Abstract

Optical spectrum is one of the most important characteristics of a laser radiation. Many spectral techniques are proposed and developed for spectral measurement, which are based on diffraction gratings, heterodyne or stimulated Brillouin scattering (SBS) amplification. For the last one, the signal radiation is amplified via SBS when frequency difference between pump wave and signal under test corresponds to Brillouin shift in a fiber (~10 GHz) within the bandwidth of the Brillouin gain window (~20 MHz) [1]. A spectrum under test can be reconstructed through scanning of the pump wave frequency. One should note that unamplified signal components being outside the SBS gain window propagate to the output with amplified components and thereby can deteriorate the dynamic range in the optical spectrum analyzer (OSA). This fact complicates the detection a weak spectral component in the presence of stronger one at a closely spaced frequency. To suppress the influence of unamplified signal the technique of polarization pulling is can be applied [2]. In this case, the specific polarization state matching is used. In the present paper, another way of separation of amplified and unamplified signals is proposed. We suggest to modulate intensity of tunable pump radiation similarly to lock-in detection.

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