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Abstract
Highly transparent Dy:CaF2 ceramics with various doping concentrations were fabricated by a hot isostatic pressing (HIP). Their optical properties, emission cross-sections, and saturable absorptivities were evaluated to examine their use as a new candidate 3-μm laser functional medium. Based on those evaluations, we demonstrated passively Q-switched operation of a 2.92-μm Er:YAP laser using a Dy:CaF2 ceramic as a saturable absorber.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Dysprosium-doped transparent materials have attracted considerable attention for use in mid-IR devices. A notable example is a laser medium employing the emission properties of Dy3+ ions at wavelengths around 3 or 4 μm [1,2]. There are various molecular absorption lines in the 3–5 μm wavelength region where atmospheric absorption is relatively insignificant. Coherent sources at such wavelengths are in demand for applications in remote sensing of CO2, CO, hydrocarbons and water isotopes [3,4]; interferometric plasma diagnosis; and laser processing of glass and plastic materials [5]. Dy-doped 3-μm solid-state lasers have been realized in Dy:BaY2F8 [6], Dy:LaF3 [7], and Dy:BaYb2F8 [8] with pumping at 1.1 or 1.3 μm wavelengths. Recently, watt-level Dy3+-doped fluoride fiber lasers at 2.9–3.3 μm were reported for 2.8-μm fluoride fiber laser pumping [9–11]. While 4.3-μm Dy-doped lasers have been demonstrated in Dy:YLF [12], Dy:CaGa2F4 [13], and Dy:PbGa2F4 [14] with pumping at 1.7 or 1.3 μm wavelengths, their output powers were less than 0.1 W. Room-temperature Dy-doped lasers, filling the gap in laser wavelengths between Er, Ho, Cr (<3.0μm), and Er-doped (>3.4 μm) lasers, are also a promising source at >4 μm, as an alternative to Fe:ZnS and Fe:ZnSe lasers [15,16] which are not suitable for room-temperature operation. CaF2 crystals, which we focus on in this study, have been extensively studied as a mid-IR laser host material due to their high thermal conductivity (9.7 W/mK), low phonon energy (∼495 cm−1), and low nonlinear refractive coefficient [17]. In addition, doping of trivalent rare-earth ions in the crystal matrix leads to inhomogeneous spectral broadening due to heterovalent substitution at Ca2+ sites [18]. The broad spectral linewidth of RE3+-doped CaF2 is favorable for ultrashort pulse generation and wavelength tunable lasers. High-quality polycrystalline transparent oxide [19,20] and fluoride [17] ceramics have recently become available and possess numerous advantages compared with single crystals, such as excellent mechanical strength and superior thermal properties, opening up the possibility of efficient high-power mid-IR lasers. Polycrystalline ceramics can also be mass-produced cheaply as large-volume crystals. In 2019, the first laser oscillation of Nd:CaF2 ceramics at 1.06 μm was realized by our research group [21], where the ceramic was fabricated by reactive sintering and a HIP method.
In this work, we fabricated high-quality Dy3+-doped CaF2 ceramics and evaluated their optical properties to demonstrate their potential as a new candidate >3-μm laser gain medium. We focused on the nonlinear absorption properties of Dy:CaF2 ceramics at around 3 μm for the first time. We demonstrated passively Q-switched operation of a 2.92-μm Er:YAlO3 (Er:YAP) laser using Dy:CaF2 ceramics as a saturable absorber.
2. Fabrication of Dy:CaF2 ceramics and their optical properties
Dy:CaF2 ceramics with Dy3+ concentrations of 0.5, 1, 2, and 3 at.% were fabricated by a HIP method. High-purity fluoride compound CaF2 (3 μm particle size, > 3 N purity) and DyF3 particles were used as starting materials. The raw materials were ball milled in ethanol. A spray dryer was then used to dry the slurry and granulate the mixed powders. The granulated powders (∼30 μm) were pressed into disks with metal molds, followed by a cold isostatic press at 147 MPa. The obtained powder compacts were presintered at 1000°C for 2 h under a nitrogen atmosphere and then isostatically pressed at 1100°C for 3 h under argon (176 MPa) using the HIP equipment. The as-synthesized ceramic samples were polished on both sides to a size of Ø12 mm × 3 mm. A photograph of the fabricated transparent ceramics is shown in the inset of Fig. 1. Detailed information about the fabrication technique, grain size, and optical isotropy was reported in our previous paper [21]. Transmission spectra of the Dy:CaF2 ceramics shown in Fig. 1 were measured using UV–vis (UV-3600Plus, Shimadzu) and FT-IR (Frontier, Perkin Elmer) spectrometers. The predicted maximum transmittance (Fresnel reflection) of an undoped CaF2 crystal is shown as a dotted line. Ground-state absorption bands of Dy3+ (6H15/2 →) were observed at peak wavelengths around 0.8 μm (6F7/2), 0.9 μm (6H5/2), 1.1 μm (6H7/2, 6F9/2), 1.3 μm (6H9/2, 6F11/2), 1.7 μm (6H11/2), and 2.8 μm (6H13/2). The scattering coefficient for each ceramic calculated using the absorption baseline and Fresnel loss by a power-law fitting is plotted against wavelength in Fig. 2. The scattering coefficients at 3 μm and 4 μm, which are potential lasing wavelengths, were less than 0.05 cm−1 and 0.04 cm−1, respectively. Optical losses besides Fresnel reflection lower than 1.5% are expected at wavelengths longer than 3 μm in 3 mm-long Dy:CaF2 ceramics. The absorption coefficient spectra are also plotted in Fig. 2 based on the scattering coefficient. These Dy3+ concentrations are highly reliable because the absorption coefficient is proportional to the concentration. The absorption coefficient at 1.7 μm and 2.8 μm wavelengths, which are potential pump wavelengths for lasing, were 1.2 cm−1 and 2.6 cm−1, respectively. The one-pass absorptivities are 30% (1.7 μm) and 55% (2.8 μm) in the Dy:CaF2 ceramics with 3 mm thickness.
Fig. 1. (a) Transmission spectra and a photograph of the Dy:CaF2 ceramics with various doping concentrations. Dotted line indicates the predicted maximum transmittance of undoped CaF2 crystal.
Fig. 2. Absorption (solid line) and scattering (dotted line) coefficient spectra of Dy:CaF2 ceramics with various doping concentrations.
3. Emission properties of Dy:CaF2 ceramics
To discuss the potential of lasing in the 3–4-μm wavelength region, the emission properties of the Dy:CaF2 ceramics were evaluated. Figure 3(a) shows the emission spectra (cross-section) for excitation by a 1.7-μm laser diode (ALC-1680-02250-CMT, Akela Laser) and detection using an optical spectrum analyzer (OSA205C, Thorlabs). These emission cross-sections were calculated by the Füchtbauer–Ladenburg equation [22] using the emission spectrum and a radiative lifetime derived from the Judd–Ofelt theory [23,24]. Broadband emission that can be assigned to 6H13/2 → 6H15/2 transitions was observed in the range of 2.6–3.4 μm with a cross-section of about 6×10−21 cm2 at 2830-nm wavelength. Such relatively broad emission or absorption owing to the disordered structure in Dy:CaF2 is comparable to that for Dy:ZBLAN glass [10], even for a larger cross-section. The reason why highly doped samples exhibited a broader emission band at longer wavelengths than samples with a low level of doping may be clustering of Dy3+ ions [25]. No significant emission was observed, unfortunately, around the 4-μm wavelength region, although a radiative transition centered at 4.25 μm (6H11/2 → 6H13/2) was expected. This result means that the non-radiative process is dominant in this transition and 4-μm lasing would not be expected for a Dy:CaF2 medium. Host materials with phonon energies less than 495 cm−1, e.g., YLF [12], PbGa2F4 [14], and chalcogenide glass [2] are required to obtain 4-μm lasing by suppressing multi-phonon relaxation. The emission spectrum excited by a 2.8-μm Er:ZBLAN fiber laser using a 3.1-μm long-pass filter is shown in Fig. 3(b). An emission line similar to that for the 1.7-μm excitation was confirmed for 2.8-μm excitation.
Fig. 3. (a) Emission cross-sections for Dy:CaF2 ceramics excited by a 1700-nm laser diode. The absorption cross-section of the 3-at.% sample is plotted as a dashed line. (b) Emission spectrum with 2800-nm excitation measured using a 3100-nm long-pass filter.
The fluorescence lifetime of the 3-μm emission for 1.7-μm excitation, which corresponds to the upper state lifetime of 3-μm lasing, was measured using a 3.0–3.5-μm band-pass filter and an InAs photodetector (C12492-210, Hamamatsu Photonics). The fluorescence lifetimes of the 0.5, 1, 2, and 3-at.% samples were 430, 340, 290, and 210 μms, respectively. These values are comparable to those reported for Dy:CaF2 single crystals [25] and strong concentration quenching due to Dy3+ clustering was observed. This demonstrates that it may be possible to construct a 3.1-μm laser using 3-at.% Dy:CaF2 ceramics of 3-mm length, and 2.8-μm cw pumping with a spot diameter of 100-μm. Assuming the use of a 3% transmittance output coupler, the required incident pump power for laser oscillation is estimated to be 0.4 W, from the determined scattering coefficient, absorption, emission cross-section, and fluorescence lifetime. The realization of a 3-μm Dy:CaF2 ceramic laser is more likely for pumping with an Er:ZBLAN fiber laser, using which we demonstrated a cw output power of >30 W at 2.80–2.84 μm wavelengths [26,27]. A Dy:CaF2 ceramic laser exhibiting a broadband gain spectrum with a transform limited pulse width of 40–50 fs is favorable for ultrashort pulse generation. It would also be a promising gain medium for a tunable laser [9] covering wavelengths of 2.9 μm to 3.4 μm which was estimated by positive range of gain coefficient as shown in Fig. 4, allowing hydrocarbon and ammonia analysis [28].
Fig. 4. Gain cross-section spectra of 6H13/2 → 6H15/2 transition for 3 at. % Dy:CaF2 ceramics with the population inversion P ranging from 0 to 1 in interval of 0.1.
4. Demonstration of the Dy:CaF2 saturable absorber
We next focused on the potential use of Dy:CaF2 ceramics as a saturable absorber (SA) in the mid-IR region. The nonlinear absorption properties of the 0.5-at.% Dy:CaF2 ceramic at a wavelength of 2920 nm were measured by a z-scan method with a self-made cw Er:YAlO3 (Er:YAP) laser [29] with a maximum power of 2.0 W. The transmittance excepting Fresnel reflection is plotted in Fig. 5. The fitting curve is shown as a red line, based on a simple two-level saturable absorber model [30]. The modulation depth and nonsaturable loss were 5.1% and 3.0% respectively, and the modulation ratio in the total absorption was 63%. This saturable absorption property compares favorably with typically used SAs for mid-IR lasers. The commercial semiconductor SA SA-2800-10-x, BATOP and monolayer graphene, for example, have modulation depths (and modulation ratios in total absorptivity) of 6% (ratio: 60%) and 1.5% (ratio: 67%), respectively [30]. The nonlinear absorption of Dy:CaF2 ceramics exhibits full saturation of over several MW/cm2 intensity with a saturation intensity of 0.23 ± 0.01 MW/cm2. This value is lower than that for monolayer graphene of 0.6–0.7 MW/cm2 at the near-infrared region [30]. Note that the modulation depth for Dy:CaF2 ceramics would be larger than 5.1% at wavelengths around the absorption peak of 2830 nm where the absorbance is almost 2 times that at 2920 nm, as shown in Fig. 2.
Fig. 5. Nonlinear transmission of 0.5-at.% Dy:CaF2 ceramic measured by a cw Er:YAP laser at 2920 nm wavelength. Fresnel reflection losses are removed in the plots and a fitting curve is shown as a red line.
As a demonstration for the practical use of a Dy:CaF2 SA, we explored Q-switching of an Er:YAP laser at 2920 nm wavelength. Figure 6 shows the setup for a passively Q-switched Er:YAP single-crystal laser using the 0.5-at.% Dy:CaF2 ceramic SA, which is essentially the same as that in our previous report on a graphene Q-switched Er:YAP laser [29]. The Dy:CaF2 SA without any coating was inserted between the Er:YAP medium and output coupler. The important advantage of using Dy:CaF2 is that it exhibits extremely low absorption at a pump wavelength of 976 nm, as shown in Fig. 2. Thus, stable loss modulation was expected even for a short linear cavity, owing to the high transparency to residual pump light.
Fig. 6. Schematic diagram of the setup for a passively Q-switched Er:YAP laser using a Dy:CaF2 ceramic as a saturable absorber.
The output power and pulse waveform were measured by a thermopile power meter (3A, Ophir) and a HgCdTe detector (PVI-4TE-3-TO8-AL2O3-35, Vigo). Stable pulsed operation at 2920 nm wavelength was achieved in the pump power range from 4.9 to 11.7 W, and the average output power increased from 5.2 to 169 mW with increasing pump power. Figure 7(a) shows a typical waveform of the pulse train under 11.7-W pumping, which is the highest pump power for stable pulsed operation. A maximum average output power of 169 mW was obtained with a 204 kHz repetition rate, resulting in 0.83 μmJ pulse energy. The pulse duration was 220 ns FWHM as shown in Fig. 7(b), which corresponds to a 3.7 W peak power. The pulse duration and repetition rate are plotted against the pump power in Fig. 8. The pulse duration decreased from 2050 to 220 ns and the repetition rate increased from 44 to 204 kHz with increasing pump power from 4.9 to 11.7 W. This pump power relation is reasonable for the passive Q-switching regime [31]. Although the pulse energy and peak power were still low because of the unoptimized cavity loss, passive Q-switching using the Dy:CaF2 ceramic SA have been successfully demonstrated at the 3-μm wavelength region for the first time to our knowledge. If the Dy:CaF2 absorption is fully saturated, then the modulation depth will be 5.1% and the estimated pulse duration about 13 ns in an ideal 18-mm long cavity [32]. One reason why the experimentally obtained shortest pulse duration was 17 times longer than this value may be the long recovery time of saturable absorption in Dy:CaF2. The recovery time should be comparable to the fluorescence lifetime, which was 430 μms in the case of two-level SA without any excited state absorption even if stimulated emission took place. The slower recovery time compared to pulse period (4.9 μms for 204 kHz repetition) may result in a small modulation depth. There are concerns of cavity loss due to Fresnel reflection on the ceramics and the cooling technique of SA in current system. Nonetheless, a Dy:CaF2 ceramic is a promising SA for various solid-state lasers emitting in the wavelength range of 2.5–3.2 μm, e.g. Er:YAG [33], Er:Lu2O3 [20], Er:Y2O3 [34], Er:ZBLAN fiber [26], and Cr:ZnSe [15] lasers, owing to the broadband absorption properties shown in Fig. 2. In addition, the absence of absorption at a wavelength of 0.97-μm enables the construction of a simple linear cavity like a microchip for a Q-switched Er-doped solid-state laser without considering pump light absorption. The desired saturable absorption performance would be obtained by controlling the Dy3+ concentration or thickness in Dy:CaF2 ceramics.
Fig. 7. (a) Typical output temporal waveform for a Q-switched Er:YAP laser with a Dy:CaF2 ceramic SA under 11.7-W pumping. (b) Temporal waveform of a pulse in the pulse train.
Fig. 8. Pulse duration and repetition rate for Dy:CaF2 Q-switched Er:YAP laser as a function of pump power.
5. Conclusion
In summary, we fabricated Dy:CaF2 ceramics with a high optical quality, exhibiting a scattering coefficient of <0.05 cm-1 in the mid-IR region, by a HIP treatment. The detailed optical properties and emission cross-section around a wavelength of 3 μm were determined for various Dy3+ doping concentrations. A >3-μm Dy:CaF2 ceramic laser is possible by pumping with a 2.8-μm fiber laser, though 4-μm lasing would not be expected due to multi-phonon relaxation. Dy:CaF2 ceramics exhibiting a broadband gain spectrum would be a promising medium for a tunable laser covering 2.9 to 3.4 μm wavelength region. We also investigated the nonlinear absorption properties of Dy:CaF2 ceramics around a wavelength of 3 μm. We have successfully demonstrated the practical use of Dy:CaF2 ceramics as a SA in passively Q-switched Er:YAP laser at 2.92 μm. Dy:CaF2 ceramics are a promising SA for various solid-state lasers emitting in the wavelength range of 2.5–3.2 μm due to their broadband absorption properties.
Funding
Japan Society for the Promotion of Science (15KK024, 18H01204, 20K05374); National Institute for Fusion Science (KBAH028, KLEH087, UFEX5003, ULHH040); Amada Foundation (AF-2018228-C2, AF-2019221-B3); Murata Science Foundation (H31009); Nippon Sheet Glass Foundation for Materials Science and Engineering (R2-No.5).
Acknowledgments
The authors would like to thank Prof. Shigeki Tokita for preparing the mid-IR spectrum analyzer.
Disclosures
The authors declare no conflicts of interest.
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