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.
This article's lead section may be too short to adequately summarize the key points. (November 2016) |
A laser diode (LD, also injection laser diode or ILD or semiconductor laser or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction.
 | |
| Type | semiconductor, light-emitting diode |
|---|---|
| Working principle | semiconductor, carrier generation and recombination |
| Invented | Robert N. Hall, 1962; Nick Holonyak, Jr., 1962 |
| Pin configuration | Anode and cathode |
Driven by voltage, the doped p–n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generates light with the same phase, coherence and wavelength.
The choice of the semiconductor material determines the wavelength of the emitted beam, which in today's laser diodes range from infrared to the ultraviolet (UV) spectrum. Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc reading/recording, laser printing, laser scanning and light beam illumination. With the use of a phosphor like that found on white LEDs, laser diodes can be used for general illumination.
laser diode (plural laser diodes)
