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Abstract
We demonstrate a novel thermally tunable grating optical filter based on suspended silicon ridge waveguide with low power consumption, fast response and relatively high mechanical stability. An enhanced power efficiency of ~300 pm/mW is achieved by employing isolation trench. Periodic lateral tethers and anchors are utilized to support the overall waveguide for maintaining mechanical robustness. 10%–90% rising time and falling time are measured to be 78 μs and 52 μs, respectively. Simulations and experiments both indicate that the demonstrated device has a relatively homogeneous temperature distribution, which is essential for practical integrated photonic devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon photonics is a promising technology for photonic integrated circuits and has been considered as an ideal platform for on-chip integration, due to its advantages of high-density chip-scale integration, low cost, low power consumption and complementary metal-oxide-semiconductor (CMOS) compatibility [1]. Up to now, various building blocks in integrated photonic devices have been demonstrated, such as electro-optic modulators [2], MUX/DEMUX filters [3], photodetectors [4] and optical switches [5]. To achieve flexible and reconfigurable optical networks, tunable optical filters are expected and being increasingly developed. Widely adopted tunable filtering configurations include microring resonators [6–12], microdisk resonators [13], Bragg gratings [14–21], Mach-Zehnder interferometers (MZI) [22] and Fabry-Perot microcavities [23]. Among these varieties, thermally tunable silicon grating filters have attracted more interests and have been extensively investigated in the past decades for its high flexibility in wavelength selection and the relatively high thermo-optic coefficient of silicon (~1.8 × 10−4 /K at room temperature for a wavelength of 1.55 μm) [18,24].
The main challenge for the thermally tunable silicon grating filters is high power consumption. Recently, several approaches have been proposed to improve the heating performance. One effective way is to define deep air trenches beside two sides of the waveguide [25], the air trench structure impedes heat generated by metal heater laterally flow to the surroundings, thus improving the power efficiency. However, such a structure is unable to prevent heat to flow to the substrate which is regarded as a major heat sink, hence the enhancement of tuning efficiency is limited. Another new approach is to introduce free standing waveguide with undercut structure [26]. The air trench lies beneath the waveguide, hindering the heat vertically flow to the substrate. Therefore, the power efficiency can be greatly improved. Nevertheless, such isolation trench structures may lead to a fragile waveguide. The deformation of the waveguide may become serious while the waveguide is being heated, resulting in a poor mechanical stability. To maintain the mechanical robustness, vertical dielectric support pillars are utilized to secure the suspended waveguide [27], however, the tuning speed is slow due to the low thermal conductivity of dielectric pillars.
In this paper, we report a high-performance silicon thermally tuned grating filter by combining undercut structure with periodic lateral tethers and anchors. So that a high thermal tuning efficiency of around 300 pm/mW is experimentally achieved, keeping with relatively high mechanical stability at the same time. Furthermore, a fast temporal response of 78 μs rising time and 52 μs falling time is also achieved during tuning process, this is due to a high thermal conductivity (~144 W/m) both for the silicon tethers and anchors, which facilitates lateral flow of the heat from tethers to the substrate. In addition, a relatively uniform temperature distribution is obtained by reasonably design the structural dimensions, such a uniformity can effectively avoid irregular tuning behavior.
2. Device design and theoretical simulation
The sectional structure of the proposed device is shown in Fig. 1(a). The whole structure is designed based on a common silicon-on-insulator (SOI) platform. The thicknesses of the buried oxide (BOX) layer and the top silicon layer are 2 and 220 nm, respectively. The geometrical parameters are reasonably selected to achieve a single mode operation, the inner ridge width W1 is fixed at 500 nm, the slab width W and height h are 1.2 μm and 120 nm, respectively. Figure 1(b) shows the top view of the waveguide with periodic tethers, anchors and vertical Bragg gratings. The total structure can be divided into many repeated units along the longitudinal direction, the beginning and the end of each unit are neighboring tethers. Tethers are utilized to connect anchors and central waveguide. The length l0 of 3 μm is sufficient to maintain mechanical robustness. The width w of 0.4 μm and the interval between adjacent tethers ΔL of 10 μm are required to get a relatively uniform temperature distribution, which is significant in integrated thermally tuned devices. To partially retain the SiO2 underneath the anchors while totally remove that below the waveguide, the width of anchor l is fixed at 6 μm, much larger than the waveguide width. Moreover, arc transitions are produced at the connection between the slab and tethers with a radius of curvature of near 200 nm, which can effectively decrease thermal deformation without causing extra optical loss in operation. Furthermore, sidewall Bragg gratings on the inner ridge are selected to control the transmission spectrum, which can be fabricated together with the inner ridge, thus simplifying the fabrication steps. The grating period is fixed at 300 nm and the effective refractive index of fundamental TE mode neff is calculated to be 2.593 using 2D simulation tool MODE Solution [28], so that the central wavelength λB is approximately 1556 nm according to Bragg condition λB = 2neff . In addition, the period number is 2000, corresponding to 600 μm total grating length. Figure 1(c) displays the cross section of the waveguide with an SiO2 upper cladding layer of 800 nm. The oxide layer is to effectively avoid metal-induced light absorption. The fundamental TE mode profile of the demonstrated rib waveguide is shown in Fig. 1(d). It is apparent that the mode profile is far away from the slab sidewall, which indicates that the periodic tethers connecting anchors and slab sidewall will not introduce noticeable extra optical loss. The overlap of mode field’s profile and waveguide sidewall is quite low, making it possible to achieve weaker coupling coefficient and narrower bandwidth compared with strip waveguide [29].
Fig. 1 (a) Three-dimensional schematic diagram of the device based on suspended ridge waveguide (without SiO2 upper cladding layer and metal). (b) Top view of the rib waveguide Bragg grating with periodic lateral tethers and anchors. (c) Cross section of the unperturbed ridge waveguide with a thick SiO2 upper cladding layer. (d) Simulated fundamental TE mode profile of the rib waveguide.
By using 3D Finite-difference time-domain (FDTD) method, bandwidth and coupling coefficient of the gratings on ridge waveguide and gratings on typical strip waveguide (500 nm width and 220 nm height) for comparison are both simulated. As shown in Fig. 2, both the bandwidth and coupling coefficient of the designed waveguide Bragg gratings are much smaller than the strip waveguide gratings, which confirms that the optical field in strip waveguide has a larger overlap with the sidewall. For the following demonstration, the corrugation widths are selected at 60 nm and 80 nm for the two designs, and the corresponding bandwidths at the first nulls are around 7.4 nm and 11.7 nm, respectively.
Fig. 2 Simulated (a) bandwidth (wavelength interval between the first nulls) (b) coupling coefficient varies with corrugation width for the gratings on the designed rib waveguide and the gratings on typical strip waveguide.
Based on our previous work [30], temperature distribution and thermal deformation distribution of the designed device are simulated using commercial software COMSOL Multiphysics. The simulated results with 9 mW heating power is presented in Fig. 3. As shown in the inset of Fig. 3(a), the highest temperature (~360K) locates at the center of each repeated unit while the lowest temperature (~355K) occurs at the connection between the tether and waveguide, indicating a relatively uniform temperature distribution. Additionally, it can be seen that the temperature decreases sharply from waveguide to tethers, the anchors are kept at room temperature when the waveguide is being heated, indicating that the designed structure has potential advantages in integration due to ultra-low thermal crosstalk. It is also observed from Fig. 3(b) that the designed device has a homogenous thermal deformation distribution, and the maximum value is less than 3 nm. Such a small deformation promotes good stability of the structure during the tuning process. The simulated results suggest that the designed device has excellent thermal properties, which is important to realize efficient thermal tuning.
Fig. 3 (a) Simulated temperature distribution of the demonstrated structure. A repeated unit is shown in the inset. (b) Simulated thermal deformation distribution of the device.
3. Fabrication
The fabrication procedure of the devices is shown in Fig. 4. The inner ridge and sidewall Bragg grating are first defined by electron beam lithography (EBL), and the photoresist pattern is transferred to the top silicon layer through a dry etching process with SF6 and C4F8 chemistry using a PlasmaLab 100 machine. Then the slab, tethers and anchors are formed together in another EBL and ICP etching processes. By using lateral undercut etching method [31], as shown in Fig. 4(c), the air trench is produced in the isotropic wet etching process using concentrated hydrofluoric (HF) acid. In this step, the temperature, etching time and concentration of HF need to be strictly controlled. The pretreated structure is put into the adequate HF solution with mass fraction of 40% for 60 s at room temperature, after that, the SiO2 beneath the anchors is exclusively partially removed when that below the waveguide is completely removed because the size of anchor is much larger than that of waveguide. Therefore, the isolation trench is produced and the released waveguide could be supported by remaining unetched oxide layer. Afterwards, an 800 nm SiO2 cladding layer is deposited on the top by plasma enhanced chemical vapor deposition (PECVD) to isolate metal absorption. Finally, 40 nm/160 nm Ti/Au heaters are deposited through EBL and lift-off processes.
Fig. 4 Fabrication circuit of the device: (a) the definition of inner ridge and Bragg gratings. (b) the definition of outer ridge, tethers and anchors. (c) removal of the oxide layer beneath the waveguide to release the structure. (d) deposition of upper cladding layer by PECVD. (e) deposition of Ti/Au heater.
Figure 5 shows the scanning electron microscope (SEM) images of the fabricated device. Focusing grating couplers (FGCs) are used to couple light into and out of the designed device. Since the grating couplers are designed for the typical strip waveguide (500 nm width and 220 nm height), double layer linear tapers are adopted to efficiently couple light between strip waveguide and the designed ridge waveguide. The taper shown in Fig. 5(c) can be formed together with the ridge waveguide through two step etching processes during the fabrication, corresponding to Fig. 4(a) and Fig. 4(b), respectively. Furthermore, the total length of the taper is 30 μm, which is sufficient to ensure that the transition loss is negligible [32]. The designed arcs are displayed in Fig. 5(d). Since the maximal thermal stress locates at the connection between the slab and the tether, which enables a large thermal deformation. The arc transitions are helpful to reduce the thermal stress and the corresponding thermal deformation, thus improving the structural stability. The struts shown in Fig. 5(e) were utilized to support the suspended strip waveguide and taper that would also be released in the wet etching process, 0.6 μm width of the strut is enough to ensure the mechanical stability without causing notable optical loss.
Fig. 5 (a) SEM images of the overall structure. (b) Focusing grating coupler (FGC) for vertically coupling between fiber and the strip waveguide. (c) Schematic of the double layer linear taper. (d) Magnified sidewall Bragg grating (SBG), tethers and anchors, the arc transitions are used to reduce the thermal deformation in operation. (e) SEM images of the double layer linear taper and the struts, the struts are used to support the released taper and the strip waveguide.
4. Measurement results
4.1 thermal tuning efficiency
Static performances of the two devices are first studied. The spectral responses are measured using Amonics ALS-18 amplified spontaneous emission (ASE) source with center wavelength of 1550 nm and ANDO AQ6319 optical spectrum analyzer (OSA) together with a power supply. The TE-polarized light from the single mode fiber with a polarization controller (PC) is coupled to the waveguide by the FGC, and the transmitted light is then fed into the OSA. The transmission spectra of the two devices with varying heating powers are shown in Fig. 6(a) and Fig. 7(a), respectively. It is obvious that the center wavelength locates at ~1550 nm at room temperature, the slight deviation of the tested center wavelength with the designed value (~1556 nm) is mainly due to the deviation of waveguide dimensions. The measured bandwidths between first nulls are around 8 nm and 11 nm for corrugation widths of 60 nm and 80 nm, respectively, which are in good agreement with the predictions displayed in Fig. 2(a). In addition, it is clear that the spectral shape is kept unchanged while tuning, indicating that the temperature distribution is homogenous, hence the irregular thermal tuning behavior is avoided. Here we note that the measured ~25dB attenuation in the filter stopband is smaller than expected, this is mainly due to the high fiber chip coupling loss and the low output power from the ASE source.
Fig. 6 Measured results of the device with 60 nm corrugation width. (a) Transmission spectra under different heating powers. (b) Wavelength shift as a function of heating power.
Fig. 7 Measured results of the device with 80 nm corrugation width. (a) Transmission spectra under different heating powers. (b) Wavelength shift as a function of heating power.
The thermal tuning efficiency of the two devices are then calculated and presented in Fig. 6(b) and Fig. 7(b), respectively. The blue circles denoted the processed resonance shift with different heating power. It can be observed that the tuning efficiency (wavelength shift per unit power) are approximately 287 nm/mW for corrugation width of 60 nm and 326 nm/mW for corrugation width of 80 nm, respectively. In general, the two devices have similar tuning efficiencies, the deviation of the measured values is mainly caused by two factors: one is the small difference of measured electrical power, the other is the ripples of measured optical powers of resonant wavelength under different heating powers. Here the thermal tuning efficiency of the device is evaluated as 300 pm/mW.
4.2 temporal response
Dynamic characteristics of the demonstrated device are also investigated. Figure 8 shows the measurement setup for temporal response. A 2 kHz square waveform electrical signal with an electrical heating power of 23.6 mW is applied to the metal pad of the device (with 60 nm corrugation width). The probe wavelength λs of 1550 nm is excited by Alnair Labs TLG-200 narrow linewidth tunable laser source, and then injected to fiber. The modulated light from the output FGC is detected by photodetector (PD) and then recorded by an oscilloscope.
The measured temporal response is exhibited in Fig. 9. It can be concluded that the 10%–90% rising time and decaying time are 78 μs and 52 μs, respectively. The difference between the rising and falling times is mainly caused by the relative detuning between the probe wavelength and the Bragg wavelength. In addition, the transient process of heating and cooling is also slightly different. Clearly, the response speed is much faster than that of the conventional undercut structures [10,33]. The main reason is that the silicon tethers and anchors both have a high thermal conductivity (~144 W/m), as a result, accelerating the heat laterally flow through the periodic tethers and then to the underlying silicon substrate which is the major heat sink. Such a fast response enables implementation in more reconfigurable devices and systems without considering the restraint of tuning speed.
Fig. 9 (a) Measurement results of temporal response, the blue line represents original square waveform electrical signal, the orange line represents temporal response recorded by an oscilloscope. (b) Zoom in view of Fig. 9(a) with time range from 2 ms to 3 ms, the falling time τf is ~52 μs, the rising time τr is ~78 μs.
5. Discussion
In the above experiment, the central wavelength is slightly deviated from the designed value, the major issue is the fabrication errors, because the wavelength-selective device is highly sensitive to dimensional variations. Both the deviations of width and thickness can cause a spectral shift by changing the effective refractive index of guiding mode. Since the thermal tuning method is generally used for accurate compensation in practice [29], the practical central wavelength of the proposed device can be well calibrated. In addition, we use titanium as the adhesion metal between gold and SiO2 during the fabrication, nevertheless, the titanium has a low thermal conductivity, resulting in a slow response speed. We can use nickel with a much higher thermal conductivity to substitute titanium, leading to a faster temporal response.
Table 1 summarizes the tuning efficiency and the temporal response for several reported thermally tuned optical filter configurations in the literature, mainly including microring resonators (MRs) [10–12], microdisk resonators (MDRs) [13], Bragg gratings (BGs) [17,21,26] and contra-directional couplers (contra-DCs) [19]. Among the various types of filters, microring resonators are usually more power efficient for their compact footprint, the typical value is ~250 pm/mW [34], and the Bragg grating filters often require more power to realize effective tuning for their much larger size. The enhanced power efficiency of the two filter configurations can be achieved by use of undercut structure, however, undercut structures significantly reduce the response speed and mechanical robustness [10]. In our work, the power efficiency is drastically enhanced by employing air trench, fast response and relatively high mechanical stability are simultaneously achieved by utilizing periodic lateral tethers and anchors. The demonstrated device has unique structural features as periodicity and repeatability, making it applicable to many integrated optical devices such as thermo-optic switches and thermo-optic modulators.
Table 1. Performance comparison of recent thermally tuned filter works
6. Conclusion
We proposed and demonstrated a thermally tunable optical filter based on silicon waveguide Bragg gratings with high tuning efficiency, fast temporal response and relatively high mechanical stability. The undercut structure is utilized to substantially improve the thermal tuning efficiency. Meanwhile, the periodic lateral tethers and anchors are designed to support the suspended silicon ridge waveguide, significantly increasing the mechanical stability. The relatively uniform temperature distribution is realized by optimizing the device structure. Furthermore, small thermal stress and corresponding ultra-low thermal deformation are achieved by employing arc transitions. We experimentally measured the tuning efficiency of ~300 pm/mW, which is relatively high among analogous devices. The response time is 78 μs for the rising time and 52 μs for the decaying time, respectively. The response speed is faster than that of the previously reported structures with undercut structures. The proposed device also has an ultra-low crosstalk with adjacent devices due to its special structural features. Such thermal tuning structure may find many potential applications in integrated photonic devices.
Funding
National Natural Science Foundation of China (NSFC) (61675073); Fundamental Research Funds for the Central Universities (2016YXZD004)
Acknowledgments
The authors would like to thank all of the engineers at the Center of Micro-Fabrication and Characterization (CMFC) of WNLO and Dr. Yuan Yu’s team for their support and valuable suggestions in the experiment.
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