Crystal surface burned again

When the output energy after the PC2 became lower, I tried to tune the pump mirrors to maximize the output. During the tuning, I found the energy really became bigger, unfortunately the left side of the crystal was burned again. The similar phenomenon happened before, I think the measured energy was not the laser pulse but the spontaneous emission. When ASE goes higher, the pump beam was focused smaller, which would damage the crystal surface. Next tuning time, it's better to measure the pulse shape using the diode and make sure no ASE anymore.

I also found the Ti:sapphire crystal was not installed well, I put more Indium layer to hold the crystal tightly. Then I realigned the amplifier #1, changed the times of optical pass from 6 to 7.

Creating the pinhole for the spatial filter

I optimized the amplifier #2 alignment and obtained the normal output energy. Then the beam was delivered into the spatial filter. We already installed a piece of window for burning the pinhole last week. The seed beam from oscillator was blocked, so the ASE beam from first and second amplifiers was used to burn the hole. After about one and half hours, the beam could be watched from the spatial filter other side. The beam profile looked good by eyes, the real profile should be measured using the SPIRICON LBA camera.

YAG #1 and YAG #2 delivering mirrors burned

The amplifier #2 worked well, the next step is to send the beam to the spatial filter, before that we must drill a pinhole using the laser beam. We chose a piece of used window to replace the old window with a bad pinhole. We tried to use ASE from Amplifier #2 to burn the pinhole, due to its low energy, it will take more than 2 hours to do it. So we left the laser running, however, after half an hour, we found there was no light emission. Checking the 532nm reflected mirrors one by one, we found two of them were burned. After changing them, it took one more day to realign the first amplifier. So far we already got the cleaning pulse from the amplifier #1, but the output energy is very low. We need time to optimize this multi-pass amplifier and then test the amplifier #2 and drill the pinhole.

Measuring the AMP2 beam profile

In order to measure the beam profile after the amplifier #2, we flip up the flip #3 before the mirror M16 ( see setup schematics ) to send the laser beam into a wedge. The laser beam was splitted by the wedge, and small part of laser beam was delivered into a SPRICON LBA-PC laser beam analyzer for diagnostics.

The beam profile, as shown in the right picture, looked very ugly. The diffraction patten implied the Ti:sapphire crystal or the reflected mirror was somehow damaged. After carefully observation, we found the crystal surface has been burnt two spots. Some anti-reflection coating areas probably were stripped by the strong pump laser.

So we replaced the damaged crystal with a new one, the measured beam profile was shown in the left picture. The light distribution looked homogeneous, no any diffraction was found.

Beam delivering mirrors were damaged again

After replacing the Ti:sapphire crystal of the first amplifier, we tested the CPA part 1 and part 2 this week. The output energies measured from test points were perfectly achieved what we expected. The crystal was not burned anymore after running 3 days, the output energy after first amplifier kept very stable from morning to the end of the work day. Unfortunately we found the surface of the beam delivering mirrors M18 and M19 was burned several dots. It's better to find what caused this damage before we change the mirrors. So we try to send the beam to the CCD camera to check the beam profile next week.

Why femtosecond lasers are not be used widely in industry?

In principle, femtosecond lasers provide a solution for most of micromachining, such as machining Teflon or glasses. The extremely high peak power means that nonlinear effects allow strong absorption even in transparent materials, enabling difficult materials to be machined. At the same time the very short pulses avoid thermal damage.

Unfortunately, femtosecond lasers have significant disadvantages. To date, most femtosecond lasers give high pulse energies at comparatively low repetition rates. The extremely high peak power tends to create a plasma at focus. The fireball is comparatively long-lived and significant thermal damage can result from the long-lived plasma. If the pulse energy is reduced to eliminate these effects, the material removal rate becomes extremely slow. Femtosecond lasers also tend to be complex, expensive, and high maintenance, making them unattractive for industrial use except where they are the only solution and the user fully understands their limitations. Therefore, while femtosecond systems are valuable research tools, they are not widely used in industry.

Digested Laser Focus World Vol. 43 (June, 2007)