Optical signal-to-noise ratio measuring device and optical signal-to-noise ratio measuring method

An optical signal-to-noise ratio (OSNR) measuring device comprising:

• a photo detector that converts an optical signal to an electrical signal;
• a light intensity monitor that acquires a signal intensity from the electrical signal;
• a first band pass filter that acquires a noise intensity of a predetermined frequency band from the electrical signal;
• an analog digital converter that converts the noise intensity digitally;
• and
• a processor that computes an OSNR of the optical signal based on the signal intensity and the converted noise intensity, wherein
• the predetermined frequency band is a frequency band including a folding noise that occurs when the electrical signal is digitally converted, and
• an upper limit frequency of the frequency band of the noise intensity acquired by the first band pass filter is in a higher band than a Nyquist frequency that is half of a sampling frequency by the analog digital converter.