A fiber laser is a type of laser that utilizes rare earth element-doped glass fiber as its gain medium. One of the most critical components of high-power ytterbium-doped fiber laser systems is the ytterbium-doped fiber.
However, as the output power of fiber lasers increases, various stability issues, such as transverse mode instability (TMI), stimulated Raman scattering (SRS), and thermal damage, have become increasingly prevalent.
A fiber laser is mainly composed of a pump source, gain medium (active fiber), and resonator.
The principle of a fiber laser with a resonant cavity structure involves injecting the pumping semiconductor laser LD’s power into ytterbium-doped double-clad fiber (YDF) through fiber grating (high emission grating HR and low reflection grating OC) via forward and backward beam combiners. After the pump light is absorbed by the rare earth ions in YDF, a particle number inversion distribution is formed, which generates spontaneous emission light. Subsequently, under the action of the fiber grating pair (hr-oc), the stimulated emission light amplifies and generates laser light, which is output through OC and output optical cable QBH.
The amplifier structure of a fiber laser principle is similar to the resonant cavity, with the difference being that the seed source laser at the front stage reduces the power requirements of the system on the unit devices, allowing for higher power to be obtained.
Resonator fiber laser
Amplifier structure fiber laser
TMI effect of lateral mode instability
Lateral mode instability is the sudden transition of a high-power fiber laser from a steady-state fundamental mode output to a non-steady-state high-order mode output as the output power increases or exceeds a certain threshold. This instability leads to a decline in beam quality and limits the increase of fiber laser output power. In severe cases, this can cause the laser, which is commonly known as “the fastest knife, the most accurate ruler, and the brightest light,” to fail to live up to its name.
Schematic diagram of transverse mode instability effect
After the onset of mode instability, the power between the fundamental mode and the higher-order mode becomes continuously coupled, but the total power remains constant.
If there is a mechanism, such as bending mode filtering, the fundamental mode experiences minimal loss, whereas the higher-order mode incurs significant bending loss. As a result, the high-order mode, represented by the green line, is filtered out, and the output terminal exhibits fundamental mode jitter in the time domain.
20 μm, bending loss of 0.065NA fiber
Physical mechanism of mode instability
The mode instability is caused by the coupling of the thermal effect and the fiber mode, which differs from the traditional high-energy laser.
As a result, the factors that influence the mode instability are not solely related to waste heat, but also to the mode characteristics of the fiber.
Factors influencing waste heat in optical fibers include:
- Optical fiber doping characteristics, such as doping concentration and doping region radius.
- Darkening effects on signal characteristics, including signal light power, signal intensity noise, initial high-order mode ratio of signal, signal light wavelength, and signal intensity modulation.
- Pump characteristics, including pump power, pump wavelength, and pump intensity modulation.
- Pumping mode, such as forward pumping, backward pumping, side pumping, and bidirectional pumping.
- Optical fiber material.
The factors that influence the mode of optical fiber include:
- Fiber core diameter and cladding diameter, as well as the fiber core numerical aperture.
- High-order mode loss.
- Cooling capacity of the system.
- The polarization maintaining characteristics of the optical fiber.
- The width of the signal light.
The methods used to suppress mode instability primarily involve enhancing thermal management and mode control abilities.
Enhancing thermal management capability involves:
- Improving gain saturation by reducing the core cladding ratio, changing the wavelength of the semiconductor pump source, changing the injection direction of the pump light, increasing the injection signal power, pumping in the same band, and changing the signal wavelength.
- Reducing the optical fiber heat source.
- Enhancing the thermal optical performance of the optical fiber.
Adding mode control capability involves:
- Improving bending loss by reducing the bending radius, reducing the core numerical aperture, optimizing the fiber winding method, reducing the core diameter, and increasing the signal light wavelength.
- Optimizing fiber design.
- Increasing the spectral width of the signal.
Stimulated Raman scattering
Stimulated Raman scattering (SRS) is a process where photons interact with the medium during the transmission of laser in the matrix, converting the laser to long wave.
SRS has emerged as one of the primary nonlinear effects influencing the power enhancement of fiber lasers.
The stimulated Raman scattering of ytterbium-doped fiber is mainly dependent on core diameter, fiber length, doping concentration, and pumping mode.
1. Influence of core diameter on output
When the pump power increases to a certain value, stimulated Raman scattering occurs in the fiber laser, and the output laser power begins to decrease.
When forward pumping is used and the fiber length is constant (L = 15 m), the core diameter increases and the power threshold of SRS is greatly increased.
To minimize the impact of stimulated Raman scattering, optical fibers with larger core diameters can be used.
2. Influence of fiber length on output
When the core diameter is kept constant at 20 μm, the threshold of SRS decreases sharply as the fiber length increases.
Reducing the length of the optical fiber can result in higher output power.
3. Influence of doping concentration on output
With forward pumping, the threshold pumping power of stimulated Raman scattering decreases as the doping concentration increases, and the maximum output laser power also decreases.
In fibers with high doping concentration, the interaction distance between the high-power laser and the fiber is longer, increasing the likelihood of stimulated Raman scattering.
In practical high-power double-clad fiber lasers, selecting a fiber with low doping concentration can lead to higher output laser power.
In the future, China’s fiber lasers will continue to develop towards higher power and beam quality, thanks to progress in large mode area (LMA) gain fiber technology, high-power and high-brightness semiconductor pump sources, and high-power pump coupling technology.
1. What is the internal structure of the fiber laser?
Regarding the fiber laser, it is comprised of three main components: optical, mechanical, and electrical.
The optical components consist of three essential parts, namely the pump source, resonant cavity, and gain medium, as mentioned in my report.
The pump source is a semiconductor laser, while the resonant cavity is constructed using a grating beam combiner. Finally, the gain medium is an active fiber.
2. What are the reasons for the high melting point temperature of the internal cavity of the laser? How to reduce the temperature?
There are several reasons for the high melting point temperature of the internal cavity of a laser, and it is a complex problem to address.
These factors may include fiber matching, welding quality, and pump absorption conversion.
To address fiber matching, we typically select the same type of fiber laser with a similar core diameter and at least a closer cladding diameter to reduce welding matching loss.
Welding quality is another consideration, and there are numerous optimization areas in our welding machine’s various welding parameter settings. Parameters can be adjusted through size optimization and other methods.
The pump conversion rate is also critical, and we must optimize our pump source and active fiber selection.
Additionally, better overall heat dissipation design can yield superior outcomes.
3. Why do two lasers of the same brand and model perform differently when cutting the same plate?
Cutting is a relatively complex process, and the output characteristics of fiber laser are also diverse. Factors such as power and spectrum vary for each laser and can impact the cutting process.
Moreover, the cutting head, nozzle, plate, and other components are also crucial in achieving the desired results. With numerous comprehensive variables involved, it’s difficult to attain identical cutting effects for two lasers.
However, significant efforts are being made to enhance the overall tolerance of the laser. As a result, the tolerance of the cutting process has improved, and relatively consistent cutting effects can now be achieved, though complete consistency remains a challenge at present.