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Arsenic sulfide single mode fibres for 1.3 μm amplification.
Journal of Non-Crystalline Solids 326&327 (2003) 446–450 www.elsevier.com/locate/jnoncrysol
Arsenic sulfide single mode fibres for 1.3 lm amplification. Preparation – gain potential – power stability limits J. Kobelke *, S. Jetschke, A. Schwuchow, J. Kirchhof, K. Schuster Institute for Physical High Technology, Winzerlaer Strasse 10, D-07745 Jena, Germany
Abstract Praseodymium doped arsenic sulfide based single mode fibres were prepared for 1.3 lm amplification. Despite of the high maximum gain coefficient (0.5 dB mW1 ) the pump power stability limits the suitability of this glass matrix type for amplifier and laser applications. A 1.2 mol% germanium doped single mode fibres show a maximum pump power density limit of <15 kW cm2 . Investigations by pulsed pumping show that material degradation effects consist of reversible and irreversible portions, depending on the power pump density. By increasing the germanium concentration to more than 10 mol% the power stability limit can be remarkably improved. This indicates that the power sensitivity is determined by the main component arsenic sulfide in first place. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.70.Km; 42.81.Cn; 52.70.Kz; 61.80.Ba; 82.50.Hp; 82.50.Bc
1. Introduction In the past years praseodymium doped chalcogenide glass fibres were considered to be wellsuited candidates for highly efficient amplifiers for the 1.3 lm telecommunication window. Different chalcogenide glass matrix types were tested for rare earth doped fibres due to their low phonon energy: germanium sulfide based glasses: Ge–S [1], Ge–Ga–S [2]; gallium sulfide based glasses: Ga– La–S [3,4], Ga–Na–S [5]; arsenic sulfide based glasses: As–S [6], As–Ge–S [7]. Whereas pure germanium sulfide glasses show a high transparency and high frequency UV edge, their rare earth sol-
*
Corresponding author. Tel.: +49-3641 206 259; fax: +493641 206 299. E-mail address: [email protected] (J. Kobelke).
ubility is rather low. Caused by their relatively high crystallization tendency, the fibre preparation of germanium sulfide based glasses succeeds only in singular cases [1]. Gallium sulfide based glasses possess a very high rare earth solubility, excellent UV edge and low fibre attenuation in the near infrared range. A controllable crystallization tendency, especially in the case of Ga–La–S, allowed the preparation of low loss fibres. Itoh et al. [5] demonstrated the highly efficient amplification at 1.3 lm with a Pr:Ga–Na–S fibre. Their gain coefficient of 0.81 dB mW1 shows the potential of chalcogenide glasses. Manufacturing and handling difficulties have obviously limited the continuation of the development of amplifier fibres based on that modified gallium sulfide glass which suffers from high corrosion sensitivity. Arsenic sulfide glasses show a very good glass stability, the preparation of low loss fibres is practiced for many
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00442-3
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450 10
As40S30Se30
Additional loss / dBm-1
years [7]. On the other hand the rare earth solubility of pure arsenic sulfide glasses is extremely low. Only rare earth concentrations below 100 wt% can be added without dramatically increasing of optical losses. To overcome this disadvantage codopants like gallium and germanium were added into the glass up to concentrations of few mol%. The goal was to find an optimization between a good fibre drawing behaviour, a low crystallization tendency and a sufficient rare earth solubility up to more than 1000 wt ppm [8].
447
As37.9Ge1.3Ga0.5S60.3 +750 ppm Pr
3+
1
0.1
As27Ge13S60 1000
As41S59
1200
1400
1600
Wavelength / nm
2. Experimental As–(Ge, Ga)–S glasses for fibre manufacturing (core glass, inner cladding glass, outer cladding glass) were prepared by melting the elements in quartz glass ampoules [8]. Fibres with core diameters 2.4–6.25 lm were manufactured by combining the methods of double-crucible and rod-in-tube drawing [9]. Praseodymium doped arsenic sulfide glass (750 wt% Pr3þ ), codoped with 1.2 mol% germanium and 0.4 mol% gallium was prepared as core material. The inner clad glass contains additionally 2.4 mol% germanium and no gallium. The numerical aperture of the single mode fibre was 0.43. The finally drawn single mode fibre has the following dimensions: The diameters for core, inner clad, outer clad are about 2.4, 15, 190 lm, respectively. The fibre was coated with UV curable acrylate of approximately 40 lm thickness to protect it from corrosion and mechanical damage. To investigate the influence of the dopants on the power stability, fibres with an increased germanium concentration in the core up to 11 mol% were prepared. The fibre attenuation was measured by the cutback-method in the wavelength range 300–1750 nm with an Anritsu optical spectrum analyzer. At our first measurements of amplification we could not see any amplification at around 1300 nm. In opposite the measured signal light at 1320 nm at the end of the fibre decreased immediately after the switching on of the pump light. Additionally to these observations at high pump power densities we found a slight decrease of transmission for both the 1018 and 1320 nm wavelength if the fibre is
Fig. 1. Additional loss of various fibre compositions under fluorescent lamp illumination.
illuminated by room- or daylight. To investigate this effect for different glass compositions, selected unstructured fibres were transversally illuminated with a 22 W fluorescent lamp. The emission of the lamp ranges from 0.4 to 1 lm with power maximum wavelength at approximately 0.6 m. The distance to the fibre was about 0.06 m and the illuminated fibre length was 1 m. The additional loss caused by the fluorescence lamp was estimated by measuring the transmission spectra with and without illumination (Fig. 1). To investigate in detail the processes of amplification, degradation and recovery the experimental setup for amplification measurements, described by Jetschke et al. [10] was used. A single mode fibre (length: 2 m) was pumped in a pulsed regime (MOPA: wavelength 1018 nm, pulse duration 1ms to 150 ms, repetition rate 1 Hz). The signal light (1320 nm) was generated in continuous wave (cw)-regime by a LED. The launched pump power varied between 0.75 and 150 mW, the launched signal power was 3.6 lW. A low pass filter removed the residual pump light at the end of the fibre. The time resolved signal was measured by an InGaAs-receiver with a bandwidth of 100 kHz.
3. Results The investigations for additional loss caused by the illumination with UV/visible light were made
448
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450
on unstructured uncoated fibres. For the four selected glass compositions we found different intensities of degradation and saturation times. Whereas selenium containing fibres (As40 S30 Se30 ) degrade completely in a few seconds, pure arsenic sulfide fibres (As41 S59 ) show a degradation saturation time of about 1 min. The germanium gallium codoped arsenic sulfide fibre (1.3 mol% Ge, 0.5 mol% Ga) with 750 wt ppm praseodymium shows a degradation saturation time of about 5 min whereas the time for the fibre with the highest germanium concentration (13 mol% Ge) is about 15 min. The final values of the additional loss (Fig. 2) at the pump wavelength 1.02 lm and the saturation time are summarized in Table 1. The observed fluorescence lamp induced loss was
Transmitted power / rel. u.
0
50
100
150
Fibre (b) : pump pulse - 150 mW, 150 ms: degradation: 0.05 dB)
1.0 0.75 mW, 1 ms 0.75 mW, 5 ms
0.8 0.6
Fibre (a) : pump pulses - power, width
0.4 0.2 75 mW, 1 ms
0.0
0
2
4
6
8
10
12
Time / ms
Fig. 2. Transmission functions for pump pulses with several peak powers and widths: fibre (a) with 750 wt% Pr3þ , 1.2 mol% Ge and core diameter 2.4 lm and fibre (b) with 11 mol% Ge and core diameter 6.5 lm.
completely reversible but the recovery lasted hours in all four cases. The function of 1.3 lm amplifiers based on arsenic sulfide glasses is clearly limited by degradation under the influence of pump power. The occupation inversion of the Pr3þ ions builds up after switching on the pump light. The fluorescence and total signal are increasing by amplification, but during further pumping they start to decrease again. This decrease is more distinct with higher pump power and pulse width. It is also observable in the fluorescence signal. After switching off the pump light the fluorescence intensity subsides exponentially. The total signal decreases below the starting level, but relaxes before the next pump pulse is launched. This trend is more intensively observed up with increasing the pump power. We observed it up to 75 mW pulse power with pulse duration of 1 ms (Fig. 3). The influences of pump pulse power and width show that the saturation of the transient absorption is finished in a few milliseconds for a praseodymium doped fibre with a germanium concentration of 1.2 mol%. The saturation trend is similarly for different pump powers up to 75 mW for this fibre. To investigate the effect of codopants on the power stability of the glass we tested arsenic sulfide based fibres with different germanium concentrations. The increasing of the germanium concentration from 1.2 to 11 mol% allows one to decrease the degradation rate of the fibre intensively. Whereas the transparency of a low 1.2 mol% ger-
Table 1 Final values of additional loss at 1.02 lm (accuracy: 0.1 dB/m) and attenuation saturation time (accuracy: 1 min) caused by fluorescence lamp illumination of the uncoated fibres Glass composition
Additional loss @1.02 lm (dB/m)
Saturation time (min)
As40 S30 Se30 As41 S59 As37:9 Ge1:3 Ga0:5 S60:3 (750 wt% Pr3þ ) As27 Ge13 S60
4.7 1.3 1.2
<1 1 5
0.9
15
Signal / mV
1500 Pump pulse: 0.75 mW, 1 ms
1250 1000
St F
250 0 0
1
2
3
4
Time / ms
Fig. 3. Measured fluorescence F and total signal St for pump pulses with launched peak power 0.75 mW.
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450
manium fibre decreases at a pump power of 0.75 mW (power density: about 13 kW cm2 ) by about 0.1 dB within approximately 1 ms, a 11 mol% germanium containing fibre degrades significantly slower. The degradation rate at a pump power of 150 mW (power density: about 450 kW cm2 ) is about 0.05 dB over 150 ms.
4. Discussion Because of the time dependent fibre loss it was very difficult to point out the presence of praseodymium amplification with cw-pumping. Therefore the amplification properties were estimated under pulsed pumping. Gain measurement tests with cw-pumping at 1.02 lm and a pump power down to 0.75 mW show no signal amplification. This is obviously caused by a strong additionally loss induced by the pump light. For more detailed study of this attenuation behaviour the usual amplifier setup was changed to a pulsed pumping regime up to 150 mW. By using the model for description of the processes of simultaneous amplification and degradation, described by Jetschke et al. [10] we determined the signal gain during simultaneous light induced attenuation. This calculation of the gain coefficient considers the effects of signal degradation. We yield the true signal light intensity, a supposition for approximation of the gain, by subtraction of fluorescence and signal light intensity, influenced by pump light, respectively. For pump pulse power <1 mW a maximum gain coefficient of 0.5 dB mW1 was achieved, which decreases with increasing the pump power. By increasing the pump pulse power over 20–75 mW the gain coefficient decreases to about 0.02 dB mW1 . Obviously the decreased slope of the gain coefficient at higher pump power up to 75 mW is not caused by gain saturation effects, but by power induced attenuation of the core glass. The increase of the germanium concentration of the core glass over 10 mol% allows one to decrease the degradation drastically to a transmission loss rate of about 0.6 dB m1 s1 at signal wavelength. However, we suspect that this degradation rate is still too high in a cw-pump regime.
449
Additionally, unfortunately the background loss of the high germanium containing fibre is significantly higher, about 20 dB m1 at signal wavelength. Similar material degradation effects we observed by the investigation of side light irradiated uncoated fibres. Here we measured an additionally loss in the sub-dB m1 -range which increased up to shorter wavelengths. This increase can be explained by photodarkening effects [11,12]. The dependence on the wavelength of the transmitted light indicates an obvious shift of the absorption edge to higher wavelength. Fritzsche [13] has described such photoinduced changes in chalcogenide glass. They have been attributed to modifications of physical and chemical states through excitation of electron–hole pairs, and can be reversible or irreversible. The analysis of the power stability limits shows that both reversible and irreversible material degeneration processes increase the transmission loss of the arsenic sulfide based fibre material.
5. Summary A 1.3 lm amplification tests show the principle high gain efficiency of arsenic sulfide based glasses due to their low phonon energy. We demonstrate a maximum gain coefficient of 0.5 dB mW1 with a pulsed pumping regime using a Pr3þ doped arsenic sulfide based single mode fibre. Unfortunately the advantage of the effective conversion of pump into signal light is failed by an increased degradation of the fibre by pump light depending on its intensity. The fibres with a higher germanium concentration up to 11 mol% show an improved, but obviously not sufficient power stability. Praseodymium doped arsenic sulfide based fibres with germanium concentration of more then a few mol% show higher background losses due to scattering effects caused by a likely lower rare earth solubility. In summary the advantages of the prepared glass material with regard to crystallization stability and low underground loss are restricted by the material degradation during pumping of the fibre for amplifier applications.
450
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG).
References [1] D.R. Simons, A.J. Faber, H. de Waal, Opt. Lett. 20 (5) (1995) 468. [2] B. Frumarova, M. Frumar, J. Oswald, J. Non-Cryst. Solids 213&214 (1997) 58. [3] P.C. Becker, M.M. Broer, V.G. Lambrecht, A.J. Bruce, G. Nykolak, Proceedings of the OSA Topics Meeting on Optical Amplifiers and Application, Santa Fe, NM, vol. 5, Post deadline paper PD, 1992, p. 20. [4] Y.D. West, T. Schweizer, D.J. Brady, D.W. Hewak, Fiber Integr. Opt. 19 (2000) 229.
[5] K. Itoh, H. Yanagita, H. Tawarayama, K. Yamanaka, E. Ishikawa, K. Okada, H. Aoki, Y. Matsumoto, A. Shirakawa, Y. Matsuoka, H. Toratani, J. Non-Cryst. Solids 256&257 (1999) 1. [6] Y. Ohishi, A. Mori, T. Kanamori, K. Fujiura, S. Sudo, Appl. Phys. Lett. 65 (1) (1994) 13. [7] N.S. Kapany, R.J. Simms, Infrared Phys. 5 (1965) 69. [8] M. Scheffler, J. Kirchhof, J. Kobelke, A. Schwuchow, in: Proceedings of the XIth International Symposium on Non-Oxide Glasses, Sheffield, UK, 1998, p. 258. [9] J. Kobelke, J. Kirchhof, M. Scheffler, A. Schwuchow, SPIE 3416 (1998) 55. [10] S. Jetschke, A. Schwuchow, J. Kobelke, K. Schuster, J. Kirchhof, SPIE 4645 (2002) 26. [11] A.M. Andriesh, N.A. Enachi, I.P. Culeac, T.N. Copaci, V.A. Binchevici, J. Non-Cryst. Solids 189 (1995) 147. [12] A. Ganjoo, K. Shimakawa, K. Kitano, E.A. Davis, J. NonCryst. Solids 299–302 (2002) 917. [13] H. Fritzsche, Semiconductors 32 (1998) 850.
Arsenic sulfide single mode fibres for 1.3 lm amplification. Preparation – gain potential – power stability limits J. Kobelke *, S. Jetschke, A. Schwuchow, J. Kirchhof, K. Schuster Institute for Physical High Technology, Winzerlaer Strasse 10, D-07745 Jena, Germany
Abstract Praseodymium doped arsenic sulfide based single mode fibres were prepared for 1.3 lm amplification. Despite of the high maximum gain coefficient (0.5 dB mW1 ) the pump power stability limits the suitability of this glass matrix type for amplifier and laser applications. A 1.2 mol% germanium doped single mode fibres show a maximum pump power density limit of <15 kW cm2 . Investigations by pulsed pumping show that material degradation effects consist of reversible and irreversible portions, depending on the power pump density. By increasing the germanium concentration to more than 10 mol% the power stability limit can be remarkably improved. This indicates that the power sensitivity is determined by the main component arsenic sulfide in first place. Ó 2003 Elsevier B.V. All rights reserved. PACS: 42.70.Km; 42.81.Cn; 52.70.Kz; 61.80.Ba; 82.50.Hp; 82.50.Bc
1. Introduction In the past years praseodymium doped chalcogenide glass fibres were considered to be wellsuited candidates for highly efficient amplifiers for the 1.3 lm telecommunication window. Different chalcogenide glass matrix types were tested for rare earth doped fibres due to their low phonon energy: germanium sulfide based glasses: Ge–S [1], Ge–Ga–S [2]; gallium sulfide based glasses: Ga– La–S [3,4], Ga–Na–S [5]; arsenic sulfide based glasses: As–S [6], As–Ge–S [7]. Whereas pure germanium sulfide glasses show a high transparency and high frequency UV edge, their rare earth sol-
*
Corresponding author. Tel.: +49-3641 206 259; fax: +493641 206 299. E-mail address: [email protected] (J. Kobelke).
ubility is rather low. Caused by their relatively high crystallization tendency, the fibre preparation of germanium sulfide based glasses succeeds only in singular cases [1]. Gallium sulfide based glasses possess a very high rare earth solubility, excellent UV edge and low fibre attenuation in the near infrared range. A controllable crystallization tendency, especially in the case of Ga–La–S, allowed the preparation of low loss fibres. Itoh et al. [5] demonstrated the highly efficient amplification at 1.3 lm with a Pr:Ga–Na–S fibre. Their gain coefficient of 0.81 dB mW1 shows the potential of chalcogenide glasses. Manufacturing and handling difficulties have obviously limited the continuation of the development of amplifier fibres based on that modified gallium sulfide glass which suffers from high corrosion sensitivity. Arsenic sulfide glasses show a very good glass stability, the preparation of low loss fibres is practiced for many
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0022-3093(03)00442-3
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450 10
As40S30Se30
Additional loss / dBm-1
years [7]. On the other hand the rare earth solubility of pure arsenic sulfide glasses is extremely low. Only rare earth concentrations below 100 wt% can be added without dramatically increasing of optical losses. To overcome this disadvantage codopants like gallium and germanium were added into the glass up to concentrations of few mol%. The goal was to find an optimization between a good fibre drawing behaviour, a low crystallization tendency and a sufficient rare earth solubility up to more than 1000 wt ppm [8].
447
As37.9Ge1.3Ga0.5S60.3 +750 ppm Pr
3+
1
0.1
As27Ge13S60 1000
As41S59
1200
1400
1600
Wavelength / nm
2. Experimental As–(Ge, Ga)–S glasses for fibre manufacturing (core glass, inner cladding glass, outer cladding glass) were prepared by melting the elements in quartz glass ampoules [8]. Fibres with core diameters 2.4–6.25 lm were manufactured by combining the methods of double-crucible and rod-in-tube drawing [9]. Praseodymium doped arsenic sulfide glass (750 wt% Pr3þ ), codoped with 1.2 mol% germanium and 0.4 mol% gallium was prepared as core material. The inner clad glass contains additionally 2.4 mol% germanium and no gallium. The numerical aperture of the single mode fibre was 0.43. The finally drawn single mode fibre has the following dimensions: The diameters for core, inner clad, outer clad are about 2.4, 15, 190 lm, respectively. The fibre was coated with UV curable acrylate of approximately 40 lm thickness to protect it from corrosion and mechanical damage. To investigate the influence of the dopants on the power stability, fibres with an increased germanium concentration in the core up to 11 mol% were prepared. The fibre attenuation was measured by the cutback-method in the wavelength range 300–1750 nm with an Anritsu optical spectrum analyzer. At our first measurements of amplification we could not see any amplification at around 1300 nm. In opposite the measured signal light at 1320 nm at the end of the fibre decreased immediately after the switching on of the pump light. Additionally to these observations at high pump power densities we found a slight decrease of transmission for both the 1018 and 1320 nm wavelength if the fibre is
Fig. 1. Additional loss of various fibre compositions under fluorescent lamp illumination.
illuminated by room- or daylight. To investigate this effect for different glass compositions, selected unstructured fibres were transversally illuminated with a 22 W fluorescent lamp. The emission of the lamp ranges from 0.4 to 1 lm with power maximum wavelength at approximately 0.6 m. The distance to the fibre was about 0.06 m and the illuminated fibre length was 1 m. The additional loss caused by the fluorescence lamp was estimated by measuring the transmission spectra with and without illumination (Fig. 1). To investigate in detail the processes of amplification, degradation and recovery the experimental setup for amplification measurements, described by Jetschke et al. [10] was used. A single mode fibre (length: 2 m) was pumped in a pulsed regime (MOPA: wavelength 1018 nm, pulse duration 1ms to 150 ms, repetition rate 1 Hz). The signal light (1320 nm) was generated in continuous wave (cw)-regime by a LED. The launched pump power varied between 0.75 and 150 mW, the launched signal power was 3.6 lW. A low pass filter removed the residual pump light at the end of the fibre. The time resolved signal was measured by an InGaAs-receiver with a bandwidth of 100 kHz.
3. Results The investigations for additional loss caused by the illumination with UV/visible light were made
448
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450
on unstructured uncoated fibres. For the four selected glass compositions we found different intensities of degradation and saturation times. Whereas selenium containing fibres (As40 S30 Se30 ) degrade completely in a few seconds, pure arsenic sulfide fibres (As41 S59 ) show a degradation saturation time of about 1 min. The germanium gallium codoped arsenic sulfide fibre (1.3 mol% Ge, 0.5 mol% Ga) with 750 wt ppm praseodymium shows a degradation saturation time of about 5 min whereas the time for the fibre with the highest germanium concentration (13 mol% Ge) is about 15 min. The final values of the additional loss (Fig. 2) at the pump wavelength 1.02 lm and the saturation time are summarized in Table 1. The observed fluorescence lamp induced loss was
Transmitted power / rel. u.
0
50
100
150
Fibre (b) : pump pulse - 150 mW, 150 ms: degradation: 0.05 dB)
1.0 0.75 mW, 1 ms 0.75 mW, 5 ms
0.8 0.6
Fibre (a) : pump pulses - power, width
0.4 0.2 75 mW, 1 ms
0.0
0
2
4
6
8
10
12
Time / ms
Fig. 2. Transmission functions for pump pulses with several peak powers and widths: fibre (a) with 750 wt% Pr3þ , 1.2 mol% Ge and core diameter 2.4 lm and fibre (b) with 11 mol% Ge and core diameter 6.5 lm.
completely reversible but the recovery lasted hours in all four cases. The function of 1.3 lm amplifiers based on arsenic sulfide glasses is clearly limited by degradation under the influence of pump power. The occupation inversion of the Pr3þ ions builds up after switching on the pump light. The fluorescence and total signal are increasing by amplification, but during further pumping they start to decrease again. This decrease is more distinct with higher pump power and pulse width. It is also observable in the fluorescence signal. After switching off the pump light the fluorescence intensity subsides exponentially. The total signal decreases below the starting level, but relaxes before the next pump pulse is launched. This trend is more intensively observed up with increasing the pump power. We observed it up to 75 mW pulse power with pulse duration of 1 ms (Fig. 3). The influences of pump pulse power and width show that the saturation of the transient absorption is finished in a few milliseconds for a praseodymium doped fibre with a germanium concentration of 1.2 mol%. The saturation trend is similarly for different pump powers up to 75 mW for this fibre. To investigate the effect of codopants on the power stability of the glass we tested arsenic sulfide based fibres with different germanium concentrations. The increasing of the germanium concentration from 1.2 to 11 mol% allows one to decrease the degradation rate of the fibre intensively. Whereas the transparency of a low 1.2 mol% ger-
Table 1 Final values of additional loss at 1.02 lm (accuracy: 0.1 dB/m) and attenuation saturation time (accuracy: 1 min) caused by fluorescence lamp illumination of the uncoated fibres Glass composition
Additional loss @1.02 lm (dB/m)
Saturation time (min)
As40 S30 Se30 As41 S59 As37:9 Ge1:3 Ga0:5 S60:3 (750 wt% Pr3þ ) As27 Ge13 S60
4.7 1.3 1.2
<1 1 5
0.9
15
Signal / mV
1500 Pump pulse: 0.75 mW, 1 ms
1250 1000
St F
250 0 0
1
2
3
4
Time / ms
Fig. 3. Measured fluorescence F and total signal St for pump pulses with launched peak power 0.75 mW.
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450
manium fibre decreases at a pump power of 0.75 mW (power density: about 13 kW cm2 ) by about 0.1 dB within approximately 1 ms, a 11 mol% germanium containing fibre degrades significantly slower. The degradation rate at a pump power of 150 mW (power density: about 450 kW cm2 ) is about 0.05 dB over 150 ms.
4. Discussion Because of the time dependent fibre loss it was very difficult to point out the presence of praseodymium amplification with cw-pumping. Therefore the amplification properties were estimated under pulsed pumping. Gain measurement tests with cw-pumping at 1.02 lm and a pump power down to 0.75 mW show no signal amplification. This is obviously caused by a strong additionally loss induced by the pump light. For more detailed study of this attenuation behaviour the usual amplifier setup was changed to a pulsed pumping regime up to 150 mW. By using the model for description of the processes of simultaneous amplification and degradation, described by Jetschke et al. [10] we determined the signal gain during simultaneous light induced attenuation. This calculation of the gain coefficient considers the effects of signal degradation. We yield the true signal light intensity, a supposition for approximation of the gain, by subtraction of fluorescence and signal light intensity, influenced by pump light, respectively. For pump pulse power <1 mW a maximum gain coefficient of 0.5 dB mW1 was achieved, which decreases with increasing the pump power. By increasing the pump pulse power over 20–75 mW the gain coefficient decreases to about 0.02 dB mW1 . Obviously the decreased slope of the gain coefficient at higher pump power up to 75 mW is not caused by gain saturation effects, but by power induced attenuation of the core glass. The increase of the germanium concentration of the core glass over 10 mol% allows one to decrease the degradation drastically to a transmission loss rate of about 0.6 dB m1 s1 at signal wavelength. However, we suspect that this degradation rate is still too high in a cw-pump regime.
449
Additionally, unfortunately the background loss of the high germanium containing fibre is significantly higher, about 20 dB m1 at signal wavelength. Similar material degradation effects we observed by the investigation of side light irradiated uncoated fibres. Here we measured an additionally loss in the sub-dB m1 -range which increased up to shorter wavelengths. This increase can be explained by photodarkening effects [11,12]. The dependence on the wavelength of the transmitted light indicates an obvious shift of the absorption edge to higher wavelength. Fritzsche [13] has described such photoinduced changes in chalcogenide glass. They have been attributed to modifications of physical and chemical states through excitation of electron–hole pairs, and can be reversible or irreversible. The analysis of the power stability limits shows that both reversible and irreversible material degeneration processes increase the transmission loss of the arsenic sulfide based fibre material.
5. Summary A 1.3 lm amplification tests show the principle high gain efficiency of arsenic sulfide based glasses due to their low phonon energy. We demonstrate a maximum gain coefficient of 0.5 dB mW1 with a pulsed pumping regime using a Pr3þ doped arsenic sulfide based single mode fibre. Unfortunately the advantage of the effective conversion of pump into signal light is failed by an increased degradation of the fibre by pump light depending on its intensity. The fibres with a higher germanium concentration up to 11 mol% show an improved, but obviously not sufficient power stability. Praseodymium doped arsenic sulfide based fibres with germanium concentration of more then a few mol% show higher background losses due to scattering effects caused by a likely lower rare earth solubility. In summary the advantages of the prepared glass material with regard to crystallization stability and low underground loss are restricted by the material degradation during pumping of the fibre for amplifier applications.
450
J. Kobelke et al. / Journal of Non-Crystalline Solids 326&327 (2003) 446–450
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG).
References [1] D.R. Simons, A.J. Faber, H. de Waal, Opt. Lett. 20 (5) (1995) 468. [2] B. Frumarova, M. Frumar, J. Oswald, J. Non-Cryst. Solids 213&214 (1997) 58. [3] P.C. Becker, M.M. Broer, V.G. Lambrecht, A.J. Bruce, G. Nykolak, Proceedings of the OSA Topics Meeting on Optical Amplifiers and Application, Santa Fe, NM, vol. 5, Post deadline paper PD, 1992, p. 20. [4] Y.D. West, T. Schweizer, D.J. Brady, D.W. Hewak, Fiber Integr. Opt. 19 (2000) 229.
[5] K. Itoh, H. Yanagita, H. Tawarayama, K. Yamanaka, E. Ishikawa, K. Okada, H. Aoki, Y. Matsumoto, A. Shirakawa, Y. Matsuoka, H. Toratani, J. Non-Cryst. Solids 256&257 (1999) 1. [6] Y. Ohishi, A. Mori, T. Kanamori, K. Fujiura, S. Sudo, Appl. Phys. Lett. 65 (1) (1994) 13. [7] N.S. Kapany, R.J. Simms, Infrared Phys. 5 (1965) 69. [8] M. Scheffler, J. Kirchhof, J. Kobelke, A. Schwuchow, in: Proceedings of the XIth International Symposium on Non-Oxide Glasses, Sheffield, UK, 1998, p. 258. [9] J. Kobelke, J. Kirchhof, M. Scheffler, A. Schwuchow, SPIE 3416 (1998) 55. [10] S. Jetschke, A. Schwuchow, J. Kobelke, K. Schuster, J. Kirchhof, SPIE 4645 (2002) 26. [11] A.M. Andriesh, N.A. Enachi, I.P. Culeac, T.N. Copaci, V.A. Binchevici, J. Non-Cryst. Solids 189 (1995) 147. [12] A. Ganjoo, K. Shimakawa, K. Kitano, E.A. Davis, J. NonCryst. Solids 299–302 (2002) 917. [13] H. Fritzsche, Semiconductors 32 (1998) 850.