Basics of Semiconductor Laser (Working Principle, History, Applications)

Since the invention of the world’s first semiconductor laser in 1962, semiconductor lasers have undergone tremendous changes, greatly promoting the development of other science and technology, and are considered one of the greatest human inventions of the twentieth century.

In recent decades, the development of semiconductor lasers has been more rapid, and has become one of the fastest-growing laser technology in the world.

The application of semiconductor lasers covers the entire field of optoelectronics and has become the core technology of optoelectronics science today.

Due to the advantages of small size, simple structure, low input energy, long life, easy modulation and low price of semiconductor lasers, it is now very widely used in the field of optoelectronics, has been highly valued by countries around the world.

Semiconductor laser manufacturing technology

Semiconductor laser manufacturing technology

The semiconductor laser is a miniaturized laser with Pn junction or Pin junction composed of direct band gap semiconductor material as the working material.

There are dozens of semiconductor laser working substances, and the semiconductor materials that have been made into lasers include gallium arsenide, indium arsenide, indium antimonide, cadmium sulfide, cadmium telluride, lead selenide, lead telluride, aluminum gallium arsenic, indium phosphorus arsenic, etc.

There are three main excitation methods for semiconductor lasers, ie:

  • Electric injection type
  • Light pumping type
  • High energy electron beam excitation type

Most semiconductor lasers are excited by electrical injection, i.e., by applying a forward voltage to the Pn junction to produce excited emission in the junction plane region, i.e., a forward-biased diode.

Therefore, the semiconductor laser is also called semiconductor laser diode.

For semiconductors, since the electrons jump between energy bands rather than between discrete energy levels, the jump energy is not a definite value, which makes the output wavelength of semiconductor lasers 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 band 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

Schematic diagram of the laser structure

Semiconductor laser manufacturing technology has gone through a variety of 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 of them.

The biggest drawback of semiconductor lasers is that the laser performance is greatly affected by temperature, and the divergence angle of the beam is large (generally between a few degrees and 20 degrees), so it is poor in directionality, monochromaticity and coherence.

However, with the rapid development of science and technology, semiconductor laser research is advancing in the direction of depth, the performance of semiconductor lasers is constantly improving.

Semiconductor laser as the core of the semiconductor optoelectronics technology in the 21st century information society will make greater progress and play a greater role.

Working principle of semiconductor laser

The semiconductor laser is a coherent radiation source, to enable it to produce laser light, there must be three basic conditions :

1. Gain 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 energy band regions.

The number of electrons at the bottom of the conduction band in the higher energy state is much larger than the number of holes at the top of the valence band in the lower energy state, which is achieved by adding forward bias to the homojunction or heterojunction and injecting the necessary carriers into the active layer to excite the 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 are compounded with holes.

2. To actually obtain the relevant stimulated radiation

Excited radiation must be made in the optical resonant cavity to get multiple feedback and the formation of laser oscillation.

The resonant cavity of a laser is formed by using the natural solution surface of a semiconductor crystal as a reflector, usually with a highly reflective multilayer dielectric film on the non-emitting end and a reflective reduction film on the emitting side.

For F-p cavity (Fabry-Perot cavity) semiconductor lasers, the F-p cavity can be easily formed by using the natural solution plane of the crystal perpendicular to the p-n junction plane.

3. In order 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 output from the cavity surface, etc., constantly increase the optical field in the cavity.

This requires a strong enough current injection, i.e., sufficient particle number reversal, and the higher the degree of particle number reversal, the greater the gain obtained, i.e., the requirement must meet a certain current threshold condition.

When the laser reaches the threshold value, the light with a specific wavelength can resonate in the cavity and be amplified, finally forming a laser and continuous output.

It can be seen that in semiconductor lasers, the dipole leap of electrons and holes is the basic light emission and light amplification process.

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 take full advantage 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

The semiconductor lasers of the early 1960s were homogeneous junction lasers, which were pn junction diodes made on a single material.

Under the forward high current injection, electrons were continuously injected into the p region and holes were continuously injected into the n region.

As a result, the carrier distribution is reversed in the original pn junction depletion zone, and because the electron migration rate is faster than the hole migration rate, radiation and compounding occurs in the active zone, emitting fluorescence, and under certain conditions, the laser occurs, which is a semiconductor laser that can only work in the form of a pulse.

The second stage of semiconductor laser development is the heterostructure semiconductor laser, which is composed of two different bandgap semiconductor material thin layer, such as GaAs, GaAlAs, the first is a single heterostructure laser (1969).

Single heterojunction injection lasers (SHLD) within the p-zone of GaAsP-N junction to reduce the threshold current density, the value of which is an order of magnitude lower than that of homojunction lasers, but single heterojunction lasers still can not operate continuously at room temperature.

Since the late 1970s, semiconductor lasers have clearly developed in two directions, one is information-based lasers for the purpose of transmitting information, and the other is power-based lasers for the purpose of increasing optical power.

Driven by applications such as pumped solid-state lasers, high-power semiconductor lasers (continuous output power of 100mw or more, pulsed output power of 5W or more, can be called high-power semiconductor lasers).

In the 1990s has made a breakthrough, marked by a significant increase in the output power of semiconductor lasers, foreign kilowatt-class high-power semiconductor lasers have been commercialized, the output of domestic sample devices have reached 600W.

If we look at the expansion of laser wavelengths, first the infrared semiconductor laser, followed by 670nm red semiconductor laser into a large number of applications, followed by the introduction of wavelengths of 650nm, 635nm, blue-green, blue semiconductor lasers have also been developed successfully, 10mW-scale violet and even ultraviolet semiconductor lasers, but also in the intensified development.

In the late 1990s, the rapid development of surface-emitting lasers and vertical-cavity surface-emitting lasers has been considered for a variety of applications in ultra-parallel optoelectronics.

Devices at 980 nm, 850 nm and 780 nm have been made 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 matured earlier and progressed faster, due to its wide wavelength range, simple production, low cost, easy mass production, and due to the small size, light weight, long life.

Therefore, its variety of rapid development, the application range has now more than 300 kinds .

1. Application in industry and technology

1)Fiber optic 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 disc access

Semiconductor lasers have been used for optical disc memory, and their biggest advantage is the large amount of stored sound, text and graphic information.

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, monitoring atmospheric pollution, automobile exhaust, etc.

In industry, it can be used to monitor the process of vapor phase precipitation.

4) Optical information processing

Semiconductor lasers have been used in optical information management systems.

Surface emitting semiconductor lasers 2D arrays are ideal light sources for optical parallel processing systems and will be used in computers and optical neural networks.

5) Laser microfabrication

Q-switched semiconductor lasers produce high energy ultra-short light strokes for cutting and punching of 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.

The use of blue and green laser can greatly improve the printing speed and resolution.

8) Laser barcode scanner

Semiconductor laser barcode scanners have been widely used for merchandising, as well as for book and file management.

9) Pumped solid-state lasers

This is an important application of high power semiconductor laser, using it to replace the original atmosphere lamp, can constitute an all-solid-state laser system.

10) High definition laser TV

In the near future, semiconductor laser TV sets without cathode ray tubes can be put on the market, which utilize red, blue and green lasers and are estimated to consume 20% less power than existing TV sets.

2. Application in medical and life science research

1) Laser surgery treatment

Semiconductor laser has been used for soft tissue excision, tissue joining, coagulation and vaporization. It has been widely used in general surgery, plastic surgery, dermatology, urology, obstetrics and gynecology, etc.

2) Laser kinetic treatment

The photosensitive substances with affinity for tumors are selectively gathered in cancerous tissues and irradiated by semiconductor laser to produce reactive oxygen species in cancerous tissues, aiming at necrosis without any damage to healthy tissues.

3) Life science research

The use of semiconductor laser “optical tweezers”, which can capture live cells or chromosomes and move them to any location, has been used to promote cell synthesis, cell interaction, and other research, and as a diagnostic technique for forensic forensics.

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