Two-octave spectral broadening of subnanojoule Cr:forsterite femtosecond laser pulses in tapered fibers

Spectral broadening of femtosecond Cr:forsterite laser pulses is enhanced due to the use of tapered fibers. Supercontinuum generation with unamplified subnanojoule femtosecond Cr:forsterite laser pulses is observed for the first time. With 40-fs 0.6-nJ pulses of 1.25-μm Cr:forsterite laser radiation coupled into a tapered fiber having a taper waist diameter of about 2 μm and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber in the regime of anomalous group-velocity dispersion. This result opens the way for the creation of compact femtosecond Cr:forsterite laser plus tapered fiber systems for optical metrology and biomedical applications. Received: 23 October 2001 / Accepted: 16 January 2002 / Published online: 14 March 2002     

Spectral broadening of 40-fs Ti:sapphire laser pulses in photonic-molecule modes of a cobweb-microstructure fiber

Photonic-molecule modes of a cobweb-microstructure fiber allow efficient nonlinear optical spectral broadening of nanojoule femtosecond light pulses. Spectral widths of approximately 200 nm are achieved at the output of a 6-cm sample of such a fiber for 40-fs Ti:sapphire laser pulses with an energy of a few nanojoules coupled into photonic-molecule modes. Higher values of the group index and a lower group-velocity dispersion, attainable with higher-order photonic-molecule modes, allow the efficiency of spectral broadening of femtosecond laser pulses to be increased relative to the efficiency of spectral broadening in the fundamental photonic-molecule mode. Received: 9 June 2002 / Revised version: 29 June 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/939-3959, E-mail: zheltikov@top.phys.msu.su     

Highly nonlinear photonic-crystal fibers for the spectral transformation of Cr: forsterite laser pulses

We demonstrate novel photonic-crystal fibers (PCFs) fabricated of a highly nonlinear glass. Dispersion profiles and nonlinearity of these fibers are tailored with an array of submicron holes in the fiber core. With the PCF structure designed to provide a nonlinearity on the order of 10³ W⁻¹ km⁻¹ at the radiation wavelength of 1 μm and a fundamental-mode dispersion profile with zero group-velocity dispersion around 1.19 μm, unamplified femtosecond Cr: forsterite laser pulses are efficiently frequency-converted into the 540-1000-nm wavelength range through solitonic spectral-transformation mechanisms and four-wave mixing.     

New simplified coupling scheme for the delivery of 20 MW Nd:YAG laser pulses by large core optical fibers

The feasibility of transmitting 20 MW, 5 ns laser pulses from a frequency doubled Nd:YAG laser through a standard 1500 μm multi mode optical fiber is demonstrated. A new coupling scheme employing an optical homogenizer prevents breakdown in air without the use of a vacuum chamber. At the same time a very homogeneous flat top beam profile on the fiber surface is achieved. The new scheme therefore clearly simplifies fiber coupling of high power laser pulses. Experiments on the delivery of more than 20000 pulses with 110 mJ mean energy without fiber damage have been performed. Received: 2 August 2000 / Revised version: 18 August 2000 / Published online: 22 November 2000     

All-fiber actively Q-switched fiber laser tuned by a pair of temperature controlled fiber Bragg gratings

We report an all-fiber actively Q-switched erbium-doped fiber laser, where the linear laser cavity mirrors are composed of two fiber Bragg gratings (FBGs). The laser oscillation wavelength could be tuned by this pair of temperature controlled FBGs. The Q-switching is achieved by an all-fiber phase modulation device. Using this system, we could obtain stable Q-switched laser pulses output, which could be optimized by tuning the reflection wavelengths of the two FBGs to be adjacent to each other. Instead of being modulated by the FBG filter in high-speed oscillation, this fiber laser system is operating in the Q-switched regime using an all-fiber phase modulator, producing a more stable laser output spectrum.     

外光注入半导体激光器实现时钟分频

从理论和实验上研究了利用光注入半导体激光器对高重复速率光脉冲产生的周期振荡和时钟分频现象.结果表明，光注入半导体激光器引起的二倍周期振荡是使注入脉冲重复频率分频的直接原因.通过耦合速率方程，数值模拟了半导体激光器在外光注入时输出光的时间序列和功率谱，并且分析了激光腔内各种周期振荡的特征.研究表明，当注入光使半导体激光器出现稳定的二倍周期振荡，且注入光的重复频率为此振荡频率的二倍时，时钟分频即可产生实验中，采用重复频率为6.32GHz的光脉冲注入Fabry-Perot激光器，实现了3.16GHz时钟分频信号 关键词： 周期振荡 时钟分频 光谱侧带 光注入     

Subpicosecond UV pulse generation for a multiterawatt KrF laser

A 180-fs UV pulse has been generated based on a hybrid synchronously pumped mode-locked dye laser for a multiterawatt KrF laser system. The pulse width was measured by the single shot autocorrelation technique with the three-photon fluorescence of the XeF C-A transition. The pulse width broadening due to dispersive media was investigated. The results show that the observed pulse width broadening from 210 fs to 390 fs through the entire system is explained mostly by the linear dispersion of the optical elements for near-transform-limited input pulses.     

Cascade compression induced by nonlinear barriers in propagation of optical solitons

We investigate the nonlinear tunneling of optical solitons through both dispersion and nonlinear barriers by employing the exact solution of the generalized nonlinear Schrödinger equation with variable coefficients. The extensive numerical simulations show that the optical solitons can be efficiently compressed when they pass through adequate engineered nonlinear barriers. A cascade compression system in a dispersion decreasing fiber with nonlinear barriers on an exponential background is proposed and the cascade compression of optical pulses is further investigated in detail. Finally, the stability to various initial perturbations of the cascade compressed optical soliton and the interaction between two neighboring compressed solitons were investigated too.     

Mode-locked fiber laser based on an attenuation-imbalanced nonlinear optical loop mirror

We propose a mode-locked figure-eight fiber laser by utilizing an attenuation-imbalanced nonlinear optical loop mirror (AI-NOLM) for the first time. Stable self-starting passively mode-locked pulses as short as 296 fs are obtained. We have also confirmed that the pulse width of the laser can be varied by changing the amount of attenuation in the AI-NOLM.     

相敏式激光啁啾色散光谱技术在高吸收度情况下的应用

研究了相敏式激光啁啾色散光谱法在高吸收度情况下的应用.用窄频半导体激光器作为光源,利用一工作于载波抑制模式的铌酸锂电光强度调制器调制单频激光,在单频激光两侧产生两个边频分量,并通过两边频分量产生外差干涉信号.利用外差干涉的相位波动来测量甲烷气体位于1653.7 nm附近的折射率波动,通过气体折射率与吸收系数之间的Kramers-Kronig关系计算甲烷气体浓度.传统的波长调制光谱法受限于郎伯-比尔定律,在应用于高吸收度的情况时,存在灵敏度下降的问题,甚至出现随气体浓度上升输出信号反而下降的现象.实验结果显示,相同实验条件下,波长调制光谱法的线性测量范围为38.1—1500 ppm·m,线性测量的动态范围仅为16 d B;而相敏式激光啁啾色散光谱法在很大的吸收度范围内均具有线性输出,检出限低至47.3 ppm·m,线性测量范围上限为174825 ppm·m,具有超过35 d B的动态范围.     

Dispersion measurements for femtosecond-pulse generation and applications

Dispersive phenomena provide limits in the generation, measurement and applications of femtosecond optical pulses. Optical elements such as prisms, gratings, air, mirrors, filters, and laser crystals all contribute to the total dispersion in optical systems. Interferometric techniques enable measurements of dispersion of individual elements, and new techniques allow dispersion measurement inside the cavity of operating modelocked femtosecond tunable lasers. These techniques provide access to detailed information about dispersion errors which can distort pulses and produce undesirable effects in modelocked lasers. In this article, we review techniques for the measurement of dispersion in optical components and systems. In several cases, we compare measurements made with intracavity dispersion techniques with extracavity measurements and point out common features.     

Transient gain in photonic crystals

Using the optical response formula for photonic crystals and the Bloch equations, we analyze coherent emission in arbitrary two-dimensional photonic crystals. The transient emission depends not only on the photon density of the states, but also on the intensity and duration of the pump laser pulses. A transient gain exists if the pump pulse area is a certain value and the transition frequency is tuned to the band edge. The sharp structure of the density of states near the band edge leads to a dramatic enhancement of the transient emission intensity and an abrupt change in the oscillation of the emission field. PACS 42.70.Qs; 32.80.-t; 42.25.Bs     

Pulse compression of nanojoule pulses in the visible using microstructure optical fiber and dispersion compensation

Experimental results on the pulse compression of 1-nJ, 150-fs pulses from a tunable, 76-MHz Ti:sapphire laser oscillator operating at around 750 nm are reported. The length of the pulses can be compressed to nearly one tenth by applying a high-delta, single-mode microstructured optical fiber exhibiting zero group-delay dispersion at 767 nm, and by a prism-pair/chirped-mirror compressor. The experimental results are verified by theoretical investigations modeling the pulse propagation taking into account non-linear self-phase modulation and fiber dispersion. Received: 10 June 2002 / Revised version: 15 November 2002 / Published online: 12 February 2003 RID="*" ID="*"Corresponding author. Fax: +36-1/375-4553, E-mail: szipoecs@sunserv.kfki.hu     

Coherent recombination of laser beams with interferometrical phase control

A HeNe laser beam is split into four component beams which are subsequently transmitted through four polarization-preserving monomode fibres. At the end of the fibres the emerging beams are coherently recombined in pairs in two steps using inverse beamsplitter cubes. The unavoidable phase differences within each pair of beams, which are caused by temperature and pressure fluctuations in the fibre environment, are detected interferometrically at the fibre ends. Using homodyne techniques, these differences are measured and compensated for by lengthening the optical path in the individual fibres with piezo-electric stretching. With appropriate adjustment of the relative phases the emerging beams can be superimposed onto each other so as to re-form one single coherent beam.     

Transmission of return-to-zero pulses in an optical split-step system based on reflecting fiber gratings

A new variety of the “soliton management” in heterogeneous optical media is proposed. The system is composed as a periodic chain of nonlinear fibers with negligible intrinsic group-velocity dispersion (GVD), alternating with sections of unchirped fiber Bragg gratings (FBGs) operating in the reflection regime. Losses due to incomplete reflection are compensated by linear amplifiers. The model may apply to fiber-optic telecommunication links with periodically installed FBG modules, and it may be used for the design of laser setups. By means of extended simulations, we identify small regions in the underlying parameter space where this model, featuring the periodic separation of the Kerr nonlinearity and FBG-induced GVD (hence the name of the “split-step” system), supports stable transmission of RZ (return-to-zero) pulses, i.e., quasi-solitons. The effect of nonzero fiber’s GVD on the stable transmission regime is considered too. Moderately unstable (partly usable) transmission regimes are found in larger regions of the parameter space; they may be of two different types, with the average nonlinearity either undercompensating or overcompensating the GVD. Interactions between the stable RZ pulses are also studied, leading to the identification of a minimum separation between them necessary for the suppression of interaction effects.     

Flat Supercontinuum Generation at 1550 nm in a Dispersion-Flattened Microstructure Fibre Using Picosecond Pulse

The generation of a flat supercontinuum of over 80nm in the 1550nm region by injecting 1.6ps 10 GHz repetition rate optical pulses into an 80-m-long dispersion-flattened microstructure fibre is demonstrated. The fibre has small normal dispersion with a variation smaller than 1.5 （ps·nm^-1·km^-1） between 1500 and 1650nm. The generated supercontinuum ranging from 1513 to 1591 nm has the flatness of ±1.5 dB and it is not so flat in the range of several nanometres around the pump wavelength 1552nm. Numerical simulation is also used to study the effect of optical loss, fibre parameters and pumping conditions on supercontinuum generation in the dispersion-flattened microstructure fibre, and can be used for further optimization to generate flat broad spectra.     

Multi-Edge-Written Long-Period Fibre Gratings with Low PDL by Using High-Frequency CO2 Laser Pulses

A novel long-period fibre grating （LPFG） with low polarization-dependent loss （PDL） is fabricated by using a multi-edge exposure method with high frequency CO2 laser pulses. The experimental results show that the PDL of a triple-edge-written LPFG with a peak amplitude of-16.5 dB can be as low as 0.22 dB. These hovel LPFGs can find important applications in optical communication and sensing.     

Measuring chromatic dispersion of optical fiber using time-of-flight and a tunable multi-wavelength semiconductor fiber laser

We have developed a tunable multi-wavelength semiconductor fiber laser (SFL) for chromatic dispersion measurements of optical fiber based on the time-of flight (TOF) or pulse-delay technique. The SFL incorporates a programmable high-birefringence fiber Sagnac loop to select the separation of the lasing wavelengths between 3.2 and 1.6 nm. The SFL emits 5 and 11 wavelengths for separations of 3.2 and 1.6 nm, respectively, all within the C-band and with an output power uniformity within 3.2 dB. Results from TOF measurements are compared with standard phase-shift techniques; the percent differences between the two methods are within ±1.34% for measurements on various lengths of standard single mode fiber.     

频率啁啾光脉冲在行波半导体激光放大器中的传播特性

本文就行波半导体激光放大器(TW-SLA)中载流子引起的群速度色散对激光脉冲传播特性的影响作了理论研究.由于载流子引起的群速度色散效应,使远离饱和区工作的TW-SLA会对有频率啁啾的光脉冲产生展宽或压缩.本文就TW-SLA参数、频率啁啾参数跟脉冲增宽因子的关系作了理论研究.结果表明,线性的TW-SLA具有对低能量光脉冲压缩的可能性.因此,在光学通信系统中,它可用作对光纤损耗及色散的补偿元件.     

Operation of Kerr-lens mode-locked Ti:sapphire laser in thenon-soliton regime

A Kerr-lens mode-locked Ti:sapphire laser operating in a non-soliton regime is demonstrated. Dispersive wave generation is observed as a result of third order dispersion in the vicinity of zero dispersion. The characteristics of the Ti:sapphire laser operating in a positive dispersion regime are presented, where the oscillator directly generates pulses with duration continuously tunable from 0.37~ps to 2.11~ps, and 36~fs pulses are achieved after extracavity compression. The oscillation is numerically simulated with an extended nonlinear Schr?dinger equation, and the simulation results are in good agreement with the experimental results.