Continuous-wave (cw) lasing of Yb:YLF was first reported in 2001, where a simple 400 mW diode-pumped system produced 50 mW of output power in the 1025–1050 nm range [1]. The benefits of cryogenic cooling were already discussed in this initial work [1], and lasing at cryogenic temperatures was shown only a year later [2]. Over the past 20 years, output power of cryogenic (${\rm{Yb}}:{\rm{LiY}}{{\rm{F}}_4}$) laser systems evolved from a 100 mW to 500 W level [2–7]. Interestingly, as is underlined by Kawanaka et al. in their pioneering work [1], the Yb:YLF gain media possess broad emission bands, even at cryogenic temperatures [8–10]. In particular, the a axis of Yb:YLF, which has a transition centered on 1016–1017 nm with a full-width at half-maximum (FWHM) of 11 nm at 125 K (typical operation temperature of Yb:YLF amplifiers under thermal load [11]), could ideally support generation and amplification of sub-250 fs pulses [3]. Because of its broad emission profile, almost all cryogenic amplification studies with Yb:YLF so far had employed the a axis, and amplification of sub-picosecond pulses with energies up to 300 mJ, and an average power up to 100 W was already demonstrated [12–16].

However, the c axis of Yb:YLF has stronger but narrower transitions [8–10]. Specifically, the peak at 995.2 nm has very high gain (small-signal, single-pass gain above 50) [8], but it has a rather narrow width of only 0.5–0.75 nm at 78 K, which extends to around 2 nm at 125 K. In a recent study, Manni et al. demonstrated first cryogenic amplification using this line, where 1 µs long 60 nJ pulses at 995 nm were amplified to a 1 mJ level (repetition rate, 40 kHz; average power, 40 W) [17]. The c axis of Yb:YLF has another strong line centered on 1019 nm which, to the best of our knowledge, has not yet been explored in any cryogenic amplification study. The 1019 nm line of Yb:YLF has a FWHM of 3.7 nm at 78 K, which extends to a FWHM of 6.5 nm at 125 K [8]. This line is not as broad as the 1017 nm transition of the E//a axis, but it has around a three times higher emission cross section (ECS) [8]. In this Letter, to the best of our knowledge, we report the first amplification results with the 1019 nm transition of the E//c axis in Yb:YLF. With the advantage of higher gain, we have demonstrated an extraction efficiency of 78% and an average power of 370 W from the amplifier which, to the best of our knowledge, are record results for a cryogenic Yb:YLF system.

Figure 1(a) shows a simplified schematic for the Yb:YLF regenerative amplifier. A 2 kW, 960 nm diode module was used as the pump source. Imaging lenses (f1–f3) were employed to achieve a 2.1 mm diameter pump spot at the center of the Yb:YLF crystal. A 2 cm long 1% Yb-doped YLF crystal with 3 mm long undoped endcaps diffusion-bonded on both ends was used as the gain medium. The crystal was indium bonded from the top side to a cold head, which was cooled to cryogenic temperatures by liquid nitrogen. The c axis of the crystal was positioned vertically to benefit from its higher thermal conductivity [18]. The crystal absorbed around 85% of the incident pump light at 960 nm. The bow-tie type regenerative ring cavity had a total length of 3 m and consisted of two dichroic mirrors (DM) with a radius of curvature of 20 m and two flat high reflectors. The reflectivity of the cavity mirrors covered the 990–1230 nm spectral range. Two thin-film polarizers (TFPs), a half-wave plate (HWP) and a water-cooled 8 mm aperture beta barium borate (BBO) Pockels cell (PC) with a rise (fall) time of 8 ns (3.5 ns) were used for seeding the amplifier. Two additional HWPs were implemented to employ the E//c axis for amplification. The cold cavity had calculated beam diameters of around 2.2 and 2.15 mm at the center of the Yb:YLF crystal and PC, respectively. The system is seeded by stretched (2 ns) pulses with 30 nJ energy from a fiber front-end [19]. The total passive loss of the cold cavity is measured to be around 5%: 2.5% from the Yb:YLF dewar (crystal and dewar windows), 1% depolarization loss, and the rest from all the remaining elements. A simpler standing-wave cavity, as shown in Fig. 1(b), was also used to benchmark the cw laser performance of the system. The beam parameters of the standing-wave cw cavity were almost identical to the regenerative amplifier.

 

Fig. 1. (a) Experimental setup of the diode-pumped cryogenic Yb:YLF regenerative amplifier. (b) Setup for a simple cw standing-wave Yb:YLF laser with identical cavity beam parameters. The inset picture shows the Yb:YLF crystals orientation.

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We present the cw lasing results first, since the limitations observed for the simpler cw cavity are quite informative in understanding the limitations observed in the regenerative amplifier performance. For that purpose, Fig. 2(a) shows the cw laser efficiency data taken using a 40% transmitting output coupler (OC). The laser had a cw lasing threshold of 45 W and a slope efficiency of 76%, and generated an output power of 506 W at an absorbed pump power of 780 W (850 W incident). The estimated temperature of the Yb:YLF crystal (${{\pm 10}}\;{\rm{K}}$) is also shown in Fig. 2(a) [20]. The free-running cw laser wavelength was 995 nm for an absorbed pump just below 300 W. Then the wavelength switched to the 1019 nm line for pump powers above 300 W [6]. The change in lasing wavelength with absorbed power is due to the induced variation in crystal temperature: at higher temperatures due to Boltzmann redistribution of level populations, the gain cross section at the 1019 nm line becomes larger than that of 995 nm transition (see Figs. 2 and 5 in [8]).

 

Fig. 2. (a) Measured cw laser performance of the cryogenic Yb:YLF laser using a 40% transmitting OC. Measured average temperature of the crystal is also shown. The free-running laser wavelength switched from 995 to 1019 nm at an absorbed pump power of around 300 W. (b) Near-field beam profiles of the cw Yb:YLF cryogenic laser at different output power levels.

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The achievable cw laser power was limited by thermal lensing effects as is also visible from the bending of the cw efficiency curve. For absorbed pump power levels above 500 W, the thermally induced lensing in the crystal gradually deteriorated the output beam profile, as shown in Fig. 2(b) [6]. Once the absorbed pump reached a 750 W level, the thermal lens become rather dynamic, and it was not possible to optimize the laser with further re-alignment. (A reduction in output power is also observed.) As can be seen from the behavior of laser output near-field for different output power levels, the output beam elongates along the horizontal direction, the a axis. (The beam size increases from around 2.1 to 3.4 mm.) On the other hand, the beam shape did not change significantly in the vertical direction (stayed around ${\sim}{{2}}\;{\rm{mm}}$), the c axis. The thermal coefficient of refractive index (dn/dT) is negative for both axes of Yb:YLF [18], which provides a significant advantage to YLF as a laser host for power scaling, compared to other well-known host materials like YAG [21]. Basically, the negative contribution of dn/dT is cancelled by other contributions to thermal lensing such as surface bulging, and one ends up with a small overall thermal lens. For the c axis, the dn/dT parameter of YLF has a larger value (${-}{2.5}\;{\rm{ppm/K}}$ in the c axis versus ${-}{{1}}\;{\rm{ppm/K}}$ in the a axis, for a temperature of 150 K: Fig. 23 in [18]). We believe that the larger negative value of dn/dT of the c axis is the main reason for the better thermal lensing performance observed in the vertical axis (c axis) in this Letter. As a side note, with Yb:YAG, due to its positive dn/dT parameter, a much stronger thermal lens exists at similar absorbed pump power levels. As a result, we could only achieve a cw power of 100 W from an identical laser cavity with Yb:YAG (using a 25 mm long 1% Yb-doped YAG sample, pumped with a similar diode module at 940 nm). A short cavity with better immunity to thermal lensing was required to achieve higher powers from a Yb:YAG rod (365 W cw power at 480 W of absorbed power). Hence, the cw output power, obtainable with cryogenic Yb:YLF from a single rod [6], is already significantly higher than what can be achieved with cryogenic Yb:YAG systems under similar conditions.

 

Fig. 3. (a) Measured output power of the Yb:YLF regenerative amplifier as a function of absorbed pump power at a 50 kHz repetition rate. The data are taken for different cavity round trips (n) of 40, 50, and 60, using seed energy of 30 nJ. The measured average temperature of the crystal is also shown. (b) Sample near-field beam profiles of the Yb:YLF regenerative amplifier at different output power levels showing the increased role of thermal lensing on the beam profile.

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Figure 3(a) shows the measured performance of the Yb:YLF regenerative amplifier for cavity round trips (n) of 40, 50, and 60, at a repetition rate of 50 kHz. First of all, as we also observed in cw laser experiments, for low absorbed pump power and hence low crystal temperatures, the gain at 995 nm is very strong [8], and the system started parasitic cavity-dumped lasing at 995 nm with a sub-10 ns pulse width, even when seeded with 30 nJ pulses at 1019 nm. With the heating of the crystal, the parasitic 995 nm lasing was fully suppressed for absorbed pump power levels above 170 W (for larger $n$ the suppression starts earlier). The regenerative amplifier produced up to 373 W of average power at an absorbed pump power of 550 W (incident power 708 W) for the case with 60 round trips. A linear fit to the data shows that the efficiency of the amplifier is as high as 78%, which is rather close to the quantum defect limited operation (94%: 960 nm/1019 nm). Note that, at this repetition rate, the system operates in saturated mode, as there is only a minor difference in amplifier performance for cavity round trips of ${{n}} = {{40}}$, 50, and 60. We also tested the variation of output power with seed energy, and saw a decrease of only 15% in output power when the seed energy was lowered from 40 to 0.2 nJ at a fixed round-trip number of 50. With an even stronger seed, the parasitic lasing at 995 nm is suppressed earlier, and the number of round trips of the regenerative amplifier could be reduced to around 20 to increase system stability. Similar to the cw case, the regenerative amplifier performance was limited due to the thermal lensing effect. Figure 3(b) shows the output beam profile of the amplifier at different average output power levels. The output beam had a rather symmetric ${{\rm{TEM}}_{00}}$ beam profile for power levels below 250 W. At these power levels, the beam quality (${{\rm{M}}^2}$) of the output was measured to be below 1.2 for both axes [6,16]. For output power levels above 300 W, the thermally induced lensing in Yb:YLF created a multimode output beam. Additionally, the thermal lens was rather dynamic and created system stability issues at these power levels (250–350 W). Interestingly, the thermal lensing limited the output power earlier in the regenerative amplifier compared to the cw cavity. We believe this is related to the larger number of round trips required for the regenerative amplifier (in the cw laser with 40% OC, the photon on average has four passes through the crystal). Despite that, in its current form, safe long-term operation of the system was feasible for an output power up to 250 W, and higher power might be sustainable via active intracavity beam pointing control.

Figure 4 shows the measured optical spectrum of the seeder, as well as the amplifier output at different average power levels. The seeder was centered on 1019.5 nm and had HWHM of 3.3 nm. With amplification, the spectrum first narrowed down to a width of 0.9 nm at 100 W power level, but then started to broaden again due to increased crystal temperature and a smoother gain profile. Figure 4 also contains the measured ECS of the c axis of Yb:YLF at selected temperatures, where we can easily observe the effect of temperature in smoothing out the gain profile [8]. At an output power level of 300 W, the spectra were centered on 1019.15 nm with a FWHM bandwidth of 1.25 nm, which could support sub-1.5 ps pulse duration. It is clear that, via usage of intracavity filters, sub-1 ps pulse durations might be achievable from the 1019 nm line of the E//c axis of Yb:YLF in future studies.

 

Fig. 4. Optical spectrum of seed pulse and regenerative amplifier output pulse at average output power levels of 10, 100, 200, and 300 W. The data are taken at a pulse repetition rate of 50 kHz, using 30 nJ seed energy and with 50 round trips (${{n}} = {{50}}$). The measured ECS for the E//c axis of Yb:YLF is also shown for crystal temperatures of 78, 100, 125, and 150 K.

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In Fig. 5, we show the measured amplifier performance at 25, 50, and 100 kHz repetition rates for a fixed cavity round trip number of 50. As expected, we could achieve similar amplification performance for each repetition rate (similar average powers, reduced pulse energy at higher repetition rates) [22]. For the 25 kHz repetition rate, we stopped at 250 W (10 mJ energy) on purpose to stay away from laser induced damage to the cavity optics in this first trial as replacement elements were not available on site. For the ${\sim}{1.6}\;{\rm{mm}}$ beam, the 10 mJ energy corresponds to a peak fluence of ${{1}}\;{\rm{J/c}}{{\rm{m}}^2}$. Due to gain narrowing, the pulse width of the output pulses was reduced from around 2 to 1.25–1.5 ns, and we know from earlier experience that the fluence values could be safely scaled up to ${{2}}\;{\rm{J/}}\;{\rm{cm}}^2$ for such pulses: hence, ideally, pulse energies of 20 mJ are safely achievable from this system.

 

Fig. 5. Measured output power performance of the Yb:YLF regenerative amplifier at 25, 50, and 100 kHz repetition rates. The data are taken at 50 cavity round trips, using 30 nJ seed energy. The inset shows the variation of measured average output power of the regen at 220 W for 30 min (50 kHz case).

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To finalize, the inset graph in Fig. 5 shows the measured average output power of the amplifier for a 30 min interval at an average power level of around 220 W. The data are taken without enclosing the cavity. The power fluctuations were measured to be below 3% rms in the first 8 min. The changing level of liquid nitrogen in the Yb:YLF dewar created the fluctuations over longer time scales, which can be resolved by installing a liquid nitrogen auto-refill system or a closed-cycle cooler. (The small dip in the curve is a realignment effort.)

In summary, we have presented, to the best of our knowledge, the first Yb:YLF amplifier employing the 1019 nm transition of the c axis. Compared to the a axis, the higher gain enabled a system with record average power and extraction efficiency. The thermally induced lensing in Yb:YLF is shown to be the main limitation for further power scaling. As the estimated B-integral of the system is still low (below 0.5), in future studies, the usage of longer Yb:YLF crystals with lower doping (0-5-0.75% Yb-doped 3–4 cm long) could reduce the thermal lensing issue induced in this Letter. Further average power scaling might be feasible via usage of periodic resonators with multiple gain elements [22,23] or via usage of zig-zag slab or thin disk geometries.