Introduction

In fiber-optic communications, wavelength-division multiplexing (WDM) is used to increase the capacity of links by enabling multiple channels to be transmitted by a single optical fiber. WDM systems were initially designed with fixed channel spacing that varied between 20 nm in coarse WDM (CWDM) to 0.8 and 0.4 nm in dense WDM (DWDM) [1]. However, WDM systems are now evolving to support flexible channel grids to improve spectral efficiency. These novel systems are often referred to as elastic optical networks [2]. Among the key enabling technologies required to implement elastic optical networks are bandwidth and wavelength-tunable filters and switches [2]. Bandwidth-tunable filters have been demonstrated using different technologies such as cholesteric liquid crystal (CLC) mirrors [3], MEMS actuated silicon microtoroidal resonators [4], and integrated silicon-on-insulator (SOI) based structures [5–9].

Various configurations have been proposed to implement bandwidth tunable filters. By tuning the coupling coefficients between ring resonators and bus waveguides, the overall bandwidth of the drop response can be tuned [9]. Also, a combination of ring resonators in a Mach Zehnder Interferometer (MZI) results in a flat-top response with tunable bandwidth [7]. Both of these devices provide limited bandwidth tunability, the former having a maximum bandwidth of 0.7 nm, whereas the maximum achievable bandwidth for the latter is limited by the free spectral range of the ring resonators, in a recently reported demonstration the maximum achievable bandwidth is 1.36 nm [7].

Another approach is to cascade two identical bandpass filters and to adjust the bandwidth of the overall response by tuning the overlap of the response of the two filters. The maximum achievable bandwidth for this configuration is limited to the bandwidth of the two bandpass filters. This configuration has been implemented using high-order ring resonators [5], CLC mirrors [3], and contra-directional grating assisted couplers (CDGAC) [6].

In [10] we proposed an alternative approach to implement bandwidth tunable filters by cascading two band stop filters. The concept was demonstrated using integrated Bragg gratings in a MZI configuration. Using the same concept, we recently implemented an add-drop filter using CDGACs, and reported the experimental results of the passive configuration [11]. In this article we present the design, fabrication, and measurement results of a bandwidth and wavelength tunable SOI filter based on CDGACs, and experimentally demonstrate the bandwidth and wavelength-tunability of the device. Within the wavelength range of 1555 nm to 1573 nm, the transmission bandwidth of the device can be tuned from 1.1 nm to 11.7 nm, which to the best of our knowledge is the largest bandwidth tunability of an integrated filter experimentally reported in the literature. The proposed configuration provides a larger bandwidth tunability than previously reported structures. Moreover, it enables the implementation of elastic optical networks.

Structure

The proposed configuration consists of a cascade of two band stop filters that results in a bandpass response. Figures 1(a) and 1(b) depict two add-drop filters with different drop channel wavelengths (${{\displaystyle \lambda }}_1$ and ${{\displaystyle \lambda }}_3$). The cascade configuration depicted in Fig. 1(c) produces a bandpass response at wavelength${{\displaystyle \lambda }}_2$. The long pass and short pass edges of the bandpass response can be adjusted by tuning the central wavelength of the band stop filters.

 

Fig. 1 a) and b) add-drop filters with different pass bands, c) cascade of two add-drop filters from the thru port to make a bandwidth tunable filter.

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In this design the band stop filters are implemented with CDGACs. Figure 2(a) depicts the top view of a CDGAC. A CDGAC is made of two waveguides of different widths that are in close vicinity, and are patterned with a periodic corrugation [12]. Depending on the period and depth of the periodic corrugations, a certain range of wavelengths couples from one waveguide to the other in the opposite propagation direction. The corrugation on each sidewall for every period consists of a recessed and a protruded section. The width of each recessed and protruded section is $\Delta W/2$, which adds to a total of $\Delta W$corrugation width. The reason for choosing a combination of recessed and protruding sections is to maintain an almost constant effective index for the grating.

 

Fig. 2 a) Top view of a CDGAC, b) Measurement result of the CDGAC with a 150 nm gap between the waveguides and a maximum grating width of 24 nm on the narrow waveguide, and of 50 nm on the wide waveguide, and a period of $\Lambda$ = 312 nm.

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The devices were fabricated on a SOI wafer with a 220 nm thick silicon device layer, a 3.0 µm buried oxide layer and a 2.0-3.0 µm top oxide cladding. The drop, through and add port responses (to excitation at the input port) of a CDGAC with waveguide widths of 450 nm and 600 nm patterned with gratings of 312 nm period on both waveguides is depicted in Fig. 2(b). The grating widths ($\Delta W$ in Fig. 2(a)) are apodized in a Gaussian manner to reduce the side-lobe level in the filter response. The maximum grating width on the narrow waveguide is 24 nm, and it is 50 nm on the wide waveguide. The gap between the two waveguides (g) is 150 nm. The drop port has a bandpass response while the through port has the complementary band stop response, and the crosstalk to the add port is below −20 dB.

The central wavelength of the CDGAC stop band depends on the period and the average effective index of the grating. Therefore, by cascading two CDGACs with sequential but not overlapping responses, a bandpass response is generated. The cascade configuration depicted in Fig. 1(c) is implemented as shown in Fig. 3 by means of two CDGACs. The through port of the first CDGAC is connected to the input port of the second CDGAC, while the add port of the first CDGAC is connected to the drop port of the second CDGAC. As a result, the through port of the overall configuration has a bandpass response. The device is tuned using the thermo-optic effect. Thus, the length of the buffer region between the two CDGAC filters must be large enough to provide thermal isolation between the heater of each filter. This constraint assures that the two CDGAC filters can be tuned independently. In this design the length of the buffer region between the two CDGACs is 100µm Both gratings have 1000 periods. The period of the first grating is 312 nm, which corresponds to a stop band with a central wavelength of 1552 nm, whereas the period of the second grating is 318 nm and creates a stop band centered at 1568.6 nm. To simplify the discussion, in this document we refer to the CDGAC with a period of 312 nm as the short-period grating, and we call the CDGAC with a period of 318 nm the long-period grating.

 

Fig. 3 Top view of the wavelength and bandwidth-tunable filter consisting of cascade of two CDGACs with a buffer region between them.

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The devices were characterized using a tunable laser and a power meter. The laser wavelength was scanned from 1540 nm to 1580 nm to capture the device response. Figure 4 depicts the through and drop port responses of the filter with a maximum grating width of 50 nm on the narrow waveguide and of 100 nm on the wide waveguide, a 150 nm gap between the waveguides, and periods of 312 nm and 318 nm for the CDGACs. The drop port response has two channels centered at 1552 nm and 1568.6 nm. The drop channel with the shorter drop wavelength arises from the short-period CDGAC, while the CDGAC with the larger period (i.e. long-period CDGAC) is associated with the drop channel with the longer central wavelength. The 3-dB bandwidth of the first drop channel is 10.1 nm and that of the second channel is 10.8 nm. The passband of the through response has a 3-dB bandwidth of 5.8 nm centered at 1560 nm with an insertion loss of 2.6 dB and an extinction ratio of 31 dB. By increasing the coupling coefficient of the CDGACs, the width of their stop band and the extinction ratio of their response would also increase. This would lead to an increase in the width of the rejection bands in the through port response. The coupling coefficient can be increased by increasing the grating width ($\Delta W$) or decreasing the gap between the two waveguides (g). Furthermore, by improving the apodization function of the CDGACs, the ripples on the long pass and short pass edges of the band pass response of the through port would decrease and the pass band would be more flat. In order to demonstrate the wavelength and bandwidth tunability of the pass band, metal heaters were fabricated on the devices using a lift-off process.

 

Fig. 4 Through and drop port response of the back to back configuration when no voltage is applied to the heaters.

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Tunability measurements

The fabricated aluminum heaters are 2 µm wide and 300 nm thick. The heaters on both CDGACs are identical except for their length, which is equal to the length of CDGAC. The first step of the tuning measurement was to characterize each grating coupler individually. Figure 5(a) depicts the through port response of the filter when current is circulating only in the heater on the long-period CDGAC. In order to calculate the thermal tunability of the CDGAC, we use a modified temperature dependent Sellmeier equation for the refractive index of silicon [13] in a commercial Eigen mode solver (Lumerical EME solver). Based on the simulation results the spectrum shift per degree of temperature change of the CDGAC is $d\lambda /dT=0.074nm{/}^{0}C$. The equivalent temperature change for different heater current is estimated using this equation, and shown in Figs. 5(a) and 5(b). As the current circulating in the heater increases, the temperature of the CDGAC increases and the long pass edge of the bandpass response shifts toward longer wavelengths. Consequently, the bandwidth and central wavelength of the pass band increases. The fact that the short pass edge of the bandpass response remains unchanged proves that the short-period CDGAC is thermally isolated from the heater on the long-period CDGAC.

 

Fig. 5 a) and b) Through port response when current is only applied to the heater on long(short)-period CDGAC ($\Delta T$is the temperature change at the long(short)-period CDGAC), c) and d) central wavelength (solid) and 3-dB bandwidth (dotted) vs. applied current of respectively the graph in (a) and (b).

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Alternatively, it is possible to pass a current only through the heater on the short-period CDGAC, which shifts the short pass edge of the bandpass response toward longer wavelengths, and decreases the bandwidth of the passband, while the central wavelength of the passband increases (Fig. 5(b)). In this case, the long pass edge of the response is not affected by the heater on the short-period CDGAC, which again is a sign of the isolation between the short-period CDGAC heater and the long-period grating coupler. It can be concluded that the fabricated filter provides the flexibility to independently tune the short pass and long pass edges of the band pass response. The 3-dB bandwidth and central wavelength of the through port response of the cascade structure as a function of applied currents is summarized in Figs. 5(c) and 5(d). Both graphs show that the change in central wavelength and bandwidth has a quadratic relation with the applied current. This is expected since the temperature change is proportional to the power consumption of the heater, which is proportional to $R{I}^2$. According to the measurement results, by providing a current of 80 mA to the heater on the short-period CDGAC the spectrum shifts by 5.14 nm. Considering that $d\lambda /dT=0.074nm{/}^{0}C$, this corresponds to a ${73}^{0}C$temperature change in the waveguide region. Based on finite element simulations of the heaters, an 80 mA current should produce a ${82}^{0}C$temperature change at the waveguide level. Therefore, there is a good agreement between the simulations and experimental results. The heating efficiency of the bandwidth tuning provided by the heater on short period CDGAC is 26 mW/nm, and for the second heater it is 22 mW/nm. A bandwidth tuning efficiency of 24 mW/nm was reported for a CDGAC based bandwidth tunable filter with aluminum heaters of the same width and thickness as this work [6].

The aluminum heaters were designed with identical width and thickness, and a slight difference of 9 µm in lengths. Therefore, we expect that by applying the same current to the heaters the 3-dB bandwidth of the pass band would remain constant, while the central wavelength is tuned. Figure 6(a) shows the through port response when the same current is applied to both heaters. The 3-dB bandwidth and central wavelength of the pass band as a function of the applied current is summarized in Fig. 6(b). Based on the results, for the same applied current there is a slight difference in the equivalent temperature of the two heaters, and thus there is a maximum change of 6% in 3-dB bandwidth. The heating efficiency of the filter with respect to the tuning of the central wavelength is 44 mW/nm. Central wavelength tuning requires approximately twice as much power as bandwidth tuning, which is expected since both heaters are being used in this situation. The power consumption of the device can be improved by increasing the thermal isolation between the silicon waveguides and underlying silicon substrate [14]. A tuning efficiency of 0.2 mW/nm has been reported by etching away the silicon substrate beneath silicon waveguides [14].

 

Fig. 6 a) Through port response when the same current is applied to the heaters ($I_1=I_2$) on short-period and long-period CDGAC, $\Delta T_1$ ($\Delta T_2$) is the temperature change at short(long)-period CDGAC, b) 3-dB bandwidth (dotted) and central wavelength (solid) of the passband for different applied current.

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Based on the experimental results, within a wavelength range from 1555 nm to 1573 nm, the device can be used as a bandwidth and wavelength tunable bandpass filter. We have demonstrated a transmission bandwidth tunability from 1.1 nm to 11.7 nm. The wavelength range within which the device can be used as a bandpass filter can be increased by increasing the coupling coefficient of the CDGACs and consequently increasing the width of the rejection bands in the through port response of the cascade filter. The maximum achievable bandwidth of the device is limited by the breakdown current of the heater of the long-period CDGAC. With a more robust heater, the maximum bandwidth of the device could be increased. The minimum achievable bandwidth depends on the breakdown current of the heater on short-period CDGAC.

In addition to the previously described functionalities, the proposed configuration is capable of tuning the bandwidth of the passband, whilst holding the central wavelength changes below 2%. We demonstrate this functionality using CDGACs with a maximum grating width of 24 nm on the narrow waveguide and of 50 nm on the wide waveguide, and a 100 nm gap between the waveguides in the configuration depicted in Fig. 3. The period of the short-period CDGAC is 312 nm, and the period of the long-period grating remains 318 nm. Figure 7(a) depicts the through port response of this structure for different heater temperatures. The 3-dB bandwidth and central wavelength of the response is summarized in Fig. 7(b). $I_1$is the current applied to the heater on short-period CDGAC and $I_2$ is the current applied to the heater on long-period CDGAC. The results show a 3-dB bandwidth tuning from 4.1 nm to 12.6 nm, while the central wavelength shifts by only 0.4 nm.

 

Fig. 7 a) Through port response for different temperature at the CDGACs, $\Delta T_1$ ($\Delta T_2$) is the temperature change at the short(long)-period CDGAC, b) 3-dB bandwidth (dotted) and central wavelength(solid) of the passband for different applied current.

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Conclusion

We presented the design, fabrication and measurement results of a bandwidth and wavelength tunable filter. The device consists of two contra-directional grating assisted couplers (CDGAC) with different periods that are cascaded such that the through port of the first CDGAC is connected to the input port of the second CDGAC, while the add port of the first CDGAC is connected to the drop port of the second CDGAC. The prototype can be used as a bandpass filter over a wavelength range of 18 nm (from 1555 nm to 1573 nm). This wavelength range can be increased by increasing the coupling coefficient of the CDGACs and consequently increasing the width of the rejection bands in the through port response of the cascade filter. The width of the bandpass response of the cascade configuration is tunable by individually tuning the central wavelength of the CDGACs. We demonstrate the tunability of the device with the thermo-optic effect using metal heaters integrated on chip.

The passband of the through port has a 3-dB bandwidth of 5.8 nm with an insertion loss of 2.6 dB and an extinction ratio of 31 dB. The transmission bandwidth can be tuned from 1.1 nm to 11.7 nm. The maximum achievable bandwidth of the device is limited by the breakdown current of the heater of the CDGAC with the longer period while the minimum achievable bandwidth depends on the breakdown current of the heater of the CDGAC with the shorter period. Moreover, we demonstrated that the central wavelength of the pass band can be tuned over more than 4 nm. There is no limit for the tuning range of the central wavelength except for the breakdown current of the metal heaters.

Future work will include the optimization of apodization function and of number of periods in each CDGACs to increase the extinction ratio and reduce the side lobes in the response of the CDGACs. This will improve the extinction ratio and the shape of passband. Moreover, the tuning range could be increased by using more resistant materials, such as tungsten or nichrome, to implement the heaters.