Aspheric Lenses - Lightpath Technologies - #4

Choosing the Right Aspheric Lens

For example, if the half max NA for a laser is used with the above foi mula you .vii get i.fie fiat max yjeyr dismelei ~-:eie is no simple ■.v.iy lo coir/e-:. from hah' mix number or a l/e² beam diameter to a full beam diameter fcr .1 sticclic source because it depends on the i':tenii';y p'o'iie of the souioy iseli. A reasonable iioproxriyiiion. though, for most edge emitting ;: ode Niseis is 10 assume ,1 Cjjjssia-i bean' profile. Usng if- s beam profile, you can convert the beam diameters as follows:

For Diode Collimation

One of the most common uses for aspheric lenses is in the

■......■■.........nr.; '■ i i srs. With ovp-- 't'i syindard

lenses in LightPath's catalog Lo choose von-. however '.his ca-hea coiifua.iv: L<iL.k ^The guide below will clear up some of the quest,0us about choosing the best lens to use with ,i spentir laser for a particular application.

Due :o Ihc ■.'-■ay '-a' the- laser cavity is constructed in edge emitting

diode iii i" ■. i ■ 'i i ■ ■ ■ i i..... flivi ■ i.......... " riTdtry ■ -,o the

o verge/ire s ty-i ca !y KJcri'sd n both the \ and _y .ixes separately.

The axis with the.........i.....■■ i..... Ih fast axis" and the

nxs y/itn the smailc'- divergence s called the "slow axis'".

When selecting a lens to rolli-nate tie ascs first rois.de'- the Numerical Aperture of the lens. If the applicator rcq..re; a '-ig/.-amount of the laiei glit to be coupled thoi.-gji -^,,e system.,: lens with a high cnougi NA l-us: lie ci-ose'i. ine l\A oi a lens is a measure of the maximum amount of divergence that the lens can ::_l;otu^,"e '™ ye .sc Ide.ty a leis shout: he used iha: ias NA nigher than the NA of the laser's fast axis. If not, the laser will ■clip" the lens ca.syg some of the ligh: to be ■//listed Tb convert the laser NA to the divergence angle (and vice-versa], use this formula:

For Fiber Coupling

Another common use for aspheric lenses is to couple laser light into cpt.ca 'be-s. Choosing ye rich'. Ic's or lenses to do the coupling is n-poriant to ma ntair high effìt. ercy ir ire optca system.The guide oelow is ntereed to show i-ow best :o do this wi-'c using or-thc-she If components. Thy guide assures ya; III» input aser light :as already been collimated (not diverging).

When selecting a lens to focus light into a fiber first consider what focal length lens ; neec.ed ..c'.'s nyv.st Hie 'ormula give'i previous y

i- most c_:;s.cs. n- I sirce ye NA e^: the laser s deyicc in air. Therefore, solving for the equation is simplified to;

¡1 is n-por'.anf to no'.e that <!> : ye half л-'gie of ye divergence cone and is given at the marginal ray (not l/e" or half angle half так).

After the minimum NA necessary for v-e ers is с etc Tni red. next tons tie- wh.u beam diameter is preferred. Although ray trying is accessary to precisely determine the жал- diameter for a given NA source у.'!-- a particular lens, it can be apo'-oximated with the following formula:

Solving for EFL it becomes:

_ Beam Diameter 2-NA

where NA is the numerical aperture of the fiber Ilia: is usee lor the

it is inoortans to r.ote that the I H vaLe rl-at is calculated above is :;-.:■ minimum ^: ' I eded to couple the light completely into the fiber. I orig.y I I-1 erses can be used, but the spot size on the fiber

"M rn lens poysde th;u s Lirger than the ivninjn v,, ue specifisc

where EFL is the effective focal length of the lens and NA is the numerical aperture o'' ye source (rot the NA of the lens).

Important Note:

Some idse: iiian..'it\.ie-y g.ve '"<-* ^A sourte it¹ drTe-enf

terms, such as half max (50% point) or I, e² (87% point). Whatever tyoc of-u 'ti e—.ered r-to y-e -Vr\ NA of soi.rco

w . be the s.ine type of nurbe- jj* foi the beam dametei..