1. Introduction

Optical communication systems have evolved to use multiple channels with independent wavelength carriers to increase their capacity for data transmission [1, 2]. When a wavelength-division-multiplexed (WDM) optical signal arrives at its destination, it is necessary to demultiplex or extract a certain wavelength by using a wavelength filter [3]. Among many experimental results, there are several results drawing attention for the practical WDM system application. Thermally tunable silicon ring resonator devices have exhibited tuning over a free spectral range of 11 nm [4]. The ring resonator was incorporated in a Mach–Zehnder interferometer to increase the tuning range [5]. By using three cascaded ring resonators, the side-mode suppression ratio (SMSR) was increased to 40 dB and a flat passband was obtained in the transmission spectrum [6].

A silicon photonic wavelength multiplexer was demonstrated for dense optical interconnections with a tuning range of 2.5 nm and a 3-dB bandwidth of 0.4 nm [7]. A widely tunable MEMS device was reported with a tuning range of over 1000 nm but with a wide bandwidth [8]. Guided-mode resonant thermo-optic tunable filters were demonstrated with a tuning range of 15 nm and a bandwidth of several nm [9]. Even after various previous research works, it is still hard to find a low-cost compact tunable filters commercially available. The tunable filter required for WDM applications should have strong reflectivity and a proper bandwidth to filter a certain wavelength from 0.8-nm spaced WDM signal

Compared to the previous technology, polymeric Bragg grating devices based on a fluorinated acrylate polymer have demonstrated important progress. Because polymer material has a highly efficient refractive index tuning capability based on its large thermo-optic (TO) effect and low heat conductivity, a tunable Bragg grating made of polymer material could achieve a tuning range of over 30 nm based on a simple device structure [10–12]. The tuning range was recently further extended through the optimization of the thermal distribution [13].

In this work, to produce a narrow-bandwidth wavelength filter, an apodized grating structure is incorporated in the polymer waveguide Bragg grating device. The apodized grating was initially demonstrated in fiber grating to reduce the side lobe of uniform grating, in which the index modulated volume grating was apodized by controlling the laser intensity [14, 15]. Apodized grating was also demonstrated in terms of volume holographic polymer Bragg grating by locally varying the intensity of the laser beam [16]. However, compared to the index modulated volume holographic grating, a surface relief polymeric grating can be applied to a broader range of materials which does not require photosensitivity. Hence, to form the apodized structure in polymeric surface relief gratings, we propose a shadow mask method during the plasma etching of the polymer grating pattern. Then, the depth of the surface relief grating could be gradually varied along the propagation direction. A reflection spectrum with a suppressed side lobe and a flat top passband is demonstrated along with a wide tuning range.

2. Design of apodized grating imbedded in polymer waveguide

The reflectivity and bandwidth of the Bragg grating is dependent on the length of the grating as well as the effective index change, determined by the grating depth of the surface relief type grating. The reflection bandwidth of the uniform grating is proportional to the coupling coefficient between the forward and backward propagating waves. Then, the reflection bandwidth is roughly the reciprocal of the penetration depth along the grating length [17]. Hence, for a long grating with a small index change, narrow bandwidth grating is achievable. However, the long length of grating may cause broadening of the reflection spectrum because of the effective index variation along the waveguide affected by the waveguide dimension change. Moreover, it is difficult to produce uniform heat distribution by using a thin film heater to tune the reflection wavelength. Hence, it is necessary to reduce the length of the grating by increasing its index modulation. However, in a uniform grating, the increased index modulation will degrade the bandwidth in the reflection spectrum, which will cause channel crosstalk problem.

To obtain a high reflectivity grating with a narrow bandwidth, in this work, an apodized grating is incorporated as shown in Fig. 1. The grating is inscribed between the core and cladding layers. As the depth of the surface relief grating changes gradually along the propagation direction, the reflectivity of the grating also changes gradually. A metal heater is placed over the grating pattern for the TO tuning of the Bragg reflection wavelength.

 

Fig. 1 Schematic diagram of tunable filters that incorporate an apodized surface relief grating.

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To design the apodized Bragg grating, the transmission matrix method was utilized along with an effective index calculation. For various grating structures, the reflection spectra were calculated as shown in Fig. 2. In the uniform grating device of Fig. 2(a), the effective index difference obtained by modulating the grating depth was 5 × 10⁻⁴, and a reflectivity of 95% was obtained for a grating length of 4 mm. The 3-dB bandwidth was 0.44 nm to be suitable as a WDM filter, while a 20-dB bandwidth was too wide to be accepted because it will cause significant crosstalk between adjacent wavelength channels separated by 0.8 nm.

 

Fig. 2 Design results of the reflection spectra for various index modulation profiles: (a) uniform grating, (b) Gaussian apodized grating with a gradually changing index modulation, and (c) combination of uniform grating with a length L_u and Gaussian apodized grating with a length L_a.

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In the results for the Gaussian apodized grating as shown in Fig. 2(b), the 20-dB bandwidth was reduced significantly; however, the length of grating to achieve 95% reflectivity became longer than 10 mm, which could cause nonuniform index modulation during the TO wavelength tuning. Hence, to reduce the length, we propose a modified grating structure as shown in Fig. 2(c). The proposed structure is a combination of a uniform grating of length L_u and an apodized grating of length L_a. The structure could be implemented by using a shadow mask during the grating etching process. To optimize the reflectivity and bandwidth for a short grating length, for various combinations of L_u and L_a, the reflectivity and bandwidths of the reflection spectrum were calculated as shown in Fig. 3. Longer values of L_u provided higher reflectivity for a given grating length, and the bandwidth became narrower for longer values of L_a. For a combined grating with L_u = 0.8 mm, L_a = 2.5 mm, and a total length of 6.6 mm, the proposed grating device exhibited a reflectivity of 96.6% with 3-dB and 20-dB bandwidths of 0.43 nm and 0.88 nm, respectively. The reflection spectrum of the proposed grating device is shown in Fig. 2(c). The proposed surface relief grating is not an ideal apodized structure because the average effective index of grating is changed proportionally to the apodization profile [18, 19]. Hence, the reflection spectrum exhibits a side lobe on one side. The side lobe could be further suppressed by the phase-shifted apodization and the phase modulation coding [20, 21].

 

Fig. 3 (a) Reflectivity and (b) bandwidth of the combined gratings calculated for various uniform grating length L_u and Gaussian apodized grating length L_a.

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The polymer waveguide is designed with low-loss fluorinated polymers with refractive indices of 1.455 and 1.430 for the core and cladding layers, respectively. When the contrast is large, a small modulation depth of the surface relief grating produces a large effective index change and strong reflectivity. To design a single-mode polymer waveguide, the effective index method was carried out, and the results are shown in Fig. 4(a). The thickness of the waveguide core was 2.5 μm with an effective index of 1.44475, and the remaining core layer thickness at the lateral cladding was 1.3 μm resulting in an effective index of 1.43775. Then, using the two effective indices, the final effective index was calculated as a function of the waveguide width, as shown in Fig. 4(b). From these results, one can observe that the waveguide strictly supports the single mode. Moreover, with a small core thickness variation for a grating depth of 200 nm, one can obtain an effective index change of 1 × 10⁻³, which is sufficient for obtaining a highly reflective grating for a short device length.

 

Fig. 4 High-contrast single-mode waveguide design results based on effective index calculation.

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3. Fabrication and characterization of the tunable filter

To fabricate the tunable wavelength filter with an apodized grating, two kinds of ZPU polymers from ChemOptics Co. with a refractive index contrast of 0.025 were used for core and cladding materials. A rib-type polymer waveguide was fabricated with a core dimension of 4 μm × 2.7 μm and a planar guide thickness of 1.3 μm. The fabrication procedure is outlined in Fig. 5. First, the lower cladding material with a refractive index of 1.43 was coated on a silicon wafer. On the lower cladding, a Bragg grating with a period of 546.8 nm was defined by using laser interferometry with a TSMR photoresist [22]. To produce the apodization of the grating depth, a shadow mask was covered on the grating area during the grating pattern transfer onto the lower cladding through oxygen plasma etching. As shown in Fig. 6, the shadow mask was placed over the sample with a gap controlled by using the spacers, which surrounded the sample to prevent oxygen plasma infiltration from the side. The central area of the grating was etched by 172 nm. The area covered by the shadow mask was exposed to fewer reactive ions, and the etching thickness was gradually decreased deep inside the shadow mask. As a result, an apodized Bragg grating pattern was formed on the lower cladding layer, as shown in the photograph of Fig. 6(c). The grating depth profile in Fig. 6(d) was measured by using AFM along the straight line covered by a shadow mask with an opening width of about 2.5 mm. The measured depths were fitted with the envelope function shown in Fig. 2(c) with L_u = 3.3 mm and L_a = 3.0 mm.

 

Fig. 5 Schematic fabrication procedure for polymer waveguide tunable filters with apodized grating.

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Fig. 6 Shadow mask process for producing apodized grating depth during oxygen plasma etching. The mask is placed above the sample with a gap as shown in (a) side view and (b) top view, and (c) shows the fabricated apodized Bragg grating on a quarter silicon wafer. (d) the depths of apodized grating measured across the line corresponding to the mask opening of about 2.5 mm.

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Over the apodized grating pattern, a 2.7-μm-thick core layer was formed by spin coating another ZPU polymer with a refractive index of 1.455, followed by the formation of a rib-type waveguide by using photolithography and dry etching. A ZPU polymer with a refractive index of 1.43 was spin coated to be an upper cladding with a thickness of 9 μm over the rib waveguide. On the upper cladding, for the TO wavelength tuning, a micro-heater was fabricated. The sample was finished by dicing the end facet and attaching single-mode fibers for characterization.

To characterize the fabricated tunable filter, a superluminescent laser diode with a center wavelength of 1550 nm and a bandwidth of 60 nm was used. The input polarization was adjusted to TE polarization throughout the measurement. A signal reflected by the apodized grating was passed through a circulator and measured with an optical spectrum analyzer (OSA). The transmission and reflection spectra are shown in Fig. 7. The Bragg reflection peak was initially located at 1576.7 nm. For a 7-mm-long apodized Bragg grating, the reflectivity was approximately 95%. The 3-dB and 20-dB bandwidths of the reflection spectrum were 0.36 nm and 0.72 nm, respectively. The side-mode suppression ratio was lower than −25 dB, and the width of the flat-top passband defined as a 0.5-dB bandwidth was 0.18 nm.

 

Fig. 7 Transmission and reflection spectra of the apodized tunable filter, where the reflection spectrum has a 3-dB bandwidth of 0.36 nm, a 20-dB bandwidth of 0.72 nm, and a flat-top 0.5-dB bandwidth of 0.18 nm.

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To measure the tunability of the wavelength filter based on the thermo-optic effect, a thin film heater was connected to a power supply, and the reflection spectra were measured by increasing the heating power step by step as shown in Fig. 8(a). The tuning range was over 14 nm for a heating power of 509 mW. The shape of the reflection spectrum was maintained throughout the tuning, while a slight broadening was observed as the tuning range increased to over 10 nm. This occurred because the heater placed on top of the sample produces a steep gradient of heat distribution as well as a refractive index gradient. The heat gradient problem could be eliminated by placing the heater under the waveguide, and the tuning range could be greatly extended [14]. As shown in Fig. 8(b), the peak wavelength of the tunable filter was linearly proportional to the heating power, with a tuning efficiency of 27 nm/W.

 

Fig. 8 Wavelength tuning results of apodized tunable filter with a length of 7 mm: (a) reflection spectra and (b) peak wavelength shift as a function of the applied thermal power that exhibits a tuning efficiency of 27 nm/W.

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4. Conclusion

By using a shadow mask process during the fabrication of a polymeric Bragg reflector, we produced an apodized grating device with a high reflectivity and a narrow bandwidth. To obtain high reflectivity from a short grating, which was necessary for integrated optic tunable filters, a combined structure of a uniform and apodized grating device was designed. The proposed Bragg grating produced high reflectivity of over 95% and narrow bandwidths of 0.36 nm for 3 dB and 0.72 nm for 20 dB. By applying thermal power to the integrated heater, the peak wavelength was tuned to over 14 nm for 500 mW without considerable change in the reflection spectral shape during the tuning. The narrow 20-dB bandwidth and the flat-top passband are important characteristics for WDM signal filtering, and the compact tunable wavelength filter based on polymer waveguide technology could be a good solution.