We theoretically investigate the generation, transmission and detection of the label signal in the optical SCM label switching system and clarify that the degradation of the label signal results from the fiber chromatic dispersion and the incomplete filtering out of the payload carrier. Our theoretical results show that the signal degradation is mainly caused by the fading effect and the expansion of the code edges. We also quantitatively analyze these theoretical results. The deep suppression of the optical carriers at receiver can greatly reduce the fading effect, and the electrical filtering and heterodyne detection techniques can improve the performance of the label signal. The time shift of the code edges leads to the closure of the label codes, which greatly limits the transmission distance as well as the radio frequency to generate the label subcarriers especially for the label with a high bit rate. SSB SCM can not only eliminate the closure of the label codes, but also reduce the fading effect. To our knowledge, this is the first time to develop a detail theoretical model to well explain the experimental results for SCM label switching systems.
A novel InP-based micromechanied tunable photodetector with the structure of OMITMC (One-Mirror-Inclined Three-Mirror-Cavity)is presented. The tuning characteristic of the device is analysed in the way of electrical actuation. Through simulation of the filter transmission spectra and the quantum efficiency, the characteristics of high tunability, narrow bandwidth and high quantum efficiency are analysed. As a result, the photodetector is tuned with 30nm,with a quantum efficiency of 59% and a linewidth of 1.2nm, when actuated by 10 volts.
In this letter,a InP-based micromechanical tuneable filter by electrostatic actuation is introduced. Through simulation of the DBR reflectance spectra and the F-P cavity transimission spectra,the filter performance is studied. By means of finite element method,the tunning characteristics of the filters based on several typical micromechanical structures are presented. The dynamical behavior of the top mirror is investigated.In addition,the flatness of the top mirror are compared among these structures. It would be valuable reference for InP-based micromechanical F-P cavity device design and optimization.
We have demonstrated a wet etching technique capable of fabricating air-gap cavity. The process utilizes nonselective and selective etchants respectively to form air-gap cavity. The etching characteristics of GaAs/Al<sub>0.8</sub>Ga<sub>0.2</sub>As/AlAs structure in different nonselective and selective etchants are investigated. The volumetric 3:2:20 ratios of H<sub>3</sub>PO<sub>4</sub>/H<sub>2</sub>O<sub>2</sub>/H<sub>2</sub>O solution and volumetric 600:1 ratio of DI water/buffered oxide [mixture of 7:1 NH<sub>4</sub>F(36%)-HF(6.4%)] solution are better nonselective and selective etchants respectively. We have used this process technique to form a tunable air-gap cavity optical filter. The measure results of the transmission spectral show that the air-gap cavity has high optical quality. These simple etching processes can be applied to fabricate the air-gap cavity devices.
The multiple access interference is the main reason that limits the UWB system capacity. The flexibility of pulse selection is a prominent property which is different from the conventional communications. In this paper, a new multiple-access scheme: Waveform Division Multiple Access (WDMA) based on the orthogonal wavelet function shapes is presented. In this scheme, the scaling function φ(t) and the wavelet function ψ(t) representing the UWB pulses are equally assigned to different users to realize multiple access. Because of the better orthogonality of φ(t) and ψ(t), the multiuser interference will be suppressed more effectively. The performance analysis and simulation results show that WDMA system can accommodate more users than TH-PPM UWB and DS-BPSK UWB system.
In this paper, a hybrid DS/TH multiple access scheme is proposed for UWB impulse radio system on the basis of TH-PPM theoretical research. This scheme makes use of time hopping for multiple access and BPSK for data modulation. It utilizes the characteristics of PN sequence signal of DS to suppress further the multi-user interference of TH-PPM system. Results of performance analysis show that DS/TH scheme has higher system capacity than TH-PPM scheme.