Optical devices for mode-division multiplexing
Student thesis: Doctoral Thesis
Related Research Unit(s)
With the ever growing Internet-driven traffic, standard single-mode fibers are rapidly approaching the information capacity limit of ~ 100 Tb/s per fiber, set by the fiber fuse phenomenon and the fiber nonlinearity. To meet the rapidly growing demand for bandwidth, there is a worldwide interest in pursuing new technologies to further increase the signal-carrying capacity of optical fibers. One approach is mode-division multiplexing (MDM), where the spatial modes in a few-mode fiber are made to carry independent signal channels. MDM can expand the transmission capacity of a fiber in proportion to the number of modes carrying independent signal channels. To build an MDM transmission system, many mode-dependent optical devices are required, such as laser sources for direct generation of high-order modes, mode converters for conversion between the fundamental mode and high-order modes, and mode (de)multiplexers for spatially combining or separating different mode channels. This thesis presents several studies on optical devices for MDM applications, which is divided into two parts. The first part considers two fiber devices: a mode converter based on a long-period fiber grating (LPFG) and a transverse-mode selectable fiber laser, and the second part considers two waveguide mode (de)multiplexers: one based on symmetric directional couplers and the other based on asymmetric directional couplers. In the first part, an LPFG-based mode converter is investigated, which allows the fundamental mode of a few-mode fiber to couple to selected high-order modes and vice versa. The device is analyzed by the coupled-mode theory and fabricated by a direct CO2-laser writing method in a two-mode fiber. The fabricated mode converters are capable of generating the cylindrical vector modes or the LP11 modes. A typical 15-period grating shows a mode conversion efficiency higher than 99% over a bandwidth of 34.0 nm in the C-band. The transmission characteristics of the gratings are insensitive to temperature variations and the mode conversion efficiency is polarization-insensitive. A passively mode-locked fiber laser for direct generation of higher-order modes is studied next. The laser incorporates a two-mode fiber Bragg grating (FBG) for the selection of both the transverse mode and the operation wavelength. When operating at the continuous-wave state, the laser can generate the LP01 mode, the LP11 mode, and a mix of them. In the mode-locked state, it generates picosecond pulses at a fundamental repetition rate of 6.58 MHz for both the LP01 and LP11 modes with a signal-to-noise ratio higher than 56 dB. The different operation states of the laser are characterized by varying the pump power to a value below the mode-locking threshold, at the mode-locking threshold, and above the mode-locking threshold. In addition to MDM, the laser could find applications in industrial material processing, medical treatment, and imaging. In the second part, two waveguide mode (de)multiplexers are studied in detailed, both of which were fabricated with polymer materials to take advantage of the spin-coating process to form multilayer structures. The first mode (de)multiplexer is based on two cascaded symmetric waveguide directional couplers arranged in the horizontal and vertical directions, respectively. The two couplers were designed to provide complete power transfer from the central core to the side cores for the LP11a and LP11b modes, respectively, with the LP01 mode staying in the central core. A typical fabricated device, which has a total length of 18.5 mm, shows coupling ratios higher than ~96% for both couplers in the wavelength range 1530 - 1570 nm (the C-band), regardless of the polarization state of light. When it functions as a demultiplexer, the crosstalks to the LP11-mode channels are lower than -15.6 and -13.4 dB for the TE and TM polarizations, respectively, while the crosstalks to the LP01-mode channel are negligible. The performance of the device is polarization-insensitive. The propagation losses for the three modes are about 2.0 dB/cm. As the three cores of the waveguides are identical, the mode patterns after demultiplexing are maintained. This device can be connected to two-mode fiber leads for mode routing applications. The second waveguide mode (de)multiplexer is based on two collocated asymmetrical directional couplers formed with two dissimilar single-mode cores placed alongside a central rectangular three-mode core in the horizontal and vertical directions, respectively. The device allows the LP11a and LP11b modes of the central core to completely couple to the LP01 modes of the two side cores, respectively, with the LP01 mode staying in the central core. The device was fabricated with polymer materials and has a total length of 9.0 mm. The coupling ratios vary from 91% to 99% for the two LP11 modes and the crosstalks among the mode channels in the side cores vary from -23.2 to -14.6 dB in the wavelength range 1530 - 1570 nm. The crosstalks in the central core are negligible. The propagation losses of the three modes are lower than 3 dB/cm. The device is insensitive to the polarization state of light and temperature variations, and can be connected directly to single-mode fiber leads for mode (de)multiplexing applications.
- Optical fiber communication, Optoelectronic devices, Multiplexing, Design and construction