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.     

Thermal-lens measurement in a side-pumped 1.3 μm Nd:YVO4 bounce laser

We investigated thermal-lensing effects in a side-pumped 1.3 μm Nd:YVO₄ slab laser amplifier having a bounce geometry. The thermal-lens power during 1.3 μm laser action was 1.3-times larger than that without laser action. Excited-state absorption is the main cause for increased heat loading during laser operation. The heat-loading formula in end-pumped 1.3 μm vanadate lasers having low Nd doping can be extended to account for heat generation in diode-side-pumped vanadate bounce lasers having high Nd doping.     

Diffusion of excited state molecules in FIR- and CO2-lasers

Both standing waves in laser oscillators and spatially inhomogenous cross sections of laser beam and pumprate cause a non-uniform distribution of excited state molecules in longitudinal and transversal direction, respectively. This spatial hole burning however is smoothed by diffusion of the excited molecules. The effect of diffusion is investigated theoretically for an optically pumped far infrared laser as well as the corresponding CO₂ pump laser. It is found, that the remaining spatial hole burning in the direction of wave propagation is negligible within CO₂ lasers but not within FIR lasers. Concerning the transversal direction it can be shown that in the FIR laser diffusion takes no effect, whereas the transversal distribution of the excited molecules in the CO₂ laser is significantly influenced by diffusion.FIR ring lasers avoid longitudinal spatial hole burning, which leads to the common assumption that they use the active medium more efficient than conventional standing wave lasers, hence delivering higher output powers. This expected advantage is levelled out to a great extent by diffusion.     

Laser stabilization to an atomic transition using an optically generated dispersive line shape

We report a simple and robust technique to generate a dispersive signal which serves as an error signal to electronically stabilize a monomode continuous-wave laser emitting around an atomic resonance. We explore nonlinear effects in the laser beam propagation through a resonant vapor by way of spatial filtering. The performance of this technique is validated by locking semiconductor lasers to the cesium and rubidium D₂ lines and observing long-term reduction of the emission frequency drifts, making the lasers well adapted for many atomic physics applications.     

On the opto-voltaic measurements in CO and CO2 lasers

We observed and compared the opto-voltaic signals in CO and CO₂ lasers. The signals are obtained capacitively from the water cooling jacket as a low voltage source not influencing the current circuit. We observed from measurement that the output power and the so-called optovoltaic input power have a distinct relationship depending on laser current and cavity parameters. It will be shown that opto-voltaic detection is a very sensitive method especially for CO lasers.     

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.     

Spectral characteristics of a Yb-doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic laser

We report the experimental observation of random wavelength emission and intensity-dependent central-wavelength shift in a diode-pumped Yb³⁺-doped Y₂O₃ ceramic laser. We show experimentally that, like conventional lasers, the emission of the laser has fixed well-defined transverse modes; however, its instantaneous emission wavelengths change randomly with time. The central wavelength of the laser emission also shifts with the intracavity light intensity. A model was developed to describe the spectral behavior of Yb³⁺-doped lasers. We show that the observed random wavelength emission and central lasing wavelength shift of the laser could be well explained based on the strong reabsorption of light in the gain medium. PACS 42.55.Rz; 42.60.Mi; 42.55.Xi     

Manufacturing of Nd:Gd3Ga5O12 ridge waveguide lasers by pulsed laser deposition and ultrafast laser micromachining

Laser radiation is used both for the deposition of the laser active thin films and for the micro structuring to define wave guiding structures for the fabrication of waveguide lasers. Thin films of crystalline and amorphous neodymium doped Gd₃Ga₅O₁₂ are grown on single crystal yttrium aluminium garnet by pulsed laser deposition using excimer laser radiation.Manufacturing of the laser active waveguides by micro structuring is done using femtosecond laser ablation of the deposited films. The structural and optical properties of the films and the morphology of the structured waveguides are determined in view of the design and the fabrication of compact and efficient diode pumped waveguide lasers. The resulting waveguides are polished, provided with resonator mirrors, pumped using diode lasers and the waveguide lasers are characterized. The spectroscopic properties of the amorphous waveguide are investigated and an infrared waveguide laser is demonstrated. To our knowledge, there have been no reports by other groups of the successful operation of a structured waveguide laser fabricated by this technique or of a waveguide laser made from amorphous neodymium doped Gd₃Ga₅O₁₂.     

Two-photon pumping of random lasers by picosecond and nanosecond lasers

We experimentally demonstrated two-photon pumping of random lasers using picosecond and nanosecond pump lasers. The picosecond laser pumping experiment was performed with 400 ps laser pulses at 770 nm, and the gain media was a Coumarin 480D dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser pulse energy below 500 μJ. The nanosecond laser pumping experiment was performed with 7 ns laser pulses at 1064 nm, and the gain media was a Rhodamine 640 dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser energy ∼18 mJ. Our results suggest that there exists an optimal pulse duration of the pumping laser in two-photon pumped random lasing that leads to minimum photodamage of the gain media and still keeps a high pumping efficiency. PACS 33.50.Dq; 42.55.Mv; 42.55.Zz     

Simultaneous Q-switching and mode-locking in the CW Nd: YAG laser

The theory of transient mode-locking for an active modulator in a laser with a homogeneously broadened line is presented. The theory is applied to simultaneously Q-switched and mode-locked Nd:YAG lasers and good agreement between theory and experiment is obtained. The main conclusion is that under usual Q-switched operating conditions the mode-locking process does not have sufficient time to build up to steady-state conditions. We also present a method to overcome this problem by allowing the laser to prelase before the Q-switch is opened. Mode-locked pulses whose width approaches the steady-state value are obtained. The transient mode-locking theory presented here also applies to actively mode-locking TEA CO₂ lasers and to other types of simultaneously pulsed and mode-locked lasers.     

Frequency stabilization and measurements of 543 nm HeNe lasers

In this paper we report our investigations on the frequency stabilization and frequency measurements of 543 nm HeNe laser. It contains following four different works. (1) Using a metal laser tube we have built an iodine-stabilized 543 nm HeNe laser by the Frequency-Modulation (FM) spectroscopy. The signal-to-noise ratio of the hyperfine spectrum reached 2 × 10^–12 at 1 s sampling time. (2) We have built a compact iodine-stabilized 543 nm HeNe laser system using the third-harmonic locking technique. Stability better than 1 × 10^–12 for sampling time >1 s is obtained. We also suggest the b₁₀ line for the future recommendation. (3) We constructed the Lamb-dip stabilized He-²⁰Ne and He-²²Ne lasers and measured their frequency stability, reproducibility, and absolute frequencies. The results suggest that the Lamb-dip stabilized lasers are appropriate for secondary wavelength standards. We have also deduced the isotope shift of Ne atom at 543 nm. (4) We have developed two two-mode stabilized 543 nm HeNe lasers using the bang-bang control method. The Allan variance is 1 × 10^–11 at 1 s sampling time.     

CO2 laser drives extreme ultraviolet nano-lithography — second life of mature laser technology

It was shown both theoretically and experimentally that nanosecond order laser pulses at 10.6 micron wavelength were superior for driving the Sn plasma extreme ultraviolet (EUV) source for nano-lithography for the reasons of higher conversion efficiency, lower production of debris and higher average power levels obtainable in CO₂ media without serious problems of beam distortions and nonlinear effects occurring in competing solid-state lasers at high intensities. The renewed interest in such pulse format, wavelength, repetition rates in excess of 50 kHz and average power levels in excess of 18 kiloWatt has sparked new opportunities for a matured multi-kiloWatt CO₂ laser technology. The power demand of EUV source could be only satisfied by a Master-Oscillator-Power-Amplifier system configuration, leading to a development of a new type of hybrid pulsed CO₂ laser employing a whole spectrum of CO₂ technology, such as fast flow systems and diffusion-cooled planar waveguide lasers, and relatively recent quantum cascade lasers. In this paper we review briefly the history of relevant pulsed CO₂ laser technology and the requirements for multi-kiloWatt CO₂ laser, intended for the laser-produced plasma EUV source, and present our recent advances, such as novel solid-state seeded master oscillator and efficient multi-pass amplifiers built on planar waveguide CO₂ lasers.     

Effect of the active medium temperature on the operation of fast-flow CO2 lasers

A new formula is obtained for estimating the output power of fast-flow CO₂ lasers. It is shown that a higher specific output power of these CO₂ lasers in comparison with sealed-off CO₂ lasers is caused mainly by the higher saturation intensity of the former. It is concluded that the temperature of the laser active medium affects almost only the charge stability and that, under stable discharge conditions, the same output power can be obtained at different temperatures of the active medium.     

Intensities of two-photon transitions in the 2ν2 band of NH3

We determine theoretically all the coincidences between the ¹⁴NH₃ν₂= 0−2 band and the spectrum of two CO₂ lasers; they can be induced either by a small frequency tuning of lasers or by a Stark tuning of molecular levels. For each two-photon transition we report the line intensity, the preferred laser lines and the necessary Stark field or laser tuning. The theoretical line intensities are compared to the results of an optoacoustic Doppler-free experiment.     

Narrow bandwidth operation of high-power broad-area diode laser using cascaded phase-conjugate injection locking

A broad-area laser is injection-locked by another broad-area laser that is also injection-locked by a single-mode diode laser. Two double-phase conjugate mirrors of photorefractive BaTaO₃ are used to couple the master laser beams to the first slave laser, and the first slave laser output to the second slave laser. One of the double-phase conjugate mirrors is built up with the beams from two broad-area lasers. Two slave lasers are oscillating in single longitudinal mode at 808.5 nm and the spectral width is the same as that of the master laser. Final single-mode output power from the second slave broad-area laser is 840 mW, which is limited by the power of the injection beam. This work verifies the possibility of the multi-stage cascaded injection locking of high-power diode lasers with phase-conjugate injection. Received: 18 November 1998 / Revised version: 29 January 1999 / Published online: 7 April 1999     

Characterization of laser processing of single‐crystal natural diamonds using micro‐Raman spectroscopic investigations

The application of lasers for processing diamond has revolutionized the diamond industry and its applications in microelectronics, microelectromechanical system (MEMS) and microoptoelectromechanical system (MOEMS) technologies. The process quality can be evaluated using spectroscopic techniques. In the present investigation, four different types of Q‐switched solid‐state lasers (with different beam parameters), namely, a lamp‐pumped Nd:YAG laser operating at 1064 nm, a lamp‐pumped Nd:YAG laser operating at second harmonically generated 532 nm, a diode‐pumped Nd:YVO₄ laser operating at 1064 nm and a diode‐pumped Nd:YAG laser operating at 1064 nm, have been employed for the processing of a single‐crystal, gem‐quality, natural diamond. The main objective behind the selection of these lasers with different beam parameters was to study the effect of wavelength, pulse width, pulse energy, peak power and beam quality factor (M² factor) on various aspects of processing (such as microcracking, material loss and cut surface quality) and their relative merits and demerits. The overall weight loss of the diamond and formation of microcracks during processing have been studied for the above four cases. The characteristics of the graphite formed during processing, elemental analysis, surface morphology of the cut surface and process dynamics have been studied using micro‐Raman spectroscopy and scanning electron microscopy (SEM). We observed that laser cutting of single‐crystal diamonds used for industrial applications can be accomplished without microcracking or surface distortion using Q‐switched Nd:YAG lasers. This allows direct processing without extensive postgrinding and polishing stages. Very efficient diamond processing is possible using diode‐pumped lasers, which results in the lowest possible breakage rate, good accuracy, good surface finish and low weight loss. From the micro‐Raman and SEM studies, it is concluded that the surface quality obtained is superior when diode‐pumped Nd:YVO₄ laser is used, owing to its extremely high peak power. The maximum graphite content is observed while processing with lamp‐pumped Nd:YAG laser at 532 nm. An overall comparison of all the laser sources leads to the conclusion that diode‐pumped Nd:YAG laser is a superior option for the efficient processing of natural diamond crystals. Copyright © 2006 John Wiley & Sons, Ltd.     

气体激光器中的类透镜效应分析

本文讨论了类负透镜介质一般曲面镜光学谐振腔,并用来分析气体激光器中的热透镜效应。对平凹腔连续波二氧化碳激光器进行了模式分析。结果表明,热透镜效应对输出功率和频率特性影响相当大,对基模输出激光器的影响最为突出。最后,简单地讨论了离子气体激光器和某些其它激光器中的可能热透镜效应。     

Visible and near-infrared chemical lasers

We analyze the basic thermodynamic, kinetic, radiative, and spectroscopic properties of halogen and interhalogen molecules which are potentially suitable for the construction of visible and near-IR chemical lasers. We discuss the problem of chemical buildup of electronically excited states (EES) of donor molecules (or atoms). We propose a number of new kinetic schemes with formation of EES of donor particles, for the development of lasers based on electronic transitions (ET). We obtain analytic relations for the lasing condition and for the laser efficiency of a system of a “donor—acceptor” type as applied to ET chemical lasers. Actual results of research into the gain properties and energy characteristics of potential visible-band chemical NF-IF and N₂-IF lasers. The possibility of using atoms in chemical lasers is discussed. Translated from Trudy Fizicheskogo Instituta im. P. N. Lebedeva Akademiya Nauk SSSR, Vol. 194, pp. 171–211, 1989.     

Deterministic chaos in laser with injected signal

Dynamic behaviour of an homogeneously broadened laser with injected signal is analyzed for a model in which the polarization is adiabatically eliminated. Detuning between the atomic resonant frequency, the cavity resonance and the frequency of the external signal is considered. We show that a transition to chaos via intermittency is possible for parameters appropriate for CO₂ lasers.     

Wide-aperture CO2 lasers pumped by electron-beam-controlled discharge

Three powerful wide-aperture CO₂ lasers with the wavelength λ = 1.06 μm (a laser pumped by electron-beam-controlled discharge, a high-pressure continuously tunable laser, and a repetitively pulsed laser) are created and studied. A method for the volume discharge generation proposed at the High-Current Electronics Institute is used for the development of the lasers. Original Text ? Astro, Ltd., 2006.