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An Office of Science User Facility

Matter in Extreme Conditions (MEC)

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Laser Systems at MEC

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 Short Description

 

 

MEC provides a range of laser capabilities through two distinct laser systems. Most of the experiments that require high energy pulses in the nanosecond regime are typically performed using the long pulse laser system, whereas research involving high peak intensity lasers, such as high energy density physics, secondary X-ray or particle sources, or sub-picosecond time resolved phenomena use the short pulse laser system. The following diagram represents the laser chain for both of these laser systems.

 

Figure 1 Diagram of laser systems at MEC.
(left) Short pulse laser system, (right) Long pulse laser system

 

For more information contact granados@slac.stanford.edu or the Point of Contact for the experiment.
 

 Long Pulse Laser System

 


Long Pulse Laser System

 

The long pulse laser at MEC is designed to provide high-energy shapeable pulses at 527 nm. The front end of the system consists of a CW fiber oscillator operating at 1054nm. The output of the oscillator is injected into a programmable electro-optical (EO) modulator and shaped to the desired pulse length and shape. The EO modulator has a rise time of 200 ps with a time jitter of approximately 20 ps. The pulse duration can be adjusted from 2 ns up to 200 ns.

The output of the pulse shaper is fiber coupled to a multi-pass Nd:Glass amplifier capable of producing pulses of 100s of mJ of energy (depending on pulse shape), and temporally filtered using a pulse slicer, which minimizes the effects of amplified spontaneous emission in the pre-amplifiers. The pulse is subsequently amplified to the J-level and split into two arms. Each arm is further amplified to more than 50 J, and finally frequency doubled to 527nm using large aperture KDP crystals. After the second harmonic stages, the maximum pulse energy per arm is 25 J for pulses that are 25 ns or longer.  

 

 

 

The repetition rate of the long pulse laser is limited to one shot every 7 minutes, mainly due to the geometry of the laser amplifier heads and their cooling system. The near-field profile of the produced beam is 42 mm in diameter. Two isolators prevent back reflections from targets, while the 527 nm and the remaining 1054 nm are separated using a pair of dichroics in each leg.

Pulse Shapes

​For flat-top pulses, the energy scaling of each arm is approximately 1.0 J per ns. Figure XX shows the trend of pulse energy varying the pulse duration. For pulses that are 20 ns or longer the output energy increases with much lower slope, achieving a maximum of around 25 J per arm for 50 ns flat-top pulses.

For flat-top pulses, the energy scaling of each arm is approximately 1.0 J per ns. Figure XX shows the trend of pulse energy varying the pulse duration. For pulses that are 20 ns or longer the output energy increases with much lower slope, achieving a maximum of around 25 J per arm for 50 ns flat-top pulses.

Arbitrary waveforms can be generated on demand. The next set of figures serve as examples of different waveforms utilized in experiments at MEC, measured at full energy at the output of the long pulse laser.

 

This is an example of 10 ns waveforms with different types of slopes (flat-top, ramp and exponential). Users specify the desired waveform before their experiment, whereas the final optimization of the pulse shape can be also performed during the experiments to optimize the results.

The minimum pulse duration that the pulse shaper can generate is approximately 2 ns, above is an example of a ramp waveform with 2 ns, 3 ns and 10 ns duration.

 

 

 

Shot to shot stability

Flat-top waveforms are widely employed for shock experiments. The typical variation in amplitude for pulses between 5 and 20 ns is approximately +/- 10%, while the leading edge of this waveform is around 250 ps.

​Flat-top waveforms are widely employed for shock experiments. The typical variation in amplitude for pulses between 5 and 20 ns is approximately +/- 10%, while the leading edge of this waveform is around 250 ps.



Depending on the waveform in use, the stability of the output of the long pulse laser vary considerably. For example, for flat-top pulses, the RMS shot-to-shot pulse energy variation is less than 10%, while for ramp or exponential waveforms can be up to 25%.

 

 

The output energy is controlled by varying the angle of half waveplates placed before the main amplifier heads, in combination with thin film polarizers located after the same amplifiers. For each pulse shape the output energy vs angle is calibrated. The following figure depicts an example of a waveplate calibration performed for 10 ns flat-top pulses.

 

 

 

Focusing optics and far field beam profiles

The two arms are typically focused with 250-500 mm focal length lenses, and phase plates are used when large spot sizes are needed for experiments.

When the long pulse laser beams are focused using simply a lens, the far field profile is as shown in the following figure. The minimum spot size achieved is approximately 10 um in diameter. Placing the sample of interest after the focal point of the laser beam enables using a roughly Gaussian beam with 100s um diameter.


MEC also provides a set of phase plates to generate a flat-top beam profile on sample with spot diameters ranging from 100 to 500 um.

Beam combination

Presently MEC offers the possibility of combining the two output beams of the long pulse laser into a single laser beam. The combiner is composed of a thin film polarizer and a half wave plate, with a total efficiency of 80% although the output polarization in this arrangement is not defined.

Long pulse laser diagnostics

The diagnostic toolbox for the long pulse laser includes characterization of the pulse temporal profile and energy on every shot and, upon request, equivalent plane cameras for far-field images of the spot on target. ​

 

 Short Pulse Laser System

 

 

Short Pulse Laser System

The short pulse laser is a Chirped Pulse Amplified Ti:Sapphire laser, laser at 800nm, with pulse length as short as 40fs, and pulse energies up to 1J. The laser system consists of a master oscillator, pulse strecher and regenerative amplifier, multipass amplifier and a vacuum compressor. The laser is located within the MEC hutch.

 


The oscillator is locked to an RF reference signal produced by the linac, and its output is stretched to 160 ps and amplified in a commercial regenerative amplifier.

 

XPW

The output of the amplifier is recompressed and used as seed for the pre-pulse cleaning stage, which consists in a cross-polarized-wave generator (XPW) that enhances the pulse contrast and reduce the pre- pulse and amplified spontaneous emission (ASE) on the laser. Currently the extinction ratio of the polarizers in the XPW limits the pulse contrast to approximately 107 measured after full amplification. The XPW output is again stretched to 160 ps and amplified in a six passes bow tie amplifier to approximately 25 mJ at 120 Hz.

 

Multi-pass amplifier 1

This amplification stage operates at the full rate of the accelerator and can be used as pump for other devices such as THz or OPA stages, increasing the wavelength range available for experiments.

 

MPA 1 performance ​ ​
Max energy (mJ)​ 28.5 mJ​
​Set point 20 mJ​
​Number of passes ​6
​Energy stability (over 5,000 shots) ​<1% rms
Repetition rate ​120 Hz
  

 

Multi-pass amplifier 2
The second multi-pass amplifier produces up to 1.5 J pulses at a repetition rate of 5 Hz.
 
MPA 2 performance ​ ​
Max energy (mJ)​ 2 J​
​Set point 1.7 J​
​Number of passes ​4
​Energy stability (over 5,000 shots) ​<2% rms
Repetition rate ​5 Hz
  
 

Currently, the construction of another multi-pass amplifier is being completed, producing up to 10 J per pulse at a repetition rate of 1 shot every 7 minutes.

 

Wavefront correction

After amplification, a deformable mirror in combination with a wavefront sensor corrects the distortions produced in the laser chain and sent to the compressor. The spot on sample is optimized afterwards using a second feedback loop that corrects the distortion introduced by the compressor and focusing optics in the experimental chamber.

​Before Correction ​After Correction
0.576 um RMS distortion​ ​22 nm RMS distortion
​2.502 um P-V 100 nm P-V​
 

 

 


Pulse duration

The final output has a pulse duration of approximately 40-55 fs, with a beam diameter of 52 mm, and an energy of 1 J. The compressor can be adjusted to produce between 40 fs and 8 ps pulses. For pulses of longer duration, it’s possible to provide the stretched pulse (160 ps in duration).


Peak Intensity

The spot size after optimization using the deformable mirror depends mainly on the focusing element in use. For example, intensities in the order of 1019 W/cm2 have been achieved using F10 and F2 parabolas, as shown in the following figures. The pointing stability for a focused 10 um beam was measured to be better than <2 um RMS.

 

Pulse contrast

Measurements utilizing third order autocorrelation techniques revealed a contrast better than 107 in the picosecond range, with an ASE pedestal at 108 . To further increase this contrast, MEC provides a second harmonic stage which in combination with dichroics can achieve more than 1014 contrast, at a cost of pulse energy (currently limited to around 200 mJ at 400 nm) and beam quality.

 

Timing jitter and synchronization with the LCLS X-ray beam

Taking full advantage of the temporal resolution of femtosecond X-ray pulses and femtosecond optical lasers in pump-probe experiments requires timing measurement of a few 10 fs or better. LCLS has a pulse-to-pulse timing jitter relative to the accelerator radio-frequency (RF) distribution of approximately 60 fs RMS, integrated over a bandwidth of 0.1-100 kHz. The MEC short pulse laser oscillator is locked to the accelerator RF, with a distribution with similar or better timing jitter. Drifts in the laser beam path and RF distribution need to be controlled to approximately the 1 ps level.

The mode-locked seed laser oscillator operates at 68 MHz, the seventh sub-harmonic of the 476 MHz RF reference frequency of LCLS, which defines the fine (sub-picosecond) timing of laser pulses in the absence of configuration changes to the laser system.

The noise performance of the LCLS laser locking system was measured to have an RMS laser-to-RF-reference timing jitter of 25 fs between 100 Hz and 100 kHz. Below 100 Hz, phase noise is dominated by the noise of the linac RF reference.

 

Time Tool

To surpass the limitations of this jitter, experiments involving the fastest physical phenomena (e.g. transitions of core shell electrons) measure the relative timing between the laser and X-rays on a shot-by-shot basis by means of X-ray/optical cross-correlation techniques.

 
                                               Time Tool at MEC

 

Diagnostics for the short pulse laser system

The short pulse laser diagnostic toolbox includes a variety of devices that are accessible on-line and on demand. Here is a list of the common diagnostics already installed in the toolbox.

  • Single-shot diagnostics:
    • Pulse duration (single shot autocorrelation)
    • Time tool (synchronization)
    • Pulse energy
    • Near-field beam profile
    • Wavefront
    • Estimated spot on target or equivalent plane (future capability)
    • Spectrum
  • On-demand diagnostics
    • SPIDER
    • 3rd order autocorrelation
    • Far-field beam profile
    • Motorized power meters

 

 

         Diagram of diagnostic setup for the short pulse laser system

SLAC SLAC National Accelerator Laboratory, Menlo Park, CA
Operated by Stanford University for the U.S. Dept. of Energy