There are increasing demands for high power continuous-wave (CW) solid-state mid-infrared lasers at 3~5 μm, for their applications in laser radar, spectroscopy, infrared pumping, infrared countermeasures, laser communication, etc. Currently, the reported solid-state lasers in CW regime at 4-μm can be roughly divided into two major categories. The one is based on the nonlinear effect, including optical parametric oscillators (OPO), difference frequency generation (DFG), and the other is based on the doped ions population inversion for amplified oscillation, including using 3-μm lasers to pump Fe:ZnSe/ Fe:ZnS crystal or using 888 nm diode to pump Ho:InF fiber. Compared with these lasers, Fe:ZnSe/Fe:ZnS lasers, enjoy the impressive advantages of high stability, high power, high efficiency and wide wavelength-tuning range, etc. As such, lots of efforts have been made in the development of Fe:ZnSe/Fe:ZnS lasers in the past decade. The lack of high power CW pump source seems to be the main obstacle for 4-μm laser power scaling. However, how to promotion of OTO efficiency is more important for obtaining higher output power when pump power is limited, and is crucial to power scaling in the future if the much higher pump power is available.

The research group led by associated Prof. Yanlong Shen from State Key Laboratory of Laser Interaction with Matter, focused on a high power, high efficiency and high stability Fe:ZnSe laser through proposing the comprehensive optimization of basic components of the laser, including designing a shorter cavity, customization of a pair of special coatings into the Fe:ZnSe crystal working facets for lower cavity loss, and a longer wavelength pump source for higher pump absorption. The obtained overall conversion efficiency, which approached the theoretical limit, was the highest in the CW solid-state laser at 4-μm. The research results are published in Chinese Optics Letters, Vol. 22, Issue 8, 2024: Yanlong Shen, Yingchao Wan, Feng Zhu, Tongxing Chai, Yousheng Wang, and Ke Huang. High power compact continuous-wave Fe:ZnSe laser at 4-μm with >50% overall conversion efficiency [J]. Chinese Optics Letters, 2024, 22(8): 081405.

In this work, an experiment was constructed to verify the innovation proposal. A home-made 2.9 μm CW fiber laser was used as pump source mainly based on the following considerations. One is that the longer wavelength of pump results in a promotion of the quantum efficiency compared to pumping at the shorter wavelengths, and the other is that the absorption cross-section at 2.9 μm of Fe:ZnSe at low temperature (LT) of ~77 K is larger than 2.8 μm. Based on the particle rate equation and power transfer equation, the optimal parameters of cavity coatings were analyzed and calculated. For more compact cavity design, both working facets of the crystal were polished carefully via a unique technique to reduce the residual wedge as far as possible. Two series of special coatings were deposited uniformly onto the polished ultra-high parallelism crystal facets. The purpose of integration of the resonant cavity and the gain medium, was to minimize the pumping loss and the laser oscillation loss of the resonant link, for achieving high conversion efficiency laser output, as shown in Fig. 1.

The experimental results show that the above series of innovations of the laser played a significant role in the high conversion efficiency output of mid-infrared 4-μm laser based on Fe:ZnSe crystal, and were in line with the theoretical simulation results. Meanwhile, the integrated module of gain medium and resonator can considerably improve the stability of laser output power. This work achieves a fairly high-efficiency of nearly theoretical limit laser generation, which would drive the development of mid-infrared 4-μm CW solid-state laser to higher power level. Much attention would be paid to the high-power, high-stability narrow-linewidth mid-infrared CW laser and spectrum control technology for applications of trace gas component diagnosis and infrared detector effect mechanism research in the future.

Schematic layout of the 2.9 μm fiber-laser-pumped CW Fe:ZnSe laser in backward-pumping (BP) scheme. Output powers versus launched pump power with a beam profile at the maximum output power. The long-term stability of BP over a 1 h period.