Photonic generation of multi-frequency phase-coded microwave signal based on a dual-output Mach-Zehnder modulator and balanced detection

Dexin Wu; Xiaoxiao Xue; ShanGyuan Li; Xiaoping Zheng; Xuedi Xiao; Yu Zha; Bingkun Zhou

doi:10.1364/OE.25.014516

1. Introduction

Pulse compression is an important technology to address the contradiction between detection range and range resolution in radar applications. Phase coding of the transmitted signals is one widely used method in pulse compression [1]. Compared to its electrical counterparts, photonic generation of phase-coded microwave signals has many advantages including larger bandwidth and immunity to electromagnetic interferences [2–26]. One method to generate phase-coded signal is based on optical pulse shaping and frequency-to-time mapping [3,4], but the system is complex and lossy due to the coupling between free space and fiber. Besides, the temporal aperture of generated microwave signal is usually limited, which limit the detection range. Another typical optical approach to generate phase-coded microwave signal is to heterodyne two coherent optical wavelengths with one wavelength phase modulated [5–9]. In order to separate the two optical wavelengths, optical filters [5–7] or a length of polarization maintaining fiber [8,9] are used, which limits the radio frequency tunable range. Many other schemes to generate phase-coded microwave signals without the use of wavelength-dependent devices have also been proposed [10–22]. However, few of these systems have realized the generation of simultaneous multi-frequency phase-coded microwave signal.

Compared to single-frequency phase-coded signals, multi-frequency phase-coded signals can provide more functionalities, such as satisfying the requirement of multifunctional radars, reducing the risk of false detections in adverse conditions, having the ability for frequency agility to resist jamming, and the elimination of the Doppler blind speed caused by the moving target detection (MTD) radar [27,28]. So more and more researchers turn to the researches in the optical generation of simultaneous multi-frequency phase-coded microwave signals. In [23–25], multi-frequency phase-coded microwave signals are generated by exploiting a direct digital synthesizer and a Mach–Zehnder modulator (MZM) to apply an intermediate frequency modulation to the modes of a mode-locked laser (MLL). Since the phase coding is performed in the electrical intermediate frequency, the coding rate is quite limited. Ref [26]. proposed an approach to generating multi-frequency phase-coded microwave signals by using a polarization modulation (PolM) together with balanced detection. The light injected into the PolM should have a state of linear polarization controlled precisely to be at 45° with respect to one principal axis of the PolM. Considering polarization components are hardly manufactured with regular integration technologies, one will find difficult in applying it to system on chip.

In this paper, we propose and demonstrate a photonic scheme that generates multi-frequency phase coding signal. A dual-output Mach-Zehnder modulator (DOMZM) driven by an electrical coding data modulates a coherent multi-wavelength light source (CMWL), and a balanced photodetector (BPD) demodulates the output of the DOMZM, then a multi-frequency phase-coded microwave signal can be generated. Experiments generate two two-frequency phase-coded signals, one is 5GHz/10GHz signal with a coding rate of 2 Gb/s, and the other is 10GHz/20GHz signal with a coding rate of 4 Gb/s. Their autocorrelation results show a good pulse compression capability. Each frequency of two-frequency signal has similar performances with the other in terms of PSR and the FWHM of the main lobe. The proposed scheme is simple, main components used are light source, DOMZM and BPD, which are expected to be integrated on a chip [29–31]. And we think it will find a wide application in future radars, for example, reducing false detections in adverse conditions, jamming resistance and elimination of the Doppler blind speed, and so on.

2. Principle

Figure 1 shows the schematic of the proposed system. A CMWL is injected into a DOMZM driven by an electrical OOK data, and the output signal of the DOMZM is detected by a BPD, then the multi-frequency phase-coded signal is generated. Let $E_{0} (t)$ represent the CMWL signal and $V (t)$ represent the electrical OOK data, then the two output signals of the DOMZM are

\begin{array}{l} E_{1} (t) = \frac{1}{\sqrt{2}} E_{0} (t) [\exp (j π V (t) / V_{π} + j φ) + \exp (- j π V (t) / V_{π})] \\ E_{2} (t) = \frac{1}{\sqrt{2}} E_{0} (t) [\exp (j π V (t) / V_{π} + j φ) - \exp (- j π V (t) / V_{π})] \end{array}

where

V_{π}

is the half-wave voltage of the DOMZM, and

φ

is the phase difference between the two arms of the DOMZM, which can be controlled by adjusting the bias voltage of the DOMZM. The output of the BPD can be expressed as

\begin{array}{l} I (t) \propto E_{1} {(t)}^{*} E_{1} (t) - E_{2} {(t)}^{*} E_{2} (t) \\ = \frac{1}{2} {| E_{0} (t) |}^{2} {2 + 2 \cos [\frac{2 π}{V_{π}} V (t) + φ]} - \frac{1}{2} {| E_{0} (t) |}^{2} {2 - 2 \cos [\frac{2 π}{V_{π}} V (t) + φ]} \\ = 2 {| E_{0} (t) |}^{2} \cos [\frac{2 π}{V_{π}} V (t) + φ] \end{array}

When

φ =- π / 2

, we have

I (t) \propto 2 {| E_{0} (t) |}^{2} \sin [\frac{2 π}{V_{π}} V (t)]

Let

V (t) = ϕ (t) V_{0}

, where

V_{0}

is the amplitude of the electrical coding data, and

ϕ (t)

is ‘1’ or ‘-1’, then

I (t) \propto 2 ϕ (t) {| E_{0} (t) |}^{2} \sin (\frac{2 π}{V_{π}} V_{0})

And CMWL signal

E_{0} (t)

can be written as

E_{0} (t) = A_{0} \exp (j ω_{0} t) + A_{1} \exp (j (ω_{0} + Δ ω) t) + \dots + A {}_{n}e x p (j (ω_{0} + n Δ ω) t)

where

ω_{0}

is the angular frequency of the first line of the CMWL,

Δ ω

is the angular frequency interval of the CMWL,

n

is the number of the angular frequency, and

A_{i}

is the amplitude of the

i

th line with an angular frequency of

ω_{0} + i Δ ω

(i = 1, 2, \dots, n)

. By using Eq. (5) to replace the

E_{0} (t)

in Eq. (4), we have

I (t) \propto 2 ϕ (t) \sin (\frac{2 π}{V_{π}} V_{0}) [\sum_{k = 0}^{n} A_{k} + 2 \sum_{k = 1}^{n} \sum_{m = 0}^{n - k} A_{m} A_{m + k} \cos (k Δ ω t)]

Neglecting DC term in Eq. (6), we can get the output RF signal as

I (t) \propto 4 ϕ (t) \sin (\frac{2 π}{V_{π}} V_{0}) \sum_{k = 1}^{n} \sum_{m = 0}^{n - k} A_{m} A_{m + k} \cos (k Δ ω t)

From Eq. (7) it can be seen that the generated RF signal has multiple angular frequencies of

k Δ ω

(k = 1, 2, \dots, n)

, and its relative phase changes with the sign of

ϕ (t)

. In the case where

ϕ (t)

changes from ‘1’ to ‘-1’ or from ‘-1’ to ‘1’, the phase of output RF signal varies by ‘

π

’. As a result, we can use Eq. (7) to generate multiple-frequency phase-coded signal. Since no wavelength-dependent devices are used in the Fig. 1, the system has a good radio frequency tunability. The amplitude

V_{0}

of the electrical coding data only affects the amplitude of the generated signal instead of its ‘0’ or ‘

π

’ phase.

Fig. 1 Conceptual diagram of the proposed phase-coded system. CMWL, coherent multi-wavelength laser; DOMZM, dual-output Mach–Zehnder modulator; BPD, balanced photodetector.

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3. Experiment results and discussion

The experimental setup is shown in Fig. 2. For simplicity, the CMWL is generated by modulating an optical carrier with a MZM onto which a single-frequency microwave signal is applied, which actually makes a dual sideband modulated laser. The optical carrier is 1550nm and from a continuous wave laser (CW, Agilent N7714A). The single-frequency microwave signal is generated by a microwave signal generator (MSG, Agilent E6432A). The used MZM (Eospace, AZ-DV5-40-PFA-PFA-LV-SLB60) has a bandwidth of 40GHz and a half-wave voltage of 3.4V. The CMWL is amplified by an erbium-doped fiber amplifier (EDFA, Amonics AEDFA-PA-35-B-FA), and then injected into the DOMZM (Eospace, AX-1x2-CK5-2-PFU-SFU) with a bandwidth of 40GHz and a half-wave voltage of 3.8V. A pulse pattern generator (PPG, Anritsu MP1758A) is used to generate the electrical OOK data fed to the DOMZM. The BPD ( $u^{2} t$ photonics, BPDV2150R) has a bandwidth of 40GHz. The optical spectrum of CMWL is measured using an optical spectrum analyzer (OSA, Advantest Q8384). The generated multi-frequency phase-coded microwave signal is measured using a spectrum analyzer (PSA, Agilent E4446A) or a sampling oscilloscope (OSC, Agilent 86100C), and we use a computer to process the readout from the OSC.

Fig. 2 Experimental setup of the proposed system. CW, continuous wave laser; MSG, microwave signal generator; EDFA, erbium-doped fiber amplifier; PPG, pulse pattern generator; OSA, optical spectrum analyzer; OSC, oscilloscope; PSA, spectrum analyzer; Trig, trigger.

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The frequency and power of the MSG is set to be 5GHz and 5dBm. The electrical OOK data (‘1’ or ‘-1’) is set to be 2Gbit/s and have a peak-to-peak voltage of 1.9V. The bias voltage of the used MZM and DOMZM is set to 0.8V and 4.6V respectively. Since the 5GHz signal-frequency signal is applied, the experimental CMWL will be a dual sideband modulated laser with a frequency interval of 5GHz, as shown in Fig. 3(a). According to Eq. (7), the output signal of BPD will be a two-frequency phase-coded signal with 5GHz and 10GHz carriers.

Fig. 3 (a) Spectrum of the dual sideband modulated laser with a frequency interval of 5GHz. (b) Electrical spectrum of the simultaneously generated 5GHz and 10GHz phase-coded signal.

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The measured electrical spectrum of generated signal is shown in Fig. 3(b). From the electrical spectrum, we can see that a two-frequency phase-coded signals with 5GHz and 10GHz carriers is indeed generated simultaneously. In order to select components with frequencies of 5GHz and 10GHz from the generated signal, two digital bandpass filters with their bandwidth of 4GHz and respective central frequencies of 5GHz and 10GHz are used for signal processing by the computer.

Figure 4 shows a partial waveform of the generated phase-coded signal at 5GHz and 10GHz and the corresponding recovered phase information from the generated waveform using Hilbert transform. As can be seen, a phase coding of ‘0’ or ‘ $π$ ’ is achieved. In order to evaluate the pulse compression capability of generated phase-coded signal. The autocorrelation is calculated and the results are shown in Fig. 5. We can see that the PSR of the generated signal at 5GHz and 10GHz are 12.1 and 11.1dB, and their FWHM of the main lobe are 0.50 and 0.51ns, respectively, which are very close to the theoretical value 0.50ns at the coding rate of 2Gbit/s.

Fig. 4 (a) Phased-coded 5-GHz signal, and (b) the recovered phase information from (a). (c) Phased-coded 10-GHz signal, and (d) the recovered phase information from (c). The interval of the CMWL is 5GHz, and the coding rate is 2Gbit/s.

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Fig. 5 (a) Autocorrelation of the 5GHz phase-coded signal, and (b) the zoom-in view of the main lobe. (c) Autocorrelation of the 10GHz phase-coded signal, and (d) the zoom-in view of the main lobe. The interval of the CMWL is 5GHz, and the coding rate is 2Gbit/s.

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To verify the frequency tunability of the system, we adjust the frequency of MSG to be at 10GHz, set the electrical OOK data to be 4Gbit/s. The dual sideband modulated laser with a frequency interval of 10GHz and the measured electrical spectrum of generated signal are shown in Fig. 6(a) and Fig. 6(b), respectively. Two digital bandpass filters are set to be centered at 10GHz and 20GHz, respectively, with same bandwidth of 8GHz to pick up components with frequencies of 10GHz and 20GHz from the generated signal. Figure 7 shows a partial waveform of the generated phase-coded signal with two frequencies of 10GHz and 20GHz and their corresponding recovered phase information from the generated waveform.

Fig. 6 (a) Spectrum of the dual sideband modulated laser with a frequency interval of 10GHz. (b) Electrical spectrum of the simultaneously generated 10GHz and 20GHz phase-coded signal.

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Fig. 7 (a) Generated phased-coded signal of 10GHz, and (b) the recovered phase information from (a). (c) Generated phased-coded signal of 20GHz, and (d) the recovered phase information from (c). The interval of the CMWL is 10GHz, and the coding rate is 4Gbit/s.

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We can also see from Fig. 7 that the phase coding of ‘0’ or ‘ $π$ ’ is also well achieved. The results of autocorrelation are shown in Fig. 8. The PSR of the generated signal at 10GHz and 20GHz are 11.3 and 10.1dB, and their FWHM of the main lobe are 0.27 and 0.28ns, respectively, which are also close to the theoretical value 0.25ns at the coding rate of 4Gbit/s.

Fig. 8 (a) Autocorrelation of the generated 10GHz phase-coded signal, and (b) the zoom-in view of the main lobe. (c) Autocorrelation of the generated 20GHz phase-coded signal, and (d) the zoom-in view of the main lobe. The interval of the CMWL is 10GHz, and the coding rate is 4Gbit/s.

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In the experiment, we have demonstrated the simultaneous generation of two-frequency phase-coded signals. In order to generate phase-coded signal with more carrier frequencies simultaneously, a CMWL with more wavelength should be used. The maximal number of generated phase-coded signals is mainly limited by the bandwidth of BPD. Suppose the frequency interval of CMWL be $Δ f$ , the coding rate $C$ , and the bandwidth of BPD $B$ , then the maximal carrier frequency number of generated phase-coded signal $N$ is given by

N = ⌈ \frac{B - C}{Δ f} ⌉ - 1

where

⌈ x ⌉

is the largest integer no greater than

x

. It should be noted that in order to prevent the overlapping between adjacent main lobes in electrical domain,

Δ f

and

C

should satisfy

C \leq \frac{Δ f}{2}

4. Conclusion

We proposed a photonic scheme to generate multi-frequency phase-coded microwave signal based on a DOMZM and balanced detection. Experiments generate two two-frequency phase-coded signals, one is 5GHz/10GHz signal with a coding rate of 2Gb/s, and the other is 10GHz/20GHz signal with a coding rate of 4Gb/s. Their autocorrelation results show a good pulse compression capability. Each frequency of two-frequency signal has similar performances with the other in terms of PSR and the FWHM of the main lobe. In order to generate a phase-coded signal with more carrier frequencies, a CMWL with more wavelength can be used. The generated multi-frequency phase-coded signal is promising in multifunctional radars and reducing the risk of false detections in adverse conditions. Besides, it has the ability for frequency agility to resist jamming, and the elimination of the Doppler blind speed caused by the MTD radar.

Funding

National Nature Science Foundation of China (NSFC) (6169190011, 6169190012, 61420106003, 61621064); Chuanxin Funding; Beijing Natural Science Foundation (4172029).

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Photonic generation of multi-frequency phase-coded microwave signal based on a dual-output Mach-Zehnder modulator and balanced detection

Abstract

1. Introduction

2. Principle

3. Experiment results and discussion

4. Conclusion

Funding

References and links

Cited By

Figures (8)

Equations (9)

Optics Express