In reality, the waves do not remain coherent for an infinite interval and after the coherence time the phase difference drifts. More generally, if the phase difference of two waves remains the same during this "coherence time", they are coherent during that time. If the phase difference of the two waves is the same, they are coherent. Looking at it this way, the signals are said to have a “coherence time”. But if you look at a small time segment (a few cycles), they are close to being coherent. In the broad spectrum of the graph, the signals aren’t coherent. As with everything, real life is somewhere in the middle. The figures above represent the ideal and the worst case. The relative difference is always fluctuating. The relative difference between any two matching points is exactly the same. Instead think of a signal that looks sinusoidal in any small piece but the phase is slowly drifting (Fig. Also, the two frequencies aren’t always exactly the same. However, no real wave is perfectly sinusoidal. When the two waves are always shifted by the same amount (Fig. With two perfect sine waves of the same frequency, the relative phase never changes. How stable is the phase between the two waves (electrical, light, etc.)?.These are two common ways of looking at phase coherence, but what do they really mean? Let’s take a closer look…Ī better way to look at the problem might be to split it into two simpler questions. These are both great definitions, although I have a feeling you may be thinking to yourself: In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency.Often, it is interpreted as signals/systems operating at the same frequency with a consistent phase relationship. In many ways, the term 'Phase-coherent' is not strictly defined.Two definitions that we like are listed below. So, What is Phase Coherence? …More Importantly, Does it Affect You?įirst, let’s look at that first question: What is phase coherence?Īfter little research, it seems that there are several definitions out there.
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