Open Access
Add to BibSonomy
Abstract
We propose and experimentally demonstrate a spurious level and phase noise improved Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) based on a self-injection-locking (SIL) technique. The scheme applies a dual-loop FDML-OEO structure, in which a long optical fiber delay loop is used to injection-lock the OEO with a short oscillating optical fiber delay loop. SIL is achieved so long as the delay of the long loop is tuned at the integral multiple of the oscillation loop. The spur suppression ratio of the wideband linear frequency modulated (LFM) signal generated by the FDML-OEO can be improved by 14 dB under SIL. Furthermore, the modification of the spur suppression ratio depending on the injection power is also demonstrated. The phase noise of the proposed OEO is -127.5 dBc/Hz at 10 kHz offset, which is much improved comparing with a free-running OEO.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optoelectronic oscillator (OEO) attracts much attention in generating high-purity wideband tunable microwave signals and it can find various applications in radar, sensing, signal processing and communication systems [1–4]. Attributing to the ultra-low loss optical fiber delay line, the feedback loop with high-quality factor can be obtained, which ensures a low phase noise. However, the introduction of a long fiber loop to reduce the phase noise also brings some problems, such as multimode oscillation and mode hopping [5,6]. Various techniques have been proposed to improve the stability of OEO, such as using high-quality-factor (Q-factor) microwave photonic filter (MPF) [7–10], multiloop structure [11,12], self-injection-locking (SIL), phase-locked-loop [13,14], and Parity-time symmetry [15,16]. Among them, SIL method implemented by coupling a part of the signal from the oscillation loop and injecting back with longer delay to lock the oscillator itself is one of the effective methods to improve the stability performance of OEO [17–22].
Recently, Fourier domain mode-locked optoelectronic oscillator (FDML-OEO), through which a wide-band linear-frequency-modulated (LFM) microwave signal with large time-bandwidth product can be directly generated, has received much attention [23]. Different FDML-OEOs have been demonstrated and published, such as harmonically FDML-OEO [24], dual-chirp FDML-OEO [25], FDML-OEO based on stimulated Brillouin scattering [26] and polarization manipulated FDML-OEO [27]. However, in most of the previous work, investigations of FDML-OEO mainly focus on the generation of frequency and bandwidth tunable LFM signal. Generally, to obtain an LFM signal with large time-bandwidth product, long fiber delay should be involved in the FDML-OEO loop, resulting in a closely-spaced longitudinal modes. Since the optical or radio frequency (RF) filter is not narrow enough to filter out the desired longitudinal mode, strong mode competition will exist in the oscillation loop, making single mode operation difficult. In this case, the short-term frequency stability of a conventional FDML-OEO is usually poor, which limits their potential usage in radar system. Actually, the parameters such as spur suppression ratio and phase noise of the LFM signal are very important for the practical application. For example, in radar system, the dynamic range is directly dependent on the spur suppression of the transmitting signal. Meanwhile, the moving target detecting ability of the radar system is influenced by the phase noise of the signal. Thus, there is an urgent need but a significant challenge to investigate the spur suppression ratio and phase noise improving method of the FDML-OEO.
In this paper, we proposed and experimentally demonstrate a spur suppression ratio and phase noise improved FDML-OEO based on SIL technique. Dual-loop structure is adopted in the FDML-OEO, in which a long loop is used to injection-lock the oscillator itself. SIL is achieved only as the delay of the long loop is integral multiple of the oscillation loop. An experiment is conducted and the results demonstrate that the spur suppression ratio of the generated signal is much improved under SIL. The improvements of the generated LFM signal under different central frequencies are measured. The relationship between the injection strength and spur suppression ratio is also investigated, as well as the phase noise of the proposed OEO under single frequency operation.
2. Principle
The schematic of the proposed FDML-OEO based on SIL is shown in Fig. 1. A continuous wave (CW) light wave that is driven by a triangular current emits a frequency sweeping optical signal. The light wave is then sent into a phase modulator (PM). The modulated light is reflected by a phase-shifted fiber Bragg grating (PS-FBG) via an optical circulator (OCir). Thanks to the narrow transmission window within the reflection band, phase-modulation to intensity-modulation conversion can be achieved and a scanning single-passband microwave photonic filter (MPF) is built. The light signal is then split into two branches through an optical coupler (OC). After being delayed by two optical fibers with different lengths, the two optical signals are detected by two photodetectors (PDs), respectively. The signals are then combined by an electrical combiner and amplified by an electrical amplifier (EA) to compensation the loop loss. Finally, the signal is transmitted back to the RF port of the PM to close the OEO loop. The proposed OEO has a dual-loop structure, in which the short loop is the oscillation loop and the long loop is used for injection locking. Different from a conventional dual-loop structure that the open-loop gains of both loops are maintained beyond 0 dB, the gain of the self-injection loop in the proposed OEO is kept below 0 dB to prevent oscillating by manipulating the optical attenuation in each loop.
It has been demonstrated that mode locking in frequency domain can be obtained only as the periodic of the sweeping time of the laser is synchronized with the round-trip time of the OEO loop [23]. Assuming the round-trip time of the OEO loop is ${T_{loop{\kern 1pt} {\kern 1pt} time}}$ and the period of the driving signal of the laser is ${T_{driving{\kern 1pt} signal}}$, then we have:
where n is an integer. To achieve SIL, the self-injection signal should have the same frequency as the oscillator, thus the round-trip time of the long loop ${T_{long{\kern 1pt} {\kern 1pt} loop}}$ should satisfy:3. Results and discussions
An experiment is performed to study the operation of the proposed dual-loop FDML-OEO based on SIL. A distributed feedback (DFB) laser diode with a maximum output power of 16 dBm is used as the light source. The laser is driven by a laser controller and its initial wavelength is set to be 1550.128 nm. The driving signal applied onto the laser is a triangular waveform with the period synchronized with the round-trip time of the OEO loop. A PM with a 3 dB bandwidth of 40 GHz is used in the FDML-OEO loop. A phase-shift fiber Bragg grating (PS-FBG) with a notch wavelength of 1551.112 nm and 3 dB notch bandwidth of 70 MHz is employed as the notch filter. Two spools of optical fiber with about 1 km and 5 km in length are applied as the delay lines in the dual-loop FDML-OEO. To reduce the effect of dispersion-induced power fading, a spool of near-zero dispersion single mode fiber is involved in the long loop. An optical tunable delay line (OTDL) is inserted in the oscillation loop to precisely control the delay difference between the two loops. To change the injection strength and control the optical power, a tunable optical attenuator (ATT) is employed in the injection loop. Another ATT in the short loop is also used to control the optical power and reduce the harmonic components. The two PDs are with the 3 dB bandwidth of 14 GHz and responsivity of 0.8 A/W. Two electrical amplifiers with combined gain of over 50 dB are cascaded to compensate the loss of the electrical signal. The signal is then fed back to the PM via a power splitter to form a closed loop. The temporal and frequency waveforms of the generated electrical signal are observed by a digital storage oscilloscope (Keysight DSO-X 93204A) and an electrical signal analyzer (ESA, Keysight N9040B), respectively.
First, the output of the FDML-OEO without SIL is measured. The measurement is implemented with the long delay loop disconnected. By setting the period of the triangular driving signal to be equal to the round-trip time, FDML-OEO is achieved. Figure 2(a) shows the spectrum of the generated LFM signal with the central frequency of 4.5 GHz and bandwidth of 1 GHz. Figure 2(b) is the frequency spectrum with a span of 1 MHz and a RBW of 1 kHz. Since the LFM signal is periodical, its Fourier spectrum is discrete. The frequency spacing between the adjacent modes is 162.219 kHz, which is equal to the reciprocal of the oscillation loop delay. The corresponding temporal waveform is presented in Fig. 2(c), and the inset depicts a zoomed-in section of the generated waveform. The instantaneous frequency distribution of frequency-scanning signal is shown in Fig. 2(d), which is obtained using short-time Fourier transform. The corresponding temporal waveform and instantaneous frequency distribution of frequency-scanning signal of the FDML-OEO under SIL are also measured and shown in Fig. 2(e) and Fig. 2(f). The unsymmetrical amplitude of the temporal waveform in Fig. 2(c) and Fig. 2(e) is mainly caused by the optical power fluctuation during the frequency tuning of the laser.
Fig. 2. (a) Frequency spectrum of the generated LFM signal. (b) Frequency spectrum of the generated LFM signal with a span of 1 MHz and RBW of 1kHz. (c) Temporal waveform of the generated LFM signal, the inset shows the zoomed-in view of a section of the waveform. (d) The instantaneous frequency distribution of the generated LFM signal. (e) Temporal waveform of the generated LFM signal under SIL. (f) The instantaneous frequency distribution of the generated LFM signal under SIL
Then the spur suppression ratio improvement based on SIL is investigated. For comparison, the frequency spectra before and after SIL are tested. Figure 3(a) and (b) depict the frequency spectrum of the FDML-OEO without SIL when the central frequency of the generated LFM signal is tuned at 5.1 GHz with different frequency spans. The measured spur suppression ratio is 42 dB, as shown in Fig. 3(b). When the long loop is connected, as shown in Fig. 3(c), the spur suppression ratio and stability of the generated signal is deteriorated due to the delay mismatch between two loops. To achieve precise delay match, the delay difference between the two loops should be pre-controlled within a certain range due to the limited optical delay tunning range of the OTDL. Then, through precisely controlling the loop delay in terms of the OTDL, the delay of the injection loop can be tuned exactly five times longer than the oscillation loop and consequently, SIL is achieved with improved spur suppression ratio, as demonstrated in Fig. 3(d). Note that the attenuation is adjusted to ensure that the optical power of the long loop in FDML-OEO is just below the threshold of oscillation. The results verified that the injection-locked FDML-OEO exhibits high quality presentation only as delay match is satisfied.
Fig. 3. (a) Frequency spectrum of free-running FDML-OEO with the central frequency of 5.1 GHz. (b) Zoomed-in view of (a) with a span of 1 MHz. (c) Frequency spectrum with a span of 1 MHz when there is a mismatch between two loop delays. (d) Frequency spectrum with a span of 1 MHz when the delays of the two loops are matched.
The frequency tunability of the proposed OEO is then investigated. Figure 4(a) presents the optical reflection (blue line) and transmission (red line) spectrum of the PS-FBG used in the OEO. The spectra of the generated LFM signals with different central frequencies of the proposed FDML-OEO based on SIL are presented in Fig. 4(b). The bandwidth of LFM signal is fixed at 1 GHz and the frequency is tuned by changing the frequency of the laser. The result demonstrates the tunability of the proposed FDML-OEO. The tunable range of the central frequency is limited by the 3 dB reflection bandwidth and the notch location of the PS-FBG. Better tunable range can be achieved when PS-FBG with wider reflection bandwidth is applied.
Fig. 4. (a) the optical reflection (red line) and transmission (blue line) spectrum of the PS-FBG used in the OEO. (b) Frequency tunning result of the proposed FDML-OEO.
The spur suppression ratio performance of the OEO based on SIL under different operating frequencies is also measured. Figure 5(a) and (c) present the frequency spectra of free-running FDML-OEO when the central frequencies are tuned at 3.25 GHz and 4.1 GHz by changing the initial wavelength of the sweeping laser source, respectively. Figure 5(b) and (d) are their corresponding frequency spectra under SIL. It can be found that the spur suppression ratio modifications are both up to 14 dB.
Fig. 5. Experimental results of the FDML-OEO under different central frequencies. (a) Frequency spectrum of free-running FDML-OEO with a central frequency of 3.25 GHz. (b) Frequency spectrum of the FDML-OEO based on SIL with a central frequency of 3.25 GHz. (c) Frequency spectrum of free-running FDML-OEO with a central frequency of 4.1 GHz. (d) Frequency spectrum of the FDML-OEO based on SIL with a central frequency of 4.1 GHz.
According to Eq. (3), the performance of the FDML-OEO based on SIL depends on the injection strength. Thus, the relationship between the spur suppression ratio improvement and injection power is also investigated, as depicted in Fig. 5. The measurement is implemented by changing the optical attenuation in the injection loop. Since the loop gain depends on the optical power, to ensure that no oscillation can be sustained in the injection loop, the optical injection power is always kept below the oscillation threshold during the measurement. Figure 6(a) shows the frequency spectra of the FDML-OEO with the optical injection power of -12 dB and -15 dB, respectively. As can be seen, the spur suppression ratio is clearly improved when the injection power is -12 dB. The spur suppression ratio improvement under different injection power is plotted in Fig. 6(b), which demonstrates that the spur suppression ratio improves as the injection power increases.
Fig. 6. (a) Frequency spectra of the FDML-OEO based on SIL for different injection strengths. (b) The relationship between injected power and spur suppression ratio improvement.
The single sideband (SSB) phase noise of the FDML-OEO before and after SIL is also measured. The result is recorded under single frequency operation condition, as shown in Fig. 7. The phase noise of free-running FDML-OEO is -121.6 dBc/Hz at 10 kHz offset. Some spurs at the integer multiples of 162.219 kHz corresponding to the mode spacing of the OEO could be observed. The phase noise reaches to -127.5 dBc/Hz at 10 kHz offset when SIL is achieved. Moreover, the sidemode spurs are also well suppressed. The spurs around 30 kHz in Fig. 6 are mainly caused by the spurious modes generated by the long loop. Since the performance of the proposed OEO is closely related to the delay of the injection loop, better improvement effect of spur suppression ratio and phase noise can be expected by using longer injection loop. Note that vibration and thermal isolation, as well as the phase locking techniques could be applied to improve the long-term stability of the proposed FDML-OEO.
4. Conclusion
In conclusion, a spur suppression ratio and phase noise improvement method for FDML-OEO based on SIL technology is proposed and experimentally demonstrated. By applying a dual-loop structure and manipulating the delay difference, SIL is achieved. An experiment is performed and a spur suppression ratio improvement of over 14 dB of the wideband LFM signal generated by the FDML-OEO can be obtained. The spur suppression ratio improvement under different central frequencies, as well as different injection strengths has also been demonstrated. The phase noise of the OEO based on SIL under single frequency operation is modified to be -127.5 dBc/Hz at 10 kHz offset. The proposed FDML-OEO based on SIL shows great application potential in future radar and communication system.
Funding
National Natural Science Foundation of China (61701532); Natural Science Foundation of Hubei Province (2018CFB311, 2018CFB539, 2020CFB288).
Disclosures
The authors declare no conflicts of interest.
References
1. X. S. Yao and L. Maleki, “Optoelectronic microwave oscillator,” J. Opt. Soc. Am. B 13(8), 1725–1735 (1996). [CrossRef]
2. B. Yin, S. Wu, M. Wang, W. Liu, H. Li, B. Wu, and Q. Wang, “High-sensitivity refractive index and temperature sensor based on cascaded dual-wavelength fiber laser and SNHNS interferometer,” Opt. Express 27(1), 252–264 (2019). [CrossRef]
3. L. Huo, Y. Dong, C. Lou, and Y. Gao, “Clock extraction using an optoelectronic oscillator from high-speed NRZ signal and NRZ-to-RZ format transformation,” IEEE Photonics Technol. Lett. 15(7), 981–983 (2003). [CrossRef]
4. H. Tsuchida, “Simultaneous prescaled clock recovery and serial-to-parallel conversion of data signals using a polarization modulator-based optoelectronic oscillator,” J. Lightwave Technol. 27(17), 3777–3782 (2009). [CrossRef]
5. Y. Jiang, G. Bai, L. Hu, H. Li, Z. Zhou, J. Xu, and S. Wang, “Frequency locked single-mode optoelectronic oscillator by using low frequency RF signal injection,” IEEE Photonics Technol. Lett. 25(4), 382–384 (2013). [CrossRef]
6. S. Guan, Q. Cen, F. Yin, K. Xu, and Y. Dai, “Low spurious optoelectronic oscillator achieved by frequency conversion filtering without deteriorating phase noise,” Opt. Express 28(12), 18529–18537 (2020). [CrossRef]
7. W. Li and J. Yao, “A wideband frequency tunable optoelectronic oscillator incorporating a tunable microwave photonic filter based on phase-modulation to intensity-modulation conversion using a phase-shifted fiber Bragg grating,” IEEE Trans. Microwave Theory Tech. 60(6), 1735–1742 (2012). [CrossRef]
8. F. Jiang, J. H. Wong, H. Q. Lam, J. Zhou, S. Aditya, P. H. Lim, K. E. K. Lee, P. P. Shum, and X. Zhang, “An optically tunable wideband optoelectronic oscillator based on a bandpass microwave photonic filter,” Opt. Express 21(14), 16381–16389 (2013). [CrossRef]
9. J. Zhang, L. Gao, and J. Yao, “Tunable optoelectronic oscillator incorporating a single passband microwave photonic filter,” IEEE Photonics Technol. Lett. 26(4), 326–329 (2014). [CrossRef]
10. H. Tang, Y. Yu, Z. Wang, L. Xu, and X. Zhang, “Wideband tunable optoelectronic oscillator based on a microwave photonic filter with an ultra-narrow passband,” Opt. Lett. 43(10), 2328–2331 (2018). [CrossRef]
11. X. S. Yao and L. Maleki, “Multiloop optoelectronic oscillator,” IEEE J. Quantum Electron. 36(1), 79–84 (2000). [CrossRef]
12. L. Huang, Q. Yu, L. Deng, S. Fu, M. Tang, M. Cheng, M. Zhang, M. Choi, D. Chang, G. Lei, and D. Liu, “Novel dual-loop optoelectronic oscillator based on self-polarization-stabilization technique,” Opt. Express 25(18), 21993–22003 (2017). [CrossRef]
13. Z. Zhou, C. Yang, Z. Cao, Y. Chong, and X. Li, “An Ultra-Low phase noise and highly stable optoelectronic oscillator utilizing IL-PLL,” IEEE Photonics Technol. Lett. 28(4), 516–519 (2016). [CrossRef]
14. Y. Zhang, D. Hou, and J. Zhao, “Long-term frequency stabilization of an optoelectronic oscillator using phase-locked loop,” J. Lightwave Technol. 32(13), 2408–2414 (2014). [CrossRef]
15. J. Zhang and J. Yao, “Parity-time–symmetric optoelectronic oscillator,” Sci. Adv. 4(6), eaar6782 (2018). [CrossRef]
16. C. Teng, X. Zou, P. Li, W. Pan, and L. Yan, “Wideband frequency-tunable parity-time symmetric optoelectronic oscillator based on hybrid phase and intensity modulations,” J. Lightwave Technol. 38(19), 5406–5411 (2020). [CrossRef]
17. T. Sun, L. Zhang, A. Poddar, U. Rohde, and A. Daryoush, “Limits in timing jitters of forced microwave oscillator using optical self-IL PLL,” IEEE Photonics Technol. Lett. 29(2), 181–184 (2017). [CrossRef]
18. R. Fu, X. Jin, Y. Zhu, X. Jin, X. Yu, S. Zheng, H. Chi, and X. Zhang, “Frequency stability optimization of an OEO using phase-locked-loop and self-injection-locking,” Opt. Commun. 386, 27–30 (2017). [CrossRef]
19. T. Sun, L. Zhang, and A. S. Daryoush, “High-resolution X-band frequency synthesizer using SIL PLL optoelectronic oscillator,” IEEE Trans. Sonics Ultrason. 67(1), 217–223 (2020). [CrossRef]
20. L. Zhang, A. K. Poddar, U. L. Rohde, and A. S. Daryoush, “Analytical and experimental evaluation of SSB phase noise reduction in self-injection locked oscillators using optical delay loops,” IEEE Photonics J. 5(6), 6602217 (2013). [CrossRef]
21. K. H. Lee, J. Y. Kim, W. Y. Choi, H. Kamitsuna, M. Ida, and K. Kurishima, “Low-Cost Optoelectronic Self-Injection-Locked Oscillators,” IEEE Photonics Technol. Lett. 20(13), 1151–1153 (2008). [CrossRef]
22. K. H. Lee, J. Y. Kim, and W. Y. Choi, “A 30-GHz Self-Injection-Locked Oscillator Having a Long Optical Delay Line for Phase-Noise Reduction,” IEEE Photonics Technol. Lett. 19(24), 1982–1984 (2007). [CrossRef]
23. T. Hao, Q. Cen, Y. Dai, J. Tang, W. Li, J. Yao, N. Zhu, and M. Li, “Breaking the limitation of mode building time in an optoelectronic oscillator,” Nat. Commun. 9(1), 1839 (2018). [CrossRef]
24. T. Hao, J. Tang, W. Li, N. Zhu, and M. Li, “Harmonically Fourier domain mode-locked optoelectronic oscillator,” IEEE Photonics Technol. Lett. 31(6), 427–430 (2019). [CrossRef]
25. T. Hao, J. Tang, N. Shi, W. Li, N. Zhu, and M. Li, “Dual-chirp fourier domain mode-locked optoelectronic oscillator,” Opt. Lett. 44(8), 1912–1915 (2019). [CrossRef]
26. T. Hao, J. Tang, W. Li, N. Zhu, and M. Li, “Fourier domain mode locked optoelectronic oscillator based on the deamplification of stimulated Brillouin scattering,” OSA Continuum 1(2), 408–415 (2018). [CrossRef]
27. S. Zhu, X. Fan, B. Xu, W. Sun, M. Li, N. Zhu, and W. Li, “Polarization manipulated Fourier domain mode-locked optoelectronic oscillator,” J. Lightwave Technol. 38(19), 5270–5277 (2020). [CrossRef]
28. H. Chang, “Phase Noise in Self-Injection-Locked Oscillators-Theory and Experiment,” IEEE Trans. Microwave Theory Tech. 51(9), 1994–1999 (2003). [CrossRef]
