Showing papers in "IEEE Journal of Quantum Electronics in 1984"
2,467 citations
TL;DR: In this article, the authors present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime.
Abstract: We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.
645 citations
TL;DR: In this article, the development of a theory of four-wave mixing in photo-refractive crystals is described and the theory is solved in the undepleted pumps approximation with linear absorption and without using the undebleted pump approximation for negligible absorption, and the results are used to analyze several photorefractive phase conjugate mirrors.
Abstract: The development of a theory of four-wave mixing in photo-refractive crystals is described. This theory is solved in the undepleted pumps approximation with linear absorption and without using the undepleted pumps approximation for negligible absorption. Both the transmission and reflection gratings are treated individually. The results are used to analyze several photorefractive phase conjugate mirrors, yielding reflectivities and thresholds. The use of photorefractive crystals as optical distortion correction elements and experimental demonstrations of several of the passive phase conjugate mirrors are described.
574 citations
TL;DR: The visible and spectral profiles from the cancerous and normal tissues are substantially different; showing characteristic principal and secondary maxima that are assigned to fluorphors in different environments in the two types of cell media.
Abstract: The visible fluorescence spectra have been measured from cancerous and normal rat kidney and prostate tissues, and from cancerous rat and mouse bladder tissues. The spectral profiles from the cancerous and normal tissues are substantially different; showing characteristic principal and secondary maxima. These peaks are assigned to fluorphors such as flavins and porphyrins in different environments in the two types of cell media.
535 citations
TL;DR: The extent of thermal destruction of tissue by visible and near-infrared lasers is governed by heat deposition in the tissue, heat transfer, and temperature-dependent rate reactions.
Abstract: The extent of thermal destruction of tissue by visible and near-infrared lasers is governed by heat deposition in the tissue, heat transfer, and temperature-dependent rate reactions. Often the thermal response has been analyzed by linear models with constant coefficients that presume exponential absorption of the laser irradiation with depth. However, for weakly absorbing tissues, light scattering dominates the optical properties. These properties may be altered by thermal damage of the tissue. Also, thermal properties for tissue vary with temperature and water content. Typical therapeutic laser irradiations vaporize water in the tissue causing sharp increases in temperature beyond 100°C, and continued irradiation causes ablation of the tissue. Models for all these events are discussed.
488 citations
TL;DR: The second edition of Dainty's widely read book on laser and astronomical speckle as discussed by the authors is a beautiful soft-bound book identical to the first edition (sans typographical errors), except for a rewritten Chapter 7 on "Stellar Speckle Interferometry" and a new Chapter 8 on "Recent Developments" in the overall field of speckles.
Abstract: This is the second edition of a widely read book on laser and astronomical speckle. I have, through the years, extensively used the material in the first edition for teaching purposes and for “meaty” homework problems. The book is very readable, with good introductory material for helping the uninitiated “over the hurdles,” and yet, with a development in each subject to the state of the art in depth (as of 1975). The second edition is a handsome soft-bound book identical to the first edition (sans typographical errors), except for a rewritten Chapter 7 on “Stellar Speckle Interferometry” and a new Chapter 8 on “Recent Developments” in the overall field of speckle. Both are by J. C. Dainty. The other chapters are 1) “Introduction” by J . C. Dainty; 2) “Statistical Properties of Laser Speckle Patterns” by J. W .
439 citations
TL;DR: In this article, the authors analyzed the electronic dipole moment in quantum-well structures and derived the linear gain taking into account the intraband relaxation, and showed that the effects of the intrusion relaxation are 1) shift of the gain peak toward shorter wavelength with increasing injected carrier density even in quantum well structures, 2) increase of gain spectrum width due to softening of the profile, and 3) reduction in the maximum gain by 30-40 percent.
Abstract: The linear gain and the intervalence band absorption are analyzed for quantum-well lasers. First, we analyze the electronic dipole moment in quantum-well structures. The dipole moment for the TE mode in quantum-well structures is found to be about 1.5 times larger at the subband edges than that of conventional double heterostructures. Also obtained is the difference of the dipole moment between TE and TM modes, which results in the gain difference between these modes. Then we derive the linear gain taking into account the intraband relaxation. As an example, we applied this analysis to GaInAs/InP quantum-well lasers. It is shown that the effects of the intraband relaxation are 1) shift of the gain peak toward shorter wavelength with increasing injected carrier density even in quantum-well structures, 2) increase of the gain-spectrum width due to the softening of the profile, and 3) reduction in the maximum gain by 30-40 percent. The intervalence band absorption analyzed for quantum-well lasers is nearly in the same order as that for conventional structures. However, its effect on the threshold is smaller because the gain is larger for quantum wells than conventional ones. The characteristic temperature T 0 of the threshold current of GaInAs/InP multiquantum-well lasers is calculated to be about 90 K at 300 K for well width and well number of 100 A and 10, respectively.
415 citations
TL;DR: In this article, a detailed theoretical description of the thermal, stress, and beam propagation characteristics of a slab laser is presented, which includes consideration of the effects of the zig-zag optical path.
Abstract: Slab geometry solid-state lasers offer significant performance improvements over conventional rod geometry lasers. We present a detailed theoretical description of the thermal, stress, and beam propagation characteristics of a slab laser. Our analysis includes consideration of the effects of the zig-zag optical path which eliminates thermal and stress focusing and reduces residual birefringence.
314 citations
TL;DR: In this paper, the spectrum of an IM and a combined intensity-frequency modulated (IM-FM) monochromatic light source has been generated and the amplitudes of the modulated carrier and the first three pairs of sidebands are plotted showing the influence of IM on an FM signal.
Abstract: The spectrum of an intensity modulated (IM) and a combined intensity-frequency modulated (IM-FM) monochromatic light source has been generated. The amplitudes of the modulated carrier and the first three pairs of sidebands are plotted showing the influence of IM on an FM signal. The effects of first order chromatic dispersion on the baseband amplitude response and harmonic distortion are determined. The manner in which modulation type and depth, modulating frequency, wavelength, and fiber length alter harmonic distortion is presented. Numerical examples giving the amplitude response of a single-mode fiber system as well as the magnitude of the second- and third-harmonic distortion caused by chromatic dispersion are presented. Based on this material, the limits placed on analog transmission due to chromatic dispersion may be assessed.
294 citations
TL;DR: Theory and experiments on optical feedback effects in index-guided single-mode semiconductor lasers are presented in this paper, where evidence is found for the existence of a characteristic parameter C which indicates the relative strength of the optical feedback.
Abstract: Theory and experiments on optical feedback effects in index-guided single-mode semiconductor lasers are presented. Evidence is found for the existence of a characteristic parameter C which indicates the relative strength of the optical feedback. Near the transition ( C \approx 1.0 ) from low to high feedback, the feedback-induced low-frequency intensity noise shows a maximum. At higher feedback hysteresis and instabilities are dominant, whereas the feedback-induced noise is low again.
264 citations
TL;DR: In this article, a small signal method is used to measure the carrier lifetime as a function of injected carrier density, and the results are used to determine the radiative and nonradiative recombination rates for AlGaAs LED's and 1.3 μm InGaAsP lasers.
Abstract: A small signal method is used to measure the carrier lifetime as a function of injected carrier density, and the results are used to determine the radiative and nonradiative recombination rates for AlGaAs LED's and 1.3 μm InGaAsP lasers. For AlGaAs LED's the radiative recombination constant decreases with injected carrier density and the rate equation contains a small nonradiative Cn3term. The low internal efficiency of 1.3 μm InGaAsP lasers is found to be primarily caused by two factors: a radiative coefficient B(n) which strongly decreases with the injected carrier density, and CHHS Auger recombination having a recombination coefficient of 1-2 \times 10^{-29} cm6/s. A recombination term representing carrier leakage is observed in some devices, but it is not the principal cause of low internal efficiency.
TL;DR: In this paper, the effect of external optical feedback on the linewidth of a single-mode injection laser is considered theoretically, and a set of three rate equations with Langevin noise sources is used to obtain the power spectrum.
Abstract: The effect of external optical feedback on the linewidth of a single-mode injection laser is considered theoretically. A set of three rate equations with Langevin noise sources is used to obtain the power spectrum. If the high-frequency structure in the power spectrum is ignored, the line shape is Lorentzian and exhibits broadening or narrowing depending on the external-cavity phase shift. Particular attention is paid to the line narrowing after including the effect of carrrier-induced index changes.
TL;DR: In this article, a detailed theoretical analysis of stability for a semiconductor laser in an external cavity is presented, where the limits of stable operation are determined as a function of the external cavity parameters and the linewidth enhancement factor α.
Abstract: A detailed theoretical analysis of stability is presented for a semiconductor laser in an external cavity. The limits of stable operation are determined as a function of the external cavity parameters and the linewidth enhancement factor α. Instability is related to jumps of the laser frequency between external cavity modes (frequency bistability) or to feedback-induced intensity pulsations due to the carrier density dependence of the refractive index. The limit of bistability is derived from the steady-state solutions of the rate equations and the intensity pulsation limit is obtained from a small-signal analysis. This analysis also gives the location of zeros in the system determinant and the resulting FM noise spectrum. For practical applications we emphasize the determination of the stable tuning range for the phase in the external cavity and the classification of the possible types of instability for various feedback levels.
TL;DR: In this article, the influence of quantum phase fluctuations which affect single frequency semiconductor lasers in various coherent detection systems is discussed in terms of photocurrent autocorrelation and spectral density functions.
Abstract: The influence of quantum phase fluctuations which affect single frequency semiconductor lasers in various coherent detection systems is discussed in terms of photocurrent autocorrelation and spectral density functions. The general treatment given in this paper can be applied in diverse practical cases and points out the problems of phase correlation and phase matching between the two mixed optical beams. In the more general case the photocurrent spectrum is found to be composed of discrete and quasi-Lorentzian parts whose energies and spectral spreads are discussed as a function of the laser line width, the phase matching and the phase correlation between the two coherently combined fields.
TL;DR: In this paper, the quantum limits on simultaneous phase and squared-amplitude measurements made via optical heterodyne detection on a single-mode radiation field are established from a fully quantum mechanical treatment of heterodyning with ideal photon detectors.
Abstract: The quantum limits on simultaneous phase and squared-amplitude measurements made via optical heterodyne detection on a single-mode radiation field are established. The analysis proceeds from a fully quantum mechanical treatment of heterodyning with ideal photon detectors. A high mean field uncertainty principle is proven for simultaneous phase and squared-amplitude observations under the condition that the signal and image band states are independent, and the image band has zero mean. Operator representations are developed which show that no such principle applies when arbitrary signal/image band dependence is permitted, although the mean observations are no longer functions of the signal field alone. A multimode two-photon coherent state illustrating this behavior at finite energy is exhibited. Potential applications for the resulting improved accuracy measurements are briefly described.
TL;DR: In this paper, the traveling wave electrodes are laterally shifted periodically to reverse the direction of the applied electric field within the optical waveguide which exactly compensates for the polarity reversal caused by the microwave-optical walkoff.
Abstract: We propose and analyze a new technique for achieving velocity match between the traveling wave electrical drive and guided optical signal for modulators in substrates for which there is an inherent mismatch. The traveling wave electrodes are laterally shifted periodically to reverse the direction of the applied electric field within the optical waveguide which exactly compensates for the polarity reversal caused by the microwave-optical walkoff. Consequently, the electrooptically induced phase shifts of each section add in phase and several sections can be used to reduce the required drive voltage at the design frequency. This artificial velocity-matching technique moves the mismatch-limited bandwidth to an arbitrarily high design frequency. In addition, we extend the new concept of phase reversal and the previously suggested technique of intermittent interaction by proposing electrode structures with large inactive to active aspect ratios. This generalization provides increased flexibility for manipulating the total available bandwidth to, for example, allow efficient modulation by a train of arbitrarily short electrical pulses. These techniques are ideally suited for several proposed integrated optic devices, including picosecond samplers and gates, which require strong overmodulation at a single high frequency.
TL;DR: In this article, a model based on vitreous surgery is described which indicates that radiation at 2.9 μm (HF laser) in short duration pulses, shorter than the thermal relaxation time (1.7 μs) of its 1μm thick absorption depth in water, will minimize thermal diffusion, and also take advantage of the large amount of heat removed by the phase change of water into steam.
Abstract: Laser surgery may be improved by modifications based on similar processes in industrial applications. A major problem in materials processing is minimizing heat diffusion from the site of laser exposure. The same problem exists in the surgery of tissue with a CO 2 laser. A model based on vitreous surgery is described which indicates that radiation at 2.9 μm (HF laser) in short duration pulses, shorter than the thermal relaxation time (1.7 μs) of its 1μm thick absorption depth in water, will minimize thermal diffusion, and also take advantage of the large amount of heat removed by the phase change of water into steam. This model suggests that, for deep cuts, many pulses are preferable to a single long duration exposure, and that more delicate surgery may be possible with such short pulse, shallow absorption depth types of energy delivery. For coagulation (hemostasis control) as well as ablation, two simultaneous wavelengths are required: 2.9μm for cutting, and another with less absorption (such as Nd: YAG or argon lasers) for more penetration and heating of deeper layers and blood vessels. Among the secondary benefits of the use of the HF laser at 2.9μm is the ready availability of flexible optical fibers for a delivery system.
TL;DR: In this article, a series of theoretical studies of photon fluctuations in the light output of semiconductor injection lasers is presented by means of a simulation of the dynamic behavior of the laser based on numerical solutions of noise driven laser rate equations.
Abstract: This paper is the first of a series devoted to theoretical studies of photon fluctuations in the light output of semiconductor injection lasers. Statistics on laser photon fluctuations are collected by means of a simulation of the dynamic behavior of the laser based on numerical solutions of noise driven laser rate equations. This first paper in the series introduces the noise driven rate equations for a single-cavity laser, explains the method used for their numerical solution, and discusses some approximate analytical results. The second paper presents results of photon counting statistics for the single-cavity laser collected from numerical solutions of the time dependent equations. Additional papers in this series will concentrate on coupled-cavity and distributed feedback lasers. Comparison of these statistical results will show clearly the advantage of coupled-cavity and distributed feedback laser designs over conventional single (Fabry-Perot) cavity injection lasers.
TL;DR: In this article, the axial mode selection process in Q-switched laser via injection seeding is shown to be quite different from the frequency locking in CW lasers via injection locking.
Abstract: We have achieved stable Fourier transform limited single axial mode operation of an unstable resonator Nd:YAG oscillator by injection seeding of an external signal. A detailed theoretical and experimental treatment of axial mode selection via injection seeding is presented. Our study shows that the axial mode selection process in Q -switched lasers via injection seeding is quite different from the frequency locking in CW lasers via injection locking.
TL;DR: In this paper, a discussion of second harmonic generation with imperfect phase matching in the high conversion limit is presented, where phase modulation of the fundamental pulse, dispersion, central frequency wavevector mismatch, and fundamental depletion are included.
Abstract: -A discussion of second harmonic generation with imperfect phase matching in the high conversion limit is presented. Phase modulation of the fundamental pulse, dispersion, central frequency wavevector mismatch, and fundamental depletion are included. The results show that even small amounts of phase modulation or central frequency wavevector mismatch can limit harmonic generation and must be considered at high conversion efficiency. Both spatial and temporal phase matching parameters are more restrictive at high conversion efficiency because of the narrowing of the central phase matching peak. If the harmonic conversion process is overdriven, severe distortion of the harmonic and transmitted fundamental pulses can result. Cascade harmonic generation of neodymium laser radiation to 266 nm is presented as an example.
TL;DR: In this article, the authors derived analytical expressions for the power spectral densities of intensity and frequency noise of single mode semiconductor lasers in the presence of an optical feedback and obtained a behavior which, at high frequencies significantly differs from the usual one, and is in good agreement with recent experimental results.
Abstract: We derive analytical expressions for the power spectral densities of intensity and frequency noise of single mode semiconductor lasers in the presence of an optical feedback. By explicitly taking into account the spontaneous emission processes into the laser mode, we obtain a behavior which, at high frequencies significantly differs from the usual one, and is in good agreement with recent experimental results. In particular, we are able to show in which way the intensity and frequency noise, besides being influenced by the external cavity length, are affected by the presence of the well known resonant peaks in the noise spectra of the solitary laser and how a substantial lowering and flattening in the noise spectra can be obtained with external cavity round trip times shorter than the inverse of the peak resonant frequency. We also present experimental results in good agreement with the theoretical ones.
Abstract: An analysis of bistable Fabry-Perot action in reflection is presented. Optimum values for the cavity reflective coatings and cavity length are determined. Reasons are given why reflection-mode operation rather than transmission-mode operation should be preferred for device applications. Two prototype devices, an all-optical SR (Set-Reset) flip-flop and an all-optical clock, are described.
TL;DR: The analysis and design of two-section and multisection coupled-cavity lasers are treated in two parts in this paper, where the poles of a linear transfer function are used to obtain mode wavelengths and wavelength dependent threshold gains.
Abstract: The analysis and design of two-section and multisection coupled-cavity lasers are treated in two parts. In this first part, the focus is on two-section laser design and control using threshold gains. In the second part numerical analysis of the transient behavior is given. The present treatment begins by using the poles of a linear transfer function for the coupled-cavity laser to obtain mode wavelengths and wavelength dependent threshold gains. A general wave scattering matrix describes the intercavity coupling, so that simple waveguide discontinuities or complex arrays of discontinuities can be modeled. Numerical examples are given. Design relationships obtained primarily from the scattering analysis, together with numerical examples, show the dependence and interdependence of the laser mode wavelengths and threshold gain minima on various parameters such as cavity and coupler lengths, indexes of refraction, and gains. These relationships show optimum or preferred coupling gap lengths and cavity length ratios for various design criteria. For example, for optimum mode and spurious rejection stability a short ( \lsim40 \mu m) cavity can be coupled via a gap approximately an integer number of half wavelengths long to a second longer cavity ( \sim 100- 250 \mu m). Conversely, for maximum tunability (or sensitivity to input changes) together with good mode selectivity, two medium length cavities ( \sim 100-250 \mu m) differing in length by a small amount ( \lsim40 \mu m) should be chosen.
TL;DR: In this paper, the effect of mirror facets on lasing properties of distributed feedback (DFB) InGaAsP/InP laser diodes was studied theoretically and experimentally.
Abstract: The effect of mirror facets on lasing properties of distributed feedback (DFB) InGaAsP/InP laser diodes was studied theoretically and experimentally. A DFB laser with a window region was found to be suitable to examine the effect of mirror facets. The effective reflectivity of a window structure was calculated to be very small, typically as small as 0.03 percent for a few tens of micrometers of the window region. These small effective reflectivities were experimentally confirmed. Three kinds of DFB lasers, i.e., a double-window (DW), a single-window (SW), and a Fabry-Perot (FP), were discussed. Two modes with a separation corresponding to a so-called stopband appeared in a DW-DFB laser, in which almost no reflection at both ends was estimated. On the other hand, the threshold and the resonant wavelength of an SW- and an FP-DFB laser were found to be sensitive to the phases of corrugation at the facets. It turned out, however, that the mirror facet contributed to the single-mode operation due to an asymmetric resonant spectrum and to the reduction in the threshold. Although a low-threshold-current FP-DFB laser was experimentally obtained, the coincidence between the gain peak and the Bragg wavelengths was essential in this type. The SW-DFB laser seemed the most promising among the three types in terms of the stability of the single-mode operation.
TL;DR: In this paper, a comparison of parallel waveguides can be performed either by expressing the coupling of energy between individual, primary guides (coupled-mode approach) or by directly analyzing a structure consisting of all the guides and their surrounding media.
Abstract: The analysis of parallel waveguides can be performed either by expressing the coupling of energy between individual, primary guides (coupled-mode approach) or by directly analyzing a structure consisting of all the guides and their surrounding media (normal-mode approach). A comparison of these two approaches, as well as the derivation of relations enabling the conversion of data, when possible, from one approach to the other, are presented in this paper. Finally, the results are used to investigate the dependence of the coupling coefficient on the spacing between the guides. Although the derivations are performed for 2-D (slab) configurations, the results pertain to 3-D guides as well, by applying an effective refractive-index method. It is also shown that normal-mode analysis predicts incomplete transfer of energy for strongly coupled guides, even for perfectly symmetric configurations.
TL;DR: In this paper, a general theory for electrooptic shock (Cerenkov) radiation in transparent crystals having a linear electro-optic effect is presented, which obtains expressions for the radiated electric field, power spectrum, and total energy both 2D and 3D geometries are discussed as well as the proper focusing conditions.
Abstract: A general theory is presented for electrooptic shock (Cerenkov) radiation in transparent crystals having a linear electrooptic effect Significant amounts of shock radiation require subpicosecond pulses of suitably focused laser radiation to produce a moving pulse of polarization in the medium The theory obtains expressions for the radiated electric field, power spectrum, and total energy Both 2D and 3D geometries are discussed as well as the proper focusing conditions Numerical examples are given based on the nonlinear material LiTaO 3
TL;DR: In this article, visible elastic light scattering and fluorescence emission from carious and non-carious regions of teeth were measured and compared, and the spectral shape of the fluorescence excited by a coherent source was similar to emission produced by incoherent (lamp) excitation.
Abstract: The visible elastic light scattering and fluorescence emission from carious and noncarious regions of teeth were measured and compared. Carious teeth regions scattered light less effectively than noncarious regions for the spectral region 480-600 nm. The spectral shape of the fluorescence excited by a coherent source was similar to emission produced by incoherent (lamp) excitation. For both modes of excitation the emission peaks at approximately 550 nm. Differences in the fluorescence and light scattering spectra from carious and noncarious regions at a given wavelength was found not to be constant across the spectrum. Laser-induced fluorescence spectra for gamma irradiated teeth (total dose of 2000 rad) showed differences from control samples; the emission was blue shifted relative to the unirradiated samples. In addition, the relative visible absorption spectra for a tooth was measured in the visible region.
TL;DR: In this article, the authors evaluated the contribution of a few components of a mode-locked dye laser cavity to the dispersion of pulses of about 100 fs or shorter and found that the dispersive contributions should influence the mode-locking behavior for pulses of up to 100 fs.
Abstract: The contribution to dispersion of a few components of a mode-locked dye laser cavity has been evaluated. The chirp produced by the self-phase modulation within the saturable absorber has also been evaluated and compared with that resulting from the dispersive components. Cavity mirrors and blocks of transparent materials (glass or quartz) of a length exceeding 1 mm give the largest dispersion. The effect of self-phase modulation, at least for high saturation of the absorber, seems to play a comparable role. The dispersive contributions should influence the mode-locking behavior for pulses of about 100 fs or shorter.
TL;DR: In this article, the complete expression for the third-order nonlinear susceptibility (chi(3) expression is given, including all 48 terms, and the diagrammatic and perturbative approaches to its derivation are matched and compared.
Abstract: The complete expression, including all 48 terms, for the third-order nonlinear susceptibility \chi^{(3)} is written down explicitly. The diagrammatic and perturbative approaches to its derivation are matched and compared. The use of nontime-ordered (one-sided) diagrams is discussed and shown to give incomplete results.
TL;DR: In this article, an intensifier based fluorescence imaging system for localization of carcinoma in situ of the lung, aided by digital background subtraction in a video system, was developed, where a ratio fluorometer and spectrum analyzer were added for quantitative, noncontact measurements of fluorescence intensity and spectrum of HPD fluorescence and autofluorescence of tissue.
Abstract: Hematoporphyrin derivative (HPD) is selectively retained in malignant tumors for several days after intravenous injection. It fluoresces in the red (600-720 nm) when excited by shorter wavelengths, especially violet (near 400 nm). While fluorescence efficiency is not high, it can be detected at parts per million concentrations when excited by a low-background source such as a krypton ion laser (413 nm), and the fluorescence detected by an image intensifier or photomultiplier tube. We have developed an intensifier based fluorescence imaging system for localization of carcinoma in situ of the lung, aided by digital background subtraction in a video system. A ratio fluorometer and spectrum analyzer have been added for research and for quantitative, noncontact measurements of fluorescence intensity and spectrum of HPD fluorescence and autofluorescence of tissue. Research is continuing to determine the optimum time after injection for best contrast, and the maximum allowable dosage of HPD, limited by the induced temporary photosensitivity of the skin. In diagnosis of epithelial or superficial tumors, the monochromatic radiation and suitability for coupling through fiberoptics, as well as reasonable power (typically 15 mW violet is used) are the principal advantages of the laser.