Excess Phase Noise Research Articles

Experimental demonstration of bi-directional laser ranging and data communication for space gravitational wave detection

In this study, we designed ranging parameters and refined a loop filtering algorithm to solve the problem of parameter compatibility with the phasemeter and low ranging accuracy under bidirectional communication conditions. This enables the ranging and communication system to be integrated directly into the phasemeter as an auxiliary module. Experimental results compare fiber measurement lengths under different clock sources, indicating that the system achieves an ranging accuracy of 82.1 cm with an rms error of 9.14 cm. Additionally, the data communication rate reaches 19.5 kbps with a bit error rate below 10−6, all while ensuring that the implementation of the laser ranging and data communication system utilizes only 1% of the optical power to avoid introducing excessive phase noise to the scientific interferometer.

Optimizing Ti/TiN Multilayers for UV, Optical and Near-IR Microwave Kinetic Inductance Detectors

Microwave Kinetic Inductance Detectors (MKIDs) combine significant advantages for photon detection like single photon counting, single pixel energy resolution, vanishing dark counts and µs time resolution with a simple design and the feasibility to scale up into the megapixel range. But high quality MKID fabrication remains challenging as established superconductors tend to either have intrinsic disadvantages, are challenging to deposit or require very low operating temperatures. As alternating stacks of thin Ti and TiN films have shown very impressive results for far-IR and sub-mm MKIDs, they promise significant improvements for UV, visible to near-IR MKIDs as well, especially as they are comparably easy to fabricate and control. In this paper, we present our ongoing project to adapt proximity coupled superconducting films for photon counting MKIDs. Some of the main advantages of Ti/TiN multilayers are their good control of critical temperature (Tc) and their great homogeneity of Tc even over large wafers, promising improved pixel yield especially for large arrays. We demonstrate the effect different temperatures during fabrication have on the detector performance and discuss excess phase noise observed caused by surface oxidization of exposed Si. Our first prototypes achieved photon energy resolving powers of up to 3.1 but turned out to be much too insensitive. As the work presented is still in progress, we also discuss further improvements planned for the near future.

180 nm NMOS voltage-controlled oscillator for phase-locked loop applications

<p>The voltage-controlled oscillator (VCO) is the primary device in the phase-locked loop (PLL) to produce the local oscillator frequency. The excessive phase noise of VCOs is the primary cause of PLL performance loss. This paper proposes the design and optimization of low phase noise and low power consumption for a 180 nm N-channel metal-oxide semiconductor NMOS VCO for PLL applications with P-channel metal-oxide semiconductor PMOS varactors and spiral inductors. At 2 V supply voltage, the optimized NMOS VCO has a power consumption of 21 mW, a phase noise of -130 dBc/Hz at 1 MHz offset and a total harmonic distortion (THD) of 3.9%. The proposed design is verified by PSpice simulations. A new criterion is proposed for optimizing NMOS LC oscillators.</p>

Yoctosecond Timing Jitter Sensitivity in Tightly Synchronized Mode-Locked Ti:Sapphire Lasers

Higher sensitivity in timing jitter measurement has great importance in studies related to precise measurements. Timing jitter noise floors contribute one of the main parts in existing measurements. In this article, a phase error signal is obtained by superposition of outputs of two optical heterodyne discrimination apparatus to suppress the noise floor. Excess phase noise of the electrical amplifier is avoided. We demonstrate 2.6 × 10−14 fs2/Hz (~160 ys/√Hz) timing jitter noise floor between two identical 99 MHz repetition-rate mode-locked Ti:sapphire lasers after their repetition rates are tightly synchronized. The performance is extensible to reach an integrated timing jitter resolution of one attosecond.

Phase noise of frequency doublers in optical clock lasers.

Frequency doublers are widely used in high-resolution spectroscopy to shift the operation wavelength of a laser to a more easily accessible or otherwise preferable spectral region. We investigate the use of a periodically-poled lithium niobate (PPLN) waveguide frequency doubler in an optical clock. We focus on the phase evolution between the fundamental (1396 nm) and frequency-doubled (698 nm) light and its effect on clock performance. We find that the excess phase noise of the doubler under steady-state operation is at least two orders of magnitude lower than the noise of today's best interrogation lasers. Phase chirps related to changes of the optical power in the doubler unit and their influence on the accuracy of optical clocks are evaluated. We also observe substantial additional noise when characterizing the doubler unit with an optical frequency comb instead of using two identical waveguide doublers.

Carrier-phase estimation for simultaneous quantum key distribution and classical communication using a real local oscillator

The simultaneous quantum and classical communication (SQCC) protocol is an appealing protocol since it allows continuous-variable quantum key distribution (CV-QKD) and classical communication to be implemented simultaneously by using the same communication infrastructure and on a single wavelength. However, the main challenge of the SQCC protocol is its low tolerance of phase noise, limiting the transmission distance under the real local oscillator (LO) design. To eliminate the phase drift without the aid of the reference signal, the classical carrier phase estimation (CPE) algorithm in the field of coherent optical communication is employed for phase recovery and data extraction. Besides, the power of the residual phase noise in classical communication and the power of the excess phase noise in CV-QKD are derived, including the effects of the employed laser linewidth, the transversal filter length, and the classical signal power. On this basis, the performances of the SQCC protocol under the trusted and the untrusted phase noise model are discussed, respectively. Our work confirms the feasibility of the CPE algorithm in the SQCC protocol and demonstrates the conditions required for this protocol to operate on a coherent optical communication system.

Continuous-Variable Quantum Key Distribution With Self-Reference Detection and Discrete Modulation

We proposed the optimized continuous-variable quantum key distribution protocol with self-reference and discrete modulation, which incorporates the advantages of both self-reference detection scenario and discrete modulation of quantum states. The impact of excess phase noise resulting from the self-reference intradyne detection on the secret key rate has been evaluated. We have comprehensively analyzed the optimized protocol performance and compared it with the other three continuous-variable quantum key distribution protocols proposed so far and confirmed that the optimized protocol that we proposed enables the quantum key distribution over longer distances and immunity against the side-channel attack.

Development of low phase noise microwave frequency synthesizers for reducing Dick effect of Cs fountain clocks

The Dick effect is one of the main factors limiting the short-term frequency stability of Cs fountain clocks, which is especially decided by the phase noise of the local oscillator at smaller offset frequency. Here we report on the development of a 9.192 GHz microwave frequency synthesizer with low phase noise to be used as the local oscillator for Cs fountain clocks. The synthesizer is based on frequency multiplication and synthesis from an ultra-low phase noise 5 MHz Oven Controlled Crystal Oscillator (OCXO). The key component of the frequency multiplication is a non-linear transmission-line (NLTL) as a frequency comb generator. Based on the principle of the NLTL, we carefully optimized the input power, the input and output impedances of the NLTL to suppress its excess phase noise. The measured results show that the absolute phase noises of 9.192 GHz signal are -64 dBc/Hz, -83 dBc/Hz, -92 dBc/Hz and -117 dBc/Hz at 1 Hz, 10 Hz, 100 Hz and 1 kHz offset frequencies, respectively. The residual phase noise of the synthesizer is − 82 dBc/Hz at 1 Hz offset frequency. The frequency stability limit due to the absolute phase noise via Dick effect is theoretically estimated to be 7.0 × 10−14τ−1/2.

Research on the spectral phase correction method for the atmospheric detection in open space.

During the atmospheric detection process in open space, the excessive phase noise is introduced into the signal, due to the atmospheric turbulence, which causes the intensity and phase fluctuation. In the previous study, a spectral data processing method based on the co-frequency and dual-wave has been used to reduce the influence of the scintillation noise from the atmospheric turbulence in open space, while the influence of the phase noise remains to be solved. So the wavelength modulated signal is theoretically analyzed at first. On studying the relationship between the dual-waves in one cycle to eliminate the phase fluctuation and reduce the phase fluctuation caused by the atmospheric turbulence, a new method of the spectral phase correction for the open space atmospheric detection has been proposed. An atmospheric detection experiment on the phase correction in the open space based on co-frequency and dual-wave has been carried out. The results show that the maximum fluctuation of the spectral signal processed with this method is 1.06%, while the power spectral density fluctuation is suppressed below 50Hz, and the Allan analysis result is 8.8 × 10-8(1s). Compared with the traditional concentration inversion method using 2f-wavelength modulation and the classical light intensity elimination, the proposed phase correction method can effectively reduce the fluctuation of random noise caused by the short-term atmospheric turbulence and the laser flashing to improve the stability of the concentration measurement, which has practical engineering value.

Comb Spacing Multiplication Enabled Widely Spaced Flexible Frequency Comb Generation

We experimentally demonstrate a new approach for the generation of widely spaced flexible optical frequency combs in a highly nonlinear fiber (HNLF) using relatively low-speed components. A cavity-less 10-GHz optical pulse source is passively multiplied to 50 GHz via the fractional temporal Talbot effect. The repetition rate multiplied pulse train then propagates in an HNLF together with a continuous-wave probe laser to achieve comb spacing multiplication through cross-phase modulation. Numerical study shows that the timing jitter of the initial 10-GHz optical pulse induces insignificant excess phase noise at high-frequency offset relative to the optical comb lines, which favors a low phase noise wideband frequency comb generation. The spectral linewidth of the central 17 comb lines are characterized, exhibiting well-preserved linewidth quality as the probe laser. Furthermore, the tunability of the generated comb, relevant from a practical perspective, is analyzed by means of conversion efficiency. Finally, the central 11 comb lines are individually modulated by Nyquist-shaped 32 GBaud 16 quadratic-amplitude modulation signals, demonstrating the applicability of the comb source in providing optical carriers for a coherent transmitter.

Parallelized Kalman Filters for Mitigation of the Excess Phase Noise of Fast Tunable Lasers in Coherent Optical Communication Systems

Numerical and experimental investigations are carried out on the performance of parallelized Kalman filters applied for mitigation of the excess phase noise of fast tunable lasers. Based on the characterization of the phase noise of a sampled grating distributed Bragg reflector (SG-DBR) laser, the proposed carrier phase recovery (CPR) scheme using Kalman filters is introduced. By performing simulations of data transmission with various advanced modulation formats in the presence of the excess phase noise, the Kalman filter based CPR scheme shows its ability to overcome the excess phase noise and this method is suitable for parallel processing. Then the results are further demonstrated by 12.5 Gbaud QPSK and 16-QAM transmission experiments employing the SG-DBR laser. We find that the Kalman filters have better performance than the 2nd-order phase-locked loop in parallel systems due to a better phase noise tolerance. The bit error rate performance is also examined in the whole tuning range (∼30 nm) of the tunable laser, which further proves the feasibility of the proposed scheme.

Transmission-line resonators for the study of individual two-level tunneling systems

Parasitic two-level tunneling systems (TLS) emerge in amorphous dielectrics and constitute a serious nuisance for various microfabricated devices, where they act as a source of noise and decoherence. Here, we demonstrate a new test bed for the study of TLS in various materials which provides access to properties of individual TLS as well as their ensemble response. We terminate a superconducting transmission-line resonator with a capacitor that hosts TLS in its dielectric. By tuning TLS via applied mechanical strain, we observe the signatures of individual TLS strongly coupled to the resonator in its transmission characteristics and extract the coupling components of their dipole moments and energy relaxation rates. The strong and well-defined coupling to the TLS bath results in pronounced resonator frequency fluctuations and excess phase noise, through which we can study TLS ensemble effects such as spectral diffusion, and probe theoretical models of TLS interactions.

Excess carrier-envelope phase noise generation in saturable absorbers.

Attosecond spectroscopy and precision frequency metrology depend on the stabilization of the carrier-envelope phase (CEP) of mode-locked lasers. Unfortunately, the phase of only a few types of lasers can be stabilized to jitters in the few-hundred millirad range. In a comparative experimental study, we analyze a femtosecond Ti:sapphire laser and three mode-locked fiber lasers. We numerically demodulate recorded time series of the free-running carrier-envelope beat note. Our analysis indicates a correlation between amplitude and frequency fluctuations at low Fourier frequencies for essentially all lasers investigated. While this correlation typically rolls off at frequencies beyond 100kHz, we see clear indications for a broadband coupling mechanism in one of the fiber lasers. We suspect that the observed coupling mechanism acts to transfer intracavity power fluctuations into excess phase noise. This coupling mechanism is related to the mode-locking mechanism employed and not to the gain medium itself. We further verify this hypothesis by numerical simulations, which identify resonances of the saturable absorber mirror as a possible explanation for the coupling mechanism. Finally, we discuss how to avoid a detrimental influence of such resonances.

Effect of nonlinear gain on the phase noise of Y-branch lasers

Effect of nonlinear gain on the phase noise of modulated grating Y-branch (MGY) lasers is investigated by experiments and simulations. The phase noise is first characterized by measuring the frequency modulation noise spectrum of the MGY laser, and some interesting phenomena are observed. In order to understand the underlying physic mechanism of those phenomena, simulations are performed with taking Langevin noise sources and nonlinear gain terms into account. Simulated results show that, in the presence of the nonlinear gain, fluctuations of side-modes can bring excess phase noise to the lasing mode, especially when the side-modes are on the long-wavelength side of the main mode. Furthermore, it has been found that the phase noise hopping phenomenon can be induced by a bi-stable state of the laser, which is also closely related to the nonlinear gain. The investigation is helpful for explaining the complicated phase noise characteristics of the MGY laser.

Robust and compact entanglement generation from diode-laser-pumped four-wave mixing

Four-wave-mixing processes are now routinely used to demonstrate multi-spatial-mode Einstein-Podolsky-Rosen entanglement and intensity difference squeezing. Diode-laser-pumped four-wave mixing processes have recently been shown to provide an affordable, compact, and stable source for intensity difference squeezing, but it was unknown if excess phase noise present in power amplifier pump configurations would be an impediment to achieving quadrature entanglement. Here, we demonstrate the operating regimes under which these systems are capable of producing entanglement and under which excess phase noise produced by the amplifier contaminates the output state. We show that Einstein-Podolsky-Rosen entanglement in two mode squeezed states can be generated by a four-wave-mixing source deriving both the pump field and the local oscillators from a tapered-amplifier diode-laser. This robust continuous variable entanglement source is highly scalable and amenable to miniaturization, making it a critical step toward the development of integrated quantum sensors and scalable quantum information processors, such as spatial comb cluster states.

A 3.5 GHz Digital Fractional-N PLL Frequency Synthesizer Based on Ring Oscillator Frequency-to-Digital Conversion

A 3.5 GHz digital fractional-N PLL in 65 nm CMOS technology is presented that achieves phase noise and spurious tone performance comparable to those of a high-performance analog PLL. It is enabled by a new second-order frequency-to-digital converter that uses a dual-mode ring oscillator and digital logic instead of a charge pump and ADC. It also incorporates a new technique to reduce excess phase noise that would otherwise be caused by component mismatches when the DCO input is near integer boundaries. The PLL's largest in-band fractional spur is -60 dBc, its worst-case reference spur is -81 dBc, and its phase noise is -93, -126, and -151 dBc/Hz at offsets of 100 kHz, 1 MHz, and 20 MHz, respectively. Its active area is 0.34 mm 2 and it dissipates 15.6 mW from a 1 V supply.

Excess Phase Noise Characterization in Multifrequency Remote Clock Distribution Based on Femtosecond Frequency Combs

Remote distribution of optical frequency references, based on multifrequency sources such as femtosecond frequency combs, holds many advantages over its single-frequency counterpart. However, characterizing the excess noise caused by the transmission links or external perturbations in a multifrequency scheme posts new challenges. We have experimentally demonstrated direct measurement of excess phase noise spectrum in both free-space and fiber-optic transfer of a frequency comb using a multiheterodyne technique. In fiber-optic distribution, we focused on the excess phase noise under a single-tone acoustic perturbation. Increased overall noise power and a change of phase noise spectrum have been observed. In free-space distribution, a fractional instability of 3 × 10−14 at 1 s was observed for a 60 m outdoor atmospheric transmission, and large phase modulation due to air fluctuations causes a sizable line broadening.

Theoretical and experimental study of suppressing stimulated Brillouin scattering and phase noise in interferometric fiber sensing systems with phase modulation.

The effect of phase modulation on the phase noise in remote interferometric fiber sensing systems is analyzed in both theory and experiment. A matching condition between the modulation frequency and optical delay difference is derived to choose the matching modulation frequency. The matching modulation frequency for an interferometer with 5 m optical-path difference is 60 MHz and its integer harmonic frequencies. By matching the condition, the phase modulation method can suppress the stimulated Brillouin scattering in an interferometric fiber sensing system effectively without bringing excess phase noise, which is verified by the experiment. The results indicate that the maximum input power of the system can be increased effectively, which has great potential in the design of remote interferometric fiber-sensing systems.

Towards low timing phase noise operation in fiber lasers mode locked by graphene oxide and carbon nanotubes at 1.5 µm.

We investigate the timing phase noise of fiber lasers mode locked by graphene oxide (GO) and carbon nanotubes (CNTs), respectively, integrated in a linear cavity fiber laser in the reflecting operation. Due to the shorter decay time of the GO and CNTs, weaker slow saturable absorber effects are expected and mode-locked lasers based on these two saturable absorbers exhibit low excess timing phase noise coupled from the laser intensity noise. Compared with a reference laser mode locked by semiconductor saturable absorber mirror (SESAM), GO based laser obtains a timing phase noise reduction of 7 dB at 1 kHz and a timing jitter reduction of 45% experimentally whereas CNTs based laser obtains a timing phase noise reduction of 3 dB and a timing jitter reduction of 29%. This finding suggests that saturable absorbers with short decay time have the potential for achieving mode locking operation with low timing phase noise, which is important for applications including frequency metrology, high-precision optical sampling, clock distribution and optical sensing.

Experimental demonstration and devices optimization of NRZ-DPSK amplitude regeneration scheme based on SOAs.

We propose and experimentally demonstrate a novel scheme which can simultaneously realize wavelength-preserving and phase-preserving amplitude noise compression of a 40 Gb/s distorted non-return-to-zero differential-phase-shift keying (NRZ-DPSK) signal. In the scheme, two semiconductor optical amplifiers (SOAs) are exploited. The first one (SOA1) is used to generate the inverted signal based on SOA's transient cross-phase modulation (T-XPM) effect and the second one (SOA2) to regenerate the distorted NRZ-DPSK signal using SOA's cross-gain compression (XGC) effect. In the experiment, the bit error ratio (BER) measurements show that power penalties of constructive and destructive demodulation at BER of 10-9 are -1.75 and -1.01 dB, respectively. As the nonlinear effects and the requirements of the two SOAs are completely different, quantum-well (QW) structures has been separately optimized. A complicated theoretical model by combining QW band structure calculation with SOA's dynamic model is exploited to optimize the SOAs, in which both interband effect (carrier density variation) and intraband effect (carrier temperature variation) are taken into account. Regarding SOA1, we choose the tensile strained QW structure and large optical confinement factor to enhance the T-XPM effect. Regarding SOA2, the compressively strained QW structure is selected to reduce the impact of excess phase noise induced by amplitude fluctuations. Exploiting the optimized QW SOAs, better amplitude regeneration performance is demonstrated successfully through numerical simulation. The proposed scheme is intrinsically stable comparing with the interferometer structure and can be integrated on a chip, making it a practical candidate for all-optical amplitude regeneration of high-speed NRZ-DPSK signal.