BKO-070 Cach Tu Nhieu Xa

8/14/2019 BKO-070 Cach Tu Nhieu Xa

1/7

VLKT Th nghim vt l BKO-070

Kho st s nhiu x ca chm laser qua cch t phng

xc nh bc sng ca laser

Dng c :1. Ngun pht tia laser bn dn ;2. Cch t nhiu x phng ;3. Thu knh hi t (2 ip) v hp bo v ;4. Cm bin photodiode silicon ;5. B khuch i v ch th cng vch nhiu x6. Thc trc vi (Panme), chnh xc 0,01mm ;7. H thng gi th nghim .

I. C s l thuyt

A. Nhiu x nh sng qua mt khe hpNhiu x nh sng l hin tngcc tiasng b

lch khi phng truyn thng khi i qua cc vt cn( l trn nh, khe h hp, ...). Di y ta kho sthin tng nhiu x ca chm tia sng song song ngvi cc sng phng .

Chiu mt chm tia sng song song, n sc, kt

hp c bc sng vung gc vi mt phng ca

khe hp AB c rng b (Hnh 1). Sau khi truyn quakhe, cc tia sng b nhiu x theo mi phng khc nhau.

Nhng tia sng nhiu x c cng gc lch s truyn

song song ti giao thoa vi nhau ti v cc. quan stnh giao thoa ca cc tia nhiu x song song, ta tmt thu knh hi t L pha sau khe hp AB hi tcc tia nhiu x ny ti im M trn mt tiu ca thuknh. Khi im M c th sng hoc ti tu thuc gitr ca gc .

Thc vy, ta hy v cc mt phng song song

0 1 2, , , ... cch nhau 2/ v vung gc vi

chm tia nhiu x. Cc mt phng ny chia mtphng ca khe AB thnh cc di sng hp c rng:

AB1 = B1 B2 = ........... =

sin.2

S di sng c trn mt khe hp bng :

sin.b.2

sin.2/

bn == (1)

V cc tia nhiu x tng ng t hai di sng k tip

truyn ti im M c hiu quang l bng 2/ , nn daong sng ca chng ngc pha v kh ln nhau. T suy ra cc kt qu sau :

- Nu khe hp cha mt s chn di:

k.2n = (vi k l s nguyn dng), th daong sng do mi cp di sng k tip truynti im M s kh ln nhau v im M s l mtim ti - gi l cc tiu nhiu x .

V tr cc cc tiu nhiu x trn mt tiu cathu knh L c xc nh bi h thc :

k2sin.b.2 =

vi k= 1 , 2 , 3 ,...

hay b/.ksin = (2)

Theo (2), khi k= 0 th = 0 : cc tia sng

truyn thng qua khe hp AB v hi t ti tiuim F ca th knh L. Cc tia sng ny ccng quang l nn ti tiu im F, chng daong cng pha v tng cng ln nhau. Do im F rt sng v gi l cc i nhiu x gia.

- Nu khe hp cha mt s l di :( )1k2n += , th dao ng sng do mi cp di

sng k tip truyn ti im M s kh ln nhau, chcn li dao ng sng ca mt di sng d rakhng b kh. Khi M l mt im sng v

c gi l cc i nhiu x bc k (k 0).

Cng sng ca cc cc i nhiu x bc knhhn nhiu so vi cc i gia. V tr cc cc inhiu x bc k trn mt tiu ca thu knh Lc xc nh bi h thc :

( )1k2sin.b.2 +=

vi k= 1, 2, 3,....

hay b.2/).1k.2(sin += (3)

E

A

B1 M

L

B.. .

2 1 0

O

Hnh 1


2/72

V tr cc cc i v cc tiu nhiu x trn mn nhE (t ti mt tiu ca thu knh L) v s phn b

cng sngIca cc cc i nhiu x ph thuc

gi tr ca sin nh hnh 2.

Nhn thy cc i gia c rng ln gp i v ccng sng ln hn nhiu so vi cc cc i nhiux khc. Da vo l thuyt, ngi ta tnh c t l

gia cng sngI1,I2 , ... ca cc cc i nhiu x

th k = 1, 2,... so vi cng sng I0 ca cc i

gia :

047,0/ =01

II ; 016,0/ =02

II ;..... (4)

Cng thc (2) v (3) chng t v tr cc cc i v

cc tiu nhiu x trn mn nh E khng ph thuc vtr ca khe hp AB. V th, nu gi c nh thu knh Lv dch chuyn khe hp AB song song vi chnh n,th nh nhiu x trn mn E khng thay i.

B.Nhiu x qua cch t phngTp hp mt s ln khe hp ging nhau nm

song song v cch u nhau trn cng mt mtphng gi l cch t phng. Khong cch dgia haikhe hp k tip nhau gi l chu kca cch t.

Chiu chm sng song song n sc kt hp c

bc sng vung gc vi mt cch t phng gm

N khe hp (Hnh 3). Mi khe hp c rng bng bv chu k ca cch t bng d . Khi s ng thixy ra hin tng nhiu x nh sng gy ra bi mikhe hp v hin tng giao thoa ca cc chm tianhiu x t N khe hp truyn ti mt tiu ca thuknh L. V vy nh nhiu x trn mn nh E trongtrng hp ny tr nn phc tp hn so vi mt khe .

Trc tin ta nhn thy ti nhng im ng vi ccgc nhiu x tho mn iu kin (2) :

b/.ksin = vi k= 1, 2,..... (5)

th mi khe hp ca cch t phng u cho cctiunhiu x : cc cc tiu nhiu x ny cgi l cc tiu chnh.

By gi ta xt s giao thoa ca cc chm tianhiu x t N khe hp truyn ti nhng v tr

nm trong khong gia cc cc tiu chnh.Nhn xt thy hiu quang l gia cc cp tianhiu x tng ng t hai khe k tip truyn tiim M trn mt tiu F ca thu knh hi t Lbng :

sin.dLL 12 = (6)

T suy ra nhng tia nhiu x c gc lch

tho mn iu kin :

d k.sin . = vi ....3,2,1,0k=

hay d/.ksin = (7)s gy ra ti im M cc dao ng sng cngpha v chng tng cng ln nhau. Khi , Ms l im sng v gi l cc i chnhbc k.D dng nhn thy cc i chnhtrung tm ngvi k = 0 v sin= 0 nm ti tiu im F ca

thu knh L.. Hn na, do d > b nn gia hai cctiu chnh s c mt s cc i chnh (Hnh 4).

Ngi ta cng chng minh c rng gia haicc i chnh k tip, cn c mt scc i ph

I

I0

I1= 0,045.I0

3

2

b I2= 0,016.I0

2

b

b0

+

b

+2

b

Hnh 2

0I

b

d 0

+

d

+

b

Hnh 4 : N = 2 , d = 3b

sin

d

b

Hnh 3


3/73

ngn cch bi cc cc tiu ph. V tr cc cc tiu ph xcnh bi gc lch tho mn iu kin :

d.N/ksin = (8)

vi ,...3,2,1k = tr cc gi tr N, 2N, 3N,....

V cc cc i ph c cng sng rt nh nn

khng v trn hnh 4. Kt qu l nh nhiu x qua ccht phng c s khe N kh ln s gm nhng vch sngsong song nm ti cc v tr xc nh theo iu kin (7).Cc vch sng ny ngn cch nhau bi nhng khongti v c cng sng gim t cc i trung tm ra

xa n v c hai pha ( xt trong phm vi gia hai cc

tiu chnh bc 1 b/+ v b/ ) .

Trong th nghim ny, ta s nghin cu hin tngnhiu x ca chm laser chiu qua mt cch tphng, kho st s phn b cng sng trn nh

nhiu x ca n , t xc nh bc sng calaser.

C. S lc v ngun sng laserLaser l t vit tt ca cm t ting Anh - Light

Amplification by stimulated Emission of Radiation (Khuchi nh sng bng pht x cm ng) ch cc bc x in tkt hp ,c n sc rt cao, c cng ln, v tnh nh

hng cao, c to ra trong nhng iu kin c bit.

Khi chiu bc x in t n sc c tn s vo mtcht, electron ca cc nguyn t mc nng lng c bn

E1hp th bc x v chuyn ln mc nng lng kch thchE2 cao hn (E2 > E1 ). Nhng electron ch tn ti mcnng lng kch thch E2trong khong thi gian ngn (10

-3s

10-8s) - gi l thi gian sng , sau chng li chuynv mc nng lng c bn E1 vpht x bc x.

Qu trnh chuyn mc nng lng khi hp th hoc pht

x u tun theo h thc Einstein :

= h v = E2 - E1 (9)vi h = 6,625.10-34 J.s l hng s Planck v =h lphoton ca bc x in t c tn s v.

R rng xc sut xy ra hp th t l vi mt

electron N1 mc nng lng c bn E1 v xc sut xy rapht x t l vi mt electron N2 mc nng lng kchthch E2.Thng thng N2N1 th xc sut xy ra pht x ln hn xc

sut xy ra hp th. Khi qu trnh pht x c tnh kt hpv gi lpht x cm ng, trong cc bc x cm ngccng tn s, cng pha, cng hng v cng phn cc

vi bc x kch thch. iu kin cn xy ra pht x cm ngl c s o mt ht, ngha l N2>N1 . Mi trng cht

trng thi c s o mt ht gi l mi trng kch hot.

to ra trng thi o mt ht, ngi ta s

dng mi trng kch hot trong nguyn t c ba(hoc bn) mc nng lng E1 , E2 , E3 sao cho thi

gian sng3 ca nguyn t mc E3 rt nh so vi thi

gian sng2

mc E2 . ng thi dng phng php

"bm in" (phng in qua mi trng kch hot) hoc

"bm quang" (dng ngun sng thch hp c cng mnh chiu vo mi trng kch hot) cc

nguyn t b kch thch chuyn t mc E1 ln E3 .

Nhng v 3N1 v dn ti hin tng phtx cm ng .

Trong th nghim ny, ta dng laser bn dn - gil diode laser. Khi cho dng in mt chiu c cng thch hp chy qua lp tip xc p-n to ra t cht

bn dn c bn GaAs, tia laser s c pht ra do

qu trnh ti hp p-n to ra cc photon. Nguyntc to ra trng thi o mt trong diode laser nhsau : Cc electron trong vng ho tr chuyn ln vng

dn nh mt qu trnh "bm" (kiu in hoc kiuquang). Kt qu l gia cc mc nng lng thp cavng dn v cc mc nng lng cao vng ho tr

c s o mt electron. Trng thi o mt electron l trng thi khng cn bng, n ch tn ti

trong khong thi gian rt ngn 10-13 s v gyra hiu ng laser.

II. Trnh t th nghimA. B thit b kho st nhiu x ca

tia laserThit b gm mt diode laser DL, pht ra

chm tia laser mu chiu vung gc vo mtphng ca cch t CT. Chm tia laser b nhiux, truyn qua thu knh hi t TK ( c tiu cf= 500mm) v cho nh nhiu x trn mt tiuca thu knh (Hnh 5) . xc nh v tr cccc i nhiu x v kho st s phn b cng sng ca chng, ta dng mt cm binquang in silicon Q t trong mt hp kn,pha trc c mn chn sng c khe h rngkhong 0,2 - 0,3 mm. Hp cm bin c gntrn u trc ca Panme P, nn c th dichuyn c theo phng ngang. Cng tialaser ri vo cm bin Q, chuyn i thnhcng dng in, chy qua mt in tr sun

220. Hiu in th ri trn in tr sun ny

c o v ch th bi Milivon k in t MV, c

li vo l mt cm 5 chn C.B.Kim tra v iu chnh chun trccho hthng.


4/74

kt qu o c chnh xc, trc ht ta cnkim tra v iu chnh chun trccho h thng, tc liu chnh sao cho chm tia laser ti p thng gcvo bng mn nh, ng v tr trung tm ca cmbin Q. Mun vy ta hy thc hin theo cc bcsau :

1. Vn Panme P a cm bin Q v v tr trungtm ( 12,5 mm trn thn thc kp ca Panme).

2. Nhc bn trt c gn cch t ra khi gi quanghc v t xung mt bn. Cm phch in cangun laser DL vo in ~220V v bt cng tc K1ca n, ta nhn c chm tia laser mu . Quanst cm bin Q xem chm tia Laser c chiu ngvo tm l trn trn mt cm bin hay khng. Nulch, ni nh cc con c trn khp a nng xoayngun laser DL sao cho tia sng ri ng vo tm l

v vung gc vi b mt l . Vi hai php xoay quanh2 trc v 2 php tnh tin dc theo 2 trc ca khpvn nng, ta hon ton c th iu chnh chun trcchnh xc cho h thng.

3. t bn trt c gn cch t tr li gi quanghc. iu chnh v tr cch t nh khp ni a nngca n, sao cho tia laser di ng vo tm ( hnhvung) cch t. Tip tc iu chnh xoay cch t saocho tia phn x t mt cch t quay ngc tr ling vo l ra ca tia Laser. Dch chuyn bn trt

dc theo gi quang hc n v tr sao cho thu knhTK cch mt cm bin Q ng 500mm th cht liv gi c nh khong cch ny trong sut qu trnh o.

Nu h thng c iu chnh ng th : Trn mt cm bin Q xut hin mt dy chmsng sc nt, nm trn ng thng nm ngang i quatm l. Nu ta y bn trt dc theo gi quang hc, vtsng ca chm Laser trn cch t khng thay i v tr.

C. Quy 0 v iu chnh nhy ca Milivon k

in t MV.

1. Cm phch ly in ca Milivon k in tMV vo in ~220V. t nm chn thang o ca MV v tr10mV v vn nm bin tr Rfca n v v tr tn cng

bn tri. Bm kho K trn mt MV, ch khong 3pht b khuch i n nh. Tin hnh iuchnh s 0 cho Milivon k in t MV bng cch :che sng hon ton khe h ca cm bin quangin Q, vn t t nm bin tr "qui 0" ( lpngay di ng h ch th ) kim ng h MV

ch ng s 0 .2. iu chnh nhy thch hp cho Milivonk in t MV, ta vn t t cn ca panme Psao cho cc i sng gia (c cng sngln nht) ca nh nhiu x lt vo ng giakhe h ca cm bin quang in Q. Khi kimca Milivon k in t MV lch mnh nht. Vnnm xoay ca bin tr Rf sao cho kim caMilivon k in t MV lch ti vch cui thango ( 80 hoc 90 ). (Nu khng t c lch

ny, th phi vn chuyn mch thang o ca MVsang v tr 1 mV ng vi nhy ln nht cun, v tin hnh iu chnh theo cch trn ).

D. Kho st s phn b cng sng trong

nh nhiu x laser :V cng sng trong nh nhiu x laser t

l vi cng Ica dng quang in, tc t l

vi hiu in th U ri trn in tr sun R, nnta c th kho st s phn b cng sngtrong nh nhiu x laser bng cch kho st s

bin thin ca hiu in th U theo v trxcacc cc i chnh nm gia hai cc tiu chnh

ng vi b/sin = .

Mun vy, ta vn t t cn panme P dchchuyn khe h ca cm bin quang in Qtrong khong gia hai cc tiu chnh bc 1 trnnh nhiu x. Mi ln ch dch chuyn mtkhong nh bng 0,05mm. c v ghi gi tr hiu

in th U ng vi mi v trxtrn panme P vobng 1. Cn c cc s liu ny, v th

U= f ( x ).

xc nh chnh xc v tr nh ca cc cci nhiu x, ta dch chuyn panme P theo mtchiu tng 0,01mm ti nhng im ln cn haipha ca cc nh ny tm thy gi tr cc ica hiu th U.

Cch c s o trn panme P: Khi quay cnthc trn mt vng th trc vt gn vi n tnhtin mt on bng 0,50mm dc theo ng chunngang ca mt thc kp thng khc trn thn

vt. Trn thc trn c 50 chia, mi chiatng ng vi dch chuyn trc vt l 0,01 mm -gi l chnh xcca panme. Thc kp gm

B

K1CT

A

DL

A

R 0

QP

K

C

QQ 1 10 100

K

+

Hnh 5

G

MV


5/75

hai dy vch so le, cch nhau 0,50mm nm hai bnng chun ngang . Dy trn ng vi cc gi trnguyn ca mm ( N = 0,1, 2... mm ), dy di ng vicc gi tr bn nguyn ca mm ( N = 0.5, 1.5, 2.5, ...mm ) .

- Nu mp thc trn nm st bn phi vch chia

N ca thc milimt pha trn ( dy nguyn), cnng chun ngang trng vi vch th n ca thctrn, th :

x = N + 0,01.n (mm) (10)- Nu mp thc trn nm st bn phi vch chia

N ca thc milimt pha di ( dy bn nguyn), cn

ng chun ngang trng vi vch th n ca thctrn, th :

x = N + 0,01.n = N + 0,5 + 0,01n (mm) (11)trong N l vch dy nguyn nm k bn tri vch N.

E. Xc nh bc sng ca chm tia laser :1.Sau khixc nh c cc i sng giang vi k= 0, vnt tpanme P o khong cch a gia haicc i nhiu x bc nhtng vi k = 1 nm i xng

hai bn cc i sng gia. Thc hin php o ny 3ln. c v ghi gi tr ca a trn thc panme vobng 2 .

2. p dng cng thc (7) i vi cc i chnh bc 1trong nh nhiu x, ta suy ra cng thc xc nh bc

sng ca chm tia laser :

= sin.d (12)

i vi cc i chnh bc 1 (hnh 6) , gc kh nh

nn c th coi gn ng :

f2/atgsin = (13)

Thay (13) vo (12) , ta tm c h thc :

f2/d.a= (14)

Theo cng thc (14) , ta c th xc nh c

bc sng ca chm tia laser nu cho bit trc

chu k d ca cch t phng.

III. Cu hi kim tra

1. nh ngha nhiu x nh sng. M t nhnhiu x ca chm tia sng song song chiuqua mt khe h hp .2. So snh nh nhiu x ca chm tia sngsong song chiu qua mt cch t phng vi nhnhiu x ca chm tia sng song song chiuqua mt khe h hp. Nu r cc cng thc xcnh v tr cc cc tiu chnh v ca cc cc ichnh trong nh nhiu x .

3. Khi xc nh bc sng ca tia laser nhiu

x qua cch t, ti sao khng o trc tipkhong cch gia cc i chnh bc 1 v cci gia (ng vi k = 0), m li o khong cch

a gia hai cc i chnh bc 1 (ng vi k = 1 )

?5. Khi kho st s phn b cng sng trongnh nhiu x laser, ti sao ta ch xt trongkhong gia hai cc tiu chnh bc 1 (ng vi

k = 1 ) v phi kim tra li v tr nh ca cc

cc i chnh bng cch ch dch chuyn panme Ptng 0,01mm (m khng dch chuyn tng0,05mm nh lc u) theo mt chiu ?

a

Hnh 6

k=+1

k= -1

F0

L


6/7

6

Bo co th nghim

Kho st s nhiu x ca chm tia laser qua cch t phng

xc nh bc sng ca tia laser

Xc nhn ca thy gio

Trng

Lp ...................T .....................

H tn .........................................

I. Mc ch th nghim

...............................................................................................................................................................................

...............................................................................................................................................................................

...............................................................................................................................................................................

II. kt qu th nghim

A.Kho st s phn b cng sng trong nh nhiu x laser

Bng 1- chnh xc ca thc panme : ................................ ( mm )- chnh xc ca Milivon k in t MV ...................... (mV)

x (mm) i(A) x (mm) i(A) x (mm) i(A) x (mm) i(A)

V th I= f ( x )


7/7

7

B.Xc nh bc sng ca chm tia laser

Bng 2- Chu k ca cch t phng : d = ........... .(mm-1 ) - chnh xc ca panme : .............(mm)

- Tiu c ca thu knh hi t : f = .......... (mm ) - chnh xc ca thc milimt :...........(mm)

Ln o a ( )mm a ( )mm ( m ) ( m )

1

2

3

TB

1. Tnhsai s tng i ca php o :

................................................................a

aff

dd

==++==

2. Tnh gi tr trung bnh ca php o :

...........................................................f2

ad ==

= ( )m

3. Tnh sai s tuyt i ca php o :

................................................................... === ( )m

4. Vit kt qu o ca php o:

= = ............................ .......................... ( )m

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BKO-070 Cach Tu Nhieu Xa