Fiber laser

A fiber laser is a special form of solid-state laser . The doped core of a glass fiber forms the active medium in a fiber laser . It is therefore a glass laser with fiber optic properties. The laser radiation that is guided through the laser-active fiber experiences a very high gain due to its great length.

Double clad fiber construction

Fiber lasers are generally optically pumped by coupling radiation from diode lasers parallel to the fiber core in its cladding or in the latter itself . Jacketed fibers (Engl. Double clad fibers ) allow for higher performance; The pump radiation is distributed from the thick cladding and reaches the active fiber core.

The most common doping element for the laser-active fiber core is erbium (medicine, communications technology), followed by ytterbium and neodymium for high-performance applications. For cost reasons, usually only the middle part of the glass fiber contains doping.

Fiber lasers have unique properties such as: B. electrical-optical efficiencies up to over 30%, outstanding beam quality (with M² <1.1 for single-mode fiber laser construction, M² <1.2 for double-clad fibers), long service life (> 20,000 h) and a compact, maintenance-free and insensitive construction. The pulse operation extends into the fs range and can reach a high peak intensity.

construction

Fiber laser assembly (version # 1)

Fiber laser assembly (version # 2)

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

The fiber typically consists of several layers. The main part is mostly made of quartz glass, e.g. B. 0.25 mm thick, surrounded with a thin protective layer of plastic. The active core is much thinner, e.g. B. 10 microns, and consists of doped quartz glass, z. B. with a few percent aluminum and a few parts per thousand of rare earths. The refractive index of the layers decreases from the inside to the outside; this is how the light guidance property arises.

The resonator can be constructed in different ways: either it consists of two additional mirrors, which can be, for example, the two mirrored fiber end faces, or of fiber Bragg gratings (FBG) that are inserted into the by means of ultraviolet radiation (e.g. an excimer laser 248 nm) Waveguides (an attached passive glass fiber) can be inscribed. This results in lateral differences in refractive indices with high and low refractive index ranges in the fiber core, which reflect radiation of a certain wavelength depending on the period length. The advantage here is that there are no additional coupling losses at these gratings and the FBGs only selectively reflect the desired wavelengths. This enables a narrow-band laser operation .

After exiting the active fiber, the laser beam mostly arrives in a glass fiber or in a light guide cable containing such , which for example directs the radiation to a focusing optics of a laser material processing machine.

A fiber laser device also contains the power supply and cooling for the pump laser diodes.

Strong fiber lasers have a small fiber laser or a laser diode of various designs as seed laser to generate the input power for a downstream fiber amplifier (optically pumped active fiber). The separation of the laser into seed laser and amplification has the advantage that the laser activity can be better controlled. This concerns the wavelength stability, the beam quality and the power stability or pulsability. There is often an optical isolator between the seed laser and the amplifier fiber .

history

As early as 1961, Elias Snitzer dealt with beam propagation in glass fibers and recognized the advantages of glass lasers realized with it. In the course of his research, he first described a jacket-pumped fiber amplifier in 1988 and is therefore considered the founder of this technology.

In the course of development, the optical performance was continuously increased - around 1990 the first commercial devices in the watt range were available. These were based on erbium-doped fiber amplifiers, which were connected downstream of a small laser oscillator.

Areas of application

Thanks to their robust design, high beam quality and efficiency, fiber lasers are suitable for many applications.

Low-power fiber lasers are used for data transmission in glass fibers - similar arrangements (fiber amplifiers) are used for signal regeneration.
Fiber lasers in the power range of a few watts can be used, among other things, for medical purposes or for labeling components by changing their color.
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 lasers ( femtosecond lasers ).

Advantages and disadvantages

The main advantages of the fiber laser are the high beam quality of the laser radiation generated, the high efficiency of the conversion process (depending on the doping, optically-optically more than 85% can be achieved), the good cooling due to the large surface of the fiber, the compact and maintenance-free structure and the effective manufacturing technology through the use of fiber-integrated components.

In general, fiber lasers must be pumped through the end faces or through spliced fiber-coupled radiation sources. This requires diode lasers of high beam quality. These are expensive and are subject to aging. By using individual diodes, reliability and pump beam quality can be increased considerably compared to the use of diode bars. Such lasers are commercially available with high beam qualities up to the high multi-kW range.

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

With an appropriate optical design, however, fiber lasers can also be linearly polarized and manufactured as single frequency lasers .

The peak performance is limited by the small cross-section of the fiber. The generation of pulses of short duration results in high peak performance. The associated high intensities can lead to the destruction of the fiber. In particular, the fiber end faces set limits to the power that can be coupled out. With photonic structures (air inclusions), the active core and also the pump jacket can be optimized for higher performance, in that the core diameter can be larger with the same beam quality and the acceptance angle of the pump radiation increases.

10 kW singlemode fiber laser

The improvement of the fiber-coupled pump laser diodes, photonic structures in the laser and pump areas of the active fiber as well as the coupling of several individual fiber lasers have made it possible to advance into the kilowatt range with continuously operating fiber lasers. This also made fiber lasers interesting for material processing, especially as they have a significantly higher beam quality than conventional diode-pumped solid-state lasers. An 18 kW fiber laser was presented at the LASER 2005 trade fair. Due to the modular structure and the associated scalability of the power, it was already possible in 2007 to build a 36 kW fiber laser.

The current maximum output power for fiber lasers is 100 kW (multimode) and 10 kW (single mode). 50 kW laser output power with high beam quality is used, for example, in shipbuilding (welding of thick metal plates) and for military purposes.

The beam quality of the emitted radiation ( beam parameter product <2.5 mm × mrad at 4… 5 kW and 11.7 mm × mrad at 17 kW laser power) is up to four times better than that of a comparable Nd: YAG laser (15–25 mm × mrad at 4 kW), its output opens up numerous fields of application in material processing, such as B. high quality cutting, soldering and welding of metals. With a corresponding beam expansion through defocusing, hardening of large metal surfaces is also possible. Due to the high beam quality, comparatively large working distances (e.g. metal welding at a distance of about 1 meter) are possible, which opens up completely new possibilities in automated production (processing hard-to-reach areas, beam deflection with mirror scanners).

Individual evidence

↑ Jenoptik.com, optoelectronics group . Jenoptik AG website. Retrieved October 19, 2012.

^ E. Snitzer: Cylindrical Dielectric Waveguide Modes . In: Journal of the Optical Society of America . tape 51 , no. 5 , April 1, 1961, pp. 491-498 , doi : 10.1364 / JOSA.51.000491 .

↑ E. Snitzer, H. Po, F. Hakimi, R. Tumminelli, B. C: Erbium fiber laser amplifier at 1.55 μm with pump at 1.49 μm and Yb sensitized Er oscillator . In: Optical Fiber Communication . Optical Society of America, 1988, pp. PD2 ( [1] ).

↑ F. Schneider: High-speed cutting of automotive steels with fiber lasers . In: Annual Report . 2008, p. 67 .

↑ Hartmut Bartelt et al.: Guide light cleverly with structured optical fibers. In: Photonics. No. 3, 2007, p. 82ff (full text, registration required)