Fiber Laser

Fiber Laser

A fiber laser is a semiconductor-based laser that uses an optical fiber as the active gain medium. This type of laser is also referred to as a fiber amplifier, semiconductor optical amplifier (SOA), or a laser-doped fiber amplifier (LDFA). The gain medium in such devices is typically erbium, ytterbium, neodymium, dysprosium, holmium or thulium doped fiber. It can be a CW laser or a pulsed one with nanosecond, picosecond, or even femtosecond pulse durations. The peak power of such devices can be several kilowatts for short pulses. The wavelengths used are in the near-IR, which makes them well absorbed by metals. They can be used for a variety of materials processing applications such as welding, cutting, drilling and marking.

Optical fibers can be used as a medium for light amplification and provide very high output powers, which can be very useful in laser systems for material processing. This is because of their relatively low loss at the operating wavelength and large dispersive bandwidth. They can be mode-locked and can produce multiwavelength outputs by introducing doping elements in the core, such as erbium, ytterbium, or neodymium for generating single-mode emission at different wavelengths, or thulium to generate a continuous-wave laser with a long repetition rate.

The optical-fiber structure makes it possible to achieve very high pulse energies in such devices, whereas conventional solid-state lasers need bulk optic components for that purpose. Consequently, they can have much lower weight and dimensions, and can be more flexible. This is important in industrial applications, for example, when it comes to repositioning the laser beam.

Besides the advantages of low weight and dimensions, the optical fiber structure makes it easy to combine the various components of a fiber laser, including amplifiers and control circuitry. This simplifies the construction of high-power fiber lasers, such as a 33 kW system developed last year for the U.S. Navy to shoot down an unmanned aerial vehicle (UAV).

The lasers are pumped by laser diodes, which convert electricity into photons and then into laser light. This is because the positive electrode of a diode has more spare electrons than the negative one, which creates an electric field that causes free electrons to jump between them. This in turn generates light, and the photons can be emitted from a laser diode.

This is how a laser diode works: It can convert electrical energy into light with an average power of up to 500 W and a pulse energy of up to 50 mJ. Pulsed fiber lasers can have higher average powers and pulse energies, depending on their application.

The nonlinear effects brought on by Brillouin and Raman scattering can limit the maximum achievable power of such fiber lasers, but they can be overcome by using certain types of fiber designs. These include double-clad fibers, and polarization-maintaining fibers or Faraday rotators to ensure the polarization of the pump light is correctly aligned with that of the laser output. This allows higher pump intensities to be sustained over longer fiber lengths, which is very useful for upconversion lasers and other fiber lasers that operate on "difficult" laser transitions that require very high pump intensities.

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