Bc sng chun Laser

Laser, ngha l khuch i nh sng bng bc x cng bc. Laser l mt trong

nhng pht minh khoa hc quan trng nht trong th k XX. n nay vi nhng tnh
cht c trng u vit ca n, laser ngy cng c ng dng nhiu trong nghin cu
khoa hc, cng ngh. S dng bc sng chun laser n nh tn s lm chun o
lng n v di mt l mt trong nhng ng dng ca laser.
Tnh cht c trng ca tia Laser
Cng tia Laser ln gp bi ln cng tia sng nhit
So snh cng ca bc x laser khi cng sut bnh thng vi tia sng nhit:
Vi laser kh HeNe pht cng sut 1mW ch lin tc vi bc sng
633nm c nng lng h = 10-9 J th s phton laser pht trong mt giy s l:

Vi ngun nhit c nhit T = 1000o K bc x t mt din tch DA = 1cm2 v

cng pht sng trong vng nhn thy c vi rng ph Dn = 104 nm th s phton
nhit tnh theo cng thc:

So snh ta thy s phton laser ln hn rt nhiu.

nh phng ca Laser l cao
Ngun sng nhit bc x theo mi phng trong khng gian. Cn ngun laser
do c cu ca bung cng hng quang hc ch pht cc dao ng ngang v chng
tp trung trong mt mt phng phn cc. Cng sut pht c phn b u v ng
pha trong ton b khu ca ngun.
Vi chm laser sng phng, bc x t mt bung cng hng vi gng c
ng knh d (hoc din tch

sau gng chm tia laser s tn x, do hin tng nhiu x, di mt gc nhiu

x.

chm tia s bc x trong mt gc khi:

 Gi tr gc khi rt nh so vi gc khi bc x ca mt ngun nhit l c
2psteradians.
nh phng cao cho s tp trung nng lng trong mt gc khi nh v to
nn cng ln.
n sc (monochromaticity)
Theo nh ngha n sc ca mt chm tia c c trng bng rng vch
ca chm. Khi rng vch ca chm tia bng khng th chm c n sc cao
nht. Trong trng hp gn ng vi bung cng hng quang hc, rng vch c
th xc nh bng cng thc.

Vi cng sut pht P = 1mW,

 vng bc sng s c

y l rng rt b.

Tnh kt hp ca Laser (Coherence of laser beam).

Mt trong cc tnh cht quan trng v c bit nht ca laser l tnh kt hp.
Mt bc x laser bt k u c tnh kt hp biu hin n sc (kt hp thi gian)
v tnh ng pha ca mt sng (kt hp khng gian).
Sng c tnh kt hp khng gian khi bt k thi im no, nh sng c pha
khng i trn khp mt sng ca n.
Tng t ti mt thi im cho trc dc theo mt sng chuyn ng nu pha l
ging pha m sng c sau khi i qua mt khong cch L vi thi gian L/c, d L
c nh th no th sng c xem c tnh kt hp hon ton.
Cc laser hot ng ch n mode dc hay ngang c biu hin trong cc
sng n sc v ng pha nn chng c bc kt hp khng gian v thi gian cao, mt
cch t ng.
Vi cc tnh cht c trng nh trn, cc ngun bc x laser, c bit l laser kh
He-Ne c dng ph bin th hin n v di mt. Hin nay, phn ln cc
Vin o lng quc gia ca cc nc tin tin u dng laser kh He-Ne n nh tn
s bng Iodine bc sng 633nm lm chun u quc gia cho n v di mt.
Do c ch hot ng ca laser kh He-Ne vi bung cng hng Faby-Perot,
xut hin "ch lm Lamb" (Lamb dip). Hiu ng ny do Lamb ch ra bng l thuyt
sau c thc nghim xc nhn li. Trn cngtua ca ng cong khuych i
(Gain line) xut hin mt ch lm.

Trong Laser kh do s chuyn ng nhit ca cc nguyn t kh nn xut hin

nhiu nhm nguyn t chuyn ng vi cc vn tc khc nhau. Gi s bc x laser c
tn s w w0 (tn s tm) i trong bung cng hng theo mt phng no , n
s tng tc vi cc nhm nguyn t c tc v ngc li vi phng ca bc x s
dn n lm gim nghch o tch lu v ng cong khuch i c s sinh hc
(hole - burning). Hon ton tng t vi vi bc x i theo chiu ngc li s gp
cc nhm nguyn t c vn tc v, kt qu trn ng cng tua ng cong khuch
i xut hin hai hc i xng so vi tm vch. rng ca hc bng m rng t
nhin trong bc x Laser.

Khi w = w0 bc x laser s ch tng tc vi cc nhm nguyn t c tc v =

0 v s sinh ra mt hc tm, chnh l Lamb dip nh thng gi.
Chnh nh hiu ng trn ngi ta s dng n n nh tn s. V rng hc
nh nn v tr ca hc xc nh kh chnh xc ch cc tiu ca hc cho php n nh
tn s pht Laser. Vi ngun Laser kh He-Ne ngi ta xc nh tn s pht ch
cc tiu ca hc ni trn vi chnh xc 10-9

ng Iodine c lp bn trong hc laser , bc x quang hc l rt mnh

(10 mW). iu ny lm cho cc tn hiu hp th bo ho rt hp, laser c iu
chnh v gi c nh ti mt trong by vch hp th ca Iodine di s kim sot ca
thit b in t.