Walter Schupita

Radiation-trapping in cylindrical and spherical geometries

Abstract We solve the Holstein radiation-trapping equation numerically for cylindrically and spherically symmetric geometries. For Doppler and Lorentz lineshapes, we give analytical fitting equations for the trapping factors and for the shapes of the 10 lowest-order modes. For Voigt profiles, we modify the classical Walsh-interpolation formula so that it is now applicable at all opacities and for all modes. The results are checked by Monte Carlo simulation and by comparison with existing approximations. Our fits and interpolations yield 2–20% accuracy.

Direct frequency reading laser spectroscopy: ν3 Fundamental and Stark effect of CH3F

Abstract The frequencies of 63 transitions in the ν 3 fundamental have been measured with a tunable sideband laser. The following excited-state constants have been derived: ν 0 = 1048.61000 (44) cm −1 , ( A ′ − A ″) = −293.77 (90) MHz; B = 25 197.570 (20) MHz, D J = 56.61 (22) kHz, D JK = 521.6 (56) kHz, D ′ K − K ″ K = −97.7 (70) kHz. Resolution is limited by the laser stability, i.e., ⋍1 MHz. The accuracy in absolute frequency is generally 13 MHz ( Δ ν ν = 4 × 10 −7 ) while the relative error in measurement is believed to be ≤5 MHz. On the Q (6,6) and Q (6,4) transitions the Stark effect was observed at moderate field strengths. Stark as well as source modulation has been applied.

Lamp-pumped thallium atomic line filter at 535.046 nm

We present a new atomic line filter concept for the detection of frequency-doubled Nd:BEL laser radiation. Advantage is taken of the thallium 6(2)P(3/2) metastable energy level whose large lifetime can be preserved even in small vapor cells. The filter is pumped intermittently by a low-power spectral lamp and thereby combines the benefits of active and passive operation. Analysis of a proposed final design shows that simple engineering solutions exist, and excellent filter properties can be deduced. We have experimentally demonstrated the new operating principle in a preliminary setup.

RAD-TRAP 2, a program for the solution of the Holstein equation of radiation trapping

Abstract RAD-TRAP computes the solution of the Holstein equation of radiation trapping for three important geometries: plane-parallel slab, long cylinder, and sphere. The new version 2 offers the direct computation of the steady-state distribution of excited atoms and the computation of the emergent spectra; effects like self-reversal can now be studied. It also includes a new algorithm for a more efficient, highly accurate computation of the cylinder case. The new version also runs on IBM-PCs.

RAD-TRAP, a program for the computation of the eigenvalues and eigenfunctions of the Holstein radiation-trapping equation

Abstract Radiation trapping is described by the Holstein equation, a Fredholm integral equation of the second kind. By combining analytical and numerical techniques, the presented code efficiently computes the eigenvalues and eigenfunctions of this equation for three important geometries: plane-parallel slab, long cylinder, and spherically symmetric geometry; all practically occuring spectral lineshapes are considered. The program is written in standard Fortran.

Quantum efficiency and signal bandwidth of thallium atomic line filters

Abstract We derive a new, simplified method for the computation of trapping effects in coupled three-level atomic systems. The linearity of the Holstein radiation trapping equation allows the reduction of the general rate equation to the solution of the basic two-level Holstein equation and to an algebraic eigenvalue problem. The method is applied to the numerical simulation of the quantum efficiency and the signal bandwidth of thallium atomic line filters. With a pumping scheme that achieves population inversion between the ground state and the metastable state, 90% quantum efficiency and 10 MHz signal bandwidth can be achieved, while in noninverting pumping schemes, even quite high pump intensities result in less than 60% quantum efficiency and 8 MHz signal bandwidth. A tradeoff between quantum efficiency, signal bandwidth and pump power is possible.

McTrap, a program for the computation of radiation trapping in 3-level atoms including bleaching effects

McTrap computes radiation trapping including bleaching effects and particle diffusion in a two-dimensional cylinder geometry. It combines Monte Carlo simulations of radiation trapping and analytical solutions of the diffusion equation for a highly efficient computation of the distribution of excited atoms. The spectral lineshapes can be freely chosen. Commonly occurring special cases, like two-level atoms, one-dimensional geometries (plane-parallel slab and infinite cylinder) etc. can also simulated. The program runs on workstations and on PCs.

Radiation trapping in a saturated atomic vapor in cylindrical geometries

We treat radiation trapping in a saturated atomic vapor in a cylindrical vapor cell. When saturation (i.e. stimulated emission) sets in, the Holstein equation of radiation trapping becomes nonlinear, and the absorption coefficient can become inhomogeneous. We first present a new method for the efficient computation of the reabsorption rate with an inhomogeneous distribution of absorbers in a cylinder. We then use this method as the basis of an algorithm for the computation of trapping in a saturated vapor.

Radiation trapping with partial frequency redistribution: Comparison of approximations and exact solutions

Abstract We treat radiation trapping with partial frequency redistribution. We solve the generalized Holstein equation by reducing it to an algebraic eigenvalue problem. In contrast to the complete frequency redistribution case, the eigensolutions depend not only on space but also on frequency. We compare results obtained with exact angle-averaged frequency redistribution functions to various commonly used approximations. For pure Doppler broadening, the assumption of CFR gives less than 10% error. For Doppler plus natural broadening, the well-known Jefferies-White approximation can lead to serious errors at high opacities.

Tunable single‐sideband generation in the infrared

Tunable single sidebands in the infrared were generated by electro‐optically mixing a 14–18‐GHz tunable microwave and a fixed‐frequency CO2 laser signal, both circularly polarized, in a resonant modulator. With 10 W microwave drive power, a peak single‐sideband conversion efficiency of 280 μW/W laser power was achieved. Sideband discrimination was 30 dB throughout the tuning range.