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Abstract
We proposed and demonstrated an unprecedented high-efficiency Brillouin random fiber laser (BRFL) by fiber length optimization in a half-open linear cavity. In terms of the trade-off between Brillouin gain saturation and weak distributed Rayleigh feedback strength, optimal laser efficiency associated to proper fiber length in a BRFL was theoretically predicted. As a proof-of-concept, a unidirectional-pumped BRFL with a half-open linear cavity was experimentally conducted, in which a fiber Bragg grating at one end of gain fiber served as a high-reflection mirror while Rayleigh scattering enabled distributed feedback for random lasing resonance. Results show that the optimal fiber length of ∼3.4 km in the BRFL offers sufficient Rayleigh scattered random feedback whilst alleviating the Brillouin gain saturation to a large extent. Consequently, an optimal laser efficiency of 77.0% in the BRFL was experimentally demonstrated, which reaches the state-of-the-art high record. Laser characteristics, including the linewidth, statistics and frequency jitter were also systematically investigated. It is believed that such efficient BRFL could provide a promising platform for inspiring new explorations of laser physics as well as potentials in long-haul coherent communication and fiber-optic sensing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Random fiber lasers (RFLs) have been increasingly attracting a great deal of attention, thanks to their unique lasing resonance formation [1], i.e., randomly distributed Rayleigh feedback which typically replaces conventional fixed fiber cavity configuration. Diverse fiber-optic gain mechanisms such as Raman scattering [2], Brillouin scattering [3] and rare-earth-doped fiber amplification [4] have been intensively attempted to carry out RFLs with either a full-open [5] or a half-open random cavity configuration [6]. Owing to their unique intensity/phase noise and power scaling capability, RFLs have shown huge potentials in fiber sensing [7–9] as well as optical communication [10–13], especially, the high conversion efficiency of RFLs is desirable not only for low-cost manufacture requirement but also being critical to high power RFLs [14,15]. Whereas in fact, due to the extremely low Rayleigh scattering coefficient in conventional single mode fibers (SMFs), RFLs with Rayleigh scattered random feedback usually suffer from moderate laser efficiency and a high laser threshold, which basically impose requirements of ultra-high pump power as well as high-power operation of fiber device and components. For instance, Raman-based RFLs with high efficiency have been achieved through reducing the length of the passive fibers [16,17], however, at a cost of extremely high threshold power up to tens of watts.
Brillouin random fiber lasers (BRFLs) utilize distributed Brillouin scattering as the gain mechanism, which offers a much lower threshold of three orders of magnitude compared to that of Raman scattering [18]. However, the BRFLs with typical Rayleigh scattered random feedback along silica fibers usually suffer from an unsatisfactory laser efficiency. To address this issue, several approaches have been implemented: 1) Optimization of Brillouin gain fibers. Conventional SMFs with randomly distributed birefringence are eventually frustrated to maintain the identical light polarization for pump-Stokes Brillouin interaction, leading to the local polarization mismatch as well as the reduced Brillouin gain coefficient [19]. Thus, the utilization of polarization maintaining fibers (PMFs) instead of the SMFs as Brillouin gain fibers of BRFLs has been demonstrated to improve the laser performance, including laser efficiency, intensity noise and frequency stability [20,21]. 2) Optimization of random feedback medium. To overcome the weak Rayleigh scattering originated from intrinsic nonhomogeneous refractive index change along kilometer-long optical fibers, special fiber media with artificially enhanced refractive index modification can be used to significantly improve the random feedback strength, even with a much shorter physical length of centimeters, for a high-efficiency random lasing resonance. For example, a weak fiber Bragg grating (FBG) array along 50-m optical fibers was alternatively employed to provide the random feedback of a BRFL, as same as 500-m Rayleigh scattered fibers [22]. Furthermore, a random fiber grating with approximate 50-mm long can even replace kilometer-long fibers as a random feedback medium, benefiting a BRFL with high laser efficiency of up to 30% [23–25]. 3) Optimization of random cavity configuration. It has been demonstrated that BRFLs with half-open ring cavity exhibit a relatively higher laser efficiency and lower threshold than that of linear open cavity [20]. Meanwhile, BRFLs with bidirectional pump scheme have been proposed to upgrade laser characteristics, including the laser linewidth and frequency noise [21]. In spite of numerous efforts, the state-of-the-art efficiency of BRFLs still remains as low as 30% [25], which is far away from their quantum-limit conversion efficiency (i.e., > 99%). One of the main reasons relies on the Brillouin gain saturation is inevitable regarding the pump depletion along long-span fibers, which however can be mitigated by using short-length fibers [26]. As a consequence, a trade-off between Brillouin gain saturation and Rayleigh scattering strength along optical fibers emerges in Rayleigh scattering-based BRFLs, which hence turns out to be critical to the further enhancement of the laser efficiency.
In this paper, we proposed and demonstrated a high-efficiency BRFL based on a half-open linear cavity, in which the random lasing resonance was built up by feedback consisting of distributed Rayleigh scattering and mirror-based one-end reflection. Laser efficiency performance in the BRFL with respect to fiber length optimization was theoretically predicted, which balanced the Brillouin gain saturation and distributed Rayleigh feedback strength. As a proof-of-concept, the BRFLs with the gain fibers of different-length SMFs were conducted, in which an FBG was placed at one fiber end for a half-open linear cavity. Experimental results show that efficient random lasing resonance with moderate gain saturation and sufficient Rayleigh scattered random feedback occurs as the gain fiber length was selected as around 3.4 km, which is in good agreement with simulations. The maximum laser efficiency of 77.0% was measured, which is much higher than that of all previous reports. In addition, the BRFL features, including linewidth, temporal dynamic, and frequency stability, were comprehensively characterized.
2. Theory and simulations
As the gain mechanism of the BRFLs, Brillouin interaction between two counter-propagating pump and Stokes seed powers along optical fibers can be described by coupled wave equations. The power evolution of the pump light (${{P}_{p}}$), and the Stokes light (${{P}_{s}}$) in the Brillouin gain fibers can be written as coupled ordinary differential equations (ODEs) [27]:
Instead of the mirror-based feedback, Rayleigh backscattered light along optical fibers can provide distributed feedback for random lasing resonance. The power evolution between the Rayleigh backscattered light (${{P}_{R}})$ and the launched light, which serves as the pump (${{P}_{{RP}}}$) for Rayleigh backscattered light, can be mathematically coupled as [3,29]:
It predicts that the Rayleigh scattering power grows as the increase of the fiber length and gradually turns to saturation due to the fiber loss.
In the simulation, the initial energy of Stokes seed photon, which originates from the thermal noise, can be approximately defined as ${{E}_{{seed}}}({{{\nu }_{{Stokes}}}} )\approx 2{kT}({{{\nu }_{{Stokes}}}/{{\nu }_{B}}} )$ [31], where ${k}$ and ${T}$ are Boltzmann's constant and the fiber temperature, respectively. This initial energy comes from the combination of phonons and photons in silica fibers, which means that energy is limited by the phonon lifetime of ∼10 ns. Hence, the power level of the Stokes seed is estimated in the range of-50 to -60 dBm. Considering the pump-depletion-induced gain saturation, the Brillouin gain versus the fiber length is simulated under the injection pump power of 20 dBm and the Stokes seed power of -60 dBm, as depicted in Fig. 1(a). It shows that the Brillouin gain goes rise as increased fiber length up to ∼2.4 km while undergoing a decline as the fiber length further increases up to 10 km. According to the boundary conditions, ${{P}_{{RP}}}(0 )= {{P}_{s}}(0 )$, ${{P}_{{RP}}}(0 )$ is around 20 dBm from the simulation of Fig. 1(a). As the 20-dBm Stokes light launches into the Rayleigh scattering fibers, the total Rayleigh backscattered Stokes power sharply rises along with the growth of the fiber length (less than 2.3 km), however, the growth trend of the backscattered power gradually slows down as further increasing the fiber length due to the increased fiber loss, as shown in Fig. 1(b). The specific simulation parameters are shown in Table 1.
Fig. 1. Simulations of (a) Brillouin gain (${{P}_{P}}(0 )= \textrm{20}\; {dBm}$, ${{P}_{s}}({L} )={-} \textrm{60}\; {dBm}$) as a function of the fiber length; (b) Rayleigh backscattered power as the launched Stokes power of ${{P}_{{RP}}}(0 )= 20\; {dBm}$.
Table 1. Parameters used in the simulations
BRFLs can be realized by employing Brillouin scattering as the gain mechanism and distributed Rayleigh scattering along optical fibers as random feedback. Figure 2(a) shows the schematic of a BRFL with a half-open linear cavity, in which a spool of single-mode optical fibers is used as both Brillouin gain and Rayleigh scattering media. A mirror as a point reflector is placed at one fiber end to form a half-open linear cavity. As the injected pump through one fiber end reaches the stimulated Brillouin scattering (SBS) threshold, the Stokes light can be backward generated and amplified via the SBS process along the propagation, which is then backscattered and accumulated. With a one-end mirror, the Rayleigh scattered Stokes is reflected and recirculated as the Stokes seed for lasing resonance. Once the Brillouin gain compensates for the total loss, the Stokes random laser will be emitted at the same fiber end as the pump injection. It should be pointed out that, the BRFL can operate under two cases of pump schemes, including bidirectional and unidirectional pump, which exhibit distinct characteristics of the Brillouin gain saturation. Normally, both the pump and Stokes light can be reflected by the mirror placed at one fiber end, leading to the bidirectional propagating pump for Brillouin amplification in the BRFL, as shown in Fig. 2(b). Otherwise, when a narrowband mirror (e.g., FBG) is utilized to primarily reflect Stokes light whilst being spectrally transparent to the pump wavelength, the Brillouin pump can boost the Stokes light in one direction, as shown in Fig. 2(c). Note that, owing to the trade-off between the saturated Brillouin gain and Rayleigh feedback strength along kilometer-long fibers, the fiber length turns out to be critical to the laser power as well as the laser efficiency of the BRFLs.
Fig. 2. (a) Schematic of BRFL based on a half-open linear cavity with (b) bidirectional and (c) unidirectional pump scheme
Considering the BRFL, the boundary conditions should be satisfied as ${{P}_{s}}$ is exactly equal to ${{P}_{{RP}}}$, the random laser output power can be given by:
Therefore, the conversion efficiency ${{P}_{s}}(0 )/{{P}_{p}}(0 )$ of the proposed BRFL with two pump schemes was simulated, in which the reflection of the mirror was considered as 100%, as shown in Fig. 3. Due to Brillouin gain saturation, both the conversion efficiencies of bidirectional- and unidirectional-pump BRFLs decrease after increasing up to 81.4%. Results show that the conversion efficiency of bidirectional-pump BRFL turns to decline as the fiber length is greater than around 1.3 km. In the case of the unidirectional-pump BRFL, the conversion efficiency increases to the maxima along the fiber length of around 2.6 km and then gradually descends as the fiber length is further extended. The main reason relies on that, the Brillouin gain which the Stokes light obtained under the condition of the bidirectional pump is essentially achieved along the two-fold fiber length, leading to the deteriorative gain saturation compared to that of the unidirectional pump.
3. Experimental setup
The experimental setup of the proposed BRFL based on the half-open linear cavity and measuring configurations for characterizing the random fiber laser emission are illustrated in Fig. 4. The 1550-nm pump light from a tunable narrow-linewidth laser (NKT, Adjustik E15) was amplified by an erbium-doped fiber amplifier (EDFA). A polarization controller (PC) was used to adjust the polarization of the pump light before injecting into the BRFL. Through a single-mode optical fiber circulator (SM-CIR), the pump light was launched into SMFs which served as both Brillouin gain and Rayleigh scattering medium with a typical fiber loss of 0.2 dB/km at the wavelength of 1550 nm. As the pump power exceeded the SBS threshold, the Stokes light can be backward generated via the SBS interaction, which is then partially Rayleigh scattered along propagation. With the assistance of the fixed reflection from a narrowband FBG (whose center wavelength is roughly matched to the Stokes wavelength at around 1550 nm) or a wavelength-independent mirror, the Rayleigh scattered Stokes light was recirculated and then amplified via the SBS to form a complete lasing resonance above the threshold. Finally, the Stokes laser was emitted through port 3 of the SM-CIR and then characterized by measuring configurations. Note that, two kinds of pump schemes for half-open BRFL cavity were conducted based on: (#1) FBG reflection for unidirectional pump, and (#2) mirror reflection for the bidirectional pump. With regard to the FBG with a bandwidth of around 0.2 nm, the wavelength of pump light can be precisely tuned out of the reflection band while the Stokes wavelength with a Brillouin upshift of ∼0.1 nm locates at the center of the FBG reflection. Hence, the pump light can mostly pass through the FBG without strong reflection, yielding the BRFL with unidirectional pump scheme. Otherwise, by means of wavelength-independent mirror feedback, the residual pump light can also be reflected in the counter direction of the gain fiber for sequentially amplifying the Rayleigh scattered Stokes, which hence forms the bidirectional-pumped BRFL.
Fig. 4. Experimental setup of the proposed BRFL based on the half-open linear cavity and the measurements of (a) optical power and spectrum, (b) intensity dynamics and (c) laser linewidth.
4. Results and discussion
Figure 5 shows the laser efficiency and threshold power versus the fiber length in BRFLs with two different configurations. As shown in Fig. 5(a), both of the BRFLs with mirror and FBG feedback show increasing laser efficiency up to 74.9% as the fiber length is selected from 1.0 km to 2.0 km, which mainly relies on that the Brillouin gain saturation even under bidirectional pump scheme is not dominant along relatively short fibers. As the fiber length of BRFL is longer than 3.4 km, the remarkable reduction of laser efficiency based on mirror feedback emerges compared to that of BRFL with FBG feedback, which is consistent with our simulations. It is mainly caused by that the BRFL with bidirectional pump suffers from the severe gain saturation along the long gain fibers. In Fig. 5(b), the BRFL with mirror feedback shows a lower laser threshold than that of FBG-based BRFL, resulting from that the higher total Brillouin gain along the round trip of Stokes light in case of the BRFL with bidirectional pump. Note that, the measured maximum laser efficiency of the BRFL is roughly lower than that of simulations nearly by 4%, which is mainly caused by the gain reduction in polarization-mismatched SBS along SMFs and additional insertion loss of components, such as the imperfect reflection of either the mirror or FBG.
Regarding the feedback from one-end FBG, the BRFL with different pump wavelengths was experimentally investigated. By tuning the pump wavelength, the wavelengths of pump light can flexibly fall in or out of the reflection band of the FBG while the Stokes wave keeps within the 3-dB bandwidth. Figure 6(a) shows the power difference between the Stokes (Ps) and residual pump (Prp) at the output of the FBG-based BRFL under different pump powers. Here, the power difference is defined as Pdif = Ps [dBm] - Prp [dBm]. The maximum reflection difference was measured at the pump wavelength ${{\lambda }_{p}}$ of 1549.86 nm, as shown in the inset of Fig. 6(a), indicating that the majority of the pump directly passes through the FBG, which hence leads to the unidirectional pump scheme. In this scenario, Pdif is strongly dependent with the saturated SBS gain under different pump powers, leading to an increased Pdif as increased pump power from 45.19 mW to 77.27 mW. As ${{\lambda }_{p}}$ is lower or higher than 1549.86 nm, Pdifturns out to be dominant by the FBG-induced reflection of the Stokes and pump. The decrease of Pdifdepends on the reduced Ps (${{\lambda }_{p}}$<1549.86 nm) or the increased Prp (${{\lambda }_{p}}$>1549.86 nm). In particular, Prp will be further increased as the pump power increases up to 77.27 mW. Correspondingly, the laser efficiency and threshold power of BRFL versus pump wavelength have been drawn in Fig. 6(b). As the pump wavelength is less than 1549.86 nm, the reflection of Stokes light is too weak to establish a strong lasing resonance, leading to lower laser efficiency and higher laser threshold. By upshifting the pump wavelength greater than 1549.86 nm, the reflection of pump light gradually climbs by the FBG, which essentially switches the unidirectional pump scheme to the bidirectional one, and hence leads to the decline of the laser efficiency and threshold.
Fig. 6. (a) Power difference between Stokes and pump at the output of the FBG-based BRFL (FBG 3-dB bandwidth: 0.2 nm, ${{\lambda }_{{FBG}}} = \textrm{1550}\textrm{.02 nm}$, Inset: FBG reflected spectrum). (b) Laser efficiency and threshold power versus pump wavelength in the proposed FBG-based BRFL with fiber length of 3.4 km.
The laser output power and spectrum were measured by an optical spectrum analyzer (OSA, YOKOGAWA AQ6370B). In Fig. 7(a), the laser wavelength with a high optical signal-to-noise ratio (OSNR) of ∼70 dB was monitored as 1549.95 nm, corresponding to a Brillouin upshift of 0.09 nm from the pump. As shown in Fig. 7(b), when the pump power surpasses the threshold power of 18.16 mW, the laser output grew proportionally as the increase of pump power with an unprecedented high slope efficiency of 77.0%.
Fig. 7. (a) Spectrum and (b) output power of the BRFL with the FBG feedback versus input pump power (fiber length: 3.4 km, pump wavelength: 1549.86 nm).
In order to explore the intensity dynamics and statistical characteristics of random lasing emission, the temporal waveforms of the laser output at different pump powers have been measured by a photodetector (PD) and an oscilloscope, as illustrated in Fig. 4(b). Figure 8 shows the comparison of the BRFL intensity dynamics below and above the laser threshold. When the pump power is lower than the lasing threshold of 18.16 mW, a typical asymmetric intensity distribution of the emitted Stokes caused by Brillouin scattering but without random lasing resonance and the corresponding probability distribution were depicted in Figs. 8(a) and 8(b), respectively. The phase portraits were obtained by a two-dimensional intensity plot of ${{I}_{N}}$ (N = 1, 2, …) versus ${{I}_{{N} + 1}}$ with a special delay of one-step intervals. Figure 8(c) reveals the chaotic nature of SBS, originating from the thermal noise in optical fibers. On the other hand, the temporal intensity trajectory and probability density distribution of the Stokes emission with the stable establishment of the random lasing oscillation above the laser threshold were illustrated in Figs. 8(d) and 8(e). In this case, the probability distribution of the temporal intensity traces followed an approximate Gaussian distribution. In Fig. 8(f), the phase portrait of the stable laser output shows the confined cycle signature.
Fig. 8. Intensity dynamics of Stokes emission. (a) Temporal trace, (b) probability distribution and (c) phase portrait of intensity dynamics of SBS noise below the laser threshold; (d) temporal trace, (e) probability distribution, (f) phase portrait of the BRFL with 3.4 km SMF. (Oscilloscope sampling rate: 10 MSa/s, PD bandwidth: 350 MHz)
The linewidth of the BRFL was measured by a delayed self-heterodyne (DSH) method consisting of a Mach-Zehnder interferometer. In Fig. 4(c), the BRFL output was split into two parts by a 90/10 coupler. The light from the 10% port was modulated by an acousto-optic modulator (AOM) with a frequency down-shift of 80 MHz, and the light from the 90% port was sent into 100-km delay-SMF in order to decoherent the light beams in two arms for the beat heterodyne. The optical beat signal was converted by a PD and visualized by an electrical spectrum analyzer (ESA). In Fig. 9(a), the 20-dB linewidth of the 3.4-km SMF BRFL was measured at 13.1 kHz, which corresponds to a 3-dB linewidth of ∼0.7 kHz. Moreover, the 3-dB linewidth of the BRFLs with different fiber lengths were also carried out, exhibiting the declined trend towards 0.4 kHz as the increase of the fiber length up to 10 km, as shown in Fig. 9(b). It mainly attributes to the acoustic damping in the SBS interaction as well as the Rayleigh scattering-induced the phase noise suppression, which is consistent with the theoretical prediction [32].
Fig. 9. Linewidth measurement of FBG-feedback BRFL. (a) DSH-based beat spectrum with 100-km delay fiber; (b) 3-dB linewidth with different fiber lengths. (ESA resolution bandwidth, 100 Hz; sweep time, 19 ms; average time, 10)
The frequency jitter of the proposed BRFL was measured through the beat signals between the pump and Stokes laser, as shown in Fig. 10. By using a 10/90 coupler, one part of pump light passed through a variable optical attenuator (VOA) and then beat with Stokes laser output via another 50/50 coupler. The beat signals, located around the center of the Brillouin shift frequency (∼11 GHz), were captured by a PD and analyzed by an ESA.
To investigate the stability of Stokes random lasing, the frequency jitter of the BRFLs with 1-km, 3.4-km and 6-km SMF have been measured at the same laser output power of 25 mW. The peak frequency of beat signals at every 1 millisecond within a time window of 1 second were recorded, as plotted in Fig. 11. Due to disturbance of ambient mechanical vibration and thermal variance, Brillouin gain fluctuates along with different positions of the gain fibers, giving rise to prevalent frequency jitters of BRFLs. It should be mentioned that the frequency jitter range of the BRFL with longer fiber length was expected to be aggravated from 4.2 MHz at 1-km gain fiber to 6.1 MHz at 6.0 km, which attributes that longer-span fibers is more susceptible to external disturbance. Accordingly, the standard deviation of the frequency drift, as a statistical parameter, increasingly changes from 0.91 (at 1.0 km) to 2.07 (at 6.0 km) with a linear dependence on the fiber length.
Finally, we compile recent investigations of BRFLs with different configurations, as shown in Table 2. Restricted by the Brillouin gain saturation, the laser efficiency of BRFLs without the optimization of fiber length is usually as low as around 10% or even less. Despite the utilization of gain fibers with high Brillouin gain coefficient (e.g., PMF) and special fiber components with enhanced distributed feedback strength, such as the weak FBG array and random fiber grating, the laser efficiency of BRFLs still remains below 30%. Thanks to gain saturation optimization as per the fiber length, the laser efficiency of the proposed BRFL, associated with a half-open linear cavity with uni-directional pump scheme, reached unprecedently as high as 77.0%. Noted that, the FBG-based reflector locates at the opposite fiber end with respect to the injecting Brillouin pump (so-called backward pump scheme) in our proposed half-open linear random cavity configuration, which is also critical to its high efficiency. Otherwise, if the FBG-based reflector is placed at the same fiber end as the injecting pump (so-called forward pump scheme), the backward 1st Stokes with the highest power could be totally reflected back to the gain fibers as copropagating with the injecting pump. Consequently, the 2nd or even high-order Stokes/anti-Stokes components would emerge via the combination of the SBS and four-wave mixing effect as increasing the pump power, which is detrimental to the 1st Stokes laser efficiency.
Table 2. Comparison of laser efficiency of BRFL with different configurations
To improve the laser efficiency of BRFLs, several aspects can be further considered: 1) Gain fiber. Since the Brillouin gain coefficient is dependent on the fiber parameters [32], such as effective mode field diameter, nonlinear coefficient of optical fibers, etc. Instead of SMFs, specialty optical fibers with higher Brillouin gain coefficient can be utilized to enhance the laser efficiency of BRFL. Moreover, identical linear polarization of pump and Stokes along fibers could guarantee polarization-matched SBS with optimized SBS gain to a large extent. Hence, the utilization of PMFs as gain fibers is favorable to upgrade BRFL efficiency. 2) Random feedback medium. Due to intradiscal homogeneities of reflective index variation along the fiber core, the weak Rayleigh scattering, albeit with a natural distributed feedback mechanism for random lasing resonance, basically a moderate laser efficiency. Alternately, random feedback media with artificially introduced index modulation inside the fiber core can be regarded as a promising solution, including direct UV exposure [36], fiber taper [33,37] and femtosecond laser-fabricated random grating [4,38] as well as fiber grating array [39]. It is worth noting that the limitation of the output power of our proposed BRFL currently relies on the applied pump power in terms of the power handling of the fiber components (e.g., the used optical circulator with the highest optical power operation at 300 mW). Providing that the fiber components with much higher power handling capability are used, the power scaling limitation of BRFLs would also rely on the generation of the 2nd Stokes. To address this issue, the higher output power of the proposed BRFL with half-open linear cavity is expected to be achieved by using shorter gain fibers, albeit with the cost of an increased laser threshold as well as a high pump power requirement. It suggests that the proposed high-efficiency BRFL could be promising candidate for long-haul coherent communication as well as fiber sensing networks.
5. Conclusion
To summarize, a recorded high-efficiency BRFL with a half-open linear configuration has been proposed and experimentally demonstrated. By balancing Brillouin gain saturation and distributed Rayleigh feedback strength in terms of the fiber length, the laser efficiency of the FBG-based BRFL under optimal fiber length of ∼3.4-km SMF was found as 77.0%. Laser properties such as linewidth, intensity dynamics and frequency jitters were systematically characterized. These findings would inspire a new exploration of laser physics and promote wide applications in remote fiber sensing networks and long-haul coherent communication.
Funding
National Natural Science Foundation of China (61905138); Science and Technology Commission of Shanghai Municipality (20ZR1420800); Industrial Foresight Technology Research and Development Program (BE2022055-4); Shanghai Professional Technology Platform (19DZ2294000); 111 Project (D20031); State Key Laboratory of Advanced Optical Communication Systems and Networks (2022GZKF004).
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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