## 3 Conversion Gain Results  
Simulation setups of simultaneous all-optical up-conversion of two IF signals are shown in Figures 5 and 6. In this work, two IF signals (IF1 and IF2) are used to evaluate simultaneous up-conversion at the SOA-MZI output. The first laser source (LS1) producing a continuous wave (CW) signal (IF1 signal) at the wavelength \(\lambda_{1}=1545\) nm was intensity-modulated by the first Mach-Zehnder Modulator (MZM1) driven by an electrical subcarrier carrying QPSK (quadrature phase shift keying) data at IF1 frequency \(f_{1}=1\) GHz. Besides, the second laser source (LS2) producing a CW signal (IF2 signal) at \(\lambda_{2}=1546\) nm was intensity-modulated by the second (MZM2) driven by an electrical subcarrier carrying QPSK data at \(f_{2}=2\) GHz. The generation and detection module at the electrical port of the MZMs was used to generate the QPSK data. For the standard modulation mode in co- and counter-directions, as displayed in Figures 5a and 6b, the IF1 and IF2 signals were combined by the first optical coupler (OC1) and injected at the upper port (UP) of the SOA-MZI. In co- and counter-directions for the differential modulation, the IF1 signal was injected at the UP and the IF2 signal at the lower port (LP), as seen in Figures 5 and 6b. Figure 5: _Cont._Figure 5: Block diagram of the simulation setup of QPSK frequency mixing for co-direction configuration. OAtt: Optical Attenuator, OF: Optical Filter, LNA: Low-Noise Amplifier, MZM: Mach–Zehnder Modulator, OC: Optical Coupler, BER: Bit Error Rate, QAM: Quadrature Amplitude Modulation, PD: Photodiode, and \(V_{b}\): Bias voltage. (**a**) Standard modulation mode, (**b**) Differential modulation mode. Figure 6: _Cont._  
The sampling signal at a repetition rate of 15.6 GHz was generated by an optical pulse source (OPC) [22] that exhibited a wavelength of 1550 nm. Its electrical spectrum showed harmonics at \(H_{n}=nf_{s}\). The signal was an input to the middle port (MP) of the SOA-MZI using different optical signal mode fibers. The position of this signal was exchangeable, as given in Figures 5 and 6. In order to make frequency up-conversion, the average optical sampling power was adjusted to approximately \(-1\) dBm in the co-direction schema and \(-3\) dBm in the counter-direction one (Figure 3) by using the second optical attenuator OAtt2. In [22], the sampling frequency is chosen to be 7.8 GHz, which is close to the cutoff frequency of the SOA-MZI. The increase of the sampling frequency from 7.8 to 19.5 GHz improves the quality and the efficiency of the optical transmission system based on an SOA-MZI as explained in [25]. As a result, by representing the theorem of [PERSON] on the sampling frequency to avoid any aliasing on the sampled signals, the selected sampling frequency \(f_{s}>=2~{}f_{max}=\)2\(\
eq\)7.8 = 15.6 GHz, where the electrical spectrum of the sampling signal that exhibits harmonics ranging from \(H_{1}=f_{s}\) up to \(H_{10}=10f_{s}\), can also lead to the better system characteristics. At the SOA-MZI output, the optical filter (OF) is tuned at \(\lambda_{s}=1550\) nm. The sampled signal is photo-detected with the 200 GHz photodiode (PD) with a responsivity of 0.85 A/W and amplified by a 33 dB-gain low-noise amplifier (LNA) and displayed on an electrical spectrum analyzer (ESA) to obtain mixing spectrums or used the BER_EL-M-QAM module to demodulate QPSK data and find its EVM. The used PD is called uni-traveling carrier (UTC) PDs that are high-power and high-linearity PDs. Besides, they are demonstrated in an upsurge of applications, such as RoF, mm-wave, production of low phase noise microwave signals, and antenna link systems based on optical fibers [4; 38; 39]. Furthermore, UTC-PDs can be used for frequency mixing for higher frequency range up to 300 GHz. The simulations were carried out by using a VPI Transmission Maker [28].