We have developed a 1 kHz regenerative amplifier using an Yb:Y2O3 ceramic thin disk as the gain medium. Furthermore, the thermal conductivity and heat generation property of Yb:Y2O3 ceramic were investigated. In the developed regenerative amplifier, a laser beam is bounced off the thin disk six times in each round trip. The output energy is over 2 mJ, spectral bandwidth is 1.8 nm at FWHM, and pulse duration after pulse compression is 0.9 ps.
© 2016 Optical Society of America
For high-repetition high-power lasers, Yb doped laser materials are suitable because of their properties such as broad absorption and emission bandwidths, long fluorescence lifetime, and low quantum defect that leads to low heat generation [1–6]. For the pumping source, a fiber-coupled high-power laser diode (LD) is appropriate. Moreover, laser ceramics have been available for several Yb3+-doped materials [7,8]. Hence, a device with large size, high doping level, and composite of ceramics with different dopant can be easily manufactured . For example, large size Y2O3 ceramic with sufficient optical quality is easily obtained compared with single crystal, because the sintering temperature 1700 °C is much lower than melting point 2430 °C . In terms of host materials, rare earth isotropic sesquioxides offer the advantages of broader band spectra and higher thermal conductivities than the widely studied material YAG [3–5]. Table 1 lists the spectral and thermal properties of Yb-doped laser materials [5,6,10]. In comparison with Yb-doped monoclinic double tungstates, which are known as wide band materials, rare earth sesquioxides are better in terms of thermal conductivity, which is important from a viewpoint of building a high-power laser system. On the other hand, their absorption and emission cross sections are lower than those of Yb:YAG and tungstates. This will be overcome by designing a pumping system and a laser cavity with multi-pass configuration.
From the view point of constructing a high-average-power laser system, water-cooled thin disks of Yb-doped gain medium offer promising candidate . The heat generated in a laser medium is one of the serious problems hindering the development of high-average-power laser, because it causes the thermal lens effect, spectral shift, and sometimes it destroys the gain medium. Using a rod gain medium, heat flows to radial direction and it leads the refractive index distribution in radial direction. To control temperature, it is effective to use a thin disk type laser medium bonded to a heat sink; the laser beam is injected onto the thin disk and reflected at the back surface, which is coated as a mirror. For heat removal, the mirror surface of the thin disk is bonded to a water-cooled heat sink. With this dimension, heat flows to thickness direction, so the thermal lens effect is originated by spatial profile of a pump laser. Thus, pumping with flat-top profile reduces the thermal lens effect. On the other hand, to build a regenerative amplifier with high loss optical components such as Pockels cell and polarizer, it is important to obtain higher gain. Because of efficient heat removal of a thin disk, higher excitation density will be possible than a rod medium. A large beam diameter on gain medium is also easily obtained using thin disk.
A regenerative amplifier using diode pumped thin disk was firstly reported in 1997 for Yb:YAG  and have been used for several ten millijoule class kilohertz laser systems [13–17]. Recently, 220 mJ at 1 kHz with the pulse duration of 1.9 ps was obtained . On the other hand, using Yb:Y2O3 thin disk, CW oscillation with over 70 W [19,20] and mode locked oscillation with pulse duration of 0.5 ps and average power of 7.4 W  were reported. Here we report our observations of the thermal properties of Yb:Y2O3 ceramics and the construction of a regenerative amplifier using a Yb:Y2O3 thin disk. Because the emission cross-section of Yb:Y2O3 is two times smaller than Yb:YAG, we constructed the regenerative amplifier with 6-bounce at the thin disk. The laser system is a 1 kHz chirped pulse amplification system, which owns the front-end system with Yb:YAG laser system jointly . Because the emission wavelength of Yb:Y2O3 is close to that of Yb:YAG, both laser system can share the front end system and additional amplifiers. Hence, the output of Yb:Y2O3 regenerative amplifier can be amplified using Yb:YAG if higher power is required.
2. Thermal property of Yb:Y2O3 ceramics
To estimate the temperature rise of a thin disk, the thermal conductivities of Yb-doped Y2O3 ceramics were measured using the laser flash method . It is known that thermal conductivity is decrease with dopant concentration , so we measured ceramics with variousdopant concentrations supplied by Konoshima Chemical Co., Ltd. (Konoshima). The results are listed in Table 2 along with the other parameters measured in the same sample. The data obtained at 0 at% is the nominal data of non-doped ceramics made by Konoshima. The measured thermal conductivities are plotted in Fig. 1 along with crystal data [3,22]. There is no meaningful difference between the ceramics and the crystals. The fitting curves in Fig. 1 were calculated with Gaumé et al.'s expression for the thermal conductivity of partially substituted compounds . From the fitting curve, we can estimate the thermal conductivity of Yb:Y2O3 ceramics of various dopant concentrations. It seems decrease of the thermal conductivity of Yb:Y2O3 with concentration is faster than that of Yb:YAG. For 5 at% Yb:Y2O3, the thermal conductivity is estimated to be 6.9 W/m·K and it is close to our measured value for 10 at% Yb:YAG of 6.4 W/m·K.
We measured the surface temperature of a thin disk pumped with CW LD using an infrared thermography (NEC/Avio TH6300). The pumping and cooling system of the thin disk is explained in next section. The temperature at the center of the pumped area of the 5 at%-doped Yb:Y2O3 ceramic thin disk is shown in Fig. 2. The calculation was performed using the one-dimensional heat conduction equation considering the absorption, quantum defect, and thermal conductivity of Yb:Y2O3 and the heat sink. As can be seen in the figure, the temperature rose nonlinearly and disagreed with the calculation. We measured temperature of other 5 at% Yb:Y2O3 thin disks with different heat sink materials and bonding methods, and the temperature rises were same tendency. An influence of spectral shift of the pump LD was negligible, because the absorption decreased from 96% to 94% with increasing the pump flux from 0.1 to 3.5 kW/cm2 using our pumping system. So, we consider there is additional non-linear heat generation in Yb:Y2O3 ceramics, which is different from the heat generated due to quantum defect. We could not elucidate the details of the process of additional heat generation, but it can be ascribed to energy transfer to impurities , color center and lattice distortion , or concentration quenching to upper state that decays nonradiatively [26,27]. Moreover, the thin disk cracked under a pump irradiation flux of approximately 3 kW/cm2; therefore, we limited the pump flux to below 2.5 kW/cm2 in this study.
3. Yb: Y2O3 regenerative amplifier
We built a regenerative amplifier using a 5 at%-doped Yb:Y2O3 ceramic thin disk produced by Konoshima. The diameter of the disk was 10 mm and it was polished to a thickness of 0.2 mm with a small wedge angle of 0.1°. The disk was bonded onto a water-cooled copper heat sink with an adhesive. The laser incident surface of the disk has an anti-reflection coating and the opposite surface bonded to the heat sink has a high reflection coating for the pump and seed laser. Both coats is high resistant coating using Al2O3/SiO2 multilayers . The radius of curvature of the thin disk was 5 m.
The pumping laser of thin disk was a fiber-coupled CW LD having an output power of 180 W and wavelength of 976 nm, however the wavelength was around 972 nm in used power range in this work. Because the Yb:Y2O3 disk was too thin to absorb an adequate amount of power by 1-bounce pumping, we constructed a 12-bounce pumping configuration, as shown in Fig. 3. The pump laser beam from fiber-coupled LD with a fiber core diameter of 400 µm and numerical aperture of 0.22 was shaped to a diameter of 2.0 mm on the thin disk through two lenses and a parabolic mirror with a focal length of 60 mm. The incident angle of the pump laser onto the thin disk was 30°. The pump light reflected at the thin disk was transferred another five times using the parabolic mirror and 5-sets of prism pairs for each pass. After six bounces, the pump light was reflected by a flat turning mirror and was returned along the same path. The optical elements were arranged such that the sum of the 12-bounce pump light assumed a nearly flat-top pattern at the thin disk when the output pattern of the pump LD was ideal flat-top. The diameter of pumped area was 2.0 mm at FWHM. Using this pumping system, absorbance was estimated to be 96% for a 0.2-mm-thick 5 at%-doped Yb:Y2O3 disk when the pump flux was 2.5 kW/cm2.
The front-end laser system of the seed laser for this regenerative amplifier is commonwith our Yb:YAG laser system, which is detailed in another paper . It was constructed using a 1030 nm mode-locked oscillator, pulse stretcher, Yb-doped fiber pre-amplifier and 1 kHz pulse picker. The pulse chirp at the stretcher is 0.4 ns/nm. The output power of the fiber pre-amplifier is typically 2 W at 80 MHz and the spectral band width is 6 nm at FWHM. After the 1 kHz pulse picker, the seed laser is collimated with a lens pair.
Fig. 4 shows the schematic layout of the developed regenerative amplifier. An isolator formed by a thin film polarizer, half-wave plate and Faraday rotator is placed before thecavity for amplified laser output. A laser beam with s-polarization is injected into the cavity by the second thin film polarizer. It then passes a quarter-wave plate twice, is reflected by an end mirror, and subsequently, passes through a thin film polarizer with p-polarization. Next, a BBO Pockels cell inside the cavity is switched on so that the seed laser continues to make round trips in the cavity with p-polarization until the Pockels cell is switched off. To increase the gain of one round trip, we designed a six-bounce cavity at the thin disk for each round trip using ray transfer matrix (ABCD matrix) . Figure 4(b) shows the distance of the optical elements, and the calculated beam diameter on each optical elements. For the pump flux from 0 to 2.5 kW/cm2, the focal length of the thin disk was changed from 2.5 to 2.4 m. This value included distortion of the heat-sink and thermal lens effect of the thin disk. In calculation we treated the thin disk a concave mirror with a focal length of 2.5 m. The optical system is designed such that the beam diameter is less than 2 mm on the thin disk for each bounce and under half the diameter of the BBO aperture at the Pockels cell. The stability, indicated with ((A + D)/2)2, is better than 0.2 for thin disk focal length of 2.2~2.8 m.
Figure 5(a) shows the output power of the regenerative amplifier with pump power. The number of round trips were optimized for each pump power. We achieved an output power of up to 2 W with a pump power of 62 W. The repetition rate was 1 kHz, which corresponds to a pulse energy of 2 mJ. Figure 5(b) shows the pulse amplification based on the number of round trips made. The gain in one round trip is 1.16. To estimate the gain for a single bounce, the loss in one round trip has to be eliminated from this value. The power drop of the seed laser in the non-pumped condition is shown in the inset of Fig. 5(b). The measured absorption for a single bounce at the thin disk is 3%, so the transmission in one round trip six bounce without considering absorption is calculated to be 86% from the power drop. By eliminating the loss from the gain value for a round trip, the gain for single bounce is estimated to be 1.05. Given this low gain value, the multi-pass configuration used in the cavity is effective for the developed Yb:Y2O3 regenerative amplifier. Figure 5(c) shows the long-term output power measured with a thermopile power sensor (Gentec-EO XLP12-3S-H2). There is no serious power drift in the laser output for around 8 h. The inset shows the energy stability of each pulses, which was measured using a PIN photodiode detector (New Focus 2031) without averaging. From this data, the pulse energy stability of our laser was estimated to be 1.0% RMS. The pump flux was limited to 2.5 kW/cm2 by the damage threshold of thin disk. This value is lower than operation condition shown in Ref. 20, which used 2 at% Yb:Y2O3, so we consider the additional heat generation is few in low dopant Yb:Y2O3. Also, the higher thermal conductivity in lower dopant concentration is an advantage to increase allowed pump power. To obtain higher output, the cavity design with larger spot size on the thin disk is also effective.
As shown in Fig. 4(b), beam size in the cavity satisfies the designed value both under CW oscillation and the regenerative amplification condition. The output beam quality wasevaluated by observing its spatial profile after passing it through a f = 300 mm focal lens. The beam size observed using CCD is plotted in Fig. 6 with the beam pattern at the focus spot. The beam propagation factors were Mx2 = 1.11 and My2 = 1.07 for the horizontal and vertical axes of laser beam, respectively.
The output beam was collimated with a 1:1 lens pair and pulse-compressed using a gold-coated diffraction grating pair with 1740 grooves/mm. The transmission of the compressor was 79%. Figure 7 shows the output spectrum and the autocorrelation trace after compression. The spectrum showed a good fit with the Gaussian profile with a center wavelength of 1031 nm and a spectral width of 1.8 nm at FWHM. It was narrowed from the 6 nm value of the seed laser caused by the gain-narrowing effect. Furthermore, the autocorrelation trace showed a good fit with the Gaussian profile. The pulse width was estimated to be 0.9 ps at FWHM, which is similar to the Fourier transform limit value of 0.87 ps calculated from the spectrum. These values are better than the 1.2 nm and 1.3 ps achieved with our Yb:YAG system . They will be improved further via reduction of the gain-narrowing effect by decreasing the number of round trips in the regenerative amplifier with increasing a seed energy or gain of one round trip. The energy of seed laser can be increased using a fiber amplifier with kilohertz repetition after the pulse picker. Injecting the seed laser with the pulse energy of 1 µJ, the output spectral width calculated to be 2.1 nm, which is correspond to 0.7 ps. To increase the gain per one round trip, increasing the bounce number at the thin disk is effective, because the gain of Yb:Y2O3 thin disk is small and the energy loss in the regenerative amplifier is considered mainly due to Pockels cell and polarizer. Increasing the bounce number will be also efficient to increase the output power.
In this study, we observed thermal properties of Yb:Y2O3 ceramics and built a 1 kHz regenerative amplifier. The thermal conductivities of Yb:Y2O3 ceramics are approximately the same as those of the corresponding crystals. For the regenerative amplifier, a thin disk of 5 at%-doped Yb:Y2O3 ceramic is used as the gain medium. The 6-bounce regenerative amplifier outputs 2 mJ at 1031 nm with a band width of 1.8 nm. The compressed pulse duration is 0.9 ps. It will be improved by reducing the gain-narrowing effect by pre-amplifying the seed laser and increasing the bounce number at the thin disk.
The authors thank Japan Fine Ceramics Center for measuring the thermal conductivities. The multilayer coat for thin disk was fabricated by Shimadzu Co. A disk bonding to a heat sink is performed by Institut für Strahlwerkzeuge. This research was supported by the Photon Frontier Network Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT).
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