Development of all-fiber coherent beam combining optical system toward higher output of the fiber laser

In this paper, the authors report that they have developed a low-noise and high gain all-fiber using a coherent beam combining (CBC) system. The CBC optical amplifier system has a feature in which only correlated signal light is combined; however, amplified spontaneous emission (ASE) is not added. Hence, the combined output power increases about twice the signal output power. On the other hand, the power of ASE does not change compared to a conventional amplifier. Thus, the gain and the noise figure can be improved compared to the conventional amplifier. In this research, because the excitation wavelength of erbium doped fiber amplifier is 1480 nm, the minimum value of the theoretical noise figure is 5 dB. As an example, the optical amplifier system proposed in this paper had the small-signal gain of 37.25 dB and the noise figure of 5.60 dB, whereas the conventional optical amplifier system had the small-signal gain of 34.26 dB and the noise figure of 7.44 dB. As a result, we successfully decreased the noise figure by 1.84 dB and increased the small-signal gain by 2.99 dB by using the all-fiber CBC optical amplifier system.