Diffraction gratings

By M. Ravi Kiran

Introduction • Diffraction grating can be understood as an optical unit that separates

polychromatic light into constant monochromatic composition.

• Uses are tabulated below

FIELD USEQuantum Mechanics Verification of Hydrogen spectrum

Astrophysics Composition and processes in stars and planetary atmospheres

chemistry Concentration of chemical species in samples

Telecommunications Increase the capacity of fiber optic networks using WDM

When an Electromagnetic radiation falls on a Diffraction Grating, the electric fieldand Phase are modified in a predictable manner.

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 maximumQ– 1st secondary maximumθ = nλ/d

Intensity of the beam is governed by

I = I0 { sin β / β }2

Where β = (π / λ) d sin θ

Diffraction Pattern

Two Slit Interference :

Slit width b

Distance between the slits d

I = I0 { sin β / β }2 cos2 µ

Where β = (π/λ).b sin θ

µ = (π/λ).d sin θ

Intensity distribution is similar to single slit and the spacing between the fringes is determined by (λ/d) and width of the envelop by λ/b.

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

αβ0

Β-1β1

d

Diffracted light

Reflected light

Grating normal

+ -

Incident light

Diffracted light

α

β1

β0

Β-1

Incident light

Grating normal

Diffracted ray

+ -

+-

A Reflection grating A transmission grating

Light diffracted in the same direction of the incident ray +ve angle

α > 0, β1 >0

β0 < 0, β-1 < 0

• Wave front propagation through the grating

A1

A2

B1

B2

B3

A3

B4

A4

d

Path difference = A2A3 ~ B2B3 = d sinα + d sin β

αβ α

β

Grating equation: mλ= d(sinα + sinβ) Gmλ= sinα + sinβ Gmλ= 2cosK sinØ

G – groove frequency = 1/d λ – wavelength of the diffracted lightK – deviation angle = ½(α-β)Ø – scan angle = ½(α+β)

Classical diffraction:

Littrow configuration : α=β mλ= 2dsinα

Conical diffraction:Gmλ= cosε (sinα + sinβ)

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• Anamorphic magnification• Free spectral range• Energy distribution

• Scattered and stray light

scattered light

instrumental stray light• Signal to noise ratio.

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

• 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.

O

A

B

r

r,’

α

βWGrating Normal

Incident light

Diffracted light

Source

Image

f/no. input= r/W

f/no. output = r’/W

r/r’ determines the exit slit width

• The more the f/number the less is the spectral aberrations.

ANAMORPHIC MAGNIFICATION

• It is the ratio of the width of the collimated diffracted beam to the collimated incident beam.

ENERGY DISTRIBUTION

• The distribution of the incident field power of a given wavelength diffracted by a grating to different spectral orders.

• This is also called the grating efficiency

SCATTERED AND STRAY LIGHT

• The light apart from the energy that is absorbed by the grating and the energy that is diffracted is scattered light.

• Scattered light in front of grating surface --- Diffuse scattered light, in dispersion plane --- In-plane scatter. Ghosts are scattered light due to periodic errors in the groove spacing.

• Instrumental stray light is the diffracted light due to the light in the atmosphere but not the incident light.

SIGNAL TO NOISE RATIO

• Ratio of the diffracted energy to unwanted light energy.

The above mentioned characteristics depend on the following parameters of the grating.

1. Groove profile

2. Groove frequencies

3. Groove pattern

4. Substrate shapes

5. Surface irregularities

And these parameters depend on the method of manufacturing :

Ruled Gratings or Holographic Gratings

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

Property ruled grating Interference grating

Surface irregularities yes no

Ruling errors Yes no

Groove placement errors

Yes No

Groove frequency Better Good

Groove pattern Need not be equally spaced

Equally spaced

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.

Aberrations

• Defocus - is the blurring of the image along the tangential plane

• Astigmatism is the blurring of the image along the Sagittal plane, this occurs generally when the element is placed off- axis.

Spectral resolution is an important imaging property and is maximum when the incident ray is focused into a line parallel to the grooves called the tangential focus and perpendicular to the grooves called the sagittal focus.

Aberrations are reduced by choosing the exact positions of the entrance slit and the exit slit.

Efficiency characteristics

• Absolute efficiency is the ratio of the diffracted light to the energy of the incident light.

• Relative efficiency is the ratio of the energy of the diffracted light to the energy from the light reflected from a polished surface.

• Blazing is the control over the magnitude and variation of diffracted energy with the change in wavelength. This control is generally obtained by getting control over the blazing angle or the groove angle.

θ

θα

β

Efficiency curve

Graph between absolute efficiency or relative efficiency with respect to wavelength or sometimes λ/d.

λB

m1

m1< m2< m3

m2

Depends on • m (diffraction order)• angles of incidence and diffraction• λ/d• polarization P- Plane => no anomalies S- Plane => anomalies.

P-plane is TE polarized lightS-plane is TM polarized light

λBis the blaze wavelength where highest efficiency is recorded

Efficiency for triangular and sinusoidal grooves

Triangular grooves ( blaze angle)

Very low BA θ < 50

Low B A 50 < θ < 100

Medium B A 100 < θ < 180

Special low anomaly 180 < θ < 220

High BA 220 < θ < 380

Very high B A θ > 380

Sinusoidal grooves (modulation)

µ = groove height/ spacing

very low µ < 0.05

low 0.05 < µ < 0.15

Medium 0.15 < µ < 0.25

High 0.25 < µ < 0.4

Very high µ > 0.4

• Maximum efficiency is obtained through triangular grooves.

Applications

Gratings as Principle used

FILTERS Plane gratings blazed for the wavelength of unwanted shorter wavelength radiation

ELECTRON MICROSCOPE CALIBRATION

Replica gratings made from master gratings so that a space is left between the grooves.

LASER TUNING Plane reflection grating used in littrow mode

BEAM DIVIDERS Symmetrically shaped grooves and laminar transmission gratings

Grating spectrometers

• Czerny-turner spectrograph

Entrance slit

Exit slit

Grating

Detector

collimator

Camera