator. The electrical volume conductivity of sol-gel silica cladding increases approximately 30 times when the calcining time of the cladding layer is critically reduced to 45 minutes, which increases the in-device EO coefficient of the 600-nm-thick EO polymer film in modulators and reduces the lower halfwave voltage (Vπ) of the modulators. The lowest driving voltage (Vπ) of the TiO2 slot waveguide modulator is 2.0 V for an electrode length (Le) of 10 mm and wavelength of 1550 nm (VπLe = 2.0 V·cm) for the low-index guest-host EO polymer SEO125. The optical propagation loss is reduced to 7 dB/cm.

CMOS-compatible 2-bit optical spectral quantization scheme using a silicon-nanocrystal-based horizontal slot waveguide

All-optical analog-to-digital converters based on the third-order nonlinear effects in silicon waveguide are a promising candidate to overcome the limitation of electronic devices and are suitable for photonic integration. In this paper, a 2-bit optical spectral quantization scheme for on-chip all-optical analog-to-digital conversion is proposed. The proposed scheme is realized by filtering the broadened and split spectrum induced by the self-phase modulation effect in a silicon horizontal slot waveguide filled with silicon-nanocrystal. Nonlinear coefficient as high as 8708 W⁻¹/m is obtained because of the tight mode confinement of the horizontal slot waveguide and the high nonlinear refractive index of the silicon-nanocrystal, which provides the enhanced nonlinear interaction and accordingly low power threshold. The results show that a required input peak power level less than 0.4 W can be achieved, along with the 1.98-bit effective-number-of-bit and Gray code output. The proposed scheme can find important applications in on-chip all-optical digital signal processing systems.

Mesoporous sol-gel silica cladding for hybrid TiO2/electro-optic polymer waveguide modulators

We report the efficient poling of an electro-optic (EO) polymer in a hybrid TiO(2)/electro-optic polymer multilayer waveguide modulator on mesoporous sol-gel silica cladding. The mesoporous sol-gel silica has nanometer-sized pores and a low refractive index of 1.24, which improves mode confinement in the 400-nm-thick EO polymer film in the modulators and prevents optical absorption from the lower Au electrode, thereby resulting in a lower half-wave voltage of the modulators. The half-wave voltage (Vπ) of the hybrid modulator fabricated on the mesoporous sol-gel silica cladding is 6.0 V for an electrode length (Le) of 5 mm at a wavelength of 1550 nm (VπLe product of 3.0 V·cm) using a low-index guest-host EO polymer (in-device EO coefficient of 75 pm/V).

Compact-sized high-modulation-efficiency silicon Mach-Zehnder modulator based on a vertically dipped depletion junction phase shifter for chip-level integration

We present small-sized depletion-type silicon Mach-Zehnder (MZ) modulator with a vertically dipped PN depletion junction (VDJ) phase shifter based on a CMOS compatible process. The fabricated device with a 100 μm long VDJ phase shifter shows a VπLπ of ∼0.6  V·cm with a 3 dB bandwidth of ∼50  GHz at -2  V bias. The measured extinction ratios are 6 and 5.3 dB for 40 and 50  Gb/s operation under 2.5  Vpp differential drive, respectively. On-chip insertion loss is 3 dB for the maximum optical transmission. This includes the phase-shifter loss of 1.88  dB/100  μm, resulting mostly from the extra optical propagation loss through the polysilicon-plug structure for electrical contact, which can be readily minimized by utilizing finer-scaled lithography nodes. The experimental result indicates that a compact depletion-type MZ modulator based on the VDJ scheme can be a potential candidate for future chip-level integration.

Slow-light Mach-Zehnder modulators based on Si photonic crystals

Mach-Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p-n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p-n diode. This length is shorter than commercial LiNbO₃ modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index n_g. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 μm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.

A 40 Gbit/s optical link on a 300-mm silicon platform

We demonstrated 40 Gbit/s optical link by coupling a silicon (Si) optical modulator to a germanium (Ge) photo-detector from two separate photonic chips. The optical modulator was based on carrier depletion in a pn diode integrated in a 950-µm long Mach-Zehnder interferometer. The Ge photo-detector was a lateral pin diode butt coupled to a silicon waveguide. The overall loss, which is mainly due to coupling (3 grating couplers times ~4 dB) was estimated to be lower than 18 dB. That also included modulator loss (4.9-dB) and propagation loss (<1 dB/cm). Both optoelectronic devices have been fabricated on a 300-mm CMOS platform to address high volume production markets.

Fringe-field carrier-depletion modulators with high modulation efficiency and low free carrier absorption

A high-speed carrier-depletion silicon modulator based on a fringe field pn junction design is presented. Due to the strong fringe field, the size of heavily doped regions can be reduced and away from the waveguide core, whereas large modulation efficiency is still accomplishable. The VπL is 1.8 V-cm and the phase shifter loss is 1.3 dB/mm. The figure of merit (FOM), defined by the product of VπL and phase shifter loss, is estimated to be 23.4 dB-V. The modulation speed and depth are 11.8 GHz and 8.1 dB, respectively, which is mainly limited by the mobility of poly-Si.

Low power 50 Gb/s silicon traveling wave Mach-Zehnder modulator near 1300 nm

The wavelength band near 1300 nm is attractive for many telecommunications applications, yet there are few results in silicon that demonstrate high-speed modulation in this band. We present the first silicon modulator to operate at 50 Gbps near 1300 nm. We demonstrate an open eye at this speed using a differential 1.5 V(pp) signal at 0 V reverse bias, achieving an energy efficiency of 450 fJ/bit.

Low loss 40 Gbit/s silicon modulator based on interleaved junctions and fabricated on 300 mm SOI wafers

We demonstrate high-speed silicon modulators based on carrier depletion in interleaved pn junctions fabricated on 300 mm-SOI wafers using CMOS foundry facilities. 950 µm-long Mach Zehnder (MZ) and ring resonator (RR) modulator with a 100 µm radius, were designed, fabricated and characterized. 40 Gbit/s data transmission has been demonstrated for both devices. The MZ modulator exhibited a high extinction ratio of 7.9 dB with only 4 dB on-chip losses at the operating point.

Slope efficiency and spurious-free dynamic range of silicon Mach-Zehnder modulator upon carrier depletion and injection effects

We investigate the performances of a silicon PN-junction Mach-Zehnder modulator for analog application. The slope efficiency and spurious-free dynamic range (SFDR) of such a modulator upon carrier depletion and carrier injection effects are characterized and compared. Input RF frequency-dependence measurements show that the depletion-type modulator is usually with ~20 dB ∙ Hz(2/3) higher SFDR comparing to the injection-type modulator, yet with an order-of-magnitude lower slope efficiency. For the depletion-type and injection-type modulators, the measured maximum SFDRs are respectively ~95 dB ∙ Hz(2/3) and 75 dB∙Hz(2/3), with maximum slope efficiency of 0.3 V(-1) and 8 V(-1<). We numerically model the SFDR by using the experimentally extracted effective refractive index change, which shows good agreement with the measurements.

High-speed, low-loss silicon Mach-Zehnder modulators with doping optimization

We demonstrate a high-speed silicon Mach-Zehnder modulator (MZM) with low insertion loss, based on the carrier depletion effect in a lateral PN junction. A 1.9 dB on-chip insertion loss and a VπLπ < 2 V·cm were achieved in an MZM with a 750 μm-long phase shifter by properly choosing the doping concentration and precisely locating the junction. High-speed modulations up to 45-60 Gbit/s have been demonstrated with an additional 1.6 dB optical loss, indicating a total insertion loss of 3.5 dB. A high extinction ratio of 7.5 dB was also realized at the bit rate of 50 Gbit/s with an acceptable insertion loss of 6.5 dB.

Compact and fast photonic crystal silicon optical modulators

We demonstrate the first sub-100 μm silicon Mach-Zehnder modulators (MZMs) that operate at >10 Gb/s, by exploiting low-dispersion slow-light in lattice-shifted photonic crystal waveguides (LSPCWs). We use two LSPCW-MZM structures, one with LSPCWs in both arms of the MZM, and the other with an LSPCW in only one of the arms. Using the first structure we demonstrate 10 Gb/s operation with a operating bandwidth of 12.5 nm, in a device with a phase-shifter length of only 50 μm. Using the second structure, owing to a larger group index as well as lower spectral noise, we demonstrate 40 Gb/s operation with a phase-shifter length of only 90 μm, which is more than an order-of-magnitude shorter than most 40 Gb/s MZMs.

High-contrast 40 Gb/s operation of a 500 μm long silicon carrier-depletion slow wave modulator

In this Letter, we demonstrate a highly efficient, compact, high-contrast and low-loss silicon slow wave modulator based on a traveling-wave Mach-Zehnder interferometer with two 500 μm long slow wave phase shifters. 40  Gb/s operation with 6.6 dB extinction ratio at quadrature and with an on-chip insertion loss of only 6 dB is shown. These results confirm the benefits of slow light as a means to enhance the performance of silicon modulators based on the plasma dispersion effect.