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     

多孔微结构光纤中飞秒激光脉冲超连续谱的产生

报道了利用800nm飞秒激光脉冲在多孔微结构光纤中产生超连续谱展宽的现象，连续谱展宽范围为440—890nm.基于标量波近似理论对微结构光纤包层的有效折射率和基模的有效面积以及光纤的色散特性进行了计算，发现微结构光纤具有特殊的控制色散和波导特性的能力，对超连续谱展宽的机理进行了初步解释.本文的理论分析和实验结果有较好的一致性，认为即使包层由无序填充气线组成的多孔微结构光纤也可以出现超连续谱展宽效应. 关键词： 多孔微结构光纤 超连续谱 有效折射率 色散     

Three-dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism

Three-dimensional reversible laser micromachining of polymer materials based on two-photon photochromism is demonstrated. Unamplified 60-fs, 0.5-nJ pulses of 790-nm Ti:sapphire laser radiation are used to induce a refractive-index change in a polymethyl methacrylate (PMMA) sample doped with spiropyran molecules through a two-photon absorption process. Waveguides are written in the bulk of spiropyran-doped PMMA samples by scanning these samples with respect to a tightly focused Ti:sapphire laser beam. Laser-induced fluorescence is used for on-line monitoring of the laser-micromachining process. The structures written in photochromic samples can be erased and reconfigured due to the reversibility of the photochromic effect. Received: 23 April 2003 / Published online: 6 June 2003 RID="*" ID="*"Corresponding author. Fax: +7-095/939-51-74, E-mail: zheltikov@top.phys.msu.su     

Spectral superbroadening of subnanojoule Cr:Forsterite femtosecond laser pulses in a tapered fiber

Spectral superbroadening of subnanojoule femtosecond Cr:forsterite laser pulses is demonstrated for the first time in experiments with a tapered fiber. Coupling 40-fs 0.6-nJ pulses of 1.25-μm Cr:forsterite laser radiation into a tapered fiber with 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. These experimental results open new horizons for the creation of compact femtosecond systems based on Cr:forsterite lasers and tapered fibers for optical metrology and biomedical applications.     

Frequency doubling of multi-terawatt femtosecond pulses

We present an experimental and theoretical study of the influence of the spatial beam quality (fluence and phase distributions) on the second-harmonic generation in KDP crystals pumped by 180-fs pulses at 790 nm. Conversion efficiency and beam focusability are investigated experimentally and theoretically by the numerical analysis of the second harmonic, considering effects due to the cubic nonlinearity, beam diffraction, group-velocity walk-off, and dispersion of the pulses. It was found that the uniform intensity and phase distributions of the fundamental beam are essential to obtain a high focal intensity of the second-harmonic beam. PACS 42.65.Ky; 42.65.Re     

Reverse propagation of femtosecond pulses in optical fibers

We present a numerical technique for reversing femtosecond pulse propagation in an optical fiber, such that given any output pulse it is possible to obtain the input pulse shape by numerically undoing all dispersion and nonlinear effects. The technique is tested against experimental results, and it is shown that it can be used for fiber output pulse optimization in both the anomalous and normal dispersion regimes.     

Frequency conversion of ultrashort pulses

Problems of frequency conversion of ultrashort pulses are discussed. Efficient frequency conversion methods with large spectral bandwidth are reviewed with special emphasis placed on dispersively compensated schemes employing phase matchable nonlinear optical crystals.     

Spectral compression of femtosecond pulses in photonic crystal fibers

We demonstrate efficient spectral compression of femtosecond pulses near the zero-dispersion wavelength in nonlinear photonic crystal fibers (PCFs). The highest measured compression factor is 21, in which case the spectral brightness increases by a factor of 5. We numerically model the pulse propagation and find good agreement with the experiment. We argue that the fibers studied allow for spectral narrowing of more than 2 orders of magnitude. With dispersion-shifted PCFs, efficient spectral compression can take place across the visible and near-infrared part of the spectrum.     

Fabrication of magneto-optical microstructure by femtosecond laser pulses

We investigate femtosecond laser direct writing (FLDW) in the fabrication of magneto-optical (MO) microstructures. The experimental results show that FDLW can introduce positive refractive index change in the MO materials. With the increase of the writing intensity of femtosecond laser pulses, refractive index change increases, whereas Verdet constant of the damaged area decreases nonlinearly. With suitable writing intensity, we obtain a single-mode waveguide in which Verdet constant is 80% of the bulk MO glass.     

Generation of broadband femtosecond visible pulses in dispersion-micromanaged holey fibers

Submillimeter-scale dispersion micromanagement (DMM) is used to generate coherent and stable femtosecond visible pulses in holey fibers as short as 10 mm. The longitudinal variation of the phase-matching conditions for Cerenkov radiation and four-wave mixing explains the results well. We have converted up to 20% of the total input energy to a low-noise solitary wave with a bandwidth up to 50 nm in the range 385-625 nm by using holey fibers with various DMM designs.     

Micromachining of circular ring microstructure by femtosecond laser pulses

In this paper, integration of interference phenomenon into femtosecond laser micromachining was reported as the femtosecond laser pulses were reshaped spatially to perform ablation. The generation of circular interference pattern was demonstrated by overlapping infrared femtosecond laser pulses. The interference pattern was subsequently focused on a copper substrate to ablate microstructures of concentric circular rings. The present technique is expected to open up new applications in the areas of rapid fabrication of micro-Fresnel lenses, hybrid microlenses and lens arrays.     

Delivery of nanojoule femtosecond pulses through large-core microstructured fibers

We investigate femtosecond-pulse propagation through large-core microstructured fibers. Although these fibers are highly multimode, excitation of the fundamental mode is readily achieved, and coupling to higher-order modes is weak even when the fiber is bent or twisted. For prechirped input pulses with energies as large as 3 nJ, pulses as short as 140 fs were produced at the output of the fiber. Such a system could prove to be extremely useful for applications such as in vivo multiphoton microscopy and endoscopy that require delivery of femtosecond pulses and collection of fluorescence.     

Stretcher-free high energy nonlinear amplification of femtosecond pulses in rod-type fibers

We report on the study of direct amplification of femtosecond pulses in an 80 mum core diameter microstructured Yb-doped rod-type fiber amplifier in the nonlinear regime. The system includes a compact single grating compressor for the compensation of the small dispersion in the amplifier. With a 1250 line/mm (l/mm) grating-based compressor, pulses as short as 49 fs with 870 nJ pulse energy and 12 MW peak power are obtained. Alternatively, the use of a 1740 l/mm grating allows the production of higher quality pulses of 70 fs, 1.25 microJ pulse energy, and 16 MW peak power.     

Characteristics of supercontinuum generationin tapered fibers using femtosecond laser pulses

Drawing single mode fibers over a flame generates tapered fibers with waist diameters of approximately 1–3 micrometers and waist lengths of up to 90?mm. We demonstrate how the profile of such tapered fibers can be determined. We then characterize the white light that is generated in a variety of such fibers, showing its dependence on waist length and waist diameter and demonstrating its dependence on pulse parameters such as pulse duration, spectral position, and pulse power. A comparison with theoretical calculations using a nonlinear Schrödinger equation model including Kerr nonlinearities is given. Furthermore, we show XFROG spectrograms of the pulses propagating through tapered fibers, confirming the model of soliton splitting in the anomalous dispersion regime.     

飞秒激光在双折射微结构光纤中模式控制的四波混频效应的实验研究

利用飞秒激光脉冲在长度为10cm，包层具有大空气比的双折射微结构光纤中通过高阶模相位 匹配的四波混频获得了波长可调谐的反斯托克斯波.实验中脉冲宽度为35fs，中心波长820nm ，单脉冲能量4nJ的飞秒激光脉冲耦合到长轴直径为5μm，短轴为46μm的双折射微结构光 纤中.在高阶模传输情况下，通过调制耦合光的偏振方向，获得了具有不同中心波长的反斯 托克斯波.通过对比分析，讨论了输入光的偏振态对双折射微结构光纤中高阶模式下四波混 频效应的影响情况.理论计算分析很好的解释了实验结果. 关键词： 微结构光纤 飞秒脉冲激光 四波混频     

Efficient spectral conversion and temporal compression of femtosecond pulses in SF6

An uniquely high conversion efficiency of fundamental radiation from a Ti:sapphire laser to a supercontinuum is achieved through filamentary propagation in sulfur hexafluoride (SF6) to generate a uniform over-octave spectrum. Two different parts of the supercontinuum, firstly around the fundamental wavelength of 800?nm and secondly within the newly generated frequency range around 550?nm, are shown to be compressible down to minimal pulse duration of about 10?fs demonstrating a potential of the method for single-cycle pulses generation.     

Spectral broadening of femtosecond laser pulses in fibers with a photonic-crystal cladding

Changes in the spectra of femtosecond laser pulses propagating through fibers with a cladding having the structure of a two-dimensional photonic crystal are experimentally investigated. It is demonstrated that the waveguide properties of defect modes of photonic-crystal fibers provide an opportunity to considerably increase the efficiency of spectral broadening of short laser pulses as compared with conventional fibers.     

Near-infrared continuum generation of femtosecond and picosecond pulses in doped optical fibers

We report broad continuum generation in GeO₂-doped silica fibers induced by femtosecond and picosecond pulses generated by novel high-power laser sources. Over 0.5 W of average power in a broad-band picosecond continuum is generated in the near-infrared part of the spectrum from 800 to 1500 nm. PACS 42.60.By; 42.55.Xi; 42.65.Wi; 42.81.Qb     

Bragg gratings photoinduced in ZBLAN fibers by femtosecond pulses at 800 nm

Fiber Bragg gratings were written in thulium-doped and undoped single-mode ZBLAN fibers by focusing femtosecond laser pulses on the fiber core through a phase mask. Maximum index modulation of the order of 1 x 10(-3) was induced in both types of fibers. Measurements of the transverse refractive index changes across the core and cladding regions indicate that the grating formation originates from a negative index change.     

Phase modulation conversion and conservation in optical parametric amplifier of femtosecond laser pulses

The influence of the pump pulse, group-velocity mismatch, dispersion and self-phase modulation & cross phase modulation on the phase transfer in the OPA process were analyzed. The results show that very sharp phase modulation can be maintained in mid infrared range by optical parametric amplifier. The period and depth of the periodic modulation can also be maintained due to small group-velocity mismatching in the calculated wavelength range.