A silicon photonic waveguide switch using adiabatic couplers with lateral comb-drive actuators is designed, fabricated, and tested for microelectromechanical optical matrix switch. One of the waveguides of the adiabatic coupler is moved laterally by three actuators for varying the coupler gap, which enables to switch the path of the waveguides. The coupler waveguides with 250 nm thickness consist of a movable tapered waveguide from 400 nm to 500 nm in width and 50 μm in length and a straight waveguide of 400 nm in width. The three actuators are ultra-small electrostatic comb-drive and move the two movable tapered waveguides. The switch’s transmission characteristics were measured as a function of the coupler gap. Around a coupler gap of 109 nm, the port isolation of 16.7 dB was obtained. The switch’s insertion loss was roughly estimated to be less than 1 dB. The switching time was 36.7 μsec under the present experimental condition. Moreover, 64 switches were arrayed in a 125 μm period square mash waveguide and an 8 x 8 matrix switch was composed. The matrix switch was also tested.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
A Silicon photonic waveguide circuits are promising for merging with silicon integrated electronic circuits based on the compatible silicon micro-fabrication technology . Silicon waveguide devices are widely studied to shrink the sizes of conventional silica waveguide devices using the high refractive index of silicon [1–5]. Moreover, it is expected that the silicon microelectromechanical systems (MEMS) technology is combined with the integrations because of the same fabrication basis. MEMS for silicon photonics are not only useful for peripheral applications [6,7] but also applicable to tunable and reconfigurable devices in silicon waveguide circuits. Movable structures applicable to silicon nano-photonics can be formed on silicon wafer by shrinking mechanical components as small as silicon photonic components [8,9]. In general, as the mechanical response time of MEMS is linearly decreased with the decrease of size, the response time of silicon photonic MEMS component is decreased from the order of millisecond obtained commonly in conventional scale (~1mm) MEMS to the order of microsecond.
Optical switch has been an essential component in silicon photonic waveguide circuits. Using the large thermal coefficient of the refractive index of silicon, the thermo-optic silicon waveguide switches using Mach-Zehnder interferometer with incorporated micro heaters were studied [2,10]. For conventional thermo-optic switch, the response time was usually around 100 μsec and the power consumption was a few tens of milliwatts. In the recent progresses, the response time and the power consumption of thermos-optic switches were improved by the heat isolation structures [11–13]. More recently, as large-scale switch, the 8 × 8 switches using double-Mach-Zehnder elements with phase shifters were demonstrated in a broadband wavelength region . In the case of electro-optic switches, a response time of the order of 1 nsec can be obtained by injection current. The injection current consumes a power of submiliwatts for phase shifter while the power consumption of electrostatic MEMS actuator is relatively negligible. The 32 × 32 electro-optic Mach-Zehnder interferometer switches was recently reported . On the other hand, a silicon waveguide MEMS switch using a directional waveguide coupler was studied, where the coupler gap was varied by a comb-drive actuator . A multiple switch consisting of six directional waveguide coupler switches was also demonstrated . Moreover, a directional coupler switch with a nanolatch mechanism was also reported using the comb-drive actuators . More recently, to construct an optical cross-connect with a large number of input/output ports, a matrix (50 x 50) switch consisting of a mesh waveguide and an array of MEMS switches was demonstrated . The two laterally coupled directional couplers were moved by vertical electrostatic actuators. Furthermore, an advanced larger matrix (64 x 64) switch using vertically coupled adiabatic waveguide couplers was reported . Using the adiabatic couplers, a wide range of transmission wavelength was obtained and the vertical electrostatic actuator generated a fast switching time less than 1μs. The fabrication process was based on surface micro-machining. And furthermore, the MEMS switches were improved to increase the matrix size up to 128 x 128 switches  and made insensitive to the polarization by the two-level waveguide crossbars . Although the demands for silicon photonic matrix switch are increasing, the different types of silicon photonic MEMS matrix switches are limited.
In this paper, an adiabatic waveguide coupler switch using silicon on insulator (SOI) wafer is designed, fabricated and tested. In order to switch the silicon waveguide paths, the gap of the coupler is varied laterally with comb-drive actuators. Moreover, the switches are arrayed to form a matrix (8 x 8) switch and the basic characteristics are obtained. Compering to the MEMS switch using the vertical adiabatic coupler [19–22], the lateral adiabatic coupler is utilized to construct switch element. A comparable performance can be expected from the theoretical calculations for the lateral adiabatic coupler. The coupler gap of the proposed switch is controllable in a wider range due to the electrostatic comb actuator, which enable to adjust the transmission wavelength region when it is operated at analog voltage. The digital operation of the switch is also feasible by equipping a stopper of the actuator. An advantage of the proposed switch is the simple fabrication consisting of the single mask lithography with the two etching processes on SOI wafer, which tolerates severe mask alignments in nanometer region. The switching time is longer in the proposed switch due to the larger mass of the movable part. In the case of large matrix switch, the optical characteristics such as insertion loss are dependent on the design and fabrication of the long mesh waveguides, which are not optimized in this experiment.
2. Design and fabrication
Figure 1(a) shows the schematic diagram of an 8 x 8 matrix switch consisting of an array of the waveguide coupler switches. The input and output ports are connected by a mesh waveguide in the matrix switch. Each switch cell of the matrix switch has the same structure of waveguide coupler switch. To route lightwave from one of the input ports to one of the output ports, the waveguide path is changed at the cross point of the input and output waveguides by turning on the switch located at the cross point.
A switch cell in the matrix switch is magnified and shown in Fig. 1(b). The switch cell consists of two waveguide couplers, three comb-drive actuators, and a waveguide crossing. The area of switch cell is 125 μm square. The first waveguide coupler located close to a horizontal waveguide transfers the lightwave from the input port to the movable waveguide of the coupler in the switch-on state. The transferred lightwave propagates to the second waveguide coupler and couples to the vertical waveguide without passing the waveguide crossing. In the switch-off state, the lightwave from the input waveguide passes through the crossing without coupling to the movable waveguide of the coupler as the coupler gap is large. The crossing used in the experiment is shown in Fig. 1(c), which consists of two elliptical waveguides intersecting at right angle. The loss of the elliptical waveguide is reported to be small (<0.1 dB) . The dimensions of the elliptical waveguides are shown in Fig. 1(c).
For testing of the proposed waveguide coupler switch, we use the same switch structure without the waveguide crossing as shown in Fig. 1(d). The input lightwave is transferred to the drop port through the two waveguide couplers in the switch-on state while the lightwave passes through the horizontal waveguide to the through port in the switch-off state. The waveguide couplers are adiabatic to allow a wide transmission wavelength region in the switch-on state. Since the coupler length of the adiabatic coupler is longer than a directional coupler with the same coupler gap, the movable coupler waveguide of the adiabatic coupler is supported by the eight arms connected to the three comb-drive actuators. Each actuator has the same structure and consists of two comb pairs and four springs. Applying a voltage to the electrode of the three comb-drive actuators, the movable coupler waveguides of the adiabatic couplers approach to the fixed waveguides located horizontally and vertically. The displacement d of actuator generates a decrease of the coupler gaps due to the oblique displacement to the coupler gaps.
The schematic diagram of the adiabatic waveguide coupler is shown in Fig. 1(e). The adiabatic waveguide coupler consists of a tapered waveguide and a bus waveguide that is a part of the mesh waveguide. The tapered waveguide varies the width from 360 nm near the input port to 440 nm at the end of the coupler, while the bus waveguide keeps 400 nm in width. The adiabatic waveguide coupler is 50 μm in length. Those waveguides are 250 nm in thickness since all the parts of switch are made of a 250 nm thick top silicon layer of SOI wafer. The ends of the tapered waveguides of the adiabatic waveguide couplers are connected to the bent waveguides to be supported and then connected to the actuators as shown in Figs. 1(b) and (d). The inner radii of the bent waveguides of the tapered waveguide are 10 μm near the input port and 100 μm at the drop port, respectively. The lightwave transferred from the horizontal input waveguide to the movable coupler waveguide propagates to the second adiabatic waveguide coupler, and then transfers to the vertical bus waveguide at the drop port.
Due to the long coupler, the two tapered waveguides are supported by the six beams from the three actuators. The three actuators are comb-drive electrostatic actuators with the sizes of 30 μm in width and 40 μm in length. The force of each actuator is designed to be 380 nN at the voltage of 20 V and the spring constant of each actuator is 0.30 N/m. The comb finger is 2.0 μm in length to obtain the displacement of about 700 nm. The initial gap of the coupler is 500 nm and is decreased to less than 100 nm by the motion of actuators. The total mass of the movable parts of each switch cell is evaluated from the sizes and the silicon density to be approximately 5.8x10−13 kg and the total spring constant is 0.90 N/m. Therefore, the resonant frequency of the mechanical oscillation for switching motion is calculated from the mass and the spring constant to be 198 kHz.
To design the adiabatic waveguide coupler, the simulation using the finite-difference time-domain (FDTD) method was carried out. The design values are as shown in Fig. 1(e). The light intensity distribution at a gap of 100 nm and a wavelength of 1550 nm is shown in Fig. 2(a). The input lightwave gradually transfers from the input waveguide to the tapered coupler waveguide and propagates finally to the drop port. The calculated intensities at the drop and through ports of the adiabatic waveguide coupler are plotted as a function of the gap between the coupler waveguides in Fig. 2(b). The intensity at the drop port increases gradually with the decrease of the coupler gap. Around the gap of 100 nm, the switch-on condition is obtained at the drop port. Figure 3 shows the calculated transmittance from the input port to the drop port of the adiabatic waveguide coupler as a function of the wavelength at the gap of 100 nm. The wavelength dependence of the transmittance is small in the calculated region.
The fabrications of the waveguide coupler switch and the matrix switch were carried out by electron beam lithography. An SOI wafer with a 250-nm-thick upper silicon layer and a 3-μm-thick buried oxide layer on a 675-μm-thick silicon substrate was used. The switch structures were patterned by an electron-beam drawing machine (JEOL, JBX-6300SK). The upper silicon layer was etched using a fast atom beam (Ebara, FAB-60ML). The oxide layer was etched by hydrofluoric acid vapor (SPTS, uEtch) to obtain the freestanding structures of the proposed device.
The output intensity was measured using a tunable laser (Agilent 81682A) and two lensed single-mode fibers to couple light to the end surfaces of the input and output ports. The input and output waveguides were widened near the ports and the waveguide ends were formed by cleaving the silicon wafer. The lightwave was coupled to the facets of the cleaved waveguide ends by the lensed fibers. To calibrate the displacement of comb-drive actuator as a function of voltage and to find the mechanical resonant frequency, a scanning electron microscope was used.
3. Results and discussion
Figure 4(a) shows the electron micrograph of the fabricated adiabatic waveguide coupler switch. The movable coupler waveguides of the adiabatic waveguide couplers are supported at their waveguides ends, where lightwave does not propagate in the switch-on state. However, since the total length of the two movable coupler waveguides is longer than 100μm, the center part of the waveguide is supported by an elliptical waveguide so as not to stick down. All the silicon dioxide layers under the waveguides, comb-drive actuators, supporting beams and springs are etched clearly as shown in Fig. 4(a). The adiabatic waveguide coupler is magnified as shown in Figs. 4(b) and 4(c) at the both ends of the coupler. The width at the narrowest part (position A in Fig. 4(a)) of the movable waveguide is measured to be 336 nm from the magnified electron micrograph, which is narrower by 24 nm than the design value of 360 nm. The other end of the coupler (position B in Fig. 4(a)) is 424 nm wide, which is 16 nm narrower than the design value of 440 nm. The bus waveguide is 392 nm, which is narrower by 8 nm than the design value of 400 nm. The narrowing is caused by the errors of the patterning and etching. The initial gap of the coupler is 533 nm as shown in Figs. 4(b) and 4(c).
The displacement of an actuator of the switch was measured from electron micrographs. The measured displacement of the actuator is plotted as a function of the applied voltage in Fig. 5. The maximum displacement of 470 nm is obtained at the voltage of 18.6 V before contacting. To obtain the gap of 109 nm in the switch-on state, the displacement of 424 nm is generated at the voltage of 18.0 V. The measured voltage dependence of the actuator displacement is explained by the theoretical calculation  as shown in Fig. 5.
The output intensities at the drop and through ports are measured as a function of the gap between the movable coupler waveguide and the bus waveguide, and the normalized intensities are shown in Fig. 6. The measurement was carried out at the laser power of 4 mW at the wavelength of 1550 nm. The intensity at the through port in the switch-off state was 140.6 nW, which corresponded to −44.5 dB including the fiber coupling loss. The normalized intensities are also plotted in linear scale as a function of the gap in the inset of Fig. 6. The switch-on state is obtained around the gap of 109 nm at the applied voltage of 18.0 V from the intensity minimum at the through port. An intensity oscillation is observed at the drop port in the switch-on state, which was repeatable when the measurement was repeated. This may be caused by the fabrication errors. The intensity oscillation is less than 1 dB. The port extinction of the waveguide coupler (1 x 2) switch is defined as IDmax/IDmin for the drop port and ITmax/ITmin for the through port, where IDmax, IDmin, ITmax, and ITmin are the maximum and minimum intensities at the drop and through ports, respectively. The port isolation of 1 x 2 switch is the intensity ratio between the ports under the worst-case condition, which is defined as ID (drop port intensity at ITmin)/ITmin at the switching point in this experiment. The port extinctions are 35.6 dB and 16.7 dB at the drop and through ports, respectively. The port isolation between the two output ports is approximately 16.7 dB. The intensity IToff at through port in switch-off state and the intensity IDon at drop port in switch-on state were measured repeatedly by setting the output fibers for a number of the fabricated switches. The averaged ratio of IDon /IToff was about −0.4 dB. If the lightwave is transmitted from the input port to the drop port without loss, the ratio IDon /IToff is equal to one. It was considered that this ratio showed a rough estimate of the insertion loss of the switch since output waveguides for drop and through ports were similar and the input lightwave was just transmitted to the through port by a short straight waveguide in the switch-off state.
The wavelength dependence of the adiabatic waveguide coupler switch was measured in a switch-on state. Figure 7 shows the relative intensities measured as a function of the wavelength at the drop and through ports in the switch-on state. The switch-on state was obtained at the wavelength of 1550 nm and the output intensities were measured by varying the wavelength of the laser from 1522 nm to 1630 nm. The port isolation between the two ports is better than 10 dB in the measured wavelength region.
The switching time was also measured by applying a step voltage to the fabricated switch. Figures 8(a) and 8(b) show the output intensities at the drop port as a function of time with the applied step voltage waveforms in the step rise and step fall conditions, respectively. Since the conductivity of the top silicon layer used in the experiment was small (resistivity ~10 kΩcm), we irradiated ultraviolet light (365 nm) at the irradiance of 0.9 mW/cm2 to increase the conductivity of the silicon layer. The rise time from 10% to 90% intensities is 36.7 μsec and the fall time is 21.4 μsec. Without the light irradiation, the rise time was longer than 300 μsec and the fall time was longer than 150 μsec. The mechanical resonant frequency for the lateral motion of actuator was measured to be 162.33 kHz, which was smaller than the theoretical value of 198 kHz due to the excess etching. The inverse of the frequency, which is an estimate of the response time, is 6.2 μsec. The waveform of the output intensity seems to be overdamping under the experimental condition, which is probably determined by the circuit time constant. The circuit time constant is dependent on the resistance of wiring (top silicon layer) and the capacitance of comb electrodes. Therefore, we believe that the response time can be decreased further by increasing the conductivity of the silicon layer.
A matrix switch was also fabricated using the design shown in Fig. 1(a). The 64 adiabatic waveguide coupler switches were arrayed as 8 x 8 matrix arrangement and the parallel input and output waveguides were connected. Figure 9(a) shows the electron micrograph of the fabricated 8 x 8 matrix switch. Each mash of the matrix waveguide is 125 μm square, which is equal to the diameter of conventional single-mode optical fiber. The total area of the switch array is 1 mm square. Figure 9(b) shows a magnified image of a single switch. Each switch has three actuators and two adiabatic waveguide couplers. The electrode for the actuators is seen brighter due to the accumulation of electron charge.
The fiber coupling losses for the respective ports largely scattered (−15 dB~-30 dB) because of the different conditions of the cleaved facets of input/output waveguides and the alignment of the lensed fibers. It was difficult to determine directly the insertion losses of the matrix switch and then the relative values (extinction ratios) were measured. The extinction ratio RD = IDon /IDoff at the output (drop) port for each switch was measured and plotted as a function of the number N of switch cells passed from the input port to the output port in Fig. 10. Since the minimum intensities at the output ports were nearly equal to the background intensity, the extinction ratio was considered to be almost equal to the relative intensity at the output port. Although the measured extinction ratios at the respective ports still scattered due to the different fiber coupling losses in our measurements, the linear correlation coefficient was 0.47 and the probability of decorrelation was less than 2.5%. The linear equation was obtained as rough estimate. When the optical path becomes longer, the extinction ratio decreases gradually as shown by the dotted line (RD = −0.52N + 21.2 (dB)), which is obtained by the least-square method. The dotted line shows that the relative output intensity at output port decreases by approximately −0.52 dB for each switch cell, which corresponds roughly to a total loss of one crossing and a straight waveguide with an elliptical waveguide support. Therefore, the total insertion loss of single switch cell including the crossing is roughly estimated from the added loss of −0.52 dB and −0.4 dB, where the latter value was obtained in the measurement of the ratio of IDon /IToff for single switch.
The durability of the matrix switch was tested by repeating the switch-on and switch-off states for a long time. No obvious changes of the output intensity were found after switching 1 billion cycles at a frequency of 2.5 kHz. From those fabrications and measurements, the feasibility of the matrix switch based on the gap variable adiabatic waveguide coupler switch with comb-drive actuators was confirmed.
The silicon photonic waveguide switch using the adiabatic couplers was fabricated on a silicon on insulator wafer. Due to the simple fabrication, only one mask and two etching processes were needed. Varying the gap of two adiabatic couplers, the optical path was switched. The tapered waveguides of the adiabatic couplers were moved by the three actuators. The adiabatic couplers were 50 μm in length. The three actuators were ultra-small electrostatic comb-drive. The output intensities of the switch were measured as a function the coupler gap. Around the coupler gap of 109 nm, the port isolation of 16.7 dB was obtained. The insertion loss of single switch was roughly estimated to be less than 1 dB. The rise time of the switch was 36.7 μsec and the fall time was 21.4 μsec under the present experimental condition, which could be shortened by decreasing the resistivity of the top silicon layer since the inverse of the mechanical resonant frequency of the switch was 6.2 μsec.
The proposed switches were arrayed with the period of 125 μm to form an 8 x 8 matrix switch. The operation of the matrix switch was demonstrated. Extinction ratio was measured for each propagation path. Optimizing the driving circuit of switch and the loss of mech waveguide, it is considered that the matrix switch based on the proposed mechanism will be feasible.
Authors thank Drs. Y. Kanamori, T. Sasaki, Y. Zhai and W. Wang and other researchers for their useful suggestions and discussions. The fabrications were carried out in MNC of Tohoku University.
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