We report the efficient generation of 100-W single-frequency radiation with diffraction-limited beam quality at the 1064-nm wavelength by use of a master-oscillator fiber power-amplifier system, consisting of a diode-pumped monolithic nonplanar ring laser as the master oscillator and an Yb-doped large-mode-area fiber as the power amplifier. The emission spectrum, the intensity noise behavior, and further power-scaling possibilities to the >200-W level, which are determined by the threshold of stimulated Brillouin scattering in the fiber amplifier, are discussed.

100-W single-frequency master-oscillator fiber power amplifier.

Here we demonstrate theoretically that stimulated Brillouin scattering (SBS) can induce a phase shift of the optical carrier relative to its sidebands due to the waveguiding effect of the optical fiber on the acoustic wave. This causes conversion of frequency modulation to intensity modulation, which results in an increase in the relative intensity noise and degradation of the modulation response of directly modulated lasers after propagation in an optical fiber, in agreement with our experimental observations. Suppression of SBS can be achieved at low frequencies and high modulation powers due to the laser adiabatic chirp.

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Degradation of modulation and noise characteristics of semiconductor lasers after propagation in optical fiber due to a phase shift induced by stimulated Brillouin scattering

We studied an anomalous result in the measured stimulated Brillouin scattering threshold for single-mode optical fibers with different refractive index profiles and effective areas. It was found that, in addition to the optical effective area, the refractive-index-dependent interaction between the optical and the acoustic fields must be considered in order to account for the differences in the stimulated Brillouin scattering threshold between fiber types. A detailed analytical formalism describing the acousto-optic interaction was derived to accommodate the influence of the index profile design on Brillouin scattering.

Brillouin gain analysis for fibers with different refractive indices.

We report on theoretical and experimental investigations on spontaneous and stimulated Brillouin scattering during operation of a high-power single-frequency polarization-maintaining ytterbium doped fiber amplifier. For different amplifier configurations with co- and counter-propagating seed and pump radiation the evolution of Brillouin scattering spectra was investigated with a heterodyne detection scheme. Spontaneous Brillouin gain spectra at low powers were additionally investigated using a pump-probe technique. The data obtained from these experiments have been compared with a theoretical model based on coupled intensity equations. A Brillouin scattering suppression has been investigated theoretically and experimentally with externally applied temperature gradients along the fiber resulting in up to 3.5 dB suppression and 115 W of amplifier output power.

Brillouin scattering spectra in high-power single-frequency ytterbium doped fiber amplifiers.

We report a master-oscillator fiber power-amplifier system consisting of a diode-pumped monolithic nonplanar ring laser as the master oscillator and a Yb-doped large-mode-area double-clad fiber as the power amplifier. The system emits up to 20.1  W of single-frequency radiation at a wavelength of 1064  nm with diffraction-limited beam quality M2≤1.3. The optical emission spectrum and amplitude-noise behavior are investigated. Furthermore, the power-scaling possibilities are discussed.

Single-frequency master-oscillator fiber power amplifier system emitting 20 W of power.

We experimentally investigate the optical noise induced by stimulated Brillouin scattering (SBS) in single-mode fibers. The noise, which is caused by the random nature of spontaneous Brillouin scattering, is induced in the transmitted wave as well as in the backward Brillouin wave. In fibers where the Brillouin gain spectra consist of several resonances, the induced intensity noise of the transmitted wave has a wide spectrum and it may be the dominant noise in the receiver for a broad radio frequency band (0-1 GHz). This unexpected result is explained by a multiple scattering process which is caused by stimulated and by thermally excited phonons. The SBS induced noise may cause a performance degradation in analog optical systems which require high optical powers and low noise. Using fibers with a single Brillouin resonance may limit the noise to lower frequencies.

M. Horowitz

Papers

Broad-band transmitted intensity noise induced by Stokes and anti-Stokes Brillouin scattering in single-mode fibers