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Photonics

Four-Wave Optical Parametric Amplification in a Raman-Active Gas

Yuichiro Kida, Totaro Imasaka

Published Aug 31, 2015• 1 citation
Publisher

Abstract

Four-wave optical parametric amplification (FWOPA) in a Raman-active medium is experimentally investigated by use of an air-filled hollow fiber. A femtosecond pump pulse shorter than the period of molecular motion excites the coherent molecular motion of the Raman-active molecules during the parametric amplification of a signal pulse. The excited coherent motion modulates the frequency of the signal pulse during the parametric amplification, and shifts it to lower frequencies. The magnitude of the frequency redshift depends on the pump intensity, resulting in intensity-dependent spectral characteristics that are different from those in the FWOPA induced in a noble-gas-filled hollow fiber.

DOI
10.3390/photonics2030933

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Physical Review A•Oct 18, 2004•60 citations
Four-wave optical parametric amplification (FWOPA) in a Raman-active medium is experimentally investigated by use of an air-filled hollow fiber. A femtosecond pump pulse shorter than the period of molecular motion excites the coherent molecular motion of the Raman-active molecules during the parametric amplification of a signal pulse. The excited coherent motion modulates the frequency of the signal pulse during the parametric amplification, and shifts it to lower frequencies. The magnitude of the frequency redshift depends on the pump intensity, resulting in intensity-dependent spectral characteristics that are different from those in the FWOPA induced in a noble-gas-filled hollow fiber.
Summary form only given. The development of the Raman gas-filled hollow core photonic crystal fibers allow to exacerbate the light-matter interaction with extremely high efficiency. Among the nonlinear optical effect that were extensively explored we count stimulated Raman scattering, with demonstrations like over 5-octaves wide Raman comb using H2-filled inhibited coupling guiding HC-PCF pumped with a picosecond pulsed laser [1]. Furthermore, Raman-active gases are known to exhibit very weak susceptibility outside the Raman frequencies, and thus are considered to be poor media for the generation of new frequencies through parametric four-wave mixing (FWM).Here we demonstrate, for the first time to our knowledge, the generation of pairs of sidebands via parametric FWM in strongly driven Raman molecular gas. The experimental set-up used a 1030 nm Yb-fiber laser with average power of 15 W, a repetition rate of 1 MHz and pulse duration of 27 ps. The light is then coupled into a photonic microcell composed by a D2-filled inhibited coupling tubular-lattice single-ring HC-PCF [2]. The fiber exhibits a low-loss broadband guidance with a minimum of 10 dB/km around the pump wavelength and operates in almost a single mode regime with a mode field diameter of 28 μm. The D2 molecule has a vibrational Raman resonance around 89.8 THz, and rotational resonances of 12.4 THz for the ortho-D2 and 7.1 THz for the para-D2. The pressure of D2 was set at 40 bar to experimentally maximize the Raman lines from ortho-D2. The results are shown in the figure 1. The generated spectrum presents around 100 laser lines over a 200 THz wide spectral span with a dense cluster whose frequency-difference is under 2 THz by a ro-vib SRS/FWM combination. Four FWM sidebands can be identified (plotted in red color), generated in both sides of two para-rotational lines reproduced eight times around the pump radiation and three times around the first vibrational Stokes.
Recent progress has been made in the theoretical and experimental study of the amplification of optical signals without population inversion (AWI) of the participating electronic levels. Induced coherence among appropriate levels in the active medium is the necessary condition for obtaining AWI. Similarly, the generation of wave packet states in atoms and molecules also relies on the coherent superposition of several states. It is feasible, therefore, that AWI is possible in a system which has been excited into a wave packet state. We have thus demonstrated experimentally such amplification in an extended sample of sodium dimers and we term this process wave packet amplification (WPA). An important characteristic of WPA is that the gain is doubly phase-sensitive—it is modulated at both the wave packet oscillation period and the optical period. The former time-dependence is a result of the periodic revivals of the pump-induced electronic polarization and the latter optical phase-sensitivity is a manifestation of the coherent nature of the molecular wave packet. The experiments to be described use a time-resolved upconversion method to detect the change in energy of a 50 fs probe pulse after it passes through a sodium heat pipe that has been pumped with another 50 fs pulse. The maximum saturated energy gain in this system is four and corresponds to all of the available energy being transferred from the wave packet to the probe pulse.
Experimental results on parametric optical processes in saturable ytterbium-doped fibers (YDFs) at the wavelength of 1064 nm are reported. Significant photoinduced change of the refractive index under saturation of the fiber-optical absorption in the near-infrared range enables us to consider this medium as an effective optical Kerr ( χ ′′′ ) medium. For pumping, it needs continuous-wave Nd:YAG laser power of 10-mW scale, but the saturation process has the characteristic time ≤1ms, governed by the metastable level lifetime in YDF. We investigate the nearly degenerate collinear configuration of the optical parametric amplification (OPA). Here we focus on unidirectional transformation of the initial amplitude modulation in the incident pump wave to the output phase modulation as a manifestation of OPA in a narrowband case. This effect results from the conventional coupled-wave equations for the signal and idler waves—if one considers them as the modulation sidebands. It can also be treated as a result of the self-phase modulation of the periodically modulated pump in the Kerr medium. OPA gain of about 0.64 with the net gain ≈0.29 was experimentally estimated for the utilized 2-m-long YDF in the frequency band ≈200Hz.
Abstract We present results of a theoretical analysis of an optical parametric amplification (OPA) in a collinear nearly degenerate four-wave mixing configuration close to the resonance of a two-level transition in χ ′ ′ ′ media. It is performed both for homogeneously and for inhomogeneously broadened spectral line using a standard set of Bloch equations in the semiclassical approximation. Unlike the traditional approach of a coupled propagation of the signal/idler waves, we treat OPA as a coupled propagation of the amplitude and phase modulations of the pump wave with the signal/idler waves as the modulation sidebands. We show that for low frequency off-sets between the pump and the signal/idler waves, OPA process is basically manifested in a unidirectional transformation of the input amplitude modulation of the pump wave into the output phase modulation. The frequency band of the OPA is determined by detuning the pump wave from the resonance transition. The obtained results are applied to evaluate the expected OPA gain for the acetylene-filled micro-structured fiber with the resonance vibration-rotational transitions in the 1500–1550 nm spectral range.
Backward stimulated Raman scattering in gases provides a promising route to compression and amplification of a Stokes seed-pulse by counter-propagating against a pump-pulse, as has been already demonstrated in various platforms, mainly in free-space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report the first unambiguous observation of efficient noise-seeded backward stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber. At high gas pressures, when the backward Raman gain is comparable with, but lower than, the forward gain, we report quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm from a narrowband 532-nm-pump. The efficiency increases to 65% when the backward process is seeded by a small amount of back-reflected forward-generated Stokes light. At high pump powers the backward Stokes signal, emitted in a clean fundamental mode and spectrally pure, is unexpectedly always stronger than its forward-propagating counterpart. We attribute this striking observation to the unique temporal dynamics of the interacting fields, which cause the Raman coherence (which takes the form of a moving fine-period Bragg grating) to grow in strength towards the input end of the fiber. A good understanding of this process, together with the rapid development of novel anti-resonant-guiding hollow-core fibers, may lead to improved designs of efficient gas-based Raman lasers and amplifiers operating at wavelengths from the ultraviolet to the mid-infrared.
Hollow-core photonic-crystal fibers are shown to offer the unique possibility of coherent excitation and probing of Raman-active vibrations in molecules by isolated air-guided modes of electromagnetic radiation. A $3\text{\ensuremath{-}}\mathrm{cm}$ section of a hollow photonic-crystal fiber is used to prepare isolated air-guided modes of pump and probe fields for a coherent excitation of $2331{\text{\ensuremath{-}}\mathrm{cm}}^{\ensuremath{-}1}$ QQQ-branch vibrations of molecular nitrogen in the gas filling the fiber core, enhancing coherent anti-Stokes Raman scattering through these vibrations by a factor of 15 relative to the regime of tight focusing.