Gate-tunable frequency combs in graphene-nitride microresonators.

Optical frequency combs, which emit pulses of light at discrete, equally spaced frequencies, are cornerstones of modern-day frequency metrology, precision spectroscopy, astronomical observations, ultrafast optics and quantum information1-7. Chip-scale frequency combs, based on the Kerr and Raman nonlinearities in monolithic microresonators with ultrahigh quality factors8-10, have recently led to progress in optical clockwork and observations of temporal cavity solitons11-14. But the chromatic dispersion within a laser cavity, which determines the comb formation15,16, is usually difficult to tune with an electric field, whether in microcavities or fibre cavities. Such electrically dynamic control could bridge optical frequency combs and optoelectronics, enabling diverse comb outputs in one resonator with fast and convenient tunability. Arising from its exceptional Fermi-Dirac tunability and ultrafast carrier mobility17-19, graphene has a complex optical dispersion determined by its optical conductivity, which can be tuned through a gate voltage20,21. This has brought about optoelectronic advances such as modulators22,23, photodetectors 24 and controllable plasmonics25,26. Here we demonstrate the gated intracavity tunability of graphene-based optical frequency combs, by coupling the gate-tunable optical conductivity to a silicon nitride photonic microresonator, thus modulating its second- and higher-order chromatic dispersions by altering the Fermi level. Preserving cavity quality factors up to 106 in the graphene-based comb, we implement a dual-layer ion-gel-gated transistor to tune the Fermi level of graphene across the range 0.45-0.65 electronvolts, under single-volt-level control. We use this to produce charge-tunable primary comb lines from 2.3 terahertz to 7.2 terahertz, coherent Kerr frequency combs, controllable Cherenkov radiation and controllable soliton states, all in a single microcavity. We further demonstrate voltage-tunable transitions from periodic soliton crystals to crystals with defects, mapped by our ultrafast second-harmonic optical autocorrelation. This heterogeneous graphene microcavity, which combines single-atomic-layer nanoscience and ultrafast optoelectronics, will help to improve our understanding of dynamical frequency combs and ultrafast optics.

High-uniformity and high-performance waveguide Ge photodetectors for the O and C bands.

This paper presents the test results for high-performance and high-uniformity waveguide silicon-based germanium (Ge) photodetectors (PDs) for the O band and C band. Both wafer-scale and chip-scale test results are provided. The fabricated lateral p-i-n (LPIN) PDs exhibit a responsivity of 0.97 A/W at a bias of -2V, a bandwidth of 60 GHz, and a no-return-to-zero (NRZ) eye diagram rate of 53.125 Gb/s. Additionally, an average dark current of 22.4 nA was obtained in the vertical p-i-n (VPIN) PDs at -2V by optimizing the doping process. The device can reach an average responsivity of 0.9 A/W in the O band. The standard deviation in a wafer with a dark current and responsivity is as low as 7.77 nA and 0.03 A/W at -2V, respectively.

PIC-integrable high-responsivity germanium waveguide photodetector in the C + L band.

We report the demonstration of a germanium waveguide p-i-n photodetector (PD) for the C + L band light detection. Tensile strain is transferred into the germanium layer using a SiN stressor on top surface of the germanium. The simulation and experimental results show that the trenches must be formed around the device, so that the strain can be transferred effectively. The device exhibits an almost flat responsivity with respect to the wavelength range from 1510 nm to 1630 nm, and high responsivity of over 1.1 A/W is achieved at 1625 nm. The frequency response measurement reveals that a high 3 dB bandwidth (f3dB) of over 50 GHz can be obtained. The realization of the photonic-integrated circuits (PIC)-integrable waveguide Ge PDs paves the way for future telecom applications in the C + L band.

High-speed compact folded Michelson interferometer modulator.

We propose and experimentally demonstrate a novel compact folded Michelson interferometer (FMI) modulator with high modulation efficiency. By folding the 0.5 mm-long phase shift arms, the length of the modulation area of the FMI modulator is only 0.25 mm. Meanwhile, the traveling wave electrode (TWE) is also shorter, which decreases the propagation loss of the RF signal and contributes to a small footprint. The Vπ-L of the present device is as low as 0.87 V·cm at -8 V bias voltage. The minimum optical insertion loss is 3.7 dB, and the static extinction ratio (ER) is over 25 dB. The measured 3-dB electro-optical (EO) bandwidth is 17.3 GHz at a -6 V bias. The OOK eye diagram up to 40 Gb/s is demonstrated under 2 V driver voltage.

80Gb/s NRZ Ge waveguide electro-absorption modulator.

We demonstrate a Ge electro-absorption modulator (EAM) in L band with a 3 dB electro-optical bandwidth beyond 67 GHz at -3 V bias voltage. The Eye diagram measurement shows a data rate of over 80 Gbps for non-return-to-zero on-off keying (NRZ-OOK) modulation at a voltage swing of 2.3 Vpp and the wavelength of 1605 nm. Through the comparison of multi-device results, it is proved that the introduction of the annealing process after CMP can increase the mean static extinction ratio of the EAM from 7.27 dB to 11.83 dB, which confirms the manufacturability of the device. The dynamic power consumption of the device is 6.348 fJ/bit. The performance of our device is comprehensive. The Ge EAM device also has excellent performance as a photodetector (PD) in the C and L communication bands. The responsivity of the device is 1.04 A/W at the wavelength of 1610 nm, resulting in ∼0.87 mW of static power consumption at -3 V bias voltage under 0.28 mW of optical input and the 3 dB opto-electric bandwidth of the devices are beyond 43 GHz at -3 V bias.

Silicon nitride assisted 1×64 optical phased array based on a SOI platform.

We demonstrate a 1×64 optical phased array (OPA) based on a silicon on insulator (SOI) platform with integrated silicon nitride. The input port of the OPA is fabricated using a silicon nitride waveguide due to its advantage of allowing more optical power. The phase shifter is a silicon waveguide with heater because of the higher thermo-optic coefficient of silicon. And a double layer silicon nitride assisted grating is used in the emitter to reduce the emission strength and then increase the length of emitter to reduce the spot size. The length of the grating emitter is 1.5 mm and the measured field of view of this optical phased array is 35.5°×22.7° with spot size of 0.69°×0.075°.

Multifunctional optoelectronic device based on graphene-coupled silicon photonic crystal cavities.

We present a hybrid device based on graphene-coupled silicon (Si) photonic crystal (PhC) cavities, featuring triple light detection, modulation, and switching. Through depositing single-layer graphene onto the PhC cavity, the light-graphene interaction can be enhanced greatly, which enables significant detection and modulation of the resonant wavelength. The device is designed to generate a photocurrent directly by the photovoltaic effect and has an external responsivity of ∼14 mA/W at 1530.8 nm (on resonance), which is about 10 times higher than that off-resonance. Based on the thermo-optical effect of silicon and graphene, the device is also demonstrated in electro-optical and all-optical modulation. Also, due to the high-quality (Q) factor of the resonate cavity, the device can implement low threshold optical bistable switching, and it promises a fast response speed, with a rise (fall) time of ∼0.4 µs (∼0.5 µs) in the all-optical switch and a rise (fall) time of ∼0.5 µs (∼0.5 µs) in the electro-optical hybrid switch. The multifunctional photodetector, modulator, and optical bistable switch are achieved in a single device, which greatly reduces the photonic overhead and provides potential applications for future integrated optoelectronics.

Nanometric Precision Distance Metrology via Hybrid Spectrally Resolved and Homodyne Interferometry in a Single Soliton Frequency Microcomb.

Laser interferometry serves a fundamental role in science and technology, assisting precision metrology and dimensional length measurement. During the past decade, laser frequency combs-a coherent optical-microwave frequency ruler over a broad spectral range with traceability to time-frequency standards-have contributed pivotal roles in laser dimensional metrology with ever-growing demands in measurement precision. Here we report spectrally resolved laser dimensional metrology via a free-running soliton frequency microcomb, with nanometric-scale precision. Spectral interferometry provides information on the optical time-of-flight signature, and the large free-spectral range and high coherence of the microcomb enable tooth-resolved and high-visibility interferograms that can be directly read out with optical spectrum instrumentation. We employ a hybrid timing signal from comb-line homodyne, microcomb, and background amplified spontaneous emission spectrally resolved interferometry-all from the same spectral interferogram. Our combined soliton and homodyne architecture demonstrates a 3-nm repeatability over a 23-mm nonambiguity range achieved via homodyne interferometry and over 1000-s stability in the long-term precision metrology at the white noise limits.

Increasing wavelength-controlled steering range of an optical phased array by using two subarrays.

We demonstrate an optical phased array that consists of two subarrays based on the silicon on insulator (SOI) platform, each subarray including 16 independent channels. The demonstrated field of view of the optical phased array is 36.6∘×32.6∘ with a spot size of 1.68∘×0.0673∘. A steering range of 32.6° is achieved by combining two subarrays with different periods and tuning the wavelength from 1500 nm to 1600 nm. In another dimension, the steering is realized by introducing phase differences between channels.

Dual-layer waveguide grating antenna with high directionality for optical phased arrays.

Development of the waveguide grating antenna with high directionality is significantly important for the optical phased array. A Si3N4/Si dual-layer structure with the grating pattern on the Si3N4 layer is proposed to improve the directionality of the waveguide grating antenna. High directionality of more than 89% can be achieved, and the length of the waveguide grating antenna is longer than 4 mm.

Broadband and compact polarization beam splitter based on an asymmetrical directional coupler with extra optimizing designs.

In this paper, a novel, to the best of our knowledge, polarization beam splitter (PBS) based on an asymmetrical directional coupler (DC) was proposed, which consists of a strip waveguide (WG) and a ${{\rm Si}_3}{{\rm N}_4}$Si3N4 loaded horizontal slot WG. By carefully adjusting the geometric parameters of the DC, the phase match condition between these two WGs can be satisfied for the transverse magnetic (TM) polarization, while the coupling efficiency of the transverse electric (TE) polarization is frustrated due to the large phase mismatch. The extra optimizing designs include adding filters to the output ports as well as introducing the tapered structure into the DC, which is settled by the particle swarm optimizing (PSO) algorithm so that the performance of the proposed PBS is improved over a broadband range. Numerical simulations show that the bandwidths for the extinction ratio (ER) $ \gt {20}\;{\rm dB}$>20dB, 30 dB, and 40 dB are 160 nm, 95 nm, and 50 nm, respectively, with insertion loss (IL) $ \lt {1}\;{\rm dB}$<1dB for the wavelength of 1.49-1.58 µm. The analysis of the deviations demonstrates that the proposed PBS allows high fabrication tolerances.

Controllable optomechanical coupling and Drude self-pulsation plasma locking in chip-scale optomechanical cavities.

We demonstrate the controllable optomechanical coupling and Drude self-pulsation plasma locking in chip-scale optomechanical cavities. The optomechanical coupling between the optical and mechanical degrees-of-freedom is dependent on the intracavity energy via the coupled fiber position. With the deterministic optomechanical stiffening, the interaction between optomechanical oscillation and self-pulsation can be controlled. Intracavity locking with 1/6 subharmonics is obtained over a wide optical detuning range of 190.01-192.23 THz. These results bring new insights into implementations of nonlinear dynamics at mesoscopic scale, with potential applications from photonic signal processing to nonlinear dynamic networks.

Quasi-phase-matched multispectral Kerr frequency comb.

We study a new type of Kerr frequency comb where the momentum conservation law is fulfilled by azimuthal modulation of the waveguide dispersion. The concept can expand the parametric range in which a Kerr frequency comb is obtained. In a good agreement with the theoretical analysis, we demonstrate a multispectral Kerr frequency comb covering important fiber-optic communication bands. Comb coherence and absence of a sub-comb offset are confirmed by continuous-wave heterodyne beat note and amplitude noise spectra measurements. The device can be used for achieving broadband optical frequency synthesizers and high-capacity coherent communication.

Stabilized chip-scale Kerr frequency comb via a high-Q reference photonic microresonator.

We stabilize a chip-scale Si3N4 phase-locked Kerr frequency comb via locking the pump laser to an independent stable high-Q reference microresonator and locking the comb spacing to an external microwave oscillator. In this comb, the pump laser shift induces negligible impact on the comb spacing change. This scheme is a step toward miniaturization of the stabilized Kerr comb system as the microresonator reference can potentially be integrated on-chip. Fractional instability of the optical harmonics of the stabilized comb is limited by the microwave oscillator used for a comb spacing lock below 1 s averaging time and coincides with the pump laser drift in the long term.

Ultra-compact and broadband Si photonics polarization rotator by self-alignment process.

An ultra-simple polarization rotator is demonstrated on SOI platform with self-aligned process to enhance performance repeatability and manufactural yield. The polarization rotation is essentially achieved by the symmetry breaking of a channel waveguide with a single-sided slab. The two-step lithography enabling this structure is fully compatible with the mainstream process flow of Si photonic integration. A polarization conversion efficiency of 93% is obtained at 1560nm in less than 10µm light propagation length. The merit of flat-band operation (> 100nm) by using asymmetric waveguide for polarization rotation is inherited.

Analysis of the polarization rotation effect in the inversely tapered spot size converter.

Inversely tapered spot size converter (SSC) is widely used to connect silicon waveguide with fiber in silicon photonics. However, the tapered structure may cause polarization rotation and further generate interference fluctuation in the transmission spectrum even of a straight waveguide. We analyzed the light propagation in a straight waveguide with SSC at the both ends with coupling matrix and transmission matrix methods. The analysis results matched with the phenomena we observed in the transmission spectrum. Combining the analysis with the measurement results, we calculated the polarization rotation efficiency of the SSC in different samples and analyzed the origin of the polarization rotation effect. Finally, we discussed the influence of the effect to the DP-QPSK signal and proposed several methods to release the impact.

Molecular-absorption-induced thermal bistability in PECVD silicon nitride microring resonators.

The wavelength selective linear absorption in communication C-band is investigated in CMOS-processed PECVD silicon nitride rings. In the overcoupled region, the linear absorption loss lowers the on-resonance transmission of a ring resonator and increases its overall quality factor. Both the linear absorption and ring quality factor are maximized near 1520 nm. The direct heating by phonon absorption leads to thermal optical bistable switching in PECVD silicon nitride based microring resonators. We calibrate the linear absorption rate in the microring resonator by measuring its transmission lineshape at different laser power levels, consistent with coupled mode theory calculations.

Silicon photonics packaging with lateral fiber coupling to apodized grating coupler embedded circuit.

We report a novel lateral packaging approach using laser welding technique with angle polished fiber coupling to grating coupler embedded silicon photonic circuit. Measurements show the relax alignment tolerance for fiber packaging process. The packaging excess loss of 1.2 dB is achieved. The use of angle polished fiber for lateral fiber coupling enables an alternative way for cost-effective deployment of silicon photonics packaging in telecommunication systems.

PN-type carrier-induced filter with modulatable extinction ratio.

We demonstrate the first PN-type carrier-induced silicon waveguide Bragg grating filter on a SOI wafer. The optical extinction ratio of this kind of filter can be efficiently modulated under both reverse and forward biases. The carrier-induced Bragg grating based on a PN junction is fabricated on the silicon waveguide using litho compensation technology. The measured optical bandwidth and the extinction ratio of the filter are 0.45 nm and 19 dB, respectively. The optical extinction ratio modulation under the reverse bias is more than 11.5 dB and it is more than 10 dB under the forward bias. Only 1-dB optical transmission loss is realized in this Bragg grating under a reverse bias. The shifting rates of the central wavelength under forward and reverse biases are ~-1.25 nm/V and 0.01 nm/V, respectively. The 3-dB modulation bandwidth of this filter is 5.1 GHz at a bias of -10 V.

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.