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InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section
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Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section I.S. Amiri∗, M.M. Ariannejad, H. Ahmad Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history: Received 22 March 2016 Accepted 1 August 2016 Available online xxx Keywords: Microring resonator InGaAsP/InP waveguide Resonance wavelength shift
a b s t r a c t In this research, we use the time-domain travelling wave (TDTW) method to model and simulate a microring resonator (InGaAsP/InP) waveguide based on resonance wavelength shifts. A change of the refractive index of the cladding results in a change of the effective index of the mode propagating within the microring resonator, where it can be observed by a shift of the resonance wavelength, thus providing a tuning mechanism. The total length of the microring resonator is 100 μm, with a varying refractive index along 40 μm of the cladding section. The Q-factor of the microring resonator is 4.5 × 105 , showing high performance of the system. The largest resonance wavelength shift is 3.7 nm and is obtained when the medium of the cladding is air. © 2016 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.
1. Introduction Recent research efforts have focused on the study and analysis of microring resonators based on wavelength-selective reflective elements [1]. The two primary characteristics of these systems, namely resonance-splitting and enhanced notch depth have been investigated experimentally and the mutual mode coupling as a result of such an amalgamation has been verified in microring resonators. It has been shown that the free-spectral range (FSR) is normally large enough to restricts the number of channels adopted for normal operation when small-radius rings are considered [2]. In this aspect, optical resonators have found many applications such as wavelength filtering, routing, switching, modulation, and multiplexing/demultiplexing [3,4]. Dielectric micro resonators in particular are highly advantageous as they have very high Q-factors as a result of the resonance appearing as a very straight line shape, making it highly suitable for detecting minor variations in the ambient refractive index. The spectral behavior of the ring resonator has enabled multiple filtering features which are useful for optical sensing and lasers applications to be realized [5,6]. Micro resonators can also be realized in the form of integrated optical waveguides [7–11] and employed as optical-waveguide sensors [12] such as directional coupler sensors [13], Mach–Zehnder interferometric sensors [14], grating-coupled waveguide sensors [15–17], which have shown substantial performance capabilities as biological and chemical sensors. Sensing in most optical-waveguides is a function of the interaction of the evanescent waves with the absorbed analyte on the sensor or mounted guides. Detection of low amount or minute concentration of measurand requires a suitable interplay with a certain length of the waveguide, such as a large dimensional straight waveguide in the scale of centimeters in order to acquire an obtainable phase shift and to detect a ∗
Corresponding author. E-mail address: isafi[email protected] (I.S. Amiri).
http://dx.doi.org/10.1016/j.cjph.2016.08.002 0577-9073/© 2016 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.
Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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I.S. Amiri et al. / Chinese Journal of Physics 000 (2016) 1–8 Table 1 InGaAsP/InP waveguide layers. Layers
Thickness (μm)
Refractive index
InP substrate InGaAsP InGaAsP etch stop layer InGaAsP InP cap
0.5 0.3 0.05 0.3 0.71
n1 n2 n3 n2 n1
= = = = =
3.18 3.31 3.39 3.31 3.18
reliable signal-to-noise ratio (SNR). However, this creates a limitation, as these waveguides are expensive and complex to fabricate, in addition to being bulky. In this regard, microring resonators provide a highly potential alternative. The shape of the microring resonator allows for a maximum surface area to be exposed to the sensing constituent, while at the same time maintaining a small footprint. Even exposure to the masured constituent modifies the effective index of the guiding mode. Furthermore, the time-domain travelling wave (TDTW) method applied to the InGaAsP/InP microring resonator model can be used to stimulate the resonance wavelength shift in the spectral domain of the device. In this work, the TDTW method is used to simulate a microring resonator (InGaAsP/InP) waveguide exploiting resonance wavelength shifts as a tuning mechanism. Changes in the refractive index of the cladding can be observed by a shift of the resonance wavelength, thus providing a tuning mechanism. 2. Proposed microring resonator The proposed structure composed of five layer including the materials namely In (indium), Ga (gallium), As (arsenide), P (phosphide) and InP (indium phosphide). The core section of the waveguide comprises of In, Ga, As and P. The microring resonator is simulated by InGaAsP/InP waveguide with core having refractive index of 3.31 surrounded by InP (n = 3.18) cladding and a low index top cladding (air). The InGaAsP assures optical isolation of waveguides from the InP substrate. A change of the refractive index of the cladding layer results in a change of the effective index of the mode propagating within the ring resonator that can be observed by a shift of the resonance wavelengths. The nonlinear refractive index is 2.2 × 10−17 m2 /W for the wavelengths around 1.55 μm. InGaAsP/InP photonic semiconductors are an ideal platform for nonlinear because they exhibit some strong nonlinear effects such as Kerr nonlinearity. Employing the nonlinear Kerr effect in a high-Q microring resonator, has a chance to achieve wideband optical comb generation with ultra-low power consumption [18,19]. Moreover, a resonator in silicon or III–V compound semiconductor might be desired for the optical comb generation due to benefiting from strong nonlinear Kerr effects [20,21]. To maximize the nonlinearity we need to optimize the nonlinear parameter, where the effective area of nonlinear interaction depends on the waveguide geometry [22]. The used InGaAsP/InP waveguide layers sequence, from the bottom to the top is presented in Table 1. The microring resonator structure is shown in Fig. 1. The cladding section acts as sensing section, where the refractive index of the medium can be changed results on a resonant wavelength shift of the generated wavelengths. The design assures a monomodal propagation of light in the waveguide and, due to a good confinement in the resonator, very low bending losses. The input optical field (Ei ) of the Gaussian pulse is given by Eq. (1) [23]
Ei ( z = 0, t ) =
P0 exp
−
t2 2T02
,
(1)
where, P0 is the input power and the pulse width T0 (related to the pulse full width at half maximum by TFWHM ≈ 1.665 T0 ) increases with z (the pulse broadens) according to Eq. (2) as
T (z ) = 1 +
z 2 1/2 LD
T0 ,
(2)
where, LD is the dispersion length. Consequently, the peak power changes, due to the group velocity dispersion (GVD), are given by Eq. (3):
P(z ) =
1+
P0
z 2 1/2
(3)
LD
When Gaussian pulse with power of 10 mW and center wavelength of 1.55 μm propagates within the microring resonator, the resonant output is formed for each round-trip [24–26]. The parameter of the add-drop microring resonator is presented in Table 2. The fractional coupler intensity loss is γ , and α is the linear absorption coefficient. When the device operates in nonlinear regime due to the Kerr nonlinearity, exactly in the spectral region showing the split resonant modes, the refractive index sensing performance is strongly improved with respect to the linear regime. The Kerr coefficients for the waveguide Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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3
Fig. 1. Schematic diagram of the microring resonator system simulated by InGaAsP/InP waveguide with a low refractive index top cladding. Table 2 Parameters of the microring resonator system.
κ1
R 16 μm
0.1
κ2 0.15
n0 3.31
n2 (m2 W−1 ) 2.2×10
−17
Aeff (μm2 )
α (dBmm−1 )
γ
1.94
0.5
0.1
is 2.2×10−17 m2 /W at 1.55 μm [27]. The effective area for the InGaAsP waveguide is 1.94 μm2 , therefore the nonlinear phase shift of the waveguides can be expressed in Eq. (4) as
ϕNL ( InGaAsP ) = kLn2 |Ein |2 = kLn2 (P/Aeff ) = 4.59 × 10−5 rad
(4)
The signals will go through the constructive and destructive interferences within the microring resonator system. Therefore, the periodic resonances can be obtained. Slightly different resonance spacing of the spectra, resulting from the different medium used as cladding. The difference in the resonance signals is crucial to achieve large wavelength shifts in the microring resonator tunable lasers [28,29]. The ring to be on resonance when the phase ϕ is a multiple of 2π , or when the wavelength of the light fits a whole number of times inside the optical length of the ring, therefore the reference resonance wavelength can be expressed by Eq. (5),
λre f erence =
neff L , m
m = 1, 2, 3 . . .
(5)
A shift of the resonance wavelength λreference is essentially caused by a change of the effective index of the resonant mode neff . Referring to Eq. (5), we get Eq. (6) as
λre f erence =
neff L m
,
m = 1, 2, 3 . . .
(6)
Where, m is the order of the resonant mode. neff is influenced by the refractive index of the cladding section, which is altered upon sensing [30]. Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 2. (a) Propagating mode profile of the microring resonator waveguide, where the cladding section is filled by InP and air respectively, (b) 3D propagation modes profile, (c) cross section propagating modes profile.
3. Result and discussion Propagation analysis of the input Gaussian pulse is done inside the microring resonator, its effective core area by varying the refractive index of the cladding section of the waveguide as shown in Fig. 1. This section has length of 40 μm. Therefore, the system of microring resonators has a cladding section of 40 μm, where the refractive index varies. The total length of the whole system is 100 μm. The propagation profile of the Gaussian input pulse is shown in Fig. 2, where the cladding section is filled by InP and air respectively. The refractive index of the InP is 3.18, while the refractive index of the air is 1. The higher contrast between the two refractive indices causes more confinement of the input pulse propagating inside the core section. The propagation mode profile, the 3D view of the propagation and the cross sectional profiles are shown in Fig. 2(a–c) respect to two different refractive indices of the air and InP. From Fig. 2(c) we have calculated the cross sections of the mode propagation within the waveguide, in the case, when the cladding is filled by InP and air respectively. There is smaller cross section of 1.77 μm2 , for the mode propagation within Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 3. Throughput resonance wavelength results, black color shows the InP is used as cladding, and blue color shows the air is used as cladding. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Throughput resonance wavelength results, black color shows the InP is used as cladding, and blue color shows the air is used as cladding. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the cladding section with lower refractive index (1 for air) compare to when the cladding is filled by the InP which has a cross section of 1.94 μm2 . It shows more confinement of the light propagating inside the waveguide section. The throughput output signals of the microring resonator are shown in Fig. 3, where it shows the outputs signals for the system with cladding of InP and air respectively within 140 nm wavelengths range (1470–1610 nm). There are resonance wavelength shifts with respect to the changes of the refractive index of the cladding section. The full width at half maximum (3 dB bandwidth) and free spectral range (FSR) of the pulses are 3.4 pm and 6.85 nm (corresponds to 0.85 GHz in frequency domain). The Q-factor, which is the ratio of the resonant wavelength to the 3-dB bandwidth (FWHM), is 4.5 × 105 . This shows high performance of the microring resonator. We have observed different locations of the resonance wavelengths therefore; the resonance wavelength shift occurs respect to the reference wavelength (cladding is filled by InP) depending on the refractive index of the material used as cladding. A resonance wavelength shift of 3.7 nm can be seen from Fig. 4 with respect to the results shown in Fig. 3. Basically we have replaced the cladding by air, which improves the contrast of refractive indices, therefore, the effective index is changed, where it causes shifting the resonance wavelength. Despite of shape shifting from asymmetrical to more symmetrical beam profiles due to replacement of air clad with InP clad (observe in Fig. 2), the simultaneous presentation of spectral shifting shown in Fig. 4 emphasize on small red shifting towards higher wavelength. However, this shift is at nanoscale shifting as to the Figs. 3 and 4, respectively, but it is fairly important to state that the refractive index of InP can be variable upon to the amount of absorbed photon with different energy [31] which makes the spectral shifting dependents on photon energy absorption. The drop port output signals show the same concept as described above. The Fig. 5 shows the drop port output signals. The resonance wavelength shift (࢞λ) can be calculated in different cases, when the cladding of the waveguide filled by different mediums having different refractive indices. The refractive indices vary between a ranges of 1–3.18. Fig. 6(a) shows Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 5. Drop port resonance wavelength results, black color shows the InP is used as cladding, and blue color shows the air is used as cladding. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. (a) Resonance wavelength shift (nm) versus the refractive index variation of the cladding (40 μm), (b) dispersion (ps/nm) versus wavelength (nm).
the refractive index (n) versus the resonance wavelength shift (nm). The dispersion versus the wavelength is shown in Fig. 6(b), clearly indicating that by increasing of the wavelength, the dispersion is reduced. Following the discussion of photon absorptivity for InP as an outer-clad of the microring resonator structure discussed in Fig. 4, the amount of resonance wavelength shifting obviously indicates a non-linear response to changing the refractive index. Although, it must be reminded that the Fig. 6a is simulated only under the refractive index variation wherein never functionalized under photon absorptivity. The changes of the refractive index of the cladding section causes changes on the mode effective index and propagating constant. Fig. 7(a) shows the refractive index versus the effective index propagation mode and Fig. 7(b) shows the effective mode propagation constant versus the refractive index. Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 7. (a) Mode effective index versus refractive index, (b) effective propagation constant (beta) versus refractive index.
From Fig. 7(a), the lower refractive index is corresponding to lower effective index, therefore higher resonance wavelength shift respect to the reference wavelength.
4. Conclusion The characterization of dielectric micro resonators using the time-domain travelling wave (TDTW) method to model and simulate a microring resonator (InGaAsP/InP) waveguide based on resonance wavelength shifts is proposed and discussed. The microring resonators can be used as sensor devices based on an evanescent wave interacting with the surrounding cladding of the system simulated by waveguide. To keep the waveguide single mode and total internal reflection guiding is required to have very small bend radii. The microring resonator is simulated by InGaAsP/InP waveguide with InGaAsP core (n = 3.31) surrounded by InP (n = 3.18) cladding and a low index top cladding (air). The propagating signals inside the microring resonator will undergo constructive and destructive interferences, therefore signals have a comb shape will be generated as periodic resonances. The difference in the refractive index of the cladding section causes changing the effective index of the waveguide, thus providing a tuning mechanism by observing the large wavelength shifts in the microring resonator system. The microring resonator has a Q-factor of 4.5 × 105 , which show high performance and quality of the system, and a resonance wavelength shift of 3.7 nm. The largest resonance wavelength shift is 3.7 nm and is obtained when the medium of the cladding is air. Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Acknowledgments IS Amiri would like to acknowledge the grant number UMRG (RP029B-15AFR), GA010-2014 (ulung), and RU007/2015 from the University of Malaya (UM). References [1] J.K. Poon, J. Scheuer, A. Yariv, Wavelength-selective reflector based on a circular array of coupled microring resonators, IEEE Photonics Technol. Lett. 16 (2004) 1331–1333. [2] Z. Zhang, M. Dainese, L. Wosinski, M. Qiu, Resonance-splitting and enhanced notch depth in SOI ring resonators with mutual mode coupling, Opt. Express 16 (2008) 4621–4630. [3] M. Soltanian, I.S. Amiri, S.E. Alavi, H. Ahmad, All optical ultra-wideband signal generation and transmission using mode-locked laser incorporated with add-drop microring resonator (MRR), Laser Phys. Lett. 12 (2015) 065105. [4] I.S. Amiri, A. Nikoukar, J. Ali, GHz frequency band soliton generation using integrated ring resonator for WiMAX optical communication, Opt. Quantum Electron. 46 (2013) 1165–1177. [5] D. Dai, L. Liu, S. Gao, D.X. Xu, S. He, Polarization management for silicon photonic integrated circuits, Laser Photonics Rev. 7 (2013) 303–328. [6] P. Schindler, A. Agmon, S. Wolf, R. Bonk, L. Meder, M. Meltsin, et al., Ultra-dense, single-wavelength DFT-spread OFDMA PON with laserless 1.2 Gb/s ONU ready for silicon photonics integration, J. Light. Technol. 33 (2014) 1650–1659. [7] E. Krioukov, D. Klunder, A. Driessen, J. Greve, C. Otto, Integrated optical microcavities for enhanced evanescent-wave spectroscopy, Opt. Lett. 27 (2002) 1504–1506. [8] E. Krioukov, D. Klunder, A. Driessen, J. Greve, C. Otto, Sensor based on an integrated optical microcavity, Opt. Lett. 27 (2002) 512–514. [9] F. Vollmer, D. Braun, A. Libchaber, M. Khoshsima, I. Teraoka, S. Arnold, Protein detection by optical shift of a resonant microcavity, Appl. Phys. Lett. 80 (2002) 4057–4059. [10] S. Arnold, M. Khoshsima, I. Teraoka, S. Holler, F. Vollmer, Shift of whispering-gallery modes in microspheres by protein adsorption, Opt. 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Larsen, Monitoring of living cell attachment and spreading using reverse symmetry waveguide sensing, Appl. Phys. Lett. 86 (2005) 071101. [17] H. Ahmad, N. Saat, S. Harun, S-band erbium-doped fiber ring laser using a fiber Bragg grating, Laser Phys. Lett. 2 (2005) 369. [18] T. Kippenberg, S. Spillane, K. Vahala, Kerr-nonlinearity optical parametric oscillation in an ultrahigh-Q toroid microcavity, Phys. Rev. Lett. 93 (2004) 083904. [19] P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, T. Kippenberg, Optical frequency comb generation from a monolithic microresonator, Nature 450 (2007) 1214–1217. [20] I.S. Amiri, J. Ali, Optical quantum generation and transmission of 57–61 GHz frequency band using an optical fiber optics, J. Comput. Theor. Nanosci. (JCTN) 11 (2014) 2130–2135. [21] I.S. Amiri, A. Nikoukar, Quantum Information Generation Using Optical Potential Well, Proceedings of the Network Technologies & Communications (NTC) Conference, Singapore, 2010–2011, 10.5176/2251-2233_NTC10. [22] C. Koos, P. Vorreau, P. Dumon, R. Baets, B. Esembeson, I. Biaggio, et al., Highly-nonlinear silicon photonic slot waveguide, in: Proceedings of the 2008 National Fiber Optic Engineers Conference, 2008 PDP25. [23] G.P. Agrawal, Nonlinear Fiber Optics, Academic press, 2007. [24] IS Amiri, SE Alavi, N Fisal, ASM Supa’at, H Ahmad, All-optical generation of two IEEE802.11n signals for 2×2 MIMO-RoF via MRR system, IEEE Photonics J. 6 (2014) 7903611. [25] S.E. Alavi, I.S. Amiri, M. Khalily, A.S.M. Supa’ at, N. Fisal, H. Ahmad, et al., W-band OFDM for radio-over-fibre direct-detection link enabled by frequency nonupling optical up-conversion, IEEE Photonics J. 6 (2014) 7903908. [26] S.E. Alavi, I.S. Amiri, S.M. Idrus, A.S.M. Supa’at, Generation and wired/wireless transmission of IEEE802.16 m signal using solitons generated by microring resonator, Opt. Quantum Electron. 47 (2014) 975–984. [27] I.S. Amiri, S.E. Alavi, S.M. Idrus, A.S.M. Supa’at, J. Ali, P.P. Yupapin, W-band OFDM transmission for radio-over-fiber link using solitonic millimeter wave generated by MRR, IEEE J. Quantum Electron. 50 (2014) 622–628. [28] I.S. Amiri, S. Soltanmohammadi, A. Shahidinejad, J. Ali, Optical quantum transmitter with finesse of 30 at 800-nm central wavelength using microring resonators, Opt. Quantum Electron. 45 (2013) 1095–1105. [29] I.S. Amiri, J. Ali, Single and multi optical soliton light trapping and switching using microring resonator, Quantum Matter 2 (2013) 116–121. [30] G. Agrawal, Applications of Nonlinear Fiber Optics, Academic Press, 2001. [31] L.I. Berger, Semiconductor Materials, CRC Press, 1996.
Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
JID: CJPH
[m3Gsc;October 8, 2016;2:27]
Chinese Journal of Physics 0 0 0 (2016) 1–8
Contents lists available at ScienceDirect
Chinese Journal of Physics journal homepage: www.elsevier.com/locate/cjph
Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section I.S. Amiri∗, M.M. Ariannejad, H. Ahmad Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history: Received 22 March 2016 Accepted 1 August 2016 Available online xxx Keywords: Microring resonator InGaAsP/InP waveguide Resonance wavelength shift
a b s t r a c t In this research, we use the time-domain travelling wave (TDTW) method to model and simulate a microring resonator (InGaAsP/InP) waveguide based on resonance wavelength shifts. A change of the refractive index of the cladding results in a change of the effective index of the mode propagating within the microring resonator, where it can be observed by a shift of the resonance wavelength, thus providing a tuning mechanism. The total length of the microring resonator is 100 μm, with a varying refractive index along 40 μm of the cladding section. The Q-factor of the microring resonator is 4.5 × 105 , showing high performance of the system. The largest resonance wavelength shift is 3.7 nm and is obtained when the medium of the cladding is air. © 2016 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.
1. Introduction Recent research efforts have focused on the study and analysis of microring resonators based on wavelength-selective reflective elements [1]. The two primary characteristics of these systems, namely resonance-splitting and enhanced notch depth have been investigated experimentally and the mutual mode coupling as a result of such an amalgamation has been verified in microring resonators. It has been shown that the free-spectral range (FSR) is normally large enough to restricts the number of channels adopted for normal operation when small-radius rings are considered [2]. In this aspect, optical resonators have found many applications such as wavelength filtering, routing, switching, modulation, and multiplexing/demultiplexing [3,4]. Dielectric micro resonators in particular are highly advantageous as they have very high Q-factors as a result of the resonance appearing as a very straight line shape, making it highly suitable for detecting minor variations in the ambient refractive index. The spectral behavior of the ring resonator has enabled multiple filtering features which are useful for optical sensing and lasers applications to be realized [5,6]. Micro resonators can also be realized in the form of integrated optical waveguides [7–11] and employed as optical-waveguide sensors [12] such as directional coupler sensors [13], Mach–Zehnder interferometric sensors [14], grating-coupled waveguide sensors [15–17], which have shown substantial performance capabilities as biological and chemical sensors. Sensing in most optical-waveguides is a function of the interaction of the evanescent waves with the absorbed analyte on the sensor or mounted guides. Detection of low amount or minute concentration of measurand requires a suitable interplay with a certain length of the waveguide, such as a large dimensional straight waveguide in the scale of centimeters in order to acquire an obtainable phase shift and to detect a ∗
Corresponding author. E-mail address: isafi[email protected] (I.S. Amiri).
http://dx.doi.org/10.1016/j.cjph.2016.08.002 0577-9073/© 2016 The Physical Society of the Republic of China (Taiwan). Published by Elsevier B.V. All rights reserved.
Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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I.S. Amiri et al. / Chinese Journal of Physics 000 (2016) 1–8 Table 1 InGaAsP/InP waveguide layers. Layers
Thickness (μm)
Refractive index
InP substrate InGaAsP InGaAsP etch stop layer InGaAsP InP cap
0.5 0.3 0.05 0.3 0.71
n1 n2 n3 n2 n1
= = = = =
3.18 3.31 3.39 3.31 3.18
reliable signal-to-noise ratio (SNR). However, this creates a limitation, as these waveguides are expensive and complex to fabricate, in addition to being bulky. In this regard, microring resonators provide a highly potential alternative. The shape of the microring resonator allows for a maximum surface area to be exposed to the sensing constituent, while at the same time maintaining a small footprint. Even exposure to the masured constituent modifies the effective index of the guiding mode. Furthermore, the time-domain travelling wave (TDTW) method applied to the InGaAsP/InP microring resonator model can be used to stimulate the resonance wavelength shift in the spectral domain of the device. In this work, the TDTW method is used to simulate a microring resonator (InGaAsP/InP) waveguide exploiting resonance wavelength shifts as a tuning mechanism. Changes in the refractive index of the cladding can be observed by a shift of the resonance wavelength, thus providing a tuning mechanism. 2. Proposed microring resonator The proposed structure composed of five layer including the materials namely In (indium), Ga (gallium), As (arsenide), P (phosphide) and InP (indium phosphide). The core section of the waveguide comprises of In, Ga, As and P. The microring resonator is simulated by InGaAsP/InP waveguide with core having refractive index of 3.31 surrounded by InP (n = 3.18) cladding and a low index top cladding (air). The InGaAsP assures optical isolation of waveguides from the InP substrate. A change of the refractive index of the cladding layer results in a change of the effective index of the mode propagating within the ring resonator that can be observed by a shift of the resonance wavelengths. The nonlinear refractive index is 2.2 × 10−17 m2 /W for the wavelengths around 1.55 μm. InGaAsP/InP photonic semiconductors are an ideal platform for nonlinear because they exhibit some strong nonlinear effects such as Kerr nonlinearity. Employing the nonlinear Kerr effect in a high-Q microring resonator, has a chance to achieve wideband optical comb generation with ultra-low power consumption [18,19]. Moreover, a resonator in silicon or III–V compound semiconductor might be desired for the optical comb generation due to benefiting from strong nonlinear Kerr effects [20,21]. To maximize the nonlinearity we need to optimize the nonlinear parameter, where the effective area of nonlinear interaction depends on the waveguide geometry [22]. The used InGaAsP/InP waveguide layers sequence, from the bottom to the top is presented in Table 1. The microring resonator structure is shown in Fig. 1. The cladding section acts as sensing section, where the refractive index of the medium can be changed results on a resonant wavelength shift of the generated wavelengths. The design assures a monomodal propagation of light in the waveguide and, due to a good confinement in the resonator, very low bending losses. The input optical field (Ei ) of the Gaussian pulse is given by Eq. (1) [23]
Ei ( z = 0, t ) =
P0 exp
−
t2 2T02
,
(1)
where, P0 is the input power and the pulse width T0 (related to the pulse full width at half maximum by TFWHM ≈ 1.665 T0 ) increases with z (the pulse broadens) according to Eq. (2) as
T (z ) = 1 +
z 2 1/2 LD
T0 ,
(2)
where, LD is the dispersion length. Consequently, the peak power changes, due to the group velocity dispersion (GVD), are given by Eq. (3):
P(z ) =
1+
P0
z 2 1/2
(3)
LD
When Gaussian pulse with power of 10 mW and center wavelength of 1.55 μm propagates within the microring resonator, the resonant output is formed for each round-trip [24–26]. The parameter of the add-drop microring resonator is presented in Table 2. The fractional coupler intensity loss is γ , and α is the linear absorption coefficient. When the device operates in nonlinear regime due to the Kerr nonlinearity, exactly in the spectral region showing the split resonant modes, the refractive index sensing performance is strongly improved with respect to the linear regime. The Kerr coefficients for the waveguide Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 1. Schematic diagram of the microring resonator system simulated by InGaAsP/InP waveguide with a low refractive index top cladding. Table 2 Parameters of the microring resonator system.
κ1
R 16 μm
0.1
κ2 0.15
n0 3.31
n2 (m2 W−1 ) 2.2×10
−17
Aeff (μm2 )
α (dBmm−1 )
γ
1.94
0.5
0.1
is 2.2×10−17 m2 /W at 1.55 μm [27]. The effective area for the InGaAsP waveguide is 1.94 μm2 , therefore the nonlinear phase shift of the waveguides can be expressed in Eq. (4) as
ϕNL ( InGaAsP ) = kLn2 |Ein |2 = kLn2 (P/Aeff ) = 4.59 × 10−5 rad
(4)
The signals will go through the constructive and destructive interferences within the microring resonator system. Therefore, the periodic resonances can be obtained. Slightly different resonance spacing of the spectra, resulting from the different medium used as cladding. The difference in the resonance signals is crucial to achieve large wavelength shifts in the microring resonator tunable lasers [28,29]. The ring to be on resonance when the phase ϕ is a multiple of 2π , or when the wavelength of the light fits a whole number of times inside the optical length of the ring, therefore the reference resonance wavelength can be expressed by Eq. (5),
λre f erence =
neff L , m
m = 1, 2, 3 . . .
(5)
A shift of the resonance wavelength λreference is essentially caused by a change of the effective index of the resonant mode neff . Referring to Eq. (5), we get Eq. (6) as
λre f erence =
neff L m
,
m = 1, 2, 3 . . .
(6)
Where, m is the order of the resonant mode. neff is influenced by the refractive index of the cladding section, which is altered upon sensing [30]. Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 2. (a) Propagating mode profile of the microring resonator waveguide, where the cladding section is filled by InP and air respectively, (b) 3D propagation modes profile, (c) cross section propagating modes profile.
3. Result and discussion Propagation analysis of the input Gaussian pulse is done inside the microring resonator, its effective core area by varying the refractive index of the cladding section of the waveguide as shown in Fig. 1. This section has length of 40 μm. Therefore, the system of microring resonators has a cladding section of 40 μm, where the refractive index varies. The total length of the whole system is 100 μm. The propagation profile of the Gaussian input pulse is shown in Fig. 2, where the cladding section is filled by InP and air respectively. The refractive index of the InP is 3.18, while the refractive index of the air is 1. The higher contrast between the two refractive indices causes more confinement of the input pulse propagating inside the core section. The propagation mode profile, the 3D view of the propagation and the cross sectional profiles are shown in Fig. 2(a–c) respect to two different refractive indices of the air and InP. From Fig. 2(c) we have calculated the cross sections of the mode propagation within the waveguide, in the case, when the cladding is filled by InP and air respectively. There is smaller cross section of 1.77 μm2 , for the mode propagation within Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 3. Throughput resonance wavelength results, black color shows the InP is used as cladding, and blue color shows the air is used as cladding. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Throughput resonance wavelength results, black color shows the InP is used as cladding, and blue color shows the air is used as cladding. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the cladding section with lower refractive index (1 for air) compare to when the cladding is filled by the InP which has a cross section of 1.94 μm2 . It shows more confinement of the light propagating inside the waveguide section. The throughput output signals of the microring resonator are shown in Fig. 3, where it shows the outputs signals for the system with cladding of InP and air respectively within 140 nm wavelengths range (1470–1610 nm). There are resonance wavelength shifts with respect to the changes of the refractive index of the cladding section. The full width at half maximum (3 dB bandwidth) and free spectral range (FSR) of the pulses are 3.4 pm and 6.85 nm (corresponds to 0.85 GHz in frequency domain). The Q-factor, which is the ratio of the resonant wavelength to the 3-dB bandwidth (FWHM), is 4.5 × 105 . This shows high performance of the microring resonator. We have observed different locations of the resonance wavelengths therefore; the resonance wavelength shift occurs respect to the reference wavelength (cladding is filled by InP) depending on the refractive index of the material used as cladding. A resonance wavelength shift of 3.7 nm can be seen from Fig. 4 with respect to the results shown in Fig. 3. Basically we have replaced the cladding by air, which improves the contrast of refractive indices, therefore, the effective index is changed, where it causes shifting the resonance wavelength. Despite of shape shifting from asymmetrical to more symmetrical beam profiles due to replacement of air clad with InP clad (observe in Fig. 2), the simultaneous presentation of spectral shifting shown in Fig. 4 emphasize on small red shifting towards higher wavelength. However, this shift is at nanoscale shifting as to the Figs. 3 and 4, respectively, but it is fairly important to state that the refractive index of InP can be variable upon to the amount of absorbed photon with different energy [31] which makes the spectral shifting dependents on photon energy absorption. The drop port output signals show the same concept as described above. The Fig. 5 shows the drop port output signals. The resonance wavelength shift (࢞λ) can be calculated in different cases, when the cladding of the waveguide filled by different mediums having different refractive indices. The refractive indices vary between a ranges of 1–3.18. Fig. 6(a) shows Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 5. Drop port resonance wavelength results, black color shows the InP is used as cladding, and blue color shows the air is used as cladding. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. (a) Resonance wavelength shift (nm) versus the refractive index variation of the cladding (40 μm), (b) dispersion (ps/nm) versus wavelength (nm).
the refractive index (n) versus the resonance wavelength shift (nm). The dispersion versus the wavelength is shown in Fig. 6(b), clearly indicating that by increasing of the wavelength, the dispersion is reduced. Following the discussion of photon absorptivity for InP as an outer-clad of the microring resonator structure discussed in Fig. 4, the amount of resonance wavelength shifting obviously indicates a non-linear response to changing the refractive index. Although, it must be reminded that the Fig. 6a is simulated only under the refractive index variation wherein never functionalized under photon absorptivity. The changes of the refractive index of the cladding section causes changes on the mode effective index and propagating constant. Fig. 7(a) shows the refractive index versus the effective index propagation mode and Fig. 7(b) shows the effective mode propagation constant versus the refractive index. Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Fig. 7. (a) Mode effective index versus refractive index, (b) effective propagation constant (beta) versus refractive index.
From Fig. 7(a), the lower refractive index is corresponding to lower effective index, therefore higher resonance wavelength shift respect to the reference wavelength.
4. Conclusion The characterization of dielectric micro resonators using the time-domain travelling wave (TDTW) method to model and simulate a microring resonator (InGaAsP/InP) waveguide based on resonance wavelength shifts is proposed and discussed. The microring resonators can be used as sensor devices based on an evanescent wave interacting with the surrounding cladding of the system simulated by waveguide. To keep the waveguide single mode and total internal reflection guiding is required to have very small bend radii. The microring resonator is simulated by InGaAsP/InP waveguide with InGaAsP core (n = 3.31) surrounded by InP (n = 3.18) cladding and a low index top cladding (air). The propagating signals inside the microring resonator will undergo constructive and destructive interferences, therefore signals have a comb shape will be generated as periodic resonances. The difference in the refractive index of the cladding section causes changing the effective index of the waveguide, thus providing a tuning mechanism by observing the large wavelength shifts in the microring resonator system. The microring resonator has a Q-factor of 4.5 × 105 , which show high performance and quality of the system, and a resonance wavelength shift of 3.7 nm. The largest resonance wavelength shift is 3.7 nm and is obtained when the medium of the cladding is air. Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002
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Please cite this article as: I.S. Amiri et al., Tunable multi-wavelength generation using InGaAsP/InP microring resonator with detectable resonance wavelength shift due to a sensing cladding section, Chinese Journal of Physics (2016), http://dx.doi.org/10.1016/j.cjph.2016.08.002