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Characteristics of the single-wavelength ring cavity Brillouin fiber laser operated in L-band region
Optik 147 (2017) 43–51
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Original research article
Characteristics of the single-wavelength ring cavity Brillouin fiber laser operated in L-band region M.S.A. Hurera a,b , N.A.M. Ahmad Hambali a,b,∗ , M.M. Shahimin c , M.H.A. Wahid a,b a
School of Microelectronic Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra, 02600 Arau, Perlis, Malaysia Semiconductor Photonics & Integrated Lightwave Systems (SPILS), School of Microelectronic Engineering, Universiti Malaysia Perlis, Pauh Putra, 02600 Arau, Perlis, Malaysia c Department of Electrical and Electronic Engineering, Faculty of Engineering, National Defence University of Malaysia (UPNM), Kem Sungai Besi, 5700 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 28 April 2017 Received in revised form 3 August 2017 Accepted 8 August 2017 Keywords: Single-wavelength Brillouin fiber laser L-band region Stimulated brillouin scattering Osnr Flatness
a b s t r a c t We experimentally demonstrated the characteristics of a single-wavelength Brillouin fiber laser that operated in the L-band region. The 4 km of a single mode fiber plays a significant role as a nonlinear gain medium to generate stimulated Brillouin scattering effect and causes the frequency shift with ∼ 0.08 nm spacing. At a pump power of 350 mW, a Brillouin pump wavelength of 1580 nm and 50% of output coupling ratio, the Brillouin threshold power at 6.3 mW is obtained. The highest average Brillouin Stokes power of 4.07 mW has been produced at 10 mW of Brillouin pump power at 50% of output coupling ratio. The tuning range of 30 nm is recorded without any unwanted oscillating modes appeared from 1570 nm to 1600 nm. The 70% of the output coupling ratio is found to produce flattened output for BS power and signal to noise ratio. The highest value for optical signal to noise ratio is recorded at 41.36 dB, with 50% of output coupling ratio and at 1600 nm of Brillouin pump wavelength. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Ever since stimulated Brillouin scattering (SBS) effect is discovered, it has become a major research due to its light wave effect on the optical systems. The backward signal propagation is produced by the SBS effect. SBS is a nonlinear effect subsequent from the interaction between intense pump light and acoustic waves in a gain medium. From the interaction, the backward propagating frequency-shifted light and Rayleigh scattering are generated, and inelastic scattering process shifted the frequency downward [1]. Raman scattering and Brillouin scattering are categorized as inelastic scattering of a photon by molecules and large scale, low-frequency phonons respectively that result in changes of energy [2]. The pump and Stokes waves intervention have a tendency to raise the amplitude of the acoustic waves; then the interaction with the pump is emphasized by the Stokes wave. This condition can cause exponential growth and recreate the Stokes wave [2]. SBS are frequently applied in laser amplifiers and oscillators to increase the quality of the laser beam and to shorten the laser pulse width. A single mode fiber (SMF) which plays an important role as a gain medium, has been used as a standard to produce the SBS effect in many research. A laser signal, known as a Brillouin fiber laser (BFL), can be generated from the
∗ Corresponding author at: School of Microelectronic Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra, 02600 Arau, Perlis, Malaysia. E-mail addresses: [email protected], [email protected] (N.A.M.A. Hambali). http://dx.doi.org/10.1016/j.ijleo.2017.08.063 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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Fig. 1. Single wavelength BFL structure that operated in the L-band region.
Brillouin Stokes (BS) effect as a seed signal by using nonlinear Brillouin gain in an optical fiber. In the gain medium, electrostriction works as both; the mechanism leading to a third-order nonlinear optical response and as a coupling mechanism that leads to SBS effect [3,4]. The interaction between intense pump light and acoustic waves in a fiber caused the SBS effect. This nonlinear effect leads to backward propagating frequency-shifted light [5]. For several years, research on BFL has developed a myriad of techniques in order to enhance their characteristics, such as ultra-narrow linewidth [6], tuning range [7], low threshold power [8] and low noise [9]. Two techniques of single-wavelength BFL, utilizing a ring-cavity, has been experimentally demonstrated in [10]. Both experimental setups consist of 25 km long SMF as the gain medium. In this experiment, the output spectrum for the conventional BFL and the proposed BFL structure are compared. From the results, the BS signal power of −0.5 dBm is found, which is 5.7 dBm higher compared with the conventional structure. A ring-cavity BFL by using similar structure and components as proposed in [10] is demonstrated in [11]. A polarization controller (PC) is employed in this structure to adjust the polarization of the pump and the BS signals. Through this experimental work, 3.6 mW of Brillouin threshold power is obtained with 50% of output coupling ratio. At the injected pump power of 40 mW, the BS signal with a signal power of 22 mW is obtained. The performance of ring cavity BFL with the effect of output coupling ratio is reported in [7]. As stated, the investigation of different output coupling ratios performance has been carried out. The output BS power of 7.3 mW and threshold power of 0.9 mW respectively, is obtained by using 90% of output coupling ratio. However, the optical signal to noise ratio (OSNR) and the flatness of BS signal powers are not discussed explicitly in this paper. A compact single wavelength BFL is demonstrated by inserting a Bismuth-based Erbium-doped fiber (Bi-EDF) and photonic crystal fiber (PCF) into the fiber resonator [12]. However, this structure only focusses on 1574 nm and known as a single wavelength Brillouin-Erbium fiber laser (BEFL). However, this structure suffers from the presence of unwanted oscillating modes at peak wavelength around 1574 nm. Furthermore, BFL structure reported in [7,10–12] are focused on C-band region of the optical communication window. A simple tunable L-band multiwavelength Brillouin-Erbium fiber laser (MWBEFL) utilizes a short passive erbium doped fiber (PEDF) as an absorber medium is reported in [13]. At 100 mW of 1480 nm pump power and 4 mW of Brillouin pump (BP) power, a wide tuning range of 24.4 nm has been achieved. High threshold power of 33 mW of first-order Brillouin Stokes is generated from a dual cavity MWBEFL laser in L-band region [14]. Meanwhile, the threshold value of 15.9 mW is used to generated first order BS signal from L-band MWBEFL that utilizing a double-pass Brillouin pump preamplified technique [15]. However, reported paper in [8] and [13–15] only focused on the multi-wavelength generation of BEFL and operated on L-band region of the optical communication window. Furthermore, the selection of band also plays an important role in the communication system. Nowadays, C-band region has extensively had been used for numerous applications. C-band region has been exploited by many industries, particularly for communication, and remote sensing. Thus, the C-band region is almost occupied. However, owing to the overcrowding and susceptibility of the nonlinear effect, the addition of L-band region wavelength is extremely desirable [8]. L-band region are preferred mainly because of its low attenuation [16], its ability to expand single wavelength and its dense wavelength division multiplexing (DWDM) which allowed multiple signals to share a single fiber [16]. The usage of the L-band region is still new. Thus the region is not crowded, and various features of research can be done and utilized. In this paper, we report the experimental demonstration the of a single-wavelength ring cavity BFL structure that operated in L-band region. A short SMF length of 4 km is used as a gain medium. The characteristic of single-wavelength ring cavity BFL structure based on amplified spontaneous emission (ASE), threshold, output BS power and OSNR are studied. A lowest Brillouin threshold power around 6.3 mW is produced at 1580 nm of BP wavelength and 50% of output coupling ratio. The flat amplitude of BS signal and OSNR are achieved at 70% of output coupling ratio, respectively. At 50% of output coupling ratio, higher average OSNR about 37.12 dB is recorded. Meanwhile, at 50% of output coupling ratio, higher average BS peak power value about 4.07 mW is recorded. 1.1. Experimental setup Fig. 1 shows the structure of L-Band single-wavelength ring cavity Brillouin fiber laser (BFL) which consists of tuneable laser sources (TLS), EDFA, an optical circulator, 4 km long of single mode fiber (SMF), an optical coupler and optical spectrum analyzer (OSA). The EDFA components consist of 35 m long of Erbium doped fiber (EDF), wavelength division multiplexing
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Fig. 2. Output ASE spectra profile across L-band region at different values of PP at 100 mW, 250 mW and 350 mW and the BP signal source is disabled.
(WDM) coupler and 980 nm pump power. The optical WDM coupler is used to combine 980 nm pump power (PP) signal and input BP signal from TLS. Meanwhile, Erbium-doped fiber (EDF) act as the gain medium to amplify the BP signal. The EDF type specification uses the 35 m long of Fibercore M-12 (980/125) with optimized for 980 nm LD pumping and 125 m of fiber diameter. The peak absorption for this Fibercore M-12 (980/125) is 19.31 dB/m with 12.64 dB/m absorption at 979 nm. The SMF plays a significant role as a nonlinear gain medium in order to produce BS signal which is backward propagating frequency-shifted light. The SMF has a dispersion value of 16.75 ps/nm/km and attenuation value of 25 dB. The BP signal is provided by utilizing an external cavity TLS with a linewidth of approximately 20 MHz. A maximum BP power of 10 mW is injected from TLS. The TLS provided BP wavelength across L-band region which starts from 1570 nm until 1600 nm. Four set of optical couplers with different output coupling ratio at 50%, 60%, 70% and 80% are used. The output signal is characterized utilizing an OSA with a resolution of 0.015 nm. The injected BP signal from TLS is propagated into EDF, and then it is amplified. The BP signal and PP signal multiplex together through WDM coupler. The BP signal is sent through an optical circulator to the nonlinear gain medium. In the gain medium, the nonlinear stimulated Brillouin scattering (SBS) effect plays the important role to generate the BS signal with constant spacing and narrow linewidth at room temperature [17]. SBS effect is obtained from the interaction between the intense pump light and acoustic waves in a gain medium that resulted in a backward propagating BS signal once the Brillouin threshold is reached [2]. Consequently, a backward propagating BS signal with ∼0.08 nm (10 GHz) spacing is produced. The frequency downshifts of scattered light are attributed to the Doppler shift related to a grating moving at the acoustic velocity [2] Then, BS signal is propagated again through optical circulator and passed to the optical coupler. The coupling ratio withdraws a percentage of signals to output, and another percentage circulates back to another end of SMF. The output signal is then extracted by using OSA.
2. Results and discussion Fig. 2 shows the output ASE spectra profile across L-band region. At first, the BP signal source is disabled in order to study the output spectrum attributed only to the PP signal. The PP injected to the structure is varied to three different values at 100 mW, 250 mW, and 350 mW. Meanwhile, the output signal is extracted from 50% of output coupling ratio. The finding from this experiment demonstrates that the ASE power and noise floor have been raised up with an increment of PP. The most stable ASE profile is produced at 350 mW of PP. It can also be claimed that ASE spectra at 350 mW of PP show a higher gain profile in comparison with 100 mW and 250 mW, respectively. The higher gain profile might lead the single-wavelength BFL structure to produce the BS signal that experiencing higher gain. In order to study the Brillouin threshold power, the graph in Fig. 3 is plotted. For Fig. 3, the BP wavelength is set at 1580 nm. The BP power is varied from its minimum value of 1 mW to the maximum value of 10 mW. Meanwhile, PP is set again at 350 mW. When a single BP signal from TLS is launched into SMF, BS signal grows rapidly through SBS. The BS signal shifted at 0.08 nm spacing from the origin of injected BP signal. In order to achieve threshold condition, BP power is tuned manually from minimum power of 1 mW to maximum power of 10 mW. The Brillouin threshold power is obtained by varying the BP power from its minimum until the BS signal at threshold condition appears. After the BS signal acquires threshold condition, it is backscattered through SMF and increases linearly with an increase in BP power. From Fig. 3, it can be found that the Brillouin threshold power at 6.3 mW is obtained from 50% of output coupling ratio. The Brillouin threshold power can be defined as the input pump power at which the backscattered power initiates to increase rapidly and depleted [18]. Furthermore, a Brillouin threshold power is the most widely used parameter for characterizing SBS by utilizing a number of laser cavity structures as reported in [19,20]. The Brillouin threshold power can be explained by taking into account the interaction between pump and Stokes waves in order to adequately represent the theory of the single wavelength BFL. The single wavelength BFL is based on the amplification
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Fig. 3. The BS signal power as a function of the BP power at 1580 nm of BP wavelength and 50% of output coupling ratio.
of Brillouin gain. It is necessary to comprehend the basic theory between the interactions of pump intensity (Ip ) and the Stokes wave intensity (Is ) in the SBS effect. Relationship between these two interactions is governed by the following equations [2]. dIs = −gB Ip Is + ˛Is dz
(1)
dIp = −gB Ip Is − ˛Ip dz
(2)
where ␣ is the fiber loss for the gain medium and gB is the Brillouin gain coefficient. In case of small Brillouin shift, the fiber loss can be approximated by ␣p ≈ ␣s ␣. In the absence of fiber losses ␣=0, both intensities can be simplified as I s − I p = Constant
(3)
In a condition where the Stokes power is much smaller than the pump power, the term on the right hand side in Eq. (2), −gB Ip Is , can be neglected. At this condition, it can be assumed that the pump power is not depleted. Consequently, Eq. (2) can be summarized as, dIp = −˛Ip dz
(4)
The clarification of Eq. (4) is supported by Eq. (5) [2] below, where Ip (z) and Ip (0)are pump intensities at length z and at z = 0 respectively. Ip (z) = Ip (0) exp(−˛z)
(5)
Thus, Stokes intensity can be presented as Is (0) = Is (L) exp[gB Ip(0)Leff − ˛L]
(6)
In this equation, Leff is the effective length of interaction and L is the fiber length. Due to pump absorption, Leff is slightly less than L. Hereby, Eqs. (5) and (6) can be used to calculate the Brillouin threshold powers which are given by Ps (0) = Ps (L) exp(−˛L) exp[gB Pp(0)Leff /Aeff ]
(7)
P p (L) = P p (0) exp(−˛L)
(8)
and
Meanwhile, the Brillouin threshold power can be estimated by [1], Pth ∼ = 21bAeff /gB Leff
(9)
where, gB is the peak value of the Brillouin gain, Leff is the effective length of interaction and Aeff is the effective core of the fiber. The numerical factor of 21 is estimated according to the specific value of Brillouin gain linewidth. The value of polarization factor b lies among 1 and 2 depending on the relative polarization of pump and Stokes waves [1]. The Brillouin threshold power is then investigated for each BP wavelength at 50% of output coupling ratio. In order to study the characteristics of Brillouin threshold power for each BP wavelength across L-band region, Fig. 4 is plotted. In this study, the BP power and PP are fixed at 10 mW and 350 mW, respectively. While BP wavelength is turned over the entire L-band wavelength at a step of 5 nm, which from 1570 nm to 1600 nm. It is obtained that the point at 1570 nm, 1575 nm and 1580 nm of BP wavelength, the minimum Brillouin threshold power at 6.15 mW, 6.61 mW and 6.3 mW is produced, respectively. This trend is attributed to the highest Brillouin gain which led to sufficient signal power for producing low Brillouin threshold power [21]. Meanwhile, the maximum Brillouin threshold power of 8.12 mW is generated at 1590 nm. Furthermore, at 1585 nm, 1595 nm and 1600 nm, the Brillouin threshold power at 7.80 mW, 7.57 mW and 7.94 mW is produced, respectively. In addition, the average Brillouin threshold power of 7.23 mW for the whole tuning range from 1570 nm to 1600 nm is
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Fig. 4. Brillouin threshold power versus BP wavelength across L-band region.
Fig. 5. A magnified view of output optical spectra of the BS signal at three conditions, below, around and above the threshold condition at 50% of output coupling ratio.
Fig. 6. The output BS powers at 80%, 70%, 60% and 50% of output coupling ratio throughout the L-band region.
recorded. Three conditions of BS signal are observed, which at below the threshold, around the threshold, and above the threshold as shown in Fig. 5. As shown in Fig. 5, the BP power is injected at 3.16 mW, 6.17 mW, and 7.08 mW. Meanwhile, BP wavelength is set at 1570 nm. For below the threshold condition the BS signal power growths slowly and dominated by spontaneous emission process. At this situation, the BS power is lower than the BP power and not depleted. Once the input BP power is increased from minimum power, the BS signal power is increased by the stimulation process when it achieves a power condition which known as a Brillouin threshold (around the threshold). The BS signal becomes noticeable when the injected BP power achieves this condition. At this condition, BS signal is dominated by stimulated emission process more than by spontaneous emission process. Above the Brillouin threshold condition, the BS signal power increases with increasing BP power. The output BS powers and tunability are investigated throughout the L-band region as illustrated in Fig. 6. The outputs coupling ratios are tuned at 50%, 60%, 70% and 80%. The SMF remains at a length of 4 km. The BP power is fixed at 10 mW
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Fig. 7. A magnified view of output optical spectra of the BS signal at injected BP power of 10 mW and 50% of output coupling ratio.
Fig. 8. The evolution of OSNR against BP wavelength at different output coupling ratio across L-band region.
while BP wavelength is turned over the entire L-band region at a step of 5 nm, which from 1570 nm to 1600 nm. From the plotted graph in Fig. 6, it is found that 50% of output coupling ratio has the capability to produce maximum BS power. In this case, 50% of the BS signal is directed to the OSA for measurement and other remaining 50% of BS signal propagated inside the laser structure. At 50% of output coupling ratio, average BS power at 4.07 mW is produced. However, when the output coupling ratio is extended from 50% to 80%, the BS power slightly decreased. The average BS power of 3.97 mW, 3.21 mW, and 2.41 mW is generated at 60%, 70% and 80% of output coupling ratio, respectively. Moreover, it is found that by optimized output coupling ratios, the values of BS power decreases with an increment of output coupling ratios. This trend is related to the fact that cavity loss increases with the increment percentage of the output coupling ratio [22] which result in the decrement of the output BS power at 60%, 70% and 80% of output coupling ratio. Furthermore, 70% of output coupling ratio is found to produce flattened output peak BS power across L-band region. For 50%, 60% and 70% of output coupling ratios, the maximum BS power occur around 1575 nm, 1580 nm and 1585 nm which indicates the highest Brillouin gain. Meanwhile, the BS power is dropped at 1590 nm, 1595 nm and 1600 nm of BP wavelength. This observation is a result of the reduction of the Brillion gain. However, unstable of BS power is recorded at 80% of output coupling ratio. This condition can be related to increment of the cavity loss in the BFL structure. A magnified view of output optical spectra of the BS signal at injected BP power of 10 mW, BP wavelength of 1580 nm and 50% of output coupling ratio are plotted in Fig. 7. In order to further analyze the output coupling ratio effect on enhancement of OSNR, the evolution of OSNR against BP wavelength at different output coupling ratio is plotted in Fig. 8. The OSNR is analyzed by comparing the peak power and the highest noise floor level [8] as shown in Fig. 7. The higher average OSNR of 37.12 dB is obtained at 50% of output coupling ratio. Meanwhile, the average OSNR of 36.44 dB, 34.59 dB, and 34.61 dB and is obtained from 60%, 70%, 80%, respectively. In the interim, the highest value of OSNR of 41.36 dB is recorded at 50% of output coupling ratio and 1600 nm of BP wavelength. The values of OSNR around 1585 nm to 1600 nm produced slightly different, especially at 50% and 60% of output coupling ratios. It is obtained that the L-band single wavelength BFL structure capable of producing a stable and flat OSNR at 70% of output coupling ratio. In addition, it proved that the OSNR performance considerably depends on the output coupling ratio.
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Fig. 9. Variation of BS signal powers and OSNR values at optimized output coupling ratio versus the BP wavelength. (a) 50% of output coupling ratio, (b) 60% of output coupling ratio, (d) 70% of output coupling ratio and (d) 80% of output coupling ratio.
To further analyze, the BP wavelength dependence on the BS power and OSNR are plotted in the graph in Fig. 9(a) to 9(d). It illustrates a variation of BS power and OSNR at optimized BP wavelength across L-band region for 50%, 60%, 70% and 80% of output coupling ratios. The BP power is fixed at 10 mW. From plotted graph in 9(a), the OSNR has minor decrement until 1585 nm; then slight increment happens at 1590 nm before remaining stable and increases again at 1600 nm. Meanwhile, the BS power is increased from 1570 nm to 1575 nm, but at 1580 nm until 1600 nm it declined slightly. However, by increasing the BP wavelength from 1585 nm to 1600 nm, the OSNR increases abruptly and is not affected by the low BS power. The highest and lowest BS power are identified at 1585 nm and 1600 nm, which contributes values of 4.58 mW and 2.91 mW, respectively. Meanwhile, the highest and lowest OSNR values are recorded at 1585 nm and 1600 nm, which produced the values of 33.39 dB and 41.36 dB, respectively. Correlation between BP power and OSNR at output coupling ratio of 60% are plotted in Fig. 9(b). Both BS power and OSNR show the stability with a minor change. However, at 1595 nm, the BS power is declining until 1600 nm, and OSNR is rising until 1600 nm. It shows by increasing the BP wavelength, the BP power slightly decreases linearly whereas the OSNR slightly increases. However, its shows less than 3-dB and maintains its small value regardless of the BP wavelength variation. In conclusion, the 60% of output coupling ratio generated slightly flattened results for both output BS power and OSNR. The highest BP power and OSNR are identified at 1570 nm and 1600 nm of BP wavelength, which are recorded at 4.94 mW and 38.19 dB, respectively. Increasing the values of OSNR around 1585 nm to 1600 nm is attributed to the decreasing effect of the noise floor of the generated BS signal as depicted in Fig. 9(a) and Fig. 9(b) [23]. While the high value of BS power is attributed to the creation of high gain at highest low BP wavelength. The values of OSNR is not affected by the low BP power at BP wavelengths around 1585 nm to 1600 nm. The 70% output coupling ratio is found has the ability to generate flattened output BS power and OSNR across L-band region as plotted in Fig. 9(c). The BS power and OSNR are fluctuating in small changes; overall the signal is flatness. Stable gain across L-band will be a reason for flatness BS power and OSNR, which influences the output stability. Fig. 10 shows a flatness of output optical spectra in term of peak BS powers and ONSR when 70% of output coupling ratio is attached to the structure. In this case, average BS power of 3.21 mW and average OSNR of 34.59 dB are recorded. Furthermore, for 80% of output coupling ratio, the OSNR value is consistent along 1570 nm to 1590 nm and had a slightly increment at 1595 nm before declining at 1600 nm. In addition, BS signal has the ability to be tuned freely over the entire L-band wavelength which from 1570 nm to 1600 nm without disturbance from any appearing unwanted oscillating modes. Furthermore, for all cases of output coupling ratios, a tuning range of 30 nm is obtained from this structure. Furthermore, characteristics of the single-wavelength ring cavity BFL operated across L-band region in terms of Brillouin threshold power, variation of BS signal powers and OSNR values for different output coupling ratios have never been reported in details. In prior work [24], only 50% and 80% of output coupling ratios are stated where the flatness of OSNR and BP power
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Fig. 10. Flatness of optical spectrum for the single-wavelength BFL at 70% of output coupling ratio.
across L-band region are not generated. In addition, single-wavelength BFL structures reported in [7,10–12] are focused only on C-band region and presented works in [13–15] are dedicated to the multi-wavelength generation of BEFL operation in merely L-band region. 3. Conclusion As presented in this paper, we experimentally demonstrated a single-wavelength ring cavity BFL that operated across L-band region. The characteristics of single-wavelength ring cavity BFL is affected by varying BP power, BP wavelength and output coupling ratio. The flatness of OSNR and BP power across L-band region depend on the optimization of output coupling ratio. The 70% of output coupling ratio is identified to has the ability to produce a flatness output BP power and OSNR. The flatness of BS signal provides more stability and enough quality to use in a communication system. Moreover, the tuning range of 30 nm spans is recorded without any unwanted oscillating modes appearance within 1570 nm to 1600 nm. The Brillouin threshold power at 6.3 mW is obtained at a pump power of 350 mW, BP wavelength of 1580 nm and 50% of output coupling ratio, respectively. At 10 mW of BP power and 50% of output coupling ratio, the highest average BS power of 4.07 mW is achieved. At 50% of output coupling ratio and 1600 nm of BP wavelength, the highest OSNR is recorded at 41.36 dB. Furthermore, due to the congestion of C-band wavelength usage, the addition of L-band wavelength is exceedingly desirable to be used in a communication system and the requirement for future fiber communications generations. Whereas, with the potential to generate a low threshold power and flatness of BS signal, the single wavelength ring BFL is an optional system that operates in the L-band region of the optical communication window. Acknowledgements The authors would like to thank the School of Microelectronics Engineering, Universiti Malaysia Perlis (UniMAP) especially SPILS for their support in this work. This work is fully funded by the Ministry of Higher Education, Malaysia under research grants.# FRGS/9003-00532# and # FRGS/9003-00529# References [1] Robert W. Boyd, Nonlinear Optics, 2007, New York. [2] G.P. Agrawal, Applications of Nonlinear Fibre Optics, 2001. [3] Kaijun Che, Deyu Tang, Changlei Guo, Huiying Xu, Changyan Ren, Pan Zhang, Shuisen Jiang, Lujian Chen, Dan Zhang, Zhiping Cai, External cavity lasing pumped stimulated Brillouin scattering in a high Q microcavity, Opt. Lett. 42 (2017) 935–938. [4] T. Eltaif, Multiwavelength based on nonlinear optics of intensity and phase modulators, Opt. Eng. 56 (2016) 016104. [5] S.P. Singh, N. Singh, Nonlinear effects in optical fibers: origin, management and applications, Prog. Electromagn. Res. 73 (2007) 249–275. [6] Tao Zhu, Baomei Zhang, Leilei Shi, Shihong Huang, Ming Deng, Jianguo Liu, Xiong Li, Tunable dual-wavelength fibre laser with ultranarrow linewidth based on rayleigh backscattering, Opt. Express 24 (2016) 1324–1330. [7] N.A.M.A. Hambali, M.A. Mahdi, M.H. Al-Mansoori, M.I. Saripan, A.F. Abas, M. Ajiya, Effect of output coupling ratio on the performance of ring-cavity Brillouin fiber laser, Laser Phys. 20 (2010) 1618–1624. [8] M. Ajiya, J.A. Oladapo, N.A.M.A. Hambali, Lasing threshold characteristics of multi-wavelength Brillouin–erbium laser in the L-band region assisted by delay interferometer, J. Nonlinear Optic. Phys. Mater. 25 (2016) 1650024. [9] A.W. Al-Alimi, N.A. Cholan, M.H. Yaacob, M.A. Mahdi, Enhanced multiwavelength generation in Brillouin fibre laser with pump noise suppression technique, Laser Phys. 26 (2016) 1–7. [10] M.R. Shirazi, S.W. Harun, K. Thambiratnam, M. Biglary, New Brillouin fibre laser configuration with high output power, Microwave Opt. Technol. Lett. 49 (2007) 2656–2658. [11] P. Zhang, S. Hu, S. Chen, Y. Yang, C. Zhang, A high-effciency Brillouin fibre ring laser, Chin. Opt. Lett. 7 (2009) 495–497. [12] S. Shahi, S.W. Harun, K. Dimyati, H. Ahmad, Brillouin fibre laser with significantly reduced gain medium length operating In L-band region, Prog. Electromagnet. Res. Lett. 8 (2009) 143–149. [13] T.F. Al-Mashhadani, M.H. Al-Mansoori, M.Z. Jamaludin, F. Abdullaha, A.K. Abass, N.I.M. Rawia, Tunable multiwavelength L-band Brillouin-Erbium fibre laser utilizing passive EDF absorber section, Opt. Fibre Technol. 19 (2013) 593–597. [14] S.W. Harun, N. Tamchek, M.K. Abd-Rahman, P. Poopalan, H. Ahmad, Brillouin/erbium-doped fibre laser with multiple wavelength generation in L-band, IEICE Trans. Commun. 85 (2002) 1386–1388.
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Original research article
Characteristics of the single-wavelength ring cavity Brillouin fiber laser operated in L-band region M.S.A. Hurera a,b , N.A.M. Ahmad Hambali a,b,∗ , M.M. Shahimin c , M.H.A. Wahid a,b a
School of Microelectronic Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra, 02600 Arau, Perlis, Malaysia Semiconductor Photonics & Integrated Lightwave Systems (SPILS), School of Microelectronic Engineering, Universiti Malaysia Perlis, Pauh Putra, 02600 Arau, Perlis, Malaysia c Department of Electrical and Electronic Engineering, Faculty of Engineering, National Defence University of Malaysia (UPNM), Kem Sungai Besi, 5700 Kuala Lumpur, Malaysia b
a r t i c l e
i n f o
Article history: Received 28 April 2017 Received in revised form 3 August 2017 Accepted 8 August 2017 Keywords: Single-wavelength Brillouin fiber laser L-band region Stimulated brillouin scattering Osnr Flatness
a b s t r a c t We experimentally demonstrated the characteristics of a single-wavelength Brillouin fiber laser that operated in the L-band region. The 4 km of a single mode fiber plays a significant role as a nonlinear gain medium to generate stimulated Brillouin scattering effect and causes the frequency shift with ∼ 0.08 nm spacing. At a pump power of 350 mW, a Brillouin pump wavelength of 1580 nm and 50% of output coupling ratio, the Brillouin threshold power at 6.3 mW is obtained. The highest average Brillouin Stokes power of 4.07 mW has been produced at 10 mW of Brillouin pump power at 50% of output coupling ratio. The tuning range of 30 nm is recorded without any unwanted oscillating modes appeared from 1570 nm to 1600 nm. The 70% of the output coupling ratio is found to produce flattened output for BS power and signal to noise ratio. The highest value for optical signal to noise ratio is recorded at 41.36 dB, with 50% of output coupling ratio and at 1600 nm of Brillouin pump wavelength. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction Ever since stimulated Brillouin scattering (SBS) effect is discovered, it has become a major research due to its light wave effect on the optical systems. The backward signal propagation is produced by the SBS effect. SBS is a nonlinear effect subsequent from the interaction between intense pump light and acoustic waves in a gain medium. From the interaction, the backward propagating frequency-shifted light and Rayleigh scattering are generated, and inelastic scattering process shifted the frequency downward [1]. Raman scattering and Brillouin scattering are categorized as inelastic scattering of a photon by molecules and large scale, low-frequency phonons respectively that result in changes of energy [2]. The pump and Stokes waves intervention have a tendency to raise the amplitude of the acoustic waves; then the interaction with the pump is emphasized by the Stokes wave. This condition can cause exponential growth and recreate the Stokes wave [2]. SBS are frequently applied in laser amplifiers and oscillators to increase the quality of the laser beam and to shorten the laser pulse width. A single mode fiber (SMF) which plays an important role as a gain medium, has been used as a standard to produce the SBS effect in many research. A laser signal, known as a Brillouin fiber laser (BFL), can be generated from the
∗ Corresponding author at: School of Microelectronic Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra, 02600 Arau, Perlis, Malaysia. E-mail addresses: [email protected], [email protected] (N.A.M.A. Hambali). http://dx.doi.org/10.1016/j.ijleo.2017.08.063 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
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Fig. 1. Single wavelength BFL structure that operated in the L-band region.
Brillouin Stokes (BS) effect as a seed signal by using nonlinear Brillouin gain in an optical fiber. In the gain medium, electrostriction works as both; the mechanism leading to a third-order nonlinear optical response and as a coupling mechanism that leads to SBS effect [3,4]. The interaction between intense pump light and acoustic waves in a fiber caused the SBS effect. This nonlinear effect leads to backward propagating frequency-shifted light [5]. For several years, research on BFL has developed a myriad of techniques in order to enhance their characteristics, such as ultra-narrow linewidth [6], tuning range [7], low threshold power [8] and low noise [9]. Two techniques of single-wavelength BFL, utilizing a ring-cavity, has been experimentally demonstrated in [10]. Both experimental setups consist of 25 km long SMF as the gain medium. In this experiment, the output spectrum for the conventional BFL and the proposed BFL structure are compared. From the results, the BS signal power of −0.5 dBm is found, which is 5.7 dBm higher compared with the conventional structure. A ring-cavity BFL by using similar structure and components as proposed in [10] is demonstrated in [11]. A polarization controller (PC) is employed in this structure to adjust the polarization of the pump and the BS signals. Through this experimental work, 3.6 mW of Brillouin threshold power is obtained with 50% of output coupling ratio. At the injected pump power of 40 mW, the BS signal with a signal power of 22 mW is obtained. The performance of ring cavity BFL with the effect of output coupling ratio is reported in [7]. As stated, the investigation of different output coupling ratios performance has been carried out. The output BS power of 7.3 mW and threshold power of 0.9 mW respectively, is obtained by using 90% of output coupling ratio. However, the optical signal to noise ratio (OSNR) and the flatness of BS signal powers are not discussed explicitly in this paper. A compact single wavelength BFL is demonstrated by inserting a Bismuth-based Erbium-doped fiber (Bi-EDF) and photonic crystal fiber (PCF) into the fiber resonator [12]. However, this structure only focusses on 1574 nm and known as a single wavelength Brillouin-Erbium fiber laser (BEFL). However, this structure suffers from the presence of unwanted oscillating modes at peak wavelength around 1574 nm. Furthermore, BFL structure reported in [7,10–12] are focused on C-band region of the optical communication window. A simple tunable L-band multiwavelength Brillouin-Erbium fiber laser (MWBEFL) utilizes a short passive erbium doped fiber (PEDF) as an absorber medium is reported in [13]. At 100 mW of 1480 nm pump power and 4 mW of Brillouin pump (BP) power, a wide tuning range of 24.4 nm has been achieved. High threshold power of 33 mW of first-order Brillouin Stokes is generated from a dual cavity MWBEFL laser in L-band region [14]. Meanwhile, the threshold value of 15.9 mW is used to generated first order BS signal from L-band MWBEFL that utilizing a double-pass Brillouin pump preamplified technique [15]. However, reported paper in [8] and [13–15] only focused on the multi-wavelength generation of BEFL and operated on L-band region of the optical communication window. Furthermore, the selection of band also plays an important role in the communication system. Nowadays, C-band region has extensively had been used for numerous applications. C-band region has been exploited by many industries, particularly for communication, and remote sensing. Thus, the C-band region is almost occupied. However, owing to the overcrowding and susceptibility of the nonlinear effect, the addition of L-band region wavelength is extremely desirable [8]. L-band region are preferred mainly because of its low attenuation [16], its ability to expand single wavelength and its dense wavelength division multiplexing (DWDM) which allowed multiple signals to share a single fiber [16]. The usage of the L-band region is still new. Thus the region is not crowded, and various features of research can be done and utilized. In this paper, we report the experimental demonstration the of a single-wavelength ring cavity BFL structure that operated in L-band region. A short SMF length of 4 km is used as a gain medium. The characteristic of single-wavelength ring cavity BFL structure based on amplified spontaneous emission (ASE), threshold, output BS power and OSNR are studied. A lowest Brillouin threshold power around 6.3 mW is produced at 1580 nm of BP wavelength and 50% of output coupling ratio. The flat amplitude of BS signal and OSNR are achieved at 70% of output coupling ratio, respectively. At 50% of output coupling ratio, higher average OSNR about 37.12 dB is recorded. Meanwhile, at 50% of output coupling ratio, higher average BS peak power value about 4.07 mW is recorded. 1.1. Experimental setup Fig. 1 shows the structure of L-Band single-wavelength ring cavity Brillouin fiber laser (BFL) which consists of tuneable laser sources (TLS), EDFA, an optical circulator, 4 km long of single mode fiber (SMF), an optical coupler and optical spectrum analyzer (OSA). The EDFA components consist of 35 m long of Erbium doped fiber (EDF), wavelength division multiplexing
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Fig. 2. Output ASE spectra profile across L-band region at different values of PP at 100 mW, 250 mW and 350 mW and the BP signal source is disabled.
(WDM) coupler and 980 nm pump power. The optical WDM coupler is used to combine 980 nm pump power (PP) signal and input BP signal from TLS. Meanwhile, Erbium-doped fiber (EDF) act as the gain medium to amplify the BP signal. The EDF type specification uses the 35 m long of Fibercore M-12 (980/125) with optimized for 980 nm LD pumping and 125 m of fiber diameter. The peak absorption for this Fibercore M-12 (980/125) is 19.31 dB/m with 12.64 dB/m absorption at 979 nm. The SMF plays a significant role as a nonlinear gain medium in order to produce BS signal which is backward propagating frequency-shifted light. The SMF has a dispersion value of 16.75 ps/nm/km and attenuation value of 25 dB. The BP signal is provided by utilizing an external cavity TLS with a linewidth of approximately 20 MHz. A maximum BP power of 10 mW is injected from TLS. The TLS provided BP wavelength across L-band region which starts from 1570 nm until 1600 nm. Four set of optical couplers with different output coupling ratio at 50%, 60%, 70% and 80% are used. The output signal is characterized utilizing an OSA with a resolution of 0.015 nm. The injected BP signal from TLS is propagated into EDF, and then it is amplified. The BP signal and PP signal multiplex together through WDM coupler. The BP signal is sent through an optical circulator to the nonlinear gain medium. In the gain medium, the nonlinear stimulated Brillouin scattering (SBS) effect plays the important role to generate the BS signal with constant spacing and narrow linewidth at room temperature [17]. SBS effect is obtained from the interaction between the intense pump light and acoustic waves in a gain medium that resulted in a backward propagating BS signal once the Brillouin threshold is reached [2]. Consequently, a backward propagating BS signal with ∼0.08 nm (10 GHz) spacing is produced. The frequency downshifts of scattered light are attributed to the Doppler shift related to a grating moving at the acoustic velocity [2] Then, BS signal is propagated again through optical circulator and passed to the optical coupler. The coupling ratio withdraws a percentage of signals to output, and another percentage circulates back to another end of SMF. The output signal is then extracted by using OSA.
2. Results and discussion Fig. 2 shows the output ASE spectra profile across L-band region. At first, the BP signal source is disabled in order to study the output spectrum attributed only to the PP signal. The PP injected to the structure is varied to three different values at 100 mW, 250 mW, and 350 mW. Meanwhile, the output signal is extracted from 50% of output coupling ratio. The finding from this experiment demonstrates that the ASE power and noise floor have been raised up with an increment of PP. The most stable ASE profile is produced at 350 mW of PP. It can also be claimed that ASE spectra at 350 mW of PP show a higher gain profile in comparison with 100 mW and 250 mW, respectively. The higher gain profile might lead the single-wavelength BFL structure to produce the BS signal that experiencing higher gain. In order to study the Brillouin threshold power, the graph in Fig. 3 is plotted. For Fig. 3, the BP wavelength is set at 1580 nm. The BP power is varied from its minimum value of 1 mW to the maximum value of 10 mW. Meanwhile, PP is set again at 350 mW. When a single BP signal from TLS is launched into SMF, BS signal grows rapidly through SBS. The BS signal shifted at 0.08 nm spacing from the origin of injected BP signal. In order to achieve threshold condition, BP power is tuned manually from minimum power of 1 mW to maximum power of 10 mW. The Brillouin threshold power is obtained by varying the BP power from its minimum until the BS signal at threshold condition appears. After the BS signal acquires threshold condition, it is backscattered through SMF and increases linearly with an increase in BP power. From Fig. 3, it can be found that the Brillouin threshold power at 6.3 mW is obtained from 50% of output coupling ratio. The Brillouin threshold power can be defined as the input pump power at which the backscattered power initiates to increase rapidly and depleted [18]. Furthermore, a Brillouin threshold power is the most widely used parameter for characterizing SBS by utilizing a number of laser cavity structures as reported in [19,20]. The Brillouin threshold power can be explained by taking into account the interaction between pump and Stokes waves in order to adequately represent the theory of the single wavelength BFL. The single wavelength BFL is based on the amplification
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Fig. 3. The BS signal power as a function of the BP power at 1580 nm of BP wavelength and 50% of output coupling ratio.
of Brillouin gain. It is necessary to comprehend the basic theory between the interactions of pump intensity (Ip ) and the Stokes wave intensity (Is ) in the SBS effect. Relationship between these two interactions is governed by the following equations [2]. dIs = −gB Ip Is + ˛Is dz
(1)
dIp = −gB Ip Is − ˛Ip dz
(2)
where ␣ is the fiber loss for the gain medium and gB is the Brillouin gain coefficient. In case of small Brillouin shift, the fiber loss can be approximated by ␣p ≈ ␣s ␣. In the absence of fiber losses ␣=0, both intensities can be simplified as I s − I p = Constant
(3)
In a condition where the Stokes power is much smaller than the pump power, the term on the right hand side in Eq. (2), −gB Ip Is , can be neglected. At this condition, it can be assumed that the pump power is not depleted. Consequently, Eq. (2) can be summarized as, dIp = −˛Ip dz
(4)
The clarification of Eq. (4) is supported by Eq. (5) [2] below, where Ip (z) and Ip (0)are pump intensities at length z and at z = 0 respectively. Ip (z) = Ip (0) exp(−˛z)
(5)
Thus, Stokes intensity can be presented as Is (0) = Is (L) exp[gB Ip(0)Leff − ˛L]
(6)
In this equation, Leff is the effective length of interaction and L is the fiber length. Due to pump absorption, Leff is slightly less than L. Hereby, Eqs. (5) and (6) can be used to calculate the Brillouin threshold powers which are given by Ps (0) = Ps (L) exp(−˛L) exp[gB Pp(0)Leff /Aeff ]
(7)
P p (L) = P p (0) exp(−˛L)
(8)
and
Meanwhile, the Brillouin threshold power can be estimated by [1], Pth ∼ = 21bAeff /gB Leff
(9)
where, gB is the peak value of the Brillouin gain, Leff is the effective length of interaction and Aeff is the effective core of the fiber. The numerical factor of 21 is estimated according to the specific value of Brillouin gain linewidth. The value of polarization factor b lies among 1 and 2 depending on the relative polarization of pump and Stokes waves [1]. The Brillouin threshold power is then investigated for each BP wavelength at 50% of output coupling ratio. In order to study the characteristics of Brillouin threshold power for each BP wavelength across L-band region, Fig. 4 is plotted. In this study, the BP power and PP are fixed at 10 mW and 350 mW, respectively. While BP wavelength is turned over the entire L-band wavelength at a step of 5 nm, which from 1570 nm to 1600 nm. It is obtained that the point at 1570 nm, 1575 nm and 1580 nm of BP wavelength, the minimum Brillouin threshold power at 6.15 mW, 6.61 mW and 6.3 mW is produced, respectively. This trend is attributed to the highest Brillouin gain which led to sufficient signal power for producing low Brillouin threshold power [21]. Meanwhile, the maximum Brillouin threshold power of 8.12 mW is generated at 1590 nm. Furthermore, at 1585 nm, 1595 nm and 1600 nm, the Brillouin threshold power at 7.80 mW, 7.57 mW and 7.94 mW is produced, respectively. In addition, the average Brillouin threshold power of 7.23 mW for the whole tuning range from 1570 nm to 1600 nm is
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Fig. 4. Brillouin threshold power versus BP wavelength across L-band region.
Fig. 5. A magnified view of output optical spectra of the BS signal at three conditions, below, around and above the threshold condition at 50% of output coupling ratio.
Fig. 6. The output BS powers at 80%, 70%, 60% and 50% of output coupling ratio throughout the L-band region.
recorded. Three conditions of BS signal are observed, which at below the threshold, around the threshold, and above the threshold as shown in Fig. 5. As shown in Fig. 5, the BP power is injected at 3.16 mW, 6.17 mW, and 7.08 mW. Meanwhile, BP wavelength is set at 1570 nm. For below the threshold condition the BS signal power growths slowly and dominated by spontaneous emission process. At this situation, the BS power is lower than the BP power and not depleted. Once the input BP power is increased from minimum power, the BS signal power is increased by the stimulation process when it achieves a power condition which known as a Brillouin threshold (around the threshold). The BS signal becomes noticeable when the injected BP power achieves this condition. At this condition, BS signal is dominated by stimulated emission process more than by spontaneous emission process. Above the Brillouin threshold condition, the BS signal power increases with increasing BP power. The output BS powers and tunability are investigated throughout the L-band region as illustrated in Fig. 6. The outputs coupling ratios are tuned at 50%, 60%, 70% and 80%. The SMF remains at a length of 4 km. The BP power is fixed at 10 mW
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Fig. 7. A magnified view of output optical spectra of the BS signal at injected BP power of 10 mW and 50% of output coupling ratio.
Fig. 8. The evolution of OSNR against BP wavelength at different output coupling ratio across L-band region.
while BP wavelength is turned over the entire L-band region at a step of 5 nm, which from 1570 nm to 1600 nm. From the plotted graph in Fig. 6, it is found that 50% of output coupling ratio has the capability to produce maximum BS power. In this case, 50% of the BS signal is directed to the OSA for measurement and other remaining 50% of BS signal propagated inside the laser structure. At 50% of output coupling ratio, average BS power at 4.07 mW is produced. However, when the output coupling ratio is extended from 50% to 80%, the BS power slightly decreased. The average BS power of 3.97 mW, 3.21 mW, and 2.41 mW is generated at 60%, 70% and 80% of output coupling ratio, respectively. Moreover, it is found that by optimized output coupling ratios, the values of BS power decreases with an increment of output coupling ratios. This trend is related to the fact that cavity loss increases with the increment percentage of the output coupling ratio [22] which result in the decrement of the output BS power at 60%, 70% and 80% of output coupling ratio. Furthermore, 70% of output coupling ratio is found to produce flattened output peak BS power across L-band region. For 50%, 60% and 70% of output coupling ratios, the maximum BS power occur around 1575 nm, 1580 nm and 1585 nm which indicates the highest Brillouin gain. Meanwhile, the BS power is dropped at 1590 nm, 1595 nm and 1600 nm of BP wavelength. This observation is a result of the reduction of the Brillion gain. However, unstable of BS power is recorded at 80% of output coupling ratio. This condition can be related to increment of the cavity loss in the BFL structure. A magnified view of output optical spectra of the BS signal at injected BP power of 10 mW, BP wavelength of 1580 nm and 50% of output coupling ratio are plotted in Fig. 7. In order to further analyze the output coupling ratio effect on enhancement of OSNR, the evolution of OSNR against BP wavelength at different output coupling ratio is plotted in Fig. 8. The OSNR is analyzed by comparing the peak power and the highest noise floor level [8] as shown in Fig. 7. The higher average OSNR of 37.12 dB is obtained at 50% of output coupling ratio. Meanwhile, the average OSNR of 36.44 dB, 34.59 dB, and 34.61 dB and is obtained from 60%, 70%, 80%, respectively. In the interim, the highest value of OSNR of 41.36 dB is recorded at 50% of output coupling ratio and 1600 nm of BP wavelength. The values of OSNR around 1585 nm to 1600 nm produced slightly different, especially at 50% and 60% of output coupling ratios. It is obtained that the L-band single wavelength BFL structure capable of producing a stable and flat OSNR at 70% of output coupling ratio. In addition, it proved that the OSNR performance considerably depends on the output coupling ratio.
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Fig. 9. Variation of BS signal powers and OSNR values at optimized output coupling ratio versus the BP wavelength. (a) 50% of output coupling ratio, (b) 60% of output coupling ratio, (d) 70% of output coupling ratio and (d) 80% of output coupling ratio.
To further analyze, the BP wavelength dependence on the BS power and OSNR are plotted in the graph in Fig. 9(a) to 9(d). It illustrates a variation of BS power and OSNR at optimized BP wavelength across L-band region for 50%, 60%, 70% and 80% of output coupling ratios. The BP power is fixed at 10 mW. From plotted graph in 9(a), the OSNR has minor decrement until 1585 nm; then slight increment happens at 1590 nm before remaining stable and increases again at 1600 nm. Meanwhile, the BS power is increased from 1570 nm to 1575 nm, but at 1580 nm until 1600 nm it declined slightly. However, by increasing the BP wavelength from 1585 nm to 1600 nm, the OSNR increases abruptly and is not affected by the low BS power. The highest and lowest BS power are identified at 1585 nm and 1600 nm, which contributes values of 4.58 mW and 2.91 mW, respectively. Meanwhile, the highest and lowest OSNR values are recorded at 1585 nm and 1600 nm, which produced the values of 33.39 dB and 41.36 dB, respectively. Correlation between BP power and OSNR at output coupling ratio of 60% are plotted in Fig. 9(b). Both BS power and OSNR show the stability with a minor change. However, at 1595 nm, the BS power is declining until 1600 nm, and OSNR is rising until 1600 nm. It shows by increasing the BP wavelength, the BP power slightly decreases linearly whereas the OSNR slightly increases. However, its shows less than 3-dB and maintains its small value regardless of the BP wavelength variation. In conclusion, the 60% of output coupling ratio generated slightly flattened results for both output BS power and OSNR. The highest BP power and OSNR are identified at 1570 nm and 1600 nm of BP wavelength, which are recorded at 4.94 mW and 38.19 dB, respectively. Increasing the values of OSNR around 1585 nm to 1600 nm is attributed to the decreasing effect of the noise floor of the generated BS signal as depicted in Fig. 9(a) and Fig. 9(b) [23]. While the high value of BS power is attributed to the creation of high gain at highest low BP wavelength. The values of OSNR is not affected by the low BP power at BP wavelengths around 1585 nm to 1600 nm. The 70% output coupling ratio is found has the ability to generate flattened output BS power and OSNR across L-band region as plotted in Fig. 9(c). The BS power and OSNR are fluctuating in small changes; overall the signal is flatness. Stable gain across L-band will be a reason for flatness BS power and OSNR, which influences the output stability. Fig. 10 shows a flatness of output optical spectra in term of peak BS powers and ONSR when 70% of output coupling ratio is attached to the structure. In this case, average BS power of 3.21 mW and average OSNR of 34.59 dB are recorded. Furthermore, for 80% of output coupling ratio, the OSNR value is consistent along 1570 nm to 1590 nm and had a slightly increment at 1595 nm before declining at 1600 nm. In addition, BS signal has the ability to be tuned freely over the entire L-band wavelength which from 1570 nm to 1600 nm without disturbance from any appearing unwanted oscillating modes. Furthermore, for all cases of output coupling ratios, a tuning range of 30 nm is obtained from this structure. Furthermore, characteristics of the single-wavelength ring cavity BFL operated across L-band region in terms of Brillouin threshold power, variation of BS signal powers and OSNR values for different output coupling ratios have never been reported in details. In prior work [24], only 50% and 80% of output coupling ratios are stated where the flatness of OSNR and BP power
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Fig. 10. Flatness of optical spectrum for the single-wavelength BFL at 70% of output coupling ratio.
across L-band region are not generated. In addition, single-wavelength BFL structures reported in [7,10–12] are focused only on C-band region and presented works in [13–15] are dedicated to the multi-wavelength generation of BEFL operation in merely L-band region. 3. Conclusion As presented in this paper, we experimentally demonstrated a single-wavelength ring cavity BFL that operated across L-band region. The characteristics of single-wavelength ring cavity BFL is affected by varying BP power, BP wavelength and output coupling ratio. The flatness of OSNR and BP power across L-band region depend on the optimization of output coupling ratio. The 70% of output coupling ratio is identified to has the ability to produce a flatness output BP power and OSNR. The flatness of BS signal provides more stability and enough quality to use in a communication system. Moreover, the tuning range of 30 nm spans is recorded without any unwanted oscillating modes appearance within 1570 nm to 1600 nm. The Brillouin threshold power at 6.3 mW is obtained at a pump power of 350 mW, BP wavelength of 1580 nm and 50% of output coupling ratio, respectively. At 10 mW of BP power and 50% of output coupling ratio, the highest average BS power of 4.07 mW is achieved. At 50% of output coupling ratio and 1600 nm of BP wavelength, the highest OSNR is recorded at 41.36 dB. Furthermore, due to the congestion of C-band wavelength usage, the addition of L-band wavelength is exceedingly desirable to be used in a communication system and the requirement for future fiber communications generations. Whereas, with the potential to generate a low threshold power and flatness of BS signal, the single wavelength ring BFL is an optional system that operates in the L-band region of the optical communication window. Acknowledgements The authors would like to thank the School of Microelectronics Engineering, Universiti Malaysia Perlis (UniMAP) especially SPILS for their support in this work. This work is fully funded by the Ministry of Higher Education, Malaysia under research grants.# FRGS/9003-00532# and # FRGS/9003-00529# References [1] Robert W. Boyd, Nonlinear Optics, 2007, New York. [2] G.P. Agrawal, Applications of Nonlinear Fibre Optics, 2001. [3] Kaijun Che, Deyu Tang, Changlei Guo, Huiying Xu, Changyan Ren, Pan Zhang, Shuisen Jiang, Lujian Chen, Dan Zhang, Zhiping Cai, External cavity lasing pumped stimulated Brillouin scattering in a high Q microcavity, Opt. Lett. 42 (2017) 935–938. [4] T. Eltaif, Multiwavelength based on nonlinear optics of intensity and phase modulators, Opt. Eng. 56 (2016) 016104. [5] S.P. Singh, N. Singh, Nonlinear effects in optical fibers: origin, management and applications, Prog. Electromagn. Res. 73 (2007) 249–275. [6] Tao Zhu, Baomei Zhang, Leilei Shi, Shihong Huang, Ming Deng, Jianguo Liu, Xiong Li, Tunable dual-wavelength fibre laser with ultranarrow linewidth based on rayleigh backscattering, Opt. Express 24 (2016) 1324–1330. [7] N.A.M.A. Hambali, M.A. Mahdi, M.H. Al-Mansoori, M.I. Saripan, A.F. Abas, M. Ajiya, Effect of output coupling ratio on the performance of ring-cavity Brillouin fiber laser, Laser Phys. 20 (2010) 1618–1624. [8] M. Ajiya, J.A. Oladapo, N.A.M.A. Hambali, Lasing threshold characteristics of multi-wavelength Brillouin–erbium laser in the L-band region assisted by delay interferometer, J. Nonlinear Optic. Phys. Mater. 25 (2016) 1650024. [9] A.W. Al-Alimi, N.A. Cholan, M.H. Yaacob, M.A. Mahdi, Enhanced multiwavelength generation in Brillouin fibre laser with pump noise suppression technique, Laser Phys. 26 (2016) 1–7. [10] M.R. Shirazi, S.W. Harun, K. Thambiratnam, M. Biglary, New Brillouin fibre laser configuration with high output power, Microwave Opt. Technol. Lett. 49 (2007) 2656–2658. [11] P. Zhang, S. Hu, S. Chen, Y. Yang, C. Zhang, A high-effciency Brillouin fibre ring laser, Chin. Opt. Lett. 7 (2009) 495–497. [12] S. Shahi, S.W. Harun, K. Dimyati, H. Ahmad, Brillouin fibre laser with significantly reduced gain medium length operating In L-band region, Prog. Electromagnet. Res. Lett. 8 (2009) 143–149. [13] T.F. Al-Mashhadani, M.H. Al-Mansoori, M.Z. Jamaludin, F. Abdullaha, A.K. Abass, N.I.M. Rawia, Tunable multiwavelength L-band Brillouin-Erbium fibre laser utilizing passive EDF absorber section, Opt. Fibre Technol. 19 (2013) 593–597. [14] S.W. Harun, N. Tamchek, M.K. Abd-Rahman, P. Poopalan, H. Ahmad, Brillouin/erbium-doped fibre laser with multiple wavelength generation in L-band, IEICE Trans. Commun. 85 (2002) 1386–1388.
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