Microwave Photonics Applications Research Articles

Lithium niobate photonic-crystal electro-optic modulator

Modern advanced photonic integrated circuits require dense integration of high-speed electro-optic functional elements on a compact chip that consumes only moderate power. Energy efficiency, operation speed, and device dimension are thus crucial metrics underlying almost all current developments of photonic signal processing units. Recently, thin-film lithium niobate (LN) emerges as a promising platform for photonic integrated circuits. Here, we make an important step towards miniaturizing functional components on this platform, reporting high-speed LN electro-optic modulators, based upon photonic crystal nanobeam resonators. The devices exhibit a significant tuning efficiency up to 1.98 GHz V−1, a broad modulation bandwidth of 17.5 GHz, while with a tiny electro-optic modal volume of only 0.58 μm3. The modulators enable efficient electro-optic driving of high-Q photonic cavity modes in both adiabatic and non-adiabatic regimes, and allow us to achieve electro-optic switching at 11 Gb s−1 with a bit-switching energy as low as 22 fJ. The demonstration of energy efficient and high-speed electro-optic modulation at the wavelength scale paves a crucial foundation for realizing large-scale LN photonic integrated circuits that are of immense importance for broad applications in data communication, microwave photonics, and quantum photonics.

Thermally-Reconfigurable Silicon Photonic Devices and Circuits

Reconfigurable photonic integrated devices and circuits are very important for many applications. Among various mechanisms for realizing reconfigurability, thermo-optic effect is popular because of the availability for many materials, the design simplicity and the fabrication ease. In particular, silicon has a large thermo-optic coefficient as well as a high heat conductivity, and thus it is promising to realize efficient thermally-reconfigurable silicon photonic integrated devices and circuits. Recent progress is reviewed in this paper. First, thermally-tunable silicon photonic filters based on different structures are summarized, including microring resonators (MRRs) and MRRs-assisted Mach-Zehnder interferometers (MZIs). Second, silicon photonic switches based on MZIs and MRRs are reviewed. Finally, a review is given for thermally-reconfigurable silicon photonic integrated circuits developed for the applications in quantum photonics, microwave photonics, and optical interconnects.

Modeling a Dual-Parallel Silicon Modulator for Sinc-Shaped Nyquist Pulse Generation

Sinc-shaped Nyquist pulses have wide applications in optical communications and microwave photonics. The quality of the generated Nyquist pulses has a significant effect on the overall performance of the communication system and the signal processing system. Generating high-quality Nyquist pulses with integrated silicon modulators is however still challenging due to the highly nonlinear response and fabrication-induced discrepancies of the device. We present a model for a silicon dual-parallel modulator with which the best possible quality of the generated optical frequency comb (OFC) and sinc-shaped Nyquist pulses can be obtained. In the model, the phase and amplitude deviations are taken into consideration. Device induced waveform and spectrum distortion can be well described. Based on the guidance of the model, a root-mean-square error of 2.8% is obtained between the experimentally generated signals and the ideal pulses, which proves the high accuracy of the model and its importance for high-quality Nyquist pulse generation.

Dynamic behavior of frequency combs in frequency-shifting loops

The understanding of frequency-shifting loops’ (FSLs) transient response is fundamental for their implementation in microwave photonic systems. We developed a numerical model, enabling us to describe the specific dynamics of the frequency comb generated in a FSL seeded by a CW laser. The model, based on laser rate equations, predicts the temporal evolution of the power of the individual comb lines during on/off cycles of the injection power, for FSLs operating below the lasing threshold. To prove the validity of the model, we performed experimental measurements of the power of the individual comb lines with time. Numerical simulations based on the model, for different gain media and pumping rates, are in excellent agreement with the experimental results. This paper provides guidance for the concrete implementation of FSLs for microwave photonic applications.

Nonlinear Characteristics of Uni-Traveling Carrier Photodiode With InGaAs/GaAsSb Type-II Multiple Quantum Wells Absorber

The linearity of high power photodetectors is the limiting factor to the dynamic range performance of analog optical links and microwave photonics applications. In this work, we characterized the third order intermodulation distortion (IMD3) of a uni-traveling carrier photodiode with InGaAs/GaAsSb type-II multiple quantum wells absorber. The 10 μm diameter photodiode shows a third order output intercept point of 36.6 dBm at 200 MHz and 12.9 dBm at 25 GHz, under reverse bias of −5 V and photo current of 15 mA. Combining with the measured device parameters, an equivalent circuit model based simulation is carried out, and the general trend of the simulated IMD3 is consistent with the measurement results. By separately involving the nonlinear mechanisms, the source of the nonlinearity is studied in detail.

Delay-induced instability in phase-locked dual-polarization distributed-feedback fiber lasers

We study experimentally and theoretically a dual-polarization fiber laser submitted to time-delayed frequency-shifted optical feedback. In addition to the usual frequency-locking regime that is expected when the frequency shift is close to the polarization beat frequency, we observe at high pumping rates a dynamical pulsing regime inside the locking range. This regime is experimentally evidenced in a full-fibered experiment based on an erbium-doped distributed-feedback fiber laser in which polarization beat frequency is about 1 GHz. A rate-equation model including the frequency-shifted feedback term reproduces well the experimental bifurcation maps, provided that both the time delay and a phase-amplitude coupling parameter ($\ensuremath{\alpha}$ factor) are taken into account. The impact on microwave-photonics applications is discussed.

Heterogeneous III-V on silicon nitride amplifiers and lasers via microtransfer printing

The development of ultralow-loss silicon-nitride-based waveguide platforms has enabled the realization of integrated optical filters with unprecedented performance. Such passive circuits, when combined with phase modulators and low-noise lasers, have the potential to improve the current state of the art of the most critical components in coherent communications, beam steering, and microwave photonics applications. However, the large refractive index difference between silicon nitride and common III-V gain materials in the telecom wavelength range hampers the integration of electrically pumped III-V semiconductor lasers on a silicon nitride waveguide chip. Here, we present an approach to overcome this refractive index mismatch by using an intermediate layer of hydrogenated amorphous silicon, followed by the microtransfer printing of a prefabricated III-V semiconductor optical amplifier. Following this approach, we demonstrate a heterogeneously integrated semiconductor optical amplifier on a silicon nitride waveguide circuit with up to 14 dB gain and a saturation power of 8 mW. We further demonstrate a heterogeneously integrated ring laser on a silicon nitride circuit operating around 1550 nm. This heterogeneous integration approach would not be limited to silicon-nitride-based platforms: it can be used advantageously for any waveguide platform with low-refractive-index waveguide materials such as lithium niobate.

Photonic microwave generation in the X- and K-band using integrated soliton microcombs

Microwave photonic technologies, which upshift the carrier into the optical domain to facilitate the generation and processing of ultrawide-band electronic signals at vastly reduced fractional bandwidths, have the potential to achieve superior performance compared to conventional electronics for targeted functions. For microwave photonic applications such as filters, coherent radars, subnoise detection, optical communications and low-noise microwave generation, frequency combs are key building blocks. By virtue of soliton microcombs, frequency combs can now be built using CMOS compatible photonic integrated circuits, operated with low power and noise, and have already been employed in system-level demonstrations. Yet, currently developed photonic integrated microcombs all operate with repetition rates significantly beyond those that conventional electronics can detect and process, compounding their use in microwave photonics. Here we demonstrate integrated soliton microcombs operating in two widely employed microwave bands, X- and K-band. These devices can produce more than 300 comb lines within the 3-dB-bandwidth, and generate microwave signals featuring phase noise levels below 105 dBc/Hz (140 dBc/Hz) at 10 kHz (1 MHz) offset frequency, comparable to modern electronic microwave synthesizers. In addition, the soliton pulse stream can be injection-locked to a microwave signal, enabling actuator-free repetition rate stabilization, tuning and microwave spectral purification, at power levels compatible with silicon-based lasers (<150 mW). Our results establish photonic integrated soliton microcombs as viable integrated low-noise microwave synthesizers. Further, the low repetition rates are critical for future dense WDM channel generation schemes, and can significantly reduce the system complexity of photonic integrated frequency synthesizers and atomic clocks.

Half-open-cavity multiwavelength Brillouin-erbium random fiber laser with improved performance

Half-open-cavity multiwavelength Brillouin-erbium random fiber lasers (RFLs) with improved performance in terms of the generated Stokes order number and wavelength tuning range have been proposed and demonstrated. Our contrastive experiments reveal that the position of the used erbium-doped fiber (EDF) gain is crucial for the performance improvement of the Brillouin-erbium RFLs with half-open-cavity configuration. When the EDF is incorporated in the double-pass port of the circulator used, eighteen Stokes orders have been generated, which demonstrates the highest order obtained from Brillouin-erbium RFLs to the best of our knowledge. The Brillouin order enhancement is attributed to the sufficient gain for the randomly reflected Stokes wavelengths with the double-pass amplification. The broadest wavelength tunability of 54 nm is achieved in the case with the EDF located at the port 1 of the circulator, owing to its strongest suppression ability for self-lasing cavity modes. The significant performance upgrade that is obtained with these RFLs, whilst maintaining a simple structure, gives rise to promising potential applications in optical communications, microwave photonics, precision metrology and fiber-optic sensing.

Effect of oxygen partial pressure on nonlinear optical and electrical properties of BNT–KNNG composite thin films

Composite thin films of 1–x [Bi0.5Na0.5TiO3] – x [K0.5Na0.5NbO3 + 1 wt% Gd2O3] (BNT–KNNG); (x = 0.01) have been deposited at various O2 pressures from 0.1 to 10 Pa by pulsed laser deposition, and their crystal structure, surface morphology, optical, dielectric, and ferroelectric properties were investigated. X-ray diffraction analysis of thin films deposited at 0.1 Pa revealed a single phase of BNT–KNNG and further (> 0.1 Pa), film crystallinity gradually increased with a rise in O2 pressure. The improvement in the refractive index and a reduction in optical bandgap are observed with O2 pressure and are estimated to be 2.28–2.42 and 4.08–3.65 eV, respectively. The third-order nonlinear optical coefficients estimated using the Z-scan technique are found to be enhanced with O2 pressure. The film deposited at 10 Pa exhibited a higher nonlinear refractive index (n2 = 6.188 × 10− 6 cm2/W) and a strong absorption coefficient (β = 1.043 cm/W). The temperature-dependent dielectric response displayed two structural phase transitions from rhombohedral to tetragonal phase at 165 oC and tetragonal to cubic phase at 298 oC. The enhanced dielectric (er = 411, tanδ = 0.156 @ 1 kHz), Microwave dielectric (er = 317 and tanδ = 0.0074 @ 10 GHz), and ferroelectric (Pr = 25.31 µC/cm2, EC = 42.62 kV/cm @ 1 kHz) properties with low leakage current are observed for the film deposited at 10 Pa which followed a space charge limited conduction behavior. The obtained microwave and nonlinear optical properties of BNT–KNNG composite films are suitable for tunable microwave and optical photonic device applications.

Broadband optical single sideband generation using an ultra high shape-factor self coupled ring resonator

In this study, we realize an integrated generation of optical Single SideBand with Carrier (SSB+C) signal for microwave photonics applications. It has been achieved using a Micro Ring Modulator (MRM) combined with a cavity based wavelength selective filter. MRM, when applied with an RF input, results in a Double SideBand with Carrier (DSB+C) signal where one sideband is suppressed by applying this signal to the filter. The filter has been created using a single resonance-split self-coupled cavity with an extremely high shape factor of 0.69. The sideband suppression ratio between DSB+C and SSB+C ranges from 16 dB to 55 dB for a frequency range of 4 GHz to 20 GHz. Tunable suppression ratio of 21 dB has been achieved at a fixed frequency of 15 GHz. Dynamic range performance of the generated signal has been evaluated at a noise floor of -156 dBm. The dynamic range remains stable in the range of 1 GHz - 5 GHz at ∼ 80 dB.Hz2∕3.

WS2 based 523 MHz mode-locked erbium-doped fiber laser for microwave photonic application

Microwave photonics is an important application for mode-locked lasers and requires the lasers to have high repetition rate and relatively low timing jitter. Although many mode-locked lasers based on two-dimensional (2D) materials have been reported, their applications in high-quality microwave photonics have not been achieved yet. We demonstrate a linear-cavity mode-locked Erbium-doped fiber laser with tungsten disulfide (WS2) saturable absorber (SA). By optimizing the WS2 SA fabrication, SA with low loss and moderate modulation depth is obtained. The laser based on this WS2 SA is demonstrated to have a fundamental repetition rate of 523 MHz, a central wavelength of 1558.6 nm, a 3-dB bandwidth of 7.4 nm and a timing jitter of 28 fs. With this laser, microwave photonic application of frequency synthesis is obtained with a tuning range up to 10 GHz and an extinction ratio greater than 60 dB in the whole range. We believe this is the first work that utilizes a 2D material SA based mode-locked laser for high-quality microwave photonic applications. This work indicates the potential of 2D material SA for high-speed and high-precision photonic and microwave systems.

Optoacoustics—Advances in high-frequency optomechanics and Brillouin scattering

The fields of cavity optomechanics and Brillouin scattering, linked by common underlying physical mechanisms, involve the interaction of light waves with mechanical vibrations at the micro- and nanoscale. Exciting fundamental research in both classical and quantum regimes as well as opportunities for applications in microwave photonics, frequency conversion, narrow-linewidth lasers, optomechanical sensors, electro-optic transducers, coherent light storage, and Brillouin spectroscopy have stimulated significant interest in the last decade. This special issue brings contributions to fundamental aspects regarding the Brillouin interaction such as novel waveguide structures, novel guiding mechanisms, the interplay between Brillouin and other nonlinear phenomena, and applications in sensing and light storage, as well as an introductory tutorial to the research field. Here, we provide a brief introduction to the topics covered in the issue.

Brillouin integrated photonics

A recent renaissance in Brillouin scattering research has been driven by the increasing maturity of photonic integration platforms and nanophotonics. The result is a new breed of chip-based devices that exploit acousto-optic interactions to create lasers, amplifiers, filters, delay lines and isolators. Here, we provide a detailed overview of Brillouin scattering in integrated waveguides and resonators, covering key concepts such as the stimulation of the Brillouin process, in which the optical field itself induces acoustic vibrations, the importance of acoustic confinement, methods for calculating and measuring Brillouin gain, and the diversity of materials platforms and geometries. Our Review emphasizes emerging applications in microwave photonics, signal processing and sensing, and concludes with a perspective for future directions. Acousto-optical interactions within integrated optics platforms are reviewed with a discussion of the useful chip-based devices such as lasers, amplifiers, filters, isolators and more besides that can result.

Wideband OPLL photonic integrated circuit enabling ultrahigh dynamic range PM RF/photonic link

The RF/photonic link is the basic element of microwave photonics. Previous RF photonic links often rely on optical intensity modulation. The inherent modulation nonlinearity leads to inadequate spurious free dynamic range (SFDR) for many sought-after microwave photonic applications in radar. A new phase-modulated (PM) link with an optical phase-locked loop (OPLL) demodulator could afford a promising solution for the SFDR. However, the efficacy of the PM link approach remained unsubstantiated as previous OPLL implementations had too restricted of a bandwidth for realistic applications. Here, we present a new OPLL photonic integrated circuit (PIC) chip that offered quadrupled bandwidth enhancement over the state of the art. The OPLL PIC represents the first OPLL linear optical phase demodulator with sufficient bandwidth for realistic microwave photonic applications. With help of the OPLL PIC, a PM RF/photonic link demonstrated a record-breaking SFDR of ∼129.3 dB·Hz2/3 and a 3 dB SFDR bandwidth of ∼1 GHz. The combined SFDR and bandwidth performance amounts to around 1 order of magnitude improvement over the prior state of the art.

Feasibility of quasicritical coupling based on LP modes and its application as a filter with tunable bandwidth and stable insertion loss.

The feasibility of the quasicritical coupling based on the high order scalar modes in tapered fiber was presented and discussed in detail theoretically. As its applications, the bandwidth evolution of the coupling process both in the Through port and the Drop port were also included in the calculations and demonstrated in the experiment. As a result, tunable bandwidth filters with stable insertion loss were realized by changing the gap between the few-mode tapered fiber and microcavity working under a state of quasicritical coupling. The bandwidth at the Through port is from 4.8 MHz to 327.1 MHz and that at the Drop port is from 15.3 MHz to 327.1 MHz, with the insertion loss fluctuation less than 10%. The combination of stable efficiency, narrow band, and bandwidth fine tunability may benefit its application in narrow-linewidth lasers, microwave photonics and nonlinear optics.

Optical Frequency Comb and Nyquist Pulse Generation With Integrated Silicon Modulators

We demonstrate nine-line optical frequency comb (OFC) and sinc-shaped Nyquist pulse generation based on two cascaded silicon Mach–Zehnder modulators. The OFC has a flatness of 1.83 dB, a frequency spacing of 5 GHz, and a corresponding Nyquist pulsewidth of ∼22 ps. Moreover, multi-wavelength operation and repetition rate tunability have also been demonstrated, which indicates the potential of our work to generate an on-chip high-speed pulse train demanded in optical communications and microwave photonics applications.

Tunable slow and fast light in a magneto-optic micro-ring resonator

We propose a magneto-optic micro-ring resonator (MO-MRR) structure to perform the slow-to-fast light transition and obtain the tunable delay time from +4 ns to −4 ns. We quantify the power and bandwidth variations with the applied magnetic field and locate a broadening bandwidth area in the fast-light region. We optimize the azimuth angle of input light, i.e. 45°-linearly polarized light, to significantly reduce the required magnetic induction for the status transition by 71%. The results of the MO-MRR scheme support the implementation of the MO-induced tunable integration chip for microwave photonic applications.

100 GHz free spectral range-tunable multi-wavelength fiber laser using single–multi–single mode fiber interferometer

Six single–multi–single (SMS) mode fiber-based Mach–Zehnder Interferometers (MZI) with different spooling radii are demonstrated as spatial mode filtering elements for multi-wavelength laser generation. Free spectral range (FSR) tuning of the multi-wavelength output generated is realized by alternating between the six MZIs, which have different macro-bending losses. Additional FSR tuning is also realized by changing the spooling radius of the two-mode step index fiber section within the MZI setups from 80 mm to 30 mm, giving an FSR tuning range of 0.89–0.99 nm. All generated multi-wavelength outputs show high stability over a test period of 100 min. The proposed multi-wavelength lasers are highly suitable for various microwave photonics applications the field of such as microwave signal source generation and microwave photonic filtering.

Tunable Brillouin and Raman microlasers using hybrid microbottle resonators

Abstract We realized a tunable Brillouin laser and a tunable Raman laser by using ultrahigh-quality-factor (Q) hybrid microbottle resonators. The whispering-gallery-mode microresonator-based Brillouin and Raman lasers possess unique advantages (e.g. low threshold, narrow line width, and flexible lasing wavelength region) and various potential applications. Efficient tuning of the Brillouin and Raman microresonator lasers is desirable in many cases. However, the corresponding lasing wavelength tunability is rarely reported. In this work, we realized the ultralow-threshold Brillouin and Raman lasers based on the proposed hybrid microbottle resonators with Q factors over 108. Meanwhile, by feeding the control light through the axial direction of the hybrid microbottle resonators, a Brillouin lasing wavelength tuning range of 2.68 nm and a Raman lasing wavelength tuning range of 2.32 nm are realized, which are one order of magnitude and almost once larger than those reported in the previous works, respectively. Such tunable microlasers could find significant applications in light sources, microwave photonics, and optical sensing.