We propose and demonstrate a simple technique to improve the carrier-to-interference ratio (CIR) of optical single sideband with carrier modulated signals for fiber-radio applications. The proposed scheme is demonstrated via experiment and simulation with a two-tone test showing that an improvement in CIR of 9 dB can be achieved.
©2006 Optical Society of America
Broadband wireless access networks operating in the millimeter-wave (mm-wave) or sub-mmwave regions with optical fiber backbones have the potential to provide users with untethered connectivity . However the transport of mm-wave modulated optical signals is subject to fibre chromatic dispersion effects that severely limit the transmission distance . It is wellestablished that this shortcoming can be overcome by distributing optical single sideband with carrier (OSSB+C) modulated signals . Another important issue in a multi-carrier radio environment is the overall system linearity. It has been shown that the nonlinear behavior of the external modulator severely limits the overall system dynamic range  and this condition is further aggravated with fiber transmission and other optical impairments such as cross phase modulation, if high launch power is used . Therefore it is essential to minimize the intermodulation distortions at the transmitter before the optical signals are transported to the customers. A number of techniques have been proposed to improve the dynamic range of optical analog links including optical feedforward , parallel modulator  and predistortion  schemes. However little work has been done on improving the carrier-to-interference ratio (CIR) for OSSB+C signals.
In this paper, we propose a technique to improve the CIR of OSSB+C signals by removing the major contributor to third-order intermodulation distortions (IMD). We demonstrate experimentally and via simulations the feasibility of the proposed technique using a two-tone test at 37.5 GHz.
2. Major IMD contributor
OSSB+C modulated signals are generated using a dual-electrode Mach-Zehnder modulator (DE-MZM) with a 90° phase shift applied to one of the arms . Due to the nonlinear transfer function of the DE-MZM, optical components in addition to the optical carrier and the desired sidebands are also generated, as shown in Fig. 1, when the DE-MZM is driven by two rf tones at f1 and f2 (with angular frequency of ω̣l and ω2). Figure 1 shows all the possible components generated taking into account second-order nonlinearity. Upon detection, these optical components will generate additional components in the RF domain. Apart from the desired channels at f1 and f2, third-order IMD at 2f1-f2 and 2f2-f1 are also generated within the desired band. To identify the major contributor to these IMD components, the CIR is calculated using typical experimental parameter values, with and without the additional optical components generated from the nonlinear characteristics of the DE-MZM and the results are tabulated in Table I. From Table I, it can be seen that when all the optical components are included in the calculation, a CIR of 14.73 dB is obtained. An improvement of > 3dB is achieved with the removal of ωc-2ω1+ω2 and ωc +ω1-ω2 while the CIR improves significantly (by > 17 dB) when the components at ωc-ω1+ω2 and ωc+ω1-ω2 are removed. Hence, it is obvious that the optical components at ωc-ω1+ω2 and ωc+ω1-ω2 are the major contributors to the third-order IMD in the RF domain.
3. Proposed technique
We propose a technique to improve the CIR performance of OSSB+C modulated signals by removing the major IMD contributors, i.e. the optical components at ωc-ω1+ω2 and ωc+ω1-ω2. The proposed scheme is shown in Fig. 2 where the optical carrier is first split into two paths, with one driving the DE-MZM to generate the OSSB+C signal. A narrowband filter is used at the output of the DE-MZM to remove the optical carrier, together with any components that lie within the vicinity of the carrier i.e. ωc-ω1 +ω2 and ωc+ω1-ω2. The output of the filter is then recombined with the clean optical carrier from the bottom arm before photodetection. Here the amplitude and the phase of the clean optical carrier are adjusted to optimize the components at f1 and f2.
4. Experimental and simulation analysis
A proof-of-concept experiment verifying the proposed technique with two RF tones at 37.50 and 37.51 GHz is shown in Fig. 3. An optical carrier at 1556.54 nm with an output power of +3 dBm was split into two paths using a polarization-maintaining (PM) fiber coupler. Two RF tones at 18.750 GHz and 18.755 GHz were frequency-doubled separately to 37.50 and 37.51 GHz, respectively, before being combined to drive the DE-MZM. At the output of the DE-MZM, the OSSB+C signal entered a 3-port optical circulator in conjunction with a fiber Bragg grating (FBG) to remove the optical carrier at 1556.54 nm. The transmission profile of the FBG is shown in the inset of Fig. 3. The optical signals from both arms were recombined using another PM fibre coupler before photodetection. The amplitude and phase of the clean optical carrier from the bottom arm were adjusted accordingly. Shown in the insets of Fig. 3 are the measured optical spectra at three different locations within the configuration. It can be seen that the optical carrier from the top arm was suppressed by >30 dB.
Figures 4(a) and 4(b) show the measured RF spectra of the detected signals at 37.50 and 37.51 GHz without and with the proposed technique, respectively. Figure 4(a) depicts a measured CIR of ~33 dB and when the proposed scheme was applied, the CIR was increased to ~42 dB. An improvement of ~9 dB was achieved with the linearization scheme.
We also carried out a simulation analysis using VPI Transmission Maker™ to further justify the proposed linearization scheme for CIR improvement in mm-wave fiber-radio system incorporating OSSB+C modulation scheme. The simulation model was constructed from the experimental setup with the parameters used closely matched to the experiment. Shown in Figs. 5(a) and 5(b) are the obtained RF spectra of the recovered signals at 37.50 and 37.51 GHz without and with the proposed scheme. The simulation results match the experimental results shown in Figs. 4(a) and 4(b), depicting a CIR improvement of ~9 dB.
We have proposed and demonstrated experimentally a technique to improve the CIR performance of OSSB+C modulated signals for mm-wave fiber-radio systems. The technique is based on the removal of the optical components at ωc-ω1+ω2 and ωc+ω1-ω2 which are identified as the major contributors to the third-order IMD in the RF domain. The feasibility of the proposed scheme was demonstrated experimentally and via simulation using a two-tone test at 37.5 GHz the results of which showed that an improvement of 9 dB can be achieved.
This work is supported by the Australian Research Council (ARC) Discovery Grant DP0452223.
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