# 680 W thulium-doped all-fiber single-oscillator laser emitting at 2036 nm in continuous-wave operation

Félix Sanson, Christophe Louot, Achille Bogas-Droy, Stefano Bigotta, Caterina Clemente, Nicolas Dalloz, Arnaud Motard, Inka Manek-Hönninger, Anne Hildenbrand-Dhollande

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Félix Sanson, Christophe Louot, Achille Bogas-Droy, Stefano Bigotta, Caterina Clemente, et al., 680 W thulium-doped all-fiber single-oscillator laser emitting at 2036 nm in continuous-wave operation. *Optics Continuum*, 2025, 4 (7), pp.1426. 10.1364/optcon.566656. hal-05252308

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# 680 W thulium-doped all-fiber single-oscillator laser emitting at 2036 nm in continuous-wave operation

FÉLIX SANSON,<fnref n="1" />,<fnref n="2" /> CHRISTOPHE LOUOT,<fnref n="1" />,* ID

ACHILLE BOGAS-DROY,<fnref n="1" /> STEFANO BIGOTTA,<fnref n="1" />

CATERINA CLEMENTE,<fnref n="1" /> NICOLAS DALLOZ,<fnref n="1" /> ID

ARNAUD MOTARD,<fnref n="1" />,<fnref n="3" /> INKA MANEK-HÖNNINGER,<fnref n="2" />

AND ANNE HILDENBRAND-DHOLLANDE<fnref n="1" />

**Abstract:** We report on a monolithic single-oscillator Tm3+-doped fiber laser emitting 680 W at 2036 nm in continuous-wave operation mode with an optical efficiency of 44%. To the best of our knowledge, this is the highest reported output power for an all-fibered single-oscillator architecture. The experimental results are compared to simulations, and the laser output performances are presented in terms of optical spectrum, beam profile, and beam propagation quality.

© 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

## 1. Introduction

Many applications require lasers with beam propagation over long distances, for example telecommunications [1], LIDARS [2], laser weapon systems [3], or directed infrared countermeasures [4,5]. Therefore, there is a need of high-power laser sources delivering several hundreds of watts with a good beam quality and at a wavelength with good atmospheric transmission. The transmission window around 2 µm is particularly interesting and well-adapted and can be achieved with lasers based on active media doped with rare-earth ions such as thulium [6–9], holmium [10–12], or by a codoping of both of them [13,14]. Fiber lasers [15,16] are often privileged thanks to their ability to produce high output power and their potential of an alignment-free architecture. First, single-oscillator lasers in which a thulium-doped fiber was placed in a mirror cavity were developed [17]. A maximum output power of 885 W at 2.05 µm was achieved in 2009 in a free-space pumping system with multimode fibers that allow for transporting more power [18]. The current absolute record confirms the tendency that MOPA systems are more convenient for power upscaling, with a laser output power of 1.05 kW and a slope efficiency of 53% in 2010 [19] and more recently with 1.1 kW and 50.7% achieved at 1950nm in 2023 [20].

However, military applications demand for compact, robust and efficient architectures, and it appears that monolithic single-oscillator all-fiber laser sources are perfect candidates. The absorption cross section of the thulium ion shows that 793 nm diodes are well-suited for pumping thulium-doped fibers. Regarding the emission cross section, the emitted wavelength can be selected by fiber Bragg gratings (FBGs) typically between 1.94 and 2.05 µm and even up to 2.09 µm, with a good efficiency [8]. In order to reach emission at longer wavelengths exceeding 2.05 µm, similar architectures have been developed with holmium (Ho<fnref n="3" />⁺) ions, such as Ho<fnref n="3" />⁺-doped fibers pumped at 1.94 µm [12,21] or Tm<fnref n="3" />⁺, Ho<fnref n="3" />⁺-codoped fibers pumped at 793 nm [13]. Up to date, the highest output power from a single-oscillator thulium (Tm<fnref n="3" />⁺) doped fiber laser source pumped at 793 nm has been reported by Walbaum et al. in 2016, with 567 W of output power at 1.97 µm and with 49.4% slope efficiency [22].

<footnote n="1">¹French-german research Institute of Saint-Louis, F-68300 Saint-Louis, France</footnote>

<footnote n="2">²Université de Bordeaux, CNRS CEA, CELIA UMR5107, F-33045 Talence, France</footnote>

<footnote n="3">³Now at: DOTA, ONERA, Université Paris-Saclay, F-91123 Palaiseau, France</footnote>

<footnote>*christophe.louot@isl.eu</footnote>
In this contribution, we show the results of a monolithic single-oscillator thulium-doped fiber laser in continuous-wave regime. We performed simulations for different opto-geometric parameters to predict the performances of the laser. In this way, we were able to determine the best set of parameters to maximize its output power.

## 2. Laser optimization by simulation

To perform this study, we used a home-made algorithm based on thulium rate equations [23] to simulate several components and configurations to optimize the laser performances. The parameters of interest in our study were the active fiber length (between 1 m and 13 m) and active fiber core diameter (20 µm and 25 µm). Moreover, pump power in fully co-propagating, fully counter-propagating and from both sides, with equal pump power, has been simulated as well. The configuration of the components was then chosen to be as close as possible to the parameters resulting as optimum by the simulations to develop the optimized single-oscillator laser source. As a matter of example, the results of the simulated slope efficiency as a function of the active fiber length are shown in Fig. 1. Simulations were performed from 1 m to 13 m, every 1 m, of a 20/400 thulium-doped fiber with a pump power up to 1580 W. Both slope efficiency and maximum power were extracted and taken into account in the choice of the best parameters. For this particular example, an active fiber length of 10 m was chosen for the final setup, as the plotted curve flattens after this value around 40% of slope efficiency. Most of the pump power is also absorbed after 10 m of active fiber, meaning there is no need to use a longer length of active fiber.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Simulated slope efficiency and maximum 2 µm output power as a function of 20/400 thulium-doped fiber length.</figcaption></figure>

This approach was also used to choose the most appropriate active fiber core diameter and pump geometry among the available options. A compromise between a single mode emission and a good laser efficiency was calculated as shown in Table 1. The comparison was done between two core diameters, 20 µm (V = 2.93) and 25 µm (V = 3.67) that are standard dimensions for commercial 400 µm clad-diameter fibers (L = 10 m). The slope efficiency is better (43.9% vs. 40.1%) for a 25 µm core, as it offers a better overlap between the cladding-propagating pump and the larger core. Moreover, the larger diameter does not allow additional linearly polarized (LP) modes to be propagated with respect to the 20 µm core diameter. The slope efficiencies are also shown in Table 2 regarding the different pump geometries. The results show that pumping from both sides of the active fiber enhances slightly the slope efficiency that co- and counter-propagating pumping (43.9% vs. 43.1%).
<table><caption>Table 1. Simulated slope efficiency for different active fiber core diameter (400 µm cladding, L = 10 m).</caption><tr><td>Active core diameter</td><td>20 µm</td><td>25 µm</td></tr><tr><td>Slope efficiency</td><td>40.1%</td><td>43.9%</td></tr></table>

<table><caption>Table 2. Simulated slope efficiency for different pump geometries (25 µm active core, 400 µm cladding, L = 10 m).</caption><tr><td>Pump geometry</td><td>Co-propagating</td><td>Counter-propagating</td><td>Both sides</td></tr><tr><td>Slope efficiency</td><td>43.1%</td><td>43.1%</td><td>43.9%</td></tr></table>

### 3. Experimental setup

The experimental setup of the monolithic single-oscillator fiber laser is depicted in Fig. 2(a). It consists of a 10 m silica double-clad fiber (exail) composed of a 25 µm Tm³⁺-doped core (numerical aperture NA = 0.095) with an absorption of 3.64 dB/m at 793 nm, which is surrounded by a 65 µm germanium-doped pedestal and a 400 µm pseudo-octagonal-shaped cladding. A microscope image of the cross section is shown in Fig. 2(b). The pseudo-octagonal cladding was used to enhance the pump absorption and hence, the laser efficiency. The fiber is pumped on both sides, in accordance to the simulations, by eight (four on each side) commercially available laser diodes at 793 nm for a total injected pump power of 1580 W. The cavity is delimited on one side by a high reflectivity (HR) FBG with a reflectivity of R = 99% (FWHM = 3 nm) at 2036 nm and on the output side by a low reflectivity (LR) FBG with a reflectivity of R = 20% (FWHM = 1 nm) at 2036 nm. All these passive components were made on a passive double clad fiber matched with the active fiber (geometry, NA). Both laser ends are cleaved at 8° in order to avoid the core propagation of Fresnel reflections generated at the fiber/air interface. The cooling of the pumping diodes and the components was realized with separated closed-loop water circuits at a temperature of 15 °C, whereas the active fiber was totally immersed in a water bath that was kept at a temperature of 10 °C.

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) Schematic of the monolithic Tm-doped fiber laser; (b) Cross section of the pseudo-octagonal Tm-doped active fiber.</figcaption></figure>
**4. Results and discussion**

The results of the laser output power measurements are presented in Fig. 3. The slope efficiencies of the laser, measured from both sides, are compared to the simulated output from our simulation algorithm. The forward output power is represented by red dots and the backward output power is indicated by dark blue triangles. The corresponding simulation results are plotted in black and cyan, respectively.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Measured and simulated 2 µm output and backward powers of the source presented in Fig. 2(a) as a function of the injected pump power at 793 nm.</figcaption></figure>

A maximum output power of 680 W at 2036 nm has been achieved. This is, to the best of our knowledge, the highest output power ever reported for a thulium-doped all-fiber single oscillator in the continuous-wave regime. Note that our simulation is in very good agreement with this experimental result and matches correctly the measured data points. The slope efficiency of 44% is slightly lower than values so far reported for some continuous-wave thulium-doped fiber lasers, which might be due to the relatively high output power on the backward output of our laser. Indeed, on the HR FBG side, 80 W of laser power were measured with a slope efficiency of 5%. Our simulations underestimate the backward output, with only half the power actually measured. This difference might arise from either the FBG performances that are degrading with a temperature increase or by spectral broadening, both due to the power increase.

We measured the output spectrum at low output power (40 W), which is just above the laser threshold, and at the maximum output power of 680 W. In Fig. 4, both spectra are displayed in blue and green, respectively. The measurements were performed with an AQ6376 Yokogawa Optical Spectrum Analyzer, with a resolution of 0.1 nm.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Normalized spectrum of laser emission at minimum (40 W, blue line) and maximum (680 W, green line) output power.</figcaption></figure>

At low power, the spectrum is centered at 2035.6 nm with a full width at half maximum (FWHM) of 0.7 nm. At maximum power, two peaks are observed, the first one matching with the
previous peak and a second one appears at 2036.4 nm, leading to a broadening of the spectrum from 0.7 nm to 1.6 nm (starting around 140 W). This observation is certainly linked to the power increase, which might induce a temperature change of the FBG and, thus, a dilatation of the material changing the path of the grating, which leads to a spectral shift of the reflected wavelength. Another explication might be a spectral broadening arising from nonlinear effects provoked by the very high intra-cavity power [24].

Beam profile measurements were performed using a Pyrocam IV camera (Ophir), where special care has been taken to avoid any thermal lensing by the optics used to attenuate the beam intensity on the camera. The measured beam profiles for an output power of 168 W, 418 W, and 680 W, respectively, are shown in Fig. 5. Note that for each measurement, the color scale has been adjusted to ensure, on the one hand, capturing a signal level high enough for an exploitable image and, on the other hand, to avoid camera saturation due to a too high signal level. Therefore, there is no absolute scale and the color bar is not displayed in this Figure.

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Laser output beam at low (a), mid (b) and high (c) output power.</figcaption></figure>

Globally, the beam profile stays nearly Gaussian with increasing power. However, at the maximum output power of 680 W, the beam seems to evade slightly on one side forming a croissant-like profile. We suppose that this phenomenon might be due to some blooming effect [25] on the camera detector where water molecules of the surrounding air are heated that locally modify the refractive index which deforms the detected beam. Some speckle can also be seen, especially on low power beams. This can be explained by the minor mismatching between the active fiber, with pedestal, and the passive fiber, without one. There are several solutions available to reduce the beam degradation, by using passive fibers with pedestal [26] or using a cladding light stripper to evacuate higher modes propagation.

Finally, the beam propagation quality has been measured with a Pyrocam IV camera (Ophir) up to 418 W, with an end-cap to avoid fiber end burning. This camera has a bigger detector head and allows for more accurate measurements with a larger beam diameter. The results of the measured $M^2$ values are shown in Fig. 6.

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Evolution of $M_x^2$ and $M_y^2$ values with laser output power. The error bars were defined to take in account an error of 5% on the beam diameter measurement.</figcaption></figure>
The $M_x^2$ and $M_y^2$ are plotted separately, to be able to observe the evolution of the $M^2$ with the power increase in both directions independently. Globally, the values remain close between 1.3 and 1.8 with a slight increase until 418 W. This measurement is in accordance with previously obtained results on similar architectures, for continuous-wave thulium doped fiber lasers [17,22].

**5. Conclusion**

In this paper, we presented the results of a monolithic single-oscillator thulium-doped fiber laser in continuous-wave regime. We achieved a maximum output power of 680 W for an optical-to-optical efficiency of 44%, at 2036 nm. The power values measured at the laser output are in accordance with the performed simulations. However, the backward output power was twice the expected power derived from the simulations. The difference most likely comes from spectral broadening due to the heating of the FBG. The $M^2$ parameter was measured between 1.3, just above the laser threshold, and 1.8 at 418 W output power. To the best of our knowledge, the output power measured in this work is the highest reported up to date for a thulium-doped all-fiber single oscillator in continuous-wave regime.

**Funding.** Direction Générale de l'Armement.

**Disclosures.** The authors declare no conflicts of interest.

**Data availability.** Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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