Fiber lasers were born in the 1960s. With the development of glass fiber materials, the maturation of high-power fiber optic devices, and the commercialization of high-brightness and high-power semiconductor pumping sources, fiber laser technology has been developed rapidly. Nowadays, fiber laser technology has profoundly changed our world and life.
I. Basic principles and classification of fiber lasers
In fiber lasers, the fiber acts as a gain medium. The earliest fiber laser is single-clad. The pump laser and the output laser are transmitted in the core. Due to the small numerical aperture of the core, the pump brightness is relatively high, and the output power is relatively low. It is further enlarged to meet the application.
After the advent of double-clad fiber lasers, fiber laser technology has undergone revolutionary development. The numerical aperture of the inner cladding is larger. Low-brightness pumped light can be transmitted in the inner cladding, and the high-brightness laser is transmitted in the fiber core, which requires pumping brightness. It is the pumping optical power that can be coupled into the inner cladding, which can be greatly increased, thereby greatly increasing the output power of the double-clad fiber laser.
According to the working mode, fiber lasers can be divided into continuous fiber lasers and pulsed fiber lasers.
At present, the output power of continuous fiber lasers has exceeded 10,000 watts. Industrialized high-power continuous fiber lasers are mainly used in laser cutting, welding, cladding, etc.; pulsed fiber lasers are divided into nanoseconds, picoseconds, and femtoseconds. Pulsed fiber lasers are nanosecond pulsed fiber lasers. The pulse energy can reach tens of millijoules. They are mainly used in laser marking and laser cleaning. Ultrafast fiber lasers are currently developing rapidly, mainly in the field of laser cold processing and hard material processing, fine processing, and other industrial applications.
According to the cavity shape, fiber lasers can be divided into linear cavity fiber lasers and ring cavity fiber lasers.
The linear cavity fiber laser is a standing wave cavity structure, and the high-power double-clad fiber laser generally is formed by a linear cavity structure; the ring cavity fiber laser is formed by a traveling wave cavity structure, usually using single-mode fiber and devices, and can be used as a light source and seed for optical communication laser.
According to the output longitudinal mode, fiber lasers can be divided into single-frequency and non-single-frequency fiber lasers.
The single-frequency fiber laser has a single longitudinal mode output with high coherence, but the threshold of the SBS effect is low. The multi-longitudinal fiber laser has a relatively wide spectrum and contains multiple longitudinal modes. The coherence is relatively poor, but it is not easily affected by the SBS effect.
According to the system structure, fiber lasers can be divided into single oscillator structure fiber lasers and main oscillation power amplification (MOPA) structure fiber lasers.
The characteristics of single oscillator structure fiber lasers are simple structure, including only one laser resonator. At present, the output power of a single continuous type (CW) laser oscillator has reached ~6 kW; the characteristics of the fiber laser with MOPA structure are: including a laser oscillation The maximum output power of the quasi-single-mode continuous fiber laser based on MOPA structure has reached ~20 kW.
Non-linear fiber laser: Use the nonlinear effect of optical fiber to provide gain to generate laser, such as Raman fiber laser, Brillouin (SBS) fiber laser, fiber parametric oscillator (OPO), etc.
Divided by doped rare-earth ions:
Ytterbium-doped fiber laser: radiation band 1 μm; simple energy level structure, no excited-state absorption; high quantum conversion efficiency; absorption spectrum matching commercial high-power semiconductor laser (LD) pump source; high output power (single-mode continuous light output~ 20 kW).
Erbium-doped and ytterbium-co-doped fiber lasers: radiation band is 1.5 μm, matching the communication band; human eye safety; using the energy conversion process between the energy levels of erbium ions and ytterbium ions to improve the absorption of pumped light, thereby improving laser efficiency.
Thulium-doped fiber laser: radiation band 2 μm; human eye safety; theoretically 200% quantum efficiency due to cross-level relaxation between energy levels; maximum output power has been broken tile (continuous); there is huge potential in output in high power/high energy laser.
The glass matrix material of the optical fiber and the doped rare-earth ions jointly determine the output wavelength of the laser. The working band of the fiber laser has covered the visible light, near-infrared, and mid-infrared bands.
The glass matrix materials of laser gain fiber include quartz glass, silicate glass, phosphate glass, germanate glass, and fluoride glass. The most widely used laser gain fiber is quartz glass fiber, which has mature technology and superior performance. Achieve a wide range of commercialization; wide wavelength transmission range coverage (0.38-2.1 μm), low transmission loss (minimum 0.2 dB/km); wide refractive index adjustment range, easy to achieve large numerical aperture; high mechanical strength, good bending performance. Cutting, welding, and coating processes are mature, and it is easy to realize high-strength and high-quality coupling between optical fibers.
II. The development of fiber laser cutting-edge technology
1. Frontier technology of high power continuous fiber laser
High-power continuous fiber lasers are mainly divided into single-fiber single-mode high-power fiber lasers and multimode high-power fiber lasers based on laser beam combining. Among them, the single-fiber single-mode high-power fiber laser generally uses a single fiber oscillator structure or MOPA structure. Using single-fiber single-mode lasers, spectral synthesis and coherent synthesis can achieve laser output in the order of tens to hundreds of kilowatts. During the beam combining process, the laser brightness will be reduced, but still, maintain a higher brightness; in industrial applications. Generally, high brightness is not required. You can use a single module to obtain a certain power, such as a 1000-3000W single-mode laser, and then use a fiber combiner to perform incoherent power synthesis to achieve high power.
From an international perspective, the US IPG company launched a single-mode 10 kW, less-mode 20 kW continuous fiber laser, combined multi-mode 500 kW continuous fiber laser.
From a domestic perspective, the National Defense Science and Technology University, Haifu Photonics, and the Chinese Academy of Engineering Physics have single-fiber kilowatt-class continuous fiber lasers; Chuangxin Laser has launched a multi-mode combined 35 kW continuous fiber laser; Ruike Laser, Fibo Laser, Hai Rich Photon, Shanghai Guanghui, Dake Laser, etc. have launched multimode 10-30 kW lasers. In 2017, Haifu Photonics reported a single-fiber 8 kW fiber laser directly pumped using semiconductors. In 2018, the Chinese Academy of Engineering Physics reported a single-fiber 10 kW fiber laser based on domestic fiber. In 2018, the National Defense Science and Technology University reported a 5.2 kW continuous laser oscillator based on domestic grating devices.
2. Frontier technology of single frequency narrow-linewidth fiber laser
The single-frequency narrow-linewidth fiber laser is a single longitudinal mode output, has excellent monochromaticity and coherence, and can be used in the field of radar sensing. Because of its strong prospectiveness, its development has received great attention. The main development direction of this technology is ultra-narrow linewidth, ultra-low noise, special band, high energy, high power, etc.
For ultra-narrow linewidth single-frequency fiber lasers, in 2004, NP photonics reported a 2 kHz linewidth single-frequency fiber laser. The single-frequency fiber laser based on the DBR short cavity structure can achieve a spectral linewidth in the order of kilohertz, which has been achieved commercial use. In 2014, domestic research began to keep up with the international pace. The South China University of Technology used a virtual ring-shaped folding cavity to reduce the hole-burning effect and extend the life of the phonon in the cavity. A 600 Hz ultra-narrow linewidth single-frequency fiber laser was obtained.
For single-band fiber lasers with special wavelengths, in 2012, NP Photonics reported a 976 nm single-band mirror-doped phosphate fiber laser. In the same year, a 1.2 μm single-frequency holmium-doped ZBLAN fiber laser was reported. The development of this laser is quite difficult. In 2013, the University of Sydney reported for the first time a mid-infrared 2.9 μm single-frequency fiber laser using a Ho/Pr co-doped ZBLAN fiber. In 2015, domestic Tianjin University and Haifu Photonics reported a 1.95 μm single-frequency doped quartz fiber laser, and a year later, they reported a 930 nm single-frequency neodymium-doped quartz fiber laser. NKT Photonics' single-frequency fiber laser products have performed well in recent years. NP Photonics, Fibertek, and Addvalue have also been committed to the production and development of single-frequency fiber laser products. At present, single-frequency semiconductor lasers are a strong competitor of single-frequency fiber lasers.
If you want to further increase the power of narrow-linewidth fiber lasers, you will encounter various nonlinear limitations. Stimulated Brillouin scattering (SBS) is the main factor limiting the power/energy increase of single-frequency fiber lasers. The usual methods to suppress stimulated Brillouin scattering include: increasing the core mode field area, using special fibers to reduce the overlap of the light field and the sound field, increasing the unity gain, reducing the fiber length, applying temperature and stress gradients, and reducing the SBS gain.
2.1 High energy pulsed single frequency fiber laser
In 2012, NP Photonics first reported the millijoule-level single-frequency nanosecond pulsed fiber laser.
2.2 High power continuous single frequency fiber laser
In 2007, Corning reported that a 502 W, 1064 nm single-frequency continuous fiber laser used a special active fiber. By changing the doping composition and concentration of the fiber core, the overlap of the sound field and the light field was reduced, and the SBS threshold was greatly increased.
In 2009, Northrop Grumman Co. reported a 608 W, 2040 nm single-frequency continuous fiber laser (the highest power in the 2 μm band).
In 2014, the US Air Force Laboratory reported an 811 W, 1064 nm single-frequency continuous fiber laser, which is the highest power single-frequency 1 μm output reported so far. In the same year, the National University of Defense Technology reported that a 411 W, 1064 nm single-frequency continuous fiber laser applied a longitudinal stress field to the main amplifier active fiber to increase the SBS threshold. This is the highest power of all-fiber structure fiber lasers reported so far.
2.3 High power narrow linewidth continuous fiber laser
By modulating the phase of the single-frequency fiber laser, the spectral linewidth of the single-frequency fiber laser can be appropriately broadened, but the single longitudinal mode is still maintained. The power amplification of the narrow linewidth fiber laser after the spectral broadening can achieve the order of high-power continuous fiber laser with several kilowatts.
In 2016, the Massachusetts Institute of Technology reported a 3.1kW narrow linewidth continuous fiber laser. This laser includes bulk optical elements such as lenses and dichroic mirrors. In 2018, the National University of Defense Technology reported a 3.94 kW narrow linewidth continuous fiber laser with an all-fiber structure, but mode instability appeared. In 2019, the University of Science and Technology of China and the Chinese Academy of Engineering Physics reported a narrow linewidth continuous fiber laser with an all-fiber structure of 3.7 kW and 1064 nm.
3. High-power nanosecond pulsed fiber laser
For nanosecond pulsed fiber lasers, there are relatively many studies based on industrial application requirements. It is mainly based on a Q-switched pulse laser or a modulated pulse laser as the seed laser. Based on the MOPA structure, the pulse energy/power is amplified step by step through a cascaded fiber amplifier.
In 2005, the University of Michigan reported a nanosecond fiber laser with a 1 μm band, 87 mJ, 24 MW peak power, and a spatial structure. In 2013, the University of Jena reported that the 1 μm band single-mode, 26 mJ high-energy nanosecond fiber laser (spatial structure) uses a patented special fiber. The disadvantage is that the gain fiber cannot be coiled and bent.
At present, IPG has 5000 W, 100 mJ, 100 ns fiber laser products. Domestic Tianjin University and Haifu Photonics reported an all-fiber structure 11 mJ high-energy nanosecond fiber laser in 2013. Haifu Photonics can provide industrial-grade nanosecond fiber laser products with an all-fiber structure with an average power of 1200 W and energy of 30-50 mJ. In the same year, the University of Central Florida reported a megawatt-level peak power nanosecond fiber laser in the 2 μm band, using a thulium-doped photonic crystal fiber as the main amplifier active fiber.
4. Ultrafast fiber laser
Ultrafast fiber lasers generally achieve picosecond and femtosecond laser pulses through mode-locked lasers, and will develop toward ultrashort pulses, new wavebands, high energy, and high power.
In 2010, the University of Arizona reported a 14 fs ultrafast femtosecond fiber laser. In 2011, Jena University reported a GW-class peak power femtosecond fiber laser. In 2013, Tianjin University reported an ultrafast femtosecond fiber laser of <25 fs. In 2018, the University of Jena reported a dry-watt-level average power femtosecond fiber laser, which is the largest average power femtosecond fiber laser known to date.
In 2019, the Shanghai Institute of Optics and Mechanics used domestic microstructured fiber to realize a picosecond fiber amplifier with an average power of 272 W in the 1 μm band. In the same year, the Beijing University of Technology reported a 27 μJ femtosecond fiber laser with pulse energy. The main amplifier stage uses Ytterbium-doped photonic crystal fiber and a grating is used to compress the pulse width to 173 fs. In 2014, Peking University reported a 910 nm, 935 nm mode-locked fiber laser. Laval University has achieved good research results in the mid-infrared band. In 2015, a 3 μm band mode-locked fiber laser was reported. Anyang Laser has made many outstanding achievements in the research and development of ultrafast fiber lasers.
Many companies at home and abroad are committed to the development and production of ultrafast fiber laser products, mainly to improve their cost performance and stability to meet the needs of industrial processing.
5. Mid-infrared fiber laser
For mid-infrared fiber lasers, the most current research is based on ZBLAN fiber-based 3 μm mid-infrared fiber lasers. In the future, the main development direction is in high power, all-optical fiber, mid-infrared mode-locking, mid-infrared super continuity, etc.
In 2009, Kyoto University reported a 24 W mid-infrared 3 μm band fiber laser (space structure, liquid cooling). In 2011, Laval University reported a 20 W, 3μm-band fiber laser (all-fiber structure, passive cooling). Among them, welding ZBLAN fibers to achieve an all-fiber structure is a major breakthrough. In 2018, Laval University reported a 41.6 W mid-infrared 3 μm band fiber laser, which is the largest power all-fiber 3 μm band fiber laser that has been reported.
6. Super-continuum fiber laser
The super-continuum fiber laser is a new type of laser, which has the broad spectrum characteristics of ordinary light sources (spontaneous emission) and the directivity, high spatial coherence, and high brightness of monochromatic laser light sources. The generation of supercontinuum usually means that after the narrow-band laser is incident on a nonlinear medium, the incident laser undergoes various nonlinear effects (such as modulation instability, self-phase modulation, cross-phase modulation, four-wave mixing, soliton self-frequency shift, and Under the combined influence of stimulated Raman scattering, etc.) and dispersion, the spectrum is greatly broadened. The main development direction is in high power, mid-infrared band, and wider supercontinuum spectrum coverage.
In terms of high power, in 2015, the National University of Defense Technology reported a multi-mode high-power 200 W supercontinuum laser based on laser beam combining. In 2018, Sichuan University reported a high-power 215 W supercontinuum laser based on photonic crystal fiber. Beijing University of Technology also reported some good results.
In terms of mid-infrared, in 2011, the University of Michigan reported a mid-infrared supercontinuum fiber laser based on ZBLAN fiber. In 2019, Jilin University reported a mid-infrared super-continuum fiber laser based on fluorotellurite glass fiber with a spectral range of 1-4 μm.
7. Other fiber lasers
In 2019, the Shanghai Institute of Optics and Mechanics reported a new type of ytterbium-doped mode-locked Raman fiber laser, which uses an "8" self-cavity structure for mode-locking. In the same year, Tsinghua University reported a 4 kW co-band pumped random fiber laser, which was the highest power reported in this type of laser.
III. Summary
In general, fiber lasers have a long history of development, but they are full of vitality; it has a simple structure and a wide variety of types; various types of fiber lasers are researched in related fields, and cutting-edge technologies have advanced by leaps and bounds. There are still many issues to be studied in terms of fiber laser materials, structure, performance, and so on.