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Abstract
We report laser operation of two Tb3+-activated gain media, Tb:LiYF4 and LiTbF4, in yellow or/and green spectral region. A record-high slope efficiency of 63% among Tb3+-lasers and maximum output power of 1.17 W (incident power of 2.79 W) at around 544 nm were obtained with a c-cut 15%Tb:LiYF4 crystal. The yellow laser characteristics in σ-polarization were studied. A slope efficiency of 21% at 582 nm was achieved. More importantly, we succeeded in laser operation of LiTbF4 for the first time to the best of our knowledge. Laser oscillation at around 544 nm yielded a maximum slope efficiency of 45%. This points toward the possibility of producing high-energy pulsed lasers using LiTbF4, which features a high active-ion concentration as well as relatively long lifetime.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Driven by the appearance of more powerful blue-emitting semiconductor pump sources, lanthanide-activated lasers emitting directly in the visible spectral region have been rapidly developed in recent years owing to their advantages over the conventional frequency-converted visible lasers [1]. They are free from non-linear crystals, thus the cavity design is simple and the energy losses during the frequency-conversion operation are circumvented, which limit the total efficiency of frequency-converted lasers.
Tb3+ is a highlighted active ion for semiconductor pumped visible lasers. As is illustrated in Fig. 1, the upper laser level of 5D4 allows for in-band pumping from the ground state at around 485 nm and several visible emission transitions are derived from this manifold. Low non-radiative losses from the upper laser state are expected due to the absence of cross-relaxation channels and negligible multiphonon relaxation in common laser gain materials. Tb3+-activated lasers have been demonstrated for the green and yellow emission transitions in various fluoride host materials [2,3]. Laser operation in the green spectral region is the most efficient since the green emission transition is predominant. The yellow laser emitting at around 585 nm also attracts attention, not only because laser radiation in the yellow-orange spectral region (560-600 nm) is difficult to obtain by means of conventional frequency-conversion operation, but also due to their utility in many fields, such as medical treatment and flow cytometry [4,5]. It is reported that by applying lasers emitting at 560-600 nm for flow cytometry of red fluorescent proteins, much larger sensitivity indices could be achieved compared to the conventional 532-nm or 633-nm lasers, yet more powerful yellow laser sources are still in demand [6,7].
A critical drawback of Tb3+ as an active ion for visible lasers is the weak absorption cross-section of the spin-forbidden pump transition of 7F6→5D4, which is usually a few 10−22 cm2 in fluoride hosts. The weak absorption cross-section can be compensated by extending the length or/and increasing the active-ion concentration of the gain medium in order to enhance the integral absorption efficiency. The former scheme can maintain the doping concentration at a relatively low level, which usually means less non-radiative losses. The latter approach is feasible for Tb3+, as concentration quenching induced by cross-relaxation is not available from the 5D4 state. Spectroscopic studies reveal that the fluorescence decay curves from 5D4 still maintain a single-exponential behavior and the quantum efficiencies are relatively high in some highly concentrated Tb3+-based materials including LiTbF4 [8,9]. Nevertheless, a long gain medium would rely on pumping with near-Gaussian beams (otherwise the mode matching would be poor) and is not favorable for plane-parallel resonators that generally require a short cavity length. On the other hand, the short gain medium can be used for powerful fiber-coupled LD pumping, which is more economically favorable than the near-Gaussian beam sources, and allows for compact resonator designs.
Furthermore, Tb3+ is a good candidate for Q-switched laser applications owing to the long upper-state lifetime. It is known that the maximum output pulse energy of a Q-switched laser depends on the total useful energy stored in the medium [10]. An active medium with a high concentration of Tb3+ would thus be promising for producing high-power laser pulses via Q-switching. In fact, lasing in a highly concentrated matrix of TbF3 (202×1020 cm−3) was observed by C. Kränkel et al., whereas the maximum output power was limited to ∼20 mW [3].
In this paper, we study the visible laser performances of two Tb3+-based crystals, a 30-mm 15%Tb:LiYF4 (Tb:YLF) and a 6-mm LiTbF4 (TLF). This subject is motivated by the prospect of Tb3+-activated visible lasers and the research interest of lasing in a gain medium with high active-ion concentration. To be more specific, realizing laser operation in such a highly concentrated active medium is meaningful for extending the application regime of Tb3+-lasers and paves the road for further high-performance Q-switched lasers. The laser experiment began with the more conventional long crystal of Tb:YLF. A maximum slope efficiency of 63% was observed at high output transmittance and 1.17 W of green laser radiation could be extracted from the cavity in the best configuration. Moreover, we demonstrated yellow laser operation in σ-polarization in this matrix, yielding a maximum slope efficiency of 21% at around 582 nm. Tb3+-laser oscillation in TLF was then realized based on the setup for the Tb:YLF lasers. This is the first demonstration of lasing in TLF and the first detailed laser characterization of a highly concentrated Tb3+-based material to the best of our knowledge. A maximum slope efficiency of 45% was recorded at around 544 nm.
2. Experimental results and discussion
2.1 Experimental setup and spectroscopic properties
The two Tb3+-based crystals were commercially ordered and polished for the $({001} )$ planes (c-cut). The Tb:LYF (supplier: EKSMA Optics) has a nominal Tb3+ doping ratio of 15 at%, length of 30 mm along the c-axis, and anti-reflection coatings ($T > 97.5\%$, 450-680 nm) on both surfaces. However, no coatings were applied for the 6-mm TLF crystal. By passing a He-Ne laser beam through the crystals, we observed many more scattering centers in TLF than in Tb:YLF. This means the worse optical quality of TLF, probably due to the more difficult crystal growth process. A frequency-doubled optically pumped semiconductor laser (2ω-OPSL, Coherent Genesis CX-STM) was employed as pump source, which provided a near diffraction-limited beam (, beam radius = 1.1 mm, polarization ratio > 100:1) and maximum optical output of 3 W. The center wavel ${M^2} < 1.1$ ength of 2ω-OPSL was measured to be 488.0 nm (ADVANTEST Q8381A optical spectrum analyzer, spectral resolution 0.1 nm, also used for the other emission spectroscopic measurements in this study), which matches well with the peak wavelength of the pump transition (487.4 nm, σ-polarization, SHIMADZU UV-3600Plus spectrophotometer, spectral resolution 0.2 nm) in both materials. The absorption efficiency at the pump wavelength were calculated to be 54% and 62% for Tb:LYF and TLF, respectively, which are the same as the measured values at low pumping powers.
The spontaneous emission and laser spectra of Tb:YLF as well as TLF are presented in Fig. 2. The emission spectra of Tb:YLF and TLF are highly analogical since they are isostructural. The lasing wavelengths are consistent with the peak wavelengths of the respective emission transitions.
Fig. 2. Laser spectra of Tb:YLF and TLF at maximum output power. The emission cross-section values are taken from [11].
TLF features a large concentration of Tb3+ of 136×1020 cm−3 which may lead to severe luminescence quenching that thoroughly prohibit lasing. Although cross-relaxation channels do not exist from the upper laser level of 5D4, the excited population can terminate at quenching centers, such as impurity ions and lattice defects, via active resonance energy transfer process among the highly concentrated Tb3+ ions [12]. Therefore, it is necessary to characterize the fluorescence lifetime of 5D4 in this material. The decay curves were recorded by a Thorlabs DET36A/M Silicon Detector (rise time of 14 ns) under excitation of a 488-nm laser pulse with duration of ∼1 ms (Fig. 3). Through deconvolution from the excitation pulse and a single-exponential fit of the obtained decay curve, the fluorescence lifetime of TLF was derived to be 3.3 ms. A comparable value of 3.8 ms can be found in the Ref. [8]. A comparison of the fluorescence lifetime with the radiative lifetime reported in [8] results in a quantum efficiency of 66%. The fluorescence lifetime of 15%Tb:YLF was determined to be 4.9 ms, which is almost the same as that of 16%Tb:YLF reported in the literature (5.0 ms) [2]. It has been reported that the fluorescence lifetime of Tb3+-doped YLF almost maintains a constant of about 5 ms at doping level lower than 25% [13]. Thus, we can assume that the fluorescence lifetime of 15%Tb:YLF is close to its radiative lifetime.
The resonator setup for both gain materials are schematically shown in Fig. 4. In most cases, a pump focal length that results in comparable Rayleigh range with the crystal length was employed to obtain better pump efficiency. In this regard, $f = 500\;\textrm{mm}$ and $f = 200\;\textrm{mm}$, which give Rayleigh range of 34.5 mm and 5.5 mm according to ray transfer analysis, are suitable for Tb:YLF (30 mm) and TLF (6 mm), respectively. The gain crystals were water-cooled in a copper crystal holder at a constant water temperature of 12°C. A half waveplate was used to match the pump polarization to the crystallographic a-axis. The residual pump radiation was filtered by a longpass filter (> 500 nm).
2.2 Green laser operation of Tb:YLF at 544 nm
We first carried out laser experiments for the 5D4→7F5 transition using Tb:YLF. This transition peaks at around 544 nm in σ-polarization with emission cross-section of 1.9×10−21 cm2 (see also Fig. 2). The input mirror is AR coated ($T = 99\%$) at the pump wavelength and HR coated ($T < 0.2\%$) from 545 nm to 662 nm. Three output couplers with output transmittance (${T_{oc}}$) of 1.2%, 1.9%, and 22% at the lasing wavelength were available. They have the same radius of curvature of 75 mm. The output couplers with ${T_{oc}}$ of 1.2% and 1.9% are PR coated ($T < 2\%$) at the pump wavelength of 488.0 nm as well, hence a total absorption efficiency of ca. 76% could be obtained benefiting from the second-pass absorption.
The output characteristics of the Tb:YLF green lasers are presented in Fig. 5. A low laser threshold pump power (${P_{thres.}}$) of 140 mW was obtained at ${T_{oc}} = 1.2\%$. A rough Findlay-Clay analysis from the ${P_{thres.}}$ and ${T_{oc}}$ data results in round trip losses of ∼1.6% within the cavity. This value is quite reasonable considering the long active medium length of 30 mm. The optimal position of the focusing point of pump was found to be ca. 5 mm inside the gain medium and the optimized cavity length was found to be 74 mm. In this configuration, the beam diameters of the pump and laser beams inside the crystal were calculated to be around 77-86 µm and 68-86 µm, respectively. These values reveal an effective overlap between the pump and laser beam mode. The slope efficiency with respect to absorbed pump power (${\eta _{slope}}$) at ${T_{oc}} = 1.2\%$ was determined to be 45% while a higher ${\eta _{slope}}$ of 59% as well as a larger peak output power of 1.17 W can be fulfilled with the ${T_{oc}} = 1.9\%$ output coupler. At the maximum output power of the ${T_{oc}} = 1.9\%$ resonator, the optical-to-optical efficiency with respect to absorbed pump power (${\eta _{opt,abs}}$) was 55%. These results are overall comparable with the laser performance carried out by P. W. Metz et al. in similar conditions (4.7-mm 16%Tb:LiYF4 at ${T_{oc}} = 1.0\%$: ${P_{out}} = 158\textrm{ mW}$, ${\eta _{slope}} = 55\%$, and ${\eta _{opt,abs}} = 53\%$; 21-mm 28%Tb:LiLuF4 at ${T_{oc}} = 1.6\%$: ${P_{out}} = 1.13\;\textrm{W}$, ${\eta _{slope}} = 58\%$, and ${\eta _{opt,abs}} = 52\%$ at incident power less than 2 W) [2]. Nevertheless, the previously reported laser demonstration with the 28%Tb:LiLuF4 crystal suffered a thermal roll-over of the output efficiency at incident power larger than 2 W and the ${\eta _{opt.,abs}}$ declined to ∼44% at incident power of about 3 W, while thermal effects were not observed in our experiments. This can be accounted for by the larger pump mode radius in our setup (77 µm vs. 45 µm), since the effective thermal focal length is proportional to the second power of the pump beam radius [14]. The highest slope efficiency of 63% was realized at an output transmittance of 22%. This indicates that it is feasible to obtain even higher output efficiency at high pump power, if possible, with relatively large ${T_{oc}}$.
2.3 Yellow laser operation of Tb:YLF at 582 nm
The yellow emission transition of 5D4→7F5 in Tb:YLF gives a cross-section of 1.1×10−21 cm2 in π-polarization at 578.5 nm and 5.2×10−22 cm2 in σ-polarization at 581.9 nm [11]. Therefore, lasing at 581.9 nm can only be realized with a c-cut crystal. The input mirror used for the yellow laser experiment has $T > 80\%$ from 540 nm to 545 nm, which can suppress lasing in the green spectral region, and high reflectance ($R > 99.5\%$) from 575 nm to 640 nm. Unfortunately, the transmittance of the input mirror at the pump wavelength is 60%, thus the maximum available incident power was around 1.8W. The output mirrors are the same as those used for the green laser experiments, which are PR coated ($T = 1\sim 3\%$) from 400 nm to 660 nm.
Laser operation at 578.5 nm with an a-cut Tb:LiLuF4 crystal is documented, yielding a slope efficiency of 25% and maximum output power of 0.5 W [15]. Although the emission cross-section at 581.9 nm (σ) is about one-half as that at 578.5 nm (π), we obtained a comparable ${\eta _{slope}}$ of 21% at the output wavelength of 581.9 nm as is shown in Fig. 6. By decreasing the pump focal length to 250 mm, the pump beam radius was reduced from approximately 77 µm to 39 µm. This leads to a lower threshold pump power whereas smaller slope efficiency because of the less suitable Rayleigh range (8.6 mm for $f = 250\;\textrm{mm}$ vs. 34.5 mm for $f = 500\;\textrm{mm}$). Finally, a maximum output power of 106 mW was achieved in the $f = 250\;\textrm{mm}$ and ${T_{oc}} = 0.9\%$ configuration.
2.4 Green laser operation of TLF at 544 nm
The experimental setup for TLF is similar to that for the Tb:YLF green laser, except that a shorter pump focal length (200 mm) was applied. By taking into consideration the second-pass absorption, a total absorption efficiency of ca. 82% (single-pass absorption efficiency of 62%) could be obtained in this configuration. The input-output data of the TLF laser emitting at around 544 nm is exhibited in Fig. 7. Despite the fact that the pump beam radius was smaller than that in the Tb:YLF laser experiment (31 µm vs. 77 µm), TLF started to lase at a higher absorbed pump power (455 mW vs. 140 mW, ${T_{oc}} = 1.2\%$ for both). This can be assigned to the larger internal losses originating from the absence of AR coatings, the higher density of scattering centers, and the lower quantum efficiency compared to Tb:YLF.
Fig. 7. Laser output characteristics of LiTbF4 at around 544 nm. Notice that for comparison, the output powers of the 50% and 10% duty cycle were multiplied by a factor of 2 and 10, respectively.
Upon cw pumping, a slope efficiency of 36% was achieved. Nevertheless, the output power exhibited a significant roll-over at absorbed pump power over 1 W. Such a phenomenon could be eliminated via q-cw pumping with a chopper (500 Hz), thus it could be attributed to thermal effects. By applying q-cw pumping with a duty cycle of 10%, the ${\eta _{slope}}$ was increased to 45% and ${P_{thres.}}$ decreased to 350 mW, while the resonator setup was not changed at all. The largest output power of 315 mW was obtained with 50% duty cycle. The significant thermal effects mainly come from the small pump area and low thermal conductivity of TLF. It is well known that the thermal conductivity of a crystalline material usually decreases with the increasing doping ratio of luminescent ions [16]. For example, the thermal conductivity of Y3Al5O12 and Tb3Al5O12 at room temperature were reported to be 10.1 W·m−1·K−1 and 6.5 W·m−1·K−1, respectively [17,18]. Thus, a lower thermal conductivity of TLF than the undoped YLF (6.5 W·m−1·K−1, [19]) is well anticipated. An attempt to lessen such thermal effects was made by increasing the pump focal length to 250 mm, which results in a larger pump beam radius of 39 µm. In this case, the thermal roll-over onset at a higher power of ∼1.3 W whereas the ${\eta _{slope}}$ decreased to 29% and ${P_{thres.}}$ increased to 565 mW owing to the less suitable Rayleigh range. Due to the much higher laser threshold, we did not succeed in yellow laser operation using TLF as gain medium. The FWHM of the laser spectra (0.6 nm, see also Fig. 2) was found to be the same as that of Tb:YLF at 544 nm.
Our results show that laser operation can be fulfilled with a high active-ion concentration gain medium with a passable output efficiency. Nevertheless, the strong thermal effects in this highly concentrated active medium hampered achieving higher output power. This can be solved by employing thin-disk geometries equipped with an active cooling system [20], considering the relatively high absorption efficiency of TLF. Our demonstration of TLF laser paves the road for several further subjects. First, the relatively high absorption efficiency of TLF results in a much shorter active-medium length suitable for lasing compared to Tb:YLF at low doping ratio, which makes it possible to build plane-parallel resonators with TLF. Furthermore, the short crystal length is more favorable for fiber-coupled LD pumping. Single-mode LD pumped Tb3+ lasers have been reported recently, while the output power of the single-mode pump source is limited to a few hundreds of milliwatts [21]. Thus, higher laser performances can be expected by using the more powerful multimode fiber-coupled LD pumps and TLF is a suitable gain medium in this regard. Finally, Tb3+ active media are known to be promising candidates for Q-switched lasers due to their long upper-state lifetime. According to the modelling of Q-switched laser, the maximum output pulse energy is a product of the total useful energy stored in the medium (${E_u}$) and the energy extraction efficiency (${n_E}(z)$): ${E_{\max }} = {E_u} \times {n_E}(z)$ [10]. The energy extraction efficiency is related to the internal cavity losses and the pump energy density inside the gain medium, both of which are irrelevant with the material properties. Therefore, gain materials with large energy storage capability are favorable for generating high energy pulses. Via a classical rate equation modelling of the ground-state, upper-state, and lower-state population (necessary parameters were referred from [8,11]) in the experimental condition at maximum pump power, we estimated a population of the 5D4 state around 1×1019 cm−3 in TLF after equilibrium. This leads to a high energy storage capacity of ca. 5×104 W·cm−3 at 544 nm in TLF. We can thus deduce that TLF is promising for producing high energy laser pulses via Q-switching and is favorable for a compact resonator design.
3. Conclusion and outlook
In conclusion, we achieved comparable or superior laser performance using a c-cut 15%Tb:LiYF4 crystal to the previous Tb3+ laser demonstrations using Tb:LiLuF4. A record-high slope efficiency among Tb3+-lasers of 63% was obtained. Moreover, the thermal roll-over occurred in the previous laser demonstration of Tb:LiLuF4 was circumvented by optimizing the pump beam diameter while maintaining a good mode matching between the pump and laser. We were able to generate an output power of 1.17 W at around 544 nm with a high optical-to-optical efficiency with respect to absorbed power of 55%. We also succeeded laser operation at around 582 nm. Yellow laser emission with a maximum output power of 106 mW can be produced by Tb:YLF. More importantly, we succeeded in the first laser operation of LiTbF4 featuring a high active-ion concentration. In the laser experiment of TLF, the maximum slope efficiency was found to be 45% at output coupling transmittance of 1.2% and 315 mW of green laser radiation could be generated. Although these results were obtained in q-cw pumping conditions due to the strong thermal effects in TLF, we believe that this limitation can be lessened via active cooling. The laser demonstration of TLF points toward potential applications such as thin-disk, micro-chip, and fiber-coupled LD pumped Tb3+-lasers. Finally, we accentuate the prospect of TLF for generating high-energy Q-switched lasers owing to its high active-ion concentration (136×1020 cm−3) and relatively long lifetime (3.3 ms). The high-energy pulsed lasers emitting in the visible are useful for various fields such as topological mapping and micromachining. Q-switched laser experiments with the Tb3+-activated materials are in progress.
Funding
Japan Society for the Promotion of Science (15KK0245, 18H01204).
Acknowledgments
This work was performed with the support and under the auspices of the National Institute for Fusion Science Research program (ULHH040 and URSX204). The authors thank Northrop Grumman Corporation for providing the TLF crystal. The authors thank Shigeki Tokita and Hiroaki Furuse for their contributions to the preliminary experiments of this study.
Disclosures
The authors declare no conflicts of interest.
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