H-WDM EDFA Configurations and Modeling

Figure 5.1 depicts an HFC or FTTC/FTTH trunk and access lightwave system, which is merely an example of a system, in which various 980- nm-pumped H-WDM EDFA configurations with associated optical components as shown in the insets are used

to simultaneously amplify hybrid digital/analog WDM signals. As illustrations in Figure 5.1, the H-WDM EDFAs can act as a booster jus t right after the transmitter in the headend (HE) or central office (CO) site, an in- line amplifier, and a splitting- loss-compensated power amplifier just right before the optical splitter in the distribution network. These H-WDM signals are detected at each optical network unit (ONU) through an optical demultiplexer by individual optical receiver. The 980- nm pumped EDFA’s have been selected since they exhibit almost quantum- limited noise performance as compared with the 1480-nm pumped EDFA’s. There are four optical pumping blocks (P1, P2, P3, and P4), a 980-nm pump-passed path block (PP), two optical isolator blocks (I1 and I2), and two erbium-doped gain fiber blocks (L1 and L2) in Figure 5.1 used to construct various amplifier configurations. Each pumping block consists of a 980-nm pump laser diode and a 980/1550-nm WDM coupler. The 980-nm pump-passed path block as indicated with the dashed line consists of a pump-passed path and two 980/1550- nm WDM couplers in the P2 and P3 without including the pump lasers.

Table 5.1 list nine kinds of H-WDM EDFA configurations, their compositions using the related blocks, and their configuration symbols. Among them, five amplifier configurations are incorporating with a midway optical isolator (I1), which is used to suppress the backward ASE noise generated from the second EDF stage (L2). They are the dual- forward (FFI), dual-backward (BBI), forward-and-backward with a 980- nm pump-passed path (FBIp), forward-and-backward without the pump-passed path (FBIu), and backward-and- forward (BFI) pumping configurations. For the pump-blocked FBIu

case using a midway optical isolator as the ASE suppressor, the forward (backward) 980-nm pump power will be drastically attenuated or blocked by the high forward insertion loss (the high reverse optical isolation) of the midway isolator. For the pump-passed FBIp case, the residual pump power is able to bypass the midway isolator to the opposite EDF side through the lower 980-nm path. In the mean time, the backward ASE was suppressed by the optical isolator I1 located in the upper optical path. In contrast, there are four amplifier configurations without using the midway optical isolator in dual- forward (FF), dual-backward (BB), forward-and-backward (FB), and backward-and-forward (BF) pumping configurations.

Figure 5.2 shows the optical channel allocation of H-WDM signals, where the

AM-VSB SCM analog channel is located at long wavelength (here, 1558.17 nm) and ten multiple digital baseband channels are located at short wavelength region (here, the most right digital channel is at 1551.72 nm). The output power and noise figure of the AM-SCM analog channel are PA and NFA, respectively. The output power and noise figure of the digital baseband channels are PD, i, and NFD, i, respectively, where i = 1, 2,…, 10 . The channel spacing between the digital WDM channels is ∆λD. The worst differential channel output power among the digital channels is ∆P_D (= P_Max – P_Min), where P_Max and PMin are the highest and lowest channel output powers, respectively, among the digital channels. Three kinds of channel spacing ∆λD of 1.6 nm (i.e., 200 GHz), 0.8 nm (100 GHz), and 0.4 nm (50 GHz) for the digital channels with the longest wavelength channel at 1551.72 nm are separately considered. The P_D,i is usually 15-20 dB lower than P_A in H-WDM systems due to the low output power used in digital lightwave transmitters. The above channel allocation and the guard band (here, about 6.5 nm) arrangement are used to reduce the stimulated Raman scattering effect induced channel crosstalk between analog and digital channels. Because both high input power level and low noise figure characteristics are a must for the AM-VSB signals to achieving the stringent system performance requirements of high carrier-to-noise ratio (CNR) and low composite second order (CSO) / composite triple beat (CTB). The satisfied CNR and CSO/CTB performances of the AM-VSB signals at each ONU are about ≥ 50 dB and ≥ 60/60 dBc, respectively. For the digital baseband optical channels, the design consideration of such H-WDM EDFA is required to offer a good channel output power uniformity while maintaining proper electrical SNR of about 16 dB (the shot noise limit) and 22 dB (the thermal noise limit) for achieving the bit-error-rate (BER) performance of 1 × 10^-9 with low power penalty. The achievement of AM-VSB’s system performance is difficult than the digital baseband signals. Therefore, the major design criterion of H-WDM EDFA considered here is to provide a high output power of ≥ 60 mW and low noise figure of ≤ 4 dB for the analog channel with an input power of +3 dBm while keeping the differential channel output power among digital channels, each with an input power of -15 dBm, to be

≤ 0.2 dB.

The simulation tool used in this work is the Lucent Technologies OASIX Optical Amplifier Simulation System (Version 2.01), in which the EDFA model used in this work

is based on the model by Giles and Desurvire [89]. The EDF and EDFA parameters used in the simulations are summarized in Tables 5.2 and 5.3, respectively. The insertion loss of each WDM coupler is assumed to be the same with 0.5 dB at both 980 and 1550 nm bands.

The insertion loss of each optical isolator is assumed to be 0.7 dB. In addition, the forward insertion loss of the used optical isolator at pump band is assumed to be infinite for all configurations. The output power of each 980-nm pump laser diode is 150 mW. The input signal power levels of each EDFA configuration for the analog channel and each digital channel are set to be +3 dBm at 1558.17 nm and –15 dBm at 1550 nm region, respectively, in all simulations. A total of eleven H-WDM channels (ten digital baseband channels and one analog channel) are considered.

There are only two optical connectors used and the others are splicing points within the proposed EDFA. Assume that the optical connectors used at the input and output ports of the H-WDM EDFA are all angled physical contact (APC) type with low backreflection of ≤ -60 dB. The return losses of each splicing point and all other optical components used within the EDFA are ≤ -60 dB. Therefore, the optical reflection induced system performance degradations of such as CNR and BER can be neglected. In addition, the Rayleigh backscattering, from the transmission fibre between two H-WDM EDFA’s with built- in optical isolators, induced system performance degradation is also trivial. This effect has been examined in a recent four-channel amplified H-WDM transmission experiment with a heterogeneous traffic of 10-Gb/s, 2.5-Gb/s, 64-QAM, and AM-VSB signals over a 100-km single fiber link [90].