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Frontiers of Optoelectronics

Front Optoelec    2012, Vol. 5 Issue (3) : 334-340     DOI: 10.1007/s12200-012-0270-1
Characteristic control of long period fiber grating (LPFG) fabricated by infrared femtosecond laser
Xiaoyan SUN1,2, Peng HUANG1, Jiefeng ZHAO1, Li WEI3, Nan ZHANG1, Dengfeng KUANG1, Xiaonong ZHU1()
1. Institute of Modern Optics, Nankai University, Key Laboratory of Optical Information Science and Technology, Ministry of Education, Tianjin 300071, China; 2. College of Mechanical and Electrical Engineering, Central South University, State Key Laboratory of High Performance Complex Manufacturing, Changsha 410083, China; 3. Department of Physics & Computer Science, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5, Canada
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Long period fiber gratings (LPFGs) with different spectral characteristics were fabricated with 1 kHz, 50 fs laser pulses. The contrast of resonant rejection band can be significantly increased by a proper amount of axial stress along a fiber during laser writing or post-processing with lower energy density laser irradiation. Variations of focal condition, pulse energy of laser irradiation and the number of grating periods lead to the generation of resonance rejection band of LPFGs from single-peak to multi-peak plus larger out-of-band loss. The out-of-band loss is primarily caused by Mie scattering from the laser processed cites, and it can be reduced by decreasing the duty cycle of grating pitch instead of lowing down the actual power of laser irradiation.

Keywords long-period fiber grating (LPFG)      infrared femtosecond laser      out-of-band loss      Mie scattering      micro-nano-fabrication     
Corresponding Authors: ZHU Xiaonong,   
Issue Date: 05 September 2012
 Cite this article:   
Xiaonong ZHU,Jiefeng ZHAO,Li WEI, et al. Characteristic control of long period fiber grating (LPFG) fabricated by infrared femtosecond laser[J]. Front Optoelec, 2012, 5(3): 334-340.
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Xiaonong ZHU
Jiefeng ZHAO
Dengfeng KUANG
Xiaoyan SUN
Fig.1  Schematic diagram of experimental setup for the fabrication of LPFGs. During laser writing, the fiber is translated along its axis and normal to the incoming laser beam focused by a 25× microscopic objective. A super continuum light source (Koheras SuperK) and an optical spectrum analyzer (OSA, Ando AQ6315E) are used for in situ monitoring the transmission spectrum of the LPFGs during laser writing
Fig.2  (a) Transmission spectra of LPFGs fabricated with 0 to 3 N axial stresses applied along the fiber respectively; (b) growth of transmission spectra as a function of length for 3 and 2 N pulling forces, respectively. In (b), for both forces of 3 and 2 N, the black curves are recorded when the grating length is 20 mm, and the red curves are measured when the grating length is 25 mm
Fig.3  Comparison of transmission spectra of LPFGs with (hollow round dots) and without (solid squares) post processing
Fig.4  Transmission spectra of LPFGs fabricated with a 10× objective and single pulse energy is 0.75 μJ
Fig.5  Evolution of LPFG transmission spectra for different total number of grating periods, indicated on the right of each plot. The traces are better viewed row by row, first starting from the one on the top left = 2, then to its right = 4, and then move to the bottom left = 6, next to it is the bottom right = 8, and then go back to the top left plot but now = 10 and so on, up to the spectrum associated with = 32 given in the bottom right plot
Fig.6  Microscopic image of light scattering from LPFGs when light from broadband light source is coupled into and propagating through fiber core
Fig.7  Transmission spectra of LPFGs written by 2 μJ laser pulses energy for three different duty cycles (Note that the grating period is 500 μm for all the gratings shown here, and thus 100/400 duty cycle means laser on for 100 μm and laser off for 400 μm)
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