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An approach for the multiple-frequency millimeter-wave (mm-wave) signals generation is proposed and demonstrated, specifically, which can be applied to a radio-over-fiber (RoF) system with multiple base stations (BSs). In this scheme, optical double sideband (ODSB) modulation is achieved using a Mach-Zehnder modulator (MZM) to generate the two-sideband signals. New frequencies of the optical signals are obtained by using four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). At the BSs, two different frequencies are achieved using a comb optical filter (COF), and which then input a photodiode (PD) to generate the mm-wave signals with the frequencies of 20, 40 or 60 GHz for different BSs, by mixing of these frequencies components. Experimental results verify that the proposed multiple-frequency mm-wave signals generation scheme for a RoF system with multiple base stations can work properly.
©2011 Optical Society of America
1. Introduction
With the broadband penetration and the ongoing growth of data traffic among residential and business customers, the ultimate bottleneck to the end user terminals is contributed by the last mile wired and wireless access systems with limited bandwidth capacity. In order to avoid such a bottleneck, and to exploit the potential of both wired and wireless technologies, researchers, carriers and service providers are all actively seeking a converged network architecture to deliver various services for both fixed and mobile users. Therefore, radio-over-fiber (RoF) technology has been considered a potential candidate for supporting future broadband access networks [1]. It has the advantages of low loss, wide-band, light weight and immunity to electromagnetic interference (EMI) due to optical fibers [1,2]. To provide broad-band services in a RoF system, radio frequency (RF) signals and millimeter-wave (mm-wave) signals are distributed over optical fibers from central station (CS) to base station (BS). However, as in the RF and mm-wave frequency range, fiber chromatic dispersion [3], complexity and cost of equipments become several limiting factors [4,5]. It is strongly required that the high-quality and high-efficiency RF and mm-wave sources are realized by optical approaches, especially 60 GHz mm-wave signals which have recently gained much attention for their huge bandwidth over 7 GHz unlicensed mm-wave band. And the advantages of 60 GHz signals include the spectral availability to achieve multi-gigabit data rate, high efficiency and low power consumption [2,6–9]. Several approaches for mm-wave frequency generation have been proposed in the last few years for the RoF system, including optical mixing methods that employ cross-gain modulation (XGM) [10], cross-phase modulation (XPM) [7,11], and four-wave mixing (FWM) in semiconductor optical amplifier (SOA) [12], all-optical frequency conversion methods using an electro-optic phase modulator and nonlinear method that employ stimulated Brillouin scattering (SBS) [13]. On the other hand, it is highly desired that these mm-wave signals can be processed directly in the optical domain without additional optical to electrical and electrical to optical conversion. In addition, the multiple-frequency mm-wave signals generation for the RoF system with multiple BSs is seldom reported, although it is considered very worthwhile for further studies.
To exploit such a bandwidth advantage of 60 GHz mm-wave by optical approaches, in this paper, we present a novel method by using FWM in an SOA at the CS and filtering out signal components at different remote sites to generate a particular RF signal by mixing of these signal components in the photodiode (PD). The new RoF system with multiple BSs is employed by SOA, Mach-Zehnder modulator (MZM) and tunable optical filter (TOF). Our method has many distinct advantages, such as low power consumption, flexible modulation, and supporting multiple BSs.
2. Operational principle of the proposed approach
The RoF system with multiple BSs of the proposed multiple-frequency mm-wave signals generation is shown in Fig. 1 . The CS of the proposed multiple-frequency mm-wave generation method for the RoF system with multiple stations is shown in the upper portion of Fig. 1. The laser with the frequency of f via a polarization controller (PC) is modulated using an MZM through radio frequency (RF) with the frequency of f ’ to perform optical double sideband (ODSB) modulation (Mod.), and the two sidebands are f 1 and f 2 respectively. The ODSB signals are separated into two branches using an optical splitter (OS 50:50), and the frequencies f 1 and f 2 of the ODSB are then filtered out using the fiber Bragg-based TOF 11 and TOF 12 respectively. The date is modulated on the sideband with frequency of f 1 using another MZM. The modulated signal with the frequency of f 1 is coupled using the optical coupler (OC 50:50) with the sideband with the frequency of f 2 to optical fiber channel consisted of transmission fiber and erbium-doped optical fiber amplifier (EDFA). After a variable optical attenuator (VOA) the power of the input signals can be properly adjusted. The signals with the frequencies f 1 and f 2 input a SOA after a PC. In the SOA, new frequencies f 3 and f 4 are generated by four-wave mixing (FWM). All optical signals with the frequencies f 1, f 2, f 3 and f 4 are then distributed to multiple BSs.
Fig. 1 RoF system with multiple base stations of the proposed multiple-frequency mm-wave signals generation. CS: central station, BS: base station, PC: polarization controller, RF: radio frequency, ODSB: optical double sideband, OS: optical splitter, OC: optical coupler, TOF: tunable optical filter, MZM: Mach-Zehnder modulator, SOA: semiconductor optical amplifier, VOA: variable optical attenuator, EDFA: erbium-doped optical amplifier, PD: photodiode, EA: electric amplifier.
The multiple BSs of the proposed multiple-frequency mm-wave generation method for the RoF system are shown in the lower portion of Fig. 1. The signals from the CS are divided into different branches for every BS using an OS, different frequencies of the signals are separated by a de-multiplexer at the BS. At each BS, two of three frequency components inputted are chosen and sent into a PD to generate a particular mm-wave signal by mixing of these frequency components. In this scheme, there are 4 different frequencies of f 1, f 2, f 3 and f 4, we obtain 6 groups of two frequencies combination, such as [ f 1, f 2 ], [ f 1, f 3 ], [ f 1, f 4 ], [ f 2, f 3 ], [ f 2, f 4 ], and [ f 3, f 4 ]. Assume that the RF frequency would be used repeatedly, this scheme with 4 frequencies can support 6 BSs (case 1); if every RF frequency is only used once, this scheme with 4 frequencies can only support 3 BSs (case 2). The generated mm-wave signal then inputs an antenna to transmit. For example, at the 1st BS, the frequencies of f 1, f 2 and f 4 are split out, and the frequencies of f 1 and f 4 input a PD to obtain the mm-wave with the frequency of f 4- f 1, this mm-wave signal is amplified through an electric amplifier (EA) and emitted via an antenna. At the receiving side, the data from the receiving antenna is modulated on the frequency of f 2 using an MZM and transmitted back to the CS. For another BS, any three frequencies of the signals can be filtered out using a splitter (de-multiplexer), two of these can also be used to generate the corresponding mm-wave signal, and the data from the antenna is modulated on another new frequency by RF with the frequency of f ’, such as the 2nd,…, j th. Note that j is 6 and 3 for case 1 and 2 respectively in this scheme with 4 frequencies.
3. Experimental setup, results and discussion
3.1 Experimental setup
To verify that the proposed scheme for the multiple-frequency mm-wave generation can work properly, the experimental setup for three frequencies mm-wave generation is shown in Fig. 2 . The ODSB modulation is achieved using an MZM through a RF modulation with the frequency of f ’, the 3 dB bandwidth (BW) of the MZM is 20 GHz, Vpi = 5 V, the insertion loss of the MZM α IL = 6 dB, and the local oscillator with the frequency of 10 GHz and the electrical power of 22 dBm are applied. The two frequencies of the ODSB signals are filtered out using the TOF 21 and TOF 22, the data is modulated on the one sideband, the modulated signals and the other sideband signals are coupled into the optical fiber channel. After an EDFA, these signals via a VOA and PC then input a SOA (CIP SOA-S-OEC), where they generate four different frequencies through the FWM effect. These signals are distributed to the BSs, any two frequencies of the one branch can be obtained using a comb optical filter (COF 1) (or a WDM de-multiplexer), and the mm-wave signal is then generated using the PD 1. Similarly, another two frequencies of the other branches can also be obtained using the COF, such as the COF 2 and 3, and the other mm-wave signals are also achieved using the PD 2 and 3 respectively.
Fig. 2 Experimental setup for the proposed scheme. OSA: optical spectrum analyzer,ESA: electrical spectrum analyzer, BER-T: bit error rate tester, COF: comb optical filter.
3.2 Results and discussion
The wavelength of the laser is set at 1552.52 nm, the average power is 1 mW, and the linewidth of the LD is 10 MHz. The optical spectra measured after an MZM are shown in Fig. 3 , we can see that (1) the central wavelength of 1552.52 nm is clearly suppressed; (2) the first pair of the sidebands signals is with high optical signal-to-noise ratio (OSNR), and the frequency interval between the sidebands is 20 GHz; (3) the second pair of the sidebands signals are with very low OSNR. These results indicate that the ODSB signals have been obtained using the MZM and RF with the frequency f ’ of 10 GHz.
The spectra of the modulated and un-modulated signals before the SOA have been measured successfully, as shown in Fig. 4 . It can be clearly found that (1) the spacing between the modulated and un-modulated signal is just 20 GHz; (2) the powers of the modulated and the un-modulated signals are about −16 dBm and −7 dBm respectively.
The SOA is biased at 220 mA and can provide small signal gain of about 13 dB, and the power of the input signal is set 4 dBm. The signals after the fiber channel, an EDFA with a gain of about 15 dB and a VOA, the optical spectra of the FWM in the SOA have been measured, as shown in Fig. 5 . It can be seen that four wavelengths have been successfully generated through the FWM effect between the two input signals. The modulated and un-modulated signals have been indicated in Fig. 5, and a high OSNR has been achieved.
Figures 6 –8 show the measured electrical spectra after the PDs, where the mm-wave signals of 60, 40 and 20 GHz have been shown in Fig. 6, 7 and 8 respectively. Any two frequencies of the signals were split at one BS, and input a PD to generate the mm-wave signal through frequency beating effect. As demonstrated in Fig. 2, the frequencies of f 1 and f 4 were split at the 1st BS, the mm-wave signal with the frequency f 4- f 1 = 4f ’ has been achieved, that is to say, it was obtained that the mm-wave signal with the frequency of 60 GHz, as shown in Fig. 6. Similarly, another mm-wave signal with the frequency of 40 GHz has been obtained at another BS. Moreover, we can see that the mm-wave signal with frequency of 40 GHz is very high, as shown in Fig. 7. The mm-wave signal with the frequency of 20 GHz has been obtained at the BS, and we can also see that the mm-wave signal with frequency of 20 GHz is very good, as shown in Fig. 8. And these multiple-frequency mm-wave signals can be applied to different BSs.
The bit error rate (BER) and eye diagrams performances of the RoF system with the proposed multiple-frequency mm-wave signals generation have been measured for different transmission optical fiber distances (TOFD) of 20, 30, 40 and 50 km, as shown in Fig. 9 . When the data of 2.5 Gb/s was considered, we can see that the performances (BER = 10-8) have been obtained for the TOFD of 20, 30, 40 and 50 km with the receiving powers are −21.8, −20.4, −19.7 and −18.5 dBm respectively, and the corresponding eye diagrams at the receiving optical power of −21 dBm for different TOFD of 20, 30 and 40 km are shown in Fig. 9(a)–9(c). It is noted that the BER performance of the proposed method degrades with the increasing of the TOFD. For example, at a BER of 10-8, the performance has a penalty of about 0.7 dB between 20 km and 30, even about 3.3 dB between 20 km and 50 km, which is induced by the high-order chromatic dispersion of the fiber.
Fig. 9 Bit error rate vs. received power and eye diagrams for different transmission fiber distance.
4. Conclusion
The multiple-frequency mm-wave signals generation method has been demonstrated successfully, and the RoF system architecture with the multiple-frequency mm-wave signals has been proposed. By using an MZM, an ODSB signal has been generated using a low-frequency RF driving signal. Different sideband signals were filtered out using the TOFs, and data was then modulated on one of the sideband signals. The signals via the fiber channel then input the SOA to generate new frequencies components through the FWM effect. At any BS, two frequencies of the signals were split using the COF to generate beating frequency in the PD, in order to obtain the mm-wave signals. The mm-wave signals with the frequencies of 20, 40 and 60 GHz have been generated for different BSs in this work, and the BER and eye diagrams of the modulated signals at 2.5 Gb/s have been demonstrated. It was concluded that the system performance degradation is induced by the high-order chromatic dispersion of the fiber. And the multiple-frequency mm-wave signals using the proposed method can be applied to the RoF system with multiple BSs to improve the capacity of the RoF system.
Acknowledgments
This work is supported by the Science Foundation of China under Contract No. 60807028. The authors would like to thank Dr. Zinan Wang, Dr. Xingwen Yi, Mr. Heng Zhou, and Dr. Sathishkumar Perumal from UESTC for their inspiring discussions and help, and anonymous reviewers for their valuable comments that improve the clarity and quality of this paper.
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