Fundamental gain suppression mechanisms in a continuous wave hydrogen fluoride overtone laser

Abstract

When lasing occurs on the overtone, a rotational nonequilibrium computer model showed that the fundamental gains are determined by three independent mechanisms. First, overtone lasing decreases the gains of the P (7) and PiC/) lines whose upper or lower levels are directly involved in PioC/) overtone lasing. Second, overtone lasing reduces the rate at which the low/ v = 2 states are populated by rotational relaxation and increases the rate at which the low J v = 0 states are populated by rotational relaxation, resulting in suppression of the low J fundamental gains whose upper or lower levels are not directly involved in overtone lasing. Third, overtone lasing reduces the rate at which the HF(0, /) and HF(1, J) states are populated by the various collisional deactivation processes. The computer model gave reasonable agreement with the measured fundamental zero power gain profiles, Fabry-Perot power, and spectra. The model overpredicted the fundamental gain suppression (Aa) for the P (8,9) and P2(8,9) lines whose upper or lower levels were directly involved in overtone lasing and underpredicted the suppression for lines PI (4) and PI (4, 5). The model predicted the suppression for lines PI (5-7) and P2(6,7) reasonably well. When the rotational relaxation rate was increased by a factor of 10, the model was in reasonable agreement with the measured suppression, A a, of all P (4-9) and P2(4-9) lines. However, with the increased rotational relaxation rate, the model's prediction of the experimental zero power gain and residual fundamental gain profiles was not adequate. OMPARISON of residual fundamental amplification ratio (RF-AR) data obtained at relatively high medium saturation with two 99.7% reflective mirrors (asal = 0.00010015 and Ls = 30 cm) with the zero power amplification ratio (ZP-AR) data indicated that the gains of the low J lines P (4-6) and P2 (4-6) were suppressed between 41 and 96% and the gains of the high J lines PI (79) and P2(7-9) were suppressed between 3 and 44%.u The 1 -> 0 lines were suppressed more than the 2-> 1 lines. The maximum suppression occurred between 2 and 6 mm downstream from the nozzle exit plane, near the center of the 9-mm overtone beam. There was minimal suppression of lines P2 (8, 9). The low J fundamental gains were suppressed more than the high J fundamental gains even though their upper or lower levels were not directly involved in overtone lasing. Residual fundamental gain (RFG) measurements at low medium saturation with overtone mirrors of 99.7/98.0% reflectivity (asat = 0.000386787) showed weak suppression on lines ^i(7), P2(5), and PI (6) at axial positions between 2 and 6 mm downstream from the nozzle exit plane.' '2 There was no suppression measured for any of the other lines. RFG measurements performed at an increased level of medium saturation with 99.8/99.86% reflective overtone mirrors (asal — 0.000056716) resulted in essentially the same suppression obtained with the 99.7/99.7% mirrors.1-2 The objectives of this work were to determine why the low J fundamental gains were suppressed more than the high J fundamental gains even though their upper or lower levels were not directly involved in overtone lasing and to predict the RFG as a function of medium saturation. Since there are many kinetic processes involved in the population and depopulation of the upper and lower levels of the fundamental transitions, a detailed rotational nonequilibrium model (ORNECL)3'4 of the laser flow was used. When ORNECL was baselined to ZP-AR data, it was necessary to decrease the rotational relaxation (RR) rate5'6 by a factor of 10 and to include the multiquantum HF-H2 vibration-to-vibration (VV) transfer reactions.