Pulsed lasers emitting wavelengths near 400 nm can selectively ablate dental calculus without damaging underlying and surrounding sound dental hard tissue. Our results indicate that calculus ablation at this wavelength relies on the absorption of porphyrins endogenous to oral bacteria commonly found in calculus. Sub- and supragingival calculus on extracted human teeth, irradiated with 400-nm, 60-ns laser pulses at ≤8 J/cm 2 , exhibits a photobleached surface layer. Blue-light microscopy indicates this layer highly scatters 400-nm photons, whereas fluorescence spectroscopy indicates that bacterial porphyrins are permanently photobleached. A modified blow-off model for ablation is proposed that is based upon these observations and also reproduces our calculus ablation rates measured from laser profilometry. Tissue scattering and a stratified layering of absorbers within the calculus medium explain the gradual decrease in ablation rate from successive pulses. Depending on the calculus thickness, ablation stalling may occur at <5 J/cm 2 but has not been observed above this fluence.
A desired outcome of scaling and root planing is the complete removal of calculus and infected root tissue and preservation of healthy cementum for rapid healing of periodontal tissues. Conventional periodontal treatments for calculus removal, such as hand instrument scaling and ultrasonic scaling, often deeply scrape the surface of the underlying hard tissue and may leave behind a smear layer. Pulsed lasers emitting at violet wavelengths (specifically, 380 to 400 nm) are a potential alternative treatment since they can selectively ablate dental calculus without ablating pristine hard tissue (i.e., enamel, cementum, and dentin). In this study, light and scanning electron microscopy are used to compare and contrast the efficacy of in vitro calculus removal for several conventional periodontal treatments (hand instruments, ultrasonic scaler, and Er:YAG laser) to calculus removal with a frequency-doubled Ti:sapphire (λ = 400 nm). After calculus removal, enamel and cementum surfaces are investigated for calculus debris and damage to the underlying hard tissue surface. Compared to the smear layer, grooves, and unintentional hard tissue removal typically found using these conventional treatments, calculus removal using the 400-nm laser is complete and selective without any removal of pristine dental hard tissue. Based on these results, selective ablation from the 400-nm laser appears to produce a root surface that would be more suitable for successful healing of periodontal tissues.
The laser ablation rate of subgingival dental calculus irradiated at a 400-nm-wavelength, 7.4-mJ pulse energy, and 85- and 20-deg irradiation angles is measured using laser triangulation. Three-dimensional images taken before and after irradiation create a removal map with 6-μm axial resolution. Fifteen human teeth with subgingival calculus are irradiated in vitro under a cooling water spray with an ∼300-μm-diam, tenth-order super-Gaussian beam. The average subgingival calculus removal rates for irradiation at 85 and 20 deg are 11.1±3.6 and 11.5±5.9 μm/pulse, respectively, for depth removal and 4.5±1.7×105 and 4.8±2.3×105 μm3/pulse, respectively, for volume removal. The ablation rate is constant at each irradiation site but varies between sites because of the large differences in the physical and optical properties of calculus. Comparison of the average depth- and volume-removal rates does not reveal any dependence on the irradiation angle and is likely due to the surface topology of subgingival calculus samples that overshadows any expected angular dependence.
A noncontact profilometer (laser triangulation) was used to measure the removal rates of subgingival dental calculus irradiated with a frequency-doubled Ti:sapphire laser (60-ns pulse duration, 400-nm wavelength, 10-Hz repetition rate, 7-mJ pulse energy). Profilometer traces before and after irradiation were used to create a removal map with 4-μm axial and 15-μm transverse resolution. Twenty-three teeth (15 with calculus and 8 pristine) were irradiated at 90° and 45° under a cooling water spray with a super-Gaussian beam (~300-μm diameter). Subgingival calculus was selectively removed at 5.6 and 4.0 J/cm2 for 90° and 45°, respecetively, within a range of rates, between 2 to 9 μm/pulse. These ablation rates were constant during these exposures. For comparison, pristine cementum irradiated for 10 min at the same peak fluence and pulse repetition rate showed only craters, 15 to 50 μm deep, corresponding to an equivalent
removal rate three orders of magnitude smaller than that obtained for calculus. Pristine enamel was not removed under the same irradiation conditions.
A Nd:YAG laser-pumped, frequency-doubled Ti:sapphire laser is used for selective ablation of calculus. The laser provides ≤25 mJ at 400 nm (60-ns pulse width, 10-Hz repetition rate). The laser is coupled into an optical multimode fiber coiled around a 4-in.-diam drum to generate a top-hat output intensity profile. With coaxial water cooling, this is ideal for efficient, selective calculus removal. This is in stark contrast with tightly focused Gaussian beams that are energetically inefficient and lead to irreproducible results. Calculus is well ablated at high fluences ≥2J/cm2; stalling occurs below this fluence because of photobleaching. Healthy hard tissue is not removed at fluences ≤3 J/cm2.
Stanley Skupsky, Robert McCrory, R. Bahr, Thomas Boehly, T. Collins, Robert Craxton, J. Delettrez, William Donaldson, R. Epstein, V. Goncharov, R. Gram, D. Harding, Paul Jaanimagi, Robert Keck, James Knauer, S. Loucks, Frederic Marshall, P. McKenty, David Meyerhofer, Samuel Morse, O. Gotchev, P. Radha, Sean Regan, Wolf Seka, V. Smalyuk, John Soures, Christian Stoeckl, R. Town, M. Whitman, Barukh Yaakobi, Jonathan Zuegel, Richard Petrasso, J. Frenje, D. Hicks, C. Li, Frederick Seguin
Direct-drive laser-fusion ignition experiments rely on detailed understanding and control of irradiation uniformity, the Rayleigh-Taylor instability, and target fabrication. LLE is investigating various theoretical aspects of a direct-drive NIF ignition target based on an 'all-DT' design: a spherical target of approximately 3.5-mm diameter, 1 to 2 micrometers of CH wall thickness, and an approximately 350-micrometers DT-ice layer near the triple point of DT. OMEGA experiments are designed to address the critical issues related to direct-drive laser fusion and to provide the necessary data to validate the predictive capability of LLE computer codes. The cryogenic targets planned for OMEGA are hydrodynamically equivalent to those planned for the NIF. The current experimental studies on OMEGA address all of the essential components of direct- drive laser fusion: irradiation uniformity and laser imprinting, Rayleigh-Taylor growth and saturation, compressed core performance and shell-fuel mixing, laser- plasma interactions and their effect on target performance, and cryogenic target fabrication and handling.
The emission of hard x-rays, coincident with 3(omega) L/2 emission, has been measured in laser-interaction experiments on the OMEGA laser system, indicating fast electrons due to the two-plasmon-decay instability. The purpose of the present experiment was to determine the preheat level due to these electrons. In a calibration experiment a flat target containing titanium and vanadium layers was irradiated and the preheat was determined using two signatures: the intensity of K(alpha) lines from the high-Z target layers and the spectrum of hard x-rays measured by an array of BaF2 scintillation detectors. The intensity of both signals can be directly related to the preheat energy; therefore, the two preheat measurements can be cross calibrated. This comparison lends support to the determination of preheat in spherical target experiments, where only the hard x-ray diagnostic is available. For CH shell implosions and square laser pulses the preheat energy deposited in the target amounts to approximately 0.1 percent of the laser energy. The maximum rise in temperature due to the preheat is approximately 10 eV. Finally, the temperature of fast electrons is deduced from the measured spectrum of MeV protons accelerated from the surface of the target; the result agree with the temperature deduced from hard x-rays, namely, T approximately 100 to 200 keV.
An all-solid-state pulse-shaping system based on a single- frequency master oscillator, preamplifier, and aperture- coupled-stripline electrical waveform generator has been developed and implemented in the OMEGA laser fusion facility.
We have developed a compact laser system capable of amplifying nanosecond-scale pulses form a few picojoules to 20J. The system has a 40-mm clear aperture and a 37-mm working aperture for high-energy output. We measured less than 1 wave phase distortion over full 37-mm aperture for a pulse with 18-J output energy at a shot repetition rate of one shot every 10 min. In experiments with a 30-mm diam beam, a flat-top spatial profile with 4 percent rms over the entire beam diameter was demonstrated for a 1-ns pulse with 20_j output energy. The amplifier has a net gain up to 1013 and fits easily on a 5-ft X 14-ft optical table.
An optical pulse shaping system based on an aperture-coupled stripline electrical waveform generator is discussed. Shaped electrical and optical pulses have been produced. The measured pulse shapes agree well with our models.
The dual-tripler scheme for enhancing the bandwidth of third-harmonic generation, proposed by Eimerl et al., has been experimentally demonstrated for the conversion of 1054- nm radiation to 351 nm. The results are in excellent agreement with theory and show that the spacing between the triplers must be carefully controlled. Designs are given for the addition of a second tripler to OMEGA and the NIF. An approximately threefold increase in bandwidth is predicted for both laser systems.
We present the requirements, design, and experimental results for a negative feedback-controlled Nd:YLF regenerative amplifier for the OMEGA laser system. This externally synchronizable region boosts the energy of temporally shaped optical pulses from the subnanojoule to the submillijoule energy level with a measured long-term output energy stability of 0.2 percent rms. To our knowledge this represents the highest energy stability ever demonstrated for a millijoule-level laser system, either flashlamp pumped or diode pumped. In addition to the excellent stability and reproducibility, the regen output is very insensitive to the injected pulse energy and the temporal distortions due to the negative feedback are immeasurable. Four regenerative amplifiers equipped with this negative feedback system have operated flawlessly on OMEGA over the past two year period.
We have developed a diode-pumped, single-mode laser that provides stable amplitude output over many hours. The laser is capable of operating with three types of output: (1) as a single-mode, Q-switched laser, (2) as a single-mode, cw laser; and (3) as a Q-switched laser operating with two adjacent, equal-amplitude cavity modes to provide sinusoidally modulated output. No laser realignment is required to change the laser to these various outputs.
Previous measurements have demonstrated that the greatest limitation on the pulse-shape bandwidth of the Laboratory for Laser Energetics' OMEGA laser's driver line occurs within the electrical pulse-shaping subsystem during transmission through the photoconductive switches. A comparison of the spectral content of the shaped pulses before and after the switch indicated that the switch 3dB bandwidth was approximately 3 GHz. Since this was the limiting bandwidth of the system, it was necessary to measure and improve the microwave transmission through the switches. This challenge required a novel measurement scheme, described in this paper, to allow accurate optimization of the switch parameters that affect bandwidth. The switches were characterized and their 3dB transmission bandwidth was expanded to over 5 GHz, thereby improving them beyond the system bandwidth limits.
Infrared lasers have been investigated for the modification and ablation of dental hard tissue. Understanding the laser- tissue interaction relies on accurate determination of the optical properties of the tissue. Biological tissues absorb strongly in the infrared region of the spectrum at wavelengths resonant with vibrational absorption bands. In these regions scattering is negligible, therefore only the absorption coefficient and the reflectance are necessary to describe the laser energy deposition in the tissue. Measurement of absorption coefficients is a seemingly simple problem that presents many difficulties when a brittle material such as dental enamel is highly absorbing in the spectral region of interest. Disintegration of dental enamel ensues as a result of fracture before the sample is thin enough to allow direct transmission experiments to determine absorption coefficients at highly absorbing wavelengths (9.3 and 9.6 micrometer). Time resolved radiometric measurements combined with computer modeling of heat deposition and redistribution was used to estimate the absorption coefficient of enamel at various wavelengths in the infrared. This technique allows determination of absorption coefficients in regions not accessible to direct transmission measurements. Problems with the radiometric method and solutions to those problems are presented. The absorption coefficient at 9.6 micrometer was determined to be 5000 cm-1 plus or minus 1000 cm-1.
We have modeled the output of a feedback stabilized regenerative amplifier (regen). We solve the rate equations including upper- and lower-laser-level lifetimes explicitly. The complete regen dynamics including the losses due to the feedback stabilizer are modeled. We provide a prescription for determining the injection-pulse shape required to produce a given output-pulse shape from this region. The model shows excellent agreement to measured regen output. This model of the regen along with our code RAINBOW, completely describes the temporal dynamics of the OMEGA laser system allowing OMEGA users to specify on-target pulse shapes in advance.
We present the design and preliminary experimental data for the multicolor fiducial laser to be used as a primary timing reference for UV and x-ray streak cameras on the 40-kJ UV OMEGA laser system.
Accurate determination of the optical properties of dental enamel is important for modeling the energy redistribution of laser radiation incident on the tissue. The high absorption coefficients of dental enamel in the mid-IR preclude the use of conventional transmission methods for the determination of optical properties, therefore non- conventional methods must be used. Enamel was irradiated with 9.3 micrometers , 9.6 micrometers , 10.3 micrometers , and 10.6 micrometers light from a CO2 laser with a pulse duration of approximately 100 microsecond(s) . Samples were irradiated with fluences up to 2 J/cm2. These fluences produce temperature excursions below the melting point for the tissue. Surface temperatures were monitored using time resolved radiometric measurements with 1 microsecond(s) temporal resolution. A 1D heat conduction model was used to estimate the temperature increase in enamel. By varying the absorption coefficient, the model temperature versus time profile was fitted to the experimental profile. The absorption coefficients of enamel were found to fall well below values in the literature determined using angular-resolved reflectance measurements at the same wavelengths. The new absorption coefficients were used in our heat conduction model to determine temperature excursions below the surface of the tooth. This study is a vital step in determining the optimum laser parameters for caries preventive treatments.
The operational conditions of the OMEGA pulse-shaping require an extremely reliable and low-maintenance master oscillator. We have developed a diode-pumped, single- frequency, pulsed Nd:YLF laser for this application. The laser generates Q-switched pulses of approximately 160-ns duration and approximately 10 (mu) J energy content at the 1053-nm wavelength with low amplitude fluctuations and low temporal jitter. Amplitude and frequency feedback stabilization systems have been used for high long-term amplitude and frequency stability.
Time-resolved infrared (IR) radiometry was used to measure surface temperatures during pulsed Er:YSGG
(l=2.79 mm) and Er:YAG (l=2.94 mm) laser irradiation of dental enamel. Scanning electron microscopy (SEM)
was used to determine the melting and vaporization thresholds and to characterize other changes in the
surface morphology. The magnitude and temporal evolution of the surface temperature during multiplepulse irradiation of the tissue was dependent on the wavelength, fluence, and pre-exposure to laser pulses. Radiometry and SEM micrographs indicate that ablation is initiated at temperatures well below the melting and vaporization temperatures of the carbonated hydroxyapatite mineral component (1200 °C). Ablation occurred at lower surface temperatures and at lower fluences for Er:YAG than for Er:YSGG laser irradiation: 400 °C vs. 800 °C and above 7 J/cm2 vs. 18 J/cm2, respectively. However, the measured surface temperatures were higher at l=2.79 mm than at l=2.94 mm during low fluence irradiation (<7 J/cm2). Spatially dependent absorption in the enamel matrix is proposed to explain this apparent contradiction.
We review the basic laser ablation processes of dental hard tissue for wavelengths ranging from the IR to the UV. The underlying tissue removal mechanisms extend from water- mediated explosive, to thermomechanical, to plasma-mediated processes. This discussion is based on a literature review of the current state of hard tissue removal under various irradiation conditions combined with some new data using surface temperature measurements. The most effective tissue removal mechanism is the water-mediated explosive process in the IR at wavelengths between 3 and 10 micrometers . Highly controlled tissue removal at low ablation rates can be obtained in the near IR (around 1 micrometers ) using plasma-mediated ablation, provided the irradiation parameters are chosen appropriately. Similarly small ablation rates combined with good tissue specificity characterize the ablation in the UV region of the spectrum. The ablation mechanism in the UV is largely dominated by photothermal processes, although photochemical and thermomechanical processes may contribute.
Dental hard tissues can be ablated efficiency by (lambda) equals 3 micrometers laser irradiation with minimal subsurface thermal damage. However, the potential of lasers operating in the region of the infrared for caries preventive treatments has not been investigated. In this study, the caries inhibition potential of Er:YAG ((lambda) equals 2.94 micrometers ) and Er:YSGG ((lambda) equals 2.79 micrometers ) laser radiation on dental enamel was evaluated at various irradiation intensities. Pulsed IR radiometry and scanning electron microscopy (SEM) were used to measure the time-resolved surface temperatures during laser irradiation and to detect changes in the surface morphology. The magnitude and temporal evolution of the surface temperature during multiple pulse irradiation of the tissue was dependent on the wavelength, irradiation intensity, and the number of laser pulses. Radiometry and SEM micrographs indicated that ablation was initiated at temperatures of approximately 300 degree(s)C for Er:YAG and 800 degree(s)C for Er:YSGG laser irradiation, well below the melting and vaporization temperatures of the carbonated hydroxyapatite mineral component (m.p. equals 1200 degree(s)C). Nevertheless, there was marked caries inhibition for irradiation intensities below those temperature thresholds, notably 60% and 40% inhibition was achieved after Er:YSGG and Er:YAG laser irradiation, respectively. These results indicate that the Er:YSGG laser can be used effectively for both preventive dental treatments and for hard tissue removal.
Several studies in our laboratories have demonstrated that CO2 laser treatment of dental enamel can inhibit subsequent caries-like progression from 10 - 85% compared to controls. The roles of several of the laser parameters in the observed inhibition are still unknown. The aim of the present study was to examine the roles of pulse duration and repetition rate by the use of in vitro caries experiments, single and multiple pulse temperature measurements, and SEM observations, each following CO2 laser irradiation of dental enamel. We used pulse durations of 100 and 500 microsecond(s) at wavelengths of 9.3, 9.6, 10.3 and 10.6 micrometers , fluences of 2.5 or 5 J/cm2 per pulse, and 25 pulses per treatment window on human enamel. To study repetition rate we used 26 pulses, wavelength of 9.3 micrometers , at 1, 10, 25 or 40 Hz, with a fluence of 2.5 or 3.5 J/cm2 per pulse. A complex relationship among pulse duration, wavelength and fluence was demonstrated. Increasing the repetition rate improved the inhibition up to a plateau at 25 Hz. Optimum caries inhibition in enamel appears to be achieved by pretreatments that produce surface temperatures in the range of 800 - 1000 degree(s)C. Optimization of pulse duration, wavelength and repetition rate should allow for effective clinical caries inhibition with short treatment times and minimal change to surface topography of enamel.
The optical pulse-shaping system for the 60-beam 30-kJ (UV) OMEGA fusion laser is capable of producing complex temporally shaped optical pulses for amplification and delivery to fusion targets. The pulse-shaping system consists of optical modulators driven by an optically activated electrical waveform generator. The electrical waveform generator consists of Si photoconductive switches, and variable impedance microstrip lines. Complex optical pulse shapes with 50 to 100 ps structure have been produced.
An optical pulse-shaping system will be implemented on the OMEGA laser that is capable of producing complex optical pulse shapes. The pulse-shaping system relies on photoconductive switches that are activated with a pulse compressed by the stimulated Brillouin scattering (SBS) process. The SBS activation pulse provides overall system bandwidth and eliminates laser prepulse noise. The SBS process is modeled in detail to understand the performance and limitations of the OMEGA pulse-shaping system. Experimental results and numerical simulations are presented in the SBS pulse-steepening and pulse-compression regimes. Good qualitative agreement is obtained between theory and experiments.
The microwave transmission properties of a high-purity (greater than or equal to 40 k(Omega) (DOT)cm) single-crystal-silicon, photoconductive (PC) switch were measured while the switch was optically activated. The switch was 2.3 mm wide (the width of the microstrip electrode), 2 mm long, and 0.5 mm thick with a 0.5-mm photoconductive gap and was mounted in a 50-(Omega) microstrip transmission line. The switch was irradiated uniformly with a 150-ns FWHM pulse from a Nd:YAG laser (wavelength equals 1.064 micrometer). The insertion loss of the optically activated PC switch was nearly constant minus 0.7 dB across the measurement system bandwidth (9 GHz). Under these illumination conditions, the switch exhibited negligible bandwidth limitations. The microstrip structure by itself had an insertion loss that increased from minus 0.4 dB at 1 GHz to minus 1.4 dB at 9 GHz.
KEYWORDS: Absorption, Dental caries, Temperature metrology, Minerals, Pulsed laser operation, Teeth, Tissues, Carbon dioxide lasers, Scanning electron microscopy, Control systems
Several studies in our laboratories have demonstrated that CO2 laser treatment of dental enamel can inhibit subsequent caries-like progression from 10 - 85% compared to controls. The reasons for these observed effects are still unclear. The aim of the present study was to combine and interpret results from in vitro caries experiments, single and multiple pulse temperature measurements, SEM observations, and crystallographic measurements, each following CO2 laser irradiation of dental enamel or synthetic carbonated apatite. Optimum caries inhibition in enamel appears to be achieved by pretreatments that produce surface temperatures in the range of 600 - 1000 degree(s)C. For clinical application the surface enamel heating must not lead to consequent pulp chamber temperature rises of > 4 degree(s)C. To meet these conditions a pulsed laser with a sufficiently high absorption coefficient ((lambda) equals 9.3 or 9.6 micrometers ), pulse width near the thermal relaxation time of enamel (50 - 100 microsecond(s) ), minimum useful number of pulses (approximately 25), low repetition rate (approximately 10 Hz), and low but effective fluence (approximately 1-5 J/cm2/pulse), works well.
We present time-resolved reflection and surface temperature measurements of dental hard tissue irradiated with 100-microsecond(s) CO2 laser pluses at two different wavelengths (9.6 and 10.6 micrometers ). The fluences in these single-pulse experiments ranged from 1 to 6 J/cm2. Time-resolved temperature measurements indicate enhanced energy coupling (absorption) for the 9.6-micrometers laser light at fluences >= 6 J/cm2 compared to irradiation at 1 J/cm2. At the same time we observe a marked reduction in surface reflection. These results are consistent with a transition from strong, relatively narrow band absorption at low temperatures (low fluences) to weaker and broader band absorption (lower absorption coefficient) at higher temperatures. Near an absorption resonance the decreased absorption coefficient leads directly to reduced reflection losses and thus increased energy coupling to the target.
Surface temperatures were monitored using pulsed photothermal radiometry (PPTR) during multiple pulse carbon dioxide laser irradiation ((lambda) equals 9.3, 9.6, 10.3 and 10.6 micrometers ). Permanent changes in the optical properties (reflectance and absorption) were observed at fluences greater than 2 J/cm2 for dentin and 5 J/cm2 for enamel. The laser irradiation changes the thermal and the optical properties of these tissues, substantially changing the energy deposition for subsequent laser pulses. The temperature response of enamel and dentin and the reflectance of dentin changed considerably with successive laser pulses. After 10 to 50 pulses the surface stabilized and no further changes were noted. Scanning electron micrographs of the laser conditioned surfaces showed large crystals of modified hydroxyapatite (approximately equals 500 nm) devoid of the organic matrix. Presumably, the water and the interwoven biopolymer matrix had been carbonized nd vaporized. Caries inhibition measurements after multiple pulse irradiation of enamel indicate that the stable laser conditioned surface is more resistant to acid dissolution than untreated enamel.
An electrical pulse-generation system using two optically activated Si photoconductive switches can generate shaped electrical pulses with multigigahertz bandwidth. The Si switches are activated by an optical pulse whose leading edge is steepened by stimulated Brillouin scattering (SBS) in CCl4. With the bandwidth generated by the SBS process, a laser having a 1- to 3-ns pulse width is used to generate electrical pulses with approximately 80-ps rise times (approximately 4-GHz bandwidth). Variable impedance microstrip lines are used to generate complex electrical waveforms that can be transferred to a matched load with minimal loss of bandwidth.
KEYWORDS: Temperature metrology, Carbon dioxide lasers, Sensors, Teeth, Absorption, Scanning electron microscopy, Photomicroscopy, Signal detection, Black bodies, Pulsed laser operation
Human and bovine enamel samples were irradiated with a pulsed CO2 laser at (lambda) equals 9.3, 9.6, 10.3, and 10.6 micrometers with 5 - 10 J/cm2 pulses of 50 - 500 microsecond(s) duration in order to determine the required energy densities needed to fuse the enamel surface. The resulting temperature rise at the sample surface ranged from 500 to 1500 degree(s)C, as measured by a HgCdZnTe detector. The temperature was significantly higher at 9.3 and 9.6 micrometers than at 10.3 and 10.6 micrometers for the same absorbed energy. Scanning electron micrographs of the irradiated enamel revealed surface changes that were consistent with the surface temperature observations. The temperature rise at the ceiling of the pulse chamber determines the risk of pulpal necrosis. This temperature was measured using thermocouples and a thermal imaging camera for different pulse repetition rates and number of pulses. These results indicate that the more efficient absorption at (lambda) equals 9.3 and 9.6 micrometers may be used to fuse enamel at lower laser energies, therefore requiring less energy, significantly reducing the risk of pulpal necrosis during laser treatment.
The angular distribution of scattered laser light was measured at 543 nm (HeNe), 632 nm (HeNe), and 1053 nm (YLF) through 50 to 100 micrometers sections of enamel and dentin. The scattering distributions were strongly forward peaked at all three wavelengths, indicating that all samples were highly transmissive. The scattering distributions are very sensitive to surface scattering, and a large percentage of the incident light is internally reflected at the exit surfaces of these materials, masking the true scattering distributions. True bulk scattering distributions were measured in a bath of index-matching fluid. For 1 micrometers radiation there was no measurable absorption and only minimal highly forward-directed scattering in 100 micrometers enamel sections. In dentin there was somewhat more forward-directed scattering with less than 10% absorption in 100 micrometers sections. This information should be taken into consideration in any laser irradiation of dental surfaces at these wavelengths.
Thomas Boehly, Robert Craxton, Robert Hutchison, John Kelly, Terrance Kessler, Steven Kumpan, Samuel Letzring, Robert McCrory, Samuel Morse, Wolf Seka, Stanley Skupsky, John Soures, Charles Verdon
Funding for the OMEGA laser Upgrade project has been approved and the design of a 30-kJ, 351-nm, 60-beam laser system has begun. This system will provide a unique capability to validate high-performance, direct-drive laser-fusion targets. We highlight various design features of a system that will attain 1% - 2% irradiation uniformity with versatile pulse shaping, thus providing a wide range of irradiation options for direct-drive experiments. The Nd:glass laser system consists of 60 beamlines using 20-cm disk power-amplifiers, frequency- tripling, phase conversion with diffractive optics, and smoothing by spectral dispersion (SSD). The initial pulse shape will be achieved by co-axial propagation of two beams having different pulse widths. These 'main' and 'foot' pulses will occupy different portions of the laser aperture. In addition, electro-optic technology will be used for shaping and truncation of these pulses.
Pulse train timing jitter, or phase noise, is an important problem for systems which require multiple synchronized lasers. The authors report on sources observed in their system and show the results of a technique used to eliminate one source of timing jitter.
The achievement of high density implosions of direct-drive laser fusion targets with the 24-beam TJV-OMEGA laser
system places very stringent requirements on the irradiation uniformity on target. Non-uniformities in the irradiation pattern
must not exceed -1% mis. One of the prerequisites for establishing the level of uniformity attained is the very precise
measurement of the laser power balance. This entails that the laser power in each of the 24 beamlines be diagnosed
simultaneously with a precision of 1%, over a dynamic range of 1000. The nominal laser pulse shape is a 600-ps FWHM
Gaussian, but this can vary between individual beamlines due to beam-to-beam differences in the nonlinear processes of
frequency conversion or gain saturation in the laser amplifiers.
We have set up a pulse-shape measurement system in which we pick off a small fraction of the energy in each
OMEGA beamline, linearly attenuate it, and couple the light into a multi-mode optical fiber for transport to a 24-channel
streak camera. The signals are multiplexed in 2 groups of 12 beams each. The streak-camera output is amplified with a dual
intensifier system and recorded on a photometric quality, cooled CCD camera. In this paper, we will report on the system performance and progress to date.
We have investigated the processes involved in cutting biological tissues with surgical sapphire tips metal tips and sculpted quartz fibers. We have found that all of these devices make very inefficient use of expensive laser energy. We have further found that the cutting actions of all the tips tested rely primarily on the concept of " hot" tips operating at temperatures of 600C to 700C. The heat transfer between tip and tissue is primarily via blackbody radiation from the tip. The process is aided by the coincidence of the peak of the blackbody radiation spectrum for 600C to 700C at -3 im with the very strong water absorption peak at the same wavelength. Based on the " hot" tip concept the efficiency of these tips can be increased from the present 1 to 3 to near 100. The resulting reduced laser power requirements allow contemplation of diode laser sources with concomitant savings in complexity power and cooling requirements maintenance and lifetime improvements and significant cost savings. 1.
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