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.