Comparison of discrete L-band Raman fiber amplifier in two different configurations.
Raman fiber amplifiers (RFAs) have found numerous applications in long haul and ultra-long haul optical communication systems, due to wide gain bandwidth and wavelength flexibility of stimulated Raman scattering (SRS). Different RFA architectures were proposed to employ in an optical communication system and different classification can be adopted for RFA. When it comes to the pump direction, RFAs classify in forward or copump RFA where the pump and the signal propagates in the same direction  and backward or counter-pump RFA where the pump and the signal propagates in an opposite directions . In addition, the RFA can be pumped within the bidirectional architecture (two pump sources are used in the opposite direction to each other) [3, 4]. When it comes to the gain medium length, the RFA can be classified as distributed RFA (the transmission span is pumped in order to compensate the link-to-link losses) and discrete RFA (a box contain a short gain medium length at the receiver or the transmitter end of the communication system .
From other hand, it's hardly utilized to provide high gain level. Because long gain medium (several kilometers of optical fibers) should be used to produce sufficient Raman amplification in which the stimulated Brillouin scattering (SBS) can be initiated easily. So the power of single frequency RFA which could be achievable is usually limited [6, 7]. The SBS has injurious sequels to signal transition in optical communication systems because it restricts the optical power that can be launched into the fiber. Therefore, it is important for the optical system designers to realize the effect of SBS on performance of communication systems to be able to decrease its retrograde effects. In addition, different techniques have been proposed to repress the SBS in optical fibers [7-9].
Furthermore, several studies were proposed to investigate the RFA gain saturation due to SBS effect theoretically  and experimentally [11, 12]. In addition, OptiSystem program was proposed to reduce the computational time required to solve RFA differential equations. The average power model is adopted as a gain medium for RFA [13, 14] while, the SBS effect is not covered in this model. M. H. Ali et al., were first studied this effect utilizing bi-directional model in OptiSystem . The experiential results show good agreement with the simulation model. However, in  the authors studied this effect under constant conditions i.e. single fiber length at co-pumped single-pass scheme.
In this paper, the performance parameters of a discrete L-band counter-pumped RFA is investigated utilizing simulation software (OptiSystem-10). A comparison between two different architectures are adopted, namely, single-pass and double-pass RFA in order to investigate the gain level and the SBS effect on Raman gain saturation for both configurations.
Simulation Model and Operating Principle:
In the simulation design the bidirectional fiber model is adopted as a Raman gain medium. Figure (1) shows the simulation setup of counter-pumped discrete RFA. The input signal is provided by a tunable laser source (TLS) within wavelength band from 1550 nm to 1600 nm, power range from -30 to 0 dBm and at linewidth of 150 kHz. A dispersion compensating fiber (DCF) of 7 km length is used as RFA gain medium, which pumped in a counter pump direction by a Raman pump unit (RPU) with center wavelength at 1480 nm. Finally, two optical spectrum analyzers (OSA) are used. OSA1 is used to record the output signal and OSA2 used to record the Brillouin Stocks power. The DCF based RFA can provide several advantages as compared with the single mode fiber (SMF) because of its small effective core area and high germanium concentration. This outcomes with a high Raman gain of 10-20 dB, nonlinearity 7-8 times higher than the SMF, and a high Rayleigh scattering coefficient [16-18].
In the single-pass RFA (configuration A), the input signal is injected to the RFA architecture via port 1 of the optical circulator prior to propagate over the gain medium, and the output signal power record by OSA1 via signal port of the wavelength division multiplexer (WDM). The Raman pump power (RPP) injected to the gain medium through the pump port of the WDM. For double-pass RFA (configuration B), a broadband optical mirror (M) is inserted between point X and Y, in order to re-inject the output signal power through the Raman gain medium and record it via port 3 of the optical circulator. Two optical isolators are used in the proposed RFA, in order to block the back reflection to the TLS.
RESULTS AND DISCUSSION
The gain level as a function to the Raman pump power for both of SP- and DP-RFA is investigated as illustrated in Figure 2. For both configurations the input signal power is fixed on -30 dBm and the wavelength on 1580 nm. The RPP is changed from 50 mW to 1000 mW for SP-RFA and from 50 mW to 600 mW for DPRFA, steps of 25 mW. According to the results, both amplifiers are provided about 22 dB net gain level when the RPP is about 1000 mW and 500 mW for SP- and DP-RFA, respectively. This represents about 50% in pump power conservation, as well as, shows good agreement with the results in [20-22], which confirm the validity of our model.
The gain profile for both amplifiers is investigated in order to determine the OPP for each amplifier as depicted in Figure 3 and Figure 4. The OPP is defined as the RPP in which the higher average gain level, a wide gain bandwidth, and lower noise figure can be obtained. According to the results in Table-1, the OPP is about 600 mW and 300 mW for SP-RFA and DP-RFA, respectively.
Figure 5 shows the gain level and the Stokes power as function to the input signal power for both configurations. In this work, the input signal power is changed from -30 dBm to 0 dBm at 1580 nm within RPP of 600 mW and 300 mW for SP- and DP-RFA, respectively. At an input signal power of -30 dBm it can be seen that the maximum gain is about 12.79 dB for the SP-RFA and 14.60 dB for DP-RFA, the gain of the amplifier keeps nearly constant at first as the input signal power increased, then a gain degradation obviously begins at critical point [SBS.sub.TH].
The gain is degraded due to large input signal of a 3-dB value from its maximum unsaturated value. It is obviously shown that the gain for DP-RFA saturates earlier in comparison to SP-RFA with a higher gain level. It is clearly shown that the degradation in the gain starts exactly when the Brillouin Stokes appears.
The gain level and NF versus input signal wavelength for both amplifiers at input signal power of -30 dBm within OPP condition is illustrated in Figure 6. The input signal wavelength is tuned from 1550 nm to 1600 nm steps of 5 nm. In addition, results in Table-2 show the gain characteristics for both amplifiers at different input signal power, namely, (-30, -6 and 0) dBm. As the signal power increased the gain profile is progressed, because of the variation in saturation level for several input signal wavelengths.
The performance parameters of L-band RFA within two various architectures are investigated and compared by using OptiSystem-10 software. The double-pass of the input signal power through the Raman gain medium improved the overall gain by 16.74 % as well as conserved the RPP by 41%. The saturation power is degraded from 0 dBm in SP-RFA to -6 dBm in DP-RFA within higher gain level.
The contribution of this research paper is the deep investigation and demonstrating of the DP-DRFA within a wide amplification bandwidth by utilizing OptiSystem-10. The bidirectional fiber model is adopted as a gain medium for the proposed amplifier. The SBS effect is covered in this model, as a result, the gain saturation due to the large input signal can be investigated in the proposed amplifier.
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(1) A. K. Abass, (2) Salah Aldeen Adnan, (3) Ban K. Hadi, (4) Mohammed A. Salih
(1) A.K Abass, Lecturer, Optoelectronics Engineering Department, University of Technology, Baghdad, Iraq,
(2) Salah Al deen Adnan, Assistant Professor Optoelectronics Engineering Department, University of Technology, Baghdad, Iraq.
(3) Ban K. Hadi, student, Optoelectronics Engineering Department, University of Technology, Baghdad, Iraq,
(4) Mohammed A. Salih, Lecturer, Education College, University of Al-Iraqia, Baghdad, Iraq.
Received 12 May 2017; Accepted 5 July 2017; Available online 28 July 2017
Address For Correspondence:
Ban K. Hadi, University of Technology, Optoelectronics Engineering Department, engineering collage, Baghdad, Iraq.
Caption: Fig. 1: Simulation setup of the counter-pumped RFA: (configuration A) single-pass; and (configuration B) double-pass, with optical mirror inserted between point X and Y.
Caption: Fig. 2: Net gain for counter-pumped SP- & DP-RFA, at input signal power of -30 dBm and wavelength of 1580 nm for different pump powers.
Caption: Fig. 3: SP-RFA gain profile at several RPP within input signal power of -30 dBm, the OPP is about 600 mW.
Caption: Fig. 4: DP-RFA gain profile at several RPP within input signal power of -30 dBm, the OPP is about 300 mW.
Caption: Fig. 5: SP- & DP-RFA gain level and Stokes power versus input signal power at OPP.
Caption: Fig. 6: SP- & DP-RFA gain profile at input signal power of -30 dBm and OPP.
Table 1: RFA gain characteristics at RPP ranging from 50 mW -to-600 mW steps of 50 mW, at input signal power and wavelength of -30 dB and 1580 nm, respectively. RPP AGL dB ANF dB 3-dB BW nm Psat. dBm mW SP DP SP DP SP DP SP DP 50 -1.54 3.82 3.46 0.36 40 25 8 2 100 -1.24 5.55 4.0 1.46 40 25 7 1 150 0.87 7.32 4.04 2.05 35 25 6 -1 200 1.96 9.13 4.3 2.41 35 25 5 -3 250 3.34 11.5 4.37 2.64 30 20 4 -4 300 4.15 13.4 4.59 2.91 25 29 6 -6 350 5.58 15.3 4.74 2.95 30 20 3 -8 400 6.7 16.6 4.7 3.06 30 25 3 -9 450 7.82 19.3 4.8 3.28 30 20 1 -11 500 8.95 21.4 4.76 3.36 30 20 2 -13 550 10.1 23.4 4.8 3.2 30 20 -1 -15 600 11.2 25.5 4.75 3.4 30 20 0 -16 Table 2: Gain characteristics vs. different input signal power for SP & DP-RFA Input signal Gain flatted Flatted borders nm power dBm within 3-dB SP DP SP DP -30 30 nm 29 nm 1565-1595 1565-1594 -6 30 nm 25 nm 1565-1595 1565-1590 0 20 nm 15 nm 1570-1590 1570-1585 Input signal Average Gain Average NF dB power dBm level dB SP DP SP DP -30 11.19 13.44 4.75 2.91 -6 11.09 11.44 7.34 5.88 0 9.5 8.5 9.41 3.32
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|Author:||Abass, A.K.; Adnan, Salah Aldeen; Hadi, Ban K.; Salih, Mohammed A.|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Jul 1, 2017|
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