Wideband square-law detector having a triangular frequency characteristic as well as a transmission system and a receiver including such a detector

A receiver for polarization diversity reception of an optical signal which is modulated by an FSK baseband signal, the receiver having an intermediate frequency (IF) circuit which derives an IF signal from the received modulated optical signal and a square law detector for detecting the IF signal, the square law detector comprising:

• an input for the IF signal;
• transfer circuit means coupled to said input for processing the IF signal in accordance with a first frequency transfer characteristic in the form of a continuous function having an ascending branch up to a predetermined frequency and a descending branch thereafter, a resulting processed IF signal being produced at an output of said transfer circuit means;
• and
• square law circuit means coupled to the output of said transfer circuit means for detecting an FSK baseband component of the processed IF signal by subjecting the processed IF signal to further processing in accordance with a square law amplitude transfer characteristic, the detected FSK baseband component being produced at an output of said square law circuit means, said output being an output of said square law detector;
• said transfer circuit means and said square law circuit means together having a combined frequency transfer characteristic in the form of a continuous function having an ascending branch up to said predetermined frequency and a descending branch thereafter, the angle between the ascending and descending branches of said combined transfer characteristic being narrower than the angle between the ascending and descending branches of said first transfer characteristic.