Showing papers on "Phase noise published in 2022"

Low-noise frequency-agile photonic integrated lasers for coherent ranging

Abstract: Frequency modulated continuous wave laser ranging (FMCW LiDAR) enables distance mapping with simultaneous position and velocity information, is immune to stray light, can achieve long range, operate in the eye-safe region of 1550 nm and achieve high sensitivity. Despite its advantages, it is compounded by the simultaneous requirement of both narrow linewidth low noise lasers that can be precisely chirped. While integrated silicon-based lasers, compatible with wafer scale manufacturing in large volumes at low cost, have experienced major advances and are now employed on a commercial scale in data centers, and impressive progress has led to integrated lasers with (ultra) narrow sub-100 Hz-level intrinsic linewidth based on optical feedback from photonic circuits, these lasers presently lack fast nonthermal tuning, i.e. frequency agility as required for coherent ranging. Here, we demonstrate a hybrid photonic integrated laser that exhibits very narrow intrinsic linewidth of 25 Hz while offering linear, hysteresis-free, and mode-hop-free-tuning beyond 1 GHz with up to megahertz actuation bandwidth constituting 1.6 × 1015 Hz/s tuning speed. Our approach uses foundry-based technologies - ultralow-loss (1 dB/m) Si3N4 photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase-noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise at mid-range offsets. By utilizing difference-drive and apodization of the photonic chip to suppress mechanical vibrations of the chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm. Our results constitute a photonic integrated laser system for scenarios where high compactness, fast frequency actuation, and high spectral purity are required.

Ultrastable microwave and soliton-pulse generation from fibre-photonic-stabilized microcombs

Abstract: The ability to generate lower-noise microwaves has greatly advanced high-speed, high-precision scientific and engineering fields. Microcombs have high potential for generating such low-noise microwaves from chip-scale devices. To realize an ultralow-noise performance over a wider Fourier frequency range and longer time scale, which is required for many high-precision applications, free-running microcombs must be locked to more stable reference sources. However, ultrastable reference sources, particularly optical cavity-based methods, are generally bulky, alignment-sensitive and expensive, and therefore forfeit the benefits of using chip-scale microcombs. Here, we realize compact and low-phase-noise microwave and soliton pulse generation by combining a silica-microcomb (with few-mm diameter) with a fibre-photonic-based timing reference (with few-cm diameter). An ultrastable 22-GHz microwave is generated with -110 dBc/Hz (-88 dBc/Hz) phase noise at 1-kHz (100-Hz) Fourier frequency and 10-13-level frequency instability within 1-s. This work shows the potential of fully packaged, palm-sized or smaller systems for generating both ultrastable soliton pulse trains and microwaves, thereby facilitating a wide range of field applications involving ultrahigh-stability microcombs.

A 3.3-GHz Integer N-Type-II Sub-Sampling PLL Using a BFSK-Suppressed Push–Pull SS-PD and a Fast-Locking FLL Achieving −82.2-dBc REF Spur and −255-dB FOM

Abstract: This brief describes an integer-N-type-II sub-sampling phase-locked loop (SS-PLL) incorporating a push–pull sub-sampling phase detector to significantly suppress the spur-induced binary frequency shift keying modulation (BFSK) effect and a low-power fast-locking frequency-locked loop (FLL) to shorten the settling time. Prototyped in 65-nm CMOS, the SS-PLL at 3.3 GHz shows a reference spur of −82.2 dBc, an integrated jitter of 64.9 fs_rms (1 kHz to 40 MHz), and an in-band phase noise (PN) of −128.4 dBc/Hz at 1-MHz offset. The corresponding jitter power figure of merit (FOM) is −255 dB. The entire SS-PLL consumes 7.5 mW, with only $90~\mu \text{W}$ associated with the FLL.

Coherent fibre link for synchronization of delocalized atomic clocks.

Abstract: Challenging experiments for tests in fundamental physics require highly coherent optical frequency references with suppressed phase noise from hundreds of kHz down to μHz of Fourier frequencies. It can be achieved by remote synchronization of many frequency references interconnected by stabilized optical fibre links. Here we describe the path to realize a delocalized optical frequency reference for spectroscopy of the isomeric state of the nucleus of Thorium-229 atom. This is a prerequisite for the realization of the next generation of an optical clock - the nuclear clock. We present the established 235 km long phase-coherent stabilized cross-border fibre link connecting two delocalized metrology laboratories in Brno and Vienna operating highly-coherent lasers disciplined by active Hydrogen masers through optical frequency combs. A significant part (up to tens of km) of the optical fibre is passing urban combined collectors with a non-negligible level of acoustic interference and temperature changes, which results in a power spectral density of phase noise over 105 rad2· Hz-1. Therefore, we deploy a digital signal processing technique to suppress the fibre phase noise over a wide dynamic range of phase fluctuations. To demonstrate the functionality of the link, we measured the phase noise power spectral density of a remote beat note between two independent lasers, locked to high-finesse stable resonators. Using optical frequency combs at both ends of the link, a long-term fractional frequency stability in the order of 10-15 between local active Hydrogen masers was measured as well. Thanks to this technique, we have achieved reliable operation of the phase-coherent fibre link with fractional stability of 7 × 10-18 in 103 s.

Progresses of Pilot Tone Based Optical Performance Monitoring in Coherent Systems

Abstract: Pilot tone (PT) is a low frequency, small intensity modulation applied to high speed optical channel. Traditionally PT is used for channel identification and channel power monitoring. A pilot tone scheme suitable for optical performance monitoring (OPM) in coherent optical communication systems is described. Two effects, Stimulated Raman scattering (SRS) and dispersion fading, are discussed. An important enhancement, multiband pilot tone, is introduced, which provides sub-channel monitoring capability. With the advanced PT technologies, signal spectrum, and relative frequency offset between signal and optical filter can be monitored with sub-GHz resolution. PT also enables direct OSNR and fiber nonlinear noise monitoring with high accuracy and sensitivity.

OFDM Radar and Communication Joint System Using Opto-Electronic Oscillator With Phase Noise Degradation Analysis and Mitigation

Abstract: Orthogonal frequency division multiplexing (OFDM) signal is a superior dual-functional waveform for the integration of radar sensing and communication in intelligent transportation. But the sensitivity to phase noise is a serious issue introducing interference and causing performance degradation during demodulation. In this paper, we explore the essential mechanism of the action and generation of phase noise through theoretical analysis, where the OFDM demodulation process and power spectrum density (PSD) of phase noise is discussed in the frequency domain, and draw the conclusion that high-speed phase jitter will cause unrecoverable deterioration of OFDM demodulation. Therefore, a photonics-aided radar and communication integrated system based on Optoelectronic oscillator (OEO) is proposed. The positive feedback oscillation with long energy storage time make the phase noise pattern of OEO just suitable to against the phase noise sensitivity of OFDM. A proof-of-concept experiment is demonstrated at 24 GHz with 2 GHz bandwidth to verify the radar sensing and communication function. A two-dimensional radar imaging with a range resolution of 0.075 m and velocity resolution of 4.4 km/h, a communication capacity of 6.4 Gbps is obtained. A quantitative performance comparison is also carried out. By using an ordinary microwave source and OEO separately, the demodulation constellation and error vector magnitude (EVM) under different subcarrier spacing is measured and compared. The result is corresponding to our analysis with the EVM decreasing from 12.5% to 4.7% under subcarrier spacing of 125 kHz.

Actively mode-locked optoelectronic oscillator for microwave pulse generation

Abstract: The optoelectronic oscillator (OEO) is the most widely investigated microwave photonic hybrid system for the generation of microwave signal with high frequency and low phase noise. It has been considered a reasonable and promising solution for obtaining pure microwave signal. Generally, a conventional OEO is used for the generation of only continuous microwave signal owing to the long mode building time of the oscillation cavity, leading to limited application areas. Here, the concept of actively mode-locking in fiber laser is introduced into the OEO, in which an external microwave signal with the frequency integral multiple of the free spectral range of OEO is injected to achieve a mode-locked condition. In this case, an OEO that can directly generate pulsed microwave signal with adjustable central frequency and time duration is demonstrated. Furthermore, by changing the frequency of the injection signal, harmonic mode locking with different orders can be easily achieved, while supermodes are confined. The proposed OEO has great potential in applications such as radars, communications and metrology.

Chip-Scaled Ka-Band Photonic Linearly Chirped Microwave Waveform Generator

Abstract: Synthetic aperture radar (SAR) systems employ a Linearly Chirped Microwave Waveform Generator (LCMWG) with large time–bandwidth product (TBWP), to provide a wide range resolution. Photonics has now been recognized as a disruptive approach to achieve high performance at bandwidth of few tens of gigahertz, with light and compact architectures, due to the typical photonics benefits, such as electromagnetic interference immunity, small power consumption, small footprint, and high immunity to vibration/shock and radiation. In this article, we report on the photonic generation of a high-frequency LCMW, with a large TBWP (102–103), using a chip-scaled architecture, based on a frequency-tunable optoelectronic oscillator (OEO) and a recirculating phase modulation loop (RPML). A new configuration of the OEO employing an ultrahigh Q-factor resonator has been conceived to allow the oscillator working in Ka band at 40 GHz or even more, with very low phase noise. Key building block of the RPML is a phase modulator driven by an engineered parabolic split waveform. The ultra-large pulse compression rate (PCR) >> 102, together with large signal purity, was also obtained, making the proposed architecture particularly suitable for SAR systems with large range resolution demand, such as Earth surveillance and monitoring.

Supermode noise suppression with polarization-multiplexed dual-loop for active mode-locking optoelectronic oscillator

Abstract: The active mode-locking (AML) technique has been widely used in erbium-doped fiber lasers to generate picosecond pulse trains. Here we propose a novel active mode-locking dual-loop optoelectronic oscillator (AML-DL-OEO), which can generate microwave frequency comb (MFC) signals with adjustable comb spacings. Based on this scheme, the order of harmonic mode-locking is dramatically decreased for a certain AML driving frequency compared with a single-loop AML-OEO. Thus, the supermode noise caused by harmonic mode-locking can be efficiently suppressed. In addition, the sidemodes are well suppressed by the dual-loop architecture. An experiment is performed. MFC signals with different comb spacings are generated under fundamental or harmonic mode-locking states. AML-DL-OEO systems with different length differences between two loops are implemented to evaluate supermode noise suppression capability. The performance of the generated MFC signals is recorded and analyzed.

Thermal noise reduction in soliton microcombs via laser self-cooling

Abstract: Thermal noise usually dominates the low-frequency region of the optical phase noise of soliton microcombs, which leads to decoherence that limits many aspects of applications. In this work, we demonstrate a simple and reliable way to mitigate this noise by laser cooling with a pump laser. The key is rendering the pump laser to simultaneously excite two neighboring cavity modes from different families that are respectively red and blue detuned, one for soliton generation and the other for laser cooling.

Coherent terahertz wireless communication using dual-parallel MZM-based silicon photonic integrated circuits.

Abstract: Coherent terahertz (THz) wireless communication using silicon photonics technology provides critical solutions for achieving high-capacity wireless transmission beyond 5G and 6G networks and seamless connectivity with fiber-based backbone networks. However, high-quality THz signal generation and noise-robust signal detection remain challenging owing to the presence of inter-channel crosstalk and additive noise in THz wireless environments. Here, we report coherent THz wireless communication using a silicon photonic integrated circuit that includes a dual-parallel Mach-Zehnder modulator (MZM) and advanced digital signal processing (DSP). The structure and fabrication of the dual-parallel MZM-based silicon photonic integrated circuit are systematically optimized using the figure of merit (FOM) method to improve the modulation efficiency while reducing the overall optical loss. The advanced DSP compensates for in-phase and quadrature (IQ) imbalance as well as phase noise by orthogonally decoupling the IQ components in the frequency domain after adaptive signal equalization and carrier phase estimation. The experimental results show a reduction in phase noise that induces degradation of transmission performance, successfully demonstrating error-free 1-m THz wireless transmission with bit-error rates of 10-6 or less at a data rate of 50 Gbps.

High-power low-noise 2-GHz femtosecond laser oscillator at 2.4 µm.

Abstract: Femtosecond lasers with high repetition rates are attractive for spectroscopic applications with high sampling rates, high power per comb line, and resolvable lines. However, at long wavelengths beyond 2 µm, current laser sources are either limited to low output power or repetition rates below 1 GHz. Here we present an ultrafast laser oscillator operating with high output power at multi-GHz repetition rate. The laser produces transform-limited 155-fs pulses at a repetition rate of 2 GHz, and an average power of 0.8 W, reaching up to 0.7 mW per comb line at the center wavelength of 2.38 µm. We have achieved this milestone via a Cr2+-doped ZnS solid-state laser modelocked with an InGaSb/GaSb SESAM. The laser is stable over several hours of operation. The integrated relative intensity noise is 0.15% rms for [10 Hz, 100 MHz], and the laser becomes shot noise limited (-160 dBc/Hz) at frequencies above 10 MHz. Our timing jitter measurements reveal contributions from pump laser noise and relaxation oscillations, with a timing jitter of 100 fs integrated over [3 kHz, 100 MHz]. These results open up a path towards fast and sensitive spectroscopy directly above 2 µm.

Actively mode-locked optoelectronic oscillator for microwave pulse generation

Abstract: The optoelectronic oscillator (OEO) is the most widely investigated microwave photonic hybrid system for the generation of microwave signal with high frequency and low phase noise. It has been considered a reasonable and promising solution for obtaining pure microwave signal. Generally, a conventional OEO is used for the generation of only continuous microwave signal owing to the long mode building time of the oscillation cavity, leading to limited application areas. Here, the concept of actively mode-locking in fiber laser is introduced into the OEO, in which an external microwave signal with the frequency integral multiple of the free spectral range of OEO is injected to achieve a mode-locked condition. In this case, an OEO that can directly generate pulsed microwave signal with adjustable central frequency and time duration is demonstrated. Furthermore, by changing the frequency of the injection signal, harmonic mode locking with different orders can be easily achieved, while supermodes are confined. The proposed OEO has great potential in applications such as radars, communications and metrology.

Integrated Coherent Tunable Laser (ICTL) With Ultra-Wideband Wavelength Tuning and Sub-100 Hz Lorentzian Linewidth

Abstract: This paper describes the design, fabrication, and record performance of a new class of ultra-wideband wavelength tuning, ultra-low noise semiconductor laser, the Integrated Coherent Tunable Laser (ICTL). The ICTL device is designed for, and fabricated in, a CMOS foundry based Silicon Photonics platform, utilizing heterogeneous integration of III-V material to create the integrated gain section of the laser–enabling high-volume mass-market manufacturing at low cost and with high reliability. The ICTL incorporates three or more ultra-low loss micro-ring resonators, with large ring size, in a Sagnac loop reflector geometry, creating exceptional laser reflector performance, plus an extended laser cavity length that enables highly-coherent output; ultra-low linewidth and phase noise. This paper describes record integrated laser performance; 118 nm wavelength tuning, covering S-, C- and L-bands, with Lorentzian linewidth <100 Hz, and with excellent relative intensity noise (RIN) of ≤ −155 dBc/Hz. The remarkable performance of the ICTL device, coupled with the high volume/low cost capability of the Silicon Photonics platform enables next-generation applications including ultra-wideband WDM transmission systems, fiber-optic and medical-wearable sensing systems, and automotive FMCW LiDAR systems utilizing wavelength scanning.

A 12.5-GHz Fractional-N Type-I Sampling PLL Achieving 58-fs Integrated Jitter

Abstract: This article presents a fractional-N sampling type-I phase-locked loop (PLL). To overcome the impairments of a conventional type-I PLL, namely the frequency-tuning-dependent time offset and the narrow range of the sampling phase detector (SPD), which would prevent fractional-N synthesis, a novel digital phase error correction (DPEC) technique, operating in the background, is introduced, which provides robust low-jitter performance. Besides, a novel frequency locking method is presented, which provides fast lock and seamless hand-off to main PLL operation. The PLL has been fabricated in a 28-nm CMOS technology process, and it synthesizes frequencies from 11.9 to 14.1 GHz, achieving an rms jitter of 58.2 and 51.7 fs (integrated into the 1 kHz-100 MHz bandwidth) for a fractional-N and integer-N channel, respectively. The reference spur is as low as -73.5 dBc, while the worst case near-integer fractional spurs are lower than -63.2 dBc. With a power consumption of 18 mW, the jitter-power figure of merit is -252.1 dB (fractional-N) and -253.3 dB (integer-N). The locking time is below 9 μs for a 1-GHz frequency step. The synthesizer occupies 0.16 mm², including decoupling capacitors.

Intra-Data Center 120Gbaud/DP-16QAM Self-Homodyne Coherent Links with Simplified Coherent DSP

Abstract: The first 120Gbaud-based C-band self-homodyne 800Gb/s coherent links using low-latency FEC are experimentally demonstrated. A minimum coherent DSP is proposed to compensate fiber dispersion, phase mismatch between signal and local oscillator, and transceiver I-Q impairments. © 2022 The Author(s)

Radiation-Tolerant Digitally Controlled Ring Oscillator in 65-nm CMOS

Abstract: This article presents a radiation-tolerant digitally controlled complementary metal–oxide–semiconductor (CMOS) ring oscillator design suitable for all-digital phase-locked loop (ADPLL) implementations. To address the challenges presented by harsh radiation environments, a wide tuning range oscillator architecture is presented with superior single-event effect (SEE) tolerance. The proposed oscillator circuit is characterized experimentally in a 65-nm technology and shown to achieve a significant reduction in SEE sensitivity up to a linear energy transfer (LET) of 63.5 MeVmg $^{-1}$ cm2, remain free from harmonic oscillation errors under irradiation, and withstand a total radiation dose exceeding 1.5 Grad. At the design frequency of 1.28 GHz, the oscillator dissipates 7 mW of power while achieving a phase noise of −105 dBc/Hz at 1 MHz offset, corresponding to a figure of merit (FOM) of 159 dB.

Chip Design of an All-Digital Frequency Synthesizer with Reference Spur Reduction Technique for Radar Sensing

Abstract: 5.2-GHz all-digital frequency synthesizer implemented proposed reference spur reducing with the tsmc 0.18 µm CMOS technology is proposed. It can be used for radar equipped applications and radar-communication control. It provides one ration frequency ranged from 4.68 GHz to 5.36 GHz for the local oscillator in RF frontend circuits. Adopting a phase detector that only delivers phase error raw data when phase error is investigated and reduces the updating frequency for DCO handling code achieves a decreased reference spur. Since an all-digital phase-locked loop is designed, the prototype not only optimized the chip dimensions, but also precludes the influence of process shrinks and has the advantage of noise immunity. The elements of novelties of this article are low phase noise and low power consumption. With 1.8 V supply voltage and locking at 5.22 GHz, measured results find that the output signal power is −8.03 dBm, the phase noise is −110.74 dBc/Hz at 1 MHz offset frequency and the power dissipation is 16.2 mW, while the die dimensions are 0.901 × 0.935 mm2.

A Study of BER and EVM Degradation in Digital Modulation Schemes Due to PLL Jitter and Communication-Link Noise

Abstract: A phase-locked-based frequency synthesizer – ubiquitously used to generate local oscillation in a communication transceiver – exhibits phase noise and jitter which considerably degrades bit-error rate (BER) and error vector magnitude (EVM) of a digital communication link. This paper presents an analytical study of EVM and BER degradation for a variety of widely used digital modulation schemes. Phase noise and jitter of a generic integer-N phase-locked loop (PLL) as a local oscillator feeding an RF mixer are derived, while accounting for the reference spurs and cyclo-stationarity of the mixer operation as well as the additive noise of the communication link. This jitter model is then utilized to directly study its impact on digital modulation constellations. Specifically, the EVM and BER degradation due to the PLL jitter in communication systems incorporating M-ary phase-shift keying (M-PSK) and ${4^{M}}$ quadrature amplitude modulation ( ${4^{M}}$ QAM) are analyzed. Comparison between analytical models and system-level simulations verifies an excellent accuracy of these models.

Dual-laser self-injection locking to an integrated microresonator

Abstract: Diode laser self-injection locking (SIL) to a whispering gallery mode of a high quality factor resonator is a widely used method for laser linewidth narrowing and high-frequency noise suppression. SIL has already been used for the demonstration of ultra-low-noise photonic microwave oscillators and soliton microcomb generation and has a wide range of possible applications. Up to date, SIL was demonstrated only with a single laser. However, multi-frequency and narrow-linewidth laser sources are in high demand for modern telecommunication systems, quantum technologies, and microwave photonics. Here we experimentally demonstrate the dual-laser SIL of two multifrequency laser diodes to different modes of an integrated Si3N4 microresonator. Simultaneous spectrum collapse of both lasers, as well as linewidth narrowing and high-frequency noise suppression , as well as strong nonlinear interaction of the two fields with each other, are observed. Locking both lasers to the same mode results in a simultaneous frequency and phase stabilization and coherent addition of their outputs. Additionally, we provide a comprehensive dual-SIL theory and investigate the influence of lasers on each other caused by nonlinear effects in the microresonator.

A 529-μW Fractional-N All-Digital PLL Using TDC Gain Auto-Calibration and an Inverse-Class-F DCO in 65-nm CMOS

Abstract: This paper presents an ultra-lower-power (ULP) digital-to-time-converter (DTC)-assisted fractional-N all-digital phase-locked loop (ADPLL) suitable for IoT applications. A proposed hybrid time-to-digital converter (TDC) extends the vernier-TDC input range with little power overhead in order to overcome the stability issue in the conventional architectures. The hybrid TDC also facilitates a background gain calibration to achieve a stable in-band phase noise insensitive to process, voltage, and temperature (PVT) variations. The implementation of a buffer-cascaded DTC simplifies the design complexity of the fractional-N operation. The ADPLL also features a 200 $\mu \text{W}$ low-phase-noise inverse-class-F (class-F⁻¹) digitally controlled oscillator (DCO) without the need of two-dimensional (2-D) capacitor tuning for frequency alignment of the fundamental and 2^nd-harmonic. Fabricated in 65-nm CMOS, the ULP ADPLL prototype achieves 868fs_rms jitter in a fractional-N channel when consuming only 529 $\mu \text{W}$ , corresponding to a figure-of-merit (FoM) of −244dB.

Planar substrate integrated waveguide‐based resonators and its usage to compact X‐band small phase noise oscillator

Abstract: This study denotes a procedure of a small phase noise (SPN) oscillator employing three different substrate integrated waveguide (SIW) resonators as a frequency stabilization‐component. The SIW resonator as modifying phase noise (PN) is investigated and branchline coupler in the feedback loop is presented. The design technique to attain SPN usage for optimized efficiency is proposed. Then, three different X‐band SIW oscillators using a packaged GaAs FET transistor are planned. Due to the PN and compacting, the miniaturized conventional SIW (MCSIW) oscillator is selected. This MCSIW oscillator operates at 8.15 GHz, mentions a PN of −169 dBc/Hz at 1‐MHz frequency‐offset.

Radio-frequency line-by-line Fourier synthesis based on optical soliton microcombs

Abstract: Radio-frequency (RF) waveform synthesis has broad applications in ultrawide-bandwidth wireless communications, radar systems, and electronic testing. Photonic-based approaches offer key advantages in bandwidth and phase noise thanks to the ultrahigh optical carrier frequency. In this work, we demonstrate Fourier synthesis arbitrary waveform generation (AWG) with integrated optical microresonator solitons. The RF temporal waveform is synthesized through line-by-line amplitude and phase shaping of an optical soliton microcomb, which is down-converted to the RF domain through dual-comb optical coherent sampling. A variety of RF waveforms with tunable repetition cycles are shown in our demonstration. Our approach provides not only the possibility of precise Fourier synthesis at microwave and millimeter-wave frequencies, but also a viable path to fully integrated photonic-based RF AWG on a chip.

Harmonically mode-locked optoelectronic oscillator with ultra-low supermode noise

Abstract: A harmonically mode-locked optoelectronic oscillator (OEO) based on a dual-loop architecture is proposed to generate a microwave pulse train with ultra-low supermode noise. Through using a dual-loop architecture, the unwanted longitudinal modes can be weakened by the “Vernier effect” at the early stage of the oscillation, which is beneficial for suppressing the supermode noise originating from phase locking among the unwanted modes in a harmonically mode-locked OEO. The proposed scheme is numerically and experimentally demonstrated. In the experiment, microwave pulse trains with repetition rates of 179 kHz and 186 kHz are generated through 2nd- and 5th-order harmonic mode locking, where the supermode noise suppression ratios are measured to be beyond 60 dB and 55 dB, respectively. The effective suppression of supermode noise can greatly improve the output power stability and the correlation between different pulses. Under 2nd-order harmonic mode locking, the output power drifts of the single-loop and the dual-loop architectures are measured to be 38.13 dB and 0.97 dB, respectively.

Flicker Phase-Noise Reduction Using Gate–Drain Phase Shift in Transformer-Based Oscillators

Abstract: This article presents a wide-band suppression technique of flicker phase noise (PN) by means of a gate–drain phase shift in a transformer-based complementary oscillator. We identify that after naturally canceling its second-harmonic voltage by the complementary operation itself, third-harmonic current entering the capacitive path is now the main cause of asymmetry in the rising and falling edges, leading to the $1/f$ noise upconversion. A complete $1/f^{3}$ PN analysis for the transformer-based complementary oscillator is discussed. By tuning gate–drain capacitance ratio, a specific phase-shift range is introduced at the gate and drain nodes of the cross-coupled pair to mitigate the detrimental effects of ill-behaved third-harmonic voltage, thus lowering the flicker PN. To further reduce the area and improve the PN in the thermal region, we introduce a new triple-8-shaped transformer. Fabricated in 22-nm FDSOI, the prototype occupies a compact area of 0.01mm² and achieves $1/f^{3}$ PN corner of 70kHz, PN of −110dBc/Hz at 1MHz offset, figure-of-merit (FoM) of −182dB at 9GHz, and 39% tuning range (TR). It results in the best FoM with normalized TR and area (FoM_TA) of −214dB at 1MHz offset.

Phase noise estimation based white light scanning interferometry for high-accuracy surface profiling.

Abstract: White light scanning interferometry (WLSI) has been an extremely powerful technique in precision measurements. In this work, a phase noise estimation based surface recovery algorithm is proposed, which can significantly improve the measurement accuracy by decreasing the noise level in phase map coming from the systemic and environmental disturbances. The noise existed in phase map is firstly researched in spectrum domain and defined as the linear combination of complex terms at each angular wavenumber. Afterwards, based on the theoretical linearity of the phase distribution, the surface features can be redefined through establishing the function with respect to phase noise. By applying least square estimation (LSE), a spectral coefficient is defined to determine the optimal estimation of phase noise that represents the best statistical consistency with the actual case, from which a more accurate surface after removing most phase noise will then be generated. In order to testify the noise elimination ability of the proposed method, a nano-scale step height standard (9.5nm±1.0nm) is scanned, and the measurement result 9.49nm with repeatability 0.17nm is successfully achieved. Moreover, a leading edge of an aero-engine blade is also tested to investigate the potential of this method in industrial inspections. The measurement comparison with AFM is also displayed.

High Precision Phase-OFDR Scheme Based on Fading Noise Suppression

Abstract: In recent years, Optical Frequency Domain Reflectometry (OFDR) has been able to realize strain measurement with high sensitivity and high spatial resolution (SR) along the sensing fiber. Compared with the conventional cross-correlation method, OFDR based on phase-measuring method has the potential to obtain strain information with better SR. However, the accuracy of this method is severely affected by coherent fading. In this paper, a method combining multi-frequency detection and nearest neighbor analysis is proposed to solve this problem. By comparing the sub-phase curves in different wavelength regions, we successfully eliminate the interference of fading noise and obtain the undistorted phase signal. Finally, in the case of an effective wavelength scanning range of 3.6 nm (corresponding to the ranging SR of 0.22 mm), we realized the deformation measurement with standard deviation of 0.015 μm and SR of 1.1 mm, as well as the strain measurement with standard deviation of 0.55 μϵ and SR of 5.6 cm. The experimental results also show that compared with the conventional multi-frequency average method, the proposed scheme can achieve similar phase measurement accuracy under less frequency division, so as to obtain better SR in the same scanning range.

Optical linewidth of soliton microcombs

Abstract: Abstract Soliton microcombs provide a versatile platform for realizing fundamental studies and technological applications. To be utilized as frequency rulers for precision metrology, soliton microcombs must display broadband phase coherence, a parameter characterized by the optical phase or frequency noise of the comb lines and their corresponding optical linewidths. Here, we analyse the optical phase-noise dynamics in soliton microcombs generated in silicon nitride high-Q microresonators and show that, because of the Raman self-frequency shift or dispersive-wave recoil, the Lorentzian linewidth of some of the comb lines can, surprisingly, be narrower than that of the pump laser. This work elucidates information about the physical limits in phase coherence of soliton microcombs and illustrates a new strategy for the generation of spectrally coherent light on chip.

A Low Phase Noise K-Band VCO With a Threshold Voltage-Based Current Source

Abstract: This letter presents a low-phase-noise voltage-controlled oscillator (VCO) with a threshold-voltage ( $V_{\mathrm {TH}}$ )-based current source. The proposed current source is insensitive to the supply voltage, and the flicker noise of the transistor in the current source is decoupled, leading to lower phase noise to the resonator. In addition, a small-size differential inductor is adopted to diminish the resonator loss and to increase the quality factor. Fabricated in a 130-nm CMOS technology, the proposed VCO demonstrates a tuning range of 23.8–26.3 GHz (10.4%). It achieved a phase noise of −74.73 dBc/Hz at 100 kHz and −107.36 dBc/Hz at 1 MHz offset frequencies from the 24.005-GHz carrier frequency, respectively. The figure of merit (FoM) reaches −188 dBc/Hz.

Novel Feed-Forward Technique for Digital Bang-Bang PLL to Achieve Fast Lock and Low Phase Noise

Abstract: This paper presents a novel technique to reduce the locking time in Digital Phase-Locked Loop (DPLL) based on Bang-Bang Phase Detector (BB-PD). The implemented 65-nm CMOS fractional-N frequency synthesizer generates an output signal between 3.7 and 4.1 GHz from a 52 MHz reference clock and improves the trade-off between phase noise, due to the loop quantization, and locking time, exploiting a digital locking loop that avoids look-up table (LUT) and finite state machine-based (FSM) locking schemes. Measurements show that the output signal spot noise at 20 MHz from the carrier is −150.7 dBc/Hz while the best locking time, for a coarse step of 364 MHz, is 115 $\mu \text{s}$ , overcoming the locking time limitations and avoiding cycle slips that usually affect the 1-bit phase detector PLL.