The fiber laser is a solid-state laser in which laser light and pump light are guided in optical fibers (“glass fibers”). The laser active medium is the internal cross-sectional area of the glass fiber, which is doped with a rare-earth element (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), i.e. often there are no open beam routes for pump or laser light (see Figure 1). As a result, the fiber laser is relatively unaffected by contamination or vibration. 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 very high. Keeping the peak power of the laser pulses below about 10 – 20kW, results in a high overall service life of several tens of thousands of hours. 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 far better suited for marking and 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 just a few milliwatts right up to a maximum of around 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 (“single-pass amplifier”). The fiber is often in coil form - therefore in a small volume, a large amplifier range and thus high gain can be realised.
The pulse peak power of fiber lasers for marking and engraving applications is typically 10kW - 20kW but with a mean output power of 10W – 100W. Due to the high beam quality and the more accurate focal capabilities, small structures can be engraved or high-resolution markings and images can be marked.
The large surface and low volume - of the glass fibers used, allows for 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 heat waste. The overall service life costs are significantly lower compared to YAG lasers that have already been available for some time and have been utilised in similar applications.
Compared to YAG lasers, fiber lasers have lower pulse peak powers (10-20kW fiber lasers, 30 – 100kW for YAG) and higher pulse durations. This can be detrimental when marking some plastic types and also when attempting the high-quality deep 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, this creates high peak intensities which can ultimately lead to serious damage of the fiber (formation of colour centres).
Pulsed fiber lasers have at least partially replaced the previously well established YAG lasers of the last 10 years. The compact, robust and relatively easy-to-cool structure of the fiber lasers, combined with the long service life and low long term service life costs, have made this possible. Significant manufacturing processes in the construction of fiber lasers were acquired and adapted from the telecommunications industry - e.g. splicing, i.e. welding the end faces of two glass fibers, wherein the contact surface has very high purity and low attenuation.