Abstract

We demonstrated the power scaling of a passively Q-switched Pr3+:LiYF4 (YLF) laser at 523, 607, and 640 nm with a Co2+:MgAl2O4 (MALO) saturable absorber and analyzed the experimental results with a numerical simulation based on rate equations. A maximum pulse energy of 33.5 µJ was obtained with a pulse width of 30.9 ns and a repetition rate of 64.0 kHz at 640 nm. We demonstrated ultraviolet pulse generation at 261 and 320 nm by intracavity frequency doubling and obtained 63.3-ns pulses with a pulse energy of 7.0 µJ and a repetition rate of 64.6 kHz at 320 nm as well as 356-ns 261-nm pulses with a pulse energy of 0.2 µJ at a repetition rate of 82.0 kHz. To the best of our knowledge, a 261-nm pulse is the shortest wavelength laser directly generated from the intracavity second-harmonic generation of passively Q-switched lasers.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Trivalent praseodymium (Pr3+) -doped materials hold potential for efficient, visible solid-state lasers because of their intense radiative transitions in the visible spectral range. Furthermore, they can easily be extended to ultraviolet (UV) lasers by a single step of second-harmonic generation (SHG), while UV lasers based on near infrared (NIR) lasers require at least two steps of frequency conversion. Moreover, Pr3+-based visible lasers have been attracting much attention because of the recent development of high-power indium gallium nitride (InGaN)-based blue laser diodes (LDs) that emit around ∼450 nm, which corresponds to the absorption peak wavelength of Pr3+-doped materials. Laser operations have already achieved with Pr3+-doped fluoride crystals [18], oxide crystals [9,10], and fluoride glass fibers [11]. In particular, Pr3+:LiYF4 (Pr3+:YLF) laser operations have been obtained in the cyan (491 nm), green (523 and 546 nm), orange (604 and 607 nm), red (640, 670, and 698 nm), and deep red (721 nm) regions [17]. Output power of ∼5 W is currently available from a single-emitter InGaN blue LD [12]. We recently obtained the highest continuous wave (CW) output powers of 3.7 W at 607 nm and 6.7 W at 640 nm using four ∼5-W single-emitter LDs for pumping [7].

In addition to CW operation, investigations on visible pulse lasers are necessary to initiate new applications. In particular, passively Q-switching with saturable absorbers can generate ns laser pulses with a compact system. Therefore, many investigations on saturable absorbers in the visible region have been carried out. After Abe et al. [13] in 2013 first demonstrated passively Q-switched Pr3+:YLF lasers with a tetravalent chromium-doped Y3Al5O12 (Cr4+:YAG), such 2D topological insulators as WS2, MoS2, and MoSe2 [14], semiconductor saturable absorber mirrors [15,16], or graphene [17] were used as a saturable absorber in the visible region. The highest pulse energy of 11.3 µJ with a pulse width of 49 ns was achieved in a Pr3+:YLF laser at 640 nm with a Cr4+:YAG crystal, which has a high damage threshold and low saturation intensity [18]. One attractive function of such pulse operations in the visible region is efficient UV laser generation. UV pulses at 320 nm with a pulse energy of 1.5 µJ and a pulse width of 50 ns were obtained by the intracavity frequency doubling of a passively Q-switched Pr3+:YLF laser [19].

A divalent cobalt-doped MgAl2O4 spinel (Co2+:MALO) crystal has been recognized as a saturable absorber in the 1.5-µm region and used for the Q-switching of Er3+ lasers [20]. In 2017, Demesh et al. first utilized a Co2+:MgAl2O4 (MALO) as a saturable absorber in the visible region and demonstrated a passively Q-switched Pr3+:YLF laser at 523, 607, and 640 nm [21]. Note that Co2+:MALO can even be used as a saturable absorber for a green laser for which a Cr4+:YAG does not show saturable absorption. In our experiments, we demonstrated the power scaling of a passively Q-switched Pr3+:YLF laser at 523, 607, and 640 nm with a Co2+:MALO saturable absorber utilizing four ∼5-W blue InGaN LDs as pump sources. We also generated UV pulses at 261 and 320 nm by intracavity frequency doubling. To the best of our knowledge, the 261-nm pulse is the shortest wavelength laser directly generated from the intracavity second-harmonic generation of passively Q-switched lasers. Furthermore, we compared the output performance of the passively Q-switched pulse by a Co2+:MALO with that obtained by a Cr4+:YAG.

2. Passively Q-switched Pr3+:YLF laser at 523, 607, and 640 nm

We first performed passively Q-switched Pr3+:YLF laser operations at 523, 607, and 640 nm with a Co2+:MALO saturable absorber. We utilized two (111)-cut Co2+:MALO crystals (EKSMA Optics) whose absorption spectra were measured by a dual-beam absorption spectrometer (UV3600 Plus, Shimadzu). We experimentally estimated ground-state absorption (GSA) cross section ${\sigma _{gs}}$ and excited-state absorption (ESA) cross section ${\sigma _{es}}$ of the Co2+:MALO at 607 and 640 nm by Z-scan measurements [22] and calculated the saturated transmission of the Co2+:MALO. The calculated saturation parameters in the absorption at 523, 607, and 640 nm are summarized in Table 1. “Measured transmission” was obtained by the transmission of the saturable absorber measured by a spectrophotometer. The actual transmission was corrected with the Fresnel reflection. “Saturated transmission” was obtained by the ratio ${\sigma _{gs}}$/ ${\sigma _{es}}$.

Tables Icon

Table 1. Saturation characteristics of Co2+:MALO at 523, 607, and 640 nm

The laser oscillator setup for a diode-pumped Pr3+:YLF laser is depicted in Fig. 1. The V-shaped cavity consisted of three mirrors: a plane dichroic mirror (DM1), a concave dichroic mirror of a 75-mm curvature radius (DM2), and a plane output coupler (OC). The OC transmissions were 2.7% at 523 nm, 11.4% at 607 nm, and 9.1% and 21.4% at 640 nm. The two dichroic mirrors have high transmission (T > 98.0%) at 444 nm and high reflectivity (R > 99.9%) from 520 to 650 nm. Only for the 607-nm operation, we used a different plane dichroic mirror with a lower transmission at the pump wavelength (T = 87.1%) for suppressing the lasing at 640 nm. As a gain medium, we employed a 12-mm-long 0.3 at.% Pr3+-doped YLF crystal rod (5-mm diameter) that was cut perpendicular to the crystal’s a-axis (AC Materials). The facets of the Pr3+:YLF crystal were both polished but uncoated. The crystal was mounted on a water-cooled copper heat sink.

 figure: Fig. 1.

Fig. 1. Experimental setup of a diode-pumped Pr3+:YLF laser Q-switched by a Co2+:MALO saturable absorber.

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As pump sources, we utilized four InGaN blue LDs (Nichia) at 444 and 442 nm, which correspond to an absorption peak of Pr3+:YLF for the π- and σ-polarization. The pump beams of two LD beams were combined by a polarizing beam splitter (PBS) and focused into a Pr3+:YLF crystal from both faces. The maximum incident and absorbed pump power were 14.9 and 10.9 W (for 607-nm operation, 14.7 and 10.2 W). The beam quality, ${M^2}$, of the pump beams was measured to be ∼2.0 × 20-25 (fast × slow axis). For the 442-nm LDs, we used optical isolators for blocking the unabsorbed pump beam from the other side because the absorption coefficient of Pr3+:YLF at 442 nm for σ-polarization is less than half of that for the π-polarization. In fact, ∼1.5 W of the transmitted pump power was directed to the LD of the other side.

As shown in Fig. 1, the Co2+:MALO was placed in the vicinity of the OC. The cavity lengths were 144, 131, and 131 mm at 523, 607, and 640 nm when they were optimized for output power. The cavity mode size for each operating wavelength was calculated by a ABCD matrix method for the fundamental spatial mode, and the calculated mode radius in the Pr3+:YLF was ∼60 × 60 µm2, ∼95 × 95 µm2, and ∼100 × 100 µm2 (vertical × horizontal), and the mode radius in the Co2+:MALO was ∼50 × 50 µm2, ∼60 × 60 µm2, and ∼60 × 60 µm2 at 523, 607, and 640 nm. Since Pr3+:YLF has a smaller stimulated emission cross section at 523 nm (0.3 × 10−19 cm2) than 607 nm (1.4 × 10−19 cm2) and 640 nm (2.2 × 10−19 cm2) [1], the output performance at the wavelength was greatly sensitive to a slight change in the cavity loss induced by aberrated thermal lensing (fourth- and sixth- (and even higher) order wavefront distortion due to thermally induced refractive index profile) in the crystals [23]. Furthermore, the Pr3+:YLF crystal exhibits strong negative thermal lensing for π-polarized 523-nm laser emission. Consequently, the laser output power was maximized when the mode radius in the Pr3+:YLF at 523 nm was smaller than at 607 and 640 nm, due to the requirement for thermal lens suppression [7].

Before the Q-switching experiment, we performed a CW operation by removing the Co2+:MALO from the cavity. At 640 nm, the threshold pump power was 619 mW and 768 mW, the slope efficiency was 50.9% and 40.0%, and the maximum output power was 5.3 W and 4.1 W at TOC = 9.1% and TOC = 21.4%, respectively. In the other laser transitions, the threshold pump power was 871 mW and 826 mW, the slope efficiency was 34.1% and 23.8%, and the maximum output power was 3.1 W and 1.1 W at 607 nm and 523 nm, respectively.

Next, we inserted Co2+:MALO into the cavity and performed passively Q-switching. The measured average power, pulse width, and repetition frequency as a function of absorbed pump power are shown in Figs. 24. Using the OC of TOC = 11.4 (21.4)%, the highest peak power of 0.76 and 1.08 kW was obtained at the maximum absorbed pump power with an SA1 (see in Table 1) at 607 and 640 nm. The width of the passively Q-switched pulses can generally be shortened by employing lower initial transmissions of the saturable absorber, resulting in larger modulation depths. The pulse width can also be shortened by adopting higher transmission OCs, since for most Q-switched lasers the pulse width and the pulse energy are not significantly changed from those around the lasing threshold because the pulse width becomes shorter due to the larger modulation depth of the SA1 transmission. In fact, when we used SA1 as a saturable absorber and an OC whose transmission is 9.1% at 640 nm for an absorbed pump power of >10 W, a pulse with a much higher peak power was generated; unfortunately, it damaged the plane dichroic mirror (DM1).

 figure: Fig. 2.

Fig. 2. (a) Average power, (b) pulse width, and (c) repetition frequency of passively Q-switched Pr3+:YLF laser with Co2+:MALO (SA2) and output coupler of 9.1% at 640 nm as a function of absorbed pump power. Dashed lines are results of numerical simulation.

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 figure: Fig. 3.

Fig. 3. (a) Average power, (b) pulse width, and (c) repetition frequency of passively Q-switched Pr3+:YLF laser with Co2+:MALO (SA2) and output coupler of 11.4% at 607 nm as a function of absorbed pump power. Dashed lines are results of numerical simulations with two different ESA cross sections.

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 figure: Fig. 4.

Fig. 4. (a) Average power, (b) pulse width, and (c) repetition frequency of passively Q-switched Pr3+:YLF laser with Co2+:MALO (SA2) and output coupler of 2.7% at 523 nm as a function of absorbed pump power.

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We also performed passively Q-switching at 523 nm. Co2+:MALO is advantageous since it enables Q-switched green lasers and Cr4+:YAG does not. At this wavelength, we failed to reach the lasing threshold with SA1 because of its relatively low initial transmission and the small stimulated emission cross section of Pr3+:YLF compared to those at 607 and 640 nm. Using the SA2 with higher initial transmission, we successfully generated green Q-switched pulses. However, the plane dichromic mirror (DM1) was damaged by the intracavity green pulses at an absorbed pump power of ∼5.5 W. Figure 4 shows the performance of the green Q-switched laser up to a 5.3-W absorbed pump power that is slightly below the damage threshold of the DM1. The maximum pulse energy of 6.6 µJ and the maximum peak power of 27 W obtained at 523 nm was mainly much lower than those of the other two wavelengths for three reasons. First, the small mode size in the Pr3+:YLF crystal led to a relatively low mode matching efficiency of ∼50%. Second, the insertion loss of the Co2+:MALO significantly reduced the output performance due to the small stimulated emission cross section at 523 nm. Third, the absorption of the Co2+:MALO could not be quickly bleached due to low intracavity laser intensity. The pulse width was inevitably longer at 523 nm because of the small modulation depth of the transmission in the Co2+:MALO at 523 nm. Even though we have not yet fully examined the Co2+:MALO transmission at 523 nm when it was fully bleached, the measured initial transmission of 97.5% indicates that the maximum obtainable modulation depth is not higher than 2.5%.

Table 2 shows the summary of the passively Q-switched Pr3+:YLF laser with Co2+:MALO at 523, 607, and 640 nm. In our experiment, we measured ${M^2}$ only at the highest pump laser power of 11 W. In our cavity design, the fundamental cavity mode size (∼100 µm in radius) is always larger than that of the pump laser beam size. Therefore, the thermal aberration induced by the pump laser always degrades the ${M^2}$ value and the degradation increases as the pump laser power increases. In the worst case, the higher cavity mode may start to oscillate and increase the laser mode size in the laser crystal. We did not observe such change in the beam size in our experiment up to the pump laser power of 11 W since the heat-induced lens effect of an YLF crystal is small. The beam quality for the 640-nm laser is better than other wavelengths since we used a higher TOC mirror (>20%) and the saturable absorber with a lower initial transmission, where the laser threshold became higher also for higher-order cavity modes.

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Table 2. Summary of characteristics of passively Q-switched Pr3+:YLF laser with Co2+:MALO at 523, 607, and 640 nm

We further scrutinized the passively Q-switched lasers by comparing the experimental results with numerical simulations based on a rate equation model. Passively Q-switched lasers can be expressed by the following three equations for a spatially zero-dimensional model:

$$\frac{{d\phi }}{{dt}} = \frac{{c\phi }}{{{l_c}}}\{{{\sigma_{st}}\Delta N{l_g} - {\sigma_{gs}}{n_{gs}}{l_{SA}} - {\sigma_{es}}({{n_{tot}} - {n_{gs}}} ){l_{SA}}} \}- \frac{\phi }{{{\tau _c}}} + S$$
$$\frac{{d\Delta N}}{{dt}} ={-} c\phi {\sigma _{st}}\Delta N - \frac{{\Delta N}}{{{\tau _f}}} + \frac{{{N_{tot}} - \Delta N}}{{{N_{tot}}}}\frac{{{\eta _Q}{\eta _{St}}{\eta _m}{P_{abs}}}}{{h{\nu _L}V}}$$
$$\frac{{d{n_{gs}}}}{{dt}} ={-} {\sigma _{gs}}c\phi {n_{gs}}\frac{{{A_g}}}{{{A_{SA}}}} + \frac{{{n_{tot}} - {n_{gs}}}}{{{\tau _{SA}}}}.$$

These are the rate equations of the three parameters: intracavity photon number density $\phi $, the population inversion density in gain medium $\Delta N$, and the ground-state population density of saturable absorber ${n_{gs}}$. The other parameters and values for the calculation are summarized in Table 3. Since the ground-state population in gain medium varies only within 2%, we treated the pumping rate ${P_{abs}}$ as a constant value.

For reproducing the experiments by a numerical model, we first determined an unknown laser parameter: the intracavity loss at the zero-dimensional gain medium model so that the calculated results of the CW laser showed good agreement with those of our experiments. At an output coupling TOC of 21.4%, we increased the intracavity loss, which is presumably increased by the loss due to a higher upper state density during lasing [2,7], to 6.5% from 0.3% at TOC = 9.1%, as shown in the Table 3. We need more detailed study to clarify the increase in the residual absorption at high TOC. Anyway, with these adjusted intracavity losses, we reproduced the output laser performance for the CW laser both in the lasing threshold and the slope efficiency by the numerical mode.

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Table 3. Parameters and values used in rate equations of passively Q-switched Pr3+:YLF laser with Co2+:MALO saturable absorber

The results of the numerical simulation are shown as dashed lines in Figs. 2 and 3 for the Q-switched lasers. In this numerical analysis, with the adjusting parameter selections described above, we obtained fairly good agreement in average power and pulse width using the GSA and ESA cross-section values (Table 1). However, the calculated repetition frequencies were always significantly different from those obtained in the experiments. In the experiment, the repetition frequency saturated when the absorbed power exceeded ∼5.5 W, while the rate equation model always exhibited a linear increase of the repetition frequency with respect to the absorbed pump power. This trend was also obvious in the previous report on a passively Q-switched Pr3+:YLF laser pumped by a frequency-doubled optically pumped semiconductor laser (2ω-OPSL) [21]. To the best of our knowledge, Nd3+:YAG/Cr4+:YAG Q-switched lasers do not show such a ceiling in repetition frequency. Note also that the Pr3+:YLF lasers Q-switched by Cr4+:YAG also exhibited a similar tendency [24]. We speculate that the charge transfer in transition metal ions by intense Q-switched pulse at the laser wavelength of 639 nm causes some nonlinear process which our rate-equation model does not consider. Further detailed investigations are necessary to fully understand the laser performance.

At 607-nm operation, the ground-state reabsorption (3H41D2) is involved, resulting in a high intracavity loss. Therefore, the maximum pulse energy at 607 nm decreases and the optimum TOC must be higher than that at 640 nm. The predicted pulse width by the model using the ESA cross section estimated by a Z-scan measurement [22] in Table 1 was ∼25 ns, which is much shorter than in the experiments. This discrepancy indicates that the experimentally estimated ESA cross section was overestimated. The calculated pulse width showed good agreement with the experimental results when the ESA cross section was set in a range of 9.5–10.5×10−19 cm2.

3. UV pulse generation at 261 and 320 nm by intracavity frequency doubling of passively Q-switched Pr3+:YLF laser

We extended the Q-switched visible lasers to UV-pulsed lasers at 261 and 320 nm by intracavity frequency doubling. The experimental setup is depicted in Fig. 5. The Z-shaped cavity consisted of four mirrors: a plane dichroic mirror (DM1), a concave dichroic mirror with a 75-mm curvature radius (DM2), a concave output coupler with a 75-mm curvature radius (OC), and a plane high reflection mirror (HR). The dichroic mirrors were identical as those shown in Fig. 1. The OC had a transmission of 78.1% at 261 nm and 83.7% at 320 nm as well as a reflectivity of >99.8% at fundamental wavelengths. The HR had a high reflectivity of >99.9% at both the fundamental and second-harmonic wavelengths. The Pr3+:YLF crystal and the pump system are the same as those in the experiments at the fundamental wavelength, and we used the Co2+:MALO (SA2) as a saturable absorber. As SHG nonlinear crystals (NLCs), we used a 7.0-mm-long Type-I β-BaB2O4 (BBO) crystal (θ = 48.9°) to convert the 523-nm light into 261 nm and an 8.0-mm-long Type-I LiB3O5 (LBO) crystal (θ = 90°, φ = 53.6°) to convert the 640-nm light into 320 nm. The Pr3+:YLF, the Co2+:MALO, and the NLC were placed at the cavity’s three focal points. For the lasers at 523 and 640 nm, the calculated mode radii were ∼60 × 55 µm2 (horizontal × vertical) and ∼115 × 95 µm2 in the Pr3+:YLF, ∼45 × 40 µm2 and ∼50 × 50 µm2 in the Co2+:MALO, and ∼110 × 85 µm2 and ∼65 × 65 µm2 in the NLC. The cavity length was ∼300 mm for both wavelengths.

 figure: Fig. 5.

Fig. 5. Experimental setup of intracavity frequency doubling a diode-pumped Pr3+:YLF laser passively Q-switched by a Co2+:MALO saturable absorber.

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We established a rate equation model to evaluate the laser performance at 320 nm. We only applied it to the laser at 640 nm since the absorption cross sections of Co2+:MALO at 523 nm were unknown. The time evolution of intracavity photon number density $\phi $ is described by the following equation [19]:

$$\frac{{d\phi }}{{dt}} = \frac{{c\phi }}{{{l_c}}}\{{{\sigma_{st}}\Delta N{l_g} - {\sigma_{gs}}{n_{gs}}{l_{SA}} - {\sigma_{es}}({{n_{tot}} - {n_{gs}}} ){l_{SA}}} \}- \frac{\phi }{{{\tau _c}}} + S - \frac{{{c^2}}}{{2{l_c}}}{\gamma _{SHG}}h{\nu _\omega }{A_{NL}}{\phi ^2}.$$
${\gamma _{SHG}}$ indicates the nonlinear conversion coefficient. Assuming perfect phase matching, ${\gamma _{SHG}}$ is given as
$${\gamma _{SHG}} = \frac{{2{\omega ^2}d_{eff}^2{l_{NLC}}k}}{{\pi n_{NL}^3{c^3}{\varepsilon _0}}}{h^{\prime}}({{B^{\prime}},\xi } ).$$
The population inversion density in gain medium $\Delta N$ and the ground-state density of saturable absorber ${n_{gs}}$ were calculated by Eqs. (2) and (3). The other parameters used for the calculation are summarized in Table 4.

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Table 4. Parameters and values used in rate equations of intracavity frequency doubling of passively Q-switched Pr3+:YLF laser with Co2+:MALO saturable absorber

The measured average power, pulse width, and repetition frequency as a function of the absorbed pump power are shown in Figs. 6 and 7. The characteristics of the Q-switched pulses are summarized in Table 5. At 320 nm, the average power showed a linear increase with respect to the absorbed pump power. The pulse width was longer than the experimental result in a previous work [19] that used a Cr4+:YAG saturable absorber. The output pulse width was 50 ns, even though the transmission modulation depth of the saturable absorber was almost the same because we used a Z-cavity in this experiment and its cavity length was longer than that of a V-cavity in a previous experiment [19]. In the numerical analysis, we adjusted the intracavity loss to obtain better agreement of the SHG performance. We assumed intracavity loss of 6.0%, which corresponds to the transmission loss at HR and OC for the 640-nm light.

 figure: Fig. 6.

Fig. 6. (a) Average power, (b) pulse width, and (c) repetition frequency of UV pulse at 320 nm as a function of absorbed pump power. Dashed lines are results of numerical simulation.

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 figure: Fig. 7.

Fig. 7. (a) Average power, (b) pulse width, and (c) repetition frequency of UV pulse at 261 nm as a function of absorbed pump power.

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Tables Icon

Table 5. Summary of characteristics of UV pulses at 261 and 320 nm

Figure 7 depicts the output characteristics of the Q-switched pulse at 261 nm. To the best of our knowledge, this is the first pulsed DUV laser at 261 nm from a passively Q-switched Pr3+:YLF laser at 261 nm. As shown in Fig. 7 and Table 5, both the pulse energy and the peak power of the 261-nm pulse were much lower than those of 320 nm since the Q-switched pulses at 523 nm before frequency doubling (Fig. 4) had a much lower pulse energy and a lower peak power, which significantly lowered the conversion efficiency.

4. Comparison of passively Q-switched Pr3+:YLF laser with Cr4+:YAG

Finally, we compared the characteristics of the Co2+:MALO and Cr4+:YAG saturable absorbers in the visible region. Q-switching operations with Co2+:MALO and Cr4+:YAG was compared again with the setup in Fig. 1 at 640 nm. In this experiment, we utilized a 1.3-mm long Cr4+:YAG crystal with the initial transmission of 94.6% and the saturated transmission of 96.3%. By utilizing OC of 21.4%, we obtained 60.9-ns pulses with a highest pulse energy of 14.3 µJ and a repetition rate of 126.8 kHz. Therefore, within the various conditions we used for the experiments, Pr3+:YLF lasers Q-switched by the Co2+:MALO produced higher pulse energy, higher peak power, and higher average power than those obtained by the Cr4+:YAG Q-switched Pr3+:YLF laser. However, output performance is always varied by the doping concentration and the length of a saturable absorber, and the cavity design such as a spot size at the saturable absorber, while the saturation characteristics are determined by GSA cross section and recovery time of the saturable absorber.

To compare the saturation characteristics between two saturable absorbers, we observed a change in the Q-switched pulse train by altering the position of the saturable absorber in the cavity, which accordingly changed the intensity (or fluence) at the saturable absorbers. To achieve passive Q-switching, a saturable absorber needs to be saturated before the laser gain; otherwise the laser operates in CW. The saturation intensity of Pr3+:YLF at 640 nm is ∼37 kW/cm2, and that of Co2+:MALO is estimated to be ∼560 kW/cm2 using a GSA cross section of 11×10−19 cm2 and a recovery time of 500 ns [22]. Based on these values, the mode radius in Co2+:MALO (${w_{SA}}$) needs to be four-times smaller than that in Pr3+:YLF (∼100 × 100 µm2). Thus, ${w_{SA}}$ may need to be smaller than ∼25 µm for stable Q-switching. Experimentally obtained pulse trains for different conditions are presented in Fig. 8. We confirmed that the Q-switch operation can be stabilized by adjusting the mode diameter in the saturable absorber. Passively Q-switching was practically obtained even though ${w_{SA}}$ was ∼60 µm. If the Co2+:MALO was shifted from the focal point, the Q-switching became unstable. Based on the calculated cavity mode radius, we conclude that the mode radius in Co2+:MALO needs to be smaller than approximately 0.6 times of that in Pr3+:YLF to obtain stable Q-switching.

 figure: Fig. 8.

Fig. 8. Pulse train of passively Q-switched Pr3+:YLF laser with Co2+:MALO (Sample 2) (a) OC-SA : 3 mm, ${w_{SA}}$: 63×60 µm2 (b) OC-SA : 6 mm, ${w_{SA}}$: 65×61 µm2 (c) OC-SA : 9 mm, ${w_{SA}}$: 69×64 µm2

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Cr4+:YAG’s saturation intensity was estimated to be ∼10 kW/cm2 using a GSA cross section of 7.5×10−18 cm2 and a recovery time of 4 µs [22]. Therefore, Cr4+:YAG has a lower saturation intensity than Pr3+:YLF gain, and Q-switching can theoretically be obtained even if ${w_{SA}}$ is twice as large as that in Pr3+:YLF. In the experiment, Q-switching became unstable when ${w_{SA}}$ (∼120 µm) almost equaled that in Pr3+:YLF (Fig. 9(b)). Thus, the mode radius in Cr4+:YAG needs to be smaller than that in Pr3+:YLF for stable Q-switching.

 figure: Fig. 9.

Fig. 9. Pulse train of passively Q-switched Pr3+:YLF laser with Cr4+:YAG (a) OC-SA : 3 mm, ${w_{SA}}$: 63×06 µm2 (b) OC-SA : 30 mm, ${w_{SA}}$: 115×112 µm2 (c) OC-SA : 40 mm, ${w_{SA}}$: 143×142 µm2

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These results indicate that using a Cr4+:YAG saturable absorber with a relatively low saturation intensity is suitable for a microchip laser, in which we cannot focus the beam into the saturable absorber. However, in the Cr4+:YAG crystal, the remaining Cr3+ in the octahedral site absorbs light in the orange-red region and does not contribute to the saturable absorption. This issue only takes place for the visible absorption, while the absorption around 1µm is purely due to the Cr4+ in the tetrahedral site. Since the amount of such a parasitic absorption center depends on the crystal growth conditions, the absorption spectra of Cr4+:YAG differ from sample to sample [22]. The optimization of growth processes, including the post-annealing process for controlling the valence of Cr ions, is required for improving the Cr4+:YAG crystal as a saturable absorber in the visible region. On the other hand, since Co2+:MALO has a higher saturation intensity, we have to place it in the focal point to obtain Q-switching, which is not useful to achieve a microchip laser with Co2+:MALO. However, Co2+:MALO can work as a saturable absorber in a wider spectral range than Cr4+:YAG. In particular, Q-switching in the green range enables DUV pulse generation by a single step of frequency conversion. More detailed scrutiny of the saturation characteristics in this range is required to estimate the performance of the Q-switched pulses, which can be applied to a demonstration of Q-switching with other gain mediums, such as terbium (Tb3+)-doped materials [25].

5. Conclusion

We reported the power scaling of passively Q-switched Pr3+:YLF lasers at 523, 607, and 640 nm with a Co2+:MALO saturable absorber pumped by four ∼5-W blue InGaN laser diodes. At 640 nm, we obtained the highest pulse energy of 33.5 µJ and the highest peak power of 1.08 kW with a pulse width of 30.9 ns and a repetition frequency of 64.0 kHz. In the other spectral ranges, the maximum pulse energies were 6.6 and 30.7 µJ at 523 and 607 nm. We also demonstrated UV pulse generation by intracavity frequency doubling. The maximum pulse energies were 0.2 and 7.0 µJ at 261 and 320 nm. Moreover, we demonstrated numerical analysis based on the rate equation and confirmed good agreement with experimental results of average power and pulse width at 320, 607, and 640 nm. Further power scaling can be achieved by increasing the pump power with strategies to overcome thermal fracture by employing a pump system that leads to lower pump density or a Pr3+-doped oxide. However, unlike such other gain media as Nd3+-doped crystal, a shorter fluorescence lifetime of Pr3+-doped crystal is the main issue for the peak power scaling of Q-switched pulses.

Disclosures

The authors declare no conflicts of interest.

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3. P. W. Metz, K. Hasse, D. Parisi, N. O. Hansen, C. Kränkel, M. Tonelli, and G. Huber, “Continuous-wave Pr3+:BaY2F8 and Pr3+:LiYF4 in the cyan-blue spectral region,” Opt. Lett. 39(17), 5158–5161 (2014). [CrossRef]  

4. K. Hashimoto and F. Kannari, “High-power GaN diode-pumped continuous wave Pr3+ -doped LiYF4 laser,” Opt. Lett. 32(17), 2493–2495 (2007). [CrossRef]  

5. T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+: LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm and 639.5 nm,” Opt. Lett. 36(6), 1002–1004 (2011). [CrossRef]  

6. B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015). [CrossRef]  

7. H. Tanaka, S. Fujita, and F. Kannari, “High-power visibly emitting Pr3+:YLF laser end pumped by single-emitter or fiber-coupled GaN blue laser diodes,” Appl. Opt. 57(21), 5923–5928 (2018). [CrossRef]  

8. A. Sottile and P. W. Metz, “Deep red diode-pumped Pr3+:KY3F10 continuous-wave laser,” Opt. Lett. 40(9), 1992–1995 (2015). [CrossRef]  

9. M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009). [CrossRef]  

10. M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011). [CrossRef]  

11. H. Okamoto, K. Kasuga, I. Hara, and Y. Kubota, “Visible–NIR tunable Pr3+-doped fiber laser pumped by a GaN laser diode,” Opt. Express 17(22), 20227–20232 (2009). [CrossRef]  

12. S. Masui, Y. Nakatsu, D. Kasahara, and S. Nagahama, “Recent improvement in nitride lasers,” Proc. SPIE 10104, 101041H (2017). [CrossRef]  

13. R. Abe, J. Kojou, K. Masuda, and F. Kannari, “Cr4+-Doped Y3Al5O12 as a Saturable Absorber for a Q-Switched and Mode-Locked 639-nm Pr3+-Doped LiYF4 Laser,” Appl. Phys. Express 6(3), 032703 (2013). [CrossRef]  

14. Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016). [CrossRef]  

15. M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, “SESAM mode-locked red praseodymium laser,” Opt. Lett. 39(24), 6939–6941 (2014). [CrossRef]  

16. K. Iijima, R. Kariyama, H. Tanaka, and F. Kannari, “Pr3+:YLF mode-locked laser at 640 nm directly pumped by InGaN-diode lasers,” Appl. Opt. 55(28), 7782–7787 (2016). [CrossRef]  

17. S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018). [CrossRef]  

18. H. Tanaka, K. Iijima, Y. Kiyota, and F. Kannari, “Power scaling, Q-switching and frequency conversion of Pr3+:YLF laser directly pumped by InGaN blue diode lasers,” in 2017 European Conference on Lasers and Electro-Optics and European Quantum Electronics Conference, (Optical Society of America, 2017), paper CA_1_5.

19. H. Tanaka, R. Kariyama, K. Iijima, and F. Kannari, “50-kHz, 50-ns UV pulse generation by diode-pumped frequency doubling Pr3+:YLF Q-switch laser with a Cr4+:YAG saturable absorber,” Appl. Opt. 55(23), 6193–6198 (2016). [CrossRef]  

20. K. V. Yumashev, “Saturable absorber Co2+:MgAl2O4 crystal for Q switching of 1.34-µm Nd3+:YAlO3 and 1.54-µm Er3+:glass lasers,” Appl. Opt. 38(30), 6343–6346 (1999). [CrossRef]  

21. M. Demesh, D.-T. Marzahl, A. Yasukevich, V. Kisel, G. Huber, N. Kuleshov, and C. Kränkel, “Passively Q-switched Pr:YLF laser with a Co2+:MgAl2O4 saturable absorber,” Opt. Lett. 42(22), 4687–4690 (2017). [CrossRef]  

22. H. Tanaka, “Trivalent Prasedymium-doped Yttrium Fluoride Visible Lasers Pumped by Gallium Nitride Series Laser Diodes,” PhD thesis, Keio University, Japan.

23. W. A. Clarkson, “Thermal effects and their mitigation in end-pumped solid-state lasers,” J. Phys. D: Appl. Phys. 34(16), 2381–2395 (2001). [CrossRef]  

24. H. Tanaka, R. Kariyama, K. Iijima, K. Hirosawa, and F. Kannari, “Saturation of 640-nm absorption in Cr4+:YAG for an InGaN laser diode pumped passively Q-switched Pr3+:YLF laser,” Opt. Express 23(15), 19382–19395 (2015). [CrossRef]  

25. P. W. Metz, D.-T. Marzahl, G. Huber, and C. Kränkel, “Performance and wavelength tuning of green emitting terbium lasers,” Opt. Express 25(5), 5716–5724 (2017). [CrossRef]  

References

  • View by:

  1. A. Richter, E. Heumann, G. Huber, V. Ostroumov, and W. Seelert, “Power scaling of semiconductor laser pumped Praseodymium-lasers,” Opt. Express 15(8), 5172–5178 (2007).
    [Crossref]
  2. P. W. Metz, F. Reichert, F. Moglia, S. Müller, D. T. Marzahl, C. Kränkel, and G. Huber, “High-power red, orange, and green Pr3+:LiYF4 lasers,” Opt. Lett. 39(11), 3193–3196 (2014).
    [Crossref]
  3. P. W. Metz, K. Hasse, D. Parisi, N. O. Hansen, C. Kränkel, M. Tonelli, and G. Huber, “Continuous-wave Pr3+:BaY2F8 and Pr3+:LiYF4 in the cyan-blue spectral region,” Opt. Lett. 39(17), 5158–5161 (2014).
    [Crossref]
  4. K. Hashimoto and F. Kannari, “High-power GaN diode-pumped continuous wave Pr3+ -doped LiYF4 laser,” Opt. Lett. 32(17), 2493–2495 (2007).
    [Crossref]
  5. T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+: LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm and 639.5 nm,” Opt. Lett. 36(6), 1002–1004 (2011).
    [Crossref]
  6. B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
    [Crossref]
  7. H. Tanaka, S. Fujita, and F. Kannari, “High-power visibly emitting Pr3+:YLF laser end pumped by single-emitter or fiber-coupled GaN blue laser diodes,” Appl. Opt. 57(21), 5923–5928 (2018).
    [Crossref]
  8. A. Sottile and P. W. Metz, “Deep red diode-pumped Pr3+:KY3F10 continuous-wave laser,” Opt. Lett. 40(9), 1992–1995 (2015).
    [Crossref]
  9. M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
    [Crossref]
  10. M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
    [Crossref]
  11. H. Okamoto, K. Kasuga, I. Hara, and Y. Kubota, “Visible–NIR tunable Pr3+-doped fiber laser pumped by a GaN laser diode,” Opt. Express 17(22), 20227–20232 (2009).
    [Crossref]
  12. S. Masui, Y. Nakatsu, D. Kasahara, and S. Nagahama, “Recent improvement in nitride lasers,” Proc. SPIE 10104, 101041H (2017).
    [Crossref]
  13. R. Abe, J. Kojou, K. Masuda, and F. Kannari, “Cr4+-Doped Y3Al5O12 as a Saturable Absorber for a Q-Switched and Mode-Locked 639-nm Pr3+-Doped LiYF4 Laser,” Appl. Phys. Express 6(3), 032703 (2013).
    [Crossref]
  14. Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
    [Crossref]
  15. M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, “SESAM mode-locked red praseodymium laser,” Opt. Lett. 39(24), 6939–6941 (2014).
    [Crossref]
  16. K. Iijima, R. Kariyama, H. Tanaka, and F. Kannari, “Pr3+:YLF mode-locked laser at 640 nm directly pumped by InGaN-diode lasers,” Appl. Opt. 55(28), 7782–7787 (2016).
    [Crossref]
  17. S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
    [Crossref]
  18. H. Tanaka, K. Iijima, Y. Kiyota, and F. Kannari, “Power scaling, Q-switching and frequency conversion of Pr3+:YLF laser directly pumped by InGaN blue diode lasers,” in 2017 European Conference on Lasers and Electro-Optics and European Quantum Electronics Conference, (Optical Society of America, 2017), paper CA_1_5.
  19. H. Tanaka, R. Kariyama, K. Iijima, and F. Kannari, “50-kHz, 50-ns UV pulse generation by diode-pumped frequency doubling Pr3+:YLF Q-switch laser with a Cr4+:YAG saturable absorber,” Appl. Opt. 55(23), 6193–6198 (2016).
    [Crossref]
  20. K. V. Yumashev, “Saturable absorber Co2+:MgAl2O4 crystal for Q switching of 1.34-µm Nd3+:YAlO3 and 1.54-µm Er3+:glass lasers,” Appl. Opt. 38(30), 6343–6346 (1999).
    [Crossref]
  21. M. Demesh, D.-T. Marzahl, A. Yasukevich, V. Kisel, G. Huber, N. Kuleshov, and C. Kränkel, “Passively Q-switched Pr:YLF laser with a Co2+:MgAl2O4 saturable absorber,” Opt. Lett. 42(22), 4687–4690 (2017).
    [Crossref]
  22. H. Tanaka, “Trivalent Prasedymium-doped Yttrium Fluoride Visible Lasers Pumped by Gallium Nitride Series Laser Diodes,” PhD thesis, Keio University, Japan.
  23. W. A. Clarkson, “Thermal effects and their mitigation in end-pumped solid-state lasers,” J. Phys. D: Appl. Phys. 34(16), 2381–2395 (2001).
    [Crossref]
  24. H. Tanaka, R. Kariyama, K. Iijima, K. Hirosawa, and F. Kannari, “Saturation of 640-nm absorption in Cr4+:YAG for an InGaN laser diode pumped passively Q-switched Pr3+:YLF laser,” Opt. Express 23(15), 19382–19395 (2015).
    [Crossref]
  25. P. W. Metz, D.-T. Marzahl, G. Huber, and C. Kränkel, “Performance and wavelength tuning of green emitting terbium lasers,” Opt. Express 25(5), 5716–5724 (2017).
    [Crossref]

2018 (2)

H. Tanaka, S. Fujita, and F. Kannari, “High-power visibly emitting Pr3+:YLF laser end pumped by single-emitter or fiber-coupled GaN blue laser diodes,” Appl. Opt. 57(21), 5923–5928 (2018).
[Crossref]

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
[Crossref]

2017 (3)

2016 (3)

2015 (3)

2014 (3)

2013 (1)

R. Abe, J. Kojou, K. Masuda, and F. Kannari, “Cr4+-Doped Y3Al5O12 as a Saturable Absorber for a Q-Switched and Mode-Locked 639-nm Pr3+-Doped LiYF4 Laser,” Appl. Phys. Express 6(3), 032703 (2013).
[Crossref]

2011 (2)

T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+: LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm and 639.5 nm,” Opt. Lett. 36(6), 1002–1004 (2011).
[Crossref]

M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
[Crossref]

2009 (2)

H. Okamoto, K. Kasuga, I. Hara, and Y. Kubota, “Visible–NIR tunable Pr3+-doped fiber laser pumped by a GaN laser diode,” Opt. Express 17(22), 20227–20232 (2009).
[Crossref]

M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

2007 (2)

2001 (1)

W. A. Clarkson, “Thermal effects and their mitigation in end-pumped solid-state lasers,” J. Phys. D: Appl. Phys. 34(16), 2381–2395 (2001).
[Crossref]

1999 (1)

Abe, R.

R. Abe, J. Kojou, K. Masuda, and F. Kannari, “Cr4+-Doped Y3Al5O12 as a Saturable Absorber for a Q-Switched and Mode-Locked 639-nm Pr3+-Doped LiYF4 Laser,” Appl. Phys. Express 6(3), 032703 (2013).
[Crossref]

Cai, Z.

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

Camy, P.

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

Cheng, Y.

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

Clarkson, W. A.

W. A. Clarkson, “Thermal effects and their mitigation in end-pumped solid-state lasers,” J. Phys. D: Appl. Phys. 34(16), 2381–2395 (2001).
[Crossref]

Demesh, M.

Doualan, J. L.

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

Fechner, M.

M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
[Crossref]

Fibrich, M.

M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

Fujimoto, Y.

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
[Crossref]

Fujita, S.

Gaponenko, M.

Guina, M.

Gün, T.

Hansen, N. O.

P. W. Metz, K. Hasse, D. Parisi, N. O. Hansen, C. Kränkel, M. Tonelli, and G. Huber, “Continuous-wave Pr3+:BaY2F8 and Pr3+:LiYF4 in the cyan-blue spectral region,” Opt. Lett. 39(17), 5158–5161 (2014).
[Crossref]

M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
[Crossref]

Hara, I.

Härkönen, A.

Hashimoto, K.

Hasse, K.

Heuer, A.

Heumann, E.

Hirosawa, K.

Huber, G.

P. W. Metz, D.-T. Marzahl, G. Huber, and C. Kränkel, “Performance and wavelength tuning of green emitting terbium lasers,” Opt. Express 25(5), 5716–5724 (2017).
[Crossref]

M. Demesh, D.-T. Marzahl, A. Yasukevich, V. Kisel, G. Huber, N. Kuleshov, and C. Kränkel, “Passively Q-switched Pr:YLF laser with a Co2+:MgAl2O4 saturable absorber,” Opt. Lett. 42(22), 4687–4690 (2017).
[Crossref]

M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, “SESAM mode-locked red praseodymium laser,” Opt. Lett. 39(24), 6939–6941 (2014).
[Crossref]

P. W. Metz, F. Reichert, F. Moglia, S. Müller, D. T. Marzahl, C. Kränkel, and G. Huber, “High-power red, orange, and green Pr3+:LiYF4 lasers,” Opt. Lett. 39(11), 3193–3196 (2014).
[Crossref]

P. W. Metz, K. Hasse, D. Parisi, N. O. Hansen, C. Kränkel, M. Tonelli, and G. Huber, “Continuous-wave Pr3+:BaY2F8 and Pr3+:LiYF4 in the cyan-blue spectral region,” Opt. Lett. 39(17), 5158–5161 (2014).
[Crossref]

T. Gün, P. Metz, and G. Huber, “Power scaling of laser diode pumped Pr3+: LiYF4 cw lasers: efficient laser operation at 522.6 nm, 545.9 nm, 607.2 nm and 639.5 nm,” Opt. Lett. 36(6), 1002–1004 (2011).
[Crossref]

M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
[Crossref]

A. Richter, E. Heumann, G. Huber, V. Ostroumov, and W. Seelert, “Power scaling of semiconductor laser pumped Praseodymium-lasers,” Opt. Express 15(8), 5172–5178 (2007).
[Crossref]

Iijima, K.

Ishii, O.

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
[Crossref]

Jelínková, H.

M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

Kajikawa, S.

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
[Crossref]

Kannari, F.

H. Tanaka, S. Fujita, and F. Kannari, “High-power visibly emitting Pr3+:YLF laser end pumped by single-emitter or fiber-coupled GaN blue laser diodes,” Appl. Opt. 57(21), 5923–5928 (2018).
[Crossref]

H. Tanaka, R. Kariyama, K. Iijima, and F. Kannari, “50-kHz, 50-ns UV pulse generation by diode-pumped frequency doubling Pr3+:YLF Q-switch laser with a Cr4+:YAG saturable absorber,” Appl. Opt. 55(23), 6193–6198 (2016).
[Crossref]

K. Iijima, R. Kariyama, H. Tanaka, and F. Kannari, “Pr3+:YLF mode-locked laser at 640 nm directly pumped by InGaN-diode lasers,” Appl. Opt. 55(28), 7782–7787 (2016).
[Crossref]

H. Tanaka, R. Kariyama, K. Iijima, K. Hirosawa, and F. Kannari, “Saturation of 640-nm absorption in Cr4+:YAG for an InGaN laser diode pumped passively Q-switched Pr3+:YLF laser,” Opt. Express 23(15), 19382–19395 (2015).
[Crossref]

R. Abe, J. Kojou, K. Masuda, and F. Kannari, “Cr4+-Doped Y3Al5O12 as a Saturable Absorber for a Q-Switched and Mode-Locked 639-nm Pr3+-Doped LiYF4 Laser,” Appl. Phys. Express 6(3), 032703 (2013).
[Crossref]

K. Hashimoto and F. Kannari, “High-power GaN diode-pumped continuous wave Pr3+ -doped LiYF4 laser,” Opt. Lett. 32(17), 2493–2495 (2007).
[Crossref]

H. Tanaka, K. Iijima, Y. Kiyota, and F. Kannari, “Power scaling, Q-switching and frequency conversion of Pr3+:YLF laser directly pumped by InGaN blue diode lasers,” in 2017 European Conference on Lasers and Electro-Optics and European Quantum Electronics Conference, (Optical Society of America, 2017), paper CA_1_5.

Kariyama, R.

Kasahara, D.

S. Masui, Y. Nakatsu, D. Kasahara, and S. Nagahama, “Recent improvement in nitride lasers,” Proc. SPIE 10104, 101041H (2017).
[Crossref]

Kasuga, K.

Kisel, V.

Kiyota, Y.

H. Tanaka, K. Iijima, Y. Kiyota, and F. Kannari, “Power scaling, Q-switching and frequency conversion of Pr3+:YLF laser directly pumped by InGaN blue diode lasers,” in 2017 European Conference on Lasers and Electro-Optics and European Quantum Electronics Conference, (Optical Society of America, 2017), paper CA_1_5.

Kojou, J.

R. Abe, J. Kojou, K. Masuda, and F. Kannari, “Cr4+-Doped Y3Al5O12 as a Saturable Absorber for a Q-Switched and Mode-Locked 639-nm Pr3+-Doped LiYF4 Laser,” Appl. Phys. Express 6(3), 032703 (2013).
[Crossref]

Kränkel, C.

Kubota, Y.

Kuleshov, N.

Leinonen, T.

Luo, S.

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

Luo, Z.

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

Marzahl, D. T.

Marzahl, D.-T.

Masuda, K.

R. Abe, J. Kojou, K. Masuda, and F. Kannari, “Cr4+-Doped Y3Al5O12 as a Saturable Absorber for a Q-Switched and Mode-Locked 639-nm Pr3+-Doped LiYF4 Laser,” Appl. Phys. Express 6(3), 032703 (2013).
[Crossref]

Masui, S.

S. Masui, Y. Nakatsu, D. Kasahara, and S. Nagahama, “Recent improvement in nitride lasers,” Proc. SPIE 10104, 101041H (2017).
[Crossref]

Metz, P.

Metz, P. W.

Moglia, F.

Moncorgé, R.

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

Müller, S.

Nagahama, S.

S. Masui, Y. Nakatsu, D. Kasahara, and S. Nagahama, “Recent improvement in nitride lasers,” Proc. SPIE 10104, 101041H (2017).
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[Crossref]

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M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

Okamoto, H.

Ostroumov, V.

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Peng, J.

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

Petermann, K.

M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
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B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
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[Crossref]

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M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

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Südmeyer, T.

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M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

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Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

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[Crossref]

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Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

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Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

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Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

Xu, H.

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

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Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

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S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
[Crossref]

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Yoshida, M.

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
[Crossref]

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Zhang, H.

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

Zhu, C.

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

Appl. Opt. (4)

Appl. Phys. B (2)

M. Fibrich, H. Jelínková, J. Šulc, K. Nejezchleb, and V. Škoda, “Visible cw laser emission of GaN-diode pumped Pr:YAlO3 crystal,” Appl. Phys. B 97(2), 363–367 (2009).
[Crossref]

M. Fechner, F. Reichert, N. O. Hansen, K. Petermann, and G. Huber, “Crystal growth, spectroscopy, and diode pumped laser performance of Pr, Mg:SrAl12O19,” Appl. Phys. B 102(4), 731–735 (2011).
[Crossref]

Appl. Phys. Express (1)

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[Crossref]

IEEE Photonics Technol. Lett. (1)

B. Qu, B. Xu, S. Luo, Y. Cheng, H. Xu, Z. Cai, P. Camy, J. L. Doualan, and R. Moncorgé, “InGaN-LD-pumped continuous-wave deep red laser at 670 nm in Pr3+:LiYF4 crystal,” IEEE Photonics Technol. Lett. 27(4), 333–335 (2015).
[Crossref]

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W. A. Clarkson, “Thermal effects and their mitigation in end-pumped solid-state lasers,” J. Phys. D: Appl. Phys. 34(16), 2381–2395 (2001).
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Nanoscale (1)

Z. Luo, D. Wu, B. Xu, H. Xu, Z. Cai, J. Peng, J. Weng, S. Xu, C. Zhu, F. Wang, Z. Sun, and H. Zhang, “Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers,” Nanoscale 8(2), 1066–1072 (2016).
[Crossref]

Opt. Commun. (1)

S. Kajikawa, M. Yoshida, O. Ishii, M. Yamazaki, and Y. Fujimoto, “Visible Q-switched pulse laser oscillation in Pr-doped double-clad structured waterproof fluoride glass fiber with graphene,” Opt. Commun. 424(7), 13–16 (2018).
[Crossref]

Opt. Express (4)

Opt. Lett. (7)

Proc. SPIE (1)

S. Masui, Y. Nakatsu, D. Kasahara, and S. Nagahama, “Recent improvement in nitride lasers,” Proc. SPIE 10104, 101041H (2017).
[Crossref]

Other (2)

H. Tanaka, “Trivalent Prasedymium-doped Yttrium Fluoride Visible Lasers Pumped by Gallium Nitride Series Laser Diodes,” PhD thesis, Keio University, Japan.

H. Tanaka, K. Iijima, Y. Kiyota, and F. Kannari, “Power scaling, Q-switching and frequency conversion of Pr3+:YLF laser directly pumped by InGaN blue diode lasers,” in 2017 European Conference on Lasers and Electro-Optics and European Quantum Electronics Conference, (Optical Society of America, 2017), paper CA_1_5.

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Figures (9)

Fig. 1.
Fig. 1. Experimental setup of a diode-pumped Pr3+:YLF laser Q-switched by a Co2+:MALO saturable absorber.
Fig. 2.
Fig. 2. (a) Average power, (b) pulse width, and (c) repetition frequency of passively Q-switched Pr3+:YLF laser with Co2+:MALO (SA2) and output coupler of 9.1% at 640 nm as a function of absorbed pump power. Dashed lines are results of numerical simulation.
Fig. 3.
Fig. 3. (a) Average power, (b) pulse width, and (c) repetition frequency of passively Q-switched Pr3+:YLF laser with Co2+:MALO (SA2) and output coupler of 11.4% at 607 nm as a function of absorbed pump power. Dashed lines are results of numerical simulations with two different ESA cross sections.
Fig. 4.
Fig. 4. (a) Average power, (b) pulse width, and (c) repetition frequency of passively Q-switched Pr3+:YLF laser with Co2+:MALO (SA2) and output coupler of 2.7% at 523 nm as a function of absorbed pump power.
Fig. 5.
Fig. 5. Experimental setup of intracavity frequency doubling a diode-pumped Pr3+:YLF laser passively Q-switched by a Co2+:MALO saturable absorber.
Fig. 6.
Fig. 6. (a) Average power, (b) pulse width, and (c) repetition frequency of UV pulse at 320 nm as a function of absorbed pump power. Dashed lines are results of numerical simulation.
Fig. 7.
Fig. 7. (a) Average power, (b) pulse width, and (c) repetition frequency of UV pulse at 261 nm as a function of absorbed pump power.
Fig. 8.
Fig. 8. Pulse train of passively Q-switched Pr3+:YLF laser with Co2+:MALO (Sample 2) (a) OC-SA : 3 mm, ${w_{SA}}$: 63×60 µm2 (b) OC-SA : 6 mm, ${w_{SA}}$: 65×61 µm2 (c) OC-SA : 9 mm, ${w_{SA}}$: 69×64 µm2
Fig. 9.
Fig. 9. Pulse train of passively Q-switched Pr3+:YLF laser with Cr4+:YAG (a) OC-SA : 3 mm, ${w_{SA}}$: 63×06 µm2 (b) OC-SA : 30 mm, ${w_{SA}}$: 115×112 µm2 (c) OC-SA : 40 mm, ${w_{SA}}$: 143×142 µm2

Tables (5)

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Table 1. Saturation characteristics of Co2+:MALO at 523, 607, and 640 nm

Tables Icon

Table 2. Summary of characteristics of passively Q-switched Pr3+:YLF laser with Co2+:MALO at 523, 607, and 640 nm

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Table 3. Parameters and values used in rate equations of passively Q-switched Pr3+:YLF laser with Co2+:MALO saturable absorber

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Table 4. Parameters and values used in rate equations of intracavity frequency doubling of passively Q-switched Pr3+:YLF laser with Co2+:MALO saturable absorber

Tables Icon

Table 5. Summary of characteristics of UV pulses at 261 and 320 nm

Equations (5)

Equations on this page are rendered with MathJax. Learn more.

d ϕ d t = c ϕ l c { σ s t Δ N l g σ g s n g s l S A σ e s ( n t o t n g s ) l S A } ϕ τ c + S
d Δ N d t = c ϕ σ s t Δ N Δ N τ f + N t o t Δ N N t o t η Q η S t η m P a b s h ν L V
d n g s d t = σ g s c ϕ n g s A g A S A + n t o t n g s τ S A .
d ϕ d t = c ϕ l c { σ s t Δ N l g σ g s n g s l S A σ e s ( n t o t n g s ) l S A } ϕ τ c + S c 2 2 l c γ S H G h ν ω A N L ϕ 2 .
γ S H G = 2 ω 2 d e f f 2 l N L C k π n N L 3 c 3 ε 0 h ( B , ξ ) .

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