Intensity graph for laser double-slit experiment?

Question

For incoherent light sources, they are passed through a single slit first. The interference pattern of the single slit then modulates the intensity of the double slits' interference pattern.

But what about lasers? They don't need to be passed through a single slit first, so would their intensity curve be modulated by a single slit 'envelope'?

Here is the graph we expect if passed through a single slit only (right), and a single followed by double slits (left)

Farcher · Answer

No.

The left hand graph is the intensity pattern for a single slit all by itself.

The right hand graph is the intensity pattern for a two parallel slit arrangement with the two slits each with the same width as that of the single slit which produced the left hand intensity pattern.
The double slit intensity pattern is an equally spaced double slit interference pattern modulated in intensity by a single slit diffraction pattern.

The effect of changing the width of the slits is described in this answer Young's double slit experiment, the slit width.

When using a laser the additional single slit is not needed because it provides light with a single coherent wavefront whereas that is not so with other light sources as described in the answers to the question Why must the single slit in a double slit experiment be narrow?

Intensity graph for laser double-slit experiment?

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