Orthogonally Polarized Pr:LLF Red Laser at 698 nm with Tunable Power Ratio

Abstract

A continuous-wave (CW) orthogonally polarized single-wavelength red laser (OPSRL) at 698 nm with a tunable power ratio within a wide range between the two polarized components was demonstrated using two Pr³⁺:LiLuF₄ (Pr:LLF) crystals for the first time. Through control of the waist location of the pump beam in the active media, the output power ratio of the two polarized components of the OPSRL could be adjusted. Under pumping by a 20 W, 444 nm InGaN laser diode (LD), a maximum total output power of 4.12 W was achieved with equal powers for both polarized components, corresponding to an optical conversion efficiency of 23.8% relative to the absorbed pump power. Moreover, by a type-II critical phase-matched (CPM) BBO crystal, a CW ultraviolet (UV) second-harmonic generation (SHG) at 349 nm was also obtained with a maximum output power of 723 mW. OPSRLs can penetrate deep tissues and demonstrate polarization-controlled interactions, and are used in bio-sensing and industrial cutting with minimal thermal distortion, etc. The dual-polarized capability of OPSRLs also supports multi-channel imaging and high-speed interferometry.

1. Introduction

Lasers operating at 698 nm play a crucial role not only in high-precision atomic clock research [1] but also in non-destructive, real-time potato quality assessment, including portable measurement applications [2]. Orthogonally polarized single-wavelength lasers (OPSLs) have been widely used in laser display, interference imaging, precision measurement, THz-wave generation, etc. [3,4,5,6,7]. In particular, OPSLs in the visible spectrum range have important application prospects. For instance, CW high-power UV SHG lasers can be achieved by type-II CPM technology [8,9,10,11,12]. Additionally, an OPSRL at 698 nm holds potential for dual-polarization bio-sensing and precision material processing due to its deep tissue penetration and polarization-controlled interactions. All-solid-state trivalent rare-earth ion (RE³⁺)-doped lasers with orthogonal polarization can be generated either via Stark-level transitions within a single active medium or through two birefringent crystals whose optical axes are orthogonally oriented. When using a single active medium, it is necessary to insert loss elements into the cavity to suppress gain competition between the two polarization components, such as etalon [13,14,15], a birefringence filter [16,17], and a polarization beam splitter [18,19]. These elements result in significant losses, substantially reducing conversion efficiency and complicating laser system design. Furthermore, the single active medium cannot achieve OPSL operation, limiting its use in homodyne detection, which requires frequency-difference-independent measurement ranges and minimized nonlinear errors [20,21]. One of the other challenges of these OPSLs is the adjustability of the power ratio between the two polarized components. The use of two active media can overcome the above-mentioned problems, and it can avoid gain competition by adjusting the power ratio between the two laser transitions over a wide range.

Pr³⁺:LiYF₄ (Pr:YLF) is the most excellent active medium in the visible emission range due to its high luminescence efficiency compared with other RE³⁺-doped laser crystals [22,23,24,25,26,27,28]. Pr:LLF is also a promising gain medium due to its higher density and improved thermomechanical properties, which originate from the heavier Lu³⁺ ions in the host lattice, compared to Pr:YLF. These properties result in enhanced thermal conductivity, reduced thermal lensing effects, and greater stability under high-power laser operation [29]. Although Pr:LLF and Pr:YLF have the same crystal structure, resulting in similar spectral characteristics, the superior thermal management of Pr:LLF makes it more suitable for demanding applications that require efficient heat dissipation [30]. Pr³⁺ can provide different transition spectra in the visible spectral range, mainly including green (³P₁ → ³H₅ transition/523 nm), orange (³P₀ → ³H₆ transition/607 nm), red (³P₀ → ³F₂ transition/640 nm), and deep-red (³P₀ → ³F₃ transition/698 nm, ³P₀ → ³F₄ transition/720 nm) regions, and the corresponding energy level structures are shown in Figure 1. Visible single-wavelength (SW) emissions at 523, 607, 640, and 722 nm and dual-wavelength (DW) emissions at 546/550 nm and 607/640 nm in Pr:LLF have been reported [13,31,32]. However, to the best of our knowledge, the deep-red laser at 698 nm in the ³P₀ → ³F₃ transition in Pr:LLF has not been reported until now.

In this work, an OPSRL at 698 nm with a tunable power ratio within a wide range between the two polarized components was achieved using two Pr:LLF crystals. Under 17.3 W absorbed pump power at 444 nm, the CW total power output reached 4.12 W for both polarized components. In addition, by a type-II CPM BBO, a CW UV SHG at 349 nm was also obtained with a maximum power output of 723 mW, which can be applied in high-precision semiconductor inspection and biomedical applications, such as Raman spectroscopy, flow cytometry, and confocal microscopy, as well as in advanced material processing including the micro-machining of polymers, ceramics, and optical components, where sub-micron resolution and cold ablation are crucial for minimizing the heat-affected zone [33,34].

2. Experimental Setup

The schematic setup of the OPSRL at 698 nm is shown in Figure 2. The pumping source was an InGaN LD (LSR444SD, Laser Components), providing a linearly polarized beam at 444 nm and a maximum output power of 20 W with a beam quality factor (M²) of 46. To make the absorption of the two laser crystals more balanced, the polarized direction of the pump beam was at a 45° angle to the c-axis of both laser crystals. A lens (L) of 50 mm focal length with anti-reflection (AR) at 444 nm focused the collimated pump beam onto the active media. A plane mirror was utilized as the input coupler of the resonator (M₁), which was AR-coated at 444, 600–650, and 721 nm, with high reflectivity (HR) at 698 and 349 nm. The active medium was a combination of two a-cut and 0.1% doped Pr:LLF crystals (AM₁ and AM₂), which was AR-coated at 349, 444, and 698 nm. The lengths of AM₁ and AM₂ were 4 mm and 10 mm, respectively, and both featured a 3 × 3 mm² cross-section. The gap between AM₁ and AM₂ was approximately 0.1 mm. Both active media were encapsulated in indium foil and installed on a 15° water-cooled copper radiator. The c-axes of AM₁ and AM₂ were perpendicular to each other, so the π-polarized emissions at 698 nm produced by the two active media were orthogonal. A plano-concave output coupler (M₂, radius of curvature = −500 mm) was AR-coated for 600–650 nm and 721 nm, with a transmittance of 3.2% at 698 nm. Output couplers with 1.5%, 3.2% (M₂) and 5.0% transmittance were utilized, and M₂ yielded optimal laser output.

3. Results and Discussion

Based on a four-level laser system with dual active media, the output power (P_out,i) for each polarized component in a CW OPSRL can be given by [35]

$$P_{out,i}={\displaystyle \frac{A_i{T}_{oc}{hv}_i}{2{\sigma }_i}}\left({\displaystyle \frac{2{\sigma }_i{N}_i{W}_i{l}_i}{L_i}}-W_i-{\displaystyle \frac{1}{{\tau }_i}}\right),$$

(1)

where i = 1 and 2 denote S- and P-polarized components (S- and P-wave), respectively. T_oc is the output coupler transmittance, hν_i is the energy of the laser photon, σ_i is the cross-section for stimulated emission, N_i is Pr³⁺ density (ions/cm³), l_i is the active medium length, τ_i is the radiation lifetime of the upper level, and L_i is the cavity round-trip loss. A_i is governed by the thermal effects of the two active media, with its value derivable through ABCD matrix computations. The focal length (f_i) of the thermal lens of each laser crystal can be written as [36]

$${\displaystyle \frac{1}{f_i}}={\displaystyle \frac{{\xi }_i{P}_i{\alpha }_i{\left(dn/dT\right)}_i}{{\pi K}_{c,i}}}\int {\displaystyle \frac{exp(-{\alpha }_{i}z)}{{\omega }_p^2\left(z\right)}}dz,$$

(2)

where ξ_i is the heat conversion coefficient, P_i is the pump power incident at the AM₁ entrance face, α_i is the absorption coefficient—thus, $P_1=P$ and $P_2=P·\mathrm{e}\mathrm{x}\mathrm{p}(-{\alpha }_1{l}_1)$—P is the pump power, (dn/dT)_i is the thermo-optic coefficient, K_c,i is the thermal conductivity, and ${\omega }_p\left(z\right)$ is the pump beam size in the active media, which can be given by

$${\omega }_p^2\left(z\right)={\omega }_{p0}^2\left\{1+{\left[{\displaystyle \frac{{\lambda }_p{M}_p^2}{{n_i\pi \omega }_{p0}^2}}\left(z-z_0\right)\right]}^2\right\},$$

(3)

where z₀ is the pump beam waist location, and the location where z₀ = 0 is located is in the gap between the two active media. When z is negative, the pump light is focused inside AM₁. Conversely, when z is positive, the pump light focuses inside AM₂. ω_p0, λ_p, and Μ² are the waist radius, wavelength, and quality factor of the pump beam, respectively, and n_i is the index of refraction of the active medium. W_i is the pump parameter, which can be obtained from [35]

$$W_i={\displaystyle \frac{{\eta }_i{P}_i{\alpha }_i}{{h{v}_{p}N}_i{l}_i}}\int {\displaystyle \frac{exp(-{\alpha }_{i}z)}{{\pi \omega }_p^2\left(z\right)}}dz,$$

(4)

where η_i is the quantum efficiency and hν_p is the energy of the pump photon. With Equations (1)–(4) and the parameters in the experiment—T_oc = 0.035, hν_i = 2.87 × 10⁻¹⁹ J, σ_i = 5.0 × 10⁻²⁰ cm², l₁ = 5 mm, l₂ = 12 mm, L_i = 0.6%, τ_i = 37.9 μs, ξ_i = 36.4%, P = 20 W, α_i = 1.44 cm⁻¹, λ_p = 444.2 nm, M² = 46, n_i = 1.45, ω_p0 = 200 μm, η_i = 0.64, hν_p = 4.51 × 10⁻¹⁹ J, and N_i = 1.44 × 10¹⁹/cm³—the output powers of the two polarized components were calculated as a function of the waist location of the pump beam, as shown in Figure 3. It can be observed in Figure 3 that the power of the S-polarized component first increased monotonically with the waist location of the pump beam, reaching its maximum at z = −2.0 mm, and then decreased monotonically. On the other hand, the power of the P-polarized component increased monotonically with the waist location of the pump beam, peaking at z = 5.0 mm before monotonically decreasing. It was also found that an intersection point existed between the two curves, indicating that at z = 1.5 mm, the output power of the two components reached equilibrium. It should be noted that the total output power reached its maximum when the power levels of the two polarized components were approximately equal. The movement of the pump beam waist in either direction from this point resulted in a gradual decrease in total output power, since optimal excitation of the two active media occurred exclusively at this location. At other locations, only one active medium was efficiently excited for lasing.

Figure 4 shows the experimentally obtained output power of the OPSRL against the waist location of the pump beam under a pump power of 20 W. The absorption efficiency of AM₁ was 43.8%, resulting in an absorption of 8.8 W. The remaining power of 11.2 W was absorbed by AM₂, with an absorption efficiency of 76.3%, yielding an absorption of 8.5 W. A peak total power output of 4.12 W was produced at z = 1.5 mm, with each polarized component measured at 2.06 W, yielding a total optical conversion efficiency of 23.8% relative to the absorbed power. The measured experimental data were in good agreement with the theoretical fitting curve in Figure 3. The output powers of the two polarized components versus the absorbed pump power for different waist locations of the pump beam were studied, as shown in Figure 4. The OPSRL was separated using a polarization beam splitter, and the output power of each polarized component was measured separately. Figure 4a shows the output powers of the two polarized components versus the absorbed pump power at z = 1.5 mm. The experimental data indicated that the output powers of P- and S-waves remained relatively close across all pump power levels. This phenomenon was attributed to the two active media reaching nearly identical excitation levels. At an absorbed pump power of 17.3 W, the total output powers of the two polarized components reached 4.12 W with a corresponding total slope efficiency of 26.9% with respect to the absorbed pump power. The laser spectrum of the OPSRL was measured using a spectrum analyzer with a resolution of 0.01 nm (Ocean Optics, HR2, Orlando, FL, USA), as shown in the inset of Figure 4a. The linewidth was 0.33 nm (FWHM) with a peak wavelength of 698.4 nm.

The total power stability of the OPSRL was measured with a power meter (FieldMate, Coherent Inc., Saxonburg, PA, USA) at maximum output power, exhibiting a fluctuation of less than 2.8% (RMS) over a one-hour period, as shown in Figure 5. The X- and Y-axis beam radii of the OPSRL were measured, as shown in Figure 6. The M² factors of the OPSRL were measured, yielding ${\mathrm{M}}_x^2=1.18$ and ${\mathrm{M}}_y^2=1.23$. The OPSRL profile is also shown in the inset of Figure 6, which is nearly distributed as a Gaussian function. Figure 4b shows the output characteristics of the two polarized components at z = −2.0 mm. It can be seen that the threshold power of the P-wave exceeds 4.5 W, and its slope efficiency is significantly lower compared to the S-wave. A balance between the two polarized components could not be achieved at any pump power level. This was because only AM₁ had been efficiently excited at z = −2.0 mm. At an absorbed pump power of 17.3 W, the total output powers of the two polarized components reached 3.40 W (2.46 W at S-wave and 0.94 W at P-wave) with a corresponding total slope efficiency of 23.9% with respect to the absorbed pump power. Although the power output of the two polarized components cannot be balanced at any pump power, it can achieve a larger power ratio. The M² factors of the S- and P-waves, measured via the knife-edge technique at maximum output power, were 1.16 and 1.21, respectively.

A folded V-shaped resonator was employed to achieve an efficient UV SFG, illustrated in Figure 2b. The pump source, input coupler (M₁), and active media (AM₁ and AM₂) are the same as those in Figure 2a. To ensure uniform excitation across both active media, the waist location of the pump beam was set at z = 1.5 mm. The completely reflecting reflector (M₃) was a concave mirror with a −200 radius of curvature, which was coated with HR at 698 and 349 nm, and AR at 600–650 and 721 nm. The UV output coupler (M₄) was a concave mirror with a -50 radius of curvature, which was coated with HR at 698 nm and AR at 349, 600–650, and 721 nm. A type-II CPM cut BBO crystal (θ = 49.7° with d_eff = 0.835 pm/V) served as the SHG crystal. The UV output performance is shown in Figure 7. The maximum UV power reached 723 mW with a threshold of 1.12 W. The UV laser spectrum at 349 nm is shown in the inset of Figure 7. The linewidth was 0.28 nm with a peak wavelength of 349.2 nm. The UV power stability was 2.1% (Figure 5). The UV laser beam radii along the X- and Y-axes versus the transmission distance are shown in Figure 8. The M² factors in the two directions were 1.22 (X-axis) and 1.31 (Y-axis), respectively. The profile of the UV beam is also shown in the inset of Figure 8. The spectrum of the UV laser at the maximum pump power is shown in the inset of Figure 6. The linewidth was 0.25 nm with a peak wavelength of 349.2 nm.

4. Conclusions

An OPSRL at 698 nm with a tunable power ratio within a wide range between the two polarized components was demonstrated using two Pr:LLF crystals. Through control of the location of the pump beam waist in the two active media, the power ratio of the two polarized components of the OPSRL could be adjusted. Under the pumping of an InGaN LD of 20 W at 444 nm, the CW total output power of the two polarized components reached 4.12 W. The corresponding optical conversion and slope efficiencies with respect to the absorbed pump power were 23.8% and 26.9%, respectively. Moreover, by a type-II CPM BBO crystal, a CW UV SHG at 349 nm was also obtained with a maximum power output of 723 mW. OPSRLs can penetrate deep tissues and demonstrate polarization-controlled interactions, and are used in bio-sensing and industrial cutting with minimal thermal distortion, etc. The dual-polarized capability of OPSRLs also supports multi-channel imaging and high-speed interferometry. In addition, the UV laser obtained through frequency doubling can be applied in fields such as high-precision semiconductor detection, biomedicine, and advanced material processing. This is the first time that an SW laser output with orthogonal polarization has been achieved in the visible region. Due to the simpler setup and higher laser output efficiency, it opens a pathway for a high-power SW visible laser emission path with orthogonal polarization. We believe that the method presented in this paper has the potential to be extended to other laser crystals for obtaining tunable SW and DW lasers.