Recording of long-period gratings in special single-mode fibers by a CO2 laser

Materials Science and Engineering C 26 (2006) 448 – 452 www.elsevier.com/locate/msec

Recording of long-period gratings in special single-mode fibers by a CO2 laser Petr Cı´sarˇovsky´ a,*, Jirˇ´ı Dunovsky´ b, Ladislav Kolarˇ´ık a, Filip Todorov a, Ivan Kaxı´k c, Vlastimil Mateˇjec c a

c

Faculty of Mechanical Engineering, Czech Technical University, Technicka´ 4, 166 07 Prague 6, Czech Republic b Faculty of Transportation Science, Czech Technical University, Horska´ 3, 128 03 Prague 2, Czech Republic Institute of Radio Engineering and Electronics, Academy of Sciences of the Czech Republic, Chaberska 57, 182 51 Prague 8, Czech Republic Available online 19 December 2005

Abstract The paper deals with the fabrication of long-period gratings (LPG) in special optical fibers with an inverted parabolic-index profile of the cladding by periodically exposing the fibers to radiation of a high-power CO2 laser. Details on the fiber fabrication and preparation of LPGs with a period of about 1 mm in the fibers are presented. The fabricated LPGs were evaluated on the basis of their transmission spectra determined by measuring the output spectral density of the fiber with the LPGs. It is shown that the position of the band and its strength depend on the laser beam velocity, beam power and the interaction time of radiation with the fiber. D 2005 Elsevier B.V. All rights reserved. Keywords: Long-period grating; CO2 laser; Fiber

1. Introduction In recent years long-period fiber gratings (LPG) have developed into a new and rapidly expanding branch of fiber grating science and applications [1,2]. LPGs are usually formed in single-mode optical fibers by introducing into the core periodic modulation of the refractive index with a typical modulation depth on the order of 10 4. The periods of the gratings are on the order of 100 Am and the grating length range from one to a few centimetres. The basic function of the LPG is that at discrete wavelengths k p, at which the phasematching condition [3] is fulfilled, it can couple the fundamental core mode propagating along the core with the effective index n 01co, to co-propagating cladding modes with the effective indexes n opcl. As the cladding modes are lossy, e.g. due to scattering at the cladding/fiber surroundings interface, the transmission spectrum of the LPG contains one or several narrow attenuation bands with a FWHM of 5– 10 nm centered at the wavelengths k p [3– 6]. The number and positions of these bands depend on the grating period K and effective

* Corresponding author. E-mail address: [email protected] (P. Cı´sarˇovsky´). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.10.027

indexes of the core and cladding modes, respectively. The effective indexes are dependent on the refractive-index profiles of the core and the cladding, external refractive index at the cladding interface, and are generally wavelength-dependent. The dependence of the effective indexes on various physical quantities is utilized in LPG sensors of temperature, strain, bend and external refractive index [3 – 9]. Chemically or biochemically induced changes of the external refractive index of LPG can also be employed for chemical or biochemical sensing [10]. The method that is most frequently used for LPG fabrication is based on photoinduced permanent index changes in a GeO2doped silica fiber core [2– 12]. UV light in a wavelength ranges around 250 nm; e.g. from an excimer KrF (248 nm), frequencydoubled argon-ion (244 nm) or quadrupled Nd: YAG (266 nm) lasers, and exposure through an amplitude chrome-plated mask with a rectangular transmittance function or a point-by-point exposure technique are used for this purpose. UV-laser-induced LPGs have mainly been fabricated in commercial step-index single-mode fibers with a GeO2-doped core and uniform pure silica cladding. Only few exceptions from this picture have been reported, e.g. an LPG fabricated in a fiber with an eccentric core in a silica cladding [7], in a polarisation-maintaining fiber [11] or in a fiber in which an

P. Cı´sarˇovsky´ et al. / Materials Science and Engineering C 26 (2006) 448 – 452

intermediate part of the silica cladding was replaced with a polymer ring [8] or with a special air-filled microstructure [12]. Recently, a new method of fabricating LPGs in optical fibers has been developed [13 – 17]. This method is based on releasing residual stress in a drawn fiber by direct exposure of the fiber to focused light of a powerful 10.6-Am CO2 laser which locally heats the fiber. The releasing of the stress causes, due to the photoelastic effect, changes of the refractive index of the core or the cladding [13,14]. The residual stress in the fibers is given by a combination of thermal and mechanical stresses and can be controlled through the composition of the fiber core and cladding as well as through the tension during the fiber drawing [13 – 15]. By using CO2 lasers, LPGs can be fabricated without masks, only by controlling the movement of the focused laser light along or across the fiber. Although such LPGs can be prepared without loading the fiber with hydrogen, it has been found that hydrogen loading enhances the LPG quality [16]. In contrast to UV-photoinduced LPGs, the CO2-laser-induced LPGs exhibit strong dependence of the transmission resonances on axial rotational orientation of the bent fiber [17]. By using pulsed and CW CO2 lasers, LPGs have been fabricated in standard dispersion-shifted fibers [16], fibers with a silica core and fluorine-doped cladding [15] or in fibers doped in the core or in the cladding with boron and germanium oxides [14]. This paper presents results on the fabrication of LPGs in single-mode fibers with an inverted graded-index (IGI) profile of parabolic type of the fiber cladding by exposing the fibers to intense light of a CO2 laser. Transmission spectra of the prepared LPGs are discussed in relation with the processing conditions used at their fabrication. 2. Experimental 2.1. Fabrication and characterization of fibers for LPG inscription The fibers were drawn from preforms prepared by the MCVD method. In the preform preparation, an inverted parabolic-index profile of the fiber cladding was produced by the application of thin silica glass layers with a controllably increased content of boron oxide onto the inner wall of a rotating silica substrate tube. After the application of the cladding glass layers, one glass layer composed of silica slightly doped with boron oxide was applied and then the tube was collapsed into a rod, the preform. Final preforms were prepared from the rods by nearly complete removal of the silica Table 1 Main characteristics of the CO2 laser used Wavelength [nm] Maximum power P max [W] Maximum beam velocity V max [mm/s] Focal depth [mm] Spot-size diameter [mm] Working area [mm2]

10,600 50 1524 34.5 0.1 610  457

449

Fig. 1. A principal scheme of the set-up for the LPG inscription. 1 — stage for the X – Y movements of the laser head, 2 — laser beam, 3 — laser head with optics, 4 — path of the laser beam, 5 — fiber holder.

substrate tube by using etching in liquid phase. For this purpose the prepared preform was dipped into an aqueous solution containing hydrofluoric and nitric acids. The rod rotated during the etching and the process of silica removal was optically controlled. The fibers were drawn from the preforms under a constant drawing velocity and at different drawing temperatures in order to control mechanical stresses in the fiber induced during drawing. Several drawing temperatures in a range of 1800– 2000 -C were tested. Fibers with an outer diameter of 120 Am were drawn and coated with an UV curable acrylate jacket (De Solite, NL) during drawing. Tomographic refractive-index profiles of the prepared preforms were evaluated on the basis of deflection functions measured for several angular positions by using commercial refractive-index profilers P101 (York Technology, GB). Refractive-index profiles of the fibers were also measured by using a S14 device (York Technology, GB). 2.2. LPG inscription Fiber segments with a maximum length of 8 m were used for LPG inscription. Before the inscription, the polymeric jacket was chemically removed from the segment in a length of about 4 cm. This bare part of the fiber was fixed by clamps of a metal holder onto a special platform in order to prevent movements of the fiber and its easy breakage in air. The rest of the segment was coiled onto a plastic bobbin, which was also mounted on the platform. LPGs were fabricated in the bare part of the segment by using a mark laser system ILS-III (Laser Tools and Technics Corp.). This system is equipped with a continuous CO2 laser

Table 2 Beam power and velocity used at LPG processing Regime

Pr

Vr

I1 I2 I3

0.15 0.10 0.15

0.30 0.40 0.20

P. Cı´sarˇovsky´ et al. / Materials Science and Engineering C 26 (2006) 448 – 452

450

Fiber coil L ~ 8m

Patchcord SMF 1300/1550 nm

Long-period grating

Optical source LED 1550nm

Spectrum analyser

FC connectors Fiber holder Fig. 2. A scheme of the set-up for spectral characterization of LPGs.

head that can move in X – Y directions and can be focused in Z direction by using a precise computer control. The main parameters of the system used are given in Table 1. A principal scheme of the LPG inscription is shown in Fig. 1. By means of optics in the laser head the laser beam was focused onto the fiber fixed in the holder. By using X – Y movement controllers the laser head scanned the fiber in a meander-like path. After the irradiation, the fiber was cooled in air. No constant tension was applied on the fiber during the inscription. The laser beam velocity, beam power and number of passes of the beam across the fiber were varied in order to find suitable processing conditions. Three processing regimes denoted I1, I2 and I3, which differed in the values of the relative beam power P r ( P r = P / P max) and the relative beam velocity Vr (Vr = V / V max, V max = 1524 mm/s), were used for thermal treatment of the fiber. The values of P r and Vr used in these regimes are listed in Table 2. 2.3. Characterisation of LPGs

3. Results and discussion Refractive-index profiles of the prepared preforms and fibers are shown in Figs. 3 and 4. The tomographic profile of the preform in Fig. 3, which was determined from measurements in 9 angular projections, shows that the preforms had suitable circular symmetry. It was found that the refractiveindex profile of the cladding shown in Fig. 3 can be approximated by an inverted parabolic-index profile with parameters n 0 = 1.4525 and 2D = 0.0074 [18]. The modulations on the curves in Fig. 3 are related to refractive-index changes across the cladding layers. These changes are caused by diffusion of boron oxide during the application of each layer and during the collapse of the tube. The same processes are responsible for a quasi step-index profile of the core. -3

0.001

2x10

0.000

1x10

-3

Refractive-index difference

Refractive-index difference

The prepared gratings were characterized by the transmission spectra of the fibers with the prepared LPGs. For this purpose, an experimental set-up schematically shown in Fig. 2 was used. In the set-up, commercially available FC connectors and a patchcord of a single-mode fiber were used for launching

light from a broadband LED (the maximum of emission at 1550 nm) into the tested fiber with the LPG fixed on the platform. The output signal from the fiber was analysed by using a spectral analyser ANDO in a spectral range of 1400– 1700 nm with a wavelength step of 0.3 nm. The transmission spectrum of the LPG was calculated from the output signal of the LPG fiber and the output signal of the fiber without any LPG.

-0.001 -0.002

Silica

-0.003 -0.004

9 projections

-0.005 -0.006

Silica part

0 -3

-1x10

-3

-2x10

-3

-3x10

-3

-4x10

-3

-5x10

-3

-6x10

-0.007 -6

-4

-2

0

2

4

6

Distance from the center [mm]

-3

-7x10

-60

-40

-20

0

20

40

60

Radius [µm] Fig. 3. Refractive-index profiles of the preform for drawing fibers for the LPG inscription measured in angular directions; the refractive-index difference is related to the refractive index of silica.

Fig. 4. Typical refractive-index profile of the fiber drawn from the preform prepared by the MCVD method and etched in liquid phase by HF.

P. Cı´sarˇovsky´ et al. / Materials Science and Engineering C 26 (2006) 448 – 452

Λ=0.98 mm

Λ=0.98 mm

0

LPG Transmission [dB]

LPG Transmission [dB]

0

-5

1 beam pass 2 beam passes 3 beam passes Pr=0.30, Vr=0.15

-5

2 times [Pr=0.10, Vr=0.40 Pr=0.10, Vr=0.40 Pr=0.15, Vr=0.30 -10

-10

1400

1450

1500

1550

451

1600

1400

1450

1650

The refractive-index profile of the fiber in Fig. 4 shows features similar to those shown in Fig. 3. Analysing the curve in Fig. 4 carefully, one can recognize a very thin silica layer remaining after the liquid-phase etching of the rods prepared by the MCVD method. One can expect that for this type of the profile, such a layer does not critically influence light propagation in the fiber cladding. A set of experiments was carried out with the aim of determining the influence of the drawing temperature on the process of the LPG inscription. The maximum attenuation LPG bands were observed on fibers drown at 1900 -C. This temperature is low enough to induce internal stresses in the fiber sufficient for the LPG inscription and is high enough to prevent serious degradation of mechanical properties of the fiber due to high internal stresses. Some results on properties of the prepared LPGs are shown in Figs. 5 –8. The curves in Fig. 5 show that the position and form of the LPG band changed when the inscription process was repeated and the beam passed several times across the same part of the fiber. In these experiments, using the processing regime I1 carried out the inscription. On the basis of Fig. 5 one can draw a conclusion that by repeating the pass

Pr=0.15, Vr=0.30] 1550

1600

1650

Wavelength [nm]

Wavelength [nm] Fig. 5. The effect of multiply exposing the fiber to a high-power beam (the I1 regime) on the transmission spectra of LPGs.

1500

Pr=0.15, Vr=0.30] 3 times [Pr=0.10, Vr=0.40

Fig. 7. The effect of periodic pretreatment of the fiber within the I2 process followed by the I1 process on the transmission spectra of the LPGs.

of the beam across the fiber, the strength of the attenuation LPG band decreased. No attenuation LPG bands were observed in the measured wavelength range when the beam passed three times across the fiber (see Fig. 5, dotted line). In Fig. 6 it is shown that a rapid thermal pretreatment of the fiber (denoted as I2) yielded a different picture. In these experiments the strongest LPG band was achieved when the beam passed across the fiber once during the I2 process and three times during the I1 process (see Fig. 6, dotted line). A strong and broad LPG band was achieved when the combination of the I2 and I1 regimes was repeated three times (see Fig. 7). In this case the beam passed six times across the fiber. In Figs. 5 –7 the dashed line presents LPGs in a different fiber than that presented by solid and dotted lines. That is the reason for the shift of the position of the band towards higher wavelengths. However, by combining the process I2 with the process I3 a more stronger attenuation band was obtained (see Fig. 8, dashed line). In this case the time of interaction of the laser beam with the fiber was longer that for the I1 regime.

5 5 0

0

-5

Pr=0.10, Vr=0.40 Pr=0.15, Vr=0.30

-10

Pr=0.10, Vr=0.40 2 passes [Pr=0.15, Vr=0.30]

-15

-20

Pr=0.10, Vr=0.40 3 passes [Pr=0.15, Vr=0.30] 1400

1450

1500

1550

1600

1650

Wavelength [nm] Fig. 6. The effect of the pretreatment of the fiber by exposing it to a low-power beam (the I2 regime) and then to a multiple exposure with a high power (the I1 process) on the transmission spectra of LPGs.

LPG Transmission [dB]

LPG Transmission [dB]

Λ=0.98 mm

-5

-10

-15

Λ=0.980 mm Λ=0.980 mm Λ=1.035 mm Pr=0.10, Vr=0.40

-20

Pr=0.15, Vr=0.20

-25 1400

1450

1500

1550

1600

1650

Wavelength [nm] Fig. 8. Results of repeated preparation of LPGs in the I1 and I3 processes (the solid and dashed lines) compared with results showing the effect of a period change (the dotted line).

452

P. Cı´sarˇovsky´ et al. / Materials Science and Engineering C 26 (2006) 448 – 452

This figure also shows that repeatability of the process is not optimal, namely as the band strength is concerned (compare the solid and dashed lines in Fig. 8). The shift of the position of the band towards lower wavelengths for an LPG with a period of 1.035 mm is shown in Fig. 8, dotted line. The above results show that the LPG inscription by means of CO2 lasers is a process with complex dynamics that depends on the fiber preparation and properties, beam power and beam velocity. The experiments carried out within this work have shown that the inscription process can be controlled by the number of beam passes, beam power and beam velocity. In some experiments high periodic degradation of the fiber surface in places where the interaction with laser radiation occurred was observed. This degradation was not circularly symmetric. In this case the final LPG bands are related both to refractive-index changes in the fiber core and to effects caused by surface corrugation. In this case the coupling between the fundamental core and cladding modes is much more complex than for structures without corrugation. One can expect that surface degradation can be, to some extent, produced at each exposure of the fiber surface to laser radiation and that this may be one reason for lower repeatability of LPGs inscribed by CO2 lasers. Sensitivity of LPGs inscribed in some of the produced fibers to temperature changes was tested and promising results were obtained [19]. 4. Conclusions LPGs were prepared in single-mode fibers with inverted parabolic-index profiles of the cladding by exposing the fibers to the radiation of a high-power CO2 laser. A relatively broad attenuation band could be observed in the transmission spectra of the fibers with LPGs in a wavelength range of 1400– 1700 nm, whose position and strength could be varied by changing parameters of the inscription process. Further research will be focused on precise determination of processing variables controlling the preparation repeatability.

Acknowledgements This research was supported by the Grant Agency of the Czech Republic (contract No. 102/03/0475) and Czech Technical University in Prague (contract No. 1084062). The authors would like to thank Jiri Kanka for spectral measurements. References [1] A.D. Persey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, E.J. Friebele, J. Lightwave Technol. 15 (8) (1997) 1442. [2] R. Kashyap, Fiber Bragg Gratings, Academic Press, London, 1999. [3] H.J. Patrick, A.D. Kersey, F. Bucholtz, J. Lightwave Technol. 16 (9) (1998) 1606. [4] B.H. Lee, Y. Liu, S.B. Lee, S.S. Choi, Opt. Lett. 22 (23) (1997) 1769. [5] V. Bhatia, A.M. Vengsarkar, Opt. Lett. 21 (9) (1996) 692. [6] C.C. Ye, S.W. James, R.P. Tatam, Opt. Lett. 25 (14) (2000) 1007. [7] H.J. Patrick, Electron. Lett. 36 (21) (2000) 1763. [8] C.G. Atherton, A.L. Steele, J.E. Hoad, IEEE Photonics Technol. Lett. 12 (1) (2000) 65. [9] A.A. Abramov, A. Hale, R.S. Windeler, T.A. Strasser, Electron. Lett. 35 (1) (1999) 81. [10] M.P. Delisa, Z. Zhang, M. Hioach, S. Pilevar, C.C. Davis, J.S. Sirkis, W.E. Bentley, Anal. Chem. 72 (13) (2000) 2895. [11] O. Duhem, M. Douay, Electron. Lett. 36 (5) (2000) 416. [12] R.P. Espindola, R.S. Windeler, A.A. Abramov, B.J. Eggleton, T.A. Strasser, D.J. DiGiovanni, Electron. Lett. 35 (4) (1999) 327. [13] C.-S. Kim, Y. Han, B.H. Lee, W.-T. Han, U.-C. Paek, Y. Chung, Opt. Commun. 185 (2000) 33. [14] B.H. Kim, Y. Park, T.-J. Ahn, D.Y. Kim, B.H. Lee, Y. Cg, U.C. Paek, W.T. Han, Opt. Lett. 26 (21) (2001) 1657. [15] S. Yamasaki, M. Akiyama, K. Nishide, A. Wada, R. Yamauchi, IEICE Trans. Electron. E83-C (3) (2000) 440. [16] D.D. Davis, T.K. Gaylord, E.N. Glytsis, S.G. Kosinski, S.C. Mettler, A.M. Vengsarkar, Electron. Lett. 34 (3) (1998) 302. [17] G.D. VanWiggeren, T.K Gaylord, D.D. Davis, E. Anemogiannis, B.D. Garrett, M.I. Braiwish, E.N. Glytsis, Electron. Lett. 36 (16) (2000) 1354. [18] V. Mateˇjec, M. Choma´t, I. Kaxı´k, J. Cˇtyroky´, D. Berkova´, M. Hayer, Sens. Actuators, B, Chem. 51 (1998) 340. [19] M. Chomat, D. Berkova, V. Matejec, I. Kasik, J. Kanka, R. Slavik, A. Jancarek, P. Bittner, in: Proc. of Abstracts of Int. Conf. MADICA 2004, Nov. 29 – Dec. 1, Tunis, Tunisia, vol. 88, 2004.