High‐speed imaging of laser intensity distribution using an acousto‐optic deflector

An acousto‐optic laser deflector was used to image the intensity distribution of a continuous wave laser of microsecond order. Undeflected light was blocked while the deflected light was directed to a CCD camera, which allowed imaging of the incident laser intensity distribution of microsecond order. Experiments using a test pattern showed that there was no distortion in the deflected image. The resolution limit, estimated from diffractive effects, was about 0.2 mrad. Using this method of high‐speed imaging, density variations accompanying laser‐induced breakdown in air were visualized by shadowgraphy and interferometry. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(3): 55–61, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20386     

Visualization of high‐speed phenomena using an acousto‐optic laser deflector

An acousto‐optic laser deflector was used for visualization of high‐speed phenomena, such as shock waves and density perturbations accompanying an impulse discharge, or shock waves generated by laser‐induced breakdown in air. Using a continuous wave laser as the light source, shadowgraphs of shock waves and density perturbations were obtained at shutter speeds down to 1µs. Results showed that shock waves propagated at a speed of 417 m/s in the case of an impulse discharge, and 485 m/s in the case of laser‐induced breakdown. Prebreakdown phenomena such as leaders progressing from the high‐voltage electrode were also visualized. Compared to conventional high‐speed imaging techniques, this method is useful when using a laser light source, since the acousto‐optic crystal can accommodate high‐intensity laser light. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(3): 9–15, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20279     

Laser‐induced discharge in a rod–plane electrode in the presence of a negative dc electric field

Discharge induction experiments were performed between a plate and a rod on a plate electrode configuration with a gap length of 1 m using a high‐power CO₂ laser and a dc voltage generator. The electrodes are parallel planes. Each plate electrode consists of a central flat part having a diameter of 3 m with a circular edge on a cross section. The length of the rod is 10 cm. A chain of plasma beads of length 10 cm was created on the tip of the rod by the CO₂ laser and used for artificial triggering of negative high‐voltage sparkover. The behavior of streamer, leader, and return stroke was observed by an image converter camera. It was found that a positive upward traveling leader can be triggered from the tip of the rod on the lower plane to the upper plane by the chain of plasma beads created by the CO₂ laser. This apparatus is useful for study for realization of laser‐induced lightning. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 19–27, 2000     

Neutron production rate and plasma characteristics of spherically convergent beam fusion

Experimental and simulation results of a spherical glow discharge for a portable neutron source are presented. The experimental device is a 45‐cm‐diameter, 31‐cm‐high stainless‐steel cylindrical chamber, in which a spherical mesh‐type anode 30 cm in diameter is installed. The spherical grid cathode consists of 2.0‐mm‐diameter stainless‐steel wire, which is made into an open spherical grid of 5‐cm diameter. The system is maintained at a constant pressure of 1 to 15 mTorr by feeding hydrogen or deuterium gas. The basic characteristics of breakdown voltages versus pressure and electrostatic potential profiles were measured for hydrogen discharge. Using deuterium, a steady‐state neutron production of 10⁴ s^–1 was observed at a discharge of 40 kV, 2 mA. Motions of ions and electrons in the device were simulated by using a particle code, which is one‐dimensional in coordinate system and two‐dimensional in velocity space. It was confirmed by both the measurement and simulation that a virtual anode is formed in the central part inside the grid cathode. © 2001 Scripta Technica, Electr Eng Jpn, 135(2): 1–8, 2001     

Recent progress in high-power blue-violet lasers

The property of GaInN-AlGaN heterostructures and GaInN multiple quantum well (MQW) gain GaInN laser diodes with low internal loss are described. GaInN blue-violet laser diodes have been developed as a light source for optical disk recording. However, the threshold current density of these diodes has been difficult to reduce and remains high at around 3-4 kA/cm/sup 2/. This is thought to be due to the large transparency current density Jt and the large optical internal loss /spl alpha//sub i/. Recently, the internal loss was successfully reduced to 13.6 cm/sup -1/ by optimizing the design of the near active region and achieved stable continuous operation under 50-mW continuous wave at 70/spl deg/C. Other laser characteristics such as far-field patterns and laser noise have also been improved for optical disk use.     

Characteristics of a saturated 18.9-nm tabletop laser operating at 5-Hz repetition rate

We report the characteristics of a saturated high-repetition rate Ni-like Mo laser at 18.9 nm. This table-top soft X-ray laser was pumped at a 5-Hz repetition rate by 8-ps 1-J optical laser pulses impinging at grazing incidence into a precreated Mo plasma. The variation of the laser output intensity as a function of the grazing incidence angle of the main pump beam is reported. The maximum laser output intensity was observed for an angle of 20/spl deg/, at which we measured a small signal gain of 65 cm/sup -1/ and a gain-length product g/spl times/l>15. Spatial coherence measurements resulting from a Young's double-slit interference experiment show the equivalent incoherent source diameter is about 11 /spl mu/m. The peak spectral brightness is estimated to be of the order of 1/spl times/10/sup 24/ photons s/sup -1/ mm/sup -2/ mrad/sup -2/ within 0.01% spectral bandwidth. This type of practical, small scale, high-repetition soft X-ray laser is of interest for many applications.     

High magnification imaging with a laser-based hard X-ray source

The usefulness of laser-based X-ray sources for imaging is assessed. A very bright point source of 20-keV X-rays, with a diameter of 10 μm, has been created by the interaction of a high contrast femtosecond laser pulse with solid targets. This X-ray source has been used for a feasibility study of high magnification imaging. Images of mammography phantoms have been obtained and spatial resolution of 35 μm has been demonstrated with object magnification up to ten. A new laser technology recently introduced allows the construction of very intense, compact and cost effective short pulse laser systems and makes now the ultrafast laser based hard X-ray sources very attractive for medical application     

Passive optical limiting of picosecond-nanosecond laser pulsesusing highly nonlinear organic liquid cored fiber array

We present a detailed account of theoretical and experimental studies of nonlinear molecular photonic processes accompanying the propagation of short intense laser pulses through an organic liquid cored fiber array. The theory takes into account two-photon absorption, excited state absorption, and dynamical evolution of the molecular energy level populations and the laser intensity. The coupled dynamical equations for the molecular energy-level populations and laser intensity are solved numerically using experimentally observed molecular and optical parameters. Experimentally obtained optical limiting results are analyzed along with nonlinear absorption and pump-probe studies of other nonlinear optical processes occurring in the fiber core. It is found that in the nanosecond time regime, besides excited state and two-photon absorption, thermal and density effects also contribute significantly to the optical limiting performance of the constituent fiber     

A TEA‐CO2 laser oscillation by an inductive energy storage pulsed‐power generator using a wire‐fuse opening switch

We have carried out experiments on TEA‐CO₂ laser oscillation using the inductive energy storage pulsed‐power generator, which has a copper wire fuse as an opening switch. Maximum laser output energy of about 1 J/pulse was obtained in the case of a fuse length of 5 cm and energy storage inductance of 8 μH. The laser output energy depends on the energy storage inductance and the parameters of the fuse. In this paper, the dependencies of laser output energy on inductance and fuse length, and a comparison between the inductive and capacitive system were described. Furthermore the laser efficiency was discussed by calculating the electron energy distribution of laser main discharge region. © 2000 Scripta Technica, Electr Eng Jpn, 132(1): 15–21, 2000     

Resolution and signal processing technique of surface charge density measurement with electrostatic probe

When an electrostatic probe is used to measure the surface charge on an insulating plate of constant thickness, the measuring system is regarded a shift-invariant system and the relation between the surface charge density and the probe output can be treated in the spatial frequency domain. The distribution of the surface charge density on an insulating plate just after occurrence of a surface discharge is measured by a Pockels probe, which is regarded as a kind of electrostatic probe without the guard electrode, and restored by Wiener inverse filter. The performance of a Pockels probe and a conventional electrostatic probe are compared quantitatively in terms of the spatial resolution. In the case that the measured object is 3 mm thickness PMMA plate and is charged up to 10 nC/cm/sup 2/ in atmospheric air, it is estimated that the spatial resolution of the Pockels probe with 0.2 mm gap is 1.5 mm and that of the conventional electrostatic probe with the grounded guard electrode with 3 mm gap is 2.2 mm.     

Noncontact surface charge density profiles measurement on top andbottom of a charged film

Equivalent surface charge density profiles on both the top and bottom surfaces of corona-charged films were measured without any contact by using a surface potential voltmeter with a horizontal resolution of about 2 mm and a laser displacement meter with a vertical resolution of 10 μm. A personal computer controls the sample position with a resolution of a few micrometers, which depends on a digital X -Y stage and a backside ground electrode position (distance from the bottom of the charged film). Surface potential profiles at a few different distances between the ground electrode (back) and the sample film were recorded on a floppy disk, and equivalent surface charge density profiles on the top and the bottom of the film were calculated and plotted automatically by using a personal computer. Some interesting electrostatic phenomena related to their charge behavior are reported as are its applications     

Repetition rate scaling up to 100 kHz of a small-scale (50 W) kinetically enhanced copper vapor laser

We report on the pulse repetition frequency (PRF) scaling of a small-scale (25 mm bore and 0.61 m long) copper vapor laser (CVL). When operated as an elemental CVL, the laser had a stable output power of 15 W at 15 kHz PRF (9.6% efficiency). After the addition of small quantities of hydrogen and hydrogen chloride to the neon buffer gas, the maximum recorded stable output power increased to 41 W at 25 kHz PRF (1.4% efficiency). This represents a record stable specific output power of 0.14 W cm/sup -3/. Pulse repetition frequency scaling of the laser was demonstrated up to 100 kHz where the output power was 9.0 W. By operating the laser at elevated input powers, transient output powers of over 50 W were achieved between PRFs of 25-40 kHz. These results are the highest recorded specific output powers (0.17 W cm/sup -3/) for a CVL with this tube diameter.     

Plasma-induced quantum well intermixing for monolithic photonic integration

Plasma-induced quantum well intermixing (QWI) has been developed for tuning the bandgap of III-V compound semiconductor materials using an inductively coupled plasma system at the postgrowth level. In this paper, we present the capability of the technique for a high-density photonic integration process, which offers three aspects of investigation: 1) universality to a wide range of III-V compound material systems covering the wavelength range from 700 to 1600 nm; 2) spatial resolution of the process; and 3) single-step multiple bandgap creation. To verify the monolithic integration capability, a simple photonic integrated chip has been fabricated using Ar plasma-induced QWI in the form of a two-section extended cavity laser diode, where an active laser is integrated with an intermixed low-loss waveguide.     

基于激光诱导荧光法诊断大气压低温等离子体射流中OH自由基和O原子的时空分布

由于传统辐射光谱法无法对大气压低温等离子体射流中OH自由基和O原子进行定量检测,本文利用自主研制的纳秒脉冲激励针筒型等离子体射流装置,基于单光子和双光子激光诱导荧光法分别对OH自由基和O原子的时空分布进行诊断。结果发现,OH自由基和O原子的寿命时间分别为1ms和3ms,远大于脉冲放电持续时间;采用拟合衰减曲线法,得到OH自由基的绝对密度为1012~1013cm-3;发现离喷嘴口越远,OH自由基和O原子密度越低。然而,即使距离喷嘴口数cm的地方,仍然存在大量的OH自由基和O原子;OH自由基和O原子的密度随激励频率和脉冲电压幅值的增加而升高,随H_2O含量和O_2含量的升高而出现先增大后减小的趋势。其中,当氦气中H_2O含量为0.012%时,OH自由基密度达到最大值。当氦气中O2含量为0.5%时,O原子密度达到最大值。本文研究为调控和优化低温等离子体射流中OH自由基和O原子密度提供重要科学依据。     

Theory of the strain-symmetrized silicon-based Ge-Si superlatticelaser

A unipolar p-i-p silicon-based intersubband laser consisting of a symmetrically strained GeSi superlattice on a relaxed Si_0.5Ge _0.5 buffer layer is modeled and analyzed. The strain-symmetrization removes the limitation on the size of the superlattice. The procedure for calculating the in-plane energy dispersion is extended to a superlattice. Analysis of the in-plane energy dispersion shows that the population inversion is local-in-k-space. For an 11 ML/11 ML superlattice (15.4 Å/15.4 Å), interminiband lasing between HH2 and HH1 is predicted at λ=2.2 μm. From the envelope functions and material properties, the miniband lifetimes and laser gain are calculated. For a current density of 10 kA/cm², a gain of G_L=96/cm is calculated. Alternate structures with larger expected gains are considered. Quantum-parallel, quantum-cascade, and quantum-staircase lasing are examined     

一种实现激光束对试样二维扫描的方法

应用多面体或电磁振镜扫描器结合步进电机驱动试样的同步平移,构成了激光束对交联聚乙烯电缆绝缘料试样片的二维扫描检查,通过测量激光束在试样处的直径和试样的一次行距,来计算和调整扫描复盖率,并对试样平移的运行状态进行了测试.该扫描系统成功地用于JS-1型激光扫描自动颗粒测量仪中,该系统较高的同步运行精度使测量仪的重复测量时的重复率误差小于5%.     

A desktop magnetic resonance imaging system

Modern magnetic resonance imaging (MRI) systems consist of several complex, high cost subsystems. The cost and complexity of these systems often makes them impractical for use as routine laboratory instruments, limiting their use to hospitals and dedicated laboratories. However, advances in the consumer electronics industry have led to the widespread availability of inexpensive radio-frequency integrated circuits with exceptional abilities. We have developed a small, low-cost MR system derived from these new components. When combined with inexpensive desktop magnets, this type of MR scanner has the promise of becoming standard laboratory equipment for both research and education. This paper describes the development of a prototype desktop MR scanner utilizing a 0.21 T permanent magnet with an imaging region of approximately 2 cm diameter. The system uses commercially available components where possible and is programmed in LabVIEW software. Results from 3D data sets of resolution phantoms and fixed, newborn mice demonstrate the capability of this system to obtain useful images from a system constructed for approximately $13 500.     

Development of a laser wavefront sensor for measurement of discharges in air

A laser wavefront sensor of the Shack‐Hartmann type capable of simultaneous measurement at two wavelengths was constructed. The sensor uses a single optical path, and is suited for applications such as high‐voltage discharges in air, in which large insulation distances are necessary. The sensor was used to detect optical path differences due to an impulse discharge in air across a needle–plane electrode gap of gap length 17 mm. Optical path differences were measured at laser wavelengths of 532 and 1064 nm at 1.6 µs after insulation breakdown across the gap. From the difference in optical path differences at the two wavelengths, a line‐integrated electron density distribution in the direction perpendicular to the gap was obtained near the high‐voltage electrode. The distribution had a peak value of 1.4 × 10²⁰m^?2 and a full width at half maximum of 4.4 mm. Because of the coverage of a large observation area including the entire gap, the spatial resolution was 1.4 mm, which was inadequate to obtain the precise form of the electron density distribution. However, the results show the possibility of applying laser wavefront sensing to electron density measurement. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 146(4): 10–17, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10247     

A desktop magnetic resonance imaging system.

Modern magnetic resonance imaging (MRI) systems consist of several complex, high cost subsystems. The cost and complexity of these systems often makes them impractical for use as routine laboratory instruments, limiting their use to hospitals and dedicated laboratories. However, advances in the consumer electronics industry have led to the widespread availability of inexpensive radio-frequency integrated circuits with exceptional abilities. We have developed a small, low-cost MR system derived from these new components. When combined with inexpensive desktop magnets, this type of MR scanner has the promise of becoming standard laboratory equipment for both research and education. This paper describes the development of a prototype desktop MR scanner utilizing a 0.21 T permanent magnet with an imaging region of approximately 2 cm diameter. The system uses commercially available components where possible and is programmed in LabVIEW software. Results from 3D data sets of resolution phantoms and fixed, newborn mice demonstrate the capability of this system to obtain useful images from a system constructed for approximately $13,500.     

Measurement of streamer charge distribution on an insulator in SF6 Gas

The charge density produced by streamers on an insulator surface in SF₆ has been investigated by using a probe method with a high-speed temporal resolution. Concentric circular probes, which also act as a plane electrode, are used in this probe method. Probe signals are observed oscilloscopically and converted into the charge densities through a numerical calculation. This method reveals the charge distribution before a disturbance caused by the “back discharge.” The charge density thus obtained ranges from several nC/cm² up to about 60 nC/cm². The density depends on the pressure, voltage height and the position of the streamer. The electric field on the insulator is analyzed numerically taking into account the surface charge. The internal electric field of the streamer is found to be 40 ～ 50 kV/cm · atm when the streamer ceases its propagation. However, it partly exceeds the critical one (89 kV/cm · atm) during the propagation.