We report a mode (de)multiplexer based on the structure of cascaded vertical waveguide directional couplers, where the spatial modes of a few-mode core are coupled to various single-mode cores placed above the few-core core. Using five cascaded directional couplers, we design and fabricate a six-mode (de)multiplexer with polymer material that can spatially combine or separate the E$_{11}$, E$_{21}$, E$_{12}$, E$_{22}$, E$_{31}$, and E$_{13}$ modes. A typical fabricated device, which has a length of 29 mm, provides coupling ratios varying from 62% to 90% and modal crosstalks from μ28.2 to μ11.6 dB in the wavelength range from 1530 to 1565 nm with weak polarization dependence. The proposed structure is simple and scalable and could be developed into a range of mode-controlling devices for mode-division-multiplexing applications based on few-mode fibers.

We propose a low-power all-optical switch based on the structure of a graphene-buried balanced Mach-Zehnder interferometer (MZI), where the signal light is switched between the two output ports of the MZI by the heat generated from graphene’s absorption of the pump light. We use orthogonal polarizations for the pump and the signal light to maximize pump absorption and minimize graphene-induced signal loss. Our experimental device fabricated with polymer waveguides buried with 5-mm long graphene shows a pump absorption of 10.6 dB (at 980 nm) and a graphene-induced signal loss of 1.1 dB (at 1550 nm) and can switch the signal light with a pump power of 6.0 mW at an extinction ratio of 36 dB. The actual pump power absorbed by graphene for activating switching is estimated to be 2.2 mW. The rise and fall times of the switch are 1.0 and 2.7 ms, respectively. The switching characteristics are weakly sensitive to ambient temperature variations. Our device can be butt-coupled to single-mode fibers and could find applications in fiber-based and on-chip all-optical signal processing.

We propose an ultra-broadband mode filter based on the structure of a phase-shifted long-period grating formed along a few-mode waveguide terminated with two linear tapers. To demonstrate the operation principle, we design a mode filter to reject the fundamental mode of a three-mode waveguide that supports the E11, E21, and E12 modes. Our typical experimental device fabricated with polymer material, which has a length of 26 mm, provides polarization-insensitive 10-dB rejection of the E11 mode against the E21 and E12 modes with bandwidths of ~190nm (1440-1630 nm) and ~140nm (1450-1590 nm), respectively. The proposed mode filter could serve as a general platform for the development of mode filters and mode-dependent-loss compensators for mode-division-multiplexing applications.

We propose a three-dimensional waveguide structure for the realization of mode-selective switches, where the high-order spatial modes of a few-mode waveguide can be switched to various single-mode waveguides with thermo-optically controlled vertical asymmetrical directional couplers. To demonstrate the idea, we design and fabricate two specific devices with a polymer material for a waveguide that supports three spatial modes. The two devices differ in the application of the thermo-optic effect to deactivate or activate the directional couplers. For one device, the switching powers measured at the wavelength 1550 nm are lower than 20.6 mW and the switching times are shorter than 4.4 ms. The extinction ratios are higher than 15.6 dB across the C-band measured at fixed switching powers. For the other device, the switching powers measured at 1550 nm are lower than 22.6 mW, the switching times are shorter than 2.9 ms, and the extinction ratios are higher than 14.1 dB in the C-band. The performances of the devices are weakly sensitive to the polarization state of light. Our proposed mode-selective switches, which have a scalable configuration and require low power consumption, could be developed into various active mode-controlling devices for applications in reconfigurable mode-division-multiplexing networks.

We present an ultra-broadband and low-modal-crosstalk mode multiplexer based on cascaded vertical directional couplers formed by adiabatic-tapered waveguides without mode conversion. The input and output ports of the mode multiplexer are designed in the same planar layer to meet the standard package requirement. We design and fabricate a three-mode mode multiplexer, which serves to spatially combine and separate the E ₁₁ , E ₂₁ and E ₁₂ modes in the few-mode-core outputs without mode conversion with polymer material, where adiabatic tapers are utilized in the design of directional coupler to extend the operation wavelength and two pairs of directional couplers serve to suppress the modal crosstalks. We fabricate a typical device to demonstrate the idea. The fabricated device has a length of 29 mm, which provides coupling ratios higher than 97% and modal crosstalks lower than −19.6 dB over the C+L band and beyond with weak polarization dependence. The proposed mode multiplexer is fiber capable and allows to connect with the few-mode fibers directly by the use of fan-in and fan-out technology. The proposed mode multiplexer is simple, scalable, and effective to extend the operation wavelength as well as suppressing the modal crosstalks, which provides a powerful device to manipulate the modes in a mode division multiplexing system and an effective method to realize an all-optical chip for simplifying the Multi-Input Multi-Output signal processing with low power consumption.

We present a mode multiplexer based on vertical directional couplers that are formed by adiabatic-tapered waveguides. We design and fabricate the device via the micro-fabrication processing to (de)multiplex the E₁₁, E₂₁, and E₁₂ modes from the few-mode bus waveguide. Our experimental device shows a coupling ratio higher than 98.6% and 97.0% for the E₂₁ and E₁₂ modes, respectively, over the C + L band and beyond. The modal cross talk of this device can be lower than –17.1 dB, –18.4 dB, and –15.1 dB caused by the unintended E₁₁, E₂₁, and E₁₂ modes, respectively. This mode multiplexer can work over a broader wavelength range with weak polarization sensitivity, which could be used in the mode-division-multiplexing systems where mode (de)multiplexing is required in the expanded communication wavelength window other than the C-band.