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1 29 OSA/OFC/NFOEC 29 Bandwidth-Variable Bandpass Filter based on Dispersion Engineered Tapered Fiber with External Polymer Cladding Kuei-Chu Hsu 1, Nan-Kuang Chen 2,3, Sen-Yih Chou 1,4, Shien-Kuei Liaw 5, Yinchieh Lai 1, and Sien Chi 6 1 Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan 3, R.O.C. 2 Department of Electro-Optical Engineering, National United University, Miaoli, Taiwan 36, R.O.C. 3 Optoelectronics Research Center, National United University, Miaoli, Taiwan 36, R.O.C. 4 Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu, Taiwan 3, R.O.C. 5 Graduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 16, R.O.C. 6 Department of Electro-Optical Engineering, Yuan Ze University, Chungli, Taiwan 32, R.O.C. jessica.eo91g@nctu.edu.tw Abstract: A simple thermo-optic bandwidth-variable fiber-based bandpass filter is demonstrated by using a tapered standard single mode fiber covered with a dispersive polymer layer. The 3 db spectral bandwidth can be tuned from 25 nm with a Gaussian shape to 5 nm with a flat-top shape. 28 Optical Society of America OCIS codes: (6.234) Fiber optics components, (6.231) Fiber optics. 1. Introduction Fiber-based filters have been widely developed and used due to their compact size and low insertion loss in fiber-optic systems. Bandpass fiber-based optical filters can provide different bandwidths ranging from less than 1 nm to more than 1 nm. Among them, wide-bandwidth tunable bandpass filters can be helpful for the applications of shaping the spectra of the super wideband light sources (i.e., super continuum, LEDs, or fiber ASE). To achieve wide operation bandwidth of fiber-based optical filtering, several kinds of design and fabrication methods have been reported. These include the approaches by utilizing the long period fiber gratings inscribed on specially designed fibers, or by utilizing the complicated photonic bandgap fibers [1-3]. However, some special filtering functions such as filter bandwidth tunability are still difficult to be achieved without resorting to complex design and manufacturing techniques, which make them not suitable for practical use. In this work, a simple bandwidth-variable wide-band bandpass fiber filter is realized by utilizing a tapered standard single mode fiber covered by a thick dispersive polymer layer around its uniform waist. To obtain the bandpass filtering function, the fundamental-mode cut-on and cut-off mechanism produced by carefully engineered material and waveguide dispersion are employed [4-5]. The optical polymer OCK-433 (Nye Lubricants) is used to cover the tapered fiber as the external cladding and the passband of the fiber filter is defined by the two cross points of the refractive index dispersion (RID) curves for the dispersive polymer and the tapered fiber. We find experimentally that the bandwidth can be thermo-optic tunable with the 3 db spectral bandwidth ranging from 25 nm (with a Gaussian shape) to 5 nm (with a flat-top shape). Numerical simulation based on the beam propagation method (BPM) is also performed to investigate the dispersion engineering effects and to determine the optimal diameter of the tapered fiber for yielding an optimal banpass performance. 2. Device structure and fabrication The bandpass fiber filter with thermo-optic tunable bandwidth is achieved by tapering a standard telecommunication single mode fiber (SMF-28) and imbedding it inside the cured dispersive polymer. The tapered fibers are fabricated by the hydrogen flame-brushing technique to guarantee a uniform waist. Figure 1 displays the structure of the bandpass fiber filter. The total elongation length of the tapered fiber is around 4.5 cm and the length of the uniform waist is around 1 cm. Its diameter ρ can be controlled by tuning the parameters during the tapering process. The OCK-433 polymer with a thermo-optic coefficient dn D _/ dt = o C is applied to the tapered fiber and then thermally cured. A TE-cooler is used to control the polymer temperature in order to change its refractive index for tuning the cut-on and cutoff wavelengths. Figure 1 shows the RID curves of the Ge-doped core, the pure silica, and the dispersive polymer at different temperatures. The two cross points of the curves of pure silica and the dispersive polymer roughtly define the passband of the fiber filter, since for lights to be guided inside the fiber, the refractive index of the external cladding can not be greater that the refractive index of the waveguide core. Because the Ge dopant in the core /9/$ IEEE Authorized licensed use limited to: THE LIBRARY OF CHINESE ACADEMY OF SCIENCES. Downloaded on September 19, 29 at 8:2 from IEEE Xplore. Restrictions apply.

2 29 OSA/OFC/NFOEC 29 will be diffused into the original cladding during tapering, the effective RID curves of the tapered fiber without the polymer will be somewhere between those of the original core and the original cladding. The actual cut-on and cut-off wavelengths should be the cross points of the effective indices of the fundamental modes for the tapered fiber and the dispersive polymer. The fundamental-mode cut-on and cutoff wavelengths can be tuned upward or downward depending on the thermo-optic coefficients (dn/dt) of the dispersive polymer materials and the applied heating temperature. According to the plot of Fig. 1, a higher temperature results in a wider bandwidth, and the bandwidth can be tuned over hundreds of nanometers. Refractive index Pure silica Polymer at 32 o C Polymer at 35 o C Ge-doped core Fig. 1. Diagram of a tapered optical fiber structure with a uniform waist coated by dispersive polymer. Refractive index dispersion curves of the core and cladding of SMF-28 and the dispersive polymer o C 36 o C 37 o C 38 o C 39 o C o C 36 o C 37 o C 38 o C 39 o C Fig. 2. Experimental spectral responses of the tunable bandpass fiber filter for ρ = 22 μm and ρ = 14 μm at different temperatures. (Resolution: 5 nm) 3. Measurements and characteristics To measure the cut-on and cut-off spectral response of the fabricated filter, a halogen lamp source is used as the light source and the transmission spectra are recorder by the optical spectrum analyzer. For the present demonstration, another suitable bare tapered fiber acted as a spatial mode filter for higher order modes is connected before the tested fiber taper in order to ensure the fundamental mode input at all wavelengths. For practical devices, true single mode fibers at the operating wavelengths should be used to avoid the input excitation of higher order modes. Figures 2 and 2 show the experimental spectral responses of the tunable bandpass fiber filter for ρ = 22 μm and 14 μm at different temperatures. The index difference between the tapered fiber and the dispersive polymer at higher temperatures is larger than that at the lower wavelength. Thus the optical fields are strongly guided inside the passband, causing a flat-top spectral shape. On the contrary, the filter at the lower temperature creates a Gaussian-shaped spectral profile and experience Authorized licensed use limited to: THE LIBRARY OF CHINESE ACADEMY OF SCIENCES. Downloaded on September 19, 29 at 8:2 from IEEE Xplore. Restrictions apply.

3 29 OSA/OFC/NFOEC 29 relatively higher losses even inside the passband. The 3-dB bandwidth at 39 o C is around 4 nm with the insertion loss lower than.5 db for ρ = 22 μm tapered diameter. At 35 o C the spectral response displays a Gaussian shape with 3-dB bandwidth around 25 nm and the insertion loss around 2.5 db at central wavelengths, as shown in Fig. 2. The central wavelengths of the passband are different at different temperatures, and this is attributed to the asymmetric shift of the cut-on and cut-off wavelengths when the temperature is varied. To further analyze the fundamental-mode cut-on and cut-off characteristics in relation to waveguide dispersion effects, numerical simulation by the BPM algorithm is performed to simulate the optical field propagation within the tapered fibers covered with the dispersive polymer and to predict the filter performance theoretically. Figures 3 and 3 show the simulation loss spectra assuming the fundamental mode input, with the fiber taper diameter of 1, 2, 3 and 4 μm respectively. The fiber transition length is 1.8 mm, the uniform waist length is 1 mm, and the temperature is at 35 o C. Both the cut-on and cut-off spectral responses have the following same tendency: when the waist diameter increases, the band-edge becomes steeper. However, since the optical fields at shorter wavelengths are confined tighter than the longer wavelength, when the taper diameter gets larger than 3 μm, the bandpass phenomena disappear entirely and the device becomes a short-wavelength pass filter. The optimal bandpass operation of this filter with a flat-top spectral shape occurs when the diameter of the tapered fiber is around 2 μm ρ = 1 μm ρ = 2 μm ρ = 3 μm ρ = 4 μm ρ = 1 μm ρ = 2 μm ρ = 3 μm ρ = 4 μm Fig. 3. Simulation results of transmission spectra at different waist diameters of 1, 2, 3 and 4 μm for spectral range at 8 nm nm. 4. Conclusion A fiber-based bandpass optical filter with ultra-wide and variable spectral bandwidth is demonstrated. This thermo-optic bandwidth-variable filter is fabricated by using a tapered standard single mode fiber covered with a dispersive polymer around the tapered waist to form a pass-band through the dispersion engineering design. By suitably adjusting the material and waveguide dispersion, the bandpass fiber filters can provide a flat-top or Gaussian shaped specral passband such that the passband width can be tuned from 25 nm with a Gaussian shape to 5 nm with a flat-top shape. Due to its simple fabrication procedure and thermo-optic tunability, the proposed fiber-based filter is very cost-effective and thus promising for practical use. 5. References [1] M. J. Kim, Y. M. Jung, B. H. Kim, W. T. Han, and B. H. Lee, Ultra-wide bandpass filter based on long-period fiber gratings and the evanescent field coupling between two fibers, Opt. Express. 15, (27). [2] B. W. Liu, M. L. Hu, X. H. Fang, Y. F. Li, L. Chai, J. Y. Li, W. Chen, and C. Y. Wang, Tunable bandpass filter with solid-core photonic bandgap fiber and Bragg fiber, IEEE Photon. Technol. Lett. 2, (28). [3] S. K. Varshney, K. Saitog, N. Saitoh, Y. Tsuchida, M. Koshiba, and R. K. Sinha, Strategies for realizing photonic crystal fiber bandpass filters, Opt. Express. 16, (28). [4] N. K. Chen, K. C. Hsu, S. Chi, and Y. Lai, Tunable Er3+-doped fiber amplifiers covering S and C+L bands over nm based on discrete fundamental-mode cutoff filters, Opt. Lett. 31, (26). [5] N. K. Chen, S. Chi, and S. M. Tseng, Wideband tunable fiber short-pass filter based on side-polished fiber with dispersive polymer overlay, Opt. Lett. 29, (24). Authorized licensed use limited to: THE LIBRARY OF CHINESE ACADEMY OF SCIENCES. Downloaded on September 19, 29 at 8:2 from IEEE Xplore. Restrictions apply.

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