Theory and Operation of Laser Diodes - Laser diodes are electrically a PIN diode. - The active region of the laser diode is in the intrinsic (I) region. - Laser diodes use the double-hetero-structure implementation. - Laser diodes are fabricated using direct band-gap semiconductors. - The active layer of laser diodes often consists of quantum wells. - Forward electrical bias across the laser diode causes charge carriers to be injected into the depletion region. - Holes are injected from the -doped into the -doped semiconductor. - Diode lasers can also be powered by optical pumping. - Optically pumped semiconductor lasers (OPSL) use a III-V semiconductor chip as the gain medium. - OPSLs offer advantages in wavelength selection and lack of interference. - Spontaneous emission occurs when an electron and a hole recombine, producing a photon. - Spontaneous emission below the lasing threshold produces similar properties to an LED. - Spontaneous emission is necessary to initiate laser oscillation. - Spontaneous emission is one of several sources of inefficiency once the laser is oscillating. - Spontaneous emission is carried away as phonons in conventional semiconductor junction diodes. - Photon-emitting semiconductors used in laser diodes are direct bandgap semiconductors. - Silicon and germanium are not direct bandgap semiconductors. - Compound semiconductors, such as gallium arsenide and indium phosphide, can emit light. - Compound semiconductors have alternating arrangements of two different atomic species. - Compound semiconductors have a critical direct bandgap property that allows photon emission. - Electrons and holes can coexist without recombining for a certain time. - Stimulated emission occurs when a nearby photon causes recombination by stimulated emission. - Stimulated emission generates another photon of the same frequency, polarization, and phase. - Stimulated emission causes gain in an optical wave. - Stimulated emission is responsible for the laser diode's coherent and monochromatic output.

Optical Cavity and Laser Modes - Gain region surrounded by optical cavity to form laser - Optical waveguide made on crystal surface to confine light - Crystal ends cleaved to form smooth, parallel edges for resonator - Photons travel along waveguide, reflected from end faces - Amplification occurs through stimulated emission, with some loss due to absorption and incomplete reflection

Properties and Formation of Laser Beams - Light contained within thin layer - Structure supports single optical mode perpendicular to layers - Wide waveguide supports multiple transverse optical modes - Narrow waveguide supports single transverse mode for diffraction-limited beam - Multiple longitudinal modes may still be supported - Beam diverges rapidly due to diffraction - Lens used to form collimated beam - Cylindrical lenses and other optics used for circular beam - Single spatial mode lasers result in elliptical beam shape - Long axis of ellipse is perpendicular to chip plane

History and Advancements in Laser Diodes - Coherent light emission from gallium arsenide diode demonstrated in 1962 - US groups led by Robert N. Hall and Marshall Nathan - Debate on whether IBM or GE invented first laser diode - Gallium arsenide suggested as good candidate for laser diode - First visible wavelength laser diode demonstrated by Nick Holonyak, Jr. - Liquid phase epitaxy (LPE) invented in early 1960s - LPE used for high-quality heterojunction semiconductor laser materials - Molecular beam epitaxy and organometallic chemical vapor deposition replaced LPE - Challenge to obtain low threshold current density at room temperature - Introduction of heterojunctions in diode lasers using aluminum gallium arsenide

Types of Laser Diodes and Reliability - Simple laser diode structure is inefficient and can only achieve pulsed operation without damage. - Double heterostructure (DH) lasers have a layer of low bandgap material sandwiched between two high bandgap layers, confining the active region and improving amplification. - Quantum well lasers have a thin layer acting as a quantum well, allowing for greater efficiency and concentration of electrons in energy states that contribute to laser action. - Quantum cascade lasers use the difference between quantum well energy levels for the laser transition, enabling laser action at longer wavelengths. - Interband cascade lasers produce coherent radiation over a large part of the mid-infrared region of the electromagnetic spectrum. - Separate confinement heterostructure (SCH) lasers have additional layers outside the thin quantum well layer to effectively confine the light. - VECSELs are similar to VCSELs and have one mirror external to the diode structure. - External-cavity diode lasers are tunable lasers and use double heterostructures diodes of the Al(1-x)As type. - Laser diodes have the same reliability and failure issues as LEDs and are subject to catastrophic optical damage (COD) at higher power. - Advances in reliability remain proprietary to developers and cannot be revealed through reverse engineering. - Surface-emitting lasers like VCSELs are prone to COD due to thermal runaway.

Applications of Laser Diodes - Laser diodes can be arrayed to produce high power outputs. - They are used in solid-state lasers for drilling and burning. - Laser diodes are widely used in telecommunications for fiber optics communication. - They are used in barcode readers and laser pointers. - Laser diodes are used in CD players, CD-ROMs, and DVD technology. - They are used in measuring instruments like rangefinders. - Laser diodes are used in the printing industry for scanning and printing plate manufacturing.