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Microwave photonic filters (MPF) are among the most crucial components required for meeting the demands of high-speed and high-frequency signal processing of modern telecommunication systems. For the first time, here, we propose an MPF based on a double wavelength spacing Brillouin-Raman fiber laser (BRFL) to realize a versatile bandpass filter. The proposed setup generated 100 optical taps output with ~ 20 GHz spacings as the light source of the MPF. A dispersive medium of 5 km long single mode fiber and 3.46 km long dispersion compensating fiber were used to characterize the effect of the time delay on the central frequency of the MPF passband. A reconfigurable 3-dB bandwidth from 150 MHz to 1.12 GHz centered around 10.3 GHz is achievable by varying the BRFL optical channel numbers using 10 to 100 optical taps with 20 GHz spacing. The 3-dB bandwidth and Q-factor of the proposed MPF were investigated experimentally and found to be in good agreement with simulation, demonstrating the potential of the BRFL as a laser source for the realization of flexible and versatile radio frequency transversal filters with potentially reduced cost and complexity. [ABSTRACT FROM AUTHOR]

Microwave photonics is a promising and rapidly developing interdisciplinary field. It combines microwave and photonic techniques to generate, transmit, process, and manipulate microwave signals by using the advantages of broadband, high frequency, and low loss provided by photonics. As an important branch of microwave photonics, the microwave photonic filter (MPF) can overcome the limitations set by traditional electronic technology and can realize advanced signal processing in modern communication systems due to its higher performance, selectivity, and flexibility. This review provides a comprehensive overview of MPFs, including fundamental principles, typical structures, and key applications. Additionally, the microwave photonic integration is a very important tendence because of its advantages of small size, light weight, low power consumption, and low cost. The recent advances in integrated MPF are also reviewed. [ABSTRACT FROM AUTHOR]

In signal processing of the growing semaphore, a microwave photonic filter (MPF) is capable of dealing with high-frequency signals, offering the advantages of high bandwidth, easy tuning, and more. This paper presents an efficient tunable microwave photonic filter that features a wideband tuning capability and narrow-band filtering effect based on the lithium niobate on insulator (LNOI) material platform. A multi-mode waveguide race-track type microring resonator has been designed, along with a thermal electrode that utilizes the thermo-optical effect of lithium niobate to adjust the microring resonator. The packaged device has been tested, with a wideband tunable range of 4.7~38.2 GHz achieved. This allows for cross-band continuous tuning across the C-band to Ka-band range. When supplied with 29.1 mW of electric power, the thermal tuning efficiency reaches 9.2 pm/mW, enabling high-frequency tuning of up to 38.2 GHz. The filter possesses a high resolution, exhibiting a 3 dB bandwidth of 662 MHz. [ABSTRACT FROM AUTHOR]

Abstract Microwave photonic sensors are promising for improving sensing resolution and speed of optical sensors. In this paper, a high-sensitivity, high-resolution temperature sensor based on microwave photonic filter (MPF) is proposed and demonstrated. A micro-ring resonator (MRR) based on silicon-on-insulator is used as the sensing probe to convert the wavelength shift caused by temperature change to microwave frequency variation via the MPF system. By analyzing the frequency shift with high-speed and high-resolution monitors, the temperature change can be detected. The MRR is designed with multi-mode ridge waveguides to reduce propagation loss and achieves an ultra-high Q factor of 1.01 × 106. The proposed MPF has a single passband with a narrow bandwidth of 192 MHz. With clear peak-frequency shift, the sensitivity of the MPF-based temperature sensor is measured to be 10.22 GHz/°C. Due to higher sensitivity and ultra-narrow bandwidth of the MPF, the sensing resolution of the proposed temperature sensor is as high as 0.019 °C. Graphical Abstract

To meet the requirement of radio frequency signal processing tasks among various frequency bands, here we have demonstrated a reconfigurable microwave photonic bandpass filter by flipping the optical notch spectrum of a cascaded microring resonator (MRR) filter pool chip through phase modulation. We experimentally characterize the cascaded MRR filter pool chip on the Si3N4 platform and evaluate the system performance of the established microwave photonic filter (MPF). In the demonstrated microwave photonic filter system, the center frequency can be tuned within 5.8--18.2 GHz and the bandwidth can be tuned within 2.1--3.5 GHz. In particular, the MPF system features a response with a shape factor of ∼2.2. With the advantages of reconfigurability and good shape factor, the demonstrated reconfigurable RF bandpass filter shows a potential application in future software-defined RF frontends.

A bandwidth-tunable multiband microwave photonic filter (MPF) using a strongly coupled microring resonator (MRR) with optical phase modulation is demonstrated theoretically and experimentally. Two notches of the MRR adjacent to the optical carrier break the intrinsic balance of the phase-modulated signal and then two sub-MPFs are formed thanks to the conversion from phase modulation to intensity modulation (PM-IM). The frequency responses of the two sub-MPFs are superimposed to form a passband with a fixed center frequency. Due to the periodicity of the MRR transmission spectrum, multiple passbands separated by a certain spacing appear in the frequency domain and combine to form a multiband MPF, whose bandwidth can be adjusted by tuning the wavelength of the carrier. The tunable bandwidth and shape factors ranging from 0.73 GHz to 2.73 GHz and 5.06 to 1.38 are experimentally performed. The number of passbands can be reconfigured. The proposed multiband MPF has the potential to be employed in modern multi-standard wireless communication systems.

In this paper, a novel microwave photonic filter (MPF) based on a single longitudinal mode Brillouin laser achieved by parity time (PT) symmetry mode selection is proposed, and its unparalleled ultra-narrow bandwidth as low as to sub-kHz together with simple and agile tuning performance is experimentally verified. The Brillouin fiber laser ring resonator is cascaded with a PT symmetric system to achieve this MPF. Wherein, the Brillouin laser resonator is excited by a 5 km single mode fiber to generate Brillouin gain, and the PT symmetric system is configured with Polarization Beam Splitter (PBS) and polarization controller (PC) to achieve PT symmetry. Thanks to the significant enhancement of the gain difference between the main mode and the edge mode when the polarization state PT symmetry system breaks, a single mode oscillating Brillouin laser is generated. Through the selective amplification of sideband modulated signals by ultra-narrow linewidth Brillouin single mode laser gain, the MPF with ultra-narrow single passband performance is obtained. By simply tuning the central wavelength of the stimulated Brillouin scattering (SBS) pumped laser to adjust the Brillouin oscillation frequency, the gain position of the Brillouin laser can be shifted, thereby achieving flexible tunability. The experimental results indicate that the MPF proposed in this paper achieves a single pass band narrow to 72 Hz and the side mode rejection ratio of more than 18 dB, with a center frequency tuning range of 0–20 GHz in the testing range of vector network analysis, which means that the MPF possesses ultra high spectral resolution and enormous potential application value in the domain of ultra fine microwave spectrum filtering such as radar imaging and electronic countermeasures. [ABSTRACT FROM AUTHOR]

Microwave photonic sensors are promising for improving sensing resolution and speed of optical sensors. In this paper, a high-sensitivity, high-resolution temperature sensor based on microwave photonic filter (MPF) is proposed and demonstrated. A micro-ring resonator (MRR) based on silicon-on-insulator is used as the sensing probe to convert the wavelength shift caused by temperature change to microwave frequency variation via the MPF system. By analyzing the frequency shift with high-speed and high-resolution monitors, the temperature change can be detected. The MRR is designed with multi-mode ridge waveguides to reduce propagation loss and achieves an ultra-high Q factor of 1.01 × 106. The proposed MPF has a single passband with a narrow bandwidth of 192 MHz. With clear peak-frequency shift, the sensitivity of the MPF-based temperature sensor is measured to be 10.22 GHz/°C. Due to higher sensitivity and ultra-narrow bandwidth of the MPF, the sensing resolution of the proposed temperature sensor is as high as 0.019 °C. [ABSTRACT FROM AUTHOR]

In signal processing of the growing semaphore, a microwave photonic filter (MPF) is capable of dealing with high-frequency signals, offering the advantages of high bandwidth, easy tuning, and more. This paper presents an efficient tunable microwave photonic filter that features a wideband tuning capability and narrow-band filtering effect based on the lithium niobate on insulator (LNOI) material platform. A multi-mode waveguide race-track type microring resonator has been designed, along with a thermal electrode that utilizes the thermo-optical effect of lithium niobate to adjust the microring resonator. The packaged device has been tested, with a wideband tunable range of 4.7~38.2 GHz achieved. This allows for cross-band continuous tuning across the C-band to Ka-band range. When supplied with 29.1 mW of electric power, the thermal tuning efficiency reaches 9.2 pm/mW, enabling high-frequency tuning of up to 38.2 GHz. The filter possesses a high resolution, exhibiting a 3 dB bandwidth of 662 MHz.

Microwave photonics is a promising and rapidly developing interdisciplinary field. It combines microwave and photonic techniques to generate, transmit, process, and manipulate microwave signals by using the advantages of broadband, high frequency, and low loss provided by photonics. As an important branch of microwave photonics, the microwave photonic filter (MPF) can overcome the limitations set by traditional electronic technology and can realize advanced signal processing in modern communication systems due to its higher performance, selectivity, and flexibility. This review provides a comprehensive overview of MPFs, including fundamental principles, typical structures, and key applications. Additionally, the microwave photonic integration is a very important tendence because of its advantages of small size, light weight, low power consumption, and low cost. The recent advances in integrated MPF are also reviewed.

• Using fiber ring microwave passband filter to demodulate the FBG can greatly improve the measurement resolution and speed. • By choosing opposite dispersion of LCFBGs , cascaded FR-MPFs based sensor can achieve magnification factor of 432. • The proposed system can realize vector magnetic field measurement. The sensitivity is 418.16 kHz/deg. A vector magnetic field sensor based on enhanced Vernier effect is experimentally demonstrated. The Vernier effect is generated from two microwave photonic filters (MPFs) with close free spectral range (FSR), which is constructed by the fiber ring (FR) and the optical carrier microwave interferometry (OCMI). Linear chirped fiber Bragg grating (LCFBG) is inserted in the FR to act as the dispersion element to provide time delay. By attaching the fiber Bragg grating (FBG) on the magnetostrictive material (MA), the magnetic field can be transferred into the wavelength shift, as well as the time delay of the FR and the frequency response of the FR-MPF. By choosing opposite dispersion of LCFBGs and closer FSRs, cascaded FR-MPFs based sensor can greatly improve the sensitivity. The sensitivities for the magnetic field intensity and direction are 127.71 kHz/Oe, the magnification factor of which is 432, and 418.16 kHz/deg. The proposed scheme has merits of high sensitivity and interference immunity, which is suitable for high precision measurement. [ABSTRACT FROM AUTHOR]

In this paper, a novel microwave photonic filter (MPF) based on a single longitudinal mode Brillouin laser achieved by parity time (PT) symmetry mode selection is proposed, and its unparalleled ultra-narrow bandwidth as low as to sub-kHz together with simple and agile tuning performance is experimentally verified. The Brillouin fiber laser ring resonator is cascaded with a PT symmetric system to achieve this MPF. Wherein, the Brillouin laser resonator is excited by a 5 km single mode fiber to generate Brillouin gain, and the PT symmetric system is configured with Polarization Beam Splitter (PBS) and polarization controller (PC) to achieve PT symmetry. Thanks to the significant enhancement of the gain difference between the main mode and the edge mode when the polarization state PT symmetry system breaks, a single mode oscillating Brillouin laser is generated. Through the selective amplification of sideband modulated signals by ultra-narrow linewidth Brillouin single mode laser gain, the MPF with ultra-narrow single passband performance is obtained. By simply tuning the central wavelength of the stimulated Brillouin scattering (SBS) pumped laser to adjust the Brillouin oscillation frequency, the gain position of the Brillouin laser can be shifted, thereby achieving flexible tunability. The experimental results indicate that the MPF proposed in this paper achieves a single pass band narrow to 72 Hz and the side mode rejection ratio of more than 18 dB, with a center frequency tuning range of 0–20 GHz in the testing range of vector network analysis, which means that the MPF possesses ultra high spectral resolution and enormous potential application value in the domain of ultra fine microwave spectrum filtering such as radar imaging and electronic countermeasures.

We propose and demonstrate a widely tunable microwave photonic filter with controlled frequency agility, bandwidth reconfigurability and flexible switching functions. It is based on the self-interference microring aided by an asymmetric Mach–Zehnder interferometer, breaking the fundamental microring footprint limit and achieving a doubled free spectral range (123.2 GHz) with a strong out-of-band rejection. Simulation results show that a bandpass microwave photonic filter can be tuned across a 60 GHz frequency range with a varied 3-dB bandwidth from 2.13 GHz to 40.7 GHz in a cascaded dual-ring topology. Its RF out-of-band rejection ratio is improved by more than 60 dB in the high frequency region due to a perfect residual phase cancellation. A dual-band notch microwave photonic filter with an extinction ratio of about 50 dB and a frequency tunable range of about 27.7 GHz is also demonstrated, showing its broadly applicability. Our method highlights its suitability to improve chip-scale wideband and high-performance RF receiver systems. • Our design breaks the fundamental microring footprint limit and achieves a doubled FSR using dissipative tuning. • A bandpass MPF can be tuned across a 60 GHz frequency range with a varied 3-dB bandwidth from 2.13 GHz to 40.7 GHz. • Our design enables an RF out-of-band rejection ratio of more than 60 dB due to a perfect residual phase cancellation. [ABSTRACT FROM AUTHOR]

A microwave photonic filter (MPF) with an adaptive algorithm is proposed and theoretically analyzed. The structure combines the finite impulse response (FIR) MPF with the adaptive algorithm. The FIR MPF includes variable optical attenuators (VOAs) and variable optical delay lines (VDLs). The least mean square (LMS) algorithm can calculate the weight coefficient of each tap in FIR MPF, which can influence the VOA to realize the adaptive filtering process. This adaptive MPF can extract the desired signal from noise precisely. The noise signal can be well suppressed and the signal-to-noise ratio (SNR) can be developed from 4.15 to 25.5 dB. In addition, this adaptive MPF can be suitable for a wide range of signals. [ABSTRACT FROM AUTHOR]

We demonstrate a reconfigurable filter based on the stimulated Brillouin scattering (SBS) effect in fiber with 3 dB bandwidth from 20 to 500 MHz. We analyze the influence of pump lines and pump spacing on the filter and propose an SBS‐based filter with low passband fluctuation by vectorial modulating multi‐lines pumps. The filter passband fluctuation is decrease to 0.49 dB by precise controlling the pumps' lines, pumps' spacing, and vector signal modulated rate. The proposed single bandpass microwave photonic filter also successfully demonstrates a wide‐band tuning range from 1 to 20 GHz and high out‐of‐band rejection ratio of over 40 dB. The SNR performance of the proposed SBS filter is analyzed and shown the potential capability in the fields of microwave signal processing in satellite communications, electronic warfare, and radar systems. [ABSTRACT FROM AUTHOR]

An optoelectronic oscillator (OEO) for arbitrary microwave waveform generation based on Fourier domain mode-locking (FDML) is proposed. A reconfigurable optical waveform generator (OWG) is implemented via external electro-optical modulation, which serves as the optical source for the proposed OEO. The optical waveform from the OWG can be seen as a series of frequency-scanning optical wavelengths in the frequency domain. At any instant, the number of wavelengths can be from 0 to infinite. By setting the period of the optical waveform or its multiple equal to the cavity round-trip time, an FDML OEO implemented by a frequency-scanning microwave photonic filter (MPF) is realized based on the phase-modulation-to-intensity-modulation conversion using a phase modulator cascaded with a phase-shifted fiber Bragg grating. The scanning characteristics of the MPF changes according to the optical waveform, which can be utilized to generate low-phase-noise arbitrary microwave waveforms using the inherent low-phase-noise merit of an OEO. An experiment is performed. Linearly chirped microwave waveforms and non-linearly chirped microwave waveforms with a scanning bandwidth of 2 GHz, 2-level frequency hopping (FH) microwave waveforms with an FH range of 500 MHz, and 240- and 250-bit binary and quaternary phase-coded microwave waveforms are generated from the OEO. To the best of our knowledge, this is the first time that an OEO is proposed for arbitrary microwave waveform generation. [ABSTRACT FROM AUTHOR]

In this article, we have proposed and demonstrated a sensing interrogation scheme of microwave photonic filter (MPF) based on multi‐wavelength fiber laser (MWFL). A Mach‐Zehnder interferometer (FMZI) acts as the wavelength selective component in the laser cavity as well as the sensing element. An erbium doped fiber amplifier (EDFA) and a semiconductor optical amplifier (SOA) are employed as hybrid gain medium to suppress the homogeneous broadening of erbium doped fiber. The MWFL is employed as the optical source to implement the MPF together with a dispersive medium. The free spectrum range of interference pattern of the FMZI changes with the temperature variation, thus changing the central frequency of MPF's passbands. The sensitivities are −12.23 and 6.11 MHz/°C, respectively, when the sensing optical fiber with the length of 2.2 and 1.1 m are used. [ABSTRACT FROM AUTHOR]

A continuously tunable microwave photonic filter with large dispersion is proposed by using bilaterally modulated and linearly chirped fibre Bragg gratings (LCFBG). A pair of LCFBGS were reverse-inserted into two arms of a Mach–Zehnder interferometer (MZI) to make a dual-tap band-pass filter. The free spectral range (FSR) can be adjusted by setting the basic time difference between two taps, adjusting the wavelength of the optical carrier and reflecting the microwave signal to different physical positions in the LCFBGs. A continuous tunability of the filter is validated. In our scheme, the FSR is adjusted from 33.6 MHz to 37.6 MHz with sub-megahertz accuracy and the adjustable range covering the whole range of over 100 MHz. [ABSTRACT FROM AUTHOR]

A multi-passband microwave photonic filter (MPF) with selectable passband frequency, spanning 0-20 GHz frequency range, is proposed, and experimentally demonstrated, in which passband frequency can be flexibly selected within a maximum passband number to four. The scheme is based on the generation of tunable optical comb lines using a broadband optical source sliced by cascaded optical comb filters, achieved by connecting an in-line birefringence fiber filter and a reconfigur- able Lyot filter in series, such that various filter tap spacing and spectral combinations are obtained for the configuration of the MPF. The proposed MPF can operate with four different passband states, namely, single-, dual-, triple-, and quadruple-passband, only by adjusting polarization states of the cascaded optical comb filters. All these passbands are with a 3-dB bandwidth varying from 200 to 460 MHz and more than 20-dB sidelobe suppression. [ABSTRACT FROM AUTHOR]

A new demodulation technique for polarization‐maintaining photonic crystal fiber pressure sensor with high resolution based on a simple photonic microwave filter has been proposed and demonstrated. Unlike the conventional methods that use optical interferometry and optical spectrum analysis, the proposed method adopts microwave photonics technique. The relationship between load and the notch frequency of the filter is studied. By measuring the notch frequency of the filter, the pressure can be determined and the sensing sensitivity is greatly increased. The sensitivity is as high as 15.625 MHz/N which satisfied well with the theoretical simulation. [ABSTRACT FROM AUTHOR]

A reconfigurable and application-specific tunable microwave photonic filter having finite impulse response is proposed. The filter exploits the differential mode group delays of linearly polarized modes in a multi-mode fiber to achieve single and multiple high-frequency passband response. The filter taps are realized using a single continuous wave laser source and differential delays among modes that are propagated through a single spool of graded index multi-mode fiber. Depending upon the application requirement, different values of free spectral range of the microwave photonic filter are obtained by employing fiber spools of different lengths. The reconfigurability of the microwave photonic filter is achieved by varying the number of linearly polarized modes. It has been shown that the passband ripple can be reduced by employing higher order spatial modes and unequal power ratio among different modes. The proposed filter enables application-specific free spectra range tunability and reconfigurability along with added advantages of easy implementation and passband optimization. [ABSTRACT FROM AUTHOR]

An approach to achieving a highly selective and stable dual-passband microwave photonic filter (MPF) is presented. The dual-passband MPF is realized based on the phase-to-intensity modulation conversion by stimulated Brillouin scattering (SBS). The core of the proposed scheme is a two-tone pump generation by a directly modulated microcavity laser instead of two individual lasers or electro-optic modulators (EOMs), which makes the system relatively simple and stable. The center frequency of the MPF can be tuned from 0 to 20 GHz by adjusting the modulation frequency of the microcavity laser, where the center frequencies of the two passbands are related. The 3-dB bandwidth and the out-of-band rejection ratio of each passband are about 38 MHz and lager than 28.5 dB, respectively. The frequency stability of the MPF is also verified experimentally. [ABSTRACT FROM AUTHOR]

A stopband and passband switchable microwave photonic filter based on the integrated tunable dual ring coupled Mach-Zehnder interferometer has been demonstrated. By using the optical single sideband modulation, the switchable stopband and passband optical responses of the optical filter are one-to-one mapping to the RF spectra. The center frequency and bandwidth of the switchable microwave photonic filter are both reconfigurable. A frequency tuning range of 4-25 GHz is obtained, and the bandwidth tuning ranges of the passband and stopband MPFs are 4.54-9.72 GHz and 3.65-6.35 GHz, respectively. In addition, a RF rejection ratio about 20 dB is achieved. Compared with the switchable MPFs implemented by nonintegrated devices, the proposed switchable MPF provides a simple and low cost method to process radiofrequency signals.

In this work, we report a Micro-Electro-Mechanical System (MEMS)-Semiconductor Optical Amplifier (SOA) active interferometer as a low cost and miniaturized tunable broadband sliced source and optical loss auto-equalizer for single pass band microwave photonic filter (MPF). A low finesse Fabry-Perot MEMS interferometer is formed from a movable deeply etched micro-mirror in front of a cleaved fiber facet. The interferometer slices the amplified spontaneous emission (ASE) output of the SOA, while the output is feedback to the SOA again for amplification and optical loss auto-equalization through the SOA gain saturation effect. This configuration improved the uniformity of the MPF passband attenuation along the tuning range down to 3.72 dB only. The filter center frequency can be finely tuned electrically for ±241.6 MHz with a MEMS electrostatic comb actuator driving the mirror. The center frequency is controlled by the distance between fiber facet and micro-mirror. The operation of the proposed MEMS-SOA based MPF is experimentally verified as a proof of concept from 1.2 to 4.5 GHz and can be easily extended to higher frequencies. A tuning resolution of 6.04 MHz/ $\mu \text{m}$ is achieved with stopband attenuation larger than 20 dB and 3-dB bandwidth mean value of 141 MHz. [ABSTRACT FROM AUTHOR]

Machine learning technique is employed to design a microwave photonic filter (MPF) consisting of an optical comb generator and a phase modulator (PM), to realize centralized management of radio signal delivery in optical networks. The proposed comb generator successfully generates up to 103 optical carriers with adjustable wavelength spacing and number of optical carriers. It exhibits large tunability, flat band response and high tone-to-noise ratio. Thus, the proposed optical comb-based filter is capable to provide flexible tunability with respect to its center frequency and 3-dB bandwidth. In our experimental setup, different wavelength spacing settings and number of optical carriers are fed into the PM for modulation and their corresponding frequency responses are measured by a network analyzer. The experimental data have been analyzed to correlate with the simulation results and theoretical predictions. Besides, a subcarrier multiplexing (SCM) technique can be applied to a multi-user optical system incorporating the proposed filter since its frequency response is varied by the accumulated dispersion. After measuring the characteristic of the proposed filter, a set of frequency responses is collected and fed into the convolutional neural network (CNN) model to obtain the inverse mapping between frequency response to the wavelength spacing and fiber length. As a result, the well-trained model can successfully predict the wavelength spacing and fiber length with high accuracy. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate tunable microwave photonic filters (MPFs) with all-optical control, low-power consumption, ultra-high rejection ratios and large tuning ranges based on three cascaded silicon opto-mechanical microring resonators (MRRs). By injecting low pump powers, the nonlinear effects (mainly including the opto-mechanical effect) could be efficiently excited to manipulate the transmissions of the silicon device. Consequently, the bandwidths of the cascaded MRRs and the frequency interval between the optical carrier and the notch peak could be flexibly adjusted. Moreover, the extinction ratios of the cascaded MRRs could be largely increased when the three resonances are adjusted to be aligned, which contributes to achieving MPFs with high rejection ratios. In the experiment, by injecting a low-power of 3.4 mW, the central frequency and 3dB-bandwidth of the MPF could be tuned from 8.5 GHz to 38.5 GHz and from 8.8 GHz to 14.5 GHz, respectively. It should be noted that the MPF rejection ratios could realize beyond 60 dB. Furthermore, an MPF response with approximately equiripple stopband could be obtained. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate an all-optical microwave filter with tunable central frequency and bandwidth based on the silicon cascaded photonic crystal (PC) cavities. As the light-matter interaction is largely enhanced in the PC nanocavity with small mode volume, its transmission spectrum could be efficiently adjusted. Hence, with injecting ultra-low pump powers to control the transmissions of the two cascaded PC cavities, the central frequency and bandwidth of the microwave photonic filter (MPF) could be simultaneously tuned. The size of the cascaded PC nanocavities is only $200~\mu \text{m}^{\mathbf {2}}$. The proposed MPF with significant features of all-optical control, ultra-high tuning efficiencies and large rejection ratios is beneficial to build on-chip energy-efficient all-optical microwave systems. [ABSTRACT FROM AUTHOR]

In this paper, a novel approach to achieving a wideband tunable dual-passband microwave photonic filter (MPF) is proposed based on optical-injected distributed feedback (DFB) semiconductor lasers and a dual-output Mach–Zehnder modulator (DOMZM). The fundamental concepts for realizing the MPF are the wavelength-selective amplification effect and the period-one oscillation state under optically injected DFB lasers. These effects provide a widely tunable range of center frequency, along with high flexibility and low insertion loss. The proposed MPF is experimentally demonstrated, showing that the dual-passband center frequency in the MPF can be tuned independently from 19 to 37 GHz by adjusting the detuning frequency and injection ratio. Meanwhile, the insertion loss of the system is about 15 dB when there is no optical or electrical amplifier in the MPF link. The out-of-band suppression ratio of the MPF is more than 20 dB, which can be improved by adjusting the power of the two optical signals. [ABSTRACT FROM AUTHOR]

The temperature sensing scheme based on fiber ring microwave photonic filter (MPF) with erbium doped fiber amplification (EDFA) is proposed and experimentally demonstrated. The fiber ring is used as the time delay element, as well as a sensing element, and by introducing an EDFA to compensate the loss in the fiber ring, the number of the effective sampling taps and thus the suppression ratio are increased. When the EDFA power is set at 0.74 mW, the 3 dB bandwidth of the passband and suppression ratio of the MPF are estimated to be 82.5 kHz, and 15 dB, respectively. The superior sharp passband characteristics make the fiber ring MPF with an EDFA very advantageous for sensing. In our experiment, temperature sensing by using the fiber ring MPF with an EDFA has been carried out for demonstration. The temperature variations induce refractive index changes of the fiber, which will change the time delay of each tap of the fiber ring MPF, and as a result, the FSR of the MPF will be changed. By tracing the peak point shift of the MPF's passband, the temperature variations can be monitored. Our experimental results show that there is a good linear relationship between temperature change and frequency shift, and by measuring passbands with different frequencies, different sensitivity can be obtained. In our experiment, the sensitivity is estimated to be −5.89, −11.74, and − 17.7 kHz/°C, respectively, when the passbands at 0.8, 1.6, and 2.4 GHz are adopted for temperature monitoring. [ABSTRACT FROM AUTHOR]

We propose an all-optical virtual private network (VPN) supporting dynamic bandwidth allocation (DBA) in an orthogonal frequency division multiple access-based passive optical network. A microwave photonic bandpass filter (MP-BPF) is used to transmit the VPN signal without electrical conversion. The DBA is implemented by adjusting a free spectral range of the MP-BPF with corresponding subcarrier allocation. A RF clipping-tone (CT) is used to stabilize the optical channel suffered from phase induced-intensity noise and Rayleigh back-scattering noise. The DBA is experimentally verified at different two DBA scenarios in 20-km single-fiber loopback link in terms of channel error vector magnitude, spectral efficiency after adaptive modulation. Due to the CT-based channel stabilization, achievable spectral efficiency could be improved, and the feasibility of the proposed system is successfully demonstrated.

Integrated optical signal processors have been identified as a powerful engine for optical processing of microwave signals. They enable wideband and stable signal processing operations on miniaturized chips with ultimate control precision. As a promising application, such processors enables photonic implementations of reconfigurable radio frequency (RF) filters with wide design flexibility, large bandwidth, and high-frequency selectivity. This is a key technology for photonic-assisted RF front ends that opens a path to overcoming the bandwidth limitation of current digital electronics. Here, the recent progress of integrated optical signal processors for implementing such RF filters is reviewed. We highlight the use of a low-loss, high-index-contrast stoichiometric silicon nitride waveguide which promises to serve as a practical material platform for realizing high-performance optical signal processors and points toward photonic RF filters with digital signal processing (DSP)-level flexibility, hundreds-GHz bandwidth, MHz-band frequency selectivity, and full system integration on a chip scale.

We demonstrate a microwave photonic single notch filter based on a two-tap delay line structure. Obviously different from previous two-tap filters with periodic notch response, our modified two-tap filter can realize microwave photonic filter (MPF) with single notch response. In the proposed two-tap delay line structure, one tap acts as a phase tunable allpass MPF, while the other tap performs as a frequency tunable bandpass MPF. A single notch MPF is realized by selecting one period of the conventional two-tap filter response. The single notch frequency is tunable over a broad bandwidth by properly setting the phase of the allpass MPF as well as the center frequency of the bandpass MPF. The proposed MPF is theoretically analyzed and experimentally verified. [ABSTRACT FROM AUTHOR]

Optical injection is an effective way to generate a tuned microwave photonic filter (MPF). However, the tuning range of the optical injection-based MPF is limited by the nonlinear dynamics and the nonideal roll-off optical filter. To enlarge the tuning range of the MPF, a polarization modulated optical signal injection distributed feedback semiconductor laser is employed. The method is free from the nonlinear dynamics and the nonideal roll-off optical filter. The experimental result shows that the proposed MPF can be tuned from 0 to 40 GHz by directly tuning the center wavelength of the optical carrier. The tuning range can be increased if polarization modulator and photodetector with larger bandwidth are exploited. To the best of our knowledge, this is the largest frequency tuning range of optical injection-based MPF. [ABSTRACT FROM AUTHOR]

A reconfigurable and tunable multi-tap bandpass microwave photonic filter based on a hybrid-gain -assisted multi-wavelength fiber ring laser (HMFRL) is proposed and experimentally demonstrated. The HMFRL containing a hybrid gain medium and a high birefringence fiber loop mirror serves as the multiple taps generator for the microwave photonic filter. In order to realize a bandpass filter, the multiple taps are phase modulated, then the modulated signal is launched into a coil of dispersion compensating fiber to introduce different time delays for each tap. As a result, a bandpass response is obtained at the output of a high speeding photodetector. By adjusting the bias of the semiconductor optical amplifier from 344 to 450 mA, the number of multiple taps can be increased without optical signal-to-noise ratio degradation. Thus, a multi-tap bandpass microwave photonic filter with bandwidth reconfiguring from 449 to 274 MHz is achieved. In addition, by changing the length of polarization maintaining fiber in the high birefringence fiber loop mirror, the wavelength spacing of the multiple taps can be adjusted, making the bandpass microwave photonic filter's free spectral range tunable. [ABSTRACT FROM AUTHOR]

In this paper, a method for optical length measurement that employs a multi-passband microwave photonic filter (MPF) is analysed and demonstrated experimentally. The MPF consists of a sliced broadband light source, a phase modulator, a dual-parallel dispersion medium and a photodetector (PD). By cascading two Mach–Zehnder interferometers (MZIs) with different arm length difference, the MPF with eight passbands has been obtained. The variation of the length difference of MZI will lead the change of the free spectral range (FSR) of the interference spectrum, as well as the frequency of the passbands. By changing the arm length of the MZI, a high length measuring sensitivity of −2.420 GHz/mm has been achieved. The MPF has broad application prospects in the field of multi-parameter sensing. Moreover, the sensor reveals the advantages of highly sensitive, easy to implement and is able to overcome the influence of the environmental perturbation, for example temperature. [ABSTRACT FROM AUTHOR]

A stopband and passband switchable microwave photonic filter based on the integrated tunable dual ring coupled Mach–Zehnder interferometer has been demonstrated. By using the optical single sideband modulation, the switchable stopband and passband optical responses of the optical filter are one-to-one mapping to the RF spectra. The center frequency and bandwidth of the switchable microwave photonic filter are both reconfigurable. A frequency tuning range of 4–25 GHz is obtained, and the bandwidth tuning ranges of the passband and stopband MPFs are 4.54–9.72 GHz and 3.65–6.35 GHz, respectively. In addition, a RF rejection ratio about 20 dB is achieved. Compared with the switchable MPFs implemented by nonintegrated devices, the proposed switchable MPF provides a simple and low cost method to process radiofrequency signals. [ABSTRACT FROM AUTHOR]

We propose a novel single-notch microwave photonic filter (MPF) using a nonsliced amplified spontaneous emission (ASE) source and a laser diode (LD). The nonsliced ASE is used for implementation of a phase tunable bandpass MPF, where the participation of an LD enables the simultaneous realization of an allpass MPF, without adding more devices into the system. By destructive interfering between the bandpass MPF and the allpass MPF, a single-notch MPF can be achieved with tunable center frequency. Compared with previous approach based on ASE source, the proposed approach has a broader tunable frequency range and a much simpler configuration since it is free from both the wavelength-division-multiplexer (WDM) and polarization processing. The proposed method is theoretically analyzed and experimentally demonstrated. [ABSTRACT FROM AUTHOR]

A new technique to achieve a single bandpass microwave photonic filter (MPF) with wide tunability based on stimulated Brillouin scattering is proposed. The MPF cascades the Brillouin frequency shifter and amplification based on a dual‐fiber structure. The experimental results present a single bandpass MPF located at 21.4 GHz with a 3‐dB bandwidth of 38 MHz and an out‐of‐band rejection ratio over 30 dB. The proposed single bandpass MPF also successfully demonstrates a wide‐band tuning over 40 GHz, which is achieved by employing a frequency shifter based on carrier suppressed single‐sideband modulation scheme in the structure. [ABSTRACT FROM AUTHOR]

We demonstrate a fully tunable microwave photonic bandpass filter fabricated with Si3N4/SiO2 waveguides on a chip. The filter is based on coupled resonator optical waveguide architecture comprising a chain of eight coupled ring resonators, each with an optical roundtrip length of 21 cm (free spectral range of 1.4 GHz), with an adjustable resonance frequency, and with adjustable power coupling. The passband shows a 3-dB bandwidth of 72 MHz with at least 51-dB out-of-band rejection and features simultaneously a flat-top, a shape factor of 1.9, and a group delay variation smaller than 2 ns. [ABSTRACT FROM AUTHOR]

A fast-switching single passband microwave photonic finite impulse response filter (MPF) using an incoherent broadband optical source (BOS) and an integrated spectrum shaper is proposed and experimentally demonstrated. The key component of our proposed MPF is the silica-on-silicon integrated spectrum shaper, which is used to slice the spectrum of the BOS. The performance of the integrated spectrum shaper is measured in terms of time delay, power consistency, and switching speed. A number of spectral shapes with the free spectrum range from 1.88 to 0.058 nm can be flexibly switched by changing the driving voltage of the optical switches. With the dispersion compensating fiber as dispersion medium, a fast-switching MPF with the switching speed up to 1.4-KHz is demonstrated in a wide tunable range from 1 to 12 GHz and the sidelobe suppression is up to 35 dB, which shows a great agreement with the theoretical result. [ABSTRACT FROM AUTHOR]

• A simple single passband microwave photonic filter with enhanced flat top and shape factor based on a tunable optical bandpass filter and two fiber Bragg gratings is experimentally. • The 3 dB bandwidth is enlarged from 3.8 GHz to 5.1 GHz. • The flatness and the shape factor of the proposed filter are enhanced. A single passband microwave photonic filter with enhanced flat top and shape factor is experimentally demonstrated. By utilizing a tunable optical bandpass filter (TOBF) and two fiber Bragg gratings (FBG), the transfer spectra of the filtered sidebands is modified. In order to show the advantages of the proposed photonic filter, theoretical analyses, simulation and experiments are carried out to compare with the setup based on a TOBF. On the same conditions, the 3 dB bandwidth is enlarged from 3.8 GHz to 5.1 GHz. The 1 dB fluctuation is enlarged from 1.5 GHz to 3 GHz. The shape factor is minimized from 2.76 to 1.88. [ABSTRACT FROM AUTHOR]

In this paper, we realize a series of 2-GHz wide rectangular optical filters based on the stimulated Brillouin scattering (SBS) effect in optical fiber with a frequency-swept pump and gain feedback compensation scheme. The frequency sweep pump period is a key parameter to achieve acceptable signal performance in the frequency-swept pump based SBS gain filter. We evaluate the system performance of the signal, which is fed into the SBS filter based on frequency-swept pump with different sweep periods ranging from 0.1 to 100 μs, in terms of eye diagrams and bit-error rate (BER) after 12.5 and 25 km of fiber transmission, respectively. Both 500-Mb/s and 1-Gb/s on-off-keying (OOK) signals are used for real-time signal quality evaluation. The optimum eye diagram and BER performance are achieved after 25 km transmission under a 1 μs frequency sweep pump period, as well as after 12.5-km transmission under a 0.5 μs sweep period. The experimental results show that the optimal frequency sweep pump period is always around 1/120 of the signal propagation time in the fiber for different transmission length, providing a guideline on the design of the frequency-swept pump based SBS gain filter and its applications.

We propose and experimentally demonstrate a photonic approach to estimate the angle-of-arrival (AOA) of broadband microwave signals. Using an integrated polarization-division multiplexing Mach–Zehnder modulator and a differential group-delay module, a microwave photonic notch filter is constructed. The relative time delay in reception of the signal at two separate antenna elements can be obtained by measuring the transmission notches over the signal spectra. The AOA is calculated from the relative time delay and the antenna spacing. The experiment results show that the measurement error is less than ±0.35 ps when the relative time delay changes from −14 to 16 ps.

We propose and demonstrate a microwave photonic band-pass filter (MPF), which features a single light source, with continuously and independently tunable center frequency and bandwidth. The center frequency and bandwidth can be adjusted by tuning the variable optical delay line and the position of a spatial filter, respectively. Using a balanced photodetector, the baseband, which is ineradicable in some conventional designs, and the common-mode noise are canceled in our proof-of-concept experiment. Alternatively, a notch with continuously tunable frequency can also be added in the MPF. The experimental measurements of the frequency response of this MPF are demonstrated, which have a good agreement with the theoretical simulations.

A novel microwave photonic filter (MPF) based on carrier-suppressed single sideband (CS-SSB) injected distributed feedback (DFB) laser is proposed. The proposed MPF system exploits the frequency selective gain feature of optical injected DFB lasers, which can be tuned by adjusting the optical injection parameters or the working conditions of the laser. Therefore, the proposed MPF system can be tunable. The proposed system is verified by a proof-of-concept experiment, and the results show that the passband of the proposed MPF can be tuned by tuning the injection ratio and bias current of the slave laser, where the tuning range of 9–39 GHz is realized. At the same time, the out-of-band rejection of the proposed MPF can reach to 32 dB. Moreover, a dual-passband MPF response is also realized by the proposed system with a couple of master lasers, and the two passbands can be tuned independently from 10 to 39 GHz.

In this paper, a novel approach to achieving a wideband tunable dual-passband microwave photonic filter (MPF) is proposed based on optical-injected distributed feedback (DFB) semiconductor lasers and a dual-output Mach–Zehnder modulator (DOMZM). The fundamental concepts for realizing the MPF are the wavelength-selective amplification effect and the period-one oscillation state under optically injected DFB lasers. These effects provide a widely tunable range of center frequency, along with high flexibility and low insertion loss. The proposed MPF is experimentally demonstrated, showing that the dual-passband center frequency in the MPF can be tuned independently from 19 to 37 GHz by adjusting the detuning frequency and injection ratio. Meanwhile, the insertion loss of the system is about 15 dB when there is no optical or electrical amplifier in the MPF link. The out-of-band suppression ratio of the MPF is more than 20 dB, which can be improved by adjusting the power of the two optical signals.