Fiber laser

A fiber laser is a special form of the solid state laser. The doped core of an optical fiber forms the active medium in a fiber laser. It is a glass laser with fiber optic properties. The laser radiation, which is passed through the laser-active fiber because of the large length experiences a very high gain.

Fiber lasers are optically pumped generally by being coupled parallel to the fiber core into the cladding, or in that even radiation from diode lasers. Double -clad fibers (german double clad fibers ) allow higher benefits; from the thick coat enters the pump radiation distribution in the active fiber core.

The most common impurity element for the laser-active fiber core is erbium (medicine, communications ), followed by Ytterbium and neodymium for high performance applications. For cost reasons, generally only the central portion of the optical fiber contains a doping.

Fiber lasers have unique properties, such as electro- optical efficiencies of up to about 30 %, excellent beam quality ( M ² <1.1 in the single-mode fiber laser structure, M ² <1.2 for double -clad fibers ), long lifetime ( > 20,000 h) and a compact, maintenance-free and resistant construction. The pulse operation extends to the fs range and can achieve a high peak intensity.

Construction

A fiber laser consists of one or more pump laser diodes, a coupling optics ( discrete or spliced ​​to the jacket fiber- coupled diode laser ) and a resonator.

The fiber is typically composed of multiple layers. The main body is usually made ​​of quartz glass, for example, 0.25 mm thick, is surrounded by a thin protective layer of plastic. The active core is much thinner, for example 10 microns, and consists of doped quartz glass, for example a few per cent of aluminum and a few thousandths of rare earths. The refractive index of the layers increases from the inside to the outside; this creates the light guiding property.

The resonator may be constructed in different ways: either it consists of two additional mirrors, which may be, for example, the two mirrored fiber ends, or fiber Bragg gratings ( FBG), by means of ultraviolet radiation (such as an excimer laser of 248 nm ) in the waveguide ( an attached dark fiber ) will be enrolled. In the fiber core created by the lateral refractive index difference of high and low refractive index regions, which is reflected depends on the period length of the radiation of a certain wavelength. The advantage of this is that in these lattices no additional coupling losses and the FBG selectively reflect only the desired wavelengths. Thus, a narrow-band laser operation is possible.

After emerging from the active fiber of the laser beam usually passes into a glass fiber or in a fiber optic cable containing such that radiation propagating as a focusing optical system of a laser material processing machine.

A fiber laser device further includes the power and cooling for the pump laser diodes.

Strong fiber lasers have a small fiber laser or a laser diode as a seed laser for generating different versions of the input power for a subsequent amplifier fiber ( active fiber optically pumped ). The separation of the laser and seed laser amplification has the advantage that the laser action can be controlled better. This affects the wavelength stability, beam quality and power stability or pulsability. Frequently between seed laser and amplifier fiber is an optical isolator.

History

The concept of the fiber laser has been around for more than forty years. Already in 1961, Elias Snitzer dealt with the beam propagation in optical fibers and recognized the benefits that this unrealized glass laser. In the course of his research he described in 1988 for the first time a cladding pumped fiber amplifier, and thus is considered the founder of this technology.

During the development of the optical powers were continually increase - by 1990, the first commercial devices in the watt range were available. These were based on a small laser oscillator downstream, erbium -doped fiber amplifiers.

Areas of application

Due to its robust construction, high beam quality and the efficiency of fiber lasers are suitable for many applications.

• Fiber laser with small power are used for data transmission in optical fibers - for signal regeneration similar arrangements ( fiber amplifier ) are used.
• Fiber lasers in the power range from a few watts can be used by a color change, among others, for medical purposes or for labeling of components.
• High power systems are used for example for welding and cutting.
• The non-linearity of the material at high field strengths is suitable for passively mode-locked laser ( femtosecond laser).

Pros and Cons

Significant advantages of the fiber laser, the beam quality of the laser radiation produced a high efficiency of the conversion process (depending on the doping can optically optically over 85 % is reached), the good cooling by the large surface area of the fiber, of the compact and maintenance-free construction and effective manufacturing technology by using fiber- integrated components.

In general, fiber lasers must be pumped through the end faces, or by spliced ​​fiber-coupled radiation sources. These diode lasers with high beam quality are required. These are expensive and subject to aging. Through the use of individual diodes reliability and pump beam quality over the use of diode bars can greatly increase. Such lasers are commercially available at high beam qualities up to the high multi- kW range.

Due to the large gain of the fiber frequency-selective elements do not work very well. Due to the high output coupling of the resonator does not have high quality. On the other hand you have a high proportion of amplified spontaneous emission (English Amplified Spontaneous Emission, ASE).

By appropriate optical design but fiber lasers can also be linearly polarized and manufactured as a single-frequency laser.

Due to the small cross section of the fiber, the peak power is limited. In the generation of short duration pulses to high peak powers arise. The high intensities associated can lead to the destruction of the fiber. In particular, the fiber end faces set of coupled out performance limits. By photonic structures ( air pockets ) can be of the active core and the pump cladding to higher performance optimization by the core diameter with the same beam quality can be greater and the acceptance angle of the pump radiation is increased.

The improvement of the fiber-coupled pump laser diodes, photonic structures in the laser and pump regions of the active fiber and the coupling of several single - fiber lasers have made it possible to advance with continuous fiber lasers in the kilowatt range. This fiber lasers have been interesting for material processing, especially since they have a much higher beam quality than conventional diode-pumped solid-state lasers. The trade fair LASER 2005 a 18- kW fiber laser was introduced. The modular structure and the associated scalability of the performance in 2007, it was already possible to build a 36 - kW fiber laser.

Current maximum output power in fiber lasers are 50 kW ( multimode) and 10 kW ( single-mode ). 50 kW laser output power with high beam quality, for example, used in shipbuilding ( welding of thick metal plates) and for military purposes.

The beam quality of the radiation emitted ( beam parameter product of <2.5 mm x 4 mrads at ... 5 kW and 11.7 mm × mrad at 17 kW laser power ) up to four times better than that of a comparable Nd: YAG laser ( 15-25 mm × mrad at 4 kW), his performance brings to light so many fields of application in material processing, such as high-quality cutting, soldering and welding of metals. With appropriate beam expansion by defocusing also curing of large metal surfaces is possible. Due to the high beam quality are comparatively large working distances (eg metal welding in about 1 meter distance ) is possible, what new possibilities in automated production opened ( edit hard to reach places, beam deflection with mirror scanners ).