# Development of New Spectral Amplitude Coding OCDMA Code by Using Polarization Encoding Technique
## 1 Introduction  
Optical code division multiple access (OCDMA) is a type of multiplexing technology that is utilized in optical communication systems. It enables the simultaneous transmission of several signals over a single optical fiber, with each signal being identified by a distinct code. OCDMA has the benefit of allowing for high levels of multiplexing, which enables the simultaneous transmission of several signals across a single fiber [1]. This can be helpful in situations when it is necessary to send a lot of data across a long distance, such as in broadband networks. OCDMA systems can also be used in a variety of other applications, including military communications, satellite communication, and biomedical imaging. A spectral amplitude coding (SAC) encoding and decoding technique was developed to eliminate multiple access interference (MAI) within the OCDMA system. Various types of codes were introduced for the SAC technique to counter the problems faced in the previous codes [2]. Each code has its own characteristics, some of which are suitable for local area networks (LAN) and others suitable for metropolitan area networks (MAN) [3, 4]. Previous codes, such as Hadamard, modified frequency hoppings (MFH), modified double weight (MDW) and enhanced double weight (EDW) code, have their limitations and drawbacks within OCDMA systems. The previous codes were designed to reduce the crosstalk within OCDMA systems. Hadamard is one of the earliest codes developed in OCDMA systems. To reduce the crosstalk in Hadamard, the code was designed to have a long code length, N, and a small code weight, W [5]. MFH, on the other hand, has larger weight with shorter code length. To design MFH is somewhat difficult and complex compared to the DW family code and Hadamard [6]. There are various types of detection techniques used in the OCDMA system, such as complementary techniques, AND subtraction, single photodiode detector (SPD) [7] and direct detection technique [1; 8; 9; 10]. In this paper, only AND subtraction and the direction detection technique were used. Based on the research paper by [PERSON] et al. [11], the polarization technique applied differs from the current research. Their technique does not compress the OCDMA code. This is because the polarization is applied for each user, which is not efficient for a higher number of users. By using multiwavelength optical orthogonal (MWOOC) code, the crosstalk between all users has been reduced, but it has longer codewords similar to Hadamard [12]. The longer the codewords, the more susceptible the system will be to noise and interference, as they may be more affected by changes in the optical signal. This can lead to decreased accuracy in decoding the data and potentially lower data rates. In addition, longer codes may require more complex coding and decoding circuitry, which can increase the cost and complexity of the OCDMA system. A research paper by [PERSON] et al. [13] provides a two-dimensional spatial/polarization encoding approach for an OCDMA system in multi-core fiber. Each user is divided into a separate core. A polarization encoding technique is used to increase the number of users supported and utilize each code word with two users with different polarization [14]. However, the distance is much shorter when using multi-core fiber instead of single-mode fiber [15]. The design is much more complex to apply in single-mode fiber. The main objective of this research is to develop a new DW family code called flexible double weight (FDW) OCDMA code by using a novel polarization encoding technique to overcome the problem faced in previous systems. FDW code is much easier to design compared to Hadamard, MFH, MDW and EDW code. FDW code has shorter code length, and the number weight can be either odd (EDW) or even (MDW). By using the polarization encoding technique, the code is compressed to support more users with shorter code length.