- Email: [email protected]
Performance analysis of single sideband modulation technique using Mach Zehnder Modulator based on different phase angles of electrical hybrid coupler
Optik 128 (2017) 93–100
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Performance analysis of single sideband modulation technique using Mach Zehnder Modulator based on different phase angles of electrical hybrid coupler Dhananjay Patel ∗ , Vinay Kumar Singh, U.D. Dalal Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat, India
a r t i c l e
i n f o
Article history: Received 7 September 2015 Received in revised form 29 September 2016 Accepted 1 October 2016 Keywords: Optical millimeter wave generation Optical single sideband generation (OSSB) Radio over fiber (RoF)
a b s t r a c t This paper provides an insight on the generation and the performance analysis of two different optical single sideband (OSSB) modulation techniques depending on the phase angle of the electrical hybrid coupler. It takes into account the problem of the chromatic dispersion in the single mode fibers in Passive Optical Networks (PON), which severely degrades the performance of the system. The RF signal at the input of a dual drive Mach Zehnder Modulator (MZM) is divided in two parts equally with an electrical hybrid coupler providing a phase difference of 90◦ and 120◦ respectively to generate two different SSB techniques. The conventional OSSB generation technique with 90◦ phase difference suppresses either lower or upper first order sideband but 2nd order harmonics still exists in the spectrum. The 2nd and higher order harmonics generated due to non-linearity of the MZM depends upon the modulation index. The increase in modulation index leads to increased harmonic distortion at the receiver. The SSB with 120◦ hybrid coupler suppresses lower first order and upper second order sideband (or upper first order and lower second order sideband). It is shown that, the amount of suppression of the second order sideband (lower or upper) increases with the increase in the Extinction Ratio (ER) of the MZM. Comparing with the conventional OSSB technique, the suppression of second order sideband in the SSB with 120◦ hybrid coupler reduces the effect of chromatic dispersion in single mode fibers thereby increasing the transmission distance. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction With the advent in the technology, the high end applications such as high speed data transfer, HD video transmission, distributed antenna systems for indoor and outdoor applications, cable television networks etc., demand large bandwidth and data rates in Giga bits per second [1]. Radio over Fiber technology comprises of transmitting the RF signal over a long distance from the central office to the base station providing high bandwidth and low transmission loss [1,2]. A lot of research is carried out to meet the bandwidth of these applications by optimizing the existing systems using different coding techniques, varying the modulation formats and using different upconversion techniques viz., direct or external modulation. The Standard Single Mode Fiber (SMF) exhibits chromatic dispersion and self phase modulation at higher optical powers due to nonlinearities [3]. When the signal is transmitted over the SMF, all the spectral components experiences different
∗ Corresponding author at: Dhananjay Patel, Electronics Department, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat 395007, India. E-mail addresses: [email protected], [email protected] (D. Patel). http://dx.doi.org/10.1016/j.ijleo.2016.10.002 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
94
D. Patel et al. / Optik 128 (2017) 93–100
Fig. 1. Simulation setup to generate OSSB modulation and its RoF transmission.
phase shifts which depends upon the length of the fiber, the dispersion coefficient and the signal frequency [4]. This leads to the degradation in received optical power. The external optical modulation techniques using MZM such as Double Sideband Full Carrier (DSBFC), Double Sideband Suppressed Carrier (DSBSC), Single Sideband, tandem single sideband and multicarrier modulation have been reported in [5–11]. The DSBFC signal undergoes power fading due to fiber dispersion after being transmitted over a short distance. This reduces the receiver sensitivity and thus limits the transmission distance. This effect becomes more prone with the increase in RF frequency [4]. The DSBSC generated by applying a phase difference of with equal bias and switching voltage to MZM [12] provides less periodic fading of the RF signal and improves the receiver sensitivity [13]. However, among all the techniques, OSSB provides the most robust operation against the chromatic dispersion effect of the fiber and thus helps in transmitting the signal over a long distance [4]. Thus RoF with OSSB modulation scheme improves optical spectral efficiency and also reduces the effect of chromatic dispersion of the fiber. A high SNR is required for the long haul transmission systems. The SNR can be improved by increasing the power of the transmitted signal within the set limits. The increase in signal power increases the modulation index which makes the MZM non-linear. Also with the increase in modulation index of the MZM the powers of the higher order harmonics increases which lead to the harmonic distortion at the receiver. At high modulation index, due to the presence of second and higher order harmonics, the conventional SSB signal looks like a quasi-DSBFC signal. The power fading of such a signal at the receiver due to the effect of chromatic dispersion will be high [14,15]. In order to overcome the problem of chromatic dispersion and to reduce the power fading at the receiver we compare two SSB modulation techniques based on the phase shift of the RF modulating signal applied at the two electrodes of the MZM. One is the conventional optical SSB technique generated by biasing the MZM at quadrature and maintaining a phase difference of /2 of the applied RF signal between the two electrodes. The other technique is implemented by maintaining a proper DC bias and applying an RF phase shift of ±2/3 between the two electrodes. The SSB signal generated comprises of the first lower −order and second higher order sideband or vice versa, depending on the ±2/3 phase shift, of the RF signal respectively. The elimination of either of the second-order sidebands reduces the effect of chromatic dispersion, thereby increasing the transmission distance. The paper is structured as follows: Sections 2 and 3 deals with the generation of the conventional SSB and the new SSB technique. Section 4 provides the simulation setup and analysis of both the SSB techniques considering different Extinction ratio of the MZM, the modulation index and different lengths of the fiber. Finally Section 5 concludes the paper. 2. Optical millimeter wave generation using OSSB modulation with 90о hybrid coupler If an MZM is applied with input optical field Ei (t), with 1 (t) and 2 (t) as the phase difference in upper and lower electrodes respectively, then the output optical field Eo (t) is given by [16] Eo (t) =
Ei (t) j1 (t) + ej2 (t) ] , [e 2
where, is a scaling factor due to finite extinction ratio of the MZM.
(1)
D. Patel et al. / Optik 128 (2017) 93–100
95
Fig. 2. Optical spectrum of OSSB. (a) SSB with 90о hybrid coupler. (b) SSB with 120о hybrid coupler.
The Eq. (1) can be expanded using Bessel function as B Eo (t) = 2
n=∞
Jn (˛) e
j (ωc +nωRF )t+ n 2
j n .
2
e
j εn− n 2 +e
,
(2)
n=−∞
where B is the amplitude of the CW laser, ωc is the center frequency. ε = v1
v2
Vdc V
is the normalized bias where V is the half
wave voltage and Vdc is the MZM DC bias. ˛ = V or V is the modulation index with v1 , v2 and ωRF are the amplitudes and angular frequency with phase angle of the applied RF signal. Jn (x) is the Bessel function of the first kind of ordern. The Eq. (2) represent optical double sideband full carrier signal (DSBFC).
96
D. Patel et al. / Optik 128 (2017) 93–100
Fig. 3. Effect of DC bias voltage on 2nd order sideband optical power in SSB with 120о hybrid coupler.
The conventional OSSB modulation scheme is implemented by applying an RF signal at the two arms of the MZM with phase difference of 90◦ . The MZM is biased at quadature with DC bias maintained at V /2. Applying the same in Eq. (2), generates an OSSB signal as follows
B EOSSB (t) = 2
∞
Jn (˛) e
j((ωc +nωRF )t+ n 4 )
e
j n 4
+ je
−j n 4
.
(3)
n=−∞
The Eq. (3) represents the optical carrier at the center frequency of ωc along with its sidebands at ωc + nωRF . The amplitude of the carrier and the sidebands depends upon the modulation index and the value of the corresponding Bessel function. It can be seen that the lower first order sideband is suppressed if the phase angle is 90◦ , while the upper first order sideband is suppressed if the phase angle is −90◦ . At high modulation index the optical spectrum of the conventional OSSB scheme looks like quasi DSBFC due to the increased powers of the second order harmonics. Thus the signal experiences high chromatic dispersion. This will reduce the overall spectral efficiency and the transmission distance over the fiber. 3. Optical millimeter wave generation using OSSB modulation with 120о hybrid coupler This OSSB modulation technique is implemented by maintaining a phase difference of 120◦ by an electric hybrid coupler. The optical signals in upper and lower arm of the MZM have a phase difference ofn ∗ 2 − , where n represents the order 3 of the sidebands and is the additional phase difference introduced by proper DC bias [17,18]. When = 3 and n = −1 the lower first order optical sideband is suppressed due to – phase difference between the two electrodes of MZM. Also, the upper second order sideband with n = 2 is suppressed due to phase difference. Thus, an OSSB signal is generated with lower first order optical sideband and upper second order sideband suppressed. Similarly, when = − 3 , the upper first order optical sideband and lower second order sideband is suppressed. When an additional phase difference = ± 3 is introduced by proper DC bias, one of the second order sidebands get suppressed. The value of DC bias required to generate the new SSB technique is given by 1 (t) = ∗
V + v
1 (t) dc V
.
Considering only the DC bias, when no RF signal is applied and substituting the value of 1 (t) = Vdc =
V . 3
(4) , 3
in Eq. (4) we get (5)
D. Patel et al. / Optik 128 (2017) 93–100
97
Fig. 4. Effect of ER on 2nd order sideband optical power.
Thus, the OSSB scheme can be realized by 120◦ hybrid coupler with an MZM, along with proper DC bias to introduce the in Eq. (6), the SSB additional phase difference of 3 between the two arms of the MZM. By substituting Vdc = V3 and = ± 2 3 signal using 120◦ hybrid coupler is generated. The optical field of the SSB scheme is given by B 120 EOSSB (t) = 2
n=∞
Jn (˛) Re
sB (t) ej(
(ωc +nωRF )t+ n 3
) e
j n 3
+e
j( −n 3 )
.
(6)
n=−∞
Substituting n = −1 and n = 2 in Eq. (6) eliminates the −1st and +2nd sidebands. Setting the normalized DC bias Vdc = introduces an additional phase difference of ± 3 , eliminating either of the second order sidebands.
V 3
,
4. Simulation setup and analysis Fig. 1 illustrates the simulation setup for generation of the OSSB modulation technique using MZM with a 90◦ /120◦ hybrid coupler. This setup is used to analyze the impact of the fiber chromatic dispersion using OptiSystem 13.0.3. The input RF signal of frequency18 GHz is applied to the hybrid coupler to generate the required phase shift for the conventional and the new OSSB technique. The hybrid coupler provides a phase shift of /2 and 2/3 while the DC bias voltage applied to the MZM is maintained at V2 , V3 for both OSSB modulation techniques respectively. A 1552.54 nm CW laser of line width of 10 MHz and −2 dBm optical power acts as optical input to the MZM. The optical spectrum at the output of MZM is observed on optical spectrum analyzer. The optical signal propagates over a varying fiber length up to 25 km with dispersion coefficient of 16.75 ps/nm/km and attenuation of 0.2 dB/km. An optical amplifier is used to overcome the fiber loss. At the receiver a photodetector with Responsitivity of 0.8 A/W down converts the signal back to electrical domain. The detected RF signal is observed on RF spectrum analyzer. Fig. 2 shows the simulated optical spectrum of OSSB signals using 90о and 120о hybrid coupler. It can be inferred that the lower first order sideband suppression in both the scheme is almost the same but the higher second order sideband of OSSB with 120о hybrid coupler is reduced by 22.2 dBm as compared to the conventional one. This suppression of higher order sideband reduces the effect of chromatic dispersion and the transmission distance can be increased. A simulation is performed to evaluate the amount of DC bias that has to be applied for the generation of the SSB with suppressed +2nd order sideband. Fig. 3 illustrates the 2nd order sideband power as the function of normalized DC biased voltage. With the increase in Vdc , the 2nd order sideband optical power reduces linearly and reaches to its lowest value whenVdc = V3 . Beyond this value, the optical power again increases. The result obtained for the calculation of the normalized DC voltage by analytically (Eq. (5)) and by simulation shows excellent agreement. Thus, the OSSB scheme realized by 120о hybrid coupler provides maximum suppression of 2nd order sideband at Vdc = V3 .
98
D. Patel et al. / Optik 128 (2017) 93–100
Fig. 5. Received RF Power v/s fiber distance for (a) ˛ = 0.125. (b)˛ = 0.25. (c)˛ = 0.5.
D. Patel et al. / Optik 128 (2017) 93–100
99
The power splitting at the Y-branches of MZM is unequal which leads to a finite ER [13,19]. At low ER, the suppression of unwanted 1st order sideband in OSSB is less and thus the optical spectrum of the OSSB looks like DSBFC. Thus the increased powers of the unwanted sidebands lead to periodic power fading due to dispersion of fiber and transmission distance reduces. With the increase in ER, the MZM becomes more balanced due to the equal power distribution at its input terminals, thereby increasing the unwanted 1st order sideband suppression. Thus the generated SSB signal will be more immune to the effect of chromatic dispersion. To analyze the effect of ER on the power of the 2nd order optical sidebands, the ER is varied from 5 to 25 dB. Fig. 4 illustrates the 2nd order sideband optical power as a function of the ER at the output of MZM for both the OSSB techniques. In case of OSSB with 90о hybrid coupler, the 2nd order optical sideband power is independent of ER, while the 2nd order optical sideband power of OSSB with 120о hybrid coupler reduces with the increase in ER. It is observed from the figure that as ER increases from 5 to 25 dB, the 2nd order optical SB power reduces from −28.4 to −48.5dBm. Thus, increasing the ER makes MZM more ideal and provides better suppression of 1st order unwanted sideband and also one of the 2nd order sideband in OSSB technique with 120о hybrid coupler. The RoF technology consists of transmitting a high frequency RF signal over the fiber from the central office to the base station. The performance of RoF system is hampered by the chromatic dispersion. The effect of chromatic dispersion on the DSBFC signal is higher compared to that on the SSB signal. Thus, the SSB modulation is employed which is more robust to the chromatic dispersion effects and the RF power degradation at the receiver reduces. In order to improve the SNR, the power of the transmitted signal has to be increased which in turn raises the modulation index of the MZM. At high modulation index, the presence of higher-order harmonics in conventional SSB becomes more significant and thus the signal undergoes more power fading along the transmission. On the contrary, the OSSB modulation technique with 120о hybrid coupler suppresses one of the second order sideband proves to be more immune to the chromatic dispersion and also reduces the second order harmonic distortion at the receiver. The transmission performance was compared by sending the two signals over a fiber with varying lengths up to 25 km. The received RF power is calculated using an RF spectrum analyzer. Fig. 5a–c illustrates the received RF power against the transmission distance for different ˛. The ER of the MZM is kept constant at 25 dB. At low ˛ = 0.125, the power of the wanted first order sideband for both the SSB technique is low compared to its optical carrier. This reduces the SNR and the impact of the chromatic dispersion on the signal will be higher. Hence the received RF power degrades. With the increase in ˛, the power of the wanted first order sideband increases increasing the SNR. At ˛ = 0.25, substantial power of wanted first order sidebands is obtained which makes the signal resilient to the periodic power fading effect due to chromatic dispersion. The SSB with 120◦ hybrid coupler outperforms against the conventional SSB due to the elimination of one of the second order sideband. Thus the received RF power of the SSB with 120◦ hybrid coupler is higher as compared to the conventional one as shown in Fig. 5(b). At ˛ = 0.5, the power of the 2nd order harmonics in the SSB with 90◦ hybrid coupler increases, which increases the power fading effect at the receiver. Hence degradation in received RF power over a fiber transmission is observed in Fig. 5(c). On a contrary, for the SSB with 120◦ hybrid coupler technique, the received RF power remains constant. The simulated results shows a good agreement with the theoretical analysis in Sections 2 and 3 stating that, the OSSB modulation technique with 120о hybrid coupler gives better system performance. The suppression of one of the 2nd order sidebands reduces the effect of the chromatic dispersion and thus the signal can be transmitted over a long fiber span before degradation. 5. Conclusions The performance of the OSSB modulation using the MZM with 90◦ and 120◦ hybrid coupler is analyzed in detail. According to the theoretical analysis and simulation results, the OSSB modulation technique with 120◦ hybrid coupler suppresses one of the unwanted 2nd order sidebands proves to be more robust against the chromatic dispersion. The suppression of 2nd order sideband increases with the increase in ER. This suppression helps in reducing the effect of chromatic dispersion of the fiber and improves the receiver sensitivity. Thus, RoF systems employing the OSSB with the 120◦ hybrid coupler proves to have better system performance against the chromatic dispersion as compared to the OSSB with 90◦ hybrid coupler. Acknowledgement Authors are thankful to Media Lab Asia, Department of Electronics and Information Technology (DEITY) for sponsoring this research work under the Visvesvaraya PhD scheme of DEITY, 2014. References [1] Chun-Ting Lin, Chen J, Peng-Chun Peng, Cheng-Feng Peng, Wei-Ren Peng, Bi-Shiou Chiou, S. Chi, Hybrid optical access network integrating fiber-to-the-home and radio-over-fiber systems, Photonics Technol. Lett. IEEE 19 (April (8)) (2007) 610–612. [2] J. Beas, G. Castanon, I. Aldaya, A. Aragon-Zavala, G. Campuzano, Millimeter-wave frequency radio over fiber systems: a survey, Commun. Surv. Tutor. IEEE 15 (4) (2013) 1593–1619, Fourth Quarter. [3] F. Ramos, J. Marti, Frequency transfer function of dispersive and nonlinear singlemode optical fibers in microwave optical systems, IEEE Photonics Technol. Lett. 12 (5) (2000) 549–551.
100
D. Patel et al. / Optik 128 (2017) 93–100
[4] G.H. Smith, D. Novak, Z. Ahmed, Overcoming chromatic-dispersion effects in fiber-wireless systems incorporating external modulators, Microw. Theory Tech. IEEE Trans. 45 (August (8)) (1997) 1410–1415. [5] Linghao Cheng, S. Aditya, A. Nirmalathas, An exact analytical model for dispersive transmission in microwave fiber-optic links using Mach-Zehnder external modulator, Photonics Technol. Lett. IEEE 17 (July (7)) (2005) 1525–1527. [6] Jianxin Ma, Yu J, Chongxiu Yu, Xiangjun Xin, Junying Zeng, Chen L, Fiber dispersion influence on transmission of the optical millimeter-waves generated using LN-MZM intensity modulation, Lightwave Technol. J. 25 (November (11)) (2007) 3244–3256. [7] Ming-Fang Huang, Jianjun Yu, Zhensheng Jia, Gee Kung Chang, Simultaneous generation of centralized lightwaves and double/single sideband optical millimeter-wave requiring only low-frequency local oscillator signals for radio-over-fiber systems, Lightwave Technol. J. 26 (August (5)) (2008) 2653–2662. [8] Hairul A. Abdul-Rashid, Hean-Teik Chuah, Moncef B. Tayahi, Malik T. Al-Qdah, Ruen-Chung Lee, Chromatic dispersion power penalties in orthogonal subcarrier – optical tandem single sideband systems, IEICE Electron. Express 2 (9) (2005) 305–310, http://dx.doi.org/10.1587/elex.2.305. [9] Hairul A. Abdul-Rashid, Hean-Teik Chuah, Moncef B. Tayahi, Malik T. Al-Qdah, Ruen-Chung Lee, An orthogonal subcarrier based optical tandem single sideband system, IEICE Electron. Express 1 (16) (2004) 490–494, http://dx.doi.org/10.1587/elex.1.490. [10] H.A. Abdul-Rashid, M.B. Tayahi, H.T. Chuah, Improving spectral efficiency in single subcarrier multiplexed systems by spectral overlapping using orthogonal carriers. Communications, 2003. APCC 2003. The 9th Asia-Pacific Conference on; 10/2003. [11] Hairul A. Abdul-Rashid, Moncef B. Tayahi, Novel technique for tandem single sideband communication systems using orthogonal carriers (TSSB-OC). ITCom 2002: The Convergence of Information Technologies and Communications; 07/2002. [12] Jianjun Yu, Zhensheng Jia, Lilin Yi, Yikai Su, Gee-Kung Chang, Ting Wang, Optical millimeter-wave generation or up-conversion using external modulators, Photonics Technol. Lett. IEEE 18 (January (1)) (2006) 265–267. [13] Chun-Ting Lin, Chen J.J, Sheng-Peng Dai, Peng-Chun Peng, Sien Chi, Impact of nonlinear transfer function and imperfect splitting ratio of MZM on optical up-conversion employing double sideband with carrier suppression modulation, Lightwave Technol. J. 26 (August (15)) (2008) 2449–2459. [14] Patel Dhananjay, Vinay Kumar Singh, U.D. Dalal, Assessment of fiber chromatic dispersion based on elimination of second-order harmonics in optical OFDM single sideband modulation using Mach Zehnder Modulator, Fiber Integr. Opt. 35.4 (2016) 181–195. [15] Dhananjay Patel, Vinay Kumar Singh, U.D. Dalal, Analysis of second order harmonic distortion due to transmitter non-linearity and chromatic and modal dispersion of optical OFDM SSB modulated signals in SMF-MMF fiber links, Opt. Commun. 383 (2016) 294–303. [16] Oluyemi Omololu Omomukuyo, Orthogonal frequency division multiplexing for optical access networks, in: Ph.D. Thesis, University College London, 2013, April. [17] Min Xue, Shilong Pan, Yongjiu Zhao, Optical single-sideband modulation based on a dual-drive MZM and a 120◦ hybrid coupler, Lightwave Technol. J. 32 (October (19)) (2014) 3317–3323. [18] S. Pan, M. Xue, Optical vector network analyzer based on optical single-sideband modulation, Optical Communications and Networks (ICOCN), 2013 12th International Conference on, pp. 1–3, 26–28 July 2013. [19] Y. Yamaguchi, S. Nakajima, A. Kanno, T. Kawanishi, M. Izutsu, H. Nakajima, High extinction-ratio characteristics over 60 db Mach-Zehnder modulator with asymmetric power splitting y-branches on x-cut Ti:LiNbO3, Microoptics Conference (MOC), 2013 18th, pp. 1,2, 27–30 Oct. 2013.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Performance analysis of single sideband modulation technique using Mach Zehnder Modulator based on different phase angles of electrical hybrid coupler Dhananjay Patel ∗ , Vinay Kumar Singh, U.D. Dalal Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat, India
a r t i c l e
i n f o
Article history: Received 7 September 2015 Received in revised form 29 September 2016 Accepted 1 October 2016 Keywords: Optical millimeter wave generation Optical single sideband generation (OSSB) Radio over fiber (RoF)
a b s t r a c t This paper provides an insight on the generation and the performance analysis of two different optical single sideband (OSSB) modulation techniques depending on the phase angle of the electrical hybrid coupler. It takes into account the problem of the chromatic dispersion in the single mode fibers in Passive Optical Networks (PON), which severely degrades the performance of the system. The RF signal at the input of a dual drive Mach Zehnder Modulator (MZM) is divided in two parts equally with an electrical hybrid coupler providing a phase difference of 90◦ and 120◦ respectively to generate two different SSB techniques. The conventional OSSB generation technique with 90◦ phase difference suppresses either lower or upper first order sideband but 2nd order harmonics still exists in the spectrum. The 2nd and higher order harmonics generated due to non-linearity of the MZM depends upon the modulation index. The increase in modulation index leads to increased harmonic distortion at the receiver. The SSB with 120◦ hybrid coupler suppresses lower first order and upper second order sideband (or upper first order and lower second order sideband). It is shown that, the amount of suppression of the second order sideband (lower or upper) increases with the increase in the Extinction Ratio (ER) of the MZM. Comparing with the conventional OSSB technique, the suppression of second order sideband in the SSB with 120◦ hybrid coupler reduces the effect of chromatic dispersion in single mode fibers thereby increasing the transmission distance. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction With the advent in the technology, the high end applications such as high speed data transfer, HD video transmission, distributed antenna systems for indoor and outdoor applications, cable television networks etc., demand large bandwidth and data rates in Giga bits per second [1]. Radio over Fiber technology comprises of transmitting the RF signal over a long distance from the central office to the base station providing high bandwidth and low transmission loss [1,2]. A lot of research is carried out to meet the bandwidth of these applications by optimizing the existing systems using different coding techniques, varying the modulation formats and using different upconversion techniques viz., direct or external modulation. The Standard Single Mode Fiber (SMF) exhibits chromatic dispersion and self phase modulation at higher optical powers due to nonlinearities [3]. When the signal is transmitted over the SMF, all the spectral components experiences different
∗ Corresponding author at: Dhananjay Patel, Electronics Department, Sardar Vallabhbhai National Institute of Technology, Surat, Gujarat 395007, India. E-mail addresses: [email protected], [email protected] (D. Patel). http://dx.doi.org/10.1016/j.ijleo.2016.10.002 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
94
D. Patel et al. / Optik 128 (2017) 93–100
Fig. 1. Simulation setup to generate OSSB modulation and its RoF transmission.
phase shifts which depends upon the length of the fiber, the dispersion coefficient and the signal frequency [4]. This leads to the degradation in received optical power. The external optical modulation techniques using MZM such as Double Sideband Full Carrier (DSBFC), Double Sideband Suppressed Carrier (DSBSC), Single Sideband, tandem single sideband and multicarrier modulation have been reported in [5–11]. The DSBFC signal undergoes power fading due to fiber dispersion after being transmitted over a short distance. This reduces the receiver sensitivity and thus limits the transmission distance. This effect becomes more prone with the increase in RF frequency [4]. The DSBSC generated by applying a phase difference of with equal bias and switching voltage to MZM [12] provides less periodic fading of the RF signal and improves the receiver sensitivity [13]. However, among all the techniques, OSSB provides the most robust operation against the chromatic dispersion effect of the fiber and thus helps in transmitting the signal over a long distance [4]. Thus RoF with OSSB modulation scheme improves optical spectral efficiency and also reduces the effect of chromatic dispersion of the fiber. A high SNR is required for the long haul transmission systems. The SNR can be improved by increasing the power of the transmitted signal within the set limits. The increase in signal power increases the modulation index which makes the MZM non-linear. Also with the increase in modulation index of the MZM the powers of the higher order harmonics increases which lead to the harmonic distortion at the receiver. At high modulation index, due to the presence of second and higher order harmonics, the conventional SSB signal looks like a quasi-DSBFC signal. The power fading of such a signal at the receiver due to the effect of chromatic dispersion will be high [14,15]. In order to overcome the problem of chromatic dispersion and to reduce the power fading at the receiver we compare two SSB modulation techniques based on the phase shift of the RF modulating signal applied at the two electrodes of the MZM. One is the conventional optical SSB technique generated by biasing the MZM at quadrature and maintaining a phase difference of /2 of the applied RF signal between the two electrodes. The other technique is implemented by maintaining a proper DC bias and applying an RF phase shift of ±2/3 between the two electrodes. The SSB signal generated comprises of the first lower −order and second higher order sideband or vice versa, depending on the ±2/3 phase shift, of the RF signal respectively. The elimination of either of the second-order sidebands reduces the effect of chromatic dispersion, thereby increasing the transmission distance. The paper is structured as follows: Sections 2 and 3 deals with the generation of the conventional SSB and the new SSB technique. Section 4 provides the simulation setup and analysis of both the SSB techniques considering different Extinction ratio of the MZM, the modulation index and different lengths of the fiber. Finally Section 5 concludes the paper. 2. Optical millimeter wave generation using OSSB modulation with 90о hybrid coupler If an MZM is applied with input optical field Ei (t), with 1 (t) and 2 (t) as the phase difference in upper and lower electrodes respectively, then the output optical field Eo (t) is given by [16] Eo (t) =
Ei (t) j1 (t) + ej2 (t) ] , [e 2
where, is a scaling factor due to finite extinction ratio of the MZM.
(1)
D. Patel et al. / Optik 128 (2017) 93–100
95
Fig. 2. Optical spectrum of OSSB. (a) SSB with 90о hybrid coupler. (b) SSB with 120о hybrid coupler.
The Eq. (1) can be expanded using Bessel function as B Eo (t) = 2
n=∞
Jn (˛) e
j (ωc +nωRF )t+ n 2
j n .
2
e
j εn− n 2 +e
,
(2)
n=−∞
where B is the amplitude of the CW laser, ωc is the center frequency. ε = v1
v2
Vdc V
is the normalized bias where V is the half
wave voltage and Vdc is the MZM DC bias. ˛ = V or V is the modulation index with v1 , v2 and ωRF are the amplitudes and angular frequency with phase angle of the applied RF signal. Jn (x) is the Bessel function of the first kind of ordern. The Eq. (2) represent optical double sideband full carrier signal (DSBFC).
96
D. Patel et al. / Optik 128 (2017) 93–100
Fig. 3. Effect of DC bias voltage on 2nd order sideband optical power in SSB with 120о hybrid coupler.
The conventional OSSB modulation scheme is implemented by applying an RF signal at the two arms of the MZM with phase difference of 90◦ . The MZM is biased at quadature with DC bias maintained at V /2. Applying the same in Eq. (2), generates an OSSB signal as follows
B EOSSB (t) = 2
∞
Jn (˛) e
j((ωc +nωRF )t+ n 4 )
e
j n 4
+ je
−j n 4
.
(3)
n=−∞
The Eq. (3) represents the optical carrier at the center frequency of ωc along with its sidebands at ωc + nωRF . The amplitude of the carrier and the sidebands depends upon the modulation index and the value of the corresponding Bessel function. It can be seen that the lower first order sideband is suppressed if the phase angle is 90◦ , while the upper first order sideband is suppressed if the phase angle is −90◦ . At high modulation index the optical spectrum of the conventional OSSB scheme looks like quasi DSBFC due to the increased powers of the second order harmonics. Thus the signal experiences high chromatic dispersion. This will reduce the overall spectral efficiency and the transmission distance over the fiber. 3. Optical millimeter wave generation using OSSB modulation with 120о hybrid coupler This OSSB modulation technique is implemented by maintaining a phase difference of 120◦ by an electric hybrid coupler. The optical signals in upper and lower arm of the MZM have a phase difference ofn ∗ 2 − , where n represents the order 3 of the sidebands and is the additional phase difference introduced by proper DC bias [17,18]. When = 3 and n = −1 the lower first order optical sideband is suppressed due to – phase difference between the two electrodes of MZM. Also, the upper second order sideband with n = 2 is suppressed due to phase difference. Thus, an OSSB signal is generated with lower first order optical sideband and upper second order sideband suppressed. Similarly, when = − 3 , the upper first order optical sideband and lower second order sideband is suppressed. When an additional phase difference = ± 3 is introduced by proper DC bias, one of the second order sidebands get suppressed. The value of DC bias required to generate the new SSB technique is given by 1 (t) = ∗
V + v
1 (t) dc V
.
Considering only the DC bias, when no RF signal is applied and substituting the value of 1 (t) = Vdc =
V . 3
(4) , 3
in Eq. (4) we get (5)
D. Patel et al. / Optik 128 (2017) 93–100
97
Fig. 4. Effect of ER on 2nd order sideband optical power.
Thus, the OSSB scheme can be realized by 120◦ hybrid coupler with an MZM, along with proper DC bias to introduce the in Eq. (6), the SSB additional phase difference of 3 between the two arms of the MZM. By substituting Vdc = V3 and = ± 2 3 signal using 120◦ hybrid coupler is generated. The optical field of the SSB scheme is given by B 120 EOSSB (t) = 2
n=∞
Jn (˛) Re
sB (t) ej(
(ωc +nωRF )t+ n 3
) e
j n 3
+e
j( −n 3 )
.
(6)
n=−∞
Substituting n = −1 and n = 2 in Eq. (6) eliminates the −1st and +2nd sidebands. Setting the normalized DC bias Vdc = introduces an additional phase difference of ± 3 , eliminating either of the second order sidebands.
V 3
,
4. Simulation setup and analysis Fig. 1 illustrates the simulation setup for generation of the OSSB modulation technique using MZM with a 90◦ /120◦ hybrid coupler. This setup is used to analyze the impact of the fiber chromatic dispersion using OptiSystem 13.0.3. The input RF signal of frequency18 GHz is applied to the hybrid coupler to generate the required phase shift for the conventional and the new OSSB technique. The hybrid coupler provides a phase shift of /2 and 2/3 while the DC bias voltage applied to the MZM is maintained at V2 , V3 for both OSSB modulation techniques respectively. A 1552.54 nm CW laser of line width of 10 MHz and −2 dBm optical power acts as optical input to the MZM. The optical spectrum at the output of MZM is observed on optical spectrum analyzer. The optical signal propagates over a varying fiber length up to 25 km with dispersion coefficient of 16.75 ps/nm/km and attenuation of 0.2 dB/km. An optical amplifier is used to overcome the fiber loss. At the receiver a photodetector with Responsitivity of 0.8 A/W down converts the signal back to electrical domain. The detected RF signal is observed on RF spectrum analyzer. Fig. 2 shows the simulated optical spectrum of OSSB signals using 90о and 120о hybrid coupler. It can be inferred that the lower first order sideband suppression in both the scheme is almost the same but the higher second order sideband of OSSB with 120о hybrid coupler is reduced by 22.2 dBm as compared to the conventional one. This suppression of higher order sideband reduces the effect of chromatic dispersion and the transmission distance can be increased. A simulation is performed to evaluate the amount of DC bias that has to be applied for the generation of the SSB with suppressed +2nd order sideband. Fig. 3 illustrates the 2nd order sideband power as the function of normalized DC biased voltage. With the increase in Vdc , the 2nd order sideband optical power reduces linearly and reaches to its lowest value whenVdc = V3 . Beyond this value, the optical power again increases. The result obtained for the calculation of the normalized DC voltage by analytically (Eq. (5)) and by simulation shows excellent agreement. Thus, the OSSB scheme realized by 120о hybrid coupler provides maximum suppression of 2nd order sideband at Vdc = V3 .
98
D. Patel et al. / Optik 128 (2017) 93–100
Fig. 5. Received RF Power v/s fiber distance for (a) ˛ = 0.125. (b)˛ = 0.25. (c)˛ = 0.5.
D. Patel et al. / Optik 128 (2017) 93–100
99
The power splitting at the Y-branches of MZM is unequal which leads to a finite ER [13,19]. At low ER, the suppression of unwanted 1st order sideband in OSSB is less and thus the optical spectrum of the OSSB looks like DSBFC. Thus the increased powers of the unwanted sidebands lead to periodic power fading due to dispersion of fiber and transmission distance reduces. With the increase in ER, the MZM becomes more balanced due to the equal power distribution at its input terminals, thereby increasing the unwanted 1st order sideband suppression. Thus the generated SSB signal will be more immune to the effect of chromatic dispersion. To analyze the effect of ER on the power of the 2nd order optical sidebands, the ER is varied from 5 to 25 dB. Fig. 4 illustrates the 2nd order sideband optical power as a function of the ER at the output of MZM for both the OSSB techniques. In case of OSSB with 90о hybrid coupler, the 2nd order optical sideband power is independent of ER, while the 2nd order optical sideband power of OSSB with 120о hybrid coupler reduces with the increase in ER. It is observed from the figure that as ER increases from 5 to 25 dB, the 2nd order optical SB power reduces from −28.4 to −48.5dBm. Thus, increasing the ER makes MZM more ideal and provides better suppression of 1st order unwanted sideband and also one of the 2nd order sideband in OSSB technique with 120о hybrid coupler. The RoF technology consists of transmitting a high frequency RF signal over the fiber from the central office to the base station. The performance of RoF system is hampered by the chromatic dispersion. The effect of chromatic dispersion on the DSBFC signal is higher compared to that on the SSB signal. Thus, the SSB modulation is employed which is more robust to the chromatic dispersion effects and the RF power degradation at the receiver reduces. In order to improve the SNR, the power of the transmitted signal has to be increased which in turn raises the modulation index of the MZM. At high modulation index, the presence of higher-order harmonics in conventional SSB becomes more significant and thus the signal undergoes more power fading along the transmission. On the contrary, the OSSB modulation technique with 120о hybrid coupler suppresses one of the second order sideband proves to be more immune to the chromatic dispersion and also reduces the second order harmonic distortion at the receiver. The transmission performance was compared by sending the two signals over a fiber with varying lengths up to 25 km. The received RF power is calculated using an RF spectrum analyzer. Fig. 5a–c illustrates the received RF power against the transmission distance for different ˛. The ER of the MZM is kept constant at 25 dB. At low ˛ = 0.125, the power of the wanted first order sideband for both the SSB technique is low compared to its optical carrier. This reduces the SNR and the impact of the chromatic dispersion on the signal will be higher. Hence the received RF power degrades. With the increase in ˛, the power of the wanted first order sideband increases increasing the SNR. At ˛ = 0.25, substantial power of wanted first order sidebands is obtained which makes the signal resilient to the periodic power fading effect due to chromatic dispersion. The SSB with 120◦ hybrid coupler outperforms against the conventional SSB due to the elimination of one of the second order sideband. Thus the received RF power of the SSB with 120◦ hybrid coupler is higher as compared to the conventional one as shown in Fig. 5(b). At ˛ = 0.5, the power of the 2nd order harmonics in the SSB with 90◦ hybrid coupler increases, which increases the power fading effect at the receiver. Hence degradation in received RF power over a fiber transmission is observed in Fig. 5(c). On a contrary, for the SSB with 120◦ hybrid coupler technique, the received RF power remains constant. The simulated results shows a good agreement with the theoretical analysis in Sections 2 and 3 stating that, the OSSB modulation technique with 120о hybrid coupler gives better system performance. The suppression of one of the 2nd order sidebands reduces the effect of the chromatic dispersion and thus the signal can be transmitted over a long fiber span before degradation. 5. Conclusions The performance of the OSSB modulation using the MZM with 90◦ and 120◦ hybrid coupler is analyzed in detail. According to the theoretical analysis and simulation results, the OSSB modulation technique with 120◦ hybrid coupler suppresses one of the unwanted 2nd order sidebands proves to be more robust against the chromatic dispersion. The suppression of 2nd order sideband increases with the increase in ER. This suppression helps in reducing the effect of chromatic dispersion of the fiber and improves the receiver sensitivity. Thus, RoF systems employing the OSSB with the 120◦ hybrid coupler proves to have better system performance against the chromatic dispersion as compared to the OSSB with 90◦ hybrid coupler. Acknowledgement Authors are thankful to Media Lab Asia, Department of Electronics and Information Technology (DEITY) for sponsoring this research work under the Visvesvaraya PhD scheme of DEITY, 2014. References [1] Chun-Ting Lin, Chen J, Peng-Chun Peng, Cheng-Feng Peng, Wei-Ren Peng, Bi-Shiou Chiou, S. Chi, Hybrid optical access network integrating fiber-to-the-home and radio-over-fiber systems, Photonics Technol. Lett. IEEE 19 (April (8)) (2007) 610–612. [2] J. Beas, G. Castanon, I. Aldaya, A. Aragon-Zavala, G. Campuzano, Millimeter-wave frequency radio over fiber systems: a survey, Commun. Surv. Tutor. IEEE 15 (4) (2013) 1593–1619, Fourth Quarter. [3] F. Ramos, J. Marti, Frequency transfer function of dispersive and nonlinear singlemode optical fibers in microwave optical systems, IEEE Photonics Technol. Lett. 12 (5) (2000) 549–551.
100
D. Patel et al. / Optik 128 (2017) 93–100
[4] G.H. Smith, D. Novak, Z. Ahmed, Overcoming chromatic-dispersion effects in fiber-wireless systems incorporating external modulators, Microw. Theory Tech. IEEE Trans. 45 (August (8)) (1997) 1410–1415. [5] Linghao Cheng, S. Aditya, A. Nirmalathas, An exact analytical model for dispersive transmission in microwave fiber-optic links using Mach-Zehnder external modulator, Photonics Technol. Lett. IEEE 17 (July (7)) (2005) 1525–1527. [6] Jianxin Ma, Yu J, Chongxiu Yu, Xiangjun Xin, Junying Zeng, Chen L, Fiber dispersion influence on transmission of the optical millimeter-waves generated using LN-MZM intensity modulation, Lightwave Technol. J. 25 (November (11)) (2007) 3244–3256. [7] Ming-Fang Huang, Jianjun Yu, Zhensheng Jia, Gee Kung Chang, Simultaneous generation of centralized lightwaves and double/single sideband optical millimeter-wave requiring only low-frequency local oscillator signals for radio-over-fiber systems, Lightwave Technol. J. 26 (August (5)) (2008) 2653–2662. [8] Hairul A. Abdul-Rashid, Hean-Teik Chuah, Moncef B. Tayahi, Malik T. Al-Qdah, Ruen-Chung Lee, Chromatic dispersion power penalties in orthogonal subcarrier – optical tandem single sideband systems, IEICE Electron. Express 2 (9) (2005) 305–310, http://dx.doi.org/10.1587/elex.2.305. [9] Hairul A. Abdul-Rashid, Hean-Teik Chuah, Moncef B. Tayahi, Malik T. Al-Qdah, Ruen-Chung Lee, An orthogonal subcarrier based optical tandem single sideband system, IEICE Electron. Express 1 (16) (2004) 490–494, http://dx.doi.org/10.1587/elex.1.490. [10] H.A. Abdul-Rashid, M.B. Tayahi, H.T. Chuah, Improving spectral efficiency in single subcarrier multiplexed systems by spectral overlapping using orthogonal carriers. Communications, 2003. APCC 2003. The 9th Asia-Pacific Conference on; 10/2003. [11] Hairul A. Abdul-Rashid, Moncef B. Tayahi, Novel technique for tandem single sideband communication systems using orthogonal carriers (TSSB-OC). ITCom 2002: The Convergence of Information Technologies and Communications; 07/2002. [12] Jianjun Yu, Zhensheng Jia, Lilin Yi, Yikai Su, Gee-Kung Chang, Ting Wang, Optical millimeter-wave generation or up-conversion using external modulators, Photonics Technol. Lett. IEEE 18 (January (1)) (2006) 265–267. [13] Chun-Ting Lin, Chen J.J, Sheng-Peng Dai, Peng-Chun Peng, Sien Chi, Impact of nonlinear transfer function and imperfect splitting ratio of MZM on optical up-conversion employing double sideband with carrier suppression modulation, Lightwave Technol. J. 26 (August (15)) (2008) 2449–2459. [14] Patel Dhananjay, Vinay Kumar Singh, U.D. Dalal, Assessment of fiber chromatic dispersion based on elimination of second-order harmonics in optical OFDM single sideband modulation using Mach Zehnder Modulator, Fiber Integr. Opt. 35.4 (2016) 181–195. [15] Dhananjay Patel, Vinay Kumar Singh, U.D. Dalal, Analysis of second order harmonic distortion due to transmitter non-linearity and chromatic and modal dispersion of optical OFDM SSB modulated signals in SMF-MMF fiber links, Opt. Commun. 383 (2016) 294–303. [16] Oluyemi Omololu Omomukuyo, Orthogonal frequency division multiplexing for optical access networks, in: Ph.D. Thesis, University College London, 2013, April. [17] Min Xue, Shilong Pan, Yongjiu Zhao, Optical single-sideband modulation based on a dual-drive MZM and a 120◦ hybrid coupler, Lightwave Technol. J. 32 (October (19)) (2014) 3317–3323. [18] S. Pan, M. Xue, Optical vector network analyzer based on optical single-sideband modulation, Optical Communications and Networks (ICOCN), 2013 12th International Conference on, pp. 1–3, 26–28 July 2013. [19] Y. Yamaguchi, S. Nakajima, A. Kanno, T. Kawanishi, M. Izutsu, H. Nakajima, High extinction-ratio characteristics over 60 db Mach-Zehnder modulator with asymmetric power splitting y-branches on x-cut Ti:LiNbO3, Microoptics Conference (MOC), 2013 18th, pp. 1,2, 27–30 Oct. 2013.