This paper focuses on the studies on the polarization- modulator-based single-side-band modulator (PSSBM) as well as its implementation in generation of frequency-locked multicarrier. The principle and properties of PSSBM have been analyzed in detail with theoretical and simulation results. Then, the PSSBM-based frequency-locked multicarrier generator (PSMCG) with recirculating frequency shifting loop has also been demonstrated via simulation. The results have a good agreement with the theoretical analysis. According to the results, multiple frequency-locked carriers with high quality can be achieved based on the proposed PSMCG. The generated carriers have the potential applications in different scenarios such as optical transmission, microwave photonics and so on.
© 2014 Optical Society of America
Optical frequency comb is broadly applied to optical arbitrary waveform generation , precise optical metrology , high accuracy optical sensor , ultrafast optical signal processing , photonic microwave signal processing , high data rate optical fiber transmission [6–9], and so on. In those applications except optical transmission, optical combs must have the equal frequency spacing and fixed phase-relation between the comb lines. Hence, optical comb generators in optical transmission can be referred as frequency-locked multicarrier generator (MCG) . To achieve the high-speed large capacity optical transmission for the increasing demand of data traffic, there are some technologies such as coherent wavelength-division-multiplexing (CoWDM) , and especially coherent optical orthogonal-frequency-division- multiplexing (CO-OFDM) or direct-detect (DD) OFDM superchannels with different modulation formats [12–16] investigated widely in recent years.
In those superchannels, MCG is a key component. There are many methods to implement the MCG, such as typical overdrive single and cascaded Mach-Zenhder modulators (MZMs) and/or phase modulators (abbr., PM-MZM-based) [17,18], mode-locked lasers (MFL) , supercontinuum , cascaded polarization modulators (CPolM) , CW-seeded Shock-Wave Mixer (CSWM)  and the recirculating frequency shifter (RFS)-based MCG [6–8,12,13, 23–32]. Among those methods, the PM-MZM-based method can generate high tone-to-noise- ratio (TNR) carriers with the required high-power electrical amplifiers which decide the number of generated carriers. As for the MFL, it can generate multiple carriers but the stable operation should be guaranteed and the frequency spacing is difficult to tune in a wide range. The weak-point of the supercontinuum is the poor wavelength tunability. Although the CPolM can achieve high-flatness output carriers, the TNR is low and carrier number should be improved if more carriers are required. Although there are amazing 1000 combs demonstrated based on the CSWM, the precise dispersion control and in-line regeneration should be applied. The advantages of the RFS-based MCG, having been studied extensively, are of easing control of carrier number and frequency spacing, low radio frequency (RF) drive power, and high stability. There are two basic configurations of RFS-based MCG, one is single-side-band (SSB) modulator-based MCG (SMCG) [6,7, 12,13,23–28], the other is multi-side-band (MSB) modulator-based (MMCG) [8, 29,30]. Although the MMCG can achieve hundreds of optical carriers, the output stability is poor compared with the SMCG configuration. However, the quality of output carriers, such as the number and TNR of carriers, should be further improved by applying the SMCG configuration. One can use the configuration of multiseed-SMCG [13,27] and dual- [25,26] or multi-loop  SMCG to achieve the generation of hundreds of output carriers. Meanwhile, one can also use the improved configuration of SMCG  based on the suppressed-third-crosstalk scheme  to obtain higher TNR. However, the SMCG implemented by the conventional SSB modulator (CSSBM) based on the integrated LiNbO3 dual-parallel MZM or IQ modulator, has the requirement of three DC bias controls to make the modulator operate at the right point. Thus, the auto bias control is necessary  due to the property of the modulator, sensitive to the changes of environmental conditions .
In this paper, aim to achieve SSB frequency shifting so as to implement high quality multicarrier generation with recirculating frequency shifting loop, we propose a polarization-modulator-based SSB modulator (PSSBM) with only one DC bias control required. The detailed theoretical analysis and simulation results show that the proposed configurations can be used to achieve the desired frequency shift and generation of multiple frequency-locked multicarriers.
2. Theoretical analysis
The schematic of PSSBM is shown in Fig. 1.It consists of a power splitter/combiner, two polarization modulators (PolMs), a phase modulator (PM), two polarization controllers (PC), a polarizer (Pol), an RF signal and a DC bias. The input RF signal f(t) with its frequency of fm is split into two equal parts with π/2 phase shift and then drives respective PolMs. As illustrated in Fig. 1, when the frequency of input seed laser Ein(t) is f0, the novel PSSBM can achieve a desired frequency-spacing SSB-shift with frequency up-shift (f0 + fm) or down-shift (f0 − fm) of seed laser by applying the DC bias control and tuning the two PCs. In general, the input linearly polarized seed laser from a tunable laser (TLS) can be represented as Ein(t) = E0exp(jω0t). The light-wave with its state of polarization (SOP) is aligned at an angle of α1 tuned by PC1 relative to one principal axis of the PolM and then projected to the two orthogonal directions. At the output of each PolM, the two modulated light waves are combined with a power combiner and then sent to the polarizer via PC2. The principal axis of the polarizer is aligned at α2 relative to the x axis of the PolMs by PC2. The two angles α1 and α2 can be tuned by the PCs before the PolMs and Pol. We also represent the input RF signals used to drive PolM1 and PolM2 as f1(t) = Vppcos(ωmt) and f2(t) = Vppsin(ωmt) respectively. VPP is the amplitude of the RF drive signals. Based on the modulation property of polarization modulator shown in , the outputs of the two PolMs can be expressed as followsEq. (2),37] when compared with our previous work . Then, the normalized transfer function of PSSBM can be expressed mathematically as followsEq. (3) should be satisfied as follows24]. Thus, the desired SSB frequency shift can be achieved by the proposed PSSBM. Besides, the unwanted carrier will be also fully-suppressed by tuning the PCs under the ideal case. After using the Jacobi-Anger expansion, Eq. (5) can be expressed asEq. (6) has the same expression as in 
Figure 2 shows the amplitude variation coefficient corresponding to the converted power of desired SSB signal as a function of angle α and drive voltage Vpp. There are three features that can be found from these figures: (1) Given a fixed Vpp, the amplitude variation coefficient of desired signal shows a periodic change with the variation of angle α. The extremum will appear at α = kπ/4 corresponding to the two cases. (2) Given a fixed angle α, the amplitude variation coefficient of desired signal shows a symmetric change with the variation of Vpp where the extremum appears nearby Vpp = 0.6Vπ. (3) The amplitude variation coefficient shows a contrary variation of the two cases. Hence, the maximum conversion efficiency of PSSBM can be obtained by tuning α = π/4 and Vpp = 0.6Vπ.
3. Modeling results
3.1 Modeling results of PSSBM
To validate our proposed PSSBM scheme, we do the simulation through VPI software . Figure 3 shows the output spectra of the PSSBM with frequency up- and down-shifted respectively with different SOPs under case1 (θ = 0, ϕ = 0) and case2 (θ = π/2, ϕ = 0). The global parameters we used here are, fm = 12.5GHz and Vpp = 0.27 Vπ of RF signal, λ0 = 1552.52 nm and Pin = 0 dBm for the input seed laser, and VDC = −Vπ/2. In these figures, Fn = f0 + nfm (n is an integer) represents the generated components of PSSBM (under the condition, the frequency down-shifted signal is the desired SSB signal F−1). The α1 and α2 equal 0 and −π/2 in Fig. 3(a), and 0 and −π/3 in Fig. 3(b) respectively. Obviously, the desired SSB frequency shift can be achieved as shown in Fig. 3(a) when the condition shown in Eq. (4) is satisfied. However, if the condition does not meet the requirement of the SSB frequency shift, the output will contain other unwanted crosstalk components as shown in Fig. 3(b), such as 0th-order F0 and 2nd-order F ± 2. Figures 3(i) and 3(ii) display the SOPs of the input (red point) and output (blue point) signals. We can see that the simulation results are in agreement with the above theoretical analysis where the SOPs of the input and output signals should be kept orthogonal. For case2 (θ = π/2, ϕ = 0), the same feature has been shown in Figs. 3(c) and 3(d). To achieve desired SSB frequency shift, the SOPs of the input and output signals should be kept identical as shown in Figs. 3(iii) and 3(iv). If not, the other unwanted components F0, F ± 2 and F ± 4 are existed.
3.2 Modeling results of PSMCG
As analyzed above, the PSSBM can be achieved. In this section, we mainly give the configuration and simulation results of the proposed PSSBM-based frequency-locked multicarrier generator (PSMCG) with recirculating frequency loop as shown in Fig. 4. The principle of PSMCG is the same as the typical SMCG with recirculating frequency loop . The seed light from the TLS is fed into the PSSBM via a polarization-maintained optical coupler (PMOC). After the desired SSB frequency shifted by the PSSBM, the generated signals are amplified by the optical amplifier (OA) and then filtered by the optical bandpass filter (OBF) and finally coupled into the PSSBM again to form a recirculating frequency loop so as to generate the desired multiple frequency-locked carriers. PC1 and PC2 are used to adjust the SOPs of seed light and signals in the loop to maximize the output power. The generated carriers can be observed by the optical spectrum analyzer (OSA). If the high-flatness of output signals is required, the waveshaping devices, such as wavelength-selective-switch (WSS) shown in the dash-box in Fig. 4, can be used to further improve the output quality. By changing the bias control voltage of PSSBM, the frequency up- or down- converted carriers can be obtained.
The output spectra of PSMCG have been also simulated through the VPI software. Figure 5 shows the output spectra of PSMCG with 50 and 80 carriers corresponding to the bandwidth of 5nm and 8nm respectively. Here, the drive voltage Vpp = 0.27 Vπ, and the optical bandpass filter used in simulation is an ideal rectangle model with proper bandwidth which covers the desired 50- and 80-carrier frequency ranges. The assumed stopband attenuation is 40dB which is a common parameter and can be realized by the commercial products. The optical amplifier we used is an EDFA with black-box model which is operated at saturation mode with 23 dBm output power and 4 dB noise figure (NF). By sweeping the PC in the loop, the final 50- and 80-carrier outputs with an acceptable flatness have been obtained by the PSMCG as shown in Figs. 5(a) and 5(b) respectively. The insets show the SOPs of the output signals. Although the generated power of TNR is just 20 dB for 80-carrier output, there is no doubt that the proposed PSSBM can be used to achieve the generation of high-quality frequency-locked multicariers under further improved configuration. However, to implement the proposed scheme in practice, the polarization extinction ratio, which is the most critical property of the polarization modulator, is desired to be as high as possible. Meanwhile, some practical factors should be considered to improve the TNR of the generated carriers, including the SOPs of the input and output signals, the crosstalk of the undesired sideband components, the DC bias control, the gain and noise figure of the optical amplifier and the qualities of the optical bandpass filter etc.
In conclusion, we have theoretically studied the proposed polarization-modulator-based SSB modulator as well as the frequency-locked multicarrier generator by utilizing it with recirculating frequency shifting loop, including the theoretical analysis and simulation of basic principle and output properties. Compared with the conventional integrated LiNbO3-based SSBM, the proposed PSSBM will only require two phase modulators and one DC bias control and also can be fabricated with integrated structure. Meanwhile, the typical operation frequency range of PolM is larger than 40 GHz which means the output carriers with large tunable frequency spacing can be achieved. According to the theoretical analysis and simulation results, the proposed PSSBM would find some potential applications in various scenarios.
This work is supported by the National Basic Research Programme of China (973) Project (No. 2012CB315603), the Open Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), National Natural Science Foundation of China (NSFC) under Grant No. 61307092, the Planned Science and Technology Project of Guangzhou (Grant No. 2012J5100028), the Fundamental Research Funds for the Central Universities and the Program for New Century Excellent Talents in University (NCET-12-0679) in China.
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38. VPIsystemsTM, “VPltransmissionMakerTM”.