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Among other lasers, I work with Frequency-doubled pulsed YAG in my lab. One of our laser cavities is acting up and I wanted to share the debugging process as it might be informative to anybody building/repairing/restoring a similar laser.
The laser is an old Quantel "Brilliant" and part of a two-oscillator "Twins" system. The laser operates at 10 Hz, producing 5ns, 5mm diameter, 150mJ @532nm pulses (after frequency doubling, 1064nm at perhaps 300mJ).
- The cavity consists of a single Nd:Yag rod pumped by a single flashlamp.
- The rod faces polished at a slight angle (Brewster's angle, I believe)
- It has a guassian output coupler
- The back mirror is slightly concave (i don't know the focal length, maybe 1000mm).
- The Q-switch is an electro-optic Pockels cell.
The problem:
I was running a PIV experiment and I started noticed that the laser energy was starting to fade (my images were getting progressively darker) - at first I suspected the doubling crystal alignment was slipping. After half a day, the energy had significantly dropped, so I decided to check under the hood.
What I checked:
Using an Ophir power meter, the laser power measured 0.5W (@ 10 Hz that's ~50mJ per pulse). This is much too low! The other laser measured a normal 1.4W.
I took some laser burns before (1064) and after (532) the frequency doubler with both lasers. The second laser produced a normal looking burn, while the trouble laser produced a splotchy looking spot at both locations --- that means the frequency doubling optics (and beam combining optics) are OKAY.
Therefore the trouble must be in the cavity.
The laser is old, and there were a good number of flashes on the flashlamp. With the brilliant system, pulse energy is controlled only by Q-switch delay; therefore, every lamp flash is at full energy. I suspected that the lamp might be dying.
I opened the pump chamber and replaced the flashlamp with a new one. The laser rod, UV filter, and reflectors did not show any signs of green beasties growing, so I was hopeful my problem would be solved! It was not. :yabbem:
1064 burns looked the same as before and only a slight increase in 532nm laser output (~60mJ) (Picture 1064nm burn spots attached)
The next thing to check is the back mirror, Pockels cell, 1/4 waveplate, and the polarizer. Unfortunately, the laser is in a cramped "woodshed" built around a wind tunnel. It is very dusty in the area, so I won't venture into Q-switch cavity and disassemble the Q-switch until I move the laser head to an optics bench in a cleaner environment. However, I did peek in the cavity and the back mirror, waveplate, and polarizer look okay (no obvious damage). I tried to use a small mirror to inspect the Pockels cell (P.C.), but the cavity is much too cramped to get a proper look without pulling the P.C. out completely.
I talked with an engineer at Quantel and the suspicion is damage on the P.C. or back mirror (I think the back mirror is okay)
I will update when I find out more!
The laser is an old Quantel "Brilliant" and part of a two-oscillator "Twins" system. The laser operates at 10 Hz, producing 5ns, 5mm diameter, 150mJ @532nm pulses (after frequency doubling, 1064nm at perhaps 300mJ).
- The cavity consists of a single Nd:Yag rod pumped by a single flashlamp.
- The rod faces polished at a slight angle (Brewster's angle, I believe)
- It has a guassian output coupler
- The back mirror is slightly concave (i don't know the focal length, maybe 1000mm).
- The Q-switch is an electro-optic Pockels cell.
The problem:
I was running a PIV experiment and I started noticed that the laser energy was starting to fade (my images were getting progressively darker) - at first I suspected the doubling crystal alignment was slipping. After half a day, the energy had significantly dropped, so I decided to check under the hood.
What I checked:
Using an Ophir power meter, the laser power measured 0.5W (@ 10 Hz that's ~50mJ per pulse). This is much too low! The other laser measured a normal 1.4W.
I took some laser burns before (1064) and after (532) the frequency doubler with both lasers. The second laser produced a normal looking burn, while the trouble laser produced a splotchy looking spot at both locations --- that means the frequency doubling optics (and beam combining optics) are OKAY.
Therefore the trouble must be in the cavity.
The laser is old, and there were a good number of flashes on the flashlamp. With the brilliant system, pulse energy is controlled only by Q-switch delay; therefore, every lamp flash is at full energy. I suspected that the lamp might be dying.
I opened the pump chamber and replaced the flashlamp with a new one. The laser rod, UV filter, and reflectors did not show any signs of green beasties growing, so I was hopeful my problem would be solved! It was not. :yabbem:
1064 burns looked the same as before and only a slight increase in 532nm laser output (~60mJ) (Picture 1064nm burn spots attached)
The next thing to check is the back mirror, Pockels cell, 1/4 waveplate, and the polarizer. Unfortunately, the laser is in a cramped "woodshed" built around a wind tunnel. It is very dusty in the area, so I won't venture into Q-switch cavity and disassemble the Q-switch until I move the laser head to an optics bench in a cleaner environment. However, I did peek in the cavity and the back mirror, waveplate, and polarizer look okay (no obvious damage). I tried to use a small mirror to inspect the Pockels cell (P.C.), but the cavity is much too cramped to get a proper look without pulling the P.C. out completely.
I talked with an engineer at Quantel and the suspicion is damage on the P.C. or back mirror (I think the back mirror is okay)
I will update when I find out more!