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Development of a Spectral Filtering Technique for Lidar Applications Through Generation of a 420 nm Coherent Beam

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

Published Apr 9, 2026• 0 citations
Publisher

Abstract

Methods to separate and filter Mie, Rayleigh and Raman scattering are required in many atmospheric lidar systems and other laser diagnostic applications. This work reports on progress towards a novel filtering technique to spectrally separate components of a backscattered signal through coherent light generation in an atomic vapor cell. Specifically, this work characterizes 420 nm (blue) light generation in a heated rubidium cell arising from the mixing of 776 nm and 780 nm pump beams. The work finds the optimal pump frequencies and cell temperature and detects a signal using pump beams as low as 0.7 μW. Progress towards using the technique to detect scattered light in a lidar application is described.

DOI
10.1051/epjconf/202636201033/pdf

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Methods to separate and filter Mie, Rayleigh and Raman scattering are required in many atmospheric lidar systems and other laser diagnostic applications. This work reports on progress towards a novel filtering technique to spectrally separate components of a backscattered signal through coherent light generation in an atomic vapor cell. Specifically, this work characterizes 420 nm (blue) light generation in a heated rubidium cell arising from the mixing of 776 nm and 780 nm pump beams. The work finds the optimal pump frequencies and cell temperature and detects a signal using pump beams as low as 0.7 μW. Progress towards using the technique to detect scattered light in a lidar application is described.
Methods to separate and filter Mie, Rayleigh and Raman scattering are required in many atmospheric lidar systems and other laser diagnostic applications. This work reports on progress towards a novel filtering technique to spectrally separate components of a backscattered signal through coherent light generation in an atomic vapor cell. Specifically, this work characterizes 420 nm (blue) light generation in a heated rubidium cell arising from the mixing of 776 nm and 780 nm pump beams. The work finds the optimal pump frequencies and cell temperature and detects a signal using pump beams as low as 0.7 μW. Progress towards using the technique to detect scattered light in a lidar application is described.
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.
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.
Precise wind velocity measurements are of interest for a variety of applications including weather forecasting and detection of clean air turbulence and wind shear. This work investigates a concept known as spectral hole burning to create an actively tunable atomic vapor filter in order to improve the resolution of LIDAR wind velocity profiling. By adjusting the frequency and intensity of a pump beam in an alkali vapor cell, steep slopes in the transmission profile may be used to monitor shifts in frequency of a probe beam. A three-level atomic model to simulate the spectral hole is described and validated experimentally through pump-probe spectroscopy. The model then optimizes parameters including cell length, cell temperature, pump intensity and pump frequency to find the steepest transmission slope obtainable. At this location, frequency resolutions of 0.69 MHz are predicted per 1% change in transmission, corresponding to a 0.27 m/s velocity sensitivity. From this information, the feasibility and limitations of using spectral hole burning as a controllable atomic filter for LIDAR velocimetry measurements are discussed.
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.
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.