Optical Standards and Measurement

The early development of lasers was marked not only by the explosive proliferation of laser oscillation on different atomic and molecular transitions, but also by efforts to stabilize them and narrow their spectral line width. This was driven by the realization that the very attribute that makes the laser so remarkable is the one that still left room for spectacular improvement: spectral purity. The fundamental quantum limit on spectral purity far exceeds that of any common laser subject to fluctuations in its optical cavity. As we saw in the example given in Chapter 14, the theoretical spectral line width of a 1mW laser with a 1m long cavity is on the order of 3 × 10⁻⁴ Hz, or a fractional line width of 5 × 10⁻¹⁹!