Showing papers in "Laser Physics Letters in 2007"

Ultrafast 2D-IR vibrational echo spectroscopy: A probe of molecular dynamics

Abstract: Ultrafast two dimensional infrared (2D-IR) vibra- tional echo spectroscopy has emerged as a powerful method for the study of molecular dynamics under thermal equilibrium con- ditions occurring on ultrafast time scales. Here, we describe ex- perimental details of 2D-IR vibrational echo spectroscopy in- cluding the experimental setup, pulse sequence, heterodyne de- tection, and extraction of the mainly absorptive part of the 2D- IR spectrum. As an experimental example, the measurements of the hydrogen bond dynamics of neat water and water in a high concentration of NaBr solution are presented and compared. The experiments are performed on OD stretching vibration of dilute HOD in H2O to eliminate contributions from vibrational exci- tation transport. A new experimental observable for extracting dynamical information that yields the frequency-frequency cor- relation function is presented. The observable is the inverse of the center line slope (CLS) of the 2D spectrum, which varies from a maximum of 1 to 0 as spectral diffusion proceeds. The CLS is the inverse of the slope of the line that connects the maxima of the peaks of a series of cuts through the 2D spectrum that are parallel to the frequency axis associated with the first radiation field-matter interaction. Comparisons of the dynamics obtained from the data on water and the concentrated NaBr solutions show that the hydrogen bond dynamics of water around ions are much slower than in bulk water.

Compact 0.56 Petawatt laser system based on optical parametric chirped pulse amplification in KD*P crystals

Abstract: 560 TW peak power has been achieved experimentally using a Cr:forsterite master oscillator at 1250 nm, a stretcher, three optical parametrical amplifiers based on KD*P crystals providing 38 J energy in the chirped pulse at 910 nm central wavelength, and a vacuum compressor providing 43 fs pulse duration. To our knowledge, it is a world-record OPCPA system and one of the five most powerful laser systems currently available.

Oscillation spectral range of Yb-doped fiber lasers

Abstract: In this brief review we consider the main factors determining the oscillation spectral range of cladding-pumped ytterbium-doped fiber lasers (YDFLs) and the results obtained for lasers emitting at various wavelengths. Like erbium-doped fiber amplifiers we suggest dividing the oscillation spectral range of YDFLs into three bands, namely: convenient (C-band), short (S-band), and long (L-band). Polymer-coated double-clad fibers with the inner cladding having a size of more than 100 μm allows one to get efficient operation in the convenient range (C-band): 1060 - 1130 nm. To get an oscillation within the S-band (976 - 980 nm and 1020 - 1060 nm) it is necessary to use active fibers with a small square of the inner cladding. Heating of the active fiber gives the possibility to get lasing within the L-band λ > 1130 nm). Another way to get an emission in this spectral range is by the application of long-wave pumping by a C-band YDFL. Also, we indicate some features that require further study.

Passive mode-locking with carbon nanotube saturable absorber in Nd:GdVO4 and Nd:Y0.9Gd0.1VO4 lasers operating at 1.34 μm

Abstract: A flash-lamp pumped passively mode-locked neodymium-doped vanadate lasers operated at 1.34 μm wavelength have been developed using a novel saturable absorber consisted of single-wall carbon nanotubes incorporated in a polymer matrix. Laser pulses with the output energy up to 50 μJ and the duration of 30 ps were generated.

Reversible photo-darkening and resonant photobleaching of Ytterbium-doped silica fiber at in-core 977-nm and 543-nm irradiation

Abstract: We report experimental results showing novel features in photo-darkening and photo-bleaching of a heavily-doped Ytterbium silica fiber exposed to in-core 977-nm and 543-nm irradiation. It is shown that pumping of the fiber at the resonant 977-nm wavelength leads to an increase of absorption in the spectrally wide range (400–1000 nm, photo-darkening) and a simultaneous decrease of the resonant (Yb3+) absorption coefficient near ~ 1 μm (resonant photo-bleaching). Such a character of the Ytterbium fiber spectra transformations allows us to propose that they are a signature of the 977-nm light-induced Yb3+ → Yb2+ conversion process, with the spectrally wide absorption, or photo-darkening, to arise owing to the Yb2+ centers formation and the resonant photo-bleaching to stem from the corresponding decrease of Yb3+ ions concentration. It is as well demonstrated that an exposure of the Ytterbium fiber, where the aforementioned transformations have occurred, to the 543-nm light results in a partial return of the initial fiber properties – bleaching of the spectrally wide excess loss in the visible and increasing of the resonant 977-nm (Yb3+) absorption peak. Therefore, we reveal a reverse process, Yb2+ → Yb3+, possibly taking place in the last case.

Single frequency single polarization DFB fiber laser

Abstract: On the base of the phase shifted fiber Bragg grating of 4 cm length written in the active non-PM Yb-doped fiber, a resonator with distributed feedback (DFB) has been formed. The single frequency single polarization unidirectional generation with the output power up to 4.5 mW has been obtained. The spectral and energy performances of the single frequency DFB fiber laser depending on the pump laser parameters and ways of pump wave coupling have been investigated. The competition between orthogonal polarization states has been also studied, and the single polarization regime of 0.5 mW power has been achieved.

Optical coherence tomography imaging depth enhancement by superficial skin optical clearing

Abstract: Non-invasive imaging of human skin and human skin tissues by optical coherence tomography (OCT) is a field exciting a great deal of interest especially in terms of day-to-day medical and biomedical diagnostics. We develop rapid scanning OCT system for non-invasive real-time imaging of topical skin tissues in vivo. We investigate the possibility of OCT image enhancement along the transcutaneous diffusion into the skin an optical clearing agent. Diffusion of optical clearing agents, e.g. such as glycerol, temporary increases transparency of topical skin tissues that allows for unrestricted sounding radiation permeating deeper into the skin. Increasing transparency of superficial skin tissues significantly improves depth of imaging, image contrast and spatial resolution. Thus, topical optical clearing of skin enable applying low power light source (0.5 – 0.2 mW) and achieve the probing depth up to 1.5 – 1.7 mm. Current approach is able providing images of superficial human blood vessels 0.2 – 1 mm in diameter that are not visible by conventional OCT at normal conditions. The presented results illustrate, for the first time of our knowledge, the images of topical skin vessels with 12 μm spatial resolution before and after optical clearing.

Bose-Einstein condensation and gauge symmetry breaking

Abstract: The fundamental problem is analyzed, the relation between Bose-Einstein condensation and spontaneous gauge symmetry breaking. This relation is largerly misunderstood in physics community. Numerous articles and books contain the statement that, though gauge symmetry breaking helps for describing Bose-Einstein condensation, but the latter, in principle, does not require any symmetry breaking. This, however, is not correct. The analysis is based on the known mathematical theorems. But in order not to overcomplicate the presentation and to make it accessible to all readers, technical details are often omitted here. The emphasis is made on the following basic general facts: Spontaneous breaking of gauge symmetry is the necessary and sufficient condition for Bose-Einstein condensation. Condensate fluctuations, in thermodynamic limit, are negligible. Their catastrophic behavior can arise only as a result of incorrect calculations, when a Bose-condensed system is described without gauge symmetry breaking. It is crucially important to employ the representative statistical ensembles equipped with all conditions that are necessary for a unique and mathematically correct description of the given statistical system. Only then one is able to develop a self-consistent theory, free of paradoxes.

An innovative solid-state laser for engine ignition

Abstract: A compact high peak power, passively Q-switched, longitudinally diode-pumped laser was specially constructed for laser ignition. Nd:YAG was chosen as laser active medium and Cr4+:YAG as the saturable absorber medium. For pumping, a laser diode emitting at 808 nm with an output power of up to 300 W and pulse duration of up to 500 μs was employed. Experimental studies were carried out to find the optimum laser design parameters. These are the output coupler reflectivity, initial transmission of the saturable absorber, doping concentration of the Nd:YAG, oscillator length and the pump light distribution within the Nd:YAG crystal. Single pulses at 1064 nm with energies of 6 mJ and durations under 1.5 ns were achieved in a TEM00 mode.

Performance of diode-pumped passively Q-switched mode-locking Nd:GdVO4/KTP green laser with Cr4+:YAG

Abstract: We demonstrate a diode-pumped passively Q-switched and mode-locking intracavity frequency-doubled Nd:GdVO4/KTP green laser with a Cr4+:YAG saturable absorber. Nearly 95% modulation depth for the mode-locked pulses inside the Q-switched envelope has been obtained. The Q-switched envelope pulses with the pulse width (FWHM) of 42 ns and the repetition rate of 8.33 kHz were obtained, at the incident pump power of 6 W. The repetition rate of the mode-locked green pulses inside the Q-switched envelope was 417 MHz.

Passive harmonically mode‐locked erbium‐doped fiber laser with scalable repetition rate up to 1.2 GHz

Abstract: Passive harmonically mode-locked fiber ring laser with scalable repetition rate up to 1.2 GHz has been experimentally demonstrated. Two types of stabilization mechanisms for passive harmonic mode-locking are identified at different repetition rates. Through investigating the dynamics of pulse generation, it is found that a new operation mode, harmonic mode-locking of multipulse bunching, favors higher order harmonic. The supermode suppression rates of harmonically mode-locked pulse trains at 439 MHz and 1.145 GHz are 40 dB and 30 dB, respectively.

Quantification of glucose diffusion in arterial tissues by using optical coherence tomography

Abstract: Several investigations suggest that the early stages of atherosclerosis are modulated by the selective permeability of the vascular tissue to pro-inflammatory molecules of different molecular weights. Up to date, a few experiments have been performed to study the permeability of arterial tissue to different molecules. This is primarily due to an absence of an experimental technique capable of depth-resolved, accurate and sensitive assessment of arterial permeability. In this paper, we report our pilot results on nondestructive quantification of glucose diffusion in animal arteries in vitro by using optical coherence tomography (OCT) technique. Permeability of glucose in animal's aorta was estimated to be 1.43±0.24×10-5 cm/sec from five independent experiments. Obtained results suggest capability of OCT technique for highly sensitive, accurate, and nondestructive monitoring and quantification of agents' diffusion in vascular tissues.

An overview on S-band erbium-doped fiber amplifiers

Abstract: An erbium-doped fiber amplifier (EDFA) for S-band signal amplification is designed by using a depressed cladding erbium-doped fiber (DC-EDF). The fiber's characteristics are described in terms of the effects of the fiber spooling diameter on the amplifier's performance. In this experiment, the spooling diameter required for optimum amplifier gain was around 5–7 cm. By using a typical two-stage configuration (with a 27 m long DC-EDF and a 260 mW pump laser power), the maximum small signal gain obtained was about 32 dB. Yet, by employing a double pass amplifier configuration with a shorter DC-EDF length and a lower pump laser power (15 m and 135 mW, respectively), a similar maximum small signal gain of approximately 30 dB was achieved. This improvement in gain characteristics however, incurred an increased noise figure penalty of about 1 dB in comparison to single-pass amplifier configurations. In order to reduce the amplifier's noise figure while maintaining its high gain, a partial double-pass S-band EDFA configuration was introduced. This configuration provides a high 26.9 dB gain and an improved noise figure comparable to a single pass configuration. Gain clamping in S-band EDFAs are also demonstrated by utilizing a fiber Bragg grating to form an oscillating laser at around 1530 nm. This technique enables good gain clamping with a gain variation of less than 1 dB.

Stable multi-wavelength erbium-doped fiber laser based on a photonic crystal fiber Sagnac loop filter

Abstract: A multi-wavelength erbium-doped fiber laser based on a Sagnac loop fiber which is formed by a birefringent and highly nonlinear photonic crystal fiber (PCF), a 3-dB optical coupler and two polariza ...

Appearance of coherent artifact signals in femtosecond transient absorption spectroscopy in dependence on detector design

Abstract: The appearance of coherent artifact signals in transient absorption spectroscopy employing a spectrally integrated detection system is studied. The influence of the detection design on the shape of the observed signal is detailed and the experimentally very important case, in which the shape of the coherent artifact is strongly influenced by the presence of the sample itself, is considered – leading to the situation that the artifact signal cannot be accounted for by simple comparison of the kinetics obtained for the solvent only. Finally, an estimate of the relative contribution of the artifact to the overall transient absorption changes is presented facilitating the interpretation of short time transients in the presence of artifact contributions and allowing to estimate the excited state absorption cross-section for a known pump-intensity dependence of the artifact signal.

Multiwavelength Brillouin/Erbium-Ytterbium fiber laser

Abstract: A multi-wavelength Brillouin/Erbium-Ytterbium doped fiber laser which operates in the 1535 nm region is proposed and demonstrated. The system employs both linear and nonlinear gain from a 4 meter Erbium-Ytterbium doped fiber and an 8 km single mode fiber respectively to generate an optical comb with a spacing of approximately 0.084 nm. A stable output laser comb of more than 22 lines was obtained with a Brillouin pump of 2 dBm and a 1058 nm pump of 175 mW. A maximum peak power of -4.2 dBm was obtained at a wavelength of 1535.16 nm at these pump power settings while the spectral linewidth of the laser is approximately 8 Hz.

Supercontinuum generation in a microstructured optical fiber by picosecond self Q-switched mode-locked Nd:GdVO4 laser

Abstract: An effective supercontinuum (SC) generation is demonstrated by injecting picosecond self Q-switched mode-locked Nd:GdVO4 laser pulses into a 1-m long microstructured fiber. The laser is operated at wavelength 100-nm away from the longer zero-dispersion of this dual zero-dispersion wavelength microstructured fiber. The phenomena of modulation instability, stimulated Raman effect, and dispersive wave can be sequentially observed from experimental results, leading to spectral broadening as pumping increases.

High-order stimulated Raman scattering in CVD single crystal diamond

Abstract: We report on the first observation of stimulated Raman scattering (SRS) in single crystal diamond synthesized by chemical vapor deposition (CVD). An efficiency of 45% of energy conversion from the pumping Nd3+:Y3Al5O12 laser radiation to all Stokes and anti-Stokes components is achieved. This makes single crystal CVD diamond an attractive χ(3) nonlinear crystalline material as a Raman laser converter.

Stable acousto-optics Q-switched Nd:YVO4 laser at 500 kHz

Abstract: Up to 1 MHz acousto-optics Q-switching operation was obtained in a grazing-incidence Nd:YVO4 laser. At 500 kHz repetition rate, 17.7 W of output power was obtained with 38 W of pump power when 32.4% transmissivity output coupler was used. The beam quality factors were measured as M2x < 4 in the x-axis direction and M2y < 2 in the y-axis direction. For high sable acousto-optics pulse operation, 5% transmissivity output coupler was used and 9.15 W of output power was obtained at 38 W of pump power. The peak-to-peak instability of pulse amplitude was less than ±3.3% at 489.6 kHz repetition rate.

Picosecond optical parametric oscillator pumped synchronously, intracavity, by a mode-locked Nd:YVO4 laser

Abstract: The operation of a picosecond synchronously intracavity pumped optical parametric oscillator (OPO) is reported. A magnesium doped lithium niobate crystal (MgO:PPLN), periodically poled, is used as the optical parametric oscillator crystal coupling the pump and the resonant signal cavities. The active medium of the pump cavity is a a diode pumped passively mode-locked Nd:YVO4 crystal. Continuous mode-locked operation was achieved, tunable from 1531 to 1554 nm by adjusting the OPO crystal from 31 to 55°C. The spectral width of the generated radiation was 2 nm.

A c-cut Nd:GdVO4 solid-state laser passively Q-switched with Co2+:LaMgAl11O19 lasing at 1.34 µm

Abstract: We investigated a high power diode-laser pumped passively Q-switched c-cut Nd:GdVO4 solid-state laser, lasing at 1.34 μm, with a Co2+:LaMgAl11O19 saturable absorber. With the optimum output coupler transmission T=5.5% and pump power of 11.5 W, the static output power of 3.01 W was obtained. The passively Q-switching operation with Co:LMA (initial transmission T0=90%) as saturable absorber, shows that the shortest output pulse width, the highest output power, the highest pulse repetition and the highest pulse peak power were 32 ns, 266 mW, 277 kHz, and 187 W, respectively.

In vivo Raman spectroscopic analysis of the influence of IR radiation on the carotenoid antioxidant substances beta‐carotene and lycopene in the human skin. Formation of free radicals

Abstract: Resonance Raman spectroscopy was used as a fast and non-invasive optical method for the determination of the influence of IR radiation on the degradation of carotenoid antioxidant substances beta-carotene and lycopene in the human skin. It was found that the beta-carotene and lycopene concentration in the skin are decreased almost immediately subsequent to IR irradiation. The magnitude of degradation of carotenoids in the skin was determined at 27% for beta-carotene and 38% for lycopene on average. A strong correlation between the individual level of beta-carotene and lycopene in the skin of volunteers and the magnitude of destruction of carotenoids beta-carotene and lycopene in the skin was found to be R2=0.78 and R2=0.89 correspondingly. Taking into consideration the absence of absorption in the infrared range of spectra and the relative heat stability of carotenoids up to 50° C, the action of free radicals, which can be produced subsequent to IR irradiation in the skin, can be the main purpose for the degradation of carotenoid substances in the skin.

Multiple wavelength Brillouin fiber laser from injection of intense signal light

Abstract: A multiple wavelength Brillouin fiber laser (BFL) is demonstrated using an injection of intense light at 1480 nm. The light generates Raman gain at around 1560 nm to amplify the Brillouin-Rayleigh backscattered light in the medium and allows the generation of multiple lines by self-feedback-seeding effects. Three Stokes are obtained in the BFL with the maximum power of the first Stokes at approximately -6 dBm from the injection of 1480 nm light at 150 mW. The line spacing is measured to be approximately 0.08 nm. There is no significant change in the power of the first Stokes with the injection of 1480 nm light; however the powers of additional Stokes are highest at Brillouin pump of 1560 nm.

Resonance Raman spectroscopy for the detection of carotenoids in foodstuffs. Influence of the nutrition on the antioxidative potential of the skin

Abstract: Various fruit, vegetables and plants contain a high amount of carotenoids such as beta-carotene, lycopene, lutein, zeaxanthin, and other antioxidants in different concentrations and combinations. These substances used by nature are an effective defense mechanism against the destructive action of free radicals, which are the result of the negative influence of the environment. It was found that the peel of fruit and vegetables usually contains a higher amount of carotenoids than the pulp. Humans cannot synthesize carotenoids by itself, thus, they should be applied systemically with a supplementation rich in these substances. The diet, based on the consumption of a high amount of carotenoid rich products, increased the concentration of carotenoids in the human skin. Taking into consideration the results of previous studies, there is a strong correlation between the appearance of the skin concerning furrows and wrinkles and the level of carotenoid lycopene obtained in the skin. The present results demonstrate that a diet rich in products, which contain a high amount of carotenoids, can be an efficient strategy to reduce skin aging. In the present study, resonance Raman spectroscopy was used as a fast non-invasive optical method for the determination of the carotenoid antioxidant substances in fruit, vegetables and for the determination of the influence of a carotenoid rich diet on the antioxidant level of the human skin.

Experiments of a diode-pumped Nd:GdVO4/LT-GaAs Q-switched and mode-locked laser

Abstract: A diode-pumped passively Q-switched mode-locked (QML) Nd:GdVO4 laser with a low temperature GaAs (LT-GaAs) saturable absorber is presented. The maximal Q-switched mode-locked average output power was 798 mW with the Q-switched envelop having a repetition rate of 125 kHz. The mode-locked pulse trains inside the Q-switched pulse envelope had a repetition rate of similar to 750 MHz. The laser properties of the operational parameters on the pump power were also investigated experimentally.

Wavelength-spacing tunable multi-wavelength erbiumdoped fiber laser incorporating a semiconductor optical amplifier

Abstract: A multi-wavelength erbium-doped fiber laser incorporating an semiconductor optical amplifier (SOA) based on a tunable Mach-Zehnder interferometer (as a comb filter) is proposed. Wavelength-spacing can be continuously tuned and multi-wavelength lasing with wavelength spacings of 0.4 nm, 0.8 nm, and 1.6 nm is demonstrated. The output of the proposed multi-wavelength laser is rather stable at room temperature and the output spectra can be adjusted by controlling the bias current of the SOA and the power of the pump laser diode.

Experimental determination of threshold dose in photodynamic therapy in normal rat liver

Abstract: Using normal rat liver we investigated the depth of necrosis induced by photodynamic therapy when different light doses and photosensitizer (Photogem®) concentrations. All experiments were done with a fluence rate of 250 mW/cm2. Photosensitizer concentration was varied from 1.0, 1.5, 2.0, and 5.0 mg/kg of body weight and it was administered through the left tail vein. For each photosensitizer concentration the light dose was varied from 10, 50, 100, 150, and 200 J/cm2. Each experimental point was done using five animals. The depth of necrosis analysis allows us to determine the threshold dose and compare its value with the existent results in the literature. Our result suggested a value about 3 times higher than the conventionally adopted value. It indicates the dependence of such value with the employed concentration photosensitizer. The use of simple models to understand basic features of the PDT (Photodynamic Therapy) may contribute to the solid establishment of dosimetry in PDT enhancing its use in the clinical management of cancers and others lesions.

Cr2+:ZnSe laser pumped by 1.66 µm or 1.97 µm radiations

Abstract: Cr:ZnSe crystal newly grown by Bridgman method was investigated as an active material for laser coherently pumped either by flashlamp-pumped Er:YAP (wavelength 1.66 ?m) or by diode pumped Tm:YAP (wavelength 1.97 ?m) laser radiations. The maximal Cr:ZnSe laser output pulse energy, for the case of flashlamp Er:YAP laser pumping, was 14 mJ (slope efficiency 66%) with the length of pulse 120 ?s. The generated laser radiation was tunable from 2.15 ?m to 2.60 ?m with only 2.5 times drop in laser energy. In the case of diode-pumped Tm:YAP laser pumping the maximal obtained Cr:ZnSe laser output power in cw regime was 195 mW for the absorbed pump power 300 mW (slope efficiency 66%). The generated laser radiation was tunable from 2.12 ?m up to 2.38 ?m.

Stimulated Brillouin scattering phenomena in a nanosecond linearly polarized Yb-doped double-clad fiber amplifier

Abstract: A comprehensive observation of the stimulated Brillouin scattering (SBS) phenomena in a high power nanosecond linearly polarized Yb-doped double-clad fiber amplifier is presented. According to the behavior of SBS, the amplification process is divided into four regions. All the temporal, spectral and power characteristics of SBS in both forward and backward directions in each region are described in detail. Cascaded SBS with up to 30 orders of Stokes waves and 10 orders of anti-Stokes waves is observed. The fiber damage caused by the giant SBS pulses is reported. And the influence of the linewidth, pulse duration, feedback and polarization on SBS is discussed.

Above-threshold detachment by a two-color bicircular laser field

Abstract: Above-threshold detachment of electrons from negative ions by a bicircular laser field is analyzed within the strong-field approximation. A bicircular field consists of two counter-rotating circularly polarized fields with angular frequencies rω and sω with integer r and s. The energy and angle resolved spectra of the detached electron are invariant with respect to a rotation by the angle 360°/(r+s). The results obtained are explained in terms of the interference of contributions to the T matrix from different complex solutions of the saddle-point equation.