We demonstrate a high average power ~4 ps output from a phase conjugate laser system based on a diode-side-pumped Nd:Gd0.6Y0.4VO4 bounce amplifier. An average output power of 16.2 W with a peak power of 210 kW was achieved. A corresponding extraction efficiency of 23% was measured.
©2008 Optical Society of America
High average-power picosecond sources have been attracting intense attention for a variety of commercial and scientific applications, such as nonlinear frequency conversion processes, nonlinear microscopy and microfabrication [1, 2].
Nd-doped vanadate bounce amplifiers, wherein an intense inversion population can be produced below the pump face, have been successfully demonstrated to generate high average-power picosecond outputs without a conventional regenerative amplifier configuration at ultra-high efficiencies [3-6]. To date, Nawata et al. have demonstrated highly intense, high-average-power, near-diffraction-limited picosecond output from a Nd-doped yttrium vanadate (Nd:YVO4) bounce amplifier with a photorefractive phase conjugate mirror in combination with a pulse selector formed by a RbTiPO4 (RTP) Pockels cell . A maximum output power of >25 W with a peak power of >5 MW was achieved at a pulse repetition frequency of 0.3-1 MHz. The pulse width of the output from the system was measured to be as much as ~8 ps, and was limited by the finite gain bandwidth of the Nd:YVO4 amplifier.
Mixed vanadate technology, in which, for example, a fraction of Gd ions in the gadolinium vanadate (GdVO4) are substituted by Y ions, allows for the customization of the fluorescence spectrum, thereby yielding adjustable laser parameters such as gain bandwidth and stimulated emission cross-section [8-11]. Recently, we successfully demonstrated actively and passively Q-switched Nd-doped mixed gadolinium yttrium vanadate (Nd:GdxY1-xVO4) lasers with custom-made pulse repetition frequency and peak power ranges [12. 13]. The Nd-doped mixed vanadates are promising candidates for much shorter picosecond pulse generation, i.e., ~5 ps pulse, at high efficiencies.
In this paper, we demonstrate high average-power sub-5 ps (4 ps) output from a diode-side-pumped phase conjugate Nd:Gd0.6Y0.4VO4 amplifier, for the first time. The maximum average power of 16 W was obtained at a pump power of 70 W. The corresponding peak power of the output was 0.2 MW at the pulse repetition frequency of 20 MHz.
2.1 Experimental setup
Figure 1 shows the experimental setup used in this study for the phase-conjugate amplifier system. The amplifiers used for the experiment were a-cut, 2 mm×5 mm×20 mm, 1.5at.% Nd-doped Gd-rich mixed vanadate Nd:Gd0.6Y0.4VO4 (Hortek Crystal Co., Ltd) slab and a 1.5at.% Nd-doped GdVO4 slab. The two end faces of the amplifiers were AR coated for a wavelength of 1 µm, and they were wedged to the normal of the pump face to prevent self-lasing within the crystal. A continuous-wave (CW) 808 nm pump diode array output was line-focused by a cylindrical lens CLD (f=12.7 mm) on the pump face of the amplifiers. The maximum pump power was 56 W.
The master laser used was a commercial 20 mW continuous-wave mode-locked Yb-doped fiber laser, having a pulse width of 5.1 ps and a pulse repetition frequency (PRF) of 20 MHz. Its lasing frequency was 1063.5 nm. A polarizing beam splitter (PBS), a Faraday rotator (FR), and a half-wave plate (HWP1) formed an optical isolator to prevent feedback to the master laser. The master laser beam was focused using cylindrical lenses, HCL1 (f=400 mm) and VCL (f=100 mm), so that the master laser beam spatially matched the ellipsoidal gain volume. The amplified master laser beam was retro-reflected, and it backed to the amplifier by 4f imaging optics formed by a prism mirror and a spherical lens L1 (f=100 mm). The external incident angles of the master laser beam and the amplified master laser beam with respect to the pump surface were 15° and 18°, respectively.
After passing through the amplifier twice, the amplified beam was collimated by two cylindrical lenses, VCL (f=100 mm) and HCL2 (f=200 mm), and was directed toward a phase-conjugate mirror based on a Rh:BaTiO3 crystal by imaging optics including two spherical lenses L2 (f=150 mm) and L3 (f=75 mm). The polarization of the amplified beam was rotated using a half-wave plate, HWP, and it lay in the extraordinary plane of the Rh:BaTiO3 crystal to maximize the two-wave mixing gain . The phase conjugation of the amplified beam was automatically fed back to the amplifier. After passing through the amplifier twice, it was ejected as an output by a PBS.
The 1000-ppm Rh-doped BaTiO3 crystal used was 0°-cut relative to the normal to the c-axis with dimensions of 6 mm×5 mm×4 mm. The crystal surfaces were AR-coated for 1 µm. The crystal was mounted on a copper block cooled by a water re-circulating chiller, and the temperature of the mount was maintained at ~16 °C. A self-pumped phase conjugate mirror was formed by the Rh:BaTiO3 crystal and an external loop cavity with 4f imaging optics (f=100 mm). The angle of the external loop cavity was 15° and its length was 400 mm. As indicated in our previous publications, a phase conjugate reflectivity of ~50% was typically achieved with this system.
2.2 Experimental results
Figure 2 shows the average output power in the Nd:Gd0.6Y0.4VO4 and Nd:GdVO4 amplifiers as a function of the pump power. Below the pump power of 30W, the Nd:Gd0.6Y0.4VO4 showed low output power when compared to Nd:GdVO4. Above the pump power of 30W, the Nd:Gd0.6Y0.4VO4 exhibited almost the same slope efficiency as that of Nd:GdVO4. The output power in the Nd:Gd0.6Y0.4VO4 and Nd:GdVO4 amplifiers reached 7.2 W and 8.5 W at the maximum pump level (56 W), respectively. As shown in Fig. 3, the output of the Nd:GdVO4 had a pulse duration of 6.3 ps, while the master laser had a pulse-duration of 5.1 ps. We assumed Gaussian-pulse shapes (Fourier transform product =0.441) for all autocorrelations, and thus, the corresponding Fourier-transform limit was 2.4ps. Pulse broadening was induced by the narrowing of the lasing frequency (Fig. 4) due to the finite gain-band of the Nd:GdVO4 .
On the other hand, the output in the Nd:Gd0.6Y0.4VO4 had almost the same spectral bandwidth (0.7 nm) as that of the master laser output, though its center frequency was shifted towards the red so that it matched spectrally the gain-band of the Nd:Gd0.6Y0.4VO4. The pulse-duration of the output was estimated to be 4.1 ps.
The peak power estimated by using the measured pulse width of the output as a function of pump power is shown in Fig. 5. The maximum peak power of the output in Nd:Gd0.6Y0.4VO4 reached 87 kW, which was 1.2-times higher than that in Nd:GdVO4. The mixed vanadate is capable of generating higher peak power picosecond pulses when compared to conventional Nd:GdVO4.
2.3 Power scalability of Nd:Gd0.6Y0.4VO4 amplifier
For further power scaling of the system, we replaced the pump diode with a three-bar diode stack. As shown in Fig. 6, the output power was almost proportional to the pump power, and reached a maximum of 16.2 W at a pump power of 70 W, and a corresponding energy extraction efficiency of ~23% was measured. Above this pump level, severe fractional thermal loading of the amplifier impacted the performance of the system, and saturation of the output power was seen. To achieve the efficient output from the system, the amplifier beam must be relayed to be a ϕ1-3mm spot onto the PCM as related in our previous publication . The saturation was mainly due to inadequate recollection of the amplified beam into the PCM by the strong thermal lens effects in the amplifier. The measured pulse width of the output was ~4 ps (Fig. 7), and the maximum peak power of the output was estimated to be ~210 kW.
The output also exhibited a near Gaussian profile, as shown in Fig. 8, and the corresponding beam-propagation factors, M 2 x and M 2 y, were <1.4, and <1.3, respectively.
The pulse width τ of the down-chirped pulse transmitted in the positive dispersive material is given by 
where τ 0 is the pulse width of the incident pulse, Δω is the spectral bandwidth of the incident pulse, β is the second-order phase dispersion, respectively. The β of the 4 mm thick BaTiO3 determined from the Sellmeier equation , then, is estimated to be ~-1×10-25 s2. Assume that the master laser pulse with the pulse width of 5.1 ps and spectral bandwidth of 0.7 nm shows the negative frequency chirp. After passing though the BaTiO3 crystal, the master laser pulse will be compressed. And then, its pulse width will be expected to be ~4.5 ps in accordance with Eq. (1). Therefore, the unexpected pulse narrowing effect might be induced by that the negative frequency chirp of the master laser was compensated by the positive group velocity dispersion in the system including phase conjugate mirror formed by the BaTiO3 crystal. To understand fully the pulse narrowing effect observed in the output, frequency-resolved pulse analysis such as FROG will be needed.
To overcome the difficulty in modeling the bounce amplifier, wherein the small-signal gain depends on the incident angle of the input beam, we used the partitioned amplifier model as described in our previous report . In the full overlap region B, all the beams (the master laser I1, the reflected amplified beam I2, the phase conjugation I3, and the amplified output I4) overlap spatially, as shown in Fig. 9. In the additional regions A and C, the non-overlapping beams separately experience different gains. We assumed that the length of the B region was approximately 75% of the gain-length seen by the master laser beam. The phase conjugate reflectivity and the internal loss of the relay optics between the amplifier and the phase conjugate mirror were also assumed to be 50% and 10%, respectively. The simulation was performed using the continuous-wave gain saturation formula, and the physical parameters used are listed in Table 1. The red and blue solid curves in Fig. 2 show the simulated output powers in both the Nd:Gd0.6Y0.4VO4 and Nd:GdVO4 amplifiers as a function of the pump power.
Since the saturation gain intensity (4.8 kW/cm2) of the Nd:Gd0.6Y0.4VO4 amplifier was approximately higher by a factor of two than that of Nd:GdVO4 (2.0 kW/cm2), the Nd:Gd0.6Y0.4VO4 exhibited low output power at low pump level when compared to Nd:GdVO4. There is good agreement between the simulations and the experiments. We also simulated the output power under three-diode stack pumping, and obtained good consistency with experiments (Fig. 6).
We have demonstrated a picosecond master-oscillator, power-amplifier system based on a diode-side-pumped phase conjugate mixed gadolinium yttrium vanadate Nd:Gd0.6Y0.4VO4 amplifier, for the first time. The maximum average power of 16.2 W was achieved at a pump power of 70 W, and a corresponding peak power of >200 kW was estimated. The output also showed a pulse-duration of 4 ps as well as near-diffraction-limited spatial form. We believe that the mixed vanadate is promising and has great potential to produce higher peak power picosecond pulses in comparison with conventional vanadate crystals. This system, capable of achieving high quality and very intense sub-5 ps pulses, has potential for applications in various fields, including laser microfabrication and nonlinear optics.
The authors acknowledge support from a Scientific Research Grant-in-Aid (19018007, 18360031) from the Ministry of Education, Science and Culture of Japan and the Japan Society for the Promotion of Science.
References and links
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