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Abstract
By analyzing the temperature-dependent transmission spectrum of a Ta2O5 micro-ring resonator, the thermal-optical coefficient at the optical communication wavelength regime has been determined. The temperature-dependent resonance wavelength shift of the resonator with radius 50 µm was measured as 7.8 pm/K, and the thermal-optical coefficient at ∼1543 nm was estimated to be 6×10−6 /K. Furthermore, a power-dependent transmission of the micro-ring resonator was performed to characterize the effects of local heating and nonlinear absorption. No significant power-dependent wavelength shift and asymmetric response were observed in the optical wavelength up to 125 mW. This suggests the absence of nonlinear absorption and thermal expansion effects in the Ta2O5 ring resonator. The Ta2O5 waveguide is potentially a superior candidate for applications in optical communication systems or high-power nonlinear waveguides due to its thermal stability, thermal insensitivity, and absence of nonlinear absorption.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
To achieve ultra-high-speed data processing and transmission in optical communication networks, using high-data-rate modulators integrated with passive wavelength-division multiplexing (WDM) devices is a popular approach to realize high-capacity data processing. Recently, Si photonics has been widely developed into versatile functional devices for on-chip high-speed data transmission, including Si-based ring resonators [1–4], Si modulators based on the Mach–Zehnder interferometer (MZI) [5–9], and Si-based array waveguide gratings (AWGs) [10]. However, a high Si thermal-optical coefficient (dn/dt = 1.86×10−4 /K) renders these devices are thermally sensitive [11]. Therefore, the introduction of local heating due to an increase in the injected optical power to those Si-based devices or a change of the environment temperature has a significant effect on the optical properties. In this case, the optical properties of the modulator, filter, and AWG devices will change. To eliminate or minimize the thermal effect in Si-based devices, three approaches have been investigated. Firstly, the basic concept is to directly control temperature by directly heating the device or using an external metal heater [12,13]. However, this will cause an increase in the power consumption of the system. Another approach is to deposit a polymer with negative thermal-optical property as a cladding layer on the Si core to compensate for the positive thermal-optical property of the Si [14,15]. The disadvantage of this approach is that it is not CMOS compatible. In addition, the polymer is very sensitive to humidity and the optical properties are difficult to control. Guha et al. proposed an athermal Si-based MZI device by carefully designing the geometric structure of the two arms of the interferometer [16]. The waveguide width of the two arms are different and the effective index variation with temperature is different in each arm. By careful selection of the length of the arms of MZI, it is possible to mutually compensate for the thermal sensitivity of the two arms. The central wavelength shift with temperature of the athermal Si-based MZI structure is ∼0.005 nm/K [16]. However, to realize such a performance, these devices should be carefully designed and fabricated with strict tolerance.
In the preceding paragraph, we examined the solution of decreasing the thermal sensitivity of Si waveguide devices. However, if an alternative material can be identified with a smaller thermal-optical coefficient than Si, then thermally-insensitive waveguide devices can be realized without external temperature control or a complex geometric designing. Tantalum pentoxide (Ta2O5) has recently been utilized to realize a wide range of waveguide device functions [17–19]. Its low loss property and high optical nonlinearity have been shown to facilitate versatile nonlinear waveguide applications such as ultrafast all-optical modulation and nonlinear wavelength conversion [20–23]. Furthermore, Chu et al. measured the thermal-optical coefficient of Ta2O5 large-core waveguide at 633 nm using a free-space MZI setup [24]. The thermal-optical coefficient was determined to be only 2×10−6 /K, which is much smaller than that of Si and Si3N4 [25,26]. In addition, the fabrication process for Ta2O5 waveguides is also CMOS compatible [27–29], which implies that thermally insensitive Ta2O5 waveguide devices can be easily integrated with Si-photonics platforms. However, the thermal-optical property of Ta2O5 in the communication regime has seldom been addressed.
In this work, we utilized a high-quality Ta2O5 based micro-ring resonator to characterize the thermal-optical coefficient of a Ta2O5 waveguide in the communication wavelength regime. The advantage of using a ring resonator is that this device is not only sensitive to refractive index variation, but it is also more compact compared with the MZI. By measuring the temperature-dependent transmission spectrum of the Ta2O5 micro-ring resonator, the thermal-optical coefficient of the Ta2O5 waveguide was determined. Furthermore, we also measured the properties of the Si-based micro-ring resonator for comparison with the Ta2O5 case. In addition, by measuring the power-dependent transmission spectrum for both the Si and Ta2O5 ring resonators, we demonstrated the outstanding thermal stability, thermal insensitivity, and absence of nonlinear absorption of Ta2O5 waveguides.
2. Device fabrication and experimental setup
Figure 1(a) shows an illustration of the Ta2O5 micro-ring resonator. The fabrication process of this device is described below. First, the Ta2O5 film with a thickness of 700 nm is deposited on a thermally oxidized Si wafer using an RF sputtering technique. After deposition, the Ta2O5 film is annealed at 650°C in an O2 environment to compensate for the oxygen deficiency of the as-grown Ta2O5 film during the sputtering process. The details of the material properties used can be found in our previous work [18]. The micro-ring and straight waveguide pattern are defined by e-beam lithography. The width of the bus waveguide is set as 700 nm for single mode operation. The width and ring radius of the micro-ring resonator are set as 1.5 µm and 50 µm, respectively. Such geometric design is utilized for the ring resonator to achieve anomalous dispersion for nonlinear wavelength conversion applications [22]. To increase power coupling between the lens fiber and the waveguide device, the ends of the waveguide are tapered from 700 nm down to 300 nm at the waveguide facet, where the length of the tapered region is 200 μm. After lithography, the defined patterns of photo resistor were annealed for reflow to smooth the sidewall of the pattern and the mask for the Ta2O5 [30]. The Ta2O5 film is then etched using reactive ion etching with CHF3 plasma. After the etching process, the resist mask is removed using an oxygen plasma. The SiO2 cladding with a thickness of 2 µm is deposited on the Ta2O5 waveguides by plasma-enhanced chemical vapor deposition. Finally, the waveguide chip is diced and polished at both facets.
Fig. 1. (a) Illustration and SEM images of the Ta2O5 micro-ring resonator. (b) Experimental setup for measuring the transmission spectrum of the Ta2O5 micro-ring resonator.
To measure the thermal-optical coefficient of the Ta2O5, a temperature-controlled setup is applied as shown in Fig. 1(b). The light source is a broadband source ranging from 1540 nm to 1570 nm. A polarization controller is utilized to adjust the input beam as TE polarized light. The input beam is then coupled into the waveguide using a lens fiber with a mode diameter of 2 µm. The substrate temperature of the waveguide chip is controlled by a thermoelectric temperature controller. Finally, the output beam is collected by the lens fiber and the temperature-dependent transmission spectra of the ring resonator are displayed on an optical spectrum analyzer.
3. Results and discussions
3.1 Optical properties of the Ta2O5 ring resonator
Figure 2 shows the transmission spectrum of a Ta2O5 micro-ring resonator at a measurement temperature of 20°C. The polarization is set as TE. Although the ring resonator supports high-order TE modes in the cavity, the measured transmission spectrum of the Ta2O5 ring resonator is dominated by the fundamental TE mode. The free spectral range of the fundamental TE mode is ∼3.39 nm at approximately 1543 nm. To determine the optical properties of the Ta2O5 ring resonator, the transmission spectrum of an all-pass-type Ta2O5 micro-ring resonator is fitted using Eq. (1) [31],
Fig. 2. Transmission spectrum of the Ta2O5 ring resonator at a substrate temperature of 20°C and the red line represents the corresponding fitting result.
3.2 Estimation of thermal-optical coefficient of Ta2O5 waveguides
To further extract the thermal-optical coefficient of Ta2O5 waveguide in the communication regime, we performed temperature-dependent measurements on the Ta2O5 ring resonator. The temperature-dependent transmission spectrum of the Ta2O5 micro-ring resonator is shown in Fig. 3(a). When the substrate temperature is increased from 20°C to 40°C, the wavelength of the resonance dip is red-shifted from 1543.430 nm to 1543.586 nm. The wavelength of the resonance dip has a linear relationship with the substrate temperature, and the slope is estimated to be 7.8 pm/K (refer to Fig. 3(b)). Conclusively, the resonance wavelength of the Ta2O5 micro-ring resonator is mainly attributed to either thermal-optical or thermal expansion effects. The thermal expansion effect causes a change in the perimeter of the ring resonator when the substrate temperature is varied, and the resonance wavelength will change accordingly. On the other hand, when the temperature of the substrate is increased, the thermal-optical effect will directly alter the refractive index of Ta2O5 and SiO2 materials. Therefore, the effective index (or group index) of an optical waveguide will be impacted, and the resonance wavelength of the micro-ring resonator will shift accordingly. To precisely calculate the thermal-optical coefficient of the present Ta2O5 waveguide, the contribution of the thermal-optical and thermal expansion effects on the temperature-dependent measurement results of the Ta2O5 micro-ring resonator have been considered. The resonance condition in the ring resonator is attained when the phase change (ϕ) of the round-trip in this structure is an integer multiple of 2π, and the simplified formula is shown in Eq. (2),
where L is the perimeter of the ring resonator that expands when the substrate temperature is increased due to the thermal expansion effect. We set the thermal expansion coefficient as 2×10−6 /K [32] to estimate the thermal optical coefficient of Ta2O5 because the thermal expansion of Si is similar to that of Ta2O5 [32,33]. The resonance order for the resonance dip at ∼1543 nm is estimated to be 386. In this case, the temperature dependence on the effective index of the Ta2O5 ring resonator can be determined using Eq. (2) and the result is shown in Fig. 3(c). When the substrate temperature is increased from 20°C to 40°C, the effective index of the resonator is increased from 1.896376 to 1.896491. It can be deduced that the thermal-optical coefficient of the Ta2O5 ring resonator in the communication regime is ∼5.75×10−6 /K.Fig. 3. (a) Temperature-dependent transmission spectrum of the Ta2O5 ring resonator. (b) Relationship between the resonance wavelength and the substrate temperature of the Ta2O5 micro-ring resonator. (c) Temperature-dependent effective index of the Ta2O5 micro-ring resonator deduced from Eq. (2).
The preceding analyses on the thermal-optical coefficient of Ta2O5 waveguide are based on solving the variation of the resonance condition as a function of the substrate temperature. By analyzing the position of the resonance wavelength, the temperature-dependent effective index of the Ta2O5 waveguide can be determined. However, these analyses only consider a single resonance dip in the transmission spectrum of the ring resonator. For comparisons, we have fitted the temperature-dependent transmission spectra of the Ta2O5 micro-ring resonator using Eq. (1). In this case, we fit the transmission spectrum with a small wavelength span and consider at least two resonance dips. By increasing the substrate temperature from 20°C to 40°C, the resonance dip at ∼1543 nm is red-shifted with a value of 156 pm. The wavelength shift contributed by the thermal expansion is estimated to be 62 pm and the remainder of the shift originate from the thermal-optical effect. From our fitting results, it is determined that the group index variation (Δng) in Eq. (1) is 1.36×10−4 when the substrate temperature is increased from 20°C to 40°C. The corresponding thermal-optical coefficient of Ta2O5 is determined to be ∼6.8×10−6 /K. This parameter is estimated for the Ta2O5 waveguide based on the effective index (neff) and the group index (ng). It can be seen that the extracted values for the thermal-optical coefficient are similar for these two methods.
We also characterized the thermal-optical properties of a Si-based micro-ring resonator for comparison with the Ta2O5 micro-ring resonator. The dimensions of the Si core is 450×220 nm2 and the radius of the ring resonator is 2.5 µm. The Si core is covered with SiO2 to form a channel waveguide structure. The temperature-dependent transmission spectrum for TE polarization of the Si micro-ring resonator is shown in Fig. 4(a). By increasing the substrate temperature from 41°C to 215°C, the resonance wavelength is red-shifted from 1542.3 nm to 1557.5 nm. The slope of the wavelength shift for the Si ring resonator is 87.3 pm/K, which is much higher than that obtained for the Ta2O5 case. In addition, the temperature-dependent effective index of the Si waveguide is determined using Eq. (2) to solve for the resonance condition of the ring resonator. When the substrate temperature is increased from 41°C to 215°C, the effective index of the Si waveguide is enlarged from 2.454647 to 2.477976 (refer to Figs. 4(b) and 4(c)), which is attributed to the thermal-optical coefficient of 1.34×10−4 /K for the Si waveguide. It is evident that the Ta2O5 waveguide exhibits greater thermal insensitivity compared with the Si waveguide. If Ta2O5 is utilized for passive WDM devices for communication applications, it can facilitate minimal crosstalk in the case of severe temperature variation.
Fig. 4. (a) Temperature-dependent transmission spectrum of a Si ring resonator. (b) Relation between the resonance wavelength and the substrate temperature of the Si micro-ring resonator. (c) Temperature-dependent effective index of the Si micro-ring resonator deduced using Eq. (2).
3.3 Power-dependent transmission spectrum of the Ta2O5 ring resonator
In the previous section, we examined the thermal-optical properties of the Ta2O5 micro-ring resonator by directly changing the substrate temperature. In this section, we will examine the local heat induced by injecting a high-power laser into waveguide devices. In particular, operating the ring resonator at the resonance wavelength will cause an accumulation of power in the resonator due to constructive interference. In this case, if there is any linear or nonlinear absorption inside the ring cavity, the local heat induced by absorption will influence the optical properties of the ring resonator. To examine whether the aforementioned effects exist in the Ta2O5 ring resonator, we utilize a tunable laser source to scan the ring resonator and track the transmittance. The optical power delivered by the tunable laser source is much higher than that of the broadband light source that was used in the previous section. Figure 5(a) shows the power-dependent transmission spectrum of the Ta2O5 ring resonator with coupled input power in the range of 0.25 mW to 2.5 mW. It is evident that the transmission spectrum obtained using the tunable laser source seems the same in all cases. This implies that there is no severe nonlinear absorption loss in the Ta2O5 ring resonator. A similar approach is applied to the Si micro-ring resonator and the resulting power-dependent transmission spectrum is shown in Fig. 5(b). When the tunable laser is coupled into the Si ring resonator with a power of 90 µW, the scanned transmission spectrum becomes asymmetric. This is due to the occurrence of two-photon absorption (TPA) in the Si ring resonator [34–36]. When the tunable laser is scanned into the Si ring resonator, the closer the approach to the resonance wavelength, the more power is accumulated in the ring resonator. Therefore, PA is more significant when the tunable laser is scanned close to the resonance dip. The local heating induced by absorption as well as the free-carriers induced by TPA will influence the refractive index of Si, thereby, considerably distorting the scanned transmission spectrum.
Fig. 5. Power-dependent transmission spectra of (a) Ta2O5 micro-ring resonator and (b) Si micro-ring resonator.
In comparison, Ta2O5-based waveguide devices exhibit better thermal stability compared with Si-based waveguide devices. In addition, the Ta2O5 micro-ring resonator is a low-loss, high-Q ring cavity (loaded Q is 55000). Under resonance condition, the buildup factor at the resonance wavelength is ∼50 [37]. This implies that the power can be enhanced up to 50 times in the ring resonator compared with the bus waveguide at the resonance wavelength. Therefore, the resonance power in the Ta2O5 ring resonator can be as high as 125 mW when a power of 2.5 mW is coupled to the bus waveguide. Our primary results indicate that Ta2O5 waveguide devices exhibit outstanding thermal insensitivity and thermal stability. Nonlinear absorption is not a primary concern in Ta2O5 based waveguide devices, even for operation at ultrahigh peak powers.
4. Conclusion
The thermal-optical coefficient of a Ta2O5 waveguide in the communication wavelength regime were successfully characterized by analyzing the temperature-dependent transmission spectrum of a Ta2O5 micro-ring resonator. For a resonator with a radius of 50 µm, the temperature-dependent resonance wavelength shift was 7.8 pm/K. By analyzing the temperature-dependent resonance condition in this ring resonator, the thermal-optical coefficient of the Ta2O5 waveguide was estimated to be 6×10−6 /K. In addition, a Si micro-ring resonator was utilized for comparison with the Ta2O5 micro-ring resonator. For the Si ring resonator with a ring radius of 2.5 µm, the temperature-dependent resonance wavelength shift was 87.3 pm/K and the thermal-optical coefficient was 1.34×10−4 /K. Furthermore, we also measured the power-dependent transmission spectra for both the Ta2O5 and Si ring resonators. For the Si micro-ring resonator, an asymmetric transmission spectrum is observed with a coupled power of only 90 µW. The thermal instability of the Si ring resonator is due to TPA and TPA-induced free-carrier plasma dispersion effects. In contrast to Ta2O5 ring resonator, the transmission spectrum did not exhibit any difference when the coupled power was increased up to 2.5 mW. Under the resonance condition for the Ta2O5 high Q ring resonator, the buildup factor for the resonance wavelength was estimated to be ∼50. Even when a resonant power of up to 125 mW is accumulated in the ring cavity, the scanned transmission spectrum of the Ta2O5 micro-ring resonator was not considerably affected by thermal-optical and thermal expansion effects. Our primary results prove that Ta2O5 waveguides possess outstanding thermal stability and the absence of nonlinear absorption. Thus, they can potentially be utilized to develop either passive WDM devices or for nonlinear waveguide applications.
Funding
Ministry of Science and Technology, Taiwan (MOST) (MOST 106-2112-M-110-006-MY3, MOST 106-2221-E-110-050-MY3, MOST-106-2112-M-110-011).
Acknowledgments
This work is partially supported by the Ministry of Science and Technology of the Republic of China, Taiwan under Contract No. MOST 106-2112-M-110-006-MY3, MOST 106-2221-E-110-050-MY3and MOST-106-2112-M-110-011-. Dr. Wu is now in Taiwan Semiconductor Manufacturing Company Limited (TSMC).
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