A perfect sinusoidal is described (by definition) by a single frequency. But if you take a sinusoidal, and start adding random phase jumps, its spectrum will start broadening.
In Laser Physics, the emitters of a fluid gain medium will collide at random times, creating abrupt phase jumps, which will broaden the emission spectrum (collision broadening).
<a href="//commons.wikimedia.org/wiki/User:Berto" title="User:Berto">Jacopo Bertolotti</a>
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