1. Introduction

The ultrahigh-peak-power lasers with a high repetition rate are in high demands as one of the most useful tools for the recent advanced application fields such as light-matter interaction in a relativistic regime.[1,2] High field lasers with high energy (~kJ) are demonstrated for such application fields. Compact high-field laser systems with low energy and short laser pulse duration are another approach, which have been developed successfully by using a chirped pulse amplification technique with a Ti:sapphire laser crystal.[3,4] Considerable efforts have been spent in the development of high-power neodymium pump laser sources, which are used in the high-field Ti:sapphire laser systems. Such high output power neodymium system is still large in size with complicated optical scheme. High repetition rate operation has not been achieved due to flash lamps. By using laser diodes as a pump source in such high-field lasers, downsized laser systems with high repetition rate could be achieved to accelerate high-field science research, which opens up various novel application fields. The power conversion efficiency of a pump source would also be improved much more than flash-lump pumped second-harmonic laser sources.

Ytterbium-doped materials are one of the most promised laser materials in the next generation of the diode-pumped high-power lasers. This is due to its high storage energy, low quantum defect and a good spectral overlap between its absorption and emission of commercial high-power laser diodes.[5–9] In addition, some Yb-doped materials, such like Yb:glass, Yb:LiYF₄, Yb:KGd(WO₄)₂, Yb:KY(WO₄)₂, and Yb:YCa₄O(BO₃)₃ have a broad emission spectral range, which is necessary to generate and amplify ultrashort laser pulses below 50 fs.[10–14] Especially, a Yb:LiYF₄ (Yb:YLF) crystal has been focused on for the development of ultrahigh-peak-power lasers, which is due to its low nonlinear refractive index, high thermal conductivity and smooth emission spectrum. Smooth spectral shape is necessary for easier spectral shaping with etalons, for example [15].

A diode-pumped Yb:YLF oscillator has been demonstrated at room temperature in continuous-wave operation mode at very low cavity loss of < 2%.[16] Its slope efficiency and effective spectral tuning range were 50% and 25 nm, respectively. This tuning range, however, is much narrower than the emission spectral range (~100 nm) even when the pump intensity was considerably high (100 kW/cm²). This is because reabsorption of the emission by lower levels in the laser transition became significant, leading to a considerable loss of gain. Increase of the pump intensity is one of the practical ways to improve the laser gain and the gain range by bleaching the absorption. Further developments of high brightness laser diodes could improve the gain in future. Cooling of the Yb:YLF crystal at low temperature is another effective way of active population control to reduce the reabsorption. Our spectroscopic research at low temperature indicated that characteristics for the high-field laser materials would be dramatically improved even in current diode-pump; the laser gain would be increased and the effective spectral gain width would be broadened.

In the present work, a diode-pumped laser oscillator with an Yb:YLF crystal at liquid nitrogen temperature has been demonstrated for the first time to our knowledge. An estimated small signal gain was dramatically improved to 21 cm^-1 and the tuning range was broadened to 35 nm due to absorption spectral narrowing. An all solid-state high-field laser will thus be achieved using a cooled Yb:YLF crystal.

2. Emission and absorption spectra at liquid nitrogen temperature

In the previous study, it was shown that spectral features of emission and absorption were almost unchanged at temperature below 80K.[16] Liquid nitrogen temperature (77K), therefore, has been chosen in laser construction and operation because of its easy handling and low cost. Emission and absorption spectra have been measured to estimate the effect at liquid nitrogen temperature, especially in gain magnitude and spectral gain range. Figs. 1 (a) and (b) show emission and absorption spectra of a 60 at. % Yb:YLF crystal at room temperature (293 K) for π- and σ-polarization, respectively. For π- and σ-polarization, observed optical polarization is parallel to the c- and a-axis of the crystal, respectively. Emission and absorption spectra overlap each other in the almost whole spectral range for both polarizations. A free-running laser oscillation was observed for π-polarization by focusing two single-emitter diode lasers tightly on the crystal with an intensity of more than 50 kW/cm². The oscillation wavelength was 1040 nm, which was far from the emission peak. A tuning range was measured to be 25 nm by use of a birefringence filter, which was much narrower than the emission spectral range.

 

Fig. 1. Emission and absorption spectra of an Yb:YLF crystal for (a) 293 K, π-polarization, (b) 293 K, σ -polarization, (c) 77 K, π -polarization and (d) 77 K, σ -polarization.

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The crystal was cooled at liquid nitrogen temperature of 77 K to reduce the absorption of lower laser levels. At 77 K, the spectral overlap was almost vanished for both polarizations, as shown in Figs. 1 (c) and (d). This leads to dramatic increase of the laser gain at any emission spectral range. The peak emission cross sections for π-polarization were 1.8 × 10^-20 cm² and 1.7 × 10^-20cm²at 995 nm and 1017 nm, respectively. Both of them are one order of magnitude higher compared to the room temperature value of 1.7 × 10^-21 cm² at 1040 nm, resulting in the laser gain more than one order of magnitude under the similar pump power. Since the peak emission cross section was 1.6 × 10^-20 cm² for σ-polarization, a comparable laser gain is expected. In addition, the laser gain spectral range would be broadened to the whole emission spectral range of ~35 nm for both polarizations, which enables the laser amplification of 40-fs laser pulses under the assumption of the sech² field envelop.

3. Cooled Yb:YLF oscillator

Fig. 2 shows a schematic diagram of our diode-pumped oscillator with a cooled Yb:YLF. The cavity was x-type configuration with a flat high reflector, a flat output coupler and two dichroic mirrors with 100-mm radius of curvature. The distance between the output coupler and the facing dichroic mirror was 700 mm, which was the same as that between the high reflector and the other dichroic mirror. The distance between two dichroic mirrors was ~100 mm. The angle of off-axis reflection of a lasing beam on dichroic mirrors was set at 9 degrees to minimize its astigmatism. An YLF crystal with 60 at. % Yb³⁺ dope with 0.3 mm thickness was used. The cross section of the Yb:YLF crystal was 5 mm × 5 mm. The crystal was sandwiched between two thermally conductive 1.5-mm-thick copper plates with a 2-mm-diameter hole, which attached to a 40-mm-diameter copper sample stage of a He cryostat with an electrical heating unit (CRT-M310-OP, Iwatani Int. Co.). Thin indium foils with a 100-□m thickness were used between copper and crystal contact surfaces to obtain a high thermal conductivity. The copper plate temperature T_c was measured as an index of the crystal temperature with an Au-Fe thermocouple, and was kept at liquid nitrogen temperature T_c = 77 K. The temperature fluctuation was stabilized within 0.5 K during the laser operation. The inside of the cryostat was evacuated below 3 × 10^-4 torr. The crystal and two cryostat windows were set at the Brewster angle for both the lasing and pump beams. A high brightness single-emitter cw laser diode (OptPower Corporation, H01-A001-940-CT) was used as a pump source at 936 nm. The diode laser beam was tightly focused upon the laser crystal by two lenses and a telescope with a cylindrical lens pair. The observed focusing spot size with a knife edge was 40 × 100 μm. Pump intensity could thus be changed to as much as 40 kW/cm², which was more than the saturation intensity of 28 kW/cm² of the crystal at liquid nitrogen temperature. The pump laser beam was linearly polarized along p-polarization for the crystal and windows, which was parallel to either c- (π-polarization) or a-axis (σ-polarization) by rotating the crystal.

 

Fig. 2. Schematic diagram of a diode-pumped Yb:YLF laser oscillator at liquid nitrogen temperature.

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4. Output characteristics

Free-running laser oscillation could not be observed at room temperature due to a low pump intensity and an increased cavity loss of two additional cryostat windows. The laser oscillation was, however, observed at T_c = 77 K for both π- and σ-polarizations. The oscillation wavelength was 1017 nm for π -polarization where the emission cross section peak was shown in Fig. 1(c). The other emission cross section peak at 995 nm was not the oscillation wavelength in free-running operation mode due to the absorption. The laser-beam profile was Gaussian with a diameter of 2 mm at the output coupler. Output power was measured as a function of an absorbed pump power by use of two output couplers with transmittances of 1.6% and 8.4%.

Fig. 3 shows the result for π-polarization. The threshold absorbed pump power became 23 mW at 77 K much lower than 270 mW at room temperature in our previous work [16]. This is because the cooled Yb:YLF crystal was operated as a four-level laser material. The slope efficiency was 7% and 25% with two different output couplers, which are lower than our previous result of 50% at room temperature [16]. This is because the estimated cavity loss of L=0.06 became large due to the additional quartz windows. Both the cavity loss and a small signal gain were estimated from the laser threshold condition. [17] The estimated small signal gain for π-polarization was g_Tc=77K = 21 cm^-1 at the absorbed pump power of 500 mW, which was 15 times as high as that at room temperature (g_Tc=293K =1.4 cm^-1). The experimental result was agreed with the gain estimation from the increased emission cross section at 1017 nm for the π-polarization A saturation fluence was then decreased to 11 J/cm² from 114 J/cm² at room temperature, which is lower than most of damage thresholds of optics in most chirped-pulse amplifiers with a pulse duration of ~2 ns at longest. An efficient energy extraction can, therefore, be achieved with obtainable optics.

 

Fig. 3. Laser output power for π -polarization as a function of an absorbed pump power.

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Fig. 4. Tuning range of a Yb:YLF laser oscillator at 77 K for π - and σ -polarization.

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For σ-polarization, the oscillation wavelength was 1017 nm. The small signal gain was 15 cm^-1 at the absorbed pump power of 500 mW. That was reasonable due to the comparable emission cross section for σ-polarization with that for π-polarization.

A laser gain spectral range was measured by tuning an oscillation wavelength with a birefringence filter inside a cavity. The result is shown in Fig. 4. The red and blue circles represent the laser output power for π- and σ-polarization at T_C = 77 K, respectively, when the pump intensity was 30 kW/cm², which corresponded to the absorbed pump powers of 480 mW and 720 mW in π- and σ-polarization, respectively. The tuning range was limited to 25 nm for π-polarization at room temperature even at 100 kW/cm² pump intensity without additional windows.[16] At T_c = 77 K, the tuning ranges for both polarizations were obtained to be 35 nm. The broadened spectral gain range enables an amplification of ultrashort laser pulses below 40 fs duration. In such amplification, active spectral controls should be necessary to prevent a spectral gain narrowing. When using the spectral shaping with etalons [15], which is one of the effective controls, σ-polarization will be preferred to π-polarization due to its smoother emission spectral shape.

The emission wavelength of laser diodes should be matched to the absorption spectra of a laser material. A peak absorption cross section was increased at 77 K for both polarizations. Pumping at 960 nm for π-polarization might be preferable because the absorption cross section was the highest. The emission wavelength of laser diodes, however, must be controlled within ±1 nm, that implies the diode temperature stabilization within ±3 °C. This means the use of an expensive electrically controlled chiller. Pumping at 940 nm for σ-polarization permits the wavelength drift of ±10 nm. The corresponding temperature drift is ±30 °C and this requires less stringent temperature control of laser diodes.

The measured absorption of the pump diode laser was increased at low temperature for both polarizations; the absorption coefficient at 936 nm pump wavelength was changed to 14 cm^-1 and 26 cm^-1 at 77 K from 11 cm^-1 and 17 cm^-1 at room temperature for π- and σ-polarizations, respectively. The increased absorption leads to a thinner laser crystal, which reduces a B-integral to improve the spatial beam quality, which is especially required in a regenerative amplifier. The thinner materials also lead to higher thermal conductivity by using a proper scheme such like a thin disk laser [8]. The thermal effect for a beam profile, such as thermal lensing and thermal birefringence, must be considered in high-field lasers with an average power of hundreds of watts. The measured thermal conductivity of Yb:YLF was improved to 23 W/mK at 77 K from 4 W/mK at room temperature. The thermal effects would thus be suppressed at low temperature.

5. Conclusions

An Yb:YLF crystal has been investigated as a diode-pumped high-field laser material of next generation. A diode-pumped Yb:YLF laser oscillator has been developed at liquid nitrogen temperature. A small signal gain was improved dramatically and the gain spectral range was broadened. Using the cooled Yb:YLF crystal, a diode-pumped high-field laser with a high repetition rate could be successfully developed with commercially obtainable laser diodes and optics, which makes a progress of advanced application fields.