- Email: [email protected]
Compact continuously tunable microwave photonic filters based on cascaded silicon microring resonators
Optics Communications 363 (2016) 128–133
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Compact continuously tunable microwave photonic filters based on cascaded silicon microring resonators Li Liu, Mengying He, Jianji Dong n 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 11 June 2015 Received in revised form 16 September 2015 Accepted 27 October 2015
We propose and experimentally demonstrate a photonic approach to achieving tunable bandpass microwave photonic filters (MPFs) based on cascaded microring resonators (CMRRs). The optical spectrum of the silicon CMRRs could offer two bandpass response to separately filter the optical carrier and one of the sidebands generated by the phase modulation. Thus we could achieve a bandpass MPF. Moreover, as the central frequencies and bandwidths of the two bandpass response can be tuned by adjusting the laser wavelength and voltages applied on one MRR, the central operating frequency or 3-dB bandwidth of the MPF can be continuously tuned in wide ranges respectively. A proof-of-concept experiment illustrates a central frequency tuning range from 19 GHz to 40 GHz, and a wide bandwidth tuning range from 5.5 GHz to 17.5 GHz. & 2015 Elsevier B.V. All rights reserved.
Keywords: Microwave photonic filter (MPF) Cascaded microring resonators (CMRRs) Integrated optics
1. Introduction Microwave photonic filters (MPFs) are of great importance to process microwave signals in the radar, satellite and wireless communication systems, due to its huge bandwidth, electromagnetic immunity as well as tunability and reconfigurability [1– 5]. To date, many tunable MPFs have been proposed in order to process random and unpredictable microwave signals [6–10]. It means that the central operating frequency or bandwidth of MPFs is required to be tuned [11,12]. Among these schemes, bandpass filters attract sustained attentions and play a critical role in microwave photonic systems to enable the selectivity of desired spectral contents and signal monitoring [13]. Numerous bandpass MPFs have been previously demonstrated utilizing stimulated Brillouin scattering [14–16], variable optical carrier time shift (VOCTS) method [17], multiple sources and dispersion medium [18]. Compared to these fiber optic systems, silicon-based waveguides can offer distinct advantages of increased stability, compactness, a very high index contrast and the availability of CMOS fabrication technology [19]. In recent years, some MPFs based on silicon photonics technology have been demonstrated with good performance [5,12]. However, different problems still exist which are urgent to be overcome. For example, the design and operation of the device in Ref. [12] are a little complex and difficult because a number of parameters are required to be considered. In addition, n
Corresponding author. E-mail address: [email protected] (J. Dong).
http://dx.doi.org/10.1016/j.optcom.2015.10.058 0030-4018/& 2015 Elsevier B.V. All rights reserved.
the reconfigurability of the schemes based on stimulated Brillouin scattering (SBS) is limited due to the intrinsic narrow bandwidth of the nonlinear effect [13–15]. Besides, in our previous scheme [5], the central frequency and bandwidth of the MPF cannot be tuned continuously. Therefore, it is highly required to develop an integrated device to realize compact MPFs whose central frequency or bandwidth could be continuously tuned in wide ranges. In this paper, we propose and demonstrate a tunable MPF based on cascaded microring resonators (CMRRs). In the experiment, by adjusting the laser wavelength and voltages on only one MRR, the central operating frequency or 3-dB bandwidth of the MPF can be continuously tuned in wide ranges respectively. Our scheme offers an integrated and compact solution for tunable MPFs.
2. Operation principle When a radio frequency (RF) is modulated onto an optical carrier (f0) by a phase modulator (PM) under small signal modulation, there are an optical carrier (f0) and both sidebands (f0 þ f1 and f0 f1) in the frequency domain. The proposed MPF is based on the three CMRRs with different radii, i.e., MRR1 (resonant wavelengths of λ1 and λ′1 ), MRR2 (resonant wavelengths of λ2 and λ′2) and MRR3 (resonant wavelengths of λ3 and λ′3), as shown in Fig. 1 (a). By properly designing the radii of the CMRRs, λ2 could be close to λ3 to merge as the right bandpass but both of them depart from λ1 in one free spectral range (FSR1), shown as the blue line. Therefore, an optical processor of two bandpass response could be
L. Liu et al. / Optics Communications 363 (2016) 128–133
129
FSR1
FSR2
Output
MRR1
λ'1 λ'3
λ'2
λ2 λ3
λ1
MRR2
MRR3
Input
FSR1: reconfigurability of 3-dB bandwidth f0
Carrier
RF power
Optical power
f1 One sideband
Freq
Freq
f1
FSR2: tunability of central frequency f0 f2 f3
RF power
Optical power
f0
Freq
Freq
f2 f3 Fig. 1. Schematic diagram of the tunable MPF. (a) Schematic structure and response of the three CMRRs. (b and c) Illustration of the reconfigurability of 3-dB bandwidth, where (b) and (c) are the optical spectrum of CMRRs and RF spectrum of MPF respectively. (d and e) Tunability of central frequency, where (d) and (e) are the optical spectrum of CMRRs and RF spectrum of MPF respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
achieved in order to separately process the optical carrier (f0) and one sideband (f0 þf1), and the other sideband (f0 f1) out of the bandpass response would be suppressed. In another FSR (named with FSR2), λ′1 could be designed closely to λ′3 to merge as the right bandpass but both of them separating from λ′2, shown as the green line. It should be noted that MRR2 has been fabricated with metal electrodes to allow tuning its resonance (λ2 or λ′2). Fig. 1(b) and (c) illustrates the reconfigurability of 3-dB bandwidth of the MPF. As shown in Fig. 1(b), assume that f0 is located at the resonant peak of the MRR1 (λ1 in FSR1). After detecting by the square-law photodetector (PD), the optical signal is converted to electrical signal and the frequency response of the RF signal reveals a bandpass filter which is shown as the blue line in Fig. 1(c). Obviously, the central frequency of RF response is f1. If we adjust the voltage applied on MRR2 to tune λ2, the bandwidth of the right merged bandpass can be changed, shown as the red line in Fig. 1 (b). In this way, the bandwidth of RF response is changed accordingly while keeping the central frequency constant with f1, as shown in Fig. 1(c). On the other hand, Fig. 1(d) and (e) illustrates the tunability of central frequency of the MPF. In Fig. 1(d), by
adjusting the voltages applied on MRR2 to shift the left resonant peak, i.e. λ′2 in FSR2, and meanwhile aligning the laser wavelength with this peak, one may tune the central frequency of RF response from f2 to f3 with constant bandwidth, shown as the green and red lines in Fig. 1(e). Therefore, a tunable bandpass MPF has been successfully built by tuning the resonance of MRR2, but the central frequency and 3-dB bandwidth could not be tune simultaneously. The frequency response of the bandpass MPF can also be deduced analytically. Assume that the optical carrier is modulated in a phase modulator (PM) by the RF signal. In the case of phase modulation, the output optical field of the PM can be described by
Eout (t ) = E0e j[ω0t + m cos(ωRF t )]
(1)
where E0 is amplitude of the input optical field, ω0 is the angular frequency of the input optical signal, ωRF is angular frequency of the RF signal and m is the phase modulation depth. Neglecting the high order side-band, the output optical field of the PM under a small signal model can be described by
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L. Liu et al. / Optics Communications 363 (2016) 128–133
Eout (t ) = E0[jJ1 (m)e j(ω0 − ωRF )t + J0 (m)e jω0t + jJ1 (m)e j(ω0 + ωRF )t ]
(2)
where Jn is the nth-order Bessel function of the first kind. After the signal is transmitted in the CMRRs with an amplitude transmission function of H(ω), each frequency component will multiply a different frequency weight of H(ω), so the output optical field can be described as
E(ω) = 2πE0[jJ1 (m)H (ω0 − ωRF ) + J0 (m)H (ω0) + jJ1 (m)H (ω0 + ωRF )] (3) After detecting of the square-law PD, and neglecting the J12 term, we obtain the alternative current (AC) term of the current, which is equal to the product of E(ω) and its conjugation.
iAC ∝ 4π 2jE02J0 (m)J1 (m)[H *(ω0)H (ω0 + ωRF ) − H (ω0)H*(ω0 − ωRF )]
(4)
To experimentally demonstrate the compact MPF, we design and fabricate the CMRRs on a commercial silicon-on-insulator (SOI) wafer. The thickness of the top silicon and the buried oxide layer of the SOI wafer are 220 nm and 2 μm, respectively. We employ deep ultraviolet (DUV) photolithography using a 248-nm stepper to define the waveguide patterns, and then anisotropic dry etch of silicon is employed. Boron and phosphorus ion implantations are performed to form the highly p-type and n-type doped regions. Also the slab layer is etched outside the p–i–n junctions to confine the current flow around the ring waveguide. Finally, contact holes are etched and aluminum is deposited to form the metal connection. The whole fabrication process is done using CMOScompatible processes. Fig. 2(a) shows the micrographs of the onchip CMRRs. As the resonant wavelengths of the CMRRs are required to meet the conditions in Fig. 1(a), the radii of the three MRRs are designed to be 54.44 mm, 57.03 mm and 59.88 mm, respectively. The zoom-in region of MRR2 is shown as Fig. 2(b) with which fabricated on a pair of metal electrodes to allow tuning. When MRR2 is applied on voltages, the refractive index and transmission loss of the waveguide would be changed due to the free-carrier plasma dispersion effect. Thus the resonant wavelength of MRR2 would be blue-shift. The waveguide widths of both bus waveguides and bending waveguides are about 500 nm. The coupling gap between the bus waveguide and bending waveguide is about 330 nm. We employ the vertical grating coupler to couple the optical signal from fiber to silicon waveguide, and the zoom-in grating coupler is shown in Fig. 2(c). The period of the grating coupler is 610 nm. The trench width changes linearly from 180 nm
to 340 nm and the etch depth is 70 nm. Fig. 3(a) shows the measured transmission spectrum of the CMRRs using an optical spectrum analyzer (AQ6370B). The maximum extinction ratio is about 30 dB and the transmission loss of the CMRRs is around 2 dB after subtracting the 16-dB coupling loss of the grating couplers. It is clear to see that two bandpass response appear in two regions which are located in the second and fourth FSRs respectively. Fig. 3(b) is the zoom-in spectrum of the second FSR. The overlapped resonant wavelength for MRR1 and MRR2 is around 1554.00 nm and the corresponding resonant peak of MRR3 is 1554.20 nm, so the frequency interval of the two bandpass response is about 25 GHz. The zoom-in spectrum of the fourth FSR is shown as Fig. 3(c). The overlapped resonant wavelength for MRR1 and MRR3 is around 1557.84 nm and the corresponding resonant peak of MRR2 is 1557.69 nm, therefore the frequency interval of the two bandpass response is about 19 GHz.
3. Experimental results and discussion A proof-of-concept experiment for the tunable MPFs has been performed as shown in Fig. 4. A tunable laser source (TLS) emits a continuous-wave (CW) beam, which has a precisely tuning resolution of 1 pm. The CW beam is modulated by a PM, which has a modulation bandwidth of 40 GHz. Although the PM can be instead of an intensity modulator, it is known to all that the PM works better because of its bias-drift-free operation and low insertion loss. The vector network analyzer (VNA) (Anritsu 37369D) has a maximum sweeping frequency of 40 GHz. The RF signal from the VNA is amplified by an electrical amplifier (EA) and then applied to the PM. A forward erbium-doped fiber amplifier (EDFA) is used to boost the input optical power. As the silicon waveguide operates only in transverse electrical (TE) mode, a polarization controller (PC) is required. Then the modulation signal is sent into the CMRRs by the vertical grating coupler. After amplification by the backward EDFA, the optical signal is converted to electrical signal by the PD and analyzed by the VNA with 40 GHz bandwidth. It should be noted that the primary motivation is to make all the rings tunable, so all three MRRs are fabricated with heaters to achieve tunable center frequency and reconfigurable bandwidth. However, due to the absence of probe pin array, only one MRR could be tuned every time. Hence as a proof-of-concept
Cascaded microring resonators
25 µm
MRR1
MRR2
MRR3
Grating coupler MRR2 25 µm 10 µm Fig. 2. (a), (b) and (c) Micrographs of the CMRRs, zoom-in region of MRR2 and the grating coupler, respectively.
L. Liu et al. / Optics Communications 363 (2016) 128–133
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Optical power (dBm)
-20 -10
-30 -20
-40 -30
-50 -40
-60 -50 1552 1552
1556 1554
1560 1556
1558 1564
Wavelength (nm) -20 -10
25 GHz
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Optical power (dBm)
-20 -10 -30 -20 -40 -30 -50 -40
19 GHz
-30 -20 -40 -30 -50 -40 -60 -50
-60 -50 1,555 1553.45
1,556 1554.00
1,562.5 1557.05
1,557 1554.55
1,563.5 1557.75
1,564.5 1558.45
Wavelength (nm)
Wavelength (nm)
Fig. 3. (a) Measured spectrum of the CMRRs. (b) and (c) The zoom in spectra of the second FSR and fourth FSR, respectively.
experiment, we have designed another operation scheme with one tunable MRR and a tunable laser as a temporary solution, as shown in Fig. 1. By tuning the spectrum of the CMRRs, we can achieve a bandpass MPF whose central frequency and bandwidth are tunable respectively. In the experiment, considering the measured frequency intervals of the two bandpass response of the CMRRs, we choose the second FSR shown as Fig. 3(b) to realize the reconfigurability of the 3-dB bandwidth, and the fourth FSR shown as Fig. 3(c) to realize the tunability of the central frequency respectively. For reconfigurability of the 3-dB bandwidth of the MPF, the bandwidth of the merged bandpass (belongs to MRR1 and MRR2) in Fig. 3(b) should be varied. In this case, we adjust the voltages applied on MRR2 to tune the merged spectrum around 1554.00 nm and meanwhile fix the laser wavelength at 1554.20 nm. Therefore, the 3-dB bandwidth could be tuned while
VNA EA
EDFA2 ATT Chip PC1
PM
EDFA1
PC2
PD
TLS
Fig. 4. Experimental setup of the proposed scheme. TLS: tunable laser source, PC: polarization controller, PM: phase modulator, EDFA: erbium doped fiber amplifier, ATT: attenuator, PD: photodetector, EA: electrical amplifier, VNA: vector network analyzer.
Central frequency (GHz)
Normalized response (dB)
0 -10 -20
VMRR2=0V VMRR2=0.83V VMRR2=0.86V VMRR2=0.90V
-30 -40
0
10
20
30
Frequency (GHz)
40
40
25
30
20
20
15
10
6
10
14
Rejection ratio (dB)
Optical path Electrical path Coupling fiber
10 18
BW3dB (GHz)
Fig. 5. (a) Measured bandpass MPFs with tunability of 3-dB bandwidth. (b) Features of central frequency and rejection ratio.
0
BW3dB (GHz)
-10 -20
VMRR2=0V VMRR2=0.86V VMRR2=0.90V VMRR2=0.93V
-30 -40 0
10
20
30
40
8
24
7
21
6
18
5
20
25
30
35
Rejection ratio (dB)
L. Liu et al. / Optics Communications 363 (2016) 128–133
Normalized response (dB)
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15 40
Frequency (GHz)
Frequency (GHz)
Fig. 6. (a) Measured bandpass MPFs with tunability of central frequency. (b) Features of 3-dB bandwidth and rejection ratio.
keeps the central frequency approximately constant. Fig. 5 (a) shows that the bandwidth could be tuned from 5.5 GHz to 17.5 GHz with the constant central frequency of 25 GHz when we apply voltages of 0.00 V, 0.83 V, 0.86 V and 0.90 V on MRR2, respectively. The measured current is 0.7 mA when the voltage is 0.9 V, namely in this case the power consumption of the electrode is 0.63 mW. When the bandwidth of the merged left bandpass is varied, the frequency interval and extinction ratio of the two bandpass response basically do not change. Therefore, in this case, the central frequency of the MPF is approximate to a constant and the rejection ratio changes in a small range, as shown in Fig. 5(b). The rejection ratio is defined as the ratio of the maximum value and minimum value in the high-frequency passband. It should be noted that there is an unexpected bandpass in the low frequency. As mentioned above, given an optical carrier injected to a PM fed by an RF signal, it is well known that the generated sidebands are out of phase. Superficially, there is only a single bandpass response when we send this modulation signal into the chip, as shown in Fig. 1(b)–(d). However, the phase difference of the two sidebands is actually influenced by the phase spectrum of the CMRRs [20]. For example, in Fig. 1(b), when the optical carrier is aligned at the separate resonant peak (λ1), the two sidebands will experience opposite phase changes around λ1 due to the existence of the MRR1's phase spectrum. Hence, the two sidebands in the low frequency are not out of phase any more thus to generate an unexpected bandpass (named as the low bandpass). In the future, we could use some dispersive mediums, such as single-mode fiber (SMF) and dispersion compensating fiber (DCF) with proper lengths to relieve the phase changes of the two sidebands. Thus the low bandpass could be suppressed as expected. In this paper, our major target is to achieve tunable MPFs in the higher frequencies (named as the high bandpass). On the other hand, to realize tunability of the central frequency of the high bandpass, the location of the separate resonant peak (belongs to MRR2) in Fig. 3(c) should be tuned. Therefore, we adjust the voltages applied on MRR2 to tune its resonant peak to be blue-shifted around 1557.69 nm and meanwhile align the laser wavelength with this peak. In this way, the central frequency of the MPF is tunable with constant bandwidth. As shown in Fig. 6(a), the central frequency of the MPF could be tuned from 19 GHz to 40 GHz when we apply voltages of 0.00 V, 0.86 V, 0.90 V and 0.93 V on MRR2, respectively. Theoretically, the MPF is able to tune a higher frequency of its central frequency but restrained by the bandwidth of VNA in the experiment. Fig. 6(b) further demonstrates the 3-dB bandwidth and rejection ratio of the frequency response. It is clear that the 3-dB bandwidth is approximately a constant around 6.5 GHz and for most frequencies, the rejection ratio is larger than 20 dB. Therefore, the central frequency of the MPF is tunable from 19 GHz to 40 GHz with constant bandwidth of
6.5 GHz and over 20 dB rejection ratio. When the frequency is lower than 25 GHz, the rejection ratio decreases which is caused by the crosstalk induced by the overlap of the high bandpass and the low bandpass.
4. Conclusions A compact and tunable MPF has been proposed and demonstrated based on the CMRRs. In the experiment, the central frequency or 3-dB bandwidth of the MPF can be tuned respectively by adjusting the laser wavelength and voltage applied on one MRR. Experimental results show a central frequency tuning range from 19 GHz to 40 GHz, and a wide bandwidth tuning range from 5.5 GHz to 17.5 GHz. Our scheme offers an integrated MPF scheme which is significant in the microwave photonic systems.
Acknowledgments 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 China (Grant no. NCET-11-0168), a Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant no. 201139), the National Natural Science Foundation of China (Grant nos. 11174096 and 61475052), and the Opened Fund of the State Key Laboratory on Advanced Optical Communication System and Network (Grant no. 2015GZKF03004). The authors would like to thank Prof. Y. Yu for providing the VNA.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Compact continuously tunable microwave photonic filters based on cascaded silicon microring resonators Li Liu, Mengying He, Jianji Dong n 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 11 June 2015 Received in revised form 16 September 2015 Accepted 27 October 2015
We propose and experimentally demonstrate a photonic approach to achieving tunable bandpass microwave photonic filters (MPFs) based on cascaded microring resonators (CMRRs). The optical spectrum of the silicon CMRRs could offer two bandpass response to separately filter the optical carrier and one of the sidebands generated by the phase modulation. Thus we could achieve a bandpass MPF. Moreover, as the central frequencies and bandwidths of the two bandpass response can be tuned by adjusting the laser wavelength and voltages applied on one MRR, the central operating frequency or 3-dB bandwidth of the MPF can be continuously tuned in wide ranges respectively. A proof-of-concept experiment illustrates a central frequency tuning range from 19 GHz to 40 GHz, and a wide bandwidth tuning range from 5.5 GHz to 17.5 GHz. & 2015 Elsevier B.V. All rights reserved.
Keywords: Microwave photonic filter (MPF) Cascaded microring resonators (CMRRs) Integrated optics
1. Introduction Microwave photonic filters (MPFs) are of great importance to process microwave signals in the radar, satellite and wireless communication systems, due to its huge bandwidth, electromagnetic immunity as well as tunability and reconfigurability [1– 5]. To date, many tunable MPFs have been proposed in order to process random and unpredictable microwave signals [6–10]. It means that the central operating frequency or bandwidth of MPFs is required to be tuned [11,12]. Among these schemes, bandpass filters attract sustained attentions and play a critical role in microwave photonic systems to enable the selectivity of desired spectral contents and signal monitoring [13]. Numerous bandpass MPFs have been previously demonstrated utilizing stimulated Brillouin scattering [14–16], variable optical carrier time shift (VOCTS) method [17], multiple sources and dispersion medium [18]. Compared to these fiber optic systems, silicon-based waveguides can offer distinct advantages of increased stability, compactness, a very high index contrast and the availability of CMOS fabrication technology [19]. In recent years, some MPFs based on silicon photonics technology have been demonstrated with good performance [5,12]. However, different problems still exist which are urgent to be overcome. For example, the design and operation of the device in Ref. [12] are a little complex and difficult because a number of parameters are required to be considered. In addition, n
Corresponding author. E-mail address: [email protected] (J. Dong).
http://dx.doi.org/10.1016/j.optcom.2015.10.058 0030-4018/& 2015 Elsevier B.V. All rights reserved.
the reconfigurability of the schemes based on stimulated Brillouin scattering (SBS) is limited due to the intrinsic narrow bandwidth of the nonlinear effect [13–15]. Besides, in our previous scheme [5], the central frequency and bandwidth of the MPF cannot be tuned continuously. Therefore, it is highly required to develop an integrated device to realize compact MPFs whose central frequency or bandwidth could be continuously tuned in wide ranges. In this paper, we propose and demonstrate a tunable MPF based on cascaded microring resonators (CMRRs). In the experiment, by adjusting the laser wavelength and voltages on only one MRR, the central operating frequency or 3-dB bandwidth of the MPF can be continuously tuned in wide ranges respectively. Our scheme offers an integrated and compact solution for tunable MPFs.
2. Operation principle When a radio frequency (RF) is modulated onto an optical carrier (f0) by a phase modulator (PM) under small signal modulation, there are an optical carrier (f0) and both sidebands (f0 þ f1 and f0 f1) in the frequency domain. The proposed MPF is based on the three CMRRs with different radii, i.e., MRR1 (resonant wavelengths of λ1 and λ′1 ), MRR2 (resonant wavelengths of λ2 and λ′2) and MRR3 (resonant wavelengths of λ3 and λ′3), as shown in Fig. 1 (a). By properly designing the radii of the CMRRs, λ2 could be close to λ3 to merge as the right bandpass but both of them depart from λ1 in one free spectral range (FSR1), shown as the blue line. Therefore, an optical processor of two bandpass response could be
L. Liu et al. / Optics Communications 363 (2016) 128–133
129
FSR1
FSR2
Output
MRR1
λ'1 λ'3
λ'2
λ2 λ3
λ1
MRR2
MRR3
Input
FSR1: reconfigurability of 3-dB bandwidth f0
Carrier
RF power
Optical power
f1 One sideband
Freq
Freq
f1
FSR2: tunability of central frequency f0 f2 f3
RF power
Optical power
f0
Freq
Freq
f2 f3 Fig. 1. Schematic diagram of the tunable MPF. (a) Schematic structure and response of the three CMRRs. (b and c) Illustration of the reconfigurability of 3-dB bandwidth, where (b) and (c) are the optical spectrum of CMRRs and RF spectrum of MPF respectively. (d and e) Tunability of central frequency, where (d) and (e) are the optical spectrum of CMRRs and RF spectrum of MPF respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
achieved in order to separately process the optical carrier (f0) and one sideband (f0 þf1), and the other sideband (f0 f1) out of the bandpass response would be suppressed. In another FSR (named with FSR2), λ′1 could be designed closely to λ′3 to merge as the right bandpass but both of them separating from λ′2, shown as the green line. It should be noted that MRR2 has been fabricated with metal electrodes to allow tuning its resonance (λ2 or λ′2). Fig. 1(b) and (c) illustrates the reconfigurability of 3-dB bandwidth of the MPF. As shown in Fig. 1(b), assume that f0 is located at the resonant peak of the MRR1 (λ1 in FSR1). After detecting by the square-law photodetector (PD), the optical signal is converted to electrical signal and the frequency response of the RF signal reveals a bandpass filter which is shown as the blue line in Fig. 1(c). Obviously, the central frequency of RF response is f1. If we adjust the voltage applied on MRR2 to tune λ2, the bandwidth of the right merged bandpass can be changed, shown as the red line in Fig. 1 (b). In this way, the bandwidth of RF response is changed accordingly while keeping the central frequency constant with f1, as shown in Fig. 1(c). On the other hand, Fig. 1(d) and (e) illustrates the tunability of central frequency of the MPF. In Fig. 1(d), by
adjusting the voltages applied on MRR2 to shift the left resonant peak, i.e. λ′2 in FSR2, and meanwhile aligning the laser wavelength with this peak, one may tune the central frequency of RF response from f2 to f3 with constant bandwidth, shown as the green and red lines in Fig. 1(e). Therefore, a tunable bandpass MPF has been successfully built by tuning the resonance of MRR2, but the central frequency and 3-dB bandwidth could not be tune simultaneously. The frequency response of the bandpass MPF can also be deduced analytically. Assume that the optical carrier is modulated in a phase modulator (PM) by the RF signal. In the case of phase modulation, the output optical field of the PM can be described by
Eout (t ) = E0e j[ω0t + m cos(ωRF t )]
(1)
where E0 is amplitude of the input optical field, ω0 is the angular frequency of the input optical signal, ωRF is angular frequency of the RF signal and m is the phase modulation depth. Neglecting the high order side-band, the output optical field of the PM under a small signal model can be described by
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Eout (t ) = E0[jJ1 (m)e j(ω0 − ωRF )t + J0 (m)e jω0t + jJ1 (m)e j(ω0 + ωRF )t ]
(2)
where Jn is the nth-order Bessel function of the first kind. After the signal is transmitted in the CMRRs with an amplitude transmission function of H(ω), each frequency component will multiply a different frequency weight of H(ω), so the output optical field can be described as
E(ω) = 2πE0[jJ1 (m)H (ω0 − ωRF ) + J0 (m)H (ω0) + jJ1 (m)H (ω0 + ωRF )] (3) After detecting of the square-law PD, and neglecting the J12 term, we obtain the alternative current (AC) term of the current, which is equal to the product of E(ω) and its conjugation.
iAC ∝ 4π 2jE02J0 (m)J1 (m)[H *(ω0)H (ω0 + ωRF ) − H (ω0)H*(ω0 − ωRF )]
(4)
To experimentally demonstrate the compact MPF, we design and fabricate the CMRRs on a commercial silicon-on-insulator (SOI) wafer. The thickness of the top silicon and the buried oxide layer of the SOI wafer are 220 nm and 2 μm, respectively. We employ deep ultraviolet (DUV) photolithography using a 248-nm stepper to define the waveguide patterns, and then anisotropic dry etch of silicon is employed. Boron and phosphorus ion implantations are performed to form the highly p-type and n-type doped regions. Also the slab layer is etched outside the p–i–n junctions to confine the current flow around the ring waveguide. Finally, contact holes are etched and aluminum is deposited to form the metal connection. The whole fabrication process is done using CMOScompatible processes. Fig. 2(a) shows the micrographs of the onchip CMRRs. As the resonant wavelengths of the CMRRs are required to meet the conditions in Fig. 1(a), the radii of the three MRRs are designed to be 54.44 mm, 57.03 mm and 59.88 mm, respectively. The zoom-in region of MRR2 is shown as Fig. 2(b) with which fabricated on a pair of metal electrodes to allow tuning. When MRR2 is applied on voltages, the refractive index and transmission loss of the waveguide would be changed due to the free-carrier plasma dispersion effect. Thus the resonant wavelength of MRR2 would be blue-shift. The waveguide widths of both bus waveguides and bending waveguides are about 500 nm. The coupling gap between the bus waveguide and bending waveguide is about 330 nm. We employ the vertical grating coupler to couple the optical signal from fiber to silicon waveguide, and the zoom-in grating coupler is shown in Fig. 2(c). The period of the grating coupler is 610 nm. The trench width changes linearly from 180 nm
to 340 nm and the etch depth is 70 nm. Fig. 3(a) shows the measured transmission spectrum of the CMRRs using an optical spectrum analyzer (AQ6370B). The maximum extinction ratio is about 30 dB and the transmission loss of the CMRRs is around 2 dB after subtracting the 16-dB coupling loss of the grating couplers. It is clear to see that two bandpass response appear in two regions which are located in the second and fourth FSRs respectively. Fig. 3(b) is the zoom-in spectrum of the second FSR. The overlapped resonant wavelength for MRR1 and MRR2 is around 1554.00 nm and the corresponding resonant peak of MRR3 is 1554.20 nm, so the frequency interval of the two bandpass response is about 25 GHz. The zoom-in spectrum of the fourth FSR is shown as Fig. 3(c). The overlapped resonant wavelength for MRR1 and MRR3 is around 1557.84 nm and the corresponding resonant peak of MRR2 is 1557.69 nm, therefore the frequency interval of the two bandpass response is about 19 GHz.
3. Experimental results and discussion A proof-of-concept experiment for the tunable MPFs has been performed as shown in Fig. 4. A tunable laser source (TLS) emits a continuous-wave (CW) beam, which has a precisely tuning resolution of 1 pm. The CW beam is modulated by a PM, which has a modulation bandwidth of 40 GHz. Although the PM can be instead of an intensity modulator, it is known to all that the PM works better because of its bias-drift-free operation and low insertion loss. The vector network analyzer (VNA) (Anritsu 37369D) has a maximum sweeping frequency of 40 GHz. The RF signal from the VNA is amplified by an electrical amplifier (EA) and then applied to the PM. A forward erbium-doped fiber amplifier (EDFA) is used to boost the input optical power. As the silicon waveguide operates only in transverse electrical (TE) mode, a polarization controller (PC) is required. Then the modulation signal is sent into the CMRRs by the vertical grating coupler. After amplification by the backward EDFA, the optical signal is converted to electrical signal by the PD and analyzed by the VNA with 40 GHz bandwidth. It should be noted that the primary motivation is to make all the rings tunable, so all three MRRs are fabricated with heaters to achieve tunable center frequency and reconfigurable bandwidth. However, due to the absence of probe pin array, only one MRR could be tuned every time. Hence as a proof-of-concept
Cascaded microring resonators
25 µm
MRR1
MRR2
MRR3
Grating coupler MRR2 25 µm 10 µm Fig. 2. (a), (b) and (c) Micrographs of the CMRRs, zoom-in region of MRR2 and the grating coupler, respectively.
L. Liu et al. / Optics Communications 363 (2016) 128–133
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Optical power (dBm)
-20 -10
-30 -20
-40 -30
-50 -40
-60 -50 1552 1552
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1,557 1554.55
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1,564.5 1558.45
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Wavelength (nm)
Fig. 3. (a) Measured spectrum of the CMRRs. (b) and (c) The zoom in spectra of the second FSR and fourth FSR, respectively.
experiment, we have designed another operation scheme with one tunable MRR and a tunable laser as a temporary solution, as shown in Fig. 1. By tuning the spectrum of the CMRRs, we can achieve a bandpass MPF whose central frequency and bandwidth are tunable respectively. In the experiment, considering the measured frequency intervals of the two bandpass response of the CMRRs, we choose the second FSR shown as Fig. 3(b) to realize the reconfigurability of the 3-dB bandwidth, and the fourth FSR shown as Fig. 3(c) to realize the tunability of the central frequency respectively. For reconfigurability of the 3-dB bandwidth of the MPF, the bandwidth of the merged bandpass (belongs to MRR1 and MRR2) in Fig. 3(b) should be varied. In this case, we adjust the voltages applied on MRR2 to tune the merged spectrum around 1554.00 nm and meanwhile fix the laser wavelength at 1554.20 nm. Therefore, the 3-dB bandwidth could be tuned while
VNA EA
EDFA2 ATT Chip PC1
PM
EDFA1
PC2
PD
TLS
Fig. 4. Experimental setup of the proposed scheme. TLS: tunable laser source, PC: polarization controller, PM: phase modulator, EDFA: erbium doped fiber amplifier, ATT: attenuator, PD: photodetector, EA: electrical amplifier, VNA: vector network analyzer.
Central frequency (GHz)
Normalized response (dB)
0 -10 -20
VMRR2=0V VMRR2=0.83V VMRR2=0.86V VMRR2=0.90V
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40
40
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Optical path Electrical path Coupling fiber
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Fig. 5. (a) Measured bandpass MPFs with tunability of 3-dB bandwidth. (b) Features of central frequency and rejection ratio.
0
BW3dB (GHz)
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Frequency (GHz)
Fig. 6. (a) Measured bandpass MPFs with tunability of central frequency. (b) Features of 3-dB bandwidth and rejection ratio.
keeps the central frequency approximately constant. Fig. 5 (a) shows that the bandwidth could be tuned from 5.5 GHz to 17.5 GHz with the constant central frequency of 25 GHz when we apply voltages of 0.00 V, 0.83 V, 0.86 V and 0.90 V on MRR2, respectively. The measured current is 0.7 mA when the voltage is 0.9 V, namely in this case the power consumption of the electrode is 0.63 mW. When the bandwidth of the merged left bandpass is varied, the frequency interval and extinction ratio of the two bandpass response basically do not change. Therefore, in this case, the central frequency of the MPF is approximate to a constant and the rejection ratio changes in a small range, as shown in Fig. 5(b). The rejection ratio is defined as the ratio of the maximum value and minimum value in the high-frequency passband. It should be noted that there is an unexpected bandpass in the low frequency. As mentioned above, given an optical carrier injected to a PM fed by an RF signal, it is well known that the generated sidebands are out of phase. Superficially, there is only a single bandpass response when we send this modulation signal into the chip, as shown in Fig. 1(b)–(d). However, the phase difference of the two sidebands is actually influenced by the phase spectrum of the CMRRs [20]. For example, in Fig. 1(b), when the optical carrier is aligned at the separate resonant peak (λ1), the two sidebands will experience opposite phase changes around λ1 due to the existence of the MRR1's phase spectrum. Hence, the two sidebands in the low frequency are not out of phase any more thus to generate an unexpected bandpass (named as the low bandpass). In the future, we could use some dispersive mediums, such as single-mode fiber (SMF) and dispersion compensating fiber (DCF) with proper lengths to relieve the phase changes of the two sidebands. Thus the low bandpass could be suppressed as expected. In this paper, our major target is to achieve tunable MPFs in the higher frequencies (named as the high bandpass). On the other hand, to realize tunability of the central frequency of the high bandpass, the location of the separate resonant peak (belongs to MRR2) in Fig. 3(c) should be tuned. Therefore, we adjust the voltages applied on MRR2 to tune its resonant peak to be blue-shifted around 1557.69 nm and meanwhile align the laser wavelength with this peak. In this way, the central frequency of the MPF is tunable with constant bandwidth. As shown in Fig. 6(a), the central frequency of the MPF could be tuned from 19 GHz to 40 GHz when we apply voltages of 0.00 V, 0.86 V, 0.90 V and 0.93 V on MRR2, respectively. Theoretically, the MPF is able to tune a higher frequency of its central frequency but restrained by the bandwidth of VNA in the experiment. Fig. 6(b) further demonstrates the 3-dB bandwidth and rejection ratio of the frequency response. It is clear that the 3-dB bandwidth is approximately a constant around 6.5 GHz and for most frequencies, the rejection ratio is larger than 20 dB. Therefore, the central frequency of the MPF is tunable from 19 GHz to 40 GHz with constant bandwidth of
6.5 GHz and over 20 dB rejection ratio. When the frequency is lower than 25 GHz, the rejection ratio decreases which is caused by the crosstalk induced by the overlap of the high bandpass and the low bandpass.
4. Conclusions A compact and tunable MPF has been proposed and demonstrated based on the CMRRs. In the experiment, the central frequency or 3-dB bandwidth of the MPF can be tuned respectively by adjusting the laser wavelength and voltage applied on one MRR. Experimental results show a central frequency tuning range from 19 GHz to 40 GHz, and a wide bandwidth tuning range from 5.5 GHz to 17.5 GHz. Our scheme offers an integrated MPF scheme which is significant in the microwave photonic systems.
Acknowledgments 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 China (Grant no. NCET-11-0168), a Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant no. 201139), the National Natural Science Foundation of China (Grant nos. 11174096 and 61475052), and the Opened Fund of the State Key Laboratory on Advanced Optical Communication System and Network (Grant no. 2015GZKF03004). The authors would like to thank Prof. Y. Yu for providing the VNA.
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