# Microwave Photonic Frequency Conversion Based on a Wavelength Swept Laser
## 1 Introduction  
The microwave frequency converter is one of the key components in microwave and millimeter-wave signal processing, including the transmitters and receivers of radio over fiber (RoF), the radar system and the reconnaissance and countermeasure systems [1; 2; 3; 4; 5]. The basic function of the microwave frequency converter is to convert the microwave signal input to the required frequency. For example, in a high-speed railway communication system, the frequencies of the signal between the mobile terminal and the receiving end of the base station are different because of the Doppler effect [4; 5]. The Doppler frequency shift needs to be compensated for and the microwave frequency converter can provide much convenience in this area. Conventional frequency converters can be created by combining electrical mixers with tunable microwave sources or digital signal processing (DSP) systems [6]. However, electrical mixers exhibit the problems of electro-magnetic interference (EMI), limited isolation and limited bandwidth [7]. The DSP systems are of high cost and face the problem of the electronic bottleneck when dealing with high frequency microwave signals. To overcome the shortage of traditional frequency converters, photonic interrogations have been introduced to improve the electro-magnetic compatibility (EMC) performance. The microwave photonic frequency converter benefits from the wideband tunability of optical local oscillation (LO). The most investigated microwave photonic frequency converters are based on external modulation. The conversion principle is similar to that of the electrical mixer. The optical LO is achieved by external modulation with a tunable Radio frequency (RF) source. The external modulation structure can be a cascaded modulation structures [6; 7; 8; 9; 10; 11; 12] and the integrated parallel modulator structures [13; 14; 15; 16; 17]. However, the external modulator-based frequency conversion structures require complicated bias optimization, polarization or dispersion control. Recently, researchers proposed the use of the mode-locked pulse laser as the optical source for the frequency up-conversion system [18; 19; 20]. [PERSON] et al. used the hybrid mode-locked multi-section distributed feedback (MS-DFB) laser as the optical source to realize sub harmonic up-conversion without external modulators [18]. [PERSON] et al. used the mode-locked laser (MLL) to realize a 60 GHz up-conversion [19]. In those MLL based structures, the pulse train was used to sample the input signals. Thus, the pulse should be narrow and an ultra-high repetition frequency (45 GHz) greatly increases the costs. Besides the MLL, the wavelength swept laser (WSL) is also good candidate. The WSL is widely used in optical sensing and optical coherence tomography (OCT) [21]. The WSL is of a broadband quality and its frequency changes linearly with time [22; 23; 24; 25; 26], which makes it a perfect frequency conversion carrier when combined with the dispersion control device. In this paper, we proposed a new all optical microwave photonic frequency conversion interrogation with flexible tunability and a simple structure based on a WSL. We used a WSL as the optical carrier to sample the input signal. By tuning the optical characteristic of the WSL or the dispersion control device, the input signal could be up-converted to higher or down-converted to lower frequency with an arbitrary conversion coefficient and without an electrical tunable RF source. We simulated the influence of the WSL on the conversion system. In the experiment, we realized the up-conversion and down-conversion by switching the sign of the chirp rate. The 5 GHz input microwave signal was up-converted to 5.577 GHz, at the highest, by adjusting the length of the single mode fiber (SMF) array. The conversion performance for 8 GHz and 16 GHz signals was also tested. The conversion spectrum had multi-longitudinal modes due to the low duty cycle of the WSL pulse. The conversion performance could be further improved by introducing a WLS with a high duty cycle.