Multi-wavelength emissions have been demonstrated in many disordered laser crystals. Improving the emission controllability is crucial for their practical applications. However, it is difficult because the closely adjacent laser components cannot be effectively adjusted by the traditional resonator design. In this paper, the anisotropy of laser emission in a monoclinic, disordered crystal Nd:LuYSiO5 (Nd:LYSO) is reported for the first time. By selecting crystal orientation, high power laser emission with different wavelengths and polarizations were obtained. For X-cut sample, 1076 nm single-wavelength laser output reached 7.56 W, which will be a useful light source for detecting carbonyl-hemoglobin and nitrite after frequency doubling. For Y- and Z-cut samples, 1076, 1079 nm dual-wavelength laser output reached 10.3 W and 7.61 W, with parallel and orthogonal polarizations, respectively, which are convenient to be used as the generation sources of 0.78 THz wave by type-I or type-II difference frequency. The output characteristic is well explained by a theoretical analysis on the stimulated emission cross-section. The present work reveals that the intrinsic anisotropy in disordered laser crystal can be utilized to elevate the emission controllability. Accordantly, the material’s application scopes can be extended.
© 2014 Optical Society of America
For many well-known Nd-doped laser crystals such as Nd:YAG, Nd:YVO4 and Nd:YAP, single wavelength operation is performed in most occasions. In recent years, multi-wavelength solid-state lasers have drawn increasing attentions because of their applications in many fields, such as THz wave generation, color display, medical treatment, etc. To reach simultaneously multi-wavelength emissions, various frequency controlling methods have been utilized, including special designed cavity mirror [1–3], saturable absorber [4, 5], polarization beam splitter (PBS) [6, 7], two-crystal linear cavity  and intracavity etalon . Besides, many disordered Nd-doped crystals were also developed as multi-wavelength laser materials, such as Nd:CNGG, Nd,Gd:YSGG, Nd:LuSGG and Nd:LaBO2MO4 [10–13]. Since their lattice structures include multiple inequivalent Nd3+ sites, multi-wavelength simultaneous operations are more liable to be realized in the disordered Nd-doped crystals compared with those traditional laser crystals. Nevertheless, the intensity and polarization of each wavelength component are often uncontrollable, at the same time single wavelength operation is difficult. These problems have plagued the practical applications of disordered laser crystals. For anisotropic laser mediums, i.e., with intermediate or lower symmetry, selecting orientation has been tried to solve these questions recently [14–18]. Currently, the output power and conversion efficiency are still at low levels in this newly emerged area; the fully characterization of the emission anisotropy to low symmetric crystals is challenging [19–21].
Generally, oxyorthosilicates allow large activator concentration and possess good mechanical, chemical, and thermal durability . The mixed oxyorthosilicate crystal LuYSiO5 (LYSO) is optically biaxial and belongs to the monoclinic C2/c space group, which has the same structure with Lu2SiO5 (LSO) or Y2SiO5 (YSO). Since its first growth in 1997 , researches about LYSO have been focused on scintillator applications [23–25] and Yb-doped lasers [26–29]. Nd-doped LYSO was first grown in 2010 , and the corresponding high efficiency continue-wave and passively Q-switched laser output were reported in 2011 [31, 32]. The passively mode locking of Nd:LYSO crystal was realized with a semiconductor saturable absorber mirror (SESAM), and 8.9 ps pulse width was achieved . Recently, dual-wavelength synchronously Q-switched solid-state laser with multi-layered graphene as saturable absorber was demonstrated . Of all the previous results, only multi-wavelength laser emissions were obtained, and the total output power was just on watt class level. In this paper, we determined the refractive principal-axes and the refractive index of Nd:LYSO crystal. The anisotropy of laser emission along different orientations was demonstrated and well explained by the fluorescence emission spectra. With an X-cut sample, more than 7.5 W, single-wavelength laser operations at 1076 nm was obtained, which will find applications in detecting carbonyl-hemoglobin and nitrite after frequency doubling . With a Y-cut sample the maximum dual-wavelength output power of 10.3 W was obtained at an absorbed pump power of 20.5 W, corresponding an optical conversion efficiency of 50.2% and slope efficiency of 54.8%.
2. Optical principal axes and refractive index
For Nd:LYSO crystal the angles between the crystallographic axes are: βab = 90°, βbc = 90° and βac = 102.8° . As a biaxial crystal, Nd:LYSO have three different principal axes of the optical indicatrix. In its monoclinic structure, one of the principal axes is collinear with the two fold axis of the crystal, i.e., the crystallographic axis b. The other two principle axes lie in (010) face at certain angles with the crystallographic axes a and c. With a XPT-6 type polarizing microscope we performed extinction experiment to a b-cut crystal sample (~2 mm thick), and found in (010) face the angle between a principal axis of the optical indicatrix and a-axis is 23.3°, and the angle between the other principal axis and c-axis is 10.5°. The definition of X, Y, and Z principal axes of the optical indicatrix follows the principle of nX<nY<nZ. Utilizing a prism coupling device (Model 2010, Metricon), the principle refractive indices at 632.8 nm were measured to be nX = 1.7915, nY = 1.7933, nZ = 1.8144. It indicates that Nd:LYSO is a positive biaxial crystal. At the same time, the relationship between the refractive principal axes and the crystallographic axes can be determined ultimately, as shown in Fig. 1(a), b-axis and X-axis are opposite, γaY = 23.3° and γcZ = 10.5°. In (010) plane (i.e. YZ plane), the calculated results of the optical indicatrix at 632.8 nm was obtained and shown in Fig. 1(b), which exhibited a good agreement with the measured data.
3. Polarized absorption spectra and fluorescence spectra
With a 1 at.% Nd3+ doped LYSO crystal, we processed X, Y and Z-cut samples with dimensions of 3 × 3 × 10 mm3. The length at the transmission direction is 10 mm, and their arrises are kept along optical principal axes. At room temperature the polarized absorption spectra were recorded by a JASCO V570 model UV/VIS/NIR spectrophotometer. Based on the formula below:Fig. 2(a). Here the three polarizations orientation was parallel to the principal axes in the XYZ-frame; the propagation direction (k-vector) was perpendicular to the plane which composed by any two orientations in the X-, Y- or Z-polarization. It can be seen that the absorption peak of X or Z polarized light is at ~810 nm. For Y polarized light the absorption peak is at 815 nm, at the same time its absorption coefficient and absorption cross-section are much smaller than those of other two polarizations. The detailed optical parameters of polarized absorption spectra for 4F5/2 + 2H9/2 excited state are listed in Table 1.
Using the crystal samples mentioned above, we measured the polarized fluorescence spectra with an Edinburgh FS920 High Sensitive Fluorescence Spectrometer (<0.09 nm resolution, 1200 g/mm grating). Then, the stimulated emission cross section can be calculated with the method used by A. Brenier . The polarized emission cross section spectra are shown in Fig. 2(b), and the peak information for 4F3/2 → 4I11/2 transition around 1.08 μm is also listed in Table 2: the maximum emission peak is at 1079 nm for X polarization, while at 1076 nm for Y and Z polarizations.
4. Laser emission performance
The laser experiment was carried out in a concave-plane resonator and the resonator length was ~1.5 cm. The pump source is a commercial fiber-coupled laser-diode (LD) with the central wavelength around 809 nm. The core size of the fiber is 100 μm in radius with a numerical aperture of 0.22. As the pump power increased from 0.3 to 29 W, the peak wavelength shifted from 808.17 to 809.91 nm, and the spectral width exhibited a small variation, i.e. 2.4 to 3 nm. As seen from Fig. 2(a), the FWHM (full width at half maximum) of absorption peak are ~7 nm for X- and Z-polarization and ~10 nm for Y-polarization, which is much broader than the pump spectral width. Thus the dominated effect on the absorption efficiency is mainly caused by the peak shift, rather than the changing of the diode spectrum width. The absorption peak of X and Z polarization is at ~810 nm and the peak of Y polarization is at 815 nm, yet for all of the three polarizations the left bottom of the absorption peak locates at ~805 nm. Thus, when the incident pump power increased and the center pump wavelength changed from 808.17 to 809.91 nm, the absorption coefficients for all of the three polarizations will increase. Three Nd:LYSO crystals cut along X, Y, and Z optical axes were used as the laser medium. All samples were processed in dimensions of 3 × 3 × 10 mm3, and their two end faces were anti-reflective (AR) coated at 808 & 1080 nm. The input concave mirror was high transmission (HT) coated at 808 nm and high reflective (HR) coated at 1080 nm, with a curvature radius of 200 mm. Three plane mirrors with different transmissions T of 2%, 10% and 24% at 1080 nm were used as the output couplers successively. To remove the heat generated under high pump power levels, the laser crystal was wrapped with indium foil and mounted in a water-cooled copper block, and the temperature of cooling water was controlled to be 20 °C.
Figure 3 shows the output power as a function of the incident pump power, and the main data were also listed in Table 3. From the theory of laser-diode end-pumped solid state laser, it can be known that the pump threshold Pth∝1/(σeηα), and the slope efficiency ηs∝ηα, where σe is the stimulated emission cross-section, ηα = 1-e-αd is the pump absorption efficiency, and α, d are the pump absorption coefficient and the length of the laser medium. Here we suppose that the experimental conditions and the resonator loss are the same for all of the samples. Based on the data in Table 1, we can obtain that at the pump wavelength 810 nm, ηα are 51%, 74%, and 60% for our 10 mm long X, Y, Z-cut crystals, respectively. These results are basically in line with those measured at the largest pump level of 28.5 W in the laser experiment, which are 50.5%, 71.9%, and 54.4%. Since our pump source is non-polarized, the pump absorption coefficient of X-cut sample is chosen as an average value of Y and Z polarizations. The similar treatment is used for Y and Z-cut samples. As seen in Fig. 2(a) and Table 1, for Y-polarized light the absorption peak of Nd:LYSO is at 815 nm, at the pump wavelength of 809 nm the absorption coefficient reduces to about one-half, which has been used in above calculation. Combining with Table 2, the 1/(σeηα) values for X, Y, Z-cut samples are determined to be 0.33, 0.2, 0.25 × 1020 cm−2, respectively. At high pump levels, 1076 and 1079 nm dual wavelengths might operate simultaneously, while at pump threshold, the one that has larger emission cross section will oscillate prior to the smaller one. So from Table 2 we chose the Y-polarization emission cross section of 1076 nm for X-cut sample, and X-polarization emission cross section of 1079 nm for Y and Z-cut samples, i.e. 5.99, 6.64, 6.64 × 10−20 cm2, respectively. From the above discussions, we can conclude that Pth,Y<Pth,Z<Pth,X for pump threshold, and ηs,Y>ηs,Z>ηs,X for slope efficiency. As seen in Table 3, the experimental results are accordant with the conclusion on the whole, for all of the three output couplers with different transmittances. We can also draw the conclusion that Pout,Y>Pout,Z >Pout,X for the maximum output power, and ηo,Y >ηo,Z>ηo,X for the optical conversion efficiency in Table 3. The laser performance of Z-cut sample seems not as good as expected; we speculate its optical quality is inferior to the other two samples, which introduces larger intracavity loss during the laser operation. For the T = 2% OC, the output power is 7.56, 9.46 and 7.61 W at the maximum pump power of 28.5 W for the X, Y and Z-cut samples, respectively. When the T = 10% OC is used, the maximum output power of 10.3 W is obtained from the Y-cut sample, corresponding to a slope efficiency of 45.9% and an optical conversion efficiency of 36.1%. With the largest absorption efficiency of 71.9% under the largest pump power, the calculated slope efficiency and optical conversion efficiency were 54.8% and 50.2%, respectively. Compared with previous reports [31–34], the present output power and efficiency has been improved greatly by selecting crystal orientations.
As seen in Fig. 3, overall the laser performance of T = 2% OC was better than the other two output couplers, so we chose this condition to introduce the laser spectra of different crystal samples. With a spectrum analyzer (HR4000CG-UV-NIR, Ocean Optics Inc.), the spectra of the output laser at different pump powers were recorded in Fig. 4. A detailed discussion on the laser output performance of different crystal orientation is given below. For the X-cut crystal (with sides parallel to Y- and Z- axis, respectively), the laser emitting at a single wavelength of 1076 nm until the incident pump power increased to 28 W. As seen in Fig. 4(a), even at the highest pump power of 28.5 W the proportion of 1079 nm component was still very small, and this wavelength was not found when the output coupler of 10% and 24% were used. It can attribute to the smaller emission cross section at 1079 nm in both the cases of Y-polarization and Z-polarization. For the Y-cut crystal (with sides parallel to X- and Z- axis, respectively), only the mode at 1079 nm oscillated at the threshold as shown in Fig. 4(b). The mode at 1076 nm appeared when the incident pump power was over 5.8 W (8.07 W and 18.4 W for T = 10% and 24%, respectively). Increasing the pump power to 28.5 W, the component of 1076 nm increased but still small, which is due to the larger emission cross-section at 1079 nm. Using a Glan-Taylor prism as the polarizer, both wavelengths were found to be X-polarized. It can be attributed to the larger emission cross section of X-polarization than that of the Z-polarization at both 1076 and 1079 nm. Figure 4(c) showed the laser spectra of the Z-cut sample (with sides parallel to X- and Y-axis) at T = 2%. The laser always operated in dual-wavelength, and the two wavelengths were found to be orthogonal-polarized, i.e. X-polarization for the 1079 nm and Y-polarization for the 1076 nm. So the individual component at 1076 or 1079 nm can be obtained just by a simple splitting through a polarizer. The different polarization of the two wavelengths can also be explained by the contrast of the emission cross sections: at the wavelength of 1076 nm, the cross section of the Y-polarization is larger than that of the X-polarization, so the 1076 nm light operated in Y-polarization; however, the situation is inversed at 1079 nm. Though the emission cross section at 1079 nm (//X, 6.64 × 10−20 cm2) is larger than that at 1076 nm (//Y, 5.99 × 10−20 cm2), the component of 1076 nm increased and exceeded the 1079 nm gradually, as increasing the pump power. We speculate this phenomenon is related to the difference of the thermal conductivity. According to our measurements, the thermal conductivities are 3.38 W/mK in Y direction and 2.52 W/mK in X direction, respectively. So the emission of Y-polarized 1076 nm component generates less thermal-induced loss and aberration at high pump powers, which is advantageous for its use in larger pump power.
In conclusion, we have determined the orientation of the dielectric frame in the monoclinic crystal, Nd:LYSO. The anisotropy of laser performance along different dielectric principal axes was examined, which exhibited good agreements with the measured polarized absorption and fluorescence characterizations. By using the samples processed along different dielectric principal axes, we have obtained high power and high efficiency 1.08 μm emissions with different components and polarizations. The 1076 nm single-wavelength laser will be particularly useful for detecting carbonyl-hemoglobin and nitrite by frequency doubling, and the 1076, 1079 nm dual-wavelength lasers with parallel or orthogonal polarizations have potential application for generating 0.78 THz wave. The output power, conversion efficiency, and the controllability of wavelength and polarization have achieved significant breakthroughs. The present work has set an excellent example for controlling laser emission with selecting crystal orientations, which will enlighten the exploitation and utilization of disordered laser materials.
This work is supported by the National Natural Science Foundation of China (61178060), Program for New Century Excellent Talents in University (NCET-10-0552), Natural Science Foundation for Distinguished Young Scholar of Shandong Province (2012JQ18), and Independent Innovation Foundation of Shandong University (2012TS215).
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