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An all-optical single sideband (OSSB) frequency upconverter based on the cross-phase modulation (XPM) effect is proposed and experimentally demonstrated to overcome the power fading problem caused by the chromatic dispersion of fiber in radio-over-fiber systems. The OSSB frequency upconverter consists of an arrayed waveguide grating (AWG) and a semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) and does not require an extra delay line used for phase noise compensation. The generated OSSB radio frequency (RF) signal transmitted over single-mode fibers up to 20 km shows a flat electrical RF power response as a function of the fiber length. The upconverted electrical RF signal at 48 GHz shows negligible degradation of the phase noise even without an extra delay line. The measured phase noise of the upconverted RF signal (48 GHz) is −74.72 dBc/Hz at an offset frequency of 10 kHz. The spurious free dynamic range (SFDR) measured by a two-tone test to estimate the linearity of the OSSB frequency upconverter is 72.5 dB·Hz2/3.
© 2016 Optical Society of America
1. Introduction
As the demand for global mobile data traffic increases rapidly, broadband wireless communication systems that offer high-speed data rates are required. The radio-over-fiber (RoF) system, which uses optical fiber as a transmission medium, is an attractive candidate for such a broadband wireless communication system. The optical fiber has ultra-low loss and ultra-wide bandwidth characteristics that are good for long distance transmissions of microwave or millimeter-wave modulated optical signals. The generation of optical signals modulated by microwave or millimeter-wave signals is one of the most important techniques for the implementation of an RoF central office. Various techniques have been studied with regard to the generation of optical signals modulated by microwave or millimeter-wave signals [1, 2]. When a generated optical signal in the form of a double-sideband signal is transmitted over an optical fiber from a central office to a base station, each spectral component experiences a different amount of phase shift due to the fiber chromatic dispersion phenomenon. At the base station, a photodetector generates two beat components at the desired radio frequency with different amounts of phase shift, resulting in the fluctuation of the electrical RF power as a function of the fiber-link distance, the RF frequency, and the fiber-dispersion parameter [1, 2].
The generation of an optical single-sideband (OSSB) RF signal is one of the solutions for overcoming the power fading problem caused by fiber chromatic dispersion, as there is a single beat component in the electrical RF signal. Various techniques for the generation of OSSB RF signals using a dual-electrode Mach-Zehnder modulator (MZM) biased at a quadrature point [2], a single-electrode MZM with a fiber Bragg grating (FBG) [3], and an electro-absorption modulator (EAM) [4] have been studied. However, these techniques use external modulators, which have a limited RF frequency range due to the limited bandwidth of the external modulators. Recently, OSSB frequency upconversion techniques using a semiconductor optical amplifier (SOA) with a large bandwidth have been reported. The OSSB RF signal can be generated through nonlinearity of the cross-gain modulation (XGM) [5], cross-polarization modulation (XPolM) [6], or cross-phase modulation (XPM) [7] in an SOA. The technique using the XGM effect [5] requires a delay line to offset the path difference to improve the phase noise characteristics and an extra erbium-doped fiber amplifier (EDFA) to improve the conversion efficiency. The technique using the XPM effect [7] also requires a delay line for compensation of the path difference to improve the phase noise characteristics. While the OSSB frequency upconverters utilizing the delay line show improved phase noise characteristics, their phase noise characteristics are susceptible to ambient temperature variations since the temperature coefficient of the optical fiber used as the delay line is different from that of the waveguides in SOAs. The method using the XPolM effect [6] needs several polarization controllers (PCs) and a polarization beam splitter (PBS) to control the state of polarization.
In this paper, an all-optical single-sideband frequency upconversion method which utilizes the XPM effect in a semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) is proposed and experimentally demonstrated. The proposed scheme consists of an arrayed waveguide grating (AWG) and the SOA-MZI. Since it does not require an extra delay line to offset the path difference in the optical LO signals, its phase noise characteristics are less susceptible to ambient temperature variations. The proposed scheme has low polarization dependence due to the low polarization dependence of SOA-MZI [8]. It can be potentially applied to WDM systems due to the broad bandwidth characteristic of the XPM effect in SOA-MZI [9], which can be also obtained in a highly nonlinear dispersion shifted optical fiber in a nonlinear optical loop mirror [10]. The electrical RF signal generated from the proposed OSSB frequency upconverter shows no serious degradation of the phase noise performance. The spurious free dynamic range (SFDR) as measured by a two-tone test satisfies the requirements of fiber infrastructures in personal communication systems.
2. Operational principle of the all-optical SSB frequency upconverter utilizing the XPM effect in an SOA-MZI
Figure 1 shows the operational principle of the OSSB frequency upconverter utilizing the XPM effect in an SOA-MZI. Figure 1(a) shows a schematic of a RoF downlink implemented using the proposed OSSB frequency upconverter. The proposed OSSB frequency upconverter consists of an AWG, an SOA-MZI, and an optical bandpass filter (OBPF). The SOA-MZI consists of two SOAs which serve as a phase modulator and phase shifters (PSs) in the Mach-Zehnder interferometric structure with 3-dB directional couplers. The transfer function of the 2 by 2 3-dB directional coupler in the SOA-MZI can be expressed as [11]
where l represents the coupling length and β is the propagation constant in the waveguides of the directional coupler.
Fig. 1 Operational principle of OSSB frequency upconversion utilizing the XPM effect in an SOA-MZI: (a) Block diagram of the RoF downlink using the proposed OSSB frequency upconverter. (b) Relative phases of the two tones of the optical LO signal in each port of the SOA-MZI. (c) Simulated transfer curve at port 6. (d) Simulated transfer curve at port 7.
Two tones (left and right) of an optical LO signal which is generated by optical carrier suppression (OCS) are directed to port 2 (P2) and port 3 (P3) of the SOA-MZI. In the SOA-MZI, the phase of the optical signal components that pass through the upper SOA (SOA 1) is modulated by the optical IF signal injected into port 1 (P1) through the XPM effect, while those passing through the lower SOA (SOA 2) are not modulated by the optical IF signal. Using the transfer function of the 2 by 2 3-dB directional coupler, the electric fields of the SOA-MZI output signals at ports 6 and 7 can be expressed as follows:
Here, Δϕ1and Δϕ2 represent the phase shifts induced by the phase shifters (PS 1 and PS 2), respectively, and ΔϕIF is the phase shift caused by the optical IF signal through the XPM effect in SOA 1. Figure 1(b) schematically shows the relative phases of the two tones of the optical signal at the input and output ports of the SOA-MZI, assuming that the phase difference between Δϕ1 and Δϕ2 is approximately π/2. The magnitudes of the two tones in the output signals Eo1 and Eo2 change as ΔϕIF is changed. Figures 1(c) and (d) show the simulated transfer curves (i.e., the magnitudes of the two tones of the optical signal at output ports 6 and 7 of the SOA-MZI as a function of ΔϕIF) of the OSSB upconverter. The relationship between ΔϕIF and the optical IF power is nonlinear because the phase shift takes place when SOA 1 becomes saturated. Therefore, considering the transfer curve at port 6, a region of optical IF power exists which corresponds to a ΔϕIF value of approximately π/2, where the left optical tone does not changes and the right tone changes sharply.The procedure used to generate an OSSB RF signal is as follows: The optical LO signal is generated in the form of two optical sidebands separated by an LO frequency (Fig. 2(a)) using OCS at the optical LO block. The two optical sidebands are separated by the AWG as the left tone and the right tone (Fig. 2(c), (d)) and are then injected into P2 and P3 of the SOA-MZI shown in Fig. 1(a), respectively. The optical IF signal is generated in the form of a double- sideband at the optical IF block (Fig. 2(b)) and is injected into P1 of the SOA-MZI in a co-propagating configuration, as shown in Fig. 1(a). The phase of the optical signal components that pass through the upper path (SOA 1) is modulated by the optical IF signal. When the upper path signal and the lower path (SOA 2) signal are combined, intensity-modulated optical signal components are generated. When the power of the optical IF signal is at a magnitude that can produce a phase shift (ΔϕIF) close to π/2, for the optical RF signal taken from port 6 the intensity of the right optical LO tone is modulated by the optical IF signal, while the intensity of the left optical LO tone is not modulated by the optical IF signal, as shown in Fig. 2(e). Similarly, for the optical RF signal taken from port 7, the intensity of the left optical LO tone is modulated by the optical IF signal, while that of the right optical LO tone is not modulated by the optical IF signal, as shown in Fig. 2 (f). The generated optical RF signal is transmitted through an optical fiber to the base station (BS) and is converted into an electrical RF signal by a photodetector (PD). Because the generated optical RF signal is in the form of an OSSB signal, the electrical RF signal has single beating components at different frequencies (fRF+ = fLO + fIF and fRF- = fLO-fIF) and thus power fading caused by fiber chromatic dispersion does not take place.
Fig. 2 Optical spectra at each point of the all-optical SSB frequency upconverter shown in Fig. 1 (a). (e) and (f) show the optical RF signals at ports 6 and 7 as observed at the output of the OBPFs, respectively.
3. Experiment and results
Figure 3 shows the experimental setup of the OSSB frequency upconverter utilizing the XPM effect in an SOA-MZI. At the optical LO block, an optical LO signal with two tones separated by fLO (60 GHz) is generated using a Mach-Zehnder modulator (MZM) with a DC bias at Vπ for the generation of a double-sideband suppressed optical carrier (DSB-SC) signal driven by an electrical signal at a frequency of fLO/2 (30 GHz). The generated optical LO signal is amplified by an erbium-doped fiber amplifier (EDFA 1) and its optical power is controlled by a variable optical attenuator (VOA 1). An optical IF signal with an IF frequency of 3 GHz is generated in the form of a double-sideband signal using an electro-absorption modulator (EAM). The generated optical IF signal is amplified by EDFA 2 and its optical power is controlled by VOA 2. The wavelengths of the laser sources for the optical LO and IF signals are 1550.1 nm and 1552.52 nm, respectively. An AWG (Kylia, 48 channels with 25 GHz spacing) is used to direct the left and right optical LO tones to ports 2 and 3 of the SOA-MZI, respectively. For the operation of the proposed OSSB frequency upconverter, the phase of the optical output at SOA1 and SOA2 is controlled by PS1 and PS2, respectively. The phase shifters are built in SOA-MZI (CIP, 40G-2R-ORP) and controlled by external dc voltages. The optical IF signal is injected into port 1 of the SOA-MZI and modulates the phase of the optical signal that passes through SOA 1 through the XPM effect. The optical signals from the two different paths (SOA 1 and SOA 2) are combined by a directional coupler, producing an intensity-modulated optical RF signal at port 6 of the SOA-MZI. Note that signal paths for the left and right optical LO tones are identical and thus an extra optical fiber delay line for phase noise performance improvement is not necessary. The generated OSSB RF signal filtered by a tunable optical bandpass filter (Santec, OTF-350) having a tunable wavelength range of 1530 ~1610 nm and a tunable bandwidth range of 0.1 ~15 nm to reject amplified spontaneous emission (ASE) noise is transmitted through a single-mode fiber (SMF) to the BS. At the BS, the transmitted OSSB RF signal is converted into an electrical RF signal by the PD and is then amplified by a low-noise amplifier (LNA).
Figure 4 shows the optical power levels of the left and right tones of the output RF signal measured at port 6 of the SOA-MZI as a function of the optical IF power. The power of the optical LO tones at the input of the SOA-MZI is set to −8 dBm for this measurement. When the power of the optical IF signal is set to −10 dBm, an OSSB RF signal having the right tone modulated by the IF signal and an unmodulated left tone can be generated.
Fig. 4 Measured optical power levels of the left and right tones of the optical RF signal at port 6 of the SOA-MZI as a function of the unmodulated optical IF power
Figure 5 shows the optical spectra at each node of the OSSB frequency upconverter. Figure 5(a) shows the spectra of the optical LO signal with a fLO value of 60 GHz. Figures 5(b) and (c) show the left and right tones of the optical LO signal separated by the AWG and directed to port 3 (P3) and port 4 (P4) of the SOA-MZI, respectively. Figure 5(d) shows the spectra of the optical IF signal with a fIF value of 3 GHz in the form of an ODSB signal. Figure 5(e) shows the spectra of the generated OSSB RF signal from port 6 (P6) of the SOA-MZI measured at the output of the OBPF.
Fig. 5 Measured optical spectra at each node: (a) Optical LO signal at the input of the AWG. (b) Left tone of the optical LO signal at P2 of the SOA-MZI. (c) Right tone of the optical LO signal at P3 of the SOA-MZI. (d) Optical IF signal. (e) OSSB RF signal from port 6 (P6) of the SOA-MZI measured at the output of the OBPF.
Figure 6(a) shows the measured electrical spectrum of the RF signal at the output of the LNA, having a LO frequency component (60 GHz) and RF frequency components (57 GHz and 63GHz). Figure 6(b) shows the normalized value of the electrical RF signal power at the base station as a function of the SMF length when the OSSB RF signal with a RF frequency of 63GHz is transmitted, along with that for the transmission of an ODSB signal with the same frequency simulated using the equation shown in [2, 12], with a dispersion parameter (D) of 17 ps/(km-nm) and a carrier wavelength (λC) of 1550.1 nm. As shown in Fig. 6(b), while the power of the electrical RF signal for the transmission of the ODSB signal over the SMF has many power suppression points, the OSSB RF signal generated by the OSSB frequency upconverter has no power suppression points for a SMF length up to 20 km.
Fig. 6 (a) Electrical RF signal measured at the output of the LNA. (b) Normalized power of the electrical RF signal measured at the BS as a function of the SMF length for transmission of the OSSB signal along with that (simulation) for transmission of the ODSB signal.
Figure 7(a) shows the normalized conversion efficiency of the OSSB frequency upconverter as a function of the LO frequency. The conversion efficiency is defined as the ratio of the electrical RF signal power to the electrical IF signal power. The frequency responses of MZM, PD, electrical signal generator, and ESA are compensated for the conversion efficiency measurement. The frequency and power of the electrical IF signal are 3 GHz and −1 dBm, respectively, and the conversion efficiency is measured for the LO signal frequencies ranging from 47 GHz to 71 GHz. The conversion efficiency of the proposed OSSB frequency upconverter is measured to be approximately 7.5 dB at the LO frequency 60 GHz. The conversion efficiencies of other OSSB frequency upconverters utilizing XGM [5] and XPolM [6] are reported to be 29 dB and −9 dB, respectively. The substantially high conversion efficiency of the OSSB frequency upconverter utilizing XGM is attributed to optimized carrier-to-sideband ratio [5]. The normalized conversion efficiency measured as a function of the LO frequency has an almost flat response with a ripple of approximately 0.5 dB. The operation bandwidth of the proposed OSSB frequency upconverter with respect to the LO or RF signal frequency cannot be measured since the bandwidth of ESA (HP8565E with 11974V) used for the measurement of the frequency response of the electrical RF signal is limited to 75 GHz.
Fig. 7 (a) Normalized conversion efficiency as a function of the LO frequency. (b) Normalized value of the electrical RF signal power (upper sideband signal) as a function of the IF frequency.
Figure 7(b) shows the normalized value of the electrical RF signal power (upper side band signal) as a function of the IF frequency. The frequency and electrical power of the LO signal are 60 GHz and 10 dBm, respectively, and the measurement is investigated for the IF signal frequencies ranging from 1 GHz to 14 GHz. The 3 dB bandwidth of the proposed OSSB frequency upconverter is approximately 10 GHz. The bandwidth of the XPM effect is related to the carrier life time in an SOA [13].
Figure 8 shows the phase noises of the upconverted RF signal and the LO and IF signals as a function of the offset frequency (from 10 Hz to 1 MHz). Because the bandwidth of the phase-noise measurement equipment in the electrical spectrum analyzer (HP8565E) is limited to 50 GHz, the phase noise of the upconverted RF signal with a frequency of 48 GHz generated by mixing a 45 GHz LO signal with a 3 GHz IF signal is measured. The phase noise of the upconverted RF signal follows that of the LO signal for frequencies below 100 kHz, while it follows that of the IF signal for frequencies which exceed 100 kHz, indicating that there is no serious degradation of the noise performance of the OSSB frequency upconverter associated with the LO signal path length difference [14]. The phase noise of the upconverted RF signal at an offset frequency of 10 kHz is approximately −74.72 dBc/Hz, which is limited by that of the LO signal source. The phase noise of the electrical LO (22.5 GHz) signal is measured to be −84.37 dBc/Hz and that of the optical LO signal (45 GHz) is measured to be −77.53 dBc/Hz at a 10 kHz frequency offset. The phase noise degradation associated with the frequency doubling is measured to be −6.84 dB at a 10 kHz frequency offset. This is very similar to the theoretical value of 6 dB, indicating that there exists no serious degradation of phase noise performance.
The linearity of the OSSB frequency upconverter is investigated by measuring the spurious free dynamic range (SFDR) through a two-tone test. An optical IF signal having two IF frequencies of 2.9975 GHz (fIF1) and 3.0025 GHz (fIF2) and an optical LO signal having a LO frequency of 45 GHz (fLO) are used as the input signals. A low-noise amplifier is used to amplify the electrical RF signal after the PD. Figure 9(a) shows the electrical spectra of the upconverted RF signal, consisting of the fundamental and third-order intermodulation distortion (IMD3) signals. The frequencies of the fundamental signals are 47.9975 GHz (fLO + fIF1) and 48.0025 GHz (fLO + fIF2), while the frequencies of the IMD3 signals are 47.9925 GHz (fLO + 2fIF1-fIF2) and 48.0075 GHz (fLO + 2fIF2-fIF1). Figure 9(b) shows the electrical power of the fundamental and IMD3 components as a function of the electrical IF power for the SFDR measurement. The noise floor measured with a 1 Hz resolution bandwidth is −126.5 dBm/Hz. The slope of the fundamental signal is 0.93, and that of the IMD3 signal is approximately 2.6. The measured SFDR is 72.5 dB-Hz2/3, which satisfies the SFDR requirement for fiber infrastructure components in personal communication systems [15].
Fig. 9 (a) Electrical spectra of the upconverted RF signal for the two-tone test. (b) Electrical power levels of the fundamental and third-order intermodulation distortion components as a function of the electrical IF power.
4. Conclusion
An all-optical single-sideband frequency upconversion method utilizing the XPM in an SOA-MZI is proposed and experimentally demonstrated. The optical RF signal was generated in the form of an OSSB signal by the proposed OSSB frequency upconverter. The generated OSSB RF signal undergoes no power fading associated with the chromatic dispersion of the SMF. The proposed upconverter shows no serious degradation of the phase noise performance associated with the LO signal path length difference even in the absence of an extra delay line. The phase noise of the upconverted RF signal at an offset frequency of 10 kHz is approximately −74.72 dBc/Hz. The SFDR as measured by a two-tone test is 72.5 dB-Hz2/3, which satisfies the SFDR requirement for fiber infrastructure components in personal communication systems.
5. Funding
This work was supported in part by a grant (NRF-2015R1A2A1A15055838) from the Brain Research Program through the NRF of Korea.
References and links
1. C. Lim, A. Nirmalathas, M. Bakaul, P. Gamage, K. L. Lee, Y. Yang, D. Novak, and R. Waterhouse, “Fiber-wireless networks and subsystem technologies,” J. Lightwave Technol. 28(4), 390–405 (2010). [CrossRef]
2. G. H. Smith, D. Novak, and Z. Ahmed, “Overcoming chromatic-dispersion effects in fiber-wireless systems incorporating external modulators,” IEEE Trans. Microw. Theory Tech. 45(8), 1410–1415 (1997). [CrossRef]
3. Z. Li, H. Chi, X. Zhang, and J. Yao, “Optical single sideband modulation using a fiber-Bragg-grating-based optical Hilbert transformer,” IEEE Photonics Technol. Lett. 23(9), 558–560 (2011). [CrossRef]
4. M.-T. Zhou, A. B. Sharma, Z.-H. Shao, and M. Fujise, “Optical single-sideband modulation at 60 GHz using electro-absorption modulators,” in Proc. MWP2005 (2005), pp. 121–124.
5. H.-J. Kim and J.-I. Song, “All-optical single-sideband upconversion with an optical interleaver and a semiconductor optical amplifier for radio-over-fiber applications,” Opt. Express 17(12), 9810–9817 (2009). [CrossRef] [PubMed]
6. S.-H. Lee, H.-J. Kim, and J.-I. Song, “Broadband photonic single sideband frequency up-converter based on the cross polarization modulation effect in a semiconductor optical amplifier for radio-over-fiber systems,” Opt. Express 22(1), 183–192 (2014). [CrossRef] [PubMed]
7. H.-J. Kim, S.-H. Lee, and J.-I. Song, “Generation of a 100-GHz optical SSB signal using XPM-based all-optical frequency upconversion in an SOA-MZI,” Microw. Opt. Technol. Lett. 57(1), 35–38 (2015). [CrossRef]
8. H.-J. Song, J.-S. Lee, and J.-I. Song, “Signal Up-conversion by Using a Cross-Phase-Modulation in All-Optical SOA-MZI Wavelength Converter,” IEEE Photonics Technol. Lett. 16(2), 593–595 (2004). [CrossRef]
9. H.-J. Song, J.-S. Lee, and J.-I. Song, “Error-Free Simultaneous All-optical Upconversion of WDM Radio-over-Fiber Signals,” IEEE Photonics Technol. Lett. 17(8), 1731–1733 (2005). [CrossRef]
10. Z. Jia, J. Yu, and G. K. Chang, “All-optical 16×2.5 Gb/s WDM signal simultaneous up-conversion based on XPM in an NOLM in ROF systems,” IEEE Photonics Technol. Lett. 17(12), 2724–2726 (2005). [CrossRef]
11. R. Ramaswami, K. N. Sivarajan, and G. H. Sasaki, Optical Networks: A Practical Perspective (Morgan Kaufmann, 2010), Chap 3.
12. J. Marti, J. M. Fuster, and R. I. Laming, “Experimental reduction of chromatic dispersion effects in lightwave microwave/millimetre-wave transmissions using tapered linearly chirped fibre gratings,” Electron. Lett. 33(13), 1170–1171 (1997). [CrossRef]
13. J.-H. Seo, Y.-K. Seo, and W.-Y. Choi, “1.244-Gb/s Data Distribution in 60-GHz Remote Optical Frequency Up-Conversion Systems,” IEEE Photonics Technol. Lett. 18(12), 1389–1391 (2006). [CrossRef]
14. H.-J. Kim and J.-I. Song, “Analog performance of an all-optical frequency upconverter utilizing an electro-absorption modulator for radio-over-fiber applications,” in Proc. ICMMT2010 (2010), pp. 1575-1577. [CrossRef]
15. J. C. Fan, C. L. Lu, and L. G. Kazovsky, “Dynamic range requirements for microcellular personal communication systems using analog fiber-optic links,” IEEE Trans. Microw. Theory Tech. 45(8), 1390–1397 (1997). [CrossRef]
