Applied Physics B最新文献

Laser-induced fluorescence is a widely used technique for measuring the concentrations of gaseous species in reactive environments. To determine absolute number densities from laser-induced fluorescence signals, the collisional quenching rate of the excited state molecule needs to be known. The methylidyne (CH) radical is an important species in combustion, catalysis, and plasma applications, the latter two of which require laser-induced fluorescence measurements at lower temperatures. Quantitative detection of CH is also needed for photofragmentation laser-induced fluorescence measurements, where CH is produced by photolysis of a larger molecule, such as the methyl radical (CH(_{3})), by a pump laser, and then is excited by a probe laser to induce fluorescence. We have measured the collisional quenching rates of CH(A) by methanol, methane, oxygen, nitrogen, and acetone at temperatures between 300 and 600 K. The CH(A) quenching rate by methanol, which is highly relevant in catalysis, has not previously been studied. The quenching rates for acetone, which is used as a precursor to photolytically produce methyl, and methane have been studied but not at elevated temperatures. We find that methanol and acetone both have high quenching rate coefficients of (2.2cdot10^{-10}) to (2.5cdot10^{-10}) cm(^3)/s with only a small temperature dependence. We also find that the quenching rate of methane has a significant temperature dependence ranging from (2.5cdot10^{-11}) cm(^3)/s at 300 K to (5.0cdot10^{-11}) cm(^3)/s at 600 K. The quenching rates determined in this work are important for laser-induced fluorescence studies of catalysis, plasmas, and combustion processes.

Y-branch distributed Bragg reflector (DBR) diode lasers with a stable narrowband emission in simultaneous dual-wavelength operation with spectral distances below 3.2 nm are presented. The Y-branch laser consists of two laser branches with different DBR gratings serving as wavelength-selective rear-side mirrors. Therefore, two emission wavelengths with a spectral distance defined by the DBR grating periods can be generated simultaneously. A Y-coupler combines the two ridge waveguide (RW) branches into a single straight output RW. Devices with a spectral distance of 0.6 nm and 2.0 nm emitting around 785 nm are manufactured. Selecting the operation parameters carefully, stable narrowband emission for both wavelengths is obtained. Resistors serving as heaters implemented next to the DBR gratings allow for wavelength adjustment and a tuning of the spectral distance. At an optical output power of 100 mW, the spectral distance can be shifted from 0 to 1.55 nm (0–0.76 THz) for the former device or from 1.00 to 3.15 nm (0.49–1.54 THz) for the latter device, respectively. This makes the Y-branch DBR diode laser particularly interesting for the generation of THz beat-note signals, needed to generate THz radiation via photo-mixing.

A fringe projection system, in special application scenarios, is used for three-dimensional (3D) shape measurement through a transparent medium. The light refraction caused by the medium gives rise to erroneous 3D data in conventional fringe projection methods. In this work, we propose ray-tracing-based 3D profilometry using fringe projection. The method uses phase information for seeking the homologous points between camera images and projector images pixel by pixel. Equations of light rays emitted from each point pair are identified with the law of flat refraction. A midpoint of skew lines common perpendicular algorithm is developed for calculating the intersections of these equations, which are 3D shape data without refraction error. For validation, a fringe projection system through a transparent glass was set up and applied for 3D shape measurements. The results verify the effectiveness and accuracy of the proposed ray-tracing-based 3D profilometry.

This manuscript presents a quantum-cascade-laser-absorption-spectroscopy (QCLAS) diagnostic for the partial pressure and internal temperatures (rotational and vibrational) of nitric oxide (NO) in hypersonic flows. Two quantum-cascade lasers (QCLs) were used to measure four transitions of NO near 1887 cm(^{-1}) and 1930 cm(^{-1}) at 25 or 100 kHz using scanned-wavelength direct absorption. Tests were performed in the Purdue High-Pressure Shock Tube (HPST) using an NO–Ar mixture to confirm the accuracy of the diagnostic. The diagnostic was then applied to characterize the Hypersonic Shock Tunnel (HST) at Sandia National Laboratories. In the HST, two flow cutters were used to direct the measurement line-of-sight through the quasi-uniform core flow exiting the nozzle, thereby avoiding measurement complications associated with the thick boundary layers at the nozzle exit. In the HST, tests were performed with air velocities of 3, 4, and 5 km/s where the rotational and vibrational temperature of NO varied from 150 to 850 K and the partial pressure of NO was near 20 Pa. Additionally, dry bottled air and humid room air were used as test gases to quantify the impact of water contamination on the vibrational non-equilibrium of NO. Comparisons with two CFD predictions using unique rate constants for vibrational relaxation are also presented. The vibrational non-equilibrium of NO was more pronounced for 3 km/s tests, and water had a negligible impact on the thermal non-equilibrium of NO. Lastly, the measured rotational temperature of NO agreed well with CFD predictions, the measured partial pressure of NO was consistently above CFD predictions, and the vibrational temperature had moderate agreement with CFD predictions for 4 and 5 km/s tests, and poor agreement for 3 km/s tests.

We prepared a double-crystal LiTaO₃ (LT) electro-optic (EO) Q-switch and investigated its overall Q-switching performances. The LT EO Q-switch is fabricated from two x-cut LT crystals to utilize the maximum effective EO coefficient. The static and dynamic quarter-wave voltage were measured to be 1200 V and 950 V respectively, which are both considerably lower than those of other commercial EO Q-switches. Besides, the LT Q-switch can operate at a wide temperature range with high temperature stability. The dynamic-to-static ratios at sub-freezing temperatures were up to 80 percent of that at room temperature. Furthermore, the piezoelectric ringing in the LT Q-switch was found to be comparable to that of RTP Q-switches, and it had been successfully operated at a repetition rate of 1 kHz with stable Q-switched laser output. The only disadvantage is the low extinction ratio which is mainly caused by the optical inhomogeneity and can be improved by improving the optical quality of LT crystals. The double-crystal LT EO Q-switches are particularly attractive for Q-switched lasers because of their excellent comprehensive performances.

External cavity diode laser (ECDL) can continuously alter its output optical frequency by tuning the external cavity. Wide mode-hop-free (MHF) range and linear optical frequency tuning are two critical characteristics of the ECDL. However, using an uncoated laser diode (LD) limits the MHF range due to the influence of internal cavity formed by the LD. To obtain linear optical frequency, a triangular wave voltage is generally used to scan the external cavity. Nevertheless, the inherent hysteresis characteristic of piezoelectric transducer (PZT) in the external cavity introduces nonlinearity in the optical frequency. The limited MHF range and nonlinear optical frequency can greatly affect the performance and accuracy of applications with uncoated ECDL as the light source. Therefore, it is crucial to address these two issues of uncoated ECDL. This study introduces a hybrid method incorporating synchronous tuning with proportional-integral-derivative (PID) feedback control. Synchronous tuning facilitates simultaneous changes in both the internal and external cavities output modes, thereby extending the MHF range. PID feedback control utilizes the displacement signal of prism in the external cavity to establish a closed-loop control system, compensating for the PZT’s hysteresis. Through PID feedback control, the uncoated ECDL’s optical frequency tuning nonlinearity is effectively suppressed while its MHF range is extended. Experimental results validate the efficacy of this hybrid method within the triangular wave scanning frequency of 50 Hz.

We propose and numerically demonstrate all-optical logical operations using frequency-switched spiking encoding based on excited photonic spiking distributed feedback semiconductor lasers (DFBs). The results reveal that, DFBs can exhibit the neuron-like dynamic behaviors under optical injection with different intensity and detuning frequency. Utilizing the spiking responses induced by frequency switching, binary data can be encoded into spikes and stable spiking response sequence with 8 orders of magnitude faster than that from biological neurons can be achieved. Furthermore, the reconfigurable all-optical logical operations including OR, AND, XOR can be implemented in a single photonic spiking neuron, and the effects of injection coefficient, duration time and the time delay between two continuous trigger signals on XOR logical operation are discussed. This proposed frequency-encoding-based spiking DFB neuron can pave a new way for all-optical logical operations and future photonic spiking neural network.

In this work, iron-oxide nanoparticle formation in the spray-flame synthesis (SFS) process of the standardized SpraySyn 2.0 burner was investigated in situ using laser-induced incandescence (LII). For the evaluation of these measurements, prior LII-experiments within iron-oxide aerosols (Fe₃O₄ and α-Fe₂O₃) with known primary particle size distribution and morphological properties were performed to determine the thermal accommodation coefficient (TAC) α, which led to approx. α = 0.08. The applicability of the TAC results within the flame was validated using spectrally and temporally resolved measurements in the flame at 65 mm HAB employing a spectrograph. Data for a bimodal particle size distribution, obtained from Transmission Electron Microscopy (TEM), were used in the LII-evaluation. The validated TAC was then used to evaluate the primary particle size evolution from in situ Time-Resolved (TiRe) LII-measurements using PMTs along the centre axis of the burner, ranging from 10 mm to 50 mm HAB. These measurements reveal a relatively constant effective particle diameter along HAB with d_p,eff ≈ 300 nm. To further investigate particle formation in SFS, 2-dimensional time-resolved LII-measurements in the SFS flame were performed, showing a clear particle formation region up to approx. 30 mm HAB, from where on a constant particle mass is observed.

The investigation of the thermo-optic characteristics of a material is vital in understanding the nonradiative relaxation processes occurring within the sample. Pyrromethene 567 a member of the laser dye family, has gained significant attention due to its excellent photo-physical characteristics, such as excellent laser efficiency, photostability, and quantum yield. The investigation of the photothermal studies in Pyrromethene 567 remains unexplored. Herein, a dual-beam thermal lens technique was performed to analyze the thermal lensing behaviour of Pyrromethene 567 in various solvents. The thermal diffusivity of Pyrromethene 567 in different solvents was calculated. There were no previous reports measuring the thermal diffusivity of Pyrromethene 567 dye. Thermal lens spectroscopy offers several advantages over conventional techniques because of its high sensitivity. The work also discusses the factors that influence the thermal lens signal in Pyrromethene 567. The influence of detector positioning, chopper frequency, and sample positioning, on thermal lens measurements was discussed. Additionally, the thermal lens experiments should be performed at intensities where diffraction patterns due to spatial self-phase modulation are absent to minimise experimental errors.

Using a non-holographic optical setup, we employed a Mueller–Stokes polarimeter to measure both the linear electro-optic and second-order electrogyration effects. The second-order electrogyration effect was observed in a (Bi_{12}SiO_{20}) (BSO) crystal with a (110) cut. This response was found for the electric field applied both parallel and perpendicular to the [001] direction, where the values for the second-order electrogyration are (4.86times 10^{7} , text {pm}^{2}/text {V}^{2}) and (1.87times 10^{7} , text {pm}^{2}/text {V}^{2}), respectively. Additionally, the linear electro-optic coefficient (gamma _{41}) was measured to be (1.17 , text {pm/V}) for (660.5 , text {nm}).