1. Introduction

In optical communication systems supporting the rapidly expanding 5G communication networks, the wavelength division multiplexing (WDM) technology has evolved to use 48 wavelength channels in the C-, L-, and O-bands, with 100 GHz spacing and transmission speed of 10 Gbps per channel [1]. In such systems, tunable wavelength lasers based on polymer waveguide Bragg gratings have been deployed, owing to the advantages of wide tuning range and simple device structure [2–5]. The optical signals carried by different wavelengths, generated by the tunable wavelength lasers, are multiplexed and then transmitted through a single optical fiber based on WDM technology. WDM signals are then demultiplexed at the receiver site using wavelength filters such as thin-film filters and arrayed waveguide gratings.

In the early WDM optical communication systems with few channels, thin-film filters were commonly used to filter a certain wavelength from the multiplexed wavelength signals [6]. Thin-film filters are easy to use owing to their simplicity and high yield; however, as the demands on the number of wavelength channels increase, the filters require a large number of dielectric film layers to reduce the bandwidth for narrower channel spacing. Therefore, their production yield was decreased because of the local film thickness variation and density fluctuation. Arrayed waveguide grating (AWG) devices, which can demultiplex different wavelength signals onto many output fibers, are better suited for systems handling many wavelength channels [7]. However, because the AWGs split the signals spatially, if the input wavelength is changed by a tunable laser, the output fiber must be reconfigured manually to other paths. Tunable wavelength filters (TWFs) are devices that can filter one wavelength signal, selectively, without spatial demultiplexing. Even if the transmitting wavelength is changed on purpose by a tunable transmitter, a certain wavelength can be filtered and detected without reconfiguring output paths.

Among the various TWFs, the simplest configuration consists of a fiber Bragg grating and piezoelectric transducer (PZT). However, this configuration is not appropriate for a compact device, and the PZT often exhibits long-term reliability issues [8]. The ring resonator in silicon optical waveguides provides a compact device structure [9]; however, it has a narrow free spectral range and several ring resonators will have to be cascaded to filter a single wavelength from the WDM signal [10]. High-speed TWFs have been demonstrated in lithium niobate optical waveguides based on the acousto-optic effect [11]; however, they do not satisfy the requirements of WDM wavelength filters.

Compared to other technologies, polymeric optical waveguides have large index tunability owing to the high thermo-optic (TO) effect, as well as strong heat confinement. Therefore, a tunable Bragg reflector made of polymeric material can tune the reflection wavelength over 40 nm, with a simple device structure facilitating high-yield production [12]. Variable optical attenuators and digital optical switches exploiting the high TO effect of polymeric waveguide have also been commercialized [13–15].

Thus far, the TWF research of our group had focused on achieving narrow bandwidth, large side mode suppression ratio (SMSR), and low insertion loss. Apodized Bragg gratings were produced by using a shadow mask to reduce the bandwidth and improve the SMSR [16]. Although the apodized grating enhanced the device performance significantly, it required an external circulator to receive the reflected signal. For reducing the device form factor by discarding the external circulator, we adopted a tilted Bragg grating along with an asymmetric Y-branch waveguide [17]. In our early demonstration, the bandwidth and SMSR were not satisfied because of unexpected mode couplings.

In this work, we propose a novel TWF based on cascaded tilted Bragg gratings, to satisfy the requirements of 5G WDM optical communication systems accommodating 16 wavelength channels with 100 GHz spacing. For this purpose, the TWF should have a tunable range covering the entire wavelength, 20 dB bandwidth narrower than 1.6 nm, and SMSR exceeding 25 dB. Such ideal performance has not been reported yet. The proposed cascaded device can improve the SMSR by suppressing the undesired mode couplings and reduce the 20 dB bandwidth through the use of double-reflection Bragg gratings. We designed a cascaded tilted Bragg grating device, optimized the fabrication procedure for the polymeric optical waveguide device, and demonstrated that the TWF could satisfy all the requirements as the dense WDM wavelength filters.

2. Design of two-stage cascaded tilted Bragg grating tunable filters

In the Bragg grating waveguide device, the filtered wavelength signal returns to the input port. Hence, it is necessary to use a circulator to redirect the reflected signal to a detector. However, using a tilted Bragg grating along with a mode-sorting asymmetric Y-branch, as shown in Fig. 1(a), a tunable filter that does not require an external circulator is constructed. The tilted grating produces coupling between the odd and even modes of the waveguide. The input light launched through a narrow waveguide evolves onto the odd mode at the tilted grating, which is then reflected back as an even mode, which evolves onto a wide waveguide in the returning path. In an ideal case, the light signal transforms in a sequence of narrow waveguide mode, odd mode, even mode, and wide waveguide mode, and we define this sequence as a NOEW mode conversion sequence. However, owing to fabrication errors, spurious mode couplings are also produced to degrade the SMSR and crosstalk [18–21]. In order to improve the device performance, a cascaded tilted Bragg grating device, as shown in Fig. 1(b), is investigated in this work. The reflected signal from the first stage is inserted to the narrow waveguide of the second asymmetric Y-branch through a waveguide taper. Then, the second tilted grating reflects the odd mode input onto an even mode output that will evolve through the wide waveguide connected to the output port.

 

Fig. 1. (a) Tunable wavelength filter (TWF) consisting of tilted Bragg grating and asymmetric Y-branch of narrow and wide waveguides, and (b) two-stage cascaded TWF proposed in this work.

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In the design of polymeric waveguide, core and cladding materials with refractive index of 1.455 and 1.430, respectively, were used. The large difference in refractive index is advantageous for obtaining high reflectance in the Bragg grating and small bending radius for reducing the chip size. In a rib-type waveguide with a core thickness of 2.5 µm and a lateral core layer thickness of 0.5 µm, a width of waveguide less than 4 µm satisfies the single-mode condition, as shown in Fig. 2(a). The waveguide over the tilted Bragg grating has a width of 8 µm so as to support both odd and even modes. In the asymmetric Y-branch waveguide, the mode-evolution crosstalk was calculated by using the beam propagation method, which resulted in less than −30 dB of crosstalk for a device structure with a branch angle 0.3°, narrow waveguide width of 3 µm, and wide waveguide width of 4 µm. The crosstalk increased as the angle increased, because of the scattering into higher-order modes.

 

Fig. 2. Design results of the proposed device: (a) effective indices of the modes calculated as a function of waveguide width, (b) reflectivity of the tilted Bragg grating obtained by considering the even–even (EE) and odd–even (OE) mode couplings in the single-reflection device and cascaded double-reflection device, (c) reflection spectra of the TWF produced by the EE and OE mode coupling in the single and double-reflection devices, and (d) bandwidth of the TWF obtained from the reflection spectrum.

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The transmission matrix method was used to calculate the reflection spectrum of the tilted Bragg grating. The reflectivity of a unit element of the tilted grating was obtained by considering the mode overlap integral between the even and odd modes with different mode profiles. The crosstalk of asymmetric Y-branch was reflected in the calculation, and it allowed even mode to be produced by the incomplete adiabatic mode evolution. The reflected powers produced by the ideal narrow-odd-even-wide (NOEW) and spurious narrow-even-even-wide (NEEW) mode conversions were added to obtain the final reflected spectrum. The effect of narrow-odd-odd-wide (NOOW) and narrow-even-odd-wide (NEOW) mode conversions were negligible. A detailed explanation on the various mode-conversion processes could be found in our earlier publication [20]. When the grating had a thickness modulation of 0.2 µm, from a core thickness of 3.0 µm, the effective index was modulated by 0.0008. The reflection spectrum from a 5 mm long tilted grating was obtained as shown in Fig. 2(b). Compared to a single reflection device, the double-reflection cascaded tilted grating provided a greatly enhanced SMSR, much narrower bandwidth, and lower adjacent-channel crosstalk. However, the reflectivity of the double-reflection device was smaller than that of single-reflection device, as shown in Fig. 2(c). To enhance the reflectivity, it was necessary to increase the length and modulation depth of the grating. When the reflectivity of the grating was increased to achieve higher reflectivity, the bandwidth of the reflection spectrum was also affected as shown in Fig. 2(d). However, the double-reflection cascaded tilted grating device could produce a 20-dB bandwidth of 1.6 nm even for high reflectivity. Hence, the crosstalk to the adjacent channel with a channel spacing of 0.8 nm will be maintained below −20 dB.

3. Fabrication of a polymeric waveguide Bragg grating device

Fluorinated acrylate-based low-loss polymer materials produced by ChemOptics Co., ZPU13-455 (n = 1.455 @ 1550 nm) and ZPU13-430 (n = 1.430 @ 1550 nm), were used as the core and cladding layers, respectively. The schematic of the fabrication process is summarized in Fig. 3. The residual surface oxide on the silicon substrate was removed and oxygen plasma treatment was applied for five seconds. ZAP1020 was coated to improve adhesion. The lower cladding material of ZPU13-430 was spin coated, UV cured in a nitrogen chamber, and then hard baked for an hour on a hot plate at 150 °C, following which the cladding thickness became 8.4 µm.

 

Fig. 3. Schematic of fabrication procedure of the proposed two-stage cascaded tunable wavelength filter (TWF).

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For fabricating the Bragg grating, AZ-MIR-703 photoresist diluted with PGMEA was spin coated to 400 nm thickness. To fabricate the grating over a large area, a holographic interference pattern of a 442 nm He–Cd laser was collimated to a diameter of 8 cm. The beam was irradiated onto the mirror and the sample target area, and then the sample was shifted and exposed once again. After developing the photoresist, the Bragg grating was created over a wide area, as shown in Fig. 4(a). The prepared grating had a thickness of 400 nm and period of 538 nm, as shown in the scanning electron microscopy (SEM) image in Fig. 4(b). To transfer the grating pattern to the lower cladding layer, oxygen plasma dry etching was performed for 15 s, following which the grating pattern was inscribed to a depth of 236 nm, as shown in Fig. 4(c).

 

Fig. 4. (a) Photograph of the Bragg grating fabricated using the holographic interference method for producing wide-area grating pattern. (b) Scanning electron microscopy (SEM) image of the grating made of photoresist and (c) SEM image of Bragg grating inscribed on the lower cladding layer after oxygen plasma dry etching.

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The core polymer of ZPU13-455 was coated over the grating pattern to have a thickness of 2.5 µm. AZ5214 photoresist was coated for waveguide patterning. The optical waveguide was aligned with the Bragg grating at an angle of 86.3°. For the precise alignment, we prepared an alignment key made of SU-8 before coating the lower cladding. A rib waveguide with a core height of 2.5 µm and lateral cladding thickness of 0.5 µm was formed through oxygen plasma dry etching. The cladding polymer was coated over the waveguide pattern, and a polymer waveguide of total thickness 17 µm was produced. A microheater was formed by depositing Cr–Au of thickness 10–100 nm, patterning with AZ5214 photoresist, and wet etching. Finally, the sample was diced and polished for optical coupling.

4. Characterization of the two-stage cascaded TWF

The fabricated device was aligned between the high NA optical fibers to achieve low coupling loss. The reflection spectrum of the fabricated TWF was measured using a superluminescent diode with a center wavelength of 1550 nm and 3-dB bandwidth of 60 nm. The reflectivity was obtained by subtracting the reference transmission spectrum measured by aligning the optical fibers without the device. TE-polarized light was incident on the narrow optical waveguide of the TWF device, as shown in Fig. 5(a). After the double reflections in the cascaded device, the output light was observed by the optical spectrum analyzer, as shown in Fig. 5(b), in which the bandwidth was too narrow to be considered as a proper Bragg reflection, and the insertion loss was larger than expected. To find the reason of narrow bandwidth, we measured the signals reflected from ports A and B, as indicated in Fig. 5(a). Ports A and B are two-mode waveguides so that odd and even modes can be excited simultaneously by adjusting the position of the optical fiber laterally, and measure the reflected signals cross-coupled to the other modes, as shown in Fig. 5(c). The two reflection spectra exhibited slight differences in the peak wavelength, which indicated that the two gratings were not identical. To adjust the difference in the Bragg wavelength, a bias power of 12.6 mW was applied to the microheater of the first stage. As a result, the output spectrum was modified as shown in Fig. 5(d), and the insertion loss was improved to 3.4 dB. The 0.5 dB, 3 dB, and 20 dB bandwidths were measured as 0.28 nm, 0.51 nm, and 1.05 nm, respectively.

 

Fig. 5. (a) Reflection spectrum measurement setup. (b) Initial output reflection spectrum of the two-stage cascaded tunable-wavelength filter (TWF). (c) Reflection spectra measured through ports A and B, as indicated in the figure, in which the two reflection spectra are not overlapping. (d) Output reflection spectrum of the device after slight bias power application at the first Bragg reflector.

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While maintaining the initial bias power, the heating power on both electrodes was increased by 15.8 mW in each step to produce a wavelength tuning of 0.8 nm per step. The reflection spectra were measured as shown in Fig. 6(a). The tuning range was limited to 12 nm before the reflection peak decreased. The SMSR was maintained at over 30 dB, and the adjacent channel crosstalk was in the range of −21.4 dB to −27.6 dB. In the spectrum of wide tuning, SMSR was degraded by the presence of adjacent side lobe caused by the other polarization, which should be improved by reducing the polarization dependence. When maximum tuning was achieved by applying 206 mW, the core temperature was increased by 56.8 °C, according to the calculation, with the TO coefficient of the ZPU polymer as −1.8 × 10⁻⁴. The peak wavelength of the reflection spectra, with respect to the applied power, was obtained as shown in Fig. 6(b), and the tuning efficiency became 54 nm/W. In the case of large tuning, a reflection peak decrease was observed, which was due to the temperature gradient between the heater and substrate and resultant refractive-index gradient. The odd mode, in particular, had lower confinement than the even mode and became more vulnerable to the refractive-index gradient; thus it could radiate easily for larger thermal tuning. To increase the tuning range, in our next experiment, a bottom-electrode structure will be adopted, in which the temperature gradient will be negligible [22]. The polymer device should be packaged with a thermo-electric cooler to maintain the substrate temperature during the device operation.

 

Fig. 6. (a) Reflection spectra obtained by applying the heating power on the integrated microheaters in each step and (b) peak wavelength of the reflection spectra for the applied thermal power.

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The device should provide polarization independent operation to be useful for the practical application. The polarization dependence of Bragg reflection filter was demonstrated in our previous publication [23], in which LFR polymer was used. LFR polymer was developed for reducing the absorption loss of the polymer by increasing the amount of fluorine compounds. In addition to that, the polymer was designed to have low volume shrinkage during the UV curing, which results in a very low optical birefringence. The device made of LFR polymer exhibited wavelength dependence of 0.08 nm and polarization dependent loss of 0.1 dB. However, the current device was fabricated by using ZPU polymer instead because LFR is not processable in our lab, then the device showed somewhat higher polarization dependence of 0.2 nm.

5. Conclusion

TWF suitable for 5G WDM optical communication systems, which required narrow bandwidth with a flat-top passband, wide wavelength tuning range, large SMSR, and low adjacent-channel crosstalk, was demonstrated in this work. Because of these strict requirements, it was difficult to find a technology that could satisfy these specifications. The two-stage cascaded tilted Bragg grating device proposed in this work, for the first time in our knowledge, satisfied all these requirements. The fabricated device exhibited 0.5 dB bandwidth of 0.28 nm, 20 dB bandwidth of 1.05 nm, SMSR of 35 dB, and adjacent-channel crosstalk of −25 dB. The tuning range was limited to 12 nm because of the thermal gradient created by the top electrode, which could be improved greatly by adopting a bottom-electrode structure. The proposed polymeric TWF can be integrated with polymeric tunable lasers, which have already been commercialized, to produce tunable transceivers that can be used in the next-generation WDM communication systems.