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Abstract
Efficient laser action was demonstrated in the long-wavelength emission sideband of Yb:YCa4O(BO3)3 crystal by effectively suppressing lasing in the main emission band with specific mirror coatings. With an X-cut crystal, a maximum output power of 13.0 W at wavelengths around 1065 nm with E//Z was generated with an optical-to-optical efficiency of 45.1%; with a Z-cut crystal, the highest output power around 1085 nm with E//X amounted to 15.5 W, the optical-to-optical efficiency being 44.4%. The most efficient laser oscillation realized with Y-cut crystal consisted of two orthogonal polarization components, E//Z and E//X, that occurred in different wavelength regions centered at about 1065 and 1084 nm, producing a total output power of 15.0 W with an optical-to-optical efficiency of 46.3%.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The crystal of ytterbium ion doped yttrium calcium oxyborate, Yb:YCa4O(BO3)3 (Yb:YCOB), has been recognized as one of the most promising Yb-ion laser crystals. It proves to be advantageous over other Yb-ion crystals having different structures in laser performance, particularly in power scaling, Q-switching, and mode-locking. With a simple compact plano-concave resonator and under conditions of longitudinal diode pumping, continuous-wave (cw) output power can readily be scaled at room temperature to 15−20 W level, very rare for Yb-ion laser crystals [1].
Yb:YCOB is a monoclinic crystal having a very low symmetry (space group of Cm and point group of m), it has been known since the early days of Yb-ion lasers [2]. During the past several years, a lot of work has been conducted on its various laser performances. With a semiconductor saturable absorber mirror, passive mode-locking was achieved, producing laser pulses with duration as short as 35 fs [3]; Kerr lens mode-locking was also realized, with pulse duration being 73 fs [4]. In thin-disk laser applications, a wide tuning range of 997−1092 nm was obtained, while the cw output power produced could reach a level of 100 W [5, 6]. In passive Q-switching operation with Cr4+:YAG saturable absorber, the pulse energy, generated under conditions of cw pumping, could amount to 0.5−1.3 mJ, which was higher than achieved with other Yb-ion crystals by factors of roughly 3−10 [1, 7]. With GaAs semiconductor crystal utilized as saturable absorber, passively Q-switched laser action was also demonstrated, the average output power and pulse energy generated being 5.7 W and 1.02 mJ, respectively, also the highest ever produced from Yb-ion lasers of this type [8, 9]. The single pulse energy of a compact acousto-optically Q-switched Yb:YCOB laser, produced at low repetition rate of 0.1 kHz, reached 5.3 mJ [10]. Recently a multi-watt Yb:YCOB microchip laser has been demonstrated, producing 8.35 W of cw output power at wavelengths around 1040 nm with a slope efficiency of 70% [11]. Laser operation in a 1084−1090 nm region was also achieved with this microchip laser under very low output coupling (1%) conditions [11].
The laser action demonstrated in the work mentioned above and in earlier experiments [12–15], usually occurred in a wavelength range of about 1020−1060 nm, except for the case of utilizing a very low output coupling. This wavelength range is connected with the main emission band of the Yb:YCOB crystal for practical laser operation [5]. Apart from this main emission band, there exists a long-wavelength emission sideband that is peaked at about 1084 nm [5]. Such a long-wavelength emission sideband seems quite unique; it is found existing only in Yb:YCOB and its isomorphic crystal of Yb:GdCa4O(BO3)3 [16], and results from the great crystal-field splitting of the 2F7/2 ground-state of the Yb ion in these oxyborates (> 1000 cm−1) [16, 17]. Given the fact that currently most practical laser operations, which can be obtained with Yb-ion crystals, are limited to a wavelength region of roughly 1000−1060 nm, it is of interest and importance to investigate the lasing properties of Yb:YCOB crystal in its long-wavelength emission sideband, in order to evaluate the potential of power scaling and to explore other unusual laser behavior.
In this paper we report on our results obtained from such a comprehensive investigation. Our studies indicate that through properly suppressing the high-gain, main-emission-band laser oscillation, efficient laser action could be realized over a long-wavelength region extending from 1059 to 1086 nm; with output power produced amounting to 13−15 W. It is also revealed that the lasing properties in the long-wavelength emission sideband depend largely on crystal orientation, showing strong anisotropy.
2. Description of experiment
To study the lasing properties of the monoclinic Yb:YCOB crystal, three crystal samples, cut along its X, Y, and Z principal optic axes, were prepared. They were 4.4 mm long, with a square aperture of 3.0 mm × 3.0 mm. The Yb ion concentration was 15 at. %, which proves to be the most appropriate for end-pumping by a high-power diode laser [1]. In our laser experiment a very compact plano-concave resonator was employed. The concave output coupler had a radius of curvature of 15 mm, while the cavity length was 13 mm. In order to suppress laser oscillation in the main emission band, the plane mirror, which served as the reflector for the laser radiation in the long-wavelength sideband, was coated for high transmittance at wavelengths of 950−1055 nm (T > 90%), and for high reflectance at wavelengths of 1070−1150 nm (R > 99.5%). The pump source utilized was a 50-W fiber-coupled diode laser emitting at 976 nm (bandwidth less than 0.5 nm), the fiber core diameter was 100 μm and the NA was 0.22. The pump radiation was focused first by a re-imaging unit (transformation ratio of 1:1.2; working distance of 30 mm) and then delivered through the plane reflector into the uncoated crystal sample, which was positioned close to the reflector inside the resonator. The pump beam spot radius was about 60 μm in the Yb:YCOB crystal; while the confocal parameter (2zR) was estimated to be approximately 2.3 mm (M2 ≈17; refractive index n ≈1.70). The crystal sample, which was fitted into a copper holder, was cooled with cycling water maintaining at a temperature of 5 °C.
3. Results and discussion
With the help of the specially coated plane reflector mentioned above, laser oscillation in the main emission band was effectively suppressed, and laser action emitting in the long-wavelength sideband could readily be achieved under output coupling conditions of T = 0.5%−20%, with lasing wavelengths ranging from about 1059 to 1086 nm. The unsaturated or small-signal absorption fraction for the pumping radiation, was measured as 0.91, 0.89, and 0.93, for the X-, Y-, and Z-cut Yb:YCOB crystal samples used, respectively. Due to the strong absorption saturation of the Yb:YCOB crystal for the 976-nm pumping radiation, as was discussed previously [1], it is difficult to estimate the exact amount of the absorbed pump power under lasing conditions, so the lasing properties will be discussed with respect to incident pump power (Pin).
3.1 Lasing properties of X-cut crystal
With the X-cut crystal sample utilized, the laser action achieved was linearly polarized with the optical electric field parallel to the Z axis (E//Z), independent of either the pumping level or the output coupling used. This was simply because of the fact that the effective gain cross-section for E//Z is greater than for E//Y, at any wavelength in the emission sideband (actually also in the main emission band) and for any excitation level [17]. Depending upon the output coupling used, the lasing wavelengths covered a wide range of approximately 1059−1085 nm.
Figure 1 shows the output power as a function of Pin for different output couplings of T = 2%, 5%, 10%, and 20%. Utilizing a still smaller output coupling, e.g., T = 0.5%, could lead to less efficient laser operation. In the case of T = 2%, lasing threshold was measured to be Pin = 0.27 W; while for T = 5% the threshold pump power increased to Pin = 0.41 W. One sees that above lasing threshold, the output power produced in the two cases proved to be very close; this situation continued until Pin ≈13 W, above which the laser action with T = 2% became less efficient, making T = 5% be the optimal output coupling for power scaling. One can note that in excess of Pin ≈23 W, the laser efficiency for T = 5% also became lowered. Despite this, however, a maximum output power of 13.0 W could be generated at Pin = 28.8 W, resulting in an optical-to-optical efficiency of 45.1%. The slope efficiency (η), determined for an intermediate pump region of 7−23 W, amounted to 53%. As can be seen in Fig. 1, even under a relatively large output coupling of T = 20%, fairly efficient laser action could still be achieved with a slope efficiency of η = 31%, generating a maximum output power of 7.1 W. This suggests the potential of Q-switching laser action to be realized in the long-wavelength emission sideband. It is in general necessary to utilize a sufficiently large output coupling in Q-switched laser operation to prevent any damage to the intra-cavity elements.
Fig. 1 Output power versus incident pump power for different output couplings, measured with the X-cut Yb:YCOB crystal.
Depending on the output coupling utilized, the lasing spectrum was found to fall into two separate wavelength regions: 1059−1070 nm and 1080−1085 nm. The way that the lasing spectrum varies with pumping level proved to be quite different for low and high output couplings.
Figure 2 illustrates the evolution behavior of the lasing spectrum with the pumping level, measured for the case of T = 2%. At low pumping levels (Pin < 6 W), the laser action was found to occur mainly in the shorter wavelength region (~1063−1070 nm); with increasing pump power, the long-wavelength components would get strengthened, becoming comparable to the short-wavelength ones, and then becoming dominant at Pin ≈15 W. Upon increasing the pump power continually, the short-wavelength components would again get enhanced; and eventually the two wavelength regions joined together at Pin = 24.4 W, giving rise to a rather broad lasing band extending from 1068.6 to 1085.2 nm. Such a broad lasing band attainable under certain conditions might be of significance for mode-locking laser action.
Fig. 2 Evolution of lasing spectrum with pumping level, measured with the X-cut Yb:YCOB crystal under conditions of T = 2%.
From the lasing spectrum shown in Fig. 2 for T = 2%, one sees the strong dependence on pumping level. By contrast, however, the lasing spectrum was found to change only slightly with pump power in the cases of T = 5%, 10%, and 20%. Figure 3 shows a typical lasing spectrum for each case, measured at an intermediate pumping level of Pin = 15.4 W. Differing from that for T = 2%, the lasing spectrum presented here consists of only one lasing band, covering a wavelength range of 1059.2−1066.1 nm. One can notice that with the output coupling increased from T = 5% to T = 20%, the resulting lasing spectrum only shifts slightly toward short-wavelength side.
Fig. 3 Lasing spectrum for T = 5%, 10%, and 20%, measured at Pin = 15.4 W with the X-cut Yb:YCOB crystal.
3.2 Lasing properties of Y-cut crystal
The laser action achieved with the Y-cut crystal sample turned out to be quite different from that for the X-cut one, exhibiting very complex polarization state varying behavior upon changing the output coupling and/or the pumping level. Only under high output couplings (T ≥ 10%) could laser action of single polarization state (E//Z) be achieved. This, clearly, resulted from the closeness in effective gain cross-sections for E//X and for E//Z, in the long-wavelength sideband (> 1060 nm) under conditions of low excitation levels [17].
Figure 4 depicts the output power versus Pin, produced under different output coupling conditions with the Y-cut Yb:YCOB crystal. In the case of T = 2%, linearly polarized laser action with E//X, at lasing wavelengths around 1083 nm, reached threshold at Pin = 0.25 W; with increasing pump power this single-polarization state could be maintained until Pin = 6.3 W, at which the second polarization component with E//Z, at lasing wavelengths around 1069 nm, started to set in, and the laser action entered a region where the two orthogonal polarization components could exist simultaneously. As indicated in the figure, this coexistence region could extend to the highest pumping level applied. With a high output coupling that was not lower than 10%, the laser action obtained consisted of only the E//Z polarization state, emitting at wavelengths of 1059−1063 nm. The highest output power generated in the case of T = 10%, was measured to be 12.0 W at Pin = 30.7 W, the optical-to-optical efficiency being 39.1%.
Fig. 4 Output power versus incident pump power, measured with the Y-cut Yb:YCOB crystal for T = 2%, 10%, and 20% (a), and for T = 5% (b).
As for the X-cut crystal, the optimum output coupling that could lead to the most efficient laser action, proved also to be T = 5%. Figure 4(b) shows the output characteristics of the laser operation achieved under this optimal output coupling, in which the output powers of the two orthogonal polarization components are presented. One sees that the second, E//Z component appeared at Pin = 2.47 W, a much lower pumping level than in the case of T = 2%. As a result, the coexistence region nearly extended over the whole operational range. One can notice the distinct ways in which the two polarized output components depend on the pump power. As the pumping level was raised, the output of the E//Z component initially increased almost linearly; then became saturated, reaching its maximum of 6.2 W at Pin = 25.1 W (interestingly, the outputs of the two polarized components were very close at this pump power); after that it dropped rapidly to a small amount of 0.78 W at the highest pumping level of Pin = 32.4 W. In contrast to this, the output of the E//X component increased monotonically with pump power, reaching 14.22 W at the highest pump power, where the total output power produced amounted to 15.0 W, the corresponding optical-to-optical efficiency was 46.3%. Such a distinction in laser behavior could be understood. At sufficiently high pump level, due to the increased overall losses resulted from thermal effects, the E//Z oscillation would be forced to shift to shorter wavelengths in the region around 1065 nm, and thus would suffer from higher losses caused by the transmittance of the plane mirror forming the resonator, leading to a laser efficiency dropping with pump power. For the E//X polarization, the laser oscillation, upon raising pump power, would also shift slightly toward short-wavelength side in the region around 1084 nm, but would not experience additional transmittance losses of the plane mirror, and its output could be able to increase monotonically with pump power.
Figure 5 shows the lasing spectra measured at an intermediate pumping level, Pin = 15.4 W, in different cases of T = 2%−20%. For high output couplings of T = 10% and T = 20%, laser action in single polarization state occurred over a narrow wavelength range of about 1059−1063 nm, which proved to be very similar to the results illustrated in Fig. 3 for the X-cut crystal. On the other hand, however, in the cases of low output couplings, T = 2% and T = 5%, laser action could occur in two orthogonal polarization states, emitting in two separate wavelength regions. This feature is unique for the Y-cut crystal, differing from the spectral lasing properties of the X-cut crystal. It needs to be pointed out that in the case of T = 2%, the laser action obtained with the X-cut crystal occurred also in two similar separate wavelength regions (Fig. 2), but in a single polarization state (E//Z).
Fig. 5 Lasing spectrum for different output couplings, measured at Pin = 15.4 W with the Y-cut Yb:YCOB crystal.
3.3 Lasing properties of Z-cut crystal
In comparison with the X- and Y-cut crystals, the Z-cut crystal exhibits the simplest lasing behavior. With the output coupling changed over a wide range from T = 0.5% to T = 20%, the laser action always occurred in a single polarization state with E//X, and in the same wavelength region extending from 1082 to 1086 nm. For laser action realized with the Z-cut Yb:YCOB crystal, the effective gain cross-section, and hence the net overall gain for E//X is always greater than for E//Y, regardless of the level of excitation [17]. Furthermore, with the help of the specific dielectric coatings on the plane mirror, laser oscillation in the short-wavelength region of λ < 1080 nm could be completely suppressed.
The output characteristics of laser action achieved with the Z-cut Yb:YCOB crystal are illustrated in Fig. 6, for different output couplings of T = 2%−20%. Similar to the situation of the X-cut crystal, the laser operations under conditions of T = 2% and T = 5% are seen to be almost equally efficient at low pumping levels; with the pump power increased above roughly 10 W, the laser action for T = 2% would become less efficient, and T = 5% proved to be the optimal. A maximum output power of 15.5 W was produced at Pin = 34.9 W, with an optical-to-optical efficiency of 44.4%. The highest output powers, which were generated under high output coupling conditions of T = 10% and T = 20%, were 11.7 and 6.5 W, respectively.
Fig. 6 Output power versus incident pump power for different output couplings, measured with the Z-cut Yb:YCOB crystal.
For the Z-cut Yb:YCOB crystal, the wavelength range over which laser action occurred was found depending only slightly upon either the output coupling or the pumping level. Figure 7 shows several typical lasing spectra for different output couplings, which were measured at an intermediate pumping level of Pin = 15.4 W. One sees, upon raising the output coupling from T = 2% to T = 20%, the lasing wavelengths changed merely slightly, covering a range of roughly 1082−1086 nm.
Fig. 7 Lasing spectrum for different output couplings, measured at Pin = 15.4 W with the Z-cut Yb:YCOB crystal.
3.4 Comparison
From the results and discussions presented in the previous subsections, one sees the lasing properties of the Yb:YCOB crystal in the long-wavelength emission sideband depend largely on the crystal orientation. In order to make a clear comparison, we list in Table 1 the primary results obtained with all the three X-, Y-, and Z-cut crystal samples, under operational conditions of optimal output coupling (Topt) as well as of high output couplings. These include: the maximum output power, Pout,max; polarization state, Pol.; optical-to-optical efficiency, ηopt; slope efficiency, η; and lasing wavelengths, λ. Also presented in the last part of the table are the corresponding results reported previously for a Y-cut Yb:YCOB crystal laser operating in the main emission band [1].
Table 1. Parameters Characterizing the Lasing Properties of the X-, Y-, and Z-cut Yb:YCOB crystals
One notices from Table 1 that under the optimal output coupling, Topt = 5%, linearly polarized output power attainable at wavelengths around 1065 nm could reach 13.0 W with the X-cut crystal, while that achievable at wavelengths around 1085 nm could amount to 15.5 W with the Z-cut crystal; with the Y-cut crystal, laser action in two orthogonal polarizations occurred in two separate wavelength regions. With high output couplings (usually required for Q-switching) of T = 10% and T = 20%, linearly polarized laser action at wavelengths of 1059−1065 nm could be achieved with X- or Y-cut crystal, producing maximum output power in a 7−12 W level; whereas the laser action with the Z-cut crystal occurred in a long-wavelength region of 1082−1086 nm, with linearly polarized output power that could reach a similar level. It may also be interesting to note that the laser action, realized with the X-cut crystal under output couplings of T = 5%−20%, or with the Y-cut crystal under output couplings of T = 10%−20%, occurred in a wavelength region of 1059−1066 nm, which turned out to be very close to the lasing wavelengths of Nd-ion lasers operating on the most common 4F3/2→4I11/2 transition. According to a previous study on the laser performance of Yb:YCOB crystal in the main emission band [1], the laser action, achieved under the optimal output coupling, Topt = 10%, and under high output couplings up to T = 60%, proved to take place in a wavelength range of 1021−1045 nm.
During the laser operation the crystal surface temperature was kept at 5 °C. It was found experimentally that the crystal temperature had only a limited effect on the laser action. For laser oscillation at wavelengths around 1084 nm, the rate at which the output power decreased with rising temperature was estimated to be less than 0.01 W/°C (at an output level of 5 W).
4. Summary
In conclusion, we have conducted an investigation into the anisotropic lasing properties in the long-wavelength emission sideband of the Yb:YCa4O(BO3)3 crystal. By suppressing lasing in the main emission band, efficient laser action was achieved in the long-wavelength sideband, with X-, Y-, and Z-cut crystals. Under the optimal output coupling of 5%, linearly polarized output power of 13.0 W at wavelengths around 1065 nm, and 15.5 W at wavelengths around 1085 nm, was produced with X- and Z-cut crystals; while with Y-cut crystal, the laser action occurred in two orthogonal polarizations, and in two separate wavelength regions. Under high output couplings of 10% and 20%, linearly polarized laser action could be realized with X- or Y-cut crystal, with lasing wavelengths ranging from 1059 to 1065 nm, and with the highest output power in a 7−12 W level; with Z-cut crystal, laser action occurred at wavelengths of 1082-1086 nm, with highest output power in a similar level. We can also conclude, from a practical point of view, that to make single-polarization lasers operating at wavelengths around 1065 nm/1085 nm, X-cut/Z-cut crystals should be chosen; whereas if a laser operating in two orthogonal polarizations is to be made, a Y-cut crystal should be employed.
Funding
National Natural Science Foundation of China (11574170 and 51602166); Natural Science Foundation of Shandong Province, China (ZR2016FQ01).
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