Absolute frequency measurements of wavelength standards 532 nm, 543 nm, 633 nm and 1540 nm

This paper describes the results of absolute frequency measurements of primary wavelength standards 633 nm, 543 nm, 532 nm, (iodine stabilized) and 1540 nm (acetylene stabilized) in CMI. The values obtained with Menlo Systems femtosecond frequency comb in CMI are compared with previous measurements of the same standards in BIPM, BEV and MPQ. Measured sub-Doppler linewidths and relative intensities of several hyperfine spectral components of iodine molecule are also presented.     

High-repetition-rate, intracavity-pumped KTP OPO at 1572 nm

An efficient, eye-safe, high-repetition-rate, intracavity optical parametric oscillator (IOPO) inside an acousto-optically Q-switched Nd:YVO₄ laser end pumped by a 30-W fiber-coupled diode laser was demonstrated. The pumping, acousto-optically Q-switched Nd:YVO₄ laser gives 3-W average output power at 1064-nm wavelength at 40-kHz repetition rate. An additional separating mirror, x-cut KTP crystal and output coupler highly reflective at 1064-nm and partially transparent at 1572-nm wavelengths form a flat–flat IOPO resonator of 35-mm length. We have achieved 3-ns-duration pulses for 20-mm-long KTP and 4-ns-duration pulses for 30-mm-long KTP, respectively. More than 8-kW-peak-power pulses with an average power of 1.5 W at the signal wavelength for 40-kHz repetition rate were demonstrated. Due to the intracavity spatial cleaning effect, a near diffraction limited signal beam was achieved despite a relatively worse beam quality of the pumping beam. Conversion efficiencies of 50% with respect to Q-switched output at 1064-nm wavelength and 11% with respect to diode pump power were achieved. PACS 42.55.Xi; 42.60.Gd; 42.65.Yj     

1318.8 nm/1338.2 nm simultaneous dual-wavelength Q-switched Nd:YAG laser

A diode-side-pumped simultaneous dual-wavelength Q-switched Nd:YAG laser around 1.3 μm is demonstrated. With the pumping power of 480 W, a peak power of 43 kW was obtained at the repetition rate of 5 kHz. The maximum average output power is up to 43 W with the slope efficiency of 20.5%, and the pulse duration is 200 ns. This dual-wavelength Q-switched laser with high peak power has a balanceable and stable proportion of dual-wavelength output. PACS 42.55.Xi; 42.60.Gd     

Time-resolved hydrino continuum transitions with cutoffs at 22.8 nm and 10.1 nm

Spectra of low energy, high current pinch discharges in pure hydrogen, oxygen, nitrogen, and helium were recorded in the EUV region, and continuum radiation was only observed from hydrogen [www.blacklightpower.com/pdf/GEN3_Harvard.pdf; Int. J. Hydrogen Energy 35, 8446 (2010); Cent. Eur. J. Phys. 8, 318 (2010)]. The continuum radiation bands at 10.1 and 22.8 nm and going to longer wavelengths for theoretically predicted transitions of hydrogen to lower-energy, so called “hydrino” states, was observed first at blacklight power, Inc. (BLP) and reproduced at the Harvard center for astrophysics (CfA). Considering the low energy of 5.2 J per pulse, the observed radiation in the energy range of about 120 eV to 40 eV and reference experiments, no conventional explanation was found to be plausible, including electrode metal emission, Bremsstrahlung radiation, ion recombination, molecular or molecular ion band radiation, and instrument artifacts involving radicals and energetic ions reacting at the CCD and H₂ re-radiation at the detector chamber. To further study these continuum bands assigned to hydrinos, time resolved spectra were performed that showed a unique delay of the continuum radiation of about 0.1 μs and a duration of < 2   μs following the high-voltage pulse consistent with the mechanism of recombination to form the optimal high-density atomic hydrogen in the pinch that permits the H–H interactions to cause the hydrino transitions and corresponding emission.     

Sub-50 nm UV-curing nanoimprint based on fluoropolymer,CYTOP, mold

In addition to the advantages of conventional fluoropolymers (i.e., chemical resistance, optical transparency, especially low surface energy), CYTOP, a commercially available cyclic fluoropolymer, can be dissolved in a special fluorinated solvent, thus it can be coated on the substrate by a spin-coating or solution casting method. In this paper, we introduced CYTOP as a rapid prototyping mold material for UV-curing nanoimprint lithography. The CYTOP mold is fabricated by a thermal nanoimprint technique on a quartz substrate or a direct solution casting on a master mold. Nanostructures with different geometries and down to sub-50 nm feature size are faithfully duplicated by CYTOP molds through a UV-curing imprint process. The CYTOP mold preserves its lower surface energy property after 20 repeated imprint cycles and there is no peeling off problem or contamination and damage on CYTOP mold observed. However, it is found that the high aspect ratio nanostructures on the CYTOP mold are tilted or deformed after separation with the master mold.     

Megawatt peak power, 1 kHz, 266 nm sub nanosecond laser source based on single-crystal fiber amplifier

We report the realization of a UV source based on the fourth harmonic generation with LBO/BBO of a Nd:YAG passively Q-switched oscillator amplified in a single-crystal fiber. With careful optimization of the nonlinear components and parameters, we obtain 530 mW average power at 266 nm with pulses of 540 ps at the repetition rate of 1 kHz, which represents a 22.7 % total conversion efficiency from IR to UV and nearly 1 MW peak power. The beam quality M ² is measured to be below 2.     

Zone-plate interferometry at 13 nm wavelength

We report on interferometry using a two-zone-plate common-path interferometer operating at a wavelength of 13 nm. The interferometer was set up with a laser-driven high-harmonic source emitting radiation with the high degree of spatial and temporal coherence necessary for interferometry. The interferometer is suited for investigations of the coherence properties of the light source employed, as well as for simultaneous measurements of the real and imaginary part of the complex index of refraction in the 100 eV regime. This is demonstrated in a proof of principle experiment with a piece of Zr-foil as the phase-shifting and absorbing sample. PACS 41.50.+h; 42.65.Ky; 42.87.Bg; 07.60.Ly     

A laser system providing tunable, narrow-band radiation from 35 nm to 2 μm

We describe a laser system that readily provides radiation tunable from 2 μm in the infra-red to 35 nm in the extreme ultraviolet spectral range. The broad spectral range is covered through a range of non-linear processes such as Raman shifting and high-order harmonic generation. Pulses with duration of tens of picoseconds are obtained. The relative bandwidth of the radiation is δλ/λ=10^-4, comparable with what can be achieved by using high-resolution monochromators at state-of-the-art synchrotron beamlines. We discuss different methods for characterising the radiation in this wide wavelength regime. We also discuss the capabilities of the system from the measured parameters. Received: 12 December 2000 / Revised version: 8 March 2001 / Published online: 27 April 2001     

Laser induced incandescence determination of the ratio of the soot absorption functions at 532 nm and 1064 nm in the nucleation zone of a low pressure premixed sooting flame

In this work, the two-excitation wavelength laser induced incandescence (LII) method has been applied in a low-pressure premixed methane/oxygen/nitrogen flame (equivalence ratio 2.32) to determine the variation of the ratio of the soot absorption functions at 532 nm and 1064 nm E(m,532 nm)/E(m,1064 nm) along the flame. This method relies on the comparison of LII signals measured upon two different excitation wavelengths (here 532 nm and 1064 nm) and with laser fluences selected in such a way that the soot particles are equally laser-heated. The comparison of the laser fluences at 532 nm and 1064 nm leads to an easy determination of E(m,532 nm)/E(m,1064 nm). The reliability of the method is demonstrated for the first time in a low pressure flame in which the soot nucleation zone can be spatially resolved and which contains soot particles acting differently with the laser fluence according to their residence time in the flame. The method is then applied to determine the profile of E(m,532 nm)/E(m,1064 nm) along the flame. A very important decrease of this ratio is observed in the region of nascent soot, while the ratio remains constant at high distance above the burner. Implication on temperature determination from spectrally resolved measurement of flame emission is studied.     

3 GHz, fundamentally mode-locked, femtosecond Yb-fiber laser

We demonstrate a fundamentally mode-locked Yb-fiber laser with 3?GHz repetition rate and ～206 fs pulse duration. The laser incorporates two enabling technologies: a 1?cm heavily Yb-doped phosphate glass fiber as the gain medium and a high-dispersion (-1300 fs²) output coupler to manage cavity dispersion. The oscillator self-starts and generates up to 53?mW average power.     

High-efficiency direct-pumped Nd:YLF laser operating at 1321 nm

We present a high-efficiency Nd: LiYF₄ (Nd:YLF) laser operating at 1321 nm pumped directly into the emitting level, ⁴F_3/2. The linear polarization of the pump diode laser was maintained by a short fiber. At the absorbed pump power of 7.3 W, as high as 3.6 W of continuous-wave output power at 1321 nm is achieved. The slope efficiency with respect to the absorbed pump power was 0.52. To the best of our knowledge, this is the first demonstration of such a laser system. Comparative results obtained for the pump with a diode laser at 806 nm, into the highly absorbing ⁴F_5/2 level, are given in order to prove the advantages of 880 nm wavelength pumping.     

Tunable diode laser spectroscopy of benzene near 1684 nm with a low-temperature VCSEL

We describe the application of a long-wavelength vertical-cavity surface-emitting laser (VCSEL) with extended tuning range to the detection of benzene vapor at atmospheric pressure. A benzene absorption feature centered at 1684.24 nm was accessed by reducing the heat sink temperature of a VCSEL designed for room-temperature operation to −55°C. This allowed us to increase the injection current and thus to extend a single-scan tuning interval up to 46.4 cm⁻¹ or 13.2 nm around a central wavelength of 1687.4 nm. Five absorption lines of methane in the 5903–5950 cm⁻¹ range could be acquired within single laser scans at a repetition rate of 500 Hz. A benzene absorption feature between 5926 and 5948 cm⁻¹ was recorded for concentration measurements at atmospheric pressure using a single-pass 1.2 m absorption cell. A 50 ppmv mixture of CH₄ in N₂ was introduced into the cell along with benzene vapor to calibrate benzene concentration measurements. Benzene mixing ratios down to ∼90 ppmv were measured using a direct absorption technique. The minimum detectable absorbance and detection limit of benzene were estimated to be ∼10⁻⁴ and 30 ppmv, respectively. Using the wavelength modulation technique, we measured a second harmonic sensor response to benzene vapor absorption in air at atmospheric pressure as a function of modulation index. We conclude that a low-temperature monolithic VCSEL operating near 1684 nm can be employed in compact benzene sensors with a detection limit in the sub-ppm range.     

Random fiber laser operating at 1,115 nm

We experimentally demonstrate a random fiber laser operating at 1,115 nm using a LD-pumped Yb-doped fiber laser as the pump source. We achieve about 270 mW lasing output in a 50 km standard communication optical fiber with slope efficiency more than 28 %. A new wavelength is provided for the application of random distributed feedback fiber lasers as light sources.     

Laser emission at 1077 nm in Nd3+-doped calcium aluminosilicate glass

Laser emission at 1077 nm (⁴F_3/2⁴I_11/2) was demonstrated in Nd³⁺-doped low silica content calcium aluminosilicate (LSCA) glass under 810-nm pumping by a Ti:sapphire laser. The slope efficiency of the laser emission was found to be 34% with a 20-mW threshold power. The excited-state absorption (ESA) and stimulated emission were measured between 850 and 1520 nm. The results showed two ESA bands from 931 to 1070 nm (⁴F_3/2²D_3/2,²G_9/2,⁴G_11/2,²K_15/2) and from 1160 to 1414 nm (⁴F_3/2,⁴G_9/2⁴G_7/2,²K_13/2). The stimulated emission for the laser transition was found to be 45-nm broad with a peak value of 1.8×10^-20 cm². Thermal and mechanical properties of the LSCA glass were compared to the ED-2 and LHG-8 glasses. While the mechanical properties of the LSCA glass are superior to those of the commercial glasses, its temperature coefficient of optical path length is the highest among the three hosts. PACS 42.60.Pk; 42.70.Ce; 78.66.Jg     

Femtosecond photodissociation dynamics of HNO3 after exitation of the S3-state at 200 nm

The photodissociation dynamics of HNO₃ in the electronic S₃ (2 ¹ A ^′ ) state leading to the fragments OH and NO₂ was investigated in real time. HNO₃ was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO₂ by three-photon ionization). A 2 ¹ A ^′ lifetime of 60–80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO₂ ^* leading to products NO(X ² Π) and O(³ P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO₂ from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO₂. Received: 12 November 1999 / Published online: 13 July 2000     

224 nm segmented hollow-cathode silver ion laser

The segmented hollow-cathode discharge arrangement is used the first time to excite the 224 nm Ag II laser transition. Quasi-continuous output power of 45 mW is obtained during the 300 s current pulses at optimal discharge conditions (10 hPa of He+4% Ar buffer gas) for discharge current of 3 A. No power saturation is observed up to this current value. An average output power of 0.75 mW is reached using pulse repetition frequency of 190 Hz. The longitudinal mode structure of the TEM₀₀ transversal mode is measured by means of a scanning confocal Fabry–Perot interferometer. Two-mode operation is found to be dominant at high current values. Attempts and suggestions are made to prolong the lifetime of the laser tube. PACS 42.55.Lt; 42.60.Lh; 52.80.-s     

Multi-millijoule, nonlinear preamplifier for high intensity femtosecond Yb:YAG chirped-pulse amplification lasers at 1030 nm

We report the highest energy broadband laser pulses at a center wavelength of 1030 nm based on optical parametric chirped-pulse amplification (OPCPA). We have demonstrated amplification of 1030 nm femtosecond laser pulses from a broadband Yb oscillator to over 6.5 mJ with a total gain of greater than 10⁷ achieved in a single pass through only 56 mm of gain material at a 10 Hz repetition rate. The amplified spectral bandwidth of 10.8 nm affords recompression to a 230 fs pulse duration following amplification. As an alternative to the regenerative amplifier (RA) this system is one of the more promising candidates for realizing compact, high intensity, direct diode-pumped, high repetition rate femtosecond Yb:YAG chirped-pulse amplification (CPA) in laser systems.     

980 nm Yb-doped single-mode fiber laser and its frequency-doubling with BIBO

A quasi-three-level Yb-doped single-mode fiber laser at 980 nm by adopting two 0° fiber ends as cavity mirrors generated a total output power of 1.32 W with the slope efficiency of 75.3%. The fiber length was 36.5 cm close to the optimal theoretical fiber length. The corresponding optical conversion efficiency was 66% from the incident pump power at 946 nm to the laser power at 980 nm. Through frequency-doubling with BIBO crystal, a total output power of 15 mW at 490.8 nm was obtained.     

Evaluation of hybrid ytterbium–neodymium laser amplification at 1053 nm

We present here the modeling of a hybrid neodymium/ytterbium laser chain. The laser chain is modeled in energy, spectrum and gain for a relevant choice of ytterbium host materials. Special attention is given to spectral matching of both technologies. The model performance is benchmarked by an experimental setup of a Yb:glass regenerative-amplifier. Results indicate Yb:CaF₂ to be the best material for Nd:glass coupling.     

Linewidth studies on Cd+ 636.0 nm and 537.8 nm lines excited in a He-Cd hollow cathode discharge

Hollow cathode (HC) lasers usually operate in a single axial mode without any optical selection. This is attributed to the large homogeneous linewidth of the gain curve due to the relatively high filling pressure of these lasers. Collisional and Doppler broadening (Δν_C and Δν_D) of the Cd⁺ 636 nm and Cd⁺ 538 nm lines (laser transitions of the HeCd⁺ laser) excited in a HC discharge tube were determined using a Fabry–Perot interferometer technique. It was found that in the pressure range 7–25 mbar Δν_D was nearly constant, while, as expected, Δν_C increased linearly with pressure. The broadening constants were α(636 nm)= (47±2) MHz/mbarand α(538 nm)=(11.8±0.5) MHz/mbar. The first constant is large enough to explain single-mode operation of the red HeCd⁺ laser; but in the case of the green laser, the exact reason for the single-mode operation remained unclear. Received: 23 November 2000 / Revised version: 30 March 2001 / Published online: 7 June 2001