A laser diode is a semiconductor device that emits coherent light via stimulated emission, which is more complex and responsive than a light-emitting diode (LED). ‘Laser’ stands for Light Amplification by Stimulated Emission of Radiation.
What is a Laser Diode?
A laser diode is defined as a diode that can generate laser light when electrically pumped with current. It consists of a p-n junction with an additional intrinsic layer in between, forming a p-i-n structure. The intrinsic layer is the active region where the light is generated by the recombination of electrons and holes.
The p-type and n-type regions are heavily doped with impurities to create excess carriers, and the intrinsic layer is left undoped or lightly doped for optical amplification. The ends of the intrinsic layer are coated with reflective materials—one fully reflective, the other partially reflective. An optical cavity is formed that traps light and enhances stimulated emission.
An incoming photon triggers stimulated emission. It makes an excited electron drop to a lower energy level and emit an identical photon—matching in frequency, phase, polarization and direction. Photons in the cavity multiply exponentially, forming a coherent light beam that exits the partially reflective end.
The laser light wavelength depends on semiconductor bandgap and optical cavity length, emitting across the EM spectrum from infrared to ultraviolet.
How Does a Laser Diode Work?
A laser diode works by applying a forward bias voltage across the p-n junction, which causes current to flow through the device. Current injects electrons (n-type) and holes (p-type) into the intrinsic layer. They recombine here, releasing energy as photons.
Some photons emit spontaneously in random directions. Cavity photons stimulate others to emit in phase. Stimulated photons bounce between reflective ends, triggering more emission and forming population inversion—more excited electrons than non-excited ones.
When population inversion hits a threshold, we get steady-state laser output. Stimulated emission rate matches photon loss from transmission or absorption. The output power of the laser diode depends on the input current and the efficiency of the device.
Output power hinges on device temperature: higher temperatures reduce efficiency and raise threshold current, requiring cooling systems for optimal performance.
What are the Types of Laser Diodes?
Laser diodes are classified into different types based on their structure, mode of operation, wavelength, output power, and application. Some of the common types are:
- Single-mode laser diodes: These have a narrow active region that supports only one optical mode, resulting in a highly focused beam with low divergence and high coherence. They have low output power and narrow spectral width. We use these lasers for high-precision, high-accuracy applications: fiber optic communication, spectroscopy, and sensing.
- Multi-mode laser diodes: These have a broad active region that supports multiple optical modes, resulting in a wider beam with high divergence and low coherence. They have high output power and broad spectral width. We use these lasers for high-intensity, high-brightness applications: laser cutting, welding, printing, and illumination.
- Master oscillator power amplifier (MOPA) laser diodes: These combine a single-mode laser diode as an oscillator with a multi-mode laser diode as an amplifier to increase the output power without compromising on the spectral width or coherence. We use these lasers for high-power, narrow-spectrum applications: LiDAR, range finding, and medical imaging.
- Vertical cavity surface emitting laser (VCSEL) diodes: These emit light perpendicular to the surface of the device, rather than parallel to it, as in conventional edge-emitting laser diodes. They have a short optical cavity with distributed Bragg reflectors (DBRs) at both ends to provide feedback. They have low threshold current, high efficiency, circular beam profile, and easy integration with other devices. We use these lasers for applications: optical interconnects, data communication, sensing, and optical mice.
- Distributed feedback (DFB) laser diodes: These have a periodic structure embedded in the active region that acts as a grating to provide feedback and wavelength selection. They have narrow spectral width, high stability, low noise, and tunability. We use these lasers for applications: fiber optic communication, spectroscopy, and metrology.
- External cavity diode lasers (ECDLs): These use an external optical component such as a grating or a prism to provide feedback and wavelength selection instead of an internal cavity. They have high tunability, narrow spectral width, low noise, and high coherence. We use these lasers for applications: spectroscopy, metrology, atomic physics, and quantum optics.
What are the Applications of Laser Diodes?
Laser diodes have wide applications across fields thanks to advantages like compact size, low power consumption, high efficiency, long lifetime, and versatility. Some of their applications are:
- Optical storage: We use laser diodes to read and write data on optical discs: CDs, DVDs, and Blu-ray discs.They use different wavelengths of light to store different amounts of data on different layers of discs. For example, CDs use red laser diodes with 780 nm wavelength, DVDs use blue-violet laser diodes with 405 nm wavelength, and Blu-ray discs use blue laser diodes with 450 nm wavelength.
- Optical communication: We use laser diodes to transmit data over long distances via fiber optic cables. They modulate their intensity or frequency according to the data signal and send pulses of light through thin glass fibers that carry them with minimal loss or interference. They use different wavelengths of light to multiplex multiple channels of data on a single fiber, increasing its capacity. For example, fiber optic communication systems use infrared laser diodes with wavelengths ranging from 800 nm to 1600 nm.
- Optical scanning: We use laser diodes to scan barcodes, UPC codes and other patterns with devices: barcode readers, scanners and printers. They emit a beam of light that reflects off the pattern onto a photodetector that converts it into an electrical signal. They use visible or near-infrared wavelengths of light depending on the type and color of the pattern. For example, barcode scanners use red laser diodes with 650 nm wavelength.
- Optical sensing: We use laser diodes to measure physical parameters: distance, speed, temperature, pressure, and concentration.We do this with devices: LiDAR, radar, thermometers, pressure sensors, and gas analyzers.They emit a beam of light that interacts with the target object or medium and returns back to a detector that analyzes its properties. They use different wavelengths of light depending on the type and range of measurement. For example, lidar systems use near-infrared laser diodes with 905 nm or 1550 nm wavelength.
- Optical display: We use laser diodes to project images and info via projectors, TVs, monitors and holograms.They emit beams of red, green, and blue light that combine to form different colors and shapes according to the input signal. They use visible wavelengths of light depending on the resolution and brightness of the display. For example, laser projectors use red laser diodes with 635 nm wavelength, green laser diodes with 520 nm wavelength, and blue laser diodes with 445 nm wavelength.
- Optical surgery: We use laser diodes for medical procedures—cutting, cauterizing, ablation, coagulation, photocoagulation—with surgical lasers and endoscopes. They emit beams of light that penetrate the tissue and cause thermal or photochemical effects depending on the power and duration of exposure. They use different wavelengths of light depending on the type and depth of treatment. For example, ophthalmic lasers use green laser diodes with 532 nm wavelength to treat retina and macular diseases.
Advantages of Laser Diodes
Laser diodes have several advantages over other types of lasers, such as:
- Compact size: Laser diodes are very small and lightweight, making them easy to integrate with other devices and systems.
- Low power consumption: Laser diodes require low voltage and current to operate, reducing energy cost and heat generation.
- High efficiency: Laser diodes convert a large fraction of the electrical input into optical output, resulting in high brightness and intensity.
- Long lifetime: Laser diodes have a long operational life, lasting for thousands of hours without degradation or failure.
- Versatility: Laser diodes can produce light in various wavelengths, modes, and patterns, allowing for a wide range of applications and customization.
Disadvantages of Laser Diodes
Laser diodes also have some disadvantages, such as:
- Temperature sensitivity: Laser diodes are sensitive to temperature changes, which can affect their performance and reliability. They may require cooling systems or temperature controllers to maintain optimal conditions.
- Optical Feedback: Laser diodes are prone to optical feedback, which destabilizes them, generates noise or causes damage—isolators or filters are often used to block stray reflections.
- Mode hopping: Laser diodes may exhibit mode hopping—a sudden shift in output wavelength or mode from temperature, current, or optical feedback fluctuations. This can affect the coherence and stability of the output beam.
- Cost: Laser diodes can be expensive, especially for high-power or tunable devices. They may also require additional components or circuits to drive and control them.
BU-LASER
BU-LASER offers semiconductor laser diodes with TO can package from 375nm to 980nm, and we also offer professional OEM& ODM service of the laser diode modules for different applications. If you are interested in the laser diodes and laser diode modules, please contact our sales person at song@bu-laser.com.
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