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Optics & Laser Technology

Green to blue light conversion through resonant four wave mixing in rubidium vapor

Robert T. Randolph, Eric Finberg, Richard B. Miles

Published Dec 22, 2025• 0 citations

Abstract

This work experimentally demonstrates the conversion of coherent and incoherent green 532 nm light to collimated blue 420 nm light through resonant four-wave mixing in rubidium vapor. The 532 nm light co-propagates with 795 nm and 1320 nm CW pump beams, simultaneously taking advantage of resonances between the 5 P, 10 S, 6 P and 5 S ground states of rubidium. Using a coherent green beam, the intensity of the collimated 420 nm signal is characterized under varying polarizations, cell temperatures, frequency detunings and pump intensities. A 2 d image is up-converted from green to blue light to extend the technique to spatially incoherent light. This excitation pathway has the potential to be used as a narrow linewidth ( < 1 GHz), tunable bandpass filter for lidar or other remote sensing applications. • Narrow linewidth of 532 nm signal up-converted to blue light using four wave mixing. • Demonstration of image up-conversion using hot Rb vapor. • A third applied IR pump beam increases conversion efficiency.

DOI: 10.1016/j.optlastec.2025.114508

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This work experimentally demonstrates the conversion of coherent and incoherent green 532 nm light to collimated blue 420 nm light through resonant four-wave mixing in rubidium vapor. The 532 nm light co-propagates with 795 nm and 1320 nm CW pump beams, simultaneously taking advantage of resonances between the 5 P, 10 S, 6 P and 5 S ground states of rubidium. Using a coherent green beam, the intensity of the collimated 420 nm signal is characterized under varying polarizations, cell temperatures, frequency detunings and pump intensities. A 2 d image is up-converted from green to blue light to extend the technique to spatially incoherent light. This excitation pathway has the potential to be used as a narrow linewidth ( < 1 GHz), tunable bandpass filter for lidar or other remote sensing applications. • Narrow linewidth of 532 nm signal up-converted to blue light using four wave mixing. • Demonstration of image up-conversion using hot Rb vapor. • A third applied IR pump beam increases conversion efficiency.

Pulsed collimated blue light at 420.3 nm is generated in hot Rb vapor by upconverting the 778.10 nm pumping beam through four wave mixing process. The energy conversion efficiency exceeds 1% when a 45 cm-long, 170°C heated Rb cell is used. The influence of cell temperature, wavelength, and energy of a pumping laser are fully examined. The efficiency of the photon conversion is found to be more sensitive to the blue detuning of the pump light and less sensitive to the red detuning of the pump light. This phenomenon can be explained by stimulated hyper-Raman scattering involved in the four-wave mixing process.

This work introduces an approach to generate coherent 420 nm (blue) light utilizing narrowband, CW pump beams at 795 nm, 532 nm and 1320 nm in a rubidium vapor cell. The frequencies giving the strongest signal intensity are found. The method is compared to other transition pathways that generate 420 nm light in rubidium vapor, namely using the 776 nm and 780 nm absorption lines. The approach discussed in this work has the potential to be used in a diagnostic measurement capability as a means to spectrally filter a component of a scattered signal.

Pulsed collimated blue light at 420.3 nm is generated in hot Rb vapor by upconverting the 778.10 nm pumping beam through four wave mixing process. The energy conversion efficiency exceeds 1% when a 45 cm-long, 170 degrees C heated Rb cell is used. The influence of cell temperature, wavelength, and energy of a pumping laser are fully examined. The efficiency of the photon conversion is found to be more sensitive to the blue detuning of the pump light and less sensitive to the red detuning of the pump light. This phenomenon can be explained by stimulated hyper-Raman scattering involved in the four-wave mixing process.

Utilizing two-photon excitation in hot Rb vapor we demonstrate the generation of collimated optical fields at 420 and 1324 nm. Input laser beams at 780 and 776 nm enter a heated Rb vapor cell collinear and circularly polarized, driving Rb atoms to the 5D5/2 state. Under phase-matching conditions coherence among the 5S1/2 → 5P3/2 → 5D5/2 → 6P3/2 transitions produces a blue (420 nm) beam by four-wave mixing. We also observe a forward and backward propagating IR (1324 nm) beam, due to cascading decays through the 6S1/2 → 5P1/2 states. Power saturation of the generated beams is investigated by scaling the input powers to greater than 200 mW, resulting in a coherent blue beam of 9.1 mWpower, almost an order of magnitude larger than previously achieved. We measure the dependences of both beams in relation to the Rb density, the frequency detuning between Rb ground-state hyperfine levels, and the input laser intensities. (C) 2014 Optical Society of America

Utilizing two-photon excitation in hot Rb vapor we demonstrate the generation of collimated optical fields at 420 and 1324 nm. Input laser beams at 780 and 776 nm enter a heated Rb vapor cell collinear and circularly polarized, driving Rb atoms to the 5D(5/2) state. Under phase-matching conditions coherence among the 5S(1/2)→5P(3/2)→5D(5/2)→6P(3/2) transitions produces a blue (420 nm) beam by four-wave mixing. We also observe a forward and backward propagating IR (1324 nm) beam, due to cascading decays through the 6S(1/2)→5(1/2) states. Power saturation of the generated beams is investigated by scaling the input powers to greater than 200 mW, resulting in a coherent blue beam of 9.1 mW power, almost an order of magnitude larger than previously achieved. We measure the dependences of both beams in relation to the Rb density, the frequency detuning between Rb ground-state hyperfine levels, and the input laser intensities.

We report a study on the transfer of intensity correlation properties in frequency upconversion via four-wave mixing (FWM) in rubidium (Rb) vapor. The 5S1/2−5P3/2−5D5/2 two-photon transition in Rb85 leads to collimated blue light (CBL) generation at 420 nm by absorbing two input photons of wavelengths 780 nm and 776 nm. The g(2)(τ) intensity correlation measurement of the CBL field reveals that the intensity correlations of the input fields are transferred to the CBL field due to strong atomic coherence in the FWM loop. We also demonstrate that the measured values of g(2)(τ) of the CBL field are reduced by the effect of Doppler broadening of Rb vapor.