基于光纤的光学频率传递研究

随着光钟研究的发展, 光钟的稳定度和不确定度均达到10^-18量级. 通过光纤可以实现光钟频率信号的高精度传输, 有望用于未来“秒”定义的复现. 演示了百公里级实验室光纤上的光学频率传递. 对于在实验室70 km光纤盘上实现的光频传递, 光纤相位噪声抑制在1-250 Hz傅里叶频率范围内均接近于光纤延时极限, 对应传输稳定度(Allan偏差)为秒级稳定度1.2×10^-15, 10000 s稳定度为1.4×10^-18. 实验室100 km光纤的光频传递秒级稳定度也达到了5×10^-15. 提出了光纤噪声用户端补偿的方案, 可以简化星形传递网络中心站的复杂度. 在25 km光纤上演示了该传递方案, 实现的传输稳定度接近传统前置补偿传递方案.     

掺Er光纤飞秒激光器中电光晶体对激光器参数的影响

由于受增益介质上能级寿命的影响,掺Er光纤光梳的梳齿线宽一般在百kHz量级.为了实现光梳梳齿线宽的压窄,一种有效的方法是在激光器中增加快速响应的电光晶体,使光纤光梳的伺服锁定带宽提高到百kHz以上,为光纤光梳的快速伺服锁定提供反馈机构.这其中,高品质的飞秒激光器是核心.基于此,本文主要研究了掺Er光纤飞秒激光器中电光晶体对激光器参数的影响.通过计算电光晶体的折射率、色散、相位延迟等参数,分析了电光晶体对激光器参数的影响,并在实验上获得了电光晶体电压对激光器重复频率和载波包络偏移频率的影响,进而通过电光晶体实现了对光纤光梳重复频率和载波包络偏移频率的锁定.通过锁定光纤飞秒激光器与窄线宽激光器的拍频信号,验证了电光晶体的引入使激光器的伺服锁定带宽提高到了236 kHz,为窄线宽飞秒光学频率梳的建立提供了技术基础.     

Broadband phase noise suppression in a Yb-fiber frequency comb

We report a simple technique to suppress high-frequency phase noise of a Yb-based fiber optical frequency comb using an active intensity noise servo. Out-of-loop measurements of the phase noise using an optical heterodyne beat with a cw laser show suppression of phase noise by ≥7?dB out to Fourier frequencies of 100?kHz with a unity-gain crossing of ～700?kHz. These results are enabled by the strong correlation between the intensity and phase noise of the laser. Detailed measurements of intensity and phase noise spectra, as well as transfer functions, reveal that the dominant phase and intensity noise contribution above ～100?kHz is due to amplified spontaneous emission or other quantum noise sources.     

200 km沙漠链路高精度光纤时频传递关键技术研究

针对沙漠环境实地链路存在的温度变化大、室外风力、地表振动等多种复杂噪声来源,通过对系统反馈补偿带宽、反馈补偿强度、光功率等时频传递系统关键参数的优化配置,研究了不同反馈补偿参数下复杂链路噪声的有效抑制技术.全链路的频率传递稳定度8×10~(-14)@1s,1×10~(-16)@1000 s,千秒尺度下时间信号传递的时间方差仅为1.2 ps.实现了氢钟信号在200 km量级沙漠环境实地链路的无损传输.该验证实验在基于短基线干涉测量的卫星测轨系统中发挥了重要作用.     

Phase-noise-compensated optical frequency domain reflectometry with measurement range beyond laser coherence length realized using concatenative reference method

A novel type of optical frequency domain reflectometry with a measurement range much longer than the laser coherence length is proposed and experimentally demonstrated. To reduce the influence of laser phase noise, the measurement signal is compensated by using reference signals generated from a single auxiliary interferometer supported by a newly proposed compensation process. The compensation is accomplished numerically with a computer for each section of the delay fiber length in an auxiliary interferometer after only one data acquisition. By using the proposed technique, it is confirmed experimentally that the laser phase noise is well compensated even beyond the coherence length.     

基于光纤色散相位补偿的高分辨率激光频率扫描干涉测量研究

为了实现更高分辨率的激光频率扫描干涉测量, 增大光源的扫频范围以及减小扫描频率的非线性成为关键. 采用外腔式大带宽扫频光源结合光纤辅助干涉仪构建的外部时钟频率采样非线性校正是目前较为常用的方法. 本研究发现随着扫频带宽和测量范围的增加, 光纤辅助干涉仪与测量光路中存在的色散失配导致频谱出现严重展宽, 极大的降低了测量的分辨率. 本文建立了辅助干涉仪和测量干涉仪色散失配影响的理论模型, 利用该模型分析了扫频带宽和测量范围与测量分辨率的变化关系, 与实验结果相一致, 并进而提出了基于峰值演化消畸变的色散相位补偿方法, 有效地提高了测量的分辨率, 在2.53 m 处实现了接近理论值的64.5 μm的测量分辨率. 该色散失配模型及补偿方法为提高大尺寸激光频率扫描干涉仪的测量分辨率及测距范围提供了参考.     

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.     

基于串行最小频移键控的8进制调制的研究

提出了一种新型的基于串行结构的最小频移键控调制技术,产生了相位连续,频谱宽度窄,每码元时间有两次π/2相移,类似于传统最小频移键控调制信号,给出了该调制方案的理论推导,并将其应用于8进制的多维多阶调制系统中。理论和仿真分析了不同的8进制调制方案在频谱特性,残留色散容限,自相位调制容限的不同,还分析了其受到系统滤波带宽的影响。最后仿真实现了通过350km的色散完全补偿的光纤120Gb/s的数据传输。     

交叉相位调制提高半导体激光器混沌载波发射机带宽方法

提出半导体激光器混沌交叉相位调制(XPM)光反馈提高混沌载波发射机带宽方法,建立了有外腔光纤传输反馈XPM控制下的激光动力学物理模型.理论导出XPM作用下激光双反馈频率失谐公式,指出XPM产生的非线性相移影响了激光器增益和线宽增强因子,其光纤二阶非线性效应使激光振幅和相位变化更加丰富,而非线性相移的出现进一步增加了新的频率分量并使频谱展宽.数值结果表明,XPM使激光器混沌带宽增加到4倍以上,使激光混沌张弛振荡频率增加到2.85倍,其光纤长度、入纤光功率、面镜反射系数、光纤二阶非线性系数等都能影响激光混沌带宽.     

A novel scheme of optical Meyer wavelet filter based on time lenses in nonlinear multi-span optics fiber communication system

In this work, a novel scheme to realize optical Meyer wavelet filter based on time lenses and Mach–Zehnder modulators in optical domain was proposed and the transfer functions of Meyer wavelet decomposition and reconstruction processed in frequency domain were derived to realize Meyer wavelet de-noising for optical signal. The filter could be used to reduce the nonlinear noise induced by the interaction of EDFA’s ASE noise, fiber’s dispersion and nonlinearity in high bit rate multi-span optical communication systems without photo-electric conversion. The bit error rate curves of the optical bit sequence without and with optical Meyer wavelet filter were plotted to show the effectiveness of the optical Meyer wavelet filter, which can achieve better result and improve the optical communication quality in dispersion compensation links.     

50 GHz optical true time delay beamforming in hybrid optical/mm-wave access networks with multi-core optical fiber distribution

We propose and experimentally validate an optical true time delay beamforming scheme with straightforward integration into hybrid optical/millimeter(mm)-wave access networks. In the proposed approach, the most complex functions, including the beamforming network, are implemented in a central office, reducing the complexity and cost of remote antenna units. Different cores in a multi-core fiber are used to distribute the modulated signals to high-speed photodetectors acting as heterodyne mixers. The mm-wave carrier frequency is fixed to 50 GHz(VBand), thereby imposing a progressive delay between antenna elements of a few picoseconds. That true time delay is achieved with an accuracy lower than 1 ps and low phase noise.     

有源循环式光脉冲复制系统的输出特性研究

改进并验证了一种基于有源循环光纤延迟线的模拟光脉冲信号复制技术.实验系统采用低增益掺铒光纤放大器补偿循环损耗.进行了光脉冲信号复制的仿真和实验,在掺铒光纤放大器增益为6.774 dB,光滤波器－3 dB,带宽为0.8 nm时,测得第500个复制光脉冲的信噪比为31 dB.仿真和实验结果表明,选用小增益掺铒光纤放大器可以实现低于3 dB的噪音系数,降低掺铒光纤放大器的放大自发辐射噪音并提高输出脉冲序列的信噪比,有望成为提高光脉冲复制器性能的一种新方法.在系统中插入窄带光滤波器等手段也是改善信噪比的有效措施.     

基于单模光纤的交叉相位调制型频率分辨光学开关超短脉冲测量

利用单模光纤中的光弹效应和交叉相位调制(XPM)效应,提出了一种频率分辨光学开关法测量超短脉冲的新方案.在本方案中,单模光纤的前一部分产生可变延迟,后一部分作为非线性介质产生非线性效应.该方案只需一根单模光纤,无须复杂的光路校准,结构简单,损耗低;光纤中的XPM效应易发生,无须相位匹配.对提出的方案进行了数值模拟,采用基于矩阵的主元素广义投影算法,恢复出待测脉冲的幅度和相位信息,并研究了光纤长度和待测超短脉冲的脉冲宽度对测量结果的影响.结果表明:测量准确度随着光纤长度的增加而提高,选取长度为2 km的光纤,就可以实现对超短脉冲的准确测量;本文方案适用于脉冲宽度不小于80 fs的超短光脉冲的测量.     

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.     

Full phase stabilization of a Yb:fiber femtosecond frequency comb via high-bandwidth transducers

We present full phase stabilization of an amplified Yb:fiber femtosecond frequency comb using an intracavity electro-optic modulator and an acousto-optic modulator. These transducers provide high servo bandwidths of 580 kHz and 250 kHz for f(rep) and f(ceo), producing a robust and low phase noise fiber frequency comb. The comb was self-referenced with an f-2f interferometer and phase locked to an ultrastable optical reference used for the JILA Sr optical clock at 698 nm, exhibiting 0.21 rad and 0.47 rad of integrated phase errors (over 1 mHz-1 MHz), respectively. Alternatively, the comb was locked to two optical references at 698 nm and 1064 nm, obtaining 0.43 rad and 0.14 rad of integrated phase errors, respectively.     

Robust laser frequency stabilization by serrodyne modulation

We report the relative frequency stabilization of a distributed feedback erbium-doped fiber laser on an optical cavity by serrodyne frequency shifting. A correction bandwidth of 2.3 MHz and a dynamic range of 220 MHz are achieved, which leads to a strong robustness against large disturbances up to high frequencies. We demonstrate that serrodyne frequency shifting reaches a higher correction bandwidth and lower relative frequency noise level compared to a standard acousto-optical modulator based scheme. Our results allow us to consider promising applications in the absolute frequency stabilization of lasers on optical cavities.     

高速光通信系统中光纤光栅色散补偿研究

通过对光栅制作过程的优化设计，解决了光纤光栅温度稳定性、纹波系数、带宽、偏振模色 散补偿等关键技术，所制作光纤光栅已经达到温度系数小于00005 nm/℃，带宽大于14 nm，纹波系数小于50 ps，色散量超过 -1000 ps/nm的先进水平. 采用琼斯矩阵本征值法较 精确地测量了光栅的偏振模色散，并对其进行了补偿，光纤光栅色散补偿器的偏振模色散由 补偿前的91406 ps改善为补偿后的01521 ps. 在此基础上，成功地建立了一个稳定可靠 、速率为40 Gb/s，传输链路为122 km G 关键词： 高速光通信系统 普通单模光纤 光纤光栅 色散补偿     

Ultra-broadband OFDM signal generation for 1-Tb/s super-channel coherent optical OFDM transmission

A 320.6-GHz multi-band optical OFDM signal generation scheme consisting of continuous 4,104 subcarriers is demonstrated for 1-Tb/s coherent optical OFDM (CO-OFDM) transmission. The 36 uncorrelated orthogonal bands are generated using a recirculating frequency shifter (RFS) with single laser by adjusting the loop delay of RFS to be an integer multiple of OFDM symbol period. The principle and performance of this generation scheme is studied theoretically and experimentally. The 1-Tb/s CO-OFDM signal is successfully received after 600-km SSMF fiber transmission without any dispersion compensation.     

Research on WDM optical fiber transmission system based on fiber Raman amplifier

After wavelength division multiplexing (WDM) optical fiber transmission system based on fiber Raman amplifier (FRA) is investigated in detail, the influence of the collocation of dispersion compensation fiber (DCF), the dispersion coefficient, dispersion slope (DS), effective core area, nonlinear index, length of FRA, launch power and the bandwidth of Bessel filter on bit error rate (BER) is deduced. The influence of Rayleigh backscattering noise on optical signal noise ratio (OSNR) is also investigated, which affects the performance of long haul transmission badly. The result indicates that the broadband long haul transmission can be realized through the reasonable design of the fiber. The result is useful to the optimal design of the WDM optical fiber transmission system based on FRA.     

Frequency comb-referenced narrow line width diode laser system for coherent molecular spectroscopy

We analyze in detail the frequency noise properties of a grating enhanced external cavity diode laser (GEECDL). This system merges two diode laser concepts, the grating stabilized diode laser and the diode laser with resonant optical feedback, thus combining a large tuning range with an excellent short-term frequency stability. We compare the frequency noise spectrum of a GEECDL to that of a grating stabilized diode laser and demonstrate a 10-fold reduction of the frequency noise linear spectral density. The GEECDL is phase locked to a similar laser and to a fs-frequency comb with a servo loop providing an open-loop unity-gain frequency of only 237 kHz, which is a tenth of the bandwidth typically required for grating stabilized diode lasers. We achieve a residual rms phase error as small as 72 mrad (≈ 200 mrad) for stabilization to a similar laser (to the fs-frequency comb). We demonstrate that the novel diode laser can phase-coherently track a stable optical reference with an instability of 1.8×10^-16 at 1 s. This laser system is well suited for applications that require phase locking to a low-power optical reference under noisy conditions. It may also be considered for the implementation of optical clock lasers. PACS 42.55.Px; 42.60.Jf; 42.50.Gy