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Optics Continuum

Bidirectional GHz bandwidth amplified spontaneous emission generation and high output power single-pass optical amplification through a hot Cs vapor cell

Mingyu Lee, Siheon Ryu, Seokjin Kim, Seongjin Hong, Yong Soo Lee, Dong‐Il Yeom, Kyunghwan Oh

Published Jan 13, 2022• 3 citations
Publisher

Abstract

We report the characteristics of GHz bandwidth amplified spontaneous emission (ASE) from a hot Cs atom vapor cell, where the optical feedback was inhibited. When pumped by an 852 nm laser, both forward and backward ASE output near 894 nm showed a nonlinear increase in its power without a pump power threshold. A continual decrease in spectral width down to 4.7 GHz was experimentally observed as the ASE output power increased. Using the same vapor cell, we injected a 1mW signal to configure a single-pass optical amplifier, and we monitored the forward output both in temporal and spectral domains. We found the signal laser efficiently suppressed the ASE and obtained a large amplification factor over 700 at the pump power of 1.2 W.

DOI
10.1364/optcon.444241

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arXiv:1701.00232•Journal of the Optical Society of America B•Nov 7, 2017•22 citations
We report the characteristics of GHz bandwidth amplified spontaneous emission (ASE) from a hot Cs atom vapor cell, where the optical feedback was inhibited. When pumped by an 852 nm laser, both forward and backward ASE output near 894 nm showed a nonlinear increase in its power without a pump power threshold. A continual decrease in spectral width down to 4.7 GHz was experimentally observed as the ASE output power increased. Using the same vapor cell, we injected a 1mW signal to configure a single-pass optical amplifier, and we monitored the forward output both in temporal and spectral domains. We found the signal laser efficiently suppressed the ASE and obtained a large amplification factor over 700 at the pump power of 1.2 W.
We have developed a laser source generated by frequency doubling of a seed laser at 1560 nm using a periodically poled lithium niobate crystal. The seed laser passes through a 30 W fiber amplifier. The laser source at 780 nm is used to perform Bloch oscillations of ultra-cold atoms in an optical lattice. In this paper we investigate the decoherence effects induced by the amplified spontaneous emission (ASE) in the gain medium. We show that the ASE drastically reduces the efficiency of Bloch oscillations. We totally filter out the ASE around the D2 line using a heated rubidium vapor cell. We precisely determine the power spectral density of the ASE by measuring the fraction of atoms transferred from the hyperfine level F=1 to F=2, when the laser is switched on.
We report high-efficiency optical amplification with near-extreme-limit gain from a diode-pumped Cs vapor cell. We used wavelength-division multiplexing to couple 852 nm pump and 895 nm seed lasers to achieve nearly overlapping spatial modes in the Cs vapor cell. We investigated the amplification factor as a function of the focal length of the lens focusing on the combined pump and seed signals and determined the optimal focal length under our experimental conditions. The small-signal amplification factor from the Cs vapor cell reached >30 dB at 240 mW pump power, and the optimal optical amplification factor per pump power was 4171/W.
Optical amplification by the stimulated emission of Cs(6p 2P3/2)-Ar atomic pairs, observed in pump-probe experiments over a ∼290 GHz-wide spectral region lying to the red of the Cs D2 line (852.1 nm), has been realized by photoexciting thermalized, ground state Cs-Ar atoms in the 834-849 nm wavelength interval. When the gain medium is pumped at the peak of the CsAr B2Σ1/2+←X2Σ1/2+ transition at 836.7 nm, maximum gain occurs between 852.2 nm and 852.3 nm and >28% of the energy stored in the upper laser level is extracted with 8 ns (FWHM) probe pulses in a single pass. From the measured rate of saturation of the extracted pulse energy with increasing probe intensity, the product of γ0L and Esat, the saturation pulse energy, is measured directly to be 400 ± 20 µJ and the lower limit for the saturation intensity (Isat) of this amplifier is estimated to be 10 kW-cm−2 at 852.2 nm. Circularly polarizing the optical pump beam increases the optical-to-optical conversion efficiency by 20%, and the storage lifetime of the upper laser level is observed from temporally-resolved gain spectra to be 5 ± 1 ns. Pump excitation spectra also reveal a significant contribution from Ar-Cs-Ar (3-body) photoassociation and suprathermal Ar atoms generated by the dissociation of the CsAr B2Σ1/2+ complex. Multipass-amplifier geometries with broad-bandwidth probe signals are expected to yield upper state energy extraction efficiencies above 50%. This alkali-rare gas amplifier demonstrates the efficiencies available with the storage of energy in, and optical extraction from, excited atomic collision pairs.
A semiclassical dynamic theory of the nonlinear propagation of a few interacting intense light pulses is applied to study the nonlinear counterpropagation of amplified spontaneous emission (ASE) induced by three-photon absorption of short intense laser pulses in a chromophore solution. Several important results from the modeling are reached for the ASE process developing in the regime of strong saturation. Accounting for ASE in both forward and backward directions with respect to the pump pulse results in a smaller efficiency of nonlinear conversion for the forward ASE compared with the case in which forward emission is considered alone, something that results from the partial repump of the absorbed energy to the backward ASE component; the overall efficiency is nevertheless higher than for the forward emission considered alone. The efficiency of nonlinear conversion of the pump energy to the counterpropagating ASE pulses is strongly dependent on the concentration of active molecules so that a particular combination of concentration versus cell length optimizes the conversion coefficient. Under certain specified conditions, the ASE effect is found to be oscillatory; the origin of oscillations is dynamical competition between stimulated emission and off-resonant absorption. This result can be considered one of the possible explanations of the temporal fluctuations of the forward ASE pulse [Nature 415, 767 (2002)].
(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.
Population inversion on the 5D5/2−6P3/2 transition in rubidium atoms produced by cw laser excitation at different wavelengths has been analyzed by comparing the generated mid-infrared radiation at 5.23 μm originating from amplified spontaneous emission (ASE) and isotropic blue fluorescence at 420 nm. Using a novel method for detection of two-photon excitation via ASE, we have observed directional co- and counter-propagating emission at 5.23 μm. Evidence of a threshold-like characteristic is found in the ASE dependences on laser detuning and Rb number density. The power dependences of the backward- and forward-directed emission can be very similar; however, their spectral dependences are not identical. A new type of temporally and spatially coherent radiation is suggested.