KEYWORDS: Erbium, Ions, Absorption, Waveguides, Silicon, Optical amplifiers, Amplifiers, Simulation of CCA and DLA aggregates, Laser optics, Signal attenuation
We present an extensive study of an Er doped Silicon Rich Silicon Oxide (SRSO) based material used for the realization of
optical waveguide amplifiers in which Si-nanoclusters (Si-ncls) are formed by thermal annealing. In particular we focus our
attention on the confined carrier absorption (CCA) mechanism within the Si-ncls and on the fraction of Er ions coupled to
them. Experimental data are used for accurate modeling of Si-ncls sensitized EDWAs (Erbium Doped Waveguide
Amplifiers) longitudinally pumped by visible broad area lasers.
Although the material requires further optimization to be effectively deployed, accurate numerical simulations of Si-ncls
sensitized EDWAs, based on this material and longitudinally pumped by visible broad area lasers at 660 nm, point out
significant benefits provided by the nanoclusters sensitization. Our model, based on the Finite Element Method, performs the
modal analysis of the guiding structure, and then allows to study the propagation of pump and signal electric fields along the
waveguide amplifier; the rate equations for the coupled Er/Si-ncls system account for their coupling ratio.
Numerical results, based on measured material parameters, point out that resonant pumping at 660 nm provides significant
benefits in terms of gain enhancement, with respect to standard EDWAs, even at low Er/Si-ncls coupling ratio. This feature
suggests that a careful design can lead to the realization of compact integrated amplifiers and lasers, compatible with CMOS
technology.
The use of broadband efficient sensitizers for Er3+ ions relaxes the expensive conditions needed for the pump source and
raises the performances of the optical amplifier. Within this context Si nanoclusters (Si-nc) in silica matrices have
revealed as optimum sensitizers and open the route towards electrically pumped optical amplifiers. Up to date two have
been the main limiting issues for achieving absolute optical gain, the first one is the low quantity of erbium efficiently
coupled to the Si-nc while the second is the carrier absorption mechanism (CA) within the Si-nc, which generates
additional losses instead of providing amplification.
In this work we will present a detailed study of the optical properties of a set of samples prepared by confocal reactive
magnetron co-sputtering of pure SiO2 and Er2O3 targets. The material has been optimised in terms of the increasing of
Er3+-related PL intensity and lifetime as well as the decreasing down to 3 dB/cm of the propagation losses in the rib-loaded
waveguides outside the absorption peak of erbium. Our signal enhancement results show that we have been able
to reduce the CA losses to less than 0.2 dB/cm at pump fluxes as high as 1020 ph/cm2 s. Around 25% of the optically
active erbium population has been inverted through indirect excitation (pumping with a 476nm laser line), leading to
internal gain coefficients of more than 1 dB/cm.
The use of broadband efficient sensitizers for Er3+ ions relaxes the expensive conditions needed for the pump source and
raises the performances of the optical amplifier. Within this context Si nanoclusters (Si-nc) in silica matrices have
revealed as optimum sensitizers and open the route towards electrically pumped optical amplifiers. In this work we
present insertion losses and pump/probe measurements, which have been carried out on rib-loaded waveguides
containing Er3+ ions coupled to Si-nc. These samples have been prepared by a multi-wafers reactive magnetron cosputtering
of a pure silica target topped with Er2O3 pellets.
Our objective with this research is to characterize, understand and optimize the main factors that are preventing net
optical gain in these samples, i.e. low excitable erbium fraction through the nanoclusters. Evidences of signal
enhancement and partial inversion of the Er3+ ions excited via Si-nc will be presented and discussed.
A study of the non-linear optical properties of Si-nc embedded in SiO2 has been performed by using the z-scan method in the nanosecond and femtosecond ranges. Substoichiometric SiOx films were grown by plasma-enhanced chemical-vapor deposition (PECVD) on silica substrates for Si excesses up to 24 at. %. An annealing at 1250 °C for 1 hour was performed in order to precipitate Si-nc, as shown by EFTEM images. Z-scan results have shown that, by using 5-ns pulses, the non-linear process is ruled by thermal effects and only a negative contribution can be observed in the non-linear refractive index, with typical values around -10-10 cm2/W. On the other hand, femtosecond excitation has revealed a pure electronic contribution to the nonlinear refractive index, obtaining values in the order of 10-12 cm2/W. Simulations of heat propagation have shown that the onset of the temperature rise is delayed more than half pulse-width respect to the starting edge of the excitation. A maximum temperature increase of ΔT = 123.1 °C has been found after 3.5 ns of the laser pulse maximum. In order to minimize the thermal contribution to the z-scan transmittance and extract the electronic part, the sample response has been analyzed during the first few nanoseconds. By this method we found a reduction of 20 % in the thermal effects. So that, shorter pulses have to be used to obtain just pure electronic non-linearities.
Complementary metal-oxide-semiconductor-compatible tunable Fabry-Perot microcavities filled with liquid crystals (LCs) were realized and studied in the near-infrared region. The microcavities were produced by chip bonding technique, which allows one to infill LC between two [SiO2/Si]n λ/4 (λ = 1.5 μm) Dielectric Bragg Reflectors separated by 950 nm thick SiO2 posts. The Dielectric Bragg reflectors were realized on Si or SiO2 substrates Liquid crystals with positive and negative dielectric anisotropy were used, i.e. MerckE7 (Δε=13.8) and Merck-6608 LC (Δε=-4.2). Mirror-integrated electrodes allow an external bias to induce an electrical field and to tune the LC properties and, hence, the microcavity resonance. Electric-field-induced shifts of the second-order cavity modes of ~120 nm and ~50 nm were obtained for Merck-E7 and Merck-6608 LC, with driving potentials of 5 V and 10 V, respectively. The transmittance at
the cavity resonance is typically in the order of 10%. Simulation of cavities allows to identify surface roughness of the Dielectric-Bragg-Reflectors as the major origin of the transmission losses. The switching behavior of microcavities filled with E7 were studied as function of applied fields. Both switch-on ton and switch-off toff times were measured and were found to be lower than 5 ms.
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