What is Laser?

Since the United States successfully developed the world’s first ruby laser in 1960 and China also successfully developed the first domestic ruby laser (born in Changchun Institute of Optics and precision machinery, Chinese Academy of Sciences) in 1961, laser technology is considered to be the second in the 20th century, following quantum physics, radio technology, atomic energy technology, semiconductor technology. Another major new scientific and technological achievement after computer technology.

Today, we have lasers for CD and DVD players in our homes, laser printers in our offices and bar code scanners in shopping malls.People use laser to treat myopia, send e-mail and browse videos through optical fiber network.Whether we realize it or not, each of us uses laser every day, but how many people really understand what laser is and how it works?

Laser is a kind of light that does not exist in nature and is emitted due to excitation. It has the characteristics of good directivity, high brightness, good monochromaticity and good coherence.

The generation mechanism of laser can be traced back to the hypothesis put forward by Einstein when explaining the blackbody radiation law in 1917, that is, the absorption and emission of light can pass through three basic processes: stimulated absorption, stimulated radiation and spontaneous emission.As we all know, the luminescence of any kind of light source is related to the motion state of particles in matter.When a particle (atom, molecule or ion) at a low energy level absorbs external energy (light) at an appropriate frequency and is excited to transition to the corresponding high energy level (stimulated absorption), it always tries to transition to a lower energy level and release the excess energy in the form of photons.

If light is spontaneously released without the action of external photons (spontaneous emission), the released light is ordinary light (such as electric lights, neon lights, etc.), which is characterized by the inconsistency of light frequency, direction and pace.

However, if the excess energy is released in the form of photons (stimulated radiation) during the transition from high-energy level to low-energy level under the direct action of external photons, the released photons are completely consistent with the external incident photons in terms of frequency, phase and propagation direction, which means that the external light has been strengthened, which is called light amplification.

Figure: laser generation mechanism: (left) stimulated absorption, (middle) spontaneous emission, (right) stimulated emission

The generation of laser needs to meet three conditions: particle number inversion, cavity feedback and meeting the threshold condition.Through stimulated absorption, the number of particles at the high energy level is more than that at the low energy level (particle number inversion). It is also necessary to make parallel reflection surfaces that can reflect photons at both ends of the active region to form a resonant cavity, and make the gain greater than the loss, that is, the number of newly generated photons at the same time is greater than the number of scattered absorbed photons.Only when these three conditions are met can laser be produced.

Characteristics of laser

Laser is known as magical light because it has four characteristics that ordinary light does not have at all.

——Ordinary light sources (sun, incandescent lamp or fluorescent lamp) emit light in all directions, and the luminous direction of laser can be limited to a solid angle of less than a few milliradians, which increases the illumination in the irradiation direction by ten million times.The diffusion diameter of the laser is less than 1 meter every 200 kilometers, if it reaches 3.8 away from the earth×When the moon is 105km away, the light beam spreads less than 2km, while the ordinary searchlight spreads tens of meters thousands of meters away.

1.Good directivity

Laser collimation, guidance and ranging use the characteristic of good directivity.

——Laser is the brightest light source in modern times. Only the strong flash at the moment of hydrogen bomb explosion can be compared with it.The brightness of the sun is about 1.865×109Cd / m2, and the output brightness of a high-power laser can be 7 ~ 14 orders of magnitude higher than that of sunlight.

2.High brightness

Although the total energy of laser is not necessarily large, due to the high concentration of energy, it is easy to produce high pressure and high temperature of tens of thousands of degrees Celsius or even millions of degrees Celsius at a micro point.Practical applications such as laser drilling, cutting, welding and laser surgery make use of this characteristic.

——Light is an electromagnetic wave.The color of light depends on its wavelength.The light emitted by ordinary light sources usually contains various wavelengths, which is a mixture of various colors.Sunlight includes visible light in seven colors: red, yellow, green, green, blue and purple, as well as invisible light such as infrared light and ultraviolet light.

3.Good monochromaticity

The wavelength of a laser is only concentrated in a very narrow spectral band or frequency range.For example, the wavelength of He Ne laser is 632.8 nm, and its wavelength variation range is less than 1 / 10000 nm.The good monochromaticity of laser provides a very favorable means for precision instrument to measure and stimulate some scientific experiments such as chemical reactions.

——Interference is an attribute of wave phenomenon.Based on the characteristics of high directivity and high monochromaticity, laser is bound to be a light with excellent coherence.This characteristic of laser makes holography a reality.

4.Good coherence

Type of laser

In the light source, the realization of energy level particle number inversion is the premise of light amplification, that is, the prerequisite of laser generation.To realize the inversion of particle number, we need to use the power of external light to make a large number of particles at low energy level jump to high energy level. This process is called “excitation”.

The laser we usually call is a device that excites the particles in the light source to produce stimulated radiation transition, realizes the inversion of particle number, and then generates light amplification through stimulated radiation.Although there are many kinds of lasers, their mission is to obtain lasers through excitation and stimulated radiation.Therefore, the laser is usually composed of three parts: the activation medium (i.e. the working material that can produce particle number inversion after being excited), the excitation device (i.e. the energy that can make the particle number inversion of the activation medium, the pump source) and the optical resonator (i.e. the two planar mirrors that can make the beam oscillate repeatedly and amplify many times).

Figure: working principle of laser

Since we can excite many different kinds of atoms in many different ways, we can (theoretically) make many different kinds of lasers.

There are many kinds of lasers, among which the most famous are solid, gas, liquid dyes, semiconductor and fiber lasers.The solid-state laser medium is similar to Ruby rod or other solid crystalline materials, and the flash tube wound on it pumps its energetic atoms.In order to work effectively, solids must be doped, which is a process of replacing some atoms with impurity ions to have appropriate energy levels to produce a laser with a certain accurate frequency.Solid state lasers produce high-power beams, usually very short pulses.In contrast, gas lasers use inert gas (so-called excimer lasers) or carbon dioxide (CO2) as a medium to produce continuous bright light.CO2 laser has powerful function and high efficiency. It is often used in industrial cutting and welding.Liquid dye lasers use the solution of organic dye molecules as the medium. The main advantage is that they can be used to produce a wider optical frequency band than solid-state and gas lasers, and can even be “tuned” to produce different frequencies.

According to the wavelength, the wavelength range covered includes far-infrared, infrared, visible light, ultraviolet and far ultraviolet. Recently, X-ray lasers andγX-ray apparatus;

According to different excitation modes, there are light excitation (light source or ultraviolet excitation), gas discharge excitation, chemical reaction excitation, nuclear reaction excitation, etc;

According to different output modes, there are continuous, single pulse, continuous pulse and ultrashort pulse, etc;

In terms of power output, the continuous output power is as small as micro watt level and up to megawatt level.The energy output of the pulse can range from micro Joule to more than 100000 Joule, and the pulse width ranges from millisecond to picosecond and even femtosecond (1 / 1000 trillion).

Various lasers meet different application requirements.For example, laser processing and some military lasers require high-power laser or high-energy laser (so-called high-power laser).Some hope to shorten the pulse time as far as possible in order to engage in the research of some express processes.Some also put forward high requirements for improving the monochromaticity of light, improving the mode of output light, improving the light intensity distribution of light spot and requiring adjustable wavelength.These requirements urge laser researchers to continue to explore, so that the exploration depth and application breadth of laser have been unprecedented developed.

Booming laser applications

The so-called laser technology is the general name of exploring and developing various methods of generating laser and exploring and applying these characteristics of laser for the benefit of mankind.

50Over the years, laser technology and applications have developed rapidly, and have been combined with many disciplines to form a number of application technology fields, such as photoelectric technology, laser medicine and photon biology, laser processing technology, laser detection and measurement technology, laser holographic technology, laser spectral analysis technology, nonlinear optics, ultrafast laser science, laser chemistry, quantum optics, lidar,Laser guidance, laser isotope separation, laser controlled nuclear fusion, laser weapons, etc.The emergence of these cross technologies and new disciplines has greatly promoted the development of traditional and emerging industries.

1.Application of laser in information field

Semiconductor laser and fiber amplifier are two key technologies of optical fiber communication.

The laser emitted by semiconductor laser not only has good monochromaticity and coherence, but also the light wave frequency is 10000 times higher than the microwave frequency. Therefore, the optical fiber communication with laser as the carrier of information transmission and optical fiber as the information transmission line has not only good communication quality, strong anti-interference ability and good confidentiality, but also the communication capacity is 10000 times higher than that of microwave communication.

The use of laser technology for optical storage has revolutionized the storage of information.The recording density of a CD audio disc is equivalent to 10 million bit / cm2, and can record 78 minutes of music programs, which is several orders of magnitude larger than that of a compact disc.

Image: laser and lens of a disc in a CD or DVD player.The small circle on the lower right is a semiconductor laser diode, while the larger blue circle is a lens that reads light after the laser is reflected from the smooth surface of the optical disc.

In addition, laser printer, laser fax machine, laser Phototypesetting, laser large screen color TV, optical fiber cable TV and atmospheric laser communication have been widely used.

2.Application of laser in holography

As a wave phenomenon, light is characterized by wavelength (related to color), amplitude (related to the intensity of light) and phase (representing the relationship between the starting point of wave and reference time).

People can only record the wavelength and amplitude by using the photosensitive photography method, so no matter how realistic it is, looking at the picture is always different from looking at the real scene.

Laser has high coherence and can obtain all the information of interference wave space including phase.Therefore, using laser holography, all the information of the photographed object is recorded on the negative film, and the lifelike three-dimensional image of the photographed object can be reproduced through light diffraction.

Hologram has the characteristics of three-dimensional imaging and can be recorded repeatedly, and each small holographic negative can reproduce the complete three-dimensional image of the object. It can be widely used in scientific research such as precision interferometry, nondestructive testing, holographic photoelasticity, micro strain analysis and vibration analysis.

Among them, holographic interferometry has been widely used to study the combustion process of gas, the vibration mode of mechanical parts, the bonding quality of honeycomb structure and the inspection of subcutaneous defects of automobile tires.Moreover, the use of holography as an anti-counterfeiting mark for commodities and credit cards has formed an industry. Photographing precious works of art with holography not only makes people feel on the spot, but also provides a reliable and realistic basis for the repair of works of art.The developing holographic TV will also add a new enjoyment to people’s life.

3.Application of laser in medical field

The application of laser in medicine is divided into two categories: laser diagnosis and laser therapy. The former takes laser as the information carrier and the latter takes laser as the energy carrier.

In the aspect of laser diagnosis, the laser can penetrate deep into the tissue for diagnosis, directly reflect the tissue condition, and provide a sufficient basis for doctors’ diagnosis.

In laser therapy, laser technology has become an effective means of clinical treatment and a key technology for the development of medical diagnosis.It solves many difficult problems in medicine, such as small incision in laser surgery, little or no damage to tissue, and few toxic and side effects.At present, the clinical application fields of laser include myopia correction, retinal repair, tooth decay repair, molecular minimally invasive surgery, etc. at present, the excellent application research of laser medicine is mainly reflected in the following aspects: photodynamic therapy for cancer;Laser treatment of cardiovascular diseases;Excimer laser keratoplasty;Laser cosmetology;Laser fiber endoscopy;Laser laparoscopic surgery;Laser thoracoscopic surgery;Laser arthroscopic surgery;Laser lithotripsy;Laser surgery;Application of laser in anastomosis;Application of laser in oral and maxillofacial surgery and dentistry;Weak laser therapy, etc.At present, laser therapy has maintained a sustained and strong development momentum in many aspects, such as basic research, new technology development, new equipment development and production.

Figure: application of laser in stomatology

4.laser processing

Using the high intensity (brightness) of the laser, the focused laser beam can emit 100J light energy within 1 ms, which is enough to melt or vaporize the material in a short time, so as to process the materials with different characteristics that are difficult to process, such as welding, drilling, cutting, heat treatment, lithography, etc.

Laser processing has the advantages of high precision, small distortion, non-contact and energy saving. Its application fields can almost cover the whole machinery manufacturing industry, including mining machinery, petrochemical industry, electric power, railway, automobile, shipbuilding, metallurgy, medical equipment, aviation, machine tool, power generation, printing, packaging, mold, pharmaceutical and other industries.The wear and corrosion of key parts and precision equipment can be repaired and optimized by laser cladding technology, which has become a powerful tool to turn corruption into magic.

5.precision measurement

Precision measurement makes use of the characteristics of good laser monochromaticity, strong coherence and good directivity.Compared with other rangefinders, laser ranging has the advantages of long detection distance, high precision, anti-interference, good confidentiality, small volume and light weight.The rangefinder sends out optical pulse, which is reflected by the measured target, and then returns to the receiving system to measure the time interval between transmission and reception.

Laser has both high brightness and high coherence, which makes the Doppler effect of light can be applied in velocity measurement.Lidar is a radar system that emits laser beam to detect the position, velocity and other characteristics of the target.In terms of working principle, there is no fundamental difference between lidar and microwave radar: transmit detection signal (laser beam) to the target, and then compare the received signal reflected from the target (target echo) with the transmitted signal. After proper processing, the relevant information of the target can be obtained, such as target distance, azimuth, altitude, speed, attitudeEven shape and other parameters, so as to detect, track and identify aircraft, missiles and other targets. It plays an important role in the military field and has become a powerful weapon for environmental monitoring.

In addition, the detection of gravitational wave also uses laser interferometry to directly detect the gravitational wave in the medium and low frequency band, and observe the gravitational wave radiation generated by the merging of double black holes and celestial bodies with maximum mass ratio, as well as other cosmic gravitational wave radiation processes.

Laser is one of the most important inventions of mankind in the 20th century. The application of laser technology has been widely penetrated into all aspects of industry, agriculture, military, medicine and even society. It is playing a more and more important role in the progress of human society and is miraculously changing our world.