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Simultaneous generation of ultra-wideband LFM and phase-coded LFM microwave waveforms based on an improved frequency-sweeping OEO
Journal Pre-proof Simultaneous generation of ultra-wideband LFM and phase-coded LFM microwave waveforms based on an improved frequency-sweeping OEO Ranran Liu, Anle Wang, Pengfei Du, Xiong Luo, Yalan Wang, Jianghai Wo, Haida Yang, Jin Zhang
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S0030-4018(19)31030-2 https://doi.org/10.1016/j.optcom.2019.124938 OPTICS 124938
To appear in:
Optics Communications
Received date : 5 September 2019 Revised date : 10 November 2019 Accepted date : 12 November 2019 Please cite this article as: R. Liu, A. Wang, P. Du et al., Simultaneous generation of ultra-wideband LFM and phase-coded LFM microwave waveforms based on an improved frequency-sweeping OEO, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.124938. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Simultaneous Generation of Ultra-wideband LFM and Phase-coded LFM Microwave Waveforms Based on an Improved Frequency-sweeping OEO Ranran Liu, Anle Wang*, Pengfei Du*, Xiong Luo, Yalan Wang, Jianghai Wo, Haida Yang, Jin Zhang Air Force Early Warning Academy, Wuhan 430019, China *Corresponding author: [email protected], [email protected]
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Abstract: Simultaneous generation of linear frequency-modulated (LFM) and phase-coded LFM microwave waveform with a large time-bandwidth-product (TBWP) based on an improved frequency-sweeping optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The key significance of the frequency-sweeping OEO is that a near-zero-dispersion single-mode-fiber (SMF) is introduced into the loop, which can not only avoid the limitation of high frequency oscillation caused by dispersion, but also construct long OEO delay loop to realize large temporal duration. A fiber-Bragg- grating Fabry-Perot (FBG-FP) with large reflection bandwidth is also adopted to construct microwave photonic filter (MPF) with wide tuning range of central frequency. Thus, the limitations on temporal duration and oscillation frequency are solved and a LFM microwave waveform with large TBWP can be generated. Simultaneously, by modulating the optical sideband from the MPF and beating the phase-modulated signal with a portion of frequency-sweeping laser light, a phase-coded LFM microwave waveform can be generated. In the experimental demonstration, a LFM microwave waveform with a chirp rate of 0.736 GHz/μs and a large TBWP of 173,856 is generated, and reconfiguring central frequency and bandwidth is also completed. Simultaneously, a phase-coded LFM microwave waveform with a bandwidth of 8 GHz and a TBWP of 173,856 is also experimentally demonstrated. Keywords: Microwave photonics; Frequency-sweeping OEO; Linear frequency-modulated (LFM) microwave waveform; Phase-coded LFM microwave waveform; Large time-bandwidth product (TBWP). 1. Introduction [7,8], which is difficult to satisfy the requirements of Increasing environment complexity and weapons modern radar systems. diversity promote radars towards longer detection Photonic-assisted techniques to generate distance, higher range resolution and multi-functional microwave waveforms have been proposed and integration, which put forward requirements of not developed due to its advantages of high carrier only generating radar waveforms with large time frequency, large bandwidth and low loss [9,10]. bandwidth product (TBWP) but also simultaneous For example, optical pulse shaping followed by generation of multiple waveforms for modern radar space-to-time mapping or frequency-to-time systems [1-4]. To our knowledge, the linear frequency mapping are the most popular schemes to modulated (LFM) microwave waveform with large generate microwave waveforms [11-14]. The TBWP is mostly common used in pulse compression main components in the above schemes are radar system. However, the LFM microwave various spatial light modulators or optical waveform adopts simple frequency modulation, spectral shapers. The advantage of these which will can be easily intercepted by enemy radar schemes lies in obtaining waveforms with larger systems. In order to further enhance the bandwidth, while the major limitation is the anti-reconnaissance and anti-jamming capability as unchangeable parameters of waveform, which well as low probability of intercept performance in means less reconfiguration for radar systems. modern radar systems, the phase-coded LFM Another popular scheme is based on microwave waveforms provide a good strategy in electro-optical phase modulation [15,16], in complex electromagnetic environment [5,6]. which a parabolic or binary phase-coded Therefore, simultaneous generation of LFM and electrical signal is employed to modulate the phase-coded LFM microwave waveforms with a phase of one of the two wavelengths and then by large TBWP has a significant value for current beating the modulated wavelength and another development trend of radar systems, especially under wavelength at a photodetector (PD), a LFM or the situation of increasingly complex battlefield phase-coded microwave waveform can be environment. At present, waveform generation generated. In the implementation, the two methods based on electrical-domain techniques are wavelengths must be phase-correlated. The limited by the performance of electronics devices. The approach has the capability of producing a generated microwave waveforms suffer from low microwave signal with a wide tuning frequency, frequency, narrow bandwidth and poor tunability but the TBWP is limited due to the modulator
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photodetector, a phase-coded LFM microwave waveform can be generated. Compared with a pure LFM signal, the generated phase-coded LFM microwave waveform has a multiplicated TBWP. The proposed technique is experimentally verified. A LFM microwave waveform and a phase-coded LFM microwave waveform with a bandwidth of 8 GHz and a large TBWP of 173,856 are demonstrated. Their central frequencies and bandwidths can be tuned independently. To our knowledge, this may be the first time to realize simultaneous optical generation of LFM and phase-coded signal with large time duration, which may provide a waveform generation method for further radar system. 2. Principle A
ASG
B
T
EDFA2
PM2
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PM1
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which has limited electrical input power and low modulation coefficient. The deficiency can be solved by splitting the parabolic waveform to realize LFM signal generation with large bandwidth, and the temporal duration can be improved by phase-coding. However, the signal quality at the joint points of splitting parabolic segments requires to be improved [17,18]. Optical frequency multiplication techniques are developed to enhance the central frequency and bandwidth of generated microwave waveforms which can combine the advantage of photoelectric technology [19,20]. However, the phase noise of the generated signal cannot be guaranteed, especially with a high multiplication factor. What is more, the quality is restricted to an external microwave source. The schemes based on sweeping-frequency optoelectronic oscillating are current research focus, which can generate LFM waveforms with low phase noise [21,22]. In reference [21], frequency-sweeping electrical filter is constructed to replace the narrow-band electrical filter in the traditional optoelectronic oscillator (OEO) loop, by which the central frequency scans rapidly and periodically. However, the bandwidth of the generated LFM signal depends on the bandwidth of the driving signal, which is still limited by the electronic technology bottleneck. In reference [22], a microwave photonic filter (MPF) composed of phase modulator (PM) and phase-shifted fiber Bragg grating (PS-FBG) is adopted and LFM waveforms with bandwidth of 7.5GHz and temporal duration of 22.22 μs are generated. In addition, some other ingenious methods of frequency-sweeping OEO technique have also been proposed to generate LFM microwave waveforms [23,24]. Clearly, current schemes based on frequency-sweeping OEO can only generate LFM microwave waveforms. In summary, there is still no photonic technology that can generate multiple microwave waveforms with a large TBWP at the same time. In this paper, we propose and experimentally demonstrate a novel scheme to simultaneously generate LFM and phase-coded LFM microwave waveforms with large TBWP based on the improved frequency-sweeping OEO. In the OEO loop, the near-zero-dispersion single-mode-fiber (SMF) and the fiber Bragg grating Fabry-Perot (FBG-FP) are employed to enlarge the temporal duration and bandwidth, respectively. Thus, a LFM microwave waveform with a large TBWP can be generated. Meanwhile, an optical sideband filtered out by MPF in the OEO loop is then modulated in phase with a binary phase-coded electrical signal. By beating the phase modulated signal with a portion of the frequency-sweeping laser light at a high-speed
AWG
T E-SA Phase-Coded LFM
Near-zerodispersion SMF
PD1
LFM
Optical Path
Electrical Path
Fig.1. Schematic diagram of the proposed simultaneous generation of LFM and Phase-coded LFM microwave waveforms with large TBWP based on an improved frequency-sweeping OEO. The schematic diagram of the proposed system for simultaneous generation of LFM and phase-coded LFM microwave waveforms with large TBWP is shown in Fig.1. The scheme can be divided into two parts: an improved frequency-sweeping OEO loop (shown on pink board) and an optical phase-coded LFM signal generation subsystem (shown on yellow board). The former is composed of a frequency-sweeping laser, a PM, an FBG-FP, a PD and an electrical amplifier (EA). In order to generate signals with high frequency and wide bandwidth, traditional construction solution for OEO loop is by combination of SMF and dispersion compensation fiber (DCF), in which the DCF is employed to break the upper oscillation frequency limitation caused by fiber dispersion [25]. To avoid the large loss brought by DCF, a low-loss near-zero-dispersion SMF is introduced into the loop, which is not only aimed to avoid the limitation of high frequency oscillation caused by dispersion, but also to construct a long OEO delay loop to realize large temporal duration. A frequency-sweeping light wave from a distributed feedback (DFB) laser driven by a laser diode controller (LDC) is divided into two parts by an optical coupler (OC1). One of part entering the OEO loop is firstly modulated by a PM1 and then sent into an optical filter through an optical circulator. The optical filter used here is an FBG-FP, which has much
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where EB is the amplitude of the transmitted light, f o is the fixed transmitted frequency of the MWP. f o f c t f e t , in which f e t is the frequency of the LFM microwave waveform generated by OEO loop. As can be seen from the above, the temporal duration of LFM microwave waveform is T , the chirp rate is and its TBWPLFM T B T 2 . A binary phase-coded electrical signal generated by AWG at Point C can be expressed as P 1 M t kT 0 t PT (5) sC t = k 0 k 0 otherwise
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where P is the length of the binary encoding signal, T is the temporal duration of the one bit, and M k is the value of the one bit, which can be +1 or -1. Mathematically, by applying the phase-coding signal to PM2, the light waves at the output of the PM2 at Point D can be expressed as V sC t j 2 fo t + V
u D t EB e
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larger reflection bandwidth than traditional PS-FBG and ultra-narrow notch. Then, the light from the output of OC1 is amplified by an erbium-doped fiber amplifier (EDFA1) to compensate the power loss caused by optical filter and transmitted by a long near-zero-dispersion SMF. The light is fed into photodetector (PD1) and the output electrical signal from PD1 is amplified by the EA1. When the OEO loop is closed by applying the amplified electrical signal to PM1 and the loop gain is sufficiently high to compensate the loop loss, oscillation will start and a LFM microwave waveform with a large TBWP will be generated with the frequency sweeping of the DFB. The second part is an optical phase-coded LFM signal generation subsystem. The transmitted light of the FBG-FP is amplified by an EDFA2 and is then modulated in PM2 by a binary phase-coded electrical signal generated by an arbitrary waveform generator (AWG). The output phase-modulated signal and the other part light from of the OC1 are combined by an OC2 and then fed into a high-speed PD2. The generated microwave signal from PD2 is amplified by the EA2 and sent into spectrum analyzer or oscilloscope. Note that the principle for generating LFM microwave waveform based on frequency-sweeping OEO can be found in reference [15] and thus only the generation of phase-coded LFM waveform is theoretically analyzed here. When the triangular driving signal within one period is given by
where V is the amplitude of the electrical signal, and V is the half-wave voltage of the PM2. By beating the light waves at the output of the PM2 and the frequency-sweeping laser light at a high-speed photodetector, a phase-coded LFM microwave waveform with an increased length can be obtained, which is given by
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t id t I0 t
0t T /2
T /2t T
(1)
where T is the temporal duration and equal to the duration of the LFM generated in the OEO loop, I0 is the maximum current value and I 0 / T . The frequency of the swept laser within in one period can be expressed as
f t fc t c f c I 0 t
0t T /2
T /2t T
(2)
where f c is the central frequency of the laser, is the driving coefficient and is related to the amplitude of the triangular current signal and the sweeping range of the laser. The frequency of the swept laser at Point A can be expressed as
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u A t EA e j 2 fc t t
(3)
where is the amplitude of the EA frequency-sweeping laser light. An optical sideband transmitted by FBG-FP at Point B is given by
uB t EB e j 2 fot
(4)
2 V i (t ) cos 2 ( f c t f o )t sC (t ) V 2 V cos 2 f e t t sC (t ) V
(7)
It can be seen from (7) that a LFM microwave signal with a frequency of f e t is generated. Its phase changes with the sC t , which can be a binary code or Barker code. As can be seen, while the sC t is a N-bit Barker code, the new signal of the i t has the same bandwidth as the LFM microwave waveform generated by the frequency-sweeping OEO loop, and the TBWP of the phase-coded LFM microwave waveform has increased to N times of the LFM microwave waveform and TBWP=NT B=N T 2 N TBWPLFM . Thus, compared with a LFM microwave waveform, the newly obtained phase-coded LFM microwave waveform has the same bandwidth, but the temporal duration increases N times. The TBWP of the generated microwave waveform is significantly enlarged. 3. Experimental results and discussion To identify the ability of the proposed scheme for simultaneously generating LFM and phase-coded LFM microwave waveforms with
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Fig.2. Performance of the constructed MPF. (a) Optical reflection and transmission spectrum of the FBG-FP. (b) Frequency response of the MPF with the central frequency tuned from about 10 GHz to about 20 GHz with a tuning step of about 5 GHz. The inset shows the zoom-in view of the frequency response when the central frequency is tuned at about 15 GHz. LFM waveform with large TBWP is firstly demonstrated. Fig.3a shows the temporal waveform of the triangular driving signal, which is used to drive the DFB laser. It has a frequency of 46.015 kHz corresponding to a temporal duration of 21.732 μs the generated LFM microwave waveform in the time domain with a temporal duration of 21.732 μs , which has the same period with the triangular driving current and matches the frequency-sweeping oscillation theory [15]. Fig.3 (b) shows the generated microwave waveform in the time domain and the inset shows a real-time capture of a section of the LFM microwave waveform. Fig.3c shows the measured spectrum of the generated microwave waveform. It has a frequency range from 16 to 24 GHz which corresponds to a bandwidth of 8 GHz and a central frequency of 20 GHz. Thus, the bandwidth and central frequency all surpass the signals reported in ref. [20], which is due to the improvement effect of near-zero-dispersion SMF and FBG-FP on OEO loop. The TBWP of the generated LFM microwave waveform is as large as 173,856. Fig.3d shows the instantaneous frequency of LFM waveform, which is obtained by the short-time Fourier transform (STFT) of the generated microwave waveform. As can be seen, the waveform is periodic and the instantaneous frequency is linearly increasing and declining periodically. The chirp rate can be calculated to be 0.736 GHz/μs . A slight frequency nonlinearity of the generated LFM microwave waveform is observed, which might result from the nonlinear response of the laser diode to the triangular driving signal. In the future, the linearity of the LFM can be improved by driving the DFB laser with an inversely proportional electrical signal. In order to show the waveform reconfigurability of the proposed system, the periodic triangular driving signal is changed by either modifying its direct current bias voltage or the slope, which corresponds to the reconfiguring of central frequency or bandwidth of the generated waveform, respectively. Fig. 3 (e-f) give the experiment results. Fig. 3e shows the tuning range of the central frequency. As can be seen, the central
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larger TBWP, an experimental setup is constructed according to the Schematic in Fig.1. A triangular driving signal with the frequency of 46.015 kHz is generated by analog signal generator (Keysight N5183B). A frequency-sweeping laser consists of an LDC (Thorlabs CLD1015) with TEC driver and a DFB (Emcore 1752A) with a highest output power of 17dBm. The PM (PHOTLINE MPZ-LN-40) has a 3-dB bandwidth of 32 GHz and a half-wave voltage of 7 V. The PDs (Finisar HPDV2120R) have a 3-dB bandwidth of 50 GHz and a responsivity of 0.5 A/W. Two cascaded amplifiers (EA1, SHF S807B and EA2, SHF S804B) with the respective frequency gain ranges of 60 kHz-55 GHz and 90 kHz-60 GHz are used to amplify the electrical signal. The length of the near-zero-dispersion SMF (YOFC HIPOSH) used to construct the OEO loop is 3 km and the final time delay of the loop is 21.732 μs .The insertion loss of the fiber is 0.21 dB/km, which is obviously lower than the DCF fiber (1.4 dB/km, YOFC DCF-G.655C/250) used in our previous work. Thus, compared with traditional loop construction mode by combination of SMF and DCF, the new optical fiber have advantage of low loss. The AWG (Keysight, M9502A) is used to generate the binary phase-coded electrical signal. An optical spectrum analyzer (OSA, YOKOGAWA AQ6370D) and an electrical spectrum analyzer (E-SA, Keysight N9040B) are used to monitor the optical and electrical spectrum, respectively. A high-speed digital oscilloscope (Agilent Technologies DSO-X 93204A) with a sampling rate of 80 GSa/s is used to measure the electrical signal in time domain. The reflection and transmission spectrum of the FBG-FP are given in Fig.2 (a). Its notch is located at around 1550.528 nm, and its reflection is 1.25 nm, which corresponds to 156 GHz, which in principle determines the frequency tunable range of the OEO to be over 39 GHz. As depicted in the inset of Fig.2 (b), the 3-dB bandwidth of the passband of the FBG-FP is measured to be about 100 MHz. To investigate the frequency response of the MPF in the frequency-sweeping OEO, the open loop response between PM and EA is measured using a Vector Network Analyzer (VNA, Agilent N5244A).In order to display the tunability of the MPF, different frequency responses with central frequency from 10 GHz to 20 GHz are both tested as shown in Fig.2 (b). Clearly, due to the employment of near-zero-dispersion SMF, upper oscillation frequency limitation is broken through and frequency as high as 23 GHz can be oscillating in the OEO loop.
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Journal Pre-proof frequency is tuned from 16 GHz to 20 GHz with a bandwidth of 6 GHz by changing the direct current (DC) bias of the triangular driving signal. Fig. 3f shows the tuning range of the bandwidth. It can be also seen that the bandwidths can be tuned from 2 GHz to 6 GHz with the central frequency of 20 GHz. The frequency tuning range could be further enhanced by extending the bandwidths of the optical and electrical components. (d)
(b)
(e)
Figure. 3c. The experimental result is consistent with the previous theoretical derivation and analysis. Fig.4d shows the phase information recovered from the generated waveform by Hilbert transform. The degree of phase change is basically consistent with the theoretical value of . The time duration of each code is also about 10.866 μs . The TBWP of the generated phase-coded LFM microwave waveform is calculated to be 173,856, which is about twice of the LFM. Considering the binary phase-coded electrical signal with 13 Barker code, the temporal duration of the microwave waveform will be increased by 13 times. Thus, a phase-coded LFM waveform with increased TBWP is simultaneously generated with the LFM based on the proposed system.
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Fig.4 Experimental results of generating phase-coded LFM microwave waveforms. (a) Temporal waveform of the binary phase-coded electrical signal with the frequency of 46.015 kHz corresponds to a temporal duration of 21.732 μs . (b) Temporal waveform of the phase-coded LFM microwave waveform, the inset shows a section of the waveform. (c) Calculated frequency-to-time relations of the generated phase-coded LFM microwave waveform. (d) Recovered phase information from the temporal waveform in (b). In order to see the pulse compression ability, we calculated the autocorrelation of the generated LFM and phase-coded LFM microwave waveforms respectively, and the results are shown in Fig. (5). For the LFM, the width of the compressed pulse is around 0.145 ns, corresponding to a pulse compression ratio of 149,876. For the phase-coded LFM, the width of the compressed pulse is around 0.16 ns, corresponding to a pulse compression ratio of 135,825.
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Fig.3. Experimental results of generating LFM microwave waveform. (a) Temporal waveform of the triangular driving current with a temporal duration of 21.732 μs . (b) Temporal waveform of the generated LFM microwave waveform, the inset shows a section of the waveform. (c) Spectrum of the generated LFM microwave waveform with the bandwidth of 8 GHz. (d) Calculated frequency-to-time relations of the generated LFM microwave waveform. (e) The carrier frequency of the generated LFM microwave waveform is tuned from 16 GHz to 20 GHz with a bandwidth of 6 GHz. (f) With a fixed carrier frequency of 20 GHz, the bandwidth of the generated LFM microwave waveform is tuned from 2 GHz to 6 GHz. Generating phase-coded LFM waveform is also experimentally demonstrated based on the proposed system. Fig.4a shows the temporal waveform of the binary phase-coded electrical signal applied to the PM2, which is generated by the AWG. It has a peak-to-peak voltage of 7 V and a temporal duration of 21.732 μs . As the half-wave voltage of the PM is 7 V, the generated chirped microwave waveform will experience a phase shift. Figure.4b shows the generated microwave waveform in the time domain. The inset shows a time section of the generated phase-coded LFM signal. Figure.4c shows the instantaneous frequency of the generated waveform. It has a frequency range from 16 to 24 GHz which has the same frequency distribution of the LFM signal in
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microwave waveform. (b) Autocorrelation of the
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phase-coded LFM microwave waveform. 4. Conclusion In this paper, a novel scheme to simultaneous generating LFM and phase-coded LFM microwave waveforms with large TBWP based on an improved frequency-sweeping OEO is proposed and experimentally demonstrated. The key significance to increase the TBWP of LFM waveform is to use a near-zero-dispersion SMF and an FBG-FP with large reflection bandwidth. At the same time, a phase-coded LFM microwave waveform can be generated independently. The proposed technique is verified experimentally. LFM and phase-coded LFM microwave waveforms with different bandwidths and carrier frequencies are generated simultaneously. The generated microwave waveforms exhibited good reconfigurability, which lays a foundation for realizing multi-functional integrated radar systems. Acknowledgements This work was supported by Wuhan Science and Technology Project (2019010701011416) and the Natural Science Foundation of Hubei Province (2018CFB411, 2018CFB539, and 2018CFB331).
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zoom-in display). (a) Autocorrelation of the LFM
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Fig.5 The compressed pulse by autocorrelation (inset:
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References [1] Zou X, Zou F, Cao Z, Lu B, Yan X, Yu G, Deng X, Yao J, A Multifunctional Photonic Integrated Circuit for Diverse Microwave Signal Generation, Transmission, and Processing, Laser & Photonics Review. 1800240 (2019) 1. [2] Wu J, Wang K, Gu Y, Research on technology of microwave-photonic-based multifunctional radar, in Cie International Conference on Radar. (2017). [3] Xiu-Liang L I, Lin F U, The technology of multifunctional integration and resource management for shipborne integrated multifunctional radar systems, Radar & Ecm. 30 (2010) 7. [4] Han L, Wu K, 24GHz Integrated Radio and Radar System Capable of Time-Agile Wireless Communication and Sensing, Transactions on Microwave Theory & Techniques. 60 (2012) 619. [5] Deng H , Zhang J , Chen X , et al. Photonic Generation of a Phase-Coded Chirp Microwave Waveform With Increased TBWP[J]. IEEE Photonics Technology Letters, 2017:1-1. [6] Seleym A . Complementary phase coded LFM waveform for SAR[C]// Integrated Communications Navigation and Surveillance (ICNS). IEEE, 2016. [7] Kwon H, Kang B, Linear frequency modulation
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[20] Guo Q, Zhang F, Zhou P, Dual-band linear frequency modulation signal generation by optical frequency quadrupling and polarization multiplexing, IEEE Photonics Technology Letters. 29 (2017) 1320. [21] Q. Z. Cen, Y. T. Dai, F. F. Yin, Y. Zhou, J. Q. Li, J. Dai, L. Yu and K. Xu, Rapidly and continuously frequency-scanning opto-electronic oscillator, Optics Express. 25 (2017) 635. [22] T. F. Hao, Q. Z. Cen, Y. T. Dai, J. Tang, W. Li, J. Yao, N. H. Zhu and M. Li, Breaking the limitation of mode building time in an optoelectronic oscillator, Nature Communication. 9 (2018) 1839. [23] Maleki L , Savchenkov A A , Eliyahu D , et al. RF photonic components for miniature Doppler radar[C]// 2014 IEEE Avionics, Fiber-Optics and Photonics Technology Conference (AVFOP). IEEE, 2015. [24] Pei Z , Fangzheng Z , Shilong P . Generation of Linear Frequency-Modulated Waveforms by a Frequency-Sweeping Optoelectronic Oscillator[J]. Journal of Lightwave Technology, 2018, 36(18):3927-3934. [25] Wo J , Wang A , Zhang J , et al. Wideband frequency tunable optoelectronic oscillator based on a dispersion compensated microwave photonic filter[J]. Optical and Quantum Electronics, 2017, 49(12):411.
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*Author Contributions Section
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Anle Wang conceived and designed the experiments, and Ranran Liu performed the experiments. Ranran Liu, Pengfei Du, Xiong Luo and Jianghai Wo conducted analytical calculations and carried out theoretical derivations. Ranran Liu, Xiong Luo and Haida Yang analyzed the data. Ranran Liu, Anle Wang, Pengfei Du and Yalan Wang wrote the paper. Anle Wang, Jianghai Wo and Jin Zhang supervised the project.
PII: DOI: Reference:
S0030-4018(19)31030-2 https://doi.org/10.1016/j.optcom.2019.124938 OPTICS 124938
To appear in:
Optics Communications
Received date : 5 September 2019 Revised date : 10 November 2019 Accepted date : 12 November 2019 Please cite this article as: R. Liu, A. Wang, P. Du et al., Simultaneous generation of ultra-wideband LFM and phase-coded LFM microwave waveforms based on an improved frequency-sweeping OEO, Optics Communications (2019), doi: https://doi.org/10.1016/j.optcom.2019.124938. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
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Simultaneous Generation of Ultra-wideband LFM and Phase-coded LFM Microwave Waveforms Based on an Improved Frequency-sweeping OEO Ranran Liu, Anle Wang*, Pengfei Du*, Xiong Luo, Yalan Wang, Jianghai Wo, Haida Yang, Jin Zhang Air Force Early Warning Academy, Wuhan 430019, China *Corresponding author: [email protected], [email protected]
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Abstract: Simultaneous generation of linear frequency-modulated (LFM) and phase-coded LFM microwave waveform with a large time-bandwidth-product (TBWP) based on an improved frequency-sweeping optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The key significance of the frequency-sweeping OEO is that a near-zero-dispersion single-mode-fiber (SMF) is introduced into the loop, which can not only avoid the limitation of high frequency oscillation caused by dispersion, but also construct long OEO delay loop to realize large temporal duration. A fiber-Bragg- grating Fabry-Perot (FBG-FP) with large reflection bandwidth is also adopted to construct microwave photonic filter (MPF) with wide tuning range of central frequency. Thus, the limitations on temporal duration and oscillation frequency are solved and a LFM microwave waveform with large TBWP can be generated. Simultaneously, by modulating the optical sideband from the MPF and beating the phase-modulated signal with a portion of frequency-sweeping laser light, a phase-coded LFM microwave waveform can be generated. In the experimental demonstration, a LFM microwave waveform with a chirp rate of 0.736 GHz/μs and a large TBWP of 173,856 is generated, and reconfiguring central frequency and bandwidth is also completed. Simultaneously, a phase-coded LFM microwave waveform with a bandwidth of 8 GHz and a TBWP of 173,856 is also experimentally demonstrated. Keywords: Microwave photonics; Frequency-sweeping OEO; Linear frequency-modulated (LFM) microwave waveform; Phase-coded LFM microwave waveform; Large time-bandwidth product (TBWP). 1. Introduction [7,8], which is difficult to satisfy the requirements of Increasing environment complexity and weapons modern radar systems. diversity promote radars towards longer detection Photonic-assisted techniques to generate distance, higher range resolution and multi-functional microwave waveforms have been proposed and integration, which put forward requirements of not developed due to its advantages of high carrier only generating radar waveforms with large time frequency, large bandwidth and low loss [9,10]. bandwidth product (TBWP) but also simultaneous For example, optical pulse shaping followed by generation of multiple waveforms for modern radar space-to-time mapping or frequency-to-time systems [1-4]. To our knowledge, the linear frequency mapping are the most popular schemes to modulated (LFM) microwave waveform with large generate microwave waveforms [11-14]. The TBWP is mostly common used in pulse compression main components in the above schemes are radar system. However, the LFM microwave various spatial light modulators or optical waveform adopts simple frequency modulation, spectral shapers. The advantage of these which will can be easily intercepted by enemy radar schemes lies in obtaining waveforms with larger systems. In order to further enhance the bandwidth, while the major limitation is the anti-reconnaissance and anti-jamming capability as unchangeable parameters of waveform, which well as low probability of intercept performance in means less reconfiguration for radar systems. modern radar systems, the phase-coded LFM Another popular scheme is based on microwave waveforms provide a good strategy in electro-optical phase modulation [15,16], in complex electromagnetic environment [5,6]. which a parabolic or binary phase-coded Therefore, simultaneous generation of LFM and electrical signal is employed to modulate the phase-coded LFM microwave waveforms with a phase of one of the two wavelengths and then by large TBWP has a significant value for current beating the modulated wavelength and another development trend of radar systems, especially under wavelength at a photodetector (PD), a LFM or the situation of increasingly complex battlefield phase-coded microwave waveform can be environment. At present, waveform generation generated. In the implementation, the two methods based on electrical-domain techniques are wavelengths must be phase-correlated. The limited by the performance of electronics devices. The approach has the capability of producing a generated microwave waveforms suffer from low microwave signal with a wide tuning frequency, frequency, narrow bandwidth and poor tunability but the TBWP is limited due to the modulator
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photodetector, a phase-coded LFM microwave waveform can be generated. Compared with a pure LFM signal, the generated phase-coded LFM microwave waveform has a multiplicated TBWP. The proposed technique is experimentally verified. A LFM microwave waveform and a phase-coded LFM microwave waveform with a bandwidth of 8 GHz and a large TBWP of 173,856 are demonstrated. Their central frequencies and bandwidths can be tuned independently. To our knowledge, this may be the first time to realize simultaneous optical generation of LFM and phase-coded signal with large time duration, which may provide a waveform generation method for further radar system. 2. Principle A
ASG
B
T
EDFA2
PM2
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which has limited electrical input power and low modulation coefficient. The deficiency can be solved by splitting the parabolic waveform to realize LFM signal generation with large bandwidth, and the temporal duration can be improved by phase-coding. However, the signal quality at the joint points of splitting parabolic segments requires to be improved [17,18]. Optical frequency multiplication techniques are developed to enhance the central frequency and bandwidth of generated microwave waveforms which can combine the advantage of photoelectric technology [19,20]. However, the phase noise of the generated signal cannot be guaranteed, especially with a high multiplication factor. What is more, the quality is restricted to an external microwave source. The schemes based on sweeping-frequency optoelectronic oscillating are current research focus, which can generate LFM waveforms with low phase noise [21,22]. In reference [21], frequency-sweeping electrical filter is constructed to replace the narrow-band electrical filter in the traditional optoelectronic oscillator (OEO) loop, by which the central frequency scans rapidly and periodically. However, the bandwidth of the generated LFM signal depends on the bandwidth of the driving signal, which is still limited by the electronic technology bottleneck. In reference [22], a microwave photonic filter (MPF) composed of phase modulator (PM) and phase-shifted fiber Bragg grating (PS-FBG) is adopted and LFM waveforms with bandwidth of 7.5GHz and temporal duration of 22.22 μs are generated. In addition, some other ingenious methods of frequency-sweeping OEO technique have also been proposed to generate LFM microwave waveforms [23,24]. Clearly, current schemes based on frequency-sweeping OEO can only generate LFM microwave waveforms. In summary, there is still no photonic technology that can generate multiple microwave waveforms with a large TBWP at the same time. In this paper, we propose and experimentally demonstrate a novel scheme to simultaneously generate LFM and phase-coded LFM microwave waveforms with large TBWP based on the improved frequency-sweeping OEO. In the OEO loop, the near-zero-dispersion single-mode-fiber (SMF) and the fiber Bragg grating Fabry-Perot (FBG-FP) are employed to enlarge the temporal duration and bandwidth, respectively. Thus, a LFM microwave waveform with a large TBWP can be generated. Meanwhile, an optical sideband filtered out by MPF in the OEO loop is then modulated in phase with a binary phase-coded electrical signal. By beating the phase modulated signal with a portion of the frequency-sweeping laser light at a high-speed
AWG
T E-SA Phase-Coded LFM
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LFM
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Fig.1. Schematic diagram of the proposed simultaneous generation of LFM and Phase-coded LFM microwave waveforms with large TBWP based on an improved frequency-sweeping OEO. The schematic diagram of the proposed system for simultaneous generation of LFM and phase-coded LFM microwave waveforms with large TBWP is shown in Fig.1. The scheme can be divided into two parts: an improved frequency-sweeping OEO loop (shown on pink board) and an optical phase-coded LFM signal generation subsystem (shown on yellow board). The former is composed of a frequency-sweeping laser, a PM, an FBG-FP, a PD and an electrical amplifier (EA). In order to generate signals with high frequency and wide bandwidth, traditional construction solution for OEO loop is by combination of SMF and dispersion compensation fiber (DCF), in which the DCF is employed to break the upper oscillation frequency limitation caused by fiber dispersion [25]. To avoid the large loss brought by DCF, a low-loss near-zero-dispersion SMF is introduced into the loop, which is not only aimed to avoid the limitation of high frequency oscillation caused by dispersion, but also to construct a long OEO delay loop to realize large temporal duration. A frequency-sweeping light wave from a distributed feedback (DFB) laser driven by a laser diode controller (LDC) is divided into two parts by an optical coupler (OC1). One of part entering the OEO loop is firstly modulated by a PM1 and then sent into an optical filter through an optical circulator. The optical filter used here is an FBG-FP, which has much
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where EB is the amplitude of the transmitted light, f o is the fixed transmitted frequency of the MWP. f o f c t f e t , in which f e t is the frequency of the LFM microwave waveform generated by OEO loop. As can be seen from the above, the temporal duration of LFM microwave waveform is T , the chirp rate is and its TBWPLFM T B T 2 . A binary phase-coded electrical signal generated by AWG at Point C can be expressed as P 1 M t kT 0 t PT (5) sC t = k 0 k 0 otherwise
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where P is the length of the binary encoding signal, T is the temporal duration of the one bit, and M k is the value of the one bit, which can be +1 or -1. Mathematically, by applying the phase-coding signal to PM2, the light waves at the output of the PM2 at Point D can be expressed as V sC t j 2 fo t + V
u D t EB e
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larger reflection bandwidth than traditional PS-FBG and ultra-narrow notch. Then, the light from the output of OC1 is amplified by an erbium-doped fiber amplifier (EDFA1) to compensate the power loss caused by optical filter and transmitted by a long near-zero-dispersion SMF. The light is fed into photodetector (PD1) and the output electrical signal from PD1 is amplified by the EA1. When the OEO loop is closed by applying the amplified electrical signal to PM1 and the loop gain is sufficiently high to compensate the loop loss, oscillation will start and a LFM microwave waveform with a large TBWP will be generated with the frequency sweeping of the DFB. The second part is an optical phase-coded LFM signal generation subsystem. The transmitted light of the FBG-FP is amplified by an EDFA2 and is then modulated in PM2 by a binary phase-coded electrical signal generated by an arbitrary waveform generator (AWG). The output phase-modulated signal and the other part light from of the OC1 are combined by an OC2 and then fed into a high-speed PD2. The generated microwave signal from PD2 is amplified by the EA2 and sent into spectrum analyzer or oscilloscope. Note that the principle for generating LFM microwave waveform based on frequency-sweeping OEO can be found in reference [15] and thus only the generation of phase-coded LFM waveform is theoretically analyzed here. When the triangular driving signal within one period is given by
where V is the amplitude of the electrical signal, and V is the half-wave voltage of the PM2. By beating the light waves at the output of the PM2 and the frequency-sweeping laser light at a high-speed photodetector, a phase-coded LFM microwave waveform with an increased length can be obtained, which is given by
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t id t I0 t
0t T /2
T /2t T
(1)
where T is the temporal duration and equal to the duration of the LFM generated in the OEO loop, I0 is the maximum current value and I 0 / T . The frequency of the swept laser within in one period can be expressed as
f t fc t c f c I 0 t
0t T /2
T /2t T
(2)
where f c is the central frequency of the laser, is the driving coefficient and is related to the amplitude of the triangular current signal and the sweeping range of the laser. The frequency of the swept laser at Point A can be expressed as
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u A t EA e j 2 fc t t
(3)
where is the amplitude of the EA frequency-sweeping laser light. An optical sideband transmitted by FBG-FP at Point B is given by
uB t EB e j 2 fot
(4)
2 V i (t ) cos 2 ( f c t f o )t sC (t ) V 2 V cos 2 f e t t sC (t ) V
(7)
It can be seen from (7) that a LFM microwave signal with a frequency of f e t is generated. Its phase changes with the sC t , which can be a binary code or Barker code. As can be seen, while the sC t is a N-bit Barker code, the new signal of the i t has the same bandwidth as the LFM microwave waveform generated by the frequency-sweeping OEO loop, and the TBWP of the phase-coded LFM microwave waveform has increased to N times of the LFM microwave waveform and TBWP=NT B=N T 2 N TBWPLFM . Thus, compared with a LFM microwave waveform, the newly obtained phase-coded LFM microwave waveform has the same bandwidth, but the temporal duration increases N times. The TBWP of the generated microwave waveform is significantly enlarged. 3. Experimental results and discussion To identify the ability of the proposed scheme for simultaneously generating LFM and phase-coded LFM microwave waveforms with
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Fig.2. Performance of the constructed MPF. (a) Optical reflection and transmission spectrum of the FBG-FP. (b) Frequency response of the MPF with the central frequency tuned from about 10 GHz to about 20 GHz with a tuning step of about 5 GHz. The inset shows the zoom-in view of the frequency response when the central frequency is tuned at about 15 GHz. LFM waveform with large TBWP is firstly demonstrated. Fig.3a shows the temporal waveform of the triangular driving signal, which is used to drive the DFB laser. It has a frequency of 46.015 kHz corresponding to a temporal duration of 21.732 μs the generated LFM microwave waveform in the time domain with a temporal duration of 21.732 μs , which has the same period with the triangular driving current and matches the frequency-sweeping oscillation theory [15]. Fig.3 (b) shows the generated microwave waveform in the time domain and the inset shows a real-time capture of a section of the LFM microwave waveform. Fig.3c shows the measured spectrum of the generated microwave waveform. It has a frequency range from 16 to 24 GHz which corresponds to a bandwidth of 8 GHz and a central frequency of 20 GHz. Thus, the bandwidth and central frequency all surpass the signals reported in ref. [20], which is due to the improvement effect of near-zero-dispersion SMF and FBG-FP on OEO loop. The TBWP of the generated LFM microwave waveform is as large as 173,856. Fig.3d shows the instantaneous frequency of LFM waveform, which is obtained by the short-time Fourier transform (STFT) of the generated microwave waveform. As can be seen, the waveform is periodic and the instantaneous frequency is linearly increasing and declining periodically. The chirp rate can be calculated to be 0.736 GHz/μs . A slight frequency nonlinearity of the generated LFM microwave waveform is observed, which might result from the nonlinear response of the laser diode to the triangular driving signal. In the future, the linearity of the LFM can be improved by driving the DFB laser with an inversely proportional electrical signal. In order to show the waveform reconfigurability of the proposed system, the periodic triangular driving signal is changed by either modifying its direct current bias voltage or the slope, which corresponds to the reconfiguring of central frequency or bandwidth of the generated waveform, respectively. Fig. 3 (e-f) give the experiment results. Fig. 3e shows the tuning range of the central frequency. As can be seen, the central
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larger TBWP, an experimental setup is constructed according to the Schematic in Fig.1. A triangular driving signal with the frequency of 46.015 kHz is generated by analog signal generator (Keysight N5183B). A frequency-sweeping laser consists of an LDC (Thorlabs CLD1015) with TEC driver and a DFB (Emcore 1752A) with a highest output power of 17dBm. The PM (PHOTLINE MPZ-LN-40) has a 3-dB bandwidth of 32 GHz and a half-wave voltage of 7 V. The PDs (Finisar HPDV2120R) have a 3-dB bandwidth of 50 GHz and a responsivity of 0.5 A/W. Two cascaded amplifiers (EA1, SHF S807B and EA2, SHF S804B) with the respective frequency gain ranges of 60 kHz-55 GHz and 90 kHz-60 GHz are used to amplify the electrical signal. The length of the near-zero-dispersion SMF (YOFC HIPOSH) used to construct the OEO loop is 3 km and the final time delay of the loop is 21.732 μs .The insertion loss of the fiber is 0.21 dB/km, which is obviously lower than the DCF fiber (1.4 dB/km, YOFC DCF-G.655C/250) used in our previous work. Thus, compared with traditional loop construction mode by combination of SMF and DCF, the new optical fiber have advantage of low loss. The AWG (Keysight, M9502A) is used to generate the binary phase-coded electrical signal. An optical spectrum analyzer (OSA, YOKOGAWA AQ6370D) and an electrical spectrum analyzer (E-SA, Keysight N9040B) are used to monitor the optical and electrical spectrum, respectively. A high-speed digital oscilloscope (Agilent Technologies DSO-X 93204A) with a sampling rate of 80 GSa/s is used to measure the electrical signal in time domain. The reflection and transmission spectrum of the FBG-FP are given in Fig.2 (a). Its notch is located at around 1550.528 nm, and its reflection is 1.25 nm, which corresponds to 156 GHz, which in principle determines the frequency tunable range of the OEO to be over 39 GHz. As depicted in the inset of Fig.2 (b), the 3-dB bandwidth of the passband of the FBG-FP is measured to be about 100 MHz. To investigate the frequency response of the MPF in the frequency-sweeping OEO, the open loop response between PM and EA is measured using a Vector Network Analyzer (VNA, Agilent N5244A).In order to display the tunability of the MPF, different frequency responses with central frequency from 10 GHz to 20 GHz are both tested as shown in Fig.2 (b). Clearly, due to the employment of near-zero-dispersion SMF, upper oscillation frequency limitation is broken through and frequency as high as 23 GHz can be oscillating in the OEO loop.
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(b)
(e)
Figure. 3c. The experimental result is consistent with the previous theoretical derivation and analysis. Fig.4d shows the phase information recovered from the generated waveform by Hilbert transform. The degree of phase change is basically consistent with the theoretical value of . The time duration of each code is also about 10.866 μs . The TBWP of the generated phase-coded LFM microwave waveform is calculated to be 173,856, which is about twice of the LFM. Considering the binary phase-coded electrical signal with 13 Barker code, the temporal duration of the microwave waveform will be increased by 13 times. Thus, a phase-coded LFM waveform with increased TBWP is simultaneously generated with the LFM based on the proposed system.
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Fig.4 Experimental results of generating phase-coded LFM microwave waveforms. (a) Temporal waveform of the binary phase-coded electrical signal with the frequency of 46.015 kHz corresponds to a temporal duration of 21.732 μs . (b) Temporal waveform of the phase-coded LFM microwave waveform, the inset shows a section of the waveform. (c) Calculated frequency-to-time relations of the generated phase-coded LFM microwave waveform. (d) Recovered phase information from the temporal waveform in (b). In order to see the pulse compression ability, we calculated the autocorrelation of the generated LFM and phase-coded LFM microwave waveforms respectively, and the results are shown in Fig. (5). For the LFM, the width of the compressed pulse is around 0.145 ns, corresponding to a pulse compression ratio of 149,876. For the phase-coded LFM, the width of the compressed pulse is around 0.16 ns, corresponding to a pulse compression ratio of 135,825.
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Fig.3. Experimental results of generating LFM microwave waveform. (a) Temporal waveform of the triangular driving current with a temporal duration of 21.732 μs . (b) Temporal waveform of the generated LFM microwave waveform, the inset shows a section of the waveform. (c) Spectrum of the generated LFM microwave waveform with the bandwidth of 8 GHz. (d) Calculated frequency-to-time relations of the generated LFM microwave waveform. (e) The carrier frequency of the generated LFM microwave waveform is tuned from 16 GHz to 20 GHz with a bandwidth of 6 GHz. (f) With a fixed carrier frequency of 20 GHz, the bandwidth of the generated LFM microwave waveform is tuned from 2 GHz to 6 GHz. Generating phase-coded LFM waveform is also experimentally demonstrated based on the proposed system. Fig.4a shows the temporal waveform of the binary phase-coded electrical signal applied to the PM2, which is generated by the AWG. It has a peak-to-peak voltage of 7 V and a temporal duration of 21.732 μs . As the half-wave voltage of the PM is 7 V, the generated chirped microwave waveform will experience a phase shift. Figure.4b shows the generated microwave waveform in the time domain. The inset shows a time section of the generated phase-coded LFM signal. Figure.4c shows the instantaneous frequency of the generated waveform. It has a frequency range from 16 to 24 GHz which has the same frequency distribution of the LFM signal in
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microwave waveform. (b) Autocorrelation of the
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phase-coded LFM microwave waveform. 4. Conclusion In this paper, a novel scheme to simultaneous generating LFM and phase-coded LFM microwave waveforms with large TBWP based on an improved frequency-sweeping OEO is proposed and experimentally demonstrated. The key significance to increase the TBWP of LFM waveform is to use a near-zero-dispersion SMF and an FBG-FP with large reflection bandwidth. At the same time, a phase-coded LFM microwave waveform can be generated independently. The proposed technique is verified experimentally. LFM and phase-coded LFM microwave waveforms with different bandwidths and carrier frequencies are generated simultaneously. The generated microwave waveforms exhibited good reconfigurability, which lays a foundation for realizing multi-functional integrated radar systems. Acknowledgements This work was supported by Wuhan Science and Technology Project (2019010701011416) and the Natural Science Foundation of Hubei Province (2018CFB411, 2018CFB539, and 2018CFB331).
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zoom-in display). (a) Autocorrelation of the LFM
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Fig.5 The compressed pulse by autocorrelation (inset:
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*Author Contributions Section
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Anle Wang conceived and designed the experiments, and Ranran Liu performed the experiments. Ranran Liu, Pengfei Du, Xiong Luo and Jianghai Wo conducted analytical calculations and carried out theoretical derivations. Ranran Liu, Xiong Luo and Haida Yang analyzed the data. Ranran Liu, Anle Wang, Pengfei Du and Yalan Wang wrote the paper. Anle Wang, Jianghai Wo and Jin Zhang supervised the project.