Short-pulse Source Research Articles

14-MeV neutron pulse source

In order to study the neutron measurement techniques set up in laser interaction experiments with D–T filled targets, we have designed and operated a 14-MeV neutron short pulse source. The device is a sealed neutron gas tube with a hot cathode supplied by an adjustable high-voltage pulse generator. Its principle consists in discharging a coaxial cable, held at the generator voltage, into the high-impedance neutron head. We then obtain a neutron pulse train with each pulse duration determined by the length of the cable. This source generates a total routine yield of 107 neutrons. A small sized head will be inserted into interaction chambers.

Dispersion compensation by active predistorted signal synthesis

Techniques for the synthesis of an optical signal predistorted to compensate for fiber dispersion are discussed theoretically. A scheme for very high bit rate (>10 Gbit/s) time-division-multiplexed transmission is proposed which neither requires extremely short pulse sources nor suffers from their inherent dispersion limitations. The rudimentary aspects of the techniques have been verified experimentally by demonstrating both enhanced and degraded transmission of a 4-GHz modulated signal at 1.55 μm over 10-30 km of optical fiber.

Diffuse reflectance pulse radiolysis of opaque samples

Flash photolysis and pulse radiolysis techniques1 have contributed substantially to our understanding of the harmful and beneficial effects of radiation. The development of extremely short pulsed excitation sources means that time resolution is no longer a serious limitation to the application of these methods2,3. However, these methods have previously been restricted to transparent samples because changes in transmission of analysing light were measured following pulsed excitation. Recently4–6, we have established that photochemical processes within opaque and highly scattering systems can be studied by observing changes in diffusely-reflected analysing light following laser excitation. Many of the most important radiation-induced processes involve non-transparent, heterogeneous systems, for example, most biological systems. We report here the first successful ‘diffuse-reflectance pulse radiolysis’ experiments in which we demonstrate that time-dependent absorption spectra of short-lived species can now be measured for opaque samples following pulse radiolysis.

Energy Transfer Mechanism and Amplified Spontaneous Emission Characteristics of Dye Mixture Solutions

The excitation energy transfer mechanism in a dye laser system is discussed. The rate constants of the radiative and radiationless deactivation processes of the donor dye are estimated for two kinds of donor-acceptor dye mixture solutions. It is found that the suitable energy transfer mechanism depends on the characteristics of the excitation source and of the fluorescence of the donor dye. When a dye mixture solution is pumped by a short-pulse excitation source, the excitation energy is transferred most effectively by resonant transfer. Experimental results are also reported on the amplified spontaneous emission characteristics of dye mixture solutions.

Nd:YAG Laser Photodisruptors

Commercial laser photocoagulators consist of: (1) a high power, short-pulse laser source, usually a Q-switched or mode-locked Nd:Y AG laser producing invisible nearinfrared radiation at 1064 nm; (2) an aiming/focusing laser system, using a continuous-wave He-Ne laser producing visible, red light at 632.8 nm; (3) a condensing lens that focuses the infrared treatment beam into a small focal spot at the working distance of the photodisruptor slit lamp; (4) an attachment system that connects the laser sources to the slit-lamp microscope to aim and focus the He-Ne laser (and thus the Nd:Y AG treatment beam) on target tissues. Basic principles, design, safety, and applications of photodisruptors are described elsewhere. I,2 In general, commercial photodisruptors may be differentiated on the basis of their: (1) pulse lengths, (2) aiming/focusing systems, and (3) attachment systems. 1 (1) Q-switched lasers produce single pulses or bursts of 1 to 9 pulses, each pulse of which is between 2 and 14 ns (l ns = 10-9 sec), depending on the commercial photodisruptor (Table 1). Mode-locked lasers produce a pulsetrain of seven to nine spikes over roughly a 30 ns interval. Each spike is 30 psec in duration (l ps = 10-12 sec). (2) With a single-beam focusing system, the operator moves the slit lamp forward and backward to minimize the size of a single red spot on the target tissue. With a doublebeam focusing system, the operator moves the slit lamp forward and backward until the two red spots fuse into

Chemiluminescence mapping. I. Experimental method and initial measurments on the D+F 2 and F+D 2 reactions

Measurements by a new experimental chemiluminescence method of the nascent DF product vibrational distribution confirm earlier findings for the F + D 2 → DF(ν⩽4) + D reaction. The distribution for D + F 2 → DF(ν⩽15) + F shows a larger fraction (≈ 78%) of the reaction exothermicity channeled into product vibration than is observed by conventional chemiluminescence measurements on the parallel H + F 2 system (58%). The new method, termed chemiluminescence mapping for its simultaneous recording of spectrally and temporally resolved chemiluminescence, differs from the earlier arrested relaxation and measured relaxation techniques by the introduction of a short duty cycle pulsed molecular reagent source, a modified deuterium dissociation source, and signal averaged (time resolved), detection of the DF infrared emission. The chemiluminescence mapping technique results for D + F 2 and F + D 2, are presented; apparent deviation from the energy distribution in the H + F 2 system is discussed.

AN H2 (D2)/F2 CHEMICAL LASER INITIATED WITH A NOVEL TEFLON SURFACE SPARK UV FLASH

The performance characteristics and the optimization parameters of a newly developed HF (DF) laser initiated by a novel surface spark UV flash using Teflon are presented. We have obtained an output energy of 0.65 J/pulse (11 J/1, 1.6 µsec FWHM) for HF laser, 0.25 J/pulse (4.2 J/1, 2.0 µsec FWHM) for DF laser with the improved maintenance of the light source. An intense short-pulse surface spark UV source driven by a coaxial Marx generator is realized experimentally in order to improve the power loading, efficiency, and lasing performances at higher operational gas pressure.

Subnanosecond periodicallyQ-switched neodymium glass laser

A train of pulses of approx.5 x 10/sup -10/ sec duration and approx.2 x 10/sup -3/ J energy was generated by a periodically Q-switched neodymium glass laser. The width of the emission spectrum was approx.3.0 nm and the contrast of the emitted radiation exceeded 10/sup 3/. Pulse generation started after 10--20 passes through the resonator. The pulses could be synchronized with external apparatus to within approx.10/sup -9/ sec. Stable generation of pulse trains was observed when the pumping rate exceeded considerably the threshold value. A laser of this kind could be used as a source of short pulses which could then be amplified effectively in high-power neodymium-glass systems.

SELF-STABILIZATION AND NARROWING OF OPTICAL PULSES FROM GaAs JUNCTION LASERS BY INJECTION CURRENT FEEDBACK

Self-induced intensity pulsations of continously operating GaAs junction lasers have been frequency-stabilized and narrowed by regenerative feedback of oscillations in the injection current. The spectral width of the pulse rate has been reduced by a factor of 20 to less than 30 kHz while the optical pulse width was reduced by at least a factor of 2 to less than 180 psec. These results suggest that a well-stabilized source of short optical pulses can be constructed from GaAs injection lasers without external microwave circuitry.

A PM of New Design for High Speed Physics

Total transit time of the order of 1 ns with a standard deviation less than 0.1 ns, IR* better than 0.3 ns FWHM** are typical characteristics of a new type of PM designed and developed at LEP. These performances are obtained with the following applied voltages: between photocathode and oultiplier entrance :1 KV ; across the multiplier: 1 KV to 1.5 KV ; between multiplier output and collector : 2 KV The overall dimensions of the tube are maximum. diameter 40 mm, length 80 mm. The paper explains how these performances are achieved, due to a new design, comapared to the classical approach of PM. The predicted data, deduced from a computer programme (dealing with electron multiplication in channels) are compared to the experimental figures obtained from a coincidence study through a short pulse source, a time to amplitude converter and a multichannel analyzer. Some typical applications are evoked such as : high energy; physics, quantum. electronics.

Verification of Attenuation and Ambient Noise in the Ocean

The attenuation of acoustic energy in the ocean at a frequency of 40 kc was measured by means of a bottom-mounted array of 7 hydrophones in 1800 ft of water. The source of short pulses was a ship-mounted projector. Measurements were made by determining the peak amplitude of the received pulses as a function of range. Spherical transducers were used to minimize the effect of patterns on the measurements. These measurements agreed with the unpublished values obtained by APL, Washington. In addition, the ambient sea noise as a function of sea state was measured. The values obtained, when corrected for the attenuation over the depth of 1800 ft, agree with published values within 12 dB from sea state 3 down to below sea state 1, where system noise became predominant.