Optics Communications 282 (2009) 3780–3784

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Optics Communications

journal homepage: www.elsevier .com/ locate/optcom

Gain and gain-flatness improved photonic crystal fiber Raman amplifier basedon a dual-pass amplification configuration with single pump laser

Yuan Li a, Yange Liu a,*, Jiaqi Zhao a, Bing Zou a, Jianbo Xu a, Weijun Tong b, Huifeng Wei b

a Key Laboratory of Opto-Electronic Information and Technology, Ministry of Education and Institute of Modern Optics, Nankai University, Tianjin 300071, Chinab Yangtze Optical Fiber and Cable Co., Ltd., Wuhan, China

a r t i c l e i n f o

Article history:Received 20 March 2009Received in revised form 9 June 2009Accepted 9 June 2009

Keywords:Raman fiber amplifier (RFA)Photonic crystal fiber (PCF)Double-passGain flattening

0030-4018/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.optcom.2009.06.028

* Corresponding author.E-mail address: ygliu@nankai.edu.cn (Y. Liu).

a b s t r a c t

A gain and gain-flatness improved L-band dual-pass Raman fiber amplifier (RFA) utilizing a photoniccrystal fiber (PCF) as gain medium is demonstrated. By introducing complementary gain spectra of typicalforward and backward pumping single-pass RFA using the same PCF, we finally achieve average net gainlevel of 22.5 dB with a ±0.8 dB flattening gain in 20-nm bandwidth from 1595 nm to 1615 nm, which israre in RFAs with only one single pump and no flattening filter. Compared with the single-pass pump con-figurations, gain level, flatness and bandwidth are greatly improved by using the dual-pass amplificationconfiguration. The limitation of this configuration caused by multi-path interference (MPI) noise andstimulated Brillouin scattering (SBS) is also discussed.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Rapid developments of wavelength division multiplexing trans-mission systems have contributed a lot to the capability of compo-nents in networks, such as intensity of channels and signalbandwidth. As one of the enabling technologies for long-distanceand high-capacity fiber optical transmission systems, RFA is apromising choice for extending the operational range of opticaltransmission systems to L- and S-bands, which are out of the band-width of erbium-doped fiber amplifiers (EDFAs). Especially withthe progress in pump laser sources, higher power and wider oper-ational wavelength range make RFAs more efficient and practical.For most RFAs using dispersion compensating fiber (DCF) and sin-gle pump, signal reflection and pump reusing are common tech-niques to enhance gain, and gain-clamping methods andflattening filters are used to flatten the gain. Tang et al. used wide-band reflector to reflect both signals and single pump, achieving a3-dB bandwidth of 22 nm in C-band with a clamped peak net gainof 14.0 dB [1]. Dung demonstrated that the configuration of for-ward pumping reflective RFA was better than backward for highernet gain and lower noise figure (NF) [2]. Ahn et al. achieved 3-dBbandwidth of more than 25 nm with peak net gain of 24.0 dB usingfiber loop mirror (FLM) [3]. Mechanically induced long-period fibergratings used as flattening filter produced flat gain with averagenet gain of 12.8 dB and ±0.8 dB flattening gain over 40 nm in S-band, but sacrificing more than 5.0 dB gain [4]. Similarly, maxi-

ll rights reserved.

mum of 15.0 dB gain was sacrificed to achieve flat gain in 50 nmusing a Chirped fiber Bragg grating (CFBG) as filter [5]. Althougheffective, this method sacrificed gain level badly, thus more tech-niques were being researched to achieve flat gain in bandwidthas broad as possible. Some schemes are demonstrated to improvegain performance. In Ref. [6], second-order pumping scheme wasproved to be better than first-order pumping scheme, for reducinggain fluctuation from 2.5 dB to 0.3 dB over 10 nm in C-band.

In recent years, PCF has attracted lots of attention due to itsflexibly designable structure and unique optical characteristics. Ithas been already demonstrated that PCF can greatly enhance non-linear effects, and therefore, it provides an optimal solution as Ra-man amplification fiber, which can achieve high gain efficiency anddispersion compensation in desired waveband. The Raman gaincharacteristics and coefficient have been investigated in Refs. [7–10], and experiments have been carried out in Refs. [11,12] dis-cussing gain and noise performance. Ref. [11] focused on testifyingthe Raman gain characteristics of the proposed PCF, and Ref. [12]demonstrated the improved system performance by inserting asection of PCF in a distributed RFA. To date, PCFs have not beenwidely used in RFA schemes mainly because of high attenuationand mode loss when connecting with standard single mode fiber(SMF). So far to the author’s knowledge, only few experimentalstudies have been done to investigate the gain flattening of PCF-based RFAs.

In this paper, a gain and flattening gain bandwidth improveddual-pass RFA utilizing pure silica PCF as gain medium is proposedand discussed. Under single high pump power, the gain spectra ofthe typical forward and backward pumping single-pass RFAs using

Y. Li et al. / Optics Communications 282 (2009) 3780–3784 3781

the same PCF present some special complementary characteristics,based on which we introduce the dual-pass configuration. Withinthe bandwidth of 20 nm from 1595 to 1615 nm, average net gainof 22.5 dB with flatness of ±0.8 dB is achieved, which is presentlythe best performance with such flat and broad gain spectrum inPCF-based RFA with only one single pump and no flattening filter,to the author’s knowledge. Compared with the single-pass config-urations, gain level is improved by more than 60%, and gain-flat-ness in the same bandwidth is improved from ±2.0 dB for theforward scheme and ±1.4 dB for the backward scheme to only±0.8 dB.

Fig. 2. The microscope image of the PCF’s section structure.

2. Experimental setup

The elementary configurations of the investigated single-passRFAs are depicted in Fig. 1a and b, which are widely used in fiberamplifiers, and Fig. 1c depicts the proposed dual-pass Ramanamplification configuration. A commercial tunable laser with awavelength range from 1530 nm to 1630 nm is used as the inputsignal source, and the signal power is tunable too. The linewidthof the laser is about 0.2 MHz. After being circulated through anoptical circulator, the signal is coupled into the gain fiber by a1495/1600 nm wavelength division multiplexer (WDM). Here, weuse the circulator to protect the tunable laser from the excess lightcaused by the high power pump. The pump light is provided by aRaman laser with a maximum power of 6.0 W at 1495 nm. InFig. 1c, a fiber loop mirror (FLM) consisting of a 3-dB coupler ofL-band is connected to one end of the WDM in order to reflect sig-nals so that signals can be amplified twice. The gain medium usedin the work is a 2.0-km PCF, with pure silica core and five layers ofhexagonally distributed air holes, as shown in Fig. 2. The PCF is fab-ricated by the Yangtze Optical Fiber and Cable Co., Ltd. of China.The normalized hole diameter d/K is 0.7 and the hole diameter dis 3.9 lm. The attenuation coefficient is 1.7 dB/km at 1495 nmand 1.4 dB/km at 1600 nm, respectively. The splicing loss betweenPCF and SMF is estimated to be 1.1 dB for each splicing point. AYokogawa optical spectrum analyzer (OSA) is utilized to monitorthe output signals and calculate the gain and NF. The total lossfor signals around 1600 nm is 5.2 dB for the single-pass configura-

Fig. 1. (a) The single-pass forward pumping PCF Raman amplifier configuration. (b) The spumping PCF Raman amplifier configuration.

tions and 12.5 dB for the dual-pass configuration (containing theinserting losses of circulator and FLM).

3. Results and discussion

Firstly, we study the net gain as a function of different signalwavelength in the single-pass configurations. The power of inputsignal is fixed at �7.0 dBm, and pump power is set to be 4.0 W.The PCF is pumped forward and backward, respectively, and weget the gain spectra of these two pumping structures, plotted inFig. 3. Firstly, these results present that both of the gain spectrashow the shape of two peaks, and the frequency shifts of thesetwo peaks are in good agreement with the Raman gain spectrumof fused silica in Ref. [13], which exhibits a broad peak at

ingle-pass backward pumping PCF Raman amplifier configuration. (c) The dual-pass

Fig. 3. The gain spectra of the three FRA configurations when pump power is 4.0 W.Black square, forward pumping configuration, input signal power, �7.0 dBm; redtriangle, backward pumping configuration, input signal power, �7.0 dBm; blue dot,dual-pass configuration, input signal power, �17.0 dBm. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

Fig. 4. The performance of on–off gain versus pump power in the dual-passscheme.

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440 cm�1 (13.2 THz) and a narrow peak at 490 cm�1 (14.7 THz) forpure silica glass. However, as for traditional fiber, doping germano-silicate into pure silica will significantly change the intensity of Ra-man scattering light, the shape of the Raman spectrum, and alsothe Raman gain coefficient (RGE) [14,15]. For the RGE of fibers withgermano-silicate (such as GeO2) doped in core region, such as highnonlinear fiber (HNF) and dispersion compensation fiber (DCF), thepeak at 490 cm�1 disappears and the peak at 440 cm�1 is enhanced[16]. The PCF we use is made of pure silica, thus we can expect atwo-peak Raman gain shape, as shown in Fig. 3. Secondly, the re-sults presented in Fig. 3 indicate apparent differences in positionand gain level, although bandwidth in which signal can be ampli-fied (net gain is more than 8.0 dB over 30 nm) is almost the samein these two structures. There is a wavelength shift of about5 nm between the two spectra, and the gain level difference ofthe peak in longer wavelength is about 3.0 dB (15.8 dB and12.5 dB, respectively), which is mainly contributed by the differentpumping schemes. Since common RFAs using DCF show onesmooth gain peak in a broad bandwidth, these differences arenot quite noticeable. However, in our experiment, when sure silicaPCF is pumped, we can clearly notice the tiny variations betweenthe spectra. Furthermore, because of the accumulation of nonlineareffects caused by the intense pump power, the interaction betweensignal and Raman scattering light may differs in intensity, positionand wavelength range for the forward and backward pumpconfigurations.

Due to the 5-nm wavelength shift, we believe the two spectracan compensate each other if the signal undergoes both forwardand backward amplifying and therefore gain level can be enhancedand flattened at the same time. The gain spectrum of the dual-passconfiguration under the same pump power (4.0 W) is exhibited inFig. 3 too. Input signal power is set to be �17.0 dBm to avoid sat-uration. As backward gain compensates forward gain when signalis reflected by the FLM and goes though the PCF again, the dual-pass configuration performs better gain and gain-flatness. Withinthe bandwidth as broad as 20 nm, from 1595 nm to 1615 nm, thegain level is 22.5 dB with a ±0.8 dB gain fluctuation, and the opticalsignal to noise ratio (OSNR) is larger than 35.0 dB. As far as theauthor’s knowledge, none of such flat and broad gain spectrumhas been ever achieved in PCF–RFAs with only single pump. Com-pared with the single-pass schemes, gain level, flatness and band-

width are greatly improved. For net gain, the peak value growsfrom 15.1 dB and 13.8 dB in the two single schemes to 23.3 dB inthe dual-pass scheme, and average net gain level increases morethan 60%. Within the bandwidth from 1595 nm to 1615 nm, theflatness is improved from ±2.0 dB for the forward scheme and±1.4 dB for the backward scheme to only ±0.8 dB for the dual-passscheme. Both of the 3-dB bandwidths for the backward and thedual-pass schemes are 25 nm, broader than the 14 nm for the for-ward scheme.

The gain performance of the dual-pass RFA has also been inves-tigated. The input signal power is fixed to be �22.0 dBm and threedifferent wavelengths are chosen for the measurements. Seen fromFig. 4, the signals at 1600 nm and 1610 nm acquire almost thesame on–off gain at different pump power, whereas a little smallerthan gain of the signal at 1605 nm, at higher pump power. Thegains of three signals are in good agreement with the gain spec-trum shown in Fig. 3. All the three signals do not saturate at pumppower as high as 4.0 W, which makes the proposed RFA a promis-ing candidate for high power signal amplification.

The gain saturates differently at different pump power levels.Fig. 5 plots the net gain versus input signal power at two differentpump powers, 2.8 W and 4.0 W, at a fixed wavelength of 1605 nm.It is obvious that signal saturates more quickly at higher pumppower, and several reasons contribute to this phenomenon. Onone hand, high pump power and large signal result in high path-average power which increases the coupling between pump lightand signal light. Consequently, pump power exhausts more deeplywhen signal power is intense enough [17]. On the other hand, asmentioned in Ref. [18], stimulated Brillouin scattering (SBS) limitsthe power level of larger signals in RFAs. Because of the high gainefficiency in dual-pass RFA, a higher SBS builds in the amplifier andthus degrades signals. Considering the reasons discussed above,appropriate input signal power should be chosen to avoid SBS.The saturation powers of input signal are �12.9 dBm and1.1 dBm at pump powers of 4.0 W and 2.8 W, respectively. It is no-ticed that when pump power is 4.0 W, we get unstable output sig-nal power with input signal beyond saturation power.

At these two chosen pump power levels, we investigate the per-formance of net gain and total noise figure (NF) as a function of in-put signal wavelength, depicted in Fig. 6. Compared with the gainspectrum measured when input signal power is �12.0 dBm andpump power is 2.8 W, much flatter gain spectrum and lower NFare achieved when input signal power is �17.0 dBm and pumppower is 4.0 W. The optical signal to noise ratio (OSNR) is larger

Fig. 5. The gain characteristics of dual-pass scheme under different pump powers.

Fig. 6. The gain and NF performance of the dual-pass scheme under different pumppowers. Blue dot, pump power is 4.0 W; red triangle, pump power is 2.8 W. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

Fig. 7. Optical spectra of the amplified signal limited by the SBS.

Y. Li et al. / Optics Communications 282 (2009) 3780–3784 3783

than 35.0 dB at either pump power. Although the gain performanceis acceptable, we have to admit that the NF, to some extent, de-creases the overall performance of the dual-pass RFA. MPI noise in-duced by reflected Rayleigh backscattering is the major source ofnoise in the dual-pass system, which was validated in Refs.[1,19]. Since Rayleigh backscattering is proportional to the inputsignal power, higher input signal power leads to larger NF.

With large signals injected into the system, SBS stokes can beobserved in the output end of OC, depicted in Fig. 7, and thereforedegrade signal performance. We fix the pump power at 4.0 W andadjust input signal power gradually. When input signal power is�15.5 dBm, first-order Brillouin stokes appear. As shown inFig. 5, the gain level starts to drop just at this input signal power.Go on to increase input signal power, the peak of signal increasesslowly and energy is transferred to the first-order stokes whichmakes it rise quickly. When the input signal power is adjusted to�13.5 dBm, we can see an obvious second-order SBS stokes, andthe signal saturates already. The measured Brillouin wavelengthshifts of first-order stokes and second-order stokes are 0.088 nmand 0.092 nm, respectively. It is obvious that the dual-pass RFA isnot suitable for large signal amplification, and for practical use,multi-channel will distribute energy among several signals andthus reduce SBS [18].

4. Conclusion

In conclusion, we demonstrate a high gain and gain flatteningdual-pass RFA with single large pump power based on the investi-gation of single-pass forward and backward pumped RFAs usingthe same PCF as gain medium. The complementary gain spectraof the two single-pass configurations enable high and flat gain of22.5 dB with flatness of ±0.8 dB within a bandwidth of 20 nm inthe dual-pass amplification configuration, which is rare in RFAwith only one single pump and no flattening filter. Compared withthe two elementary single-pass configurations, the gain level, flat-ness and bandwidth of the dual-pass configuration are greatly im-proved. The MPI noise, however, limits the performance of theproposed dual-pass RFA, especially when pump power is intense,and SBS is the major limitation for amplified large signals. Fartherwork is required to improve the performance of the dual-pass con-figuration and make it become a practical usage.

Acknowledgements

This work was supported by the Tianjin Natural Science Foun-dation under Grant No. 06YFJZJC00300, the National Natural Sci-ence Foundation of China under Grant Nos. 10774077 10674074,50802044 and 60736039. The authors thank Ming Feng and Yon-gNan Li for their help in experiment, and JiangBing Du for revisingthe English usage.

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