Since the birth of fiber lasers, people have been pursuing higher power and narrower pulses. Ytterbium-doped fiber is widely used in ultrafast fiber laser systems because of its low quantum defects after heating and high output efficiency under 980nm pump light.
However, as the output power of ytterbium-doped fiber continues to increase, transverse mode instability occurs when the output power is close to the kilowatt level. Transverse mode instability (TMI) refers to the phenomenon that the energy output by the laser is dynamically converted between the fundamental mode and the high-order mode when the average output power exceeds a certain threshold in the fiber amplifier. This phenomenon will seriously deteriorate the beam quality of the laser. , Therefore, this phenomenon is the main factor hindering the increase of the average power of large mode field high power ultrafast fiber lasers.
The gain band of thulium-doped fiber is around 2 μm, and studies have shown that it has a higher TMI threshold, but it has not been observed in experiments. Recently, Professor Limpert’s research group at the University of Jena in Germany built a bidirectional cascade pumping system based on thulium-doped optical fibers, as shown in Figure 1. The device uses bidirectional cascaded pumping to solve the problem of low pump energy utilization in thulium-doped fibers, and uses a dry helium cavity (VC in the figure) to avoid losses caused by water absorption in the atmosphere.
Fig.1 Schematic diagram of bidirectional cascade pumping device
The seed light with a pulse width of 600ps, a frequency of 80MHz, and a power of 100w is amplified through a 1.2m-long photonic crystal thulium-doped fiber. Figure 2 a) is the curve of output power versus pump power, and the green curve is the simulation result. Figure 2b) is the curve of the root mean square of the laser spot fluctuation versus the output power, the red is the numerical fitting curve, and the blue curve is the first derivative of the red curve. When the value of the first derivative is >0.1‰/W, TMI can be considered to occur, and the maximum value in Figure 2b) shows that TMI does not occur at this time.
Figure 2 Output result of Experiment 2
Figure 3 shows the light spots measured in the experiment. It can be seen from Figure 3a) that although the spot is distorted, it is still stable and no TMI is observed. At this time, the maximum heat load in the fiber has reached 350W/m. When the output power exceeds 400W, the basement film in the fiber begins to distort, and its spectrum is shown as the red line in Figure 3b). The spectrum shows interference between the basement film and the high-order mode, indicating that a high-order mode appears at this time.
Figure 3 Experiment 1 output spot
Subsequently, the research group slightly improved the above-mentioned device and further increased the pump power. The schematic diagram of the device is shown in Figure 4. This device can provide up to 2090W of pump power, and the laser system did not appear TMI in the initial operation. However, TMI was observed after 20 minutes of continuous operation at maximum power; the TMI threshold reached a stable level after 60 minutes of continuous operation. Figure 5 records the observed TMI phenomenon, which is the first time that TMI has been observed in a thulium-doped fiber.
Figure 4 Schematic diagram of experiment 2 device
Fig.5 Instability of transverse mode in thulium-doped fiber
The blue data points in Figure 6a) are the experimental data when no TMI is observed, and the red points are the measured data after TMI is observed. It can be seen that after the emergence of TMI, the efficiency of the laser system decreases. Figure 6b) is the heat load simulation result at the highest output power, and the heat load at the highest point is 200W/m.
Figure 6 Output result of Experiment 2
Figure 7a) records the power density spectrum of the random change of the beam. After the appearance of TMI, the power density spectrum (red line in a) increases greatly in the range of 300Hz to 1000Hz. The red line in Fig. 7b) shows that the spot fluctuation RMS value increases sharply after exceeding the TMI threshold (847W).
Fig.7 Power density spectrum and root mean square diagram of beam fluctuation
The above two independent experiments investigated the transverse mode instability of thulium-doped fibers under extreme thermal loading conditions. The first experiment did not observe the TMI phenomenon in the thulium-doped fiber under the heat load of up to 350W, which proved that the TMI threshold of the thulium-doped fiber was much higher than that of the ytterbium-doped fiber.
The second experiment, run continuously at the highest output power, observed TMI for the first time in a thulium-doped fiber. Thulium-doped fiber can simultaneously obtain high average power and high peak power in the 2μm band. Such an ultrafast laser will be very useful in high-order harmonic generation, particle acceleration, mid-infrared generation, life science and material processing.