The Recent Development of Rare Earth-Doped Borate Laser Crystals

As a laser host, borates possess favourable chemical and physical characteristic and higher damnification threshold. Ln2Ca3B4O12 (Ln = La, Gd, or Y) double borate family crystals,Ca3(BO3)2 (CBO) and LaB3O6 crystals are the potential laser host materials. They have the suitable hardness and good chemical stability and are moisture free. Furthermore, they melt congruently and can be grown by Czochralski method [1-5], so the high optical quality crystal with large dimension can be easily grown.

Where L is the initial length of the sample at room temperature and L  is the change in length when the temperature changes T  . We can calculate the thermal expansion coefficient from the slope of the linear fitting of the linear relationship between TT  and the temperature. In this case, the linear thermal expansion coefficients for different crystallographic directions c-, and b-axes are 4.69×10 -5 K -1 , 1.37×10 -5 K -1 , respectively [14] . The thermal expansion coefficients of the a and b axes are comparable. Thermal expansion www.intechopen.com coefficient along the c-axis is about two times larger than those of b and a axes. Although the thermal expansion property of YVO 4 crystal has little different from that of CBO crystal, the CBO crystal has no cleavage plane. It is well known that the higher the consistency of atom in the crystal structure, the larger the heat expansion coefficient, and vice versa. Obviously, it was demonstrated from the Fig.2.4 that the consistency of atom along c axis is higher than that along b axis, which is comparable to a axis. Therefore, the heat expansion coefficient along c axis is much larger than those along b axis and a axis. It has high transmittance in the 190-3300 nm optical ranges. [14] Fig.2.6 shows the absorption spectrum measured at room temperature in the 300~950 nm ranges [15] . There are three main strong absorption peaks in the spectrum centered at 588, 751 and 808 nm, respectively, corresponding to the transitions from the 4 I 9/2 ground state. The introduction of Na + as charge compensator results in the disorder in the local crystal fields acting on the optically active ions. [16] Therefore, the absorption and emission peaks can become broadening. [17 ] The full width at half maximum (FWHM) of the absorption peak at 808 nm is about 19 nm, which is larger than that of Nd 3+ :GdVO 4 (FWHM is 4 nm) [18] . The absorption coefficient and absorption cross section of Nd 3+ :CBO crystal at 808 nm are 0.93 cm -1 and 2.12×10 -20 cm 2 , respectively, which are compared with those of Nd 3+ :GdVO 4 (2.0×10 -20 cm 2 for π spectrum) [20] and Nd 3+ :YVO 4 (2.0×10 -20 cm 2 for π spectrum) [19] . Therefore, Nd 3+ :CBO crystal is suitable for GaAlAs laser diode pumping.   2 , which is smaller than those of NdAl 3 (BO 3 ) 4 (1.43×10 -19 cm 2 ) [26] and Nd 3+ :La 2 (WO 4 ) 3 (11.2×10 -20 cm 2 ) crystals [27] , but larger than that of Nd 3+ :LaB 3 O 6 (3.46×10 -20 cm 2 ) crystal [21] . Fig.2

The spectrum characteristics of Er 3+ :Ca 3 (BO 3 ) 2 crystal
Figure.2.9 shows the room temperature (RT) polarized absorption spectra in the 200-1600 nm spectra region of Er3 + in the CBO crystal [28] . It consists of a number of groups of lines corresponding to transitions between the ground state 4 I 15/2 and higher energy states inside the 4f 11    The maximum values of the emission cross section centered at about 1530 nm are 9.67×10 −21 cm 2 for the π spectrum and 7.43×10 −21 cm 2 for the spectrum, which can be compared with those reported for other Er 3+ doped laser crystals [9.3×10 −21 cm 2 for Er 3+ :LaGaO 3 , [29] 4.5×10 −21 cm 2 for Er 3+ :YAG (yttrium aluminum garnet), [30] and 3.1×10 −21 cm 2 for Er 3+ :YAlO 3 [30] ]. The wavelength dependence of the gain cross-section for several values of population inversion P (P = 0,0.1,0.2,…,1) is shown in Fig.2.13. A wide tunable wavelength range from 1530 to 1650 nm is expected when the population inversion P is larger than 0.5, which is encountered in a free-running laser operation.

The spectrum characteristics of Dy 3+ :Ca 3 (BO 3 ) 2 crystal
Fig.2.14 shows the room temperature absorption spectrum of Dy 3+ :Ca 3 (BO 3 ) 2 crystal, which consists of nine groups of bands. [36] They are associated with the observed transitions from the 6 H 15/2 ground state. The wavelengths corresponding to the transitions are listed in Table  2-7, which also displays the integrated absorbance, the measured and calculated line strengths of Dy 3+ : CBO crystal. The room temperature emission spectrum is presented in Fig.2

The crystal structure
An ORTEP drawing of the structure fragment of crystal Yb:Ca 3 Y 2 (BO 3 ) 4 is shown in Fig.3.1a. Fig.3.1b shows the packing diagram of cell units of Yb:Ca 3 Y 2 (BO 3 ) 4 crystal. [6] In Yb 3 ＋ :Ca 3 Y 2 (BO 3 ) 4 crystal structure, cations occupy three independent sites statistically, which is similar to Ca 3 La 2 (BO 3 ) 4 [39] and Ba 3 La 2 (BO 3 ) 4 [40] . The basic structure of Yb:Ca 3 Y 2 (BO 3 ) 4 is composed of three sets of M-oxygen distorted polyhedrons, and three sets of BO 3 planar triangles. Ca 2+ and Y 3+ ions occupy three independent sites statistically. M1、M2 and M3 were suggested to stand for these three independent sites respectively. They are coordinated by eight oxygen ions to form the distorted polyhedron. From the value of the electronic density of each independent site, and the ion charges of Ca 2+ and Y 3+ ions as well as the ratio of their atomic number in the formula, we can calculate their ratio in each site. Concretely, the method for calculating the ratio of Yb/Ca is as follows: we suggest the average atomic number of M n atom: statistical distribution of Ca 2+ , Y 3+ and Yb 3+ ions might lead to the increase of width of spectra of this crystal. As a matter of fact, this was confirmed by the next part of this report. Table 3-1 presents the atomic coordinates and thermal parameters.

The crystal growth
The crystal was grown by Czochralski method. The crystal growth was carried out in a DGL-400 furnace with KGPF25-0. Thoroughly mixed and pressed mixtures of the stoichiometric composition were slowly heated to 500 0 C at a rate of 50 0 C/h and further to the synthesis temperature at the rate of 150 0 C/h in a Pt-crucible. Then the sintered compound was melt in the Ir-crucible under N 2 atmosphere, at a temperature which was 50 0 C higher than the crystallization temperature, and was kept at this temperature for one hour. Seeding was performed on the Ir-wire. The nitrogen gas pressure is 0.04MPa. Pulling rate and the rotation rate was 1.3~1.5mm/h and 12-20 r.p.m, respectively. When the growth process was ended, the crystal was drawn out of the melt surface and cooled down to room temperature at a rate of 10~30 0 C/h. The transparent single crystals with a size up to φ20 mm×55 mm was obtained (as shown in Fig.3.2). Fig.3.3 shows the interference fringe of the grown Yb 3+ :Ca 3 Y 2 (BO 3 ) 4 crystal, the optical homogeneity is 4×10 −5 , it means the crystal has excellent quality. Table 3-2 presents the parameters of crystal growth. In order to estimate the solubility of the doping ion in the crystal, it is customary to use an "effective segregation coefficient" defined as [41] : k e =C s /C l , C s is the doped-ion concentration in the crystal and C l is the doped-ion concentration in the melt, since the concentration of Yb 3+ ion in Yb 3+ :CYB was measured to be 1.56wt% by electron probe microanalysis method ,the effective segregation coefficient of Yb 3+ ion in CYB crystal was calculated to be 0.97. Fig. 3 [43] . The room temperature emission spectrum of Ca 3 Y 2 (BO 3 ) 4 :Er 3+ crystal excited at 530 nm is presented in Fig.3.7, in which there is a broad emission band ranged from 1429.4 nm to 1662.8 nm. The FWHM of this emission band is 126 nm. The factor contributing to this broad emission is the disordered structure of the crystal, namely, Ca 2+ and Y 3+ ions are statistically situated in three different lattices in the crystal structure determined by us. This broad emission will benefit the energy storage. Therefore, this crystal should be useful as a tunable infrared (in the eye-safe region at 1.54 m) laser crystal.
Based on the measured absorption spectra of three commutative perpendicularity directions and J-O theory, the J-O parameters are calculated to be Ω 2 =1.214×10 -20 cm 2 , Ω 4 =1.585×10 -20 cm 2 , Ω 6 =1.837×10 -20 cm 2 . The oscillator strength, radiative transition probability A, radiative lifetime rad and the fluorescent branching ratio β are also calculated, which are shown in   4 :Yb 3+ crystal, in which there is a broad absorption band ranged from 850 nm to 1000 nm, corresponding to the transition from 2 F 7/2 2 F 5/2 . [6] The FWHM at 977 nm is about 12 nm and the cross-section is about 1.9×10 -20 cm 2 , which is benefit to the pumping of commercial laser diode.  , 4 G 9/2 , 2 G 9/2 + 4 F 9/2 + 2 H 9/2 , 4 F 5/2 , 2 H 11/2 , 4 S 3/2 , 4 F 9/2 , 4 I 9/2 , 4 I 13/2 , respectively [56] . The room temperature emission spectrum of Ca 3 Gd 2 (BO 3 ) 4 :Er 3+ crystal excited at 530nm is presented in Fig.3.11, in which there is a broad emission band ranged from 1460 nm to 1600 nm. The FWHM of this emission band is 126 nm, which is resulted from the statistical distribution of Ca 2+ , Gd 3+ and Er 3+ ions. Based on the measured absorption spectra of three commutative perpendicularity directions and J-O theory, the J-O parameters are calculated to be Ω 2 = 4.01×10 -20 cm 2 , Ω 4 =0.98×10 -20 cm 2 , Ω 6 =1.72×10 -19 cm 2 . Comparing the parameters with those of the other Er 3+ doped crystal, we found that the parameters are larger. The oscillator strength, radiative transition probability A, radiative lifetime rad and the fluorescent branching ratio β are also calculated, which are shown in Tables 3-6~3-8. The stimulated emission cross-section at 1535nm is calculated to be 6.0×10 -21 cm 2 . Fig.3.12 shows the fluorescence lifetime of Er:CGB crystal under the excitation of 530 nm. The lifetime measured is about 792 s, so the luminescent quantum efficiency of the 4 I 13/2 manifold is estimated to be 20%.     Fig.3.14, in which there is a broad emission band ranged from 930 nm to 1100.7 nm. The FWHM of this emission band is 72.6 nm and its peak is located at 1020nm, which is resulted from the statistical distribution of Ca 2+ , Gd 3+  An passively mode-locked Yb:Y 2 Ca 3 (BO 3 ) 4 (Yb:CYB) laser with a partially reflective semiconductor saturable-absorber mirror was achieved. The 244 fs pulses with a repetition rate of ~55 MHz were obtained at the central wavelength of 1044.7 nm. The measured average output power amounted to 261 mW. This was the first demonstration of femtosecond laser in Yb:CYB crystal. Fig.3.15 shows the experimental setup of the laser oscillator. The input mirror M1 was a flat mirror coated with high reflection (HR) in a broad band from 1010 to 1060 nm and high transmission (HT) at 976 nm. The two folding mirrors, M2 and M3, were concave and had the radii of curvature of 1000 and 500 mm, respectively. Both of which were HR-coated placed near normal incidence (~3º). A SESAM with a reflection of 96% at 1040 nm was employed, which had a modulation depth of 1.6% and saturation fluence of 70 µJ/cm 2 . Fig.3.16 presents the continuous wave and mode locking average output power versus the absorbed pump power. We can see that the threshold absorbed pump power was 1.9 W and a maximum output power of 783 mW was obtained under the absorbed pump power of 7.0W. The laser oscillation was achieved with the threshold absorbed pump power of 2.8 W when the output coupler was replaced by the SESAM. Within the range of absorbed pump power from threshold to 4.6W, a metastable regime rapidly alter-nating between Q-switched mode locking and continuous wave (CW) mode locking was observed. Fig.3.17 presents the central wavelength and FWHM of the emission spectrum for mode locking operation. The spectrum was red-shifted obviously at a range from 1041.5 to 1044.7 nm with the absorbed pump power increased from threshold to 7.0 W, which was possibly attributed to the reabsorption effect for quasi-three-level system as the short wavelength part of the absorption spectrum overlaps the emission spectrum. Fig.3.18 shows the pulse train of the cw mode-locked laser with the repetition rate of ~55MHz. Fig.3.19 presents the autocorrelation trace of the 244 fs pulse with the average output power of 261 mW at the central wavelength of 1044.7 nm. The corresponding spectrum had a FWHM of 8.1 nm centered at 1044.7 nm, with a time bandwidth product of 0.54. In this job, a partially reflective SESAM was used as the output coupler that would lower the positive dispersion. When the absorbed pump power was fixed at 7.0 W, the adjustment of the Yb:CYB crystal and SESAM in such a resonator (by either moving or rotating that could vary the amount of www.intechopen.com material that the light went through) is critical for the stability of mode-locking operation and pulse duration: an average output power of 375 mW could be obtained but the mode locking was unstable; the duration also fluctuated in a wide range from ~1000 to 244 fs. The minimum pulse width is 79 ns at the repetition rate of 1.7 kHz. The pulse energy and peak energy are calculated to be 231 J and 2.03 kW, respectively. In Q-switched modelocking case, the average output power of 64 mW with a mode-locked pulse repetition rate of 118 MHz and Q-switched pulse energy of 48 J is generated under the absorbed pump power of 6.1W. Fig.3.20 presents the CW and Q-switched average output power versus absorbed pump power. The CW lasers were operated with the absorbed pump power of up to 0.8 W and 1.3 W, respectively, for T = 1% and T = 5% output coupler. When the absorbed pump power reaches 6.1W, the T = 5% output coupler provides the best performance with an output power of 992 mW, which is much higher than 760 mW by using T = 1% output coupler. Fig.3.21 gives the emission spectra of the Yb:CYB laser with plano-concave cavity configuration. (a) is in the CW situation with T = 5% output coupler. (b)-(d) are in the Qswitching situation with T = 5% output coupler, (e) is in the Q-switching situation with T = 1% output coupler the emission spectra of the Yb:CYB laser. Fig.3.22 shows the pulse width versus absorbed pump power. At the pulse repetition rate of 1.7 kHz and the absorbed pump power 6.1 W, the pulse widths of 79 ns and 114 ns are detected by used of T = 1% and 5% coupler, respectively. Fig.3.23 presents the pulse energy and pulse peak power versus the absorbed pump power at the pulse repetition rate of 1.7kHz. The pulse energy of 231 J and peak energy of 2.03 kW can be obtained with the T = 5% output coupler. Fig.3.24 gives the single pulse profile of the A-O Q-switched Yb:CYB lasers with the pulse width of 76 ns at the absorbed pump power of 6.1 W, and the inset corresponding to the temporal pulse trains with the repetition rate of 1.7 kHz. The beam quality M2 is measured to be about 1.4 by using the knife-edge scanning method.   Pulse width (ns)

Absorbed pump power (W)
Q-S T=1% 1.7kHz Q-S T=1% 5kHz Q-S T=1% 10kHz Q-S T=5% 1.7kHz Q-S T=5% 5kHz Q-S T=5% 10kHz  nm. Similar to that in the Q-switched mode, the spectrum shifts to the long side with the increase of the absorbed pump power owing to the reabsorption effect. When the absorbed pump power reaches 6.1 W, a new branch in the short side (1040 nm) appears. The CW and Q-switched average output power versus absorbed pump power for T = 1% and 5% are plotted in Fig.3.27, (a) is T = 1% and (b) is T = 5%. The average output powers of 64 mW and 87 mW are obtained for T = 1% and T = 5% output coupler, respectively. Fig.3.28 shows the pulse energy of the Q-switched envelope versus the absorbed pump power at repetition rate of 1.7 kHz. In Fig.3.29, (a) is the oscilloscope traces of Q-switched pulse train with the T = 5% output coupler and the repetition rate of 1.7 kHz under the absorbed pump power of 6.1 W in the same situation. (b) is the typical QML pulse envelope of T = 5% in the same situation. (c) is the expanded traces of mode-locked train. The repetition rate of the periodic modelocked pulses is about 118 MHz, which matches exactly with the axial mode interval. The output beam density distribution is close to the fundamental transverse mode (TEM00) and the quality parameter M2 factor is about 1.6.    The Yb 3+ -doped Y 2 Ca 3 B 4 O 12 diode-pumped laser operation in both continuous-wave (CW) and passively Q-switched modes was reached. The differential slopes of the CW output power are in the 22-40 % range under different experimental conditions. Continuous tuning of the laser wavelength is obtained in the 1020-1057 nm range, in agreement with the broad emission spectra. In pulsed regime the repetition rate occurs up to 1.6 kHz and pulse energies of 30-75 µJ with about 40 ns duration are obtained. Fig.3.30 demonstrates the polarized emission spectra of the CYB:Yb 3+ crystal used for laser experiments. The two orthogonal polarizations of the eigenstates are labeled as H and V. The main peak at 976.3 nm has 6 nm full width at half maximum (FWHM) and is suitable for diode pumping. Fig.3.31 displays the spectral distribution of the laser emission in CW and passive Q-switch modes. We can see that the more intense one corresponds to the H polarization with a broadband peaking near 1040 nm. The time evolution of the fluorescence was displayed with a 9410 Lecroy oscilloscope. The decay time was found to be 1 ms. Fig.3.32 presents repetition rates and pulse energies obtained in passive Q switching the Ca 3 Y 2 (BO 3 ) 4 :Yb 3+ laser. Lasing was obtained in H polarization in agreement with the polarized emission spectra, near 1045 nm (with the 97.5% transmission coupler) and up to 1 W power. Fig.3. 33 shows the tunability of the laser emission obtained from rotation of a birefringent filter. Fig.3.34 presents the laser output power versus pump power obtained under different experimental conditions. The obtained slope efficiencies were in the 29%-40% range with the 5 cm radius curvature output coupler and 22% with the 7.5 cm coupler. Table 3-10 shows the pulse energy obtained in passive Q-switching different Yb 3+ -doped hosts. In particular, we can see that the performances for Yb 3+ doped GGG and GAB crystals obtained with similar experimental conditions were better than for Yb 3+ :CYB, with no instability of the pulsed regime and less thermal problems. A plausible explanation is the lower laser emission cross section in Yb 3+ :CYB and a too low absorbed pump power (61% absorption) of our sample.     Fig.3.35 represents the experimental setup of the CW Yb:CGB laser oscillator. M1 was a plane mirror with antireflection coating at the pump wavelength and high-reflection coating at a broad band from 1040 to 1070 nm. A concave mirror with 75-mm curvature radius and ~99% reflectance from 1040 to 1070 nm was used as the output coupler M2. Fig.3.36 shows the absorbed pump power and absorption efficiency versus incident pump power for the two Yb:CGB samples, with the same cross section of 3×3 mm 2 but different lengths of 2 and 5 mm (described as sample 1 and 2, respectively). It can be seen that the absorption efficiency of sample 1 was around 40% if the incident pump power was below 4.0 W. But the efficiency decreased dramatically from 40% to 30% when the incident pump power was increased from 4.0 to 8.0 W. Then the efficiency was stable again, varying within a narrow range of 30% ~ 32 %. That was possibly attributed to the saturation of pump absorption to some extent, resulting from the depletion of the population in ground state. The similar phenomenon was observed when sample 2 was tested. Fig.3.37 depicts the relationship between the output power (P out ) and absorbed pump power for the two samples. The laser operation was realized with threshold absorbed pump powers of 0.4 and 0.9 W for sample 1 and 2, respectively. The maximum output power was 1.4 W by using sample 2 under the absorbed pump power of 6.8 W, with a slope efficiency of 23.7% and an optical conversion efficiency of 20.6%. The sample 1 exhibited higher slope efficiency of 30.3% and optical conversion efficiency of 27.0% with the output power of 1.0 W under the absorbed pump power of 3.7 W. Fig.3.38 gives the emission wavelengths versus absorbed pump power for the two Yb:CGB samples. Fig.3.39 and Fig.3.40 shows the emission spectra of the simultaneous multi-wavelength Yb:CGB laser with sample 1 and sample 2, repectively. It can seen that the emission wavelengths at each stage were almost same in intensity. That is advantageous to the practice terahertz-wave generation. If the quintuple-wavelength simultaneous emission is employed, multiple terahertz waves can be generated theoretically through difference frequency nonlinear interaction. Furthermore, it is interesting to note the separation of emission peaks varied from 1.0 to 2.0 nm with the absorbed pump power and Yb:CGB crystals, which means the multi-wavelength CW Yb:CGB laser could support the tunable terahertz-wave generation from 0.27 to 2.16 THz as calculated from Fig.3.38. Our experiment also showed that the reabsorption effect in quasi-three-level laser systems depended on the length of laser medium and the level of pump intensity. In addition, this effect had a great influence on the laser characteristics such as output power, optical efficiency and emission wavelength.

The crystal growth
Nd 3+ -doped LaBO crystal with size up to ф20 mm×35 mm was grown using the Czochralski technique by Dr.Guohua Jia [4] . When the crystal was cut into laser bulk, it split into the cleavage crystal with the size of 2.5 mm×9 mm×35 mm as shown in Fig.4.1. The room temperature absorption spectrum (Fig.4.2) consists of 10 groups of bands, which are associated with the observed transitions from the 4 I 9/2 ground state. The absorption spectrum of the LaB 3 O 6 :Nd 3+ crystal reaches its maximal value at about 800 nm and its FWHM is about 16 nm. The absorption cross-section was measured to be abs = 3.37 × 10 −20 cm 2 . [7] This stronger absorption band corresponding to the transition 4 I 9/2 2 H 9/2 is very favorable for commercial GaAlAs diode pumping [4] . The room temperature emission spectrum with the light perpendicular to <1 1 1> planes is presented in Fig.4.3. The 4 F 3/2 the emission cross-section at 1062 nm of LaB 3 O 6 :Nd 3+ is 3.46 × 10 −20 cm 2 , which is a little smaller than that of other Nd 3+ doped crystals. The emission cross-section and branching ratio (β) of the 4 F 3/2 4 I 9/2 transition are centered at 891 nm. The values of the emission cross-section at 891 nm and the branching ratio of this transition are 4.07 × 10 −21 cm 2 and 0.336, respectively. Fig. 4.4 shows the room temperature fluorescence decay curve of LaB 3 O 6 :Nd 3+ crystal from which the fitting result of single exponential decay is 44.465 ns.

3+ -doped LaB 3 O 6 crystal
A method utilizing an unprocessed Nd 3+ -doped LaB 3 O 6 crystal cleavage microchip as the solid-state laser gain medium was proposed by Prof. Huang [10] . Pumped by a Ti:sapphire laser at 871 nm, 1060 nm continuous-wave laser emission with slope efficiency of 23% has been achieved in an unprocessed microchip directly obtained from a cleavage Nd 3+ :LaB 3 O 6 crystal. Fig.4.5 shows the infrared laser output power at 1060 nm as a function of absorbed pump power at 871 nm. A maximum output power of 112 mW was obtained when the absorbed pump power was 580 mW. The laser performance of the unprocessed cleavage Nd 3+ :LaBO microchip cannot compare with those of other microchip lasers yet, such as widely investigated Nd 3+ :YAG and Nd 3+ :YVO 4 . [91~94]

Summary
The growth, thermal, optical and spectrum characteristics and laser characteristics of rare earth-doped Ln 2 Ca 3 B 4 O 12 (Ln = La, Gd, or Y) double borate family laser crystals, Ca 3 (BO 3 ) 2 and LaB 3 O 6 laser crystals were reviewed.
From a passively mode-locked Yb:Y 2 Ca 3 (BO 3 ) 4 (Yb:CYB) laser, the 244 fs pulses with a repetition rate of ~55 MHz were obtained at the central wavelength of 1044.7 nm. The measured average output power amounted to 261 mW. Q-switching and Q-switched modelocked Yb:Y 2 Ca 3 B 4 O 12 lasers with an acousto-optic switch were also demonstrated. In the Qswitching case, an average output power of 530 mW was obtained at the pulse repetition rate of 10.0 kHz under the absorbed pump power of 6.1 W. The minimum pulse width is 79 ns at the repetition rate of 1.7 kHz. The pulse energy and peak energy are calculated to be 231 J and 2.03 kW, respectively. In Q-switched mode-locking case, the average output The differential slopes of the CW output power are in the 22-40% range under different experimental conditions. Continuous tuning of the laser wavelength is obtained in the 1020-1057 nm range, in agreement with the broad emission spectra. Also, the diodepumped multi-wavelength continuous-wave laser operation of the disordered Yb:Ca 3 Gd 2 (BO 3 ) 4 (Yb:CGB) crystal was reached. An output power of 1.4 W was obtained when quadruple wavelengths were emitted simultaneously, corresponding to a slope efficiency of 23.7%.
Finally, the laser property of microchip Nd 3+ :LaB 3 O 6 crystal are reviewed. Pumped by a Ti:sapphire laser at 871 nm, 1060 nm continuous-wave laser emission with slope efficiency of 23% has been achieved in an unprocessed microchip directly obtained from a cleavage Nd 3+ :LaB 3 O 6 crystal. A maximum output power of 112 mW was obtained when the absorbed pump power was 580 mW.