Fiber lasers are solid-state lasers in which laser light and pump light are guided in optical (glass) fibers. The laser active medium is the internal cross-sectional area of the glass fiber, which is doped with a rare-earth element, which is often ytterbium.
The energy is supplied by laser diodes, whose light (often 915nm or 977nm) is brought to the doped glass fiber via optical fibers. The optical fibers are interconnected via splicing (welding of glass), where often there are no open beam routes for pump or laser light (see Figure 1). As a result, the fiber laser is relatively insensitive to contamination and vibrations. As the pump diodes are spatially separated from one another and each has its own heat sink, the service life of the pump diodes is high. As long as the peak power of the laser pulses is kept below about 10 – 20kW, a high overall service life of tens of thousands of hours can be achieved. There are continuously emitting fiber lasers (“cw” = continuous wave) as well as pulsed fiber lasers. Only pulsed fiber lasers will be discussed below, as they are much better suited for laser marking and laser engraving applications. The pulse durations are typically around 100 nanoseconds - shorter pulses of a few nanoseconds are achievable, but only at significantly lower pulse energy.
The pulsed fiber lasers in the “MOPA” design consist of a master oscillator (also seed laser) and a fiber-coupled power amplifier. The former is either a diode laser or a "laser on a chip" with an average power of a few milliwatts to a maximum of about 150mW. The laser emits pulses with a defined pulse shape. The “laser on a chip” houses a laser on a single chip - laser-active medium, reflectors and other optical components are often not only integrated but constructed monolithically. The amplifier consists of a ytterbium-doped glass fiber, which is supplied with energy via fiber-coupled pump diodes. If a laser pulse is to be generated, the pump diodes first charge (population inversion) the amplifier fiber. Before it discharges by spontaneous emission, the seed laser emits a pulse that is amplified a few hundredfold to a thousandfold as it passes through the fiber. The amplification takes place in a single pass. The fiber is often in coil form - therefore in a small volume, a large amplifier range and thus high gain can be achieved.
The pulse peak power of fiber lasers for marking and engraving applications is typically 10kW - 20kW. This at a mean output power of 10W – 100W. Due to the high beam quality and the associated good focusability, small structures can be laser engraved or high-resolution laser markings and images can be marked.
The large surface and simultaneously low volume of the glass fibers used allows effective cooling and thus a very compact and maintenance-free structure. The relatively high efficiency (electrical - optical up to more than 20%) ensures low energy costs and low waste heat. Overall service life costs are significantly lower compared to YAG lasers that have been available longer and cover similar applications.
Compared to YAG lasers, fiber lasers have lower pulse peak powers. Fiber lasers are around 10-20kW with higher pulse durations, while 30 – 100kW is the peak for a YAG laser. This can be detrimental when laser marking some plastics and in the high-quality deep laser engraving of metals.
The small cross-section of the glass fibers used limits the peak power of the fiber lasers. If pulses with short duration and high pulse energy are generated, high peak intensities arise, which can lead to the destruction of the fiber (formation of colour centres).
Pulsed fiber lasers have at least partially substituted the previously established YAG lasers during the last 10 years. The compact, robust and relatively easy-to-cool structure of the fiber lasers, in combination with a long service life and low overall service life costs, have made this possible. Significant manufacturing processes in the construction of fiber lasers were adapted from the telecommunications industry such as splicing - the welding of the end faces of two glass fibers, wherein the contact surface has very high purity and low attenuation.