Measurement of the linewidth of a continuous-wave laser with a cavity-length modulation technique

We introduce a Fabry-Perot cavity-length modulation technique for measuring the linewidth of a continuous wave (cw) laser. We calculate the peak intensity of a cw laser transmitted through a Fabry-Perot cavity as a function of mirror speed. By fitting the experimental data to the results of the calculation, we determine the linewidth of a frequency-stabilized cw laser. The linewidth of a cw ring dye laser measured in the 570-590-nm wavelength range is approximately 170 +/- 20 kHz. We also demonstrate the use of this technique to measure the reflectivity of a high-reflectance mirror.