Fiber lasers are essentially different from other types of lasers. In a fiber optic laser, the active medium that produces the laser is actually dispersed within the fiber itself. This distinguishes it from the laser that passes through the optical fiber, which simply transfers the beam from the laser resonator to the guiding light. Optical fiber lasers are by far the most widely recognized laser types in all lasers.
Fiber laser has established it in the industrial laser families, especially in high power number kilowatt products, obviously, restrictions earlier fiber laser has been fully understood, it also contributed to the development of laser wider. The extensibility of the fiber laser is used to enlarge the output power of multi-mode fiber laser to above 50 kw, and increase the power of single-mode fiber laser to 10 kw.
Figure 1. Q-switched fiber laser module
Pulse fiber laser
Pulse nanosecond lasers are the first type to be developed, and their success in the commercial market has replaced a large portion of the target laser. The average power of the laser has now been extended to up to 500 watts. On the other hand, for the further study of this kind of optical fiber laser technology led to a number of fiber lasers can generate a more narrow nanosecond pulse width, higher brightness, and the repetition rate is as high as several MHZ. Polarization-preserving fiber lasers are now being developed for efficient conversion to 532 nm. Recently, the millisecond quasi-continuous wave (QCW) laser with longer pulse width has been developed. Based on the in-depth study of optical fiber laser, the advantages of the laser laser are almost entirely applicable to the whole range of laser machining of macroscopic and micro industries.
FIG. 2 nanosecond laser with an average power of 500 watts
Nanosecond pulse fiber laser
The versatile laser marking machine and the mirror scanning system are constantly innovating, and the functions of the two are also converging, which further enhances the application capacity. The q-switched fiber laser with single pulse energy of 1mJ can be marked on many different materials, including from ceramics to metal alloys and low melting point polymers. The average power goes up to 50 watts, and at the pulse rate of 50kHz, the monopulse energy is 1mJ, and the M2 is kept at 1.6 without degradation, which can quickly remove the metal material in the engraving application. The new type of marking laser has a very short pulse closing time which is less than 3s, which improves the processing effect in the application of some sensitive materials. The repetition rate increases to 200kHz and the pulse cycle decreases to 5s. It is believed that in such a high repetition frequency, the plasma near the pulse interval leads to the non-continuous processing of the beam to the target. One of the important examples is that the oxide layer is produced by the stainless steel, which is visible to the naked eye; And even after repeated high temperature and pressure treatment, the oxide layer cannot be removed. If the target object is a high-reflective material that is prone to problems, the remote optical amplifier (RA) can redouble the peak power to 20 kilowatts.
Q-switched fiber laser with high average power
FIG. 3 relation diagram of high power pulsed laser, brightness and pulse energy.
Further development has enabled the compact 500-watt average power system to provide a pulse width of less than 100 nanoseconds. The laser in this range has a three-level MOPA configuration of all optical fiber. The fiber-optic tail connector (AOM) is used to control pulse width ranging from 30 to 2, 000 nanoseconds, with a peak output of up to 1 megawatt in this range. The output light is in the form of "fiber to air" or "fiber to process fiber."
High pulse energy - up to 50mJ, less than 100 nanosecond pulses, can achieve high peak power on the workpiece, although some compromises must be made when the pulse energy increases (see figure 3). These lasers, through a sound wave or thermal shock mechanism, can effectively remove various types of surface film layers, thus minimizing the heat input to the parts. Square fiber is now used in such lasers, which can greatly improve the processing efficiency in some applications (see fig.4 and 5).
MOPFA laser
Pulse fiber laser is the second large category is called the seeds of MOPFA semiconductor diode laser fiber master oscillator power amplifier, they and the Q - switching differences between fiber laser pulse rise time may be faster, may be more short pulse duration, diversification of pulse width, pulse repetition frequency can be as high as several megahertz.
According to the combination of parameters shown in table 3, the power of peak power and power density can be greatly improved, so as to handle the tasks of marking and micro-processing. The pulse width can be reduced to less than 10 nanoseconds to achieve higher energy density. In microprocessing, a small amount of material is required to be precisely removed in a limited area, in which case the laser can be treated with this type of laser. The market needs a new high luminance short pulse laser which has good pulse to pulse stability. But before develop the new type of laser, the only solution for a specific micro machining process is: the high cost of diode pump solid state laser, or low efficiency of flash lamp pumped solid state laser.
Double frequency green optical fiber laser
This type of optical fiber laser has some unique functions, although it is not strictly a green light fiber laser (because its active medium does not directly release 532 nanometer laser beams). The laser source provides a narrow range of pulse duration and a frequency of up to 600 kHz. A one-way second harmonic generator USES a 20 mm LBO crystal. A high speed digital servo loop adopts the reactive piezoelectric mirror. The laser source of high spectral luminance facilitates efficient conversion, achieving 84% conversion efficiency and more than 20% of the electro-optical conversion efficiency, and the feasibility of upgrading to high power of 355 and 266 nanometers.
Continuous wave (CW) laser - modulation type
The pulse rise time of the diode pump is 5s, so the minimum pulse duration (or time modulation) is about 10s.
Using simple control techniques can be used to control these lasers at 10-100%, so the modulation frequency is as high as 50 kHz can be achieved. During microcutting, modulation can minimize heat input of components. A wide range of control over the occupied space and M2 is equal to 1.05 - the combination of these two factors may achieve the slit width smaller than 20 microns, using traditional optical elements.
Quasi-continuous wave (QCW) laser
Although the cw fiber laser has the ability to achieve high energy pulse, but mainly through the use of higher than the average power needed to mode or long pulse duration to complete, and that both are flawed. In recent years, the range of continuous wave laser has been expanded to develop lasers with higher peak power and higher pulse energy, as shown in table 5.
The laser has a number of joule-pulse energy, super-long pulses, and the ability to be coupled to a larger diameter, making the diameter of the solder point up to 0.5mm. Early experiments show that these low - duty solder joints are similar in any way to the flash pump solids laser.
Improvement of fiber laser
The reasons for the rapid development of fiber optic lasers can be summarized as follows:
Low in-depth scientific research can be realized to increase the heat load of the fiber, and does not produce any thermal decay or light absorption effect and the influence of stimulated Raman scattering (SRS), after these tend to be as limiting factors of fiber laser, now only for high average power laser is concerned.
As a pump source, single emitter pump diodes have a more reliable advantage over semiconductor diodes and semiconductor diodes. The life of the pump diode is usually greater than 100,000 hours, and it is not needed to be replaced during the service life of the whole laser processing system. The high strength of aging tests further improved reliability.
Increases the power of the diode pump and the efficiency of the pump to bring higher average power.
Because of its own nature, optical fiber is largely self-cooling, thus reducing the thermal lens effect and simplifying the design of the laser. These benign thermal factors mean that its cooling requirements are not as harsh as the other lasers designed by semiconductor diodes and semiconductor diodes.
Multiple optical fiber can be selected and plug and play optical fiber can get various types of spatial energy distribution; Small single mode gaussian fiber is used for cutting and drilling, and the larger diameter multi-mode fiber is used for welding or surface treatment. A continuous wave is equipped with a single mode fiber laser optical fiber with a diameter of 50 microns for example, you can simply from cutting laser to laser welding, the need to do is simply replace the terminal focusing optical element.
A fiber adapter capable of transmitting up to 25 kilowatts of power has been obtained. Some fiber-optic laser power beam switches, which can be used to connect up to six channels for fiber-optic cables, can be switched for less than 10 milliseconds. A visible red alignment beam is available for each channel.
More high-speed electronic components, advanced interfaces, control and network software have been developed.
Development in microprocessing and marking fields
As we have seen, now there are a variety of different pulse fiber laser, when used with advanced galvanometer scanners, can be used to many processing such as cutting, drilling and cladding. Proved, it can be used in those usually performed by high power density infrared laser beam of material removal applications, and is suitable for laser micro processing technology and marking, fiber laser has a higher brightness means material removal process can be improved significantly. This fact, coupled with the increasing accuracy of the mirror scanner, means that the standard laser can now perform tasks previously considered "precision microprocessing". Although the wavelength is directly proportional to the brightness and the ability to focus, in some cases it has been able to achieve the characteristic dimensions, accuracy and precision previously achieved only through a laser of 532nm and 355nm.
Fiber laser spot welding
As we know, it is easier to have a single emitter diode than a semiconductor diode or a semiconductor diode stack in the low to medium range modulation single emitter. Recently, because of the special pump is the latest development of semiconductor diode and enhance the ability of pulse has developed high brightness fiber laser, it can produce very close to the traditional flash lamp pumped by laser pulse, as shown in table 5. Will this feature binding energy and the characteristics of optical fiber with a diameter of different matching, can make this kind of laser for low heat input, low duty ratio, effect is the same with the traditional flash lamp pumped laser spot welding, but it has all the advantages of the fiber laser is well known, such as the smaller area, less maintenance and electro-optical efficiency of 10 times. Because of the flexibility of pump source, it is easy to realize the adjustment of pulse waveform in the application of medical equipment welding, without the limitation of traditional flash type.
High average power fiber laser is used
Although the 10-kilowatt single-mode laser has been made, this combination of high average power and high brightness clearly exceeds the current demand for material processing. In the traditional gas-assisted laser cutting, it is necessary to provide auxiliary gas to the front end of the cutting, so there is a limitation: the high width ratio can be achieved only within a certain range.
Fiber laser is widely considered the brightness of the highest and infrared laser beam focused laser source, along with some other well known the advantage of optical fiber laser, its applications in sheet metal cutting, won the rapid growth of market share, particularly in Europe. Now it is widely accepted and agreed that, for metal cutting thickness less than 8 mm, compared with CO2 laser, in obtaining the same cutting quality, the same power can obtain higher cutting speed, the same cutting speed only needs less power. Are some Suggestions in the scientific literature, points out that the different wavelengths of laser in the process of cutting more than 10 mm thick steel plate when there are essential differences in the performance of, obviously this is true remains to be investigated, but the optical fiber laser equipment integrator has begun to make substantial improvement of thick plate cutting quality.