I've seen the beam from the Subaru Telescope's adaptive optics laser on several occasions, and those of the Keck II and Gemini North lasers many more times, but today was the first time I had the opportunity to actually see the source of Subaru Telescope's beam.I accompanied a couple co-workers and a couple students to the laser enclosure in the dome. After donning a hair net and face mask and taking off my shoes and jacket, I went through an antechamber containing computers, electronics and cooling equipment, and into the laser room.
The Associate Professor in charge of the laser and related optics explained that because optical fibers have trouble with lasers that are too powerful, splitting each pulse into four helps keep the fiber happy, and also keeps the bandwidth of the light narrow.
So 1/4 of the light passes through both beam splitters without delay, 1/4 goes through only the short delay loop and gets the short delay, 1/4 goes through only the long delay loop and gets the long delay, and 1/4 goes through both delay loops and gets the short delay plus the long delay.
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