This talk focuses on the high frequency characteristics of red VCSELs. After a short description of important fabrication issues the modulation behaviour of GaInP surface emitting lasers is discussed on the basis of the laser rate equations. The influence of the geometric dimensions of the laser structure and of the operating conditions is investigated. From the S-parameter analysis a modulation coefficient of 3 GHz/(mA)<sup>1/2</sup> for VCSELs with a 7 µm aperture and a differential gain of 1.15•10<sup>-16</sup> cm<sup>2</sup> are deduced. A more detailed analysis reveals, that the modulation behaviour of red VCSELs nearly solely depends on their photon density inside the quantum wells as expected from the rate equations. These results imply that for a certain range of geometries diffusion and diffraction have a second order influence on the high frequency characteristics of red VCSELs. The K-factor analysis indicates very short carrier transfer and relaxation times around 5 ps and a maximum frequency of 25 GHz. Large signal modulation issues such as the properties of the eye diagram are also addressed. From the device characteristics it is concluded that the GaInP-VCSEL is suitable for data communication applications. Low cost fabrication makes the red VCSEL an attractive candidate for both automotive and high-speed data communication.
We report in this paper the principal function of the electrically controlled variable optical attenuator (VOA) using polymer dispersed liquid crystal (PDLC) and describe the fabrication procedure on silicon v-groove. We have fabricated three VOA with a pitch of 2 mm on a single silicion v-groove chip with total dimensions of 12 mm x 10 mm. We have achieved a cell-dependent contrast ratio from 8 dB to 14 dB by applying a control voltage U<sub>RMS</sub> (squared wave voltage, f = 10 kHz) from 0 to 30 V. We measured also a cell-dependent polarization dependent loss (PDL): < 3.6 dB for two cells and < 1.6 dB for one cell depending on the control voltage. The strong variation of the PDL and contrast ratio is due to the non-optimized PDLC processing parameters. Due to the large pitch size there is no crosstalk. The estimated power consumption is very low (< 1 μW), so the described fabrication procedure meets the requirements low cost, small power consumption and compact size. We have used this three VOA and proper chosen delay lines to build up a liquid crystal phase shifter (LCPS) for optically generated RF-signals at f<sub>RF</sub> = 2 GHz. Using the vector sum of two signals a continuously 360° phase shift of the RF-signal is demonstrated. We will present the theory and measurement results of 360° phase shifting. This LCPS can be used to control individually the phase and amplitude of each antenna element.
With the recent advances in CMOS technologies, the MOSFETs offer competitive low noise performance at high frequencies comparable with their bipolar counterparts and become attractive candidates even for challenging high frequency applications with their low cost and high integration level. Therefore, the transistor model accuracy becomes a crucial factor for predicting the RF circuit performance accurately in a broad frequency range. An overview of a high frequency noise modeling approach based on a direct parameter extraction technique is presented in this paper. Moreover, the presented parameter extraction methods are evaluated by means of broadband noise parameter and S-parameter measurements. A temperature noise model predicts all four noise parameters at any frequency and can be used to determine the dominant noise source of the small-signal equivalent circuit. The model can be verified by comparing the measured noise parameters with the simulation results over a broad frequency range. Finally, a practical circuit example of an amplifier using a 0.12 μm CMOS technology at 24 GHz is given.
In this paper tuneable optical filters based on liquid crystal Fabry-Perot interferometers (LCFPI) are presented. Liquid crystal (LC) devices are lightweight and suitable for compact arrays with a large number of pixels, as shown in high resolution flat panel displays. The fabricated filters offer a high finesse (119) and a wide tuning range. The devices are coupled to standard single mode fibers by fiber collimators. For all filters the layer structure of a standard passive LC display is used, adding only two reflective layers. Dielectric mirrors (R = 0.98) are used to achieve high finesse and low insertion losses (-4.8 dB). The cost can be further reduced by using thin gold layers, acting as electrodes and mirrors (R = 0.9) at the same time. The finesse of the Gold-FPIs is about 30 and the measured insertion loss is -10 dB. Additionally, a twisted nematic (TN) structure is investigated. Using this orientation, the polarization dependence of the device is reduced with increasing tuning voltage.