Laser Diode: An efficient source of coherent light

 Introduction

A laser diode , also known as semiconductor laser, is a semiconductor device that emits coherent light when an electric current passes through it. It is constructed from a p-n semiconductor junction similar to that found in a light-emitting diode. The key difference is that the semiconductor laser has an optical cavity between two parallel reflectors on the surfaces of the p-n junction. This laser cavity forms a coherent light beam with a very narrow wavelength spectrum.

Working Principle of semiconductor laser
When a semiconductor diode is forward-biased, electrons from the n-type and holes from the p-type material recombine in the junction region. Some of the energy released by the recombining charged carriers is emitted as photons. This is similar to the process that occurs in LEDs. However, in a semiconductor laser the optical cavity provides feedback of the electromagnetic radiation emitted during recombination. Photons within the laser cavity stimulate the emission of additional photons with the same phase, wavelength, and polarization. This leads to amplification by stimulated emission and laser action. The amplified light exits through one of the reflecting facets of the cavity.

Types of Semiconductor Laser
Some common types of semiconductor laser are:

Visible semiconductor laser: These diodes emit visible light typically in the range of 400-700nm. Red semiconductor laser emitting at 635-650nm are commonly used in laser pointers and barcode scanners.

Infrared semiconductor laser: Infrared semiconductor laser emit light in the near-infrared region from 700nm to 1mm. They find applications in optical fiber communication systems, remote controls, and industrial heating and drilling.

Vertical-cavity surface-emitting laser (VCSEL): VCSELs emit laser light perpendicular to the wafer surface. They are increasingly used in applications like scanning, imaging, printing and data storage due to advantages like low power consumption, robustness, and possibility of array formation.

Edge-emitting semiconductor laser: The conventional edge-emitting semiconductor laser emits light parallel to the wafer surface from cleaved edges of the chip. They offer high output power and are suitable for applications requiring high-speed modulation like telecommunications.

Applications of Laser Diodes
Due to their compact size, efficiency and reliability, semiconductor laser have widespread applications:

Optical storage: Semiconductor laser are extensively used to read and write data in optical drives like CD, DVD, Blu-ray players/writers. Their short wavelengths allow very high density storage.

Optical communication: Infrared semiconductor laser installed in fiber optic links transmit data over long distances for telecommunication networks and Internet backbone.

Laser printing: Laser printers use semiconductor laser to scan and expose photoconductive drums pixel by pixel for high-quality printing.

Laser pointing devices: Red semiconductor laser in the form of laser pointers are commonly used in presentations, surveys and instrumentation.

Optical sensors: Semiconductor laser enable sensing applications likebarcode scanners, map readers, intrusion detection systems, medical equipment etc.

Manufacturing: Infrared and UV semiconductor laser are employed for material processing like scribing, welding, drilling and marking in industries like electronics, automotive and aviation.

Entertainment: Laser shows, laser tag and lighting displays take advantage of visible semiconductor laser for dynamic colorful illumination.

Advantages of Laser Diodes
The key advantages of semiconductor laser compared to other light sources are:

- High efficiency: Semiconductor laser can achieve wall-plug efficiencies exceeding 50%, much better than LEDs or arc lamps.

- Narrow spectral width: Semiconductor laser provide highly monochromatic and coherent light ideal for precision applications.

- Compact size: Advanced semiconductor manufacturing enables semiconductor laser to be extremely small in size, allowing integration into portable devices.

- Long lifetime: Reliability of semiconductor laser is high with lifetimes approaching or exceeding 10,000 hours of continuous operation.

- Fast modulation: Semiconductor laser can be directly modulated at high speeds up to 40Gbps or more using small drive currents. This makes them well-suited for fiber-optic communication links.

- Low power consumption: Efficiency of semiconductor laser results in very low power consumption per optical output, important for battery-powered portable equipment.

Challenges with Semiconductor Laser
Despite many advantages, semiconductor laser also face some technical challenges:

- Temperature sensitivity: Emission wavelength and output power of semiconductor laser fluctuate significantly with changes in operating temperature, requiring thermal control.

- Safety issues: High power semiconductor laser can cause permanent eye damage if beams are viewed directly and require appropriate safety precautions.

- Non-linearity: Laser power does not scale proportionately with injection current and begins to roll-over at high drive levels due to droop in internal efficiency.

- Beam divergence: Semiconductor laser give divergent beams that require collimation for many applications using optical components like lenses and mirrors.

- Catastrophic optical damage: High power semiconductor laser are prone to sudden failure if operated outside safe limits due to physical damage like mirror fracture or facet contamination.

- Multi-mode operation: Some semiconductor laser experience multi-wavelength operation due to spatial hole burning or filamentation in aperture, degrading performance.

Conclusion
In conclusion, semiconductor laser are extremely efficient sources of coherent light that have revolutionized many applications through their compactness, reliability and ability to directly modulate at high speeds. Continued research and development in materials, structures and packaging will help overcome existing challenges and expand the use of semiconductor laser in newer technologies of the future.