全固化自锁模飞秒Ti:S激光器实验研究

首次在国内系统报道了以腔内LBO倍频Nd:YVO₄激光器为泵浦源的全固化自锁模飞秒钛宝石激光器的实验结果.设计了一种热不敏腔内LBO倍频线性折叠腔结构,获得到了25.5%的光转换效率,泵浦功率为22W时得到了5.6W的基模绿光输出功率;以该激光器为泵浦源,在线性Z型腔的基础上,使用了一种能够通过改变腔内凹面聚焦镜折叠角消除象散的方法,直接由钛宝石激光器得到了脉冲宽度为22fs、功率为300mW的光脉冲.整个激光系统稳定性好,噪音明显低于Ar³⁺激光器的泵浦情况.     

Tunable picosecond infrared pulses generated by stimulated electronic Raman scattering of a mode-locked Ti:Sapphire laser in potassium vapor

A down-conversion to the mid-infrared region by using Stimulated Electronic Raman Scattering (SERS) in potassium vapor is described. The pump radiation is a frequency-doubled regeneratively amplified Ti:Sapphire laser with a pulse duration of 2 ps, pulse energy of 0.2 mJ, and repetition rate of 10 Hz. With the pumping frequency tuned around the potassium 4s-5p transition, nearly transform-limited infrared radiation tunable between 2.2 and 3.4 m has been generated with a peak infrared energy of 12 µJ, corresponding to a quantum efficiency of 17%, and with a pulse duration of 2 ps. The present tuning range could be extended by extending the tuning range of the pump laser. In comparison, intense infrared radiation of 90 µJ energy but with a very narrow tunability around 2.9 m has also been generated by SERS in barium vapor.     

High-efficiency Nd:YVO4 laser emission under direct pumping at 880 nm

We report a high-efficiency Nd:YVO₄ laser pumped by an all-solid-state Q-switched Ti:Sapphire laser at 880 nm in this paper. Output power at 1064 nm with different-doped Nd:YVO₄ crystals of 0.4-, 1.0- and 3.0-at.% under the 880 nm pumping was measured, respectively. Comparative results obtained by the traditional pumping at 808 nm into the highly absorbing ⁴F_5/2 level were presented, showing that the slope efficiency and the threshold with respect to the absorbed pump power of the 1.0-at.% Nd:YVO₄ laser under the 880 nm pumping was 17.5% higher and 11.5% lower than those of 808 nm pumping. In a 4-mm-thick, 1.0-at.% Nd:YVO₄ crystal, a high slope efficiency of 75% was achieved under the 880 nm pumping, with an optical-to-optical conversion efficiency of 52.4%.     

Comparative study of spectral narrowing of a pulsed Ti:Sapphire laser using pulsed and CW injection seeding

We report a comparative study of a pulsed as well as continuous-wave (cw) injection seeding of a Ti:Sapphire laser pumped by aQ-switched frequency-doubled Nd³⁺:YAG laser for achieving narrow spectral bandwidth. The results have indicated that the Ti:Sapphire laser using either a pulsed or a cw injection seeding could achieve efficient energy extraction in a narrow spectral bandwidth. In the case of pulsed injection seeding, the injection energy required for the complete injection seeding critically depended upon the timing of the Ti:Sapphire laser with respect to the delayed onset of the slave laser. On the other hand, in the case of cw injection seeding, the spectral bandwidth of the Ti:Sapphire laser was efficiently narrowed down to approximately 0.01 cm^–1 with an injection power of less than 1 mW. In both types of injection seeding, characteristics observed experimentally were compared with those obtained by a numerical simulation code based on the one-dimensional rate-equation model.     

Tunable Ti: Sapphire regenerative amplifier for femtosecond Chirped-Pulse Amplification

We describe a tunable Ti:Sapphire regenerative amplifier which is used to amplify 120 fs pulses from a self-mode-locked Ti:Sapphire laser to energies in the range of 7–12 mJ from 760 nm to 855 nm. We have used three sets of cavity mirrors in the regenerative amplifier to vary the output wavelength of the laser.On leave from Institute of Laser Engineering, Osaka University, 2-6, Yamada-oka, Osaka 565, Japan (Fax: +81-6/877-4799)     

Efficient 1047 nm CW laser emission of Nd:YLF under direct pumping into the emitting level

We demonstrate a 1047 nm Nd:LiYF₄ (Nd:YLF) laser by directly pumping into the upper lasing level with a tunable Ti:Sapphire laser. The results obtained for direct upper laser level pumping at 863, 872 and 880 nm of Nd:YLF were compared with traditional 806 nm pump band excitation. Highly efficient 1047 nm continuous-wave (CW) laser emission under direct pumping at 880 nm in an 8 mm thick, 1.0 at.% Nd:YLF crystal is obtained. The slope efficiency is improved from 55.6% for traditional pumping at 806 nm to 76.3% for direct pumping at 880 nm.     

Optically pumped high-pressure DF-CO2 transfer laser

Pulsed sealed off CO₂ laser operation at 19 atm gas pressure has been demonstrated by pumping DF/CO₂/He mixtures with radiation from a pulsed DF laser. DF acts as an absorber for the pumping radiation, and the CO₂ upper laser level is populated via subsequent energy transfer. CO₂ pulse energy at 19 atm was 0.5 mJ corresponding to a photon conversion efficiency of 4% measured relative to the number of absorbed pumping photons.     

Terahertz time-domain reflection spectroscopy for high-Tc superconducting cuprates

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for reflectivity measurement with a temperature-controllable cryostat. For emission and detection of THz radiation, a Ti:Sapphire pulsed laser and photoconductive antennas are used. Two wire-grid polarizers enable us to carry out the polarized reflectivity measurements. Using our THz-TDS system, we measured the c-axis polarized reflectivity spectra for La_2−xSr_xCuO₄ single crystals (x = 0.10 and 0.13) and observed sharp Josephson plasma edges for both samples below T_c. The reflectivity spectra of the x = 0.10 sample were in good agreement with those reported previously, which confirms the validity of the system. For the x = 0.13 sample, we discuss the anomolous features of the optical spectra.     

Q-switched Er:YAG lasers resonantly pumped by Erbium fiber laser

The study describes efficient, acousto-optic Q-switching of Er:YAG laser at the 1645 nm eye-safe wavelength. For longitudinal pumping at wavelength 1532 nm, linear-polarized 10 W Erbium fiber laser radiation was used. The investigated Er:YAG crystals were 25 and 40 mm long and their Erbium concentration was 0.20 and 0.25%, respectively. For giant pulse generation, a fused silica acousto-optic modulator was inserted inside the Er:YAG laser oscillator. For a maximum incident pump power of 7.95 W, pulse energy up to 4.1 mJ was generated with pulse duration 34 ns at 500-Hz repetition rate; the corresponding peak power was 119 kW.     

Ultra fast melting process in femtosecond laser crystallization of thin a-Si layer

In this paper, we investigated the mechanism of crystallization induced by femtosecond laser irradiation for an amorphous Si (a-Si) thin layer on a crystalline Si (c-Si) substrate. The fundamental, SHG, THG wavelength of a Ti:Sapphire laser was used for the crystallization process. To investigate the processed areas we performed Laser Scanning Microscopy (LSM), Transmission Electron Microscopy (TEM) and Imaging Pump-Probe measurements. Except for 267 nm femtosecond laser irradiation, the crystallization occurred well. The threshold fluences for the crystallization using 800 nm and 400 nm femtosecond laser irradiations were 100 mJ/cm² and 30 mJ/cm², respectively. TEM observation revealed that the crystallization occurred by epitaxial growth from the boundary surface between the a-Si layer and c-Si substrate. The melting depths estimated by Imaging Pump-Probe measurements became shallower when the shorter wavelength was used.     

Nd:YAG waveguide laser with 1.3 W output power, fabricated by direct femtosecond laser writing

Tracks of modified material were written with femtosecond-laser pulses in neodymium-doped YAG crystals. Due to a stress-induced change of the refractive index, waveguiding beside the tracks and between two adjacent tracks with a distance of approximately 25 μm was observed. Loss measurements resulted in guiding losses of about 1.6 dB/cm for the double track waveguide. Spectroscopic investigations of the ⁴ F _3/2→⁴ F _11/2 transmission lines of the neodymium ions, which are close to the modified region, revealed a small stress-induced red shift of the lines. Laser oscillation of single-track waveguides and double-track waveguides was demonstrated with Ti:Sapphire laser pumping at a wavelength of 808 nm. Best laser performance with about 1.3 W output power at 2.25 W launched pump power was achieved using a double-track waveguide with a separation of 27 μm at an outcoupling transmission of 95%.     

Pulsed inductive discharge CO2 laser

A pulsed inductive discharge CO₂ laser with a wavelength of 10.6 μm has been created for the first time. The excitation system of a cylindrical pulsed inductive discharge (pulsed inductively coupled plasma) in the gas mixture of CO₂:N₂:He was developed. The temporal and energy parameters of the laser radiation were investigated. The maximum inductive discharge CO₂ laser radiation energy of 104 mJ was achieved. An average power of 3.2 W was obtained at laser generation energy of 65 mJ and pulse repetition rate of 50 Hz. In the cross-section, the laser radiation had the ring shape with an external diameter of 34 mm and thickness of 4-5 mm. The measured divergence of laser radiation was 12 mrad.     

Broad-linewidth laser absorption measurements of oxygen between 211 and 235 nm at high temperatures

Absorption coefficient data are presented for molecular oxygen at temperatures between 1100 and 2000 K and discrete wavelengths between 211 and 235 nm. Measurements were made behind reflected shock waves using broad-linewidth ultraviolet laser radiation generated from a frequency-quadrupled, tunable, pulsed Ti:Sapphire laser. Test mixtures consisting of 15% O₂, 15% He and balance Ar were used to minimize the influence of vibrational relaxation on the reflected shock temperature. The experimental results are in good agreement with theoretical calculations and confirm that discrete features from the Schumann–Runge system dominate between 211 and 235 nm at temperatures higher than 1100 K.     

Flashlamp-pumped Ti:Sapphire laser with different rods grown by Czochralski and Verneuil methods

The design and the development of a flashlamp-pumped Ti:Sapphire laser is described. Design criteria are discussed and performance improvements using different types of fluorescent UV converters or filters, such as organic dyes or doped glass are presented. We have tested different laser rods at various Ti-concentrations obtained by Verneuil or Czochralski growth techniques. The maximum laser output energy of 540 mJ with a differential efficiency up to 1% was achieved by using only a pyrex filter surrounding the laser rod.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Suppression of gain narrowing in multi-TW lasers with negatively and positively chirped pulse amplification

A laser scheme implementing a combination of negatively and positively chirped pulse amplification (NPCPA) is demonstrated. This method of amplification suppresses spectral narrowing typically appearing in chirped pulse amplification (CPA) lasers thus supporting pulse spectrum much broader than a conventional CPA. With a NPCPA Ti:Sapphire laser we have achieved laser pulses of 50 nm spectral width and 150 mJ energy without any additional spectral correction. The scheme appears as an easy and reliable solution to preserve spectral bandwidth in Ti:Sapphire lasers, especially at high power levels up to the Petawatt regime. PACS 42.60 By, Lh; 42.65 Re     

钛宝石泵浦掺钕碲酸盐玻璃激光器

室温下钛宝石激光泵浦接钛碲酸盐块体玻璃激光器运载成功，激光波长1066nm，激光阈值4．20mJ。斜率效率14．7％。     

Continuous wave laser operation of Yb<Superscript>3+</Superscript>:YVO<Subscript>4</Subscript>

We have demonstrated the continuous wave laser operation of Yb³⁺:YVO₄. For Ti:Al₂O₃ laser pumping at 985 nm, a maximum slope efficiency of 41.1% and a threshold pump power of 76 mW were obtained. The maximum output power was 433 mW at a laser wavelength of 1037 nm.Using a cw diode laser around 974 nm as a pump source, a slope efficiency of 10.9% and a maximum output power of 152 mW were achieved at a laser wavelength of 1039 nm. The laser threshold pump power was 608 mW with respect to the absorbed pump power. The effective emission cross-sections for the ²F_5/2²F_7/2 transition were determined using the Füchtbauer–Ladenburg equation. The maxima of the effective absorption and emission cross-sections were found at 984.5 nm (6.74×10^-20 cm²) in -po larization and 985.5 nm (4.28×10^-20 cm²) also in -p olarization. The upper laser level lifetime was measured with suppression of radiation trapping and is around 318 s. PACS 42.55.Rz; 42.55.Xi; 42.70.Hj     

Lasing and fluorescence properties of dye-doped xerogel

Dye-doped SiO₂-xerogels are prepared for application as laser active medium. An optical surface and a sealing of the samples are achieved by covering them with silicon oil and an optical glass window. All optical properties were compatible with the assumption that the active chromophores are dissolved in the liquid which fills the pores of the xerogel. A replacement of the dye molecules destroyed by the pumping radiation via diffusion was observed. For stilbene 3 a diffusion coefficient ofD = 0.35 × 10^–9 m²/s was determined. Thus a durable operation of the laser with low repetition rates is possible.Notation as in [10]     

Orange,red and deep-red flashlamp-pumped Pr3+:LiYF4 laser with improved output energy and efficiency

Three laser transitions (607.2 nm, 639.5 nm, 720.9 nm) in a flashlamp-pumped Pr³⁺:LiYF₄ laser at room temperature are investigated. Cerium ions (Ce³⁺) doped into the fused-quartz envelope of the flashlamp efficiently transfer strong UV radiation from pumping light into the absorption region of Pr³⁺ ions ( = 420–480 nm), so that the slope efficiency of all three laser emissions could be increased by almost 100% compared to excitation with a pure quartz flashlamp. At a pump energy of 30 J the output energy of these three laser emissions reached 4.7 mJ, 87 mJ and 30 mJ.     

The growth of nanoscale periodic and dot-like structures on the surface of stainless steel with femtosecond laser pulses in the dry and wet ambient environment

The present work deals with growth of nanoscale periodic and dot-like structures on the surface of stainless steel (SS) by the irradiation of femtosecond laser pulses. For this purpose Ti: Sapphire femtosecond laser pulses (wavelength of 800 nm, pulse length of 25 fs and pulse repetition rate of 1 kHz) were employed in a dry (air) and liquid confined (deionized water and ethanol) environments. The targets were exposed to 1000 succeeding pulses for various fluences ranging from 50 to 150 mJ?cm^?2. Nanoscale structures including ripples, and dots were observed by SEM analysis. The growth and dependence of structure-formation on the ambient environment and laser fluence in both central as well as peripheral ablated areas is systematically investigated. The development of nanostructures and nanoripples is correlated with structural analysis carried out by micro Raman spectroscopy.