Lasers and LEDs Lasers and LEDs Lasers produce narrow beams of - PDF document

Lasers and LEDs 1 Lasers and LEDs 2 Observations about Lasers Observations about Lasers and LEDs and LEDs � Lasers and LEDs often have pure colors Lasers and LEDs often have pure colors Lasers and LEDs Lasers and LEDs � Lasers produce narrow beams of intense light Lasers produce narrow beams of intense light � Lasers are dangerous to eyes Lasers are dangerous to eyes � Reflected laser light has a funny speckled look Reflected laser light has a funny speckled look Lasers and LEDs 3 Lasers and LEDs 4 3 Questions about 3 Questions about Question 1 Question 1 Lasers and LEDs Lasers and LEDs � How does laser light differ from regular light? How does laser light differ from regular light? � How does laser light differ from regular light? How does laser light differ from regular light? � � How does a laser produce coherent light? How does a laser produce coherent light? � How does an LED produce its light? How does an LED produce its light? Lasers and LEDs 5 Lasers and LEDs 6 Light: Photons and Waves Light: Photons and Waves Spontaneous Emission Spontaneous Emission � Electrons obey the Pauli exclusion principle Electrons obey the Pauli exclusion principle � Excited atoms normally emit light Excited atoms normally emit light spontaneously spontaneously � � Each wave mode can have only one unique electron. Each wave mode can have only one unique electron. � These photons are uncorrelated and independent These photons are uncorrelated and independent � That result gives structure to atoms and materials That result gives structure to atoms and materials � Each photons has its own wave mode Each photons has its own wave mode � Photons don’t obey the Pauli exclusion principle Photons don’t obey the Pauli exclusion principle Ph Ph d d ’ ’ b b h P h P li li l l i i i i i l i l � These independent waves are incoherent light These independent waves are incoherent light � Each wave mode can have many photons Each wave mode can have many photons � A radio wave has many photons in a single wave A radio wave has many photons in a single wave � Most light sources produce photons randomly Most light sources produce photons randomly � Each photon usually has its own wave mode Each photon usually has its own wave mode � But laser But laser light is an exception! light is an exception! 1

Lasers and LEDs 7 Lasers and LEDs 8 Stimulated Emission Stimulated Emission Question 2 Question 2 � Excited atoms can be Excited atoms can be stimulated stimulated into duplicating into duplicating � How does a laser produce coherent light? How does a laser produce coherent light? passing light passing light � These photons are correlated and identical These photons are correlated and identical � The photons all have the same wave mode Th The photons all have the same wave mode Th h h ll h ll h h h d d � This single, giant wave is This single, giant wave is coherent light coherent light Lasers and LEDs 9 Lasers and LEDs 10 Laser Amplification Laser Amplification Laser Oscillation Laser Oscillation � Light can be amplified using stimulated emission Light can be amplified using stimulated emission � A laser medium can amplify its own light A laser medium can amplify its own light � Excited atom Excited atom- -like systems can act as a laser medium like systems can act as a laser medium � A laser medium in a resonator acts as an oscillator A laser medium in a resonator acts as an oscillator � That medium will duplicate any photons that have the That medium will duplicate any photons that have the � It duplicates its one of its own spontaneous photons It duplicates its one of its own spontaneous photons right wavelength, polarization, and orientation right wavelength, polarization, and orientation right wavelength polarization and orientation right wavelength polarization and orientation � Duplicated photons leak from semitransparent mirror � D pli Duplicated photons leak from semitransparent mirror D pli t d ph t n l t d ph t n l k fr m k fr m mitr n p r nt mirr r mitr n p r nt mirr r � This duplication is perfect: the photons are true clones This duplication is perfect: the photons are true clones � The photons from this oscillator are identical The photons from this oscillator are identical � This light amplification is the basis for lasers This light amplification is the basis for lasers Lasers and LEDs 11 Lasers and LEDs 12 Properties of Laser Light Properties of Laser Light Examples of Lasers Examples of Lasers � Coherent Coherent – – identical photons identical photons � Gas lasers (powered by discharges) Gas lasers (powered by discharges) � Controllable wavelength/frequency Controllable wavelength/frequency – – colors colors � Helium Helium- -neon lasers (red, green, yellow) neon lasers (red, green, yellow) � Carbon dioxide lasers (infrared) Carbon dioxide lasers (infrared) � Controllable spatial structure Controllable spatial structure – – narrow beams narrow beams � Solid state lasers (powered by current or light) S lid Solid state lasers (powered by current or light) S lid l l ( ( d b d b li h ) li h ) � Controllable temporal structure Controllable temporal structure – – short pulses short pulses � Diode lasers (red, blue, infrared) Diode lasers (red, blue, infrared) � Energy storage and retrieval Energy storage and retrieval – – intense pulses intense pulses � Ruby lasers (red) Ruby lasers (red) � Giant interference effects Giant interference effects � Nd:YAG Nd:YAG lasers (infrared) lasers (infrared) � But apart from all this, laser light is still just light But apart from all this, laser light is still just light � Ti:Sapphire Ti:Sapphire lasers (infrared) lasers (infrared) 2

Lasers and LEDs 13 Lasers and LEDs 14 Question 3 Question 3 Light Light- -Emitting Diodes Emitting Diodes � How does an LED produce its light? How does an LED produce its light? � LEDs are Light LEDs are Light- -Emitting Diodes Emitting Diodes � They conduct current only in one direction They conduct current only in one direction � Each charge releases energy on crossing Each charge releases energy on crossing pn pn- -junction junction � That energy is often emitted as a photon of light � Th t n r That energy is often emitted as a photon of light Th t n r i i ft n mitt d ft n mitt d ph t n f li ht ph t n f li ht � For LEDs to emit higher For LEDs to emit higher- -energy photons, energy photons, � they must be designed to have larger band gaps they must be designed to have larger band gaps � they must be supplied with larger voltage drops they must be supplied with larger voltage drops � Laser diodes are LEDs that can amplify light. Laser diodes are LEDs that can amplify light. Lasers and LEDs 15 Summary about Lasers and LEDs Summary about Lasers and LEDs � Lasers produce coherent light by amplification Lasers produce coherent light by amplification � Coherent light contains many identical photons Coherent light contains many identical photons � Laser amplifiers and oscillators are common Laser amplifiers and oscillators are common � LEDs are incoherent, light LEDs are incoherent, light- -emitting diodes emitting diodes 3