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We propose and experimentally demonstrate an all-optical upconverter for the generation of an optical single-sideband (OSSB) signal in radio-over-fiber (RoF) systems. The OSSB signal, which is required for overcoming the fiber chromatic dispersion problem in RoF systems, is generated by using an all-optical SSB upconverter consisting of an optical interleaver and a semiconductor optical amplifier. With this upconversion technique, OSSB radio frequency (RF) signals with an RF frequency ranging from 15 GHz to 42.5 GHz are generated by mixing an optical intermediate frequency (IF) signal (1 GHz) with an optical local oscillator signal and transmitted over a 46 km standard single-mode fiber. The OSSB RF signal generated by this upconversion technique shows negligible dispersion-induced carrier suppression effects, which are usually observed for an optical double-sideband RF signal. The all-optical SSB upconverter provides high conversion efficiency of up to 29 dB and a sufficiently large spurious free dynamic range of 82 dB·Hz2/3 for microcellular personal communication system applications.
©2009 Optical Society of America
1. Introduction
Radio-over-fiber (RoF) technology has been considered a potential candidate for supporting future broadband wireless networks. Optical fibers are an attractive transmission medium for high data rate millimeter-wave communication systems because of their wide bandwidth and low loss characteristics. However, as the carrier frequency of RoF systems is increased, the length of the fiber link can be severely limited by fiber chromatic dispersion [1]. In RoF systems, an radio frequency (RF) modulated optical signal (optical RF signal) is generated at a central office (CO) and distributed to base stations (BSs) via an optical fiber. When an optical double-sideband (ODSB) modulation technique is used to generate the optical RF signal, the generated optical double sideband (ODSB) RF signal has one carrier and two sidebands. At the BSs, the transmitted ODSB RF signal is converted to an electrical DSB RF signal by a photodiode (PD). The electrical DSB RF signal consists of two beat components that originate from the two sidebands. During the transmission of the ODSB RF signal via an optical fiber, the two sidebands experience different phase shifts due to the fiber chromatic dispersion. As a result, the power of the detected RF signal at the BSs fluctuates according to the phase difference between the two sidebands when the fiber length or the carrier frequency is changed, thereby degrading the performance of the RoF system.
The use of an optical single-sideband (OSSB) signal is a useful way of overcoming the fiber chromatic dispersion problem in RoF systems. Various methods have been proposed to generate an OSSB RF signal in RoF systems [1–4]. The OSSB RF signal is generated by using a dual-electrode Mach-Zehnder modulator biased at the quadrature point with a phase shift of π/2 [1], an optical filter [2,3], and two electro-absorption modulators [4]. The generated OSSB RF signal has only one sideband with an optical carrier and, thus, the dispersion-induced carrier suppression (DICS) does not appear even when the fiber length or the carrier frequency is changed [1,2]. However, these methods suffer from a relatively low level of receiver sensitivity originating from a large difference in the power of the optical carrier component and sideband component [5,6]. In addition, the frequency of the OSSB RF signal generated by these methods depends on the performance of external optical modulators, such as the Mach-Zehnder modulator and electro-absorption modulator.
This paper demonstrates an all-optical single-sideband upconversion technique. The technique utilizes a frequency upconverter consisting of an optical interleaver and a semiconductor optical amplifier (SOA), and these features suppress the fiber chromatic dispersion problem in the RoF system. This technique shows no DICS effects and has a high conversion efficiency, immunity to polarization change of the optical intermediate frequency (IF) signal, and a small difference in the power of the optical carrier component and the power of the sideband component. It also shows a sufficiently large spurious free dynamic range (SFDR) to satisfy the minimum value required for applications in microcellular personal communication systems.
2. Operational principle of the all-optical SSB upconversion
Fig. 1 shows the operational principle of the all-optical SSB upconversion technique for the generation of an optical RF signal in an OSSB format. Figure 1(a) shows an RoF system configured with an all-optical SSB upconverter. Figure 1(b) shows a schematic block diagram of an all-optical SSB upconverter based on optical demultiplexing and cross-gain-modulation-based (XGM-based) wavelength conversion, while Fig. 1(c) shows optical spectra at each node of the all-optical SSB upconverter. The optical local oscillator (LO) signal (λLO) and the optical IF signal (λIF) are generated at the CO (node A, Input). The optical LO signal consists of two optical signal tones separated by the LO frequency (ƒLO). The optical IF signal is generated by conventional DSB modulation with an IF frequency of ƒIF. The optical LO and IF signals are directed to the optical demultiplexer and the XGM-based wavelength converter, respectively. The optical demultiplexer directs the left tone of the optical LO signal to the upper optical fiber (node B) and the right tone of the optical LO signal to the wavelength converter (node C). The wavelength converter copies two sidebands (ƒIF) of the optical IF signal to the right tone of the optical LO signal (node D). The output of the wavelength converter is then combined with the left tone of the optical LO signal by the optical combiner (node E). An optical bandpass filter is used to produce an optical RF signal that consists of the left tone of the optical LO signal and the right tone of the optical LO signal with the two sidebands (ƒIF) (node F, Output). The wavelengths of the optical LO and optical RF signals are the same. The OSSB RF signal is transmitted via an optical fiber to the BS and then converted to an electrical RF signal by a photodetector. The converted RF signal has only one beat component for each upper sideband (USB, ƒLO + ƒIF) and lower sideband (LSB, ƒLO−ƒIF); thus, there are no DICS effects due to the destructive interference between the two beat components observed in the ODSB signals.
Fig. 1 Principle of all-optical SSB upconversion: (a) an RoF system configured by using the all-optical SSB upconverter; (b) schematic diagram of the all-optical SSB upconverter; (c) optical spectra at each node of the all-optical SSB upconverter
3. Experiment and results
A schematic diagram of the experimental setup for the all-optical SSB upconversion is shown in Fig. 2 . An optical LO signal (λLO = 1556.036 nm) is generated in the LO block by using MZM1, which is biased at Vπ for an optical carrier suppression method, with the electrical LO source (ƒLO/2 = 12 GHz). The EDFA1, OBPF1, and VOA1 are placed at the output of MZM1 to provide different signal powers. The optical IF signal (λIF = 1559 nm) is generated in the IF block by using MZM2, which is biased at Vπ/2, with the electrical IF signal (ƒIF = 1 GHz). The VOA2 at the output of MZM2 is also used to control the optical IF power. The generated optical LO signal shown in Fig. 3(a) is demultiplexed by the 25/50 GHz optical interleaver. The left tone of the optical LO signal is directed to the odd port of the optical interleaver and then amplified by EDFA2, as shown in Fig. 3(b). The right tone of the optical LO signal is directed to the even port of the optical interleaver and fed to the SOA (CIP, model number: SOA-NL-OEC-1550) along with the optical IF signal by means of an optical combiner. The two sidebands (ƒIF) of the optical IF signal are copied to the right tone of the optical LO signal by the XGM effect in the SOA. Figure 3(c) shows the optical spectra of the signal at the output of the SOA. Furthermore, as shown in Fig. 3(d), the optical signals shown in Fig. 3(b) and Fig. 3(c) are combined and then directed to OBPF2. The OSSB RF signal is selected by the OBPF2, transmitted via an optical fiber to the BS, and then converted to the electrical SSB RF signal by the PD. Figure 3(e) shows the spectra of the OSSB RF signal at the input of the PD. The noise pedestal shown in Fig. 3 (e) originates from ASE noises of the SOA, EDFA1, and EDFA2. The other source of optical noise is shown in Fig. 3(b). The suppression of the right-tone of the optical LO signal by the interleaver is imperfect that thus there remains the right-tone signal having the optical power of approximately −18 dBm, which is much larger than the noise pedestal shown in Fig. 3 (b). The suppression ratio of the optical interleaver used for the suppression of the right-tone of the optical LO signal is around 25 dB in the experiment. This unwanted signal can act as a noise and the resulting optical signal to noise ratio is approximately 28 dB. An optical attenuator is used before the PD to prevent nonlinear detection and possible damage of the PD. Figure 3(f) shows RF spectra of the detected OSSB RF signal at the output of the PD with a resolution bandwidth of 10 kHz. As shown in Fig. 3(f), the USB and the LSB of the electrical RF signal are generated at frequencies of 25 GHz and 23 GHz, respectively. This all-optical SSB upconversion scheme produces an inverted modulation signal since it utilizes the XGM effect.
Fig. 2 Experimental setup for the all-optical SSB upconversion (DFB-LD: distributed feedback laser diode; MZM: Mach-Zehnder modulator; VOA: variable optical attenuator; SOA: semiconductor optical amplifier; EDFA: erbium-doped fiber amplifier; PD: photodiode, ESA: electrical spectrum analyzer)
Fig. 3 Measured optical and electrical spectra: (a) input of the optical interleaver; (b) output of EDFA2; (c) output of the SOA; (d) input of OBPF2; (e) input of the PD; (f) output of the PD
Fig. 4(a) shows the conversion efficiency as a function of the optical LO power for different EDFA2 currents. The conversion efficiency is defined as the ratio of the electrical RF power of the up-converted signal (USB: upper-sideband or LSB: lower-sideband) to the electrical IF power measured before the all-optical SSB upconverter. The optical power and the electrical IF power used for the measurements are −14 dBm and 10 dBm, respectively. The conversion gain increases as the optical LO power is increased. However, when the optical LO power is increased beyond −7 dBm, the conversion efficiency saturates and then decreases due to the optical gain saturation of the SOA [7]. A very high conversion efficiency in the range of 12 dB to 29 dB is achieved, which is substantially larger than those of other XGM-based all-optical upconversion schemes [7,10]. The high conversion efficiency is due to the optical gain of the SOA and EDFA2. As the bias current of the EDFA2 is increased, the conversion efficiency is increased because the optical gain of the EDFA2 is increased. Fig. 4(b) shows the carrier-to-sideband ratio (CSR) as a function of the EDFA2 currents. The CSR, which is defined as the ratio of the optical carrier power (left-tone in Fig. 3 (e)) to the optical sideband power (right-tone in Fig. 3 (e)), is negative because the power of the optical sideband is larger than that of the optical carrier. As the EDFA2 current is increased, the magnitude of the CSR decreases, indicating that the power of the optical carrier becomes similar to that of the optical sideband.
Fig. 4 (a) Conversion efficiency as a function of the optical LO power for different EDFA2 currents; (b) CSR as a function of the EDFA2 current
The effect of the fiber chromatic dispersion on the all-optical SSB conversion is investigated by inserting a 46 km-long single mode fiber (SMF) between the CO and the BS. For the characterization, the following values are used: an optical LO power of −5 dBm, an optical IF power of −14 dBm, an SOA bias current of 30 mA, and an EDFA2 bias current of 95 mA. The LO frequency is scanned from 14 GH to 41.5 GHz with the IF frequency fixed at 1 GHz. The power (USB) of the electrical RF signal is measured with respect to the RF frequencies, with and without the 46 km SMF. For a comparison of the effect of fiber chromatic dispersion on the transmission of OSSB-modulated signals and ODSB-modulated signals, the electrical power (USB) of the ODSB-modulated signal are also calculated in accordance with the procedure shown in [8] by using a fiber dispersion parameter (D) of 17 ps/(km-nm) and a center wavelength (λ0) of 1550 nm. As shown in Fig. 5 , while the ODSB modulation shows multiple carrier suppression points of the electrical RF signal due to the fiber chromatic dispersion effect, the all-optical SSB upconversion shows no carrier suppression of the electrical RF signal after transmission over the 46 km SMF. Measurements show that the electrical power of the USB remains almost flat for the LO frequency range of 14 GHz to 41.5 GHz (limited by the experimental setup). The use of an appropriate optical demultiplexer and photodetector can generate an electrical RF signal with the frequency of more than several hundred gigahertz.
Fig. 5 Normalized value of the electrical RF signal (USB) power as a function of the frequency of the RF signal
While the response of the electrical RF signal power with respect to the RF frequency is almost flat for a large frequency range, the corresponding response with respect to the IF frequency is not. Figure 6 shows the normalized value of the electrical RF signal power (USB) as a function of the IF frequency for different SOA currents. The LO frequency is fixed at 24 GHz. As the SOA current is increased, the electrical RF signal power and the IF bandwidth also increase. The electrical RF signal power increases because the increased optical gain of the SOA due to the increased bias current causes an increase in the efficiency of the XGM effect [9]. The IF bandwidth increases because the carrier lifetime of the SOA, which limits the response of the XGM effect, is reduced as the SOA bias current is increased. For an SOA current of 300 mA, the IF bandwidth is approximately 4.5 GHz. The results indicate that the number of channels can be limited by the IF bandwidth when subcarrier multiplexing is used.
Fig. 6 Normalized value of the electrical RF signal (USB) power as a function of the IF frequency for different SOA currents
The linearity of the all-optical SSB upconverter is investigated by means of a two-tone test. A low noise amplifier with a gain of 30 dB and a noise figure of 2.5 dB is used to amplify the electrical RF signal detected by the PD. Two IF frequencies (ƒIF1 = 0.9975 GHz and ƒIF2 = 1.0025 GHz) and an LO frequency (ƒLO) of 24 GHz are used. Figure 7(a) shows the electrical spectra of the OSSB RF signal measured with a resolution bandwidth of 10 kHz at the center frequency of 25 GHz. As shown in Fig. 7(a), the fundamental signals along with the third order intermodulation (IMD3) signals are generated by the upconverter. The frequencies of the fundamental signals are 24.9975 GHz (ƒLO + ƒIF1) and 25.0025 GHz (ƒLO + ƒIF2), and the frequencies of the IMD3 signals are 24.9925 GHz (ƒLO + 2ƒIF1-ƒIF2) and 25.0025 GHz (ƒLO + 2ƒIF2-ƒIF1). The electrical powers of the fundamental and the IMD3 signals as a function of the electrical IF power are measured to investigate the SFDR characteristics. Figure 7(b) shows the electrical powers of the fundamental and third-order harmonic components of the OSSB RF signal as a function of the electrical IF power for the SFDR measurement. In the experiment, the SFDR is defined as the difference of the fundamental and IMD3 signal powers when the IMD3 signal power is equal to that of the noise floor. The measured noise floor of the 1 Hz resolution bandwidth is −126 dBm/Hz. The slopes of the fundamental and IMD3 signal power are 1 and 3, respectively. The SFDR of approximately 82 dB·Hz2/3 is comparable to those reported for other all-optical frequency upconversion schemes [10,11] and satisfies the minimum SFDR requirement (72 dB·Hz2/3) for microcellular personal communication systems [12].
Fig. 7 (a) Electrical spectra of the OSSB RF signal measured with a resolution bandwidth of 10 kHz at the center frequency of 25 GHz; (b) electrical powers of fundamental and third-order harmonic components of the OSSB RF signal as a function of the electrical IF power
4. Conclusion
An all-optical single-sideband upconversion technique with an optical interleaver and an SOA is proposed and experimentally demonstrated. This technique shows no DICS effects; it also shows a high conversion efficiency, a small power difference of the optical carrier and sideband, and a large SFDR. The all-optical SSB upconverter generates OSSB RF signals that have a carrier frequency of up to 42.5 GHz with a minimal chromatic dispersion effect.
Acknowledgments
This work is supported in part by a grant from KOSEF (2009-0080277) and the Bio-imaging Research Center at the Gwangju Institute of Science and Technology.
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