a noise-figure improvement of about 1.7 to 7.5 dB over the flat gain region from 1568 to 1598 nm, as compared to those of the standard double-pass amplifier [ Fig. 1(a)]. The amplifier has also achieved a flat gain output of about 34.2 dB with a gain variation of less than 1.3 dB over 30 nm at the wavelength from 1568 to 1598 nm. The corresponding noise figure varies from 5.0 to 5.7 dB at the flat-gain region. Figure 3 shows the measured gain (shaded symbols) and noise figure (clear symbols) at 1580 nm, with respect to the variation of input signal power. The symbols of square and rectangular represent the characteristics of standard amplifier of Figure 1(a) and amplifier with EDF1 ϭ 7 m of Figure 1(b), respectively. P1, P2, and P3 are fixed at 20.6, 15.6, and 50.6 mW, respectively. As shown in the figure, the proposed amplifier shows a noise figure reduction of 2.3 dB at small input-signal power, as compared to that of the conventional amplifier. The placement of the forward pumped EDF1 section immediately before the port 1 of optical circulator OC1, as shown in Figure 1(b), prevents the amplified signal and backward ASE from propagating into the input part of the amplifier (EDF1). Therefore, the population inversion of the EDF1 is not affected by the intense lights and, hence, the noise figure can be kept low. Figure 4 shows the gain spectra of the proposed double-pass amplifier (EDF1 ϭ 7 m) at different pumppower combinations. As is apparent from the figure, the flattest spectrum is obtained when pump powers P1, P2, and P3 are fixed at 20.6, 15.6, and 50.6 mW, respectively. The gain varies from 33.6 dB to 34.9 dB within the flat gain region between 1568 to 1598 nm. However the lowest noise-figure spectrum is shown by the combination of pump powers of 20.6 mW, 26.6 mW, and 50.6 mW. The corresponding noise figure varies from 4.8 to 5.6 dB in the flat gain region. The new double-pass amplifier with a high flat-gain and low noise figure can be useful as inline amplifiers in WDM transmission systems.
CONCLUSIONA new double-pass L-band EDFA with enhanced noise-figure characteristics has been demonstrated using a short length of forward-pumped EDF, which is placed in front of a double-pass amplifier. Since the OC1 prevents the amplified signal and backward ASE from propagating into the EDF1, the population inversion of the input part of the amplifier was hardly affected by the intense lights; therefore, the noise figure can be kept low. The new double-pass system with EDF1 ϭ 7 m has achieved a flat-gain output at about 34.2 dB with gain variation less than 1.3 dB at the flat-gain region from 1568 to 1598 nm. The corresponding noise figure varies from 5.0 to 5.7 dB, which is improved by 1.7 to 7.5 dB over the flat-gain region, compared to that of the conventional double-pass amplifier. sive method of moments (ORMoM) [4] or direct matrix manipulation (DMM) [5]. In this paper, an original alternative solution is presented. The proposed multistructure MoM technique is based on the simultaneous analysis of different str...
