Richard A. Chodzko

Annular (HSURIA) resonators: some experimental studies including polarization effects

A repetitively pulsed CO(2) laser facility was developed for testing annular resonators. The large-aperture device exhibits generally uniform gain over an annular region of 18-cm o.d. and 10-cm i.d. The half-symmetric unstable resonator with internal axicon (HSURIA) was tested at equivalent Fresnel numbers up to 4.5. This resonator design incorporates a W-axicon mirror beam compactor that transforms a cylindricalmode region into an annular-mode region. Two HSURIA configurations were evaluated: (a) with a conical end mirror and (b) with a flat end mirror in the annular leg. With the conical end mirror, the aligned resonator produced a predominantly higher-order azimuthal mode with an on-axis null in the far field. The output was strongly linearly polarized with the electric-field vector tangential to the optic axis in both the near and far fields. The higher-order tangentially polarized mode appears to be the result of a geometric polarization scrambling effect caused by the conical end mirror. The boundary conitions for the conical or W-axicon mirrors imply that the radial electric field has a 180 degrees phase shift on reflection, whereas the tangential component is unchanged. Thus, a tangentially polarized mode is self-reproducing, but a linearly polarized mode is not. To eliminate the polarization scrambling effect in the HSURIA, the conical end mirror was replaced with a flat end mirror. The HSURIA with a flat end mirror produced a central spot in the far field that indicated an l = 0 mode with no spatial variations in polarization. Beam quality was measured in terms of the ratio n(2) of the theoretical (geometric-mode) power transmitted through an aperture of the central lobe diameter to the observed power; n(2) values as low as 1.2 were obtained. The variation of beam quality with tilt of the flat end mirror indicated a factor of 2 degradation in n(2) for a 20-microrad tilt, which is in good agreement with theory.

Annular converging wave cavity.

A new type of resonator that generates an annular geometric mode by use of spherical mirrors has been developed. The four-element cavity consists of an external confocal unstable resonator with a double-sided 45 degrees coupling mirror and a flat feedback mirror. The flat feedback mirror is placed on the plane wave side of the confocal cavity to form an annular mode between the feedback mirror and the coupling mirror. A plane annular wave (matched to an annular gain medium) is fed back into the unstable resonator that alternately converges and diverges to produce a diverging annular output beam. Experiments were performed on a cw HF laser. Observations of mode patterns on the flat feedback mirror and the convex mirror and of far-field beam quality were made. Far-field measurements indicated near-diffraction-limited beam quality for a peak on-axis intensity mode on the convex mirror. A nearly uniform annular mode was observed on the flat feedback mirror.

Multiple-selected-line unstable resonator

A new type of unstable resonator has been developed that permits multiple-selected-line operation. Selected-line operation is obtained with a diffraction grating that is oriented at an angle that is different from the Littrow angle. Secondary feedback mirrors at the appropriate angles provide an independent threeelement cavity for each wavelength selected. The off-Littrow configuration is less efficient than a singleline cavity at the Littrow angle because there are secondary zero-order beams at each wavelength as well as a primary zero-order beam at all selected wavelengths. In the limit of a high gain this loss in efficiency is shown to be significantly reduced. Two types of multiple-selected-line unstable resonators are considered: an edge-coupled design, which produces an annular beam with a 45 degrees output coupling mirror, and a continuously coupled design, which uses the primary zero-order beam as output. Experiments have been carried out with a multiple-selected-line unstable resonator on a cw HF laser. Selected-line operation was obtained on the P(2) (5) and P(1) (6) HF transitions in a continuously coupled design.

Zero-power gain measurements in a CW HF laser using a pulsed-probe laser

Zero-power gain measurements in a supersonic diffusion-type CW HF laser have been made using a pulsed HF probe laser. A peak zero-power gain of 10 percent/cm was measured on a P_{2}(3) vibrational-rotational transition and 7 percent/cm on a P_{1}(6) transition for flow rates corresponding to 1.8 kW of closed-cavity power. The variation of zero-power gain with rotational quantum number J fits a simple rotational equilibrium model indicating increasing rotational temperatures with increasing distance x from the nozzle exit plane. The model also indicates a total inversion for x \leq 0.2 in for the P_{2}(J) transitions and a partial inversion further downstream.

Zero-power-gain measurements in CW HF(DF) laser by means of a fast scan technique

An improved technique for measuring zero power gain in a CW HF-DF chemical laser has been developed in which a CW HF(DF) single-line, frequency-stabilized TEM00mode probe laser is used. Through use of a flat rotating mirror and focusing elements, the streamwise distribution of zero-power gain was scanned at a rate of ∼1 mm/μs with a spatial resolution of ∼1 mm. The zero-power-gain profile was observed for two arc-driven chemical-laser nozzles with both HF and DF active species. The nozzles consisted of a 36-slit array with perforated tube H2injectors and a 55-slit array with uniform H2injection. Results are presented for the variation of zero power gain with axial distance, the magnitude and location of peak gain, and the gain cutoff location for a number of transitions. Peak gain values of 15 percent/cm and 5.5 percent/cm were measured with HF and DF active species, respectively, with the 55-slit nozzle array.

Coherent combination of multiline lasers

Coupled unstable resonators and phase-matched amplifiers with a master oscillator (MOPA) are studied in the framework of coherent beam combination. Coherent combination is accomplished on multiline CW HF lasers by both approaches. It is shown that the MOPA approach requires a master oscillator with good beam quality in order to diagnose the mutual coherence of the two amplified beams and to demonstrate the narrowing of the far-field central lobe. The coupled-unstable-resonators approach is found to be more difficult due to the coupling-path alignment requirement. Attention is focused on a procedure for equalizing external paths for optimum far-field brightness of multiline phased arrays.

Semimonolithic Nd:YAG ring resonator for generating cw single-frequency output at 1.06 μm

We report 2.1 W of unidirectional single-frequency laser output from a novel Nd:YAG ring laser longitudinally pumped by a laser diode bar. The resonator requires only a single discrete optic and magnet, in addition to the laser medium, for stable multiwatt single-frequency operation. Undoped YAG is diffusion bonded to the Nd:YAG laser crystal at the pump face to eliminate residual spatial hole burning and improve thermal conductivity. This new laser design can be scaled to higher powers through more facile diode access and is useful for cw intracavity doubling.

Experiments with active phase matching of parallel-amplified Multiline HF laser beams by a phase-locked Mach-Zehnder interferometer

Active phase matching of multiline HF laser beams by means of a phase-locked Mach-Zehnder interferometer was demonstrated by locking the interferometer to the central interference fringe at zero optical path length difference. The central fringe could be found by varying the spectral content of the input beam. Laser amplification in one leg of the interferometer decreased fringe visibility without adversely affecting locking. Single-line fringe patterns produced by an array spectrometer (while the interferometer was operated in its scanning mode) were analyzed to show that no significant dispersion occurred in the amplifier. The techniques developed have potential for measuring dispersion mismatch between larger parallel amplifiers. These experiments demonstrated in principle that a number of multiline HF amplified beams can be recombined and phase-matched to produce a high beam quality output beam.

Measurements of the anomalous dispersion of HF in absorption

We report quantitative measurements of the anomalous refractive index of the P_{1}(6),P_{1}(7) , and P_{1}(8) vibration-rotational transitions of hydrogen fluoride in absorption. The experimental technique uses the displacement of spatial fringes produced by a Mach-Zehnder interferometer containing the absorption cell. A frequency scanned, CW HF probe laser served as the light source and the spectrally selective element. In addition, we measured the absorption coefficient of the three transitions. The experimental results are in good agreement with calculated values derived from the transition-matrix theory of HF.

Mutual coherence of two coupled multiline continuous-wave HF lasers

Two multiline cw HF lasers employing unstable resonators were coupled by injecting 20% of each laser output into the other laser. The mutual coherence of the two output beams was measured by recording the visibility V of the interference fringes generated when the beams are overlapped. Both single-line and multiline interference patterns were observed. The output from the two lasers in the present experiment (20% coupling) was completely coupled, indicating that the achievement of stable coupling is not dependent on careful adjustment of the length of each laser resonator.