Tunable microwave photonic filter based on chirped fiber gratings working with a single optical carrier at constant wavelength

This paper presents a tunable transversal filter working with a single optical carrier at constant wavelength. The filter consists of a set of chirped gratings whose time delay is tuned with respect to the emission wavelength of a fiber laser by a piezoelectric actuator; extra lengths of fiber are inserted in the filter arms in order to avoid interferences between signals reflected in different gratings. Two and three taps filters are experimentally demonstrated, the filters transfer function is electronically tuned within the free spectral range.     

Rapid tuning of a dye laser

An electrooptically tunable Lyot filter was inserted into a dye laser resonator as a first stage element. The laser emission was tuned with a speed of 35 nm/μsec over a wavelength range of 20 nm.     

Coexistence and noncoexistence of modes in a multi-mode laser

Mode competition effects in a cw multi-mode laser with a broad homogeneous emission line are studied theoretically, taking account of a Lorentzian gain profile. Explicit formulas are presented for the frequency range in which simultaneous oscillation of modes takes place, and the tuning range for a selected mode (the latter being privileged by a lower loss). Moreover, a novel spectroscopic technique introduced and successfully applied recently, is analysed. The experimental procedure consists in first restricting the oscillation range by means of filter and afterwards observing the additional spectral broadening of the laser output, which is caused by placing a narrow-band absorber inside the laser cavity. A useful formula allowing to estimate the sensitivity of this technique is derived.     

X-ray emission from a laser irradiated target in the presence of high electric field

The temporal characteristics of the X-ray emission from a laser irradiated Ai target in a 20 kV/cm electric field are measured with electronic detectors. Spatial and spectral distributions of the X-ray radiation are evaluated using a pinhole camera and filter technique.     

相位调制边带失衡问题研究

 高功率激光系统中一般利用相位调制器对单纵模激光进行频谱展宽以避免受激布里渊散射，当后继系统中的滤波器存在中心波长偏差时，则会导致相位调制产生的边带失衡，使光信号出现较为严重的高频起伏。基于前端系统模拟平台对此问题进行了数值模拟研究，结果表明滤波后的高频起伏随滤波器带宽变窄和中心波长偏差增大而加剧。分析表明0.2 nm带宽的滤波器出现0.05 nm左右的波长偏差时，将引起24.3%左右的顶部起伏。实验中根据分析将滤波器带宽从0.2 nm加大到0.4 nm，成功将脉冲顶部起伏从25.2%减小到了4.4%，对前端系统的研制起到了很好的指导作用。     

Tunable dual-wavelength fiber laser based on a polarization-maintaining fiber Bragg grating and a Hi-Bi fiber optical loop mirror

We demonstrate experimentally the operation of a linear cavity dual-wavelength fiber laser using a polarization maintaining fiber Bragg grating (PM-FBG) as an end mirror that defines two closely spaced laser emission lines. The PM-FBG is also used to tune the laser wavelengths. The total tuning range is ∼8 nm. The laser operates in a stable dual-wavelength mode for an appropriate adjustment of the cavity losses for the generated wavelengths. The high birefringence (Hi-Bi) fiber optical loop mirror (FOLM) is used as a tunable spectral filter to adjust the losses. The FOLM adjustment was performed by the temperature control of the Hi-Bi fiber.     

弱耦合光纤光栅外腔半导体激光器

报道作者对单频窄宽光纤光栅外腔半导体激光器的一些研究结果。理论上分析了外腔的引入对激光器的阈值增益和光谱线宽的影响，实验上用自行研制的光纤布拉格反射滤波器（ＦＢＲ）与普通多纵模半导体激光耦合，在１．５５μｍ波段，得到边模抑制比大于２５ｄＢ，线宽小于６０ｋＨｚ的激光输出。     

Dynamics of a filtered-feedback laser: influence of the filter width

The behavior of a semiconductor laser subject to filtered optical feedback is studied in dependence on the width of the filter. Of special interest are pure frequency oscillations where the laser intensity is practically constant. We show that frequency oscillations are stable in a large region of intermediate values of the filter width, where the dispersion of the filter is able to compensate for the well-known phase-amplitude coupling of the semiconductor laser. Our stability diagram covers the entire range from a very narrow filter, when the system behaves like a laser with monochromatic optical injection, to a very broad filter, when the laser effectively receives conventional (i.e., unfiltered) optical feedback.     

罗斯滤片法测量激光打靶X射线辐射通量

利用一套罗斯滤片系统测量激光轰击固体金属Ti平面靶产生的X射线辐射通量,系统包括两个相同的X射线探测器及相应滤膜。罗斯滤片法的优点在于利用相邻核素对X射线相似的阻止率,可滤出一个窄的能带并去除高能部分的干扰,获得了Ti平面靶K壳层X光辐射产额。实验结果表明:硬X射线能段在4.5~4.9keV之间的K壳特征辐射占优,连续谱所占份额较低(与晶体谱仪一致);随着激光能量的增加,特征辐射增加;激光强度接近2×1015 W/cm2时,能量转换效率出现峰值。     

空气中激光等离子体通道的三次谐波光谱特性研究

对不同条件下强激光在空气中形成等离子体通道的三次谐波光谱特性进行了研究。单脉冲能量12 mJ,脉宽30 fs,重复频率10 Hz,中心波长795 nm的飞秒激光脉冲经0.5 m焦距的凹面镜聚焦,在空气中形成了等离子体通道, 并在前 向观测到谱线半峰全宽（FWHM）为15 nm的三次谐波。随着脉冲啁啾的变化,三次谐波的光谱出现红移或兰移,当激光脉冲附带+1.3×105fs2的二阶色散时,三次谐波谱线红移且谱峰强度增长了两倍。同时 ,通过改变可编程声光色散滤波器（AOPDF）光谱调制的位置（Hole position）,三次谐波的光谱也发生频移。     

Axial mode tuning of a single frequency Yb:YAG thin disk laser

We describe an arrangement for an Yb:YAG thin disk laser, which enables narrow bandwidth operation in single-frequency mode at freely selectable wavelengths within the broad tuning range of the laser. This is facilitated by a combination of a double-stage birefringent filter and an etalon inside the laser cavity. We investigate the wavelength selection characteristics of the single elements as well as their combination. A simple procedure is implemented for a computer-based automation of wavelength tuning. The reflectivity of the partially reflecting resonator mirror is optimised, and the laser pump power is adapted for best tuning performance. Single-frequency emission is achieved in a frequency range of 9.75 THz (wavelength range 1020 nm to 1055 nm). Each axial laser mode in this range can be selected individually. The axial mode separation of 0.47 GHz corresponds to wavelength steps of 1.7 pm at 1030 nm. PACS 42.55.Xi; 42.60.Fc; 42.60.Lh     

Modulation-free optical locking of an external-cavity diode laser to a filter cavity

Optical locking to a filter cavity is an effective method to eliminate the limitations of an external-cavity diode laser, such as broad spontaneous emission backgrounds and frequency jitters. Stable operation of the optical locking requires simultaneous control of the feedback phase and the diode-laser frequency. Frequency dither is usually used to extract the two error signals, but this causes extra frequency modulation in the output beam. A modulation-free method for deriving the error signals by modulating the laser-cavity coupling strength is demonstrated with a violet diode laser. A modulation-free linewidth upper limit of about 7 kHz for a 1 s measurement is realized by the method.     

An efficient background suppression ratio and output power using a novel fiber loop mirrors technique

This paper demontrates a novel configuration of a linear cavity erbium-doped fiber laser using fiber loop mirrors to effectively suppress the noise generated by the amplified spontaneous emission (ASE). A tunable filter is deployed in each fiber loop mirror to create a narrow passband for the selected mode. The fiber laser produces high background suppression ratio (BSR) of more than 75 dB from 1525 to 1565 nm at the output coupling of 95%. There is no residual ASE detected at the output to verify the effectiveness of the noise filtering in the proposed fiber laser. Low threshold powers are obtained in the range of 2.5–3 mW and the peak power variation is smaller than 1.6 dB.     

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.     

Imaging of the reaction zone in a 100 kW oil-burning furnace by use of a broad-band excimer laser

The reaction zone in the hostile combustion environment of a 100 kW oil-burning furnace has been imaged by laser-induced fluorescence using a broad-band XeCl-excimer laser. Upon excitation, the averaged images obtained by using an interference filter around 320 nm (FWHM of 10 nm) show three distinct areas along the direction of the gas flow. An intense emission spreads around the spray axis and is attributed to the fluorescence of large hydrocarbons in the unburned fuel. Approximately 12 cm downstream of the nozzle, a narrow dark region is displayed suggesting the preheat zone of the combustion process where large hydrocarbons are considerably degraded. The third distinct region is characterized by a strong onset of the fluorescence intensity localized downstream of the dark region. This feature is strongly suppressed by replacing the interference filter by a broad-band transmission filter passing light from 350 to 500 nm. Since OH strongly absorbs at the laser wavelength and its fluorescence is significantly lower above 345 nm, the findings imply that the major contribution to the observed intensity in this region originates from the OH radical. This molecule reaches its maximum concentration immediately downstream of the flame front. However, a contribution from other flame species fluorescing around 320 nm cannot be ruled out. Nevertheless, the combined spatial and spectral information obtained imply that the reaction zone of the combustion process can be localized accurately. The results are compared with simultaneously performed numerical simulations of the burner and are in reasonable agreement.     

滤片-XRD探测系统响应时间测量

 研究了软X射线能谱仪探测道系统(系统包括X射线二极管（XRD）、SUJ-50-10电缆和不同频带示波器)的响应时间。实验利用上海激光联合实验室的20TW激光器激光(激光能量约20J，脉冲宽度约1ps)打金箔靶产生的X光，用XRD探测系统测量，记录示波器有TK684C，TK694C和WM8500等。将实验数据进行了线性拟合和比对分析。滤片XRD探测系统的响应时间随偏压升高而加快，随传输电缆长度的增加而变慢，因此测量快信号过程时，应提高探测器偏压，缩短传输电缆，选择宽频带高采样率示波器，以便减少系统的响应时间，减小信号失真程度。     

应用复振幅滤波器实现高斯光束均匀化

激光束的光强分布为高斯分布。在激光应用的许多领域，要求光束的光强为均匀分布。本文提出了一种利用复滤波器实现高斯光束均匀化的新方法。给出了复滤波器的制作方法和实验结果。实验表明，该方法是一种能量转化率较高、容易实现、均匀化程度较为理想的高斯光束均匀化的方法。     

Refractive-index dispersion measurement of bulk optical materials using a fiber raman laser widely tunable in the visible and near-infrared

We propose a simple, highly sensitive fiber-optic autocollimation method for refractive-index dispersion measurement of solid-state and liquid bulk optical materials using a double-pass fiber Raman laser with Littrow-prism-tuned emission. The optical fiber is a key element of the scheme and serves simultaneously as a point laser source for the test, as a highly sensitive point receiver (or spatial filter) of the autocollimation backreflectance signal and as a medium for nonlinear frequency conversion and generation of a broadband continuum spectrum. When the Raman medium is a graded-index multimode fiber with powerful pumping (over 100 kW) using the second harmonic of a Q-switched Nd:YAG laser (λ_p=532nm), we obtain widely tunable (0.54-1.01 μm) generation in both the visible and near-IR ranges. The results obtained in the refractive-index dispersion measurements are fitted to the Sellmeier dispersion equation and the standard deviation of the experimental data from the analytical curve does not exceed 5x10^-5.     

Negative-ion emission during laser ablation of multicomponent materials

The time of flight technique coupled with an electrostatic energy filter has been used for composition and energy-distribution analysis of the ion species emitted during laser ablation of multicomponent materials Y-Ba-Cu-O, Pb-Sn-Te. The negative-ion output and kinetic-energy distribution as a function of the laser wavelength and the laser fluence on the target were measured. A high output of the negative ions of matrix elements comparable with the positive-ion emission was detected. The barium negative-ion formation was observed in spite of the negative electron affinity of alkaline earth elements in the ground electronic state. The mechanism of negative-ion formation based on the ternary collisional recombination in the laser plasma is analyzed.     

染料掺杂液晶填充毛细管的激光发射特性研究

本文提出利用染料掺杂液晶填充PI光控取向膜毛细管获得可调谐激光.采用532 nm YAG倍频脉冲激光器抽运,实验及理论研究了有聚酰亚胺(PI)取向膜和无PI取向膜毛细管的激光发射特性,对比分析了两种情形的激光产生阈值以及随温度变化的特性.结果表明:经PI光控取向处理的染料掺杂胆甾相液晶毛细管的激光发射模式具有分布反馈模式和回音壁模式,同时,激光产生阈值为4.5 m J·mm~(-2);发现当温度升高时,发射光谱发生"蓝移",中心波长调谐范围为5.9 nm,温度升高到43℃时,形成质量非常好的回音壁模式,其自由光谱范围为1.05 nm.经PI光控取向处理的染料掺杂向列相液晶激光发射模式为随机激光,并且较无PI取向时激光发射峰减少.