The self-image effects of the multi-port multimode couplers based on weakly guided graded Ti:LiNbO<sub>3</sub> optical waveguides are analyzed through the three dimensional (3D) full-vector beam propagation method (FVBPM). By applying the perfect matching layer (PML) boundary condition, we can efficiently calculate the various performances such as insertion loss, crosstalk, and power splitting ratio of the multimode couplers. Through this model, the effect of the design and fabrication parameters such as the thickness and width of the Ti-strip and diffusion time and temperature on performances of the multimode couplers can be easily investigated. By comparing with traditional MMI couplers based on step-index optical waveguides, some salient features are observed and the related design rules should be revised. Finally, some related issues such as the bending loss and high order modes are discussed.
As the development of the technology, fiber-to-the-home (FTTH) becomes a feasible solution to meet the increasing demand on bandwidth. Due to the massive number of end users, cheap and reliable components become the bottleneck to deploy the new technology. Triplexer is one of the key components in the FTTH and is used by every end user. Currently, the available triplexers are either based on bulk optics or fiber optics with large size and high price due to manual labor involved. Planar lightwave circuit (PLC) is a possible technology for massive production and cost reduction. However, it is very challenging to design such bi-directional triplexer on PLC. The first challenge is that three channels, at λ=1310nm, 1490nm, and 1555nm, are separated unevenly over a very large wavelength range; Secondly, the bandwidths of the three channels, Δλ=100nm, 20nm, and 10nm, are very different. In the paper, we proposed a novel design by combining both coarse WDM and dense WDM. In the design, a multi-mode interference (MMI) device is used for coarse WDM to separate the 1310nm from the other two channels. The dense WDM for the remaining two channels is performed by an array waveguide gratings (AWG). The MMI and AWG are built on the same wafer with monolithic integration. Initial simulation results show it is a very promising device.
Modal characteristics of the one-dimensional (1D) photonic crystal waveguides (PCWs) are investigated thoroughly. By employing the transfer matrix method, we can put the design parameters related to the general multiplayer structure into a compact analytical expression, which serves as the basis for analysis of the band-gap structure of the general 1D photonic crystals (PCs) and the modal characteristics of the general 1D PCWs. The band structure of 1D PCs and modal properties of 1D PCWs, such as the effective index, the modal field profile, the dispersion, the confinement loss, and the confinement factor, are all calculated and simulated. With the help of the band-gap map of the 1D PCs, four guiding regimes for the 1D PCWs are recognized, in accordance with the index of the guiding core. It is shown that the modal characteristics for each regime behave differently from the point of view of guiding mechanism. Furthermore, some related issues such as PCFs are discussed.
In wavelength division multiplexing (WDM) systems, such as the fiber-to-the-home (FTTH) service (e.g., wavelength 1.31/1.55/1.49 mm in the EPON and wavelength 1.31/1.545/1.5x mm in the APON), coarse and dense WDM are two key technologies. In general, it takes two steps to accomplish this mixing (coarse/dense) WDM functions. In order to make the WDM device compact and low loss, it is necessary to integrate these two WDM functions into one step. In this paper, we propose two typical designs of the coarse/dense multiplexers, which multiplex the coarse/dense-WDM (C/D-WDM) signals simultaneously. We believe that, although their operation principle is simple, the proposed structures simplify the design of integrated photonic devices and will be needed in future WDM systems, specially for the WDM filter in the FTTH service.
The proposed devices consist of a grating-assisted symmetrical or asymmetrical Mach-Zehnder interferometer with the wideband coupler (WBC) and/or the 3dB coupler. Depending on different operation principles, the WBC coupler operates as a 3dB coupler in two wavelength bands or operates as a 3dB coupler in one wavelength band and a cross-state coupler in another one. In this paper, firstly, the operation principles of the proposed C/D-WDM Multiplexers are demonstrated though an example of multiplexing three signals (l1/l2/l3) in two wavelength bands, where l1 (e. g., 1.31mm) belongs to the first band and l2/l3 (e. g., 1.55/1.5x mm) belong to the second band. Then, the design procedure and simulation results of those C/D-WDM multiplexers and related components such as the WBC coupler and planar gratings, which are based on the silica material, are given. Finally, some related issues such as polarizatin dependence and material dispersion are discussed.
A novel design of photonic crystal fiber (PCF) for broadband dispersion compensation is proposed. By changing the radius of the air holes placed in close proximity of the guiding core, the PCF fiber can realize effective broadband dispersion compensation. Through the proper design, the PCF fiber is shown to provide large normal dispersion up to several hundred ps/nm/km. Two examples of the PCF fibers for dispersion compensation of the standard single mode fiber and "True wave-RS" fiber are given. For example, by decreasing the diameter of second ring of air holes, the proposed PCF fiber is shown to provide large normal dispersion (up to -811 ps/nm/km), nearly ten times of conventional dispersion compensating fibers, and compensate conventional single-mode fibers within ±0.05 ps/nm/km over a wavelength range of the conventional C-band.