Patent ID: 7457549
Filing Date: 2008-11-25
Classification: H04B

Abstract:
1. A sub signal modulation apparatus comprising: a sub signal outputting unit outputting a sub signal different from a main signal; a variable optical attenuator variably attenuating the main signal light, into which the main signal has been modulated, at a response speed lower than ½ of a speed of the main signal; an attenuation determining unit determining, in accordance with the sub signal output from said sub signal outputting unit, an attenuation of said variable optical attenuator for the main signal light; and a stabilizing controller controlling said attenuation determining unit such that an amplitude of the attenuation of said variable optical attenuator is stabilized to have a constant ratio to a mean intensity of the main signal light, wherein said stabilizing controller includes: a mean intensity measuring section measuring a mean intensity of a signal light output from said variable optical attenuator; a sub signal component extracting section extracting a sub signal component from the signal light output from said variable optical attenuator; an amplitude measuring section measuring an amplitude of the sub signal component extracted by said sub signal component extracting section; and a controlling unit controlling the determining of the attenuation in said attenuation determining unit such that the amplitude measured by said amplitude measuring section has a constant ratio to the mean intensity measured by said mean intensity measuring section, wherein said attenuation determining unit outputs an electric signal having an intensity variation based on the sub signal from said sub signal outputting unit, which signal serves as a determination signal for the determining of the attenuation, to said variable optical attenuator; and said controlling unit includes: an amplifier amplifying the sub signal component measured by said amplitude measuring section at a gain corresponding to the constant ratio, a subtractor obtaining a difference between the mean intensity measured by said mean intensity measuring section and an amplitude of the sub signal component amplified by said amplifier; and a signal provider providing said attenuation determining unit with a signal for controlling an intensity of the determination signal that is to be output to said variable optical attenuator such that the difference obtained by said subtractor is substantially zero, wherein said signal provider integrates the difference from said subtractor and provides said attenuation determining unit with an integrated result as the controlling signal, and said attenuation determining unit multiplies the sub signal from said sub signal outputting unit by the controlling signal and provides said variable optical attenuator with a multiplied result as the determination signal.