Single-shot measurement of temperature and pressure using laser-induced thermal gratings with a long probe pulse

We report accurate and precise single-shot measurements of temperature and pressure using laser-induced thermal grating spectroscopy (LITGS). The use of a flashlamp-pumped dye laser (FLPDL) as a probe for the induced gratings is described and a fast method of data analysis using Fourier methods is reported. Thermal gratings were induced by 8 ns pump pulses from a frequency-doubled Nd:YAG laser. The signals were produced by scattering 2-s-long pulses, of energy 200 mJ, from the FLPDL. LITGS signals were obtained from NO₂/N₂ mixtures at pressures in the range 1–40 bar and temperatures between ambient and 400 K. Temperature values were derived from the data with an accuracy of 0.42% and a single-shot precision of 0.16%. Pressure values were obtained with an accuracy of 5.7% and a precision of 1.4% limited by uncertainties in the gas kinetic parameters. PACS 63.62.Cf; 82.33.Vx     

紫外超短脉冲激光的单次测量

 利用瞬态光栅衍射法研制了一台单次频率分辨光学开关激光参数测量仪,可以对脉宽范围在10 ps以内的紫外超短脉冲激光进行单次测量。利用该仪器对重复频率为10 Hz的放电泵浦和单次运行的电子束泵浦KrF准分子激光器的输出脉冲分别进行了单次测量,结果表明：对于放电泵浦,当系统工作在零啁啾附近时,脉冲波形和光谱分布较规则,相位分布起伏较小,当系统偏离零啁啾状态时,脉冲波形和光谱分布不再规则,并且相位分布为抛物线结构;对于单次运行的电子束泵浦,脉冲波形具有多峰结构,光谱具有复杂的调制,脉宽约为2 ps,带宽约为1.3 nm,相位分布为抛物线结构。     

紫外超短脉冲激光的单次测量

利用瞬态光栅衍射法研制了一台单次频率分辨光学开关激光参数测量仪,可以对脉宽范围在10 ps以内的紫外超短脉冲激光进行单次测量。利用该仪器对重复频率为10 Hz的放电泵浦和单次运行的电子束泵浦KrF准分子激光器的输出脉冲分别进行了单次测量,结果表明：对于放电泵浦,当系统工作在零啁啾附近时,脉冲波形和光谱分布较规则,相位分布起伏较小,当系统偏离零啁啾状态时,脉冲波形和光谱分布不再规则,并且相位分布为抛物线结构;对于单次运行的电子束泵浦,脉冲波形具有多峰结构,光谱具有复杂的调制,脉宽约为2 ps,带宽约为1.3 nm,相位分布为抛物线结构。     

Single-shot measurement of a carrier-envelope phase by use of a time-dependent polarization pulse

We propose a method for single-shot measurement of the carrier-envelope phase of high-intensity laser pulses. The method is based on observation of the electrons' spatial distribution ionized by a time-dependent polarization pulse generated by a combination of replicas of the measuring pulse. The dependence of the electrons' angular distribution on carrier-envelope phase, pulse width, delay between two combining components, and a peak intensity is calculated. Important experimental issues such as broadening of the angular distribution, Gouy phase, difference between the two replicas, and asymmetric pulse shape are also discussed.     

Single-shot UV-laser ablation of polyimide with variable pulse lengths

Single-shot laser ablation of polyimide has been investigated with UV-Ar⁺-laser radiation ( = 270-315 nm) for pulse lengths between 140 ns and 5 µs. The irradiated polymer surface was studied with respect to its morphology and ablated depth by means of atomic force microscopy. The dependence of the ablation threshold on laser pulse-length and intensity can be tentatively interpreted on the basis of a thermal process and a (thermal or non-thermal) mechanism which diminishes the activation energy for the desorption of species from the surface.     

Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling

We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.     

Single-shot terahertz pulse characterization via two-dimensional electro-optic imaging with dual echelons

A single-shot measurement of terahertz electromagnetic pulses is implemented using two-dimensional electro-optic imaging with dual echelon optics. The reported embodiment produces sequentially delayed multiprobe beamlets, routinely providing a time window of >10 ps with ~25 fs temporal step sizes. Because of its simplicity and robustness, the technique is ideally suited for real-time ultrashort relativistic electron bunch characterization.     

On the noise of high-transimpedance amplifiers for long-wavelength pulse OTDRs

Noise measurements on high-transimpedance amplifiers suitable for long-wavelength OTDRs give results higher than is predicted by normal noise models. Consequently, we have developed two useful techniques to measure independently the noise contribution of the JFET and the feedback resistor to the overall amplifier noise. p ]Our results show that the noise of the JFET is in accordance with an accurate theoretical model for such a device. In contrast, the noise from the feedback resistor is much higher than is predicted from the normal resistance-capacitance model for such a component. This increase results from the distributed nature of high-ohmic resistors. Our results indicate that both choice of resistor manufacturer and individual selection of a resistor from a specific manufacturer are warranted. By selecting a low-noise resistor we demonstrate a 500-M transimpedance amplifier with an input equivalent noise current of 13.8 pA. In comparison, the same amplifier with a noisy resistor had an input equivalent noise current of 23 pA. p ]We use our results to show that a reasonable value of the input equivalent noise current of a low-noise photodiode-amplifier combination is 20 pA.     

Single-shot line scan imaging using stimulated echoes

A new high-speed MRI method is described for single-shot line scan imaging (LSI) based on stimulated echoes (STE). To allow for multislice imaging, the technique comprises a series of slice-selective preparation pulses (each corresponding to the first RF pulse of a STE sequence), a slab-selective refocusing pulse (second RF pulse), and multiple line-selective read pulses (third RF pulses). An alternative version employs packages of two slice-selective pulses followed by multiple line-selective read pulses. Experimental applications deal with human brain imaging on a clinical MRI system at 2.0 T. The technique offers user-selectable trade-offs between volume coverage (1-15 sections) and in-plane spatial resolution (1-5 mm linear pixel dimension) within total acquisition times of less than 500 ms. Although LSI yields a lower signal-to-noise ratio than Fourier imaging, single-shot LSI with STEs is free from resonance offset effects (e.g., magnetic field inhomogeneities and susceptibility differences) that are typical for echo-planar imaging. Moreover, the technique exhibits considerable robustness against motion and provides access to arbitrary fields-of-view, i.e., localized imaging of inner volumes without aliasing artifacts due to phase wrapping.     

Single-shot time-frequency imaging spectroscopy using an echelon mirror

We demonstrate single-shot time-frequency imaging spectroscopy with an echelon mirror for measuring ultrashort laser pulses as well as ultrafast responses of materials using the same optical setup. The echelon mirror produces a spatially encoded time delay for the probe pulse whereby both the probe and pump pulses are focused on samples with small spot size. Using the optical Kerr gate apparatus, we successfully mapped the time-frequency images of ultrashort laser pulses and subsequently evaluated the chirp characteristics with the phase-retrieval procedure on a single-shot basis. By simply replacing the Kerr medium with samples, we could also visualize the phonon-polariton oscillations in ferroelectric LiNbO3.     

Single-shot profilometry of rough surfaces using hyperspectral interferometry

The combination of white light interferometry with hyperspectral imaging ("hyperspectral interferometry") is a recently proposed technique for single-shot measurement of 3D surface profiles. We consider for the first time its application to speckled wavefronts from optically rough surfaces. The intensity versus wavenumber signal at each pixel provides unambiguous range information despite the speckle-induced random phase shifts. Experimental results with samples undergoing controlled rigid body translation demonstrate a measurement repeatability of 460 nm for a bandwidth of approximately 30 nm. Potential applications include roughness measurement and coordinate measurement machine probes where rapid data acquisition in noncooperative environments is essential.     

Ultrashort optical pulse monitoring using asynchronous optical sampling technique in highly nonlinear fiber

An asynchronous optical sampling scheme based on four-wave mixing （FWM） in highly nonlinear fiber （HNLF） is experimentally demonstrated. Based on this scheme, 10-GHz input pulse train with 1.8-ps pulse width is successfully sampled in 100-m HNLF. A single pulse at 10 GHz with 2.3-ps pulse width is rebuilt by using a 50-MHz frequency tunable free-running fiber laser as the sampling pulse source （SPS）. 40-GHz pulse train is used as the input signal. The rebuilt waveforms, together with the low-jitter eye diagram, are also presented.     

Single-shot femtosecond electron diffraction with laser-accelerated electrons: experimental demonstration of electron pulse compression

We report the first experimental demonstration of longitudinal compression of laser-accelerated electron pulses. Accelerated by a femtosecond laser pulse with an intensity of 101? W/cm2, an electron pulse with an energy of around 350 keV and a relative momentum spread of about 10?2 was compressed to a 500-fs pulse at a distance of about 50 cm from the electron source by using a magnetic pulse compressor. This pulse was used to generate a clear diffraction pattern of a gold crystal in a single shot. This method solves the space-charge problem in ultrafast electron diffraction.     

Gas temperature measurements using widely tunable long-wavelength VCSEL

A method for gas temperature measurements with a widely tunable laser diode is presented. The method involves rapidly switching the laser frequency between two distantly spaced absorption lines chosen for optical thermometry. Direct absorption spectroscopy using a single-mode VCSEL was employed to probe the R10 and R22 lines of the 2ν₁+2ν₂ ⁰+ν₃ combination band of CO₂ near 6355.9 and 6363.7 cm^-1 sequentially. A specially designed 0.5-m cryogenic gas cell was filled with 10 mbar CO₂ at room temperature and cooled to 150 K with liquid N₂. The VCSEL was modulated with a 10-kHz ramp superimposed on a 1-kHz square waveform to scan two 0.04 cm^-1 intervals sequentially. The gas temperatures obtained with the VCSEL in the 150–300 K range are in a good agreement with those derived from gas pressure ratios. The maximum relative error of temperature measurements using the VCSEL was ± 3%. A compact VCSEL-based sensor can be developed for gas temperature and concentration measurements in the Martian atmosphere. The method proposed can be used for many applications including in situ monitoring of combustion processes. PACS 42.62.Fi; 42.55.Px; 39.30.+w     

Single-shot acquisition of time-resolved fluorescence spectra using a multiple delay optical fiber bundle

We present a technique for simultaneous acquisition of time-resolved fluorescence spectra using a customized optical fiber bundle with multiple collection fibers of different length. These fibers were aligned at the output plane of a spectrograph corresponding to different wavelengths. Fluorescence signal dispersed by the spectrograph was converted into a pulse train owing to time delays introduced by the length differences between each fiber and subsequently detected by a photomultiplier and digitizer. The performance of the technique was tested with standard fluorescent dyes, and the results are in good agreement with literature values.     

Single-shot sonogram generation for femtosecond laser pulse diagnostics by use of two-photon absorption in a silicon CCD camera

We present an experimental technique capable of single-shot recording of an ultrashort laser pulse sonogram by use of two-photon absorption in a conventional silicon CCD camera. The quadratic spectral phase, introduced into a 100-fs pulse by a grating stretcher, was measured and found to be in good agreement with the analytically calculated value. The nonlinear response of silicon allows sonogram characterization in a wavelength range from 1 to 2 mum .     

Calcium imaging using fluorescence lifetimes and long-wavelength probes

We describe imaging of calcium concentrations using the long-wavelength Ca²⁺ indicators, Calcium Green, Orange, and Crimson. The lifetimes of these probes were measured using the frequency-domain method and were found to increase from 50% to severalfold in response to calcium. The two-dimensional images of the calcium concentration were obtained using a new apparatus for fluorescence lifetime imaging (FLIM). We also describe procedures to correct for the position-dependent frequency response of the gain-modulated image intensifier used in the FLIM apparatus. Importantly, the FLIM method does not require the probe to display shifts in the excitation or emission spectra. Using the FLIM method, calcium imaging is possible using probes which display changes in lifetime in response to calcium. Consequently, calcium imaging is possible with excitation wavelengths ranging from 488 to as long as 620 nm, where autofluorescence and/or photochemical damage is minimal. These probes are also suitable for calcium measurements of single cells using lifetime-based flow cytometry.     

Scattering noise and measurement artifacts in a single-shot cross-correlator and their suppression

We studied the optical scattering noise, measurement artifacts, and their suppression in single-shot measurements of prepulse contrast. The measurement noise due to air scattering and the two kinds of measurement artifacts resulting from multiple reflections of the generated correlation signal and correlation between the pulse under test and the internally reflected sampling pulse were experimentally explored. The scattering noise and measurement artifacts significantly degrade the measurement fidelity. We demonstrate the clean measurement of prepulse contrast in a single-shot cross-correlator by attenuating the main peak of the correlation beam with a dot mirror and designing the correlating process based on a periodically-poled lithium niobate crystal embedded in an unpoled wafer. The standard time-scanning measurements of pulse contrast are also performed to confirm the experimental results of our single-shot measurement.