英文原文
1.5 Experimental Setup
Due to the many concepts and variations involved in performing the experiments in this project and also because of their introductory nature, Project 1 will very likely be the most time consuming project in this kit. This project may require as much as 9 hours to complete. We recommend that you perform the experiments in two or more laboratory sessions. For example, power and astigmatic distance characteristics may be examined in the first session and the last two experiments (frequency and amplitude characteristics) may be performed in the second session.
A Note of Caution
All of the above comments refer to single-mode operation of the laser which is a very fragile device with respect to reflections and operating point. One must ensure that before performing measurements the laser is indeed operating single-mode. This can be realized if a single, broad fringe pattern is obtained or equivalently a good sinusoidal output is obtained from the Michelson interferometer as the path imbalance is scanned. If this is not the case, the laser is probably operating multimode and its current should be adjusted. If single-mode operation cannot be achieved by adjusting the current, then reflections may be driving the laser multimode, in which case the setup should be adjusted to minimize reflections. If still not operating single-mode, the laser diode may have been damaged and may need to be replaced.
Warning
The lasers provided in this project kit emit invisible radiation that can damage the human eye. It is essential that you avoid direct eye exposure to the laser beam. We recommend the use of protective eyewear designed for use at the laser wavelength of 780 nm.
Read the Safety sections in the Laser Diode Driver Operating Manual and in the laser diode section of Component Handling and Assembly (Appendix A) before proceeding.
1.5.1 Semiconductor Diode Laser Power Characteristics
1.Assemble the laser mount assembly (LMA-I) and connect the laser to its power supply. We will first collimate the light beam. Connect the laser beam to a video monitor and image the laser beam on a white sheet of paper held about two to ten
centimeters from the laser assembly. Slowly increase the drive current to the laser and observe the spot on the white card. The threshold drive current rating of the laser is supplied with each laser. Increase the current to about 10-20 mA over the threshold value.
With the infrared imager or infrared sensor card, observe the spot on the card and adjust the collimator lens position in the laser assembly LMA-I to obtain a bright spot on the card. Move the card to about 30 to 60 centimeters from the lens and adjust the lens 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 co-workers from accidental exposure to the laser beam.
2.Place an 818-SL 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 1.2. 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.
