II. BACKGROUND
3.3: Methods
3.3.1: The FEL Pulse Stretcher
to help bridge the gap between the multimillion-dollar FEL and much cheaper and efficient bench top laser sources, as they become available.
The determination of the ablation threshold for the pulse stretched FEL is critical for carrying out any analysis of the ablation mechanism at 6.45 µm. There are many methods available for this determination; however, it is difficult to define what the actual threshold is. This definition; however, is critical to the decision of which determination method will be used. For the purposes of this paper, we will define the threshold of ablation as the amount of energy necessary to cause an ablation plume, i.e. ejected material, to be seen by visual inspection with one single micropulse with a 50%
probability (ED50) [5].
The ablated crater depth is another important metric for analysis of the ablation mechanism at this wavelength. It provides an easy method for analyzing the effects of ablation at 6.45 µm on a target tissue. To employ this metric, we measured the depth of a created crater given a defined number of pulses delivered at a constant radiant exposure.
when tuned to either 6.1 or 6.45 µm, has a beam diameter of about 12 mm. Therefore, it was necessary to telescope the beam down, with a 300 mm focal length curved mirror and a 100 mm focal length lens, by a factor of 3:1 in order to enable the use of a reasonable sized grating. The grating (ML303, Optometrics, Ayer, MA) was blazed at 10.6 µm with 150 lines/mm. Once the pulse passes through the telescope and is dispersed by the grating, it is then retro-reflected back to the grating to halt the dispersive effect. The beam is then translated vertically and returned through the system to be spatially reconstructed.
A diagram of the pulse stretcher used in this study is shown in Figure 3.1. The resulting delay per unit wavelength is a function of the dispersive power of the grating, the grating angle, and the length of the dispersive section of the stretcher. The resultant stretch was determined with a first-order geometrical approximation and confirmed with an autocorrelation technique as described by Kozub et al.[8]. The dispersive path is changed by moving the first retro-reflector; the minimum and maximum pathlength is determined by the beam size and blockage by optical mounts in the setup. The resultant delay interval can be varied from 30-200 picoseconds (FWHM) at either 6.1 or 6.45 µm.
A representation of the relative intensities for the stretched pulses from 1-200 ps is illustrated in Figure 3.2. The exiting pulse has the same divergence, waist size, and spectral content as the input pulse, but with a variable micropulse width. Losses in the pulse stretcher are due to both the efficiency of the grating and atmospheric losses. The pulse stretcher is currently open to the air and has a total pathlength of roughly 1.75 meters. The total efficiency of this stretcher is on the order of 30%. This allows for 3-4
1ps
FWHM * (ps/nm) Retro Mirror
Mirror Grating
Mirror Lens
Mirror
Curved Mirror Mirror
Figure 3.1 A diagram of the pulse stretcher is shown. After passing through a 3:1 telescope, the pulse is dispersed by a grating and is then retro-reflected back to the grating to halt the dispersive effect; the beam is then translated and returned through the system to be spatially reconstructed. The angle of the grating causes the longer wavelengths to travel a greater distance through the device than the shorter wavelengths;
giving the output pulse a frequency-dependent time delay (chirp) and a temporal width proportional to the bandwidth of the incoming pulse. The resulting delay per unit wavelength is a function of the dispersive power of the grating, the grating angle, and the length of the dispersive section of the stretcher. The exiting pulse has the same divergence, diameter, and spectral content as the input pulse with a variable micropulse width. Losses are mostly determined by the efficiency of the grating. The pulse stretcher currently in use with the Vanderbilt FEL has a total efficiency of about 30% and is capable of stretching FEL pulses at 6.45 and 6.1 µm from 30 to 200 picoseconds.
0 0.005 0.01 0.015 0.02
0 100 200 300 400 500 600 700
Time (ps)
Intensity (a.u.) .
1 ps 100 ps 200 ps
Figure 3.2 The relative intensity and duration of the 1, 100, and 200 ps micropulses are shown with two consecutive micropulses. The value on the y-axis is in arbitrary units and is scaled to 0.02 out of 1 to illustrate the relative intensity differences.
mJ per pulse to be delivered to the sample, which is enough to reach two to four times the ablation threshold of water, depending on wavelength.
Since the spectral content and hence the micropulse width varies considerably over the macropulse as shown in Figure 3.3, we quote the average micropulse width over the macropulse. Before each ablation experiment, the fast spectrum of the FEL macropulse was recorded, and the resulting average micropulse bandwidth was multiplied by the chip-rate to give the average micropulse duration over the macropulse from the stretcher. By tuning the FEL for the desired bandwidth, and adjusting the stretcher chirp-rate, it is possible to attain any pulse width between 30 and 200 picoseconds [8]. To attain 1 ps pulses with the same spatial characteristics as the stretched pulses, a mirror was placed after the telescope but before the grating, in order to bypass the stretcher. The output beam was viewed on a 2-D pyroelectric array (Spiricon Inc.) to verify the proper spatial reconstruction. The focused spotsize at the sample was measured using a scanning knife-edge technique.