Low V(π) Silicon photonics modulators with highly linear epitaxially grown phase shifters

Integrated barium titanate electro-optic modulators operating at CMOS-compatible voltage

We propose monolithically integrated electro-optical modulators based on thin-film x-cut barium titanate that exhibit large modulation bandwidth and operate at voltages compatible with complementary metal-oxide-semiconductor technology. The optical and radio frequency parameters of the modulator are systematically simulated, calculated, and optimized, respectively. Our simulation includes the evaluation of single-mode conditions, the separation distance between the electrode edge and the waveguide edge, bending loss, optical field distribution, and half-wave voltage-length product for optical parameters, and characteristic impedance, attenuation constant, radio frequency effective index, and -3d B modulation bandwidth for radio frequency parameters. By engineering both the microwave and photonic circuits, we have achieved high electro-optical efficiencies and group-velocity matching simultaneously. Our numerical simulation and theoretical analysis show that the half-wave voltage-length product was 0.48 V·cm, and the -3d B modulation bandwidths with a device length of 5 mm and 10 mm were 262 GHz and 107 GHz, respectively. Overall, our study highlights the potential of the proposed modulators for low driving voltage and high-performance optical communication systems.

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.

Compact thin film lithium niobate folded intensity modulator using a waveguide crossing

A small footprint, low voltage and wide bandwidth electro-optic modulator is critical for applications ranging from optical communications to analog photonic links, and the integration of thin-film lithium niobate with photonic integrated circuit (PIC) compatible materials remains paramount. Here, a hybrid silicon nitride and lithium niobate folded electro-optic Mach Zehnder modulator (MZM) which incorporates a waveguide crossing and 3 dB multimode interference (MMI) couplers for splitting and combining light is reported. This modulator has an effective interaction region length of 10 mm and shows a DC half wave voltage of roughly 4.0 V, or a modulation efficiency (Vπ ·L) of roughly 4 V·cm. Furthermore, the device demonstrates a power extinction ratio of roughly 23 dB and shows .08 dB/GHz optical sideband power roll-off with index matching fluid up to 110 GHz, with a 3-dB bandwidth of 37.5 GHz.

Heterogeneously-Integrated Optical Phase Shifters for Next-Generation Modulators and Switches on a Silicon Photonics Platform: A Review

The realization of a silicon optical phase shifter marked a cornerstone for the development of silicon photonics, and it is expected that optical interconnects based on the technology relax the explosive datacom growth in data centers. High-performance silicon optical modulators and switches, integrated into a chip, play a very important role in optical transceivers, encoding electrical signals onto the light at high speed and routing the optical signals, respectively. The development of the devices is continuously required to meet the ever-increasing data traffic at higher performance and lower cost. Therefore, heterogeneous integration is one of the highly promising approaches, expected to enable high modulation efficiency, low loss, low power consumption, small device footprint, etc. Therefore, we review heterogeneously integrated optical modulators and switches for the next-generation silicon photonic platform.

Low Vπ thin-film lithium niobate modulator fabricated with photolithography

Thin-film lithium niobate (TFLN) modulators are expected to be an ideal solution to achieve a super-wide modulation bandwidth needed by the next-generation optical communication system. To improve the performance, especially to reduce the driving voltage, we have carried out a detailed design of the TFLN push-pull modulator by calculating 2D maps of the optical losses and V_π for different ridge waveguide depths and electrode gaps. Afterwards the modulator with travelling wave electrodes was fabricated through i-line photolithography and then characterized. The measured V_π for a modulator with 5-mm modulation arm length is 3.5 V, corresponding to voltage-length product of 1.75 V·cm, which is the lowest among similar modulators as far as we know. And the measured electro-optic response has a 3-dB bandwidth beyond 40 GHz, which is the limitation of our measurement capability. The detailed design, fabrication and measurement results are presented.

Verified equivalent-circuit model for slot-waveguide modulators

We formulate and experimentally validate an equivalent-circuit model based on distributed elements to describe the electric and electro-optic (EO) properties of travelling-wave silicon-organic hybrid (SOH) slot-waveguide modulators. The model allows to reliably predict the small-signal EO frequency response of the modulators exploiting purely electrical measurements of the frequency-dependent RF transmission characteristics. We experimentally verify the validity of our model, and we formulate design guidelines for an optimum trade-off between optical loss due to free-carrier absorption (FCA), electro-optic bandwidth, and π-voltage of SOH slot-waveguide modulators.

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

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

Microwave Photonic ICs for 25 Gb/s Optical Link Based on SiGe BiCMOS Technology

The design, simulation and experimental results of the integrated optical and electronic components for 25 Gb/s microwave photonic link based on a 0.25 µm SiGe:C BiCMOS technology process are presented. A symmetrical depletion-type Mach-Zehnder modulator (MZM) and driver amplifier are intended for electro-optical (E/O) integrated transmitters. The optical divider and combiner of MZM are designed based on the self-imaging theory and then simulated with EM software. In order to verify the correctness of the theory and material properties used in the simulation, a short test (prototype) MZM of 1.9 mm length is produced and measured. It shows an extinction ratio of 19 dB and half-wave voltage-length product of Vπ ∙ L = ~1.5 V·cm. Based on these results, the construction of the segmented modulator with several driver amplifier units is defined. The designed driver amplifier unit provides a bandwidth of more than 30 GHz, saturated output power of 6 dBm (output voltage of Vpp = 1.26 V), and matching better than −15 dB up to 35 GHz; it dissipates 170 mW of power and occupies an area of 0.4 × 0.38 mm2. The optical-electrical (O/E) receiver consists of a Ge-photodiode, transimpedance amplifier (TIA), and passive optical structures that are integrated on a single chip. The measured O/E 3 dB analog bandwidth of the integrated receiver is 22 GHz, and output matching is better than −15 dB up to 30 GHz, which makes the receiver suitable for 25 Gb/s links with intensity modulation. The receiver operates at 1.55 μm wavelength, uses 2.5 V and 3.3 V power supplies, dissipates 160 mW of power, and occupies an area of 1.46 × 0.85 mm2.

Compact wavelength- and bandwidth-tunable microring modulator

Fabrication errors currently hold back the large-scale adoption of silicon micro-ring modulators (MRMs). The ability to correct their spectral features post-fabrication is required to enable their commercialization. Here, we report and demonstrate an MRM that uses a tunable two-point coupling scheme, which maintains the MRM's compact footprint (60 µm×45 µm) and allows one to tune the MRM's operating wavelength and adjust the optical bandwidth (and/or extinction ratio). This means that one can compensate for fabrication errors and thereby improve the yields. We confirm the modulator's operation by showing NRZ and PAM-4 modulation, up to 28 Gb/s and 19.9 Gb/s, respectively. Also, the proposed tunable MRM maintains the microring's free-spectral range (FSR), which proves its compatibility for configurable and high-bandwidth DWDM applications.

Experimental quantification of the free-carrier effect in silicon waveguides at extended wavelengths

We examine the electro-optic effect at wavelengths ranging from 1.31 to 2.02 μm for: (1) an Electronic Variable Optical Attenuator (EVOA); and (2) a Micro-Ring Resonator (MRR). For the EVOA, simulations were performed to ascertain the relationship between free-carrier concentration and optical attenuation, and are in agreement with our observation of an increase in attenuation with increasing wavelength. MRRs were fabricated for use around wavelengths of 2 μm to explore the sensitivity of operation to bus-to-ring coupling gap and p-n junction offset. Trends observed in the experiment are replicated by simulation, calibrated using the observations of the EVOA operation. The previously proposed efficiency increase of operation around 2 μm compared to more traditional wavelengths is demonstrated. Future development of devices for these wavelengths, supported by amplification using Thulium Doped Fiber Amplifier (TDFA) technology, is a promising route to aid in the alleviation of increasing demands on communication networks.

Phase tuning by length contraction

Typical integrated optical phase tuners alter the effective index. In this paper, we explore tuning by geometric deformation. We show that tuning efficiency, Vπ L, improves as the device size shrinks down to the optimal bend radius, contrary to conventional index-shift based approaches where Vπ L remains constant. We demonstrate that this approach is capable of ultra-low power tuning across a full FSR in a low-confinement silicon nitride based ring resonator of 580 μm radius. We demonstrate record performance with V_FSR = 16 V, Vπ L = 3.6 V dB, Vπ Lα = 1.1 V dB, tuning current below 10 nA, and unattenuated tuning response up to 1 MHz. We also present optimized designs for high confinement silicon nitride and silicon based platforms with radius down to 80 μm and 45 μm, respectively, with performance well beyond current state-of-the-art. Applications include narrow-linewidth tunable diode lasers for spectroscopy and non-linear optics, optical phased array beamforming networks for RF antennas and LIDAR, and optical filters for WDM telecommunication links.

Coherent modulation up to 100 GBd 16QAM using silicon-organic hybrid (SOH) devices

We demonstrate the generation of higher-order modulation formats using silicon-based inphase/quadrature (IQ) modulators at symbol rates of up to 100 GBd. Our devices exploit the advantages of silicon-organic hybrid (SOH) integration, which combines silicon-on-insulator waveguides with highly efficient organic electro-optic (EO) cladding materials to enable small drive voltages and sub-millimeter device lengths. In our experiments, we use an SOH IQ modulator with a π-voltage of 1.6 V to generate 100 GBd 16QAM signals. This is the first time that the 100 GBd mark is reached with an IQ modulator realized on a semiconductor substrate, leading to a single-polarization line rate of 400 Gbit/s. The peak-to-peak drive voltages amount to 1.5 V_pp, corresponding to an electrical energy dissipation in the modulator of only 25 fJ/bit.

High-efficiency strip-loaded waveguide based silicon Mach-Zehnder modulator with vertical p-n junction phase shifter

We demonstrate a silicon Mach-Zehnder modulator (MZM) based on hydrogenated amorphous silicon (a-Si:H) strip-loaded waveguides on a silicon on insulator (SOI) platform, which can be fabricated by using a complementary metal-oxide semiconductor (CMOS) compatible process without half etching of the SOI layer. Constructing a vertical p-n junction in a flat etchless SOI layer provides superior controllability and uniformity of carrier profiles. Moreover, the waveguide structure based on a thin a-Si:H strip line can be fabricated easily and precisely. Thanks to a large overlap between the depletion region and optical field in the SOI layer with a vertical p-n junction, the MZM provides 0.80- to 1.86-Vcm modulation efficiency and a 12.1- to 16.9-dBV loss-efficiency product, besides guaranteeing a 3-dB bandwidth of about 17 GHz and 28-Gbps high-speed operation. The αV_πL is considerably lower than that of conventional high-speed modulators.

Silicon photonic transceiver circuit for high-speed polarization-based discrete variable quantum key distribution

We demonstrate a silicon photonic transceiver circuit for high-speed discrete variable quantum key distribution that employs a common structure for transmit and receive functions. The device is intended for use in polarization-based quantum cryptographic protocols, such as BB84. Our characterization indicates that the circuit can generate the four BB84 states (TE/TM/45°/135° linear polarizations) with >30 dB polarization extinction ratios and gigabit per second modulation speed, and is capable of decoding any polarization bases differing by 90° with high extinction ratios.

High-speed resonantly enhanced silicon photonics modulator with a large operating temperature range

We present a novel resonant Mach-Zehnder modulator whose arms are each loaded with five identical resonators. Size and power consumption are aggressively reduced compared to conventional modulators based on linear phase shifters. At the same time, a large optical bandwidth of 3.8 nm is maintained. We experimentally demonstrate open eye diagrams at 30 Gbps with a signal Q-factor remaining within a factor of 2 of its peak value in an operational temperature range spanning 55°C.

Dual phase-shift Bragg grating silicon photonic modulator operating up to 60 Gb/s

We demonstrate PAM-4 and OOK operation of a novel silicon photonic modulator. The modulator design is based on two phase-shifts in a Bragg Grating structure driven in a push pull configuration. Back-to-back PAM-4 modulation is demonstrated below the FEC threshold at up to 60 Gb/s. OOK modulation is also shown up to 55 Gb/s with MMSE equalization and up to 50 Gb/s without equalization. Eye diagrams and BER curves at different bit rates are provided for both PAM-4 and OOK modulations. To our knowledge, this structure is the fastest silicon photonic modulator based on Bragg gratings, reaching modulation speed comparable to the fastest Mach-Zehnder modulators and micro-ring modulators.