掺铒光纤中亚稳态粒子振荡和慢光时间延迟关系研究

由于光纤慢光在实际中的应用价值引起广泛关注. 技术手段上利用相干布居振荡效应实现光速可控更具有优势. 本文主要介绍了利用相干布居振荡效应 (CPO) 实现掺铒光纤中的光速减慢传输, 通过改变掺杂浓度、光纤长度、入射信号光及抽运光功率等参量, 具体研究了亚稳态粒子振荡和时间延迟的关系. 研究结果表明: 选择高浓度光纤、增加光纤长度、关闭抽运光源, 选择适当强度的信号光可以有效地增大时间延迟. 关键词： 掺铒光纤 时间延迟 相干布居振荡 慢光     

Slowdown of Group Velocity of Light Using Coherent Population Oscillation

We investigate the reduction of the group velocity propagation resulting from the steep change of the refractive index by the coherent population oscillation in erbium-doped optical fibre. The largest delay is measured to be about 8.75ms corresponding to a group index of 1.312×10^6. The time delay or advancement depends on the pump intensity. Influences of the ion density on the fractional delay and the group velocity reduction of light propagation are studied. Based on our discussion the optimization parameters should be selected in order to obtain more appropriate time delay and the slowdown of group velocity.     

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.     

Analytical performance evaluation of an adjustable PMD compensator using high-birefringence linearly chirped FBG

System performance of a high-speed WDM transmission system in the presence of PMD can be improved significantly by using an adjustable PMD compensator. We have analytically evaluated the performance of a tunable PMD compensator based on high-birefringence linearly chirped fiber Bragg grating. The device can adjust differential group delay in a linearly continuous way without affecting the wavelength outside the bandwidth. Various properties of the device such as relative group delay, differential group delay, reflectivity, etc. are investigated in terms of wavelength and grating length. Results show that under stretched condition the device generates a time delay between fast and slow polarization axes, which is adjustable from 0 to 55 ps and is tunable within 2.4 nm wavelength range.     

Intensity dependence of the transient two-wave mixing by population grating in Er-doped fiber

Theoretical analysis of the transient two-wave mixing (TWM) of two continuous waves with rectangular phase modulation counter-propagating through the Er-doped optical fiber is presented. Approximate analytic solutions obtained for undepleted interacting waves of low power (neglecting large-scale saturation of the fiber optical absorption) and of high power (with significant spatially uniform large-scale absorption saturation) are complemented by numerical simulation for a broad region of the recording light power. The experimental dependence of the TWM efficiency on the recording power obtained for a single-mode fiber with ?640 ppm erbium concentration and with initial optical density ≈8.4 at λ = 1526 nm demonstrated the initial, growing and decaying parts predicted theoretically. The maximum experimentally observed relative TWM signal amplitude proved to be approximately 3 times lower than expected.     

室温掺铒光纤放大器中实现参量控制无损耗光速减慢传输

本文对掺铒光纤放大器中的光速减慢传输系统进行深入研究,提出一种直接利用掺铒光纤放大器中抽运光 强度和掺铒光纤长度,通过优化控制参量来降低信号光强度损耗系数,从而可以实现无强度损耗光速减慢传输, 研究结果表明:当抽运光功率为3.5 mW时,信号光强度损耗系数近似为零;当抽运光关闭时,掺铒光纤长度为 0.1 m时,信号光强度损耗系数近似为零.     

Light slowdown in Bragg waveguide

A Bragg waveguide is analyzed from the viewpoint of obtaining slow light. It is shown that, for this system, a complete mathematical analysis of the pulse propagation with allowance for leakage is possible. Calculations are presented that show that, in a TiO₂/SiO₂-based Bragg waveguide, one can obtain a group index of ∼1000 with a spatial decay length of ∼3 mm for a nanosecond-scale pulse. Distortion of the pulse due to the group index dispersion proves to be acceptable, in this case, for the pulse propagation length of about 3 mm, which corresponds to the fractional pulse delay ∼10. We also analyze the propagation of the light pulse in the Bragg waveguide with a quantum well inside and show possibility of obtaining a group index of ∼10000.     

Simultaneous slow and fast light in C60 solution

Recently, many researchers have reported their work about sub- and superluminal propagation. And many experiments have demonstrated the group velocity transition between slow and fast light. In this letter, the authors reported simultaneous slow and fast light in a single light beam which could be explained by the photorefractive lens-like effect and the spatial dispersion qualitatively. In our experiment, a focused light beam at 532 nm normally incident upon a thin quartz cell full of C₆₀ solution and spatial diffraction rings were observed behind the cell. Recurring to a pinhole diaphragm, we measured the group velocity of light of different position on the diffraction rings. The experimental results demonstrated that slow and fast could exist in a single beam at the same time. We could acquire slow or fast light just by adjusting the position of the detector and keeping the experimental parameters, such as: modulation frequency and incident intensity, unchanged. Our experiment put forward a new way to control the group velocity in nonlinear optical material.     

Efficient anti-stokes signal generation through degenerate four wave mixing in an all solid photonic bandgap fiber

We reported the generation of visible red light through degenerate four wave mixing (FWM) in an all solid photonic bandgap fiber (PBGF), which was achieved by pumping the fiber with a 800 nm Ti:sapphire-based femtosecond pulse laser. At a fiber length of 30 m, a broadband anti-Stokes spectrum range from 620 nm to 740 nm was obtained at the highest pump power, the spectrum evolution as a function of pump power and propagation distance had been measured. Furthermore, the intensity-dependent parametric gain characteristic is also calculated, which accords well with the experimental results.     

Light propagation in the multi-mode double-clad erbium: ytterbium co-doped fiber: theory and experiment

The pump light and forward- and backward-amplified spontaneous emissions (ASE±) propagation in the multi-mode double-clad erbium: ytterbium co-doped fiber are analyzed numerically and measured experimentally. And the light propagation in hexagonal inner cladding based on geometrical optics are analyzed comprehensibly. We obtained that the suitable length of the fiber in optical fiber lasers is 2–2.5 m, and the 2 m long fiber emits at 1535 and 1543 nm simultaneously, and the peak-value wavelength of ASE+ changes to the long wavelength with an increase in the pump power.     

Medium power single-mode single-wavelength fiber laser

Medium-power, single-mode, single-wavelength fiber laser working at room temperature using a polarization-maintaining erbium-ytterbium co-doped fiber as the gain medium, and an un-pumped elliptical core erbium-doped fiber as a saturable absorber to reduce linewidth and mode hopping of the lasing wavelength is reported. The effects of length, erbium ion concentration, and polarization-maintaining property of the saturable absorber were explored. The output power of the laser was more than 100 mW and the lasing line was stable for more than 3 h with an intensity fluctuation of less than 0.2 dB. The laser linewidth (FWHM = Full width at half maximum) was 7.5 MHz and the signal to noise ratio was more than 50 dB. The output of the laser was measured using an optical spectrum analyzer (OSA) of resolution 1.25 GHz and a scanning Fabry-Perot spectrum analyzer (SFPSA) of resolution 6.7 MHz.     

Electrically controlled slow/fast propagation of 12.5-GHz light pulses in lithium niobate waveguide Bragg grating

Experiments on slow/fast light propagation in the Bragg grating recorded in the optical waveguide formed in a photorefractive copper-doped lithium niobate wafer are reported. The change from slow to fast light propagation in the 8-mm-long grating with spectral FWHM of ≈0.14 nm and maximum Bragg reflection of ≈90% was achieved by tuning the carrier wavelength around the central Bragg value of ≈1553.36 nm. Effective electrooptical control of the 80-ps pulse shift (fractional delay/advancement of up to 0.4) was demonstrated by applying an external voltage of ≈80 V to the electrodes deposited on the waveguide sides.     

Simulative demonstration of soliton pulse stability over the nonlinear regime in a birefringent optical fiber

In this paper, the results of numerical analysis are demonstrated for sech pulse (soliton) propagation in a birefringent optical fiber using computer modeling and simulation. Here, the initial pulse is polarized linearly and guided into the fiber at an angle of 45° to its polarization axes. The birefringence-induced time delay of 200 and 440 ps between X and Y polarization components has been reported at a fiber length of 631.72 km (10 soliton periods) by considering linear and nonlinear regimes, respectively. The Kerr nonlinearity, which stabilizes solitons against spreading due to GVD, also stabilizes them against splitting due to birefringence. A similar fact is true for the birefringent walk-off. Above a certain soliton order (N_th), the evolution scenario is qualitatively different and two orthogonally polarized components of the soliton move with a common group velocity despite their different modal indices or polarization mode dispersion (PMD) at a fiber length of 631.72 km (10 soliton periods) and 1264.344 km (20 soliton periods) over a nonlinear regime at θ≠45°. The physical effect responsible for this type of behavior is the cross-phase modulation (XPM) between the two polarization components.     

Tunable erbium doped fiber ring laser using fiber Bragg grating-assisted add-drop filter

A stable and low costless tunable erbium doped fiber ring laser using fiber Bragg grating-assisted add-drop filter is proposed and demonstrated. A stable laser output is obtained with a 4 nm tuning range. The power fluctuation, full-width at half maximum and SMSR are measured to be less than 0.50 dB, smaller than 0.015 nm and better than 55 dB in this tuning range.     

Strain and temperature discrimination technique by use of A FBG written in erbium doped fiber

We propose and demonstrate strain and temperature discrimination technique using a single fiber Bragg grating (FBG) written in the core of an erbium doped fiber. We observed that amplified spontaneous emission power varying linearly from the erbium doped fiber with temperature which determines temperature changes and strain is estimated by subtracting the wavelength shift due to temperature change, from the measured shift corresponding to the dip in the transmission spectrum of the FBG. A simple and compact FBG sensor is presented with improved rms errors of 21.2 μ? and 1 °C over ranges of 0–800 μ? and 40–95 °C, respectively. The sensor is shown to have strain and temperature sensitivity of 0.8 pm/μ? and 12 pm/°C.     

Nonlinear performance in silicon nitride slow light photonic crystal waveguides with elliptical holes

We propose a slow-light photonic crystal waveguide, which uses a combination of circular and elliptical air holes arranged in a hexagonal lattice with the background material of silicon nitride (refractive index n = 2.06). Large value of normalized delay bandwidth product (NDBP = 0.3708) is obtained. We have analyzed nonlinear performance of the structure. With our proposed geometry strong SPM is observed at moderate peak power levels. Proposed photonic crystal waveguide has slow light applications such as reduction in length and power consumption of all-optical and electro-optic switches at optical frequency.     

掺铒光纤中方波信号高次谐波的快慢光特性

光纤中方波信号的慢光技术在全光通信和光纤传感等领域具有重要的应用前景. 提出了谐波相对延时量的度量方法, 分别采用速率方程和相干布居振荡理论, 对掺铒光纤中方波信号的基波和高次谐波的快慢光特性进行了研究. 在无抽运光输入情况下, 改变入射光功率, 入射探测光的基波最大相对延时量能达到20%, 且存在实现最大相对延时量的入射光功率为8 mW; 在有抽运光输入的情况下, 改变信号光增益, 入射探测光的基波相对超前量同样能达到-20%, 且随着信号光增益的增大而增加. N次谐波(N=1, 3, 5, 7, …)在频率f/N(f为基波信号最大延时量对应的调制频率)处有最大相对延时量, 且它们的最大延时量相同, 频率处于相干布居振荡引起的光谱烧孔带宽内.     

Superluminal pulse propagation in an erbium fiber laser

It is well known that saturating gain media can give rise to superluminal pulse propagation velocity. In most mode-locked lasers the effect is unmeasurably small and has not yet been directly demonstrated. We present experiments on the initial transient of an erbium fiber laser that show a dynamic shift in the propagation velocity to a value larger than the medium’s linear group velocity as the pulse builds up. Received: 14 December 1998 / Revised version: 21 May 1999 / Published online: 25 August 1999     

损耗可控条件下掺铒光纤中光速减慢现象的研究

由于相干布居振荡效应,光脉冲通过处于吸收状态的掺铒光纤介质时,将导致群速度传输减慢. 由掺铒光纤的吸收特性可知,此时必伴随着强烈的吸收损耗,这势必会给实际的慢光信号测量及光通信领域带来诸多的困难. 从铒离子基态粒子速率方程出发,对损耗可控条件下的光速减慢传输进行了数值仿真研究,并加以实验验证. 研究表明：抽运光强度越高,信号光损耗系数越小;抽运掺铒光纤长度越长,信号光损耗系数越小. 通过优化抽运光强度、掺铒光纤长度等参量,可以实现损耗可控条件下的光脉冲群速度减慢传输. 关键词： 时间延迟 吸收损耗 掺铒光纤     

Electromagnetic energy flow near metal nanoparticles—II: Algorithms for the calculation of the light scattering of multi-spheres and photon energy transport via linear chains of Ag nanoparticles

We present a new algorithm to calculate the near-field distribution of scattered light of multiple nanospheres based on recursive order-of-scattering (OS) and the matrix inversion approaches, which avoids the divergent problem encountered in origin OS method at the resonance condition. Using this method, we investigate the light-transport properties of linear chains of Ag nanospheres. We found a maximum 3 dB damping length of 1.4 μm of the light propagation when the first sphere of the linear Ag spheres with the radius R=25 nm was illuminated. The optimal configurations that favor the photon energy transport are investigated as well.