All-fibre flat-top comb filter based on high-birefringence photonic crystal fibre loop m irror

L IU Shuo,L IU Yan-ge,L IU Run-yu,L IHong-xiao,XIE Hong-bin,WANG Shu

(Key Laboratory of Opto-electronic Info rm ation and Technology,M inistry of Education,Institute of M odern Optics,N ankai University,Tianjin300071,China)

1 Introduction

Wavelength-division-multiplexing(WDM) isan attractive fibre-opticalcommunication technique.Optical comb filters are key components to control light signals for such techniques. Optical comb filters can be used for signal dropping or adding wavelengths in optical ne tworks.The flat-top is one of the essential features for comb filters in highspeed optical communication systems.The s moother the top of the filter,the smaller the impact to the selected signal and thus the more stable the system becomes.A fibre Sagnac loop mirror filter,which has very good compatibilitywith fibre and stable perfor mance,polarization independence and low dispersion,has attracted considerable interest in recent years due to its potential application in WDM systems[1,2],multiwavelength fiber lasers[3-6]and fibre amplifiers[7,8].

The flat-topped filter based on the fiber Sagnac loop mirror has been studied by other research groups. In Ref.[9],a flat-top comb filter was developed using a fifth-order Solc Sagnac filterwhich includes five sections of high birefringence fibre in a Sagnac fibre loop mirror.In Ref.[10],an all-fibre flat-top comb filter was fabricated by cascading two high birefringence Sagnac fibre loop mirrors.However,these schemes are difficult to construct.In addition,they are composed of conventional high birefringence fibre,whose temperature stability is not ideal.

In this paper,an all-fibre flat-top comb filter based on a high birefringence photonic crystal fibre loop mirror is proposed and demonstrated.The filter is composed of a 3 dB fibre coupler,single mode fibres and two sectionsof high birefringence photonic crystal fibres.Compared to the filters in Ref.[9,10],the proposed configuration is more compact.Further more,the filter has better temperature stability than the filters consisting of the conventional high birefringence fibre. The temperature sensitivity of the proposed filter based on the photonic crystal fibre is about 0.047 5 nm/℃,which is only 4.36% of the termerpatre sensitivity of the filter based on a Panda polarization maintaining fibre loop mirror.

2 Experiment setup and operation principle

Fig.1 shows the experiment setup which consistsof a Broadband Source(BBS),a fibre Sagnac loop mirror and an Optical Spectrum Analyzer(OSA)with a maximum resolution of 0.01 nm.The fiber Sagnac loop mirror is formed by a 3 dB fibre coupler,two Polarization Controllers(PC),and two sections of high birefringence photoniccrystal fibres whose lengths are 2 m and 4 m,respectively.The beat length of the photonic crystal fibre is about 4.8 mm at1 550 nm.The 3 dB fibre coupler and the PCs are allmade of single mode fibre.The BBS has an operating wavelength range from 1 525 nm to 1 565 nm that provides the input and an OSA is used to measure the output spectrum of the filter.

In the fibre loop mirror,additionalphase difference is generated between forward-and backwardpropagating lightsbecause ofthe birefringence effect. This accounts for coherent optical interference between the input and output.

When there are two sections of birefringence fibre in the fibre loop mirror(Fig.1),the transmittivity can be calculated as[11]

whereAi=1,2.θ1,θ2andθ3denote the rotation angles of the optical signals in the single mode fibres and polarization controllers,liis the length of HBF(i),noandneare the refractive indices of the ordinary(o)and extraordinary(e)light,λis the wavelength andkis the power-splitting ratio of the coupler.

For the 3 dB coupler,we have

It can be seen from Eq.(2)that the lengths of the fibre used in the fibre loop mirror andθidetermine the trans mittivity.After the lengths are determined by adjusting the state of PCs,differentθican be produced,so that different filter characteristics can be achieved.Under certain circumstances,we may achieve a flat-top comb filter.

3 Results and discussion

From Fig.2 to Fig.6,we present the numerical simulation results calculated from Eq.(2)and corresponding exper imental results. It can be seen that different types of filters can be realized with different values ofθi. It follows from Fig.6(a)and Fig.6(b)that whenθ1+θ3=0.08π andθ2=0.64π,and the results correspond to a flat-top comb filter with an extinction ratio of about 20 dB. It also follows that the experimental and numerical results are in good agreement.

Fig.2 Trans mittivity of high birefringence fibre loop mirrorwhenθ1+θ3=0.5π,θ2=0.1π.

Fig.3 Transmittivity of high birefringence fibre loop mirrorwhenθ1+θ3=0.38π,θ2=0.64π.

Fig.4 Transmittivity of high birefringence fibre loop mirrorwhenθ1+θ3=0.32π,θ2=0.64π.

Fig.5 Transmittivity of high birefringence fibre loop mirrorwhenθ1+θ3=0.64π,θ2=0.64π.

Fig.6 Transmittivity of high birefringence fibre loop mirrorwhenθ1+θ3=0.08π,θ2=0.64π.

The difference between the experiment results and the numerical s imulation results,such as larger insertion loss,uns mooth spectra and deflections of peak/notch wavelengths may be due to:1)low power light source;2)loss and interference caused by splicing points;and 3)return loss.

For the case of flat-top filter,we also investigate its stability at room temperature.The spectra of the filter are repeatedly scanned by the OSA over intervalsof 5 min at room temperature and are shown in Fig.7.The excellent stability of this flat-top filter at room temperature can be seen from the spectra as a lmost identical.

Since the photonic crystal fibre has excellent temperature stability,we expect that the proposed flat-top filter will also have excellent temperature stability. The filter was placed in a temperature controlled oven and the wavelength shift of the notch around 1 550 nm was recorded,and the results are shown in Fig.8.

Fig.7 Spectra of flat-top comb filter based on high birefringence photonic crystal fibre loop mirror recorded every 5 min at room temperature.

W ithin the range of temperature change of 16℃,the notch wavelength shifts by only 0.76 nm corresponding to a temperature sensitivity of about 0.047 5 nm/℃.

Fig.8 Wavlength shift of flat-top comb filter based on high birefringence photonic crystal fibre loop mirror due to the change of temperature.

In order to compare the temperature stability of a flat-top filter based on a conventional high birefringence fibre loop mirror with a high birefringence photonic crystal fibre loop mirror,a fibre loop mirror with two sections of Panda polarization maintaining fiberwas assembled.The lengths of Panda polarization maintaining fibre were 2.9 m and 5.8 m.The beat length of the Panda polarization maintaining fibre is about 2.9 mm at 1 550 nm.After the careful adjustment,the fibre loop mirror shows the following flat-top filter characteristics(Fig.9).

Fig.9 Spectrum of flat-top comb filter based on Panda polarization maintaining fibre loop mirror.

Through experiment observation,the stability of the flat-top filter based on a Panda polarizationmaintaining fibre loop mirror at room temperature is found to be less stable.The spectra of the filterwere repeatedly scanned by the OSA with an interval of 5 min at room temperature and the results are shown in Fig.10. It can be seen that the spectrum has large fluctuations.

Fig.10 Spectra of flat-top comb filter based on Panda polarization maintaining fibre loop mirror recorded every 5 min at room temperature.

Similarly,the temperature stability of the filter based on the Panda polarization maintaining fibre loop mirrorwas also measured and the experimental resuts are shown in Fig.11.W ithin a range of temperature change of less than 8℃,the notch wavelength shifts 8.72 nm,so that the temperature sensitivity of the filter is 1.09 nm/℃.The lack of temperature stability will affect any signal tranmission significantly and hence this filter cannot be directly used in a practical fibre-optical communication system.Compared with a flat-top filter based on a conventional high birefringence fibre loop mirror,the proposed flat-top filter based on a high birefringence photonic crystal fiber loop mirror has a better temperature stability,which is only 4.36% of the temperature sensitivity of the for mer.

Fig.11 Wavlength shift of flat-top comb filter based on Panda polarizaitonmaintaining fibre loop mirror due to the change of temperature.

4 Conclusions

In this paper,a flat-top filter based on a high birefringence photonic crystal fibre loop mirror is proposed and realized.The improved temperature stability of this filter is also demonstrated by experiments.If developments can be made to improve the extinction ratio,s moothness and other parameters,this filter can be expected to be used in practical fibre-optical communication systems in the future.

[1] HUANG YL,L IJ,MA X R.High extinction ratioMach-Zehnder interferometer filter and implementation of single-channel optical s witch[J].Opt.Comm un.,2003,222(1-6):191-195.

[2] CVECEK K,SPONSEL K,STEPHAN C,et al..Phase-preserving amplitude regeneration for a WDM RZ-DPSK signal using a nonlinear amplifying loop mirror[J].Opt.Express,2008,16(3):1923-1928.

[3] LEE YW,JUNG J H,LEE B H.Multiwavelength-s witchable SOA-fiber ring laser based on polarization-maintaining fiber loop mirror and polarization beam splitter[J].IEEE Photonics Technol.Lett.,2004,16(1):54-56.

[4] SOHN K R,TAEK K.Multiwavelength all-fiber ring laser using side-polished fiber comb filter and mechanically for med long-period fiber gratings[J].IEEE Photonics Technol.Lett.,2005,17(2):309-311.

[5] SUN G Y,MOON D S,L IN A X,et al..Tunablemultiwavelength fiber laser using a comb filter based on erbium-ytterbium co-doped polarization maintaining fiber loop mirror[J].Opt.Express,2008,16(6):3652-3658.

[6] WANG J,ZHENG K,PENG J,et al..Theory and experiment of a fiber loop mirror filter of two-stage polarization-maintaining fibers and polarization controllers formultiwavelength fiber ring laser[J].Opt.Express,2009,17(13):10573-10583.

[7] L I SH P,CH IANG K S,GAMBL INGW A.Gain flattening of an erbium-doped fiber amplifier using ahigh-birefringence fiber loop mirror[J].IEEE Photonics Technol.Lett.,2001,13(9):942-944.

[8] J IA D F,WANG Y Y,BAO H M,et al..Experimental studies on dual-wavelength all-optical gain-clamped erbium-doped fiber amplifier[J].J.Optoelectronics Laser,2007,18(3):273-275.

[9] HAN Y,L IQ,L IU XM,et al..Architecture of high-order all-fiber birefringent filters by theuse of the Sagnac interferometer[J].IEEE Photonics Technol.Lett.,1999,11(1):90-92.

[10] MA X R,WU ZH F,KA I G Y,et al..Polarization-independent all-fiber flat-top comb filter[J].Opt.Fiber Technol.,2006,12(1):1-9.

[11] L IU Y G,L IU B,FENG X H,et al..High-birefringence fiber loop mirrors and their applications as sensors[J].Appl.Optics,2005,44(12):2382-2390.