【文章內(nèi)容簡介】 igmatic distance will be zero.Different laser structures will have different angular beam divergences and, thus, different astigmatic distances. If you have access to several different laser types (gain guided, index guided), it may be instructive to characterize their astigmatic distances. Frequency Characteristics of Diode LasersIn order to study frequency characteristics of a diode laser, we will employ a Michelson interferometer to convert frequency variations into intensity variations. An experimental setup for examining frequency and, also, amplitude characteristics of a laser source is illustrated in Figure .1. In this experiment, it is very possible that light may be coupled back into the laser, thereby, destabilizing it. An optical isolator, therefore, will be required to minimize feedback into the laser. A simple isolator will be constructed using a polarizing beam splitter cube and a quarterwave plate. We orient the quarterwave plate such that the linearly polarized light from the polarizer is incident at 45176。 to the principal axes of the quarterwave plate so that light emerging from the quarterwave plate is circularly polarized. Reflections change leftcircular polarized light into rightcircular or vice versa so that reflected light returning through the quarterwave plate will be linearly polarized and 90176。 rotated with respect to the polarizer transmission axis. The polarizer, then, greatly attenuates the return beam. In assembling the isolator, make sure that the laser junction (xz plane in Figure ) is parallel to the surface of the table (the notch on the laser diode case points upward) and the beam is collimated by the lens. The laser beam should be parallel to the surface of the optical table. Set the polarizer and quarterwave (λ/4) plate in place. Place a mirror after the λ/4 plate and rotate the λ/4 plate so that maximum rejected signal is obtained from the rejection port of the polarizing beam splitter cube as shown in Figure . When this signal is maximized, the feedback to the laser should be at a minimum.2. Construct the Michelson interferometer as shown in Figure . Place the beam steering assembly (BSAII) on the optical table and use the reflected beam from the mirror to adjust the quarterwave plate orientation. Set the cube mount (CM) on the optical breadboard, place a double sided piece of adhesive tape on the mount, and put the nonpolarizing beam splitter cube () on the adhesive tape. Next, place the other beam steering assembly (BSAI) and the detector mount (M818BB) in location and adjust the mirrors so that the beams reflected from the two mirrors overlap at the detector. When long path length measurements are made, the interferometer signal will decrease or disappear if the laser coherence length is less than the two way interferometer path imbalance. If this is the case, shorten the interferometer until the signal reappears. If this does not work, then check the laser for singlemode operation by looking for the fringe pattern on a card or by scanning the piezoelectric transducer block (PZB)in BSAII and monitoring the detector output which should be sinusoidal with PZB scan distance. If the laser does not appear to be operating singlemode, realign the isolator and/or change the laser operating point by varying the bias current. Additionally, to ensure singlemode operation, the laser should be DC biased above threshold before applying AC modulation. Overdriving the laser can also force it into multimode operation.3. The Michelson interferometer has the property that depending on the position of the mirrors, light may strongly couple back toward the laser input port. In order to further reduce the feedback, slightly tilt the mirrors as illustrated in Figure . If still unable to obtain singlemode operation, replace the laser diode.4. Place a white card in front of the detector and observe the fringe pattern with the infrared imager. Slightly adjust the mirrors to obtain the best fringe pattern. Try to obtain one broad fringe.5.