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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

420nm alkali blue laser based on two-photon absorption

Yannan Tan, Yimin Li, Tong Liu, Faquan Gong, Chunyan Jia, Shu Hu, Baodong Gai, Jingwei Guo, Wanfa Liu

Published Feb 3, 2015• 0 citations
Publisher

Abstract

Based on two-photon absorption, a 420nm blue laser of alkali Rb vapor was demonstrated, and a dye laser was used as the pumping laser. Utilizing the energy level structure of Rb atom, lasering mechanism and two-photon absorption process are analyzed. Absorbing two 778.1nm photons, Rb atoms were excited from 52 S1/2 to 52 D5/2, then relaxed to 62 P3/2 with mid infrared photon radiation. 420nm blue laser was achieved by the transition 62 P3/2→52 S1/2. To improve efficiency of the blue laser, two-photon resonant excitation pumped alkali vapor blue lasers are proposed, which will be good beam quality, high efficiency and scalable blue lasers. The development of diode pumped alkali vapor blue laser is expected.

DOI
10.1117/12.2065373

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Frontiers of Physics•Mar 6, 2023•7 citations
Based on two-photon absorption, a 420nm blue laser of alkali Rb vapor was demonstrated, and a dye laser was used as the pumping laser. Utilizing the energy level structure of Rb atom, lasering mechanism and two-photon absorption process are analyzed. Absorbing two 778.1nm photons, Rb atoms were excited from 52 S1/2 to 52 D5/2, then relaxed to 62 P3/2 with mid infrared photon radiation. 420nm blue laser was achieved by the transition 62 P3/2→52 S1/2. To improve efficiency of the blue laser, two-photon resonant excitation pumped alkali vapor blue lasers are proposed, which will be good beam quality, high efficiency and scalable blue lasers. The development of diode pumped alkali vapor blue laser is expected.
Achieving population inversion through multi-photon cascade pumping is almost always difficult, and most laser medium work under 1-photon excitation mechanism. But for alkali atoms such as cesium, relatively large absorption cross sections of several low, cascading energy levels enable them properties such as up conversion. Here we carried out research on two-photon excitation alkali fluorescence. Two photons of near infrared region are used to excite alkali atoms to n 2 D5/2, n 2 D3/2 or higher energy levels, then the blue fluorescence of (n+1) 2 P3/2,(n+1) 2 P1/2→n 2 S1/2 are observed. Different pumping paths are tried and by the recorded spectra, transition routes of cesium are deducted and concluded. Finally the possibility of two-photon style DPALs (diode pumped alkali laser) are discussed, such alkali lasers can give output wavelengths in the shorter end of visual spectroscopy (400-460 nm) and are expected to get application in underwater communication and material laser processing.
Based on an analysis of kinetic processes in the continuous-wave (CW) single-wavelength two-photon pumped Rb-vapor laser (Rb-TPAL), a three-dimensional computational model is established using the rate equation. Based on the application of the model to cavity-enhanced Rb-TPAL [Yuanet al.,Optics Express29, 4858 (2021)], the calculated and measured dependences of the laser power on the pump power show good agreement. We simulate the Rb-TPAL pumped by a high-power CW laser diode and analyze the effects of temperature, vapor cell length, pump waist, vapor cell transmission, and output mirror reflectance on the characteristics of the blue laser. The results suggest that the output power and efficiency of blue laser can be improved by optimizing the above parameters; however, the particles in the52D5/25^{2}\text{D}_{5/2}52D5/2​level face difficulty to transition to the62P3/26^{2}\text{P}_{3/2}62P3/2​level, and this limits the further improvement of blue laser efficiency; therefore, an approach is proposed to improve the efficiency of Rb-TPAL. This study can be useful for designing a two-photon pumped alkali metal laser with high-power output.
Alternative wavelengths for optically pumped alkali vapor lasers have been developed using single photon excitation of higher lying P-states, stimulated Raman processes, two-photon excitation of S and D states, and electric quadruple excitation on S-D transitions. Two photon excitation of Cs 72D leads to competing and cascade lasing producing red and infrared lasers operating on the D-P transitions, followed by ultraviolet, blue, the standard near infrared DPAL transitions operating on P-S transitions. The S-D pump transitions are fully bleached at pump intensities exceeding 1 MW/cm2, allowing for lasing transitions that terminate on the ground state. The kinetics of these systems are complex due to competition for population inversion among the many optical transitions. An optically pumped mid-infrared rubidium pulsed, mirrorless laser has also been demonstrated in a heat pipe along both the 62P3/2 - 62S1/2 transition at 2.73 μm and the 62P1/2 - 62S1/2 transition at 2.79 μm with a maximum energy of ~100 nJ. Performance improves dramatically as the rubidium vapor density is increased, in direct contradiction with the prior work. No scaling limitations associated with energy pooling or ionization kinetics have been observed. Practical application for infrared counter measures depends on the further development of blue diode pump sources. Finally, stimulated electronic Raman scattering and hyper-Raman processes in potassium vapor near the D1 and D2 lines have been observed using a stable resonator and pulsed laser excitation. First and second order Stokes and anti-Stokes lines were observed simultaneously and independently for a pump laser tuning range exceeding 70 cm-1. When the pump is tuned between the K D1 and D2 lines, an efficient hyper-Raman process dominates with a slope efficiency that exceeds 10%. Raman shifted laser may be useful as a target illuminator or atmospheric compensation beacon for a high power diode pumped alkali laser.
Abstract : The Doppler-broadened two-photon absorption (TPA) cross-section for the 52S1/2- 52D5/2 transition in Rb is measured using direct absorption methods. A detailed model of the intensity profile was developed to account for a focused Gaussian beam propagating through a two-photon absorption medium. A peak absorbance of 24% was observed for an intensity of 6.28 kW/cm2 at the focus, a Rb density of 4.6 x 1015 cm-3 concentrations from 1.61-8.52 x 10(15) cm-3 line. Extracting the hyperfine-broadened TPA cross-section from 87 test configurations, while varying the pump power, alkali concentration and focal length, yielded an error weighted average of 6.75x10-21 cm4/W. This cross-section is sufficient for a pulsed dye laser to bleach the pump transition in the Two-Photon Pumped Alkali Laser (TPAL) that lases at 420 nm and 5.2 mum. Additionally, optically pumped atomic rubidium lasers pumped in the blue satellite of the D2 line from the ground Rb-Ar or Rb-Kr collision pair to the dissociative B2 sum 1/2 produce laser emission at 780.2 nm. Lasing is achieved for the Rb-Ar system and 757.1 - 760.4 nm for the Rb-Kr system. Slope efficiencies exceed 0.25% using a heat pipe configuration. The gain is very high with photon build-up times of 1-3.7 ns.
The four-wave mixing (FWM) process is an important method for generating collimated blue light from alkali vapor. In this paper, we introduced high-pressure He gas to broaden the absorption linewidth of Rb atoms for the pump laser. By using an optical parametric oscillator (OPO) that outputs tunable wavelength pump laser, we performed two-photon excitation of the Rb-He mixture and achieved redshifted and asymmetric broadened collimated light (CL) around 420 nm. We demonstrated the participation of Rb-He excimers in the FWM process for the first time, to the best of our knowledge, from both spectral and temporal dimensions, and revealed the underlying mechanism. Experiments showed that the peak of the ∼420 nm CL intensity appeared at higher heating temperatures, influenced by changes in the optical depth of the Rb-He mixture. This study provides both theoretical and experimental support for the promotion of a collimated blue light scheme in alkali metal -buffer gas systems, balancing the pump laser absorption and FWM efficiency.
Diode-pumped alkali lasers, possessing high efficiency and narrow linewidth, can provide feasible solutions for wavelength ranges difficult to reach by commercial lasers. In this study, we investigate a generation of coherent blue light (CBL) via four-wave mixing (FWM)-based up-conversion processes in cesium (Cs) vapor. A bichromatic pumping scheme with 852- and 917-nm lasers drives the Cs atoms to the 6 D 5/2 excited level, followed by cascaded decay of 6 D 5/2 → 7 P 3/2 → 6 S 1/2 , producing 456-nm CBL under phase matching conditions. The fluorescence in multiple bands from blue to near- and far-infrared in the FWM process is demonstrated under different experimental conditions. To optimize the experimental parameters, we investigate the dependence of 456-nm CBL on the vapor temperature, frequency, and intensity of the two pump lasers. A maximum power of 2.94 mW is achieved with pump powers of 430 mW (for 852 nm) and 470 mW (for 917 nm). The corresponding conversion efficiency is 1.5%/W, three-fold higher than those in previous studies. Our results can contribute to fundamental research on atom—photon interactions and quantum metrology.