With single-mode fiber lasers reaching 10KW and multimode fiber lasers reaching 50KW, fiber lasers are breaking out of the industrial field and entering military applications, becoming a candidate for high-energy laser weapons deployed on the battlefield.
In the early days of laser technology, the best way to obtain high-power laser output was to extract energy from large volumes of laser material. There are still some applications that use this approach, such as the National Ignition Facility (NIF) at Lake Trent National Laboratory, which uses large glass amplifiers to amplify pulses to 1.8 M. But for many industrial applications, ytterbium-doped fiber has become the ideal choice for high-power laser media.
Fiber lasers have come a long way in terms of power since Elilas Snitzer invented the first fiber laser in 1963. In June 2009, IPG Photonics released a continuous-wave single-mode fiber laser with an output power of 10KW at the Munich Laser Show and the Solid-State Laser and Semiconductor Laser Conference hosted by the Directed Energy Professionals Society (DEPS). Bi Shiner, vice president of industrial markets at IPG Photonics, said that IPG has produced multimode fiber lasers with output powers up to 50kW, and Raytheon has tested their potential applications as laser weapons. However, IPG's main business is still for industrial material processing applications, from cutting silicon wafers for solar cells to robotic welding of metal plates.
Why choose fiber?
Similar to other diode-pumped lasers, fiber lasers essentially convert low-quality pump lasers into higher-quality laser outputs, which can be used in many fields such as medical treatment, material processing and laser weapons. In terms of achieving high-power output, fiber lasers have two important advantages: one is the process from pump light to high-quality output light, which has high conversion efficiency; the other is good heat dissipation capacity.
The reason why fiber lasers can achieve high efficiency is mainly due to diode pumping, careful selection of gain doping media and optimized fiber design. The optical fiber used in high-power fiber lasers contains an inner core doped with gain medium and an outer core that confines the pump light. The pump light can enter the outer core through the end face of the fiber, or be coupled into the outer core along the side of the fiber in a direction nearly parallel to the fiber axis (see Figure 1). The latter method is called "side pumping", but it does not mean that the pump light enters the laser cavity laterally like a bulk laser. Once the pump light is introduced into the outer core, it will repeatedly pass through the inner core along the fiber to achieve efficient pumping. Subsequently, the stimulated radiation is conducted along the inner core and continuously accumulates energy to output high-intensity laser light.
Most fiber lasers have dopants, which is because the selective mirror can obtain a small quantum loss (the energy difference between the pump photon and the output photon). When using 975nm pump light to produce 1035nm output light, the quantum loss value is only 6%. By comparison, the quantum loss of a neodymium-doped laser pumped at 808 nm and outputting at 1064 nm is as high as 20%. Smaller quantum losses allow the optical-optical pumping efficiency of fiber-doped lasers to exceed 60%, which, combined with the 50% electro-optical conversion efficiency of the pump diode, means that the total conversion efficiency of the fiber laser can reach 30%.
The fiber structure has a large surface area per unit volume, which helps the fiber laser dissipate heat, but even with water cooling, heat dissipation will limit its performance. Five years ago, researchers hoped to output higher powers by increasing the doping level and the size of the inner core, but Johan Nilsson of the University of Southampton said that at high average powers, because residual heat is difficult to remove from the fiber, "the thermal effect limitation is back."
