High power gas laser - Applications and future developments

The inability of edge-emitting gain-guided (Chap. 5) and most laser-array oscillators (Chap. 6), as well as surface-emitting laser arrays to operate single frequency, and at brightness and power level above single-element diode lasers, provided an impetus to researchers to investigate alternative structures. In recent years, the Master-Oscillator Power-Amplifier (MOPA) architecture, which was already well known in the fields of large high-power gas, dye, and solid-state laser systems [1.272, 2.32, 2.37, 2.38, 4.16][8.1]–[8.3], as well as optical telecommunications [8.4]–[8.7], has emerged as a preferred approach for obtaining high-power, single-frequency, beam-quality, diode-laser sources. The mode discrimination can be considerably simpler in a MOPA diode-laser array, as compared to a diode-laser array oscillator. In a MOPA architecture, the output of a low-power, single-frequency laser oscillator is injected unidirectionally into an optical amplifier of greater output-power capacity. Here, what is meant by unidirectional coupling is that a negligible light level from the amplifier is injected back into the oscillator. This is also referred to as nonreciprocal coupling. In principle, the unidirectional coupling means that the frequency characteristics and power output levels can be set independently of each other. Of course, the advantage here is that the spectral-and spatial-mode instability with drive current, which is pervasive in diode-laser array oscillators, can be avoided. A critical requirement for all MOPA designs is that the light levels injected back into the oscillator from the amplifier be sufficiently low that the frequency stability of the oscillator is maintained and self oscillation of the power amplifier is avoided.

Gas lasers have shown to be capable of delivering tens of terrawatt aspeak power or tens of kilowatt as average power. The efficiencies of most high power gas lasers are relatively high compared with other types of lasers. For instance molecular lasers,oscillating on low lying vibrational levels, and excimer lasers may have intrinsic efficiencies above 10%. The wavelengths of these gas lasers cover the range from the far infrared to the ultra-violet region, say from 12000 to 193 nm. The most important properties are the scalability, optical homogeneity of the excited medium, and the relatively low price per watt of output power. The disadvantages may be the large size of the systems and the relatively narrow line width with limited tunability compared with solid state systems producing the same peak power.

The success of high-power fiber lasers is fueled by maturation of active and passive fibers, combined with the availability of high-power fiber-based components. In this contribution, we first overview the enormous potential of rare-earth doped fibers in spectral coverage and recent developments of key fiber-based components employed in high-power laser systems. Subsequently, the emerging functional active and passive fibers in recent years, which exhibit tremendous advantages in balancing or mitigating parasitic nonlinearities hindering high-power transmission, are outlined from the perspectives of geometric and material engineering. Finally, novel functional applications of conventional fiber-based components for nonlinear suppression or spatial mode selection, and correspondingly, the high-power progress of function fiber-based components in power handling are introduced, which suggest more flexible controllability on high-power laser operations.Graphical abstract

Nearly every new large-scale laser system application at LLNL has requirements for beam control which exceed the current level of available technology. For applications such as inertial confinement fusion, laser isotope separation, laser machining, and laser the ability to transport significant power to a target while maintaining good beam quality is critical. There are many ways that laser wavefront quality can be degraded. Thermal effects due to the interaction of high-power laser or pump light with the internal optical components or with the ambient gas are common causes of wavefront degradation. For many years, adaptive optics based on thing deformable glass mirrors with piezoelectric or electrostrictive actuators have be used to remove the low-order wavefront errors from high-power laser systems. These adaptive optics systems have successfully improved laser beam quality, but have also generally revealed additional high-spatial-frequency errors, both because the low-order errors have been reduced and because deformable mirrors have often introduced some high-spatial-frequency components due to manufacturing errors. Many current and emerging laser applications fall into the high-resolution category where there is an increased need for the correction of high spatial frequency aberrations which requires correctors with thousands of degrees of freedom. The largest Deformable Mirrors currently availablemore » have less than one thousand degrees of freedom at a cost of approximately $1M. A deformable mirror capable of meeting these high spatial resolution requirements would be cost prohibitive. Therefore a new approach using a different wavefront control technology is needed. One new wavefront control approach is the use of liquid-crystal (LC) spatial light modulator (SLM) technology for the controlling the phase of linearly polarized light. Current LC SLM technology provides high-spatial-resolution wavefront control, with hundreds of thousands of degrees of freedom, more than two orders of magnitude greater than the best Deformable Mirrors currently made. Even with the increased spatial resolution, the cost of these devices is nearly two orders of magnitude less than the cost of the largest deformable mirror.« less

Current laser systems based on high-power laser diode bars need active cooling either water cooling or the use of thermo-electric coolers to ensure an adequate operating temperature for a reasonable lifetime. Here is a solution with a bonded fin heat sink and forced ventilation introduced, a diode laser bar with an improved efficiency and a low thermal resistance as well as an optical system for a highly efficient fibre coupling. With this system it is possible to couple 25 Watt continuous wave power from a single laser diode bar on a passive heat sink into a fibre with 200 μm core diameter. The basis for this performance is a heat sink with an exceptionally low thermal resistance. Several new features are introduced to reach a low overall gradient between the laser diode temperature and the ambient temperature. In addition, it does geometrically fit to the layout of the optical design. Shape and aspect ratio of both heat sink and housing of the laser system are matched to each other. Another feature is the use of hard-soldered or pressed bars to achieve a thermo-mechanically stable performance. The long-term thermal characteristic was tested. The operation temperature comes to saturation after about 30 minutes. Therefore it can be used for continuous wave operation at 25 Watt output power. At a quasi continuous operation at 70 percent duty cycle a peak power of 30 Watt out of the fibre is possible. From this technology results a compact fibre coupled laser system what is simple to drive compared with current high power laser systems, because there is no need to control the operating temperature. This gives way for more compact driver solutions. Fields of application are laser marking systems and material processing, where a simple driver system is requested. Also medical applications need this requirement and a compact cooling too so that mobile integrated solutions become possible. Further developments allow multiple laser diode systems for specific industrial applications demanding more power. Our measurements show the potential for direct air-cooled laser systems with 100 Watt power out of the fibre.

The rudimental microparticle contaminants and airborne molecular contaminants (AMCs) will obviously reduce the lifetime of the lens widely used in the high peak power laser system. An inline contaminants sensor based on the optical microfiber (OM) is here proposed. Due to Van Der Waals force and electrostatic attraction, contaminants are easily adhered to the surface of OM, which will cause an obvious perturbation to the evanescent field transmitted in the OM. The additional loss, caused by the adhered contaminants, has been theoretically analyzed and simulated. The corresponding experiments have also been carried out, and the experimental results agree well with the simulation. The inline containments sensor based on OM has potentially wide sensing range for many kinds of determinate absorptive materials.

High Power Diode Laser Bars serve as the foundation for many high-power laser systems used in manufacturing and research. Efficient heat removal is essential for reducing cost per watt by increasing available output power or extending the effective lifetime of these devices. In this paper we will introduce a novel method for electrically isolated water impingement cooling of high-power diode laser bars operating in the 975nm region. We will present results from 58 emitter diode laser bars operating in the 975nm region with CW output power levels exceeding 500W and a thermal resistivity of less than 0.12 K/W.

Effects related to the use of high power and repetition rate lasers in ablation of metals and silicon were investigated.Numeric simulation and experiments were performed to find out conditions of the efficient use of laser energy.Accumulation of defects lowers the ablation threshold when a high pulse-repetition rate is applied.An optimum beam waist exists for certain pulse energy to maximize the ablation but the waist does not allow achieving a high processing accuracy.Energy efficiency of the laser processing falls down when pulse energy exceeds the material dependant limit.Plasma shielding was assumed to be the main limiting factor in processing efficiency of metals with the high power picosecond lasers.Intelligent control of process parameters is required to keep optimal conditions for the material removal by laser ablation.

High average power operation of the Mercury Laser induces dynamic aberrations to the laser beam wavefront. Analysis of recent data indicates that up to 4 waves of low order aberration (mainly focus error or power, with spatial resolution < 0.5 cm{sup -1}) could be expected at each pass. Because of the magnitude of the wavefront error, the logical position is to place a deformable mirror (DM) at the M11 position, where the DM will correct the beam between passes 1 & 2 and 3 & 4. Currently, there are only two established commercial vendors offering complete adaptive optic (AO) systems that can accommodate the Mercury beam size (45 x 75 mm) which are compatible with high damage threshold coatings. Xinetics (MA, USA) offers a complete AO system along with a Shack-Hartmann wavefront sensor. The Xinetics DM is based on lead magnesium niobate (PMN) technology. A number of US aerospace firms as well as NIF use Xinetics PMN technology for their DMs. Phasics (Paris, France) offers a complete AO solution with its proprietary SID-4, a four-way shearing interferometric wavefront sensor capable of high resolution (over 100 x 100 sampling points). The Phasics system includes a bimorph deformable mirror from Night-n-Opt (Moscow, Russia) that uses lead zirconate titanate (PZT) technology. Various high power laser laboratories around the world such as LULI (France), HELEN (UK), and GEKKO (Japan) are using the PZT-based bimorph DM in their system. While both DM technologies are equivalent and have been deployed in high-energy laser systems, the PZT based bimorph DM offers two distinct features that makes it more attractive for high average power laser systems. The bimorph DM uses two layers of PZT actuators with the outer layer acting as power correctors, capable of correcting up to 20 waves of power. The Xinetics DM offers a maximum stroke of 4 waves. In addition, Night-N-Opt has also designed a water-cooled DM with a silicon based substrate (as opposed to a glass substrate) specifically for high average power laser systems--an option that is currently not available for PMN based DMs.

Introduction: The labial frenum is a fold of mucous membrane that attaches the lip and the cheek to the alveolar mucosa, the gingiva, and the underlying periosteum. In some cases, its presence can cause a midline diastema, periodontal diseases related to food impaction, or retention of biofilm, among others. In such cases, lip frenectomy is indicated as treatment, which can be performed with a scalpel (conventional method), an electric scalpel, or a surgical laser. Objective: To show a clinical case performed at Laser Extension Project in Dentistry, Federal University of Maranhão grounded in a literature review. Case Presentation: A laser frenectomy was performed on a female patient, aged 20, who had a diastema between the upper central incisors and an indication for frenum removal. The high-power diode laser is excellent for procedures in soft tissue because its wavelength is well absorbed by hemoglobin and other pigments; its use also allows a reduction in the amount of anesthetic and medicines used. The parameters used were 2 W, in a continuous mode, 808 nm infrared emission; with delivery of the beam through optical fiber 300 μM; energy of 120 J; 20 pps. Conclusion: the high power diode laser allowed a satisfactory result, the procedure was safe, the technique was a simple one and of reduced clinical time, as mentioned in the literature. It is worth noting that the technique is dependent on the skill of the professional performing it.

An effort was under taken to mitigate thermal lensing and have an easy and compact means of monitoring thermal lensing in a high power laser system. Utilizing the negative temperature coefficient of refractive index in CaF2 with the nearly opposing positive temperature coefficient of refractive index in SiO2, a multi-element focusing objective was fabricated to reduce the amount of thermal lensing experienced with a multi-kilowatt, cw fiber laser. In addition, an all-passive optical design beam analyzer was developed to accurately measure the focus shift of an all SiO2 lens objective in contrast to the CaF2/SiO2 multi-element lens to compare their perspective thermal lensing values.An effort was under taken to mitigate thermal lensing and have an easy and compact means of monitoring thermal lensing in a high power laser system. Utilizing the negative temperature coefficient of refractive index in CaF2 with the nearly opposing positive temperature coefficient of refractive index in SiO2, a multi-element focusing objective was fabricated to reduce the amount of thermal lensing experienced with a multi-kilowatt, cw fiber laser. In addition, an all-passive optical design beam analyzer was developed to accurately measure the focus shift of an all SiO2 lens objective in contrast to the CaF2/SiO2 multi-element lens to compare their perspective thermal lensing values.

A 30 kW, continuous wave (CW), Photo-initiated, Impulse-enhanced, Electrically-excited (PIE) CO2 laser is presented. This high power laser system is unique in that it is also capable of modulated optical output over a wide range of frequencies featuring multi-kilowatt peak power with high average power. The primary focus of this paper is documentation of the enhanced performance achievable from this laser when operated under a burst mode excitation technique. Excitation of the system, using a burst mode process, results in a dramatic change in the output characteristics of the laser. Control of burst frequency and duty cycle provides a convenient means to alter the time-varying nature of the output beam. Low burst frequencies result in clearly defined pulse profiles, falling to zero in between pulses. Increasing burst frequency results in a significant offset or quasi-CW time-varying output. At low burst durations, optical output has a triangular profile. Increasing burst duty cycles produce square optical pulse shape profiles. High frequency, large duty cycle operation produces an optical output approaching that observed in CW. The significance of these results are manifest in a major enhancement in deep penetration welding at reduced average laser powers. Welding performance, featuring a 400% increase in penetration as compared to CW welding, has been demonstrated at the same average power level. Reductions in average laser power and plasma suppression gas requirements could translate into major savings in high power laser welding applications utilizing this unique excitation process. Another attractive aspect of burst mode welding is its potential for deep penetration alloying of inexpensive base materials through incorporation of a computer controlled wire-feed/MIG system.A 30 kW, continuous wave (CW), Photo-initiated, Impulse-enhanced, Electrically-excited (PIE) CO2 laser is presented. This high power laser system is unique in that it is also capable of modulated optical output over a wide range of frequencies featuring multi-kilowatt peak power with high average power. The primary focus of this paper is documentation of the enhanced performance achievable from this laser when operated under a burst mode excitation technique. Excitation of the system, using a burst mode process, results in a dramatic change in the output characteristics of the laser. Control of burst frequency and duty cycle provides a convenient means to alter the time-varying nature of the output beam. Low burst frequencies result in clearly defined pulse profiles, falling to zero in between pulses. Increasing burst frequency results in a significant offset or quasi-CW time-varying output. At low burst durations, optical output has a triangular profile. Increasing burst duty cycles produce square optica...

High-power laser propagation through the inner optical path will produce a significant thermal effect on the beam-splitter mirror which will cause phase aberrations. Based on the three-dimensional transient heat conduction equation and the thermal elastic stress-strain equation, a simulation model of reflector mirror was built with three-dimensional finite element method (FEM). The temperature increase and thermal displacements of two kinds of mirror substrates (Al₂O₃ crystal and spinel) were especially investigated with different laser intensity, output duration and absorption coefficient. The effects of mirror thermal distortion on laser beam phase aberrations were also evaluated on both reflection and transmission directions. The experiments of high–power laser propagation through two kind materials of beam-splitter mirrors samples with diameters of 50mm and thicknesses of 5mm were carried out to measure the thermal effects induced by the absorbed laser energy. Both two kinds mirror samples were deposited the same film layer of same reflectance. A high power semiconductor laser was expanded to a beam of 35mm diameter, and double Shack-Hartmann wavefront sensors were used to detect both reflection and transmission thermal distortions of the mirror samples. The measurements showed that reflection aberrations of spinel mirror sample were larger than those of Al₂O₃ crystal mirror sample while its transmission aberrations were slightly less than Al₂O₃ crystal mirror sample. The results of experiments and simulations showed a very good consistency.

This paper describes results of the first phase of a Gas Research Institute (GRI) funded research program. The overall purpose of this research is to advance a fundamental change in the method currently used to drill and complete natural gas wells applying the U.S. Defense Department's StarWars laser technology. Results of tests conducted at the U.S. Air Force and the U.S. Army's high power laser research facilities are presented. Initial testing on reservoir rocks conducted with the Mid Infrared Advanced Chemical Laser (MIRACL) at the U.S. Army's High Energy Laser Systems Test Facility at White Sands, New Mexico, showed the potential for laser drilling. The laser beam drilled at a speed that indicates penetration could be increased by more than 100 times current rates. Planned testing for the next phase of this research is to use the Chemical Oxygen-Iodine Laser (COIL) high powered laser invented in 1977 by the U.S. Air Force Research Laboratory in Albuquerque, New Mexico, for air-to-air defense. COIL has gained notoriety as an airborne laser tactical weapon capable of tracking and destroying missiles. An annotated bibliography was developed for the GRI that comprehensively reviews published laser-rock interaction, current drilling research, and technical data related to laser applications in non-reservoir rocks.1 Developing commercial applications of StarWars laser technologies has commenced not only in the U.S., but also in Russia and the former Soviet Union.

Diffractive optical elements offer many advantages over conventional optical elements in high power laser systems; e.g., focussing beam sampling and splitting elements.1 Typical benefits include the ability to perform multiple optical functions with one element, high laser damage thresholds,2 reduced nonlinear phase retardation and the possibility of mass production using VLSI fabrication techniques. Here, we present the design procedure and experimental results for a 16- level on-axis near-field Gaussian to flat-top generator with low fidelity and an harmonic separation filter for purging 3λ0 and 2λ 0 wavelengths from a laser beam to leave only the desired λ0.