M. J. Shaw

A high performance excimer pumped Raman laser

An electron beam pumped large aperture KrF laser operating in a short pulse multiplexed mode has been used to pump a methane Raman laser to produce a single high intensity pulse at 268 nm. With an output beam divergence of 20 μrad and final amplifier conversion efficiency of greater than 50%, intensity at the focus of an F/3 lens was greater than 1017 W/cm2. Prepulse intensity was less than 10−10 of peak intensity.

A 1 TW KrF laser using chirped pulse amplification

Abstract Chirped pulse amplification (CPA) and recompression have been used in a large aperture KrF laser system. The power focused onto target in a 300 fs pulse reached 1 TW with an irradiance of ≈ 10 19 W/cm 2 .

Measurement of the nonlinear refractive index of air and other gases at 248 nm

Abstract The self-phase modulation of 10 ps laser pulses at 248 nm passing through a focus in a gas cell has been used to determine the nonlinear refractive indices of air, nitrogen, oxygen, methane, argon and neon and to put an upper limit on the value for helium.

Vibrational relaxation of KrF* and XeCl* by rare gases

A steady-state, chemiluminescence technique has been used to measure effective rate constants for vibrational relaxation as a function of vibrational level for KrF* in collisions with He, Ne, and Ar and XeCl* with Ar. The effective rate constants reported include contributions to relaxation due to intersystem crossing between theB andC states, in addition to direct relaxation within theB state. The relevance of these results to the understanding of previous measurements in KrF and XeCl lasers is discussed.

Ultrabroad bandwidth multifrequency raman generation

We report on the modeling of transient stimulated rotational Raman scattering in H(2) gas. We predict a multifrequency output, spanning a bandwidth greater than the pump frequency, that may be generated without any significant delay with respect to the pump pulses. The roles of dispersion and transiency are quantified.

Titania—a 1020 W cm−2 ultraviolet laser

The Titania laser system, based around a 42 cm e-beam pumped KrF amplifier, is currently being installed at the Rutherford Appleton Laboratory and will come on line as a user facility in 1996. Like Sprite, its predecessor, it will operate in both CPA (249 nm) and Raman (268 nm) short-pulse modes, delivering up to 10 TW to target in high-quality beams. With brightness expected to reach 10 21 W cm -2 sterad -1 , it will be the worlds brightest ultraviolet laser. The design of the Titania system includes a number of novel features. The multi-pass Ti :sapphire front-end amplifier uses an unusual beam-folding scheme. The Raman system will involve the first application of Raman multiplexing, combining high KrF efficiency with low transport cost. Reflective coatings with very high damage thresholds are being developed for the CPA compressor gratings and the UV transport optics. A windowless configuration for the final Raman amplifier is presently under analysis, to allow the performance of this maximally stressed component to be extended substantially. Finally the design of the Titania e-beam machine, featuring novel split-cathode diodes, has resulted in unusually high efficiency of electron transport into the laser gas. The lasers infrastructure has involved sophisticated mechanical and electrical design, and a computerized diagnostic, control and safety package is being developed to allow one-man operation of the whole 1000 m 2 installation.

Picosecond gain and saturation measurements in a KrF laser amplifier depumped by amplified spontaneous emission

The input/output characteristic of a large aperture (270‐mm diam) electron‐beam‐pumped KrF laser amplifier is obtained for pulses of 6‐ and 40‐ps duration. The off‐resonance absorption coefficient is estimated by measurement of loss at 268 nm. The gain‐to‐loss ratio is found to decrease from 8 to 6 as the specific pump rate is increased from 0.3 to 0.65 MW cm−3. Gain depumping due to amplified spontaneous emission (ASE) is treated by a simple 1D code and found to give good agreement with experiment. A 3D code incorporating wall reflections gives even better agreement. The spatial distribution of gain and ASE is recorded using visible and ultraviolet imaging.

Ultrahigh-brightness KrF laser system for fast ignition studies

The main requirements for a fast igniter laser beam are reviewed and shown to favour short wavelength and ultrahigh brightness. These requirements are met by the new KrF laser system at Rutherford Appleton Laboratory called TITANIA. TITANIA uses two schemes to enhance the laser beam brightness. The first is chirped pulse amplification which is used to enhance brightness by compressing the pulse into the femtosecond region. In this mode TITANIA produces in the region of 250 mJ on target in 700 fs. The second mode of operation uses a Raman technique for beam combining and beam clean-up which is designed to give a single beam of 80 joules on target in a pulselength of 60 ps. In this scheme the KrF wavelength is Raman shifted to 268 nm. The Raman amplifiers will use gaseous rather than solid windows and experiments which demonstrate their feasibility will be described. A concept for a reactor scale fast igniter beam using the Raman technique will be discussed.

Gain depletion due to amplified spontaneous emission in multi-pass laser amplifiers

Calculations of the effects of amplified spontaneous emission (ASE) in laser amplifiers designed to amplify sub-picosecond pulses are presented. A one-dimensional code is used with some simple approximations to account for aspect ratio variation in cases of cylindrical symmetry. Gain depletion due to ASE is compared for various multi-pass geometries. Optimisation for short pulse amplification of a telescopic, beam-expanding, 4-pass amplifier is considered.

The bi-directional amplifier in the constant intensity approximation and its application to KrF lasers

Bi-directional and folded laser amplifiers are analysed in the constant intensity approximation. Simple analytical expressions are developed for gain, output intensity and extraction efficiency. The analysis is applied to electron-beam pumped KrF lasers and expressions for saturation intensity, small-signal gain coefficient and intrinsic efficiency in terms of gas composition and pump power per unit volume are given. The problem of fluorine burn-up in KrF lasers is discussed. Under certain conditions the analysis is shown to be equally applicable to oscillators and comparison is made with previous experimental results and computer code predictions.