Using a discrete thermal model to obtain a linear frequency ramping in a frequency-modulated continuous-wave system

The lasing wavelength of a single-section distributed feed- back DFB laser diode can be modulated by modulating the drive cur- rent. This makes it possible to utilize the DFB laser diode in a frequency- modulated continuous wave FMCW range and velocity measuring system. In FMCW, the frequency of the laser is ramped, and the fre- quency difference between the reflected wave and a local-oscillator wave is monitored. For maximum performance the frequency ramping should be linear. Due to thermal phenomena, a linear ramping of the current seldom results in a linear ramping of the optical frequency. We have derived a discrete thermal model, using resistors and capacitors, of our laser module. The thermal model was then used as a starting point to model the frequency behavior of the laser and to derive modulation cur- rents that resulted in a linear frequency ramping at some different modu- lation frequencies. © 2005 Society of Photo-Optical Instrumentation