## 3 Experiment Results and Discussion  
In order to demonstrate the feasibility of the proposed large dynamic frequency up-conversion scheme, an experiment is carried out. An optical carrier from a distributed feedback LD (Emcore 1754W, EMCORE Corporation, Alhambra, CA, USA) with a wavelength of 1545.42 nm and a power of 15 dBm is sent to an optical coupler with a variable splitting ratio to form the upper and lower paths. Two optical carriers with different optical power are input into DD-MZMs (FTM793 FZ, Shenzhen HLT Optical Technology Co., Ltd., Shenzhen, China). The DD-MZMs have a 3-dB bandwidth of 25 GHz and a half-wave voltage of 3.5 V. Two bias controllers (OZ Optics, MBC-MINI-PD-3A, OZ Optics Ltd., Westbrook, ON, Canada) are used to control DD-MZMs to work at the minimum point. The output CS-DSB signals are amplified by two EDFAs (Maxray Photonics, EDFA-PA-35-M, Hefei, China) and then sent to the BPD (u\({}^{2}\)t, BPDV2150R, Berlin, Germany) to realize the photoelectric conversion. The up-conversion signal is measured by an electrical spectrum analyzer (ESA, Agilent E4440A, Santa Clara, CA, USA). Firstly, we analyze the frequency up-conversion performance of the system. A 2 GHz single-tone IF signal with a power of 0 dBm generated from a microwave signal generator (Agilent E8267D, Santa Clara, CA, USA) is divided into two parts: one is sent into DD-MZM\({}_{1}\), and the other is sent into DD-MZM\({}_{2}\). The 16 GHz LO signal with a power of 10 dBm generated from the other microwave signal generator (Agilent E8257D, Santa Clara, CA, USA) is also divided into two parts and sent into two DD-MZMs. All the DD-MZMs work stably at the minimum transmission point. The optical spectrum from DD-MZM\({}_{1}\) is shown in Figure 2. (Here, due to the limited resolution of the optical spectrum analyzer, the frequency of the intermediate frequency signal is set as 5 GHz). It can be found that there is still a little optical carrier remaining, and this is due to the limited extinction ratio of the modulators. The modulated signal light in input to the BPD for photoelectric conversion and the measured electrical spectrum is shown in Figure 3. After photoelectric conversion, an up-converted signal with a frequency of 18 GHz and a power of \(-\)26.7 dBm is obtained. By means of the carrier suppression modulation, the LO signal output from the system is effectively suppressed. An LO isolation of 31.4 dB is achieved. If the extinction ratio of the modulator is higher, the residual LO signal can be further suppressed. The 3-dB bandwidths of DD-MZM and BPD are 25 GHz and 43 GHz, respectively. With the frequency of the IF signal being unchanged, the proposed large dynamic frequency up-conversion system can achieve a maximum up-conversion frequency of 27 GHz, namely, up-conversion from S-band to Ka-band. Then, the nonlinearity optimization is investigated. A two-tone IF signal with frequencies of 2 GHz and 2.01 GHz is produced by two microwave signal generators, and the LO signal remains unchanged. A 3-dB power divider and a tunable attenuator are utilized to realize the function of a VEC. By adjusting the distribution ratio of the optical carrier power and the IF signal power input to the two paths, the IMD3 cancellation condition of Equation (10) is matched. By disconnecting the optical path to the lower port of BPD, the measured electrical spectrum of the upper path is shown in Figure 4a. It can be found that in addition to the up-conversion two-tone signal of 18 GHz and 18.01 GHz, there are also strong IMD3 components at 17.99 GHz and 18.02 GHz, with the signal-to-interference ratio (SIR) of 54.3 dB. Figure 4b shows the measured electrical spectrum of the signal output from the lower path after optimizing the power distribution ratio. By comparing Figure 4a,b, the power of the IMD3 component output from the upper path is basically equal to the power of the IMD3 component output from the lower path.