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Abstract
We report optical amplification with an optical-to-optical conversion efficiency of 70 ± 1% from a diode-pumped Cs vapor cell. When pump (852 nm; D2-line) and signal (895 nm; D1-line) lasers with a narrow spectral width of ∼2 MHz are resonant on the hyperfine states (F = 3 or 4) of the 6S1/2 state, we observe that the amplification factors are significantly changed according to the hyperfine-state combination of the pump and signal lasers. We find that the optical frequencies of the pumping and signal lasers need to be controlled near the hyperfine state of 6S1/2 (F = 4) to obtain an efficient diode-pumped alkali amplifier (DPAA). To realize highly efficient optical gain conditions, both the spatial modes of the pump and signal lasers are made to overlap in the Cs vapor cell with the use of a single-mode optical fiber. An amplification factor of 430 ± 15 is achieved under the following conditions: cell temperature of 90 °C, signal power of 0.1 mW, and pump power of 200 mW. We believe that our results can aid in the development of highly efficient diode-pumped alkali-vapor lasers and amplifiers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Highly efficient laser sources have attracted intense attention in many fields outside of pure scientific research, for e.g., in commercial manufacturing, medical, and military applications. Since W. F. Krupke first proposed the concept of the diode-pumped alkali-vapor laser (DPAL) in 2001 [1], DPALs have been continuously studied over the last 18 years because of their many advantages including high quantum efficiency, high power, and high beam quality [2–37]. In particular, a high quantum efficiency of >95% is an important reason as to why DPALs have attracted attention [3–5]. The efficiency and average power of DPALs have begun to gradually improve with the development of narrowband high-power diode laser pumps [6–16]. Furthermore, to increase the operating power of high-power DPALs, another method has been proposed using a diode-pumped alkali amplifier (DPAA) because it simplifies the use of multiple pump sources and suitably addresses the problem of excessive heat released into the gain medium [17–20].
An understanding of the gain characteristics of alkali-vapor media is essential for the maximization of the efficiency of high-power DPALs. The key parameters of an alkali-vapor medium are the composition ratio of the alkali atoms and buffer gases, vapor cell temperature, and active medium length. In this regard, many studies have investigated the optical-to-optical conversion or slope efficiency of an alkali-vapor medium with respect to the pump power according to the abovementioned key parameters. The optical-to-optical conversion efficiency (OOCE) has been reported to be as high as ∼63% with the use of narrowband pump lasers [14,15]. Thus far, DPAL and DPAA experiments have based on the D1 and D2 lines of an alkali atom, which form the components of a fine-structure doublet. The DPAL medium produced population inversion when optically pumped with a narrowband laser diode via the collisional mixing process between the upper energy states (P1/2 and P3/2 states) of spin–orbit splitting by hydrocarbon molecules. However, there is a hyperfine structure with the total nuclear angular momentum in each of these D1 and D2 transitions of the alkali atoms. But in the case of Cs atoms, the hyperfine splitting between the two hyperfine states (F = 3 and 4) of the 6S1/2 state of Cs is ∼9.2 GHz, which frequency is comparable with the collisional broadening of 10 GHz. To date, studies on DPALs and DPAAs have never considered the hyperfine structure, because the collisional broadening owing to the collisional mixing process is larger than the hyperfine splitting of alkali atoms [17].
In this work, we investigate the optical amplification of a small signal utilizing the hyperfine states of the 6S1/2 state of Cs in a vapor cell with ethane buffer gas to realize a highly efficient DPAA with Cs atoms. In particular, we compare the optical amplification for four cases relating to the two hyperfine states in the D2 transition for the pumping laser and the D1 transition for the signal laser. For highly efficient amplifier operation, the spatial modes of the pump and signal lasers are completely overlapped and matched in size along the entire length of the Cs vapor cell. Under the optimal transition condition, we investigate the dependence of the optical amplification ratio and optical-to-optical conversion efficiency on the vapor cell temperature, pumping power, and signal power. In addition, the gain coefficient is numerically calculated as a function of the vapor cell temperature and compared with the experimental result under the condition for effective optical pumping and amplification.
2. Experimental setup for DPAA from Cs atom vapor with ethane buffer gas
Figure 1(a) shows the energy-level diagram of 6S1/2–6P1/2 (D1-line) and 6S1/2–6P3/2 (D2-line) transitions of 133Cs atoms for DPAA via a collisional mixing process with ethane (C2H6) buffer gas. The D1 and D2 lines were used for amplification and optical pumping, respectively. The energy difference between the hyperfine states (F = 3 and 4) of the 6S1/2 state is approximately 9.2 GHz. To optically amplify the signal beam from the Cs vapor cell, the ethane buffer gas is required to provide collisional population transfer to an upper lasing level that creates population inversion and gain in the Cs vapor [4]. While the natural linewidths of the D1 and D2 transitions are narrow at 4.2 MHz and 5.2 MHz, respectively, the collisional broadening of ethane buffer gas in the collisional mixing process between the 6P1/2 and 6P3/2 states is estimated to be approximately 2 MHz/Torr [3]; this means that the collisional broadening of 10 GHz owing to the 500-Torr ethane buffer gas is comparable with the hyperfine splitting of the 6S1/2 state.
Fig. 1. Experimental configuration for diode-pumped alkali amplifier (DPAA) using Cs vapor and ethane buffer gas. (a) Energy-level diagram of the hyperfine structure of the D1 and D2 transitions of 133Cs atoms. (b) Experimental setup for highly efficient DPAA in the Cs vapor cell (pump laser (wavelength 852 nm); signal laser (wavelength 895 nm); OI, optical isolator; PBS, polarization beam splitter; HWP, half-wave plate; BF, bandpass filter).
The experimental setup for DPAA from Cs vapor with ethane buffer gas is shown in Fig. 1(b). In our experiment, the Cs vapor cell for DPAA comprised a 20-mm-long vapor cell with antireflection-coated windows containing 133Cs along with 500-Torr ethane buffer gas. To maintain a warm atomic vapor, the vapor cell temperature was stabilized by means of a temperature-controlled oven, and the cell windows were heated to prevent Cs condensation on them. Two distributed Bragg reflector (DBR) lasers were operated independently at wavelengths of 852 (for the pump laser) and 895 (for the signal laser) nm. For efficient amplifier operation, the pump and signal lasers were coupled into a single-mode optical fiber for spatial-mode overlapping in the Cs vapor cell and completely overlapped with each other along the entire length of the cell. The pump and signal beams were simultaneously focused into the vapor cell by means of a lens with a 200-mm focal length. The pump and signal laser linewidths were ∼2 MHz, with their beam diameters being 1.2 mm. The polarizations of both lasers were perpendicularly linear. To investigate the amplification factors according to the hyperfine-state combination of the pump and signal lasers, we monitored the optical frequencies of the pump and signal lasers via the saturated absorption spectra (SAS) of the D1 and D2 transitions for the signal and pump lasers. The pump-laser power was set to be adjustable from 10 mW to 200 mW to investigate the dependence of the DPAA gain on the pump power.
3. Experimental results and discussion
We investigated the optical amplification of a small signal from the Cs vapor cell with ethane buffer gas according to four cases of the two hyperfine states (F = 3 and 4) of the 6S1/2 state in the D2 transition for the pumping laser and those of the D1 transition for signal laser. The optical amplification means the ratio of the output power of the DPAA to the input power of the signal laser. Figure 2(a) depicts the four cases for optical amplification in the Cs vapor cell, wherein the red arrows indicate the pumping transition and the blue arrows the gain transition due to the signal laser. As mentioned above, we did not consider the hyperfine states of the excited 6P1/2 and 6P3/2 states.
Fig. 2. Optical amplification in Cs vapor cell according to four combinations of two hyperfine states (F = 3 and 4) of 6S1/2 state. (a) Four configurations for optical amplification in the Cs vapor cell, wherein the red arrows indicate pumping transition and blue arrows the gain transition. (b) Optical amplification as a function of the pumping power for the four combinations.
For the four cases shown in Fig. 2(a), we investigated the optical amplification as a function of the pumping power under the experimental conditions of a small-signal power of 0.1 mW and Cs vapor cell temperature of 90 °C. As shown in Fig. 2(b), the optical amplification is significantly different according to the four combinations of the two hyperfine states (F = 3 and 4) of the 6S1/2 state. Although all the optical amplifications of the four cases increase with increase in the pumping power, the increment ratio of the optical gain is the highest for case I followed by those of cases II, III, and IV. The optical gain coefficients at the pumping power of 200 mW are measured to be 430, 280, 90, and 70 for the four cases.
The main cause for strong amplification of the 6S1/2(F = 4) − 6P3/2 pump transition is because it has the cycling transition of 6S1/2(F = 4) − 6P3/2(F′ = 5) within its pump transition. In the case of the 6S1/2(F = 4) − 6P3/2(F′ = 5) cycling transition, when the optical frequency of pump laser are resonant on the cycling transition, the population of the 6P3/2(F′ = 5) state cannot spontaneously decay to the 6S1/2(F = 3). These cycling transitions may be an example of a simple two-level atomic system. So, many atoms of the ground state of 6S1/2(F = 4) can be effectively pumped to the excited state of 6P3/2(F′ = 5) without loss to the other ground state of 6S1/2(F = 3). However, in the hyperfine states except the cycling transition of the 6S1/2 − 6P3/2 transition, the population of the 6S1/2(F = 5) state may be depleted to the 6S1/2(F = 4) state. The atoms in the 6S1/2(F = 4) state weakly interact with the pump laser and faintly contribute to the optical pumping to the 6P3/2 state, because the spectral width of the pump laser of 2 MHz is narrower than the frequency difference of 9.2 GHz between the hyperfine states (F = 3 and 4) of the 6S1/2 state.
The optical-frequency dependence of the signal laser is significantly smaller than that of the pump laser, because there is not a cycling transition in the 6S1/2(F = 3 and 4) − 6P1/2(F′ = 3 and 4) transition. The reason that the amplification of the 6S1/2(F = 4) − 6P1/2 transition slightly increases is the effective dipole moment difference between the 6S1/2(F = 3) − 6P1/2(F′ = 3 and 4) and 6S1/2(F = 4) − 6P1/2(F′ = 3 and 4) transitions [38]. The relative strength of the F = 4 → F′=3, 4 transition is 1.3 times larger than that of the F = 3 → F′=3, 4 transition. From the results of Fig. 2(b), we find that the optical frequency of the pumping field needs to be controlled near the hyperfine state of 6S1/2 (F = 4) to obtain the maximum optical gain.
To elucidate the optical amplification according to the hyperfine states of the pump and signal lasers, we measured the absorption spectra of the D1 and D2 transitions in the 133Cs vapor cell with ethane buffer as a function of the detuning frequency from the 6S1/2(F = 3 and 4) − 6P1/2 and 6P3/2 resonances, as shown in Fig. 3(a) and 3(b), respectively. To avoid saturation of the absorption spectra, the cell temperature was set to less than 74 °C, and the probe laser power was attenuated to less than 0.1 mW. The blue curves in Fig. 3 indicate the SAS of the D1 and D2 transitions in the pure 133Cs vapor cells to monitor the optical frequencies of the pump and signal lasers. From the observed SAS in the pure Cs cell without a buffer gas, the hyperfine states (F = 3 and 4) of the 6S1/2 state were clearly distinguished. Meanwhile, the red curves indicate the transmittances of both lasers in the Cs vapor cell with 500-Torr ethane buffer. From the measured absorption spectra, we note that the collisional broadening of ethane buffer gas in the collisional mixing process is approximately 10 GHz.
Fig. 3. Absorption spectra of (a) D1 and (b) D2 transitions in 133Cs vapor cell with ethane buffer, wherein the red curves indicate linear absorption spectra and blue curves the saturated absorption spectra.
In the case of the absorption spectrum of the D1 transition in Fig. 3(a), we cannot observe the peak separation of the absorption spectrum, because the frequency difference between the hyperfine states (F = 3 and 4) of the 6P1/2 state is ∼1.2 GHz larger than that of the 6P3/2 state. However, the absorption degrees at the 6S1/2(F = 4) state are significantly larger than that at the 6S1/2(F = 3) state. In the case of the absorption spectrum of the D2 transition in Fig. 3(b), we weakly observe two peaks corresponding to the frequency difference of 9.2 GHz between the hyperfine states (F = 3 and 4) of the 6S1/2 state. The absorption difference of the two hyperfine states is related to the transition strength, which is in turn related to the optical pumping efficiency. From the results of Figs. 2 and 3, we can infer that the linewidth and optical frequency of the pumping and signal lasers are important factors for realizing a highly efficient diode-pumped cesium vapor amplifier.
The temperature of the Cs vapor cell forms one of the key experimental parameters for the diode-pumped cesium vapor amplifier. The vapor-cell temperature is related to the atomic density and the collision ratio of Cs atoms with the buffer gas, which affects the small-signal amplification. Thus, we next investigated the dependence of the small-signal amplification and gain on the cell temperature (Fig. 4). Here, optical amplification (A) is defined as the ratio of the output power (Pout) to the input power (Pin) of the signal laser, wherein Pin = 0.1 mW in our case. We measured Pout as a function of the cell temperature in the range from 60 °C to 90 °C. In our experiment, we heated both windows of the Cs vapor cell to prevent Cs condensation on them. The cell temperature was measured at the surface of the vapor cell.
Fig. 4. Dependence of (a) small-signal amplification and (b) gain on vapor cell temperature. The blue circles indicate the experimental G values and the red curve the theoretical result in a three-level atomic system.
Figure 4(a) presents amplification A as a function of the cell temperature for a pump power of 200 mW. With increase in the cell temperature, we note that the increase in A is nearly exponential. Parameter A rapidly increases from 80 °C and reaches a value of 430 at 90 °C. In our experimental setup, the heating sources were positioned at the windows of the vapor cell, and the temperature difference between the heating source and cold point was estimated to be approximately 40 °C. Therefore, we could not observe saturated amplification at greater temperatures, as we wanted to prevent chemical reactions between Cs and ethane [4].
From the result of Fig. 4(a), the gain coefficient (G) of DPAA in Cs atoms can be estimated as shown in Fig. 4(b). In general, assuming constant gain of the medium, gain coefficient G can be expressed as [17]
where L denotes the length of the gain medium corresponding to the Cs vapor gain length of 20 mm in our case, except for the window thickness of the vapor cell. Under pump and signal powers of 200 mW and 0.1 mW, respectively, the blue circles in Fig. 4(b) indicate the G values as a function of the cell temperature. The G value can be estimated as 3 cm−1 at the cell temperature of 90 °C. The red curve in Fig. 4(b) indicates the theoretical result corresponding to the expression GT = ΔN·σ considering the three-level atomic system (6S1/2, 6P1/2, and 6P3/2 states), wherein ΔN and σ denote the population inversion and the resonant cross-section broadened by the buffer gas for the optical amplification transition (D1), respectively. The σ-value of 7.8 × 10−13 cm2 was used for the calculation of the GT value considering the pressure broadening and the intrinsic cross-section of Cs [17]. Considering the three-level pump scheme, we can assume that the population inversion on the lasing transition (D1) is proportional to the vapor number density, when the pump intensity is significantly larger the saturation intensity (1.6 mW/cm2) of the D2 transition. We calculated the vapor number density from the vapor pressure of Cs atoms according to the cell temperature [38]. As can be inferred from the red curve in Fig. 4(b), the GT values are larger than the experimental results, but the change tendency of G according to the cell temperature is consistent for both sets of results. The main causes of the observed discrepancies between the experimental and theoretical results are the inaccurate temperature measurement of Cs vapor and approximations of the simple three-level model relative to the actual atomic system. However, owing to the complete matching of the pump and signal beams, the experimental gain coefficient reaches about 3 cm−1, even though this is smaller than the GT value of 4 cm−1 at the cell temperature of 90 °C.The optical amplification from the Cs vapor cell depends on the signal power. Figure 5(a) shows the optical amplification as a function of the signal power in the range from 0.1 mW to 34 mW for a pump power of 200 mW and cell temperature of 90 °C. As the signal power is increases from 0.1 to 5.0 mW, the small-signal amplification exponentially decreases from 430 to 20.6. Although the dependence of the amplification on the signal power is similar to that described in a previous study for a pumping power of 12 W [17], we could obtain a highly efficient amplifier operation. The main reasons underlying the observed highly efficient amplifier operation are as follows: (1) Spatial-mode overlapping of the pump and signal lasers with the use of the single-mode optical fiber, and (2) the hyperfine transitions of the pump and signal lasers with narrow linewidths for realizing the optimal gain condition.
Fig. 5. Diode-powered alkali amplification (DPAA) as function of signal power and optical-to-optical conversion efficiency (OOCE). (a) Optical amplification as a function of the signal power; (b) DPAA output power as a function of the signal power; (c) OOCE of Cs vapor medium with respect to the pump power as a function of the cell temperature.
Meanwhile, we note that the optical-to-optical conversion efficiency of an alkali-vapor medium with respect to the pump power is important for realizing high quantum efficiency of a DPAA. Thus, we investigated the output power of the DPAA as a function of the signal power. The results are shown in Fig. 5(b). Although the optical amplification shown in Fig. 5(a) decreases with increase in the signal power, the DPAA output power increases and the optical-to-optical conversion efficiency (OOCE) for a fixed pump power of 200 mW reaches a large value of ∼70%, where OOCE is defined as the ratio of the output power of the DPAA to the input power of the pump laser. Furthermore, we compared the gain extraction efficiencies for cell temperatures of 84.5 °C (red squares) and 90.0 °C (blue circles). The change tendency of the OOCE according to the signal power is different between the two cases. Although we have reached the amplification factor of 430 and the OOCE of 70%, respectively, it is difficult to get high values of the OOCE and the amplification factor at once under the same experimental condition.
Figure 5(c) shows the OOCE as a function of the cell temperature in the range from 82 °C to 90 °C for a pump power of 200 mW. Interestingly, the OOCE at 84.5 °C is larger than that at 90 °C. Therefore, we could confirm that the temperature of the atomic vapor cell affects not only the small-signal amplification but also the OOCE.
However, our DPAA using the Cs vapor cell with antireflection-coated windows was realized for a weak pump power of 200 mW. In DPAA, the amplified spontaneous emission (ASE) efficiency increases as the pump power increases [19]. Under our experimental condition, we assumed that the ASE effect is weak, because the transition due to two-photon absorption or two-stepwise excitation is negligible. It is difficult to measure the noise properties due to ASE of single-pass amplifier in populated inverted media. On the other hand, the spectral property measurement of narrowband signal laser requires a spectrometer with megahertz-level spectral resolution; even so, that of the amplified narrowband signal light by a commercial optical spectrum analyzer is not easy. However, to investigate the spectral properties of the amplified signal laser, we have achieved the spectroscopy in a pure Cs vapor cell using the amplified signal laser from DPAA. Figure 6 shows the linear absorption (red curve) and the saturated absorption (blue curve) spectra of the laser transition in the 6S1/2(F = 4) − 6P1/2(F′ = 3 and 4) transition, when the optical frequency of the input signal laser is scanned. From the results of Fig. 6, we confirmed that the spectral feature of the amplified light is same with that of the input signal laser. Also, when the pump-laser power was set to be adjustable from 10 mW to 200 mW and the amplified signal power for spectroscopy was fixed to 0.5 mW, we couldn’t find the dependence of the spectral feature of the amplified signal light on the pump-laser power, because of our experimental condition of weak pump power. Furthermore, when the input signal power is tuned on and off, we can see that the output DPAA is strongly correlated with the input signal.
Fig. 6. Linear absorption spectrum (LAS; red curve) and saturated absorption spectrum (SAS; blue curve) of the amplified signal light in the 6S1/2(F = 4) − 6P3/2(F′ = 3 and 4) transition of the warm 133Cs atoms.
4. Conclusion
We experimentally investigated the optical amplification of small signals from a vapor cell filled with Cs atoms and ethane buffer gas to realize a highly efficient DPAA. We measured and compared the optical amplification for four cases of the two hyperfine states corresponding to the D2 transition for the pumping laser and the D1 transition for the signal laser. For pump and signal lasers with a narrow spectral width of 2 MHz being resonant on the hyperfine state (F = 4) of the 6S1/2 state, we determined the efficient gain conditions for DPAA with Cs atoms. In our experiment, to achieve a highly efficient DPAA, the pump and signal lasers were coupled into a single-mode optical fiber for spatial-mode overlapping in the Cs vapor cell and completely overlapped with each other and matched in size along the entire length of the cell. Furthermore, we investigated small-signal amplification as a function of the Cs vapor cell temperature, signal power, and pump power. From our experimental results, we found that the amplification factor and OOCE of the DPAA with Cs atoms reached 430 and 70%, respectively, under various experimental conditions. Although our DPAA using a Cs vapor cell was realized for a weak pump power of 200 mW, we could achieve a highly efficient DPAA. We believe that our results can contribute to a better understanding of the properties of highly efficient DPALs and DPAAs subjected to high pump powers.
Funding
Research fund of high efficiency laser laboratory of agency for defense development of Korea (UD160069BD).
References
1. W. F. Krupke, “Diode Pumped Alkali Laser,” U.S. Patent No. 6,643,311 (2001).
2. W. F. Krupke, R. J. Beach, V. K. Kanz, and S. A. Payne, “Resonance transition 795-nm rubidium laser,” Opt. Lett. 28(23), 2336–2338 (2003). [CrossRef]
3. F. Gao, F. Chen, J. Xie, D. Li, L. Zhang, G. Yang, J. Guo, and L. Guo, “Review on diode-pumped alkali vaporlaser,” Optik 124(20), 4353–4358 (2013). [CrossRef]
4. W. F. Krupke, “Diode pumped alkali lasers (DPALs)—A review (rev1),” Prog. Quantum Electron. 36(1), 4–28 (2012). [CrossRef]
5. B. V. Zhdanov and R. J. Knize, “Review of alkali laser research and development,” Opt. Eng. 52(2), 021010 (2012). [CrossRef]
6. W. F. Krupke, R. J. Beach, V. K. Kanz, and S. A. Payne, “DPAL: A new class of CW, near-infrared high power diode pumped alkali (vapor) lasers,” Proc. SPIE 5334, 156–167 (2004). [CrossRef]
7. R. J. Beach, W. F. Krupke, V. K. Kanz, and S. A. Payne, “End-pumped continous-wave alkali vapor lasers: experiment, model, and power scaling,” J. Opt. Soc. Am. B 21(12), 2151–2163 (2004). [CrossRef]
8. T. Ehrenreich, B. Zhdanov, T. Takekoshi, S. P. Phipps, and R. J. Knize, “Highly efficient optically pumped Cs vapor laser,” Opt. Commun. 260(2), 696–698 (2006). [CrossRef]
9. R. H. Page, R. J. Beach, V. K. Kanz, and W. F. Krupke, “Multimode-diode-pumped gas (alkali-vapor) laser,” Opt. Lett. 31(3), 353–355 (2006). [CrossRef]
10. Y. Wang, T. Kasamatsu, Y. Zheng, H. Miyajima, H. Fukuoka, S. Matsuoka, M. Niigaki, H. Kubomura, and H. Kan, “Cesium Vapor Laser Pumped by a Volume-Bragg-Grating Coupled Quasi-Continuous-Wave Laser Diode Array,” Appl. Phys. Lett. 88(14), 141112 (2006). [CrossRef]
11. Y. Wang, M. Niigaki, H. Fukuoka, Y. Zheng, H. Miyajima, S. Matsuoka, H. Kubomura, T. Hiruma, and H. Kan, “Approaches of output improvement for cesium vapor laser pumped by a volume-Bragg-grating coupled laser-diode-array,” Phys. Lett. A 360(4-5), 659–663 (2007). [CrossRef]
12. T. A. Perschbacher, D. A. Hostutler, and T. M. Shay, “High-efficiency diode-pumped rubidium laser: experimental results,” Proc. SPIE 6346, 634607 (2007). [CrossRef]
13. B. V. Zhdanov and R. J. Knize, “Diode pumped 10 Watts continous wave cesium laser,” Opt. Lett. 32(15), 2167–2169 (2007). [CrossRef]
14. B. V. Zhdanov, A. Stooke, G. Boyadjian, A. Voci, and R. J. Knize, “Laser diode array pumped continuous wave Rubidium vapor laser,” Opt. Express 16(2), 748–751 (2008). [CrossRef]
15. B. V. Zhdanov, A. Stooke, G. Boyadjian, A. Voci, and R. J. Knize, “Rubidium vapor laser pumped by two laser diode arrays,” Opt. Lett. 33(5), 414–415 (2008). [CrossRef]
16. S. Wu, T. Soules, R. Page, S. Mitchell, C. Scott, V. Kanz, and R. Beach, “Hydrocarbon-free resonance transition 795-nm rubidium laser,” Opt. Lett. 32(16), 2423–2425 (2007). [CrossRef]
17. B. V. Zhdanov and R. J. Knize, “Efficient diode pumped cesium vapor amplifier,” Opt. Commun. 281(15-16), 4068–4070 (2008). [CrossRef]
18. D. A. Hostutler and W. L. Klennert, “Power enhancement of a Rubidium vapor laser with a master oscillator power amplifier,” Opt. Express 16(11), 8050–8053 (2008). [CrossRef]
19. Z. Yang, H. Wang, Q. Lu, W. Hua, and X. Xu, “Modeling of an optically side-pumped alkali vapor amplifier with consideration of amplified spontaneous emission,” Opt. Express 19(23), 23118 (2011). [CrossRef]
20. B. Shen, B. Pan, J. Jiao, and C. Xia, “Kinetic and fluid dynamic modeling, numerical approaches of flowing-gas diode-pumped alkali vapor amplifiers,” Opt. Express 23(15), 19500 (2015). [CrossRef]
21. J. Yablon, Z. Zhou, M. Zhou, Y. Wang, S. Tseng, and M. S. Shahriar, “Theoretical modeling and experimental demonstration of Raman probe induced spectral dip for realizing a superluminal laser,” Opt. Express 24(24), 27444 (2016). [CrossRef]
22. Q. Zhu, B. L. Pan, L. Chen, Y. J. Wang, and X. Y. Zhang, “Analysis of temperature distributions in diode pumped alkali vapor lasers,” Opt. Commun. 283(11), 2406–2410 (2010). [CrossRef]
23. J. Zweiback, A. Komashko, and W. F. Krupke, “Alkali vapor lasers,” Proc. SPIE 7581, 75810G (2010). [CrossRef]
24. B. V. Zhdanov, M. K. Shaffer, and R. J. Knize, “Demonstration of a diode pumped continuous wave Potassium laser,” Proc. SPIE 7915, 791506 (2011). [CrossRef]
25. A. V. Bogachev, S. G. Geranin, A. M. Dudov, V. A. Yeroshenko, S. M. Kulikov, G. T. Mikaelian, V. A. Panarin, V. O. Pautov, A. V. Rus, and S. A. Sukharev, “Diode-pumped caesium vapor laser with closed-cycle laser-active medium circulation,” Quantum Electron. 42(2), 95–98 (2012). [CrossRef]
26. N. D. Zameroski, G. D. Hager, W. Rudolph, and D. A. Hostutler, “Experimental and numerical modeling studies of a pulsed rubidium optically pumped alkali metal vapor laser,” J. Opt. Soc. Am. B 28(5), 1088–1099 (2011). [CrossRef]
27. Z. N. Yang, H. Y. Wang, Q. S. Lu, L. Liu, Y. D. Li, W. H. Hua, X. J. Xu, and J. B. Chen, “Theoretical model and novel numerical approach of a broadband optically pumped three-level alkali vapour laser,” J. Phys. B: At., Mol. Opt. Phys. 44(8), 085401 (2011). [CrossRef]
28. R. J. Knize, B. V. Zhdanov, and M. K. Shaffer, “Photoionization in alkali lasers,” Opt. Express 19(8), 7894–7902 (2011). [CrossRef]
29. L. Ge, W. Hua, H. Wang, Z. Yang, and X. Xu, “Study on photoionization in a rubidium diode-pumped alkali laser gain medium with the optogalvanic method,” Opt. Lett. 38(2), 199–201 (2013). [CrossRef]
30. Z. Yang, L. Zuo, W. Hua, H. Wang, and X. Xu, “Experimental measurement of ionization degree in diode pumped rubidium laser gain medium,” Opt. Lett. 39(22), 6501–6504 (2014). [CrossRef]
31. W. Zhang, Y. Wang, H. Cai, L. Xue, J. Han, H. Wang, and Z. Liao, “Theoretical study on temperature features of a sealed cesium vapor cell pumped by laser diodes,” Appl. Opt. 53(19), 4180–4186 (2014). [CrossRef]
32. F. Chen, D. Xu, F. Gao, C. Zheng, K. Zhang, Y. He, C. Wang, and J. Guo, “Experimental investigation on a diode-pumped cesium-vapor laser stably operated at continuous-wave and pulse regime,” Opt. Express 23(9), 12414–12422 (2015). [CrossRef]
33. B. V. Zhdanov, G. Venus, V. Smirnov, L. Glebov, and R. J. Knize, “Continuous wave Cs diode pumped alkali laser pumped by single emitter narrowband laser diode,” Rev. Sci. Instrum. 86(8), 083104 (2015). [CrossRef]
34. M. Endo, R. Nagaoka, H. Nagaoka, T. Nagai, and F. Wani, “Output power characteristics of diode-pumped cesium vapor laser,” Jpn. J. Appl. Phys. 54(12), 122701 (2015). [CrossRef]
35. G. A. Pitz, D. M. Stalnake, E. M. Guild, B. Q. Olike, P. J. Moran, S. W. Townsend, and D. A. Hostutler, “Advancements in flowing diode pumped alkali lasers,” Proc. SPIE 9729, 972902 (2016). [CrossRef]
36. M. D. Rotondaro, B. V. Zhdanov, M. K. Shaffer, and R. J. Knize, “Narrowband diode laser pump module for pumping alkali vapors,” Opt. Express 26(8), 9792–9797 (2018). [CrossRef]
37. S. Hong, B. Kong, Y. S. Lee, S. Song, S. Kim, and K. Oh, “Pulse control in a wide frequency range for a quasi-continuous wave diode-pumped cesium atom vapor laser by a pump modulation in the spectral domain,” Opt. Express 26(20), 26679–26687 (2018). [CrossRef]
38. D. A. Steck, “Cesium D Line Data,” https://steck.us/alkalidata/cesiumnumbers.1.6.pdf
