Properties of off-axis helical long-period fiber gratings *

In this paper, a new four-electrode arc discharge device with large constant temperature region is designed, which is used to prepared high-quality off-axis helical long-period fiber grating. The larger constant temperature heating area is more conducive to releasing the stress of optical fiber, so that the prepared device is less off-axis. In order to show that low off-axis is a key parameter of high-quality off-axis helical long-period fiber grating, the effects of single mode fiber on transmission spectrum of off-axis helical long-period fiber grating under different coupling lengths, pitches, core refractive indexes, cladding refractive indexes, core diameters, cladding diameters and off-axis quantity are simulated by using beam propagation method. Since traditional methods are difficult to measure the off-axis helical long-period fiber grating with small off-axis quantity, the off-axis quantity of the prepared device is estimated by using the method of spectral comparison and back-thrust off-axis quantity in this work. The off-axis helical long-period fiber grating is prepared by using the established processing device. The off-axis quantities of the prepared devices are about 0.12, 0.13 and 0.16 µ m, respectively, according to the comparison between the simulated transmission spectrum and the actual spectrum. Finally, experiments on the torsional resistance and repeatability of the off-axis helical long-period fiber grating prepared by the device are carried out. The experimental results show that the prepared grating has certain torsional resistance and good spectral repeatability.


Abstract
In this paper, a new four-electrode arc discharge device with large constant temperature region is designed, which is used to prepared high-quality off-axis helical long-period fiber grating.The larger constant temperature heating area is more conducive to releasing the stress of optical fiber, so that the prepared device is less off-axis.In order to show that low off-axis is a key parameter of high-quality off-axis helical long-period fiber grating, the effects of single mode fiber on transmission spectrum of off-axis helical long-period fiber grating under different coupling lengths, pitches, core refractive indexes, cladding refractive indexes, core diameters, cladding diameters and off-axis quantity are simulated by using beam propagation method.Since traditional methods are difficult to measure the off-axis helical long-period fiber grating with small off-axis quantity, the off-axis quantity of the prepared device is estimated by using the method of spectral comparison and back-thrust offaxis quantity in this work.The off-axis helical long-period fiber grating is prepared by using the established processing device.The off-axis quantities of the prepared devices are about 0.12, 0.13 and 0.16 µm, respectively, according to the comparison between the simulated transmission spectrum and the actual spectrum.Finally, experiments on the torsional resistance and repeatability of the off-axis helical long-period fiber grating prepared by the device are carried out.The experimental results show that the prepared grating has certain torsional resistance and good spectral repeatability.

Fig. 1 .
Fig. 1.(a) Four electrodes arc discharge OAH-LPFG processing device in large constant temperature region; (b) OAH-LPFG structure; (c) a cross section of OAH-LPFG; (d) structure of the four-electrode machining device; (e) structure of the two-electrode machining device.
Fig. 2. (a) Temperature map taken by infrared thermal imager when arc discharge is not carried out on four electrodes; (b) fourelectrode arc discharge heating fiber diagram; (c) temperature map taken by infrared thermal imager when the optical fiber is heated by four-electrode arc discharge; (d) fluctuation of the maximum temperature in the optical fiber heating region during the four-electrode arc discharge.

Fig. 6 .
Fig. 6.Influence of OAH-LPFG parameters on the uncoupled region of transmission spectrum: (a) Coupling length L c ; (b) pitch ; (c) core refractive index n co ; (d) cladding refractive index n cl ; (e) core diameter d co ; (f) cladding diameter d cl .Relationship between the uncoupled region of transmission spectrum and the off-axis quantity d: (g) OAH-LPFG mode; (h) OAH-LPFG mode.
Fig. 10.(a) A microscopic image of a cross section of Corning single mode fiber; (b) 3D refractive index profile of corning single-mode fiber measured at optical wavelength 532 nm; (c) transmission spectra of 10 samples of off-axis helical long-period grating.

图
Fig. 12.(a) Cross section of single-mode fiber; (b) longitudinal cross-section of single-mode fiber after torsion; (c) cross-sectional diagram of eccentric fiber; (d) longitudinal cross-section of the eccentric fiber after torsion.

图
Fig. 13.(a) Breakage of single-mode fiber during torsion, and the maximum fluctuation is 4.54 rad/m; (b) breakage of eccentric fiber during torsion, and the maximum fluctuation is 4.01 rad/m.

Fig. 14 .
Fig. 14.(a) Transmission spectra of coupling peaks with different torsion angles in clockwise direction; (b) dependence of clockwise wavelength on the distortion rate; (c) transmission spectra of coupling peaks with different torsion angles in counterclockwise direction; (d) dependence of counterclockwise wavelength on the distortion rate.

Table 2 .
Comparison of OAH-LPFG prepared by different preparation methods