All-fiber-photonics-based ultralow-noise agile
frequency synthesizer for X-band radars
JUAN WEI,
1,2,†
DOHYEON KWON,
1,†
SHUANGYOU ZHANG,
1
SHILONG PAN,
2,3
AND JUNGWON KIM
1,
*
1
School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea
2
Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of Aeronautics and Astronautics (NUAA),
Nanjing 210016, China
3
e-mail: pans@nuaa.edu.cn
*Corresponding author: jungwon.kim@kaist.ac.kr
Received 2 October 2017; revised 15 November 2017; accepted 16 November 2017; posted 16 November 2017 (Doc. ID 307777);
published 13 December 2017
We propose and demonstrate an agile X-band signal synthesizer with ultralow phase noise based on all-fiber-
photonic techniques for radar applications. It shows phase noise of −145 dBc∕Hz (−152 dBc∕Hz) at 10 kHz
(100 kHz) offset frequency for 10 GHz carrier frequency with integrated RMS timing jitter between 7.6
and 9.1 fs (integration bandwidth: 10 Hz– 10 MHz) for frequencies from 9 to 11 GHz. Its frequency switching
time is evaluated to be 135 ns with a 135 pHz frequency tuning resolution. In addition, the X-band linear-
frequency-modulated signal generated by the proposed synthesizer shows a good pulse compression ratio approxi-
mating the theoretical value. In addition to the ultrastable X-band signals, the proposed synthesizer can also
provide 0–1 GHz ultralow-jitter clocks for analog-to-digital converters (ADC) and digital-to-analog converters
(DAC) in radar systems and ultralow-jitter optical pulse trains for photonic ADC in photonic radar systems.
The proposed X-band synthesizer shows great performance in phase stability, switching speed, and modulation
capability with robustness and potential low cost, which is enabled by an all-fiber-photonics platform and can be
a compelling technology suitable for future X-band radars.
© 2017 Chinese Laser Press
OCIS codes: (140.4050) Mode-locked lasers; (350.4010) Microwaves; (280.5600) Radar; (060.2310) Fiber optics; (320.7160)
Ultrafast technology.
https://doi.org/10.1364/PRJ.6.000012
1. INTRODUCTION
It is expected that photonic techniques will have enormous
potential in next-generation radar systems [1]: recently, a radar
system that takes advantages of photonics has indeed shown
great performance in field demonstration [2]. The heart of
the photonics-based radar is the mode-locked laser (MLL)
for tunable microwave signal generation and photonic ana-
log-to-digital conversion. Microwave signals with ultralow
phase noise can benefit radar systems mainly in the following
three aspects. First, lower phase noise can improve detection
sensitivity for targets’ velocity based on the Doppler effect [3].
For example, when reducing the phase noise of a 10 GHz
oscillator at 70 Hz offset frequency from −70 to −80 dBc∕Hz,
the probability of detection increases from 95% to 100% in a
coherent radar for detection of a target with velocity of 4 km/h.
On the other hand, if the phase noise is deteriorated to
−58 dBc∕Hz, the probability of detection would degrade to
zero [4,5]. The phase noise of the oscillator set the minimum
echo power level required for accurate detection [6,7]. Second,
lower phase noise improves imaging quality for a synthetic
aperture radar because the integrated phase noise of the oscil-
lator manifests itself as a deterioration of the impulse response
function [8]. Third, lower phase noise can reduce error vector
magnitude (EVM) in orthogonal frequency division multiplex-
ing (OFDM) communication, where the EVM is directly deter-
mined by the integrated phase noise of the carrier over a certain
offset frequency range [9,10]. As the integration of radar, com-
munications, and electronic warfare functions into one system
sharing the same radio frequency (RF) aperture and supporting
subsystems is the trend for future operation platforms, the com-
munication quality is critical [11–15]. Besides, OFDM is also
used in radar processing to overcome the typical drawbacks of
correlation operation processing [16,17].
Aside from low phase noise, agile tunability of the oscillator
frequency is also critical, for example, for electronic warfare sys-
tems. The probability of interception in electronic reconnais-
sance could be degraded by frequency hopping, which often
requires that the frequency changes 100 times or more in
1s[18]. Moreover, hopping frequencies are useful in a variety
of radar systems as well [19–21]. Typically, in a modern radar
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Vol. 6, No. 1 / January 2018 / Photonics Research
Research Article
2327-9125/18/010012-06 Journal © 2018 Chinese Laser Press