Method and apparatus for measuring optical power of very weak light, and optical communication system using the same

An optical communication system for transmitting information between a first communication device and a second communication device through a plurality of communication channels, wherein

• the first communication device comprises:
• a modulator for modulating a series of optical pulses according to original information to be transmitted, at given modulation timing to transmit the series of modulated optical pulses through a first communication channel;
• and
• a timing supplier for supplying the modulation timing to the modulator, wherein the modulation timing is changeable, and
• the second communication device comprises:
• a photon detector for detecting photon arrival of the series of optical pulses through the first communication channel according to driving timing having time intervals each corresponding to a constant period of time;
• a counter for counting a number of photons detected by the photon detector;
• a measurement section for measuring optical power of the series of optical pulses from a time average of the number of photons counted by the counter in each of the time intervals while phase-shifting the driving timing in steps of a predetermined amount of phase within a predetermined phase range which is substantially coincident with a period of the optical pulses;
• and
• a timing controller for controlling the modulation timing of the first communication device through a second communication channel based on the optical power measured by the measurement section.

An optical receiver for communicating with an optical transmitter through a plurality of communication channels, comprising:

• a photon detector for detecting photon arrival of a series of optical pulses according to driving timing having time intervals each corresponding to a constant period of time, wherein the series of optical pulses is transmitted by the optical transmitter through a first communication channel;
• a counter for counting a number of photons detected by the photon detector;
• a measurement section for measuring optical power of the series of optical pulses from a time average of the number of photons counted by the counter in each of the time intervals while phase-shifting the driving timing in steps of a predetermined amount of phase within a predetermined phase range which is substantially coincident with a period of the optical pulses;
• and
• a timing controller for controlling the modulation timing of the optical transmitter through a second communication channel based on the optical power measured by the measurement section.

A method for measuring optical power of a series of optical pulses, comprising:

• shifting driving timing of a photon detector, wherein the driving timing has time intervals each corresponding to a constant period of time;
• detecting photon arrival of the series of optical pulses by driving the photon detector according to the driving timing;
• counting a number of photons detected by the photon detector;
• and
• measuring optical power of the series of optical pulses from a time average of the number of photons counted in each of the time intervals while phase-shifting the driving timing in steps of a predetermined amount of phase within a predetermined phase range which is substantially coincident with a period of the optical pulses.

A computer-readable medium storing a computer program for measuring optical power of a series of optical pulses, comprising:

• shifting driving timing of a photon detector, wherein the driving timing has time intervals each corresponding to a constant period of time;
• detecting photon arrival of the series of optical pulses by driving the photon detector according to the driving timing;
• counting a number of photons detected by the photon detector;
• and
• measuring optical power of the series of optical pulses from a time average of the number of photons counted in each of the time intervals while phase-shifting the driving timing in steps of a predetermined amount of phase within a predetermined phase range which is substantially coincident with a period of the optical pulses,
• wherein the number of photons counted is stored in a memory at the driving timing for each phase shift while sequentially shifting the driving timing, wherein the optical power is measured from the time average of the number of photons stored in the memory.