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COMPONENTS OF OPTICAL INSTRUMENTS

Chapter 7UV, Visible and IR Instruments

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TopicsA. GENERAL DESIGNSB. SOURCESC. WAVELENGTH SELECTORSD. SAMPLE CONTAINERSE. RADIATION TRANSDUCERSF. SIGNAL PROCESSORS AND READOUTG. FIBER OPTICSH. TYPES OF OPTICAL INSTRUMENTI. PRINCIPLES OF FOURIER TRANSFORM

OPTICAL MEASUREMENTS

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A. General Designs5 components

Absorption

FluorescencePhosphorescenceScattering

EmissionChemiluminescence

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B. Sources

General Requirements• Adequate power • Stability • Utility voltage: 115 V ac

– 115 V ac desired/ appropriate dc voltage– Double polarity single polarity smooth dc

signal constant dc signal• These conversions are performed within the

Power Supply.

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Power Supply

• Components of a Power Supply and their Effects on the 115-V line voltage

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Power Supply• Transformer

– Function: decreases or increases the ac voltage

– Components: induction/ inductor coils

• Primary inductor coil• Secondary inductor coil

– Principle of operation• A varying magnetic field induces a

voltage in any conductor in its field.

• A changing current in the primary winding produces a changing magnetic field flux in the seconday coil, which induces a changing voltage across the secondary coil.

– Vout = 115 V x N2/N1

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Power Supply

• Rectifier– Function: convert double

polarity current to single polarity current

– Components: semiconductor diodes

– Operational principle: block current in one direction while allowing it to flow in the opposite direction

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Power Supply• Filtering

– RC circuit– C: capacitor– R:resistor

– Capacitor: two conductors separated by an thin layer of dielectric substance/ insulator with no mobile current carrying charged species

– Capacitor charging by a dc sourceQ = C x V• Q: quantity of charge• C: capacitance (farads/F)• A one-farad capacitor stores one coulomb of charge per applied volt.

– Charge and discharge rate

RCtinit eIi /−=

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Power Supply• Voltage regulators

– Function: maintain output voltage at constant desired level

– Component: Zener diode

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B-1& 2 Continuum and Lines Sources• Line source: emit a few discrete lines• Continuum source: emit a continuum spectrum

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B-3 Laser Sources

• Light Amplification of Stimulated Emission of Radiation

• Useful characteristics– High Intensity– Narrow bandwidth– Coherent

• Applications– High-resolution spectroscopy– Kinetic studies of short lived events (10-9 to 10-12 sec)– Low concentrations– Raman– FT IR

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Components of lasers• Lasing medium

– Gas (He-Ne, Argon)– Solid (ruby)– Solution (dye)

• Pumping system (radiation, electrical discharge)• Mirrors (amplifying cavity)

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Mechanism of Laser Action• Pumping-Excitation• Population of a Metastable

Excited State• Spontaneous emission

– Normal radiativerelaxation

• Stimulated emission– excited species struck

by photons that have same energies emit photons that travel in phse with the stimulating photon

• Coherence

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Mechanism of Laser Action• Population inversion

required!– Normal distribution:

number of particles in the lower energy state higher than that in the higher energy state

– Population inversion: number of particles in the higher energy state higher than that in the lower energy state

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How Population Inversion Methods

• Three levels system– More than 50% of lasing

species must populate level Ey

• Four level system– Because the transition from Ex

to E0 is fast population inversion is more readily achieved between Ey and Ex

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Laser cavity

• Optical Cavity– Lasing is an optical feedback– Wave is amplified as it moves back and forth within

the cavity– All lasers cavities are types of interferometer

100% refelecting < 5-90%reflecting

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Examples of Lasers• Solid State Lasers

– Ruby (Al2O3 + 0.05% Cr(III))– Nd-YAG

• Gas Lasers– He-Ne (neutral atom)– Argon (ion laser) Ar+

– CO2 (molecular)– N2

• Dye Lasers– Fluorescent dyes– Tunable (20 to 50 nm)

• Semiconductor Diode Lasers– Light Emitting Diodes (LED)