Richard B. Kay

Effects of thermalization on Q-switched laser properties

The conventional rate equations for a Q-switched laser are augmented to explicitly include the effects of time- and level-dependent pumping, thermalization among the sublevels in the upper and lower multiplets, and multiplet relaxation in a homogeneously broadened four-level laser medium. To make the numerical computations more generally valid, we introduce a number of dimensionless variables. We show that the initial set of five coupled differential equations can be reduced to a simple set of two coupled equations for the inversion density and photon flux. Via numerical modeling, we have investigated the manner in which both thermalization and lower multiplet relaxation affect Nd:YAG laser characteristics such as output energy and temporal waveform. Our numerical results confirm earlier predictions that the Q-switched Nd:YAG laser output energy increases monotonically by a factor of 3.33 as one progresses from the assumption of slow to rapid thermalization and by an additional factor of 1.46 if one further assumes a terminal multiplet relaxation which is fast relative to the resonator photon decay time. We also find that the laser pulsewidth is substantially broadened when the resonator photon decay time is comparable to the thermalization, and to a lesser extent, the terminal multiplet relaxation times.

Numerical model estimating the capabilities and limitations of the fast Fourier transform technique in absolute interferometry.

Anumerical model was developed to emulate the capabilities of systems performing noncontact absolute distance measurements. The model incorporates known methods to minimize signal processing and digital sampling errors and evaluates the accuracy limitations imposed by spectral peak isolation by using Harming, Blackman, and Gaussian windows in the fast Fourier transform technique. We applied this model to the specific case of measuring the relative lengths of a compound Michelson interferometer. By processing computer-simulated data through our model, we project the ultimate precision for ideal data, and data containing AM-FM noise. The precision is shown to be limited by nonlinearities in the laser scan.

Stimulated Raman scattering in hydrogen pumped with a tunable, high power, narrow linewidth alexandrite laser

The conversion efficiencies and the linewidth of the Stokes components which result from stimulated Raman scattering (SRS) in hydrogen gas pumped with a high power alexandrite laser have been studied. The pump laser was operated at 742 nm giving first and second Stokes components at 1074 nm and 1935 nm respectively. The pump laser could be operated either in the normal Q-switched configuration with a linewidth of the order of 0.2-0.3 nm or with injection seeding which narrowed the linewidth to 1-2 pm. Measurements of the Stokes components under these varying conditions reveal that there is no effect on the conversion efficiency by the narrowing of the pump linewidth and that the linewidth of the first Stokes component is broader than the expected linewidth of the injection seeded pump. Modelling of the conversion from the pump to the Stokes components shows a strong dependence of this conversion on a specific, resonant four wave mixing process. The study gives a complete set of information about the radiation resulting from SRS in molecular hydrogen pumped with a high power, narrow linewidth, tunable solid-state laser.

Laser transmitter development for NASA's Global Ecosystem Dynamics Investigation (GEDI) lidar

The Global Ecosystems Dynamics Investigation (GEDI) Lidar, to be installed aboard the International Space Station in early 2018, will use 3 NASA laser transmitters to produce 14 parallel tracks of 25 m footprints on the Earths surface. A global set of systematic canopy measurements will be derived, the most important of which are vegetation canopy top heights and the vertical distribution of canopy structure. Every digitized laser pulse waveform will provide 3-D biomass information for the duration of the mission. A total of 5 GEDI-HOMER lasers are to be built (1 ETU + 3 Flight + 1 spare) in-house at NASA-GSFC, and is based on a well-studied architecture, developed over several years in the Lasers and Electro-Optics Branch.

Lifetest of the High Output Maximum Efficiency Resonator (HOMER) laser for the SAFFIRE instrument on NASA's DESDynI project

We update the status of a diode-pumped, Nd:YAG oscillator that is the prototype laser for NASAs DESDynl mission. After completing TRL-6 testing, this laser has fired over 5.5 billion shots in lifetesting.

Q-switched rate equations for diode side-pumped slab and zigzag slab lasers including Gaussian beam shapes

A set of rate equations accounting for the transverse spatial distributions of both the laser mode and inversion density is developed for side-pumped Q-switched oscillators. The equations can be reduced to a simple form by assuming a Gaussian laser mode, a one-dimensional Gaussian pump beam and a few additional basic assumptions, allowing for rapid numerical solution. The derived spatial rate equations are then used to model an experimentally well-characterized Nd/sup +3/:YAG zigzag slab laser to demonstrate their simplicity and accuracy in predicting oscillator performance.

Injection-seeded diode-pumped regenerative ring Nd:YAG amplifier for spaceborne laser ranging

We report on a small, all solid state, regenerative ring amplifier designed as a prototype for space application. Features include dual side pumping of a 2 mm*2 mm*10 mm Nd:YAG slab and a triangular ring cavity design, which minimizes the number of optical components. The pump head was fitted with twin single-bar diode arrays consisting of 960 individual stripes, temperature tuned to the Nd:YAG pump bands at 809 nm and capable of operation up to 100 Hz. The single-pass gain of this head was measured at 13% for a pump pulse energy of 18.4 mJ from the diode arrays and the cavity round trip transit time was 3.1 ns. The ring has amplified 170 ps pulses with 10-13-10-14 J of energy by a factor of more than about 106 while preserving both the temporal and spatial shape of the pulses. The seeding threshold was about 10-15 J and a maximum saturated output energy of about 40 mu J was obtained when seeded with pulses greater than 1 pJ. Comparison with modelled performance is also given.

Multiphoton interactions in rutile

Laser-induced photoluminescence and photoconductivity in rutile crystal are described. Nd:YAG laser photons were employed which have energies of 1.16 eV (1.06 μm). This energy is just slightly more than ⅓ of the TiO2 band gap. Intensities between 105 and 106 W/cm2 were employed. The photoconductivity is shown to be produced by competing single- and two-photon events. The first-order and second-order photoconductivity cross sections are found to be 3.6 × 10−26 cm2 and 1.54 × 10−50 cm4 sec, respectively. A thermoluminescence study revealed traps with thermal ionization energies between 0.4 and 0.9 eV below the conduction band. Traps with photoionization energies ~1 and ~2 eV below the conduction band are believed to be responsible for the observed results and may or may not be due to alumina impurities known to exist in the samples.

Derivation of the Frantz-Nodvik Equation for Diode-Side-Pumped Zigzag Slab Laser Amplifiers With Gaussian Laser Mode and Pump Beam Shapes

The theory for Gaussian beam pulse propagation in a zigzag slab amplifier with a Gaussian pump distribution is detailed. Provisions are made for amplification of an input signal with an elliptical Gaussian spatial mode by modifying the time-dependent photon transport equations as described by Eggleston Frantz, and Injeyan. A comparison is made with experimental results of a diode-side-pumped zigzag slab amplifier element amplifying a near-Gaussian beam.

100 mJ Q-switched, high efficiency, high brightness Nd:YAG oscillator: Wavefront analysis

A diode-pumped, 100 mJ/pulse, Nd:YAG oscillator employing a dual pump head design aligned orthogonally in an unstable resonator configuration producing aperture-free TEM00 beams is presented.