Time domain coherent Raman techniques

Summary Exploiting the polarization properties of coherent Raman scattering in liquids, new techniques of polarization-controlled 3-colour CARS are described. It is shown that the nonresonant, resonant-isotropic and resonant-anisotropic scattering components can be alternatively eliminated for special geometries with magic angles of the polarization of the detected anti-Stokes signal. Perturbations via the optical Kerr effect are discussed. The potential to study the molecular reorientational motion and isolated dephasing channels is demonstrated. Paper presented at the ?XI European CARS Workshop”, Florence, Italy, 23–25 March, 1992.     

Gas spectroscopy and temperature measurement by coherent Raman anti-stokes scattering

The feasibility of Raman spectroscopy and temperature measurements in gases by Coherent Anti-Stokes Scattering is discussed and demonstrated experimentally. Results are presented for H₂ gas at room temperature and also for H₂ liberated by pyrolysis in a Bunsen flame.     

Near-field acoustics of clustered rocket engines

This paper presents an approach for the prediction and characterization of the near-field acoustic levels from closely spaced clustered rocket engines. The calculations are based on the method proposed by Eldred wherein the flow field from the clustered rockets is divided into two zones. Zone 1 contains the isolated nozzles that produce noise independently and extends up to a distance where the individual flows completely mix to form an equivalent single nozzle flow. Zone 2 is occupied by the single mixed stream starting from the station where the jets merge. The acoustic fields from the two zones are computed separately on the basis of the NASA-SP method developed for a single equivalent nozzle. A summation of the spectra for the two zones yields the total effective sound pressure level for the clustered engines. Under certain conditions of nozzle spacing and flow parameters, the combined sound pressure level spectrum for the clustered nozzles displays a double peak. Test cases are presented here to demonstrate the importance of hydrodynamic interactions responsible for the double peak in the sound spectrum in the case of clustered rocket nozzles, and the role of ground reflections in the case of noninterfering jets.     

Multipoint temperature and oxygen-concentration measurements using rotational coherent anti-Stokes Raman spectroscopy

A novel technique for coherent anti-Stokes Raman spectroscopy (CARS) measurements in multiple points is presented. With a system of cylindrical lenses, each laser beam is split into several focused beams, yielding separate planar boxcars configurations. Spectrally resolved CARS signals are detected at different heights on the CCD chip. With dual-broadband rotational CARS the setup is demonstrated for quantitative measurements of temperature- and oxygen-concentration profiles. The technique was demonstrated for three points only, but it can be extended to more points by use of special optics; this choice must be based on a sufficient signal-to-noise ratio in all points for the actual measurement condition.     

Combined coherent anti-Stokes Raman spectroscopy and linear Raman spectroscopy for simultaneous temperature and multiple species measurements

The simultaneous application of pure rotational coherent anti-Stokes Raman spectroscopy (CARS) and vibrational linear Raman spectroscopy (LRS) for the measurement of temperature and species concentrations in combustion systems is demonstrated. In addition to the standard rotational CARS experimental setup, only one detection system (spectrometer and intensified CCD camera) for the collection of the LRS signals was applied. The emission of the broadband dye laser used for CARS was shifted to the deep red to avoid interferences with the LRS signals located in the visible region. First experimental results from a vaporizing propane spray using an engine injection system are shown.     

Effects of rocket engines on laser during lunar landing

In the Chinese moon exploration project “ChangE-3”, the laser telemeter and lidar are important equipments on the lunar landing vehicle. A low-thrust vernier rocket engine works during the soft landing, whose plume may influence on the laser equipments. An experiment has first been accomplished to evaluate the influence of the plume on the propagation characteristics of infrared laser under the vacuum condition. Combination with our theoretical analysis has given an appropriate assessment of the plume?s effects on the infrared laser hence providing a valuable basis for the design of lunar landing systems.     

Optical fibre excitation by partially coherent sources

Sources for optical fibre excitation have previously been assumed to be either totally coherent or incoherent. This paper formulates the modal excitation problem for partially coherent sources. The modal excitation coefficients are given in terms of the source complex degree of coherence and numerical results and simple analytical expressions appropriate to multimode step index fibres are presented. The assumption of equal modal power when excitation is by a very incoherent source is examined and the following simple criterion developed: for highly incoherent sources, modes with eigenvaluesU < the reciprocal of the coherence length, measured in units of fibre radius, are approximately equally excited, while the remaining modes carry little power.     

Optimization of a tracer-based PLIF diagnostic for simultaneous imaging of EGR and temperature in IC engines

A tracer-based planar laser-induced fluorescence (TB-PLIF) imaging diagnostic using 3-pentanone has been optimized for use in IC engines. The diagnostic utilizes dual-wavelength excitation of 3-pentanone in the ultraviolet to make simultaneous measurements of exhaust gas residual mole fraction and temperature. A merit function based optimization of the diagnostic precision was performed which allowed for selection of optimal excitation wavelengths for the conditions of interest. Optimized system performance was validated in a motored optical engine over a wide range of in-cylinder temperatures and pressures. In-cylinder results verify the utility of the uncertainty estimates. Differences in magnitude between the estimated and measured precision were determined to be due to errors in parameter values used in the calculations. The observed 2.1% temperature precision at a temperature of 600 K was compared with previous TB-PLIF temperature measurements and shown to be approximately a factor of 2 better than previous results.     

Optical fibre excitation by polychromatic partially coherent sources

The general formalism describing modal fields excited in optical fibres by partially coherent sources is given. The limiting cases of spectrally pure, quasi-monochromatic, coherent and totally incoherent sources are discussed.     

液体火箭发动机中声腔抑制不稳定燃烧的声学分析

发展了声腔的分析和数值模型,对液体火箭发动机不稳定燃烧的抑制作用进行了评定,通过迭代计算研究了二维流动和温度分布变化对声腔调 谐和稳定性能的影响,对不同的声腔几何尺寸和温度梯度的稳定性计算结果表明,燃烧带有较大开口面积的声腔会更大程度地改变振荡的空间分布,这种改变而且影响了驱动和抑制燃烧的机理,讨论了在声腔设计安排中正确选择声腔的几何尺寸,且比较了不同长度和不同直径声腔的阻尼特性,通过考察声吸收系统的方法来最优化系统的阻尼,得到了可供设计参考的结论。     

Analysis of coherent Raman propagation effect

A theoretical analysis of coherent Raman propagation described by two classes of the well-known hyperbolic secant solution and elliptic solutions are presented.     

Detection of acetylene by electronic resonance-enhanced coherent anti-Stokes Raman scattering

We report the detection of acetylene (C₂H₂) at low concentrations by electronic resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS). Visible pump and Stokes beams are tuned into resonance with Q-branch transitions in the v₂ Raman band of acetylene. An ultraviolet probe beam is tuned into resonance with the – electronic transition of C₂H₂, resulting in significant electronic resonance enhancement of the CARS signal. The signal is found to increase significantly with rising pressure for the pressure range 0.1–8 bar at 300 K. Collisional narrowing of the spectra appears to be important at 2 bar and above. A detection limit of approximately 25 ppm at 300 K and 1 bar is achieved for our experimental conditions. The signal magnitudes and the shape of the C₂H₂ spectrum are essentially constant for UV probe wavelengths from 233.0 to 238.5 nm, thus indicating that significant resonant enhancement is achieved even without tuning the probe beam into resonance with a specific electronic resonance transition. PACS 42.65.Dr; 42.62.Fi; 42.65.-k     

Single-molecule detection of biomolecules by surface-enhanced coherent anti-Stokes Raman scattering

We report on the applicability of combining surface-enhanced Raman scattering (SERS) with coherent anti-Stokes Raman scattering for high-sensitivity detection of biological molecules. We found that this combination of techniques provides more than 3 orders of signal enhancement compared with SERS and permits monitoring of biological molecules such as deoxyguanosine monophosphate (dGMP) and deoxyadenosine monophosphate at the single-molecule level. This combined technique also improved detection sensitivity for angiotensin peptide. As this is believed to be the first report of detection of dGMP at the single-molecule level, we suggest that this approach can serve as a new tool for biological studies.     

Excitation profiles of resonant coherent Raman scattering by impurity molecules

The amplitudes and the excitation profiles of coherent resonant Raman scattering (coherent anti‐Stokes and Stokes Raman scattering (CARS and CSRS)) by impurity centers in crystals and/or molecules in solutions are studied, taking into account the coherence of the excited mode and the inhomogeneity of the environment. The CARS and CSRS excitation profiles can directly be related to one‐photon absorption when using the transform theory known from spontaneous resonant Raman scattering. The enlargement of the amplitude of the active mode reveals itself in an enhancement of the vibrational overtones. At large amplitudes, a splitting in the profiles caused by the contribution of two turning points of a strong coherent vibration is observed. The inhomogeneous broadening is different in the excitation profiles of CARS and CSRS. The overlapping contributions to spectrally narrow and broad transitions lead to Fano peculiarities. The effects are illustrated by model calculations. Copyright © 2011 John Wiley & Sons, Ltd.     

Mitigating heat dissipation in Raman lasers using coherent anti-stokes Raman scattering

We present a novel technique that intrinsically mitigates the quantum-defect heating in Raman lasers. The basic principle of this so-called "coherent anti-Stokes Raman scattering (CARS)-based heat mitigation" is to suppress the phonon creation in the Raman medium by increasing the number of out-coupled anti-Stokes photons with respect to the number of out-coupled Stokes photons. We demonstrate with the aid of numerical simulations that for a hydrogen and a silicon Raman laser, CARS-based heat mitigation efficiencies of at least 30% and 35%, respectively, can be obtained.     

Raman lasing and cascaded coherent anti-Stokes Raman scattering of a two-phonon Raman band

Raman lasing of a two-phonon Raman band in the anti-Stokes side is demonstrated. Two femtosecond light pulses with identical wavelengths are irradiated onto a SrTiO3 crystal in a cross-beam configuration. Under low excitation power, several wave-mixing signals with identical wavelengths are emitted. When the power exceeds a critical value, cascaded coherent anti-Stokes Raman scattering (CARS) signals are emitted, the frequency step of which is coincident with that of the strongest two-phonon Raman band of 2TO2.     

Gas-phase temperature measurement in the vaporizing spray of a gasoline direct-injection injector by use of pure rotational coherent anti-Stokes Raman scattering

Pure rotational coherent anti-Stokes Raman spectroscopy is applied for quantitative gas-phase temperature measurements in the vaporizing spray of an automotive fuel injector. Interferences from elastically scattered stray light are greatly reduced by use of a polarization technique and spectral filtering in a double monochromator. The applicability of this technique to probing low-temperature sprays is successfully demonstrated.