Photopyroelectric measurement of thermal effusivity of liquids by sample's thickness scan

A method based on the sample's thickness scan of the amplitude of the photopyroelectric (PPE) signal is proposed as an alternative for thermal effusivity measurement of liquids. The proposed method uses a combined amplitude-phase information and needs the knowledge of the absolute values of the sample's thickness and phase of the signal. The accuracy of the method is similar with that of previously reported frequency-scanned methods, provided an accurate control of the cell's (sample's) thickness is performed. A 479 Ws^1/2/m²K room temperature value for the thermal effusivity of silicon oil was found, with a 0.1 μm step thickness control.     

Optimum design of erbium-ytterbium codoped fibres for large-signal, high-pump-power applications

A comprehensive numerical fibre amplifier model has been used to optimise erbiumytterbium codoped active fibre for maximum gain and quantum conversion efficiency at large signal operation. The optimum cut-off wavelength of the LP₁₁ mode has been found to increase from 800 nm at low pump powers (≈ 50 mW) to 1400 nm at pump powers higher than 500 mW. While at low pump powers fibres with higher numerical aperture give higher quantum conversion efficiency, at high pump levels better large signal performance is achieved with fibres having lower numerical aperture. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998.     

Picosecond near- to mid-infrared optical parametric oscillator using KTiOAsO4

The operation and characterization of a high-repetition-rate singly-resonant picosecond optical parametric oscillator based on the non-linear material KTiOAsO4 and synchronously pumped by a Kerr-lens-mode-locked Ti:sapphire laser at 81 MHz is described. By utilizing non-critical type II phase-matching in a 10 mm crystal, average near-infrared output powers of 403 mW have been generated at 31% extraction efficiency. The oscillator exhibits a pump power threshold of 230 mW and with the available mirror set can provide signal tuning over 1.116–1.281 μm and idler tuning over 2.260–3.160 μm by tuning the pump wavelength over 770–896 nm. Without dispersion compensation, near-transform-limited signal pulses with durations of 1.01–1.03 ps and idler pulses with 1.61–2.91 ps duration have been obtained for 1.2 ps input pump pulses. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal lens spectrometry under excitation of a divergent pump beam

Thermal lens spectrometry (TLS) under excitation of a divergent pump beam is discussed in both conventional and microscopic TLS instruments. A refined thermal lens (TL) model was proposed for calculating the TL signal of a “finite TL element.” Experiments as well as comparison with numerical simulations demonstrated that the effective sample length for a certain pump beam profile was about six times the confocal distance of the pump beam for laser-excited case and 1.5 mm for incoherent light source-excited case. For laser-excited conventional TLS instrument or thermal lens microscope (TLM), an empirical formula of the optimum pump beam waist radius for maximum detection sensitivity was obtained at a given sample length. At larger pump beam waist radius of 7 μm, the TL signal was found 2.5 times lower compared to the diffraction limit; however, the resulting two orders of magnitude lower power density in the sample could be quite desirable for the detection of photolabile analytes. By investigating the influence of a finite TL element on the TL signal, we found that an optimal distance between the probe beam waist and the sample was needed to assure the maximum detection sensitivity and good response linearity. Under the optimal detection scheme, limit of detection of the laser-excited TLM at 4 mW power was evaluated to be 8.6 × 10^?9 M for 100-μm-thick ferroin solution, corresponding to an absorbance of 9.5 × 10^?7 absorbance units.     

Photopyroelectric calorimetry of liquids; recent development and applications

This paper is a synthesis of the main photopyroelectric (PPE) calorimetric techniques proposed in the last years for accurate measurements of dynamic thermal parameters of liquids. The area of interest is restricted to liquids because, due to the perfect sample-sensor thermal contact, accurate quantitative results can be obtained and consequently, intimate processes occurring in liquids (as molecular associations, structural changes in nanofluids, adulteration and spoilage of liquid foodstuffs, etc.) can be studied. The paper describes the possibilities offered by the two main used PPE detection configurations, “back” and “front”in investigating two dynamic thermal parameters: thermal diffusivity and effusivity. In the paper we analyzed only the information contained in the phase of the PPE signal, due to the fact that the phase, as a source of information, leads to more accurate results than the amplitude. A study of the accuracy of the investigations when using the chopping frequency, or sample’s thickness as scanning parameters is also made. Some basic applications concerning high-resolution measurements of thermal diffusivity and effusivity of some “special”liquid samples (magnetic nanofluids, adulterated vegetable oils) are described. A new possibility of using the thickness scan of the phase of the PPE signal of a liquid, in order to obtain the thermal parameters of a solid is analyzed.     

Yb-doped strictly all-fiber laser actively Q-switched by intermodal acousto-optic modulation

We show an actively Q-switched ytterbium-doped strictly all-fiber laser. Cavity loss modulation is achieved in a tapered optical fiber by core-to-cladding mode-coupling induced by travelling flexural acoustic waves. When the acoustical signal is switched-off, the optical power losses within the cavity are reduced, and then a laser pulse is emitted. Trains of Q-switched pulses were successfully obtained at repetition rates in the range 1–10 kHz, with pump powers between 59 and 88 mW, at the optical wavelength of 1064.1 nm. Best results were for laser pulses of 118 mW peak power, 1.8 μs of time width, with a pump power of 79 mW, at 7 kHz repetition rate.     

Pulsed photoacoustic technique to study nonlinear processes in liquids: Results in toluene

Pulsed photoacoustic measurements have been carried out in toluene at 532 nm wavelength using a Q-switched frequency doubled Nd:YAG laser. The variation of photoacoustic signal amplitude with incident laser power indicates that at lower laser powers one photon absorption takes place at this wavelength while a clear two photon absorption occurs in this liquid at higher laser powers. The studies made here demonstrate that pulsed photoacoustic technique is simple and effective for the investigation of multiphoton processes in liquids.     

Multiple frequency radiofrequency-optical resonance in a four-level model of rubidium atoms with optical pumping

We consider a four-level model for alkali metal atoms with optical pumping by nonresonant light under conditions when magnetic dipole transitions are induced between energy sublevels of the hyperfine structure in the ground state. We present the dependences of the observed signal as a function of the frequency detuning of the applied rf fields relative to the resonant value, calculated in the density matrix formalism. We note the absence of a light shift in the radiofrequency-optical resonance signal, independent of the amplitude of the rf field and the optical and thermal relaxation rates. We show that when using a modulation technique for phase detection of the signal, its maximum discrimination ability is observed under conditions for simultaneous modulation of the pump light intensity and the frequency of the rf field, which in principle does not occur in the classical two-level model for optically oriented atoms in magnetic resonance. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 326–329, May–June, 2006.     

High-gain and low-noise-figure erbium-doped fiber amplifier employing dual stage quadruple pass technique

A high-gain and low-noise-figure (NF) erbium-doped fiber amplifier (EDFA) was demonstrated utilizing a new technique called the dual-stage quadruple pass (DSQP) with filters. An efficient amplification occurs at the signal wavelength of 1550 nm when it travels along the DSQP amplifier. The highest gain of 62.56 dB with a low noise figure of 3.98 dB was achieved for an input signal power of −50 dBm and pump powers of 10 and 165mW in the second and first stage amplifiers respectively.     

Four-magnon decay of magnetostatic surface waves in yttrium iron garnet films

The four-magnon instability of magnetostatic surface waves (MSSWs) in yttrium iron garnet epitaxial films is investigated experimentally. It is shown that four-magnon instability for MSSWs with wave numbers 30–600 cm⁻¹ is a decay instability and develops for values of the wave magnetization close to the threshold level for second-order parametric instability of a homogeneous transverse pump wave. When the supercriticality of the MSSW power is 15–20 dB, the generated parametric spin waves themselves become unstable with respect to the four-magnon interaction, so that kinetic instability develops in the film. It is shown that the pump signal transmitted through the signal and the length of the “nonlinear” part of the film, where a MSSW is capable of exciting parametric spin waves, increase as the pump power is increased. Fiz. Tverd. Tela (St. Petersburg) 38, 330–338 (February 1997)     

Low-threshold mid-infrared optical parametric oscillation in periodically poled LiNbO3 synchronously pumped by a Ti:sapphire laser

We report the generation of tunable high-repetition-rate optical pulses in the mid-infrared using synchronously pumped parametric oscillation in periodically poled LiNbO₃ (PPLN). Using a Kerr-lens-mode-locked Ti:sapphire laser as the pump source and a PPLN crystal incorporating grating periods of 21.0–22.4 μm, we have achieved wavelength conversion in the -–4 6μm spectral range in the mid-infrared. The use of a semi-monolithic cavity design and hemispherical focusing has permitted pulse generation in the strong idler absorption region of PPLN, resulting in a simple, compact, all-solid-state configuration with a pump power threshold as low as 17 mW and mid-infrared idler powers of up to 64 mW at 9% extraction efficiency. Signal output powers of up to 280 mW at 35% extraction efficiency are available over the -–1.004 1.140μm spectral range at 80.5 MHz and pulse repetition rates at harmonics of the fundamental frequency up to 322 MHz have also been obtained. Received: 5 December 2000 / Revised version: 23 January 2001 / Published online: 27 April 2001     

A power-scaling strategy for longitudinally diode-pumped Tm:YLF lasers

We describe a power-scaling strategy for longitudinally diode-pumped Tm:YLF lasers based on optimisation of the thulium doping level and the gain geometry. Carefully designed laser experiments at lower powers are shown to allow a simple means for accurately predicting the thermal loading and hence the power-scaling properties of this system, including the effects of ground-state depletion, cross relaxation and energy-transfer upconversion. An optimised doping level of 2 at. % and a slab geometry are shown experimentally to allow scaling of a single gain unit to output powers of ∼70 W, limited by the available pump power, and a strategy for scaling well beyond 100-W output is discussed. PACS 42.55.-f; 42.55.Xi; 42.60.By     

Heat treatment-induced bond layer diffusion and re-crystallization in copper carbon interface systems measured by modulated IR radiometry

Copper-carbon interface systems with additional Mo bond layers in the range of 25 nm to 200 nm have been analyzed with respect to their effective thermal depth profiles before and after heat treatment using modulated IR radiometry. Comparing the inverse calibrated modulated IR phase lags before and after heat treatment, several effects can be identified: – (1) The effusivity of the interface layer, which – due the contact resistance between the two elements copper and carbon – is rather low before heat treatment, increases considerably with heat treatment. – (2) This effect is accompanied by an increase of the thermal diffusion time of the interface layer, relying on the diffusion of Mo and Cu particles. – (3) The sputter-deposited copper films, which before heat treatment can be characterized as effective multi-layer structures, re-crystallize with heat treatment and show modulated IR phases, which are characteristic for thermally homogeneous thin films. – (4) The thermal diffusion times of the Cu films decrease considerably with heat treatment due to increased thermal diffusivities, and – (5) the thermal effusivities of the Cu films increase with heat treatment.     

Thermo-optic effects affecting the high pump power end pumped solid state lasers: Modeling and analysis

A complete solution of the thermal effect for continuous wave laser diode array fiber coupled high pump power end pumped actively solid state laser is presented. In this paper, the thermal loading resulted from energy transfer up-conversion effect for laser operated under different pulse repetition frequency has been taken into account in the overall thermal effects. The thermal loading transfer the absorbed pump power into non-uniform temperature distribution in the laser rod, and thus results in thermal strain, thermal stress, thermal dispersion of index, and thermally induced diffraction losses. For a practical laser cavity configuration containing the internal optical elements, we use spherical aberration theory to give a better explanation of experimental results via calculating diffraction losses quantitatively. According to analysis of thermal focal length experimental data, the results show that our calculated value of thermal loading under fixed Nd³⁺ doped concentration laser crystal is not a constant but a function of pump power instead. The theoretical predicted curve of thermal focal length based on our model is in good agreement with experimental measurements. At low pump power, the thermal loading is a linear function varying with pump power, but it begins to saturate at pump powers exceeding 6 W. This phenomenon is consistent with our physical intuitions.     

多泵浦光纤喇曼放大器的模拟分析

通过使用综合理论模型对不同配置条件下的多泵浦分布式光纤喇曼放大器的增益谱进行了数值模拟研究,该理论模型包含了瑞利散射、放大自发辐射和不同交互效应,包括泵浦与泵浦、泵浦与信号和信号与信号之间的交互与能量转移.模拟研究了泵浦源功率、泵浦源波长间隔及光纤损耗对喇曼增益谱的影响,结果表明设计多泵浦光纤喇曼放大器需要对泵浦源功率、泵浦源波长间隔光纤损耗谱进行综合考虑,需对泵浦源功率、泵浦源波长间隔进行合理配置.在本文的泵浦源波长设定条件下,考虑实际光纤的损耗谱特性,为了获得大的增益带宽和小的增益不平坦度,短波长泵浦源和最长波长的泵浦源需要更高的泵浦功率,中间波长泵浦源的功率应较低.     

New trend for optical signal-to-noise ratio of disturbed Raman fiber amplifier

In a distributed Raman fiber amplifier (DRFA), Raman amplification allows a lower signal launch powers to transverse the span above the noise floor while still increasing the optical signal-to-noise ratio (OSNR). It improves the noise figure and reduces the nonlinear penalty of fiber systems. In this paper, we demonstrate a new trend of OSNR at different pump configurations: forward, backward and bidirectional pumping for DRFAs as a function of fiber length. We also present the variation of OSNR with both input pump power and input signal power. It is found that forward pumping provides the highest OSNR, reaching its maximum value of 37 dB. However, backward pumping provides the smallest OSNR that has its maximum of 22 dB and the bidirectional pumping provides the moderate OSNR between the others having its peak of 26 dB.     

Photothermal techniques applied to the study of thermal properties in biodegradable films

The objective of the present work was to determine the thermal diffusivity and effusivity of biodegradable films by using photothermal techniques. The thermal diffusivity was studied by using the open photoacoustic cell technique. On the other hand the thermal effusivity was obtained by the photopyroelectric technique in a front detection configuration. The films were elaborated from mixtures of low density polyethylene (LDPE) and corn starch. The results showed that at high moisture values, the thermal diffusivity increased as the starch concentration was higher in the film. However at low moisture conditions (<9%) the thermal diffusivity diminished when the starch content in the sample was increased. On the other hand the thermal effusivity has a behavior in opposition to the thermal diffusivity. The thermal effusivity increased with the increase of the starch content in the film, at low extrusion moisture conditions (6.55%). As the moisture and starch concentration in the films were increased, the thermal effusivity diminished.     

Measurement of thermophysical properties of YBa2Cu3O7−x thin film using picosecond thermoreflectance technique

Simultaneous determination of thermophysical properties of YBa₂Cu₃O_7?x thin film has been carried out using the optical pump–probe method. The thermal diffusivity and thermal effusivity were analyzed from the same picosecond thermoreflectance data. The thermal conductivity and specific heat were then derived from the measured values of the thermal diffusivity and effusivity. The thermal diffusivity, thermal effusivity and thermal conductivity obtained compared favorably with those reported in the literature.     

Theoretical and experimental investigation of thermal effects in a high power Yb-doped double-clad fiber laser

In this paper, based on the thermal conduction equations and the steady-state rate equations, a theoretical and numerical analysis of thermal effects is investigated for a single end pumped Yb-doped double-clad fiber laser (YDDC). The distributions of signal power and temperature for different pump powers and fiber parameters are compared. According to the results, the parameters of the optical-cavity have been optimized and an effective method has been adopted to reduce the thermal effects in an experimental investigation. As a result, an output power of 621 W has been obtained with a slope efficiency of 78%.     

Laser induced surface damage, thermal transport and microhardness studies on certain organic and semiorganic nonlinear optical crystals

N-alkyl-2,6-dimethyl-4(1H)-pyridinones, salts of 4-dimethylaminopyridine and 2-amino-5-nitropyridine are considered to be potential candidates for nonlinear optical (NLO) applications, in particular for the generation of blue-green laser radiation. Single crystals were grown following the slow evaporation technique at constant temperature. Single-shot laser-induced surface damage thresholds in the range 3–10 GW/cm² were measured using a 18 ns Q-switched Nd:YAG laser. The surface morphologies of the damaged crystals were examined under an optical microscope and the nature of damage identified. The Vicker’s microhardness was determined at a load of 98.07 mN. The thermal transport properties, thermal diffusivity (α), thermal effusivity (e), thermal conductivity (K) and heat capacity (C_p), of the grown crystals were measured by an improved photopyroelectric technique at room temperature. All the results are presented and discussed. PACS 42.70.Mp; 61.66.Hq; 67.80.Gb; 42.65.-k