Wavelength Monitoring Circuit

A wavelength monitoring circuit comprising:

• an input light split circuit configured to split an input optical signal;
• an optical delay unit configured to give a time delay difference to two beams of split light from the input light split circuit;
• a light split stage including a light trifurcation unit configured to split one of optical signals output from the optical delay unit into first split light, second split light, and third split light, and to output the first split light as a first light output and a light bifurcation unit configured to split another one of the optical signals output from the optical delay unit, wherein, when the second split light and the third split light have an identical phase, split light beams from the light bifurcation unit have a phase difference of 90 degrees, and, when the second split light and the third split light have a phase difference of 90 degrees, the split light beams from the light bifurcation unit have an identical phase;
• and
• a light multiplexing stage including a first light multiplexing circuit configured to obtain a second light output, which is interference light, by multiplexing the second split light from the light trifurcation unit and one of the split light beams from the light bifurcation unit, and a second light multiplexing circuit configured to obtain a third light output, which is interference light, by multiplexing the third split light from the light trifurcation unit and another one of the split light beams from the light bifurcation unit, and connecting to the light split stage via waveguides having an equal length, wherein
• a wavelength of the input optical signal is determined, based on an electrical output obtained from the second light output and an electrical output obtained from the first light output, or an electrical output obtained from the third light output and the electrical output obtained from the first light output.