All-optical generation of binary phase-coded microwave pulses without baseband components based on a dual-parallel Mach&#x2013;Zehnder modulator

Chunqi Song; Mingzheng Lei; Jinwang Qian; Zhennan Zheng; Shanguo Huang; Xinlu Gao

doi:10.1364/OE.27.020064

1. Introduction

Pulse compression technology has been widely used in modern radar system to simultaneously obtain high detection resolution and large detection range [1]. Phase-coded microwave waveform (PCMW), as one of the most commonly used signals in pulse compression, has attracted extensive attention. Conventionally, PCMWs are generated by electrical methods. However, PCMWs generated by electrical systems are hardly to meet the increasing frequency and bandwidth requirements due to the inherent limitations of electrical elements. To overcome the inherent limitations, photonics methods are proposed to generate PCMWs. Compared to traditional electrical systems, photonic systems have many excellent advantages, such as: high operating frequency, large operating bandwidth, fine reconfigurability and immunity to electromagnetic interference [2,3].

In recent years, numerous photonics systems for PCMWs generation have been demonstrated [4–21]. According to operation principle, photonics generation of PCMWs can be divided into two main categories: one is based on the spectral shaping and wavelength-to-time mapping (SS-WTT) technology [4–6], the other is based on the optical heterodyne technology [7–21]. The schemes based on the SS-WTT technology have excellent flexibility and reconfigurability to generate arbitrary user-defined PCMWs. However, restricted by the optical bandwidth and optical dispersion, the generated PCMWs suffer the limited time duration. The schemes based on the optical heterodyne technology mainly utilize the electro-optic modulators, which can generate PCMWs with arbitrary time duration and thus has more application value. For example, PCMWs can be generated by using a Mach-Zehnder interferometer incorporating a phase modulator (PM) [7,8] or a Sagnac interferometer incorporating a PM [9,10]. These schemes can generate PCMWs with arbitrary phase. However, these systems have poor stability since the spatial separation of the optical waves. PCMWs can also be generated based on a polarization modulator (PoIM) [11–14] or a polarization sensitive phase modulator (PS-PM) [21]. PCMWs are generated by modulating a pair of orthogonally polarized optical waves via a PoIM or a PS-PM without spatial separation structure. However, these systems are suffering the complexity of the orthogonally polarized optical wave generation and the polarization direction alignment. Furthermore, these schemes using PM, PoIM and PS-PM can only generate continuous-waved PCMWs. The system based on a dual-parallel Mach–Zehnder modulator (DPMZM) can be used for binary PCMW pulses generation [15,16]. However, the generated binary PCMW pulses have baseband components caused by intensity modulation. The generation of binary PCMW pulses based on a dual-drive Mach–Zehnder modulator (DDMZM) can eliminate baseband components [17,18]. However, the input microwave carrier power of these systems need to be accurate and high enough to eliminate baseband components. This puts forward higher requirements for electrical power amplifiers and will introduces higher-order sidebands interferences. A dual-polarization DPMZM (DP-DPMZM) are also demonstrated to generate binary PCMW pulses [19,20]. These systems are highly reconfigurable. However, these systems are sensitive to the polarization state change and complex to operate.

In fact, binary phase-coded microwave pulse signals can satisfy most of the detection demands. This means the structure of the PCMWs generation system can be greatly simplified and the performance of the generated signal can be greatly improved. In this paper, we proposed such a photonics system based on a DPMZM for binary phase-coded microwave pulses generation. Unlike the DPMZM-based systems mentioned in [15,16], the sub-MZMs of the DPMZM in our system are dual-drive rather than push-pull. Thus we generate phase-coded pulse by phase modulation instead of intensity modulation, the baseband components of the generated pulse are eliminated. In general, the proposed system has the following merits: first, the generated binary phase-coded microwave pulses have no baseband components due to the switching of the pulse is not achieved by intensity modulation; second, the system is very simple and stable since it is a single path system and only based on a single integrated modulator; third, the system has a wide frequency tuning range because it is an all-optical system and no filters are used. In addition, the optical polarization direction, the coding signal amplitude and the microwave carrier power do not need to be precisely controlled. Compared with other binary phase-coded microwave pulses generation systems, our system has obvious advantages in simple structure, easy operation, stable output and large frequency tuning range. The corresponding theoretical analysis, simulation demonstration and experimental verification are conducted.

2. Principle and simulation

Schematic of the proposed photonics system for binary phase-coded microwave pulses generation is shown in Fig. 1. The system consists of a laser diode (LD), a DPMZM, an Erbium-doped-fiber-amplifier (EDFA) and a photodetector (PD). In the system, a continuous-wave (CW) light generated by the LD is injected into the DPMZM. Three direct-current voltages are used to control the bias points of the DPMZM. Two identical three-level coding signals are applied respectively to two arms of the sub-MZM1 biased at the maximum transmission point to realize the phase modulation. A cosine microwave signal is applied to one arm of the sub-MZM2 biased at the minimum transmission point to realize the carrier-suppressed double sideband (CS-DSB) modulation. The parent-MZM of the DPMZM is biased at the maximum transmission point. After being compensated for the insertion loss via the EDFA, the modulated optical signal output from the DPMZM is injected into the PD. Through the PD heterodyne detection, accurate π phase shift binary phase-coded microwave pulses without baseband components can be generated.

Fig. 1 Schematic of the proposed photonics system for binary phase-coded microwave pulses generation. OC: optical coupler.

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Mathematically, the CW light with the angular frequency _$ω$ generated by the LD can be given by _{$E_{L D} \propto e^{j ω t}$}. When two identical coding signals _{$V_{c} c (t)$} are applied respectively to two arms of the sub-MZM1, and a cosine microwave signal _{$V_{R F} \cos (Ω t)$} is applied to one arm of the sub-MZM2, the output of the DPMZM can be expressed as

E_{D P M Z M} \propto (e^{j \frac{π V_{c} c (t)}{V_{π R F}} + j \frac{π V_{D C 1}}{V_{π D C}}} + e^{j \frac{π V_{c} c (t)}{V_{π R F}}}) e^{j ω t} + (e^{j \frac{π V_{D C 2}}{V_{π D C}}} + e^{j \frac{π V_{R F} \cos (Ω t)}{V_{π R F}}}) e^{j \frac{π V_{D C 3}}{V_{π D C}}} e^{j ω t}

where _{$V_{c}$} is the amplitude of the two identical coding signals _{$c (t)$}, _{$V_{R F}$} and _$Ω$ are the amplitude and the angular frequency of the cosine microwave signal, _{$V_{D C 1}$} and _{$V_{D C 2}$} are the bias voltage of the sub-MZM1 and sub-MZM2, _{$V_{D C 3}$} is the bias voltage of the parent-MZM, _{$V_{π R F}$} and _{$V_{π D C}$} are the radio-frequency and the direct-current half-wave voltage of the DPMZM. In the small signal model, _{$J_{0} (m_{R F}) \approx 1$} and ignoring the higher-order sidebands, the Jacobi-Anger expansion of expression (1) can be expanded as

\begin{matrix} E_{D P M Z M}^{'} \propto \cos (\frac{φ_{1}}{2}) e^{j (m_{c} c (t) + \frac{φ_{1}}{2})} e^{j ω t} + \cos (\frac{φ_{2}}{2}) e^{j (\frac{φ_{2}}{2} + φ_{3})} e^{j ω t} \\ + J_{1} (m_{R F}) e^{j (\frac{π}{2} + φ_{3})} e^{- j Ω t} e^{j ω t} + J_{1} (m_{R F}) e^{j (\frac{π}{2} + φ_{3})} e^{j Ω t} e^{j ω t} \end{matrix}

where _{$m_{c} = π V_{c} / V_{π R F}$} is the modulation index of the two coding signals, _{$m_{R F} = π V_{R F} / V_{π R F}$} is the modulation index of the cosine microwave signal, _{$φ_{i} = π V_{D C i} / V_{π D C}$} is the phase shift of the MZM, and _{$J_{n} (\cdot)$} represent nth-order Bessel function of the first kind. Only considering the beating between optical carrier and sidebands, the output of the PD can be obtained by

\begin{matrix} I_{P D} \propto \cos^{2} (\frac{φ_{1}}{2}) + \cos^{2} (\frac{φ_{2}}{2}) + 2 J_{1}^{2} (m_{R F}) \\ + 2 \cos (\frac{φ_{1}}{2}) \cos (\frac{φ_{2}}{2}) \cos (m_{c} c (t) + \frac{φ_{1}}{2} - \frac{φ_{2}}{2} - φ_{3}) \\ + 4 \cos (\frac{φ_{1}}{2}) J_{1} (m_{R F}) \cos (m_{c} c (t) + \frac{φ_{1}}{2} - \frac{π}{2} - φ_{3}) \cos (Ω t) \\ + 4 \cos (\frac{φ_{2}}{2}) J_{1} (m_{R F}) \cos (\frac{φ_{2}}{2} - \frac{π}{2}) \cos (Ω t) \end{matrix}

When _{$φ_{1} = 0$}, _{$φ_{2} = π$} and _{$φ_{3} = 0$} (i.e. sub-MZM1 is biased at maximum transmission point to realize the phase modulation, sub-MZM2 is biased at minimum transmission point to realize the CS-DSB modulation and parent-MZM is biased at maximum transmission point), the baseband components and interference signals in expression (3) are eliminated, the expression (3) can be rewritten as

I_{P D}^{'} \propto 1 + 2 J_{1}^{2} (m_{R F}) + 4 J_{1} (m_{R F}) \sin (m_{c} c (t)) \cos (Ω t)

The final alternating-current output of the system can be described as

I_{P D - A C}^{'} \propto \sin (m_{c} c (t)) J_{1} (m_{R F}) \cos (Ω t)

According to (5), accurate π phase shift binary phase-coded microwave pulses without baseband components can be generated by encoding the _{$c (t) = 0, \pm 1$}. The spectrums output form the DPMZM and the waveforms output from the PD under different coding levels are illustrated in Fig. 2. The phase-coded is achieved by controlling the signs of the coding signal rather than the amplitude. The coding signal amplitude and the microwave carrier power do not need to be precisely controlled. It should be noted that this scheme cannot generate non-binary phase-coded microwave pulses. Because essentially this scheme controls the polarity of microwave signals by phase modulation to intensity modulation, the microwave signals are not actually phase modulated. If non-binary phase-coded microwave pulses want to be generated, a filter can be added to filter out the single sideband modulation signal. The binary phase-coded microwave pulses can be given by

Fig. 2 The spectrums output form the DPMZM and the waveforms output from the PD under different coding levels.

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I_{P D - A C}^{'} \propto {\begin{cases} 0 c (t) = 0 \\ \sin (m_{c}) J_{1} (m_{R F}) \cos (Ω t) c (t) = + 1 \\ \sin (m_{c}) J_{1} (m_{R F}) \cos (Ω t + π) c (t) = - 1 \end{cases}

A simulation demonstration based on Fig. 1 is conducted to verify the system performance. In the simulation, the generation of 13-bit binary Barker-code phase-coded microwave pulses are demonstrated. First, the coding signal is set at 2-Gbit/s coding signal rate and 1.00 V coding signal amplitude, and the cosine microwave signal is set at 14-GHz microwave carrier frequency and 0.28 V microwave carrier amplitude. Figures 3(a) and 3(d) show the waveform and the spectrum of the generated 2-Gbit/s 14-GHz 13-bit binary Barker-code phase-coded microwave pulse sequence. A single phase-coded pulse of the sequence and the phase information extracted from the single phase-coded pulse are shown in Figs. 3(b) and 3(c), respectively. The autocorrelation of single phase-coded pulse is given in Fig. 3(e). Then, the coding signal is adjusted at 4-Gbit/s coding signal rate and 1.25 V coding signal amplitude, and the cosine microwave signal is adjusted at 16-GHz microwave carrier frequency and 0.42 V microwave carrier amplitude. The results of the generated 4-Gbit/s 16-GHz 13-bit binary Barker-code phase-coded microwave pulse are shown in Figs. 4(a)–4(e). As shown in Figs. 3 and 4, accurate π phase shift binary phase-coded microwave pulses without baseband components can be generated under different coding signal rates, different coding signal amplitudes, different microwave carrier frequencies and different microwave carrier amplitudes, and a good pulse compression performance can be achieved. Here, the phase information extraction is based on the Hilbert transform. Firstly, Hilbert transform is conducted on the original phase-coded microwave pulse signal. Then, the original phase-coded microwave pulse signal and its Hilbert transform signal is combined to construct the complex exponent analytic signal. Finally, the phase information is extracted from the complex exponent analytic signal to realize phase demodulation.

Fig. 3 (a) The waveform of generated 2-Gbit/s 14-GHz 13-bit binary Barker-code phase-coded microwave pulse sequence; (b) a single phase-coded pulse of the sequence; (c) the phase information extracted from the single phase-coded pulse; (d) the spectrum of phase-coded microwave pulse sequence; (e) the autocorrelation result of the single phase-coded pulse.

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Fig. 4 (a) The waveform of generated 4-Gbit/s 16-GHz 13-bit binary Barker-code phase-coded microwave pulse sequence; (b) a single phase-coded pulse of the sequence; (c) the phase information extracted from the single phase-coded pulse; (d) the spectrum of phase-coded microwave pulse sequence; (e) the autocorrelation result of the single phase-coded pulse.

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3. Experiment

A proof-of-concept experiment is conducted based on Fig. 1. A CW light with a 1550 nm wavelength and a 15 dBm power is provided by a LD (Coherent Solutions MTP1000). The CW light is launched into a DPMZM (Fujitsu FTM7960EX). The half-wave voltage of the DPMZM is about 3.5 V. The bias points of the DPMZM are controlled by a bias control module (PlugTech MBC-IQ-03). The sub-MZM1, the sub-MZM2 and the parent-MZM are biased at the maximum transmission point, the minimum transmission point and the maximum transmission point, respectively. Two identical three-level coding signals generated by an arbitrary waveform generator (AWG) (Agilent M8190A) are applied to two arms of the sub-MZM1. A cosine microwave signal generated by a microwave signal generator (MSG) (Anritsu 68047C) is applied to one arm of the sub-MZM2, while the other arm of the sub-MZM2 is null. The modulated optical signal output from the DPMZM is amplified by an EDFA (Conquer KG-EDFA-P) and then is detected by a PD (Optilab PD-30). The output of the PD is recorded by a digital storage oscilloscope (DSO) (Agilent X93204A) and an electrical spectrum analyzer (ESA) (Agilent N9020A).

The cosine microwave signal is first set at 14-GHz microwave carrier frequency and 2 dBm microwave carrier power. And the coding signals are set at 2-Gbit/s coding signal rate and 250 mV coding signal amplitude. The pattern of the coding signals are “+1 +1 +1 +1 +1 −1 −1 +1 +1 −1 +1 −1 +1” with a duty cycle of 1/4. Figures 5(a) and 5(d) show the waveform and the spectrum of the generated phase-coded microwave pulse sequence. A single phase-coded pulse of the sequence and the phase information extracted from the single phase-coded pulse are shown in Figs. 5(b) and 5(c). Clearly, accurate π phase shift 2-Gbit/s 14-GHz 13-bit binary Barker-code phase-coded microwave pulse without baseband components is successfully generated. For evaluation the pulse compression performance, the autocorrelation result of the single phase-coded pulse is shown in Fig. 5(e). The pulse compression ratio (PCR) and the main-side lobe ratio (MSR) are 12.7 and 11.1 dB, which are well accord with the theoretical values 13 and 11.2 dB, respectively. A good pulse compression performance can be achieved.

Fig. 5 (a) The waveform of generated 2-Gbit/s 14-GHz 13-bit binary Barker-code phase-coded microwave pulse sequence; (b) a single phase-coded pulse of the sequence; (c) the phase information extracted from the single phase-coded pulse; (d) the spectrum of phase-coded microwave pulse sequence; (e) the autocorrelation result of the single phase-coded pulse.

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Then, the cosine microwave signal is adjusted at 16-GHz microwave carrier frequency and 5 dBm microwave carrier power. And the coding signals are adjusted at 4-Gbit/s coding signal rate and 375 mV coding signal amplitude. The results of the generated phase-coded microwave pulse are shown in Figs. 6(a)–6(e). As shown, accurate π phase shift 4-Gbit/s 16-GHz 13-bit binary Barker-code phase-coded microwave pulse without baseband components is also successfully generated, and a good pulse compression performance can also be achieved (PCR: 12.1, MSR: 9.4 dB).

Fig. 6 (a) The waveform of generated 4-Gbit/s 16-GHz 13-bit binary Barker-code phase-coded microwave pulse sequence; (b) a single phase-coded pulse of the sequence; (c) the phase information extracted from the single phase-coded pulse; (d) the spectrum of phase-coded microwave pulse sequence; (e) the autocorrelation result of the single phase-coded pulse.

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The above results are further verified that the proposed system can generate accurate π phase shift binary phase-coded microwave pulses without baseband components under different coding signal rates, different coding signal amplitudes, different microwave carrier frequencies and different microwave carrier powers, and the generated phase-coded pulses have a good pulse compression performance. The results are in good agreement with theoretical analysis and simulation demonstration. The proposed binary phase-coded microwave pulses generation system has high quality and performance.

The BER (bit error rate) results at different received optical powers and microwave carrier powers are provided in Figs. 7(a) and 7(b). Here, coding signals with repeating 127-bit pseudo-random bit sequences (PRBS) coding signal pattern and a 375 mV coding signal amplitude are applied. Figure 7(a) shows the BER results at different received optical powers when microwave carrier power stays as 5 dBm. Figure 7(b) shows the BER results at different microwave carrier powers when received optical power stays as 5 dBm. In Figs. 7(a) and 7(b), accurate π phase shift binary phase-coded microwave pulses with a BER of 0 can be generated when the received optical powers and the microwave carrier powers are enough. The proposed system provides high sensitivity.

Fig. 7 (a) The BER results at different received optical powers (microwave carrier power stays as 5 dBm); (b) the BER results at different microwave carrier powers (received optical power stays as 5 dBm).

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4. Conclusion

An all-optical system for the generation of binary phase-coded microwave pulses without baseband components is proposed. The proposed system has an extremely simple and stable all-optical structure, leading to the advantages of simple structure, easy operation, stable output and large frequency tuning range. The corresponding theoretical analysis, simulation demonstration and experimental verification are conducted. The generation of the binary phase-coded microwave pulses without baseband components under different coding signal rates, different coding signal amplitudes, different microwave carrier frequencies and different microwave carrier powers are proved in the simulation and the experiment. Good pulse compression and BER performance of the generated phase-coded pulses can be achieved. The experimental results are in good agreement with the theoretical analysis and the simulation demonstration. The proposed binary phase-coded microwave pulses generation system shows high quality and performance and has a great application value.

Funding

National Natural Science Foundation of China (NSFC) (61690195, 61575028, 61605015, 61622102, 61821001, 61801038).

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All-optical generation of binary phase-coded microwave pulses without baseband components based on a dual-parallel Mach–Zehnder modulator

Abstract

1. Introduction

2. Principle and simulation

3. Experiment

4. Conclusion

Funding

References

Cited By

Figures (7)

Equations (6)

Optics Express