Frequency Comb Widening To One Octave
To be usable, the comb must be widened to at least an octave: that is, the highest-frequency must be at least double the lowest frequency. One of three techniques may be used:
- supercontinuum generation by strong self-phase modulation in nonlinear photonic crystal fiber
- a Ti:sapphire laser using intracavity self-phase modulation
- the second harmonic can be generated in a long crystal so that by consecutive sum frequency generation and difference frequency generation the spectrum of first and second harmonic widens until they overlap.
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—William Butler Yeats (1865–1939)