Showing papers on "Phase noise published in 2021"

Beamforming Through Reconfigurable Intelligent Surfaces in Single-User MIMO Systems: SNR Distribution and Scaling Laws in the Presence of Channel Fading and Phase Noise

Abstract: We consider a fading channel in which a multi-antenna transmitter communicates with a multi-antenna receiver through a reconfigurable intelligent surface (RIS) that is made of ${N}$ reconfigurable passive scatterers impaired by phase noise. The beamforming vector at the transmitter, the combining vector at the receiver, and the phase shifts of the ${N}$ scatterers are optimized in order to maximize the signal-to-noise-ratio (SNR) at the receiver. By assuming Rayleigh fading (or line-of-sight propagation) on the transmitter-RIS link and Rayleigh fading on the RIS-receiver link, we prove that the SNR is a random variable that is equivalent in distribution to the product of three (or two) independent random variables whose distributions are approximated by two (or one) gamma random variables and the sum of two scaled non-central chi-square random variables. The proposed analytical framework allows us to quantify the robustness of RIS-aided transmission to fading channels. For example, we prove that the amount of fading experienced on the transmitter-RIS-receiver channel linearly decreases with ${N}~\gg ~1.$ This proves that RISs of large size can be effectively employed to make fading less severe and wireless channels more reliable.

First demonstration of 6 dB quantum noise reduction in a kilometer scale gravitational wave observatory

Abstract: Photon shot noise, arising from the quantum-mechanical nature of the light, currently limits the sensitivity of all the gravitational wave observatories at frequencies above one kilohertz. We report a successful application of squeezed vacuum states of light at the GEO 600 observatory and demonstrate for the first time a reduction of quantum noise up to 6.03±0.02 dB in a kilometer scale interferometer. This is equivalent at high frequencies to increasing the laser power circulating in the interferometer by a factor of 4. Achieving this milestone, a key goal for the upgrades of the advanced detectors required a better understanding of the noise sources and losses and implementation of robust control schemes to mitigate their contributions. In particular, we address the optical losses from beam propagation, phase noise from the squeezing ellipse, and backscattered light from the squeezed light source. The expertise gained from this work carried out at GEO 600 provides insight toward the implementation of 10 dB of squeezing envisioned for third-generation gravitational wave detectors.

Real-Time Demonstration of Homodyne Coherent Bidirectional Transmission for Next-Generation Data Center Interconnects

Abstract: We experimentally propose and demonstrate homodyne coherent detection in a short-distance data center interconnect with bidirectional fiber transmission The transmitter in the proposed system sends the modulated signal and a continuous wave (CW) tone originating from the same laser diode (LD) over the 2 lanes of a duplex fiber to the remote receiver for coherent detection Thus, this system allows the use of an uncooled LD with a large linewidth and reduces the complexity of digital signal processing (DSP) compared to that in a classical coherent system A successful real-time demonstration with 600-Gb/s DP-64QAM using uncooled large-linewidth DFB lasers is conducted Automatic stabilization against polarization fluctuations of the transmitted tone is achieved by the proposed polarization-tracking integrated coherent receiver in compact silicon photonics (SiP), tracking up to 300 rad/s without a performance penalty The phase noise caused by mismatch and the link loss budget limitations are discussed and analyzed This study shows that the proposed system is a potentially attractive solution for future 800G and 16T intra-data-center optical interconnects

Optically referenced 300 GHz millimetre-wave oscillator

Abstract: Optical frequency division via optical frequency combs has enabled a leap in microwave metrology, leading to noise performance never explored before. Extending this method to the millimetre-wave and terahertz-wave domains is of great interest. Dissipative Kerr solitons in integrated photonic chips offer the unique feature of delivering optical frequency combs with ultrahigh repetition rates from 10 GHz to 1 THz, making them relevant gears for performing optical frequency division in the millimetre-wave and terahertz-wave domains. We experimentally demonstrate the optical frequency division of an optically carried 3.6 THz reference down to 300 GHz through a dissipative Kerr soliton, photodetected with an ultrafast uni-travelling-carrier photodiode. A new measurement system, based on the characterization of a microwave reference phase locked to the 300 GHz signal under test, yields attosecond-level timing-noise sensitivity, overcoming conventional technical limitations. This work places dissipative Kerr solitons as a leading technology in the millimetre-wave and terahertz-wave field, promising breakthroughs in fundamental and civilian applications. A 300 GHz signal is generated by the combination of a low-noise stimulated Brillouin scattering process, dissipative Kerr soliton comb and optical-to-electrical conversion. A phase noise of −100 dBc Hz−1 is achieved at a Fourier frequency of 10 kHz.

Nonlinear Coherent Optical Systems in the Presence of Equalization Enhanced Phase Noise

Abstract: Equalization enhanced phase noise (EEPN) occurs due to the interplay between laser phase noise and electronic dispersion compensation (EDC) module. It degrades significantly the performance of uncompensated long-haul coherent optical fiber communication systems. In this work, a general expression accounting for EEPN is presented based on Gaussian noise model to evaluate the performance of multi-channel optical communication systems using EDC and digital nonlinearity compensation (NLC). The nonlinear interaction between the signal and the EEPN is analyzed. Numerical simulations are carried out in nonlinear Nyquist-spaced wavelength division multiplexing (WDM) coherent transmission systems. Significant performance degradation due to EEPN in the cases of EDC and NLC are observed, with and without the consideration of transceiver (TRx) noise. The validation of the analytical approach has been done via split-step Fourier simulations. The maximum transmission distance and the laser linewidth tolerance are also estimated to provide important insights into the impact of EEPN.

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

Abstract: In this letter, 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 constant. Finally, our results validate the feasibility of using low-precision hardware at the IRS when BSs are equipped with low-resolution ADCs.

2.4-GHz Highly Selective IoT Receiver Front End With Power Optimized LNTA, Frequency Divider, and Baseband Analog FIR Filter

Abstract: High selectivity becomes increasingly important with an increasing number of devices that compete in the congested 2.4-GHz industrial, scientific, and medical (ISM)-band. In addition, low power consumption is very important for Internet-of-Things (IoT) receivers. We propose a 2.4-GHz zero-intermediate frequency (IF) receiver front-end architecture that reduces power consumption by 2 $\times $ compared with state-of-the-art and improves selectivity by >20-dB without compromising on other receiver metrics. To achieve this, the entire receive chain is optimized. The low-noise transconductance amplifier (LNTA) is optimized to combine low noise with low power consumption. State-of-the-art sub-30-nm complementary metal–oxide–semiconductor (CMOS) processes have almost equal strength complementary field-effect transistors (FETs) that result in altered design tradeoffs. A Windmill 25%-duty cycle frequency divider architecture is proposed, which uses only a single NOR-gate buffer per phase to minimize power consumption and phase noise. The proposed divider requires half the power consumption and has 2 dB or more reduced phase noise when benchmarked against state-of-the-art designs. An analog finite impulse response (FIR) filter is implemented to provide very high receiver selectivity with ultralow power consumption. The receiver front end is fabricated in a 22-nm fully depleted silicon-on-insulator (FDSOI) technology and has an active area of 0.5 mm2. It consumes 370 $\mu \text{W}$ from a 700-mV supply voltage. This low power consumption is combined with a 5.5-dB noise figure. The receiver front end has −7.5-dBm input-referred third-order-intercept point (IIP3) and 1-dB gain compression for a −22-dBm blocker, both at maximum gain of 61 dB. From three channels offset onward, the adjacent channel rejection (ACR) is ≥63 dB for Bluetooth Low-Energy (BLE), BT5.0, and IEEE802.15.4.

Geometric Shaping of 2-D Constellations in the Presence of Laser Phase Noise

Abstract: In this article, we propose a geometric shaping (GS) strategy to design 8, 16, 32, and 64 -ary modulation formats for the optical fibre channel impaired by both additive white Gaussian (AWGN) and phase noise. The constellations were optimised to maximise generalised mutual information (GMI) using a mismatched channel model. The presented formats demonstrate an enhanced signal-to-noise ratio (SNR) tolerance in high phase noise regimes when compared with their quadrature amplitude modulation (QAM) or AWGN-optimised counterparts. By putting the optimisation results in the context of the 400ZR implementation agreement, we show that GS alone can either relax the laser linewidth (LW) or carrier phase estimation (CPE) requirements of 400 Gbit/s transmission links and beyond. Following the GMI validation, the performance of the presented formats was examined in terms of post forward error correction (FEC) bit-error-rate (BER) for a soft decision (SD) extended Hamming code (128, 120), implemented as per the 400ZR implementation agreement. We demonstrate gains of up to 1.2 dB when compared to the 64 -ary AWGN shaped formats.

Optical Heterodyne Analog Radio-Over-Fiber Link for Millimeter-Wave Wireless Systems

Abstract: Optical heterodyne analog radio-over-fiber (A-RoF) links provide an efficient solution for future millimeter wave (mm-wave) wireless systems. The phase noise of the photo-generated mm-wave carrier limits the performance of such links, especially, for the transmission of low subcarrier baud rate multi-carrier signals. In this work, we present three different techniques for the compensation of the laser frequency offset (FO) and phase noise (PN) in an optical heterodyne A-RoF system. The first approach advocates the use of an analog mm-wave receiver; the second approach uses standard digital signal processing (DSP) algorithms, while in the third approach, the use of a photonic integrated mode locked laser (MLL) with reduced DSP is advocated. The compensation of the FO and PN with these three approaches is demonstrated by successfully transmitting a 1.95 MHz subcarrier spaced orthogonal frequency division multiplexing (OFDM) signal over a 25 km 61 GHz mm-wave optical heterodyne A-RoF link. The advantages and limitations of these approaches are discussed in detail and with regard to recent 5G recommendations, highlighting their potential for deployment in next generation wireless systems.

Oscillator Flicker Phase Noise: A Tutorial

Abstract: A deep understanding of how to reduce flicker phase noise (PN) in oscillators is critical in supporting ultra-low PN frequency generation for the advanced communications and other emerging high-speed applications. Unfortunately, the current literature is either full of conflicting theories and ambiguities or too complex in mathematics, hiding the physical insights. In this brief, we comprehensively review the evolution of flicker noise upconversion theories and clarify their controversial and confusing parts. Two classes of such upconversion mechanisms in voltage-biased $LC$ -tank oscillators (nMOS-only and complementary) are specifically compared and numerically verified using a commercial simulation model of 28-nm CMOS. We identify that non-resistive terminations of both 2nd and 3rd harmonic currents contribute to oscillation waveform asymmetries that lead to the flicker noise upconversion. Further, we discuss three $1/f^{3}$ PN reduction mechanisms: waveform shaping, narrowing of conduction angle, and gate-drain phase shift.

Theoretical Analysis of Phase Noise Induced by Laser Linewidth and Mismatch Length in Self-Homodyne Coherent Systems

Abstract: The intra- and inter- data center links face continuously increasing pressure on the transmission capacity while having to meet strict constraints in cost and power consumption. For such cost-sensitive data center applications, self-homodyne coherent (SHC) system as a promising solution has attracted increasing attention. In this article, the influence of phase noise (PN) for the SHC system is thoroughly analyzed by studying the PN probability distribution characteristic and bit-error rate (BER) performance in theory. The correctness of the theorical PN analysis is also verified by simulation. Further, the PN characteristics in SHC systems are investigated and discussed with and without the implementation of carrier phase recovery (CPR). A unique property is found for SHC systems that the system performance is only related to the product of linewidth and mismatch length, independent of the symbol rate in the absence of CPR. For SHC-16QAM systems, in order to limit the OSNR penalty to below 1-dB, the product of laser linewidth and delay mismatch should be kept below 0.18 ${\bf{MHz}} \cdot {\bf{m}}$ . Under such a conduciton, no CPR algorithm is reuiqred. Alternatively, in presence of CPR, the phase noise tolerance of SHC and conventional coherent systems are similar.

Analysis and Compensation of Phase Noise in Mm-Wave OFDM ARoF Systems for Beyond 5G

Abstract: Fifth-generation mobile networks (5G) are the solution for the demanding mobile traffic requirements, providing technologies that fulfill the requisites of different type of services. The utilization of the millimeter-wave (mm-wave) band is the straightforward technique to achieve high bit rates. Moreover, analog radio-over-fiber (ARoF) brings outstanding benefits such as low cost, low power consumption, and high spectral efficiency, among others. Thereby, mm-wave ARoF is a strong candidate to pave the way for common public radio interface (CPRI) in the fronthaul for the future 5G architecture. As orthogonal frequency-division multiplexing (OFDM) is the adopted waveform in the 5G standard, it should be also utilized in mm-wave ARoF systems for 5G. However, phase noise is one of the most degrading factors in mm-wave OFDM ARoF systems. Therefore, in this work, an analysis of the phase noise is carried out through an experimental setup up. The configuration of this setup enables to gradually modify the final phase noise level of the system. Furthermore, an original and novel algorithm to compensate the phase noise in OFDM receivers is proposed. The performance of this algorithm is experimentally evaluated through the setup for different phase noise levels and different subcarrier spacings. The obtained results show the effectiveness of the proposed algorithm under those conditions, highlighting the viability of mm-wave OFDM ARoF for 5G and beyond.

Tutorial on optoelectronic oscillators

Abstract: Microwave photonic approaches for the generation of microwave signals have attracted substantial attention in recent years, thanks to the significant advantages brought by photonics technology, such as high frequency, large bandwidth, and immunity to electromagnetic interference. An optoelectronic oscillator (OEO) is a paradigmatic microwave photonic oscillator that produces microwave signals with ultra-low phase noise, thanks to the high-quality-factor of the OEO cavity that is achieved with the help of optical energy storage elements, such as low-loss optical fiber or a high-quality-factor optical resonator. Different OEO architectures have been proposed to generate spectrally pure single-frequency microwave signals with ultra-low phase noise. Multiple oscillation mode control methods have been proposed in recent years to obtain different kinds of microwave signals. With the rapid development of photonic integration technologies, prototypes of integrated OEOs have been demonstrated with compact size and low power consumption. Moreover, OEOs have also been used for sensing, computing, and signal processing. This Tutorial aims to provide a comprehensive introduction to the developments of OEOs. We first discuss the basic principle and the key phase noise property of OEOs and then focus on its developments in spectrally pure low phase noise signal generation and mode control methods, its chip-scale integration, and its applications in various fields.

A low-noise photonic heterodyne synthesizer and its application to millimeter-wave radar.

Abstract: Microwave photonics offers transformative capabilities for ultra-wideband electronic signal processing and frequency synthesis with record-low phase noise levels. Despite the intrinsic bandwidth of optical systems operating at ~200 THz carrier frequencies, many schemes for high-performance photonics-based microwave generation lack broadband tunability, and experience tradeoffs between noise level, complexity, and frequency. An alternative approach uses direct frequency down-mixing of two tunable semiconductor lasers on a fast photodiode. This form of optical heterodyning is frequency-agile, but experimental realizations have been hindered by the relatively high noise of free-running lasers. Here, we demonstrate a heterodyne synthesizer based on ultralow-noise self-injection-locked lasers, enabling highly-coherent, photonics-based microwave and millimeter-wave generation. Continuously-tunable operation is realized from 1-104 GHz, with constant phase noise of -109 dBc/Hz at 100 kHz offset from carrier. To explore its practical utility, we leverage this photonic source as the local oscillator within a 95-GHz frequency-modulated continuous wave (FMCW) radar. Through field testing, we observe dramatic reduction in phase-noise-related Doppler and ranging artifacts as compared to the radar’s existing electronic synthesizer. These results establish strong potential for coherent heterodyne millimeter-wave generation, opening the door to a variety of future applications including high-dynamic range remote sensing, wideband wireless communications, and THz spectroscopy. Photonics-based radars offer intriguing potential but face tradeoffs in tunability, complexity, and noise. Here the authors present microwave generation in a photonics platform by heterodyning of two low-noise, self-injection-locked lasers, and demonstrate its advantages in an FMCW radar system.

Design and Analysis of Low Power and High Frequency Current Starved Sleep Voltage Controlled Oscillator for Phase Locked Loop Application

Abstract: This paper presents a current starved sleep voltage-controlled oscillator(VCO) for the Phase Locked Loop (PLL) at high frequency with low power. The PLL’s significance is still vital in many communication systems today, such as GPS system, clock data recovery, satellite communication, and frequency synthesizer. The PLL design for low voltage applications has many challenges, such as leakage power, supply voltage fluctuations. The VCO is an electronic oscillator which produces oscillating frequency for the control voltage. Current starved VCO is popular among the oscillators because it offers the right balance between low area, wide tuning range. VCO’s power consumption is most significant and has an impact on the performance of the low power PLL. This work proposes the many inverter delay techniques (stack delay cell, sleepy stack delay cell, sleep delay cell) techniques in current starved VCO, resulting in reduced leakage power consumption. Introducing the sleep transistor between the pull-up MOSFET and supply voltage in an inverter induces a reverse bias, causing the reduction in sub-threshold leakage current when both are in off condition. The current starved sleep VCO has been designed using CMOS 90nm technology and investigated at the operating frequency of 1 GHz and wide tuning range from 0.5GHz-5.8GHz. It is to be observed that the power dissipation of the VCO is 8.12μ W, which is 2.3X lesser to the conventional VCO, the phase noise of -115dBc/Hz @1MHz and figure of merit value of -228.2dBc/Hz.

Dispersive-wave induced noise limits in miniature soliton microwave sources.

Abstract: Compact, low-noise microwave sources are required throughout a wide range of application areas including frequency metrology, wireless-communications and airborne radar systems. And the photonic generation of microwaves using soliton microcombs offers a path towards integrated, low noise microwave signal sources. In these devices, a so called quiet-point of operation has been shown to reduce microwave frequency noise. Such operation decouples pump frequency noise from the soliton’s motion by balancing the Raman self-frequency shift with dispersive-wave recoil. Here, we explore the limit of this noise suppression approach and reveal a fundamental noise mechanism associated with fluctuations of the dispersive wave frequency. At the same time, pump noise reduction by as much as 36 dB is demonstrated. This fundamental noise mechanism is expected to impact microwave noise (and pulse timing jitter) whenever solitons radiate into dispersive waves belonging to different spatial mode families. Here the authors explore the noise spectrum of soliton microcomb when the pump is decoupled from the solitons motion by balancing the Raman shift with the emitted dispersive wave. Based on the analysis of the phase noise and the soliton repetition rate, they identify the uncorrelated thermal fluctuations as underlying mechanism.

A 77-GHz 8RX3TX Transceiver for 250-m Long-Range Automotive Radar in 40-nm CMOS Technology

Abstract: An automotive 77-GHz long-range radar (LRR) with an 8-channel receiver (RX) and a 3-channel transmitter (TX) in 40-nm CMOS technology is presented. It integrates a 38.5-GHz phase-locked loop (PLL), a transmitter power detector (DET) and power calibration loop, a crystal oscillator (XO), built-in-self-test (BIST) circuits, an SRAM, an eFuse, a temperature compensation calibration loop with a lookup table (LUT) and a temperature sensor, a serial peripheral interface (SPI), and a multiple-input multiple-output (MIMO) control logic. The receiver shows a noise figure (NF) of 8.7 dB and input-referred 1-dB compression point (IP1-dB) of −7.4 dBm. The NF and IP1-dB under the worst conditions are 14 dB and −10 dBm, respectively. The transmitter shows output power of 14.1 dBm and phase noise of −116 dBc/Hz at a 12.5-MHz offset frequency, which corresponds to the frequency of 250-m objects for the fast-chirp frequency-modulated continuous wave (FMCW) radar. The proposed radar module utilizes two transmitter channels for horizontal detection. A 2 $\times $ 8 time-division-multiplexing MIMO (TDM-MIMO) technique provides a detection range of 250 m.

Photonics-assisted joint radar and communication system based on an optoelectronic oscillator.

Abstract: This paper reports a photonics-assisted joint radar and communication system for intelligent transportation based on an optoelectronic oscillator (OEO). By manipulating the optical multi-dimensional processing module inserted in the OEO loop, two phase-orthogonal integrated signals are generated with low phase noise and high frequency, as the communication data loaded on the overall polarity of radar pulses. At the receiver, single-channel matched filtering and two-channel IQ data fusion are utilized to retrieve the communication data and the range profile, without any performance deterioration of either. In this way, the contradiction between the performance of two functions existing in the previous scheme is solved, and the integrated performance can be further optimized as bandwidth increases. A proof-of-concept experiment with 2 GHz bandwidth at 24 GHz, which is the operating frequency of short-range automotive radar, is carried out to verify that the proposed system can meet the requirement of the intelligent vehicles in the short-range scene. A communication capacity of 335.6 Mbps, a range profile with a resolution of 0.075 m, and a peak-to-sidelobe ratio (PSLR) of 20 dB is demonstrated under the experimental condition. The error vector magnitude (EVM) curve and constellation diagrams versus received power are measured, where the EVM is -8 and -14.5 dB corresponding to a power of -14 and 6 dBm, respectively.

A 2-D Mode-Switching Quad-Core Oscillator Using E-M Mixed-Coupling Resonance Boosting

Abstract: This article proposes a quad-core millimeter-wave (mm-wave) oscillator using E-M mixed-coupling resonance boosting. The mixed-coupling resonator is introduced to generate four reconfigurable resonances. The magnetic coupling and electric coupling are achieved by the symmetrically coupled transformer and coupling capacitor, respectively. A 2-D mode switch array is introduced to couple the quad-core and achieve the quad-mode switching without degrading the performance of the selected mode. Therefore, the reduction of phase noise and the extension of tuning range are achieved simultaneously. Meanwhile, the digitally controlled tail-resistor array is used to optimize the flicker noise up-conversion in wideband. Prototyped in 40-nm CMOS, the proposed oscillator exhibits a 73.2% tuning range from 18.6 to 40.1 GHz. The best phase noise at 1-MHz offset is −108.5 dBc/Hz, corresponding to an figure-of-merit (FoM) of 184.4 dBc/Hz and FoMT of 201.7 dBc/Hz. The variation range of FoM over the whole frequency range is 3 dB. The 1/ $f^{3}$ phase noise corner is 220–620 kHz.

Mobile Communications Beyond 52.6 GHz: Waveforms, Numerology, and Phase Noise Challenge

Abstract: In this article, the 5G New Radio (NR) physical layer evolution to support beyond 52.6 GHz communications is addressed. The performance of both OFDM based and DFT-s-OFDM based networks are evaluated with special emphasis on the phase noise (PN) induced distortion. It is shown that DFT-s-OFDM is more robust against PN under 5G NR Release 15 assumptions, specifically regarding the supported phase tracking reference signal (PTRS) designs. To further improve the PN compensation capabilities, the PTRS design for DFT-s-OFDM is revised, while for the OFDM waveform a novel block PTRS structure is introduced, providing similar link performance as DFT-s-OFDM with enhanced PTRS design. We demonstrate that the existing 5G NR Release 15 solutions can be extended to support deployments at 60 GHz bands with the enhanced PTRS structures. In addition, DFT-s-OFDM based downlink for user data could be considered for beyond 52.6 GHz communications to further improve system power efficiency and performance with higher order modulation and coding schemes. Finally, network link budget and cell size considerations are provided, showing that at certain bands with specific transmit power regulation, the cell size can eventually be downlink limited.

A Comprehensive Phase Noise Analysis of Bang-Bang Digital PLLs

Abstract: This work introduces an accurate linearized model and phase noise spectral analysis of digital bang-bang PLLs, that includes both the reference and the digitally-controlled oscillator (DCO) noise contributions. A time-domain analysis of bang-bang PLLs is leveraged to derive closed-form expressions for the integrated jitter, leading to a precise estimation of the binary phase detector (BPD) equivalent gain. The theoretical predictions differ by less than 1% from the simulation results obtained using a behavioral model, in all typical cases: dominant reference noise, dominant DCO noise, and comparable contributions. An accurate discrete-time model that takes into account the time-variant effect arising from the multirate nature of a digital phase-locked loop (DPLL) is used, along with the provided estimation of the jitter, to predict the output and input-referred phase noise spectra. An excellent match with the simulated spectra is achieved for all the different operating conditions.

Highly Tunable Heterodyne Sub-THz Wireless Link Entirely Based on Optoelectronics

Abstract: This article presents the experimental demonstration of a fully photonics-based heterodyne subterahertz (sub-THz) system for wireless communications. A p-i-n photodiode is used as a broadband transmitter to upconvert the signal to the sub-THz domain and a photoconductive antenna downconverts the received wave to an intermediate frequency around 3.7 GHz. The optical signals used for photomixing are extracted from two independent optical frequency combs with different repetition rates. The optical phase locking reduces the phase noise of the sub-THz signal, greatly improving the performance of the system when phase modulation formats are transmitted. The sub-THz carrier is tuned between 80 and 320 GHz in 40-GHz steps, showing a power variation of 21.8 dB. The phase noise at both ends of the communication link is analyzed and compared with the phase noise of the received signal with different wireless carriers. As a proof-of-concept, a 100-Mbit/s binary-phase-shift-keying signal is successfully transmitted over 80-, 120-, and 160-GHz carriers, achieving a bit error rate below 10−5 in the first two cases. These results show the great potential of THz communications driven by photonics to cover an extensive portion of the THz range without relying on electronic components that limit the operating range of the system to a concrete frequency band.

Spectral phase transitions in optical parametric oscillators

Abstract: Driven nonlinear resonators provide a fertile ground for phenomena related to phase transitions far from equilibrium, which can open opportunities unattainable in their linear counterparts Here, we show that optical parametric oscillators (OPOs) can undergo second-order phase transitions in the spectral domain between degenerate and non-degenerate regimes This abrupt change in the spectral response follows a square-root dependence around the critical point, exhibiting high sensitivity to parameter variation akin to systems around an exceptional point We experimentally demonstrate such a phase transition in a quadratic OPO We show that the divergent susceptibility of the critical point is accompanied by spontaneous symmetry breaking and distinct phase noise properties in the two regimes, indicating the importance of a beyond nonlinear bifurcation interpretation We also predict the occurrence of first-order spectral phase transitions in coupled OPOs Our results on non-equilibrium spectral behaviors can be utilized for enhanced sensing, advanced computing, and quantum information processing

Perspective on the next generation of ultra-low noise fiber supercontinuum sources and their emerging applications in spectroscopy, imaging, and ultrafast photonics

Abstract: A new generation of ultrafast and low-noise supercontinuum (SC) sources is currently emerging, driven by the constantly increasing demands of spectroscopy, advanced microscopy, and ultrafast photonics applications for highly stable broadband coherent light sources. In this Perspective, we review recent progress enabled by advances in nonlinear optical fiber design, detail our view on the largely untapped potential for noise control in nonlinear fiber optics, and present the noise fingerprinting technique for measuring and visualizing the noise of SC sources with unprecedented detail. In our outlook, we highlight how these SC sources push the boundaries for many spectroscopy and imaging modalities and focus on their role in the development of ultrafast fiber lasers and frequency combs with ultra-low amplitude and phase noise operating in the 2 μm spectral region and beyond in the mid-IR.

Two-octave-wide (3-12 µm) subharmonic produced in a minimally dispersive optical parametric oscillator cavity.

Abstract: We report a subharmonic (frequency-divide-by-2) optical parametric oscillator (OPO) with a continuous wavelength span of 3 to 12 µm (-37dB level) that covers most of the molecular rovibrational "signature" region. The key to obtaining such a wide spectral span is the use of an OPO with a minimal dispersion-through the choice of intracavity elements, the use of all gold-coated mirrors, and a special "injector" mirror. The system delivers up to 245 mW of the average power with the conversion efficiency exceeding 20% from a 2.35 µm Kerr-lens mode-locked pump laser.

Gradient-Free Training of Autoencoders for Non-Differentiable Communication Channels

Abstract: Training of autoencoders using the back-propagation algorithm is challenging for non-differential channel models or in an experimental environment where gradients cannot be computed. In this paper, we study a gradient–free training method based on the cubature Kalman filter. To numerically validate the method, the autoencoder is employed to perform geometric constellation shaping on differentiable communication channels, showing the same performance as the back-propagation algorithm. Further investigation is done on a non–differentiable communication channel that includes: laser phase noise, additive white Gaussian noise and blind phase search-based phase noise compensation. Our results indicate that the autoencoder can be successfully optimized using the proposed training method to achieve better robustness to residual phase noise with respect to standard constellation schemes such as Quadrature Amplitude Modulation and Iterative Polar Modulation for the considered conditions.

X -Band Feedback Type Miniaturized Oscillator Design With Low Phase Noise Based on Eighth Mode SIW Bandpass Filter

Abstract: A standard substrate integrated waveguide (SIW)-based bandpass filter (BPF) suffers from bulkiness that limits its application in the implementation of compact size low phase noise oscillator. Instead, in this letter, an eighth mode SIW (EMSIW) resonator-based BPF is utilized to realize a miniaturized low phase noise oscillator for $X$ -band applications. Additionally, an equivalent circuit model of EMSIW-BPF is presented. A compact feedback-type oscillator prototype using EMSIW-BPF is designed and fabricated for the first time with a circuit area of just 1092 mm2. The experimental results of the prototype show a phase noise of −126.13 dBc/Hz at 1 MHz offset from 9.97 GHz oscillation frequency with an output power of 3.17 dBm.

Spurious level and phase noise improved Fourier domain mode-locked optoelectronic oscillator based on a self-injection-locking technique.

Abstract: We propose and experimentally demonstrate a spurious level and phase noise improved Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) based on a self-injection-locking (SIL) technique. The scheme applies a dual-loop FDML-OEO structure, in which a long optical fiber delay loop is used to injection-lock the OEO with a short oscillating optical fiber delay loop. SIL is achieved so long as the delay of the long loop is tuned at the integral multiple of the oscillation loop. The spur suppression ratio of the wideband linear frequency modulated (LFM) signal generated by the FDML-OEO can be improved by 14 dB under SIL. Furthermore, the modification of the spur suppression ratio depending on the injection power is also demonstrated. The phase noise of the proposed OEO is -127.5 dBc/Hz at 10 kHz offset, which is much improved comparing with a free-running OEO.

An 8.2-to-21.5 GHz Dual-Core Quad-Mode Orthogonal-Coupled VCO with Concurrently Dual-Output using Parallel 8-Shaped Resonator

Abstract: Software-defined radio transceivers, wireless infrastructure equipment, and test equipment require the local oscillator (LO) to cover a very wide range of the output frequencies while meet the phase noise performance. The most straightforward method for a wide tuning range is to adopt multiple separated oscillators. However, this method suffers from the bulky area and relatively complicated multiplexing functions. The authors in [1] introduces triple-coupled inductors that create multiple resonant frequency in a high-order resonator. Nevertheless, the impedance magnitude looking into either port of the high-order resonator shows higher peak value at the lower resonant frequency, which makes it difficult to sustain stable oscillation at higher resonant frequency when the coupling factor is large. The VCO in [2] exploits series 8-shaped coils to generate two oscillation modes without coupling between each other while occupying compact area. However, the quality factor of the 8-shaped inductor is considerably lower than a typical octagonal inductor thereby degrading the oscillator phase noise. In order to solve the above-mentioned issues, a dual-core quad-mode orthogonal-coupled VCO using parallel 8-shaped resonator is proposed. Each core in the proposed VCO have two modes, with one mode being equivalent to the 8-shaped inductor, which allows both cores to be orthogonal to each other by proper configurations.

A 2–20-GHz Ultralow Phase Noise Signal Source Using a Microwave Oscillator Locked to a Mode-Locked Laser

Abstract: In this article, we develop the theory of a special type of optoelectronic phase-locked loop (PLL). The output signal of this type of PLL is in the electrical domain and its reference oscillator, typically a mode-locked laser, operates in the optical domain. The PLL uses a balanced optical microwave phase detector (BOMPD). In order to model the optoelectronic PLL, the nonlinear characteristic function and the gain of the BOMPD are derived analytically. Using the results of the phase detector analysis, the theory of an optoelectronic PLL using such a phase detector is developed. Based on the theoretical analysis, a broadband optoelectronic frequency synthesizer with a programmable frequency range from 2 to 20 GHz is designed and implemented. Phase noise measurements show that the optoelectronic PLL frequency synthesizer achieves an integrated rms-jitter (1 kHz–100 MHz) of less than 4 fs in the frequency range from 5 to 20 GHz with a typical value of 4 fs and a minimum of 3 fs. This is the first reported wideband PLL frequency synthesizer achieving sub-10-fs integrated rms-jitter (1 kHz–100 MHz) in the frequency range from 3 to 20 GHz. A comparison with best-in-class laboratory-grade frequency synthesizers in this frequency range shows that this synthesizer achieves lower phase noise than any electronic frequency synthesizer for offset frequencies larger than 2 kHz.