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We demonstrated a quasi-continuous wave (QCW) Nd:YAG planar waveguide laser amplifier with the single pulse output energy of 944 mJ. At the pulse repetition frequency (PRF) of 100 Hz, 1064 nm seed laser was coupled into the planar waveguide laser amplifier with the single pulse energy of 128 mJ and the pulse duration of 192 μs. With the total pump energy of 1582 mJ, the extracted energy of 816 mJ was obtained, leading to an optical to optical efficiency of 52% and an electrical to optical efficiency of 28%. To the best our knowledge, it is up to now the highest efficiency and output energy in Nd:YAG planar waveguide laser amplifier reported. At the maximum output, the beam quality factors M2 were measured to be 3.44 and 4.81 in the guided direction and unguided direction respectively, and the polarization degree was 98.6%. Higher average power could be obtained with the better performance of seeder oscillator.
© 2016 Optical Society of America
1. Introduction
Planar waveguide laser combines the both advantages of slab laser [1,2] and fiber laser [3,4]. As we all know, the optical to optical efficiency of fiber laser is close to quantum efficiency [5,6]. However, high power density will introduce nonlinear effect, which limited the power scaling. Slab laser could achieve high power output, but the efficiency is unsatisfied [7,8]. Because of the special structure of gain medium, planar waveguide laser has the potential of producing high optical conversion efficiency and high power laser together. Therefore planar waveguide laser has attracted much attention during the past two decades [9–11], and it is one of the research hotspots currently [12–15].
Planar waveguide is the simplest one-dimensional waveguide, of which the width (unguided direction) is much larger than the thickness (guided direction), and a planar waveguide gain medium is essentially a flat gain fiber. It uses a high aspect ratio sandwich structure consisting of a higher index active core surrounded by lower index claddings. The doped zone, as the core, is typically 5 to 200 μm thick, and the pump power is absorbed in this thin sheet. Therefore, the planar waveguide gain medium offers high gain and high extraction efficiency. Meanwhile, the large surface-area-to-volume ratio provides efficient cooling of the gain medium and minimizes the adverse thermal effects.
Because of the excellent physical, chemical and optical properties, YAG is one of the most popular hosts for planar waveguide. An asymmetrical Nd:YAG planar waveguide was used to obtain 280 mJ 1064 nm QCW laser output at the pulse repetition frequency (PRF) of 1 kHz in a positive unstable resonator in 2008 [16]. The waveguide reported was 58 mm × 10 mm × 1 mm in dimension with 3 μm thick Al2O3 coating on the bottom side facet. The active core made by 1.0 at.% Nd-doped YAG was 0.2 mm in thickness, while the upper cladding was a 0.8 mm -thick undoped YAG layer. The pump light was focused into the waveguide at the oblique angle of 5°. The pump sources were operated at the duty cycle of 20%. The output power of 280 W was obtained with a slope efficiency of 38%, and meanwhile the beam quality M2 was 1.5 in the unguided direction.
In order to obtain high power and maintain good beam quality, master oscillator power amplifier (MOPA) configuration is adopted usually. Lockheed Martin released a quasi-continuous wave MOPA laser based on their self-imaging planar waveguide in 2011 [17]. The MOPA laser system consisted of a passively Q-switched Nd:YAG oscillator that was successively amplified in three Nd:YAG planar waveguide amplifiers. Each of the amplifiers was 2-passed by the injected 1064 nm light, with the final system output being 375 W at 20 kHz PRF with 1.5 ns pulse width. The electrical to optical efficiencies for the three individual stages were 16.5%, 19.1% and 18.4% respectively. The maximum optical to optical efficiency was about 35%. The three waveguides were about 4 cm, 8 cm and 13 cm in length successively.
The highest optical to optical efficiency on Nd:YAG planar waveguide was 58% from a thermal-bonded Nd:YAG planar waveguide oscillator [18]. However, the result was only demonstrated in a low power system. The waveguide was end-pumped by a fiber-coupled laser diode operating at 808 nm. A maximum output power of 2.90 W at 1064 nm was achieved with an M2 value of 2.6 in the guided direction for a pump power of 5.0 W.
In this paper, the performance of Nd:YAG planar waveguide laser amplifier was investigated, and we achieved the high conversion efficiency and high output energy together. The single pulse energy of 944 mJ and the corresponding optical to optical efficiency of 52% were demonstrated in a QCW Nd:YAG planar waveguide laser amplifier at the PRF of 100 Hz. To the best our knowledge, it is up to now the highest efficiency and output energy in Nd:YAG planar waveguide laser amplifier reported. Besides, higher pulse repetition frequency and longer pulse duration of the seeder laser would result in higher average output power. In order to achieve high optical to optical efficiency and high output in high average QCW planar waveguide laser amplifier, we must overcome some difficulties, such as: the enhancement of pump coupling efficiency, the suppression of amplified spontaneous emission (ASE) and the improvement of extraction efficiency.
2. Experimental setup
The schematic of the Nd:YAG planar waveguide laser amplifier setup is shown in Fig. 1. The seed laser was a 1064 nm free functional oscillator with Nd:YAG rods. It operated at the PRF of 100 Hz and the pulse width was 192 μs. A Faraday isolator was followed to protect the seeder from the backward laser. The spot size of seeder beam was enlarged by two spherical lenses f1 and f2, and then launched into the planar waveguide from one end facet with a cylindrical lens fx. The focused direction of seeder beam accorded with the guided direction of planar waveguide, and a single pass amplification was introduced. Pump light were coupled into the waveguide from the two end facets. Two copper micro-pipe heat sinks were welded to the face facets to ensue effective cooling.
Fig. 1 Schematic of Nd:YAG planar waveguide laser amplifier. f1, spherical lens, f = 200 mm; f2, spherical lens, f = 500 mm; fx, cylindrical lens, f = 300 mm; Fy1, cylindrical lens, f = 77 mm; Fy2, cylindrical lens, f = 66 mm; Fx, cylindrical lens, f = 77 mm.
The Nd:YAG planar waveguide was 59 mm (L) × 10 mm (W) × 1 mm (T) in size, as illustrated in Fig. 2. The central 50 mm × 10 mm × 100 μm was 1.5at.% Nd3+ doped YAG layer, and two undoped YAG layers of 450 μm thickness were bonded onto it from the bottom and top respectively, acting as inner claddings. SiO2 films of 3 μm thickness were coated onto the top and bottom facets as the outer cladding. This simple double cladding waveguide had the capacity to enhance the pump power and further scale the laser output. Undoped YAGs were bonded on each ends to decrease the thermal effect. One end of the waveguide was cutted 90° with regard to the face facet, and the other end was 80°. The end facets were anti-reflective coated at both 808 nm and 1064 nm. This structure of planar waveguide united simple adjustment with the suppression of amplified spontaneous emission and parasitic oscillation (ASE/PO). With the right trapezoid cross section in the guided direction, the amplified spontaneous emission would go out through the end facet or the side facet.
Fig. 2 Schematic of the structure of the Nd:YAG planar waveguide. L, the length direction; T, thickness, the guided direction.
For the 2 × 10 bar space-combined diode laser array (DLA), the spot size and half divergence were 18.9 mm and 0.2° (1/e2) in the fast axis, and 10 mm and 4° (1/e2) in the slow axis respectively. Pump light from diode laser array were obliquely launched into the planar waveguide from two end facets respectively. The pump beam was focused in the fast axis and imaged in the slow axis. Two plane-convex cylindrical lenses were adopted to image the pump spot to the unguided direction of the planar waveguide. TracePro software was adopted to simulate the focusing of the pump light in the fast axis from LDA via a plane-convex cylindrical lens with a focal length of 77 mm. Figure 3 shows the intensity distribution of the simulated focused spot of the pump beam, and the spot size was nearly 600 μm. The measured intensity distribution of the focused pump spot in the fast axis is illustrated in Fig. 4. The curve in the inset section shows one dimensional distribution after compression. These two matched with each other well. The focused spot size was about 600 μm, so the half divergence angle was 0.10 rad. The NA of cladding was calculated to be 0.429 according to the refractive index of YAG and SiO2, therefore this pumping transformation ensured the efficient coupling. For oblique end pumping, appropriate angle could ensue efficient pump coupling, and also protect the DLAs against the unabsorbed pump light from the other end. Besides, more DLAs could be adopted to augment higher pump power.
3. Results and discussion
Parasitic oscillation (PO) was the most annoying effect in the planar waveguide laser, which limited the power scaling and even destroyed the laser medium under certain circumstance. Therefore, PO was firstly tested. When one DLA pumped from one end in pulsed mode, a PIN diode together with an oscilloscope was employed to measure the parasitic oscillation signal. When the pump energy increased to a certain value, the PO occurred inside the planar waveguide. The direction of PO lasing was along the length of the waveguide only, and in other direction all around the waveguide no signal was detected. Besides, the intensity of PO from the 80°-cut end was stronger than that from the other end.
The direction of PO showed that the oscillation occurred along the length of the waveguide. The results indicates that the feedback was induced by the right-angled end facet and bonding interface or the scattering probably. When the pulse width of pump source was 87 μs, the threshold of the DLA drive current was 87 A. The small signal gain coefficient g0 could be calculated by [19]
Where, ηstore is the storage efficiency, ηabs is the absorption efficiency, ηu is the upper state efficiency defined as the ratio of the power emitted at the laser transition to the power absorbed into the pump bands [19], Ep is the pump energy, ρs is the saturation energy density, and V is the volume of the gain zone. In this case, the average small signal gain coefficient g0 was about 1.68 cm−1, indicating that the single pass small signal gain G0 = exp(g0·Leff) was up to 4.5 × 103 with the Leff of 5 cm. In order to suppress PO, the product of the reflectivity of the two facets should be less than 5 × 10−8 theoretically, and this was such a harsh term. Therefore, it is very certain that the right-angled end facet gave the feedback. In the future work, we will optimize the structure to further increase the threshold of the PO.
Forward pumped amplification performance was observed subsequently. The pump pulse width was the same as that of the seeder pulse, and they were synchronized to each other. A maximum energy of 504 mJ was produced, and meanwhile the corresponding optical to optical efficiency ηoo was 50.0%. Figure 5(a) shows the output energy as a function of the pump energy at different seeder energy. These curves illustrated that the output energy increased almost linearly with the pump energy. The corresponding efficiencies were given in Fig. 5(b). It was obvious that ηoo was enhanced with the rise in seeder power and pump power. The increment of ηoo was reduced gradually with the pump power, until ηoo trended to a certain value. This trend was in agreement with the theoretical analysis. When the seeder energy declined from 128 mJ to 30 mJ, the extracted energy at maximum pump energy dropped from 376 mJ to 348 mJ gradually, and ηoo dropped from 50.0% to 46%. Significantly, it was indicated that ASE was completely suppressed by these high power seeder input. For the seeder energy of 30 mJ, the input signal intensity was about 6 times of the saturation intensity.
Fig. 5 Output energy (a) and optical-optical efficiency (b) versus pump energy under forward pumping at strong seeder
Figure 6 shows the output energy and efficiency at lower seeder energy. It was apparent from the chart that the extracted energy increased with the improvement of seeder energy and pump energy. With the rise of pump energy, the ηoo increased rapidly in the beginning, then slowed down, and finally level off, or even decreased gradually at smaller seeder signal energy. For the seeder energy of 1 mJ, the input intensity was about 500 W/cm2. Because of the forward pumping and single pass amplification, the highest gain was positioned at the input end. Due to the small seeder energy, just a little of the stored energy at the input end was extracted, so the unexhausted gain here was high enough to induce ASE.
Fig. 6 Output energy (a) and optical-optical efficiency (b) versus pump energy under forward pumping at lower seeder.
Backward pumping was followed with another DLA. Figure 7 illustrates the output energy and ηoo under forward pumping and backward pumping at the seeder energy of 1 mJ. When the energy of the seeder signal varied from 128 mJ to 30 mJ, the extracted energy and ηoo were almost the same as those under forward pumping. At lower seeder energy, it differed greatly. The ηoo maintained its growth when the pump energy increased from 0 to the maximum, and there was an upward trend. As shown in Fig. 7, it was obviously that when the pump energy was lower than 300 mJ, the output energy and efficiency were almost the same with each other. The maximum ηoo was 30.7% under forward pumping, which occurred at the pump energy of 444 mJ. When the pump energy increased to 752 mJ, ηoo decreased to 27.9%. The maximum optical to optical efficiency was 39.0% under backward pumping, which occurred at the maximum pump energy, and higher pump energy will introduce higher efficiency. Therefore, the backward pumping was more suitable than the forward pumping at lower seeder intensity. Besides, higher pump power or higher seeder power would introduce a higher efficiency when backward pumped.
Fig. 7 Output energy and optical-optical efficiency versus pump energy under forward pumping and backward pumping when 1 mJ seeded.
Dual end pumping architecture was adopted then. The maximum energy of 892 mJ was produced, and the corresponding optical-optical efficiency was 51.7%. The output energy and ηoo are revealed in Fig. 8. The output energy increased linearly with the pump energy. The efficiency rise gradually from 44.8% to 51.7%, and the rate of enhancement decreased. With respect to the absorbed pump energy, the effective ηoo was 56.2%.
Fig. 8 Output energy and corresponding efficiency versus pump energy under dual end pumping with the pulse duration of 192μs.
In order to increase the output energy, the pump pulse width was increased from 192 μs to 207 μs with the same PRF of 100 Hz. Parasitic oscillation signal was not detected in the period of the 15 μs pumping before seed laser with the drive current of 200 A. Here, the maximum output energy of 944 mJ was produced at the pump energy of 1582 mJ, and the corresponding optical-optical efficiency was 51.6%, as shown in Fig. 9. The effective optical-optical efficiency and extraction efficiency were 56.2% and 78% respectively. The electrical to optical efficiency of this planar waveguide amplifier was 28%. The peak power was calculated to be 5 kW. The beam quality factor M2 of the output laser was measured by a Spiricon M2-200 laser beam analyzer. At the maximum energy, M2 was measured to be 3.44 and 4.81 in the guided direction and unguided direction respectively. Figure 10 shows the fitting curves and measured spot size as a function of distance after lens. With the help of a thin film polarizer, the polarization degree was measured to be 98.6% at this maximum output energy. Comparing the efficiency for different pump width at the same seeder energy, the optical-optical efficiency at 207 μs was slightly lower than that at 192 μs for any drive current, which was due to the storage efficiency.
Fig. 9 Output energy and corresponding efficiency versus pump energy under dual end pumping with the pulse duration of 207μs.
If the pulse width of the seeder signal is increased, the output pulse energy would be increased linearly, while the peak power stays the same. In addition, the output energy remains unchanged with higher pulse repetition frequency improved. Because the seed laser was based on Nd:YAG rods, longer pulse width and higher PRF would reduce the output energy and worsen the beam quality owing to the adverse thermal effect. So the experimental performance at longer pulse width and higher PRF was not provided here. However, for this planar waveguide laser amplifier, we believe it has the same performance at the PRF of 500 Hz, even at 1000 Hz, except for the beam quality in the unguided direction. With higher pulse repetition frequency, the beam quality in the unguided direction becomes worse due to the thermal effect. While the beam quality in the guided direction stays the same level owing to the waveguide effect. It should be noted that, according to Figs. 8 and 9 the output energy increased linearly with the pump energy, therefore single pulse energy of 2 J could be obtained with stronger pump energy.
4. Conclusion
In conclusion, an end pumped planar waveguide Nd:YAG laser amplifier was demonstrated at the pulse repetition frequency of 100 Hz. Efficient pump coupling, suppression of ASE/PO, and efficient extraction were discussed and realized. The output energy of 944 mJ was produced with the pulse duration of 192 μs. The corresponding total optical-optical efficiency was 52%, and the extraction efficiency was 78%. To the best of our knowledge, it has the highest efficiency and output energy in planar waveguide Nd:YAG laser amplifier. At the maximum output, the beam quality M2 were measured to be 3.44 and 4.81 respectively in guided direction and unguided direction, and the polarization degree was measured to be 98.6%. In the future, we aim to increase the output energy to 2 J at the PRF of 500 Hz with this planar waveguide.
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