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Photonics-based radar with balanced I/Q

de-chirping for interference-suppressed

high-resolution detection and imaging

XINGWEI YE,

1,†

FANGZHENG ZHANG,

1,2,†

YUE YANG,

AND SHILONG PAN

1,3

Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, Nanjing University of Aeronautics

and Astronautics, Nanjing 210016, China

e-mail: zhangfangzheng@nuaa.edu.cn

e-mail: pans@nuaa.edu.cn

Received 28 September 2018; revised 9 December 2018; accepted 24 December 2018; posted 2 January 2019 (Doc. ID 346958);

published 11 February 2019

Photonics-based radar with a photonic de-chirp receiver has the advantages of broadband operation and real-time

signal processing, but it suffers from interference from image frequencies and other undesired frequency-mixing

components, due to single-channel real-valued photonic frequency mixing. In this paper, we propose a photonics-

based radar with a photonic frequency-doubling transmitter and a balanced in-phase and quadrature (I/Q)

de-chirp receiver. This radar transmits broadband linearly frequency-modulated signals generated by photonic

frequency doubling and performs I/Q de-chirping of the radar echoes based on a balanced photonic I/Q frequency

mixer, which is realized by applying a 90° optical hybrid followed by balanced photodetectors. The proposed

radar has a high range resolution because of the large operation bandwidth and achieves interference-free de-

tection by suppressing the image frequencies and other undesired frequency-mixing components. In the experi-

ment, a photonics-based K-band radar with a bandwidth of 8 GHz is demonstrated. The balanced I/Q de-chirping

receiver achieves an image-rejection ratio of over 30 dB and successfully eliminates the interference due to the

baseband envelope and the frequency mixing between radar echoes of different targets. In addition, the desired de-

chirped signal power is also enhanced with balanced detection. Based on the established photonics-based radar,

inverse synthetic aperture radar imaging is also implemented, through which the advantages of the proposed radar

are verified.

https://doi.org/10.1364/PRJ.7.000265

1. INTRODUCTION

Detection and imaging of objects by radar has critically impor-

tant applications in both civil and security areas. By transmit-

ting radio frequency (RF) signals that feature low loss in rain,

fog, and smoke, radars can break the limitation of natural illu-

mination and are superior to optical sensors in terms of oper-

ation in darkness and bad weather [1]. To achieve a high range

resolution required in emerging applications such as pilotless

automobiles, smart navigators, and unmanned aerial vehicles,

a radar transceiver should be able to generate and process RF

signals with large instantaneous bandwidths [2], which is quite

a challenge for state-of-art electronics.

Fortunately, the generation and processing of broadband RF

signals can be implemented by microwave photonic technolo-

gies [3,4]. By converting the RF signal into the optical domain,

the fractional bandwidth of the optical signal is about 4 orders

of magnitude less than that of the original RF signal, which

dramatically relieves the difficulties in achieving a flat response

over a large bandwidth. Microwave photonic technologies have

other advantages, including ultralow transmission loss, small

size, and immunity to electromagnetic interference. Until

now, various photonic techniques have been proposed for radar

applications, including photonic generation of broadband radar

signals [5–7], microwave photonic phase shifters and filters

[8–10], photonic frequency mixing [11–15], photonic analog-

to-digital conversio n [16–18], phase-stable RF signal transmis-

sion [19–21], and photonic true time delay RF beam forming

[22–24]. Some of these techniques have realized remarkable

superiority when compared with the state-of-art electronics.

For example, with the precise phase shift brought by an optical

hybrid coupler, a photonic microwave mixer in Ref. [15]

achieved an image rejection and mixing spur suppression of

more than 60 dB, which far outweighs the performance of

the electronic mixers [25,26]. Such kinds of photonic tech-

niques can provide promising options to construct a photonics-

based radar, which can overcome the bandwidth limitations of

traditional radar.

Research Article

Vol. 7, No. 3 / March 2019 / Photonics Research 265

Recently, photonics-based radars by transmitting photonic-

generated broadband linearly frequency-modulated (LFM) sig-

nals and receiving radar echoes with photonic de-chirping were

proposed [7,27–33]. In these radar systems, a large operation

bandwidth over 10 GHz can be easily achieved, leading to an

ultrahigh resolution up to 1.3 cm [29]. In addition, the band-

width compression property of the broadband photonic de-

chirping receiver makes it possible for real-time signal process-

ing. Based on this photonics-based radar architecture, high

resolution and real-time target detection and imaging were suc-

cessfully demonstrated [30–32], and photonics-based multiple-

input-multiple-output radar and multiband radar were also

demonstrated [33]. In spite of the huge advantages, there

are still problems limiting the performance of the current pho-

tonic de-chirping radar receivers. First, the single-channel pho-

tonic frequency-mixing radar receiver only outputs a real-

valued de-chirped signal, of which the spectrum is symmetric

around zero frequency. This causes image-frequency interfer-

ence that makes it im possible to distinguish the targets on

the two sides of the observational reference point. This problem

was ignored in previous photonics-based radar demonstrations

in which only the targets on one side of the reference point were

considered. In practical radar application s, determination of the

sign (positive or negative) of the de-chirped frequency is highly

desirable to avoid image-frequency interference. Second, when

performing radar echo de-chirping, the photonic frequency

mixing between radar echoes reflected from different targets

generates interference components in the de-chirped signal

spectrum, whi ch not only conceals the real targets but also pro-

duces false targets. Third, the baseband signal or the envelop of

the radar pulses exists in the de-chirped signal, r esulting in in-

terference in the low-frequency range of the de-chirped signal

spectrum as well as a reduced power efficiency. Therefore, the

photonics-based radar transceiver should be improved to reject

as much interference as possible in the optical and analog do-

main so that the advantages of microwave photonic techniques

could be further exploited to simplify the digital radar signal

processing and false target discrimination.

In this paper, we propose a photonics-based broadband ra-

dar based on frequency doubling and a balanced in-phase and

quadrature (I/Q) de-chirp receiver. In the transmitter, photonic

frequency doubling of an intermediate frequency (IF)-LFM sig-

nal is implemented to generate a broadband transmitted signal

so as to achieve a high r ange resolution. In the receiver, bal-

anced I/Q de-chirping detection is performed based on a

90° optical hybrid followed by two balanced photodetectors

(BPDs), to acquire a complex de-chirped signal. This I/Q

de-chirping scheme can determine the sign of the de-chirped

frequencies, and hence distinguish the targets on both sides

of the observational reference point. Besides, the frequency

components due to frequency mixing between echoes of differ-

ent targets and the baseband background signal are removed by

balanced detection. Thus, the undesired interference and the

false targets are eliminated. Furthermore, the power of the

de-chirped frequency components is enhanced by balanced

detection compared with the system using single-channel

detection. A photonics-based K-band radar with a bandwidth

of 8 GHz (18–26 GHz) is established. Performance of the

broadband LFM signal generation and the balanced I/Q de-

chirp processing are investigated separately. Inverse synthetic

aperture radar (ISAR) imaging is also demonstrated to show

the advantages of the proposed photonics-based radar.

2. PRINCIPLE

Figure 1 shows the structure of the proposed photonics-based

radar. A continuous wave (CW) light from a laser diode (LD) is

equally split into two branches by an optical coupler (OC,

OC1), which are used as the optical carriers in the transmitter

and the receiver, respectively. The carrier in the upper branch is

modulated by an IF-LFM signal at a Mach–Zehnder modulator

(MZM, MZM1), in which the IF-LFM signal is generated by

an electrical signal generator and MZM1 is biased at the peak

transmission point. Assuming the center angular frequency, the

bandwidth, and the chirp slope of the IF-LFM signal are ω

B, and k, respectively, the optical field at the output of MZM1

can be expressed as

tcosβ cosω

t  πkt

 expjω

t

≈ −J

β exp jω

t − 2ω

t − 2πkt



 J

β expjω

t

− J

β expjω

t  2ω

t  2πkt

, (1)

where ω

is the angular frequency of the laser source, β is the

modulation index, J

· is the nth order Bessel function, and t

satisfies −B∕2k ≤ t ≤ B∕2k. In obtaining Eq. (1), high or-

der (>2) terms are neglected assuming that the modulation in-

dex of MZM1 is small. As a result, only the optical carrier and

the 2nd-order modulation sidebands are obtained after

MZM1, as illustrated by the spectrum at point A in the inset

of Fig. 1. This optical signal is divided into two branches by

another OC (OC2). The signal in the upper branch is sent

to a photod etector (PD) to perform the optical-to-electrical

conversion. Because of the heterodyning between the optical

carrier and the frequency-sweeping 2nd-order sidebands, a

Fig. 1. Schematic diagram of the proposed photonics-based radar.

LD, laser diode; ESG, electrical signal generator; OC, optical coupler;

MZM, Mach–Zehnder modulator; OBPF, optical bandpass filter; PA,

power amplifier; LNA, low-noise amplifier; PD/BPD, (balanced)

photodetector; ADC, analog-to-digital conversion; DSP, digital signal

processing. (Insets, optical spectra at several key points in the system.)

266 Vol. 7, No. 3 / March 2019 / Photonics Research

Research Article

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