COMPONENTS OF OPTICAL INSTRUMENTS Chapter 7 UV, Visible and IR Instruments Topics A. GENERAL DESIGNS B. SOURCES C. WAVELENGTH SELECTORS D. SAMPLE CONTAINERS E. RADIATION TRANSDUCERS F. SIGNAL PROCESSORS AND READOUT G. FIBER OPTICS H. TYPES OF OPTICAL INSTRUMENT I. PRINCIPLES OF FOURIER TRANSFORM OPTICAL MEASUREMENTS 1
A. General Designs 5 components Absorption Fluorescence Phosphorescence Scattering Emission Chemiluminescence 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. 2
Power Supply Components of a Power Supply and their Effects on the 115-V line voltage 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. V out = 115 V x N 2 /N 1 3
Rectifier Power Supply 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 Filtering RC circuit C: capacitor R:resistor Power Supply Capacitor: two conductors separated by an thin layer of dielectric substance/ insulator with no mobile current carrying charged species Capacitor charging by a dc source Q = 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 i I t RC init e / = 4
Voltage regulators Power Supply Function: maintain output voltage at constant desired level Component: Zener diode B-1& 2 Continuum and Lines Sources Line source: emit a few discrete lines Continuum source: emit a continuum spectrum 5
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 Components of lasers Lasing medium Gas (He-Ne, Argon) Solid (ruby) Solution (dye) Pumping system (radiation, electrical discharge) Mirrors (amplifying cavity) 6
Mechanism of Laser Action Pumping-Excitation Population of a Metastable Excited State Spontaneous emission Normal radiative relaxation Stimulated emission excited species struck by photons that have same energies emit photons that travel in phse with the stimulating photon Coherence 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 7
How Population Inversion Methods Three levels system More than 50% of lasing species must populate level E y Four level system Because the transition from E x to E 0 is fast population inversion is more readily achieved between E y and E x Optical Cavity Laser 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 8
Examples of Lasers Solid State Lasers Ruby (Al 2 O 3 + 0.05% Cr(III)) Nd-YAG Gas Lasers He-Ne (neutral atom) Argon (ion laser) Ar + CO 2 (molecular) N 2 Dye Lasers Fluorescent dyes Tunable (20 to 50 nm) Semiconductor Diode Lasers Light Emitting Diodes (LED) 9