Short Pulse Laser System
The short pulse laser at MEC is a double-CPA laser designed to operate at >25 TW peak power, with < 40 fs pulse duration and > 1 J per pulse. A schematic of the laser system is shown in Figure 1. A commercial CPA (Legend) system, seeded by an oscillator that is RF locked to LCLS, generates mJ-level compressed pulses, which are passed through an OPA pulse cleaning system before being stretched again for further amplification. The first bowtie amplifier operates at 120 Hz, matching the LCLS x-ray pulse rate, outputting >10 mJ per pulse. A second bowtie stage amplifies to >1.5 J at 5 Hz repetition rate. After compression, this yields > 1 J in 40 fs, for 25 TW peak power.
Figure 1. Diagram of the MEC short pulse laser system.
Pulse Duration
The final output has a pulse duration of approximately 35-55 fs, with a beam diameter of 65 mm, and an energy of 1 J. The compressor grating spacing can be adjusted to produce between 35 fs and 8 ps pulses. For pulses of longer duration, stretched pulses with 150 ps duration have been provided successfully.
Figure 2. (legacy information: please contact) Wizzler measurement of the pulse shape of the MEC short pulse laser indicating compression to nearly transform-limited pulse at 23 nm bandwidth.
Wavefront Correction
After amplification, a 90 mm clear aperture deformable mirror (DM; cf. Figure 3), using feedback from an online wave front sensor (WFS), corrects the distortions produced in the laser chain. Focal spot and wave front measurements in the chamber allow for pre-correction of wave front distortion from the compressor and focusing optics. This system is necessary for producing the highest intensity at focus.
Figure 3. A photo of the deformable mirror of the MEC short pulse laser is shown.
| Before Correction | Before Correction | After Correction |
|---|---|---|
| RMS | 0.6 µm | 22 nm |
| P-V | 2.5 µm | 100 nm |
Table 3. Demonstrated wavefront correction of the DM/WFS closed-loop system
Focusing
Short pulse experiments use non-standard beam delivery, so users should contact MEC scientists to discuss the best focusing options and expected performance for their experiment. An inventory of off-axis parabolic mirrors (OAPs) is given in table 4. Note that the standard full-power (1 J, <50 fs) beam size is roughly 65 cm, and so OAP 4 is incompatible with this mode. Recent example measurement of the focus from OAP 3 is given in figure 4.
MEC OAP Inventory for Run 20
| Name | CA (mm) | Effective focal length(mm) | Off-axis angle (deg) | coating | notes |
|---|---|---|---|---|---|
| OAP1 | 99 | 311.66 | 17.55 | Dielectric | Incompatible with standard target mount |
| OAP2 | 99 | 311.66 | 17.55 | Dielectric | Incompatible with standard target mount |
| OAP3 | 99 | 330.4 | 35.31 | Silver | Primary OAP for full intensity |
| OAP4 | 50 | 500.58 | 34.41 | Silver | For long focal length and low power. Only compatible with reduced beam size and power. |
Table 4. The OAPs owned by MEC. OAP 3 is standard for full intensity experiments. OAP 1 and 2 have too tight of an angle to use with the standard target frame, but may be used in certain customized experiments. OAP 4 is intended for frequency doubled or uncompressed experiment in which the beam size is smaller than standard. Typical use is to produce an f/10 focus.
Figure 4. A false color log scale focal spot image of the focus from OAP with full wavefront correction. The measurement was taken at reduced power but full power inferred intensity is shown. Lineouts are shown in linear scale. The right image depicts a false color image in linear scale.
Pulse Contrast
Nearly all user experiments using the multi-TW beam require excellent contrast to be successful. To achieve highest baseline pulse contrast, the short pulse laser system employs a double-CPA front end. After initial chirped pulse amplification of the oscillator pulses at 120 Hz, the beam is recompressed and passed through a nonlinear filter (with an OPA and SFG-based pulse cleaner [1] replacing XPW starting in Run 17) and then stretched again for final amplification. This filter removes prepulses and pedestal energy that builds up primarily in the laser front end. Measurements utilizing third order autocorrelation techniques revealed a contrast better than 107 in the > 3 picosecond range using a double CPA pulse cleaning system. To further increase this contrast, MEC can provide a 2nd harmonic stage that, in combination with dichroics, should theoretically achieve better than 1014 contrast beyond 3 ps, while sacrificing pulse energy (output ~300 mJ at 400 nm).
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, referred to as the time tool, pictured in Figure 5. Ultrafast ionization by the x-ray beam in YAG windows allows its synchronization relative to the MEC laser to be precisely set. The ionization causes the window to become opaque showing definite before/after timing. By angling a window relative to the x-ray beam, the time tool gives synchronization to within a few fs. The time tool is cross-referenced to the target plane using another YAG window at the laser focus position, with a small f-stop temporarily inserted to spread the laser focus at the target plane. The time tool measures an RMS jitter of ~ 100 fs between the MEC laser and the x-ray beam.
Figure 5. Image of the MEC timing tool.
Figure 6. Synchronization of laser to the X-ray the time tool. Left: geometry of x-ray and probe laser on YAG crystal. Right/Top: laser shadography of a target window on two consecutive laser shots, arriving just before (left) and just after (right) an X-ray pulse (square shaped beam); Right/bottom: The time tool shows arrival time of the ionizing X-ray pulse inline, demonstrating that the synchronization jitter between the two consecutive pulses shown here is less than 100 fs.
Diagnostics for the short pulse laser system
The short pulse laser diagnostic suite 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.
- On-shot diagnostics:
- Time tool (synchronization)
- Pulse energy
- Near-field beam profile
- Wavefront
- Equivalent plane focal spot
- Spectrum
- On-demand diagnostics (off-line)
- SPIDER
- 3rd order autocorrelation
- Far-field beam profile
- Motorized power meters