# Design and Analysis of an O+E-Band Hybrid Optical Amplifier for CWDM Systems
## 1 Introduction
Such an arrangement is generally known as a hybrid optical amplifier (HOA) and has been shown to enhance system capacity and performance [11]. HOAs can be realized using different combinations of series or parallel amplifiers, as discussed in [12]. [PERSON] et al. proposed a C+L-band HOA based on hafnia-bismuth-erbium-doped fiber (HBEDF) and zirconia-erbium-doped fiber (ZEDF) as the gain medium [13]. For the design proposed in [13], a gain of 14.6 dB, gain fluctuation of 1.8 dB, and NF fluctuating in the range of 4.3-7.9 dB over a wavelength range of 1530-1600 nm was observed. HOAs that have been demonstrated in other studies include the HOA based on distributed Raman-EDFA that was used for amplifying a 120 Tbps optical signal over 630 km of SMF [14], Raman-EDFA used for a 54 Tbps optical signal over 9150 km of SMF [15], and Raman-SOA that was used for a 107 Tbps optical signal over 300 km of SMF [16]. [PERSON] et al. proposed an HOA based on a combination of two fiber parametric wavelength converters and an EDFA for S-band amplification [17]. An NF as low as 4 dB was observed when the conversion efficiency was kept higher than 10 dB for the first stage. [PERSON] et al. designed an HOA based on SOA-EDFA-Raman for the transmission of 40 DWDM channels at a rate of 10 Gbps over 240 km of SMF operating at the edge of the L- and U-bands [18]. A gain of 31 dB with a flatness of 0.8 dB and an NF of 5.7 dB was observed for a 1611.8-1620.5 nm wavelength range. [PERSON] et al. demonstrated an erbium-ytterbium co-doped waveguide amplifier (EYDWA)-Raman HOA having a gain of 25 dB with a flatness of 2.78 dB and an NF of less than 6 dBover a 1539.7-1562.7 nm wavelength range [19]. [PERSON] et al. proposed an HOA based on an erbium-ytterbium co-doped fiber amplifier (EYDFA) and backpropagating Raman amplifier [20]. A gain of 26 dB was achieved with a flatness of 1.37 dB over a wavelength range of 1545-1565 nm. In [21], an HOA based on a Raman-EDFA configuration was proposed for WDM transmissions. The HOA exhibited a gain of 46 dB and an NF of 3 dB over a 1530-1600 nm wavelength range. [PERSON] reported designs of HOAs based on a three-stage holmium-doped fiber amplifier (HDFA) and a thulium-doped fiber amplifier (TDFA) [22] and a two-stage HDFA and TDFA employing a shared pump [23]. A small signal gain of 70 dB over a 2009-2098 nm wavelength range and an NF of 7.5 dB [22], 49.1 dB, and 6.5 dB at 2051 nm [23] were observed. [PERSON] et al. reported on an E+S-band HOA based on bismuth-doped fiber (BDF) and EDF, which produced a small signal gain and output power of 27 dB and 24.5 dBm, respectively over a 1431-1521 nm wavelength range [24]. [PERSON] et al. demonstrated a three-stage S-band HOA based on an L-band EDFA placed between two optical parametric amplifier (OPA)-based wavelength converters [25]. An average gain of 18.6 dB with a flatness of 1.2 dB and an NF of 5.1 dB was observed. [PERSON] et al. optimized a C+L-band HOA based on an EDFA-Raman amplifier using neural network models, where gain flatness decreased from 6.7 dB to 1.9 dB [26]. A survey of the previous work related to HOAs has been summarized in Table 1 and compared with the main results obtained from our proposed work. The missing information in the studies presented in Table 1 is represented by dashes. In this paper, we proposed a wideband flat-gain O+E-band HOA based on a parallel configuration of PDFA-SOA. Wideband fiber Bragg grating (WFBG) has been used to separate the O- and E-band signals to input them separately into the PDFA and the SOA, respectively. In our previous study, we optimized the values of the doping concentration of Pr\({}^{3+}\) and the length of the PDF [6; 27]. In this work, we optimized the pump wavelength and the injection current to analyze the performance of the proposed HOA. A small signal gain of 24 dB with a flatness of around 1 dB and an NF of 4-5.9 dB over a 1270-1450 nm wavelength range was observed.