About laser

The core of the PALS facility is the high-power iodine laser system Asterix IV. It is capable of delivering up to 1 kJ of energy at the fundamental wavelength 1.315 µm. This energy may be split to few auxiliary beams with controlled time shift from the main beam. All beams may be frequency doubled (wavelength 657 nm) or tripled (438 nm). At a pulse of about 350 ps, the laser produces power of 3 TW. The full energy shot can be fired each 25 minutes. The laser delivers a beam with superior spatial profile quality, and exhibits high stability of the output beam energy over a sequence of shots.

The auxiliary beam may be split, if required, into two beams with an energy ratio of 2 : 3 or 1 : 5. Both these beams may be independently timed to arrive at the target up to 12 ns before or after the main pulse. While using the auxiliary beam, output energy of the main beam is correspondingly lowered.

The Asterix IV/PALS laser is characteristic by a high spatial profile quality across the output beam. The below image shows a camera-recorded footprint of the output beam, and the vertical and horizontal line outs of this footprint.

Asterix IV / PALS is a gas laser using neutral iodine atoms to produce near-infrared light at the wavelength of 1.315 µm. The iodine atom is obtained from a parent perfluoroisopropyl iodide molecule C₃F₇I in a photochemical process called photodissociation (or photolysis). In this reaction, UV radiation produced by powerful flashlamps is used to free the iodine atom from the chemical bonding. The iodine emerges from the photodissociation reaction as an electronically excited atom, which inherently provides population inversion with respect to the lower-lying iodine ground state, constituting thereby conditions for lasing action.

The lasing action occurs between fine-structure-split levels ²P_1/2 and ²P_3/2 of the fundamental configuration 5s²5p⁵ of the neutral iodine atom. The electronic configuration of these two levels is described using a spectroscopic notation corresponding to so-called LS coupling which is applicable to atoms in which the spin-orbit interaction is small. The upper left index denotes the multiplicity of the electronic spin, the capital letter gives the orbital angular momentum, and the right lower index expresses the total value of the electronic angular momentum.

The light emitted by the transition ²P_1/2 to ²P_3/2 is characterised by six very closely spaced spectral lines, in spectroscopic nomenclature called components. These components arise because the magnetic field of the iodine nucleus (nuclear spin 5/2) splits the upper and lower levels into 2 and 4 so-called hyperfine levels, while the selection rules limit the number of possible transitions between these levels to six. Among the individual spectral components the strongest one belongs to the transition F = 3 -> 4, where F labels the total angular momentum of the hyperfine level.

Along with isopropyl C₃F₇I (labelled also as i-C₃F₇I), the active medium of the Asterix IV/PALS laser contains Ar which acts as a buffer gas. The mutual ratio of C₃F₇I and Ar in the mixture is different for individual laser modules, but Ar is always the largely predominant component and the typical sum pressure is on the order of 1 to 3 atmospheres (1 000 to 3 000 mbar). Ar does not participate in the lasing but has three other practical functions.

First it broadens, via atomic collisions, the spectral width of the emitted radiation. The individual spectral components thus “melt” into one spectral peak, as seen from the image. This increases the efficiency of the system (as all the spectral components participate in the lasing) and reduces the small-signal gain, inhibiting potential parasitic oscillations of the laser chain.

Secondly, the presence of Ar helps to improve the transverse uniformity of pumping, since the partial pressure of the active gas C₃F₇I is low despite the high total pressure (required for the spectral broadening).

Thirdly, Ar acts as a heat sink carrying away a part of the thermal energy dumped into the active medium by the flashlamps. This reduces the rate of formation of certain undesired products during the photolysis.

Asterix IV/PALS is a single-beam laser system. It consists of an oscillator section that generates the initial pulse, followed by a succession of five power amplifiers in which the pulse is boosted to a high energy. This arrangement is called master oscillator power amplifier (MOPA). The size of the amplifiers gradually increases along the chain, and the beam diameter is increased from the initial 3 mm at the output of the oscillator to 290 mm at the output of the final amplifier. The beam expansion is achieved in spatial filters regularly placed between the amplifiers. The Faraday rotator prevents the light reflected from the target from counter-propagating the chain and damaging the initial stages of the system.

In addition to the 290 mm main beam, the laser delivers a 148 mm auxiliary beam obtained by splitting off a fraction of the A4 amplifier using a semi-transparent mirror. It can be separately synchronized to the main pulse and can be employed for instance to drive backlighter or prepulse plasmas.

A shot of the laser system begins in the oscillator section, which produces the initial weak pulse of light that is then admitted to the chain of power amplifiers. The oscillator section consists of the master oscillator and of the preamplifier. The master oscillator generates a sequence of several nearly identical pulses.

The length of the laser pulse supplied by the oscillator section is determined by the pressure of the active medium in the oscillator and by the power fed into a device called acousto-optical mode coupler that is an integral oscillator’s part. The length of the pulse (~800ps) is reduced as the pulse propagates through the laser chain and the early pulse saturates the active medium, inhibiting amplification of the late pulse. In a similar fashion the pulse is shortened also in the saturable absorber (see the detailed Asterix IV/PALS scheme), where its leading edge is discriminated until the dye becomes bleached. The pulse length is thus reduced on its way through the system by a factor of ~2, resulting in the pulse duration about 350 ps.

The Asterix IV/PALS laser chain includes besides preamplifier five power amplifiers. They boost the pulse supplied by the oscillator section up to an energy of one kilojoule. The size of the individual amplifiers gradually increases along the laser chain. The final amplifier is nearly 13 m long (see the picture) and provides the laser beam of 29 cm in diameter. A fraction of second before the laser shot, the amplifiers are energized by discharging the large capacitor banks placed in the underneath floor into the flashlamps that surround the amplifier’s tube containing the gas active medium. The intense flash of incoherent ultraviolet light produced by the flashlamps creates in the amplifier’s tube abundance of excited iodine atoms that are “ready” to add their extra energy to the laser pulse supplied by the oscillator section.

The advantage of using a gaseous active medium in the iodine power amplifiers is that this cannot be optically damaged or cracked, in contrast to solid-state lasers. The use of gaseous active medium allows furthermore at Asterix IV/PALS to produce the quality of the laser beam superior to the beam quality achieved by most of solid-state lasers.

The amplifiers are built as arrays of modules each consisting of a quartz tube surrounded by a flashlamp block. For example, the final amplifier is composed of eight modules about 1 m long, each carrying 12 flashlamps mounted on two half-shell units easing access and maintenance. The table shows the active length, the beam diameter, and the typical partial pressures of the working and the buffer gases for each amplifier.

The flashlamps energizing the amplifiers consist of quartz tubes and are filled by xenon. They are driven at 30 to 40 kV – depending on the desired laser output energy – through a discharge circuit resulting in white light pulses with maximum spectral intensity at 250-300 nm and duration of 2-10 μs. Yellow-green light near 500 nm also dissociates the iodine molecules I₂ that spontaneously form in the active medium.

Using short UV light pulses to pump the active medium has two principal advantages.

First, the efficiency of generation of the inverted iodine atoms from the parent alkyliodide molecules is fairly large – nearly 50 % – since fewer “adverse” secondary products are generated during the photolysis. This makes it possible to easily regenerate the lasing medium after each shot, allowing a low-cost operation of the system.

Second, the short pumping creates population inversion only in a few microseconds. Amplification may thus take place in a homogeneous active medium prior the acoustic waves, which develop over a tens-of-microsecond time scale as a result of local heating of the wall of the amplifier tube by the flashlamp light, destroy the uniformity of the medium. This allows producing an extremely uniform profile of the output beam.

The flashlamps design ensures the electric discharge to always occur near the flashlamp axis, minimising the load of the quartz tube and hence the risk of its damage or disruption. The electric current flowing through the discharge is led back along the flashlamp by four current rods surrounding the tube and connected to the ground potential. The metallic reflector serving to couple the pump UV light from the flashlamp to the active medium is floating. Thus the field inside the flashlamp is fully determined by the electrodes and the surrounding current rods, giving it the axial symmetry.

In all amplifiers the Xe filling of the flashlamps is regularly exchanged to remove the molecular impurities which reduce performance and lifetime. All flashlamps are cooled by a closed nitrogen loop which re-establishes their thermal balance over several-minutes (this system additionally cools the amplifier’s quartz tube). Typically 600 – 1 000 full energy shots are possible before the flashlamps need to be inspected and replaced.

The laser medium is replenished from cryogenic units located in the underneath floor, which store the C₃F₇I liquid at a temperature providing the saturated vapour pressure at which the amplifier operates. The system removes the post-shot products such as I₂ that would otherwise strongly quench the population inversion. The regeneration of the medium of the A3, A4, and A5 amplifiers is enhanced by local heating which generates pressure gradients and allows a full energy shot every 25 minutes.

A spatial filter is a telescope-like device consisting of two lenses that have a common focus, with a pinhole located at the plane of this focus. The laser beam is focused by the first lens through the pinhole and expands out again until it reaches the second lens which re-collimates it anew.

In the Asterix IV/PALS laser, in total six vacuum spatial filters are spaced along the amplifier chain. Five of them serve for an efficient coupling of the laser beam between adjacent amplifiers, while the sixth, largest, spatial filter transports the beam between the output plane of the last amplifier A5 and the frequency conversion DKDP crystals. All spatial filters employed in the Asterix IV/PALS chain are assembled from thick stainless-steel tubes equipped at each end with a plano-convex lens constituting the vacuum interface. The lenses are mounted on flexible bellows segments making it possible to optically adjust the whole assembly during the initial setup of the laser chain.

The job is carried through spatial filtering.

On its focal plane, the input lens produces a diffraction pattern which is a Fourier transform of the light distribution from the object plane. The diffraction pattern is constituted by the spatial frequency spectrum of the object, where a specific frequency is proportional to its distance from the axis of the system. Thus high spatial frequencies, corresponding to small-scale intensity modulations of the object, appear at a large distance from the axis. When a screen with a pinhole is inserted at the focal plane, these high spatial frequencies are eliminated. On the other hand, the low-frequency components constituting the smooth beam profile appear near the axis, and pass through the pinhole unperturbed. The output lens of the spatial filter performs the inverse Fourier transform, projecting the filtered diffraction pattern onto the image plane.

In Asterix IV/PALS, the pinholes of the final spatial filters are typically 2 mm in diameter. It should be also noted that compared to solid state lasers the issue of optical damage by intensity filaments in the beam is less critical, as Asterix IV/PALS employs gas — hence “indestructible” — active medium.