Minimization of amplified spontaneous emission noise in upstream SuperPON 512, 10 Gbit/s. A.J. Sakena* a, M.Y. Jamro b and J.M. Senior b a Faculty of Engineering, Universiti Malaysia Sarawak, 94300, Kota Samarahan, Sarawak, Malaysia. b Department of Electronic Communication and Electrical Engineering, Faculty of Engineering and Information Science, University of Hertfordshire, College Lane, AL10 9AB, Hatfield, United Kingdom. ABSTRACT We demonstrated the effect of presenting optical band pass filter for point to multipoint architecture such SuperPON. The position of optical filter and the range of optical filter bandwidth to minimise amplified spontaneous emission noise for upstream SuperPON with 512 at transmission speed of 10 Gbit/s will also be exp lored. Keywords: Optical band pass filter, SuperPON, optical filter bandwidth, amplified spontaneous emission. 1. INTRODUCTION The optical amplifier application as an in-line amplifier for point to multipoint architecture, associated with system s parameter values are required further investigation as mentioned in ITU-TG 663(04/2000) 1. The point to multipoint architecture such SuperPON employed optical amplifier in the architecture to enhance the budgeting power is one of the ideal systems to be examined. Since the SuperPON is an amplified system, it is suffered from amplified spontaneous emission (ASE) noise. The downstream transmission of SuperPON present less challenging problems compare to upstream transmission. Nowadays, the challenge arises from the time response of optical amplifier with micro second bursts of data at the upstream SuperPON is consider solved by installing burst mode receiver at the optical line termination unit () 2. Unfortunately, the accumulation of ASE noise at the upstream of SuperPON still needs attentions. The pioneer SuperPON architecture of 2048, had discovered the accumulation of noise at the upstream due to the semiconductor optical amplifier (SOA) was placed in between splitter 1 16 and 1 128 3. The possibility of deploying SuperPON 8192 had also been investigated by considering the minimum noise in the several architectures 4. The high speed upstream SuperPON of 10 Gbit/s with of 1024 also discussed the noise issue which considers the gain at the pre amplifier r eceiver must operate more than 20 db to achieve best performance 5. In this paper, the accumulation of ASE noise will be handled by placing optical band pass filter. The position of optical band pass filter and the optical filter bandwidth (OFB) will also be investigated. 2. DESCRIPTION OF SIMULATION MODEL The simulation of SuperPON 512 has done using Virtual Photonic Integration, (VPI). Figure 1 shows the SuperPON model which is designed based on the current PON standard which defines the maximum distribution distance of 20 km 6. The 512 optical network units () are generated from combination of splitter 32 1 and 16 1. In the simulation fiber is replaced with the attenuator to examine the noise effect accurately. The dispersion is neglected due to the SuperPON using single mode fiber with narrow spectral width of semiconductor laser 7. The upstream SuperPON architecture for 512 is designed based on downstream architecture 8, where an optical amplifier was placed in front the splitter at feeder length. This respective architecture 8 provides minimum noise accumulation. * ajsakena@dominomail.unimas.my; phone +6082 672316; fax +6082 672317
10 km Figure 1: Simulation model for upstream SuperPON 512, 100 km. For the upstream, erbium doped fiber amplifier () is also used. The gain of an was modelled by choosing the suitable value of Er 3+ ion concentration at the particular overlapping factor. The signal from does not need to boost due to the in-line optical amplifier, employed high gain (=20 db) and it sufficient enough to eliminate the fiber loss and splitter loss in SuperPON. The optical amplifier produces gain and ASE simultaneously. At the optical line termination unit (), the receiver is using sensitivity of -20 dbm without pre amplifier. Thus, the optical amplifier cost could be reduced. 2.1 ASE noise and optical band pass filter The ASE noise had been studied and minimized using several approaches. A variable polarization beam splitter had been used to reduce ASE power and noise figure concurrently 9. The accumulation of ASE noise in WDM multistage amplified system could be rejected by using uniform fiber grating 10. Another approach is using the cascaded optical fiber grating couplers to reduce ASE noise and improved signal to noise ratio in for WDM signals 11. The optical band pass filter was used to minimize the noise in the upstream SuperPON due to the optical amplifier generates wide band noise. For point-to-point transmission, rectangular band pass filter is most suitable 1. A precaution must be taken in point to multi point transmission such SuperPON because the optical spectrum from the splitter perform wide band signal. Here, optical band pass filter with Gaussian function was used to avoid a part of signals missing. The SuperPON model with 512 has been simulated with transmission speed of 10 Gbit/s. In order to assess the effect of optical band pass filter, the following steps has applied; a) Simulation with and without optical band pass filter. The different spectrum power is displayed using optical spectrum analyser. b) The optical band pass filter is placed in different position to obtain the effective placement. It is evaluated based on the eye opening ratio and Q factor. c) The variation OFB will be applied to identify the minimum ASE power. 2.2 Simulation procedure of presenting optical band pass filter 10 km OSA: Power Spectrum Figure 2: Measurement of presenting optical filter for SuperPON architecture.
At first the measurement of spectrum powers are taken without optical filter. The transmits the signal through optical fiber and splitter. The signal is amplified and reaches together with ASE noise. In the simulation, has not receiver module to avoid receiver noises are included. The optical spectrum analyser is used to obtain the spectrum power of signal and ASE noise. The effect of presenting optical filter is discovered by placing the filter at any place in front the amplifier. The OFB of 80 GHz had been applied in this simulation. 2.3 Simulation procedure of optical band pass filter placement The SuperPON was designed to provide the efficient optical access network especially in term of system performance and cost saving. The main objective of placing the filter at different positions is to achieve a better signal and save the filter cost. In this simulation two different positions which provide minimum cost are identified. The simulation of optical band pass filter placement was constructed as shown in figure 3 and figure 4. In figure 3, the optical band pass filter is placed in front of before optical fiber. It is known as position 1, (P1). The alternative position of filter is known as position 2, (P2) and depicted in figure 4 where the filter is placed just before s. The in each figure has identical receiver module and oscilloscope to measure the eye opening and Q_factor. Distance 10 to 20 km P1 Distance 10 to 20 km P2 Bandpass filter at Position 1 Figure 3: SuperPON with optical band pass filter at position 1 Eye Opening Factor and Q_dB Eye Opening Factor and Q_dB Band pass filter at Position 2 Figure 4: SuperPON with optical band pass filter at position 2 The optical filter positions are evaluated by determining eye opening factor and Q-factor. The eye opening is defined as an eye closure Gaussian which, expressed by the following equation 12, ( P -3σ ) ( P + 3σ ) top top base base EyeOpeningFactor = (1) P top P base where P top is average logic power level one, P base is average logic power level zero, σ top is standard deviation of level one, σ base is standard deviation of level zero. The Q-factor is obtained from extraction of the Gaussian probability density function with power mean and unit standard deviation. It is calculated according to the following expression Ptop P base Q factor = (2) σ top + σ base For more comfy the Q_factor is expressed in decibel unit or known as Q_dB, the equation (2) is written as Q_dB = 20* (Log 10 (Q)) (3) For optical fiber communication, the good transmission system is obtained when bit error rate (BER) is lower than BER 1E-9 with Q factor more than 6 7. In this section simulation will be done by fixing the OFB at any values of filter bandwidth, for example 80 GHz. The variation distribution distance from 10 km up to 20 km will be considered.
2.4 Simulation procedure for determining a range of OFB The most suitable position of optical band pass filter will be used. In order to prevent the noise, it is necessary to determine a range of OFB. The measurement of ASE power at the variation of OFB is taken. The minimum value of ASE power indicates the good value of OFB. The simulat ion procedure based on mathematical approach is depicted in figure 5. There are two identical SuperPON architectures. The first SuperPON has with ASE noise and the second SuperPON has without ASE noise. The subtraction process of both architectur es nullifies the total losses and left the ASE power. The OFB is varied to measure the ASE noise power and to obtain the optimum range of bandwidth. Here, the operation of optical band pass filter is more likely a gate. Distance 10 km P2 Distance 10 km with ASE noise ON Band pass filter at Position 2 P2 + - ASE power measurement with ASE noise OFF Band pass filter at Position 2 Figure 5: Simulation procedure for obtaining the ASE power at various values of OFB. The simulation parameters are summarized in the Table 1. The transmitter power at the has been identified after several simulations in which cannot be shown here and has been obtained based on the desired, BER of 1E-9. Table 1: Typical value s of the various parameters used in the simulation of SuperPON 512. Parameter Value Unit Transmitter power 10 Gbit/s 1.9 mw Extinction Ratio 30 db Wavelength 1530 nm Receiver Sensitivity (BER 1E-10, Q 6.38, Responsivity = 1 A/W ½ ) -20 dbm Noise Figure 3 db 2 Stages Splitter Loss (1 16) (1 32) 12.04 15.05 db db Attenuation 0.2 db/km Feeder Length Distance (DD) DD- Loss 10-20 2-4 km db
3. RESULTS AND DISCUSSIONS This section describes the simulation results and discussions of the effect presenting optical band pass filter, the most suitable position of optical band pass filter and the optimum range of OFB. 3.1 The effect of presenting optical band pass filter The effect of presenting optical band pass filter is plotted in figure 6. It shows the total spectral power, ASE and signal under different condition: (a) without filter and (b) with filter. The graphs present power in dbm versus wavelength (nm). It shows that the signal beat noise at the operating wavelength of 1530 nm, where the effect of ASE noise is higher. Obviously, in figure 6 (a) the ASE noise increase when the wavelength increase and reach up to -50 dbm at 1530 nm. The ASE noise decreases after 1530 nm step by step, with three stages of wavelengths, 1530-1540 nm, 1540-1580 nm and 1580-1660 nm. These figures show that filtering method provides a minimum ASE noise at the particular value of OFB, e.g 80 GHz as shown in figure 6 (b). Figure 6(a): SuperPON 512 without optical band pass filter. Figure 6(b): SuperPON 512 with optical band pass filter. 3.2 The position of optical band pass filter The relative position of the optical band pass filter within the upstream SuperPON architecture affects the system performance. The optical band pass filter has a detrimental effect on the ASE noise performance of the upstream SuperPON when placed towards either end of or at the. Figure 7: EOF of SuperPON 512 against distribution distance loss, db. Figure 8: Q_dB against distribution distance loss, db.
Figure 7 and 8 show the effect of different position of optical band pass filter for upstream SuperPON 512 at transmission speed of 10 Gbit/s. The measured EOF of SuperPON 512 as a function of distribution distance with and without optical band pass filter is shown in figure 7. The EOF slope of three sets measurements are decreased when the distance of SuperPON increased. The position 2 provides the better position of optical band pass filter where a good signal received by. The performance of overall upstream SuperPON archit ecture was measured by plotting Q_dB with and without optical band pass filter against the distribution distance as shown in figure 8. It confirms that the position 2 is the most suitable place for installing optical band pass filter. 3.3 The optimum range of OFB The measured ASE power for SuperPON 512 ON, 100 km with transmission speed of 10 Gbit/s is plotted against the OFB. The ASE power below than zero microwatt is considered as background power. For above zero microwatt the ASE power is foreground pow er. The 3 db filter bandwidth occurs at nearly 40 GHz. From figure, it clear that there exists a range of OFB hereafter called optimum range of OFB where ASE power has minimum value. The optimum value of OFB begins at 50 GHz up to 160 GHz. The SuperPON s signal could be congested if OFB below than 50 GHz because the gate width too narrow. The ASE power reduction of ~-0.02 µm is achieved at the OFB of 160 GHz. If the OFB exceeds more than 160 GHz, the upstream SuperPON will suffer from ASE of inferior quality. The spectrum power of SuperPON at 50 GHz is shown in figure. It is showing the effect of ASE noise in minimum condition. Figure 9: ASE noise power (µw) versus OFB (GHz) Figure 10: Spectrum power of upstream SuperPON at OFB, 50 GHz (nm) 4. CONCLUSION The minimization of ASE noise in upstream SuperPON with low number of at 10 Gbit/s using optical band pass filter had been demonstrated. The placement of optical band pass filter results the most suitable place is at P2, near to the. It was found that the range of OFB of 50 GHz up to 160 GHz can be used to minimize the ASE noise power. REFERENCES 1. ITU-T G.663 (04/2000) Application related Aspects of Optical Amplifier devices and sub systems. 2. Brigati, S. Colombara, P. D'Ascoli, L. Gatti, U. Kerekes, T. Malcovati, P, A SiGe BiCMOS burst-mode 155-Mb/s receiver for PON, IEEE Journal of Solid-State Circuits, Vol: 37, Issue: 7, July 2002, pp 887 894. 3. Moss, S. E. Senior, J. M. Qiu, X. Z. Vandewege, J, Modelling the Upstr eam Semiconductor Optical Amplifier Cascade in the PLANET SuperPON, Proceedings of the European Conference on Networks and Optical Communications 1999, pp 127-133. 4. Mestdagh, D., Martin, C., The SuperPON Concepts and It technical challenges, Broadband Communication, April, 1996.
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