Remote sensing requires efficient lasers that are tunable over a short wavelength range around a particular atmospheric absorption feature of interest. High efficiency usually implies lanthanide series lasers. Although lanthanide series lasers have sufficient tuning capability, they must operate at preselected atmospheric absorption features. Often, there is no commonly available laser that operates at the requisite wavelength. This type of problem can be addressed using compositional tuning to create a laser at a preselected wavelength where none existed before. Quantum mechanics is an invaluable tool to predict the effects of compositional tuning. Quantum mechanical predictions are confirmed with spectroscopic measurements. Laser performance data for a laser that operates at 0.9441 μm, a preselected water vapor absorption feature, are featured.
With the goal of obtaining high energy at 946 nm,
we have built and characterized a power amplifier that
will be used in conjunction with our Three Module Oscillator
and PreAmplifier to further increase the available laser energy.
Previously we obtained >75 mJ Q-switched energy in TEM00
mode at 946 nm in an all solid state diode pumped system
using Nd:YAG.1 This transition is extremely useful to the
remote sensing community, considering the presence of strong
atmospheric water vapor absorption lines in this region. In
order to obtain the higher energies that are necessary for
such measurements, we have built a power amplifier. Gain
was measured for varying input probe energies and the
experimental results compared well with model predictions.
An innovative approach to obtaining high energy at 946 nm has yielded 101 mJ of laser energy with an optical-to-optical slope efficiency of 24.5%. A single gain module resonator was evaluated, yielding a maximum output energy of 50 mJ. In order to obtain higher energy a second gain module was incorporated into the resonator. This innovative approach produced unsurpassed output energy of 101 mJ. This is of utmost importance since it demonstrates that the laser output energy scales directly with the number of gain modules. Therefore, higher energies can be realized by simply increasing the number of gain modules within the laser oscillator. The laser resonator incorporates two gain modules into a folded “M-shaped” resonator, allowing a quadruple pass gain within each rod. Each of these modules consists of a diode (stack of 30 microlensed 100 Watt diode array bars, each with its own fiber lens) end-pumping a Nd:YAG laser rod. The diode output is collected by a lens duct, which focuses the energy into a 2 mm diameter flat to flat octagonal pump area of the laser crystal. Special coatings have been developed to mitigate energy storage problems, including parasitic lasing and amplified spontaneous emission (ASE), and encourage the resonator to operate at the lower gain transition at 946 nm.
We present new femtosecond degenerate four-wave (DFWM) measurements on a C70 film in the wavelength range 0.9- 1.6 micrometers in conjunction with previously measured data in C60 and C70 in the ranges 0.74-1.7 (mu) and 0.74-0.88 micrometers , respectively. The new data reveal tow closely spaced peaks in the DFWM spectrum which we interpret as two-photon states with excitation energies of 2.41 +/- 0.05 eV and 2.64 +/- 0.03 eV. We relate these results to nonlinear optical spectra obtained by others in C60 and C70 films. In particular, we compare third-harmonic generation, electro-absorption and DFWM and emphasize the relative advantages of DFWM for two-photon spectroscopy.
We have extended the wavelength range of our previous study of the third order nonlinear optical susceptibility tensor (chi) (3)(-(omega) , (omega) , (omega) , -(omega) ) of a thin C60 film to 1.7 micrometers . We use time-resolved degenerate four-wave-mixing with femtosecond pulses to measure both the phase and magnitude of (chi) 1111. Our data are well defined in terms of a single two-photon resonance at a fundamental wavelength of 930 nm. From our fit parameters, we predict for (chi) 1111 in the zero-frequency limit a value of (9 +/- 3) X 10-13 esu and at the resonance maximum a value of i(3.9 +/- 0.6) X 10-12 esu.
We use time resolved degenerate four-wave-mixing with femtosecond pulses to measure magnitude, phase, and dispersion of all nonzero components of the third order nonlinear optical susceptibility tensor (chi) (3)(-(omega) ; (omega) , (omega) , -(omega) ) of a polycrystalline C70 film. Rise and fall times of the nonlinearities measured are short compared to the (112 plus or minus 5 fs) pulses employed. Accordingly, the cw symmetry relation (chi 1111) equals 2 (chi 1212) plus (chi 1221) is experimentally found to be satisfied. The magnitude of (chi 1221) is measured to be (5.04 plus or minus 0.19) 10-13 esu relative to fused silica independent of wavelength. The ratio (chi 1212)/(chi 1221) is wavelength dependent and varies between 1.87 plus or minus 0.12 and 1.44 plus or minus 0.09. The magnitudes of phase angles for (chi 1111) and (chi 1212) are (120 plus or minus 22) degree(s) and (105 plus or minus 21) degree(s), respectively. The intensity dependence of the observed signals is cubic for intensities up to 20 GW/cm2 at all wavelengths. Good agreement between data derived from degenerate four-wave-mixing and third-harmonic generation in C70 as well as in C60 films is found.