Butler matrix enabled multi-beam optical phased array for two-dimensional beam-steering and ranging

Based on the wavelength transparency of the Butler matrix(BM)beamforming network,we demonstrate a multibeam optical phased array(MOPA)with an emitting aperture composed of grating couplers at a 1.55μm pitch for wavelength-assisted two-dimensional beam-steering.The device is capable of simultaneous multi-beam operation in a field of view(FOV)of 60°×8°in the phased-array scanning axis and the wavelength-tuning scanning axis,respectively.The typical beam divergence is about 4°on both axes.Using multiple linearly chirped lasers,multibeam frequency-modulated continuous wave(FMCW)ranging is realized with an average ranging error of 4 cm.A C-shaped target is imaged for proof-of-concept 2D scanning and ranging.

Nonvolatile waveguide transmission tuning with electrically-driven ultra-small GST phase-change material

Low-power reconfigurable optical circuits are highly demanded to satisfy a variety of different applications. Conventional electro-optic and thermo-optic refractive index tuning methods in silicon photonics are not suitable for reconfiguration of optical circuits due to their high static power consumption and volatility. We propose and demonstrate a nonvolatile tuning method by utilizing the reversible phase change property of GST integrated on top of the silicon waveguide. The phase change is enabled by applying electrical pulses to the lm-sized GST active region in a sandwich structure. The experimental results show that the optical transmission of the silicon waveguide can be tuned by controlling the phase state of GST.

16 × 16 silicon Mach–Zehnder interferometer switch actuated with waveguide microheaters

We experimentally demonstrate a 16 × 16 reconfigurably nonblocking optical switch fabric using a Benes architecture. The switch fabric consists of 56 2 × 2 Mach–Zehnder interferometer based elementary switches, with each integrated with a pair of waveguide microheaters. The average on-chip insertion loss is ~5.2 dB for both of the 'all-cross' and the 'all-bar' states, with a loss variation of 1 dB over all routing paths. The cross talk for all switching states is better than -30 d B. The switching time of the switch element is about 22 μs. The switching functionality is verified by transmission of a 40 Gb∕s quadrature phase-shift keying optical signal.

Ultra-low-loss multi-layer 8×8 microring optical switch

Microring-based optical switches are promising for wavelength-selective switching with the merits of compact size and low power consumption.However,the large insertion loss,the high fabrication,and the temperature sensitivity hinder the scalability of silicon microring optical switch fabrics.In this paper,we utilize a three-dimensional(3D)microring-based optical switch element(SE)on a multi-layer Si_(3)N_(4)-on-SOI platform to realize highperformance large-scale optical switch fabrics.The 3D microring-based SE consists of a Si∕Si_(3)N_(4) waveguide overpass crossing in the bottom and the top layers,and Si_(3)N_(4) dual-coupled microring resonators(MRRs)in the middle layer.The switch is calibration-free and has low insertion loss.With the 3D microring-based SEs,we implement an 8×8 crossbar optical switch fabric.As the resonance wavelengths of all SEs are well aligned,only one SE needs to be turned on in each routing path,which greatly reduces the complexity of the switch control.The optical transmission spectra show a box-like shape,with a passband width of~69 GHz and an average on-state loss of~0.37 dB.The chip has a record-low on-chip insertion loss of 0.52-2.66 dB.We also implement a non-duplicate polarization-diversity optical switch by using the bidirectional transmission characteristics of the crossbar architecture,which is highly favorable for practical applications.100 Gb/s dual-polarization quadrature-phase-shift-keying(DP-QPSK)signal is transmitted through the switch without significant degradation.To the best of our knowledge,this is the first time that 3D MRRs have been used to build highly scalable polarization-diversity optical switch fabrics.

Low temperature sensitivity on-chip Fouriertransform spectrometer based on dual-layer Si_(3)N_(4)spiral waveguides

On-chip Fourier-transform spectrometers(FTSs)based on Mach–Zehnder interferometer(MZI)arrays suffer from severe central wavelength and fringe contrast variation due to fabrication errors.Even though a calibration matrix can be employed to correctly retrieve the input spectra,environmental temperature variation greatly degrades the retrieving performance.In this paper,we devise a dual-layer Si_(3)N_(4) waveguide interferometer to reduce the temperature sensitivity.The beating of the even and odd supermodes in the dual-layer waveguide generates periodic intensity fluctuations in the spectrum.Since these two modes have similar modal profiles,their thermal sensitivity and propagation loss are relatively balanced,leading to a low temperature sensitivity and a high interference extinction ratio.We designed and fabricated a passive FTS based on a 32-channel dual-layer Si_(3)N_(4) waveguide array.Experimental results show that the temperature sensitivity is reduced to 10 pm/°C,which is almost half that of single-layer Si_(3)N_(4) MZI-based FTSs.With this chip,we accurately reconstructed various types of optical spectra,including single and two sparse laser lines,and broadband optical spectra.Our method can fit a wide wavelength range,which is a promising technology to improve the practical applications of on-chip FTSs.

Integrated optical delay lines:a review and perspective[Invited]

Optical delay lines（ODLs） are one of the key enabling components in photonic integrated circuits and systems.They are widely used in time-division multiplexing, optical signal synchronization and buffering, microwave signal processing, beam forming and steering, etc. The development of integrated photonics pushes forward the miniaturization of ODLs, offering improved performances in terms of stability, tuning speed, and power consumption. The integrated ODLs can be implemented using various structures, such as single or coupled resonators, gratings, photonic crystals, multi-path switchable structures, and recirculating loop structures.The delay tuning in ODLs is enabled by either changing the group refractive index of the waveguide or changing the length of the optical path. This paper reviews the recent development of integrated ODLs with a focus on their abundant applications and flexible implementations. The challenges and potentials of each type of ODLs are pointed out.

4×4 nonblocking optical switch fabric based on cascaded multimode interferometers

We experimentally demonstrate a 4 × 4 nonblocking silicon thermo-optic(TO) switch fabric consisting of three stages of tunable generalized Mach–Zehnder interferometers. All 24 routing states for nonblocking switching are characterized. The device's footprint is 4.6 mm × 1.0 mm. Measurements show that the worst cross talk of all switching states is-7.2 dB. The on-chip insertion loss is in the range of 3.7–13.1 dB. The average TO switching power consumption is 104.8 mW.

Optical generation of UWB pulses utilizing Fano resonance modulation

In this paper,we reported an integrated method to generate ultra-wideband(UWB)pulses of different orders based on a reconfigurable silicon micro-ring resonator-coupled Mach–Zehnder interferometer.Under proper operating conditions,the device can produce Fano resonances with a peak-to-valley extinction ratio of above 20 dB.UWB monocycle and doublet signals with picosecond pulse widths are produced when the microring resonator is modulated by square and Gaussian electrical pulses,respectively.With our Fano resonance modulator on silicon photonics,it is promising to foresee versatile on-chip microwave signal generation.

Microwave frequency upconversion employing a coupling-modulated ring resonator

We present a method to generate a frequency-doubled microwave signal by employing a coupling-modulated ring resonator. Critical coupling is achieved when the resonator intrinsic loss is perfectly balanced by the external coupling enabled by a Mach–Zehnder interferometer coupler. The high suppression of the carrier leads to a clean two-tone optical signal with the frequency interval two times larger than that of the input microwave frequency.The beating of the two-tone signal at a photodiode generates the frequency upconverted microwave signal.A theoretical model is established to analyze the modulation process and the microwave signal generation.Experimental results show that the electrical harmonic suppression ratio is around ~20 dB(29 dB) for an input microwave signal with 5 dBm(10 dBm) power.

Continuous-variable quantum key distribution with on-chip light sources

Integrated quantum key distribution(QKD)systems based on photonic chips have high scalability and stability,and are promising for further construction of global quantum communications networks.On-chip quantum light sources are a critical component of a fully integrated QKD system;especially a continuous-variable QKD(CVQKD)system based on coherent detection,which has extremely high requirements for the light sources.Here,for what we believe is the first time,we designed and fabricated two on-chip tunable lasers for CV-QKD,and demonstrated a high-performance system based on these sources.Because of the high output power,fine tunability,and narrow linewidth,the involved on-chip lasers guarantee the accurate shot-noise-limited detection of quantum signals,center wavelength alignment of nonhomologous lasers,and suppression of untrusted excess noise.The system’s secret key rate can reach 0.75 Mb/s at a 50 km fiber distance,and the secure transmission distance can exceed 100 km.Our results mark a breakthrough toward building a fully integrated CV-QKD,and pave the way for a reliable and efficient terrestrial quantum-secure metropolitan area network.

Wavelength-mode pulse interleaver on the silicon photonics platform

We report an 8-channel wavelength-mode optical pulse interleaver on a silicon photonic chip.Wavelength-and mode-division multiplexing techniques are combined to increase the repetition rate of the pulses without adding the complexity of a single dimension.The interleaver uses a cascaded Mach–Zehnder interferometer architecture as a wavelength-division(de)multiplexer,an asymmetric directional coupler as a mode(de)multiplexer,and various lengths of silicon waveguides as delay lines.A pulse sequence with a time interval of 125 ps is implemented with the repetition rate being eight times that of the initial one.The demonstrated wavelength-mode multiplexing approach opens a new route for the generation of high-speed optical pulses.

Silicon non-blocking 4×4 optical switch with automated polarization adjustment

We demonstrate a polarization-insensitive silicon 4×4 optical switch based on Mach–Zehnder interferometer(MZI)switch elements.On-chip polarization controllers are integrated before the switch fabric to automatically adjust an arbitrary input polarization to the transverse electric mode.The 4×4 switch fabric is based on a dilated double-layer network architecture to completely cancel the first-order crosstalk.Thermo-optic phase shifters are integrated in the MZI switch elements and the polarization controllers for adjustment of the switching state and polarization,respectively.We develop a polarization control algorithm based on a gradient descent method for automated polarization control.The polarization recovery time is less than 4 ms,and the measured polarization-dependent loss is~2 d B.The scheme provides a new solution for realizing polarization-insensitive silicon optical switches.