Lithium niobate on insulator (LNOI) thin film maintains the attractive material properties of lithium niobate, while also offering a stronger optical confinement and a high optical element integration density. The structure of integrated chips needs to be redesigned because the LNOI chip is quite different from the single crystal LN one. Beam splitter has been widely used in integrated optical devices, such as MZM modulator, AM modulator and light switch. In this paper, two kinds of beam splitters, including directional coupler (DC) and Multi-Mode interference (MMI) structure were designed by using finite difference time domain. The gap and coupling length of directional coupler were analyzed. An MMI beam splitter structure was designed and simulated. The key parameters of beam splitter such as free spectral range, splitting ratio and propagation loss were compared. The analysis of the two beam splitter structures can supply guidance for the design of the LNOI devices.
The integrated electro-optic modulator plays an important role in the broadband wireless communication and phase-coded radar etc. The electro-optic modulator using a lithium niobite (LN) ridge waveguide is easy to be integrated and has excellent electro-optic response. However, its fabrication and coupling still face great challenge. In this paper, an intensity modulator (IM) based on heterogeneous platform with LN and silicon nitride (Si3N4) is designed. The optical mode field distribution is simulated as the waveguide size changes. The parameters such as the width and height of loadinged strip are optimized to ensure that the light power is highly concentrated in the LN layer (over 90%). The propagation loss of the IM with different Y-junction shapes is analyzed. In addition, the influence of different electrode parameters on half-wave voltage is discussed systematically. The results demonstrate that the designed IM has a low half wave voltage of 2.1V, characteristic impedance of 53Ω and propagation loss of -0.2dB. The proposed IM has the advantages of convenient fabrication and coupling, which provides an alternative modulation unit for multi-level or large-scale modulation integrated chips.
A triple-frequency microwave photonic link is proposed based on a polarization-multiplexing dual-parallel Mach- Zehnder modulator (PM-DPMZM). The lower sub-DPMZM is biased at the maximum transmission point to obtain the 2nd-order RF sidebands. Meanwhile, the RF signal modulates the upper sub-DPMZM through an electrical 90° hybrid coupler. The upper sub-DPMZM works at the carrier-suppressed single-sideband (CS-SSB) modulation to obtain the +1st order RF sideband. An optical band pass filter is used to filter out the +2nd-order RF sideband, and only the +1storder and -2nd-order RF sidebands are output for the frequency beating at a photodetector (PD). The frequency triple signal of 3ωRF is obtained correspondingly. The experimental link is built. The results show that the RF signal from 8 to 12 GHz is tripled, and frequency tripled signal at 24 to 36 GHz is acquired. The minimal spur suppression ratio of triple-frequency signal is 20dB. A relatively low-frequency signal can be used to generate a high-frequency signal with well quality by this triple-frequency link. The proposed method can be applied in various microwave photonic transmitting systems.
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