how phase fluctuations of laser is converted to intensity fluctuation by the use of PLC-MZI in application of QRNG? what is the working principle behind it?
The unbalanced interferometer interferes the laser light from two different moments in time. One arm of the interferometer is longer than the other, so the light arriving at the detector has travelled farther and is from the laser output further back in time. Since the phase varies randomly in time, the light from two different times may or may not be in phase. If they are in phase they constructively interfere producing a bright signal. If they are out of phase they destructively interfere producing a weak signal. Over time the signal strength will vary rapidly and randomly as determined by the phase noise of the laser.
This works particularly well with a gain switched pulsed laser where the delay leg is equal to the inter pulse period. As each pulse builds from random spontaneous noise, there is expected to be no phase relation between different pulses.
The advantage of this approach is that it is fundamentally a quantum phenomenon and is guaranteed to produce true randomness, but, unlike other schemes, it doesn’t compare one photon at a time. The signal is strong and easily detected, effects of detector noise can be suppressed, and the result is analog, so digitizing the signal strength generates more than one bit at a time. This means it is generates random bits much more rapidly than single photon schemes.