Tunable Microwave Photonic Filter Research Articles

Tunable microwave photonic filters based on double-sideband modulation and linear chirped fibre Bragg gratings

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.

Microwave Photonic Filter by Using Recirculating Frequency Shifter to Generate Optical Frequency Comb

In this paper, a continuously tunable microwave photonic filter (MPF) with optical frequency comb (OFC) generated by recirculating frequency shifter (RFS) based on an inphase/quadrature (I/Q) modulator is proposed and experimentally demonstrated. A chirped fiber Bragg grating (CFBG) is inserted into the RFS loop to act as an optical bandpass filter. The central frequencies of two passbands of the MPF in 20 GHz range have been tuned linearly from 7.67 GHz to 9.04 GHz, and 15.34 GHz to 18.03 GHz, respectively by changing the frequency of the driving microwave signal onto the I/Q modulator from 20 GHz to 17 GHz. When the frequency of the driving microwave signal is decreased, OFC with smaller tone spacing is generated, and then the MPF's passbands move to higher frequency. So that, the proposed MPF's passbands with high frequencies can be achieved by applying driving microwave signal with low frequency to generate the OFC. By varying the bandwidth of the CFBG, the number of comb lines can be changed, which results in different bandwidth of the MPF's passband. Furthermore, in our experiment by carefully adjusting bias voltage of the Mach-Zenhder modulator, when the frequencies of driving microwave signal onto the I/Q modulator is 20 GHz and 18 GHz, one passband in the 20 GHz range can be suppressed at the frequency of 15.34 GHz, and 8.52 GHz, respectively.

Design and optimization of a microwave photonic filter exploiting differential mode group delay of a multi-mode fiber

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.

Silicon nitride programmable photonic processor with folded heaters.

General-purpose programmable photonic processors rely on the large-scale integration of beamsplitters and reconfigurable phase shifters, distributed within unit cells or photonic gates. With their future evolution threatened by several hardware constrains, including the integration density that can be achieved with current mesh topologies, in this work, we present a unit cell topology design to increase the integration density of waveguide mesh arrangements based on folded Mach-Zehnder Interferometers. We report the design details of a 40-unit cell waveguide mesh integrated in a 11mm x 5.5 mm silicon nitride chip achieving, to the best of our knowledge, the highest integration density reported to date for a general-purpose photonic processor. The chip is electrically interfaced to a PCB and we report examples of reconfigurable optical beamsplitters, basic tunable microwave photonic filters with high peak rejection (40 dB approx.), as well as the dynamic interconnection and routing of 5G digitally modulated signals within the photonic mesh.

All-optical gain optoelectronic oscillator based on a dual-frequency integrated semiconductor laser: potential to break the bandwidth limitation in the traditional OEO configuration.

A novel photonic method, to the best of our knowledge, to generate high-frequency micro/millimeter-wave signals based on the optoelectronic oscillator (OEO) with all-optical gain is proposed in this paper. The core device is the monolithically integrated dual-frequency semiconductor laser (MI-DFSL), in which the two DFB laser sections are simultaneously fabricated on one chip. Attributing to the combined impact of the photon-photon resonance effect and the sideband amplification injection locking effect, one widely tunable microwave photonic filter with a high Q value and narrow 3-dB bandwidth can be realized. In this case, the generated microwave signals would largely break the limitation in bandwidth once making full use of the optical amplifier to replace the narrow-band electrical amplifiers in traditional OEO configuration to provide the necessary gain. No additional high-speed external modulator, high-frequency electrical bandpass filters or multi-stage electrical amplifiers are required, highly simplifying the framework and reducing the power consumption. Moreover, this simple and compact structure has the potential to be developed for photonic integration. In the current proof-of-concept experiment, microwave signals with wide tuning ranges from 14.2 GHz to 25.2 GHz are realized. The SSB phase noises in all tuning range are below -103.77 dBc/Hz at 10 kHz and the best signal of the -106.363 dBc/Hz at 10 kHz is achieved at the frequency of 17.2 GHz.

Microwave photonic injection locking frequency divider based on a tunable optoelectronic oscillator.

We present a novel broadband divide-by-2 microwave photonic injection locking frequency divider (ILFD) based on a dual-loop optoelectronic oscillator (OEO). In the proposed scheme, a tunable microwave photonic filter is used to replace the traditional electrical filter, which makes sure a large tuning range of the ILFD. The microwave photonic ILFD whose center frequency tracks the tunable frequency of the free-running OEO, links up with every single locking range together. Thus the frequency range is only determined by the tunable OEO. In the experiment, a wide operating frequency range from 4.51 GHz to 34.88 GHz is realized. Furthermore, a divide-by-3 ILFD is experimentally demonstrated with the help of a frequency mixer.

Cascaded Microwave Photonic Filters for Side Mode Suppression in a Tunable Optoelectronic Oscillator applied to THz Signal Generation & Transmission

We demonstrate experimentally an optoelectronic oscillator (OEO) in which high side-mode suppression is achieved by cascading a phase modulator-based single passband tunable microwave photonic (MWP) filter with an optoelectronic feedback loop-based infinite impulse response (IIR) MWP filter. The OEO provides an RF oscillation that can be tuned from 6.5 GHz to 17.8 GHz with a phase noise lower than -103 dBc/Hz. Experimental results show that inclusion of the IIR section leads to a 20 dB reduction of phase noise close to the carrier and an increase of 10 dB in side mode suppression, compared to the equivalent OEO without an IIR section. The OEO was used to drive an optical frequency comb generator to generate a THz signal at 242.6 GHz by optical heterodyning; inclusion of the IIR section increases suppression of the side modes neighboring the THz carrier. A radio over fiber link was then implemented at a 242.6 GHz carrier frequency, with transmission of a 24 Gbps signal over 40 km of fiber and a 30 cm wireless path at a bit error rate below the forward error correction limit. The proposed system may be applied to frequency reconfigurable THz links and radars.

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.

Ultra-High Peak Rejection All-Optical Microwave Filter Based on the Opto-Mechanical Effect

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.

Tunable microwave-photonic filtering with high out-of-band rejection in silicon

The ever-increasing demand for high speed and large bandwidth has made photonic systems a leading candidate for the next generation of telecommunication and radar technologies. The photonic platform enables high performance while maintaining a small footprint and provides a natural interface with fiber optics for signal transmission. However, producing sharp, narrow-band filters that are competitive with RF components has remained challenging. In this paper, we demonstrate all-silicon RF-photonic multi-pole filters with ∼100× higher spectral resolution than previously possible in silicon photonics. This enhanced performance is achieved utilizing engineered Brillouin interactions to access long-lived phonons, greatly extending the available coherence times in silicon. This Brillouin-based optomechanical system enables ultra-narrow (3.5 MHz) multi-pole response that can be tuned over a wide (∼10 GHz) spectral band. We accomplish this in an all-silicon optomechanical waveguide system, using CMOS-compatible fabrication techniques. In addition to bringing greatly enhanced performance to silicon photonics, we demonstrate reliability and robustness, necessary to transition silicon-based optomechanical technologies from the scientific bench-top to high-impact field-deployable technologies.

Tunable Microwave Photonic Filter Based on LNOI Polarization Beam Splitter and Waveguide Grating

A tunable microwave photonic filter (MPF) based on a polarization beam splitter (PBS) and waveguide grating true-time delay line (TTDL) is proposed and demonstrated experimentally. Taking advantage of the high birefringence property of lithium niobate (LN), the PBS is achieved by a simple structure in a short length. The TTDL consists of an LNOI waveguide grating and flexible electrodes covering along the grating. Owing to the electro-optic effect of LN, a wide tuning range and high tuning accuracy of delay time can be achieved simply by controlling the charging position of the electrodes. With the proposed PBS and TTDL, the MPF exhibits low drive voltage, low power consumption, and high response rate in a simple construction.

Hundred megahertz microwave photonic filter based on a high Q silicon nitride multimode microring resonator

Narrowband microwave photonic filters based on a microring resonator are difficult to achieve because low cavity loss and low coupling loss should be satisfied simultaneously. Here, a high Q (∼2.6×106) multimode microring resonator is proposed to achieve an ultra-narrow band tunable microwave photonic filter. Combining the ultra-low loss of the silicon nitride waveguide and the ultra-low coupling coefficients of the multimode ring resonator, very narrow optical bandwidths between 72.5 MHz to 275 MHz were obtained for different order modes, which match the simulation results well. Furthermore, by introducing the two switchable modulation methods supported by the dual-drive Mach–Zehnder modulator, we achieved a narrowband passband/stopband switchable microwave photonic filter, whose 3 dB bandwidths are 180 MHz and 120 MHz, respectively. The filter frequency can be tuned from 2 ∼ 18 GHz by altering the laser wavelength, and a high out of band RF rejection ratio about 27 dB was obtained for the passband filter due to the high-quality factor. Besides, a high RF rejection ratio of about 51 dB was achieved for the stopband filter by using the RF cancellation technology.

A Tunable Dual-Passband Microwave Photonic Filter Based on Optically Injected Distributed Feedback Semiconductor Lasers and Dual-Output Mach-Zehnder Modulator

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.

Microwave photonic notch filter based on polarisation multiplexing and cross gain modulation in a semiconductor optical amplifier

A coherent interference-free tunable microwave photonic notch filter with a pair of negative and positive coefficients generated by employing polarisation multiplexing and cross gain modulation in a semiconductor optical amplifier is proposed and demonstrated through simulation results. The negative and positive coefficients of the notch filter are realised through two orthogonal polarisation states of a single continuous wave laser diode. The free spectral range of microwave photonic filter is tuned by varying the differential group delay of a birefringent single-mode fibre. The proposed filter shows good tunability, easy implementation and is cost-effective.

A tunable microwave photonic filter based on gain-switched optical pulses

A novel approach to realize a microwave photonic filter (MPF) based on gain-switched optical pulses is proposed and demonstrated. The proposed MPF using directly modulation-based optical source is more flexible and simple than using an external modulation-based optical frequency comb source. A theoretical model describing the principle of the proposed MPF is derived, which is confirmed by experimental results. Under different modulation and injection parameters, we present a numerical simulation of the frequency response of the proposed MPF. With no discernible tones, the proposed MPF is demonstrated by a proof-of-concept experiment. A single passband is achieved, which can be continuously tuned from 1 to 30 GHz. Further, self-phase modulation is explored in the optical domain to broaden the spectrum of gain-switched optical pulses and narrow down the bandwidth of the MPF passband.

Tunable THz Signal Generation and Radio-Over-Fiber Link Based on an Optoelectronic Oscillator-Driven Optical Frequency Comb

We propose and demonstrate experimentally a photonic THz signal generation technique based on a tunable optoelectronic oscillator (OEO), and its application in a radio over fiber (RoF) link. The OEO's frequency is tuned by varying the bandwidth of a tunable optical bandpass filter (TOBF) that is cascaded with a phase modulator (PM). The resulting tunable microwave photonic filter is used to generate OEO oscillations from 6.58 GHz up to 18.36 GHz (with a phase noise of ≤−103 dBc/Hz at 10 kHz offset from the carrier frequency). The OEO is subsequently used to drive an optical comb, generating 22 comb lines with a frequency spacing of 17.33 GHz covering a bandwidth of 360 GHz within a 20 dB envelope. By selecting two optical comb lines with a wavelength selective switch (WSS) and beating them in a uni-traveling carrier photodiode (UTC-PD), THz signals are generated at 101.5 GHz and 242.6 GHz with phase noise of −90 dBc/Hz and −78 dBc/Hz, respectively at 10 kHz offset from carrier frequency. Tunable mm-wave and THz signals can be generated either by changing the OEO oscillation frequency or the selected comb lines. Using the OEO driven OFCG, we implemented a RoF link at 242.6 GHz with a data rate of 24 Gbps over a wireless distance of 30 cm and with a bit error rate (BER) below the hard decision forward error correction (FEC) limit of 3.8 × 10−3. This method allows the creation of an all-photonic frequency reconfigurable THz signal generator and RoF system.

Instantaneous microwave frequency measurement based on non-sliced broadband optical source

We propose and experimentally demonstrate a reconfigurable instantaneous frequency measurement (IFM) system. A non-sliced broadband optical source (BOS) is used to realize two tunable microwave photonic filters (MPFs) with equivalent infinite taps. The two MPFs allow us to have an amplitude comparison function (ACF) which maps the unknown frequency to microwave power. The IFM system is reconfigurable by simply adjusting two variable optical delay lines (VODLs). The principle behind the proposed IFM systems is theoretically analyzed. Experimentally, unknown frequency measurement in a frequency range of 8–16.5 GHz and 11.5–20 GHz with a measurement error of less than 110 MHz is realized.

Tunable notch microwave photonic filter based on interferometry of a single low-incoherence source.

We present a method to realize a single-notch microwave photonic filter (MPF) based on interferometry of a single, low-coherence broadband optical source (BOS). Normally, a notch MPF based on low-coherence interferometry requires independent control of two optical sources located in different wavebands. In this work, we use a single BOS to accurately perform destructive interference between a bandpass and an all-pass MPF. A frequency and bandwidth freely tunable single-notch MPF can thus be realized. The proposed method is theoretically analyzed and experimentally demonstrated. A notch depth of more than 30 dB and a continuously tunable frequency range from 2 to 18 GHz was demonstrated in the proof-of-concept experiment.

Tunable and reconfigurable microwave photonic filter based on Fourier domain optical processor and cascaded fiber rings

A tunable and reconfigurable microwave photonic filter (MPF) based on a Fourier domain optical processor (FD-OP) and cascaded fiber rings is proposed and demonstrated. A stable and multiple wavelengths source can be realized by using a broadband source and a waveshaper. The multiple wavelengths source with different wavelength spacings is achieved by programming the waveshaper. Two fiber rings of the same structure are cascaded together to achieve a high main sidelobe suppression ratio (MSSR) and a high Q factor. The FD-OP is induced to add additional phase shifts to achieve tunability. When the number of taps is 60, by introducing cascaded two-fiber rings and the FD-OP, the MSSR is 59.356 dB, the maximum Q factor is 470.895 and the 3dB bandwidth of the filter is 0.019 GHz. The simulating results show that the center frequency continuously tuned from 4.47–13.41 GHz. The tunability and reconfigurability of the MPF are well realized.

Wideband tunable single passband microwave photonic filter based on optical injection

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.