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Manuscript Title: SUBMMW: a theoretical model to predict CW sub-millimeter wave laser performance.
Authors: K. Smith
Program title: SUBMMW
Catalogue identifier: ACYC_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 15(1978)85
Programming language: Fortran.
Computer: IBM-360/91.
Operating system: OS-360.
RAM: 53K words
Word size: 8
Keywords: Laser physics, Submillimeter wave, Far infrared, Gas laser, Small signal gain, Output power, Absorption coefficient.
Classification: 15.

Nature of problem:
SUBMMW assumes that the energy levels of the lasing molecule are those of a symmetric top. The molecule is pumped by an intense laser source, for example, the P(20) line of carbon dioxide, from a rotational sub- level of one vibrational level to a higher vibrational level and then decays to a lower rotational level within the higher vibrational level emitting sub-millimeter wave radiation. Given the parameters of the pump radiation, the constants associated with the lasing molecule, for example methyl fluoride, and the relaxation times, SUBMMW calculates small signal gain, saturated gain, output power and absorption coefficients. Consequently, the code can be used for any laser in which two lasing fields interact with a three level molecule.

Solution method:
The computer program solves the time-independent rate equation in a manner first developed by Tucker. The theoretical model upon which SUBMMW is based is described in detail in Smith and Thomson. The principal improvement over Tucker's model is the adaptation from the formalism of De Temple and Danielewicz of terms representing absorption from the two travelling-wave components of the pump standing wave (which is usually measured in watts) rather than the FIR standing wave (which is usually present in the oscillator in milliwatts). The rate equations can be solved analytically leading to explicit formulae to be calculated for the gain and the absorption coefficient. These formulae involve an integral which must be evaluated with considerable care.

Restrictions:
This model of the C.W.SMMW laser programmed in SUBMMW involves the interaction of two laser fields with a three-level molecular system. Consequently, the model would have to be generalized to include
(a) the effects of buffer gases;
(b) to forecast the performance of pulsed systems;
(c) the system of Hacker et al. which involves five laser fields interacting with six molecular levels.