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Scientific RepoRts | 7:40223 | DOI: 10.1038/srep40223
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Photonic crystal nanocavity
assisted rejection ratio tunable
notch microwave photonic lter
Yun Long, Jinsong Xia, Yong Zhang, Jianji Dong & Jian Wang
Driven by the increasing demand on handing microwave signals with compact device, low power
consumption, high eciency and high reliability, it is highly desired to generate, distribute, and process
microwave signals using photonic integrated circuits. Silicon photonics oers a promising platform
facilitating ultracompact microwave photonic signal processing assisted by silicon nanophotonic
devices. In this paper, we propose, theoretically analyze and experimentally demonstrate a simple
scheme to realize ultracompact rejection ratio tunable notch microwave photonic lter (MPF) based on
a silicon photonic crystal (PhC) nanocavity with xed extinction ratio. Using a conventional modulation
scheme with only a single phase modulator (PM), the rejection ratio of the presented MPF can be tuned
from about 10 dB to beyond 60 dB. Moreover, the central frequency tunable operation in the high
rejection ratio region is also demonstrated in the experiment.
Silicon photonics has become one of the most promising photonic integration platforms owing to its small foot-
print, reduced power consumption, and availability of complementary metal-oxide-semiconductor (CMOS) fab-
rication technology
1
. Because of its unprecedented small size for potential large scale integration, silicon photonic
crystal (PhC) nanocavity is of great importance to accelerate the success of silicon photonics
2–6
. In traditional
scenario, large scale photonic integration is usually driven by the digital applications, such as high capacity opti-
cal communications and optical interconnects technologies. In recent years, due to the strong requirements in
handling analog signals with low power, high ecient and high reliability, the use of integrated photonics tech-
nology to generate, distribute, process and analyze microwave signals has also attracted more and more research
interests
7–9
.
Microwave photonic lter (MPF), which can be employed to process microwave signals in the optical domain
by using photonic devices, is a key element in microwave photonic systems. Several approaches to realizing MPFs
have been proposed and demonstrated based on bulky ber based devices
10–13
. ese congurations are rela-
tively expensive, power consuming, and decient in exibility and stability. Compared to these ber devices,
silicon-on-insulator (SOI) based waveguides can oer distinct advantages of increased stability and reliability,
low cost, small footprints, and compatibility with other integrated optoelectronic devices. Recently, some MPFs
based on SOI microring resonator, microdisk resonator, and Mach–Zehnder interferometer have been proposed
and demonstrated showing superior characteristics
14–18
.
Tunability of MPFs is highly desirable in practical applications to facilitate exible performance. Previously
reported tunable or recongurable MPFs mainly focus on tuning the bandwidth and central frequency
10,13,19–21
.
Besides tuning of bandwidth and central frequency, the stop-band rejection ratio tunability is also highly desira-
ble for a bandstop microwave lter. Specically, in cognitive systems, the rejection ratio of a microwave lter need
to be tunable for handing dierent needs of interference rejection
22,23
. In electrical domain, when constructing
a microwave lter with continuous rejection ratio tunability, resonators with tunable coupling to a transmission
line are usually utilized
22–27
. e operation frequency of these lters are limited to a few GHz. To increase the
operation frequency, a MPF have been proposed to construct rejection ratio tunable microwave lters in optical
domain, showing superior performance
28
. It is based on a Sagnac loop assisted by a polarization beam splitter
and a linearly chirped ber Bragg grating. e rejection ratio of the MPF can be tuned from 0 to 37 dB. A central
frequency tuning range from 2.5 to 13 GHz is also observed. However, the conguration of this rejection ratio
tunable MPF is relatively complicated, and the obtained maximum rejection ratio is not very large owing to the
Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University
of Science and Technology, Wuhan 430074, Hubei, China. Correspondence and requests for materials should be
addressed to J.W. (email: jwang@hust.edu.cn)
Received: 27 September 2016
Accepted: 01 December 2016
Published: 09 January 2017
OPEN
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Scientific RepoRts | 7:40223 | DOI: 10.1038/srep40223
limited maximum split ratio of the polarization beam splitter. Moreover, the application of this MPF is also lim-
ited by its periodic response which results from large loop path dierence between the two taps. So far, achieving
a rejection ratio tunable single stop-band notch MPF with wide rejection ratio and central frequency tuning
ranges is still challenging.
In this paper, we propose a simple scheme to realize rejection ratio tunable single stop-band notch MPF with
wide tuning range based on a silicon PhC nanocavity with xed extinction ratio. We use the combination of a
conventional phase modulator (PM), a tunable bandpass lter (TBF), and a silicon PhC nanocavity to manipulate
the phase and amplitude of optical sidebands. Lower sideband (LSB) and upper sideband (USB) with anti-phase
are generated by a PM. e TBF is used to modify the relative amplitude of LSB and USB, resulting an asymmetric
amplitude modulation signal. e PhC nanocavity is used as an optical lter. By adjusting the location of TBF
to achieve dierent asymmetric amplitude modulation, a wide rejection tuning range from 11.8 to 62.1 dB is
achieved experimentally. A wide central frequency tuning range from 12.9 to 32.3 GHz in high rejection ratio
operating region of the proposed rejection ratio tunable MPF is also observed in the experiment.
Results
Concept and operation principle. Figure1 summarizes the operation principle of the proposed rejection
ratio tunable notch MPF based on a silicon PhC nanocavity. An optical carrier is modulated by a radio frequency
(RF) signal with a conventional phase modulation. It is well known that the phase dierence between the gen-
erated LSB and USB is π. If the modulated signal is applied directly to a photodiode (PD), due to the anti-phase
relation between the LSB and USB, no RF signal could be obtained. Here the modulated signal is sent to a TBF
rst, and the USB signal is attenuated aer the TBF. e output eld is then applied to the PhC nanocavity. Note
that the resonant frequency of the PhC nanocavity is aligned to the frequency of LSB signal, thus the LSB signal
will be ltered by the PhC nanocavity. By tuning the location of the TBF, the relative amplitude of the LSB and
USB can be tuned exibly. Since the anti-phase condition of the LSB and USB, the amplitude dierence of the
LSB and USB will directly map to the rejection ratio of the MPF aer detecting by the PD. Dierent amplitude
Figure 1. Schematic illustration of the proposed notch MPF with rejection ratio tunability.
Figure 2. Measured transmission spectrum of the TBF.
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Scientific RepoRts | 7:40223 | DOI: 10.1038/srep40223
dierence between LSB and USB will lead to dierent rejection ratio of the obtained MPF. is is a photonic
implementation of a rejection ratio tunable notch microwave lter, which exhibits rejection tunability from 0 dB
to innite rejection in principle.
Theory. Providing a continuous wave (CW) is launched into a PM, the optical eld at the output of the PM
can be expressed as
=
ωβ ω
At ee() (1)
jtjtsin( )
LRF
where β = πV
RF
/V
π
is the modulation index. ω
L
and ω
RF
are the angular frequencies of the launched optical carrier
and microwave signal applied on the PM, respectively. V
RF
is the amplitude of the microwave signal. V
π
is the
half-wave voltage of the PM. Based on the Jacobi–Anger expansions, Eq.(1) can be expanded to be
Figure 3. (a–d) Calculated MPF response when the central wavelength of the TBF is 1554.262 nm,
1554.292 nm, 1554.322 nm and 1554.352 nm, respectively.
Figure 4. (a) Scanning electron microscope image of the PhC nanocavity and (b) its transmission spectrum.
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Scientific RepoRts | 7:40223 | DOI: 10.1038/srep40223
ββ β=+ −
ωωωωω+−
At Je Je Je() () () ()
(2)
jt jt jt
01
()
1
()
LLRF LRF
where J
n
is the nth order Bessel function of the rst kind. e the modulated signal is then sent to TBF and PhC
nanocavity. At the output of PhC nanocavity, assuming the eld transmission of the TBF and PhC nanocavity at
ω is T
TBF
(ω) and T
cavity
(ω), respectively, the signal can be described as
βωω
βωωωω
βωωωω
=
+++
−−−
ω
ωω
ωω
+
−
At JeTT
Je TT
Je TT
() () () ()
() ()()
() ()()
(3)
jt
TBFLcavity L
jt
TBFL RF cavity LRF
jt
TBFL RF cavity LRF
0
1
()
1
()
L
LRF
LRF
T
cavity
(ω) in the above equation is expressed by coupled mode theory
3
ω
ωω ττ
ωω ττ
=
−+ −
−+ +
T
j
j
()
()1/ 1/
()1/ 1/
(4)
cavity
iv
iv
0
0
where ω
o
is the resonant frequency of the PhC nanocavity. 1/τ
i
is the photon lifetime reduction associated with
the temporal coupling coecients between PhC nanocavity and the input waveguide. 1/τ
v
is the photon lifetime
reduction in free space (vertical direction).
We further calculate the RF response with dierent TBF location to show the realization of the proposed rejec-
tion ratio tunable MPF. In the simulation, the transmission spectrum of TBF is tted from the measured trans-
mission curve, which is shown in Fig.2. We use a super-gaussian function
ω =
−
ωω
π
−
×.×
T e()
TBF
()
center
2348910
9
4
to t the
response of the TBF in the simulation. ω
center
is the central angular frequency of the TBF. e parameters 1/τ
i
and
1/τ
v
used to calculate PhC nanocavity eld transmission are extracted from the measured PhC nanocavity trans-
mission spectrum. e MPF response with dierent TBF locations is shown in Fig.3(a–d). e carrier light
wavelength is 1554.153 nm, and the central wavelength of TBF is increased from 1554.262 nm to 1554.352 nm
with a step of 0.03 nm, in keeping with the experiment setup. It can be seen that when the central wavelength of
TBF is 1554.292 nm, an very large rejection ratio MPF is obtained.
Experiment. e scanning electron microscope image of the fabricated PhC nanocavity is shown in Fig.4(a).
e PhC nanocavity consists of a PhC membrane with a line of three holes missing. e lattice constant is 420 nm,
and the hole radius is 126 nm. Positions of the three holes adjacent to the cavity are optimized to obtain high Q
factor. e three holes adjacent to the cavity are laterally shied by 0.175a, 0.025a, 0.175a, respectively, where a
is the lattice constant. Figure4(b) shows the measured transmission spectrum of the fabricated PhC nanocavity.
e resonant wavelength of the cavity is around 1554.313 nm.
Figure5 depicts the experimental setup. A tunable laser diode (TLD) emits a CW light. An electric amplier
(EA) is used to amplify the RF signal from vector network analyzer (VNA). e CW light is modulated by a PM to
produce an optical double sideband signal. e TBF is used to modify the USB of the signal to obtain a modied
asymmetric optical optical double sideband signal. e output eld is then applied to the PhC nanocavity. Aer
the device, the optical signal is converted to electric signal by a PD and analyzed by the VNA. e measured opti-
cal spectra aer the TBF and the corresponding MPF responses are shown in Fig.6. e carrier light wavelength
is 1554.153 nm. Figure6(a–d) depict the optical spectra when the central wavelength of the TBF is 1554.262 nm,
1554.292 nm, 1554.322 nm and 1554.352 nm, respectively. Figure6(e–h) show the corresponding MPF responses.
e experimental results agree well with the simulation. As shown in Fig.6(a) and (e), when the USB signal is
Figure 5. Experimental setup of bandpass MPF based on a silicon PhC nanocavity. Solid lines: optical
path; dash lines: electrical path; TLD: tunable laser diode; PM: phase modulator; EDFA: erbium-doped ber
amplier; VOA: variable optical attenuator; PC: polarization controller; PD: photodetector; EA: electrical
amplier; VNA: vector network analyzer.
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Scientific RepoRts | 7:40223 | DOI: 10.1038/srep40223
Figure 6. (a–d) Optical spectra aer the TBF when the central wavelength of the TBF is 1554.262 nm,
1554.292 nm, 1554.322 nm and 1554.352 nm, respectively. (e–h) e corresponding MPF response. (i) Rejection
ratio as a function of TBF central wavelength.