Diffraction Gratings

Optical dispersive ntsiab · Spectroscopy · Lasers · Telecom · Source: PhotonLab

Diffraction gratings yog cov khoom siv kho qhov muag tseem ceeb hauv ntau hom kev siv, suav nrog spectrometers, lwm yam khoom siv tshuaj ntsuam xyuas, kev sib txuas lus, thiab laser tshuab. Lub grating muaj cov qauv microscopic periodic groove uas faib cov teeb meem teeb meem mus rau ntau txoj kab teeb los ntawm kev sib txawv, ua rau lub teeb ntawm cov wavelengths sib txawv kom nthuav tawm nyob rau hauv ntau cov lus qhia. Qhov no ua rau lub diffraction grating functionally zoo ib yam li ib tug dispersive prism, txawm hais tias ib tug prism cais wavelengths los ntawm wavelength - nyob ntawm refraction es tsis yog diffraction (Daim duab 1).

Daim duab 1. Ib tug dispersive prism cais wavelengths los ntawm refraction (sab saum toj), thaum lub diffraction grating cais wavelengths los ntawm diffraction vim nws nto qauv (hauv qab).

Lub teeb teeb meem ntawm lub grating yog diffraced raws li qhov sib npaug grating:

Ntawm no,myog ib tug integer piav txog qhov diffraction (los yog spectral) kev txiav txim,λyog lub wavelength ntawm lub teeb,dyog qhov sib nrug ntawm grooves ntawm grating, yog lub kaum sab xis ntawm qhov xwm txheej, thiab yog lub kaum sab xis ntawm lub teeb tawm ntawm lub grating. Kev cuam tshuam ntawm kev sib txawv ntawm qhov sib txawv wavefronts tshwm sim ntawm qhov sib npaug ntawm qhov sib npaug ntawm lub wavelength, uas yog vim li cas "m" tshwm hauv Equation (1).mtxhais qhov kev txiav txim diffraction:m= 1 yog hu ua thawj-kev txiav txim siab,m= 2 yog thib ob- xaj qhov sib txawv, thiab lwm yam (Daim duab 2). Yogm= 0, lub teeb yog ncaj qha cuam tshuam los ntawm lub grating los yog kis los ntawm nws, nyob ntawm seb nws yog ib qho kev xav los yog kis grating; qhov no yog hu ua zero-order diffraction. Tsis zoo li lub prism dispersive, luv wavelengths ib txwm nyob ze rau lub teeb ncaj qha los yog kis tau tus mob -qhov no, qhov kev txiav txim thib 0. Qee qhov sib tshooj muaj nyob ntawm qhov kev txiav txim sib txawv. Tag nrho cov ces kaum yog ntsuas los ntawm lub grating ib txwm (perpendicular mus rau lub grating).

Daim duab 2. Qee qhov teeb pom kev ncaj qha raws li 0th- xaj qhov sib txawv, thaum lwm qhov ntawm qhov teeb meem teeb pom kev sib txawv rau thawj- cov ces kaum nyob ntawm qhov wavelength. Cov nyiaj me me tau muab cais ua ntau dua -angle 2nd- thiab 3rd-order diffraction.

Yuav tsum xav txog dab tsi thaum xaiv lub grating

Thaum xaiv ib lub grating, nws yog ib qho tseem ceeb uas yuav tsum tau hais kom meej lub wavelength ntau, lub blaze wavelength (lub wavelength nrog lub siab tshaj plaws efficiency nyob rau hauv lub diffracted spectrum), thiab lub blaze lub kaum sab xis. Lub blaze lub kaum sab xis piav qhia thawj -kev txiav txim siab lub kaum sab xis ntawm blaze wavelength. Ntawm lub kaum sab xis no, thiab nyob rau hauv Equation (1) yog sib npaug, thiab qhov teeb meem teeb meem yog diffracted rov qab rau hauv cov kev taw qhia los ntawm qhov uas nws tuaj. Cov mob no tseem hu ua Littrow configuration. Kev khiav haujlwm ze rau lub kaum sab xis no muab qhov ua tau zoo tshaj plaws.

Groove density, lossis groove zaus, feem ntau yog teev. Nws yog qhov sib txawv ntawm qhov sib txawv ntawm qhov zawjd. Cov cuab yeej tseem ceeb ntawm lub tshuab kho qhov muag yog nws qhov tawg, tab sis qhov no nyob ntawm ob qho tib si grating lub zog thiab siv cov grating li cas. Yog tias tsis muaj lwm cov ntsiab lus, lub grating ib leeg tsis tuaj yeem muab txoj cai xws li "ib qho kev sib hloov sib raug rau qee qhov sib cais wavelength." Lub grating lub zog daws teeb meem kuj yuav raug teev tseg, thiab nws cuam tshuam rau qhov kev daws teeb meem spectral. Txawm li cas los xij, qhov kev daws teeb meem no nyob ntawm ob qho tib si grating thiab lub kaw lus nkag thiab tawm slits. Lub grating daws lub zogRnyob ntawm qhov kev txiav txim spectralmthiab tus naj npawb ntawm grooves nyob rau hauv illuminationN:

Feem ntau yog li ntau qhov grooves tau teeb pom kev zoo tias qhov nkag thiab tawm qhov hlais - tsis yog qhov grating - txwv qhov kev daws teeb meem. Kev ua tau zoo nkhaus kuj tuaj yeem siv los txheeb xyuas cov theem diffraction hla txhua lub wavelengths siv hauv daim ntawv thov.

Lub grating yuav tsum tsawg kawg yog loj npaum li qhov teeb meem lub teeb lub khob hliav qab los yog beam; txwv tsis pub lub teeb los ntawm cov npoo yuav ploj. Yog li ntawd, ib qho grating yuav tsum tau ua kom tsis muaj zog los tiv thaiv lub teeb tsis pom kev los ntawm kev thaws ib puag ncig ntawm lub cev thiab tsim cov teeb liab cuav.

Hom Gratings

Reflection Gratings vs. Transmission Gratings

Ob pawg dav tshaj plaws yog kev xav gratings thiab kis gratings. Daim duab 1 thiab 2 qhia txog kev xav gratings: qhov tseem ceeb tsom iav nrog cov grooves microscopic. Tag nrho cov kev txiav txim diffracted qhia los ntawm cov grating ntawm cov ces kaum sib txawv. Ib qho kev sib kis grating zoo li lub lens nrog microscopic grooves: txhua qhov kev txiav txim siab tsis sib haum dhau los ntawm cov grating, tab sis yog offset los ntawm lub kaum sab xis muab rau hauv Equation (1).

Ruled Gratings vs. Holographic Gratings

Ob qho tib si kev xav thiab kev sib kis gratings tuaj yeem muab faib ntxiv rau hauv kev txiav txim gratings thiab holographic gratings, uas txawv ntawm qhov zawj profiles raug tsim. Nyob rau hauv txoj cai gratings, grooves yog mechanically engraved los yog txiav rau hauv cov khoom. Nyob rau hauv holographic gratings, grooves yog qhia optically: ib tug photosensitive khoom hu ua photoresist yog tso rau ntawm ib tug substrate thiab raug rau ib tug optical interference qauv. Tom qab ntawd cov photoresist tseem raug tshem tawm, tawm hauv cov qauv grating. Ruled gratings feem ntau muaj daim duab peb sab grooves (Daim duab 3), thaum holographic gratings feem ntau muaj sinusoidal grooves (Figures 3 thiab 4).

Daim duab 3. Ruled diffraction gratings feem ntau muaj triangular grooves. Arrow on grating: direction of blaze.

Daim duab 4. Holographic diffraction gratings feem ntau muaj sinusoidal grooves.

Echelle Gratings

Ib qho echelle grating yog tus cwj pwm los ntawm qhov sib txawv ntawm qhov loj dua lossis qis qis dua li cov gratings zoo tib yam - feem ntau txog 10 ×, thiab qee zaum txog 100 ×. Illuminating echelle grating ntawm lub kaum sab xis siab ntawm qhov xwm txheej ( ) ua rau muaj kev sib cais siab, kev daws teeb meem siab, thiab tsis tshua muaj polarization- nyob ntawm qhov ua tau zoo. Cov gratings no zoo haum rau cov ntawv thov uas xav tau kev daws teeb meem siab, xws li cov cuab yeej siv hnub qub rhiab heev thiab cov tshuab ua raws atomic- theem kev daws teeb meem.

Plane Gratings vs. Concave Gratings

Txhua hom grating saum toj no tuaj yeem muab faib ua dav hlau gratings thiab concave gratings, piav qhia lawv cov duab tag nrho. Lub dav hlau gratings yog tiaj thiab ntau dua. Yog hais tias lub grooves yog ncaj thiab sib npaug spaced, lub grating yog tiaj, thiab qhov teeb meem teeb yog collimated, ces tag nrho cov diffracted teeb tseem collimated. Qhov no muaj txiaj ntsig zoo hauv ntau daim ntawv thov vim hais tias cov khoom tsom ntawm lub kaw lus ua ywj pheej ntawm wavelength. Piv nrog rau concave gratings, dav hlau gratings feem ntau txo qhov system complexity. Concave gratings yog nkhaus, yog li lawv yog converge los yog diverge lub teeb. Qhov no muaj txiaj ntsig zoo rau kev txo qis tag nrho cov khoom siv kho qhov muag hauv lub cev, tab sis cov khoom tsom mus rau qhov nyob ntawm lub wavelength.

Grating Applications

Monochromators

Ib tug monochromator siv lub concave los yog dav hlau grating ua ke nrog iav los xaiv ib tug nqaim band los ntawm qhov teeb meem teeb. Yog hais tias ib qho dawb - lub teeb ci nkag mus rau hauv lub cuab yeej zoo li no, nws lim tawm tag nrho cov wavelengths tshwj tsis yog lub hom phiaj nqaim nqaim. Daim duab 5 qhia tau hais tias yuav ua li cas ib tug monochromator rotates lub grating kom sib txawv wavelengths dhau los ntawm kev tawm slit thaum tag nrho lwm cov wavelengths raug thaiv.

Daim duab 5. Plane-grating monochromator (sab saum toj) thiab concave-grating monochromator (hauv qab) ob qho tib si tig lub grating kom scan diffraction txiav txim hla txoj kab tawm thiab txiav txim siab qhov twg wavelengths yuav tawm ntawm lub cuab yeej.

Spectrometers

Zoo li monochromators, spectrometers cais cov wavelengths ntawm qhov chaw dav dav, tab sis lawv tsis muaj qhov txav. Hloov chaw, txhua qhov sib cais wavelengths yog duab ib txhij mus rau ib qho chaw ntes (Daim duab 6). Txhua lub wavelength yog duab mus rau ntau qhov sib txawv ntawm pixels, tso cai rau lub cuab yeej los txiav txim siab npaum li cas ntawm txhua qhov wavelength muaj nyob hauv qhov chaw dav dav. Spectrometers feem ntau yog siv thaum xav tau ceev spectral tsom xam, vim hais tias tsis muaj yuav tsum tau luam theej duab wavelengths hla ib tug ntes, txuag lub sij hawm.

Daim duab 6. Plane-grating spectrometer (sab saum toj) thiab concave-grating spectrometer (hauv qab) ob leeg siv lub grating ruaj los cais qhov xwm txheej wavelengths mus rau txawv pixels ntawm ib tug detector array.

Laser Tuning

Muaj ntau txoj hauv kev los siv lub diffraction grating los kho lub laser lub spectral tso zis los yog nqaim nws cov zis band. Lub grating tuaj yeem tig tau kom tsuas yog ib qho kev txiav txim siab tshwj xeeb tawm ntawm lub laser; lub grating tuaj yeem nyob ruaj ruaj thaum daim iav tig los lim cov zis band; los yog lub grating tuaj yeem hloov daim iav hauv lub laser kom nqaim qhov tso zis band (Daim duab 7).

Daim duab 7. Peb qhov kev teeb tsa no qhia ntau txoj hauv kev uas siv cov grating los kho lub laser tso zis wavelength los yog nqaim cov zis wavelength ntau.

Laser Pulse Stretching, Compression, thiab Amplification

luv laser pulses, xws li ultrafast pulses, feem ntau muaj lub siab ncov lub zog uas tuaj yeem ua rau puas tsuaj rau cov txheej txheem kho qhov muag thiab cov khoom. Txhawm rau zam qhov no, ib khub ntawm diffraction gratings qee zaum siv los ncab cov mem tes, ua kom nws ntev thiab txo nws lub zog siab. Cov mem tes ncab tuaj yeem dhau los ntawm lub tshuab hluav taws xob kho qhov muag thiab tau txais lub zog yam tsis muaj kev puas tsuaj rau cov khoom siv kho qhov muag. Lwm tus khub grating hauv qhov kev teeb tsa rov qab tuaj yeem ua rau lub sijhawm pulse tom qab kev nthuav dav, ua kom luv, siab - lub zog mem tes ntawm lub hom phiaj (Daim duab 8).

Daim duab 8. Gratings tuaj yeem siv rau hauv cov tshuab laser mem tes ob qho tib si txhawm rau ua kom lub sijhawm mem tes ntau ntxiv -tiv thaiv laser- cuam tshuam kev puas tsuaj- thiab txo lub sijhawm mem tes, ua rau siab - lub zog pulses ntawm lub hom phiaj.