Comparative Analysis of Laser Systems for the Micro-Processing of Materials
The use of laser technologies in the electronics, automotive, aeronautical, shipbuilding, nuclear industries, precision instrument-making and other areas of anthropologic activity has become one of the progressive technological trends in the last 20–25 years, and their use in the defense industry has significantly expanded. These technologies include dimensional cutting, welding, tailoring, hole piercing, heat treatment, alloying, surface treatment, selective sintering, micro processing, high-resolution marking and engraving of materials. The introduction of laser technology almost always improves the quality and productivity of processing, ensures ecological cleanliness of production, and in many cases technical and economic results are achieved that cannot be obtained by other processing methods. A special place in the manufacture of electronic products and precision instrument-making is precision micro-machining.
Gas and solid-state technological laser systems
Copper vapor lasers and laser systems (CVL and CVLS)
Currently, the CVL and CVLS with radiation wavelengths λ = 510.6 and 578.2 nm, nanosecond pulse duration (τpulse = 20–40 ns), large gain of the active medium (AC) (k = 10–102 Db / m), removal average power from one active element (AE) to 750 W, relatively high pulse repetition rates (f = 5–30 kHz) and low pulse energy (W = 0.1–10 mJ) remain the most powerful pulsed sources of coherent radiation in visible spectrum [1–14]. With the given parameters of the CVL and under the condition of the formation of radiation of a single-beam structure with diffraction quality, the peak power density in the focused spot (d = 5–20 μm), even at relatively small values of the average power (Rrad = 1–20 W), reaches very high values: ρ = 109–1011 W / cm2. With such levels of visible radiation density, effective micromachining in the evaporative mode is ensured both by high-heat conducting Cu, Al, Ag, Au, and high-melting W, Mo, Ta, Re and other metals Ni, Ti, Zr, Fe and their alloys, steels, many semiconductors and dielectrics: silicon, polycrystalline diamond, sapphire, graphite, carbides and nitrides, and transparent materials [15–19]. Furthermore, to ensure high quality of the cut, characterized by minimal heat-affected zone (≤3 μm) and roughness (≤1 μm), the instability of the axis of the radiation beam pattern should be three to four orders of magnitude less than the diffraction divergence [θdiffr / (103–104)].
At low radiation power levels (1–10 W), CVL is usually constructively performed as a separate generator with one AE and an optical resonator. To obtain medium (20–100 W) and especially high (units – tens of kW) radiation power levels, CVLS are used, operating according to the master oscillator – power amplifier (MO – PA) scheme with one or several AE as PA. In CVLS, in comparison with CVL operating in the mode of a single generator, higher powers and efficiency and, therefore, micromachining productivity are achieved [6, 10]. These pulsed lasers are also used as efficient pumping sources of nonlinear crystals (NC) of the BBO, KDP, DKDR type, which convert the visible CVL radiation into the second harmonic – 255.3, 289.1 and 272.2 nm, i. e., into the ultraviolet region spectrum with an efficiency of 10–25%. Such tunable laser systems are preferred for the micro-processing of organic materials and polymers [5, 6, 10, 20, 21].
In the technology of processing materials, CO2 lasers with a wavelength of λ = 10.6 μm are traditionally widely used. High-power CO2 lasers and solid-state IR lasers (1–30 kW) are mainly used for high-speed cutting, tailoring and welding ferrous metals and stainless steel with a thickness of 1–30 mm However, due to the high reflection coefficient (>95%), high-conducting metals such as Cu, Al, Au and Ag are not efficiently treated with CO2 and other IR lasers. The most well-known manufacturers of commercial laser technology complexes (LTC) created on their basis include the following companies: Bystronic (Switzerland), Trumpf and Rofin-Sinar Laser (Germany), Koike and Mitsubishi Electric (Japan), Salvagnini (Italy), Rukhservomotor (Belorus), Hankwang (South Korea), Hans Laser (China), TechnoLaser (Shatura, Moscow Region), NTO IRE-Polus (Fryazino, Moscow Region), IPG Photonics Corporation, Shokin NPP “Istok” (Fryazino, Moscow region), LLC “Research and Production Center “Lasers and Laser Technologies” (Zelenograd) [20, 22–32].
Solid-state rod lasers
A widespread solid-state laser based on a rod of yttrium – aluminum garnet with neodymium (Nd: YAG) with λ = 1064 nm and frequency doubling on nonlinear crystals with λ = 532 nm, due to the occurrence of thermal deformations in the working rod has radiation divergences greater than the diffraction limit. Low-power Nd: YAG lasers are massively used for marking and engraving of parts, assemblies and finished products during their manufacture, those that are powerful ones – for welding and cutting metals, including aluminum [22–32].
Disk and fiber lasers
In terms of the aggregate parameters for micro-processing of materials, the closest to CVL are high-performance (efficiency up to 20%) pulsed solid-state disk ytterbium lasers (Yb : YAG) with diode pumping and emission wavelength λ = 1030 nm produced by such famous German companies as Trumpf and Rofin-Sinar Laser. Disk lasers provide the best solution for a wide range of industrial applications. The disk is a simple and easily excited laser AE that allows for generating high-quality radiation in its parameters. With a large radiating surface of a disk laser, the power density is not critical, even at high peak powers. In fiber ytterbium lasers (efficiency up to 30%), compared to the disk ones, an increase in peak power negatively affects the radiation quality and operational reliability of the resonator primarily. Another noticeable disadvantage of a fiber laser is its high sensitivity to reflected radiation, which occurs when interacting with highly reflective materials (copper, bronze, aluminum, etc.). Fiber lasers are designed and manufactured by such companies as IPG Photonics Corporation (USA), Lumera Laser (USA), Light Conversion Ltd. (Lithuania), NTO “IRE – Polyus”, Avesta Project and Laser-Form (Russia). High-power, multi-kilowatt, disk and fiber lasers are used mainly for high-speed precision cutting and tailoring, welding and hardening of metallic materials [20, 22–32].
Lasers of this class are also produced in combination with NCs, while due to the doubling and tripling of the frequency of the main radiation, UV-radiation is generated with wavelengths λ = 0.515 and 0.343 μm. The pico- and femtosecond values of the pulse duration at a repetition frequency from kilo to megahertz were achieved (see table). With high-frequency pico- and femtosecond lasers, due to the short exposure time of a radiation to a substance, the highest quality of micromachining is achieved – the minimum (submicron) heat-affected zone (HAZ) with almost no melt formation . Disk short-pulse lasers are mainly used in cases where it is impossible to achieve high quality processing with other lasers, e. g., in the automotive industry for drilling microholes in stainless steel for injector nozzles or for manufacturing medical stents for expanding blood vessels. These lasers expensive during production, to ensure stable output radiation parameters, require additional stringent conditions of protection against external influences. At the same time, our experience shows that by several times cheaper CVL with nanosecond pulse durations often provides micro-processing of materials with high productivity, acceptable quality and profitability several times [6, 15–20].
Excimer gas lasers on inert gas halide compounds (ArF, KrF, XeCl, XeF) and inert gas dimers (Ar, Kr) operate similar to CVLs in a pulsed mode with nanosecond duration, but have shorter wavelengths of radiation (λ = 157; 193; 248; 282; 308; 351 nm), i. e. generate in the near UV range (see Table.) [20, 29, 30]. They are widely used in semiconductor production for photolithography and labeling, as well as the processing of plastics, ceramics, crystals, and biological tissues. But because of the relatively large divergence and lower pulse repetition operating frequencies (PRFs) (up to 1–5 kHz), the quality and productivity of materials processing decreases. At the same time, higher-frequency CVL, through the use of nonlinear crystals, provides a productive and high-quality micro-processing in the ultraviolet spectrum region [5, 10].
Diode lasers are small and can be produced in large quantities at relatively low costs. Most diode lasers are generated in the near infrared region (λ = 800–1000 nm). They are reliable and durable, but the output power of a single element is limited and have a large radiation divergence. Diode lasers are used in many areas of human activity, mainly in the telecommunications and optical memory sector, and also commonly used as pump sources for solid-state and fiber lasers. The developed technology of addition of single diodes to diode arrays allows increasing the average laser power to 1–3 kW, which is sufficient for high-performance and high-quality welding, e. g., aluminum parts.
State and development of CVL and CVLS
A comparative analysis of pulsed CVL and CVLS generating in the visible region of the spectrum with other known types of technological lasers, carried out in the introduction, shows that, from the set of output radiation parameters, these lasers remain promising for precision micromachining of materials [5, 6, 10, 15–19]. It should also be emphasized that besides microtreatment, the important areas of application for these lasers are selective isotope separation technology, spectral studies of the composition of solid, liquid and gaseous substances, image intensifiers of micro-objects, nanotechnology, etc. [5, 6, 10–12, 20].
The CO2, solid-state disk and fiber, excimer, nitrogen and diode lasers are widely used in modern technological equipment. The use of pulsed CVL in specialized equipment, despite the unique combination of its output parameters, is still extremely limited due to the small number of commercial models on the market with high reliability and radiation quality. This situation seems to have developed for several reasons. Firstly, many Russian (USSR) research institutes carried out mainly large-scale theoretical and experimental studies of physical processes in CVL, rather than industrial developments. Secondly, in advanced foreign countries (USA, England, France, Japan), the main efforts were directed to the research and development of high-power CVLS of MO – PA type in providing laser isotope separation programs using AVLIS technology for the needs of nuclear energy. At the same time, the development of the most popular commercial CVLs, which include lasers of small (1–20 W) and medium (30–100 W) power levels, remained aside. Thirdly, over the past 10–15 years, the laser market has been represented by a relatively large number of varieties of CVL with a low level of reliability and quality of radiation, which has reduced user demand for this type of laser. Despite the current situation, a number of domestic and foreign organizations continue to work on the creation of new commercial models of CVL and CVLS and, on their basis, of modern technological equipment for micro-processing of materials and isotope separation, medical and research equipment. These include JSC “Shokin NPP “Istok” (Fryazino Moscow Region), LLC NPP VELIT (Istra, Moscow Region), CJSC Clean Technologies (Izhevsk), Lebedev Physical Institute of RAS, Oxford Lasers (England), Macquarie University (Australia) and Pulse Light (Bulgaria). As for powerful CVLS designed for isotope separation technology, the Lawrence Livermore National Laboratory (USA) with the average power increased to 72 kW and the RRC Kurchatov Institute are the leaders. The research continues at the A. N. Prokhorov Institute of General Physics of RAS (Moscow), TSU and the Institute of Optics and Atmosphere of the Siberian Branch of RAS (Tomsk), the Institute of Semiconductor Physics (Novosibirsk), NPO Mechatron LLC, St. Petersburg State Polytechnical University, St. Petersburg National Research University of Information Technologies, Mechanics and Optics, N. E. Bauman MSTU and the Joint Institute for High Temperatures of RAS (Moscow).
Implementing the advantages of pulsed CVL
In order to implement the advantages of pulsed radiation of CVL in the technology of micro-processing of materials and other modern technologies, it was necessary to create a new generation of highly efficient industrial CVL and CVLS use them as a base for modern precision technological equipment. The goal was achieved by performing the following tasks:
development of efficient, durable and stable parameters of industrial sealed laser AE on copper vapor with an average radiation power of 1–100 W;
development and research of highly efficient and reliable schemes for the execution of a high-voltage IP modulator with a nanosecond duration of pump pulses;
studies of highly selective optical resonators and systems for the formation of single-beam diffraction quality and stable parameters in CVL and CVLS to achieve high peak power densities (109–1012 W / cm2);
studies of AC properties of the pulsed CVL and the development of methods and electronic devices for the operational management of power and PRF radiation;
development on the basis of a new generation of sealed-off AEs, high-voltage modulators for IPs, high-selective optical systems and operational methods for controlling the emission parameters of industrial technological CVL and CVLS with radiation power up to 100 W with high reliability, efficiency, quality and stability of the radiation parameters;
creation of modern automated laser technological installations (ALTI) of the “Caravella” type based on industrial CVL and CVLS and modern precision XYZ three-coordinate tables for productive and high-quality precision laser micromachining of the materials;
determination of optimum densities of peak and average radiation power of CVL for effective micromachining of foil (0.01–0.2 mm) and thin-sheet (0.3–1 mm) materials;
Industrial sealed-off AEs of the pulsed CVL
In the development process, the design of self-heating AE with the intravacuum arrangement of the thermal insulator proposed by the staff of the P. N. Lebedev Physics Institute, RAS, and JSC “Shokin NPP “Istok” (Fryazino. Moscow Region) was taken as the basis for the design of both the first AE models of pulsed CVL and the new generation of industrial sealed-off AEs: “Pendant”, small (1–20 W) and “Crystal”, medium (30–100 W) power levels with an operating temperature of the discharge channel 1600–1700 °C. The design, technology of outgassing and cleaning after outgassing of the developed industrial sealed-off AEs “Pendant” and “Crystal” are identical and differ only in the size of functional units and the time of outgassing and cleaning. The power of each individual AE model is ultimately determined by the diameter and length of the discharge channel. The diameter and length of the discharge channel of AE “Pendant” with an average radiation power of 1 W is 7 and 140 mm, a power of 5 W – 12 and 340 mm, 20 W – 14 and 625 mm, AE “Crystal” with a power of 30 W – 20 and 930 mm, 55 W – 32 and 1230 mm, 100 W – 45 and 1520 mm. High efficiency, power, durability, quality and stability of radiation parameters of the industrial sealed-off AEs “Pendant” and “Crystal” are achieved through the implementation of a set of scientific, technical and technological solutions:
The parameters of AE radiation for power consumption, pressure of the neon and hydrogen buffer gas, repetition frequency, and pump pulse parameters were optimized. As a pump generator, a thyratron IP was used with the high-voltage modulator made according to the capacitive voltage doubling scheme with magnetic compression units of nanosecond current pulses and an anode reactor, which according to the results of research today is the most reliable and easy-to-use AC laser [6, 34, 35]. Compared with the classical scheme, the duration of the current pulses generated is shortened by 2 times, from 250–300 ns to 120–150 ns, which leads to an increase in radiation power by 2 times The efficiency is 1.5 times due to an increase in the optimal operating temperature and, as a result, the concentration of copper vapor in the AC is approximately 2–2.5 times, the service life of the thyratron switch (more than 2000 hours) and switching power (up to 5–10 kW) by reducing the power loss in it.
AE “Pendant”, AE “Crystal” (GL‑205) are produced with illuminated exit windows. The basis of optimization to achieve maximum operating values of efficiency and radiation power was the results of a complex of unique experimental studies. The design and manufacturing technology of AE GL‑205G on gold vapor is identical to the GL‑205A model on copper vapor, and differs only in the composition of the active substance, the increased operating temperature of the discharge channel (Tc ~ 1700 °C) and, therefore, the radiation wavelength (λ = 578.2 nm) (see table), which somewhat reduces the AE”s service life.
High-selective optical resonators and systems for the formation of single-beam diffraction-quality radiation in CVL and CVLS
In this work, special attention was paid to the study of the optical resonators of the CVL, since they determine the characteristics and quality of the output radiation. In the course of these studies, the dynamics of the formation and structure of the radiation of a CVL were revealed. It has been established that in CVL in the mode of a separate generator with an optical resonator, the structure of the output radiation is multibeam, and each beam has its own spatial, temporal and energy characteristics. At the same time, the intensity distribution of the CVL radiation in the focusing plane has a stepped, uneven character, which in principle is not suitable for high-quality micro-processing and prevented the creation of modern CVL-based technological equipment. The greatest interest was aroused by studies of CVL with optical systems with high spatial selectivity: with one convex mirror, with a telescopic unstable resonator (UR) and UR with two convex mirrors, as a result of which the conditions for the formation of one diffraction-quality beam radiation with high stability of parameters [6, 15, 16]. Based on the laws of geometric optics, taking into account the diffraction limit of divergence, formulas are derived for calculating the divergence of the output laser beam for these three optical systems and the conditions for the formation of single-beam diffraction quality with stable parameters are determined.
The formula for calculating the radiation divergence during the operation of the CVL in the generator mode with one convex mirror has the following form
The smaller the radius of curvature of the mirror (R) and the diameter of the discharge channel aperture (Dk), and the longer the distance from the mirror to the output aperture of the discharge channel (l), the closer the divergence (θ) to the diffraction limit – θdiffr= 2.44λ / Dk. When the radii R are one-two orders of magnitude smaller than the distance l, the beam divergence becomes close to the diffraction one (θ = 2–4 θdiffr), and the peak power density in the spot of the focused radiation beam reaches 109 W / cm2, sufficient for productive micromachining of foil materials and cutting solders (0.02–0.1 mm). For processing thicker materials, a CVL with one convex mirror is used as the MO in the CVLSs of the type “MO – spatial collimator filter (SCF) – PA”, when using AE “Crystal” of GL‑205A and GL‑205B models as the PA, the radiation power increases more than an order of magnitude (30–60 W) [6, 16].
The use of CVL in the generator mode with telescopic UR as MO in CVLS of type “MO – SCF – PA” was the basis for creating modern industrial ALTI “Caravella‑1” with a radiation power of 10–15 W for precision micromachining of metallic materials up to 0.5 mm thick and non-metallic up to 1–1,2 mm; with UR with two convex mirrors as MO in CVLS – the basis for creating the most powerful industrial ALTI “Caravella‑1M” (20–25 W) for precision micro-machining of metallic materials up to 1 mm thick and non-metallic up to 1.5 … 2 mm electronics article ; with a telescopic UR and SCF at its output – the basis for creating compact low-power modern industrial ALTIs “Caravella‑2” and “Caravella‑2M” (5–8 W) for precision micromachining of metallic materials up to 0.3 mm thick and non-metallic to 0,5–0.7 mm electronics article .
In CVLS operating according to the scheme “MO – PA”, maximum efficiencies (2–3%), power (≥100 W) and density of peak radiation power (up to 1013 W / cm2) are ensured
Operational power control and PRF radiation in the CVLS of the “MO – PA” type according to a given algorithm is carried out due to the high-speed desynchronization of the optical signal (radiation pulse) of the MO from the PA active medium absorption zone to its amplification or transparency zone and vice versa.
In the process of a large amount of experimental work carried out, the most promising technological directions for creating special equipment based on laser elements on metal vapors were identified for use in micro-processing of the materials with nanosecond radiation in order to scale them to a wide production level.