Asymmetric long period fiber gratings fabricated by use of CO2 laser to carve periodic grooves on the optical fiber

Abstract

An asymmetric long period fiber grating ͑LPFG͒ with a large attenuation of −47.39 dB and a low insertion loss of 0.34 dB is fabricated by use of focused CO 2 laser beam to carve periodic grooves on one side of the optical fiber. Such periodic grooves and the stretch-induced periodic microbends can effectively enhance the refractive index modulation and increase the average strain sensitivity of the resonant wavelength of the LPFG to −102.89 nm/ m . The resonant wavelength and the peak attenuation of the LPFG can be tuned by ϳ12 nm and ϳ20 dB, respectively, by the application of a stretching force. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2360253͔ Long period fiber grating ͑LPFG͒ is one of the widely used passive optical fiber devices. Various LPFG fabrication techniques have been demonstrated, including ultraviolet laser irradiation, 1 CO 2 laser heat, 2,3 hydrofluoric acid etching corrugation, 4 and application of periodic microbend. 5 The strain sensitivity obtained for the CO 2 -laser-induced LPFGs without physical deformation is usually very low, only −0.45 nm/ m . 2,3 In this letter, a technique of fabricating asymmetric LPFG by use of focused CO 2 laser beam to carve periodic grooves on one side of the optical fiber is presented. The LPFGs obtained exhibit a large peak transmission attenuation of −47.39 dB and a low insertion loss of 0.34 dB. Moreover, the average strain sensitivity of resonant wavelength of the LPFG is increased to −102.89 nm/ m .Our experimental setup is shown in Fig. 1. A CO 2 laser ͑SYNRAD 48-1͒ with a maximum output power of 10 W, a light-emitting diode light source, and an optical spectrum analyzer ͑HP 70004A͒ were used. The optical fiber ͑Corning SMF-28͒ was situated in the focal plane of the CO 2 laser beam. One of the fiber ends was fixed and a small weight of ϳ5 g was used at the free end of the fiber to avoid the weight-induced macrobend and to provide a tensile strain in the fiber. The focused CO 2 laser beam scanned repeatedly for M times along the X direction at a location, corresponding to the first grating period, of the fiber via a two-dimensional optical scanner under the computer control. Then the laser beam was shifted by a grating period along the Y direction and scanned repeatedly for M times to generate the next grating period. This scanning and shifting process was carried out for N times ͑N is the number of grating periods͒ until the final grating period was created. The above mentioned process was repeated for K cycles until a high quality LPFG was produced. The repeated scanning of the focused CO 2 laser beam created a local high temperature in the fiber, which led to the gasification of SiO 2 on the surface of the fiber. As a result, periodic grooves were carved on the fiber as shown in Fig. 2. Such grooves induce periodic refractive index modulation along the fiber axis due to the photoelastic effect, thus creating a LPFG. The typical depth and width of the grooves obtained in our LPFGs were ϳ15 and ϳ50 m, respectively. The depth of the grooves dep...

Review of long period fiber gratings written by CO2 laser

“…Compared with the UV-laser exposure technique, the CO 2 laser irradiation technique is much more flexible and low cost because no photosensitivity and any other pretreated process are re-quired to write a grating in the glass fibers. [16][17][18][19][20][21][22][23][24][25][26][27][28][29] Moreover, the CO 2 laser irradiation process can be controlled to generate complicated grating profiles via the well-known point-topoint technique without any expensive masks. This technique could be, hence, used to write LPFGs in almost all types of fibers including pure-silica photonic crystal fibers ͑PCFs͒.…”

“…5-15 Since Hill et al 1 and Vengsarkar et al 5 wrote the first FBG and LPFG in conventional glass fibers in 1978 and 1996, respectively, the fabrications and applications of in-fiber gratings have achieved rapid developments. Various fabrication methods, such as ultraviolet ͑UV͒ laser exposure, 5-15 CO 2 laser irradiation, [16][17][18][19][20][21][22][23][24][25][26][27][28][29] electricarc discharge, [30][31][32][33][34][35][36][37] femtosecond laser exposure, [38][39][40][41][42][43] mechanical microbends, [44][45][46][47][48][49][50][51] etched corrugations, [52][53][54][55][56][57] and ion beam implantation, [58][59][60] have been demonstrated to write LPFGs in different types of optical fibers. Numerous LPFGbased devices have ...…”

Tapered fiber Mach–Zehnder interferometer for simultaneous measurement of refractive index and temperature

“…2,4 Besides the approach with evanescent waves from nanometer fiber tips coated with gold particles, 5 a majority of fiber sensors for refractive index measurement utilized fiber gratings, i.e., long-period gratings ͑LPGs͒ and fiber Bragg gratings ͑FBGs͒. Recent work that reported refractive index measurement with LPGs are either gold coated, 6 arc-induced phase shifted, 7 asymmetric, 8 or inscribed in air-and water-filled photonic crystal fibers. 9 Reported refractive index measurement with FBGs includes a metal-coated grating in a special single-mode fiber of larger core ͑26 m͒ and thinner cladding ͑30 m͒, 10 a tilted FBG with gold coating in single-mode fiber, 11 and cladding mode resonances of etched-eroded FBG.…”

Post-treatment techniques for enhancing mode-coupling in long period fiber gratings induced by CO2 laser

“…An asymmetrical LPFG with 20 periods and a grating pitch of 400 m were written in a standard singe mode fiber (SMF) by the use of the CO 2 -laser-carving technique reported in [13]. Periodic grooves were carved on the single-side surface of the fiber, as shown in Fig.…”

“…In other words, they are so-called preprocessing techniques [12]. Authors recently reported a novel technique for writing an asymmetric LPFG by means of carving periodic grooves on the surface of an optical fiber with a focused CO 2 laser beam [13]. Mode coupling in such an asymmetrical CO 2 -laser-induced LPFG with periodic grooves is very sensitive to the tensile strain applied and the change in external temperature.…”