If EM fields do not operate on each other, how do we generate and manipulate laser pulses?

摘要

Generating or shaping a light pulse requires complete knowledge of all the parameters of the complex pulse amplitude. But they are not directly available to us. Our gathered data is only proportional to the square modulus of the EM fields as reported to us by various detectors (including non-linear processes). These detector responses are "colored" by their unique quantum "preferences". EM fields are not directly observable to us. They do not operate on (interfere with) each other either. Classical optics recognizes that light beams of different frequencies and of orthogonal polarizations do not "interfere" with each other. The success of Michelson's Fourier Transform spectrometry relies on this non-interference of different frequencies. Yet, beat signal is a result of simultaneous actions of different frequencies on a fast detector. We are promoting the hypothesis of non-interference of light beams as a generalized principle irrespective of the similarities or dissimilarities of their parameters. All superposition effects can become manifest only when the interacting medium is capable of summing all the induced stimulations simultaneously. Accordingly, an array of CW beams with multiple frequencies cannot create temporal pulses as implied by the time-frequency Fourier theorem in free space or in a medium that does not interact with the fields. Dipole undulations induced by the superposed EM fields within appropriate media are always at the root of creating laser pulses, and this brings conceptual congruence between the processes in generating pulses whether they are due to spiking or mode-locking. To illustrate our position, we present a series of experiments: (ⅰ) Simple superposition of three coherent frequencies did not generate mode locking. (ⅱ) A series of ordinary CW He-Ne laser beams can produce mode-lock like current pulses by a high speed detector. (ⅲ) A Q-switched ps diode laser shows 100fs train of autocorrelation spikes but it is not mode locked.