1. What is a fiber laser?
A fiber laser refers to a laser that uses glass fibers doped with rare earth elements as a gain medium.
It can be developed based on fiber amplifiers: under the action of pump light, high power density is easily formed in the fiber, causing “population inversion” of laser energy levels in the laser material.
When an appropriate positive feedback loop (forming a resonant cavity) is added, laser oscillation output can be formed.
2. Types of fiber lasers
Based on the types of fiber materials, fiber lasers can be divided into:
(1) Crystalline fiber lasers.
The working material is laser crystal fiber, including ruby monocrystalline fiber lasers and Nd3+:YAG monocrystalline fiber lasers, among others.
(2) Nonlinear fiber optic lasers.
The main types include stimulated Raman scattering fiber lasers and stimulated Brillouin scattering fiber lasers.
(3) Rare earth doped fiber lasers.
The fiber's base material is glass, and rare earth element ions are doped into the fiber to activate it, thus creating a fiber laser.
(4) Plastic fiber lasers.
Laser dyes are doped into the core or cladding of plastic fibers to create fiber lasers.
3. Advantages of fiber lasers
As representatives of third-generation laser technology, fiber lasers have the following advantages:
- Glass fibers have low manufacturing costs and the technology is mature, bringing the advantages of miniaturization and intensification due to the flexibility of the fibers.
- Glass fibers do not require a strict phase match to the incident pump light as crystals do. This is due to the broad absorption band caused by the non-uniform broadening induced by Stark splitting in the glass matrix.
- Glass materials have an extremely low volume/surface area ratio, providing rapid heat dissipation and low loss. Therefore, they have a high upconversion efficiency and a low laser threshold.
- Fiber lasers offer a wide range of laser output wavelengths, thanks to abundant rare earth ion energy levels and the variety of rare earth ions available.
- Tunability: This is due to the broad energy levels of rare earth ions and the broad fluorescence spectrum of glass fibers.
- The resonant cavity of a fiber laser contains no optical lenses, which offers the advantages of no adjustments or maintenance and high stability. This is something that traditional lasers cannot match.
- Fiber delivery allows the laser to easily handle various multidimensional arbitrary space processing applications, simplifying the design of mechanical systems.
- Fiber lasers can withstand harsh working environments and have high tolerance to dust, vibration, shock, humidity and temperature.
- There is no need for thermoelectric cooling or water cooling, simple air cooling is sufficient.
- High electro-optical efficiency: The overall electro-optical efficiency reaches more than 20%, significantly saving energy consumption during operation and reducing operating costs.
- High power: Commercially available fiber lasers have reached powers of up to 60,000 watts.
4. High-power fiber lasers and jacket-pumped technology
The advent of dual fibers is undoubtedly a major advance in the field of fibers, making the manufacture of high-power fiber lasers and high-power optical amplifiers a reality.
Since E Snitzer first described cladding-pumped fiber lasers in 1988, cladding-pumped technology has been widely applied to fiber lasers and amplifiers, becoming the preferred method for producing high-performance fiber lasers. power.
Casing pumping technology consists of four layers:
①fiber core;
②inner lining;
③outer coating;
④protective layer.
The pump light is attached to the inner casing (which generally adopts an irregular structure, including elliptical, square, plum blossom, D-shape, hexagonal, etc.), the light is reflected back and forth between the inner and outer casings (generally designed to be circular) and is absorbed by the single-mode fiber core after multiple crossings.
This structure does not require the pump light to be a single-mode laser and can pump the entire length of the fiber, therefore a set of high-power multimode laser diodes can be chosen as the pump source, indirectly coupling more than 70% of the pump energy to the fiber core, significantly improving pumping efficiency.
The characteristics of the coating pumping technology determine the following excellent performance of this type of laser:
(1) High power
A group of multimode pump diode modules can output 100 watts of optical power, and the parallel configuration of multiple multimode pump diodes allows the design of high-power output fiber lasers.
(2) No need for thermoelectric coolers
This high-power, wide-area multimode diode can operate at high temperatures, requiring only simple air cooling, which is low in cost.
(3) Wide pumping wavelength range
The active cladding fiber doped with erbium/ytterbium rare earth elements in high-power fiber lasers has a wide and flat light wave absorption range (930-970nm), so pump diodes do not require any type of wavelength stabilization device.
(4) High efficiency
The pump light passes through the single-mode fiber core several times, so its utilization is high.
(5) High reliability
Multimode pump diodes are much more stable than single-mode pump diodes. Its large geometric area results in low optical power density and low current density across the active area, providing the pump diodes with a reliable operational lifetime of over 1 million hours.
At present, technologies for achieving cladding-pumped fiber lasers can be divided into three main categories: single-end linear cavity pumping, linear cavity double-end pumping, and fiber ring cavity double cladding fiber lasers. Different types of dual fiber lasers can be expanded from these three basic types.
An OFC-2002 paper adopted a framework to obtain a new type of cladding-pumped fiber laser with 3.8W output power, 1.7W threshold, and tilt efficiency up to 85%.
In terms of product technology, the American company IPG emerged, which developed a 700W erbium-doped dual fiber laser and announced the launch of a 2,000W fiber laser.
5. Applications of fiber lasers
(1) Application tagging
The pulsed fiber laser, with its excellent beam quality, reliability, longer maintenance-free time, higher overall electro-optical conversion efficiency, pulse repetition frequency, smaller size, simpler and more flexible use without water cooling, and smaller operating cost, makes it the only option for high-speed, high-precision laser marking.
A fiber laser marking system may consist of one or two 25W fiber lasers, one or two scanning heads to guide the light to the workpiece, and an industrial computer to control the scanning heads. This design is four times more efficient than using a 50W laser split across two scanning heads.
(2) Material processing applications
Processing materials with fiber lasers is a heat treatment process based on parts of the material that absorb laser energy. Laser light with a wavelength of about 1um is easily absorbed by metals, plastics and ceramic materials.
(3) Material bending applications
Fiber laser shaping or bending is a technology used to change the curvature of metal or hard ceramic plates.
Concentrated heating and rapid self-cutting lead to plastic deformation in the area heated by the laser, permanently altering the curvature of the target part.
(4) Laser cutting applications
With the continuous increase in power, fiber lasers are being applied on a large scale in industrial cutting. For example, using a fast-cutting continuous fiber laser for microcutting stainless steel arterial tubes.
Due to their high beam quality, fiber lasers can achieve a very small focus diameter and consequently a small cutting width, setting new standards in the medical device industry.
Furthermore, fiber lasers have an irreplaceable position in the field of optical communication because their wavelength covers two main communication windows at 1.3μm and 1.5μm.
The successful development of high-power dual fiber lasers has led to a rapid expansion in market demand in the area of laser processing.
The specific scope and required performance of fiber lasers in the field of laser processing are as follows:
- soft welding and sintering: 50-500W;
- polymer and composite cutting: 200W-1kW;
- deactivation: 300W-1kW;
- fast printing and marking: 20W-1kW;
- metal hardening and coating: 2-20kW;
- glass and silicon cutting: 500W-2kW.
Furthermore, with the development of ultraviolet fiber Bragg grating writing and cladding pumping technology, wavelength conversion fiber lasers with UV, blue, green, red and near-infrared output are widely used in the data storage, color display and medical fluorescence diagnosis as practical. solid state light sources.
Fiber lasers with far-infrared wavelength output, due to their compact and flexible structure, adjustable energy and wavelength, are also applied in fields such as laser medicine and bioengineering.
(6) New fiber laser technology
Early research into lasers focused mainly on short-pulse output and expanding the tunable wavelength range.
Today, the rapid development and progress of Dense Wavelength Division Multiplexing (DWDM) and Optical Time Division Multiplexing technologies are accelerating and stimulating the advancement of multi-wavelength fiber laser technology and supercontinuum fiber lasers.
Meanwhile, the emergence of multi-wavelength fiber lasers and supercontinuum fiber lasers provides an ideal solution for implementing low-cost DWDM or OTDM Tb/s transmission.
From the point of view of its technological implementation, the use of EDFA-amplified spontaneous emission, femtosecond pulse technology and superluminescent diodes has been reported.
Conclusion
As representatives of third-generation laser technology, fiber lasers possess unparalleled technical superiority over other lasers.
However, in the short term, we believe fiber lasers will focus primarily on high-end applications. With the popularization of fiber lasers, the reduction in costs and the increase in production capacity, they could eventually replace a large part of high-power CO2 lasers and the vast majority of YAG lasers around the world.
