In this paper, we have set up a diode pumped rubidium MOPA system with a chain of two amplifiers. The experimental results show an amplified laser power of 26W with amplification factor of 16.3 and power extraction efficiency of 53% for a single amplifier, and an amplified laser power of 11W with amplification factor of 7.9 and power extraction efficiency of 26% for a chain of two amplifiers. The reason for lower performance of cascade amplification is mainly due to the limited total pump power, which will be not sufficient for efficient pumping when assigned from a single amplifier into two amplifiers. The situation could be well improved by increasing the seed laser power as well as the pump power for each amplifier to realize high efficient saturated amplification. Such MOPA configuration has the potential for scaling high beam quality alkali laser into high powers.
© 2015 Optical Society of America
As a hybrid kind of optically pumped gaseous laser, diode pumped alkali lasers (DPALs) show great potential for extremely high power operation due to their many advantages, such as the high quantum efficiency, the feasibility of electrically driven diode pumping, the intrinsic capacity of convective thermal management, and the lightweight and compact configurations [1–5]. Since first proposed by Krupke et al. , a great number of DPALs have demonstrated high efficiencies. In 2012, a Cesium DPAL with 1kW output power and 48% optical conversion efficiency has been demonstrated . Like traditional solid-state and fiber laser systems, there are two basic ways for DPAL power scaling, one is to extract power from a single resonator, the other is to use the master oscillator power amplifier (MOPA) configuration. As compared with a single resonator, the MOPA configuration is beneficial for keeping good beam quality and reducing complexities of thermal management. Due to the power scaled potential of diode pumped alkali amplifiers (DPAAs), scientists have successfully realized some demonstrations. In 2008, Zhdanov et al. have realized the first Cs DPAA with a small signal amplification factor of 145 , and Hostutler et al. have realized the first Rb DPAA with 7.9dB amplification . In 2010, Zhdanov et al. have realized a more scalable transversely pumped Cs DPAA . In theoretical field, Bailiang Pan et al. have set up models for both static and flowing gas DPAAs [11,12]. We have considered the effect of amplified spontaneous emission (ASE) in DPAAs . And Parkhomenko et al. have set up a model for transversely pumped alkali vapor amplifiers .
Till now, the DPAA experiments that reported all have used a single amplifier for demonstration, and in power scaling of alkali lasers, a chain of two or more amplifiers may be needed for cascade power enhancement. In this paper, we report our experimental results of a Rb DPAA with a chain of two stage amplifiers.
2. Experimental setup
A schematic of the experimental setup is shown in Fig. 1. The maximal total diode pump power is ~108W, with central wavelength of 780.2nm that matched the Rb D2 transition and narrowed spectral linewidth of ~0.2nm (FWHM) by using volume Bragg gratings. Due to the limitation number of diode pumps, the pre-collimated pump light is split by a fixed beam splitter to pump the resonator and the two amplifiers with splitter ratio of 1:8. The 2cm long Rb cell inside the resonator contains buffer gases of 200torr ethane and 300torr helium at room temperature, with outer surfaces of the two windows AR coated. It is placed in an heating oven with temperature adjustment precision of ± 1°C. The resonator consists of a concave mirror (R = 99%) as back reflector, and a plane mirror (R = 30%) as output coupler. The pump light that used for pumping the chain of two amplifiers is further assigned by using a combination of a half wave plate and a polarized beam splitter (PBS). The three 10cm focal length lenses are used for focusing the pump light inside the Rb cells, and the spot sizes at the focal points for the three Rb cells are nearly the same, about ~2.5mm2 (~90% power included). Two identical rubidium cells used as amplifiers have a length of 2 cm and are filled with methane serving as a buffer gas. The PBS that arranged behind the second amplifier is used to separate the amplified 795nm Rb laser and the residual 780.2nm pump light.
3. Results and discussions
3.1 Rb MOPA with single amplifier
In this section, we present the performance of Rb MOPA with a single amplifier. The experimental configuration corresponds to the “master oscillator” and “First Stage Amplifier” parts in Fig. 1. The resonator produces seed laser power of 1.6W. First, we study the temperature (Rb atom concentration) dependence of the amplifier. Figure 2 shows the amplification factor versus the amplifier temperature at fixed pump power of 60W. The results shows an maximal amplification factor of 11.1 at temperature of 135°C, which corresponds to a Rb concentration of 4.7 × 1013cm−3. The temperature dependence for the amplifier is similar to that obtained for the oscillators [15–17], with the optimal temperature (alkali concentration) corresponding to the best balance between pump absorption and strong spontaneous emission .
Figure 3 shows the results of the amplified laser power and the amplification factor versus the seed laser power under fixed pump power of 72W:
The seed laser power is adjusted from 0.3 to 1.6W by a combination of half wave plate and PBS in front of the amplifier, and the pump power is fixed as 72W. It could be seen that, at the minimal seed laser power of 0.3W, the amplifier could realize a maximal amplification factor of 36. As the seed laser power increases, the amplified laser power increases linearly with increased power extraction fraction and reduced amplification factor. At the maximal seed laser power of 1.6W, the amplifier could produce 26W output, with amplification factor of 16.3. The optical conversion efficiencies are 34% and 53% relative to the total and the absorbed pump powers respectively. The linear relation between amplified and seed laser powers as well as the low pump power conversion efficiency indicates that the amplifier doesn’t work in saturated state. For further enhancement of the power extraction efficiency, the seed laser power should be increased in order to reach saturation in the amplifier, and the seed and pump modes overlap should be further optimized.
It should be noticed that, the existence of the seed laser could dramatically enhance the pump absorption fraction. In our experiment, at temperature of 135°C, the amplifier absorbs 32W without seed laser and 46W when the seed laser is present (~1.6W), with 44% increase of pump absorption. This phenomenon is in agreement with theoretical estimation, and could be explained as follows. In DPALs, the alkali atom works in recycling mode  52S1/2→52P3/2→52P1/2→52S1/2 to convert pump photons into laser photons, when the seed laser is absent, the cycling step 52P1/2→52S1/2 is dominated by spontaneously emission. And when the seed laser is present, the cycling step 52P1/2→52S1/2 is dominated by stimulated emission, which is much faster than the spontaneously emission process. Thus, the presence of seed laser accelerates the total atomic recycling rate, including the pump absorption step 52S1/2→52P3/2, thus for the case of constant alkali concentration, the pump absorption fraction is dramatically enhanced.
3.2 Rb MOPA with a chain of two amplifiers
In this section, we present the performance of Rb MOPA with a chain of two amplifiers. The seed laser power could be adjusted from 0.2 to 1.4W by using the combination of a half wave plate and a PBS. The total pump power for the two amplifiers is fixed as 64W, and the pump powers that assigned for each amplifier is adjusted by using another group of half wave plate and PBS after the beam splitter. Figure 4 shows the results of the final amplified laser power for different pump assignment and seed laser power:
It could be seen that, for each pump power assignment, the amplified laser power increases nearly linearly as seed laser power increases. Different pump assignment leads to different MOPA performance. At pump assignment ratio of 2:1, the maximal amplified laser power is 11W with amplification factor of 7.9 and power extraction efficiency of 26% (relative to absorbed pump power). These efficiencies for the other two pump assignment cases are 22% and 14% respectively. It is obviously that, at the condition of limited total pump power, the power extracted efficiency of MOPA with a chain of two amplifiers (26%) is much lower than using a single stage amplifier (53%), although the total pump power for two amplifiers (64W) is a little lower than for a single amplifier (72W). The main reason is that, at limited pump power, the use of a chain of two amplifiers leads to a dispersion and reduction of the pump intensity as compared with the case of a single amplifier. For the three-level alkali laser which needs intense pumping, the reduced pump intensity will lead to a decrease of the overall efficiency . In fact, an extreme case could explain this phenomenon: as we further increase the pump assignment ratio, for example from 2:1 to 3:1 to 4:1 etc., and finally we assign all the pump power into the first amplifier just as discussed in section 3.1, although the first amplification will give the same result as in Fig. 3, but the final output laser power will be much lower than the single amplifier case because the second Rb cell becomes an totally absorbing medium in this case.
The above experimental results and the comparison with a single amplifier indicate that, the low efficiency for our demonstration of a chain of amplifiers is mainly due to the limited total pump power. For cascade power amplifications, each amplifier stage should work in saturated region, that is, by conducting sufficient strong pumping and intense seed laser injection. In the next step, we will improve our experimental conditions for a further improvement of the alklia MOPA performance.
In this paper, we have demonstrated a diode pumped Rb MOPA with a chain of two amplifiers. By using a single amplifier, a maximal amplified laser power of 26W is achieved with amplification factor of 16.3 and power extraction efficiency of 53%. By using a chain of two amplifiers, a maximal amplified laser power of 11W is achieved with amplification factor of 7.9 and power extraction efficiency of 26%. The reason for the relatively worse performance of the cascade amplification experiment is analyzed and clarified. In the next step, we will improve the experimental conditions to ensure each amplifier work in saturated amplification state, thus to realize high efficient alkali MOPAs. Such configuration of chain of amplifiers is potential to be used for scaling high quality alkali laser beam into high powers.
This work was supported by the National Natural Science Foundation of China (No. 11272343).
References and links
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