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 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, this 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 much 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 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 (“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 realized.