Thin film Optical fiber

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THIN FILM OPTICALFILTERS

By: Er.Amit Mahajan

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Thin film optics2

Thin-film optics is the branch of optics that deals withvery thin structured layers of different materials

In order to exhibit thin-film optics, the thickness of

the layers of material must be on the order of thewavelengths of visible light (about 500nm) Optical layers at this scale can have remarkable

properties These properties can further be enhanced by use of

difference in refractive indices between layers, airand substrate

These properties are used to make the Thin FilmOptical Filters


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Thin film filter3

Thin-film filter is a very accurate colour filter used toselectively pass light of a small range of colours whilereflecting other colours.

A thin film filter uses the principle of interference. Layers of an optical coating is made on glass substrate

This coating reinforces the certain wavelength andinterfere with other wavelengths

By controlling the thickness of coating, we can select thewavelength to be filtered out

The filtered energy is sometimes absorbed by thecoating and sometimes reflected, depending onoperation used


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Working principle of Thin filmFilter

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1. The amplitude reflectance of light at any boundary between twomedia is given by (1)/(1+), where is the ratio of the opticaladmittances or ratio of the refractive indices at boundary

2. There is a phase shift of 180 when the reflectance takes place in amedium of lower refractive index than the adjoining medium, and zero

if the medium has a higher index than the one adjoining it.3. The third is that if light is split into two components by reflection at

the top and bottom surfaces of a thin film, then the beams willrecombine in such a way that the resultant amplitude will be thedifference of the amplitudes of the two components if the relative

phase shift is180, or the sum of the amplitudes if the relative phaseshift is either zero or a multiple of 360.


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Basic thin film filters5

According to the above three principles, a basic filter is formed

At both the upper and lowerboundaries of the coating film,

the reflection takes place in amedium of lower refractiveindex than the adjoiningmedium. To ensure that the relativephase shift is 180 so that thebeams cancel, the opticalthickness of the film should bemade one quarter wavelength.


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Practical Filters using thinfilm optics6

There are many filters that can be constructed usingbasic thin film optics which are:

Edge filters Band-pass filters Narrow-band filters Broadband filters Multicavity filters Phase dispersion filters


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Edge filter: Thin film absorptionfilter

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The coating material is absorption material

It makes absorption edge in wavelength characteristics

Mainly these are longwave-pass filters

Silicon is used which exhibit fast transition from opacity totransparency

To decrease reflection losses, Germanium(multiple layer) isused for filtering which has edge at 1.65m and RI of 4.0

Other single layer materials include cerium dioxide, giving anultraviolet rejectionvisible transmitting filter


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Edge filters: Interference filters9

The basic type of interference edge filter is thequarter-wave stack with characteristics given below

Principle of operation is interference of two waves

The system of figure can be used either as alongwave-pass filter with an edge at 5.5m or a

shortwave-pass filter with an edge at 3.3m


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Band-pass filters: Broadband passfilters

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Most common filter is made by joining two filters, oneis short-pass and other is long-pass

They are fabricated by both side coating of a glasssubstrate


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Narrow-band filters12

The simplest type of narrowband thin-film filter is based on theFabryPerot interferometer

Fabry-Parot Interferometer

The FabryPerot interferometer consists of two identical parallel

reflecting surfaces spaced apart a distance d

In collimated light, thetransmission is low for all

wavelengths except for aseries of very narrowtransmission bandsspaced at intervals thatare constant in terms of

wavenumber


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Contd13

Multiple-beam interference in the spacer or cavity layer causesthe transmission of the filter to be extremely high over anarrow band of wavelengths around that for which the spaceris a multiple of one half wavelength thick


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Application of Narrow-band filter inAstronomy

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It is used to detect faint Astronomical objects Faintness is due to starlight scattering with dust The sky light causes overall fogging in the

photographic plates The signal received from the astronomical objects is

photons Narrowband filter is used to capture only object light

and filter out the excess light This excess light is from sun and other stars We consider photons as Poissons distribution for

analysis of received photons and exposure time


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Filter positions15

The use of an interference filter centred on 656.3 nmgreatly increases the contrast between the nebulaeand the night sky


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Basic Astronomical filters16

There are mainly five category filters used in astronomywhich are UBVRI

U-Ultraviolet, B-Blue, V-Visible, R-Red, I-Infrared


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Case study: SPITZER SPACETELESCOPE17

The Spitzer Space Telescope is the fourth element in

NASAs family of Great Observatories The spacecraft carries an 85-centimeter telescope and

three cryogenically-cooled science instruments capableof performing imaging and spectroscopy from 3 to 180m

The telescope used in SST is a Ritchey-Chrtien design The 85-cm diameter primary mirror and the rest of the

telescope structure are fabricated entirely of beryllium


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Specifications18


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Infrared Array Camera(IRAC)

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The Infrareds Array Camera (IRAC) was built by theNASA Goddard Space Flight Centre (GSFC)

Light from the telescope is reflected into the IRACstructure by the pickoff mirrors


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Some photographs taken by Spitzer20

Photo of infant stars in Triffid Nebula bySpitzer


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Photo of hot planet HD 189733b, where water vapours were found in itsatmosphere


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Contd22

Photo of cluster of stars called Eye of God in infrared bySpitzer


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Thin film Optical fiber

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