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High quality factor silicon oxynitride optical waveguide ring resonators
Optical Materials 85 (2018) 138–142
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
High quality factor silicon oxynitride optical waveguide ring resonators ∗
T
Yue Jia, Xiaoyu Dai, Yuanjiang Xiang , Dianyuan Fan International Collaborative Laboratory of 2D Materials for Optoelectronic Science & Technology of Ministry of Education, Engineering Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SON Q factor Ring resonator LSCVD Bragg-grating coupler
In this manuscript, we report the design, manufacturing and characterization of silicon oxynitride (SiON) based optical waveguide ring resonators (OWRR) which achieve higher quality factors with respect to the state-of-theart waveguide ring resonators. To improve the coupling efficiency, Bragg grating couplers are used at both ends of the designed resonators instead of the conventional directional couplers. In the proposed design, the core of OWRR structure was realized in the optimized refractive silicon oxynitride films deposited by liquid source chemical vapor deposition (LSCVD), under controllable composition ratio, time slot duration, and deposition temperature. The SiON based resonators have achieved a measured waveguide loss of 4.07 dB/cm and a quality factor of 0.93 × 105 in the transverse electric (TE) mode, and this by the way outperforms the results fulfilled with the state-of-the-art waveguide ring resonators.
1. Introduction Optical waveguide ring resonators (OWRR) play an important role in the construction of many optical devices such as sensing systems [1], optical de-multiplexing systems [2], optical filters [3,4], and optical modulators [5], due to their excellent transmission characters, like wavelength selectivity, tunability [6,7], special compact size, and flexible structure. Recently, it turns out that silicon oxynitride (SiOxNy) materials, which are always written as SiON for short, represent an excellent choice for fabricating the OWRRs due to their controllable high wide range refractive index ranging from 1.45 (SiO2) to 2.0 (Si3N4) [8–11]. Accordingly, it can be used to produce medium refractive index contrast optical waveguide [12–14]. Additionally, due to their low transmission losses, this kind of SiON waveguide is beneficial in reducing the coupling loss, scattering loss, and tolerance sensitivity of the OWRRs [15]. Moreover, compared to SiO2 waveguide, its eminent optical transparence of SiON ranges from 210 nm to beyond 2000 nm, this makes it possible to manufacture low loss optical waveguides [16]. Furthermore, SiON optical waveguides can be lightly produced by standard photolithography and reactive ion etching (RIE) technique [17]. Some SiON optical waveguide devices, such as integrated spectrometers [18], adaptive gain equalizers [19] or micro ring resonators [20], rings [21], and elliptic couplers [22] have been also reported. High quality factor (Q) ring resonators are considered as a cornerstone constituent in optical facilities, for instance, low power modulators, high sensitivity sensors, and narrow pass band filters [23,24].
∗
One of the important requirements that should be satisfied in ring resonators is the higher quality factor, because high quality factor value can enhance many waveguide parameters of the resonators [25,26]. For the sake of improving the Q factor of the ring resonators, many efforts have been done, including enlarging the light maintenance within the ring as well as reducing the bending loss of the device. Among the effective methods which can be used to decrease the bending loss, reduce the scattering loss of OWRRs by smoothing the sidewall roughness, and increase the radius of the ring resonators, show excellent performance especially for cases that seek ultra-high quality micro ring resonators [27]. This paper describes a type of optical waveguide ring resonators which are deposited with SiON core layers and indicates the optical features of the resulting OWRRs. We introduce OWRRs with SiON films, produced by the Liquid source CVD (LSCVD) method, in which Bragg grating couplers are employed at both the output and input terminals to couple light to the resonators directly. LSCVD is the preferred process for the non-volatile molecular precursors with a very low vapor pressure. Serious OWRRs with different radius are introduced, depositions and fabrications are also used to optimize these high quality and low loss OWRRs. A propagation loss of 4.07 dB/cm and a quality factor of 0.93 × 105 are achieved from OWRR with 150 μm radius and 0.2 μm gap width. This study provides a method for developing high quality OWRRs and their optimization by utilizing LSCVD deposition of the SiON film and demonstrates their feasibility on the low energy, small footprint applications of photonic integration platforms.
Corresponding author. E-mail address: [email protected] (Y. Xiang).
https://doi.org/10.1016/j.optmat.2018.08.031 Received 23 April 2018; Received in revised form 7 August 2018; Accepted 11 August 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 85 (2018) 138–142
Y. Jia et al.
Fig. 1. (a) Change of coupling efficiency versus thickness; (b) Change of coupling efficiency versus etching depth; (c) Change of coupling efficiency versus grating period; (d) Change of coupling efficiency versus slot width.
2. Experimental
1.45, respectively, and the wavelength is set to 1550 nm. It is observed that most of the light in the waveguide extends deeply into the SiO2 layer. Such a field distribution can be attributed to both the high refractive index contrast and the ultra-thin SiON. The deposited core layer SiON is sandwiched between the bottom cladding of silicon dioxide (SiO2) and the over cladding of a poly (methyl methacrylate) (PMMA). The lower cladding layer is deposited by LSCVD with a thickness 5.5 μm above the Si substrate. The SiON layer with a thickness of 0.32 μm and controlled refractive index upon SiO2 layer is fabricated using a normative parallel-plate LSCVD system. The liquid of this LSCVD system is SiN-X source (SiN-2). By controlling the flow rates of N2 and N2O, the refraction index of the SiON films could be alterative from 1.45 to 2.0. SiON film is grown with the thickness of 0.32 μm, and refractive index of 1.81 [28,29] at temperature of 150 °C with the flow rate of N2 and N2O which is 100 and 10 sccm, respectively. Finally, we chose SiN-2 in a flow rate of 1.0 sccm and deposition time of 6.5 min to get this refractive index by serious examining the refractive index with different SiN-2 flow rate and time. The patterns of OWRRs, and grating couplers are all transferred onto the resist layer upon SiON by applying the electron beam lithography (EBL) technique. Direct writing and high resolution of EBL allow to get the high accuracy and small feature size of the OWRRs. The samples are developed by oxylene for one minute and exposure baked for 5 min at 145 °C to re-flow the resist surface further to decrease the
The light coupling efficiency from a grating to a single mode fiber (SMF) is calculated with the assumption that it is the same in the reverse direction, from fiber to the grating. A wavelength of 1550 nm TE polarized light is sent through the Bragg grating coupler to the waveguide ring resonator. The output power is obtained from a normative SMF which makes ten degrees angle with the vertical axis on top of the Bragg grating. The variation of the computed coupling efficiency for the Bragg grating couplers versus the SiO2 thickness and the core etching depth are shown in Fig. 1 (a) and (b). It is obvious from the results that, the optimum SiO2 thickness is 5.5 μm, and the optimum core SiON thickness is 0.32 μm. The change of the computed coupling efficiency for Bragg grating couplers with extensive periods and widths are showed in Fig. 1 (c) and (d). From the results, it can be noticed that the coupling efficiencies are extremely dependent on the period and width. When the grating period is about 1.2 μm and the duty cycle is set to 0.5, the coupling efficiency can approach a maximum value of 50.1%. The schematic cross section of the designed waveguide, showing the layer structures and the devised parameters of the prospected SiON waveguide, is described in Fig. 2 (a). The cross sectional distribution of the optical field intensity in basic TE mode field simulated by the beam propagation method using Rsoft is shown in Fig. 2 (b). We set the refractive indices of the SiON, SiO2 and cladding film as 1.81, 1.45, and
Fig. 2. (a) Schematic cross section and prospected design parameters of SiON/SiO2 waveguide; (b) Simulated TE mode profile. 139
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Fig. 3. SEM micrograms of the fabricated SiON waveguide. (a) Top view; (b) Enlarged scene of the coupling region; (c) cross sectional; (d) Grating coupler.
10 min at 90 °C for layer consolidation. The fabricated optical waveguide ring resonators are checked by scanning electron microscope (SEM) whose images were showed in Fig. 3, including top view of OWRR, enlarged view of the coupling region, and cross section of the grating coupler. From the views, we can affirm that the devices have ideal rectangle also smooth and little roughness surface. The prospected optical waveguide ring resonator dimensions in this study are chosen as 0.32 μm thickness and 2 μm wide, according to the simulation results. The radii of the rings are set from 50 μm to 250 μm. The gaps between the ring and bus are maneuvered 0.2 μm. The Bragg grating couplers are founded at both ends of SiON OWRRs with a period of 1.2 μm. Finally, We collected the intensity of the transmitted light in the spectral region by utilizing a wavelength step of 1 dBm optical detector. 3. Results and discussion The SiON films without obvious cracking are produced on a lower cladding SiO2 substrate followed by the preparation of 5.5 μm SiO2 deposited on Si substrate both using the LSCVD method. The low absorption of SiON materials made the low loss waveguides come true. The smooth sidewalls and the rectangular cross section of the manufactured OWRRs are the main source of the scattering loss reduction. Fig. 4 demonstrates at 1550 nm wavelength, a propagation loss of a 0.32 μm high and 2.0 μm wide SiON core based waveguide using the cut back method. The measured results indicate that we can obtain a propagation loss of 4.07 dB/cm from this kind of SiON waveguides, and the error in this loss measurement is 0.3 dB/cm. This propagation loss is relatively small when compared to the silicon-organic-hybrid waveguide with a slot or strip structure [30], and is comparable to the Si3N4 waveguide [23]. The transmission spectra of the resonators are measured using a
Fig. 4. Propagation loss of 2 μm width waveguides.
sidewall roughness. The waveguide pattern was then designated by optical contact lithography and transferred to the SiON layer by reactive ion etching (RIE) using hybrid gases of trifluor-methane (CHF3) and oxygen (O2) with the flow rate of 30 and 5 sccm. This anisotropic etching process produces upright sidewalls to guarantee outstanding reproducibility of the geometry and accurate control, consequently, comparison of effective transverse refractive index of waveguide structures. After etching to reach the proposed depths, we removed the residual resist using RIE in which the O2 plasma removing only the resist but leaving the exposed SiON waveguide untouched. After stripping photoresist mask, the OWRR pattern is spin coating 5 μm PMMA acetone solvent as an upper cladding layer, followed by baking 140
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Fig. 5. (a) Transmission spectrum of the ring resonators; (b) High transmission spectrum of the OWRR (G = 0.2 μm, R = 150 μm) showing quality factor of 0.93 × 105.
property quality factors of the OWRRs indicated that the SiON based waveguides have the ability to match high sensing and varied modulation function demands. The transmission spectrums of the proposed OWRRs with various radii have been metered using the tunable laser, followed the calculations for Q factors of the resonators. Fig. 6 shows the variation of the quality factor of the OWRR versus its radius. At a certain value of loss, increasing radii will accumulate energy represented by higher Q factor in resonators, along with the growing of radii also energy is accumulating, at the same time loss will augment to bring lower Q factor [31]. In this experiment, the measured quality factors are increasing as the radius increases until an inflection point at 150 μm where it arrives to a maximum of 0.93 × 105. 4. Summary We have investigated the device, production, and optical characterization of a serious high quality SiON OWRRs. High quality SiON core layer has successfully deposited by utilizing LSCVD. This technique has the advantages of low temperature deposition and convenient refractive index optimization. Various radius OWRRs with Bragg grating couplers have been manufactured and displayed a preparatory average waveguide loss of 4.07 dB/cm and a high-quality factor of 0.93 × 105. Therefore, the low temperature deposited SiON with a high optical quality is a promising photonic integration platform for various photonic integration applications. We firmly believe that LSCVD deposited SiON ring resonators will be promising candidates for complementary metal oxide semiconductor (CMOS) compatible components.
Fig. 6. Quality factors of the OWRRs with gaps of 0.2 μm and various radius from 50 μm to 250 μm.
Bragg coupling system. Light from a tunable laser source was coupled into the waveguide in TE mode through a polarization maintaining lensed fiber, and the wavelength is scanned with a 5 p.m. step. The output light from the waveguide is collected using another fiber and connected to a photo detector to obtain the transmission spectrum. Fig. 5 (a) gives the transmission spectrum of the resonators within the wavelength range of 1595.5–1599.9 nm. Five sharp resonance peaks are observed, and the FSR of them is 0.9 nm. The main positive attribute of OWRRs quality factor Q is defined by Eq. (1) as follow:
Q=
λm ΔλFWHM
Acknowledgments This work is partially supported by the National Natural Science Foundation of China (Grant Nos. 61505111 and 11604216). This work is supported by professor Shiyoshi Yokoyama's laboratory.
(1)
where λm is the resonant wavelength. △λFWHM is the full wavelength width at half maximum. By this calculation, a quality factor of 0.93 × 105 is obtained from the data of Fig. 5 (b), which is shown as the highest spectral resolution of the OWRR and its radius and gap are 150 μm and 0.2 μm, respectively. The full width at half-maximum (FWHM) of the spectrum is 0.02 nm. This OWRR channel length is 1.5 cm, and during the measurement it got propagation loss of 4.07 dB/ cm in the waveguides from the cut-back method, so we can obtain a coupling efficiency from the Bragg grating of 7.2 dB. The peak of the transmission is – 32.2 dB, the top roof – 20.8 dB, and the extinction ratio is 11.4 dB, this almost reaches the critical condition of the resonators. From FSR of 0.9 nm, we got ng = 3, and loss a = 4.07 dB/cm, the intrinsic Q factor (Q = 2*π*ng/λ/a) is equal to 1.9 × 105. For critical coupling the Q factor is half of that, so 0.95 × 105 is very close to the experimentally measured values. The proposed resonance
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142
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
High quality factor silicon oxynitride optical waveguide ring resonators ∗
T
Yue Jia, Xiaoyu Dai, Yuanjiang Xiang , Dianyuan Fan International Collaborative Laboratory of 2D Materials for Optoelectronic Science & Technology of Ministry of Education, Engineering Technology Research Center for 2D Material Information Function Devices and Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
A R T I C LE I N FO
A B S T R A C T
Keywords: SON Q factor Ring resonator LSCVD Bragg-grating coupler
In this manuscript, we report the design, manufacturing and characterization of silicon oxynitride (SiON) based optical waveguide ring resonators (OWRR) which achieve higher quality factors with respect to the state-of-theart waveguide ring resonators. To improve the coupling efficiency, Bragg grating couplers are used at both ends of the designed resonators instead of the conventional directional couplers. In the proposed design, the core of OWRR structure was realized in the optimized refractive silicon oxynitride films deposited by liquid source chemical vapor deposition (LSCVD), under controllable composition ratio, time slot duration, and deposition temperature. The SiON based resonators have achieved a measured waveguide loss of 4.07 dB/cm and a quality factor of 0.93 × 105 in the transverse electric (TE) mode, and this by the way outperforms the results fulfilled with the state-of-the-art waveguide ring resonators.
1. Introduction Optical waveguide ring resonators (OWRR) play an important role in the construction of many optical devices such as sensing systems [1], optical de-multiplexing systems [2], optical filters [3,4], and optical modulators [5], due to their excellent transmission characters, like wavelength selectivity, tunability [6,7], special compact size, and flexible structure. Recently, it turns out that silicon oxynitride (SiOxNy) materials, which are always written as SiON for short, represent an excellent choice for fabricating the OWRRs due to their controllable high wide range refractive index ranging from 1.45 (SiO2) to 2.0 (Si3N4) [8–11]. Accordingly, it can be used to produce medium refractive index contrast optical waveguide [12–14]. Additionally, due to their low transmission losses, this kind of SiON waveguide is beneficial in reducing the coupling loss, scattering loss, and tolerance sensitivity of the OWRRs [15]. Moreover, compared to SiO2 waveguide, its eminent optical transparence of SiON ranges from 210 nm to beyond 2000 nm, this makes it possible to manufacture low loss optical waveguides [16]. Furthermore, SiON optical waveguides can be lightly produced by standard photolithography and reactive ion etching (RIE) technique [17]. Some SiON optical waveguide devices, such as integrated spectrometers [18], adaptive gain equalizers [19] or micro ring resonators [20], rings [21], and elliptic couplers [22] have been also reported. High quality factor (Q) ring resonators are considered as a cornerstone constituent in optical facilities, for instance, low power modulators, high sensitivity sensors, and narrow pass band filters [23,24].
∗
One of the important requirements that should be satisfied in ring resonators is the higher quality factor, because high quality factor value can enhance many waveguide parameters of the resonators [25,26]. For the sake of improving the Q factor of the ring resonators, many efforts have been done, including enlarging the light maintenance within the ring as well as reducing the bending loss of the device. Among the effective methods which can be used to decrease the bending loss, reduce the scattering loss of OWRRs by smoothing the sidewall roughness, and increase the radius of the ring resonators, show excellent performance especially for cases that seek ultra-high quality micro ring resonators [27]. This paper describes a type of optical waveguide ring resonators which are deposited with SiON core layers and indicates the optical features of the resulting OWRRs. We introduce OWRRs with SiON films, produced by the Liquid source CVD (LSCVD) method, in which Bragg grating couplers are employed at both the output and input terminals to couple light to the resonators directly. LSCVD is the preferred process for the non-volatile molecular precursors with a very low vapor pressure. Serious OWRRs with different radius are introduced, depositions and fabrications are also used to optimize these high quality and low loss OWRRs. A propagation loss of 4.07 dB/cm and a quality factor of 0.93 × 105 are achieved from OWRR with 150 μm radius and 0.2 μm gap width. This study provides a method for developing high quality OWRRs and their optimization by utilizing LSCVD deposition of the SiON film and demonstrates their feasibility on the low energy, small footprint applications of photonic integration platforms.
Corresponding author. E-mail address: [email protected] (Y. Xiang).
https://doi.org/10.1016/j.optmat.2018.08.031 Received 23 April 2018; Received in revised form 7 August 2018; Accepted 11 August 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 85 (2018) 138–142
Y. Jia et al.
Fig. 1. (a) Change of coupling efficiency versus thickness; (b) Change of coupling efficiency versus etching depth; (c) Change of coupling efficiency versus grating period; (d) Change of coupling efficiency versus slot width.
2. Experimental
1.45, respectively, and the wavelength is set to 1550 nm. It is observed that most of the light in the waveguide extends deeply into the SiO2 layer. Such a field distribution can be attributed to both the high refractive index contrast and the ultra-thin SiON. The deposited core layer SiON is sandwiched between the bottom cladding of silicon dioxide (SiO2) and the over cladding of a poly (methyl methacrylate) (PMMA). The lower cladding layer is deposited by LSCVD with a thickness 5.5 μm above the Si substrate. The SiON layer with a thickness of 0.32 μm and controlled refractive index upon SiO2 layer is fabricated using a normative parallel-plate LSCVD system. The liquid of this LSCVD system is SiN-X source (SiN-2). By controlling the flow rates of N2 and N2O, the refraction index of the SiON films could be alterative from 1.45 to 2.0. SiON film is grown with the thickness of 0.32 μm, and refractive index of 1.81 [28,29] at temperature of 150 °C with the flow rate of N2 and N2O which is 100 and 10 sccm, respectively. Finally, we chose SiN-2 in a flow rate of 1.0 sccm and deposition time of 6.5 min to get this refractive index by serious examining the refractive index with different SiN-2 flow rate and time. The patterns of OWRRs, and grating couplers are all transferred onto the resist layer upon SiON by applying the electron beam lithography (EBL) technique. Direct writing and high resolution of EBL allow to get the high accuracy and small feature size of the OWRRs. The samples are developed by oxylene for one minute and exposure baked for 5 min at 145 °C to re-flow the resist surface further to decrease the
The light coupling efficiency from a grating to a single mode fiber (SMF) is calculated with the assumption that it is the same in the reverse direction, from fiber to the grating. A wavelength of 1550 nm TE polarized light is sent through the Bragg grating coupler to the waveguide ring resonator. The output power is obtained from a normative SMF which makes ten degrees angle with the vertical axis on top of the Bragg grating. The variation of the computed coupling efficiency for the Bragg grating couplers versus the SiO2 thickness and the core etching depth are shown in Fig. 1 (a) and (b). It is obvious from the results that, the optimum SiO2 thickness is 5.5 μm, and the optimum core SiON thickness is 0.32 μm. The change of the computed coupling efficiency for Bragg grating couplers with extensive periods and widths are showed in Fig. 1 (c) and (d). From the results, it can be noticed that the coupling efficiencies are extremely dependent on the period and width. When the grating period is about 1.2 μm and the duty cycle is set to 0.5, the coupling efficiency can approach a maximum value of 50.1%. The schematic cross section of the designed waveguide, showing the layer structures and the devised parameters of the prospected SiON waveguide, is described in Fig. 2 (a). The cross sectional distribution of the optical field intensity in basic TE mode field simulated by the beam propagation method using Rsoft is shown in Fig. 2 (b). We set the refractive indices of the SiON, SiO2 and cladding film as 1.81, 1.45, and
Fig. 2. (a) Schematic cross section and prospected design parameters of SiON/SiO2 waveguide; (b) Simulated TE mode profile. 139
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Fig. 3. SEM micrograms of the fabricated SiON waveguide. (a) Top view; (b) Enlarged scene of the coupling region; (c) cross sectional; (d) Grating coupler.
10 min at 90 °C for layer consolidation. The fabricated optical waveguide ring resonators are checked by scanning electron microscope (SEM) whose images were showed in Fig. 3, including top view of OWRR, enlarged view of the coupling region, and cross section of the grating coupler. From the views, we can affirm that the devices have ideal rectangle also smooth and little roughness surface. The prospected optical waveguide ring resonator dimensions in this study are chosen as 0.32 μm thickness and 2 μm wide, according to the simulation results. The radii of the rings are set from 50 μm to 250 μm. The gaps between the ring and bus are maneuvered 0.2 μm. The Bragg grating couplers are founded at both ends of SiON OWRRs with a period of 1.2 μm. Finally, We collected the intensity of the transmitted light in the spectral region by utilizing a wavelength step of 1 dBm optical detector. 3. Results and discussion The SiON films without obvious cracking are produced on a lower cladding SiO2 substrate followed by the preparation of 5.5 μm SiO2 deposited on Si substrate both using the LSCVD method. The low absorption of SiON materials made the low loss waveguides come true. The smooth sidewalls and the rectangular cross section of the manufactured OWRRs are the main source of the scattering loss reduction. Fig. 4 demonstrates at 1550 nm wavelength, a propagation loss of a 0.32 μm high and 2.0 μm wide SiON core based waveguide using the cut back method. The measured results indicate that we can obtain a propagation loss of 4.07 dB/cm from this kind of SiON waveguides, and the error in this loss measurement is 0.3 dB/cm. This propagation loss is relatively small when compared to the silicon-organic-hybrid waveguide with a slot or strip structure [30], and is comparable to the Si3N4 waveguide [23]. The transmission spectra of the resonators are measured using a
Fig. 4. Propagation loss of 2 μm width waveguides.
sidewall roughness. The waveguide pattern was then designated by optical contact lithography and transferred to the SiON layer by reactive ion etching (RIE) using hybrid gases of trifluor-methane (CHF3) and oxygen (O2) with the flow rate of 30 and 5 sccm. This anisotropic etching process produces upright sidewalls to guarantee outstanding reproducibility of the geometry and accurate control, consequently, comparison of effective transverse refractive index of waveguide structures. After etching to reach the proposed depths, we removed the residual resist using RIE in which the O2 plasma removing only the resist but leaving the exposed SiON waveguide untouched. After stripping photoresist mask, the OWRR pattern is spin coating 5 μm PMMA acetone solvent as an upper cladding layer, followed by baking 140
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Fig. 5. (a) Transmission spectrum of the ring resonators; (b) High transmission spectrum of the OWRR (G = 0.2 μm, R = 150 μm) showing quality factor of 0.93 × 105.
property quality factors of the OWRRs indicated that the SiON based waveguides have the ability to match high sensing and varied modulation function demands. The transmission spectrums of the proposed OWRRs with various radii have been metered using the tunable laser, followed the calculations for Q factors of the resonators. Fig. 6 shows the variation of the quality factor of the OWRR versus its radius. At a certain value of loss, increasing radii will accumulate energy represented by higher Q factor in resonators, along with the growing of radii also energy is accumulating, at the same time loss will augment to bring lower Q factor [31]. In this experiment, the measured quality factors are increasing as the radius increases until an inflection point at 150 μm where it arrives to a maximum of 0.93 × 105. 4. Summary We have investigated the device, production, and optical characterization of a serious high quality SiON OWRRs. High quality SiON core layer has successfully deposited by utilizing LSCVD. This technique has the advantages of low temperature deposition and convenient refractive index optimization. Various radius OWRRs with Bragg grating couplers have been manufactured and displayed a preparatory average waveguide loss of 4.07 dB/cm and a high-quality factor of 0.93 × 105. Therefore, the low temperature deposited SiON with a high optical quality is a promising photonic integration platform for various photonic integration applications. We firmly believe that LSCVD deposited SiON ring resonators will be promising candidates for complementary metal oxide semiconductor (CMOS) compatible components.
Fig. 6. Quality factors of the OWRRs with gaps of 0.2 μm and various radius from 50 μm to 250 μm.
Bragg coupling system. Light from a tunable laser source was coupled into the waveguide in TE mode through a polarization maintaining lensed fiber, and the wavelength is scanned with a 5 p.m. step. The output light from the waveguide is collected using another fiber and connected to a photo detector to obtain the transmission spectrum. Fig. 5 (a) gives the transmission spectrum of the resonators within the wavelength range of 1595.5–1599.9 nm. Five sharp resonance peaks are observed, and the FSR of them is 0.9 nm. The main positive attribute of OWRRs quality factor Q is defined by Eq. (1) as follow:
Q=
λm ΔλFWHM
Acknowledgments This work is partially supported by the National Natural Science Foundation of China (Grant Nos. 61505111 and 11604216). This work is supported by professor Shiyoshi Yokoyama's laboratory.
(1)
where λm is the resonant wavelength. △λFWHM is the full wavelength width at half maximum. By this calculation, a quality factor of 0.93 × 105 is obtained from the data of Fig. 5 (b), which is shown as the highest spectral resolution of the OWRR and its radius and gap are 150 μm and 0.2 μm, respectively. The full width at half-maximum (FWHM) of the spectrum is 0.02 nm. This OWRR channel length is 1.5 cm, and during the measurement it got propagation loss of 4.07 dB/ cm in the waveguides from the cut-back method, so we can obtain a coupling efficiency from the Bragg grating of 7.2 dB. The peak of the transmission is – 32.2 dB, the top roof – 20.8 dB, and the extinction ratio is 11.4 dB, this almost reaches the critical condition of the resonators. From FSR of 0.9 nm, we got ng = 3, and loss a = 4.07 dB/cm, the intrinsic Q factor (Q = 2*π*ng/λ/a) is equal to 1.9 × 105. For critical coupling the Q factor is half of that, so 0.95 × 105 is very close to the experimentally measured values. The proposed resonance
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