1. Introduction

In order to handle the massive data traffic in the ever expanding data services such as video streaming and social networks, the use of high speed wavelength division multiplexing (WDM) optical communication technology has been rapidly increasing. A wavelength tunable filter is an essential component for selecting a certain wavelength channel from the WDM optical signal [1, 2]. Various tunable wavelength filters have been demonstrated based on thin film filters [3–5], Fabry-Perot cavity devices [6, 7], fiber Bragg gratings [8, 9], arrayed waveguide grating device [10], etc. Among them, Bragg reflector device is the most suitable for dense WDM optical communication because of its unlimited free spectral range (FSR), narrow bandwidth, and flat-top passband [11]. Moreover, the Bragg reflector in a polymer waveguide has a wide wavelength tuning range owing to the excellent thermo-optic (TO) effect of the polymer material [12, 13].

For an ordinary Bragg reflector, an external circulator device, which poses difficulty in the integration on a single chip, is indispensable to separate the reflected signal. We have demonstrated a channel-drop filter including separate input and output ports without the use of an external circulator [14]. However, a phase modulator was required for adjusting the phase difference between the reflected waves from the two Bragg gratings. In addition, it was difficult to produce the same Bragg grating under the two waveguides.

In this work, we propose a channel-drop filter with a simple structure by using a tilted Bragg grating and a mode sorting waveguide. The titled Bragg grating has been applied to add-drop filters [15–19]; however, it was not employed for tunable devices because direct tuning of the Bragg wavelength requires significant change in the refractive index. Polymers have a significantly larger TO effect as compared to any other material and hence, it enables the tuning of the Bragg reflection wavelength with a simple electrode. For optimizing the device performance, the coupling efficiency between odd and even modes in the tilted Bragg grating and the mode evolution efficiency in the mode sorting waveguide were optimized. The reflectance of the fabricated filter was higher than 95%, and a wavelength tuning over 10 nm was achieved with a channel crosstalk of less than −20 dB.

2. Operating principle and device design

A Bragg grating normally returns the reflected signal to the same path. In this work, we utilize a mode sorting waveguide along with a tilted Bragg grating as shown in Fig. 1 to return the reflected signal onto another path. When the light is launched at the input port, which is connected to the narrow waveguide, it evolves into the odd mode at the two-mode waveguide section due to the mode evolution in the asymmetric Y-branch. Further, the reflection by the tilted Bragg grating induces coupling between the odd and even modes. Consequently, the input odd mode is reflected back as an even mode output and the even mode evolves onto the wide waveguide, which is connected to the output waveguide.

 

Fig. 1 Schematic of a channel-drop filter consisting of an asymmetric Bragg reflector and a mode sorting waveguide.

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In practice however, the tilted grating can also introduce odd–odd and even–even mode coupling. By calculating the overlap integral between the even and odd modes, including the perturbation effect by the tilted Bragg grating, the mode coupling coefficients were obtained as shown in Fig. 2(a). For 1550 nm wavelength, the refractive index of core and cladding material was 1.455 and 1.430, respectively. When the angle of the tilted Bragg grating (θ_t) is 0°, the odd-even mode coupling disappears. For a θ_t of 2.5°, the odd-odd mode coupling is suppressed and the odd-even mode coupling efficiency exhibits a maximum value of 61%. Therefore, for this angle, the odd mode excited through the narrow waveguide can be reflected back to the wide waveguide through the odd-even mode coupling without any extra mode conversion loss.

 

Fig. 2 (a) Coupling efficiency calculated as a function of the tilt angle, θ_t using an overlap integral between the modes along with a grating perturbation. (b) Crosstalk calculated as a function of the branch angle, θ_b when an even mode is launched at the two-mode waveguide.

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The efficiency of mode evolution in an asymmetric Y-branch can be optimized through the beam propagation method (BPM) simulation. The crosstalk as a function of the branch angle (θ_b) is calculated as shown in Fig. 2(b) for the widths of narrow waveguide (W_n) and wide waveguide (W_w). It is shown that for θ_b less than 0.5°, the crosstalk can be less than −20 dB. The length of adiabatic Y-branch is 0.72 mm for the branch angle of 0.5° until the waveguide is separated by 10 μm.

3. Device fabrication and characterization

For the fabrication of the channel-drop filter, fluorinated acrylate ZPU polymers (ChemOptics Co.) were used. A schematic of the fabrication procedure is illustrated in Fig. 3. A ZPU polymer with a refractive index of 1.435 was spin-coated on a silicon wafer as a lower cladding. TSMR photoresist was spin-coated and exposed by an interference pattern produced by using a 442 nm He-Cd laser. Further, the tilted Bragg grating pattern was transferred onto the lower cladding layer by using oxygen plasma etching. The grating thickness was 180 nm with a period of 535 nm. A core layer with a thickness of 2.5 μm was formed by the spin coating of another ZPU polymer with a refractive index of 1.455. For etching the waveguide pattern, a 200-nm Cr was deposited as an etch mask. The core layer was etched by 2 μm to define a rib waveguide. The ZPU of 1.435 was spin-coated again to produce an upper cladding of 8 μm over the rib waveguide. The thin film heater for TO wavelength tuning was fabricated with a Cr-Au of 100-1000 Å to have a width of 20 μm, and a length of 6.2 mm.

 

Fig. 3 Schematic fabrication procedure of polymeric tunable channel drop filters, and the cross-section of the fabricated waveguide.

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The tilt angle of grating affects the mode coupling efficiency and it should be fabricated within a marginal error. During the interference pattern exposure, the titled Bragg grating was vertically aligned using the flat zone defined on the silicon wafer. Moreover, under the mask aligner, the waveguide pattern was carefully aligned to have a certain angle, θ_t to the flat-zone. In this way, we confirmed that the error in θ_t could be as small as 0.1°. The performance of mode sorting is affected by the sharpness of the Y-branch vertex. As shown in Fig. 4(a), the pattern sharpness is significantly improved by incorporating the metal mask as compared to the pattern formed by using the AZ5214 photoresist mask as shown in Fig. 4(b). It was because AZ5214 mask was etched during the plasma etching, while metal mask withstood the etching.

 

Fig. 4 Microscope photographs of the asymmetric Y-branch appearance after oxygen plasma etching of the lower cladding fabricated by using the mask of (a) metal and (b) photoresist.

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To characterize the mode sorting waveguide, two asymmetric X-junctions were cascaded to form a Mach-Zehnder interferometer as shown in Fig. 5(a). By applying a triangular voltage signal on the phase modulator, the signals from the two output ports were measured. For a device with a θ_b of 0.3°, W_n of 3 μm, and W_w of 4 μm, the intensity of the modulated output signals were obtained as shown in Figs. 5(b) and 5(c), which correspond to the samples with branch shapes as shown in Figs. 4(a) and 4(b), respectively. The extinction ratio of Fig. 5(b) obtained from a sharp vertex device was better than 20 dB, while that of Fig. 5(c) obtained from a poor vertex device was only 10 dB.

 

Fig. 5 (a) Schematic diagram of Mach-Zehnder device comprising of two cascaded mode sorting devices for the experimental characterization of the mode sorting efficiency, (b) optical interference signal for the device etched by using a metal mask, and (c) the output signal from the sample etched by using a photoresist mask.

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To characterize the device, a superluminescent laser diode with a center wavelength of 1550 nm and a bandwidth of 60 nm was used along with a polarization controller to adjust the TE polarization. The reflected signal at the output port was directly monitored with an optical spectrum analyzer and the signal returning to the input port was measured using an optical circulator. The input light evolved into the odd mode at the two-mode waveguide section and then the tilted Bragg grating produced the even mode reflection, which further propagated into the wide waveguide connected to the output port. This ideal wavelength filtering was produced in a sequence of narrow waveguide input, odd mode evolution, even mode reflection, and wide waveguide output, which can be defined as the Narrow-Odd-Even-Wide (NOEW) reflection. In the measured spectrum shown in Fig. 6, the NOEW reflection peak appeared at 1547.2 nm. Meanwhile, due to the crosstalk in mode sorting and undesired reflection by the tilted grating, several other reflection peaks were also observed at wavelengths of 1543.8 nm, 1547.2 nm, and 1551.0 nm. The wavelength peak at 1551.0 nm was due to the narrow-even-even-wide (NEEW) conversion and it appeared at the same output port with the NOEW reflection peak. The side mode suppression ratio (SMSR) of the output signal was −23.5 dB because of the NEEW reflection peak. The peaks at 1543.8 nm and 1547.2 nm were measuredfrom the input port with a circulator and they were due to the narrow-odd-odd-narrow (NOON) conversion and the narrow-even-odd-narrow (NEON) conversion, respectively.

 

Fig. 6 Reflection spectra measured from the output port and the input port using a circulator, and the transmission spectrum of a channel-drop filter. The strongest reflection peak caused by the NOEW conversion is important for channel dropping, while other reflection peaks also appear due to the NEEW, NOON, and NEON conversions.

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The insertion loss for the NOEW conversion was 6.0 dB. In the cut-back measurement, the propagation loss was measured to be 0.4 dB/cm. Due to the significant difference of the mode field diameters of the polymer waveguide (4.3 μm) and the optical fiber (10.5 μm), the coupling loss became 2.3 dB/facet. An additional loss by the mode evolution and the tilted grating reflection was only about 1.0 dB. The small peaks on the side of main peaks could be reduced by incorporating an apodized grating [20].

By applying electrical power on the thin film heater, the reflected wavelength was tuned, as shown in Fig. 7(a). As the tuning range was increased over 10 nm for an applied thermal power of 312 mW, the reflection peak gradually decreased. This can be explained by the reduced confinement of the odd mode under the electrode. TO coefficient of ZPU polymer is −1.8 × 10⁻⁴, then the temperature change at the waveguide core becomes 51.6 °C for 10 nm tuning. The tuning efficiency was 32 nm/W as shown in Fig. 7(b). Heater damage was not observed until the applied power increased over 800 mW. The tuning efficiency could be improved by placing the heater under the optical waveguide and also by introducing an air trench structure [21].

 

Fig. 7 (a) Reflection spectra tuned by applying thermal power and (b) peak wavelength as a function of the applied thermal power.

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4. Conclusion

A channel-drop filter based on polymer waveguides has been proposed and demonstrated by using a tilted Bragg grating and a mode sorting waveguide. The polymeric tilted grating, demonstrated for the first time in this work exhibited a good reflection of the input odd-mode into an output even-mode. The maximum reflectivity was 95% for a 6-mm long tilted Bragg grating. The mode sorting polymer waveguide with a sharp vertex was fabricated by using a metal mask. Consequently, a crosstalk of less than −20 dB was obtained with a good fabrication tolerance. The device was tunable over 10 nm by applying an electrical power of 312 mW. The tuning range and the power consumption can be improved significantly in terms of the optimization of the electrode structure that has been reported elsewhere. The proposed polymeric tunable channel drop filter has a simple device structure enabled by the specialty of the polymer material, and then the device has strong competitiveness in realizing compact and low-cost channel-drop filters for dense WDM communication systems.