- Email: [email protected]
s RoF downlink using stimulated Brillouin scattering
Optics Communications 290 (2013) 158–162
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Experimental demonstration of tunable optical single sideband modulation and 1.5 Gb/s RoF downlink using stimulated Brillouin scattering Yi Qin, Junqiang Sun n, Mingdi Du, Jianfei Liao Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2012 Received in revised form 13 October 2012 Accepted 15 October 2012 Available online 6 November 2012
A frequency-tunable optical radio-frequency signal generation and broadband data upconversion technique with single sideband modulation for radio-over-fiber (RoF) systems is experimentally demonstrated. The proposed technique is based on optical processing an optical double sideband (DSB) modulated signal by using double stimulated Brillouin scattering interaction. On the one hand, a dual-electrode Mach–Zehnder modulator is used to realize broadband data upconversion. On the other hand, a fiber Bragg grating is applied to improve the optical carrier suppression ratio and avoid the effect of residual optical harmonics. By using this method, a broadband radio-frequency signal range from 5 to 40 GHz with the undesired sideband suppression ratio over 58 dB is experimentally demonstrated. In addition, the transmission performance of the 1.5 Gb/s RoF downlink is also examined. The power penalty is less than 1 dB at the bit-error rate of 10 10 after 50 km single-mode fiber transmission. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Tunable single sideband modulation Dual-electrode Mach–Zehnder modulator Fiber Bragg grating Stimulated Brillouin scattering Radio over fiber
1. Introduction With the development of communication technology, radioover-fiber (RoF) systems have attracted considerable attention since they have many advantages such as enhanced microcellular coverage, higher capacity, lower cost, lower power, and easier installation [1–3]. These systems need high-frequency microwaves to deliver high-speed signals. Many techniques by using standard optical double-sideband (DSB) modulation to generate high-frequency microwaves have been reported. In [4], an optical interleaver was utilized to separate the spectrum of the DSB signals into first-order sideband modes and the optical carrier so that to generate 40 GHz optical millimeter-wave. Similar technique based on frequency quadrupling was also proposed [5]. These DSB modulation techniques generate two symmetric signal sidebands on both sides of the optical carrier. The fiber dispersion causes a walkoff in the relative phases of the sidebands, resulting in power penalty of the detected radio frequency (RF) signal [6]. Since the use of single-sideband (SSB) modulation can alleviate the dispersion power fading, various methods have been proposed to generate SSB modulated for RoF systems such as filtering out one of the sidebands [7,8], using the strong optical injectionlocked semiconductor lasers [9], or using the SSB modulator [10]. However, they are limited to the low level of receiver sensitivity
n
Corresponding author. E-mail address: [email protected] (J. Sun).
originating from a large difference in the power of the strong optical carrier and the weak sideband. Stimulated Brillouin scattering (SBS) has mostly been recognized as one of the most severe nonlinear impairments in fiberoptic networks [11] due to the facts that the signal energy is transferred to the backscattering signal and SBS has low threshold [12]. However, SBS has beneficial characteristics such as frequency selective amplification that can be applied to microwave photonics applications [13]. Shen et al. demonstrated the 11 GHz RoF system carrying the 10 Mb/s data using SBS [14], but the data bandwidth was restricted by the Brillouin gain profile. In [15], it was proposed an improved technique based on the simultaneous use of Brillouin gain and loss of the pump wave, amplifying one of the DSB modulated sidebands while attenuating the other one. In [16], a scheme with SSB modulation based on SBS was proposed to achieve 1.25 Gb/s data upconversion. Recently, a harmonic RF carrier generation and broadband data upconversion technique with SSB modulation using SBS was demonstrated [17]. However, the main limitation of these methods is that the generated RF signals are restricted to the Brillouin frequency [16] or the multiple of the Brillouin frequency [17]. In this paper, a frequency-tunable SBS-based optical SSB RF signal generation and broadband data upconversion technique for RoF systems is demonstrated. In principle, the technique works for arbitrarily high frequency of the RF signal, which is only limited by the bandwidths of the optical transmitter and receiver employed. On the one hand, in order to realize broadband data
0030-4018/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.10.045
Y. Qin et al. / Optics Communications 290 (2013) 158–162
upconversion, we use a dual-electrode Mach–Zehnder modulator (DEMZM) to carry the data signal on the optical carrier instead of carrying on the selectively amplified sideband [14]. This method ensures that the broadband data rate can be carried and independent of the Brillouin gain bandwidth. On the other hand, we use a fiber Bragg grating (FBG) to improve the optical carrier suppression ratio (CSR) so as to avoid the residual optical harmonics affect to the experimental results. In addition, frequency-tunable RF signal generation with SSB modulation is performed by using SBS with three cases of configuration. The detailed principle will be discussed in the next section. Finally, the 1.5 Gb/s downlink signal is examined. The power penalty is less than 1 dB at the BER of 10 10 after 50 km standard singlemode fiber (SSMF) transmission. This paper is organized as follows. In Section 2, the principle of the proposed technique is described. In Section 3, the experiment is built and demonstrated so as to verify the proposed technique. Finally, the conclusion is given in Section 4.
2. Operation principle As described by Agrawal [12], SBS generates both Brillouin gain and loss. Assume the Brillouin frequency shift is fB and the pump wave frequency is fPUMP, then the central frequencies of the Brillouin gain and loss profiles are fPUMP fB and fPUMP þfB, respectively. As a result, the Stokes wave is amplified when it is located at the Brillouin gain profile of the pump wave. Meanwhile, the anti-Stokes wave is consequently attenuated when it is located at the Brillouin loss profile of the pump wave. Fig. 1 shows the principle of the SSB modulation using SBS with three cases of configuration. The SSB modulation is realized by counterpropagating a DSB signal and a DSB–suppressed carrier (DSB-SC) signal in a length of standard single-mode fiber (SSMF) and exciting double SBS effect. During the process, the DSB-SC signal is acted
2
159
as pump waves; one sideband of the DSB signal is acted as Stokes wave and the other sideband is acted as anti-Stokes wave. Assume the local RF signal frequency is fRF and the optical carrier frequency is f0, then fp ¼9fPUMP f09. In case I, when fp 4fRF and fp ¼ fB þ fRF, the upper frequency sideband of DSB is located at the Brillouin gain profile of the pump wave and amplified by the Brillouin gain profile. Meantime, the lower frequency sideband is correspondingly located at the Brillouin loss profile of the pump wave and attenuated by the Brillouin loss profile. In such process, a SSB modulation is achieved, as illustrated in Fig. 1. In case II, when fp ofRF and fp ¼fRF fB, the upper frequency sideband of DSB is selected by the Brillouin loss profile of the pump wave and attenuated by the Brillouin loss profile, while the lower frequency sideband is selected by the Brillouin gain profile of the pump wave and amplified by the Brillouin gain profile. Then, the other kind of SSB modulation is obtained. In case III, when fp 4fRF and fp ¼ fB fRF, by using the same principle, the same result also can be achieved. Since the SBS is only imparted on the two modulation sidebands, the optical carrier is not affected by the SBS. Therefore, we use the DEMZM to carry the data signal on the optical carrier and the data bandwidth is not restricted by the Brillouin bandwidth. In principle, the proposed technique works for arbitrarily high frequency of the RF signal, which is only limited by the bandwidths of the optical transmitter and receiver employed. Furthermore, the RF signal frequency can be easily tuned by adjusting the frequency of fRF and fP, as described in Fig. 1. 3. Experiments and results In order to verify the proposed technique, an experimental proof is conducted and an experimental setup is shown in Fig. 2. First of all, we use the Case I scheme. A continuous wave (CW) light source at 1550.04 nm (f0 ¼193.5 THz) with a 3 dBm of
2
2
Fig. 1. Principle of the proposed technique using SBS with three cases of configuration. SBS process is performed by counterpropagating DSB signal and DSB-SC signal (purple line) in a length of SSMF. (a) Unprocessed DSB signal. (b) One sideband of the DSB signal is selected and amplified by Brillouin gain while the other sideband is corresponding attenuated by Brillouin loss. (c) Obtained SSB signal.
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CW
OC1 2
MZM
EDFA
FBG
PC
1 3
fe
PPG
PC
DEMZM
Central Station
11.9 km SSMF
3
Phase Shifter PD ED
OC2
2
ISO
f RF
1
EDFA
50 km SSMF
LPF DC-block
Mixer
Base Station Fig. 2. Experimental setup. CW: continuous wave; DEMZM: dual-electrode MZM; OC: optical circulator; PC: polarization controller; FBG: fiber Bragg grating; EDFA: erbium-doped fiber amplifier; PPG: pulse pattern generator; ISO: optical isolator; SSMF: standard single-mode fiber; PD: Photodiode; DC-block: DC-block capacitor; ED: Error detector.
Fig. 4. Reflective (dashed line) and transparent (solid line) response of the used FBG. Fig. 3. (a) Optical spectrum before (dashed line) and after (solid line) filtered by FBG. The dashed line is the output optical spectrum of MZM. The solid line is DSBSC signal. The optical sidebands at f0 þ 2fe and f0 2fe is acted as pump waves. The optical carrier at f0 is injected to the DEMZM. The output power of the optical source is 3 dBm, f0 ¼193.5 THz (1550.04 nm), fe ¼10.266 GHz. (b) Optical spectrum of the SSB signal (solid line) and the original DSB signal (dashed line) for 10 GHz RF signal. The original DSB signal is produced by DEMZM. The optical power of the pump waves injected into SSMF is 15 dBm.
output optical power is modulated by a MZM, which is driven by an fe ¼10.266 GHz electrical signal. The MZM is biased to completely suppress the odd-order optical sidebands [18]. Thus, three optical sidebands, i.e., f0, f0 þ2fe and f0 2fe, are obtained, as seen in the dashed line in Fig. 3(a) (comparing with Fig. 1, fP ¼ 2fe). Then they are divided into two branches by FBG with the reflective (dashed line) and transparent (solid line) response shown in Fig. 4. The optical sideband at f0 is reflected to one branch and
injected to the DEMZM. Each electrode of the DEMZM is driven by the fRF ¼10 GHz signal and the pulse pattern generator of the 1.5 Gb/s 231-1 pseudorandom bit sequence (PRBS) data signal, respectively. The biasing point is at Vp/2. Then the data signal has been carried on the optical carrier, as shown in Fig. 3(b) (dashed line). Now, the DSB modulation signal has been achieved. Meanwhile, the optical sidebands at f0 þ 2fe and f0 2fe go though the other branch, which act as the pump waves. The solid line in Fig. 3(a) shows the optical spectrum of the pump waves. The optical CSR is shown to be 38 dB below the sidebands. The suppression ratio is sufficient enough to avoid having the residual optical carrier which induce any significant Brillouin gain or loss. Then the pump waves are amplified by erbium-doped fiber amplifier (EDFA) to increase the optical power beyond the Brillouin threshold.
Y. Qin et al. / Optics Communications 290 (2013) 158–162
161
Fig. 5. Electrical spectra of (a) upconverted 10 GHz RF signal and (b) downconverted 1.5 Gb/s data signal.
70 CaseI
68
CaseII
CaseIII
USSR (dB)
66 64 62 60 58 56
Fig. 6. Measured (dots) and predicted (solid lines) BER of the RoF downlink system. Insets are the eye diagram of (a) the BTB, (b) the 50 km transmission case with the optimized polarization, (c) the BTB case with the pump polarization mismatch, and (d) the 50 km transmission case with the pump polarization mismatch.
An optical circulator (OC2) is used to counter-propagate the pump waves and the Stokes waves in the 11.9 km SSMF. The optical power of the pump waves injected into SSMF is 15 dBm. An optical isolator is added to ensure unidirectional transmission. The polarization controllers (PC) are added to optimize the polarization state. For the 1550.04 nm pump wave, the Brillouin frequency (fB) is measured to be 10.532 GHz for the 11.9 km SSMF used in the experiment. For the Case I scheme, when the fRF ¼10 GHz, the driven signal of the MZM should be fe ¼10.266 GHz. From the solid line in Fig. 3(b), measured at the port 3 of the optical circulator, we can see that the upper frequency sideband of the DSB modulation signal is amplified by the optical sideband at f0 þ2fe, while the lower frequency sideband of the DSB modulation signal is consequently attenuated by the optical sideband at f0 2fe in the SBS interaction. The SSB modulation signal can be clearly seen with 65 dB undesired sideband suppression ratio (USSR) (i.e., the power ratio of the amplified sideband and the attenuated sideband). Compared with the dashed line in Fig. 3(b), we can see that the optical carrier is not affected by the SBS. The SSB modulation signal generated from the central station is then transmitted to the base station through 50 km SSMF. Once the optical spectrum of the solid line in Fig. 3(b) is detected by the photodiode, it generates the upconverted RF spectrum of the amplitude modulated signal as shown in Fig. 5(a). It can be clearly seen that the strong peaks separated 1.5 GHz from the 10 GHz RF signal. To downconvert the optically upconverted 1.5 Gb/s data to the baseband, a mixer is used with a phase shifter controlling the phase of the RF signal, as shown in Fig. 2. An electrical low-pass filter (LPF) of the 3 GHz cutoff
0
10
20 fRF (GHz)
30
40
Fig. 7. Measured USSR vs fRF for the fixed pump power of 15 dBm. Different cases of SSB modulation configuration were used in the measurement.
frequency is attached to the IF port of the mixer to remove the high-frequency noise after the downconversion. Fig. 5(b) shows the downconverted 1.5 Gb/s data, which has the peaks of the 1.5 GHz separation, and the sidelobes of the 1.5 Gb/s data do not hamper the system performance. Fig. 6 shows the measured bit-error rate (BER) of the RoF downlink system. The power penalty is less than 1 dB at the BER of 10 10 after 50 km single-mode fiber transmission compared with the back-to-back (BTB) case. A small amount of noise is added after the 50 km fiber transmission [Fig. 6(b)] compared to the BTB case [Fig. 6(a)]. Fig. 6(c) and (d) shows the eye diagram of the BTB and the 50 km transmission case with the pump polarization mismatch. In the SBS process, the Brillouin gain or loss can be affected by the polarization state of the pump wave [19,20]. As compared with Fig. 6(a) and (c), the polarization state of the pump wave should be optimized to reduce the intensity noise involved in the recovered signal since the degree of the signal deterioration by the intensity noise and the overall signal intensity are changed depending on the polarization state of the pump wave during the experiment [19–21]. We also perform the other Cases of configuration. Fig. 7 shows the USSR versus fRF for the fixed pump power of 15 dBm. More than 58 dB of USSR is achieved from 5 to 40 GHz. The measurement from 5 to 40 GHz is limited by the 0.02 nm resolution bandwidth of the optical spectrum analyzer and the bandwidth of the DEMZM. In principle, arbitrarily high frequency of the RF signal with SSB modulation could be achieved by the proposed technique if high-frequency devices could be available.
4. Conclusion In this paper, an effective frequency-tunable SBS-based optical SSB RF signal generation and broadband data upconversion
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technique for RoF systems has been proposed and experimentally demonstrated. The proposed technique utilizes two MZMs, one is the DEMZM used to carry the broadband data rate on the optical carrier, the other one is biased to completely suppress the oddorder optical sidebands so that to utilize the low-frequency lower-cost driving signal to achieve the high-frequency signal. Furthermore, a FBG is used to improve the CSR and avoid the residual optical harmonics affect to the experimental results. The experiments show the RF signal frequency can be easily tuned by adjusting the frequency of fRF and fP, as described in Fig. 1. The transmission performance of the 1.5 Gb/s RoF downlink has also been examined. The power penalty is less than 1 dB at the BER of 10 10 after 50 km single-mode fiber transmission.
Acknowledgement This work is supported by the National Natural Science Foundation of China under Grant No. 61178002. References [1] J.C. Lin, J. Chen, P. Peng, C. Peng, W. Peng, B. Chiou, S. Chi, IEEE Photonics Technology Letters 19 (2007) 610. [2] A. Martinez, V. Polo, J. Marti, IEEE Transactions on Microwave Theory and Techniques 49 (2001) 2018.
[3] C. Lim, A. Nirmalathas, D. Novak, R. Waterhouse, G. Yoffe, Journal of Lightwave Technology 18 (2000) 1355. [4] L. Chen, H. Wen, S. Wen, IEEE Photonics Technology Letters 18 (2006) 2056. [5] J. Yu, Z. Jia, T. Wang, G.K. Chang, IEEE Photonics Technology Letters 19 (2007) 1499. [6] H. Al-Raweshidy, S. Komaki, Radio Over Fiber Technologies for Mobile Communication Networks, Artech House, Boston, MA, 2002. [7] K. Yonenaga, N. Takachio, IEEE Photonics Technology Letters 5 (1993) 949. [8] J. Yu, M. Huang, Z. Jia, T. Wang, G.K. Chang, IEEE Photonics Technology Letters 20 (2008) 478. [9] H.K. Sung, E.K. Lau, M.C. Wu, IEEE Photonics Technology Letters 19 (2007) 1005. [10] G.H. Smith, D. Novak, Z. Ahmed, IEEE Transactions on Microwave Theory and Techniques 45 (1997) 1410. [11] M. Sauer, A. Kobyakov, A. Boh Ruffin, IEEE Photonics Technology Letters 19 (2007) 1487. [12] G.P. Agrawal, Nonlinear Fiber Optics. New York, 2007. [13] X.S. Yao, IEEE Photonics Technology Letters 12 (2000) 1382. [14] Y. Shen, X. Zhang, K. Chen, IEEE Photonics Technology Letters 17 (2005) 1277. [15] M. Sagues, A. Loayssa, Optics Express 18 (2010) 17555. [16] C.S. Park, C.G. Lee, C.-S. Park, IEEE Photonics Technology Letters 19 (2007) 1828. [17] W. Li, N.H. Zhu, L.X. Wang, Optics Communication 284 (2011) 3437. [18] M. Mohamed, X. Zhang, B. Hraimel, K. Wu, Optics Express 16 (2008) 10141. [19] M. Oskar, V. Deventer, A.J. Boot, Journal of Lightwave Technology 12 (1994) 585. [20] A. Zadok, E. Zilka, A. Eyal, L. The´venaz, M. Tur, Optics Express 16 (2008) 21692. [21] J. Zhang, and M.R. Phillips, in Proceedings OFC 2005, Anaheim, CA, 2005, Paper PDP24.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Experimental demonstration of tunable optical single sideband modulation and 1.5 Gb/s RoF downlink using stimulated Brillouin scattering Yi Qin, Junqiang Sun n, Mingdi Du, Jianfei Liao Wuhan National Laboratory for Optoelectronics, School of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 April 2012 Received in revised form 13 October 2012 Accepted 15 October 2012 Available online 6 November 2012
A frequency-tunable optical radio-frequency signal generation and broadband data upconversion technique with single sideband modulation for radio-over-fiber (RoF) systems is experimentally demonstrated. The proposed technique is based on optical processing an optical double sideband (DSB) modulated signal by using double stimulated Brillouin scattering interaction. On the one hand, a dual-electrode Mach–Zehnder modulator is used to realize broadband data upconversion. On the other hand, a fiber Bragg grating is applied to improve the optical carrier suppression ratio and avoid the effect of residual optical harmonics. By using this method, a broadband radio-frequency signal range from 5 to 40 GHz with the undesired sideband suppression ratio over 58 dB is experimentally demonstrated. In addition, the transmission performance of the 1.5 Gb/s RoF downlink is also examined. The power penalty is less than 1 dB at the bit-error rate of 10 10 after 50 km single-mode fiber transmission. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Tunable single sideband modulation Dual-electrode Mach–Zehnder modulator Fiber Bragg grating Stimulated Brillouin scattering Radio over fiber
1. Introduction With the development of communication technology, radioover-fiber (RoF) systems have attracted considerable attention since they have many advantages such as enhanced microcellular coverage, higher capacity, lower cost, lower power, and easier installation [1–3]. These systems need high-frequency microwaves to deliver high-speed signals. Many techniques by using standard optical double-sideband (DSB) modulation to generate high-frequency microwaves have been reported. In [4], an optical interleaver was utilized to separate the spectrum of the DSB signals into first-order sideband modes and the optical carrier so that to generate 40 GHz optical millimeter-wave. Similar technique based on frequency quadrupling was also proposed [5]. These DSB modulation techniques generate two symmetric signal sidebands on both sides of the optical carrier. The fiber dispersion causes a walkoff in the relative phases of the sidebands, resulting in power penalty of the detected radio frequency (RF) signal [6]. Since the use of single-sideband (SSB) modulation can alleviate the dispersion power fading, various methods have been proposed to generate SSB modulated for RoF systems such as filtering out one of the sidebands [7,8], using the strong optical injectionlocked semiconductor lasers [9], or using the SSB modulator [10]. However, they are limited to the low level of receiver sensitivity
n
Corresponding author. E-mail address: [email protected] (J. Sun).
originating from a large difference in the power of the strong optical carrier and the weak sideband. Stimulated Brillouin scattering (SBS) has mostly been recognized as one of the most severe nonlinear impairments in fiberoptic networks [11] due to the facts that the signal energy is transferred to the backscattering signal and SBS has low threshold [12]. However, SBS has beneficial characteristics such as frequency selective amplification that can be applied to microwave photonics applications [13]. Shen et al. demonstrated the 11 GHz RoF system carrying the 10 Mb/s data using SBS [14], but the data bandwidth was restricted by the Brillouin gain profile. In [15], it was proposed an improved technique based on the simultaneous use of Brillouin gain and loss of the pump wave, amplifying one of the DSB modulated sidebands while attenuating the other one. In [16], a scheme with SSB modulation based on SBS was proposed to achieve 1.25 Gb/s data upconversion. Recently, a harmonic RF carrier generation and broadband data upconversion technique with SSB modulation using SBS was demonstrated [17]. However, the main limitation of these methods is that the generated RF signals are restricted to the Brillouin frequency [16] or the multiple of the Brillouin frequency [17]. In this paper, a frequency-tunable SBS-based optical SSB RF signal generation and broadband data upconversion technique for RoF systems is demonstrated. In principle, the technique works for arbitrarily high frequency of the RF signal, which is only limited by the bandwidths of the optical transmitter and receiver employed. On the one hand, in order to realize broadband data
0030-4018/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.10.045
Y. Qin et al. / Optics Communications 290 (2013) 158–162
upconversion, we use a dual-electrode Mach–Zehnder modulator (DEMZM) to carry the data signal on the optical carrier instead of carrying on the selectively amplified sideband [14]. This method ensures that the broadband data rate can be carried and independent of the Brillouin gain bandwidth. On the other hand, we use a fiber Bragg grating (FBG) to improve the optical carrier suppression ratio (CSR) so as to avoid the residual optical harmonics affect to the experimental results. In addition, frequency-tunable RF signal generation with SSB modulation is performed by using SBS with three cases of configuration. The detailed principle will be discussed in the next section. Finally, the 1.5 Gb/s downlink signal is examined. The power penalty is less than 1 dB at the BER of 10 10 after 50 km standard singlemode fiber (SSMF) transmission. This paper is organized as follows. In Section 2, the principle of the proposed technique is described. In Section 3, the experiment is built and demonstrated so as to verify the proposed technique. Finally, the conclusion is given in Section 4.
2. Operation principle As described by Agrawal [12], SBS generates both Brillouin gain and loss. Assume the Brillouin frequency shift is fB and the pump wave frequency is fPUMP, then the central frequencies of the Brillouin gain and loss profiles are fPUMP fB and fPUMP þfB, respectively. As a result, the Stokes wave is amplified when it is located at the Brillouin gain profile of the pump wave. Meanwhile, the anti-Stokes wave is consequently attenuated when it is located at the Brillouin loss profile of the pump wave. Fig. 1 shows the principle of the SSB modulation using SBS with three cases of configuration. The SSB modulation is realized by counterpropagating a DSB signal and a DSB–suppressed carrier (DSB-SC) signal in a length of standard single-mode fiber (SSMF) and exciting double SBS effect. During the process, the DSB-SC signal is acted
2
159
as pump waves; one sideband of the DSB signal is acted as Stokes wave and the other sideband is acted as anti-Stokes wave. Assume the local RF signal frequency is fRF and the optical carrier frequency is f0, then fp ¼9fPUMP f09. In case I, when fp 4fRF and fp ¼ fB þ fRF, the upper frequency sideband of DSB is located at the Brillouin gain profile of the pump wave and amplified by the Brillouin gain profile. Meantime, the lower frequency sideband is correspondingly located at the Brillouin loss profile of the pump wave and attenuated by the Brillouin loss profile. In such process, a SSB modulation is achieved, as illustrated in Fig. 1. In case II, when fp ofRF and fp ¼fRF fB, the upper frequency sideband of DSB is selected by the Brillouin loss profile of the pump wave and attenuated by the Brillouin loss profile, while the lower frequency sideband is selected by the Brillouin gain profile of the pump wave and amplified by the Brillouin gain profile. Then, the other kind of SSB modulation is obtained. In case III, when fp 4fRF and fp ¼ fB fRF, by using the same principle, the same result also can be achieved. Since the SBS is only imparted on the two modulation sidebands, the optical carrier is not affected by the SBS. Therefore, we use the DEMZM to carry the data signal on the optical carrier and the data bandwidth is not restricted by the Brillouin bandwidth. In principle, the proposed technique works for arbitrarily high frequency of the RF signal, which is only limited by the bandwidths of the optical transmitter and receiver employed. Furthermore, the RF signal frequency can be easily tuned by adjusting the frequency of fRF and fP, as described in Fig. 1. 3. Experiments and results In order to verify the proposed technique, an experimental proof is conducted and an experimental setup is shown in Fig. 2. First of all, we use the Case I scheme. A continuous wave (CW) light source at 1550.04 nm (f0 ¼193.5 THz) with a 3 dBm of
2
2
Fig. 1. Principle of the proposed technique using SBS with three cases of configuration. SBS process is performed by counterpropagating DSB signal and DSB-SC signal (purple line) in a length of SSMF. (a) Unprocessed DSB signal. (b) One sideband of the DSB signal is selected and amplified by Brillouin gain while the other sideband is corresponding attenuated by Brillouin loss. (c) Obtained SSB signal.
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CW
OC1 2
MZM
EDFA
FBG
PC
1 3
fe
PPG
PC
DEMZM
Central Station
11.9 km SSMF
3
Phase Shifter PD ED
OC2
2
ISO
f RF
1
EDFA
50 km SSMF
LPF DC-block
Mixer
Base Station Fig. 2. Experimental setup. CW: continuous wave; DEMZM: dual-electrode MZM; OC: optical circulator; PC: polarization controller; FBG: fiber Bragg grating; EDFA: erbium-doped fiber amplifier; PPG: pulse pattern generator; ISO: optical isolator; SSMF: standard single-mode fiber; PD: Photodiode; DC-block: DC-block capacitor; ED: Error detector.
Fig. 4. Reflective (dashed line) and transparent (solid line) response of the used FBG. Fig. 3. (a) Optical spectrum before (dashed line) and after (solid line) filtered by FBG. The dashed line is the output optical spectrum of MZM. The solid line is DSBSC signal. The optical sidebands at f0 þ 2fe and f0 2fe is acted as pump waves. The optical carrier at f0 is injected to the DEMZM. The output power of the optical source is 3 dBm, f0 ¼193.5 THz (1550.04 nm), fe ¼10.266 GHz. (b) Optical spectrum of the SSB signal (solid line) and the original DSB signal (dashed line) for 10 GHz RF signal. The original DSB signal is produced by DEMZM. The optical power of the pump waves injected into SSMF is 15 dBm.
output optical power is modulated by a MZM, which is driven by an fe ¼10.266 GHz electrical signal. The MZM is biased to completely suppress the odd-order optical sidebands [18]. Thus, three optical sidebands, i.e., f0, f0 þ2fe and f0 2fe, are obtained, as seen in the dashed line in Fig. 3(a) (comparing with Fig. 1, fP ¼ 2fe). Then they are divided into two branches by FBG with the reflective (dashed line) and transparent (solid line) response shown in Fig. 4. The optical sideband at f0 is reflected to one branch and
injected to the DEMZM. Each electrode of the DEMZM is driven by the fRF ¼10 GHz signal and the pulse pattern generator of the 1.5 Gb/s 231-1 pseudorandom bit sequence (PRBS) data signal, respectively. The biasing point is at Vp/2. Then the data signal has been carried on the optical carrier, as shown in Fig. 3(b) (dashed line). Now, the DSB modulation signal has been achieved. Meanwhile, the optical sidebands at f0 þ 2fe and f0 2fe go though the other branch, which act as the pump waves. The solid line in Fig. 3(a) shows the optical spectrum of the pump waves. The optical CSR is shown to be 38 dB below the sidebands. The suppression ratio is sufficient enough to avoid having the residual optical carrier which induce any significant Brillouin gain or loss. Then the pump waves are amplified by erbium-doped fiber amplifier (EDFA) to increase the optical power beyond the Brillouin threshold.
Y. Qin et al. / Optics Communications 290 (2013) 158–162
161
Fig. 5. Electrical spectra of (a) upconverted 10 GHz RF signal and (b) downconverted 1.5 Gb/s data signal.
70 CaseI
68
CaseII
CaseIII
USSR (dB)
66 64 62 60 58 56
Fig. 6. Measured (dots) and predicted (solid lines) BER of the RoF downlink system. Insets are the eye diagram of (a) the BTB, (b) the 50 km transmission case with the optimized polarization, (c) the BTB case with the pump polarization mismatch, and (d) the 50 km transmission case with the pump polarization mismatch.
An optical circulator (OC2) is used to counter-propagate the pump waves and the Stokes waves in the 11.9 km SSMF. The optical power of the pump waves injected into SSMF is 15 dBm. An optical isolator is added to ensure unidirectional transmission. The polarization controllers (PC) are added to optimize the polarization state. For the 1550.04 nm pump wave, the Brillouin frequency (fB) is measured to be 10.532 GHz for the 11.9 km SSMF used in the experiment. For the Case I scheme, when the fRF ¼10 GHz, the driven signal of the MZM should be fe ¼10.266 GHz. From the solid line in Fig. 3(b), measured at the port 3 of the optical circulator, we can see that the upper frequency sideband of the DSB modulation signal is amplified by the optical sideband at f0 þ2fe, while the lower frequency sideband of the DSB modulation signal is consequently attenuated by the optical sideband at f0 2fe in the SBS interaction. The SSB modulation signal can be clearly seen with 65 dB undesired sideband suppression ratio (USSR) (i.e., the power ratio of the amplified sideband and the attenuated sideband). Compared with the dashed line in Fig. 3(b), we can see that the optical carrier is not affected by the SBS. The SSB modulation signal generated from the central station is then transmitted to the base station through 50 km SSMF. Once the optical spectrum of the solid line in Fig. 3(b) is detected by the photodiode, it generates the upconverted RF spectrum of the amplitude modulated signal as shown in Fig. 5(a). It can be clearly seen that the strong peaks separated 1.5 GHz from the 10 GHz RF signal. To downconvert the optically upconverted 1.5 Gb/s data to the baseband, a mixer is used with a phase shifter controlling the phase of the RF signal, as shown in Fig. 2. An electrical low-pass filter (LPF) of the 3 GHz cutoff
0
10
20 fRF (GHz)
30
40
Fig. 7. Measured USSR vs fRF for the fixed pump power of 15 dBm. Different cases of SSB modulation configuration were used in the measurement.
frequency is attached to the IF port of the mixer to remove the high-frequency noise after the downconversion. Fig. 5(b) shows the downconverted 1.5 Gb/s data, which has the peaks of the 1.5 GHz separation, and the sidelobes of the 1.5 Gb/s data do not hamper the system performance. Fig. 6 shows the measured bit-error rate (BER) of the RoF downlink system. The power penalty is less than 1 dB at the BER of 10 10 after 50 km single-mode fiber transmission compared with the back-to-back (BTB) case. A small amount of noise is added after the 50 km fiber transmission [Fig. 6(b)] compared to the BTB case [Fig. 6(a)]. Fig. 6(c) and (d) shows the eye diagram of the BTB and the 50 km transmission case with the pump polarization mismatch. In the SBS process, the Brillouin gain or loss can be affected by the polarization state of the pump wave [19,20]. As compared with Fig. 6(a) and (c), the polarization state of the pump wave should be optimized to reduce the intensity noise involved in the recovered signal since the degree of the signal deterioration by the intensity noise and the overall signal intensity are changed depending on the polarization state of the pump wave during the experiment [19–21]. We also perform the other Cases of configuration. Fig. 7 shows the USSR versus fRF for the fixed pump power of 15 dBm. More than 58 dB of USSR is achieved from 5 to 40 GHz. The measurement from 5 to 40 GHz is limited by the 0.02 nm resolution bandwidth of the optical spectrum analyzer and the bandwidth of the DEMZM. In principle, arbitrarily high frequency of the RF signal with SSB modulation could be achieved by the proposed technique if high-frequency devices could be available.
4. Conclusion In this paper, an effective frequency-tunable SBS-based optical SSB RF signal generation and broadband data upconversion
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technique for RoF systems has been proposed and experimentally demonstrated. The proposed technique utilizes two MZMs, one is the DEMZM used to carry the broadband data rate on the optical carrier, the other one is biased to completely suppress the oddorder optical sidebands so that to utilize the low-frequency lower-cost driving signal to achieve the high-frequency signal. Furthermore, a FBG is used to improve the CSR and avoid the residual optical harmonics affect to the experimental results. The experiments show the RF signal frequency can be easily tuned by adjusting the frequency of fRF and fP, as described in Fig. 1. The transmission performance of the 1.5 Gb/s RoF downlink has also been examined. The power penalty is less than 1 dB at the BER of 10 10 after 50 km single-mode fiber transmission.
Acknowledgement This work is supported by the National Natural Science Foundation of China under Grant No. 61178002. References [1] J.C. Lin, J. Chen, P. Peng, C. Peng, W. Peng, B. Chiou, S. Chi, IEEE Photonics Technology Letters 19 (2007) 610. [2] A. Martinez, V. Polo, J. Marti, IEEE Transactions on Microwave Theory and Techniques 49 (2001) 2018.
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