Filaments and self-focusing of UV laser beam

Yesterday we found the small mirror surface of the beam expander was damaged, we figured out that it was caused by the laser beam filament or self-focusing. We improved the spatial filter vacuum, covered the optical tables, cleaned the windows, and filled the new gases into TWINAMP. All these behaviors might increase the laser output energy, which finally provoked the strong nonlinear effect such as filament or self-focusing.

We used the photo papers to imprint the laser beam patterns, one was put before the beam expander, the other one was put in front of the convex mirror in the beam expander box. Each print recorded about 300 laser shots at ~24 mJ. It's obvious that the beam print before the telescope was smoother than the one in front of the convex mirror. Some dark spots were recorded at the edge or in the bottom of the laser beam, which almost matched the damaged spots on the convex mirror.

Beam Profiles of Tripler and after 1st sptial filter

We measured the beam profile and wavefront of tripler (see the right figure). M²(X)= 1.58, M²(Y)=1.56; Beam width(X)= 3.3 mm, Beam width(Y)= 2.9 mm.

After the first beam expander, the beam size was enlarged about 10 times. In order to measure the whole beam, we used a lens to concentrated the beam into the CCD camera. The beam profile is shown in the below. Only the beam size was changed to bigger, the other beam characteristics were similar to the tripler one(see the bottom figure).
M²(X)=1.89, M²(Y)=1.72; Beam width(X)=20.5 mm, Beam width (Y) = 23.4 mm.

X-Ray Rainbow

via Physics News Update #805

In 1670 Isaac Newton demonstrated the composite nature of sunlight when he sent a carefully collimated sunbeam through a prism, which spread out the light into a rainbow of colors; by sending a beam of single color through a second prism (with no further spreading) Newton showed that the color was not being imposed by the prism but was intrinsic to the light itself. Now physicists using the Advanced Photon Source at Argonne National Lab, in Illinois, have spread out a beam of X-rays (which are, after all, just a more energetic version of visible light) into a rainbow of colors.

Trying to reflect X-rays from a surface is difficult because X-ray wavelengths are some 10,000 times shorter than those for visible light. Glancing reflection of only a few tenths of a degree is normally possible, and even then the beam of X-rays will suffer very little wavelength-dependent spreading. However, another phenomenon, Bragg diffraction, allows for scattering of X-rays from a crystal through large angles; in this case the incoming X-rays scatter not merely from a top layer of atoms in the crystal but from numerous atomic planes. Furthermore, if the atomic planes are not parallel to the crystal surface the diffracted X-ray beam will be spread out prismatically into a range of component wavelengths (or colors).

In the Argonne experiment an incoming beam of 9-kiloelectronvolt X-ray photons with angular spread of only 1 micro-radian (two-tenths of an arcsecond) was backwards scattered and spread out into an X-ray rainbow with an angular dispersion of 230 micro-radians (see figure).

Argonne physicist Yuri Shvyd'ko (shvydko@aps.anl.gov, 630-252-2901) says that his rainbow is not just a novelty but will have many practical applications in X-ray optics. These include compression of X-ray pulses in time and the development of X-ray monochomators (which fashion X-ray beams of pure wavelength, or color) and much higher-resolution X-ray spectrometers.

Shvyd'ko et al., Physical Review Letters, 8 December 2006

1st Spatial Filter leakage

Mike helped us fix the vacuum pump for the first Spatial Filter of TWINAMP, however we found there was a big leak from the vacuum pipes. We checked the pipes step by step, firstly found one of the O-ring was broken, then small holes around the vacuum pipes. After replacing the O-ring and gluing the leaks, the vacuum reached about 10 mTorrs.

Supervision software operates and maintains large laser systems

I dreamed I could control the large laser system by computer someday, fortunately I found the Thales Laser company already achieved it.

Supervision software operates and maintains large laser systems

Auther: Fabien Ghez

Large laser systems, such as terawatt- and petawatt-class ultrafast amplifiers or Nd:glass fusion beams, are complex to use and maintain, primarily because they are large systems that comprise several smaller ones. A terawatt ultrafast laser, for instance, consists of several optical elements such as an oscillator and its pump, a set of amplifiers, many pump lasers, beam-shaping devices, a stretcher-compressor pair, timing electronics, cooling units, and systems that interface to user experiments.

So in addition to operating the overall system, a user must also bring a capacity for tuning and maintaining all of the components individually and collectively. Laboratories often employ engineers or Ph.D. scientists whose sole or primary responsibility consists of maintaining and operating such highly complex experimental tools. To ease this burden for the average user while enabling broader application of large laser systems, computerized supervision tools have become available that combine hardware and software dedicated to system operation, tuning, and maintenance. The goal of such processing capabilities, of course, is to enable the user to spend time on the application rather than on tweaking the laser. The primary processing tasks can be divided into two areas.

The first of these areas, synchronizing laser pulses of femtosecond duration to single-shot experiments with negligible jitter, is one of the toughest tasks in laser-driven experiments. Supervision software enables the laser to be slaved to external clock references, or to be used as a main system clock. In both cases computer-driven synchronization signals set the pace for both the laser and the experiments. X-ray generation in connection with a linear electron accelerator offers a classic example of this sort of application: the laser beam collides with an electron beam and subpicosecond accuracy is required despite the very noisy environment.

In terms of the second control area, laser operation and maintenance, supervision software and hardware enable automated start-up of all pumps and other devices, monitors operation of all systems, including motors and beam analyzers, and diagnosis malfunctions, thus enabling rapid preventive maintenance while minimizing the need for system tweaking. If a large laser system must operate in an environment that would be unsafe for human operators, such as a room exposed to harmful radiation, supervision software enables remote control of operational tasks such as start-up, shutdown, the control of energy levels, and the optimization and alignment the beam, all through a fiberoptic link.

System architecture

In the Thales laser-control architecture, supervision software controls, through an RS232 connection, a laser-control system that drives all individual control units and drivers; a femtocontrol unit that drives the diagnostics; one or several synchronization units, CCD cameras and diagnostics; and all connections between computers, power supplies, electronic racks, and breadboards (see Figure).

The masterclock (synchronization unit) generates all signals to trigger pump lasers, Pockels cell, and CCD cameras in synchronization with the master RF of the oscillator. This provides the aforementioned system synchronization with the laser oscillator, reduction of jitter, and improvement of stability, whether within one laser system or among several. Of course, if the laser is only one part of a global facility, the computer can be linked to global supervision software via network connections and protocols to be determined by the user.

The femtocontrol unit is a single electronic rack that can simultaneously drive and control stepper motors, motorized mounts (x and y axis), and pump-laser interlocks. It can also make some spectral and power measurements using integrated photodiodes. Those integrated and calibrated photodiodes can be implemented in breadboards to control the power levels of different amplifiers and pump lasers.

The compressor can also be adjusted through the software, enabling pulse duration to be adjusted remotely without removing system covers (which can prove particularly useful with vacuum-chamber compressor systems, or pulse propagation in air). The software adjusts the distance between gratings by means of a translation stage.

The laser-beam profile can be monitored by the user on the computer screen through a Firewire CCD camera, which enables saving of the profile in jpeg format. This camera also enables control of beam profiles in intermediate amplifiers. And when coupled with an alignment system it can also control beam pointing.

In some experiments, precise pulse energy is required to improve efficiency. This can be achieved via an attenuation process that reduces output power without degrading other performance parameters (such as polarization, beam focusing, energy, stability, and contrast). The easiest way to implement such a solution is to modify the delay on pump lasers to reduce the gain of some amplifiers. The drawbacks of this solution are that it can degrade some parameters of the beam in a way that invalidates the experiments, and that a precise attenuation within the required range may not be possible. An alternative software-enabled solution avoids such problems by using rotating waveplates coupled to polarizers that can vary the output power from 15% to 100% without affecting other specifications.

The Thales supervision software is based on Labview 7.0 (National Instruments; Austin, TX), which in addition to user-friendliness enables compatibility with a broad range of user software systems. So when laser and experimental protocols are compatible it may even become possible to integrate laser control with experimental software, and to operate the laser and experiment as a single system.

From Laser Focus World November, 2006

On the node of a wave

A compact electron accelerator can be made by the cunning use of laser pulses to let electrons 'surf' on a plasma wave. The problem has been controlling exactly how much the electrons are accelerated.
Jerome Faure and Victor Malka at the ENSTA/CNRS laboratory near Paris injected electrons into a plasma wave created by a single intense laser pulse.

a, In Faure and colleagues' experimental scheme, the primary laser wakefield pulse ionizes helium gas to a plasma. If the parameters of pulse and plasma are chosen appropriately, the electrons of the plasma oscillate about a fixed spot. b, If a second 'tow-in' pulse travelling in the opposite direction crosses the first, a standing wave forms. Electrons are pushed left and right in the standing wave from the antinodes to the nodes. c, Some electrons — those pushed to the right — gain enough speed to get caught up in the following wave crest and are accelerated forwards. The energy gain of the electrons is determined by how far they have to surf through the plasma, and so by where exactly along the plasma the two laser pulses cross.

From Nature 444, 688-689 (7 December 2006)

2D imaging conical x-ray spectrograph

We are using a Von Hamos x-ray spectrograph to observe the x-ray imession from laser cluster interaction, which only provides the spectral information. I think we can use this novel x-ray crystal spectrograph to measure the spectral resolved 2-D image in our experiment.

Extreme luminosity imaging conical spectrograph

S. A. Pikuz, T. A. Shelkovenko, M. D. Mitchell, K. M. Chandler, J. D. Douglass, R. D. McBride, D. P. Jackson, and D. A. Hammer
Laboratory of Plasma Studies, Cornell University, 439 Rhodes Hall, Ithaca, New York 14853

A new configuration for a two-dimensional (2D) imaging x-ray spectrograph based on a conically bent crystal is introduced: extreme luminosity imaging conical spectrograph (ELICS). The ELICS configuration has important advantages over spectrographs that are based on cylindrically and spherically bent crystals. The main advantages are that a wide variety of large-aperture crystals can be used, and any desired magnification in the spatial direction (the direction orthogonal to spectral dispersion) can be achieved by the use of different experimental arrangements. The ELICS can be set up so that the detector plane is almost perpendicular to the incident rays, a good configuration for time-resolved spectroscopy. ELICSs with mica crystals of 45×90 mm2 aperture have been successfully used for imaging on the XP and COBRA pulsed power generators, yielding spectra with spatial resolution in 2D of Z pinches and X pinches. ©2006 American Institute of Physics

Rev. Sci. Instrum. 77, 10F309 (2006)

Catching the wave

In recent years the use of high-order harmonic radiation to create and control events on attosecond timescales has grown at a phenomenal rate. With the use of carrier-envelope phase (CEP) stabilization and few-cycle laser systems it is possible to probe physical processes on unprecedented timescales. Here, we report the first experimental observation of high-harmonic emission at individual half-cycles of a laser pulse. We show that these half-cycle emissions are extremely sensitive to the CEP, providing a route to a new single-shot measurement technique of the CEP. We use this technique to measure the CEP of an 8.5 fs pulse at a centre wavelength of 800 nm with an accuracy of 20 as (1 as=1x10^-18 s). With appropriate spatio-spectral filtering of the harmonic spectra, our calculations show that we can isolate emission from an individual half-cycle cutoff, which corresponds to a single isolated attosecond pulse of duration less than 300as.

Citation: "Half-cycle cut-offs in harmonic spectra and robust carrier-envelope phase retrieval", Nature Physics, Sunday 3 December 2006.

related link: Attosecond Technology