25 Laser

8/8/2019 25 Laser

1/20

Tng quan v laser

Trong nhng b phim khoa hc vin tng ni ting hi thp nin 1950, cc con qui vt

thng c miu t c kh nng pht ra nhng tia sng gy cht ngi ti mt ca

chng (xem hnh 1), nhng cho ti khi pht minh ra laser th cc chm nng lng mnhlit v tp trung nh th cng ch l tng tng m thi. Ngy nay, ngi ta c th sa

i, thm d, hay ph hy vt cht bng cch s dng cc bc x tp trung cao pht ra t

cc ngun nng lng gi l laser. Hu nh tt c nh sng m chng ta nhn thy hng

ngy, t nh sng Mt Tri, cc v sao, cc bng n nng sng v n hunh quang, cho

n cc b ti vi, u xy ra t pht khi cc nguyn t v phn t t gii phng nng

lng tha ca chng.

Hnh 1. Qui vt mt laser ca thp nin 1950

nh sng t nhin v nh sng nhn to thng thng c pht ra bi s thay i nng

lng cc mc nguyn t v phn t xy ra m khng cn c s can thip t bn ngoi.

Tuy nhin, loi nh sng th hai tn ti v xy ra khi nguyn t hay phn t vn gi nng

lng d tha ca n cho n khi b cng bc phi pht ra nng lng di dng nh

sng. Laser c ch to to ra v khuch i dng nh sng cng bc ny thnh cc

chm cng mnh v tp trung. Laser l t vit tt ca Light Amplification by theStimulated Emission ofRadiation (Khuch i nh sng bng s pht bc x cng bc).

Tnh cht c bit ca nh sng laser khin cho kthut laser trthnh mt cng c thit

yu trong hu nh mi mt i sng hng ngy, nh vin thng, gii tr, sn xut v y

khoa.

8/8/2019 25 Laser

2/20

8/8/2019 25 Laser

3/20

Maiman cng vi laser honh trng hn vn c lu truyn v s dng trong nhiu

n phm.

Mc d laser pht ra nh sng kh kin l ph bin nht, nhng cc nguyn l cbn c

th p dng c cho nhiu vng phin t. S pht x cng bc u tin thu ctrong vng vi ba ca phin t, nhng hin nay laser c mt trn th trng cn pht ra

nh sng cc tm v hng ngoi, v tin bang c thc hin theo hng to ra laser

trong vng ph tia X. Cc laser thc tc s dng hin nay c cng sut pht t di

1 miliwatt cho n nhiu kilowatt, v mt s to ra c nghn t watt trong nhng xung cc

ngn. Hnh 2 cho thy mt s loi laser in hnh, c kch thc cv ng dng rng

ri. Cc phng th nghim thuc qun i v phng th nghim khc ch to c

nhng thit b laser chim c mt ta nh, trong khi nhng laser ph bin nht s dng

dng c bn dn kch thc bng mt ht ct.

hiu c cc nguyn l cbn ca laser, iu cn thit l phi gii thch cch thc

bc x cng bc c to ra v khuch i. Nguyn lu tin trong s cc nguyn l

ny l cn thit, bi v laser vn dl mt dng c clng t v bn cht lng t ca

nng lng phi c kn gii thch hot ng ca laser. Vt l cin cho rng

nng lng c th bin thin lin tc v u n, v cc nguyn t v phn t c th c

bt k lng nng lng no. Cng trnh nghin cu ca Einstein, co trthnh cha kha

cho s pht trin ca chc lng t, cho rng nng lng tn ti trong tng n v gin

on, hay lng t, v cc nguyn t v phn t (v do l mi i tng khc) b hnch ch c nhng lng nng lng gin on nht nh.

8/8/2019 25 Laser

4/20

Hnh 2. Mt sloi laser thng dng

Thm mt vi khi nim na cng cn thit hiu c hot ng laser, bt u l

photon v mc nguyn t v xut pht t nguyn tc lng t ha:

S lng t ha nng lng trong nguyn t lm cho

nguyn tc cc mc nng lng gin on.

S chuyn t mc nng lng ny sang mc nng

lng khc phi xy ra cng vi spht x nh sng.

Mt s loi chuyn trng thi xy ra v nh hng n

lng nng lng trong schuyn .

S pht x t pht v cng bc c th xy ra do schuyn mc nng lng.

Nghch o dn c gia cc mc nng lng phi thu

c khuch i spht bc x cng bc.

Nu nh nguyn t hay phn t nm mt trng thi nng lng cao hn trng thi thp

nht, hay trng thi c bn, n c th t pht ri xung mc nng lng thp hn m

khng cn kch thch t bn ngoi. Mt kt qu c th xy ra ca s ri lm gim trng

thi nng lng l gii phng nng lng d tha (bng vi s chnh lch gia hai mcnng lng) di dng mt photon nh sng. Nguyn t hay phn t kch thch c mt

thi gian pht xc trng, l thi gian trung bnh m chng vn gic trng thi

nng lng kch thch cao hn trc khi ri xung mc nng lng thp hn v pht ra

photon. Thi gian pht x l mt nhn t quan trng trong vic to ra pht x cng bc,

loi pht x th hai m Einstein nu ra.

Cn trng thi kch thch, nu nguyn tc ri vi photon n c cng nng lng

chnh xc nh nng lng m s chuyn trng thi c th xy ra t pht, nguyn t c th

b cng bc bng photon n quay tr li trng thi nng lng thp hn v ngthi pht ra mt photon c cng nng lng chuyn trng thi. Mt photon ring l tng

tc vi mt nguyn t b kch thch do c th to ra hai photon pht x. Nu cc

photon pht xc xem l sng, th s pht x cng bc s dao ng cng tn s vi

nh sng ti, v cng pha (kt hp), kt qu l lm khuch i cng ca nh sng ban

8/8/2019 25 Laser

5/20

u. Hnh 3 minh ha s pht x t pht (a) v cng bc (b) vi hai sng kt hp nh

trng hp th hai trn.

Hnh 3. Cc qu trnh tpht v cng bc

Vn quan trng nht trong vic thu c pht x laser cng bc l di nhng iu

kin cn bng nhit ng lc hc bnh thng, dn c, hay s nguyn t hoc phn t

mi mc nng lng, khng thun li cho s pht x cng bc. Do cc nguyn t v

phn t c xu hng t ri xung cc mc nng lng thp hn nn s nguyn t hay

phn tmi mc s gim khi nng lng tng. Tht vy, di nhng iu kin bnh

thng, i vi mt s chuyn mc nng lng ng vi mt bc sng quang in hnh

(vo bc 1 electron-volt), t s ca s nguyn t hay phn ttrng thi nng lng cao

hn v s nguyn t hay phn t trng thi cbn thp hn c l l 1017. Ni cch

khc, hu nh tt c cc nguyn t hay phn tvo trng thi cbn i vi s chuyn

mc nng lng nh sng kh kin.

Mt l do khin s pht x cng bc kh thu c trnn hin nhin khi xem xt cc s

kin c kh nng xy ra quanh s phn hy ca mt electron t mt trng thi kch thch

vi s pht x nh sng sau v t pht. nh sng pht x c th d dng kch thch spht x t cc nguyn t b kch thch khc, nhng mt s c th gp phi nguyn t

trng thi cbn v b hp th ch khng gy ra pht x (hnh 3c). Do s nguyn t

trng thi kch thch t hn nhiu so vi s nguyn ttrng thi cbn nn photon pht

x c kh nng b hp th nhiu hn, b li th s pht x cng bc cng khng ng k

so vi pht x t pht (trng thi cn bng nhit ng lc hc).

8/8/2019 25 Laser

6/20

Cch lm cho pht x cng bc c th ln t l phi c s nguyn ttrng thi kch

thch nhiu hn s nguyn ttrng thi nng lng thp hn, sao cho cc photon pht

x c kh nng gy kch thch pht x nhiu hn l b hp th. Do iu kin ny l nghch

o trng thi cn bng ban u nn n c gi l s nghch o dn c. Min l cnhiu nguyn ttrng thi nng lng cao hn so vi trng thi nng lng thp hn,

th pht x cng bc s ln t v ta thu c dng thc photon. Photon pht x ban u

s kch thch s pht x ca nhiu photon hn, nhng photon ny sau li kch thch s

pht x ra nhiu photon hn na, v c th tip din. Kt qu l dng thc photon tng

ln, nh sng pht xc khuch i. Nu s nghch o dn c chm dt (dn c

trng thi cbn trnn ln t) th pht x t pht s trli l qu trnh ch yu.

Vo khong thi gian Einstein xut tng, a s cc nh vt l tin rng bt ciu

kin no khng phi trng thi cn bng nhit ng lc hc u khng bn v khng

thc duy tr lin tc. Mi n sau Th chin th hai, ngi ta mi xem xt n cc

phng php to ra s nghch o dn c cn thit duy tr pht x cng bc. Cc

nguyn t v phn t c th chim gi nhiu mc nng lng, v mc d mt s s

chuyn trng thi c kh nng xy ra hn so vi mt s s chuyn trng thi khc (do cc

quy lut ca chc lng t v v nhng l do khc), nhng s chuyn trng thi c th

xy ra gia bt k hai mc nng lng no. Yu cu ti thiu cho s pht x cng bc

v khuch i, hay hot ng laser, l t nht phi c mt trng thi nng lng cao hn

c dn c nhiu hn mt trng thi nng lng thp hn.

S nghch o dn c c thc to ra qua hai cch cbn: hoc l to ra d tha s

nguyn t hay phn tmt trng thi nng lng cao, hoc l lm gim dn smt

trng thi nng lng thp. Cng c th chn mt h khng bn mc nng lng thp,

nhng i vi hot ng laser lin tc, phi ch va lm tng dn cmc cao va

lm gim dn cmc thp. Nu qu nhiu nguyn t hay phn t tch tmc nng

lng thp th s nghch o dn c s khng cn v hot ng laser dng li.

Phng php thng dng nht to ra s nghch o dn c trong mi trng laser l cp

thm nng lng cho h kch thch cc nguyn t hay phn t ln mc nng lng

cao. Cch cp nng lng n gin bng cch dng nhit khuy ng mi trng khng

(di iu kin cn bng nhit ng lc hc) to ra s nghch o dn c, do nhit

ch lm tng nng lng trung bnh ca cc ht, ch khng lm tng s loi trng thi

8/8/2019 25 Laser

7/20

kch thch tng i so vi trng thi thp. T s ca s nguyn thai mc nng lng

(1 v 2) di trng thi cn bng nhit ng lc hc c cho bi phng trnh sau:

N2/N1 = exp[- (E2 - E1) / kT]

trong N1 v N2 tng ng l s nguyn tmc 1 v mc 2, E1 v E2 l nng lng

ca hai mc, k l hng s Boltzmann, T l nhit kelvin. Nh c ch r trong

phng trnh, trng thi cn bng nhit ng lc hc, N2 ch c th ln N1 nu nh

nhit l mt s m. Trc khi nghin cu m t hot ng maser v laser c cng

b, cc nh vt l thng xem s nghch o dn c l nhit m, l t ch nhng

iu kin khng trng thi cn bng nhit ng lc hc khng c mong i l s

c duy tr lin tc.

to ra s nghch o dn c cn thit cho hot ng laser, cc nguyn t hay phn t

phi b kch thch c chn lc ln nhng mc nng lng c bit. nh sng v dng

in l cch kch thch c chn ca a s laser. nh sng hoc cc electron c th

cung cp nng lng cn thit kch thch cc nguyn t hay phn t ln cc mc nng

lng cao c chn, v s truyn nng lng khng i hi a cc electron trc tip

ln mc nng lng cao no ca s chuyn trng thi laser. Mt s phng php khc

c th phc tp hn, nhng chng thng to ra hot ng laser tt hn. Mt phng

php thng c s dng l kch thch nguyn t hay phn t ln mc nng lng cao

hn cn thit, sau n s ri xung mc laser cao. Kiu kch thch gin tip c thc

s dng kch thch cc nguyn t trong hn hp kh xung quanh, sau chng s

truyn nng lng ca chng cho cc nguyn t hay phn tm nhn vic to ra hot

ng laser.

8/8/2019 25 Laser

8/20

Hnh 4. Biu nng lng laser ba mc v bn mc

Nh ni ti phn trc, lng thi gian m mt nguyn t hay phn t tri qua

mt trng thi kch thch l yu t quyt nh trong vic xc nh xem n s b cng bc

pht x v tham gia vo dng thc photon, hay l s mt i nng lng qua vic pht x

t pht. Cc trng thi kch thch thng c thi gian sng ch vi nano giy trc khi

chng gii phng nng lng ca mnh bng pht x t pht, mt khong thi gian khng

lu c th chu s kch thch bi mt photon khc. Do , yu cu ti cn thit cho

hot ng laser l mc nng lng cao phi c thi gian sng lu hn. Cc trng thi nh

vy tht s tn ti trong nhng cht nht nh, v thng c gi l trng thi siu bn(xem hnh 4). Thi gian sng trung bnh trc khi pht x t pht xy ra i vi trng

thi siu bn l vo bc micro giy n mili giy, mt khong thi gian kh di th gii

nguyn t. Vi thi gian sng lu ny, cc nguyn t v phn t b kch thch c th to

ra mt lng ng k pht x cng bc. Hot ng laser ch xy ra nu nh dn c

mc cao c to ra nhanh hn s phn hy ca n, duy tr c dn cmc cao nhiu

hn mc thp. Thi gian sng ca pht x t pht cng lu th nguyn t hay phn t

cng thch hp cho cc ng dng laser.

Maser m Charles Townes chng minh trong bc tin ti laser u tin tht c ngha,

v n yu cu to ra s nghch o dn c hot ng, v do chng minh vi nhiu

nh vt l cn hoi nghi rng s nghch o dn c nh vy l c th thc hin c. H

ca ng l maser hai mc, ch s dng cc mc cao v thp. Townes tin hnh mt

phng php mi l trong h nguyn t amoniac ca ng to ra s nghch o dn c

8/8/2019 25 Laser

9/20

kthut chm phn t tch cc phn t amoniac b kch thch khi cc phn ttrng

thi cbn. Cc phn ttrng thi cbn b loi b, v cc phn t b kch thch c

tch ra thit lp s nghch o dn c cn thit. Cc phng php khc, hiu qu hn,

hin nay c pht trin cho maser v laser thc t, yu cu s dng ba, bn mc nng

lng hoc nhiu hn.

Cu trc mc nng lng thit thc n gin nht i vi hot ng laser l h ba mc,

c minh ha trong hnh 4a. Trong h ny, trng thi cbn l mc laser thp, v s

nghch o dn c c thc to ra gia mc ny v mt trng thi siu bn nng lng

cao hn. a s cc nguyn t hay phn t ban u b kch thch ln trng thi nng lng

cao c thi gian sng ngn nhiu hn ln mc siu bn. T trng thi ny, chng nhanh

chng phn hy sang mc siu bn trung gian, mc c thi gian sng di hn nhiu so

vi trng thi nng lng cao (thng di hn c1000 ln). Do thi gian c tr ca mi

nguyn ttrng thi siu bn tng i lu, nn dn s c xu hng tng v a n s

nghch o dn c gia trng thi siu bn v trng thi cbn thp hn (dn s gim

lin tc i vi mc cao nht). Pht x cng bc thu c t thc t s nguyn t c

sn trng thi b kch thch (siu bn) nhiu hn so vi trng thi thp, trng thi m

s hp th nh sng c kh nng xy ra nht.

Mc d laser ba mc hot ng i vi mi mc ch thc t, nh c minh chng

bng laser u tin ca Maiman, nhng c mt s vn hn ch hiu qu ca phng

php ny. Vn trng tm xut hin do mc laser thp l mc cbn, l trng thi bnh

thng i vi a s cc nguyn t hay phn t. to ra s nghch o dn c, phn

ln electron trng thi cbn phi c a ln mc nng lng b kch thch cao, i

hi phi cung cp ng k nng lng t bn ngoi. Ngoi ra, s nghch o dn c kh

c th duy tr trong mt khong thi gian ng k, v do , laser ba mc hot ng theo

kiu xung ch khng lin tc.

Laser s dng bn mc nng lng hay nhiu hn trnh c mt s vn cp

trn, v do c s dng ph bin hn. Hnh 4b minh ha kch bn bn mc nnglng. Cu trc mc nng lng tng t nh trong h ba mc, tr vn sau khi

nguyn t ri t mc cao nht xung trng thi cao siu bn, chng khng ri ht xung

trng thi cbn qua mt bc. Do s nghch o dn c khng c to ra gia trng

thi cbn v mc cao, nn s nguyn t hay phn t phi c a ln t ngt gim

xung trong m hnh ny. Trong mt h laser bn mc in hnh, nu ch 1 hoc 2% s

8/8/2019 25 Laser

10/20

nguyn t hay phn t c tr mc laser thp (mc nm trn trng thi cbn) th ch

cn kch thch 2 n 4% trong tng s nguyn t hay phn t ln mc cao l s thu c

s nghch o dn c cn thit. Mt thun li na ca vic tch mc laser thp khi mc

cbn l cc nguyn t mc thp s tng ri xung trng thi cbn. Nu nh mc

laser thp c thi gian sng ngn hn nhiu so vi mc cao th cc nguyn t s phn hysang mc cbn tc trnh vic tch tmc laser thp. Nhiu laser c thit

k di nhng rng buc ny c th hot ng theo mode lin tc to ra chm tia khng

t qung.

Hnh 5. Pht hin maser v tr

Cc laser hot ng thc t thng phc tp hn m hnh m t trn. Mc laser cao

thng khng phi l mt mc n, m l mt nhm mc nng lng cho php nng

lng kch thch cn thit bin i trong mt phm vi rng trong khi hot ng. Mc thp

cng gm nhiu mc, v nu mi mc cao gn nhau phn hy sang mt mc thp khc,

mt laser c th hot ng nhiu s chuyn trng thi, to ra nhiu hn mt bc sng.

V d, laser helium-neon c dng ph bin nht pht ra bc sng , nhng n

cng c th hot ng nhng s chuyn trng thi khc pht ra bc x cam, vng,

xanh l v hng ngoi. Nhiu nhn t khc tn ti trong vic thit k laser thc t, nh

bn cht ca mi trng hot tnh. Hn hp kh hay nhng kt hp khc ca cc loiphn t thng c dng ci thin hiu qu bt v truyn nng lng, hoc h trs

gim dn smc laser thp.

Trc khi c bng chng mang tnh bc ngot chng t maser v laser thc s c th

to ra c, cc nh khoa hc thy thc t l maser xut hin trong t nhin tn ti

8/8/2019 25 Laser

11/20

trong khng gian v tr bn ngoi (hnh 5). Ngay c sau khi Einstein tin on s pht x

cng bc, a s cc nh vt l vn tin rng vic to ra s nghch o dn c l qu kh

nn n khng th xy ra trong t nhin. Thc ra m ni th hnh nh cc nh khoa hc

khng xem xt ng n liu vt cht c tn ti trong t nhin trng thi khc, ngoi

trng thi cn bng nhit ng lc hc hay khng. Ci gi l maser v tr gm cc ngunnh cc lp v kh bao bc quanh cc sao knh , sao chi, tn d ca sao siu mi, v

nhng m my phn tang hnh thnh sao khc. Trong m my kh bao quanh mt

ngi sao nng, bc x pht ra t ngi sao c th kch thch cc phn t kh ln cc mc

nng lng cao, ri phn hy xung trng thi siu bn. Ch cn tn ti mc laser thp

thch hp, s nghch o dn c c th xy ra v s thu c hot ng laser. Mc d qu

trnh khng ging vi cc maser v laser nhn to, v mt lng ln nng lng c

thc pht ra, nhng s pht x ca nng lng maser hoc laser sao khng b gii

hn trong mt chm. Bc x pht ra bi maser v tr truyn i xa theo mi hng ging

nh nng lng pht ra tm my kh nng ging sao no khc.

Ngoi vic to ra s nghch o dn c, cng cn mt vi nhn t khc na khuch i

v tp trung nh sng thnh mt chm. nh sng pht ra t s pht x cng bc c

to ra trong mi trng laser thng c mt bc sng ring, nhng phi c trch ra c

hiu qu t mi trng bng mt s cch bao gm s khuch i. Cng vic ny c

hon thnh trong mt hp cng hng, n phn x mt s nh sng pht x tr li mi

trng laser, v qua nhiu ln tng tc, hnh thnh hay khuch i cng nh sng.

V d, sau s pht x cng bc ban u, hai photon c cng nng lng v cng phami ht c th bt gp cc nguyn t b kch thch, ri th s pht ra nhiu photon hn c

cng nng lng v cng pha. S photon c to ra bi pht x cng bc tng ln

nhanh chng, v s tng ny t l trc tip vi khong cch m nh sng truyn trong mi

trng laser.

Hnh 6 minh ha s thu li, hay khuch i, xy ra vi chiu di ng truyn tng ln

trong hp cng hng do cc gng t hai u mang li. Hnh 6a cho thy s bt u

ca pht x cng bc, nh sng c khuch i trong hnh 6b n hnh 6g khi n bphn x t cc gng t hai u hp. Mt phn nh sng truyn xuyn qua gng

phn x mt phn pha bn phi ca hp trong mi ln truyn (hnh 6b, d v f). Cui

cng, trng thi cn bng (hnh 6h), hp bo ha bc x cng bc.

8/8/2019 25 Laser

12/20

Mc khuch i thu c trong mt laser, biu din bng thut ng li, ch lng

pht x cng bc m mt photon c th to ra khi n truyn i mt khong cch cho

trc. V d, li 1,5 /cm ngha l mt photon sinh ra thm 1,5 photon na trn mi cm

m n truyn i. H s khuch i ny tng ln theo chiu di ng truyn ca hp

laser. li thc t phc tp hn nhiu, v ngoi nhng nhn t khc, n ph thuc vonhng dao ng trong s phn b dn c gia cc mc nng lng laser cao v thp.

iu quan trng l lng khuch i tng r rt vi khong cch truyn trong mi trng

laser.

Hnh 6. Spht x cng bc trong hp laser

Trong laser c hp cng hng dc, nh thi ruby hay ng cha y kh, nh sng truyn

dc theo chiu di ca mi trng laser lm pht sinh nhiu pht x cng bc hn nh

sng truyn vung gc vi trc ca hp cng hng. Do , s pht x nh sng tp

trung dc theo chiu di ca hp, ngay c khi khng dng gng gii hn ng

truyn ca n theo hng dc. Vic t cc gng hai u ca hp laser cho php

chm tia truyn ti lui, lm tng thm s khuch i do ng truyn qua mi trng dihn. S phn x nhiu ln cng to ra chm tp trung cao (mt c trng quan trng ca

laser), do ch c nhng photon truyn song song vi thnh hp l b phn x bi hai

gng. S xp xp ny c gi l dao ng t, v n cn thit, v a s vt liu laser c

li rt thp v s khuch i y ch c th thu c vi ng truyn di qua mi

trng.

8/8/2019 25 Laser

13/20

a s laser hin nay c thit k c cc gng c hai u ca hp cng hng lm

tng qung ng nh sng truyn trong mi trng laser. Cng pht x tng ln

theo mi lt truyn ca nh sng cho ti khi n t ti mc cn bng, mc ny do cu

to hp v gng thit t. Mt gng ca hp phn x gn nh ton b nh sng ti,cn gng kia (gng ra) phn x mt s nh sng v truyn mt phn ra ngoi di

dng chm laser. Trong mt laser c li thp, gng ra c chn sao cho ch truyn

mt phn nh nh sng ra ngoi (c l ch vi phn trm) v phn xa phn nh sng tr

li hp. trng thi cn bng, cng sut laser trong hp cao hn bn ngoi, v thay i

theo phn trm nh sng truyn qua gng ra. Bng cch lm tng h s truyn ca

gng ra, s chnh lch cng sut gia bn trong v bn ngoi hp c thc lm gim

xung. Tuy nhin, ch cn gng ra phn x mt s phn nh sng trli hp, cng sut

bn trong vn cao hn bn ngoi trong chm tia xut hin.

Mt nhn thc sai lm v laser l tng cho rng tt c nh sng pht x b phn x ti

lui trong hp cho ti khi cng ca n t ti gii hn, ri th mt s thot ra ngoi

qua gng ra di dng chm tia. Trong thc t, gng ra lun lun truyn mt phn

khng i nh sng di dng chm, phn x phn cn li trvo hp. Chc nng ny

quan trng trong vic cho php laser t ti trng thi cn bng, vi cc mc cng sut

laser c bn trong ln bn ngoi u trnn khng i.

V trong thc t nh sng dao ng ti lui trong hp laser, nn hin tng cng hng tr

thnh mt nhn tnh hng ti vic khuch i cng laser. Ph thuc vo bc

sng ca bc x cng bc v chiu di hp, sng phn x t cc gng s hoc l giao

thoa tng cng v c khuch i mnh, hoc l giao thoa trit tiu v xa b hot

ng laser. V cc sng trong hp l kt hp hon ton cng pha, chng s vn l cng

pha khi phn x t mt gng. Cc sng cng s cng pha khi chm ti gng i din,

vi iu kin l chiu di hp bng mt s nguyn ln bc sng. Nh vy, sau khi thc

hin mt dao ng hon chnh trong hp, sng nh sng truyn c qung ng

bng hai ln chiu di hp. Nu khong cch l mt bi s nguyn ca bc sng, thcc sng s tng thm bin bi s giao thoa tng cng. Khi chiu di hp khng

chnh xc l bi s nguyn ca bc sng, giao thoa trit tiu s xy ra, ph hy hot

ng laser. Phng trnh sau y xc nh iu kin cng hng phi c s khuch i

mnh xy ra trong hp laser:

8/8/2019 25 Laser

14/20

N ? = 2 ? (chiu di hp)

trong N l mt s nguyn, v l bc sng. iu kin cng hng tht ra

khng quan trng v nhng s chuyn trng thi laser thc t trong

hp phn b trong mt phm vi bc sng, gi l di thng li.

Bc sng ca nh sng cc k nh so vi chiu di ca mt hp laser

in hnh, v ni chung mt qung ng truyn hon chnh trong hp

s tng ng vi vi trm ngn bc sng nh sng c khuch

i. Cng hng c th xy ra mi s gia bc sng nguyn (v d

200 000, 200 001, 200 002,...), v do bc sng tng ng rt gn

nhau, chng ri trong di thng li ca laser. Hnh 7 minh ha mt

v din hnh, trong mt vi gi tr cng hng ca N, thng

c gi l mode dc ca laser, va kht trong di thng li.

Hnh 7. Mode cng hng hp v di thng li

Cc chm laser c nhng c im chung nht nh, nhng cng khc nhau mc

rng cc kha cnh nh kch thc, s phn k, v s phn b nh sng qua ng knh

chm tia. Nhng c im ny ph thuc nhiu vo vic thit k hp laser (hp cng

hng), v h thng quang hc iu khin chm tia, c bn trong ln bn ngoi hp. Mc

d laser c th to ra mt m sng khng i khi chiu ln mt b mt, nhng nu ocng sng ti nhng im khc nhau trong tit din ngang ca chm, th s thy s

khc nhau v cng . Vic thit k hp cng hng cng nh hng ti phn k

chm tia, so mc tri rng ca chm tia khi khong cch ti laser tng ln. Gc

phn k ca chm tia l mt nhn t quan trng trong vic tnh ton ng knh ca chm

tia ti mt khong cch cho trc.

8/8/2019 25 Laser

15/20

Trong nhiu phn tho lun trc, chng ta ginh cc gng ti hai u ca hp

cng hng laser l gng hai chiu, hay gng phng. V mt khi nim th y l mt

cu hnh n gin nht, nhng trong thc t n c th rt kh c thc hin. Nu hai

gng khng thng hng chnh xc vi nhau, th s mt nh sng d tha s xy ra, lmcho laser ngng hot ng. Ngay c khi s khng thng hng chmc nh, sau mt

vi phn x lin tip, kt qu c th l s tht thot ng k nh sng t cc mt ca hp.

Nu mt hoc c hai gng c b mt cu, th s tht thot nh sng do s khng thng

hng c th gim bt hoc b loi tr. Do tnh hi t ca gng cu, nh sng b gii hn

trong hp ngay c khi cc gng khng chnh xc thng hng vi nhau, hoc nu nh

sng khng c pht ra chnh xc dc theo trc ca hp. C mt s bin tu thit k s

dng kt hp c gng phng v gng cu m bo nh sng lun lun hi t trli

pha gng i din. Mt cu hnh thuc loi ny c tn l hp cng hng bn, do nh

sng phn x t mt gng i ti gng kia s tip tc dao ng mi mi nu nh khng

c nh sng no b tht thot.

Trong mi trng laser c li thp, hp cng hng rt quan trng trong vic ti a

ha vic s dng bc x cng bc. Trong laser li cao, s mt mt mc thp t cc

mt ca hp khng c tnh quyt nh. Tht ra, cc thit k hp cng hng khng bn c

thc a chung hn v chng thng d thu nng lng t mt th tch ln hn trong

mi trng laser, mc d chng cho php nh sng tht thot. Cc gng trong laser

li cao thng thng trong sut hn cc gng trong laser c li thp hn, cho nn

mt tia sng cho trc ch c th truyn mt ln qua hp trc khi xut hin trong chm

tia. Do , s sp thng hng ca cc gng khng c tnh quyt nh nh trong laser

li thp, ni m h s phn x ca gng ra lm cho nh sng phn x nhiu ln trc khi

xut hin ra ngoi.

Chiu di hp laser v bc sng nh sng tc ng ln nhau to ra mode dc ca s

phn b nng lng trong chm tia, cn thit k hp cng hng l mt nhn t then cht

trong vic xc nh s phn b cng theo chiu rng ca chm tia, v t l m chmtia phn k. Cng ct ngang chm tia c xc nh bng mode ngang ca chm.

Nhng phn b c kh nng trong cng chm tia c gii hn bi ci gi l cc

iu kin bin nht nh, nhng thng th mt chm tia biu hin mt, hai, hoc hn hai

nh gia, vi cng khng cc ra ngoi. Cc mode khc nhau ny c gi l

mode TEM(mn), vit tt ca cc t mode ngang (Tranverse), mode in (Electric) v

8/8/2019 25 Laser

16/20

mode t (Magnetic), trong m v n l cc s nguyn. Cc s nguyn cho bit s cc

tiu, hay sim cng bng khng, gia cc ra ca chm theo hai hng vung gc

nhau (m cho mode E v n cho mode M).

Mt chm laser in hnh sng nht ti trung tm v gim dn cng v pha ngoira. y l mode bc nht n gin nht, k hiu l TEM(00) v c cng ct ngang

chm tun theo hm Gauss. Hnh 8 minh ha mt vi trong s nhiu mode TEM(mn) kh

d. Mc d mt s laser hp cng hng bn, c bit l nhng laser c thit k cho

cng sut ra cc i, hot ng mt hoc nhiu mode bc cao, nhng ngi ta thng

mun loi b nhng dao ng ny. Mode bc nht c th thu c d dng trong cc

laser li thp hp cng hng bn, v l mode c a chung v chm tia tri rng

do s nhiu x c th tin n mt gi tr cc tiu l thuyt.

Hnh 8. Mode ngang ca chm laser

Nhiu xng mt vai tr quan trng trong vic xc nh kch thc ca m laser c

th chiu ti mt khong cch cho trc. Dao ng ca chm tia trong hp cng hng

to ra mt chm hp sau phn k mt s gc ph thuc vo thit k hp cng hng,

kch thc ca l h ra, v gy ra cc hiu ng nhiu x trn chm tia. S nhiu x

thng c m t l hiu ng tri rng chm tia, hnh thnh nn cc vng nhiu x (gi

l vng Airy) bao quanh chm tia khi sng nh sng truyn qua mt l nh. Hin tng

nhiu x ny t ra gii hn vng knh ti thiu ca m sng sau khi truyn qua mt

8/8/2019 25 Laser

17/20

h quang hc. i vi laser, chm tri rng t gng ra c thc xem l qua mt l

nh, v hiu ng nhiu x trn chm tia do gng gy ra s gii hn phn k ti thiu

v kch thc m sng ca chm. i vi cc chm mode TEM(00), nhiu x thng l

nhn t gii hn s phn k ca chm. Gi tr danh ngha ca s phn k chm tia c

cho bi mi quan hn gin sau:

phn k (radian) = Hng s ? Bc sng /ng knh chm tia

Nu chm tia laser truyn qua mt h quang hc, gi tr ng knh thch hp trong

phng trnh trn l ng knh ca thnh phn cui m chm tia truyn qua. Hng s

trong phng trnh ph thuc vo s phn b cng trong chm, v c gi tr rt gn

thng nht vi nhau. Mi quan h r rng cho thy phn k chm tia tng theo bc

sng, v gim khi ng knh chm (hoc thu knh ra) tng. Ni cch khc, ng knh

chm cng nh th chm cng b phn k nhiu v cng tri rng ra theo khong cch so

vi chm ln.

Gi tr ca phn k chm tia i vi mt laser cho trc c th c ngha thc hnh

rt ln. Laser helium-neon v laser bn dn trthnh nhng cng c chun trong lnh vc

trc a. Ngi ta gi mt xung laser nhanh ti mt gng phn x gc t ti ni cn lp

bn , v tr ca xung laser phn x li c thc o chnh xc thu c khong

cch ti ni t laser. Trn nhng khong cch ngn thng thng, phn k chm tia

khng phi l vn quan trng, nhng i vi nhng php o khong cch xa, s phnk qu mc c th lm gim cng chm tia phn x, v cn trvic o c. Cc nh

du hnh ngi M trn s mnh Apollo 11 v Apollo 14 t mt ci gng phn x

gc trn Mt Trng, n s phn x nh sng t mt laser ruby xung cng sut ln t ti

i quan st MacDonald, Texas. Mc d chm tia tri ra trong bn knh 3km trn b

mt Mt Trng, nh sng phn x vn c cng mnh thu nhn c trn Tri

t. Khong cch t Mt Trng n i quan st Texas c o vi chnh xc 15cm

trong th nghim ny, nhng k t thp nin 1980, nhng tin b k thut tng

chnh xc ln di 2cm. Nhng c gng hin nay ang c thc hin s dng cc knh

thin vn cng sut ln truyn v nhn cc xung nh sng t mt vi gng phn x

t trn Mt Trng lm gim hn na sai so, c th ch khong 1mm.

Do cch to ra hot ng laser lin quan ti vic lm tng s nguyn t hay phn t ln

trng thi kch thch cao nhm to ra s nghch o dn c cn thit, nn hin nhin mt

s dng nng lng phi c a vo h laser. Cc photon c thc p dng cung

8/8/2019 25 Laser

18/20

cp nng lng cn thit trong mt qu trnh gi l bm quang hc. Bng cch chiu sng

vt liu laser vi nh sng c bc sng thch hp, nguyn t hay phn t pht x c th

c a ln mc nng lng cao, t n ri xung mc siu bn, v ri b cng bc

pht x ra nh sng. Tht may mn, trong a s laser, nh sng dng bm khng nht

thit phi c bc sng c bit, ch yu do laser c th c nhiu mc cao c th phnhy hon ton xung mc siu bn. Do , mt ngun nh sng khng t tin pht ra

mt ngng rng bc sng, nhn nng sng hay n flash, thng c thc dng

lm bm quang hc cho laser. Mt nhn t quan trng gii hn hiu sut laser l photon

ca nh sng bm phi c nng lng cao (hay c bc sng ngn hn) so vi nh sng

laser.

Bm in l mt cch kch thch khc thng c dng trong laser kh v laser bn

dn. Trong laser kh, dng in truyn qua cht kh kch thch cc nguyn t v phn t

vo mc nng lng cao cn thit bt u phn hy, hoc phn hy mt lot, to ra

pht x laser. Mt s laser kh dn mt dng khng i qua cht kh to s pht ra

laser lin tc, cn nhng laser dng xung in th to nh sng laser ra dng xung. Mt s

laser cng sut ln cn s dng chm electron a trc tip vo cht kh kch thch.

Laser bn dn hot ng theo kiu rt khc, nhng cng da trn dng in to ra s

nghch o dn c cn thit. Trong nhng dng c ny, s nghch o c to ra gia

dn c ca cc ht mang in (electron v cp electron-l trng) trong mt phng tip

gip gia cc vng cht bn dn khc nhau. S pht x nh sng trong laser bn dn tptrung trong mt phng tip gip bi s phn hi t cc u ch ca tinh th (hnh 9). Vt

liu lt mng c h s phn x cao, v phn x nh sng trli tinh th thu c

li. B mt ch cng c thc nh bng iu chnh h s phn x. u chin

hnh ca tinh thc ph mt vt liu phn x sao cho s pht x ch c th xy ra

mt u, nhc minh ha trn hnh 9. in th v dng in cn thit trong laser bn

dn thp hn nhiu so vi laser kh.

8/8/2019 25 Laser

19/20

Hnh 9. Diode laser bn dn

Cc dng truyn nng lng khc t c s dng hn to ra s chuyn trng thi laser

mnh. Cc phn ng ht nhn v ha hc c thc dng to ra s kch thch trong

mt s loi laser. Cc laser kh c th dng nhng hn hp kh khc nhau thc hinqu trnh laser. Trong laser helium-neon, cc nguyn t helium bt nng lng t s

phng in kh do mt dng in vo gy ra, ri truyn sang cc mc nng lng rt gn

nhau tn ti trong kh neon. Sau s chuyn trng thi laser xy ra trong kh neon

to ra pht x laser.

Laser vn dkhng hiu qu. Nng lng phi c cung cp cho laser, v mt s b tht

thot trong qu trnh bin i sang loi nng lng c trt t cao hn l dng nh sng

laser. Nh ni phn trn, i vi laser bm quang hc, nh sng laser ra lun lunc bc sng di hn bc sng nh sng bm. Nhng mt mt nng lng khc xy ra

trong cc qu trnh chuyn mc nng lng xy ra trong laser ba mc v bn mc. Sau

kch thch ban u ln mc cao, s chuyn trng thi laser t n ch c th gii phng

mt phn nng lng , ng ni l phn cn li b mt trong nhng qu trnh khc.

Trong mt s h thng, vi s chuyn trng thi laser nng lng cao, a phn nng

lng c dng cha cc loi laser ln mc thch hp, nm trn trng thi cbn.

Qu trnh kch thch, d bng phng php in hay phng php quang, khng c hiu

sut 100% - v nng lng cha bao gic hp th hon ton bi mi trng laser.

Tt c nhng nhn t chnh ny, v mt s nhn t th yu khng ni n y, tht sgii hn hiu sut tng th ca laser. Mc d nhng laser bn dn hiu qu nht v mt

s laser kh c th bin i gn 10% nng lng vo thnh nh sng laser, nhng laser

in hnh c hiu sut ch c1% hoc thp hn.

8/8/2019 25 Laser

20/20

Trong vi thp k k t thp nin 1960, laser khng cn l mt tng khoa hc vin

tng, mt vt him trong phng nghin cu, mt tht tin na, m l mt cng c

qu gi trong nhng ng dng khoa hc nht nh, n tr thnh mt vt thit yu trong

cng vic hng ngy, v thng dng n mc c th mua nhng ca hng tp ha, c

ngi dng n o kch thc phng dn giy ln tng. Bt k danh sch no imli nhng thnh tu cng ngh ch yu ca th k 20 cng c tn laser nm phn trn

u. S thm nhp ca laser vo mi mt i sng hin nay c thc nh gi ng

nht bng phm vi ng dng ca cng ngh laser. mt pha ngon mc ca phm vi

ny l nhng ng dng trong qun s, k c vic s dng laser lm v kh chng li s

tn cng bng tn la. mt phm vi khc l nhng hot ng thng nht nh nghe

nhc trn a CD, v in n hoc in sao cc vn bn giy. Cc thanh laser c bn hng

trm la mi thanh c xem l nhng mn ph tng khng t tin, c ngi thmc

cng s dng laser, v nhng dng co c n gin cng c gn laser.

Va k l va bnh d, laser c s dng rng ri trong iu tr y khoa v phu thut, v

trong vic ct v hn cc gin khung bng thp, cao su, v plastic dng trong xng ch

to t v dng c. Nhit t laser c dng hn im cc kim loi, v trong cc th

thut y khoa tinh vi nh dn li vng mc sau khi m tch ra trong k thut phu thut

mt ngi. Nhng th thut y khoa chnh xc cao khc nh sa cha cc mch mu

hng, ct v t chy m, thng s dng laser. Phn ln mng vin thng trn th gii

c truyn dn bng vic gi nhng tn hiu laser dng xung i hng dm ng trong

cc si cp quang, v nhng to tc mang ngha vn ha, nh nhng bc tranh thici, thng c thm nh s rn nt, hng hc v phc hi vi s h trca laser.

Cng vi my tnh in t, mch tch hp, v v tinh nhn to, cng ngh laser pht trin

ngy cng trnn quan trng trong cuc sng hng ngy ca chng ta, bin nhng gic

mnhiu nm trc y ca loi ngi thnh s tht.

Tc gi: Kenneth R. Spring, Thomas J. Fellers v Michael W. Davidson([email protected])