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Abstract
A coupled optoelectronic oscillator (COEO) based on σ-shaped fiber ring structure and intra-cavity semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated. The σ-shaped fiber ring structure is skillfully utilized in COEO to eliminate the harmful influence of polarization disturbance. The SOA is embedded for super-mode suppression due to the fast gain saturation effect. The eximious phase noise performance of COEO could be maintained by operating the SOA at the unitary gain regime. The stable operation of COEO is guaranteed by the immunity to polarization fluctuation and the greatly suppressed spurious-mode competition. As a result, a 10-GHz signal is generated featuring high spectral purity and ultra-low spurious tones as soon as the system is power-on, and can hold steady even if the polarization changes dramatically. The single sideband phase noise of the proposed COEO is about -133 dBc/Hz at 10-kHz offset frequency, and the spurious suppression ratio reaches more than 95 dB, which is 60-dB superior than the conventional COEO.
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1. Introduction
Highly spectrally pure microwave source is of prime importance in a wide range of scientific and technological fields [1–4], such as radar and electronic warfare, wireless communication and precise scientific measurement. With the development of microwave photonics technology, photonic oscillators [5–8] represented by optoelectronic oscillators (OEO) [9–11] have attracted considerable interests due to the ability of directly generating high frequency microwave signal with breakthrough phase noise performance. At the moment, a recorded ultra-low phase noise (-163 dBc/Hz @ 6kHz from a 10-GHz carrier) performance was achieved by the state-of-the-art OEO employing 16-km fiber as the microwave energy storage element [12]. In general, long fiber is required to enhance the quality factor (Q) of OEO so as to generate low phase noise radio frequency (RF) signal, which simultaneously induces serious spurious-mode noise and bulky architecture [13]. A 10-GHz OEO with 4-km fiber as energy storage element has approximately 52-kHz mode spacing, which is too narrow to filter out the spurious tones and to select single required oscillation mode by the commercial microwave band-pass filters, whose Q-factor is limited by existing microwave filtering techniques. In order to achieve more efficient mode selection while maintaining the superb phase noise performance, coupled optoelectronic oscillator (COEO) was proposed [14], where energy storage element is designed as the ultra-high Q mode-locked fiber ring with an erbium-doped fiber amplifier (EDFA) as the gain medium and an electro-optic modulator (EOM) as the mode-locker. Thanks to regenerative gain of fiber ring, the COEO with 200-m fiber ring cavity can obtain comparable phase noise performance to the OEO with more than 4-km fiber, while the mode spacing is expanded to about 1 MHz.
Despite the greatly shortened fiber and expanded mode spacing, the COEO is still a multimode cavity because of harmonically mode-locked working state under very high order. The fundamental frequency of the fiber ring cavity is usually a few MHz while the objective oscillating microwave frequency is tens of GHz. Therefore, the microwave signal generated from COEO still suffers the problem of the significant spurious-mode noise. A number of techniques have been reported to suppress the spurs and to improve the side-mode suppression ratio for COEO [15–19]. Dual loop cavity structure is also applicable to COEO [15], which is widely used in OEO for spurious mode suppression. The optical pulse power feedforward scheme was applied in COEO to form a fast power limiting effect and to reduce optical pulse amplitude fluctuations, achieving the increase of side-mode suppression ratio by nearly 40-dB under the precise tune of feedforward strength [16]. A free running single-mode electronic oscillator was embedded in the COEO configuration and made spurious mode effectively suppressed via mutual injection locking mechanism [17]. However, the free-running single mode oscillator with poor spectral purity inevitably deteriorates the phase noise performance of generated microwave signal from COEO. An optoelectronic hybrid filter was incorporated in the COEO loop and cooperated with the fiber ring, forming the cascade of two equivalent channelized radio frequency (RF) filters and then achieving superior out-of-band rejection ratio of the combined filter [18]. The super-mode suppression ratio of larger than 82-dB was obtained at the cost of complex structure. The spurious-mode noise of COEO could also be suppressed by a length of unpumped erbium-doped fiber (EDF) in the laser cavity due to the saturable absorption formed by the periodic spatial hole burning (SHB) [19]. Since the standing waves in EDF to excited the SHB effect depends on the reflections from the connectors and is usually weak, the absorption of undesirable side-mode is therefore limited. Besides, the widely-used EOMs, which are the pivotal device for mode-locking of the fiber ring in COEO, is generally polarization-sensitive due to the polarization dependence of the electro-optic effect. This would make the performance of COEO rely heavily on the alignment of the polarization state in the fiber ring with the desired polarization state of the EOM. Therefore, a polarization controller is usually inserted before EOM and COEO could generate RF signals after the search for the best polarization state by manually plucking the polarization controller. However, the external environmental influences usually make the polarization state of light unstable when the light transmits in ordinary non-polarization-maintaining fibers. It is necessary to manually align the polarization state in time for the oscillation sustention. Even though a polarizer could also be added in the fiber ring to replace the manual polarization controller and to improve modulation efficiency, it will cause power jitter due to the change of the polarization state in the fiber ring, which hinders or deteriorates the stable oscillation of COEO.
Serious super-mode competition and sensitivity to polarization disturbance greatly hinder the stable operation of COEO. In this paper, we propose and experimentally demonstrate a coupled optoelectronic oscillator with σ-shaped fiber ring structure and intra-cavity SOA, achieving reliable immunity to polarization disturbance and high spurious-mode rejection ratio while maintaining low phase noise. The optical fiber ring cavity structure of COEO is designed similar to σ-shape which can automatically achieve the alignment and maintenance of the polarization state, making the COEO immune to polarization disturbance. The SOA with fast gain saturation effect is utilized to suppress the undesirable super-modes and eliminate the mode competition in COEO, while the EDFA with low noise figure and high saturated output power is still employed to provide the needed gain for stable oscillation of the fiber ring as ever. The phase noise deterioration of COEO caused by the introduction of SOA can be effectively eliminated by operating SOA at the unitary gain regimes, which could be pre-set before the system is switched on. Moreover, the stability of COEO can be improved by the immunity to polarization disturbance and the greatly suppressed super-mode competition. Experimentally, the proposed COEO with σ-shaped fiber ring structure and intra-cavity SOA is demonstrated. A 10-GHz microwave signal is generated after the power supply of the proposed COEO is turned on. The immunity to polarization disturbances of the proposed COEO is verified via the output optical power jitter of the opening fiber loop and the RF spectrum evolution of COEO. No spurious tones can be observed under the fiber loop length around 200-m when the SOA is introduced, and up to 95-dB spurious tone suppression ratio is achieved, more than 60-dB improvement compared with the conventional COEO. The optimal single-side band (SSB) phase noise of the generated signal is as low as about −111 dBc/Hz at 1-kHz offset frequency and −133 dBc/Hz at 10-kHz offset frequency.
2. Principle
The schematic diagram of the proposed coupled optoelectronic oscillator based on σ-shaped fiber ring and intra-cavity SOA is shown in Fig. 1(a). The whole system is basically consistent with the conventional COEO, containing an actively mode-locked fiber laser ring and a RF positive feedback loop. In the fiber laser ring, the LiNbO3 Mach-Zehnder modulator (MZM) working at slow axis as the mode-locker provides periodic amplitude loss mechanism. The EDFA composed of wavelength division multiplexing (WDM), 980-nm pump laser and erbium-doped fiber (EDF) is inserted to provide the total gain. The optical bandpass filter is used to suppress the amplifier spontaneous emission (ASE) noise in the loop. Two isolators eliminating the reflected light make the unidirectional operation. The piece of dispersion-shifted optical fiber (DSF) is used to enhance the quality factor of fiber ring, improving the phase noise performance of COEO. The optical coupler (OC) extracts portion of the optical power from the fiber ring and then directly sends it to the photodetector (PD), achieving the photo-electric conversion. The RF positive feedback loop is composed of low noise amplifiers (LNA), bandpass filter, phase shifter (PS) and electric power combiner (EC). Specifically, the beat-note signal of the fiber laser by PD is amplified, filtered, phase-shifted and then feedback to the MZM. The EC is used to extract portion of the generated microwave signal as the output.
Fig. 1. (a) Schematic diagram of the proposed coupled optoelectronic oscillator. WDM: wavelength division multiplexing; EDF: erbium-doped fiber; SOA: semiconductor optical amplifier; ISO: isolator; PBS: polarization beam splitter; DSF: dispersion-shifted optical fiber; FRM: Faraday rotator mirror; OC: optical power coupler; MZM: Mach-Zehnder modulator; OBPF: optical bandpass filter; PD: photodetector; LNA: low noise amplifier; EBPF: electric bandpass filter; PS: phase shifter; EC: electric power combiner. The orange and green arrows indicate the direction of the light traveling along the fiber ring. The black double arrows and black circles represent slow-axis and fast-axis light respectively. (b) Principle of the immunity to polarization disturbance of the σ-shaped fiber ring structure. The solid red arrows indicate the polarization states of different nodes, while the dotted red arrow represents the polarization state of the previous node.
In order to implement polarization-insensitive COEO, the σ-shaped mode-locked fiber laser ring is designed to settle the polarization sensitive problem, as shown in the shaded area of Fig. 1(a). The orange and green arrows indicate the direction of light travelling in the fiber ring. The black double arrows and black circles represent slow-axis and fast-axis light respectively. Concretely, the σ-shaped fiber ring is conjointly made up of a polarization-maintaining (PM) fiber loop and a non-polarization-maintaining (non-PM) branch. The composition of the PM loop is similar to that of the conventional COEO, except for the long DSF, which is necessary to enhance quality factor but is not polarization-maintaining. The non-PM branch is constituted of a three-port polarization beam splitter (PBS), a 45° Faraday rotator mirror (FRM) and the piece of DSF. The PBS is employed to connect the PM loop and the non-PM branch. Note that the light in PM loop is required to work on slow axis due to the slow-axis working MZM, and the port-2 cannot access to the port-1 of PBS due to the large isolation degree. Besides, the polarization axis alignment of the PBS is fast axis blocked by integrating a 90° polarization rotator at the port2, which means that both port-1 and port-2 of the PBS are working on the slow axis, and the slow axis of port-3 and the slow axis of port-1 are aligned while the fast axis of port-3 is aligned with the slow axis of port-2. The slow-axis light entering the non-PM branch via port-3 from port-2 is firstly converted to fast axis light by the integrated a 90° polarization rotator and then introduced to the 45° FRM after travelling through the DSF. The FRM rotates the polarization of the light by 90° and reflects it back into the DSF. Since the time of light travelling in the fiber is instantaneous relative to that of the external environment action, the angle values of polarization rotation due to the DSF are the same when light travels back and forth in DSF. The directions of polarization rotations by the DSF are opposite which is dependent on the direction of light propagation. Thus, the polarization variations introduced by the non-PM DSF can be effectively counteracted [20], reverting back to slow axis light before returning into the PM loop. The slow-axis working light can be guaranteed in the PM loop, aligning with the working axis of MZM. Therefore, the path of the PBS is that the light in the fiber ring firstly travels from port3 to port 1, then from port1 to port2, and eventually from port2 to port3, thus forming a closed fiber loop. The polarization states of light at different nodes in the σ-shaped fiber ring are illustrated in Fig. 1(b). Besides, the optical signal travels back and forth in the DSF could further enhance the Q factor of fiber ring.
Except the polarization sensitivity issue, serious spurious tone competition also hinders the stable operation of COEO. The SOA benefitting from the fast gain saturation effect can be employed as a reliable super-mode suppressor of fiber ring. Inasmuch as the spurious tones result from the beating between the super-mode of the fiber laser cavity and result in the optical output intensity fluctuation, the gain ($R$) of output power fluctuation to input power fluctuation of optical amplifier can be employed to quantitatively discuss the super-mode noise. This can be derived from the rate equation of the small signal gain under a perturbation on the average optical power [21]. By simply considering the fluctuation of gain as $g = {g_0} + \Delta g$, the rate equation can be given by
3. Experiment and results
A proof-of-concept experiment for the proposed coupled optoelectronic oscillator with σ-shaped fiber ring and intra-cavity SOA has been implemented. The proposed COEO briefly contains an σ-shaped fiber laser loop and a RF feedback loop. In the σ-shaped fiber laser loop, the MZM (iXblue MXLN-40) with PM fiber pigtail is employed as mode-locker with the half-wave voltage and insertion loss about 5 V and 5 dB respectively. The PM EDFA is based on a 980-nm pump laser, a WDM and a 1.5-m EDF, with a small signal gain of 30 dB. The saturated output power of the PM SOA is around 16-dBm. The 3-dB bandwidth of the optical bandpass filter (OBPF) is about 4 nm with less than 1-dB insertion loss. A piece of DSF around 83-m is inserted in the non-PM branch to enhance the Q value of the fiber ring cavity, forming around 200-m optical cavity length together with the PM fiber ring. The corresponding free spectral range (FSR) is about 1-MHz, and highly harmonic mode locking for the 10-GHz repetition frequency is required. 20% optical power is extracted out from the fiber ring and then introduced to a PD (Discovery DSC40S) with bandwidth of 14-GHz and responsivity of 0.8-A/W. In the RF feedback loop, two low noise amplifiers with respectively 22-dB and 13-dB gain are used to compensate for loss and to provide RF oscillating gain. The electronic band-pass filter (K&L) centered on 10-GHz has a 3-dB bandwidth of 6-MHz. The phase shifter (MPS-DC18G-60-S) is pre-tuned, ensuring the phase match between the fiber loop and RF feedback loop of the proposed COEO. Finally, the oscillating signal with enhanced spurious-mode suppression ratio and low phase noise at 10-GHz is output and feedback to the MZM via 3-dB electric power combiner.
In order to verify the immunity to polarization disturbance of the proposed COEO, the experiment setup for optical power jitter measurement of open-loop σ-shaped fiber ring without SOA under the polarization disturbance is firstly built as shown in Fig. 2(a). An ordinary distributed feedback laser (DFB) is applied as a continuous light source. A polarization controller (PC) is inserted in the non-PM branch. The power jitter is measured via optical power meter while the polarization controllers are continually plucked for polarization fluctuations emulation over times. The optical power jitter measurement for the open-loop fiber laser ring of the conventional COEO is also carried out as a comparison, employing a 166-m DSF, a PC and a polarizer before the MZM. The polarizer before the MZM in COEO is generally used to ensure optimum modulation performance due to the polarization dependence of the electro-optic effect. As shown in Fig. 2(b), the output optical power for the open-loop σ-shaped fiber ring could maintain almost constant (slight variations in the 0.24 dB range) despite the polarization state fluctuates, while the output optical power of the open-loop fiber laser ring from the conventional COEO vibrates violently with the change of polarization state. The corresponding optical power jitter range is 26.62 dB. The almost constant power output of open-loop σ-shaped fiber ring strongly demonstrates the ability to counteract polarization fluctuation of σ-shaped structure.
Fig. 2. (a) The experiment setup for optical power jitter measurement of open-loop σ-shaped fiber ring. (b) the measured optical output power jitter of open-loop σ-shaped fiber ring (red triangle) and open-loop fiber ring (blue circle) for conventional COEO. DFB: distributed feedback laser.
In terms of the spurious tone suppression, the RF spectrum of COEO is a direct measure. Firstly, the RF spectrum of conventional COEO is measured as a reference by a spectrum analyzer (R&S FSWP50) as shown in Fig. 3(a). After the search of the optimal polarization state by manually plucking the polarization controller, the 9.999 GHz microwave oscillation signal is obtained around 100 MHz span with a resolution bandwidth of 2 kHz. Significant spurious-mode noise with around 1-MHz interval corresponding to the 200-m fiber loop can be observed near the 9.999-GHz signal, and the side-mode suppression ratio is around 33.18 dB. When the SOA is utilized in the COEO, the efficient spurious-mode suppression can be achieved immediately. The single mode RF spectrum centered at 9.998-GHz of the proposed COEO can be observed around 100 MHz span with a resolution bandwidth of 2 kHz as shown in Fig. 3(b), and the corresponding spurious-tone suppression ratio is up to 95.58 dB. As a comparison, the serious super-mode noise can be suppressed significantly with the intra-cavity SOA, and the improvement of the super-mode suppression ratio is more than 60-dB.
Fig. 3. RF spectra of (a) the conventional COEO and (b) the proposed COEO in 100-MHz span with 2-kHz resolution bandwidth (RBW).
For further investigating the immunity to polarization fluctuation of the proposed COEO, the RF spectrum evolutions of the conventional COEO and the proposed COEO with the polarization controller at the non-PM branch are respectively measured while the polarization controllers are continually plucked over time, which are obtained by R&S FSWP50. As shown in Fig. 4(a) and 4(b), the stable oscillation of the proposed COEO with low spurs can be maintained in spite of the strong polarization disturbance. Conversely, the operation of the conventional COEO is greatly affected by the change of light polarization state. The polarization misalignment will lead to several unwished phenomena, such as mode hopping and no signal oscillation. Therefore, σ-shaped cavity structure can effectively resist the effect of polarization perturbation on mode-locking state and then on the oscillating microwave signal. Thanks for the σ-shaped fiber ring structure, stable operation of COEO can be directly achieved without the need of manually adjusting the polarization state, which is usually critical in conventional COEO for finely aligning the polarization state of the light with the working axis of the modulator in time. Note that the two COEOs are both simply placed under an ordinary laboratory condition in our experiment, without any passive (such as temperature and vibration isolation) or active (such as cavity length feedback control) stabilization, which will be a key aspect in the subsequent industrialization design.
Fig. 4. The RF spectrum evolution of (a) the conventional COEO and (b) the proposed COEO when the polarization state of light in fiber ting fluctuates.
Due to the superior noise figure of EDFA compared with SOA, the residual phase noise floor of EDFA is usually lower than that of SOA [23]. The introduction of SOA as super-mode suppressor in the COEO would deteriorate the spectrum purity of the generated microwave signal, which is strongly correlated with the stimulated or spontaneous emission induced carrier density fluctuation [24]. The single sideband (SSB) phase noise of spontaneous-emission-induced fluctuation can be given as ${S_\phi }_{_{spons}}(f) = {\raise0.7ex\hbox{${hv(G - 1){n_{sp}}}$} \!\mathord{\left/ {\vphantom {{hv(G - 1){n_{sp}}} {G{P_{avg}}}}}\right.}\!\lower0.7ex\hbox{${G{P_{avg}}}$}}$, where h is Planck constant, v is optical frequency, G represents the gain of SOA and ${n_{sp}}$ is the spontaneous emission factor. The SSB phase noise of stimulated-emission-induced indirect fluctuation can be written as
Fig. 5. (a) The SSB phase noise at 10kHz offset frequency and (b) the spurious-mode suppression ratio as functions of SOA gain regime for the proposed COEO.
In order to further investigate the signal quality of the proposed COEO with the σ-shaped fiber ring structure and intra-cavity SOA, the SSB phase noises of the oscillating signals from the COEO and the conventional COEO have been exhibited and compared, and the corresponding measurement results from phase noise analyzer (R&S FSWP50) are shown in Fig. 6. For the case of the conventional COEO (blue line), which is exclusive of σ-shaped cavity structure and SOA, the SSB phase noise of the output RF signal is about -136 dBc/Hz at 10-kHz offset frequency. Due to the 200-m fiber cavity length, serious spurious mode can be observed at the 1-MHz and its harmonic offset frequency. In the proposed COEO, the σ-shaped cavity structure is designed to eliminate the influence of polarization disturbance on COEO and the SOA is employed as super-mode suppressor. The optimal phase noise of the proposed COEO (red line) is about -133 dBc/Hz at 10-kHz offset frequency when the SOA is at the unitary gain regime. Meanwhile, the super-mode can be greatly suppressed to below −130 dBc according to the SSB phase noise spectrum. This result clearly shows that the introduction of intra-cavity SOA not only can greatly improve the super-mode suppression ratio, but also roughly maintain the low phase noise performance of conventional COEO. Besides, the phase noise at 1-kHz offset frequency of the conventional COEO and the proposed COEO are respectively about -92 dBc/Hz and -111 dBc/Hz. The optical polarization disturbance insensitivity due to the σ-shaped cavity structure together with the greatly suppressed super-mode competition effectively improve the stability of COEO, which is reflected in an around 15-dB optimization of phase noise at the near carrier frequency.
Fig. 6. SSB phase noise of the 10-GHz microwave signals generated from the conventional COEO (blue line) and the proposed COEO with intra-cavity SOA and σ-shaped fiber ring structure (red line).
4. Conclusion
In conclusion, we proposed a coupled optoelectronic oscillator based on σ-shaped fiber ring structure and intra-cavity SOA, which can generate high-performance microwave signal as soon as the COEO is power on. The σ-shaped fiber ring structure is employed and makes COEOs immune to polarization disturbance automatically. The SOA serves as a super-mode suppressor for the COEO due to the fast gain saturation effect, and the phase noise performance of the COEO could roughly maintain when the SOA operates on the unitary gain regime. The stability of COEO is also improved by the polarization disturbance insensitivity and the greatly suppressed spurious-tone competition. Finally, a 10-GHz signal has been generated with the phase noise about -133 dBc/Hz at 10-kHz offset frequency, and the spurious suppression ratio reaches more than 95 dBc, an up to 60-dB improvement for the conventional COEO.
Funding
National Natural Science Foundation of China (61625104, 61971065); State Key Laboratory of Information Photonics and Optical Communications (IPOC2020ZT03); BUPT Excellent Ph.D Students Foundation (CX2020225).
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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