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Mode-insensitive optical switch is becoming more and more important as one of the key elements for mode-division multiplexing (MDM) optical communication systems. However, the different structural relationships between the waveguides and the different modes make it challenging to control more spatial modes simultaneously. Here, we put forward a four-mode thermo-optic (TO) switch based on two Y-junctions integrated with two multimode interferometers (MMIs), where the symmetric Y-junction is used to split E11, E21, E12 and E22 modes into E11 or E12 mode, and the MMIs are designed to be insensitive for E11 and E12 modes by optimizing the waveguide geometries and electrode heater location. The proposed device can realize the switching functions for the guided four modes insensitively. Our fabricated device based on polymer waveguide platform shows an extinction ratio of larger than 6.2 dB with a power-consumption of 42.0 mW, over the wavelength range of 1500 nm to 1600 nm. The proposed four-mode TO switch is suitable for MDM systems.

A polymer/silica hybrid 3D waveguide thermo-optic (TO) mode switch based on cascaded asymmetric directional couplers (ADCs) is theoretically designed and simulated, where the spatial modes of a few-mode silica waveguide can be switched to various single-mode polymer waveguides placed above the few-mode silica waveguide. A beam propagation method is employed to optimize the dimensional parameters of the mode switch to convert the LP_11a and LP_11b modes of the few-mode silica waveguide to the LP_01 mode of two single-mode polymer waveguides using the cascaded ADC 1 and ADC 2. The coupling ratios are higher than 96.4% (93.4%) and 95.1% (92.8%) for the ADC 1 and ADC 2, respectively, under the TE (TM) polarization within the wavelength range from 1530 to 1570 nm, which shows good wavelength independence. Furthermore, the monolayer graphene is introduced as the heating electrode and buried on the surface of the polymer core to increase the heating efficiency and reduce the power consumption. The power consumption for ADC 1 and ADC 2 is 16.69 mW and 17.35 mW, respectively. Compared to the traditional TO switch with an aluminum (Al) heating electrode, the heating efficiency of the presented device can be improved by ∼30%. Moreover, the response speed of the TO mode switch with a 3D waveguide structure was also significantly improved. Compared to the device with Al electrodes, the introduced graphene electrodes can improve the switching speed of the device by ∼60%. The presented TO mode switch with its small size and easy integration should find applications in reconfigurable mode division multiplexing systems.

Water-saving irrigation strategies can successfully alleviate methane emissions from rice fields, but significantly stimulate nitrous oxide (N2O) emissions because of variations in soil oxygen level and redox potential. However, the relationship linking soil N2O emissions to nitrogen (N) functional genes during various fertilization treatments in water-saving paddy fields has rarely been investigated. Furthermore, the mitigation potential of organic fertilizer substitution on N2O emissions and the microbial mechanism in rice fields must be further elucidated. Our study examined how soil N2O emissions were affected by related functional microorganisms [ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nirS, nirKand nosZ] to various fertilization treatments in a rice field in southeast China over two years. In this study, three fertilization regimes were applied to rice cultivation: a no nitrogen (N) (Control), an inorganic N (Ni), and an inorganic N with partial N substitution with organic manure (Ni+No). Over two rice-growing seasons, cumulative N2O emissions averaged 0.47, 4.62 and 4.08 kg ha−1for the Control, Niand Ni+Notreatments, respectively. In comparison to the Nitreatment, the Ni+Nofertilization regime considerably reduced soil N2O emissions by 11.6% while maintaining rice yield, with a lower N2O emission factor from fertilizer N (EF) of 0.95%. Nitrogen fertilization considerably raised the AOB, nirS, nirKand nosZgene abundances, in comparison to the Control treatment. Moreover, the substitution of organic manure for inorganic N fertilizer significantly decreased AOB and nirSgene abundances and increased nosZgene abundance. The AOB responded to N fertilization more sensitively than the AOA. Total N2O emissions significantly correlated positively with AOB and nirSgene abundances while having a negative correlation with nosZgene abundance and the nosZ/nirSratio across N-fertilized plots. In summary, we conclude that organic manure substitution for inorganic N fertilizer decreased soil N2O emissions primarily by changing the soil NO3−-N, pH and DOC levels, thus inhibiting the activities of ammonia oxidation in nitrification and nitrite reduction in denitrification, and strengthening N2O reduction in denitrification from water-saving rice paddies.