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Abstract
Yellow lasers at 590 nm have many extensive applications in our daily life, but extremely difficult to attain by traditional solid-state laser technology, owing to the absence of highly-efficient transition channels at this spectral range. In this work, we proposed a cooperative lasing mechanism to obtain the yellow light emission, with multiphonon-assisted electronic transitions and phase-matched frequency-doubling. Based on the predictable configurational coordinate model, we can calculate the multiphonon-assisted emission step-by-step. Using Yb3+-doped La2CaB10O19 crystal as an example, it is capable of producing yellow laser at 581–590 nm, with a maximum output power of 4.83 W and a high slope efficiency of 31.6%. To the best of our knowledge, it represents the highest power of solid-state yellow laser realized in one single crystal pumped by a laser diode. This power scaling can be assigned to the amplified phonon-assisted emission beyond the fluorescence spectrum, and optimized crystal angle for phase-matching condition. Such a compact, low-cost, and high-power laser device, provides an alternative candidate for the spectral “yellow-gap” where no practical solid-state laser exists at present.
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1. Introduction
Laser, first realized by Maiman in 1960, was one of the greatest inventions in twentieth century [1]. The first ruby laser is red light with a central wavelength at 694.3 nm, corresponding to a photon energy of 1.79 eV. According to the stimulated radiation theory proposed by Einstein [2], this photon energy is determined by the difference between laser up-level and low-level. Since the first ruby laser, there have been many active materials for laser emission, with the spectral range covering from ultraviolet, visible, to near-infrared region. If no practical laser source exists, nonlinear frequency conversion with various nonlinear optical crystals is helpful to fill the spectral gap with high efficiency and good wavelength tunability. This establishes a basic paradigm for laser generation and wavelength extension in traditional laser community.
Yellow laser at the wavelength of 560–600 nm, is an indispensable light for sodium guide star, optical coherence tomography (OCT), retinal photocoagulation, and the treatment of dermatology hemangioma, etc [3,4]. However, yellow lasers are not easily available at present. In 1970s, dye yellow lasers are dominant, but suffering from large size and toxic dyestuff. Solid-state diode lasers are very convenient to obtain blue and red lasers, but exist a “yellow-gap” due to the absence of semiconductor materials with suitable bandgap [5]. To date, only a few yellow diode lasers were reported with several milliwatt power [6,7]. Diode-pumped yellow lasers were realized in some active materials, including Dy3+-doped crystals and Tb3+-doped fibers [8,9]. Nevertheless, their overall conversion efficiencies were very low and the highest output power was only 500 mW [10]. In order to attain high-power yellow lasers, nonlinear optical technology was applied in recent years [11,12]. For example, a 86-W yellow laser at 589 nm was obtained using sum frequency generation from two Nd:YAG crystals, lasing at 1064 and 1319 nm [13]. A 20-W yellow laser at 588 nm was demonstrated in intracavity frequency-doubled vertical-external-cavity surface-emitting laser (VECSEL) [14]. In addition, a 7.9-W yellow laser at 588 nm was realized in intracavity frequency-doubled self-Raman Nd:YVO4 laser [15–17]. Recently, a high-power widely-tunable diamond Raman yellow laser was also reported [12]. These yellow laser sources are very useful for high-power scenarios, but suffer from complex setups, high-cost, and the bulky size of laser devices. Therefore, it is still a great challenge to design an all-solid-state, compact, low-cost, and high-power yellow laser source.
Returning to the stimulated radiation theory, the biggest obstacle for yellow laser is the absence of strong electronic transitions at this spectral range. In realistic materials, the doping active ions would interact with their surrounding lattices, and some new transition channels can be created associated with electron-phonon coupling, namely vibronic emission [18,19]. In this condition, phonon acts as an intermediary for energy exchange, that the emitting photon energy could be modulated with the creation, or annihilation, of coherent phonons simultaneously. Especially in multiphonon-coupling process, some unprecedented lasing beyond the inherent fluorescent spectrum can be designed. Based on this novel lasing paradigm, we demonstrated multiphonon-assisted lasing in Yb:YCa4O(BO3)3 (Yb:YCOB) and Yb:La2CaB10O19 (Yb:LCB) crystals [20,21], with a tunable wavelength in 1120–1200 nm [22,23]. Therefore, if we combine the multiphonon-coupling and frequency-doubling in one crystal, a compact high-power yellow laser, can be theoretically designed and technically realized.
Luckily, Yb:LCB is a non-centrosymmetric laser crystal with self-frequency-doubling capacity. In addition, its electron-phonon coupling intensity is sufficiently strong to support multiphonon-lasing, as well as frequency-doubled yellow lasing with high efficiency. First, we give a quantitative description of configuration coordinate model for Yb:LCB crystal, which is helpful to predict the phonon-assisted emission. Then, we calculate the phase-matching condition to make the optimal frequency-doubling conversion for yellow light. Via designing a monolithic coated crystal, it is capable of producing a yellow laser at 581 –590 nm, with a maximum power of 4.83 W and a high slope efficiency of 31.6%. To our best knowledge, this is the highest power of optically-pumped yellow laser, surpassing all Dy3+ and Tb3+-based yellow lasers pumped by the blue diode.
2. Theoretical analysis and calculation
First, the room temperature fluorescence spectra of 15at.% Yb:LCB crystal was collected by Edinburgh spectrometer (FLS-980). The measured sample thickness is 1 mm, which is helpful to reduce the reabsorption of Yb3+ ion. As shown in Fig. 1(a), Yb:LCB exhibits a broadband fluorescence spectrum from 920–1070 nm. Four fluorescent peaks at 976, 994, 1030, 1041 nm can be assigned to the Stark-splitting of 2F7/2 level. The zero-phonon line locates at 976 nm. In addition, there is a long fluorescence trialing beyond 1050 nm, extending to 1100 nm. This case can be explained by the spontaneous phonon-assisted vibronic emission with lattice relaxation [24]. However, at the long-wavelength spectral range beyond 1100 nm, there is no spontaneous fluorescence emission. Therefore, we cannot calculate the gain cross-section beyond 1100 nm based on the traditional ‘inside-fluorescence’ prejudge. Some new lasing mechanism needs to be developed.
Fig. 1. Multiphonon-coupling mechanism and frequency-doubling properties in Yb:LCB crystal. (a) The fluorescence spectrum of Yb:LCB crystal in 900–1300 nm. (b) Configurational coordination model of multiphonon-coupled laser. ΔQ is lattice displacement. The pale-orange region represents the spontaneous vibronic fluorescent region. (c) The type-I and type-II frequency-doubling phase-matching angle calculations of Yb:LCB at various wavelengths.
In order to calculate the wavelength of multiphonon-assisted laser, we give an analytical configurational coordinate of Yb:LCB crystal [see Fig. 1(b)]. Two basic physical parameters, Huang-Rhys factor S and average phonon energy , were used in this calculation [24]. The S parameter is fitted to be 0.996 from fluorescence spectrum. The $\hbar {\omega _p}$ is 379 cm-1 derived from in-situ Raman spectrum [21]. The lattice relaxation energy $S\hbar {\omega _p}$ is calculated to 378 cm-1. The lattice displacement ΔQ is estimated by ΔQ = $\sqrt {2S} $ [25]. The zero-phonon line of Yb:LCB locates at 976 nm, corresponding to an energy level difference of 10246 cm-1. This is also the pump wavelength of laser diode, see black solid arrow shown in Fig. 1(b). The long-wavelength photoluminescent edge set as 1100 nm, corresponding to an energy difference of 9091 cm-1.
In theory, the potential of ground state (GS) and excited state (ES) were plotted as two parabolas. With an incident pump photon, one electron at the GS is excited to high-energy ES state, and then relaxed to the center of ES with non-radiative transition. Then, it jumps back to the GS with a strong fluorescence emission. This is the traditional vibronic emission. Benefiting from strong electron-phonon coupling effect, there is a wide vibronic fluorescence spectrum, as plotted by pale-orange region in Fig. 1(b).
However, the multiphonon-coupling differs from traditional vibronic emission. The excited electron at ES can absorb the energy of quantized lattice vibrations, and then move to high-energy electron-phonon coupling states. At those high-energy states, electron jumps down to GS with weak photon emission at longer wavelength. Meanwhile, electron will return back to the center of GS with nonradiative electron-phonon coupling process, associated with the creation of phonons. In a complete cycle, there are some coherent phonons created in the multiphonon-coupling process, and the emitting photon energy was reduced consequently. As a result, the multiphonon-coupling emission can be modulated by the involved phonon numbers step-by-step. In theory, multiphonon-coupling effect is not spontaneous and multiphonon-assisted emission cannot be observed in conventional fluorescent spectrum. However, in a lasing cavity, it become possible by suppressing conventional laser oscillation and amplifying the weak multiphonon-assisted transitions. Therefore, based on this configurational coordinate, it become possible to design and create some unprecedented lasers with extended wavelengths by multiphonon-coupling effect.
On the other hand, to get the maximum frequency-doubling conversion, the crystal angle of Yb:LCB needs to be designed along the phase-matching direction. The refractive index of LCB crystal has been measured over the full transmission range. The Sellmeier's equations were refined by J. Zhang et al [26]. Then, we can calculate the theoretical phase-matching curve for second-harmonic generation (SHG) at different wavelengths. Figure 1(c) depicts the phase-matching angles in the XZ, YZ and XY plane, showing that the shortest SHG wavelengths of LCB crystals are 289 and 403 nm for type-I and type-II phase-matching condition, respectively. Therefore, Yb:LCB can support the self-frequency-doubled yellow lasing at 580 - 590 nm.
Yb:LCB is a biaxial nonlinear optical crystal with C2 space group. It has four non-zero independent second-harmonic generation (SHG) coefficients, i.e. d14, d21, d22, and d23, respectively. According to the previous study [26], they are 0.70, 0.58, -1.04 and 0.25 pm/V, respectively. In type-I phase-matching condition, the effective nonlinear coefficient deff = d21cos2θ+d23sin2θ-d14sin2θ in the XZ plane. For 1160 nm and 1180 nm SHG, the angle of Yb:LCB is calculated to be (θ = 34.8°, φ = 0°) and (θ = 34.6°, φ = 0°), respectively. At this time, the deff is about 0.18 pm/V. Benefiting from multiphonon-assisted emission and designed phase-matching angle, a high-power yellow laser at 580–590 nm can be expected in laser experiments.
3. Experimental setup
Then, we design a compact multiphonon-coupling Yb:LCB yellow laser, as shown in Fig. 2(a). The pump source is a fiber-coupled InGaAs diode laser with the center wavelength of 976 nm. The fiber diameter is 105 µm. The pump light was focused into crystal by an imaging unit with two different beam compression ratios (1:2 and 1:1), so the beam waist diameter in the crystal is 210 µm or 105 µm. A 15at.% Yb3+-doped LCB crystal was utilized in laser experiments, cut along (θ = 34.6°, φ = 0°) as the type-I phase-matching direction. The crystal dimensions are 3 × 3 × 6 mm3 and 3 × 3 mm front/end faces were polished and coated. The pump absorption ratio of Yb:LCB crystal was 45%.
Fig. 2. Experimental scheme of Yb:LCB yellow laser. (a) Multiphonon-coupling yellow laser setup, (b) Photograph of ten coated Yb:LCB laser devices.
The coating parameters for input and output mirrors are very important for realizing yellow lasers. Figure 1(a) shows that there is no effective emission at 1140–1200 nm, which is overwhelmed by much stronger spontaneous processes such as pure electronic emission at 1030 and 1041 nm. Therefore, in order to make yellow light actually lasing, conventional lasing inside fluorescent range must be suppressed, and weak vibronic emissions aside 1100 nm are amplified significantly. In our setup, a plane-plane monolithic cavity is applied in Yb:LCB laser. Both input mirror and output coupler are coated on the front and end faces of Yb:LCB crystal directly. The input mirror M1 is coated with high-transmission at 970–980 nm (T > 95%) and 1000–1100 nm (T > 90%) and high-reflection (HR, R > 99.9%) at 1120–1200 nm and 560–600 nm. The output mirror M2 is HT-coated at 1000–1100 nm and HR-coated at 1120–1200 nm (T ∼ 0.1%). In this setup, the stimulated radiation for pure electronic transitions was totally suppressed, and phonon-assisted transitions beyond 1100 nm were amplified simultaneously. The transmittance of M2 is 90% at 560–600 nm, which is favorable for improving yellow laser power. In addition, M2 is also HR-coated at 976 nm to increase the absorption of the pump light. The coated Yb:LCB crystals were depicted in Fig. 2(b).
Compared with conventional lasing at 1030 nm, the quantum defect of multiphonon-assisted lasing at 1140–1200 nm increases by 13%. Therefore, the thermal management is critical for realizing stable laser operation. To remove the waste heat in laser operation, Yb:LCB crystal was wrapped with indium foil and mounted in a copper block. The water-cooling temperature set as 5 °C.
A filter was placed behind the Yb:LCB crystal, which is HR-coated at 976 nm to reflect the residual pump light. The output power of yellow laser was collected by a power meter (Newport, Model 843-R). The output laser spectrum was recorded by two different spectrometers, yellow laser wavelength by Ocean Optics, HR4000, and near-infrared fundamental wavelengths by A.P.E.-WaveScan, S/N S09668.
4. Results and discussion
4.1 High-power yellow laser in Yb:LCB crystal
Based on the rational coating, we successfully realized yellow laser generation at 580-590 nm. It was a continuous-wave laser. The fundamental laser spectra and frequency-doubled laser spectra were plotted in Figs. 3(a) and 3(b). At the low pump power above threshold, the fundamental laser wavelength of Yb:LCB locates at 1162 nm, and the yellow light locates at 581 nm. With the increasing pump power, the laser gradually shifts to longer wavelength. With the absorbed pump power increasing from 2 W to 10 W, the laser wavelength shifts from 1162 nm to 1180 nm, and yellow laser shifts from 581 to 590 nm concurrently. The experimental photographs were depicted in Fig. 3(c), that the laser line gradually change from yellow, to golden-yellow, and finally orange light. According to our theoretical calculations, such a wide laser spectrum can be attributed to the cooperative contribution of electron broadening and phonon dispersion concurrently [21].
Fig. 3. Laser wavelengths of yellow laser. (a, b) Fundamental wavelength and the corresponding yellow laser wavelength. (c) The photographs of SHG yellow laser at 581 nm and corresponding fundamental-wave laser at 1162 nm; 583 nm and 1166 nm; 590 nm and 1180 nm.
In addition, the wavelength shift could be assigned to a synergistic effect of temperature-dependent phonon frequency shifting and population on electron levels. As depicted in Fig. 4, we simulated the temperature distribution inside Yb:LCB crystal. The surface temperature of the crystal was same to water-cooling temperature, set as T0. The heat distribution generated inside the crystal during radiative transitions were obtained by the finite element method. When the cooling temperature was 278 K, the maximum temperature Tmax at the center of the front surface of the Yb:LCB crystal was 429.7 K. Along the light propagation direction, the maximum temperature difference ΔT at the center of the front surface and the end surface is 101.5 K. Such a large temperature gradient could modulate the density of phonon states and refractive index, thus being responsible for wavelength shifting from 581 to 590 nm.
Fig. 4. Temperature distribution in Yb:LCB crystal during multiphonon-assisted laser operation with the beam diameter of 105 µm and the absorbed pump power of 10 W.
Benefiting from amplified multiphonon-coupling and efficient frequency-doubling effect, we obtained a high-power yellow laser in Yb:LCB crystal. The output powers were displayed in Fig. 5. At a beam diameter of 210 µm, the output power of yellow laser was 2.36 W under the absorbed pump power of 11.56 W. At this time, the optical-to-optical efficiency is 20.4% and the corresponding slope efficiency is 22.8%. In addition, the output power of fundamental-wave laser is 0.14 W. To further improve output power, we reduce the pump beam waist and increase the pump power density. At a beam diameter of 105 µm, the highest output power of yellow laser reaches up to 4.83 W under the pump power of 16.52 W, with the optical-to-optical efficiency of 29.2% and the slope efficiency of 31.6%. The lasing threshold reduces to 0.56 W. At this time, the output power of fundamental-wave is 0.47 W. To our best knowledge, this is the highest power of yellow laser with optically-pumped solid-state lasers in one crystal.
Fig. 5. Multiphonon-coupling yellow laser power in Yb:LCB. (a) Pump light spot size wp = 210 µm. (b) Pump light spot size wp = 105 µm.
As shown in Fig. 6, we make a comprehensive comparison for solid-state yellow lasers with one single crystal ever reported [4,10,23,27–33]. For Dy3+/Tb3+-doped crystal, the maximum output power of 500 mW was obtained in Tb:LiLuF4 crystal with an overall efficiency of 25% [10]. In this work, the experimental results of Yb:LCB crystal is record high in all-solid-state laser materials, suggesting the great potential for power scaling of self-frequency-doubled yellow lasers with multiphonon-coupling effect.
4.2 Characteristics of yellow laser in Yb:LCB
The fundamental properties of yellow lasers in Yb:LCB crystal were investigated, including polarization, beam quality, and power stability. In Fig. 7(a), it is observed that the polarization directions of 1180 nm and 590 nm are perpendicular by each other, which is consistent with type-I phase-matching condition. In addition, the spatial profile of the laser beam at different output powers were also measured by a beam quality analyzer (MicronViewer 7290A), as shown in Fig. 7(b), suggesting a near-diffraction-limited TEM00 mode. The yellow lasers hold the good beam quality. When the beam radius is 105 µm, the beam quality factor at a high output power was Mx2 = 3.37 in the horizontal direction and My2 = 4.05 in the vertical direction [Fig. 7(c)]. These values are comparable to previous intracavity frequency-doubled self-Raman yellow lasers (M2 = 2.5 ∼ 3.0) [34,35]. Finally, we measured the power stability of the SFD laser at 4.7 W output power by power meter over a period of 30 minutes [Fig. 7(d)]. It can be seen that this yellow laser is very stable with power fluctuation less than 10%. This stable self-frequency doubling laser can be understood because the plane-plane cavity length is much smaller than thermal focal length [36,37]. Therefore, Yb:LCB crystal would be a very promising laser crystal with extended laser wavelengths in near-infrared and visible regime, by employing multiphonon-coupling and self-frequency doubling effect.
Fig. 7. Yellow laser performances in Yb:LCB crystal. (a) The polarization measurement of 1180 nm laser and 590 nm yellow laser. (b) The beam profiles of FW and yellow lasers at different output powers. (c) The beam quality of yellow laser at the output power of 4 W. (d) The output power stability of yellow laser.
5. Conclusion
This compact yellow laser obtained in Yb:LCB crystal is ready for many important medicine applications, including retinal photocoagulation, nevus flammeus therapy, capillary hemangioma, and so on. Besides yellow light, this multiphonon-coupled lasing can be extended into longer wavelengths at orange, red, and deep-red range. In previous report [21], we have demonstrated multiphonon-assisted lasing at 1234 nm, corresponding to five-phonon creation involved in electron-phonon coupling process. If we set the angle of Yb:LCB crystal at (θ = 33.9°, φ = 0°), an efficient orange laser at 617 nm can be designed. Moreover, red and deep-red laser beyond 650 nm should be possible with the combination of multiphonon-coupling and self-frequency doubling in Yb:LCB. Associated with the green emission at 521 nm [38], Yb:LCB crystal can support the visible laser covering green-lime-yellow-orange-red, indicating its great potential in multi-wavelength coherent sources and applications.
In summary, we design and realize the yellow laser in Yb:LCB crystal for the first time. The highest yellow laser power reaches up to 4.83 W, which is the highest one among all known diode-pumped solid-state lasers. The optical-to-optical efficiency is 29.2% and slope efficiency is 31.6%. This low-cost and compact yellow source is ready for possible applications in modern medicine and ophthalmic surgery. The potential of this work does not end there. Besides Yb:LCB and Yb:YCOB, multiphonon-coupling mechanism is universal for all solid-state laser materials. We believe some novel and intriguing laser wavelengths could be demonstrated in Nd:YAG, Ti:sapphire, and optical fibers.
Funding
National Key Research and Development Program of China (2021YFB3601504, 2023YFF0718801); National Natural Science Foundation of China (52025021, 52372010, 92163207); Future Plans of Young Scholars at Shandong University.
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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