Open Access
Add to BibSonomy
Abstract
A Faraday anomalous dispersion optical filter (FADOF) could lock high-power diode lasers to atomic resonance lines with ultra-narrow bandwidth. However, the polarization sensitivity of the Faraday filter limits its applications since the standard diode module often employs polarization combination to increase pumping brightness. We proposed a polarization-insensitive mutual injection configuration to solve this problem and locked a standard polarization combined diode module to Rb D2-line. The laser bandwidth was narrowed from 4 nm to 0.005 nm (2.6 GHz, FWHM) with 38.3 W output and an external cavity efficiency of 80%. This FADOF-based polarization-insensitive external-cavity scheme would find many applications, such as high energy atomic gas laser pumping (alkali lasers, metastable rare gas lasers) and quantum optics, etc.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
For pumping applications, such as spin-exchange optical pumping (SEOP) [1]and high energy diode pumped gas lasers, including diode pumped alkali lasers (DPALs) [2–6], diode pumped metastable rare gas lasers (DPRGLs) [7–11], etc., the key element is the high-power wavelength-locked narrowband laser diode. Generally, the main scheme for narrowing high-power diode are holographic plane gratings or volume Bragg gratings (VBGs) in an external cavity. For the plane grating, the external cavity diode laser (ECDL) could obtain an ultra-narrow spectrum and stable frequency [12,13]. However, the cavity loss, grating damage, and optics complexity limit the power and efficiency. For the VBG, this scheme is the current state-of-art method for linewidth narrowing on high-power diode laser with high external cavity efficiency [14–19]. But VBG method needs temperature control to stabilize the wavelength, which leads complexity for laser stacks with muliti-bars [20,21]. FADOF is an optic filter that has transmission peaks centered at atomic resonant transitions. Its operation principle could be briefly summed as follows: The polarization rotation originates from the Zeeman splitting of the atomic transitions, which causes a difference in index of refraction for right and left circular polarized light. This effect is similar to the regular Faraday effect but is much enhanced in the vicinity of absorption lines. The degree of rotation is strongly wavelength dependent and thus a pair of crossed polarizers in combination with a gas cell could act as a sensitive transmission filter. When FADOF as a frequency selecting element in diode external cavity, the signal is transmitted by the Faraday filter and can create feedback for the diode emitters, resulting lasing in a narrow spectral range. Detailed descriptions of the principle of FADOFs could be found in [22–25]. As an alternative method, FADOF’s performances sets a balance between the plane grating and VBG by combining the advantages of both and shows great potential for high-power pumping conditions needing high efficiency, especially for DPALs.
Up to now, the FADOF has achieved the wavelength coverage from ultraviolet to the communication band [26–28]. FADOF has been used in external cavity low-power single emitter diodes, also called Faraday laser [29–31], and widely used in free-space optical communication, atomic clock, and quantum technology [32–40]. Besides, FADOF could also be used in the high-power diode laser. In 2018, M. D. Rotondaro et al. firstly succeeded in locking a high-power diode stack precisely to the Cs D2 transition line with a record power of 518 W and an external cavity efficiency of 80%, which showed the potential of the Faraday laser on DPAL pumping [41]. However, the high intensity would bring a large thermal load in the vapor cell, which would quickly cause damages to the cell windows. In 2021, our team alleviated this problem and finally achieved a CW output power of 18 W with an external cavity efficiency of 80%. The central wavelength of the diode module is precisely locked to the Rb D2 transition line, with a spectral bandwidth of 1.2 GHz [42]. However, due to the principle and structure of FADOF, the Faraday laser is naturally suitable for linear polarized diodes, this will limit the application of Faraday laser in higher power diode laser, particularly for standard fiber tailed diode module, which generally employs the polarization combination to further increase the brightness.
In this paper, to improve the scaling ability of high-power Faraday laser, we proposed an Rb FADOF-based ring structure to narrow and lock polarization combining diode module. We precisely locked the diode module to the Rb D2 transition line, with a spectral bandwidth of 2.6 GHz. The system achieves a CW output power of 38.3W with an external cavity efficiency of 80%.
2. Experimental design and results
We proposed a polarization-insensitive diode module by an 87Rb FADOF-based ring configuration. The experimental setup is shown in Fig. 1. The critical component in this system is the FADOF-based ring optic loop, which consists of a polarization beam splitter (PBS), a right-angle prism, two high-reflection mirrors, and the FADOF.
Fig. 1. The top view of the schematic diagram of the FADOF-based polarization insensitive mutual injection configuration diode laser. The brief structure of the polarization combing diode laser is shown in a blue dotted box. The red and blue lines respectively represent the path of the spatial overlapping two beam components, which both are useful output by polarization combining optics. The dot or double-arrow symbols represent the orthogonal polarization of incident light. A red dotted box represents the feedback loss. BS, non-polarization beam splitter; PBS, polarization beam splitter; HR, high-reflective mirror; RP, right-angle prism; HWP, half-wave plate; PM, power meter; MT1 and MT2, permanent cylindrical neodymium magnet.
The (Everbright) polarization combining diode laser consists of total 18 single emitters. The emitters in each column are connected by reflect-mirror in a 9×1 standard stepped combination configuration and further coupled by a PBS. The facet of the emitters is anti-reflective coated, with a residual reflectivity of less than 0.5%, to suppress the competition of the free-running mode. The central wavelength of the diode laser locates near 780 nm with a nominal spectral linewidth of 4 nm (FWHM). The output of diode laser is 47 W power at a driven current of 5 A.
The diode laser beam was collimated by a confocal telescope with two achromatic convex lenses, which is not shown in Fig. 1. A PBS with a high extinction ratio (103:1) divided the total output laser spectra into horizontal and vertical polarized paths. The horizontally polarized beam in the diode module was showed with a red line with pointing arrows in Fig. 1. After passing the PBS, the horizontally polarized light transformed to a vertically polarized light under the action of the FADOF, which was similar to the Faraday effect but only appears in the atomic resonance region. Finally, after passing through the entire optic loop, the beam at the non-resonance area passed through the PBS, whereas the beam at the resonance region was reflected by PBS and returned to the emitters in the diode module. For the vertically polarized light in the diode module, which was showed with a blue line with a pointing arrow in Fig. 1, the process was the same as the vertically polarized components described above. In the case of such mutual injection, each emitter of the polarized beam combination diode module was narrowed and locked at the 87Rb resonance line. The modern automatic package process of this stepped combination structure in the diode laser module could easily control the pointing error to ensure a better mutual injecting effect. By precise adjustment, each emitter could receive its feedback light.
Figure 2(a) shows how the transmission spectra were measured and configuration of the FADOF was showed in the dotted box. The alkali vapor cell was filled with pure 87Rb without buffer gas. The cell length is 20 mm, the diameter is 25.4 mm, and the windows are anti-reflection coated on the exterior surfaces. The cell is heated by a resistance wire, which has a temperature control accuracy of 1°C. The magnetic field is generated with a pair of permanent cylindrical neodymium magnets. The axial magnetic field profile is from two permanent cylindrical neodymium magnets (outer diameter 80.0 mm, inner diameter 55.0 mm, thickness 10.0 mm), which was shown in Fig. 2(b). The positive and the negative value represent the opposite direction of the axial magnetic field, and the value at the center of the vapor cell is 111 Gauss, which are measured by a digital Gaussmeter with an accuracy of 0.1 Gauss. The orthogonal polarizing beam splitters (PBS1 and PBS2) with a high extinction ratio (103:1) are placed on ends of the Rb cell.
Fig. 2. The measurement arrangement of the FADOF and the axial magnetic field profile in FADOF. (a) BS, a beam splitter; PBS1 and PBS2, two orthogonal polarizers; HR, high-reflective mirror; MT, permanent cylindrical neodymium magnet; PD, photodiode. (b) the magnetic field profile between two permanent cylindrical neodymium magnets (axial extent marked by a blue shading), separated by 30 mm with a 20 mm vapor cell placed between them (purple shading).
Figure 2(a) shows the FADOF spectrum of transmittance measuring experiment setup. A 780 nm single-frequency distributed Bragg reflection (DBR) probe laser (DBR801-780, UniQuanta) was used to measure spectral characteristics of the FADOF. The weak probe light was divided into two parts by a beam splitter (BS), where one part entered the FADOF, and the other part passed an independent vapor cell as a frequency reference. By tuning the chip temperature of the probe laser, its wavelength can be continuously tuned across the 52S1/2→52P3/2 transition of 87Rb without mode hopping. Since the cell length and the magnetic strength has been set, the cell temperature was adjusted to optimize the performance of the FADOF.
The transmission spectrum of the FADOF and the absorption of the alkali atom at typical temperatures were shown in Fig. 3, which presents the hyperfine structure resolved transitions of the 87Rb D2 line. As the temperature (Rb density) increased, the absorption at the resonant transitions dramatically enhanced (red line), and the transmittance and the number of transmitted peaks also increased obviously (blue line). At temperature of 40°C, the filter transmission peaked at the center of the resonance line which had a low transmission. At higher temperatures, the spectra shifted to the margin of the hyperfine resonances, presenting a higher transmittance and a narrow envelope.
Fig. 3. Spectral properties of the FADOF corresponding to the 52S1/2 →52P3/2 transition of 87Rb. The blue line represents the transmitted signal of the FADOF, the red line represents the transmission of the 87Rb cell used inside the FADOF, and the purple line represents the transmission of the 87Rb cell without an axial magnetic field. Subfigures represent the results for cell temperature of 40°C, 80°C, and 120°C.
The optimized temperature of the FADOF was set at 80°C. At this temperature value, the peak transmittance approaches ∼90%, and the spectral width of the central transmitted envelope is less than 3 GHz, which is enough for DPAL pumping, etc. It should be noted that the transmittance of the FADOF here don’t include the optical loss of the cell windows, which mainly come from the un-coated inner surfaces of the cell.
The narrowed and locked light was coupled out by a non-polarization BS. Three different splitting ratios (R: T = 50:50, 80:20, 90:10) practiced in this paper. The diode spectra at a driven current of 5A, measured by an optical spectra analyzer (OSA) with a resolution and accuracy uncertainty of 0.02 nm, are shown in Fig. 4. The BS with a splitting ratio of 50:50 and 80: 20 both obtain an ultra-narrowed spectrum, whereas the former had a higher side mode suppression ratio (SMSR), shown in Fig. 4(b). During this optic feedback process, the loss appears due to the decreased injecting light. For the 90:10 splitting ratio, the part of the actual back injected light will be less than 1%, which is too weak to gain a significant advantage in mode competition. The final result was that the spectrum has a prominent peak at 780.23 nm but had exceptionally high side modes. When the reflection of BS decreased, the SMSR is slightly improved, but the output power and the slope efficiency observably decreased. The thermal load in the alkali vapor cell also increased due to the increased energy entered into the alkali vapor cell from the BS. Our purpose is to find a balance between spectral quality and output power, the third concern is the less thermal load in the alkali cell, which may cause damage to the Rb cell for a long-time operation. A non-polarization beam splitter with a reflectivity of 80% in 700-1100 nm was selected to output the narrowed and locked light.
Fig. 4. (a) The narrowed spectra about beam splitters with beam splitting ratio of (R: T) 50:50, 80:20, and 90:10 at a driven current of 5.0 A and cooling temperature of T = 35°C. (b) The compared narrowed spectra in a log scale at the driven current of 5.0 A and cooling temperature of T = 35°C (90:10).
The spectra of the free-running and the locked diode laser, measured by an optical spectra analyzer (OSA) with a resolution of 0.02 nm, are presented in Fig. 5(a). The diode laser has a relatively broad-spectrum (∼ 4 nm, FWHM), which centers at ∼ 781 nm under a driven current of 5.0A and cooling temperature of 35°C. When the FADOF was added, the central wavelength was locked to the atomic line, and the spectral linewidth is dramatically narrowed. A high-resolution spectrum of the laser was measured with a scanning Fabry-Perot interferometer, see Fig. 5(b), the free spectral range is 10 GHz with a resolution of 67 MHz in the free-running case. The fine spectrum shows a trimodal characteristic due to the multiple peaks of the FADOF and the mode competition. The two side modes had a frequency deviation of 2.2 GHz (∼0.004 nm) and 4GHz (∼0.008 nm). The FWHM spectral linewidth of the center peak was as narrow as ∼2.6 GHz (0.005 nm). Although the multiple modes exist, it is enough to the Rb DPAL (due to the existence of atmospheric pressure buffer gas, the Rb atom absorption linewidth is collisionally broadened to 20∼100 GHz (FWHM)). Figure 6 shows the spectrum in the log scale. A SMSR of 20 dB with more than 90% of energy was contained in the center peak.
Fig. 5. (a) Measured spectra of the free-running and the locked diode laser. The spectra are presented in the linear scale at driven current of I = 5.0 A and cooling temperature of T = 35°C. (b) The Fabry-Perot spectrum of the FADOF-based mutual injection configuration.
Fig. 6. Narrowed total spectrum in log scale at driven current of I = 5.0 A and cooling temperature of T = 35°C.
The stability of central wavelength of this FADOF-based diode module at different conditions was studied and showed in Fig. 7, it could be seen that the central wavelength was ultra-stable and centered around 780.23 nm over a wide cooling temperature from 28 to 38°C and a current from 1.5 to 5.0 A. The wavelengths deviation error is much less than the OSA resolution and accuracy uncertainty of 0.02 nm, which means a stable wavelength within the limits of experimental error. The high wavelength stability, plus the ultra-narrowed bandwidth is crucial for highly efficient pumping of DPAL, etc.
Fig. 7. Stability of the FADOF-based mutual injection configuration diode system. The central wavelength measured by optical spectra analyzer. (a) The red squares represent the central wavelength at current range from 1.5 to 5.0 A and constant temperature of 35°C. (b) The blue squares represent the central wavelength at temperature range from 28 to 38°C and constant driven current of 3.0 A.
The output power of both the free-running and the locked diode laser is shown in Fig. 8. With a feedback fraction of ∼4%, the system realized a maximal output of 38.3 W at a driven current of 5.0 A, which shows an 80% external cavity efficiency. The slope efficiency slightly dropped from the free running 11.6 W/A to the locked case 9.2 W/A because of the intrinsic loss in the optical loop and the BS, particularly, the inner surfaces loos of the Rb cell, and the aberrations of the imaging optics.
Fig. 8. External cavity efficiency of the FADOF-based mutual injection configuration diode system. Output power of the free running (blue) and the locked (pink) LDAs with respect to driven currents.
For further scaling to high power, further improvement should be taken. Firstly, the parameters, e.g., the alkali concentration, the magnetic field, the cell length, etc., should be systematically optimized to maximize the transmittance of the FADOF, which could further increase the external cavity efficiency and reduce the thermal load. Secondly, the FADOF transmission peak enveloping under high intensity would differ to weak probe light cases due to the optical pumping effect, the performance of the FADOF needs to be re-estimated.
3. Conclusion
In conclusion, we demonstrated a 38.3 W ultra-narrow diode laser locked to the Rb D2 line with a FADOF-based mutual injection configuration. The work expanded the FADOF-based method to the case of non-polarized diode lasers, which increased the scaling ability of FADOF method in high-power diode spectrum narrowing. The FADOF-based diode lasers could be well used in many applications, including DPAL, DPRGL pumping and quantum optics etc.
Acknowledgments
We acknowledge Dr. Hao Yu (Suzhou Everbright Photonics Co., Ltd.) for the valuable technical support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. 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]
2. E. Yacoby, I. Auslender, K. Waichman, O. Sadot, B. D. Barmashenko, and S. Rosenwaks, “Analysis of continuous wave diode pumped cesium laser with gas circulation: experimental and theoretical studies,” Opt. Express 26(14), 17814–17819 (2018). [CrossRef]
3. M. Endo, R. Nagaoka, H. Nagaoka, T. Nagai, and F. Wani, “Modeling of diode-pumped cesium vapor laser bycombination of computational fluid dynamics and wave-optics,” Jpn. J. Appl. Phys. 59(2), 022002 (2020). [CrossRef]
4. G. A. Pitz, D. M. Stalnaker, E. M. Guild, B. Q. Oliker, P. J. Moran, S. W. Townsend, and D. A. Hostutler, “Advancements in flowing diode pumped alkali lasers,” Proc. SPIE 9729, 972902 (2016). [CrossRef]
5. B. V. Zhdanov, M. D. Rotondaro, M. K. Shaffer, and R. J. Knize, “Potassium Diode Pumped Alkali Laser demonstration using a closed cycle flowing system,” Opt. Commun. 354, 256–258 (2015). [CrossRef]
6. J. Han and M. C. Heaven, “Gain and lasing of optically pumped metastable rare gas atoms,” Opt. Lett. 37(11), 2157–2159 (2012). [CrossRef]
7. P. A. Mikheyev, A. K. Chernyshov, M. I. Svistun, N. I. Ufimtsev, O. S. Kartamysheva, M. C. Heaven, and V. N. Azyazov, “Transversely optically pumped Ar:He laser with a pulsed-periodic discharge,” Opt. Express 27(26), 38759–38767 (2019). [CrossRef]
8. J. Han, M. C. Heaven, P. J. Moran, G. A. Pitz, E. M. Guild, C. R. Sanderson, and B. Hokr, “Demonstration of a CW diode-pumped Ar metastable laser operating at 4 W,” Opt. Lett. 42(22), 4627–4630 (2017). [CrossRef]
9. W. T. Rawlins, K. L. Galbally-Kinney, S. J. Davis, A. R. Hoskinson, J. A. Hopwood, and M. C. Heaven, “Optically pumped microplasma rare gas laser,” Opt. Express 23(4), 4804–4813 (2015). [CrossRef]
10. Z. Yang, G. Yu, H. Wang, Q. Lu, and X. Xu, “Modeling of diode pumped metastable rare gas lasers,” Opt. Express 23(11), 13823–13832 (2015). [CrossRef]
11. T. G. Walker and W. Happer, “Spin-exchange optical pumping of noble-gas nuclei,” Rev. Mod. Phys. 69(2), 629–642 (1997). [CrossRef]
12. B. V. Zhdanov, T. Ehrenreich, and R. J. Knize, “Narrowband external cavity laser diode array,” Electron. Lett. 43(4), 221–222 (2007). [CrossRef]
13. J. F. Sell, W. Miller, D. Wright, B. V. Zhdanov, and R. J. Knize, “Frequency narrowing of a 25 W broad area diode laser,” Appl. Phys. Lett. 94(5), 051115 (2009). [CrossRef]
14. F. W. Hersman, J. H. Distelbrink, J. Ketel, J. Wilson, and D. W. Watt, “Power scaling of a wavelength-narrowed diode laser system for pumping alkali vapors,” Proc. SPIE 9729, 972905 (2016). [CrossRef]
15. A. Gourevitch, G. Venus, V. Smirnov, D. A. Hostutler, and L. Glebov, “Continuous wave, 30 W laser-diode bar with 10 GHz linewidth for Rb laser pumping,” Opt. Lett. 33(7), 702–704 (2008). [CrossRef]
16. Z. Yang, H. Wang, Q. Lu, W. Hua, and X. Xu, “An 80-W Laser Diode Array with 0.1 nm Linewidth for Rubidium Vapor Laser Pumping,” Chin. Phys. Lett. 28(10), 104202 (2011). [CrossRef]
17. A. Podvyaznyy, G. Venus, V. Smirnov, O. Mokhun, V. Koulechov, D. Hostutler, and L. Glebov, “250W diode laser for low pressure Rb vapor pumping,” Proc. SPIE 7583, 758313 (2010). [CrossRef]
18. T. Koenning, D. McCormick, D. Irvin, D. Stapleton, T. Guiney, and S. Patterson, “Narrow-line fiber-coupled modules for DPAL pumping,” Proc. SPIE 9348, 934805 (2015). [CrossRef]
19. T. Koenning, D. McCormick, D. Irwin, D. Stapleton, T. Guiney, and S. Patterson, “DPAL pump system exceeding 3kW at 766 nm and 30 GHz bandwidth,” Proc. SPIE 9733, 97330E (2016). [CrossRef]
20. T. Koenning, D. Irwin, D. Stapleton, R. Pandey, T. Guiney, and S. Patterson, “Narrow line diode laser stacks for DPAL pumping,” Proc. SPIE 8962, 89620F (2014). [CrossRef]
21. C. Ebert, T. Guiney, J. Braker, D. Stapleton, K. Alegria, and D. Irwin, “High-power pump diodes for defense applications,” Proc. SPIE 10192, 101920D (2017). [CrossRef]
22. M. D. Rotondaro, B. V. Zhdanov, and R. J. Knize, “Generalized treatment of magneto-optical transmission filters,” J. Opt. Soc. Am. B 32(12), 2507–2513 (2015). [CrossRef]
23. E. T. Dressler, A. E. Laux, and R. I. Billmers, “Theory and experiment for the anomalous Faraday effect in potassium,” J. Opt. Soc. Am. B 13(9), 1849–1858 (1996). [CrossRef]
24. J. Xiong, L. Yin, B. Luo, and H. Guo, “Analysis of excited-state Faraday anomalous dispersion optical filter at 1529 nm,” Opt. Express 24(13), 14925–14933 (2016). [CrossRef]
25. J. Halpern, B. Lax, and Y. Nishina, “Quantum theory of interband Faraday and Voigt effects,” Phys. Rev. 134(1A), A140–A153 (1964). [CrossRef]
26. Y. Ohman, “On some new auxiliary instruments in astrophysical research VI. A tentative monochromator for solar work based on the principle of selective magnetic rotation,” Stockholm Obs. Ann. 19, 9–11 (1956).
27. Q. Sun, W. Zhuang, Z. Liu, and J. Chen, “Electrodeless-discharge-vapor-lamp-based faraday anomalous-dispersion optical filter,” Opt. Lett. 36(23), 4611–4613 (2011). [CrossRef]
28. Q. Sun, Y. Hong, W. Zhuang, Z. Liu, and J. Chen, “Demonstration of an excited-state Faraday anomalous dispersion optical filter at 1529 nm by use of an electrodeless discharge rubidium vapor lamp,” Appl. Phys. Lett. 101(21), 211102 (2012). [CrossRef]
29. X. Miao, L. Yin, W. Zhuang, B. Luo, A. Dang, J. Chen, and H. Guo, “Note: Demonstration of an external-cavity diode laser system immune to current and temperature fluctuations,” Rev. Sci. Instrum. 82(8), 086106 (2011). [CrossRef]
30. Z. Tao, X. Zhang, D. Pan, M. Chen, C. Zhu, and J. Chen, “Faraday laser using 1.2 km fiber as an extended cavity,” J. Phys. B: At., Mol. Opt. Phys. 49(13), 13LT01 (2016). [CrossRef]
31. P. Chang, H. Peng, S. Zhang, Z. Chen, B. Luo, J. Chen, and H. Guo, “A Faraday laser lasing on Rb 1529 nm transition,” Sci. Rep. 7(1), 8995 (2017). [CrossRef]
32. D. Wang, Y. Wang, Z. Tao, S. Zhang, Y. Hong, W. Zhuang, and J. Chen, “Cs 455 nm Nonlinear Spectroscopy with Ultra-narrow Linewidth,” Chin. Phys. Lett. 30(6), 060601 (2013). [CrossRef]
33. J. Tang, Q. Wang, M. Duan, Y. Li, L. Zhang, J. Gan, J. Kong, Y. Wu, and L. Zheng, “Experimental study of a novel free-space optical communication system,” Opt. Eng. 33(11), 3758–3762 (1994). [CrossRef]
34. J. Tang, Q. Wang, M. Duan, Y. Li, L. Zhang, J. Gan, M. Duan, J. Kong, and L. Zheng, “Experimental study of a model digital space optical communication system with new quantum devices,” Appl. Opt. 34(15), 2619–2622 (1995). [CrossRef]
35. X. Liu, X. Chen, X. Yao, W. Yu, G. Zhai, and L. Wu, “Lensless ghost imaging with sunlight,” Opt. Lett. 39(8), 2314–2317 (2014). [CrossRef]
36. P. Chang, H. Shi, J. Miao, T. Shi, D. Pan, B. Luo, H. Guo, and J. Chen, “Frequency-stabilized Faraday laser with 10−14 short-term instability for atomic clocks,” Appl. Phys. Lett. 120(14), 141102 (2022). [CrossRef]
37. X. Shan, X. Sun, J. Luo, Z. Tan, and M. Zhan, “Free-space quantum key distribution with Rb vapor filters,” Appl. Phys. Lett. 89(19), 191121 (2006). [CrossRef]
38. K. Choi, J. Menders, P. Searcy, and E. Korevaar, “Optical feedback locking of a diode laser using a cesium Faraday filter,” Opt. Commun. 96(4-6), 240–244 (1993). [CrossRef]
39. J. Keaveney, W. J. Hamlyn, C. S. Adams, and I. G. Hughes, “A single-mode external cavity diode laser using an intra-cavity atomic Faraday filter with short-term linewidth < 400 kHz and long-term stability of <1 MHz,” Rev. Sci. Instrum. 87(9), 095111 (2016). [CrossRef]
40. Z. Tao, Y. Hong, B. Luo, J. Chen, and H. Guo, “Diode laser operating on an atomic transition limited by an isotope 87Rb Faraday filter at 780 nm,” Opt. Lett. 40(18), 4348 (2015). [CrossRef]
41. 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]
42. H. Tang, H. Zhao, R. Wang, L. Li, Z. Yang, H. Wang, W. Yang, K. Han, and X. Xu, “18W ultra-narrow diode laser absolutely locked to the Rb D2 line,” Opt. Express 29(23), 38728–38736 (2021). [CrossRef]
