Single CW Research Articles

Increase in weld penetration depth by two Nd: YAG laser beams combined

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

Ｎｄ：ＹＡＧレーザ２ビーム合成による溶込み深さの増大

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

Performance of M-ary/spread spectrum communication system in the presence of continuous wave interference

This paper describes an evaluation method and performance improvement for an M-ary/spread spectrum (SS) communication system in the presence of continuous wave interference. First, we introduce a theoretical equation for the average bit error rate of M-ary/SS receiver in a transmission line with AWGN and single CW interference. Second, we define the required jamming margin for M-ary/SS receiver to satisfy a certain level of average bit error rate. Then we analyze the theoretical value of the required jamming margin and evaluate the performance of receiver by a simulation. From this study, we found that the jamming margin relates to the spectrum spreading factor of the total system and does not relate to code length of pseudo-random noise sequence. To improve performance against CW interference, we first examine the effect of improvement when we use error correcting code as a spreading code. However, we cannot find any effect of the improvement, even if the jamming margin relates to spectrum spreading factor and even if there is a coding gain by the error correcting code. Second, we studied the performance improvement when whitening filter based on LMS algorithm is added to the receiver to reduce CW jamming. For a spreading factor between 7 and 25 dB, the jamming margin can be improved by 15 dB to 25 dB. © 1998 Scripta Technica. Electron Comm Jpn Pt 3, 81(5): 32–40, 1998

Upconversion fluorescence of Ho 3+ ions in a BaF 2 crystal

Efficient upconversion luminescence from the Ho 3+ C 3v centre in a BaF 2 crystal is reported. The red-to-green and red-to-blue upconversion is observed upon pumping with a single cw red dye laser. The upconversion mechanism is assigned as a sequential two-photon excitation process in which the excitation wavelength is resonant with either ground state absorption (GSA) or excited state absorption transitions. The efficient green upconversion with excitation into GSA results from a double-resonant excitation between the 5I 8→ 5F 5 and 5I 7→ 5F 3 transitions.

Impact of the environment on energy spreading loss in shallow water at high frequencies

Energy spreading loss (ESL) is qualitatively defined as reduction in peak echo level due to time spreading of the transmitted acoustic pulse. An analysis of the impact of shallow water propagation on ESL was performed using a time domain propagation loss program to compute the spreading of the received pulse due to multipath propagation. Typical sound speeds and a Hamilton geoacoustic model of Area Foxtrot off the U.S. eastern seaboard was used. ESL’s impact on sonar performance was determined as a function of range, depth, sound speed and geoacoustic properties. The impact of shallow water propagation on the correlation of the propagated pulses through the quantitative definition of mismatch loss (MML) was also discussed. Strong ESL existed over a reflective bottom and was generally invariant with range. ESL was correlated with TL. ESL was not as large over absorptive bottoms due to increased absorption of the bottom refracted path, and thus, reduction in the number of multipath modes. Broadband pulses were found to exhibit fewer fluctuations than narrowband signals, and the total TL loss was larger than a single cw case. To overcome the ESL, integration techniques based on an accurate prediction model are required with high temporal resolution of the echo energy.

Finite element analysis of temperature controlled coagulation in laser irradiated tissue.

The Theoretical study of thermal damage processes in laser irradiated tissue provides further insight into the design of optimal coagulation procedures. Controlled laser coagulation of tissue was studied theoretically using a finite element method with a modulating laser heat source to simulate feedback controlled laser delivery with a constant surface temperature. The effects of uncertainty in scattering and absorption properties of the tissue, thermal denaturation induced changes in optical properties, and surface convection were analyzed. Compared to a single pulse CW irradiation in which a doctor would presumably stop CW laser delivery after noticing some effect such as vaporization or carbonization, the constant surface temperature scenario provided a better overall control over the coagulation process. In particular, prediction of coagulative damage in a constant temperature scenario was less sensitive to uncertainties in optical properties and their dynamic changes during the course of coagulation. Also, subsurface overheating under surface convective conditions could be compensated for under constant temperature irradiation by lowering the surface temperature.

A longitudinal and side-pumped single transverse mode double-clad fiber laser with a special silicone coating

A single transverse mode neodymium-doped double-clad cw fiber laser at λ = 1.06 μm with 286 mW output is described. A 25.0% slope efficiency with respect to absorbed pump power has been reached with longitudinal cladding-pumping at 805 nm. We describe here, what is to our knowledge the first side pumped double-clad fiber laser. Side-pumped operation has been realised with a prism-fiber coupler. An output power of 143 mW and a slope efficiency of 21.2% with respect to absorbed pump power have been achieved. The delicate polishing of the fiber for side-pumping is not necessary, because the silicone coating can simply be removed from the fiber. Saturation effects have been estimated.

NdドープLiTaO_3単結晶の育成と室温レーザ発振( LN,LT結晶の育成と光損傷)

NdドープLiTaO_3単結晶の育成と室温レーザ発振( LN,LT結晶の育成と光損傷)

Line shapes of saturated absorption spectra in hollow cathode discharges

The saturated absorption line shapes of sputtered uranium atoms in hollow cathode discharges have been investigated using a single axial mode tunable CW laser. The spectral profiles observed under specific experimental conditions exhibit a marked deviation from the usual saturated absorption line shape function. These new features appear under the low velocity changing collision rate, high quenching rate and in the regime of strong pump saturation. A generalized line shape function based on population dynamics along with quenching mechanisms is derived and used to discuss the observed spectra. Our results suggest that the additional features in the observed spectra may be attributed to coherent effects which are manifested under suitable experimental conditions.

Applications of Organic Electro-Optic Materials in High Speed Electronic Processors

ABSTRACTThe exceptional electro-optic properties of poled polymer films, coupled with the power and flexibility of thin film fabrication and photolithographic processing, may make possible a new class of integrated optic systems combining the processing power of VLSI with a dense, high bandwidth, photonic interconnection and switching network. We report on the recent development and initial test results of two electro-optic polymer based integrated optic devices for optical interconnection applications. The first is an optical railtap for the distribution of many different optical signals from a single CW laser diode, and the second is a traveling wave Mach-Zehnder integrated optic modulator, which was modulated at frequencies up to 20 GHz.

Velocity measurement using the phase shift of a laser-induced grating

We have developed a simple and general technique for measuring the flow velocity over a wide range of values. It involves creating a cw light-induced grating and measuring the velocity-induced phase shift occurring in a known time period. It uses polarization spectroscopy and optical heterodyne detection with two beams from a single cw laser. Measurements are made of the two spatial phase components of the induced steady-state grating, and their ratio determines the velocity. Because a ratio is computed, the effects of many possible error sources cancel out. We have demonstrated this method for an approximately 1-m/sec flow of ethanol seeded with an absorbing, nonfluorescing dye.

Operation of a tunable gyrotron at the second harmonic of the electron cyclotron frequency

The University of Sydney's GYROTRON IV is a single CW source of low-power millimetre-submillimetre radiation that can be tuned across a broad frequency range from below 100 GHz to above 500 GHz. The higher frequencies are obtained when the gyrotron operates at the second harmonic of the electron cyclotron frequency. The conditions for achieving second harmonic operation are discussed.

Hyperfine structure in the 6snh (l = 5) Rydberg series of barium

The hyperfine structure in the odd-parity 6snh (9 ≤ n ≤ 40) Rydberg series of barium has been investigated, using a single cw ring dye laser and a beam of neutral atoms in the metastable 5d2 1G4 state. Owing to small fine-structure splitting, this hyperfine structure is dominated by the Fermi contact interaction of the 6s electron already at n = 12. The hyperfine-structure analysis results in a determination of the complete fine structure of the 6snh series as well as in values for the singlet–triplet mixing in the 6snh J =5 states. The 6snh series turn out to be perturbed by levels of the 5d4f configuration; J = 4 and J = 5 levels of this configuration have been observed. The Rydberg series perturbations are analyzed, using multichannel quantum-defect theory in the shifted R-matrix formalism. Far from the 5d4f perturbers, the 6snh configuration is almost purely jj coupled.

Continuous-wave self-pumped optical phase conjugation in atomic sodium vapor

Self-pumped optical phase conjugation has been demonstrated for the first time to our knowledge in a resonant atomic system. This process was implemented by using a single cw pump beam to excite a sodium-vapor oscillator. The counterpropagating optical fields inside this oscillator then combine with the pump beam in an internal fourwave-mixing interaction to yield the phase conjugate of the incident (pump) beam.

Multifrequency Doppler flowmeter: a way to rapid CW Doppler imaging

A single CW velocimeter for two-dimensional Doppler imaging yields a low frame rate because the single transducer pair must be moved in the x, y plane. The authors propose to increase the frame rate by measuring simultaneously the flow in several directions using several transmitter/receiver pairs, each with a unique emission frequency and having a separate CW velocimeter. The results obtained with a prototype composed of eight channels show that (1) the lateral resolution and the sensitivity of each channel are not affected by interference from the adjacent channels; and (2) it is possible to measure the flow simultaneously in multiple cross sections because the ultrasonic beams are independent.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical–optical double resonance and double resonance multiphoton ionization in 7Li2 produced by single cw dye laser excitation

A comparison of fluorescence detection of optical–optical double resonance transitions with ionization detection of double resonance multiphoton ionization signals was made for Li2 vapor. The spectra in both cases were excited by a single broadband (30 GHz) cw dye laser and exhibited identical features. These arise from ‘‘accidental’’ two-photon double resonance transitions via real intermediate states and consist of hundreds of distinct transitions in the region 565–645 nm studied in this work. Analysis and assignment reveal that multiphoton excitations within the singlet manifold of electronic states of Li2 together with intermediate state rotational relaxation can account for the spectra. A table of vibrational and rotational assignments of the more intense features is available which includes a comparison between calculated and observed intensities for the transitions.

High-Power RF Controls for the NBS-Los Alamos Racetrack Microtron

The high-power rf system for the National Bureau of Standards (NBS)-Los Alamos racetrack microtron (RIM)l uses waveguide power splitters2 and waveguide phase shifters to distribute rf power from a single 500-kW cw klystron to four side-coupled accelerating structures. The amplitude and phase of each structure is controlled by a feedback system that uses the waveguide variable power splitters, waveguide phase shifters, and klystron drive as the active control elements. A block diagram of this system is shown in Fig. 1. Figure 2 shows a subset of the complete system on which the measurements reported in this paper were performed. The feedback controls on the capture section use low-level rf amplitude and phase controls on the rf drive to the klystron. These controls are very fast with an open loop gain bandwidth of approximately 40 kHz. The feedback loop is identical to the feedback loop used in the chopper/buncher system described in another paper at this conference. 3

Rejection of CW Interference in QPSK Systems Using Decision-Feedback Filters

In this paper, the performance of QPSK systems using complex transversal filters with additional decision-feedback taps, in the presence of Gaussian noise and a single CW interferer, is analyzed. Both one-sided (with lagging taps) and two-sided transversal filters with additional decision-feedback taps are considered. Analytic expressions for the tap weights and the minimum mean square errors are obtained. The effect of error propagation on the error probability is discussed and an approximate solution for the error probability is obtained. The transient behavior of the filters using the LMS adaptation algorithm is analyzed. It is shown that if the filter is used for rejecting CW interference only, the one-sided decision-feedback filter is preferred.

The optical–optical double resonance spectrum of 7Li2 excited by a single tunable cw laser

The two-photon excitation spectrum of 7Li2 produced by a single tunable cw dye laser operating between 570 and 650 nm has been observed using ultraviolet fluorescence detection. The spectrum arises from optical–optical double resonance transitions from the X 1Σg+ ground electronic state to the F 1Σg+ and G 1Πg excited states via the A 1Σu+ intermediate state, in which vibrational and rotational relaxation also take place. Using the known molecular constants for these states, a theoretically predicted spectrum was found from which the prominent lines in the experimental spectrum could be assigned. The multiphoton ionization spectrum of Li2, previously observed and interpreted in terms of bound–free–bound triplet absorption bands, can be explained as OODR transitions among the above states followed by photoionization.

The fluorescence excitation spectrum of chromyl chloride (CrO2Cl2) cooled in a free jet expansion

The fluorescence excitation spectrum of CrO2Cl2 in a free jet has been studied using a single mode tunable CW dye laser. Spectra were recorded for the A4 (origin), A5, and D5 bands. Sequence congestion has been entirely eliminated and detailed rotational structure of the bands observed for the first time. Assignment of the A4 origin band indicates the excited state to be B1 in vibronic symmetry under the C2v point group. The best least squares fit rotational constants are: A″=0.1073±0.0002 (cm−1); A′=0.0910±0.0003 (cm−1), B″=0.0620±0.0004 (cm−1); B′=0.0659+0.0003 (cm−1); C″=0.0521±0.0010 (cm−1); C′=0.0524±0.0010 (cm−1). Assignment of the D5 band indicates it is based on the same electronic transition but involves a symmetric vibration in the excited state. Possible geometries for the ground and excited states are also reported.