Slow and fast light effects in semiconductor waveguides for applications in microwave photonics

A continuously tunable microwave photonic notch filter at around 30 GHz is experimentally demonstrated and 100% fractional tuning over 360deg range is achieved without changing the shape of the spectral response. The tuning mechanism is based on the use of slow and fast light effects in semiconductor optical amplifiers assisted by optical filtering.

A continuously tunable microwave photonic notch filter with complex coefficient based on phase modulation is proposed and demonstrated. The complex coefficient is generated using a Fourier-domain optical processor (FD-OP) to control the amplitude and phase of the optical carrier and radio-frequency (RF) phase modulation sidebands. By controlling the FD-OP, the frequency response of the filter can be tuned in the full free spectral range (FSR) without changing the shape and the FSR of the frequency response. The results show that the center frequency of the notch filter can be continuously tuned from 17.582 GHz to 29.311 GHz with FSR of 11.729 GHz. The shape of the frequency response keeps unchanged when the phase is tuned.

A widely tunable microwave photonic notch filter with adjustable bandwidth based on multi-wavelength fiber laser is proposed and demonstrated. The multi-wavelength fiber laser generates the multi-taps of the microwave photonic filter (MPF). In order to obtain notch frequency response, a Fourier-domain optical processor (FD-OP) is introduced to control the amplitude and phase of the optical carrier and phase modulation sidebands. By adjusting the polarization controller (PC), different numbers of taps are got, such as 6, 8, 10 and 12. And the wavelength spacing of the multi-wavelength laser is 0.4 nm. The bandwidth of the notch filter is changed by adjusting the number of taps and the corresponding bandwidths are 4.41 GHz, 3.30 GHz, 2.64 GHz and 2.19 GHz, respectively. With the additional phase shift introduced by FD-OP, the notch position is continuously tuned in the whole free spectral range (FSR) of 27.94 GHz. The center frequency of the notch filter can be continuously tuned from 13.97 GHz to 41.91 GHz.

Owing to the unique features of flexible spectral characteristics, low loss, light weight, compact footprint, and compatibility with other fiber-optic elements, fiber Bragg gratings (FBGs) have been extensively used in various microwave photonics systems. In this work, the recent development in employing advanced FBGs for various microwave photonics applications are reviewed, with emphasis on microwave arbitrary waveform generation and photonic microwave filtering. An innovative FBG sensor interrogation scheme using microwave photonics techniques is also presented.

We introduce a novel scheme based on slow and fast light effects in semiconductor optical amplifiers, to implement a microwave photonic notch filter with ~100% fractional tuning range at a microwave frequency of 30 GHz.

A new topology for a tunable microwave photonic notch filter, which achieves a wide and continuous tuning range of FSR/2, is presented. The novel principle of tuning is based on changing optical variable attenuators only; consequently, a fixed wavelength laser can be used as the optical source. Experimental filter tuning results demonstrate a wide fractional tuning range of 50%, continuous tuning capability, and a notch filter shape that does not change as it is tuned, in very good agreement with predictions.

A widely tunable microwave photonic (MWP) notch filter with high linearity based on a dual-parallel Mach—Zehnder modulator (DPMZM) is proposed and experimentally demonstrated. The motivation of this work lies in the fact that the MWP notch filter previously reported has nonlinear distortions generally induced by the nonlinear property of electro-optic devices. In this scheme, even-order modulated sidebands at the DPMZM output are effectively suppressed by properly adjusting bias voltages of the DPMZM, which are the dominant factor to induce the nonlinear distortions into the MWP notch filter. The proposed scheme is theoretically analyzed and experimentally verified.

Microresonator-based frequency combs (microcombs) are very promising for microwave photonic applications for their advantages including wide bandwidth, large line spacing, and compact volume. Recently, various microcomb-based microwave photonic systems have been reported such as high-spectral-purity microwave generation, microwave photonic signal processing, true-time-delay beamforming, etc. Outstanding performances including low phase noise, high reconfigurability, and large Nyquist bandwidth have been demonstrated, showing a bright future of microcomb-based microwave photonics.

Microwave photonics has advanced greatly as a result of breakthroughs on many fronts. Breakthroughs in fiber amplifiers, sources and receivers have all resulted in greatly increased system performance, and significant evolution of microwave photonic networks has occurred. The author discusses these elements in turn. There are many applications of microwave photonics, including fiber optic distribution of signals for cellular networks, phase array radars, and cable TV. Bandwidth requirements in these applications keep increasing, driving up the center frequency and increasing the need for microwave photonics to reduce the loss, dispersion, crosstalk and cost. This is also causing an evolution in these networks from smaller, lower frequency networks at 0.88 /spl mu/m to larger, higher frequency networks at 1.3 and 1.5 /spl mu/m.

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Recently, optical phase modulation has been widely used in microwave photonics (MWP) systems, such as radio over fiber systems, photonic microwave filters, optical microwave and millimeter-wave signal generators, and optical subcarrier frequency up-converters. An optical phase-modulated signal can be converted to an intensity-modulated signal in a dispersive optical fiber. Due to the intrinsic nonlinearity of optical phase modulation, for linear applications such as microwave signal distribution and filtering, the modulation index should be kept small to minimize the unwanted modulation nonlinearity. However, for nonlinear applications such as microwave frequency multiplication and subcarrier frequency upconversion, the modulation index should be large to maximize the frequency multiplication and upconversion efficiency. In this paper, for the first time to our knowledge, we develop a thorough theoretical framework for the characterization of phase-modulation-based MWP systems, in which the phase modulation to intensity modulation conversion is realized using a dispersive fiber. Analytical models for the distributions of single-tone and two-tone microwave signals and for microwave frequency multiplication and subcarrier frequency upconversion are developed, which are verified by numerical simulations. The analytical models for single-tone and two-tone transmissions are further confirmed by experiments. The developed analytical models provide an accurate mathematical tool in designing phase-modulation-based MWP systems. </para>

A new photonic microwave phase shifter that can achieve a full 360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> phase shift with very little RF signal amplitude variation, and which can operate over very wide frequency range, is presented. It is based on controlling the amplitude and phase of the optical carrier and the two RF modulation sidebands via the dc bias voltages to a dual-parallel Mach-Zehnder modulator (DPMZM). It has a simple structure, only requiring a single laser, modulator, and photodetector. A continuous 360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> RF phase shift without changes in the RF signal amplitude is obtained by controlling the dc bias voltages of the DPMZM. Experimental results demonstrate a very wideband operation of the RF phase shifter over an 8:1 frequency range from 2 to 16 GHz, a continuous 0-360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> RF phase shift with very low amplitude variation of <; 3 dB of over the entire frequency band, and with a phase deviation standard deviation of <; 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> .

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.

We demonstrate that the initial optical phase difference between the carrier and sidebands will strongly influence the final RF phase shift induced by filter assisted slow and fast light effects in semiconductor optical amplifiers.

The evolution towards the 6th mobile systems Generation (6G) will be characterized by several factors, among which it is possible to identify two significant trends for the topics of this paper: i) the always increasing Bandwidth (BW) and high data rates, and ii) the increasingly complex radio systems architecture. The former will lead to exploit centimeter-wave (∼ 7-15 GHz), millimeter-wave (∼ 26-43 GHz) and sub-Terahertz (∼ 90-300 GHz) frequencies to achieve ultra-high data rates on air. This means that radio hardware platforms will have to support processing volumes of the order of several hundred Gbit/s up to 1 Tbit/s. The latter will make more intensive use of massive Multiple-Input Multiple-Output (MIMO) and beamforming technologies, already introduced in the 5th mobile systems Generation (5G), which will require a very high number of radiating elements, leading to high complexity and energy consuming radio systems. In this scenario, high-speed optical transmission systems, integrated photonics for interconnection systems in high-bandwidth-density hardware platforms, co-packaged optics, advanced photonic communication systems on integrated optical waveguides, radio-over-fiber transmission systems, and microwave photonics components for implementing radio functions (currently dominated by microwave technologies) will all play a critical role, and in some cases, drive disruptive innovation for future 6G radio systems. Extending its traditional definition, in this paper we will refer to all these technologies as Microwave Photonics (MWP), intended as the set of critical photonic technologies, often associated with integrated photonic technologies, contributing to the realization of future radio systems.

A novel photonic microwave notch filter with capability of frequency tuning is proposed and experimentally demonstrated. The scheme is based on a fiber Bragg grating (FBG)-based, single longitudinal mode, wavelength-spacing tunable dual-wavelength fiber laser and a dispersive fiber delay line. By using a symmetrical S-bending technique along the FBGs, the wavelength spacing of the laser can be tuned, which enables the microwave notch frequency tuning. Experimental results show that the notch rejection of more than 30 dB and the flexible tunability of notch frequency can be readily achieved in the range of 1.2 approximately 6.7 GHz.