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Urea sensor by racetrack silicon resonator
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Urea sensor by racetrack silicon resonator MM Ariannejada, Elnaz Akbarib,c,*, Mourad Niazid, Effariza Hanafie a
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Viet Nam c Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Viet Nam d General and Laparoscopic Surgery, Department of Surgery, Aleppo University Hospital, University of Aleppo, Aleppo, Syria e Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia b
A R T IC LE I N F O
ABS TRA CT
Keywords: Racetrack resonator Urea Refractive index sensor Silicon waveguide Upper cladding
Optical Racetrack resonator waveguide which is based on silicon material possesses a good ability in sensing different materials, and urea is one of them has been simulated using a Finite different time-domain (FDTD) method. The drop port of racetrack possessed a higher spectrum shift of 7 × 10−3 nm. Urea was considered as the upper cladding in the throughput, resulting in the generation of the Qfactor and full width at half maximum (FWHM) with the highest frequency. Power changes posed a significant effect on the upper cladding material. In the time domain, throughput showed the maximum time delay, resulting in 4.9 fs, which was a unit for time difference generated by urea and H2O materials as cladding.
1. Introduction Health sensor is a compelling subject. As electronic devices are considered sensitive but not that compact like optical sensors, the development of optical sensors has opened a new opportunity for the healthcare industry products [1,2]. The urine ratio parameter control through optical sensors has a crucial function for body health [3–5]. Additionally, this device is known to be more compact compared to electronics sensors. Optical fiber can act as optical sensor that can be use in in remote-based health monitoring. According to recent studies, although optical fiber has also been used health monitoring purposes, optical waveguides could offer more compact size of sensors [6–8]. Silicon-based materials exhibit various remarkable industrial uses in electronic devices and the optical waveguide. Complementary metal-oxide-semiconductor (CMOS), can be used in the optical waveguide fabrication. Besides reducing the complexity of fabrication and cost of health sensor devices [9–11], this type of waveguide fabrication will also reduce the noise ratio, which is common in electronics-based health sensors [12–14]. As the main function of micro ring resonator (MRR) is generating frequency, any changes within the environment could influence the output spectrum. [15–17]. One of the unique designs of MRR would be the racetrack resonator. The racetrack resonator consists of two half-circles connected to a straight waveguide so that a distance between these two circles would be present. The straight waveguide would facilitate coupling effects to take place in MRR design. Meanwhile, the closed-loop system design of MRR would lead to a resonance effect, which would be relevant to the group index of materials and the ring radius. Multiple resonances could be defined by the space between the resonances, which is also known as free spectral range (FSR). In respect of the resonators’ structure, the Qfactor could be calculated by the output spectrum [18,19]. As for racetrack resonator, the
⁎
Corresponding author. E-mail address: [email protected] (E. Akbari).
https://doi.org/10.1016/j.ijleo.2019.164042 Received 13 September 2019; Received in revised form 2 December 2019; Accepted 10 December 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Please cite this article as: MM Ariannejad, et al., Optik - International Journal for Light and Electron Optics, https://doi.org/10.1016/j.ijleo.2019.164042
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
M. Ariannejad, et al.
length of coupling and its gap are highly crucial in order to achieve improved filtering of the light modes [20–22]. the fabrication of such waveguide can be possible by nano fabrication methods [22–24]. The bend waveguide can cause optical power loss that the design can improve this problem [25]. The racetrack design can optimize the Qfactor by creating a longer coupling region [26,27].The compact size of racetrack would increase free spectral range (FSR), that can affect the Qfactor value [28]. Overall, the optical racetrack could be designed to decrease the size of the health sensor by obtaining a good FSR. 2. Material and methods The refractive index sensor (RI) can be designed by silicon racetrack design. The racetrack design simply can track the refractive index changes on the upper cladding accordingly. The resonator of the racetrack could be represented by Eq. 1 below:
m⋅λ = neff ⋅L
(1)
Based on Eq. 1, the wavelength (λ ) consists of a linear relation with the order of the mode (m ) due to resonance. Among all types of waveguides, the effective refractive index (neff ) plays an important role which could be affected by the length (L ) of the waveguide. Performing changes in the material is an effective method of shifting the resonance spectrum. The racetrack resonator design is represented by Qc , while the clockwise direction, μ (t ) , is according to the laser source. Therefore, the equation of the coupling of mode according to the time-domain theory is as follows:
u (t ) = u 0 exp(−2t τc )
(2)
According to Eq. 2, the modes would be coupled in the racetrack with τc as shown in the equation below: (3)
τc = 2Qc ω0
The racetrack comprises a closed-loop ring. In Fig. 1 the starting part, which is labelled as I is the mode which could be coupled with efficient power. The light will travel along the round trip in a clockwise direction so that the average power in this system throughout the time could be represented with tr = L vg . Furthermore, the group velocity (vg ) poses an opposite effect on the roundtrip time (tr ), while the length of the resonator (L ) impacts the following the equation below:
p = u 0 (1 − e 2L
(vg τc ) )
(L vg )
(4)
Through an understanding of the parallel couple waveguides, the light could travel with resonance from point I to point II, as represented in [32],
p = p0 sin2 (ΔβZ0 2)
(5)
The length of coupling (Z0 ) in the racetrack has an important role in the optical power which circulates around the ring, resulting in the occurrence of resonance ( p0 ). By putting the point I into consideration, the coupler in this design could be directional so that the constant propagation Δβ could be defined. The Qc could be calculated according to u 0 = p0 L vg and vg = c ng , resulting in the following equation:
Qc = −
πng L λ 0 ln [ cos (ΔβZ0 2)]
(6)
Based on the above equation (with constant Z0 in Fig. 1), L and Qc have a linear relationship, which could be elaborated through the volume of light modes in the resonator-based design. The add-drop port is incorporated into the racetrack design so that the mode dispersion and wavelength range could be defined by the free spectral range (FSR), as per the equation below:
Fig. 1. The structure of racetrack with two-sided coupling as straight waveguides. 2
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
M. Ariannejad, et al.
FSR =
λ2 ng L
(7)
Silicon material poses a low bend loss effect, which may result in a sharp angle bend for the compact health sensor. It could be seen from Eq. 8 that the group index (ng ) is an important parameter, while the effective index neff poses no effect on FWHM and FSR. The equation for the group index and effective index could be defined as follows:
ng = neff − λ 0
dneff (8)
dλ
The FSR could affect the Finesse, while the FWHM would pose a reversed effect as follows:
Finesse =
FSR FWHM
(9)
The Qfactor could be calculated according to the reference wavelength λref . Meanwhile, the frequency-based Qfactor could be calculated based on the following equation:
Qfactor =
λres FWHM
(10)
The ring waveguide would lead to the main resonance, where energy would circulate inside the ring. Higher Qfactor racetrack could be achieved by optimizing its design. With that being said, FSR has an important role as the sensor in the racetrack design. 3. Results and discussion The changes in the material at the top of the structure could alter the measurement reading based on the optical waveguide. In finite different time domain (FDTD) simulation, air used as a fundamental material to identify the changes occurring in the output spectrum. Through an understanding of the urea material, water optical properties were near to the urea by having an almost similar refractive index. Provided that the silicon material is suitable for confining the light inside the waveguide, the electromagnetic field of the light is presented in Fig. 2. Moreover, the design of racetrack consisted of two-straight waveguides as parts of the coupling so that the drop and throughput port spectrum could be measured. As an upper cladding, urea contributed to significant optical power loss (Fig. 2(c)) compared to H2O (Fig. 2(b)). The most significant optical power loss identified through air as an upper cladding, as shown in Fig. 2(a). With the racetrack design, an improved coupling effect could be created so that the mode would be confined within the silicon waveguide. Moreover, the length of this straight waveguide was 1400 nm, while the gap between the couplers was 310 nm. The radius of both half-circles was optimized while the total size of the devices amounted to 26,000 nm, as seen in Table 1. The differences between the refractive index could contribute to meaningful optical waveguide design. Similar to the optical fiber which possessed a core with a higher refractive index, the optical waveguide also exhibited a core waveguide with a higher refractive index compared to the substrate and upper cladding. Notably, improved confinement of light would take place with a higher refractive index of the core material. Silicon oxide was used as a substrate for silicon waveguide the racetrack design. Additionally, air was the most suitable material for upper cladding due to its lower refractive index, which could contribute to improved mode confinement inside the waveguide and mode coupling in the racetrack waveguide design. As H2O possessed a refractive index which was almost similar to urea’s refractive index, it was used to observe the sensibility of the proposed sensor. All the materials which were involved in this simulation are presented in Table 2, with related references being included. The urea used in this simulation exhibited a concentration value of 60 gm/dl - 100 gm/dl [29]. Notably, drop-port plays the most important role in resonator designs as it could measure the filter mode and resonance frequency. Based on Fig. 3, several power changes were visible from drop-port transmission through a different material. In comparison to air and H2O, as a cladding material, the urea reduced power loss. This power loss occurred due to different refractive index as an upper cladding, which could influence the optical transmission which took place in the drop-port. By regarding air as reference material for upper cladding, the right shift spectrum (redshift) of water and urea amounted to 6 ×
Fig. 2. The electromagnetic field in (a) Air, (b) H2O, and (c) Urea. 3
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Table 1 Silicon racetrack structure parameters. Parameter
Value(nm)
Base width Gap Height Total Length Radius Coupling length
450 310 180 26000 3400 1400
Table 2 The list of materials and refractive indexes which were used in the FDTD simulation. Material
n
K
Reference
Urea H2 0 Air Si SiO2
1.335 ± 0.001 1.3154 1.00027326 3.4870 1.4657
– 0.00014925 – 1.0901e-13 –
[29] [30] [31] [32] [33]
Fig. 3. Drop-port transmission racetrack design for urea sensor.
10−3 and 7 × 10−3 respectively. Meanwhile, the shift value between H2O and urea was 1 × 10-3. This was a value which was almost similar to the aforementioned spectrums. Throughput is a straight waveguide, where one side of the waveguide was used to penetrate the source light and modes. Meanwhile, on the other side of it, the coupling would take place between the refracted modes and this straight waveguide after resonance. Furthermore, the changes in power were significant in throughput due to the low power in air compared to the power in H2O and urea. The spectrum of the power ranges from 1.55 um to 1.6 um. A redshift was also present in this wavelength region (Fig. 4). Moreover, air could be considered as a reference so that the H2O would shift for 6 × 10−3, while urea would have spectrum shift for 7 × 10−3. The shift between the H2O and urea was roughly 1 × 10−3. The closed-loop of racetrack design would generate the resonance. As a result, the parameters, such as Qfactor and FSR could be calculated according to the output spectrum (Table 3). The drop port of resonator design constantly displayed the most significant spectrum as it filtered the modes. In respect of the drop port, the highest Qfactor was obtained through the use of urea as upper cladding. Additionally, air displayed the highest FSR frequency with a lower refractive index. All these calculated parameters are displayed in Table 3 accordingly. The changes in the main parameters in the design were due to the changing refractive index as an upper layer on the top of the whole waveguide. The reflected and coupled modes to the upper straight waveguide was measured by throughput port in racetrack resonator, as shown in Table 4. The highest FSR, frequency, and Qfactor were obtained with urea as upper cladding despite the minimum FWHM level. The THz generation could be useful for communication and sensor applications. The results of the time domain could also be affected by different materials for cladding, leading to a time delay. Taking air into consideration as the references for these measurements, a reasonable power loss was mostly present in urea (1.6 × 10−5(W)). However, air displayed the highest power level (2.35 × 10-4 (W)) due to its refractive index being lower than urea. As a result, the coupling of light would be more effective. The time delay between Air as a reference and urea was 1.1 (fs), while the time delay 4
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M. Ariannejad, et al.
Fig. 4. Throughput port transmission racetrack design for urea sensor.
Table 3 Different cladding conditions and calculation of the parameters in drop-port. CLADDING
FSR (nm)
FWHM (nm)
Qfactor ×103
Finesse
Δf (THz)
AIR H2O Urea
23 22 21
1 0.9 0.5
1.55 1.72 3.10
23 24.44 42
2.87 2.74 2.62
Table 4 Different cladding conditions and calculation of the parameters in the throughput port. CLADDING
FSR (nm)
FWHM (nm)
Qfactor×103
Finesse
Δf (THz)
AIR H2O Urea
21 22 23
1 2 0.85
1.55 0.77 1.82
21 11 27.05
2.62 2.75 2.87
generated through H2O was 1.2 (fs), as shown in Fig. 5. Based on the time domain shown in Fig. 6, the throughput possessed opposite features of the drop port. As the highest power was achieved in urea (7.94 × 10−3(W)) as the upper cladding, significant time delay values were present compared to drop-port. The H2O resulted in 3 (fs) time delay, while urea contributed to 1.9 (fs) time delay. Additionally, there was a significant time difference between urea and H2O as the upper cladding due to 4.9 fs time delay.
Fig. 5. Drop port of racetrack in the time domain for urea sensor. 5
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
M. Ariannejad, et al.
Fig. 6. Throughput port of racetrack design for urea sensor.
A higher time delay could be seen from the throughput port. Furthermore, a more significant difference was present in the time delay values of the drop port which filtered the coupled modes. The urea and H2O as upper cladding material contributed to the highest time delay due to the various changes in the refractive indexes. 4. Conclusion A silicon racetrack resonator with a region of two couplers was designed in this study. The observation was conducted on the redshift in the drop port and throughput port spectrum, specifically in terms of changes in the upper cladding. As a result, the most significant redshift by 7 × 10−3 nm was present in drop port urea. On the contrary, the redshift between the urea and H2O was not significant. Moreover, the changes in power were also focused in this simulation. It was found that air as the upper cladding possessed the highest spectrum depth. The highest frequency was also present in air as the upper cladding (2.87 THz), particularly in terms of an improved mode condiment of light. In respect of the throughput, the pattern of the power changes was based on the drop port. On the other hand, the highest frequency was seen from Qfactor, and FWHM under urea as upper cladding, which occurred due to high power loss. As for drop port, the highest power in the time domain was achieved through air ((2.35 × 10−4 (W))) as the upper cladding. Based on a comparison conducted between drop port and throughput, it was found that the throughput power was higher in urea (7.94 × 103 (W)). Subsequently, the highest time delay of 4.9 (fs) was obtained, which explained the difference between urea and H2O material as upper cladding. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] D.A. Krohn, T. MacDougall, A. Mendez, Fiber Optic Sensors: Fundamentals and Applications, Spie Press, Bellingham, WA, 2014. [2] D. Benedikovic, et al., Enhanced performance of integrated silicon nanophotonic devices engineered by sub-wavelength grating structures, Integrated Optics: Design, Devices, Systems, and Applications V, International Society for Optics and Photonics, 2019. [3] S. Gundavarapu, et al., Sub-hertz fundamental linewidth photonic integrated Brillouin laser, Nat. Photonics 13 (1) (2019) 60–67. [4] F. Khozeymeh, M. Razaghi, Parallel-coupled dual Si O x N y racetrack resonators as biosensors with high improved intrinsic limit of detection, Phys. Rev. Appl. 12 (5) (2019) 054045. [5] M.R. Bryan, B.L. Miller, Silicon optical sensor arrays for environmental and health applications, Curr. Opin. Environ. Sci. Health (2019). [6] H.-Y. Lin, et al., Multiple resonance fiber-optic sensor with time division multiplexing for multianalyte detection, Opt. Lett. 37 (19) (2012) 3969–3971. [7] J. Guo, et al., Stretchable and temperature‐sensitive polymer optical fibers for wearable health monitoring, Adv. Funct. Mater. (2019) 1902898. [8] P. Zhu, et al., Distributed modular temperature-strain sensor based on optical fiber embedded in laminated composites, Compos. Part B Eng. 168 (2019) 267–273. [9] S.M. Khan, et al., CMOS enabled microfluidic systems for healthcare based applications, Adv. Mater. 30 (16) (2018) 1705759. [10] D. Benedikovic, et al., 25 Gbps low-voltage hetero-structured silicon-germanium waveguide pin photodetectors for monolithic on-chip nanophotonic architectures, Photonics Res. 7 (4) (2019) 437–444. [11] R. Hainberger, et al., Silicon-nitride waveguide-based integrated photonic circuits for medical diagnostic and other sensing applications, Smart Photonic and Optoelectronic Integrated Circuits XXI, International Society for Optics and Photonics, 2019. [12] E. Ozbay, Plasmonics: merging photonics and electronics at nanoscale dimensions, Science 311 (5758) (2006) 189–193. [13] J.D. Caldwell, et al., Photonics with hexagonal boron nitride, Nature 41578 (2019) 019–0124.
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[14] A. Nag, S.C. Mukhopadhyay, Nanoparticles-based flexible wearable sensors for health monitoring applications, Nanotechnology Characterization Tools for Environment, Health, and Safety, Springer, 2019, pp. 245–284. [15] M. Ariannejad, et al., A large free spectral range of 74.92 GHz in comb peaks generated by SU-8 polymer micro-ring resonators: simulation and experiment, Laser Phys. 28 (11) (2018) 115002. [16] M. Ariannejad, et al., Polarization dependence of SU-8 micro ring resonator, Results Phys. 11 (2018) 515–522. [17] I.S. Amiri, et al., Visible wireless communications using solitonic carriers generated by microring resonators (MRRs), Iran. J. Sci. Technol. Trans. A Sci. 42 (3) (2018) 1595–1601. [18] S. Xiao, et al., Compact silicon microring resonators with ultra-low propagation loss in the C band, Opt. Express 15 (22) (2007) 14467–14475. [19] M. Soltani, S. Yegnanarayanan, A. Adibi, Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics, Opt. Express 15 (8) (2007) 4694–4704. [20] S.T. Chu, et al., Cascaded microring resonators for crosstalk reduction and spectrum cleanup in add-drop filters, Ieee Photonics Technol. Lett. 11 (11) (1999) 1423–1425. [21] P. Koonath, T. Indukuri, B. Jalali, Monolithic 3-D silicon photonics, J. Light. Technol. 24 (4) (2006) 1796–1804. [22] S. Khani, M. Danaie, P. Rezaei, Design of a single-mode plasmonic bandpass filter using a hexagonal resonator coupled to graded-stub waveguides, Plasmonics 14 (1) (2019) 53–62. [23] Q. Li, et al., Design and demonstration of compact, wide bandwidth coupled-resonator filters on a silicon-on-insulator platform, Opt. Express 17 (4) (2009) 2247–2254. [24] N. Eti, Z. Çetin, H. Sözüer, Fully three-dimensional analysis of a photonic crystal assisted silicon on insulator waveguide bend, Int. J. Mod. Phys. B 32 (31) (2018) 1850344. [25] W. Bogaerts, et al., Compact wavelength-selective functions in silicon-on-insulator photonic wires, IEEE J. Sel. Top. Quantum Electron. 12 (6) (2006) 1394–1401. [26] H. Rong, et al., A continuous-wave Raman silicon laser, Nature 433 (7027) (2005) 725. [27] F. Xia, et al., Ultra-compact high order ring resonator filters using submicron silicon photonic wires for on-chip optical interconnects, Opt. Express 15 (19) (2007) 11934–11941. [28] Q. Xu, D. Fattal, R.G. Beausoleil, Silicon microring resonators with 1.5-μm radius, Opt. Express 16 (6) (2008) 4309–4315. [29] S.I. Ahmad, Studies on Some Biophysical Aspects of Human Renal Excretory Fluid, (2010). [30] S. Kedenburg, et al., Linear refractive index and absorption measurements of nonlinear optical liquids in the visible and near-infrared spectral region, Opt. Mater. Express 2 (11) (2012) 1588–1611. [31] P.E. Ciddor, Refractive index of air: new equations for the visible and near infrared, Appl. Opt. 35 (9) (1996) 1566–1573. [32] C. Schinke, et al., Uncertainty analysis for the coefficient of band-to-band absorption of crystalline silicon, AIP Adv. 5 (6) (2015) 067168. [33] L. Gao, F. Lemarchand, M. Lequime, Exploitation of multiple incidences spectrometric measurements for thin film reverse engineering, Opt. Express 20 (14) (2012) 15734–15751.
7
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Urea sensor by racetrack silicon resonator MM Ariannejada, Elnaz Akbarib,c,*, Mourad Niazid, Effariza Hanafie a
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Viet Nam c Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Viet Nam d General and Laparoscopic Surgery, Department of Surgery, Aleppo University Hospital, University of Aleppo, Aleppo, Syria e Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia b
A R T IC LE I N F O
ABS TRA CT
Keywords: Racetrack resonator Urea Refractive index sensor Silicon waveguide Upper cladding
Optical Racetrack resonator waveguide which is based on silicon material possesses a good ability in sensing different materials, and urea is one of them has been simulated using a Finite different time-domain (FDTD) method. The drop port of racetrack possessed a higher spectrum shift of 7 × 10−3 nm. Urea was considered as the upper cladding in the throughput, resulting in the generation of the Qfactor and full width at half maximum (FWHM) with the highest frequency. Power changes posed a significant effect on the upper cladding material. In the time domain, throughput showed the maximum time delay, resulting in 4.9 fs, which was a unit for time difference generated by urea and H2O materials as cladding.
1. Introduction Health sensor is a compelling subject. As electronic devices are considered sensitive but not that compact like optical sensors, the development of optical sensors has opened a new opportunity for the healthcare industry products [1,2]. The urine ratio parameter control through optical sensors has a crucial function for body health [3–5]. Additionally, this device is known to be more compact compared to electronics sensors. Optical fiber can act as optical sensor that can be use in in remote-based health monitoring. According to recent studies, although optical fiber has also been used health monitoring purposes, optical waveguides could offer more compact size of sensors [6–8]. Silicon-based materials exhibit various remarkable industrial uses in electronic devices and the optical waveguide. Complementary metal-oxide-semiconductor (CMOS), can be used in the optical waveguide fabrication. Besides reducing the complexity of fabrication and cost of health sensor devices [9–11], this type of waveguide fabrication will also reduce the noise ratio, which is common in electronics-based health sensors [12–14]. As the main function of micro ring resonator (MRR) is generating frequency, any changes within the environment could influence the output spectrum. [15–17]. One of the unique designs of MRR would be the racetrack resonator. The racetrack resonator consists of two half-circles connected to a straight waveguide so that a distance between these two circles would be present. The straight waveguide would facilitate coupling effects to take place in MRR design. Meanwhile, the closed-loop system design of MRR would lead to a resonance effect, which would be relevant to the group index of materials and the ring radius. Multiple resonances could be defined by the space between the resonances, which is also known as free spectral range (FSR). In respect of the resonators’ structure, the Qfactor could be calculated by the output spectrum [18,19]. As for racetrack resonator, the
⁎
Corresponding author. E-mail address: [email protected] (E. Akbari).
https://doi.org/10.1016/j.ijleo.2019.164042 Received 13 September 2019; Received in revised form 2 December 2019; Accepted 10 December 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Please cite this article as: MM Ariannejad, et al., Optik - International Journal for Light and Electron Optics, https://doi.org/10.1016/j.ijleo.2019.164042
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
M. Ariannejad, et al.
length of coupling and its gap are highly crucial in order to achieve improved filtering of the light modes [20–22]. the fabrication of such waveguide can be possible by nano fabrication methods [22–24]. The bend waveguide can cause optical power loss that the design can improve this problem [25]. The racetrack design can optimize the Qfactor by creating a longer coupling region [26,27].The compact size of racetrack would increase free spectral range (FSR), that can affect the Qfactor value [28]. Overall, the optical racetrack could be designed to decrease the size of the health sensor by obtaining a good FSR. 2. Material and methods The refractive index sensor (RI) can be designed by silicon racetrack design. The racetrack design simply can track the refractive index changes on the upper cladding accordingly. The resonator of the racetrack could be represented by Eq. 1 below:
m⋅λ = neff ⋅L
(1)
Based on Eq. 1, the wavelength (λ ) consists of a linear relation with the order of the mode (m ) due to resonance. Among all types of waveguides, the effective refractive index (neff ) plays an important role which could be affected by the length (L ) of the waveguide. Performing changes in the material is an effective method of shifting the resonance spectrum. The racetrack resonator design is represented by Qc , while the clockwise direction, μ (t ) , is according to the laser source. Therefore, the equation of the coupling of mode according to the time-domain theory is as follows:
u (t ) = u 0 exp(−2t τc )
(2)
According to Eq. 2, the modes would be coupled in the racetrack with τc as shown in the equation below: (3)
τc = 2Qc ω0
The racetrack comprises a closed-loop ring. In Fig. 1 the starting part, which is labelled as I is the mode which could be coupled with efficient power. The light will travel along the round trip in a clockwise direction so that the average power in this system throughout the time could be represented with tr = L vg . Furthermore, the group velocity (vg ) poses an opposite effect on the roundtrip time (tr ), while the length of the resonator (L ) impacts the following the equation below:
p = u 0 (1 − e 2L
(vg τc ) )
(L vg )
(4)
Through an understanding of the parallel couple waveguides, the light could travel with resonance from point I to point II, as represented in [32],
p = p0 sin2 (ΔβZ0 2)
(5)
The length of coupling (Z0 ) in the racetrack has an important role in the optical power which circulates around the ring, resulting in the occurrence of resonance ( p0 ). By putting the point I into consideration, the coupler in this design could be directional so that the constant propagation Δβ could be defined. The Qc could be calculated according to u 0 = p0 L vg and vg = c ng , resulting in the following equation:
Qc = −
πng L λ 0 ln [ cos (ΔβZ0 2)]
(6)
Based on the above equation (with constant Z0 in Fig. 1), L and Qc have a linear relationship, which could be elaborated through the volume of light modes in the resonator-based design. The add-drop port is incorporated into the racetrack design so that the mode dispersion and wavelength range could be defined by the free spectral range (FSR), as per the equation below:
Fig. 1. The structure of racetrack with two-sided coupling as straight waveguides. 2
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
M. Ariannejad, et al.
FSR =
λ2 ng L
(7)
Silicon material poses a low bend loss effect, which may result in a sharp angle bend for the compact health sensor. It could be seen from Eq. 8 that the group index (ng ) is an important parameter, while the effective index neff poses no effect on FWHM and FSR. The equation for the group index and effective index could be defined as follows:
ng = neff − λ 0
dneff (8)
dλ
The FSR could affect the Finesse, while the FWHM would pose a reversed effect as follows:
Finesse =
FSR FWHM
(9)
The Qfactor could be calculated according to the reference wavelength λref . Meanwhile, the frequency-based Qfactor could be calculated based on the following equation:
Qfactor =
λres FWHM
(10)
The ring waveguide would lead to the main resonance, where energy would circulate inside the ring. Higher Qfactor racetrack could be achieved by optimizing its design. With that being said, FSR has an important role as the sensor in the racetrack design. 3. Results and discussion The changes in the material at the top of the structure could alter the measurement reading based on the optical waveguide. In finite different time domain (FDTD) simulation, air used as a fundamental material to identify the changes occurring in the output spectrum. Through an understanding of the urea material, water optical properties were near to the urea by having an almost similar refractive index. Provided that the silicon material is suitable for confining the light inside the waveguide, the electromagnetic field of the light is presented in Fig. 2. Moreover, the design of racetrack consisted of two-straight waveguides as parts of the coupling so that the drop and throughput port spectrum could be measured. As an upper cladding, urea contributed to significant optical power loss (Fig. 2(c)) compared to H2O (Fig. 2(b)). The most significant optical power loss identified through air as an upper cladding, as shown in Fig. 2(a). With the racetrack design, an improved coupling effect could be created so that the mode would be confined within the silicon waveguide. Moreover, the length of this straight waveguide was 1400 nm, while the gap between the couplers was 310 nm. The radius of both half-circles was optimized while the total size of the devices amounted to 26,000 nm, as seen in Table 1. The differences between the refractive index could contribute to meaningful optical waveguide design. Similar to the optical fiber which possessed a core with a higher refractive index, the optical waveguide also exhibited a core waveguide with a higher refractive index compared to the substrate and upper cladding. Notably, improved confinement of light would take place with a higher refractive index of the core material. Silicon oxide was used as a substrate for silicon waveguide the racetrack design. Additionally, air was the most suitable material for upper cladding due to its lower refractive index, which could contribute to improved mode confinement inside the waveguide and mode coupling in the racetrack waveguide design. As H2O possessed a refractive index which was almost similar to urea’s refractive index, it was used to observe the sensibility of the proposed sensor. All the materials which were involved in this simulation are presented in Table 2, with related references being included. The urea used in this simulation exhibited a concentration value of 60 gm/dl - 100 gm/dl [29]. Notably, drop-port plays the most important role in resonator designs as it could measure the filter mode and resonance frequency. Based on Fig. 3, several power changes were visible from drop-port transmission through a different material. In comparison to air and H2O, as a cladding material, the urea reduced power loss. This power loss occurred due to different refractive index as an upper cladding, which could influence the optical transmission which took place in the drop-port. By regarding air as reference material for upper cladding, the right shift spectrum (redshift) of water and urea amounted to 6 ×
Fig. 2. The electromagnetic field in (a) Air, (b) H2O, and (c) Urea. 3
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Table 1 Silicon racetrack structure parameters. Parameter
Value(nm)
Base width Gap Height Total Length Radius Coupling length
450 310 180 26000 3400 1400
Table 2 The list of materials and refractive indexes which were used in the FDTD simulation. Material
n
K
Reference
Urea H2 0 Air Si SiO2
1.335 ± 0.001 1.3154 1.00027326 3.4870 1.4657
– 0.00014925 – 1.0901e-13 –
[29] [30] [31] [32] [33]
Fig. 3. Drop-port transmission racetrack design for urea sensor.
10−3 and 7 × 10−3 respectively. Meanwhile, the shift value between H2O and urea was 1 × 10-3. This was a value which was almost similar to the aforementioned spectrums. Throughput is a straight waveguide, where one side of the waveguide was used to penetrate the source light and modes. Meanwhile, on the other side of it, the coupling would take place between the refracted modes and this straight waveguide after resonance. Furthermore, the changes in power were significant in throughput due to the low power in air compared to the power in H2O and urea. The spectrum of the power ranges from 1.55 um to 1.6 um. A redshift was also present in this wavelength region (Fig. 4). Moreover, air could be considered as a reference so that the H2O would shift for 6 × 10−3, while urea would have spectrum shift for 7 × 10−3. The shift between the H2O and urea was roughly 1 × 10−3. The closed-loop of racetrack design would generate the resonance. As a result, the parameters, such as Qfactor and FSR could be calculated according to the output spectrum (Table 3). The drop port of resonator design constantly displayed the most significant spectrum as it filtered the modes. In respect of the drop port, the highest Qfactor was obtained through the use of urea as upper cladding. Additionally, air displayed the highest FSR frequency with a lower refractive index. All these calculated parameters are displayed in Table 3 accordingly. The changes in the main parameters in the design were due to the changing refractive index as an upper layer on the top of the whole waveguide. The reflected and coupled modes to the upper straight waveguide was measured by throughput port in racetrack resonator, as shown in Table 4. The highest FSR, frequency, and Qfactor were obtained with urea as upper cladding despite the minimum FWHM level. The THz generation could be useful for communication and sensor applications. The results of the time domain could also be affected by different materials for cladding, leading to a time delay. Taking air into consideration as the references for these measurements, a reasonable power loss was mostly present in urea (1.6 × 10−5(W)). However, air displayed the highest power level (2.35 × 10-4 (W)) due to its refractive index being lower than urea. As a result, the coupling of light would be more effective. The time delay between Air as a reference and urea was 1.1 (fs), while the time delay 4
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Fig. 4. Throughput port transmission racetrack design for urea sensor.
Table 3 Different cladding conditions and calculation of the parameters in drop-port. CLADDING
FSR (nm)
FWHM (nm)
Qfactor ×103
Finesse
Δf (THz)
AIR H2O Urea
23 22 21
1 0.9 0.5
1.55 1.72 3.10
23 24.44 42
2.87 2.74 2.62
Table 4 Different cladding conditions and calculation of the parameters in the throughput port. CLADDING
FSR (nm)
FWHM (nm)
Qfactor×103
Finesse
Δf (THz)
AIR H2O Urea
21 22 23
1 2 0.85
1.55 0.77 1.82
21 11 27.05
2.62 2.75 2.87
generated through H2O was 1.2 (fs), as shown in Fig. 5. Based on the time domain shown in Fig. 6, the throughput possessed opposite features of the drop port. As the highest power was achieved in urea (7.94 × 10−3(W)) as the upper cladding, significant time delay values were present compared to drop-port. The H2O resulted in 3 (fs) time delay, while urea contributed to 1.9 (fs) time delay. Additionally, there was a significant time difference between urea and H2O as the upper cladding due to 4.9 fs time delay.
Fig. 5. Drop port of racetrack in the time domain for urea sensor. 5
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Fig. 6. Throughput port of racetrack design for urea sensor.
A higher time delay could be seen from the throughput port. Furthermore, a more significant difference was present in the time delay values of the drop port which filtered the coupled modes. The urea and H2O as upper cladding material contributed to the highest time delay due to the various changes in the refractive indexes. 4. Conclusion A silicon racetrack resonator with a region of two couplers was designed in this study. The observation was conducted on the redshift in the drop port and throughput port spectrum, specifically in terms of changes in the upper cladding. As a result, the most significant redshift by 7 × 10−3 nm was present in drop port urea. On the contrary, the redshift between the urea and H2O was not significant. Moreover, the changes in power were also focused in this simulation. It was found that air as the upper cladding possessed the highest spectrum depth. The highest frequency was also present in air as the upper cladding (2.87 THz), particularly in terms of an improved mode condiment of light. In respect of the throughput, the pattern of the power changes was based on the drop port. On the other hand, the highest frequency was seen from Qfactor, and FWHM under urea as upper cladding, which occurred due to high power loss. As for drop port, the highest power in the time domain was achieved through air ((2.35 × 10−4 (W))) as the upper cladding. Based on a comparison conducted between drop port and throughput, it was found that the throughput power was higher in urea (7.94 × 103 (W)). Subsequently, the highest time delay of 4.9 (fs) was obtained, which explained the difference between urea and H2O material as upper cladding. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] D.A. Krohn, T. MacDougall, A. Mendez, Fiber Optic Sensors: Fundamentals and Applications, Spie Press, Bellingham, WA, 2014. [2] D. Benedikovic, et al., Enhanced performance of integrated silicon nanophotonic devices engineered by sub-wavelength grating structures, Integrated Optics: Design, Devices, Systems, and Applications V, International Society for Optics and Photonics, 2019. [3] S. Gundavarapu, et al., Sub-hertz fundamental linewidth photonic integrated Brillouin laser, Nat. Photonics 13 (1) (2019) 60–67. [4] F. Khozeymeh, M. Razaghi, Parallel-coupled dual Si O x N y racetrack resonators as biosensors with high improved intrinsic limit of detection, Phys. Rev. Appl. 12 (5) (2019) 054045. [5] M.R. Bryan, B.L. Miller, Silicon optical sensor arrays for environmental and health applications, Curr. Opin. Environ. Sci. Health (2019). [6] H.-Y. Lin, et al., Multiple resonance fiber-optic sensor with time division multiplexing for multianalyte detection, Opt. Lett. 37 (19) (2012) 3969–3971. [7] J. Guo, et al., Stretchable and temperature‐sensitive polymer optical fibers for wearable health monitoring, Adv. Funct. Mater. (2019) 1902898. [8] P. Zhu, et al., Distributed modular temperature-strain sensor based on optical fiber embedded in laminated composites, Compos. Part B Eng. 168 (2019) 267–273. [9] S.M. Khan, et al., CMOS enabled microfluidic systems for healthcare based applications, Adv. Mater. 30 (16) (2018) 1705759. [10] D. Benedikovic, et al., 25 Gbps low-voltage hetero-structured silicon-germanium waveguide pin photodetectors for monolithic on-chip nanophotonic architectures, Photonics Res. 7 (4) (2019) 437–444. [11] R. Hainberger, et al., Silicon-nitride waveguide-based integrated photonic circuits for medical diagnostic and other sensing applications, Smart Photonic and Optoelectronic Integrated Circuits XXI, International Society for Optics and Photonics, 2019. [12] E. Ozbay, Plasmonics: merging photonics and electronics at nanoscale dimensions, Science 311 (5758) (2006) 189–193. [13] J.D. Caldwell, et al., Photonics with hexagonal boron nitride, Nature 41578 (2019) 019–0124.
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