光通讯基础课件Chapter2-4.ppt

Mach-Zehnder (MDM) external modulators [Lithium niobate (LiNbO3) modulator].  The Application of Phase Modulator Electroabsorption (EA) external modulators The major drawbacks of an MDM modulator are high insertion loss (up to 5 dB) and relatively high modulation voltage (up to 10 V). The property of electroabsorption modulator * Prof. Yang, The college of Optic and Electronic Information Engineering, USST * 2.4 External Modulators ? Internal modulation Internal (or direct) modulation:The intensity of the radiated power is changed from maximum to minimum by simply changing the driving current . Two serious drawbacks: 1.Bandwidth is restricted by the laser diode’s relaxation frequency. 2.Chirp—the fast variations of the laser’s peak radiating frequency in response to a change in driving (modulation) current—results to produce a broadening of the light pulse. It is a serious limiting factor in high-speed communications ?External modulation This approach leaves a laser diode to radiate a continuous light wave (CW) while a change in light power occurs outside the laser diode. We need to insert an external component into the optical loop. Figure 2.22 Block diagram of external modulation Its principle of operation is as follows: The refractive index of lithium niobate changes when an electrical voltage is applied. Light splits equally when it enters the waveguide. When no voltage is applied, both halves of the incident wave have no phase shift and so they interfere constructively, forming the original wave. When voltage is applied, one-half of the incident wave experiences a phase shift of +90° because the refractive index of this portion of the wave-guide decreases, increasing the velocity of the light propagation and lessening the delay. The other half of the waveguide receives a -90°shift because its refractive index increases, reducing the velocity of the light propagation and lengthening the propagation delay. When the halves recombine, they