Optical fiber is the abbreviation of optical fiber, which is usually a cylindrical optical waveguide.
It uses the principle of total reflection to constrain the light wave in the fiber core and guide the light wave to transmit along the fiber axis.
Replacing copper wire with quartz glass changed the world.
As a medium for conducting light waves, optical fiber has been widely used since it was proposed by Gao Kun in 1966 because of its advantages such as large communication capacity, strong anti-interference ability, low transmission loss, long relay distance, good security performance, strong adaptability, small volume, light weight and rich sources of raw materials.
Gao Kun, known as the “father of optical fiber”, also won the Nobel Prize in physics in 2009.
With the increasingly perfect and practical performance of optical fiber, optical fiber has revolutionized the reform of the telecommunications industry.
It has basically replaced the copper wire and become the core part of modern communication.
Optical fiber communication system is a kind of communication system with light as information carrier and optical fiber as guided wave medium.
When optical fiber transmits information, the electrical signal is transformed into optical signal, and then transmitted inside the optical fiber.
As a new communication technology, optical fiber communication has shown incomparable advantages from the beginning, which has aroused great interest and widespread concern.
The wide application of optical fiber in communication also promotes the rapid development of fiber amplifier and fiber laser.
In addition to the communication field, optical fiber system is also widely used in medicine, sensing and other fields.
The gain medium of fiber laser is active fiber. According to its structure, it can be divided into single-mode fiber, double clad fiber and photonic crystal fiber.
Single-mode fiber is composed of core, cladding and coating. The refractive index N1 of the core material is higher than that of cladding material N2.
When the incident angle of the incident light is greater than the critical angle, the beam is fully emitted in the core, so the optical fiber can bind the beam to the core and propagate.
The inner cladding of single-mode fiber can not restrict the multi-mode pump light, and the numerical aperture of the core is low.
Therefore, the laser output can only be obtained by coupling the single-mode pump light into the core.
Early fiber lasers used this single-mode fiber, resulting in low coupling efficiency, and the laser has only milliwatt output power.
Transmission of light in optical fiber
Double clad fiber
In order to overcome the limitation of conventional single-mode single cladding Ytterbium doped (Yb3 +) fiber on conversion efficiency and output power, R. Maurer first proposed the concept of double cladding fiber in 1974.
Since then, it was not until E. Snitzer and others proposed cladding pumping technology in 1988 that high-power ytterbium-doped fiber laser/amplifier developed rapidly.
Double clad optical fiber is a kind of optical fiber with special structure.
Compared with conventional optical fiber, it has an inner cladding, which is composed of coating layer, inner cladding, outer cladding and doped core.
Cladding pumping technology is based on double clad fiber. Its core is to transmit multimode pump light in the inner cladding and laser in the fiber core, so that the pump conversion efficiency and the output power of fiber laser can be greatly improved.
The structure of double clad fiber, the shape of inner cladding and the coupling mode of pump light are the key to this technology.
The fiber j coil of double clad fiber is composed of silica (SiO2) doped with rare earth elements. In the fiber laser, it is both a laser medium and a transmission channel of laser signal.
For the corresponding working wavelength, the V parameter is generally reduced by designing its numerical aperture and core diameter to ensure that the output excitation is the fundamental transverse mode.
The transverse dimension (tens of times of the conventional core diameter) and numerical aperture of the inner cladding are much larger than the core, and the refractive index is smaller than the core, which can limit the complete propagation of laser in the core.
In this way, an optical waveguide with large cross-section and large numerical aperture is formed between the core and the outer cladding.
It can allow high-power pump light with large numerical aperture, large cross-section and multimode to be coupled to the optical fiber, and is limited to transmission within the inner cladding without diffusion, which is conducive to maintaining high power density optical pumping.
The outer cladding is composed of polymer materials with a refractive index smaller than that of the inner cladding;
The outermost layer is a protective layer composed of organic materials.
The coupling area of double clad fiber to pump light is determined by the size of inner cladding, unlike the traditional single-mode fiber, which is only determined by the core.
In this way, the double clad fiber constitutes a double-layer waveguide structure.
On the one hand, it improves the power coupling efficiency of human fiber laser, so that when the pump light is conducted in the inner cladding, It can excite the doped ion to emit laser through the fiber core for many times;
On the other hand, the output beam quality is determined by the nature of the fiber core, and the introduction of the inner cladding does not destroy the output beam quality of the fiber laser.
Structural diagram of octagonal double clad fiber
Schematic diagram of various inner cladding structures
These specially designed inner cladding can significantly improve the utilization efficiency of pump light in double clad fiber laser.
At first, the inner cladding structure of double clad fiber is cylindrical symmetry. Its manufacturing process is relatively simple and easy to be coupled with the tail fiber of pump laser diode (LD).
However, its perfect symmetry leads to a large number of spiral rays in the pump light in the inner cladding, which can never reach the core area even after enough reflections.
Therefore, it is impossible to be absorbed by the fiber core, so even if a longer fiber is used, there will still be a large number of light leakage, making it difficult to improve the conversion efficiency.
Therefore, the cylindrically symmetric structure of the inner cladding must be destroyed.
Photonic crystal fiber
In ordinary double clad fiber, the geometric size of the fiber core determines the output laser power, and the numerical aperture determines the beam quality of the output laser.
Due to the limitations of physical mechanisms such as nonlinear effect and optical damage in optical fiber, the single means of increasing the core diameter can not meet the needs of single-mode operation of large mode field double clad fiber at high power output.
The emergence of special optical fibers, such as photonic crystal fiber (PCF), provides an effective technical way to solve this problem.
The concept of photonic crystal was first proposed by E. yablonovitch in 1987, that is, dielectric materials with different dielectric constants form a periodic structure with the order of light wavelength in one-dimensional, two-dimensional or three-dimensional space, in which photonic guide bands that allow light propagation and photonic band gaps (PBG) that prohibit light propagation are generated.
By changing the arrangement and distribution period of different media, many changes in the properties of photonic crystals can be caused, so as to realize specific functions.
PCF is a two-dimensional photonic crystal, also known as microstructure fiber or porous fiber.
In 1996, J.C. Knight et al. drew the first PCF, and its light guiding mechanism is similar to the total internal reflection light guiding mechanism of traditional optical fiber.
The first PCF based on the principle of photonic band gap was born in 1998.
After 2005, the design and preparation methods of large mold field PCF began to diversify, and various shapes of structures appeared, including leakage channel PCF, rod PCF, large spacing PCF and multi-core PCF.
The mode field area of optical fiber is also increasing.
Microstructure of different photonic crystal fibers
In appearance, PCF is very similar to traditional single-mode fiber, but it shows complex hole array structure in microstructure.
It is these structural characteristics that endow PCF with many unique advantages unmatched by traditional optical fibers, such as non cut-off single-mode transmission, large mode field area, adjustable dispersion and low limiting loss, which can overcome many problems of traditional lasers.
For example, PCF can realize single-mode operation under large mode field area, significantly reduce the laser power density in the optical fiber, reduce the nonlinear effect in the optical fiber and improve the damage threshold of the optical fiber while ensuring the beam quality;
Large numerical aperture can be realized, which means that more pump light coupling and higher power laser output can be realized.
These advantages of PCF have caused a series of research upsurge all over the world, making it a new research highlight in fiber lasers and playing a more and more important role in the application of high-power fiber lasers.
Invention of fiber laser
The laser with optical fiber as laser gain medium is called fiber laser.
Like other types of lasers, it is composed of gain medium, pump source and resonator.
The fiber laser uses the active fiber doped with rare earth elements in the core as the gain medium.
Generally, semiconductor lasers are used as pump sources. The resonator is generally composed of mirrors, fiber end faces, fiber ring mirrors or fiber gratings.
According to the time-domain characteristics of fiber lasers, they can be divided into continuous fiber lasers and pulsed fiber lasers;
According to different resonator structures, it can be divided into linear cavity fiber laser, distributed feedback fiber laser and ring cavity fiber laser;
According to the different gain fiber and pumping mode, it can be divided into single cladding fiber laser (core pumping) and double cladding fiber laser (cladding pumping).
Structure principle of all fiber linear cavity fiber laser
In 1961, snizer discovered laser radiation in Nd doped glass waveguides.
In 1966, Gao Kun studied in detail the main causes of optical attenuation in optical fiber, and pointed out the main technical problems to be solved in the practical application of optical fiber in communication.
In 1970, Corning company of the United States developed optical fiber with attenuation less than 20 dB / km, which laid a foundation for the development of optical communication and optoelectronic technology industry.
This technological breakthrough has also greatly promoted the development of fiber lasers.
In the 1970s and 1980s, the maturity and commercialization of semiconductor laser technology provided reliable and diversified pump sources for the development of fiber lasers.
At the same time, with the development of chemical vapor deposition, the transmission loss of optical fiber is continuously reduced.
Fiber lasers are also developing rapidly in the direction of diversification. The fiber is doped with a variety of rare earth elements, such as erbium (Er3 +), ytterbium (Yb3 +), neodymium (Nd3 +), samarium (Sm3 +), thulium (Tm3 +), holmium (Ho3 +), praseodymium (PR3 +), dysprosium (Dy3 +), bismuth (Bi3 +).
According to different doped ions, laser output of different wavelengths can be realized. Meet different application requirements.
Emission spectrum range of quartz fiber doped with rare earth elements
Characteristics of high power fiber laser
The advantages of high-power fiber laser are as follows.
(1) Good beam quality.
The waveguide structure of the fiber determines that the fiber laser is easy to obtain single transverse mode output, and is little affected by external factors, so it can achieve high brightness laser output.
(2) High efficiency.
Fiber laser can achieve high optical to optical conversion efficiency by selecting a semiconductor laser whose emission wavelength matches the absorption characteristics of doped rare earth elements as the pump source.
For ytterbium doped high-power fiber lasers, 915 nm or 975 nm semiconductor lasers are generally selected.
Due to the simple energy level structure, few phenomena such as up conversion, excited state absorption and concentration quenching, and long fluorescence life, Yb3 + can effectively store energy to achieve high-power operation.
The overall electro-optic efficiency of commercial fiber laser is as high as 25%, which is conducive to reducing cost, energy conservation and environmental protection.
(3) Good heat dissipation characteristics.
Fiber laser uses slender rare earth doped fiber as laser gain medium, and its surface area and volume ratio are very large.
It is about 1000 times that of solid-state block laser, and has natural advantages in heat dissipation capacity.
In the case of low and medium power, there is no need for special cooling of the optical fiber.
In the case of high power, water cooling can also effectively avoid the decline of beam quality and efficiency caused by thermal effect in solid-state lasers.
(4) Compact structure and high reliability.
Because the fiber laser uses a small and soft fiber as the laser gain medium, it is conducive to compressing the volume and saving the cost.
The pump source is also a semiconductor laser with small volume and easy modularization. Generally, commercial products can be output with tail fiber.
Combined with optical fiber devices such as fiber Bragg grating, all-optical fiber can be realized as long as these devices are fused with each other.
It has high immunity to environmental disturbance, high stability, and can save maintenance time and cost.
High power fiber lasers also have insurmountable disadvantages:
First, it is easily restricted by nonlinear effects.
Due to the geometric structure of the waveguide, the effective length of the fiber laser is long, and the threshold of various nonlinear effects is low.
Some harmful nonlinear effects such as stimulated Raman scattering (SRS) and self phase modulation (SPM) will cause phase fluctuation, energy transfer in the spectrum, and even damage to the laser system, which limits the development of high-power fiber lasers.
The second is photon darkening effect.
With the increase of pumping time, the photon darkening effect will lead to the monotonous and irreversible decline of the power conversion efficiency of rare earth doped fiber with high doping concentration, which restricts the long-term stability and service life of high-power fiber laser, especially in ytterbium doped high-power fiber laser.
With the progress of high brightness fiber coupled semiconductor laser and double clad fiber technology, the output power, optical conversion efficiency and beam quality of high-power fiber laser have been greatly developed.
Driven by the huge demand in industrial processing, directional energy weapons, long-distance telemetry, lidar and other application fields, research units dominated by IPG photonics, Nufern, nlight and Trumpf Group have actively developed continuous wave and pulse wave high-power fiber lasers and launched a rich product line.
Tsinghua University, University of national defense science and technology, Shanghai Institute of Optics and precision machinery, Chinese Academy of Sciences and the Fourth Research Institute of China Aerospace Science and industry group also reported exciting results.
Fiber laser power enhancement technology
Due to the limitations of nonlinear effect, the thermal effect and material damage threshold in fiber laser, the output power of single channel fiber laser is limited, and with the increase of power, the beam quality gradually decreases.
It is necessary to adopt mode control technology and design new fiber with special structure to improve the beam quality.
J.W. Dawson et al. theoretically analyzed the output power limit of a single fiber.
The calculation shows that in a broadband fiber laser, a single fiber can obtain a near diffraction limit laser output with a maximum power of 36 kW, while for a narrow linewidth fiber laser, the maximum power is 2 kW.
In order to further improve the output power of fiber laser and amplifier, power synthesis of multi-channel fiber laser by coherent synthesis technology is an effective method. It has become an international research hotspot in recent years.
Coherent synthesis system of fiber laser
Coherent synthesis is to control the phase, frequency and polarization of each laser beam to have certain consistency, make it meet the coherence conditions, and obtain in-phase locking output.
It can obtain much higher peak intensity than simple incoherent superposition, and maintain good beam quality.
The development history of coherent synthesis technology is almost the same as that of the laser itself, and involves various types such as gas laser, chemical laser, semiconductor laser and solid-state laser.
However, due to the immaturity of various devices in the early stage, the experimental results obtained by coherent synthesis technology did not exceed the maximum output power of the corresponding single link laser at that time.
Therefore, the effect is not very obvious.
Since the 1990s, the emergence of fiber lasers has led to the rapid development of coherent synthesis technology.
In addition to the unique advantages of fiber laser and the demand for the use of 100 kilowatts tactics, several supporting devices (i.e. fiber fused cone coupler, multi-core fiber, phase modulator with pigtail and acousto-optic frequency shifter) in the process of commercial promotion of optical fiber communication have played a vital role.
The fiber-fused cone coupler and multi-core fiber make the passive phase control based on laser energy injection coupling and evanescent wave coupling very convenient.
The phase modulator with pigtail and acousto-optic frequency shifter enable the active phase control to have a control bandwidth of megahertz, which can be used to control the phase fluctuation under high-power conditions and realize phase-locked output.
Researchers have proposed many distinctive coherent synthesis schemes.
Spectral synthesis technology is an incoherent synthesis technology.
One or more diffraction gratings are used to diffract multiple sub beams into the same aperture, so as to achieve a single aperture output and obtain better beam quality.
Spectral synthesis of fiber laser can make full use of the wide gain bandwidth of ytterbium doped fiber laser to make up for the limitation of single fiber laser output power, so as to obtain high-power and high beam quality laser output.
It is one of the important technical paths of high-power fiber laser in the future.
Spectral synthetic fiber laser system
In recent years, Shanghai Institute of Optics and mechanics has carried out a lot of research on high-power fiber laser and spectral synthesis, and made important breakthroughs in device preparation, key technology breakthrough and spectral synthesis system.
In terms of narrow linewidth and high-power fiber amplifiers, the Institute adopted self-developed core devices such as fiber Bragg grating, high-power fiber combiner and cladding optical filter in 2016, based on key technologies such as fiber Bragg grating cascade filtering, linewidth control, amplification stage parameter control and fiber mode control.
It breaks through the single-mode output power limit of laser with linewidth less than 50 GHz reported by the research group of Jena University in Germany.
The near diffraction limit fiber laser output with power of 2.5 kW, linewidth of 0.18 nm (50 GHz) and center wavelength of 1064.1 nm is realized.
The laser adopts compact and stable all-optical fiber seed and three-stage amplification structure. The laser has good robustness.
The main amplifier adopts non polarization maintaining 20 μ m / 400 μ m fiber.
Increasing the available pump power can further improve the laser output power.
In the aspect of spectral synthesis, the damage threshold of metal film reflective diffraction grating is low, which is difficult to withstand the irradiation of high-power laser, and it is difficult to realize high-power spectral synthesis.
In August 2016, the spectral synthesis of 11.27 kW high beam quality was realized by using 7 narrow linewidth fiber lasers and high damage threshold polarization uncorrelated multilayer dielectric diffraction gratings (MLDG), and great progress was made in the spectral synthesis of high-power fiber lasers.
Typical applications of high power fiber lasers
Fiber laser has all-round excellent performance in industrial processing, medical treatment, remote sensing, security, scientific research and other fields because of its good beam quality, high electro-optic efficiency, compact structure and good reliability.
In the industrial field, fiber lasers can be divided into three levels according to the output power:
Low power fiber laser (< 50 watts), mainly used in microstructure processing, laser marking, resistance adjustment, precision drilling, metal engraving, etc;
Medium power fiber laser (50 ~ 500 watts) is mainly used in drilling, welding, cutting and surface treatment of thin metal plates;
High power fiber laser (> 1000 watts) is mainly used for cutting thick metal plates, metal surface coating, three-dimensional processing of special plates, etc.
The flexible characteristics of optical fiber can be well combined with the robot arm to meet the application requirements of various complex industrial environments.
In recent years, the rising 3D printing technology especially needs this high brightness laser system.
In the medical field, the ideal laser wavelength is 1.3 μ m, which can be used for diagnostic imaging;
Between 1.5 microns (absorption peak of water) and 4 microns can be used for surgery.
For medical applications, the biggest advantage of fiber laser is its compact and flexible geometry.
Short coherent wavelength light source with wide spectral range and high output power is the key to obtain high-speed and ultra-high resolution optical coherence tomography system.
Erbium-doped fiber lasers and ytterbium doped Raman fiber lasers have typical requirements for optical coherence tomography:
Compact, rugged, affordable, relatively high power and high resolution without optical calibration.
High-power erbium-doped fiber laser and thulium-doped fiber laser are very suitable for medical and surgical applications.
The researchers found that the laser can not only quickly remove and coagulate soft tissue, but also stop bleeding at the wavelength of 1.94 microns.
Moreover, due to the excellent beam quality of fiber laser, its operation has high precision.
In the field of remote sensing, the output wavelength of mid infrared fiber lasers such as erbium-doped fiber laser and thulium doped fiber laser is located in the atmospheric window and can pass through the atmosphere with low loss.
In particular, thulium doped fiber laser is easier to obtain high power output in the safe band of human eyes and has more advantages in power amplification.
Another advantage of fiber laser is its simplicity, compactness and portability, which will help to reduce the load of aviation or aerospace vehicles.
In the field of national defense and military affairs, laser is widely used in radar detection, secure communication, guidance, killing and so on.
Since the birth of fiber laser, it has become a hot candidate light source for a new generation of laser weapons with its unique advantages.
The high beam quality of fiber laser is especially suitable for long-distance transmission of energy seedlings.
Compared with other light sources, its smaller volume is conducive to the launch platform to achieve high mobility and improve the adaptability and survivability in the battlefield.
In the Afghan battlefield, the “Zeus” laser mine sweeping system of Spata company has carried out mine sweeping tasks.
Since 2009, the U.S. Navy has repeatedly destroyed UAVs, shells and small ships with fiber-optic laser system. It has been installed on warships in 2014.
In 2012, Rheinmetall, a German defense arms dealer, launched a double tube laser system with an output power of 50 kW, which intercepted and destroyed UAVs, shells and other targets in the demonstration experiment.
The laser weapon is a new concept weapon that is developing rapidly.
The laser weapon emits high-energy laser to the target surface at the speed of light. By damaging key devices such as photoelectric detection, navigation and guidance, or making the target “blind and blind”, or burning through the moving object shell, shooting it down, or detonating fuel to make it explode in the air, the damage task can be completed in a short time.
It has the advantages of energy concentration, fast transmission speed and repeated use.
It has the advantages of high efficiency cost ratio, fast fire transfer, anti electromagnetic interference and so on.
Since its birth, the development of laser weapons has experienced many ups and downs.
The maturity of solid-state laser technologies such as fiber lasers has injected new power into the development of laser weapons and become the research focus of major military powers.
At present, the United States, Britain, Russia, Germany, India and other countries have started the development of laser weapons and carried out relevant tests.
It is just around the corner for laser weapons to enter the battlefield.
In order to deal with asymmetric threats such as UAVs and sneak attack boats and improve the ship’s near defense capability, the US Navy officially began to develop the “laser weapon system” (laws) in 2010.
In September 2014, the system began to be deployed on the amphibious dock transport ship “Ponce” and conducted a one-year operational test and evaluation.
The laws system is led by Raytheon, and Boeing and Lockheed Martin also participate in some of the work.
Laws rely on existing commercial technologies and components to the greatest extent in order to reduce R & D and procurement costs.
The laws prototype consists of six industrial fiber lasers. In operation, the laser beams emitted by these six lasers are combined by beams to obtain a laser beam with a power of 30 kW.
The use cost of laser weapon system is low.
According to the estimation of relevant departments, the marginal cost of single irradiation is only $1, which is in sharp contrast to the price of tens of thousands or hundreds of thousands of dollars per missile.
In 2016, the US Naval Research Bureau launched the development of a new shipborne high-energy laser weapon system, with an output power of 150 kW, which is five times that of the laws system prototype previously tested on board.
The project took 12 months and US $53 million to develop the “laser weapon system demonstration prototype” in three stages:
The first stage mainly completes the initial design;
The second stage is ground test;
The third phase will be tested on the Navy self-defense test ship.
In 2014, China Academy of Engineering Physics and Shanghai Institute of Optics and mechanics jointly developed the “low altitude guard” system.
In the demonstration and verification experiment, more than 30 small aircraft such as fixed wing, multi rotor and helicopter were successfully shot down, and the shooting down rate was 100%.
The system has a launch power of nearly 10000 watts and a low altitude effective protection area of 12 square kilometers.
It can accurately intercept a variety of aircraft such as fixed wings (with a radius of 2 kilometers and 360 degree airspace) within 5 meters. It is fast, accurate and free of collateral damage.
In 2015, Lockheed Martin destroyed a truck a mile away with a 30 kW laser weapon (Athena).
In March 2017, the company said that it had completed the research and development of 60 kW laser weapon system and was transporting “beam forming” laser weapons to the U.S. Army Command Center in Alaska.
Jackson, the company’s chief technologist, said in a statement: “optical fiber laser weapons are bringing revolutionary changes to directional energy systems. This test also brings us closer to the development of lightweight fixed laser weapon systems that can be used in military aircraft, helicopters, ships and trucks.”
Research has proved that the powerful directional kinetic energy laser is now light enough, small enough and reliable enough to be deployed on tactical platforms on the ground, sea and air for defense.
In a word, from the development trend of the whole laser technology, fiber laser technology represents the development direction of high-power and high brightness laser.
It organically integrates waveguide fiber technology and semiconductor laser pumping technology.
The high-power fiber laser with fiber as the carrier is expected to meet the urgent needs of high-power and high-efficiency lasers in the fields of advanced laser manufacturing and military defense in the future.
It is a cutting-edge technology of great strategic significance to the national economy and national security.
High power fiber laser also shows great application potential in energy exploration, large scientific devices, space science, environmental science and other fields.
It will become a powerful tool for human beings to understand and transform the world.