Frequency Stabilization of Laser Diodes in an Aggressive Thermal Environment

摘要

Mobile free-space laser communication systems must reconcile the requirements of low size, weight, and power with the ability to both survive and operate in harsh thermal and mechanical environments. In order to minimize the aperture size and amplifier power requirements of such systems, communication links must exhibit performance near theoretical limits. Such performance requires laser transmitters and receiver filters and interferometers to maintain frequency accuracy to within a couple hundred MHz of the design frequency. We demonstrate an approach to achieving high frequency stability over wide temperature ranges by using conventional DFB lasers, tuned with TEC and current settings, referenced to an HCN molecular frequency standard. A HCN cell absorption line is scanned across the TEC set-point to adjust the DFB laser frequency. Once the center of the line is determined, the TEC set-point is offset as required to obtain frequency agility. To obtain large frequency offsets from an HCN absorption line, as well as continuous laser source operation, a second laser is offset from the reference laser and the resulting beat tone is detected in a photo-receiver and set to the desired offset using a digital frequency-locked loop. Using this arrangement we have demonstrated frequency accuracy and stability of better than 8 MHz RMS over an operational temperature range of 0℃ to 50℃, with operation within minutes following 8 hour soaks at -40℃ and 70℃.