We demonstrate a hitless wavelength-selective switch (WSS) based on InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) quadruple series-coupled microring resonators. The WSS is driven by the electric-field-induced change in refractive index in the FACQW core layer caused by the quantum-confined Stark effect (QCSE) for high-speed operation. The WSS with high-mesa waveguides is fabricated on a molecular beam epitaxy-grown wafer by dry etching. The fabricated WSS consists of four microrings, each with a round-trip length of 350 μm and five directional couplers with shallow grooves. A boxlike spectral response and hitless switching with higher extinction ratios than a double series-coupled microring resonator are successfully demonstrated. In addition, we propose the improvement of switching characteristics by controlling the coupling efficiencies at the directional couplers.
© 2013 OSA
Hitless wavelength-selective switches (WSSs) are promising devices for use in reconfigurable optical add–drop multiplexers (ROADMs) [1,2] in next-generation photonic networks. In particular, high-order series-coupled microring resonators are suitable for hitless WSSs because of their boxlike spectral response and high extinction ratio. To date, various high-order microring-based WSSs have been proposed and demonstrated using materials, such as silica-based dielectrics [3–13]. They are mainly driven by the thermooptic (TO) effect and their switching speed is limited even though the tuning range of wavelength switching is wide and the switching characteristics are stable. Silicon microring resonators have also been intensively studied because of their large-scale integrality, high-speed operation, and capability of being fabricated by CMOS-compatible process [14–20]; however, it is difficult to integrate them with semiconductor active devices, such as laser diodes (LDs) and semiconductor optical amplifiers (SOAs).
Our proposed solution to these problem is to utilize the electric-field-induced change in refractive index caused by the quantum-confined Stark effect (QCSE) in a multiple quantum well (MQW) . Compound-semiconductor-based WSSs are feasible for integration with LDs and SOAs and have a great potential for high-speed and low-power-consumption operation. This is because the QCSE is a very fast phenomenon and has been applied to high-speed and low-power-consumption photonic devices such as electroabsorption modulators . Several types of compound-semiconductor-microring-based filters and switches have been proposed and developed [23–27]. Recently, our group has also demonstrated an InGaAs/InAlAs MQW double series-coupled microring hitless WSS driven by the electric-field-induced change in refractive index [28,29]. Even though high-speed operation and low power consumption are expected in this device, the boxlike filtering response and extinction ratio could still be improved. To the best of our knowledge, a high-order series-coupled microring WSS based on the QCSE has never been demonstrated experimentally, except for in the above report.
In this study, we propose the hitless WSS with the InGaAs/InAlAs MQW quadruple series-coupled microring resonators  and discuss its improved switching characteristics in detail. An improved boxlike spectral response and hitless switching with higher extinction ratios than a double series-coupled microring resonator are successfully demonstrated. In addition, we propose the improvement of switching characteristics by controlling the coupling efficiencies at the directional couplers. For the MQW in the core layer, we employ a five-layer asymmetric coupled quantum well (FACQW) [31,32] to induce a large change in refractive index.
2. Transfer function of quadruple series-coupled microring resonator
The transfer function of a series-coupled ring resonator can be obtained using transfer matrix methods [4, 33–35]. In this section, we briefly describe the derivation of the transfer function of a quadruple series-coupled microring resonator with buslines.
Figure 1 shows the calculation model for the transfer function of the quadruple series-coupled ring resonator. The model consists of four microrings and five coupling regions between waveguides. The transfer matrix is given by
3. Device design and fabrication
3.1. Design of ring resonator
Figure 2 shows a schematic top view of the proposed WSS. The WSS is composed of four racetrack-shaped microring resonators and busline waveguides. The busline waveguides and microring resonators are series-coupled laterally. The coupling efficiencies of couplers C1 to C4 are denoted as K1 to K4, respectively. The proposed WSS is designed so that C1 = C5, C2 = C4, and R1 = R2 = R3 = R4, that is, the coupling efficiencies are designed so that K1 = K5≡Ka and K2 = K4≡Kb, and the round-trip length of all the microrings is set to L.
A schematic cross-sectional view of the waveguide is shown in Fig. 3(a). The waveguide structure is the same as that of the previously reported double-microring WSS . The waveguide consists of a core layer with 12 periods of an InGaAs/InAlAs FACQW , 50 nm In0.52AlGa0.24As0.24 separate confinement heterostructure layers, and p/n-doped InP cladding layers (n = 3.17). The total thickness of the core layer is approximately 300 nm. To reduce the absorption loss caused by the p-doped upper cladding layer, a 200-nm-thick undoped InP layer is inserted close to the core layer. The waveguide is buried in benzocyclobutene (BCB) (n = 1.543 at λ = 1550 nm). The width of the waveguide, w, is 1.45 μm. The average refractive index of the FACQW layer is calculated to be 3.394 using a formula for calculating the average refractive index of dielectric multilayers  The equivalent refractive index of the waveguides neq is calculated to be 3.260. The effective refractive index neff is assumed to be 3.843, which is obtained using the measured free spectral range (FSR) of a single microring resonator with the same waveguide structure.
As mentioned above, the multiple In0.53Ga0.47As/ In0.52Al0.48As FACQW  is used as the waveguide core layer of the WSS. This structure exhibits a unique QCSE behavior, that is, the absorption peak intensity near the band edge increases without a redshift with increasing an applied electric field. This behavior leads to exhibiting a large electrorefractive index change [32,37]. Using the InGaAs/InAlAs FACQW as the waveguide core layer of a microring resonator, a high-speed and low-voltage wavelength-tunable filter is expected to be realized. The wavelength of the absorption edge of the FACQW is set at approximately 1420 nm, which is 130 nm less than the operation wavelength of 1550 nm, which contributes to the reduction in the propagation loss and the independence of the field-induced change in refractive index on the wavelength in the 1550-nm-wavelength region.
In the coupling regions, we employ directional couplers with a shallow gap  to control the coupling efficiencies easily. Figure 3(b) shows a schematic cross-sectional view of the coupling region. The width wg and the depth dg of the gap are 0.3 and 1.25 μm, respectively. The round-trip length of each ring resonator is 350.2 μm, which corresponds to an FSR of 1.79 nm. The coupling efficiencies at the directional couplers are controlled by the lengths of the coupling regions, l1 to l3. The designed parameters for proposed WSS are summarized in Table 1.
3.2. Theoretical switching characteristics
Figure 4 shows the theoretical switching characteristics at the drop port of the proposed WSS, assuming that the electric-field-induced change in refractive index of the FACQW core layer is 0.0017, which is comparable to that of the fabricated WSS, as discussed in Sect. 4.1. It was calculated using the transfer function described in Sect. 2 and the device parameters in Table 1, assuming that the coupling loss η2 and propagation loss are 0.177 dB/coupler and 2.1 dB/mm, respectively, and the coupling efficiencies are constant.
The hitless WSS can be obtained using the electric-field-induced change in refractive index in the FACQW core layer by controlling the individual resonant wavelengths of the series-coupled microring resonators, Rings 1 to 4. When the resonant wavelengths of all the microrings in the series-coupled microring resonator are matched, the resonant wavelength channel is transmitted to the drop port. This is the initial ON state. When the refractive index of the waveguides in Rings 3 and 4 is changed by 0.0017 and the resonant wavelengths of the microrings are not matched, all the wavelength channels are transmitted to the through port and no spectral response appears in the drop port. This is the OFF state. Finally, the refractive index of the waveguides in Rings 1 and 2 is changed by 0.0017 and the resonant wavelengths of all the microrings are matched again, and a new spectral peak in the drop-port response appears at other wavelength channels. This is the final ON state. As described above, the resonant wavelength can be shifted to another wavelength channel without blocking other wavelength channels. This is the hitless wavelength selective switching operation. The principle and optimum design of a high-order series-coupled microring WSS are discussed in detail in [3,7,38].
An epitaxial wafer was grown by solid-source molecular beam epitaxy (MBE). The waveguide structures were fabricated by electron-beam lithography, photolithography, and inductively coupled plasma reactive ion etching (ICP-RIE) using a Br-based gas. Since the microrings and buslines are electrically isolated by the gap in the directional couplers, reverse voltages are only applied to the microrings. Figure 5(a) shows a microscopy image of the top of the fabricated WSS. Each microring and busline has an electrode. They are electrically isolated and reverse voltages can be applied to them independently. Figure 5(b) shows a scanning electron microscopy (SEM) image of the sidewall of the waveguide, showing that the sidewall is smoothly formed.
4. Measured switching characteristics
4.1. Measured electric-field-induced change in refractive index in FACQW core layer
To evaluate the electric-field-induced change in refractive index in the FACQW core layer of the microring resonator, we measured the through-port spectral responses of a single-microring resonator fabricated on the same epitaxial wafer under reverse voltages, as shown in Fig. 6(a). Its round-trip length is the same as that of the WSS. The coupling efficiencies in this single microring denoted as KS1 and KS2, were designed to be 0.542. A marked shift in wavelength and no increase in propagation loss were observed. The electrodes were formed only on the microring resonators. Therefore, the microrings are electrically separated from the other waveguides. Figure 6(b) shows the evaluated change in the refractive index of the core layer of the microring waveguide Δncore at resonant wavelengths of 1545.4, 1549.1, and 1552.7 nm as functions of the applied dc reverse bias voltage Va, considering the optical confinement factor (0.527). Considering the filling factor p of the FACQW in the core layer (p = 0.574), the change in refractive index of the FACQW at V = −10 V was evaluated to be approximately 6.6 × 10−3. This result shows that the change in refractive index of the FACQW is almost constant in this wavelength region. The operation range of wavelength of the proposed WSS is the full C band. The wavelength of the absorption edge of the FACQW is approximately 1420 nm that is sufficiently away from the C-band, and the driving voltage and the absorption loss are almost constant in the C band .
This change in refractive index is mainly caused by the QCSE in the FACQW core layer. The TO effect and a carrier injection effect are negligible, because the dark current is very small (approximately 10 μA/350 μm2 at −18V) .
Unfortunately, this change in refractive index in the epitaxial wafer is only one-quarter of the largest measured change . The reason for this is considered to be the fabrication error in the thickness of each layer in the FACQW and the larger thickness fluctuation in the FACQW .
4.2. Static switching characteristics of WSS
Figure 7(a) shows the measured through-port and drop-port spectral responses of the WSS for transverse electric (TE)-polarized input light without applied reverse voltages. The FSR was 1.81 nm, which is comparable to that expected from the designed round-trip length (1.79 nm), and the peak heights were more than 35 dB. The shapes of the drop-port peaks are not symmetrical. This means that the round-trip lengths of the microrings are not identical. Therefore, the adjustment of the round-trip lengths of the microrings is required for the initial ON state, as mentioned later. Figure 7(b) shows a comparison among the drop-port spectral responses of the fabricated quadruple and double series-coupled WSSs and a single-microring filter. The double series-coupled WSS and the single microring filter have the same waveguide structure and round-trip length as those of the quadruple one. An improved boxlike response is successfully obtained in the proposed quadruple series-coupled microring resonator compared with the other resonators.
Figure 8 shows the measured wavelength-switching characteristics of the drop-port spectral response of the WSS. As mentioned above, the round-trip lengths of the four microrings are not identical, and that of Ring 2 was slightly smaller than those of the other microrings. For the initial ON state at λ = 1548.8 nm, therefore, a reverse voltage V2 of 4.0 V was applied to Ring 2 to adjust them, and the voltages V1, V3, and V4 were set to 0 V for the initial ON state, where V1 to V4 are the reverse voltages applied to Rings 1 to 4, respectively. Next, V3 and V4 of 12.0 V were applied to Rings 3 and 4 for the OFF state, respectively. Finally, V1 of 12.0 V and V2 of 13.5 V were applied to Rings 1 and 2 for the final ON state at λ = 1549.5 nm, respectively. The difference between V1 and V2 is not 4.0 V because the electric-field-induced change in refractive index of the FACQW core layer is nonlinear and at a higher reverse voltage, the change in refractive index is larger, as shown in Fig. 6(b). Hitless switching of approximately 0.7 nm owing to the change in the refractive index of the FACQW core layer was successfully demonstrated, as shown in Fig. 8. The full-width at half-maximum (FWHM) of the bandwidth was approximately 0.35 nm. The extinction ratios of the drop-port response of the initial and final ON states were 20.4 and 18.1 dB, respectively, and they were greatly improved compared with those of the double-series microring WSS . The total insertion losses for the through port and the drop port were both approximately 32 dB, as shown in Fig. 7(a). It includes the fiber coupling losses at both facets of approximately 17 dB and the propagation loss in the 2 mm-long buslines from one facet to another of approximately 4 dB, Therefore, the insertion loss of the quadruple series-coupled microring is evaluated to be approximately 11 dB.
In the proposed WSS, the pass band has a ripple on the top because the microrings are more strongly coupled, and it does not satisfy the maximally flat pass band condition  where the Butterworth filter response is obtained, as discussed in Sect. 3.2.
Unfortunately, the wavelength shift of the WSS was not large because the change in the refractive index of the FACQW core layer was smaller than expected, as discussed in the previous subsection. To obtain a larger wavelength shift, we need to use the Vernier effect with slightly different round-trip lengths of the microrings.
4.3. Dynamic switching characteristics of WSS
To evaluate the switching speed of the WSS, we measured the temporal response of the drop port of the single microring resonator with the same waveguide structure as the WSS. An ac square voltage wave with an amplitude of 4.0 V (−9 to −13 V) was applied to the microring. The measured result is shown in Fig. 9. The rise and fall times were 1.5 and 2.0 ns, respectively. The switching speed is much higher than that of the devices based on the TO effect, showing that the WSS was driven by the electric-field induced change refractive index in the FACQW waveguides.
The quality factor (Q factor) of each microring was evaluated to be approximately 5000 from the measured pass band width (0.35 nm). The switching speed of a microring resonator was determined by both the capacitance-resistance (CR) time constant and the Q factor of the resonator. In this WSS, it is considered to be dominated by the CR time constant of the device. However, if the capacitance is greatly reduced and the device is operated under a condition where the Q factor of the resonator is dominant, the switching speed is expected to be approximately 40 GHz.
If the resonant wavelengths of the four microrings are matched with each other, the WSS can be driven with one driving signal. In our current WSS, however, they are slightly different due to fabrication errors. In order to match them, the adjustment of the resonant wavelengths is required using bias voltages, as discussed in Sect. 4.2. Therefore, the applied voltages for the four microrings need to be optimized and controlled separately in this WSS. The dynamic switching characteristics of the WSS have not been measured owing to the lack of a suitable multichannel voltage source. To drive the WSS with a single driving signal, a wavelength-trimming mechanism for each microring, such as microheaters and/or UV trimming of SiON  should be introduced.
5. Improvement of switching characteristics by controlling coupling efficiency
5.1. Change in coupling efficiency at directional couplers during switching
Figure 10(a) shows the change in the depth of the dip at a resonant wavelength in the through-port spectral response of the single microring when reverse voltages are applied to the busline. The single microring resonator is the same as that used in Sect. 4.1. With increasing the reverse voltage applied to the busline, the dip depth decreased. The origin of this change in dip depth is the change in coupling efficiency at the directional couplers. That is, the change in the effective refractive index of the waveguide on the side of the microring in the directional couplers leads to the change in the propagation constant of the waveguide, resulting in the change in coupling efficiency.
The coupling efficiencies can be evaluated using the depth and full width of the dip in the spectral responses of the through and drop ports, as discussed in . Figure 10(b) shows the evaluated changes in the coupling efficiencies Kb1 and Kb2 of the single microring as a function of the reverse voltage applied to each busline. As shown in the figure, Ks1 decreased by 0.12 and Ks2 increased by 0.2 when the applied reverse voltage was changed from 0 to 14 V.
The reason for these coupling efficiency changes can be explained as follows. In the fabrication process of the WSS, the resist patterns for the gaps in the directional couplers are formed by electron-beam lithography and those for the waveguides are formed by photolithography. In this device, it is considered that the positions of all the electron-beam resist patterns for the gaps in Fig. 6(a) were shifted slightly downward from the center of the directional coupler. Figure 11 shows an SEM image of the cross section of the coupler between the microring and the busline of the single-microring resonator. Although the sidewalls of the waveguides are almost vertical, the widths of the waveguides in the drop port and the microring are smaller and larger than the designed width (1.45 μm), respectively. This off-centered position of the groove results in the degradation of the symmetry of the directional couplers. That is, for example, the width of the drop busline is smaller than that of the microring waveguide in Directional Coupler 2 (DC2). When the voltage VDrop is applied to the drop-port busline, its equivalent refractive index increases. As a result, the symmetry of DC2 is recovered, leading to the increase in coupling efficiency Ks2. In the case of Directional Coupler 1 (DC1), the situation is opposite and Ks1 decreases when the voltage VThrough is applied to the through-port busline. This change in coupling efficiency has an effect on the switching characteristics of the WSS.
Figure 12 shows the switching characteristics calculated using the transfer matrix method explained in Sect. 2, considering the dependence of the coupling efficiencies on the applied voltage shown in Fig. 10(b). It reproduces well the measured characteristics shown in Fig. 8.
5.2. Improvement of switching characteristics
By closely examining the static switching characteristics in Fig. 8, we find that the transmission at the center wavelength for the final ON state is slightly smaller than that for the initial ON state. This decrease in transmission is caused not only by the excess loss due to the change in absorption in the microrings but also by the decrease in coupling efficiency between the microring and busline at the drop port when a reverse voltage is applied to the microring, as discussed in the previous subsection.
If this change in coupling efficiency is used positively, the switching characteristics can be improved, as shown in Fig. 13. For the final ON state, when a reverse voltage of 12 V was applied to the busline on the drop-port side in addition to the four microrings, the transmission increased by 1.7 dB owing to the increase in coupling efficiency at C5. It became almost comparable to the transmission in the initial ON state. On the other hand, for the OFF state, when a reverse voltage of 12 V was applied to the busline on the through-port side, the transmission decreased owing to the decrease in coupling efficiency at C1. That is, the extinction ratio improved by 3.0 dB. Therefore, by suitably controlling the coupling efficiencies in the coupling regions, the loss in the final ON state and the extinction ratio in the OFF state can be improved.
We have demonstrated a hitless WSS based on the InGaAs/InAlAs FACQW quadruple series-coupled microring resonators. The WSS was driven by the electric-field-induced change in refractive index of the FACQW core layer caused by the QCSE for high speed operation. The WSS was fabricated on an MBE-grown wafer by ICP dry etching. The fabricated WSS consists of four high-mesa microring waveguides with a round-trip length of 350 nm, and five directional couplers with shallow grooves. An improved boxlike spectral response compared with that of a double series-coupled microring resonator and hitless switching were successfully demonstrated by controlling the voltages applied to the four microring resonators. The FSR and FWHM of the filter peaks at the drop port were approximately 1.9 and 0.35 nm, respectively. The wavelength shift was approximately 0.7 nm at a driving voltage of 12 V. The extinction ratios of the drop-port response of the initial and final ON states were 20.4 and 18.1 dB, respectively. The temporal switching response showed that the WSS is able to be operated much faster than TO-driven devices. In addition, we proposed the improvement of the transmittance for the final ON state and the extinction ratio by controlling the coupling efficiencies at the directional couplers. The experimental results show that the proposed WSS is very promising for use in ROADMs.
The authors express sincere thanks to Dr. T. Kawanishi and Dr. A. Kanno of National Institute of Information and Communications Technology (NICT). This work was partly supported by SCOPE, Ministry of Internal Affairs and Communications, a Grant-in-Aid for Scientific Research B (No. 24360025) from the Ministry of Education, Culture, Sports, Science and Technology, and A-STEP (No. AS251Z02303J) from Japan Science and Technology Agency (JST).
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