Widely flatness gain bandwidth with double pass parallel hybrid fiber amplifier

A Widely flatness gainbandwidth with double pass parallel hybrid fiber amplifier is experimentally demonstrated in this study. The proposed design combines serial erbium–Raman fiber amplifier in one branch and Raman fiber amplifier in the second branch. Multiple Raman pump units with a maximum power of 800 mW (250 mW of 1410 nm, 225 mW of 1480 nm, and 325 mW of 1495 nm) are utilized. Pump recycling technique is applied to achieve acceptable pumping efficiency. A maximum flatness gain bandwidth of 80 nm (1525–1605 nm) and average gain level of 22.5 dB are obtained at a small input signal power of -25\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-25$$\end{document} dBm and optimum pump power values. While, a wider flatness gain of 90 nm (1520–1610 nm) and average gain level of 11.5 dB are achieved at a large input signal power of -5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-5$$\end{document} dBm.


Introduction
Raman fiber (RFA) and erbium-doped fiber (EDFA)amplifiers have been combined as a hybrid-fiber amplifier (HFA) by many researchers. The new HFA demonstrates enhanced amplifier performance in terms of large fiber span, wide gain flatness bandwidth, and improved pump efficiency (Ali et al. 2013(Ali et al. , 2015Abdullah et al. 2018;Kumar and Goyal 2018). A serial combination was performed first between the two amplifiers to form a serial hybrid-fiber amplifier (SHFA) (Masuda et al. 1998;Seo et al. 2007;Almukhtar et al. 2019Almukhtar et al. , 2020. SHFA generally has one single path with two amplification stages. Input signal is inserted into the first amplifier for the first amplification and then amplified signal is inserted as an input signal into the second amplifier. Saturation will likely occur and be effective, especially if the peak bandwidth of both amplifiers includes the same signal wavelength, due to the cascading process. Although the SHFA scheme can achieve high gain level, narrow flatness gain bandwidth and early gain saturation that led to limit the input signal power range can also occur. A flatness gain bandwidth of 76 nm (1531.5-1607.5 nm) was recorded. Double-pass SHFA with a maximum pump power of 1.4 W obtained aflatness gain bandwidth of 70 nm (1530-1600 nm) (Masuda et al. 1998). Double-pass SHFA with a maximum pump power of 1.4 W was presented by Seo et al. (2007). Flatness gain bandwidth of 70 nm (1530-1600 nm) was obtained. Complex fiber Bragg grating arrays introduced (Liaw and Huang 2008) into the SHFA configuration obtained an enhanced gain variation of 65 nm (1530-1595 nm) with a small gain variation of 0.2 dB. A partial double pass EDFA with hybrid gain medium was proposed by Almukhtar et al. (2019). Up to 17.2 dB gain level that uned over flat gain bandwidth of 55 nm from 1545 to 1600 nm was presented at large input signal power of −10 dBm. However, no flatness gain bandwidth was recorded at small input signal power of −30 dBm and high noise fiugere of 10.6 dB was recrded. At same large input signal power of −10 dBm, a gain level of 19 dB and noise figure of 9.3 dB were achieved by Almukhtar et al. (2020). However, flatness gain bandwidth was reduced to 45 nm (1555-1600) and no study was conducted at low input signal power. Parallel HFA (PHFA) was proposed by many researchers to avoid gain saturation and limited gain dynamic range caused by the cascading process in SHFA Liaw et al. , 2009Ali et al. 2014Ali et al. , 2020. The cascading effect was absent, but the signal was divided into two different individual amplifiers for re-accumulation in the PHFA. The input signal was divided according to its wavelength (C-band to EDFA and L-band to RFA) in previous PHFA configurations . A single Raman pump power of 1480 nm with a percentage ratio of 29:1 was delivered to the RFA and EDFA. The C-and L-bands demonstrated an average gain of 14 and 13.6 dB, respectively, . Which enhanced the gain level variation from 9.8 to 0.5 dB. However, dividing the signal according to the wavelength (C-band for EDFA and L-band for RFA) can present two different power saturation values ). EDFA typically generates faster gain saturation compared with RFA. Therefore, a large input signal power can induce degradation in the EDFA branch. The input signal power was divided into two PHFA branches instead of dividing the input signal wavelength to prevent this limitation . Gain control technique is used in PHFA to control the amount of input signal power to each amplifier. A wide flatness gain bandwidth of 60 nm (1530-1590 nm) and an average gain level of 14 dB were observed using 800 mW of Raman pump power at 1480 nm . The implementation of double-pass PHFA achieved an enhancement of 38.5% and 18.7% in gain level and pump power, respectively ). However, two major limitations still exist, namely, phase mismatch between amplified signals after these two branches due to the long signal propagation in DCF and possible absence of dispersion compensation in EDF. Another DCF fiber is connected serially to the EDFA branch in PHFA to overcome these two limitations Abass et al. 2018b;Ali et al. 2019). The PHFA design with multiple Raman pump power units (800 mW of 1480 nm and 300 mW of 1495 nm) was reported by Abass et al. (2018a). However, a gain level of only 13.38 dB and a flatness gain bandwidth of 62.5 nm were achieved although high pump power was used. The gain control technique was proposed in the PHFA setup with a maximum Raman pump power of 1.4 W for multiple pump power units (Abass et al. 2018b). The flatness gain bandwidth and the average gain slightly increased to 65 nm and 16 dB, respectively. Thirty-two input channels were examined in the PHFA design under the same parameters presented by (Abass et al. 2018b) and the system showed acceptable amplification performance in terms of dispersion issue and phase matching . Low and limited flatness gain levels were the main degradation products in previous studies. A dual stage double pass PHFA was reported by Al . wide flatness gain bandwidth of 80 nm (1525-1605) was achieved at large input signal power of −10 dBm but in the expense of gain level that is reletively low of about 12 dB. In addition, the flatness gain bandwidth was reduced to 30 nm (1535-1565 nm). A double-pass PHFA is experimentally demonstrated in this study. Only 800 mW of Raman pump power (250 mW for 1410 nm, 225 mW for 1480 nm, and 325 for 1495 nm) is used.Such relatively low pump power of 800 mW was utilized due to the using of pump reflection that make double pass pump technique and reduced the required pump power inside the amplifier. A broad flatness gain bandwidth of 80 nm (1525-1605 nm) with an average gain of 22.5 dB are presented at a small input signal power of −25 dBm, whereas a wider flatness gain of 90 nm (1520-1610 nm) with an average gain of 16.5 dB and 11.6 dB is achieved for a large input signal powers of −10 and −5 dBm respectively. Figure 1 illustrates the experimental setup of the proposed double-pass PHFA. Tunable laser source (TLS) 150 nm bandwidth (1480-1630 nm) with a maximum power of 14 dBm was used as the input signal power. Variable optical attenuator was connected directly after TLS to control the input signal power range. A three-port circulator (Cir1) was utilized to measure the amplified signal at port 3 and to avoid any reflection back to the TLS for protection issue. The input signal was divided equally using 50:50 coupler and then directed to amplifier branches. EDFA, which consists of 3 m of normal EDF concentration, was connected serially to 7 km of DCF (dispersion parameter of −110 ps/nm/km , total loss of 4.4 dB, effective area of 18.5 μm 2 , and a nonlinear coefficient of 14.5 × 10 −10 W −1 ) and pumped by the residual pump power of 1480 nm. DCF was pumped by two Raman pump units with a maximum power of 575 mW (250 mW of 1410 nm and 325 mW of 1495 nm) as RFA. An optical Cir2 was connected to the end of the first branch to achieve double-pass amplification in this branch. Only RFA, which consists of 7 km DCF with the same specifications as the first DCF, was pumped by 225 mw of 1480 nm and inserted in the second amplifier branch. Another circulator (Cir3) was connected to achieve double-pass amplification in the second branch and recycle the signal back to RFA. A wave selctive coupler (WSC1 and WSC2) was used to extract the pump power. After WSC2 a pump reflecter was used to refelct the residual pump power into the DCF in order to achieve good pump efficiency and to reduced the required pump power within the setup. wave An optical spectrum analyzer (OSA) was utilized to record amplified optical signals.

Theoretical background for gain and noise figure
the proposed hybrid fiber amplifier is a result of combining of double pass serial EDFA, RFA in the first branch and RFA in the second branch as dipected in Fig. 1. The total gain of the proposed amplifier can be expressed as follows: where P in (mW) is the input signal power, P out (mW) is the output signal power, (G EDFA ) (DP) and (G RFA ) (DP) are the double pass gain factor of the Erbium and Raman, respectively, C : coupler losses, Cir : circulator losses. Therefore, the gain factor of the proposed douplepass PHFA can be written as a function of as follows: In addition, G EDFA and G RFA were calculated in Becker et al. (1999) and Agrawal (2005) respectively as the output signal power ratio to the input signal power: where P (0) is the inserted signal power to both of EDFA and RFA and P (L) represents the amplified signal after these two amplifiers.
On the other side, the noise figure of the EDFA was obtained by Becker et al. (1999): where P ASE (E) : EDFA noise, h: Planck's constant, : input signal frequency in Hz, B o : ASE bandwidth in Hz while the noise figure in RFA was estimated by Bristiel et al. (2004) as follows: where P ASE (R) is the noise generated in Raman amplifier. Corresponding to Fig. 1, the proposed amplifier has serial of Erbium-Raman amplifier in the first branch. In this conext, the noise figure of the serial hybrid fiber amplifier was calculated by Becker et al. (1999) and Bristiel et al. (2004): Therefore, the noise figure of the first amplifier branch can be written as follows: While the noise figure for the second amplifier branch is: Finally the total hybrid noise figure of the proposed ampilifier can be obtianed by add Eqs. (9) and (10):

Results and discussions
Amplified spontaneous emission (ASE) of the proposed double-pass PHFA in the absence of the input signal power is shown in Fig. 2. ASE is important in any amplifier design because it generally reflects the obtained amplification band. Both pump power values of 1410 and 1495 nm were optimized at 250 and 325 mW, respectively. Optimization is then performed for the pump power of 1480 nm to achieve a wide flatness spectrum. The pump power of 1480 nm varied from 150 to 325 mW with a step of 50 mW, as shown in Fig. 2a. The ASE spectrum clearly showed better flatness of 225 mW at a pump power of 1480 nm that can achieve a wide gain flatness of the proposed amplifier. The effect of each pump power unit on the ASE spectrum is illustrated in Fig. 2b. The spectrum recorded at optimum pump power values of 250, 325, and 225 mW corresponded to 1410, 1495, and 1480 nm, respectively. Making 1410 nm off, the contribution gain at the 1610 nm region was reduced, while the first-order Raman gain at 1595 nm was highly exaggerated by making 1495 nm off. By comparison, all the spectra were affected, especially in the C-band region (1520-1570 nm),when 1480 nm was off due to the absorption peak in EDF. The effect of EDF minimized and the ASE spectrum showed poor reduction when the distance from this region increased.
The gain spectrum and the noise figure of the proposed double-pass PHFA is shown in Fig. 3. The results were obtained at the optimum pump power values of 250, 325, and 225 mW for 1410, 1495, and 1480 nm, respectively. The gain spectrum was recorded within 110 nm (1520-1630 nm). A flatness gain bandwidth of 80 nm (1525-1605 nm) demonstrated a maximum gain variation of 3 dB at a small input signal power of −25 dBm, as shown in Fig. 3a.
Several Raman amplification peaks of 1510 (first-order Raman peak gain produced by a pump power of 1410 nm), 1595 (first-order Raman peak gain provided by a pump power of 1495 nm), and 1580 (first-order Raman peak gain released by a pump power of 1480 nm) and the contribution gain of 1610 nm (tail gain related to a pump power of 1410 nm) promoted the EDFA peak gain (1530-1570 nm) to achieve a wide flatness gain bandwidth. Both RFA and EDFA were still unsaturated when a low signal power was used. Therefore, the EDFA gain can be reduced by optimizing the pump power of 1480 nm to achieve satisfactory amplifier performance. In addition, an acceptable noise figure value of nearly 5 dB was recorded within the entire gain bandwidth likely due to the residual pump power extraction. The gain spectrum was examined at a large input signal power of −5 dBm to demonstrate the effect of the gain saturation, as shown in Fig. 3b. A high flatness gain bandwidth of 90 nm (1525-1615 nm) was obtained, while a lower gain level of 11.5 dB was recorded on the other side. The degradation in the average gain level was due to the deep saturation in EDFA while saturating Raman amplification peaks of 1510, 1595, and 1610 nm because of their serial connection. A high noise figure of 9 dB was recorded within the gain bandwidth due to the EDFA and  Table 1 summarizes the results of the performance evaluation of our work and previous studies. Figure 4a-f show the output spectrum of the amplified signal at different wavelengths (1525-1605 nm) measured via an OSA and at a fixed signal power of −25 dB. The optical signal-to-noise ratio (OSNR) recorded for both input and amplified signal verified the gain and noise figure illustrated in Fig. 3a.
Experimental data for the gain dynamic range used as a function of the input signal power are shown in Fig. 5. The gain spectrum was recorded as a function of the input signal power variation from −25 to −5 dBm with a step of 5 dBm. The gain dynamic range is the input signal power at which the maximum gain variation is 3 dB. The gain dynamic range was examined at different signal wavelengths in the flatness gain bandwidth period to illustrate the proposed amplifier performance. The gain was clearly saturated at an input signal power of −14 dBm for input signal wavelengths of 1525, 1540, and 1560 nm (Fig. 5a-c) due to the higher effect of EDFA in such signal wavelengths. By comparison, a wider gain dynamic range was observed and the gain was saturated at an input signal power of −11 dBm for a signal wavelength of 1580 nm (Fig. 5d)  the effect of RFA in such a wavelength in the second amplifier branch. In addition, the EDFA gain provided in this branch has no tail. Although the proposed amplifier showed a narrow gain dynamic range at signal wavelengths of 1595 and 1605 nm (Fig. 5e, f) with an effective RFA in the first amplifier branch, the gain was saturated at −14 dBm due to the tail gain of EDFA that added to the RFA gain.

Conclusion
An experimental examination of double-pass PHFA related to flatness gain bandwidth, noise figure, and input dynamic range is presented in this study. The proposed structure is tested under small and large input signal power regimes to demonstrate its performance. A suitable wavelength of Raman pump unit is determined in the obtained wide flatness gain. Tunable flatness gain bandwidths of 80 and 90 nm are achieved for small and large input signal power values, respectively, when an optimum Raman pump power of 800 mW is used. In addition, an acceptable gain dynamic range is obtained because EDFA and RFA amplifiers demonstrate different peak gain wavelengths.