Laser diode linewidth measurement using high-Q spherical microresonators

A novel technique to measure the emission linewidth of laser diodes (LDs) utilising whispering gallery modes (WGMs) in high-Q microspheres (diameter≃200μm) is proposed and demonstrated. Laser light from a distributed feedback (DFB) LD operating at λ≃1537.1nm is coupled into silica glass microspheres of Q≃10⁷ (estimated experimentally). WGM spectra are recorded and then used to estimate the DFB LD emission linewidth, yielding a value of 3.8 MHz. The estimated value is in good agreement, within experimental error, with a linewidth value of 6.0 MHz independently measured using the self-homodyne technique, strongly supporting the validity of this novel method.