High-Power Mid-Infrared Ultrafast Sources at 2 - 5μm Based on Dual-Wavelength Source - Part 10

4. Conclusion

To obtain high-power tunable short-wavelength mid-infrared ultrashort pulses, this paper utilizes an erbium-doped fiber laser (EDF-LAD) and employs nonlinear amplification and compression methods.

Part of the energy is broadened to 1.03 μm via a highly nonlinear fiber, serving as seed light for Ytterbium-doped CPA (chromatic amplification and compression). The remaining pulse is used as seed light for erbium-doped CPA. The erbium-doped CPA and ytterbium-doped CPA boost the energies of the 1.55 μm and 1.03 μm pulses to 140 nJ and 0.95 μJ, respectively, with pulse widths of 290 fs and 260 fs. Coupled with the 1.55 μm high-energy pulse into an 8.5 cm long dispersion-shifted fiber, a spectral sidelobe with a tunable spectrum from 1300–1900 nm and an average power of 50–400 mW is generated, providing a high-energy signal pulse for the DFG (dispersion-shifted spectral generation) process.

Based on the distributed Fourier method to solve the three-wave mixing equation, the influence of the pump pulse and signal pulse energies on the idler pulse energy during the DFG process was studied. The idler energy variation with pump energy can be divided into linear, exponential, and saturation regions. It was proposed that the output idler energy can be optimized by adjusting the delay between incident pulses.

In the experiment, based on a high-power dual-wavelength tunable light source, a mid-infrared ultrafast light source with a center wavelength of 3.06 μm, an average power of 3.06 W, and a pulse energy of 90 nJ was obtained using a 3 mm PPLN crystal (www.wisoptic.com). The center wavelength of this mid-infrared light source is tunable between 2-5 μm, and the average power is > 1 W, representing the highest average power result in this wavelength range.

The pump and signal optical paths in the experiment had relatively long optical paths and both contained high-power amplification stages and fiber coupling modules. These factors collectively led to poor stability of the final output power. The most critical factor is that as the pump pulse and signal pulse propagate in their respective optical paths, the refractive index change caused by fiber temperature variations alters the optical path difference between the two pulses, affecting the time synchronization of the pump and signal pulses and ultimately impacting the difference frequency efficiency.

A locking system will be added in subsequent experiments to stabilize the output power. Due to the lack of a beam and pulse width measurement device in the mid-infrared band, only the beam and pulse quality of the near-infrared laser were measured in this experiment. The circularity of the focused pump and signal beams was higher than 95%.

Since this experiment uses collinear difference frequency generation to produce mid-infrared ultrafast laser, the beam quality of the mid-infrared laser is greatly influenced by the near-infrared beam quality; therefore, it is reasonable to believe that the mid-infrared laser also possesses high beam quality.

Furthermore, based on the pump and signal pulse parameters in the experiment, we simulated the difference frequency generation process and calculated the mid-infrared femtosecond pulse width to be approximately 200 fs, which is close to the transform-limited pulse width corresponding to the output spectrum. This high-energy, high-power mid-infrared femtosecond source has a wide spectral tuning range and high peak power, and is expected to be used in research fields such as gas molecule detection, combustion diagnostics, and high-order harmonic generation.