Glass ceramic activated by rare earth ions are nanocomposite systems that exhibit specific morphologic, structural and spectroscopic properties allowing to develop interesting new physical concepts, for instance the mechanism related to the transparency, as well as novel photonic devices based on the enhancement of the luminescence. At the state of art the fabrication techniques based on bottom-up and top-down approaches appear to be viable although a specific effort is required to achieve the necessary reliability and reproducibility of the preparation protocols. In particular, the dependence of the final product on the specific parent glass and on the employed synthesis still remain an important task of the research in material science. Glass-ceramic waveguides overcome some of the efficiency problems experienced with conventional waveguides. These two-phase materials are composed of nanocrystals embedded in an amorphous matrix. The respective volume fractions of the crystalline and amorphous phases determine the properties of the glass ceramic. They also represent a valid alternative to widely used glass hosts such as silica as an effective optical medium for light propagation and luminescence enhancement. Looking to application, the enhanced spectroscopic properties typical of glass ceramic in respect to those of the amorphous structures constitute an important point for the development of integrated optics devices, including optical amplifiers, monolithic waveguide laser, novel sensors, coating of spherical microresonators, and up and down converters for solar energy exploitation.
Silica microspheres were made by melting the tip of a standard telecom fiber and were coated with a 70SiO2 - 30 HfO2
sol-gel derived glass activated by 0.3 mol % of Er3+ ions. The samples were coated using a dip coating apparatus. The
thickness of the coating was estimated to be around 1 μm. The whispering gallery modes of the coated resonator were
studied using a full taper - microsphere coupling setup. Upon excitation at 1480 nm sharp peaks at wavelengths 1540-
1565 nm were observed. They were attributed to the whispering gallery modes of the microsphere falling in the
wavelength range of the erbium emission.
Coating of spherical microresonators is a very promising technique for optimizing their optical
properties. Optical coatings are constituted by glasses, polymer, and glass ceramics, passive or
activated by luminescent species, Glass ceramic activated by rare earth ions are nanocomposite
systems that exhibit specific morphologic, structural and spectroscopic properties allowing to
develop interesting new physical concepts, for instance the mechanism related to the transparency,
as well as novel photonic devices based on the enhancement of the luminescence. At the state of art
the fabrication techniques based on bottom-up and top-down approaches appear to be viable
although a specific effort is required to achieve the necessary reliability and reproducibility of the
preparation protocols. In particular, the dependence of the final product on the specific parent glass
and on the employed synthesis still remain an important task of the research in material science.
Looking to application, the enhanced spectroscopic properties typical of glass ceramic in respect to
those of the amorphous structures constitute an important point for the development of integrated
optics devices, including coating of spherical microresonators. Here we present a review regarding
spherical microresonators coated by glass and glass-ceramic film activated by Er3+ ions. Er3+ ions
appear to be embedded in a crystalline or amorphous environment and the lifetime dynamic is
influenced by the geometry and by the morphology of the system. Photoluminescence results and
morphologic properties are discussed for both amorphous and glass ceramic films.
In the microwave domain and among many other advantages, optics represents an elegant solution to increase the
quality Q factor in a system. Different types of optical resonators lead to Q factors above 109, and these resonators can
be used as an alternative to optical delay lines to set up the frequency in optoelectronic oscillators (OEO). However,
microwave-optics is also a complex field, and if the use of optical resonators in high spectral purity frequency
generation systems like OEO has been already demonstrated, many aspects of these OEOs are still incompletely
understood, especially the contribution to the oscillator phase noise of the different optical and microwave elements
used in the oscillator system. In order to improve the phase noise of a fiber ring resonator based OEO, this oscillator has
been theoretically studied in term of white frequency noise. In this paper, we present a theoretical study that has lead us
to optimize a fiber ring resonator and the experimental phase noise results obtained for an OEO based on an optimized
optical resonator. The OEO thermal stability is also investigated in this paper.
Microwave optical systems for frequency generation are described in this paper. The goal is to reach high spectral
purity in the microwave frequency range using ultra high Q optical resonators. The resonators investigated are of two
types : resonant (passive) fiber rings and WGM tridimensional resonators. They all feature ultra high optical Q factors, in
excess of 108 or 109 near 1550 nm. These resonators also sustain a large number of optical resonances, and the
microwave signal is stabilized on two (or more) resonances of this optical comb. Different problems have to be
overcome in order to reach a functional system, such as : resonator design and coupling, laser stabilization on a
resonance, overall system design, noise optimization... This paper gives an overlook on these problems, and on some
solutions we found to work towards a compact and efficient microwave opto-electronic oscillator (OEO). A first result is
presented on a 10 GHz OEO based on a resonant fiber ring.
Phase noise of microwave free running sources has always been an important problem in various applications. This noise generates an increased bit error rate in a telecommunication link and degrades the sensitivity of a radar (particularly in the case of Doppler or FM-CW radar). Reducing this noise contribution is a difficult challenge for microwave engineers and circuit designers. The main contributor to this noise is well known to be the microwave transistor and finally an improvement of the oscillator phase noise will result from an optimization of the transistor phase noise. The 10 kHz to 1 MHz offset frequency range is the most important frequency range for many microwave oscillators applications. An improvement of the transistor (or oscillator) phase noise in this frequency range cannot be obtained without a good knowledge of the noise mechanisms involved in the device. In this frequency range, two different mechanisms may be at the origin of the phase noise. The first one involves the conversion to high frequencies of the transistor baseband noise (or 1/f noise) through the devices nonlinearities. The second one is due to the direct superposition of the transistor high frequency noise. This noise is simply added to the carrier, and this contribution may be described using the amplifier noise figure. In this paper, the evidence of the transistor high-frequency noise contribution in residual phase noise data is demonstrated. This behavior is observed in several bipolar devices in which the low-frequency noise contribution has been carefully minimized using an optimized bias network. Then, the phase noise behavior is correlated to nonlinear noise figure measurements. This study has been carried on numerous different microwave transistors, including FET and bipolar devices. An increase of the noise figure with the microwave signal level has been observed in each case.
Phase noise in microwave transistors is studied both theoretically and experimentally using residual phase noise measurements. The experimental approach allows the exploration of many interesting features of phase noise generation in these devices, such as the dependence of phase noise versus microwave power or transistor low frequency loading, meanwhile nonlinear simulation is still necessary to optimise the microwave load and the whole oscillator circuit. The different behaviours described are illustrated in various microwave circuits, and particularly dielectric resonator oscillators, with some of them featuring state of the art performance.
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