Excited state absorption spectroscopy for thulium-doped zirconium fluoride fiber

The excited state absorption (ESA) transitions at 1050 and 1420 nm play a fundamental role in thulium-doped fiber amplifiers (TDFA). We present a novel setup to measure the spectral cross-sections of these transitions in amplifier fibers and the results of this measurement in case of thulium-doped fluorozirconate (ZBLAN) fibers. Besides a standard system for fiber attenuation measurements and a long-pass filter, we use only components from the fiber amplifier setup, including the active fiber. No special parts are needed. We show that this fiber optic method delivers reliable results for different lengths of the doped fibers. The oscillator strengths of the measured transitions are calculated and compared to values published in the literature.     

光纤放大器放大自发辐射特性与高温易损点位置

在高功率光纤放大器实验中,时常发现增益光纤抽运注入熔接点后10—50 cm处容易发生光纤烧毁现象.为了对该现象进行理论预测,基于光纤激光器速率方程模型和增益光纤的热传导模型,从种子功率、抽运功率和抽运吸收三个方面对掺镱双包层光纤放大器中的放大自发辐射(ASE)和温度特性进行研究.结果表明,在放大倍率较高、ASE较为严重等情况下,光纤放大器中的最高温度点一般不在抽运注入的熔接点处,而在距离熔接点10—50 cm处,与实验中发现光纤烧毁的位置基本符合.从光纤放大器的ASE抑制、最高温度点温度控制角度出发,对光纤放大器在种子功率、抽运功率、抽运吸收、放大倍率和抽运波长等方面的设计给出了指导性的建议.     

Amplifying properties of neodymium glass fiber

The amplfying properties of silcate—neodymium-glass fiber quantum amplifiers are investigated. The propagation features of laser emission in multimode light guides are studied. The important role played by short-lived color centers (SCC) in the amplification process is demonstrated. Stimulated decay of SCC by absorption of neodymium radiation, which leads to an abrupt increase of the gain of the glass fiber amplifier and to a substantial shortening of the duration of the amplified giant pulses, is observed.Work performed at the Quantum Electronics Laboratory of the Tbilisi State University.Quantum Radiophysics Division, Lebdev Physics Institute. Translation of Preprint No. 201, Lebedev Physics Institute of the Academy of Sciences of the USSR, Moscow, 1989.     

Modeling and Investigation of High-Power Optical-Fiber Transmitters for Lidar Applications

In this work, we develop a high-power optical-fiber transmitter for lidar applications. The device can be utilized for measurements of the methane concentration and determination of the parameters of methane absorption line at wavelengths near 1,650 nm. The transmitter is based on a fiber Raman amplifier with an active fiber length converting the pumping radiation with a wavelength of 1,540 nm to the desired wavelength region of about 1,650 nm. We conduct the theoretical modeling of the Raman amplifier operation and demonstrate the advantage of the two-stage amplifier design over the single-stage design. In the modeling, the total power of a single Raman amplifier in the twostage design is 4.5 W with a pumping power of 5 W and an active fiber length of 1,000 m. Then we experimentally investigate the developed single Raman amplifier and demonstrate a satisfactory agreement between its parameters and the results of the theoretical modeling. The achieved average power of the transmitter at the output stage is over 7 W with two single Raman amplifiers joined in parallel. The distributed-feedback (DFB) master laser utilized in the transmitter is modulated by a linear-frequency signal with simultaneous application of a nonsynchronous high-frequency signal to eliminate the noise due to stimulated Brillouin scattering.     

Suppression of stimulated Brillouin scattering in high-power single-frequency fiber amplifiers

The effect of temperature variation along a high-power fiber amplifier on the stimulated Brillouin scattering (SBS) threshold is considered theoretically. We show that for an end-pumped rare-earth-doped double-clad fiber the inhomogeneous distribution of temperature, which is caused by the absorption of pump radiation, may result in total suppression of SBS even for output powers well above 200 W.     

High-peak-power second-harmonic generation of single-stage Yb-doped fiber amplifiers

A high-peak-power and high-repetition-rate fiber laser architecture is successfully demonstrated using a single-stage fiber amplifier. Nonlinear optical effects in a fiber amplifier degrade the monochromaticity of amplified laser pulses. In general, it is difficult for a non-monochromatic laser pulse to realize high-order harmonic generation with bulk nonlinear optical crystals. To overcome this problem, a single-stage amplifier architecture and a gain fiber with a high cladding absorption coefficient are employed. Furthermore, single-stage amplification enables the use of a multi-longitudinal mode electro-optically (EO) Q-switched micro seed laser. This architecture can generate a peak power of 100 kW at 50 kHz and an average power of 10 W. A second-harmonic conversion efficiency of 51% is obtained using this architecture and a LiB₃O₅ (LBO) crystal.     

室温条件下掺铥光纤放大器中光波群速减慢的研究

从掺铥离子光纤的速率方程和传输方程出发,建立了掺铥离子光纤放大器中光速减慢的理论模型,分析并讨论了介质的增益与泵浦光功率之间的关系。当掺铥离子光纤处于吸收区域时,粒子布居振荡导致光脉冲经历了饱和吸收过程,此时光脉冲传输延迟;当掺铥离子光纤处于增益区域时,粒子布居振荡导致光脉冲经历了增益饱和过程,此时脉冲传输超前。依据该理论模型进行了理论仿真计算,同时进行了室温条件下掺铥离子光纤中光波群速减慢传输的研究。     

In-situ measurement of the surface temperature during holographic recording

Photo-induced processes in organic materials mostly occur on molecular levels. Excited molecules may split to form radicals, starting a polymerization process with diffusing monomers. The azo-dyes perform an optically induced cis-trans isomerization. During pattern formation like a holographic grating, the local temperature increase, especially in thin films, is up to date a subject of estimation from absorption and dissipation data. However, the exact knowledge of the surface temperature would help a lot in understanding the resulting refractive index and thickness patterns during holographic exposure. In this paper, in-situ pyrometer measurements are presented. As examples, different photosensitive materials, varying from a photopolymer to polycrystalline azo dyes, are used in order to outline the magnitude of this effect and demonstrate the feasibility of this technique.     

飞秒激光激发下本征GaP非线性吸收及Kerr效应的实验研究

采用掺镱光子晶体光纤飞秒激光放大器为抽运源,搭建了偏振分辨的Z-scan实验系统.实验研究了块状本征磷化镓(GaP)晶体的非线性吸收和非线性折射率系数.研究证明,在中心波长1 μm飞秒激光的作用下,本征GaP晶体的非线性吸收主要为三光子吸收.而且还表明本征GaP的非线性吸收和非线性折射率系数具有很强的各项异性、偏振相关性及饱和特性. 关键词： 磷化镓 Z-scan Kerr效应 三光子吸收     

固体激光器泵浦输出波长在1428nm的单级拉曼光纤激光器

介绍了一种作为拉曼光纤放大器中的泵浦源使用的1428nm波长的拉曼单级频移光纤激光器,采用固体激光器产生的1342nm波长作为泵浦波长,得到240mW的1428nm输出,光一光转换效率在28％左右。     

OH sensor based on ultraviolet, continuous-wave absorption spectroscopy utilizing a frequency-quadrupled, fiber-amplified external-cavity diode laser

The development of an all-solid-state cw laser system for optical absorption measurements of the OH radical in the UV spectral range is described. The tunable output of a 1064-nm external-cavity diode laser is amplified by use of a Nd:doped, double-clad fiber amplifier. The amplified near-IR radiation is frequency doubled by a periodically poled lithium niobate crystal and then quadrupled in a beta-barium borate crystal. The design and operation of the system and measurements of OH absorption in the (2, 0) band of the A(2)?(+)- X(2)? electronic transition are discussed.     

Resonant fluorescence line narrowing and gain spectral hole burning in erbium-doped fiber amplifier

Gain spectral hole burning and resonant fluorescence line narrowing have been performed at low temperature in standard aluminosilicate erbium-doped fiber amplifier to demonstrate the nature of the line broadening. Comparison of the hole-width measurements with resonant fluorescent line narrowing data shows a good agreement at 77 K, working temperature for which both experiments are feasible and have been performed simultaneously. The FWHM results reported here compare well with the earlier line profile measurements performed on aluminosilicate glass preform of the same chemical composition as the fiber. The pump power dependences are reported and indicate that they influence both the depth and line width, which may induce a limitation for wavelength division multiplexing techniques in the long-haul transmission regime of telecommunications.     

3-Dimensional thermal analysis of Yb-doped gain guided and index-antiguided fiber amplifier

In this paper, the thermal characteristics of gain-guided and index-antiguided (GG-IAG) fiber amplifier were investigated with a 3-D heat flow model, based on rate equations. From the simulation results, it is found that the longitudinal heat conduction has a very important influence on the GG-IAG fiber amplifier temperature distribution, and the GG-IAG fiber amplifier would be subjected to a more serious thermal problem. Finally, the influence of different pump directions, cooling methods, cladding radius and negative refractive index step on the temperature distribution has also been studied.     

On the Photophysics of 3,5,6-Trichlorosalicylic Acid: Spectroscopic Study Combined with Hartree-Fock and Density Functional Theory Calculations

The present contribution reports a detailed photophysical study of a simple salicylic acid derivative viz., 3,5,6-Trichlorosalicylic acid (TCSA) based on steady state absorption, emission and time-resolved emission spectroscopy. Anomalous “dual” emission coupled with a large Stokes shift and negligible solvent polarity dependence marks the spectroscopic signature for Excited State Intramolecular Proton Transfer (ESIPT) reaction. Variation of medium polarity and pH of the medium have been implemented as fruitful tools to decipher the photophysics of TCSA. Quantum chemical calculation by ab initio Hartree-Fock and Density Functional Theory methods yields consistent results to follow experimental findings with distinct illustration of the inoperativeness of GSIPT reaction as well as occurrence of ESIPT process. A rigorous comparison of our experimental and theoretical measurements of TCSA with the parent compound salicylic acid, 5-chlorosalicylic acid and 3,5-dichlorosalicylic acid reveals the impact of chlorine substitution on the photophysics of the studied molecular systems with simultaneous exploration of the complexities induced in TCSA with respect to salicylic acid.     

Simulation and experimental characterization of Raman/EDFA hybrid amplifier with enhanced performance

We present simulation and experimental characterization of a hybrid amplifier comprising of a Raman amplifier and an erbium doped fiber (EDF) amplifier, with enhanced performance. The incorporation of a pumped EDF section in a fiber Raman amplifier (FRA) employing a dispersion compensating fiber is demonstrated to provide superior performance than a sole FRA system. The hybrid amplifier is characterized in terms of single channel gain and noise figure, and the results of measurements are shown to be in close agreement with the simulated results. Polarization-dependent gain (PDG) and multi-channel measured and simulated gain characterization of the Raman/EDFA hybrid amplifier are also presented.     

An inhomogeneous theoretical model for analysis of PbSe quantum-dot-doped fiber amplifier

Optical fiber doped with lead salts quantum dots (QDs) of different radii, has previously been proposed as a wide-band amplifier in the optical communication range. The emission and absorption spectra of QDs are size-dependent. Optical fiber amplifier doped with QDs of different sizes is an inhomogeneous gain medium. This inhomogeneity is the principle of operation of wide-band quantum-dot-doped fiber amplifiers (QDFAs). To study the effects of inhomogeneity such as spectral hole burning on the amplifier behavior, a fiber amplifier with an arbitrary size distribution of QDs is taken into consideration. The light propagation equations and rate equations for the inhomogeneous medium are presented. To investigate the gain spectrum, the amplifier bandwidth and the spectral hole burning effect on the amplifier behavior, the system of governing equations for the steady-state regime of operation are solved and numerical results are presented.     

Fiber laser intracavity absorption spectroscopy for in situ multicomponent gas analysis in the atmosphere and combustion environments

Intracavity absorption spectroscopy with a broadband Er³⁺-doped fiber laser is applied for the measurements of several molecular species revealing quantitative information about the gas concentration, temperature and chemical reactions in flames. The spectral range of measurements extends from 6200 cm⁻¹ to 6550 cm⁻¹ with the proper choice of the fiber length and by moving an intracavity lens. With a pulsed laser applied in this experiment, the sensitivity to absorption corresponds to an effective absorption path length of 3 km assuming the cavity is completely filled with the sample. For a cw laser, the effective absorption path length is estimated to be 50 km. Absorption spectra of various molecules such as CO₂, CO, H₂O, H₂S, C₂H₂ and OH were recorded separately in the cell and/or in low-pressure methane and propane flames. The presented measurements demonstrate simultaneous in situ detection of three molecular products of chemical reactions at different flame locations. Variation of the relative strengths of OH absorption lines with the temperature enables the estimation of the local flame temperature. The sensitivity of this laser does not depend on the broadband cavity losses and it can be used for in situ measurements of absorption spectra in hostile environments such as contaminated samples, flames or combustion engines. The presented technique can be applied for various diagnostic purposes, such as in environmental, combustion and plasma research, in medicine and in the determination of stable isotope ratios.     

Experimental study on tandem pumped fiber amplifier

We present the experimental results of a 1083 nm fiber amplifier tandem pumped by 1030 nm fiber laser. The output characteristics of the tandem pumped amplifier with cladding-pump and core-pump schemes are both investigated. The 1083 nm signal laser has not been efficiently amplified when cladding-pumped by 1030 nm laser for the weak absorption of the gain fiber. The core-pump scheme works well with the amplifier. The output properties with different gain fiber length are experimentally investigated. The maximum output power is 2.4 W with power conversion efficiency of 60%.     

High-peak-power femtosecond pulse compression with polarization-maintaining ytterbium-doped fiber amplification

We report the generation of 140 fs pulses with a peak power of up to 270 kW using a fiber pulse source based on a polarization-maintaining ytterbium-doped fiber amplifier and a semiconductor saturable absorber mirror mode-locked fiber laser seed. The seed laser pulses were amplified and chirped in the fiber amplifier and subsequently compressed in an external transmission grating pair. The use of a polarization-maintaining amplifier addresses nonlinear polarization-induced limitations to the obtainable compressed pulse duration and quality that can arise if isotropic fiber amplification is used. Numerical simulations of the system support the experimental measurements and also confirm the role of fiber dispersion in obtaining high-quality compressed pulses.