The microring resonator (MRR) plays an important role in signal processing because high-quality bandpass filtering can be obtained at its drop port. To promote the signal-to-noise ratio, a high rejection ratio is significantly demanded. However, it is still challenging to promote the rejection ratio of the MRR-based bandpass filter. To solve this problem, we propose to use an all-pass filter to enhance the rejection ratio of the MRR-based bandpass filter. Experimental results show that the improved rejection ratio is as high as 47.7 dB, which is improved by 23.6 dB compared with that of the MRR. Meanwhile, the bandwidth of the MRR-based bandpass filter is reduced from 2.61 to 1.14 GHz due to the constructive interference in the passband. In addition, the center frequency of this ultrahigh rejection MRR can be continuously tuned from 6.26 to 46.25 GHz. The quality factor (Q) of the MRR is improved from 7.4×104 to 1.7×105. During the adjustment, the rejection ratio of the bandpass filter exceeds 40 dB. The proposed approach can be used to achieve optical bandpass filters with high performance.
KEYWORDS: Orthogonal frequency division multiplexing, Digital signal processing, Signal processing, Wireless communications, Sensing systems, Optical transmission
A photonic integrated wireless sensing and communication (ISAC) system with photonic frequency multiplication and frequency conversion of orthogonal frequency division multiplexing (OFDM) signals is proposed and demonstrated. We designed and simulated a 16QAM-OFDM modulation photonic ISAC system, which is operated at 90 GHz with a bandwidth of 12.5 GHz. High-speed communication with a bit rate of 50 Gbit/s and high-resolution sensing with a range resolution of 1.2 cm is achieved. In addition, the OFDM ISAC system has the characteristics of a low sampling rate and high accuracy due to the use of optical frequency conversion.
We proposed and demonstrated a photonic integrated sensing and communication (ISAC) system with multi-target detection capability. In addition to wireless communication, the proposed system implements accurate range and Doppler measurements with the help of polarization-multiplexing, frequency-doubling, and other optical methods. Besides, a frequency-domain method is applied to decouple range and Doppler frequencies. Thus, the system has the advantages of high resolution, real-time processing, and less calculation.
To cope with the bandwidth limitation of traditional electronic radars, many photonic radar signal generation and processing methods have been demonstrated, through which the range resolution has been improved to several millimeters. In recent years, the period-one oscillation of a semiconductor laser under optical injection has been proposed to generate frequency-modulated radar signals. Advantages of this method include large bandwidth, reconfigurable capability, and compact structure. In this report, we introduce the recent progress on radar signal generation and high resolution imaging based on period-one dynamics of semiconductor lasers. The generated frequency modulated signal bandwidth is as large as 18.5 GHz (1.5-20 GHz). The corresponding range resolution is measured to be 8.1 mm. Based on this signal generation scheme, high-resolution through the wall radar imaging is successfully achieved.
KEYWORDS: Signal processing, Near field, Radar, Synthetic aperture radar, Near field optics, Digital signal processing, Image processing, Image resolution, Modulation
A photonics-based inverse synthetic aperture radar (ISAR) for near-field RCS calculation is proposed. The proposed radar implements the function of radar range profile target recognition, 2D ISAR image and near-field RCS calculation. Firstly, a photonics-based ISAR is proposed with a frequency-quadrupled linear frequency-modulated continuous wave (LFMCW) signal covers 18-26 GHz. A 2D ISAR image with 2 cm*2 cm resolution is obtained. Secondly, a method of calculating the RCS from the ISAR image is also proposed. According to the Parseval theorem, by summing the pixel values of target area 2D ISAR image and multiplying by the scaling factor, the mean RCS value of target in the radar working band can be obtained. In the experimental demonstration, the standard metal spheres with diameters of 10 cm, 15 cm and 20 cm are tested 1.2 m away from the radar. The testing results show that the difference between the RCS test value and the theoretical value is less than 1.31 dB.
A grating-lobe-suppressed two-dimensional (2D) optical phased array (OPA) is proposed to realize optical beam steering based on unequally-spaced technique, in which the element distribution is optimized by a modified genetic algorithm to achieve a minimum peak side-lobe level (PSLL). Numerical simulations of 2D unequally-spaced OPAs are carried out. The results show that by optimizing the element distribution in an unequally-spaced OPA using the modified genetic algorithm, the grating lobes and side lobes can be well suppressed. Specifically, the PSLL of the far-field pattern reaches as low as 0.20 in 0° beam direction for a 2D 10×10 OPA with the inter-element spacing between 1 μm and 3 μm, which is much better than the traditional unequally-spaced OPA. The relationship between the optimized PSLL and the practical fabrication accuracy is also investigated. The results indicate that the proposed OPA can allow a certain fabrication deviation. Besides, the additional amplitude weighting method is applied to the optimized 2D unequally-spaced OPAs, and the simulation results show that the optimized unequally-spaced OPAs with amplitude weighting can achieve a better grating-lobe suppression. For the 2D 10×10 OPA, the PSLL can be further reduced to 0.18 in 0° beam direction. The proposed 2D low grating-lobe OPAs are expected to benefit many practical applications such as light detection and ranging, high-resolution display, and free-space optical communications.
When a light is properly injected to a semiconductor laser (SL), periodic one (P1) dynamics can be excited and a microwave signal with a specific frequency can be generated after optical-to-electrical conversion. By varying the injection strength, the detuning frequency, or both, a continuously tunable microwave signal over a frequency range up to 100 GHz can be generated. Based on this principle, we implemented wideband linear frequency modulation (LFM) signal generation, which is highly required in high-resolution radars. While, this method suffers from a poor spectrum purity of the generated LFM signal, because the generated microwave using optically injected SL has a relatively large 3-dB linewidth on the order of megahertz. To overcome this problem, we applied a feedback to the optically injected SL with the round-trip time delay matched to the period of sweeping frequency, through which the spectrum purity of the obtained wideband LFM signals is greatly improve. Applying the generated broadband LFM signals, high-resolution radar target detection is demonstrated, which shows the good potential of the proposed wideband LFM signal generator in practical applications.
A 2-bit photonic digital-to-analog conversion unit is proposed and demonstrated based on polarization multiplexing. The proposed 2-bit digital-to-analog converter (DAC) unit is realized by optical intensity weighting and summing, and its complexity is greatly reduced compared with the traditional 2-bit photonic DACs. Performance of the proposed 2-bit DAC unit is experimentally investigated. The established 2-bit DAC unit achieves a good linear transfer function, and the effective number of bits is calculated to be 1.3. Based on the proposed 2-bit DAC unit, two DAC structures with higher (>2) bit resolutions are proposed and discussed, and the system complexity is expected to be reduced by half by using the proposed technique.
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