- Email: [email protected]
Optical-biased modulator employing a single silicon micro-ring resonator
Optics Communications 368 (2016) 58–62
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Invited Paper
Optical-biased modulator employing a single silicon micro-ring resonator Siqi Yan, Jianji Dong n, Aoling Zheng, Yuan Yu Wuhan National Laboratory for Optoelectronics & School of Optical and Electrical Information, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2015 Received in revised form 22 January 2016 Accepted 30 January 2016
We propose and experimentally demonstrate an optical-biased modulator employing a single silicon micro-ring resonator. By adjusting optical bias, the micro-ring modulator is capable of generating several modulation formats, namely, on–off keying, binary phase shift keying and reversed on–off keying, at the speed of 0.4 Gbit/s with extinction ratio higher than 5 dB. Compared to the previous reported bias control approaches, the optical bias proposed in this study is a novel mechanism, which can be easily conducted without complicated integrated structures or redundant electrical devices. Meanwhile, optical bias can also effectively protect the vulnerable integrated silicon devices from possible damage induced by high direct current voltage. & 2016 Elsevier B.V. All rights reserved.
Keywords: Integrated optics Modulators Optoelectronics
1. Introduction Integrated silicon optical modulator, which has long been a focus of the scientific community, is vitally important as one of the main functional elements for the future optical interconnection system [1]. Remarkable progress in silicon modulators is achieved in recent years, such as ultra-high speed silicon modulator [2], ultralow power consumption silicon modulator [3] and integrated silicon modulator with novel structures [4]. In previous researches, several approaches were proposed to control the bias of integrated modulators, such as using micro-heaters [5], tuning the signal's wavelength [6] and employing electrical bias [7]. Among the methods mentioned above, the employment of micro-heater on a silicon modulator greatly adds to the complexities during the fabrication process while tuning the signal wavelength may cause trouble in the high speed wavelength division multiplex (WDM) communication systems. Furthermore, as the most commonly used method at the present, electrical bias relies on some electrical devices such as bias tee to accomplish bias control in integrated modulators, which seriously impedes the all-optical integration and causes the system to be bulky. Meanwhile, because most integrated optical devices are extremely vulnerable to high voltage, the voltage of DC bias must be carefully adjusted or it can induce serious damage to the integrated optical modulators. In this letter, we propose and experimentally demonstrate an optical-biased modulator employing a single micro-ring resonator n
Corresponding author. E-mail address: [email protected] (J. Dong).
http://dx.doi.org/10.1016/j.optcom.2016.01.091 0030-4018/& 2016 Elsevier B.V. All rights reserved.
(MRR). We can obtain non-return-zero on–off keying (NRZ-OOK), binary phase shift keying (BPSK) and reversed NRZ-OOK signals at fixed wavelength in a single MRR modulator by adjusting the power of a continuous wavelength (CW) as the bias. The fundamental physics of the optical bias is the optical induced thermal effect, which causes the red shift of resonance in the MRR [8]. Compared to the previous methods of altering bias in the integrated silicon modulators, the employment of optical bias neither requires additional integrated structures such as micro-heaters nor depends on external electrical devices such as bias tee. Meanwhile, the optical bias can protect the modulators completely from potential harm induced by high DC voltage. Hopefully, the optical manipulation of the modulator bias proposed in this study can overcome most disadvantages of previous methods. With ultra-small footprint, the optical-biased modulator can also reduce the interconnect systems complexity and pave the way of all-optical integrated modulator as a novel way of on-chip optical manipulation.
2. Operation principle Fig. 1 gives an overview about the working principle of opticalbiased modulator. Assume that the wavelength of input continuous wavelength (CW) light is aligned with the MRR resonant wavelength. Meanwhile, another CW light whose wavelength is aligned with another resonant wavelength acting as the optical bias. When no optical bias is applied, the transmission of through port of MRR is positively varied as the voltage applied on the MRR is increased, due to the plasma dispersion effect induced by
S. Yan et al. / Optics Communications 368 (2016) 58–62
59
Voltage
0
Time
RF Signal
Power
MRR
Power
Power
Time
Phase
Time Power
Time
Power
Reverse OOK BPSK Positive OOK
0
Time
Time
Optical bias Fig. 1. Overview about the working principle.
external voltage [9]. In this case, the CW light is positively modulated by electrical frequency (RF) signal. Thus positive NRZOOK signal can be obtained, as the blue waveforms show. When we add an optical bias, the input light will heat the high quality factor (Q) MRR remarkably owing to its strong resonance phenomenon. Meanwhile, significant red-shift of the initial transmission spectrum is generated due to the large thermo-optic coefficient in silicon. In this way, the transfer function of the modulator, i.e., the transmission of the MRR at the resonant wavelength with different external voltages, can be altered to obtain different signal formats. Via choosing the power of optical bias properly, we expect the modulator to be capable of generating both BPSK and reversed NRZ-OOK signal, as the waveforms in pink and brown color show respectively. To verify the feasibility of the optical bias, we calculate the redshift of the MRR transmission spectrum under different optical bias power. In the calculations, we believe that the thermal effect caused by high power light is the dominant phenomenon under these circumstances, despite the fact that the free carriers generated from two photons absorption may induce blue shift when the heating just starts [10]. According to theoretical model of optical induced thermal effects in the high-Q micro-cavity community, the red-shift response of the MRR can attain a steady red-shift state finally under given optical power. The Eqs. (1) and (2) gives the relationship between the input optical power and the corresponding amount of red-shift [8].
0 = Ih
1
(
λp − λr 2 Δλ / 2
)
− K ΔT +1
λr = λ 0 (1 + aΔT )
(1)
(2)
where Ih is the optical power that heats the cavity λP , λ 0 and λr
represent the input wavelength, the cold cavity resonance-wavelength and resonance wavelength after red-shift respectively, α is the temperature coefficient of resonance-wavelength, ΔT is the temperature difference between mode volume and the surrounding, K (J/s °C) is the thermal conductivity between the cavity mode volume and the surrounding. For a given pump light with the wavelength fixed at the cold cavity resonance-wavelength, one stable solution of ΔT for Eq. (1) can be calculated, inducing a stable red-shift of the resonance described as Eq. (2). In the calculations, we assume that Q factor of the MRR is 15,000 with the resonant wavelength of 1550 nm, indicating a resonance width of 0.1 nm, the temperature coefficient of resonance wavelength is 5.34*10 5 (1/°C) and thermal conductivity is 3.81(J/s °C). The red shift of the MRR dependent on the optical bias power is shown in Fig. 2. It should be noted that the high optical power coupled to the waveguide will cause the two photon absorption (TPA) and generate excessive free carriers. These free carriers may induce free carrier dispersion (FCD) and free carrier absorption (FCA). The FCD will contribute to the modulation, which may only alter the external voltage applied in the modulation. Meanwhile, the FCA will result in nonlinear loss, which will increase the insertion loss of the modulator and can be compensated by optical amplifier. Therefore, both FCD and FCA will not have serious impact on the performance of the modulator. In addition, due to the external voltage, the free carriers will be swept out and the lifetime of the free carriers will be very short comparing to the modulation speed. Thus, we believe that the nonlinear effect induced by the high power will not significantly affect our experiment results. We can see that, the red-shift caused by thermo-optic effect can be as large as 0.42 nm when the optical bias power reaches 10 dBm. Meanwhile, the red-shift is continuously varied with the optical bias power. Furthermore, with the help of the plasma dispersion model in silicon, the transfer function of the MRR modulator under different optical bias powers can be
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S. Yan et al. / Optics Communications 368 (2016) 58–62
CW input T (dB) 0 -10 -20 0
Red-shift (nm)
0.4 T (dB) 0
0.3
TLS 1 1.2
-20
RF signal
TLS 2 0
0.6 Voltage (V)
T (dB)
1.2
High power EDFA
0
-10
0.1 0
OC
Optical bias
-10
0.2
Grating coupler
PC1 0.6 Voltage (V)
MRR
PC2
AWG
-20 0.6 Voltage (V)
0
1.2
BPF
0
2
4 6 Optical bias power (dBm)
8
10
CSA
VOA
Grating coupler
EDFA2
Fig. 2. Calculation results of optical–thermal effect and modulation curves. OSA
simultaneously obtained, as shown in the insets. The calculated curves under 0 dBm, 15 dBm and 20 dBm display positive OOK, BPSK and reversed OOK modulation, respectively. Next we fabricated the MRR modulator on the commercial Silicon-on-Isolator (SOI) platform to experimentally demonstrate our proposal. The 220 nm top silicon layer was etched through to form strip waveguides consisting of a ring waveguide and two straight waveguides, with boron and phosphorus ion implantations performed to form the highly p-type and n-type doped regions near the ring waveguide to modulate the input signal, which employed carrier-injection mode according to our design. Also the slab layer was etched outside the p–i–n junctions to confine the current flow around the ring waveguide. Finally, contact holes were etched and aluminum was deposited to form the signal– ground–signal (S–G–S) electrode connection, as shown in Fig. 3(a). Fig. 3(b) shows the microscope images of the zoom in ring region of the MRR. The MRR in our work possess a ring waveguide with the radius of 12 m and two straight waveguides with the width of 500 nm. Meanwhile, the gap between the add port and the ring is 250 nm and the gap between the drop port and the ring is 280 nm.
3. Experimental results and discussion The schematic diagram for the proposed optical-biased modulator is shown in Fig. 4. A CW light is emitted by tunable laser source (TLS) 1 as input and TLS 2 acts as optical bias. Meanwhile, TLS 1 and TLS 2's wavelengths are respectively fixed at two different resonant wavelengths of the MRR, i.e., 1558.3 nm and 1548.8 nm based on our measurement. Both the powers of CW light emitted from two TLSs are 10 dBm. In particular, the light from TLS 2 is amplified by a high-power erbium doped fiber amplifier (EDFA) to induce significant thermo-optical effect in the MRR. All the optical bias powers mentioned below are measured at the output of the EDFA. The measured Q factor of the fabricated MRR is about 12,000, which is sufficient for effective red-shift according to our previous simulations. Afterwards, both CW input and bias input are coupled to the fabricated MRR modulator via the vertical grating coupler. The insertion loss of grating coupler is 7 dB for each side. The output light is coupled out by another grating coupler. At last, bias light is filtered out by a band pass
S
G
Fig. 4. Schematic diagram for optical-biased modulator.
filter (BPF) before the output signal is recorded and analyzed by communication signal analyzer (CSA Tektronix 8000B) and optical spectrum analyzer (OSA, YOKOGAWA AQ6370C), respectively. We first measure the optical-biased modulator's performance under DC voltages and optical bias. Fig. 5(a) shows the measured normalized transmission spectra of the MRR modulator with different DC voltages. The 5 dB insertion loss mainly results from the fabrication error such as sidewall roughness. The Q factor and extinction ration (ER) of the fabricated MRR is estimated to be approximately 12,000 and 16 dB respectively with no external voltage applied. When the applied DC voltages vary from 0.6 V to 0.9 V, the transmission spectrum experiences a maximum of 0.69 nm blue-shift. Besides, the measured extinction ratio of the signal from the transmission spectrum is about 10 dB when the input CW light's wavelength is fixed at 1558.3 nm. Next we measure the transmission spectrum under various optical bias power, as shown in Fig. 5(b). The refractive index of silicon increases as the device's temperature increases with higher optical power injection, thus the transmission spectrum is red-shifted. Red-shift of 0.71 nm is achieved when the bias power is 30 dBm and it can be continuous tuning by the optical bias power. In the meantime, the ER of the MRR experiences a slightly decrease due to the TPA effect on silicon's transmission loss. Finally, the transfer functions of the modulator are obtained, as shown in Fig. 5(c). When no optical bias is employed, the blue curve depicts the common positive NRZ-OOK modulation. When optical bias power is 24 dBm, the measured modulation curve is shown as pink curve, which is the BPSK modulation. When the optical bias power is increased to 28 dBm, modulation curve is reversed compared to the initial case, implying that reversed NRZ-OOK signal can be generated. Therefore, the experiment results above demonstrate that all the signal formats mentioned above, i.e., positive NRZ-OOK, BPSK and reversed NRZ-OOK can be switched conveniently by only changing optical bias power. The dynamic response of the optical-biased modulator is also measured to directly verify our scheme. Electrical NRZ-OOK RF signal is generated by an arbitrary waveform generator to drive the MRR modulator, shown in Fig. 6(a) and (e), with the bit rates of
S through add
drop
Fig. 3. Microscope image of the modulator. (a) General view, (b) zoom in region of the MRR.
0
-5
(a)
-10
0V 0.6V 0.7V 0.8V 0.9V
-15 Blue shift
-20
Transmittance (dB)
Transmittance (dB)
S. Yan et al. / Optics Communications 368 (2016) 58–62
1558.5 1558 1557.5 Wavelength (nm) Transmittance (dB)
0
(c)
0 -5
61
(b) 0 mw 24dBm 26dBm 28dBm Red shift 30dBm
-10
-15 -20
1558 1559 1558.5 Wavelength (nm)
No opitcal bias Optical bias=24dBm Optical bias=28dBm
-5
-10 -15 -20
0
0.2
0.4 0.6 Voltage (V)
0.8
1
Fig. 5. Static performance. (a) Blue shift under external voltage. (b) Red shift under optical bias. (c) Transfer functions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0.2 Gbit/s and 0.4 Gbit/s, respectively. The peak to peak voltage is 0.7 V, from 0.2 V to 0.9 V. When the optical bias is not applied, the positive NRZ-OOK modulated signal is obtained, shown in Fig. 6 (b) and (f), with the ER of 6.6 dB and 5.7 dB for 0.2 Gbit/s and 0.4 Gbit/s cases. When we adjust the power of the optical bias to 25.8 dBm, BPSK signal at the bit rates of 0.2 Gbit/s is acquired, shown in Fig. 6(c). For 0.4 Gbit/s input RF signal, the optical bias power required for BPSK signal is 24.9 dBm and the output waveforms are depicted in Fig. 6(g). When the optical bias is increased to 29.1 dBm, output is reversed NRZ-OOK signal at the bit rate of 0.2 Gbit/s with the ER of 5.4 dB, shown in Fig. 6(d). Meanwhile, the optical bias power employed for 0.4 Gbit/s input signal is adjusted to 28.1 dBm, the ER is 5.1 dB, depicted in Fig. 6 (h). All the ERs of the signal's formats mentioned above are higher than 5 dB. Therefore, we can ensure that the optical bias is able to switch the output signal formats between positive NRZ-OOK, BPSK
and reversed NRZ-OOK successfully by tuning the optical power at the highest bit rates of 0.4 Gbit/s according to our time domain measurement. Besides, it should be specially pointed out that “thermal lock” technology, which ensure the system does not jump out of the resonance [8], is not required in our experiments because the modulation can be accomplished when the red-shift is stable. We also believe that this stable state is hard to be break even if the MRR is modulated by RF signal because the modulation speed here is very high compared to the thermal effect’s response time (usually at millisecond level). We have noticed that there are significant differences between the optical bias powers required in the experiments and simulation results. In the calculations, we assume the optical power is directly injected to the MRR without considering the loss caused by vertical grating couplers and the fabrication roughness, which can be as much as 20 dB in total in our experiments. Thus the
Input @0.4Gbit/s 10ns/div
Input @0.2Gbit/s 20ns/div
(a)
(e) No optical bias
(b)
No optical bias
(f) Optical bias=25.8dBm
(c)
Optical bias=24.9dBm
(g) Optical bias=28.1dBm
Optical bias=29.1dBm
(d)
(h)
Fig. 6. Time domain measurement results. (a)–(d): input and three output signal formats of 0.2 Gbit/s bite rate. (e)–(h): input and three output signal formats of 0.4 Gbit/s bite rate.
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S. Yan et al. / Optics Communications 368 (2016) 58–62
actual optical power employed in our experiments is much higher than our simulations. Fortunately, the power consumption can be greatly reduced if the coupling efficiency of the grating couplers is enhanced, such as using the inverted taper couplers rather than vertical grating couplers in our experiments [11]. Additionally, power consumption can benefit from higher Q of the MRR for stronger resonance under same optical bias power and smaller red-shift required for the same ER. Besides, when the RF driving signal's speed is increased to 0.8 Gbit/s, the modulated signal's quality is seriously deteriorated. Although our MRR can only work at about 0.4 Gbit/s, it is feasible to improve optical-bias modulator's speed by optimizing both electrical and optical characteristics of the device, which has been put in great efforts in recent decades [12–16], such as to employ carrier-depletion effect in a reversed PN diode or to employ coupled-ring-resonator structure. It should be clarified that in our opinion, the optical bias is not likely to lower the modulation speed, which is intrinsically limited by its electrical or optical characteristics. More importantly, with application of the achievements in high speed modulator above, the working mechanism of optical-biased can still be utilized as long as resonantbased modulator is employed. In addition, as the previous research illustrated [17], the high pump power may interact with the modulated signal in resonant cavities under critical coupling condition and generate idler signal as a result of the four wavelength mixing (FWM). The efficiency of the conversion could be close to 0 dB, which may possibly deteriorate the signal quality of our proposed scheme. Fortunately, the generated idler signal will be filtered out by the BPF along with the pump signal, which makes the modulated signal free from the interference. Meanwhile, the dispersion property of the waveguide in our scheme is not specially designed for the anomalous dispersion. This will significantly decrease the conversion efficiency and in turn ensure our modulated signal to avoid potential impact from the FWM.
4. Conclusions In conclusion, we propose and experimentally demonstrate a novel optical-biased modulator employing a single micro-ring resonator. By adjusting optical bias power, the MRR modulator is capable of generating positive OOK, BPSK and reversed OOK at the highest bit rate of 0.4 Gbit/s with good extinction ratio. The optical bias proposed in this work can replace the electrical DC bias thus to protect the optical devices from potential damage induced by high DC voltage. Meanwhile, excess electrical devices such as bias tee can be removed by optical bias to reduce the optical interconnection system's complexity and pave way of the future's alloptical integration.
Acknowledgment This work was partially supported by the National Basic Research Program of China (Grant No. 2011CB301704), the Program for New Century Excellent Talents in Ministry of Education of the People's Republic of China (Grant no. NCET-11-0168), a Foundation for Author of National Excellent Doctoral Dissertation of the People's Republic of China (Grant no. 201139), the National Natural Science Foundation of China (Grant no. 11174096 and 61475052). The authors would like to thank Dr. Xi Xiao and Dr. Qi Yang in the State Key Laboratory of Optical Communication Technologies and Networks, Wuhan Research Institute of Posts &Telecommunications for the assistance in the experiments.
References [1] G.T. Reed, G. Mashanovich, F. Gardes, D. Thomson, Silicon optical modulators, Nat. Photonics 4 (2010) 518–526. [2] H. Xu, X. Li, X. Xiao, P. Zhou, Z. Li, J. Yu, et al., High-speed silicon modulator with band equalization, Opt. Lett. 39 (2014) 4839–4842. [3] E. Timurdogan, C.M. Sorace-Agaskar, J. Sun, E.S. Hosseini, A. Biberman, M. R. Watts, An ultralow power athermal silicon modulator, Nat. Commun. 5 (2014). [4] X. Li, X. Feng, K. Cui, F. Liu, Y. Huang, Integrated silicon modulator based on microring array assisted MZI, Opt. Express 22 (2014) 10550–10558. [5] P. Dong, W. Qian, H. Liang, R. Shafiiha, D. Feng, G. Li, et al., Thermally tunable silicon racetrack resonators with ultralow tuning power, Opt. Express 18 (2010) 20298–20304. [6] E.L. Wooten, R.L. Stone, E.W. Miles, E.M. Bradley, Rapidly tunable narrowband wavelength filter using LiNbO3 unbalanced Mach–Zehnder interferometers, J. Lightwave Technol. 14 (1996) 2530–2536. [7] R.A. Soref, B.R. Bennett, Electrooptical effects in silicon, IEEE J. Quantum Electron. vol. 23 (1987) 123–129. [8] T. Carmon, L. Yang, K. Vahala, Dynamical thermal behavior and thermal selfstability of microcavities, Opt. Express 12 (2004) 4742–4750. [9] G.T. Reed, A.P. Knights, Silicon Photonics: an Introduction, John Wiley & Sons, West Sussex, England, 2004. [10] T.J. Johnson, M. Borselli, O. Painter, Self-induced optical modulation of the transmission through a high-Q silicon microdisk resonator, Opt. Express 14 (2006) 817–831. [11] M. Pu, L. Liu, H. Ou, K. Yvind, J.M. Hvam, Ultra-low-loss inverted taper coupler for silicon-on-insulator ridge waveguide, Opt. Commun. 283 (2010) 3678–3682. [12] N.-N. Feng, S. Liao, D. Feng, P. Dong, D. Zheng, H. Liang, et al., High speed carrier-depletion modulators with 1.4 V-cm VπL integrated on 0.25 μm silicon-on-insulator waveguides, Opt. Express 18 (2010) 7994–7999. [13] F. Gardes, A. Brimont, P. Sanchis, G. Rasigade, D. Marris-Morini, L. O'Faolain, et al., High-speed modulation of a compact silicon ring resonator based on a reverse-biased pn diode, Opt. Express 17 (2009) 21986–21991. [14] Y. Li, L. Zhang, M. Song, B. Zhang, J.-Y. Yang, R.G. Beausoleil, et al., Coupledring-resonator-based silicon modulator for enhanced performance, Opt. Express 16 (2008) 13342–13348. [15] X. Xiao, H. Xu, X. Li, Y. Hu, K. Xiong, Z. Li, et al., 25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions, Opt. Express 20 (2012) 2507–2515. [16] J.-B. You, M. Park, J.-W. Park, G. Kim, 12.5 Gbps optical modulation of silicon racetrack resonator based on carrier-depletion in asymmetric pn diode, Opt. Express 16 (2008) 18340–18344. [17] A.C. Turner, M.A. Foster, A.L. Gaeta, M. Lipson, Ultra-low power parametric frequency conversion in a silicon microring resonator, Opt. Express 16 (2008) 4881–4887.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Invited Paper
Optical-biased modulator employing a single silicon micro-ring resonator Siqi Yan, Jianji Dong n, Aoling Zheng, Yuan Yu Wuhan National Laboratory for Optoelectronics & School of Optical and Electrical Information, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2015 Received in revised form 22 January 2016 Accepted 30 January 2016
We propose and experimentally demonstrate an optical-biased modulator employing a single silicon micro-ring resonator. By adjusting optical bias, the micro-ring modulator is capable of generating several modulation formats, namely, on–off keying, binary phase shift keying and reversed on–off keying, at the speed of 0.4 Gbit/s with extinction ratio higher than 5 dB. Compared to the previous reported bias control approaches, the optical bias proposed in this study is a novel mechanism, which can be easily conducted without complicated integrated structures or redundant electrical devices. Meanwhile, optical bias can also effectively protect the vulnerable integrated silicon devices from possible damage induced by high direct current voltage. & 2016 Elsevier B.V. All rights reserved.
Keywords: Integrated optics Modulators Optoelectronics
1. Introduction Integrated silicon optical modulator, which has long been a focus of the scientific community, is vitally important as one of the main functional elements for the future optical interconnection system [1]. Remarkable progress in silicon modulators is achieved in recent years, such as ultra-high speed silicon modulator [2], ultralow power consumption silicon modulator [3] and integrated silicon modulator with novel structures [4]. In previous researches, several approaches were proposed to control the bias of integrated modulators, such as using micro-heaters [5], tuning the signal's wavelength [6] and employing electrical bias [7]. Among the methods mentioned above, the employment of micro-heater on a silicon modulator greatly adds to the complexities during the fabrication process while tuning the signal wavelength may cause trouble in the high speed wavelength division multiplex (WDM) communication systems. Furthermore, as the most commonly used method at the present, electrical bias relies on some electrical devices such as bias tee to accomplish bias control in integrated modulators, which seriously impedes the all-optical integration and causes the system to be bulky. Meanwhile, because most integrated optical devices are extremely vulnerable to high voltage, the voltage of DC bias must be carefully adjusted or it can induce serious damage to the integrated optical modulators. In this letter, we propose and experimentally demonstrate an optical-biased modulator employing a single micro-ring resonator n
Corresponding author. E-mail address: [email protected] (J. Dong).
http://dx.doi.org/10.1016/j.optcom.2016.01.091 0030-4018/& 2016 Elsevier B.V. All rights reserved.
(MRR). We can obtain non-return-zero on–off keying (NRZ-OOK), binary phase shift keying (BPSK) and reversed NRZ-OOK signals at fixed wavelength in a single MRR modulator by adjusting the power of a continuous wavelength (CW) as the bias. The fundamental physics of the optical bias is the optical induced thermal effect, which causes the red shift of resonance in the MRR [8]. Compared to the previous methods of altering bias in the integrated silicon modulators, the employment of optical bias neither requires additional integrated structures such as micro-heaters nor depends on external electrical devices such as bias tee. Meanwhile, the optical bias can protect the modulators completely from potential harm induced by high DC voltage. Hopefully, the optical manipulation of the modulator bias proposed in this study can overcome most disadvantages of previous methods. With ultra-small footprint, the optical-biased modulator can also reduce the interconnect systems complexity and pave the way of all-optical integrated modulator as a novel way of on-chip optical manipulation.
2. Operation principle Fig. 1 gives an overview about the working principle of opticalbiased modulator. Assume that the wavelength of input continuous wavelength (CW) light is aligned with the MRR resonant wavelength. Meanwhile, another CW light whose wavelength is aligned with another resonant wavelength acting as the optical bias. When no optical bias is applied, the transmission of through port of MRR is positively varied as the voltage applied on the MRR is increased, due to the plasma dispersion effect induced by
S. Yan et al. / Optics Communications 368 (2016) 58–62
59
Voltage
0
Time
RF Signal
Power
MRR
Power
Power
Time
Phase
Time Power
Time
Power
Reverse OOK BPSK Positive OOK
0
Time
Time
Optical bias Fig. 1. Overview about the working principle.
external voltage [9]. In this case, the CW light is positively modulated by electrical frequency (RF) signal. Thus positive NRZOOK signal can be obtained, as the blue waveforms show. When we add an optical bias, the input light will heat the high quality factor (Q) MRR remarkably owing to its strong resonance phenomenon. Meanwhile, significant red-shift of the initial transmission spectrum is generated due to the large thermo-optic coefficient in silicon. In this way, the transfer function of the modulator, i.e., the transmission of the MRR at the resonant wavelength with different external voltages, can be altered to obtain different signal formats. Via choosing the power of optical bias properly, we expect the modulator to be capable of generating both BPSK and reversed NRZ-OOK signal, as the waveforms in pink and brown color show respectively. To verify the feasibility of the optical bias, we calculate the redshift of the MRR transmission spectrum under different optical bias power. In the calculations, we believe that the thermal effect caused by high power light is the dominant phenomenon under these circumstances, despite the fact that the free carriers generated from two photons absorption may induce blue shift when the heating just starts [10]. According to theoretical model of optical induced thermal effects in the high-Q micro-cavity community, the red-shift response of the MRR can attain a steady red-shift state finally under given optical power. The Eqs. (1) and (2) gives the relationship between the input optical power and the corresponding amount of red-shift [8].
0 = Ih
1
(
λp − λr 2 Δλ / 2
)
− K ΔT +1
λr = λ 0 (1 + aΔT )
(1)
(2)
where Ih is the optical power that heats the cavity λP , λ 0 and λr
represent the input wavelength, the cold cavity resonance-wavelength and resonance wavelength after red-shift respectively, α is the temperature coefficient of resonance-wavelength, ΔT is the temperature difference between mode volume and the surrounding, K (J/s °C) is the thermal conductivity between the cavity mode volume and the surrounding. For a given pump light with the wavelength fixed at the cold cavity resonance-wavelength, one stable solution of ΔT for Eq. (1) can be calculated, inducing a stable red-shift of the resonance described as Eq. (2). In the calculations, we assume that Q factor of the MRR is 15,000 with the resonant wavelength of 1550 nm, indicating a resonance width of 0.1 nm, the temperature coefficient of resonance wavelength is 5.34*10 5 (1/°C) and thermal conductivity is 3.81(J/s °C). The red shift of the MRR dependent on the optical bias power is shown in Fig. 2. It should be noted that the high optical power coupled to the waveguide will cause the two photon absorption (TPA) and generate excessive free carriers. These free carriers may induce free carrier dispersion (FCD) and free carrier absorption (FCA). The FCD will contribute to the modulation, which may only alter the external voltage applied in the modulation. Meanwhile, the FCA will result in nonlinear loss, which will increase the insertion loss of the modulator and can be compensated by optical amplifier. Therefore, both FCD and FCA will not have serious impact on the performance of the modulator. In addition, due to the external voltage, the free carriers will be swept out and the lifetime of the free carriers will be very short comparing to the modulation speed. Thus, we believe that the nonlinear effect induced by the high power will not significantly affect our experiment results. We can see that, the red-shift caused by thermo-optic effect can be as large as 0.42 nm when the optical bias power reaches 10 dBm. Meanwhile, the red-shift is continuously varied with the optical bias power. Furthermore, with the help of the plasma dispersion model in silicon, the transfer function of the MRR modulator under different optical bias powers can be
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S. Yan et al. / Optics Communications 368 (2016) 58–62
CW input T (dB) 0 -10 -20 0
Red-shift (nm)
0.4 T (dB) 0
0.3
TLS 1 1.2
-20
RF signal
TLS 2 0
0.6 Voltage (V)
T (dB)
1.2
High power EDFA
0
-10
0.1 0
OC
Optical bias
-10
0.2
Grating coupler
PC1 0.6 Voltage (V)
MRR
PC2
AWG
-20 0.6 Voltage (V)
0
1.2
BPF
0
2
4 6 Optical bias power (dBm)
8
10
CSA
VOA
Grating coupler
EDFA2
Fig. 2. Calculation results of optical–thermal effect and modulation curves. OSA
simultaneously obtained, as shown in the insets. The calculated curves under 0 dBm, 15 dBm and 20 dBm display positive OOK, BPSK and reversed OOK modulation, respectively. Next we fabricated the MRR modulator on the commercial Silicon-on-Isolator (SOI) platform to experimentally demonstrate our proposal. The 220 nm top silicon layer was etched through to form strip waveguides consisting of a ring waveguide and two straight waveguides, with boron and phosphorus ion implantations performed to form the highly p-type and n-type doped regions near the ring waveguide to modulate the input signal, which employed carrier-injection mode according to our design. Also the slab layer was etched outside the p–i–n junctions to confine the current flow around the ring waveguide. Finally, contact holes were etched and aluminum was deposited to form the signal– ground–signal (S–G–S) electrode connection, as shown in Fig. 3(a). Fig. 3(b) shows the microscope images of the zoom in ring region of the MRR. The MRR in our work possess a ring waveguide with the radius of 12 m and two straight waveguides with the width of 500 nm. Meanwhile, the gap between the add port and the ring is 250 nm and the gap between the drop port and the ring is 280 nm.
3. Experimental results and discussion The schematic diagram for the proposed optical-biased modulator is shown in Fig. 4. A CW light is emitted by tunable laser source (TLS) 1 as input and TLS 2 acts as optical bias. Meanwhile, TLS 1 and TLS 2's wavelengths are respectively fixed at two different resonant wavelengths of the MRR, i.e., 1558.3 nm and 1548.8 nm based on our measurement. Both the powers of CW light emitted from two TLSs are 10 dBm. In particular, the light from TLS 2 is amplified by a high-power erbium doped fiber amplifier (EDFA) to induce significant thermo-optical effect in the MRR. All the optical bias powers mentioned below are measured at the output of the EDFA. The measured Q factor of the fabricated MRR is about 12,000, which is sufficient for effective red-shift according to our previous simulations. Afterwards, both CW input and bias input are coupled to the fabricated MRR modulator via the vertical grating coupler. The insertion loss of grating coupler is 7 dB for each side. The output light is coupled out by another grating coupler. At last, bias light is filtered out by a band pass
S
G
Fig. 4. Schematic diagram for optical-biased modulator.
filter (BPF) before the output signal is recorded and analyzed by communication signal analyzer (CSA Tektronix 8000B) and optical spectrum analyzer (OSA, YOKOGAWA AQ6370C), respectively. We first measure the optical-biased modulator's performance under DC voltages and optical bias. Fig. 5(a) shows the measured normalized transmission spectra of the MRR modulator with different DC voltages. The 5 dB insertion loss mainly results from the fabrication error such as sidewall roughness. The Q factor and extinction ration (ER) of the fabricated MRR is estimated to be approximately 12,000 and 16 dB respectively with no external voltage applied. When the applied DC voltages vary from 0.6 V to 0.9 V, the transmission spectrum experiences a maximum of 0.69 nm blue-shift. Besides, the measured extinction ratio of the signal from the transmission spectrum is about 10 dB when the input CW light's wavelength is fixed at 1558.3 nm. Next we measure the transmission spectrum under various optical bias power, as shown in Fig. 5(b). The refractive index of silicon increases as the device's temperature increases with higher optical power injection, thus the transmission spectrum is red-shifted. Red-shift of 0.71 nm is achieved when the bias power is 30 dBm and it can be continuous tuning by the optical bias power. In the meantime, the ER of the MRR experiences a slightly decrease due to the TPA effect on silicon's transmission loss. Finally, the transfer functions of the modulator are obtained, as shown in Fig. 5(c). When no optical bias is employed, the blue curve depicts the common positive NRZ-OOK modulation. When optical bias power is 24 dBm, the measured modulation curve is shown as pink curve, which is the BPSK modulation. When the optical bias power is increased to 28 dBm, modulation curve is reversed compared to the initial case, implying that reversed NRZ-OOK signal can be generated. Therefore, the experiment results above demonstrate that all the signal formats mentioned above, i.e., positive NRZ-OOK, BPSK and reversed NRZ-OOK can be switched conveniently by only changing optical bias power. The dynamic response of the optical-biased modulator is also measured to directly verify our scheme. Electrical NRZ-OOK RF signal is generated by an arbitrary waveform generator to drive the MRR modulator, shown in Fig. 6(a) and (e), with the bit rates of
S through add
drop
Fig. 3. Microscope image of the modulator. (a) General view, (b) zoom in region of the MRR.
0
-5
(a)
-10
0V 0.6V 0.7V 0.8V 0.9V
-15 Blue shift
-20
Transmittance (dB)
Transmittance (dB)
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1558.5 1558 1557.5 Wavelength (nm) Transmittance (dB)
0
(c)
0 -5
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(b) 0 mw 24dBm 26dBm 28dBm Red shift 30dBm
-10
-15 -20
1558 1559 1558.5 Wavelength (nm)
No opitcal bias Optical bias=24dBm Optical bias=28dBm
-5
-10 -15 -20
0
0.2
0.4 0.6 Voltage (V)
0.8
1
Fig. 5. Static performance. (a) Blue shift under external voltage. (b) Red shift under optical bias. (c) Transfer functions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0.2 Gbit/s and 0.4 Gbit/s, respectively. The peak to peak voltage is 0.7 V, from 0.2 V to 0.9 V. When the optical bias is not applied, the positive NRZ-OOK modulated signal is obtained, shown in Fig. 6 (b) and (f), with the ER of 6.6 dB and 5.7 dB for 0.2 Gbit/s and 0.4 Gbit/s cases. When we adjust the power of the optical bias to 25.8 dBm, BPSK signal at the bit rates of 0.2 Gbit/s is acquired, shown in Fig. 6(c). For 0.4 Gbit/s input RF signal, the optical bias power required for BPSK signal is 24.9 dBm and the output waveforms are depicted in Fig. 6(g). When the optical bias is increased to 29.1 dBm, output is reversed NRZ-OOK signal at the bit rate of 0.2 Gbit/s with the ER of 5.4 dB, shown in Fig. 6(d). Meanwhile, the optical bias power employed for 0.4 Gbit/s input signal is adjusted to 28.1 dBm, the ER is 5.1 dB, depicted in Fig. 6 (h). All the ERs of the signal's formats mentioned above are higher than 5 dB. Therefore, we can ensure that the optical bias is able to switch the output signal formats between positive NRZ-OOK, BPSK
and reversed NRZ-OOK successfully by tuning the optical power at the highest bit rates of 0.4 Gbit/s according to our time domain measurement. Besides, it should be specially pointed out that “thermal lock” technology, which ensure the system does not jump out of the resonance [8], is not required in our experiments because the modulation can be accomplished when the red-shift is stable. We also believe that this stable state is hard to be break even if the MRR is modulated by RF signal because the modulation speed here is very high compared to the thermal effect’s response time (usually at millisecond level). We have noticed that there are significant differences between the optical bias powers required in the experiments and simulation results. In the calculations, we assume the optical power is directly injected to the MRR without considering the loss caused by vertical grating couplers and the fabrication roughness, which can be as much as 20 dB in total in our experiments. Thus the
Input @0.4Gbit/s 10ns/div
Input @0.2Gbit/s 20ns/div
(a)
(e) No optical bias
(b)
No optical bias
(f) Optical bias=25.8dBm
(c)
Optical bias=24.9dBm
(g) Optical bias=28.1dBm
Optical bias=29.1dBm
(d)
(h)
Fig. 6. Time domain measurement results. (a)–(d): input and three output signal formats of 0.2 Gbit/s bite rate. (e)–(h): input and three output signal formats of 0.4 Gbit/s bite rate.
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actual optical power employed in our experiments is much higher than our simulations. Fortunately, the power consumption can be greatly reduced if the coupling efficiency of the grating couplers is enhanced, such as using the inverted taper couplers rather than vertical grating couplers in our experiments [11]. Additionally, power consumption can benefit from higher Q of the MRR for stronger resonance under same optical bias power and smaller red-shift required for the same ER. Besides, when the RF driving signal's speed is increased to 0.8 Gbit/s, the modulated signal's quality is seriously deteriorated. Although our MRR can only work at about 0.4 Gbit/s, it is feasible to improve optical-bias modulator's speed by optimizing both electrical and optical characteristics of the device, which has been put in great efforts in recent decades [12–16], such as to employ carrier-depletion effect in a reversed PN diode or to employ coupled-ring-resonator structure. It should be clarified that in our opinion, the optical bias is not likely to lower the modulation speed, which is intrinsically limited by its electrical or optical characteristics. More importantly, with application of the achievements in high speed modulator above, the working mechanism of optical-biased can still be utilized as long as resonantbased modulator is employed. In addition, as the previous research illustrated [17], the high pump power may interact with the modulated signal in resonant cavities under critical coupling condition and generate idler signal as a result of the four wavelength mixing (FWM). The efficiency of the conversion could be close to 0 dB, which may possibly deteriorate the signal quality of our proposed scheme. Fortunately, the generated idler signal will be filtered out by the BPF along with the pump signal, which makes the modulated signal free from the interference. Meanwhile, the dispersion property of the waveguide in our scheme is not specially designed for the anomalous dispersion. This will significantly decrease the conversion efficiency and in turn ensure our modulated signal to avoid potential impact from the FWM.
4. Conclusions In conclusion, we propose and experimentally demonstrate a novel optical-biased modulator employing a single micro-ring resonator. By adjusting optical bias power, the MRR modulator is capable of generating positive OOK, BPSK and reversed OOK at the highest bit rate of 0.4 Gbit/s with good extinction ratio. The optical bias proposed in this work can replace the electrical DC bias thus to protect the optical devices from potential damage induced by high DC voltage. Meanwhile, excess electrical devices such as bias tee can be removed by optical bias to reduce the optical interconnection system's complexity and pave way of the future's alloptical integration.
Acknowledgment This work was partially supported by the National Basic Research Program of China (Grant No. 2011CB301704), the Program for New Century Excellent Talents in Ministry of Education of the People's Republic of China (Grant no. NCET-11-0168), a Foundation for Author of National Excellent Doctoral Dissertation of the People's Republic of China (Grant no. 201139), the National Natural Science Foundation of China (Grant no. 11174096 and 61475052). The authors would like to thank Dr. Xi Xiao and Dr. Qi Yang in the State Key Laboratory of Optical Communication Technologies and Networks, Wuhan Research Institute of Posts &Telecommunications for the assistance in the experiments.
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