Wired/wireless access integrated RoF-PON with scalable generation ...

Wired/wireless access integrated RoF-PON with scalable generation of multi-frequency MMWs enabled by tunable optical frequency comb Yu Xiang, Ning Jiang,* Chen Chen, Chongfu Zhang, and Kun Qiu Key Lab of Optical Fiber Sensing and Communication Networks (Ministry of Education), and School of Communication and Information Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan, 611731, China * [email protected]

Abstract: In this paper, a novel wired/wireless access integrated radioover-fiber passive optical network (RoF-PON) system that utilizes scalable multiple-frequency millimeter-wave (MF-MMW) generation based on tunable optical frequency comb (TOFC) is proposed. The TOFC is performed by cascading a phase modulator (PM) and two intensity modulators (IMs), and with proper selection of the peak-to-peak voltage of the PM, a flat and effective optical comb with tens of frequency lines is achieved. The MF-MMWs are generated by beating the optical comb line pairs with an interval about 60GHz. The feasibility and scalability of the proposed wired/wireless access integrated RoF-PON scheme are confirmed by the simulations of simultaneous distribution of wired and wireless data with the proposed multiple frequency MMW generation technology. © 2013 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.2360) Fiber optics links and subsystems; (060.4250) Networks; (060.5625) Radio frequency photonics.

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#189643 - $15.00 USD Received 29 Apr 2013; revised 5 Aug 2013; accepted 11 Aug 2013; published 15 Aug 2013 (C) 2013 OSA 26 August 2013 | Vol. 21, No. 17 | DOI:10.1364/OE.21.019762 | OPTICS EXPRESS 19762

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1. Introduction Radio over fiber (RoF) system has been widely studied for the ability of seamlessly integrating the broadband optical fiber communication with the highly-mobile wireless radio communication [1,2]. In last decade, the study on photonic generation of 60GHz millimeterwave (MMW) and its application in the access network has attracted great attention. Up to present, several RoF passive optical network (RoF-PON) schemes based on the photonic generation of MMW technologies, such as optical heterodyne, four wave mixing, optical injection, etc., have been reported [3–6]. While most of these schemes work at a single or fixed frequency, which may cause crosstalk between different optical network units (ONUs), therefore the development of multiple-frequency RoF-PON is highly desirable and valuable. Photonic generation of multiple-frequency MMW (MF-MMW) is a good candidate for the application of multiple-frequency RoF-PON, since that it can provide different optical MMWs for different ONUs. Several optical MF-MMW generation approaches have recently been developed. J. Li et al. proposed an ultra-wideband MF-MMW generation using fiber optical parametric amplifier [7]. M. Mohamed and colleagues proposed an OFDM MMW ultra-wideband generation scheme by optical frequency quadrupling technology [8]. J. Yu and associates demonstrated a novel MF-MMW generation scheme using multichannel recirculating frequency shifter loop [9]. In our recent works, we have proposed to use the FWM effects in SOA and the polarization demultiplexing/multiplexing technology to generate MF-MMWs, and used the MF-MMWs to achieve a multiple frequency RoF-PON [10,11]. In this work, we propose and demonstrate a novel wired/wireless access integrated RoFPON system with scalable MF-MMWs around the radio frequency of 60GHz, in virtue of a tunable optical frequency comb (TOFC) generator which has been extensively applied in the wideband optical communication in recent years [12,13]. The paper is organized as follows. Section 2 describes the principle and theoretical analysis of the proposed wired/wireless access integrated RoF-PON. In section 3, the investigations on the properties of TOFC and the performance of the RoF-PON are presented. Finally, a conclusion is given in section 4. 2. Principle and theoretical analysis of the proposed wired/wireless access integrated RoF-PON system based on TOFC

Fig. 1. Schematic of the proposed wired/wireless integrated RoF-PON using TOFC generator.

Figure 1 presents the schematic architecture of the proposed wired/wireless access integrated RoF-PON system. The multi-frequency MMW is generated by a TOFC generator which is shown in the inset (a). The TOFC generator is composed of one laser diode, RF source, a phase modulator (PM), two intensity modulators (IMs), and two power control modules. Here, a distributed-feedback (DFB) laser is used as the optical source, and a sinusoidal RF #189643 - $15.00 USD Received 29 Apr 2013; revised 5 Aug 2013; accepted 11 Aug 2013; published 15 Aug 2013 (C) 2013 OSA 26 August 2013 | Vol. 21, No. 17 | DOI:10.1364/OE.21.019762 | OPTICS EXPRESS 19763

source is applied to drive the cascaded PM and IMs. A tunable electrical amplifier and electric attenuator are employed to control the input RF signal voltage of the PM, so as to dynamically revise the scalability of the advanced TOFC generator. A phase shifter (PS) is adopted to adjust the phase of the input electrical signal of the two IMs. Under these conditions, optical comb lines generated by the TOFC generator are used as the source for the proposed multiple-frequency RoF-PON system. The output of optical comb lines [see the inset (b)] is injected into a fiber Bragg grating (FBG) with the center frequency of f0. Consequently, the comb line at the frequency of f0 is reflected and then modulated by the wired signal with one IM [see the inset (c)]. The rest of comb lines passing through the FBG are modulated by wireless signal with other IMs [see the inset (d)]. After that, the modulated optical signals including both wired and wireless messages [see the inset (e)] are combined and transmitted to the optical distribution network (ODN) through standard single mode fiber (SSMF). In the ODN, an arrayed-waveguide grating (AWG) is utilized to filter out the signals on different comb lines and an optical splitter is used to divide the wired signal for different terminal nodes. The wired signal from optical splitter and a pair of wireless signal on the two comb lines are delivered to the terminal node. The grid of the two wireless signal comb lines is chosen according to the need of the terminal, which determinates the user’s MMW frequency. The wired signal and the pairs of comb lines with wireless signal [see the insert (f)] are simultaneously sent to different terminal. The RF signal is generated by using a linear photodiode (PD) to perform the optical heterodyning from the wireless signal [see the insert (g)]. The baseband wired signal is directly obtained from the optical signal. In the TOFC generator, the DFB laser works in a CW mode, and its output can be described as Ec ( t ) = Ec exp( jωc t ) , where Ec is the envelope of intensity and ωc is the operation angular frequency. Considering the successive modulations at the PM and IMs, the output comb of TCG can be expressed as: E (t ) =

∞ 1 Ec  {J n (mPM π ) + 2 exp( j β ) J n [( mPM + mIM )π ] 4 n =−∞

(1)

+ exp( j 2 β ) J n [( mPM + 2mIM )π ]} × j exp[ j (ωc + nω )t ], n

where ω is the angular frequency of the sinusoidal RF signal, mPM is the PM electrical input voltage normalized by its half-wave voltage, while mIM and β are the IM electrical input voltages and the IM bias voltages normalized by their half-wave voltages, respectively. In addition, n is the comb line index and J n ( x ) is the first kind Bessel function of nth order. In order to effectively generate the multiple-frequency MMWs, two wireless bands are selected from the multiple wireless bands: one is selected from the left part, while the other is selected from the right part. We assume that the ith band in the left is selected where –n≤i≤n and it can be given as E Left ( t ) = A cos{[ωc + 2π × f d ( −15 + i )]t + φ0 ],

(2)

where A is the amplitude of the optical comb line generated by TOFC generator, φ0 is phase, f d = 2 GHz is the frequency interval of the wireless bands. Similarly, the jth band in the right region can be written as E Right (t ) = A cos{[ωc + 2π × f d (15 + j )]t + φ0 }.

(3)

Subsequently, these two bands are taken to perform the optical heterodyning in a PD and the output current of the PD within its limited bandwidth can be described as


I ( t ) = R E Left ( t ) + E Right ( t )

2

= 2 RA2 cos{[ωc + 2π × 2 × 109 (15 + j )]t − [ωc − 2π × 2 × 109 (15 − i )]t + φ0 − φ0 }

(4)

= 2 RA {1 + cos{2π × 10 [60 + 2( j − i )]t}}, 2

9

where R is the responsivity of PD. As can be seen from Eq. (4), a MMW signal with a frequency around 60GHz has been generated. The frequency range of the MF-MMW is [602nfd, 60 + 2nfd]GHz with a frequency grid of 2GHz. Thus totally 4n + 1 MMWs with different frequency can be generated. 3. Results and discussion In our scheme, the wavelength of the DBF laser is set at 1552.52nm, and the RF signal is a sinusoidal signal with a frequency of 2GHz. The half-wave voltages of the PM and two IMs are 3.2V and 4.6V, respectively. The PPV of PM driving signal is controlled at 6V. The IMs are of identical single-arm chirp-free LiNbO3 type. Under these conditions, tens of flat f0centered frequency lines can be obtained by the TOFC generator. Figure 2 shows the flatness contour plot for the 39 f0-centered comb lines as a function of the indices mIM and β. Here the electrical normalized input voltage applied to the PM is set to mPM = 3 . It is obvious that there is an optimize point with mIM = 0.54 and β = 0.52, where the flatness of the obtained central 39 comb lines is less than 0.1 dB, i.e., the frequency lines are highly flat [see the red point]. It is worth mentioning that, in the TOFC generator adopted here, about 12dB insertion loss would be induced by the cascade of PM and IMs, and moreover, the number of comb lines would be less and the range of bandwidth would be comparatively narrower with respect to those of the TOFC generators used in [12,13]. However, it hardly affects the application in the proposed RoF-PON, since that it not only can provide sufficient flexibility and stability, but also can satisfy the demands of frequency bands around 60GHz. For the sake of better performance, in the following investigations the values of mIM and β are set to the optimize values.

Fig. 2. Flatness contour plot with comb lines generated by advanced TCG

Figure 3 displays the analytical and simulation spectra for the output optical comb generated by the advanced TOFC generator. It is indicated that the simulation results agree with the theoretical analysis well, even though the flatness of the simulation result (0.4dB) is a little larger than that of the theoretical calculation result (0.1dB). We attribute this deterioration in flatness to the intrinsic difference of the two IMs, while in the theoretical calculation we set the IMs to identical.


Fig. 3. Output optical comb spectra generated by advanced TCG, where (a) and (b) represent the theoretical analysis result and simulation result, respectively.

To get rid of the affection of the signals outside of the flat region, a band-pass filter is adopted, and then the spectrum of the TOFC becomes as shown in Fig. 4(a). The optical comb lines are then modulated with wired and wireless signal. The spectra for the modulated signals are shown in Fig. 4(b), where the blue curve represents the spectrum for the modulated wireless signal, while the green one represents that for the modulated wired signal. Figure 5(a) shows the spectrum of modulated wired and wireless signals after optical coupler. We can see that the power difference between wired signal and wireless signals is about 3dB. At the ODN, the pairs of wireless signal comb lines with a frequency interval around 60GHz and the wired signal are sent to different ONUs, where a linear PD is adopted to perform the optical heterodyning, to generate the MMW for wireless communication and another PD is used to drop the wired signal directly. Taking the cases of j-i = −1, 0, 1 for instance, three MMWs with frequencies of 58GHz, 60GHz and 62GHz are obtained simultaneously as shown in Fig. 5(b).

Fig. 4. (a) Optical comb spectrum after rectangular filter, (b) Modulated optical comb spectrum by wired data and wireless data.

Fig. 5. (a) Optical comb spectrum modulated wired data and wireless data after combiner, (b) Electrical spectra of generated MMWs near 60GHz

Figure 6 depicts the bit-error rate (BER) performance of a 1.25Gbps wired access in our proposed wired/wireless integrated ROF-PON system for both back-to-back (B2B) and 20km SSMF transmission cases. It is indicated that the power penalty at a BER of 10−9 is 0.55 dB after transmission over 20km SSMF. The insets (a) and (b) show the eye diagrams for B2B and 20km SSMF transmission cases, which indicates that a geed performance is maintained. Similarly, Fig. 7 shows the BER performance for a 1.25Gbps wireless access, where the 58GHz, 60GHz and 62GHz MMWs in Fig. 5(b) are adopted to investigate the performance of wireless access. It is demonstrated that the performance variations for all MMW channels are


similar. The power penalties at a BER of 10−9 after 20km SSMF transmission are 0.3dB, 0.4dB and 0.3dB for 58GHz, 60GHz and 62GHz, respectively, and the communication performance is not greatly deteriorated with respect to that of the B2B case, as shown in the insets Figs. 7(a)-7(f).

Fig. 6. BER performance of the 1.25Gbps wired access for the B2B and 20km SSMF transmission cases. The insets (a) and (b) denotes the corresponding eye diagrams for the cases of B2B and 20km SSMF transmission at the BER of 10−9.

Fig. 7. BER performance of the 1.25Gbps wireless access at 58GHz, 60GHz and 62GHz for the B2B and 20km SSMF transmission cases. The insets (a)-(f) denotes the corresponding eye diagrams at the BER of 10−9.

4. Conclusion We have proposed a novel integrated RoF-PON system for both wired and wireless access, by utilizing multiple-frequency millimeter generation based on an advanced TOFC generator. By properly selecting operation conditions of the TOFC generator, a flat TOFC with 39 frequency lines is achieved, and then a set of MF-MMWs around 60GHz are generated by beating the optical comb line pairs. Based on this, a simulation utilizing the MF-MMWs to distribute a wired and a wireless signal simultaneously demonstrate that the proposed wired/wireless access integrated RoF-PON is achievable, and a prominent performance can be maintained. The proposed RoF-PON scheme provides a potential way for the integration of optical communication and wireless communication in the next-generation broadband optical access networks. Acknowledgments This work is supported in part by the Fundamental Research Funds for the Central Universities No. ZYGX2011J009, the Young Scientist Fund of National Natural Science Foundation of China No. 61101095, 863 high technique program of China No. 2012AA011304 and Natural Science Foundation of China No. 61171045. The authors would also thank Dr. Feng Wen, Dr. Yun Ling, Dr. Xingwen Yi, M. E. Zhenming Yu and all anonymous reviewers for improving the clarity and quality of this paper.
