Fabrication of micro-ridge long-period gratings inscribed on polarization-maintaining fibers
© Kwon et al.; licensee Springer. 2014
Received: 12 October 2013
Accepted: 6 January 2014
Published: 20 January 2014
We experimentally investigated a simple and new technique for the fabrication of micro-ridge long-period gratings (MRLPGs) based on polarization-maintaining fibers (PMFs). The cladding region of the PMFs was etched periodically using a wet etching technique resulting in the periodic formation of micro-ridges on the surface of the PMF. The PMF-based MRLPGs has two resonant peaks because of the birefringence of the PMF. The extinction ratios of two resonant peaks of the PMF-based MRLPGs were effectively improved by increasing the applied strain because of the photoelastic effect.
Long-period fiber gratings (LPGs) have attracted much attention in optical communication systems and optical sensors because of their many advantages, such as low cost, ease of fabrication, and electromagnetic immunity [1–3]. Since the cladding modes coupled from the guided core mode in the LPGs are directly interfaced with external environments, the LPGs have high sensitivity to ambient perturbation change such as temperature, strain, and ambient index [1–3]. In general, UV excimer lasers and frequency-doubled argon lasers are conventionally exploited to fabricate the LPGs based on the variation of the photoinduced refractive index [1–3]. For specialty fibers without photosensitivity, such as photonic crystal fibers, however, it is not easy to induce the refractive index change with UV excimer lasers and frequency-doubled argon lasers. Recently, the LPGs inscribed on a dispersion-shifted fiber (DSF) by etching its silica-based cladding with the hydrofluoric acid (HF) solution after taking the metal coating process was proposed . However, it is difficult to symmetrically deposit the metal layer on the silica-based cylindrical cladding of the DSF. In this paper, we propose a new fabrication technique of the micro-ridge long-period gratings (MRLPGs) using both wet etching and double polymer coating methods. In addition, a polarization-maintaining fiber (PMF), for the first time to our knowledge, is implemented to make the MRLPGs. The birefringence of the PMF generates two resonant peaks in the transmission spectrum of the PMF-based MRLPGs. The applied strain changes the extinction ratio of two resonant peaks but not their wavelengths because of the photoelastic effect. It means that the proposed PMF-based MRLPGs have the great potential for the application to strain sensors.
where is the averaged effective index of the cladding mode, Λ is a grating period, and and κco are the averaged self-coupling coefficients of the cladding and core modes, respectively. From Equation 2, it is evident that the resonant wavelengths should be determined by the birefringence of the PMF .
Results and discussion
We proposed and experimentally demonstrated a fabrication method for the PMF-based MRLPG using the double coating and the wet etching processes, which has the great potential for mass production. The transmission characteristics of the PMF-based MRLPG with variations in strain were measured. Two resonant peaks of the PMF-based MRLPG were observed in the transmission spectrum of the PMF-based MRLPG because of the birefringence of the PMF. The extinction ratios of two resonant peaks of the PMF-based MRLPG were enhanced by increasing the applied strain without variation in their resonant wavelengths because of the photoelastic effect. The variation of the extinction ratios of two resonant peaks at wavelengths of 1,395 and 1,471 nm were measured to be −10.16 and −14.13 dB, respectively, when the applied strain was 840 μϵ. We believe that the experimental results are very useful for applications to fiber optic sensors, optical switch filters, etc.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2000999).
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