1. Introduction

Tunable optical wavelength filters are one of the essential components for wavelength division multiplexed optical communication system [1, 2]. Various methods have been investigated to produce tunable wavelength filters based on thin film filters [3–5], Fabry-Perot cavity devices [6, 7], and fiber Bragg gratings [8, 9]. The thin film filter has the merits of thermal stability and good reproducibility; on the other hand, it requires a large thermal tuning power and has slow tuning speed. The Fabry-Perot device provides a narrow bandwidth and low polarization dependence compared to the thin film filter; however, mechanical instability and hysteresis degrade its overall long-term stability. Lastly, the fiber grating device is rather bulky and has a small tuning range, but its reflection spectrum is close to an ideal wavelength filter.

Polymer is highly efficient in thermally tuning its refractive index. Therefore, widely tunable wavelength filters can be demonstrated by thermo-optic tuning of polymeric Bragg reflector waveguide, and its long-term reliability was verified [10–14]. The polymer Bragg reflector has single electrode for wavelength tuning compared to the sampled grating device which requires at least 3 control electrodes [15]. It becomes important in the implementation of WDM system. The simple direct refractive index tuning in polymer waveguides can achieve the wavelength tuning range over 30 nm, which is not possible in Bragg reflectors based on other material [16].

In an ordinary Bragg reflection device, the filtered signal is reflected backward to the input optical waveguide. Hence, an external magneto-optic circulator is necessary to separate the filtered signal from the input waveguide. It increases the size of the device and the cost. To extract the filtered signal into another output port, multimode interference coupler or a directional coupler was incorporated along with Bragg reflectors [12, 13, 17]. However, because of strict fabrication tolerance, experimental demonstration of such a device with good performance has not been reported yet.

In this work, an asymmetric X-junction device is incorporated along with dual Bragg reflectors to facilitate the fabrication of channel-drop filters. The asymmetric X-junction exhibits good mode sorting efficiency with large fabrication tolerance [18]. The channel-drop filter has a 3-dB bandwidth of 0.39 nm and a 20-dB bandwidth of 2.6 nm. The wavelength tuning over 10 nm is achieved with a channel crosstalk less than −20 dB.

2. Design and fabrication of the tunable channel-drop filter

The channel dropping wavelength filter proposed in this work consists of dual Bragg reflectors and an asymmetric X-junction. The asymmetric X-junction operates as a 3-dB coupler based on a mode sorting effect between the wider and the narrower waveguides. Its schematic diagram is illustrated in Fig. 1. When light is launched at one of the input waveguides, it is converted into the even and odd modes at the junction waist with equal intensities. The even mode gradually changes into the wider waveguide mode, while the odd mode gradually changes into the narrower waveguide mode. Tapered waveguides are used to change the width of the narrow and wide waveguides into that of a normal waveguide.

 

Fig. 1 Schematic diagram of channel-drop filters consisting of dual Bragg reflectors and a mode sorting asymmetric X-junction.

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The two Bragg reflectors placed on the normal waveguide section reflect the filtered signal back to the asymmetric X-junction. The relative phase between the two reflected lights is adjustable by the phase control electrode, which in turn determines the propagation direction of the output signal, as shown in Fig. 2. When the two modes are in-phase, the filtered signal propagates to the lower output port. When they are out-of-phase, it propagates back to the upper port used as an input port as shown in Fig. 2(c). The total length of device is 30 mm. The length of X-junction taper structure from the junction waist to the end of wider or narrower waveguide is 3.2 mm, and the waveguide taper changing the width is 500 μm. According to the BPM simulation, when the junction angle is 0.2° and the widths of wide and narrow waveguides are 4 μm and 3 μm, respectively, the inherent loss becomes 0.11 dB.

 

Fig. 2 BPM simulation results of the mode sorting phenomenon on the asymmetric X-junction; (a) mode evolution for even and odd mode excited at the junction waist, respectively, (b) the crosstalk calculated as a function of junction angle for the even mode excitation, and (c) mode sorting by the two Bragg reflected modes with in-phase and out-of phase, respectively.

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The mode evolution in the X-junction was simulated by using BPM simulation as shown in Fig. 2(a). In an ideal case, when the even mode is launched at the junction waist, it gradually evolves into the wider waveguide mode. In this case, the crosstalk is defined as the ratio of the optical power output from the narrow waveguide over the wide waveguide. As summarized in Fig. 2(b), a crosstalk of −30 dB was achieved in the device with 3-μm narrow waveguide, 4-μm wide waveguide, and a junction angle less than 0.2°. For a smaller junction angle, a lower crosstalk could be obtained. As shown in Fig. 2(b), for a wide range of waveguide width variation, the crosstalk remains to be below −20 dB, which represents the large tolerance in fabrication error of the waveguide widths. The asymmetric X-junction based on adiabatic mode evolution has no significant wavelength dependence as long as the waveguide maintains the single mode condition.

To produce high reflectivity of the Bragg reflector for a short grating length and a compact device size, polymeric materials with a refractive index contrast of 0.02 were used. The dimension of polymer waveguide was chosen as 3.5 μm × 2.5 μm for single mode condition, which was subjected to a large fiber coupling loss. A mode size converter will be required in an optimized device for practical use [19].

ZPU polymers from ChemOptics Co. were used for this experiment. Fabrication procedures of our devices were schematically illustrated in Fig. 3. ZPU with a refractive index of 1.435 was spin-coated on a silicon wafer as a lower cladding. Bragg grating was defined by using a laser interferometry with TSMR photoresist, and the interference pattern was transferred onto the lower cladding layer by using an oxygen plasma etching [20]. Core layer of 2.5 μm thick was formed by spin coating of another ZPU polymer of 1.455. Rib type waveguide was formed via photolithography with AZ5214 photoresist and dry-etching. The upper cladding was spin-coated to produce a cladding of 8 μm over the rib. Phase control electrode and two wavelength tuning electrodes were fabricated. SEM photographs of the fabricated device is shown in Fig. 4 for the top view of (a) the asymmetric X-junction and a bird’s eye view of (b) the Bragg grating.

 

Fig. 3 Schematic fabrication procedures of polymer waveguide channel-drop filters.

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Fig. 4 SEM photographs of the fabricated device: (a) the asymmetric X-junction and (b) the Bragg grating. The two dotted-circles in (a) indicate the parts of asymmetric X-junction magnified in the insets.

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3. Characteristics of the tunable channel-drop filter

To measure the characteristics of mode sorting X-junction, we combined two asymmetric X-junction with a mirror symmetry to form a Mach-Zehnder (MZ) interferometer as shown on the inset of Fig. 5(a). A distributed feedback laser of 1550 nm was used as a light source. For the applied phase control voltage signal, the measured intensity of the output signal is shown in Fig. 5(a) and 5(b). For the device with a junction angle of 0.3°, the extinction ratios of the upper and lower ports were 20 dB and 15 dB, respectively. More precise fabrication of asymmetric X-junction structure would be important to reduce the crosstalk further. For π phase shift in MZ interferometer, the refractive index changes by 1.93 × 10⁻³ for a phase controller length of 400 μm. The applied thermal power for π phase shift was 3.64 mW, then the refractive index tuning efficiency became 0.212 × 10⁻³ /W/m for a unit heater length.

 

Fig. 5 Experimental characterization of the mode sorting device using a Mach-Zehnder device consisting of two cascaded X-junction device, as depicted in the inset of (a): for the applied signal on the phase modulator shown in (a), the optical output signal, shown in (b), exhibited large extinction ratio by efficient mode sorting with a low crosstalk as −20 dB.

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To evaluate the channel-drop filter, a superluminescent laser diode with a center wavelength of 1550 nm and a bandwidth of 60 nm was used. The input polarization was maintained as TE polarization throughout the measurement. The transmission and reflection spectra of the Bragg reflector were measured, as shown in Fig. 6(a). The initial Bragg reflection peak was located at 1543.97 nm. The insertion loss was 6.7 dB which includes the fiber coupling loss, propagation loss, mode evolution loss by asymmetric X-junction, and mode conversion loss by taper structure. The fiber coupling loss by mode mismatch is calculated as 4.5 dB. The total propagation loss is 1.2 dB for the total device length of 3 cm assuming the propagation loss of 0.4 dB/cm in this high contrast waveguide. Therefore, the excess loss by mode evolution loss and mode conversion loss is 1.0 dB. It may be reduced by incorporating a high NA fiber with a small mode size or a mode size converter in the polymer waveguide. The transmission dip was −12 dB relative to the insertion loss level of the device and the maximum reflectivity was about 94% for a 6-mm long Bragg grating. The 3-dB and 20-dB bandwidths of reflection spectrum were 0.39 nm and 2.6 nm, respectively. The side mode suppression ratio was less than −20 dB. The small dip in the transmission spectrum was caused by scattering of the guided mode into the higher order modes.

 

Fig. 6 Measurement results of channel-drop filters: (a) Transmission and reflection spectra of the Bragg reflector, (b) reflection spectra tuned by applying thermal power, and (c) peak wavelength shift as a function of the applied thermal power.

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For tuning the Bragg reflector, the two heaters were connected to supply equal amounts of heat on the two Bragg reflectors. For an applied power of 377 mW, the filterd wavelength was tuned by 10 nm as shown in Fig. 6(b). The peak wavelength was shifted linearly with a tuning efficiency of 28.7 nm/W as shown in Fig. 6(c). The refractive index tuning efficiency was calculated as 0.197 × 10⁻³ /W/m, which was close to that of the phase controller. The efficiency can be improved by modifying the position of heater or by incorporating the air-trench structure, which improves the uniformity of thermal distribution and increases heat isolation efficiency. The tuning range may be extended by incorporating other fluorinated polymers with the larger TO coefficient, such as LFR series polymers available from ChemOptics.

4. Conclusion

A channel-drop filter based on polymer waveguide has been proposed and demonstrated by using dual Bragg reflectors and an asymmetric X-junction waveguide. The maximum reflectivity of Bragg reflectors was 94% for a 6-mm long Bragg grating. The 3-dB and 20-dB bandwidths were 0.39 nm and 2.6 nm, respectively. The side mode suppression ratio was less than −20 dB. The crosstalk in the mode sorting asymmetric X-junction was less than −20 dB. The wavelength tuning range was over 10 nm for 377 mW of thermal power, and the wavelength shifted linearly with a tuning efficiency of 28.7 nm/W. Without using an external circulator, the proposed device will be useful in realizing compact and low-cost channel-drop filters for WDM communication system.