Marc De Micheli

Quantum photonics at telecom wavelengths based on lithium niobate waveguides

Integrated optical components on lithium niobate play a major role in standard high-speed communication systems. Over the last two decades, after the birth and positioning of quantum information science, lithium niobate waveguide architectures have emerged as one of the key platforms for enabling photonics quantum technologies. Due to mature technological processes for waveguide structure integration, as well as inherent and efficient properties for nonlinear optical effects, lithium niobate devices are nowadays at the heart of many photon-pair or triplet sources, single-photon detectors, coherent wavelength-conversion interfaces, and quantum memories. Consequently, they find applications in advanced and complex quantum communication systems, where compactness, stability, efficiency, and interconnectability with other guided-wave technologies are required. In this review paper, we first introduce the material aspects of lithium niobate, and subsequently discuss all of the above mentioned quantum components, ranging from standard photon-pair sources to more complex and advanced circuits.

High performance mode adapters based on segmented SPE:LiNbO3 waveguides

We propose a new mode adapter which allows more efficient launching of the optical power selectively in the fundamental mode of a multimode waveguide. Theoretical and experimental results confirm that such a mode adapter increases the performances in terms of coupling efficiency, coupling tolerances and transmitted power with respect to previously proposed solutions. Proof of principle of device operation is obtained with a simple Coupled Mode Theory model. Experimental results are obtained at a wavelength of 840 nm in Lithium Niobate Soft Proton Exchanged waveguides and agree very well with theoretical predictions.

Réalisation sur LiNbO3 de guides d'ondes présentant une forte variation d'indice et de très faibles pertes

RésuméDans cet article les auteurs reportent les différentes améliorations qu’ils ont apportées au precédé d’échange protonique, et terminent par la présentation du double échange, processus qui leur a permis de réaliser sur LiNbO3, des guides présentant de fortes variations d’indice et de très faibles pertes.AbstractIn this paper we report how we improved the proton exchange process and we present a new process, the double exchange, which allowed us to realized high index low loss waveguides on LiNbO3.

Calculation of hybrid modes in uniaxial planar optical waveguides: application to proton exchange lithium niobate waveguides

Abstract A rigorous electromagnetic model which considers both guided and semi-leaky modes of planar uniaxial optical waveguides with nondiagonal dielectric tensors is proposed. This model permits an excellent fit with the experimental results obtained from different high Δ n e proton exchanged LiNbO 3 waveguides and allows us to determine the rotation of the principal axes which occurs for layers exchanged at low temperature.

Increased pump acceptance bandwidth in spontaneous parametric downconversion process using Bragg reflection waveguides.

In this paper we show that by suitably tailoring the dispersion characteristics of a Bragg reflection waveguide (BRW) mode, it is possible to achieve efficient photon pair generation over a large pump bandwidth while maintaining narrow signal bandwidth. The structure proposed consists of a high index core BRW with a periodically poled GaN core and periodically stratified cladding made up of alternate layers of Al(0.02)Ga(0.98)N and Al(0.45)Ga(0.55)N. Such photon-pair generators should find applications in realizing compact and stable sources for quantum information processing.

Highly Confining Proton Exchanged Waveguides on Z-Cut LiNbO 3 With Preserved Nonlinear Coefficient

We propose a new proton exchange process to reproducibly realize on lithium niobate highly confining (δne = 0.1) planar waveguides without degrading the nonlinear coefficient of the crystal. Waveguides were fabricated with different acidic baths and the crystallographic changes, the index profiles and the local SHG response were measured. We show that for certain fabrication conditions, characteristic of the so-called high-index soft proton exchange process, the high-index modification can be obtained without degrading the crystal nonlinear coefficient. This result should open the way to the fabrication of very efficient nonlinear devices.

Proton exchange, anneal proton exchange, and reverse proton exchange waveguides in Er:LiNbO3

We report the fabrication, characterization and comparison of waveguides in Z-cut Erbium-doped LiNbO3, obtained by Proton Exchange, Anneal Proton Exchange and Reverse Proton Exchange (RPE). We found that even in the very low proton concentration (α-phase) waveguides, the radiative lifetime reduction, hence the fluorescence quenching, of the 4I13/2 yields 4I15/2 laser transition due to OH phonons is dramatic. At the contrary, in the RPE waveguides we measured a nearly unaffected lifetime as the ordinary polarized modes are supported by a surface layer nearly free of proton. As it is known that the ordinary polarization is largely advantageous in terms of pump absorption efficiency and relative signal amplification, such waveguides appear to be a workable solution for amplification/lasing around 1.5 μm, provided that the losses due to leakage through the index barrier can be sufficiently reduced.

Highly efficient coupling in lithium niobate photonic wires by the use of a segmented waveguide coupler.

The purpose of this article is to show that efficient light coupling in lithium niobate waveguides presenting a strongly confined mode, such as photonic wires, is possible with the use of a periodically segmented waveguide coupler. The coupler consists in an input periodically segmented waveguide whose mode size is adapted to the mode of a standard single-mode fiber coupled to a photonic wire whose mode size is of the same order of the wavelength. The periodic segmentation of the input waveguide allows fulfilling the phase matching condition necessary to achieve an efficient light transfer between these waveguides. The coupling efficiency is typically 5 times higher than the butt-coupled configuration.

Characteristics of hybrid modes in proton-exchanged lithium niobate waveguides

A rigorous numerical model, verified by experimental results, gives an explanation of the particular electromagnetic behaviors observed in x-cut proton-exchanged lithium niobate waveguides. This approach, which allows an exact calculation of the weights of the coupled ordinary and extraordinarywaves that make up the hybrid modes, provides deeper insight into the study of the strains induced by the proton-exchange process in the waveguide itself, showing that the optical axis of the exchanged layer is not parallel to the waveguide plane.

All-in-one measurement setup for fast and accurate linear characterization of guided-wave optical devices

We present an all-in-one, computer-controlled, measurement setup able to evaluate propagation losses, effective group index, and mode size of integrated optical devices and optical fibers. The possibility to use a single setup, instead of three separate ones, enables faster measurements, improvements in terms of reproducibility and precision, and reduction of systematic errors. Control of the operating conditions, easier system upgrade besides cost and laboratory space savings are other additional features of this system. To confirm proper operation and versatility of the proposed setup, different samples are successfully characterized, and results are presented and discussed.