Ultrafast measurement of two-photon absorption by loss modulation

We demonstrate a direct and sensitive technique for measuring two-photon absorption (TPA). An intensity-modulated femtosecond laser beam passes through a sample exhibiting TPA. A TPA signal at twice the modulation frequency is then generated and subsequently measured by a lock-in amplifier. The absolute TPA cross section of Rhodamine 6G at 800nm is found to be (15.3+/-2.0)x10(-50) cm(4) s/photon and agrees well with previously published results obtained with much higher intensity [J.Chem.Phys.112, 9201 (2000)]. Our method may be especially useful in measuring nonlinear absorptions of nonfluorescent materials.     

Nondestructive measurement of loss performance in channel waveguide devices with phase modulator

A novel method using a phase modulator is proposed for the nondestructive measurement of propagation loss in a channel waveguide. The phase modulator was fabricated on a Ti-diffused LiNbO3 crystal. Tuning the effective optical length of a waveguide by application of an electrical field on the phase modulator, the contrast of the resultant Fabry-Perot resonator in a channel waveguide device can be measured. The associated propagation attenuation coefficients are determined from the experimental results. Coordination of the phase modulator with an optical channel waveguide device not only allows measurement of the bending loss, but improvement in the relative intensity noise owing to the reduction of the reflected power.     

Accurate method for the measurement of absorption cross sections of solid-state saturable absorbers

A rigorous approach taking into account both the temporal and spatial distributions of the pulsed probe beam used for transmission measurements of solid-state saturable absorbers is applied to characterise these absorbers accurately. The use of this method can significantly influence the resulting value for the ground-state absorption cross section. It also permits us to avoid the introduction of an artificial excited-state absorption in systems where, according to energy-level diagrams, no such loss should occur. This is illustrated in the case of passive Q-switchers for 1.5-μm lasers such as Co²⁺-doped LaMgAl₁₁O₁₉ and MgAl₂O₄ and Co²⁺- or Cr²⁺-doped ZnSe and ZnS. Received: 21 November 2001 / Revised version: 22 January 2002 / Published online: 14 March 2002     

Accurate measurement of ultralow loss in a high-finesse Fabry-Perot interferometer using the frequency response functions

We describe the accurate measurement of ultralow loss in a high-finesse Fabry-Perot interferometer using a diode-pumped Nd:YAG laser locked to the longitudinal mode with an active frequency-stabilization technique. By measuring the resonance full width and the free spectral range with the frequency response functions, and by measuring the transmission efficiency on resonance, the finesse and the loss at 1064 nm are accurately measured to be 78100 ± 1200 (reflectance of 99.99598 ± 0.00006%) and 1 5.9 ±2.0 × 10^–6 (15.9 ± 2.0 ppm), respectively.     

Accurate measurement of pressure

This paper briefly describes the fundamental principles of the instruments used for accurate measurement of hydrostatic pressure and in particular the use of piston gauges as primary pressure standards. Different methods for the calibration of secondary standards have been discussed and in particular, emphasis has been given to the calibration of secondary piston gauges against the primary standards by the cross-float method along with the evaluation of uncertainties attached to different correction factors associated with the measurement of pressure from these gauges. The importance of secondary pressure standards in the region 0.1 GPa to several GPa has also been defined.     

Simultaneous reconstruction of absorption and scattering images by multichannel measurement of purely temporal data

We present what is believed to be the first simultaneous reconstruction of the internal scattering and absorbing properties of a highly scattering medium by use of purely temporal data. These results are also the first acquired with the multichannel time-resolved imaging system developed at University College London.     

Resonant absorption and scattering in evanescent fields

Received: 14 July 1998     

Resonances in electron scattering and photon absorption

A short survey is given of the development of ideas about resonances in atomic scattering processes and their connection with the theory of resonant states in nuclei, impurity resonances in solids, ion-atom scattering and recombination in plasmas. A detailed discussion of the experimental situation for atomic resonances is then given, followed by a review of the theory of resonance reactions as applied to them. Special attention is given to effective range and quantum defect methods, and to Fano's configuration interaction theory. Theoretical results for line positions, shapes and widths are compared with experimental data and the need for more angular distribution data is emphasized.     

Analysis on absolute measurement of optical coating absorption loss based on photothermal detuning technique

Based on the photothermal detuning technique, the three linear relations among reflectance （or trans- mittance）, temperature rise, and pump beam power are studied to achieve the absolute measurement of absorption loss. The relation between temperature and reflectance and the calculation accuracy of tem- perature rise on sample surface have great impacts on the measured result. The influences of parameters involved in the method and the relation between temperature and reflectance and the temperature model of sample surface are also studied. The results show that the absolute absorption loss of optical coatings can be achieved by the proposed method. The measurement accuracy depends on the temperature model and the relation between temperature and reflectance. The linear relation between reflectance and temper- ature rise has largest slope when probe beam wavelength is 632.8 nm and incident angle is 28°. The linear relation slope between temperature rise and pump beam power decreases with the growing of beam size and modulation frequency. The results provide theoretical and experimental supports for the perfection and further application of the photothermal detuning technique.     

Rotational Raman scattering of hydrogen and deuterium for calibrating Thomson scattering devices

Rotational Raman scattering cross sections of hydrogen and deuterium were measured and calculated for ruby and neodymium laser wavelengths. They are suited for calibrating Thomson scattering of KeV plasmas.     

Modal redistribution and power loss in on-fibre devices

The mode coupling and optical power loss taking place in on-fibre devices are investigated. These devices are fabricated by removing the cladding of a multimode fibre and replacing it with a new material such as electro-optic or magneto-optic. Under an application of an external field the index of this modified cladding increases, leading to coupling among guided modes and coupling to radiation modes, resulting in optical power loss. The coupling among guided modes and the power loss are calculated as a function of the change in the index under the field and the length of the active region for the case when HE_1m modes are excited individually or in a group. Results indicate that on-fibre devices may be used as low-loss devices for applications involving sensors and delay lines.     

Oxygen saturation-dependent absorption and scattering of blood

We report on the scattering properties of oxygenated and deoxygenated whole blood from 250 to 1000 nm. We determine the complex refractive index of oxygenated and deoxygenated hemoglobin using a Kramers-Kronig analysis and optical coherence tomography measurements. Combining these data with Mie theory, the scattering properties are calculated. The strong oxygen saturation dependent scattering effects should be taken into account in the data analysis of optical oxymetry.     

Coherence gating in optical heterodyne detection measurements of scattering and absorption in highly scattering media

Laser-Induced Fluorescence (LIF) from the S₁ state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O₂, and N₂ respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N₂ but decreased markedly for O₂. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.     

The measurement of the absorption spectra of chlorine e 6 photosensitizer and hemoglobin in whole blood by multiple light scattering

The potential of using multiple light scattering methods to monitor the state and concentration of photosensitizer and hemoglobin in whole blood is demonstrated. Samples of whole blood with various contents of chlorine e ₆ photosensitizer were studied in the spectral region of 620–850 nm. On the basis of experimentally measured coefficients of diffuse reflection and transmission, absorption spectra of blood with and without the photosensitizer and the concentrations of oxy-, deoxy-, and methemoglobin in the samples are calculated. On the basis of this information, the absorption spectrum of chlorine e ₆ in whole blood is reconstructed and its state and concentration are estimated for the first time. It is found that chlorine e ₆ in whole blood occurs mainly in the monomer form.     

Doppler-angle measurement in highly scattering media

We describe a dual-channel optical low-coherence reflectometer for accurate measurement of Doppler angles in highly scattering media. Accurate fluid-flow velocity estimation requires measurement of the Doppler shift and angle. Estimated values of the Doppler angle and average fluid-flow velocity from experimental data are in good agreement with preset values.     

Quantal theory of Coulomb absorption in heavy-ion scattering

The component in the heavy-ion optical potential due to the Coulomb coupling to inelastic channels has been calculated using the on-energy-shell approximation for the intermediate-channel Green's functions. Closed expressions were derived for the Coulomb polarization potential representing coupling to all orders in the K = 0 rotational band. As a test of this general aproach for coupling to higher states, elastic-scattering calculations were performed with a truncated expression which included reorientation in the 2⁺ state and coupling to the 4⁺ state to all orders. Comparison with coupled-channels calculations indicate the increasing importance of off-shell effects with increasing coupling strength. An analytical estimate of offshell effects is presented. Limits on the range of validity of the optical-potential approach are determined.     

The role of absorption in large-angle elastic scattering

The measurements of energy loss distributions obtained in electron scattering experiments at high momentum transfer are presented for Xe, Ar and methane. The spectra show a large variety of intensity distributions, clearly different from those obtained in measurements at the dipole limit. The fraction of the intensity present in the energy loss distribution compared to the elastic peak is significant, but is in line with the reduction of the elastic cross section due to absorption. It is argued that, if similar effects are present in the condensed phase, they should be dealt with in any quantitative analysis of electron transport in matter, as is often done using Monte Carlo simulations. This argument is worked out in some detail for Reflection Electron Energy Loss Spectroscopy.     

Second-order Raman scattering and infra-red absorption in SnS2

Summary The second-order Raman scattering and infra-red absorption spectra of 2H−SnS₂ are reported. The temperature, excitation wavelength and polarization dependences of Raman spectra are studied; resonance effects are put in evidence. The assignment of the second-order bands is discussed on the basis of the appliable selection rules. The role of phonons of high symmetry points different from Γ in second-order spectra is also taken into account. Work partially supported by M.P.I.     

电子器件散粒噪声测试方法研究

本文分析了超导量子干涉器(SQUID)和超导-绝缘-超导(SIS)约瑟夫森结散粒噪声测试方法的应用局限性,提出了常规器件的散粒噪声测试方案.针对常规电子器件散粒噪声特性,研究了噪声测试基本条件,并建立了低温测试系统.通过采用双层屏蔽结构和超低噪声前置放大器,实现了较好的电磁干扰屏蔽和极低的背景噪声.在10 K温度下对常规二极管散粒噪声进行了测试,通过理论和测试结果对比分析,验证了测试系统的准确和可信性. 关键词： 散粒噪声 电子器件 噪声测试