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Broadband optical single sideband generation using an ultra high shape-factor self coupled ring resonator
Journal Pre-proof Broadband optical single sideband generation using an ultra high shape-factor self coupled ring resonator Awanish Pandey, Vadivukkarasi Jeyaselvan, Shankar Kumar Selvaraja
PII: DOI: Reference:
S0030-4018(19)31184-8 https://doi.org/10.1016/j.optcom.2019.125224 OPTICS 125224
To appear in:
Optics Communications
Received date : 23 September 2019 Revised date : 9 December 2019 Accepted date : 30 December 2019 Please cite this article as: A. Pandey, V. Jeyaselvan and S.K. Selvaraja, Broadband optical single sideband generation using an ultra high shape-factor self coupled ring resonator, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2019.125224. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Broadband optical single sideband generation using an ultra high shape-factor self coupled ring resonator Awanish Pandey*,1 , Vadivukkarasi Jeyaselvan*,1 , Shankar Kumar Selvaraja1 Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, 566012
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In this study, we realize an integrated generation of optical Single SideBand with Carrier (SSB+C) signal for microwave photonics applications. It has been achieved using a Micro Ring Modulator (MRM) combined with a cavity based wavelength selective filter. MRM, when applied with an RF input, results in a Double SideBand with Carrier (DSB+C) signal where one sideband is suppressed by applying this signal to the filter. The filter has been created using a single resonance-split self-coupled cavity with an extremely high shape factor of 0.69. The sideband suppression ratio between DSB+C and SSB+C ranges from 16 dB to 55 dB for a frequency range of 4 GHz to 20 GHz. Tunable suppression ratio of 21 dB has been achieved at a fixed frequency of 15 GHz. Dynamic range performance of the generated signal has been evaluated at a noise floor of -156 dBm. The dynamic range remains stable in the range of 1 GHz - 5 GHz at ∼ 80 dB.Hz2/3 . Keywords: Integrated optics, Integrated microwave photonics, Micro-ring resonator 1. Introduction
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Low-loss transmission of Radio Frequency (RF) signals over optical fibers has put Radio over Fiber (RoF) technology at the forefront of future microwave communication [1]. It functions by modulating the RF signal on Email address: [email protected] (Awanish Pandey) () * These authors contributed equally to the work.
Preprint submitted to Optics Communications
January 7, 2020
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an optical carrier, carrying out the required signal processing tasks in the optical domain, transmitting the signal, and finally demodulating it back to RF domain at receiver end [2]. The modulation scheme utilized to imprint the RF signal over an optical carrier determines the performance of the RoF link to a substantial degree [3]. The conventional intensity/phase modulators produce DSB+C signal where the optical carrier is flanked by sidebands on either side. The spectral properties of the sideband are governed by the modulator DC bias, modulation frequency and the modulation scheme employed. Performance of DSB+C signal is limited by the chromatic dispersion in optical fiber resulting in frequency dependant power penalty [4]. The straightforward solution to alleviate power penalty is to suppress one of the sidebands of the DSB+C signal resulting in a SSB+C modulated signal [5]. SSB+C signal remains the preferred choice for RoF technology and has been investigated extensively towards realizing photonic links [6, 7]. To further harvest the advantages of RoF, integrated photonics based opportunities are being explored for cost-effective, compact, low-loss, and lightweight solutions [8]. Several schemes/techniques have already been exploited for on-chip SSB+C signal generation such as Stimulated Brillouin Scattering (SBS), Fiber Bragg Grating (FBG), Micro Ring Resonator (MRR), and Whispering Gallery Mode (WGM) resonators [8, 13, 14, 15, 12, 9, 10, 11]. Typically, bulk-modulators to generate a DSB+C signal followed by an integrated wavelength filter is used to suppress one sideband. MRRs are a preferred choice for wavelength filters due to its simple operation and implementation [12]. Even with MRRs, the filters are primarily realized using higher-order resonators that adds additional complexity on cavity design and fabrication. The only fully-integrated solution is demonstrated in [9] that is bandwidth limited (30 GHz) and requires complex quadrature hybrid coupler. To achieve an integrated yet efficient solution for RoF, the realization of a simple SSB+C signal modulation unit that ideally shows frequency independent performance and does not require integration of complex structures is necessary. In this letter, we propose and demonstrate an integrated and configurable generation of SSB+C signal by cascading a Micro Ring Modulator (MRM) with a wavelength selective filter realized with a Self-Coupled Micro Ring Resonator (SCMRR). SCMRR works on the principle of degenerate resonance moe splitting in a resonant cavity [16]. The splitting is completely tunable and can be as high as the Free Spectral Range (FSR) itself. By adjusting the splitting, we realize a wavelength filter with high Shape Factor 2
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(SF). Using the high SF filter in combination with with MRM, we demonstrate generation of SSB+C signal. Suppression ratio, and dynamic range performance of the generated SSB+C signal has been reported to assess the quality of the generated signal.
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Figure 1: Generation of SSB+C signal using a MRM and SCMRR. One sideband of DSB+C signal produced by MRM is suppressed by SCMRR. The generated SSB+C signal is finally collected from the back Drop Port (BDP). κ3 refers to the coupling co-efficient of the self-coupling region in the SCMRR.
2. Working Principle
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A schematic for the generation of SSB+C signal from a DSB+C signal is shown in Fig. 1. An optical carrier is modulated by a MRM, driven by RF frequency (ωrf ) and DC bias (V ), that results in a DSB+C signal. The sidebands are located at ωc + ωrf (Upper Side Band (USB)) and ωc − ωrf (Lower Side Band (LSB)). The DSB+C signal is then transmitted through the SCMRR to suppress lower sideband (LSB). An extensive discussion on the design rules, parameters, and the working principle of SCMRR can be found in [16]. The SCMRR is optimized, by controlling the Self Coupling Co-efficient (SCC) explained in ref. [16], to exhibit a box-type function that allows transmission of carrier and USB whereas exhibiting transmission null at LSB. The SCMRR output is a SSB+C signal with LSB suppressed that is finally collected by an optical fiber with the help of a grating coupler. 3. Experiment and Results The spectral response of MRM and SCMRR is shown in Fig. 2. MRM has PN junction radially doped along the ring waveguide. It is operated in 3
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Figure 2: Spectral characterization of MRR and SCMRR, (a) variation of MRM resonance with different amount of reverse bias, (b) transmission spectra of BDP. Both MRR and SCMRR show an insertion loss of 25 dB (grating couplers included), and (c) variation of PBR and resonance split in SCMRR as a function of κ3 .
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carrier depletion mode resulting in red-shift of the resonance wavelength with bias as shown in Fig. 2(a). The resonant wavelength tunability of the MRM is 20 pm/V. Transmission response at Back Drop Port (BDP) of SCMRR for κ3 = 1% is shown in Fig. 2(b). The resonances have split and are characterized by Extinction Ratio (ER), PassBand Ripple (PBR) and the resonance split. Since the carrier and USB should not suffer any additional insertion loss due to SCMRR, PBR must be maintained at a minimum value. Also, to achieve broadband operation, the resonance split should be maximum to accommodate higher modulation frequency sidebands. A higher modulation frequency that exceeds the resonance-split value would invariably fall on the resonance roll-off and suffer enhanced insertion loss. Both the parameters were optimized through engineering the optical power in the degenerate modes of SCMRR by modifying the SCC. Variation of PBR and Split is reported in Fig. 2(c). PBR for BDP reduces from 12 dB at κ3 = 9% to 1 dB at κ3 = 1%. Since smaller self-coupling results in reduced PBR at BDP, it has been utilized to further process thr DSB+C signal. However, since split decreases at smaller κ3 from 130 GHz κ3 = 9% to 25 GHz nm at κ3 = 1%, bandwidth and insertion loss of the generated SSB+C signal must be negotiated depending upon the requirement for particular RoF application. Table 1: Shape Factor (SF) for different types of cavity configurations.
Year Architecture 2018 [17] MRRs 2017 [18] MRRs 2016 [19] MRRs 2015 [20] MRRs 2014 [21] MRRs, MZM 2013 [22] MRRs This work SCMRR
No. of elements 8 9 4 2-4 3, 2 5 1
SF 0.2 - 0.73 0.49 0.22 - 0.44 0.40, 0.60 0.80 0.40 0.69
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Effective suppression of LSB necessitates a box-like transfer function of the wavelength selective filter. The transfer function profile of the filter has been qualified by the parameter SF. It is defined as the ratio of 1 dB to 10 dB linewidth of resonance and corresponds to one for an ideal square shaped response. The 1 dB and 10 dB linewidth are calculated from the minima of PBR. A high-SF is necessary from SCMRR so that LSB is efficiently suppressed. For example, a perfect square shape response would have the maximum SF of 1 as both the linewidths would be equal. Lower SF leads 5
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smaller out-of-band filter roll-off that leads to leakage of LSB optical power and hence degrades the suppression performance. The optical length and busto-ring coupling coefficients along with κ3 were further optimized to achieve resonance at BDP with a SF of 0.69. It should be mentioned that such a high SF has not yet been attained using a single cavity and has only been realized with complicated higher-order architectures. Table 1 summaries some of the recent approaches using MRR to realize a high SF configuration. A SF of 0.8 was achieved in [21] but it utilizes complex assembly of 3 MRRs along with 2 Mach-Zehnder interferometers and hence offers a complicated method than a much simpler SCMRR. Figure 3 shows the experimental setup to demonstrate the generation of SSB+C signal. A tunable laser source serves as an input to MRM. MRM is driven by a combined DC and RF sources that result in DSB+C signal. The MRM output is passed through an amplifier (SOA) and fed to SCMRR that suppresses one of the sideband and results in SSB+C signal. SSB+C signal is given to an optical amplifier (EDFA) followed by an optical filter to suppress spontaneous emission noise. Finally, the signal is analyzed in optical domain with a spectrum analyzer (2.5 GHz resolution) whereas it is fed to a 20 GHz photodetector and then an electrical spectrum analyzer to analyze the signal in RF domain.
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Figure 3: Experimental setup for SSB+C generation and characterization. PC: Polarization Controller, EDFA: Erbium Doped Fiber Amplifier, SOA: Semiconductor Optical Amplifier, TC: Temperature Controller, BPF: BandPass Filter, OSA: Optical Spectrum Analyzer, PD: PhotoDetector, ESA: Electrical Spectrum Analyzer.
The analysis of generated SSB+C signal is shown in Fig. 4. Figure 4(a) shows the generated SSB+C signal imposed on the DSB+C signal from the MRM modulated at 20 GHz. As is evident, the carrier and one sideband are preserved whereas the other sideband has been completely suppressed 6
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along with the harmonics. The suppression is quantified by Optical Sideband Suppression Ratio (OSSR) that measures the insertion loss difference between the optical carrier and LSB. Figure 4(b) shows the variation of OSSR with the modulation frequency. OSSR increases with the modulation frequency from 16 dB at 4 GHz to 56 dB at 20 GHz. The slope of the increase in OSSR is primarily dictated by the out-of-band spectral roll-off of SCMRR at BDP. Higher modulation frequency pushes the LSB at the null of BDP and hence the OSSR increases.
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Figure 4: Analysis of generated SSB+C signal, (a) generated SSB+C signal at 20 GHz, (b) OSSR variation as a function of modulation frequency, (c) thermal tuning of OSSR at a particular frequency of 15 GHz, and (d) variation in OSSR at different modulator bias.
Depending upon the bias of the MRM, the sideband extinction of the modulated output varies, and hence it becomes essential to achieve tunable 7
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OSSR at a particular frequency. We accomplish this by thermal tuning of the resonance of SCMRR. Figure 4(c) shows the variation in sideband suppression as a function of temperature. Increasing temperature red-shifts the resonance that pushes thr LSB sideband out of the transmission band of the filter that results in higher suppression. The OSSR varies from 14 dB at 00 Cto 35 dB at 12 0 C variation from the initial temperature of 28 0 C of SCMRR. Figure 4(d) shows the variation of OSSR at a fixed temperature and frequency but at different bias points after MRM and MRM followed by SCMRR. The output after SCMRR shows a constant OSSR swing of 11 dB establishing that the operating point of MRM has no effect on the suppression ratio at a particular frequency.
Figure 5: Received signal at PD with varying modulation frequency.
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The received RF signal at the ESA, measured after applying the SSB+C signal to a 20 GHz PD, is shown in Fig. 5 for a frequency range of 1 GHz - 13 GHz. The signals have been regenerated back in the RF domain with extinction ranging from 20 dB to 35 dB. The highest measured frequency response was limited by the spectrum analyzer (13.5 GHz). The performance of microwave link in the presence of multiple signals and non-linear elements is characterized by two-tone measurement to evaluate the Spurious Free Dynamic Range (SFDR) parameter [23]. Two tones (f1 and f2 ) placed at 40 MHz are mixed with a 50:50 RF combiner and fed to the MRM. The Third Order Inter-Modulation (3IMD) products 2f1 - f2 , 2f2 - f1 ) resulting from the non-linearity of modulator needs to be suppressed below a particular level to avoid degrading the RoF transmission. We calculated the variation of power in 3IMD products and the fundamental tones at the output as a function of two-tone power using ESA with a phase noise floor of -156 dBm. The result for tones at 1 GHz and 1 GHz + 40 MHz is plotted in Fig. 6(a) where SFDR calculation has been defined. A dynamic range of 76 dB.Hz2/3 was obtained. The variation in SFDR for a frequency range 8
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Figure 6: Dynamic range measurement, (a) at 1 GHz when the tones are 40 MHz apart, (b) variation of SFDR as function of two tone frequency.
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of 5 GHz is shown in Fig. 6(b). It ranges from 76 dB.Hz2/3 at 1 GHz to 86 dB.Hz2/3 at 2 GHz with a dynamic range at other frequencies is between these limits. We have demonstrated an integrated approach to generate SSB+C signal from DSB+C signal. It was achieved by generating a DSB+C signal from a ring modulator and then suppressing one sideband using a wavelength selective filter. The filter was realized with a SF of 0.69 using a self-coupled cavity. A suppression ratio till 55 dB for a frequency range of 20 GHz was achieved. Suppression tunability of 21 dB was also attained at a fixed frequency. The dynamic range performance in the range of 80 dB.Hz2/3 was also reported. The scheme proposed is simple to realize and can be utilized for RoF application in the future.
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References
[1] C. Lim, Y. Tian, C. Ranaweera, A. Nirmalathas, E. Wong, and K. L. Lee, J. Lightwave Technol. 37(6) (2018) 1647-1656.
9
Journal Pre-proof
[2] K. Xu, R. Wang, Y. Dai, F. Yin, J. Li, Y. Ji, and J. Lin, Photon. Res. 2(4) (2014) 54-63.
of
[3] J. Yu, Z. Jia, L. Yi, Y. Su, G. K. Chang, and T. Wang, IEEE Photon. Technol. Lett. 18(13) (2006) 1418-1420.
pro
[4] G. H. Smith, D. Novak, and Z. Ahmed, IEEE Trans. Microwave Theory Tech. 45(8) (1997) 1410-1415. [5] B. Hraimel, X. Zhang, Y. Pei, K. Wu, T. Liu, T. Xu, and Q. Nie, J. Lightwave Technol. 29(5) (2011) 775-781. M3E.4.
re-
[6] R. Puerta, S. Rommel, J. J. Vegas Olmos, and I. Tafur Monroy, in Proc. Opt. Fiber Commun. Conf. Expo., 2017, Paper M3E.4. [7] Y. Xu, X. Li, J. Yu, and G. Chang, Opt. Express 24(20) (2016) 2283022835.
urn al P
[8] J. S. Fandi˜ no, P. Mu˜ noz, D. Dom´enech, and J. Capmany, Nat. Photonics 11(2) (2017) 124. [9] B. M. Yu, J. M. Lee, C. Mai, S. Lischke, L. Zimmermann, and W. Y. Choi, Photon. Res. 6(1) (2018) 6-11. [10] S. X. Chew, X. Yi, S. Song, L. Li, P. Bian, L. Nguyen, and R. A. Minasian, J. Lightwave Technol. 34(20) (2016) 4705-4714 [11] S. Song, X. Yi, S. X. Chew, L. Li, L. Nguyen, and R. Zheng, Opt. Eng. 55(3) (2015) 031114. [12] A. Pandey, and S. K. Selvaraja, Opt. Express 27(6) (2019) 8476-8487. [13] S. R. Blais and J. Yao, IEEE Photon. Technol. Lett. 18(21) (2006) 22302232.
Jo
[14] Y. Shen, X. Zhang, and K. Chen, IEEE Photon. Technol. Lett. 17(6) (2005) 1277-1279. [15] A. A. Savchenkov, A. B. Matsko, W. Liang, V. S. Ilchenko, D. Seidel, and L. Maleki, IEEE Trans. Microwave Theory Tech. 58(11) (2010) 31673174. 10
Journal Pre-proof
[16] A. Pandey and S. K. Selvaraja, Opt. Lett. 42(14) (2017) 2854-2857.
of
[17] T. Dai, G. Wang, X. Guo, C. Bei, J. Jiang, W. Chen, Y. Wang, H. Yu, and J. Yang, IEEE Photon. Technol. Lett. 30(11) (2018) 1044-1047.
pro
[18] P. Wang, T. Yang, T. Dai, G. Wang, J. Zhang, W. Chen, and J. Yang, IEEE Photon. J. 9(6) (2017) 1-10. [19] T. Dai, A. Shen, G. Wang, Y. Wang, Y. Li, X. Jiang, and J. Yang, Opt. Lett. 41(20) (2016) 4807-4810. [20] H. Ito, N. Ishikura, and T. Baba, J. Lightwave Technol. 33(2) (2015) 304-310.
re-
[21] S.-H. Jeong, D. Shimura, Y. Tanaka, and K. Morito, in Proceedings of the European Conference on Optical Communication (ECOC, 2014), Paper We1.4.3
urn al P
[22] H. Yan, X. Feng, D. Zhang, K. Cui, F. Liu, and Y. Huang, IEEE Photon. Technol. Lett. 25(15) (2013) 1462-1465.
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[23] B. Masella, B. Hraimel, and X. Zhang, J. Lightwave Technol. 27(15) (2009) 3034-3041.
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Awanish Pandey: Conceptualization, Methodology, Software, Investigation, Writing- Original draft preparation. Vadivukkarasi jeyaselvan.: Methodology, Software, Investigation, Writing- Original draft preparation. Shankar Kumar Selvaraja: Conceptulaization, Writing- Reviewing and Editing, Supervision.
PII: DOI: Reference:
S0030-4018(19)31184-8 https://doi.org/10.1016/j.optcom.2019.125224 OPTICS 125224
To appear in:
Optics Communications
Received date : 23 September 2019 Revised date : 9 December 2019 Accepted date : 30 December 2019 Please cite this article as: A. Pandey, V. Jeyaselvan and S.K. Selvaraja, Broadband optical single sideband generation using an ultra high shape-factor self coupled ring resonator, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2019.125224. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Broadband optical single sideband generation using an ultra high shape-factor self coupled ring resonator Awanish Pandey*,1 , Vadivukkarasi Jeyaselvan*,1 , Shankar Kumar Selvaraja1 Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, 566012
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In this study, we realize an integrated generation of optical Single SideBand with Carrier (SSB+C) signal for microwave photonics applications. It has been achieved using a Micro Ring Modulator (MRM) combined with a cavity based wavelength selective filter. MRM, when applied with an RF input, results in a Double SideBand with Carrier (DSB+C) signal where one sideband is suppressed by applying this signal to the filter. The filter has been created using a single resonance-split self-coupled cavity with an extremely high shape factor of 0.69. The sideband suppression ratio between DSB+C and SSB+C ranges from 16 dB to 55 dB for a frequency range of 4 GHz to 20 GHz. Tunable suppression ratio of 21 dB has been achieved at a fixed frequency of 15 GHz. Dynamic range performance of the generated signal has been evaluated at a noise floor of -156 dBm. The dynamic range remains stable in the range of 1 GHz - 5 GHz at ∼ 80 dB.Hz2/3 . Keywords: Integrated optics, Integrated microwave photonics, Micro-ring resonator 1. Introduction
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Low-loss transmission of Radio Frequency (RF) signals over optical fibers has put Radio over Fiber (RoF) technology at the forefront of future microwave communication [1]. It functions by modulating the RF signal on Email address: [email protected] (Awanish Pandey) () * These authors contributed equally to the work.
Preprint submitted to Optics Communications
January 7, 2020
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an optical carrier, carrying out the required signal processing tasks in the optical domain, transmitting the signal, and finally demodulating it back to RF domain at receiver end [2]. The modulation scheme utilized to imprint the RF signal over an optical carrier determines the performance of the RoF link to a substantial degree [3]. The conventional intensity/phase modulators produce DSB+C signal where the optical carrier is flanked by sidebands on either side. The spectral properties of the sideband are governed by the modulator DC bias, modulation frequency and the modulation scheme employed. Performance of DSB+C signal is limited by the chromatic dispersion in optical fiber resulting in frequency dependant power penalty [4]. The straightforward solution to alleviate power penalty is to suppress one of the sidebands of the DSB+C signal resulting in a SSB+C modulated signal [5]. SSB+C signal remains the preferred choice for RoF technology and has been investigated extensively towards realizing photonic links [6, 7]. To further harvest the advantages of RoF, integrated photonics based opportunities are being explored for cost-effective, compact, low-loss, and lightweight solutions [8]. Several schemes/techniques have already been exploited for on-chip SSB+C signal generation such as Stimulated Brillouin Scattering (SBS), Fiber Bragg Grating (FBG), Micro Ring Resonator (MRR), and Whispering Gallery Mode (WGM) resonators [8, 13, 14, 15, 12, 9, 10, 11]. Typically, bulk-modulators to generate a DSB+C signal followed by an integrated wavelength filter is used to suppress one sideband. MRRs are a preferred choice for wavelength filters due to its simple operation and implementation [12]. Even with MRRs, the filters are primarily realized using higher-order resonators that adds additional complexity on cavity design and fabrication. The only fully-integrated solution is demonstrated in [9] that is bandwidth limited (30 GHz) and requires complex quadrature hybrid coupler. To achieve an integrated yet efficient solution for RoF, the realization of a simple SSB+C signal modulation unit that ideally shows frequency independent performance and does not require integration of complex structures is necessary. In this letter, we propose and demonstrate an integrated and configurable generation of SSB+C signal by cascading a Micro Ring Modulator (MRM) with a wavelength selective filter realized with a Self-Coupled Micro Ring Resonator (SCMRR). SCMRR works on the principle of degenerate resonance moe splitting in a resonant cavity [16]. The splitting is completely tunable and can be as high as the Free Spectral Range (FSR) itself. By adjusting the splitting, we realize a wavelength filter with high Shape Factor 2
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(SF). Using the high SF filter in combination with with MRM, we demonstrate generation of SSB+C signal. Suppression ratio, and dynamic range performance of the generated SSB+C signal has been reported to assess the quality of the generated signal.
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Figure 1: Generation of SSB+C signal using a MRM and SCMRR. One sideband of DSB+C signal produced by MRM is suppressed by SCMRR. The generated SSB+C signal is finally collected from the back Drop Port (BDP). κ3 refers to the coupling co-efficient of the self-coupling region in the SCMRR.
2. Working Principle
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A schematic for the generation of SSB+C signal from a DSB+C signal is shown in Fig. 1. An optical carrier is modulated by a MRM, driven by RF frequency (ωrf ) and DC bias (V ), that results in a DSB+C signal. The sidebands are located at ωc + ωrf (Upper Side Band (USB)) and ωc − ωrf (Lower Side Band (LSB)). The DSB+C signal is then transmitted through the SCMRR to suppress lower sideband (LSB). An extensive discussion on the design rules, parameters, and the working principle of SCMRR can be found in [16]. The SCMRR is optimized, by controlling the Self Coupling Co-efficient (SCC) explained in ref. [16], to exhibit a box-type function that allows transmission of carrier and USB whereas exhibiting transmission null at LSB. The SCMRR output is a SSB+C signal with LSB suppressed that is finally collected by an optical fiber with the help of a grating coupler. 3. Experiment and Results The spectral response of MRM and SCMRR is shown in Fig. 2. MRM has PN junction radially doped along the ring waveguide. It is operated in 3
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Figure 2: Spectral characterization of MRR and SCMRR, (a) variation of MRM resonance with different amount of reverse bias, (b) transmission spectra of BDP. Both MRR and SCMRR show an insertion loss of 25 dB (grating couplers included), and (c) variation of PBR and resonance split in SCMRR as a function of κ3 .
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carrier depletion mode resulting in red-shift of the resonance wavelength with bias as shown in Fig. 2(a). The resonant wavelength tunability of the MRM is 20 pm/V. Transmission response at Back Drop Port (BDP) of SCMRR for κ3 = 1% is shown in Fig. 2(b). The resonances have split and are characterized by Extinction Ratio (ER), PassBand Ripple (PBR) and the resonance split. Since the carrier and USB should not suffer any additional insertion loss due to SCMRR, PBR must be maintained at a minimum value. Also, to achieve broadband operation, the resonance split should be maximum to accommodate higher modulation frequency sidebands. A higher modulation frequency that exceeds the resonance-split value would invariably fall on the resonance roll-off and suffer enhanced insertion loss. Both the parameters were optimized through engineering the optical power in the degenerate modes of SCMRR by modifying the SCC. Variation of PBR and Split is reported in Fig. 2(c). PBR for BDP reduces from 12 dB at κ3 = 9% to 1 dB at κ3 = 1%. Since smaller self-coupling results in reduced PBR at BDP, it has been utilized to further process thr DSB+C signal. However, since split decreases at smaller κ3 from 130 GHz κ3 = 9% to 25 GHz nm at κ3 = 1%, bandwidth and insertion loss of the generated SSB+C signal must be negotiated depending upon the requirement for particular RoF application. Table 1: Shape Factor (SF) for different types of cavity configurations.
Year Architecture 2018 [17] MRRs 2017 [18] MRRs 2016 [19] MRRs 2015 [20] MRRs 2014 [21] MRRs, MZM 2013 [22] MRRs This work SCMRR
No. of elements 8 9 4 2-4 3, 2 5 1
SF 0.2 - 0.73 0.49 0.22 - 0.44 0.40, 0.60 0.80 0.40 0.69
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Effective suppression of LSB necessitates a box-like transfer function of the wavelength selective filter. The transfer function profile of the filter has been qualified by the parameter SF. It is defined as the ratio of 1 dB to 10 dB linewidth of resonance and corresponds to one for an ideal square shaped response. The 1 dB and 10 dB linewidth are calculated from the minima of PBR. A high-SF is necessary from SCMRR so that LSB is efficiently suppressed. For example, a perfect square shape response would have the maximum SF of 1 as both the linewidths would be equal. Lower SF leads 5
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smaller out-of-band filter roll-off that leads to leakage of LSB optical power and hence degrades the suppression performance. The optical length and busto-ring coupling coefficients along with κ3 were further optimized to achieve resonance at BDP with a SF of 0.69. It should be mentioned that such a high SF has not yet been attained using a single cavity and has only been realized with complicated higher-order architectures. Table 1 summaries some of the recent approaches using MRR to realize a high SF configuration. A SF of 0.8 was achieved in [21] but it utilizes complex assembly of 3 MRRs along with 2 Mach-Zehnder interferometers and hence offers a complicated method than a much simpler SCMRR. Figure 3 shows the experimental setup to demonstrate the generation of SSB+C signal. A tunable laser source serves as an input to MRM. MRM is driven by a combined DC and RF sources that result in DSB+C signal. The MRM output is passed through an amplifier (SOA) and fed to SCMRR that suppresses one of the sideband and results in SSB+C signal. SSB+C signal is given to an optical amplifier (EDFA) followed by an optical filter to suppress spontaneous emission noise. Finally, the signal is analyzed in optical domain with a spectrum analyzer (2.5 GHz resolution) whereas it is fed to a 20 GHz photodetector and then an electrical spectrum analyzer to analyze the signal in RF domain.
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Figure 3: Experimental setup for SSB+C generation and characterization. PC: Polarization Controller, EDFA: Erbium Doped Fiber Amplifier, SOA: Semiconductor Optical Amplifier, TC: Temperature Controller, BPF: BandPass Filter, OSA: Optical Spectrum Analyzer, PD: PhotoDetector, ESA: Electrical Spectrum Analyzer.
The analysis of generated SSB+C signal is shown in Fig. 4. Figure 4(a) shows the generated SSB+C signal imposed on the DSB+C signal from the MRM modulated at 20 GHz. As is evident, the carrier and one sideband are preserved whereas the other sideband has been completely suppressed 6
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along with the harmonics. The suppression is quantified by Optical Sideband Suppression Ratio (OSSR) that measures the insertion loss difference between the optical carrier and LSB. Figure 4(b) shows the variation of OSSR with the modulation frequency. OSSR increases with the modulation frequency from 16 dB at 4 GHz to 56 dB at 20 GHz. The slope of the increase in OSSR is primarily dictated by the out-of-band spectral roll-off of SCMRR at BDP. Higher modulation frequency pushes the LSB at the null of BDP and hence the OSSR increases.
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Figure 4: Analysis of generated SSB+C signal, (a) generated SSB+C signal at 20 GHz, (b) OSSR variation as a function of modulation frequency, (c) thermal tuning of OSSR at a particular frequency of 15 GHz, and (d) variation in OSSR at different modulator bias.
Depending upon the bias of the MRM, the sideband extinction of the modulated output varies, and hence it becomes essential to achieve tunable 7
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OSSR at a particular frequency. We accomplish this by thermal tuning of the resonance of SCMRR. Figure 4(c) shows the variation in sideband suppression as a function of temperature. Increasing temperature red-shifts the resonance that pushes thr LSB sideband out of the transmission band of the filter that results in higher suppression. The OSSR varies from 14 dB at 00 Cto 35 dB at 12 0 C variation from the initial temperature of 28 0 C of SCMRR. Figure 4(d) shows the variation of OSSR at a fixed temperature and frequency but at different bias points after MRM and MRM followed by SCMRR. The output after SCMRR shows a constant OSSR swing of 11 dB establishing that the operating point of MRM has no effect on the suppression ratio at a particular frequency.
Figure 5: Received signal at PD with varying modulation frequency.
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The received RF signal at the ESA, measured after applying the SSB+C signal to a 20 GHz PD, is shown in Fig. 5 for a frequency range of 1 GHz - 13 GHz. The signals have been regenerated back in the RF domain with extinction ranging from 20 dB to 35 dB. The highest measured frequency response was limited by the spectrum analyzer (13.5 GHz). The performance of microwave link in the presence of multiple signals and non-linear elements is characterized by two-tone measurement to evaluate the Spurious Free Dynamic Range (SFDR) parameter [23]. Two tones (f1 and f2 ) placed at 40 MHz are mixed with a 50:50 RF combiner and fed to the MRM. The Third Order Inter-Modulation (3IMD) products 2f1 - f2 , 2f2 - f1 ) resulting from the non-linearity of modulator needs to be suppressed below a particular level to avoid degrading the RoF transmission. We calculated the variation of power in 3IMD products and the fundamental tones at the output as a function of two-tone power using ESA with a phase noise floor of -156 dBm. The result for tones at 1 GHz and 1 GHz + 40 MHz is plotted in Fig. 6(a) where SFDR calculation has been defined. A dynamic range of 76 dB.Hz2/3 was obtained. The variation in SFDR for a frequency range 8
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Figure 6: Dynamic range measurement, (a) at 1 GHz when the tones are 40 MHz apart, (b) variation of SFDR as function of two tone frequency.
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of 5 GHz is shown in Fig. 6(b). It ranges from 76 dB.Hz2/3 at 1 GHz to 86 dB.Hz2/3 at 2 GHz with a dynamic range at other frequencies is between these limits. We have demonstrated an integrated approach to generate SSB+C signal from DSB+C signal. It was achieved by generating a DSB+C signal from a ring modulator and then suppressing one sideband using a wavelength selective filter. The filter was realized with a SF of 0.69 using a self-coupled cavity. A suppression ratio till 55 dB for a frequency range of 20 GHz was achieved. Suppression tunability of 21 dB was also attained at a fixed frequency. The dynamic range performance in the range of 80 dB.Hz2/3 was also reported. The scheme proposed is simple to realize and can be utilized for RoF application in the future.
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References
[1] C. Lim, Y. Tian, C. Ranaweera, A. Nirmalathas, E. Wong, and K. L. Lee, J. Lightwave Technol. 37(6) (2018) 1647-1656.
9
Journal Pre-proof
[2] K. Xu, R. Wang, Y. Dai, F. Yin, J. Li, Y. Ji, and J. Lin, Photon. Res. 2(4) (2014) 54-63.
of
[3] J. Yu, Z. Jia, L. Yi, Y. Su, G. K. Chang, and T. Wang, IEEE Photon. Technol. Lett. 18(13) (2006) 1418-1420.
pro
[4] G. H. Smith, D. Novak, and Z. Ahmed, IEEE Trans. Microwave Theory Tech. 45(8) (1997) 1410-1415. [5] B. Hraimel, X. Zhang, Y. Pei, K. Wu, T. Liu, T. Xu, and Q. Nie, J. Lightwave Technol. 29(5) (2011) 775-781. M3E.4.
re-
[6] R. Puerta, S. Rommel, J. J. Vegas Olmos, and I. Tafur Monroy, in Proc. Opt. Fiber Commun. Conf. Expo., 2017, Paper M3E.4. [7] Y. Xu, X. Li, J. Yu, and G. Chang, Opt. Express 24(20) (2016) 2283022835.
urn al P
[8] J. S. Fandi˜ no, P. Mu˜ noz, D. Dom´enech, and J. Capmany, Nat. Photonics 11(2) (2017) 124. [9] B. M. Yu, J. M. Lee, C. Mai, S. Lischke, L. Zimmermann, and W. Y. Choi, Photon. Res. 6(1) (2018) 6-11. [10] S. X. Chew, X. Yi, S. Song, L. Li, P. Bian, L. Nguyen, and R. A. Minasian, J. Lightwave Technol. 34(20) (2016) 4705-4714 [11] S. Song, X. Yi, S. X. Chew, L. Li, L. Nguyen, and R. Zheng, Opt. Eng. 55(3) (2015) 031114. [12] A. Pandey, and S. K. Selvaraja, Opt. Express 27(6) (2019) 8476-8487. [13] S. R. Blais and J. Yao, IEEE Photon. Technol. Lett. 18(21) (2006) 22302232.
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[14] Y. Shen, X. Zhang, and K. Chen, IEEE Photon. Technol. Lett. 17(6) (2005) 1277-1279. [15] A. A. Savchenkov, A. B. Matsko, W. Liang, V. S. Ilchenko, D. Seidel, and L. Maleki, IEEE Trans. Microwave Theory Tech. 58(11) (2010) 31673174. 10
Journal Pre-proof
[16] A. Pandey and S. K. Selvaraja, Opt. Lett. 42(14) (2017) 2854-2857.
of
[17] T. Dai, G. Wang, X. Guo, C. Bei, J. Jiang, W. Chen, Y. Wang, H. Yu, and J. Yang, IEEE Photon. Technol. Lett. 30(11) (2018) 1044-1047.
pro
[18] P. Wang, T. Yang, T. Dai, G. Wang, J. Zhang, W. Chen, and J. Yang, IEEE Photon. J. 9(6) (2017) 1-10. [19] T. Dai, A. Shen, G. Wang, Y. Wang, Y. Li, X. Jiang, and J. Yang, Opt. Lett. 41(20) (2016) 4807-4810. [20] H. Ito, N. Ishikura, and T. Baba, J. Lightwave Technol. 33(2) (2015) 304-310.
re-
[21] S.-H. Jeong, D. Shimura, Y. Tanaka, and K. Morito, in Proceedings of the European Conference on Optical Communication (ECOC, 2014), Paper We1.4.3
urn al P
[22] H. Yan, X. Feng, D. Zhang, K. Cui, F. Liu, and Y. Huang, IEEE Photon. Technol. Lett. 25(15) (2013) 1462-1465.
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[23] B. Masella, B. Hraimel, and X. Zhang, J. Lightwave Technol. 27(15) (2009) 3034-3041.
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Awanish Pandey: Conceptualization, Methodology, Software, Investigation, Writing- Original draft preparation. Vadivukkarasi jeyaselvan.: Methodology, Software, Investigation, Writing- Original draft preparation. Shankar Kumar Selvaraja: Conceptulaization, Writing- Reviewing and Editing, Supervision.