Electro-optical modulator which provides optical bias stabilization during operation

Claim 1

An electro-optical modulation circuit for modulating the optical output of an electro-optical converter in accordance with a received modulating signal while maintaining a selected bias level of said optical output which is independent of the received modulating signal; said modulation circuit comprising: an opto-electrical converter optically coupled to said electro-optical converter; a negative feedback path having an input and an output and including there-between said electro-optical converter, said opto-electrical converter and an automatic gain control amplifier in that order, the output of said automatic gain control amplifier being the output of said negative feedback path; a control amplifier having a first input for receiving a DC bias reference voltage corresponding to said selected optical bias level, a second input coupled to the output of the negative feedback path for receiving an output signal of said path, and an output at which said control amplifier is adapted to produce a bias control signal corresponding to a combination of the signals at the inputs thereof; combining means having a first input coupled to the output of said control amplifier and having an output coupled to the input of said negative feedback path so as to complete a negative feedback DC control loop which includes said negative feedback path, said control loop being operative to stabilize the optical output of said electro-optical converter at said selected bias level; and signal cancelling circuit means for deriving from the received modulating signal (i) a modulating current signal corresponding thereto and supplying said modulating current signal to a second input of said combining means, and (ii) a cancelling signal which is supplied to a third input of said control amplifier, said cancelling signal being the negative of a low frequency (including DC) component of the output signal of said negative feedback path caused by the modulating current signal supplied to the input of said negative feedback path via said combining means; whereby the bias control signal produced by said control amplifier corresponds to said biasing reference voltage independently of any low frequency (including DC) component of the received modulating signal.