Tunable single- and dual-wavelength lasers around 1.4 μm in Nd:LuGdAG crystal

Abstract

We present the first diode-pumped tunable single- and dual-wavelength (DW) laser operation near 1.4 μm spectral region in Nd:LuGdAG (Nd:LGAG) crystal on the ⁴F_3/2 → ⁴I_13/2 transition. Three distinct lasing wavelengths at 1414 nm, 1426 nm and 1437 nm were generated by adjusting a Lyot filter (LF) in the cavity, respectively. The maximum continuous-wave (CW) power output of 3.64 W at 1414 nm was attained under an absorbed pump power of 18.7 W, exhibiting a slope efficiency of 23.7% and optical conversion efficiency of 19.5%. Further, three pairs of DW lasers operating at 1414 nm and 1426 nm, 1414 nm and 1437 nm, 1426 nm and 1437 nm were also achieved, respectively. The DW operation at 1414 nm and 1437 nm yielded 2.82 W total CW output power, attaining 15.1% total optical conversion efficiency. Single- and DW lasers in the 1410–1440 nm spectral range have important application in fields such as optical communication and medicine.

1. Introduction

Nd-doped solid-state lasers predominantly employ three principal emission bands in the near-infrared spectrum: 0.9 μm corresponding to the three-level ⁴F_3/2 → ⁴I_9/2 transition, and four-level configurations at 1.1 μm (⁴F_3/2 → ⁴I_11/2) and 1.3/1.4 μm (⁴F_3/2 → ⁴I_13/2). Multiple Nd³⁺-doped crystals including Nd:YVO₄ [1,2], Nd:YAG [3–5], Nd:GdVO₄ [6,7], Nd:YLF [8,9], Nd:YAP [10,11], Nd:CALGO [12] and Nd:GSAG [13–15] have demonstrated solid-state laser functionality. Conventionally, the 1.3 μm emission band in Nd-doped crystals originates from the ⁴F_3/2 → ⁴I_13/2 transition. However, this splitting phenomenon arises from the crystal field splitting effect, which partitions the energy levels into multiple Stark sublevels. Exemplified by Nd:YAG, such crystal fields induce over a dozen distinct emission peaks within the ⁴F_3/2 → ⁴I_13/2 manifold, with corresponding 1.4 μm region emissions having been documented in multiple studies [16–19]. Laser sources operating near 1.4 μm exhibit inherent eye-safe characteristics, enabling their deployment in diverse technical domains including optical communications, coherent LIDAR systems, dermatological treatments, advanced laser medicine, and ophthalmic therapies [20–25]. Among neodymium-doped laser crystals, Nd:LGYAG has been widely adopted in solid-state lasers due to its excellent optical quality and weak thermal lensing effect [26,27]. In the case of Nd:LGAG, Lu³⁺ and Gd³⁺ ions substitute for the totality of the Y³⁺ ions of the Nd:YAG but with a proportion Lu³⁺and Gd³⁺ of about 30%. While The Nd:LGAG lasers at 1.1 [28], 0.9 [29] and 1.3 μm [30] have been implemented successfully in prior studies, systematic research on CW laser generation at 1.4 μm in the Nd:LGAG has not been reported until now. Fig 1 demonstrates the emission cross-section of the Nd:LGAG from 1250 nm to 1500 nm at room temperature on the ⁴F_3/2 → ⁴I_13/2 transition, which was calculated via the Füchtbauer-Ladenburg (F-L) formula [31]. It can be shown in Fig 1 that the strongest peak was 1332 nm. In addition, there were five peaks at 1316 nm, 1348 nm, 1414 nm, 1426 nm and 1437 nm.

Fig 1. Emission spectrum of the Nd:LGAG in 1250 −1500 nm.

In this study, we achieved three-wavelength tunability at 1414, 1426 and 1437 nm in Nd:LGAG on the ⁴F_3/2 → ⁴I_13/2 transition. Additionally, three pairs of DW tunability at 1414 nm and 1426 nm, 1414 nm and 1437 nm, 1426 nm and 1437 nm were also realized. Extensive implementation potential of DW laser systems has been identified across diverse technical domains including LIDAR systems [32], medical diagnostics [33], optical holography [34,35], precision spectral analysis [36], metrological sensing [37,38], nonlinear frequency conversion for UV/visible generation [39,40], and THz wave synthesis via difference frequency generation [41–43]. Especially, DW lasers around 1.4 μm enable simultaneous superficial epidermal heating and deep dermal collagen stimulation for non-invasive skin tightening and vascular coagulation, leveraging minimal thermal damage to surrounding tissues [44].

2. Experimental setup

The schematic diagram for the laser experiment was displayed in Fig 2. The pump system employs a 20 W 808 nm laser diode (LD) with a NA of 0.22 and a fiber core diameter of 400μm. The radius of the pump spot was about 200 μm in the active medium. The laser spot radius in the active medium was about 190 μm. Two identical coupling lenses (L₁ and L₂) with a focal length of 100 mm were utilized, featuring anti-reflection (AR) at 808 nm on both surfaces. The measured transmittance of the optical coupling system exceeded 98%. A Nd:LGAG (1.0 at.% doping, 6 mm length) functioned as the active medium with AR at 808 nm and 1410-1440 nm, which was sealed in indium foil and affixed to red copper mounts equipped with water cooling, maintained at 15°C.

Fig 2. Schematic setup for the laser experiment.

Inset: LF.

The cavity input coupler was a planar mirror (M₁) with AR for 808 nm and 1060–1350 nm, and high reflectivity (HR) at 1410–1440 nm. The cavity output coupler was a concave mirror (M₂) with a radius of curvature of –200 mm, a transmittance (T_oc) of 3.5% at 1410–1440 nm, and AR at 1060–1350 nm. Two other couplers (T_oc = 2.0% and 5.0%) were also carried out, with the M₂ demonstrated the optimal output performance. A quartz-based LF (4.0 mm thickness) was employed for wavelength tuning, positioned within the resonator at θ_B (Brewster angle) as depicted in Fig 2 inset. The tuning angle (α) was an angle between the optical axis of the LF (C) and the incident light projection on the LF surface.

3. Results and discussion

The single-pass transmittance for different wavelengths transmitted through the LF was expressed as [45]

$$T_{Lyot,i}=\text{1}-\frac{\text{4}{cos}^{\text{2}}\alpha {sin}^{\text{2}}{\theta }_B}{\text{1}-\text{4}{cos}^{\text{2}}\alpha {cos}^{\text{2}}{\theta }_B}(\text{1}-\frac{{cos}^{\text{2}}\alpha {sin}^{\text{2}}{\theta }_B}{\text{1}-{cos}^{\text{2}}\alpha {cos}^{\text{2}}{\theta }_B}){sin}^{\text{2}}\left(\frac{{\delta }_i}{\text{2}}\right),$$ (1)

where i = 1, 2 and 3 represents the 1414 nm, 1426 nm and 1437 nm three wavelengths, respectively, δ_i = 2πd(n_o–n_e)(1–cos²αcos²θ_B)/λ_isinθ_B is an optical phase difference, n_o and n_e are the refractive indices of o- and e-light, respectively, d is a thickness of the filter. With Eq. (1) and the parameters: n_o = 1.5443, n_e = 1.5534, d = 4 mm and θ_B = 57.2^o, the round-trip transmittance$(T_{Lyot,i}^{\text{2}})$ of the different laser wavelength (λ_i) was calculated as a function of the tuning angle as displayed in Fig 3. It can be shown from Fig 3 that $T_{Lyot,i}^{\text{2}}$ can be controlled by regulating the LF surface around its normal axis. Thus, tuning between emission wavelengths can be realized by controlling the LF.

Fig 3. Round-trip transmittance (T2 Lyot,i) versus the tuning angle (α).

When tuning angle was rotated to about 38^o, 39^o, and 40^o, the corresponding emissions at 1414 nm, 1426 nm, and 1437 nm were achieved, and their output-input performances were displayed in Fig 4. At an absorbed pump power of 18.7 W (corresponding to an incident power of 20 W) with T_oc = 3.5%, the laser demonstrated output powers of 3.64 W at 1414 nm, 3.01 W at 1426 nm, and 2.21 W at 1437 nm. The corresponding lasing thresholds were 2.70 W, 3.01 W and 3.21 W, with slope efficiencies of 23.7%, 19.4% and 14.5%, respectively. At T_oc= 2.0%, the laser demonstrated slope efficiencies of 18.3%, 15.2%, and 10.3% at 1414 nm, 1426 nm, and 1437 nm respectively, with corresponding threshold powers of 1.72 W, 2.12 W and 2.65 W. When T_oc was increased to 5.0%, the slope efficiencies changed to 17.4%, 14.6%, and 8.5%, while the threshold powers increased to 4.5 W, 5.7 W and 6.3 W for the respective wavelengths. The laser spectra at 1414 nm, 1426 nm, and 1437 nm at the maximum pumping were displayed in the Fig 5. The corresponding wavelength peaks (1415.85 nm, 1426.28 nm and 1436.94 nm) exhibited spectral line width (FWHM) values of 0.30 nm, 0.33 nm and 0.35 nm, respectively.

Fig 4. Output powers of the three single-wavelengths versus absorbed pump power.

Fig 5. Laser spectra of the three single-wavelengths.

The power stabilities of the three laser wavelengths were measured with a precision power meter. The power fluctuations (RMS) at the maximum output powers were about 2.7%, 3.6% and 3.9% in 1 hour, respectively, as shown in Fig 6. The insets (a)-(c) of Fig 6 show the measured radii and the beam quality factors (M²) of the 1414 nm, 1426 nm, and 1437 nm beams, respectively. The beam quality factors (M²) of the 1414 nm, 1426 nm, and 1437 nm wavelengths were measured using the knife-edge technique. The corresponding values in both transverse directions at maximum output power were less than 1.16, 1.12 and 1.25, respectively.

Fig 6. Power stabilities of the three laser wavelengths.

Insets (a), (b) and (c) show the X- and Y-axes radii as functions of Z-axis position for the 1414 nm, 1426 nm and 1437 nm beams, respectively.

For a four-level laser system operating with CW, the oscillation threshold of each emission wavelength in a DW operation was given by [46]

$$P_{tha,i}=\frac{-ln(\text{1}-T_{oc})+L_i{+L}_{\text{0}i}}{\text{2}{l}_c{\eta }_{q,i}}\frac{h{\nu }_p}{{\sigma }_i{\tau }_i}\frac{\text{1}}{\iiint r_p(r,z)s_i(r,z)d\upsilon },$$ (2)

where T_oc is the cavity transmittance for the laser emission wavelengths, $L_i=\text{1}-T_{Lyot,i}^{\text{2}}$ is the round-trip loss, which is caused by the LF, L_0i is the cavity round trip passive loss, l_c is the length of the Nd:LGAG, η_q,_i is the quantum efficiency, hν_p is the photon energy of the pump beam, σ_i is the stimulated emission cross-section, τ_i is the upper energy level lifetime, r_p(r,z) is the pump beam distribution of the normalized intensity in the Nd:LGAG, and s_i(r,z) is the cavity mode distribution of the normalized intensity for the emission wavelength. r_p(r,z) and s_i(r,z) can be written by [47], respectively,

$$r_p(r,z)=\frac{\alpha e^{-\alpha z}}{\pi {\omega }_p^{\text{2}}(z)(\text{1}-e^{-\alpha z})}\Theta ({\omega }_p^{\text{2}}(z)-r^{\text{2}}),$$ (3)

and

$$s_i(r,z)=\frac{\text{2}}{\pi {\omega }_{i\text{0}}^{\text{2}}{l}_c}exp\left(\frac{\text{2}{r}^{\text{2}}}{{\omega }_{i\text{0}}^{\text{2}}}\right),$$ (4)

where α is the absorption coefficient, $\Theta$is the Heaviside step function, ω_0i is the radius of the laser spot, and the size of the pump beam in the Nd:LGAG given by

$${\omega }_p^{\text{2}}(z)={\omega }_{p\text{0}}^{\text{2}}\{\text{1}+{\left[\frac{{\lambda }_p{M}_p^{\text{2}}}{{n\pi \omega }_{p\text{0}}^{\text{2}}}(z-z_{\text{0}})\right]}^{\text{2}}\},$$ (5)

where ω_p0 is the radius of the pump spot, λ_p is the pump wavelength, M2 pis the quality factor of the pump beam. With Eqs. (1)-(5) and the parameters in our experiment: T_oc = 3.5%, σ₁ = 1.04 × 10⁻²⁰ cm², σ₂ = 0.84 × 10⁻²⁰ cm², σ₃ = 0.98 × 10⁻²⁰ cm², α = 3.5 cm^-1, l_c = 5 mm, ω_p = 200 μm, ω_0i = 190 μm, n = 1.83, M² = 3.5,η_q,i ≈ 0.57, τ_i ≈ 262μs, hν_p = 2.45 × 10⁻¹⁹J, and L_0i=0.5% was measured using the Findlay-Clay method [48], the threshold was calculated as a function of tuning angle α for the three laser wavelengths, as displayed in Fig 7. It can be seen that the threshold can be controlled by regulating the LF. It can be observed that intersecting points exist between any two threshold power curves, indicating that the pump power required for lasing threshold is identical for both wavelengths at these intersection points. Consequently, DW lasers could be achieved when the pump powers were precisely regulated to these power levels.

Fig 7. Tuning angle versus the threshold power.

When α was regulated to about 3.5^o, 5.5^o, and 16.5^o, the three pairs of the DWs at 1414 nm and 1426 nm, 1414 nm and 1437 nm, and 1426 nm and 1437 nm were generated, respectively, and their output-input performances were displayed in Fig 8. At an absorbed power of 18.7 W, the total powers were 2.82 W (1.44 W at 1437 nm and 1.38 W at 1414 nm), 2.75 W (1.50 W at 1426 nm and 1.25 W at 1414 nm) and 2.12 W (1.06 W at 1426 nm and 0.96 W at 1437 nm) for the three pairs of DWs, respectively. The corresponding threshold powers were 3.58 W, 4.21 W and 4.72 W, respectively. The total optical conversion efficiencies with respect to the absorbed power were 15.1%, 14.7% and 11.3%, respectively. The laser spectra of the three pairs of DWs were displayed in the Fig 9. The corresponding peak wavelengths were 1413.83 nm and 1426.41 nm, 1413.95 nm and 1437.04 nm, 1426.44 nm and 1437.02 nm, respectively. For the three pairs of DWs, Their corresponding M² factors were 1.12 and 1.15, 1.18 and 1.24 and 1.22 and 1.27, respectively, and their power stabilities were about 2.5% and 2.9%, 2.8% and 3.8%, and 3.5% and 4.2%, respectively.

Fig 8. Output powers of DWs versus absorbed pump power.

Fig 9. Laser spectra of the three pairs of DWs.

Compared with the previously reported the 1.4 μm single-wavelength laser on Nd:GSAG crystal (slope efficiency of 13.6%, optical conversion efficiency of 11.5% [15]), the single-wavelength system in this study achieved a slope efficiency of 23.7% and an optical conversion efficiency of 19.5%. In terms of DW laser output, the total optical conversion efficiency has increased from 9.2% to 15.1%. These data fully demonstrate the significant progress of this laser system in the optical conversion efficiency at the 1.4 μm spectral region.

4. Conclusion

In conclusion, diode-pumped tunable single- and DW laser operation near 1.4 μm spectral region in Nd:LGAG on the ⁴F_3/2 → ⁴I_13/2 transition was demonstrated for the first time. By regulated an intracavity LF, the three single-wavelengths at 1414 nm, 1426 nm and 1437 nm were obtained, respectively. The maximum CWoutput power of 3.64 W at 1414 nm was attained under an absorbed pump power of 18.7 W, exhibiting a slope efficiency of 23.7% and optical conversion efficiency of 19.5%. Further, three pairs of DW lasers operating at 1414 nm and 1426 nm, 1414 nm and 1437 nm, and 1426 nm and 1437 nm were also achieved, respectively. The DW operation at 1414 nm and 1437 nm yielded 2.82 W total CW output power, attaining 15.1% total optical conversion efficiency. This study proposes a new method for generating tunable single- and DW lasers, which can be applied to other active medium to achieve laser output of different spectral regions.

Data Availability

All relevant data are within the manuscript.

Funding Statement

This work has been supported by the National Natural Science Foundation of China (Grant Nos. 62175209 and 62241506).