英文原文
大课间1.5 Experimental Setup
Due to the many concepts and variations involved in performing the experiments in this project and also becau 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 ssions. For example, power and astigmatic distance characteristics may be examined in the first ssion and the last two experiments (frequency and amplitude characteristics) may be performed in the cond ssion.
A Note of Caution
小学生自我简介All of the above comments refer to single-mode operation of the lar which is a very fragile device with respect to reflections and operating point. One must ensure that before performing measurements the lar is indeed operating single-mode. This can be realized i
dancedf 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 ca, the lar is probably operating multimode and its current should be adjusted. If single-mode operation cannot be achieved by 国画入门学习adjusting the current, then reflections may be driving the lar multimode, in which ca the tup should be adjusted to minimize reflections. If still not operating single-mode, the毛森 lar diode may have been damaged and maydecide名词>垃圾分类感悟 need to be replaced.
Warning
The lars provided in this project kit emit invisible radiation that can damage the human eye. It is esntial that you avoid direct eye exposure to the lar beam. We recommend the u of protective eyewear designed for u at the lar wavelength of 780 nm.
Read the Safety ctions in the Lar Diode Driver Operating Manual and in the lar diode ction of Component Handling and Asmbly (Appendix A) before proceeding.
1.5.1 Semiconductor Diode Lar Power Characteristics
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1. Asmble the lar mount asmbly (LMA-I) and connect the lar to its power supply. We will first collimate the light beam. Connect the lar beam to a video monitor and image the lar beam on a white sheet of paper held about two to ten centimeters from the lar asmbly. Slowly increa the drive current to the lar and obrve the spot on the white card. The threshold drive current rating of the lar is supplied with each lar. Increa the current to about 10-20 mA over the threshold value.
With the infrared imager or infrared nsor card, obrve the spot on the card and adjust the collimator lens position in the lar asmbly 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 lar 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 clor or further from the lar, the output can be considered collimated. Alternatively, the lar beam may be collimated by focusing it at a distance as far away as possible. Protect fellow co-workers from accidental exposure to the lar beam.
2. Place an 818-SL detector on a post mount (asmbly 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 lar. Reduce the drive current to a few milliamperes below threshold and, again, check to e that room light is not the dominant signal at the detector by blocking the lar light.
3. Increa the current and record the output of the detector as a function of lar drive current. You should obtain a curve similar to Figure 1.2. If desired, the diode temperature may also be varied to obrve the effects of temperature on threshold current. When examining lar diode temperature characteristics, the lar diode driver should be operated in the constant current mode as a safeguard against excessive currents that damage the diode lar. Note that as the diode temperature is reduced, the threshold decreas. Start all measurements with the diode current off to prevent damage to the lar by preventing drive currents too high above threshold. To prevent destruction of the
lar, do not exceed the stated maximum drive current of the lar.
1.5.2 Astigmatic Distance Characteristics
The lar diode astigmatic distance is determined as follows. A lens is ud to focus the lar 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 lar beam (Figure 1.9a) which through differentiation expos the actual power profile (Figure 1.9b) which, in turn, permits determination of the beam diameter (W). A beam diameter profile is obtai
ned by measuring the beam diameter while varying the lar position. Figure 1.9c illustrates the two beam diameter profiles of interest: one for razor edge travel in the direction perpendicular to the lar diode junction plane and the other for travel in the direction parallel to the junction plane. The astigmatic distance for a lar diode is the displacement between the minima of the two profiles. This method is known as the knife edge technique.
