Rayleigh-Based Raman Fiber Laser With Passive Erbium-Doped Fiber for Secondary Pumping Effect in Remote L-Band Erbium-Doped Fiber Amplifier

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1 University of Malaya From the SelectedWorks of Faisal Rafiq Mahamd Adikan June, 2012 With Passive Erbium-Doped Fiber for Secondary Pumping Effect in Remote L-Band Erbium-Doped Fiber Amplifier Faisal Rafiq Mahamd Adikan, University of Malaya Available at:

2 With Passive Erbium-Doped Fiber for Secondary Pumping Effect in Remote L-Band Erbium-Doped Fiber Amplifier Volume 4, Number 3, June 2012 M. H. Abu Bakar F. R. Mahamd Adikan M. A. Mahdi DOI: /JPHOT /$ IEEE

3 With Passive Erbium-Doped Fiber for Secondary Pumping Effect in Remote L-Band Erbium-Doped Fiber Amplifier M. H. Abu Bakar, 1;2 F. R. Mahamd Adikan, 2 and M. A. Mahdi 1 1 Wireless and Photonics Networks Research Center, Faculty of Engineering, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia 2 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia DOI: /JPHOT /$31.00 Ó2012 IEEE Manuscript received May 7, 2012; revised May 29, 2012; accepted May 31, Date of publication June 6, 2012; date of current version June 19, This work is partly funded by the Ministry of Higher Education and Universiti Putra Malaysia under research grant RU and Graduate Research Fellowship scheme. The authors would also like to acknowledge financial supports from University of Malaya under High Impact Research Grant A Corresponding author: M. H. Abu Bakar ( mhab@ieee.org). Abstract: This work focuses on a secondary pumping scheme for remote L-band erbiumdoped fiber amplifier (R-EDFA) generated by integrating Rayleigh-based ultralong Raman fiber laser (ULRFL) with a section of passive erbium-doped fiber (EDF). A selective wavelength reflector combined with Rayleigh feedback in the transmission span forms the ULRFL cavity while the passive EDF deployed at the end of the span converts the ULRFL into a secondary C-band pump. Optimum ULRFL wavelength is determined by tuning the selective wavelength reflector across the C-band region. Combination of R-EDFA gain, Raman amplification, and passive EDF provides gain improvement as high as 15.5 db over conventionally pumped R-EDFA with minimal optical signal-to-noise ratio penalty. Index Terms: Laser amplifiers, fiber lasers, fiber nonlinear optics. 1. Introduction The introduction of L-band transmission window has provided several advantages over the C-band window. Apart from the additional bandwidth it provides, the L-band window is also free of zero dispersion wavelength in older dispersion-shifted fiber (1550 nm) thus reducing the effect of fourwave mixing which is prevalent in dense wavelength division multiplexing (DWDM) transmission [1]. The application of L-band window is further supported by the fact that the emission spectrum of erbium extends to that region and combined with its reasonably flat gain profile, made erbiumdoped fiber amplifier (EDFA) an obvious choice for amplification of L-band signals. The downside to the use of EDFA in L-band is the low gain coefficient attributed to the location of L-band at the tail end of the emission spectrum. This necessitates the use of longer erbium-doped fiber (EDF) length to produce substantial gain in the region. Consequently, several enhancement schemes were proposed to maximize the pumping efficiency in discrete L-band EDFA in order to cater to the longer EDF length. The secondary pumping scheme, which utilizes internally or externally generated light source as an additional pump source has been reported in several Vol. 4, No. 3, June 2012 Page 1042

4 Fig. 1. (a) The proposed experimental setup. Single-mode fiber (SMF), erbium-doped fiber (EDF), Raman pump unit (RPU), wavelength-division multiplexing (WDM), tunable bandpass filter (TBF), and isolator (ISO). (b) Absorption spectrum of OFS-LSL EDF used in this experiment. articles [2], [3]. Feedback of backward ASE was able to produce flat gain of 24 db [2] while 25 db gain was attained through ring-laser feedback [3]. In remote EDFA (R-EDFA), pump power is more critical due to the loss suffered during the long distance pump delivery. Unfortunately, there are very few researches done on enhancing L-band R-EDFA with the typical approach in this issue is to employ R-EDFA/Raman hybrid amplifier [4]. Attempts were made to translate the secondary pumping scheme to the R-EDFA by using stimulated Raman scattering or SRS as the secondary pump which saw gain reduction noted due to amplification of SRS in the R-EDFA [5] while the other study was impaired by the lack of Raman amplification in L-band [6]. Recent work on random lasing in ultralong cavity provides an interesting avenue for supplementary pump source in R-EDFA [7] [11]. Here, we propose a new secondary pumping scheme for EDFA/Raman hybrid amplifier by utilizing ultralong Raman fiber laser (ULRFL) based on Rayleigh feedback that is generated in the pump delivery cum transmission line. The ULRFL cavity is formed by incorporating a physical reflector at one end while Rayleigh backscattering (RBS) is utilized as virtual distributed mirror at the other end. A section of unpumped or passive EDF is placed at the end of the span to convert the ULRFL into a C-band pump for the amplification of L-band signal [12] [14]. Optimization of the ULRFL wavelength is also performed in order to produce the best gain and noise performance at the output of the amplifier span. This work enhances typical R-EDFA/ Raman hybrid approach by enabling the utilization of SRS not only for Raman amplification but also as an additional pump source. Vol. 4, No. 3, June 2012 Page 1043

5 Fig. 2. R-EDFA gain and noise figure with variation in wavelength. 2. Experimental Setup Fig. 1(a) depicts the experimental setup of the amplifier span, which was pumped by a 1455 nm Raman pump unit (RPU) at its maximum power of 1580 mw. The amplifier span configuration is based largely on typical forward-pumped R-EDFA/Raman hybrid amplifier [5], [6]. The pump power was delivered to the R-EDFA by first going through the circulator and then over the 41 km Truewave-RS single-mode fiber (SMF) that also functioned as the transmission line. The attenuation coefficient of this SMF at 1455 nm is 0.27 db/km. A C/L coupler was employed at the end of the SMF to allow both 1455 nm pump and the C-band SRS to pass through its pump leg. A 1480/1550 wavelength-division multiplexing (WDM) coupler, WDM1 was attached to the end of the pump leg to separate the 1455 nm pump light from the C-band SRS. At point A, the 1455 nm pump light was measured using optical power meter (OPM) to be around 29 mw. The pump then continued to propagate to another 1480/1550 WDM coupler, WDM2 that was used to couple the pump light to the L-band signal and into the R-EDFA. Isolator, ISO1 was placed at the 1550 leg of WDM2 to block backward ASE from the R-EDFA from reaching the tunable laser source (TLS). The gain medium was an OFS-LSL EDF with a length of 23 meters. The absorption spectrum of the EDF is shown in Fig. 1(b), with absorption coefficient at 1455 nm of around 3 db/m. C-band absorption coefficient ranges from about 16 db/m at the peak wavelength of 1530 nm to about 4 db/m at 1565 nm. Another isolator, ISO2 was also deployed at the EDF output to reduce backscattering and to inhibit the propagation of SRS through the reflect leg of C/L coupler. The use of C/L coupler allowed only light in the L-band region to go back into the SMF thus minimizing broadband noise from C-band ASE. At WDM1, a tunable bandpass filter (TBF) with 3-dB bandwidth of 0.2 nm was integrated with an optical mirror to realize a selective wavelength reflector to reflect specific wavelength portion of the SRS to become seed signal for the generation of ULRFL. The L-band signal coming from the R-EDFA then went back through the SMF and both signal and ULRFL were routed by the circulator to the section of 30 m passive EDF that was of similar type to the R-EDFA gain medium. The gain and noise performances were evaluated at point B, C and D using optical spectrum analyzer (OSA). 3. R-EDFA Characteristics The performance of the R-EDFA measured at point B is shown in Fig. 2. The preliminary work is performed only with 0 dbm input signal as it is considered in this study to represent multiple DWDM signals of lower power. Average gain of only 5.5 db is observed due to the low inversion level caused by the limited pump power. Resultantly, higher noise figure is also noted around 5.5 to 8 db. The noise figure curve is slanted forward due to the decreasing ratio of emission to absorption in that area [15]. Vol. 4, No. 3, June 2012 Page 1044

6 Fig. 3. (a) The ULRFL spectra and (b) the power and spectral width at different ULRFL wavelengths. Interpolated curve lines are added with blue and red lines representing 3-dB and 20-dB width, respectively. (c) Interpolated curve line showing power development of ULRFL at 1555 nm. 4. Generation of ULRFL The ULRFL generated in the ultralong cavity was observed at point C. The integration of a selective wavelength reflector facilitates the generation of ULRFL, as longer cavity length is needed to produce enough accumulated RBS to induce oscillation without any physical reflector [11]. In order to have ULRFL usable for pumping the passive EDF, the wavelength has to be in the absorption spectrum of erbium. The ULRFL can then be used to excite the passive EDF and produce another round of amplification for the L-band signal. Therefore, the selective wavelength reflector also allows the ULRFL wavelength to be tuned along the absorption wavelength range simply by controlling the TBF. Fig. 3(a) illustrates the laser spectrum as the TBF was tuned from 1545 to 1565 nm at 5 nm step. The selective wavelength reflector reflects a narrow portion of the SRS back into the SMF. The reflected light becomes a seed signal that is amplified by SRS in the fiber. As the amplified light propagates in the SMF, it is backreflected by RBS in the fiber, which acts as a virtual distributed mirror. The backreflected light is then amplified by SRS as well. This eventually creates the oscillation effect needed to induce lasing in the ultralong cavity. Through this method, the readily available SRS can be exploited to generate the ULRFL thus eliminating the need for Vol. 4, No. 3, June 2012 Page 1045

7 Fig. 4. SBS effect observed during the experiment, the signal wavelength is set at 1570 nm. Fig. 5. (a) Transmission gain and (b) OSNR before passive EDF at different ULRFL wavelengths. additional physical pump unit. The ULRFL is known to have a broadened spectrum that is contributed by the random RBS effect, which leads to interactions between multiple stochastic modes contributed by four-wave mixing [10]. Due to the shape of the Raman gain profile, higher output power is attained as the laser is tuned nearer to the peak Raman gain shift at 1555 nm [Fig. 3(b)]. Laser output power of 86.3 mw is measured at 1555 nm while at 1545 nm, only 45 mw is obtained, which is influenced by the proximity to the Raman gain peak. The 3-dB spectral width of the laser at all wavelengths is kept around 0.2 nm as the spectral development of the laser is constrained by the 3-dB bandwidth of the physical reflector [7]. However, broader 20-dB bandwidth is observed with higher laser output power due to broader spectral wings attributed to higher interactions between the laser modes [9]. Fig. 3(c) presents the power development of the 1555 nm ULRFL with different pump power, which shows the lasing threshold achieved around 1165 mw. 5. Integration of Passive EDF The cumulative gain of the whole R-EDFA span is referred to as the transmission gain. The transmission gain prior to the passive EDF is measured at point C. Unfortunately, due to the combination of R-EDFA gain and Raman amplification in the transmission line, stimulated Brillouin scattering (SBS) effect can be observed at this point (Fig. 4) with OSA resolution bandwidth of 0.1 nm. As a result, part of the original signal power is siphoned to the SBS Stokes signal and affected the accuracy of the gain and noise measurements. In typical DWDM transmission, the total input power is composed of smaller individual signals therefore the effect of SBS is not significant. For single channel input, it is possible to suppress SBS effect through several techniques such as phase modulation [16]. Fig. 5 depicts the transmission gain and optical signal-to-noise ratio (OSNR) measured at point C. SBS-affected range is found from 1570 to 1582 nm at all ULRFL wavelengths due to the close proximity to the second Raman gain shift at 1567 nm. Smaller SBS effect can also be seen around Vol. 4, No. 3, June 2012 Page 1046

8 Fig. 6. Raman distributed gain at different ULRFL wavelengths. Fig. 7. (a) Transmission gain and (b) OSNR after passive EDF at different ULRFL wavelengths nm due to the third Raman gain shift. However, the transmission gain with the 1555 nm ULRFL at the third Raman gain shift region exhibits lesser SBS effect as the Raman gain is more concentrated toward the generation of the 1555 nm laser. Overall, the gain values at SBSunaffected areas hovers between 3.6 to 4.3 db at the shorter wavelengths and declines to around 0 to 0.6 db at 1605 nm. The gain shape conforms to the typical Raman gain profile in SMF. Higher gain values at the shorter wavelengths are attained with ULRFL nearer to the peak Raman gain since the gain profile is compressed forward toward the laser. However, this behavior contributes to lower gain values at the end of the L-band window. The OSNR presented in Fig. 5(b) demonstrates worse values as the ULRFL wavelength is varied nearer to the L-band region. This is attributed to higher spontaneous emission in the amplification bandwidth due to the shift of the Raman gain profile to accommodate the ULRFL generation. Nevertheless, the lowest OSNR at SBS-unaffected region found with 1565 nm ULRFL is still more than 44 db. Although the input signal is located away from the peak Raman shift wavelength, it is still able to benefit from Raman amplification in the SMF that helps offset some of the fiber loss. Raman distributed gain in the span is obtained from the difference between the measured output power at point C and the calculated unamplified output power with total SMF attenuation of 8.5 db. In Fig. 6, all ULRFL wavelengths recorded values more than 6 db from 1583 to 1598 nm that decreases afterward to less than 4 db at 1605 nm. Fig. 7(a) shows the transmission gain after passive EDF. Better values are attained with ULRFL at 1550 to 1560 nm, which produces average gain of more than 15 db. The absorption and emission properties of erbium are integral toward the power conversion efficiency and the suppression of backward ASE at the EDF input, which ultimately translate to better gain performance [17]. In addition, the suppression of backward ASE at the EDF input when using pump in C-band wavelengths has been reported to contribute better power conversion efficiency compared to 1480-nm band pump by more than 2 times [17]. In this particular experiment, the gain performance is also attributed to the Vol. 4, No. 3, June 2012 Page 1047

9 Fig. 8. (a) Transmission gain and (b) OSNR after passive EDF for 1550 to 1560 nm ULRFL. Fig. 9. (a) Transmission gain and (b) OSNR after passive EDF with 1554 nm ULRFL at different input signal levels. ULRFL power in 1550 to 1560 nm range that is substantially larger and resulted in more pump power for the passive EDF. The OSNR however, is better with ULRFL at shorter wavelengths [Fig. 7(b)]. This is contributed by the lower ground state absorption in that area [17]. The minimum OSNR with 1545 nm ULRFL is 43 db in comparison to 37 db that is acquired with ULRFL at 1565 nm. Based on the outcome of the preliminary assessment, the experiment is repeated with the ULRFL wavelength tuned at 1 nm step from 1550 to 1560 nm in order to ascertain the exact optimum ULRFL wavelength range. It can be observed in Fig. 8(a) that the lowest gain performance is obtained at 1560 nm ULRFL while the best performance can be found with ULRFL between 1553 to 1557 nm with very little difference between them. In terms of OSNR, better values are again noted in Fig. 8(b) with ULRFL farther from the L-band amplification bandwidth. Nevertheless, the OSNR is still more than 40 db even at the worst OSNR performance (1560 nm ULRFL). Therefore, the gain performance is used to determine the optimum ULRFL wavelength range, which is from 1553 to 1557 nm. A detailed evaluation is then performed at various signal powers with the TBF fixed at 1554 nm. Gain value as high as 45.3 db is obtained at 1570 nm with 30 dbm input [Fig. 9(a)]. The gain value declines at higher signal power with 5 dbm input giving out an average gain of 20 db. The lowest OSNR of about 23 db is from 30 dbm signal, which is expected due to the low input OSNR. With 5 dbm signal, OSNR around 40 db is acquired. The effect of SBS is not noticeable at the third Raman gain shift for 5 to 30 dbm signal as the Raman gain in that region is lower and concentrated on the ULRFL at 1554 nm. The use of passive EDF section contributes to the good gain flatness observed for 5 to 15 dbm signals as the gain at the longer wavelength range is augmented by the amplification in the passive EDF. The performance of the amplifier span is then compared to a conventionally pumped R-EDFA that has similar configuration to the proposed amplifier architecture minus the selective wavelength reflector and the passive EDF section. In Fig. 10(a), gain improvement as high as 15.5 db is noted at the longer wavelength region for smaller signal levels. For larger signal powers, the gain is Vol. 4, No. 3, June 2012 Page 1048

10 Fig. 10. (a) Transmission gain improvement and (b) OSNR penalty compared to conventionally pumped R-EDFA at different input signal levels. enhanced by as high as 13 db. The higher gain improvements observed at the longer wavelengths allow the amplifier to compensate for the lower Raman gain in that area. Lower improvement is observed at shorter wavelengths for all signal powers and with 30 dbm input, the lowest gain improvement is 6 db at 1572 nm. This behavior is attributed to the use of ULRFL as C-band pump, which favors the longer wavelength region. The OSNR penalty is minimal with the worst value of 5.5 db found with 0 dbm input. Lower OSNR penalty is noted for smaller signal powers with the value held to less than 1 db. 6. Conclusion A new pumping scheme for R-EDFA has been demonstrated in this work, which combines conventional R-EDFA and Raman amplifications with a secondary pumping scheme. By using a single RPU, a ULRFL is generated as secondary pump and is integrated with a passive EDF section. Together, they provide additional amplification that translates to significant gain improvement with minimal OSNR penalty over conventional pumping scheme. By tuning the selective wavelength reflector across the C-band, the optimum wavelength range is able to be determined, which takes into account the interrelation between absorption and emission properties of erbium as well as the ULRFL power. This method provides additional amplification on top of typical R-EDFA/Raman hybrid configuration. Moreover, since this pumping scheme utilizes C-band SRS, it can be a viable solution for transmission systems that lack L-band optimized Raman amplification. References [1] M. Jinno, T. Sakamoto, K. Jun-Ich, S. Aisawa, K. Oda, M. Fukui, H. Ono, M. Yamada, and K. Oguchi, B1580-nm band, equally spaced 8 10 Gb/s WDM channel transmission over 360 km (3 120 km) of dispersion-shifted fiber avoiding FWM impairment,[ IEEE Photon. Technol. Lett., vol. 10, no. 3, pp , Mar [2] M. A. Mahdi and H. Ahmad, BGain enhanced L-band Er 3þ -doped fiber amplifier utilizing unwanted backward ASE,[ IEEE Photon. Technol. Lett., vol. 13, no. 10, pp , Oct [3] M. A. Mahdi and H. Ahmad, BLong-wavelength-bandEr 3þ -doped fiber amplifier incorporating a ring-laser as a seed signal generator,[ IEEE J. Sel. Topics Quantum Electron., vol. 7, no. 1, pp , Jan./Feb [4] K. Hogari, K. Toge, N. Yoshizawa, and I. Sankawa, BLow-loss submarine optical fibre cable for repeaterless submarine transmission system employing remotely pumped EDF and distributed Raman amplification,[ Electron. Lett., vol. 39, no. 15, pp , Jul [5] M. H. Abu Bakar, M. A. Mahdi, M. Mokhtar, A. F. Abas, and N. M. Yusoff, BInvestigation on the effect of stimulated Raman scattering in remotely-pumped L-band erbium-doped fiber amplifier,[ Laser Phys. Lett., vol. 6, no. 6, pp , Aug [6] M. H. Abu Bakar, A. F. Abas, M. Mokhtar, H. Mohamad, and M. A. Mahdi, BUtilization of stimulated Raman Scattering as secondary pump on hybrid remotely-pump l-band Raman/erbium-doped fiber amplifier,[ Laser Phys., vol. 21, no. 4, pp , Apr [7] S. A. Babin, A. E. El-Taher, P. Harper, D. V. Churkin, S. I. Kablukov, E. V. Podivilov, and S. K. Turitsyn, BUltra-long Raman laser with a feedback based on the Rayleigh scattering,[ in Proc. Eur. Conf. CLEO Eur.-EQEC, 2009, p. 1. [8] A. R. Sarmani, M. H. Abu Bakar, F. R. Mahamd Adikan, and M. A. Mahdi, BLaser parameter variations in a Rayleigh scattering-based Raman fiber laser with single fiber Bragg grating reflector,[ IEEE Photon. J., vol. 4, no. 2, pp , Apr Vol. 4, No. 3, June 2012 Page 1049

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