Effects of different Raman pumping schemes onstimulated Brillouin scattering in a linear cavity

M. R. Shirazi,1,* M. Biglary,1 S. W. Harun,2 K. Thambiratnam,1 and H. Ahmad1

1Photonics Research Center, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia2Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

*Corresponding author: behshirazi@yahoo.com

Received 23 January 2008; revised 25 March 2008; accepted 7 May 2008;posted 9 May 2008 (Doc. ID 91934); published 2 June 2008

The effects of backward, forward, and bidirectional Raman pumping schemes on stimulated Brillouinscattering (SBS) is investigated in this study. By using a linear cavity, we utilize residual Brillouin pump(BP) and Raman pump (RP) power after each transmission through a 25km single-mode fiber (SMF) usedas a gain medium. The SBS threshold power is reduced in the forward, backward, and bidirectionalRaman pumping schemes by 2.5, 1.75, and 2:75dB, respectively when the 1480nm RP power is fixedat 150mW and the BP wavelength is 1580nm. Surprisingly, it is revealed that the SBS threshold re-duction depends strongly and solely on Raman gain and it is independent of the Raman pumpingschemes. In addition, the effect of Raman amplification on SBS is more effective at the SBS threshold,especially in the bidirectional and forward schemes. © 2008 Optical Society of America

OCIS codes: 290.5900, 290.5860, 060.2320.

Raman and Brillouin scattering are inelasticprocesses in which part of the optical wave poweris absorbed by the transmission medium while theremaining energy is reemitted as a wave (Stokeswave) with a down-shifted frequency called aStokes shift. The growth of Brillouin and RamanStokes waves is governed by the Brillouin-gain gBand the Raman-gain gR. For silica fibers, the peakvalues of gB and gR are about gB ¼ 6 × 10−11m=Wand gR ¼ 7 × 10−14m=W and occur for the Brillouinand Raman Stokes shifts by about 13THz and10GHz, respectively. In addition, stimulated Bril-louin scattering (SBS) and stimulated Raman scat-tering (SRS) have threshold pump powers abovewhich pump power transfer to the Stokes waveincreases rapidly [1]. For signal transmission inoptical fibers, SBS limits the launched power to afew milliwatts because of its low threshold in com-parison to SRS [2]. However, SBS has been advanta-

geously utilized in the past few years for someapplications, such as Brillouin fiber lasers [3,4].

The effect of SBS on the properties of Raman ampli-fiers has been studied in previous works [5]. Thesestudies show that SBS causes saturation of Ramangain with forward or copropagating Raman pumping,where the Raman pump (RP) light is injected in thesame direction as the signal under amplification.Although forward pumping is used in some Ramanamplification schemes, the most common type ofRaman amplifiers currently in use for relative-intensity noise relies on backward or counterpropa-gating Raman pumping in which the RP travels inthe opposite direction of the signal [6]. It has beenindicated that by means of multiwavelength Ramanpumping, it is possible to achieve flattening of Ramangain and broadening of Raman amplification band-width even with backward pumping. However, ithas also been reported thatwithout the use of forwardRaman amplification, flattening of noise figure can-not be realized. This results in what is known as bi-directional Raman amplification where the forwardand backward Raman pumping operates simulta-neously [7]. For further explanation of RP schemes

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and details of definitions, the reader may also refer toRef. [8]. There are still some debates on the advan-tages of either forward or backward Raman pumpingschemes [9]. Since SRS and SBS are third order non-linear optical effects, factors that increase Ramangain also facilitate in the generation of SBS [10].Therefore, when using optical fibers such as disper-sion compensating fibers (DCFs) where the effectivearea of the fiber core is small and the Raman gainis high in comparison with the standard SMFs,SBS readily occurs. To suppress SBS generation,we have to keep the light intensity below the SBSthreshold power, or broaden the Brillouin pump line-width [10,11]. The effect of Raman amplification onSBS threshold power has only been studied in back-ward or forward Raman pumping using differentfibers [12,13].In this work, the effects of the backward, forward,

and bidirectional Raman pumping on SBS perfor-mance in a linear cavity are studied. We use a linearcavity to utilize residual Brillouin pump (BP) and RPpower after each transmission through a 25km SMF(gain medium). The results show that the reductionin SBS threshold power is interestingly related lin-early to Raman gain, and this relationship is inde-pendent of Raman pumping schemes. In addition,Raman gain effect on the SBS is highly increasedaround the threshold especially in forward and bi-directional Raman pumping.Figure 1 shows the experimental setup, where a

25km SMF used as a gain medium located betweenPorts 2 of the two optical circulators (OC1 and OC2)to form a linear cavity. A (2 × 2) 1=99 coupler locatedbetween Port 3 and Port 1 of OC1 helps to inject BPinto the cavity, to send a feedback signal, and at thesame time, to extract part of the output. TheBPsignalis performed using an external cavity tunable lasersource (TLS) amplified by an erbium-doped fiber am-plifier (EDFA) to obtain the maximum power of12dBm at 1580nm wavelength with a 15MHzlinewidth.Thegeneratedbackward-propagatingBril-louin Stokes and the transmitted BP signal reflectedby OC2 are coupled from Port 2 to 3 in the circulatorOC1 and 99% of that signal is reinjected back into thesystem by the coupler. Wavelength-division multi-

plexing couplersWDM1andWDM2areused simulta-neously to inject Ramanpump light into the system inbidirectional Raman amplification. However, to de-crease loss in the cavity for forward Raman pumping,we remove WDM2 in the setup and connect Port 3 tothe Port 1 of OC2. Similarly, WDM1 is removedfor backward Raman pumping and BP is injected di-rectly into the cavity through the 1=99 coupler. TheSMF has a cutoff wavelength of 1161nm with a zerodispersion wavelength of 1315nm, a mode field dia-meter of 9:36 μm, and an attenuation coefficient of0:19dB=km at 1580nm. The backward-propagatingSBS is coupledoutvia the couplerand is characterizedusing an optical spectrumanalyzer (OSA)with a reso-lution of 0:015nm. WDM3 is incorporated in front ofthe OSA to suppress the residual Raman pump lightin the output. In addition, anOSA is used in the setupinstead of a power meter to avoid additional noisesduring measurement. Errors can originate from themeasuring instruments, i.e., TLS, OSA in the setup,and the power meter used to measure the power at1480nm, each with, respectively, 0.05, 0,05, and0:01dB of optical powermeasurement error. The totalerror is calculated using the following equation [14]:

Total error ¼ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiXU2

3

s;

where U is the error or uncertainty of each individualinstrument. For the current setup, the total error inour measurement is about 0:08dBm.

Figure 2 shows the output Brillouin Stokespeak power and the transmitted BP peak poweragainst the input signal BP power, with andwithout 1480nm RPs, in various pumping schemes.Figures 2(a)–2(c) represent backward, forward, andbidirectional Raman pumping, respectively. Beforethe transmitted BP power starts to saturate quicklyaround −5dBm in the schemes, the output power ofthe SBS increases in accordance with the increaseof theBPpower. The SBS threshold power, PSBS, is de-fined as an input BP power at which the transmittedBP peak power equals the output peak power of theSBS [15]. To decrease loss, we remove WDM2 and

Fig. 1. Experimental setup for bidirectional Raman pumping. To decrease loss,WDM2 andWDM1 are removed from the configuration forforward and backward Raman pumping, respectively.

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WDM1 from the configuration for forward and back-ward Raman pumping, respectively. Therefore, whenthe RP power is off, we have different SBS thresholdpowers, i.e., 5.25, 5.6, and 6:75dBm, due to differentlosses for backward, forward, and bidirectionalschemes, respectively. In the experiment, the RPpower is varied from 0 to 150mW. It should also benoted that the SBS threshold power decreases asthe RP power is increased.

The SBS threshold reduction, ΔSBS induced byRaman pumping, is examined and compared for dif-ferent pumping schemes. It is at a maximum withthe bidirectional Raman pumping scheme and at aminimum with the backward. It is also proportionalto theRPpower, as shown inFig. 2. TheSBSthresholdpower is reduced by 1.75, 2.5, and 2:75dB in back-ward, forward, and bidirectional schemes, respec-tively, by the same 150mW RP power at 1480nmand a BP wavelength of 1580nm. To further investi-gate the effect of Raman pumping on SBS, we definethe Raman on–off gain (gain on–off) on the peakpower of Brillouin Stokes as the increase in BrillouinStokes peak power when Raman pumping operates.From Fig. 2, the gain can be measured easily for agiven Brillouin pump power with RP power, I ¼75mW and I ¼ 150mW relative to I ¼ 0mW in thedifferent Raman pumping schemes [Fig. 3(a)]. As ex-pected, adecrease inPSBS is observedwithan increasein the RP power, especially in the case of the forwardand bidirectional Raman pumping schemes. Becauseboth SBS and SRS are related to the third ordernonlinear susceptibility, the reduction of SBS thresh-old with an increase of the Raman pump means SBSdominates over SRS in a competition between them.In other words, SRS can effectively amplify BrillouinStokes only around the Brillouin threshold power, asalready shown in Fig. 2. Therefore, the Raman on–offgain must have a maximum around the SBS thresh-old. The gain starts to show saturation with the ap-pearance of the SBS and is eliminated completelybeyond the SBS threshold power. The gain on theSBS peak is also higher with a wider region for boththe forwardand thebidirectional pumping schemes incomparison to the backward pumping scheme.

Finally, from Figs. 2 and 3(a), the relationship be-tween the SBS threshold reduction and the Ramanpeak gain on–off effect on the SBS threshold canbe deduced as shown in Fig. 3(b). It is interestingthat the relationship is almost linear and simpleindependent of the Raman pumping schemes.

In conclusion, the reduction in the SBS thresholdpower under different Ramanpumping schemes is in-vestigated experimentally in a linear cavity with25km SMF. The results show that SBS threshold de-creaseswith the increase ofRaman-gain effect onSBSand this decrease is much faster for bidirectional andforwardpumping than for backwardpumpingdue to alowerRamangain on theSBS threshold. It is revealedthat the amount of SBS threshold reduction dependsstrongly and solely on the gain effect on it, indepen-dent of Raman pumping scheme.

Fig. 2. Output transmitted BP power, solid lines, and BrillouinStokes power, dashed lines, as a function of the input BP powerin the case of (a) backward, (b) forward, and (c) bidirectionalRaman pumping. The squares, triangles, and circles representthe cases where RP power is 0, 75, and 150mW, respectively.

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Fig. 3. (a) shows effect of Raman on–off gain on the peak power ofBrillouin Stokes against various signal input BP and RP powers.From Figs. 2 and 3(a), we can immediately deduce the SBS thresh-old reduction (SBS TR) against peak Raman on–off gain effect onthe Brillouin threshold, as shown in (b). Different Raman pumpingschemes are indicated by markers, i.e., backward (circle), bidirec-tional (triangle), and forward (square). Solid lines and solid mar-kers represent RP power at 150mW, whereas dashed lines andhollow markers represent RP power at 75mW.

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