# Coupling of Modes in Step-Index Plastic Optical Fibers by Using D-Shape Technique
## 1 Introduction  
Plastic optical fibers (POFs) offer several advantages over glass fiber that make them viable and versatile for various applications where flexibility, cost, ease of installation, resistance to harsh environments, and short-range performance are essential considerations [1; 2; 3]. Step-index POFs (SI POFs), here named POFs, are widely used in several fields, ranging from home networks, fiber to the home (FTTH), Local Area Networks (LANs), the automotive industry, aviation, Unmanned Aerial Vehicles (UAVs), Internet of Things (IoT), and medical applications, among others [4; 5; 6]. In such systems, it is important to elevate the data transmission rate as high as possible according to the sensor numbers used and the field of application. The development of POFs started in the decade of the 1960s but only became viable in the 1980s [1]. A POF is a multimodal optical fiber with high attenuation due to its intrinsic characteristics. A POF is mainly used in the visible spectrum region, although applications are also in the near-infrared region [1]. Poly Methyl Methacrylate (PMMA) is the most common material used in the manufacturing of POFs [7; 8] and presents typical values of attenuation in the visible spectrum; for example, in the red region at 650 nm, the attenuation is 130 dB/Km, which is much more significant than 0.2 dB/Km at 1550 nm for glass optical fibers (GOFs) [3]. To achieve high data rates in POFs, one must establish mode coupling and minimize modal dispersion [7; 9]. By promoting a uniform energy distribution between the modes, mode coupling minimizes the effects of modal dispersion, which is a crucial limitation of the bandwidth of multimode fibers. In shorter lengths of fiber, where balancing the distribution of modes is more difficult to achieve naturally, mode coupling helps to stabilize the bandwidth, slowing its decline [10; 11]. Mode coupling occurs when the light energy is redistributed among the different modes as it propagates through the fiber; this phenomenon helps to equalize the arrival time at the receiver of the various modes, thereby reducing modal dispersion. The coupling of modes passes through two steps. The first step is the equilibrium mode distribution (EMD), which balances the energy distribution between the different propagation modes supported by the POF. In this condition, the POF has a single attenuation value per unit length [12]. The modes interact and stabilize after the EMD but exchange energy until they reach the steady-state distribution (SSD). Upon reaching the SSD, the behavior of the modes changes regardless of the conditions of the light entering the fiber [13]. The POF reaches the EMD at a certain length, termed the Coupling Length (LC), and it attains SSD at another length, denoted as the Mode Stabilization Length (Zs). The SSD condition is ideal for achieving high transmission rates in POFs because in this condition, there is a reduction in modal dispersion and an improvement in signal integrity. The ZS must be decreased to achieve SSD in short TOF segments and allow for higher data rates over shorter distances. In this work, if we consider the need for increased transmission rates in embedded systems, small and lightweight systems, laboratory instrumentation, and critical military applications (aircraft, ships, combat vehicles, and campaign communication systems), the transmission of high data rates in short TOF stretches may be necessary. Considering the advantages of the POF over conventional glass fibers, this study proposes using the D shape technique, widely used in fiber sensor systems [14; 15], to achieve SSD in short-length POFs. This approach's merit is its simplicity and low cost, as it does not require additional components. This paper is divided into six sections, starting with the introduction. The coupling between the modes is then discussed in Section 2, and the components and experimental setup used to perform the measurements are shown in Section 3.