Ever-increasing capacity requirements of optical interconnects drive the emergence and fast development of mode-division-multiplexing (MDM) transmission on-chip, where efficient mode control and conversion components become indispensable. Here, we propose an on-chip silicon TM 0 - to - TM 1 mode-order converter by leveraging shallowly etched rectangular slots on the top surface of silicon nanowire. The mode conversion region consists of two rectangular slots on the same side of a silicon nanowire and a smaller one between them to realize the efficient mode-order conversion from input TM 0 to output TM 1 mode with the help of multimode interference and accumulated phase difference. By studying the etching pattern on the silicon nanowire in detail, we have achieved an on-chip TM 0 - to - TM 1 mode-order converter with a high conversion efficiency of 97.5% and low modal crosstalk < − 23 dB in a conversion length of 11.8 µm; further, the insertion loss is only 0.29 dB at the wavelength of 1.55 µm. Moreover, the device working bandwidth and fabrication tolerance are also analyzed. Note that the proposed shallowly etched slots on the silicon nanowire can also be further developed to achieve the on-chip TM 0 - to - TM 2 mode-order conversion. With these characteristics, such a device could boost the development of MDM transmission on-chip with more TM-polarized mode channels.

A mode-order converter is an indispensable component in some typical on-chip mode-division-multiplexing (MDM) systems. Here, we propose a compact and high-performance TM-polarized mode-order converter using shallowly etched structures on the silicon waveguide. This device consists of a rhombus etching part at the waveguide center and two same triangle etching parts on both sides of the waveguide along the propagation direction to achieve the mode conversion from the input ${{\rm TM}_0}$ to the output ${{\rm TM}_2}$ mode, where both the effective waveguide structure and the corresponding effective index distribution can be changed. By optimizing the dimensions of the rhombus and triangle etching parts as well as the relative position between them, we have realized the efficient ${{\rm TM}_0}$ to ${{\rm TM}_2}$ mode conversion in a conversion length of only 6 µm, which distinguishes from mainly reported TE-polarized mode-order converters, and the obtained mode conversion efficiency (CE), cross talk (CT), and insertion loss (IL) are $\sim{94}\% $, $ \lt - {15}\;{\rm dB}$, and $\sim{0.5}\;{\rm dB}$, respectively, at the wavelength of 1.55 µm. Meanwhile, the allowable bandwidth can be extended to 128 nm by keeping ${\rm CE} \gt {94}\% $, where the mode CT and IL are $ \lt - {15}\;{\rm dB}$ and ${\rm IL} \lt {0.7}\;{\rm dB}$, respectively, in the same wavelength range. Also, the device fabrication processes and tolerance analyses have been done. We hope this device could be beneficial for the capacity improvement of the on-chip MDM systems.

Higher-order mode conversion plays a vital role in on-chip multimode applications. Here, we propose an efficient silicon-based higher-order mode conversion scheme that can achieve the mode-order conversion from T E 0 to T E 1 ( T E 0 − T E 1 ) mode and from T E 0 to T E 2 ( T E 0 − T E 2 ) mode, respectively. These mode conversion functions are realized by etching subwavelength grating (SWG) slots on a silicon waveguide, where the design of the SWG slot pattern matches with the required T E 1 ( T E 2 ) higher-order mode. According to the calculations, the mode conversion length is only 3.3 µm (4.32 µm) with the mode conversion efficiency (CE) 92.1% (95.6%), mode crosstalk (CT) − 17.5 d B ( − 16.7 d B ), and insertion loss 0.65 dB (0.5 dB) for the T E 0 − T E 1 ( T E 0 − T E 2 ) mode converter at λ = 1.55 µ m . Moreover, device bandwidths over 110 nm ( C E > 90 % and C T < − 15 d B ) and other higher-order mode conversions can also be achieved using the proposed scheme (e.g., T E 0 − T E 3 and T E 0 − T E 4 mode converters). We believe such a device scheme would help increase the transmission capacity of on-chip multimode multiplexing transmissions by leveraging more mode channels.

We propose and experimentally demonstrate a photonic approach to estimate the time-difference-of-arrival (TDOA) and the angle-of-arrival (AOA) of a microwave signal. TDOA and AOA are estimated from the carrier power difference of the two outputs of the Mach-Zehnder modulator (MZM) using only one dual-drive 1 \times 2 MZM. Experimentally, the TDOA of a microwave signal at 3 GHz from 27.78 to 166.67 ps is measured with maximum measurement errors of +(-) 2.24 ps; correspondingly, the AOA from 60o to 85.2o is measured with maximum measurement errors of +(-)0.4o.

A low-loss and compact TM-pass polarizer is demonstrated based on improved hybrid plasmonic grating (HPG) on a Z-cut lithium-niobate-on-insulator (LNOI) platform. In this work, we introduced the silica partition structures surrounding an LN (LiNbO3) ridge waveguide to support two parallel metal gratings, achieving the optimal coupling coefficient for HPG. Compared with the traditional HPG structure, the transverse electric (TE) mode suffers a higher confinement and the transverse magnetic (TM) mode transmission loss exhibits a significant reduction in the improved HPG structure. The simulation shows that the polarizer achieves a broad bandwidth of 150 nm (1500 to 1650 nm) with an extinction ratio (ER) over 20 dB and an insertion loss varying from 0.18 to 0.3 dB, including a maximum ER of 36 dB at the wavelength 1550 nm. In addition, the length of the demonstrated polarizer is only 10 μm, making it compact for integration. The proposed polarizer shows a good application prospect for integrated photonics in LNOI.