May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
2103
Flexible compensation of dispersion-induced power fading for multi-service RoF links based on a phase-coherent orthogonal lightwave generator Beilei Wu,1,2 Ming Zhu,2 Mu Xu,2 Jing Wang,2 Muguang Wang,1 Fengping Yan,1,* Shuisheng Jian,1 and Gee-Kung Chang2 1
Institute of Lightwave Technology, Key Lab of All Optical Network and Advanced Telecommunication of EMC, Beijing Jiaotong University, Beijing, China 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA *Corresponding author: [email protected] Received March 18, 2015; revised April 7, 2015; accepted April 11, 2015; posted April 14, 2015 (Doc. ID 236292); published April 30, 2015
A novel technique to simultaneously compensate dispersion-induced power fading for multi-service radio-overfiber (RoF) links is proposed. At the central office (CO), a phase-coherent orthogonal lightwave generator (POLG) consisting of a polarization rotator (PR) and a single-driver Mach–Zehnder modulator (MZM) is used. By adjusting a polarization controller (PC) in the base station (BS), the phase difference between the orthogonal polarization carrier and two sidebands can be controlled, and the frequency response can be flexibly shifted for individual radio frequency (RF) service. We experimentally shift the destructive interference to the constructive interference at various frequencies for 25 km and 30 km fiber links. The performances of two services carrying 1 Gb/s on–off keying (OOK) data at 9 GHz and 16.6 GHz over a 30 km fiber in the proposed system show receiver sensitivity improvement of ∼12 dB at the bit error rate (BER) of 10−3 compared with the conventional double sideband (DSB) modulation case. © 2015 Optical Society of America OCIS codes: (060.2360) Fiber optics links and subsystems; (060.5625) Radio frequency photonics; (350.4010) Microwaves; (130.4110) Modulators. http://dx.doi.org/10.1364/OL.40.002103
Radio-over-fiber (RoF) technologies have been considered as the efficient delivery of multimedia services to wireless base stations (BSs) for their wideband and low-loss of optical fiber transmission [1]. In a RoF system, the radio signal is usually modulated on an optical carrier based on double-sideband (DSB) modulation using a Mach–Zehnder modulator (MZM) [2]. However, optical DSB signals suffer from power fading induced by fiber chromatic dispersion (CD) [3]. The single sideband (SSB) and optical carrier suppression (OCS) modulations are used to eliminate the power fading [4–6]. SSB modulation can be achieved by using a dual-port external modulator with a nonideal 90° hybrid coupler, which introduces frequency dependent phase error [7]. Optical filters can also be used to remove one sideband. However, this technique would be difficult to implement for lower radio frequency (RF) services and limit the flexibility. On the other hand, OCS modulation is not suitable for vector-modulated signal transmission since the phase of the signal is doubled after the square-law detection. Recently, two techniques based on a dual-parallel MZM (DPMZM) [8] and an integrated modulator consisting of a polarization modulator (PolM) and a polarizer [9] were proposed to compensate for dispersion-induced power fading. However, the former cannot be efficiently adopted in multi-service RoF systems because each RF signal requires an individual DPMZM to shift the response to the maximum. The latter needs PolM, a special designed modulator that would restrict RoF-based optical and wireless access networks from low-cost and large-scale deployment. In this Letter, a simple method based on a phasecoherent orthogonal lightwave generator (POLG) [10] is proposed and experimentally demonstrated to 0146-9592/15/092103-04$15.00/0
overcome the dispersion-induced power fading. In contrast to the published research works, the new technique can be used widely in multi-service RoF systems, and the phase difference between the orthogonal polarization carrier and two sidebands of each service is flexibly adjusted by tuning a polarization controller (PC) placed in the BS. In this way, maximum frequency responses can be achieved simultaneously at multiple frequencies independent of the fiber length. We experimentally shift the destructive interference to the constructive interference at various frequencies for 25 km and 30 km standard single mode fiber (SSMF) links, and deliver two services carrying 1 Gb/s on–off keying (OOK) data around fading frequencies (9 GHz and 16.6 GHz) over a 30 km fiber. The experimental results show that, by employing the proposed method based on POLG, the receiver sensitivity at the bit error rates (BER) of 10−3 can be improved by nearly 12 dB after a 30 km transmission compared with a conventional DSB modulation case. Figure 1(a) illustrates the conceptual diagram of the proposed method to compensate power fading for multi-service RoF systems based on a simple POLG. At the central office (CO), the light emitted from a laser diode (LD) is injected into the POLG. POLG is the key device in our experiment which consists of a polarization rotator (PR) and a single-driver LiNbO3 MZM. The PR, which contains a quarter-wave plate and a rotatable polarizer, can align the light’s polarization direction at an angle of θ to the principal axis of LiNbO3 crystal of MZM [noted as the y-axis in Fig. 1(b)], as shown in Fig. 1(b1). Since the half-wave voltage V π in the x-direction is around 3.58 times that in the y-axis because of the electro-optical property of the LiNbO3 crystal [11], maximum modulation efficiency is achieved when the © 2015 Optical Society of America
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dispersive fiber link is given as Hjω expjzβ2 ω0 ∕2) [13]. Thus, we have 8 1 2 < θ0 2 zβ2 ω0 θ 1 zβ ω2 2ωm ω0 ω2m ; : 1m 12 2 0 θ−1m 2 zβ2 ω20 − 2ωm ω0 ω2m
Fig. 1. (a) Conceptual diagram of the proposed multi-service RoF system with power fading compensation based on simple POLG; (b) example of the spectrum evolution of location 1–4 for one service.
polarization direction of the input lightwave is parallel to the principal axis of MZM, and the modulation at x-axis is negligible. As shown in Fig. 1(a), the aggregate services on multiple frequency, ωm , m 1; 2; …N are electrically coupled and modulated on the MZM biased at the minimum transmission point. Thus, the component of injected light on the y-axis is modulated in the form of OCS while the x-axis component of the light is not modulated. As illustrated in Fig. 1(b2), for one service, −1st-order optical side bands will be generated, and the OC will be suppressed in the y-axis direction. Meanwhile, the OC in the x-axis direction will be reserved. As a result, the orthogonal lightwaves are generated. Naturally, the x-axis component is phase coherent with the y-axis component since it originates from the same light source. Note that a polarization maintaining fiber patch cord with a specially designed connector key [12] can be used to set θ at a certain angle in place of the PR to further reduce the cost of the system. For better observation and easier tuning, we use the PR in the experiment. Ignoring the modulation and polarization dependent loss in the x-axis, the optical field at the output of the MZM is X sin θ E t1 ; E 0 ejω0 t cos θJ 1 βm ejwm t e−jwm t m
(1)
where E 0 and ω0 are the amplitude and the angular frequency of the optical carrier. βm is the phase modulation index for each service, J n βm is the nst-order Bessel function of the first kind. Note that only the carrier and the 1st sidebands are considered under smallsignal modulation condition. After the dispersive fiber, the optical field can be expressed as E t2
X m
E 0 ejω0 t
jθ0
sin θe ; cos θJ 1 βejwm tjθ1m e−jwm tjθ−1m (2)
where θo , θ−1m , and θ1m are the dispersion-induced phase shifts of the optical carrier (OC) in the x-axis direction, the lower first-order and the upper first-order sideband of the signal at ωm in the y-axis direction, respectively [see Fig. 1(b3)]. The transfer function of a
3
where β2 Dλ2 ∕2πc and z is the transmission distance, D is the dispersion coefficient of the fiber (D −17 ps∕nm∕km for SSMF), c is the velocity of light, and λ is the wavelength of optical light. In the BS, for one path of the system, the signal is injected into a PC and a polarizer (Pol). The angle between the principal axis of the Pol and the principal axis of MZM is defined as α, and the phase difference between the orthogonal light waves is defined as φ. Note that α and φ can be controlled and adjusted by the PC. Assuming α 45°, as shown in Fig. 1(b4), the optical field at the output of polarizer can be expressed as 9 8 1 2 = < sin θejφj2zβ2 ω0 1 2 2 E 0 ejω0 t cos θJ 1 βejwm tj2zβ2 ω0 2ωm ω0 ωm E t3 ∝ ; : −jwm tj1zβ2 ω2 −2ωm ω0 ω2m m 2 0 e ) ( X jφj 12zβ2 ω20 cos θJ β jω0 t sin θe 1 E0 e : 1 2 2 ej2zβ2 ω0 ωm 2 cosωm t − zβ2 ωm ω0 m X
(4) Then the optical signal is sent to a photodetector (PD), and the photocurrent of the generated microwave signal can be expressed as iPD ∝
X m
sin 2θE 20 J 1 β cosωm t − zβ2 ωm ω0
1 × cos φ − zβ2 ω2m S m t; 2
(5)
where the first term is the desired signal and S m t denotes frequency terms that arise from beating among subcarriers. It can be seen that the generated current at ωm is in proportion to a coefficient η cosϕ − zβ2 ω2m ∕2 such that the current varies periodically as the square of the angular frequency ωm . To realize the maximum point of the frequency response of any frequency of interest, we should let 1 jηj cos φ − zβ2 ω2m 1; 2
(6)
that is, 1 φm − zβ2 ω2m kπ; 2
k 0; 1; 2…:
(7)
For one channel of multi-service RoF link, the frequency of the modulation signal (ωm ), the length of the link (z), and the dispersion coefficient of the fiber (β2 ) are known. The condition shown in Eq. (7) can be easily satisfied by changing the PC. As a result, this channel will have a maximum response around ωm .
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
Finally, signals are emitted to the user equipment (UE). By making use of the polarization dependence of the MZM, the special DSB modulation is implemented in which the polarization direction of the OC is orthogonal with that of the 1st-order sidebands. Because of the orthogonality of the carrier and sidebands, their phase differences can be tuned by the PC, and the compensation of power fading is consequently achieved for different services. The experimental setup for the POLG-based RoF system is depicted in Fig. 2(a). A continuous wave light originated from a LD with a wavelength of 1553.98 nm is fed into the POLG. Figures 3(a)–3(c) display optical spectra of the modulated signal when a 9 GHz microwave was applied to the MZM. As expected, the OC is reserved in the x-axis, while the OCS modulation is realized in the y-axis as shown in Figs. 3(a) and 3(b), respectively. Figure 3(c) shows the light’s polarization direction being set at θ 45° by adjusting the PR. After transmission over the fiber link, the optical signal is sent to the BS, including an erbium-doped fiber amplifier (EDFA), a variable optical attenuator (VOA), a PC, a Pol, and a PD with a 3 dB bandwidth of 40 GHz. The pre-amplifier is used in the experiment because of the high input power required by the 40 GHz PD. As a comparison, the experimental setup for the conventional DSB modulation is shown in Fig. 2(b), in which the MZM is biased at the quadrature point. First, the transmittance responses using the proposed compensation technique after 25 km and 30 km SSMF fiber links are measured by a vector network analyzer with 20 GHz bandwidth and depicted in dashed lines in Fig. 3(d)–Fig. 3(e), respectively. Frequency responses using conventional DSB modulation are also measured and depicted in solid lines. It can be observed that fading frequencies include 11.77 GHz in a 25 km link and 9.83/17.33 GHz in a 30 km link with DSB. By using the POLG technique and adjusting the PC in the BS, maximum responses are successfully acquired at above mentioned frequencies. Next, the 1 Gb/s OOK from the pseudo random binary sequence (PRBS) pattern generator is up-converted to 9 GHz, 9.5 GHz, or 16.6 GHz microwave frequencies and modulated onto the optical carrier successively. Because of the lack of wireless equipment at such frequencies, wireless transmission between the BS and the UE is not implemented in this experiment. The output of the BS is connected to a local oscillator (LO) with the same frequency at the UE to down-covert the OOK signal.
Fig. 2. Experimental setup for (a) a POLG-based RoF system (b) a conventional DSB-based RoF system.
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Fig. 3. (a)–(c) Optical spectra after the MZM for θ 90°, 0°, and 45°. (d) and (e) Frequency responses of 25 km and 30 km SSMF, respectively. The responses with a conventional DSB link are shown in solid lines and the ones with the proposed POLG-based link are shown in dashed lines.
The experimental BER measurements for the OOK signal at different microwave frequencies after 30 km transmission are illustrated in Fig. 4. The received optical power is measured before the EDFA. With the proposed dispersion compensation scheme, the BER performances are greatly improved for all three RF signals. Compared with the conventional DSB cases, the receiver sensitivity of the proposed method at the BER of 10−3 has a 12 dB improvement after the 30 km transmission. Furthermore, the three curves using POLG technique at 9/9.5/16.6 GHz show identical performances, indicating high stability and a frequency-independent feature of the proposed compensation technique. Eye diagrams are also provided in Fig. 4 for signals at 9/9.5/16.6 GHz in DSB and POLG links. Note that the BER performance of the signal at 9.5 GHz with conventional DSB modulation cannot be measured by the BER tester since it is severely impaired by the power fading effect.
Fig. 4. BER versus received power for OOK signal transmitted over 30 km fiber using conventional DSB modulation and POLG technique. Eye diagrams (20 ms/div) of signals at 9/9.5/16.6 GHz are inserted.
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can be shifted flexibly to any desired frequency independent of the fiber length. Therefore, the new method greatly increases the flexibility in choosing RFs and transmission distances, and ensures all signal achieve the best performance at the same time. The experimental BER performances for various RF signals indicate significant improvements using the proposed compensation method. This work was supported in part by the Georgia Institute of Technology and in part by the National Natural Science Foundation of China (NSFC) (Grant Nos. 61275091 and 61327006). The work of B. Wu was supported in part by the China Scholarship Council for Scholarship.
Fig. 5. Electrical spectra of the received OOK signal at (a) 9 GHz, (b) 9.5 GHz, (c) 9.8 GHz, and (d) 16.6 GHz using DSB modulation (black dashed lines) and POLG technique (blue solid lines).
Figure 5 shows the electrical spectra of the received RF signals at different frequencies after 30 km transmission. It can be observed clearly that without any compensation, spectra are notched around 9.83 GHz and 17.33 GHz, and directly affect the signal performances. As shown in Fig. 5(c), we also measure the spectrum of the signal carried at 9.8 GHz which suffers the most power loss because it is nearly centered at the lowest fading point. With dispersion compensation, by adjusting PC, spectra are recovered to maximum power for all frequencies. In conclusion, a simple technique based on phasecoherent orthogonal lightwave generator to overcome power fading for multi-service RoF systems is proposed and demonstrated. By changing the PC in each service link, the maximum point of the frequency response
References 1. T. Kuri and K. Kitayama, J. Lightwave Technol. 21, 3167 (2003). 2. Z. Jia, J. Yu, G. Ellinas, and G. K. Chang, J. Lightwave Technol. 25, 3452 (2007). 3. J. P. Yao, F. Zeng, and Q. Wang, J. Lightwave Technol. 25, 3219 (2007). 4. B. Hraimel, X. Zhang, Y. Pei, K. Wu, T. Liu, T. Xu, and Q. Nie, J. Lightwave Technol. 29, 775 (2011). 5. G. H. Smith, D. Novak, and Z. Ahmed, IEEE Trans. Microw. Theory Techn. 45, 1410 (1997). 6. J. Ma, J. Yu, C. Yu, X. Xin, J. Zeng, and L. Chen, J. Lightwave Technol. 25, 3244 (2007). 7. R. C. Lee, H. A. Abdul-Rashid, M. T. Al-Qdah, H. T. Chuah, M. B. Tayahi, and S. Lanka, Microw. Opt. Technol. Lett. 48, 1574 (2006). 8. S. Y. Li, X. P. Zheng, H. Y. Zhang, and B. K. Zhou, Opt. Lett. 36, 546 (2011). 9. H. Zhang, S. Pan, M. Huang, and X. Chen, Opt. Lett. 37, 866 (2012). 10. J. Zheng, J. Wang, J. Yu, M. Zhu, Z. Dong, X. Wang, T. Su, J. Liu, N. Zhu, and G. K. Chang, Opt. Lett. 39, 5263 (2014). 11. M. Lawrence, Rep. Prog. Phys. 56, 363 (1993). 12. J. Zheng, H. Wang, W. Li, L. Wang, T. Su, J. Liu, and N. Zhu, Opt. Lett. 39, 1366 (2014). 13. X. Qi, J. Liu, X. Zhang, and X. Xie, IEEE J. Quantum Electron. 46, 1170 (2010).
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Flexible compensation of dispersion-induced power fading for multi-service RoF links based on a phase-coherent orthogonal lightwave generator Beilei Wu,1,2 Ming Zhu,2 Mu Xu,2 Jing Wang,2 Muguang Wang,1 Fengping Yan,1,* Shuisheng Jian,1 and Gee-Kung Chang2 1
Institute of Lightwave Technology, Key Lab of All Optical Network and Advanced Telecommunication of EMC, Beijing Jiaotong University, Beijing, China 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA *Corresponding author: [email protected] Received March 18, 2015; revised April 7, 2015; accepted April 11, 2015; posted April 14, 2015 (Doc. ID 236292); published April 30, 2015
A novel technique to simultaneously compensate dispersion-induced power fading for multi-service radio-overfiber (RoF) links is proposed. At the central office (CO), a phase-coherent orthogonal lightwave generator (POLG) consisting of a polarization rotator (PR) and a single-driver Mach–Zehnder modulator (MZM) is used. By adjusting a polarization controller (PC) in the base station (BS), the phase difference between the orthogonal polarization carrier and two sidebands can be controlled, and the frequency response can be flexibly shifted for individual radio frequency (RF) service. We experimentally shift the destructive interference to the constructive interference at various frequencies for 25 km and 30 km fiber links. The performances of two services carrying 1 Gb/s on–off keying (OOK) data at 9 GHz and 16.6 GHz over a 30 km fiber in the proposed system show receiver sensitivity improvement of ∼12 dB at the bit error rate (BER) of 10−3 compared with the conventional double sideband (DSB) modulation case. © 2015 Optical Society of America OCIS codes: (060.2360) Fiber optics links and subsystems; (060.5625) Radio frequency photonics; (350.4010) Microwaves; (130.4110) Modulators. http://dx.doi.org/10.1364/OL.40.002103
Radio-over-fiber (RoF) technologies have been considered as the efficient delivery of multimedia services to wireless base stations (BSs) for their wideband and low-loss of optical fiber transmission [1]. In a RoF system, the radio signal is usually modulated on an optical carrier based on double-sideband (DSB) modulation using a Mach–Zehnder modulator (MZM) [2]. However, optical DSB signals suffer from power fading induced by fiber chromatic dispersion (CD) [3]. The single sideband (SSB) and optical carrier suppression (OCS) modulations are used to eliminate the power fading [4–6]. SSB modulation can be achieved by using a dual-port external modulator with a nonideal 90° hybrid coupler, which introduces frequency dependent phase error [7]. Optical filters can also be used to remove one sideband. However, this technique would be difficult to implement for lower radio frequency (RF) services and limit the flexibility. On the other hand, OCS modulation is not suitable for vector-modulated signal transmission since the phase of the signal is doubled after the square-law detection. Recently, two techniques based on a dual-parallel MZM (DPMZM) [8] and an integrated modulator consisting of a polarization modulator (PolM) and a polarizer [9] were proposed to compensate for dispersion-induced power fading. However, the former cannot be efficiently adopted in multi-service RoF systems because each RF signal requires an individual DPMZM to shift the response to the maximum. The latter needs PolM, a special designed modulator that would restrict RoF-based optical and wireless access networks from low-cost and large-scale deployment. In this Letter, a simple method based on a phasecoherent orthogonal lightwave generator (POLG) [10] is proposed and experimentally demonstrated to 0146-9592/15/092103-04$15.00/0
overcome the dispersion-induced power fading. In contrast to the published research works, the new technique can be used widely in multi-service RoF systems, and the phase difference between the orthogonal polarization carrier and two sidebands of each service is flexibly adjusted by tuning a polarization controller (PC) placed in the BS. In this way, maximum frequency responses can be achieved simultaneously at multiple frequencies independent of the fiber length. We experimentally shift the destructive interference to the constructive interference at various frequencies for 25 km and 30 km standard single mode fiber (SSMF) links, and deliver two services carrying 1 Gb/s on–off keying (OOK) data around fading frequencies (9 GHz and 16.6 GHz) over a 30 km fiber. The experimental results show that, by employing the proposed method based on POLG, the receiver sensitivity at the bit error rates (BER) of 10−3 can be improved by nearly 12 dB after a 30 km transmission compared with a conventional DSB modulation case. Figure 1(a) illustrates the conceptual diagram of the proposed method to compensate power fading for multi-service RoF systems based on a simple POLG. At the central office (CO), the light emitted from a laser diode (LD) is injected into the POLG. POLG is the key device in our experiment which consists of a polarization rotator (PR) and a single-driver LiNbO3 MZM. The PR, which contains a quarter-wave plate and a rotatable polarizer, can align the light’s polarization direction at an angle of θ to the principal axis of LiNbO3 crystal of MZM [noted as the y-axis in Fig. 1(b)], as shown in Fig. 1(b1). Since the half-wave voltage V π in the x-direction is around 3.58 times that in the y-axis because of the electro-optical property of the LiNbO3 crystal [11], maximum modulation efficiency is achieved when the © 2015 Optical Society of America
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OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
dispersive fiber link is given as Hjω expjzβ2 ω0 ∕2) [13]. Thus, we have 8 1 2 < θ0 2 zβ2 ω0 θ 1 zβ ω2 2ωm ω0 ω2m ; : 1m 12 2 0 θ−1m 2 zβ2 ω20 − 2ωm ω0 ω2m
Fig. 1. (a) Conceptual diagram of the proposed multi-service RoF system with power fading compensation based on simple POLG; (b) example of the spectrum evolution of location 1–4 for one service.
polarization direction of the input lightwave is parallel to the principal axis of MZM, and the modulation at x-axis is negligible. As shown in Fig. 1(a), the aggregate services on multiple frequency, ωm , m 1; 2; …N are electrically coupled and modulated on the MZM biased at the minimum transmission point. Thus, the component of injected light on the y-axis is modulated in the form of OCS while the x-axis component of the light is not modulated. As illustrated in Fig. 1(b2), for one service, −1st-order optical side bands will be generated, and the OC will be suppressed in the y-axis direction. Meanwhile, the OC in the x-axis direction will be reserved. As a result, the orthogonal lightwaves are generated. Naturally, the x-axis component is phase coherent with the y-axis component since it originates from the same light source. Note that a polarization maintaining fiber patch cord with a specially designed connector key [12] can be used to set θ at a certain angle in place of the PR to further reduce the cost of the system. For better observation and easier tuning, we use the PR in the experiment. Ignoring the modulation and polarization dependent loss in the x-axis, the optical field at the output of the MZM is X sin θ E t1 ; E 0 ejω0 t cos θJ 1 βm ejwm t e−jwm t m
(1)
where E 0 and ω0 are the amplitude and the angular frequency of the optical carrier. βm is the phase modulation index for each service, J n βm is the nst-order Bessel function of the first kind. Note that only the carrier and the 1st sidebands are considered under smallsignal modulation condition. After the dispersive fiber, the optical field can be expressed as E t2
X m
E 0 ejω0 t
jθ0
sin θe ; cos θJ 1 βejwm tjθ1m e−jwm tjθ−1m (2)
where θo , θ−1m , and θ1m are the dispersion-induced phase shifts of the optical carrier (OC) in the x-axis direction, the lower first-order and the upper first-order sideband of the signal at ωm in the y-axis direction, respectively [see Fig. 1(b3)]. The transfer function of a
3
where β2 Dλ2 ∕2πc and z is the transmission distance, D is the dispersion coefficient of the fiber (D −17 ps∕nm∕km for SSMF), c is the velocity of light, and λ is the wavelength of optical light. In the BS, for one path of the system, the signal is injected into a PC and a polarizer (Pol). The angle between the principal axis of the Pol and the principal axis of MZM is defined as α, and the phase difference between the orthogonal light waves is defined as φ. Note that α and φ can be controlled and adjusted by the PC. Assuming α 45°, as shown in Fig. 1(b4), the optical field at the output of polarizer can be expressed as 9 8 1 2 = < sin θejφj2zβ2 ω0 1 2 2 E 0 ejω0 t cos θJ 1 βejwm tj2zβ2 ω0 2ωm ω0 ωm E t3 ∝ ; : −jwm tj1zβ2 ω2 −2ωm ω0 ω2m m 2 0 e ) ( X jφj 12zβ2 ω20 cos θJ β jω0 t sin θe 1 E0 e : 1 2 2 ej2zβ2 ω0 ωm 2 cosωm t − zβ2 ωm ω0 m X
(4) Then the optical signal is sent to a photodetector (PD), and the photocurrent of the generated microwave signal can be expressed as iPD ∝
X m
sin 2θE 20 J 1 β cosωm t − zβ2 ωm ω0
1 × cos φ − zβ2 ω2m S m t; 2
(5)
where the first term is the desired signal and S m t denotes frequency terms that arise from beating among subcarriers. It can be seen that the generated current at ωm is in proportion to a coefficient η cosϕ − zβ2 ω2m ∕2 such that the current varies periodically as the square of the angular frequency ωm . To realize the maximum point of the frequency response of any frequency of interest, we should let 1 jηj cos φ − zβ2 ω2m 1; 2
(6)
that is, 1 φm − zβ2 ω2m kπ; 2
k 0; 1; 2…:
(7)
For one channel of multi-service RoF link, the frequency of the modulation signal (ωm ), the length of the link (z), and the dispersion coefficient of the fiber (β2 ) are known. The condition shown in Eq. (7) can be easily satisfied by changing the PC. As a result, this channel will have a maximum response around ωm .
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
Finally, signals are emitted to the user equipment (UE). By making use of the polarization dependence of the MZM, the special DSB modulation is implemented in which the polarization direction of the OC is orthogonal with that of the 1st-order sidebands. Because of the orthogonality of the carrier and sidebands, their phase differences can be tuned by the PC, and the compensation of power fading is consequently achieved for different services. The experimental setup for the POLG-based RoF system is depicted in Fig. 2(a). A continuous wave light originated from a LD with a wavelength of 1553.98 nm is fed into the POLG. Figures 3(a)–3(c) display optical spectra of the modulated signal when a 9 GHz microwave was applied to the MZM. As expected, the OC is reserved in the x-axis, while the OCS modulation is realized in the y-axis as shown in Figs. 3(a) and 3(b), respectively. Figure 3(c) shows the light’s polarization direction being set at θ 45° by adjusting the PR. After transmission over the fiber link, the optical signal is sent to the BS, including an erbium-doped fiber amplifier (EDFA), a variable optical attenuator (VOA), a PC, a Pol, and a PD with a 3 dB bandwidth of 40 GHz. The pre-amplifier is used in the experiment because of the high input power required by the 40 GHz PD. As a comparison, the experimental setup for the conventional DSB modulation is shown in Fig. 2(b), in which the MZM is biased at the quadrature point. First, the transmittance responses using the proposed compensation technique after 25 km and 30 km SSMF fiber links are measured by a vector network analyzer with 20 GHz bandwidth and depicted in dashed lines in Fig. 3(d)–Fig. 3(e), respectively. Frequency responses using conventional DSB modulation are also measured and depicted in solid lines. It can be observed that fading frequencies include 11.77 GHz in a 25 km link and 9.83/17.33 GHz in a 30 km link with DSB. By using the POLG technique and adjusting the PC in the BS, maximum responses are successfully acquired at above mentioned frequencies. Next, the 1 Gb/s OOK from the pseudo random binary sequence (PRBS) pattern generator is up-converted to 9 GHz, 9.5 GHz, or 16.6 GHz microwave frequencies and modulated onto the optical carrier successively. Because of the lack of wireless equipment at such frequencies, wireless transmission between the BS and the UE is not implemented in this experiment. The output of the BS is connected to a local oscillator (LO) with the same frequency at the UE to down-covert the OOK signal.
Fig. 2. Experimental setup for (a) a POLG-based RoF system (b) a conventional DSB-based RoF system.
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Fig. 3. (a)–(c) Optical spectra after the MZM for θ 90°, 0°, and 45°. (d) and (e) Frequency responses of 25 km and 30 km SSMF, respectively. The responses with a conventional DSB link are shown in solid lines and the ones with the proposed POLG-based link are shown in dashed lines.
The experimental BER measurements for the OOK signal at different microwave frequencies after 30 km transmission are illustrated in Fig. 4. The received optical power is measured before the EDFA. With the proposed dispersion compensation scheme, the BER performances are greatly improved for all three RF signals. Compared with the conventional DSB cases, the receiver sensitivity of the proposed method at the BER of 10−3 has a 12 dB improvement after the 30 km transmission. Furthermore, the three curves using POLG technique at 9/9.5/16.6 GHz show identical performances, indicating high stability and a frequency-independent feature of the proposed compensation technique. Eye diagrams are also provided in Fig. 4 for signals at 9/9.5/16.6 GHz in DSB and POLG links. Note that the BER performance of the signal at 9.5 GHz with conventional DSB modulation cannot be measured by the BER tester since it is severely impaired by the power fading effect.
Fig. 4. BER versus received power for OOK signal transmitted over 30 km fiber using conventional DSB modulation and POLG technique. Eye diagrams (20 ms/div) of signals at 9/9.5/16.6 GHz are inserted.
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OPTICS LETTERS / Vol. 40, No. 9 / May 1, 2015
can be shifted flexibly to any desired frequency independent of the fiber length. Therefore, the new method greatly increases the flexibility in choosing RFs and transmission distances, and ensures all signal achieve the best performance at the same time. The experimental BER performances for various RF signals indicate significant improvements using the proposed compensation method. This work was supported in part by the Georgia Institute of Technology and in part by the National Natural Science Foundation of China (NSFC) (Grant Nos. 61275091 and 61327006). The work of B. Wu was supported in part by the China Scholarship Council for Scholarship.
Fig. 5. Electrical spectra of the received OOK signal at (a) 9 GHz, (b) 9.5 GHz, (c) 9.8 GHz, and (d) 16.6 GHz using DSB modulation (black dashed lines) and POLG technique (blue solid lines).
Figure 5 shows the electrical spectra of the received RF signals at different frequencies after 30 km transmission. It can be observed clearly that without any compensation, spectra are notched around 9.83 GHz and 17.33 GHz, and directly affect the signal performances. As shown in Fig. 5(c), we also measure the spectrum of the signal carried at 9.8 GHz which suffers the most power loss because it is nearly centered at the lowest fading point. With dispersion compensation, by adjusting PC, spectra are recovered to maximum power for all frequencies. In conclusion, a simple technique based on phasecoherent orthogonal lightwave generator to overcome power fading for multi-service RoF systems is proposed and demonstrated. By changing the PC in each service link, the maximum point of the frequency response
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