422IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 19, NO. 6, JUNE 2009
Instantaneous Microwave Frequency Measurement
Using an Optical Phase Modulator
Xiaomin Zhang, Hao Chi, Xianmin Zhang, Member, IEEE, Shilie Zheng, Xiaofeng Jin, and
Jianping Yao, Senior Member, IEEE
Abstract—A novel technique for instantaneous microwave fre-
and demonstrated. In the proposed system, a microwave signal
with its frequency to be measured is modulated on two optical
wavelengths at the phase modulator, with the phase-modulated
optical signals sent to a dispersive element, and detected at two
photo-detectors. Due to the chromatic dispersion of the dispersive
element,thetwomicrowavesignals willexperiencedifferent power
fading, leading to different power versus frequency functions.
A fixed relationship between the microwave frequency and the
microwave powers is established. By measuring the microwave
powers, the microwave frequency is estimated. Compared with the
techniques using an intensity modulator, the proposed approach
is simpler with less loss. Since no bias is needed the system has a
better stability, which is highly expected for defense applications.
Experimental verification is presented.
Index Terms—Microwave frequency measurement, microwave
photonics, phase modulation.
mate the frequency of an unknown microwave signal over a
wide bandwidth. Conventional techniques for instantaneous
microwave frequency measurement are thought to be bulky,
limited in bandwidth, and suffer from electromagnetic inter-
ference. Due to the advantages such as high bandwidth, light
weight, low loss, and immunity to electromagnetic interference,
photonic technology for microwave signal processing has been
considered a promising solution recently –. A number of
approaches have been proposed for the measurement of a mi-
crowave frequency in the optical domain –. A microwave
frequency can be measured using an intensity modulator in
ICROWAVE receiver for radar and other electronic
warfare applications require the capability to esti-
Manuscript received December 05, 2008; revised March 04, 2009. First
published May 26, 2009; current version published June 05, 2009. This work
was supported by the National Natural Science Foundation of China (60871011
and 60801003), the Program for New Century Excellent Talents in University
(NCET-05-518), the Specialized Research Fund for the Doctoral Program of
Higher Education of China under Grant 20060335074, and the Y. C. Tang
Disciplinary Development Fund of Zhejiang University.
X. Zhang, H. Chi, X. Zhang, S. Zheng, and X. Jin are with the Department
of Information and Electronic Engineering, Zhejiang University, Hangzhou
310027, China (e-mail: firstname.lastname@example.org).
J. Yao is with the Department of Information and Electronic Engineering,
Zhejiang University, Hangzhou 310027, China and also with the Microwave
Photonics Research Laboratory, School of Information Technology and Engi-
neering, University of Ottawa, Ottawa, ON K1N 6N5, Canada.
Color versions of one or more of the figures in this letter are available online
Digital Object Identifier 10.1109/LMWC.2009.2020046
conjunction with a dispersive element. In a microwave pho-
tonic transmission system employing double-sideband (DSB)
modulation, the received microwave power is dependent on the
microwave frequency due to the chromatic dispersion in the
optical link. The frequency-dependent power variation of the
received RF signal in an intensity-modulation-based system has
been employed to measure the frequency of a microwave signal
. An improved method with adjustable measurement range
and resolution was recently presented in . Novel approaches
based on dc optical power monitoring using low-speed optical
power meters with reduced system cost were demonstrated in
, . Microwave frequency measurement using two cas-
caded Mach-Zehnder modulators (MZMs), a RF delay line and
a single photodetector were also demonstrated to estimate the
frequency of a microwave signal , .
The major difficulty associated with the use of an MZM for
microwave frequency measurement is the need for a sophisti-
cated electrical circuit to control the dc bias, to stabilize the op-
eration of the MZM. In this letter, a novel technique to mea-
sure the frequency of a microwave signal using an optical phase
proach is that the phase modulator is not biased, which elimi-
nates the bias drifting problem, a feature that is highly expected
in defense systems , . In addition, a phase modulator
is simpler and has smaller insertion loss. The proposed tech-
nique is experimentally demonstrated. An excellent agreement
between the theoretical predictions and experimental results is
II. PRINCIPLE OF OPERATION
The system configuration of the proposed technique is shown
length multiplexer, an optical phase modulator, a length of dis-
persivefiber serving asthedispersiveelement, a wavelengthde-
multiplexer, and two photo-detectors (PDs). The light waves at
different wavelengths from the two LDs are combined by the
multiplexer and sent to the phase modulator. An unknown RF
signal is applied to the phase modulator to phase-modulate the
two optical carriers. The modulated optical signals propagate in
the dispersive fiber, which are then separated by the demulti-
plexer and converted to electrical signals at the two PDs. Since
the dispersion coefficients are different for the two carriers, the
detected RF powers are different for the two channels. The dif-
ference in the detected RF powers will be used to determine the
frequency of the unknown RF signal.
1531-1309/$25.00 © 2009 IEEE
ZHANG et al.: INSTANTANEOUS MICROWAVE FREQUENCY MEASUREMENT USING AN OPTICAL PHASE MODULATOR 423
Fig. 1. Schematic of the proposed approach for instantaneous microwave fre-
quency measurement. (LD: laser diode; PM: phase modulator; PD: photode-
tector; MUX/DEMUX: multiplexer/demultiplexer).
The electrical power of a phase modulated microwave signal
traveling in a dispersive link is a function of the microwave fre-
quency, which is given by 
dispersion of the fiber link in ps/nm,
is the frequency of the modulating microwave
signal to be measured.
In order to determine the microwave frequency, two different
wavelengths are needed. Assume that the powers from the two
then we have the power ratio between the two wavelength chan-
is the light velocity in a vacuum, is the accumulated
is the wavelength of the
are thecorresponding accumulateddispersions of the
As can be seen the power ratio is independent of the input
RF powers. For a system with given wavelengths and accumu-
lated dispersions, the microwave frequency can be calculated if
the power ratio is known. Therefore, by simply measuring the
microwave powers at the outputs of the two channels, the mi-
crowave frequency can be estimated.
First, we investigate the proposed technique by simulations.
The simulations are performed based on a system with the two
optical wavelengths at 1520 and 1630 nm, and the accumulated
dispersions of 362 and 512 ps/nm, which correspond to the dis-
persions of 25 km standard single mode fiber. Fig. 2(a) shows
the calculated RF power distribution versus the microwave fre-
quency obtained based on (1). Fig. 2(b) shows the distribution
of the power ratio versus the microwave frequency, calculated
based on (2). To estimate the microwave frequency based on
the power ratio without ambiguity, a monotone interval should
tion correspond to different spectral ranges. For most of the ap-
plications, the first monotone interval is usually selected, with
the upper and the lower limits determined by two factors. The
upper limit is determined by the first notch of the power ratio
curve, which is given by
are the wavelengths of the two optical carriers,
Fig. 2. Simulation results. (a) RF power versus microwave frequency
? ???? ??, ?
? ??? ?????; dashed: ?
? ? ??? ?????); (b) Power ratio versus microwave frequency.
? ???? ??,
It is seen that the power ratio distribution at the lower fre-
quency range is flat due to a small difference between the de-
tected RF powers. The flat power ratio distribution would lead
to a large measurement error since a small measurement error
in the microwave powers would cause a large power ratio error.
The lower limit
is thus determined by the predetermined
measurement error tolerance. The total measurement range of
the system with a given measurement accuracy is
is the larger dispersion, and is the corre-
On the other hand, if the second monotone interval is chosen,
the upper and lower limits would be determined by the posi-
tionsof thefirst notchesof thetwo powerdistribution functions.
However, a bandpass filter should be used to eliminate the fre-
quencies from outside the frequency range.
An experiment based on the setup shown in Fig. 1 is im-
plemented. Two wavelengths from two tunable laser sources
(Agilent 81940A) with each having an output power of 6 dBm
are multiplexed and then sent to the phase modulator. A mi-
crowave signal generated by a vector network analyzer (VNA,
Agilent 8720ET) is applied to the phase modulator via the RF
port to modulate the two optical wavelengths. The microwave
frequency is tunable from 50 MHz to 20.05 GHz. The modu-
lated optical carriers are sent to the 25.6 km single mode fiber.
At the output of the fiber, the two phase-modulated signals are
seperated by the demultiplexer and then sent to the two PDs.
The RF powers of the two microwave signals are measured by
the VNA. The RF frequency is thus estimated.
In the experiment, two power distribution functions are first
measured using the VNA, with the two wavelengths set at 1520
and 1620 nm. The power ratio function is calculated, which is
shown in Fig. 3(a). Then, we tune the frequency of the input mi-
crowave signal and record the measured frequency. The results
424 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 19, NO. 6, JUNE 2009 Download full-text
Fig. 3. Experimental results (?
ratiofunction;(b)measuredfrequencyversus inputfrequency; (c)measurement
errors for a measurement range of 7.3–15.05 GHz.
? ???? ??, ?
? ???? ??). (a) Power
Fig. 4. Experimental results (?
ratiofunction;(b)measuredfrequencyversus inputfrequency; (c)measurement
errors for a measurement range of 11.75–17.95 GHz.
? ???? ??, ?
? ???? ??). (a) Power
are shown in Fig. 3(b) as circles. The measurement errors cal-
culated by comparing the measured frequencies and the input
frequency are shown in Fig. 3(c). It can be seen that for a given
measurement accuracy, say
is 7.3–15.05 GHz.
If a third wavelength is used, a second power ratio function
For example, if we introduce a third wavelength at 1540 nm,
a second power ratio function calculated according to the RF
Fig. 4(b) shows the frequency measurement results. The mea-
surement errors are calculated, which are shown in Fig. 4(c).
In this case, the measurement range is 11.75–17.95 GHz with
a measurement accuracy of
tween the measurement range and the measurement accuracy.
Higher measurement accuracy results in a smaller measurement
range, while a lower measurement accuracy would lead to a rel-
atively large measurement range. For example, in the first case
, the measurement range
. There is a tradeoff be-
of the experiment, the measurement resolution is
corresponding to a measurement range of 11.0–15.0 GHz. The
measurement accuracy achieved here is comparable to that of
the previous works –, which is acceptable for applications
where a rough but instantaneous microwave frequency estima-
tion is required. We believe that the noise generated in the PD
is the major source of measurement error. The influence of the
wavelength stability on the system performance is also studied.
It is estimated thata wavelength driftof 0.1 nm would lead toan
additional measurement error of about 5 MHz for the first case
of the experiment.
We have proposed and experimentally demonstrated a novel
technique for instantaneous microwave frequency measurement
signal was estimated by measuring the microwave powers at
the outputs of the two PDs, with the two microwave signals
carried by two different optical wavelengths that were experi-
encing different power fading. The key significance of the pro-
posed approach is the use of an optical phase modulator. Since
no bias was needed, the biasing drifting problem existing in an
MZM-based system was completely eliminated, leading to im-
proved system stability. This feature is highly expected in the
defense systems. In addition, the use of an optical phase mod-
ulator makes the system simpler with lower loss, which is also
desirable for defense applications.
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