Most conventional magnetic materials used in the electronic devices are ferrites, which are composed of micrometer-size grains. But ferrites have small saturation magnetization, therefore the performance at GHz frequencies is rather poor. That is why functionalized nanocomposites comprising magnetic nanoparticles (e.g. Fe, Co) with dimensions ranging from a few nm to 100 nm, and embedded in dielectric matrices (e.g. silicon oxide, aluminium oxide) have a significant potential for the electronics industry. When the size of the nanoparticles is smaller than the critical size for multidomain formation, these nanocomposites can be regarded as an ensemble of particles in single-domain states and the losses (due for example to eddy currents) are expected to be relatively small.Here we review the theory of magnetism in such materials, and we present a novel measurement method used for the characterization of the electromagnetic properties of composites with nanomagnetic insertions. We also present a few experimental results obtained on composites consisting of iron nanoparticles in a dielectric matrix.

extinction ratio could be increased further by a proper choice of splitting ratios of the couplers [10]. The wavelength resolution limit of the OSA also limits a precise measurement of the actual extinction ratio. The minimum possible change in the FSR during tuning of the MZI by this technique was somewhat difficult to estimate due to the following reason: the smallest real-time change in the FSR during tuning was much less than the resolution of the OSA. Further, the effect of this small change was difficult to record during the tuning process because of short-duration smallscale fluctuations that take place in the online response of the MZI in real time. The effect of the increase in loss during this process was also visible in the output spectrums (compare the reference level RL in Figs. 3 and 4). The difference between the peak-topeak powers at the two complementary output ports increases due to the losses that occurred during the tuning process. This can be exploited to our advantage: if the first coupler is not a perfect 3-dB one, then by introducing appropriate loss in one of the arms of the MZI, equal power distributions at the output ports can be achieved. CONCLUSIONWe have presented a simple technique to tune the channel spacing and channel wavelengths of an all-fiber MZI-based wavelength interleaver. This technique involves selective heating and controlled stretching of one of the arms of the MZI. We have demonstrated the applicability of the technique by fabricating a singlestage MZI-based wavelength interleaver with a stable wavelength response. Using this technique, the results of successfully realized wavelength interleavers with channel wavelengths conforming to ITU grids and channel-spacing values of 25 and 50 GHz were also presented.

We studied the possibilities of LTCC (low temperature cofired ceramics) technology to fabricate fiber-pigtailed transmitter arrays using surface-emitting sources, such as VCSELs, equipped with vertical-mounted multimode fiber pigtails. The designed LTCC module can be mounted vertically on a printed-circuit-board (PCB), thus providing small, essentially one-dimensional PCB footprint. The fiber is aligned and supported using a hole structure through the layers, and the light source is flip-chipped on the other side of the substrate. Thus, this kind of fiber pigtailed module allows for the use of detachable electrical interface between the fiber optic media and applications electronics. Therefore, in this case, the alignment tolerances required for the connections are looser than when using conventional optical connections. In addition, the small size of the module increases the positioning freedom of transmitter and receiver modules on PCBs.To evaluate the feasibility of the system, a 4-layer LTCC substrate for a 5-channel transmitter was designed and realized using VCSEL chips and 62.5/125-micron fibers. Each transmitter, sized 5 mm × 5 mm, included discrete passives and a laser-driver chip. Metallic tubes were used as strain relief boots for the fiber assembly, and the substrate was mounted vertically on a test board. Optical alignment tolerances were examined by measurements and simulations, and the tolerances of fiber-mounting holes were evaluated by preparing test structures. The alignment accuracy seemed adequate even for the first prototypes. Nevertheless, stringent requirements for the LTCC process control are necessary to achieve the needed accuracy.

We investigate the electromagnetic properties of assemblies of nanoscale ϵ-cobalt crystals with size range between 5 to 35 nm, embedded in a polystyrene matrix, at microwave (1-12 GHz) frequencies. We investigate the samples by transmission electron microscopy imaging, demonstrating that the particles aggregate and form chains and clusters. By using a broadband coaxial-line method, we extract the magnetic permeability in the frequency range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance (FMR) with respect to an externally applied magnetic field. We find that the zero-magnetic field ferromagnetic resonant peak shifts towards higher frequencies at finite magnetic fields, and the magnitude of complex permeability is reduced. At fields larger than 2.5 kOe the resonant frequency changes linearly with the applied magnetic field, demonstrating the transition to a state in which the nanoparticles become dynamically decoupled. In this regime, the particles inside clusters can be treated as non-interacting, and the peak position can be predicted from Kittel's FMR theory for non-interacting uniaxial spherical particles combined with the Landau-Lifshitz-Gilbert equation. In contrast, at low magnetic fields this magnetic order breaks down and the resonant frequency in zero magnetic field reaches a saturation value reflecting the interparticle interactions as resulting from aggregation. Our results show that the electromagnetic properties of these composite materials can be tuned by external magnetic fields and by changes in the aggregation structure.

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