Efficient and tunable diode-pumped femtosecond Yb:glass lasers

Diode-pumped Yb:phosphate and Yb:silicate glass lasers have been passively mode locked for the first time to the authors' knowledge. Reliable self-starting mode locking without critical cavity alignment has been achieved with intracavity semiconductor saturable-absorber mirrors and soliton mode locking. We generated pulses as short as 58 fs with the Yb:phosphate laser and 61 fs with the Yb:silicate laser at average output powers of 65 and 53 mW, respectively. The pulse repetition rate was 112 MHz. Additionally, we demonstrated tunability of femtosecond pulses from 1025 to 1065 nm for the Yb:phosphate and from 1030 to 1082 nm for the Yb:silicate glasses. The highest mode-locked output power was 405 mW, with 183-fs pulses from the phosphate glass. The diode pump power was 1.68 W, corresponding to 24% optical-to-optical efficiency. The highest cw output power was 510 mW at the same incident pump power.     

Femtosecond hybrid mode-locked semiconductor laser and amplifier dynamics

We describe the generation of femtosecond high power optical pulses using hybrid passive-active mode-locking techniques. Angle stripe geometry GaAs/AlGaAs semiconductor laser amplifiers are employed in an external cavity including prisms and a stagger-tuned quantum-well saturable absorber. An identical amplifier also serves as an optical power amplifier in a stretched pulse amplification and recompression sequence. After amplification and pulse compression this laser system produces 200 fs, 160 W peak power pulses. We discuss and extend our theory, and supporting phenomenological models, of picosecond and subpicosecond optical pulse amplification in semiconductor laser amplifiers which has been successful in calculating measured spectra and time-resolved dynamics in our amplifiers. We have refined the theory to include a phenomenological model of spectral hole-burning for finite intraband thermalization time. Our calculations are consistent with an intra-band time of approximately 60 fs. This theory of large signal subpicosecond pulse amplification will be an essential tool for understanding the mode-locking dynamics of semiconductor lasers and for analysis of high speed multiple wave-length optical signal processing and transmission devices and systems based on semiconductor laser amplifiers.     

Conical emission by femtosecond pulses with different spectral bandwidths

Conical emission (CE) has been investigated experimentally by laser pulses with different pulse durations and spectral bandwidths. The results show that the overall CE curve will shift as the varying of spectral bandwidth of pump laser pulse. But for pump laser pulses which have same spectral bandwidth but different pulse duration, the CE angles will be same at the spectral region close to the pump wavelength while will be different at the spectral region far away from the pump wavelength. We have also fitted the measured CE angles with X-wave model. The calculated curves and the measured CE curves match reasonably well. The best fits indicate that the group velocity of the filament pulse may be greatly controlled by controlling the spectral bandwidth of pump laser pulse.     

Multi-stage pulse compression by use of cascaded quadratic nonlinearity

In this paper, the multi-stage compression of picosecond pulses by cascaded quadratic nonlinearity is studied theoretically, and the dependence of pulse compression on phase-mismatch, laser intensity, and crystal characteristics has been discussed in detail. We demonstrate that the multi-stage pulse compression is much more efficient than the single-stage with a same total crystal length. Pulses as short as ∼150 fs can be generated by compressing 30-ps initial pulses in a two-stage configuration under the realistic crystal and laser conditions, and shorter pulses of ∼30 fs may be obtained by three-stage compression. Pulse compression performances with BiBO and BBO crystals are compared and discussed finally.     

Red-Shift Conical Emission by Femtosecond Pulses at Low Input Power

Red-shift conical emission （CE） is observed by femtosecond laser pulse propagating in BK7 at a low input power （compared to those input powers for generation of blue-shift CE）. With the increasing input power the blue-shift CE begins to appear whereas the red-shift CE ring （902 nm in our experiment） disappears accompanied by the augment of the central white spot size synchronously. The disappearing of red-shift CE in our experiment is explained such that the increase of axial intensity is much higher than that of ring emission and the augment of the central white spot size with the increasing input laser power.     

Stable mode-locked fiber laser using 49 cm long bismuth oxide based erbium doped fiber and slow saturable absorber

A stable mode-locked fiber laser employing a 49 cm long bismuth oxide based erbium doped fiber (Bi-EDF) by using a slow saturable absorber is demonstrated. Near transform limited short pulses with a repetition rate of 8.27 MHz are obtained at a wavelength of 1560 nm with a maximum spectral width of 15 nm. Results indicate that pulse characteristics are strongly dependent on pump power rather than spectral width. Moreover the Time-Bandwidth products (TBWP), pulse duration, the energy fluctuation and timing jitter decrease with increasing pump power. The pulse width is continuously varied from 1.2 ps to less than 300 fs. It produces stable mode locking with a maximum spectral width of 15 nm, minimum timing jitter of 4 ps and energy fluctuations of 2.5%. The pulse train was amplified using a two-stage amplifier up to 447 mW average power corresponding to peak powers of 177.3 kW.     

Recombination emissions and spectral blueshift of pump radiation from ultrafast laser induced plasma in a planar water microjet

We report spectroscopic investigations of an ultrafast laser induced plasma generated in a planar water microjet. Plasma recombination emissions along with the spectral blueshift and broadening of the pump laser pulse contribute to the total emission. The laser pulses are of 100 fs duration, and the incident intensity is around 10¹⁵ W/cm². The dominant mechanisms leading to plasma formation are optical tunnel ionization and collisional ionization. Spectrally resolved polarization measurements show that the high frequency region of the emission is unpolarized whereas the low frequency region is polarized. Results indicate that at lower input intensities the emission arises mainly from plasma recombinations, which is accompanied by a weak blueshift of the incident laser pulse. At higher input intensities strong recombination emissions are seen, along with a broadening and asymmetric spectral blueshift of the pump laser pulse. From the nature of the blueshifted laser pulse it is possible to deduce whether the rate of change of free electron density is a constant or variable within the pulse lifetime. Two input laser intensity regimes, in which collisional and tunnel ionizations are dominant respectively, have been thus identified.     

Theory of multiple harmonic generation in reflection from a metal surface

A microscopic theory is presented for multiple harmonic generation (MHG) from a metal surface in the jellium approximation and for laser intensities below the critical value for plasma formation. Calculations are presented for the second and up to the sixth harmonic, in the case of a gold surface and for laser wavelengths 800 and 1064 nm, with femtosecond and picosecond laser pulses. The peaks of the harmonic pulses are delayed relative to the peak of the incident laser pulse by about 20 to 30 fs due to the finite electron-relaxation times. The calculated relative peak intensities of the various harmonics are most sensitive to the values of the relaxation times, and MHG provides a way of determining these times. An erratum to this article can be found at .     

Highly Efficient Self-Starting Femtosecond Cr:Forsterite Laser

We report a highly efficient and high power self-starting femtosecond Cr：forsterite laser pumped by a 1064-nm Yb doped fibre laser. Five chirped mirrors are used to compensate for the intra-cavity group-delay dispersion, and the mode-locking is initiated by a semiconductor saturable absorber mirror （SESAM）. Under pump power of 7.9 W, stable femtosecond laser pulses with average power of 760mW are obtained, yielding a pump power slope efficiency of 12.3%. The measured pulse duration and spectral bandwidth （FWHM） are 46 fs and 45 nm; the repetition rate is 82 MHz.     

All-solid-state cw mode-locked picosecond KTiOAsO4 (KTA) optical parametric oscillator

4 (KTA) optical parametric oscillator (OPO) synchronously pumped by a cw diode-pumped mode-locked Nd:YVO₄ oscillator–amplifier system. The laser system (pumped by 84 W of cw 808-nm diode radiation) generates 7-ps-long pulses at 1.064 μm with a repetition rate of 83.4 MHz and an average power of 29 W. The OPO, synchronously pumped by the 1.064-μm laser pulses, consists of a 15-mm-long KTA crystal (cut for type II noncritical phase-matching) in a folded signal resonant linear resonator. The average powers of the 1.54-μm signal radiation and the 3.47-μm idler radiation are 14.6 W and 6.4 W, respectively. The total OPO output of 21 W corresponds to an internal efficiency of 75%. The experimental investigations include measurements of the OPO output power (and its dependence on the pump power, the transmission of the output coupler, and the resonator length) and of the pulse properties (such as pulse duration and spectral width). The measured results are in good agreement with the predictions of a numerical analysis based on a split-step Fourier method. Received: 4 May 1998     

Active phase control and frequency chirp effects on supercontinuum generation in high birefringence photonic crystal fiber

An acoustic-optics programmable dispersive filter (AOPDF) was first employed to actively control the linearly polarized femtosecond pump pulse frequency chirp for supercontinuum (SC) generation in a high birefringence photonic crystal fiber (PCF). By accurately controlling the second order phase distortion and polarization direction of incident pulses, the output SC spectrum can be tuned to various spectral energy distributions and bandwidths. The pump pulse energy and bandwidth are preserved in our experiment. It is found that SC with broader bandwidth can be generated with positive chirped pump pulses except when the chirp value is larger than the optimal value, and the same optimal value exists for the pump pulses polarized along the two principal axes. With optimal positive chirp, more than 78% of the pump energy can be transferred to below 750 nm. Otherwise, negative chirp will weaken the blue-shift broadening and the SC bandwidth.     

基于多通单元的高能量耗散孤子锁模光纤振荡器

为了在有限抽运功率条件下探索基于大模场面积光 子晶体光纤的耗散孤子锁模振荡器的能量提升潜力, 本文利用多通单元将基于掺镱大模场面积光子晶体光纤锁模振荡器的腔长延展, 消除了有限抽运功率的限制, 使得该系统能够在较低平均功率水平下获得更高的单脉冲能量. 实验上构建了重复频率低至15.58 MHz的高能量光子晶体光纤锁模脉冲振荡器, 并通过分别使用6 nm带宽和12 nm带宽的两种不同带宽的光谱滤光片, 能够直接输出平均功率分别为3.73 W和4.9 W的啁啾脉冲, 对应单脉冲能量分别为239 nJ和314 nJ. 经过光栅对去啁啾后, 最窄脉冲宽度分别为56 fs和75 fs, 对应峰值功率均超过3 MW. 关键词： 多通单元 耗散孤子 飞秒 光纤激光器     

Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control

The spectral width of a 5-kHz Ti:sapphire laser system was broadened by spectral control in a regenerative amplifier consisting of broadband chirped mirrors. The dispersion over the wide spectral range was compensated by a deformable mirror along with a genetic algorithm, resulting in a pulse width of 15 fs. The pulse width is the shortest, to our knowledge, in chirped pulse amplification systems with a regenerative amplifier. The phase distortion of broadband frequency doubling in addition to the Ti:sapphire laser was compensated by using the self-diffraction intensity in sapphire as the feedback signal into the genetic algorithm, resulting in a pulse width of 7.5 fs. The average power of the second harmonic was 1 W with a fundamental input of 7 W.     

Cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber

Copropagating fundamental-wavelength and second-harmonic femtosecond pulses of Cr: forsterite laser radiation are used to study cross-phase-modulation-induced instabilities and frequency shifts in a photonic-crystal fiber. Parametric instability of the second-harmonic probe pulse induced through cross-phase modulation by the fundamental-wavelength pump pulse gives rise to distinct sidebands in the spectrum of the probe field transmitted through the fiber. The wavelength of these sidebands was tuned in our experiments within approximately 100 nm by varying the peak power and the delay time of the pump pulse, suggesting a convenient way of controlled parametric spectral transformation of ultrashort laser pulses.This revised version was published online in March 2005. In the previous version, the published online date was missing     

超宽带近红外和蓝光飞秒激光脉冲产生的实验研究

增益介质中泵浦光束提供的软光阑效应对于实现克尔透镜稳定锁模及其超宽带光谱脉冲的产生具有非常重要的作用.实验上首先对半导体泵浦全固化钛宝石飞秒激光器的锁模动态特性进行了研究,在4 W绿光泵浦状态下获得了平均输出功率为570 mW、中心波长在794 nm～835 nm范围内调谐、光谱带宽最大可达135 nm的近红外光脉冲输出,其相应的时域变换极限脉冲宽度均小于10 fs.另外,将光束聚焦在超薄BBO晶体上,获得了中心波长在418 nm～429 nm之间调谐、光谱宽带时域变换极限小于15 fs的蓝光飞秒脉冲.     

Depolarization of white light generated by ultrashort laser pulses in optical media

We report measurements of the extinction ratio (ER) of white light generated upon irradiation of BK7 glass by ultrashort (36 fs) laser pulses with incident power approximately 10(3) times larger than the critical power for self-focusing. At low incident powers, the continuum is symmetric about the incident laser wavelength; at high powers it becomes broader and distinctly asymmetric towards the blue side. We observe that ER degrades by 100-fold after the onset of multiphoton-induced free-electron generation (at incident intensity approximately 2 TW cm-2), which also corresponds to the onset of asymmetry in white-light spectra.     

High-energy femtosecond photonic crystal fiber laser

We report the generation of high-energy high-peak power pulses in an all-normal dispersion fiber laser featuring large-mode-area photonic crystal fibers. The self-starting chirped-pulse fiber oscillator delivers 11 W of average power at 15.5 MHz repetition rate, resulting in 710 nJ of pulse energy. The output pulses are dechirped outside the cavity from 7 ps to nearly transform-limited duration of 300 fs, leading to pulse peak powers as high as 1.9 MW. Numerical simulations reveal that pulse shaping is dominated by the amplitude modulation and spectral filtering provided by a resonant semiconductor saturable absorber.     

Femtosecond 5-W Yb:KGW slab laser oscillator pumped by a single broad-area diode and its application as supercontinuum source

We demonstrate a novel and very simple design for an efficient high-power femtosecond Yb:KGW slab laser oscillator. The laser is capable of producing output powers of more than 5 W at 44-MHz repetition rate and pulse widths as short as 161 fs with a nearly diffraction limited beam and an optical-to-optical efficiency of more than 28%. The laser is pumped by a single broad-area laser diode and employs simple pump optics consisting of three lenses. The laser output power is distributed between two beams and single or combined outputs are discussed. In combination with a tapered fiber the laser was used to generate femtosecond supercontinua with up to 1.3-W average power and spectra spanning from 420 to over 1750 nm.     

Ti:Sapphire-pumped femtosecond optical parametric oscillators based on KTiOPO4 and RTiOAsO4

The design, configuration and operation of Ti:Sapphire-laser-pumped femtosecond Optical Parametric Oscillators (OPOs) using KTP and RTA are described. The results presented include configurations capable of producing transform-limited pulses shorter than 40 fs and operating with pump powers as low as 50 mW. Tunability within the 1–3 µn spectral region is demonstrated using pump-tuning alone.     

Manifestations of sub- and superluminality in filamented femtosecond laser pulse in fused silica

Filamented propagation of femtosecond laser pulses in fused silica is studied using microscopy of Femtosecond Time-resolved Optical Polarigraphy (FTOP). Both super- and subluminal translation of axial intensity maxima in a 100 fs time scale has been directly recorded. The observed spatio-temporal periodicity of the pulse shape in longitudinal and transversal directions is tentatively interpreted as two projections on X and Y axes of the same fringe pattern formed by the interference of the axial part of the beam and spontaneously generated conical beam.