# Design and Analysis of an O+E-Band Hybrid Optical Amplifier for CWDM Systems
## 1 Introduction  
IP traffic is continuously increasing globally due to the proliferation of various applications and technologies in our daily lives such as fifth-generation mobile networks, cloud computing, web applications, and the Internet of Things (IoT) [1, 2]. The tendency of increased bandwidth demands and, therefore, network capacity will persist in the future. Urgent steps are required to meet the enormous bandwidth demands since the commonly employed C-band for commercial optical links is facing its capacity limits [2]. Fiber-optic communication is mainly conducted in the wavelength region where optical fibers have relatively low transmission loss [2]. This low-loss wavelength region ranges from 1260 nm to 1650 nm and is divided into six wavelength bands referred to as the O-, E-, S-, C-, L-, and U-bands, which range from 1260-1360 nm, 1360-1460 nm, 1460-1530 nm, 1530-1565 nm, 1565-1625 nm, and 1625-1675 nm, respectively [2]. Moreover, the O-band has smaller fiberdispersion, which enables high-speed optical transmission in this band without using any dispersion compensation schemes [3]. Similarly, the E-band has had a small transmission loss compared to the O-band since the invention of dehydration techniques during glass production [3]. Different solutions are available to fix the above-mentioned capacity limits of optical communication systems, which include (a) the optimal use of advanced multi-level modulation formats, (b) spatial division multiplexing (SDM)-based systems, and (c) exploiting the relatively low attenuation and unused optical windows for transmission beyond the C-band [2,4]. The first two approaches can be considered for addressing the issues of capacity and bandwidth demand at the expense of certain issues, which have been discussed in [2,4]. The third approach that exploits the high bandwidth provided by SMFs is generally called optical multiband transmission (OMBT) and is the most viable and efficient solution [4]. The OMBT technique relies on the use of the low-attenuation optical bands of SMF for data transmission, resulting in an 11-fold expansion of the available bandwidth of the C-band and 5 times the available bandwidth of the C+L-band [5]. The first phase of the OMBT scheme targets the implementation of communication outside the C-band such as the L-band using commercial off-the-shelf components [5]. In the second phase, the remaining bands, such as the O-, E-, S-, and U-bands, will be considered for data transmission [5]. The upgrade to the L-band is carried out based on erbium-doped fiber amplifiers (EDFAs), thus enabling the addition of a massive bandwidth of around 60 nm to the current 35 nm window of the C-band [2,5]. Therefore, the main challenge for the realization of OMBT for the second phase is the design and realization of innovative, efficient, and low-cost photonic components, particularly optical sources and wideband amplifiers compatible with the O-, E-, S-, and U-bands. In optical fiber communication, optical signals suffer from various impairments such as fiber attenuation, component insertion losses, fiber dispersion, and nonlinearities [6]. The capacity of optical networks can be increased by providing a sufficient power budget or by increasing the signal power through the use of amplifiers at regular intervals along the fiber link [7,8]. Initially, electronics-based semiconductor amplifiers were employed to boost the power of optical signals. However, these amplifiers had some issues such as the requirement of optical-to-electrical and then electrical-to-optical conversions, low reliability, complexity, bulkiness, and cost inefficiency [9]. All-optical fiber amplifiers have been extensively researched and various doped fiber amplifiers based on the rare-earth dopants ytterbium, praseodymium, thulium, and holmium have been proposed [6]. Another way to realize wideband optical amplifiers is to combine multiple optical amplifiers in parallel or in series, where each amplifier operates over a distinct spectral region [10].