Research Article Frequency Scanning Multibeamforming Method Based on CFBG Photonic Microwave Oscillation

In this paper, a two-loop photoelectric oscillator based on chirped fber Bragg grating (CFBG) is used to construct a swept source, which acts on the frequency scanning array antenna to realise multibeamforming. Te simulation results of the designed beamforming system have shown that it can realise wide-range beam scanning and has ultralow phase noise.


Introduction
Beamforming is one of the key technologies for radar systems. Te principle of phased array antenna to realise beam scanning relies on the phase shift between diferent signals arriving at each radiating elements [1]. Tere are two major electrical signal delay devices: one is to use metal wires, and the delay is adjusted by controlling the length of the wires; the other is to use phase shifters, and the wavefront control is realised by regulating the phase of diferent units. Te disadvantages of metal wires are bulky volume, heavy weight, and susceptibility to electromagnetic interference. Phase shifters are only suitable for narrow bands [2]. For large instantaneous bandwidth, the "squint" phenomenon makes it difcult to meet the demand of modern radar systems. In order to realise wideband angular scanning of phased-array radars, true time delay (TDD) could be used instead of phase shift to achieve precise control of time delay in broadband scenario [3].
With the development of microwave photonic technology, especially optical true time delay (OTTD) technology, the performance of phased-array radar has been further improved [4]. Te basic principle of OTTD is to convert the microwave signal into an optical signal by a photoelectric modulator, and then use the optical method to control the delay, and fnally use a photoelectric conversion device to restore the microwave signal. Compared with the electrical phase shifters, OTTD technology has the advantages such as wideband, stable transmission, light weight, and no radiation interference [5].
Optically controlled beamforming has become a hot topic in the recent years. Te optical time delay based on wavelength tuning and dispersive element helps to improve the processing bandwidth and scanning accuracy. In the beamforming system based on fber dispersion, the OTTD is realised by changing the output wavelength of the laser device. Te OTTD technologies realise the optically controlled beamforming for phased array antennas. However the introduction of the OTTD devices results in a large insertion loss [6]. In addition, due to the complexity of the system, and the optoelectronic integration technology is relatively preliminary compared to electrical integration, the current application scope is limited.
Te frequency scanning array antenna is a special case of series-fed phased array antennas, where beam steering is controlled by tuning the frequency of the exciter [7]. For the frequency scanning array antenna, the transmission line of a certain length is used to replace the phase shifter, so the insertion loss is relatively small. Te frequency band generated by the traditional frequency source is relatively narrow, and the range of beam scanning is therefore limited.
Te aim of this paper is to propose a beamforming system based on photonic microwave frequency step scanning. Te microwave photonic oscillation and CFBG tuning have been adopted in the system. Te system can realise wide range of beam scanning and has ultralow phase noise, and it has the advantages such as less control elements, continuously adjustable frequency, compact size, and low cost.

Frequency Scanning Beamforming Principle
Te traditional phased array antenna beamforming is achieved by adjusting the amplitude and phase excitation of each antenna element through an electric phase shifter. A schematic diagram of the phased array antenna with N elements is shown in Figure 1.
From Figure 2, the pattern of the phased array antenna can be expressed as where λ is the wavelength in free space, d is the spacing of adjacent elements, θ is the exit angle of the beam, ∆ϕ B is the phase diference between adjacent elements, and j is the imaginary unit. Te frequency scanning array antenna uses the transmission lines to replace the shifters, as shown in Figure 2.
Te phase diference ∆ϕ B between adjacent elements could be expressed as where λ g is the wavelength of the radio wave in the transmission line and L is the length of transmission line between adjacent elements. Te pattern of the phased array antenna in (1) can be rewritten as For a frequency scanning array antenna, the beam direction changes in accordance with the frequency of the excitation signal. In the following section, a CFBG-based frequency scanning beamforming system is presented with detailed simulation analysis.

CFBG-Based Frequency Scanning
Beamforming System

System Composition.
A low-loss, high-efciency frequency scanning beamforming system based on microwave photonics is designed. Te system design block diagram is shown in Figure 3. Te frequency source (inside the dashed boxed) is given by a CFBG-based two-loop photoelectric oscillator [8], which is mainly composed of a light source, a polarizer, a polarization combiner, single mode fbers, a photodiode, a radio frequency amplifer, a chirped grating, and a Mach-Zehnder external modulator (MZM) [9][10][11][12][13]. Te tuneable photoelectric oscillator can be tuned by using a chirped fber Bragg grating (CFBG) as a photonic flter to select the frequency of the oscillation mode [14][15][16]. Te carrier signal f 0 provided by the laser source is input to the modulator. A signal with frequency f is obtained at the output end. By reasonably designing τ 1 and τ 2 , the phase noise generated by the excitation signal at the ofset center frequency of 10 kHz can reach about − 140.5 dBc/Hz [8], compared to − 105 dBc/Hz with the conventional phaselocking technique. By adjusting the dispersion characteristic of the chirped grating, the oscillation frequency of the photon RF oscillator could be changed.
In the system, the optical carrier output by the light source is modulated by the MZM to produce two or more optical waves of diferent frequencies at the output of the modulator. Te output of the modulator could be expressed as follows [17]: where ω 0 represents the angular frequency of laser carrier, ω represents the angular frequency of RF modulated wave, E 0 is the amplitude, V ac is the amplitude of the AC drive voltage, V dc is the DC ofset voltage, V π is the half wave voltage of the modulator, and ∆θ is the phase diference where η is the degree of unbalance of the MZM, β is the phase modulation coefcient, V B is the bias voltage of the modulator, V π is the half wave voltage of the modulator, V 0 is a constant bias voltage without charge, and V Ph is the amplitude of the oscillation signal. J 0 and J 2m are Bessel functions.
In the system, the chirped grating is connected to the photoelectric oscillator to form a photon flter. Te frequency of the oscillation mode could be selected to achieve frequency adjustment. Te frequency response of the photon flter corresponding to the grating is given as [16,17] In (6), α � tan(π/2V B − V 0 − V π /V π ), λ 0 represents the wavelength of the laser source, c is the speed of light, and D represents the grating dispersion value. When the system satisfes extreme value will occur, and the frequency of peak point is According to (8), the central frequency of photoelectric oscillation is determined by three parameters: wavelength of light source, grating dispersion, and bias voltage. Te dispersion characteristic of the grating can be adjusted to achieve large-bandwidth frequency tuning, thereby realizing large-scale scanning of the antenna beam.

Simulation Analysis.
Te simulation analysis of CFBGbased frequency scanning beamforming system in X band is presented in this section.
Te spectrum generated by the CFBG-based frequency scanning source is shown in Figure 4, where the dispersion value D � 850 ps/nm, the light source wavelength λ 0 � 1550nm, the bias voltage V B � 2.4V, and the fber length is 0.6 km and 1.6 km, respectively. It can be seen that the oscillation frequency of the two-loop photoelectric oscillator is stabilized at 10.9 GHz, and the edge touch suppression reaches 75 dB. As shown in Figure 5, the phase noise can reach − 132 dBc/Hz@10 kHz.
When the CFBG dispersion parameter changes in the swept source, the beam pointing angle changes accordingly. As shown in Figure 6, when the source oscillation frequency f g is stabilized at 10 GHz, the beam is directed to the normal direction. When it is stable at 10.9 Hz, the beam pointing angle is θ B � 10.4 ∘ .
Te oscillation frequency of the two-loop photoelectric oscillator varies with the grating dispersion value. Te variation curve of the oscillation frequency with the dispersion value in the X-band is displayed in Figure 7. It can be seen that the grating dispersion value and the oscillation frequency are almost linearly related in the X-band, and the oscillation frequency gradually decreases as the value of the grating increases.
Adjust the dispersion value in the step of 100 ps/nm to obtain diferent oscillation frequencies in the X-band range and to obtain continuous beam scanning. Te simulation results are shown in Figures 8 and 9. It can be seen that the designed CFBG-based frequency scanning beamforming system can achieve continuous beam scanning within ±30 ∘ .

Multibeamforming System.
Te frequency scanning beamforming system constructed in the abovementioned sections applies to point-frequency or narrow-band systems. In practice where multibeam search and precise positioning are required, a two-plane scanning structure can be adopted, as shown in Figure 10. Te second plane is added to the frequency scanning system. In the designed system, n frequency swept subarrays are used to implement n beams in space. Multibeam formation at diferent locations can be achieved by adjusting the length of the transmission line between the subarrays. Te system enables full-scale, widerange scanning of the beam.

Simulation Analysis.
In order to achieve spatial 4-beam scanning, that is, n � 4 in Figure 10, use diferent lengths for transmission lines between adjacent subarrays. Take L 1 � 0.03 m, L 2 � 0.05 m, L 3 � 0.06 m, and L 4 � 0.08 m. When the output frequency of the swept source is stabilized at 10.9 GHz, the four beams generated are shown in Figure 11.
With the change in the fber Bragg grating dispersion value, the two-loop photoelectric oscillator will generate diferent frequency output signals to act on the frequency scanning array, which can realise the synchronous scanning of spatial 4-beams. When the CFBG dispersion value of the swept source frequency changes from 850 ps/nm to 900 ps/ nm, the simulation result diagram of the 4-beams is shown in Figure 12. It can be seen that the beams are defected with the change of the swept source frequency.       International Journal of Optics

Conclusions
In this paper, the CFBG-based two-loop photoelectric oscillator is used to construct a swept source, which can obtain high-quality, low-phase-frequency shift spectrum. Te swept source is applied to the frequency scanning system to obtain multibeamforming scanning. Te detailed design structure and implementation method are given. Te simulation results have shown that large scale continuous beam scanning could be obtained. Te system has ultralow phase noise and advantages such as less control elements, continuously adjustable frequency, compact size, and low cost.

Data Availability
Te data used to support the fndings of this study are available from the authors upon reasonable request.

Conflicts of Interest
Te authors declare that there are no conficts of interest regarding the publication of this paper. International Journal of Optics 7