David found the compressor grating became dirty, that caused the output energy lower. He suggested to order a new grating or clean the grating. We ordered a new grating from Spectra-Physics, the model number is 9800-2680, the size is 30x110x16. The grating cleaning procedure is listed below.
Direct Contact Method:
Spectra-Physics RGL provided the direct contact method and they have used it on gratings with
minor dirt or oil contamination.
1. Select a pan large enough to accommodate the grating as well as being able to allow unrestricted movement around the grating. A 20cm x 20cm x 5cm (8" x 8" x 2") aluminum baking pan was used here.
2. Heat water temperature to 32 C (90F) +/-1C.
3. Mix together Woolite 2 capfuls of (or equivalent) with water.
http://www.fabriclink.com/Woolite/Home.html in case one is not familiar with Woolite.
4. Place grating in the soapy water pan.
5. The water should cover the top surface of the grating by 10-12mm.
6. Adjust water level in pan accordingly and maintain water temperature of 32-34 C.
7. Grating is to be lightly stroked with a cotton ball in the direction that is parallel to the groove.
7.1. Note: "Stroking" means to drag the cotton ball across grating, applying no physical pressure, allowing just the weight of the wet cotton to contact the grating surface.
8. Remove the grating from the soapy water pan and rinse with distilled water. Make sure that the water is flowing down the grooves, not across the grooves.
9. Rinse the grating with isopropyl alcohol (rubbing alcohol 70% by volume) with the alcohol going down the grooves not across. Be careful for the alcohol will dissolve the ink on the grating. I would advise that one write the groove density on the grating with a pencil.
10. Blow dry the grating with nitrogen (or clean "air") starting at the top allowing the alcohol to run down the grooves. As with the rinsing, blow down the grooves not across them. When testing this method at SPMV, dry nitrogen at a pressure of 50 psi through a 6mm dia. nozzle was used. This dried the grating well.
Bake Out Method
The bake out method is generally used when the grating exhibits a film or 'fog' on the surface of the grating. The cause and/or composition of this film is unknown. However, baking out the grating usually results in the elimination of the film.
Place gratings in the oven with the gold surface up. Take care when placing the gratings so that the gold surface does not come in contact with anything.
Ramp up oven over at a 2 hour rate to 105 C.
Bake the gratings for 2 hours at 105 C.
Ramp down the oven at a 2 hour rate to room temperature.