Single-Frequency Fiber Lasers by Zhongmin Yang & Can Li & Shanhui Xu & Changsheng Yang

Author:Zhongmin Yang & Can Li & Shanhui Xu & Changsheng Yang
Language: eng
Format: epub
ISBN: 9789811360800
Publisher: Springer Singapore

4.2 Raman and Brillouin Fiber Lasers

In principle, the Raman and Brillouin scattering effects inside the optical fiber can both be utilized to achieve single-frequency lasers, given that the nonlinear gain bandwidth or the associated filter bandwidth is narrow enough to support only one longitudinal mode to survive. In this way, the main difference between the nonlinear fiber laser and the RE ions doped fiber laser is the origin of the optical gain, while the configuration of the laser cavity should be similar. By constructing a Raman fiber laser (RFL), one can achieve single-frequency lasing at wavelengths well beyond the gain bandwidth of RE ions, depending only on the availability of appropriate pump sources. However, since the gain provided by the SRS effect is quite small, the typical cavity length required for efficient operation of the RFLs is at least 100 m, and it is then difficult to realize single-longitudinal mode oscillation [3, 4]. On the contrary, the SBS gain is much larger than that of the SRS, while the bandwidth of the SBS gain is generally several tens of megahertz and is exceedingly smaller than that of the SRS gain (>10 terahertz). In addition, the dynamic process of the SBS has the advantages of a low-pass filter which can reduce the intensity and frequency noise of the pump laser, for instance, the linewidth of the generated Stokes signal is typically several orders of magnitude narrower than that of the pump source. In view of this, Brillouin fiber laser (BFL) has drawn much attention thanks to its merit of realizing low noise narrow linewidth single-frequency laser sources since its first demonstration [5].

To date, both linear and ring cavity configuration were examined for the BFLs. Generally, the linear BFL employs FBG or optical circulator to make up a F-P cavity which allows the Brillouin to pump light and the SBS light to oscillate inside the resonator. However, it suffers from the problems such as the low output power and the generation of high-order Stokes and anti-Stokes light compared with the ring cavity BFLs, which also shows a comparatively simplicity [6, 7]. Figure 4.1 shows a typical structure of the ring cavity BFL, in which only an optical fiber coupler is used to launch the pump laser and couple a part of the light out of the ring cavity. The lasing mechanism is that the pump light reflects by a co-propagating index grating that induced by the thermally excited acoustic wave in the fiber and the produced counterpropagating Stokes light is correspondingly frequency downshifted according to the Doppler effect.

Fig. 4.1Typical structure of the ring cavity BFL

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