Gain dependence of the linewidth of Brillouin amplification in optical fibers

Abstract

We report a novel, pump-power dependent, linewidth broadening effect in stimulated Brillouin amplification of a continuous-wave probe by a pulsed pump. This behavior is different from the case of two interacting continuous-wave pump and probe fields, where the shape of the logarithmic Brillouin gain spectrum is independent of the pump power. Studying this effect numerically and experimentally and also analytically, we find that for a given width of the pump pulse the Brillouin linewidth grows linearly with the Brillouin logarithmic gain with a slope, which inversely depends on the pulse width. Thus, for example, in a standard single-mode fiber, a 15ns pump pulse, strong enough to generate a gain of 0.5dB, broadens the logarithmic lineshape by ~1.5MHz, while a 45ns pulse, providing the same gain, increases the linewidth by only ~0.5MHz. Since the rising and falling slopes of the shape of the Brillouin gain spectrum are also gain dependent, this effect might challenge the calibration of Brillouin distributed slope-assisted sensing techniques.

Gain dependence of measured spectra in coherent Brillouin optical time-domain analysis sensors

“…3. The general trends and linewidth broadening agree well with the results presented, 4 and hence independently confirming the existence of this effect.…”

“…We started by confirming experimentally the existence of the gain dependence of the Brillouin linewidth in conventional BOTDA systems. 4 Notice that the setup in Fig. 1(b) can be used for conventional BOTDA measurements by simply removing the phase modulator and RF demodulation module.…”

“…1, 2 However, recently, a previously unknown effect in stimulated Brillouin scattering (SBS) interaction has been discovered, by which the linewidth of the BGS has been found to depend on the pump pulse power. 4 This unexpected finding means that BFS measurement error can appear in techniques such as the slope-assisted BOTDA that rely on the shape of the BGS. 5 We have recently proposed a dynamic BOTDA sensor tolerant to Brillouin gain changes in the sensing fiber.…”

Gain Dependence of the Phase-Shift Spectra Measured in Coherent Brillouin Optical Time-Domain Analysis Sensors

“…We perform this investigation employing large pump power and Brillouin gain because no significant shape change was observed when moderate gain was used in our previous investigation. In addition to our study of shape alterations in coherent BOTDA, we also independently confirm the existence of the Brillouin gain linewidth dependence on the power pump pulses in conventional direct-detection BOTDA sensors [7], [8], which linearly broadens the BGS linewidth with the gain. Furthermore, we compare both methods, direct-detection BOTDA and coherent BOTDA, in terms of the amount of spectrum shape change with increasing Brillouin gain.…”

“…Then, changes of the Brillouin frequency shift (BFS) are converted to changes of the detected probe power by the slope of the Brillouin gain spectrum (BGS) [1], [2], [6]. However, recently, a previously unknown effect in stimulated Brillouin scattering (SBS) interaction has been discovered, by which the linewidth of the BGS has been found to depend on the pump pulse power [7], [8]. This unexpected finding means that BFS measurement errors can appear in techniques such as the slope-assisted BOTDA that rely on the shape of the BGS [9].…”