Laser diodes are semiconductor devices that emit coherent light when electric current passes through them. Amplification of light by stimulated photon emission produces a monochromatic, directional, coherent, and high-intensity beam. The laser diode is a component similar to the light-emitting diode.
LASER is a light amplification process, which stands for Light Amplification by Stimulated Emission of Radiation.
Different kinds of lasers exist based on the material they are used to generate, such as gas lasers, liquid lasers, solid-state lasers, fiber lasers, chemical lasers, etc.
They all include three elements- medium, excitation, and amplification.
Symbol of Laser Diode
Principle of a Laser diode
Albert Einstein formulated the principle of stimulated emission in 1917. It states that an excited electron or molecule can emit energy in the form of light. This stimulated emission is triggered by supplying energy to an ideally light-amplifying material (the so-called laser-active material) and thus shifting it to a higher energy level, an energetically excited state.
Construction of a Laser diode
An active zone of the diode is quasi-neutral between the two doped regions called p and n-regions. The regions create a junction through which current can flow when a voltage is applied. Electrical contacts are placed on the p-type and n-type areas to allow the injection of current into the diode. The active zone consists of different semiconductor materials, for example, gallium arsenide (GaAr).
The active layer is surrounded by two mirrors that form an optical cavity. One mirror is highly reflective, and the other is partial. When current passes through the pn-junction, photon energy is emitted. The active region typically consists of a material that can undergo stimulated emission.
The energy is bounced back and forth within the cavity.
Voltage-Current characteristic curve
In the forward-biased region, where the anode is more positive than the cathode, the current (mA) through the laser diode increases rapidly with a small voltage (V) increase. The figure above shows that the current increases significantly beyond a certain threshold voltage, leading to lasing action and light emission.
The rise in temperature of the laser diode increases the threshold current.
Laser production -Three main components
The following three components involved in laser production are
- The energy source
- Laser active medium (gain medium)
- Resonator
The energy sources
The energy source pumps light into a laser-active medium (sometimes also called gain medium). It varies depending on the laser type. The energy source can be electrical, optical, or chemical, depending upon the type of laser. Common pumping methods include electrical discharge, chemical reactions, flash lamps, and other types.
The laser-active medium
When excited by light, the laser-active medium emits a light beam (photons) of a specific wavelength. Active medium or gain medium amplifies the light through a process called stimulated emission. For example, the gain medium in a semiconductor diode is GaAs. In the CO2 laser, the gain medium is a mixture of CO2, Nitrogen, Helium, etc.
The resonator
In a laser diode, the resonance effect is achieved by cavities, where two mirrors are placed parallel to the p-n junction of the diode at a precisely defined distance. Two mirrors are placed at opposite sides on the active region. One mirror is highly reflective, and the other is partial. These two mirrors provide optical feedback placed at either end of a semiconductor gain medium.
The resonator serves two purposes: it provides a feedback loop that helps in selecting a specific wavelength. Second, it increases optical gain through mirrors surrounding the gain medium.
The light is allowed to bounce and forth between them; hence, stimulated emission increases light intensity, and a coherent and narrow beam of laser light emerges.
Classification of Semiconductor Laser Diodes
Homojunction Laser diode
The semiconductor material used for both the active region and the cladding layers is the same. This results in a single structure, but it may have limitations in terms of performance and efficiency. There arises an issue such as carrier leakage and high threshold currents.
Single Heterojunction Lasers
Diodes utilize different semiconductor materials for the active and cladding regions in this case. The active region has a higher band gap semiconductor material compared to the cladding layers. This results in the confinement of carriers within the active region; hence, leakage carriers are reduced.
Double Heterojunction
They use two Heterojunction interfaces, once between the active region and lower band gap cladding layer and another between the active region and higher band gap upper cladding layer. This design further enhances carrier confinement, resulting in lower threshold currents, higher efficiency, and good performance.
Materials used for Laser Diodes
A semiconductor laser uses semiconductor material from compounds like gallium-arsenide (GaAs), gallium nitride (GaN), Indium phosphide (InP), or other similar materials as the active substance. Due to differences in the structure of materials, the specific processes in manufacturing lasers of different types are special.
The wavelength of light emitted from a laser diode is directly related to the material of the active region, a region where the maximum stimulated emissions take place.
GaAlAs(gallium-aluminum-arsenide),-A p-n junction can emit a specific wavelength range from 750 nm to 900 nm. It is crucial in medical applications for treating specific issues.
InGaAsP(indium gallium arsenide phosphide) is mainly used for the manufacture of components that emit wavelengths around 1300 nm and 1550 nm. Its applications involve optical communication systems, laser pointers, etc.
InGaAlP(indium gallium aluminum phosphorous) is used for semiconductor lasers in the range visible from 630 nm. These lasers are suitable for data transmission with synthetic plastic fibers. An example is barcode scanners.
Difference between Laser Diodes (LD) and Light Emitting Diodes (LED)
A laser diode (LD) is a semiconductor closely related to the light-emitting diode (LED) in form and function. However, they have distinct differences in their operation, characteristics, and applications.
A Laser diode produces monochromatic, coherent light through the process of light amplification. Light-emitting diodes emit light as electrons recombine with holes in a semiconductor material; energy is released in the form of photons. The emitted light is not coherent and has a broader spectral bandwidth.
Laser diodes emit high-intensity light. On the other hand, Light-emitting diodes emit low-intensity light.
Laser diodes are used in high-precision applications such as laser printing, communication, and barcode scanners. Light-emitting diodes are used widely in consumer electronics, indicators, and displays such as TVs, computers, etc.
Advantages
- It is helpful when a particularly large number of frequencies (or wavelengths)are to be transmitted simultaneously with a single fiber optic cable.
- Laser diodes are extremely small and require little effort to work with. So, they find it suitable to integrate into various applications and systems.
- Laser diodes are generally more energy-efficient than other types of diodes.
- Laser diodes operate at a relatively low electrical current; their life span is longer.
- Laser diodes are generally cost-effective to manufacture and operate compared to other types of lasers.
- A beneficial property of laser diodes is their high modulation bandwidth. An almost linear change in the output power can be achieved by modulating the electrical current flowing through the diode.
- Due to the high bandwidth and the short resonator length, modulations up to the gigahertz range are possible.
- Diode lasers are the most efficient radiation sources in the near-infrared range, with efficiencies of over 50%.
- Available in a wide range of wavelengths across the electromagnetic spectrum, from ultraviolet to infrared.
Disadvantages
- Laser diodes have a shorter life span when compared to any other light source.
- Laser diodes are sensitive to voltage fluctuations and over-current.
- A highly divergent, non-circular, symmetrical beam of light causes reduced range, and loss of intensity is a disadvantage in various applications.
- Diode lasers are sensitive to back reflections and electrostatic discharge.
- Unfortunately, many laser diodes show noticeable signs of aging, which leads to a significant decrease in radiation intensity.
- It is difficult to find semiconductors suitable for certain wavelengths.
- The performance of laser diodes is influenced by electromagnetic interference.
Laser Diode Applications
Laser diodes are used in several fields, particularly in:
- Industrial application involving – direct material processing (laser welding, remelting, hardening) with high-performance diode lasers (power up to the kilowatt range)
- Barcode scanner-used to read the barcode on products that helps to track inventory in shopping malls.
- Used in laser printers, laser pointers, and light barriers.
- Optical storage – read and writes data on storage devices like DVD.
- Scientific application – especially in spectroscopy, chemical analysis, trace analysis, and quantum optics.
- Instrumentation field – used to find its application in electronic devices of precise distance, speed, guidance, and pointing measurement.
- Telecommunications – when many particular frequencies are to be transmitted simultaneously with a fiber optic cable.
- Medical field –Laser diodes are employed in various medical areas. Laser technology is used in surgery, dermatology, ophthalmology, and dental treatment.
- Defense development – used in the production of missile guidance systems.
