Arthur N. Chester

Visible and uv Laser Oscillation at 118 Wavelengths in Ionized Neon, Argon, Krypton, Xenon, Oxygen, and Other Gases

Laser oscillation has been observed at one hundred and eighteen wavelengths in ionized neon, argon, krypton, xenon, oxygen, and other gases in the spectral range 2677 A to 7993 A. Of these lines, ninety-six have been definitely identified, and arise from singly, doubly, and triply ionized atoms. A 2-m, pulsed dc discharge was employed. Measured and calculated wavelengths and level classifications are tabulated. The majority of the laser lines observed are shown by comparison with calculated relative line strengths to be the strong lines predicted by L-S coupling that possess lower levels optically connected to the ion ground state. The rules ΔS = 0, ΔJ = ΔL = +1 are reasonably well obeyed, although violations of L-S restrictions on core change and multiplicity are also observed. Evidence of upper level population by electron collision with ground-state ions is presented. The time dependence of laser output under pulsed excitation is discussed.

Iterative Diffraction Calculations of Transverse Mode Distributions in Confocal Unstable Laser Resonators

A theoretical investigation has been undertaken to study the transverse modes of two-dimensional positive branch, confocal unstable resonators. Mode amplitude and phase information is obtained from a numerical-iterative type calculation that uses the Fresnel integral for propagating the cavity radiation back and forth between resonator mirrors. Near- and far-field distributions for empty cavity resonators are presented for various resonator Fresnel numbers and magnifications, along with results of resonator mode stability and diffraction losses when cavity perturbations such as mirror misalignment and/or a uniformly saturable gain medium are included. In addition, the diffractive calculations are compared with results obtained from geometric models.

Resonator theory for hollow waveguide lasers.

A numerical technique has been developed for analyzing the transverse modes of waveguide lasers with external mirrors. Propagation outside the guide is computed with the Fresnel-Kirchhoff diffraction integral and within the guide by decomposing the fields into the characteristic modes of the guide structure. The transverse modes of the entire waveguide-mirror system fall into a number of distinct classes: TE(0m), TM(0m), EH(1m), EH(2m), etc. For each class of modes, the, corresponding guide modes form a complete and orthogonal set and may be used as basis vectors to describe those modes. This reduces the mode analysis of the waveguide resonator to the diagonalization of a small (5 x 5 or 10 x 10) complex matrix. Guide losses, coupling losses, and mode shapes will be discussed for a number of interesting cases, with the Fresnel number of the waveguide ranging from 0.1 to 1.0 and with various values of mirror curvature and position. It will be shown that some vales of resonator parameters are particularly advantageous for achieving single mode operation.

Mode losses in hollow‐waveguide lasers

We have developed a theory to solve for the lowest‐order mode of hollow dielectric laser resonators with external mirrors. The results agree well with previously reported experimental results. In general, the optimum position (or positions) for a mirror of fixed curvature can be calculated, but as yet no simple rule has been found for predicting these positions.

Mode Selectivity and Mirror Misalignment Effects in Unstable Laser Resonators

It is known that single transverse mode operation of an unstable laser resonator can be achieved using large output coupling and large Fresnel number operation. However, it has not been recognized that there is an upper limit on usable Fresnel number, determined by such effects as mirror alignment stability. We have analyzed a variable-reflectivity unstable resonator using mode solutions obtained by Zucker. The differential loss between the lowest order modes leads to conditions on the Fresnel number and the output coupling if single mode operation is to be realized. The Zucker solution is extended by a perturbation analysis to examine mode distortion caused by mirror misalignment. The on-axis intensity in the far field of the laser output is shown to be a sensitive function of mirror misalignment, and the mirror alignment tolerance permitted varies inversely as the Fresnel number. In practice, this places an upper limit on the usable Fresnel number. The results of this theory are shown to be consistent with laser experiments using unstable resonators.

Study of the HF chemical laser by pulse-delay measurements

A simple analytical model is presented to describe the delay τ c between flashlamp initiation and laser-pulse onset in the flash-photolysis HF chemical laser. The predictions of the model agree well with experiments concerning the functional dependence of τ c on pressure, flashlamp intensity, optical cavity loss, and the absolute magnitude of τ c . This detailed agreement between theory and laser performance has not been achieved in previous chemical laser work. The pulse-delay measurements demonstrate the presence of complete vibrational inversion at the time of laser-pulse initiation. In the Appendixes, the fractional photodissociation of MoF 6 is calculated for the experimental geometry used, the effects of flashlamp heating are considered, and a general expression for the temperature dependence of HF laser gain is derived and plotted. A complete bibliography of laser transitions in HF and DF is included.

Gain thresholds for diffuse parasitic laser modes.

High power, high gain lasers are vulnerable to parasitic oscillations, which are unwanted laser modes that rely on stray wall reflections for their optical feedback. Since parasitics reduce power in the intended laser mode and can cause thermal damage to structural hardware, it is important to know the gain threshold required to support parasitics in various laser medium geometries. When the walls are diffusely reflecting, as in many high power laser devices, a diffuse parasitic mode that fills essentially the entire gain medium can form. A differential equation is derived for the radiant intensity of a diffuse parasitic in the absence of diffraction. An expansion in angular functions leads to a Helmholtz-like equation and suitable boundary conditions at the walls. These equations are solved to give the radiant intensity within a rectangular laser region with specified reflectivities at the bounding walls. Limiting cases are discussed and numerical results are given for the diffuse parasitic gain threshold in rectangular gain regions bounded by two, four, or six partially reflecting walls. A computer code is provided for calculating the gain threshold for other combinations of wall reflectivities.

Three-Dimensional Diffraction Calculations of Laser Resonator Modes

The numerical technique of Fox and Li for computing laser resonator modes is applied to the case of three-dimensional laser resonators without circular or rectangular symmetries. The computation techniques are explained, and results are presented for several specific resonators, both stable and unstable. The effect of laser medium shock waves on the refractive index of the optical cavity is approximated by a thin sheet near one resonator mirror. The near-field burn pattern of the laser output beam exactly follows the phase pattern of the shock fronts, in good qualitative agreement with experimental results reported for gas dynamic lasers. The far-field output beam demonstrates pronounced astigmatism, being considerably broadened at right angles to the flow direction, and it suggests a breakup of the far-field pattern into several separate intensity spots. The optical phase of the resonator mode is quite smooth, even in the worst cases studied, suggesting the possibility of phase compensation by suitable optics.

“Complete” and “Partial” Vibrational Inversion in Chemically Pumped Molecular Lasers

A generalized inversion density is defined and applied to a molecular laser having simultaneous inversion on many different vibrational–rotational transitions. This inversion density is shown to be a measure of the maximum quantum efficiency and the molar flow efficiency of a flowing chemical laser. Calculations are carried out using previously measured vibrational population distributions, and the limiting performance of completely inverted and partially inverted molecular lasers is compared quantitatively.

1980 Morris N. Liebmann Award

The IEEE 1980 Morris N. Liebmann Award will be presented to Dr. Anthony J. DeMaria at the 1980 Conference on Laser and Electro-Optical Systems in San Diego, CA, USA. Dr. DeMaria is cited for his contributions to the initiatidn and demonstration of the first picosecond optical pulse generator. A professiona biography of Dr DeMaria is provided.