Photonic microwave filters with ultra-high noise
rejection [Invited]
Xihua Zou (邹喜华)*, Peixuan Li (李沛轩), Wei Pan ( 潘 炜), and Lianshan Yan (闫连山)
School of Information Science and Technology, Southwest Jiaotong University, Chengdu 611756, China
*Corresponding author: zouxihua@swjtu.edu.cn
Received November 2, 2018; accepted December 7, 2018; posted online March 1, 2019
As a key figure-of-merit for high-performance microwave filters, the out-of-band noise rejection is of critical
importance in a wide range of applications. This paper overviews the significant advances in photonic microwave
filters (PMFs) having ultra-high rejection ratios for out-of-band noise suppression over the last ten years.
Typically, two types of PMFs, the bandpass and bandstop ones, are introduced with fundamental principles,
detailed approaches, and then cutting-edge results for noise rejection. Ultra-high noise rejection ratios of ∼80 dB
and >60 dB have been demonstrated for single-passband and single-stopband PMFs, respectively, which are
comparable with the state-of-the-art electronic filters operating in stringent conditions. These PMFs are also
characterized by wide frequency coverage, low frequency-dependent loss, and strong immunity to electromag-
netic interference due to the intrinsic features from the advanced photonics technology.
OCIS codes: 060.5625, 070.2615, 070.1170.
doi: 10.3788/COL201917.030601.
Assisted by advanced photonic techniques, photonic
microwave filters (PMFs) enable the processing of micro-
wave signals in the optical domain, harnessing advantages
including large instantaneous bandwidth, low frequency-
dependent loss, flexible tunability, and strong immunity
to electromagnetic interference
[1–7]
. Thus, PMFs have been
regarded as a promising alternative for pure electronic fil-
ters when facing speed and bandwidth limitations origi-
nating from the electronic bottleneck
[7]
. On the other
hand, PMFs still have some weaknesses to be overcome,
such as the out-of-band noise rejection and spectral reso-
lution, compared with electronic filters. For instance, the
conventional electronic cavity filters used in cellular base
stations require an out-of-band rejection ratio (ORR) of
no less than 80 dB
[8]
.
Fortunately, over the past y ears, a host of significant
advances in PMFs have been demonstrated, involving
multiple functionalities, enhanced figure-of-merit
improvements, and device integration. Here, the out-
of-band noise rejection, as a s ignificant figure-of-merit,
has been greatly improved to beyond that of pure
electronic filters. Accordingly, this paper summarizes
the advances in PMFs with ultra-high out-of-band
noise rejection. Specifically, we focus on the PMFs
with a 3 dB bandwidth below 10 GHz, which is typically
expected to provide high resolution in t he microwave
photonics field.
Principles of PMFs. B asically, t he PMFs can be
categorized as two types operating at the coherent and
incoherent regimes
[7]
,asshowninFigs.1(a) and 1(b).
Multi-tap delay lines or dispersive elements are employed
to introduce time delays among taps in the optical do-
main. Consequently, classic finite-impulse response
(FIR) or infinite-impulse response (IIR) can be generated
forPMFsas
[7]
H ðΩÞ¼
P
a
n
e
−jnΩT
P
b
n
e
−jnΩT
; (1)
where Ω denotes the radio-frequency (RF) angular fre-
quency, a
n
and b
n
are the weighted coefficients of the
nth tap, and T is the time delay between two adjacent taps.
Figure
1(c) shows another structure for designing
PMFs, where the optical response of an optical filter is
faithfully mapped to the microwave response. Here,
different modulation formats can be used, including
the single-sideband and double-sideband intensity
modulation (IM)
[2]
or phase modulation (PM)
[7]
.
Based on these three structures, PMFs fueled by differ-
ent kinds of optical sources, electro-optic modulators,
optical delay lines, and optical filters, have been
Fig. 1. Three structures for PMF design. (a) PMF operating at
the coherent regime; (b) PMF operating at the incoherent
regime; (c) PMF based on the mapping of optical response.
(E/O, electro-optic conversion; PD, photodetector.)
COL 17(3), 030601(2019) CHINESE OPTICS LETTERS March 10, 2019
1671-7694/2019/030601(6) 030601-1 © 2019 Chinese Optics Letters