William J. Alford

Intracavity Frequency Doubling of a Diode-Pumped, External Cavity, Surface Emitting Semiconductor Laser

We present a compact, robust, solid-state blue-light (490-nm) source capable of greater than 5 mW of output in a TEM(00) mode. This device is an optically pumped, vertical external-cavity surface-emitting laser with an intracavity frequency-doubling crystal.

Measured laser parameters for reactor‐pumped He/Ar/Xe and Ar/Xe lasers

We have measured laser parameters for reactor pumping of He/Ar/Xe gas mixtures lasing predominantly at 2.03 μm and Ar/Xe mixtures lasing predominantly at 1.73 μm. Gains as high as ∼3%/cm have been measured in He/Ar/Xe at pump powers of ∼200 W/cm3 . Both systems exhibit small distributed losses. Intrinsic laser energy efficiencies as high as 2.4% (3.0%) have been observed for He/Ar/Xe (Ar/Xe). These efficiencies are the highest reported for reactor‐pumped lasers.

Amplitude and phase beam characterization using a two-dimensional wavefront sensor

We have developed a two-dimensional Shack-Hartman wavefront sensor that uses binary optic lenslet arrays to directly measure the wavefront slope (phase gradient) and amplitude of the laser beam. This sensor uses an array of lenslets that dissects the beam into a number of samples. The focal spot location of each of these lenslets (measured by a CCD camera) is related to the incoming wavefront slope over the lenslet. By integrating these measurements over the laser aperture, the wavefront or phase distribution can be determined. Since the power focused by each lenslet is also easily determined, this allows a complete measurement of the intensity and phase distribution of the laser beam. Furthermore, all the information is obtained in a single measurement. Knowing the complete scalar field of the beam allows the detailed prediction of the actual beams characteristics along its propagation path. In particular, the space-beamwidth product, M2, can be obtained in a single measurement. The intensity and phase information can be used in concert with information about other elements in the optical train to predict the beam size, shape, phase and other characteristics anywhere in the optical train. We present preliminary measurements of an Ar+ laser beam and associated M2 calculations.

Wavelength Variation of the Second-Order Nonlinear Coefficients of KNbO3, KTiOPO4, KTiOAsO4, LiNbO3, LiIO3, B-BaB2O4, KH2PO4, and LiB3O5 Crystals: A Test of Miller Wavelength Scaling

We measure second-order nonlinear coefficients using optical parametric amplification and second-harmonic generation over a range of wavelengths for the crystals KNbO3,KTiOPO4,KTiOAsO4,LiNbO3,LiIO3,β-BaB2O4,KH2PO4, and LiB3O5. Combining our new measurements with previously reported values, we compare the wavelength variation of individual dijk’s with Miller scaling, and we conclude that Miller scaling is a useful approximation for these crystals.

The effects of He addition on the performance of the fission‐fragment excited Ar/Xe atomic xenon laser

The intrinsic power efficiency of the atomic xenon laser depends upon the electron density because of the mixing of the laser levels by electron collisions while the electron density in high‐pressure particle‐beam excited plasmas increases with increasing gas temperature. Therefore, in order to reduce the amount of electron collisional mixing when operating at high‐energy loadings (≳100’s J/1‐atm) mixtures having a high‐heat capacity are required. In particle‐beam excited Ar/Xe mixtures, which typically yield the highest intrinsic laser efficiencies, increasing the gas pressure to increase the heat capacity is not always practical due to the high‐stopping power of the gas mixture. For this reason we have experimentally and theoretically investigated adding He to Ar/Xe mixtures in studies of a fission‐fragment excited atomic xenon laser. Adding He increases the heat capacity without appreciably perturbing the favorable kinetics resulting in efficient operation of the laser in Ar/Xe mixtures. We find that w...

Absolute measurement of the effective nonlinearities of KTP and BBO crystals by optical parametric amplification

Absolute magnitudes of the effective nonlinearity, deff, were measured for seven KTP and six BBO crystals. The d(eff), were derived from the parametric gain of an 800-nm signal wave in the sample crystals when they were pumped by the frequency-doubled, spatially filtered light from an injectionseeded, Q-switched Nd:YAG laser. The KTP crystals, all type II phase matched with propagation in the X-Z plane, had d(eff) values ranging from 1.97 to 3.50 pm/V. Measurements of gain as a function of phase velocity mismatch indicate that two of the KTP crystals clearly contain multiple ferroelectric domains. For five type I phase-matched BBO crystals, d(eff) ranged from 1.76 to 1.83 pm/V, and a single type II phase-matched BBO crystal had a d(eff) of 1.56 pm/V. The uncertainty in our measurements of d(eff) values is ±5% for KTP and ±10% for BBO.

Frequency shifts in injection-seeded optical parametric oscillators with phase mismatch.

We have observed a frequency shift in the output signal pulses relative to the seed frequency in an injection-seeded, singly resonant, critically phase-matched, pulsed optical parametric oscillator in which phase mismatchwas intentionally introduced. The observed shifts can be large compared with the linewidth of the signal pulse, are approximately linear in phase mismatch, and increase with increasing pump fluence. We observe frequencyshifts of as much as +/-400 MHz for our 532-nm-pumped, potassium titanyl phosphate ring optical parametric oscillator. For zero phase mismatch, we observe nearly transform-limited linewidths of less than 130 MHz. Wecompare the experimental data to a simple analytic model that overestimates the shifts because it ignores pumpdepletion. We also compare our measurements with a numerical model that calculates the two-dimensional, transient electric fields and the resultant spectral distributions while explicitly including walk-off, diffraction, and pump depletion. We find good agreement between the experimental data and the results of this model.

Frequency-doubling broadband light in multiple crystals

We compare frequency doubling of broadband light in a single nonlinear crystal with doubling in five crystals with intercrystal temporal walk-off compensation and with doubling in five crystals adjusted for offset phase-matching frequencies. Using a plane-wave dispersive numerical model of frequency doubling, we study the bandwidth of the second harmonic and the conversion efficiency as functions of crystal length and fundamental irradiance. For low irradiance, the offset phase-matching arrangement has lower efficiency than a single crystal of the same total length but gives a broader second-harmonic bandwidth. The walk-off-compensated arrangement gives both higher conversion efficiency and broader bandwidth than a single crystal. At high irradiance, both multicrystal arrangements improve on the single-crystal efficiency while maintaining a broad bandwidth.

Beam tilt and angular dispersion in broad-bandwidth, nanosecond optical parametric oscillators

We show that the signal and idler beams generated by certain types of unseeded, nanosecond optical parametric oscillators are tilted and angularly dispersed and have anomalously large bandwidths. This effect is demonstrated in both laboratory measurements and a numerical model. We show how the optical cavity design influences the tilts and how they can be eliminated or minimized. We also determine the conditions necessary to injection seed these parametric oscillators.

Application Of High-Speed Photography To Time-Resolved Wavefront Measurement

For long-pulse (150 microseconds), side-pumped lasers, such as dye or reactor-pumped lasers, a significant transverse index gradient can develop. In order to design an appropriate laser resonator for efficient power extraction, a detailed space- and time-resolved determination of the index perturbation is required. We have applied high-speed photography to the measurement of wavefront errors in a pulsed laser gain region using a fast framing camera. We utilized both a derivative of a Hartmann test technique and a shearing interferometer, with good agreement between the two. The measured wavefronts have a predominantly parabolic index shape except near the walls, with a roughly linear time dependence during the pumping pulse.