Analysis of a loss-compensated recirculating delayed self-heterodyne interferometer for laser linewidth measurement

A loss-compensated recirculating delayed self-heterodyne interferometer (LC-RDSHI) for laser linewidth measurement is theoretically analyzed. An analytical result for the output spectrum of the LC-RDSHI is obtained. It is found that the spectrum from a LC-RDSHI is equivalent to a spectrum from a conventional delayed self-heterodyne interferometer with equivalent time delay and frequency shift, but modified by a periodical function, which could significantly influence the laser linewidth measurement. The parameters of a LC-RDSHI must be optimized to permit an accurate and direct measurement of laser linewidth from the output spectrum.     

Laser frequency stabilization by means of optical self-heterodyne beat-frequency control

We propose and demonstrate a novel frequency-stabilization scheme that provides linewidth narrowing and at the same time is compatible with high-speed frequency agility. The method relies on sensing and control of a heterodyne beat signal derived from a fiber interferometer and functions in the absence of a fixed reference frequency. Our demonstration utilizes a short-external-cavity diode laser equipped with an intracavity electro-optic crystal for frequency correction. The stabilization method is shown to suppress impressed laser frequency modulation by nearly 2 orders of magnitude. Without further modification of our scheme we also demonstrate laser frequency tuning control.     

Delay self-heterodyne measurement of narrow linewidth laser frequency drift characteristic

Resonator integrated optic gyro (RIOG), which employs narrow linewidth laser, is a high-accuracy inertial rotation sensor based on the Sagnac effect. The performance of RIOG is greatly affected by the frequency drift of narrow linewidth laser. A simple, effective method to measure the relative frequency drift of narrow linewidth laser based on delayed self-heterodyne technique is proposed in this paper. The measurement range and sensitivity can easily be satisfied by setting the length difference of the fiber segments between two interferometer arms. The relationship between the length difference and the frequency drift is derived based on the given principle of measuring the relative frequency drift. Then the laser frequency drift measuring setup is established and the experiment results demonstrate that a center frequency drift rate is less than 2 MHz/6.7 s under the room-temperature. Moreover, the measuring setup is applied to test the modulation coefficient of piezoelectric-transducer (PZT), and the modulation coefficient of 9.62 MHz/V is obtained, which satisfies the requirements of RIOG closed-loop operation.     

Laser frequency locking by direct measurement of detuning

We present a new method of laser frequency locking in which the feedback signal is directly proportional to the detuning from an atomic transition, even at detunings many times the natural linewidth of the transition. Our method is a form of sub-Doppler polarization spectroscopy, based on measuring two Stokes parameters (I2 and I3) of light transmitted through a vapor cell. It extends the linear capture range of the lock loop by as much as an order of magnitude and provides frequency discrimination equivalent to or better than those of other commonly used locking techniques.     

Excess noise in intracavity laser frequency modulation

The response to perturbations and to stochastic noise of a laser below threshold subjected to an intracavity periodic frequency modulation is theoretically studied. It is shown that, when the modulation frequency is close to the cavity axial mode separation but yet detuned from exact resonance, the laser exhibits a strongly enhanced sensitivity to external noise, which includes large transient energy amplification of perturbations in the deterministic case and enhancement of field fluctuations in presence of a continuous stochastic noise. This large excess noise is due to the nonorthogonality of Floquet laser modes which makes it possible continuous energy transfer from the forcing noise to transient growing perturbations.     

RF spectral modulation caused by delayed interference and photodetection of optical noise

We demonstrate that an RF spectral fringe appears when a nonpolarized optical noise and its delayed replica are combined and photo-detected without any polarization control. A theoretical analysis explains the visibility of the fringe and its insensitivity to polarization.     

Laser frequency stabilization and shifting by using modulation transfer spectroscopy

The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the87 Rb atom with D2 line transition F = 2 → F = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro–optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.     

Magnetic Barkhausen noise measurement by resonant coil method

This paper describes a powerful new technique for nondestructive evaluation of ferromagnetic material. A method has been developed for measuring magnetic Barkhausen signals under different coil resonance frequencies. The measurements allow one to establish the behavior relating the power spectral density maximum and the resonant coil frequency. Time-frequency analysis of Barkhausen signals puts in evidence the tuning regions for each coil, and allows clear identification of each contribution to the Barkhausen signal spectrum. This concept was used in order to evaluate the relation between the degree of plastic deformation in carbon steel samples, and the power spectral density maximum at different resonance frequencies. This result also makes it possible to the selectively modify measurement sensibility to the magnetic Barkhausen signal by using different resonance frequencies.     

Laser frequency noise immunity in multiplexed displacement sensing

Digitally enhanced interferometry (DI) can be used to distinguish between interferometric signals and simultaneously monitor in-line object displacements with subnanometer sensitivity. In contrast to conventional interferometry-where these signals interfere with each other and degrade performance-we experimentally show that by using DI, each of these signals can be isolated and measured at the same time. We present what we believe to be the first demonstration of DI's signal multiplexing capabilities, showing simultaneous length sensing of three sections of an optical fiber. The cross talk between length measurements was less than 2.6×10(-3) with a displacement noise floor of 200 pm/√Hz, which corresponds to a strain sensitivity of less than 80 picostrain(p?) in each sensor. We also enhance our system's displacement sensitivity at low frequencies by combining information from multiple lengths to suppress errors due to laser frequency noise.     

Contribution of frequency modulation to speech recognition in noise

Cochlear implants allow most patients with profound deafness to successfully communicate under optimal listening conditions. However, the amplitude modulation (AM) information provided by most implants is not sufficient for speech recognition in realistic settings where noise is typically present. This study added slowly varying frequency modulation (FM) to the existing algorithm of an implant simulation and used competing sentences to evaluate FM contributions to speech recognition in noise. Potential FM advantage was evaluated as a function of the number of spectral bands, FM depth, FM rate, and FM band distribution. Barring floor and ceiling effects, significant improvement was observed for all bands from 1 to 32 with the additional FM cue both in quiet and noise. Performance also improved with greater FM depth and rate, which might reflect resolved sidebands under the FM condition. Having FM present in low-frequency bands was more beneficial than in high-frequency bands, and only half of the bands required the presence of FM, regardless of position, to achieve performance similar to when all bands had the FM cue. These results provide insight into the relative contributions of AM and FM to speech communication and the potential advantage of incorporating FM for cochlear implant signal processing.     

Low frequency wind noise contributions in measurement microphones

In a previous paper [R. Raspet, et al., J. Acoust. Soc. Am. 119, 834-843 (2006)], a method was introduced to predict upper and lower bounds for wind noise measured in spherical wind-screens from the measured incident velocity spectra. That paper was restricted in that the predictions were only valid within the inertial range of the incident turbulence, and the data were from a measurement not specifically designed to test the predictions. This paper extends the previous predictions into the source region of the atmospheric wind turbulence, and compares the predictions to measurements made with a large range of wind-screen sizes. Predictions for the turbulence-turbulence interaction pressure spectrum as well as the stagnation pressure fluctuation spectrum are calculated from a form fit to the velocity fluctuation spectrum. While the predictions for turbulence-turbulence interaction agree well with measurements made within large (1.0 m) wind-screens, and the stagnation pressure predictions agree well with unscreened gridded microphone measurements, the mean shear-turbulence interaction spectra do not consistently appear in measurements.     

Simulation and measurement of frequency modulation insubmillimeter-wave gyrotron

Frequency modulation of the gyrotron output has been numerically simulated employing the approximation of weakly relativistic, mono-velocity electron beam as well as a quasi-empirical approach. The simulation results are compared to a first experiment in the 300 GHz region performed with a submillimeter-wave gyrotron named “Gyrotron FU IV”. The modulation amplitude of 30 MHz has been achieved by a 120 V modulation of the accelerating voltage resulting in the variation of the relativistic electron mass and in the corresponding modulation of the electron cyclotron frequency. Calculated amplitudes of frequency modulation are in reasonable agreement with the experimental results     

Radiated noise measurement method of underwater low noise target by a vertical nested array

A radiated noise measurement method of underwater acoustic targets with relatively low power by a vertical nested array is studied. Firstly, the array gain requirements for different frequency bands are obtained through comparative analysis of the background noise level and the radiated noise level of the target. Secondly, an ultra wideband constant-beamwidth beamforming method with high robustness is established for the waveguide environment based on convex optimization. The radiated noise meas...     

Generation and noise analysis of a wide-band optical -frequency comb based on recirculating frequency shifter

By optimizing the gain configuration and length of the loop, a 90-tone optical frequency comb （OFC） is successfully generated based on recirculating frequency shifter structure. The peak-to-peak power fluctuation of the 90-tone OFC is 4.26 dB and the tone-to-noise ratio is higher than 19.17 dB. To further analyze the noise accumulation feature of the tones when travelling around the loop, linewidth of the tones is measured by delayed self-heterodyne interferometer structure. The result shows the linewidth of the tones deteriorates little during the recirculating process, indicating that the generated OFC is an ideal multi-wavelength source for high-speed communication svstems.     

Photonic RF frequency measurement combining SSB-SC modulation and birefringence

A new technique to instantaneously measure microwave frequency and amplitude is presented and experimentally demonstrated. The architecture described is based on a single sideband modulation with suppressed carrier (SSB-SC) and the measurement of the Stokes parameters at the output of a birefringent medium. The results show errors below 50 MHz in the 59% of measurements carried out in a range from 1 to 11 GHz and an average power error of 0.34 dB. The technique shows the potential to increase the resolution and the range by increasing the differential time delay of the birefringent medium.     

Frequency response measurement of high-speed photodetectors using the spectrum power method in a delay self-heterodyne system

We report on a photoionization scheme for controlled loading of ⁸⁸Sr⁺ ions into a radio-frequency (rf) ion trap. The two-step doubly-resonant process drives the ⁸⁸Sr atom on the 5s^{2 1} S ₀ – 5s5p¹ P ₁ transition at 461 nm using a frequency-doubled diode laser, and then excites the atom to the autoionizing (4d²+5p²) ¹ D ₂ state with 405 nm light from a free-running diode laser. This method of trap loading is quantitatively compared to electron bombardment loading, and shown to reduce the Sr vapour pressure required to load by four orders of magnitude. It also provides more than an order of magnitude reduction in the day-to-day variation of the voltages required to compensate micromotion. We additionally introduce a two-step atomic source consisting of an oven and hotplate, which reduces the number of impurities evaporated towards the trap. This is shown to significantly reduce fluctuations in required micromotion compensation voltages. The demonstrated reduction in flux and increased source purity are expected to improve the reliability of trap operation for precision ion-trap experiments. PACS 32.80.Fb; 32.80.Pj; 39.10.+j     

一种重构辐射噪声调制谱的方法

在进行辐射噪声信号的仿真研究中,希望利用经滤波、特征提取等处理所得到的反映目标特性的线谱、连续谱、调制谱来重构辐射噪声,以便于研究辐射噪声在声场中的传播特性。本文提出了利用迭代算法估算调制深度谱的算法,给出了时域辐射噪声的重构模型,以实测数据的调制谱为出发点,来重构其时域信号,并对重构前后的调制谱进行了误差分析,取得了较好的效果。     

Efficient modulation frequency doubling by induced modulation instability

We show that inducing modulation instability with a weak modulation whose frequency is such that its second harmonic falls within the band of instability may lead to asynchronous Fermi-Pasta-Ulam recurrence and efficient transfer of power from the pump into the second harmonic of the modulation, resulting in a periodic modulation at the second harmonic with extinction ratios in excess of 30 dB.     

位相差值法测量大气湍流格林伍德频率

为了利用位相差值法实现大气湍流格林伍德频率的准确测量, 研究了位相差值法的有效性、哈特曼探测器采样点数和采样频率的选取方法。首先给出了位相差值法的基本原理和测量噪声去除方法, 然后分析了哈特曼探测器采样点数和格林伍德频率的统计平均次数对测量精度的影响, 结果显示, 当采样点数大于400、统计平均次数大于400时, 可以实现大气湍流格林伍德频率的准确测量。研究了测量噪声的影响, 结果表明:去除噪声后, 测量值的偏离误差从30%降低到0.6%。研究了算法的重复性精度, 得到测量值偏离量的RMS值为1.9 Hz, 占理论值的3%, 说明测量方法非常稳定。依据上述结果, 对8~108 Hz的湍流进行测量和分析, 结果显示, 当不考虑空气扰动时, 测量值与理论值基本一致。最后, 研究了哈特曼探测器采样频率和格林伍德频率之间的关系:哈特曼探测器的采样频率越高, 能够准确测量的格林伍德频率也越高, 并得到了定量的经验公式。上述结果表明, 在满足采样点数、采样频率以及统计平均次数等条件下, 位相差值法可以实现大气湍流格林伍德频率的准确测量。该研究工作为大气湍流的格林伍德频率测量提供了应用依据。     

应用于碳同位素丰度测量的激光频率刻度系统研究

对应用于激光吸收光谱法碳同位素丰度测量中的激光频率实时刻度系统进行研究. 采用不同自由光谱范围的共焦法布里-珀罗干涉仪对测量过程中的激光扫描频率进行实时测量. 分别采用线性内插法和多项式拟合法对激光频率每次扫描过程中的频率非线性进行分析,通过对4976–4980 cm^-1波段CO₂吸收谱的多次测量平均的实验结果与HITRAN-2008数据库相应的吸收峰数据进行比较,得到两种方法的激光频率刻度精度均可达到10^-4 cm^-1,线性内插法的刻度精度要好于多项式拟合法. 验证了激光频率实时刻度系统在碳同位素丰度测量中应用的可行性. 关键词： 激光频率实时刻度 共焦法布里-珀罗干涉仪 同位素丰度测量 激光吸收光谱法