Luis Romero Cortés

Integrated waveguide Bragg gratings for microwave photonics signal processing

Integrated Microwave photonics (IMWP) signal processing using Photonic Integrated Circuits (PICs) has attracted a great deal of attention in recent years as an enabling technology for a number of functionalities not attainable by purely microwave solutions. In this context, integrated waveguide Bragg grating (WBG) devices constitute a particularly attractive approach thanks to their compactness and flexibility in producing arbitrarily defined amplitude and phase responses, by directly acting on coupling coefficient and perturbations of the grating profile. In this article, we review recent advances in the field of integrated WBGs applied to MWP, analyzing the advantages leveraged by an integrated realization. We provide a perspective on the exciting possibilities offered by the silicon photonics platform in the field of MWP, potentially enabling integration of highly-complex active and passive functionalities with high yield on a single chip, with a particular focus on the use of WBGs as basic building blocks for linear filtering operations. We demonstrate the versatility of WBG-based devices by proposing and experimentally demonstrating a novel, continuously-tunable, integrated true-time-delay (TTD) line based on a very simple dual phase-shifted WBG (DPS-WBG).

On-chip generation of high-dimensional entangled quantum states and their coherent control

Optical quantum states based on entangled photons are essential for solving questions in fundamental physics and are at the heart of quantum information science. Specifically, the realization of high-dimensional states (D-level quantum systems, that is, qudits, with D > 2) and their control are necessary for fundamental investigations of quantum mechanics, for increasing the sensitivity of quantum imaging schemes, for improving the robustness and key rate of quantum communication protocols, for enabling a richer variety of quantum simulations, and for achieving more efficient and error-tolerant quantum computation. Integrated photonics has recently become a leading platform for the compact, cost-efficient, and stable generation and processing of non-classical optical states. However, so far, integrated entangled quantum sources have been limited to qubits (D = 2). Here we demonstrate on-chip generation of entangled qudit states, where the photons are created in a coherent superposition of multiple high-purity frequency modes. In particular, we confirm the realization of a quantum system with at least one hundred dimensions, formed by two entangled qudits with D = 10. Furthermore, using state-of-the-art, yet off-the-shelf telecommunications components, we introduce a coherent manipulation platform with which to control frequency-entangled states, capable of performing deterministic high-dimensional gate operations. We validate this platform by measuring Bell inequality violations and performing quantum state tomography. Our work enables the generation and processing of high-dimensional quantum states in a single spatial mode.

On-chip programmable ultra-wideband microwave photonic phase shifter and true time delay unit

We proposed and experimentally demonstrated an ultra-broadband on-chip microwave photonic processor that can operate both as RF phase shifter (PS) and true-time-delay (TTD) line, with continuous tuning. The processor is based on a silicon dual-phase-shifted waveguide Bragg grating (DPS-WBG) realized with a CMOS compatible process. We experimentally demonstrated the generation of delay up to 19.4 ps over 10 GHz instantaneous bandwidth and a phase shift of approximately 160° over the bandwidth 22-29 GHz. The available RF measurement setup ultimately limits the phase shifting demonstration as the device is capable of providing up to 300° phase shift for RF frequencies over a record bandwidth approaching 1 THz.

Programmable Fiber-Optics Pulse Repetition-Rate Multiplier

We develop a strikingly simple fiber-optics pulse repetition-rate multiplication approach, based on dispersion-induced temporal self-imaging (TSI) effect, in which the rate multiplication factor can be electrically programmed to be any desired integer value. The multiplier setup is composed of an electrooptic phase modulator followed by a dispersive medium. In contrast to the conventional TSI-based rate-multiplication techniques, the required dispersion is fixed in this method and does not depend on the multiplication factor. Programming the output repetition rate will be then assigned to a preprocessing step where the overall phase of the input pulses is modulated by a phase modulator. Herein, we derive in detail a condition for the required dispersion and introduce an analytical equation for the temporal phase modulation. To validate the derived expressions, proof-of-concept experiments have been carried out and we have achieved high-quality tunable rate multiplication, by factors ranging from 1 to 4, of mode-locked fiber lasers with input repetition rates of 10 and 4.85 GHz, respectively.

Observation of spectral self-imaging by nonlinear parabolic cross-phase modulation.

We report an experimental demonstration of spectral self-imaging on a periodic frequency comb induced by a nonlinear all-optical process, i.e., parabolic cross-phase modulation in a highly nonlinear fiber. The comb free spectral range is reconfigured by simply tuning the temporal period of the pump parabolic pulse train. In particular, undistorted FSR divisions by factors of 2 and 3 are successfully performed on a 10 GHz frequency comb, realizing new frequency combs with an FSR of 5 and 3.3 GHz, respectively. The pump power requirement associated to the SSI phenomena is also shown to be significantly relaxed by the use of dark parabolic pulses.

Time-delay to intensity mapping based on a second-order optical integrator: application to optical arbitrary waveform generation

We propose and validate experimentally a time-delay to intensity mapping process based on second-order optical integrators. This mapping provides dynamic control of the intensity modulation profile of a waveform based on a purely passive and linear process. In particular, we can realize linear intensity control by tuning the time-delay between two optical pulses launched into a second-order optical integrator. We suggest and experimentally prove the use of this mapping process for reconfigurable optical arbitrary waveform generation.

On-chip ultra-wideband microwave photonic phase shifter and true time delay line based on a single phase-shifted waveguide Bragg grating

We propose and experimentally demonstrate a novel, ultra-broadband and continuously tunable integrated microwave photonic processor that can operate both as a RF phase shifter (PS) and a true-time-delay (TTD) line. The device is implemented with a silicon phase-shifted waveguide Bragg grating (PS-WBG) realized with a CMOS compatible process. The grating features a sharp phase shift localized within the reflection band that can be exploited in conjunction with optical single sideband modulation with full carrier (OSSB+C) to generate either PS or TTD on the modulated RF signal, depending on the relative frequency position of the carrier and the sideband with respect to the phase shift. Proof-of-concept experiments show the generation of a delay up to 25.7 ps over an 8 GHz bandwidth, and a phase shift of approx. 90 degrees over the bandwidth 16-20 GHz is also demonstrated. The phase shifter demonstration is ultimately limited by the available measurement setup, as the device is capable of providing up to 300 degrees phase shift for RF frequencies above 16 GHz over a record bandwidth approaching 1 THz.

SNR enhancement in high-resolution phase-sensitive OTDR systems using chirped pulse amplification concepts

Phase-sensitive optical time-domain reflectometry (φOTDR) is widely used for the distributed detection of mechanical or environmental variations with resolutions of typically a few meters. The spatial resolution of these distributed sensors is related to the temporal width of the input probe pulses. However, the input pulse width cannot be arbitrarily reduced (to improve the resolution), since a minimum pulse energy is required to achieve a good level of signal-to-noise ratio (SNR), and the pulse peak power is limited by the advent of nonlinear effects. In this Letter, inspired by chirped pulse amplification concepts, we present a novel technique that allows us to increase the SNR by several orders of magnitude in φOTDR-based sensors while reaching spatial resolutions in the centimeter range. In particular, we report an SNR increase of 20 dB over the traditional architecture, which is able to detect strain events with a spatial resolution of 1.8 cm.

PROUD-based method for simple real-time in-line characterization of propagation-induced distortions in NRZ data signals.

A simple, in-line method for real-time full characterization (amplitude and phase) of propagation distortions arising because of group velocity dispersion and self-phase modulation on 10-20 Gbps transmitted NRZ optical signals is reported. It is based on phase reconstruction using optical ultrafast differentiation (PROUD), a linear and self-referenced technique. The flexibility of the technique is demonstrated by characterizing different data stream scenarios. Experimental results were modeled using conventional propagation equations, showing good agreement with the measured data. It is envisaged that the proposed method could be used in combination with DSP techniques for the estimation and compensation of propagation distortions in fiber links, not only in conventional IM/DD systems, but also in coherent systems with advanced modulation formats.

Sub-harmonic periodic pulse train recovery from aperiodic optical pulse sequences through dispersion-induced temporal self-imaging

Temporal self-imaging effects (TSIs) are observed when a periodic pulse train propagates through a first-order dispersive medium. Under specific dispersion conditions, either an exact, rate multiplied or rate divided image of the input signal is reproduced at the output. TSI possesses an interesting self-restoration capability even when acting over an aperiodic train of pulses. In this work, we investigate and demonstrate, for the first time to our knowledge, the capability of TSI to produce periodic sub-harmonic (rate-divided) pulse trains from aperiodic sequences. We use this inherent property of the TSI to implement a novel, simple and reconfigurable sub-harmonic optical clock recovery technique from RZ-OOK data signals. The proposed technique features a very simple realization, involving only temporal phase modulation and first-order dispersion and it allows one to set the repetition rate of the reconstructed clock signal in integer fractions (sub-harmonics) of the input bit rate. Proof-of-concept experiments are reported to validate the proposed technique and guidelines for optimization of the clock-recovery process are also outlined.