www.elsevier.com/locate/optcom

Optics Communications 272 (2007) 102–106

Novel in-line fiber polarization beam splitter usinghigh-birefringence fiber Bragg grating

Yu-Lung Lo a,b,c,*, Bo-Rong Chue a, Ying-Hao Chen a

a Department of Mechanical Engineering, National Cheng Kung University, Tainan, Taiwan, ROCb Institute of Nanotechnology and Microsystem Engineering, National Cheng Kung University, Tainan, Taiwan, ROC

c Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan, ROC

Received 3 January 2006; received in revised form 23 October 2006; accepted 14 November 2006

Abstract

This study proposes a novel fiber-type polarization beam splitter (PBS) based on a high-birefringence fiber Bragg grating (Hi-BiFBG). The reflective and transmitted modes of the Hi-Bi FBG are used to separate the p- and s-waves of an optical signal. The exper-imental results show that a 36 dB extinction ratio can be obtained in the reflective mode. However, in the transmitted mode, the extinc-tion ratio is just 4.78 dB since the reflectivity of the Hi-Bi FBG is only 34.75%. It is shown analytically that the extinction ratio in thetransmitted mode can be improved to 20 dB by specifying a Hi-Bi FBG with a reflectivity of 99%. Finally, it is shown that the Braggwavelength of the in-line fiber PBS can be tuned electrically to comply with the working wavelength.� 2006 Elsevier B.V. All rights reserved.

Keywords: Fiber Bragg grating; High-birefringence fiber; Polarization beam splitter

1. Introduction

The polarization state of light guided by an optical fiberis an important characteristic in fiber optic communica-tions, optical gyroscopes and interferometric sensors.Obtaining a high accuracy in signal processing or opticalmeasurement applications requires the use of light withwell-controlled polarization. In-line fiber polarization beamsplitters (PBSs) provide a convenient means of separatinglaunched light into its p-mode and s-mode polarizationcomponents, and are more suitable for optical communica-tion systems than bulk-type PBS devices. In-line fiber polar-izers are commonly applied in fiber interferometric sensorsand are frequently used in polarization-mode-dispersionand high-coherence transmission applications.

0030-4018/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2006.11.066

* Corresponding author. Address: Department of Mechanical Engineer-ing, National Cheng Kung University, Tainan, Taiwan, ROC. Tel.: +8866 2757575x62123; fax: +886 6 2352973.

E-mail address: loyl@mail.ncku.edu.tw (Y.-L. Lo).

Various in-line fiber-based polarization-controllingcomponents have been developed. For example, Ma andTseng [1] presented a liquid crystal clad fiber polarizerbased on a side-polished fiber. In their polarizer, a single-mode fiber was glued in a curved V-shaped groove withina silicon wafer and was then side-polished to a proximityof 0.1 lm from the core. After side-polishing, liquid crys-tals were dropped onto the polished region. It was shownexperimentally that the polarizer achieved an extinctionratio of 42 dB. Diez et al. [2] developed an in-line fiberpolarizer based on resonant excitation of the hybrid plasmamode. The device was fabricated by tapering a standardstep-index fiber to form an adiabatic taper with a uniformwaist. A gold film was then evaporated on one side of thewaist to form an asymmetric optical waveguide. In [3,4],the authors demonstrated polarizers with extinction ratiosexceeding 30 dB formed by fabricating long-period gratingswithin Hi-Bi optical fibers. It was shown that the transmis-sion spectra of the in-line polarizers were characterized bya splitting of the loss peaks, thereby enabling the realiza-tion of wavelength-selective polarization filters.

Y.-L. Lo et al. / Optics Communications 272 (2007) 102–106 103

Davis et al. [5] developed a PBS featuring polarizationgratings written onto a zero-twist nematic liquid-crystaldisplay. The PBS proved capable of separating diffractedlight into two orthogonally polarized orders that either lin-early or circularly polarized. Sakamoto et al. [6] presentedan All-PANDA-fiber polarization beam splitter/combinerand showed that by coupling two PANDA fibers of appro-priate lengths at a suitable alignment angle, the launchedlight could be separated into two orthogonal polarizationswith a crosstalk of less than �25 dB. Zhou et al. [7]reported a near-ideal in-fiber polarizer implemented using45� tilted fiber Bragg grating structures inscribed in ahydrogenated Ge-doped fiber. The results showed that apolarization-extinction ratio of 33 dB could be achievedover a 100 nm operation range near a working wavelengthof 1550 nm.

The present study develops a novel in-line fiber PBSbased on a Hi-Bi FBG. By fusing the Hi-Bi FBG to a cir-culator, the device is capable of splitting the incident lightinto two orthogonal polarizations regarding to one specificBragg wavelength. Furthermore, by depositing a thin metalfilm onto the Hi-Bi FBG [8,9], one specific Bragg wave-length of the in-line fiber PBS can be tuned electrically toenable the device to function over a wider range of workingwavelengths.

2. In-line fiber polarization beam splitter

Fig. 1 presents a schematic illustration of the proposedin-line fiber PBS. Since the two orthogonal axes of theHi-Bi FBG have different refractive indices, the reflectiveoptical spectrum has two different wavelength peaks inthe two orthogonal polarizations. When monochromaticlight with a wavelength corresponding to one of the Braggwavelengths passes though the Hi-Bi FBG, s- (or p-) polar-ization light is reflected, while p- (or s-) polarization light istransmitted. In other words, the PBS separates thelaunched light into different polarization modes via itsreflective and transmitted modes, respectively. As shownin Fig. 1, the in-line fiber PBS is coated with a thin metalfilm such that the one specific Bragg wavelength in theHi-Bi FBG can be tuned by the application of an electricalvoltage in order to comply with the working wavelength.

In general, a FBG selectively reflects an optical signal inaccordance with its Bragg wavelength. The conventionalBragg equation for a FBG has the form

Fig. 1. Novel in-line fiber PBS.

kB ¼ 2neffKB ð1Þwhere neff is the effective refractive index of the optical fiberand KB is the pitch of the Bragg grating. Since the fast andslow axes of a Hi-Bi optical fiber have two different refrac-tive indices, Eq. (1) implies that the FBG will selectively re-flect incident light at two different Bragg wavelengths, i.e.kBF and kBS, corresponding to the fast axis and the slowaxis, respectively.

To characterize the extinction ratio of the in-line fiberPBS, a Gaussian function was used to simulate the reflec-tive spectrums associated with the fast and slow axes ofthe Hi-Bi FBG [10]. The overall reflectivity spectrum ofthe Hi-Bi FBG was obtained simply by adding the twospectrums corresponding to the two orthogonal axes,respectively. For analytical convenience, an assumptionwas made that the intensity of the launched light was unity,with 50% of the light being in the p-polarization mode and50% in the s-polarization mode. Therefore, the intensitiesof the p- and s-polarization light reflected from the in-linefiber PBS can be expressed respectively as

IPRðkÞ ¼ 50% R exp �ðk � kBFÞ2

r2

( )

ISRðkÞ ¼ 50% R exp �ðk � kBSÞ2

r2

( )

r ¼ 2pkBS;BF

� 2pkBS;BF þ h

ð2Þ

where R is the reflectivity of the Hi-Bi FBG, kBS, BF is thewavenumber corresponding to the Bragg wavelength inkBS, BF (kBS, BF = 2p/kBS, BF), r is the peak width parameter,and h is the half bandwidth of the Gaussian distribution atthe point where the intensity is 1/e of the maximum. Inpractical Hi-Bi fibers, the characteristic Dk = (kBS � kBF)is small, and hence the problem of crosstalk arising as a re-sult of the Gaussian distribution of the Bragg wavelengthsfrom the Hi-Bi FBG must be taken into account. In analyz-ing the crosstalk problem in the current in-line fiber PBS,the extinction ratio in the reflective mode, i.e. ERR, is de-fined as

ERR ¼ 10j logðIPR=ISRÞj ð3Þ

From Eqs. (2) and (3), it can be inferred that the extinctionratio in the reflective mode is independent of the reflectivity,R, of the Hi-Bi FBG. This study simulated the variation inthe extinction ratio in the reflective mode with h as a func-tion of Dk. The two reflective Bragg wavelengths were as-signed values of kBF = 1541.96 nm and kBS = 1542.44 nm,respectively. The simulation results presented in Fig. 2 showthat the extinction ratio improves as the bandwidth of thetwo Bragg wavelengths is reduced or as the separation ofthe two Bragg wavelengths is increased.

In the in-line fiber PBS, the p-polarization light isreflected by the Hi-Bi FBG at kBF while the s-polarizationlight is transmitted. Similarly, the s-polarization light isreflected at kBS while the p-polarization light is transmitted.

Fig. 2. Variation of ERR with h as a function of Dk for in-line fiber PBS.

0 10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

Reflectivity (%)

ER

T

(dB

)

h=0.1 nm

h=0.5 nm

0 10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

Reflectivity (%)

ER

T (

dB)

h=0.1 nm

h=0.5 nm

a

b

Fig. 3. Variation of ERT with reflectivity as a function of h for:(a) Dk = 0.48 nm and (b) Dk = 0.7 nm.

104 Y.-L. Lo et al. / Optics Communications 272 (2007) 102–106

Therefore, the extinction ratio in the transmitted mode ofthe in-line fiber PBS can be defined as

ERT ¼ 10j logðIST=IPTÞj ð4Þwhere IST and IPT are the intensities of the s- and p-polar-ization transmitted light and can be expressed as

IPTðkÞ ¼ 50%� R exp �ðk � kBFÞ2

r2

( )50%

ISTðkÞ ¼ 50%� R exp �ðk � kBSÞ2

r2

( )50% ð5Þ

Unlike the extinction ratio in the reflective mode, in whichthe values of Dk and h play key roles, the extinction ratio inthe transmitted mode of the in-line fiber PBS is determinedprimarily by the reflectivity of the Hi-Bi FBG.

Applying Eqs. (3) and (4), Fig. 3 plots the variation of theextinction ratio in the transmitted mode with the reflectivityof the Hi-Bi FBG. Fig. 3a corresponds to conditions ofDk = 0.48 nm and h = 0.1–0.5 nm, while Fig. 3b considersconditions of Dk = 0.7 nm and h = 0.1–0.5 nm. The resultsindicate that in both cases, the extinction ratio in the trans-mitted mode improves significantly as the reflectivity of theHi-Bi FBG is increased. Furthermore, Fig. 3b shows thatfor the case of Dk = 0.7 nm, the curves corresponding to dif-ferent values of h are virtually superimposed. This resultimplies that the bandwidth of the Bragg wavelength hasonly a negligible effect on the extinction ratio in the trans-mitted mode at higher values of Dk. Also, regardless in val-ues of h, a Hi-Bi FBG with a reflectivity of 99% is stillrequired in order to achieving an extinction ratio of 20 dBin the transmitted mode of the novel in-line fiber PBS.

Generally, commercial PBSs are required to provide anextinction ratio of 20 dB. According to the analytical resultspresented in Fig. 3, a 20 dB extinction ratio can be realizedin the transmitted mode of the proposed in-line fiber PBS byusing a Hi-Bi FBG with a reflectivity of 99%.

3. Experimental setup and results

A FBG was fabricated in a hydrogen-loaded Hi-Bi opti-cal fiber using a 248 nm KrF excimer laser. During the

writing process, a phase mask with a pitch of 1070 nmwas positioned between the laser source and the Hi-Bi opti-cal fiber to diffract the laser beam. The writing processinduced regular periodic changes in the refractive indicesof the Hi-Bi optical fiber. In order to understand the effectof reflectivity on the extinction ratios in the reflective andtransmitted modes of the fabricated in-line fiber PBS, alower reflectivity of 34.75% from the Hi-Bi FBG was delib-erately chosen. Therefore, the experimental parameters ofthe Hi-Bi FBG were as follows: kBF = 1541.96 nm, kBS =1542.44 nm, Dk = 0.48 nm, h = 0.118 nm, and reflectivity= 34.75%.

Fig. 4 presents a schematic illustration of the basicexperimental setup. As shown, white light from an ampli-fied spontaneous emission (ASE) light source (SDOES1117C) was passed through a 2 · 2 coupler andlaunched into the Hi-Bi FBG. The polarization states ofthe reflected and transmitted light at the two Bragg wave-lengths were characterized using linear polarizers. Theunwanted reflective light from the fiber ends was eliminated

Fig. 4. Experimental setup for in-line fiber PBS.

intensity of 1541.96nmintensity of 1542.44nm

Fig. 6. Polar plots of transmitted peak intensity as function of polarizerangle.

Y.-L. Lo et al. / Optics Communications 272 (2007) 102–106 105

by using matching gel at the ends of each fiber in the exper-imental setup.

Fig. 5 presents the polar plots of the reflective peakintensities corresponding to the Bragg wavelengths of1541.96 nm and 1542.44 nm as a function of the polarizerangle. Fig. 6 presents the corresponding plots for the trans-mitted peak intensities. Note that all of the data wereextracted using an optical spectrum analyzer. It is observedthat the extinction ratio in the reflective mode is 34.78 dBat 1541.96 nm and 36.65 dB at 1542.44 nm. Although thereflectivity of the current Hi-Bi FBG is only 34.75%, theresults indicate that the extinction ratio in the reflectivemode nevertheless attains a value of 36 dB. Hence, it isclear that the poor reflectivity of the Hi-Bi FBG has noeffect on the extinction ratio of the in-line fiber PBS inthe reflective mode. Therefore, a good quality in-line fiberpolarizer can be easily realized using the reflective modeof the Hi-Bi FBG. However, as expected from the analyti-cal results presented in Fig. 3, the extinction ratio in thetransmitted mode is only 4.78 dB at 1541.96 nm and4.75 dB at 1542.44 nm. This poor performance is attributedmainly to the low reflectivity of the Hi-Bi FBG.

intensity of 1541.96nm

intensity of 1542.44nm

Fig. 5. Polar plots of reflected peak intensity as function of polarizerangle.

According to Figs. 2 and 3, for conditions of R =34.75%, Dk = 0.48 nm, and h = 0.118 nm, the extinctionratios in the reflective and transmitted modes are 70 dBand 2 dB, respectively. Clearly, these analytical estimatesdeviate significantly from the experimental results of34.78 dB and 4.78 dB at 1541.96 nm, respectively. The dis-crepancy between the two sets of results arises primarily asa result of constructing the analytical model simply by add-ing the two Gaussian spectra corresponding to the twowavelengths. Nonetheless, the simple analytical model pro-vides a basic qualitative understanding of the parameterswhich exert the greatest influence on the extinction ratiosin the reflective and transmitted modes, respectively.Recently, Wuilpart et al. [11] analyzed the polarizationproperties of uniform fiber Bragg gratings written intohighly birefringent fibers. The analytical model [11] is pos-sible to be applied to analyze the characteristics of the in-line fiber PBS developed in this study.

An electrically resistive coating [8,9] was applied to theHi-Bi FBG to enable the tuning of one of the Bragg wave-lengths of the in-line fiber PBS to the working wavelength.Fig. 7 shows that the maximum attainable shift in theBragg wavelength is 1.5 nm. From inspection, the shiftingefficiency is found to be 0.6482 (V/nm).

4. Conclusions and discussion

This study has developed a novel in-line fiber PBS basedon a Hi-Bi FBG. The analytical results have shown that theextinction ratio in the reflective mode of the PBS isimproved by increasing the difference between the refrac-tive indexes of the fast and slow axes in the FBG or byreducing the bandwidth of the Bragg wavelengths. By con-trast, the extinction ratio in the transmitted mode of the in-line fiber PBS is determined primarily by the reflectivity of

y = 0.6482x + 1546

1546

1546.2

1546.4

1546.6

1546.8

1547

1547.2

1547.4

1547.6

1547.8

1548

0 0.5 1 1.5 2 2.5 3

Voltage (V)

Wav

elen

gth

(nm

)

Fig. 7. Variation of Bragg wavelength with applied voltage in in-line fiberPBS.

106 Y.-L. Lo et al. / Optics Communications 272 (2007) 102–106

the Hi-Bi FBG. Although the results obtained from theanalytical model deviate significantly from the experimen-tal results, the model nevertheless provides a useful toolfor basic qualitative analysis.

A commercial PBS is generally required to provide anextinction ratio of 20 dB. The experimental results haveshown that the extinction ratio of the current in-line fiberPBS is 36 dB in the reflective mode even though the reflec-tivity of the Hi-Bi FBG is just 34.75%. Furthermore, if thereflectivity of the Hi-Bi FBG is increased to the order of99%, the extinction ratio in the transmitted mode canachieve the desired value of 20 dB. Under such conditions,the insertion loss in the reflective and transmitted modes ofthe proposed in-line fiber PBS can be significantlyimproved. As a result, this paper has demonstrated the fea-sibility of a novel in-line fiber PBS based on a Hi-Bi FBG.

Acknowledgements

This study was supported in part by the Ministry ofEducation Program for Promoting Academic Excellenceof Universities under Grant No. A-94-E-FA08-1-4.The funding received from the Advanced OptoelectronicTechnology Center, National Cheng Kung Universityunder Projects sponsored by the Ministry of Educationand the National Science Council (NSC 95-219-M-009-008) of Taiwan, ROC, is also gratefully acknowledged.

References

[1] S.P. Ma, S.M. Tseng, J. Lightwave Technol. 15 (1997) 1554.[2] A. Diez, M.V. Andres, D.O. Culverhouse, IEEE Photon. Technol.

Lett. 10 (1998) 833.[3] B. Ortega, L. Dong, W.F. Liu, J.P. de Sandro, L. Reekie, S.I.

Tsypina, V.N. Bagratashvili, R.I. Laming, IEEE Photon. Technol.Lett. 9 (1997) 1370.

[4] A.S. Kurkov, M. Douay, O. Duhem, B. Leleu, J.F. Bayon, L.Rivoallan, Electron. Lett. 33 (1997) 616.

[5] J.A. Davis, J. Adachi, C.R. Fernandez-Pousa, I. Moreno, Opt. Lett.26 (2001) 587.

[6] A. Sakamoto, H. Sasaki, D. Tanaka, R. Matsumoto, S. Okude, K.Nishide, and A. Wada, in: Proceeding of the 14th InternationalConference on O.F.S., p. 612, 2000.

[7] K. Zhou, G. Simpson, X. Chen, L. Zhang, I. Bennion, Opt. Lett. 30(2005) 1285.

[8] H.G. Limberger, N.H. Ky, D.M. Costantini, R.P. Salathe,C.A.P. Muller, G.R. Fox, IEEE Photon. Technol. Lett. 10(1998) 361.

[9] J.A. Rogers, P. Kuo, A. Ahuja, B.J. Eggleton, R.J. Jackman, Appl.Opt. 39 (2000) 5109.

[10] Y.L. Lo, IEEE Photon. Technol. Lett. 10 (1998) 1003.[11] M. Wuilpart, C. Caucheteur, S. Bette, P. Me’gret, M. Blondel, Opt.

Commun. 247 (2005) 239.