Application Note for LN Modulators

P2/P5

3, How the Intensity Modulator Works

Part. 1

Basic structured LN modulator comprises of 1) two waveguides 2) two Y-junctions 3) RF/DC electrode. Optical signals coming from the LD is launched into the LN modulator through the PM fiber, then it is equally split into two waveguide at the first Y-junction on the substrate. When the voltage IS NOT applied to the RF electrode, the two signals are re-combined at the second Y-junction and coupled into a single output as two separated signals are in phase. In this case output signals from the LN modulator is recognized as "ONE". When voltage IS applied to the RF electrode, due to the electro-optic effects of LN substrate, refractive index is changed, and the phase of the optical signal in one arm is advanced though retarded in the other arm. When two signals are re-combined at the second Y-junction, they are transformed into higher order mode and lost as a radiation mode. In the case two signals are completely out of phase, all signals are lost into the substrate and the output signal from the LN modulator is recognized as "ZERO". The voltage difference which induces this "ZERO" and "ONE" is called the driving voltage of the modulator, and is one of the important parameters in deciding modulator's performance.

Part. 2

The transfer function of Mach-Zehnder modulator is expressed as I(t)=aIqcos2(V(t)π/2Vπ), where I(t)=transmitted intensity, a=insertion loss, Iq=Input intensity from LD, V(t)=applied voltage, Vπ=driving voltage. It is necessary to set the static bias on the transmission curve through Bias electrode. It is common practice to set bias point at 50% transmission point, Quadrature Bias point. As shown here, electrical digital signals are transformed into optical digital signal by switching voltage to both end from quadrature point. DC drift is the phenomena, where this transmission curve gradually shift in the long term.

P2/P5