Single passband microwave photonic filter using continuous-time impulse response

Multiplexed sensing interrogation technique based on a flexibly switchable multi-passband RF filter by using a Solc-Sagnac loop

In this paper, we have proposed and experimentally demonstrated a multiplexed sensing interrogation technique based on a flexibly switchable multi-passband RF filter with a polarization maintaining fiber (PMF) Solc-Sagnac loop. A high-order Solc-Sagnac loop can be used as a spectrum slicer as well as sensing heads, and a multi-passband microwave photonic filter (MPF) can be achieved together with a dispersive medium. Environmental parameter variations will cause a frequency shift of the corresponding passband of the MPF, so by employing only one Sagnac loop, it is possible to monitor several environmental parameters simultaneously. In this article, we have demonstrated and analyzed the performance of the flexibly switchable multi-passband MPF by using a second-order Solc-Sagnac loop. To demonstrate the temperature sensing capabilities of our interrogation system, we have applied temperature changes individually to Sensor Head 1 (L _{P M F 1} ≈0.97m) only, Sensor Head 2 (L _{P M F 2} ≈2.97m) only, and both Sensor Head 1 and 2 in the experiment. By monitoring frequency shift of the MPF's passbands, the sensitivities for Sensor Head 1 and Sensor Head 2 have been estimated to be -0.275 ± 0.011 MHz/℃ and -0.811 ± 0.013 MHz/℃ respectively, which show a good sensing linearity and stability. By utilizing the longer length of the sensing PMF, higher sensitivity can be achieved. By using Solc-Sagnac loop with higher order, more sensors can be multiplexed.

Microwave Photonic Signal Processing and Sensing Based on Optical Filtering

Microwave photonics, based on optical filtering techniques, are attractive for wideband signal processing and high-performance sensing applications, since it brings significant benefits to the fields by overcoming inherent limitations in electronic approaches and by providing immunity to electromagnetic interference. Recent developments in optical filtering based microwave photonics techniques are presented in this paper. We present single sideband modulation schemes to eliminate dispersion induced power fading in microwave optical links and to provide high-resolution spectral characterization functions, single passband microwave photonic filters to address the challenges of eliminating the spectral periodicity in microwave photonic signal processors, and review the approaches for high-performance sensing through implementing microwave photonics filters or optoelectronic oscillators to enhance measurement resolution.

Software-defined microwave photonic filter with high reconfigurable resolution

Microwave photonic filters (MPFs) are of great interest in radio frequency systems since they provide prominent flexibility on microwave signal processing. Although filter reconfigurability and tunability have been demonstrated repeatedly, it is still difficult to control the filter shape with very high precision. Thus the MPF application is basically limited to signal selection. Here we present a polarization-insensitive single-passband arbitrary-shaped MPF with ~GHz bandwidth based on stimulated Brillouin scattering (SBS) in optical fibre. For the first time the filter shape, bandwidth and central frequency can all be precisely defined by software with ~MHz resolution. The unprecedented multi-dimensional filter flexibility offers new possibilities to process microwave signals directly in optical domain with high precision thus enhancing the MPF functionality. Nanosecond pulse shaping by implementing precisely defined filters is demonstrated to prove the filter superiority and practicability.

Passband switchable microwave photonic multiband filter

A reconfigurable microwave photonic (MWP) multiband filter with selectable and switchable passbands is proposed and experimentally demonstrated, with a maximum of 12 simultaneous passbands evenly distributed from 0 to 10 GHz. The scheme is based on the generation of tunable optical comb lines using a two-stage Lyot loop filter, such that various filter tap spacings and spectral combinations are obtained for the configuration of the MWP filter. Through polarization state adjustment inside the Lyot loop filter, an optical frequency comb with 12 different comb spacings is achieved, which corresponds to a MWP filter with 12 selectable passbands. Center frequencies of the filter passbands are switchable, while the number of simultaneous passbands is tunable from 1 to 12. Furthermore, the MWP multiband filter can either work as an all-block, single-band or multiband filter with various passband combinations, which provide exceptional operation flexibility. All the passbands have over 30 dB sidelobe suppression and 3-dB bandwidth of 200 MHz, providing good filter selectivity.

Tunable microwave photonic notch filter based on sliced broadband optical source

A microwave photonic filter is demonstrated with both tunable center frequency and bandwidth. This filter is switchable from all-pass, bandpass to notch filter, and the notch filter is a result of the subtraction of a bandpass filter from an all-pass filter based on a balanced photodetector. The all-pass filter is achieved based on a single wavelength radio over fiber link, and the bandpass one is acquired by using the spectrum-sliced broadband optical source. Theoretical analysis and experimental results show that both the center frequency and the bandwidth of the notch filter can be widely tuned.

Switchable and tunable microwave frequency multiplication based on a dual-passband microwave photonic filter

In this paper, a novel approach to implement switchable and tunable microwave frequency multiplication has been proposed and experimentally demonstrated. High order harmonics of microwave signal with external modulation technique can be selected by using a novel switchable dual-passband microwave photonic filter (MPF) based on a modified fiber Mach-Zehnder interferometer (FMZI) and a dispersive medium. By adjusting the polarization controllers in the modified FMZI, the passbands of the MPF can switch between lower frequency, higher frequency or dual-passband states, and by changing the length of the variable optical delay line (VODL) in the modified FMZI, the central frequencies of these passbands can also be tuned. Therefore, tunable and switchable microwave signal frequency multiplication can be achieved. The experimental results show that by modulating a driving signal with frequency of 2.5 GHz, a signal with frequency of 7.5 GHz, which is three times of the driving frequency, the other one with the frequency of 15 GHz, which is six times of the driving frequency can be generated and freely switchable between two frequencies and dual frequency states by simply adjusting the polarization controllers in the modified FMZI.

Widely tunable single bandpass microwave photonic filter based on Brillouin-assisted optical carrier recovery

A widely tunable single bandpass microwave photonic filter (MPF) based on Brillouin-assisted optical carrier recovery in a highly nonlinear fiber (HNLF) with only one optical filter is proposed and experimentally demonstrated. The fundamental principle lies in the fact that the suppressed optical carrier of the phase modulated optical signal could be recovered by the stimulated Brillouin scattering (SBS) amplification effect. When phase modulated optical signals go through an optical filter with a bandpass response, the optical carrier and the upper sidebands suffer from the suppression of the optical filter because they fall in the stopband of that. In our system, the optical carrier could be recovered by the SBS operation around 38 dB. The MPF is achieved by one-to-one mapping from the optical domain to the electrical domain only when one of phase modulated sidebands lies in the bandpass of the optical filter. It shows an excellent selectivity with a 3-dB bandwidth of 170 MHz over a tuning frequency range of 9.5-32.5 GHz. The out-of-band suppression of the MPF is more than 20 dB. Moreover, the MPF shows an excellent shape factor with 10-dB bandwidth of only 520 MHz. The frequency response of the MPF could be widely tuned by changing the frequency difference between the frequency of the optical carrier and the center frequency of the bandpass of the optical filter. A proof-of-concept experiment is carried out to verify the proposed approach.

Widely tunable single-bandpass microwave photonic filter based on polarization processing of a nonsliced broadband optical source

We propose a new scheme of microwave photonic filter (MPF) based on the polarization processing of a broadband optical source (BOS), which features single-bandpass response and a wide span of operation bandwidth. The BOS is orthogonally polarized by a polarization division multiplexing emulator (PDME) with a tunable time delay between the two polarization states and incident at ± 45° to one principle axis of a polarization modulator (PolM). The PDME cascades a PolM, and a polarizer realizes a microwave modulation making the phase of the carrier able to be tuned while ± 1st sidebands remain unchanged, which after propagating in a dispersive medium results in a tunable single-bandpass response in the RF domain. We experimentally verify the MPF. By adjusting the time delay and the optical spectrum bandwidth, the passband center frequency is continuously tuned from DC to 20 GHz and the 3 dB passband bandwidth changes while the optical spectrum bandwidth ranges from 1 to 4 nm.

Shifted dispersion-induced radio-frequency fading in microwave photonic filters using a dual-input Mach-Zehnder electro-optic modulator

A simple microwave photonic processor structure with single passband response, and widely tunable capability, is demonstrated. It is based on the principle of shifted dispersion-induced radio-frequency (RF) fading by using a dual-input Mach-Zehnder electro-optic modulator (EOM) that is fed from a broadband optical source with unbalanced input fiber lengths into the upper and lower arms of the EOM, in combination with a dispersive medium. This topology consequently produces a spectral response equivalent to the curve of the dispersion-induced RF fading that is shifted from the conventional baseband location to high frequencies. Therefore, an equivalent single passband is formed without the requirement of the conventional tap coefficients. Experimental results verify the structure and demonstrate a continuously tunable microwave filter exhibiting shape invariance and a single passband. In addition, the filter response sidelobe suppression is also significantly improved by applying a Gaussian windowed profile to the broadband optical source.

An ultrawide tunable range single passband microwave photonic filter based on stimulated Brillouin scattering

A single passband microwave photonic filter with ultrawide tunable range based on stimulated Brillouin scattering is theoretically analyzed. Combining the gain and loss spectrums, tuning range with 44GHz is obtained without crosstalk by introducing two pumps. Adding more pumps, Tuning range multiplying with the multiplication factor equaling to the total quantity of pump can be achieved, which has potential application in microwave and millimeter wave wireless communication systems.

Integrable microwave filter based on a photonic crystal delay line

The availability of a tunable delay line with a chip-size footprint is a crucial step towards the full implementation of integrated microwave photonic signal processors. Achieving a large and tunable group delay on a millimetre-sized chip is not trivial. Slow light concepts are an appropriate solution, if propagation losses are kept acceptable. Here we use a low-loss 1.5 mm-long photonic crystal waveguide to demonstrate both notch and band-pass microwave filters that can be tuned over the 0-50-GHz spectral band. The waveguide is capable of generating a controllable delay with limited signal attenuation (total insertion loss below 10 dB when the delay is below 70 ps) and degradation. Owing to the very small footprint of the delay line, a fully integrated device is feasible, also featuring more complex and elaborate filter functions.

Single passband microwave photonic filter with wideband tunability and adjustable bandwidth

A new and simple structure for a single passband microwave photonic filter is presented. It is based on using an electro-optical phase modulator and a tunable optical filter and only requires a single wavelength source and a single photodetector. Experimental results are presented that demonstrate a single passband, flat-top radio-frequency filter response without free spectral range limitations, along with the capability of tuning the center frequency and filter bandwidth independently.

Harmonic distortion in microwave photonic filters

We present a theoretical and experimental analysis of nonlinear microwave photonic filters. Far from the conventional condition of low modulation index commonly used to neglect high-order terms, we have analyzed the harmonic distortion involved in microwave photonic structures with periodic and non-periodic frequency responses. We show that it is possible to design microwave photonic filters with reduced harmonic distortion and high linearity even under large signal operation.

Widely tunable single-bandpass microwave photonic filter employing a non-sliced broadband optical source

We demonstrate a novel microwave photonic filter based on a non-coherent broadband optical source and the variable optical carrier time shift (VOCTS) method. Optical slicing which is essential conventionally is not employed in our scheme. Nevertheless, equivalent "electrical slicing" is performed by VOCTS, generating a passband free from the carrier-suppression effect. The baseband response is eliminated by using carrier-suppression or phase modulation. Single bandpass is also achieved due to the continuous-time sinusoidal impulse response. Detailed theoretical analyses are presented and agree with the experiments quite well.

Tunable chirped microwave photonic filter employing a dispersive Mach-Zehnder structure

A Mach-Zehnder structure with modulation in one arm and dispersive time delay in the other is proposed to implement highly flexible single-bandpass chirped microwave photonic filters based on broadband optical sources. Both the amplitude response and the time delay slope can be fully reconfigured via control of the optical spectra and dispersion. The passband can also be widely tuned without changing the shape. A chirped filter with a bandwidth of ∼4 GHz, a delay slope of ∼-0.6 ns/GHz, and a tunability up to 40 GHz is demonstrated experimentally.