In order to enhance the transmission capacity of a single fiber, it has been proposed a dense wavelength division multiplexing (DWDM), an alternative technique – the space division multiplexing (SDM) technique, which uses a multi-core fiber or few-mode fiber (FMF).
FMF is a core area is large enough to take advantage of parallel data streams of serveral independent space mode transmission optical fiber. The ideal case, is proportional to the capacity and the number of patterns in the FMF. However, in order to extend the transmission distance, it is necessary to use a few mode fiber amplifier. Unlike amplifier, less controlled mode gain mode fiber amplifier for free space optical communications and high-power laser applications, and to ensure that all SDM channels are optimized.
At present, the University of Florida in the United States of Optics and Photonics Institute (CREOL) and NECL Labs researchers in the United States, has developed a modal gain in the less mode erbium-doped fiber amplifier (FM-EDFA) control, in order to improve the mode sub the method of multiplexing optical communication network feasibility.
Control Modal Dependence Gain
The Modal Dependence Gain (MDG) is activated by the distribution of the concentration of the dopant ions, the function of the overlap intergral between the distribution of the transverse strength of the pump light and the signal light. Through the fiber design to control the distribution and the signal intensity distribution of the dopant ions, and the modal dependence gain can be tuned. Complete doped fiber production can be achieved by controlling the content of the pumped mode dynamic control of the MDG.
Few mode erbium-doped fiber amplifier is constituted by a 15-meter long few mode erbium-doped fiber, the doped core diameter of the core 16? M support 1550nm wavelength of the signal in the vicinity of the two modes the group-LP01 (s) and the LP11 (s)and 980 nm of the four modes of the pump wavelength near group-LP01 mode (p), the LP11 (p), the LP21 (p) and LP02 (p).
Like the LP11 (s), so that the mode groups of the LP11 (p) and LP21 (p), having the same propagation constant in the two types of the degenerate modes by their angle-dependent parity is defined as even mode andodd mode (see Figure).
In order to demonstrate the use of the amplifer means to the MDG control, using a spatial filter (mode converter) and an optical variable attenuator (OVA) of the pump source light source is divided into the N-way, to produce the N-pump mode model. N signal pattern through the beam combiner space beam combiner and input to the signal light path through the dichroic mirror. By tuning each OVA, it is possible to adjust the content of the power of the single pumpming mode, there by enabling control of the overlap between the pump light and the signal light and the MDG.
Use less mode erbium-doped fiber amplifier in the receiver before, the researchers successfully through the 50km long few mode fiber transmission 26.4Tbit / s mode-division multiplexing DWDM signal.The signal received by three synchronous detection, coherent receiver mode coupling modal group delay and dispersion, multiple input multiple output (MIMO) by a digital equalizer to compensate.
The experiment proved researchers MDG control for all channels, generating the bit error rate (BER) of less than 3.8 × 10-3, the forward error correction will be sufficient for error-free transmission. Less mode erbium-doped fiber amplifier is expected to become key components of the transmission system of the future long-distance SDM CREOL PhD,, Neng Bai said, “We look forward to working with the industry to achieve low complexity, less high-efficiency mode erbium-doped fiber amplifier. “