光电系毕业论文外文翻译半导体激光器(适用于毕业论文外文翻译+.doc

毕 业 设 计（论 文） 英文翻译 姓名 学号 所在学院理学院 专业班级 级光信 班 指导教师 日期 年 月 日 英文原文1.5 Experimental Setup Due to the many concepts and variations involved in performing the experimentsin this project and also because of their introductory nature Project 1 will very likelybe the most time consuming project in this kit. This project may require as much as 9hours to complete. We recommend that you perform the experiments in two or morelaboratory sessions. For example power and astigmatic distance characteristics maybe examined in the first session and the last two experiments frequency andamplitude 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 avery fragile device with respect to reflections and operating point. One must ensurethat 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 goodsinusoidal output is obtained from the Michelson interferometer as the path imbalanceis scanned. If this is not the case the laser is probably operating multimode and itscurrent should be adjusted. If single-mode operation cannot be achieved by adjustingthe current then reflections may be driving the laser multimode in which case thesetup should be adjusted to minimize reflections. If still not operating single-modethe 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 damagethe 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 wavelengthof 780 nm. Read the Safety sections in the Laser Diode Driver Operating Manual and in thelaser diode section of Component Handling and Assembly Appendix A beforeproceeding.1.5.1 Semiconductor Diode Laser Power Characteristics1. Assemble the laser mount assembly LMA-I and connect the laser to its powersupply. We will first collimate the light beam. Connect the laser beam to a videomonitor and image the laser beam on a white sheet of paper held about two to tencentimeters from the laser assembly. Slowly increase the drive current to the laser andobserve the spot on the white card. The threshold drive current rating of the laser issupplied with each laser. Increase the current to about 10-20 mA over the thresholdvalue. With the infrared imager or infrared sensor card observe the spot on the card andadjust the collimator lens position in the laser assembly LMA-I to obtain a bright spoton the card. Move the card to about 30 to 60 centimeters from the lens and adjust thelens position relative to the laser to obtain a spot where size does not vary stronglywith the position of the white card. When the spot size remains roughly constant asthe card is moved closer or further from the laser the output can be consideredcollimated. Alternatively the laser beam may be collimated by focusing it at adistance as far away as possible. Protect fellow co-workers from accidental exposureto the laser beam.2. Place an 818-SL detector on a post mount assembly M818 and adjust its positionso that its active area is in the center of the beam. There should be adequate opticalpower falling on the detector to get a strong signal. Connect the photodetector to thepower meter 815. Reduce the background lighting room lights so that the signalbeing detected is only from the laser. Reduce the drive current to a few milliamperesbelow threshold and again check to see that room light is not the dominant signal atthe detector by blocking the laser light.3. Increase the current and record the output of the detector as a function of laser drivecurrent. You should obtain a curve similar to Figure 1.2. If desired the diodetemperature may also be varied to observe the effects of temperature on thresholdcurrent. When examining laser diode temperature characteristics the laser diodedriver should be operated in the constant current mode as a safeguard againstexcessive currents that damage the diode laser. Note that as the diode temperature isreduced the threshold decreases. Start all measurements with the diode current off toprevent 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 ofthe laser.1.5.2 Astigmatic Distance Characteristics The laser diode astigmatic distance is determined as follows. A lens is used tofocus the laser beam at a convenient distance. A razor blade is then incrementallymoved 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 thelaser beam Figure 1.9a which through differentiation exposes the actual powerprofile Figure 1.9b which in turn permits determination of the beam diameter W.A beam diameter profile is obtained by measuring the beam diameter while varyingthe laser position. Figure 1.9c illustrates the two beam diameter profiles of interest:one for razor edge travel in the direction perpendicular to the laser diode junctionplane and the other for travel in the direction parallel to the junction plane. Theastigmatic distance for a laser diode is the displacement between the minima of thesetwo profiles. This method is known as the knife edge technique.1. Assemble the components shown in Figure 1.8 with the collimator lens LC in therotational stage assembly RSA-I 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 incollimating the laser in the previous setup. The approximate placement of all thecomponents are shown in the figure. Make sure that the plane of the diode junctionxz plane in Figure 1.1 is parallel with the table surface.2. Due to the asymmetric divergence of the light the cross-section of the beamleaving the laser and further past the spherical lens is elliptical. The beam thus hastwo distinct focal points one in the plane parallel and the other in the planeperpendicular to the laser diode junction. There is a point between the two focalpoints where the beam cross-section is circular. With the infrared imager and a whitecard roughly determine the position where the beam cross-section is circular. Figure 1.9 Procedure for finding astigmatic distance.3. Adjust the laser diode to lens distance such that the razor blades are located in thexy plane where the beam cross-section is circular.4. Move the laser diode away from the lens until minimum beam waist is reached atthe plane of razor blades. Now move the laser diode about 200 m further away fromthe lens.5. Move razor blade 1 in the x direction across the beam through the beam spread xand record the x position and detected intensity at each increment 100 mincrements. The expected output is shown in Figure 1.9. The derivative of this curveyields the intensity profile of the beam in the x direction from which the beamdiameter is determined.6. Repeat with razor blade 2 for y in the y direction.7. Move the laser closer to the lens in increments 50 m through a total of at leastthan 500m. Repeat Steps 5 and 6 at each z increment recording the z position.8. Using the collected data determine the beam intensity profiles in the x and ydirections as a function of the lens position z. This is done by differentiating each dataset with respect to position. Then calculate the beam diameter and plot as a functionof z. The difference in z for the minimum in x and y is the astigmatic distance of thelaser diode. Use of computer software especially in differentiating the data is highlyrecommended. If the laser junction is not parallel to the table surface then for eachmeasurement above you will obtain an admixture of the two beam divergences andthe measurement will become imprecise. If the laser is oriented at 45 to the surfaceof the table the astigmatic distance will be zero. Different laser structures will have different angular beam divergences and thusdifferent astigmatic distances. If you have access to several different laser types gainguided index guided it may be instructive to characterize their astigmatic distances.1.5.3 Frequency Characteristics of Diode Lasers In order to study frequency characteristics of a diode laser we will employ aMichelson interferometer to convert frequency variations into intensity variations. Anexperimental setup for examining frequency and also amplitude characteristics of alaser source is illustrated in Figure 1.10.1. In this experiment it is very possible that light may be coupled back into the laserthereby destabilizing it. An optical isolator therefore will be required to minimizefeedback into the laser. A simple isolator will be constructed using a polarizing beamsplitter cube and a quarterwave plate. We orient the quarterwave plate such that thelinearly polarized light from the polarizer is incident at 45 to the principal axes of thequarterwave plate so that light emerging from the quarterwave plate is circularlypolarized. Reflections change left-circular polarized light into right-circular or viceversa so that reflected light returning through the quarterwave plate will be linearlypolarized and 90 rotated with respect to the polarizer transmission axis. The polarizerthen greatly attenuates the return beam. In assembling the isolator make sure that the laser junction xz plane in Figure1.1 is parallel to the surface of the table the notch on the laser diode case pointsupward and the beam is collimated by the lens. The laser beam should be parallel tothe surface of the optical table. Set the polarizer and quarterwave /4 plate in place.Place a mirror after the /4 plate and rotate the /4 plate so that maximum rejectedsignal is obtained from the rejection port of the polarizing beam splitter cube asshown in Figure 1.11. When this signal is maximized the feedback to the laser shouldbe at a minimum.2. Construct the Michelson interferometer as shown in Figure 1.12. Place the beamsteering assembly BSA-II on the optical table and use the reflected beam from themirror to adjust the quarterwave plate orientation. Set the cube mount CM on theoptical breadboard place a double sided piece of adhesive tape on the mount and putthe nonpolarizing beam splitter cube 05BC16NP.6 on the adhesive tape. Next placethe other beam steering assembly BSA-I and the detector mount M818BB inlocation and adjust the mirrors so that the beams reflected from the two mirrorsoverlap at the detector. When long path length measurements are made the interferometer signal willdecrease or disappear if the laser coherence length is less than the two wayinterferometer path imbalance. If this is the case shorten the interferometer until thesignal reappears. If this does not work then check the laser for single-mode operationby looking for the fringe pattern on a card or by scanning the piezoelectric transducerblock PZBin BSA-II and monitoring the detector output which should be sinusoidalwith PZB scan distance. If the laser does not appear to be operating single-moderealign the isolator and/or change the laser operating point by varying the bias current.Additionally to ensure single-mode operation the laser should be DC biased abovethreshold before applying AC modulation. Overdriving the laser can also force it intomultimode operation.3. The Michelson interferometer has the property that depending on the position of themirrors light may strongly couple back toward the laser input port. In order to furtherreduce the feed-back slightly tilt the mirrors as illustrated in Figure 1.13. If stillunable to obtain single-mode operation replace the laser diode.4. Place a white card in front of the detector and observe the fringe pattern with theinfrared imager. Slightly adjust the mirrors to obtain the best fringe pattern. Try toobtain one broad fringe.5. Position the detector at the center of the fringe pattern so that it intercepts no morethan a portion of the centered peak.6. By applying a voltage to the piezoelectric transducer block attached to the mirrorpart PZB in one arm of the interferometer i.e. BSA-II maximize the outputintensity. The output should be stable over a time period of a minute or so. If it is notverify that all components are rigidly mounted. If they are then room air currents maybe destabilizing the setup. In this case place a box cardboard will do over the setupto prevent air currents from disturbing the interferometer setup.7. Place the interferometer in quadrature point of maximum sensitivity betweenmaximum and minimum outputs of the interferometer by varying the voltage on thePZB.8. The output signal of the interferometer due to frequency shifting of the laser isgiven by I 2/c L where L is the difference in path length between thetwo arms of the interferometer and is the frequency sweep of the laser that isinduced by applying a current modulation. Remember that in a Michelsoninterferometer L is twice the physical difference in length between the arms sincelight traverses this length difference in both directions. L values of 3-20 cmrepresent convenient length differences with the larger L yielding higher outputsignals. Before we apply a current modulation to the laser note that the interferometeroutput signal I should be made larger than the detector or laser noise levels byproper choice of L and current modulation amplitude di. Also recall from Section 1.3that when the diode current is modulated so is the laser intensity as well as itsfrequency. We can measure the laser intensity modulation by blocking one arm of theinterferometer. This eliminates interference and enables measurement of the intensitymodulation depth. We then