Thermo-Optical Tuning Cascaded Double Ring Sensor with Large Measurement Range

In this paper, a thermo-optic tuning optical waveguide sensor system based on a cascaded double micro-ring resonator is investigated. The system consists of a micro-ring resonator with the microheater as a reference ring and a micro-ring resonator with removing the upper cladding layers as a sensing ring, combined with a microfluidic control. The refractive index change of the sample is measured by the electric power change of the microheater. The experimental results show that the sensitivity of the thermo-optic tuning is 34.231 W/RIU (refractive index units), and the measurement range is 4.325 × 10−3 RIU, almost eight times larger than that of the cascaded double micro-ring resonator without thermo-optic tuning for the intensity interrogation.


Introduction
Integrated optical sensor devices have been widely used in micro-ring resonator [1,2], photonic crystal (PhC) [3], and Mach-Zehnder interferometers (MZI) [4] due to their high integration and miniaturization. The bandgap of PhC sensors is formed by the periodic dielectric structure [3]. However, a high-precision photolithography process is required to make the feature pattern of the device, which is very unfavorable for commercialization. Based on the MZI structure sensor, highly sensitive power sensors and a stable system are required in the intensity interrogation to achieve the high sensitivity and the low level of noise [4]. Micro-ring resonators made of silicon on insulator (SOI) materials have been used widely in biosensing due to their high Q value and high sensitivity. SOI has a very high refractive index contrast that allows us to bend the waveguide very tightly with a radius of a few micrometers. Due to the high evanescent field on the surface of the silicon waveguide, the sensitivity of the SOI sensor is high [5,6]. The fabrication process of the SOI sensor is compatible with the complementary metal oxide semiconductor (CMOS) process. This means the SOI sensors can be made a large array for different functions, not only to detect the different type biomolecules [7][8][9][10] but, also, to use some sensors as a reference to monitor the temperature change [11,12].
Recently, a new type of electric tracking-assisted dual micro-ring optical sensor was reported [13]. The dual micro-ring sensor includes a sensing ring and a reference ring with an electrical controller. The shift of the resonance wavelength is converted into the change of the electric power by tuning the thermal heater. The radii of both micro-rings are designed with 10 µm, and the sensitivity of the sensing system 2 of 9 is 15 mW/RIU (refractive index units), based on transverse electric (TE) polarized light. Such sensing systems can provide a low detection limit of 3.9 × 10 −6 refractive index units (RIU) and have been used in biological detection. For the intensity interrogation of the cascaded double micro-ring resonators, a low-cost broadband light source and the optical power meters were used without measuring the spectrum [14][15][16]. However, the measurement range of the intensity interrogation is limited by the linear region of the output power with the refractive change of the analyte, and the detection limit is limited by the sensitivity of the power meter, which is an order of magnitude lower than the wavelength interrogation. Another intensity-sensing scheme uses a cascade micro-ring configuration based on the rib waveguide and tracks the maximum spectral position by using peak tracking methods [17,18]. This scheme was demonstrated to obtain an entire free spectral range of the probing micro-ring with the same radii of micro-rings. A significant shortcoming of this sensor scheme is that the limit of detection limit of this device is affected by the intensity peak width and provides a low detection limit (LOD) of 4.6 × 10 −5 RIU. We address the detection limit by setting different radii micro-rings to produce a Vernier effect to improve. The waveguide was chosen as the slab waveguide for the transverse magnetic (TM) mode, and the sensor sensitivity is effectively improved in our scheme.
In order to achieve the low-cost, large measurement range and high sensitivity on-chip sensor, a thermo-optic tuning cascaded double ring (TTCDR) sensor is proposed in this paper, with a microheater on the reference ring for converting the output intensity changes into the electric power changes. The two micro-rings are designed to have different radii to produce a Vernier effect to increase the sensitivity. For a certain output intensity of the TTCDR sensor, the curve of the electric power changes with the different concentrations that the NaCl solution is plotted. The sensitivity of the sensor is 34.231 W/RIU for the TM mode. Utilizing the thermo-optic effect, the measurement range is almost eight times larger than that of the traditional cascaded double ring sensor without a microheater for the intensity interrogation.

Operating Device and Principle
Intensity interrogation and wavelength interrogation are two typical sensing methods for the cascaded micro-ring resonator sensors [14][15][16][17][18][19][20]. Intensity interrogation is spectrometer-free, which requires a low-cost broadband light source and a power meter. The refractive index change of the sample can be detected by the change of the output intensity of the sensor.
A schematic illustration of the TTCDR sensor is shown in Figure 1. As shown in Figure 1, the sensor is comprised of two main components: a sensing ring without an upper-cladding layer and a reference ring with a microheater. There is a titanium nitride (TiN) microheater on the reference ring for thermo-optic tuning. We adopt a low-cost broadband light source as the light source (Conquer, OS-ASE-M2-C-0-100-0-S-FA). This source was provided by Beijing Conquer Optical Technology Co. Ltd., Beijing, China. The light source has the advantages of a high output power, spectral line width, low degree of polarization, high power stability, and good average stability, which can meet the stringent performance requirements of the broadband light source (BLS) for sensing and testing.
Sensors 2020, 20, x FOR PEER REVIEW 2 of 8 sensitivity of the sensing system is 15 mW/RIU (refractive index units), based on transverse electric (TE) polarized light. Such sensing systems can provide a low detection limit of 3.9 × 10 −6 refractive index units (RIU) and have been used in biological detection. For the intensity interrogation of the cascaded double micro-ring resonators, a low-cost broadband light source and the optical power meters were used without measuring the spectrum [14][15][16]. However, the measurement range of the intensity interrogation is limited by the linear region of the output power with the refractive change of the analyte, and the detection limit is limited by the sensitivity of the power meter, which is an order of magnitude lower than the wavelength interrogation. Another intensity-sensing scheme uses a cascade micro-ring configuration based on the rib waveguide and tracks the maximum spectral position by using peak tracking methods [17,18]. This scheme was demonstrated to obtain an entire free spectral range of the probing micro-ring with the same radii of micro-rings. A significant shortcoming of this sensor scheme is that the limit of detection limit of this device is affected by the intensity peak width and provides a low detection limit (LOD) of 4.6 × 10 −5 RIU. We address the detection limit by setting different radii micro-rings to produce a Vernier effect to improve. The waveguide was chosen as the slab waveguide for the transverse magnetic (TM) mode, and the sensor sensitivity is effectively improved in our scheme. In order to achieve the low-cost, large measurement range and high sensitivity on-chip sensor, a thermo-optic tuning cascaded double ring (TTCDR) sensor is proposed in this paper, with a microheater on the reference ring for converting the output intensity changes into the electric power changes. The two micro-rings are designed to have different radii to produce a Vernier effect to increase the sensitivity. For a certain output intensity of the TTCDR sensor, the curve of the electric power changes with the different concentrations that the NaCl solution is plotted. The sensitivity of the sensor is 34.231 W/RIU for the TM mode. Utilizing the thermo-optic effect, the measurement range is almost eight times larger than that of the traditional cascaded double ring sensor without a microheater for the intensity interrogation.

Operating Device and Principle
Intensity interrogation and wavelength interrogation are two typical sensing methods for the cascaded micro-ring resonator sensors [14][15][16][17][18][19][20]. Intensity interrogation is spectrometer-free, which requires a low-cost broadband light source and a power meter. The refractive index change of the sample can be detected by the change of the output intensity of the sensor.
A schematic illustration of the TTCDR sensor is shown in Figure 1. As shown in Figure 1, the sensor is comprised of two main components: a sensing ring without an upper-cladding layer and a reference ring with a microheater. There is a titanium nitride (TiN) microheater on the reference ring for thermo-optic tuning. We adopt a low-cost broadband light source as the light source (Conquer, OS-ASE-M2-C-0-100-0-S-FA). This source was provided by Beijing Conquer Optical Technology Co. Ltd., Beijing, China. The light source has the advantages of a high output power, spectral line width, low degree of polarization, high power stability, and good average stability, which can meet the stringent performance requirements of the broadband light source (BLS) for sensing and testing.  The BLS is connected to the attenuator and tunable pass band filter (TPBF) in Figure 1. Then, the light is coupled into the input and output of the sensor chip by the grating couplers. The input (port 1) and output (port 2 and port 3) shared the same fiber array for easy reuse. Finally, the output power was collected by the power meter (Agilent 81634A). The current source (KEITHLEY 2400) is controlled by PC for the microheater and connected to the sensor through a probe. The output through and drop of the sensor were received by detector 1 and detector 2, respectively, and finally, the computer received the data. The windowed sensing ring was in direct contact with the substance to be measured through the microfluidic channel to sense the change in the refractive index and realize sensing.
The peak wavelength shift in the envelope function of the cascaded double ring resonators transmission is magnified by a Vernier effect factor. The free spectrum range (FSR) of the output transmission FSR output is given as [15]: where F is the Vernier effect amplification factor, F = FSR re f FSR sen −FSR re f , and FSR sen and FSR re f are the FSR of the reference ring and sensing ring, respectively. The operating principle of the TTCDR sensing system is shown in Figure 2. When the refractive index of the analyte changes ∆n, the output intensity of the TTCDR sensor changes ∆I. The sensitivity of the cascade double micro-ring sensor (CMRR) is defined as S CMRR = ∆I/ ∆n. In Figure 1, in order to obtain the high sensitivity, the bandwidth of the TPBF is equal to half of FSR the of the cascade double rings transmission envelope. The spectrum of the BLS is I In (λ), and λ is the wavelength. The output intensity I out of the TTCDR sensor can be expressed as [15]: where T re f and T sen are the transmission spectra of the reference ring and the sensing ring port, respectively. The thermo-optic tuning principle is shown in Figure 2. When the refractive index changes by ∆n, the transmission spectrum shifts from a black line to a blue line, as shown in Figure 2c When the refractive index of the analyte changes, the electric power of the microheater is scanned, until the ∆I out /∆P elec is equal to the initial value and ∆I out = 0. Therefore, by measuring, the power change of the microheater ∆P elec can detect the variation of the sample refractive index ∆n.
intensity Iout of the TTCDR sensor can be expressed as [15]: where and are the transmission spectra of the reference ring and the sensing ring port, respectively.

Fabrication of TTCDR Chip and Characterization
The wafer preparation and technical assistant was by the Integrated Circuit Advanced Process Center (ICAC) of the Institute of Microelectronics of Chinese Academy of Sciences (IMECAS), Beijing, China. The fabrication started by spin-coating a thin film of photoresist on the SOI wafer. Waveguides, grating couplers, and micro-rings were fabricated by the stepper, then growing 2-µm SiO 2 upper cladding, and the sensing window was opened by photolithography. Finally, the resistance heating metal TiN and the conductive electrode Al layer are coated by the sputtering. The micro-ring chip was fabricated on a silicon insulator platform with a 220-nm-thick silicon top layer and a 2-µm-thick buried oxide layer. The SOI micro-ring sensor is based on the evanescent field. The TE mode used for sensing has one order lower sensitivity compared to the TM mode, due to the smaller mode-field overlap with the sample solution [21]. The silicon strip waveguide is designed with the width of 550 nm for the TM mode.
The whole chip was covered by the SiO 2 upper cladding layer, except that the sensing ring is exposed to the reagent sample by removing the upper cladding layer in the sensing window. The optical microscope image of the sensor chip is shown in Figure 3a, and the size of the whole chip is 2 cm × 2 cm. The scanning electron microscopy (SEM) images of the sensing ring and the directional coupler between the bus waveguide and the ring are shown in Figure 3b,c, respectively. The image of the TTCDR sensing system consists of the microfluidic channels, the sensor chip, the microheater probes, and the fiber arrays, as shown in Figure 4a. In Figure 4b is a SEM image of the TiN microheater.
Sensors 2020, 20, 5149 5 of 9 exposed to the reagent sample by removing the upper cladding layer in the sensing window. The optical microscope image of the sensor chip is shown in Figure 3a, and the size of the whole chip is 2 cm × 2 cm. The scanning electron microscopy (SEM) images of the sensing ring and the directional coupler between the bus waveguide and the ring are shown in Figure 3b,c, respectively. The image of the TTCDR sensing system consists of the microfluidic channels, the sensor chip, the microheater probes, and the fiber arrays, as shown in Figure 4a. In Figure 4b is a SEM image of the TiN microheater.

Intensity Interrogation with Thermo-Optic Tuning
To produce the Vernier effect, the radii of the reference ring and the sensing ring are 123 μm and 121 μm, respectively. The optical transmission spectra of the drop port was measured in Figure 5 with the refractive index of 1.8 × 10 −3 RIU (1.0% NaCl solution, in black) and 2.7 × 10 −3 RIU (1.5% NaCl solution, in blue). The refractive index of the NaCl solution varies by 1.8 × 10 −3 per 1% concentration of variation [22]. The concentration of NaCl with their refractive index is listed in Table  1. The red curve in Figure 5 shows the spectrum of the input BLS through the TPBF. The FSRoutput of the transmission curve envelope is 11.92 nm. To achieve the highest sensitivity, the input BLS with a 3-dB bandwidth of ~6 nm and a center wavelength of 1543.81 nm was chosen by the TPBF. The peak of the transmission spectral envelope has a blue shift with increasing the concentration of NaCl for their refractive index from 1.8 × 10 −3 RIU (1.0%) to 2.7 × 10 −3 RIU (1.5%).

Intensity Interrogation with Thermo-Optic Tuning
To produce the Vernier effect, the radii of the reference ring and the sensing ring are 123 µm and 121 µm, respectively. The optical transmission spectra of the drop port was measured in Figure 5 with the refractive index of 1.8 × 10 −3 RIU (1.0% NaCl solution, in black) and 2.7 × 10 −3 RIU (1.5% NaCl solution, in blue). The refractive index of the NaCl solution varies by 1.8 × 10 −3 per 1% concentration of variation [22]. The concentration of NaCl with their refractive index is listed in Table 1. The red curve in Figure 5 shows the spectrum of the input BLS through the TPBF. The FSR output of the transmission curve envelope is 11.92 nm. To achieve the highest sensitivity, the input BLS with a 3-dB bandwidth of 6 nm and a center wavelength of 1543.81 nm was chosen by the TPBF. The peak of the transmission spectral envelope has a blue shift with increasing the concentration of NaCl for their refractive index from 1.8 × 10 −3 RIU (1.0%) to 2.7 × 10 −3 RIU (1.5%).
NaCl solution, in blue). The refractive index of the NaCl solution varies by 1.8 × 10 per 1% concentration of variation [22]. The concentration of NaCl with their refractive index is listed in Table  1. The red curve in Figure 5 shows the spectrum of the input BLS through the TPBF. The FSRoutput of the transmission curve envelope is 11.92 nm. To achieve the highest sensitivity, the input BLS with a 3-dB bandwidth of ~6 nm and a center wavelength of 1543.81 nm was chosen by the TPBF. The peak of the transmission spectral envelope has a blue shift with increasing the concentration of NaCl for their refractive index from 1.8 × 10 −3 RIU (1.0%) to 2.7 × 10 −3 RIU (1.5%).     [22].

Concentration of NaCl Solution
Refractive Index Units In order to eliminate the influence of the BLS fluctuations, the output intensity of drop port 3 is normalized by the output power through port 2. The normalized output power changes with the electric power of the microheater periodically, as shown in Figure 6a, under a refractive index of 1.8 × 10 −3 RIU (1.0% NaCl solution). When the concentration of the NaCl solution with their refractive index changes from 1.8 × 10 −3 RIU (1.0%) to 7.2 × 10 −3 RIU (4.0%), the responses of the sensor are measured four times at each the refractive index changes of the concentration of NaCl solutions, as shown in Figure 6b without the electric power of the microheater. The normalized output power decreases with increasing the concentration of the NaCl solution for their refractive index. The output intensity changes periodically with the different concentration of NaCl solution for their refractive index. In Figure 6b, the experimental results showed that the measurement range for the TTCDR sensor Range 2 = 4.325 × 10 −3 RIU was almost eight times larger than that of the traditional cascaded double ring sensor Range 1 = 0.54 × 10 −3 RIU for the intensity interrogation. Range 1 is determined by ∆I/∆n, and Range 2 is determined by ∆I/∆n and ∆I = 0. In the intensity interrogation, the sensing is realized by the slope of intensity variation (∆I/∆n) [14], and the intensity detection range needs to be in the linear region of the power curve (Figure 6b) to obtain high sensitivity in the sensing experiment. Thermal tuning can solve the problem of the nonlinear area of the power curve, the detection range can cover the total range of power curve, and every point on the curve is linear.
1 is determined by ΔI/Δn, and Range 2 is determined by ΔI/Δn and ΔI = 0. In the intensity interrogation, the sensing is realized by the slope of intensity variation (ΔI/Δn) [14], and the intensity detection range needs to be in the linear region of the power curve (Figure 6b) to obtain high sensitivity in the sensing experiment. Thermal tuning can solve the problem of the nonlinear area of the power curve, the detection range can cover the total range of power curve, and every point on the curve is linear. The choice of the initial point is very important in the experiment. We set the starting point at the maximum slope of the power curve instead of the peak intensity of reference [17,18]. Since the slope of the power curve is equal to zero at the peak intensity, it is very difficult to find the peak position accurately. This makes the uncertainty of the peak point relatively large and affects the detection limit of the sensor. In Figure 6b, the ΔI/Δn of the initial point m is not equal to 0. The shift of the transmission curve due to the variation of the different concentration solutions can be converted to the electric power change of the heater by keeping the slope of the output power curve and the output power as the constant, as shown in Figure 7.
The sensitivity of the thermo-optical tuning cascade micro-ring is 34.231 W/RIU, as shown in Figure 7. Accordingly, the limit of detection (LOD) for the measurement of the refractive index of the sensor is calculated based on the standard deviation σ = 0.105 mW divided by the sensitivity S: LOD = 3σ/S = 9.202 × 10 −6 RIU. The standard deviation σ is obtained from the response measurement from 1.5% to 1.0%. The intensity interrogation method of the traditional cascaded double ring sensor can be improved by using thermo-optic tuning. No matter where the initial operating point is located, the electric power of the microheater is linear with the refractive index change of the NaCl solution. The choice of the initial point is very important in the experiment. We set the starting point at the maximum slope of the power curve instead of the peak intensity of reference [17,18]. Since the slope of the power curve is equal to zero at the peak intensity, it is very difficult to find the peak position accurately. This makes the uncertainty of the peak point relatively large and affects the detection limit of the sensor. In Figure 6b, the ∆I/∆n of the initial point m is not equal to 0. The shift of the transmission curve due to the variation of the different concentration solutions can be converted to the electric power change of the heater by keeping the slope of the output power curve and the output power as the constant, as shown in Figure 7. The micro-ring resonator sensor based on the SOI waveguide has high refractive index contrast. Most of the electric field is distributed in the cladding of the waveguide. Due to the small overlap with the mode field of the analyte, the TE mode used for sensing has a low sensitivity. The TM mode electric field is distributed on the surface of the waveguide, and the strong surface electric field can make the sensing substance have a greater influence on the effective refractive index of the waveguide, and it is easier to achieve a high sensitivity [21]. Sensors are made of rib waveguide materials [18], the electric field is mainly distributed inside the core layer, and the sensitivity of the sensor is relatively low: S = 3σp/LOD = 3.021 W/RIU (σp = 4.638 × 10 −5 mW and LOD = 4.606 × 10 −5 RIU). In our TTCDR sensing system, the cascaded ring resonators are constructed by the slab waveguides. The two micro-rings are designed to have different radii to produce a Vernier effect so as to increase the sensitivity. The experiments showed a high sensitivity of S = 34.231 W/RIU for the TM mode. The LOD was also improved: LOD = 9.202 × 10 −6 RIU. Keeping ΔIout/ΔPelec and Iout as the constants are the The sensitivity of the thermo-optical tuning cascade micro-ring is 34.231 W/RIU, as shown in Figure 7. Accordingly, the limit of detection (LOD) for the measurement of the refractive index of the sensor is calculated based on the standard deviation σ = 0.105 mW divided by the sensitivity S: LOD = 3σ/S = 9.202 × 10 −6 RIU. The standard deviation σ is obtained from the response measurement from 1.5% to 1.0%. The intensity interrogation method of the traditional cascaded double ring sensor can be improved by using thermo-optic tuning. No matter where the initial operating point is located, the electric power of the microheater is linear with the refractive index change of the NaCl solution.
The micro-ring resonator sensor based on the SOI waveguide has high refractive index contrast. Most of the electric field is distributed in the cladding of the waveguide. Due to the small overlap with the mode field of the analyte, the TE mode used for sensing has a low sensitivity. The TM mode electric field is distributed on the surface of the waveguide, and the strong surface electric field can make the sensing substance have a greater influence on the effective refractive index of the waveguide, and it is easier to achieve a high sensitivity [21]. Sensors are made of rib waveguide materials [18], the electric field is mainly distributed inside the core layer, and the sensitivity of the sensor is relatively low: S = 3σ p /LOD = 3.021 W/RIU (σ p = 4.638 × 10 −5 mW and LOD = 4.606 × 10 −5 RIU). In our TTCDR sensing system, the cascaded ring resonators are constructed by the slab waveguides. The two micro-rings are designed to have different radii to produce a Vernier effect so as to increase the sensitivity. The experiments showed a high sensitivity of S = 34.231 W/RIU for the TM mode. The LOD was also improved: LOD = 9.202 × 10 −6 RIU. Keeping ∆I out /∆P elec and I out as the constants are the determination condition of the electric power change, instead of using the intensity peak tracking method in order to reduce the uncertainty of the sensor. In addition, the detection limit and sensitivity of the micro-ring resonator can also be improved by using a high-resolution current source.

Conclusions
An optical TTCDR sensing system based on the SOI substrate has been fabricated by the CMOS process. The TTCDR sensing system includes the BLS, attenuator, TPBF, power meters and sensing chip integrated with a grating coupler, a reference ring, a microheater, and a sensing ring. The analyte can be measured by the electric power change of the microheater without requiring the spectral measurement to reduce the cost. The experimental results show that the sensitivity is 34.231 W/RIU, the detection limit is 9.202 × 10 −6 RIU, and the measurement range is 4.325 × 10 −3 RIU. The TTCDR sensing system has a great potential for biosensing applications.