Ultrafast fiber laser technology 35 hollow fiber compression produces a nanofocal less periodic pulse in the 1.9μm band

Short-wave infrared nanofocal low-period laser sources can drive two-color plasma to generate terahertz pulses with higher efficiency, and can also be generated in non-oxide crystals by optical difference frequency. Mid-infrared femtosecond pulse of 5μm. Thulium-doped fiber laser system can produce a thulium-doped fiber pulse with a central wavelength of about 2μm. In 2022, Limpert's research group in Jena, Germany, coherent synthesis of the output of four thulium-doped fiber amplifiers [1] finally obtained 85fs pulses with an energy of 1.65mJ and a repetition frequency of 100 KHZ. The limitation of single fiber on single pulse energy and average power is broken through, as shown in Figure 1.

FIG. 1 Schematic diagram of coherent synthesis device of four thulium-doped fibers [1]

In order to further shorten the pulse width, the Limpert research group took the above device as the front end and used hollow fiber for compression in 2023 [2]. The structure of the compression unit is shown in Figure 2, consisting of two vacuum chambers for input and output and a high-pressure chamber for nonlinear broadening filled with argon gas. To reduce water vapor absorption, the air pressure in both vacuum chambers is maintained. 1mabr. The bottom side of the high pressure chamber is equipped with water cooling for heat dissipation, avoiding harmful thermal effect under high power. The hollow fiber is placed in a V-shaped groove of a long straight line to avoid loss caused by bending. The diameter of the hollow fiber core is 500μm, the length is 1.05m, and the internal nonlinear gas is argon. The theoretical maximum passing efficiency is 89.5%.

FIG. 2 Schematic diagram of hollow fiber compression device [2]

Gradually increase the gas pressure in the cavity, and the corresponding output result is shown in Figure 3. When the air pressure is lower than 3bar, the output power is around 139W and the beam quality remains good (Figure 3a). When the air pressure is higher than 3bar, the output power begins to decline, and the beam quality deteriorates significantly. The spot deviates significantly from the Gaussian beam when the air pressure is 4.25bar, as shown in FIG. 3b. Figure 3c analyzes the spectral width of output under different air pressures. When the air pressure exceeds 3bar, the spectrum no longer expands significantly with the increase of air pressure, and the corresponding transformation limit pulse basically remains unchanged. Considering the above factors, the author finally selected 3bar pressure for the follow-up experiment.

FIG. 3 Output results of different air pressure in hollow core fiber [2]

The spectrum and autocorrelation curve measured at 3bar pressure are shown in FIG.4. The spectrum covers 1.2μm-2.4μm. After using a pair of chirped mirrors to compensate the dispersion, the pulse width decreases to 10.2fs, the average power is 132W, the main peak energy ratio is 66%, and the peak power is up to 80GW. Figure 5 shows the stability test results. The front-end output relative intensity noise is 0.75%, concentrated in the frequency range of 20 Hz to 50 kHz. After the nonlinear pulse compression, the main noise contribution is in the low frequency range of 2kHz. These noises come from the water cooling and mechanical vibration of the vacuum pump, proving that no additional noise is introduced in the compression process and ensuring the stability of the light source.

FIG. 4 Spectrum and autocorrelation measurement results under 3bar pressure [2]

Figure 5 Short-term stability test [2]

A high energy femtosecond pulse with a central wavelength of 1.9μm and a width of 10.2fs is obtained by hollow fiber compression. The pulse width is less than two cycles, the pulse energy is 1.3mJ, and the peak power is 80GW. The average power of the light source is 132W, which is the highest power level of the short-wave infrared pulse. The high energy and high power driving light source will greatly promote the development of laser technology in the middle infrared band.

References:

[1] Tobias Heuermann, Ziyao Wang, Mathias Lenski, Martin Gebhardt, Christian Gaida, Mahmoud Abdelaal, Joachim Buldt, Michael Müller, Arno Klenke, and Jens Limpert, "Ultrafast Tm-doped fiber laser system delivering 1.65-mJ, sub-100-fs pulses at a 100-kHz repetition rate," Opt. Lett. 47, 3095-3098 (2022)

[2] Ziyao Wang, Tobias Heuermann, Martin Gebhardt, Mathias Lenski, Philipp Gierschke, Robert Klas, Jan Rothhardt, Cesar Jauregui, and Jens Limpert, "Nonlinear pulse compression to sub-two-cycle, 1.3 mJ pulses at 1.9 μm wavelength with 132 W average power," Opt. Lett. 48, 2647-2650 (2023)

Source: Light wave