Showing papers on "Phase noise published in 2020"

Entanglement on an optical atomic-clock transition

Abstract: State-of-the-art atomic clocks are based on the precise detection of the energy difference between two atomic levels, which is measured in terms of the quantum phase accumulated over a given time interval1–4. The stability of optical-lattice clocks (OLCs) is limited both by the interrupted interrogation of the atomic system by the local-oscillator laser (Dick noise5) and by the standard quantum limit (SQL) that arises from the quantum noise associated with discrete measurement outcomes. Although schemes for removing the Dick noise have been recently proposed and implemented4,6–8, performance beyond the SQL by engineering quantum correlations (entanglement) between atoms9–20 has been demonstrated only in proof-of-principle experiments with microwave clocks of limited stability. The generation of entanglement on an optical-clock transition and operation of an OLC beyond the SQL represent important goals in quantum metrology, but have not yet been demonstrated experimentally16. Here we report the creation of a many-atom entangled state on an OLC transition, and use it to demonstrate a Ramsey sequence with an Allan deviation below the SQL after subtraction of the local-oscillator noise. We achieve a metrological gain of $$4.{4}_{-0.4}^{+0.6}$$ decibels over the SQL by using an ensemble consisting of a few hundred ytterbium-171 atoms, corresponding to a reduction of the averaging time by a factor of 2.8 ± 0.3. Our results are currently limited by the phase noise of the local oscillator and Dick noise, but demonstrate the possible performance improvement in state-of-the-art OLCs1–4 through the use of entanglement. This will enable further advances in timekeeping precision and accuracy, with many scientific and technological applications, including precision tests of the fundamental laws of physics21–23, geodesy24–26 and gravitational-wave detection27. A many-atom state of trapped 171Yb atoms that are entangled on an optical atomic-clock transition overcomes the standard quantum limit, providing a proof-of-principle demonstration towards entanglement-based optical atomic clocks.

Broadband Microwave Frequency Conversion Based on an Integrated Optical Micro-Comb Source

Abstract: We report a broadband microwave frequency converter based on a coherent Kerr optical micro-comb generated by an integrated micro-ring resonator. The coherent micro-comb displays features that are consistent with soliton crystal dynamics with a free spectral range of 48.9 GHz. We use this to demonstrate a high-performance millimeter-wave local oscillator for microwave frequency conversion. We experimentally verify the microwave performance up to 40 GHz, achieving a ratio of −6.8 dB between output radio frequency power and intermediate frequency power and a spurious suppression ratio of >43.5 dB. The experimental results show good agreement with theory and verify the effectiveness of microwave frequency converters based on coherent optical micro-combs, with the ability to achieve reduced size, complexity, and potential cost.

Photonic RF Arbitrary Waveform Generator Based on a Soliton Crystal Micro-Comb Source

Abstract: We report a photonic-based radio frequency (RF) arbitrary waveform generator (AWG) using a soliton crystal micro-comb source with a free spectral range (FSR) of 48.9 GHz. The comb source provides over 80 wavelengths, or channels, that we use to successfully achieve arbitrary waveform shapes including square waveforms with a tunable duty ratio ranging from 10% to 90%, sawtooth waveforms with a tunable slope ratio of 0.2 to 1, and a symmetric concave quadratic chirp waveform with an instantaneous frequency of sub GHz. We achieve good agreement between theory and experiment, validating the effectiveness of this approach towards realizing high-performance, broad bandwidth, nearly user-defined RF waveform generation.

Ultralow-noise photonic microwave synthesis using a soliton microcomb-based transfer oscillator.

Abstract: The synthesis of ultralow-noise microwaves is of both scientific and technological relevance for timing, metrology, communications and radio-astronomy. Today, the lowest reported phase noise signals are obtained via optical frequency-division using mode-locked laser frequency combs. Nonetheless, this technique ideally requires high repetition rates and tight comb stabilisation. Here, a microresonator-based Kerr frequency comb (soliton microcomb) with a 14 GHz repetition rate is generated with an ultra-stable pump laser and used to derive an ultralow-noise microwave reference signal, with an absolute phase noise level below −60 dBc/Hz at 1 Hz offset frequency and −135 dBc/Hz at 10 kHz. This is achieved using a transfer oscillator approach, where the free-running microcomb noise (which is carefully studied and minimised) is cancelled via a combination of electronic division and mixing. Although this proof-of-principle uses an auxiliary comb for detecting the microcomb’s offset frequency, we highlight the prospects of this method with future self-referenced integrated microcombs and electro-optic combs, that would allow for ultralow-noise microwave and sub-terahertz signal generators. In order to satisfy a wide range of modern microwave applications, improved methods are needed to produce low-noise microwave signals. Here the authors demonstrate ultra-low noise microwave synthesis via optical frequency division using a transfer oscillator method applied to a microresonator-based comb on the path to future self-referenced integrated sources.

Recent advances in optoelectronic oscillators

Abstract: An optoelectronic oscillator (OEO) is a microwave photonic system that produces microwave signals with ultralow phase noise using a high-quality-factor optical energy storage element. This type of oscillator is desired in various practical applications, such as communication links, signal processing, radar, metrology, radio astronomy, and reference clock distribution. Recently, new mode control and selection methods based on Fourier domain mode-locking and parity-time symmetry have been proposed and experimentally demonstrated in OEOs, which overcomes the long-existing mode building time and mode selection problems in a traditional OEO. Due to these mode control and selection methods, continuously chirped microwave waveforms can be generated directly from the OEO cavity and single-mode operation can be achieved without the need of ultranarrowband filters, which are not possible in a traditional OEO. Integrated OEOs with a compact size and low power consumption have also been demonstrated, which are key steps toward a new generation of compact and versatile OEOs for demanding applications. We review recent progress in the field of OEOs, with particular attention to new mode control and selection methods, as well as chip-scale integration of OEOs.

Parity-time symmetry in wavelength space within a single spatial resonator.

Abstract: We show a parity-time (PT) symmetric microwave photonic system in the optical wavelength space within a single spatial resonator, in which the gain and loss modes can perfectly overlay spatially but are distinguishable in the designated parameter space To prove the concept, a PT-symmetric optoelectronic oscillator (OEO) in the optical wavelength space is implemented The OEO has a single-loop architecture, with the microwave gain and loss modes carried by two optical wavelengths to form two mutually coupled wavelength-space resonators The operation of PT symmetry in the OEO is verified by the generation of a 10-GHz microwave signal with a low phase noise of -1293 dBc/Hz at 10-kHz offset frequency and small sidemodes of less than -6622 dBc/Hz Compared with a conventional spatial PT-symmetric system, a PT-symmetric system in the wavelength space features a much simpler configuration, better stability and greater resilience to environmental interferences

A 19.5-GHz 28-nm Class-C CMOS VCO, With a Reasonably Rigorous Result on 1/ f Noise Upconversion Caused by Short-Channel Effects

Abstract: Class-C operation is leveraged to implement a $K$ -band CMOS voltage-controlled oscillator (VCO) where the upconversion of $1/f$ current noise from the cross-coupled transistors in the oscillator core is robustly contained at a very low level. Implemented in a bulk 28-nm CMOS technology, the 12%-tuning-range VCO shows a phase noise as low as −112 dBc/Hz at 1-MHz offset (−86 dBc/Hz at 100 kHz offset) from a 19.5 GHz carrier while consuming 20.7 mW, achieving a figure of merit (FoM) of −185 dBc/Hz. The design is complemented by a theoretical investigation of $1/f$ noise upconversion caused by short-channel effects in the cross-coupled transistors, obtaining the first instance of a closed-form phase noise expression in the $1/f^{3}$ region.

Towards a Scaleable 5G Fronthaul: Analog Radio-over-Fiber and Space Division Multiplexing

Abstract: The introduction of millimeter wave (mm-wave) frequency bands for cellular communications with significantly larger bandwidths compared to their sub-6 GHz counterparts, the resulting densification of network deployments and the introduction of antenna arrays with beamforming result in major increases in fronthaul capacity required for 5G networks As a result, a radical re-design of the radio access network is required since traditional fronthaul technologies are not scaleable In this article the use of analog radio-over-fiber (ARoF) is proposed and demonstrated as a viable alternative which, combined with space division multiplexing in the optical distribution network as well as photonic integration of the required transceivers, shows a path to a scaleable fronthaul solution for 5G The trade-off between digitized and analog fronthaul is discussed and the ARoF architecture proposed by blueSPACE is introduced Two options for the generation of ARoF two-tone signals for mm-wave generation via optical heterodyning are discussed in detail, including designs for the implementation in photonic integrated circuits as well as measurements of their phase noise performance The proposed photonic integrated circuit designs include the use of both InP and SiN platforms for ARoF signal generation and optical beamforming respectively, proposing a joint design that allows for true multi-beam transmission from a single antenna array Phase noise measurements based on laboratory implementations of ARoF generation based on a Mach–Zehnder modulator with suppressed carrier and with an optical phase-locked loop are presented and the suitability of these transmitters is evaluated though phase noise simulations Finally, the viability of the proposed ARoF fronthaul architecture for the transport of high-bandwidth mm-wave 5G signals is proven with the successful implementation of a real-time transmission link based on an ARoF baseband unit with full real-time processing of extended 5G new radio signals with 800 MHz bandwidth, achieving transmission over 10 km of 7-core single-mode multi-core fiber and 9 m mm-wave wireless at 255 GHz with bit error rates below the limit for a 7% overhead hard decision forward error correction

Photon-driven broadband emission and frequency comb RF injection locking in THz quantum cascade lasers

Abstract: We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83 THz (Δf/f=50%) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low CW threshold (115 A·cm-2) and broadband comb operation (Δf/f=25%) make these sources extremely appealing for on-chip frequency comb applications.

High-speed Gaussian-modulated continuous-variable quantum key distribution with a local local oscillator based on pilot-tone-assisted phase compensation.

Abstract: A high-speed Gaussian-modulated continuous-variable quantum key distribution (CVQKD) with a local local oscillator (LLO) is experimentally demonstrated based on pilot-tone-assisted phase compensation. In the proposed scheme, the frequency-multiplexing and polarization-multiplexing techniques are used for the separate transmission and heterodyne detection between quantum signal and pilot tone, guaranteeing no crosstalk from strong pilot tone to weak quantum signal and different detection requirements of low-noise for quantum signal and high-saturation limitation for pilot tone. Moreover, compared with the conventional CVQKD based on homodyne detection, the proposed LLO-CVQKD scheme can measure X and P quadrature simultaneously using heterodyne detection without need of extra random basis selection. Besides, the phase noise, which contains the fast-drift phase noise due to the relative phase of two independent lasers and the slow-drift phase noise introduced by quantum channel disturbance, has been compensated experimentally in real time, so that a low level of excess noise with a 25 km optical fiber channel (with 5 dB loss) is obtained for the achievable secure key rate of 7.04 Mbps in the asymptotic regime and 1.85 Mbps under the finite-size block of 107.

Performance Analysis of THz Wireless Systems in the Presence of Antenna Misalignment and Phase Noise

Abstract: This work aims to present the theoretical framework for the outage performance assessment of the joint impact of antenna misalignment and local oscillator (LO) phase noise (PHN) on terahertz (THz) wireless systems. In more detail, a closed form expression for the outage probability and a tight high signal to noise ratio (SNR) approximation were extracted. Our results reveal the detrimental impact of antenna misalignment and PHN and the importance of taking them into consideration when analyzing the performance of THz wireless systems. Furthermore, it was observed that the impact of the LO hardware impairments is more significant to the system performance compared to the antenna misalignment.

Deep-learning-assisted, two-stage phase control method for high-power mode-programmable orbital angular momentum beam generation

Abstract: High-power mode-programmable orbital angular momentum (OAM) beams have received substantial attention in recent years. They are widely used in optical communication, nonlinear frequency conversion, and laser processing. To overcome the power limitation of a single beam, coherent beam combining (CBC) of laser arrays is used. However, in specific CBC systems used to generate structured light with a complex wavefront, eliminating phase noise and realizing flexible phase modulation proved to be difficult challenges. In this paper, we propose and demonstrate a two-stage phase control method that can generate OAM beams with different topological charges from a CBC system. During the phase control process, the phase errors are preliminarily compensated by a deep-learning (DL) network, and further eliminated by an optimization algorithm. Moreover, by modulating the expected relative phase vector and cost function, all-electronic flexible programmable switching of the OAM mode is realized. Results indicate that the proposed method combines the characteristics of DL for undesired convergent phase avoidance and the advantages of the optimization algorithm for accuracy improvement, thereby ensuring the high mode purity of the generated OAM beams. This work could provide a valuable reference for future implementation of high-power, fast switchable structured light generation and manipulation.

An mm-Wave Synthesizer With Robust Locking Reference-Sampling PLL and Wide-Range Injection-Locked VCO

Abstract: In this article, a two-stage millimeter (mm)-wave frequency synthesizer with low in-band noise and robust locking reference-sampling techniques is presented. Using a two-stage scheme allows separately dealing with the low phase noise (PN) frequency synthesis in the first stage and the mm-wave frequency multiplication in the second stage, achieving the best overall power efficiency. In the first stage, a voltage domain reference-sampling phase detector (RSPD)-locked loop (RSPLL) is adopted to achieve both low PN and robust locking without additional frequency locking loop. A reference reshaping buffer is implemented to improve the phase detector gain and in-band PN. The reference rising/falling time is programmable to achieve optimal RSPLL performance even under external disturbances. The second stage employs an injection-locked voltage-controlled oscillator (ILVCO) for 4 $\times $ frequency multiplication. A low-power digital frequency tracking loop (FTL) detecting actual frequency errors is implemented in order to achieve wide operation range for the ILVCO while using a high ${Q}$ tank with low power. The prototype synthesizer was fabricated in a 45-nm partially depleted silicon on insulator (PDSOI) CMOS technology. The first stage 9-GHz RSPLL achieves 144-fs integrated jitter with 7.2-mW power consumption, achieving a figure of merit (FoM) of −248 dB and the overall mm-wave synthesizer achieves 251-fs integrated jitter with 20.6-mW power consumption at 35.84 GHz, achieving an FoM of −238.9 dB.

Physical Layer Authentication Jointly Utilizing Channel and Phase Noise in MIMO Systems

Abstract: In this paper, we propose a physical layer authentication scheme in heterogeneous coexist multiple-input-multiple-output (MIMO) systems. This scheme utilizes two physical layer features in terms of location-specific channel gains and transmitter-specific phase noise caused by imperfect oscillators to identify transmitters. Three properties of the proposed scheme: covertness, robustness, and security, are analyzed in detail. By using a maximum-likelihood estimator (MLE) and extended Kalman filter (EKF), we estimate channel gains and phase noise, and formulate variances of estimation errors. We also quantize the temporal variations of channel gains and phase noise through the developed quantizers. Based on quantization results and theories of hypothesis testing and stochastic process, we then derive the closed-form expressions for false alarm and detection probabilities with the consideration of quantization errors. Simulations are carried out to validate the theoretical results of the two probabilities. Based on theoretical models, we further demonstrate that the proposed scheme makes it possible for us to flexibly control authentication performance by adjusting thresholds (for channel gain, phase noise, and decision, respectively) to achieve a required authentication performance in specific MIMO applications.

A 0.46-THz 25-Element Scalable and Wideband Radiator Array With Optimized Lens Integration in 65-nm CMOS

Abstract: In this article, we present a 438–479-GHz fully integrated 25-element radiator array source based on a scalable structure of coupled standing wave oscillator cells without extra loss and parasitics from coupling networks. The bandwidth is extended using a varactor-less frequency tuning method, and EIRP is improved by increasing the size of the array using the proposed scalable coupling method and maximizing the radiation directivity using a silicon lens in an optimized radiation setup. An analysis of the radiation directivity of a chip-lens setup is presented as well as the employed approach for enhancing EIRP by maximizing the directivity. The circuit is implemented in a 65-nm CMOS process and is measured to have 40.7 GHz/8.9% frequency tuning range. The chip consumes 0.38–2.34-W power (1.18 W at 459-GHz center frequency) from a 1.2-V supply voltage. The circuit provides a maximum of −1.8-dBm radiated power and 19.3-dBm EIRP at 448 GHz using a 12.5-mm radius silicon lens. The minimum measured phase noise at 10-MHz offset is −100.6 dBc/Hz. To the best of our knowledge, this article has the largest EIRP, radiated power, and bandwidth among fully integrated silicon-based coherent sources above 350 GHz.

Photonic Flywheel in a Monolithic Fiber Resonator.

Abstract: We demonstrate the first compact photonic flywheel with sub-fs time jitter (averaging times up to 10 μs) at the quantum-noise limit of a monolithic fiber resonator. Such quantum-limited performance is accessed through novel two-step pumping scheme for dissipative Kerr soliton generation. Controllable interaction between stimulated Brillouin lasing and Kerr nonlinearity enhances the DKS coherence and mitigates the thermal instability challenge, achieving a remarkable 22-Hz intrinsic comb linewidth and an unprecedented phase noise of -180 dBc/Hz at 945-MHz carrier at free running. The scheme can be generalized to various device platforms for field-deployable precision metrology.

Uplink Achievable Rate of Intelligent Reflecting Surface-Aided Millimeter-Wave Communications with Low-Resolution ADC and Phase Noise

Abstract: In this paper, we derive the uplink achievable rate expression of intelligent reflecting surface (IRS)-aided millimeter-wave (mmWave) systems, taking into account the phase noise at IRS and the quantization error at base stations (BSs). We show that the performance is limited only by the resolution of analog-digital converters (ADCs) at BSs when the number of IRS reflectors grows without bound. On the other hand, if BSs have ideal ADCs, the performance loss caused by IRS phase noise is a constant. Finally, our results validate the feasibility of using low-precision hardware at the IRS when BSs are equipped with low-resolution ADCs.

Point-to-Point Stabilised Optical Frequency Transfer with Active Optics

Abstract: Timescale comparison between optical atomic clocks over ground-to-space and terrestrial free-space laser links will have enormous benefits for fundamental and applied science, from measurements of fundamental constants and searches for dark matter, to geophysics and environmental monitoring. However, turbulence in the atmosphere creates phase noise on the laser signal, greatly degrading the precision of the measurements, and also induces scintillation and beam wander which cause periodic deep fades and loss of signal. We demonstrate phase stabilized optical frequency transfer over a 265 m horizontal point-to-point free-space link between optical terminals with active tip-tilt mirrors to suppress beam wander, in a compact, human-portable set-up. A phase stabilized 715 m underground optical fiber link between the two terminals is used to measure the performance of the free-space link. The active optics terminals enabled continuous, coherent transmission over periods of up to an hour. We achieve an 80 dB suppression of atmospheric phase noise to $3\times10^{-6}$ rad$^{2}$Hz$^{-1}$ at 1 Hz, and an ultimate fractional frequency stability of $1.6\times10^{-19}$ after 40 s of integration. At high frequency this performance is limited by the residual atmospheric noise after compensation and the frequency noise of the laser seen through the unequal delays of the free space and fiber links. Our long term stability is limited by the thermal shielding of the phase stabilization system. We achieve residual instabilities below those of the best optical atomic clocks, ensuring clock-limited frequency comparison over turbulent free-space links.

A Block Bi-Diagonalization-Based Pre-Coding for Indoor Multiple-Input-Multiple-Output-Visible Light Communication System

Abstract: Visible Light Communication (VLC) is a promising field in optical wireless communications, which uses the illumination infrastructure for data transmission. The important features of VLC are electromagnetic interference-free, license-free, etc. Additionally, Multiple-Input-Multiple-Output (MIMO) techniques are enabled in the VLC for enhancing the limited modulation bandwidth by its spectral efficiency. The data transmission through the MIMO-VLC system is corrupted by different interferences, namely thermal noise, shot noise and phase noise, which are caused by the traditional fluorescent light. In this paper, an effective precoding technique, namely Block Bi-Diagonalization (BBD), is enabled to mitigate the interference occurring in the indoor MIMO-VLC communications. Besides, a Quadrature Amplitude Modulation (QAM) is used to modulate the signal before transmission. Here, the indoor MIMO-VLC system is developed to analyze the communication performance under noise constraints. The performance of the proposed system is analyzed in terms of Bit Error Rate (BER) and throughput. Furthermore, the performances are compared with three different existing methods such as OAP, FBM and NRZ-OOK-LOS. The BER value of the proposed system of scenario 1 is 0.0501 at 10 dB, which is less than that of the FBM technique.

Exploiting Sensitivity Enhancement in Micro-wave Planar Sensors Using Intermodulation Products With Phase Noise Analysis

Abstract: In this paper, a new technique to enhance the sensitivity of microwave resonators is introduced Double split ring resonators are implemented as the core of a loss-compensated resonator It is illustrated that regenerative oscillators when mixed together can produce higher order intermodulation products (IMP) at the output The variations in sensing tone are multiplied and exhibit considerably higher sensitivities at 3rd, 5th, and 7th IMP components compared to the main resonant frequency The sensor is also integrated into wireless platform with ultra-wideband bowtie antennas Common fluids such as Toluene, n-Heptane, IPA, Ethanol, Methanol, Acetone, and Water are tested in fluidic channel and demonstrated that the sensitivity for intermodulation products are significantly increased proportional to the order of IMP The proposed sensor is also examined with glucose concentration sensing for the range of 0– 600 mg/dL and significant variation for 7th IMP sensor is observed as opposed to saturation of conventional sensor Also, asphaltene concentrations down to 6 ppm are recognizable when precipitated from toluene solution using the proposed sensor Moreover, a rigorous analytical study is presented for phase noise of the IMP originated from reference signals

Performance Evaluation and Diversity Analysis of RIS-Assisted Communications Over Generalized Fading Channels in the Presence of Phase Noise.

Abstract: In this paper, we develop a comprehensive theoretical framework for analyzing the performance of reconfigurable intelligent surfaces (RISs)-assisted communication systems over generalized fading channels and in the presence of phase noise. To this end, we propose the Fox's H model as a unified fading distribution for a large number of widely used generalized fading channels. In particular, we derive a unified analytical framework for computing the outage probability and for estimating the achievable diversity order of RIS-aided systems in the presence of phase shifts that either are optimally configured or are impaired by phase noise. The resulting expressions are general, as they hold for an arbitrary number of reflecting elements, and various channel fading and phase noise distributions. As far as the diversity order is concerned, notably, we introduce an asymptotic analytical framework for determining the diversity order in the absence of phase noise, as well as sufficient conditions based on upper bounds and lower bounds for ensuring that RIS-assisted systems achieve the full diversity order in the presence of phase noise. More specifically, if the absolute difference between pairs of phase errors is less than $\pi/2$, RIS-assisted communications achieve the full diversity order over independent fading channels, even in the presence of phase noise. The theoretical frameworks and findings are validated with the aid of Monte Carlo simulations.

17.2 A 66fs rms Jitter 12.8-to-15.2GHz Fractional-N Bang-Bang PLL with Digital Frequency-Error Recovery for Fast Locking

Abstract: The substantial increase in mobile data-rates, enabled by the 5G standard, calls for significantly lower integrated jitter of the local oscillator with respect to previous generations, with requirements below 90fs rms for millimeter-wave frequency bands [1]. To satisfy such stringent requirements, while at the same time guaranteeing fast lock, analog PLLs have been preferred over digital implementations in recent literature [1], [2]. Digital bang-bang PLLs, on the other hand, consume less power and occupy smaller footprint due to the absence of analog loop filters. Digital bang-bang PLLs, however, generally suffer from poor locking performance, which is due to the bang-bang phase detector (BBPD) overloading in presence of large frequency errors, and from increased jitter due to quantization. To improve locking, [3] relies on additional frequency-locking loops (FLLs) to bypass the BBPD when overloaded, at the cost of additional design complexity and power. To improve on quantization noise, [4] proposes a digital PLL based on an enhanced-resolution time-to-digital converter (TDC). However, locking performances were not addressed, and its operation was limited to the integer-N mode. Achieving low-jitter in a fractional-N PLL poses an extra challenge, because of the random jitter introduced by the digital-to-time converter (DTC) and the spectrum folding of the DTC quantization noise arising from its non-linearity and memory effects [1].

A VGA Indirect Time-of-Flight CMOS Image Sensor With 4-Tap 7- $\mu$ m Global-Shutter Pixel and Fixed-Pattern Phase Noise Self-Compensation

Abstract: A video graphics array (VGA) (640 $\times $ 480) indirect time-of-flight (ToF) CMOS image sensor has been designed with 4-tap 7- $\mu \text{m}$ global-shutter pixel in 65-nm back-side illumination (BSI) process. With a 4-tap pixel structure, we achieved motion artifact-free depth map. Peak current during exposure time has been reduced by current spreading with constant delay chain in the photo-gate driver. Column fixed-pattern phase noise (FPPN) from the constant delay chain is self-compensated by the proposed time-interleaving technique with the two inversely directional clock chains in the photo-gate driver. Quantum efficiency (QE) and demodulation contrast (DC) have been optimized by using appropriate optical engineering techniques with an optimal silicon thickness. As a result, QE of 34% at 940-nm near-infrared and high DC of 86% at 100-MHz modulation frequency have been achieved. In addition, motion artifact and column FPPN are successfully removed in the depth map. The proposed ToF sensor shows depth noise less than 0.57% with 940-nm illuminator over the working distance up to 4 m, and consumes only 160 mW for VGA output at 60 frames/s.

A 3.3-mW 25.2-to-29.4-GHz Current-Reuse VCO Using a Single-Turn Multi-Tap Inductor and Differential-Only Switched-Capacitor Arrays With a 187.6-dBc/Hz FOM

Abstract: A millimeter-wave current-reuse voltage-controlled oscillator (VCO) features a single-turn multi-tap inductor and two separate differential-only switched-capacitor arrays to improve the power efficiency and phase noise (PN). Specifically, a single-branch complementary VCO topology, in conjunction with a multi-resonant Resistor-Inductor-Capacitor-Mutual inductance (RLCM) tank, allows sharing the bias current and reshaping the impulse-sensitivity-function. The latter is based on an area- efficient RLCM tank to concurrently generate two high quality- factor differential-mode resonances at the fundamental and 2nd- harmonic oscillation frequencies. Fabricated in 65-nm CMOS technology, our VCO at 27.7 GHz shows a PN of −109.91-dBc/Hz at 1-MHz offset (after on-chip divider-by-2), while consuming just 3.3 mW at a 1.1-V supply. It corresponds to a Figure-of-Merit (FOM) of 187.6 dBc/Hz. The frequency tuning range is 15.3% (25.2 to 29.4 GHz) and the core area is 0.116 mm2.

Active mode-locking optoelectronic oscillator.

Abstract: The optoelectronic oscillator (OEO) has been widely investigated to generate ultra-pure single-frequency microwave signals. In this study, we propose and experimentally demonstrate an active mode-locking optoelectronic oscillator (AML-OEO), which can generate broadband microwave frequency comb (MFC) signals. An additional intensity modulator is inserted into the OEO as an active mode-locking device for loss modulation to realize phase-locking between adjacent oscillation modes. Through the active mode-locking technique, steady multi-mode oscillation is achieved, which is difficult to realize in a conventional OEO due to the mode-competition effect. By tuning the frequency of the active modulation signal (AMS), both fundamental and harmonic AML-OEOs can be established. In the experiments, MFC signals with a frequency spacing of 195 kHz and 50.115/100.035 MHz are generated with fundamental and harmonic AML-OEOs.

Laser Phase Noise Tolerance of Uniform and Probabilistically Shaped QAM Signals for High Spectral Efficiency Systems

Abstract: We numerically and experimentally investigate the laser phase noise tolerance of probabilistically shaped (PS) and uniformly shaped (US) quadrature amplitude modulation (QAM) signals. In the simulations, we compare PS-64QAM to US-16QAM, PS-256QAM to US-64QAM, and PS-1024QAM to US-256QAM under the same information rate (IR). We confirm that a sufficient shaping gain is observed with narrow linewidth lasers, whereas degradation of the shaping gain is clearly observed when large phase noise and high order modulation formats are assumed. In our experiments, we compare polarization-division-multiplexed (PDM) 16-GBd PS-1024QAM and US-256QAM under the same IR using lasers with 0.1-kHz and 40-kHz linewidths. For carrier phase recovery (CPR), we employ a pilot-assisted digital phase locked loop. Results reveal that PS-1024QAM achieves high performance with the 0.1 kHz-laser or >5% pilot ratio, whereas US-256QAM outperforms PS-1024QAM when lasers with 40-kHz linewidth and <5% pilot ratio are used. We also evaluate the pilot ratio dependency of the required optical signal-to-noise ratio at the forward error correction limit and the achievable information rate. Additionally, we compare the performance of two types of CPR updating schemes: updating phase estimation at only the pilot symbol or at all symbols.

Spatial Modulation for Terahertz Communication Systems With Hardware Impairments

Abstract: Terahertz (THz) communication has emerged as a promising technology that efficiently alleviates the scarcity of the frequency resources, and is capable of providing Terabit-per-second (Tbps) data transmission. However, the hardware impairments, which are usually omitted in the low-rate systems, have a detrimental impact on the THz communication link. These hardware imperfections, including in-phase/quadrature (I/Q) imbalance, phase noise and nonlinearities of the power amplifier, can be approximately modelled as complex-Gaussian distortions at both the transmitter and the receiver. In this paper, the channel estimator and signal detector (CE/SD) design of the THz-band spatial modulation (THz-SM) system is investigated, where the counterparts for classical low-rate SM are no longer applicable. More specifically, a dual-stage maximum-likelihood (ML) channel estimator is proposed containing two phases of candidate acquirement and exhaustive search. Then a novel low-complexity mean-least-squares (MLS) channel estimator is developed by considering the hardware impairments in an averaging manner. Besides, the optimal detector for THz-SM is designed by taking hardware impairments into account. Simulation results validate that the proposed CE/SD design is capable of attaining reliable data transmission in THz-SM with hardware imperfections.

Real-Time Demonstration of 600 Gb/s DP-64QAM Self-Homodyne Coherent Bi-Direction Transmission with Un-Cooled DFB Laser

Abstract: We report first successful real-time self-homodyne coherent bi-direction transmission demonstration with 600-Gb/s DP-64QAM under un-cooled ∼7-MHz linewidth DFB laser. A novel coherent receiver is proposed to achieve automatic stabilization against polarization fluctuations of received LO.

19.3 A 200dB FoM 4-to-5GHz Cryogenic Oscillator with an Automatic Common-Mode Resonance Calibration for Quantum Computing Applications

Abstract: Low-power, low phase noise (PN) cryogenic frequency generation is required for the control electronics of quantum computers. To avoid limiting the performance of quantum bits, the frequency noise of a PLL should be < 1.9 kHz rms [1]. However, it is challenging for RF oscillators, as the heart of frequency synthesizers to satisfy such a requirement at cryogenic temperatures (CT), since 1) white noise in nanoscale CMOS devices is limited by temperature-independent shot noise; 2) the transistor 1/f noise is much higher, resulting in the oscillator PN being dominated by the 30dB/dec region [1].

Ultralow jitter silica microcomb

Abstract: Silica microcombs have a high potential for generating tens of gigahertz of optical pulse trains with ultralow timing jitter, which is highly suitable for higher speed and higher bandwidth information systems. So far, the accurate characterization of timing jitter in microcombs has been limited by the measurement methods—although theoretically predicted to be >20dB better performance, the true performance has not been accurately measured until now. Here, using a self-heterodyne-based measurement method with 20zs/Hz1/2 resolution, we show that 2.6-fs rms timing jitter is possible for 22-GHz silica microcombs. We identified their origins, which suggests that silica microcombs may achieve 200-as-level jitter by better intensity noise control. This jitter performance can greatly benefit many high-speed and high-bandwidth applications including analog-to-digital conversion, microwave generation, and optical communications.