a) Describe the spectrum obtained by the heterodyning of a single-mode laser wit
ID: 1770781 • Letter: A
Question
a) Describe the spectrum obtained by the heterodyning of a single-mode laser with a mode-locked laser, assuming the two fields have frequencies as indicated in the following figure.
b) Explain why there must be phase coherence among pulses equally spaced in time if the pulse is to serve as a comb for optical frequency metrology.
Frequency no,+ (n+1)a+ o Figure 14.31 Frequency comb spectrum and position of optical frequency oL with respect to the closest comb frequency nu,+0. Heterodyning results in beat notes determined by -n4 -ub.Explanation / Answer
a)The frequency spectrum of an ideal mode-locked laser is generally composed of hundreds of thousands of equidistant modes spaced by the repetition rate of the laser, thus the name optical frequency comb. In practice, however, a mode-locked laser exhibits noise on the optical phase and repetition rate of the emitted pulses.The original motivation for the invention of the frequency comb technique in laboratory has been precision laser spectroscopy of the simple hydrogen atom, which permits unique confrontations between experiment and quantum electro-dynamic theory.Optical frequency comb spectroscopy has the potential to become the ultimate tool for precision spectroscopy and trace gas detection since it allows highly sensitive simultaneous detection of many molecular species in short acquisition times.
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