Large-aperture Amplifier Research Articles

Liquid-cooled Nd:phosphate split-slab large aperture amplifier pumped by flash-lamps

High power laser facilities have been developed to achieve novel standards in light-matter interactions. The repetition rate is severely limited by the heating of the large size components, and in particular the large-aperture amplifiers. Cooling these optical components turns out to be a major challenge for increasing the repetition rate of high power laser beam lines. Here we report a practical implementation of a large aperture Nd:phosphate split-slab based amplifier using a liquid cooling system. The amplification of a 90 × 90 mm2 beam with a gain coefficient of 1.135 per pass and wave-front deformation lower than λ/3 per pass is achieved with a repetition rate of 1 shot per minute. The cooling system is sufficient to maintain the gain as well as the beam quality over a long time operation. Additionally, the pressure inside the amplifier can be controlled to minimize the optical deformation. This single liquid-cooled amplifier is a promising step towards future beam-lines able to deliver hundreds of shots per day with beam quality compatible with target focusing applications.

Influence of amplified spontaneous emission on gain lifetime in high-aperture Ti:sapphire amplifiers

We demonstrate that by lowering gain lifetime, transverse amplified spontaneous emission imposes practical limit on usable aspect ratio of large-aperture amplifiers in a high-energy Ti:sapphire system.

Coupling method for laser-diode-array end-pumped solid-state amplifiers

A new coupling method is proposed for high-power laser-diode-array (LDA) end-pumped large-aperture amplifiers. In this method, stacks in the LDA are distributed on a spherical surface, and a hollow duct is placed after them. Both numerical simulations and experimental research have been done for this method. All results show that high coupling efficiency and uniform pumping field can be obtained simultaneously using this method.

Simulation on a new coupling method of laser diode array end-pumped large-aperture amplifiers

A new coupling method is put forward for high power LDA end-pumped large-aperture amplifiers. The LDA is inscribed on a sphere and then a duct is added adjacent to it. Three-dimensional ray-tracing program is used to simulate the features of the new coupling method. Simulation results show that when the area contraction coefficient is about 30, deposition efficiency in the gain media as high as 93.7% can be achieved; When the emitting area is not less than the aperture of the duct, uniform and top-hat distribution is obtained. When several stacks are spliced to make an inscribing area, amplifiers with larger aperture can be end pumped. When stacks in LDA is placed as m×n(m≠n) arrays, uniform and top-hat distribution is also obtained. In the LDA coupling experiment, the output is top-hat distributed at the end of the duct and the coupling efficiency is 85.4%.

高功率激光二极管阵列端面抽运大口径片状放大器抽运场评价方法

高功率激光二极管阵列端面抽运大口径片状放大器抽运场评价方法

Thickness optimization of the composite gain medium for the oscillator and amplifier of the Lucia laser

Using a theory of quasi-three-level laser ions, we obtained the optimum thickness of the gain medium of Yb:YAG. For the parameters of the oscillator and the amplifier of the diode-pumped Lucia laser (Lasers Ultra-Courts et Intense et Applications), which will deliver a 100 J, 10 Hz, 10 ns pulse train the optimum thicknesses are, respectively, 1.4 and 1.6 mm, with a concentration of 10 at. %. Simulations indicate that the bending of such a thin medium is great but can be eliminated by substitution of a composite gain medium. The optimum thickness of undoped YAG is related to that of Yb:YAG and is also dependent on cooling conditions. Results show that, for a small-aperture oscillator, we can obtain both minimum bending and optical distortion at several doping concentrations. However, greater doping (approximately 20 at. %) is preferred for a large-aperture amplifier. The results reported will be helpful for the design of gain media for high-average-power thin disk Yb:YAG lasers.

Transient gain enhancement for a train of picosecond pulses in a large-aperture KrF laser amplifier

The heavily depleted steady-state gain in a large-aperture amplifier can be transiently enhanced by temporally suppressing the intra-cavity amplified spontaneous emission (ASE). In our previous experiments of amplifying two 10-ps pulses in a 29-cm-diameter KrF amplifier, we showed that output fluences of three times the saturation energy density Esat were obtained at a pulse separation of 1.7 ns for full-filling beams, whereas it took 4 ns for 5-cm-diameter beams. Our recently developed time-dependent ASE code almost reproduced this observed quick gain recovery. In this paper, we report the experimental results of amplifying six 10-ps pulses in series to the saturation level in the large-aperture amplifier. The quick gain recovery was observed also for the pulse train. Short-pulse outputs of 3Esat were obtained at a reduced pulse separation of only 1.5 ns.

Calculation of transient gain enhancement by suppressing intra-cavity ASE in a large-aperture KrF laser amplifier

In a large-aperture amplifier, steady-state gain is heavily reduced by the intra-cavity amplified spontaneous emission (ASE). However, the reduced gain could be transiently enhanced by temporally suppressing the ASE with an intense depleting short pulse. Previously, we reported the experimental observation of this transient gain enhancement in a KrF laser amplifier with a 29-cm-diameter aperture. In this paper, this transient gain enhancement is examined theoretically by using a time-dependent ASE code. From a comparison of the code calculations with the experimental results, the observed quick gain recovery and also the possibility of a transient gain higher than the steady-state value are discussed.

Transient gain enhancement for picosecond pulses by suppressing intra-cavity ASE in a large-aperture KrF laser amplifier

In a large-aperture amplifier, steady-state gain is heavily reduced by the intra-cavity amplified spontaneous emission (ASE). However, the reduced gain could be transiently enhanced by temporally suppressing the ASE with an intense depleting short pulse. This transient gain enhancement is experimentally studied in a 29-cm-diameter-aperture KrF laser amplifier. The gain depleted by a 10-ps pulse is observed to recover more quickly to the initial steady-state gain level when the aperture of the amplifier is filled with the depleting beam than when only 3% of the aperture is filled.

Simulations of Raman conversion in large aperture amplifiers

A two-dimensional computer code has been developed to model the final stage of the Raman conversion process in the SPRITE laser system at the Rutherford Appleton Laboratory. Predictions of the code are compared with those of earlier one-dimensional studies and found to be in encouragingly good agreement. The effect of varying the relative input pulse timing is studied, and the consequences of pulses with intensity and phase fluctuations in both temporal and spatial domains are examined. The role of second-order Stokes generation is briefly considered.

Performance of the NIF Prototype Beamlet

Beamlet is a full scale single beam prototype laser system, built to demonstrate the laser technology and performance of the 192 beam National Ignition Facility (NIF) fusion laser driver. Both laser systems apply multipass amplifier architectures. By passing the beam four times through the large aperture amplifier sections, the small signal gain during the first few passes is used efficiently to reduce expensive staged amplifier chains. The beamlet prototype laser integrates results of development programs for large aperture components: large aperture optical switch, polarizers, 2 x 2 multisegment amplifiers and new pulse generation and preamplification techniques. The authors report on performance test results of the recently completed 1{omega}-laser section of Beamlet.

Discharge and kinetics modeling in electron-beam controlled CO<inf>2</inf>laser amplifiers

The specific computational methods used to model the spatial and temporal behavior of large pulsed electron-beam controlled CO 2 laser amplifiers are presented. The interactions between the various physical processes which occur in these devices are related to the logical structure of the model, so that the key approximations can be identified. We show that, in most cases, the uniformity of the gain can be adequately described with a two-dimensional (2-D) analysis, which uses the discharge conditions present at a single snapshot time corresponding to the peak of the discharge power. We apply these techniques to model an experiment in which the nonuniformities play an important role; and, as a second example, we demonstrate that the modeling capabilities can show the consequences of design modifications in large-aperture amplifiers.

Passively switched double-pass active mirror system

Active mirror amplifiers have been demonstrated to be viable large aperture amplifiers for use in laser fusion systems. In addition to having a large storage efficiency (1.2-1.5%) at the highest pumping level and a thermal recovery time in the 15-30-min range, they are also scalable to large apertures and allow the propagation of circular polarization. An important additional property is that regardless of the angle of incidence, no birefringence occurs in active mirror amplifiers. Based upon a circular polarization scheme that allows passive switching, we have constructed and operated a double-passed active mirror system. For a 50-J input energy and a pulse width of 700 psec (Gaussian FWHM), we have measured a focusable output energy of approximately 230 J. Complementary single-pass experiments resulted in an output energy of approximately 140 J, therefore, using the double-passed arrangement, we have increased the focusable output energy by ~64%. In this paper we describe the short-pulse (B-limited) and long-pulse (damage-limited) staging of double-passed active mirror systems, discuss in detail the passive double-pass system, and describe its experimental verification.

Time resolved spectroscopy of large bore Xe flashlamps for use in large aperture amplifiers

Time resolved spectroscopy was performed on 13-mm xenon filled flashlamps. The use of a well-characterized system allowed the acquisition of absolute radiometric data. The data were compared to the predictions of a phenomenological flashlamp model and differences noted. Specifically the arc for equal currents is not the same for dI/dt > 0 and dI/dt < 0, the continuum is a stronger function of current density than the model indicated, and different lines, linewidths, and line intensities were found.

Penning ionization in doped CO2 TEA lasers

A quantitative model is presented which predicts the behavior of the discharge of a CO2 laser gas mixture with a low-ionization seed gas. The basis of the developed model is the assumption of the so-called Penning ionzation mechanism in which metastable nitrogen molecules ionize the seed gas. The developed model will be compared with measurements on an appropriate CO2 laser device. As a result of this study, we describe the successful operation of a simple large-aperture amplifier with a diameter of 12 cm.