FM modulation device

An FM modulation device comprising:

• a signal source for outputting an electrical signal;
• a branch portion for branching said electrical signal outputted from said signal source into a first electrical signal and a second electrical signal, said first electrical signal and said second electrical signal being set in opposite phases to each other;
• a laser light source for FM modulation (hereinafter referred to as an FM laser element) having the property of oscillating a light at a wavelength .lambda.1 in a steady state in which its inputted electrical signal remains constant, and also the property of uniquely converting a variation in amplitude of the inputted electrical signal into a variation in optical intensity and also uniquely into a variation in optical frequency, for converting said first electrical signal supplied as the inputted electrical signal into an optical signal and outputting the optical signal;
• a first optical waveguide portion for guiding the optical signal outputted from said FM laser element;
• a local light source for outputting a light at a wavelength .lambda.0 which is different from the oscillation wavelength .lambda.1 of said FM laser element by .DELTA..lambda.;
• a second optical waveguide portion for guiding the light outputted from said local light source;
• a laser light source for suppressing IM-DD component (hereinafter referred to as an IM suppressing laser element) having the property of uniquely converting a variation in amplitude of its inputted electrical signal into a variation in optical intensity, for converting said second electrical signal supplied as the inputted electrical signal into an optical signal having a wavelength .lambda.2 and outputting the optical signal;
• a third optical waveguide portion for guiding the optical signal outputted from said IM suppressing laser element;
• and
• a photodetection portion having the property of converting and outputting a variation in optical intensity of its inputted light as a variation in output current by a square-law detection characteristic (hereinafter referred to as IM-DD component), and also the property of, when two lights having different wavelengths are inputted, producing a beat component of the two lights at a frequency corresponding to a difference in wavelength between the two lights, for converting an optical intensity modulation component in the optical signal outputted from said FM laser element and the optical-intensity-modulated signal outputted from said IM suppressing laser element individually into variations in amplitude of the output current, and also outputting a beat signal of the optical signal outputted from said FM laser element and the light outputted from said local light source at a frequency corresponding to the wavelength difference .DELTA..lambda. between the two lights.