What is a Fibre Laser?
A fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.
Fibre Laser History
The high power fibre laser system has been developed recently which can be used in industrial applications. In the following, firstly the development history of fibre laser is introduced. And then the component and character of high power fibre laser system are presented. In the end, the benefit and application area is stated.
There are two technical developments that give the prerequisites for the application of fibre in material processing: the optical communication industry ensures the preparation technology for highly transmissive single-mode fibre, and the optoelectronics industry made available the high-power laser diodes necessary for the pumping of the fibre. Therefore, it makes the possibility of increasing the power in fibre lasers.
According to the scientific and technical information published in the area of high power laser, the notion of high power means continuous wave and average output powers of about 100 W and more.
How Does a Fibre Laser Work?
In principle, the fibre laser is similar to any other diode pump solid-state laser technology. The laser-active media usually consists of a glass fibre pumped by a diode.
Setup of a simple fibre laser. Pump light is launched from the left side through a dichroic mirror into the core of the doped fibre. The generated laser light is extracted on the right side
High power fibre lasers are nearly always realized with double-clad fibres, which are end or side pumped with fibre-coupled high power diode bars or other kinds of laser diodes. The pump light is launched into an inner cladding (diameter of some 100 μm, NA≈0.4) rather than into the much smaller fibre core, in which the laser light is generated. The laser light can have very good beam quality – even diffraction-limited beam quality if the fibre has a single-mode core
In most cases, the active medium is doped with rare-earth ions such as ytterbium, neodymium, praseodymium, erbium, or praseodymium, and their gain spectral ranges
The majority of rare-earth-doped high power lasers consist of Yb or Er dopants with output around 1070 nm and 1550 nm respectively. Higher efficiencies are achieved at 1070 nm compared with 1550 nm output. And one or several laser diodes are used for pumping
With the core material doped rare-earth elements and a pump-radiation coaxially superimposed, the so-called “core-pumped” fibre lasers were realized in the low-power regime (limit to 1W). This longitudinal pumping concept requires pump sources with M2 values suitable to couple to the single-mode fibre. Such single-mode diodes were, and still are available only for low power.
Optical fibres are a special kind of waveguides, which are usually made of silica glass. In the high power fibre laser system, optical fibre is used as an active medium to scale power and peak intensity. Therefore, maximizing fibre mode size and preserving a single transverse mode beam are required for the optical fibre according to the purpose.
There are many different types of fibre used in high power fibre laser systems with respect to different properties:
- With respect to optical guidance:
- Conventional index-guiding
- Photonic Crystal effective index guiding
- Photonic Crystal bandgap guiding
- With respect to modal properties:
- Single transverse mode (SM) core fibres
- Large mode area (LMA) fibres
- Effectively single-mode (ESM)
- With respect to polarization properties:
- Polarization non-preserving fibres
- Polarization preserving (high birefringence) fibres
- Single polarization fibres