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Physical Review A

Frequency-asymmetric gain profile in a seeded Raman amplifier

Kevin S. Repasky, John L. Carlsten

Published Nov 1, 1996• 7 citations
Publisher

Abstract

This paper examines the effect of index guiding on Raman gain. The slowly varying Maxwell wave equation including both the real and imaginary parts of the Raman susceptibility for a seeded Raman amplifier is explored. Using a Gauss-Laguerre mode expansion for the Stokes field, the output Stokes energy is numerically studied as a function of gain and detuning from the Raman resonance. The calculations indicate that the real part of the Raman susceptibility causes the Raman medium to act as a lens when the Stokes seed is detuned from the Raman resonance. This focusing effect leads to higher peak Stokes energy when the Stokes seed is tuned to the blue side of the Raman resonance. Specifically for Raman scattering in ${\mathrm{H}}_{2}$ with a pump laser at 532 nm and an input seed near 683 nm, the peak Stokes energy can shift by as much as 300 MHz from the Raman resonance. An experiment which confirms these predictions is also presented. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.54.4528

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Measurements of the output of a Raman amplifier when there is no input Stokes seed are presented.It is shown that a fully quantum mechanical scaled planewave theory describes the growth from spontaneous emission well in the high gain limit.However, in the low gain regime there is a significant increase in the amount of Stokes output which is not predicted by the plane wave theory.To account for the deviation from planewave growth observed at low gain, a fully quantum mechanical theory of amplification in a focused gain geometry is presented.The theory is based on a non-orthogonal mode expansion of the amplified field and incorporates the effects of both gain-guiding and diffraction.In addition to accounting for the observed Stokes output at low gain the non-orthogonal mode theory predicts that the Stokes field cart experience enhanced gain as well as have excess noise.Results from an experiment are presented which verify the existence of enhanced gain in a multipass Raman amplifier.Though the experiment does not give a direct measure of the excess noise factor the results

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Gain-guiding and quantum noise in a transient Raman amplifier

Philip R. Battle

Gain-guiding in an optical amplifier can have a significant effect on the quantum noise properties of the amplified signal. It has been demonstrated, both theoretically and experimentally, that gain-guiding in a steady state Raman amplifier can lead to excess noise in the output signal. Experiments using a Raman amplifier operating in the extreme transient regime also indicate that gain-guiding can have a significant effect on the output Stokes field. In this paper, a theoretical description of transient stimulated Raman scattering in a focused gain geometry is developed. The theory accounts for diffraction, gain-guiding, and quantum initiation.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 10, 1996•0 citations
This paper examines the effect of index guiding on Raman gain. The slowly varying Maxwell wave equation including both the real and imaginary parts of the Raman susceptibility for a seeded Raman amplifier is explored. Using a Gauss-Laguerre mode expansion for the Stokes field, the output Stokes energy is numerically studied as a function of gain and detuning from the Raman resonance. The calculations indicate that the real part of the Raman susceptibility causes the Raman medium to act as a lens when the Stokes seed is detuned from the Raman resonance. This focusing effect leads to higher peak Stokes energy when the Stokes seed is tuned to the blue side of the Raman resonance. Specifically for Raman scattering in H2{\mathrm{H}}_{2}H2​ with a pump laser at 532 nm and an input seed near 683 nm, the peak Stokes energy can shift by as much as 300 MHz from the Raman resonance. An experiment which confirms these predictions is also presented. \textcopyright{} 1996 The American Physical Society.
Measurements of the output of a Raman amplifier when there is no input Stokes seed are presented.It is shown that a fully quantum mechanical scaled planewave theory describes the growth from spontaneous emission well in the high gain limit.However, in the low gain regime there is a significant increase in the amount of Stokes output which is not predicted by the plane wave theory.To account for the deviation from planewave growth observed at low gain, a fully quantum mechanical theory of amplification in a focused gain geometry is presented.The theory is based on a non-orthogonal mode expansion of the amplified field and incorporates the effects of both gain-guiding and diffraction.In addition to accounting for the observed Stokes output at low gain the non-orthogonal mode theory predicts that the Stokes field cart experience enhanced gain as well as have excess noise.Results from an experiment are presented which verify the existence of enhanced gain in a multipass Raman amplifier.Though the experiment does not give a direct measure of the excess noise factor the results
We have studied the phase pulling and locking of an injected Stokes pulse in a transient Raman amplifier both experimentally and theoretically. In addition, we have investigated the pulse energy statistics and related spatial modes of a high gain amplified Stokes pulse. Experiments reported here show that in the unsaturated case, the phase of the amplified Stokes is pulled almost instantaneously to that of the pump, as soon as integrated gain levels of slightly greater than unity are reached. For higher input seed-Stokes levels, such that the amplified Stokes energy is saturated, the phase locking is completed early, and the phase of the amplified Stokes then follows the phase excursion of the pump. The phase pulling has been modelled theoretically for the specific parameters of our experiment. We have also used the pulse energy statistics to study the seed-Stokes control of a high gain Raman amplifier. For an input seed-Stokes beam to control a transient, high gain Raman amplifier, operated near threshold, it must dominate the self-generated Raman which builds from quantum noise. For a near-unity-Fresnel-number Raman amplifier we have measured the number of input seed-Stokes photons needed to produce an unchanged statistical signature on the amplified Stokes pulse and have found that a surprisingly large number (109) were needed. For a larger Fresnel number Raman amplifier, a near-field camera was used to study the level at which the seed-Stokes pulse controlled the spatial output of the amplifier. Control of the spatial modes seems to have a sharp threshold with input Stokes energy and was fully established with an input of 2x103 photons per mode.
The exact resonance of pump and Stokes with the Raman transition is essential for successful generation of Raman solitons.1 Even slight detuning (~1 % of the Raman linewidth) leads to rapid soliton decay. Viewed in frequency space, this is due to an asymmetric gain profile for the injected Stokes seed. Through the frequency dependence of gain and Raman dispersion, this leads to the rapid growth of an out-of-phase component of the Stokes beam, which does not participate in the transient reversal of pump depletion. The asymmetric gain profile can be compensated by suitably modifying the total phase shift of the injected seed, resulting in an enhancement of those frequency components that see lower gain. We show that almost complete soliton stability can be achieved this way.
We have previously reported experimental results of Raman amplification of a low-light-level seed-Stokes beam in the transient limit.1 Here we present results of measurements which used a 10-nsec seed-injected Nd:YAG laser. The 532-nm frequency-doubled output had a duration of 8.5 ns and was used both to generate a 4.5-ns duration seed-Stokes pulse at 683 nm and to pump the 1-m long Raman amplifier. The seed-Stokes beam was spatially filtered, apertured at the Airy minimum, and collimated before the amplifier at a diameter of 5.2 mm. Likewise, the spatial mode of the pump was optimized to be as smooth as possible and was collimated into the Raman amplifier at a diameter of 3.4 mm. With no seed Stokes incident in the amplifier, random spatial patterns characteristic of the quantum-noise initiated, self­generated Stokes light were observed over a spatially gain-narrowed beam size supporting ~ 12 spatial modes. For the Raman gains of ~105 used in these experiments the number of temporal modes was 1.8. The seed-Stokes light was used to illuminate a bar chart of line-width 0.315 mm, and either a direct image of the bar chart, or its Fourier transform, was amplified in the Raman amplifier. The original bar chart image was then observed using an intensified camera. As the level of seed-Stokes was reduced to that approaching one photon per spatial-temporal mode, we were able to observe intensity modulations approaching 100% within the amplified bar chart images. Modulation levels of a few percent persist out to input levels of ~103 photons per mode. This observation is consistent with an interpretation of interference between electric field of the seed-Stokes beam and the vacuum fluctuations present in the amplifier but is inconsistent with a photon number description where the maximum predicted modulation would be only 50%.
Gain-guiding in an optical amplifier can have a significant effect on the quantum noise properties of the amplified signal. It has been demonstrated, both theoretically and experimentally, that gain-guiding in a steady state Raman amplifier can lead to excess noise in the output signal. Experiments using a Raman amplifier operating in the extreme transient regime also indicate that gain-guiding can have a significant effect on the output Stokes field. In this paper, a theoretical description of transient stimulated Raman scattering in a focused gain geometry is developed. The theory accounts for diffraction, gain-guiding, and quantum initiation.