Laser diode frequency sweeping techniques for fiber optic sensors and systems

The use of frequency ramped (chirped) laser diode sources has increased dramatically through their use in interferometric heterodyne systems, fiber fault location systems, and fiber sensor multiplexing. Frequency chirping is typically achieved via either direct modulation of the device, modulation of the diode's bulk heatsink, or varying the diode's bias current with both methods having advantages and disadvantages. In this paper we present a direct comparison of these methods' chirping performance (sweep linearity and range) on various commercially available laser diodes. Measurements were performed using a high resolution spectrometer system capable of realtime monitoring of the temporal evolution of the spectral response of a laser diode's output. Three dimensional light-current curves were determined and mode instability was investigated. Analysis of the resultant information was used to make some inferences regarding laser diode selection.