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Abstract
We propose and experimentally demonstrate a polarization multiplexed active mode-locking optoelectronic oscillator (AML-OEO) based on a single dual-polarization binary phase-shift keying (DP-BPSK) modulator for frequency tunable dual-band microwave pulse signal generation. In order to realize mode-locking, two single-tone signals whose frequency are integer multiple of the free spectrum range (FSR) of AML-OEO are applied as active modulation signals (AMSs) at the bias ports of the DP-BPSK modulator. By dividing the AML-OEO into two loops with polarization demultiplexing, both the carrier frequency and pulse repetition frequency (PRF) of the dual-band microwave pulses are independently adjustable. In the experiment, microwave pulses with different PRFs of 162.4 kHz, 324.8 kHz and 812 kHz are generated based on fundamental, second-order harmonic and fifth-order harmonic mode-locking, respectively. In addition, the carrier frequency tunability within 4∼10 GHz is verified by inserting a frequency tunable electrical filter. The phase noise of the generated pulse signal at 10 kHz offset is better than −125 dBc/Hz.
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1. Introduction
Optoelectronic oscillator (OEO) is a well-known microwave and photonic hybrid loop system, which inserts a long fiber into the loop as an energy storage element to generate ultra-low phase noise microwave signals [1,2]. The phase noise of microwave signals generated by the OEO does not deteriorate with the increase of the frequency, which makes it extremely helpful in scenarios that require high-frequency sources with a low phase noise [3–5]. Plenty of OEOs have been proposed to generate low phase noise single-frequency microwave signals with long-term frequency stability [6–9], high side-mode suppression ratio [10,11], and broadband frequency tunability [12,13]. Meanwhile, researchers have proposed multiple OEO mode control methods to get different microwave signals, such as chirped microwave waveforms [14,15] and microwave pulses [16–18]. The mode-locking technology was introduced into OEO to realize passive mode-locking OEO [16] or active mode-locking OEO (AML-OEO) [17–20] for generating low phase noise microwave pulses, which are desired in many applications such as pulse radar, remote sensing, and broadband wireless communication. In [17], we proposed and experimentally demonstrated an AML-OEO, where an additional electro-optical modulator is inserted into the OEO as an active mode-locking device for gain modulation to realize phase-locking between adjacent oscillation modes. Through setting the frequency of the active modulation signal (AMS) to be an integer multiple of the free spectral range (FSR) of AML-OEO, steady multi-mode oscillation can be achieved. The pulse repetition frequency (PRF) of the generated microwave pulses can be tuned by employing harmonic mode-locking technology. In [18], Zhen Zeng et al. proposed an AML-OEO for microwave pulses generation by using an additional electric mixer to achieve gain modulation in the OEO cavity. Through imposing the AMS directly at the DC bias port of the electro-optical modulator in the traditional OEO, the additional electro-optical modulator or electrical mixer can be removed to simplify the structure of the AML-OEO [19]. It is also worth mentioning that modeling of the AML-OEO is beneficial to the study of its dynamic process [20]. Subsequently, a polarization multiplexed dual-loop AML-OEO was proposed to suppress the super-mode noise in the harmonic mode-locking state [21]. Basically, these structures aim to generate single-band microwave pulses. For dual-band applications like dual-band pulse radar, where one band pulse with a lower carrier frequency and PRF is responsible for early warning of far targets, and the other band pulse with a higher carrier frequency and PRF is for tracking close-range targets [22], two independent AML-OEOs are required. However, the cost and complexity would be very high.
In this letter, we propose and experimentally demonstrate a polarization multiplexed AML-OEO based on a single dual-polarization binary phase-shift keying (DP-BPSK) modulator for frequency tunable dual-band microwave pulse signals generation. In order to realize mode-locking, two single-tone signals whose frequencies are integer multiple of the FSR of AML-OEO are applied as AMSs at the bias ports of the DP-BPSK modulator. By dividing the AML-OEO into two loops with polarization demultiplexing, both the carrier frequency and PRF of the dual-band microwave pulses are independently adjustable. Since a single DP-BPSK is used to realize dual-loop oscillation signal feedback and gain modulation, the proposed scheme features a simple structure.
2. Principle
Figure 1(a) shows the schematic diagram of the polarization multiplexed AML-OEO. A laser diode (LD) emitting continuous wave is connected to a DP-BPSK modulator, which integrates two parallel single-drive Mach-Zehnder modulators (MZMs), a 90° polarization rotator (PR) and a polarization beam combiner (PBC). The power of the inputted optical signal is equally divided into two branches of the DP-BPSK by properly adjusting a polarization controller (PC1). The 90° PR makes the two optical signals in the upper and lower branches have orthogonal polarizations, polarization X and Y, while the PBC multiplexes them. After passing through a length of single-mode fiber (SMF), the polarization multiplexed optical signal is demultiplexed by another PC (PC2) and a polarization beam splitter (PBS). The PC2 is used to align the two orthogonal polarizations (X and Y) of the modulator with that of the two output ports (A and B) of the PBS. The demultiplexed optical signals are converted to electrical signals by two photodetectors (PDs), and properly amplified by two electrical amplifiers (EAs), respectively. Then, electrical band-pass filters (EBPFs) with different central frequencies are used in each loop for frequency selection. Finally, the RF signals are fed back to the RF ports of the DP-BPSK modulator to close two OEO loops, i.e., loops A and B. Different from the traditional OEO, the bias ports of the DP-BPSK modulator are driven by two single-tone signals with DC offset voltage, which are used as AMSs to modulate the gains of loops A and B. Since oscillations of the two loops are established on two orthogonal polarizations, respectively, the oscillation modes of loops A and B are independent of each other.
Fig. 1. (a) Schematic diagram of the proposed AML-OEO. (b) Principle of microwave pulses generation in one loop. LD: laser diode, PC: polarization controller, DP-BPSK: dual-polarization binary phase-shift keying modulator, MZM: Mach-Zehnder modulator, PR: polarization rotator, PBC: polarization beam combiner, Pol: Polarization, AMS: active modulation signal, SMF: single-mode fiber, PBS: polarization beam splitter, PD: photodetector, EA: electrical amplifier, EBPF: electrical band-pass filter, EC: electrical coupler.
Take loop A as an example for analysis, the output optical signal from the DP-BPSK with X polarization can be expressed as
The signal obtained after a cycle in the OEO loop can be written as
According to (3), we can get an open-loop small-signal gain curve with a period same as that of the AMS1 by properly setting the ${V_{DC}}$ and ${V_{RF}}$. Figure 1(b) shows the gain curve when the ${V_{DC}}$ and ${V_{RF}}$ are both set at ${V_\pi }/4$. Since the frequency of AMS1 is an integer multiple of the FSR of the OEO, the microwave signal at the net gain (gain > 1) region will be amplified at each loop iteration. Finally, a microwave pulse with a PRF same as the frequency of AMS1 can be generated by loop A. Similarly, a microwave pulse with a PRF same as the frequency of AMS2 can also be generated by loop B. Through simply combining the outputs of loops A and B, dual-band microwave pulse signals can be generated.
3. Results and discussions
We carried out a proof-of-concept experiment based on the setup shown in Fig. 1(a). The central wavelength and optical power of the LD are 1550.12 nm and 14 dBm, respectively. The DP-BPSK modulator has a 3 dB bandwidth of 20 GHz and a half-wave voltage of 8 V. A dual-channel signal generator (RIGOL-DG1022) is used to output two AMSs. A SMF with a length of 1200 m is used to act as the energy storage element. The PDs have a 3 dB bandwidth of 10 GHz and a responsivity of 0.8 A/W. The center frequencies of the EBPF1 and EBPF2 are 2.1 GHz and 2.45 GHz, respectively, and the bandwidths are both 30 MHz. Two low noise EAs with a gain of 40 dB are used to compensate for the loop loss. Two electrical couplers (ECs) with a bandwidth of 2∼18 GHz are employed to output the oscillating microwave signal. An electrical spectrum analyzer (ESA, R&S FSV30) and a high-speed real-time oscilloscope (OSC, RTP084, 20 GS/s) are used to measure the spectra and the temporal waveforms of the generated microwave signals, respectively.
First, the signal generator outputs two DC signals to make both sub-MZMs of the DP-BPSK modulator biased at quadrature points. By properly tuning PC2 to align the two orthogonal polarizations (X and Y) of the DP-BPSK modulator with that of the two output ports of PBS, the two OEO loops work in a free-running state. Figure 2(a) shows the spectra of the generated two single-tone RF signals by coupling the outputs of the two loops with another EC. The frequencies of the free oscillation signals in loops A and B are 2.102 GHz and 2.446 GHz, respectively, which are same as the center frequencies of the EBPFs. The FSRs of loops A and B are measured to be the same 162.4 kHz. Then, the two outputs of the signal generator are set to be single-tone signals with a frequency of ${f_s} = N{f_{FSR}}$, where the offset and RF amplitude voltage are both set to around 2 V. Fundamental mode locking of the two loops are realized when the frequencies of AMS1 and AMS2 are both 162.4 kHz. The spectral of the dual-band signal turn to be much wider as shown in Fig. 2 (b). Figure 2(c) and (d) show the spectral details of the oscillation signals in two bands. It can be seen that a stable multi-tone oscillation with a mode interval of 162.4 kHz is obtained for both loops. These modes are coherently superimposed in the time domain to form a microwave pulse train with a period of 6.158 $\mu s$ as shown in Fig. 2 (e) and (f). The full width at half maximum (FWHM) of the pulse in loops A and B is 0.075 $\mu s$ and 0.607 $\mu s$, respectively. The oscillation signal of loop A has a wider spectrum and smaller FWHM than loop B. The difference can be attributed to that the output temporal and spectral characteristics from the OEO are closely relative to the net gain in the OEO cavity.
Fig. 2. Measured spectra of the generated dual-band microwave signals of the proposed AML-OEO (a) in free-running state (b) in fundamental mode-locking state, measured spectra (c) (d) and temporal waveforms (e) (f) of the generated microwave pulses by loop A (left column) and loop B (right column) in fundamental mode-locking state. RBW: resolution bandwidth.
Second, to investigate PRF tunability of the generated microwave pulses, we adjust the frequency of AMS2 in loop B to be multiple times of the FSR. When the frequency of AMS2 is set at 324.8 kHz and 814 kHz, second-order and fifth-order harmonic mode-locking are realized, respectively. Figure 3(a) and (c) show the temporal waveforms. The pulses period in Fig. 3(a) and (c) are 3.079 $\mu s$ and 1.229 $\mu s$, respectively. It is obvious the PRF varies with the frequency of AMS. Figure 3(b) and (d) in the right column show the spectra of the generated signals. The mode spacings of the dominant oscillation modes under second-order and fifth-order harmonic mode-locking are 324.8 kHz and 814 kHz, respectively, which is consistent with the prediction. It should also be pointed out that there are spurious super-modes between the dominate oscillation modes. The power ratio between the dominate modes and the super-modes is about 20 dB. These unwanted super-modes have a non-negligible impact on the performance and power stability of the generated microwave pulses. Such as in pulse Doppler radar, these unwanted frequency components may be mixed with the Doppler shift of target mode, thus affecting the accuracy and sensitivity of velocity measurement. The vernier effect between long and short loops can be employed to suppress the unwanted super-modes [10]. Similarly, by adjusting the frequency of AMS1, microwave pulses with tunable PRF can also be got by loop A.
Fig. 3. Measured results of the generated microwave pulse train by loop B. Temporal waveforms under (a) second-order harmonic mode locking and (c) fifth-order harmonic mode locking, spectra under (b) second-order harmonic mode locking and (d) fifth-order harmonic mode locking.
Finally, we replace EBPF1 with an EBPF whose central frequency is adjustable from 2 GHz to 18 GHz with a 3 dB bandwidth of 20 MHz. Figure 4(a) shows the spectra of the dual-band signal when the center frequency of the EBPF in loop A is tuned from 4 GHz to 10 GHz with a step of 1 GHz. The maximum carrier frequency of 10 GHz in the experiment is mainly limited by the bandwidth of the PD. Figure 4(b) shows a zoom view of the spectral line at 7 GHz, where the shape of frequency comb indicates the OEO is in mode-locking state. The results verify that the carrier frequency of the dual-band microwave pulses can be independently tuned by adjusting the center frequency of the EBPF.
Fig. 4. (a) The spectra of the dual-band signal when the center frequency of the EBPF in loop A is tuned in a step of 1 GHz from 4 GHz to 10 GHz, (b) the zoom view of the spectral line at 7 GHz.
Figure 5 shows the single-sideband phase noise of the generated microwave signals, which is directly measured by the phase noise measurement module built in the ESA. It can be seen that the oscillating signals in the two loops have similar phase noise performances, since the two loops consist of a same length of SMF and link components with similar parameters (except the electrical filter). An interesting result is that the close-to-carrier phase noise of the mode-locked signal is lower than that of the free-oscillating signal, which may be attributed to the high-pass filtering effect in the mode locking process [18]. At 10 kHz frequency offset, the phase noise is better than −125 dBc/Hz.
Fig. 5. Single-sideband phase noise of the microwave signals generated by loop A at 2.102 GHz and loop B at 2.446 GHz.
In addition to the adjustable PRF and carrier frequency, small FWHM and good frequency stability are also key parameters for microwave pulses. To improve the long-term stability of the proposed AML-OEO, the effective length of the loop determining the frequency of mode locking should be carefully controlled, the feedback loop control technology used in traditional OEO can be employed [7,8]. To get microwave pulses with a small FWHM, not only the amplitude of the AMS should be properly controlled to narrow the duration of the net gain range, but also an electrical filter or a microwave photonic filter with a flat passband should be used.
4. Conclusion
In summary, we have proposed a polarization multiplexed AML-OEO for frequency tunable dual-band microwave pulse signals generation. Thanks to polarization multiplexing, the oscillating signals in the two loops have no interfere with each other, and their carrier frequency and PRF are independently adjustable. In the experiment, low phase noise dual-band microwave pulses with a tunable PRF of 162.4 kHz 324.8 kHz and 814 kHz and a tunable carrier frequency within 4∼10 GHz is generated. The proposed scheme presents a simple way to generate flexible and high-quality microwave pulses, which may find applications in pulse radar and microwave photonic systems.
Funding
National Natural Science Foundation of China (62101168, 41905024, 61901148, 61975048, 62001148).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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