Optical fiber, also known as optical fiber, is a cylindrical waveguide used to transmit light. It uses the principle of total internal reflection to confine the light wave within the fiber core and guide it along the fiber axis.
The replacement of copper wires with optical fiber changed the world. As a light transmission medium, optical fiber has been widely adopted since its proposal by Gao Kun in 1966 due to its numerous advantages such as high capacity, strong anti-interference capabilities, low transmission loss, long transmission distance, excellent safety, strong adaptability, compact size, light weight and abundant raw material resources.
Gao Kun, widely recognized as the “father of fiber optics,” was awarded the Nobel Prize in Physics in 2009.
The telecommunications industry has been transformed by the continuous improvement and practical applications of fiber optics. Fiber optics have largely replaced copper wire and are now a crucial part of modern communication.
The fiber optic communication system is a type of communication system that uses light as a carrier of information and optical fiber as a waveguide medium. When transmitting information, the electrical signal is converted into an optical signal and transmitted within the optical fiber.
As a new form of communication technology, optical fiber communication has presented incomparable advantages since its inception, attracting widespread interest and attention.
The widespread use of fiber optics in communication has also stimulated the rapid development of fiber amplifiers and fiber lasers. In addition to the field of communication, fiber optic systems are also commonly used in medicine, detection and other areas.
Optical fiber
Active fiber serves as the gain medium in fiber lasers. It can be classified into single-mode fiber, dual-mode fiber and photonic crystal fiber based on its structure.
Single-mode fiber consists of core, cladding and cladding. The refractive index (N1) of the core material is higher than that of the cladding material (N2). When the angle of incidence of the incident light is greater than the critical angle, the beam is completely emitted into the core, allowing the optical fiber to confine the beam to the core and transmit it.
However, the inner cladding of single-mode fiber cannot limit the light from the multimode pump, and the core has a low numerical aperture. As a result, laser output can only be obtained by coupling light from the single-mode pump to the core.
The first fiber lasers used single-mode fiber, leading to low coupling efficiency and only producing output power in milliwatts.
Light transmission in optical fiber
Double coated fiber
In an effort to overcome the limitations of conventional single-mode, single-cladded fiber doped with ytterbium (Yb3+) in terms of conversion efficiency and power output, R. Maurer first proposed the concept of double-cladded fiber in 1974. In However, it was not until E. Snitzer and others proposed cladding pumping technology in 1988 that high-power ytterbium-doped fiber laser/amplifier technology saw rapid development.
Dual optical fiber is a type of optical fiber with a unique structure. Compared to conventional optical fiber, it has an inner jacket consisting of a cladding layer, inner jacket, outer jacket and doped core.
Cladding pumping technology is based on double cladding fiber and aims to transmit multimode pump light into the inner cladding and laser light into the fiber core, significantly improving the pump conversion efficiency and output power of the fiber laser .
The double fiber structure, the shape of the inner coating and the light pump coupling mode are crucial to this technology.
The fiber coil of the dual fiber is composed of silica (SiO2) doped with rare earth elements. In fiber lasers, it serves as both the laser medium and the laser signal transmission channel.
To ensure that the output excitation is the fundamental transverse mode, the V parameter is reduced to the corresponding working wavelength by designing its numerical aperture and core diameter.
The transverse dimension of the inner cladding (tens of times larger than the diameter of the conventional core) and the numerical aperture are much larger than those of the core, and its refractive index is lower than that of the core, which restricts the complete propagation of the laser in the core.
This creates an optical waveguide with a large cross-section and numerical aperture between the core and outer cladding, allowing high-power pump light with a large numerical aperture, cross-section and multimode to be coupled into the optical fiber and limited. to transmission within the inner lining without diffusion. This helps maintain high power density optical pumping.
The outer coating of the double fiber is composed of polymeric materials with a lower refractive index than the inner coating. The outermost layer is a protective layer made of organic materials.
The coupling area of the double-clad fiber to the pump light is determined by the size of the inner cladding, unlike traditional single-mode fiber, which is determined solely by the core.
This creates a double-layer waveguide structure for the double-clad fiber.
On the one hand, it improves the power coupling efficiency of the fiber laser, allowing the pump light to excite the doped ions and emit laser light through the fiber core several times when driven into the inner cladding.
On the other hand, the quality of the output beam is determined by the nature of the fiber core, and the introduction of the inner cladding does not negatively affect the quality of the fiber laser output beam.
Octagonal Clad Double Fiber Structural Diagram
Schematic diagram of various inner lining structures
The specifically designed inner shell of the double-shell fiber laser can greatly increase the pump light utilization efficiency.
Initially, the inner cladding structure of the dual fiber was symmetrical cylindrical, making its manufacturing process relatively simple and easy to couple to the tail fiber of the laser diode (LD) pump.
However, its perfect symmetry resulted in a large number of spiral rays in the bomb light within the inner casing, which would never reach the central area even after multiple reflections.
As a result, these rays could not be absorbed by the fiber core, leading to light leakage, making it difficult to improve conversion efficiency even with the use of longer fibers.
Therefore, the cylindrical symmetry of the internal structure of the coating must be disturbed.
Photonic Crystal Fiber
In conventional double-jacketed fiber, the output laser power is determined by the size of the fiber core, and the numerical aperture determines the quality of the output laser beam.
However, the limitations of physical mechanisms such as nonlinear effects and optical damage in optical fiber make it impossible to meet the needs of single-mode operation of large-mode dual-field fiber with high output power by simply increasing the core diameter.
The advent of special optical fibers such as photonic crystal fiber (PCF) offers an effective solution to this problem.
The concept of photonic crystals was first proposed by E. Yablonovitch in 1987. This involves dielectric materials with varying dielectric constants forming a periodic structure on the order of the wavelength of light in one-, two-, or three-dimensional space. This creates photonic guide bands that allow light to propagate and photonic band gaps (PBG) that prohibit light propagation.
By altering the arrangement and distribution period of different media, numerous changes in the properties of photonic crystals can be achieved, enabling specific functions.
Photonic crystal fiber (PCF) is a two-dimensional photonic crystal, also known as microstructural fiber or porous fiber.
In 1996, JC Knight and others created the first PCF, and its light guidance mechanism is similar to the total internal reflection light guidance mechanism in traditional optical fiber.
The first PCF based on the photonic bandgap principle was invented in 1998.
After 2005, the design and preparation methods of large-mode field PCF have become diverse, with the emergence of various shape structures, including leaky channel PCF, rod PCF, wide-spacing PCF and multi-mode PCF. core.
The modal field area of optical fibers has also increased.
Microstructure of different photonic crystal fibers
Photonic crystal fiber (PCF) appears similar to traditional single-mode fiber, but has a complex hole array structure at the microstructure level.
These structural features grant PCF many unique advantages that traditional optical fibers cannot match, such as clipless single-mode transmission, a large mode field area, adjustable dispersion, and low limiting loss, overcoming several problems in traditional lasers.
For example, PCF can achieve single-mode operation with a large mode field area, significantly reducing the laser power density in the optical fiber, minimizing the nonlinear effect in the optical fiber, and improving the damage threshold of the optical fiber while preserving the beam quality.
It also allows for a large numerical aperture, resulting in better pump light coupling and higher power laser output.
These advantages of PCF have led to an increase in research worldwide, making it a new focus of research in fiber lasers and playing an increasingly important role in high-power fiber laser applications.
Invention of the fiber laser
A laser with optical fiber as the laser gain medium is called a fiber laser.
Like other types of laser, it consists of a gain medium, a pump source, and a resonator.
The fiber laser uses active fiber, doped with rare earth elements in the core, as a gain medium.
Typically, semiconductor lasers serve as the pump source, while the resonator is composed of mirrors, fiber end faces, fiber ring mirrors, or fiber gratings.
Based on time domain characteristics, fiber lasers can be divided into continuous fiber lasers and pulsed fiber lasers.
Based on the resonator structure, they can be divided into linear cavity fiber lasers, distributed feedback fiber lasers and ring cavity fiber lasers.
Based on the different gain and pumping fiber modes, they can be divided into single-clad fiber lasers (core pumping) and double-cladded fiber lasers (cladding pumping).
Structure principle of all fiber linear cavity fiber laser
In 1961, Snitzer discovered laser radiation in Nd-doped glass waveguides.
In 1966, Gao Kun comprehensively studied the main causes of optical attenuation in optical fibers and pointed out the main technical problems that needed to be solved for the practical application of optical fibers in communication.
In 1970, Corning Company in the United States developed optical fibers with attenuation of less than 20 dB/km, which laid the foundation for the development of optical communication and optoelectronic technology.
This technological advancement has also greatly facilitated the development of fiber lasers.
In the 1970s and 1980s, the maturity and commercialization of semiconductor laser technology provided reliable and diverse pump sources for the development of fiber lasers.
At the same time, the advancement of chemical vapor deposition has reduced the transmission loss of optical fibers.
Fiber lasers have diversified rapidly. Different rare earth elements such as erbium (Er3+), ytterbium (Yb3+), neodymium (Nd3+), samarium (Sm3+), thulium (Tm3+), holmium (Ho3+), praseodymium (Pr3+), dysprosium (Dy3+) and bismuth (Bi3+ ), are doped into the fiber to obtain laser output of different wavelengths to meet various application requirements.
Emission spectrum range of quartz fiber doped with rare earth elements
Features 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 laser facilitates the production of single transverse mode output and is not significantly affected by external factors, leading to high brightness laser output.
(2) High efficiency.
Fiber lasers can achieve high optical-to-optical conversion efficiency by using a semiconductor laser whose emission wavelength matches the absorption characteristics of doped rare earth elements as the pump source.
For high-power ytterbium-doped fiber lasers, 915 nm or 975 nm semiconductor lasers are typically selected.
The simple energy-level structure of Yb3+ leads to few phenomena such as upconversion, excited-state absorption and concentration quenching, and a long fluorescence lifetime, making it effective for storing energy and achieving high-power operation.
The overall electro-optical efficiency of commercial fiber lasers can reach 25%, contributing to cost reduction, energy conservation and environmental protection.
(3) Good heat dissipation characteristics.
Fiber lasers use a thin fiber doped with rare earths as the laser gain medium, which has a large surface area to volume ratio. This is approximately 1,000 times greater than that of solid-state block lasers and offers inherent advantages in terms of heat dissipation.
For low to medium power applications, no special cooling of the optical fiber is required. In high-power scenarios, water cooling can effectively mitigate the decline in beam quality and efficiency caused by thermal effects in solid-state lasers.
(4) Compact structure and high reliability.
The fiber laser's use of a small, flexible fiber as the laser's gain medium makes it ideal for reducing volume and reducing costs. The pump source, a semiconductor laser, is also compact in size and easily modularizable. Most commercial products can be produced using tail fiber.
By incorporating fiber optic devices such as fiber Bragg gratings, an all fiber optic system can be achieved through the fusion of these devices. This results in high immunity to environmental disturbances, high stability and reduced maintenance time and costs.
High-power fiber lasers also have insurmountable disadvantages:
First, it is easily constrained by nonlinear effects.
The fiber laser waveguide structure provides a long effective length, resulting in a low threshold for various nonlinear effects. However, harmful nonlinear effects such as stimulated Raman scattering (SRS) and self-phase modulation (SPM) can lead to phase fluctuations, energy transfer in the spectrum, and even damage to the laser system, hindering the advancement of fiber lasers. high power. .
The second is the photon dimming effect.
The high concentration of rare earth doping in fiber lasers results in a gradual and irreversible decline in power conversion efficiency due to the photon dimming effect with prolonged pumping time. This limits the long-term stability and service life of high-power fiber lasers, particularly in the case of ytterbium-doped high-power fiber lasers.
However, advances in high-brightness fiber-coupled semiconductor lasers and dual-fiber technology have significantly improved the output power, optical conversion efficiency, and beam quality of high-power fiber lasers.
The enormous demand for high-power fiber lasers in industrial processing, directional energy weapons, long-distance telemetry, lidar and other fields has driven the research efforts of companies such as IPG Photonics, Nufern, NLight and the Trumpf Group, leading to the development of high power continuous wave and pulsed wave fiber lasers with a diverse product line.
Academic institutions such as Tsinghua University, National Defense Science and Technology University, 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 in this field.
Fiber Laser Power Boost Technology
Limitations of nonlinear effects, thermal effects, and material damage limits in fiber lasers result in limited output power for single-channel fiber lasers, with a decrease in beam quality as power increases.
To improve beam quality, it is necessary to adopt mode control technology and design new fibers with special structures. JW Dawson and colleagues conducted a theoretical analysis of the output power limit of a single fiber. Calculations reveal that a broadband fiber laser can achieve a laser output close to the diffraction limit with a maximum power of 36 kW, while a narrow linewidth fiber laser can achieve a maximum power of 2 kW.
To further increase the output power of fiber lasers and amplifiers, power synthesis of multichannel fiber lasers through coherent synthesis technology is an effective method. This has become a widely researched topic in recent years.
Fiber Laser Coherent Synthesis System
Limitations imposed by nonlinear effects, thermal effects, and material damage limits on fiber lasers restrict the output power of single-channel fiber lasers and result in a decline in beam quality with increasing power.
To improve beam quality, mode control technology and special fiber structure design must be utilized. JW Dawson and colleagues conducted a theoretical analysis of the output power limit of a single fiber. The results show that a broadband fiber laser can produce a laser output close to the diffraction limit with a maximum power of 36 kW, while a narrow linewidth fiber laser can achieve a maximum power of 2 kW.
Coherent synthesis technology, which involves synthesizing power from multiple fiber lasers, is an effective method for increasing the output power of fiber lasers and amplifiers. This approach has become a topic of significant research interest in recent years.
In addition to the unique advantages of fiber lasers and the demand for 100-kilowatt systems, various supporting devices such as fiber-fused tapered couplers, multi-core fibers, phase modulators with pigtails, and acousto-optical frequency shifters have played a crucial role. role in the commercialization of fiber optic communication.
The fiber-fused tapered coupler and multi-core fibers make passive phase control through laser power injection coupling and evanescent wave coupling much more manageable.
The phase modulator with pigtails and acousto-optical frequency shifters enables active phase control with a megahertz control bandwidth, enabling control of phase fluctuations under high power conditions and achieving phase-locked output.
Researchers have proposed several distinct coherent synthesis schemes, including spectral synthesis technology, an incoherent synthesis technology that uses one or more diffraction gratings to diffract multiple subbeams in the same aperture for single-aperture output and improved beam quality.
Spectral synthesis of fiber lasers makes full use of the wide gain bandwidth of ytterbium-doped fiber lasers to overcome the limitations of single fiber laser output power, resulting in high-power, high-quality laser output. beam. This is one of the important technical paths for high-power fiber lasers in the future.
Spectral synthetic fiber laser system
The Shanghai Institute of Optics and Mechanics has conducted extensive research on high-power fiber lasers and spectral synthesis in recent years, making significant advances in the preparation of devices, key technologies and spectral synthesis systems.
In terms of narrow line width and high-power fiber amplifiers, the Institute used self-developed core devices such as fiber Bragg gratings, high-power fiber combiners and optical cladding filters in 2016. This was based on key technologies , including cascade fiber Bragg grating filtering, line width control, amplification stage parameter control and fiber mode control.
This advance has surpassed the single-mode output power limit of lasers with a linewidth of less than 50 GHz reported by the research group at the University of Jena in Germany. The Institute was able to achieve near-diffraction-limit fiber laser output with a power of 2.5 kW, a linewidth of 0.18 nm (50 GHz) and a central wavelength of 1064.1 nm.
The laser has a compact and stable all-optical fiber seed and a three-stage amplification structure, making it highly robust. The main amplifier uses a 20μm/400μm fiber that maintains non-polarization, and increasing the available pump power can further improve the laser output power.
In terms of spectral synthesis, metal film reflective diffraction gratings have a low damage threshold and are unable to withstand high-power laser irradiation, making it difficult to achieve high-power spectral synthesis. However, in August 2016, the Institute performed high-quality 11.27 kW beam spectral synthesis using 7 narrow linewidth fiber lasers and high damage threshold uncorrelated multilayer dielectric diffraction gratings (MLDG) , making significant progress in high-power fiber spectral synthesis. lasers.
Typical applications of high power fiber lasers
Fiber lasers have excellent performance in a variety of fields such as industrial processing, medical treatment, remote sensing, security and scientific research due to their good beam quality, high electro-optical efficiency, compact structure and reliability.
In the industrial sector, fiber lasers can be classified into three categories based on their output power:
Low power fiber lasers (<50 watts) are mainly used for microstructure processing, laser marking, resistance adjustment, precision drilling, metal engraving, etc.
Medium power fiber lasers (50 to 500 watts) are mainly used for drilling, welding, cutting and surface treatment of thin metal plates.
High-power fiber lasers (>1000 watts) are mainly used for cutting thick metal plates, coating metal surfaces, and three-dimensional processing of special plates, among others.
Fiber lasers have excellent performance in various fields such as industrial processing, medical treatment, remote sensing, security and scientific research due to their good beam quality, high electro-optical efficiency, compact design and reliability.
In the industrial domain, fiber lasers can be grouped into three categories based on their output power:
Low-power fiber lasers (<50 watts) are mainly used for microstructure processing, laser marking, resistance adjustment, precision drilling, metal engraving, etc.
Medium power fiber lasers (50 to 500 watts) are predominantly used for drilling, welding, cutting and surface treatment of thin metal plates.
High-power fiber lasers (> 1000 watts) are mainly used for cutting thick metal plates, coating metal surfaces and three-dimensional processing of special plates, among other applications.
Fiber lasers have exceptional performance in various fields such as industrial processing, medical treatment, remote sensing, security and scientific research due to their good beam quality, high electro-optical efficiency, compact design and reliability.
In the industrial sector, fiber lasers can be classified into three categories based on their output power:
Low-power fiber lasers (<50 watts) are mainly used for microstructure processing, laser marking, resistance adjustment, precision drilling, metal engraving, etc.
Medium power fiber lasers (50 to 500 watts) are predominantly used for drilling, welding, cutting and surface treatment of thin metal plates.
High-power fiber lasers (>1000 watts) are mainly used for cutting thick metal plates, coating metal surfaces, and three-dimensional processing of special plates, among other applications.
Compared to other light sources, the smaller volume of fiber lasers contributes to high mobility on launch pads, thus improving adaptability and survivability on the battlefield.
In Afghanistan, Spata's “Zeus” laser mine clearance system has been used to clear mines.
Since 2009, the US Navy has successfully used fiber optic laser systems to destroy UAVs, projectiles and small ships. The system was installed on warships in 2014.
In 2012, German defense weapons dealer Rheinmetall launched a 50 kW twin-tube laser system that successfully intercepted and destroyed UAVs, projectiles and other targets in a demonstration experiment.
laser weapon
The laser weapon is a new weapon concept that is rapidly developing.
It emits high-energy lasers at the speed of light at the surface of the target, causing damage to important devices such as photoelectric detection, navigation and guidance, or making the target “blind and deaf”, or burning the casing of the moving object to shoot. or detonate fuel to explode it in the air, thus completing the task of causing damage in a short space of time.
It has the benefits of energy concentration, fast transmission speed and repeatable use, as well as high cost-effectiveness, rapid fire transfer and resistance to electromagnetic interference.
Since its inception, the development of laser weapons has had its share of ups and downs. However, the maturity of solid-state laser technologies such as fiber lasers has revitalized the development of laser weapons and has become the focus of investigation for major military powers.
At present, countries such as the United States, Great Britain, Russia, Germany and India have started developing laser weapons and carried out relevant tests.
The entry of laser weapons onto the battlefield is coming.
In an effort to combat asymmetric threats such as UAVs and stealth attack boats and improve ship close defense capabilities, the U.S. Navy officially began development of the “Laser Weapons System” (LAWS) in 2010. The system was deployed at the amphibious transport dock. ship “Ponce” in September 2014 for a year-long operational test and evaluation.
LAWS is led by Raytheon, with participation from Boeing and Lockheed Martin in certain aspects of the work. The system takes full advantage of existing commercial technologies and components to minimize R&D and procurement costs.
The LAWS prototype consists of six industrial fiber lasers that, when operational, combine their laser beams to produce a 30 kW laser beam. The cost of using the laser weapons system is low, with a single shot estimated to cost just $1, in stark contrast to the tens of thousands or hundreds of thousands of dollars per missile.
In 2016, the US Department of Naval Research began development of a new ship-borne high-energy laser weapon system with an output power of 150 kW, which was five times more powerful than the prototype law system. previously tested. The project took 12 months and cost $53 million to develop the “laser weapon system demonstration prototype” in three stages: the first stage was initial design, the second stage was ground testing, and the third stage was the test in a Navy vehicle. defense test ship.
In 2014, the Chinese Academy of Engineering Physics and the 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 wings, multirotors and helicopters were successfully shot down, with a success rate of 100%. The system had a launch power of almost 10,000 watts and an effective protection area of 12 square kilometers for low altitudes. It could accurately intercept a variety of aircraft, including fixed wings, within a radius of 2 kilometers and 360-degree airspace within a radius of 5 meters. The system was fast, accurate and free from collateral damage.
In 2015, Lockheed Martin used a 30 kW laser weapon called Athena to destroy a truck a kilometer away. In March 2017, the company announced the completion of its research and development of a 60 kW laser weapon system and its shipment to the US Army Command Center in Alaska. The company's chief technologist stated that the successful tests bring us closer to developing portable laser weapons systems that can be deployed on military aircraft, helicopters, ships and trucks. Research has shown that the high-energy directional laser is now compact, light and reliable enough to be used in defense on land, sea and air platforms.
Summary
In conclusion, the development of laser technology shows that fiber laser technology is the future direction of high-power and high-brightness lasers. The combination of waveguide fiber technology and semiconductor laser pumping technology leads to the creation of high-power fiber lasers that can meet the urgent demand for high-power, high-efficiency lasers in advanced laser manufacturing and defense military.
This technology is of great strategic importance for both the economy and national security. Furthermore, high-power fiber lasers have immense application potential in various fields such as energy exploration, large scientific devices, space science, environmental science, and more. It will serve as a powerful tool for humans to understand and shape the world.
