A frequency converter, which is an important part of the receiver and transmitter in communication systems, is widely used in broadband wireless communication, radar, satellite communication, etc. Because signals are processed in the optical domain, photonics-based converters possess several advantages, such as large bandwidth, low loss, and anti-electromagnetic interference, thus providing a new solution for modern communication systems. The frequency conversion functions of frequency converters include up-conversion, down-conversion, and in-phase/quadrature (I/Q) up-conversion. Most proposed schemes may implement one of these frequency conversion functions, which present limited application significance. Therefore, researchers have proposed the generation of multiple frequency-converted signals in the same structure. Nevertheless, optical bandpass filters are widely used in these schemes to filter out unwanted optical sidebands, which severely limits the frequency coverage of the frequency converter. Meanwhile, complicated operations (including changing the input signal and adjusting the direct current (DC) bias voltage or 90° electrical phase shift of the modulator) must also be implemented in these approaches. We propose a reconfigurable dual-output microwave photonic frequency converter with high frequency tunability. Such a converter may perform multiple frequency conversion functions with dual outputs, and it is expected to meet the demands of future multifunctional and wide-bandwidth communication systems. Up- and down-converted signals, up-converted upper and lower sideband signals, and vector signals can be generated by changing the input signal in the reconfigurable structure. Moreover, the frequency-converted signals may be output separately in two channels, and the converter is filterless.
In our proposed scheme, an optical carrier generated by a laser diode is transmitted to a polarization division multiplexing dual-parallel Mach-Zehnder modulator (PDM-DPMZM). Moreover, in addition to a local oscillator (LO) signal, an intermediate frequency (IF), a radio frequency (RF), or an I/Q baseband signal generated by an arbitrary waveform generator is loaded into the PDM-DPMZM. Then, the output signal of the PDM-DPMZM is transmitted to a polarization controller (PC). The PC has a polarization rotation angle of 45° and a phase difference of 90°. The positive/negative sideband of the LO signal is eliminated by the PC in two orthogonal polarization directions. A polarization beam splitter is employed to implement polarization separation, which splits the input signal into two parts for photoelectric conversion. Then, two frequency-converted signals are separately generated by the photodetectors in two independent channels. Our reconfigurable scheme can generate up- and down-converted signals, up-converted upper and lower sideband signals, or vector signals when the input signal is changed.
The input RF signal has a uniformly spaced frequency ranging from 9 to 29 GHz, whereas the LO signal has a fixed frequency of 30 GHz. After the beat frequency, up- and down-converted signals with equal frequency intervals are obtained simultaneously and independently. The generated signals present an electrical spurious suppression ratio (ESSR) of 31 dB and a frequency range of 1-59 GHz (Fig. 4). Up-converted upper and lower sideband signals can be generated by changing the RF signal to an IF signal. The results show a highly flat power response for the generated signals (Fig. 5). To verify the feasibility of generating a vector signal with a high frequency, an I/Q baseband signal with a rate of 400 Msym/s was applied to implement I/Q up-conversion. Thus, 64-quadrature amplitude modulation (QAM) signals centered at 30 GHz can be obtained (Fig. 6). This implies that our proposed reconfigurable scheme can generate multiple frequency-converted signals by switching the input signals. Moreover, frequency-converted signals can be obtained simultaneously in two independent channels. These results are consistent with those of the theoretical analysis. The error vector magnitude (EVM) of the 64QAM signal was evaluated (Fig. 7). The EVM value is observed to fluctuate from 2.67% to 3.26% when the frequency ranges from 5 GHz to 40 GHz. This indicates the good frequency tunability of the 64QAM signals. The transmission performance was also evaluated, which considers the situations of back-to-back and 30 km single-mode fiber transmission (Fig. 8). The measured EVM values of the upper and lower sideband signals of the up-conversion frequency are both less than 4.5% with minimal fluctuations. This indicates good transmission performance and suitability for long-distance optical fiber transmission.
In this paper, we propose a reconfigurable dual-output microwave photonic frequency converter that is capable of realizing multiple frequency conversion functions, such as up-conversion, down-conversion, and I/Q up-conversion. Up- and down-converted signals, up-converted upper and lower sideband signals, and vector signals can be generated by changing the input signal. Frequency-converted signals can be generated simultaneously in two independent channels with an ESSR greater than 30 dB. Such a filterless scheme presents good frequency tunability as well as a large frequency range of 1-59 GHz. 64QAM signals centered at 5-40 GHz with EVM values less than 3.5% can also be obtained when the I/Q baseband signal is applied. Moreover, our scheme may eliminate the periodic power fading effect caused by fiber dispersion. This indicates its suitability for long-distance fiber transmission. Its spurious-free dynamic range (SFDR) is as high as 107.1 dB·Hz2/3. The performance interference caused by non-ideal factors was also evaluated and analyzed, and the results demonstrate the practicability and feasibility of our proposed scheme.
