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Abstract
A novel approach to generating multi-format microwave signals directly from an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. Through applying different low-frequency electrical waveforms to the bias port of the electro-optic Mach-Zehnder modulator, the net gain in the OEO cavity is dynamically controlled to make the OEO work at different status. Hence, different microwave signals can be generated in the oscillation process. In the proof-of-concept experiment, a single-tone microwave signal at 4.005 GHz is generated by using a direct-current driving voltage. Microwave pulse trains centered at 4 GHz and with repetition rates of 390 kHz and 3.9 MHz are generated under fundamental and 10th-order harmonic mode locking, respectively, by using sinusoidal driving voltages. Besides, rectangular microwave waveforms centered at 4 GHz and with duty cycles of 25%, 50%, and 75% are generated under gain switching status by using rectangular driving voltages. The proposed method is also able to generate other types of microwave signals in a broad frequency range, which can find applications in a multi-functional microwave photonic system.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photonics-assisted microwave signal generation has been deeply researched in recent years [1–3]. Among numerous methods, optoelectronic oscillator (OEO) is recognized as a promising candidate to generate high-quality microwave signals due to its remarkable advantages such as ultra-low phase noise and flexible tunability in a large frequency range [4–7]. Up to now, various OEO schemes have been proposed to generate a single-format microwave signal. For example, a conventional OEO is generally aiming at generating a single-tone microwave signal with an ultra-low phase noise and a high sidemode suppression ratio, which can be used as an excellent local oscillator in a frequency conversion system or a clock source in a frequency synthesizer [8–10]. Besides, Fourier domain mode locking OEO has been proposed to generate linearly-chirped microwave waveforms with ultra-large time-bandwidth products, which is beneficial for improving the range measurement resolution of a chirp radar [11][12]. Recently, time-domain mode locking OEOs have been reported to generate microwave pulse signals. In [13] and [14], passive and active locking technique have been employed to lock the phase of the longitudinal modes in the net gain spectrum, respectively, where the phase-locked oscillating modes are coherently superimposed in the time domain to form a nearly chirp-free microwave pulse train. This kind of microwave signals can find applications in pulse Doppler radars [15] and ultra-wideband systems [16].
In order to meet the multi-functional requirement in a modern microwave system, arbitrary microwave waveform generation based on an OEO has also been explored in recent years [17][18]. In [17], phase-coded and frequency-chirped microwave waveforms with a tunable carrier frequency are generated. In this scheme, optical signal from a tunable OEO is sent to an external polarization modulator, where it is modulated by specially-designed electrical signals to generate phase-coded or frequency-chirped microwave signal after polarization detection and photoelectric conversion. In [18], an OEO-based multi-format microwave signal generator is realized by employing a dual-polarization quadrature phase shift-keying (DP-QPSK) modulator, where one of the dual-parallel Mach-Zehnder modulators (DPMZMs) in the DP-QPSK modulator is used in the OEO loop to generate a single-tone oscillation signal, and the other DPMZM is used outside the OEO loop to multiply and encode the oscillation signal. In this scheme, a frequency-multiplying microwave signal, a phase-coded microwave signal and an optical frequency comb with a tunable frequency or frequency interval are generated. Nevertheless, it should be pointed out that, in these schemes, the OEO only acts as a local oscillator to generate a single-tone microwave signal. An auxiliary electro-optic modulator outside the OEO loop is needed to process (e. g. multiply or encode) the oscillation signal. Therefore, multi-format microwave signals are not directly generated by the OEO.
In this paper, we propose and experimentally demonstrate a novel method to realize multi-format microwave signal generation via dynamically controlling the net gain in an OEO cavity. The signal format can be changed by simply varying the low-frequency electrical waveform injected into the bias port of the electro-optic Mach-Zehnder modulator (MZM) in the OEO loop. In the experiment, a single-tone microwave signal at 4.005 GHz, microwave pulse trains with repetition rates of 390 kHz and 3.9 MHz, and rectangular microwave waveforms with duty cycles of 25%, 50%, and 75% are generated directly from the OEO cavity.
2. Operation principle
Figure 1 shows the schematic diagram of the OEO for multi-format microwave signal generation. It can be noticed that the architecture of the proposed scheme is similar to that of a conventional OEO, which mainly consists of a continuous-wave (CW) laser diode, an electro-optic MZM, a section of optical fiber, a photodetector, an electric bandpass filter (BPF), and a low-noise amplifier. The only difference lies in that the bias of the MZM in the proposed scheme is controlled by the electrical signal from a function generator instead of a simple direct-current (DC) voltage. Through varying the electrical signal waveform from the function generator and finely adjusting the voltage level, the net gain in the cavity can be controlled in the time domain to generate different types of microwave signal waveforms.
Fig. 1. Schematic diagram of the OEO for multi-format microwave signal generation. LD: laser diode; VOA: variable optical attenuator; MZM: Mach-Zehnder modulator; SMF: single-mode fiber; PD: photodetector; LNA: low-noise amplifier; EC: electric coupler; FG: function generator; OEO: optoelectronic oscillator.
The optical signal output from the MZM can be expressed as
Figure 2 presents the diagrammatic sketch of the microwave signal generation under three types of bias waveforms. The first one is a DC bias as shown in Fig. 2(a), where the net gain (blue dashed line) in the OEO cavity is always above the oscillation threshold (black dotted line). In such a case, a sinusoidal microwave signal (i.e., a single-tone microwave signal) in the passband of the BPF can oscillate in the OEO cavity. The second bias waveform is a sinusoidal signal with a frequency ${f_s}$ equal to an integer multiple of the free spectrum range ${f_{FSR}}$ of the OEO cavity, i.e., ${f_s} = N{f_{FSR}} = N/\tau$, as shown in Fig. 2(b), where $\tau$ is the round-trip time delay of the OEO cavity. In this situation, fundamental ($N = 1$) and harmonic ($N \ge 2$) mode locking can be achieved in the OEO cavity to generate an ultra-short microwave pulse train with a repetition rate of ${f_s}$ and a center frequency in the passband of the BPF. The third bias waveform is a rectangular one with a repetition frequency of ${f_s} = N{f_{FSR}}$, which realizes gain switching as shown in Fig. 2(c). On this condition, rectangular microwave waveform with a repetition rate of ${f_s}$ and a center frequency in the passband of the BPF can oscillate in the OEO cavity, where the duration of each pulse can be varied through simply changing the duration of the bias waveform. In the proposed OEO scheme, the oscillation signal feedback and the gain modulation are realized by using a single electro-optic MZM, which simplifies the system architecture compared with those in [17–19].
Fig. 2. Diagrammatic sketch of the microwave signal generation under three types of bias waveforms, (a) DC bias for single-tone microwave signal generation, (b) sinusoidal bias for microwave pulse train generation, and (c) rectangular bias for rectangular microwave waveform generation.
3. Experimental results
A proof-of-concept experiment is carried out to demonstrate the proposed scheme for multi-format microwave signal generation. In the experiment, CW light wave with a power of 17 dBm and a center wavelength of 1569 nm is emitted from a laser diode (INNO08104DFBM-PM). A variable optical attenuator is employed to finely control the optical power injected into the OEO cavity. A 20 Gb/s electro-optic MZM (EOSPACE) with a half-wave voltage of 6 V for the bias port is used in the OEO, where the RF port is driven by the feedback oscillating signal, and the bias port is connected to a function generator (Hantek HDG2022B). The intensity-modulated optical signal is detected by a 20 Gb/s photodetector (HP 11982A) after propagating through a spool of the single-mode fiber (YOFC) with a length of 500 m. An electric BPF with a fixed center frequency of 4 GHz and a 3-dB bandwidth of 70 MHz is used to achieve mode selection in the OEO cavity. Besides, an electric low-noise amplifier (Qotana) with a small-signal gain of 25 dB and an operation frequency range from 2 GHz to 18 GHz is employed to compensate for the power loss in the OEO cavity. The oscillating microwave signal is split into two paths by an electric coupler (GTPD-COMB50G), of which one port is used to output the oscillating microwave signal, and the other port is used to feed the oscillating microwave signal back to the MZM. An electrical spectrum analyzer (ESA, R&S FSU50, 2-50 GHz) and a high-speed real-time oscilloscope (OSC, Tektronix DP75002SX, 100 GS/s, 33 GHz) are utilized to measure the spectra and the temporal waveforms of the generated microwave signals, respectively.
Firstly, the output of the function generator is set to be a DC signal, where its voltage guarantees that the MZM is biased at its quadrature point. In such a case, the OEO works at single-tone oscillation status. Figure 3(a) and (b) present the spectrum and the temporal waveform of the generated microwave signal, respectively. It can be seen from Fig. 3(a) that single-tone oscillation is realized at 4.005 GHz, where the sidemode suppression ratio is larger than 40 dB. The existence of numerous weak sidemodes is attributed to the relatively large 3-dB bandwidth of the BPF, which has a tiny influence on the single-tone oscillation as shown in Fig. 3(b). The mode interval is measured to be 390 kHz (i.e., ${f_{FSR}} = 390{\kern 1pt} {\kern 1pt} {\kern 1pt} \textrm{kHz}$) as shown in zoom-in view of Fig. 3(a), which provides a reference for setting the driving signal frequency to realize mode locking and gain switching status.
Fig. 3. Generated microwave signal when the OEO works at single-tone oscillation status. (a): spectrum, (b): temporal waveform.
Then, the output of the function generator is set to be a sinusoidal signal with a frequency of ${f_s} = N{f_{FSR}}$ to realize mode locking, where the peak-to-peak voltage and the offset voltage are set to be 4.04 V and 2 V, respectively. Figure 4(a) shows the spectrum of the generated microwave signal when ${f_s}$ is set to be 390 kHz, which corresponds to fundamental mode locking. It can be seen that a stable multi-tone oscillation with a mode interval of 390 kHz and a smooth spectrum envelope is obtained under mode locking status. These modes are coherently superimposed in the time domain to form a microwave pulse train with a repetition rate of 390 kHz and a full width at half maximum (FWHM) of 386 ns as shown in Fig. 4(b). Figure 4(c) exhibits the spectrum of the generated microwave signal when ${f_s}$ is set to be 3.9 MHz, which corresponds to 10th-order harmonic mode locking. Microwave frequency combs with an interval of 3.9 MHz are generated, which are coherently superimposed in the time domain to form a microwave pulse train with a repetition rate of 3.9 MHz and a FWHM of 44 ns as shown in Fig. 4(d). The relatively large pulse width of the generated microwave pulse is mainly attributed to the narrow 3-dB bandwidth of the electric BPF used in the experiment. The pulse width can be further reduced if a wideband BPF is employed in the proposed OEO. In addition, the output temporal and spectral characteristics from the OEO are relative to the net gain in the OEO cavity. Therefore, through finely adjusting the net gain in the OEO cavity, the output microwave pulses can be optimized. It should also be pointed out that there are spurious frequency components between the dominate oscillation modes in Fig. 4(c), which are called supermodes. These unwanted supermodes have a non-negligible impact on the performance of the generated microwave pulse signals. For example, in a pulsed Doppler radar, the supermodes may induce interference in velocity measurement. In addition, the existence of the supermodes may lead to instability of the generated microwave pulse signals owing to the mode competition effect. Some methods can be employed in the proposed OEO scheme to suppress the supermodes and also the sidemodes under single-tone oscillation status, such as using injection locking technology [20] or multiple comb filters [21].
Fig. 4. Spectra (left column) and temporal waveforms (right column) of the generated microwave signals when the OEO works at (a)-(b) fundamental and (c)-(d) 10th-order harmonic mode locking status.
The stability of a single microwave comb tooth under mode locking is measured by using the Maxhold mode of the ESA in five minutes, which is presented in Fig. 5. The maximum frequency and power drifts of the generated microwave comb are 2.1 kHz and 0.78 dB, respectively, which are attributed to the loop length and net gain variation. It should be pointed out that the loop length variation will lead to FSR drift, which may result in loss of mode locking status. Therefore, feedback control technique should be employed to maintain stable mode locking.
Fig. 5. Measured frequency and power stability of a single microwave comb tooth when the OEO is working at mode locking status.
Finally, a rectangular waveform with a repetition rate of 390 kHz is applied to the bias port of the MZM, where the high voltage level and the low voltage level are set to be 4.04 V and 0 V, respectively. In such a case, the OEO works in the gain switching status to generate rectangular microwave waveforms with an identical repetition rate of 390 kHz. Through varying the duty cycle of the externally-applied signal, rectangular microwave waveforms with different duty cycles are generated. Figure 6(a), (c) and (e) present the measured spectra of the generated rectangular microwave waveforms with a duty cycle of 25%, 50% and 75%, respectively. Figure 6(b), (d) and (f) exhibit the corresponding temporal waveforms. The main difference between the mode locking status and the gain switching status lies in that the number of the dominated oscillation modes in the gain switching status is smaller than that in the mode locking status. In addition, it should be pointed out that the repetition rate of the generated rectangular microwave waveform can be increased through setting the repetition rate of the externally-applied rectangular signal to be an integer multiple of the free spectrum range.
The main advantage of directly generating rectangular waveforms in the OEO cavity lies in that the coherence between different rectangular microwave pulses can be maintained. Figure 7 shows the cross-correlation results of the rectangular microwave waveforms generated by the proposed OEO scheme and through external modulation with a corresponding low-speed rectangular waveform. It can be seen from Fig. 7 that the correlation peak of the signal generated by the OEO cavity is higher than that of the signal generated through external modulation. This can be attributed to that the phase relationship of different modes is fixed due to mode locking. These phase-locked modes are superimposed in the time domain to form rectangular microwave waveforms with an identical initial phase. However, in the scheme based on external modulation, it is difficult to guarantee an identical initial phase between different rectangular microwave waveforms. A good coherence between different rectangular microwave pulses can effectively improve the signal-to-noise ratio of the cross-correlation peak, which is beneficial for long-range target detection.
Fig. 6. Spectra (left column) and temporal waveforms (right column) of the generated rectangular microwave waveforms with a duty cycle of (a)-(b) 25%, (c)-(d) 50% and (e)-(f) 75% when the OEO works in the gain switching status.
Fig. 7. Cross-correlation results of the rectangular microwave waveforms generated by the proposed OEO and through external modulation with a corresponding low-speed rectangular waveform.
Figure 8 shows the single-sideband phase noise of the generated microwave signals, which is measured by using the phase noise analysis module of the ESA. It can be seen from Fig. 8 that the far-from-carrier (>500 Hz frequency offset) phase noise of the rectangular microwave waveform is close to that of the single-tone microwave signal. However, its close-to-carrier (<500 Hz frequency offset) phase noise is similar to that of the microwave pulse signal, both of which are nearly 10-dB lower than that of the single-tone microwave signal. In an actively mode-locked OEO, the close-to-carrier phase noise can be effectively suppressed due to the periodic high-pass filtering effect induced by the superposition between each mode and its delayed duplicate during mode locking. Therefore, the gain switching status is an intermediate state between the single-tone oscillation status and the mode locking status. In addition, based on the simulation results in [22], the phase noise of the generated microwave comb under mode locking should be smaller than that of the single-tone microwave signal. Nevertheless, the measured phase noise of the microwave pulse signal is larger than that of the single-tone microwave signal. This can be attributed to the limited measurement dynamic range of the ESA. The dominated oscillation tone of the microwave pulse signal in Fig. 4 is nearly 20 dB smaller than that of the single-tone microwave signal in Fig. 3. Therefore, the phase noise measurement of the microwave pulse signal at the far-from-carrier (>500 Hz frequency offset) is limited by the noise floor of the ESA.
Fig. 8. Measured single-sideband phase noise of the generated single-tone microwave signal, microwave pulse signal and rectangular microwave waveform at 4 GHz.
4. Conclusion
In summary, we have proposed and experimentally demonstrated a simple method to achieve multi-format microwave signal generation directly from an OEO cavity. The kernel of this method is using a low-frequency electrical waveform to control the bias status of the MZM in the OEO. Hence, the net gain in the OEO cavity is dynamically tuned to maintain specially-designed microwave signal oscillation. In the proof-of-concept experiment, three types of microwave waveforms (i.e., single-tone, pulse and rectangular) were generated through simply varying the externally-applied low-frequency electrical waveforms (i.e., DC, single-tone and rectangular). The generated microwave waveforms were characterized by low close-to-carrier phase noise. In addition, the center frequency of the generated microwave signals can be tuned by introducing a tunable BPF or a microwave photonic filter into the OEO cavity. The proposed scheme paves the way to directly generate multi-format microwave signals in an OEO, which can find applications in a multi-functional microwave photonic system.
Funding
National Key Research and Development Program of China (2018YFE0201900); National Natural Science Foundation of China (61927821, 61421002); Fundamental Research Funds for the Central Universities (ZYGX2020ZB012).
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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