By optimizing single-mode optical fiber for use in fiber lasers, a highly scalable output power of 4.3 kW has been achieved, and further research directions for ultrafast laser applications have been identified.
One of the pressing problems in the development of laser technologies is the increase in the power of fiber lasers, which have already “won” market share from high-power CO 2 lasers, as well as volumetric solid-state lasers. Currently, large fiber laser manufacturers are paying close attention to the development of new applications, considering further market conquest in the future. Among the high-power lasers on the market, single-mode systems have a number of features that make them the most sought-after - they have the highest brightness and can be focused down to a few microns, making them more suitable for non-contact material processing. The production of such systems is quite complex. IPG Photonics (Oxford, MA) has proposed development of a 10 kW single-mode system, but information on beam characteristics is not available and data, in particular, on any possible multimode components that may exist alongside the single-mode signal are not provided.
German scientists from the Friedrich Schiller University and the Fraunhofer Institute for Applied Optics and Precision Engineering, with financial support from the German government, and in collaboration with TRUMPF, Active Fiber Systems, Jenoptik, the Leibniz Institute for Photonic Technology, analyzed the scaling problems of such lasers and developed new fibers to overcome power limitations . The team successfully completed a series of tests, demonstrating a 4.3 kW single-mode output in which the fiber laser output power was limited only by the pump signal power.
Factors limiting the radiation power of a single-mode fiber laser
The main tasks that require careful study include the following: a) improved pumping; b) development of active fiber with low optical losses, operating only in single-mode mode; c) more accurate measurement of the received radiation. Assuming that the problem of improved pumping can be solved using ultra-bright laser diodes and appropriate pump supply methods, we will therefore consider the other two in more detail in this article.
As part of the development of active fiber for high-power single-mode operation, two sets of optimization parameters were selected: doping and geometry. All parameters must be clearly defined to achieve minimal losses, single-mode operation and powerful gain. An ideal fiber amplifier should provide a high conversion efficiency of over 90%, excellent beam quality, and an output power limited only by the available pump power. However, upgrading a single-mode system to higher powers can result in higher power densities within the core of the fiber itself, increased thermal load, and a number of nonlinear optical effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS).
Transverse modes can be enhanced depending on the size of the fiber active zone. The smaller the active cross-section of the fiber, the smaller the number of such modes - for a given ratio between the cross-sections of the fiber and the cladding. However, a smaller diameter also determines a higher power density, and when bending a fiber, for example, losses for higher modes are also added. However, with a large fiber core diameter and thermal stress, other emission modes may occur. Such modes are subject to interaction with each other during amplification, and therefore, without optimal propagation conditions, the output radiation profile may become spatially or temporally unstable.
Transverse mode instability
Ytterbium (Yb) doped fibers are the typical working medium for high-power single-mode fiber lasers, but beyond a certain threshold they exhibit a completely new effect - the so-called transverse mode instability (TMI) effect. At a certain power level, higher modes or even shell modes can suddenly appear. The energy is dynamically redistributed between them, and the quality of the beam deteriorates. A fluctuation of radiation appears at the output (the beam begins to oscillate). The TMI effect has been observed in a variety of fiber designs, from step-index fibers to photonic crystal fibers. Its threshold value depends on geometry and doping, but a rough estimate suggests that this effect occurs at output powers greater than 1 kW. During the study, the dependence of TMI on photodarkening and its connection with thermal effects inside the fiber were revealed. Moreover, the susceptibility of fiber lasers to TMI is also dependent on the modal core content.
The step index fiber geometry allows for optimization. For pumping, the following can be selected: fiber diameter, pump fiber cladding size, and other refractive indices of the fiber and cladding. All of these tuning parameters depend on the dopant concentration, that is, the Yb ion concentration can be used to control the length of the pump radiation absorption region in the active fiber. Other additives can be added to the fiber to reduce thermal effects and control the refractive index. However, there are some contradictions. To reduce nonlinear effects, the fiber must be shorter, and to reduce the thermal load, the fiber must be longer. Photodarkening is proportional to dopant concentration, so longer fibers with lower dopant concentration will definitely be better. An idea of some parameters can be obtained during the experiment. Thermal behavior, for example, can be modeled but is quite difficult to predict since photodarkening is small by definition and cannot be physically measured in accelerated tests. Therefore, direct measurements of thermal behavior in fibers can be useful for experimental design. Shown in comparison for a typical active fiber are the measured thermal load (derived from simultaneously distributed temperature measurements within the fiber amplifier) and the simulated thermal load (Figure 1).
Figure 1. Measured active fiber thermal load compared to simulated load with and without additional loss
Another important parameter for fiber design is the cutoff wavelength, which is the longest wavelength that increases the number of modes in the fiber. Higher level modes beyond this wavelength are not supported.
Testing new fibers at kilowatt power
During the experiment, two types of Yb-doped fibers were investigated. Fiber No. 1 with a core diameter of 30 microns with additional doping with phosphorus and aluminum. Fiber No. 2, with a smaller diameter of 23 microns, was less doped, but contained more ytterbium in order to achieve a higher profile coefficient compared to fiber No. 1 (Table 1).
Table 1. Parameters of tested fibers
The calculated cutoff wavelength is located around 1275 nm and 1100 nm for fibers 1 and 2, respectively. This is much closer to single-mode emission than a typical 20 µm core diameter, 0.06 numerical aperture (NA) fiber having a cutoff wavelength of ~1450 nm. The amplified laser wavelength was ultimately centered at 1067 nm.
Both fibers were tested in a high-power pumping circuit (Fig. 2). The pump diode laser and the initial signal were coupled in free space into a fiber with welded ends and connectors, washed with water for cooling. The radiation source was a phase-modulated external cavity diode laser (ECDL), the signal of which was pre-amplified to achieve an input signal power of up to 10 W at a wavelength of 1067 nm and a spectral width of 180 μm.
Figure 2. High power amplifier experimental setup used for the fiber amplifier test where the fiber was pumped at 976 nm in the counter propagation direction.
During testing of the first fiber, sudden fluctuations were observed on a millisecond scale at the 2.8 kW threshold, which can be attributed to TMI. A second 30 m fiber, at the same wavelength and spectral width, was pumped to an output power of 3.5 kW, limited by SBS rather than TMI.
In the third experiment, the emitter laser spectrum was modified to increase the fiber SBS threshold by broadening the spectrum (higher than the previous experiment). For this purpose, a second diode laser with a central wavelength of 300 μm was combined with the first. This interference resulted in temporal fluctuations that allowed the signal power to increase due to autophase modulation. Using the same main amplifier as before, very similar output power values were obtained at 90% efficiency, but they could only be increased to 4.3 kW without TMI (Table 2).
Table 2. Fiber test result
Measurement tasks
Measuring all parameters of a high-power fiber laser is one of the main tasks and requires special equipment to solve them. To obtain complete fiber characterization, dopant concentration, refractive index profiles, and fiber core attenuation were determined. For example, measuring core loss for different bending diameters is an important parameter for correlation with the TMI threshold.
Figure 3. a) Photodiode intensity trace when testing the output signal using fiber 1, below and above the TMI threshold. b) Normalized standard deviation of photodiode traces at different output powers
During testing of a fiber amplifier, the TMI threshold is determined using a photodiode by tapping a small fraction of the power. The onset of power fluctuations turned out to be quite sharp and significant (Fig. 3), the signal change was especially significant when testing fiber 1, but it was not detected when testing fiber 2 up to a power level of 4.3 kW. The corresponding relationship is shown in Figure 4a.
Figure 4. a) Fiber 2 efficiency slope up to 4.3 kW output power. b) Optical spectrum with an output power of 3.5 kW with a ratio of 75 dB from output to ASE. 180 µm spectral width with 4.3 kW output power extended to 7 nm bandwidth
Beam quality measurements are the most challenging part of fiber laser characterization and deserve separate discussion. In short, thermal-free attenuation is key and can be achieved using Fresnel reflections or low-loss optics. In the experiments presented in this review, attenuation was introduced using wedge plates and pulsed pumping on a time scale exceeding the TMI onset time.
Applications in fast-paced science
After a ten-year lull, the development of high-power single-mode fiber lasers of a new generation in the kilowatt class with excellent beam quality seems quite possible. An output power of 4.3 kW has already been achieved, limited only by the pump power, the main limitations on the path of further development have been identified and ways to overcome them are clear.
Powers of almost 1 kW have already been achieved on a single fiber when amplified by ultrafast laser pulses, so an increase to 5 kW is entirely possible through a combination of techniques. While systems are being developed for research centers such as ELI (Prague, Czech Republic), further development of reliable optical signal transmission systems remains a challenge for industrial systems.
The work done has identified a number of interesting prospects. On the one hand, this is the transfer of results to production, despite the fact that much effort is still required in this direction, and on the other hand, the technology is extremely important for increasing the parameters of other fiber-optic laser systems, for example, for femtosecond fiber amplifiers.
Based on materials from http://www.lightwaveonline.com
In previously published articles testing the technological potential, the fiber laser was analyzed for its most effective technological applications, namely cutting, welding, hardening, perforating and surface cleaning. A fiber laser can do all this.
However, it is extremely important for managers and technologists of industrial enterprises to understand, in addition to this, the economic aspects of implementing a fiber laser in modern laser technologies. So, let's discuss the economic issues about fiber laser that arise during the evaluation of technical upgrade projects.
It should be noted right away: the differences are very important, since the new fiber laser has a number of technical properties and features, due to which it is not entirely correct to transfer the experience of using classical lasers to new equipment. That is why it is advisable to begin what a fiber laser is, first of all, by outlining these features and differences.
Fiber laser:
The unique life of modern emitters (more than 100,000 hours with the possibility of extending the life at relatively low costs) and almost zero operating costs. Mandatory, taking into account the actual exclusion of part of depreciation through UST and VAT in the existing tax system. Since this can be an extremely important economic factor (i.e., part of the depreciation remains directly at your disposal because it is not used).
Minimum costs and time for premises preparation and commissioning. During the commissioning process, a fiber laser is called “installation”.
Fiber laser, its incredible versatility as a laser source. As a rule, a fiber laser is an example of a source of “pure” beam energy, so there is practically no technological specificity in it, that is, during diversification or other restructuring of production, a fiber laser can be reoriented from one technological process to another. Such a source can even be called, of course (with reservations) - liquid, in the sense that it retains value and value in itself. From here, certain laser exchange and leasing services begin to develop (on these issues, it is best to contact the manufacturer directly).
Fiber laser, its main characteristics:
Its probability of increasing power. You can buy a fiber laser with a design margin, for example, when supplied at a power of 700 W, and then simply purchase special pumping units, thereby increasing the power, for example, up to 2400 W. At the same time, in a production system (the process of installing additional blocks lasts no more than 3 hours) there is practically no need to change anything. This allows you to significantly reduce initial capital investments, as well as increase productivity at the moment necessary for your production.
Transporting radiation directly through an optical cable, the length of which ranges from 10 to 100 meters, greatly simplifies the design and layout of technological systems as a whole. You can use a huge range of industrial robotics. It is worth noting that some production tasks require only 3 components, namely a fiber laser/process head/industrial robot. Of course, in the absence of experience, the services of an integrator company will still be required, but the total costs of organizing a specific production system will be significantly reduced.
The fiber laser is a multifunctional and multi-purpose technological area for maximum loading of the laser source. Naturally, this is not quite as easy as it might seem at first glance, but it is quite possible. And because of the importance of this probability, we will discuss it further.
A question for specialists and personnel in general. A fiber laser eliminates the need for a company to maintain a whole staff of specialists with knowledge of optics, vacuum systems and electrical discharges. A fiber laser, nothing is required to operate it, since operator training takes no more than 1 week. Of course, this will not relieve the enterprise of the need for competent technologists, but this is another question that has absolutely nothing to do with the laser itself. It is quite possible to utilize existing staff and at the same time achieve a higher level of operational efficiency.
Fiber laser, its basic technologies:
These 7 points in themselves can arouse high interest in new modern equipment. To enhance the effect, some basic technologies should be listed:
- laser cutting of metals. We are talking not only about classic cutting of sheets, but also very volumetric cutting, for example, with the use of industrial robots;
- laser perforation (filter elements, meshes);
- laser welding. First of all, this is high-performance seam butt welding without the use of edge preparations and filler materials. But today, technologists are quite rapidly developing hybrid processes, that is, combined welding schemes combining a laser beam and, accordingly, an electric arc;
- laser hardening (heat treatment) is a process that provides local hardening of certain fragments of a part without a significant thermal effect on the part;1
- laser surfacing is an analogue of the action of arc surfacing, characterized by high locality and accuracy;
- laser cleaning of coatings and dirt. The most environmentally friendly cleaning method, and a non-contact one that has the potential to compete with mass technologies, such as sandblasting.
Moving directly to the economic aspects, it is worth noting that the fiber laser and its system are currently an order of magnitude more expensive than classical CO2 lasers and therefore the price of the laser itself usually constitutes a significant part of the technological system as a whole.
Fiber laser, its minimum set includes: equipment intended for performing a technological operation with a laser includes:
- fiber laser must have a specified cost of rub./kW;
- a fiber laser has a special laser processing head, which generates a radiation flow, as well as flows of other substances directly in the processing zone;
- manipulator (robotic) for moving the product or laser head, as well as for general and thorough control of the process. If you use a ready-made and universal fiber laser, then the costs will directly depend on the configuration and, of course, the brand.
Fiber laser, its minimum set for a laser technological system is as follows: 1 – laser, 2 – technological head, 3 – optical cable, 4 – manipulator.
Thus, for a technological system with a power of 1000 W, the basic amount of capital costs will be approximately 6 million rubles. RF. In fact, this is not all the costs, since it is also necessary to take into account the costs of software, integration, preparation of premises and production. Therefore, for the sake of simplicity of calculations, it would be most reasonable to assume that the cost of the overall investment - a fiber laser - will be approximately 2 prices. A similar proportion is observed in particular for laser machines designed for cutting metal. The fiber laser has a power of 2000 W. Prices range from 12 to 14 million Russian rubles. At the same time, laser cutting equipment is a rather large complex system with large dimensions. However, thanks to serial production and standard, well-tested technology, the price is noticeably reduced.
In other technological processes (for example, welding, hardening), the complex of such equipment can be much simpler, but here it is worth considering that at this stage such technologies are not at all packaged into standard serial complexes (that is, in this case there will be costs for the technology and engineering, and very significant ones at that). Therefore, the x2 coefficient for a wide class of uses with an average degree of automation (i.e., the processing process is automatic, and loading and unloading is either semi-automatic or manual) may be justified.
Economics of laser technology by analyzing 2 test production problems
Let's consider the first production problem, about a fiber laser:
So, as the first test task, let's consider the mass production of parts with cylindrical geometry, in which it is necessary to weld 2 half-bodies into a single (solid) sealed body. This is a standard task in the manufacture of various types of filters. The steel is 0.5-1 mm thick, with the average diameter of the product being 60 mm. The goal of the problem is maximum production volume at minimum cost of the product.
The production system itself is synthesized almost automatically for this task. For fast laser welding of such a product, you need a fiber laser with a power of approximately 700 W (i.e., the linear welding speed is about 50 mm/sec), you need a fairly simple welding head, a product rotator (automated) and, accordingly, a system loading and unloading workpieces. For the loading system, it is possible to use a simple tray feeder. Fiber laser, it is assumed that the products intended for welding have already been pre-assembled by workers. However, depending on the level of quality of the workpieces themselves (size calibration), a correction system for the joint of the products - the position of the welding head - may well be necessary. In general, the cost of developing and, accordingly, manufacturing such a fairly simple system amounts to approximately 5 million rubles.
We can draw a small conclusion after the text presented:
- The economic parameters of the system deteriorate significantly as the load level of equipment and, of course, personnel decreases: when producing, for example, 10% of products/parts from the maximum production process figure, the cost will simply increase 10 times. Thus, in both cases, the rather expensive equipment is underutilized and, accordingly, the personnel sit idle.
- In terms of cost, giving up automation also does nothing: the transition to non-automated technological processes will also increase the cost of products, and sharply. This will happen due to a general decrease in labor productivity.
- The use of laser technology allows you to “win” only with maximum load (or at least close to maximum) of the production system and is directly beneficial for the conditions of production itself, and large-scale production at that. The high quality of the laser processing process (i.e. reproducibility and stability) is extremely important for such productions.
It is clear that for large-scale applications, the payback on fiber laser welding can be quite fast due to a sharp increase in overall productivity.
Let's consider the second production problem, about a fiber laser:
As a rule, many real enterprises are characterized by significantly lower serial production, so the problem of loading the laser source will constantly arise.
For example, a certain enterprise manufactures a complex product that consists of a cylindrical body and a lid with a powerful fastening element must be welded to it, and 2 elements must also be welded directly to the lid itself. Inside such a product there is also a rod that operates in abrasion mode, therefore requiring strengthening, as well as a filter for liquid, made in the form of a ring to which a metal mesh is soldered. The estimated serial production of such products is 100,000 per year.
In a typical basic technology for manufacturing products, the following technological processes are used:
- production of forgings intended for a head with an eye;
- complex mechanized processing of forgings;
- cutting holes (several) in the body using a mechanical method;
- welding the necessary parts into the holes;
- welding of the head to the main body is manual arc; there is a large percentage of defects, the cause of which is, among other things, geometry violations (i.e., displacement of the axis of the head and the axis of the cylinder);
- volumetric hardening of the rod, chrome plating and grinding;
- ring mesh cutting;
- subsequent soldering of the mesh along the external and internal contours (a rather difficult to automate process with a high level of defects).
The product of this test task: 1 – body, 2 – cover, 3 – welded part, 4 – ring with holes, 5 – filter mesh. Fiber laser:
Is it possible to use a fiber laser to perform or simplify the technological process in the production of such a product? The essence of the idea is as follows: to use a fiber laser directly in the time division mode, thereby loading its resource with various operations. From a technical point of view, such a possibility exists, but we will discuss the technical aspects of this at the end of the story.
Based on the laser technology parameters of the fiber laser from the database, we estimate, first of all, that we will need a laser source with a power of 1500 W. This is, of course, the minimum power required to reliably weld the elements. Since multifunctional use of the laser is planned, the price of robotic equipment, as a rule, should be higher.
It is also necessary to mention an extremely important integral advantage: the increase in the level of product quality is an extremely important and significant competitive factor directly in the sales market, which allows us to occupy a significant share of it.
It is worth especially emphasizing that the fiber laser and its utilitarian feasibility of all planned technological processes when using it have already undergone appropriate testing and preliminary experimental data on these processes are available.
Thus: a fiber laser, its complex use of a set of laser technologies can quite realistically give a fairly large overall effect, but only on condition that the laser equipment is fully loaded!
The cost of the laser production option is calculated only with an underestimated cost of an industrial enterprise, but an honest calculation of the cost per minute clearly shows that the margin of profitability of such a project is so large and obvious that it is significantly profitable even with high overhead costs - and this is a fact!
It is also worth noting the fiber laser: the designer of the laser system may suggest dividing the technological functionality into 2 laser complexes asymmetrically (i.e., not equally) - the 1st laser complex performs exclusively cutting holes and welding work, and the 2nd performs the remaining operations for manufacturing filters and hardening of rods. Or it can leave only the first complex, which performs operations on the first two factors, due to their main contribution to the profitability of the project as a whole. Fiber laser, these decisions will definitely be determined in many ways by technical issues, namely the questions: “How exactly is multifunctionality implemented?” - “Is this really possible to implement technically?” - “What immediate problems can this lead to?” Let's consider the options and possibilities.
Fiber laser and its applications:
Using a robot with a laser head placed on its manipulator for the provided test task is a completely successful solution. First of all, the robot is capable of automatically welding the ring to the main cover on all 4 sides with minimal time spent on transitions, and during the manufacture of an elementary rotary product positioner with removal and manual installation, the loss of time directly for loading and unloading will also be minimized. Which, of course, is also true for other cutting and welding operations.
The use of universal robots has the advantage that the costs of designing and then manufacturing non-standard technological equipment and tooling are practically eliminated. Since the main burden of production training falls precisely on the preparation of certain programs for the robot, that is, its efficiency.
USE OF MULTIPLE SITES.
This solution requires the development of a separate technological station for absolutely all technological operations, which is equipped with a highly functional manipulator. Following the completion of a certain operation, the laser head, connected by an optical cable to the laser, is reinstalled at another technological station, and accordingly readjusted for another operation performed on the same or another batch of products.
Following the completion of a certain operation, the fiber laser, its laser head, connected by an optical cable to the laser, is reinstalled at another technological station, adjusted accordingly to another operation, and another operation is processed, performed on the same or another batch of products.
Fiber laser Unfortunately, it is not yet possible to have personal laser technological heads at different positions. Since undocking from the head of an optical cable in a workshop environment is strictly prohibited due to dustiness, because the slightest speck of dust from an optical fiber, when it hits an optical output, as a rule, leads to irreversible destruction of this output. A solution to this problem is eagerly awaited by all enterprises with similar equipment, and perhaps in the near future it will still be found.
APPLICATION OF OPTICAL MULTIPLEXERS
A new feature, currently rarely used. Its main essence is the following: you can purchase a certain special laser beam switch, connected by its input to the laser, and at individual posts by several outputs with technological heads. The switching of radiation occurs quite quickly between stations, and such a system can minimize the loss of time for changing products and technological transitions.
To do this, the top-level system must provide dispatch functions, as well as distribute the resources of the laser source directly according to the requests of these technological posts. Since in the calculations for formation we assumed that the loading and unloading time is at least equal to the operation time, in this case, when using such a multiplexer, only one laser will be enough to implement a test program for the production of approximately 100,000 products.
The cost of such a multiplexer is about 1-2 million rubles. In addition, it should be noted that the fiber laser can be ordered with a built-in multiplexer that has several outputs.
Perhaps the only drawback is that the multiplexer slightly degrades the quality of the radiation (i.e., at the output it is necessary to use a fiber of a much larger cross-section), but this is critical only for laser cutting. Fiber laser, its similar system is the most optimal and expedient. For a multiplexer, additional capital costs are compensated many times over thanks to the laser load level.
So: 1 – laser, 2 – optical switch, 3 – heads (technological), 4 – technological stations, 5 – central control system.
Another important issue related to the versatility of the laser heads themselves: If you plan to use an industrial robot or a multi-station area, then the laser head must have the property of versatility (that is, be able to perform various technological processes). Today, Western manufacturers do not produce such heads!
However, such equipment already exists: mass production will soon begin - a universal tunable head that can perform the entire basic range of technological operations using fiber laser radiation (welding, cutting, hardening, perforation). Adaptation of the head to any specific operation is carried out both through the automatic conversion of the optical system and through a replaceable technological attachment (i.e., its replacement), which is attached according to the principle of the well-known magnetic suspension.
Fiber laser, its advantages:
Estimates show that fiber laser has significant economic potential.
- The high profitability of fiber laser projects based on modern lasers is ensured exclusively with maximum equipment load, that is, due to the fairly significant reliability and unique resource of new lasers, it is technically possible.
- Multifunctional technological areas that share the resource of the laser source can have quite a significant future.
- Despite significant capital investments, the payback on laser equipment and laser technological systems in general can be very, very fast, up to 1-1.5 years.
A fiber laser is a laser with a fully or partially fiber-optic implementation, where the gain medium and, in some cases, the resonator are made of optical fiber.
A fiber laser is a laser with a fully or partially fiber optic implementation, where optical fiber A a gain medium and, in some cases, a resonator are made. Depending on the degree of fiber implementation, a laser can be all-fiber (active medium and resonator) or discrete fiber (fiber only resonator or other elements).
Fiber lasers can operate in continuous wave as well as nano- and femtosecond pulsed pulses.
Design laser depends on the specifics of their work. The resonator can be a Fabry-Perot system or a ring resonator. In most designs, an optical fiber doped with ions of rare earth elements - thulium, erbium, neodymium, ytterbium, praseodymium - is used as the active medium. The laser is pumped using one or more laser diodes directly into the fiber core or, in high-power systems, into the inner cladding.
Fiber lasers are widely used due to a wide selection of parameters and the ability to customize the pulse over a wide range of durations, frequencies and powers.
The power of fiber lasers is from 1 W to 30 kW. Optical fiber length – up to 20 m.
Applications of fiber lasers:
– cutting metals and polymers in industrial production,
– precision cutting,
– microprocessing metals and polymers,
– surface treatment,
– soldering,
– heat treatment,
– product labeling,
– telecommunications (fiber optic communication lines),
– electronics production,
– production of medical devices,
– scientific instrumentation.
Advantages of fiber lasers:
– fiber lasers are a unique tool that opens a new era in materials processing,
– portability and the ability to select the wavelength of fiber lasers allow for new effective applications that are not available for other types of currently existing lasers,
– superior to other types of lasers in almost all significant parameters important from the point of view of their industrial use,
– possibility to customize the pulse in a wide range of durations, frequencies and powers,
– the ability to set a sequence of short pulses with the required frequency and high peak power, which is necessary, for example, for laser engraving,
– wide choice of parameters.
Comparison of different types of lasers:
| Parameter | Required for industrial use | CO 2 | YAG-Nd lamp-pumped | Diode-pumped YAG-Nd | Diode lasers | |
| Output power, kW | 1…30 | 1…30 | 1…5 | 1…4 | 1…4 | 1…30 |
| Wavelength, µm | as less as possible | 10,6 | 1,064 | 1.064 or 1.03 | 0,8…0,98 | 1,07 |
| BPP, mm x mrad | < 10 | 3…6 | 22 | 22 | > 200 | 1,3…14 |
| Efficiency, % | > 20 | 8…10 | 2…3 | 4…6 | 25…30 | 20…25 |
| Fiber radiation delivery range | 10…300 | absent | 20…40 | 20…40 | 10…50 | 10..300 |
| Output power stability | as high as possible | low | low | low | high | very high |
| Back-reflection sensitivity | as low as possible | high | high | high | low | low |
| Occupied area, sq.m | as less as possible | 10…20 | 11 | 9 | 4 | 0,5 |
| Installation cost, relative units | as less as possible | 1 | 1 | 0,8 | 0,2 | < 0,05 |
| Cost of operation, rel. units | as less as possible | 0,5 | 1 | 0,6 | 0,2 | 0,13 |
| Cost of maintenance, rel. units | as less as possible | 1…1,5 | 1 | 4…12 | 4…10 | 0,1 |
| Frequency of lamp or laser diode replacement, hour. | as much as possible | – | 300…500 | 2000…5000 | 2000…5000 | > 50 000 |
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The study of the problem of laser cutting of metals must begin with a consideration of the physical principles of laser operation. Since further in the work all studies of the accuracy of laser cutting of thin-sheet materials will be carried out on a laser complex using an ytterbium fiber laser, we will consider the design of fiber lasers.
A laser is a device that converts pump energy (light, electrical, thermal, chemical, etc.) into the energy of a coherent, monochromatic, polarized and highly targeted radiation flux.
Fiber lasers were developed relatively recently, in the 1980s. Currently, models of fiber technological lasers with a power of up to 20 kW are known. Their spectral composition ranges from 1 to 2 μm. The use of such lasers makes it possible to provide different temporal characteristics of radiation.
Recently, fiber lasers have been actively replacing traditional lasers in such areas of application of laser technology as, for example, laser cutting and welding of metals, marking and surface treatment, printing and high-speed laser printing. They are used in laser rangefinders and three-dimensional locators, telecommunications equipment, medical installations, etc.
The main types of fiber lasers are continuous wave single-mode lasers, including single-polarization and single-frequency lasers; pulsed fiber lasers operating in Q-switching, mode-locking, and random modulation modes; tunable fiber lasers; superluminescent fiber lasers; high-power continuous multimode fiber lasers.
The operating principle of the laser is based on transmitting light from a photodiode through a long fiber. A fiber laser consists of a pump module (usually broadband LEDs or laser diodes), a light guide in which lasing occurs, and a resonator. The light guide contains an active substance (doped optical fiber - a core without a cladding, unlike conventional optical waveguides) and pump waveguides. The design of the resonator is usually determined by the technical specifications, but the most common classes can be distinguished: Fabry-Perot type resonators and ring resonators. In industrial installations, several lasers are sometimes combined in one installation to increase output power. In Fig. Figure 1.2 shows a simplified diagram of a fiber laser device.
Rice. 1.2. Typical fiber laser circuit.
1 - active fiber; 2 - Bragg mirrors; 3 - pumping block.
The main material for active optical fiber is quartz. The high transparency of quartz is ensured by the saturated states of the energy levels of atoms. Impurities introduced by doping transform quartz into an absorbing medium. By selecting the pump radiation power, in such an environment it is possible to create an inverse state of population of energy levels (that is, high-energy levels will be more filled than the ground level). Based on the requirements for the resonant frequency (infrared range for telecommunications) and low threshold pump power, as a rule, doping is performed with rare earth elements of the lanthanide group. One of the common types of fibers is erbium, used in laser and amplifier systems, the operating range of which lies in the wavelength range 1530-1565 nm. Due to the different probability of transitions to the main level from sublevels of the metastable level, the efficiency of generation or amplification differs for different wavelengths in the operating range. The degree of doping with rare earth ions usually depends on the length of the active fiber being manufactured. Within a range of up to several tens of meters it can range from tens to thousands of ppm, and in the case of kilometer lengths - 1 ppm or less.
Bragg mirrors - a distributed Bragg reflector - is a layered structure in which the refractive index of the material periodically changes in one spatial direction (perpendicular to the layers).
There are various designs for pumping optical waveguides, of which the most common are pure fiber designs. One option is to place the active fiber inside several sheaths, of which the outer one is protective (the so-called double-coated fiber). The first shell is made of pure quartz with a diameter of several hundred micrometers, and the second is made of a polymer material, the refractive index of which is selected to be significantly lower than that of quartz. Thus, the first and second claddings create a multimode waveguide with a large cross-section and numerical aperture into which the pump radiation is launched. In Fig. Figure 1.3 shows the pumping diagram of a laser based on a double-coated fiber.
Rice. 1.3. Pumping circuit for a laser based on a double-coated fiber.
The advantages of fiber lasers traditionally include a significant ratio of the resonator area to its volume, which ensures high-quality cooling, thermal stability of silicon and small sizes of devices in similar classes of power and quality requirements. A laser beam, as a rule, must be inserted into an optical fiber for subsequent use in technology. For lasers of other designs, this requires special optical collimation systems and makes the devices sensitive to vibrations. In fiber lasers, radiation is generated directly in the fiber, and it has high optical quality. The disadvantages of this type of laser are the risk of nonlinear effects due to the high radiation density in the fiber and the relatively low output energy per pulse due to the small volume of the active substance.
Fiber lasers are inferior to solid-state lasers in applications where high polarization stability is required, and the use of polarization-maintaining fiber is difficult for various reasons. Solid-state lasers cannot be replaced by fiber lasers in the spectral range of 0.7-1.0 microns. They also have greater potential for increasing pulse output power compared to fiber ones. However, fiber lasers perform well at wavelengths where there are no good enough active media or mirrors for other laser designs, and allow some laser designs like up-conversion to be implemented more easily.
These machines are based on a fiber optic laser. It is distinguished by very high radiation quality with small device dimensions. In addition, the equipment is easy to cool and does not require labor-intensive maintenance. Fiber laser engravers are widely used in such areas as:
- production of souvenirs;
- automotive industry, medical equipment manufacturing and other industries that require high-quality corrosion-resistant marking of parts;
- production of jewelry and costume jewelry;
- production of monuments and ritual products;
- decoration of furniture and interior elements.
Fiber laser engravers have a slightly higher price compared to CO 2 machines. But this circumstance is compensated by a number of advantages that such equipment has:
- higher efficiency, due to which the laser has low power consumption and good power;
- the operation of fiber laser engravers is based on the use of diodes, which are characterized by compactness, reliability and durability;
- Ultra-small beam size allows for higher engraving resolution and produces microscopic images with excellent detail.
How to choose a fiber laser engraver
When purchasing equipment, you must pay attention to the following characteristics:
- power. It must correspond to the type of material being processed, as well as the required machine performance;
- dimensions of the engraving field. They determine the maximum dimensions of the workpiece that the machine can process;
- functionality and availability of additional options.