Fiber laser - functionality and areas of application

Fiber lasers are solid-state systems. This means that the pump light and laser light are guided in optical, or glass, fibers. The active medium of the laser is the interior cross-sectional area of the glass fiber. The fiber is doped with a rare-earth element (frequently ytterbium).

Energy is provided by laser diodes, whose light (ordinarily 915nm or 977nm) is passed through optical fibers to reach the doped glass fiber. There are typically no clear beam routes for laser or pump light, resulting in the fiber laser being moderately indifferent to pollution and fluctuations. The overall lifespan of the pump diodes is high, primarily due to them having their own heat sink and being evenly distributed. The lifespan of the laser can last several tens of thousands of hours if the max power of the laser pulses is maintained below 10 – 20kW. There are two types of fiber lasers: continuously emitting, or continuous wave, and pulsed. Pulsed fiber lasers are typically better for engraving and marking applications, so they will be the only type discussed below. Pulse durations of about 100 nanoseconds are normal, though it is possible to achieve shorter pulses of a few nanoseconds at lower pulse energy.

The design of the MOPA fiber laser systems includes a master oscillator, otherwise known as a seed laser, as well as a fiber-coupled power amplifier. The power amplifier is either a diode laser or a “laser on a chip.” This has an average power ranging from just a few milliwatts to about 150mW. The laser has a specific pulse shape while the “laser on a chip” contains a laser on a single chip where the laser-active medium, reflectors, and additional optical components are usually combined, or sometimes even constructed from a single piece of material. The amplifier contains a ytterbium-doped glass fiber, which is provided energy through fiber-coupled pump diodes. To generate a laser pulse, the pump diodes charge the amplifier fiber through population inversion. Before it releases by random emission, the seed laser releases a pulse that is increased exponentially as it moves through the fiber. The amplification is completed in just one pass. The fiber is usually coiled allowing for higher gain in a small volume with a high amplifier range.

Areas of application

10kW - 20kW is usually the maximum pulse power of fiber lasers with a mean output power of 10W – 100W. Great focusability and high beam quality allow for finely detailed markings on small objects.

Advantages of the fiber laser

The glass fibers have a large surface area allowing for efficient cooling. This, and low volume, creates a small, low maintenance structure. Higher efficiency means lower cost of energy and heat wasted. The lifetime costs of a fiber laser are remarkably lower compared to YAG lasers even though YAG lasers have been around for a longer time and can do comparable applications.

Disadvantages of fiber lasers

Fiber lasers and YAG laser can differ 20-80kW in power, with fiber having a lower pulse peak and higher pulse duration. This can be harmful when it comes to plastic marking and getting fine detail in deep metal engravings.

The peak power is limited by the small area of the glass fibers in fiber laser systems. Highly concentrated pulse energy and short pulse durations can cause damage to the fiber.

Conclusion

Over the past ten years, pulsed fiber laser systems have all but replaced YAG lasers. This is largely due to their hardy and compact structure, as well as a longer lifetime and lower overall cost. Many construction methods in the development of fiber lasers were taken from the telecommunications industry, including the joining of the top and bottom surfaces of two glass fibers, where the connection surface is clean and has low attenuation.

 

 

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