A temporal convolution system for the short-time Fourier transformation (STFT) of an electrical signal based on a bidirectional chirped fiber Bragg grating (CFBG) is proposed and experimentally demonstrated. In this system, the electrical signal to be analyzed is applied to an electro-optical modulator to simultaneously modulate the temporal waveform and the spectrum of a time-stretched optical pulse, which is generated by a mode-locked laser and dispersed by a CFBG. The modulated optical signal is filtered to be several parts, added with separate time delays and sent to the other port of the same CFBG. Thus the optical signal is temporally recompressed and the spectrum of the electrical signal is able to be mapped into the time domain. The bidirectional CFBG realizes exactly complementary dispersion value for the optical pulse propagating in two opposite directions, which guarantees an optimal frequency resolution of the STFT system. An STFT experiment for a microwave signal with four different frequencies at 5 GHz, 10 GHz, 15 GHz and 20 GHz has been demonstrated.
A multi-beam optical beamforming network with low loss is proposed based on the integrated arrayed waveguide grating (AWG). By using the diffraction effect of AWG, the optical multiple beamforming architecture is reduced significantly since the true time delays for different wavelength can be realized simply by employing only one module. The 3 channels of 88 AWG for dense wavelength division multiplexing (WDM) with low loss is fabricated successfully, and then the integrated optical multiple beamformer for eight-element is demonstrated. Experimental results show that the insertion loss for each beamformer is reduced to 5.64 dB. Furthermore, the number of simultaneous beamformer achieves to be 4, which could cover the airspace from -36° to 18°.
For the true time delay (TTD) of optical fiber changes with temperature, integrated optical chips are proposed to improve the delay consistency among channels. A 5-bit TTD chip is proposed in this paper. By using cascaded switches and optical waveguides, the chip is switchable and wavelength independent. A beamforming system operating at 6GHz is established for demonstration. 7 beam directions covering ±15° is obtained with proper mainside lobe ratio.
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