A tunable wavelength filter based on an asymmetric single-mode grating fiber fabricated by anisotropic CF4 plasma etching is proposed. The reflection wavelength is shown to shift linearly with the degree of curvature of the bent fiber, affording a center wavelength shift of 3.5 nm at 1550 nm. In this region of wavelength shift, the rejection and half-power width of the measured spectra remain almost constant.
© 2002 Optical Society of America
A wavelength-selecting device with low insertion loss is a key element for wavelength division multiplexed (WDM) fiber telecommunications. Although fixed-band rejection filters are readily available in the form of Fiber Bragg Gratings (FBGs) or Long-Period Gratings (LPGs), there are few practical tunable filters. Several approaches have been reported for tuning the wavelength of fiber gratings, including thermal tuning based on temperature-induced refractive index change , piezoelectric straining , and magnetostrictive straining . However, none of these approaches represent a simple and low-cost control system for wavelength shifting. Temperature tuning is an intrinsically slow process due to its dependence on thermal conduction and equilibration, piezoelectric straining requires high voltage, and magnetostriction generally requires high magnetic fields for appreciable wavelength shifts. The present authors have demonstrated tunable-wavelength filters using single-mode Bragg grating fibers (10/125 □ m core/cladding diameter, 1550 nm grating wavelength, 10 mm grating region) with cladding thinned by reactive plasma etching . In the experiment, the center wavelength of the Bragg reflector has been shifted by two methods; changing the effective refractive index through the introduction of additional cladding, and applying tension to the grating region. In the former technique, the reflection wavelength can be controlled by changing the refractive index of the material surrounding the thinned grating region based on temperature control. However, as noted above, the use of temperature as a controlling mechanism limits the response speed of the device. In the latter technique, a grating fiber is subjected to tensile stress in order to change the characteristic period of the grating, thereby shifting the reflection wavelength. This effect can be achieved readily using a narrow fiber, however, a high tensile force and strict control are required to achieve widely tunable filtering .
In this study, the reflection wavelength is shifted by controlled bending of a single-mode grating fiber with asymmetric cross section. The fabrication of such asymmetric fiber devices can be readily achieved by plasma etching, which is an inexpensive anisotropic fabrication technology. The application of bending stress to the optical fiber is equivalent to inducing longitudinal stress in the fiber core, which results in a change in the grating period. A wide range of stress is achieved by etching one side of the fiber; bending the fiber in the direction of the etched surface compresses the fiber core, whereas bending in the opposite direction induces tensile stress. The neutral axis is free of bending stress, and the fiber may be fabricated in such a way that the fiber core does not coincide with this neutral axis.
2. Experimental procedure
FBGs were fabricated in hydrogen-loaded single-mode fibers using a scanning phase-mask technique with a KrF excimer laser operating at 248 nm. The FBG was 10 mm in length, written with a laser energy of 280 mJ/cm2 and a total scanning time of ~10 min. One side of the fiber was etched by plasma etching along a length of about 30 mm including the grating region. Etching was performed at 13 Pa and 150 W RF power using CF4 plasma. Figure 1 shows a cross section of the grating fiber after etching for 6 h. One side of the cladding was etched down close to the core.
Figure 2 shows the experimental setup for the tunable-wavelength filter consisting of the etched grating fiber and an optical circulator. Laser light from a tunable-wavelength light source is injected into the input port and reaches the grating fiber via an optical circulator. The light at the Bragg wavelength is reflected and extracted at the optical circulator. The spectra of reflected light were measured for various curvature radii on a fixed perpendicular bend using an optical spectrum analyzer.
3. Experimental results
3.1 Reflection wavelength shifts
Figure 3 shows the measured spectra of the device for various bending conditions. The positive curvature radii represent bending in the tensile direction, and negative values indicate the compressive direction. The current filter is tunable over 3.5 nm from 1545.7 to 1549.2 nm at bending radii of greater than 18 mm in both directions. This wavelength shift is approximately 1.5 times larger than that achieved at an applied voltage of 100 V by piezoelectric straining , and approximately 3.5 times larger than that at an applied magnetic field of 100 mT by magnetostrictive straining . The spectra exhibited only a very minor change in rejection and 3 dB bandwidth, confirming the applicability of this method.
Figure 4 shows the shift in the reflection wavelength as a function of the degree of curvature (1/r). The center wavelength of the reflection band is shifted in proportion to 1/r. The theoretical results in the figure were determined by calculating the deformation of the core using Eq. (1)–(3) below with parameters for the cross section of the etched fiber shown in Fig. 5.
where λs is the initial reflection wavelength, ε is the deformation of the core, Rc and Rp are the radii of curvature for tensile and compressive stress, and yc, y 1 and y 2 are the dimensions shown in Fig. 5. The experimental shift in reflection wavelength (Fig. 4) is about 0.57 times that predicted theoretically, attributed to the possibility that the grating was not bent exactly along the plane perpendicular to the grating, and potential deviations in the dimensions of the grating region along the section.
3.2 Polarization characteristics
3.2.1 Comparison with thinned-cladding grating fiber device
The polarization dependence loss (PDL) of the proposed single-mode grating fiber with asymmetric cross section (etching region 30 mm) produced by anisotropic etching was measured and compared with that of a thinned grating fiber with symmetric cross section (diameter 20 μm, etching region 10mm) produced by coaxial thinning. The reflection wavelengths of the grating fibers were 1548.34 nm and 1548.72 nm, respectively. The measured values for input at 1310 nm and 1550 nm are listed in Table 1. The PDL of the proposed fiber was 8 times larger than that of the thinned fiber at 1550 nm, and 3 times larger at 1310 nm.
3.2.2 Change in PDL with bending degree
Figure 6 shows the variation in the measured PDL of the asymmetric fiber as a function of the degree of curvature. At 1550 nm, the PDL varied significantly with the degree of bending. When a higher tensile stress was applied to the fiber core, the PDL increased with the degree of curvature at 1310 nm and 1550 nm. When compressive stress was applied, the PDL decreased with the degree of curvature at 1550 nm, but increased slightly at 1310 nm. This difference in behavior is explained as due to similarity between the reflection wavelength of the grating and the wavelength of light source for measurement (1550 nm). It is thought that many factors affect the PDL associated with the bending, including the physical effects of etching one side of the fiber and the anisotropy of the UV irradiation for grating writing. However, these factors appear to largely cancel upon compression in the 1550-nm regime.
We have presented a novel control device for tunable wavelength filtering using an asymmetric single-mode grating fiber fabricated by anisotropic etching. Control is achieved by adjusting the curvature of bending of the etched grating fiber, and the reflection wavelength can be shifted in linear proportion to the reciprocal of the curvature radius. At present, the polarization dependence loss of the novel grating fiber is significantly larger than that of symmetrically thinned grating fiber, however, the simple design and low-cost fabrication are expected to grant the proposed technology many potential applications in WDM filtering, tunable fiber lasers and dynamic add-drop functions.
This work was supported in part by Grants-in-Aid for Scientific Research from The Ministry of Education, Culture, Sports, Science and Technology and the Koshiyama Research Grant.
References and links
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