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A diode-pumped, cryogenically-cooled Yb:KYW regenerative amplifier utilizing chirped-pulse amplification and regenerative pulse shaping has been developed. An amplified pulse with an energy of 5.5 mJ and a broad bandwidth of 3.4 nm is achieved.
©2007 Optical Society of America
1. Introduction
Ytterbium (Yb3+) doped gain media are one of promising laser materials for the next generation of diode-pumped high-power lasers [1, 2]. Such Yb-doped media have numerous advantages. First, it has wide absorption bandwidth which is suitable for direct diode pumping. Second, the low quantum defect enables efficient and high repetition rate operation. Third, the simple electronic structure avoids processes such as excited-state absorption, upconversion and concentration quenching. In addition, Yb:KY(WO4)2 (Yb:KYW) and Yb:KGd(WO4)2 (Yb:KGW) [3, 4] have favorable characteristics. For example, their emission spectra are broad bandwidth compared with that of Yb:YAG, therefore the femtosecond pulse generation can be realized. Although Yb:glass has broader emission bandwidth compared with those of Yb:KYW or Yb:KGW, it has small absorption and emission cross sections and poor thermal conductivity.
So far, diode pumped Yb:KYW regenerative amplifiers with pulse energies ranging from 10 μJ to 200 μJ have been demonstrated [5, 6, 7]. However, multi-millijoule-class Yb:KYW or Yb:KGW lasers have not yet been reported so far, excepting a high-energy Ti:sapphire laser pumped Yb:KGW regenerative amplifier [8]. In the system, a flash-lamp pumped, joule-class Ti:sapphire laser with pulse duration of 150 μs and a 1Hz was used as a pump source. A high-energy (> 400 mJ) and high-intensity (> 100 kW/cm2) pumping was required to extract the multi-milijoule energy at room temperature which is rather difficult to access by direct diode pumping.
Cryogenic cooling of Yb-doped solid-state lasers can offer a number of benefits, including the enhancement of absorption and emission spectral properties, and thermal and thermo-optic properties under the four-level laser operation [9, 10, 11]. For example, cryogenically-cooled Yb:YAG is suitable for the high energy amplification with the emission cross section of over 5 times higher than that of Yb:YAG at room temperature [12]. The high thermal conductivity between 16- and 34- W/m∙K is also beneficial for high average power operation [11]. However, extremely narrow gain bandwidth of Yb:YAG is inadequate for ultrashort pulse generation. Cryogenically-cooled Yb:YLF has a broad band emission spectrum and long emission life-time of 1.8 ms [13]. This long life-time is greatly valuable for high energy amplification, but not good for multi-kHz operation. On the other hand, the reabsorption of cryogenically-cooled Yb:KYW is considerably reduced, and the emission cross section is increase to 6 × 10-20 cm2. In addition, the thermal conductivity of cryogenically-cooled Yb:KYW is also improved to 7.4 W/m∙K [11]. Therefore, the high pump intensity is not needed for laser operation. Finally, its short emission life-time of 0.3 ms is beneficial for multi-kHz operation.
By taking these advantages of cryogenically-cooled Yb:KYW, we developed a compact diode-pumped, chirped-pulse Yb:KYW regenerative amplifier. As a result, an amplified pulse with an energy of 5.5 mJ and a high optical conversion efficiency of ~14% is achieved. Due to the relatively narrow emission spectrum (~6 nm) of cooled Yb:KYW compared with that (~16 nm) of Yb:KYW at room temperature [4], regenerative pulse shaping is used to counter gain narrowing during amplification.
2. Experiments and result
A layout of the chirped-pulse Yb :KYW regenerative amplifier is shown in Fig. 1. An X-type regenerative amplifier cavity consisted of 1.0 m and 1.5 m curvature concave high reflectors and two dichroic convex mirrors with a 10 m curvature. The 20at.% Yb:KYW crystal with 2 mm thickness and 5 × 5 mm2 cross section was aligned along the a axis. The crystal which was attached to the liquid-nitrogen cryostat was placed between the dichroic mirrors. The crystal and two fused-silica windows of the cryostat were set at a Brewster angle for both the pump and the amplified beams. Output with 80-fs duration and 17 nm bandwidth at the center wavelength of 1024 nm from a mode-locked Ti:sapphire oscillator was used as a seed pulse. The seed pulse was positively chirped to 1.2 ns duration by a 1.2-km polarization-maintained single-mode fiber before amplification. The energy of the seed pulse injected into the regenerative amplifier was ~50 pJ. As a pump source, a 600 μm core fiber-coupled laser diode (LD) with the 940 nm emission wavelength was used. A duration of the LD pulse was set to be 0.5 ms which is close to the upper state life time of cooled Yb:KYW. A relatively short pump pulse is also desired towards multi-kHz repetition rate operation. At present the repetition rate of the laser system was 10-Hz which was limited by the repetition rate of the Pockels cell driver. The seed pulse is injected after reflecting off the polarizers and passing through the Pockels cell which is pulsed with a quarter wave voltage coincident with the optical pulse arrival. After 25 round trips, the pulse is rejected from the cavity, having fully depleted the gain, by once again pulsing the Pockels cell. The amplified pulse energy and conversion efficiency of the regenerative amplifier are shown in Fig. 2. A maximum pulse energy and optical-to-optical conversion efficiency of 5.5 mJ and 14.1 %, respectively, were achieved at an absorbed pump energy and intensity of 39 mJ and 20 kW/cm2, respectively. Since no saturation occurred during amplification at this pump intensity level, the output energy will be increased by using a higher power LD. It should be noted that no lasing occurred in a cavity-dumped mode at room temperature and at same LD pump intensity. It is clearly highlighted the ability of cryogenic cooling of Yb:KYW.
Fig. 2. Output pulse energy and conversion efficiency of the regenerative amplifier as a function of absorbed pump energy.
Previously, a high-energy liquid-nitrogen-cooled Yb:YLF regenerative amplifier was reported [14]. The Yb:YLF regenerative amplifier generated an output pulse energy of 30 mJ at a pump power of 93 W with a pulse duration of 4 ms. Since fluorescence lifetimes of Yb:YLF and Yb:KYW are 2 ms and 0.3 ms, respectively, Yb:KYW is more favorable for higher repetition rate operation. We have measured the output pulse energy and conversion efficiency of the Yb:YLF regenerative amplifier (20at.% doping, 2 mm crystal thickness) on equal cavity configuration and pump pulse duration of 0.5 ms. No amplification was observed with the same pump energy of 39 mJ. At last, an output pulse energy of 1.3 mJ was achieved with an absorbed pump energy of 52 mJ which was corresponding to a conversion efficiency of only 2.5%. Because cooled Yb:KYW has relatively high thermal conductivity which corresponds to about half of that of cooled Yb:YAG [11, 15], multi-kHz operation is quite feasible. Therefore we are confident that the scaling of the Yb:KYW regenerative amplifier to the multi-kHz repetition rate while maintaining the multi-milijoule output is more beneficial.
Owing to gain narrowing during amplification with high gain (~106) and emission property of cooled Yb:KYW, the spectral bandwidth of the output pulse was greatly reduced to less than 1.9 nm, as shown in Fig. 3 (dashed line). To overcome this, we used a thin etalon in the regenerative amplifier cavity [16]. With a 150 μm thick etalon the spectrum has broadened to 3.4 nm as shown in Fig. 3 (solid line). The inset of Fig. 3 shows the compressed pulse with 670 fs duration calculated from the measured amplified spectrum with the etalon. By using the etalon with proper thickness a spectrum could be broadened further. A pulse compression is currently limited to ~ 1ps due to the third-order dispersion mismatch between the fiber stretcher and grating compressor. Though, the transform-limited pulse duration after compression could be obtained by simply replacing the fiber stretcher to the anti-parallel grating based stretcher.
Fig. 3. Amplified spectra without (dashed line) and with etalons (solid line). Inset, a compressed pulse calculated from the measure amplified spectrum with the etalon.
3. Summary
We have demonstrated a high-energy, broadband, diode-pumped Yb:KYW regenerative amplifier by incorporating chirped-pulse amplification and regenerative pulse shaping. We obtained an output pulse energy of 5.5 mJ and the optical to optical conversion efficiency of ~14.1%. A multi-kHz operation could be accomplished since a thermal conductivity of Yb:KYW is dramatically enhanced by cooling at a liquid-nitrogen temperature. Such a multi-mJ, multi-kHz range, short pulse Yb:KYW laser is useful for the generation of high-average power short pulse x-ray and particle beams as well as an optical parametric chirped-pulse amplification pump source.
Acknowledgment
We thank M. Fujita of Institute for Laser Technology for his stimulating discussions.
References and links
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