In LMA fiber, the mode area is controlled by the difference in refractive indices between the core and clad, Δn. In a typical SI-type LMA fiber, however, because of the limitations of modified chemical vapor deposition (MVCD) and direct nanoparticle deposition (DND) techniques, it is difficult to control the core-clad difference in refractive indices far below 10-3, therefore is difficult to manufacture low NA (< 0.06) optical fiber.
To overcome the difficulties, a perfect SM LMA fiber (mode cross-section~2000 µm2) was developed by controlling the interval of periodic holes and diameter in photonic crystal fiber (PCF), which perfectly controls low NA value, as a new-type technique. However, if the NA is too low, the propagation property is sensitive not only to macroscopic bending, but also to microscopic bending loss. Therefore, the length of LMA is limited.
Rod-type PCF is developed to avoid increases in the bending loss accompanied with low NA, which is realized in PCF. The rod-type PCF can not be bent. The core diameter of PCF is 60~100 µm. In order to keep the fiber being straight, the rod-type PCF is supported by the quartz tube (outer clad) with a diameter of ca. 2 mm. By september 2010, the rod-type PCF with mode area of 4500 µm2 (Yb-doped, polarization maintaining, core diameter of 100 µm, pump-clad of 285 µm, outer-clad of 1.7 mm) is commercially available as a rod-type PCF with the maximal mode area. This rod-type PCF has a nearly 100 times larger mode area than a typical SMF.
So far, a rod-type Yb-doped LMAPCF achieved a femtosecond chirped pulse amplification (CPA) with an average power of 131 W, pulse duration of 220 fs, and peak power of 8.2 MW(@73 MHz).
Chirally-coupled core (CCC) fiber has a coiling structure of passive site core around the rare-earth-doped centering core (core diameter of 35 µm, NA = 0.07) as shown in Fig. In a CCC fiber, higher-order modes are strongly coupled with the side core, while the most of fundamental mode travels in the core center. The higher-order modes are largely affected by the bending loss (the loss is 90~130 dB/m) in the side core, and finally LP01 mode remains in the core center (the loss is 0.2~0.3 dB/m). Master oscillator power amplifier (MOPA) system using Yb-doped CCC fiber with the core diameter of 35 µm realizes 100 kW peak power at 100 kHz repetition rate with 100W average power and 10ns pulse duration. With using CCC fiber, a fiber laser with pulse energy of 1.0 mJ at 10~100 kHz, pulse duration of 10 ns, average power of 100 W, and beam quality of M2<1.1, and SM propagation in the core diameter of 50~100 µm are expected.
Fig. Structure of CCC fiber