A novel millimeter-wave synergetic optoelectronic oscillator based on regenerative frequency-dividing oscillation and phase-locking techniques is proposed and experimentally demonstrated. The regenerative frequency-dividing oscillator is embedded for millimeter-wave frequency division, and then synergistically oscillates with the optoelectronic oscillator (OEO) due to injection-locking effect. The phase-locking stabilization technique is skillfully utilized in millimeter-wave OEO via commercial analog phase shifter. As a result, a 40-GHz signal is generated featuring low phase noise, high stability and low spurs. The single-sideband phase noise is about −117 dBc/Hz at 10-kHz offset frequency and the spurious suppression ratio reaches more than 80 dBc. The measured overlapping Allan deviation of the proposed synergetic OEO reaches lower than 10−13 at 1024-s averaging time, which is five orders of magnitude lower than free-running millimeter-wave OEO.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Millimeter-wave (MMW) source with inherent-wide bandwidth is essential to a growing number of applications, such as time and frequency metrology [1–3], wireless communication system [4,5] and high-speed sampling . All these radio-frequency (RF) systems put forward high requirements for phase noise and frequency stability performances of millimeter-wave sources. In MMW band, electronic oscillators usually suffer from parasitic loss, which would decrease the quality factor (Q factor) and increase close-to-carrier phase noise. Besides, the phase noise will deteriorate with the multiplication factor during the frequency multiplication process for the quartz oscillator. Photonic methods, such as optical heterodyning [7,8], optoelectronic oscillator , electro-optical frequency division [10,11] and Kerr-frequency-comb oscillator , have drawn extensive efforts to explore and achieve millimeter-wave source with low phase noise and high frequency stability. In particularly, the OEO is deemed as one of the most promising and powerful methods, which can produce low phase noise microwave signal from hundreds of MHz to 100 GHz. The state-of-art OEO with recorded ultra-low phase noise (−163 dBc/Hz @ 6 kHz offset from a 10 GHz carrier) is achieved by employing 16-km optical fiber . Besides, the phase noise performance of OEO is theoretically independent of the oscillation frequency [14,15].
For the MMW-OEO, the employment of low-loss optical fiber as the ultra-high Q energy storage element ensures the low phase noise performance. However, the effective refractive index and length of fiber are sensitive to ambient temperature variations . This character has non-negligible contribution to oscillation frequency drift , and it would greatly deteriorate the long-term frequency stability. The phase-locked loop (PLL) with voltage-controlled phase shifter is one of the most practical solutions to effectively adjust the cavity length and improve the long-term frequency stability . Nevertheless, the maximum phase shift range of commercial available MMW phase shifter is too narrow to compensate the frequency drift. For MMW-OEOs, the PLL-based stabilization method is also limited by the operating frequency range of commercial frequency divider. Furthermore, the Q value of the microwave band-pass filter will decrease along with the operating frequency, and it is difficult to reject the unwanted oscillation modes for the MMW-OEO due to the long optical fiber . Therefore, improving long-term frequency stability and suppressing spurious mode are two inescapable burning questions for the application of OEO in the MMW bands.
In this letter, a novel millimeter-wave synergetic optoelectronic oscillator based on regenerative frequency-dividing oscillation and phase-locking techniques has been proposed and experimentally demonstrated. The regenerative frequency-dividing oscillator is self-oscillated and then steers the oscillation of the MMW-OEO due to injection locking effect. Meanwhile, the frequency division of MMW signal can be realized via regenerative frequency-dividing oscillator breaking the limitation of commercial frequency dividers . Thanks to the frequency division and multiplication , the MMW frequency compensation range can be greatly enlarged, and the PLL technique is skillfully utilized to stabilize the MMW-OEO via commercial available analog phase shifter in relative low frequency band. Finally, a 40-GHz signal is successfully generated featuring low phase noise, high frequency stability and low spurs by the proposed millimeter-wave synergetic OEO. The wide-range millimeter-wave phase shift around 1360 degrees can be obtained, which is about 10 times wider than commercial phase shifters. The single-sideband phase noise of the generated 40-GHz signal is about −117 dBc/Hz at 10-kHz offset frequency, and the spurious suppression ratio reaches more than 80 dBc. Compared with the free-running 40-GHz OEO, the frequency stability based on the proposed MMW synergetic optoelectronic oscillator is greatly improved from 1.2☓10−6 to 2.96☓10−13 at 1024-s averaging time in the lab room without any thermal control.
The schematic diagram of the proposed MMW synergetic optoelectronic oscillator based on regenerative frequency-dividing oscillation and phase-locking techniques is shown in Fig. 1, and the regenerative frequency-dividing oscillator (RFDO) is given in the shaded area. The whole system consists of three parts: the millimeter-wave, relative low frequency and frequency conversion modules. In the oscillation loop, the MMW module mainly plays the role in energy storage of MMW signal in optical domain via long fiber, and the low frequency module focuses on processing MMW oscillation signal, including the band-pass filtering and phase shifting. The bridge between these two different frequency bands is built by the frequency conversion module, which is composed of the RFDO, a commercial frequency divider and a frequency multiplier. Besides, it can also effectively improve the filter Q-value and phase-shift range for equivalent MMW band.
In this scheme, the RFDO is the key part with the closed-loop composed of a three-port mixer, a band-pass filter, a microwave amplifier and a phase shifter . It is suitable for frequency division of MMW input signal with ultra-low residual phase noise overcoming the frequency limitation of conventional frequency dividers. When the gain of the amplifier exceeds the isolation value between the LO and IF ports of the mixer, the RFDO can self-oscillate and then serve as a seed signal for the system via self-oscillation signal with inferior phase noise, and the limitation of nonlinear devices with boot threshold, such as commercial frequency divider and multiplier, can be overcome. Both the oscillation frequencies of the RFDO and synergetic MMW-OEO are mainly determined by the band-pass filter of RFDO, which can also be finely tuned by the phase shifter. When the MMW oscillation signal with low phase noise is injected into the RFDO, it will be frequency-divided and injection-lock the self-oscillation signal of the RFDO. The final output locked signal of RFDO would be determined by the MMW injection signal in phase noise and output frequency according to injection locking effect . Therefore, the RFDO can help to achieve the start-up gain and frequency matching for the synergetic MMW OEO.
In principle, the MMW synergetic optoelectronic oscillator embedded with the RFDO works the familiar way as conventional OEO, including fiber storage link and electronic feedback chain. The high Q MMW photonic link is employed to decrease the phase noise of the oscillation signal. Besides, the equivalent MMW feedback chain can help to improve the spurious suppression and frequency compensation range. For the proposed MMW synergetic OEO, the threshold limitation of the nonlinear frequency conversion components can be overcome by the embedded RFDO, which provide the original trigger for the synergetic oscillation. Due to the injection locking effect, the phase noise of the final MMW oscillation signal would be determined by the optoelectronic oscillation loop.
For the spurious modes suppression and frequency compensation range of the proposed MMW oscillator, it will be discussed and theoretically analyzed below. For conventional MMW-OEO, several spurious modes will exist in the oscillation loop because of the low-Q MMW band-pass filter, and the spurious signal can be considered as the frequency modulated component. When the oscillation signal with the spur is divided simultaneously, the oscillation signal and output signal can be given byEq. (1), we can conclude that the frequency spacing between the carrier and spurs keeps constant after frequency division. Hence, the spurs of the proposed MMW-OEO can be easily suppressed at relative low frequency via the equivalent high-Q MMW band-pass filter.
Thanks to the frequency division and multiplication, the equivalent wide-range MMW phase-shift can also be operated at relative low frequency by the commercial analog phase shifter. Since the phase shifter is set between the frequency divider and multiplier, the corresponding output of final MMW oscillation signal after phase shifting can be represented asEq. (2), the phase-shift range can be extended as much as N times, which is significantly larger than the one achieved by conventional MMW phase shifter. Finally, high performance MMW synergetic optoelectronic oscillator can be constructed with low phase noise, high spurious suppression ratio and large phase compensation range.
3. Experiment and results
The principle of the proposed millimeter-wave synergetic optoelectronic oscillator has been discussed and theoretically analyzed above. Then a proof-of-concept experiment was conducted. In the experiment, the RFDO is composed of a millimeter-wave mixer (Marki M90942), a band-pass filter centering on 20-GHz with the bandwidth about 30 MHz, a low-noise amplifier (LNA2) (Marki AP-0120EZP) and a phase shifter (ATM). The RFDO can self-oscillate and produce a microwave signal about 20 GHz, whose spectrum can be observed in Fig. 2(a). To verify the synergetic oscillation principle, an external MMW source with a power of approximately 5 dBm around 40 GHz (Agilent E8257D) is injected into the RFDO and the spectrum of the final output locked signal is shown in Fig. 2(b) with the same span and resolution about 200 kHz and 20 Hz. The phase noise of the self-oscillation signal of the RFDO, the injection MMW signal and injection-locked signal of the RFDO are also evaluated via a phase noise analyzer (R&S FSWP) as shown in Fig. 2(c). It is obvious that the phase noise can be significantly optimized when the RFDO is injection-locked by the external MMW signal. The phase noise of the injection-locked RFDO is determined by the phase noise of the injection MMW signal, consistent with the law that phase noise improves 20lgM with decreasing frequency and M is the frequency division factor. Finally, the injection-locking range is measured by tuning the external MMW signal frequency, and the injection-locking effect will be sustained with the output frequency following the injection signal in a certain frequency range. Just as shown in Fig. 2(d), we can find that the injection locking range is more than 500 kHz. It is larger than the frequency spacing of the oscillation modes, which means that the RFDO technique is applicable for the proposed scheme.
In the MMW synergetic optoelectronic oscillator, a continuous wave (CW) laser (Gooch & Housego EM650, 1550 nm) with an optical power about 18 dBm is connected with a 40-GHz MZM (Sumitomo) which is biased at quadrature point and driven by the MMW oscillation signal. The modulated signal is transmitted through a spool of optical fiber about 3 km and finally converted back to electrical domain by a high-speed photodetector (U2T XPDV210R, 50 GHz bandwidth). In the electric feedback path, the MMW oscillation signal is firstly amplified by a wideband low noise amplifier (LNA1) with 40-dB gain and then divided to 20-GHz signal with low phase noise by the RFDO. It will lock the self-oscillation signal of the RFDO via injection-locking effect. The final injection-locked 20-GHz signal is converted to 10 GHz by a commercial frequency divider (RF BAY INC FBS-2-20). Thereupon, the MMW signal is frequency divided by four via the RFDO and commercial frequency divider in series. After frequency division, a band-pass filter centered at 10 GHz with 10-MHz passband (K&L Microwave) is used to further suppress the spurious modes. Then an analog voltage-controlled phase shifter (PMI PS-5G18G-400-A-SFF) is settled to tune the whole cavity length according to the control signal from the PID module, which is extracted from phase comparison between the oscillation signal and a frequency reference (Agilent E8267D). A 30-dB LNA3 cascaded with a 10-dB attenuator is used to ensure that the input power exceeds the threshold level of frequency quadrupler (Marki AQA2040). Finally, the signal will be recovered back to MMW band and fed back to the optical storage link.
In order to verify the magnification of frequency compensation range, the phase shift responses of conventional analog phase shifter at different frequencies are firstly measured by connecting the RF input and output ports to a vector network analyzer (VNA, Agilent N5244A). As shown in Fig. 3, the phase shift ranges are 340 degrees and 130 degrees for 10 GHz and 40 GHz respectively. It is obvious that the value of additional phase shift is determined by the bias voltage from 0 V to 10 V. Besides, the phase shift range will decrease along with the operation frequency. For comparison, the equivalent MMW phase-shift range of the proposed feedback link is also measured by connecting the input and output ports of the RFDO and frequency quadrupler to the VNA respectively. As can be seen, the equivalent 40-GHz phase shift range is about 1360 degrees, which is about 10 times wider than the conventional phase shifter. The phase shift range can be further enlarged by increasing the frequency division and multiplication factor according to Eq. (2).
Finally, a 40-GHz MMW synergetic optoelectronic oscillator with phase locked loop as shown in Fig. 1 is successfully constructed. For comparison, a free-running conventional MMW-OEO employing the same length fiber (about 3 km) and a commercial 40-GHz band-pass filter with 30-MHz passband is also built and measured. The spectra of the free-running conventional OEO and PLL-based synergetic OEO are measured by a spectrum analyzer (R&S FSWP) as shown in Fig. 4. Due the same length of fiber, which is the determiner of phase noise, the linewidths of the free-running conventional OEO and PLL-based synergetic OEO are almost uniform. Besides, it can be seen that the oscillation signals have the same mode spacing due to the uniform fiber length. Comparison with the conventional MMW-OEO, the spurious mode of the proposed MMW synergetic optoelectronic oscillator is significantly suppressed. Besides, the insert window is the spectrum of the 10-GHz signal extracted from the PLL-based synergetic OEO via a 10-dB power splitter after the phase shifter. The mode spacing is the same between the 10-GHz and 40-GHz signals, and it is consistent with Eq. (1).
Figure 5(a) shows the SSB phase noise performance of the 40-GHz MMW oscillation signals measured by a phase noise analyzer (R&S FSWP), including the PLL-based synergetic OEO, free-running synergetic OEO, free-running conventional OEO and commercial microwave source respectively. The SSB phase noise of the proposed 40-GHz PLL-based synergetic OEO is about −117 dBc/Hz at 10-kHz offset frequency and the spurious suppression ratio reaches more than 80 dBc. The deterioration about 6-dB at high-offset frequency versus the free-running conventional OEO is attributed to the additive noise introduced by the active frequency quadrupler. The employment of the PLL whose loop bandwidth is about 300 Hz can significantly improve the phase noise of MMW-OEO at low-offset frequency, while remaining the performance at high-offset frequencies compared with the free-running synergetic OEO. Compared with the commercial MMW source (Agilent E8267D), the proposed 40-GHz PLL-based synergetic OEO is competitive with 20-dB phase noise optimization at an offset frequency of more than 300 Hz, while shares the identical phase noise performance at low-offset frequency. The bulge at the offset frequency between 100 Hz and 400Hz results from the insufficient precision of phase-locked bandwidth, which can be further optimized.
Finally, the long-term frequency stability is evaluated by overlapping Allan deviation which is calculated via the measurement results from a frequency counter (Agilent 53210A). Just as shown in Fig. 5(b), the overlapping Allan deviation of the proposed free-running oscillator is lower than 10−8 at 1-s averaging time, and it increases to 10−6 at 1024-s averaging time. When the PLL-based stabilization is implemented, the overlapping Allan deviation reaches 1.73☓10−11 at 1-s averaging time and 2.96 × 10−13 at 1024-s averaging time, respectively. Therefore, an improvement of more than 5 orders of magnitude can be achieved, and the proposed PLL-based MMW synergetic OEO works well in the lab room without any thermal control.
In conclusion, we proposed a novel MMW synergetic optoelectronic oscillator based on regenerative frequency-dividing oscillation and phase-locking techniques. The RFDO not only serves as a seed source for triggering the proposed MMW-OEO, but also implement the MMW frequency division for frequency closed-loop. The equivalent MMW band-pass filtering and phase shifting are also greatly helpful for the spurious mode suppression and frequency drift compensation. Finally, a 40-GHz signal has been generated with the phase noise about −117 dBc/Hz at 10-kHz offset frequency and the spurious suppression ratio reaches more than 80 dBc. The stability of the locked MMW synergetic OEO is improved from 1.15☓10−6 to 2.96 × 10−13 at 1024-s averaging time.
National Natural Science Foundation of China (NSFC) Program (61625104 and 61821001); Fundamental Research Funds for the Central Universities; Fund of State Key Laboratory of Information Photonics and Optical Communications (BUPT No. IPOC2017ZT01); Fund of State Key Laboratory of Advanced Optical Communication Systems Networks.
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