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.
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 delivery 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 studied. 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 powerful 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 start what a fiber laser is, first of all, by outlining these features and differences.
Fiber laser:
The unique lifespan of modern emitters (more than 100,000 hours with the possibility of extending the lifespan 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 preparing the premises 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 process 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 replacement of lamps or laser diodes, hour. | as much as possible | – | 300…500 | 2000…5000 | 2000…5000 | > 50 000 |
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Fiber lasers are understood as optically pumped solid-state lasers, the active element of which is a fiber light guide with additives of laser activators. The most promising for light-guide systems are lasers based on fibers activated by neodymium ions. Neodymium ions have two main laser lines with central wavelengths µm and µm, lying in the spectral range where losses and dispersion of light in quartz fibers are minimal.
Rice. 4.11. Dependence of the length of the relay section on the information transmission rate for a stepped fiber with attenuation for microns:
1 - for a laser diode (the characteristic decline in the BC section is due to intermode dispersion) 2 - for a sbeto-emitting diode (the characteristic decline is due to the broad spectrum of the diode in the section, and in addition to the frequency characteristic decline in the section)
The spectral characteristics of the amplification of neodymium are practically independent of external conditions; the temperature drift of the wavelength corresponding to the maximum amplification of neodymium ions is equal, while for semiconductor media this parameter is The fiber design of the emitter allows using standard connectors to effectively introduce radiation into fiber light guides, including and single-mode.
Despite these advantages and, as will be shown below, wide functionality, fiber lasers have not yet left the research stage. This is explained by the fact that when creating fiber-optic systems, many problems were solved using well-developed semiconductor emitters, especially in fairly simple systems being implemented in the first place, where one of the main advantages of semiconductor sources plays a decisive role - the possibility of direct modulation of the radiation intensity by the pump current. In solid-state lasers, in particular in lasers based on neodymium-activated media, high-speed modulation of the radiation intensity by changing the pump power is fundamentally impossible due to the relatively long longitudinal relaxation time. The inability to quickly “switch on” the inverted population limits the direct modulation frequencies to Hz values. Development of light-guide systems, especially promising systems of the near future with coherent reception and multi-channel spectral
compaction stimulates the development of fiber lasers, which can be used not only as generators, but also as light amplifiers.
Existing fiber laser designs can be divided into three groups. Fiber lasers of the first group use bundles of several long fibers and powerful pumping with pulsed gas-discharge lamps. Positive feedback in such structures is formed due to the reflection of light from the ends of the fibers and backscattering at microbends and inhomogeneities.
Rice. 4.12. Designs of fiber lasers: a - with end pumping; b - with transverse pumping for small-diameter fibers, c - with direct laying of fibers on a ruler - emitting platform - laser resonator mirror, transparent to radiation, 13 - active fiber, 5 - resonator mirror; 6 - optical glue, 8 - reflector, 9 - glass cylinder, 10, 12 - radiators; 11, 14 - LED lines
Tube pumping makes it possible to achieve high gains in a single pass, but requires the use of forced liquid cooling systems and bulky power supplies, which apparently makes the creation of small-sized devices unrealistic. Certain prospects in this sense may lie in the use of gas-discharge microlamps. The advantages of lamp-pumped designs include the possibility of using them as traveling-wave optical amplifiers and regenerative amplifiers with a fairly high (~30-40 dB) gain.
The second group of fiber laser designs uses short lengths of monocrystalline and glass fibers doped with neodymium ions. Pumping is carried out through the end of the fiber with a semiconductor laser or LED. A sufficiently high pump efficiency is achieved by matching the emission spectrum of a semiconductor emitter based on a GaAlAs GVD with one of the intense absorption lines of neodymium with a central wavelength of about
0.81 µm. The design of fiber lasers of the second group is shown schematically in Fig. 4.12, a. Due to the low gain of the active medium, the laser cavity is formed
dielectric mirrors with high reflectivity. Lasers based on monocrystalline fibers made of yttrium aluminum garnet with neodymium and glass quartz fibers with neodymium have this design. There are reports of generation with end-pumping by a krypton laser in a crystalline fiber and with pumping by an argon laser in a ruby fiber. The best results were obtained when using a crystal with a fiber geometry, 0.5 cm long and 80 μm in diameter. The external resonator (Fig. 4.12, a) was formed by mirrors with a dielectric coating, one of which had a reflectance for laser radiation with microns and only for pump radiation, the second mirror with the same high reflectivity for laser radiation reflected the pump light quite well The mirrors were located almost close to the ends of the fiber. Pumping was carried out by a surface LED with a emitting area diameter of 85 μm. The threshold pump power was
The main advantages of fiber lasers of this design are low power consumption and overall dimensions. Main disadvantages: the end pumping circuit does not allow the use of fiber segments with a length of more than 1 cm, which limits the output power. In addition, the manufacturing and alignment technology of these lasers is complex, and the presence of a pump LED at one of the ends complicates the use of the laser as an optical signal amplifier.
Multi-turn fiber lasers with transverse pumping by LED bars (Fig. represents the designs of the third group. Several turns of glass fiber are placed on the LED bar, the core of which is activated by neodymium ions. The design to a certain extent combines the advantages of fiber lasers of the first and second groups and is devoid of most of their disadvantages. The use of semiconductor emitters as pump sources makes such systems quite small in size; the use of a transverse pumping scheme and long fiber sections makes it possible to obtain a fairly large gain in one pass. Due to the small diameter of the optical fibers in a transversely pumped scheme, the use of glass fibers with a high ion concentration is effective. neodymium and, accordingly, with a high absorption coefficient of pump light. Fibers made of neodymium ultraphosphates have such properties. Multi-turn fiber placement on LED lines can be done in different ways. Thus, a piece of fiber is repeatedly pulled through a glass cylinder with a diameter of about 1 mm (Fig. 4.12, b), on the outer surface of which a reflective coating is applied to
increasing the efficiency of using pump radiation. This method is preferred for fibers with a small outer diameter (µm). Fibers of larger diameter can be laid on the LED line turn to turn (Fig. 4.12, c). Both designs can be used as traveling-wave optical amplifiers, with one end of the light guide being the amplifier input and the other being the output. The application of mirror coatings to the ends of the fibers allows lasing with a Fabry-Perot fiber resonator.
The features of laser processes in active optical fibers are determined by the presence of specific laser generation in the absence of positive feedback.
Rice. 4.13. Fiber light guide: a - with an active core and a passive cladding; b - with a passive core and an active shell (2)
This is the main difference between fiber lasers and lasers based on volumetric active elements. To explain the essence of this process, which is close to the superluminescence regime in semiconductor LEDs, let us consider some elementary section of the light guide in which an inverted population is created (Fig. 4.13, a). Spontaneous emission occurs with equal probability in all directions, but the radiation, concentrated in two cones of angles that have a common axis with the fiber and are determined by an opening angle of 20, does not leave the core. Here
where are the refractive indices of the core and cladding, respectively. This radiation excites natural oscillations (modes) of the fiber, which are amplified by stimulated emission during propagation along the fiber to the right and left (Fig. 4.13, a). The same picture is observed for any other elementary section of the active fiber core. At the output of such a fiber light source, the radiation divergence is approximately determined by the numerical aperture of the fiber
As long as the intensity of light waves propagating towards each other in an active light guide is significantly less than the value that saturates the gain, the counterpropagating waves are independent, as well as the energies transferred by different modes of the light guide. Under these conditions, the process of amplification of spontaneous emission due to stimulated emission is described by the well-known equations of a laser amplifier without saturation and taking into account spontaneous emission. The spectral power density of radiation in one mode at the output of the active section of a fiber length (Fig. 4.13, a) is equal to
Here is Planck's constant; - frequency of light vibrations; - populations of the upper and lower laser levels; - gain per unit length, where is the Einstein coefficient for the forced transition; - normalized shape of the spectral gain line; c is the speed of light. The maximum generated power can be limited either by the length of the light guide or, as in lasers with resonators, by saturation. Naturally, during the amplification process, the generation spectrum narrows compared to the luminescence spectrum due to the fact that the spectral components in the center of the line are amplified more. The width of the spectrum is determined by the gain and shape, and the emission spectrum is continuous due to the absence of a resonator.
The specific fiber laser process under consideration has three significant aspects.
1. Active fiber light guide can be used as a light source without an optical resonator.
2. When creating fiber lasers using a traditional cavity design, it is necessary to take into account that the considered process can lead to gain saturation in one pass, as a result of which the feedback will lose its meaning. In this case, the values of and must be chosen so that they are far from the value that saturates the gain.
3. In fiber optical amplifiers, the generation of light as a result of the process discussed is the main source of noise. The noise power spectral density in one mode, recalculated to the amplifier input, as follows from formula (4.12), is equal to
In a four-level system, such as the neodymium laser level circuit, usually at high gains
In volumetric amplifiers, the noise of amplified spontaneous emission has long been considered fundamentally irremovable (see, for example, work), however, in fiber amplifiers, its level can be significantly reduced when using the light guide shown in Fig. 4.13, 6. Single-mode fiber, the core of which is made of quartz glass with an additive that increases the refractive index, for example, has a cladding of glass activated by neodymium ions. Creation of an inverse population in the cladding leads to amplification of the core mode with an effective gain
where is the gain in the shell; - part of the core mode power that propagates in the cladding; P is the total power carried by this mode. The ratio changes from 0.99 to 0.1 when the fiber parameter changes from 0.6 to 2.4048. When the core begins to effectively direct the main mode by localizing its field near itself, the second mode is excited. The formula was obtained in the same way as the expression for the attenuation coefficient of a fiber with a cladding in which radiation losses occur that are inferior in quality to fiber ones. Significant disadvantages of the former are the temperature instability of the amplification line (for microns), significant losses when connecting single-mode fiber light guides to the planar light guide of the amplifier, and a high level of noise power - superluminescence radiation.
Fiber lasers open up the possibility of creating new types of FODs. The sensitive element, which is a fiber light guide, is here part of a fiber ring or linear laser resonator.
Rice. 4.14. Single-frequency fiber lasers with distributed feedback (a) and Bragg mirrors (b): 1 - active core; 2 - shell with a periodic structure
A change in the phase of light oscillations under the influence of external factors leads to a change in the generation frequencies of various modes in lasers. Information about external influences is contained in the change in the frequency of intermode beats. Based on a fiber laser with a ring resonator, which is realized by welding the ends of the light guide or by detaching them, it is quite simple to create a small-sized laser fiber gyroscope.
Stable single-frequency fiber lasers can be implemented as a distributed feedback or distributed Bragg reflection design. To do this, a fiber reflective spectral filter is created in certain sections of the fiber using one of the methods that will be described below (see paragraph 4.8) (Fig. 4.14). Such sources can be used in phase water diodes.
The use of superluminescent fiber lasers makes it possible to simplify the design of passive fiber gyroscopes and increase their sensitivity by reducing the noise level caused by the presence of volumetric elements. In ring interferometers and gyroscopes, the noise level decreases with a decrease in the coherence length of the source radiation and the number of volumetric elements (see section 3.6). In a fiber source, it is easy to ensure that the coherence length of the radiation is greater than the path difference between the counterpropagating waves of the interferometer, due to rotation and non-reciprocal effects. Superluminescent fiber lasers have a nm spectrum width and a fairly high pulse power. Such a source
connects to a fiber ring interferometer using standard couplers.
