| In order to meet the ever-increasing demand for communication traffic,the realization of all-optical networks is an inevitable trend for future networks.The current optical switching network employs a wavelength selective switch(WSS)based reconfigurable optical add/drop multiplex(ROADM)where semiconductor lasers are widely used as light source.However,compared with semiconductor lasers with high noise and high cost,the fiber lasers possess the virtue of low noise,good wavelength consistency,low cost,and the ease of integrating into all-optical networks.Narrow linewidth fiber lasers are widely used in dense wavelength division multiplexing systems,coherent optical communications,and laser radar because of their narrow linewidth and high beam quality.In addition,fiber lasers which directly output high-order transverse mode(including high-order linear poparization modes and vector modes)laser beams could be applied to a wavelength-mode co-multiplexing system which has much potential to be used in metropolitan area networks and local area networks to help promote the performance of optical communications.This paper,with the support of the Central Universities' Basic Research Funds Project,the '973' Project and the National Natural Science Foundation,has achieved the following major innovations:1.A wideband mode selective coupler that converts the fundamental mode in the single mode fiber to the high order linear polarization mode in the two-mode fiber is designed and fabricated with optimized coupler parameters.A chirped moire fiber grating acts as a narrow-band filter is also designed,optimized and fabricated.A fiber laser that outputs high-order linear-polarization mode beams is proposed and built.The laser simultaneously and stably outputs a fundamental mode and a second-order linear polarization mode(including odd and even modes)at two switchable wavelengths of 1547.88 nm and 1547.985 nm.The 20 dB linewidth measured at the two wavelengths are respectively 7.2 kHz and 6.4 kHz,corresponding to 3 dB linewidth are 360 Hz and 320 Hz.The purity of the second-order linear polarization mode measured is higher than 90%.2.The principle of orbital angular momentum generation in all-optical fibers is analyzed.A multi-layer core fiber and a broadband mode selective coupler is designed and fabricated.The laser simultaneously and stably outputs a fundamental mode and a third-order linear polarization mode(including odd and even modes)at 1549.950 nm.Using a linear polarizer,the laser is also capable of used generating a tunable vortex beam.The purities of the generated OAM modes are measured to higher than 80%respectively,and the 20 dB linewidth of the fitted output laser is 7.2 kHz,corresponding to 3 dB linewidth is 360 Hz.3.A vector few mode optical fiber that can break the degeneration of the second-order mode in the fiber is designed and fabricated.The effective refractive index differences between the modes can reach the magnitude of 10-4.The vector few mode optical fiber is used to fabricate a wideband mode selective coupler which separates radial polarization mode in the second-order mode group.The purity of the radial polarization mode measured by introducing spatial and spectral imaging techniques is 94%in the range of 1540-1550 nm.An all-fiber high-order mode output laser is proposed and constructed.At 1554.996 nm,the fundamental mode and the radially-polarized mode in the second-order mode can be output from the fiber laser stably and simultaneously.The 20 dB linewidth of the Lorentz fitting curve of the output laser beam is approximately 5.6 kHz,corresponding to 3 dB linewidth is 280 Hz.4.Based on the previous theory,an asymmetric double-cavity F-P fiber grating is fabricated as a narrow-band filter.Saturable absorber is introduced in asymmetric double-cavity F-P all-fiber linear cavity laser to achieve narrow-linewidth laser output with single polarization.The 20 dB linewidth is measured to be 8.6 kHz,corresponding to 3 dB linewidth is 430 Hz. |
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