Compressor Alignment Procedure

I concentrated on the pulse compressor these days. Several motors were used to control two big 1200 line/mm gratings. The motors were controlled by Aerotech UNIDEX 511 motion controller. The computer interface program was written in LabView. It's easy to change the grating reflected angles and distances by rotating a hand wheel. The alignment procedure is:

1). Steering the mirrors to center the beam in the grating G1.
2). Set G1 to 0 degree to check the retro reflection.
3). Set G1 to Littrow angle (28.76 in our case) by adjusting the rock.
4). Change G1 to 13.358 for deviation.
5). Set G2 angle 26.716 for 0 degree back reflection checking.
6). Set G2 angle 55.48 for Littrow checking.
7). Set G1 and G2 both 13.358 degree.

related link: UNIDEX 511

Far-field distribution of the seed beam after 3rd ampfilfier

After checking the mirrors and cleaning the dirty ones, we measured the beam profile again. By changing the distance between convex lens and CCD camera, we can measure the far-field distribution of the seed beam. We believed the beam was still good after passing the third amplifier and many reflected mirrors.

Measuring the seed beam profile after 3rd amplifier

We measured the seed beam profile after third amplifier, the CCD camera was put behind the convex mirror of the telescope.

Because the camera was closed to the mirror, we measured the intensity distribution. From the picture shown in the right, we found there were some dust or damaged parts. We should check the optical path and optical components to find what caused this ugly beam profile.

YAG #4 beam alignment

Before running the third amplifier, it's necessary to check pump laser beam qualities of YAG #3 and YAG #4. We found the some beam delivering mirrors for YAG #4 had been burned. Two mirrors were replaced by the new ones, the other two mirrors were just carefully rotated to avoid beam hit the small burned dots.

It's so excited that the spectrometer can be controlled by the desktop.

Beam profile before the third amplifier

After cleaning the delivering mirrors one by one, we measured the beam profile before inputing the third amplifier. This time, the beam looked similar to the Gaussian shape perfectly.

Femtosecond time-delay X-ray holography

Researchers have used the ultrafast X-ray pulses from a free-electron laser to image a nanoscale object in just a femtosecond. The technique, which is a new form of X-ray holography, has been pioneered by Henry Chapman from Lawrence Livermore National Laboratory and colleagues in the US, Switzerland and Germany. Being able to study materials so fast brings us one step closer to the holy grail of observing, at the same time, how all the atoms in a molecule move (Nature 448 676).

The incident FEL pulse from the left passes through a hole in a multilayer-coated detector mirror. The 'dusty mirror' consists of particles on a 20-nm-thick silicon nitride membrane backed by a multilayer-coated plane mirror. This returns the direct beam back through the hole in the detector mirror, which reflects the diffracted light onto a CCD detector. The prompt diffraction (blue, the reference wave) and delayed diffraction (red, the object wave) interfere to generate the hologram on the CCD detector.

Realigning the laser pulse stretcher

It's very hard to increase the output energy from the second amplifier, even optimizing the cavity more carefully. The problem might come from the front end, we traced the beam and found it's very weak after the stretcher. So it took time to realign the laser path and made the dim laser dot became very bright.

The first amplifier output energy without the saturater was around 5 mV (normally ~4 mV), it's about more than 2 mV with the saturater (normally ~1 mV). The output from second amplifier reached about 400 mV. The beam was delivered into the spatial filter and measured the profile using the CCD camera. The diffraction pattern indicated there was a dust or damage on some mirror surface.

Spectrum from laser osillator

A loptop was used to replace the old desktop. In the beginning the spectrometer S2000-USB could not be recognized by the laptop. Somebody suggested to try another USB port, then the signal was appeared on the screen. This indicated the laptop remembered the original port connecting the hardware.

The snapshot of the spectra is shown on the right, the blue color line is reference spectrum, the red one is the real-time spectrum.

The Control Computer Out of Order

I planed to run the whole system today, unfortunately I found the computer controlling the Ocean Optics S2000 spectrometer was down in the morning. I tried to restart the computer many times, it's no response. Because this spectrometer is used to monitor the laser spectrum, which can show if mode-locking status. The control computer must be repaired as soon as possible. The connection adapter is ADC1000-USB(S/N ADUD5565), the software is OOIBASE32.

The computer power supply was totally dead, I tried another computer. However it's not easy to run OOIBEASE32 in the new computer. I tried a laptop which the software has been install, but it cannot recognize the hardware.

Crystal surface burned again

When the output energy after the PC2 became lower, I tried to tune the pump mirrors to maximize the output. During the tuning, I found the energy really became bigger, unfortunately the left side of the crystal was burned again. The similar phenomenon happened before, I think the measured energy was not the laser pulse but the spontaneous emission. When ASE goes higher, the pump beam was focused smaller, which would damage the crystal surface. Next tuning time, it's better to measure the pulse shape using the diode and make sure no ASE anymore.

I also found the Ti:sapphire crystal was not installed well, I put more Indium layer to hold the crystal tightly. Then I realigned the amplifier #1, changed the times of optical pass from 6 to 7.

Creating the pinhole for the spatial filter

I optimized the amplifier #2 alignment and obtained the normal output energy. Then the beam was delivered into the spatial filter. We already installed a piece of window for burning the pinhole last week. The seed beam from oscillator was blocked, so the ASE beam from first and second amplifiers was used to burn the hole. After about one and half hours, the beam could be watched from the spatial filter other side. The beam profile looked good by eyes, the real profile should be measured using the SPIRICON LBA camera.

YAG #1 and YAG #2 delivering mirrors burned

The amplifier #2 worked well, the next step is to send the beam to the spatial filter, before that we must drill a pinhole using the laser beam. We chose a piece of used window to replace the old window with a bad pinhole. We tried to use ASE from Amplifier #2 to burn the pinhole, due to its low energy, it will take more than 2 hours to do it. So we left the laser running, however, after half an hour, we found there was no light emission. Checking the 532nm reflected mirrors one by one, we found two of them were burned. After changing them, it took one more day to realign the first amplifier. So far we already got the cleaning pulse from the amplifier #1, but the output energy is very low. We need time to optimize this multi-pass amplifier and then test the amplifier #2 and drill the pinhole.

Measuring the AMP2 beam profile

In order to measure the beam profile after the amplifier #2, we flip up the flip #3 before the mirror M16 ( see setup schematics ) to send the laser beam into a wedge. The laser beam was splitted by the wedge, and small part of laser beam was delivered into a SPRICON LBA-PC laser beam analyzer for diagnostics.

The beam profile, as shown in the right picture, looked very ugly. The diffraction patten implied the Ti:sapphire crystal or the reflected mirror was somehow damaged. After carefully observation, we found the crystal surface has been burnt two spots. Some anti-reflection coating areas probably were stripped by the strong pump laser.

So we replaced the damaged crystal with a new one, the measured beam profile was shown in the left picture. The light distribution looked homogeneous, no any diffraction was found.

Beam delivering mirrors were damaged again

After replacing the Ti:sapphire crystal of the first amplifier, we tested the CPA part 1 and part 2 this week. The output energies measured from test points were perfectly achieved what we expected. The crystal was not burned anymore after running 3 days, the output energy after first amplifier kept very stable from morning to the end of the work day. Unfortunately we found the surface of the beam delivering mirrors M18 and M19 was burned several dots. It's better to find what caused this damage before we change the mirrors. So we try to send the beam to the CCD camera to check the beam profile next week.

Why femtosecond lasers are not be used widely in industry?

In principle, femtosecond lasers provide a solution for most of micromachining, such as machining Teflon or glasses. The extremely high peak power means that nonlinear effects allow strong absorption even in transparent materials, enabling difficult materials to be machined. At the same time the very short pulses avoid thermal damage.

Unfortunately, femtosecond lasers have significant disadvantages. To date, most femtosecond lasers give high pulse energies at comparatively low repetition rates. The extremely high peak power tends to create a plasma at focus. The fireball is comparatively long-lived and significant thermal damage can result from the long-lived plasma. If the pulse energy is reduced to eliminate these effects, the material removal rate becomes extremely slow. Femtosecond lasers also tend to be complex, expensive, and high maintenance, making them unattractive for industrial use except where they are the only solution and the user fully understands their limitations. Therefore, while femtosecond systems are valuable research tools, they are not widely used in industry.

Digested Laser Focus World Vol. 43 (June, 2007)

Plasma mirrors generate high harmonics

For many researchers working with high-intensity lasers, plasma formation is often bad news. For others, it’s an enabling step for putting their laser systems to good use, in the form of so-called plasma mirrors. Now, collaborators at CEN Saclay (Gif-sur-Yvette, France), École Polytechnique (Palaiseau, France), and the University of Toronto (Toronto, Ont., Canada) have used a chain of plasma mirrors to create extreme-ultraviolet (EUV) pulses that they predict to be on the attosecond timescale, all with a tabletop laser.

In the highest intensity femtosecond-laser systems, a nanosecond-scale prepulse can be enough to ablate the surface of samples before most of the pulse energy arrives. Enter the plasma mirror, a simple but effective solution to increase the contrast between the prepulse and the main pulse. By placing a highly polished mirror blank in the beam at Brewster’s angle, the nanosecond-scale prepulse passes unchanged. The rising edge of the main pulse, however, converts the surface of the mirror into a highly reflective plasma, specularly reflecting only the main pulse. Once the prepulse is cleaned up by two plasma mirrors, a third forms the basis for high-order harmonic generation (HHG).

C. Thaury et al.,Nature Phys. doi:10.1038/nphys595 (2007).

Laser vision fuels energy future

Photonics.com reported:

The proposed European High Power laser Energy Research (HiPER) facility -- a device intended to demonstrate the feasibility of laser-driven fusion as an energy source -- is entering the preparation phase after completion of a two-year study by an international team of scientists.

Their conclusions have allowed the HiPER project to be selected as part of the European roadmap for future large-scale science facilities. The preparatory phase of the HiPER facility is expected to begin in January and to last for three years.

Achieving nuclear fusion using lasers is the goal of the National Ignition Facility, the latest in a series of high-power laser facilities used for research in inertial confinement fusion. Now under construction at the Lawrence Livermore National Laboratory, in Livermore, Calif., NIF is being built by the US Department of Energy as part of its Stockpile Stewardship program, and as such has a strong defense mission. "This is largely due to the fact that NIF converts its laser light to x-rays, and those x-rays are then used to implode the pellet (or perform other, classified experiments)," Dunne said. "This is meant to be analogous to the use of x-rays in thermonuclear weapons."

HiPER removes this link to defense science, Dunne said. "It uses the optical laser light directly to drive the implosion and initiate fusion. The physics associated with the interactions of lasers with matter have no relevance whatsoever to nuclear weapons, so we see this as very much a 'swords into ploughshares' undertaking."

Laser Fusion Facilities
LASER FACILITY	LOCATION	COMPRESSION ENERGY	IGNITION POWER	ESTIMATED START
National Ignition Facility	United States	1.8 MJ	NA	2009
Laser Mégajoule	France	2.0 MJ	NA	2011
FIREX-I + Gekko XII	Japan	10 kJ	1 PW (10 kJ)	2007
OMEGA EP	United States	30 kJ	2 PW (5 kJ)	2007
HiPER	Europe	200 kJ	10 PW (70 kJ)	proposed

For more information, visit: www.hiper-laser.org

Relativistic tennis with photons

Science daily news reported that a team from Advanced Photon Research Center at the Japan Atomic Energy Agency has demonstrated to generate an ultrashort and ultraintense x-ray pulse using the ordinary laser.

Sergei Bulanov of the Advanced Photon Research Center at the Japan Atomic Energy Agency in Kyoto and colleagues say they have a prototype that can generate pulses of x-ray laser light on the cheap. The researchers call their technique "relativistic tennis with photons," but a more violent analogy may better convey how it works. Suppose you throw a golf ball at a locomotive that is speeding toward you. The golf ball will bounce off it and come flying back at you with tremendous energy--just before you get run over.

The golf ball is a pulse of ordinary low-energy photons. With a tabletop setup, Bulanov and colleagues create the equivalent of a locomotive by firing a different laser into a cloud of plasma, where it creates a wake that travels at near-light speed. When the photons hit the wake, their energy increases 56-fold. They are also focused into an ultrashort, ultraintense blast by the wake, which is shaped like a miniature radar dish.

Improvement of the multipass amplifier

In order to protect the crystal, we tried many ways including decreasing the pump energy, walking away the focal point from crystal, reflecting back the pump beam, however the crystal still was burned sometimes. So this time we try to using the second beam to pump from the other side. The second was sent to dump before, now we added three mirrors to steer the beam into the crystal.

A good overview of plasma wakefield

Dr. Chan Joshi, the professor of University of California, Los Angeles, reviews recent progress in the development of plasma-based particle accelerators and considers the challenges still to be overcome to turn this concept into a practical technology for high-energy physics. This overview was published on recent CERN Courier Vol 47, No.5(2007).

(a) A simple 1D schematic of how wakefields are excited by a short-laser (top) or particle-beam (bottom) driver in a plasma. (b) 3D computer simulation of an extremely nonlinear wakefield excited by the drive beam in the "bubble" regime. The wakefield can accelerate an appropriately phased trailing beam at ultra-high gradients.