Actively doped double clad fibers is a group of very efficient brightness converting fibers. Such fibers are pumped by multimode diodes and provides lasing/amplification in a single mode. Traditionally, double clad fibers have been made as step-index fibers of different materials - most typically with a polymer outer cladding. With the PCF technology, the fibers can be made of all silica. Double clad fibers made with photonic crystal fiber technology are known as air-clad fibers and provide a range of new benefits: The extremely high numerical aperature (NA) for the pump core/inner cladding is possible, allowing efficient pumping with inexpensive / high power broad area emitting pumps. The large mode area for the single mode signal facilitates high power levels while avoiding nonlinearities and providing a good overlap between the pump guide and the signal guide area (= high pump absorption, high efficiency). The fibers have efficient mode and ray scrambling even for circular pump cores and the all glass design has a much higher damage threshold than standard double-clad fibers. Advantages of the fibers, are: Single mode with large mode area High peak power levels High pulse energy while single mode High pump NA (0.6-0.7) High pump absorption (up to 30 dB/m) Short fiber lengths All-glass design - no polymers - for excellent power handling Good thermal properties High reliability  No skew-rays or whispering gallery modes.