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Simulative investigation the impact of optical amplification techniques on single- and dual-electrode MZM and direct modulator's in single-tone RoF system
Optik 122 (2011) 371–374
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Simulative investigation the impact of optical amplification techniques on singleand dual-electrode MZM and direct modulator’s in single-tone RoF system Vishal Sharma a , Amarpal Singh b,∗ , Ajay K. Sharma c a b c
Shaheed Bhagat Singh College of Engineering & Technology, Ferozepur, Punjab, India Beant College of Engineering and Technology, Gurdaspur, Punjab, India Dr B R Ambedkar National Institute of Technology, Jalandhar, Punjab, India
a r t i c l e
i n f o
Article history: Received 1 August 2009 Accepted 4 February 2010
Keywords: Radio over fiber (RoF) system External- and direct-modulation Dual-and single-electrode MZM SOA EDFA
a b s t r a c t Since the introduction of erbium-doped fiber amplifier (EDFA) in optical fiber communication systems at 1550 nm, the fiber loss is no longer considered as the limiting factor but the fiber dispersion and chirp parameters play an important role in degrading the performance of optical communication systems. Further, the invention of external modulators of Mach-Zehnder (MS) type minimizes the transmission power penalty due to fiber chromatic dispersion as these modulators are consider almost chirp-free modulators. Also, better suppression of nonlinear distortions consists of harmonic distortions (HDs) and intermodulation distortions (IMDs) were achieved by using Mach-Zehnder external modulator [8]. In this work, a simulation comparison of external- and direct-modulation schemes using different optical amplifiers; SOA and EDFA in single-tone RoF system is reported. A single-tone channel of 20 GHz is transmitted over single-mode fiber (SMF) either externally modulated by using single-electrode (SEMZM) – and dualelectrode (DEMZM) – Mach-Zehnder modulator or by direct-modulation scheme. Better improvements in the measurement of received RF power of a 20 km linked single-tone RoF system are achieved by using SEMZM with EDFA, which means maximum optical link is achieved between Central Office (CO) and Base Station (BS). Further, this scheme provides better BER as compare to other schemes discussed in this paper. © 2010 Elsevier GmbH. All rights reserved.
1. Introduction The growth of mobile and wireless communications fuels increasing demand for multimedia services with a guaranteed quality of service. This requires realization of broadband distribution and access networks. The RoF technology has various advantages when it applies to wireless systems, especially, mobile communication radio networks. Very small base stations (access units) are equipped on the ceiling of rooms in a building and are connected to a master base station (base unit) by optical fiber cables. On large scale level, research on RoF networks has been performed for last few years because of a numerous advantages of RoF technology. Within this framework, RoF schemes can be applied for realizing seamless wireless networks since they allow for the easy distribution of microwaves and millimeter waves over long distances along optical fibers [1,2]. Several techniques for distributing and generating microwave signals via optical fiber exist. The techniques may be classified into two main categories namely Intensity Modulation-
∗ Corresponding author. E-mail address: s [email protected] (A. Singh). 0030-4026/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2010.02.021
Direct Detection (IM-DD) and Remote Heterodyne Detection (RHD) techniques [3]. In such systems, it is desirable to achieve better receiver sensitivities, higher dynamic ranges, and lower nonlinear distortions. Techniques to reduce nonlinear distortions have been investigated extensively. A method for reducing nonlinear HDs and IMDs is to use the pre-distortion method [4], counteracting the nonlinear effects of the optical modulation characteristics. However, if EAMs are used, it was found that EAM modulation characteristics are not only dependent on wavelength, but also on input optical power [5]. Thus, different pre-distortions for different wavelengths and different power levels have to be used, making the radio over fiber (RoF) system design very complicated. Another technique is to use two wavelengths for each RoF system, in which one wavelength is tunable so that EAM transfer function nonlinearities at the two wavelengths will be matched. Recently, two balanced systems with one wavelength and two fibers and two wavelengths and one fiber were demonstrated to suppress second-order HD (HD2) and second-order intermodulation distortion (IMD2) and improve dynamic range in a RoF system [6–7]. The balanced system that utilizes one wavelength and two fibers, in which two EAMs have mirrored transfer
372
V. Sharma et al. / Optik 122 (2011) 371–374
functions, is very difficult to maintain balance between two fiber transmissions, resulting in less suppression of HD2 and IMD2. Also, a feedback control system is required to maintain balance [6]. Later, this technique was improved by using two wavelengths and one fiber for each RoF system [7], which is referred to as the conventional balanced system in this letter. However, spectral efficiency is considerably reduced because two wavelengths for each RoF system are utilized. Also, the two EAMs must have very similar modulation characteristics at the two wavelengths, which may not be easy to obtain because EAM modulation characteristics depend on wavelength [4]. Alternatively, in order to reduce nonlinear distortions, low optical modulation indexes or depths (the ratio of optical signal sub-carrier to optical carrier) were usually preferred. Unfortunately, in this case the optical carrier is dominant in comparison to the optical signal sub-carrier, which leads to reduced receiver sensitivity. So, higher modulation indexes are preferred, which lead to significant increases of nonlinear distortion. In [8], the better suppression of nonlinear distortions consist of harmonic distortions (HDs) and intermodulation distortions (IMDs), both of which come from the nonlinear modulation characteristics of the optical modulators, was achieved by using single electrode Mach-Zehnder external modulator (SEMZM) with EDFA in a single-tone RoF system. In this work, we have transmitted a single-tone channel of 20 GHz, modulated either by external modulation schemes consist of DEMZM, SEMZM or by direct modulation, with EDFA and SOA at an optical link of 20 km and compared the received RF power of the transmitted single-tone channel for each case.
2. Simulation setup In our simulation setup using OPTSIMTM 3.6 simulator schematically shown in Fig. 1(a), a single-tone RF signal of varying frequency from 1 to 20 GHz is modulated by using DEMZM and SEMZM external modulators over a continuous wave (CW) laser at 1550.5 nm and 1549.5 nm of laser line width 10 MHz with CW power of 10 mW respectively and transmitted over a optical link of 20 km simultaneously. The offset voltage corresponding to the zero phase retardation in absence of any electric field on both electrodes of DEMZM modulator is set at 5 V and maximum transmitted offset voltage is set to zero in case of SEMZM modulator. The chip factor is kept at zero in the both DEMZM and SEMZM modulators as external modulators are consider almost chirp-free modulators. The Vpi voltage is fixed at 8 V and 8.2 V in SEMZM and DEMZM modulators respectively. The single-tone RF signal is splitted into two parts by using electrical splitter; one part with phase shift of 90◦ is applied to the first drive electrode while the second part is directly applied to the second drive electrode of DEMZM modulator. In the receiving section, the two channels are splitted, amplified either with an EDFA or SOA and detected by connecting narrow bandwidth electric power meters. The quantum efficiency and responsivity of PIN diode, acts as photo detector, is fixed at 0.7199 and 0.9A/W respectively. In the second setup as shown in Fig. 1(b), we have transmitted a single-tone RF signal of varying frequency from 1 to 20 GHz either modulated by using DEMZM external modulators over a continuous wave (CW) laser at 1550.5 nm or by using direct-modulation technique over a rate equation laser at 1549.5 nm of modulation index of 0.04 simultaneously. The other simulation parameters of different components used in designing the RoF system using OPTSIMTM simulator are listed in Table 1. 3. Result and discussion We first consider our simulation set-up to investigate the frequency response of single-tone RoF system (back to back) as shown graphically in Fig. 2(a, c, and e) by transmitting a singletone RF channel of 20 GHz by using dual-electrode Mach-Zehnder (DEMZM) modulator, single electrode Mach-Zehnder (SEMZM) modulator with chirp factor = 0 and direct-modulation technique with EDFA and SOA. It is observed that the received electric Power is measured as −38.998 dB with EDFA and as −52.211 dB with SOA when the single-tone RF signal is externally modulated by DEMZM. But with direct-modulation technique, it is calculated as −36.992 dB with EDFA and −57.491 dB with SOA. The RF power is calculated as
Table 1 Simulation parameters.
Fig. 1. Simulation setup for transmitting a single-tone channel over SMF of 20 km (a) using DEMZM and SEMZM simultaneously and (b) using DEMZM and direct modulation simultaneously.
Parameter
Value
Fiber loss Fiber dispersion (D) Fiber nonlinearity Fiber core effective area Fiber average beat length EDFA’s gain EDFA’s noise figure SOA’s line enhancement factor SOA’s confinement factor SOA’s bias current SOA’s saturation power Dark current PIN diode Quantum noise PIN diode Reference frequency PIN diode
0.2 dB/km 16 ps/nm/km 1.267 80 m2 5m 16 dB 4.5 dB 3 0.35 100 mA 9.15 mW 0 nA Off 1550 nm
V. Sharma et al. / Optik 122 (2011) 371–374
373
Fig. 2. Received RF power vs RF modulating frequency with (a) back-to-back direct and SEMZM, (b) direct and SEMZM at optical length of 20 km, (c) back-to-back direct and DEMZM, (d) direct and DEMZM at optical length of 20 km, (e) back-to-back SEMZM and DEMZM and (f) SEMZM and DEMZM at optical length of 20 km of single-tone RoF system.
Table 2 Received RF power. S. no.
Simulation setup
Received RF power in dB at 20 GHz EDFA with SEMZM
SOA with SEMZM
EDFA with direct modulation
SOA with direct modulation
−47.482 −49.180
−36.961 −39.711
−57.468 −54.365
1 2
SEMZM with direct modulation (back to back) SEMZM with direct modulation (20 km)
−26.623 −34.405
S. no.
Simulation setup
Received RF power in dB at 20 GHz
3 4
DEMZM with direct modulation (back to back) DEMZM with direct modulation (L = 20 km)
S. no.
5 6
EDFA with DEMZM
SOA with DEMZM
EDFA with direct modulation
SOA with direct modulation
−38.998 −47.132
−52.211 −53.898
−36.992 −40.170
−57.491 −54.479
Simulation setup
DEMZM with SEMZM external modulation (back to back) DEMZM with SEMZM external modulation (L = 20 km)
Received RF power in dB at 20 GHz EDFA with DEMZM
SOA with DEMZM
EDFA with SEMZM
SOA with SEMZM
−68.677 −74.361
−81.976 −81.067
−26.625 −34.486
−47.482 −49.184
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V. Sharma et al. / Optik 122 (2011) 371–374
−26.625 dB with EDFA and −47.482 dB with SOA using SEMZM modulator at the Base Station (BS), which is quite large as compare to the RF power received with DEMZM and direct-modulation method. Thus, an improvement of 12 dB and 10 dB in received RF power at BS is obtained in single-tone RoF system (back to back) on externally modulating the RF channel by using SEMZM on comparing with DEMZM and direct-modulation scheme amplified with EDFA. It is also observed from the Fig. 2 (a, c, and e) and Table 2 that the received RF power reduces more when amplified by using SOA amplifier but again less reduction is calculated in case of SEMZM. Secondly, we analyzed the same RoF systems at an optical link of 20 km as shown in Fig. 2 (b, d, and f) and observed the same behavior in receiving the RF power of single-tone RF channel and maximum receiving RF power is achieved with SEMZM modulator with EDFA. 4. Conclusion From the results, we have concluded that an improvement at BS in received RF power of a single-tone RF channel transmitted over an optical link of 20 km in a single-tone RoF system is achieved by using SEMZM modulator with EDFA amplifier on comparing with
DEMZM- and direct-modulated RoF system using EDFA. It is also investigated that better results in the measurement of received RF power are achieved with SOA in SEMZM modulated RoF system than that of DEMZM and direct modulated RoF systems. References [1] A. Vilcot, B. Cabon, J. Chazelas (Eds.), Microwave Photonics, Kluwer Academic Publ., 2003. [2] C.H. Lee (Ed.), Microwave Photonics, CRC Press, 2007. [3] U. Gliese, T.N. Nielsen, S. Norskov, K.E. Stubkjaer, Multifunction fibre optic microwave links based on remote heterodyne detection, IEEE Trans. Microw. Theory Tech. 46 (May (5)) (1998). [4] L. Roselli, V. Borgioni, F. Zepparelli, F. Ambrosi, M. Comez, P. Faccin, A. Casini, Analog laser pre-distortion for multiservice radio over fiber system, J. Lightw. Technol. 21 (May (5)) (2003) 1211–1223. [5] B. Liu, J. Shim, Y. Chiu, A. Keating, J. Piprek, J.E. Bowers, Analog characterization of low-voltage MQW traveling-wave electro-absorption modulators, J. Lightw. Technol. 21 (December (12)) (2003) 3011–3019. [6] S. Mathai, F. Cappelluti, T. Jung, D. Novak, R. Waterhouse, D. Sivco, A. Cho, G. Ghione, M. Wu, Experimental demonstration of a balanced electro-absorption modulated microwave photonic link, IEEE Trans. Microw. Theory Tech. 49 (October (10)) (2001) 1956–1961. [7] Y. Wu, Optical heterodyned radio over fiber link design using electro-absorption and electro-optic modulators, Ph.D. dissertation, Univ. California, San Diego, 2004. [8] Vishal Sharma, Amarpal Singh, Ajay K. Sharma, “Simulative investigation of non linear distortion in single- and two-tone RoF System using direct- and externalmodulation techniques” Optik - International Journal for Light and Electron Optics, in press, corrected proof, Available online 28 August 2009.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Simulative investigation the impact of optical amplification techniques on singleand dual-electrode MZM and direct modulator’s in single-tone RoF system Vishal Sharma a , Amarpal Singh b,∗ , Ajay K. Sharma c a b c
Shaheed Bhagat Singh College of Engineering & Technology, Ferozepur, Punjab, India Beant College of Engineering and Technology, Gurdaspur, Punjab, India Dr B R Ambedkar National Institute of Technology, Jalandhar, Punjab, India
a r t i c l e
i n f o
Article history: Received 1 August 2009 Accepted 4 February 2010
Keywords: Radio over fiber (RoF) system External- and direct-modulation Dual-and single-electrode MZM SOA EDFA
a b s t r a c t Since the introduction of erbium-doped fiber amplifier (EDFA) in optical fiber communication systems at 1550 nm, the fiber loss is no longer considered as the limiting factor but the fiber dispersion and chirp parameters play an important role in degrading the performance of optical communication systems. Further, the invention of external modulators of Mach-Zehnder (MS) type minimizes the transmission power penalty due to fiber chromatic dispersion as these modulators are consider almost chirp-free modulators. Also, better suppression of nonlinear distortions consists of harmonic distortions (HDs) and intermodulation distortions (IMDs) were achieved by using Mach-Zehnder external modulator [8]. In this work, a simulation comparison of external- and direct-modulation schemes using different optical amplifiers; SOA and EDFA in single-tone RoF system is reported. A single-tone channel of 20 GHz is transmitted over single-mode fiber (SMF) either externally modulated by using single-electrode (SEMZM) – and dualelectrode (DEMZM) – Mach-Zehnder modulator or by direct-modulation scheme. Better improvements in the measurement of received RF power of a 20 km linked single-tone RoF system are achieved by using SEMZM with EDFA, which means maximum optical link is achieved between Central Office (CO) and Base Station (BS). Further, this scheme provides better BER as compare to other schemes discussed in this paper. © 2010 Elsevier GmbH. All rights reserved.
1. Introduction The growth of mobile and wireless communications fuels increasing demand for multimedia services with a guaranteed quality of service. This requires realization of broadband distribution and access networks. The RoF technology has various advantages when it applies to wireless systems, especially, mobile communication radio networks. Very small base stations (access units) are equipped on the ceiling of rooms in a building and are connected to a master base station (base unit) by optical fiber cables. On large scale level, research on RoF networks has been performed for last few years because of a numerous advantages of RoF technology. Within this framework, RoF schemes can be applied for realizing seamless wireless networks since they allow for the easy distribution of microwaves and millimeter waves over long distances along optical fibers [1,2]. Several techniques for distributing and generating microwave signals via optical fiber exist. The techniques may be classified into two main categories namely Intensity Modulation-
∗ Corresponding author. E-mail address: s [email protected] (A. Singh). 0030-4026/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2010.02.021
Direct Detection (IM-DD) and Remote Heterodyne Detection (RHD) techniques [3]. In such systems, it is desirable to achieve better receiver sensitivities, higher dynamic ranges, and lower nonlinear distortions. Techniques to reduce nonlinear distortions have been investigated extensively. A method for reducing nonlinear HDs and IMDs is to use the pre-distortion method [4], counteracting the nonlinear effects of the optical modulation characteristics. However, if EAMs are used, it was found that EAM modulation characteristics are not only dependent on wavelength, but also on input optical power [5]. Thus, different pre-distortions for different wavelengths and different power levels have to be used, making the radio over fiber (RoF) system design very complicated. Another technique is to use two wavelengths for each RoF system, in which one wavelength is tunable so that EAM transfer function nonlinearities at the two wavelengths will be matched. Recently, two balanced systems with one wavelength and two fibers and two wavelengths and one fiber were demonstrated to suppress second-order HD (HD2) and second-order intermodulation distortion (IMD2) and improve dynamic range in a RoF system [6–7]. The balanced system that utilizes one wavelength and two fibers, in which two EAMs have mirrored transfer
372
V. Sharma et al. / Optik 122 (2011) 371–374
functions, is very difficult to maintain balance between two fiber transmissions, resulting in less suppression of HD2 and IMD2. Also, a feedback control system is required to maintain balance [6]. Later, this technique was improved by using two wavelengths and one fiber for each RoF system [7], which is referred to as the conventional balanced system in this letter. However, spectral efficiency is considerably reduced because two wavelengths for each RoF system are utilized. Also, the two EAMs must have very similar modulation characteristics at the two wavelengths, which may not be easy to obtain because EAM modulation characteristics depend on wavelength [4]. Alternatively, in order to reduce nonlinear distortions, low optical modulation indexes or depths (the ratio of optical signal sub-carrier to optical carrier) were usually preferred. Unfortunately, in this case the optical carrier is dominant in comparison to the optical signal sub-carrier, which leads to reduced receiver sensitivity. So, higher modulation indexes are preferred, which lead to significant increases of nonlinear distortion. In [8], the better suppression of nonlinear distortions consist of harmonic distortions (HDs) and intermodulation distortions (IMDs), both of which come from the nonlinear modulation characteristics of the optical modulators, was achieved by using single electrode Mach-Zehnder external modulator (SEMZM) with EDFA in a single-tone RoF system. In this work, we have transmitted a single-tone channel of 20 GHz, modulated either by external modulation schemes consist of DEMZM, SEMZM or by direct modulation, with EDFA and SOA at an optical link of 20 km and compared the received RF power of the transmitted single-tone channel for each case.
2. Simulation setup In our simulation setup using OPTSIMTM 3.6 simulator schematically shown in Fig. 1(a), a single-tone RF signal of varying frequency from 1 to 20 GHz is modulated by using DEMZM and SEMZM external modulators over a continuous wave (CW) laser at 1550.5 nm and 1549.5 nm of laser line width 10 MHz with CW power of 10 mW respectively and transmitted over a optical link of 20 km simultaneously. The offset voltage corresponding to the zero phase retardation in absence of any electric field on both electrodes of DEMZM modulator is set at 5 V and maximum transmitted offset voltage is set to zero in case of SEMZM modulator. The chip factor is kept at zero in the both DEMZM and SEMZM modulators as external modulators are consider almost chirp-free modulators. The Vpi voltage is fixed at 8 V and 8.2 V in SEMZM and DEMZM modulators respectively. The single-tone RF signal is splitted into two parts by using electrical splitter; one part with phase shift of 90◦ is applied to the first drive electrode while the second part is directly applied to the second drive electrode of DEMZM modulator. In the receiving section, the two channels are splitted, amplified either with an EDFA or SOA and detected by connecting narrow bandwidth electric power meters. The quantum efficiency and responsivity of PIN diode, acts as photo detector, is fixed at 0.7199 and 0.9A/W respectively. In the second setup as shown in Fig. 1(b), we have transmitted a single-tone RF signal of varying frequency from 1 to 20 GHz either modulated by using DEMZM external modulators over a continuous wave (CW) laser at 1550.5 nm or by using direct-modulation technique over a rate equation laser at 1549.5 nm of modulation index of 0.04 simultaneously. The other simulation parameters of different components used in designing the RoF system using OPTSIMTM simulator are listed in Table 1. 3. Result and discussion We first consider our simulation set-up to investigate the frequency response of single-tone RoF system (back to back) as shown graphically in Fig. 2(a, c, and e) by transmitting a singletone RF channel of 20 GHz by using dual-electrode Mach-Zehnder (DEMZM) modulator, single electrode Mach-Zehnder (SEMZM) modulator with chirp factor = 0 and direct-modulation technique with EDFA and SOA. It is observed that the received electric Power is measured as −38.998 dB with EDFA and as −52.211 dB with SOA when the single-tone RF signal is externally modulated by DEMZM. But with direct-modulation technique, it is calculated as −36.992 dB with EDFA and −57.491 dB with SOA. The RF power is calculated as
Table 1 Simulation parameters.
Fig. 1. Simulation setup for transmitting a single-tone channel over SMF of 20 km (a) using DEMZM and SEMZM simultaneously and (b) using DEMZM and direct modulation simultaneously.
Parameter
Value
Fiber loss Fiber dispersion (D) Fiber nonlinearity Fiber core effective area Fiber average beat length EDFA’s gain EDFA’s noise figure SOA’s line enhancement factor SOA’s confinement factor SOA’s bias current SOA’s saturation power Dark current PIN diode Quantum noise PIN diode Reference frequency PIN diode
0.2 dB/km 16 ps/nm/km 1.267 80 m2 5m 16 dB 4.5 dB 3 0.35 100 mA 9.15 mW 0 nA Off 1550 nm
V. Sharma et al. / Optik 122 (2011) 371–374
373
Fig. 2. Received RF power vs RF modulating frequency with (a) back-to-back direct and SEMZM, (b) direct and SEMZM at optical length of 20 km, (c) back-to-back direct and DEMZM, (d) direct and DEMZM at optical length of 20 km, (e) back-to-back SEMZM and DEMZM and (f) SEMZM and DEMZM at optical length of 20 km of single-tone RoF system.
Table 2 Received RF power. S. no.
Simulation setup
Received RF power in dB at 20 GHz EDFA with SEMZM
SOA with SEMZM
EDFA with direct modulation
SOA with direct modulation
−47.482 −49.180
−36.961 −39.711
−57.468 −54.365
1 2
SEMZM with direct modulation (back to back) SEMZM with direct modulation (20 km)
−26.623 −34.405
S. no.
Simulation setup
Received RF power in dB at 20 GHz
3 4
DEMZM with direct modulation (back to back) DEMZM with direct modulation (L = 20 km)
S. no.
5 6
EDFA with DEMZM
SOA with DEMZM
EDFA with direct modulation
SOA with direct modulation
−38.998 −47.132
−52.211 −53.898
−36.992 −40.170
−57.491 −54.479
Simulation setup
DEMZM with SEMZM external modulation (back to back) DEMZM with SEMZM external modulation (L = 20 km)
Received RF power in dB at 20 GHz EDFA with DEMZM
SOA with DEMZM
EDFA with SEMZM
SOA with SEMZM
−68.677 −74.361
−81.976 −81.067
−26.625 −34.486
−47.482 −49.184
374
V. Sharma et al. / Optik 122 (2011) 371–374
−26.625 dB with EDFA and −47.482 dB with SOA using SEMZM modulator at the Base Station (BS), which is quite large as compare to the RF power received with DEMZM and direct-modulation method. Thus, an improvement of 12 dB and 10 dB in received RF power at BS is obtained in single-tone RoF system (back to back) on externally modulating the RF channel by using SEMZM on comparing with DEMZM and direct-modulation scheme amplified with EDFA. It is also observed from the Fig. 2 (a, c, and e) and Table 2 that the received RF power reduces more when amplified by using SOA amplifier but again less reduction is calculated in case of SEMZM. Secondly, we analyzed the same RoF systems at an optical link of 20 km as shown in Fig. 2 (b, d, and f) and observed the same behavior in receiving the RF power of single-tone RF channel and maximum receiving RF power is achieved with SEMZM modulator with EDFA. 4. Conclusion From the results, we have concluded that an improvement at BS in received RF power of a single-tone RF channel transmitted over an optical link of 20 km in a single-tone RoF system is achieved by using SEMZM modulator with EDFA amplifier on comparing with
DEMZM- and direct-modulated RoF system using EDFA. It is also investigated that better results in the measurement of received RF power are achieved with SOA in SEMZM modulated RoF system than that of DEMZM and direct modulated RoF systems. References [1] A. Vilcot, B. Cabon, J. Chazelas (Eds.), Microwave Photonics, Kluwer Academic Publ., 2003. [2] C.H. Lee (Ed.), Microwave Photonics, CRC Press, 2007. [3] U. Gliese, T.N. Nielsen, S. Norskov, K.E. Stubkjaer, Multifunction fibre optic microwave links based on remote heterodyne detection, IEEE Trans. Microw. Theory Tech. 46 (May (5)) (1998). [4] L. Roselli, V. Borgioni, F. Zepparelli, F. Ambrosi, M. Comez, P. Faccin, A. Casini, Analog laser pre-distortion for multiservice radio over fiber system, J. Lightw. Technol. 21 (May (5)) (2003) 1211–1223. [5] B. Liu, J. Shim, Y. Chiu, A. Keating, J. Piprek, J.E. Bowers, Analog characterization of low-voltage MQW traveling-wave electro-absorption modulators, J. Lightw. Technol. 21 (December (12)) (2003) 3011–3019. [6] S. Mathai, F. Cappelluti, T. Jung, D. Novak, R. Waterhouse, D. Sivco, A. Cho, G. Ghione, M. Wu, Experimental demonstration of a balanced electro-absorption modulated microwave photonic link, IEEE Trans. Microw. Theory Tech. 49 (October (10)) (2001) 1956–1961. [7] Y. Wu, Optical heterodyned radio over fiber link design using electro-absorption and electro-optic modulators, Ph.D. dissertation, Univ. California, San Diego, 2004. [8] Vishal Sharma, Amarpal Singh, Ajay K. Sharma, “Simulative investigation of non linear distortion in single- and two-tone RoF System using direct- and externalmodulation techniques” Optik - International Journal for Light and Electron Optics, in press, corrected proof, Available online 28 August 2009.