A complex-coefficient microwave photonic filter with continuous tunability is proposed and demonstrated, which has a compact structure and stable performance without splitting the optical path and tuning the polarization state. By only controlling the DC biases of the modulator, the amplitudes and the phases of the filter taps can both be tuned. The phase difference between the two filter taps covers a full 360° range from 10 GHz to 32 GHz. Frequency responses of the proposed filter are measured within 10–20 GHz with different center frequencies.

As a key figure-of-merit for high-performance microwave filters, the out-of-band noise rejection is of critical importance in a wide range of applications. This paper overviews the significant advances in photonic microwave filters (PMFs) having ultra-high rejection ratios for out-of-band noise suppression over the last ten years. Typically, two types of PMFs, the bandpass and bandstop ones, are introduced with fundamental principles, detailed approaches, and then cutting-edge results for noise rejection. Ultra-high noise rejection ratios of ～80 dB and >60 dB have been demonstrated for single-passband and single-stopband PMFs, respectively, which are comparable with the state-of-the-art electronic filters operating in stringent conditions. These PMFs are also characterized by wide frequency coverage, low frequency-dependent loss, and strong immunity to electromagnetic interference due to the intrinsic features from the advanced photonics technology.

Investigation on the group delay performance of microwave photonic filters is presented. Analysis and simulation results show symmetrical distribution on the delayed optical signals in the impulse response is required to obtain a constant group delay performance. Experimental results for both symmetrical and asymmetrical tap distribution microwave photonic notch filters are presented showing the group delay response with <±25 ps and around 1 ns ripples, respectively.

A passband frequency-electable microwave photonic multiband bandpass filter based on a cascaded fiber Sagnac loop is presented and experimentally demonstrated. A broadband light is sliced by the cascaded fiber Sagnac loop, which serves as a spectrum slicer. After broadband light passes through the spectrum slicer, the sliced broadband light will have the varied periodical spectral characteristics that will cause a transmitted spectral distribution with uniform or multiple periods, then will be modulated by an optical phase modulator and delayed by a dispersive medium. Therefore, a frequency-band-selectable microwave photonic multiband bandpass filter with a suppressed baseband is achieved that can be switched between the single-passband, dual-passband, and triple-passband state. The presented filter exhibits a maximal out-of-band rejection ratio of about 30 dB.

A finite impulse-response microwave photonic filter is typically achieved based on spectrum-shaped optical frequency combs and a dispersive element. We propose an analytical model to describe the amplitude responses of the sidelobes. The model shows that the sidelobe suppression ratio is limited by the spectrum structure of the optical combs. By taking Gaussian-profiled combs as an example, it is both theoretically and experimentally proved that the suppression ratio can be improved by optimizing the spectral power range, which is defined as the ratio of the maximum tap weight to the minimum tap weight.

A new tunable microwave photonic notch filter with negative coefficients is presented. It is based on a polarization modulator and a polarization beam interferometer. Experimental results are presented that embody the new concept.

An optical length-change measurement technique is proposed based on an incoherent microwave photonic filter (MPF). The optical length under testing is inserted into an optical link of a single-bandpass MPF based on a polarization-processed incoherent light source. The key feature of the proposed technique is to transfer the length measurement in the optical domain to the electrical domain. In the electrical domain, the measurement resolution is extremely high thanks to the high-resolution measurement of microwave frequency response. In addition, since the MPF is a single-bandpass MPF, the optical length is uniquely determined by the central frequency of the MPF. A detailed investigation of the relation between the center frequency of the MPF and the optical length change is implemented. A measurement experiment is also demonstrated, and the experimental results show that the proposed technique has a measurement sensitivity of 1 GHz/mm with a high length-measurement resolution of 1 pm in theory. The proposed approach has the advantages of high sensitivity, high resolution, and immunity to power variation in electronic and optical links.