Highly efficient CW laser operation in 4 at. % Tm3+ and 4 at. % Y3+ codoped CaF2 crystals

Xinsheng Guo; Qinghui Wu; Linyang Guo; Fengkai Ma; Fei Tang; Cheng Zhang; Jie Liu; Bingchu Mei; Liangbi Su

doi:10.3788/COL201816.051401

Abstract

Tm : {CaF}_{2}

and

Tm, Y : {CaF}_{2}

single crystals were prepared by the temperature gradient technique. The spectral properties of

Tm, Y : {CaF}_{2}

single crystals were investigated and compared with those of

Tm : {CaF}_{2}

. It was demonstrated that codoping with

Y^{3 +}

ions could efficiently improve the spectroscopic properties.

Tm, Y : {CaF}_{2}

crystals have larger absorption cross-sections at the pumping wavelength, larger mid-infrared stimulated emission cross-sections, and much longer fluorescence lifetimes of the upper laser level (

{Tm}^{3 +} : {}^{3}{H_{4}}

level) than

Tm : {CaF}_{2}

crystals. Continuous-wave (CW) lasers around 1.97 μm were demonstrated in 4.0 at. % Tm,4.0 at. %

Y : {CaF}_{2}

single crystals under 792 nm laser diode (LD) pumping. The best laser performance has been demonstrated with a low threshold of 0.368 W, a high slope efficiency of 54.8%, and a maximum output power of 1.013 W.

${Tm}^{3 +}$ -ion-based laser materials operating at around 2.0 μm are being developed at an increasing pace in recent years^[1–5]. Lasers around this specifc region have several atmospheric transparency windows and are strongly absorbed by water and biological tissues. The unique features of the mid-infrared (IR) domain make it very attractive for many applications, such as coherent laser radar^[6], remote sensing^[7], laser ablation of varicose veins in vitro^[8], and medical surgery^[9–11]. Moreover, lasers around 2 μm are suitable pumping sources for IR optical parametric oscillation (OPO)^[12,13], which have wide applications in scientifc research, atmospheric pollution monitoring, and directional-IR countermeasure.

${CaF}_{2}$ has been proved to be an excellent optical material with broad-band transparency and low phonon energy¹⁴. Trivalent rare-earth-ion-doped ${CaF}_{2}$ ( ${RE}^{3 +} : {CaF}_{2}$ ) crystals have very broad and smooth absorption and emission spectra due to heterovalent substitution of ${Ca}^{2 +}$ and different forms of charge compensation. Such broad spectra are suitable for femtosecond pulses. In ${Nd, Y : CaF}_{2}$ crystal, Qin et al. have achieved 103 fs using a diode pumping passively mode-locked technique^[15]. In addition, pure ${CaF}_{2}$ is characterized by a high thermal conductivity of $9.7 W · {mK}^{- 1}$ and is thus comparable to that found in the case of $Y_{3} {Al}_{5} O_{12}$ (YAG). Even though the thermal conductivity of the ${CaF}_{2}$ -doped rare-earth ions is reduced^[16], it is still higher than that of glass. Many studies have been conducted based on rare–earth-doped ${CaF}_{2}$ crystals, including $Yb : {CaF}_{2}$ , $Er : {CaF}_{2}$ , and $Nd : {CaF}_{2}$ ^[17–21]. However, fewer studies have been performed using $Tm : {CaF}_{2}$ crystals in the 2 μm wavelength band, since 1.9 μm lasers based on the $Tm : {CaF}_{2}$ ceramics upon LD pumping have been demonstrated for the first time^[22], to the best of our knowledge. So far, the laser of $Tm : {CaF}_{2}$ ceramic with 4% doping only emitted an output power of 60 mW with a slope efficiency of 5.5%^[23]. The laser performance of the $Tm : {CaF}_{2}$ crystal with 1.34% doping has been reported with a slope efficiency of 41.0%^[24]. Recently, a continuous-wave (CW) output power of 453 mW with a slope efficiency of 21% from a $Tm, Y : {CaF}_{2}$ laser was obtained^[25], proving that the $Tm, Y : {CaF}_{2}$ crystal was a significant potential lasing material. However, the superiorities of $Tm, Y : {CaF}_{2}$ crystal compared with $Tm : {CaF}_{2}$ crystal were not been analyzed systematically.

In this Letter, comparative studies on spectroscopic properties between $Tm : {CaF}_{2}$ and $Tm, Y : {CaF}_{2}$ were performed for the first time, to the best of our knowledge. As we know, the quenching effect will occur due to defects and impurities introduced by increasing concentration of rare-earth ions. We found that codoping of $Y^{3 +}$ ions in $Tm : {CaF}_{2}$ can alleviate the cluster quenching effect and improve the spectral performance. 4 at. % Tm,4 at. % $Y : {CaF}_{2}$ demonstrated the laser performance with a high slope efficiency of 54.8% and a maximum output power of 1.013 W. To the best of our knowledge, this is the highest slope efficiency and largest power reported in ${Tm}^{3 +}$ -doped ${CaF}_{2}$ crystals.

Sign up for Chinese Optics Letters TOC. Get the latest issue of Chinese Optics Letters delivered right to you！Sign up now

$Tm : {CaF}_{2}$ and $Tm, Y : {CaF}_{2}$ crystals were grown by the temperature gradient technique (TGT). The raw materials of grown crystals are ${CaF}_{2}$ , ${TmF}_{3}$ , and ${YF}_{3}$ . To prevent oxidation in the growing process, 1 wt. % ${PbF}_{2}$ was added to the starting materials. All of the raw materials used for our experiment were of 99.99% purity. The weighed chemical powders were mixed thoroughly and then sealed in the graphite crucibles during the process of growth. The crystal samples were cut and then polished into a size of $10 mm \times 10 mm \times 2 mm$ for spectral measurements.

The room temperature absorption spectra were measured using a Jasco V-570 UV/visible (VIS)/near-IR (NIR) spectrophotometer. Fluorescence spectra were obtained using an FLS 980 time-resolved fluorimeter with grating blazed at 1820 nm and detected using a Hamamatsu InSb. Fluorescence decay curves measured at 1820 nm were obtained with FLS 980 spectrophotometers under an 808 nm pulse laser excitation with the frequency of 9 Hz and duration of 10 μs. All of the measurements were carried out at room temperature.

Figure 1 shows the X-ray diffraction (XRD) patterns of 4% $Tm : {CaF}_{2}$ and 4% Tm,4% $Y : {CaF}_{2}$ crystals compared with the standard pattern of the pure ${CaF}_{2}$ phase. No impurity peaks are found, and all of the diffraction peaks of the $Tm : {CaF}_{2}$ and $Tm, Y : {CaF}_{2}$ crystals are in good agreement with those of the pure ${CaF}_{2}$ phase. These suggested that ${Tm}^{3 +}$ and $Y^{3 +}$ ions have substitutionally entered the ${Ca}^{2 +}$ sites and the doping ions do not change the perovskite-like structure of the pure ${CaF}_{2}$ crystal. The structural parameters of these crystals were obtained by fitting the XRD data using the software JADE 6.0. The lattice parameters of 4% Tm,4% $Y : {CaF}_{2}$ crystal is a bit larger than those of 4% $Tm : {CaF}_{2}$ crystal. It is well in agreement with the fact that the radius of $Y^{3 +}$ is greater than that of ${Tm}^{3 +}$ .

Figure 1.XRD patterns of the $Tm : {CaF}_{2}$ and $Tm, Y : {CaF}_{2}$ crystals.

Figure 2 shows the room temperature absorption spectra of 4% $Tm : {CaF}_{2}$ and 4% Tm,4% $Y : {CaF}_{2}$ crystals in the wavelength range of 720–880 nm. There are three strong absorption peaks centered at 767, 784, and 792 nm, corresponding to the transitions from the ground state to the higher levels of ${Tm}^{3 +}$ .

Figure 2.Absorption spectra of $Tm : {CaF}_{2}$ and $Tm, Y : {CaF}_{2}$ crytals.

The absorption coefficient and absorption cross-section at the strongest absorption peak of 767 nm in 4% Tm,4% $Y : {CaF}_{2}$ crystal are enhanced from $2.85 {cm}^{- 1}$ and $0.34 \times 10^{- 21} {cm}^{2}$ to $4.54 {cm}^{- 1}$ and $4.7 \times 10^{- 21} {cm}^{2}$ , respectively, compared with 4% $Tm : {CaF}_{2}$ crystal. The larger absorption cross-section means a higher pump absorbing efficiency. The increasing of the absorption cross-section benefits from the stronger crystal field induced by codoping of $Y^{3 +}$ ions. While the crystallographic ${Ca}^{2 +}$ sites were occupied by the $Y^{3 +}$ ions, the compensation ions $F^{-}$ were introduced to fill the vacancies of cubic lattices. This leads to distorted crystallographic sites and a stronger crystal field around ${Tm}^{3 +}$ .

According to the known absorption cross-section based on the reciprocity method, the emission cross-sections were calculated and shown in Fig. 3: $σ_{em} (λ) = σ_{abs} (λ) \frac{Z_{L}}{Z_{u}} \exp (\frac{E_{Z L} - h c / λ}{k T}),$ (1)where $Z_{u}$ and $Z_{L}$ are the partition functions of the lower and upper levels, $E_{Z L}$ is the zero line energy defined as the energy gap between ${}^{3}{H_{6}}$ and ${{}^{3}F}_{4}$ manifolds, $h$ is Planck’s constant, $k$ is Boltzmann’s constant, and $T$ is temperature. Here, the zero line is 1666 nm, and $Z_{u} / Z_{L}$ is 1.512^[11]. There are three emission peaks located at 1611, 1666, and 1820 nm, respectively. The emission cross-section of 4% Tm,4% $Y : {CaF}_{2}$ crystal was calculated to be $0.45 \times 10^{- 20} {cm}^{2}$ at 1820 nm, which is increased by 40.96% compared with the value of $0.32 \times 10^{- 20} {cm}^{2}$ in 4% $Tm : {CaF}_{2}$ crystal. The emission cross-section is enhanced by the incorporation of 4% $Y^{3 +}$ ions. There are two reasons to be concerned for the phenomenon. On the one hand, part of the $f - f$ forbidden transition between ${}^{3}{F_{4}}$ and ${}^{3}{H_{6}}$ energy levels of ${Tm}^{3 +}$ ions was relieved by a stronger crystal field, as discussed above for absorption cross-sections. That is to say, higher absorption intensity results in a stronger emission in $Tm, Y : {CaF}_{2}$ crystal. On the other hand, it was pointed out that the $Y^{3 +}$ ions enter the lattice predominantly in the vicinity of the ${Tm}^{3 +}$ ions^[26]. Thus, codoped $Y^{3 +}$ ions may separate the clustered ${Tm}^{3 +}$ ions in the lattice at an appropriate distance. As a result, the probability of cross-relaxation between ${Tm}^{3 +}$ ions was increased, and there will be more population inversion on the upper energy level. The emission was improved accordingly. Table 1 shows the comparison of the emission cross-section between our work and the Tm laser in other oxide hosts reported. The value of the $Tm, Y : {CaF}_{2}$ crystal is lower than those of YAG, ${YAlO}_{3}$ (YAP), and $C_{22} H_{36} {NNaO}_{7} S$ (SSO) but higher than that of ${CaYAlO}_{4}$ . The data indicate that laser energy conversion efficiency of Tm:SSO is lower than those of YAP and ${CaF}_{2}$ . In addition, the full widths at half-maximum of the emission peaks at 1820 nm of these samples were similar, which were about 200 nm.

Figure 3.Emission spectra of the $Tm : {CaF}_{2}$ and $Tm, Y : {CaF}_{2}$ crystals.

Hosts	Emission Cross-section (cm−2)	Emission Lifetime (ms)	Pout (W)	η (%)	References
Tm:YAG	2.9×10−20	0.25	0.593	18.9	[27]
Tm:SSO	6×10−21	1.1	0.934	25.9	[28,29]
Tm:CaYAlO4	2.5×10−21	2.68–2.9	0.24	28	[30,31]
Tm:YAP	2.09×10-19	48.3	10.5	46	[32,33]
Tm:CaF2	3.2×10−21	2.99	–	–	This work
Tm,Y:CaF2	4.5×10−21	3.94	1.013	54.8	This work

Table 1. Emission Cross-section, Emission Lifetime, and Laser Parameters of Tm-doped Hosts

View all Tables

The fluorescence decay curves of these samples excited by 808 nm pulsed lasers show a single exponential decay behavior, which are shown in Fig. 4. By fitting the decay curves, the lifetimes are obtained to be 2.99 and 3.94 ms in 4% $Tm : {CaF}_{2}$ crytal and 4% Tm,4% $Y : {CaF}_{2}$ crytal, respectively. The longer lifetime in $Tm, Y : {CaF}_{2}$ crystal further proved that codoping $Y^{3 +}$ is beneficial to luminescence. Moreover, compared to other oxide-based laser materials, the lifetime of $Tm, Y : {CaF}_{2}$ crystal is relatively long, which is listed in Table 1. Also, longer lifetime also means higher quantum efficiency.

Figure 4.Fluorescence decay curves at (a) 4 at. % $Tm : {CaF}_{2}$ and (b) 4 at. % Tm,4 at. % $Y : {CaF}_{2}$ .

As the absorption and emission cross-section were calculated, the gain cross-section $σ_{g}$ could be estimated by the following equation: $σ_{g} = P \cdot σ_{abs} - (1 - P) \cdot σ_{em},$ (2)where parameter $P$ is the relative inverted population of the involved levels. The gain cross-sections of 4 at. % $Tm : {CaF}_{2}$ and 4 at. % Tm,4 at. % $Y : {CaF}_{2}$ crystals with the $P$ varying from 0 to 0.4 were estimated and illustrated in Fig. 5. Obviously, the gain cross-section becomes positive when the population inversion level reaches 10%. In particular, the value of $Tm, Y : {CaF}_{2}$ is almost two times higher than that of $Tm : {CaF}_{2}$ crystal when the value of P is equal to 0.4.

Figure 5.Gain cross-section at (a) 4 at. % $Tm : {CaF}_{2}$ and (b) 4 at. % Tm,4 at. % $Y : {CaF}_{2}$ .

CW laser operations were carried out by inserting an uncoated $Tm, Y : {CaF}_{2}$ sample inside a plane–concave laser resonator with water cooling at 13°C, and the setup was shown in Fig. 6. The pump source was a fiber-coupled 792 nm diode laser, delivering a maximum power of 30 W with a core diameter of 105 μm and a numerical aperture of 0.22. The pump beam was expanded into the gain medium by a coupling system of 1:2. M1 was a flat mirror (more than 90% transmission at the pump wavelength and more than 99% reflectivity at the lasing wavelength); M2 was a concave mirror having a radius of 100 mm with output transmissions of 2%, 5%, and 10% around 2.0 μm, respectively. The cavity length was about 90 mm. The laser sample was in dimensions of $3 mm \times 3 mm \times 7 mm$ . Laser operations were demonstrated around 1.97 μm with an LD pumping at 792 nm.

Figure 6.Schematic of the $Tm, Y : {CaF}_{2}$ crystal laser.

As shown in Fig. 7, CW laser operations around 1.97 μm were demonstrated in 4 at. % Tm,4 at. % $Y : {CaF}_{2}$ . A slope efficiency of 54.8% and a maximum output power of 1.013 W were achieved by using the 5% transmissive output coupler (OC). To the best of our knowledge, this is the highest slope efficiency and largest power reported in ${Tm}^{3 +}$ -doped ${CaF}_{2}$ crystals. A comparative study for laser properties of $Tm, Y : {CaF}_{2}$ with other oxide hosts is shown in Table 1. The $η$ value of $Tm, Y : {CaF}_{2}$ crystal is 54.8%, which is the highest in these hosts. The data indicate that the laser energy conversion efficiency of $Tm, Y : {CaF}_{2}$ is lower than those of oxide materials. The laser output power is 1.013 W, which is remarkable. The laser slope efficiencies are 49.9% and 53.6% by using the OCs with transmissions of 2% and 10%, respectively. The maximum output powers are 0.976 and 0.867 W, respectively. In order to protect the crystal, the experiments were done at low incident pump powers. Therefore, higher output power can be achieved with the increasing pump power.

Figure 7.Output powers versus absorbed pump power with output coupler transmissions of 2%, 5%, and 10%, respectively.

In summary, ${CaF}_{2}$ crystals doped with ${Tm}^{3 +}$ ions and $Y^{3 +}$ ions are obtained by the TGT. The fluorescent emissions around 1820 nm, corresponding to ${}^{3}{F_{4}} \to {}^{3}{H_{6}}$ transitions of ${Tm}^{3 +}$ , were observed under the excitation of an 808 nm LD. The emission intensity at 1820 nm increased with codoping of $Y^{3 +}$ ions. Under LD pumping, a maximum CW output power of 1.013 W and a slop efficiency of 54.8% were obtained in the 4% Tm,4% $Y : {CaF}_{2}$ crystal. Further studies will foucus on $Y^{3 +}$ doping concentration in $Tm : {CaF}_{2}$ for potential CW lasers.