The coupled optoelectronic oscillator (COEO) is typically used to generate high frequency spectrally pure microwave signal with serious sidemodes noise. We propose and experimentally demonstrate a simple scheme for supermode suppression with mutual injection locking between the COEO (master oscillator with multi-modes oscillation) and the embedded free-running oscillator (slave oscillator with single-mode oscillation). The master and slave oscillators share the same electrical feedback path, which means that the mutually injection-locked COEO brings no additional hardware complexity. Owing to the mode matching and mutually injection locking effect, 9.999 GHz signal has been successfully obtained by the mutually injection-locked COEO with the phase noise about −117 dBc/Hz at 10 kHz offset frequency. Besides, the supermode noise can be significantly suppressed more than 50 dB to below −120 dBc.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
High spectral pure microwave source known as the basis of all microwave systems, directly affects the performance of many high frequency electronic devices, such as the applications of radar, communication and surveillance [1–3]. With the development of traditional radio frequency (RF) source and microwave photonic technology, photonic oscillator has attracted intense research interests to realize high frequency microwave source and make a breakthrough in the phase noise performance [4–7]. In the 1990s, the concept of optoelectronic oscillator (OEO) was firstly proposed to produce microwave signals with low phase noise from hundreds of MHz to 100 GHz by the NASA's Jet Propulsion Laboratory . The state-of-art OEO with recorded ultra-low phase noise (−163 dBc/Hz @ 6 kHz offset frequency from a 10 GHz carrier) is achieved by employing 16-km optical fiber as the microwave energy storage element . Generally, a typical OEO consists of optical storage and RF feedback parts, including a light source, electro-optic modulator, high Q optical storage element, RF filter and amplifiers. Due to the long optical fiber in the loop, the optoelectronic oscillator is a typical multi-mode source with severe supermode noise at high offset frequency. The mode spacing is inversely proportional to the optical fiber length, which means that several kilometers of fiber corresponds to ultra-narrow mode spacing around hundreds or tens of kHz. At present, it’s impossible to select only one required oscillation mode via a RF bandpass filter. Several approaches, such as multi-loops cavity  and ultra-high-Q optical filter [11,12], have been developed to solve the problem. In addition, another effective method has been proposed to increase the mode spacing through replacing the long optical fiber by a high-Q fiber ring cavity with short fiber as the microwave resonator [13–16].
To further shorten the fiber length and reduce the system size, S. Yao designed a compact architecture, namely, coupled optoelectronic oscillator (COEO) . The optical storage element is designed based on the ultra-high Q fiber ring loop with an erbium-doped fiber amplifier (EDFA) as the gain medium and a Mach-Zehnder modulator (MZM) as the mode-locker. To enhance the Q value of the resonator and reduce the phase noise of the oscillator, hundreds of meters of fiber is employed in the EDFA-based fiber ring cavity. The microwave signal is derived from the beat-note of the fiber laser, properly filtered, amplified, phase-shifted, and finally fed back to drive the mode-locker. In principle, it resembles a regenerative mode-locked laser in the absence of fiber delay. The existence of the high-Q fiber loop is non-trivial, which makes the oscillator capable of producing low phase-noise RF signals. The ultra-narrow bandwidth of the oscillator requires the mode-locking frequency to be in synch with the oscillation frequency. It offers an attractive means to simultaneously produce spectral pure RF signals as well as low jitter optical pulse trains without an optical frequency divider . Therefore, it has been increasingly used for various applications, such as radar systems, optical communications, instrumentations, satellite communications, imaging and sampling.
The COEO with 200-m fiber ring cavity can generate ultra-low phase noise microwave signals whose performance is comparable to that achieved in the conventional OEO with more than 4-km fiber. Although the fiber length is shortened, the COEO is still a multimode cavity with a few MHz fundamental frequency and tens of GHz repetition rate. Various techniques have been reported to suppress the supermode noise for an active mode-locked fiber laser which is also suitable for the COEO, such as the incorporation of a comb filter  or the employment of a composite cavity structure . Additionally, some fast power limiting mechanisms, including nonlinear polarization rotation , self-phase modulation  and optical pulse power feedforward , have also been demonstrated.
In this paper, we propose and experimentally demonstrate a simple mutual injection locking scheme for COEO supermode noise suppression. By using conventional coupled optoelectronic oscillator as the master oscillator, the microwave feedback path is closed in electrical domain to form a slave oscillation loop. Therefore, the mutually injection-locked COEO is mainly composed of a conventional coupled optoelectronic oscillation and a free-running microwave oscillation loops. Finally, the phase noise performance of the mutually injection-locked COEO would be determined by the high-Q fiber ring resonator. Thanks to the single mode oscillation of the closed feedback loop, the supermode components can be greatly suppressed as well. In the proof-of-concept experiment, a mutually injection-locked COEO with high Q fiber loop around 200m is demonstrated. Owing to the mutual injection locking effect, the supermode noise is significantly suppressed within the whole spectrum, more than 50 dB lower than the conventional COEO. Besides, the measured single-side band (SSB) noise of the generated signal is about −117 dBc/Hz at 10-kHz offset frequency.
2. Principle of operation
Figure 1 shows the schematic diagram of the proposed mutually injection-locked coupled optoelectronic oscillator. It mainly consists of two parts: a coupled optoelectronic oscillator (master oscillator) and an embedded free-running microwave oscillator (slave oscillator), which shares the same microwave feedback path and mode selection element. In the master oscillator, the fiber laser contains an erbium-doped fiber amplifier, a LiNbO3 MZM, a polarization controller, an optical isolator, an optical bandpass filter, a piece of dispersion-shifted optical fiber and a 30% optical coupler. The EDFA based fiber laser cavity can be used as the high-Q resonator and optical source simultaneously. In the cavity, the MZM is the mode locker, and the polarization controller is used to optimize the mode-locking effect. The isolator and optical bandpass filter are used to ensure the unidirectional operation and suppress the ASE noise in the loop, respectively. The Q value of the laser ring cavity is enhanced by a piece of dispersion-shifted fiber (DSF). Portion of the optical power is extracted out of the loop by the optical coupler and then converted to electrical domain by the photodetector. The recovered signal from the beat-note of the fiber laser will be sent to the electrical feedback section, which is composed of a RF amplifier, a bandpass filter, a phase shifter and a power divider.
For the free-running RF oscillator, it shares the same positive feedback path and mode selection element with the coupled optoelectronic oscillator. The positive feedback path is closed with an extra attenuator and phase shifter inserted in the loop to optimize the closed-loop gain and tune the oscillation frequency respectively. A portion of the oscillating signal from the first RF coupler is injected into the modulator in the fiber ring laser, and fed back to the electrical loop again after circulating the high-Q optical ring loop. Both loops have sufficient gain for oscillation by itself. When the mode locking frequency of the master oscillator equals to the oscillation frequency of the slave oscillator, the mutual injection locking can be achieved between the two loops in the scheme via the shared feedback path. The actively mode-locked fiber laser has many oscillation modes with 1-MHz interval due to the 200-m optical ring loop. Meanwhile, there is only one oscillation mode for the free-running microwave oscillator, which is determined by the RF bandpass filter and phase shifter. The multiple oscillation modes from the COEO are injected into the microwave oscillator, and only the mode close to the oscillation frequency will injection-locks the electrical mode. Consequently, other modes will be drastically suppressed due to destructive interference in the loop, and the single-mode oscillation, which can survive in both loops, can be realized in the mutually injection-locked COEO.
The schematic diagram in Fig. 1 illustrates an example of a typical free-running oscillator based on the shared electrical feedback loop. The microwave oscillator could be of any type, such as those based on Gunn diodes, impact avalanche transit-time diodes, backward-wave tubes, and so on. The Q attainable in the fiber laser resonant cavity is of the order of 106 at 10 GHz and will increase with the carrier frequency. The RF signal from the microwave oscillator would be fed to the MZM in the fiber laser resonator. After travelling in the EDFA-based fiber ring loop, the RF signal recovered by the photodetector would be fed to the microwave oscillator again. Assisted by the high-Q photonic ring cavity, the return of the delayed RF signal would enforce a steady phase in an otherwise noisy free-running oscillator, thereby suppressing the phase noise of the microwave oscillator. Meanwhile, the supermode noise induced by the EDF-based ring cavity can also be suppressed simultaneously, which is also expected as a consequence of the mutual injection locking effect.
3. Experiment results and discussion
A proof-of-concept experiment for mutually injection-locked COEO based on the scheme shown in Fig. 1 has been implemented. Briefly, the proposed setup contains an EDFA based fiber laser and an embedded microwave oscillator. In the fiber laser loop, the MZM (iXblue MXLN-40) with the half-wave voltage and insertion loss about 5 V and 5 dB respectively, is used as the mode locker. A polarization controller is located in front of the MZM to align the polarization state of the light with the main axis of the modulator. A piece of DSF around 200 meters is employed in the loop to ensure the Q value of the fiber ring cavity. It will result in serious sidemodes with around 1-MHz free spectral range (FSR), and highly harmonic mode locking for the 10 GHz repetition frequency is required. In addition, the 3-dB bandwidth and insertion loss of the optical bandpass filter is about 3 nm and 1 dB, respectively. The optical amplifier is based 2.5-m EDF and a 980 nm pump laser. To guarantee the unidirectional transmission, two optical isolators are also inserted before and after the EDFA respectively. Finally, 30% of the optical power is output by an intra-cavity coupler, which is then converted back to electrical domain by the high-speed photodetector (Discovery DSC30S, 20 GHz bandwidth and 0.65 A/W responsivity).
For the microwave oscillation loop based on the shared RF feedback path, it consists of a RF amplifier, a RF bandpass filter, a phase shifter and an attenuator. In the loop, a microwave amplifier with 30 dB gain (Marki A-0120) and an attenuator with 15 dB attenuation (Pasternack PE7045-15) work together to optimize the loop gain of the embedded RF oscillator. The 10 MHz RF bandpass filter centered at 10 GHz is shared to select the oscillation mode for the COEO and free-running oscillator. To optimize the mutual injection locking effect, two RF phase shifters (ATM P1607 and PMI PS-5G18G-400-A-SFF) are used to finely tune the oscillation frequency of the microwave oscillator and the phase of the feedback signal, respectively. Two electronic power dividers (Pasternack PE2020) are used to extract the generated signal out of the oscillator and fed the returned signal into the free-running oscillator. Here 10% of the signal power is extracted from the microwave feedback loop to measure its spectrum and phase noise by a RF coupler (Pasternack PE2205-10) and a microwave signal analyzer (Agilent N9030A).
For the mutually injection-locked COEO, the oscillation mode of the free-running oscillator should match with the mode of the EDFA-based fiber laser loop. Therefore, we firstly investigate the transmission response of the EDFA based optical ring cavity, which would determine the mode frequency of the COEO, via a microwave vector network analyzer (VNA, Agilent N5244A). The output and input ports of the VNA are connected with the MZM and photodetector in an open loop for the COEO respectively. The measurement result illustrates that the amplitude modulation response is obviously periodic with the FSR about 1 MHz as shown in Fig. 2(a). In Fig. 2(b), the zoom view of the amplitude response shows that the 3 dB bandwidth of the transmission peak is about 4 kHz, which means that the corresponding quality factor of the resonator is about 2.5x106 around 10 GHz frequency.
Based on the amplitude response of the open loop stated above, the oscillation mode of the free-running oscillator can match one of the oscillation modes of the COEO by finely tuning the phase shifter in the microwave oscillation loop. Consequently, we measure its operation frequency range with different amount of phase shift and the amplitude response of the inserted RF bandpass filter as shown in Fig. 3. We can deduce that more than 10 MHz tuning range can be obtained, and it is mainly limited by the characteristics of the electrical bandpass filter. Compared with the measured modulation response of the open RF loop for the COEO in Fig. 2(a), the mode matching between the COEO and the free-running oscillator can be easily achieved in the 10MHz span. Here we finely tune the microwave phase shifters to phase synchronize the two oscillators around 9.999 GHz.
Firstly, the RF spectrum of the conventional COEO based on regenerative mode locking laser with 200-m fiber ring cavity is measured as a reference, and the corresponding sidemodes measurement result is shown in Fig. 4(a). With optimized bias voltage and polarization control, the 9.999 GHz microwave oscillation signal is obtained around 100 MHz span with a resolution bandwidth of 2 kHz. Obvious supermode noise with 1-MHz interval corresponding to the 200-m fiber loop delay can be found. When the shared RF feedback path is closed, the mode matching and mutual injection locking can be achieved by finely tuning the two embedded phase shifters. Therefore, the single mode oscillation spectrum of the mutually injection-locked COEO can be observed around 100 MHz span with a resolution bandwidth of 2 kHz as shown in Fig. 4(b). As a comparison, the serious supermode noise can be suppressed significantly with mutual injecting locking scheme.
To evaluate the signal quality, we have studied the spectra shape of the generated RF signal from the free-running oscillator, conventional COEO and mutually injection-locked COEO, respectively. Figure 5 shows the spectrum of the electrical signal around 5 kHz span measured at a resolution bandwidth of 100 Hz. It’s obviously that the generated RF signal from the mutually injection-locked COEO is purer than the free-running RF oscillator.
In order to further investigate the signal quality in detail, the SSB phase noises of the oscillating signals have also been measured, and the corresponding measurement results are shown in Fig. 6. For the case of the master oscillator, which is exactly a conventional COEO, the SSB phase noise of the output signal is about −119 dBc/Hz at 10 kHz offset frequency, and severe spurious modes can be observed with 1-MHz interval which corresponds to the high-Q optical cavity around 200-m long. When the electrical feedback loop is closed, it works as the free-running slave oscillator with the corresponding SSB phase noise about −75 dBc/Hz at 10 kHz offset frequency. Meanwhile, the mutually injection-locked COEO can be constructed as long as the mode matching condition between the master and slave oscillators is satisfied. The corresponding phase noise is about −117 dBc/Hz at 10 kHz offset frequency. This result clearly shows that the phase noise of mutually injection-locked COEO is mainly determined by the master oscillator with high-Q resonator, and it can be further improved by using longer optical fiber and low phase noise microwave components. Meanwhile, according to the SSB noise spectrum, the supermode can be greatly suppressed more than 50 dB to below −120 dBc compared with the conventional COEO.
In conclusion, we have proposed and demonstrated a novel mutually injection-locked COEO, which can generate high frequency microwave signal with low phase noise and low supermode noise simultaneously. The mutual injection-locked COEO is composed of a conventional COEO and an embedded free-running oscillator with shared electrical feedback path, which function as the master and slave oscillators respectively. Assisted by the shared electrical feedback section, the mutual injection-locking effect can be achieved, and no additional hardware complexity is brought to the proposed mutually injection-locked COEO. In the proof-of-concept experiment, 9.999 GHz signal is successfully generated with the phase noise and supermode noise suppression ratio about −117 dBc/Hz at 10-kHz offset frequency and below −120 dBc respectively.
National Natural Science Foundation of China (NSFC) Program (61501051, 61625104 and 61431003).
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