Simple Method to Generate Nanosecond UV Pulse Trains

By use of a self-seeding technique, energetic nanosecond UV pulse trains of adjustable characteristics (number of pulses, pulse energy, interpulse delay) can be easily produced from an oscillator-amplifier XeCl laser system.     

Complete Analysis of STJ Detector Performance via Absorption of Phonon Pulses

For the first time a complete analysis of the tunnel and loss parameters of superconducting tunnel junction photon detectors has been made solely by the use of nanosecond phonon pulse excitation. Previously only a partial characterization, requiring supplementary information from photo-excitation measurements, was possible. The present results have been achieved by a more realistic model for the energy spectrum of the phonon pulses and greatly improved (nanosecond) time resolution of the detected signal. The value determined for the tunnel rate is in good agreement with calculations based on the device layer structure. It is believed that the relatively high values of loss time obtained are the result of trap-enhanced recombination due to the high quasiparticle densities attained in the experiments.      

Nanosecond and femtosecond laser ablation of brass: particulate and ICPMS measurements

Femtosecond and nanosecond lasers were compared for ablating brass alloys. All operating parameters from both lasers were equal except for the pulse duration. The ablated aerosol vapor was collected on silicon substrates for particle size measurements or sent into an inductively coupled plasma mass spectrometer. The diameters and size distribution of particulates were measured from scanning electron microscope (SEM) images of the collected ablated aerosol. SEM measurements showed that particles ablated using nanosecond pulses were single spherical entities ranging in diameter from several micrometers to several hundred nanometers. Primary particles ablated using femtosecond ablation were approximately 100 nm in diameter but formed large agglomerates. ICPMS showed enhanced signal intensity and stability using femtosecond compared to nanosecond laser ablation.     

Development of tunable picosecond dye laser for multiphoton ionization of dioxin precursors in supersonic jet/time-of-flight mass spectrometry

A distributed-feedback dye laser has been developed for achieving the efficient multiphoton ionization of chlorobenzene and dichlorobenzene, that is, precursor molecules of dioxins. This tunable picosecond laser with a narrow spectral line width, that is, a nearly transform-limited pulse, provides a more efficient ionization than the nanosecond laser, which is currently in use in supersonic jet spectrometry. The advantage of picosecond over nanosecond and femtosecond lasers is also discussed on the basis of the theoretical model reported in a previous paper.     

Observations in collinear femtosecond-nanosecond dual-pulse laser-induced breakdown spectroscopy

In the work reported herein, we have combined a short-lived femtosecond laser-induced plasma (LIP) and a longer-lived nanosecond LIP in a collinear pulse configuration to examine the source(s) of atomic emission and signal-to-noise enhancement in dual-pulse laser-induced breakdown spectroscopy (LIBS). Initial studies indicate that the primary source of dual-pulse LIBS enhancement in the collinear configuration may in large part be a matter of pulse focus; focusing on the sample surface, for example, yields atomic emission enhancements whose lifetime correlates reasonably well with the femtosecond LIP emissive lifetime, suggesting that plasma-plasma coupling may play an important role at that pulse focus. At a second "optimal" focal position above the sample surface, alternatively, atomic emission and signal-to-noise enhancements correlate quite well with the nitrogen and oxygen atomic emission reductions previously seen following use of a femtosecond air spark and a nanosecond ablative pulse in the orthogonal dual-pulse configuration, suggesting that pressure or number density reductions due to femtosecond LIP formation in air may be significant at that pulse focus.     

An estimate of the jitter of test generators using time-measuring apparatus

A method of estimating the jitter of test pulse generators using time-measuring apparatus (event timers) is considered. It is shown that the covariance of results of simultaneous measurements of the period by two measuring instruments gives an estimate of the jitter of the period of the test sequence. An experimental estimate of a jitter of 0.66 psec with an error of 8 fsec is obtained. The method enables generators of test pulse trains with small jitter to be checked and enables the results to be used further to test precision time measuring instruments.     

Nanomusical systems visualized and controlled in 4D electron microscopy

Nanomusical systems, nanoharp and nanopiano, fabricated as arrays of cantilevers by focused ion beam milling of a layered Ni/Ti/Si(3)N(4) thin film, have been investigated in 4D electron microscopy. With the imaging and selective femtosecond and nanosecond control combinations, full characterization of the amplitude and phase of the resonant response of a particular cantilever relative to the optical pulse train was possible. Using a high repetition rate, low energy optical pulse train for selective, resonant excitation, coupled with pulsed and steady-state electron imaging for visualization in space and time, both the amplitude on the nanoscale and resonance of motion on the megahertz scale were resolved for these systems. Tilting of the specimen allowed in-plane and out-of-plane cantilever bending and cantilever torsional motions to be identified in stroboscopic measurements of impulsively induced free vibration. Finally, the transient, as opposed to steady state, thermostat effect was observed for the layered nanocantilevers, with a sufficiently sensitive response to demonstrate suitability for in situ use in thin-film temperature measurements requiring resolutions of <10 K and 10 μm on time scales here mechanically limited to microseconds and potentially at shorter times.     

Effect of voltage and capacitance in nanosecond pulse discharge enhanced laser-induced breakdown spectroscopy

The laser ablation fast pulse discharge plasma spectroscopy (LA-FPDPS) technique has demonstrated its validity to enhance the optical emission of laser-induced plasma. It has the potential to improve the performance of traditional LIBS measurement. Very recently, LA-FPDPS with a nanosecond pulse discharge circuit has been developed, which has a better capability to enhance the optical emission intensity of laser plasma compared with that using a microsecond pulse discharge circuit. In this paper, the effect of the discharge capacitance and discharge voltage on the optical emission of soil plasma generated by LA-FPDPS with a nanosecond pulse discharge circuit is evaluated in detail. In addition, the stability of the time delay between the laser firing and discharge, and between the discharge and optical emission, has been carefully investigated.     

纳秒脉冲激光对水下靶材的掩模微细刻蚀加工

在激光的传输光路中加入掩模板,采用纳秒脉冲激光对水下靶材进行了掩模微细刻蚀加工,与飞秒激光微细加工相比可显著提高加工效率,加工精度由掩模保证.在所构建的试验系统中,分别在空气中、纯水中和盐溶液中进行了加工试验,在水流的作用下,激光辐照的多余热量被带走,加工边缘热影响区明显减小.同时,水下激光辐照还会在加工区域造成光学击穿,产生等离子体冲击波,在水层的约束限制作用下,加工表现出了明显的冲击波力学效应.通过这种热-力学的复合效应,使金属材料表面产生材料去除和变形的加工效果.最终在质量分数为15%的NaNO3的溶液中,利用热-力学效应和化学反应的复合作用,实现了线宽120μm左右、热影响区小、成形精度较好的微细刻蚀加工.     

A pulse measurement intercomparison

A pulse measurement intercomparison, organized by the National Institute of Standards and Technology (NIST) and conducted by the authors in their respective labs, is described. The purpose was to assess the state of the art for time-domain pulse waveform measurements in the nanosecond regime, and to find problem areas in need of better metrology support. The experiment was conducted by circulating two stable pulse generators among the five labs; participants recorded the waveforms over two different time epochs: 10.24 ns, with 10 ps sampling period and 102.4 ns with 100 ps sampling period. The data records were sent to NIST for analysis and comparison. The pulse generators that were used produce a step-like waveform with nominal high and low states of 0.5 and 0 V, respectively, transition duration of approximately 200 ps, and significant frequency components out to almost 10 GHz. The settling behavior was purposely spoiled. Some significant measurement differences were found among the five labs. The overall experiment is described, along with measurement results and conclusions     

Suppression of afterpulsing in photomultipliers by gating the photocathode

A number of gating schemes to minimize the long-term afterpulse signal in photomultipliers have been evaluated. Blocking the excitation pulse by gating the photocathode was found to reduce the gate-on afterpulse background by a factor of 230 over that for nongated operation. This afterpulse or signal-induced background (SIB), which is particularly troublesome in stratospheric lidar measurements, appears as a weak exponentially decaying signal extending into the millisecond region after the photomultiplier tube (PMT) is exposed to an intense submicrosecond optical pulse. Photocathode gating is not feasible in PMTs with semitransparent bialkali photocathodes because of their slow gate response time, but is easily implemented in PMTs with opaque bialkali or semitransparent multialkali (S-20) photocathodes that can be gated with nanosecond response. In those PMTs with semitransparent bialkali photocathodes, a gated (adjacent) focus grid (if available) also produces a significant reduction in the SIB.     

High precision synchronization of shaped nanosecond pulse with compressible chirped short pulse used in front-end of ICF

A method for electrical waveform generation based on chirped pulse stacker in intensity modulation pulse shaping is provided as a means for high precision synchronization between nanosecond shaped pulse and compressible chirped short pulse in inertial confinement fusion (ICF) research. The arbitrary shaped optical pulse could be obtained, and the measured synchronization precision between ultra-short pulse and shaped optical pulse with the same or different wavelengths is better than 2.75 ps root mean square error.     

Steady-state and transient ultraviolet resonance Raman spectrometer for the 193-270 nm spectral region

We describe a state-of-the-art tunable ultraviolet (UV) Raman spectrometer for the 193-270 nm spectral region. This instrument allows for steady-state and transient UV Raman measurements. We utilize a 5 kHz Ti-sapphire continuously tunable laser (approximately 20 ns pulse width) between 193 nm and 240 nm for steady-state measurements. For transient Raman measurements we utilize one Coherent Infinity YAG laser to generate nanosecond infrared (IR) pump laser pulses to generate a temperature jump (T-jump) and a second Coherent Infinity YAG laser that is frequency tripled and Raman shifted into the deep UV (204 nm) for transient UV Raman excitation. Numerous other UV excitation frequencies can be utilized for selective excitation of chromophoric groups for transient Raman measurements. We constructed a subtractive dispersion double monochromator to minimize stray light. We utilize a new charge-coupled device (CCD) camera that responds efficiently to UV light, as opposed to the previous CCD and photodiode detectors, which required intensifiers for detecting UV light. For the T-jump measurements we use a second camera to simultaneously acquire the Raman spectra of the water stretching bands (2500-4000 cm(-1)) whose band-shape and frequency report the sample temperature.     

Review of low timing jitter mode-locked fiber lasers and applications in dual-comb absolute distance measurement

Passively mode-locked fiber lasers emit femtosecond pulse trains with excellent short-term stability. The quantum-limited timing jitter of a free running femtosecond erbium-doped fiber laser working at room temperature is considerably below one femtosecond at high Fourier frequency. The ultrashort pulse train with ultralow timing jitter enables absolute time-of-flight measurements based on a dual-comb implementation, which is typically composed of a pair of optical frequency combs generated by femtosecond lasers. Dead-zone-free absolute distance measurement with sub-micrometer precision and kHz update rate has been routinely achieved with a dual-comb configuration, which is promising for a number of precision manufacturing applications, from large step-structure measurements prevalent in microelectronic profilometry to three coordinate measurements in large-scale aerospace manufacturing and shipbuilding. In this paper, we first review the sub-femtosecond precision timing jitter characterization methods and approaches for ultralow timing jitter mode-locked fiber laser design. Then, we provide an overview of the state-of-the-art dual-comb absolute ranging technology in terms of working principles, experimental implementations, and measurement precisions. Finally, we discuss the impact of quantum-limited timing jitter on the dual-comb ranging precision at a high update rate. The route to highprecision dual-comb range finder design based on ultralow jitter femtosecond fiber lasers is proposed.     

Accurate digital time-of-flight measurement using self-interference

Most ultrasonic distance measurements are based on the determination of the time of flight. This paper presents a novel way for the measurement of the time of flight, based on the detection of the envelope zero in an ultrasonic wave. A particular wave form is produced by supplying two pulse trains subsequently to an acoustic transducer. Distance information is retrieved from the zero, so a fully digital measurement is possible, reducing signal-processing time and resulting in a fast and accurate distance measurement     

Spectrum-preserving Amplification of Ultra-broadband Optical Pulses

Abstract

An amplifier scheme which allows control of the spectrum of the amplified pulse is proposed. The spectral components of the initial pulse are sent into laterally shifted regions of the amplifying medium, which enables control of the gain of particular frequency components. By the use of a properly formed pump, spectral narrowing during amplification can be avoided. A ‘spectrum preserving’ amplification of ultra-broadband (f.w.h.m. of about 30 nm) nanosecond pulses is demonstrated.     

Formation of pulses with controlled parameters in a resonance microwave compressor employing oscillation-mode transformation

Results of an experimental study of the formation of rf pulses with controlled power, duration, repetition rate, and envelope parameters in a resonance compressor employing oscillation-mode transformation are presented. The pulse parameters are varied by using tuned intermodal coupling elements. A possibility of the formation of a series of subsecond and nanosecond rf pulses in the partial extraction of energy and nanosecond pulses of various durations in single complete extraction of energy from the compressor resonator is shown.     

Noise properties of microwave signals synthesized with femtosecond lasers

We discuss various aspects of high resolution measurements of phase fluctuations at microwave frequencies. This includes methods to achieve thermal noise limited sensitivity, along with the improved immunity to oscillator amplitude noise. A few prototype measurement systems were developed to measure phase fluctuations of microwave signals extracted from the optical pulse trains generated by femtosecond lasers. This enabled first reliable measurements of the excess phase noise associated with optical-to-microwave frequency division. The spectral density of the excess phase noise was found to be -140 dBc/Hz at 100 Hz offset from the 10 GHz carrier which was almost 40 dB better than that of a high quality microwave synthesizer.     

Design and performance analysis of transmission line-based nanosecond pulse multiplier

Conventionally, Marx generators are used for the production of short duration, high voltage pulses but since many discharge gap switches are utilized for stepping up the voltage, there are many disadvantages. Here, an alternative and much simpler technique for the multiplication of nanosecond high voltage pulses has been presented in which multiplication takes place by switching single spark gap providing voltage gain of ‘nxV’ wheren is the subsequent number of stages. Stepped up high voltage pulse with fixed voltage gain of defined shape with fast rise time and good flat top is produced without using additional pulse-forming network. Its operation has been made repetitive by switching single spark gap. Multipurpose use, low cost, small size, light weight (weighing less than 50 kg) and portability are the additional benefits of the system. The reported nanosecond pulser has been made by cascading three stages of Blumlein. To cross check its performance the parasitic impedance of the system has been evaluated to realize its adverse effect on the voltage gain and pulse shape. Also its operation has been simulated by PSPICE circuit simulator program and good agreement has been obtained between simulated and experimental results. Applications of this pulse generator include X-ray generation, breakdown tests, ion implantation, streamer discharge studies and ultra wideband generation, among others.