Optical interferometer and optical receiver

An optical receiver comprising:

• an optical interferometer for receiving a first and a second input light and for outputting a first and a second output lights, including:
• a substrate;
• a first branch portion formed on the substrate for branching the first input light into a first and a second branch lights;
• a first and a second branch lines formed on the substrate for transmitting the first and the second branch lights, respectively;
• a second branch portion formed on the substrate for branching the second input light into a third and a forth branch lights;
• a third and a fourth branch lines formed on the substrate for transmitting the third and the fourth branch lights, respectively;
• a first interference portion formed on the substrate and connected to the first and the third branch lines for receiving the first and third branch lights, causing the first and the third branch lights to interfere with each other, and outputting a first output light;
• and
• a second interference portion formed on the substrate and connected to the second and the fourth branch lines for receiving the second and fourth branch lights, causing the second and the fourth branch lights to interfere with each other, and outputting a second output light;
• wherein each of the first and the third branch lines runs in the surface of the substrate such that the first and the third branch lines provide respective optical path lengths with a constant difference for a temperature change, and each of the second and the fourth branch lines runs in the surface of the substrate such that the second and the fourth branch lines provide respective optical path lengths with a constant difference for a temperature change;
• a first balanced receiver for inputting the first output light;
• and
• a second balanced receiver for inputting the second output light.

An optical interferometer comprising:

• a substrate;
• first, second, third and fourth optical waveguides formed on the substrate, respectively;
• a first splitter that branches first input light to at least first branch light traveling through the first waveguide and second branch light traveling through the second waveguide;
• a second splitter that branches second input light to at least third branch light traveling through the third waveguide and fourth branch light traveling through the fourth waveguide;
• a first interference combiner inputting the first branched light and the third branched light to interfere with each other at a first phase;
• and
• a second interference combiner inputting the second branched light and the fourth branched light to interfere with each other at a second phase, wherein
• the first phase at the first interference combiner and the second phase at the second interference combiner are constant to a temperature change in each waveguide.