Patent ID: 8725003

Claim:
An external modulator monitoring device for monitoring an external modulator, comprising: a mixing unit to mix optical signals modulated by the external modulator and optical signals not modulated by the external modulator, so as to obtain a multiple branches of mixed optical signals; an electrical differential unit to convert the multiple branches of mixed optical signals into corresponding multiple branches of electrical signals, and perform a differential processing on voltages of the multiple branches of electrical signals to obtain differential signals; and a power detection unit to detect power of the differential signals; wherein the mixing unit is a 3×3 optical coupler to mix optical signals modulated by the external modulator and optical signals not modulated by the external modulator into three branches of mixed optical signals; the electrical differential unit is a differential filter unit comprising: three photovoltaic transformers respectively to convert the three branches of mixed optical signals into electrical signals, so as to obtain three branches of electrical signals; three filters respectively to filter the three branches of electrical signals, so as to obtain three branches of filtered electrical signals; three differential units to obtain differences between the voltages of every two branches of filtered electrical signals among the three branches of filtered electrical signals, so as to obtain three branches of differential signals, the power detection unit comprising: three squarers respectively to square the three branches of differential signals, so as to obtain three branches of square values; and an adder to obtain a sum of the three branches of square values.