Nonlinear Optical and Band-pass Filter Characteristics in Thermally Poled Fused Silica

Thermally poled fused silica with periodic interdigital electrodes can induce second-order optical nonlinear effects and generate a band-pass filter with characteristics for microwave application. The filter has a center frequency of ∼2.5 GHz, 3 dB bandwidth of ∼300MHz, and insertion loss of −5.15 dB.     

Influence of laser linewidth on performance of Brillouin optical time domain reflectometry

The effects of optical sources with different laser linewidths on Brillouin optical time domain reflectometry (BOTDR) are investigated numerically and experimentally. Simulation results show that the spectral linewidth of spontaneous Brillouin scattering remains almost constant when the laser linewidth is less than 1 MHz at the same pulse width; otherwise, it increases sharply. A comparison between a fiber laser (FL) with 4-kHz linewidth at 3 dB and a distributed feedback (DFB) laser with 3-MHz linewidth is made experimentally. When a constant laser power is launched into the sensing fiber, the fitting linewidths of the beat signals (backscattered Brillouin light and local oscillator (LO)) is about 5 MHz wider for the DFB laser than for the FL and the intensity of the beat signal is about a half. Furthermore, the frequency fluctuation in the long sensing fiber is lower for the FL source, yielding about 2 MHz less than that of the DFB laser, indicating higher temperature/strain resolution. The experimental results are in good agreement with the numerical simulations.     

400 mW ultrashort cavity low-noise single-frequency Yb³⁺-doped phosphate fiber laser

A compact, low-noise, single-frequency fiber laser by using a newly developed Yb3? heavily doped single-mode phosphate glass fiber has been demonstrated. Over 400 mW stable continuous wave single transverse and longitudinal mode laser at 1.06 μm was achieved from a 0.8 cm long active fiber. The measured slope efficiency and estimated quantum efficiency of laser emission are 72.7% and 93%, respectively. The signal-to-noise ratio is higher than 72 dB, and the linewidth of the fiber laser is less than 7 kHz, while the measured relative intensity noise is less than -130 dB/Hz at frequencies of over 1.5 MHz.     

基于光学-微波同步的低噪声微波产生方法

低噪声微波在冷原子光钟、光子雷达、大科学装置远程同步等领域具有重要的应用价值.本文介绍了一种基于光学-微波相位探测技术的低噪声微波产生方案,利用光纤环路光学-微波鉴相器,将超稳激光的频率稳定度相干传递至介质振荡器.实验采用梳齿相位参考至超稳激光的窄线宽掺铒光纤飞秒光学频率梳,结合光纤环路光学-微波鉴相器和精密锁相装置,将7 GHz介质振荡器同步至光频梳重复频率的高次谐波,同步后的光脉冲序列与微波信号的剩余相位噪声为–100 d Bc/Hz@1 Hz,定时抖动为8.6 fs [1 Hz—1.5 MHz];通过搭建两套低噪声微波产生系统,测得7 GHz微波的剩余相位噪声为–90 d Bc/Hz@1 Hz,对应的频率稳定度为4.8×10^–15@1 s.该研究结果对基于光学相干分频的低噪声微波产生提供了一种新思路.     

Frequency Sources in W-band Radar Front-end with Low Phase Noise

A W-band coherent stepped-frequency pulsed radar front-end is developed. It consists of a millimetre wave transmitting source, a mm-wave local source, a DDS with multi frequency points output and two microwave sources serving as local oscillators. All the sources are coherent with the 120 MHz referenced crystal oscillator. The mm-wave sources are realized by frequency multiplier chain, up-conversion and injection locking. The phase noise of fundamental-wave injection-locked W-band harmonic Gunn oscillator output signal achieves −98 dBc/Hz at 10 kHz offset and the spurious output is less than −50 dBc. The received intermediate frequency signal is also presented.     

Compact phase-lock loop for external cavity diode lasers

We present a compact,low-noise,and inexpensive optical phase-lock loop(OPLL) system to synchronize the frequency and the phase between two external cavity diode lasers. Based on a direct digital synthesizer technique,a programmable radio-frequency generator is implemented as the reference signal source. The OPLL has a narrow beat note linewidth below 1 Hz and a residual mean-square phase error of 0.06 rad~2 in a 10 MHz integration bandwidth. The experimental test results prove the competent performance of the system,which is promising as a low-budget choice in atomic physics applications.     

Frequency modulated and polarization maintaining fiber laser with narrow linewidth

A novel configuration of fiber laser with frequency modulation is presented. Frequency modulation, stable polarization state and narrow linewidth are realized by using the waveguide phase modulator, polarization maintaining devices and saturable absorber. It is shown that the laser output reaches 23 mW, linewidth is less than 1 kHz, polarization extinction ratio is higher than 20 dB and maximum value of frequency deviation can reach 7.5 MHz.     

PMN-PT single crystal thick films on silicon substrate for high-frequency micromachined ultrasonic transducers

In this work, a novel high-frequency ultrasonic transducer structure is realized by using PMNPT-on-silicon technology and silicon micromachining. To prepare the single crystalline PMNPT-on-silicon wafers, a hybrid processing method involving wafer bonding, mechanical lapping and wet chemical thinning is successfully developed. In the transducer structure, the active element is fixed within the stainless steel needle housing. The measured center frequency and −6 dB bandwidth of the transducer are 35 MHz and 34%, respectively. Owing to the superior electromechanical coupling coefficient (k _t ) and high piezoelectric constant (d ₃₃) of PMNPT film, the transducer shows a good energy conversion performance with a very low insertion loss down to 8.3 dB at the center frequency.     

A W-band Frequency Source with Low Spurs and Low Phase Noise

A W-band millimeter wave frequency source is developed by frequency multiplier chain and injection locking. The referenced crystal oscillator (CO) signal 120 MHz is multiplied 400 times to output 48 GHz signal. Then, it is used as a referenced source of fundamental-wave injection-locked harmonic Gunn oscillator with output power more than 10 mW at 96 GHz and spurious output less than −65 dBc. The measured phase noise is −97 and −105 dBc/Hz at 10 kHz and 200 kHz offset, respectively. At last, the influence of the flicker noise, provided by the frequency multipliers and amplifiers, is analyzed.     

低噪声光纤激光器的实验研究

光传感和光通信领域的迅速发展迫切需要相位噪声和强度噪声都很低的激光光源,为满足这一需求,设计出一种新型低噪声光纤激光器。激光器采用复合腔结构,以掺铒光纤作为工作物质,通过在未被抽运的掺铒光纤中形成的瞬态自写入光纤光栅的窄带滤波特性进行选模和压窄线宽,产生稳定的单频激光输出;经过光路改进,激光输出光谱信噪比优于62 dB;利用光电负反馈电路,弛豫振荡峰下降了约25 dB,低频段强度噪声也大为改善,有效地抑制了光源的强度噪声。激光器的输出光功率大于1 dBm,线宽小于1 kHz,边模抑制比超过50 dB。优异的低噪声特性使得该激光器在光传感和光通信领域具有重要的应用价值。     

Environmentally-stable all-normal-dispersion picosecond Yb-doped fiber laser with an achromatic quarter-wave-plate

We present an environmentally-stable picosecond Yb-doped fiber laser by employing an achromatic quarter-wave-plate (AQWP) as a pulse stabilizer in the all-normal-dispersion and polarization-maintaining linear laser cavity. It is shown that the AQWP plays a critical role to control both the polarization state and spectral filtering of the proposed mode-locked laser. The demonstrated laser generates 2-ps-long pulses with pulse energies of 1 nJ at a repetition rate of 117 MHz and parabolic spectral width of 26 nm. The fundamental RF carrier frequency exhibits Allan deviation of 3.8 × 10⁻⁸ at 1-s averaging time and phase noise of −95 dBc/Hz at 10-Hz offset frequency.     

Tunable single-frequency operation of a diode-pumped vertical external-cavity laser at the cesium D<Subscript>2</Subscript> line

We report on a diode-pumped vertical external-cavity surface-emitting laser emitting around 852 nm for Cesium atomic clocks experiments. We have designed a 7-quantum-well semiconductor structure optimized for low laser threshold. An output power of 330 mW was achieved for 1.1 W of incident pump power. Furthermore, a compact setup was built for low-power single-frequency emission. We obtained an output power of 17 mW in a single longitudinal mode, exhibiting both broad (9 nm) and continuous (14 GHz) tunability around the Cesium D₂ line. The laser frequency has been stabilized on an atomic transition with residual frequency fluctuations ∼300 kHz. Through a beatnote experiment the −3 dB laser linewidth has been measured to <500 kHz over 10 ms.     

Linewidth of a quantum-cascade laser assessed from its frequency noise spectrum and impact of the current driver

We report on the measurement of the frequency noise properties of a 4.6-??m distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6?cm^?1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below ${\approx} 1~\mbox{nA/}\sqrt{}\mbox{Hz}$ allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of ${\approx} 1.6~\mbox{MHz/}(\mbox{nA/}\sqrt{}\mbox{Hz})$ for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several $\mbox{nA/}\sqrt{}\mbox{Hz}$ , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only $350~\mbox{pA/}\sqrt{}\mbox{Hz}$ current noise, which is suitable to observe the ??550?kHz linewidth of our QCL.     

基于双环混频光电振荡器的可调谐微波频率梳产生

随着无线通信的速率提升和微蜂窝趋势,光载微波技术已经成为重要的发展趋势,而光生多载波系统是光载微波的最重要的技术之一.本文提出了一种基于双环混频光电振荡器（OEO）的可调谐光载微波频率梳产生方案,可同时实现多频段微波信号产生,从而高效低成本地为无线节点提供光生微波载波.方案采用混频双环OEO系统,通过工作在增益开关状态的直调激光器,利用其非线性动态特性产生多频率光载微波频率梳信号,并采用双路微波滤波器分别滤出两个相邻频率的微波信号,并利用二者的差频反馈注入直调激光器构成光电谐振.利用偏振双环结构抑制长谐振腔引起的边模问题,提高了输出信号的噪声特性.经过实验分析,得到了低相噪的多路微波信号,并最终实现了间隔797.4 MHz的稳定的微波频率梳信号,一阶载波相位噪声低于-101.7 dBc/Hz@10 kHz,-115.2 dBc/Hz@50 kHz.因此该方案产生的光载微波频率梳信号具有低噪声的优点,适用于光载微波通信系统.     

Frequency jitter and spectral width of an injection-seeded Q-switched Nd:YAG laser for a Doppler wind lidar

The design of a 50 Hz single longitudinal mode, diode-pumped and frequency-tripled Nd:YAG master oscillator power amplifier is described, and the first measurements of output parameters are presented. The laser oscillator is injection-seeded by a tuneable monolithic Nd:YAG ring laser and frequency stabilized by minimising the Q-switch build-up time. The laser system will be an integral part of an airborne instrument demonstrator for a first satellite based Doppler wind lidar to measure vertical profiles of one component of the atmospheric wind vector. This paper focuses on the investigation of the frequency jitter and the linewidth of the laser, which are measured on a pulse-to-pulse basis. For this purpose a compact, high accuracy beat frequency monitoring system has been developed at DLR. By operating the amplifier stage at half the repetition rate (50 Hz) of the oscillator, we could reduce the frequency stability from 10 MHz (rms) to 1.3 MHz (rms) (over a 14 s period). We have determined a mean linewidth of 15 MHz (FWHM) at 1064 nm. These measured laser parameters enable wind velocity measurements in the atmosphere (0–15 km) at an accuracy of 1 to 2 m/s. PACS 42.55.Xi; 42.60.Lh; 42.68.Wt     

Ionic conductivity studies on Sr stabilized zirconia by impedance spectroscopy

Sr stabilized zirconia has been prepared by the co-precipitation method and electrically characterized by using impedance spectroscopy in the frequency range from 42 Hz to 5 MHz. The charge carrier concentration of the material has been calculated from the conductance spectra and it is found to be of the order of 10²⁵ cm⁻³. The conductance spectra show a dc plateau and a dispersive region suggesting correlated hopping motion of the ions. The impedance analysis shows a depressed semicircle indicating non-Debye nature of the material as well as the broad nature of modulus peaks indicates non-Debye behaviour. The bulk resistance of the material has been extracted from the impedance spectra and it is found to be 3.5×10⁷ Ω·cm⁻¹ at 673 K. The low frequency dispersion of the dielectric constant implies the space charge effects arising from the electrodes. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28 – 30, 2003.     

New Electromagnetic Absorbers Composed of Left-handed and Right-handed Materials

New double-layered electromagnetic absorbers are presented in this paper. The new absorbers composed of one lossy left-handed material absorbing layer and one impedance matching layer consisted of lossless right-handed material. It is indicated that the reflection loss of below −20dB can be obtained in the frequency range 7GHz–13GHz. Power attenuation achieving −50dB of narrow frequency band electromagnetic absorbers can also be obtained by modulate permittivity of right-handed material. Furthermore, the thickness of the whole absorbing structure is only 2mm, which is particularly helpful in some practical applications. The presented results are of reference significance for accurate design of the new electromagnetic absorbers and of practical prospects for stealth technology.     

分布反馈式光纤激光器线宽特性及其展宽机理研究

针对分布反馈式光纤激光器(DFB-FL)线宽特性进行了深入研究.通过理论推导和实验验证, 分析了自脉动、相 移光栅、激射窗口宽度以及外界环境扰动等因素对DFB-FL线宽的展宽作用. 进一步提出了压缩DFB-FL线宽的有效方法和途径,给出了自注入锁模方式下对DFB-FL线宽压缩的对比实验结果, 将DFB-FL的线宽从35 kHz压缩到了10 kHz.     

Frequency dependent magnetoconductivity and conductivity study in Ni-dispersed silica nano-composite produced by sol-gel technique

The low frequency (20 Hz to 1 MHz) ac conductivity and magnetoconductivity behaviour of ceramic nanocomposite (Ni-SiO₂) at low temperature down to 77 K are reported. The frequency dependent conductivity followed the power law, σ(ω) ∝ ω ^s . The fractional exponent s is a function of temperature and was found to increase with increasing temperature. This type of variation may be attributed to small polaron hopping. A peak present in the loss tangent indicates the presence of a Debye relaxation process. The magnetoconductivity of the samples is positive, which strongly depends on frequency. A firm theoretical explanation of frequency dependent magnetoconductivity is still lacking.     

Noise spectrum of the 140-GHz gyrotron designed for controlled thermonuclear fusion installations

A method for measurement of gyrotron amplitude noise using the Mach-Zehnder interferometer as a rejection filter, the measurement facility, and results of testing a 140-GHz industrial high-power gyrotron are discussed. The method improved the sensitivity by two orders of magnitude over that of the direct detection method. The experimental relative power spectral density in a frequency range of 50–250 MHz from the line center is 7 × 10⁻²⁰–5 × 10⁻¹⁹ 1/Hz.