Temperature distribution along oversized hollow-core waveguides for CO2 laser radiation

Temperature distributions along various hollow waveguides are fully analysed experimentally when the waveguides are excited by CO₂ laser light. It is shown that the temperature decreases in an oscillatory manner along the waveguides due to the interference effect of guided modes. The distributors are well explained by a theory developed previously.     

Loss reduction of dielectric-coated metallic hollow waveguides for CO2 laser light transmission

A condition for reducing transmission loss of a dielectric-coated metallic hollow waveguide is shown theoretically when the inner metallic surface is not perfect. It is also shown that the transmission loss for CO₂ laser light is 10.6 μm wavelength is able to be estimated from an attenuation spectrum of a short waveguide. By fabricating a ZnSe coated Ag hollow waveguide of short length, we have shown that it has a lower loss than a Ge coated waveguide for CO₂ laser light.     

Theoretical prediction on the wavelength-temperature shift in pulsed TE CO2 lasers

The wavelength-temperature shift observed in pulsed TE CO₂ lasers is discussed theoretically by means of Six-temperature model rate equations for tunable TE CO₂ lasers. Numerical calculations of the temperature-wavelength shift in a pulsed TE CO₂ laser with a simple plano-concave stable resonator, whether excited by conventional low-inductance fast-discharge scheme or by a long-pulse Pulser/sustainer discharge scheme, show that the laser output wavelengths are within the 10P branch as the ambient temperature varies from 228 to 338 K, but will change as the ambient temperature varies. The laser output wavelengths will move to the transition lines with longer wavelengths in the 10P branch as the ambient temperature increases and vice versa. The calculated results also illustrate that near the ambient temperature of 310 K, the laser is more likely to operate on multi-transition lines. Considering this wavelength-temperature shift, the chilling device adopted in high-power high repetition rate TE CO₂ lasers is important in maintaining a stable laser output spectra as well as a stable laser output power. The numerical results also suggest that a frequency agile resonator is highly recommended if stable laser output spectra are required in TE CO₂ lasers.     

Low loss smart hollow waveguides with new polymer coating material

A new kind of cyclic olefin polymer COP-E48R has been selected as the dielectric material for a silver hollow glass tube. Owing to its lower extinction coefficient at the wavelength of 10.6 μm, transmission losses for the CO₂ laser light has been reduced significantly in the COP-E48R-coated silver (COP-E48R/Ag) hollow glass waveguide. By properly selecting the film thickness of COP, Er:YAG and CO₂ laser light are shown to be transmitted with low loss simultaneously or independently. Delivery properties of red and green pilot beams were also evaluated.     

New and simple method for fabricating polymer-coated silver hollow fibers with large mechanical strength

A new method is proposed for fabricating polymer-coated silver hollow glass fibers to avoid the flexibility deterioration after the curing process. Transmission properties of fibers made by the two procedures with and without curing process are compared. Little difference was observed in the transmission properties at the wavelength 2.94 μm of Er:YAG laser light and 10.6 μm of CO₂ laser light. The polymer layer is shown to be stable after 2-h, 5-W, continuous wave CO₂ laser light transmission.     

Rugged distal tips for CO2 laser medicine

Low-loss and rugged distal tips for CO₂ laser have been proposed and fabricated based on a commercially available stainless steel (St) pipe. A method of smoothening the inner surface of the St pipe with a rough inner surface is put forward. Fabrication parameters, transmission properties, and mechanical strength of the cyclic olefin polymer-coated silver hollow St distal tip are experimentally discussed.     

Design and experimental characterisation of a DC-excited CW/repetitively pulsed sealed off CO2 laser

This paper reports the design procedure and experimental study of a sealed off CO₂ laser. Simple algorithms for threshold and steady state excitation voltage calculation, resonator design and its temperature dependent operation are presented. The sealed off CO₂ laser was operated both in CW and pulsed modes and found stable both thermodynamically and optically. Frequency limits for pulsed operation regarding maximum and minimum output energy ranges are determined. Different aspects of CO₂ laser studied include threshold excitation voltages, temperature dependent efficiency, optical power saturation limitations, pulsed and steady state discharge currents for optimum gases mixture combinations. The laser has successfully been constructed, operated and tested for different applications within the limits of its maximum output power of 14 W.     

Two localized CO2 laser treatment methods for mitigation of UV damage growth in fused silica

Two methods：high-power,short-time,single-shot irradiation（Method A） and low-power,long-time,multi-shot irradiation（Method B） are investigated to mitigate the UV damage growth in fused silica by using a 10.6-μm CO2 laser.To verify the mitigation effect of the two methods,the laser induced damage thresholds（LIDTs） of the mitigated sites are tested with a 355-nm,6.4-ns Nd：YAG laser,and the light modulation of the mitigation sites are tested with a 351-nm continuous Nd：YLF laser.The mitigated damaged sites treated with the two methods have almost the same LIDTs,which can recover to the level of pristine material.Compared with Method A,Method B produces mitigated sites with low crater depth and weak light modulation.In addition,there is no raised rim or re-deposited debris formed around the crater edge for Method B.Theoretical calculation is utilized to evaluate the central temperature of the CO2 laser beam irradiated zone and the radius of the crater.It is indicated that the calculated results are consistent with the experimental results.     

分布反馈光纤激光器弯曲特性的实验研究

为了研究分布反馈光纤激光器的弯曲特性,对光纤激光器输出波长和弯曲半径的关系进行了理论分析和实验研究.理论分析表明,光纤激光器输出波长变化量与弯曲半径平方的倒数为线性相关,并且随着弯曲半径的增大布拉格波长向短波长方向移动|实验中通过对3支不同波长的光纤激光器在弯曲半径分别为10 mm、20 mm、90 mm时输出波长的测量,验证了理论分析的正确性.实验中采用参考光纤激光器法排除环境干扰,对光纤激光器的激射功率进行了耐弯曲测试,并验证了光纤激光器斜率效率优越的抗弯曲特性.结果表明,在测试的32天内,待测光纤激光器激射功率变化量均小于2 dB,为分布反馈光纤激光器做为弯曲类传感应用的研究提供了重要的参考依据.     

DESIGN OF A 532nm Nd:YVO4/KTP SINGLE-FREQUENCY LASER

A model of the laser-diode pumped solid-state laser is developed to deduce the minimum average radii of the pump beam in the solid medium, since the smaller the cavity waist, the higher the laser output power is expected to be. With an appropriate coupling system consisting of the collimating lens, prism pair and focusing lens, a diode-pumped single-frequency Nd:YVO₄/KTP intracavity frequency-doubling cw laser has been demonstrated through the precise temperature control of the Nd:YVO₄ crystal, the KTP crystal and the diode laser. The 532nm single-frequency output power of 40.4mW (in fact 55mW if the reflection loss of the triangular prism is taken into account) is obtained for an incident power of 515mW. It is derived theoretically and is verified in experiment that the frequency drift of the free-running laser can be reduced by increasing the cavity length.     

Lasers for materials processing: specifications and trends

An overview is given of the types of lasers dominating the field of laser materials processing. The most prominent lasers in this field are the CO₂ and the Nd: YAG laser. The domain of CO₂ lasers is applications which demand high laser powers (up to 30 kW are available at present), whereas the domain of Nd:YAG lasers is micro-machining applications. In the kilowatt range of laser output power, the two types of lasers are in competition. New diffusion-cooled CO₂ laser systems are capable of output laser powers of several kilowatts, with good beam qualities, while still being quite compact. The output power and beam quality of Nd:YAG lasers has been improved in recent years, so that Nd:YAG lasers are now an alternative to CO₂ lasers even in the kilowatt range. This is especially true for applications that demand optical fibre transmission of the laser beam, which is possible with Nd:YAG laser light but not with the longerwavelength light emitted by CO₂ lasers. The main problem in solid-state lasers such as Nd:YAG is the thermal lensing effect and damage due to thermal stresses. In order to reduce thermal loading, cooling has to be enhanced. Several alternative geometries have been proposed to reduce thermal loading and, by this, thermal lensing effects. There are now slab and tube geometries which allow much higher output powers than the conventionally used laser rods. A very new scheme proposes a thin slab whose cooled side is also used as one of the laser mirrors, so that thermal gradients occur mainly in the direction of the beam propagation and not perpendicular to it, as is the case in the other geometries. As well as CO₂ and Nd:YAG lasers, semiconductor laser diodes are very promising for direct use of the emitted light or as pump sources for Nd:YAG and other solid-state lasers. When packaging together thousands of single laser diodes, output powers of several kilowatts can be realized. Major problems are collimation of the highly divergent laser beams and cooling of the laser diode bars.     

内窥镜的红外激光传输用柔性空芯光纤的研制

通过理论计算确立了多功能介质-金属结构空芯光纤的结构参量,优化了液相镀膜法的有关条件,明确了具体制作参量。制作了以聚碳酸酯毛细管为基管的,高柔韧性的,可同时低损耗传输红外目标波长激光和可见导航光红外的空芯光纤。对光纤传输性能进行了测试。在2.94μm波长的Er∶YAG和0.63μm波长的红色半导体激光器的直线损耗分别为0.4 dB/m和3 dB/m。组装在医疗内窥镜中的柔性空芯光纤,在先端以0.9 cm半径135°角弯曲时,对Er∶YAG激光仍有近70%的传输效率;绿色导航光在内窥镜中的的损耗值为11 dB,绿色指示点在内窥镜的视窗中清晰可见。结果表明此种光纤在内窥镜的激光传输方面有重要的应用价值。     

Optimization of three gas compositions in a CO2 laser

Three pressures of gases (CO₂, N₂ and He) added into CO₂ laser tube are optimized for obtaining maximal laser output power by applying a genetic algorithm and solving the CO₂ laser kinetics equations. After the optimization, the laser power is increased by 96% as compared with a non-optimal case.     

Construction of a photon drag detector for evaluating the performance of a CO2 laser amplifier

In this article, the construction of P-type germanium (Ge) photon drag detector which is used to characterize the gain and output powers of a 10 W CO₂ laser and a CO₂ laser amplifier is described. Gain and laser amplifier output power versus laser input power measurements collected at 4.7 mbar and 12.0 mbar amplifier tube pressures are discussed. Moreover, measurements relating the CO₂ laser output power to the laser tube discharge current are provided at 6.9 mbar and 7.3 mbar tube pressures.     

A compact TEA CO2 laser for field-based spectrochemical analysis of geological samples

A compact capacitor-transfer TEA CO₂ laser of 180 mJ output energy and 50 ns pulse duration in half width was designed and constructed for field use. This laser is suitable for laser microprobe spectrochemical analysis on geological and mining samples. Experiments show that the use of helium as a surrounding gas suppresses the continuous emission spectrum of the plasma, thus increasing the S/N ratio. Using glass as standard samples, the minimum detectable concentrations are estimated with Zn neutral line and F ionic line to be 60 ppm and 500 ppm, respectively.     

Mathematical modeling of tunable TEA CO2 lasers

A mathematical model, based on the Landau–Teller equations of six-temperature model for the CO₂–N₂–He–CO system, to describe the process of dynamic emission in tunable TEA CO₂ lasers is introduced. In this model, the Landau–Teller equations are rewritten with regard to fine longitudinal mode frequencies in the laser resonator. These revised equations can be utilized to estimate the laser output spectra as well as other laser output pulse parameters. Examples are given to show the modeling results of non-tunable, grating tuned or injection-locking TEA CO₂ lasers.     

The research of phase-locking of more discharge tubes for two-dimensional axisymmetric-fold combination resonator CO2 laser

A method of reflection-injection phase-locking used on two-dimensional axisymmetric-fold combination resonator (ASFC) CO₂ laser has been proposed. The structure characteristics and the method of phase-locking have been introduced. The expression of output light intensity distribution has been deduced by matrix theory and Collins Formula and the output light intensity distribution has been numerically calculated and simulated. Comparing the output light intensity distribution in the case of phase-locking with that of non-phase-locking. The result shows that this method of phase-locking greatly improve the characteristics of output beam.     

Small-bore fluorocarbon polymer-coated silver hollow glass waveguides for Er:YAG laser light

Several types of fluorocarbon polymer (FCP)-coated silver hollow glass waveguides have been fabricated for Er:YAG laser delivery by using the improved wet chemical technique and dynamic coating procedure. The straight losses of 2 m long 700 μmØ and 540 μmØ waveguides are 0.4 and 1.0 dB, respectively. The transmission losses of these waveguides are below 1.5 dB even when the waveguides are bent to 180° with the bending radius of either 20 or 15 cm. The waveguides with the small diameters of 320 and 200 μm have also been developed for clinical treatment, which exhibit low enough transmission losses for Er:YAG laser light.     

AC-chopper application for CW CO2 laser

A microprocessor-based algorithm was developed to control the power supply to a CO₂ laser using an AC-chopper method. This system was connected directly to a CO₂ laser tube without the need for a dc-dc converter or the storage capacitance of a multilevel circuit. The typical CO₂ laser power supply system had a full-bridge series resonant inverter or voltage multiplier. AC-to-AC converter schemes using zero voltage switching (ZVS) can be used efficiently and economically for low and medium power applications.This paper reports the performance characteristics of a symmetrical AC-chopper technology that can maintain the quality of the ac output of a CO₂ laser tube, regardless of the amount of switching loss. The laser was operated to an output power, maximum system efficiency total gas mixture of 37.2 W, 92%, and CO₂:N₂:He=1:9:15, respectively. The laser system, AC-chopper power supply and its operation were examined.     

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO₂ laser of wide tunability by using three kinds of reference signals: the CO₂ laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.