Since the invention of the world's first semiconductor laser in 1962, semiconductor lasers have undergone enormous changes, greatly promoting the development of other sciences and technologies, and are considered one of the greatest human inventions of the 20th century.
In recent decades, the development of semiconductor lasers has been even faster, making them one of the fastest growing laser technologies in the world.
The application of semiconductor lasers covers the entire field of optoelectronics and has become the core technology of optoelectronic science today.
Due to the advantages of small size, simple structure, low input energy, long life, easy modulation and low price of semiconductor lasers, they are now widely used in the field of optoelectronics and are highly valued by countries around the world.
Semiconductor laser is a miniaturized laser with Pn junction or Pin junction composed of direct band semiconductor material as working material.
There are dozens of semiconductor laser working substances, and the semiconductor materials that have been processed into lasers include gallium arsenide, indium arsenide, indium antimonide, cadmium sulfide, cadmium telluride, lead selenide, lead telluride, lead arsenic. aluminum and gallium, indium arsenic and phosphorus, etc. .
There are three main excitation methods for semiconductor lasers, namely:
- Electric injection type
- Light pumping type
- High-energy electron beam excitation type
Most semiconductor lasers are excited by electrical injection, which means that a direct voltage is applied to the Pn junction to produce excited emission in the junction plane region, which is a forward-biased diode.
Therefore, semiconductor laser is also called semiconductor laser diode.
For semiconductors, since electrons jump between energy bands and not between discrete energy levels, the jumping energy is not a set value, which causes the output wavelength of semiconductor lasers to spread over a wide range.
They emit wavelengths in the range of 0.3 to 34 μm.
The wavelength range is determined by the energy gap of the material used, and the most common is the AlGaAs double heterojunction laser with an output wavelength of 750 to 890 nm.
Schematic diagram of the laser structure
Semiconductor laser manufacturing technology has gone through various processes, from diffusion to liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), molecular beam epitaxy (MBE), MOCVD method (metal organic compound vapor deposition ), chemical beam epitaxy (CBE) and various combinations thereof.
The biggest disadvantage of semiconductor lasers is that the laser performance is greatly affected by temperature and the beam divergence angle is large (generally between a few degrees and 20 degrees), which results in poor directionality, monochromaticity and coherence.
However, with the rapid development of science and technology, research on semiconductor lasers is advancing in the direction of depth, and the performance of semiconductor lasers is constantly improving.
Semiconductor lasers, as the core of semiconductor optoelectronic technology in the 21st century information society, will make greater progress and play a more important role.
Semiconductor laser working principle
The semiconductor laser is a source of coherent radiation, so that it can produce laser light, three basic conditions must exist:
1. Earn condition
To establish the inversion distribution of carriers in the excitation medium (active region), the electron energy in a semiconductor is represented by a series of energy bands consisting of a series of nearly continuous energy levels.
Therefore, to achieve particle number inversion in semiconductors, it is necessary to be between two regions of the energy band.
The number of electrons at the bottom of the conduction band in the higher energy state is much greater than the number of holes at the top of the valence band in the lower energy state. This is achieved by adding forward bias to the homojunction or heterojunction and injecting the necessary carriers into the active layer to excite electrons from the lower energy valence band to the higher energy conduction band.
Excited emission occurs when a large number of electrons in the particle number reversal state combine with holes.
2. To actually get the relevant stimulated radiation
To obtain multiple feedback and the formation of laser oscillation, the excited radiation must be made into the optical resonant cavity.
The resonant cavity of a laser is formed using the natural surface solution of a semiconductor crystal as a reflector, typically with a highly reflective multilayer dielectric film on the non-emitting end and a partially reflective film on the emitting side.
In the case of Fp cavity (Fabry-Perot cavity) semiconductor lasers, the Fp cavity can be easily formed using the natural solution plane of the crystal perpendicular to the pn junction plane.
3. To form stable oscillations, the laser medium must be able to provide a large enough gain
To compensate for the optical loss caused by the resonant cavity and the loss caused by the laser exit from the cavity surface, it is necessary to constantly increase the optical field in the cavity.
This requires a sufficiently strong current injection, i.e. sufficient particle number reversal. The greater the degree of particle number reversal, the greater the gain obtained, which is why it is necessary to meet a certain current limit condition.
When the laser reaches the threshold value, light with a specific wavelength can resonate in the cavity and be amplified, eventually forming a laser and producing continuously.
It can be seen that in semiconductor lasers, the dipole jump of electrons and holes is the basic process of light emission and amplification.
For new semiconductor lasers, it is now recognized that quantum wells are the fundamental driving force for the development of semiconductor lasers.
The topic of whether quantum wires and quantum dots can make the most of quantum effects has been extended into this century, and scientists have tried to make quantum dots in various materials with self-organized structures, while GaInN quantum dots have been used in semiconductor lasers.
History of semiconductor lasers
Semiconductor lasers were first developed in the early 1960s as homogeneous junction lasers, which were p-n junction diodes made from a single material. When subjected to high direct current injection, electrons were continuously injected into the p-region, and holes were continuously injected into the n-region, resulting in a reversal of the carrier distribution in the depletion zone of the original p-n junction. Because the electron migration rate is faster than the hole migration rate, the emission of radiation and compound particles occurs in the active zone, emitting fluorescence, and under certain conditions, a pulse-shaped semiconductor laser occurs.
The second stage of semiconductor laser development is the heterostructure semiconductor laser, which consists of two thin layers of semiconductor material of different bandgap, such as GaAs and GaAlAs. The first of these was a single heterostructure laser (1969). Single heterojunction injection lasers (SHLD) within the p-zone of the GaAsP-N junction to reduce the limiting current density, the value of which is an order of magnitude lower than that of homojunction lasers, but single heterojunction lasers still cannot operate continuously at room temperature.
Since the late 1970s, semiconductor lasers have clearly developed in two directions. One is the development of information-based lasers for the purpose of transmitting information, and the other is the development of power-based lasers for the purpose of increasing optical power. This has been driven by applications such as pumped solid-state lasers and high-power semiconductor lasers (continuous output power of 100 mW or more, pulsed output power of 5 W or more) are now considered high-power semiconductor lasers.
In the 1990s, there was a breakthrough in semiconductor laser technology, marked by a significant increase in the output power of semiconductor lasers. Kilowatt-class high-power semiconductor lasers have been commercialized, and the production of domestic sample devices has reached 600W. Laser wavelengths have also expanded from infrared semiconductor lasers to 670 nm red semiconductor lasers, followed by the introduction of 650 nm, 635 nm, blue-green, and blue semiconductor laser wavelengths. Violet and even ultraviolet semiconductor lasers on a 10mW scale have also been successfully developed.
In the late 1990s, the development of surface-emitting lasers and vertical-cavity surface-emitting lasers was considered for a variety of applications in ultraparallel optoelectronics. Devices at 980 nm, 850 nm, and 780 nm have become practical in optical systems. Currently, vertical cavity surface-emitting lasers are used in high-speed networks for gigabit Ethernet.
Applications of semiconductor lasers
Semiconductor lasers are a class of lasers that have matured earlier and progressed faster due to their wide wavelength range, simple production, low cost, easy mass production, small size, light weight and long service life. Therefore, its development has been rapid and the range of applications has already exceeded 300 types.
1. Application in industry and technology
(1) Optical fiber communication:
Semiconductor lasers are the only practical light source for fiber-optic communication systems, and fiber-optic communication has become the mainstream of contemporary communication technology.
(2) Optical disk access:
Semiconductor lasers have been used for optical disk memory and their biggest advantage is the large amount of sound, text and graphic information stored. The use of blue and green lasers can greatly improve the storage density of optical discs.
(3) Spectral analysis:
Far-infrared tunable semiconductor lasers have been used for environmental gas analysis, smog monitoring, automobile exhaust, etc. In industry, they can be used to monitor the vapor phase precipitation process.
(4) Optical information processing:
Semiconductor lasers have been used in optical information management systems. 2D surface-emitting semiconductor laser arrays are ideal light sources for parallel optical processing systems and will be used in computers and optical neural networks.
(5) Laser microfabrication:
Q-switched semiconductor lasers produce ultra-short, high-energy light traces for cutting and drilling integrated circuits.
(6) Laser alarm:
Semiconductor laser alarms are used for a wide range of applications, including burglar alarms, water level alarms, car distance alarms, etc.
(7) Laser Printers:
High-power semiconductor lasers have been used in laser printers. Using blue and green lasers can greatly improve printing speed and resolution.
(8) Laser Barcode Reader:
Semiconductor laser barcode readers have been widely used for merchandising as well as book and file management.
(9) Solid-state pumped lasers:
This is an important application of high-power semiconductor lasers, using them to replace the original atmospheric lamp, it can constitute an all-solid-state laser system.
(10) High definition laser TV:
In the near future, semiconductor laser TVs without cathode ray tubes could be placed on the market, which use red, blue and green lasers and are estimated to consume 20% less energy than existing TV sets.
2. Application in medical and life science research
(1) Laser surgery treatment
Semiconductor lasers have been used for soft tissue excision, tissue joining, coagulation, and vaporization. They have been widely used in general surgery, plastic surgery, dermatology, urology, obstetrics and gynecology.
(2) Kinetic laser treatment
Photosensitive substances with an affinity for tumors are selectively collected from cancerous tissues and irradiated by a semiconductor laser to produce reactive oxygen species in cancerous tissues, targeting necrosis without causing damage to healthy tissues.
(3) Life science research
The use of semiconductor laser “optical tweezers,” which can capture living cells or chromosomes and move them to any location, has been used to promote cell synthesis, cell interaction, and other research, as well as a diagnostic technique for the forensic science.
