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A novel photonic approach to generating binary and quaternary phase-coded microwave waveforms with an ultra-wide frequency tunable range is proposed and experimentally demonstrated. In the proposed system, a dual-parallel Mach-Zehnder modulator (DP-MZM) is used as an optical wavelength shifter. To generate a phase-coded microwave waveform, the coding signal is modulated on the original wavelength using a phase modulator (PM). Combining the shifted wavelength and the original wavelength, two wavelengths with a frequency space determined by the input microwave signal are obtained. Applying them to a photodetector (PD), a phase-coded microwave waveform is generated. The key significance of the approach is that both binary and quaternary phase-coded microwave waveforms can be generated with an ultra-wide frequency tunable range. An experiment is performed. The generation of binary and quaternary microwave waveforms with a microwave carrier frequency at 10 and 20 GHz is demonstrated.
© 2014 Optical Society of America
1. Introduction
In modern radar systems, pulse compression has been widely employed to increase the range resolution and improve the signal-to-noise ratio (SNR) [1,2]. Usually, in a radar receiver, pulse compression is implemented by sending a microwave waveform with a large time-bandwidth product (TBWP), to a matched filter. Phase-coded or frequency-chirp microwave waveforms are the most commonly used signals in radar system. Due to the limited speed and bandwidth of electronic circuits, phase-coded or frequency-chirped microwave waveforms generated in the electrical domain usually have a small TBWP [3,4]. Furthermore, the reconfigurability and frequency tunability are also required in modern radar systems. Thanks to the large bandwidth and high speed offered by modern photonics, photonic generation of microwave waveforms has been a topic of interest, and numerous techniques have been proposed and demonstrated.
A phase-coded or frequency-chirped microwave waveform can be generated through optical pulse shaping using a spatial light modulator [5]. The key significance of the technique is its high flexibility. However, the system is usually bulky and with a high loss due to the employment of free-space optics. To reduce the cost and simplify the architecture, microwave waveforms can also be generated using pure fiber optics [6–20]. A frequency-chirped microwave waveform can be generated based on a temporal interferometer incorporating an optically pumped linearly chirped fiber Brag grating (LCFBG), and compressed based on a time-spectrum convolution system [6,7]. A phase-coded microwave waveform can be generated based on frequency-to-time mapping [8,9]. However, the optical spectral shaper in [8] is specially designed and its spectral response is usually fixed, so the generated waveform is also fixed. A phase-coded microwave waveform can be generated using pure fiber optics based on Mach-Zehnder interferometer (MZI) [10]. The MZI in [10] can be replaced by a Sagnac interferometer (SI) to make the system have better stability [11]. To further improve the stability, a phase-coded microwave waveform can also be generated using a polarization modulator (PolM) [12]. The major limitation of the technique in [12] is the two orthogonal polarized sidebands applied to the PolM from the polarization maintaining fiber (PMF) are frequency dependent, thus the frequency cannot be tuned unless changing the length of the PMF. In order to generate a frequency tunable phase-coded microwave waveform, the PMF in [12] can be replaced by a polarization-maintaining fiber Bragg grating (PM-FBG) [13]. Another approach to generating a phase-coded microwave waveform using a PM-FBG was demonstrated in [14]. The major disadvantage of the approaches in [13,14] is that their frequency tunable ranges are limited by the bandwidths of the two orthogonal passbands of the PM-FBG. To further increase the tunable range, a phase coding system based on two PolMs was proposed [15]. Recently, we proposed a novel approach to generating a binary phase-coded microwave waveform using a single PolM [16]. By switching between the two quadrature transmission points or the maximum and the minimum transmission points, a phase-coded microwave waveform at the fundamental or doubled frequency is generated, which means the theoretical frequency tunable range is doubled to twice the bandwidth of the PolM. There were many other photonic approaches for the generation of binary phase-coded microwave waveforms proposed in recent years [17–19]. However, the phase-coded microwave waveform generation approaches discussed above are most for binary phase coding. Quaternary phase-coded microwave waveforms are also the commonly used waveforms in radar system. In radar theory, poly phase-coded microwave waveform has better Doppler tolerance compared with binary phase-coded microwave waveform, and can be implemented in receivers with limited bandwidth. Furthermore, poly phase-coded microwave waveform has much more complicated signal structure, which guarantees that it is more difficult to be intercepted, so it has better LPI (low probability of intercept). In [20], an arbitrary microwave waveform generation based on a tunable optoelectronic oscillator (OEO) was proposed. The key significance of the technique is that both binary and quaternary phase-coded microwave waveforms can be generated and the system is microwave source free due to its employment of an OEO. However, the frequency tunable range in [20] is limited by the bandwidth of the reflection band of the phase-shifted fiber Bragg grating (PS-FBG). The frequency tuning is realized by changing the optical wavelength, so there may be a wavelength drift during long term operation, which leads to a frequency drift of the generated phase-coded microwave waveforms.
In this paper, we propose and experimentally demonstrate a novel photonic technique to generate binary and quaternary phase-coded microwave waveforms with an ultra-wide frequency tunable range. Compared with the phase-coded approaches discussed above, the key significances of our method are that both binary and quaternary phase-coded microwave waveforms can be generated, and the long term frequency stability is very good because it is only determined by the frequency of the input microwave signal. The key component in the system is the DP-MZM, which functions as an optical wavelength shifter [21], where the optical wavelength is shifted from the original wavelength by applying a microwave signal to the DP-MZM. To generate a phase-coded microwave waveform, the coding signal is modulated on the original wavelength using a phase modulator (PM). Combining the shifted wavelength and the original wavelength, two wavelengths with a frequency space determined by the input microwave signal are obtained. By applying the two wavelengths to a photodetector (PD), a binary or quaternary phase-coded microwave waveform is generated. The proposed technique is experimentally evaluated.
2. Principle of operation
The schematic diagram of the proposed phase-coded microwave waveform generation system is shown in Fig. 1(a). A light wave generated from a laser diode (LD) is sent to an optical coupler (OC). The two outputs of the OC are sent to a DP-MZM via PC1 and to a PM via PC2, respectively. The DP-MZM, which functions as an optical wavelength shifter, is driven by a microwave signal generated from a microwave signal generator (MSG). The coding signal generated from an arbitrary waveform generator (AWG) is applied to the PM after amplified by an electrical amplifier (EA1). The two outputs of the DP-MZM and the PM are combined using a polarization beam combiner (PBC) via PC3 and PC4, and then the output of the PBC is sent to a polarizer via PC5 before being detected by a PD. The photocurrent from the PD is amplified by another electrical amplifier (EA2) and then monitored by a real time oscilloscope (OSC). In fact, the outputs from the PM and the DP-MZM can be directly coupled by another OC and then sent to the PD. Here, we use PBC instead of an OC to improve the stability of the system. Figure 1(b) shows the inside view of the optical wavelength shifter. The microwave signal from the MSG is split into two equal parts by an electrical coupler (EC) and then applied to the two RF ports of the DP-MZM with a 90° phase difference introduced by a phase shifter (PS). The two sub-MZMs are both biased at the minimum transmission points and the main-MZM is biased to introduce a ± 90° phase shift. At the output of the DP-MZM, a first-order sideband is dominant and other sidebands and the carrier are deeply suppressed, which realize the shift to the input optical wavelength [21].
Fig. 1 (a) Schematic diagram of the proposed phase-coded microwave waveform generation system. (b) Schematic diagram of the optical wavelength shifter. LD, laser diode; OC, optical coupler; PC, polarization controller; DP-MZM, dual-parallel Mach-Zehnder Modulator; PM, phase modulator; MSG, microwave signal generator; AWG, arbitrary waveform generator; EA, electrical amplifier; PBC, polarization beam combiner; Pol, polarizer; PD, photodetector; OSC, oscilloscope; EC, electrical coupler, PS, phase shifter.
As discussed above, the outputs of the PM and the DP-MZM are
where, andare the amplitudes of the optical signal at the outputs of the PM and the DP-MZM, andare the angular frequency of the optical carrier and the microwave signal, respectively, , is the amplitude of the coding signal, is the half-wave voltage of the PM, andis the coding signal. The polarization states of the two outputs are aligned with the two orthogonal principle axes of the PBC by tuning PC3 and PC4. Applying the two orthogonal polarized signals to the polarizer with its principal axis oriented at an angle of 45° to one principle axis of the PBC, we haveApplying the optical signal to the PD, we have the photocurrent given bywhere, is the responsivity of the PD. It can be seen from (4) that a microwave signal with a carrier frequency of, which is phase coded by the coding signal, is generated. The phase coding pattern is determined by. For example, if is a square wave, andequals to, a binary phase-coded microwave waveform is generated. If is four-level stair wave, and equals to, a quaternary phase-coded microwave waveform is generated.3. Experiment and discussion
An experiment based on the configuration shown in Fig. 1 is carried out. A light wave at 1549.4 nm from the LD (Yokogawa AQ2200) is split into two channels and sent to a DP-MZM (Sumitomo, 40-GHz bandwidth) via PC 1 and a PM (Thorlabs, 10-GHz bandwidth) via PC2, respectively. A microwave signal generated from a MSG (Agilent 83630B) is applied to the DP-MZM. A coding signal generated from an AWG (Tektronix AWG7082C) is amplified by EA1 and then sent to the PM. The two outputs from the DP-MZM and the PM are sent to the PBC with their polarization state aligned with the two orthogonal principle axes of the PBC via PC3 and PC4. The output of PBC is sent to the polarizer with its principal axis oriented at an angle of 45° to one principle axis of the PBC via PC5. The optical signal from the polarizer is injected into the PD (U2T, 30-GHz bandwidth). An electrical amplifier, EA2, is used to amplify the electrical signal at the output of the PD, and the waveform at the output of EA2 is monitored by the OSC (LeCroy SDA830Zi-A).
Figure 2 shows the optical signal at the output of the polarizer. Two specific conditions with a microwave carrier frequencies of 10 GHz and 20 GHz are shown in Fig. 2. From the inserts in Fig. 2, we can clearly see that the optical signal at the output of the DP-MZM is a single-sideband modulation with carrier suppressed, which realizes the optical wavelength conversion with a wavelength shift of the frequency of the input microwave signal. Thus, the output signals at the output of the polarizer mainly contain a carrier from the PM and a first-order sideband from the DP-MZM, and the other first-order sideband is deeply suppressed. Applying the optical signal in Fig. 2(a) to the PD, a 10-GHz phase-coded microwave waveform can be generated. Figure 3 shows the generated 10-GHz phase-coded microwave waveform and the recovered phase information using Hilbert transform. First, the coding signal is a square wave at a data rate of 500 Mb/s generated by the AWG shown in red line in the lower figure of Fig. 3(a), and the blue line is the recovered phase information from the binary phase-coded microwave waveform above. As can be seen, the blue line and the red line are very consistent, and the phase shift is π, which confirms the effective generation. Then the coding signal is changed to a stair wave with four voltage levels shown in red line in the lower figure of Fig. 3(b), and the blue line is the recovered phase information from the quaternary phase-coded microwave waveform above. As can be seen, the blue line and the red line are consistent, and the phase shifts have four levels of 0, π/2, π and 3π/2, corresponding to the four voltage levels of the coding signal.
Fig. 2 Optical signals at the output of the polarizer with a microwave signal at (a) 10 GHz, and (b) 20 GHz. Insects are the optical signals at the output of the DP-MZM (left) and PM (right).
Fig. 3 Generated 10-GHz (a) binary, and (b) quaternary phase-coded microwave waveform and the recovered phase information.
To demonstrate the pulse compression capability, a pseudo-random bit sequence (PRBS) phase-coding signal at 500 Mb/s with a length of 128 bits generated from the AWG is applied to the PM. Figures 4(b) and 4(d) show the autocorrelation of the binary and quaternary phase-coded microwave waveforms, respectively. Compressed pulses are obtained. The autocorrelation peaks have a full-width at half-maximum (FWHM) of 1.85 ns and 2.3 ns, which correspond to pulse compression ratios (PCR) of 138 and 111, respectively. The peak-to-sidelobe ratios (PSR) are 8.69 dB and 7.95 dB. To study the robustness of the generated waveforms to noise, waveforms with an additive white Gaussian noise (AWGN) are generated, and one of the waveforms with an AWGN is shown in Fig. 4(a) using red lines, where the SNR is controlled to be as low as −10 dB. Figures 4(c) and 4(e) show the correlation between the original phase-coded microwave waveforms and the phase-coded microwave waveforms with an AWGN. The PCRs and PSRs are very close to the values without an AWGN, which confirms the robustness of the pulse compression process.
Fig. 4 (a) A section of the generated 10-GHz binary phase-coded microwave waveform with and without an AWGN. Autocorrelation of the 10-GHz binary phase-coded microwave waveform (b) without noise, and (c) with an AWGN. Autocorrelation of the 10-GHz quaternary phase-coded microwave waveform (d) without noise, and (e) with an AWGN.
To verify the frequency tunability, the optical signal in Fig. 2(b) is injected into the PD, where a 20-GHz phase-coded microwave waveform can be generated. Figure 5 shows the generated 20-GHz phase-coded microwave waveforms and the recovered phase information. The coding signals are also a square wave and a stair wave with four voltage levels at 500 Mb/s. As can be seen the phase information are successfully recovered from the waveforms, and they are very close to the coding signals generated from the AWG.
Fig. 5 Generated 20-GHz (a) binary, and (b) quaternary phase-coded microwave waveform and the recovered phase information.
The pulse compression capability is also studied with the results shown in Fig. 6. Figures 6(a) and 6(c) show the autocorrelation of the binary and quaternary phase-coded microwave waveforms, respectively. The PCRs are 138 and 119, and the PSRs are 8.63 dB and 8.39 dB, respectively. We also add an AWGN to the generated phase-coded microwave waveforms to make the SNR to be as low as −10 dB. Figures 6(b) and 6(d) show the correlation between the original phase-coded microwave waveforms and the phase-coded microwave waveforms with an AWGN. The PCRs and PSRs are very close to the values without an AWGN
Fig. 6 Autocorrelation of the 20-GHz binary phase-coded microwave waveform (a) without noise, and (b) with an AWGN. Autocorrelation of the 20-GHz quaternary phase-coded microwave waveform (c) without noise, and (b) with an AWGN.
Frequency tunability is very important for a phase-coded microwave waveform generation approach. There are two key components in our experiment which limit the frequency tunable range. The first one is the DP-MZM. The DP-MZM used in the experiment has a bandwidth of 40 GHz, so using such a DP-MZM, the frequency of the generated phase-coded microwave waveforms is limited to less than 40 GHz. The second one is the PS. The microwave signal generated from the MSG is applied to the two sub-MZMs of the DP-MZM with a phase difference of 90°, which is introduced by a PS. The bandwidth of the PS will limit the frequency tunable range of the generated phase-coded microwave waveforms. In the experiment, the PS is an electrical delay line. By tuning the delays introduced by the delay line, a phase shift to the input microwave signal is introduced. The PS has a bandwidth of 20 GHz and a tunable range of 30°/GHz, so 90° phase shift can be introduced to microwave signals with a carrier frequency equals to or greater than 3 GHz. Using the PS, the frequency tunable range of the generated phase-coded microwave waveforms is limited from 3 GHz to 20 GHz. To further increase the frequency tunable range limited by the PS to be in line with that limited by the DP-MZM, a PS with larger bandwidth should be employed.
The proposed approach is based on optical interference of two arms, so the system stability is also studied. The short-term stability is acceptable. Since, the system is implemented based on discrete components, the long-term stability is still an issue. A solution is to integrate the system using a photonic integrated circuit.
4. Conclusion
A novel photonic approach to generating binary and quaternary phase-coded microwave waveforms with an ultra-wide frequency tunable range was proposed and experimentally demonstrated. The key contributions of the work were that both binary and quaternary phase-coded microwave waveforms can be generated, and the frequency tunable range was only limited by the bandwidth of the DP-MZM used in the system if a PS with enough bandwidth was employed. The proposed technique was investigated experimentally. The generation of binary and quaternary phase-coded microwave waveforms at 10 and 20 GHz was demonstrated. In fact, using a DP-MZM with 40-GHz bandwidth, the system had the potential to generate phase-coded microwave waveforms up to 40 GHz. The pulse compression performance was also evaluated, which confirmed that the generated phase-coded microwave waveforms had very good robustness to noise.
Acknowledgments
This work was supported in part by the National Basic Research Program of China (973 Program no. 2010CB328300), in part by the China 111 project (No. B08038), and in part by Innovation Funds of Space TT & C Communication.
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