Negative dispersion mirrors for dispersion control in femtosecond lasers: chirped dielectric mirrors and multi-cavity Gires-Tournois interferometers

Chirped dielectric laser mirrors have been known for years as useful devices for broadband feedback and dispersion control in femtosecond pulse lasers. First we present a novel design technique referred to as frequency domain synthesis of chirped mirrors. These mirrors exhibit high reflectivity and nearly constant group delay over 150 THz supporting generation of sub-5-fs pulses in the visible. Afterwards, multi-cavity thin-film Gires-Tournois interferometers are introduced for the first time as an alternative approach to realize "negative dispersion mirrors". These novel dielectric high reflectors exhibit reflectivities R>99.97% and a negative group delay dispersion of ⫆ǃ fs² over a bandwidth of 56 THz. Dispersive properties originate from coupled resonances in multiple 5/2 cavities embedded in the layer structure. The general structure and performance of multi-cavity Gires-Tournois interferometers are compared to that of chirped dielectic mirrors and their distinct applications are discussed.     

基于白光干涉测量色散补偿薄膜的群延迟色散

为精确测量超快激光色散补偿薄膜的群延迟色散,提出了一种基于窗口傅里叶变换和样条插值去噪算法的新型白光干涉测量方案。计算机模拟表明此方法测试精度可达0.58fs2。分析了高斯噪声和光强平均效应对测试精度的影响,并使用此方法对实验制备的Gires-Tournois干涉反射镜和啁啾镜进行了测试,在宽光谱范围内测试误差小于10fs2。该方法相比其他算法可以更快速、更精确地实现薄膜相位信息的提取,具有更高的测试精度和实用性。     

Dispersive mirrors designed with mixed metal multilayer dielectric stacks

A different approach to construct dispersive mirrors (DMs) for ultrafast applications is proposed based on the high reflectivity and constant phase property of a novel metal in ultrawide spectral band. A 200- nm bandwidth DM, a high dispersive DM, and a complementary DM are designed with mixed metal multilayer dielectric stacks. The results show that the mixed-metal multilayer dielectric DMs (MMDMs) have much less layers and total thickness compared with an all-dielectric DM under the case of comparable performance. Such an approach will save manufacturing time and remarkably improve the stress of the DM.     

Mode-locked ytterbium fiber laser tunable in the 980-1070-nm spectral range

Spectral tuning of a mode-locked Yb-doped fiber laser over a 90-nm range is reported. Using semiconductor saturable absorber mirrors in a fiber laser cavity incorporating a grating-pair dispersive delay line, we obtain reliable self-starting mode locking over the whole tuning range. The wide tuning range is achieved by optimization of reflection characteristics and bandgap energy of the multiple-quantum-well semiconductor saturable absorber and by proper engineering of the laser cavity.     

Mirror-dispersion-controlled OPA: a compact tool for sub-10-fs spectroscopy in the visible

We report on a sub-10-fs optical parametric amplifier in the visible, using noncollinear phase matching in #-barium borate, with a pulse compressor made exclusively of chirped dielectric mirrors. We employ mirrors of novel design, providing negative group delay dispersion for bandwidths of over 200 THz, up to the blue-green spectral region. The resulting setup, compact, easy to operate and reliable, is an ideal tool for ultrafast spectroscopy with extremely high time resolution. We use these pulses to observe in the time domain coherent oscillations in organic molecules with period as short as 16 fs.     

等效腔法研究低损介质镜的反射特性

利用多光束干涉原理的等效腔模型对四分之一波长多层介质镜进行了研究。证明在反射镜中心频率ωo附近，形为ρ(ω)exp[iψ(ω)]的反射系数中的ρ(ω)等于ρ(ω)，ψ(ω)与频率成正比。求得了在介质具有微弱吸收情况下，吸收引起的四分之一波长镜反射下降和反射时延的表达式，研究了反射率下降以及反射时延与膜层数目的奇偶性的关系。在此基础上还引入了一个具有一定普适性的权重因子，通过这个权重因子可以定量描述镜子中各层对多层介质镜反射率下降、反射时延等重要特性的影响。     

Mirror-dispersion-controlled sub-10-fs optical parametric amplifier in the visible

Pulses from an optical parametric amplifier in the visible are compressed to sub-10-fs duration by a delay line made exclusively from chirped dielectric mirrors. We employ what we believe are the first ultrabroadband visible chirped mirrors, which provide negative group-delay dispersion up to the blue-green spectral region. The setup is compact and reliable, and we used it to observe, for what is to our knowledge the first time in organic molecules, coherent oscillations with periods as short as 16 fs.     

Integrative optimization of chirped mirrors for intracavity dispersion compensation

A new integrative optimization of three chirped mirrors for precise broadband intracavity group delay dispersion compensation in ring cavity is proposed. Simulation demonstrates that the residual group delay dispersion ripples of these three chirped mirrors are less than 25 fs² for most of the wavelengths from 600 to 1200 nm. Pulse evolution simulation shows that these three matched chirped mirrors compensate the group delay dispersion in the Ti:Sapphire ring cavity laser well to obtain an octave-spanning spectrum.     

High-dispersive mirror for pulse stretcher in femtosecond fiber laser amplification system

We present a high-dispersive multilayer mirror for pulse stretching in a femtosecond fiber laser amplification system. The designed mirror contains 54 layers with a total physical thickness of 7.3 μm, which can provide a positive group delay dispersion (GDD) of 600 fs² and a high reflectance over 99.9% from 1010 to 1070 nm. The samples were prepared by dual ion beam sputtering. The measured transmittance matches well with the theoretical result. The GDD characteristics of samples were tested by home-made white light interferometer. The measured GDD is higher than the design results, an average GDD of +722 fs² from 1010 nm to 1070 nm. The mirrors were employed in a Yb-doped large-mode-area photonic crystal fiber amplification system. An input pulse compressed by the gratings with autocorrelation function of 83 fs is obtained with a stretched FWHM of 1.29 ps after 28 bounces between the dispersive mirrors. The results show that the multilayer dispersive mirror could be an effective and promising technique for pulse stretching in femtosecond amplification systems.     

Pseudo-spectral time-domain simulation of the transmission and the group delay of photonic devices

The pseudo-spectral time-domain (PSTD) method is applied to the simulation of photonic structures. We study transmission and group delay properties of dielectric and dispersive devices. An auxiliary differential equation technique is used to model the dispersive behavior of metallic features. Comparisons with the finite-difference time-domain method show the superiority of the PSTD method in particular when extracting phase information from a photonic device. The validity of the algorithm is demonstrated by investigating a ring resonator, Bragg gratings and metallic nano-particle arrangements.     

Sub-20-fs terawatt-class laser system with a mirrorless regenerative amplifier and an adaptive phase controller

We have developed an ultrabroadband regenerative amplifier with a mirrorless cavity to eliminate the limitation of bandwidth of dielectric coats on cavity mirrors. The large amount of material dispersion in the Pellin-Broca prisms that are used instead of cavity mirrors is compensated for with a hybrid technique that uses Brewster prism pairs in the regenerative amplifier and an adaptive phase controller of a liquid-crystal spatial light modulator. We obtained a 16-fs pulse width with an energy of 13 mJ, which is to our knowledge the highest energy obtained in the sub-20-fs regime by use of an adaptive phase controller.     

A new theory for multiprofile,buried gratings

A new theory for dielectric coated metallic gratings is presented. The formalism is well adapted to the case for which the dielectric overcoatings are separated by plane interfaces or for which the modulated interfaces do not interpenetrate. We show that the numerical program is well suited to the theoretical study of the use of gratings as beam sampling mirrors in the infrared region.     

Model-based analysis of dispersion curves using chirplets

Time-frequency representations, like the spectrogram or the scalogram, are widely used to characterize dispersive waves. The resulting energy distributions, however, suffer from the uncertainty principle, which complicates the allocation of energy to individual propagation modes (especially when the dispersion curves of these modes are close to each other in the time-frequency domain). This research applies the chirplet as a tool to analyze dispersive wave signals based on a dispersion model. The chirplet transform, a generalization of both the wavelet and the short-time Fourier transform, enables the extraction of components of a signal with a particular instantaneous frequency and group delay. An adaptive algorithm identifies frequency regions for which quantitative statements can be made about an individual mode's energy, and employs chirplets (locally adapted to a dispersion curve model) to extract the (proportional) energy distribution of that single mode from a multimode dispersive wave signal. The effectiveness of this algorithm is demonstrated on a multimode synthetic Lamb wave signal for which the ground-truth energy distribution is known for each mode. Finally, the robustness of this algorithm is demonstrated on real, experimentally measured Lamb wave signals by an adaption of a correlation technique developed in previous research.     

飞秒激光脉冲压缩用啁啾镜新设计方法

对配对啁啾镜传统的优化过程进行了改进,并且尝试了新的多个啁啾镜匹配的方法。理论上实现了在700～1050 nm光谱范围内,两个啁啾镜配对色散总和振荡小于±10 fs2,三个啁啾镜配对色散总和振荡小于±15 fs2。此配对啁啾镜所实现的精确补偿色散,是获得极短光脉冲的基础,可以用于飞秒激光的脉冲压缩。     

Nonlinear absorption of light pulses under conditions of two-photon resonance in bulk crystals and nanostructures in the femtosecond pump-probe spectroscopy mode

A theory of nonlinear absorption of femtosecond light pulses by bulk crystals and nanostructures of different dimensions in the pump-probe spectroscopy mode has been developed. Expressions for the energy absorbed from the probe pulse under conditions of two-photon resonances are derived for transitions between the discrete or band electronic states and between the size-quantization subband states in quantum wells and quantum wires. The dependences of the energy absorbed from the probe pulse on the two-photon resonance detuning and on the time delay between the pulses are analyzed. The dependence of the power absorbed from the probe pulse on the time is also obtained.     

Spectrum splitting using multi-layer dielectric meta-surfaces for efficient solar energy harvesting

We designed a high-efficiency dispersive mirror based on multi-layer dielectric meta-surfaces. By replacing the secondary mirror of a dome solar concentrator with this dispersive mirror, the solar concentrator can be converted into a spectrum-splitting photovoltaic system with higher energy harvesting efficiency and potentially lower cost. The meta-surfaces are consisted of high-index contrast gratings (HCG). The structures and parameters of the dispersive mirror (i.e. stacked HCG) are optimized based on finite-difference time-domain and rigorous coupled-wave analysis method. Our numerical study shows that the dispersive mirror can direct light with different wavelengths into different angles in the entire solar spectrum, maintaining very low energy loss. Our approach will not only improve the energy harvesting efficiency, but also lower the cost by using single junction cells instead of multi-layer tandem solar cells. Moreover, this approach has the minimal disruption to the existing solar concentrator infrastructures.     

Unravelling the effect of Ni doping on the structural,optical and dielectric properties of nanocrystalline SnO2

Nanocrystalline pristine and Ni doped (x= 0.05, 0.10 and 0.15) SnO₂ samples are synthesized via sol-gel process. An extensive microstructure, optical and dielectric studies are performed through several analytical techniques. Analysis of the x-ray diffraction (XRD) data through Rietveld refinement confirms tetragonal crystal structure with P4₂/mnm space group for all the samples without any impurity or secondary phase. Scanning electron microscopy (SEM) images exhibit smooth surface morphology with agglomeration of the particles and energy dispersive x-ray spectra (EDS) confirm elemental composition of the samples. The average crystallite/particles size of the samples is found to increase for 5% Ni doped SnO₂ but reduces on further increase in the Ni content. Fourier transform infrared (FTIR) spectra show vibrational modes of the functional groups present in the samples. UV-visible absorption spectra indicate a red shift on Ni doping in SnO_2, ascribed to the trapping of excitons by the oxygen vacancies and thereby creating extra energy states within the bandgap. The band gap is found to diminish from 3.81 eV for pristine SnO₂ to 3.57 eV for the sample with 15% Ni concentration. Frequency dependent dielectric measurements at room temperature reveal higher values of the dielectric constant at lower frequencies that can be described on the basis of Maxwell-Wagner theory. The results demonstrate that the oxygen vacancies play a crucial role and concentration of Ni has strong influence on the microstructure, optical and dielectric behaviour of SnO₂.     

Group delay dispersion compensation in an Ytterbium-doped fiber laser using intracavity Gires-Tournois interferometers

This paper reports the use of a pair of multi-cavity Gires-Tournois interferometer (GTI) mirrors in a ring cavity Ytterbium-doped fiber laser for dispersion compensation. The optical spectrum was broadened by 6 nm due to the large negative group delay dispersion introduced by the light undergoing 10 round trips between the GTI mirrors. The design of the GTI mirrors and the optimisation of the layers are discussed and fabrication results are presented.     

Low-threshold, 12-MHz, multipass-cavity femtosecond Cr4+: Forsterite laser

We report on the development of an extended-cavity, low-threshold femtosecond Cr⁴⁺:forsterite laser operating near 1270 nm. The resonator was end-pumped by a continuous-wave Yb-fiber laser at 1060 nm. The 195-MHz short cavity could be operated with absorbed threshold pump powers as low as about 300 mW. To scale up the pulse energy, the cavity length was increased by using a q-preserving multipass cavity (MPC) consisting of a flat and a curved (R = 4 m), notched high-reflector. Kerr-lens mode locking was used to generate femtosecond pulses and double-chirped mirrors were included in the cavity for dispersion compensation. With an absorbed pump power of 1250 mW, the mode-locked laser produced 4 nJ of pulse energy at an average output power of only 47 mW. The corresponding repetition rate of the extended cavity was 11.7 MHz. The pulse-width was 90 fs and the spectral bandwidth was 23 nm, corresponding to a time-bandwidth product of 0.37.     

Combined-method for designing broadband dispersion mirror based on local optimization approach and needle optimization technique

A combined-method based on local optimization and needle optimization is proposed to design broadband dispersion mirrors. The broadband dispersion mirrors are designed by using needle optimization combined with cross application of local optimization and changing high refractive index materials. We combine high efficiency of the local optimization and accuracy of needle optimization. This method can efficiently prevent optimizing process from being captured in a local minimum and obtain higher reflectivity and more smooth group delay dispersion (GDD) curves. The designed dispersion mirrors have a few numbers of layers, which do not have the thin layers that are not easily deposited.