Ultrafine electro-optical frequency comb based on cascade phase modulation with cyclic frequency shifting.

An ultrafine electro-optical frequency comb (EOFC) with plentiful comb teeth is demonstrated. Adopting a single-frequency fiber laser as a light source, cascade phase modulation based on a sinusoidal signal and a frequency-time transformation (FTT) signal is executed to generate the EOFC with high fineness. Meanwhile, a cyclic fast frequency shifting strategy is introduced to boost the number of comb teeth and the bandwidth of the EOFC. As a result, an EOFC with 12600 comb lines covering a broad bandwidth from -6.3 GHz to 6.3 GHz is established, corresponding to an ultrafine comb space of 1 MHz. Moreover, the power fluctuation of a comb tooth is less than 0.5 dBm. This state-of-the-art EOFC has significant potential in the field of precision spectroscopy.

Wideband ultra-low intensity noise reduction via joint action of gain saturation and out-of-phase polarization mixing effect from a semiconductor optical amplifier.

In this article, the vector dynamics of semiconductor optical amplifiers (SOAs) are systematically analyzed and developed to explore its mechanism of intensity noise suppression. First, theoretical investigation on the gain saturation effect and carrier dynamics is performed via a vectorial model, and the calculated result unravels desynchronized intensity fluctuations of two orthogonal polarization states. Particularly, it predicts an out-of-phase case, which allows the cancellation of the fluctuations via adding up the orthogonally-polarized components, then establishes a synthetic optical field with stable amplitude and dynamic polarization, and thereby enables a remarkable relative intensity noise (RIN) reduction. Here, we term this approach of RIN suppression as out-of-phase polarization mixing (OPM). To validate the OPM mechanism, we conduct an SOA-mediated noise-suppression experiment based on a reliable single-frequency fiber laser (SFFL) with the presence of relaxation oscillation peak, and subsequently carry out a polarization resolvable measurement. By this means, out-of-phase intensity oscillations with respect to the orthogonal polarization states are clearly demonstrated, and consequently enable a maximum suppression amplitude of >75 dB. Notably, the RIN of 1550-nm SFFL, suppressed by joint action of OPM and gain saturation effect, is dramatically reduced to -160 dB/Hz in a wideband of 0.5 MHz∼10 GHz, and the performance of which is excellent by comparing with the corresponding shot noise limit of -161.9 dB/Hz. The proposal of OPM here not only facilitates us to dissect the vector dynamics of SOA but also offers a promising solution to realize wideband near-shot-noise-limited SFFL.

Intensity-noise suppression in 1950-nm single-frequency fiber laser by bidirectional amplifier configuration.

In this Letter, a bidirectional amplifier configuration suppressing the relative intensity noise in a 1950-nm linearly polarized single-frequency fiber laser (SFFL) is proposed. The scheme to amplify the signal in a nonlinear saturated amplification regime with low gain distribution for suppressing the RIN is theoretically analyzed. By optimizing the input power level and reflectivity of the bidirectional power-amplifier, the RIN is decreased maximally by >24dB within the frequency range of 200 kHz. A stable output power of over 5.16 W with a polarization extinction ratio of 21.2 dB is obtained. Additionally, the amplified signal maintains a linewidth of 7.1 kHz nearly identical with that of the seed, both with a signal-to-noise ratio of more than 60 dB. This all-optical technique on noise suppression applied to the fiber amplifier paves the way to realize low-noise SFFL with power improvement.

Noise-sideband-free and narrow-linewidth photonic microwave generation based on anoptical heterodyne technique of low-noise fiber lasers.

Noise-sideband-free and narrow-linewidth photonic microwave generation based on an optical heterodyne technique is demonstrated experimentally. By beating a self-injection-locking low-noise single-frequency fiber laser and a Brillouin fiber laser, a 9.4 GHz microwave is produced, and its noise sidebands are completely suppressed. Additionally, the signal-to-noise ratio of the microwave signal is improved by 15 dB from 40 to 55 dB, and the linewidth is compressed from 1.6 to 0.53 kHz. The high-performance photonic microwave based on low-noise fiber lasers is a promising candidate in further applications such as wireless network, lidar, and satellite communication.

Sulfur deposition still contributes to forest soil acidification in the Pearl River Delta, South China, despite the control of sulfur dioxide emission since 2001.

Sulfur dioxide emissions have been regulated at a global scale; sulfur (S) deposition no longer contributes to soil acidification instead of an alleviation effect in temperate regions; however, it remains unclear whether S deposition still contributes to soil acidification in the tropics. The Pearl River Delta (PRD), South China, has been suffering serious soil acidification, but the contribution of S deposition was ignored because of the regulation of S emission since 2001. Here, we chose the evergreen broadleaf forests, which are the typical forest type at the regional scale in PRD to examine the contribution of S deposition and its characteristics in this acidification, based on an established urban-rural gradient in the range of 260 km. A substantial acidification was evidenced by the significant decline of soil pH from rural to urban sites, with mean pH values decreased by more than 0.60 U through the whole 40-cm depths. However, there was no significant difference in soil pH from 0-10 cm, 10-20 cm, and to 20-40 cm at each site (P > 0.05). Acid-neutralizing capacity (ANC) showed a similar trend to soil pH, with a significant decline along the urbanization gradient and no significant effect of soil depths. Soil sulfate (SO42-), as the most abundant species in ANC, contributed greatly to soil acidification for the whole 40-cm depth, as shown by the significant positive relationships between it with soil pH and base cations. Soils also exhibited the depletion of base cations with low base saturation (< 20%) and the release of Al and Fe. Our research demonstrated that the severe soil acidification in the PRD region has extended to the subsoil level (40-cm depth), and S deposition is still an important driver to this acidification. Therefore, both recovering the acidified soils and controlling the acidifying pollutants, especially S, are particularly difficult in southern China.

Intelligent Land-Vehicle Model Transfer Trajectory Planning Method Based on Deep Reinforcement Learning.

To address the problem of model error and tracking dependence in the process of intelligent vehicle motion planning, an intelligent vehicle model transfer trajectory planning method based on deep reinforcement learning is proposed, which is able to obtain an effective control action sequence directly. Firstly, an abstract model of the real environment is extracted. On this basis, a deep deterministic policy gradient (DDPG) and a vehicle dynamic model are adopted to jointly train a reinforcement learning model, and to decide the optimal intelligent driving maneuver. Secondly, the actual scene is transferred to an equivalent virtual abstract scene using a transfer model. Furthermore, the control action and trajectory sequences are calculated according to the trained deep reinforcement learning model. Thirdly, the optimal trajectory sequence is selected according to an evaluation function in the real environment. Finally, the results demonstrate that the proposed method can deal with the problem of intelligent vehicle trajectory planning for continuous input and continuous output. The model transfer method improves the model's generalization performance. Compared with traditional trajectory planning, the proposed method outputs continuous rotation-angle control sequences. Moreover, the lateral control errors are also reduced.

Near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser.

The Earth's magnetic field has significant effects that protect us from cosmic radiation and provide navigation for biological migration. However, slow temporal variations originating in the liquid outer core invariably exist. To understand the working mechanism of the geomagnetic field and improve accuracy of navigation systems, a high-precision magnetometer is essential to measure the absolute magnetic field. A helium optically pumping magnetometer is an advanced approach, but its sensitivity and accuracy are directly limited by the low-frequency relative intensity noise and frequency stability characteristics of a light source. Here, we demonstrate a near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser. The relative intensity noise is only 5 dB higher than the quantum-noise limit, and the root mean square of frequency fluctuation is ∼17 kHz after locked. This fiber laser could suppress the fluctuation of magnetic resonant frequency and improve the signal-to-noise ratio of the magnetic resonance signal detection.

kHz-order linewidth controllable 1550 nm single-frequency fiber laser for coherent optical communication.

A kHz-order linewidth controllable 1550 nm single-frequency fiber laser (SFFL) is demonstrated for the first time to our best knowledge. The control of the linewidth is realized by using a low-pass filtered white Gaussian noise (WGN) signal applied on a fiber stretcher in an optical feedback loop. Utilizing WGN signals with different signal amplitudes An and different cutoff frequencies fc, the linewidths are availably controlled in a wide range from 0.8 to 353 kHz. The obtained optical signal-to-noise ratio (OSNR) is more than 72.0 dB, and the relative intensity noise (RIN) at frequency greater than 40 MHz reaches -148.5 dB/Hz which approaches the shot noise limit (-152.9 dB/Hz). This kHz-order linewidth controllable SFFL is meaningful and valuable, for optimizing the receiver sensitivity and bit error rate (BER) performance of the coherent optical communication system based on high-order quadrature amplitude modulation (QAM).

The role of exopolymeric substances in the bioaccumulation and toxicity of Ag nanoparticles to algae.

Exopolymeric substances (EPS) have an important role in bioaccumulation and toxicity of nanoparticles (NPs) to algae, which warrants specific studies. The interaction of EPS with citrate and polyvinyl pyrrolidone (PVP) coated AgNPs (C-AgNPs and P-AgNPs, respectively) and its roles in bioaccumulation and toxicity of the AgNPs to Chlorella pyrenoidosa were investigated. The amino and aromatic carboxylic groups in the EPS were involved in the EPS-AgNP interactions. Compared with Ag(+), C-AgNPs had comparable total bioaccumulation but greater absorption by intact algae with EPS; P-AgNPs had the smallest total bioaccumulation and were mainly adsorbed on algal surfaces. With EPS removed, the total bioaccumulations and surface adsorptions for the three Ag species decreased but the cell internalizations increased; the 96 h half growth inhibition concentrations decreased, indicating EPS alleviated the algal toxicity of Ag. The cell-internalized but not the adsorbed AgNPs could contribute to the nanotoxicity. The EPS could bind both AgNPs and Ag(+), and thus inhibited the cell internalization and the nanotoxicity. However, the EPS-bound Ag on the cell surfaces would migrate along with the algae and be biologically amplified in the aquatic food chains, presenting ecological risks. These results are helpful for understanding the fate and ecological effects of NPs.

Short all Tm-doped germanate glass fiber MOPA single-frequency laser at 1.95 µm.

Based on heavily Tm-doped germanate glass fibers (TGFs), a short all-TGF MOPA laser system with uniform core parameters in each stage was demonstrated. An 11.7 W stable single-frequency laser at 1.95 µm with an optical-to-optical conversion efficiency of 20.4% is obtained from a homemade 31-cm-long double-cladding single-mode TGF. The estimated stimulated Brillouin scattering (SBS) threshold of 980 W and the measured relative intensity noise of < -130 dB/Hz for frequencies above 2 MHz are achieved in this MOPA system. Furthermore, the prospect for further power-scaling of such short MOPA laser is considered.

Broad-bandwidth near-shot-noise-limited intensity noise suppression of a single-frequency fiber laser.

A significant broad-bandwidth near-shot-noise-limited intensity noise suppression of a single-frequency fiber laser is demonstrated based on a semiconductor optical amplifier (SOA) with optoelectronic feedback. By exploiting the gain saturation effect of the SOA and the intensity feedback loop, a maximum noise suppression of over 50 dB around the relaxation oscillation frequencies and a suppression bandwidth of up to 50 MHz are obtained. The relative intensity noise of -150 dB/Hz in the frequency range from 0.8 kHz to 50 MHz is achieved, which approaches the shot-noise limit. The obtained optical signal-to-noise ratio is more than 70 dB. This near-shot-noise-limited laser source shows important implications for the advanced fields of high-precision frequency stabilization, quantum key distribution, and gravitational wave detection.

High retention of 15 N-labeled nitrogen deposition in a nitrogen saturated old-growth tropical forest.

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem 15 N labeling experiment performed in a N-rich old-growth tropical forest in southern China. We added 15 N tracer monthly as 15 NH415 NO3 for 1 year to control plots and to N-fertilized plots (N-plots, receiving additions of 50 kg N ha-1  yr-1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N-plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N-plots, respectively. Leaching below 20 cm was 50 kg N ha-1  yr-1 in the control plots and was close to the N input (51 kg N ha-1  yr-1 ), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha-1  yr-1 . However, of these only 7 and 23 kg N ha-1  yr-1 in the control and N-plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N-saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short-term N retention represent the main difference between temperate forests and the studied tropical forest.

All-optical frequency and intensity noise suppression of single-frequency fiber laser.

An all-optical frequency and intensity noise suppression technique of a single-frequency fiber laser is demonstrated. By exploiting the recursive noise reduction effect of a semiconductor optical amplifier (SOA) in a self-injection locked fiber laser, the frequency and intensity noise of the laser are remarkably suppressed in a significantly wide frequency range. In addition to the linewidth suppression from 3.5 kHz to 700 Hz, the frequency noise has been reduced by ∼25 dB. After suppression, the relative intensity noise (RIN) is within 5 dB of the shot noise limit at frequencies from 1.5 to 3 MHz, and the frequency range of the suppression reaches about 30 MHz. The relaxation oscillation peak is observed to shift to lower frequencies and is reduced by about 35 dB from -90 dB/Hz to -125 dB/Hz. It is believed that the achieved low noise makes the fiber laser a promising candidate in applications such as ultra-long haul coherent optical communication and LIDAR.

The effect of water chemistry on homoaggregations of various nanoparticles: specific role of Cl⁻ ions.

Colloidal behavior of various nanoparticles (NPs) could be quite different under the influence of complicated water chemistry. Homoaggregations of four types of nanoparticles (NPs) were investigated with a focus on the effects of electrolyte ions, pH, and natural organic matter (NOM). Results show that critical coagulation concentrations (CCCs) of NaCl and CaCl2 for anatase TiO2, rutile TiO2, or CeO2 NPs were all inversely proportional to the valence of cations, following the Schulze-Hardy Rule, and homoaggregations of the three NPs were similarly inhibited by the presence of NOM within pH 3-9. Although the homoaggregation of Ag NPs was also increased as the concentration or valence of cations increased, the relationship between CCC and valence of the cations was far away from the Schulze-Hardy Rule; moreover, NOM only slightly increased CCC of CaCl2 and surprisingly decreased CCC of NaCl for Ag NPs. Excessively-adsorbed Cl(-) on the formed Ag-AgCl colloidal nucleus was detected, which could increase the electronegativity of Ag NPs and thus limit the aggregation effect of chlorides and the dispersion effect of NOM as well. These results are expected to increase our knowledge on the colloidal behavior and fate of NPs in aquatic environments.

Heteroagglomeration of oxide nanoparticles with algal cells: effects of particle type, ionic strength and pH.

Discharged oxide nanoparticles (NPs) have been shown toxic to unicellular algae, yet the research on heteroagglomeration between NPs and cells as an important precondition of the toxicity is scarce. This study for the first time investigated heteroagglomerations between various NPs and algal cells (Chlorella pyrenoidosa) and analyzed influencing factors including pH and ionic strength (IS) through cosettling experiment, transmission electron microscopic (TEM) observation, and Derjaguin­Landau­Verwey­Overbeek (DLVO) calculation. The examined NPs included anatase and rutile TiO2, microporous and spherical SiO2, and α-form and ß-form Al2O3. The results of cosettling experiments coincided well with the TEM observations, whereas the DLVO theory could only partly explain the NP­cell heteroagglomerations. The NP­cell heteroagglomeration for rutile TiO2 and ß-form Al2O3 was weak and insensitive to pH or IS. Preferential heteroagglomeration occurred at low pH or high IS for microporous SiO2, while marked heteroagglomeration only occurred under the neutral and low IS condition for anatase TiO2. The heteroagglomeration for spherical SiO2 was insensitive to pH but increased with increasing IS, while the heteroagglomeration for α-form Al2O3 occurred at low pH and irrelevant to IS. The work will shed new light on the bionano interfacial interaction and help to understand biological effects of NPs.