- Email: [email protected]
Transverse UV-laser irradiation-induced defects and absorption in a single-mode erbium-doped optical fiber
Optical Materials 31 (2009) 1296–1299
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Transverse UV-laser irradiation-induced defects and absorption in a single-mode erbium-doped optical fiber B. Tortech a,*, Y. Ouerdane a, A. Boukenter a, J.-P. Meunier a, S. Girard b, M. Van Uffelen c, E. Régnier d, F. Berghmans c,e, H. Thienpont e a
Université Jean Monnet, Laboratoire Hubert Curien-UMR CNRS 5516, Bât F, 18 rue Benoît Lauras, 42000 Saint-Etienne, France CEA DIF, BP 12, 91680 Bruyères-le-Châtel, France c SCK-CEN, Boerentang 200, B-2400 Mol, Belgium d Draka Comteq France, Data Center IV, Route de Nozay, 91 460 Marcoussis, France e Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium b
a r t i c l e
i n f o
Article history: Available online xxxx PACS: 42.88.+h 42.81.Wg 61.80.Ba 78.55.m Keywords: Optical fiber Photoluminescence Erbium UV irradiation
a b s t r a c t Near UV-visible absorption coefficients of an erbium-doped optical fiber were investigated through an original technique based on a transverse cw UV-laser irradiation operating at 244 nm. Such irradiation leads to the generation of a quite intense guided luminescence signal in near UV spectral range. This photoluminescence probe source combined with a longitudinal translation of the fiber sample (at a constant velocity) along the UV-laser irradiation, presents several major advantages: (i) we bypass and avoid the procedures classically used to study the radiation induced attenuation which are not adapted to our case mainly because the samples present a very strong absorption with significant difficulties due to the injection of adequate UV-light levels in a small fiber diameter; (ii) the influence of the laser irradiation on the host matrix of the optical fiber is directly correlated to the evolution of the generated photoluminescence signal and (iii) in our experimental conditions, short fiber sample lengths (typically 20–30 cm) suffice to determine the associated absorption coefficients over the entire studied spectral domain. The generated photoluminescence signal is also used to characterize the absorption of the erbium ions in the same wavelength range with no cut-back method needed. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The erbium-doped fibers are a key component of optical devices such as optical amplifiers or super-luminescent sources used in fiber gyroscopes [1,2]. They are widely deployed in terrestrial and submarine applications as repeaters for long-line or highly distributed telecommunication systems. It is well known that, in space, the fibers may be exposed to a relatively large dose of electrons and protons over the spacecraft lifetime [3–5]. Therefore, the effect of energetic radiation on rare earth doped fibers is of interest for both predicting device performances in radiation environments, and identifying the specific mechanisms of color center formation in these material systems. Several laboratory experiments have already shown that the radiation exposure could potentially affect the fiber optical properties in different ways. Nevertheless, most of these studies were devoted to the Radiation Induced Attenuation (RIA) in the visible and near-infrared spectral domain when the fibers are exposed to UV-light, gamma or X-rays irradiations * Corresponding author. Tel.: +33 4 77 91 58 10; fax: +33 4 77 91 57 81. E-mail address: [email protected] (B. Tortech). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.029
[6–8]. The evolution of this RIA was especially studied around 980 nm for the erbium ions absorption band and in 1250– 1600 nm range where the host matrix and the erbium ions responses can be checked at 1300 and 1550 nm, respectively [3,9– 12]. The influence of various parameters studied: the dose-rate, cumulative dose, fiber composition (Ge, P, F, Al and pure silica), Er3+ ions concentration, etc. [13–16]. The main idea of this work is the study of an Er-doped fiber attenuation and response, in the near UV domain, during UV-laser exposure. As pointed out before, these kinds of measurements are not easy to perform for several reasons: (i) one needs a powerful light source with an adequate level in the near UV range; (ii) the probe signal must be injected, with a specific objective, into the fiber core which is only a few microns in diameter; (iii) the fiber length should be long enough when the cut-back technique is used and (iv) the attenuation coefficients are important in this spectral range. To overcome these difficulties, we developed an original technique where the near UV-visible probe signal is due to the photoluminescence ascribed to optically active centers located in the fiber core. This photoluminescence light exists only when the fiber
1297
B. Tortech et al. / Optical Materials 31 (2009) 1296–1299
2. Experimental set-up
1.0
Normalized Intensity (arb. u.)
is submitted to a transverse UV-laser exposure. It is kept guided along the fiber, and its level is sufficient enough for spectroscopic measurements. In the corresponding wavelength range, associated to the photoluminescence of the emitting centers, one can study both the influence of UV-laser exposure on the host matrix attenuation and the absorption of the erbium ions.
0.8 SMF 28 0.6 Er
0.4
3+
The experimental set-up is drawn in Fig. 1. The Er-doped fiber used has a mode field diameter of 5.1 lm at 1558 nm it was stripped over the entire length (20–30 cm) where the UV-exposure is performed. The transverse irradiation is conducted with a cw UV-laser operating at 244 nm, an output power of about 90 mW and a spot size of 0.5 mm. For the optical measurements, we connected a mini-spectrometer at each end of the fiber. These apparatus are equipped with a CCD camera in the 250–850 nm spectral range and their minimum time response is in the order of a few ms. To increase the UV-irradiated fiber length, 20 cm, we should move the sample along the laser beam. Therefore, the fiber is fixed to a tight thread which can be pulled by a continuous motor at a constant speed. During this displacement, the sample passes through a fiber guide which leads to a homogeneous irradiation along the fiber axis. In this configuration (Fig. 1), the detection system 1 records the photoluminescence spectrum which goes through the irradiated part of the sample and mainly it shows the evolution of the absorption spectra, whereas the detection system 2 should remain constant unless any variation of the laser beam position is detected, this signal is used as a reference in this experiment.
380 nm ð4 I15=2 ! 2 G11=2 Þ, 407 nm ð4 I15=2 ! 2 H9=2 Þ, 450 nm 487 nm ð4 I15=2 ! 4 F 7=2 Þ, and 525 nm ð4 I15=2 ! 4 F 3=2 Þ, ð4 I15=2 ! 2 H11=2 Þ. Even if the origin of the emitting centers is not the subject of this paper, one can notice that the most important concerns the twofold-coordinated defects such as the = X, where the symbols (=) and () stand for bonds with two distinct oxygen atoms and a pair of electrons in a lone pair orbital respectively, whereas a part of the X can be attributed to Ge atoms [17,18].
3. Results
3.1. Host matrix response during UV-laser exposure
In Fig. 2, we show typical photoluminescence spectrum recorded at one end of the erbium-doped fiber when the sample is transversally UV-laser irradiated (at 244 nm) over the laser spot size length, with a power of 90 mW and during a short exposure time (a few tenth of ms). The two detection systems present the same spectra if the fiber is kept fixed. In addition, we performed the same measurement in SMF 28 samples in order to highlight that the origin of this photoluminescence signal is principally due to the emitting centers related to the host matrix and cannot be attributed to the erbium ions. Nevertheless, in the studied spectral range, the Er3+ ions play a role through their absorption bands and some transitions [1] are pointed out: 356 nm ð4 I15=2 ! 2 G7=2 Þ, 365 nm ð4 I15=2 ! 2 G9=2 Þ,
The influence of the UV irradiation on the host matrix is evaluated in Fig. 3. In this case the fiber moves parallel to its optical axis at a constant speed of about 4 mm/s and the two synchronized detection systems record the entire spectral range in a time scale of hundreds milliseconds. During this displacement, the photoluminescence intensity recorded with detection system 1 crosses a given UV-irradiated length of the Er-doped fiber leading to an evolution which directly reflects the effect on the UV-exposure over the entire irradiated length. The detection system 2 controls the stability of the measurement during the experiment. The corresponding photoluminescence signal does not pass through an irradiated zone, so its intensity and profile should not change. This is
0.2
0.0 250
300
350
400
450
500
550
Wavelength (nm) Fig. 2. Normalized photoluminescence spectra recorded in two different optical fibers (Er-doped fiber and SMF 28) during a transverse cw UV-laser irradiation at 244 nm (90 mW).
Fig. 1. Scheme of experimental set-up.
B. Tortech et al. / Optical Materials 31 (2009) 1296–1299
t ia
ad Irr
0.35 2.17 4.90 7.63 11.3 17.2 20.3
ed
gt
n le
h
) m (c
350
400
450
500
Wavelength (nm) Fig. 3. Evolution of the UV-laser induced photoluminescence spectra along the irradiated lengths of the Er-doped fiber.
reflected in the spectra of Fig. 2. This useful method requires a short fiber length (typically 20-30 cm) and short time exposures (a few minutes) to measure the attenuation coefficients in the whole spectral domain. The intensities evolutions versus the irradiated length of the sample are plotted in Fig. 4 at three different wavelengths. These signals are easily fitted with an exponential function law such as: I = A exp(ax) + y0, where I is the intensity, x is the irradiated length, A, y0 and a are the fitting parameters. These fits lead to the corresponding attenuation coefficients a. In our experimental conditions, especially UV-laser density and translation speed of the fiber sample, we obtain for these attenuation coefficients: 0.21 cm1 at 373 nm, 0.22 cm1 at 400 nm and 0.16 cm1 at 500 nm.
mitted photoluminescence spectrum is recorded. This operation is then repeated for every 90 cm in this study, on other part of the Erdoped fiber. Therefore, the Er-doped fiber must be long enough, 10 m, and the starting measurement point should be done at the longest length far from the detection system. With this method, the guided photoluminescence signal does not cross any irradiated part of the fiber and its evolution reflects directly the absorption of the erbium ions with a tiny contribution of the host matrix. Fig. 5 shows spectra recorded at different distances (8.1 m, 2.7 m, and 0.9 m) between the detection system and the irradiation point on the Er-doped fiber under the same laser power. We observe that the intensity decreases rapidly at certain wavelengths which correspond to the specific erbium ions absorptions, whereas at other wavelengths, the intensity decreases slowly according to the host matrix attenuation. This effect is reported on Fig. 6, where the evolution of signals intensities versus their propagation length in the Er-doped fiber is plotted. The two curves at 372 nm and 407 nm correspond to the erbium ions absorption wavelengths and their evolution can be fitted with an exponential law, whereas the signal at 510 nm, with a lower attenuation coefficient reflects the host matrix effect through a quasi-linear law.
0.9 m 2.7 m 8.1 m
Intensity (abr. u.)
Photoluminescence Intensity (arb. u.)
1298
3.2. Direct measurement of the Er3+ ions absorption coefficients
Normalized Transmited Intensity (arb. u.)
The same experimental set-up can be used to determine the absorption coefficients of the erbium ions. In this case, the procedure is still based on the same photoluminescence signal as a probe signal but the UV-laser irradiation should be performed just on very short lengths, typically in the order of the laser spot size and for an exposure time of a few seconds, during which the trans-
300
350
400
450
500
Wavelength (nm) Fig. 5. Photoluminescence spectra recorded at 8.1 m, 2.7 m and 0.9 m far from the detection system and under punctual transverse UV-laser irradiation at 244 nm.
373nm 400nm 500nm
1.0
0.8
0.6
-1
0.4
α = 0.21 cm
α = 0.065 m
Intensity (arb. u.)
α = 0.16 cm
-1
0.2 -1
0
5
α = 0.27 m
372nm 407nm 510nm
α = 0.22 cm
0.0
10
15
20
25
α = 0.49 m
30
Irradiated Length (cm)
0
2
-1
4
6
-1
-1
8
10
Fiber Length (m) Fig. 4. Evolution of the normalized transmitted photoluminescence intensity along the UV-irradiated length at three different wavelengths. The symbols correspond to the experimental data and the lines correspond to exponential fits where a is the absorption coefficient.
Fig. 6. Evolution of the normalized transmitted photoluminescence intensity along the Er-doped fiber. The symbols correspond to the experimental data and the lines correspond to exponential fits where a is the absorption coefficient.
B. Tortech et al. / Optical Materials 31 (2009) 1296–1299
4. Conclusions An original technique has been developed in order to determine the absorption coefficients of an optical fiber in the 350–550 nm spectral range. The probe signal is based on the photoluminescence of optically active defects (located in the fiber) excited with a transverse UV-laser beam. This method has been applied to an Er-doped fiber and we evaluated both the influence of the UVexposure on the host matrix attenuation and the absorption coefficients of erbium ions along the fiber core. Short fiber length is needed to check and measure the UV radiation induced attenuation in this near UV domain. References [1] E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications, Wiley Interscience, New York, 1995. [2] M.J.F. Digonnet (Ed.), Rare Earth Doped Fiber Lasers and Amplifiers, second ed., M. Dekker, New York, 2001. [3] G.M. Williams, M.A. Putnam, E.J. Friebele, SPIE 2811 (1996) 30. [4] R.H. Boucher, W.F. Woodward, T.S. Lomheim, R.M. Shima, D.J. Asman, K.M. Killian, J. LeGrand, G.J. Goellner, Opt. Eng. 35 (1996) 955.
1299
[5] G.M. Williams, B.M. Wright, W.D. Mack, E.J. Friebele, SPIE 3848 (1999) 271. [6] E.J. Friebele, G.H. Sigel, M.E. Gingerich, IEEE Trans. Nucl. Sci. 25 (1978) 1261. [7] S. Girard, Analyse de la réponse des fibres optiques soumises à divers environnements radiatifs, Thèse de Doctorat, Université de Saint-Etienne, France, 2003. [8] S. Girard, J. Keurinck, A. Boukenter, J.-P. Meunier, Y. Ouerdane, B. Aza, P. Charre, M. Vié, Nucl. Instrum. Met. Phys. Res. Sect. B 215 (2004) 187. [9] T.S. Rose, D. Gunn, G.C. Valley, J. Lightwave Technol. 19 (2001) 1918. [10] R.J. Bussjager, M.J. Hayduk, S.T. Johns, E.W. Taylor, IEEE-Aerospace-Conference, Big Sky, MT, USA, 3 (2001) 3/1473. [11] M. Van Uffelen, S. Girard, F. Goutaland, A. Gusarov, B. Brichard, F. Berghmans, IEEE Trans. Nucl. Sci. 51 (2004) 2763. [12] B. Tortech, M. Van Uffelen, A. Gusarov, Y. Ouerdane, A. Boukenter, J.-P. Meunier, F. Berghmans, H. Thienpont, J. Non-Cryst. Solids 353 (2007) 477. [13] G.M. Williams, M.A. Putnam, C.G. Askins, M.E. Gingerich, E.J. Friebele, Electron. Lett. 28 (1992) 1816. [14] R.B.J. Lewis, E.S.R. Sikora, J.V. Wright, R.H. West, S. Dowling, Electron. Lett. 28 (1992) 1589. [15] J.R. Simpson, M.M. Broer, D.J. DiGiovanni, K.W. Quoi, S.G. Kosinski, Proceedings of OFC/IOOC’93, 1993. [16] C. Fukuda, Y. Chigusa, T. Kashiwada, M. Onishi, H. Kanamori, S. Okamoto, Electron. Lett. 30 (1994) 1342. [17] L. Sukja, J. Non-Cryst. Solids 149 (1992) 77. [18] K. Médjahdi, A. Boukenter, Y. Ouerdane, F. Messina, M. Cannas, J. Non-Cryst. Solids 353 (2007) 447.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Transverse UV-laser irradiation-induced defects and absorption in a single-mode erbium-doped optical fiber B. Tortech a,*, Y. Ouerdane a, A. Boukenter a, J.-P. Meunier a, S. Girard b, M. Van Uffelen c, E. Régnier d, F. Berghmans c,e, H. Thienpont e a
Université Jean Monnet, Laboratoire Hubert Curien-UMR CNRS 5516, Bât F, 18 rue Benoît Lauras, 42000 Saint-Etienne, France CEA DIF, BP 12, 91680 Bruyères-le-Châtel, France c SCK-CEN, Boerentang 200, B-2400 Mol, Belgium d Draka Comteq France, Data Center IV, Route de Nozay, 91 460 Marcoussis, France e Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium b
a r t i c l e
i n f o
Article history: Available online xxxx PACS: 42.88.+h 42.81.Wg 61.80.Ba 78.55.m Keywords: Optical fiber Photoluminescence Erbium UV irradiation
a b s t r a c t Near UV-visible absorption coefficients of an erbium-doped optical fiber were investigated through an original technique based on a transverse cw UV-laser irradiation operating at 244 nm. Such irradiation leads to the generation of a quite intense guided luminescence signal in near UV spectral range. This photoluminescence probe source combined with a longitudinal translation of the fiber sample (at a constant velocity) along the UV-laser irradiation, presents several major advantages: (i) we bypass and avoid the procedures classically used to study the radiation induced attenuation which are not adapted to our case mainly because the samples present a very strong absorption with significant difficulties due to the injection of adequate UV-light levels in a small fiber diameter; (ii) the influence of the laser irradiation on the host matrix of the optical fiber is directly correlated to the evolution of the generated photoluminescence signal and (iii) in our experimental conditions, short fiber sample lengths (typically 20–30 cm) suffice to determine the associated absorption coefficients over the entire studied spectral domain. The generated photoluminescence signal is also used to characterize the absorption of the erbium ions in the same wavelength range with no cut-back method needed. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The erbium-doped fibers are a key component of optical devices such as optical amplifiers or super-luminescent sources used in fiber gyroscopes [1,2]. They are widely deployed in terrestrial and submarine applications as repeaters for long-line or highly distributed telecommunication systems. It is well known that, in space, the fibers may be exposed to a relatively large dose of electrons and protons over the spacecraft lifetime [3–5]. Therefore, the effect of energetic radiation on rare earth doped fibers is of interest for both predicting device performances in radiation environments, and identifying the specific mechanisms of color center formation in these material systems. Several laboratory experiments have already shown that the radiation exposure could potentially affect the fiber optical properties in different ways. Nevertheless, most of these studies were devoted to the Radiation Induced Attenuation (RIA) in the visible and near-infrared spectral domain when the fibers are exposed to UV-light, gamma or X-rays irradiations * Corresponding author. Tel.: +33 4 77 91 58 10; fax: +33 4 77 91 57 81. E-mail address: [email protected] (B. Tortech). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.029
[6–8]. The evolution of this RIA was especially studied around 980 nm for the erbium ions absorption band and in 1250– 1600 nm range where the host matrix and the erbium ions responses can be checked at 1300 and 1550 nm, respectively [3,9– 12]. The influence of various parameters studied: the dose-rate, cumulative dose, fiber composition (Ge, P, F, Al and pure silica), Er3+ ions concentration, etc. [13–16]. The main idea of this work is the study of an Er-doped fiber attenuation and response, in the near UV domain, during UV-laser exposure. As pointed out before, these kinds of measurements are not easy to perform for several reasons: (i) one needs a powerful light source with an adequate level in the near UV range; (ii) the probe signal must be injected, with a specific objective, into the fiber core which is only a few microns in diameter; (iii) the fiber length should be long enough when the cut-back technique is used and (iv) the attenuation coefficients are important in this spectral range. To overcome these difficulties, we developed an original technique where the near UV-visible probe signal is due to the photoluminescence ascribed to optically active centers located in the fiber core. This photoluminescence light exists only when the fiber
1297
B. Tortech et al. / Optical Materials 31 (2009) 1296–1299
2. Experimental set-up
1.0
Normalized Intensity (arb. u.)
is submitted to a transverse UV-laser exposure. It is kept guided along the fiber, and its level is sufficient enough for spectroscopic measurements. In the corresponding wavelength range, associated to the photoluminescence of the emitting centers, one can study both the influence of UV-laser exposure on the host matrix attenuation and the absorption of the erbium ions.
0.8 SMF 28 0.6 Er
0.4
3+
The experimental set-up is drawn in Fig. 1. The Er-doped fiber used has a mode field diameter of 5.1 lm at 1558 nm it was stripped over the entire length (20–30 cm) where the UV-exposure is performed. The transverse irradiation is conducted with a cw UV-laser operating at 244 nm, an output power of about 90 mW and a spot size of 0.5 mm. For the optical measurements, we connected a mini-spectrometer at each end of the fiber. These apparatus are equipped with a CCD camera in the 250–850 nm spectral range and their minimum time response is in the order of a few ms. To increase the UV-irradiated fiber length, 20 cm, we should move the sample along the laser beam. Therefore, the fiber is fixed to a tight thread which can be pulled by a continuous motor at a constant speed. During this displacement, the sample passes through a fiber guide which leads to a homogeneous irradiation along the fiber axis. In this configuration (Fig. 1), the detection system 1 records the photoluminescence spectrum which goes through the irradiated part of the sample and mainly it shows the evolution of the absorption spectra, whereas the detection system 2 should remain constant unless any variation of the laser beam position is detected, this signal is used as a reference in this experiment.
380 nm ð4 I15=2 ! 2 G11=2 Þ, 407 nm ð4 I15=2 ! 2 H9=2 Þ, 450 nm 487 nm ð4 I15=2 ! 4 F 7=2 Þ, and 525 nm ð4 I15=2 ! 4 F 3=2 Þ, ð4 I15=2 ! 2 H11=2 Þ. Even if the origin of the emitting centers is not the subject of this paper, one can notice that the most important concerns the twofold-coordinated defects such as the = X, where the symbols (=) and () stand for bonds with two distinct oxygen atoms and a pair of electrons in a lone pair orbital respectively, whereas a part of the X can be attributed to Ge atoms [17,18].
3. Results
3.1. Host matrix response during UV-laser exposure
In Fig. 2, we show typical photoluminescence spectrum recorded at one end of the erbium-doped fiber when the sample is transversally UV-laser irradiated (at 244 nm) over the laser spot size length, with a power of 90 mW and during a short exposure time (a few tenth of ms). The two detection systems present the same spectra if the fiber is kept fixed. In addition, we performed the same measurement in SMF 28 samples in order to highlight that the origin of this photoluminescence signal is principally due to the emitting centers related to the host matrix and cannot be attributed to the erbium ions. Nevertheless, in the studied spectral range, the Er3+ ions play a role through their absorption bands and some transitions [1] are pointed out: 356 nm ð4 I15=2 ! 2 G7=2 Þ, 365 nm ð4 I15=2 ! 2 G9=2 Þ,
The influence of the UV irradiation on the host matrix is evaluated in Fig. 3. In this case the fiber moves parallel to its optical axis at a constant speed of about 4 mm/s and the two synchronized detection systems record the entire spectral range in a time scale of hundreds milliseconds. During this displacement, the photoluminescence intensity recorded with detection system 1 crosses a given UV-irradiated length of the Er-doped fiber leading to an evolution which directly reflects the effect on the UV-exposure over the entire irradiated length. The detection system 2 controls the stability of the measurement during the experiment. The corresponding photoluminescence signal does not pass through an irradiated zone, so its intensity and profile should not change. This is
0.2
0.0 250
300
350
400
450
500
550
Wavelength (nm) Fig. 2. Normalized photoluminescence spectra recorded in two different optical fibers (Er-doped fiber and SMF 28) during a transverse cw UV-laser irradiation at 244 nm (90 mW).
Fig. 1. Scheme of experimental set-up.
B. Tortech et al. / Optical Materials 31 (2009) 1296–1299
t ia
ad Irr
0.35 2.17 4.90 7.63 11.3 17.2 20.3
ed
gt
n le
h
) m (c
350
400
450
500
Wavelength (nm) Fig. 3. Evolution of the UV-laser induced photoluminescence spectra along the irradiated lengths of the Er-doped fiber.
reflected in the spectra of Fig. 2. This useful method requires a short fiber length (typically 20-30 cm) and short time exposures (a few minutes) to measure the attenuation coefficients in the whole spectral domain. The intensities evolutions versus the irradiated length of the sample are plotted in Fig. 4 at three different wavelengths. These signals are easily fitted with an exponential function law such as: I = A exp(ax) + y0, where I is the intensity, x is the irradiated length, A, y0 and a are the fitting parameters. These fits lead to the corresponding attenuation coefficients a. In our experimental conditions, especially UV-laser density and translation speed of the fiber sample, we obtain for these attenuation coefficients: 0.21 cm1 at 373 nm, 0.22 cm1 at 400 nm and 0.16 cm1 at 500 nm.
mitted photoluminescence spectrum is recorded. This operation is then repeated for every 90 cm in this study, on other part of the Erdoped fiber. Therefore, the Er-doped fiber must be long enough, 10 m, and the starting measurement point should be done at the longest length far from the detection system. With this method, the guided photoluminescence signal does not cross any irradiated part of the fiber and its evolution reflects directly the absorption of the erbium ions with a tiny contribution of the host matrix. Fig. 5 shows spectra recorded at different distances (8.1 m, 2.7 m, and 0.9 m) between the detection system and the irradiation point on the Er-doped fiber under the same laser power. We observe that the intensity decreases rapidly at certain wavelengths which correspond to the specific erbium ions absorptions, whereas at other wavelengths, the intensity decreases slowly according to the host matrix attenuation. This effect is reported on Fig. 6, where the evolution of signals intensities versus their propagation length in the Er-doped fiber is plotted. The two curves at 372 nm and 407 nm correspond to the erbium ions absorption wavelengths and their evolution can be fitted with an exponential law, whereas the signal at 510 nm, with a lower attenuation coefficient reflects the host matrix effect through a quasi-linear law.
0.9 m 2.7 m 8.1 m
Intensity (abr. u.)
Photoluminescence Intensity (arb. u.)
1298
3.2. Direct measurement of the Er3+ ions absorption coefficients
Normalized Transmited Intensity (arb. u.)
The same experimental set-up can be used to determine the absorption coefficients of the erbium ions. In this case, the procedure is still based on the same photoluminescence signal as a probe signal but the UV-laser irradiation should be performed just on very short lengths, typically in the order of the laser spot size and for an exposure time of a few seconds, during which the trans-
300
350
400
450
500
Wavelength (nm) Fig. 5. Photoluminescence spectra recorded at 8.1 m, 2.7 m and 0.9 m far from the detection system and under punctual transverse UV-laser irradiation at 244 nm.
373nm 400nm 500nm
1.0
0.8
0.6
-1
0.4
α = 0.21 cm
α = 0.065 m
Intensity (arb. u.)
α = 0.16 cm
-1
0.2 -1
0
5
α = 0.27 m
372nm 407nm 510nm
α = 0.22 cm
0.0
10
15
20
25
α = 0.49 m
30
Irradiated Length (cm)
0
2
-1
4
6
-1
-1
8
10
Fiber Length (m) Fig. 4. Evolution of the normalized transmitted photoluminescence intensity along the UV-irradiated length at three different wavelengths. The symbols correspond to the experimental data and the lines correspond to exponential fits where a is the absorption coefficient.
Fig. 6. Evolution of the normalized transmitted photoluminescence intensity along the Er-doped fiber. The symbols correspond to the experimental data and the lines correspond to exponential fits where a is the absorption coefficient.
B. Tortech et al. / Optical Materials 31 (2009) 1296–1299
4. Conclusions An original technique has been developed in order to determine the absorption coefficients of an optical fiber in the 350–550 nm spectral range. The probe signal is based on the photoluminescence of optically active defects (located in the fiber) excited with a transverse UV-laser beam. This method has been applied to an Er-doped fiber and we evaluated both the influence of the UVexposure on the host matrix attenuation and the absorption coefficients of erbium ions along the fiber core. Short fiber length is needed to check and measure the UV radiation induced attenuation in this near UV domain. References [1] E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications, Wiley Interscience, New York, 1995. [2] M.J.F. Digonnet (Ed.), Rare Earth Doped Fiber Lasers and Amplifiers, second ed., M. Dekker, New York, 2001. [3] G.M. Williams, M.A. Putnam, E.J. Friebele, SPIE 2811 (1996) 30. [4] R.H. Boucher, W.F. Woodward, T.S. Lomheim, R.M. Shima, D.J. Asman, K.M. Killian, J. LeGrand, G.J. Goellner, Opt. Eng. 35 (1996) 955.
1299
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