Polarization properties of a long-period grating written in a pure fused silica photonic crystal fiber

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Polarization properties of a long-period grating inscribed in a single-mode photonic crystal fiber are presented. A strong modulation in the spectra of polarization-dependent loss and differential group delay with periods of 2.6 and 1.3 nm is
  Polarisation properties of long-periodgrating inscribed in pure-fused-silicaphotonic crystal fibre C. Caucheteur, A.A. Fotiadi, P. Me´gret, G. Brambilla,S.A. Slattery and D.N. Nikogosyan The first measurements of polarisation properties of a long-period grating inscribed in an endlessly singlemode photonic crystal fibre arereported. Strong modulation in the spectra of polarisation-dependent loss and differential group delay with periods of 2.6 and 1.3 nm,respectively, were found. As such an effect has not been observed instandard optical fibres, it is believed that this is due to the specificmode structure of the holey fibre used for grating fabrication.  Introduction:  Though long-period fibre grating (LPFG) inscription ina photonic crystal fibre (PCF) has been known since 2002 [1], polarisation studies of such LPFGs have not been conducted untilnow. In this Letter we report on measurement of polarisation depen-dent losses (PDL) and differential group delay (DGD) spectra in anLPFG prepared in a hydrogenated PCF by a photochemical technique,namely, using the two-photon absorption (TPA) of high-intensityfemtosecond 264 nm pulses in pure fused silica [2]. Such an inscrip-tion technique is advantageous over non-photochemical ones, whichdeal with refractive index change induced by heating (using CO 2  laser light or an electric arc discharge) or by applying mechanical pressure,and it allows the production of high quality LPFGs [3]. 1200 1300 1400 1500 1600 1700–90–80–70–601551 nm    t  r  a  n  s  m   i  s  s   i  o  n ,   d   B  m l  , nm 10 m m a b  Fig. 1  Electronic microscope photograph, and transmission spectrum a  Electronic microscope photograph of ESM-12-01 fibre b  Transmission spectrum of LPFG recorded in ESM-12-01High-intensity 264 nm femtosecond pulses at irradiation intensity of 293 GW = cm 2 , total incident fluence of 13.7 J = cm 2 , annealed for 5 h at 80  C.LPFG is of 500  m m period and 1 cm long  Experimental setup:  In these experiments, we used an endlesslysinglemode PCF ESM-12-01 from Blaze Photonics (now CrystalFibre A = S), supplied by the Photonics & Photonic Materials Groupat the University of Bath, UK. The ESM-12-01 fibre, shown inFig. 1 a  has a 12  m m core diameter surrounded by four rings of holes (hole diameter 3.7  m m, hole pitch 8  m m, number of holes 54),and its outside diameter is 125  m m. The pieces of PCF used in theexperiments were 0.3–0.5 m long. To decrease the rate of hydrogenoutdiffusion from the PCF, it was pigtailed to two 0.5 m-long piecesof SMF-28 fibre, by splicing before hydrogenation. For LPFGfabrication, femtosecond UV laser pulses ( l ¼ 264 nm,  e  p ’ 200  m J, t ¼ 220 fs (FWHM), 2 w ¼ 0.3 cm (FWHM),  f   ¼ 27 Hz) [2] wereapplied. The experimental setup for LPFG fabrication was similar to those used earlier  [4, 5]. The inscription procedure and thecomparison of an LPFG recorded in PCF with a similar gratingrecorded in standard hydrogenated telecom fibre are given in [3].The length of LPFG used for polarisation measurements was 1 cm and its period,  L , was equal to 500  m m.The polarisation-dependent loss and differential group delay werededuced from the measurement of the Jones matrix in transmission. For that purpose, a tunable laser source ILX 8801, covering the C-band (1527–1567 nm), and a polarimeter Profile PAT 9000 B were used. Theinput signal was launched through a polariser controlled by the polarimeter. Measurements were realised with a 25 pm wavelengthstep. The obtained results represent the mean of five consecutivemeasurements conducted under the same experimental conditions.Connecting fibres were secured to avoid polarisation instabilities and to ensure good repeatability. The grating was kept straight and thetemperature was maintained constant during the measurements.  Results and discussion:  Fig. 1 b  shows the spectrum of transmissionloss for an LPFG inscribed in a hydrogenated ESM-12-01 fibre after thermal annealing, which was used to remove remaining hydrogenand to stabilise the grating. This spectrum was taken using non- polarised light. As the LPFG peak at 1551 nm corresponds well tothe spectral range of our tunable source, we used this peak for the polarisation studies. Fig. 2 shows transmission loss spectrafor the 1551 nm peak of the LPFG in the ESM-12-01 fibre, takenwith polarised light and corresponding to the maximum and minimumamplitude of the transmitted signal. The input angles for the maxi-mum and minimum amplitude spectra differ by 90  , thus correspond-ing well to LPFGs inscribed in standard photochemical fibre [6–8].However, a specific feature occurs in a PCF, namely, a strong periodicmodulation of spectra corresponding to the maximum and minimumamplitude of the transmitted signal, which we believe is due to the periodic hole structure (and hence to the specific mode structure) of an endlessly singlemode PCF (Fig. 1 a ). 1530154015501560–20–15–10–505 maximum amplitude minimum amplitude    t  r  a  n  s  m   i  s  s   i  o  n ,   d   B  m l  , nm Fig. 2  Transmission spectra for LPFG in PCF taken with polarised light and corresponding to maximum and minimum amplitudes of transmitted  power  Figs. 3 a  and   b  show the polarisation-dependent loss (PDL) and differential group delay (DGD) spectra, respectively, taken in a PCF.The comparison with similar PDL and DGD spectra, observed for LPFGs, prepared by both photochemical [8] and non-photochemical[6, 7] methods in a standard telecom fibre, allows us to make someimportant conclusions. First, we mention that the maximum PDL value ELECTRONICS LETTERS 9th November 2006 Vol. 42 No. 23  Downloaded 13 Nov 2006 to Redistribution subject to IET licence or copyright, see http://ietdl.org/copyright.jsp  related to the grating length for the LPFG inscribed in ESM-12-01 fibreis ’ 4 dB = cm, which is two to three times higher than values given in[6–8] for LPFGs recorded in the standard telecom fibre SMF-28. Themaximum DGD value related to grating length for an LPFG inscribed in PCF is about 1 ps = cm, which is an order of magnitude higher thanthe corresponding value given in [8] for an LPFG recorded in SMF-28.Despite the similarity between the PDL and DGD spectra in PCF and SMF-28 (e.g. the famous peak-trough-peak shape of PDL spectra), allthe spectra in Figs. 3 a  and   b  show strong modulation, which again is probably due to the regular hole structure inside the PCF. While in thePDL spectrum (Fig. 3 a ) the period of oscillations is 2.6 nm, in the DGDspectrum (Fig. 3 b ) we observed oscillations with a 1.3 nm period,exactly half the PDL spectrum periodicity. 01234    P   D   L ,   d   B 1530 1540 1550 1560 01    D   G   D ,  p  s l  , nm1530 1540 1550 1560 l  , nm a b  Fig. 3  PDL and DGD spectra for LPFG in PCF  a  PDH spectra b  PGD spectra Although detailed description of the reported effects is beyond thescope of this Letter, we believe that the quantitative behaviour observed in the experiment can be simulated on the basis of the well-developed coupled-mode theory for an LPFG described in detail in a number of  papers [9]. Since the presence of holes breaks the central symmetry of the fibre, such an LPFG model should take into account the real modestructure of the ESM-12-01 fibre [10]. Specific features of the interac-tion between the coupling modes in this fibre seem to be responsible for the reported effect. The corresponding modelling is in progress and results will be published later. Conclusion:  We have observed a strong modulation in the polarisa-tion-dependent loss and differential group delay spectra of long- period gratings prepared by high-intensity UV femtosecond pulsesin endlessly singlemode PCF.  Acknowledgments:  We are indebted to P. St. J. Russell for continuoussupport, and are grateful to Science Foundation Ireland (grant 04 = IN3 = I608) and the Interuniversity Attraction Pole programme(IAP V 18) of the Belgian Science Policy for financial support.C. Caucheteur is supported by the Fonds National de la RechercheScientifique (FNRS). # The Institution of Engineering and Technology 2006 22 August 2006  Electronics Letters  online no: 20062610doi: 10.1049/el:20062610C. Caucheteur, A.A. Fotiadi and P. Me´gret (  Faculte ´  Polytechnique de Mons, Service d’Electromagne ´ tisme et de Te ´ le ´ communications, 31, Boulevard Dolez, Mons B-7000, Belgium )G. Brambilla ( Optoelectronics Research Centre, University of  Southampton, Southampton SO17 1BJ, United Kingdom )S.A. Slattery and D.N. Nikogosyan (  Physics Department, National University of Ireland, University College Cork, Cork, Ireland  )E-mail: niko@phys.ucc.ie References 1 Kakarantzas, G., Birks, T.A., and Russell, P.St.J.: ‘Structural long- period gratings in photonic crystal fibers’,  Opt. Lett. , 2002,  27 , (12), pp. 1013–10152 Dragomir, A., Mcinerney, J.G., and Nikogosyan, D.N.: ‘Femtosecond measurements of two-photon absorption coefficients at   l ¼ 264 nm inglasses, crystals, and liquids’,  Appl. Opt. , 2002,  41 , (21), pp. 4365–43763 Brambilla, G., Fotiadi, A.A., Slattery, S.A., and Nikogosyan, D.N.: ‘Two- photon photochemical long-period grating fabrication in pure-fused-silica photonic crystal fiber’,  Opt. Lett. , 2006,  31 , (18), pp. 2675–26774 Kalachev, A.I., Pureur, V., and Nikogosyan, D.N.: ‘Investigation of long- period fiber gratings induced by high-intensity femtosecond UV laser  pulses’,  Opt. Commun. , 2005,  246 , (1–3), pp. 107–115; Errata,  Opt.Commun. ,  2005 , 251,  (1–3), p. 2295 Kalachev, A.I., Nikogosyan, D.N., and Brambilla, G.: ‘Investigation of long-period fiber gratings induced by high-intensity femtosecond UVlaser pulses’,  J. Lightwave Technol. , 2005,  23 , (8), pp. 2568–25786 Savin, S., Digonnet, M.J.F., Kino, G.S., and Shaw, H.J.: ‘Tunablemechanically induced long-period fiber gratings’,  Opt. Lett. , 2000,  25 ,(10), pp. 710–7127 Kim, M., Lee, D., Hong, B.I., and Chung, H.: ‘Performancecharacteristics of long-period fiber-gratings made from periodic tapersinduced by electric-arc discharge’,  J. Korean Phys. Soc. , 2002,  40 , (2), pp. 369–3738 Caucheteur, C., Fotiadi, A., Megret, P., Slattery, S.A., and  Nikogosyan, D.N.: ‘Polarization properties of long-period gratings prepared by high-intensity femtosecond 352-nm pulses’,  IEEE  Photonics Technol. Lett. , 2005,  17 , (11), pp. 2346–23489 Erdogan, T.: ‘Cladding-mode resonances in short- and long-period fiber grating filters’,  J. Opt. Soc. Am. A , 1997,  14 , (8), pp. 1760–177310 Uranus, H.P., Hoekstra, H.J.W.M., and Van Groesen, E.: ‘Modes of anendlessly single-mode photonic crystal fiber: a finite element investigation’. Proc. 9th Annual Symp. of the IEEE = LEOS BeneluxChapter, Gent University, Belgium, 2004, pp. 311–314 ELECTRONICS LETTERS 9th November 2006 Vol. 42 No. 23  Downloaded 13 Nov 2006 to Redistribution subject to IET licence or copyright, see http://ietdl.org/copyright.jsp
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