# POL-MUX System for Noncoherent Optical Networks
## 1 Introduction  
The demand for wider bandwidth in transport optical networks, as well as in optical access networks, increases every year. For this purpose, many multiplexing techniques have been developed, and some of them have been deployed for real data traffic. The most important multiplexing techniques in transport optical networks are the dense wavelength division multiplex (DWDM) and time division multiplex (TDM) [1]. The second generation of passive optical networks (PONs) uses both WDMs and TDMs [2]. Optical transport networks combine a DWDM with a polarization division multiplex (PDM) and coherent detection, thereby doubling the bandwidth. Although these systems are effective, it is not possible to use them for PONs due to digital coherent detection (DCD), which is the most expensive part. Nevertheless, there is a way to use a polarization multiplex without coherent detection. Other multiplexing technologies have prospects for future deployment, but many of them are difficult to enforce because of the difficulty of implementation and necessity of intervention for the active components and distribution networks. This does not apply for polarization multiplexes, which can double the bandwidth without any intervention regarding the existing optical network, and in addition, such a multiplex is partially immune to polarization mode dispersion (PMD) [3]. The polarization multiplex (POL-MUX) is based on the transmission of two different optical signals in one optical fiber using polarization division. The optical beam in a single-mode fiber spreads in two polarization planes \(x\) and \(y\) that are orthogonal to each other. With a polarization beam splitter (PBS), it is possible to align independent optical signals into both polarization planes and transmit two different polarization-divided optical signals in one fiber. The principle of polarization multiplexes can be compared with technologies used in radio systems, where radio signals are also transmitted in two orthogonal polarization planes and for receivers, two differently oriented antennas are used. Detection is possible only if the orthogonality of both signals is preserved. This is not possible in real optical fibers due to fiber imperfections (circular asymmetry of the fiber) and birefringence [4]. The basic scheme of the POL-MUX is shown in Figure 1.  
An important advantage of the POL-MUX is that the distribution network does not need to be changed by any special component, as is this case for other multiplexingtechniques. On the transmitter side, two optical signals are aligned into a fiber by a polarization beam splitter (PBS), which has two inputs with polarization filters. The first input aligns the signal into the \(x\) plane, and the second input aligns the signal into the \(y\) polarization plane. Therefore, it is necessary to adjust the polarization angle so that each signal is parallel to the polarization axis of a given port. The polarization angle and state of polarization are set by a polarization controller (PC) [5]. Multiplexed signals contain two differently modulated optical signals, which may have the same wavelength. These signals pass through an optical distribution network, at the end of which another polarization controller is used for rotation of the polarization process--each signal must be parallel to the polarization planes of the second polarization beam splitter, which is used for polarization demultiplexing. This ensures that the desired output signal is included in each polarization output [6]. Increasing the bitrate in optical networks tightens the requirements for polarization mode dispersion (PMD), which is the main limit of high-speed optical systems. Because a POL-MUX uses two separate polarization planes, a POL-MUX is not as influenced by PMD, as opposed to common telecommunication systems. This is very important for bitrates \(\geq 10\) Gbps.