【正文】 position relative to the laser to obtain a spot where size does not vary strongly with the position of the white card. When the spot size remains roughly constant as the card is moved closer or further from the laser, the output can be considered collimated. Alternatively, the laser beam may be collimated by focusing it at a distance as far away as possible. Protect fellow coworkers from accidental exposure to the laser beam.2. Place an 818SL detector on a post mount (assembly M818) and adjust its position so that its active area is in the center of the beam. There should be adequate optical power falling on the detector to get a strong signal. Connect the photodetector to the power meter (815). Reduce the background lighting (room lights) so that the signal being detected is only from the laser. Reduce the drive current to a few milliamperes below threshold and, again, check to see that room light is not the dominant signal at the detector by blocking the laser light.3. Increase the current and record the output of the detector as a function of laser drive current. You should obtain a curve similar to Figure . If desired, the diode temperature may also be varied to observe the effects of temperature on threshold current. When examining laser diode temperature characteristics, the laser diode driver should be operated in the constant current mode as a safeguard against excessive currents that damage the diode laser. Note that as the diode temperature is reduced, the threshold decreases. Start all measurements with the diode current off to prevent damage to the laser by preventing drive currents too high above threshold. To prevent destruction of the laser, do not exceed the stated maximum drive current of the laser. Astigmatic Distance CharacteristicsThe laser diode astigmatic distance is determined as follows. A lens is used to focus the laser beam at a convenient distance. A razor blade is, then, incrementally moved across the beam to obtain data for total optical power passing the razor edge vs. the razor blade position. A plot of this data produces an integrated power profile of the laser beam (Figure ) which through differentiation exposes the actual power profile (Figure ) which, in turn, permits determination of the beam diameter (W). A beam diameter profile is obtained by measuring the beam diameter while varying the laser position. Figure illustrates the two beam diameter profiles of interest: one for razor edge travel in the direction perpendicular to the laser diode junction plane and the other for travel in the direction parallel to the junction plane. The astigmatic distance for a laser diode is the displacement between the minima of these two profiles. This method is known as the knife edge technique.1. Assemble the ponents shown in Figure with the collimator lens (LC), in the rotational stage assembly (RSAI), placed roughly 1 centimeter away from the laser. The beam should travel along the optic axis of the lens. This is the same lens used in collimating the laser in the previous setup. The approximate placement of all the ponents are shown in the figure. Make sure that the plane of the diode junction (xz plane in Figure ) is parallel with the table surface.2. Due to the asymmetric divergence of the light, the crosssection of the beam leaving the laser and, further, past the spherical lens is elliptical. The beam, thus, has two distinct focal points, one in the plane parallel and the other in the plane perpendicular to the laser diode junction. There is a point between the two focal points where the beam crosssection is circular. With the infrared imager and a white card, roughly determine the position where the beam crosssection is circular.Figure – Procedure for finding astigmatic distance.3. Adjust the laser diode to lens distance such that the razor blades are located in the xy plane where the beam crosssection is circular.4. Move the laser diode away from the lens until minimum beam waist is reached at the plane of razor blades. Now,