In recent years, people's demands for massive information transmission and processing are increasing, which leads to technological innovation and development in satellite communication, radars, electronic military affairs, broadband wireless communication, and other fields. As a carrier of information, the high-frequency microwave signal is an inevitable development trend. In practical applications, there is a great demand not only for high-frequency microwave signals but also for microwave signal waveforms in radars, software radio, modern instruments, testing, and other fields. For example, triangle wave signals in radars can be generated by triangle waves, which can be used for range and velocity measurement of multiple targets by radar systems, and the signals can be combined with cross-phase modulation and be used for optical frequency conversion and pulse compression. Square waves can be used to form false target interference to pulse compression radars. Therefore, the realization of multi-waveform online flexible and adjustable waves becomes a future development trend. However, traditional electronic systems are faced with problems such as electronic bottlenecks, high complexity, large volume and weight, and poor flexibility, and they are easy to be affected by electromagnetic interference and will produce electromagnetic radiation. Nowadays, radars, satellite communication, and other systems tend to move towards multifunctional integration, which requires large working bandwidth, large capacity, flexible waveform generation, high carrier frequency stability, flexible tuning, and multi-channel and dynamic resource allocation, which cannot be realized by traditional electronic systems. Therefore, it is important for our research to explore efficient methods to generate high-frequency and high-quality microwave local oscillator signals and arbitrary waveform signals with a high-frequency bandwidth.

A function waveform signal generator based on birefringence characteristics of a polarization-maintaining fiber (PMF) is proposed and studied. The generator adopts sinusoidal microwave signals to modulate continuous light waves, which are controlled by 45° polarization and then coupled into the PMF. By using the birefringence characteristics of the PMF, the paper introduces a controllable delay difference between two orthogonal optical field components of fast and slow axes. After the final photoelectric detection, the expression of photocurrent is composed of Fourier series cosine harmonics and sine harmonics featuring a 90° broadband bridge. Thus, the model has tunable function waveform output characteristics. Analysis shows that three variables are controlled by the generator, namely, the phase shift φ, the modulation coefficient β, and the delay difference τ, and theycan be used to adjust the harmonic coefficients of the Fourier series. The optical simulation software OptiSystem is used to build the structural diagram of the system and verify the simulation. The light source used in the system is a CW laser. The values of wavelength, power, and linewidth are set, and then the light is coupled to a signal-drive Mach-Zehnder modulator (SD-MZM) for modulation. The polarization state of the modulated signals is adjusted by a polarization controller so that the polarization state of the optical field is 45° from the polarization principal axis of the PMF. Due to the birefringence characteristics of the PMF, the two orthogonal polarized optical signals transmitted by the fast and slow axes will be transmitted at different group speeds, and the resulting delay difference is related to the access length of the PMF. According to the values calculated in the table, the access length of the PMF can be adjusted to realize the tuning of delay quantity. In order to evaluate the difference between the target function waveform and the generated waveform, the root mean square error is used to compare the similarity between the approximate waveform generated by the system scheme and the theoretical waveform.

A function waveform generator based on the birefringence characteristics of a PMF is presented and analyzed. According to the simulation experiment, trapezoidal wave signals with an adjustable top and edge (Fig. 4) and triangular wave signals (Fig. 6) with an adjustable symmetry factor are obtained, which are helpful for the application of multifunctional waveforms in high-speed signal processing. At the same time, by introducing adjustable and symmetry factors, the waveforms of triangular waves, sawtooth waves, trapezoidal waves, and rectangular waves are connected, so as to realize the output of multiple waveforms in a single system. Moreover, the proposed scheme does not need to fix the modulation coefficient, which makes the scheme have a more flexible parameter configuration and enriches the diversity of the signal generator.

An optical generator for generating function waveforms with high birefringence characteristics based on SD-MZM and PMF is proposed and described. By changing the delay difference τ and the phase shift φ caused by bias, trapezoidal waveforms with an adjustable top edge and triangular waveforms with an adjustable symmetry factor are generated. Compared with similar schemes, the proposed scheme does not need to fix the modulation coefficient and has a more flexible parameter configuration. The tunable performance of waveform and parameter control methods is discussed, and the simulation verification is carried out by using OptiSystem simulation software. It is found that when the root mean square error is less than or equal to 5%, the adjustable trapezoid waves (0≤δ≤30%) and the adjustable symmetrical triangular waves (20%≤σ≤80%) with better waveform quality can be obtained.