Thermal lens determination of LD end-pumped solid-state laser with stable resonator by slit scanning method

A simple method to determine the thermal focal length of LD end-pumped solid-state laser with stable resonator is presented. The M² factor describing the quality of the beam can be obtained by scanning a slit through the multi-mode Gaussian beam field. The waist width of the beam and the corresponding TEM₀₀ under the same parameters of laser are then deduced through the law of multi-mode Gaussian beam propagation. Based on the standard matrix theory of stable resonator, the thermal focal length of the gain medium can be easily achieved. To show the application of this approach, the thermal focal length of an LD pumped Nd:YVO₄ laser is measured and the experimental results are in agreement with the theoretical prediction.     

Sub-Doppler spectroscopy of atoms excited in the regime of Rabi oscillations in a thin gas cell

We carried out theoretical investigation about velocity-selective atomic excitation on long-lived (metastable) levels of an atomic vapour in a thin cell by a monochromatic laser beam, running in the normal direction. The regime of coherent Rabi oscillations is considered on the light-induced transition from a sublevel of the ground quantum term to a metastable atomic level. On the basis of density matrix equations for the two-level system, we analysed the atomic population density of the metastable level, when the sample is irradiated by resonant monochromatic laser beam with an annular cross-section versus atomic velocities and versus the detuning, the amplitude, and the geometry of the laser beam. It is shown that, in the centre of the annular region, it can be obtained a population distribution on the metastable level as a function of the laser detuning, characterized by a sharp narrow resonance profile, whose width is reduced with respect to the thermal Doppler width roughly by the ratio between the diameter of the irradiated region and the inner thickness of the cell. We suggest high-sensitive schemes, in order to detect these sub-Doppler resonances, by probing the population of the metastable state with a second laser beam, resonant with a transition leaving from the metastable level. The case of ¹S₀ → ³P₁ spin-forbidden transition of Ca is discussed in more detail     

Measurement of στ product of solid state laser materials by an alternative method: Application to Nd3+ doped YVO4 crystal for 4 F 3/2→4 I 11/2 transition

In this paper an alternative approach for measurement of στ product for ⁴ F _3/2→⁴ I _11/2 transition of Nd³⁺ doped YVO₄ crystal is reported. In this method a microchip laser is formed by keeping a small piece of the sample in plane-plane resonator and a diode laser (808 nm) is used for pumping. The pump power induced thermal lensing effect is used to make the cavity stable. The cavity mode area is estimated by measuring the thermal lens focal length at the threshold and the average pump area is measured by Gaussian fit to the intensity profiles of the pump beam. The value of στ product of Nd:YVO₄ crystal obtained by this method is within 10% of the reported values. The advantage of this method is that it is a simple method for direct measurement of στ product of laser crystals.     

Characterization of thermal diffusivity of micro/nanoscale wires by transient photo-electro-thermal technique

In this work, a transient photon-electro-thermal (TPET) technique based on step laser heating and electrical thermal sensing is developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and nonconductive wires. In this method, the to-be-measured thin wire/tube is suspended over two electrodes and is irradiated with a step cw laser beam. The laser beam induces a transient temperature rise in the wire/tube, which will lead to a transient change of its electrical resistance. A dc current is applied to the sample, and the resulting transient voltage variation over the wire is measured and used to extract the thermophysical properties of the sample. A 25.4-μm thick Pt wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying the TPET technique, one can measure the thermal diffusivity of conductive single-wall carbon nanotube (SWCNT) bundles and nonconductive cloth fibers. For nonconductive wires, a thin (∼ nm) metallic film is coated on the outside of the wire for electrical thermal sensing. The measured thermal diffusivity for the SWCNT bundle is 2.53×10^-5 m²/s, much less than the thermal diffusivity of graphite in the layer direction. For microscale cloth fibers, our experiment shows its thermal diffusivity is at the level of 10^-7 m²/s. PACS 78.20.Nv; 42.62.-b; 65.80+n; 66.30.Xj     

High spectral resolution analysis of tunable narrowband resonant grating waveguide structures

A high spectral resolution analysis of narrowband reflection filters based on resonant grating waveguide structures is presented. A tunable high-performance dye laser with ∼ 0.15 cm^-1 line width and a beam analyzing system consisting of three simultaneously controlled CCD cameras were used to investigate grating waveguide resonances at wavelengths in the 694 nm and 633 nm ranges. A reflectivity of ∼ 91% and a line width of ∼ 0.55 nm were measured and theoretically modeled for a resonant reflection filter specifically designed for the ruby laser wavelength 694.2 nm. For a second grating waveguide structure, designed for the helium-neon laser emission wavelength 632.8 nm, we observed a thermal shift of its spectral resonance position of several nanometers, when increasing the sample temperature by some degrees. An inverse thermal shift was observed when the structure was subsequently cooled down to room temperature. Our results suggest implementation of grating waveguide devices combining a narrow line width with a tunability of the resonant response into innovative concepts for reflection filter and sensor applications. PACS 42.62.-b; 42.79.Dj; 42.79.Gn     

A slow gravity compensated atom laser

We report on a slow guided atom laser beam outcoupled from a Bose–Einstein condensate of ⁸⁷Rb atoms in a hybrid trap. The acceleration of the atom laser beam can be controlled by compensating the gravitational acceleration and we reach residual accelerations as low as 0.0027 g. The outcoupling mechanism allows for the production of a constant flux of 4.5×10⁶ atoms per second and due to transverse guiding we obtain an upper limit for the mean beam width of 4.6 μm. The transverse velocity spread is only 0.2 mm/s and thus an upper limit for the beam quality parameter is M ²=2.5. We demonstrate the potential of the long interrogation times available with this atom laser beam by measuring the trap frequency in a single measurement. The small beam width together with the long evolution and interrogation time makes this atom laser beam a promising tool for continuous interferometric measurements.     

New method for isotopic ratio measurements of atmospheric carbon dioxide using a 4.3 μm pulsed quantum cascade laser

We present a new approach to the measurement of stable isotopic ratios of carbon dioxide using a near-room-temperature pulsed quantum cascade laser and a spectral ratio method based upon dual multiple pass absorption cells. The spectral ratio method improves precision and accuracy by reducing sensitivity to variations in the laser tuning rate, power and line width. The laser is scanned across three spectral lines (near 2310 cm^-1) quantifying three CO₂ isotopologues: ¹²C¹⁶O₂, ¹³C¹⁶O₂ and ¹²C¹⁶O¹⁸O. Isotopic ratios are determined simultaneously with a precision of 0.2δ for each ratio with a one-second measurement. Signal averaging for 400 s improves the precision to better than 0.03δ for both isotopic ratios (¹³ R and ¹⁸ R). Long-term accuracy of 0.2 to 0.3δ is demonstrated with replicate measurements of the same sample over a one-month period. The fast time response of this instrument is suitable for eddy flux measurements. PACS 07.57.Ty; 42.62.Fi; 92.70.Cp; 91.67.Rx     

Thermal and optical properties of Nd3+:SrWO4: a potential candidate for eye-safe 1.517 μm Raman lasers

Nd³⁺ doped SrWO₄ single crystal with a dimension of ϕ 30 mm×80 mm and a good optical quality was grown by Czochralski method. The refractive indices in a function of wavelength and the principal coefficient of thermal expansion were precisely measured. The three principal coefficients of thermal expansion along (001), (010), (100) are 2.39×10^-5 °C^-1, 1.09×10^-5 °C^-1, and 0.97×10^-5 °C^-1, respectively. In the framework of the Judd–Ofelt theory, the intensity parameters, radiative probabilities, radiative branching ratios and radiative lifetime were calculated. We also present end-pumped self-stimulated Raman scattering in the two laser channels ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 to generate 1.18 and 1.52 μm Self-stimulated Raman scattering radiations, the latter being close to the eye-safe domain. This material is a potential material for eye-safe Raman lasers.PACS 78.55.Hx; 42.70.Hj     

Laser-Induced Diffraction Rings from an Absorbing Solution

We observed multiple diffraction rings of a cw Ar⁺ laser beam from a nitrobenzene solution of saturable absorber BDN (bis-(4-dimethylaminodithiobenzil)-nickel) caused by the spatial self-phase modulation at low incident optical intensities. We obtained 37 rings for a 200-μm thick sample at an optical intensity of 38 W/mm². The refractive-index change Δ;n and effective nonlinear refractive index n₂ were determined from the number of observed rings and by the z-scan technique. We obtained large values of Δ;n∼0.1 and n₂ = -2.9×;10^-5 cm²/W. This large nonlinearity is attributed to a thermal effect resulting from linear absorption.     

Fast electron energy deposition in aluminium foils: Resistive vs. drag heating

The high current electron beam losses have been studied experimentally with 0.7 J, 40 fs, 6 10¹⁹ Wcm^-2 laser pulses interacting with Al foils of thicknesses 10-200 μm. The fast electron beam characteristics and the foil temperature were measured by recording the intensity of the electromagnetic emission from the foils rear side at two different wavelengths in the optical domain, ≈407 nm (the second harmonic of the laser light) and ≈500 nm. The experimentally observed fast electron distribution contains two components: one relativistic tail made of very energetic (T _h ^tail ≈ 10 MeV) and highly collimated (7° ± 3°) electrons, carrying a small amount of energy (less than 1% of the laser energy), and another, the bulk of the accelerated electrons, containing lower-energy (T _h ^bulk=500 ± 100 keV) more divergent electrons (35 ± 5°), which transports about 35% of the laser energy. The relativistic component manifests itself by the coherent 2ω₀ emission due to the modulation of the electron density in the interaction zone. The bulk component induces a strong target heating producing measurable yields of thermal emission from the foils rear side. Our data and modeling demonstrate two mechanisms of fast electron energy deposition: resistive heating due to the neutralizing return current and collisions of fast electrons with plasma electrons. The resistive mechanism is more important at shallow target depths, representing an heating rate of 100 eV per Joule of laser energy at 15 μm. Beyond that depth, because of the beam divergence, the incident current goes under 10¹² Acm^-2 and the collisional heating becomes more important than the resistive heating. The heating rate is of only 1.5 eV per Joule at 50 μm depth.     

High-contrast all-optical switching with Pt:ethynyl complex

We have theoretically analyzed all-optical switching in Pt:ethynyl complex based on nonlinear excited-state absorption. A detailed analysis for Pt:ethynyl complex has been presented based on rate equation approach. It is shown that a pulsed pump laser beam at 355 nm switches the transmission of a cw probe laser beam at 633 nm through a Pt:ethynyl sample. The effect of various parameters, such as pump pulse width, peak pumping intensity, normalized parameter , transition times of S₁→S₀ and S₁→T₁ states and lifetime of triplet state, on switching characteristics has been analyzed in detail. It has been shown that the probe beam can be completely switched off (i.e. 100% modulation) by a pulsed pump laser beam at 50 kW/cm². These results have been used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses.     

The use of a diffraction grating as a beam expander in a dye laser cavity

The use of a diffraction grating as a dispersive one-dimensional beam expander with variable expansion ratio is described. The idea is applied to intracavity beam expansion in a dye laser cavity containing two gratings in a tandem configuration. By varying the beam expansion ratio up to 60 the laser linewidth may be reduced continously down to 0.1 cm^-1. Linewidth calculations based on single pass estimates are presented and found to agree with experimental results.     

Laser heating method for estimation of carbon nanotube purity

A new method of a carbon nanotube purity estimation has been developed on the basis of Raman spectroscopy. The spectra of carbon soot containing different amounts of nanotubes were registered under heating from a probing laser beam with a step-by-step increased power density. The material temperature in the laser spot was estimated from a position of the tangential Raman mode demonstrating a linear thermal shift (-0.012 cm^-1/K) from the position 1592 cm^-1 (at room temperature). The rate of the material temperature rise versus the laser power density (determining the slope of a corresponding graph) appeared to correlate strongly with the nanotube content in the soot. The influence of the experimental conditions on the slope value has been excluded via a simultaneous measurement of a reference sample with a high nanotube content (95 vol. %). After the calibration (done by a comparison of the Raman and the transmission electron microscopy data for the nanotube percentage in the same samples) the Raman-based method is able to provide a quantitative purity estimation for any nanotube-containing material. Received: 11 December 2001 / Accepted: 12 December 2001 / Published online: 4 March 2002     

Dynamic thermometry of a solid via optical diffraction method at pulsed irradiation

A method for the fast measurement of the temperature of solids under the action of a high-power light pulse is proposed and demonstrated. This method is based on the application of the Fraunhofer diffraction and is implemented for silicon samples heated to melting temperatures T _melt = 1412°C by a high-power light pulse. The current silicon temperature was determined by measuring the varying diffraction angle of the probing laser beam. The diffraction angle was varied over time because the period of the diffraction grating increased as a result of the dynamic thermal expansion of the crystal. An initial grating was formed on the surface of the silicon plate with a period of d = 4 μm. The radiation beam of a He-Ne laser with λ = 0.6328 μm was used as the probing beam; the measured signal was recorded in the pair of symmetric fifth-order diffraction maxima.     

An improved technique for the measurement of low optical absorption losses in bulk glass

A thin rod sample is thermally clamped at both ends to provide a rigid mounting and a thermal short-circuit for heating effects associated with surface losses. The ends are index matched with liquid to minimize sample preparation. A laser beam is focused through the sample and the temperature rise due to the absorbed energy is monitored at its centre. Sensitivity is limited to a few dB km^–1 optical loss with a laser of 100 mW power. The theory is discussed in detail for this particular geometry and experimental verification presented.     

Short pulse width UV laser at 355 nm based on pulse LD side-pumped ceramic Nd:YAG and BBO electro-optical Q-switched

355 nm UV laser was obtained with a pulse width of less than 5 ns and a peak power at megawatt level by adopting the 808 nm pulse laser diode (LD) side-pumped ceramic Nd:YAG and BBO crystal electrooptical Q-switched. The single-pulse energy was measured to be 24.3 mJ with 4.86 ns pulse width and 5.11 MW peak power at a repetition rate of 1Hz under a 120 A pump current. Using a volume of beam splitting mirrors, wavelength outputs at 1064, 532, and 355 nm pulse laser was obtained simultaneously with a respective average output power of 656.6, 357.1, and 260.5 mW, the beam quality factor M ₂ are (M _{x − 1064}² = 5.83, M _{y − 1064}² = 5.61), (M _{x − 532}² = 4.25, M _{y − 532}² = 4.08) and (M _{x − 355}² = 6.32, M _{y − 355}² = 6.15), corresponding to a conversion efficiency at 11% from 1064 to 355 nm.     

High power composite Nd:YAG/YAG zigzag dual-slab laser oscillator

We report a high power zigzag slab laser oscillator employing two composite Nd:YAG/YAG slab in the cavity. The CW laser output with the power of 401 W was achieved from the oscillator with the optical-optical efficiency of 32.6%. The beam quality is estimated as M _x ² ≈ 10, M _y ² ∼ 50 in the slab width and thickness direction, respectively.     

Q-switched Ho:YAlO<Subscript>3</Subscript> laser pumped by Tm:YLF laser at room temperature

A Q-switched high efficient Ho:YAlO₃ (Ho:YAP) laser pumped by a diode-pumped Tm:YLF laser at room-temperature is realized. The maximum output energy reaches 1.58 mJ under the repetition frequency of 5 kHz, when the incident pump power is 15.6 W. The pulse width is 22 ns. The wavelength is 2118 nm when the transmission of output coupler is 30%. The beam quality factor is M ² ∼ 1.39 measured by the traveling knife-edge method.     

Miniature high-repetition-rate TEA CO2 laser with surface-wire-corona preionization

In this article, an experimental study of a miniature, sealed-off, high-repetition-rate transversely excited atmospheric-pressure (TEA) CO₂ laser with a kind of surface-wire-corona preionization (SWCP) is described. We have utilized an SWCP consisting of SiO₂ dielectric tube and a fine wire strained and attached to the dielectric surface. A BN ceramic material, which has an extremely low coefficient of thermal expansion of about 5 × 10⁻⁷/°C was employed as a supporter of the resonator. A measurement on emission spectra of SWCP has been reported. By applying SWCP to the TEA CO₂ laser, efficient laser operation at an overall efficiency of 9.8% with an output energy of 150 mJ has been achieved from a small discharge volume of 25 cm³ with an active length of 230 mm. At the pulse repetition frequency of 60 Hz, the TEM₀₀ mode of laser beam with pulse width of 60 ns was obtained.     

CW single transverse mode all-fiber Tm3+-doped silica fiber laser

The CW 25.6 W output power with a slope efficiency of 30.6% respected to the pump power from a CW single transverse mode all-fiber Tm³⁺-doped Silica Fiber Laser is reported. The all-fiber laser is made up by progressively splicing the pigtail fiber, matched FBG fiber and Tm fiber. The reflective FBG and Tm³⁺-doped fiber end Fresnel reflection build up the laser resonance cavity. Due to the multi-mode FBG as the reflective mirror, the output laser spectrum is multi-peaks at high output power, but the spectrum width is less than 2 nm at 1.94 μm. We estimate the beam quality to be M ² = 2.39, clearly indicating nearly diffraction-limited beam propagation.