We propose and experimentally demonstrate a full-duplex radio-over-fiber (RoF) system with colorless upstream transmission based on orthogonal phase-correlated modulation (OPM). This new OPM scheme, which realized by a polarization rotator (PR) and a single-driver Mach-Zahnder modulator (MZM) at the central office (CO), achieves polarization-orthogonality between the optical carrier (OC) and subcarriers generated by radio frequency (RF) signals. By adjusting a polarization controller (PC) in the remote access units (RAU), different modulation schemes can be flexibly implemented, e.g. double-sideband (DSB) modulation for low RF service and optical carrier suppression (OCS) modulation for millimeter-wave (mm-wave) service. In the meantime, the OC can be reused for the upstream transmission without any filtering and additional PC. A proof-of-concept experiment is conducted to demonstrate the feasibility of proposed scheme, where downstream 800-Mb/s orthogonal frequency division multiplexing (OFDM) signal at 58 GHz as an mm-wave service and 800-Mb/s OFDM signal at 0.3 GHz as a low frequency wireless service, as well as an upstream 1-Gb/s on-off keying (OOK) are simultaneously delivered in a shared architecture. By providing heterogeneous services and colorless upstream transmission, the proposed architecture can be seamlessly integrated in wavelength division multiplexing passive optical network (WDM-PON).
© 2015 Optical Society of America
The proliferation of smart mobile devices is accelerating the evolution of wireless access networks from 3G to 4G and beyond [1, 2]. Currently, lower radio frequencies (RFs), such as Wi-Fi (IEEE 802.11), Long Term Evolution (LTE) and WiMAX (IEEE 802.16), are the dominant technologies for wireless communications because of their universal presence and mobility. On the other hand, 60-GHz millimeter-wave (mm-wave) has attracted significant interests due to its huge bandwidth. In addition, the 60-GHz wireless technology has been considered as a candidate for next-generation very high throughput (VHT) wireless personal area networks (WPANs) and wireless local access networks (WLANs) . Radio-over-fiber (RoF) technology has been proposed for its wideband and low-loss of optical fiber transmission, and considered as a promising candidate for multi-service broadband access networks . In order to reduce the cost and complexity of the networks and improve the compatibility between the RoF and the wavelength division multiplexing passive optical network (WDM-PON), it is strongly desired to simultaneously deliver the low frequency wireless and mm-wave wireless services in the shared infrastructure with colorless remote access units (RAUs).
Recently, wavelength-reuse methods have been proposed for the cost-effective upstream transmission [5, 6]. The wavelength-reuse method based on the signal erasing effect uses a gain-saturated reflective semiconductor optical amplifier (RSOA) in RAUs [7, 8]. In this case, the extinction ratio of the downstream signal is required to be low, which would limit the network performance in terms of modulation formats and data rates. In other methods, complicated wavelength-dependent optical filters or interleavers are generally required which limit the operation flexibility and ease of system integration [9–11]. In [12–14], a method of using polarization modulation-to-intensity modulation (PolM-to-IM) convertor has been proposed. However, the proposed systems suffer from the modulator chirp which interacts with dispersive effects [15, 16]. In this way, it incurs chirp-induced distortion after long distance transmission.
In this paper, a simple architecture for delivering heterogeneous services with colorless upstream transmission based on orthogonal phase-correlated modulation (OPM) is proposed and experimentally demonstrated. Making use of the polarization dependence of the single-driver Mach-Zehnder modulator (MZM), the OPM is implemented in which the polarization direction of the central OC is orthogonal with the sidebands generated by RF signals. In some cases [17, 18], OPM can be applied in microwave-signal-mixing technique and overcoming dispersion-induced power fading. In this proposed bidirectional RoF system, by adjusting polarization controllers (PCs) in the RAU, different modulation schemes can be flexibly implemented without any chirp-induced distortion, e.g. double-sideband (DSB) modulation for legacy wireless service and optical carrier suppression (OCS) modulation for mm-wave service. Meanwhile, the unmodulated OC can be selected out by a polarization beam splitter (PBS) and reused for the upstream transmission without additional filter or PC. In the experiment, 800Mb/s 16-quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM) downstream service carried at 0.3-GHz and 58-GHz are delivered simultaneously over 15-km fiber to the RAU, and then the OFDM service at mm-wave is transmitted wirelessly over 3 feet distance. Error-free transmission of 1-Gb on-off keying (OOK) upstream signals over 15 km single-mode fiber (SMF) is achieved. The receiver penalty resulting from the interference of downstream signals is less than 0.3 dB.
2. Principle of the proposed RoF system
Figure 1 illustrates the basic structure of the proposed OPM based multi-service RoF links. OPM is realized by a polarization rotator (PR) and a single-driver MZM at the central office (CO). The light emitted from a laser diode (LD) is injected into the MZM via a PR. The PR, which contains a quarter-wave plate and a rotatable polarizer, can align the light’s polarization direction at an angle of θ to the principle axis of LiNbO3 crystal of MZM (noted as y-axis in Figs. 1(a)-1(e)), as shown in Fig. 1(a). In general, the half-wave voltage Vπ in the x-direction is around 3.58 times of that in the y-axis due to the electro-optical property of the LiNbO3 crystal . As a result, maximum modulation efficiency (ME) is achieved when the polarization direction of the input lightwave is parallel to the principal axis of MZM, and the modulation for the x-axis component of the light is insignificant. As shown in Fig. 1, mm-wave service on frequency ω1 and low RF service on frequency ω2 are electrically coupled and modulated on the MZM biased at the minimum transmission point. Thus, the component of injected light on the y-axis is modulated in the form OCS while the x-axis component of the light is not modulated, as illustrated in Fig. 1(b). Moreover, the x-axis component is phase-correlated with y-axis component since they originate from the same light source and travel in the same optical path. Ignoring the modulation and polarization dependent loss in the x-axis, the optical field at the output of the MZM can be written as 
At the RAU, the orthogonal signals are split into two beams by a splitter. The first beam is injected into a PC1 and a polarizer (Pol). By adjusting PC1, the polarization direction of the OC is oriented to 45° with respect to the principle axis of Pol. Therefore, as shown in Fig. 1 (c), the optical field of the modulated light after Pol can be expressed as
For the other beam, the signal is injected into a PC2 and a PBS, and divided into two orthogonal polarizations, as shown in Fig. 1(d) and Fig. 1(e) respectively. The optical field at one output of the PBS can be expressed as (see Fig. 1 (d))20] is implemented and the frequency-doubled mm-wave signal can be generated.
The optical field at the other output of PBS is (see Fig. 1 (e))Eq. (6), only the OC is reserved with constant phase and amplitude, which implies that the selected OC signal would be chirp-free. Thus it can be re-used directly for uplink transmission without additional optical filtering.
Therefore, the orthogonal phase-correlated lights are generated by utilizing the polarization dependence of the MZM. By adjusting PCs at the RAU, DSB and OCS formats can be achieved simultaneously for different services. Meanwhile, unmodulated OC can be easily reused for upstream without any additional optical filtering or PC at the RAU.
3. Experiment setup and results
The experimental setup for the OPM-based multi-service RoF system is depicted in Fig. 2. A continuous wave light originated from a LD with a wavelength of 1545.46 nm and a linewidth of 5MHz at the CO is fed into a PR and an MZM1 (bandwidth:40GHz, insertion loss:5dB) to generate phase-correlated orthogonal lightwaves. The angle of polarization direction of the OC and the principle axis of the MZM1 can be adjusted precisely by the PR (noted as θ in Fig. 1(b)). Here, θ is set at 45° . Data1 and Data2, generated by an arbitrary waveform generator (AWG) at 2.0-GSa/s sampling rate, are 16-QAM-OFDM data at an intermediate frequency at 0.3 GHz and a bit rate of 800 Mb/s. In the experiment, Data2 is used to emulate the low RF service. The carrier frequency is relatively low due to the bandwidth limitation of the AWG. The frequency of the sinusoidal wave provided by a local oscillator (LO) is up-converted to 29 GHz from 14.5 GHz by a frequency doubler (FD). Data1 is mixed with the 29-GHz microwave and then combined with Data2 to drive the MZM1 which is biased at the minimum transmission point. The modulated light is amplified by an erbium-doped fiber amplifier (EDFA) after transmitting over 15-km SMF.
At the RAU, the downstream signal is split into two branches by the splitter. In the first branch, DSB format is implemented by tuning a PC1 and a Pol and Data2 (low RF signal) is received by a PD1 with a 3-dB bandwidth of 10 GHz. The 16-QAM-OFDM signal is recorded by an oscilloscope at 10-GSa/s sampling rate and demodulated offline. The beating of OC and Data1 tones will exceed the bandwidth of the PD1, and thus introduce no interference to the low RF signal. The optical spectrum of the DSB signal is shown as inset (a) in Fig. 2. In the second branch, a PC2 and a PBS are used to separate the optical signal into two orthogonal polarizations. For the y-axis, OCS is achieved (see Fig. 2(b)) and Data1 carried at 58-GHz is obtained after the PD2 (3-dB bandwidth: 60-GHz). A pair of 60-GHz horn antenna (15 dBi gain) with 3-ft separation is used for wireless propagation. After that, an envelope detector (ED) and a direct current (DC) block are employed to down-convert the 58-GHz wireless signal and eliminate the DC components, respectively. Electrical spectrum measured at point d is shown in Fig. 2(d). For the upstream, the x-axis lightwave, i.e. the unmodulated OC (see Fig. 2(c)) is sent to the PC3 and modulated by the MZM2 (bandwidth: 10GHz, insertion loss: 4.5dB) with 1-Gb/s OOK signal which is generated from pseudo random binary sequence (PRBS) pattern generator. PC3 is fixed to the polarization direction paralleled to the principle axis of the MZM2 for maximum ME. After the 15-km SMF transmission, the upstream signal is converted into an electrical signal by the PD3 and measured by the bit error rate tester (BERT).
For OFDM-RoF systems, nonlinear effect due to high peak-to-average power ratio of OFDM will affect the error vector magnitude (EVM) performances of received signals . Figures 3(a) and 3(b) indicate the EVM versus voltage of the input OFDM signal for mm-wave and low RF channels with BTB transmission, respectively. As the input voltage increases, EVM first decreases due to improvement of signal-to-noise ratio, and then increases due to the nonlinear effect. It can be observed that the optimum value of the OFDM input voltage is 35 mV and 25 mV for the mm-wave and low RF channels, respectively. As a result,the input voltage of Data1 and Data2 are set at 35 mV and 25 mV to modulate the MZM1 in our experiment.
The EVM performance versus received optical power of the 16-QAM-OFDM data carried on 58-GHz and 0.3-GHz are shown in Figs. 4(a) and 4(b), respectively. The received optical power is measured before the EDFA at the RAU. The insets show the clear constellations of the demodulated Data1 and Data 2 in different EVM value. After 15-km SMF transmission, negligible power penalty is observed. Both low-RF and the mm-wave services achieve good performances after optical fiber transmission.
Figure 5 shows the BER value for the upstream OOK signal versus received optical power after 15-km SMF transmission with or without downstream signals. The clear eye diagrams are recorded in both cases with BER lower than 10−9 .The power penalty of uplink data resulting from the interference of downstream signals is less than 0.3 dB at 10−9 BER.
For long-term RoF link, drifts in polarization state which result from physical changes in the fiber will incur power degradation of the signals. As a result, a commercial polarization tracker should be adopted to replace PC in the RAU to increase the stability of the device as the proposed system is applied in practice.
In summary, a simple access network architecture with colorless upstream transmission based on orthogonal phase-correlated modulation to simultaneously deliver low RF and mm-wave services is proposed and demonstrated. Compared with the existing wavelength-reuse method, the proposed scheme greatly increases the flexibility and further simplifies the design of RAU. In our design, the RAU can be operated wavelength-independently, i.e., colorless operation. Thus it is possible to integrate the proposed RoF system into WDM-PON based on OPM to cover massive amount of users. We believe the proposed RoF architecture provides a versatile and cost-effective solution for multi-service systems.
This work was supported in part by the Georgia Institute of Technology, in part by the National Natural Science Foundation of China under Grant 61275091 and Grant 61327006. The work of B. Wu was supported in part by the China Scholarship Council for Scholarship.
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