Objective   Microwave photonic filter is one of hot research topics in recent years due to their ability to achieve high bandwidth, anti-electromagnetic interference, fast tunability and reconfigurability with the advantage of optical devices. In order to realize the flexible reconfiguration of the filter response, the response can be flexibly configured by constructing a finite impulse response filter in the optical domain, where the taps can be flexibly configured. Optical frequency combs are capable of providing a larger number of combs as filter taps and are now widely used. A large number of combs allow for more taps, implying a larger quality factor and a larger time bandwidth product, which also allows for higher frequency resolution. However, in optical frequency comb-based filter schemes, simply having a large number of taps are not enough to achieve arbitrary reconfigurability of the filter shape. It is well known that positive coefficient tapped finite impulse response filters can only achieve a low-pass response, whereas bandpass, high-pass or more complex waveforms require the introduction of negative coefficients in the taps. With the help of programmable waveshaper to differentially control different combs of the optical frequency comb in the optical domain, combined with optical devices such as photodetectors, filters with positive and negative coefficients can be realized. In addition, in the process of realizing the response, the beat frequency between the comb lines of the optical frequency comb introduces unwanted spuriousness. Therefore, the operating frequency of existing optical comb-based microwave photonic filter schemes must be strictly limited to a single "Nyquist zone," which undoubtedly limits the operating frequency range of the filter. By introducing proper pre-dispersion, this spurious signal can be effectively suppressed and the operating frequency range of microwave photonic filter can be expanded.

  Methods   A high-resolution reconfigurable microwave photonic filter scheme based on optical frequency comb is proposed to address the above problem (Fig.1). By using waveshaper to realize the independent and flexible configuration of each tap, combined with balanced photodetectors, the positive and negative taps of the filter are realized, which can complete the formation of arbitrary filter waveforms without low-pass response. By using a flat optical frequency comb with a large number of combs generated by a cascaded electro-optic modulator as a light source, the filter is able to achieve high resolution in the order of tens of MHz. At the same time, the spurious signals are effectively suppressed by introducing pre-dispersion so that different values of dispersion are introduced to the carrier and sidebands (Fig.2).

  Results and Discussions   The simulation verifies that the filter has a high resolution of 93MHz (Fig.3), (Fig.4), the spurious suppression of more than 40 dB (Fig.7), and the innovative construction of low-pass, band-pass, high-pass, and band-stop filters with different center frequencies, as well as arbitrary filter shapes such as rectangular, Gaussian, and sinc (Fig.5), (Fig.6), which plays a leading role in the subsequent research of microwave photonic filter.

  Conclusions   The theory of microwave photonic filter based on optical frequency combs proposes a high-resolution reconfigurable microwave photonic filter scheme capable of realizing arbitrary filter shapes. By using the optical frequency comb generated by the cascaded electro-optic modulator method as a light source, the amplitude of the signal is flexibly configured using a waveshaper, and the signal is split into two outputs from two independent ports. In recovering the broadband radio frequency signal, coherent detection technique is used to physically realize the positive and negative taps of the filter, which ultimately accomplishes the reconstruction of arbitrary response shapes without low-pass response. A flat optical frequency comb with a large number of combs increase the number of taps and realizes the high resolution of the filter. In addition, the filter is able to effectively suppress the spurious frequency components due to the different dispersion values introduced by the carrier and sidebands. Simulations demonstrate the high-resolution response of the filter at 93 MHz, and low-pass, band-pass, high-pass, and band-stop filters with different center frequencies, as well as arbitrary filter shapes such as rectangular, Gaussian, and sinc shapes, are constructed. In addition, by introducing pre-dispersion, the filter achieves a spurious rejection ratio of more than 40 dB.