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Abstract
We have investigated room-temperature continuous-wave (cw) lasing performance of Yb:YLF oscillators in detail using rod-type crystals with low Yb-doping (2%). The laser is pumped by a low-cost, high brightness, 10 W, 960 nm single-emitter multimode diode. Laser performance is acquired in both ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ configurations, using 12 different output couplers with transmission ranging from 0.015% to 70%. We have estimated the passive loss of the Yb:YLF crystal as 0.06% per cm, corresponding to an impressive crystal figure of merit above 4000. The low-doping level not only reduces the system losses but also minimizes the thermal load as the low doped crystals enable distribution of heat load in a greater volume. Using the advantages of lower loss and improved thermal behavior, we have achieved cw output power above 4 W, cw slope efficiencies up to 78%, and a record cw tuning range covering the 993–1110 nm region (117 nm). The output power performance achieved in this initial work is limited by the available pump power, and future room-temperature Yb:YLF systems have the potential to produce higher output power levels.
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1. INTRODUCTION
Yb:YLF has been attracting the attention of researchers working in solid-state laser-cooling (optical refrigeration) [1–7] and laser/amplifier development [8–17] over the last two decades. One important reason for this interest is the capability to grow high-quality Yb:YLF crystals with minimal passive losses [6]. Especially, for the solid-state optical cooling community, extremely high purity samples with high external quantum efficiency are necessary [18]. Using Yb:YLF samples with background absorption losses as low as ${2} \times {{10}^{- 4\:}}{{\rm cm}^{- 1}}$ (0.02% per cm) [18], laser-cooling of the samples down to 91 K has been demonstrated [19]. High-quality laser crystals enable efficient lasing performance with minimal intracavity losses and, therefore, high power output.
Room-temperature (RT) lasing of Yb:YLF has been investigated in several different geometries (rod, thin-disk, and waveguide). In their initial lasing work, using a 0.3-mm-long 40.5% doped crystal, Kawanaka et al. achieved 50 mW of continuous-wave (cw) output power around 1040 nm at an absorbed pump power of 400 mW using a 1.5% output coupler [8]. After this initial demonstration, with the development of higher brightness diodes and higher quality crystals and cavity optics, the performance of Yb:YLF lasers improved significantly, and cw output power levels as high as 3.03 W [20], 2.76 W [21], and 5.87 W [22,23] were achieved at RT from rod, waveguide, and thin-disk geometries, respectively. A slope efficiency as high as 76% was demonstrated under Ti:sapphire pumping [21]; cw tuning ranges covering the 993–1076 nm region were also achieved [24]. Moreover, via cooling the crystal to cryogenic temperatures [9,14,15,25–28], cw output power above 500 W [16] and slope efficiencies as high as 82% [29] were demonstrated at the expense of reduced gain bandwidth (tuning range, 995–1020.5 nm [30]).
In this work, we have investigated cw lasing performance of 2%-Yb-doped YLF crystals in great detail at RT. We have determined that the crystals have very low passive losses of around 0.06% per cm enabling efficient laser operation. Upon pumping the laser with a high brightness 10 W single-emitter multimode diode (MMD) at 960 nm, we have achieved cw power up to 4 W, a slope efficiency of 78%, and a record cw tuning range covering the 993–1110 nm region. This tuning performance is among the broadest tuning ranges yet reported from Yb-based solid state lasers [31], and it shows the potential of Yb:YLF crystals for the development of efficient short pulse (sub-20-fs [32]) oscillators.
The paper is organized as follows: In Section 2, we present the setup we have used in lasing experiments. In Section 3, we provide details of cw laser efficiency and laser crystal loss estimate. Later, in Section 4, we report our cw tuning results and discuss the tuning performance by comparing it with the measured gain spectra. Finally, in Section 5, we conclude with a brief summary and provide an outlook for future work.
Fig. 1. Experimental setup of the diode-pumped Yb:YLF laser used in cw laser experiments. BRF, birefringent filter; OC, output coupler; MMD, 10 W, 960 nm, single-emitter multimode laser pump diode.
Fig. 2. Top view of Brewster-Brewster cut Yb:YLF crystals optimized for lasing in (a) ${\rm E}//{\rm c}$ and (b) ${\rm E}//{\rm a}$ configurations, respectively. Path of the TM-polarized intracavity laser beam is shown in blue. ${E}$, electric field vector for intracavity laser beam; ${k}$, intracavity laser beam propagation direction; ${\theta _b}$, Brewster’s angle; ${L}$, length of the laser crystal; ${W}$, width of the crystal; ${H}$, height/thickness of the crystal (not visible in the picture). Pump light (not shown in figure) is (a) TM-polarized and (b) TE-polarized to employ the $c$ axis with larger absorption cross section value.
2. EXPERIMENTAL SETUP
Figure 1 shows a schematic of the Yb:YLF laser that is used in cw laser experiments. A single-emitter MMD providing up to 10 W of pump power at 960 nm is used for pumping (LDX-3A10-960-C from RPMC Lasers). The MMD has an emitter size of ${1} \times {100}\;\unicode{x00B5}{\rm m}$ (${\rm sagittal}/{\rm fast} \times {\rm tangential}/{\rm slow}\;{\rm axes}$) and provides a quite asymmetric rectangular-shaped multi-transverse mode beam. The MMD output beam is first collimated with a 4.5 mm focal length aspheric lens (${{f}_1}$), and then, to minimize the diode beam astigmatism, a cylindrical lens with a focal length of 50 mm (${{f}_z}$, acting on the fast axis) is employed. An achromatic doublet with a focal length of 80 mm (${{f}_2}$) is utilized to focus the pump beam to a waist of around ${25}\;{\unicode{x00B5}{\rm m}} \times {100}\;{\unicode{x00B5}{\rm m}}$ inside the crystal. Brewster-Brewster cut, 2% Yb-doped 10-mm-long and 15-mm-long YLF crystals with a width of 5 mm and thickness of 2 mm are used for the lasing experiments in ${\rm E}//{\rm c}$ and ${\rm E}//{\rm a}$ configurations, respectively (Fig. 2). Due to the losses introduced by Brewster cut surfaces for the TE (s) polarized light, laser output is TM (p) polarized. Hence, for the crystal shown in Fig. 2(a), the electric field of the intracavity laser beam is parallel to the $c$ axis of the crystal (${\rm E}//{\rm c}$ configuration), and for the crystal shown in Fig. 2(b), the electric field is parallel to the $a$ axis of the crystal (${\rm E}//{\rm a}$ configuration). The crystals are mounted with indium foil in a copper holder under water cooling at 15°C. The estimated small-signal absorption of crystals for TM-polarized light at 960 nm is 81% and 94% for the 15-mm-long (${\rm E}//{\rm a}$) and 10-mm-long (${\rm E}//{\rm c}$) Yb:YLF crystals. Note that, despite the usage of a longer crystal length, the absorption of the crystal used for ${\rm E}//{\rm a}$ configuration was rather low for TM-polarized pump light. To improve the absorption of the ${\rm E}//{\rm a}$ cut crystal, pump polarization is rotated using a half-wave plate, to achieve a small-signal absorption of around 98.5% for the TE-polarized light (at the expense of 12.5% reflection loss from the crystals Brewster surface, resulting in an effective absorption of 86.2%).
A standard ${X}$-type resonator is used in cw laser experiments. The laser cavity consisted of two curved high reflecting mirrors, each with a radius of curvature of 75 mm (M1 and M2), a flat end high reflector (M3), and a flat OC. The arm lengths for the high reflector and OC end are 27 cm and 25 cm, respectively, resulting in a beam waist of ${30}\;{\unicode{x00B5}{\rm m}}\;{\times}\;{40}\;{\unicode{x00B5}{\rm m}}$ inside the crystal. The pump mirrors (M1–M2) are anti-reflection-coated for the 800–970 nm range (${R}\; \lt \;{2}\%$). All the cavity high reflectors (M1–M3) have reflective coatings covering the 1010–1200 nm spectral region with a reflectivity above 99.9% specified at 0° incidence (Layertec mirror, 102924). The mirrors M1–M2 are used at an incidence angle of around 10–15°, which slightly blueshifted the reflectivity band specified above. Twelve different output couplers with transmission ranging from 0.015% to 70% are used in the experiments to carefully scan the operation range of the lasers. An adjustable width mechanical slit is inserted near the output coupler to control the transverse output mode of the laser in the tangential axis.
A 2-mm-thick off-surface optical axis crystal quartz birefringent filter (BRF) with a diving angle of 25° is inserted at Brewster’s angle near the output coupler for smooth tuning of the laser wavelength [33]. Compared to regular on-surface optical axis BRFs, off-surface optical axis BRFs provide a broader set of filtering functions in different orders, which results in superior performance in tuning cw, femtosecond, and two-color lasers [34,35]. The TE-polarized intracavity beam has Fresnel reflection losses from the laser crystal and the BRF plate surfaces, which provide a modulation depth of around 40% in one round trip. While operating around 1000 nm, the BRF has a calculated free-spectral range of around 350 nm, a filter full width at half-maximum (FWHM) of around 40 nm, and a tuning rate of around 45 nm per degree (calculated parameters are for the second order of the BRF, around a rotation angle of 35°).
3. CONTINUOUS-WAVE LASING RESULTS
Figure 3 shows the measured efficiency of the Yb:YLF laser using output couplers with transmission ranging from 0.015% to 60% for both ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ configurations [Figs. 3(a) and 3(b), respectively]. Using ${\rm E}//{\rm a}$ configuration, the optimum cw performance is obtained with a 1% transmitting output coupler. At this output coupling, we have measured a lasing threshold of around 0.5 W, and the laser produced a cw output power of 3.1 W around 1040 nm at an absorbed pump power of 8 W (absorption, 94.8%). The laser reaches a slope efficiency of 54% (with respect to absorbed pump power) at high pump intensities. Note that the laser efficiency curves are nonlinear due to (i) the three-level structure of the gain medium [36] and (ii) the power dependence of the pump and, hence, laser transverse modes. Due to the nonlinear laser efficiency curves, the slope efficiency of the laser is also not constant and varies with pump power level [sample curves demonstrating the variation of slope efficiency with pump power are shown for 1% OC in Fig. 3(a) and 5% OC in Fig. 3(b)]. Hence, in our analysis, we have determined instantaneous slope efficiency values, by determining the slopes of the laser efficiency curves at different pump power levels.
Fig. 3. Measured cw laser performance of the room-temperature Yb:YLF laser using output couplers in the 0.015% to 60% range for (a) ${\rm E}//{\rm a}$ and (b) ${\rm E}//{\rm c}$ axes, respectively. Variation of slope efficiency with absorbed pump power level is also shown for the best performing output couplers: 1% OC for ${\rm E}//{\rm a}$ and 5% OC for ${\rm E}//{\rm c}$ configurations, respectively.
For the ${\rm E}//{\rm c}$ configuration, using the same 1% output coupler, we have measured a lasing threshold of only around 0.2 W due to higher gain in the ${\rm E}//{\rm c}$ axis [37]. At 1% output coupling, the ${\rm E}//{\rm c}$ axis lasing results in an output power of 3.67 W around 1050 nm at an absorbed pump power of 8.5 W (absorption, 91%). With the ${\rm E}//{\rm c}$ configuration, a better cw performance is achieved using the 5% transmitting output coupler. While employing the 5% OC, we have measured a cw lasing threshold of around 0.75 W, and we obtained a cw power of 4 W at an absorbed pump power of 8 W. The slope efficiency curve is again quite nonlinear: the slope efficiency starts at 30% and increases up to 78% at high pump power levels. This we believe is one of the highest slope efficiencies obtained from Yb:YLF lasers to date [21,29].
As mentioned earlier, the increase of slope efficiency with pump power is partly due to the three-level laser structure and partly due to changes in the laser transverse mode. As we will discuss later (using Fig. 6), as the pump power is increased, the laser output becomes multimode. This improves the mode-matching between the pump and laser modes and, hence, also the slope efficiency at high pump power levels. As another interesting observation, note that, for large output coupling [such as the 60% shown in Fig. 3(a)], we have observed a decrease in slope efficiency. This phenomenon is also observed in earlier work to some extent [13,16,30] and might be due to effects such as Auger upconversion, and pump excited state absorption [36]. These processes are weak in Yb-systems due to the simple energy level diagram. On the other hand, the emission observed at visible wavelengths in Yb:YLF clearly shows the presence of such mechanisms [38], and the change of rate of these processes with output coupling might affect the laser performance at large output coupling values. As a side note, in our experiments, we have observed that the greenish visible emission of Yb:YLF under tight pumping shifts to the blue region of the spectrum while using low output coupling values, which shows the role of inversion and intracavity power levels in these dynamics. Future work is required for better understanding of this phenomena.
In Fig. 4, we plot the variation of cw output power performance and free-running laser wavelength with output coupling for both configurations. Note that some of the results that are shown in Fig. 4 are not included in Fig. 3, to not over-crowd the efficiency data (out of the 12 output couplers used, we have selected 8). As we discussed earlier, the optimum output coupling is around 5% for the ${\rm E}//{\rm c}$ configuration and around 1% for the lower gain ${\rm E}//{\rm a}$ axis. The variation of performance with output coupling is rather smooth, and we could achieve similar cw performance for output coupling values in the 0.5%–5% range for ${\rm E}//{\rm a}$ axis, and 0.5%–30% range for ${\rm E}//{\rm c}$ axis. As an interesting observation, while employing ${\rm E}//{\rm c}$ configuration, rather efficient laser operation could be achieved even at an output coupling of 70%, which shows the high gain nature of the Yb:YLF material.
Fig. 4. Measured variation of free-running laser wavelength and maximum attainable cw laser power with output coupling transmission. The data are taken while pumping the room-temperature Yb:YLF laser with an incident pump power of 9.87 W.
Note that the obtainable output power levels are higher while employing ${\rm E}//{\rm c}$ configuration for almost all output coupling values due to lower lasing thresholds and higher slope efficiencies acquired in this axis. The lower lasing threshold of the ${\rm E}//{\rm c}$ axis is clearly mostly due to the higher gain in this axis, as already mentioned. On the other hand, the superior performance in terms of laser slope efficiency might be due to the better mode-matching that could be acquired in the shorter crystal length employed for ${\rm E}//{\rm c}$ axis lasing (a 10-mm-long crystal is used in ${\rm E}//{\rm c}$, compared to the 15 mm crystal used in ${\rm E}//{\rm a}$ configuration).
Fig. 5. Measured variation of the inverse of the slope efficiency with the inverse of the output coupling percentage for the Yb:YLF laser employing the ${\rm E}//{\rm c}$ axis. Using Caird analysis, making a best-fit to the measured data [blue solid curve, given by $y(x) = \;({0.003}\;{\pm}\;{0.001})x + ({0.018}\;{\pm}\;{0.001})$], we have estimated the round trip passive cavity loss to be ${0.18}\;{\pm}\;{0.05}\%$. For the slope efficiency values, a linear fit is made to the laser efficiency data, and in this fit only data points above 5 W absorbed pump power level are taken. Slope efficiency data for the 60% and 70% output couplers are considered outliers (shown with open markers in the graph) and are not included in the analysis. ${{R}_{{\rm oc}}}$, reflectivity of the output coupler.
In terms of the free-running laser wavelength, both crystal axes behave rather similarly: as the output coupling is increased, the free-running laser wavelength shifts from around 1060–1070 nm to around 1020 nm region (Fig. 4). This observation is in relatively good agreement with earlier reports in literature [13,24] and will be discussed in more detail in the next section. As a side note, we see that, at low output coupling values (below 3%), the free-running laser wavelengths are slightly longer in ${\rm E}//{\rm a}$ configuration compared to the ${\rm E}//{\rm c}$ (e.g., 1070 nm in ${\rm E}//{\rm a}$ and 1061 nm in ${\rm E}//{\rm c}$ using the 0.015% transmitting output coupler). This might be due to the lower gain in ${\rm E}//{\rm a}$ axis: the self-absorption losses push the laser wavelength more toward the longer wavelength side of the emission spectrum.
We have also utilized the measured variation of the laser slope efficiency with output coupler transmission to estimate the level of passive cavity losses (Caird analysis [39]), and we found that the round trip passive loss of the system is around ${0.18}\;{\pm}\;{0.05}\%$ (Fig. 5). Assuming around 0.06% of this loss comes from cavity optics (two bounces on M1–M2, and one bounce on M3 and OC: six overall bounces), for the 10 mm 2% Yb-doped crystal, we estimate a passive loss of around ${0.06}\;{\pm}\;{0.02}\%$ per cm (effective loss coefficient, ${0.0006}\;{{\rm cm}^{- 1}}$) for the Yb-doped crystal. Note that this loss level is lower than the passive losses that are estimated for Cr:LiCAF/Cr:LiSAF crystals (0.1%–0.2% per cm) and is similar to the values one can get in good quality Alexandrite samples (0.06% per cm [40]). On the other hand, for a more practical/useful comparison between different laser gain media, it is better to compare the figure of merit (FOM) parameter of different crystals. FOM of a crystal is defined as the ratio of absorption coefficient at the pump wavelength to the effective passive loss coefficient at lasing wavelength (this ignores self-absorption losses). The 2% Yb-doped YLF crystal we have employed in this study has an absorption coefficient of around ${2.8}\;{{\rm cm}^{- 1}}$ at 960 nm for ${\rm E}//{\rm c}$ axis at RT [37]. Overall, then the FOM of the Yb:YLF crystal is above 4000 (${2.8/0.0006}(\cong \;{4667})$. For comparison, in a carefully performed earlier study, Pirzio et al. estimated a round trip passive loss of around 0.34% for a Yb:YLF laser employing a 2.1-mm-thick 10% Yb-doped YLF crystal [13]. Under similar assumptions, we estimate a passive loss of around 0.7% per cm and a FOM of around 2000 for the 10% Yb-doped YLF sample used in [13]. We believe the higher FOM of the crystals used in this study might be due to the lower Yb-doping employed in our case.
It is educational to compare the estimated FOM of Yb:YLF with other common laser materials. For example, the FOM of commonly employed Ti:sapphire crystals are just around 250 [41], and only carefully hand-selected samples could reach FOM values up to 1000 [42] (FOM of Ti:sapphire is primarily determined by the concentration of ${{\rm Ti}^{3 +}} - {{\rm Ti}^{4 +}}$ pairs inducing absorption around 800 nm, and high FOM values could only be achieved for very low Ti concentrations). The FOM of carefully grown Cr:LiCAF crystals is around 2000, and for Cr:LiSAF and Alexandrite, the FOM could reach 3000 [43]. Having such a low-loss crystal with extremely high FOM enables construction of high-${Q}$ cavities, which has benefits in optimizing intracavity experiments such as intracavity second-harmonic generation. Of course, as discussed earlier, Yb:YLF crystals with high FOM also provide advantages to the laser-cooling community [6].
Due to the multimode pump beam, the output of the Yb:YLF laser is also multi-transverse mode, as we have also observed in earlier MMD pumped systems such as Cr:LiCAF and Cr:LiSAF [43]. Figure 6(a) shows the beam profile of the pump diode at its maximum power setting (10 W), which is quite asymmetric mainly due to the asymmetry of the diode emitter size. The Yb:YLF laser output beam profile at different output power levels is shown in Figs. 6(b)–6(e). As the pump power is increased, the pump beam itself contains higher transverse order modes, and in response the Yb:YLF laser, which starts with a ${{\rm TEM}_{00}}$ beam at low output power levels (0.3 W), becomes highly multi-transverse at increasing pump power levels. The observed variation of the output beam profile of the Yb:YLF laser enables efficient mode-matching between the pump and laser modes, and as discussed earlier, this process is partially responsible for the larger laser slope efficiencies observed at higher pump power levels. In our experiments, we have tried implementing an intracavity slit near the output coupler mirror, and by adjusting its width in the tangential plane, it was possible to push the Yb:YLF laser to produce a symmetric beam with a ${{\rm TEM}_{00}}$ profile. However, this comes at the expense of reduced mode-matching and, hence, reduced output power levels. In this mode [Fig. 6(f)], the Yb:YLF laser only produced 1.8 W of output power at 10 W incident pump power. For comparison, we have recently investigated the laser performance of a highly Yb-doped (25%) 1.5-mm-long crystal in the same laser cavity. The laser performance was inferior in this highly doped sample (around 3 W compared to 4 W in this work) [20]. However, the short crystal enabled better mode-matching between the pump and laser modes compared to the 10–15-mm-long crystals used in this study, and the laser output beam quality was superior. Hence, depending on the intended application, one needs to carefully optimize the crystals’ doping and length for control of the laser behavior.
Fig. 6. (a) Measured pump beam profile at the position of the crystal at the maximum pump power level (10 W). (b)–(e) Laser output beam profile just after the OC (near field) at several different output power levels. Without the intracavity slit, the laser output mode starts to acquire higher transverse mode orders at increasing power levels. (f) Adjusting the width of the intracavity slit in the tangential plane enables ${{\rm TEM}_{00}}$ laser output, at the expense of reduced output power: the laser power decreases from 4 W to 1.8 W at 10 W incident pump power.
In closing this section, we would like to mention that, in our cw laser experiments with Yb:YLF, we have not yet observed limitations due to thermal effects, even at the maximum pump power level (10 W) that is available from the system. This is partly due to the low Yb-doped YLF samples used in this study, which enable a good distribution of thermal load in the relatively long crystals employed in this work. Especially, we have not noticed presence of thermal lensing, which is a known advantage of the YLF host compared to other hosts like YAG [44–51]. The experimental data in Figs. 3 and 4 are taken at a crystal holder temperature of 15°C. However, we have also investigated the variation of laser output power with crystal holder temperature, in the temperature range of 10–50°C. As we can see from Fig. 7, as the temperature of the Yb:YLF crystal holder is increased, the obtainable laser output power levels slowly decrease with temperature for lasing in both axes. This decrease of output power with temperature is mainly due to the decreasing gain cross section (GCS) value (as the intrinsic lifetime of the Yb:YLF material is constant over a wide temperature range) [37,38].
Fig. 7. Measured variation of Yb:YLF laser output power with laser crystal holder temperature for ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ axes. The data are taken at an incident pump power of 9.87 W using a 5% transmitting output coupler.
4. CONTINUOUS-WAVE TUNING RESULTS
The detailed cw laser efficiency measurements we have made showed that the low Yb-doped YLF samples employed in this study have very low passive losses. Motivated by this, we have explored the cw tuning capabilities of the laser in both ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ configurations using an intracavity BRF. Tuning of the laser wavelength is facilitated simply by rotation of the birefringent plate about an axis normal to the surface, and the process does not require laser realignment. Moreover, since the BRF is inserted at Brewster’s angle inside the laser cavity, it does not require anti-reflective coatings, and its passive loss is rather low.
Figure 8 shows the measured cw tuning behavior of the room-temperature Yb:YLF laser for both ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ axes, at an incident pump power of 9.87 W. Tuning data are taken with several different output couplers with transmissions in the 0.015%–20%, range. With the ${\rm E}//{\rm a}$ polarization, using a 1% output coupler, the Yb:YLF laser is tunable in the 1000–1075 nm range, and cw power levels up to 1.8 W could be obtained. For comparison, using the same OC, we could achieve a tuning range from 995 nm to 1089 nm, and a cw power up to 2.9 W with the ${\rm E}//{\rm c}$ configuration. Clearly, due to its higher gain, the ${\rm E}//{\rm c}$ axis provides a broader tuning range when one employs the same output coupler. For pushing the long wavelength operation limit, we have used a 0.015% transmitting output coupler, where cw tuning ranges of 1005–1100 nm and 994–1110 nm are demonstrated for ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ configurations, respectively. For the ${\rm E}//{\rm a}$ polarization, the short wavelength tuning limit could be reduced down to 993 nm using output couplers with transmission values above 3%. Figure 9 shows sample optical spectra taken during the tuning experiments. To our knowledge, this study reports the first demonstration of lasing of Yb:YLF above 1075 nm [20]. This we believe is due to the low-loss cavities employed in this study.
Note that, for the ${\rm E}//{\rm c}$ configuration, while employing output couplers with transmission values above 3%, the tuning modulation depth of the BRF (40%) was not high enough to enable smooth tuning: as one can see from Fig. 8(b), the tuning curves are not always complete. As an example, using the 3% output coupler, while trying to tune the laser above 1045 nm, we have observed wavelength jumps to the gain peak (${\sim}{1020}\;{\rm nm}$). Basically, the laser preferred to operate at 1020 nm in TE polarization instead of operating at 1050 nm with TM polarization. This shows that the 40% modulation depth we have in the system is not sufficient to suppress this jump due to large differences in gain. Hence, in future studies, design of higher modulation depth systems could smoothen the tuning of the ${\rm E}//{\rm c}$ axis laser at high output coupling values.
Fig. 8. Measured tuning behavior of the cw Yb:YLF laser using output couplers in the 0.015% to 20% range for (a) ${\rm E}//{\rm a}$ and (b) ${\rm E}//{\rm c}$ configurations, respectively. The data are taken at an incident pump power of 9.87 W. The overall tuning range covers the 993–1110 nm region (117 nm).
Fig. 9. Sample optical spectra demonstrating cw tunability of the Yb:YLF laser in the 993–1110 nm region.
As one can see from Fig. 8, the tuning behavior is quite diverse at different output coupling values. First of all, as the output coupling is increased, the wavelength at which the maximum output power could be attained starts to shift to shorter wavelengths. This observation is in good agreement with the variation of free-running laser wavelength with output coupling, as outlined earlier in Fig. 4. Moreover, at increased output coupling, the obtainable tuning range gets narrower. This behavior is due to the three-level laser structure of Yb:YLF gain medium and could be understood by investigating the RT gain spectrum. Figure 10 shows the calculated variation of GCS with wavelength for both axes of Yb:YLF for inversion levels between 5% and 30%. The GCS spectra (${\sigma _g}(\lambda)$) are estimated using the measured emission spectra [37] via the following formula:
Fig. 10. Calculated room-temperature gain cross section (GCS) spectra for Yb:YLF at fractional population inversion levels ($\beta$) between 5 and 30% for (a) ${\rm E}//{\rm a}$ and (b) ${\rm E}//{\rm c}$ configurations, respectively.
Note that, at increased output coupling, one increases the overall loss of the cavity; hence, to balance the increased loss, the crystal possesses a higher level of inversion at high output coupling values. As a result, the losses and inversion of the crystal increases, and one expects a blueshift of laser wavelengths. Hence, this first observation explains the wavelength shifts we have observed in Figs. 4 and 8. As a second observation, we see that, at high inversion levels, gain curves get sharper, and the FWHM of the gain curve decreases. This second observation explains the narrower tuning range behavior we have observed at higher output coupling.
Table 1. Summary of Room-Temperature Continuous-Wave Lasing Results Obtained with Yb:YLF Material
Fig. 11. Calculated room-temperature gain cross section spectra for Yb:YLF at fractional population inversion levels ($\beta$) of 10% and 30% for ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ configurations.
As another observation, we see from Fig. 10 that the gain at the longer wavelength edge of Yb:YLF is rather small. As a result, to achieve lasing in this region, we had to use an output coupler with minimal losses (0.015%). To look at this in detail, Fig. 11 shows the GCS curve for ${\rm E}//{\rm c}$ and ${\rm E}//{\rm a}$ polarizations together in the same graph, at selected inversion levels of 10 and 30%. From Fig. 11, we clearly see that the gain of Yb:YLF around 1100 nm is more than 2 orders of magnitude lower compared to the gain around 1020 nm for both polarizations. This explains the tuning behavior we have observed for the long wavelength range (Fig. 8): the long wavelength tuning range could be improved by employing output couplers with lower transmission. As an example, for the ${\rm E}//{\rm c}$ axis, we have obtained cw tuning limits of 1074 nm, 1082 nm, 1090 nm, and 1110 nm at output coupling values of 1%, 0.6%, 0.15%, and 0.015%, respectively. We also see from Fig. 11 that, for the long wavelength side, the ${\rm E}//{\rm c}$ axis gain spectra extend to longer wavelengths compared to ${\rm E}//{\rm a}$ configuration (for a fixed inversion/pump level), and this explains the slightly broader tuning range we have achieved while employing the ${\rm E}//{\rm c}$ polarization (1110 nm in ${\rm E}//{\rm c}$ versus 1100 nm in ${\rm E}//{\rm a}$). As another observation, note from Fig. 11 that, on the short wavelength side, the GCS spectra drop sharply around 990 nm for both ${\rm E}//{\rm a}$ and ${\rm E}//{\rm c}$ axes operation due to increased role of self-absorption losses, and this matches well with the short-wavelength tuning limit that is observed in his study (993 nm).
5. SUMMARY AND OUTLOOK
We would like to finalize our paper with Table 1, which provides a summary of the cw RT lasing results obtained with Yb:YLF lasers to date. We see from Table 1 that this study reports RT lasing results using crystals with a lower amount of Yb-doping compared to earlier work. It is likely that, as a result of this low-doping level, the crystals used in this study had relatively lower passive losses. Usage of low-loss crystals with rather long length also enabled distribution of the thermal load in a greater volume and reduced thermal effects. As a consequence, in this study, we have generated cw power levels up to 4 W from the system, which is the highest reported true cw power to date in rod geometry at RT. The power level is limited by the available pump power, and we believe future diode-pumped RT Yb:YLF systems could scale the output power levels above 10 W even in simple rod geometry. Despite the asymmetric pump diode we have employed in this work, we have observed slope efficiencies up to 78%, which is similar to what is achieved with Ti:sapphire pumping (76% in [21]). Moreover, by taking advantage of the lower crystal losses, we have also extended the cw tuning range of Yb:YLF lasers considerably and achieved smooth cw tuning in the 993–1110 nm wavelength range. For comparison, cw tuning ranges of 987–1134.5 nm (147.5 nm, ${\rm Yb}:{{\rm Lu}_2}{{\rm O}_3}$ [31]), 992.5–1111 nm (118.5 nm, Yb:YAG [52]), 992–1095 nm (103 nm, Yb:LuAG [53,54]), 999–1089 nm (90 nm, Yb:CALGO [55]), 1017–1086 nm (69 nm, Yb:BOYS [56]), 1018–1072 nm (54 nm, ${\rm Yb}:{{\rm CaF}_2}$ [57]), ${\sim}{1005} - {1078}$ (73 nm, ${\rm Yb}:{{\rm SrF}_2}$ [58]), and 1018–1088 nm (70 nm, Yb:SYS [59]) are reported from other Yb-based systems. Comparing with these results, we see that the tuning range we have acquired with Yb:YLF (993–1110 nm, 117 nm) is among the broadest tuning ranges yet reported from Yb-doped solid state lasers, which shows its potential as a solid-state ultrafast laser and amplifier.
Funding
European Research Council (609920).
Acknowledgment
We thank the anonymous reviewers for their constructive suggestions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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