Photonics System: LED Signal Variation - Intensity vs. Amplitude

[Shark 774]

In a photonics system where the signal from an LED is varied and picked up by an LD, can the variation of the LED signal be either the intensity of light or the amplitude of the wave? Or is this the same thing?

Thanks.

 

[chrisbaird]

The intensity of light is proportional to the square of the amplitude of the electric component of the wave. Intensity and wave amplitude can be thought of as basically the same thing.

 

[Shark 774]

The intensity of light is proportional to the square of the amplitude of the electric component of the wave. Intensity and wave amplitude can be thought of as basically the same thing.

Ok thanks. So therefore it is possible to have negative amplitude?

 

[chrisbaird]

Unfortunately, the word amplitude can mean different things. Mathematically, the amplitude of a vector is just its length, so by definition it never goes negative because negativity is contained in the directional part of the vector and not in the length. In physics, "amplitude". Also in mathematics, amplitude can refer to the magnitude of a complex number, which is also always positive. Finally in physics, "amplitude" can mean the part of the wave's functional form left after the waving part has been isolated away. Used in this sense, the amplitude can be negative. The problem is that EM waves are complex-valued, waving, vector fields. This means that the word "amplitude" is present in all three forms in EM waves, and we must be more specific in relaying which one is meant.

In my previous comment, I meant the amplitude in the sense of what is left when the waving part is put to the side. So if the wave is E = E₀ cos(kx - ωt) then the light intensity is proportional to |E₀|^2.

 

[Shark 774]

Unfortunately, the word amplitude can mean different things. Mathematically, the amplitude of a vector is just its length, so by definition it never goes negative because negativity is contained in the directional part of the vector and not in the length. In physics, "amplitude". Also in mathematics, amplitude can refer to the magnitude of a complex number, which is also always positive. Finally in physics, "amplitude" can mean the part of the wave's functional form left after the waving part has been isolated away. Used in this sense, the amplitude can be negative. The problem is that EM waves are complex-valued, waving, vector fields. This means that the word "amplitude" is present in all three forms in EM waves, and we must be more specific in relaying which one is meant.

In my previous comment, I meant the amplitude in the sense of what is left when the waving part is put to the side. So if the wave is E = E₀ cos(kx - ωt) then the light intensity is proportional to |E₀|^2.

Ok thanks for the help!