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Abstract
We report on a quasi-continuous Er:YAG planar waveguide laser operated at 2.94 µm based on the major oscillator power amplification configuration. With the total pump peak power of 32.01 kW, a maximum output peak power of 1.14 kW was obtained at the seed injection peak power of 184.4 W operated at 400µs, 40 Hz. Furthermore, the numerical simulation results indicate that better performance of the laser could be obtained with the higher injected seed laser power. To the best of our knowledge, this is the first experimental demonstration of 2.94 µm planar waveguide laser with an Er doped host material.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High power mid-infrared lasers operated at the 2.5∼3 µm wave band have attracted much attention due to their significant applications in military countermeasures, remote sensing, atmospheric monitoring, and medicine [1,2]. Previous studies have shown that erbium-doped host materials have been utilized for efficient crystal for obtaining 2.5∼3 µm wave band mid-infrared laser [2–6]. However, the main problem that limits the improvement of laser output performance for erbium laser is the generation of a lot of heat inside the crystal under high pump power due to low quantum efficiency and high absorption caused by high doping for erbium-doped medium. In the past few decades, in order to improve the output performance of erbium lasers, on the one hand, researchers have developed new materials to reduce heat generation or increase heat dissipation rate, such as erbium-doped fluoride (Er:YLF [7], Er:BYF [8]) can effectively reduce heat generation in the medium due to their low phonon energy and low doping concentration. On the other hand, some special structures of the erbium laser such as fiber [9–12], micro laser arrays [13] and slab [14] were used to improve the heat dissipation efficiency, which is a more flexible and controllable way. As is well known, the advantage of fiber laser and slab laser is that fiber laser can provide high gain because of its small core diameter, and slab laser has high heat dissipation efficiency due to its large surface area to volume ratio. Fortunately, the planar waveguide can combine both the advantages of the slab and the fiber which have achieved great success in near-infrared lasers [15,16]. Moreover, N. Ter-Gabrielyan et al. in 2012 demonstrated an efficient Er:YAG-core, double-clad, all-crystalline eye-safe waveguide laser with a slope efficiency of 56.6% [17], fully demonstrating the advantages of the waveguide structure.
Similar to the fiber laser, which constraints the light in the core layer, the planar waveguide has high pumping power density and laser power density, which can improve the gain coefficient of erbium-doped material with low emission cross section. By the double cladding design, pump laser can go through the core layer many times, which makes relatively uniform absorption of pump energy, and extends the absorption area. Due to the absorption in the band of 2.5∼3 µm, the mid-infrared fiber generally adopts fluoride material instead of fused silica as host materials, and sacrifices the thermal conductivity and damage threshold. On the contrary, the core layer of the planar waveguide can adopt crystals such as yttrium aluminum garnet as the gain medium host, which can avoid the problem of fluoride fibers. Moreover, non-doped crystals bonded to the end face of the planar waveguide can effectively increase the damage threshold of the end face of the crystal.
In this paper, we studied the laser gain characteristics of Er:YAG planar waveguide amplifier by using the major oscillator power amplification (MOPA) structure, which obtained the average output power of 18.19 W and the peak power of up to 1.14 kW. To the best of our knowledge, this is the first reported Er:YAG mid-infrared planar waveguide laser. Based on the structure of MOPA of Er:YAG planar waveguide, the mid-infrared laser has the potential to achieve much higher power.
2. Process simulation of laser amplification
The energy level diagram for the trivalent erbium ion has been demonstrated in Ref. [18–20]. The steady-state rate equations for the population densities Ni and the laser photon flux Φ of the erbium amplifier can be given by
The pump rate $R_{02}^{pump}$ given by:
The description and numerical values of the parameters used in simulations are listed in Table 1.
Table 1. Selected Er (50%):YAG parameters used in numerical simulations
An Er:YAG planar waveguide with a size of 1 mm (Thickness, T)×8 mm (Width, W)×90 mm (Length, L) and a core erbium-doped region with a size of 80 mm (L)×8 mm (W)×80 µm (T) is used in the simulation. The laser mode cross-section Smod and the laser crystal length Lcr are the cross-sectional and length of the waveguide core erbium-doped region, respectively. The coupling efficiency of the pump light ηc≈90% is estimated by the coating parameters of each lens. The internal loss of the Er:YAG waveguide ρ0≈0.003 cm−1 is obtained by referring to the Er:YAG rod-shaped medium of the same length, and the Er:YAG waveguide average absorption coefficient α≈0.316 cm−1 is obtained by measuring the transmittance of the Er:YAG waveguide at 20°C. Due to the low emission cross-section of erbium-doped mid-infrared materials, a high pump power density is required to achieve sufficiently large population inversion. When the pump power is 35 kW (The maximum pump power that the pump source can provide in the experiment), the average pump power density of the Er:YAG planar waveguide is about 625 kW/cm3. However, the average pump power density of the Er:YAG rod amplifier with a typical size (Φ2×90 mm3) rod amplifier is only about 24 kW/cm3 at the maximum pump power of 4.5 kW(The maximum pump power of the LD side pump module we used in Ref. [22]). Further, the Er:YAG planar waveguide has a much higher average small signal gain coefficient (g0≈0.25 cm−1) than that of a Er:YAG rod (g0≈0.04 cm−1), and is more appropriate for laser amplification.
Figure 1 shows the simulated output power and extraction efficiency of the Er:YAG planar waveguide amplifier versus pump power under dual pumping at different seed laser power. The simulated results indicate that the power of the injected seed laser has a stronger effect on the output performance of the laser. As the power of the injected seed laser increases, both the slope efficiency and extraction efficiency of the laser are significantly improved. In addition, the output power of the laser increases substantially linearly with the increase of the input power. However, the laser extraction efficiency first increases rapidly and then slowly with the increase of input power when injecting larger seed laser power.
Fig. 1. Simulated output power (a) and extraction efficiency versus pump power under dual pumping at different seed laser power of the Er:YAG waveguide amplifier.
Typically, the saturation intensity Is can be given by Is= hν(Wp+1/τf)/γσ [23], where h is the Planck constant, ν is the frequency of signal laser, Wp is the pump rate which can be neglected in a four-level system, τf is the fluorescence lifetime, γ=1 + g2/g1 for a three-level system and γ=1 for a four-level system, and σ is the effective emission cross-section. However, the dynamic equations of highly doped Er:YAG laser system are much more complicated, because of its strong cross-relaxation and up-conversion process effects. Due to the quadratic terms in the rate equation, the gain cannot be written in a simple form of g = g0/(1 + I/Is) as in a three-level or four-level system. Thus, we follow the initial definition, in which the saturation density Is is the signal power which reduces the small-signal gain by a factor of one-half.
We calculated the gain under different pumping conditions through numerical method in Fig. 2(a), and marked the saturation power in our planar waveguide which reduced the small-signal gain by half. The saturation peak power of the planar waveguide amplifier is 6kW∼9 kW, the saturation density is 0.9 MW/cm2 ∼1.4 MW/cm2. Like the three-level system, the saturation density is correlated with the pump rate Wp, which is proportional to the absorbed pump power density. From the Fig. 2(a) we can see that the saturation density increases with the increase of the pump power slightly. In Fig 2(b), we calculated the extraction efficiency with an increased seed laser power under different pump conditions of our planar waveguide. From the figure, we also see that as the seed laser peak power increases in the range of 0 ∼10 kW (saturation power is 6kW∼9 kW for different pump power), the extraction efficiency continues to grow without significant saturation phenomenon.
Fig. 2. Simulated single pass average gain (a) and extraction efficiency (b) versus inject seed laser power under dual pumping at different pump power of the Er:YAG waveguide amplifier.
Figure 3 shows the simulated output power and extraction efficiency versus pump power under forward (same direction as the seed enters the waveguide) or backward (opposite to the direction in which the seed enters the waveguide) pumping of the Er:YAG waveguide amplifier at the seed laser power of 200 W. The simulated results indicate that the output power and extraction efficiency of the Er:YAG waveguide amplifier under backward pumping are higher than that of the forward pumping at same pump power. This could be explained by reason that the highest gain is positioned at the input end, and just a little of the stored energy at the input end is extracted under forward pumping due to the small power of the seed laser.
Fig. 3. Simulated output power and extraction efficiency versus pump power under forward or backward pumping of the Er:YAG waveguide amplifier
3. Experimental setup
As illustrated in Fig. 4, the dimension of the Er:YAG planar waveguide used in the experiment was 1mm (Thickness, T)×8 mm (Width, W)×90 mm (Length, L), which was manufactured by adhesive free bonding.
The central 80 mm (L)×8 mm (W)×80 µm (T) was 50 at.% Er3+ doped YAG layer, and two undoped YAG layers of 450 µm thickness were bonded onto it from the bottom and top respectively, acting as inner cladding. SiO2 films of 4 µm thickness were coated onto the top and bottom facets as the outer cladding. This simple double cladding waveguide had the capacity to enhance the pump power and further scale the laser output. Since the waveguide was end-pumped by huge power in the experiment, which would lead to large heat generation, two undoped YAG crystals of 5 mm length were bonded to its end facets to reduce the thermal effect. Then the undoped YAG end facets (1 mm×8 mm) were cut to be 80° relative to the face facet (8 mm×90 mm), to make it easier for the pump laser to enter the waveguide and suppress parasitic oscillation. Furthermore, their end facets (1 mm×8 mm) were coated with anti-reflective film at both 970 nm and 2940 nm.
The schematic of the Er:YAG planar waveguide laser amplifier is shown in Fig. 5. The seed laser was a laser diode side-pumped quasi-continuous Er:YAG laser, which could achieve a laser output with a pulse width of 100∼500 µs, a repetition frequency of 10∼100 Hz. Its maximum output peak power could reach 230 W and the beam quality M2 was measured to be 1.5 at its maximum output power. F1 and F2 were two collimated spherical lenses with focal lengths of F1 = 50 mm and F2 = 250 mm respectively, which expanded the seed laser beam into 7 mm in diameter. F(y) was a cylindrical lens with a focal length of 45 mm in the fast axis direction of the waveguide mounted on a 6-dimensional adjustment bracket, which could focus the seed laser beam spot into less than 80 µm in the fast axis and ensure the coupling efficiency of more than 90%. F1(z) and F2(z) were two cylindrical collimating lenses with focal lengths F1(z) = 240 mm and F2(z) = 142.6 mm, which could collimate the pump beam to a width of 7 mm in the width direction to ensure that the pump beam propagated uniformly within the width of the waveguide. In the fast axis direction, two cylindrical collimating lenses with focal lengths of F1(y) = 200 mm and F2(y) = 122.9 mm were used, which could converge the pump beam into 0.8 mm in this direction. The profile of the pump spot on the front end face of the Er:YAG waveguide after the coupling system is shown in Fig. 6(a). Besides, we measured the absorption of the pump energy by the Er:YAG waveguide at different temperatures by adjusting the temperature of its cooling water, and the results were shown in Fig. 6(b). The results indicated that the absorption of the Er:YAG waveguide exceeded 90% at different temperatures, which meant that the pump energy could be fully absorbed by the Er:YAG waveguide. In the experiment, the two large surfaces of the Er:YAG planar waveguide were respectively welded on two copper microchannel heatsinks to ensure efficient heat exchange, as shown in Fig. 6(c). The two heatsinks were connected to the water cooling system to conduct sufficient heat exchange with the Er:YAG planar waveguide.
Fig. 6. (a) The pump beam profile after coupling system; (b) The absorption of the Er:YAG waveguide at different temperatures; (c) The state of the Er:YAG waveguide under loading conditions
Two space-combined diode laser arrays (DLA) with the total maximum output peak power of 35 kW were used in the experiment. The spot size and half-divergence of each DLA were 12 mm and 5° (1/e2) in the slow axis, and 50 mm and 0.2°(1/e2) in the fast axis. The pump beam was coupled into the waveguide from the two end facets with different angles to protect the DLA against the unabsorbed pump energy from the other end. Figure 6(c) shows the distribution of the pump laser in the Er:YAG planar waveguide along the propagation direction under pump laser loading state.
4. Experimental results and discussion
We measured the output power versus pump peak power under dual pumping at different seed laser peak power of the Er:YAG waveguide amplifier when the pump pulse width was 400µs and the repetition frequency was 40 Hz, and the results are presented in Fig. 7.
Fig. 7. Output peak power (a), extraction efficiency and amplification (b) versus pump peak power under dual pumping at different seed laser peak power of the Er:YAG waveguide amplifier
It is apparent from the Fig. 7(a) that the output peak power increased almost linearly with the pump peak power at different seed laser power. As the peak power of the injected seed laser increased, the slope efficiency of the Er:YAG waveguide amplifier also gradually increased, and the maximum slope efficiency of 3.1% was obtained with the injection seed laser peak power of 184.4 W. When the total pump peak power was 32.01 kW, the waveguide amplifier obtained a maximum output peak power of 1.14kW corresponding to the average output power of 18.19 W. The extraction efficiency and amplification versus pump peak power at different seed laser peak power of the Er:YAG waveguide amplifier are presented in Fig. 7(b). The results show that with the injection seed laser peak power of 62.5 W, the amplification increased from 1.84 to 8.71 when the pump power increased from 3.89 kW to 32.01 kW. With the rise of pump peak power, the extraction efficiency of the Er:YAG waveguide amplifier gradually increased in the beginning, and then remained basically unchanged or dropped slightly. However, with the same pump power, the extraction efficiency of the Er:YAG waveguide amplifier gradually increased with the peak power of the injected seed laser. With the seed laser power of 62.5W, 138.1W and 184.4W, the corresponding extraction efficiency are 1.3% ∼1.6%, 2.6% ∼ 2.8% and 3.0%∼3.3%, respectively, while the simulated results shown in Fig. 2(b) are 1.1% ∼ 1.5%, 2.3% ∼ 2.7% and 3.0% ∼ 3.5%, which are very close to the experimental value. These results indicate that our theoretical simulation has an acceptable confidence level, thus we can expect that the amplifier extraction efficiency could be further improved by increasing the injection seed laser power according to the simulation.
Figure 8 illustrates the output peak power and extraction efficiency versus pump peak power under forward (same direction as the seed laser enters the waveguide) or backward (opposite to the direction in which the seed laser enters the waveguide) pumping of the Er:YAG waveguide amplifier at the injection seed laser peak power of 225 W (the pump pulse width of 400 µs and the repetition frequency of 25 Hz). The maximum output peak power of 850.2 W and 789 W was obtained at the pump peak power of 19.33 kW and 20.86 kW under backward and forward pumping respectively. It is obviously that the output peak power and extraction efficiency of the Er:YAG waveguide amplifier under backward pumping were higher than that of the forward pumping at same pump peak power, which is basically consistent with the simulation results. Therefore, simulation and experimental results showed that the backward pumping was more suitable than the forward pumping at lower seed laser intensity for single end pumping and single pass amplification.
Fig. 8. Output peak power and extraction efficiency versus pump peak power under forward or backward pumping of the Er:YAG waveguide amplifier
In order to measure the beam quality (M2) of the output beam of the waveguide amplifier, a cylindrical lens with a focal length of 20 mm was used to collimate the output beam, and then a spherical lens with a focal length of 200 mm was used to focus the output beam. As shown in Fig. 9, we recorded the laser beam size in fast and slow axis directions at a peak output power of 1.01 kW by the 90/10 knife method at different positions near the lens focus point [24], which was further fitted by the propagation equation as follow,
5. Conclusion
In conclusion, we demonstrate for the first time that Er:YAG planar waveguide laser amplifier is an effective method to obtain high power mid-infrared laser, through numerically and experimentally investigation of its output performance, and provide a theoretical basis for structure optimization of the Er-doped planar waveguide. When the total pump peak power was 32.01 kW, the Er:YAG planar waveguide amplifier obtained a maximum output peak power of 1.14 kW at the seed laser injection peak power of 184.4 W corresponding to the average power of 18.19 W under dual end pumping. In the future, the output power of the Er:YAG planar waveguide laser amplifier could be significantly improved by the seed injection peak power, and the beam quality could be improved by the optimum of the distribution of pump energy in fast and slow axis directions.
Funding
Military-civilian Integration Development Fund of China Academy of Engineering Physics (2021-JMRH-ZXKZHW).
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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