Fiber laser refers to the laser using rare earth element doped glass fiber as the gain medium.
Among them, ytterbium doped fiber is one of the most core devices of high-power ytterbium doped fiber laser system.
However, with the increase of the output power of fiber laser, various stability “killers” such as transverse mode instability (TMI) Stimulated Raman scattering (SRS) phenomenon and thermal damage have surfaced.
Fiber laser is mainly composed of pump source, gain medium (active fiber) and resonator.
Principle of fiber laser with resonant cavity structure: the power of pumping semiconductor laser LD is injected into ytterbium doped double clad fiber (YDF) through fiber grating (high emission grating HR and low reflection grating OC) through forward and backward beam combiners.
After the rare earth ions in YDF absorb the pump light, the particle number inversion distribution is formed to generate spontaneous emission light.
Then, under the action of fiber grating pair (hr-oc), the stimulated emission light is formed to amplify and generate laser light, which is output through OC and output optical cable QBH.
Amplifier structure fiber laser principle: similar to the resonant cavity, the difference is that the seed source laser at the front stage reduces the power requirements of the system on the unit devices.
And higher power can be obtained.
Resonator fiber laser
Amplifier structure fiber laser
TMI effect of lateral mode instability
Lateral mode instability refers to the sudden change of high-power fiber laser from steady-state fundamental mode output to non steady-state high-order mode output as the output power increases or exceeds a certain time after reaching a certain threshold, which will lead to the decline of beam quality and limit the increase of fiber laser output power.
In serious cases, the laser which is called “the fastest knife, the most accurate ruler and the brightest light” will not live up to its name.
Schematic diagram of transverse mode instability effect
After the mode instability occurs, the power between the fundamental mode and the higher-order mode will be continuously coupled, and the total power will not change.
When there is a mechanism such as bending mode filtering, the fundamental mode loss is small, and the bending loss of the higher-order mode is large, resulting in the high-order mode of the green line being filtered out, and the output terminal shows fundamental mode jitter in the time domain.
20 μm, bending loss of 0.065NA fiber
Physical mechanism of mode instability
Unlike the traditional high-energy laser, the mode instability is caused by the coupling of the thermal effect and the fiber mode.
Therefore, the influencing factors of the mode instability are not only related to the waste heat but also related to the mode characteristics of the fiber.
Influencing factors of waste heat of optical fiber:
- Influence of optical fiber doping characteristics (doping concentration and doping region radius);
- Influence of darkening on signal characteristics (signal light power, signal intensity noise, initial high-order mode ratio of signal, signal light wavelength, signal intensity modulation);
- Influence of pump characteristics (pump power, pump wavelength, pump intensity modulation);
- Influence of pumping mode (forward pumping, backward pumping, side pumping and bidirectional pumping);
- Optical fiber material.
Influencing factors of optical fiber mode:
- Fiber core diameter / cladding diameter, fiber core numerical aperture;
- High order mode loss;
- Cooling capacity of the system;
- Polarization maintaining characteristics of optical fiber;
- The signal light is wide.
The suppression methods for mode instability mainly start from increasing the thermal management ability and mode control ability.
Increase thermal management capability:
- Enhance gain saturation (reduce the core cladding ratio, change the wavelength of the semiconductor pump source, change the injection direction of the pump light, increase the injection signal power, pump in the same band, and change the signal wavelength);
- Reduce the optical fiber heat source;
- Enhance the thermal optical performance of the optical fiber.
Add mode control capability:
- Improve the bending loss (reduce the bending radius, reduce the core numerical aperture, optimize the fiber winding method, reduce the core diameter, and increase the signal light wavelength);
- Optimize fiber design;
- Increasing the spectral width of the signal.
Stimulated Raman scattering
Stimulated Raman scattering (SRS) is a process in which photons interact with the medium during the transmission of laser in the matrix, and the laser is converted to long wave.
Stimulated Raman scattering (SRS) has become one of the main nonlinear effects affecting the power improvement of fiber lasers.
The stimulated Raman scattering of ytterbium doped fiber mainly depends on the 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.
Under forward pumping, when the fiber length is constant (L = 15 m), the core diameter increases, and the power threshold of SRS will be greatly increased.
In order to reduce the influence of stimulated Raman scattering, the optical fiber with large diameter core can be used.
2. Influence of fiber length on output
When the core diameter is constant (20 μ m), with the increase of fiber length, the threshold of SRS will decrease sharply.
By reducing the length of the optical fiber, higher output power can be obtained.
3. Influence of doping concentration on output
Under forward pumping, with the increase of doping concentration, the threshold pumping power of stimulated Raman scattering decreases, and the maximum output laser power decreases.
In the fiber with high doping concentration, the interaction distance between the high-power laser and the fiber is longer, and the stimulated Raman scattering is more likely to occur.
In the practical high power double clad fiber laser, in order to obtain high output laser power, the fiber with low doping concentration can be appropriately selected.
In the future, thanks to the progress of large mode area (LMA) gain fiber technology, high power and high brightness semiconductor pump source and high power pump coupling technology, China’s fiber lasers will continue to develop towards higher power and higher beam quality.
1. What is the internal structure of the fiber laser?
In terms of the whole machine, the fiber laser is composed of three parts: optical, mechanical and electrical.
The components of light are the three blocks mentioned in my report: pump source, resonant cavity and gain medium.
The pump source is a semiconductor laser, the resonant cavity is composed of a grating beam combiner, and 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 many reasons for the high melting point temperature of the internal cavity of the laser, which is a very complicated process problem.
It may be caused by such factors as fiber matching, welding quality and pump absorption conversion.
For this kind of fiber matching, we will generally choose the same type of fiber laser, try to have the same core diameter, and at least the cladding diameter is closer, so as to reduce the welding matching loss.
There are also welding quality, that is, various welding parameter settings of our welding machine can have many optimization areas, and parameters can be optimized through size adjustment and other methods.
Another is the pump conversion rate.
We need to do more optimization when selecting the pump source and active fiber.
In addition, if the overall heat dissipation design is better, better results can be achieved.
3. Why do two lasers of the same brand and model perform differently when cutting the same plate?
Cutting is a process and relatively complex.
The output characteristics of the fiber laser itself are also many.
Such factors as power and spectrum are different for each laser, and these factors will have a certain impact on cutting.
At the same time, the cutting head, nozzle, plate, etc. are also involved in the cutting.
There are many comprehensive variables, so it is difficult to say that the cutting effects of the two lasers are identical.
But now we are working hard to improve the overall tolerance of the laser.
The tolerance of the cutting process has been improved.
In the face of various variables, relatively consistent cutting effects can be achieved. It is still difficult to achieve complete consistency at present.