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An all-optical frequency downconversion utilizing a four-wave mixing effect in a single semiconductor optical amplifier (SOA) was experimentally demonstrated for wavelength division multiplexing (WDM) radio-over-fiber (RoF) applications. Two WDM optical radio frequency (RF) signals having 155 Mbps differential phase shift keying (DPSK) data at 28.5 GHz were simultaneously down-converted to two WDM optical intermediate frequency (IF) signals having an IF frequency of 4.5 GHz by mixing with an optical local oscillator (LO) signal having a LO frequency of 24 GHz in the SOA. The bit-error-rate (BER) performance of the RoF up-links with different optical fiber lengths employing all-optical frequency downconversion was investigated. The receiver sensitivity of the RoF up-link with a 6 km single mode fiber and an optical IF signal in an optical double-sideband format was approximately −8.5 dBm and the power penalty for simultaneous frequency downconversion was approximately 0.63 dB. The BER performance showed a strong dependence on the fiber length due to the fiber dispersion. The receiver sensitivity of the RoF up-link with the optical IF signal in the optical single-sideband format was reduced to approximately −17.4 dBm and showed negligible dependence on the fiber length.
©2012 Optical Society of America
1. Introduction
With the rapid increase in broadband wireless multimedia services, including high definition television, ultra-high definition television, telemedicine, distance education, and video conferencing, the demand for broadband wireless access networks is increasing. Radio over fiber (RoF), in which an optical fiber having ultra-wide bandwidth and ultra-low loss characteristics is used as a transmission medium for microwave or millimeter-wave modulated optical signals, is a candidate for broadband wireless access networks. The bandwidth of RoF systems can be further increased by using wavelength division multiplexing (WDM) technology, which makes them attractive for broadband wireless networks [1].
Various nonlinear optical signal processing methods, including photonic signal generation, all-optical frequency conversion, photonic RF filtering, and photonic true-time delay for processing micro/millimeter-wave signals, are used for WDM RoF applications due to their broadband bandwidth characteristics [2–7]. All-optical frequency conversion, in which radio frequency (RF), local oscillator (LO), and intermediate frequency (IF) signals in the form of an optical signal are used, is one of most important nonlinear optical signal processing techniques for WDM RoF systems. The all-optical frequency downconversion scheme using a cross phase modulation (XPM) effect in a semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) was experimentally demonstrated for WDM RoF systems [6]. Two WDM optical IF signals were simultaneously generated by mixing two WDM optical RF signals with an optical LO signal without serious crosstalk between two channels. However, the maximum frequency of the optical RF signal that can be downconverted by the SOA-MZI was limited by the bandwidth of the SOA-MZI. The bandwidth of the SOA-MZI for all-optical frequency downconversion using the XPM effect is determined by the minority carrier lifetime [6].
In this paper, an all-optical frequency downconversion using a four-wave mixing (FWM) effect in a single SOA for WDM RoF applications is reported. The maximum frequency of the all-optical frequency downconversion using the FWM effect in the SOA is higher than that using the XPM effect in the SOA-MZI since the FWM mechanisms in the SOA such as spectral hole burning (SHB) and carrier heating (CH) have a much shorter response time compared with the carrier lifetime of the SOA-MZI [8–10]. Two WDM optical RF signals were simultaneously downconverted to two WDM optical IF signals by mixing with an optical LO signal in an SOA. Bit-error-rate (BER) measurements were performed to investigate the crosstalk between two channels during simultaneous all-optical frequency downconversion and the transmission characteristics of the optical IF signal generated by this scheme over a single mode fiber (SMF).
2. Principle of all-optical frequency downconversion using the FWM effect in a single SOA for WDM RoF systems
Figure 1 shows the principle of a WDM RoF system using an all-optical frequency downconverter based on the FWM effect in an SOA. WDM optical RF signals (ωRF1, ωRF2,···, ωRFn) are generated at the base stations (BSs), combined by the multiplexer (MUX) located at a remote node (RN), and directed via the SMF to a central office (CO). An optical LO signal (ωLO) is generated from an LO block located at the CO. The WDM optical RF signals and the optical LO signal are combined by an optical coupler and directed to the SOA. The spectra of the WDM optical RF signals and the optical LO signal are shown in Fig. 1(b). The two optical tones of the optical LO signal and the optical carrier of the optical RF signal can be expressed as [11];
where ELO and ERF–C are the amplitudes of the two optical tones of the optical LO signal and the optical carrier of the optical RF signal, respectively. ωLO1, ωLO2, and ωRF–C are the angular frequencies of the two optical tones of the optical LO signal and the optical carrier of the optical RF signal, respectively. øLO and øRF–C are the phases of the two optical tones of the optical LO signal and the optical carrier of the optical RF signal, respectively. J2(β) is the second-order Bessel function and β is the phase modulation index. The SOA is used for a nonlinear medium in which the FWM effect occurs. The optical LO signal interacts with each of the WDM optical RF signals through a third order nonlinearity such as the FWM effect in the SOA. Figure 1(c) shows the spectra of the WDM optical IF signals (ωIF1, ωIF2,···, ωIFn) at the output of the SOA. For each channel, the wavelength of the WDM optical IF signal is identical to that of the WDM optical RF signal. After the all-optical frequency downconversion process, the WDM optical IF signals are demultiplexed by a demultiplexer (DEMUX) and directed to their respective receivers (Rx1, Rx2,···, Rxn). Figure 1(d) shows the spectra of one of the optical IF signals (channel 1) at the output of the DEMUX. The powers of the converted signals (ωc1 and ωc2) within channel 1 can be expressed approximately as [11]; where r(ωLO2-ωLO1) and r(ωLO1-ωLO2) are the relative conversion efficiencies which are inversely proportional to the frequency spacing. The angular frequencies of the converted signals can be expressed as; where ƒLO and ƒRF are the frequencies of the electrical LO and RF signals, respectively. The optical IF signal is detected by a photo-detector (PD) at Rx1. At the output of the PD, the electrical IF signal is generated by beating of tones ((ωRF1– and ωc1) and (ωRF1+ and ωc2)).
Fig. 1 Principle of all-optical frequency downconversion. (a) Schematic diagram for a WDM RoF system using an all-optical frequency downconverter based on the FWM effect in an SOA. (b) Optical spectra at the input of the SOA. (c) Optical spectra at the output of the SOA. (d) Optical spectra at one of the DEMUX outputs (channel 1).
3. Experiment and results
The experimental setup for the all-optical frequency downconversion using the FWM effect in an SOA is shown in Fig. 2 . In order to generate two WDM optical RF signals, two laser diodes (LDs; LD1 and LD2) and an electro-absorption modulator (EAM) were used. The wavelengths of LD1 and LD2 (i.e. channel 1 and 2) were 1549.46 and 1551.05 nm, respectively. A differential phase-shift keying (DPSK) modulator was used to generate 155 Mbps DPSK data at 4.5 GHz from 155 Mbps non-return to zero (NRZ) data. The output of the DPSK modulator was upconverted to an electrical RF signal with 155 Mbps DPSK data at 28.5 GHz by mixing with a 24 GHz electrical LO signal. The electrical RF signal was amplified by an electrical amplifier1 (EA1), filtered by an electrical band pass filter (EBPF1) having the loss of 0.6 dB and the bandwidth of 400 MHz, and directed to the EAM. Two WDM optical RF signals were generated at the output of the EAM by modulating the optical carriers from LD1 and LD2 with the electrical RF signal and directed to the CO via the SMF (Fiber type: SMF-28, Corning). The power of the optical RF signals was adjusted by a variable optical attenuator (VOA1). An optical LO signal consisting of two optical tones separated by 24 GHz was generated by the optical carrier suppression (OCS) technique. LD3 with a wavelength of 1546.24 nm was modulated by the MZM biased at the minimum transmission point with an electrical signal source (ƒLO/2 = 12 GHz). OA1 with a gain of 35 dB, OBPF1 having the loss of 1 dB and the bandwidth of 1.1 nm, and VOA2 were used at the output of the MZM. The optical LO signal and two optical RF signals were directed to an SOA. Figure 3(a) shows the optical spectra of the signal at the input of the SOA. The SOA was biased at a current of 350 mA. At the SOA output, two optical IF signals, the optical spectra of which are shown in Fig. 3(b), were generated by the FWM effect. One of the two optical IF signals was selected by a tunable optical filter (OBPF2 or OBPF2´), amplified by OA2 with a gain of 35 dB, and then filtered by OBPF3 to select spectra used for generation of an electrical IF signal. OBPF2 and OBPF2´ had the losses of 3.3 and 3.2 dB and the bandwidths of approximately 0.5 and 0.11 nm, respectively, and were used to generate an optical IF signal in the form of an optical double-sideband (ODSB) signal and an optical single-sideband (OSSB) signal, respectively. OBPF3 having the loss of 3 dB and the bandwidth of 0.85 nm was used to remove the spontaneous emission noise from OA2. VOA3 was used to adjust the power of the optical IF signal for BER measurements. Figures 3(c) and 3(d) show the spectra of the optical IF signals at an output of OBPF2 and OBPF2´, respectively. The filtered optical IF signal was converted to the electrical IF signal by the PD. The electrical IF signal with 155 Mbps DPSK data at 4.5 GHz was amplified by EA2, filtered by EBPF2 having the loss of 2 dB and the bandwidth of 500 MHz, and amplified by EA3. Figure 4 shows the spectra of the electrical IF signal when OBPF2 was used. The DPSK demodulator converted the electrical IF signal to 155 Mbps NRZ data.
Fig. 2 Experimental setup for all-optical frequency downconversion using the FWM effect in an SOA (BERT: bit-error-rate tester, OA: optical amplifier, PC: polarization controller).
Fig. 3 Spectra at each node. (a) Optical spectra at the input of the SOA. (b) Optical spectra at the output of the SOA. (c) Optical spectra at the output of OBPF2. (d) Optical spectra at the output of OBPF2´. The resolution bandwidth (RBW) of the optical spectrum analyzer was 0.01nm.
Bit-error-rate measurements were performed to investigate crosstalk between two channels during the simultaneous all-optical frequency downconversion process. The optical LO signal power was approximately 0 dBm. The peak power of the optical RF signals for single-channel and simultaneous upconversion was approximately −10 dBm. BER measurements were carried out for optical IF signals filtered by OBPF2 (spectra of the signal shown in Fig. 3(c)) and OBPF2´ (spectra of the signal shown in Fig. 3(d)). Figure 5 shows the measured BER for the case of an optical IF signal filtered by OBPF2 as a function of the received optical power for single-channel downconversion and simultaneous downconversion. An SMF with a length of 6 km was used. For single-channel downconversion, the receiver sensitivities of channel 1 and 2 at the BER of 10−9 were approximately −8.54 and −8.42 dBm, respectively. The power penalties of channel 1 and 2 at a BER of 10−9 for simultaneous all-optical frequency downconversion were approximately 0.72 and 0.54 dB, respectively. The transmission performance of RoF uplinks with different fiber lengths using this all-optical frequency downconversion scheme was experimentally investigated for a single-channel configuration. The length of the SMF was varied from 0 to 8 km. Figure 6(a) shows the measured BER for the case of an optical IF signal filtered by OBPF2 as a function of the received optical power. The BER performance of this case depended strongly on the length of the SMF since the optical IF signal (spectra shown in Fig. 3(c)) has more than two IF beat components in the upper and lower sidebands, which makes it very sensitive to the chromatic dispersion of the SMF. As can be seen in Fig. 6(a), a worse receiver sensitivity was observed for the back-to-back transmission (0 km) due to the destructive interference between two beat signals ((ωc1 and ωRF–) and (ωc2 and ωRF+)). The destructive interference was produced since the phase difference between the two tones (ωLO1 and ωLO2) of the optical LO signal generated by the OCS method is 180°, which makes the phase difference between the tones (ωc1 and ωc2) of the optical IF signal 0°. When the length of the SMF was 8 km, a BER floor was observed. Figure 6(b) shows the measured BER for the case of an optical IF signal filtered by OBPF2´ as a function of the received optical power. Since the optical IF signal was converted to the OSSB format by OBPF2´, the effect of the fiber dispersion was eliminated. In addition, the receiver sensitivities at a BER of 10−9 were decreased to approximately −17.4 dBm.
Fig. 5 Measured BER as a function of the received optical power for single-channel downconversion and simultaneous downconversion for an optical IF signal in ODSB format (filtered by OBPF2). (a) Channel 1. (b) Channel 2.
Fig. 6 Measured BER as a function of the received optical power for different lengths of SMFs. (a) Optical IF signal in ODSB format (filtered by OBPF2) and (b) Optical IF signal in OSSB format (filtered by OBPF2´).
Note that there are several advantages of this all-optical frequency downconversion using the FWM effect in the SOA. First, this downconversion scheme provides simultaneous conversion capability, which makes the WDM system simple. Second, the maximum RF frequency that can be converted by this downconversion scheme using the FWM effect in an SOA can be extended to serveral hundreds of gigahertz, which is much higher than that of the downconversion scheme using the XPM effect in an SOA-MZI, since the characteristic time of the FWM effect, such as SHB and CH, is much shorter than that of the XPM effect [7]. Thus, this downconversion scheme can be readily applied to different RF bands. Third, the SOA is compact and can be integrated with other semiconductor-based devices on the same substrate. In addition, the optical IF signal can be amplified by the optical gain of the SOA. Finally, since the two opical LO signal tones generated by the OCS method in this downconversion scheme using the FWM effect have the same polarization, this downconversion scheme is less sensitive to the polarization states of the input optical RF signals compared with the scheme using the two opical pump signals with different polarization states were used [12, 13].
4. Conclusions
An all-optical frequency downconversion utilizing the FWM effect in an SOA for WDM RoF applications was experimentally demonstrated. This downconversion scheme offers simultaneous conversion capability, broad RF/IF bandwidth, compactness, and insensitivity to the polarization state of optical RF signals. The results show that the all-optical frequency downconverter utilizing the FWM effect in an SOA has potential for use in broadband wireless access networks employing WDM technology.
Acknowledgments
This work was supported by a WCU program (R31-2008-000-10026-0) from the NRF of Korea and by a grant (2011-0000341) and the Bio-imaging Research Center program at GIST.
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