7.4 Types of semiconductor lasers and applications

Semiconductor lasers come in various types, each with unique characteristics and applications. Edge-emitting lasers, surface-emitting lasers, and quantum cascade lasers offer different emission patterns and wavelength ranges, making them suitable for diverse uses.

These lasers play crucial roles in modern technology. From fiber-optic communications and optical storage to printing, displays, and sensing applications, semiconductor lasers have become indispensable in our daily lives and industrial processes.

Edge-emitting and Surface-emitting Lasers

Edge-emitting Laser Diodes (EELs)

• Emit light from the edge of the semiconductor chip, in the plane of the active layer
• Consist of a double heterostructure with an active layer sandwiched between two cladding layers of higher bandgap material
• Have a Fabry-Perot cavity formed by cleaved facets or mirrors at the ends of the chip
• Typically emit light with an elliptical beam profile due to the rectangular shape of the active region
• Examples include Fabry-Perot (FP) lasers and distributed Bragg reflector (DBR) lasers

Vertical-Cavity Surface-Emitting Lasers (VCSELs)

• Emit light perpendicular to the surface of the semiconductor chip
• Have a short cavity length, typically a few wavelengths, formed by distributed Bragg reflector (DBR) mirrors above and below the active layer
• Require lower threshold currents compared to edge-emitting lasers due to the small cavity volume
• Emit light with a circular beam profile, which is advantageous for coupling into optical fibers
• Can be fabricated in large arrays on a single chip, enabling parallel data transmission

Distributed Feedback (DFB) Lasers

• Incorporate a periodic structure, such as a diffraction grating, within the active layer to provide optical feedback and wavelength selectivity
• Achieve single-mode operation without the need for external gratings or filters
• Have a narrow linewidth and stable wavelength output, making them suitable for high-speed, long-distance fiber-optic communications
• Commonly used in dense wavelength division multiplexing (DWDM) systems for telecommunications

Quantum Cascade Lasers

Quantum Cascade Laser (QCL) Operation

• Emit light through intersubband transitions in a cascaded series of quantum wells
• Rely on quantum confinement and band structure engineering to create a staircase-like energy level structure
• Electrons undergo multiple radiative transitions as they cascade down the energy levels, emitting photons in the mid-infrared to terahertz range
• Can be designed to emit at specific wavelengths by adjusting the layer thicknesses and compositions
• Require cryogenic cooling for continuous-wave operation, but can operate at room temperature in pulsed mode

Applications of Quantum Cascade Lasers

• Used in gas sensing and spectroscopy applications, such as detecting trace gases and monitoring air quality
• Enable high-resolution imaging and non-destructive testing in the mid-infrared range
• Employed in free-space optical communication systems for secure, high-bandwidth data transmission
• Potential for use in medical diagnostics, such as breath analysis and non-invasive glucose monitoring

Laser Applications in Communications and Storage

Fiber-Optic Communications

• Semiconductor lasers, particularly DFB lasers, are the primary light sources for fiber-optic communication systems
• Used to transmit digital data over long distances with high bandwidth and low signal attenuation
• Wavelength division multiplexing (WDM) techniques allow multiple wavelengths to be transmitted simultaneously over a single fiber, increasing the total data capacity
• Examples include long-haul telecommunications networks, submarine cable systems, and data center interconnects

Optical Storage

• Semiconductor lasers are used in optical storage devices, such as compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs
• Data is recorded by focusing a laser beam onto a photosensitive layer, creating microscopic pits and lands that represent binary data
• Reading the stored data involves detecting the reflectivity differences between the pits and lands using a lower-power laser
• Examples include CD-ROMs for software distribution, DVD movies for home entertainment, and Blu-ray discs for high-definition video storage

Laser Applications in Printing and Displays

Laser Printing

• Laser printers use a semiconductor laser to create an electrostatic image on a photosensitive drum
• The laser beam is modulated to selectively discharge areas of the drum, forming the desired image pattern
• Toner particles are attracted to the charged areas of the drum and then transferred onto paper, creating a permanent print
• Examples include office and home printers, as well as high-volume commercial printing systems

Barcode Scanners

• Semiconductor lasers, typically VCSELs, are used in barcode scanners to read and decode printed barcodes
• The laser beam is scanned across the barcode, and the reflected light is detected by a photodiode
• The pattern of light and dark bars is decoded to extract the encoded information, such as product identification numbers
• Examples include handheld scanners in retail stores, inventory management systems, and package tracking in logistics

Laser Displays

• Laser displays use semiconductor lasers as the light source to create high-brightness, wide-color-gamut images
• Laser beams are scanned rapidly across a screen or projected onto a surface to form the display
• Advantages include high contrast ratio, wide viewing angles, and the ability to create large-scale displays
• Examples include laser projectors for home theaters, digital cinema systems, and large-venue displays for events and advertising