# Optical Amplifiers for Access and Passive Optical Networks: A Tutorial
## 2 3R--Reamplification, Reshaping, and Retiming  
The specification of gigabit passive optical networks (GPONs) considers two scenarios for reach extension. The first specification is based on optical-electrical-optical (OEO) conversion, and the second specification uses full optical signal processing and amplification. We provide a basic principle of amplifiers based on OEO conversion. In general, these amplifiers can be divided into three categories: 1R, 2R, and 3R. While the current research interest is full optical amplifiers, we discuss all three categories due to the potential usage of 3R amplifiers in xPONs [43; 44; 45; 46; 47; 48; 49]. The main signal degradation in fiber optic systems arises from amplified spontaneous emission (ASE) due to optical amplifiers, pulse spreading due to group velocity dispersion (GVD), which can be corrected by passive dispersion compensation schemes, and polarization mode dispersion (PMD). Nonlinear distortions are attributed to Kerr nonlinearity, such as cross-phase modulation, which can be responsible for time jitter in wavelength division multiplexing (WDM), or Raman amplification, which can induce channel average power discrepancies [50]. The 1R category represents the simplest amplifier of an optical signal. Only the input signal is amplified and transferred to the output. Note that an input signal is not recovered (the shape, position, and phase are exactly the same as those of the input signal). However, 1R amplifiers are simple, which presents some advantages. For example, a processed signal does not depend on the modulation format, transmission speed, or other parameters of a signal. The basic principle of 1R amplifiers is shown in Figure 1. The input signal is degraded, but the output signal is only amplified because 1R amplifiers do not consider the shape and timing of the input signal; they only consider amplification. All known optical amplifiers can be placed in the 1R category. The second category of R amplifiers works more complexly with an input signal because they are based on the 1R category and add reshaping of an input signal. The shape of a carried signal is degraded with increasing distance from the transmitter side. We consider optical networks and take into account the attenuation of optical fibers. We cannot eliminate the attenuation of optical fibers because we are not able to produce clean silica fibers without admixtures and impurities (additional details about optical fiber manufacturing are provided in [51; 52]). The standard attenuation values of the fibers are 0.35 and 0.22 dB/km for 1310 and 1550 nm, respectively. Other important factors are dispersion (additional details about dispersions are provided in [53; 54; 55]). In general, dispersion causes a carried signal to become deformed in the fiber and spread in the time domain, which produces a range restriction by decreasing the signal-to-noise ratio (SNR), transmission speeds, and improperly logical 0 or 1 decision in the receiver. A 2R amplifier is referred to as a regenerator. A regenerator has an optical signal at an input port, which is converted into an electrical signal; decisions are subsequently made. A decision entails recognition of logical 0 and 1 of the input signal. The signal  
Figure 1: Principle of 1R, 2R and 3R. is subsequently transferred to a transmit circuit. The transmitting circuit converts the signal to the optical domain and transfers to a fiber path. Note that the output signal has recovered its shape and has a higher power level (was amplified), but timing recovery does not occur (the positions of the signal samples are unchanged); refer to Figure 1.  
The 3R amplifier adds time synchronization to the basic principle of 2R. The 3R amplifier converts the input signal from the optical domain to the electrical domain, amplifies it and reshapes it. A clock rate is recovered and reconstructed before sending a time position (for example, by a comparator). This output signal is equivalent to the original signal that was transferred to the fiber.