Abstract
Keywords
1. Introduction
Traditionally, microwave signals generated in the electrical domain by electronic devices have limited bandwidth and tunability, owing to the electronic bottleneck. Having the intrinsic advantages of broadband, reconfigurability, low system complexity, and immunity to electromagnet interference, microwave photonics has attracted more and more research interests in radar systems[
A binary FSK signal is represented in terms of two frequencies. The frequency carrying bit ‘1’ is called the mark frequency, and the other frequency carrying bit ‘0’ is designated the space frequency. How to switch between the mark frequency and the space frequency is a key technique for FSK signal generation. Cao et al. reported an FSK signal generation scheme by switching the bias point of a Mach–Zehnder modulator (MZM)[
In this Letter, we propose and experimentally demonstrate a photonic-assisted approach for FSK signal generation based on specially designed carrier phase-shifted double sideband (CPS-DSB) modulation. The joint operation of the polarization-sensitive MZM and phase modulator (PM) helps to realize the switch between mark frequency and space frequency by introducing different phase shifts between the optical carrier (OC) and the -order sidebands along the two orthogonal axes of the PM. By adjusting the driven RF signals of the MZM and PM, a microwave FSK signal is generated at the output of the photodiode (PD). A theoretical analysis is presented and verified by a proof-of-concept experiment. A 0.5 Gb/s FSK signal with the carrier frequencies of 4 and 8 GHz and a 1 Gb/s FSK signal with the carrier frequencies of 8 and 16 GHz are generated successfully. The electrical spectra, waveforms, and eye diagrams of the FSK signals are investigated.
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2. Principle
The proposed schematic diagram for FSK signal generation based on the photonic-assisted CPS-DSB technique is shown in Fig. 1(a), and the spectral evolution of the polarized light wave at different positions is also depicted in Fig. 1(b). The polarization direction of the linearly polarized continuous-wave (CW) light wave emitted from a laser diode (LD) is adjusted to have an angle of to the principal axis of the polarization-sensitive MZM (parallel to the X axis) by a polarization controller (). The optical field of the light wave in the two orthogonal directions can be written as
Figure 1.(a) Proposed schematic diagram for FSK signal generation based on CPS-DSB modulation. (b) Spectral evolution of the injected CW light wave at different positions in the system.
When a microwave signal is applied on the MZM, the optical field of the light wave at the output of the MZM can be expressed as
Considering the small signal modulation condition and the bias point of the MZM, only the OC and the -order sidebands are taken into account. Then, the optical field received by the PD can be rewritten as
Then, optical-to-electrical conversion is performed in the PD. Ignoring the DC component, the electrical current can be expressed as
To optimize the amplitude of the electrical current, let , for simplicity. The generated electrical signal can be rewritten as
Recall that is the phase shift induced by the PM. For a bit ‘0’, equals zero, and the electrical signal is presented as
The above operation principle can also be explained by analyzing the relationship of the beat signals between different optical-frequency components of the modulated signal. As shown in Fig. 1(b) at position , for a bit ‘0’, the beat signal between the −1st sideband and the OC and the beat signal between the OC and the sideband are in phase, and therefore a constructive interference occurs. An equivalent intensity modulation as well as a dominant fundamental-frequency component is generated. Whereas for a bit ‘1’, a phase shift of π/2 is introduced, so the phase difference between the OC and the -order sidebands is π/2, and an equivalent phase modulation is formed[
For an FSK signal, the amplitude of the mark-frequency signal and the space-frequency signal should be identical. The amplitude of the sidebands at the output of the MZM needs to be adjustable according to bit ‘0’ and bit ‘1’. Once the condition
Figure 2.Values of A0 and A1 with the change of MI.
3. Experiment
An experiment generating a 0.5 Gb/s FSK signal with the carrier frequencies of 4 and 8 GHz and a 1 Gb/s FSK signal with the carrier frequencies of 8 and 16 GHz is carried out to verify the performance of the proposed scheme. Firstly, the polarization property of the crystal is explored. The optical signal emitted from a tunable laser source (TLS, Agilent 8164 A) centered at 1550 nm with a power of 7 dBm is sent to a polarization-sensitive MZM via . Both the MZM (JDSU) and PM (EOSPACE) are commercially available with 3 dB bandwidths of 10 GHz, and half-wave voltages of 5 V and 3 V, respectively. The optical spectra directly after the MZM biased at NTP driven by an 8 GHz sinusoidal signal are observed by an optical spectrum analyzer (OSA) with a resolution of 0.02 nm, as shown in Fig. 3. Figure 3(a) shows the total spectrum of the special O-DSB modulated signal, and Figs. 3(b) and 3(c) depict the spectra along the orthogonal axis and the principal axis, respectively. As can be seen, the OC in the orthogonal axis is hardly modulated, because the power of the -order sidebands is 27 dB lower than that of the OC. The case is similar to a polarization-sensitive PM. In the principal axis, the OC is well suppressed, and the power of -order sidebands is 20 dB higher than that of the OC. In addition, the power of the -order sidebands is 27 dB lower compared to the -order sidebands, so it is reasonable to neglect the -order and higher-order sidebands.
Figure 3.Optical spectra at the output of the MZM. (a) Total spectrum. (b) Spectrum along the orthogonal axis. (c) Spectrum along the principal axis.
To evaluate the performance of the proposed system based on CPS-DSB modulation, an 8 GHz microwave sinusoidal signal and an adjustable DC are applied on the MZM and PM by replacing the two branches of data signals. The insert losses of the MZM and PM are also polarization sensitive, where the insert loss along the principal axis is about 5 dB less than that along the orthogonal axis. To make the orthogonal OC aligned with the principal axis of the PM, just ensure that the orthogonal OC has the maximum power after passing though the PM by monitoring the optical spectrum at the output of the Pol. Another method to keep the principal axes of the MZM and PM orthogonal to each other is to use a polarization-maintaining fusion splicer to fuse the polarization-maintaining fiber pigtails of the MZM and PM with a polarization rotation of 90°. The PM is driven by the amplitude-adjustable DC acting as binary data sequences of all bit ‘0’ or all bit ‘1’ by adjusting the voltage to be 0 and 1.5 V. Figures 4(a) and 4(c) illustrate the optical spectrum and electrical spectrum, respectively, when the data sequence of all bit ‘0’ is applied. While Figs. 4(b) and 4(d) show the optical and electrical spectra of the all bit ‘1’ case. The electrical spectra are captured by an electrical spectrum analyzer (ESA, Agilent N9010A). The amplitude of the 8 GHz sinusoidal signal of the ‘0’ case and the ‘1’ case is 1.2 V and 6 V, respectively. As can be seen from Figs. 4(a) and 4(b), the MI of the ‘0’ case has a smaller MI compared with the one of the ‘1’ case. In Fig. 4(c), the power of the dominating fundamental-frequency component is 16 dB higher than that of the frequency-doubled component. In Fig. 4(d), the power of the frequency-doubled component is 24 dB higher than that of the fundamental-frequency component as well. It is also shown that the amplitude of the dominant fundamental-frequency component in Fig. 4(c) is almost the same as that of the dominant frequency-doubled component in Fig. 4(d). The above experiment results indicate that by carefully controlling the amplitudes of the RF signals applied on the MZM and the PM, the dominant fundamental-frequency component and the dominant frequency-doubled component can be designated as the space-frequency signal and the mark-frequency signal, respectively.
Figure 4.(a) Optical spectrum and (c) the related electrical spectrum of the ‘0’ case. (b) The optical spectrum and (d) the related electrical spectrum of the ‘1’ case.
Afterwards, a 1 Gb/s FSK signal with the carrier frequencies of 8 and 16 GHz is generated. A 1 Gb/s () NRZ pseudorandom bit sequence (PRBS) generated by a bit error rate tester (BERT, Agilent N4901B) with an amplitude of 1.1 V is divided into two branches. One branch is added by a 0.2 V DC to generate a unipolar ASK signal, mixed with a 11 dBm 8 GHz microwave carrier, and then enlarged to drive the MZM, while the other is amplified and led to the RF port of the PM to modulate the orthogonal OC output from the MZM. An electrical delay line is employed to synchronize the two branches of data signals. After the lights in the two polarization directions interfere with each other in a Pol and are injected into the PD for optical-to-electrical conversion, a microwave FSK signal with the carrier frequencies of 8 and 16 GHz is generated at the output of the PD. The imperfect polarization direction alignment of the devices, imperfect driven signals, as well as low extinction ratio of the Pol would lead to the amplitude imbalance of the mark-frequency and space-frequency signals, the decrease of the extinction ratio of the FSK signal, and finally deteriorate the quality of the FSK signal. Using devices with polarization-maintaining fiber pigtails will reduce the external environment induced polarization fluctuation significantly. The electrical spectrum, waveform, and eye diagram of the FSK signal are captured by an ESA and a high-speed sampling oscilloscope (Agilent, 86100B), as shown in Figs. 5(a)–5(c), respectively. As shown in Fig. 5(a), there are two carrier frequencies at 8 GHz and 16 GHz in the electrical spectrum, among which the 16 GHz carrier is generated by frequency doubling thanks to the CPS-DSB modulation. A piece of temporal waveform carrying a binary data sequence of ‘01001’ in a duration of 5 ns is illustrated in Fig. 5(b). The bit period of the baseband signal is 1 ns. The bits ‘0’ and ‘1’ are carried by the 8 GHz and 16 GHz microwave signals with periods of 125 ps and 62.5 ps, respectively. An eye diagram in a duration of 2 ns is also clearly obtained, as shown in Fig. 5(c).
Figure 5.(a) Electrical spectrum, (b) waveform, and (c) eye diagram of the 1 Gb/s FSK signal with the carrier frequencies of 8 and 16 GHz.
The photonic-assisted scheme based on CPS-DSB modulation can be considered microwave carrier-frequency and bit-rate independent in the optical domain and can support high-speed and wideband operation when, and only when, the used electrical and optoelectronic devices in the system have enough operation bandwidth. In order to further verify the tunability of frequency and bit rate of the proposal, the generation for a 0.5 Gb/s FSK signal with the carrier frequencies of 4 and 8 GHz is also implemented. The MZM is driven by a 0.5 Gb/s ASK signal at 4 GHz. The electrical spectrum, waveform, and eye diagram of the FSK signal with the carrier frequencies of 4 and 8 GHz is shown in Figs. 6(a)–6(c). As can be seen, the electrical spectrum, waveform, and eye diagram are clear and clean, which implies a good quality of generated FSK signals. Thus, the wideband operation of the scheme is well verified. The mark-frequency signal is generated by photonic-assisted frequency doubling of the space-frequency signal; therefore, the required bandwidths for the electrical and optical devices are relieved. In addition, in this scheme, the MZM is biased at the NTP, so the -order sidebands as well as the OC in the principal axis are suppressed, which will reduce the distortion from -order sidebands. Therefore, no additional filter or digital processing is needed.
Figure 6.(a) Electrical spectrum, (b) waveform, and (c) eye diagram of the FSK signal with the carrier frequencies of 4 and 8 GHz.
4. Conclusion
In conclusion, a novel photonic-assisted method for FSK signal generation based on CPS-DSB modulation has been proposed and experimentally validated. By controlling the driven RF signals of the cascaded polarization-sensitive MZM and PM, a high-speed FSK signal with wideband-frequency tunability can be generated. A 0.5 Gb/s FSK signal with carrier frequencies of 4 and 8 GHz and a 1 Gb/s FSK signal with carrier frequencies of 8 and 16 GHz were demonstrated successfully. The proposed scheme is compact and cost effective, supporting high-speed and wideband operation, which may find applications in RoF and radar systems.
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