Increased Accuracy in Molecular Filter Based Flow Field Diagnostics Through Direct Frequency Calibration Using Optical Modulators

*† A description of technique and appar atus for determining the transfer function of molecular filters used in laser -based flow diagnostics is presented. The technique employs the use of a frequency stabilized argon ion laser and an Acousto -Optic Frequency Shifter (AOFS) to accurately determine the frequency -to -optical density transfer function of molecular iodine. An argon ion laser that is locked in frequency to a selected location on a molecular filter absorption line provides a stable, low -drift optical frequency input to the AOFS. Advances in crystal technology have resulted in the development of AOFS systems that can be varied in frequency without significant loss of power (<3 dB) in the shifted laser beam. Driving the frequency shifter with a computer -controlled synthesizer provides a very accurate and precise means of controlling the Bragg -shifted laser output from the AOFS. By directing the frequency -shifted laser beam through a molecular filter and recording the laser intensity throughput as a function of the shifted optical frequency, a profile of the filter transfer function has been achieved with rms uncertainties less than 0.1 -percent of the absorption linewidth.