English: The internal cavity of a Coherent Inc. model 740 dye laser is exposed during alignment of the optics. The frequency-doubled Nd:YAG laser enters the dye cell from the left, to "pump" the dye into a fluorescent state, where it will be ready to emit stimulated radiation. The fluorescence from the dye is reflected between the end mirrors, which consists of a diffraction grating (upper left of center) on one end of the beam and a cavity dumper, in the form of an acousto-optic modulator (beyond the right edge), on the other end, which is used as the output coupler. The beam goes through several stages of expansion, folding and contraction using a variety of curved mirrors, flat mirrors and prisms, adjusting it to the optimum size for each stage of the cavity. The cavity length is 1.97 meters, although the mirrors allow for it to be mounted in a shorter case. The dye cell is placed closer to one end of the cavity, because two waves are emitted from the cell in opposite directions. In this way, the wave on the shorter end of the cavity returns to the dye cell first, using most of its energy, and the other wave is eventually extinguished. This allows full gain from only one wave and a very short pulse of light can be extracted. The laser is capable of continuous wave operation or ultrafast picosecond pulses. The laser was photographed during a mirror-alignment procedure, allowing better imaging of the beam. The external laser used for alignment is 589 nm and is much brighter than the actual beam would be. The pump laser is shown for demonstration purposes, but at very low power (30 mw) in order to match the brightness of the 50 mw yellow beam. (In normal operation the pump laser would be much brighter than the intra-cavity beam, making it very difficult to photograph.