Patent ID: 7991301

Claim:
An optical apparatus comprising: a branching unit branching an input optical signal modulated by DQSPK format and thereby outputting a first branched light and a second branched light; a first interferometer having a first phase shifting unit that shift an optical phase of part of the first branched light and thereby outputting a first interfered light; a first photo detector converting the first interfered light to a first demodulated signal; a first discriminator recovering the first demodulated signal and thereby outputting a first recovered signal; a second interferometer having a second phase shifting unit that shift an optical phase of part of the second branched light and thereby outputting a second interfered light; a second photo detector converting the second interfered light to a second demodulated signal; a second discriminator recovering the second demodulated signal and thereby outputting a second recovered signal; a first phase control unit controlling the first phase shifting unit based on the first demodulated signal and the second recovered signal; a second phase control unit controlling the second phase shifting unit based on the second demodulated signal and the first recovered signal; and a abnormality detection unit detecting abnormality based on a synchronized detection of the first demodulated signal and the first recovered signal, and a synchronized detection of the second demodulated signal and the second recovered signal, wherein, the abnormality detection unit further comprises a plurality of inversion circuits switching a polarity of the first demodulated signal, the first recovered signal, the second demodulated signal, and the second recovered signal, and the abnormality detection unit switching the polarity of the first demodulated signal, the first recovered signal, the second demodulated signal, and the second recovered signal on the synchronized detection.