It is not uncommon for the 1' and 2" amplifier heads to leak water at a very slow rate or to "discharge" a mist of water when the heads are fired. The primary cause for this condition has been the degradation of the Nd:Glass rod thrust washers and/or rod Orings used to seal the end of the Nd:Glass rods to the amplifier head.
Signs that a water leak exists:
There are several different signs and techniques that can be used to confirm if an amplifier rod is leaking. The most obvious initial sign that a small leak exists is when one of the LPL beam 527nm cameras indicates a slightly or severely distorted spatial profile around the outer circumference of the beam:
If a similar pattern (as shown above) is displayed from one of the two main 527nm profile cameras, then it can be assumed that the distortion is a result of a water mist or vapor is being discharged into the beam path after an amplifier-pair leg or legs when the amplifiers are fired. The next step is to determine which amplifier head(s) are leaking and causing the spatial distortion.
Obvious signs of a problem
There may be visible signs of water accumulation near either end of the amplifier rod, on optical surfaces that are near the end of the rod, or signs of damage on the Delrin thrust washer as shown below. These are all obvious signs of a problem that needs attention. Leaks can also exist in areas not as obvious as those shown here:
This optic used to reflect a small portion of the 1053nm beam had signs of prolonged leaks from the adjacent amplifier head as permanent water damage spots were spattered over a large portion of the optic.
Not-so-obvious signs of a problem-procedure to eliminate non-leaking ends/heads
If it is not obvious which amplifier head may be the source of the water leak, then the following steps will help in determining which end(s) of the the amplifier head(s) are leaking. Since the first sign that a problem exists may have been the camera image from one of the two 527nm cameras used to monitor the spatial profile of the four beams, it will be necessary to eliminate amplifier legs that are not the cause of beam distortion. Since each camera image displays two coincident beams simultaneously:
- block the input beam into one amplifier leg while allowing the input beam from the other parallel leg to pass through the respective amplifiers. Alternatively, the de-selecting of one amplifier pair from firing on the LPL GUI is another way of achieving the same thing.
- Since the energy into the screen that is imaged to the camera will be approx. 50% lower with only one leg firing, it will be necessary to remove or change the ND filters used with the camera.
- Fire the heads that are amplifying the non-blocked input beam and determine if the distortion in the camera image is present or not. If not, then reverse the input beam that is being blocked and fire the other parallel leg. If the beam distortion was present, then repeat the procedure but de-select one of the two amplifier-pair heads from the GUI, lower the ND level again and take another shot in effort to isolate the head with the problem.
If it remains difficult to isolate or determine which head or end is leaking, then try the following procedure:
Another not-so-obvious signs of a problem-procedure to identify leaking ends/heads
This is a procedure that can be used in the absence of an amplified or un-amplified light beam. This procedure will focus on determining if an individual head is leaking by using a technique that will indicate the presence of water discharging when the amplifier head is fired and a the laser seed beam is off. Since water droplets or water misting is quite easy to visibly see on an optical surface, this test will require that a large diameter (>3") optic be placed as close as possible to the end of each amplifier head being tested for water leaks. For the purpose of this test, an old, scratched optic was selected as the surface to be used to detect the water leak. It is not recommended to use an optic that has other practical uses than this test as water can be difficult to clean off of optical surfaces.
After the optic has been mounted in an optical mount and placed as close as possible to the amplifier end of interest, fire the head of interest at the normal operating pump voltage. After the head has been fired, remove the optic mount and look carefully for any signs of water on the optic surface that was facing the amplifier rod.
Test optic and mount placed as close a possible to the end of the amplifier head of interest.
With the oscillator/seed beam completely off, fire just the head of interest. After firing, remove optic mount and inspect:
Test optic clearly shows signs of water discharged after firing head.
Complete this procedure on each end of each amplifier head where there is suspicion of a leak, Or use several optics/mounts at different ends of amplifiers at one time to minimize the time required to identify leaks. This technique proved to be a simple and accurate way to determine the location of water leaks that only occur when the head(s) are fired.