Tuning range extension by active mode-locking of external cavity laser including a linearly chirped fiber Bragg grating

We demonstrate electrical wavelength tuning by mode locking of an external cavity laser (ECL) with linearly chirped fiber Bragg grating (LCFBG). The configuration consists of a laser chip providing the gain coupled to an LCFBG with a large chip rate of 10 or 55 nm/cm providing the counter-reaction for laser oscillation. The laser chirp is electrically modulated by a sinusoidal signal in such a way that the ECL is mode locked. By changing the modulation frequency, a wavelength tuning range of 27 nm is achieved with the 10 nm/cm LCFBG, and a partial tuning range over 41 nm is demonstrated with the 55-nm/cm LCFBG. The output pulse stream at a specific mode-locking frequency and a corresponding wavelength is obtained for both positively and negatively chirped grating. A time bandwidth product reduction is measured in the case of negatively chirp grating when compared with positively chirp grating. A simple general law between the laser parameters is given (locking frequency, tuning range, and FBG chirp value). The parameters for a 40-nm tunable source modulated at 10 GHz are given. This simple tuning mechanism is very well adapted for a structure that requires accurate wavelength monitoring.     

Effect of Grating Parameters on Mode-Locked External Cavity Lasers

This paper deals with numerical solutions of actively mode-locked fiber grating semiconductor lasers using a time-domain solution of coupled wave equations and rate equations. Simulation of linearly chirped tanh apodized fiber Bragg grating (FBG) utilized in hybrid soliton pulse source (HSPS) shows an extreme increase in the mode-locking frequency range of HSPS. Our model predicts transform-limited pulses over a frequency range of 1.6 GHz (1.8–3.4 GHz) for this grating around a system operating frequency of 2.5 GHz, with a pulsewidth of 46 ps required for a practical soliton transmission system, whereas it ranges to about 1.3 GHz (2–3.3 GHz) for the linearly chirped raised-cosine flat top and 850 MHz (2.1–2.95 GHz) for linearly chirped Gaussian apodized. Furthermore, in this study, the effects of FBG parameters, such as peak reflectivity, grating length, grating chirp, and modulation index, on output of mode-locked HSPS are also described for the first time. The numerical results indicated that although pulsewidths decreased with the increase in grating chirp, shorter grating lengths gave shorter pulses, and the modulation index and peak reflectivity of the grating did not significantly affect the pulsewidths.      

Frequency range extension of actively mode-locked lasers integratedwith electroabsorption modulators using chirped gratings

We have fabricated actively mode-locked lasers integrated with electroabsorption modulators and chirped gratings. A chirped grating with a large chirp rate of 1.45 Å/μm can be realized by using multiphase-shifted patterns. Short pulses of 4-6 ps were generated over a wide frequency range from 18.9-19.8 GHz. We observed jumps in the wavelength during detuning. These jumps arise from multiple lobes in the reflectivity spectrum. It is found that the wavelength jumps cause increases in the intensity noise. We showed that by reducing the grating length from 150 to 100 μm the sidelobes were suppressed and the detuning frequency range of over 1% could be realized. A smaller pulsewidth was obtained for the negatively-chirped gratings when compared to the positively chirped gratings     

Monolithically integrated multiwavelength sampled grating DBRlasers for dense WDM applications

For accurate control of the channel spacing in fabricating multiwavelength laser arrays or discrete multicolor lasers, we proposed a novel approach that exploits sampled grating distributed Bragg reflector (DBR) mirrors to vary the laser wave length across the wafer. This approach can realize a set of lasers with a wavelength spacing that meets the ITU recommendations for dense wavelength-division multiplexing systems and a wavelength range that can cover up to 40 nm or more. The wavelength variation across an array is achieved by changing the sampling periods of the DBR mirrors from laser to laser. The accuracy on the channel spacing of sampled grating DBR laser arrays was shown to be the same as that of conventional distributed feedback or DBR laser arrays, but their wavelengths can be better controlled for the gratings are fabricated with single holographic exposure. Arrays of 21 lasers have been successfully fabricated and have around 0.8-nm wavelength spacing with a simple tuning mechanism     

Detecting hybridization of DNA by highly sensitive evanescent field etched core fiber Bragg grating sensors

Highly sensitive fiber Bragg grating sensors were developed by etching away the cladding and part of the core of the fiber and detecting the change of Bragg wavelength due to the change of index of the surrounding medium. A sensitivity of 1394 nm/riu was achieved when the diameter of the grating core was 3.4 /spl mu/m and the index of the surrounding medium was close to the index of the core of the fiber. Assuming a detectable spectral resolution of 0.01 nm realized in the experiment, the sensor achieves a minimum detectable index resolution of 7.2/spl times/10/sup -6/. Higher sensitivity at lower surrounding index was achieved by using higher order modes excited in the Bragg grating region. The use of the fiber Bragg grating sensor was further investigated to detect hybridization of DNA. Single stranded DNA oligonucleotide probes of 20 bases were immobilized on the surface of the fiber grating using relatively common glutarahyldehyde chemistry. Hybridization of complimentary target single strand DNA oligonucleotide was monitored in situ and successfully detected. The demonstrated fiber Bragg grating sensors provide an elegant method to monitor biological changes in an in situ manner, and provide temporal information in a single experiment.     

高精度光纤光栅振动解调系统研究与应用

为实现工程系统中对振动信号的检测,提出了一种基于衍射光栅的光纤布拉格光栅(FBG)振动解调系统。解调系统通过色散成像原理获取传感器反射光谱,由所设计的嵌入式单元进行信号处理,并通过专用的上位机解调软件完成数据处理与分析。将系统封装为解调仪器,能实际应用于振动信号的探测。所设计的系统波长解调范围为1 525～1 570 nm,波长重复性可达±1 pm,分辨率为0.5 pm,可实现8通道复用测量,最高光谱解调速度可达10 kHz。实验结果表明,该FBG振动解调系统能够高精度测量不同频率与幅度的振动信号,最小可探测应变约为0.319με/Hz^1/2,在结构安全监测等领域具有重要的应用前景。     

微型光谱仪数据采集系统设计与标定

针对目前多采用CCD作为微型光谱仪的光电探测,文中介绍一种利用日本滨松公司的一款高性能线性CMOS图像传感器S11639-01设计了一款微型光谱仪。图像传感器的驱动及光谱仪数据采集系统控制均采用ARM Cortex-M7内核的32位处理器STM32H743芯片。系统硬件电路结构紧凑、数据采集分辨率高、传输实时性好。光谱仪的光路结构由入射狭缝、平场凹面光栅、线阵CMOS探测器组成,该结构相对简单,有利于光谱仪的微型化。采用美国海洋光学标准低压汞灯和卤钨灯对光谱仪进行实验测试,实验所得数据通过波长标定、光谱自动寻峰等技术进行处理,可以得到该系统光谱响应范围220 nm-950 nm,光谱波长标定误差低于0.05 nm,系统最小分辨率1.153 nm,线性度在99.898%以上。     

单片机光纤光栅传感器匹配解调系统

通过对双光纤光栅并联匹配解调原理的研究,提出了基于MSP430F1612单片机的光纤光栅传感器解调系统。利用曲线拟合的方法对压电陶瓷驱动电压与波长偏移量的数据进行拟合计算,实现光纤光栅解调的目的。本文给出了该方案软硬件设计,并对匹配解调法的发展前景进行了论述。     

Polymeric 16×16 arrayed-waveguide grating router usingfluorinated polyethers operating around 1550 nm

A 16×16 arrayed waveguide grating (AWG) router with a channel spacing of 0.8 nm (100 GHz) operating around the 1550-nm wavelength has been fabricated using newly synthesized fluorinated polyethers. It has a good processibility and a high thermal stability up to 510°C. The propagation loss of the buried-channel waveguide is about 0.4 dB/cm at the 1550-nm wavelength. The on-chip insertion loss ranges from 5.5 to 11 dB and the crosstalk is less than -27 dB. The AWG router shows good cyclic rotation property of the wavelength channels with an error smaller than 0.03 nm     

L-band erbium-doped fiber amplifier incorporating an inline fibergrating laser

Wide-band erbium-doped fiber amplifiers (EDFAs) having an inherently flat gain spectrum in the long-wavelength-band (L-band) between 1570 nm and 1600 nm have recently been realized. The L-band amplification has been achieved by simply using a longer length of EDF than in the conventional-wavelength-band (C-band) EDFAs. In the paper, we investigate the improvement of the gain flatness in an L-band EDFA co-pumped with the high-power pump light at 1.48 μm and the medium-power auxiliary pump light at 1.55-1.565 μm. We also propose a novel L-band EDFA incorporating an inline fiber grating laser (FGL) operating at the auxiliary pump wavelength, and demonstrate that the proposed EDFA has better gain flatness as well as the gain clamping characteristics     

相对光谱响应自动测试装置

本文设计的相对光谱响应自动测试装置系采用微机控制单色仪进行波长扫描， 单色仪出射的单色辐射经光纤分束器导入标准探测器和待测探测器，两探测器产生 的电流信号分别经两套程控放大器放大，进入两个数字表显示，并由微机对二表头转 换出的数字量采样和处理。该系统设计了自动、半自动和手动等功能，既可联机工作， 又可脱机工作，使用方便。 该系统不更换光栅时的应用波长范围为３６０-１０００ｎｍ，波长精度小于０．３ｎｍ， 测量重复精度０．４％     

Multiple-quantum-well Mach-Zehnder modulators: comparison of calculated and measured results for modulator properties and system performance

A measurement-based model of a /spl pi/ phase-shift multiple-quantum-well Mach-Zehnder modulator with asymmetric Y-branch waveguides is presented. Using this model, calculated and measured results for the transmission contours, /spl alpha/-parameter, intensity modulation, and time-resolved chirp are compared. The suitability of the model for system performance evaluation is demonstrated by examining the implications of the modulator chirp on 10-Gb/s transmission over nondispersion shifted fiber, and the combined effect of the modulator chirp and group delay ripple of a dispersion compensating fiber Bragg grating on 10-Gb/s dispersion compensated transmission. Calculated and measured results for the system performance are compared.     

Grating-assisted codirectional coupler filter using electroopticand passive polymer waveguides

We propose and demonstrate a band-rejection filter based on grating-assisted forward coupling between an electrooptic (EO) polymer waveguide and a passive polymer waveguide. The passive polymer waveguide is used as a low-loss optical path, and the EO polymer waveguide is used for dropping the optical signal at a given wavelength. The grating is etched on top of the EO polymer waveguide, which satisfies the phase-matching condition for the power exchange between the two asynchronous waveguides. Using the grating with a period of 63.4 μm, we achieve significant power exchange between the two waveguides with a 3-dB bandwidth of 4.4 nm at a wavelength of 1533 nm. Next, we demonstrate that photobleaching may be effectively used as a permanent tuning method of the center wavelength in fabricating the proposed device. The center wavelength for the maximum coupling can be adjusted in a range of more than 25 nm by photobleaching without extra loss. Lastly, we investigate changes of the filter characteristics induced by poling and demonstrated wavelength tuning by the poling-induced EO effect     

激光触发多级开关触发延迟及其抖动的研究

实验研究了激光触发多级多通道开关触发延迟时间及其抖动与激光脉冲能量等实验光学参量之间的关系。建立了基于Boltzmann方程的激光触发间隙零维数值模拟模型,并与基于T H Martin经验公式的过压自击穿间隙击穿时延计算程序相结合,对实验现象进行了初步的理论解释。     

Effects of laser triggering parameters on runtime and jitter of a gas switch

Parameters affecting the runtime and jitter of a laser triggered gas switch have been studied. Experiments tested a variety of switch parameters including percentage of selfbreak and switch pressure. The effects of laser beam parameters were also considered, including focal length, laser energy, laser spark length, and laser wavelength. Experiments were performed on the Tiger pulsed power machine. Measurements were taken on a spark gap switch built from the trigger section of a Rimfire switch. A Marx bank consisting of 32, 3.1 uF, capacitors that fed into a 7 nF intermediate storage capacitor was used to drive the switch into a 4 ? resistive load. The test switch was pressurized to 306 kPa (30 psig) with SF6 and operated near 1 MV. A New Wave Tempest Nd:YAG laser was used to trigger breakdown of the switch. The laser was focused at the mid-gap between the switch electrodes using lenses with focal lengths between 30 cm and 100 cm. Focused laser energy in the switch ranged from <5 mJ to 80 mJ. The effects of switch and laser beam parameters on the runtime and jitter of a laser triggered gas switch are presented. The end goal of the research is to determine optimal conditions for improved switch performance.     

Dual-Grating Spectral Beam Combination of High-Power Fiber Lasers

We describe a dual-grating spectral beam combination (SBC) system to combine multiple high-power fiber laser outputs while maintaining near-diffraction-limited beam quality. The two gratings are parallel in a grating rhomb configuration, with input and output beams that are parallel but shifted with wavelength, rather than the typical angular dispersion of a single grating. The resulting advantage of the dual-grating SBC over other beam combination systems is the relaxation of the linewidth requirement. We combined two fiber lasers with output powers of 115 W each and linewidths of about 0.15 nm ( ~40 GHz) to produce a combined beam of 190 W power with near-diffraction-limited beam quality (M ² ~ 1.18).     

基于双AWG的新型FBG连续解调方法研究

为了实现基于阵列波导光栅(arrayed waveguide grating, AWG)的光纤布拉格光栅(fiber Bragg gratin, FBG)的连续解调, 提出了一种使用两个 AWG 联合解调的方法。 在一个 AWG 相邻两通道光谱中间插入另一个 AWG 对应通道的光谱,组成最小 的光谱周期;每次测量均从三通道中选择光强度最强的两个通道,利用相对光强解调算法,根据其波长—功率关系对 FBG 中心 波长进行精确测量。 使用两个 100 GHz 的 AWG 搭建了实验平台,并对温度传感器的解调进行研究。 实验表明,在 0. 8 nm 的系 统最小动态范围周期内,实现了对 FBG 的连续精确解调,系统的解调线性度达 0. 999 1,波长精度达±4 pm。 对数据和实验结果 进行数学分析,可以将 C 波段范围分成多个波长周期,系统可以实现在 C 波段 40 nm 全周期范围内对单个 FBG 的连续解调。 该方法不但可以实现在 C 波段范围内基于 AWG 对 FBG 的连续解调,使得运用 FBG 可以连续感测外界物理量变化,提高了系 统的实用性。 而且,该方法能够准确解调出波长信息,为实现对 FBG 的连续精确解调提供了借鉴信息,有利于进一步扩大 FBG 的应用领域。     

基于OH基团二级光谱的氩大气压等离子体射流温度诊断

为了应用OH基团的二级光谱来诊断大气压等离子体射流(APPJs)的温度,首先采用线性场电极结构产生了氩大气压等离子体射流,利用增强型电荷耦合器件(ICCD)和三光栅光谱仪,拍摄了200～900nm波长范围内氩APPJs的光谱,并对光谱进行了辨认及分析,结果表明,除了氩原子的2p-1s(帕邢符号)跃迁谱线外,还存在一些分子基团的一级和二级光栅光谱,且其二级光谱的分辨率远高于一级光谱,其中OH(其相应跃迁为A2∑+→X2П)自由基尤为明显。然后选择OH(A2∑+→X2П)自由基的二级光谱诊断了OH自由基的转动温度(近似为等离子体射流的温度),并探讨了该温度随外施电压幅值、气体体积流量的变化,结果表明,在其他实验条件(如管径、电极间距和电极宽度)给定的情况下,氩APPJs的温度随外施电压幅值和气体体积流量的增大而都呈现先下降后上升的趋势,当外施电压幅值为10kV、气体体积流量为4L/min时,温度达到最小值348K。上述结果将为氩AP-PJs的参数调节及应用提供重要的帮助。     

Quantitative research on higher order harmonics in metrology beamline

The synchrotron radiation spectra of the spherical grating monochromator (SGM) working in the soft X-ray and VUV region are often contaminated by significant amounts of higher order harmonics. They cannot be suppressed completely by suitable filters. Higher order contributions in the spectral radiation standard and metrology beamline were researched using transmission grating (made in-house) and IRD AXUV100G (USA) photodiode detector. The exit beam was dispersed with the transmission grating behind the exit slit of the monochromator, and the contributions of the different orders were analyzed. The higher order distributions were quantitatively determined for three gratings with line densities of 1800, 600 and 200 l/mm. Experiment results show that in wavelengths between 5 nm and 15 nm the contributions of the higher orders to the detector signal are restricted to less than 7% even without the use of filters. In wavelength regions between 5 nm and 34 nm, the contributions of the higher orders to the detector signal are less than 14% with proper A1, Si₃N₄ and Zr filters, and after being modified by quantum efficiency of the detector, the higher order contributions are restricted to less than 6.5%. The study also shows that higher orders are almost totally suppressed by MgF₂ filter when the wavelength ranges between 115–140 nm. __________ Translated from Optics and Precision Engineering, 2007, 15(5): 640–645 [译自: 光学精密工程]     

Narrow linewidth, tunable Tm3+-doped fluoride fiberlaser for optical-based hydrocarbon gas sensing

Operation of an efficient continuous-wave (CW) thulium-doped fiber laser emitting at wavelength, λ=2.31 μm is reported. The fiber laser parameters are optimized with a view to ultimately producing a compact and efficient laser source for optical absorption based gas sensing. A number of fiber laser configurations are investigated to assess their suitability for narrow linewidth, tunable fiber laser operation emitting around λ=2.3 μm, which is a wavelength region of significant importance for hydrocarbon gas monitoring. Tuning ranges of 140 nm and linewidths of less than 210 MHz have been demonstrated with lasers with bulk external tuning grating. Preliminary hydrocarbon gas sensing investigation confirm the potential of this source for detection of ppb gas concentrations