Alignment of the completed laser proceeds as follows. The laser diode is mounted in the collimation tube, and the lens is adjusted to collimate the beam over a distance of several meters. The collimation tube is then clamped in the mirror mount as shown in Fig. 1, rotated so that the long axis of the elliptical laser beam is perpendicular to the lines on the grating. Next, the laser current is reduced just below threshold for the solitary laser diode, where the diode is most sensitive to feedback as it cannot lase by itself. Adjusting the mirror mount to direct the Littrow reflection from the grating back into the center of the collimating lens should cause a second beam to become visible near the main zeroth-order output beam. This beam is much weaker than the main one because it has made a complete round trip inside the cavity. Collapsing it into the main output beam by further grating adjustments should then produce a large increase in output power as the feedback from the grating takes the laser above threshold. The output power can be optimized by adjusting the grating angle and the focusing of the collimation lens. The lens adjustment is very critical, corresponding to only a few degrees of rotation of the threaded lens mount. While it can be accomplished by rotating the lens carefully with a small screwdriver, we have found it more convenient to glue to the lens mount a short adjustment tube, extending beyond the end of the collimation tube (Fig. 1). Once the system is lasing, the laser current can be increased to give the desired
output power, and the horizontal grating adjustment is used to tune the laser to the required wavelength.