A microelectromechanical tunable notch filter using silicon-photonic freestanding waveguides is proposed, and the basic characteristics are experimentally investigated. The proposed filter is composed of a wavelength-tunable silicon microring resonator and a busline switch. The tunable microring consists of freestanding single-mode waveguides and air-gap directional waveguide couplers. The optical path length of the microring is varied physically by a displacement of electrostatic comb-drive actuator. The busline switch consists of a gap-variable waveguide coupling mechanism, which enables coupling the tunable microring with the busline by another electrostatic comb-drive actuator. During the wavelength tuning of microring, the busline can be disconnected from the microring. Therefore, the proposed device operates as a hitless wavelength-selective switch if they are connected in series. The waveguides are 320 nm in width and 340 nm in thickness. The resonant wavelength shift of the microring is 9.96 nm at the voltage of 26 V with the actuator displacement of 1.0 μm. The coupling to busline is adjusted from the switch-off state at the gap of 600 nm to the switch-on state corresponding to the critical coupling condition at the gap of 383 nm. The whole size of the wavelength-tunable filter with hitless mechanism is about 150 μm by 80 μm. Due to the capacitive operation of the comb-drive actuators, the power consumption is negligibly small.
© 2013 OSA
Silicon waveguide technology is promising for optical telecommunication devices and optical interconnection in integrated electronic circuits [1, 2]. Since the refractive index of silicon is high (~3.5) at 1.5 μm wavelength, the area of silicon waveguide circuits becomes smaller by two orders of magnitude than that of silica waveguide circuits. Several functional devices such as modulators , switches for routing , wavelength-selective filters [5, 6] as well as fundamental silicon waveguide components, have been studied.
For wavelength-division multiplexing systems, wavelength-selective switch is a key component for routing light waves at different wavelengths. One of the promising technologies for the wavelength selective switches in planer lightwave circuits is the wavelength filter on the basis of microring resonator [7–13]. By tuning the resonant wavelength of microring, a light wave at a specific wavelength is dropped and routed from a busline waveguide. The tuning in a wide wavelength region is usually based on thermo-optic effect by heating microring waveguide [7–11]. However, the conventional tunable microring resonator blocks other wavelength channels while tuning its resonant wavelength since the microring is always connected to the busline. Recently, in order to avoid blocking, hitless wavelength-selective switches using series-coupled microring resonators were reported [14, 15]. However, there are few reports on the hitless wavelength-selective switches using silicon microring resonators. The power consumption for tuning the resonant wavelength by the thermo-optic effect was on the order of 1 mW/nm [7, 9, 11]. The power consumption should be minimized for large scale matrix switches.
On the other hand, the variable mechanisms using microelectromechanical electrostatic actuators have the advantages of low power consumption and wide-range tuning [16–18]. Silicon-photonic waveguide coupler switch using electrostatic comb-drive actuator was reported . The switching between busline and microring was also studied by moving either a freestanding waveguide or a microring [16, 17]. Lately, a wavelength-tunable microring resonator using an electrostatic microactuator was also reported . However, hitless wavelength-selective switch using wavelength-tunable silicon microring based on microelectromecahnical technology has not been studied.
In this paper, a tunable notch filter consisting of a wavelength-tunable microring resonator and a busline switch is proposed and fabricated for a hitless wavelength-selective switch, using silicon-photonic waveguide microelectromechanical structures. Using the notch filter with the two variable mechanisms, the wavelength-selective switch becomes simple and compact. The characteristics of the wavelength tuning and the busline switching by two independent actuators are investigated.
2. Principle and design
Figure 1(a) shows the schemetic diagram of the proposed notch filter composed of a wavelength-tunable microring resonator with a busline switch mechanism. The microring consists of two freestanding submicron silicon waveguides. One is a U-shaped waveguide coupling with a baseline waveguide as shown in Fig. 1(a). The second U-shaped waveguide of the microring is movable, which is connected to an electrostatic comb-drive actuator. The two U-shaped waveguides are located with two gaps at their ends. Each overlapping region of the two U-shaped waveguides operates as directional waveguide coupler with air clad. Keeping the air gaps of the couplers constant, i.e., the coupling rate is close to unity, the movable waveguide is translated to vary the circumferential length of microring by using the electromechanical actuator. The coupling length of the coupler is unchangeable since the length of the gap region is defined by the closest region of the bent parts of the waveguide. The change in physical length of the microring waveguide is two-times larger than the displacement of actuator.
In order to realize a hitless wavelength-selective switch using the notch filters, it is indispensable to introduce a busline switch mechanism, where the busline is connected to and disconnected from microrings. The resonant wavelength of microring is varied after disconnecting the busline so as to not disturb the signal waves. As shown in Fig. 1(a), the fixed waveguide is coupled with the busline waveguide in the busline switch mechanism. The gap between them can be varied by another actuator connected with the freestanding busline waveguide through the low-loss suspension bridge waveguides. Moreover, when the busline is connected to the microring, the position of the busline waveguide should be precisely controlled by adjusting the voltage of actuator for obtaining the critical coupling condition, which is necessary to realize high extinction ratio.
Figure 1(b) shows an example of the connection of the wavelength-tunable notch filters for the signal waves at three wavelengths. The signal waves at the different wavelengths are assumed to be traveling simultaneously in the busline waveguide. The respective signal waves are dropped at the respective ports if the resonant wavelengths of the microrings are tuned to the wavelengths of the signal waves as shown in the first set of wavelengths shown in Fig. 1(b). In order to change the set of wavelengths dropped at the respective ports as shown in the second set of wavelengths in Fig. 1(b), the busline waveguide is disconnected from the respective microrings and the resonant wavelengths of the microrings are changed to match the new set of wavelengths. After tuning the wavelengths of microrings, the busline waveguide is connected again to the microrings to drop the signal waves at the set wavelengths. Therefore, the signal waves in any sets of wavelengths are switched to the respective drop ports if the simple configuration shown in Fig. 1(b) is realized. Without the busline switch mechanism, the signal waves at different wavelengths are blocked while tuning the resonant wavelengths of microrings. The cascadability of the busline couplers was tested by the connections of coupler switches . The through port insertion loss of a single busline coupler was approximately −0.3 dB.
In order to examine the basic functions of the proposed system shown in Figs. 1(a) and 1(b), the wavelength-tunable microring resonator with the busline switch mechanism is designed as shown in Fig. 2(a), where the drop/add waveguide is omitted for simplicity. One end of the busline waveguide is the input port, and the other end is the through port. The waveguides of the couplers in microring are extended and narrowed at the ends to prevent the reflection and to check the leakage of uncoupled light. Figure 2(b) shows the whole mask pattern of the proposed device for fabrication. Figure 2(c) shows the magnified pattern of the microring part. The waveguides used for the device are 320 nm in width and 340 nm in thickness. The gaps of the couplers used in the microring are 250 nm and the coupling lengths are 14 μm long. These gaps and lengths were determined by a numerical simulation (Crystal Wave) using the finite-difference time-domain (FDTD) method based on a rigorous electromagnetic theory to obtain the maximum coupling coefficient nearly equal to unity. The fixed waveguide is suspended in air by two suspension bridges, where the elliptical waveguide shape is utilized to minimize the waveguide loss. The dimensions of the suspension bridge are 1.2 μm wide and 8 μm long, and the suspension arm is 0.2 μm wide and 1.5 μm long. The movable waveguide is connected to the actuator with the two suspension bridges. The two ends of the movable waveguide are physically connected by an optical isolation structure using a narrow silicon wire, which reinforces the freestanding waveguide. The light wave propagation in the optical isolation structure is prevented by the strong bends as shown in Fig. 2(c). Even if light is leaked from the microring couplers, most of the light is scattered by the bent waveguide with the bent angle of more than 90 degrees. This ensures that the leaked light would not couple to the microring again. The total length of the microring is approximately 122 μm under the initial condition.
The initial gap between the U-shaped fixed waveguide and the busline waveguide is designed to be 300 nm, which ensures an over-coupling condition on the basis of the numerical calculation. The busline is supported by the two suspension arms in order to increase the rigidity of the freestanding waveguide. In order to prevent microloading effect in the areas of thin long freestanding waveguides with narrow gaps, the large etching areas adjacent to them are filled with the mesh dummy structures as shown in Fig. 2(c).
An electrostatic comb-drive actuator (tuning actuator) is used to translate the movable waveguide of microring. The area of the actuator is approximately 25 μm in width and 45 μm in length. The comb finger is 225 nm wide, 340 nm thick, and 1.73 μm long, and the gap between each finger pair is 225 nm. Total number of comb finger pairs is 20. Doubly folded springs are utilized in the microactuator. Each of the spring elements is a straight silicon bar with the width of 250 nm, the thickness of 340 nm and the length of 15 μm, which corresponds to an equivalent spring constant of 0.41 N/m. From the calculation, the displacement is 1.0 μm at the voltage of 37 V. The resonant frequency is calculated to be about 190 kHz. The gap between the busline and microring waveguides is also controlled by a similar actuator (switching actuator) except that the motion of the actuator is not push-type but pull-type. The number of comb finger pairs is 29. The displacement of 1.0 μm is obtained at 31 V by calculation.
3. Fabrication and experiments
In the fabrication of the proposed device, a SOI wafer with 340-nm-thick top silicon layer and 2-μm-thick buried oxide layer on a 625-μm-thick silicon substrate is used as shown in Fig. 3(a). First, 350-nm-thick positive resist polymer (ZEON ZEP-520A) is coated on the SOI wafer (b), and it is exposed by using an electron-beam patterning machine (JEOL JBX-5000LS) (c). After developing the resist polymer, the top silicon layer is etched by a fast atom beam (Ebara FAB-60ML) (d). The fast atom beam consists of neutral molecular fragments extracted from a dc SF6 plasma. The resist polymer is removed by O2 plasma ashing after the fast atom beam etching (e). After the lithographic processes, the SOI wafer is cleaved to obtain a facet of input waveguide for coupling light (f). Finally, the SiO2 layer is etched by hydrofluoric acid vapor to obtain the freestanding structure of microring resonator (g).
In the measurement of the fabricated microring, a tunable infrared laser (Agilent 81682A) was used as a light source at the wavelength around 1.5 μm. For coupling the laser light to the end surfaces of the input and output ports of busline waveguide, lensed single mode fibers were used. The output intensity at the through port was measured by a detector (Agilent 8164A).
4. Results and discussion
Figure 4(a) shows an electron micrograph of the whole structure of the fabricated microring. The SiO2 layer beneath the Si top layer is undercut by HF vapor for about 2.0 μm from the edge of the top Si layer as seen in the pale color in Fig. 4(a). The waveguides of the couplers in microring are successfully formed and aligned parallel to each other with the air gap of 248 nm, which agrees well with the designed value. The spring width of the actuator is measured to be about 220 nm. The width of the waveguide is measured from a high-magnification scanning electron micrograph to be 324 nm. The surfaces of the parallel waveguides are found to be smooth with a peak roughness of 15 nm and the root-mean square roughness of 3 nm.
Figure 4(b) shows a magnified oblique view of the busline switch mechanism. From this magnified image, it is confirmed that the waveguides are freestanding. The heights of the two waveguides of the busline switch are nearly equal to each other. The gap of the busline switch is measured from scanning electron microscope to be 380 nm, which is wider than the design value due to a decrease of the width of the busline waveguide in the coupling region. It is also seen from Fig. 4(b) that the coupler waveguides of the microring are nearly parallel. These freestanding structures were successfully obtained after optimizing the structure design and compensating the influence of residual stress in the top silicon layer of SOI wafer.
The wavelength-tunable mechanism was tested by driving the tuning actuator. Applying voltage to the tuning actuator, the output intensity at the through port was measured as a function of wavelength. Figure 5 shows an example of the measured light intensities at the through port with the actuator voltage as a parameter under the condition that the busline coupling is close to the critical coupling, where the gap of the coupler is about 383 nm.
In Fig. 5, the periodic dips are observed as a function of the wavelength. The depth of the resonant dip is approximately −8 dB. The free spectral range is 3.4 nm, and thus the group index is calculated to be 5.4. Since the full-width at the half maximum of the dip around the measured wavelength is approximately 0.4 nm, the Q-value is about 3700. By increasing the actuator voltage from 5 V to 10 V, the wavelength at the resonant dip shifts by 1.09 nm. From 10 V to 20 V, it shifts by 4.66 nm, and further shifts by 1.50 nm from 20 V to 22 V. Finally the resonant wavelength shift is 2.40 nm from 22 V to 26 V as shown in Fig. 5.
The wavelength shift of the resonant dip was measured as a function of the voltage as shown in Fig. 6. The actuator displacement was also measured as a function of the voltage by observing the displacement with a scanning electron microscope. The displacement of the actuator varies quadratically in Fig. 6 as predicted by the theory of electrostatic comb-drive actuator. The maximum displacement of 1 μm is obtained at the voltage of about 26 V, and it is limited by the stopper of actuator. The displacement of the actuator at the applied voltage was larger than the calculation since the spring width became narrower than the designed value. The wavelength shift of the resonant dip decreases also quadratically as shown in Fig. 6. Therefore, the wavelength shift is almost linearly dependent on the displacement. The maximum wavelength shift is −9.96 nm at the voltage of 26.0 V.
In the next experiment, the switching function of the busline was tested using the switching actuator. Under the initial condition (at the gap of 380 nm), the coupling condition was close to the critical coupling condition of the microring resonator at the gap of 383 nm. By increasing the voltage of switching actuator to pull the busline apart from microring, the coupling between the busline and the microring was changed. Figure 7 shows the output intensity at the through port measured as a function of wavelength with the voltage applied to the switching actuator as parameter. At the voltage of 2.0 V, the deepest resonant dip is obtained. With increasing the voltage, the depth of the dips decreases as shown in Fig. 7. At the voltage of 22V, there are no dips as shown in Fig. 7, and thus the busline is disconnected completely from the microring resonator. The ripple of about 1 nm period observed outside the filtered region may be caused by a parasitic Fabry-Perot interference  between reflection points in microring. A few light spots caused by reflection were observed in the infrared image of the microring. The Fabry-Perot interference can be diminished by decreasing the fabrication errors and optimizing the structures.
Figure 8 shows the output intensity at the through port measured as a function of the gap between the microring and the busline waveguides at the wavelength of 1478.24 nm. The inset shows the magnified part in the range from 380 nm to 410 nm. The critical coupling is obtained at the gap of 383 nm. By increasing the gap from the critical coupling cndition, the coupling decreases exponentially. At the gap larger than 600 nm, the output intensity at the through port becomes constant (0 dB), and thus the coupling is disconnected. Therefore, the coupling to busline is varied from the critical coupling condition to the switch-off condition by using the switching actuator.
A tunable notch filter using silicon-photonic microelectromechanical structures was proposed. The proposed filter composed of a wavelength-tunable silicon microring resonator and a busline switch. The resonant wavelength of the microring was tuned by the tuning actuator after disconnecting the busline by the switching actuator. The optical path length of the microring was varied physically by the displacement of the tuning actuator. The resonant wavelength shift of the microring was 9.96 nm around the wavelength of 1.47 μm at the actuator displacement of 1.0 μm. The sensitivity of resonant frequency shift was 1.1 nm/V in the voltage range from 24 V to 26 V while the average sensitivity was 0.4 nm/V in the entire region of measured voltage. The coupling to the busline was varied from a switch-off condition at the gap of 600 nm to the critical coupling condition at the gap of 383 nm by using the switching actuator. Combining the tunable microring and the busline switch, the compact tunable notch filter with the busline switch mechanism was demonstrated in the size of 150 μm by 80 μm on a silicon chip.
The authors thank Y. Kanamori for his useful advices. This work was supported by SCOPE and partially by μSIC. The device fabrication was carried out in MNC in Tohoku University.
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