Jay M. Bernard

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.

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.

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.

Coupled Multiline CW HF Lasers: Experimental Performance

The experimental performance of two coupled arc-driven continuous wave HF chemical lasers employing unstable resonators has been measured. Mode-matched adjoint coupling of 20% was accomplished with a beamsplitter. Multiline interference fringes were obtained when the two laser output beams were overlapped in the near field. The mutual coherence of the two beams was inferred from measurement of the visibility of these fringes. Single-tine fringe visibility was measured as 98% +/—2% for each of the two strongest lines. These data suggest that the two outputs are nearly completely phase locked. This locking was found to be insensitive to perturbations in the lengths of the laser resonators. Mode-beating measurements also suggest complete phase locking, since the coupled system behaves tike a single laser cavity. The coherent combination of the two laser output beams was attempted by minimizing the optical path difference between these beams at an external measuring station. This was achieved by changing the external path length of one beam until maximum multiline visibility (97%) was obtained. Demonstration of the corresponding far-field peak intensity enhancement and reduction in spot size for the combined outputs was also accomplished.

Master oscillator with power amplifiers: performance of a two-element cw HF phased laser array

The experimental performance of a two-element phased array of multiline cw HF chemical lasers in the master oscillator with power amplifier (MOPA) configuration has been measured. The mutual coherence of the two amplified beams was inferred from measurements of the visibility of interference fringes obtained when the beams were overlapped in the near field. When the optical path difference for the two beams was minimized, multiline visibilities of 0.90 +/- 0.02 were measured. White light interferometry was used to equalize the optical path lengths. Spectral mismatch between the master oscillator output and the amplifiers preferred gain distribution affected neither the amplification factor nor the mutual coherence of the amplified beams. The effect of spatial coherence of the master oscillator beam on these near field measurements and the eventual requirement of far field measurements to precisely optimize path lengths are discussed. Spectra data, amplification factors, and mutual coherence measurements are shown, and the resulting phased array far field performance is presented.

Particle sizing in combustion systems using scattered laser light

Abstract This paper reviews light-scattering techniques for particle sizing, with emphasis on their use in combustion diagnostics. Submicron soot particles sizing in flames, using the diffusion-broadened spectrum of scattered light, is discussed in detail, beginning with the pioneering work of Penner and coworkers. Recent developments in the inversion of scattering data to yield soot number densities and size distributions are then reviewed. Finally, two imaging techniques for measuring particle sizes >1 μm are mentioned. One is holographic, and the other uses laser light emitted by the particle itself.

Reverse-Wave Suppressor Mirror Effects on CW HF Unstable Ring Laser Performance,

The effect of a reverse-wave suppressor (RWS) mirror on the performance of a multiline cw HF ring laser was measured and theoretically discussed. Forward-wave far-field brightness was found to be very sensitive to the tilt of a conventional RWS mirror, being reduced by a factor of 2 for a 200-microrad RWS tilt. An aberrated mirror was found to reduce this tilt sensitivity by nearly an order of magnitude. Also discussed is a possible mechanism whereby the tilt of the conventional RWS mirror reinforced a higher-order mode in the forward-wave output, yet the tilt of the aberrated mirror did not.

Reverse wave suppression in unstable ring resonator

The effectiveness of a reverse wave suppressor (RWS) mirror in an unstable ring resonator has been investigated theoretically and experimentally for the case of an inhomogeneously broadened gain medium. The theory indicates that the RWS mirror is effective when (δ/Δνh)2 ≪ 1, where Δνh is the characteristic homogeneous linewidth of the gain medium and δ = Δu(ν0/c) is a measure of the separation between competing forward and reverse waves. Unstable linear ring resonator experiments were conducted using a continuous wave HF laser. Successful suppression of the reverse wave was achieved. In these tests the ratio of forward to reverse power had an average value of 41. An unstable annular ring resonator was investigated using a pulsed CO2 laser. Reverse wave suppression was achieved when the resonator and RWS mirror were in good alignment. Suppression effectiveness and beam quality were degraded when the RWS mirror was tilted.

Continuous-wave HF selected-line integral master oscillator power amplifier

Efficient lasing on multiple-selected lines has been recently demonstrated experimentally on a cw hf chemical laser. Multiple-selected-line operation is required to both enhance atmospheric transmission and to ensure high-power extraction efficiency on multiple vibrational hf levels. Seventy-five percent of the multiline power was measured on two selected hf lines using a confocal, unstable cavity with a high-efficiency (97%) intercavity diffraction grating. This measured fraction of the multiline power is consistent with theoretical calculations, which include the effect of rotational nonequilibrium. The two-selected-line hf unstable cavity was not prone to parasitic oscillations. A novel multiple-selected-line integral master oscillator power amplifier (IMOPA) concept was also evaluated. Line selection on two hf lines was demonstrated with the IMOPA, although the hole diameter had to be made sufficiently large to prevent parasitic oscillations within the amplifier. It was concluded from our experiments and theoretical calculations that, although the IMOPA concept was demonstrated at relatively low power (400 W), parasitics may be a problem at much higher values of the single-pass gain.

Measurement of cw HF laser resonant degenerate four‐wave mixing in an absorbing HF gas

Continuous wave (cw) phase conjugation of a HF laser beam has been observed using resonant degenerate four‐wave mixing in a HF gas absorption cell. A HF laser was operated in the P1(8) transition to provide total (forward plus backward) pump beam and probe beam intensities of about 400 and 30 W/cm2, respectively, in the cell. Conjugate reflectivities were measured as a function of HF pressure and frequency detuning from line center. Reflectivities of the order of 10−4 were measured at HF gas pressures of the order of 2 Torr when the laser frequency was tuned near line center.