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INTRODUCTION

MECHANISMS

DESIGN

STATUS

The EAMs are based on the PIN structure to apply the electric field. When a PIN diode is reverse-biased, almost all the electric field drops at the intrinsic layer as its conductivity is lowest (giving highest resistance). Hence, we put the electroabsorption layer in the intrinsic layer. As a diode, the EAM has characteristic capacitance determined by the device dimension and the intrinsic layer thickness when it is reverse-baised. The capacitance can be calculated very simply by using the formula for the parallel-plate capacitor. As the capacitor stores charges, it delays the response of the circuit driven by an RF source. The time constant of the circuit with series resistance R and the capacitor C is given by RC. Hence the bigger the capacitace, the slower the whole circuit to the driving signal. That's why the device dimension should be small and the intrinsic layer thickness should not be too thin to have a high speed device.

Other than the device speed, there is another important device parameter: its efficiency to the electric field (which is supplied to the device by means of the electrical reverse bias to the PIN diode-- in this case, the EAM). This means how sharply the intensity output from the modulator changes with the applied electric field. In digital applications (e.g. for telecommunications), if the modulator efficiency is good, it requires less voltage to change the light intensity from its on-state to off-state, and vice versa. In analog applications (e.g. cable TV), good efficiency modulator gives the good RF gain, which represents how much RF power output we get with the given RF power input to the fiber-optic link.

Now a fundamental optimization issue comes: the device speed and its efficiency. Although the thick intrinsic layer is good for the device speed by decreasing the device capacitance, it is bad for the modulator efficiency! (Remember the electric field is inversely proprotional to the intrinsic layer thickness for the given electrical bias.) Hence normally the intrinsic layer thickness is determined by the device speed requirement, and this eventually limits the device' modulator efficiency.