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Compact and tunable silicon nitride Bragg grating filters in polymer
Optics Communications 321 (2014) 23–27
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Compact and tunable silicon nitride Bragg grating filters in polymer Ziyang Zhang n, Alejandro Maese Novo, Dongliang Liu, Norbert Keil, Norbert Grote Fraunhofer Heinrich-Hertz-Institut (HHI), Einsteinufer 37, 10587 Berlin, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2014 Received in revised form 22 January 2014 Accepted 25 January 2014 Available online 5 February 2014
A series of tunable filters based on silicon nitride waveguide Bragg gratings buried in polymer are studied, fabricated and analyzed. The gratings are etched completely through the waveguides to improve the peak reflectivity at short grating lengths. Reflectivity from 1% to 70% can be reached when the thirdorder grating length varies from 16 mm to 160 mm. The experimental results are in good agreement with numerical simulations. Due to its compact size and the thermal advantages of polymer, the filter can be tuned very efficiently by a micro heater buried beneath. A tuning range of 34.5 nm is demonstrated at a heat power of only 22 mW. & 2014 Elsevier B.V. All rights reserved.
Keywords: Optical polymers Silicon nitride waveguide Bragg grating Tunable filter
1. Introduction At the backbones of optical communication networks low-cost and power-efficient tunable filters are highly desired. Apart from wavelength (de)multiplexing, tunable filters are also widely used as wavelength selectors and Bragg reflectors in tunable lasers. Compact optical filters have been studied on various integration platforms, including silica planar lightwave circuits (PLC) [1], silicon on insulator (SOI) [2,3] and indium phosphide monolithic platforms [4–6]. Over the years optical polymers have matured and proven to form a viable waveguide platform for hybrid photonic integration [7,8]. Various filters and tunable lasers on polymer platforms have then been reported [9–11]. Polymer materials generally feature fairly high thermo-optic coefficients, in the range of 1 to 3 10 4 K 1, and they also possess very low thermal conductivity. This means that the thermal energy can be confined relatively long before it eventually dissipates into the heat sink. Highly power-efficient thermally tunable devices can be ideally implemented on this platform. Most polymer waveguides, however, feature only weak index contrast, limited by the synthesizing complexities and material compatibilities to only a few percent. To increase the waveguide index contrast, bring down the device footprint, and broaden the design choices, silicon nitride (SiNx) has been introduced to the polymer platform [12]. Low loss heterogeneous SiNx/polymer waveguides and related tunable filters have been demonstrated [13,14]. The previously reported filters are only based on weakly spatially modulated grating structures, i.e., the sidewall of the SiNx
n
Corresponding author. Tel.: þ 49 3031002524 E-mail address: [email protected] (Z. Zhang).
0030-4018/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2014.01.064
waveguide core is perturbated slightly to introduce a periodic index variation. This inevitably leads to a rather bulky device for a certain high peak reflectivity. In this work we introduce a strongly coupled grating structure by breaking the waveguide section completely. We show both numerically and experimentally that a series of reflectivities and bandwidths can be achieved by varying the grating lengths. Reflectivity larger than 70% can be achieved at a grating length of 160 mm, one order of magnitude shorter than those previously reported [12]. The wide bandwidth in some of the designs may also find applications in band-pass filters and coarse wavelength selectors [15,16]. Using a buried heater underneath the grating, the filter curve can be tuned efficiently. The reduced grating length leads to a much improved thermal tuning efficiency.
2. Waveguide properties The cross-section of the buried SiNx waveguide in polymer is sketched in Fig. 1(a). The width of the SiNx core measures a few micrometers while the thickness is kept below some hundred nanometers for a single mode operation. The micro heater imbedded below can effectively generate a uniform heating condition around the waveguide mode region [12]. Additional air trenches on both sides of the waveguides further confine the heat and improve the tuning efficiency. Part of the waveguide session is etched completely and filled with polymer cladding to form a strongly coupled grating structure, as illustrated in Fig. 1(b). The SiNx material deposited by low temperature plasma enhanced chemical vapor deposition (PECVD) exhibits a refractive index of 1.83 at 1550 nm. The polymer cladding shows an index of 1.45. The waveguide mode is studied by commercial software
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Z. Zhang et al. / Optics Communications 321 (2014) 23–27
FIMMWAVE and the results are summarized in Fig. 2. The single mode border is labelled as a black dashed line in Fig. 2(a) and (c) for TE and TM polarizations, respectively. The effective index (neff) and the mode confinement factor (Γ) increase rapidly when the waveguide thickness increases. A clear difference between TE and TM mode is seen, with the TM mode always being less confined and registering a lower neff. For example, a structure of 2 mm 200 nm gives a relatively good confined yet still single TE mode, with Γ 30%, while for the TM mode, the value drops quickly to 10%.
Fig. 1. Schematic of the buried SiNx waveguide grating and metal heater electrode in polymer. (a) Cross-section view; (b) lateral view showing the completely etched grating structure.
The thermal tuning of Bragg grating can be expressed by
Δλ ¼
2Λ ∂neff ΔT; M ∂T
ð1Þ
where Δλ is the wavelength tuning, Λ is the grating period, and M is the grating order. The effective index variation with respect to the temperature change can be written as ∂npoly ∂n ∂neff ¼ ð1 ΓÞ þ Γ SiN x ; ∂T ∂T ∂T
ð2Þ
The thermo-optic coefficient (TOC) of the specific polymer used in this study has been measured to be ∂npoly =∂T ¼ 1:1 10 4 =K, and for SiNx the value is determined to be ∂n SiN x =∂T ¼ þ 3:0 10 5 =K. Waveguides with higher neff and larger Γ usually lead to more compact devices. However, since the TOCs of polymer and SiNx material have opposite signs, a substantial amount of light should preferably remain in the cladding. In this way, the polymer thermo-optic properties dominate and power efficient thermal tuning can be realized. A good balance between mode confinement and wavelength tunability should be made. As a compromise, the waveguide structure in this study is chosen to be 3 mm 125 nm. The corresponding principal mode field distributions are shown in Fig. 3. The TE mode field diameter extends in the horizontal direction to 3.0 mm and in the vertical direction to 1.5 mm, while for the TM mode the values are 3.9 mm and 2.8 mm, respectively. The grating functions as a Bragg reflector when the grating order is an even number. In order to be patterned by conventional photolithography, third-order gratings are considered in this study. For a grating central wavelength of 1550 nm, the period is calculated to be 1600 nm. Since TE mode exhibits higher neff and larger Γ, higher peak reflectivity is expected. In addition, when used as a tunable reflector in a laser cavity, TE is usually the preferred polarization. Therefore we concentrate on the TE mode
Fig. 2. Mode properties with respect to waveguide geometry. The horizontal axis represents the waveguide width and the legend indicates the waveguide thickness, both in units of micrometers. (a) Effective index and (b) mode confinement factor for the TE mode. (c) Effective index and (d) mode confinement factor for the TM mode.
Z. Zhang et al. / Optics Communications 321 (2014) 23–27
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Fig. 4. (a) Microscopic image of the device; (b) detailed view of the SiNx Bragg grating.
Fig. 3. (a) TE and (b) TM mode field distributions for the buried SiNx waveguide of 3 mm 125 nm.
as a proof of concept, though the design principles apply for the TM mode as well. Take neff ¼ 1.4699 and Γ ¼14.5% in Eq. (2), and the effective TOC for the TE mode is then calculated to be 9.0 10 5 K 1. The thermo-optic behavior of the waveguide resembles a pure polymer waveguide with slightly reduced TOC. From Eq. (1), when the temperature around the mode region increases, a blue-shift of the grating spectrum is expected. The metal heater is placed 3 mm below the SiNx core. The mode field (Ex) at this distance already drops to below 5% of its central peak value. The heater can then provide efficient heating without causing excessive light absorption.
3. Device fabrication Commercially available polymer (ZPU-12 series from ChemOptics Inc.) is used as cladding material. A layer of 20-mm-thick polymer was first cured on a silicon wafer. Heater electrodes containing Ti and Au are structured on the cladding by a lift-off process. After another 3-mm-thick polymer layer is added on top of the electrode, the SiNx core layer is deposited by PECVD using pure SiH4 and NH4 gas at 100 1C. SiNx is then structured using conventional photolithography (mask aligner: Karl Suss MA6) and subsequent reactive ion etching (20 sccm pure CHF3 at 1.5 Pa and 150 W). A 5-mm-thick polymer layer is created to serve as upper cladding. Finally deep air trenches are etched to provide additional thermal buffering, as well as to expose the metal pads for electrical contacts.
Fig. 4(a) shows a microscopic image of the device. The buried heater is 10 mm wide and 250 mm long. Both sides of the electrode are turned 901 and connected to 80 mm 80 mm square pads. Round openings are etched in the middle of the pads for wire-bonding and electrical contacts. The waveguide sits symmetrically on top of the electrode with the designed 3-mm thick cladding buffer. Gratings are etched in the middle session of the waveguide, as shown in Fig. 4(b). The grating period is kept as 1.6 mm and only the number of periods, i.e., the grating length, varies on the lithography mask. The heater, air trench, and waveguide structure remain the same to facilitate characterization. Only the access waveguide lengths are adjusted accordingly with different grating lengths. It is worth noting that the degradation temperature of the chosen polymer material is 300 1C. Below this temperature low-loss amorphous silicon can be deposited by PECVD [17]. The refractive index of amorphous silicon (3.6) is the highest among optical transparent materials at 1550 nm. Future work may include extremely small footprint photonic devices utilizing amorphous silicon as core material buried in polymer. In addition, SiNx can also be deposited by sputtering. The waveguides can be readily created by a single lift-off process without any etching. The fabrication cost can then be further reduced. In the same way various high index materials such as TiO2 and Ta2O5 can be embedded in polymer to diversify the device functionalities.
4. Filter characteristics Since gratings in this study are completely etched from the waveguides, perturbation theory based on weakly coupled assumption is no longer accurate [18]. The gratings are then investigated numerically by two dimensional finite difference time domain (FDTD) simulations. The grating period is set to 1.6 mm and the duty cycle (ratio between unetched and etched sessions) remains 50%. The number of period (NP) is varied from 10 to 100, i.e., a grating length from 16 mm to 160 mm. The simulated filter spectra are shown in Fig. 5(a). The grating bandwidth drops quickly from 36 nm to 6.9 nm as NP increases from 10 to 100, while the peak reflectivity increases from 4% to 82%. The simulated structures are fabricated and prepared for characterization. Light from a broadband tunable laser source is launched via a standard single mode fiber through a polarization controller into the waveguide. The input light polarization is carefully tuned to TE orientation. The back-reflected light is collected by the same fiber, separated by an optical circulator and fed into an optical spectrum analyzer. Index matching oil is applied to the waveguide/fiber interface to suppress the Fabry– Perot effects and facilitate the coupling process.
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Z. Zhang et al. / Optics Communications 321 (2014) 23–27
Fig. 5. (a) Simulated and (b) measured Bragg grating reflection spectra as the number of periods increases from 10 to 100.
The waveguides have shown an average propagation loss of 0.96 dB/cm [12]. The buried electrode causes additional propagation loss of less than 0.1 dB/cm for the TE mode. The measured back-reflection is normalized with respect to the insertion loss of the unperturbed reference waveguide of the same length. Fig. 5 (b) summarizes the measurement data. The results resemble very much the predications from simulations, though the central wavelength shifts from the target 1550 nm to around 1556 nm. The main reason is that the grating duty cycle walks off slightly during fabrication, i.e., the undercut of the photoresist profile after developing leaves an unetched SiNx region longer than half of the grating period, resulting in an altered duty cycle that is lightly larger than 50%. The overall effective index of the grating region is then increased, leading to a red-shift of the spectra. This effect can be well compensated in the second design iteration. Detailed comparison on reflectivity and bandwidth can be seen in Fig. 6. The measured peak reflectivity is always smaller. For example, at NP ¼100, the value is 71% instead of the predicated 82%, which again can be attributed to the altering of the grating duty cycles. The bandwidth (BW), on the other hand, matches the simulation almost perfectly. At larger NP, the measured BW is even slightly narrower. At NP ¼100, the measured value is 6.3 nm, instead of 6.9 nm from simulation. This can be well understood by the non-optimal parameter settings in FDTD, such as the effective index of the two dimensional SiNx slab, the grid size and the number of time steps.
5. Thermal tuning Detailed thermal simulations have been carried out in the previous studies with a focus on the enhancement of heating efficiency by sufficient thermal buffering in the waveguide cross-
Fig. 6. Comparison between FDTD simulation and measurement data. (a) Peak reflectivity and (b) bandwidth of the gratings.
Fig. 7. Wavelength tuning of the Bragg grating (NP ¼ 30) by a micro heater.
section [19]. However, the absolute tuning power is also determined by the actual heater electrode size. The length of the electrodes in this study is fixed at 250 mm, a value that is 5–6 times shorter compared to the previous work [12,19]. A dramatic improvement of tuning efficiency is then expected. Since the number of grating period does not alter the thermooptic behavior, as indicated in Eq. (1), and the design of the heater electrode/air trenches is the same, the thermal tuning behavior for all variations of NPs should remain almost identical. As a demonstrator, NP¼ 30 is chosen and the tuning results are displayed in Fig. 7. When the heater power is raised from 0 mW to 22 mW, a continuous shift of the center wavelength of the Bragg filter over
Z. Zhang et al. / Optics Communications 321 (2014) 23–27
34.5 nm from 1556 nm to 1521.5 nm is observed. Further tuning is prohibited by the electrode failure, though the grating itself remains undamaged. The temperature change in the grating region can be calculated using Eq. (1). Assuming an ambient temperature of 25 1C, the local temperature is raised to 389 1C for the 34.5 nm wavelength tuning. Though this temperature already exceeds the polymer material degradation temperature of 300 1C, the material does not “break down” abruptly. The degradation is rather a slow aging effect. The devices have been characterized repeatedly for lab demonstrations without noticeable changes. Yet as far as long-term stability is concerned, the wavelength tuning range should be kept below 26 nm (ΔT¼ 275 1C) to prevent polymer from thermally induced aging. The wavelength shift remains a linear relation to the heating power and a tuning efficiency of 1.57 nm/mW is obtained. This efficiency records, to the best of our knowledge, among the highest value reported in the literature for the thermally tunable filters [9–13,19]. The 6-fold efficiency improvement compared to the results in [12], where a tuning range of 57 nm is demonstrated at the cost of 225 mW tuning power (0.24 nm/mW), proves the importance of device compactness in the design of highly efficient thermally tunable devices.
6. Conclusion To conclude, a series of tunable Bragg grating filters based on silicon nitride waveguide in polymer are studied, fabricated and analyzed. The grating session is completely etched to form a compact structure. By varying the grating length, the reflectivity and bandwidth can be chosen. The much shortened structures pave way for highly efficient thermal tuning. A record tuning efficiency of 1.57 nm/mW is achieved. The results may impact the design of novel filters and tunable lasers in telecom applications. This work also demonstrates the possibility of incorporating inorganic materials in polymer as waveguides. By mode engineering, the properties of the chosen polymer as well as the inorganic core can be utilized. Future work may include adding amorphous silicon in the structure. By high-resolution lithography methods, such as deep-UV stepper and electron beam lithography, more compact devices can be fabricated, and the thermal tuning efficiency can be further improved. Naturally, other specialized polymers can be chosen as the overall or partial cladding. For example, electro-optic and nonlinear polymers can be employed for fast-switches and wave mixers [20,21]. Combined with materials possessing certain bio/chemical affinity, the proposed structure may be well introduced to sensing applications.
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Acknowledgment This work was partly conducted in the framework of the CELTIC-SASER project sponsored by the German Federal Ministry of Education and Research (BMBF) and co-financed by the European Commission. The authors also wish to thank the TZL team in HHI for their assistance in the device fabrication. References [1] T. Niwa, R. Hirako, H. Hasegawa, K. Sato, M. Okuno, and T. Watanabe, in: Proceedings of the Optical Fiber Communication Conference, OSA Technical Digest, Paper OTh3D.6, 2012. [2] John E. Cunningham, Ivan Shubin, Xuezhe Zheng, Thierry Pinguet, Attila Mekis, Ying Luo, Hiren Thacker, Guoliang Li, Jin Yao, Kannan Raj, Ashok V. Krishnamoorthy, Opt. Express 18 (2010) 19055. [3] Kang Xiong, Xi Xiao, Xianyao Li, Yingtao Hu, Zhiyong Li, Tao Chu, Yude Yu, Jinzhong Yu, Opt. Commun. 285 (2012) 4368. [4] J. Kani, IEEE J. Sel. Top. Quantum Electron 16 (5) (2010) 1290. [5] Robert S. Guzzon, Erik J. Norberg, John S. Parker, Leif A. Johansson, Larry A. Coldren, Opt. Express 19 (2011) 7816. [6] S. Nicholes, M. Mashanovitch, B. Jevremovic, E. Lively, L. A. Coldren, and D. J. Blumenthal, in: Proceedings of the Optical Fiber Communication Conference/ National Fiber Optic Engineers Conference, OSA Technical Digest (CD), paper OThD1, 2011. [7] Z. Zhang, N. Mettbach, C. Zawadzki, J. Wang, D. Schmidt, W. Brinker, N. Grote, M. Schell, N. Keil, IET Optoelectron. 5 (2011) 226. [8] N. Keil, C. Zawadzki, Z. Zhang, J. Wang, N. Mettbach, N. Grote, and M. Schell, in: Proceedings of theOptical Fiber Communication Conference/National Fiber Optic Engineers Conference, OSA Technical Digest (CD), paper OWM1, 2011. [9] Y.-O. Noh, H.-J. Lee, J.-J. Ju, M.-S. Kim, S.-H. Oh, M.-C. Oh, Opt. Express 16 (2008) 18194. [10] S.H. Oh, K.H. Yoon, K.S. Kim, J. Kim, O. Kwon, D.K. Oh, Y.O. Noh, J.K. Seo, H.J. Lee, IEEE J. Sel. Top. Quantum Electron 17 (6) (2011) 1534. [11] H. Klein, C. Wagner, W. Brinker, F. Soares, D. de Felipe, Z. Zhang, C. Zawadzki, N. Keil, and M. Moehrle, in: Proceedings of the International Conference on Indium Phosphide and Related Materials (IPRM 2012), 27–30 August, pp. 20–21, 2012. [12] Z. Zhang, D. Liu, D. Felipe, A. Liu, N. Keil, N. Grote, Appl. Phys. Lett. 102 (2013) 181105. [13] D. Liu, Z. Zhang, N. Keil, N. Grote, IEEE Photon Technol. Lett. 25 (2013) 1734. [14] Ziyang Zhang, Garri Genrich, Norbert Keil, Norbert Grote, Opt. Lett. 39 (2014) 162. [15] Yanwu Zhang, Yingfeng Li, Zian He, Liying Liu, Lei Xu, Opt. Commun. 284 (2011) 1828. [16] T. Suzuki, H. Arimoto, T. Kitatani, A. Takei, T. Taniguchi, K. Shinoda, S. Tanaka, and S. Tsuji, in: Proceedings of the Optical Fiber Communication Conference/ National Fiber Optic Engineers Conference , OSA Technical Digest (CD), paper OWD7, 2011. [17] Matteo Dainese, Marcin Swillo, Lech Wosinski, Lars Thylen, Opt. Commun. 260 (2006) 514. [18] K. Ogawa, W. Chang, S. Sopori, L. Bhushan, F. Rosenbaum, IEEE J. Quantum Electron. 9 (1) (1973) 29. [19] Anjin Liu, Ziyang Zhang, David de Felipe, Norbert Keil, Norbert Grote, IEEE Photonics Technol. Lett. 26 (2014) 313. [20] C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R. Baets, B. Esembeson, I. Biaggio, T. Michinobu, F. Diederich, W. Freude, J. Leuthold, Nat. Photonics 3 (2009) 216. [21] S. Jakobs, A. Petrov, M. Eich, J.M. Hales, J.W. Perry, S. Marder, V Nazabal, P. Nemec, Proc. SPIE 8434 (2012) 84340.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Compact and tunable silicon nitride Bragg grating filters in polymer Ziyang Zhang n, Alejandro Maese Novo, Dongliang Liu, Norbert Keil, Norbert Grote Fraunhofer Heinrich-Hertz-Institut (HHI), Einsteinufer 37, 10587 Berlin, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2014 Received in revised form 22 January 2014 Accepted 25 January 2014 Available online 5 February 2014
A series of tunable filters based on silicon nitride waveguide Bragg gratings buried in polymer are studied, fabricated and analyzed. The gratings are etched completely through the waveguides to improve the peak reflectivity at short grating lengths. Reflectivity from 1% to 70% can be reached when the thirdorder grating length varies from 16 mm to 160 mm. The experimental results are in good agreement with numerical simulations. Due to its compact size and the thermal advantages of polymer, the filter can be tuned very efficiently by a micro heater buried beneath. A tuning range of 34.5 nm is demonstrated at a heat power of only 22 mW. & 2014 Elsevier B.V. All rights reserved.
Keywords: Optical polymers Silicon nitride waveguide Bragg grating Tunable filter
1. Introduction At the backbones of optical communication networks low-cost and power-efficient tunable filters are highly desired. Apart from wavelength (de)multiplexing, tunable filters are also widely used as wavelength selectors and Bragg reflectors in tunable lasers. Compact optical filters have been studied on various integration platforms, including silica planar lightwave circuits (PLC) [1], silicon on insulator (SOI) [2,3] and indium phosphide monolithic platforms [4–6]. Over the years optical polymers have matured and proven to form a viable waveguide platform for hybrid photonic integration [7,8]. Various filters and tunable lasers on polymer platforms have then been reported [9–11]. Polymer materials generally feature fairly high thermo-optic coefficients, in the range of 1 to 3 10 4 K 1, and they also possess very low thermal conductivity. This means that the thermal energy can be confined relatively long before it eventually dissipates into the heat sink. Highly power-efficient thermally tunable devices can be ideally implemented on this platform. Most polymer waveguides, however, feature only weak index contrast, limited by the synthesizing complexities and material compatibilities to only a few percent. To increase the waveguide index contrast, bring down the device footprint, and broaden the design choices, silicon nitride (SiNx) has been introduced to the polymer platform [12]. Low loss heterogeneous SiNx/polymer waveguides and related tunable filters have been demonstrated [13,14]. The previously reported filters are only based on weakly spatially modulated grating structures, i.e., the sidewall of the SiNx
n
Corresponding author. Tel.: þ 49 3031002524 E-mail address: [email protected] (Z. Zhang).
0030-4018/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2014.01.064
waveguide core is perturbated slightly to introduce a periodic index variation. This inevitably leads to a rather bulky device for a certain high peak reflectivity. In this work we introduce a strongly coupled grating structure by breaking the waveguide section completely. We show both numerically and experimentally that a series of reflectivities and bandwidths can be achieved by varying the grating lengths. Reflectivity larger than 70% can be achieved at a grating length of 160 mm, one order of magnitude shorter than those previously reported [12]. The wide bandwidth in some of the designs may also find applications in band-pass filters and coarse wavelength selectors [15,16]. Using a buried heater underneath the grating, the filter curve can be tuned efficiently. The reduced grating length leads to a much improved thermal tuning efficiency.
2. Waveguide properties The cross-section of the buried SiNx waveguide in polymer is sketched in Fig. 1(a). The width of the SiNx core measures a few micrometers while the thickness is kept below some hundred nanometers for a single mode operation. The micro heater imbedded below can effectively generate a uniform heating condition around the waveguide mode region [12]. Additional air trenches on both sides of the waveguides further confine the heat and improve the tuning efficiency. Part of the waveguide session is etched completely and filled with polymer cladding to form a strongly coupled grating structure, as illustrated in Fig. 1(b). The SiNx material deposited by low temperature plasma enhanced chemical vapor deposition (PECVD) exhibits a refractive index of 1.83 at 1550 nm. The polymer cladding shows an index of 1.45. The waveguide mode is studied by commercial software
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Z. Zhang et al. / Optics Communications 321 (2014) 23–27
FIMMWAVE and the results are summarized in Fig. 2. The single mode border is labelled as a black dashed line in Fig. 2(a) and (c) for TE and TM polarizations, respectively. The effective index (neff) and the mode confinement factor (Γ) increase rapidly when the waveguide thickness increases. A clear difference between TE and TM mode is seen, with the TM mode always being less confined and registering a lower neff. For example, a structure of 2 mm 200 nm gives a relatively good confined yet still single TE mode, with Γ 30%, while for the TM mode, the value drops quickly to 10%.
Fig. 1. Schematic of the buried SiNx waveguide grating and metal heater electrode in polymer. (a) Cross-section view; (b) lateral view showing the completely etched grating structure.
The thermal tuning of Bragg grating can be expressed by
Δλ ¼
2Λ ∂neff ΔT; M ∂T
ð1Þ
where Δλ is the wavelength tuning, Λ is the grating period, and M is the grating order. The effective index variation with respect to the temperature change can be written as ∂npoly ∂n ∂neff ¼ ð1 ΓÞ þ Γ SiN x ; ∂T ∂T ∂T
ð2Þ
The thermo-optic coefficient (TOC) of the specific polymer used in this study has been measured to be ∂npoly =∂T ¼ 1:1 10 4 =K, and for SiNx the value is determined to be ∂n SiN x =∂T ¼ þ 3:0 10 5 =K. Waveguides with higher neff and larger Γ usually lead to more compact devices. However, since the TOCs of polymer and SiNx material have opposite signs, a substantial amount of light should preferably remain in the cladding. In this way, the polymer thermo-optic properties dominate and power efficient thermal tuning can be realized. A good balance between mode confinement and wavelength tunability should be made. As a compromise, the waveguide structure in this study is chosen to be 3 mm 125 nm. The corresponding principal mode field distributions are shown in Fig. 3. The TE mode field diameter extends in the horizontal direction to 3.0 mm and in the vertical direction to 1.5 mm, while for the TM mode the values are 3.9 mm and 2.8 mm, respectively. The grating functions as a Bragg reflector when the grating order is an even number. In order to be patterned by conventional photolithography, third-order gratings are considered in this study. For a grating central wavelength of 1550 nm, the period is calculated to be 1600 nm. Since TE mode exhibits higher neff and larger Γ, higher peak reflectivity is expected. In addition, when used as a tunable reflector in a laser cavity, TE is usually the preferred polarization. Therefore we concentrate on the TE mode
Fig. 2. Mode properties with respect to waveguide geometry. The horizontal axis represents the waveguide width and the legend indicates the waveguide thickness, both in units of micrometers. (a) Effective index and (b) mode confinement factor for the TE mode. (c) Effective index and (d) mode confinement factor for the TM mode.
Z. Zhang et al. / Optics Communications 321 (2014) 23–27
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Fig. 4. (a) Microscopic image of the device; (b) detailed view of the SiNx Bragg grating.
Fig. 3. (a) TE and (b) TM mode field distributions for the buried SiNx waveguide of 3 mm 125 nm.
as a proof of concept, though the design principles apply for the TM mode as well. Take neff ¼ 1.4699 and Γ ¼14.5% in Eq. (2), and the effective TOC for the TE mode is then calculated to be 9.0 10 5 K 1. The thermo-optic behavior of the waveguide resembles a pure polymer waveguide with slightly reduced TOC. From Eq. (1), when the temperature around the mode region increases, a blue-shift of the grating spectrum is expected. The metal heater is placed 3 mm below the SiNx core. The mode field (Ex) at this distance already drops to below 5% of its central peak value. The heater can then provide efficient heating without causing excessive light absorption.
3. Device fabrication Commercially available polymer (ZPU-12 series from ChemOptics Inc.) is used as cladding material. A layer of 20-mm-thick polymer was first cured on a silicon wafer. Heater electrodes containing Ti and Au are structured on the cladding by a lift-off process. After another 3-mm-thick polymer layer is added on top of the electrode, the SiNx core layer is deposited by PECVD using pure SiH4 and NH4 gas at 100 1C. SiNx is then structured using conventional photolithography (mask aligner: Karl Suss MA6) and subsequent reactive ion etching (20 sccm pure CHF3 at 1.5 Pa and 150 W). A 5-mm-thick polymer layer is created to serve as upper cladding. Finally deep air trenches are etched to provide additional thermal buffering, as well as to expose the metal pads for electrical contacts.
Fig. 4(a) shows a microscopic image of the device. The buried heater is 10 mm wide and 250 mm long. Both sides of the electrode are turned 901 and connected to 80 mm 80 mm square pads. Round openings are etched in the middle of the pads for wire-bonding and electrical contacts. The waveguide sits symmetrically on top of the electrode with the designed 3-mm thick cladding buffer. Gratings are etched in the middle session of the waveguide, as shown in Fig. 4(b). The grating period is kept as 1.6 mm and only the number of periods, i.e., the grating length, varies on the lithography mask. The heater, air trench, and waveguide structure remain the same to facilitate characterization. Only the access waveguide lengths are adjusted accordingly with different grating lengths. It is worth noting that the degradation temperature of the chosen polymer material is 300 1C. Below this temperature low-loss amorphous silicon can be deposited by PECVD [17]. The refractive index of amorphous silicon (3.6) is the highest among optical transparent materials at 1550 nm. Future work may include extremely small footprint photonic devices utilizing amorphous silicon as core material buried in polymer. In addition, SiNx can also be deposited by sputtering. The waveguides can be readily created by a single lift-off process without any etching. The fabrication cost can then be further reduced. In the same way various high index materials such as TiO2 and Ta2O5 can be embedded in polymer to diversify the device functionalities.
4. Filter characteristics Since gratings in this study are completely etched from the waveguides, perturbation theory based on weakly coupled assumption is no longer accurate [18]. The gratings are then investigated numerically by two dimensional finite difference time domain (FDTD) simulations. The grating period is set to 1.6 mm and the duty cycle (ratio between unetched and etched sessions) remains 50%. The number of period (NP) is varied from 10 to 100, i.e., a grating length from 16 mm to 160 mm. The simulated filter spectra are shown in Fig. 5(a). The grating bandwidth drops quickly from 36 nm to 6.9 nm as NP increases from 10 to 100, while the peak reflectivity increases from 4% to 82%. The simulated structures are fabricated and prepared for characterization. Light from a broadband tunable laser source is launched via a standard single mode fiber through a polarization controller into the waveguide. The input light polarization is carefully tuned to TE orientation. The back-reflected light is collected by the same fiber, separated by an optical circulator and fed into an optical spectrum analyzer. Index matching oil is applied to the waveguide/fiber interface to suppress the Fabry– Perot effects and facilitate the coupling process.
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Fig. 5. (a) Simulated and (b) measured Bragg grating reflection spectra as the number of periods increases from 10 to 100.
The waveguides have shown an average propagation loss of 0.96 dB/cm [12]. The buried electrode causes additional propagation loss of less than 0.1 dB/cm for the TE mode. The measured back-reflection is normalized with respect to the insertion loss of the unperturbed reference waveguide of the same length. Fig. 5 (b) summarizes the measurement data. The results resemble very much the predications from simulations, though the central wavelength shifts from the target 1550 nm to around 1556 nm. The main reason is that the grating duty cycle walks off slightly during fabrication, i.e., the undercut of the photoresist profile after developing leaves an unetched SiNx region longer than half of the grating period, resulting in an altered duty cycle that is lightly larger than 50%. The overall effective index of the grating region is then increased, leading to a red-shift of the spectra. This effect can be well compensated in the second design iteration. Detailed comparison on reflectivity and bandwidth can be seen in Fig. 6. The measured peak reflectivity is always smaller. For example, at NP ¼100, the value is 71% instead of the predicated 82%, which again can be attributed to the altering of the grating duty cycles. The bandwidth (BW), on the other hand, matches the simulation almost perfectly. At larger NP, the measured BW is even slightly narrower. At NP ¼100, the measured value is 6.3 nm, instead of 6.9 nm from simulation. This can be well understood by the non-optimal parameter settings in FDTD, such as the effective index of the two dimensional SiNx slab, the grid size and the number of time steps.
5. Thermal tuning Detailed thermal simulations have been carried out in the previous studies with a focus on the enhancement of heating efficiency by sufficient thermal buffering in the waveguide cross-
Fig. 6. Comparison between FDTD simulation and measurement data. (a) Peak reflectivity and (b) bandwidth of the gratings.
Fig. 7. Wavelength tuning of the Bragg grating (NP ¼ 30) by a micro heater.
section [19]. However, the absolute tuning power is also determined by the actual heater electrode size. The length of the electrodes in this study is fixed at 250 mm, a value that is 5–6 times shorter compared to the previous work [12,19]. A dramatic improvement of tuning efficiency is then expected. Since the number of grating period does not alter the thermooptic behavior, as indicated in Eq. (1), and the design of the heater electrode/air trenches is the same, the thermal tuning behavior for all variations of NPs should remain almost identical. As a demonstrator, NP¼ 30 is chosen and the tuning results are displayed in Fig. 7. When the heater power is raised from 0 mW to 22 mW, a continuous shift of the center wavelength of the Bragg filter over
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34.5 nm from 1556 nm to 1521.5 nm is observed. Further tuning is prohibited by the electrode failure, though the grating itself remains undamaged. The temperature change in the grating region can be calculated using Eq. (1). Assuming an ambient temperature of 25 1C, the local temperature is raised to 389 1C for the 34.5 nm wavelength tuning. Though this temperature already exceeds the polymer material degradation temperature of 300 1C, the material does not “break down” abruptly. The degradation is rather a slow aging effect. The devices have been characterized repeatedly for lab demonstrations without noticeable changes. Yet as far as long-term stability is concerned, the wavelength tuning range should be kept below 26 nm (ΔT¼ 275 1C) to prevent polymer from thermally induced aging. The wavelength shift remains a linear relation to the heating power and a tuning efficiency of 1.57 nm/mW is obtained. This efficiency records, to the best of our knowledge, among the highest value reported in the literature for the thermally tunable filters [9–13,19]. The 6-fold efficiency improvement compared to the results in [12], where a tuning range of 57 nm is demonstrated at the cost of 225 mW tuning power (0.24 nm/mW), proves the importance of device compactness in the design of highly efficient thermally tunable devices.
6. Conclusion To conclude, a series of tunable Bragg grating filters based on silicon nitride waveguide in polymer are studied, fabricated and analyzed. The grating session is completely etched to form a compact structure. By varying the grating length, the reflectivity and bandwidth can be chosen. The much shortened structures pave way for highly efficient thermal tuning. A record tuning efficiency of 1.57 nm/mW is achieved. The results may impact the design of novel filters and tunable lasers in telecom applications. This work also demonstrates the possibility of incorporating inorganic materials in polymer as waveguides. By mode engineering, the properties of the chosen polymer as well as the inorganic core can be utilized. Future work may include adding amorphous silicon in the structure. By high-resolution lithography methods, such as deep-UV stepper and electron beam lithography, more compact devices can be fabricated, and the thermal tuning efficiency can be further improved. Naturally, other specialized polymers can be chosen as the overall or partial cladding. For example, electro-optic and nonlinear polymers can be employed for fast-switches and wave mixers [20,21]. Combined with materials possessing certain bio/chemical affinity, the proposed structure may be well introduced to sensing applications.
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Acknowledgment This work was partly conducted in the framework of the CELTIC-SASER project sponsored by the German Federal Ministry of Education and Research (BMBF) and co-financed by the European Commission. The authors also wish to thank the TZL team in HHI for their assistance in the device fabrication. References [1] T. Niwa, R. Hirako, H. Hasegawa, K. Sato, M. Okuno, and T. Watanabe, in: Proceedings of the Optical Fiber Communication Conference, OSA Technical Digest, Paper OTh3D.6, 2012. [2] John E. Cunningham, Ivan Shubin, Xuezhe Zheng, Thierry Pinguet, Attila Mekis, Ying Luo, Hiren Thacker, Guoliang Li, Jin Yao, Kannan Raj, Ashok V. Krishnamoorthy, Opt. Express 18 (2010) 19055. [3] Kang Xiong, Xi Xiao, Xianyao Li, Yingtao Hu, Zhiyong Li, Tao Chu, Yude Yu, Jinzhong Yu, Opt. Commun. 285 (2012) 4368. [4] J. Kani, IEEE J. Sel. Top. Quantum Electron 16 (5) (2010) 1290. [5] Robert S. Guzzon, Erik J. Norberg, John S. Parker, Leif A. Johansson, Larry A. Coldren, Opt. Express 19 (2011) 7816. [6] S. Nicholes, M. Mashanovitch, B. Jevremovic, E. Lively, L. A. Coldren, and D. J. Blumenthal, in: Proceedings of the Optical Fiber Communication Conference/ National Fiber Optic Engineers Conference, OSA Technical Digest (CD), paper OThD1, 2011. [7] Z. Zhang, N. Mettbach, C. Zawadzki, J. Wang, D. Schmidt, W. Brinker, N. Grote, M. Schell, N. Keil, IET Optoelectron. 5 (2011) 226. [8] N. Keil, C. Zawadzki, Z. Zhang, J. Wang, N. Mettbach, N. Grote, and M. Schell, in: Proceedings of theOptical Fiber Communication Conference/National Fiber Optic Engineers Conference, OSA Technical Digest (CD), paper OWM1, 2011. [9] Y.-O. Noh, H.-J. Lee, J.-J. Ju, M.-S. Kim, S.-H. Oh, M.-C. Oh, Opt. Express 16 (2008) 18194. [10] S.H. Oh, K.H. Yoon, K.S. Kim, J. Kim, O. Kwon, D.K. Oh, Y.O. Noh, J.K. Seo, H.J. Lee, IEEE J. Sel. Top. Quantum Electron 17 (6) (2011) 1534. [11] H. Klein, C. Wagner, W. Brinker, F. Soares, D. de Felipe, Z. Zhang, C. Zawadzki, N. Keil, and M. Moehrle, in: Proceedings of the International Conference on Indium Phosphide and Related Materials (IPRM 2012), 27–30 August, pp. 20–21, 2012. [12] Z. Zhang, D. Liu, D. Felipe, A. Liu, N. Keil, N. Grote, Appl. Phys. Lett. 102 (2013) 181105. [13] D. Liu, Z. Zhang, N. Keil, N. Grote, IEEE Photon Technol. Lett. 25 (2013) 1734. [14] Ziyang Zhang, Garri Genrich, Norbert Keil, Norbert Grote, Opt. Lett. 39 (2014) 162. [15] Yanwu Zhang, Yingfeng Li, Zian He, Liying Liu, Lei Xu, Opt. Commun. 284 (2011) 1828. [16] T. Suzuki, H. Arimoto, T. Kitatani, A. Takei, T. Taniguchi, K. Shinoda, S. Tanaka, and S. Tsuji, in: Proceedings of the Optical Fiber Communication Conference/ National Fiber Optic Engineers Conference , OSA Technical Digest (CD), paper OWD7, 2011. [17] Matteo Dainese, Marcin Swillo, Lech Wosinski, Lars Thylen, Opt. Commun. 260 (2006) 514. [18] K. Ogawa, W. Chang, S. Sopori, L. Bhushan, F. Rosenbaum, IEEE J. Quantum Electron. 9 (1) (1973) 29. [19] Anjin Liu, Ziyang Zhang, David de Felipe, Norbert Keil, Norbert Grote, IEEE Photonics Technol. Lett. 26 (2014) 313. [20] C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R. Baets, B. Esembeson, I. Biaggio, T. Michinobu, F. Diederich, W. Freude, J. Leuthold, Nat. Photonics 3 (2009) 216. [21] S. Jakobs, A. Petrov, M. Eich, J.M. Hales, J.W. Perry, S. Marder, V Nazabal, P. Nemec, Proc. SPIE 8434 (2012) 84340.