Three-Dimensional Optical Waveguide Devices for Broadband Mode-Division Multiplexing


Student thesis: Doctoral Thesis

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Award date6 May 2019


Single-mode optical fibers are gradually approaching their maximum transmission capacity of about 100 Tbit/s set by the nonlinear Shannon limit. To meet the rapidly growing demand for bandwidth, there is an urgent need to develop new approaches to largely increasing the transmission capacity of the optical fiber. An approach that is being actively developed worldwide is space-division multiplexing (SDM), which is based on the transmission of parallel signal channels in a multi-core fiber, a few-mode fiber (FMF), or their combination. The SDM technology based on an FMF is often referred to as mode-division multiplexing (MDM), where different spatial modes in an FMF carry different signal channels. To build an MDM transmission system, a series of broadband mode-dependent optical devices are required, which include mode (de)multiplexers, mode switches, mode routers, mode-dependent-loss compensators, mode filters, etc. Different technologies based on free-space optics, fiber optics, and waveguide optics are being actively explored for the development of mode-dependent devices for MDM applications. Waveguide devices, in particular, are compact and can be integrated to achieve more advanced functions. Conventional waveguides, however, are co-planar or two-dimensional (2D) structures, which poses significant restrictions in the design of devices for manipulating fiber modes, which have three-dimensional (3D) spatial distributions. A natural way to manipulate fiber modes is to employ 3D waveguide structures that allow the propagation of light in waveguides formed in different geometric levels. This thesis presents a series of studies on 3D waveguide devices for broadband mode manipulation, which includes a six-mode (de)multiplexer, a high-order-mode-pass mode (de)multiplexer, two thermo-optic three-mode spatial switches, a Mach-Zehnder interferometer (MZI) four-mode switch, an MZI-based four-mode router, and an ultra-broadband grating-based mode filter.

In the first study, a 3D mode (de)multiplexer based on the structure of cascaded vertical waveguide directional couplers (DC) is investigated, where the spatial modes of a few-mode core (FMC) are coupled to various single-mode cores (SMC) placed above the few-core core. By using five cascaded DCs, a six-mode (de)multiplexer is designed to spatially combine or separate the E11, E21, E12, E31, E22, and E13 modes. The experimental device fabricated with polymer material has a length of 29 mm and provides coupling ratios varying from 62% to 90% and modal crosstalks from −28.2 to −11.6 dB in the C-band (from 1530 to 1565 nm) with weak polarization dependence. The structure is simple and scalable and could be developed into a range of mode-controlling devices for MDM applications.

In the second study, a high-order-mode-pass mode multiplexer based on a hybrid-core vertical DC is investigated. This device serves to couple the fundamental mode (i.e., the E11 mode) of a three-mode core into an SMC without affecting the other two modes. The experimental device has a length of 15 mm and provides a coupling ratio for the E11 mode higher than 92.5 % in the C-band with a relative residual power of the E11 mode in the three-mode core smaller than −10.9 dB. The modal crosstalks caused by the high-order modes are smaller than −13.8 dB in the C-band. The characteristics of the device are polarization-insensitive. This device can realize the add-drop function for the fundamental mode and thus find applications in reconfigurable MDM systems.

In the third study, two designs of thermo-optic mode spatial switches based on two cascaded vertical DCs are investigated. The first design relies on the application of the thermo-optic effect to deactivate the two DCs. The switching powers required for the two DCs are 15.9 and 20.6 mW, respectively, and the extinction ratios achieved at these switching powers are higher than 17.1 and 15.6 dB across the C-band, respectively. The switching times are shorter than 4.4 ms. The second design relies on the application of the thermo-optic effect to activate the DCs. The switching powers required for the two DCs are 18.3 and 22.6 mW, respectively, and the extinction ratios achieved are higher than 14.1 and 14.5 dB across the C-band, respectively. The switching times are shorter than 2.9 ms. The performances of the two devices are weakly sensitive to the polarization state.

In the fourth study, a thermo-optic four-mode switch that operates on modulating the optical phases of a 3D balanced four-arm waveguide MZI is investigated. This device allows switching between any two of the E11, E21, E12, and E22 modes of the waveguide. The experimental device fabricated with polymer material shows an extinction ratio higher than 14 dB and a switching time shorter than 3.7 ms, measured with the E11 mode switched to any of the other modes at 1550 nm. This mode switch can operate over a wide range of wavelengths with weak polarization dependence and could be used in reconfigurable MDM systems.

In the fifth study, a broadband four-mode router based on integrating a 3D MZI four-mode switch with matching 3D waveguide branches is investigated. This device can dynamically route the four spatial modes of a four-mode waveguide into four single-mode waveguides (SMWs). The experimental device offers a switching time shorter than 4.6 ms at a switching power of ~13 mW for each arm of the MZI. 

In the sixth study, an ultra-broadband mode filter based on a specially designed phase-shift long-period grating is investigated. This device serves to filter out only the fundamental mode of a few-mode waveguide (FMW) over a broad range of wavelengths and can be used in conjunction with a mode (de)multiplexer to increase the mode-extinction ratios of the mode (de)multiplexer. The experimental device provides −10-dB rejection of the E11 mode against the E21 and E12 modes with bandwidths of ~190 nm (1440 – 1630 nm) and ~140 nm (1450 – 1590 nm), respectively. The performance of the device is insensitive to the ambient temperature and the polarization state. The device can be developed into a mode-dependent-loss compensator or a mode stripper for MDM systems.