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Optics Letters

Gain and noise characteristics of a continuous-wave Raman laser

Salim N. Jabr

Published Sep 1, 1987• 11 citations
Publisher

Abstract

The spectrum of the stimulated electronic Raman gain at 650 nm in Ne I, pumped by a 633-nm He-Ne cw laser in the same cavity, is derived. The question of the proper Raman-gain linewidth is addressed. It is shown that the gain has a nearly Doppler-free component whose width is measured as 30 +/- 10 MHz (FWHM) at 6 Torr. This value agrees reasonably well with the residual Doppler broadening but not with collisional broadening rates. A simple calculation is included that shows the quantum-limited frequency uncertainty of the 650-nm laser to be higher by a factor of 1.5 than the frequency uncertainty of the 633-nm laser at equal power.

DOI
10.1364/ol.12.000690

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A fresh look at continuous-wave lasers: How they really work!

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This lecture describes the operation principle of a continuous-wave (cw) laser. In order to keep the photon rate equation and its solution simple, usually the spontaneous-emission rate is neglected with the argument that it is so much smaller than the stimulated-emission rate. The direct consequence is that in a cw laser the gain would equal the losses. Yet, additional implications are that the light emitted by such a laser would be a pure sine wave with an infinite coherence length, its linewidth would become a delta function, its Q-factor would assume an infinite value, the coherent photon number would build up and coherence would manifest itself inside the resonator only when pumping above the laser threshold, and the threshold inversion would depend only on the total resonator losses. None of these implications holds true for any laser that mankind has ever created. Starting from vacuum fluctuations, we consider spontaneous emission directly in the photon rate equation, thereby a priori avoiding all these inconsistencies. It is then straight-forward to see that in a cw laser the gain is smaller than the losses, a cw laser does not only have a finite linewidth that can be derived in a very simple manner, but also a finite Q-factor, as well as two laser thresholds which are both different from the commonly assumed “threshold inversion‿. This work was performed in collaboration with Dr. Marc Eichhorn from the French-German Research Institute of St. Louis, France.

•Oct 1, 2013•0 citations

Stimulated collision-induced resonance Raman scattering

V.M. Arutunyan, N. Sh. Badanyan, N. V. Shakhnazaryan

The passage of strong monochromatic laser radiation through a resonance medium consisting of three-level atoms is investigated. It is shown that if collisions are taken into account in the scattered waves the resonances at combination frequencies occur to first order in the laser field intensity. A numerical analysis of the dependence of the stimulated electronic Raman scattering (SERS) components' gain factors on the pump frequency is carried out. The dispersion curves of the SERS components' gain (absorption) factors for fixed values of pump frequency and intensity are plotted.

Journal of Physics B Atomic and Molecular Physics•Oct 14, 1985•0 citations
The spectrum of the stimulated electronic Raman gain at 650 nm in Ne I, pumped by a 633-nm He-Ne cw laser in the same cavity, is derived. The question of the proper Raman-gain linewidth is addressed. It is shown that the gain has a nearly Doppler-free component whose width is measured as 30 +/- 10 MHz (FWHM) at 6 Torr. This value agrees reasonably well with the residual Doppler broadening but not with collisional broadening rates. A simple calculation is included that shows the quantum-limited frequency uncertainty of the 650-nm laser to be higher by a factor of 1.5 than the frequency uncertainty of the 633-nm laser at equal power.
The development of the Stokes spectrum and the dependence of Raman gain upon pump laser bandwidth in a narrow linewidth Raman medium are analyzed in terms of a closed set of multimode equations. The analysis predicts that the Stokes gain coefficient takes on its maximum value, which is independent of bandwidth, when the Stokes spectrum replicates the pump spectrum. It is also shown that in the absence of dispersion the spectrum of a weak Stokes wave is driven to duplicate the pump spectrum, and that for noise input the average power generated in the Stokes wave is independent of the pump bandwidth.
Dopplerons are the signature of the nonlinear response of moving atoms driven by a continuous standing-wave laser beam. They display maxima on the velocity-dependent atomic population difference at velocities nearly equal to ±(c/ω)[(δ(ω2 + 2Ω2)1/2/(2p + 1)], where Ω is the Rabi frequency associated with an individual beam field and δω is the detuning between the driving frequency and the center of the Doppler linewidth. Probing the device with a weak signal yields gain for Doppler broadenings of the magnitude of the detuning, exclusively on the atomic-resonance-frequency side, with a maximum shifted from the driving frequency by the generalized Rabi frequency associated with a standing wave, defined by (δω2 + 4Ω2)1/2. The existence of dopplerons is demonstrated by peaks occurring at frequencies shifted from the driving frequency by even harmonics of doppleron frequencies. The intensity of the peaks is shown to depend on the Doppler width, and the complexity of the gain curve is shown to increase as the pump intensity does.
This lecture describes the operation principle of a continuous-wave (cw) laser. In order to keep the photon rate equation and its solution simple, usually the spontaneous-emission rate is neglected with the argument that it is so much smaller than the stimulated-emission rate. The direct consequence is that in a cw laser the gain would equal the losses. Yet, additional implications are that the light emitted by such a laser would be a pure sine wave with an infinite coherence length, its linewidth would become a delta function, its Q-factor would assume an infinite value, the coherent photon number would build up and coherence would manifest itself inside the resonator only when pumping above the laser threshold, and the threshold inversion would depend only on the total resonator losses. None of these implications holds true for any laser that mankind has ever created. Starting from vacuum fluctuations, we consider spontaneous emission directly in the photon rate equation, thereby a priori avoiding all these inconsistencies. It is then straight-forward to see that in a cw laser the gain is smaller than the losses, a cw laser does not only have a finite linewidth that can be derived in a very simple manner, but also a finite Q-factor, as well as two laser thresholds which are both different from the commonly assumed “threshold inversion‿. This work was performed in collaboration with Dr. Marc Eichhorn from the French-German Research Institute of St. Louis, France.
The passage of strong monochromatic laser radiation through a resonance medium consisting of three-level atoms is investigated. It is shown that if collisions are taken into account in the scattered waves the resonances at combination frequencies occur to first order in the laser field intensity. A numerical analysis of the dependence of the stimulated electronic Raman scattering (SERS) components' gain factors on the pump frequency is carried out. The dispersion curves of the SERS components' gain (absorption) factors for fixed values of pump frequency and intensity are plotted.