Switchable microwave photonic filter using a phase
modulator and a silicon-on-insulator micro-ring resonator
Simin Li (李思敏)
1
, Rong Cong (丛 榕)
1
, Zhengqian He (何正前)
1
,
Tianliang Wang (王天亮)
2
, Fangzheng Zhang (张方正)
1,
*, and Shilong Pan (潘时龙)
1
1
Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of Aeronautics
and Astronautics, Nanjing 210016, China
2
Shanghai Institute of Satellite Engineering, Shanghai 201109, China
*Corresponding author: zhangfangzheng@nuaa.edu.cn
Received November 3, 2019; accepted January 10, 2020; posted online April 28, 2020
A switchable microwave photonic filter (MPF) using a phase modulator (PM) and a silicon-on-insulator micro-
ring resonator (MRR) is proposed and demonstrated. By adjusting the polarization controller between the PM
and the MRR, the filtering function of the MPF can be switched between a band-stop filter and a band-pass
filter. In a proof-of-concept experiment, an MPF with a rejection ratio of 30 dB (or 15 dB) for the band-stop (or
band-pass) response and a frequency tuning range from 9.6 to 20.5 GHz is achieved.
Keywords: microwave photonics; micro-ring resonator; microwave photonic filter.
doi: 10.3788/COL202018.052501.
Microwave photonic filters (MPFs) have shown attrac-
tive performance for broadband radio-frequency (RF)
applications because of the advantages such as broad
bandwidth, flexible tunability, reconfigurability, and anti-
electromagnetic interference
[1]
. In multifunctional RF sys-
tems, MPFs with switchable functions between band-pass
response and band-stop response are highly desirable. In
addition, switchable MPFs are useful when the systems are
operated in a complex electromagnetic environment
[2,3]
.
Until now, several switchable MPFs have been pro-
posed, which can be generally classified into three catego-
ries. The first approach is realized by changing the tap
coefficients of a multi-tap delay line filter. For example,
in an infinite impulse response (IIR) MPF based on an
amplified recirculating delay line with an in-loop electro-
optic modulator (EOM), the band-stop and band-pass
responses were implemented by biasing the EOM at the
maximum and minimum transmission points, respec-
tively
[4]
. Another switchable IIR MPF was achieved by
a phase modulator (PM), a fiber delay line loop (FDLL),
and two tunable optical band-pass filters (TOBFs), which
were in and out of the FDLL, respectively
[5]
. If the two
TOBFs were tuned on the same side of the optical carrier,
the overall response of the MPF was equal to an all-pass
response (determined by a straight-through component)
plus a band-pass response (determined by recirculating
output taps in the FDLL). Conversely, the two TOBFs
were tuned on the opposite sides of the optical carrier,
the overall response was equal to an all-pass response mi-
nus a band-pass response, and a band-stop MPF was
achieved. In these schemes, since there is a huge time delay
difference between two adjacent taps, the periodic spectral
responses of the multi-tap delay line filter normally have a
small free spectral range (FSR), which may limit the filters
for wideband applications
[6]
. The second approach is to
manipulate the amplitudes and phases of two optical
sidebands by applying an EOM and an optical filter. In
Refs. [
6] and [7], this was realized by tuning the driving
voltages of a dual-drive (DD) Mach–Zehnder modulator
(MZM) to employ it in phase modulation or single-
sideband (SSB) modulation. Specifically, a phase-shifted
fiber Bragg grating was used as the optical filter to sup-
press one of the sidebands in Ref. [
6], while both side-
bands were suppressed and amplified, respectively, by
the stimulated-Brillouin-scattering (SBS) effect in Ref. [
7].
Besides, an MPF with complementary band-pass and
band-stop responses was implemented through an inte-
grated polarization-division multiplexing MZM (PDM-
MZM) consisting of two dual-parallel MZMs (DP-MZMs)
and the SBS effect
[8]
. By controlling the bias voltages of
the two DP-MZMs, the modulated optical signals in the
two orthogonal polarizations had asymmetric double side-
bands. The SBS effect was performed as an ultra-narrow
amplification. The polarizers were employed to adjust the
polarization axis of the modulated signal to exhibit an SSB
modulation or a phase modulation. In the above scenarios,
the DD-MZM or DP-MZM is driven by two quadrature
RF signals with the same freque ncy and amplitude but
with a 90° phase offset, and therefore, an electrical
3 dB 90° hybrid exerts an important role in these systems.
Because of the small bandwidth of the 3 dB electrical 90°
hybrid, the MPFs are also limited for wideband applica-
tions. Another method to implement an MPF is mapping
an optical filter to a microwave filter
[9]
. This method em-
ploys a switchable optical filter to obtain the switchable
function. As the integration of a microwave photonic sys-
tem is becoming a major trend, the micro-ring resonator
(MRR) is regarded as the most promising optical filter sol-
ution in the photonic integrated circuits
[10]
. For a single
MRR, despite the drop port and the through port having
complementary responses, only one lineshape (band-stop
or band-pass) occurs in a certain port. In order to obtain
COL 18(5), 052501(2020) CHINESE OPTICS LETTERS May 2020
1671-7694/2020/052501(5) 052501-1 © 2020 Chinese Optics Letters