COL 11(5), 050604(2013) CHINESE OPTICS LETTERS May 10, 2013
Photonic approach to microwave frequency measurement
with extended range based on phase modulation
Xiuyou Han (
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∗
, Siteng Zhang (
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), Chao Tong (
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), Nuannuan Shi (
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Yiying Gu (
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), and Mingshan Zhao (
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Photonics Research Center, School of Physics and Optoelectronic Engineering,
Dalian University of Technology, Dalian 116024, China
∗
Corresponding author: xyhan@dlut.edu.cn
Received December 12, 2012; accepted January 25, 2013; posted online April 24, 2013
We propose and demonstrate a photonic approach to instantaneous frequency measurement with an ex-
tended range based on phase modulation. In the measurement system, two optical wavelengths and two
dispersion fiber segments are used to construct the frequency-dependent amplitude comparison functions
(ACFs). Several ACFs can be utilized jointly to determine the microwave frequency without ambiguities
beyond a monotonic region of the lone conventional ACF. The measurable range of microwave frequency
can be extended and the accuracy can be improved by selecting an ACF with a large slope. The exper-
imental results show that the errors are limited within ±140 MHz of a frequency measuremental range
from 8 to 20 GHz.
OCIS codes: 060.0060, 060.5060, 060.5625.
doi: 10.3788/COL201311.050604.
The frequency measurement of input microwave signals is
very important for modern radar warning receivers in the
field of electronic warfare (EW)
[1]
. Unlike conventional
receivers, the input signals of EW receivers are unknown.
Estimate the frequency of the unknown signal in a very
short period of time is desirable, a method known as in-
stantaneous frequency measurement (IFM). Co ns idering
the frequencies of modern radar systems range from sub-
gigahertz to millimeter waves, the conventional electrical
implementations may not meet the requirements for a
wide operating frequency range and a nearly real-time
response.
Photonic techniques can overcome these limitations,
given such advantages as a w ide instantaneous band-
width, light weight, low loss, and immunity to elec -
tromagnetic interference
[2,3]
. A number of appro aches
have been pr oposed in the past few years to measure
the microwave frequency in the optical domain
[4−14]
.
Optical power or microwave power monitoring is usu-
ally performed to determine the unknown frequency.
With optical powe r, a sinusoidal filter
[4,5]
, channeliza-
tion filter
[6]
, phase shift filter
[7]
or scanning filter
[8,9]
is
used to map the frequency information into the change
of optical power, which may reduce the system cos t,
thereby fulfilling the req uirement of a low-speed pho-
todetector (PD). With microwave power, on the other
hand, dispersion-induced microwave p ower variations are
mostly adopted to establish a fre quency-dependent am-
plitude comparison function (ACF)
[10−14]
, which can de-
rive the frequency with a wide measurable range and high
accuracy.
Intensity modulation using Mach-Zehnder modula-
tors (MZMs) is usually applied in the photonic IFM
links
[10,11]
. The major difficulty associated w ith MZMs
is the need for a sophisticated DC bias-controlled elec-
trical circuit to stabilize the operation of the MZMs
[15]
.
The phase modulators (PMs) can appropriately elimi-
nate the bias drifting problem. Recently, PMs have at-
tracted more and more attentions in microwave-photonic
systems besides the IFM links
[12−14]
. For the ACF-based
IFM technique with phase modulation, the measurable
frequency range is generally limited to a monotonic re-
gion o f ACF to avoid ambiguities
[12]
.
In this letter, a photonic approach to microwave fre-
quency measurement with an extended range based on
phase modulation is proposed. In the measurement sys-
tem, multiple photonic links with phas e modulation are
established. Using the microwave signal power fading
effect induced by the dispersive medium, several ACFs
can be used jointly to determine the microwave frequency
without ambiguities beyond a monotonic region with only
one ACF. The measurable range of the microwave fre-
quency can be extended, and the accuracy can be im-
proved with the proper selection of ACF with a large
slope. The operation principle of the photonic approach
for IFM is illustra ted, and a proof-of-concept experiment
is performed to verify the feasibility of the approach.
The s chematic diagram of the proposed pho tonic IFM
approach is shown in Fig. 1. The lightwaves with
different wave le ngths from two laser diodes (LDs) are
combined in a multiplexer a nd sent to the PM. The un-
known input microwave signal is modulated on the two
lightwaves simultaneously. After tr ansmitting through
the fir st dispersive fiber l
1
, the two lightwave signals are
split equally into two paths by a 3-dB coupler. The two
lightwave signals in the upper path are demultiplexed di-
rectly and detected by the PDs, respectively. The lower
Fig. 1. Schematic diagram of the proposed photonic approach
for microwave frequency measurement.
1671-7694/2013/050604(5) 050604-1
c
2013 Chinese Optics Letters