Radio frequency (RF) generators can be programmed to emit many differently shaped oscillatory patterns, from sinusoidal waves associated with single frequencies to square and sawtooth shapes that emerge when frequencies spanning multiple octaves are mixed. Analogous manipulation of optical fields is rather more challenging, because laser sources cannot usually produce many mutually coherent components spaced octaves apart. Chan et al. (p. 1165, published online 20 January; see the Perspective by Yavuz) have now succeeded in preparing optical sawtooth and square wave pulses by assembling discrete combs of five harmonics, from the blue to the mid-infrared, derived through Raman shifting in a hydrogen cell and demonstrate a method for characterizing such waveforms based on linear cross-correlation.
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