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2024年6月12日发(作者：银志文)

毕业设计（论文）英文翻译姓名

学号０８１１１２２１２１所在

学院理学院专业班级

２００８级光信１班指导教师

日期２０１２年４月２０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 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

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

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 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 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

θxand 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 θy 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 θx and θy 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 λ/4 plate in a mirror after the λ/4 plate and rotate

the λ/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 onally 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

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∝φ 2π/c

L ν where L is the difference in path length between thetwo arms of the interferometer

and ν 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 subtract this value from the total interferometer

output todetermine the true dI/di due to frequency modulation. Apply a low frequency

smallcurrent modulation to the laser diode. Note that when the proper range is

beingobserved 1 dv 10 5 mA 1 v diand 1 dI 0.2mA 1 I difor the amplitude change

ingdI d（Δφ） 2π Δv c dI ∝ ΔL 10 5 mA 1 di di c Δi 2πΔLv diordI ΔL

2Kπ mA 1di λ10 -5where K is a detector response constant determined by varying L.9.

With the interferometer and detection system properly adjusted vary the drivefrequency

of the laser and obtain the frequency response of the laser Figure 1.4 will

need to record two sets of data: i the modulation depth of theinterferometer output as a

function of frequency and ii the laser intensitymodulation depth. The difference of the

two sets of collected data will provide anestimate of the actual dI/di due to frequency

modulation. Also note that if the currentmodulation is sufficiently small and the path

mismatch sufficiently large the laserintensity modulation may be negligible. You may

need to actively keep theinterferometer in quadrature by adjusting the PZB voltage. Make

any necessary function generator amplitude adjustments to keep thecurrent modulation

depth of the laser constant as you vary the frequency. This isbecause the function

generator/driver combination may not have a flat frequencyresponse. The effect of this is

that the current modulation depth di is not constant andvaries with frequency. So to avoid

unnecessary calculations monitor the current.