A tunable optically pumped HBr laser has been demonstrated for the first time. As pump source for the HBr oscillator,
we developed a single-frequency Ho:YLF laser- amplifier system which was locked to the 2064 nm absorption line of
HBr. Through the implementation of an intra-cavity diffraction grating, laser oscillation was demonstrated on nineteen
molecular transition lines including both the R-branch (3870 nm to 4015 nm) and the P-branch (4070 nm to 4453 nm).
The highest output energy for the given input energy was 2.4 mJ at 4133 nm.
Simulating coherent control with femtosecond pulses on a polyatomic molecule with anharmonic splitting was
demonstrated. The simulation mimicked pulse shaping of a Spatial Light Modulator (SLM) and the interaction was
described with the Von Neumann equation. A transform limited pulse with a fluence of 600 J/m2 produced 18% of the
population in an arbitrarily chosen upper vibrational state, n =2. Phase only and amplitude only shaped pulse produced
optimum values of 60% and 40% respectively, of the population in the vibrational state, n=2, after interaction with the
ultra short pulse. The combination of phase and amplitude shaping produced the best results, 80% of the population was
in the targeted vibrational state, n=2, after interaction. These simulations were carried out with all the population initially
in the ground vibrational level. It was found that even at room temperatures (300 Kelvin) that the population in the
selected level is comparable with the case where all population is initially in the ground vibrational state. With a 10%
noise added to the amplitude and phase masks, selective excitation of the targeted vibrational state is still possible.
In this paper we present results on improved paint stripping performance with an intra-cavity generated Flattened
Gaussian Beam (FGB). A resonator with suitable diffractive optical elements was designed in order to produce a single
mode flat-top like laser beam as the output. The design was implemented in a TEA CO2 laser outputting more than 5 J
per pulse in the desired mode. The FGB showed improved performance in a paint stripping application due to its
uniformity of intensity, and high energy extraction from the cavity.
In this paper we present the design of a CO2 laser resonator that produces as the stable transverse mode a super-Gaussian
laser beam. The resonator makes use of an intra-cavity diffractive mirror and a flat output coupler, generating the
desired intensity profile at the output coupler with a flat wavefront. We consider the modal build-up in such a resonator
and show that such a resonator mode has the ability to extract more energy from the cavity that a standard cavity single
mode beam (e.g., Gaussian mode cavity). We demonstrate the design experimentally on a high average power TEA CO2
laser for paint stripping applications.
Results are presented on the influence of acoustic waves on the performance of high-repetition-rate TEA CO2 lasers. It is shown that acoustic waves generated inside the laser cavity lead to nonuniform discharges, resulting in a deterioration of the laser beam quality, decreased output energy, and an increase in pulse-to-pulse energy variation. The effect of the gas mix on the acoustic behavior is investigated, and experimental results on laser performance across a range of gas mixtures are presented. Methods to reduce the effects of acoustic waves are presented together with experimental results. The influence of acoustic damping measures on laser gain are demonstrated, showing a significant improvement in gain and output power at high pulse repetition rates.
High pressure CO2 lasers are good candidates for amplifying picosecond mid infrared pulses. High pressure CO2 lasers are notorious for being unreliable and difficult to operate. In this paper a high pressure CO2 laser is presented based on well developed LC-inversion type excitation circuit. The laser contains internal room temperature catalysts allowing closed loop or rare isotope operation. The laser was designed for 300Hz operation and could achieve this for short time periods and could be operated at up to 200Hz for extended time periods. Some of the design features and experimental results are presented in this paper.
A high power 1kW pulsed transversely excited atmospheric CO2 laser that has been developed for the paint stripping of missiles was used to test paint stripping on several metallic and composite aircraft panels to determine the rate at which
this laser could remove paint from aircraft.
In this paper we present results on the influence of acoustic waves on the output laser beam from high repetition rate TEA CO2 lasers. We show that acoustic waves generated inside the cavity lead to deterioration in beam quality, decreased output energy, and an increase in pulse to pulse energy variation. We investigate the impact of gas mix on the acoustic behaviour, and present experimental results on laser performance across a range of gas mixtures. Solutions to the acoustic wave problem are presented together with experimental results. The influence of acoustic damping measures on laser gain are demonstrated showing a significant improvement in gain and output power at high repetition rates. The link between the pre-ionisation method employed and the acoustic wave impact on laser performance is discussed.
Laser ultrasonics is currently the optimal method for non-destructive testing of composite materials in the aerospace industry. The process is based on a laser-generated ultrasound wave which propagates inside the composite. The response at the material surface is detected and converted into a defect map across the aircraft. The design and optimisation of a laser system for this application is reviewed in this paper, together with the basic science involved. This includes the optimisation of laser parameters, such as output couplers and gas mixture, and the impact these choices have on the laser chemistry. We present a theory for the catalytic recombination of the gas which shows excellent agreement with experiment. Finally, an operating laser system for this application, yielding a six-fold improvement in performance over conventional laser systems, is described.
A model is developed for the breakdown and regeneration of component gases in an industrialised TEA CO2 laser, both with and without internal catalysts, and is found to be in excellent agreement with experimental data. The laser was found to be stable at O2 levels in excess of 2%, whereas previously reported values suggest stable operation at values of less than 1%. This is thought to be related to the unusually high starting CO2 concentration of the gas mix, and the short time pulse of the laser ouput. Long term catalytic behaviour however shows a decay in the catalyst activity, corresponding to higher energy variation and lower average power.
In the multi-photon dissociation process of Carbon isotope enrichment, IR photons are used to selectively excite a molecule with the given isotopic base element. This enrichment process is very sensitive to the beam's intensity and wavelength. Because the intensity is determined by the propagation of the field, the enrichment factors are also very dependent on the field propagation. In this paper, the influence of the wavelength and intensity of the beam, on the isotope selective dissociation of a CFC compound is investigated both experimentally and theoretically. Consideration is also given to some of the factors that influence the delivery of various beams to the reactor chamber, and their subsequent propagation through the reactor. The results show that suitable beam forming can lead to an improved isotope separation process.
Laser resonators are very sensitive to intra-cavity losses and instabilities. When high power extraction is required, thermal distortions introduced from pumping the active medium cause the resonator to become unstable. We model these effects, and find a closed form solution to the wavefront change, as a function of time. Using physical optics modeling methods, we show that diffractive effects are needed to explain the onset of optical damage, and derive an analytical equation that describes the compensation of the resonator instabilities in real time for damage minimization.
A computer model for injection seeding of a high pressure CO2 laser is presented. A rate equation model is used to predict single longitudinal mode (SLM) operation through injection of a cw seed into the resonator cavity. The coupled non-linear differential equations are solved using a Runge-Kutta method. Predictions for the minimum injection power required to produce SLM pulses are made. Detuning off resonance and the effect of the output coupler reflectivity and small signal gain on SLM operation is also considered. Conditions for stable SLM operation through injection of a cw seed are suggested. Single mode operation, even in the detuned case, can be attained due to the homogeneous broadening of the gain, and the rapid growth of the pulse under high gain conditions.
A theoretical and experimental study investigated beam perturbations on propagation through a MOPA chain, including both optical and medium influences. Analytical models are presented to explain the influence of thermal aberrations on the beam, and these effects are related directly to the change in M2 (quality factor) of the beam.
Continuously tunable output in the mid-infrared was generated via four-wave mixing in a parahydrogen Raman cell. Continuously and line tunable carbon-dioxide master oscillator power amplifier chains produced the 10 micron input for the Raman cell.
A CO2 master oscillator power amplifier (MOPA) laser chain has been developed capable of producing approximately 2J per pulse at 2kHz. This leads to a unique combination of high peak and average power in a single laser system. As the laser is to be used in a process where it must be capable of prolonged periods of operation, a method had to be devised to allow constant monitoring of the beam profile and behavior of the optical components to ensure that the laser induced damage thresholds (LIDT) of the optical components are not exceeded. A photoacoustic detector (PAD), that allows determination of damage to optical components, and a M2, or beam quality detector for accurate calculation of beam profiles in the CO2 laser chain have been developed. The results obtained with these detectors will be discussed.
A continuously tunable 3-atm mixed isotope CO2 oscillator power amplifier (MOPA) chain is developed for a molecular laser isotope separation pilot plant. The characteristics of this laser such as tunability, bandwidth, and output energy are reported. A closed loop gas flow system with a catalyst is employed and its performance is reported.
To obtain economical extraction of 235U in the molecular laser isotope separation (MLIS) process, 16-μm laser beams must be generated in a parahydrogen Raman cell with high repetition rates and sufficient intensity. Because the intensities of the 16-μm laser beams are dependent on the intensity of the incoming pump laser beams, the intensity of the CO2 lasers must be kept as high as possible. The maximum intensity has, however, been found to be controlled by the onset of gas breakdown in the Raman cell at too low a level for efficient Raman conversion. Through tests, the origin of gas breakdown in a 2-kHz-repetition-rate Raman cell is identified as particle contamination. The effect of the degree of contamination is determined and compared with experimental results. Conditions are set and modifications implemented on the Raman cell to ensure efficient Raman conversion.