Electro-optic Phase Shifters Research Articles

CMOS-compatible, AlScN-based integrated electro-optic phase shifter

Abstract Commercial production of integrated photonic devices is limited by scalability of desirable material platforms. We explore a relatively new photonic material, AlScN, for its use in electro-optic phase shifting and modulation. Its CMOS-compatibility could facilitate large-scale production of integrated photonic modulators, and it exhibits an enhanced second-order optical nonlinearity compared to intrinsic AlN, indicating the possibility for efficient modulation. Here, we measure the electro-optic effect in Al0.80Sc0.20N-based phase shifters. We utilized the TM0 mode, allowing use of the r 33 electro-optic coefficient, and demonstrated V π L around 750 V cm. Since the electro-optic response is smaller than expected, we discuss potential causes for the reduced response and future outlook for AlScN-based photonics.

Hybrid 3C-silicon carbide-lithium niobate integrated photonic platform

In this paper, we demonstrate a novel hybrid 3C-silicon carbide-lithium niobate (3C-SiC-LN) platform for passive and active integrated nanophotonic devices enabled through wafer bonding. These devices are fabricated by etching the SiC layer, with the hybrid optical mode power distributed between SiC and LN layers through a taper design. We present a racetrack resonator-based electro-optic (EO) phase shifter where the resonator is fabricated in SiC while using LN for EO-effect (r33≈ 27 pm/V). The proposed phase shifter demonstrates efficient resonance wavelength tuning with low voltage-length product (Vπ.Lπ ≈ 2.18 V cm) using the EO effect of LN. This hybrid SiC-LN platform would enable high-speed, low-power, and miniaturized photonic devices (e.g., modulators, switches, filters) operable over a broad range of wavelengths (visible to infrared) with applications in both classical and quantum nanophotonics.

Deep neural network-based phase calibration in integrated optical phased arrays

Calibrating the phase in integrated optical phased arrays (OPAs) is a crucial procedure for addressing phase errors and achieving the desired beamforming results. In this paper, we introduce a novel phase calibration methodology based on a deep neural network (DNN) architecture to enhance beamforming in integrated OPAs. Our methodology focuses on precise phase control, individually tailored to each of the 64 OPA channels, incorporating electro-optic phase shifters. To effectively handle the inherent complexity arising from the numerous voltage set combinations required for phase control across the 64 channels, we employ a tandem network architecture, further optimizing it through selective data sorting and hyperparameter tuning. To validate the effectiveness of the trained DNN model, we compared its performance with 20 reference beams obtained through the hill climbing algorithm. Despite an average intensity reduction of 0.84 dB in the peak values of the beams compared to the reference beams, our experimental results demonstrate substantial agreements between the DNN-predicted beams and the reference beams, accompanied by a slight decrease of 0.06 dB in the side-mode-suppression-ratio. These results underscore the practical effectiveness of the DNN model in OPA beamforming, highlighting its potential in scenarios that necessitate the intelligent and time-efficient calibration of multiple beams.

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.

Compact and Low Power-Consumption Solid-State Two-Dimensional Beam Scanner Integrating a Passive Optical Phased Array and Hybrid Wavelength-Tunable Laser Diode

Large-scale optical phased arrays have attracted attention as devices for solid-state beam scanning employing light detection and ranging. However, conventional optical phased arrays using electro-optic phase shifters require active phase control for each antenna. Because the power consumption of the active phase shifters is proportional to the number of antennas, the total power consumption increases in a large-scale optical phased array with a narrow beam divergence. In this study, we propose a passive beam scanner that realizes two-dimensional beam scanning through the wavelength sweep of a laser light. Additionally, most studies on optical phased arrays use external laser diodes as light sources. This increases the size of the beam-scanning system. Therefore, there is a demand for integrating an optical phased array and a laser light source in a combined chip. In this study, we integrated a 64-channel passive optical phased array and a hybrid wavelength-tunable laser diode into a combined chip. The size of the device was 5 mm × 1.5 mm, and the power consumption for beam scanning was 49.3 mW. The field of view was 44.2° × 13.7°, and the full width at half maximum of the beam divergence was 0.517° × 3.67°. Compared with the device used in a previous study, our device was nine times smaller, and the total power consumption was only tens of milliwatts. This device consumes less power and achieves a fully integrated light detection and ranging.

Ultra-Compact and Efficient Microheaters on a Submicron-Thick InP Membrane

Energy-efficient electro-optic phase shifters have been widely adopted in the InP material system, but their lengths have been in millimeter scales. The strong thermo-optic effect of InP ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dn/dT = 2.0 \times 10^{-4}\;\text {K}^{-1}$</tex-math></inline-formula> ) can potentially enable compact devices, but it has been hindered by the large heat capacity of the bulky waveguide and inefficient heat transfer to the optical mode. By moving to a sub-micron-thick InP membrane, where the optical modes are more exposed and heat capacity minimized, we report here, to the best of our knowledge, first ultra-compact ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$97\;\mu \text {m}^{2}$</tex-math></inline-formula> footprint), efficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2.26\;\text {mW}/\pi$</tex-math></inline-formula> ) and broadband (100 nm) thermo-optic microheaters in the InP material system. An epitaxially grown layer as part of the waveguide is used as the heating element, enabling maximum heat transfer efficiency. The epitaxial layer structure is designed to be compatible with the membrane photonics platform that includes monolithic amplifiers and lasers. This work could contribute to the further miniturization of monolithic photonic integrated circuits (PIC) that involves phase shifters and amplifiers, and facilitate applications such as optical computing, switching, and sensing.

High-bandwidth thermo-optic phase shifters for lithium niobate-on-insulator photonic integrated circuits.

Phase shifters are key components of large-scale photonic integrated circuits. For the lithium niobate-on-insulator (LNOI) platform, thermo-optic phase shifters (TOPS) have emerged as a more stable and compact alternative to common electro-optic phase shifters (EOPSs), which are prone to anomalous behavior and drifting at low frequencies. Here, we model and experimentally characterize the influence of geometry on the performance of metal strip TOPSs. Compared to EOPSs, a 10-fold reduction of the voltage-length product is measured and bandwidths beyond 100 kHz are demonstrated, while keeping the footprint as low as 0.04 mm2. This shows the potential of TOPSs as small-scale building blocks for stable tuning and switching in LNOI photonic circuits.

Racetrack microresonator based electro-optic phase shifters on a 3C silicon-carbide-on-insulator platform.

We report, to the best of our knowledge, the first demonstration of integrated electro-optic (EO) phase shifters based on racetrack microresonators on a 3C silicon-carbide-on-insulator (SiCOI) platform working at near-infrared wavelengths. By applying DC voltage in the crystalline axis perpendicular to the waveguide plane, we have observed optical phase shifts from the racetrack microresonators whose loaded quality ($ Q $) factors are $\sim\! {30,\!000}$. We show voltage-length product (${{V}_{\pi}} \cdot {{L}_{ \pi}}$) of ${118}\;{{\rm V}\cdot{\rm cm}}$, which corresponds to an EO coefficient ${{r}_{41}}$ of 2.6 pm/V. The SiCOI platform can be used to realize tunable silicon carbide integrated photonic devices that are desirable for applications in nonlinear and quantum photonics over a wide bandwidth that covers visible and infrared wavelengths.

Comprehensive Model for Evaluating the Performance of Mach-Zehnder-Based Silicon Photonic Switch Fabrics in Large Scale

Building a large-scale Mach-Zehnder-based silicon photonic switch circuit (LS-MZS) requires an appropriate choice of architecture. In this work, we propose, for the first time to our knowledge, a single metric that can be used to compare different topologies. We propose an accurate analytical model of the signal-to-crosstalk ratio (SCR) that highlights the performance limitations of the main building blocks: Mach-Zehnder interferometers (MZI) and waveguide crossings. It is based on the cumulative crosstalk and total insertion loss of the LS-MZS. Four different architectures: Beneš, dilated Beneš, switch and select, double-layer network were studied for the reason that they are mainly referenced in the literature. We compared them using our developed SCR indicator. With reference to the state-of-the-art technology, the analysis of the four architectures using SCR showed that, on a large scale, a high number of waveguide crossings significantly affects the performance of the switch matrix. Moreover, better performance was reached using the double-layer-network architecture. Then, we presented a 2 × 2 MZI using two electro-optic phase shifters and a waveguide crossing realized in LETI’s silicon photonics technology. Measured performances were quite good: the switch circuit had a crosstalk of −31.3 dB and an insertion loss estimated to be less than 1.31 dB.

Digital Demodulation Using I/Q Signals and Optical Phase Control Applied to a Vibrometer

An efficient and cost effective optical phase detection vibrometer based on a modified closed loop homodyne Michelson interferometer is presented. Real-time phase demodulation is carried out, using an embedded platform that performs data acquisition, signal processing, PI (proportional-integral) control and the generation of signals that drive the electrooptic Pockels cell phase shifter and the piezoelectric actuator under test. Two phase quadrature signals are generated from a single interferometric output, using the interleaving action, in alternation, of a digitally generated modulating signal, and then the well-known differential-cross-multiplication technique is applied to perform the computation of the phase shift of interest. The quadrature condition is reached using the PI loop based on an error signal obtained from a Lissajous figure derived from out-of-phase signals. The vibrometer is capable of measuring nanometric displacements, and is simple, inexpensive, accurate, immune to fading and self-consistent. The new method was used to determine the displacement frequency response curves of two prototypes of multi-actuated flextensional piezoelectric actuators. Measurements were made between 500 Hz and 15 kHz, and the results agreed with those obtained by the standard SCM-Signal Coincidence Method.

Wide-Angle Beam-Steering Using an Optical Phased Array with Non-Uniform-Width Waveguide Radiators

We demonstrate wide-angle beam-steering using an optical phased array (OPA) with waveguide radiators designed with non-uniform widths to reduce the crosstalk between waveguides. The OPA consists of a silicon based 1 × 16 array of electro-optic phase shifters and end-fire radiators. The 16 radiators were configured with four different widths and a half-wavelength spacing, which can remove the higher-order diffraction patterns in free space. The waveguides showed a low crosstalk of −10.2 dB at a wavelength of 1540 nm. With phase control, the OPA achieved wide beam-steering of over ±80° with a side-lobe suppression of 7.4 dB.

Wide‐angle 2D beam‐steering with Si‐based 16 × (1 × 16) optical phased arrays

The authors demonstrate integrated 1D optical phased arrays (OPAs) for wide-angle 2D beam-steering. The 2D OPA consists of 16 1D transversal steering OPAs whose radiators have different grating periods, to assign different longitudinal angles. The transversal steering of each OPA is performed by the phase control of the p-i-n electro-optic phase shifter array. The 1D transversal steering OPAs operate serially, changing the longitudinal radiation angle. The grating radiators were designed with an array pitch of 2 μm, and a varying period in the range of 610-710 nm. The entire 16 × (1 × 16) OPAs were integrated into a small area of 26.5 mm 2 . The obtained transversal/longitudinal beam-steering range was 46°/44°, respectively, with a wavelength of 1560 nm, which enables wide 2D beam-steering.

Tolerant, broadband tunable 2 × 2 coupler circuit

We propose a circuit design for a broadband tunable 2 × 2 waveguide coupler, consisting of a two-stage Mach-Zehnder interferometer with electro-optic phase shifters in each stage. We demonstrate that such design can be configured as a tunable coupler with arbitrary coupling ratio and with a uniform response over 50-nm spectral range around 1550 nm. The design is also tolerant to fabrication variations that affect the coupling ratios of the directional couplers.

Silicon-Based Optical Phased Array Using Electro-Optic $p$ -$i$ -$n$ Phase Shifters

We present a $1\times 16$ silicon optical phased array using electro-optic phase shifters to attain high-speed operation with low power consumption. The phase shifters are constructed using a $p$ - $i$ - $n$ junction structure with the $p$ - and $n$ -doped regions formed on both sides of the $i$ -region. The $i$ -region width of the phase shifter is optimized considering the power consumption for phase-tuning and the propagation loss in the phase shifter. The fabricated $p$ - $i$ - $n$ phase shifter exhibits a fast operating speed of 20 MHz and a low phase-tuning power of 1.7 mW/ $\pi $ . From a 1-D optical phased array integrated with 2- $\mu \text{m}$ -pitch grating radiators, a wide beam-steering range of 45° is attained along the transversal direction at a $1.55~\mu \text{m}$ wavelength. Average power consumption for the beam-forming operation of the $1\times 16$ OPA is measured to be 39.6 mW and the average transition time to steer the beam is 24 ns.

A $4$ × $4$ Electrooptic Silicon Photonic Switch Fabric With Net Neutral Insertion Loss

We present a strictly nonblocking $4$ × $4$ electrooptic silicon photonic switch fabric with on-chip gain. The switch integrates 12 Mach-Zehnder cells in a 3-stage topology equipped with fast electrooptic phase shifters, and thermooptic phase trimmers. To compensate for the losses of the fabric, a 4-channel GaInAsP/InP semiconductor optical amplifier array is flip-chip attached into an etched cavity in the silicon photonic chip with butt-coupled waveguide interfaces. The chip is wirebonded to a CMOS driver that provides push-pull drive to each elementary Mach-Zehnder cell. We demonstrate an optical switch assembly with net neutral insertion loss in the C-band together with nanosecond-scale reconfiguration time.

Silicon Photonics towards Disaggregation of Resources in Data Centers

In this paper, we demonstrate two subsystems based on Silicon Photonics, towards meeting the network requirements imposed by disaggregation of resources in Data Centers. The first one utilizes a 4 × 4 Silicon photonics switching matrix, employing Mach Zehnder Interferometers (MZIs) with Electro-Optical phase shifters, directly controlled by a high speed Field Programmable Gate Array (FPGA) board for the successful implementation of a Bloom-Filter (BF)-label forwarding scheme. The FPGA is responsible for extracting the BF-label from the incoming optical packets, carrying out the BF-based forwarding function, determining the appropriate switching state and generating the corresponding control signals towards conveying incoming packets to the desired output port of the matrix. The BF-label based packet forwarding scheme allows rapid reconfiguration of the optical switch, while at the same time reduces the memory requirements of the node’s lookup table. Successful operation for 10 Gb/s data packets is reported for a 1 × 4 routing layout. The second subsystem utilizes three integrated spiral waveguides, with record-high 2.6 ns/mm2, delay versus footprint efficiency, along with two Semiconductor Optical Amplifier Mach-Zehnder Interferometer (SOA-MZI) wavelength converters, to construct a variable optical buffer and a Time Slot Interchange module. Error-free on-chip variable delay buffering from 6.5 ns up to 17.2 ns and successful timeslot interchanging for 10 Gb/s optical packets are presented.

High speed ultra-broadband amplitude modulators with ultrahigh extinction >65 dB.

We experimentally demonstrate ultrahigh extinction ratio (>65 dB) amplitude modulators (AMs) that can be electrically tuned to operate across a broad spectral range of 160 nm from 1480 - 1640 nm and 95 nm from 1280 - 1375 nm. Our on-chip AMs employ one extra coupler compared with conventional Mach-Zehnder interferometers (MZI), thus form a cascaded MZI (CMZI) structure. Either directional or adiabatic couplers are used to compose the CMZI AMs and experimental comparisons are made between these two different structures. We investigate the performance of CMZI AMs under extreme conditions such as using 95:5 split ratio couplers and unbalanced waveguide losses. Electro-optic phase shifters are also integrated in the CMZI AMs for high-speed operation. Finally, we investigate the output optical phase when the amplitude is modulated, which provides us valuable information when both amplitude and phase are to be controlled. Our demonstration not only paves the road to applications such as quantum information processing that requires high extinction ratio AMs but also significantly alleviates the tight fabrication tolerance needed for large-scale integrated photonics.

Ultralinear heterogeneously integrated ring-assisted Mach–Zehnder interferometer modulator on silicon

A linear modulator is indispensable for radio frequency photonics or analog photonic link applications where high dynamic range is required. There is also great interest to integrate the modulator with other photonic components, to create a photonic integrated circuit for these applications, with particular focus on silicon photonics integration in order to take advantage of complementary metal–oxide–semiconductor compatible foundries for high-volume, low-cost devices. However, all silicon modulators, including the highest performing Mach–Zehnder interferometer (MZI) type, have poor linearity, partially due to the inherent nonlinearity of the MZI transfer characteristic, but mostly due to the nonlinearity of silicon’s electro-optic phase shift response. In this work, we demonstrate ultralinear ring-assisted MZI (RAMZI) modulators, incorporating heterogeneously integrated III–V multiple quantum wells on silicon phase modulation sections to eliminate the nonlinear silicon phase modulation response. The heterogeneously integrated III–V/Si RAMZI modulators achieve record-high spurious free dynamic range (SFDR) for silicon-based modulators, as high as 117.5 dB·Hz2/3 at 10 GHz for a weakly coupled ring design, and 117 dB·Hz2/3 for a strongly coupled ring design with higher output power. This is a higher SFDR than typically obtained with commercial lithium niobate modulators. This approach advances integrated modulator designs on silicon for applications in compact and high-performance analog optical systems.

Low Loss Electro-Optic Polymer Based Fast Adaptive Phase Shifters Realized in Silicon Nitride and Oxynitride Waveguide Technology

We present a comprehensive study on how to design and fabricate low loss electro-optic phase shifters based on an electro-optic polymer and the silicon nitride and silicon oxynitride waveguide material systems. The loss mechanisms of phase shifters with an electro-optic (EO) polymer cladding are analyzed in detail and design solutions to achieve lowest losses are presented. In order to verify the low loss design a proof of concept prototype phase shifter was fabricated, which exhibits an attenuation of 0.8 dB/cm at 1550 nm and an electro-optic efficiency factor of 27%. Furthermore, the potential of this class of phase shifters is evaluated in numerical simulations, from which the optimal design parameters and achievable figures of merit were derived. The presented phase shifter design has its potential for application in fast adaptive multi stage devices for optical signal processing.

Aluminum nitride electro-optic phase shifter for backend integration on silicon.

An AlN electro-optic phase shifter with a parallel plate capacitor structure is fabricated on Si using the back-end complementary metal-oxide-semiconductor technology, which is feasible for multilayer photonics integration. The modulation efficiency (Vπ⋅Lπ product) measured from the fabricated waveguide-ring resonators and Mach-Zehnder Interferometer (MZI) modulators near the 1550-nm wavelength is ∼240 V⋅cm for the transverse electric (TE) mode and ∼320 V⋅cm for the transverse magnetic (TM) mode, from which the Pockels coefficient of the deposited AlN is deduced to be ∼1.0 pm/V for both TE and TM modes. The methods for further modulation efficiency improvement are addressed.