Microstructure fibers as frequency-tunable sources of ultrashort chirped pulses for coherent nonlinear spectroscopy

Microstructure fibers are shown to allow the creation of new tunable sources for femtosecond nonlinear spectroscopy. These fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr:forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp. These pulses from a microstructure fiber were used to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene solution by cross-correlating these pulses with the femtosecond second-harmonic output of the Cr:forsterite laser in boxcars geometry (XFROG CARS). PACS 42.65.Wi; 42.81.Qb     

Limiting temporal and spectral resolution in spectroscopy and microscopy of coherent Raman scattering with chirped ultrashort laser pulses

Chirped ultrashort light pulses offer new options for coherent nonlinear spectroscopy and microscopy. We show here that the temporal resolution of spectroscopy and microscopy based on coherent anti-Stokes Raman scattering (CARS) can be smoothly tuned within a broad range, with upper and lower bounds of this range controlled by the pump and probe pulse durations. The spectral resolution of CARS spectroscopy and microscopy is analyzed as a function of the duration and chirp of the pump pulses. Pulses with a periodic phase modulation can provide the limiting spectral resolution of the CARS technique, corresponding to the lower bound of uncertainty in spectral measurements, dictated by the uncertainty principle.     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

新型全光纤高能量飞秒光脉冲源的数值模拟

数值模拟了自相似脉冲的产生与压缩,得到一种产生高能量飞秒光脉冲的新方法.结果表明:利用掺铒光纤对光脉冲进行自相似传输,可得到含线性频率啁啾的高能量自相似光脉冲;自相似光脉冲经过空芯光子带隙光纤的一级线性压缩和高非线性光纤的二级非线性压缩,可获得高峰值功率的飞秒光脉冲;压缩过程中存在最佳光纤长度;喇曼自频移和自陡效应对脉冲压缩产生不利影响.     

Optimally chirped CARS using fiber stretchers

High spectral resolution coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy are demonstrated with femtosecond laser systems. We perform optimal chirping in glass fibers and demonstrate a spectral resolution enhancement to better than 26 cm⁻¹, which is limited by the bandwidth of the measured resonances. Considering the convolution with the resonance bandwidth this corresponds to a spectral resolution of approximately 2.5 cm⁻¹, which is an enhancement by a factor of 165 with respect to the use of bandwidth-limited pulses. In microscopic imaging, a water background suppression of 81.5% is achieved.     

Frequency-shifted megawatt soliton output of a hollow photonic-crystal fiber for time-resolved coherent anti-Stokes Raman scattering microspectroscopy

Hollow-core photonic-crystal fibers (PCFs) provide soliton delivery and frequency shifting of 2.8 MW femtosecond pulses with an input central wavelength of 618 nm. The frequency-shifted megawatt soliton output of the hollow PCF is used as a high-peak-power Stokes field for coherent anti-Stokes Raman scattering (CARS) microspectroscopy, providing a dynamic range of nearly four decades for anti-Stokes signal detection, thus enabling time-resolved CARS studies of ultrafast relaxation processes on time scales from tens of femtoseconds up to tens of picoseconds.     

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     

Frequency conversion of subnanojoule femtosecond pulses in microstructure fibers

We experimentally demonstrate highly efficient multiplex frequency conversion of unamplified subnanojoule femtosecond pulses of Ti:sapphire laser radiation in fused silica microstructure fibers. Nonlinear optical spectral transformation of femtosecond pulses in an array of fused silica threadlike channels in these microstructure fibers results in the generation of isolated anti-Stokes spectral components within the wavelength range of 400–500 nm. An efficiency of frequency conversion of about 20% is achieved for 800-nm pump pulses with an energy of 0.7 nJ and a pulse duration of 70 fs.     

Phase-locked ultrashort pulse trains at separate and independently tunable wavelengths

The generation and characterization of femtosecond pulses at three independently tunable visible wavelengths is reported. Selected spectral portions of a common continuum generated in sapphire are amplified in noncollinear optical parametric amplifiers. The phase relation of the pulse trains is analyzed with a nonlinear interferometer based on coherent anti-Stokes Raman scattering and is found to be locked to better than 250 mrad rms. Small spectral shifts of the pulses lead to interference behavior that is consistent with 1 kHz frequency combs.     

Raman lasing and cascaded coherent anti-Stokes Raman scattering of a two-phonon Raman band

Raman lasing of a two-phonon Raman band in the anti-Stokes side is demonstrated. Two femtosecond light pulses with identical wavelengths are irradiated onto a SrTiO3 crystal in a cross-beam configuration. Under low excitation power, several wave-mixing signals with identical wavelengths are emitted. When the power exceeds a critical value, cascaded coherent anti-Stokes Raman scattering (CARS) signals are emitted, the frequency step of which is coincident with that of the strongest two-phonon Raman band of 2TO2.     

Sensitive femtosecond coherent anti-Stokes Raman spectroscopy discrimination between dipicolinic acid and dinicotinic acid

We demonstrate that femtosecond ultraviolet and visible coherent anti-Stokes Raman spectroscopy provides the sensitivity and specificity needed to distinguish between two similar molecules of pyridinedicarboxylic acid. The Fourier transforms of the temporal measurements provide the energy difference between the ground state vibrational modes. Quantum chemical calculations provide theoretical predictions that agree well with the measurements. The present technique allows us to distinguish 10 cm(-1) frequency shifts by using pulses ten times broader than the shifts.     

Efficient anti-Stokes generation through phase-matched four-wave mixing in higher-order modes of a microstructure fiber

Phase-matched four-wave mixing in higher-order modes of microstructure fibers allows unprecedentedly high efficiencies of anti-Stokes frequency conversion to be achieved for subnanojoule femtosecond laser pulses. 70-fs pulses of 790-nm radiation were used to generate an anti-Stokes component at 520-530 nm in a higher-order mode of a microstructure fiber with a 4.8-microm core. The maximum ratio of the anti-Stokes signal energy to the energy of the pump component in the output spectrum is estimated as 1.7.     

Coherent anti-Stokes Raman scattering microscopy with a photonic crystal fiber based light source

A coherent anti-Stokes Raman scattering microscope based on a Ti:sapphire femtosecond oscillator and a photonic crystal fiber is demonstrated. The nonlinear response of the fiber is used to generate the additional wavelength needed in the Raman process. The applicability of the setup is demonstrated by imaging of micrometer-sized polystyrene beads.     

Selective excitation and suppression of coherent anti-Stokes Raman scattering by shaping femtosecond pulses

Femtosecond coherent anti-Stokes Raman scattering (CARS) suffers from poor selectivity between neighbouring Raman levels due to the large bandwidth of the femtosecond pulses. This paper provides a new method to realize the selective excitation and suppression of femtosecond CARS by manipulating both the probe and pump (or Stokes) spectra. These theoretical results indicate that the CARS signals between neighbouring Raman levels are differentiated from their indistinguishable femtosecond CARS spectra by tailoring the probe spectrum, and then their selective excitation and suppression can be realized by supplementally manipulating the pump (or Stokes) spectrum with the $\pi $ spectral phase step.     

Highly sensitive single-beam heterodyne coherent anti-Stokes Raman scattering

Single-beam coherent anti-Stokes Raman-scattering (CARS) microspectroscopy achieves a complete CARS scheme with a femtosecond laser. Here, we introduce heterodyne detection in a simple experimental extension: the optical fields driving the CARS process and the local oscillator used for heterodyning are derived from a single beam of ultrashort laser pulses by pulse shaping. The heterodyne signal is amplified by more than 3 orders of magnitude and is linearly dependent on the concentration of Raman scatterers. This dramatically increases the sensitivity of chemically selective detection at microscopic resolution while maintaining the simplicity of the single-beam setup.     

Experimental study on the Stokes effect in disordered birefringent microstructure fibers

In this paper, a 120-fs pulse transmission experiment is carried out using disordered birefringent microstructure fibers with cladding ventages. Through this experiment, it is found for the first time that remarkable Stokes and anti-Stokes waves can also be produced when the central wavelength of the incident pulse is in the normal dispersion regime of the microstructure fiber. The generation of the two waves can be explained by the four-wave mixing phase matching theory. Properties of the two waves under the action of femtosecond laser pulses with different parameters are studied. The results show that the central wavelength of anti-Stokes waves and Stokes waves produced under the two orthogonal polarization states shift by 63 nm and 160 nm, respectively. The strengths and central positions of the two waves in birefringent fibers can be controlled by adjusting the phase match condition and the polarization directions of incident pulses.     

Broadband coherent anti-Stokes Raman scattering light generation in BBO crystal by using two crossing femtosecond laser pulses

As broad as 12000 cm(-1) coherent anti-Stokes Raman scattering (CARS) light from ultraviolet to infrared was generated in a BBO crystal by using two crossing femtosecond laser pulses with 30% conversion efficiency. More than fifteenth-order anti-Stokes and second-order Stokes Raman sidebands were observed with nice Gaussian spatial mode. The effect of the crossing angle between two input beams on the spectrum and emitting angle of the Raman sidebands was studied in detail. Calculation shows that the phase-matching condition determines the frequencies and angles of the sidebands.     

Microanalytical nonlinear single-beam spectroscopy combining an unamplified femtosecond fibre laser,pulse shaping and interferometry

Nonlinear vibrational spectroscopy using a single beam of femtosecond pulses from an unamplified fibre laser oscillator is demonstrated. To achieve high spectral resolution and sensitive signal detection with the picojoule pulse energies available, pulse shaping and integrated interferometric detection is employed. The spectroscopic technique used is coherent anti-Stokes Raman scattering (CARS), which yields well-resolved spectra of molecular vibrations in the 100–350 cm^-1 domain of halomethane samples in the liquid phase. We explore the implications of phase control for the interferometric detection of weak signals. The presented combination of a fiber laser, pulse shaping and CARS microspectroscopy is a first example of simplified schemes for compact and robust nonlinear spectroscopic detection and sensing, which is demonstrated exemplarily by on-line monitoring of the chemical composition in a microfluidic flow cell. PACS 42.55.Wd; 42.62.Fi; 78.47.Fg; 42.65.Dr; 82.80.Gk; 92.20.cn     

Actively shaped supercontinuum from a photonic crystal fiber for nonlinear coherent microspectroscopy

The combination of broadband pulses from a photonic crystal fiber (PCF) pumped by a standard 100 fs oscillator and pulse shaping is successfully employed for coherently controlled nonlinear spectroscopy. The pulse shaper manages not only to compress the PCF supercontinuum in a closed-loop optimization scheme but also to manipulate the phase at the same time for quantum control applications. This approach is demonstrated by single-beam coherent anti-Stokes Raman microspectroscopy and should be, due to its simplicity, well suited for general applications in nonlinear microscopy.     

Femtosecond OPO based on LBO pumped by a frequency-doubled Yb-fiber laser-amplifier system for CARS spectroscopy

We present detailed investigations of a femtosecond green-pumped optical parametric oscillator (OPO) based on lithium triborate. As pump source, a frequency-doubled Yb-fiber laser-amplifier system is used. The OPO generates signal pulses tunable over a spectral range from 780 to 940 nm and idler pulses tunable from 1630 to 1190 nm. More than 250 mW are generated in the signal beam and more than 300 mW in the idler beam. Without dispersion compensation chirped signal pulses with a pulse duration between 100 and 250 fs are measured. Using this system for coherent anti-Stokes Raman scattering spectroscopy, vibrational resonances between 1110 and 6760 cm⁻¹ can be excited. Due to the chirped pulses, a spectral resolution of 100 cm⁻¹ is achieved, which is 2.5 times higher compared to an excitation with time-bandwidth limited pulses.