A comparative study of the gain achievement is performed in a waveguide optical amplifier whose active layer is constituted by a silica matrix containing silicon nanograins acting as sensitizer of either neodymium ions (Nd3+) or erbium ions (Er3+). Due to the large difference between population levels characteristic times (ms) and finite-difference time step (10−17s), the conventional auxiliary differential equation and finite-difference time-domain (ADE-FDTD) method is not appropriate to treat such systems. Consequently, a new two loops algorithm based on ADE-FDTD method is presented in order to model this waveguide optical amplifier. We investigate the steady states regime of both rare earth ions and silicon nanograins levels populations as well as the electromagnetic field for different pumping powers ranging from 1 to 104 mW/mm2 . Furthermore, the three dimensional distribution of achievable gain per unit length has been estimated in this pumping range. The Nd3+ doped waveguide shows a higher gross gain per unit length at 1064 nm (up to 30 dB/cm-1) than the one with Er3+ doped active layer at 1532 nm (up to 2 dB/cm-1). Considering the experimental background losses found on those waveguides we demonstrate that a significant positive net gain can only be achieved with the Nd3+ doped waveguide. The developed algorithm is stable and applicable to optical gain materials with emitters having a wide range of characteristic lifetimes.
The series of Nd3+-doped Si-rich SiO2 thin films with different excess Si content were deposited by magnetron co-sputtering of three different (SiO2, Si and Nd2O3) targets under a plasma of pure argon at 500 °C. The Si excess content in the samples was monitored via a power applied on Si cathode. The films were submitted to the rapid thermal annealing (RTA) at 900, 1000 and 1100 °C, respectively. It was observed a phase separation and a formation of Si nanoclusters embedded in oxide host. The Si excess, remaining after a RTA-1100 °C annealing, was found to be negligible, confirmed nearly complete phase separation. The Nd3+ photoluminescence (PL) property was explored as a function of Si excess and/or annealing temperature. The most efficient Nd3+ PL emission was found for the samples with about 4.7% of Si excess. These optimal samples, submitted to RTA-900 °C-1 min treatment and conventional annealing at 900°C for 1 h in nitrogen flow, demonstrated comparable Nd3+ PL intensities. This offers future application of RTA treatment to achieve an efficient emission from the materials doped with rare-earth ions.
The Nd3+-doped Silicon Rich Silicon Oxide (SRSO) layers were elaborated by reactive magnetron cosputtering.
We report on refractive index measurements of Nd3+-doped SRSO layers obtained by both m-lines method
and reflectance spectroscopy. From these measurements, the Si volume fraction and also the Nd3+-doped SRSO index
dispersion were deduced by using the Bruggeman model. At 1.06 μm, work wavelength, Nd3+-doped SRSO refractive
index was equal to 1.543 corresponding to a Si volume fraction of 6.5%.
Optical losses measurements were performed on these waveguides at different wavelengths and were about 0.3 dB/cm at
1.55 μm and 1 dB/cm at 1.06 μm. Measurements are confirmed by theoretical models showing that the losses are
essentially attributed to surface scattering.
Guided fluorescence by top pumping at 488 nm on planar waveguides was studied as a function of the distance
between the excitation area and the output of the waveguide and also as a function of the pump power. The guided
fluorescence at 945 and 1100 nm was observed until 4mm of the output of the waveguide and, of course, decreased when
the excitation area moved away from the output. The fluorescence intensity increased linearly for low pump power and
this linear increasing of the guided fluorescence of Nd3+ excited by a non resonant excitation at 488 nm confirms the
strong coupling between the Si- nanoparticles and rare earth ions.
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.
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