Tunable True-Time Delay Operation in A Dispersion-Engineered Few-Mode Fiber

[EN] We present a simple few-mode fiber design as a promising platform to implement tunable sampled true-time delay lines for radiofrequency signal processing. To the best of our knowledge, this is the first few-mode fiber ever reported featuring evenly spaced incremental dispersion values, which is an essential characteristic required for tunable operation of microwave photonics applications. The performance of the designed five-sample true-time delay line is theoretically validated in the context of microwave signal filtering, demonstrating free spectral range continuous tunability from 12.4 up to 57 GHz.This work is supported by the European Research Council (ERC) under Consolidator Grant Project 724663, and Ramon y Cajal fellowship RYC-2014-16247 for I. Gasulla.Nazemosadat-Arsanjani, SB.; Gasulla Mestre, I. (2020). Tunable True-Time Delay Operation in A Dispersion-Engineered Few-Mode Fiber. IEEE. 203-206. https://doi.org/10.23919/MWP48676.2020.9314392S20320

Mode-division multiplexing for microwave signal processing

[EN] We present an overview of different mode-division multiplexing fiber technologies engineered to provide tunable microwave signal processing, including signal filtering and optical beamforming for phased-array antennas. The exploitation of both the space and wavelength dimensions brings advantages in terms of increased compactness, flexibility and versatility.This research was supported by the ERC Consolidator Grant ERC-COG-2016 InnoSpace 724663 and the Spanish MINECO fellowship RYC-2014- 16247 for I. Gasulla.Nazemosadat-Arsanjani, SB.; Gasulla Mestre, I. (2021). Mode-division multiplexing for microwave signal processing. IEEE. 1-2. https://doi.org/10.1109/SUM48717.2021.95058021

Dispersion-tailored few-mode fiber design for tunable microwave photonic signal processing

[EN] We present a novel double-clad step-index few-mode fiber that operates as a five-sampled tunable true-time delay line. The unique feature of this design lies in its particular modal chromatic dispersion behavior, which varies in constant incremental steps among adjacent groups of modes. This property, which to the best of our knowledge has not been reported in any other few-mode fiber to date, is the key to tunable operation of radiofrequency signal processing functionalities implemented in few-mode fibers. The performance of the designed true-time delay line is theoretically evaluated for two different microwave photonics applications, namely tunable signal filtering and optical beamforming networks for phased array antennas. In the 35-nm optical wavelength tuning range of the C-band, the free spectral range of the microwave filter and the beam-pointing angle in the phased array antenna can be continuously tuned from 12.4 up to 57 GHz and 12.6 degrees up to 90 degrees, respectively.European Research Council (Consolidator Grant Project 724663); Ministerio de Ciencia, Innovacion y Universidades (Ramon y Cajal fellowship RYC-2014-16247).Nazemosadat-Arsanjani, SB.; Gasulla Mestre, I. (2020). Dispersion-tailored few-mode fiber design for tunable microwave photonic signal processing. Optics Express. 28(24):37015-37025. https://doi.org/10.1364/OE.412830S3701537025282

Elliptical-Core Highly Nonlinear Few-Mode Fiber Based OXC for WDM-MDM Networks

In order to realize an optical cross-connect (OXC) converting wavelengths and spatial modes into one-dimensional switching ports, we propose an active mode selective conversion without parasitic wavelength conversion, based on the intermodal four-wave mixing (FWM) arising in a few-mode fiber (FMF). First, we design a dispersion-engineered elliptical-core highly nonlinear FMF (e-HNL-FMF) with a graded refractive index (RI) profile, which can independently guide 3 linearly polarized (LP) spatial modes. Meanwhile, a high doping concentration of germanium in the core leads to relatively high intermodal nonlinear coefficients of 3.23 (W\ub7km)-1 between LP01 and LP11a modes and 3.14 (W\ub7km)-1 between LP01 and LP11b modes. Next, we propose an e-HNL-FMF based OXC scheme for wavelength division multiplexing-mode division multiplexing (WDM-MDM) networks. After optimizing both the e-HNL-FMF length and pump power, we can realize either active mode selective conversion over the designated wavelength-band or three-wavelength to three-mode superchannel conversion for 100 Gbaud 16-quadratic-amplitude modulation (16-QAM) signals over the C-band. Due to excellent characteristics of the e-HNL-FMF, both cost and configuration complexity of the OXC can be reduced, showing great potentials for all-optical signal processing in the future WDM-MDM networks

Vector mode inter-modal wavelength conversion in a dispersion tailored highly nonlinear few-mode fibre

We present the design and fabrication of a dispersion tailored highly nonlinear few-mode fibre with an inter-modal nonlinear coefficient of 2.81 (W \ub7 km)-1, the highest reported to date. Inter-modal wavelength conversion between the HE21 and TE01 vector modes is demonstrated in the fibre

Design, fabrication, and characterization of a highly nonlinear few-mode fiber

We present the design, fabrication, and characterization of a highly nonlinear few-mode fiber (HNL-FMF) with an intermodal nonlinear coefficient of 2.8 (W \ub7 km)−1, which to the best of our knowledge is the highest reported to date. The graded-index circular core fiber supports two mode groups (MGs) with six eigenmodes and is highly doped with germanium. This breaks the mode degeneracy within the higher-order MG, leading to different group velocities among corresponding eigenmodes. Thus, the HNL-FMF can support multiple intermodal four-wave mixing processes between the two MGs at the same time. In a proof-of-concept experiment, we demonstrate simultaneous intermodal wavelength conversions among three eigenmodes of the HNL-FMF over the C band

Hot-Cavity Spectroscopy of Dark Pulse Kerr Combs in Microresonators

Kerr frequency combs are generated through cascaded four-wave mixing in high-Q microresonators [1]. These devices are pumped with a continuous-wave laser and modulational instability (MI) is responsible for the growth of the initial comb lines. Since it is easier to satisfy the MI phase matching condition in the anomalous dispersion regime, most studies on Kerr combs have focused on anomalous dispersion microresonators. However, coherent microresonator combs can also take place in the normal dispersion regime. In these combs, phase matching is attained with the aid of the mode coupling between transverse modes of the microresonator [2]. One particularly interesting comb state that operates in the normal dispersion regime is the dark pulse Kerr comb [3]. The time domain pulses of these combs arise as interlocking switching waves that connect the upper and lower homogenous steady state solutions of the bi-stability curve in the continuous-wave-driven Kerr cavity [see Fig. (a)] [3]. These combs are of high interest as most nonlinear materials suitable for fabricating microresonators display normal dispersion in the visible and near infrared ranges. Moreover, these combs provide a much higher power conversion efficiency compared to bright-soliton combs, which makes them particularly useful for telecommunications [4]

Switching Dynamics of Dark Solitons in Kerr Microresonators

Dissipative Kerr solitons (DKS) are localized structures in optical resonators that arise from a double balance between dispersion and Kerr effect, and linear loss and parametric gain [1]. The periodic nature of DKS corresponds to frequency combs. DKS can be generated in high-Q microresonators for diverse applications, from coherent communications to precision frequency synthesis [1]. Most studies of DKS have focused on microresonator cavities operating in the anomalous dispersion regime, where the waveforms correspond to bright soliton pulses. Coherent microresonator combs can also be formed in the normal dispersion regime [2]. The time-domain waveform corresponds to a localized dark-pulse structure, interpreted as two interlocked switching waves connecting the two branches of the bi-stability curve in continuous-wave-pumped Kerr resonators [2, 3]. Each switching wave connects the two branches following an oscillating behavior. These type of Kerr combs are relevant for practical applications because they display unusually high power-conversion efficiency [4, 5], but their physical dynamics remain largely unexplored. Here, we report the discovery of deterministic switching of dark pulse Kerr combs, where the number of oscillations that appear between the switching waves can be either increased or decreased one at a time. The switching dynamics observed here have intriguing similarities to the switching behavior of bright temporal solitons in anomalous dispersion microresonators [6], and they indicate that dark pulse Kerr combs arise as a complex interplay of dark solitons circulating in the cavity

Applications of four-wave mixing and cross phase modulation in highly nonlinear fibers

Wavelength Division Multiplexing (WDM) is one of the most common techniques used in fiber-optic communication systems in which multiple optical signals at various wavelengths are combined and transmitted through a single fiber. One of the key components in WDM systems are optical amplifiers. The most widely used amplifier is the Erbium-doped fiber amplifier (EDFA). However, its operating wavelength doesn’t cover all the low loss wavelength range (1450-1650 nm) of optical fibers. Hence, to overcome the bandwidth limitation of EDFAs, alternative optical amplifiers have been investigated. One of the candidates is the fiber optical parametric amplifier (FOPA). FOPAs operate based on a fiber nonlinearity known as four-wave mixing (FWM). FWM arises from the third order nonlinear susceptibility in a fiber and occurs when at least two waves with different frequencies co-propagate in the fiber. When a signal beam at angular frequency ω_s is launched into a fiber along with a strong optical pump beam at ω_p, the signal is amplified through a parametric process and another wave, called the idler, is generated at 〖2ω〗_p-ω_s. For a modulated signal, the modulation format will be transferred to the idler; this feature enables the FOPA to be used for shifting the optical frequency of a signal in addition to amplifying it. In order to use any optical amplifier in WDM systems, it should have a flat gain spectrum over its operating range, and FOPAs aren’t an exception. A proposed method for achieving a FOPA with a flat gain spectrum is to set up the amplifier by cascading several fiber segments which have different parameters. A part of this research is allocated to optimizing the parameters of each of these fiber segments using a genetic algorithm such that the resulting FOPA has a flat gain spectrum. It is shown that by optimizing the fourth order dispersion of each fiber section a broader flat gain spectrum can be achieved. Fluctuations of the zero dispersion wavelength (ZDW) have been considered for an amplifier which has optimum fourth order dispersion; the results show that as long as the average ZDW of each fiber segment is maintained close to the optimum value, ZDW fluctuations won’t have a large effect on the flatness of the gain spectrum. The noise properties and the operation of a multi-section FOPA in the depleted pump regime have also been studied. The four wave mixing behavior in a novel fiber loop mirror, composed of two dispersion shifted fibers along with a single mode fiber in between them, is theoretically investigated. Due to the phase shift which the SMF introduces this configuration provides higher conversion efficiency for signals in the vicinity of the pump, compared to previously reported fiber loop mirrors. Furthermore, a new design for de-multiplexing and de-modulating a differential phase shift keying (DPSK) optical time-domain multiplexed signal is proposed and investigated. The scheme consists of a nonlinear medium and a dispersive fiber connected through a 3-dB coupler to form a fiber loop mirror. Most optical signal processing operations are based on optical-electrical-optical (O/E/O) methods in which the operation bit rate is often limited by the electrical response time. In addition, utilizing ultrahigh speed photodiodes, data modulators and electronic devices are costly. Therefore, all-optical signal processing is highly desired. In this research work applications of FWM and cross phase modulation (XPM) in all-optical signal processing are discussed. A multiple format conversion module based on FWM for converting non return-to-zero (NRZ) or return-to-zero (RZ) on-off keying (OOK) modulation format to carrier-suppressed return-to-zero (CSRZ)-OOK and NRZ/RZ-DPSK signals to CSRZ-DPSK is presented. Moreover, a scheme for performing all-optical logic exclusive-OR (XOR) between phase shift keying (PSK) data and OOK signals is described. This logic gate is based on XPM in a highly nonlinear fiber (HNLF). All-optical generation of 4 levels and 8 levels RZ-amplitude and phase shift keying (APSK) modulation format based on FWM and XPM in a HNLF is presented.DOCTOR OF PHILOSOPHY (EEE

Highly Nonlinear Few-Mode Fiber for Optical Parametric Amplification

A highly nonlinear dispersion-shifted few-mode fiber is designed. The dispersion properties and high nonlinearity of this fiber allow simultaneous single-pump parametric amplification of four spatial modes (LP01, LP11, LP02, LP21), with high gain and low cross-talk, over the C-band