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Pumping scheme optimisation of 980-nm pumped L-band EDFA associated with broadband noise as the secondary pump
Optics Communications 236 (2004) 167–172 www.elsevier.com/locate/optcom
Pumping scheme optimisation of 980-nm pumped L-band EDFA associated with broadband noise as the secondary pump F.R.M. Adikan a, A.S.M. Noor b, M.A. Mahdi
b,*
a
b
Department of Electrical Engineering and Telecommunication, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Photonic and Fiber Optics Systems Laboratory, Department of Computer and Communication Systems Engineering, Faculty of Engineering, University of Putra Malaysia, 43400 Serdang Selangor, Malaysia Received 7 May 2003; received in revised form 13 February 2004; accepted 9 March 2004
Abstract L-band gain improvement through usage of secondary pumping sources in the form of broadband noise (amplified spontaneous emission, ASE) is conducted. For an L-band amplifier system employing ASE to improve gain, pumping the system counter-directionally with ASE while the 980-nm pump is being used in a co-propagating configuration would yield the best overall performance in terms of gain and noise figure. For high power applications, the 980-nm and ASE sources must be counter-directionally pumped to the direction of the L-band signal. Gain improvement of 1570nm signal in between 6 and 8.5 dB is attained at 12 mW of 980-nm pump laser. Ó 2004 Elsevier B.V. All rights reserved. PACS: 42.60.D; 42.79.S Keywords: Laser amplifiers; Optical communication systems; Erbium; Optical pumping
1. Introduction Optical networks, particularly dense wavelength division multiplexing (DWDM) systems, have been – among other things – the enabling *
Corresponding author. Tel.: +60389466438; fax: +60386567127. E-mail addresses: rafi[email protected] (F.R.M. Adikan), [email protected] (A.S.M. Noor), [email protected]. edu.my, [email protected] (M.A. Mahdi).
technology behind multi-gigabit transmission systems. DWDM employs multi-wavelength optical signals as carriers. These signals – travelling over long distances – are subject to detrimental effects such as fibre attenuation and dispersion and these in turn, limit the system’s data carrying capacity. Optical amplifiers are needed in order to counter the effects of fibre attenuation, thus increasing the transmission distance as well as bit rate [1,2]. Erbium-doped fibre amplifiers (EDFAs) – with their ability to provide gain optically (transparent
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.03.030
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F.R.M. Adikan et al. / Optics Communications 236 (2004) 167–172
system) and polarisation insensitivity – are without doubt one of the most important building block for the DWDM system. Demands from the communication industry saw the introduction of L-band (long band, 1565– 1620 nm) EDFAs, enhancing the C-band (1525– 1565 nm) transmission window EDFAs. EDFAs that could provide both C- and L-band gain – called wideband EDFAs – were introduced shortly afterwards. The benchmarks used in evaluating EDFAs are the gain and noise figure. Increasing the gain and lowering the noise figure of the EDFA have been the subject of continuous and various research efforts. This is particularly the case for L-band where the gain is usually low. One advantage of L-band amplifiers over their C-band counterpart is the inherent gain flatness criteria in which a flat gain profile could be achieved using Lband amplifiers by ensuring that the population inversion in the EDF is around 40% [3] and therefore, eliminating the need for gain flattening filters. This approach however, suffers again from a relatively low maximum gain value as a direct result of low population inversion. Other techniques used in obtaining wideband amplification are through parallel or simultaneous gain blocks [4,5], the use of Mach–Zehnder equalisers [6], amplified spontaneous emission (ASE) utilisation [7,8], C-band injection [9] as well as reflected pump via fibre Bragg reflectors [10]. This paper presents a study on the effect of pumping directly a functioning L-band amplifier system with ASE. ASE is an unwanted bi-product of optical amplification. ASE is usually filtered out of the system. However, it is discovered that ASE
ASE source 1
could be used as a secondary pump source to improve L-band gain performance albeit it also introduces some noise penalty. The novelty of the technique hinges on the fact that ASE is usually an unwanted bi-product of amplification, therefore, ASE generated by a C-band portion of an EDFA system could be fed into that of the L-band, creating a wideband EDFA incorporating both C- and L-band wavelengths.
2. Experimental set-up Fig. 1 shows a single stage EDFA configuration used for the experiment. The length of erbiumdoped fibre (EDF) used is 38 m. The EDF is characterised by 440 ppm of Er3þ ion concentration, numerical aperture of 0.27, a cutoff wavelength of 920 nm and the peak absorption of 7 dB/ m at 1531 nm. The 980-nm laser diode (LD) pumps are used as the primary pump lights and the secondary pump lights; the ASE is obtained from amplifier modules. The 980-nm pump lasers can produce output power of 100 mW each. Pump lights are coupled to the system using wavelength selective couplers (primary pumps) and 3-dB couplers (secondary pumps). L-band signal used for the experiments, three wavelength settings – 1570, 1575 and 1580 nm. The power level used for the L-band signals is )30 dB m. The ASE power into the EDF is controlled using the variable optical attenuator (VOA). Table 1 shows four different pumping configurations of the EDFA configuration as depicted in Fig. 1. The four possible pumping combinations of
980nm LD2
980nm LD1 980/1550nm coupler
VOA1
ASE source 2 VOA2
INPUT
OUTPUT Isolator
3dB coupler
3dB coupler
Isolator
EDF=38m
Fig. 1. Single stage EDFA configuration that consists of 980-nm pump lasers and ASE sources.
F.R.M. Adikan et al. / Optics Communications 236 (2004) 167–172 Table 1 The 980-nm pump laser and ASE source activation status for four different amplifier configurations Amplifier
ASE source 1
980-nm LD1
980-nm LD2
ASE source 2
Type-1 Type-2 Type-3 Type-4
On X On X
On On X X
X X On On
X On X On
169
are referred to the ends of the EDF. The input and output loss are well characterised beforehand.
3. Experimental results and discussions For the experimental part of the study, the flat gain region of the L-band amplifier is first determined without seeding the ASE pump light. A 3dB flat gain is achieved with 980-nm pump setting of 12 mW for both co- and counter-propagating schemes as depicted in Fig. 2. The 3-dB flat gain is defined as the setting at which gain difference between all three wavelengths used is below 3 dB. The circle indicates the flat gain region where all three wavelengths used; 1570, 1575 and 1580 nm experience equal gain values. The 980-nm pump power setting is set necessarily low in order to create an under-pumped region within the 38-m EDF so that L-band amplification is possible. This is further expounded by the fact that to attain in-
980-nm pump and ASE are – Type 1 (980 nm copropagating, ASE co-propagating), Type 2 (980 nm co-propagating, ASE counter-propagating), Type 3 (980 nm counter-propagating, ASE copropagating) and Type 4 (980 nm counter-propagating, ASE counter-propagating). Since the configuration introduces a significant of loss at the input and output ports, thus it is impractical to discuss the effect of the ASE pump light onto the L-band signals. Experiments are carried out for all four configurations where all the measurements 40
counter-propagating
co-propagating EDF gain, dB
30 20 10
1570nm signal
0
1575nm signal
-10
1580nm signal
-20 0
20
EDF noise figure, dB
60
80
100
0
20
40
60
80
100
pump power, mW
6
30
5
25
4
20
counter-propagating 1570nm signal 1575nm signal
3
15
co-propagating
1580nm signal
2
10
1
5
0
(b)
40
pump power, mW
(a)
0
20
40
60
pump power, mW
80
100
0
0
20
40
60
80
100
pump power, mW
Fig. 2. L-band EDFA gain (a) and noise figure (b) performance without the secondary ASE pumps.
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herent flat gain, 40% population inversion is necessary. It could also be noted that pumping the EDFA in the counter-propagating manner would produce poor noise performance. Once the basic L-band EDFA configuration is characterised, measurements of gain and noise figure are taken for different ASE powers. The gain and noise figure performance of the ASE-pumped L-band EDFA is shown in Figs. 3 and 4. The EDFA is set to its flat-gain pump setting and then ASE power into the system is added. Thus, the 980-nm pump power is set at 12 mW for all the amplifier configurations. Then the ASE power is added into the system. Initially, gain of the co-pumped L-band EDFA (Type-3 and Type-4 amplifiers) is slightly higher than the counter-pumped L-band EDFA (Type-1
Gain (dB)
14 13
Type-1
12
Type-2
11
Type-3
10
Type-4
9 8 7
980nm-pumped only
6 5 4 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5
0
5
ASE power (dBm)
Fig. 3. Gain against ASE power for all possible configuration of ASE-pumped L-band amplifier.
20 18 Noise figure (dB)
16 14 12 10 8 6 4
Type-1 Type-2 Type-3 Type-4
980nm-pumped only
2 0 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 ASE power (dBm)
0
5
Fig. 4. Gain against ASE power for all possible configuration of ASE-pumped L-band amplifier.
and Type-2 amplifiers) without the additional ASE powers in the system. When the ASE power is added into the amplifier systems, the L-band signal starts to experience additional gain due to the presence of the secondary pump energy. Referring to Fig. 3, the behaviour of signal amplification can be divided into two groups; the first group consists of Type-1 and Type-4 amplifiers and the other group consists of Type-2 and Type-3 amplifiers. For the former group, the L-band signal gain is gradually improved when the ASE pump source is set at )28 dB m and beyond. The signal is finally saturated at 11.4 dB gain from )8 to 2 dB m ASE pump powers for Type-1 amplifier. On the other hand, there are no indications of saturation of the signal for Type-4 amplifier. For the latter group, the signal experiences additional gain when the ASE pump source is set at )13 dB m and beyond. Overall, gain improvements between 6.0 and 8.5 dB are obtained for all the amplifier configurations. For any amplifier configurations, the ASE photon energy in short wavelengths is transferred to longer wavelengths photon energy through radiative transition between Starks multiplets of energy level 4 I13=2 . The L-band signal experiences additional gain at lower ASE pump powers for Type-1 and Type-4 amplifiers due to the fact that the C-band ASE is amplified as it enters the EDF in the same direction of the 980-nm primary pump light. In general, the amplification of the C-band ASE is more effective than the L-band signals. Most of the primary pump energy is absorbed by the C-band ASE because of its higher gain coefficient. Therefore the intensity of the ASE becomes much stronger in the EDF region where the primary pump light dominates. The propagation of the 980-nm pump light in the EDF is limited to its effective absorption by the Er3þ ions from 4 I15=2 to 4 I11=2 energy levels. Thus it cannot travel very far in the EDF. For effective amplification, the gain medium must be populated by the excited Er3þ ions along the fibre. This property is taken over by the stronger C-band ASE that can propagate longer into the under-pumped region at the other end of the EDF. The energy transfer from short to longer wavelengths occurs while the C-band ASE propagating in the EDF. As a result, the popula-
F.R.M. Adikan et al. / Optics Communications 236 (2004) 167–172
tion inversion increases which can be evaluated by the increment of the L-band signal gain. The gain saturation is observed for Type-1 amplifier only because the C-band ASE saturates the amplifier system at the input of the EDF. Since the increment of the amplified ASE is minimal in the highly saturated regime, the gain improvement of the Lband signal is also minimal as well. When the secondary ASE pump light is injected in the opposite direction of the 980-nm pump light propagation; Type-2 and Type-3 amplifiers, it must have adequate photons in short wavelengths to transfer their energy to 1570-nm photon energy. Since the L-band gain coefficients are much lower compared to C-band signals. Therefore, the gain increment is very minimal for ASE pump powers lesser than )13 dB m which can be seen from Fig. 3. The gain increment slope of Type-3 amplifier is higher than that of Type-2 amplifier. This is due to the level of intensity of the L-band signal in the fibre region where the ASE pump light dominates. Clearly, the L-band signal is very small compared to the ASE pump light at the input of the EDF for Type-3 amplifier. On the other hand, the L-band signal is already amplified by the 980-nm pump light at the input end of the EDF for Type-2 amplifier. Therefore, the intensity of the ASE pump light must be very large at the output end of the EDF to amplify the L-band signal. As a result, Type-3 amplifier performed better than Type-2 amplifier in terms of gain improvement. The behaviour of noise figure of the L-band signal with respect to the ASE power is depicted in Fig. 4. The counter-pumped amplifiers (Type-3 and Type-4) exhibit high noise figure because the L-band signal experiences absorption at the input end of the EDF due to lack of excited ions at the upper energy level. The presence of the C-band ASE for these amplifier configurations improve the noise figure of the L-band signal. The noise figure improved by 10.5 dB for Type-4 amplifier however, this value reduced to 2.5 dB for Type-3 amplifier. It indicates that the L-band signal amplification is efficient in the backward-pumped ASE scheme. As the L-band signal amplified gradually as it propagates in the EDF, the intensity of the ASE is also gradually increased with
171
respect to the propagation direction of the L-band signal. Therefore, if higher ASE pump powers are used in the experiment; the noise figure can be improved further. For co-pumped amplifier configurations (Type-1 and Type-2), the ASE pump light degrades the noise figure of the L-band signal. The noise figure for Type-1 amplifier is severely degraded due to additional noise by the broadband ASE light. Since the output of the ASE source is not filtered-out in the L-band range, a significant amount of noise can be added into the L-band signal. As the ASE power increases, the noise figure increases proportionally. On the other hand, a minimal penalty of noise figure is obtained for Type-2 amplifier. Since the additional ASE pump light propagates in the opposite direction of the L-band signal, the noise is not directly added into the signal bandwidth. Therefore, the noise figure is nearly flat from )38 to )8 dB m ASE powers. However, the noise figure starts to degrade at high ASE powers. The remnant of ASE pump light can travel further in the fibre as its power increases. If its photon energy is stronger than the L-band signal photon energy in this fibre range, it can suppress gain of the L-band signal. Based on these findings, Type-2 and Type-4 amplifiers exhibit good performance in terms of gain and noise figure for large ASE pump light. Type-2 amplifier configuration is more suitable for pre-amplification stage where it produces low noise figure because of its forward-pumped scheme. For high power applications, Type-4 amplifier produces effective gain improvement and its backward-pumped scheme is preferable due to better power conversion efficiency. The C-band ASE generation in the L-band amplifiers cannot be omitted. Therefore, it is feasible to manipulate this unwanted ASE to be used as the improving agent for the L-band signals.
4. Conclusions The impact of propagation direction of the broadband noise (ASE) in the 980-nm pumped L-band EDFA is demonstrated. In general, the
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980-nm pump laser acts as a primary pump light that determines the effectiveness of the seeded ASE in the amplifier systems. Gain improvement is recorded for all possible amplifier configurations. The 980-nm pump laser is primarily determined the noise figure of the L-band signal; low noise figures are obtained in the co-pumped scheme and high noise figures are recorded in the counter-pumped scheme. The seeded ASE improves the noise figure of the 980-nm-counterpumped scheme and it degrades the noise figure of the 980-nm-co-pumped scheme. Based on the findings, the penalty of noise figure is very minimal for the amplifier configuration pumped by the ASE source in the opposite direction. This configuration is suitable for pre-amplification applications. On the other hand, seeding ASE source in the same direction of the 980-nm light in a counter-pumped scheme yields effective noise figure improvement. This configuration is more suitable for high power applications in which the noise figure is determined by the input-stage amplifier.
References [1] S. Sudo, Optical Fiber Amplifiers: Materials, Devices, and Applications, Artech House Inc., London, 1997. [2] M. Yamada, H. Ono, Y. Ohishi, Electronic Letters 34 (1998) 1747. [3] M. Yamada, H. Ono, T. Kanamori, S. Sudo, Y. Ohishi, Electronic Letters 33 (1997) 710. [4] M.A. Mahdi, FR Mahamd Adikan, P. Poopalan, S. Selvakennedy, H. Ahmad, Simultaneous Bi directional of C- and L-band Erbium Doped Fiber Amplifier, in: Optical Fiber Communication Conference (OFC 2000), Baltimore, MD, USA, paper TuA3, 2000. [5] H. Ono, M. Yamada, T. Kanimori, S. Sudo, Y. Ohishi, Journal of Lightwave Technology 17 (3) (1999) 490. [6] F.R. Mahamd Adikan, M.A. Mahdi, S. Thirumeni, P. Poopalan, K. Dimyati, H. Ahmad, Microwave Optical Technology Letters 28 (6) (2001) 399. [7] J. Lee, U. Ryu, S.J. Ahn, N. Park, IEEE Photonics Technology Letters 11 (1) (1999) 42. [8] M. Yamada, H. Ono, Y. Ohishi, Electronic Letters 34 (18) (1998) 1747. [9] M.A. Mahdi, F.R. Mahamd Adikan, P. Poopalan, S. Selvakennedy, W.Y. Chan, H. Ahmad, Optics Communication 176 (2000) 125. [10] J. Nilsson, S.Y. Sun, S.T. Hwang, J.M. Kim, S.J. Kim, IEEE Photonics Technology Letters 10 (11) (1998) 1551.
Pumping scheme optimisation of 980-nm pumped L-band EDFA associated with broadband noise as the secondary pump F.R.M. Adikan a, A.S.M. Noor b, M.A. Mahdi
b,*
a
b
Department of Electrical Engineering and Telecommunication, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia Photonic and Fiber Optics Systems Laboratory, Department of Computer and Communication Systems Engineering, Faculty of Engineering, University of Putra Malaysia, 43400 Serdang Selangor, Malaysia Received 7 May 2003; received in revised form 13 February 2004; accepted 9 March 2004
Abstract L-band gain improvement through usage of secondary pumping sources in the form of broadband noise (amplified spontaneous emission, ASE) is conducted. For an L-band amplifier system employing ASE to improve gain, pumping the system counter-directionally with ASE while the 980-nm pump is being used in a co-propagating configuration would yield the best overall performance in terms of gain and noise figure. For high power applications, the 980-nm and ASE sources must be counter-directionally pumped to the direction of the L-band signal. Gain improvement of 1570nm signal in between 6 and 8.5 dB is attained at 12 mW of 980-nm pump laser. Ó 2004 Elsevier B.V. All rights reserved. PACS: 42.60.D; 42.79.S Keywords: Laser amplifiers; Optical communication systems; Erbium; Optical pumping
1. Introduction Optical networks, particularly dense wavelength division multiplexing (DWDM) systems, have been – among other things – the enabling *
Corresponding author. Tel.: +60389466438; fax: +60386567127. E-mail addresses: rafi[email protected] (F.R.M. Adikan), [email protected] (A.S.M. Noor), [email protected]. edu.my, [email protected] (M.A. Mahdi).
technology behind multi-gigabit transmission systems. DWDM employs multi-wavelength optical signals as carriers. These signals – travelling over long distances – are subject to detrimental effects such as fibre attenuation and dispersion and these in turn, limit the system’s data carrying capacity. Optical amplifiers are needed in order to counter the effects of fibre attenuation, thus increasing the transmission distance as well as bit rate [1,2]. Erbium-doped fibre amplifiers (EDFAs) – with their ability to provide gain optically (transparent
0030-4018/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.03.030
168
F.R.M. Adikan et al. / Optics Communications 236 (2004) 167–172
system) and polarisation insensitivity – are without doubt one of the most important building block for the DWDM system. Demands from the communication industry saw the introduction of L-band (long band, 1565– 1620 nm) EDFAs, enhancing the C-band (1525– 1565 nm) transmission window EDFAs. EDFAs that could provide both C- and L-band gain – called wideband EDFAs – were introduced shortly afterwards. The benchmarks used in evaluating EDFAs are the gain and noise figure. Increasing the gain and lowering the noise figure of the EDFA have been the subject of continuous and various research efforts. This is particularly the case for L-band where the gain is usually low. One advantage of L-band amplifiers over their C-band counterpart is the inherent gain flatness criteria in which a flat gain profile could be achieved using Lband amplifiers by ensuring that the population inversion in the EDF is around 40% [3] and therefore, eliminating the need for gain flattening filters. This approach however, suffers again from a relatively low maximum gain value as a direct result of low population inversion. Other techniques used in obtaining wideband amplification are through parallel or simultaneous gain blocks [4,5], the use of Mach–Zehnder equalisers [6], amplified spontaneous emission (ASE) utilisation [7,8], C-band injection [9] as well as reflected pump via fibre Bragg reflectors [10]. This paper presents a study on the effect of pumping directly a functioning L-band amplifier system with ASE. ASE is an unwanted bi-product of optical amplification. ASE is usually filtered out of the system. However, it is discovered that ASE
ASE source 1
could be used as a secondary pump source to improve L-band gain performance albeit it also introduces some noise penalty. The novelty of the technique hinges on the fact that ASE is usually an unwanted bi-product of amplification, therefore, ASE generated by a C-band portion of an EDFA system could be fed into that of the L-band, creating a wideband EDFA incorporating both C- and L-band wavelengths.
2. Experimental set-up Fig. 1 shows a single stage EDFA configuration used for the experiment. The length of erbiumdoped fibre (EDF) used is 38 m. The EDF is characterised by 440 ppm of Er3þ ion concentration, numerical aperture of 0.27, a cutoff wavelength of 920 nm and the peak absorption of 7 dB/ m at 1531 nm. The 980-nm laser diode (LD) pumps are used as the primary pump lights and the secondary pump lights; the ASE is obtained from amplifier modules. The 980-nm pump lasers can produce output power of 100 mW each. Pump lights are coupled to the system using wavelength selective couplers (primary pumps) and 3-dB couplers (secondary pumps). L-band signal used for the experiments, three wavelength settings – 1570, 1575 and 1580 nm. The power level used for the L-band signals is )30 dB m. The ASE power into the EDF is controlled using the variable optical attenuator (VOA). Table 1 shows four different pumping configurations of the EDFA configuration as depicted in Fig. 1. The four possible pumping combinations of
980nm LD2
980nm LD1 980/1550nm coupler
VOA1
ASE source 2 VOA2
INPUT
OUTPUT Isolator
3dB coupler
3dB coupler
Isolator
EDF=38m
Fig. 1. Single stage EDFA configuration that consists of 980-nm pump lasers and ASE sources.
F.R.M. Adikan et al. / Optics Communications 236 (2004) 167–172 Table 1 The 980-nm pump laser and ASE source activation status for four different amplifier configurations Amplifier
ASE source 1
980-nm LD1
980-nm LD2
ASE source 2
Type-1 Type-2 Type-3 Type-4
On X On X
On On X X
X X On On
X On X On
169
are referred to the ends of the EDF. The input and output loss are well characterised beforehand.
3. Experimental results and discussions For the experimental part of the study, the flat gain region of the L-band amplifier is first determined without seeding the ASE pump light. A 3dB flat gain is achieved with 980-nm pump setting of 12 mW for both co- and counter-propagating schemes as depicted in Fig. 2. The 3-dB flat gain is defined as the setting at which gain difference between all three wavelengths used is below 3 dB. The circle indicates the flat gain region where all three wavelengths used; 1570, 1575 and 1580 nm experience equal gain values. The 980-nm pump power setting is set necessarily low in order to create an under-pumped region within the 38-m EDF so that L-band amplification is possible. This is further expounded by the fact that to attain in-
980-nm pump and ASE are – Type 1 (980 nm copropagating, ASE co-propagating), Type 2 (980 nm co-propagating, ASE counter-propagating), Type 3 (980 nm counter-propagating, ASE copropagating) and Type 4 (980 nm counter-propagating, ASE counter-propagating). Since the configuration introduces a significant of loss at the input and output ports, thus it is impractical to discuss the effect of the ASE pump light onto the L-band signals. Experiments are carried out for all four configurations where all the measurements 40
counter-propagating
co-propagating EDF gain, dB
30 20 10
1570nm signal
0
1575nm signal
-10
1580nm signal
-20 0
20
EDF noise figure, dB
60
80
100
0
20
40
60
80
100
pump power, mW
6
30
5
25
4
20
counter-propagating 1570nm signal 1575nm signal
3
15
co-propagating
1580nm signal
2
10
1
5
0
(b)
40
pump power, mW
(a)
0
20
40
60
pump power, mW
80
100
0
0
20
40
60
80
100
pump power, mW
Fig. 2. L-band EDFA gain (a) and noise figure (b) performance without the secondary ASE pumps.
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herent flat gain, 40% population inversion is necessary. It could also be noted that pumping the EDFA in the counter-propagating manner would produce poor noise performance. Once the basic L-band EDFA configuration is characterised, measurements of gain and noise figure are taken for different ASE powers. The gain and noise figure performance of the ASE-pumped L-band EDFA is shown in Figs. 3 and 4. The EDFA is set to its flat-gain pump setting and then ASE power into the system is added. Thus, the 980-nm pump power is set at 12 mW for all the amplifier configurations. Then the ASE power is added into the system. Initially, gain of the co-pumped L-band EDFA (Type-3 and Type-4 amplifiers) is slightly higher than the counter-pumped L-band EDFA (Type-1
Gain (dB)
14 13
Type-1
12
Type-2
11
Type-3
10
Type-4
9 8 7
980nm-pumped only
6 5 4 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5
0
5
ASE power (dBm)
Fig. 3. Gain against ASE power for all possible configuration of ASE-pumped L-band amplifier.
20 18 Noise figure (dB)
16 14 12 10 8 6 4
Type-1 Type-2 Type-3 Type-4
980nm-pumped only
2 0 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 ASE power (dBm)
0
5
Fig. 4. Gain against ASE power for all possible configuration of ASE-pumped L-band amplifier.
and Type-2 amplifiers) without the additional ASE powers in the system. When the ASE power is added into the amplifier systems, the L-band signal starts to experience additional gain due to the presence of the secondary pump energy. Referring to Fig. 3, the behaviour of signal amplification can be divided into two groups; the first group consists of Type-1 and Type-4 amplifiers and the other group consists of Type-2 and Type-3 amplifiers. For the former group, the L-band signal gain is gradually improved when the ASE pump source is set at )28 dB m and beyond. The signal is finally saturated at 11.4 dB gain from )8 to 2 dB m ASE pump powers for Type-1 amplifier. On the other hand, there are no indications of saturation of the signal for Type-4 amplifier. For the latter group, the signal experiences additional gain when the ASE pump source is set at )13 dB m and beyond. Overall, gain improvements between 6.0 and 8.5 dB are obtained for all the amplifier configurations. For any amplifier configurations, the ASE photon energy in short wavelengths is transferred to longer wavelengths photon energy through radiative transition between Starks multiplets of energy level 4 I13=2 . The L-band signal experiences additional gain at lower ASE pump powers for Type-1 and Type-4 amplifiers due to the fact that the C-band ASE is amplified as it enters the EDF in the same direction of the 980-nm primary pump light. In general, the amplification of the C-band ASE is more effective than the L-band signals. Most of the primary pump energy is absorbed by the C-band ASE because of its higher gain coefficient. Therefore the intensity of the ASE becomes much stronger in the EDF region where the primary pump light dominates. The propagation of the 980-nm pump light in the EDF is limited to its effective absorption by the Er3þ ions from 4 I15=2 to 4 I11=2 energy levels. Thus it cannot travel very far in the EDF. For effective amplification, the gain medium must be populated by the excited Er3þ ions along the fibre. This property is taken over by the stronger C-band ASE that can propagate longer into the under-pumped region at the other end of the EDF. The energy transfer from short to longer wavelengths occurs while the C-band ASE propagating in the EDF. As a result, the popula-
F.R.M. Adikan et al. / Optics Communications 236 (2004) 167–172
tion inversion increases which can be evaluated by the increment of the L-band signal gain. The gain saturation is observed for Type-1 amplifier only because the C-band ASE saturates the amplifier system at the input of the EDF. Since the increment of the amplified ASE is minimal in the highly saturated regime, the gain improvement of the Lband signal is also minimal as well. When the secondary ASE pump light is injected in the opposite direction of the 980-nm pump light propagation; Type-2 and Type-3 amplifiers, it must have adequate photons in short wavelengths to transfer their energy to 1570-nm photon energy. Since the L-band gain coefficients are much lower compared to C-band signals. Therefore, the gain increment is very minimal for ASE pump powers lesser than )13 dB m which can be seen from Fig. 3. The gain increment slope of Type-3 amplifier is higher than that of Type-2 amplifier. This is due to the level of intensity of the L-band signal in the fibre region where the ASE pump light dominates. Clearly, the L-band signal is very small compared to the ASE pump light at the input of the EDF for Type-3 amplifier. On the other hand, the L-band signal is already amplified by the 980-nm pump light at the input end of the EDF for Type-2 amplifier. Therefore, the intensity of the ASE pump light must be very large at the output end of the EDF to amplify the L-band signal. As a result, Type-3 amplifier performed better than Type-2 amplifier in terms of gain improvement. The behaviour of noise figure of the L-band signal with respect to the ASE power is depicted in Fig. 4. The counter-pumped amplifiers (Type-3 and Type-4) exhibit high noise figure because the L-band signal experiences absorption at the input end of the EDF due to lack of excited ions at the upper energy level. The presence of the C-band ASE for these amplifier configurations improve the noise figure of the L-band signal. The noise figure improved by 10.5 dB for Type-4 amplifier however, this value reduced to 2.5 dB for Type-3 amplifier. It indicates that the L-band signal amplification is efficient in the backward-pumped ASE scheme. As the L-band signal amplified gradually as it propagates in the EDF, the intensity of the ASE is also gradually increased with
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respect to the propagation direction of the L-band signal. Therefore, if higher ASE pump powers are used in the experiment; the noise figure can be improved further. For co-pumped amplifier configurations (Type-1 and Type-2), the ASE pump light degrades the noise figure of the L-band signal. The noise figure for Type-1 amplifier is severely degraded due to additional noise by the broadband ASE light. Since the output of the ASE source is not filtered-out in the L-band range, a significant amount of noise can be added into the L-band signal. As the ASE power increases, the noise figure increases proportionally. On the other hand, a minimal penalty of noise figure is obtained for Type-2 amplifier. Since the additional ASE pump light propagates in the opposite direction of the L-band signal, the noise is not directly added into the signal bandwidth. Therefore, the noise figure is nearly flat from )38 to )8 dB m ASE powers. However, the noise figure starts to degrade at high ASE powers. The remnant of ASE pump light can travel further in the fibre as its power increases. If its photon energy is stronger than the L-band signal photon energy in this fibre range, it can suppress gain of the L-band signal. Based on these findings, Type-2 and Type-4 amplifiers exhibit good performance in terms of gain and noise figure for large ASE pump light. Type-2 amplifier configuration is more suitable for pre-amplification stage where it produces low noise figure because of its forward-pumped scheme. For high power applications, Type-4 amplifier produces effective gain improvement and its backward-pumped scheme is preferable due to better power conversion efficiency. The C-band ASE generation in the L-band amplifiers cannot be omitted. Therefore, it is feasible to manipulate this unwanted ASE to be used as the improving agent for the L-band signals.
4. Conclusions The impact of propagation direction of the broadband noise (ASE) in the 980-nm pumped L-band EDFA is demonstrated. In general, the
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980-nm pump laser acts as a primary pump light that determines the effectiveness of the seeded ASE in the amplifier systems. Gain improvement is recorded for all possible amplifier configurations. The 980-nm pump laser is primarily determined the noise figure of the L-band signal; low noise figures are obtained in the co-pumped scheme and high noise figures are recorded in the counter-pumped scheme. The seeded ASE improves the noise figure of the 980-nm-counterpumped scheme and it degrades the noise figure of the 980-nm-co-pumped scheme. Based on the findings, the penalty of noise figure is very minimal for the amplifier configuration pumped by the ASE source in the opposite direction. This configuration is suitable for pre-amplification applications. On the other hand, seeding ASE source in the same direction of the 980-nm light in a counter-pumped scheme yields effective noise figure improvement. This configuration is more suitable for high power applications in which the noise figure is determined by the input-stage amplifier.
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