Wave-number stability of a laser diode mounted in a closed cycle helium refrigerator

The wave-number stability of a laser diode mounted in a closed cycle helium refrigerator was examined using a CO(2) laser heterodyne spectrometer. The set current supplied by a commercial laser diode supply appeared stable enough over the time periods normally used. Temperature control was achieved through both passive and active loop control. A stage of open loop control of the cold tip allowed the closed loop control of diode heat sink to be operated at higher gains than conventionally used. Long-term stability of the center of the line shape function was better than 10 MHz per hour and the 1-min average was better than +/-5 MHz. Vibration of the diode mount determined the instrument line shape in spite of steps taken to eliminate the vibrations. The line shape had a half-width of 10 MHz with broad shoulders out to 50 MHz.