Modern lasers have intrinsic pre-pulses associated with main pulses which disrupt experiments by destroying the surfaces of targets. These pre-pulses, if understood could be entered into a simulation to understand the changed surface of a target. Pre-pulse diagnosis is difficult because of the damaging effects of the impending main pulse. Using a special technique called a “water cell,” the safe characterization of a pre-pulse can be accomplished. The separation of an inherent pre-pulse from the main pulse of a high intensity short pulse laser is possible using a tube of water known commonly as a “water cell.” The water cell diagnostic setup blocks the main pulse from entering the characterization equipment, so that only the pre-pulse is characterized.
A high intensity short laser pulse, created through modern laser amplification, is always accompanied by an intrinsic pre-pulse. All laser pulses have a pre-pulse which is due to the techniques used in the creation of the main pulse. Modern laser systems use a type of amplification meant to greatly increase the intensity of a main pulse but unfortunately also increases the pre-pulse . Generally this pre-pulse can be decreased through a number of processes, but there will always be some pre-pulse which remains. Today, in a typical high intensity short pulse laser experiment the pre-pulse is on the order of 1,000 times less intense than the main pulse and the pre-pulse peak is about 20 ns in front of the main pulse. Some laser systems can reach intensities of 1020 W/cm2 with associated pre-pulse intensities of 1017 W/cm2. Below in Figure 1 is an example of an actual trace of a laser pre-pulse and main pulse.
In many experiments, the pre-pulse intensity can destroy a known target surface before the arrival of the main pulse. In physics experiments, it is essential to know the exact structure of a target at the time of the impact of the main pulse. If the exact structure is not known, it is impossible to properly extract the science from a high-intensity experiment. Unfortunately a pre-pulse at 1017 W/cm2 is intense enough to ionize atoms on the surface of a target and thus destroy the target . In this case, the surface of a target is changed before the arrival of the main pulse and therefore much of the data from the experiment is unusable, unless the characteristics of the pre-pulse and the changes caused by it are known.
We can use diagnostic tools to determine the pre-pulse characteristics, the shape and intensity, in order to understand how it changes the surface of a target. Pre-pulses are simple packets of light that can be diagnosed using a photodiode and an oscilloscope, which is an electronic device that interprets the photodiode output signals. A photodiode captures light and emits current proportional to the amount of light received. The associated voltage of this current pulse is graphed vs. time on an electronic device called an oscilloscope. The output graph can be analyzed to understand the shape and intensity of the pre-pulse.
Unfortunately a pre-pulse that can damage a target can also damage a photodiode. Thus, in order to get a pre-pulse low enough in intensity to not destroy the photodiode a trick known as a light pick-off must be employed. Pick-off light is the light that leaks through the backside of a mirror. Even an efficient mirror which reflects almost all of the light leaks a small percentage through the mirror. This light, although much lower in intensity, maintains the exact profile of a main pulse and pre-pulse. This pick-off has a pre-pulse that is low enough in intensity that it can safely be diagnosed by a photodiode.
At a lower intensity pick-off, the pre-pulse does not destroy the photodiode, but the main pulse does. The pick-off main pulse, 1000 times larger than the pick-off pre-pulse is still high enough in intensity that it can destroy a target. Thus the only way for a safe diagnosis of a laser pre-pulse is the removal of the pre-pulse from a main pulse.
One method that can be used to cut off a main pulse from a pre-pulse is that of a “plasma mirror.” Plasmas reflect light and thus can be used to stop the main pulse from hitting a photodiode. If plasma could be created in a medium after the pre-pulse passes through, then it could be used to stop the main-pulse. Plasmas are created when sufficient energy is injected into a medium. The molecules in the medium ionize and the electrons are freed from their original energy levels. Thus plasma could theoretically be created in a medium after a pre-pulse and stop a main pulse.
Laser pulses can be used to ionize water molecules and create plasma . A main pulse can actually ionize water and if the intensity was matched to the medium, the front of the main pulse will theoretically create plasma which would reflect the rest of the main pulse. A pick-off of the amplified laser can be focused to a spot in a tube of water, allowing for such a separation of the main pulse and pre-pulse. And, if tuned correctly the pre-pulse will pass through the water unchanged and focused onto a photodiode. The setup shown below in figure 2 lets a pre-pulse pass through a tube of water and be diagnosed by a photodiode and oscilloscope.
The shown setup involves two photodiodes. The first diode sees both the pre-pulse and the main pulse. It has filters in front of it that decrease the intensity of light in both of the pulses. Since the 50-50 beam splitter passes exactly half of the light to this diode, it is used to calibrate with the second “water cell” diode. As shown, the first lens (L1) focuses the pre-pulse and main pulse into the water. The other two lenses (L2 and L3) are used to focus the light onto the photodiodes. The pre-pulse will be safely diagnosed by the photodiode and oscilloscope and the main pulse will be reflected away by the plasma.
A water cell can be used to separate the main pulse and from the pre-pulse in a high intensity short laser pulse so that the pre-pulse profile and intensity can be understood. The main pulse is focused to an intensity high enough to ionize the water and create a plasma mirror. This plasma mirror reflects the main pulse away allowing for separation of the main pulse from the pre-pulse.