Performance evaluation of praseodymium doped fiber amplifiers

Optical Review - In this paper, we report the performance evaluation of praseodymium doped fiber amplifier (PDFA) operating in 1.25–1.35 μm band of wavelengths based on...     

Side-polished fiber based gain-flattening filter for erbium doped fiber amplifiers

A simple and accurate novel normal mode analysis has been developed to take into account the effect of the non-uniform depth of polishing in the study of the transmission characteristics of optical waveguide devices based on loading of a side-polished fiber half-coupler with a multimode planar waveguide. We apply the same to design and fabricate a gain-flattening filter suitable for fiber amplifiers. The wavelength dependent filtering action of the overall device could demonstrate flattening of an EDFA gain spectrum within ±0.7 dB over a bandwidth of 30 nm in the C-band. Results obtained by the present analysis agree very well with our experimental results. This present analysis should be very useful in the accurate design and analysis of any SPF-MMOW device/component including side-polished fiber based sensors.     

Optimization of core size in erbium doped holey fiber amplifiers

Holey fibers (HFs) can be used as amplifier in optical communication systems. These new fiber structures have flexibility to change the fiber parameters such as refractive index of guided mode, air filling factor (AFF), V number, group velocity dispersion (GVD) and numerical aperture, only by the change of the hole size and air-hole spacing. In this paper we will use improved fully vectorial effective index method (IFEIM) to analyze the erbium doped holey fiber amplifier (EDHFA) with hexagonal unit cell. The range that core size can change in any air-hole spacing will determine and the growth of core size in the amplifier character such as maximum gain and optimum length will study. By determination of single mode region the effect of pump power and dopant concentration growth, on the amplifier parameter will define.     

Intensity based erbium distribution for erbium doped fiber amplifiers

A method to calculate an optimum Erbium distribution to enhance the gain efficiency in EDFA is proposed. This method calculates Erbium distribution based on optical pump envelop, pump power and optical properties of Erbium ion. The intensity-based Erbium distribution is obtained for single-mode fiber and dispersion-shifted fiber types EDFA for pump power from 10 to 40 mW. All of the profiles have a Gaussian-like shape. For single-mode fiber type EDFA, high gain enhancement is obtained in a relatively short optimal length of fiber. Optimal length of EDFA, using intensity-based Erbium distribution, is increased by a factor of 1.5–3 with respect to the one using stepwise. This optimal length increasing factor is considerably smaller than that of the EDFA, using the one-fourth confinement Erbium profile.     

掺杂稀土离子晶体中的电磁感应光透明现象

对在掺杂稀土离子晶体中实现电磁感应光透明进行了实验研究。首先,以Er3＋∶YAG晶体为样品,用旋转波近似下的密度矩阵方程理论计算了探测场的吸收特性随Er3＋离子浓度的变化规律,结果表明：在探测场失谐Δp=0时,形成了一个对于探测光透明的窗口,从而在理论上论证了在掺杂稀土离子晶体中实现电磁感应光透明效应的可行性。设计了一个以Pr3＋∶Y2SiO5晶体为样品的实验激发方案,吸收光谱显示,当温度为6 K时其在共振吸收峰处可形成一个完全透明的窗口,实现了在掺杂离子晶体中的电磁感应光透明。实验还分析了工作温度、耦合场失谐对探测光透过率的影响,结果显示：当样品温度上升到15 K时,透明窗口消失;耦合场的失谐量越大,透射率越小。     

Effect of induced crystallization in rare-earth doped lithium borate glass

Rare-earth doped borate glasses and glass ceramics are investigated for their potential as photon converters. Thermal processing of the as-made glass results in the formation of nanocrystals therein. For optical activation, the glasses are doped with Eu³⁺ and Tb³⁺, both enabling an intense emission under ultraviolet excitation. Differential scanning calorimetry and X-ray diffraction are applied to analyze the crystallization behavior. Compared to the undoped glass, the glass transition temperature as well as the crystallization temperature are increased with Eu³⁺ doping. Upon thermal processing of the as-made glasses, the transmittance is significantly reduced. Preliminary photoluminescence quantum efficiency measurements yield lower values for the glass ceramics.     

Photonic crystal fiber for fundamental mode operation of multicore fiber lasers and amplifiers

A mode-selection method based on a single-mode photonic crystal fiber (PCF) in the multicore fiber (MCF) lasers is presented. The designed PCF has a central core region formed by a missing air-hole, and three air-hole rings. With an appropriate choice of the design parameters of the PCF, the power coupling between the fundamental mode (FM) of the PCF and the fundamental MCF mode can be much higher than those between the FM and the other supermodes. As a result, the fundamental MCF mode has the maximum power reflection coefficient on the right-hand side of the MCF laser cavity, and dominates the output laser power. Since the maximum power of the fundamental MCF mode will lead to the desired laser beam profile, higher the fraction of the fundamental MCF mode power contained in the total output power contributes to higher beam quality. The numerical simulations show that the effectiveness of the fundamental MCF mode-selection is higher in the MCF lasers with the PCF as a mode-selection component than in the MCF lasers based on the free-space Talbot cavity method. Additionally, for the MCF amplifiers, an approach is presented to decrease the sensitivity of the amplifier performance to the variation of Gaussian beam waist utilizing the coupling between the Gaussian beam and the FM of the PCF. The numerical results show that this method can effectively increase the design flexibility for a broad range of the Gaussian beam waist.     

Mechanism of mode coupling in multicore fiber lasers

We present what is believed to be the first experimental demonstration of a new mechanism of mode coupling in multicore fibers (MCFs) based on their indirect interaction inside the fiber via intermediate mode, analogous to the Bragg mode, which is very sensitive to bending of the fiber. Very strong coupling between the core modes regardless of large spacing (approximately 28 microm) between them has been demonstrated in the MCF laser as well as in the probe beam schemes. 70% of power conversion from one core to another with beating length of tens of centimeters in 4-core MCF is measured.     

Feedback coupling loss in single mode fiber lasers

Based on the expansion of the fundamental mode LP₀₁ in single mode fiber in terms of Laguerre-Gaussian free space modes, the feedback coupling losses for two different types of cavity mirror, i.e., a curvature mirror and a combing mirror of a lens and a plane mirror, are numerically calculated for the first time. The results show that, for the curvature mirror, the lowest coupling loss is obtained when its curvature radius matches the wavefront curvature. In particular, if a plane mirror is used as the cavity mirror, it has to be placed close to the fiber end to obtain the low coupling loss. For the combing mirror, the lowest coupling loss can be obtained when the plane mirror is placed at the back focal plane of the lens, and the variation of the coupling loss is insensitive to the mirror positions for the lens with longer focal length. Finally, the plane mirror and the combing mirror of a lens and a plane mirror are suggested to be the cavity mirror in the practical construction of the high power fiber lasers.     

Two-mode coupling model in a few mode fiber

A phenomenological two-mode coupling model in few mode fibers (FMF) is developed. It uses a mode coupling coefficient and the differential modal delay as the input parameters and the split-step approach. The model is consistent with the experimental results demonstrating distributed mode coupling effects in FMF and may be used for multipath interference estimation based on statistical analysis of fluctuations.     

Fabrication of high aluminium containing rare-earth doped fiber without core-clad interface defects

The unwanted core-clad interface defect generation and related scattering loss in rare-earth doped optical fibers containing Al-oxide is up to now an unsolved problem as it cannot be avoided without modifying fiber design or composition. Here a simple technique is proposed to eliminate the problem without affecting fiber parameters. The method comprises addition of fumed silica in appropriate amount under dispersed condition to the soaking solution containing Al and rare-earth salts during the solution impregnation stage. This facilitates the networking between Al₂O₃ and SiO₂ during sintering. The method leads to an improvement in optical properties of the fibers and enables doping of high concentration of Al-oxide even in presence of GeO₂ in the core.     

Multilayer optical memory using femtosecond-laser induced fluorescence in rare-earth ion doped glass

We report three-dimensional fluorescent memory by recording optical bits with irradiation of femtosecond laser pulses at 800 nm and by reading photoluminescence change in Eu³⁺ doped glass. We produced multi-layered micro-bit patterns and read the blue emission from the 405 and 325 nm excitations due to permanent reduction of Eu³⁺ to Eu²⁺ in sodium borate glass by scanning the irradiated region in multilayers.     

Acousto-optic polarization-dependent mode coupling in a dual-mode hollow optical fiber

We demonstrated unique polarization dependence in acousto-optic coupling in a two-mode hollow optical fiber (HOF). Local deformation of circular HOF induced by traveling acoustic wave resulted in highly birefringent optical transmission characteristics, which were experimentally analyzed for various central air-hole sizes. Potential applications for polarization-dependent devices such as polarization-dependent loss compensators and broadband polarization controllers are discussed.     

Adsorbate motions induced by vibrational mode coupling

Adsorbate motions are discussed with a primary attention focused on the coupling between a vibrational mode excited by ultrafast laser heated hot-electrons or by inelastic tunneling electrons with scanning tunneling microscope and the reaction coordinate (RC) mode. Recent experimental results have demonstrated an efficient reaction pathways involving an indirect excitation of a frustrated translational mode, rather than its direct excitation for adsorbate hopping on surfaces. Elementary processes are briefly described for hopping of CO molecules on a laser heated stepped Pt surface, where excitation of the frustrated rotation mode has been found to plays an indispensable. Calculation of the inelastic tunneling current (ITC) for excitation of the C-O stretch mode of a CO molecule is combined with a theory of anharmonic mode coupling to activate the frustrated translation mode above the barrier. The hopping rate as a function of the bias voltage agrees with the experimental result. An unified theory of single-, and two-electron processes for ITC-induced motions induced by an indirect excitation of the RC-mode via mode coupling is also applied to reproduce a crossover from hopping to desorption of a single NH₃ molecule on Cu(1 0 0) with an increase in the tunneling current.     

Frequency resolved analysis of thermally induced refractive index changes in fiber amplifiers

Temperature induced refractive index changes are an important aspect in today's fiber amplifiers with high average power. For many processes, their time dependence is critical. Here, we analyze the impact of radial heat diffusion on the optical phase. We modulated the pump power in a 10?W amplifier and measured the frequency response of the optical phase. We compared the result with the calculated frequency response of the temperature in the fiber core, which shows the same characteristics. Additionally, we analyzed the influence of fiber parameters on the temperature dynamics.     

Multi-wavelength ytterbium doped fiber laser based on longitudinal mode interference

Multi-wavelength Ytterbium-doped fiber laser (YDFL) is demonstrated using a longitudinal mode interference assisted by a four-wave mixing (FWM) effect in a ring laser cavity. The gain medium is a 16 m long of the fabricated (Ytterbium-doped fiber) YDF, which has a core composition of 0.8 wt % of Yb₂O₃, 1.8 wt % of Al₂O₃ and 23 wt % of GeO₂, Ytterbium ion fluorescence lifetime of 1.1 ms and absorption of 9.0 dB/m at the pump wavelength of 976 nm. 20 m long photonic crystal fiber (PCF) is used to provide FWM effect so that the energy of different oscillating lines can be redistributed to improve multi-wavelength operation. The proposed laser generates 12 lines of optical comb with a line spacing of approximately 0.59 nm at 1035 nm region.     

长周期光纤光栅傅里叶模式耦合理论

建立了长周期光纤光栅傅里叶模式耦合理论.在分析同向模式耦合时,发现了同向耦合模式的振幅系数间存在傅里叶变换关系.推导了长周期光纤光栅的同向耦合谱和透射谱的通用表达式.该理论是用傅里叶变换分析得出长周期光纤光栅折射率微扰的空域谱,再对该空域谱进行模式同向耦合分析,从而得到长周期光纤光栅光谱特性的通用表达式.根据该理论模拟分析了长周期光纤光栅在不同长度和微扰幅值时的光谱特性,与传统耦合模理论进行了对比分析.结果表明,该长周期光纤光栅傅里叶模式耦合理论具有简单、精确和高效的特点,与实际长周期光纤光栅的透射谱特性一致.应用该理论可分析无过耦合的任意轴向折射率微扰分布的长周期光纤光栅光谱特性.     

Bragg光纤光栅傅里叶模式耦合理论

建立了Bragg光纤光栅傅里叶模式耦合理论.在分析光纤光栅的耦合模时,发现了耦合模式的振幅系数间存在傅里叶变换关系.推导了将傅里叶变换和模式耦合融合在一起的Bragg光纤光栅反射谱和透射谱的通用表达式.该理论是用傅里叶变换得到Bragg光纤光栅折射率微扰的空域谱,再对该空域谱进行模式耦合分析计算,从而得到Bragg光纤光栅的光谱特性.根据该理论,仿真分析了Bragg光纤光栅的谱特性,与耦合模理论、直接傅里叶变换法进行了对比分析.结果表明,傅里叶模式耦合理论与传统的耦合模理论及实际Bragg光纤光栅的光谱特性一致,具有简单、清晰、直接、精确和分析效率高的特点,可分析任意轴向折射率微扰分布的Bragg光纤光栅结构.     

Principal modes in fiber amplifiers

The dynamics of the state of polarization in multimode fiber amplifiers is presented. The experimental results reveal that although the state of polarizations at the output can vary over a large range when changing the temperatures of the fiber amplifiers, the variations are significantly reduced when resorting to the principal states of polarization in single-mode fiber amplifiers and principal modes in multimode fiber amplifiers.