全光纤速度干涉仪设计及应用

针对冲击波物理与爆轰物理等研究领域中对高速运动物体进行连续速度测量的需求,设计了一种全光纤速度干涉仪.该干涉仪采用单模光纤作为光传输和延迟元件,对t和t-τ两个时刻由于速度变化而引起的多普勒差拍信号进行检测.由于两个时刻的两束光信号对应的待测物体速度变化不大,因而两者几乎有相等的频移量,从而大大降低了差拍信号频率.并且,通过光纤长度的改变,灵活调节条纹常量（τ值）,使差拍信号频率不超过记录系统的带宽,从原理上解决记录系统响应带宽受限问题,拓展测速的上限.单模光纤的采用,对漫反射光起到了较好的选模作用,使干涉仪实现了对漫反射靶的测量.实验设计了1.5 m·s-1和150 m·s-1两种条纹常量,对低速过程的霍普金森杆实验和高速过程的激光驱动实验分别进行了测试,取得较好结果,证明了该干涉测试技术的有效性.     

Absolute metrology by phase and frequency modulation for multiwavelength interferometry

A fiber interferometer for absolute distance measurements is presented whereby wavelength variation is achieved via a sinusoidal strain modulation of a fiber Bragg grating to generate a series of beat wavelengths. The interferometer employs fiber laser sources where the design is based on the use of narrow-bandwidth fiber Bragg gratings. The accuracy of the beat wavelengths is improved compared to the use of multiple wavelengths measured with conventional optical spectrum analyzers or available wavemeters. Initial measurements are presented for beat wavelengths of 254.74 mm and 27.4 m over an optical path difference of 200 mm and 3.8 m, respectively. Combined with a two (or three) wavelength interferometer, this technique has the potential for ultrahigh dynamic range metrology ranging over several meters while preserving subfringe resolution and a low system complexity.     

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.     

Narrow-linewidth chirped frequency comb from a frequency-shifted feedback Ti:sapphire laser seeded by a phase-modulated single-frequency fiber laser

Frequency-shifted feedback (FSF) lasers have emerged as powerful tools for precision distance metrology. At the output of a Michelson interferometer, the detected rf spectra of the FSF laser light contain a length-dependent heterodyne beat signal whose linewidth ultimately limits the achievable accuracy of length measurements. Here, we demonstrate a narrow-linewidth chirped frequency comb from an FSF Ti:sapphire ring laser seeded by a phase-modulated, ultra-low-phase-noise, single-frequency fiber laser. We experimentally investigate the influence of the seed laser linewidth on the resulting width and shape of the length-dependent rf beat signal. An ultranarrow heterodyne beat linewidth of <20 Hz is observed.     

Influence of radiation coherence on the effects of polarization nonreciprocity of fiber ring interferometers

In this paper, we consider the effects of polarization nonreciprocity taking into account coherence or partial coherence of radiation at the output of a fiber ring interferometer (FRI) and discuss the possibility of occurrence of additional effects of polarization nonreciprocity at the output of an FRI with a broadband radiation source due to the coherent component of radiation resulting from the coupling of orthogonal modes at the output segments of the fiber. Allowing for the coherence, we estimate the temperature instability of the interferometer signal related to the polarization nonreciprocity. The features of the polarization nonreciprocity effects for a fiber ring interferometer with polarizer or dichroism are considered. It is shown that if the conditions of conventional reciprocity theorem (no rotation) are satisfied for the interferometer, then the polarization nonreciprocity effects become hidden and cannot be detected in the interference signal without a priori information on the character of birefringence and on the orientation of anisotropic elements of the fiber loop of the interferometer and polarizer.     

大功率、高效率、高消光比铒光纤多波长超荧光光源

采用改进的反射式Mach-Zehnder干涉滤波器，对双程后向结构掺铒光纤超荧光光源(DPB SFS) 分别进行光谱分割和光谱预分割，构建了两种结构的多波长超荧光光纤光源（MW SFS），波长间隔为～0.8 nm时，在1550 nm附近（1542～1559 nm）20个波长的功率波动小于0.5 dB其中前者消光比高达27 dB；后者消光比～18 dB，在泵浦光功率为72.8 mW时，最大输出功率25.3 mW，光光转换效率高达34.8%改变Mach-Zehnder干涉仪的臂长差，采用光谱预分割技术，得到1550 nm附近波长间隔～0.4 nm、消光比～16 dB的50个波长输出     

Self-referencable frequency comb from a 170-fs, 1.5-μm solid-state laser oscillator

We report measurement of the first carrier-envelope offset (CEO) frequency signal from a spectrally broadened ultrafast solid-state laser oscillator operating in the 1.5 μm spectral region. The f-to-2f CEO frequency beat signal is 49 dB above the noise floor (100-kHz resolution bandwidth) and the free-running linewidth of 3.6 kHz is significantly better than typically obtained by ultrafast fiber laser systems. We used a SESAM mode-locked Er:Yb:glass laser generating 170-fs pulses at a 75 MHz pulse repetition rate with 110-mW average power. It is pumped by one standard telecom-grade 980-nm diode consuming less than 1.5 W of electrical power. Without any further pulse compression and amplification, a coherent octave-spanning frequency comb is generated in a polarization-maintaining highly-nonlinear fiber (PM-HNLF). The fiber length was optimized to yield a strong CEO frequency beat signal between the outer Raman soliton and the spectral peak of the dispersive wave within the supercontinuum. The polarization-maintaining property of the supercontinuum fiber was crucial; comparable octave-spanning supercontinua from two non-PM fibers showed higher intensity noise and poor coherence. A stable CEO-beat was observed even with pulse durations above 200 fs. Achieving a strong CEO frequency signal from relatively long pulses with moderate power levels substantially relaxes the demands on the driving laser, which is particularly important for novel gigahertz diode-pumped solid-state and semiconductor lasers.     

单纵模布里渊掺铒光纤激光器的实验研究

提出了一种具有多环形腔(MRC)和光纤布拉格光栅可调谐滤波器(FBG-TF)相结合的单纵模布里渊掺铒光纤激光器(BEDFL).实验中通过观察激光器输出光的拍频信号分析其光谱精细结构,分别讨论了布里渊增益、FBG-TF、MRC对BEDFL单模输出的贡献,给出了激光器的功率特性曲线和波长稳定性测试图.实验得到了在1550 nm处功率为4 dBm,信噪比(SNR)>60 dB,线宽小于1.5 kHz的稳定单纵模输出光.     

2.1 μm可调谐多波长掺钬光纤激光器

为了实现2.1 μm波段光纤激光器输出多波长激光，设计了一种基于光纤Sagnac干涉仪的可调谐多波长掺钬光纤激光器。采用1.9 μm波段掺铥光纤激光器泵浦一段长3 m的掺钬石英光纤，获得2.1 μm波段的光放大；环形腔中，由保偏光纤和偏振控制器构成的光纤Sagnac干涉仪，实现2.1 μm波段周期滤波，获得了2.1 μm波段多波长激光，输出功率1 mW~15 mW可调谐，最多可观测到6个波长的激光输出。通过调节环形腔内偏振控制器，能够实现2.1 μm波段1~6个波长的调谐。     

A dual-pass Mach–Zehnder interferometer filter using a TCF loop mirror for double-wavelength fiber lasers

A dual-pass Mach–Zehnder interferometer filter using a section of twin-core fiber (TCF) loop mirror is proposed. The filter is theoretically and experimentally studied for various interferometer arm difference when TCF length is constant. Theoretical results are validated by the experimental demonstration and in good agreement with the experimental results. And then, by using the filter in a ring fiber laser, a stable and switchable dual-wavelength lasing is obtained experimentally. The 3-dB bandwidth and the SMSR of the output laser are 0.015 nm and higher than 62.4 dB, respectively. The peak power fluctuation and wavelength shift are also monitored to be less than 0.04 dB and 0.02 nm over an hour at room temperature. Furthermore, the output laser can be switched between single and dual wavelength by carefully adjusting the PCs. The experimental results show that the filter can suppress mode competition effectively, improve the SMSR availably, and enhance the stability of the output lasing.     

Measurements of neutron interference and polarization effects caused by nuclear and magnetic interaction

The effects of simultaneous phase shift and spin rotation on neutron waves were measured with the perfect crystal neutron interferometer. Using an unpolarized beam of slow neutrons characteristic “beat” effects of the interference pattern and a polarization of the neutrons behind the interferometer could be observed.     

高功率宽带射频调制连续激光源

射频强度调制激光作为激光雷达系统的载波可以有效提高系统的抗干扰和抗散射能力,高功率宽带射频强度调制光源是实现高分辨率远距离探测的关键.本文采用在Nd:YAG激光器的耦合腔中插入一对四分之一波片的方法实现了频差调谐范围为30 MHz—1.5 GHz的双频激光输出,结合光纤振荡功率放大技术,将双频信号光功率放大为50 W.耦合腔双频种子源具有良好的功率和频率稳定性,输出功率为9.5 mW时,功率标准差为0.145 mW,稳定性为1.52%,输出双频激光的频差为250 MHz时,拍频的标准差为1.6144 MHz.种子光进行三级光纤功率放大,得到50 W双频激光输出.放大后的双频激光功率波动范围小于0.1 W,双频拍频的标准差为1.777 MHz,很好地保持了放大之前的功率稳定性和双频频差稳定性.     

基于3×3耦合器的迈克耳孙干涉仪相位特性分析

研究了基于3×3耦合器的非平衡迈克耳孙干涉仪的相位特性.由光纤耦合器的散射矩阵理论,推导出了当3×3耦合器分光比不均匀时,干涉仪三路输出信号相位差的表达式.根据实际使用的3×3耦合器各通道的插入损耗,经计算与修正得到其散射矩阵,并求出干涉仪三路输出信号的相位差分别为120.21°、120.77°和119.02°,与理想值120°的偏差在1°以内.实验测得的干涉仪三路输出信号的相位差随时间随机变化,经分析是由光偏振态随机变化引起的.相位差与理想值120°的偏离均在1°以内,符合理论分析得到的结论.     

Stable and spacing-adjustable multiwavelength Raman fiber laser based on mixed-cascaded phosphosilicate fiber Raman linear cavity

A novel multiwavelength Raman fiber laser based on the mixed-cascaded Stokes effects of phosphosilicate fiber is proposed and demonstrated experimentally. By using stimulated Raman scattering of both P(2)O(5) and SiO(2) along 1 km phosphosilicate fiber pumped with a 1064 nm double-clad fiber laser, the mixed-cascaded Raman linear cavity is formed by a pair of fiber Bragg gratings at 1239 nm, a polarization-maintaining fiber (PMF) Sagnac loop filter, and a conventional optical loop mirror. Up to 15-wavelength stable oscillations around 1320 nm are obtained with a wavelength spacing of 0.44 nm and power nonuniformity of less than 4 dB. By changing the length of the PMF in the Sagnac loop filter from 10 to 5.5 m, the wavelength spacing is adjustable from 0.44 to 0.8 nm. The extinction ratio of the laser is more than 30 dB. Excellent stability is also observed with a peak power fluctuation of less than 0.8 dB in 1 h.     

Tunable dual-wavelength fiber laser based on a single fiber Bragg grating in a Sagnac loop interferometer

We propose a simple dual-wavelength Er-doped fiber laser configuration based on a dual-wavelength fiber Bragg grating written on the splice joint between two different fibers for wavelength-selective filter in the Sagnac loop interferometer. The wavelength separation between the adjacent lasing wavelengths is 1.12 nm and the side-mode suppression ratio (SMSR) is over 55 dB. The output power variation is less than 0.8 dB over a two-minute period. Moreover, the lasing wavelength can be effectively tuned using the thermal heating method.     

基于光纤气泡和纤芯失配的Mach-Zehnder干涉液体折射率传感器

介绍了一种基于光纤气泡和纤芯失配的Mach-Zehnder干涉液体折射率传感器. 将两根纤芯经过腐蚀的普通单模光纤熔接在一起, 在熔接点处形成一个气泡, 在距气泡20 mm处级联一段20 mm的细芯光纤, 再接入一段单模光纤, 形成单模光纤-气泡-单模光纤-细芯光纤-单模光纤结构的传感器. 气泡与光纤芯径失配处的两个节点起到光纤耦合器的作用, 从而形成光纤Mach-Zehnder干涉仪. 环境液体折射率的变化,将使得传感器透射谱能量发生变化, 通过测量干涉谱波峰峰值能量从而实现对折射率的测量. 并对所制作传感器的折射率响应特性进行了实验研究, 实验结果表明干涉谱波峰峰值能量与环境液体折射率之间存在良好的线性关系, 当环境液体折射率变化范围在1.351–1.402时, 响应灵敏度为143.537 dB/RIU, 线性度0.996. 该传感器在生物化学领域有较好的应用前景. 关键词： 光纤气泡 纤芯失配 Mach-Zehnder干涉仪 折射率传感     

Picosecond flat-top pulse generation using dual-mode fiber Mach-Zehnder interferometers

We report on an optical fiber Mach-Zehnder interferometer made by offset splicing a dual-mode fiber with two single-mode fibers. The extinction ratio can be tuned from less than 1 dB to greater than 28 dB by rotating the dual-mode fiber along its axial direction. The interferometer is utilized as a pulse shaper to convert a 2.5 ps Gaussian pulse to a 5.2 ps flat-top pulse.     

Broad tunability of erbium-doped fiber ring laser based on few-mode polarization maintaining fiber Sagnac interferometer

A highly stable erbium-doped fiber (EDF) ring laser with broad tunability is proposed and experimentally demonstrated. It is accomplished by using a few-mode polarization maintaining fiber (FM-PMF) Sagnac interferometer, in which simultaneous Sagnac and modal interferences occur. Continuously tunable output is obtained by exerting the longitudinal strain on the FM-PMF in combination with adjusting a polarization controller (PC) next to the FM-PMF in the Sagnac interferometer. In addition, the adverse impact of uneven EDF gain distribution on the laser output power uniformity may be offset by the PC-dependent transmission loss of the FM-PMF Sagnac interferometer and reflected backward C-band amplifier spontaneous emission from the pumped EDF. During the whole tuning process, the tunable span covers more than 40 nm with the output power flatness better than 1 dB, 3 dB spectral linewidth narrower than 0.1 nm and optical signal-to-noise extinction ratio higher than 45 dB.     

阿秒和频相位共轭极化拍频的不对称色锁二阶相干控制

研究了级联三能级系统中孪生色锁随机光场的相位共轭阿秒和频极化拍频(ASPB)不对称新效应。从理论上分析了自关联单光子简并四波混频(DFWM)和双光子非简并四波混频(NDFWM)信号辐射场与物质间不对等效应、混合型太赫兹失谐振荡特性,及双光子非简并共振四波混频信号的延时不对称性;同时也分析了由干涉计两臂不平衡色散效应引起的阿秒和频极化拍频信号的延时不对称性,即阿秒和频极化拍频信号的最大值相对于零延时有一个小偏移,结果表明该不对称性并这不影响阿秒和频极化拍频零差探测两偶极禁戒跃迁能级分裂的总体精度。阿秒和频极化拍频作为一种消除多普勒增宽的超快调制过程,从理论上说它可以扩展到任何两偶极禁戒跃迁能级系统。     

High-performance SOA-based multiwavelength fiber lasers incorporating a novel double-pass waveguide-based MZI

In this paper, using a direct double-pass and a novel isolator-assisted double-pass waveguide-based Mach-Zehnder interferometer (MZI), high-performance SOA-based multiwavelength fiber lasers (MFL) are proposed and demonstrated experimentally. The filtering characteristics of the proposed isolator-assisted double-pass MZI are analyzed and examined theoretically in comparison with those of the single-pass and direct double-pass MZI. Using a direct double-pass waveguide-based MZI with the single-pass free spectral range (FSR) of 43 GHz, up to 115- and 104-channel simultaneous oscillations spaced at 21.5 GHz in the L-band and C-band are obtained, respectively, with a power non-uniformity of less than 3 dB and an extinction ratio of ∼30 dB. To the best of our knowledge, it is the highest lasing-channel count that has been achieved from an SOA-based MFL. To enhance the extinction ratio while maintaining the FSR, the proposed isolator-assisted double-pass MZI is then utilized in the laser cavity, and a stable 55-wavelength simultaneous oscillation spaced at 43 GHz is accordingly achieved in C-band with an extinction ratio of higher than 50 dB. Compared with the lasing linewidth of 0.058 nm with the conventional single-pass MZI, narrower linewidths of 0.038 and 0.028 nm are obtained with the isolator-assisted and direct double-pass MZI configurations, respectively. The lasers are stable with a maximum power fluctuation per channel of less than 0.8 dB during an hour’s test.