Stimulated Brillouin scattering (SBS) is one of thenonlinear effects in optical fibers that is not favoredby optical transmission engineers due to its inherentenergy transfer from carriers to the downshifted fre-quency signal. However, this disadvantage does notpermit the exploitation of SBS in distributed strainand temperature sensing [1], microwave generators
[2], optical signal processing [3–5], gyroscopes [6],and lasers [7–9].
A Brillouin fiber laser is attractive due to itsnarrowing effect of laser linewidth as reported in[10–12]. However, the Brillouin gain in optical fibersis low and, therefore, it requires a significant amountof Brillouin pump (BP) power to generate theBrillouin Stokes (BS) laser in the cavity. Thus, by in-tegrating an erbium-doped fiber in the laser cavity,this problem is efficiently rectified and a new breedof laser is formed, namely, a Brillouin–erbium fiberlaser (BEFL) [13,14]. The efficient amplification ofthe erbium-doped fiber is utilized to compensate
the cavity loss. On the other hand, the SBS effect isexploited as the frequency shifter. The downshiftedfrequency Stokes signal that circulates in the cavitycan act as a low-pass filter that can reject highfrequency component [15]. As a result, the relativeintensity noise of the Brillouin fiber laser is lowercompared to its BP, especially in the high frequencyregion.Although the Brillouin fiber laser operation is en-
hanced by the Erbium gain, the BS laser cannot bewidely tuned due to the presence of self-lasing cavitymodes. This is owing to the fact that the cavity modesgenerated from the erbium-doped fiber compete withthe BS laser within the amplification bandwidth.Therefore, the BEFL output consists of these twolasing regions. This situation leads to research activ-ities on widening the tuning range of the BS laser inthe absence of the self-lasing cavity modes as re-ported in [16–18]. The BP preamplification techni-que is unique because it amplifies the BP beforeentering the Brillouin gain medium. Therefore, theamplified BP can create a deep saturation operationin the erbium gain section that leads to a wider tun-ing range of the BS laser. However, the tuning rangestill cannot cover the whole range of the erbium gainbandwidth due to an imbalance discrepancy betweencavity gain and cavity loss [18,19]. To enhance thetuning range of BEFL, the cavity loss can be opti-mized so that the average population inversion ofthe erbium gain section can be controlled to a levelin which the discrepancy between the cavity gainand cavity loss is minimized.In this paper we present experimental results of a
BEFL for different output coupling ratios in the ringcavity. A variety of the coupling ratios is important todetermine the characteristics of the laser system.The peak power of the BP line increases with the in-crement of the output coupling ratios from 10% to95%. At the optimum coupling efficiency of 95%, asingle-wavelength BS line is achieved with a peakpower of 28:7mW. The BS lasing wavelength canbe tuned over the range of 60nm from 1520 to1580nm without any appearances of the self-lasingcavity modes in the laser system.
2. Experimental Setup
The structure of the ring-cavity BEFL system withBrillouin pump preamplification technique is shownin Fig. 1. TheBEFL system consists of a 11kmdisper-sion compensating fiber (DCF), an optical circulator(Cir), an optical coupler, and an erbium-doped fiberamplifier (EDFA). The Brillouin gain is provided bythe DCF while the Cir is utilized to guide the propa-gation of both theBrillouin pump (BP) and theBS sig-nal into and out of the Brillouin gain media,respectively. The DCF has a 20 μm2 effective area, a7:31 ðWkmÞ−1 nonlinear coefficient,−1328ps=nmdis-persion, and7:28dB total loss.Anoptical couplerwithdifferent coupling ratios provides a medium throughwhich the output of the laser cavity is connected to an
optical spectrum analyzer (OSA) that is used formon-itoring and measurement in the experiment.
The EDFA gain block is composed of an 8:0merbium-doped fiber (EDF), a 1480=1550nm wave-length division multiplexing (WDM) coupler, and a1480nm laser diode. The 1480nm pump laser witha maximum power of 135mW is used as the primarypump source for the EDF. The WDM coupler is usedto multiplex the 1480nm pump and the BP signal.
The BP signal with 200kHz narrow linewdith pro-vided by an external cavity tunable laser source(TLS) was injected into the resonator. The injectedBP signal from the TLS is guided into the EDFAby the optical circulator. First it is amplified bythe EDFA and then guided into the Brillouin gainmedia (i.e., the DCF). When the BP signal exceedsthe threshold power of the Brillouin gain media,the SBS effect is initiated and thus the first-orderBrillouin Stokes (BS) signal that propagate in the op-posite direction to the direction of the BP signal isgenerated. This BS signal with 0:08nm downshiftedfrom the BP signal is amplified by the EDFA andthen transported to the optical coupler via the circu-lator from its port 2 through port 3. For each of theselected coupling ratios of the optical coupler, part ofthe BS signal is guided into the resonator and theother part is measured by using an OSA throughthe output port of the optical coupler.
3. Results and Discussion
In the experiment, the single-wavelength BS signalpeak power for different output optical coupling ra-tios is investigated as shown in Fig. 2. The outputcoupling ratio values of 10%, 20%, 30%, 50%, 60%,70%, 80%, 95%, and 99% are used in the experiment.The output is measured using an OSA with a band-width resolution of 0:015nm. For the entire outputcoupling ratio cases, the 1480nm pump power is var-ied to its maximum power of 135mWat step of 5mW,the BP wavelength is fixed at 1550nm, while the BPpower is set to the value of 1:6mW.
Fig. 1. (Color online) Ring-cavity BEFL with Brillouin pumppreamplification technique.
Above the threshold condition, BS signal power in-creases linearly with an increment of 1480nm pumppower [13,20]. The BS signal power increases in tan-dem with the output coupling ratios from 10% to 95%and decreases at the 99% output coupling ratio. Asevidently seen in Fig. 2, the optimum Stokes signalpower is recorded at 95% output coupling ratio. Themaximum BS signal power of 28:7mW is measuredat the maximum 1480nm pump power of 135mW.This is much higher than the Stokes peak powerof about 10mW as reported in [13] and around9mW as reported in [20]. Meanwhile, for the otheroutput coupling ratios of 50%, 60%, 70%, and 80%,the BS signal powers are 13.6, 18.0, 20.6, and25:1mW, respectively. Above the optimum outputcoupling ratio, the BS signal power decreases to24:5mW for 99% output coupling ratio. The incre-ment of BS signal power is inline with the incrementof output coupling ratio from 10% to 95%. However,when the output coupling ratio is too large (95%), thecavity loss is also increased. This factor becomesdominant on the performance of fiber laser, which re-sults in the decrement of output power [21].On the other hand, the BS signal power remains
very low for 10%, 20%, and 30% output coupling ra-tios. In these cases, the characteristic of BS signalpower is saturated to specific values. The criticalpump power in which the BS signal power startsto saturate is measured around 85mW pump powerfor 10% output coupling ratio and around 100mWpump power for 20% and 30% output coupling ratio.This saturation characteristic is the evidence of en-ergy transfer from the BS signal to the second-orderBS signal, which propagates in the counterclockwisedirection. To validate this statement, the outputspectra at selected output coupling ratios are illu-strated in Fig. 3. For output coupling ratios of10%, 20%, and 30%, there are two small peaks atlonger wavelengths from the BS signal. The firstpeak represents the Rayleigh component of thesecond-order BS signal. In addition to this, the
second peak is the third-order BS signal that propa-gates in the same direction as the BS laser. Since itdoes not reach the threshold power, thus, it cannotoscillate in the ring cavity as a laser.
Without launching the BP power into the lasersystem, the appearances of the self-lasing cavitymodes around the EDF peak gain are closely moni-tored for each case of the output coupling ratios at135 mW power of the 1480 nm pump laser. Outputspectra of the EDFL self-lasing cavity modes for10%, 50%, 60%, 70%, and 95% output coupling ratiosare shown in Fig. 4. Based on the experimental re-sults, there are two self-lasing cavity mode regionswithin 1530 and 1560nm. This can be explaineddue to the average population inversion, which is de-pendent on the total cavity loss. The cavity loss de-creases with the decrement of the output couplingratio. As a result, the laser cavity operates in the re-gime of low average population inversion. Therefore,the self-lasing cavity modes are observed around1560nm for output coupling ratio of 70%, 60%,50%, and 10%. On the other hand, the self-lasing
Fig. 2. (Color online) Brillouin Stokes signal peak power against1480nm pump power at different output coupling ratios.
Fig. 3. Output spectra of power saturation because of Rayleighscatterring at 10%, 20%, 30%, 90%, and 95% coupling ratio at135mW pump power.
Fig. 4. (Color online) Output spectra of EDFL self-lasing cavitymodes at 135mW of 1480nm pump power for different couplingratios of 10%, 50%, 60%, 70%, and 95%.
cavity modes are observed around 1530nm for theoutput coupling ratio of the 95% due to the higheraverage population inversion.For the purpose of investigating the behaviors of
the BPwavelength tunability on the BS signal power,the 1480 pump power was set to its maximum powerof 135mW and the BP was fixed to 1:6mW power.The BP wavelength was tuned from 1520 to1580nm with a 1:0nm step. The resolution band-width of the OSA is set to 0:05nm. The BS signalpower with different output coupling ratios of 10%,50%, 60%, 70%, and 95% are measured.The results of BS signal power for different output
coupling ratios versus BP wavelength are shown inFig. 5. Obviously, from experimental results, the BSpeak power for wavelength tunability increases withthe increment of the output coupling ratio. Mean-while, the appearance of self-lasing cavity modes de-creases with the increment of the output couplingratios. In our work, the tuning range is defined asthe range of BPwavelength that produced the BS linesignal in the absence of self-lasing cavitymodes activ-ity [15–17]. At the optimum coupling ratio of 95%, theBS signal is generatedwithout the appearances of theself-lasing cavity modes around wavelength regionsfrom 1520 to 1580nmwithin the 60nm tuning range.This is much wider than the tuning range of about2nm as reported in [13] and around 7nm as reportedin [20]. For the 70% output coupling ratio, the BS sig-nal is observed around 1530 to 1570nm without anyappearance of self-lasing cavity modes. A tuningrange of 23nm with a BP wavelength from 1545 to1568nm is measured for the 60% output coupling ra-tio. The appearance of the self-lasing cavity modescannot be suppressed when the low BS line power cir-culated in the laser system.Only a small tuning rangearound 17nm (1550 to 1567nm) and 11nm (1556 to1567nm) are obtained at lower output coupling ratiosof 50% and 10%, respectively. The BEFL system canoperate with high efficiency without appearances ofself-lasing cavity modes around the C-band wave-
length region (1530 to 1565nm) at output coupling ra-tios of 95% and 70%. To show our observation clearly,the output spectra for 50%, 70%, and 95% coupling
Fig. 5. (Color online) Brillouin Stokes signal power versus BPwavelength at different output coupling ratios.
Fig. 6. (Color online) Output spectra of the BS signal tunabilityat different BP wavelength for 135mW of 1480nm pump powerwith (a) 95%, (b) 70%, and (c) 50% output coupling ratio.
ratioswitha5nmstepareplottedasdepicted inFig.6.The output spectrum is clean from any self-lasingcavity modes for any injected BP wavelength asshown in Fig. 6(a). The self-lasing cavity modes arenot clearly suppressed when the injected BP wave-length is detuned away from the EDF peak gainaround 1560nm for the output coupling ratio of70% and 50% as depicted in Figs. 6(b) and 6(c),respectively.In terms of noise feature, the intensity noise of the
laser output can be considered as the weighted sumof twomain contributions: the amplified spontaneousemission and the Brillouin pump fluctuations con-verted into the Stokes signal fluctuations. It hasbeen experimentally confirmed that the intensitynoise of the Stokes signals was below the Brillouinpump power noise in the spectral range 100kHz–100MHz except at the multiple free spectral rangeof the cavity [22], and it is shot noise limited to−155dB=Hz for the spectral range from 100MHz–18GHz [23]. In addition, it was theoretically proventhat the relative intensity noise of the Brillouinpump was transferred to the first-order BrillouinStokes signals after decays 20dB=decade in the highfrequency regime [24]. The maximum noise reduc-tion of 40–60dB was observed experimentally atantiresonant frequencies that are multiples of halfthe cavity free spectral range [25].
4. Conclusion
In conclusion, we have presented the characteristicsand performance of a single-wavelength BEFL sys-tem with a Brillouin pump preamplification techni-que in the ring-cavity system at different outputcoupling ratios. Through experimental analysis,the efficiency of the BEFL operation is obtained withan optimum output coupling ratio of 95%. At the op-timum output coupling ratio, a high peak power of a28:7mW single-wavelength BS signal is obtained fora BP power of 1:6mW and the 1480nm pump poweris set at 135mW. The BS signal power increases withthe increment of the output coupling ratio from 10%to 95%. For the tunability study, the increment of theoutput coupling ratio from 10% to 95% generateswider tuning ranges. At 95% of the output couplingratio, the BS signal can be tuned over the range of60nm from 1520 to 1580nm without the appearanceof the self-lasing cavity modes at the fixed 1480nmpump power of 135mW, while the BP is set to1:6mW. The 95% output coupling ratio has beenfound to produce the highest output power and thewidest tunability of the BS laser.
This work was partly supported by the Ministryof Higher Education, Malaysia and the UniversitiPutra Malaysia under research grant 05-04-08-0549RU and Graduate Research Fellowship.
References1. T. Chang, D. Y. Li, T. Koscica, H-L. Cui, Q. Sui, and L. Jia,
“Fiber optic distributed temperature and strain sensing
system based on Brillouin light scattering,” Appl. Opt. 47,6202–6206 (2008).
2. S. Molin, G. Baili, M. Alouini, D. Dolfi, and J-P. Huignard,“Experimental investigation of relative intensity noise inBrillouin fiber ring lasers for microwave photonics applica-tions,” Opt. Lett. 33, 1681–1683 (2008).
3. E. Granot, S. Sternklar, H. Chayet, S. Ben-Ezra, N. Narkiss, N.Shahar, A. Sher, and S. Tsadka, “10Gbit=s optical wavelengthconverter with a Brillouin scattering-based spectral filter,”Appl. Opt. 44, 4959–4964 (2005).
4. M. G. Herráez, K. Y. Song, and L. Thévenaz, “Optically con-trolled slow and fast light in optical fibers using stimulatedBrillouin scattering,” Appl. Phys. Lett. 87, 081113 (2005).
5. M. Lee, R. Pant, andM. A. Neifeld, “Improved slow-light delayperformance of a broadband stimulated Brillouin scatteringsystem using fiber Bragg gratings,” Appl. Opt. 47, 6404–6415(2008).
6. C.H. Rowe,U. K. Schreiber, S. J. Cooper, B. T. King,M. Poulton,and G. E. Stedman, “Design and operation of a very large ringlaser gyroscope,” Appl. Opt. 38, 2516–2523 (1999).
7. L. F. Stokes, M. Chodorow, and H. J. Shaw, “All-fiber stimu-lated Brillouin ring laser with submilliwatt pump threshold,”Opt. Lett. 7, 509–511 (1982).
8. P. Bayvel and I. P. Giles, “Evaluation of performanceparameters of single-mode all-fiber Brillouin ring lasers,”Opt. Lett. 14, 581–583 (1989).
9. J. C. Yong, L. Thevenaz, and B. Y. Kim, “Brillouin fiber laserpumped by a DFB laser diode,” J. Lightwave Technol. 21, 546–554 (2003).
10. S. P. Smith, F. Zarinetchi, and S. Ezekiel, “Narrow linewidthstimulated Brillouin fiber laser and applications,” Opt. Lett.16, 393–395 (1991).
11. A. Debut, S. Randoux, and J. Zemmouri, “Linewidth narrow-ing in Brillouin lasers: theoretical analysis,” Phys. Rev. A 62,023803 (2000).
12. A. Debut, S. Randoux, and J. Zemmouri, “Linewidth narrow-ing in Brillouin lasers,” Phys. Rev. A 62, 023803 (2000).
13. G. J. Cowle and D. Yu. Stepanov, “Hybrid Brillouin/erbiumfiber laser,” Opt. Lett. 21, 1250–1252 (1996).
14. M. H. Al-Mansoori, S. Saharudin, H. A. Rashid, M. A. Mahdi,and M. K. Abdullah, “Characterization of multiwavelengthBrillouin/erbium fiber laser based-on a linear cavity config-uration,” Appl. Opt. 44, 2827–2831 (2005).
15. J. Geng and S. Jiang, “Pump-to-Stokes transfer of relativeintensity noise in Brillouin fiber ring lasers,” Opt. Lett. 32,11–13 (2007).
16. Y. J. Song, L. Zhan, S. Hu, Q. H. Ye, and Y. X. Xia, “Tunablemultiwavelength Brillouin-erbium fiber laser with a polariza-tion-maintaining fiber Sagnac loop filter,” IEEE PhotonicsTechnol. Lett. 16, 2015–2017 (2004).
17. Z. Zhang, L. Zhan, and Y. Xia, “Tunable self-seeded multi-wavelength Brillouin-erbium fiber laser with enhanced powerefficiency,” Opt. Express 15, 9731–9736 (2007).
18. M. H. Al-Mansoori andM. A. Mahdi, “Tunable range enhance-ment of Brillouin-erbium fiber laser utilizing Brillouin pre-amplification technique,” Opt. Express 16, 7649–7654 (2008).
19. M. H. Al-Mansoori, M. A. Mahdi, and M. Premaratne, “Novelmultiwavelength L-band Brillouin-erbium fiber laser utilizingdouble-pass Brillouin pump preamplified technique,” IEEE J.Sel. Top. Quantum Electron. 15, 415–421 (2009).
20. V. Sinivasagam, M. K. Abdullah, F. Isnin, P. Poopalan, and H.Ahmad, “Stokes signal saturation in tunable BEFL system,”Electron. Lett. 34, 1751–1752 (1998).
21. X. Dong, P. Shum, N. Q. Ngo, H. Y. Tam, and X. Dong, “Outputpower characteristics of tunable erbium-doped fiber ringlasers,” J. Lightwave Technol. 23, 1334–1341 (2005).
22. J. Geng, S. Staines, Z. Wang, J. Zong, M. Blake, and S. Jiang,“Highly stable low-noise Brillouin fiber laser with ultranarrow
spectral linewidth,” IEEE Photonics Technol. Lett. 18, 1813–1815 (2006).
23. S. Molin, G. Baili, M. Alouini, D. Dolfi, and J. Huignard,“Experimental investigation of relative intensity noise in Bril-louin fiber ring lasers for microwave photonics applications,”Opt. Lett. 33, 1681–1683 (2008).
24. J. Zhou, Y. Jaouen, L. Yi, and P. Gallion, “Pump to Stokeswaves intensity noise transfer in cascaded Brillouinfiber lasers,” IEEE Photon. Technol. Lett. 20, 912–914 (2008).
25. J. Geng and S. Jiang, “Pump-to-Stokes transfer of relativeintensity noise in Brillouin fiber ring lasers,” Opt. Lett. 32,11–13 (2007).
