We propose a kind of photonic crystal fiber with flat-top fundamental mode by introducing a depressed central dip into
the core of the conventional index-guiding photonic crystal fiber. The design guidelines and characteristics of the large
flattened mode photonic crystal fiber (LFM-PCF) are discussed in detail. By appropriate design, the effective area of the
LFM-PCF can be increased by a factor of greater than 2 as compared with conventional index-guiding photonic crystal
fiber with the same hole and pitch parameters. The improved effective area, single mode operation and flat-top
fundamental mode output make LFM-PCF an ideal candidate to realize high power, high beam quality fiber amplifiers
A theoretical method to analyze four-layer large flattened mode (LFM) fibers is presented. The influence of the second cladding on the properties of four-layer LFM fiber, including the fundamental and higher-order modal fields, effective area, bending loss, and dispersion, are studied by comparison. At the same time, the reasons for the different characteristics are considered. The obtained results indicate that the effective area of the four-layer LFM fiber is about 1.6 times larger than that of the conventional standard step-index fiber and the fibers have better bend-induced filtering ability than three-layer LFM fibers.