A simple and low-crosstalk 1 × 4 silicon mode (de)multiplexer based on multimode grating-assisted-couplers is proposed. Mode transitions can be flexibly controlled by designing the grating period at the phase-matching condition. Due to the contra-directional coupling, precise control of the coupling strength and the coupling length are not needed in the system. Calculation results show that the insertion loss and the 3 dB bandwidths of the device are 0.2 dB and 3.7 nm, 0.34 dB and 7.6 nm, and 0.21 dB and 11.8 nm for the channels which (de)multiplex to the 1st, 2nd, and 3rd modes of the bus waveguide, respectively.

A silicon photonic wire filter based on an asymmetric sidewall Bragg grating in a multimode silicon-on-insulator strip waveguide is demonstrated. The operating principle is based on the contra-directional coupling between the transverse electric fundamental (TE0) and first-order (TE1) modes, which is enabled by the asymmetric spatially periodic refractive-index perturbations. An asymmetric Y-junction is cascaded at the input port of the filter so as to drop the Bragg reflection. Compared with conventional Bragg grating-based filters, this device eliminates the back reflection at the input port and the 6 dB inherent insertion loss at the drop port; moreover, a narrow 3 dB bandwidth can be obtained with a large critical dimension as a result of the weak coupling strength between the TE0 and TE1 modes inside the multimode waveguide. Experimental results show that a bandwidth of $\sim 2.8\mathrm{nm}$ is achieved by a large corrugation width of 150 nm. The insertion loss at the drop port is $-2.1\mathrm{dB}$, and the extinction ratio is $-33\mathrm{dB}$ at the through port.

In this paper, a novel design of a 1×N multimode-interference power splitter is proposed and investigated. By using the finite difference time domain method and particle swarm optimization algorithm, our proposed 1×N optical power splitter can be optimized to realize compact size, good uniformity, low insertion loss, and broad bandwidth. To verify the effectiveness of this design, the designed 1×4 and 1×8 multimode-interference power splitters are experimentally demonstrated on an silicon-on-insulator platform. The lengths of the designed 1×4 and 1×8 power splitters are 36 μm and 47.8 μm, respectively. Experimental results show that, for the fabricated 1×4 power splitter, the uniformity ranges from 0.07 to 0.89 dB and the corresponding maximum insertion loss is 0.62 dB within a bandwidth from 1520 to 1624 nm. For the fabricated 1×8 power splitter, within a bandwidth from 1519 to 1623 nm, the uniformity varies from 0.19 to 0.83 dB and a maximum insertion loss of 0.64 dB are obtained.

In this paper, an on-chip silicon polarization beam splitter using a particle-swarm-optimized counter-tapered directional coupler is proposed, designed, and fabricated. The coupling length of the proposed device is only 5 µm. As the waveguide width variation ΔW increases from −20 to 20 nm, the simulated polarization extinction ratio larger than 18.67 dB and the corresponding insertion loss lower than 0.17 dB are achieved. Measured experimental results achieved insertion loss <0.50 dB, TE polarization extinction between 16.68 to 31.87 dB, TM polarization extinction between 17.78 to 31.13 dB, over the wavelength range 1525 to 1600 nm.

Asymmetrical directional couplers aided with subwavelength sidewall corrugations are used to realize ultra-compact silicon mode (de)multiplexers at C-band. Three mode (de)multiplexers with ultra-short coupling lengths of 5.6/6.5/7.7 μ m are designed to enable low-loss mode conversions between TE₀ and TE_1/2/3 modes. They are then cascaded to build a four-channel mode-division-multiplexing (MDM) link. The four mode channels present minimal on-chip insertion losses of 0.2/0.7/0.7/0.9 dB at their peak wavelengths. Measured cross talk levels of the four mode channels are better than −18.0/−19.1/−16.0/−18.2 dB within the wavelength range from 1530 nm to 1580 nm.

Background: Biologgers have contributed greatly to studies of animal movement, behaviours and physiology. Accelerometers, among the various on-board sensors of biologgers, have mainly been used for animal behaviour classification and energy expenditure estimation. However, a general principle for the combined sampling duration and frequency for different taxa is lacking. In this study, we evaluated whether Nyquist–Shannon sampling theorem applies to accelerometer-based classification of animal behaviour and energy expenditure approximation. To evaluate the influence of accelerometer sampling frequency on behaviour classification, we annotated accelerometer data from seven European pied flycatchers (Ficedula hypoleuca) freely moving in aviaries. We also used simulated data to systematically evaluate the combined effect of sampling duration and sampling frequency on the performance of estimating signal frequency and amplitude. Results: We found that a sampling frequency higher than Nyquist frequency at 100 Hz was needed to classify fast, short-burst behavioural movements of pied flycatcher, such as swallowing food with a mean frequency of 28 Hz. In contrast, high frequency movements with longer durations such as flight could be characterized adequately using much lower sampling frequency of 12.5 Hz. To identify rapid transient prey catching manoeuvres within these flight bouts, again a high frequency sampling at 100 Hz was needed. For both the experimental data of the flycatchers and the simulated data, the combination of sampling frequency and sampling duration affected the accuracy of signal frequency and amplitude estimation. For long sampling durations, the sampling frequency equal to the Nyquist frequency was adequate for accurate signal frequency and amplitude estimation. Accuracy declined with decreasing sampling duration, especially for signal amplitude estimation with up to 40% standard deviation of normalized amplitude difference. To accurately estimate signal amplitude at low sampling duration, a sampling frequency of four times the signal frequency was necessary (two times the Nyquist frequency). Conclusions: The appropriate sampling frequency of accelerometers depends on the objective of the specific study and the characteristics of the behaviour. For studies with no constraints on device battery and storage, a sampling frequency of at least two times the Nyquist frequency will achieve relative optimal representative of signal information (i.e., frequency and amplitude). For classification and energy expenditure estimation of short-burst behaviours, 1.4 times the Nyquist frequency of behaviour is required.