Ytterbium fiber laser: device, operating principle, power, production, application

Fiber lasers are compact and durable, accurately inducted and easily dissipate heat energy. They are of different types and, having much in common with optical quantum generators of other types, have their own unique advantages.

Fiber Lasers: The Principle of Operation

Devices of this type are a variation of a standard solid-state source of coherent radiation with a working body of optical fiber, rather than a rod, plate, or disk. Light is generated by the dopant in the central part 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 dispersed.

Fiber lasers are optically pumped, most often by means of diode quantum generators, but in some cases - by the same sources. The optics used in these systems are generally fiber components, with most or all of them being connected to each other. In some cases, volumetric optics is used, and sometimes the internal fiber-optic system is combined with external volumetric optics.

The source of diode pumping can be a diode, a matrix, or a set of individual diodes, each of which is connected to a connector by a fiber-optic lightguide. The doped fiber at each end has a mirror of the cavity resonator - in practice, Bragg gratings are made in the fiber. There is no volumetric optics at the ends, unless the output ray passes into something other than the fiber. The lightguide can be twisted, so if desired, the laser cavity 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. An unalloyed outer core (sometimes called the inner shell) collects the pumped light and directs it along the fiber. Forced 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 pumping light beam. There are many non-circular forms of the outer core, including hexagonal, D-shaped and rectangular, reducing the probability of a light beam falling into the central core.

A fiber laser can have an end or side pumping. In the first case, light from one or several sources enters the end of the fiber. With lateral pumping, the light is fed to the splitter, which feeds it to the outer core. This differs from the core laser, where light enters perpendicular to the axis.

For such a solution, many design developments are required. Considerable attention is paid to bringing the pump light into the active zone in order to produce a population inversion leading to stimulated emission in the inner core. The core of the laser can have a different degree of amplification, depending on the doping of the fiber, and also on its length. These factors are configured by the design engineer to obtain the necessary parameters.

There may be power limitations, in particular, when operating within a single-mode fiber. Such a core has a very small cross-sectional area, and as a result, light of very high intensity passes through it. In this case, nonlinear scattering of Brillouin becomes more and more perceptible, which limits the output power by several thousand watts. If the output signal is high enough, the end face of the fiber may be damaged.

Features of fiber lasers

The use of fiber as a working medium gives a large interaction length, which works well with diode pumping. This geometry leads to a high efficiency of photon conversion, as well as a reliable and compact design, in which there is no discrete optics requiring adjustment or alignment.

Fiber laser, the device of which allows it to adapt well, can be adapted both for welding thick metal sheets, and for obtaining femtosecond pulses. The fiber-optic amplifiers provide one-pass amplification and are used in telecommunications as they can amplify many wavelengths simultaneously. The same gain is used in power amplifiers with a master oscillator. In some cases, the amplifier can operate with a continuous radiation laser.

Another example is the sources of spontaneous radiation with fiber amplification, in which the stimulated emission is suppressed. Another example is the Raman fiber laser with a gain in combined scattering that significantly shifts the wavelength. It has found application in scientific research, where fluorine glass fibers are used for combination generation and amplification, rather than standard quartz fibers.

Nevertheless, as a rule, the fibers are made of quartz glass with a rare-earth doping impurity 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. Using 940-nm diode pumping significantly reduces the deficit of photons. Ytterbium does not possess any of the self-quenching effects that neodymium has at high densities, so the latter is used in volumetric lasers, and ytterbium is used in fiber lasers (they both provide approximately the same wavelength).

Erbium emits in the range 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 the pump radiation and transfer this energy to erbium. Thulium is yet another alloying additive with a glow in the near infrared, which is thus an eye-safe material.

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 the energy. Thus, ytterbium is inherently more efficient, 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 - at a wavelength of 1480 or 980 nm. The higher frequency is not as effective from the point of view of the photon defect, but it is useful even in this case, because at 980 nm the best sources are available.

In general, the efficiency of a fiber laser is the result of a two-step process. First, it is the efficiency of the pump diode. Semiconductor sources of coherent radiation are very effective, with 50% efficiency of converting an electrical signal into an optical signal. The results of laboratory studies indicate that it is possible to achieve a value of 70% or more. If the output radiation of the fiber laser is accurately matched, a high pump efficiency is achieved.

Secondly, it is the optical-optical conversion efficiency. With a small defect in photons, it is possible to achieve a high excitation and extraction efficiency with an optical-optical conversion efficiency of 60-70%. The resulting efficiency is in the range of 25-35%.

Different configurations

Fiber-optic quantum generators of continuous radiation can be single- or multimode (for transverse modes). Single-mode ones produce a high-quality beam for materials that work or send a beam through the atmosphere, and multimode 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 pulses of millisecond type. Usually its working cycle is 10%. This results in a higher peak power than continuous mode (usually ten times larger), which is used, for example, for pulsed drilling. The frequency can reach 500 Hz, depending on the duration.

The Q-switching in fiber lasers acts as well as in bulk lasers. A typical pulse duration is in the range of nanoseconds to a microsecond. The longer the fiber, the longer it takes for Q-switching output radiation, which leads to a longer pulse.

The fiber properties impose some restrictions 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 that give better performance, or fiber modulators that connect to the ends of the active part.

Pulses with Q-switching 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 impulses in large plates of alloy glass are amplified to megajoules.

In fiber lasers with synchronization, the repetition frequency depends on the length of the amplifying material, as in other modes of mode locking, and the pulse width depends on the gain bandwidth. The shortest are within 50 fs, and the most typical are in the range of 100 fs.

Between the erbium and ytterbium fibers there is an important difference, as a result of which they operate in different modes of dispersion. Erbium-doped fibers are emitted at 1550 nm in the region of anomalous dispersion. This makes it possible to produce solitons. The 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 the fiber-laser pulses, in particular, for ultrafast picosecond studies. Photonic crystal fibers can be manufactured with very small nuclei to produce strong nonlinear effects, for example, for the generation of a supercontinuum. In contrast, photonic crystals can also be manufactured with very large single-mode cores to avoid non-linear effects at high powers.

Flexible photonic crystal fibers with a large core are created for applications requiring high power. One of the methods consists in deliberately bending such a fiber to eliminate any undesirable higher-order modes while maintaining only the basic transverse mode. Nonlinearity creates harmonics; By subtracting and folding frequencies, shorter and longer waves can be created. 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 by means of phase self-modulation. For example, from the initial 6 ps pulses at 1050 nm, which creates an ytterbium fiber laser, a spectrum is obtained in the range from ultraviolet to over 1600 nm. Another IR source of the supercontinuum is pumped by an erbium source at a wavelength of 1550 nm.

High power

Industry is currently the largest consumer of fiber lasers. Great demand now enjoys the power of the order of kilowatt, used in the automotive industry. The automotive industry is moving towards the production of high-strength steel cars to meet the longevity requirements and have been relatively easy to save fuel. It is very difficult for ordinary machine tools, for example, to punch holes in this kind of steel, and sources of coherent radiation make it easy.

Metal cutting by a fiber laser, in comparison with quantum generators of other types, has a number of advantages. For example, the near infrared range of waves is well absorbed by metals. The beam can be delivered over the fiber, which allows the robot to easily move focus when cutting and drilling.

Optical fiber meets the highest power requirements. The US Navy's weapon, tested in 2014, consists 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 defeat an unmanned aerial vehicle. Although the beam is not single-mode, the system is of interest, since it allows us to create a fiber laser with our own hands from standard, easily accessible components.

The highest power of the single-mode coherent source of IPG Photonics is 10 kW. The master oscillator produces a kilowatt of optical power, which is fed to the cascade of an amplifier pumped at 1018 nm with light from other fiber lasers. The whole system has the size of two refrigerators.

The use of fiber lasers has also spread to high-power cutting and welding. For example, they replaced the contact welding of sheet steel, solving the problem of deformation of the material. The power control and other parameters make it possible to cut the curves very accurately, especially the angles.

The most powerful multimode fiber laser - a device for cutting metals of the same manufacturer - reaches 100 kW. The system is based on a combination of an incoherent beam, so this is not an ultra-high-quality beam. Such durability makes fiber lasers attractive to the industry.

Drilling of concrete

Multimode fiber laser power of 4 kW can be used for cutting and drilling concrete. Why is this necessary? When engineers try to achieve seismic resistance of existing buildings, one must be very careful with concrete. When installing in it, for example, steel reinforcement, conventional impact drilling can cause cracks and weaken the concrete, but fiber lasers cut it without crushing.

Quantum generators with a Q-switched fiber are used, for example, for marking or for the production of semiconductor electronics. They are also used in range finders: hand-sized modules contain eye-safe fiber lasers, whose power is 4 kW, frequency 50 kHz and pulse width 5-15 ns.

Surface treatment

There is great interest in small fiber lasers for micro- and nanoprocessing. When removing the surface layer, if the pulse width is shorter than 35 ps, there is no splashing of the material. This eliminates the formation of depressions and other unwanted artifacts. Pulses in the femtosecond mode produce nonlinear effects that are not sensitive to the wavelength and do not heat the surrounding space, which makes it possible to work without significant damage or weakening of the surrounding areas. In addition, the holes can be cut with a large depth-to-width ratio - for example, quickly (within a few milliseconds) to make small holes in 1-mm stainless steel using 800-fs pulses at a frequency of 1 MHz.

It is also possible to surface treatment of transparent materials, for example, human eyes. To cut the flap with 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 unharmed. 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 the 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, and for general materials research. In addition, they are needed for the production of femtosecond frequency combs, required in metrology and general studies. One of the real applications in the short term will be an atomic clock for new generation GPS satellites, which will increase the accuracy of positioning.

A single-frequency fiber laser is produced with a spectral line width of less than 1 kHz. This is an impressively small device with a radiation output of 10 mW to 1 W. It finds application in the field of communication, metrology (for example, in fiber gyroscopes) and spectroscopy.

What's next?

As for other research applications, many more are being studied. For example, military development, which can be used in other areas, consisting in combining fiber-laser beams to produce a single high-quality beam by means of a coherent or spectral combination. As a result, a high power is achieved in a single-mode beam.

The production of fiber lasers is growing rapidly, especially for the needs of the automotive industry. There is also a replacement of non-fiber devices with fiber. In addition to general improvements in cost and performance, there are more and more practical femtosecond quantum generators and supercontinuum sources. Fiber lasers occupy more and more niches and become a source of improvement for other types of lasers.