Objective The next-generation radar systems make increasing demands on receivers, such as large instantaneous bandwidth for increased resolution, wide operating frequency for multi-function, and high radio frequency (RF) isolation for large-scale phased array antenna systems. These demands are enormous for electronic receiver technologies. Owing to the advantages of large bandwidth, high isolation, and immunity to electromagnetic interference, photonic-assisted microwave processing techniques provide new solutions for radar receivers. A zero-intermediate-frequency (IF) in-phase and quadrature (I/Q) receiver using microwave photonic technology, exploiting the advantages of the ultrawideband photonic processing technology and agile electronic digital processing technology, has become a competitive solution for wideband radar systems. Recent studies on microwave photonic I/Q receivers obtain the 90° phase shift between I/Q signals by adjusting the phase of signals in optical frequency. Commonly employed optical phase shift methods, such as using an optical 90° hybrid or adjusting a polarization controller, experience disturbance of temperature and stress. In these methods, a narrow bandpass optical filter or complicate electronic-optical modulation is needed for carrier-suppressed single-sideband modulation, which restricts the operation frequency of photonic I/Q receivers. Besides, the amplitude and phase imbalances of the I/Q channels induced by the IF processing devices, such as photodiodes (PDs), low-pass filters, and analog-to-digital converters (ADCs) are rarely considered recently. Our research on a simple and stable microwave photonic I/Q receiver has potential in radar detection applications.

Methods By converting input RF signal into zero IF signals with an I/Q mixer, the I/Q receiver can realize cancellation of image interference, thus doubling receiver's working bandwidth (Fig. 1). However, as it is hard for electronic devices to keep high amplitude and phase consistency in wide bandwidth, the image cancellation decreases in ultrawideband receiver. The image rejection ratio will fall to less than 20 dB with -0.3--0.7 dB amplitude imbalance and -5°--15° phase imbalance (Fig. 2). In our research, the I/Q mixer is realized with optical devices and the imbalance of devices following the optical mixer in I/Q channels is compensated with digital processing, thus realizing high image rejection in 4 GHz operation bandwidth. An optical delay line (ODL) is used for phase tuning of microwave signals carried by optical field (Fig. 3). Two continuous-wave lasers working at 1550.9 and 1550.1 nm separately are combined through a wavelength division multiplexer (WDM) and sent to a Mach-Zehnder modulator (MZM). A local oscillator (LO) signal is used as the optical carrier through the MZM. The output signal is then wavelength-demuxed and multiplexed by two WDMs. An equivalent 90° phase shift between the LO signals carried on the two optical wavelengths can be introduced by tuning the ODL on one of the wavelength channels between the two WDMs. An RF signal is applied to the combined optical signals through a second MZM. An erbium-doped fiber amplifier is used to compensate for the link loss, and then the two optical signals are separated through a WDM to obtain I/Q signals. The relative amplitude difference and transmission delay between the I/Q channels are adjusted through a tunable attenuator and another optical delay line on the two optical paths separately. After being detected by two PDs, the I/Q signals are filtered by 2 GHz low-pass microwave filters and then sampled by two ADCs working at 4-GSa/s sampling rate. We generate a series of RF signals with different frequencies from 10 to 14 GHz and set the LO signal frequency at 12 GHz. The I/Q signals are acquired twice to form the calibration and signal data groups. The amplitude and phase imbalances of the I/Q channels induced by the IF processing devices can be obtained by analyzing the calibration data group.

Results and Discussions The input 1-dB compression powers of LO and RF signals are 8.8 and 10.8 dBm, respectively, and the conversion loss of the microwave photonic link is 31 dB (Fig. 4). In our receiver, the filters, PDs, and ADCs in I/Q channels induce larger amplitude and phase imbalances (Fig. 5). In the 4-GHz operation band, the maximum amplitude imbalance is calculated to be 4 dB, and the maximum phase imbalance is about 40° obtained by analyzing the I/Q signals in the calibration data group (Fig. 6). We chose 200 data in the calibration group with different frequencies and analyze the image rejection of the receiver. The minimum image rejection is only 18 dB with the I/Q imbalance induced by IF processing devices (Fig. 6). Frequency-dependent calibration parameters can be fitted with the amplitude and phase imbalances calculated from the calibration data group. We use the calibration parameters to calibrate the data in the signal data group and compensate for the residual I/Q imbalance using an impulse response filter as that used in Chi-Hao Cheng's work. After calibration, the maximum amplitude imbalance is less than 0.4 dB, and the maximum phase imbalance is less than 1.5° (Fig. 7). After I/Q imbalance calibration and compensation, the image rejection of the receiver is more than 45 dB in its 4-GHz operation bandwidth, and maximum image rejection can reach 79 dB.

Conclusions In this study, an optical delay line-based microwave photonic I/Q mixer for zero-IF receivers is proposed and experimentally demonstrated. The 90° phase shift of the LO signal is realized by tuning the optical delay line to adjust the relative transmission delay. By tuning the 90° phase shift in the microwave frequency, we build a photonic zero-IF receiver which is more stable than that in commonly used microwave photonic methods. The amplitude and phase imbalances of the I/Q channels are also minimized using wavelength-multiplexing technology. In our microwave photonic zero-IF receiver, the amplitude and phase imbalances of the I/Q channels induced by the PDs, low-pass filters, and ADCs are calibrated and compensated. After digital I/Q imbalance compensation, the zero-IF receiver based on the proposed microwave photonic I/Q mixer achieved 0.4-dB amplitude imbalance and 1.5° phase imbalance within 4-GHz operation bandwidth centered at 12-GHz frequency, and the image rejection was more than 45 dB.