Fiber lasers are compact and robust, precisely guided and easily dissipate thermal energy. They come in many forms and, having much in common with other types of optical quantum generators, have their own unique advantages.
Fiber Lasers: How It Works
Devices of this type are a variation of a standard solid-state source of coherent radiation with a working medium of fiber, and not a rod, plate or disk. Light is generated by a dopant in the center of the fiber. The basic structure can range from simple to quite complex. The ytterbium fiber laser device is such that the fiber has a large surface to volume ratio, so heat can be relatively easily dissipated.
Fiber lasers are pumped optically, most often with the help of diode quantum generators, but in some cases with the same sources. The optics used in these systems are typically fiber components, with most or all of them connected to each other. In some cases, surround optics are used, and sometimes the internal fiber optic system is combined with external surround optics.
The source of the diode pump can be a diode, a matrix, or many individual diodes, each of which is connected to the connector by a fiber optic fiber. The doped fiber at each end has a cavity resonator mirror - in practice, Bragg gratings are made in the fiber. There is no bulk optics at the ends, unless the output beam goes into something other than fiber. The fiber can be twisted, so that if desired, the laser resonator can have a length of several meters.
Dual core structure
The structure of the fiber used in fiber lasers is important. The most common geometry is a dual core structure. The undoped outer core (sometimes called the inner shell) collects the pumped light and directs it along the fiber. The stimulated radiation generated in the fiber passes through the inner core, which is often single-mode. The inner core contains an additive of ytterbium, stimulated by a light pump beam. There are many non-circular forms of the outer core, including hexagonal, D-shaped and rectangular, which reduce the likelihood of a light beam not falling into the central core.
The fiber laser may have end or side pumping. In the first case, light from one or more sources enters the end of the fiber. With lateral pumping, the light is supplied to the splitter, which feeds it into the external core. This is different from a rod laser, where light enters perpendicular to the axis.
Such a solution requires many constructive developments. Considerable attention is paid to bringing the pumping light into the core in order to invert the population leading to stimulated emission in the inner core. The laser core may have a different degree of amplification depending on the doping of the fiber, as well as on its length. These factors are tuned by the design engineer to obtain the necessary parameters.
Power limitations may occur, particularly when operating within a single-mode fiber. Such a core has a very small cross-sectional area, and as a result, very high-intensity light passes through it. In this case, Brillouin non-linear scattering becomes more and more noticeable, which limits the output power to several thousand watts. If the output signal is high enough, the fiber end may be damaged.
Features of fiber lasers
The use of fiber as a working medium gives a long interaction length, which works well with diode pumping. This geometry leads to high photon conversion efficiency, as well as a robust and compact design that lacks discrete optics that require adjustment or alignment.
A fiber laser, the device of which allows it to adapt well, can be adapted both for welding thick sheets of metal and for receiving femtosecond pulses. Fiber optic amplifiers provide single-pass amplification and are used in telecommunications because they are capable of amplifying many wavelengths simultaneously. The same gain is used in power amplifiers with a master oscillator. In some cases, the amplifier may operate with a cw laser.
Another example is fiber-amplified spontaneous emission sources in which stimulated emission is suppressed. Another example is the Raman fiber laser with combined scattering amplification, which significantly shifts the wavelength. It has found application in scientific research, where fluoride glass fiber is used for Raman generation and amplification rather than standard silica fibers.
However, as a rule, the fibers are made of quartz glass with a rare-earth dopant in the core. The main additives are ytterbium and erbium. Ytterbium has wavelengths from 1030 to 1080 nm and can emit in a wider range. The use of 940 nm diode pumping significantly reduces the photon deficit. Ytterbium does not have any of the self-extinguishing effects that neodymium has at high densities, so the latter is used in bulk lasers, and ytterbium in fiber lasers (they both provide approximately the same wavelength).
Erbium emits in the range of 1530-1620 nm, safe for the eyes. The frequency can be doubled to generate light at 780 nm, which is not available for other types of fiber lasers. Finally, ytterbium can be added to erbium in such a way that the element will absorb pump radiation and transmit this energy to erbium. Thulium is another dopant with a near-infrared glow, which is thus safe for the eyes.
High efficiency
A fiber laser is a quasi-three-level system. The pump photon excites the transition from the ground state to the upper level. The laser transition is a transition from the lowest part of the upper level to one of the split ground states. This is very effective: for example, ytterbium with a 940-nm pump photon emits a photon with a wavelength of 1030 nm and a quantum defect (energy loss) of only about 9%.
In contrast, neodymium, pumped at 808 nm, loses about 24% of its energy. Thus, ytterbium by its nature has a higher efficiency, although not all of it is achievable due to the loss of some photons. Yb can be pumped in a number of frequency bands, and erbium can be pumped at a wavelength of 1480 or 980 nm. A higher frequency is not so effective from the point of view of a photon defect, but is useful even in this case, because at 980 nm the best sources are available.
Overall, fiber laser efficiency is the result of a two-step process. Firstly, it is the efficiency of the pump diode. Semiconductor sources of coherent radiation are very efficient, with 50% efficiency for converting an electrical signal into an optical one. The results of laboratory tests suggest that it is possible to achieve values of 70% or more. With an exact match of the output radiation of the absorption line of the fiber laser, a high pump efficiency is achieved.
Secondly, it is the optical-optical conversion efficiency. With a small defect of photons, a high degree of excitation and extraction efficiency can be achieved with an opto-optical conversion efficiency of 60–70%. The resulting efficiency is in the range of 25–35%.
Various configurations
Fiber optic quantum cw generators can be single- or multimode (for transverse modes). Singlemode produces a high-quality beam for materials that work or send a beam through the atmosphere, and multi-mode industrial fiber lasers can generate more power. This is used for cutting and welding, and, in particular, for heat treatment, where a large area is illuminated.
A long-pulse fiber laser is essentially a quasi-continuous device, typically producing millisecond pulses. Typically, its duty cycle is 10%. This leads to a higher peak power than in continuous mode (usually ten times more), which is used, for example, for pulse drilling. The frequency can reach 500 Hz, depending on the duration.
Q-switching in fiber lasers works the same as in bulk lasers. Typical pulse widths range from nanoseconds to microseconds. The longer the fiber, the longer it takes for the Q-switching of the output radiation, which leads to a longer pulse.
The properties of the fiber impose some limitations on the Q-switching. The nonlinearity of the fiber laser is more significant due to the small cross-sectional area of the core, so that the peak power should be somewhat limited. You can use either volumetric Q switches, which give higher performance, or fiber modulators, which are connected to the ends of the active part.
Q-switched pulses can be amplified in a fiber or in a cavity resonator. An example of the latter can be found in the National Nuclear Test Simulation Complex (NIF, Livermore, California), where the ytterbium fiber laser is the master oscillator for 192 beams. Small pulses in large alloy glass slabs are amplified to megajoules.
For synchronized fiber lasers, the repetition rate depends on the length of the amplifying material, as in other mode locking schemes, and the pulse duration depends on the gain bandwidth. The shortest are within 50 fs, and the most typical are in the range of 100 fs.
There is an important difference between erbium and ytterbium fibers, as a result of which they operate in different dispersion modes. Erbium-doped fibers emit at 1550 nm in the region of anomalous dispersion. This allows the production of solitons. Ytterbium fibers are in the region of positive or normal dispersion; as a result, they generate pulses with a pronounced linear modulation frequency. As a result, a Bragg grating may be needed to compress the pulse length.
There are several ways to change fiber laser pulses, in particular for ultrafast picosecond studies. Photonic crystal fibers can be made with very small cores to produce strong nonlinear effects, for example, to generate a supercontinuum. In contrast, photonic crystals can also be made with very large single-mode cores to avoid non-linear effects at high powers.
Large-core flexible photonic crystal fibers are designed for high power applications. One technique is to deliberately bend such a fiber to eliminate any unwanted higher-order modes while maintaining only the main transverse mode. Nonlinearity creates harmonics; By subtracting and adding frequencies, you can create shorter and longer waves. Nonlinear effects can also produce pulse compression, which leads to the appearance of frequency combs.
As a source of supercontinuum, very short pulses produce a wide continuous spectrum using phase self-modulation. For example, from the initial 6 ps pulses at 1050 nm generated by the ytterbium fiber laser, a spectrum is obtained in the range from ultraviolet to more than 1600 nm. Another IR source of the supercontinuum is pumped by an erbium source at a wavelength of 1550 nm.
High power
The industry is currently the largest consumer of fiber lasers. Power in the order of kilowatts used in the automotive industry is now in great demand. The automotive industry is moving towards the production of high-strength steel cars to meet the requirements of durability and are relatively lightweight for greater fuel economy. It is very difficult for ordinary machines, for example, to punch holes in this type of steel, and coherent radiation sources make it easy.
Fiber laser metal cutting, in comparison with other types of quantum generators, has several advantages. For example, the near infrared wavelength is well absorbed by metals. The beam can be delivered through the fiber, which allows the robot to easily move focus during cutting and drilling.
Fiber meets the highest power requirements. The U.S. Navy's weapons, tested in 2014, consist of 6-fiber 5.5-kW lasers combined into a single beam and emitting through a forming optical system. A 33 kW installation was used to destroy an unmanned aerial vehicle. Although the beam is not single-mode, the system is of interest because it allows you to create a fiber laser with your own hands from standard, easily accessible components.
The highest power of IPG Photonics single-mode coherent radiation source is 10 kW. The master oscillator produces a kilowatt of optical power, which is fed into the cascade of the pumped amplifier at 1018 nm with light from other fiber lasers. The whole system is the size of two refrigerators.
The use of fiber lasers has also spread to high-power cutting and welding. For example, they replaced the resistance welding of sheet steel, solving the problem of deformation of the material. Control of power and other parameters allows you to very accurately cut curves, especially angles.
The most powerful multimode fiber laser - a metal cutting machine of the same manufacturer - reaches 100 kW. The system is based on a combination of an incoherent beam, so it is not a beam of superhigh quality. This durability makes fiber lasers attractive to the industry.
Concrete drilling
A 4 kW multimode fiber laser can be used for cutting and drilling concrete. Why is this needed? When engineers try to achieve earthquake resistance of existing buildings, you need to be very careful with concrete. When installing, for example, steel reinforcement in it, conventional impact drilling can cause cracks and weaken concrete, but fiber lasers cut it without crushing.
Q-switched quantum generators are used, for example, for marking or in the manufacture of semiconductor electronics. They are also used in range finders: arm-sized modules contain eye-safe fiber lasers with a power of 4 kW, a frequency of 50 kHz, and a pulse duration of 5–15 ns.
Surface treatment
There is great interest in small fiber lasers for micro- and nano-processing. When removing the surface layer, if the pulse duration is shorter than 35 ps, there is no spraying of the material. This eliminates the formation of indentations and other unwanted artifacts. Pulses in the femtosecond mode produce non-linear effects that are not sensitive to wavelength and do not heat the surrounding space, which allows you to work without significant damage or weakening of the surrounding areas. In addition, holes can be cut with a large depth to width ratio - for example, quickly (within a few milliseconds) make small holes in 1 mm stainless steel using 800-fs pulses with a frequency of 1 MHz.
You can also perform surface treatment of transparent materials, for example, human eyes. To cut the flap during eye microsurgery, femtosecond pulses are tightly focused by a high-aperture lens at a point below the surface of the eye, without causing any damage to the surface, but destroying the eye material at a controlled depth. The smooth surface of the cornea, which is important for vision, remains intact. The flap, separated from below, can then be pulled up for surface excimer laser lens formation. Other medical applications include shallow penetration surgery in dermatology, as well as use in some types of optical coherence tomography.
Femtosecond Lasers
Femtosecond quantum generators in science are used for excitation spectroscopy with laser breakdown, time-resolved fluorescence spectroscopy, as well as for the general study of materials. In addition, they are needed for the production of femtosecond frequency combs needed in metrology and general research. One of the real applications in the short term will be atomic clocks for new generation GPS satellites, which will increase positioning accuracy.
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Fiber laser production is growing rapidly, especially for the automotive industry. Non-fiber devices are also being replaced by fiber ones. In addition to general improvements in cost and performance, increasingly practical femtosecond quantum oscillators and supercontinuum sources are emerging. Fiber lasers occupy more and more niches and become a source of improvement for other types of lasers.