Efficient output coupling of intracavity high-harmonic generation

We demonstrate a novel technique for coupling extreme-ultraviolet (XUV) harmonic radiation out of a femtosecond enhancement cavity. We use a small-period diffraction grating etched directly into the surface of a dielectric mirror. For the fundamental light, this element acts as a high reflector. For harmonic wavelengths, it acts as a diffraction grating, coupling XUV radiation out of the cavity. Using this method, we observed the third through twenty-first odd harmonics with a dramatic increase in usable power over previous results of high-harmonic generation at high repetition rates.     

Quantitative measurement of timing and phase dynamics in a mode-locked laser

We present results of an experimental study of the timing and phase dynamics in a mode-locked Ti:sapphire laser. By measuring the response of two widely spaced comb lines to a sinusoidal modulation of the pump power, we determine quantitatively the response of both the central pulse time and the phase. Because of the distinct response of the pulse energy, central frequency, and gain to the modulation, we are able to distinguish their contributions to the timing and phase dynamics.     

Validation of high-performance thin-layer chromatographic methods for the identification of botanicals in a cGMP environment

Current Good Manufacturing Practices (cGMP) for botanicals stipulates the use of appropriate methods for identification of raw materials. Due to natural variability, chemical analysis of plant material is a great challenge and requires special approaches. This paper presents a comprehensive proposal to the process of validating qualitative high-performance thin-layer chromatographic (HPTLC) methods, proving that such methods are suitable for the purpose. The steps of the validation process are discussed and illustrated with examples taken from a project aiming at validation of methods for identification of green tea leaf, ginseng root, eleuthero root, echinacea root, black cohosh rhizome, licorice root, kava root, milk thistle aerial parts, feverfew aerial parts, and ginger root. The appendix of the paper, which includes complete documentation and method write-up for those plants, is available on the J. AOAC Int. Website (http://www.atypon-link.com/AOAC/loi/jaoi).     

Frequency metrology with a turnkey all-fiber system

The repetition rate and carrier-envelope phase offset frequencies of a turnkey, all-fiber-based continuum generator were phase locked to a hydrogen maser. The frequency of the hydrogen maser was calibrated with a highly stable cesium atomic clock, and therefore a fully phase-locked optical frequency comb with well-defined absolute frequencies was obtained. In contrast with the commonly used Ti:sapphire-laser-based systems, we have accomplished a fully turnkey system with no user-adjustable parts. To evaluate the performance of this novel system, we performed absolute frequency measurements in the telecommunications region and at 1064 nm and compared them with our traditional Ti:sapphire-based comb.     

Quantitative determination of beta-asarone in calamus by high-performance thin-layer chromatography

A quantitative high-performance thin-layer chromatographic method for determination of beta-asarone in Calamus rhizome was developed and validated. The method is suitable for proper identification of Acorus calamus. Through the use of caffeine-modified silica gel as the stationary phase and toluene-ethyl acetate (93 + 7, v/v) as the mobile phase, beta-asarone is baseline separated from its isomer alpha-asarone. Scanning densitometry with absorption measurement at 313 nm allows specific, accurate, and precise quantification of beta-asarone. The working range of 40 to 200 ng absolute of the target substance is sufficient to establish whether a given sample passes the limit test of 0.5% maximum as required by the Swiss Pharmacopoeia.     

Suppression of Q-switched mode locking and break-up into multiple pulses by inverse saturable absorption

Suppression of Q-switching during mode locking is extremely important for the successful development of continuous-wave mode-locked solid-state and fiber lasers. In this paper we show that the use of inverse saturable absorption, such as two-photon absorption and free carrier absorption, can suppress Q-switching instabilities and expand the continuous-wave operating regime in solid-state and fiber lasers. Conversely, an excess of inverse saturable absorption may lead to pulse-energy limiting and induce a break-up into multiple pulses. To analyze the advantages and limitations of adding this nonlinearity, we derive and discuss the mode-locking stability criteria in the presence of inverse saturable absorption. Useful asymptotic formulas for the absorber design employing inverse saturable absorption are also derived.     

Doping-induced changes in the saturable absorption of monolayer graphene

Graphene is a broadband, fast saturable absorber well suited for passive mode-locking of lasers. The broadband absorption, ultra-short recovery time, and low cost of graphene absorbers compare favorably with traditional semiconductor saturable absorber mirrors (SESAMs). However, it remains difficult to tailor the parameters of a monolayer graphene absorber such as the modulation depth and the insertion loss; this limits the absorber??s design freedom, which is often required for mode-locking without Q-switching instability. We demonstrate in this work that, by hole-doping graphene chemically to various Fermi levels, the modulation depth and insertion loss are modified. Further control of graphene??s saturable absorption by electric-field gating and its application to active suppression of Q-switching in lasers is discussed.     

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2³S state atoms. We analyze schemes for measuring the two-photon 2³S →4³S interval, and for resonant two-photon excitation to high Rydberg states, 2³S →3³P →n³S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1¹S or 2¹S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 1¹S →2¹S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1¹S, 2¹S and 2³S states are all similar, small, and positive.     

Phase-locked widely tunable optical single-frequency generator based on a femtosecond comb

We present an arbitrary optical single-frequency generator based on a femtosecond optical frequency comb. The functions of this device are comparable to those of a radio-frequency synthesizer. However, this device operates at hundreds of terahertz. The absolute frequency accuracy of this synthesizer is approximately 1 kHz at a 282 THz carrier frequency. The stability is approximately 2 x 10(-14) at 100 s, and the tuning speed exceeds 30 GHz/s. This source demonstrates the integration of a phase-locked optical comb into a versatile and easy-to-use system for the generation of tunable, absolute optical frequencies. By using downconversion, one could generate tunable terahertz frequencies that are phase locked to a microwave reference, such as a Cs atomic clock, and high-precision interferometry could benefit greatly from the stability and accuracy of this widely tunable source.