1. Introduction

Tm³⁺-doped crystals for laser emission around 1.9 μm via the ³F₄→³H₆ transition have been widely studied due to their applications in medical, military, and optical communication [1–3]. The Tm³⁺-doped crystals exhibit a common feature that their absorption band of the ³H₆→³H₄ transition matches well with the emitting wavelengths of commercial AlGaAs diode lasers (DL), which can make the laser device compact and durable [4]. Furthermore, the cross relaxation between Tm³⁺ ions, ³H₄ + ³H₆→³F₄ + ³F₄, makes one pumping photon excite two Tm³⁺ ions to the upper laser level possible, therefore, high efficiency can be expected for the Tm³⁺ laser.

The acentric β′-Gd₂(MoO₄)₃ crystal has been known earlier as ferroelectric and ferroelastic material [5,6]. The crystal can also be used as second harmonic generator and Raman shifter in laser system [7–10]. When the crystal is doped with rare earth ions, it may be a gain medium for self-frequency doubling and self-stimulated Raman scattering (SRS) lasers. The spectroscopic and laser properties of Nd³⁺-doped β′-Gd₂(MoO₄)₃ have been studied [11,12] and its promising integration in optical devices has been evaluated [7,10]. However, to our knowledge, the detailed characterization of the Tm³⁺-doped case has not been reported.

In this work, spectroscopic properties of a Tm³⁺-doped β′-Gd₂(MoO₄)₃ crystal are investigated and a quasi-cw Tm³⁺ laser operated in the free running regime was performed under DL pumping. All the experiments were carried out at room temperature.

2. Crystal growth and spectroscopic analysis

Tm³⁺:β′-Gd₂(MoO₄)₃ single crystal under investigation was grown by the Czochralski method in air in a platinum crucible heated by a 2 kHz radio frequency furnace. The raw material was synthesized by repeated solid state reactions at about 900 °C from the starting chemicals of Tm₂O₃ (99.99%), Gd₂O₃ (99.99%), and MoO₃ (99.5%). 3% excess of the MoO₃ component over stoichiometry was added for compensating the volatilization. In order to melt homogeneously, the raw material was heated up to a temperature of about 30 °C higher than its melting point and kept there for 1 h and then the temperature was lowered to the crystallization point. The crystal was grown on a seed crystal in the [001] direction. The pulling and rotating rates were 1 to 3 mm/h and 15 to 30 rpm, respectively. After the growth, the crystal was drawn above the melt surface and cooled down to a temperature of about 250 °C at a rate of −15 °C/h, and then a slower cooling rate of −5 °C/h was conducted down to the room temperature for getting through the ferroelectric phase transition. An oriented crystal sample with dimensions of about 10 × 7 × 0.93 mm³ was cut and polished for experiments and the photograph is shown in Fig. 1 . The crystalline c axis was parallel to the long edge. The concentration of Tm³⁺ ions was measured to be 6.6 at.% by an inductively coupled plasma atom emission spectrometer (ICP-AES, Ultima 2, Jobin-Yvon), which is lower than the 8 at.% in the raw material. The melting points were measured using differential scanning calorimetry (DSC) analysis and were almost kept unvaried for the doped and undoped crystals (~1170 °C). The obtained low temperature phase β′-Gd₂(MoO₄)₃ has an orthorhombic structure with space group ${\text{C}}_{\text{2V}}^{\text{8}}\text{-Pba2}$. The lattice parameters interpolated from data of Ref [13]. are a = 10.383 Å, b = 10.416 Å, and c = 10.698 Å (Z = 4). As an approximation this crystal can be considered as an uniaxial crystal for the refractive indexes are similar along the a and b crystalline axes, i.e., n_a≈n_b = n _o, with the optic axial angle 2V≈10° [7,12]. The spectroscopic properties can be simply characterized in two different polarizations of q = σ and π, which correspond to the electrical field (E) of the light perpendicular or parallel to the c axis, respectively.

 

Fig. 1 Polished crystal sample of the Tm³⁺:β'-Gd₂(MoO₄)₃ for experiments with dimensions of about 10 × 7 × 0.93 mm³.

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The polarized absorption spectra around 800 nm, corresponding to the transition of ³H₆→³H₄ were measured with a UV-VIS-NIR spectrophotometer (Lambda 900, Perkin-Elmer) and are shown in Fig. 2 . The peak absorption cross sections are 3.92 × 10⁻²⁰ cm² at 797 nm with a full width at half the maximum (FWHM) of about 5.5 nm and 2.08 × 10⁻²⁰ cm² at 794.5 nm with a FWHM of about 13.5 nm for σ and π polarizations, respectively. The broad absorption band originates mainly from the interactions between Tm³⁺ ions and the vibrating host. It is also worth noting that a stable absorption from 794 to 798 nm with the absorption cross section about 2 × 10⁻²⁰ cm² for π polarization should be very beneficial to the tolerance of wavelength shift of DL pump source.

 

Fig. 2 Polarized absorption spectra of the Tm³⁺ doped β′-Gd₂(MoO₄)₃ crystal in a range of 750–850 nm.

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The polarized absorption spectra in a range of 1800-2100 nm, which are related to the ³H₆→³F₄ transition of Tm³⁺ ions in the crystal, are shown in Fig. 3 . The stimulated emission cross section (${\sigma }_{\text{EM}}^q$) of the ³F₄→³H₆ transition at wavelength λ, which is an important parameter relevant to the Tm³⁺ laser around 1.9 μm, was calculated by the reciprocity method (RM) from the measured absorption cross section (${\sigma }_{\text{GSA}}^q$) [14].

 

Fig. 3 Polarized absorption and emission cross sections of the Tm³⁺ doped β′-Gd₂(MoO₄)₃ crystal in a range of 1500–2100 nm.

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Then, the scaled values of $Z_{\text{l}}/Z_{\text{u}}$ was estimated to be 1.77 in our case. The polarized emission cross sections calculated by the RM method are also shown in Fig. 3.

From the absorption and emission cross sections, the gain cross section ${\sigma }_{\text{G}}^q$ can be calculated from [16]

 

Fig. 4 Gain curves of the ³F₄→³H₆ transition for the Tm³⁺:β′-Gd₂(MoO₄)₃ with different values of population inversion P (P = 0.1, 0.2, …, 0.5).

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3. Laser experiment

A simple and compact end-pumped plano-concave resonator was adopted in laser experiment. A fiber-coupled DL emitting around 795 nm was used as the pump source. The pump beam passes through an input coupler (IC) which has a 90% transmission at 795 nm and a reflectivity of R>99% in the region between 1850 and 2000 nm. The crystal without any antireflection coating was held in an aluminum mount and positioned as close as possible to the IC. The pump beam was modulated with a duty cycle of 2% and a modulation frequency of 10 Hz to reduce the thermal load in the crystal. The diameter is about 115 μm and about 67% of the incident pump power was absorbed by the crystal with thickness of 0.93 mm. Three output couplers (OC) with transmissions T _OC = 2.1%, 3.8%, and 7.1% around 1.9 μm and the same curvature radius of 100 mm were adopted in the experiment. The length of the plano-concave cavity was set close to the curvature radius of the output couplers.

The free running Tm³⁺ laser operated in a quasi-cw regime. The average output power versus average absorbed pump power was measured at different T _OC of output couplers and is shown in Fig. 5 . For the T _OC = 7.1% output coupler, the achieved maximum output power is 58 mW when the absorbed pump power is 111 mW, and the slope efficiency η with respect to absorbed pump power is 57%, with an optical-to-optical conversion efficiency as high as 52%. The lasing threshold is about 8 mW in this case. It is worth noting that the present maximum slope efficiency is much higher than the Stokes limit λ _pump/λ _laser = 42%. Therefore, the cross relaxation ³H₄ + ³H₆→³F₄ + ³F₄ took place efficiently in this laser operation. Moreover, it can be found that, with the increment of T _OC from 2.1% to 7.1%, the slope efficiency increases from 39% to 57%, and the maximum laser output power increases from 41 to 58 mW at the same absorbed pump power. It means the laser with higher output power and efficiency may be expected by adopting an output coupler with a higher transmission.

 

Fig. 5 Average output power of the Tm³⁺:β'-Gd₂(MoO₄)₃ laser versus absorbed pump power for different output coupler transmissions.

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Laser spectra were recorded using a monochromator (Triax550, Jobin-Yvon) associated with a TE-cooled Ge detector (DSS-G025TT, Jobin-Yvon) and are shown in Fig. 6 . All the measurements were carried out at the maximum absorbed pump power of 111 mW. It can be found that the lasing wavelength blue-shifts from around 1980 to 1900 nm with the increment of T _OC from 2.1% to 7.1%, which covers a spectral region of about 100 nm. This can be interpreted by the gain curves shown in Fig. 4. For a lower output coupler transmission, i.e., a lower cavity loss, a smaller value of population inversion is required to achieve laser oscillation and so the lasing wavelength is longer. The large shift of the lasing wavelength also indicates the potential of this crystal for tunable laser operation. The output laser beam is linear π polarization, which is in agreement with the larger π polarized gain cross section and is favorable for the application of nonlinear frequency conversion, such as self-stimulated Raman shift and optical parametric oscillator, to obtain valuable laser at other wavelengths.

 

Fig. 6 Free running laser spectra of the Tm³⁺:β'-Gd₂(MoO₄)₃ at the same absorbed pump power of 111 mW for different output coupler transmissions.

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4. Conclusion

A β′-Gd₂(MoO₄)₃ single crystal with Tm³⁺ concentration of 6.6 at.% was grown by the Czochralski method. The peak absorption cross sections were measured to be 3.92 × 10⁻²⁰ at 797 nm and 2.08 × 10⁻²⁰ cm² at 794.5 nm for σ and π polarizations, respectively. The gain curves for 1.9 μm laser were obtained and a broad tunable range of about 200 nm could be expected for this crystal. A maximum average output power of 58 mW for the quasi-cw laser around 1.9 μm with linear π polarization was realized under DL pumping at 795 nm, the slope and optical-to-optical conversion efficiencies were up to 57% and 52%, respectively, and the lasing threshold was only 8 mW. Consequently, the Tm³⁺-doped β′-Gd₂(MoO₄)₃ crystal is a promising multi-purpose high performance DL pumped gain medium for the 1.9 μm laser.