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Photonic True-Time Delay Beamforming Using a SwitchControlled Wavelength-Dependent Recirculating Loop
Jiejun Zhang and Jianping Yao
Microwave Photonics Research Laboratory, School of Electrical Engineering and Computer Science,
University of Ottawa, Ontario K1N 6N5, Canada
[email protected]
Abstract: A photonic true-time delay beamforming network using a switch-controlled
recirculating wavelength-dependent dispersive loop is proposed and demonstrated. By controlling
the number of round trips in the dispersive loop, tunable progressive time delays are achieved.
OCIS codes: (280.5110) Phased-array radar; (230.2035) Dispersion compensation devices; (060.5625) Radio frequency
photonics.
1. Introduction
Phased array antenna (PAA) plays a key role in modern radar systems as it can provide beam steering at a high
speed without mechanical movement with ultra-high directivity. A beamforming network is required to feed a phase
array antenna, which can be implemented using either phase shifters or true time delay lines. The advantage of using
true time delay lines is the beam is squint free, thus it is suitable for broadband applications. The time delays can be
realized using electronic delay lines, but with small bandwidth, large size and high loss. Photonics is considered a
potential solution to implement true-time delays with large bandwidth, small size and low loss [1]. In the past few
years, photonic true-time delay beamforming networks have been proposed and demonstrated based on free space
optics [2], fiber optics such as fiber Bragg gratings (FBGs) [3] and linearly chirped fiber Bragg gratings (LCFBGs)
[4], and integrated optics such as an integrated movable prism group [5] and a tunable optical filter to mimic the
spectral response of a delay line [6]. A free space optics based beamforming network is very bulky [2]. A fiberoptics based beamforming network has a smaller size [3, 4], but a tunable laser source is required, making the
system costly. An integrated optics based beamforming network has an ultra-smaller size, but only small the time
delays are achievable due to the small size of the chip [5, 6].
In this work, we introduce a fiber-optic true-time delay beamforming network using a switch-controlled
recirculating wavelength-dependent dispersive loop incorporating an LCFBG [7]. A laser array instead of a tunable
laser source is used. Thus, the system is simpler. A microwave signal to be radiated to the free space is modulated
on the multiple wavelengths from the laser array, which are sent to a switch-controlled recirculating wavelengthdependent dispersive loop [7]. Since the optical signals at different wavelengths are reflected from different
locations of the LCFBG, different time delays are achieved. The tuning of the time delays are achieved by
controlling the number of round trips in the loop, which is done by using a 2x2 optical switch. The proposed truetime delay beamforming network is experimentally demonstrated. A four true-time delay beamforming network
with a time delay difference of 2.5 ns per round trip with a maximum round trip of 10 is demonstrated.
2. Principle
The schematic diagram of the proposed true-time delay beamforming network is shown in Fig. 1(a). The optical
outputs from four laser diodes (LDs) with different wavelengths of 1 to 4 are combined at a wavelength-division
multiplexer (WDM) and sent to a Mach-Zehnder modulator (MZM), where a microwave signal, generated by an
electrical arbitrary waveform generator (AWG) to be radiated to free space, is modulated on the four wavelengths.
The optical signals at different wavelengths at the output of the MZM is then launched into a switch-controlled
wavelength-dependent recirculating loop through a 2x2 optical switch. In the loop, an LCFBG is adopted via an
optical circulator (OC) to provide different time delays for different wavelengths. An erbium-doped fiber amplifier
(EDFA) is also incorporated to compensate for the loss in the loop. Thus, that the optical signals can recirculate for
multiple round trips in the loop. The number of round trips is controlled by the 2x2 optical switch. At the output of
the loop, a second WDM is used to separate the time-delayed optical signals, which are converted to time delayed
microwave signals at the photodetectors (PDs).
If the optical switch is configured to make an optical signal at i recirculates in the dispersive loop multiple time,
a series of time delayed replicas of the optical signal will be generated, with a time interval between two adjacent
replicas determined by the wavelength-dependent time delay of the loop, given by
Ti  t0    i  r 
(1)
where t0 is the fixed time delay of the loop excluding the LCFBG;  is the dispersion coefficient of the LCFBG
and r is a reference wavelength. Now, we consider the time delay difference between two wavelengths i and i 1
recirculating in the loop after N round trips
(2)
t  NTi 1  NTi  N   i 1  i   N 
where  is the wavelength spacing between two adjacent wavelengths. Note that the optical wavelengths are
uniformly spaced. The time delay of an optical signal carried by a different wavelength relative to 1 as the round
trip number N increases is shown in Fig. 1(b). It can be seen that a time delay difference t given by (2) can be
achieved between the microwave signals carried by any two adjacent optical wavelengths. In our system, it is also
noted that as N increases, an increasing amount of time delay can be achieved, which is required to scan the beam at
different directions. The beam pointing angle can be tuned by changing the number of round trips, while the
maximum number of round trips determines the largest beam pointing direction.
N=0
LCFBG
LD3
LD4
3
1
1
EDFA
OC
WDM
LD2
AWG
2
WDM
LD1
2
3
1
MZM
2x2
Switch
4
Antenna
array
2
3
4
N=1
PD
N=2
t  0
1
2
3
4
t  
t  2
PD
t  3
N=3
PD
PD
Relative time delay to Channel 1
(a)
(b)
Fig. 1. (a) Schematic diagram of the true-time delay beamforming network. LD: laser diode; WDM: wavelength-division multiplexer; AWG:
arbitrary waveform generator; MZM: Mach-Zehnder modulator; EDFA: erbium-doped fiber amplifier; OC: optical circulator; PD: photodetector;
(b) the time delay of the signal in each channel relative to channel 1 as the number of round trips N increases.
3. Experiment
A total of four channels are used for the proof-of-concept experiment. The central wavelengths of the four LDs are
1548.9, 1549.9, 1550.9 and 1551.9 nm, with a uniform wavelength spacing of 1 nm. The LCFBG is fabricated to
have a dispersion coefficient of 2500 ps/nm within its 4-nm reflection band centered at 1550.5 nm. The spectrum of
the optical carrier and the spectral response of the LCFBG are given in Fig. 2. It can be calculated that
  2.5 ns , i.e., a true-time delay of 2.5 ns can be achieved between adjacent channels when the signal
recirculates in the loop for one round trip. The AWG, which has a sampling rate of 10 Gb/s, is configured to
generate a microwave signal to be transmitted. A 2x2 coupler is used instead of the 2x2 switch to simplify the
experiment and to study the round-trip-by-round-trip behavior of the system.
0
-10
100
(a)
Reflectivity (%)
Power (dBm)
-30
-40
-50
-60
10
80
8
60
6
40
4
20
2
Group Delay (ns)
-20
(b)
-70
-80
1548
1549
1550
1551
Wavelength (nm)
1552
0
0
1547 1548 1549 1550 1551 1552 1553 1554
Wavelength (nm)
Fig. 2. (a) Optical spectrum of the laser array, and (b) the magnitude and group delay responses of the LCFBG.
To demonstrate the operation of the system, an electrical pulse with a temporal width of 1 ns is generated by the
AWG. The waveforms detected at the outputs of the four PDs following the second WDM are shown in Fig. 3.
Multiple time-delayed replicas of the electrical pulse can be detected at the output of each channel. In addition, the
pulses from the four channels overlap in time only for N=0 and an increasing time shifts between the time delayed
pulses can be observed for N>0, indicating that a true-time delay has been achieved between the channels when the
pulse starts to recirculate in the loop. A reduced amplitude can be seen as N increases, as the EDFA cannot be
configured to fully compensate for the loop loss to avoid lasing. Nevertheless, the pulses can still recirculates for
more than 10 round trips before being imbedded in noise. The reduced amplitude may be compensated by a power
amplifier that is commonly used in phased array antenna.
More detailed waveforms are shown in Fig. 4 for different number of round trips of 0, 3 and 6. The time spacing
between two pulses from adjacent channels are measured to be 7.6 and 15.1 ns for N=3 and N=6, respectively,
which agree well with the theoretical values of 7.5 and 15 ns.
Voltage (mV)
2
N=1
N=0
1
2
3
4
1.5
N=2
N=3
1
N=4
N=5
0.5
0
-0.5
0
0.5
1
N=6
1.5
2
Time (ms)
N=7
2.5
3
3.5
Fig. 3. Measured time-delayed signals at the outputs of the four PDs when an electrical pulse is applied to the MZM.
1
2
3
4
Voltage (mV)
0.8
Voltage (mV)
(a)
1.5
1
0.5
0
-40
-20
0
20
Time offset (ns)
40
1
2
3
4
(b)
0.6
0.3
0.4
0.2
0
-40
1
2
3
4
(c)
0.25
Voltage (mV)
2
0.2
0.15
0.1
0.05
0
-20
0
20
Time offset (ns)
40
-40
-20
0
20
Time offset (ns)
40
Fig. 4. The detailed waveforms at the output of the true-time delay beamforming network. (a) N=0; (b) N=3 and (c) N=6.
4. Conclusion
We have proposed and demonstrated a photonic true-time delay beamforming network based on a switch-controlled
recirculating wavelength-dependent dispersive loop. By controlling the number of round trips that the optical signals
recirculate in the dispersive loop, different time delays were achieved. A four-channel true-time delay beamforming
network with a time delay difference of 2.5 ns per round trip and a maximum number of round trips of 10 was
demonstrated.
5. References
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controlled with phase and time delays," Appl. Opt. 35(26), 5293-5300 (1996).
[3] Y. Liu, J. Yao, and J. Yang, "Wideband true-time-delay unit for phased array beamforming using discrete-chirped fiber grating prism," Opt.
Comm. 207(1), 177-187 (2002).
[4] Y. Liu, J. Yang, and J. Yao, "Continuous true-time-delay beamforming for phased array antenna using a tunable chirped fiber grating delay
line," IEEE Photonics Technol. Lett. 14(8), 1172-1174 (2002).
[5] J. Wang, P. Hou, H. Cai, J. Sun, S. Wang, L. Wang, and F. Yang, "Continuous angle steering of an optically-controlled phased array antenna
based on differential true time delay constituted by micro-optical components," Opt. Express 23(7), 9432-9439 (2015).
[6] R. Bonjour, S. A. Gebrewold, D. Hillerkuss, C. Hafner, and J. Leuthold, "Continuously tunable true-time delays with ultra-low settling time,"
Opt. Express 23(5), 6952-6964 (2015).
[7] J. Zhang and J. Yao, "Time-stretched sampling of a fast microwave waveform based on the repetitive use of a linearly chirped fiber Bragg
grating in a dispersive loop," Optica 1(2), 64-69 (2014).