Physicist view of Diffraction grating A Multi-slit arrangement which uses diffraction to separate light wavelengths with high resolution and high intensity. The resolving power is achieved by interference of light.

Basics of diffraction Single slit interference P– 1st maximum Q– 1st secondary maximum θ = nλ/d Diffraction Pattern Intensity of the beam is governed by I = I0 { sin β / β }2 Where β = (π / λ) d sin θ

Multiple slit interference A N-slits interference pattern is the diffraction pattern and we develop diffraction gratings based on N-slit interference pattern. Intensity transmission function is I = I0 { sin β / β }2 {(sin Nµ )/ (N sin µ) }2 Where β = (π/λ).b sinθ µ = (π/λ).d sinθ Principle fringes occur at µ = n π ➔ n λ= d sinθ Secondary fringes occur at µ = 3π/2N, 5π/2N, ……

Physics of diffraction Ray Propagation through the grating Grating normal Grating normal Incident light Reflected light Incident light + + - - α Diffracted light β0 α > 0, β1 >0 Diffracted light α β1 Β-1 β0 < 0, β-1 < 0 d Β-1 β1 β0 + - Diffracted ray A Reflection grating A transmission grating Light diffracted in the same direction of the incident ray ➔ +ve angle

Wave front propagation through the grating Classical diffraction: Grating equation: mλ= d(sinα + sinβ) ➔ Gmλ= sinα + sinβ ➔ Gmλ= 2cosK sinØ B1 A1 G – groove frequency = 1/d λ – wavelength of the diffracted light K – deviation angle = ½(α-β) Ø – scan angle = ½(α+β) A4 B4 β α α β Littrow configuration : α=β ➔ mλ= 2dsinα A3 B2 A2 B3 d Conical diffraction: Gmλ= cosε (sinα + sinβ) Path difference = A2A3 ~ B2B3 = d sinα + d sin β ε – angle between the incident light path and the plane perpendicular to the grooves.

Characteristics of Diffraction Grating Dispersion: angular dispersion linear dispersion Resolving power Spectral resolution Band pass Focal length and f-number

DISPERSION Angular Dispersion is the measure of the separation between diffracted light of different wavelengths. It gives the spectral range per unit angle. Mathematically, D= ∂β/∂λ = G.m.secβ = (2/λ)tanβ --- Littrow condition Linear dispersion is the product of angular dispersion D and effective focal length r’(β) linear dispersion (l) = r’D = r’.G.m.secβ Platefactor is change in wavelength when we move along the spectrum and is given by P = 1/l = dcosβ / r’m Obliquity factor is the factor that governs the platefactor when the incident ray is not perpendicular to the grooves and is = 1/sinØ

RESOLVING POWER SPECTRAL RESOLUTION: This is the ability to separate adjacent spectral lines of average wavelength λ. Mathematically, R = λ/∆λ ∆λ -- limit of resolution, difference in wavelength of equal intensity Theoretically, it is the product of diffraction order and the total number of grooves illuminated. R = N.d.(sinα + sinβ)/λ ➔ Rmax = 2n.d/ λ SPECTRAL RESOLUTION: ∆λ is the spectral resolution and is measured by convoluting the image of the entrance aperture with the exit aperture.

BANDPASS This is the wavelength interval that passes through the exit slit. Also, the difference in wavelengths between the points of half-maximum intensity on either side of the intensity maximum. Mathematically, its estimate is given by B = w’. P where w’– exit slit width P – reciprocal of linear Dispersion. FREE SPECTRAL RANGE It is the range of wavelengths in a given spectral order for which light from adjacent orders are not superposed. Mathematically, F λ = λ 1 /m where λ 1 is the wavelength of light diffracted in the mth order. The greater the free spectral ranges the less is the filters required.

FOCAL LENGTH AND f/NUMBER If the beam diffracted from the grating of a given wavelength and order converges to a focus, then the distance between the focus and the grating centre is the focal length and the ratio of the focal length to the width of the grating. f/no. input = r/W Source A Incident light r α f/no. output = r’/W W O β Grating Normal r,’ B r/r’ determines the exit slit width Diffracted light Image The more the f/number the less is the spectral aberrations.

Ruled gratings Mechanically ruled by burnishing grooves with a diamond tool against a thin coating of evaporated metal using Ruling engines. Michelson engine servo controlled laser interferometer 20 grooves/mm to 10,800 grooves/mm Mann engine automatic interferometric servo system no ghosts and theoretical resolving power MIT ‘B’ Engine double interferometric control system based on frequency stabilized laser 20 grooves/mm to 1500 grooves/mm

The Ruling Process Substrate material BK-7 , fused silica or special grade ZeroDur polished to one tenth of wavelength with gold o aluminum coatings. Involves interferometric control ➔ requires a monochromatic source ➔ the source environment must have constant temperature and atmospheric pressure. Vibrations of the ruling engine has to nullified by passing through the diamonds. VLS gratings these gratings work on the principle that the variations in the groove spacing modifies the curvature of the diffracted wavefronts which in turn changes the focus of the spectrum.

Holographic gratings Groves are recorded using interference pattern on a photographic plate, which is a photo resist material ( molecular structure changes with the light exposure). Selected laser should be of the wavelength that the photo resist is sensitive to. Steps : 1. exposing to Interference pattern\ 2. development…..valleys at bright fringe, ridges at dark. Classification ➔ single beam : beam reflected upon itself ➔ double beam : groove pattern defined by the Intersection of the surface of the substrate and the fringe pattern.

Comparison ruled grating Equally spaced Need not be equally spaced Groove pattern Good Better Groove frequency No Yes Groove placement errors no Ruling errors yes Surface irregularities Interference grating ruled grating Property

Imaging properties The properties of the image obtained depends mostly on the aberrations in the wave front. These aberrations depend on the groove pattern. With respect to groove patterns we divide gratings into classical gratings ➔ equally spaced lines on tangent plane 1st generation gratings ➔ unequal spacing and curved 2nd generation gratings ➔ toroidal wavefronts varied line spacing ➔ grooved lines are varied uniformly

General definitions Plane grating – grating whose surface is plane and requires other optical elements for focusing or imaging. Concave grating – grating whose surface is concave and focusing is done by the grating itself. Tangential plane – the plane that contains the incident beam and the diffracted rays. Also called as dispersive plane. Sagittal plane – the plane perpendicular to tangential plane. Pole rays – the rays that fall on the grating grooves and diffract. General rays – the rays that fall outside the groove pattern.