Laser Power Fluctuations Research Articles

Forced oscillations of the keyhole in penetration laser beam welding

The dynamic behaviour of the keyhole in penetration laser beam welding is essential for the entire welding process, and is closely connected to welding seam defects and splash generation. To improve understanding of the process and to help interpret measured process signals, forced oscillations of the keyhole due to fluctuations of the CW laser power are theoretically studied. It is shown that even very small laser power fluctuations can lead to strong keyhole oscillations dominated by eigenfrequency response. Because the governing differential equation is nonlinear, bifurcation effects can occur which tend to broaden eigenfrequency peaks in the keyhole vibration spectrum. Calculated Fourier spectra are in good agreement with corresponding measurements of the ultraviolet and near infrared light emission during welding with an industrial CW laser.

Multiphase homodyne interferometry: analysis of some error sources

Some sources of error with multiphase homodyne interferometry are reviewed. A major advantage over the classic two-channel approach is that the inaccuracies that originate from laser-power fluctuations and drifts are shown to be automatically compensated for by proper adjustment of the light beams.

Frequency control of semiconductor lasers

This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency. For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three-level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.

Dichotomous noise-induced transitions in a bistable interference filter

The long-time behavior of a bistable interference filter subjected to external fluctuations is analytically investigated. The random forcing consists of Poisson dichotomous fluctuations in the incident laser power. The structure of the stationary distribution density is presented for various values of the noise parameters both as a function of the filter temperature and of its transmittance. This system illustrates many features typical of noise-induced transitions in nonlinear systems and lends itself easily to an experimental verification.

A wide-band high-sensitivity laser strainmeter

A new laser strainmeter, inspired by those used in the first gravitational waves detectors, is presented. A wide-band feedback circuit controls the optical length of the reference arm of an unequal-arm Michelson interferometer by means of a Pockels cell. No modulation is added to the Pockels cell supply, since laser power fluctuations are proved to be of negligible effect if some care is taken in balancing the two outputs of the interferometer. A reset circuit follows variations in arm length greater than λ, obscuring the servo for 50 μs. As a consequence, an analog signal combined with a digital signal allow to detect any change in the arm length greater than 10−3 λ. The frequency band extends from continuous signals to 100 kHz.

Shaping laser-scanner signals by double shadow-pulse differentiation

Scanning laser linear-dimension meters are widely used abroad in industry [i]; there have also been Soviet reports on early specimens of such systems [2]. A typical one contains an He-Ne laser, a scanning unit based on a multifaced prism and electric motor, a lens providing parallel displacement of the narrow scanning beam, and a second lens that focusses the beam after transmission through the measurement zone on the semiconductor photocell [3]. The size is determined from the pulse length provided by the shadow on intersection with the component. The shadow length is determined from a set photocurrent threshold, usually 0.5, where there is a direct proportionality between the size and the length. The 0.5 threshold provides maximum accuracy [4], and a linear amplifier has been described [5] to measure the pulse length for a fixed threshold. "However, in that system, there are stringent specifications for the accuracy of the threshold level, and any drift or fluctuation in the radiation power can cause several percent errror in the size measurement. The same effects arise from background illumination, thermal drift in the diode current, and distortion in the pulse crest due, for example, to dirt in the optics. Also, the crest is often distorted by interference fringes arising from reflections in the coherent beam at the component surfaces, which further reduces the accuracy in the threshold. The change in pulse amplitude AU arising from any of these factors results in an error in size measurement of AD = AU/U.d/2, where d is the beam diameter, which is proportional to the edge length in the pulse. For example, with AU/U = 0.i and d = 0.15 mm, the error in measuring the size is ~D = 7.5 #m. Better immunity is provided by a device for recording the edges, which is based on double differentiation of the pulse and isolating the signals for the first derivative dU/dt (Fig. ib) and the second one d2U/dt 2 (Fig. ic) [6]. Here the edge is identified from the point at which the second-derivative signal passes through zero (here the first derivative should exceed the level shown by the dashed line in Fig. Ib). That provides exact coordinate determination no matter what the pulse amplitude and eliminates effects from laser power fluctuations and other forms of interference whose spectra are narrower than the shadow pulse one. Here specifications for the detector components are formulated and tests and an implementation of the double differentiation method are described. From [5] calculations, the edge length in a scanning laser device is about tf = 2 ~sec, and the passband for the photocell system should be fl = i/tf = 0.5 MHz. To provide the maximum signal-noise ratio, the photodiode should be used in a photogalvanic circuit [7], while the amplifier should work in large-signal mode (U ~ 5 V) and provide a rise rate not less than v = 5 V/#sec. Large-signal operation and stage decoupling are provided by operational amplifiers OA. An FDK-142 fast silicon photodiode (~ = 80 nsec) was used, which provided a sensitivity S = i0 mA/im and a low noise level. It in combination with a KR544UD2A operational amplifier connected with negative current feedback provided a noise level in a i MHz passband less by a factor of 1.5-10 by comparison with FD-256 photodiodes (factor of 1.5), FD-7G (factor of 4), FD-I (factor of 8), and FD-24K (factor of 6-10). The diameter of the sensitive area is D = 15 mm, which simplifies the optical design. Tests with fast OA of various types (KR544UD2, KR574UDI, and KR754UD2) showed that in in large-signal mode (5-10 V), the best shapes were obtained for the leading and trailing edges for a pulse corresponding to a Laplace function [4] when the photodiode was used with a KR544UD2. The waveform showed no oscillation due to shock excitation of the OA, and the edges were symmetrical. The maximum passband was attained with weak illumination, flux power not more than 30 #W, in which the output current from the photodiode did not exceed 50 #A.

The effect of laser-induced heating upon the vibrational raman spectra of graphites and carbon fibres

Laser-induced sample heating has been shown to significantly shift the E 2 g vibrational Raman mode (1,580 cm −1) of graphite to lower frequency, with typical bandshifts of ~10 cm 1−20 cm −1, depending upon laser power. These shifts were only observed when small samples (particles or fibres) were examined using a Raman microprobe. The temperature sensitivity of the band position was determined to be ~ −0.03 cm −1 K −1 for a Lonza KS-5-75 graphite. A variety of carbons were studied, from amorphous carbons through graphites to carbon fibres, and all showed the same heating effect with similar bandshifts. Frequency shifts were also noted for the 1,355 cm −1 A 1 g mode, but the magnitude of these shifts was generally smaller than those of the E 2 g mode. These results have important ramifications for measurements of strain in carbon fibres, in that a Hercules HMS4 fibre showed an E 2 g mode frequency shift of −0.7 cm −1 per mW of laser beam power incident on the fibre surface with a 5 μm spot size. Laser power fluctuations can therefore cause bandshifts of magnitude comparable to those induced by fibre strain if they are not carefully controlled.

Self-optogalvanic effect of internal-mirror He-Ne lasers.

A self-optogalvanic effect in the laser positive column of internal-mirror He-Ne lasers is investigated both experimentally and theoretically. The effect of laser power fluctuation on the voltage across a laser discharge tube and its dependence on laser currents (3-7 mA) are observed. A general model based on the steady-state solution of rate equations is developed to interpret the measurements. Good agreement between experimental and calculated results is obtained.

Factors affecting the growth of single crystal fibers of the superconductor Bi 2Sr 2CaCu 2O 8

While short sections of single crystal fibers of the 85 K bismuth compound superconductor can be produced by the miniaturized laser-heated float zone process, it is much more difficult to produce extended lengths without the introduction of additional grains. Part of the problem is related to the peritectic nature of the material. Detailed studies have been performed to determine the influence of growth rate, seeding, and laser power (melt temperature) fluctuations on grain structure and second phase inclusions in fibers grown from a Bi 2.1Sr 1.8Ca 1.1Cu 2O x starting material. It was found that as the fiber diameter decreases, the maximum allowable growth rate increases due to an increase in the axial temperature gradient. Thermal fluctuations were shown to introduce CaO inclusions. Grain structure is not altered unless the fluctuations produce a large amplitude diameter variation. Several seeding techniques are discussed.

Comparison of Different Excitation Sources and Normalization Techniques in Laser-Ablation AES Using a Photodiode-Based Spectrometer

A new technique is evaluated for normalizing laser power fluctuations in laser-ablation atomic spectrometry. The technique involves measuring the light loss caused by scattering from the ablated material as it flows through a specially designed cell. The resulting measured optical density can be used to correct for variations in the amount of sample that has been ablated. This new approach is compared to the use of a matrix element as an internal standard. Three different excitation sources for AES (Ar-ICP, Ar-MIP, and He-MIP) were combined with laser ablation and evaluated with the new normalization approach. Although the overall performance is best for the Ar-ICP combination, the MIPs have some desirable characteristics (i.e., low background radiation) which in some cases lead to better results. The spectra were collected with a photodiode-based spectrometer that is designed for simultaneous multichannel detection and is therefore especially well suited to measurement and background correction of signals produced during transient sampling such as with laser ablation. The ability of such a spectrometer to deal with complex samples such as cast iron is discussed.

Simultaneous detection of laser-enhanced ionization and laser-induced fluorescence in flames: noise correlation studies

Random signal fluctuations of simultaneously detected laser-enhanced ionization (LEI) and laser-induced fluorescence (LIF) were investigated and analyzed for correlation with each other and with laser power fluctuations. A greater degree of noise correlation was found between LEI and LIF than between either technique and laser power fluctuations. A background correction method is proposed which subtracts the LIF signal of the background-inducing interfering element from the combined LEI signals from analyte and interferant. Noise correlation between LEI and LIF can be utilized to cancel the noise in the interfering background signal. Broadband spectral interference due to wing excitation of sodium LEI was used as an example to demonstrate the method. Excitation spectra and laser power dependencies of sodium LEI and LIF were also studied, and found to be identical.

Raman‐induced kerr effect spectroscopy of solutions

AbstractRaman‐induced Kerr effect spectroscopy (RIKES) as a tool for the investigation of species in solution is presented. RIKES has proved to be a particularly useful probe for Raman modes of molecules in solution because control of the polarizations of the interacting laser fields allows non‐resonant solvent contributions to the background to be minimized. New techniques to subtract laser power fluctuations from the signal have further increased the sensitivity of RIKES. Spectra of carbon tetrachloride, benzene and the hydroxide ion are presented to show the utility of RIKES for investigating solutions. The ability to decrease background noise by controlling the polarization of the pump laser was demonstrated by studying the 459 cm−1 carbon tetrachloride resonance. Concentration studies on benzene in carbon tetrachloride showed a narrowing of the 992 cm−1 mode of benzene as its concentration was decreased. The sensitivity of the technique is demonstrated with spectra recorded on 0.01 M benzene in carbon tetrachloride. The 3620 cm−1 OH− resonance is presented for a 10 M solution of NaOH in water. A nearby water resonance makes the OH− mode difficult to see at lower concentrations. Polarization tuning was not able to improve the RIKES sensitivity in this case.

Automatic impedance matching and opto-Hertzian effect in RF excited CO2 waveguide lasers

We report the successful operation of a novel automatic impedance matching technique (AIMing) and observations of the opto-Hertzian effect. The AIMing system is capable of maintaining efficient RF power delivery to a CO2 waveguide laser over a wide range of operating conditions. The opto-Hertzian signal, produced by circulating laser power fluctuations and detected in the RF reflected power, has some interesting properties and may be used for laser frequency stabilisation.

Study on recorded domain characteristics of magneto-optical TbFeCo disks

Domain characteristics (size and shape) of TbFeCo disks of several compositions are studied systematically through direct optical observation under various recording conditions. The relationships among recorded domains, magnetic properties, and recording conditions are also investigated. Magnetic films suitable for very high-density recording are found to have a compensation temperature between room temperature and 100 °C and a Curie temperature near 200 °C. In those films, the formation of domains is stable against fluctuations in laser power and external field. These results can be explained by a simple domain formation model in which coercive force, demagnetizing field, and wall energy at the domain wall are taken into account. Teardroplike domains are observed in the case of relatively long laser pulse recording. Such domains cause detection errors during the domain edge detection. It is also found that recording noise is high under certain recording conditions. The origin of this noise is supposed to be caused by inversely magnetized regions existing in a domain and irregularities in the outline of domains.

Method to compensate laser fluctuation in photon correlation spectroscopy

A method to compensate the laser power fluctuation in the photon correlation spectroscopy has been developed. In the method, as the key TTL IC element, rate multipliers are used to modulate the sequential digital photoelectron pulses by the parallel digital signal which is processed to be proportional to the inverse of the incident laser intensity. Details of the circuit design are given. To show the usefulness of the method, a case is illustrated in which the ac line noise is compensated in the observation of the Brownian motion of polystyrene latex spheres suspended in water. The bandwidth of the method is estimated to be about 180 kHz.

Dynamic phenomena in laser cutting and cut quality

The quality obtained with laser cutting, one of the most important processes of laser machining, is characterized by a nearly periodic pattern of striations that cause a certain roughness of the cut surfaces. Improvements of the cut quality, that would be of major technical importance, can only be obtained if the mechanism that leads to the formation of these striations is entirely understood. The present author has argued already in 1984 that temporal fluctuations of laser power can induce oscillations of the liquid layer formed at the momentary end of the cut and its temperature, thus causing distortions of the cut surfaces due to the movement of that layer in cutting direction. The numerical evaluation of the theoretical model yields satisfactory agreement of the “wavelength” of the striations and their dependance on the thickness of the workpiece with experimental values. So far, no attempt has been made to calculate not only the wavelength but also the depth of these striations. It is the purpose of this paper to determine the depth of the striations and to compare the numerical values with the experimental situation. Moreover, the paper is devoted to the explanation of the fact that the laser cut surfaces are showing not only one but usually two different striation patterns, one with a finer structure adjacent to the upper surface and one with a coarser pattern adjacent to the lower surface of the workpiece.

The use of discharge electric field variations to CO 2 laser stabilization

A method is described for detection of CO 2 laser power variations which requires no optical detector. It is based on electric field changes near the laser tube induced by the laser power fluctuations within the optical cavity. This phenomenon called by us “antenna effect” is used for laser stabilization. A simple and inexpensive technique for CO 2 laser power measurements, resonator adjustment, and for observation of the laser signature is described.

Scaling of state multipoles in rotationally inelastic transfer

Laser excitation of molecules in thermal cells gives rise to an anisotropic distribution of m states and thus fluorescence from these states, and from levels populated by inelastic collisions, is polarized. Neglect of fluorescence polarization can give rise to misleading conclusions concerning population transfer in the excited state. Precise measurements of the transfer of population, orientation, and alignment, through polarization ratios, can provide an excellent means for testing the various fitting laws for rotationally inelastic transfer through modified ‘‘sudden’’ scaling laws. The justification for this is fourfold: rotational line strength factors and laser power fluctuations do not affect the degree of polarization; thirdly, the effect of multiple collisions is less significant on the inelastic polarization transfer than it is on the population transfer as measured in conventional total fluorescence intensity measurements; and fourthly, the leading parameters present in fitting laws cancel out, reducing the number of fitting parameters by one. Scaling laws appropriate to polarization measurements are derived and various fitting relations are tested against Li*2/He and Li*2/Ar rotationally inelastic polarization data.

Chromosome analysis by high illumination flow cytometry.

Fluorescence measurements from metaphase chromosomes of the Chinese hamster, stained with propidium iodide excited at high illumination irradiance, completely resolve each chromosome type. The measurements are performed in a specially designed flow cytometer that achieves high irradiance (4 MW/cm2) by using high power laser output (2 W at 488 nm) focused to small spot size (1% irradiance variation over 2 microns). The coefficient of variation of each chromosome peak is near 1.5%. Saturation of the fluorescence transition and photobleaching, two consequences of high irradiance, are shown to occur. Even with a nonlinear dependence of fluorescence upon illumination irradiance, fluorescence retains a proportional response to chromosome type; each chromosome peak maintains a consistent ratio to the others at every irradiance. No perturbation of fluorescence by the optical or geometrical properties of the chromosomes is evident. The advantages of high irradiance illumination are an increase in fluorescence sufficient to reduce the statistical error in photoelectron number to a low level and reduced influence of laser power fluctuations and variable chromosome flow trajectories on the precision. These benefits improve the resolution of chromosome analysis by flow cytometry, particularly the resolution of smaller chromosomes.

Laser-power fluctuations: An analog signal-correction technique

A simple and inexpensive analog technique which corrects spectroscopic signals for pulse-to-pulse fluctuations in the output power of an excitation laser is described. The technique is applicable to both linear and nonlinear spectroscopy and, in the latter case, requires no fast electronic function generator. This design yields an improved time-constant characteristic which parallels the approach of the effective time constant toward RC. Furthermore, since this improvement does not require any preintegration, pulse shape information is preserved. A description of a photoacoustic (PA) spectrometer which incorporates the signal-correction device and which relies on the photogalvanic effect for wavelength calibration is presented as an example. Finally, signal-corrected PA spectra of the S1←S0 transition in azulene are presented.