A high-energy Ho: YAG Amplifier pumped by 1907 nm Tm-doped fiber laser

doi:10.21203/rs.3.rs-7710050/v1

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A high-energy Ho: YAG Amplifier pumped by 1907 nm Tm-doped fiber laser

https://doi.org/10.21203/rs.3.rs-7710050/v1

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published 22 Dec, 2025

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A Ho: YAG amplifier based on in-band pumping by a high-power and narrow-linewidth Tm-doped fiber laser at 1907.5 nm is domenstrated. Maximum output power of 135 W is achieved at 2090.48 nm by using a single-end-pumped configuration with the Tm-doped fiber laser. The amplifier delivers a maximum output pulse energy of 67.5 mJ with a pulse width of 30 ns at a repetition rate of 2 kHz, corresponding to a peak power of 2.25 MW. The slope efficiency relative to the absorbed pump power reaches 61.9%. These results demonstrate Tm-doped fiber lasers' high laser efficiency and electro-optical efficiency for high-energy Ho: YAG amplifier operating.

High-power pulsed solid-state lasers in the 2 μm wavelength range have been widely applied in various fields, including plastic processing, biomedical treatment, and laser therapy [1-5]. Furthermore, using 2 μm lasers as pump sources for optical parametric oscillators (OPOs) or amplifiers (OPAs) has become an attractive method for generating mid-infrared lasers [6]. Currently, the most popular 2 μm lasers are those based on Tm³⁺ and Ho³⁺ doping. However, compared to Tm³⁺-doped media, Ho³⁺-doped lasers are more widely used in high-power and high-energy pulsed laser systems due to their higher emission cross-section and longer upper-state lifetimes [7].

So far, one of the most important ways to generate 2 μm lasers is to use 1.9 µm wavelength lasers made from Tm-doped fibers or solid-state gain media to pump Ho-doped solid-state gain media. This technique is characterized by a low quantum deficit and high conversion efficiency. The primary Ho-doped solid-state gain media include Ho: YAG [8,9], Ho: YLF [10-12], and Ho: YAP [13-15], among others.

Among these crystals, the Ho: YAG crystal has emerged as good option for producing high-energy pulsed lasers, owing to its exceptional emission cross-section, extended energy level lifetime, and superior mechanical and thermal attributes. Ho: YAG crystals are typically pumped in-band by high-power 1907 nm Tm-doped (Thulium, Tm) lasers to achieve 2.09 μm laser output. Currently, there are two main methods for generating 1907 nm laser output: using Tm:YLF (Tm-doped Yttrium Lithium Fluoride) crystals or Tm-doped fibers as the gain medium. The superior conversion efficiency and beam quality of fiber lasers have established Tm-doped fiber laser-pumped Ho solid-state lasers as the dominant technical approach for high-power 2 μm laser systems.

For the efficient Ho:YAG amplifier pumped by Tm-doped fiber lasers (TDFL), Encai Ji et al reported a Ho: YAG oscillator pumped by a 1907 nm Tm-doped fiber laser, which achieved a pulse energy of 12 mJ at a repetition rate of 1 kHz[16]. In addition, W.C. Yao successfully pumped a Ho: YAG laser using a 1931 nm Tm-doped fiber laser, achieving a maximum continuous output power of 142.2 W at a wavelength of 2091 nm, corresponding to a slope efficiency of 56.7% [17]. It is worth noting that the absorption cross-section of Ho:YAG at 1907 nm (1.09×10^-20 cm²) is approximately twice that at 1931 nm (0.6×10^-20 cm²), indicating superior pumping efficiency for the 1907 nm wavelength[18]. However, the pulse energy and power output of Ho:YAG crystals pumped by Tm-doped fiber lasers are still relatively low and does not yet meet the demands of some high-energy application fields.

In this paper, we experimentally demonstrate a Ho:YAG amplifier based on in-band pumping by a high-power, narrow-linewidth TDFL at 1907.5 nm. The TDFL is composed of a low-power seed oscillator and an amplifier configuration, achieving a maximum output power of 150 W. To mitigate the thermal lens effect caused by single-end pumping, a 120 mm long, 0.3 at% doped Ho: YAG crystal is employed. The Ho:YAG amplifier produces an output power of 135 W under full power conditions. The amplifier delivers a maximum output pulse energy of 67.5 mJ with a pulse width of 30 ns at a repetition rate of 2 kHz, corresponding to a peak power of 2.25 MW. The slope efficiency relative to the absorbed pump power reaches 61.9%.

The structure of the home-built TDFL used as the pump source for the Ho:YAG amplifier is shown in Fig. 1(b). The TDFL is composed of a low-power seed oscillator and an amplifier configuration. To avoid water absorption lines while aligning the wavelength with the absorption peak of the Ho:YAG crystal, the grating center wavelength is selected to be around 1907.5 nm[19]. The single-mode Tm-doped fiber oscillator provides an output power of 10 W. For power scaling, the seed laser is injected into the main amplifier stages. The pump and seed lasers are launched into the active fiber via a (2 + 1)×1 signal-pump combiner with matched passive fibers. The gain fiber in the main amplifier is a piece of double-clad LMA-TDF with a core diameter of 25 µm and an inner cladding diameter of 250 µm. The cladding absorption coefficient of the active fiber is 3 dB/m at a pump wavelength of 793 nm. To ensure sufficient pump light absorption, the fiber length is 4 m. The active fiber is wrapped around an aluminum heat sink, which is water-cooled at 16°C for efficient heat dissipation. After amplification, a cladding pump suppression filter (CPS) removes the excess pump light. Finally, an endcap coated with a 1.9 µm anti-reflection (AR) film is used for laser output to minimize unwanted feedback from Fresnel reflections.

For cladding-pumped short-wavelength TDFL operating at wavelengths below 1940 nm, the reabsorption loss of signal light, due to the quasi-three-level nature of the system, leads to long-wavelength ASE (amplified spontaneous emission), suppressing further power scaling of the short-wavelength signal [20]. The key to increasing the power output of short-wavelength cladding-pumped TDFL lies in reducing the concentration of active ions in the fiber, which is typically achieved by increasing the fiber's core-to-cladding ratio to reduce signal light reabsorption losses [21]. However, for fibers with a core diameter of 25 µm, a larger core-to-cladding ratio may introduce thermal management issues. By simulating two commercially available 25/250 µm Tm-doped fiber, the temperature distribution was simulated in Fig. 2. The results showed that the fiber with lower doping concentration (LMA-TDF-25/250-LC) maintained a coating temperature below 80°C during operation. This is crucial for preventing thermal damage to the coating in high-power, short-wavelength amplification applications.

The amplification system is a single-end pumping configuration. The gain medium used in this amplifier is the Ho: YAG crystal was 0.3 at. %-doped with a length of 120 mm and 5 mm diameter. Both ends of the crystal are coated with high-transmission coatings for 2.09 µm and 1.9 µm. The crystal was wrapped with indium foil and mounted in a copper block, with water cooling directly at 16°C for effective heat dissipation.The pump laser is focused to the front surface of the Ho:YAG crystal by using two coupling lenses with focal lengths of F1 = 15 mm and F2 = 250 mm, resulting in a beam diameter of 1.2 mm on the crystal surface. Between F1 and F2, 45° high-reflectivity (HR) mirrors M1, M2, M4, and M5 are used to adjust the radiation of the pump light. The signal light is generated by a Ho:YAG laser, which comprises an electro-optic (EO) Q -switched oscillator with one-stage preamplifier, with capable of 70W output power. The signal laser is directed into the amplification system via a pair of high-reflectivity (HR) 2.1 µm dichroic mirrors (M3 and M6), and subsequently focused by a lens with a focal length of F3 = 250 mm. At the center of the amplifier Ho:YAG crystal, the beam diameter of the signal Ho laser is 1.6 mm. Dichroic mirrors M3, M6, and M7 are coated with special coatings at a 45° angle, providing high reflectivity at 2.09 µm under vertical polarization. Therefore, the entire amplification system operates under vertical polarization. A convex lens with a radius of curvature of R = -100 mm is used in the system to expand the beam diameter of the output laser, preventing optical damage to mirror M7.

The output power of the TDFL as a function of the pump power is shown in Fig. 3. The MOPA configuration achieved an output with an average power of 150 W under full power conditions. The slope efficiency of the amplifiers was 56%. Figure 4 shows the output spectrum of the TDFL, which has a central wavelength of 1907.5 nm and a 3 dB spectral bandwidth of 0.16 nm as measured with a resolution of 0.05 nm. The inset of Fig. 4 shows the spectrum of the maximum output power in a large region from 1900-2000nm. Obviously, there is no ASE occurred in the main amplifier output. In addition, no evidence of SRS (or SBS) was observed.

The absorption efficiency of the Ho:YAG crystal was estimated by testing the incident 1907 nm pump power and the unabsorbed pump power. With an injected pump power of 150 W, the unabsorbed pump light is 13 W. Therefore, the absorption efficiency at 1907.5 nm is estimated to be 91.3%. Due to the self-absorption effect of the Ho:YAG crystal, with 70 W input seed laser, only 50 W of signal power after transmission through the crystal was remained, experiences a loss of approximately 28.6%.

Under continuous-wave operation, the maximum output power reaches 142.3 W at an absorbed pump power of 137 W, corresponding to a slope efficiency of 64.2%, which is shown in Fig. 5. The high slope efficiency mainly results from the adoption of a longer crystal and a pump source characterized by high beam quality.

When it operated in pulse scale, maximum average powers of 138 W and 135.5 W were achieved at PRF of 3 kHz and 2 kHz, which are shown in Fig. 6, the corresponding slope efficiencies are 63.7% and 61.9%, respectively. Under 3 kHz and 2 kHz repetition rates, the maximum pulse energies were 46 mJ and 67.5 mJ, respectively.

Figure 7 describes the pulse profile under maximum output power of PRF of 2 kHz. The pulse profiles were recorded by using a biased InGaAs detector and a 1 GHz digital oscilloscope (Tektronix DPO4102B). With the pump power increasing, the pulse width remains relatively consistent. At the maximum output power, the pulse width of the Ho laser is 30 ns, corresponding to a peak power of 2.25 MW. The Ho:YAG laser's typical output spectrum is shown in the inset of Fig. 8. The central wavelength is 2090.48 nm, with a linewidth of about 0.1 nm.

A half-wave plate (HWP) and a thin-film polarizer (TFP) were employed to measure the proportions of horizontally polarized light (P-polarized) and vertically polarized light (S-polarized) at various pump power levels. This allowed us to determine the degree of polarization (DOP) of the amplifier output. As illustrated in Fig. 9, the DOP exhibits a slight decrease as the pump power increases. At the maximum output power and a pulse repetition frequency (PRF) of 2 kHz, the DOP reaches approximately 94.07%.

A high-power and high-energy Ho:YAG amplifier pumped by a narrow-linewidth Tm-doped fiber laser operating at 1907 nm is illustrated. The amplifier employs a straightforward single-end pumping architecture and delivers a maximum output power of 135 W at 2090.48 nm. Under pulsed operation, it achieves a maximum pulse energy of 67.5 mJ at a repetition rate of 2 kHz, with a pulse duration of 30 ns, corresponding to a peak power of 2.25 MW. The slope efficiency relative to the absorbed pump power reaches 61.9%. These results demonstrate the viability of the Tm-fiber-pumped Ho-laser scheme for future generation of high-energy, high-repetition-rate lasers at 2-micron.

Conflict of interest

The authors declare no competing interests.

Funding

Advanced Materials-National Science and Technology Major Project,2024ZD0606000

Author Contribution

Z.H. and Y.T. wrote the main manuscript text. J.R., J.L., K.Y., and X.Y. reviewed and revised the manuscript. Z.H. and Y.T. prepared all figures. All authors have read and agreed to publish the manuscript.

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No competing interests reported.

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You are reading this latest preprint version

A high-energy Ho: YAG Amplifier pumped by 1907 nm Tm-doped fiber laser

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