Coherent beam combination of two thulium-doped fiber laser beams with the multi-ditheringtechnique

Coherent beam combination of two thulium-doped fiber laser beams using a multi-dithering technique is presented for the first time. In the experiment, two fiber lasers centered at 1948.6 nm are coherently combined, and a phase modulator based on piezoelectric ceramics transducer is connected in one beam path to compensate for the phase errors between the two beams. When the phase control system is closed loop, the fringe contrast of the far-field intensity pattern is improved to be more than 75%, from 15% in open-loop, and the residual phase error is less than λ/20. The experimental results show that the performance of the phase control system is robust and the control bandwidth is more than 1 kHz, which indicates that the above approach can be scaled to facilitate the coherent beam combination of kilo-watt level thulium-doped fiberlaser.     

Three-dimensional coherent ladar based on FMCW and its flight demonstration

A scanning three-dimensional coherent laser radar(ladar) based on the frequency modulated continuous wave(FMCW) is proposed and demonstrated, which can obtain many three-dimensional high-quality images. The system utilizes an electro-optic modulator and an optical filter to output a linear FMCW with a bandwidth of2 GHz. The flexible and variable rotating double prism is used for beam scanning. The images of flight demonstration are formed by attitude compensation correction. The experiment result validates the performance of our system for airborne three-dimensional scanning imaging.     

Coherent Beam Combination of Two Polarization Maintaining Ytterbium Fibre Amplifiers

We investigate coherent beam combination of fibre laser beams by phase locking. Phase noise of a polarization maintaining ytterbium fibre amplifier is inspected with a fibre interferometer. In a feed back control loop, two fibre polarization maintaining ytterbium amplifiers are phase locked and coherent combined when the phase noise is properly controlled by a LiNO3 phase modulator.     

Effect of processing on dielectric properties of (0.95)PbZr0.52Ti0.48O3–(0.05)BiFeO3

An effort has been made to synthesize solid solution of a composition with x=0.05 in the system (1?x)PbZr_0.52Ti_0.48O₃–(x)BiFeO₃ by the sol–gel method. XRD patterns of the pure PZT and BFO modified PZT samples have shown single phase formation. The effects of substitution of BFO on dielectric properties of PZT have been studied in the frequency range 10 Hz to 100 kHz and temperature range from RT to 773 K. The density was optimized by sintering the BFO modified PZT samples at different temperatures in four batches, S1, S2, S3 and S4. P–E hysteresis loop measurements for all the samples have shown almost saturated polarization. It has been observed that sample S2, sintered at 950 °C, exhibits superior dielectric properties of the four samples. The occurrence of weak ferromagnetism, observed in the M–H hysteresis loop, indicates coupling between ferroelectricity and magnetism. Impedance analysis has revealed that all the samples, sintered at different temperatures, have a different grain resistance. A large change in ac conductivity around T _c has been observed in all the samples.     

Auditory processing of real and illusory changes in frequency modulation (FM) phase

Auditory processing of frequency modulation (FM) was explored. In experiment 1, detection of a tau-radians modulator phase shift deteriorated as modulation rate increased from 2.5 to 20 Hz, for 1- and 6-kHz carriers. In experiment 2, listeners discriminated between two 1-kHz carriers, where, mid-way through, the 10-Hz frequency modulator had either a phase shift or increased in depth by deltaD% for half a modulator period. Discrimination was poorer for deltaD = 4% than for smaller or larger increases. These results are consistent with instantaneous frequency being smoothed by a time window with a total duration of about 110 ms. In experiment 3, the central 200-ms of a 1-s 1-kHz carrier modulated at 5 Hz was replaced by noise, or by a faster FM applied to a more intense 1-kHz carrier. Listeners heard the 5-Hz FM continue at the same depth throughout the stimulus. Experiments 4 and 5 showed that, after an FM tone had been interrupted by a 200-ms noise, listeners were insensitive to the phase at which the FM resumed. It is argued that the auditory system explicitly encodes the presence, and possibly the rate and depth, of FM in a way that does not preserve information on FM phase.     

New speckle control technique using high-speed electro-optic modulators with resonant electrode and polarization-reversed structures

We propose a new technique for reducing speckle noise in laser displays utilizing a high-speed optical phase modulator for expanding the laser beam spectrum. By adopting standing-wave resonant electrodes and polarization-reversed structures to LiTaO₃ electro-optic modulators, high-efficiency phase modulation with a large modulation index at high microwave frequency ranges is obtainable for different color laser beams at the same time. This enables us to expand laser spectra to over 100 GHz and reduce speckle noise. The device design, fabrication, and experimental demonstration for speckle control are reported.     

The PZT optical fiber phase modulator testing with varying amplitude modulation

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Coherent Beam Combination of Three Fiber Amplifiers with Multi-dithering Technique

Coherent beam combination of three W-level fiber amplifiers with multi-dithering technique is demonstrated. The multi-dithering technique is used for phase control in two channels. In the experiment, two channels are modulated by sine wave with 70 kHz and 100 kHz respectively, and two regular commercial DSP lock-in amplifiers and an industrial computer are used for electric signal processing in the feedback loop. The fringe contrast is advanced from 12% to 81%, and 78% coherent combination efficiency is obtained when the feedback loop is closed.     

Multi-beam second-harmonic generation in beta barium borate with a spatial light modulator and application to internal structuring in poly(methyl methacrylate)

Parallel beam frequency doubling of 170 fs, NIR pulses is demonstrated by placing a thin beta barium borate (BBO) nonlinear crystal after a spatial light modulator. Computer-generated holograms applied to the spatial light modulator create 18 parallel diffracted beams at the fundamental wavelength λ=775 nm, then frequency doubled to λ=387 nm and focussed inside the poly(methyl methacrylate) (PMMA) substrate for refractive index structuring. This procedure, demonstrated for the first time in PMMA, requires careful attention to phase matching of multiple beams and opens up dynamic parallel processing at UV wavelengths where nematic liquid crystal devices are more sensitive to optical damage. By overlapping filamentary modifications, an efficient, stable volume phase grating with dimensions 5×5×2.0 mm³ and pitch Λ=15 μm was fabricated in 18 minutes and reached a first-order diffraction efficiency of 70 % at the Bragg angle.     

Improved frequency response measurement method of half-wave voltage for phase modulator

In this paper, the improvement method of the traditional optical spectrum analyzer (OSA) is demonstrated to measure the half-wave voltage of LiNbO₃ phase modulator with low RF driving. Based on calculating the measured powers ratio of the carrier and first-order side band with optical spectrum analyzer (OSA), the phase modulation index is between 0.5 and 1.43 and the voltage of driving signal is lower than 0.796 V, the accurate characterization of the phase modulator is obtained. By using this method, the half-wave voltage of a LiNbO₃ phase modulator had been measured in the frequency range of 26 GHz. And, our experimental results showed a good agreement between the measured data and those provided by the manufacturer in 9 GHz frequency range. It is an accurate and easy method to measure the half-wave voltages of a phase modulators, especially for measuring high the frequency response.     

基于双音外差的电光相位调制器半波电压自校准测量方法

电光相位调制器是光纤通信系统、微波光子系统和相干光通信系统中的关键器件之一. 作为器件本征参数, 电光相位调制器的半波电压通常利用光谱方法和电谱方法进行测量. 光谱方法受到光源线宽和光谱仪分辨率限制, 测量的分辨率较低; 电谱方法则需要光电检测之前将相位调制转换成强度调制, 电谱方法的主要困难在于需要对探测器的不平坦响应进行额外校准. 提出了利用双音外差实现电光相位调制器半波电压自校准测量新方法, 该方法利用双音电光相位调制的边带与移频光载波的外差拍频, 对外差拍频信号进行频谱分析, 获得电光相位调制器的半波电压; 通过设定双音调制信号的频率关系, 克服了探测器光电转换中的不平坦频率响应, 实现了自校准测量. 该方法可扩展探测器和频谱仪的测试频率两倍以上, 节省至少一半的带宽需求. 与光谱测量方法相比, 该方法测试分辨率大幅提高且避免了光源线宽的影响; 与传统电域测量方法相比, 该方法无须额外校准, 无驱动功率和工作波长限制, 且对测试仪器带宽需求降低一半以上. 实验证实了所提方法获得的电光相位调制器半波电压的测量结果与光谱分析法获得的结果一致, 且大幅度地提高了测量范围和分辨率. 该方法提供了非常简单的电光相位调制器微波特性化分析方法, 对其他光电子器件分析也提供了参考.     

Optical phase locking of two extended-cavity diode lasers with ultra-low phase noise for atom interferometry

We present a phase coherent laser system with ultra-low phase noise with a frequency difference of 6.9 GHz. The laser system consists of two extended-cavity diode lasers that are optically phase-locked with electrical feedback to the injection current of a slave laser. The bandwidth of the optical phase-locking loop is extended up to 8 MHz. We achieve the residual phase noise of two phase-locked lasers of below ?120 dBrad²/Hz in the offset frequency range of 100 Hz–350 kHz and a flat phase noise of ?127 dBrad²/Hz from 700 Hz to 20 kHz. These results are, to the best of our knowledge, the lowest phase noise level ever reported with two extended-cavity diode lasers.     

Frequency-octupled phase-coded signal generation based on carrier-suppressed high-order double sideband modulation

An approach for photonic generation of a frequency-octupled phase-coded signal based on carrier-suppressed high-order double sideband modulation is proposed and experimentally demonstrated. The key component of the scheme is an integrated dual-polarization quadrature phase shift keying modulator, which is used to achieve the carrier-suppressed high-order double sideband modulation. At the output of the modulator, two fourth-order optical sidebands are generated with the optical carrier suppressed. After that, a Sagnac loop incorporating a fiber Bragg grating and a phase modulator is employed to separate the two optical sidebands and phase modulate one sideband with a binary coding signal. The approach features large carrier frequency tuning range for the generated phase-coded signal from several megahertz to beyond the W-band. A proof-of-concept experiment is carried out. The 2 Gbit/s phase-coded signals with frequencies of 16.48, 21.92, and 29.76 GHz are generated.     

Millimeter wave generation through frequency 12-tupling using DP-polarization modulators

The polarization modulators are being utilized for the millimeter wave (MM-Wave) generation because of their bias drift free operation and high extinction ratio. In this work, an optical millimeter wave generation through frequency 12 tupling using dual parallel polarization modulator (DP-PolM) is demonstrated. By setting RF drive voltage and proper polarization angle of the DP-PolM, the frequency 12-tupled optical millimeter is generated with an optical sideband suppression ratio (OSSR) of 37.76 dB and radio frequency sideband suppression ratio (RFSSR) of 31.67 dB. A good agreement between simulation and theoretical approach is achieved. Further, impact of non ideal parameters on the OSSR, RFSSR and BER are also analysed and presented.     

压电式光纤相位调制器相移系数测试

介绍了压电式光纤相位调制器的相移系数理论公式,提出了基于杨氏双缝干涉原理的相移系数测量方法。对测量方法的可行性进行分析,通过实验对测量方法进行了验证。实际研制了一个基于压电陶瓷环的光纤相位调制器件,实验测得的相移系数与理论分析结果基本一致。利用研制的压电式光纤相位调制器实现了两路准直光束之间的相位锁定,在远场实现了相干合成,干涉条纹可见度从3%提升至43%。     

Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers

We demonstrate coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using a feedback loop based on an electro-optic phase modulator, an average power of 7.2?W before compression is obtained with a combining efficiency of 90%. The spatial and temporal qualities of the oscillator are retained, with a recombined pulse width of 325?fs. This experiment opens up a way to scale the peak/average power of ultrafast fiber sources.     

Phase-Locked Fibre Array for Coherent Combination and Atmosphere Aberration Compensation

We report a fibre amplifier array that not only achieves coherent beam combination by compensation of phase noises of fibre amplifier, but also accomplishes correction of atmosphere aberration. It is of master-oscillatormultiple-amplifier （MOPA） configuration, which can be phase-locked by the multidither principle or heterodyne detection principle. First laboratory experiments of atmosphere aberration compensation of a three-element fibre amplifier array are reported. The atmosphere aberration is created by a phase screen in the experiment. The phase changes of the beam, which are introduced by the fibre amplifier and the phase screen, are both detected by the heterodyne detection method. Phase modulators are controlled to compensate for the phase in the three paths. No matter whether there is a phase screen producing atmosphere aberration or not, the dim dynamic interference fringes in the far field turn to a clear and stable pattern, and the peak intensity is maximized. It is indicated that the fibre amplifier array is phase-locked, and coherent combination of the three beams is achieved. It can be used not only to obtain high power fibre laser array but also in laser space communication.     

A picosecond near-infrared laser source based on a self-seeded optical parametric generator

We report on the design and development of a new type of near-IR laser source. The source comprises of an optical parametric generator (OPG) and a second harmonic generator (SHG) pumped by an 80-MHz, 1064-nm, 7-ps Nd:YVO₄ laser. The OPG is self-seeded with a fraction of its own signal output, which significantly enhances its conversion efficiency. The SHG doubles the frequency of OPG signal to produce a coherent output. The final output beam has a tunable wavelength near 800 nm, an average power of over 1 W, and a pulse duration around 5 ps. The M²-factor of the output beam can reach 1.1 after spatial filtering. With the new laser source, we have successfully demonstrated coherent anti-Stokes Raman scattering microscopy on 1 μm polystyrene beads, which shows that it has the potential to be a substitute for a picosecond optical parametric oscillator in certain microscopy or spectroscopy applications.     

上海光源数字化低电平控制系统的硬件设计与实现

 设计了上海光源高频低电平控制系统,它是以数字化技术为基础,采用上下变频和IQ调制解调技术,实现幅度、频率和相位的反馈控制。上海光源储存环的束流设计流强为300 mA,为了抑制由此带来的Robinson不稳定性和纵向零模束流振荡,加入了高频直接反馈和零模束流反馈环路。从上下变频技术、IQ调制解调技术、时钟分配及锁相技术等方面对上海光源数字化低电平系统的硬件设计及其实现进行了阐述,给出了该系统试验、高功率下运行的测试结果,实现了设计要求的幅度控制精度±1%、频率控制精度 ±10 Hz和相位控制精度± 1°的技术指标。     

A simple image-reject mixer based on two parallel phase modulators

A simple photonic microwave image-reject mixer (IRM) using two parallel phase modulators is proposed. First, a photonic microwave mixer with phase shift ability is achieved using two parallel phase modulators (PMs), an optical bandpass filter, three polarization controllers, three polarization beam splitters and two balanced photodetectors. At the output of the mixer, two frequency downconverted signals with tunable frequency difference can be obtained. By adjusting the phase difference as 90° and utilizing an electrical 90° hybrid, the useless components can be eliminated, and the image reject operation is realized. The key advantage of the proposed scheme is the usage of PM, which avoid the DC bias shifting problem and make the system simple and stable. A simulation is performed to verify the proposed scheme, a relative ??90° or 90° phase shift can be obtained between the two output ports of the photonic microwave mixer, at the output of the IRM, 60 dB image-reject ratio is obtained.