Bandpass Microwave Photonic Filter Research Articles

Wideband-tunable microwave photonic filter using dissipative self-interference microring resonators

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.

A single passband microwave photonic filter with enhanced flat top and shape factor based on tunable optical bandpass filter and fiber Bragg gratings

A single passband microwave photonic filter with enhanced flat top and shape factor based on tunable optical bandpass filter and fiber Bragg gratings

Tunable bandpass microwave photonic filter with largely reconfigurable bandwidth and steep shape factor based on cascaded silicon nitride micro-ring resonators.

Bandpass microwave photonic filter (MPF) can be achieved based on the well-known phase to intensity conversion method by using phase modulation and single micro-ring resonator (MRR) notch filter. Since MRR could introduce residual phase in handling one optical sideband, the out-of-band radio frequency (RF) rejection ratio and the shape factor of the bandpass MPF are very limited. Here, by introducing another MRR to handle the other optical sideband, the residual phase can be greatly suppressed, thus the filter's performance can be greatly improved. The proposed bandpass MPF was both verified theoretically and experimentally. Compared with the single MRR, the out-of-band RF rejection ratio and the shape factor were improved by 20 dB and 1.67, respectively. Furthermore, the bandpass MPF's bandwidth is reconfigurable by adjusting the optical carrier's frequency or the two MRRs' amplitude coupling coefficients. The bandpass MPF's center frequency is also tunable by changing the resonant wavelengths of two MRRs in the opposite direction simultaneously. Experimentally, bandwidth reconfiguration from 0.38 GHz to 15.74 GHz, the shape factor optimization from 2 to 1.23, and frequency tuning from 4 GHz to 21.5 GHz were achieved. We believe that the proposed bandpass MPF has great potential for microwave photonic signal processing.

Ultrahigh-Q lithium niobate microring resonator with multimode waveguide.

Difficulty in etching lithium niobate (LN) results in a relatively high propagation loss, which necessitates sophisticated processes to fabricate high-quality factor (Q) microresonators. Here, we fabricate a multimode microring resonator with an intrinsic Q of 6 × 106, which exhibits a propagation loss 50 times lower than that of a single-mode LN microring fabricated under the same process. Notably, the excitation of higher-order modes in the multimode microring is effectively suppressed by utilizing the Euler bend. The highly regular transmission spectrum of the resonator demonstrates a free spectral range (FSR) of 56 GHz. Based on this microresonator, we implement a bandpass microwave photonic filter with an ultra-narrow 3 dB bandwidth of 47.5 MHz and a large tuning range of 2-26.5 GHz. It can be anticipated that the combination of existing advanced etching techniques with this work will drive the propagation loss of a LN waveguide closer to the material absorption loss, significantly facilitating the optimization of performance in applications requiring ultrahigh-Q LN microresonators, such as frequency combs, frequency conversion, electro-optic modulation, and quantum photonics.

Silicon integrated frequency‐tunable microwave photonic bandpass filter

AbstractMicrowave photonic filters have been regarded as an alternative to traditional radio‐frequency filters because of their wide bandwidth and large tunability. Integrated microwave photonic filters can integrate all necessary components into a single chip and are highly demanded for future radio‐frequency applications. Here, a highly integrated frequency‐tunable microwave photonic bandpass filter based on a silicon platform is proposed and demonstrated. The integrated filter consists of a phase modulator, four cascaded microring resonators and a photodetector. The frequency‐tunable range of the integrated filter is from 6.1 to 35.9 GHz, and the reconfigurable bandwidth is from 0.22 to 0.54 GHz. A large spurious free dynamic range of 102.1 dB Hz2/3 is obtained. This highly integrated approach holds great promise for miniaturised, flexible, and high‐performance microwave signal processing in modern radar and communication systems.

Silicon-on-Insulator Microwave Photonic Filter With Widely Tunable and Reconfigurable Flat-Top Bandpass Functionality

We report of a silicon photonics (SiP) flat-top bandpass microwave photonic (MWP) filter with large out-of-band power rejection (OBPR) of up to more than 40 dB and exhibiting central frequency tunability over a range as large as 70 GHz, while still guaranteeing a minimum full-span OBPR larger than 25 dB. Furthermore, the RF bandpass transfer function can be widely configured without sacrificing the filter performance. In the present realization, a continuous bandwidth tuning from 5 to 10 GHz (corresponding to a 100% of bandwidth variation) is demonstrated. The operation is based on optical-to-RF mapping the response of a tunable and bandwidth reconfigurable photonic integrated optical filter, realized with a high-order distributed feedback resonator (DFBR) waveguide Bragg grating structure. A high level of functional integration is provided in a compact footprint of less than 2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> by embedding within the circuit a SiP phase modulator generating the modulation sideband scanning the DFBR filter, and a tunable optical splitter optimizing the MWP filter performance. Possibility of further extending system integration for monolithic on-chip operation is also investigated.

Tunable kilohertz microwave photonic bandpass filter based on backscattering in a microresonator.

A tunable microwave photonic bandpass filter (MPBPF) with a kilohertz bandwidth based on the backscattering mode of a silica microsphere resonator is proposed and experimentally demonstrated. In this work, an ultrahigh-quality-factor microsphere resonator is used to generate a radio frequency bandpass response with a bandwidth of 600 kHz. Meanwhile, scattering-induced coupling between the clockwise mode and the counterclockwise mode is introduced to reduce the number of resonance modes, and a single backscattering mode which has a high extinction ratio is obtained. Therefore, an MPBPF with a tuning range of 40 GHz and a rejection ratio of 16.9 dB is realized. This MPBPF possesses advantages such as ultranarrow bandwidth, large tuning range, and compactness, and shows great potential for microwave photonic applications.

Energy-efficient bandwidth enhancement of Brillouin microwave photonic bandpass filters.

Stimulated Brillouin scattering has been widely utilized to realize frequency-agile narrowband and wideband microwave photonic bandpass filters by primarily utilizing its gain response. However, most demonstrated wideband Brillouin-based filters are limited in operation due to the high-power requirements for bandwidth tailoring. We propose a novel approach to realize wideband reconfigurable, Brillouin-based microwave photonic bandpass filters employing RF interferometry and advanced phase engineering. Demonstrated filters exhibit >20 dB selectivity and >700 MHz bandwidth using only 8 dB peak SBS gain (of intrinsic linewidth 30 MHz), and total optical pump power of only ∼14dBm. We also demonstrate frequency tunability up to 22 GHz. The filter passband has a very flat and highly linear phase response, thus exhibiting zero group delay which we have experimentally verified by propagating an RF pulse at 10.25 GHz. Furthermore, the filter does not suffer from added Brillouin noise in the passband, which is a major advance compared to conventional Brillouin-based microwave photonic sub-systems. This paper presents simulations, mathematical analysis, and experimental results of the proposed filter. The proposed filter demonstrates a pathway toward power-efficient Brillouin-based microwave photonic filters, utilizing SBS responses, in combination with phase manipulation for advanced filtering operations.

Tunable and Reconfigurable Microwave Photonic Bandpass Filter Based on Cascaded Silicon Microring Resonators

A tunable and reconfigurable bandpass microwave photonic filter (MPF) is demanded in scenarios where different center frequencies and bandwidths are required. Here, we propose and demonstrate a silicon-on-insulator (SOI)-based microwave photonic bandpass filter, whose center frequency and bandwidth can be adjusted. The MPF is implemented by using four cascaded microring resonators (MRRs) that fabricated based on SOI wafer. Each MRR combines the design of multi-mode waveguides and tunable coupling regions, thereby exhibiting simultaneous high frequency selectivity and excellent reconfigurability. The center frequency can be tuned by adjusting the resonant wavelengths of the MRRs, and the bandwidth can be reconfigured by adjusting the coupling coefficients as well as the resonant wavelengths of the MRRs. The operation state of each MRR can be monitored and thus the MPF can be adjusted precisely. In the experiment, the bandwidth and center frequency of the MPF are adjusted from 0.7 to 2 GHz and from 5.2 to 35.8 GHz, respectively. During the adjustment, the rejection ratio of the MPF remains above 40 dB. The link performance, including the link gain, noise figure and spurious-free dynamic range, are also charactered. The achieved MPF can be potentially applied to radar, sensor networks, and wireless communications.

Ultrahigh spectral resolution single passband microwave photonic filter.

Microwave photonic filters (MPFs) with only one ultra-narrow passband are able to provide high frequency selectivity and wide spectral range, and they are of great importance in radio-frequency (RF) signal processing. However, currently all MPFs are limited by trade-offs between key parameters such as spectral resolution and range, tunability, and stability. Here, we report the first demonstration of a single passband MPF with unprecedented performance including ultrahigh spectral resolution of 650 kHz, 0-40 GHz spectral range, and high stability of center frequency drifting within ±50 kHz. This record performance is accomplished by breaking the amplitude equality of a phase-modulated signal via a Brillouin dynamic grating (BDG) which has an ultra-narrow reflection spectrum of sub-MHz. The results point to new ways of creating high performance microwave photonic systems, such as satellite and mobile communications, radars, and remote-sensing systems.

Bandwidth‐reconfigurable microwave photonic filter based on stimulated Brillouin scattering effect spreading by vector modulation technology

AbstractWe 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.

Trace copper detection using in-line optical fiber Mach-Zehnder interferometer combined with an optoelectronic oscillator.

We experimentally demonstrate a novel optical fiber chemosensor for trace Cu2+ ions detection that is implemented by using an in-line optical fiber Mach-Zehnder interferometer (MZI) in conjunction with an optoelectronic oscillator (OEO). The MZI is fabricated by lateral offset splicing a section of D-shaped fiber between two single-mode fibers. It splices the broadband optical source into a sinusoidal-shaped light, which can form a single passband microwave photonic filter (MPF) by combining the Mach-Zehnder modulator, a segment of fiber and a photodetector. The center frequency of the MPF, determined by the free spectra range of MZI, is affected by the solution concentration. Incorporating the MPF in the OEO sensor, the oscillation frequency is determined by the solution concentration. Therefore, we can estimate the solution concentration by measuring the microwave frequency change. We carry out a proof to concept experiment. High sensitivity Cu2+ ions concentration sensing with sensitivity of 13 Hz/(μM/L) is achieved. The maximum measurement error of concentration obtained is within 1.38 μM/L. The proposed sensor has merits of high interrogation speed, simple operation, high sensitivity and accuracy, offering the potentials in a wide range of biological application scenarios.

MEMS-SOA Spectrum-Sliced Auto-Equalized Source Enabling Uniformly Tunable Microwave Photonic Filter

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.

LNOI waveguide grating based true time delay line for tunable bandpass microwave photonic filter

A precisely tunable true time delay line (TTDL) based on lithium niobate on insulator waveguide grating is proposed. Utilizing the electro-optic effect of lithium niobate, active tuning of reflected wavelength and time delay can be realized. The numerical simulation results show that, with the optimized grating structure and flexible electrode distribution, wide tuning range and high tuning accuracy of time delay can be achieved by simply controlling the charging position of the electrodes. This TTDL is applied to a tunable bandpass microwave photonic filter (TBMPF), which using PM-IM conversion to suppressed dc signal to achieve bandpass. In the TBMPF, the TTDL plays the role of introducing the dispersion for PM-IM conversion, time delay between the taps and tuning of central frequency. This TTDL can reduce the system complexity and cost of microwave photonic filter.

On-chip programmable microwave photonic filter with an integrated optical carrier processor

We demonstrate a programmable microwave photonic bandpass filter with a rectangular frequency response and a reconfigurable spectral resolution. We achieved these features through dual-sidebands processing of a phase modulated signal using a network of four optical ring resonators in a low-loss silicon nitride (Si3N4) circuit. Furthermore, we integrate a pair of optical ring resonators in the same circuit to precisely control the amplitude and phase of the optical carrier to enhance the noise performance of the filter. We achieved filtering with a tunable bandwidth from 2 to 7 GHz with optical carrier suppression up to 6 dB, a maximum RF gain of -10 dB, and a minimum noise figure of 27 dB. These experiments are expected to provide a feasible design to approach fully integrated microwave photonic filters with improved link gain and reduced noise figure.

Fiber ring laser cavity for strain sensing via beat frequency demodulation

A fiber ring laser cavity is developed for use in strain sensing that works via beat frequency demodulation. The system is constructed by using a fiber ring laser cavity which allows the strain to be monitored. An erbium-doped fiber amplifier (EDFA) is used in the laser cavity to provide the gain and a single mode fiber (SMF) is acted as a sensing head. Furthermore, a π-phase-shifted fiber Bragg grating (π-FBG) is served as a microwave photonic bandpass filter and reflector in the loop. In sensor testing, a good linear relationship is observed between the change in strain and the shift in beat frequency. Various sensitivities are obtained by measuring the different beat frequency shifts. The strain monitoring sensitivities are estimated to be −0.511 kHz/με, −0.853 kHz/με, and −1.606 kHz/με for beat frequencies of 0.429 GHz, 1.008 GHz, and 1.459 GHz, respectively. Moreover, the beat frequency signals (BFS) have signal-to-noise ratio (SNR) of 60.0 dB and the bandwidth of the strong BFS can reach 4.0 GHz. The proposed strain sensing is considered to have great potential for many optical sensing applications and attractive features such as compact size, good repeatability, high sensitivity, and low cost.

Passband-Switchable and Frequency-Tunable Dual-Passband Microwave Photonic Filter

A passband-switchable and frequency-tunable dual passband microwave photonic filter (MPF) implemented using a broadband optical source (BOS) in a delay-line configuration is proposed and experimentally demonstrated. In the proposed MPF, the BOS is sliced by a Mach-Zehnder interferometer (MZI), in which a phase modulator (PM) is incorporated in the upper arm. The phase-modulated light signal combined with an unmodulated light at the output of the MZI is sent through a dispersive delay line and detected at a photodetector (PD). The entire operation is equivalent to a microwave bandpass filter with its central frequency determined by the time delay difference between the two arms of the MZI. To implement a dual passband MPF, a second lower arm with a different arm length corresponding to a different time delay is added. The independent switchability of the passband is realized by controlling the polarization state of the light wave in each of the lower arms to be parallel or orthogonal with that of the upper arm. The independent frequency tunability is achieved by tuning the time delay of each of the lower arms. The proposed MPF is experimentally evaluated. A dual-passband MPF with independent passband switchability and independent frequency tunability is demonstrated. The results show that each of the two passbands can be switched on and off with an extinction ratio of about 20 dB. The passband frequency can also be continuously tuned with a tunable range from 1 to 18 GHz.

Frequency-definable linearly chirped microwave waveform generation by a Fourier domain mode locking optoelectronic oscillator based on stimulated Brillouin scattering.

A Fourier domain mode locking (FDML) optoelectronic oscillator (OEO) scheme based on stimulated Brillouin scattering (SBS) is proposed to generate linearly chirped microwave waveforms (LCMWs) with excellent frequency sweep linearity and a precisely controllable frequency sweep range. The kernel of the proposed scheme is a center-frequency-definable SBS-based fast-tunable microwave photonic bandpass filter based on frequency-selective phase-to-intensity conversion, which is realized by using a single laser source to generate a fast frequency-swept probe light and a tunable pump light via electro-optic frequency shifting. A proof-of-concept experiment is carried out to demonstrate the proposed scheme, where LCMWs with flexibly-tunable center frequency and bandwidth are generated. In the experiment, the maximum time-bandwidth product and chirp rate are 82000 and 0.195 GHz/µs, respectively. Most importantly, the frequency deviation is less than 1 MHz and the frequency sweep linearity is smaller than 1.7%, which indicate that the proposed SBS-based FDML OEO can generate frequency-definable LCMWs with excellent frequency sweep linearity.

Temperature-insensitive relative humidity sensing based on a carrier-suppression-effect-free tunable optoelectronic oscillator using a non-coherent broadband optical source

A relative humidity (RH) sensing scheme with high resolution and high interrogation speed based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. A carrier-suppression-effect-free single bandpass microwave photonic filter (MPF) with no baseband response is employed in the OEO to select the oscillating frequency, which is consisted of a non-coherent broadband optical source, a Mach-Zehnder interferometer (MZI) with one arm embedded a phase modulator, a dispersion compensation fiber and a photodetector. In one arm of the MZI, a section of optical fiber is coated with a polyvinyl alcohol (PVA) film acting as sensing probe, in which the PVA film can convert the RH variation to the strain on the fiber. When the RH changes, the arm-length difference and free spectral range of the MZI is changed, leading to a frequency change of the MPF as well as the oscillating signal of the OEO. By simply monitoring the frequency shift of the oscillating signal in the electrical domain, a high resolution and high interrogation speed RH measurement can be achieved. The temperature cross-sensitivity can be almost eliminated by adding a section of fiber without PVA film coating in the other arm of the MZI. An experiment is performed, and a high RH sensitivity of 2.016 MHz/%RH with a high resolution of 0.17 % is achieved.

Multiple passband microwave photonic filter with adjustable bandwidth

We propose a straightforward technique that allows adjusting the bandwidth of microwave filtered signals by using two tunable optical bandpass filters. The principle of operation of the microwave photonic filter that allows the generation of passband windows is described by a full mathematical analysis. The electrical bandwidth of each generated passband window is adjusted by optically filtering a multimode laser diode. We demonstrate that the proposed filter can generate passband windows located at 4.87 GHz and 9.63 GHz exhibiting a bandwidth of 284.54 MHz and 391.24 MHz, respectively. The electrical bandwidth of each passband window can be increased to a maximum value of 1000 MHz by means of an adequate adjustment of the optical bandwidth from 2 nm to 10 nm approximately. A potential application of this proposal resides in the fact that the microwave filtered signals can be used as electrical carriers in optical communication schemes taking advantage of the electrical bandwidth that can be tailored according to a data transmission.