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Abstract
We report, for the first time to our knowledge, on the polarized absorption and emission spectra and efficient laser action of the Yb:Sr3La2(BO3)4 disordered crystal. The strongest absorption and emission occur at about 977 nm for polarization direction parallel to the crystallographic b axis, with the peak absorption and emission cross-sections amounting, respectively, to 1.95 × 10−20 and 2.45 × 10−20 cm2. Efficient continuous-wave laser action is demonstrated with a-, b-, and c-cut crystal samples longitudinally pumped by a 976-nm diode laser, producing an output power of up to 8.2 W with an optical-to-optical efficiency of 32% with respect to incident pump power. Depending on the output coupling utilized, the laser action could exhibit a transition from single-polarization oscillation to dual-orthogonal-polarization oscillation at a certain pump level.
© 2017 Optical Society of America
1. Introduction
Nowadays, Yb-ion solid-state lasers have occupied an important position in coherent sources working in the 1-μm spectral region. This is particularly true for femto-second pulsed laser sources, since the broad emission band characteristic of most Yb-ion crystals can greatly benefit mode-locking laser action, leading to ultra-short laser pulses. Among the various host crystals for trivalent rare-earth active ions, the strontium double borates, Sr3Re2(BO3)4, with Re denoting La, Gd, Lu, and Y, are of special interests for the Yb ion. These borates belong to the orthorhombic space group of Pnma, offering three different substitution sites for Yb ions, viz., two distinct eight-coordinated sites occupied by Sr ions, and one seven-coordinated site occupied by Re ions [1]. As a consequence, when doped into these strontium double borates, the Yb ion will present a certain degree of structural disorder and additional inhomogeneous line broadening, resulting in wide absorption as well as emission bands.
Up to now, the most extensive studies of these strontium borates have been on Er, Yb co-doped crystals, including Er:Yb:Sr3Y2(BO3)4, Er:Yb:Sr3Gd2(BO3)4, Er:Yb:Sr3Lu2(BO3)4 [2–5]; spectroscopic properties have been determined, and laser operation at 1.5−1.6 μm has been realized. Nd-doped crystals such as Nd:Sr3Y2(BO3)4 [6–8], Nd:Sr3Gd2(BO3)4 [9], and Nd:Sr3La2(BO3)4 [10], have also been investigated. For Yb-doped strontium double borates, the research work was mainly focused on the Yb:Sr3Y2(BO3)4 crystal [11–13]; ultra-short laser pulses of 58 fs in duration have been generated recently through Kerr-lens mode-locking [13]. Apart from Yb:Sr3Y2(BO3)4, other Yb-doped crystals in this class received much less attention. The relevant work was limited to crystal growth and preliminary spectroscopic studies of Yb:Sr3La2(BO3)4 and Yb:Sr3Gd2(BO3)4 [14, 15].
In this paper we report, for the first time to our knowledge, on the polarized spectroscopic properties and continuous-wave (cw) laser performance of Yb:Sr3La2(BO3)4 disordered crystal. Polarized absorption and emission spectra, with polarizations parallel to the crystallographic a, b, and c axes, are presented in terms of cross-sections. Efficient cw laser action was realized under 976-nm diode pumping, producing a maximum output power in excess of 8 W. More importantly, complex polarization state variation behavior was observed in the laser operation, which was found depending largely on the crystal orientation, showing strong anisotropy in the laser properties.
2. Description of experiment
The Yb:Sr3La2(BO3)4 (Yb:SLB) crystal was grown by the Czochralski method in nitrogen atmosphere containing 2% of oxygen. The Yb-ion concentration in the melt was 5 at. %. Taking a value of 0.65 for the segregation coefficient [14], we determined the actual Yb ion concentration in the crystal to be 3.25 at. % (2.4 × 1020 cm−3). The magnitude of the segregation coefficient proves to be very close to that for either Yb:Sr3Y2(BO3)4 (0.73) or Yb:Sr3Gd2(BO3)4 (0.63) [12, 16], the other two members of the strontium double borates. The reasons responsible for such low segregation coefficient are still not very clear, but probably are related to the multiple sites for Yb-ion substitution, some of which requiring charge compensation. Three different crystal samples were utilized, which were cut along the crystallographic a, b, and c axes, 4 mm long, with a square aperture of 3 mm × 3 mm. No antireflection coatings were deposited on their end faces. To study the basic laser properties of the Yb:SLB crystal, we employed a simple compact plano-concave resonator. A plane mirror, serving as the reflector, was coated for high reflectance at 1020−1200 nm (>99.9%) and for high transmittance at 808−980 nm (>98%). The output coupler was a concave mirror having a radius-of-curvature of 25 mm, its transmission (output coupling) could be chosen from T = 0.5% to T = 20%. The physical cavity length was 23 mm. To evaluate the potential of the Yb:SLB crystal in generating high output power, the crystal sample was fitted into a copper holder which was cooled with cycling water at a temperature of 5 °C. The crystal was positioned near the plane reflector in the cavity. To pump the Yb:SLB crystal, a fiber-coupled diode laser emitting unpolarized radiation at 976 nm (emission bandwidth of less than 0.5 nm) was used. The fiber core diameter and NA were 200 μm and 0.22, respectively. The pump radiation from the fiber end was first focused by a re-imaging unit and then was coupled through the plane reflector into the Yb:SLB crystal, with a beam spot radius of about 70 μm. For the 4 mm long, a-cut Yb:SLB crystal sample (Yb-ion concentration of 3.25 at. %), the small-signal absorption for the 976-nm unpolarized pump radiation was measured to be 0.80, which turned out to be suitable for achieving efficient laser action in the presence of resonant absorption losses. The plano-concave resonator configuration utilized in the experiment, is typical for studying Yb-ion lasers; it provides a small mode size (50 μm of spot radius, nearly independent of thermal lensing) that enables efficient operation of a quasi-three-level laser suffering from resonant absorption losses. The pump to laser mode size ratio was 1.4, which could result in higher-order mode oscillation under high pump levels. In spite of the small mode size, no damage occurred to the Yb:SLB crystal samples.
3. Results and discussion
Yb:SLB is an orthorhombic biaxial crystal, its principal optic axes coincide with the three crystallographic axes. Consequently, the polarization-dependent absorption and emission spectra, as well as the polarization nature of the laser action, can be characterized with respect to the crystallographic a, b, and c axes.
The polarized absorption spectra of the Yb:SLB crystal were measured at room temperature by use of a spectrophotometer, with a spectral resolution of 0.2 nm. Figure 1(a) shows the absorption cross-section (σabs) versus wavelength (λ) over a range of 850−1100 nm, for the three different polarizations of E//a, E//b, and E//c. Shown in Fig. 1(b) are the polarized emission cross-section (σem) spectra with a spectral resolution being also 0.2 nm, which were calculated on the basis of the measured σabs(λ), along with the room-temperature fluorescence spectra, by a combination of the modified reciprocity method and the Füchtbauer-Ladenburg (F-L) method [17, 18]. The F-L method was used only for the long wavelength region (> 1050 nm), where the absorption becomes very low and the fluorescence spectrum can be regarded as representing the real emission line-shape. Therefore, there is no need to calibrate the fluorescence spectra for the spectral response of the system. The radiative lifetime and the index of refraction, required for the calculation, were taken to be τr = 0.59 ms and n = 1.73 [14]. The radiative lifetime is not likely to be dependent on the Yb-ion concentration, owing to the little or no concentration quenching effect that is characteristic of Yb-ion crystals. From Fig. 1(a) one sees the strongest absorption occurs at λ = 977.2 nm with polarization direction parallel to the b axis, giving a maximum absorption cross-section of 1.95 × 10−20 cm2, while the bandwidth amounting to 7.3 nm (FWHM). For light polarization along the a axis, the peak absorption, which occurs at λ = 976.9 nm, proves to be much weak, with the maximum absorption cross-section being only 1.19 × 10−20 cm2. In the case of E//c, a peak absorption cross-section of 1.69 × 10−20 cm2 is measured at λ = 977.3 nm. Similar to the situation of absorption, the strongest emission is also observed, as shown in Fig. 1(b), with the polarization direction parallel to the b axis; the peak emission cross-section, σem = 2.45 × 10−20 cm2, is measured at 977.5 nm, with an emission bandwidth of 8.5 nm (FWHM). For the polarization of E//a, the maximum emission cross-section amounts only to 1.46 × 10−20 cm2, which is the smallest among those for the three polarization directions. The significant difference in the magnitude of peak absorption as well as emission cross-sections for different polarization directions, implies the presence of strong anisotropy in spectroscopic properties of the disordered Yb:SLB crystal. It is worth mentioning that laser oscillation of the strongest zero-phonon emission at 977.5 nm will be very difficult to achieve, because of the presence of very strong absorption at this wavelength. Of more practical importance for laser application is the long wavelength emission band ranging from about 1020 to 1080 nm. In this region, a pronounced emission peak can be recognized, which is located, for different polarizations, at 1022 nm (E//a, σem = 0.69 × 10−20 cm2), 1020 nm (E//b, σem = 0.79 × 10−20 cm2), and 1021 nm (E//c, σem = 0.75 × 10−20 cm2). Besides this emission peak, there exists a second, less pronounced emission peak at about 1049 nm for E//a; while for E//b or E//c, the emission cross-section decreases monotonically with wavelength. The emission for E//b is found stronger (or slightly stronger) than for E//a or for E//c, valid for wavelengths up to 1070 nm.
Fig. 1 Room-temperature polarized absorption cross-section spectra (a) and emission cross-section spectra (b) determined for the Yb:SLB disordered crystal. A diagram of energy levels is illustrated as an inset to (b).
The Stark levels could be deduced very roughly from the absorption and emission spectra, for the ground state (2F7/2) and the excited state (2F5/2) of the Yb ion in the SLB crystal. The resulting energy level diagram is illustrated as an inset to Fig. 1(b).
Making use of the simple plano-concave resonator configuration described in the preceding section, we achieved efficient cw laser operation under different output coupling conditions, with all the three a-, b-, and c-cut Yb:SLB crystal samples. Illustrated in Fig. 2 are the output characteristics of the a-cut crystal laser for different output couplings. Due to the very strong absorption saturation of the Yb:SLB crystal at the pump wavelength of 976 nm, the absorbed pump power is difficult to be estimated accurately. As a result, the laser performance is discussed in terms of incident pump power (Pin). Under the lowest output coupling conditions of T = 0.5%, the laser oscillation arrived at threshold at Pin = 0.10 W, with a linear polarization of E//b; this single-polarization operation could be maintained until the Pin was increased to a certain transition pump power, Pt = 7.30 W, at which the second polarization component, E//c, started to oscillate, and the laser entered an operational regime in which two orthogonal polarization components coexisted. This coexistence region extended to the highest pump level where the output power began to saturate. Such an evolution behavior of polarization state was common for the cases of T ≤ 3%. With a larger output coupling the transition pump power became higher, as indicated in Fig. 2(a) for the case of T = 3%. In the coexistence region, the output power of the second polarization component (E//c) would initially increase with pump power, but the increasing was not always monotonic. In general, the power ratio of the two orthogonal polarization components would vary with pump level, depending upon the detailed gain competition, which, in turn, would depend on the output coupling utilized. Under output coupling conditions of T ≥ 5%, the laser oscillation, achieved with the a-cut Yb:SLB crystal, could remain in the single linear polarization of E//b over the entire pump range. The output coupling of T = 5% was found to be the optimal, which resulted in the most efficient laser operation, producing a maximum output power of 7.56 W at Pin = 25.3 W, the optical-to-optical efficiency was 30%. One notes from Fig. 2(b) that the laser action became less efficient in excess of Pin ≈20 W, a common feature for all cases of different output couplings. The reduction in laser efficiency was attributed to the thermally induced losses, which would become strengthened with increasing pump power. A slope efficiency of 33% was determined over the operational region of Pin ≤ 20 W, for the case of T = 5%; for other cases of T = 10%−20%, the slope efficiencies ranged from 31% to 22%.
Fig. 2 Output power versus incident pump power for the a-cut Yb:SLB crystal laser, measured under output coupling conditions of T = 0.5%, 3% (a); and T = 5%, 10%, 15%, 20% (b).
Figure 3 shows the laser emission spectra measured under different operational conditions. In the case of T = 0.5%, the a-cut Yb:SLB laser oscillated at wavelengths of 1061.5−1066.6 nm, when operated with linear polarization of E//b in the low pump power region; entering the coexistence regime under high pump levels, the laser would oscillate over a wider wavelength range with the short limit extending to 1056.0 nm. Similar situation occurred in the case of T = 3%. In general, the emission spectra for the two orthogonal polarization components overlapped to a large extent. The variation behavior of emission wavelengths with output coupling became simple in the cases of T ≥ 5%; upon changing the output coupling from T = 5% to T = 20%, the laser emission band shifted from 1035.6−1042.0 nm to 1022.0−1026.6 nm.
Fig. 3 Emission spectra of the a-cut Yb:SLB crystal laser, measured for T = 0.5%, 3% (a); and measured at Pin = 12.9 W in the cases of T = 5%, 10%, 15%, 20% (b).
We have seen that for the a-cut Yb:SLB laser operating under certain conditions, a coexistence region could be approached where two orthogonal polarization components could oscillate simultaneously. Such laser behavior is intrinsically connected with the spectroscopic properties of the crystal. As is known, the effective gain cross-section for a quasi-three-level laser species like Yb ion, σg(λ), is determined by the emission as well as the absorption cross-sections. It can be expressed as σg(λ) = βσem(λ) − (1−β)σabs(λ), with β denoting the fraction of the Yb-ions that have been excited to the upper manifold (2F5/2). For free-running laser operation, the oscillation will occur at those wavelengths where the effective gain cross-section, and thus the overall gain, reaches its maximum.
Depicted in Fig. 4 are polarized effective gain cross-section curves for E//b and E//c, which were calculated for β = 0.05, 0.1 and 0.2. One can, on the basis of these curves, qualitatively understand the laser behavior of the a-cut Yb:SLB crystal. Under steady-state conditions of laser oscillation, for the very low excitation level, β = 0.05, corresponding to a very small output coupling, the highest gain occurs in the emission band peaked at about 1061 nm; increasing the amount of β will make the maximum gain shift toward short-wavelength side, reaching a peak wavelength of 1023 nm for β = 0.2. One sees from Fig. 3 that the evolution of emission wavelengths upon increasing the output coupling agrees fairly well with the varying tendency of the gain maximum. Examining the curves for β = 0.05 and β = 0.1, one notes that over the wavelength range where the laser action is likely to occur, the gain cross-sections for E//b and E//c are very close, suggesting the presence of strong gain competition between the two orthogonal polarization states. This is the reason for the occurrence of a two-polarization coexistence region in the laser operation achieved with low output couplings of T ≤ 3%. One can also notice from Fig. 4 that with the parameter β increased to a large enough value, e.g., β = 0.2, the effective gain for E//b will be significantly greater than for E//c, at those wavelengths where the laser action is expected. This roughly explains the single-polarization laser operation obtained under high output coupling conditions (Fig. 2(b)).
Fig. 4 Polarized effective gain cross-section as a function of wavelength for different values of the parameter β.
Figure 5 shows the output characteristics of the b-cut Yb:SLB crystal laser operating under different output coupling conditions. In the case of T = 0.5%, the laser reached threshold at Pin = 0.10 W simultaneously in the two orthogonal polarizations of E//a and E//c; the two polarization components remained coexisting over the whole pump power range. The same laser behavior was also observed in other cases when the output coupling fell in a specific range: 0.5% < T ≤ 3%. With the output coupling further increased, the laser would start and remain oscillating in the linear polarization of E//c until a transition pump power, Pt, above which the second polarization component (E//a) would begin to oscillate, and the laser would enter the coexistence region. This was the case for T = 5% and T = 10%; with a larger output coupling, a higher Pt was measured. Only when the output coupling was increased to T ≥ 15%, could single-polarization (E//c) oscillation be maintained over the entire pump power range. Clearly, the dependence of polarization state on output coupling, demonstrated with the b-cut crystal laser, was quite distinct from the performance of the a-cut one (Fig. 2). Similar to the a-cut crystal laser, the optimal output coupling was also T = 5%. Under the optimal output coupling conditions, a maximum output power of 7.72 W was generated at Pin = 25.8 W, the optical-to-optical efficiency being 30%, while the slope efficiency was determined to be 34%.
Fig. 5 Output power versus incident pump power for the b-cut Yb:SLB crystal laser, measured under output coupling conditions of T = 0.5%, 5%, 10% (a); and T = 15%, 20% (b).
The emission spectra of the b-cut crystal laser, measured under different operational conditions, are illustrated in Fig. 6. Similar to the situation of the a-cut crystal laser, in the two-polarization coexistence region, the emission band for one polarization component overlapped with that for the other. One notes from Fig. 6(b) that the emission band for T = 20%, 1025.7−1028.6 nm, shifted to long-wavelength side, in comparison with its counterpart of the a-cut crystal laser (1022.0−1026.6 nm).
Fig. 6 Emission spectra of the b-cut Yb:SLB crystal laser, measured for T = 0.5%, 5%, 10% (a); and measured at Pin = 12.9 W in the cases of T = 15%, 20% (b).
Figure 7 shows the output power versus the incident pump power, produced from the c-cut Yb:SLB laser operating under different output coupling conditions. Through a comparison with Fig. 2, one can see the close similarity in laser performance between the a- and c-cut crystals. Such a similarity resulted from the fact that in both cases, the laser action was dominated by the absorption as well as the emission for E//b, which were stronger than for the other competing polarization component involved. However, in contrast to the case of the a-cut crystal, the transition pump power measured here was much lower for T = 0.5%, and was higher for T = 3%, leading to a very large ΔPt that amounted to 8.35 W (compared to ΔPt = 2.32 W, as shown in Fig. 2 for the a-cut crystal). What was responsible for the distinct ΔPt was the difference between the emission (absorption) spectrum for E//a and its counterpart for E//c. In essence, it is the gain competition between the E//b and E//a (or E//c) polarizations that governs the polarization evolution behavior exhibited in the laser action of the c-cut (or a-cut) crystal. Similar to the situation of a- or b-cut crystal, the most efficient laser action was also achieved with an output coupling of T = 5%, producing a maximum output power of 8.20 W; the corresponding optical-to-optical and slope efficiencies were 32% and 36%, respectively. Furthermore, no output power saturation or laser efficiency dropping occurred at the highest pump power applied, which was also the case for higher output couplings of T = 10%, 15%, and 20%. These results suggest that there still exists some room for further power scaling with the c-cut crystal. So long as power scaling is concerned, the c-cut crystal turns out to be superior to both a- and b-cut ones. For a- or b-cut crystal, the laser operation would become less efficient under high pump power levels, as shown in Figs. 2(b), 5(a), and 5(b). The superiority of c-cut crystal might be due to the more favorable thermal properties accessible for this crystal cut.
Fig. 7 Output power versus incident pump power for the c-cut Yb:SLB crystal laser, measured under output coupling conditions of T = 0.5%, 3% (a); and T = 5%, 10%, 15%, 20% (b).
Figure 8 depicts the laser emission spectra for the c-cut Yb:SLB crystal, measured under different operational conditions. For single-polarization laser operation, the emission band extending from 1035.7 to 1041.5 nm for T = 5%, proved to be very close to the corresponding one obtained in the case of the a-cut crystal (1035.6−1042.0 nm); however, the laser action occurred at wavelengths of 1026.0−1029.6 nm under output coupling conditions of T = 20%, which were obviously longer than those measured in the case of the a-cut crystal (1022.0−1026.6 nm).
Fig. 8 Emission spectra of the c-cut Yb:SLB crystal laser, measured for T = 0.5%, 3% (a); and measured at Pin = 12.9 W in the cases of T = 5%, 10%, 15%, 20% (b).
The polarization state of laser radiation is essential for many applications such as electro-optic Q-switching and frequency conversion through nonlinear optical interactions. Due to the wide absorption and emission bands, as well as the close effective gain cross-sections for different polarizations over some emission wavelength ranges, the free-running Yb:SLB laser exhibits complex polarization state varying behavior, which depends on crystal orientation, output coupling, and pump power level. As discussed above, the evolution behavior of polarization state with pump power, observed for a given output coupling, proved to be very similar for the a- and c-cut crystals, except for the parameter ΔPt which was much greater for the c-cut than for the a-cut crystal. However, the polarization properties of the laser action achieved with the b-cut crystal turned out to be quite distinct: firstly, with the output coupling ranging from 0.5% to 3%, simultaneous oscillations in the two orthogonal polarization states (E//a, E//c) could be built, starting from the lasing threshold and remaining over the whole operational pump power range; and secondly, two-polarization operation could be realized under high pump levels until the output coupling was increased to T > 10% (compared to T > 3% for the a- and c-cut crystals).
The variation behavior of polarization state with output coupling, as well as with incident pump power, exhibited by the a-, b-, and c-cut Yb:SLB crystal lasers operating in cw mode, are summarized in Table 1.
Table 1. Operational Conditions and Resulting Polarization States for the a-, b-, and c-Cut Yb:SLB Lasers
One may notice that the crystal samples utilized in the current experiment were not antireflection (AR) coated. For the resonator configuration employed in the Yb:SLB laser, the Fresnel reflection was not likely to constitute a significant loss source, it would be eliminated greatly by some etalon that could be formed within the resonator, e.g., the air etalon formed by the surface of the plane reflector and the front surface of the crystal, with an air gap amounting merely to a fraction of one millimeter. In fact, the laser action could only occur at those wavelengths where the transmission peaks of this air etalon were located, as confirmed evidently by the laser emission spectra (in particular those for higher output couplings such as in Fig. 3(b), consisting of equally spaced, discrete emission lines). Additionally, the very low threshold pump power measured, for instance, in the case of T = 0.5% for the a-cut crystal, Pin = 0.1 W (0.08 W for the absorbed), provides another evidence for the absence of the Fresnel reflection losses experienced by the intracavity circulating laser beam. Consequently, the improvement in laser performance that is to be made, with Yb:SLB crystals which are either AR coated or oriented under Brewster angle, might be very limited.
4. Summary
In conclusion, the polarized absorption and emission cross-section spectra were determined for the Yb:Sr3La2(BO3)4 disordered crystal, showing significant anisotropy. The strongest absorption and emission are found occurring at about 977 nm for polarization direction parallel to the b crystallographic axis, with peak absorption and emission cross-sections being respectively 1.95 × 10−20 cm2 and 2.45 × 10−20 cm2. Efficient continuous-wave laser action was demonstrated with a-, b-, or c-cut crystal sample longitudinally pumped by a 976-nm diode laser, producing maximum output power in the range of 7.6−8.2 W, with optical-to-optical efficiencies of 30%−32% with respect to the incident pump power. Depending upon the output coupling utilized and pump power applied, two orthogonal polarization components were able to coexist in the laser radiation generated. To achieve single-polarization laser oscillation over the entire operational region, an output coupling of T ≥ 5% proved to be necessary for a- or c-cut crystal sample; whereas for b-cut crystal the requirement would be T ≥ 15%. In terms of applications, the c-cut crystal seems to be the most desirable for power scaling in single-polarization laser operation; whereas the b-cut crystal becomes more suitable for making dual-polarization lasers capable of operating over a wide output power range.
Funding
National Natural Science Foundation of China (11574170 and 51402268); Institute of Chemical Materials, China Academy of Engineering Physics (Grant No. 32203).
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